U.S. patent application number 11/841941 was filed with the patent office on 2008-09-18 for methods and apparatus for spray forming, atomization and heat transfer.
Invention is credited to Helmut Gerhard Conrad, Wayne Conrad, Robin M. Forbes Jones, Richard L. Kennedy, Andrew Richard Henry Phillips, Richard Stanley Phillips, Ted Szylowiec.
Application Number | 20080223174 11/841941 |
Document ID | / |
Family ID | 22789643 |
Filed Date | 2008-09-18 |
United States Patent
Application |
20080223174 |
Kind Code |
A1 |
Forbes Jones; Robin M. ; et
al. |
September 18, 2008 |
METHODS AND APPARATUS FOR SPRAY FORMING, ATOMIZATION AND HEAT
TRANSFER
Abstract
The present invention is directed to methods and apparatus that
use electrostatic and/or electromagnetic fields to enhance the
process of spray forming preforms or powders. The present invention
also describes methods and apparatus for atomization and heat
transfer with non-equilibrium plasmas. The present invention is
also directed to articles, particularly for use in gas turbine
engines, produced by the methods of the invention.
Inventors: |
Forbes Jones; Robin M.;
(Charlotte, NC) ; Kennedy; Richard L.; (Monroe,
NC) ; Conrad; Helmut Gerhard; (Oshawa, CA) ;
Szylowiec; Ted; (Hampton, CA) ; Conrad; Wayne;
(Hampton, CA) ; Phillips; Richard Stanley;
(Courtice, CA) ; Phillips; Andrew Richard Henry;
(Oshawa, CA) |
Correspondence
Address: |
WILMERHALE/DC
1875 PENNSYLVANIA AVE., NW
WASHINGTON
DC
20004
US
|
Family ID: |
22789643 |
Appl. No.: |
11/841941 |
Filed: |
August 20, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10913361 |
Aug 9, 2004 |
7374598 |
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11841941 |
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09882248 |
Jun 18, 2001 |
6772961 |
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10913361 |
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60212122 |
Jun 16, 2000 |
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Current U.S.
Class: |
75/336 ;
219/121.36 |
Current CPC
Class: |
B22F 2999/00 20130101;
B22F 2999/00 20130101; C23C 4/123 20160101; B01J 2/02 20130101;
B22F 9/082 20130101; B22F 3/115 20130101; B22F 2202/05 20130101;
B22F 2202/06 20130101; B22F 9/082 20130101; B22F 2202/06 20130101;
B22F 2202/05 20130101; B22F 3/115 20130101; B22F 2202/13 20130101;
B22F 2999/00 20130101; B22F 2009/0892 20130101 |
Class at
Publication: |
75/336 ;
219/121.36 |
International
Class: |
B22F 9/14 20060101
B22F009/14 |
Claims
1. A method for atomizing a particle comprising: producing a first
molten particle; applying a rapid electrostatic charge to the first
molten particle, wherein the rapid electrostatic charge causes the
first molten particle to form at least one smaller second molten
particle; and cooling the second molten particle by producing a
non-equilibrium plasma that transfers heat from the second molten
particle to a first heat sink, wherein the first heat sink is
electrically charged or held at a potential.
2. The method of claim 1, wherein the first molten particle is
produced by melting a material in a melt chamber, and expelling the
first molten particle through at least one orifice in the melt
chamber.
3. The method of claim 1, wherein the rapid electrostatic charge is
an arc discharge or an electron beam.
4. The method of claim 1, wherein the non-equilibrium plasma is a
glow discharge or a cold corona discharge.
5. An apparatus for transferring heat between a first heat transfer
device and a workpiece comprising: the first heat transfer device,
where the first heat transfer device is electrically charged or
held at a potential, and wherein the first heat transfer device
comprises a heat sink or a heat source; the workpiece, wherein the
workpiece is mechanically separate from the first heat transfer
device; and a means for transferring heat between workpiece and the
first heat transfer device comprising a means for generating a
non-equilibrium plasma.
6. The apparatus of claim 5, wherein the non-equilibrium plasma is
a glow discharge or a cold corona discharge.
7. The apparatus of claim 5, further comprising an external means
for generating or maintaining the non-equilibrium plasma.
8. The apparatus of claim 7, wherein the external means for
generating or maintaining the non-equilibrium plasma is a
thermionic emission, an RF electromagnetic radiation, an
electromagnetic radiation, a magnetic field or an electron
beam.
9. The apparatus of claim 5, wherein the first heat transfer device
comprises a plurality of heat-transfer devices.
10. The apparatus of claim 5, further comprising a second
heat-transfer device that is mechanically and electrically separate
from the first heat-transfer device, wherein the second
heat-transfer device comprises a heat sink or a heat source, and
wherein the potential between the first heat-transfer device and
the second heat-transfer device produces a non-equilibrium
plasma.
11. A method for transferring heat between a first heat-transfer
device and a workpiece comprising producing a non-equilibrium
plasma that transfers heat between the first heat-transfer device
and the workpiece, wherein the first heat-transfer device is
electrically charged or held at a potential, wherein the first
heat-transfer device is mechanically separate from the workpiece,
and wherein the first heat-transfer device comprises a heat sink or
a heat source.
12. The method of claim 11, wherein the non-equilibrium plasma
comprises a glow discharge or a cold corona discharge.
13. The method of claim 11, further comprising an external means
for producing or maintaining the non-equilibrium plasma.
14. The method of claim 11, wherein the external means for
producing or maintaining the non-equilibrium plasma comprises a
thermionic emission, an RF electromagnetic radiation, an
electromagnetic radiation, a magnetic field or an electron
beam.
15. A preform produced by the method of one of claims 11-14.
16. The preform of claim 15 which is a near net preform.
17. An article of manufacture produced by the method of one of
claims 11-14.
18. The article of claim 17 which is a component of a gas turbine
engine.
Description
RELATED APPLICATIONS
[0001] This application claims priority to and is a divisional
application of U.S. patent application Ser. No. 09/882,248, filed
Jun. 18, 2001, which claims priority to U.S. Provisional
Application No. 60/212,122, filed Jun. 16, 2000, which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention is directed to methods and apparatus
that use electrostatic and/or electromagnetic fields to enhance the
process of spray forming preforms or powders. The present invention
also describes methods and apparatus for heat transfer using
non-equilibrium plasmas and for atomization.
BACKGROUND OF THE INVENTION
[0003] Spray forming is a process by which a stream of molten metal
is atomized by a gas stream impinging upon it. The resulting
atomized droplets are then directed to a target by the gas stream,
or the resulting atomized droplets are cooled to form a powder.
Producing powders by typical prior spray forming methods results in
a yield loss of 10-15%, and much of the loss is associated with
powder being trapped in various areas of the apparatus rather than
being delivered to the collection vessel during the process. In
producing solid workpieces, known as preforms, typical prior spray
forming methods result in a yield loss of 25-40%, and a significant
portion of the loss is usually caused by over-spray and particles
bouncing off the surface due to their angular impact relative to
the normal of the preform surface. Various methods have been
described to recover and reuse overspray powder, such as, for
example, U.S. Pat. No. 5,649,993, but these are not wholly
satisfactory.
[0004] Because many powders and preforms are susceptible to damage
to their chemical structure by air and oxygen, they are often
produced in a shield gas environment of nitrogen or argon. The flow
of shield gas, however, must be turned off to allow workers to
enter the chamber for cleanup, changeover and maintenance. Thus,
any powder or preform remaining in the chamber becomes contaminated
and unusable when air and oxygen enter the spray forming apparatus
after the flow of shield gas is turned off.
[0005] Previously, gas streams or jets have been used to direct the
path of the particles involved in the spray forming process. The
gas streams typically consist of argon or nitrogen as the means of
directing the particles, and heat is removed from the workpiece
through conduction or convection.
[0006] Current processes for making powder metal products,
particularly in materials used for critical aerospace applications,
use a conventional gas atomizing process. In this process,
high-pressure gas is directed at a molten metal stream to break it
into smaller droplets. The droplets solidify as powder. For
critical applications, the resultant powder is then blended with
batches of powder from other small melts. The blend is screened to
a small mesh size (325 mesh), canned and consolidated by extrusion
into product suitable for manufacture into an aircraft component.
This method of manufacture is not efficient because several small
melts are required for blending, melts are made in conventional
ceramic lined furnaces and hence result in oxide contamination,
several powder handling operations offer opportunity for
contamination, and many steps in the process make the production
operation costly.
[0007] Heat transfer using non-equilibrium plasmas has heretofore
been poorly understood and often incorrectly or inefficiently
applied. There is a need in the art for methods and apparatus that
improve the yield and quality of powders and preforms produced by
spray forming. The present invention is directed to these, as well
as other, important ends.
SUMMARY OF THE INVENTION
[0008] The present invention overcomes the limitations of the
conventional powder process by permitting a significantly larger
melt to be manufactured to powder, thereby eliminating the blending
steps. They also are melted and atomized in a ceramicless system,
thereby minimizing the contamination from the furnace linings. They
are atomized in vacuum, thereby eliminating the need for screening
and handling. They can either be containerized and sealed in a
vacuum or rapidly solidified to form a solid preform in vacuum,
thereby eliminating sources of handling and hence possible
contamination. Finally, the present invention will have
considerably fewer handling steps than conventional powder making,
and thus will be more cost effective.
[0009] In one embodiment, the present invention describes apparatus
comprising dispensing means, collecting means, and means for
directing molten particles from the dispensing means to the
collecting means comprising an electrostatic field and/or an
electromagnetic field. Optionally, the apparatus may further
comprise atomization apparatus and/or non-equilibrium heat transfer
apparatus.
[0010] In another embodiment, the present invention describes spray
forming methods comprising directing molten particles from
dispensing means to collecting means by producing an electrostatic
field and/or electromagnetic field between the dispensing means and
the collecting means. Optionally, the apparatus may further
comprise atomization apparatus and/or non-equilibrium heat transfer
apparatus.
[0011] In another embodiment, the present invention is directed to
apparatus comprising a melt chamber that comprises at least one
orifice; a means for expelling a molten material through the at
least one orifice in the melt chamber; and a means for applying a
rapid electrostatic charge to the molten material. Preferably, the
means for forcing the molten material through the at least one
orifice in the melt chamber is a mechanical or electromechanical
actuator or a pressure means. In a preferred embodiment, the
apparatus further comprises a means for cooling the molten
particle. Preferably, the means for cooling the molten particle
comprises a means for generating a non-equilibrium plasma.
[0012] In another embodiment, the present invention describes
methods for forming particles comprising producing a first molten
particle; and applying a rapid electrostatic charge to the first
molten particle, wherein the rapid electrostatic charge causes the
first molten particle to form at least one smaller second particle.
Preferably, the first molten particle is expelled through at least
one orifice in the melt chamber via mechanical means or by a
pressure means. In a preferred embodiment, the at least one smaller
second molten particle is cooled, preferably by a non-equilibrium
plasma.
[0013] In another embodiment, the present invention is directed to
apparatus for transferring heat between a beat-transfer device and
a workpiece comprising the heat-transfer device, wherein the
heat-transfer device is electrically charged or held at a
potential; the workpiece, wherein the workpiece is mechanically
separate from the heat-transfer device; and means for transferring
heat between the workpiece and the heat-transfer device comprising
a means for generating a non-equilibrium plasma. The heat-transfer
device can be either a heat sink or a heat source.
[0014] In yet another embodiment, the present invention is directed
to methods of transferring heat between a heat-transfer device and
a workpiece comprising producing a non-equilibrium plasma capable
of transferring heat between the heat-transfer device and the
workpiece, wherein the heat-transfer device is electrically charged
or held at a potential, and wherein the heat-transfer device is
mechanically separate from the workpiece. The heat-transfer device
can be either a heat sink or a heat source.
[0015] Accordingly, in various embodiments, non-equilibrium plasmas
are advantageously employed to effect optimal heat transfer, and
the non-equilibrium plasma must act with a heat sink/source that
has a thermal conductivity capable of removing the desired quantity
of heat. While two or more electrodes have been used in the past to
produce a plasma in a region of high heat, such as a weld zone, so
that the plasma would serve to conduct heat outward from the weld
zone, thereby increasing the surface area for heat, embodiments of
the present invention are directed to the discovery that a
non-equilibrium plasma may be used to introduce heat into a
workpiece as well as from a workpiece. It has further been
surprisingly discovered that under the correct conditions a
non-equilibrium plasma can be used to efficiently transfer heat in
a vacuum.
[0016] The novel methods of the present invention are particularly
useful in preparing any metal article, such as articles for gas
turbine engines, including, for example, airfoils, blades, discs
and blisks.
[0017] Accordingly, in one aspect, there is provided according to
the present invention an apparatus comprising: a dispensing means;
a collecting means; and a means for directing a molten particle
from the dispensing means to the collecting means comprising at
least one of an electrostatic field or an electromagnetic field. In
another aspect is provided the apparatus described above, wherein
the means for directing the molten particles from the dispensing
means to the collecting means comprises an electrostatic field or
an electromagnetic field. The apparatus may further comprise at
least one magnetic coil, and may also further comprise a means for
charging the molten particles. In one embodiment, the means for
charging the molten particles may comprise a thermionic emission
source or a tribocharging device. The dispensing means of the
apparatus may be a gas atomizer, and may further comprise a means
for transferring heat from the molten particles. The means for
transferring heat from the molten particles may comprise gas
conduction and/or convection and/or a non-equilibrium plasma.
[0018] In another aspect, there is provided according to the
present invention an apparatus comprising: a dispensing means; a
collecting means; and a means for directing a molten particle from
the dispensing means to the collecting means comprising at least
one of an electrostatic field or an electromagnetic field, and
further comprising a means for transferring heat from the
collecting means. The means for transferring heat from the
collecting means may comprise a means for generating a
non-equilibrium plasma. In a particular aspect, the means for
transferring heat from the molten particles comprises a first heat
sink, wherein the first heat sink is electrically charged or held
at a potential; and a means for transferring heat from the molten
particles to the first heat sink comprising a means for generating
a non-equilibrium plasma. The non-equilibrium plasma may be a glow
discharge or a cold corona discharge.
[0019] In another aspect, there is provided according to the
present invention an apparatus comprising: a dispensing means; a
collecting means; and a means for directing a molten particle from
the dispensing means to the collecting means comprising at least
one of an electrostatic field or an electromagnetic field, and
further comprising a means for expelling the molten particle
through at least one orifice in the dispensing means; and a means
for applying a rapid electrostatic charge to the molten material.
The means for expelling the molten particle through the at least
one orifice may comprise a mechanical or electromechanical
actuator. In one aspect, the means for expelling the molten
particle through the at least one orifice may be a pressure means
that produces a pressure in the dispensing means that is greater
than the pressure on the outside of the dispensing means. The
pressure means may cause interrupted flow of the molten particle
from the dispensing means. The rapid electrostatic charge may be an
arc discharge or an electron beam.
[0020] In another aspect, the present invention provides for a
spray forming method comprising directing molten particles from a
dispensing means to a collecting means by producing at least one of
an electrostatic field or an electromagnetic field between the
dispensing means and the collecting means. The electromagnetic
field may be produced by, for example, means comprising at least
one magnetic coil. The method according to this aspect of invention
may further comprise charging the molten particles. Charging the
molten particles may be accomplished, for example, using a
thermionic emission source or a tribocharging device. In one
aspect, the dispensing means may be a gas atomizer. According to
this aspect of the invention, the method may further comprise
transferring heat from the molten particle. Transferring heat from
the molten particles may be accomplished, for example, by gas
conduction and/or convection and/or non-equilibrium plasma. In
another aspect, the method of the invention further comprises
producing a second electromagnetic field. According to the
invention, the method may further comprise transferring heat from
the collecting means, which may be by a non-equilibrium plasma.
[0021] In another aspect, the present invention provides for a
spray forming method comprising directing molten particles from a
dispensing means to a collecting means by producing at least one of
an electrostatic field or an electromagnetic field between the
dispensing means and the collecting means, further comprising
applying a rapid electrostatic charge to the molten particle,
wherein the rapid electrostatic charge causes the molten particle
to form at least one smaller molten particle. In a particular
aspect, the rapid electrostatic charge may be an arc discharge or
an electron beam. In another aspect, the method of the invention
may further comprise transferring heat from the molten particle
comprising producing a non-equilibrium plasma that transfers heat
from the molten particle to a first heat sink, wherein the first
heat sink is electrically charged or held at a potential. The
non-equilibrium plasma may be a glow discharge or a cold corona
discharge.
[0022] In another aspect, the invention is directed to an apparatus
comprising a melt chamber comprising at least one orifice; a means
for forcing a molten material through the at least one orifice in
the melt chamber; and a means for applying a rapid electrostatic
charge to the molten material. The rapid electrostatic charge may
be an arc discharge or en electron beam. The apparatus of the
invention may further comprise a means for cooling the molten
material. In a particular aspect, the means for cooling the molten
material may comprise a first heat sink, wherein the first heat
sink is electrically charged or held at a potential; and a means
for transferring heat from the molten material to the first heat
sink comprising a means for generating a non-equilibrium plasma.
The non-equilibrium plasma may be a glow discharge or a cold corona
discharge.
[0023] In another aspect, there is provided a method for atomizing
a particle comprising producing a first molten particle, applying a
rapid electrostatic charge to the first molten particle, wherein
the rapid electrostatic charge causes the first molten particle to
form at least one smaller second molten particle. According to the
method of the invention, the first molten particle may be produced
by melting a material in a melt chamber, and expelling the first
molten particle through at least one orifice in the melt chamber.
The rapid electrostatic charge may be an arc discharge or en
electron beam. The method of the invention may further comprise
cooling the second molten particle by producing a non-equilibrium
plasma that transfers heat from the second molten particle to a
first heat sink, wherein the first heat sink is electrically
charged or held at a potential. The non-equilibrium plasma may be a
glow discharge or a cold corona discharge.
[0024] In another aspect, the invention provides for an apparatus
for transferring heat between a first heat-transfer device and a
workpiece comprising a first heat-transfer device, wherein the
first heat-transfer device is electrically charged or held at a
potential, and wherein the first heat-transfer device is a heat
sink or a heat source; a workpiece, wherein the workpiece is
mechanically separate from the first heat-transfer device; and
means for transferring heat between the workpiece and the first
heat-transfer device comprising a means for generating a
non-equilibrium plasma. The non-equilibrium plasma may be a glow
discharge or a cold corona discharge. The apparatus of the
invention may further comprise an external means for generating or
maintaining the non-equilibrium plasma. The external means for
generating or maintaining the non-equilibrium plasma may be a
thermionic emission, an RF electromagnetic radiation, an
electromagnetic radiation, a magnetic field or an electron beam.
The first heat-transfer device of the apparatus of the invention
may comprise a plurality of heat-transfer devices. In a particular
aspect, the apparatus of the invention may further comprise a
second heat-transfer device that may be mechanically and
electrically separate from the first heat-transfer device, wherein
the second heat-transfer device is a heat sink or a heat source,
and wherein the potential between the first heat-transfer device
and the second heat-transfer device produces a non-equilibrium
plasma.
[0025] In another aspect is provided a method for transferring heat
between a first heat-transfer device and a workpiece comprising
producing a non-equilibrium plasma that transfers heat between the
first heat-transfer device and the workpiece, wherein the first
heat-transfer device is electrically charged or hd at a potential,
wherein the first heat-transfer device is mechanically separate
from the workpiece, and wherein the first heat-transfer device is a
heat sink or a heat source. The non-equilibrium plasma may be a
glow discharge or a cold corona discharge. The method may further
comprise generating or maintaining the non-equilibrium plasma via
an external means. In an aspect, the external means for generating
or maintaining the non-equilibrium plasma comprises a thermionic
emission, an RF electromagnetic radiation, an electromagnetic
radiation, a magnetic field or an electron beam.
[0026] In another aspect, the invention provides for a preform
produced by the methods of the invention. The preform of the
invention may be a near net preform. There is also provided an
article of manufacture produced by the method of the invention.
[0027] These and other aspects of the present invention will become
more apparent from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a view of an apparatus of the present invention
wherein an electrostatic field directs and accelerates molten
particles to a preform.
[0029] FIG. 2 is a view of an apparatus of the present invention
wherein an electrostatic field directs and accelerates particles to
produce a near net shape preform, and a non-equilibrium plasma
controls the temperature of the molten particles.
[0030] FIG. 3 is a view of an apparatus of the present invention
wherein an electromagnetic field directs and accelerates molten
particles to a preform, a first non-equilibrium plasma controls the
temperature of the molten particles, and a second non-equilibrium
plasma controls the temperature of the preform.
[0031] FIG. 4 is a view of an apparatus of the present invention
wherein an electromagnetic field directs and accelerates molten
particles to control particle collisions and resultant particle
growth, and a non-equilibrium plasma cools the molten particles to
form a powder.
[0032] FIG. 5 is a graph showing deflection versus applied voltage
for a molten tin particle, as described in Example 2.
[0033] FIG. 6 is a view of a non-equilibrium plasma heat transfer
apparatus wherein the heat-transfer device and the electrode
producing the non-equilibrium plasma are a single element and a
dielectric fluid is used to transfer heat from the heat-transfer
device to a large thermal mass.
[0034] FIG. 7 is a view of a non-equilibrium plasma heat transfer
apparatus wherein the heat-transfer device and the electrode
producing the non-equilibrium plasma are a single element and the
heat-transfer device is coupled to a large thermal mass via a heat
pipe.
[0035] FIG. 8 is a view of a non-equilibrium plasma heat transfer
apparatus that can be used to cool powders or small workpieces
(e.g., molten particles or preforms) in a vacuum.
[0036] FIG. 9 is a view of a non-equilibrium plasma heat transfer
apparatus wherein the heat-transfer device and the electrode
producing the non-equilibrium plasma are separate elements.
[0037] FIG. 10 is a view of an apparatus wherein a vacuum and
pressure chamber serves as the dispensing means (e.g., melt
chamber) for a molten material, pulsed pressure in the head space
above the molten material produces molten particles through a
plurality of nozzles at the base of the dispensing means, and rapid
electrostatic charging is applied as the molten particles exit the
nozzles to produce smaller molten particles.
[0038] FIG. 11 is a view of an apparatus wherein a flow control rod
in a dispensing means (e.g., melt chamber) is manipulated to
produce molten particles, and rapid electrostatic charging is
applied as the molten particles exit the nozzle to produce smaller
molten particles.
[0039] FIG. 12 is a view of multiple electrostatically induced
atomizations wherein a droplet is atomized to form a plurality of
smaller droplets which are further atomized to form a plurality of
still smaller droplets. FIG. 14 is a photograph of the droplets
shown in FIG. 12.
[0040] FIG. 13 is a graph showing how primary atomized droplets are
not sensitive to high voltage levels or electrode gaps once a
critical value is reached for a particular geometry.
[0041] FIG. 14 is a photograph showing primary, secondary and
tertiary droplets produced from the experiment in Example 5. FIG.
14 is a photograph of the droplets schematically drawn in FIG.
12.
[0042] FIG. 15 shows an apparatus used for liquid metal flow
against the direction of gravity.
[0043] FIG. 15A is a schematic illustration of FIG. 15.
[0044] FIGS. 16 and 17 show drops and droplets collected from an
exemplary series of experiments described in Example 4. For each
figure, the larger drops (upper portion of the figure) are those
collected during the control experiments, and the smaller droplets
(lower portion of the figure) are those collected during
experiments using an electrostatic field according to the
invention.
[0045] FIGS. 18-25 show various views of a section of CPVC pipe,
placed in the assembly of the apparatus of the invention in such a
way as to surround the extractor ring and its supporting arm,
permitting substantially higher potential differences between
nozzle and extractor before arcing and voltage breakdown. FIGS. 18,
19 and 20 show consecutive frames of the atomization of liquid
metal against gravity without any applied mechanical force other
than that due to the head of liquid in the reservoir. FIG. 18A is a
schematic illustration of FIG. 18.
[0046] FIG. 26 shows twin electrode melting as the source for the
molten metal for electrostatic atomizing.
[0047] FIG. 27 shows electron beam melting as the source for the
molten metal for electrostatic atomizing in vacuum.
[0048] FIG. 28 shows electron beam cold hearth melting as the
source for molten metal for electrostatic atomizing in vacuum.
[0049] FIG. 29 shows ESR/CIG melting as the source for the molten
metal for electrostatic atomizing in vacuum.
[0050] FIG. 30 shows the atomized powder being collected in the
bottom of the atomizing chamber.
[0051] FIG. 31 shows electrostatically atomized powder being
collected as a solid preform after the powder is cooled via a
non-equilibrium plasma.
[0052] FIG. 32 shows electrostatically atomized powder being
collected in a can, where the can is transferred into a smaller
chamber without breaking the vacuum. In the smaller chamber, the
lid may welded to the can prior to hot working to a final
product.
[0053] FIG. 33 shows the production of a solid ingot in a mold from
a powder produced by electrostatic atomization.
[0054] FIG. 34 shows three stages of electrostatic atomizing using
plasma and one stage of electrostatic steering of the atomized
powder.
[0055] FIG. 35 is a schematic diagram of the experimental set-up
described in Example 5 for heat transfer using non-equilibrium
plasmas.
[0056] FIG. 36 is an enlarged schematic diagram showing the
dimensions of Blocks A and B described in FIG. 35.
[0057] FIG. 37 is a graph showing the temperature decay in air from
Block A with and without the non-equilibrium plasma in atmospheric
pressure, where the gap between the blocks was 1.5 inches, and the
voltage applied for the non-equilibrium plasmas was 51 keV, and
Block A was in -ve potential.
[0058] FIG. 38 is a graph showing the temperature decay in air from
Block A with and without the non-equilibrium plasma at a pressure
of 10.sup.-1 Torr, where the gap between the blocks was 1.5 inches,
and the voltage applied for the non-equilibrium plasmas was 0.7 keV
with a current maintained at 20 mA, and Block A was in -ve
potential.
[0059] FIG. 39 is a graph showing the temperature decay in air from
Block A with the non-equilibrium plasma (changing polarity of Block
A) and without the non-equilibrium plasma at a pressure of
10.sup.-1 Torr, where the gap between the blocks was 1.5 inches,
and the voltage applied for the non-equilibrium plasmas was 0.6 and
0.8 keV with a current maintained at 20 mA.
[0060] FIG. 40 is a graph showing the temperature decay in air from
Block A with and without the non-equilibrium plasma at a pressure
of 10.sup.-1 Torr, where the gap between the blocks was 4 inches,
and the voltage applied for the non-equilibrium plasmas was about
0.7 keV with a current maintained at 20 mA, where Block A was at a
-ve potential.
[0061] FIG. 41 is a graph showing the temperature decay in argon
from Block A with and without the non-equilibrium plasma at a
pressure of 10.sup.-1 Torr, where the gap between the blocks was 4
inches, and the voltage applied for the non-equilibrium plasmas was
0.6 to 0.9 keV with a current maintained at 20 mA.
[0062] FIG. 42 is a graph showing the temperature decay in helium
from Block A with and without the non-equilibrium plasma at a
pressure of 10.sup.-1 Torr, where the gap between the blocks was 4
inches, and the voltage applied for the non-equilibrium plasmas was
0.6 to 0.7 keV with a current maintained at 20 mA, where Block A
was at a -ve potential.
[0063] FIG. 43 is a graph showing the temperature decay in air from
Block A at various current with the non-equilibrium plasma at a
pressure of 10.sup.-1 Torr, where the gap between the blocks was 4
inches, and the voltage applied for the non-equilibrium plasmas was
0.5 to 1 keV with a current at 10 mA, 15 mA or 20 mA, where Block A
was at a -ve potential.
[0064] FIG. 44 is a graph showing the temperature decay in air from
Block A with and without the non-equilibrium plasma at a pressure
of 10.sup.-2 Torr, where the gap between the blocks was 4 inches,
and the voltage applied for the non-equilibrium plasmas was 1.2 to
1.6 keV with a current at 20 mA, where Block A was at a -ve
potential.
[0065] FIGS. 45A-B are graphs showing the argon control and plasma
experimental data and numerical simulation results (p.about.1E-1
Torr) for the modeling of the experimental data set presented in
FIG. 39. Control and non-equilibrium plasma curves are separated
into two graphs to make the curve fit presentation clearer. FIG.
45A shows argon without plasma and shows the experimental and model
results. FIG. 45B shows argon in the presence of a non-equilibrium
plasma and shows the experimental and model results. The .gamma.
factor necessary to relate the two model curves is
.gamma.=10.5.
[0066] FIGS. 46A-B are schematic drawings of nozzle and extractor
ring by a side view (FIG. 46A) and a view looking up through the
extractor ring toward the nozzle (FIG. 46B).
[0067] FIG. 47 shows the profiles of electric field pendent drops,
where the electric field increases from left to right.
[0068] FIG. 48 is a graph wherein the line with squares shows the
limiting charge according to the Rayleigh Criterion, and the line
with circles shows the calculated charge applied to a primary drop
using measured voltage and the geometry of the drop. Though the
graph shows that the primary drop should have been atomized into 4
to 6 times, some charge may have escaped to the environment or with
the secondary droplets.
[0069] FIG. 49 is a schematic diagram of the experimental set-up
described in Example 6 for heat transfer using non-equilibrium
plasmas.
[0070] FIG. 50 is an enlarged schematic diagram showing the
dimensions of Blocks A and B described in FIG. 49.
[0071] FIG. 51 is a showing the temperature decay in air from Block
A with and without the non-equilibrium plasma in atmospheric
pressure, where the gap between the blocks was 1.5 inches, and the
voltage applied for the non-equilibrium plasmas was 51 keV, and
Block A was in -ve potential.
[0072] FIG. 52 is a graph showing the temperature decay in air from
Block A with and without the non-equilibrium plasma at a pressure
of 10.sup.-1 Torr, where the gap between the blocks was 1.5 inches,
and the voltage applied for the non-equilibrium plasmas was 0.7 keV
with a current maintained at 20 mA, and Block A was in -ve
potential.
[0073] FIG. 53 is a graph showing the temperature decay in air from
Block A with the non-equilibrium plasma (changing polarity of Block
A) and without the non-equilibrium plasma at a pressure of
10.sup.-1 Torr, where the gap between the blocks was 1.5 inches,
and the voltage applied for the non-equilibrium plasmas was 0.6 and
0.8 keV with a current maintained at 20 mA.
[0074] FIG. 54 is a graph showing the temperature decay in air from
Block A with and without the non-equilibrium plasma at a pressure
of 10.sup.-1 Torr, where the gap between the blocks was 4 inches,
and the voltage applied for the non-equilibrium plasmas was about
0.7 keV with a current maintained at 20 mA, where Block A was at a
-ve potential.
[0075] FIG. 55 is a graph showing the temperature decay in argon
from Block A with and without the non-equilibrium plasma at a
pressure of 10.sup.-1 Torr, where the gap between the blocks was 4
inches, and the voltage applied for the non-equilibrium plasmas was
0.6 to 0.9 keV with a current maintained at 20 mA.
[0076] FIG. 56 is a graph showing the temperature decay in helium
from Block A with and without the non-equilibrium plasma at a
pressure of 10.sup.-1 Torr, where the gap between the blocks was 4
inches, and the voltage applied for the non-equilibrium plasmas was
0.6 to 0.7 keV with a current maintained at 20 mA, where Block A
was at a -ve potential.
[0077] FIG. 57 is a graph showing the temperature decay in air
block-A in plasma and without plasma at pressure 10.sup.-2 Torr,
gap between blocks was 4', and the voltage applied for plasma: 1:2
to 1.5 kev, current 20 mA.
[0078] FIG. 58 is a graph showing the results for the modeling of
the experimental data set presented in FIG. 53 relating to argon
and without plasma.
[0079] FIG. 59 is a graph showing the results for the modeling of
the experimental data set resented in FIG. 53 relating to argon and
with plasma.
[0080] FIG. 60 is a graph showing the comparison of withl/without
plasma for (Gamma)=10 in Example 7.
[0081] FIG. 61 is a graph showing how primary atomized droplets are
not sensitive to high voltage levels or electrode gaps once a
critical value is reached for a particular geometry.
[0082] FIG. 62 is a photograph showing primary, secondary and
tertiary droplets produced from the experiment in Example 8.
[0083] FIGS. 63-64 are tables illustrating two sets of experimental
data for liquid wood's metal atomization.
[0084] FIGS. 65-70 and FIGS. 74-76 are pictures illustrating a
piece of CPVD pipe placed in such a way as to surround the
extractor ring and its supporting arm.
[0085] FIGS. 71-72 are pictures illustrating drops and droplets
collected in example 8.
[0086] FIG. 73 is a picture illustrating an apparatus used for
example 8.
DETAILED DESCRIPTION OF THE INVENTION
[0087] The present invention provides methods and apparatus for
enhancing spray forming for the production of solid workpieces,
known as preforms, and for powders. It has surprisingly been found
that the amount or yield of powder collected during spray forming
can be controlled to an unexpectedly high degree by using an
electrostatic and/or electromagnetic field to direct the trajectory
of particles in the spray forming process. Additionally, the speed
and/or direction of the particles can be controlled to produce a
solid workpiece using an electrostatic and/or electromagnetic
field. Using an electrostatic and/or electromagnetic field, the
particles can be directed to various areas of the preform at
various times during the spray forming process to produce shapes.
Using an electrostatic and/or electromagnetic field, particle size
and trajectory can be controlled to avoid particle collisions, and
the resulting growth in particle size that occurs when particles
collide, or to cause particle collisions if larger particle size is
desired for any purpose. Using an electrostatic and/or
electromagnetic field, the particles can be directed to areas where
heat can be added or removed from the particles to control the
macrostructure of the preform or powder being produced. The shape
of the electrostatic and/or electromagnetic field can also be
manipulated to produce near net shapes by directing where particles
build up to form the preform at various times during the process.
Spray forming using an electrostatic field and/or an
electromagnetic field can enhance the yield of the process as well
as improve (and control) the density of the resulting preform.
[0088] The present invention describes methods and apparatus using
electrostatic fields and/or electromagnetic fields for selectively
controlling the yield, quality or density of solid workpieces
(preforms) and powders produced by spray forming. Surprisingly, the
methods and apparatus of the present invention have been
unexpectedly found to provide enhanced yields of 95-99%, and
unexpectedly provide workpieces that have a density that is 11-14%
greater than the density of conventionally-formed workpieces.
[0089] Preferably, the methods and apparatus of the present
invention comprise a source of molten particles; a means for
collecting the molten particles; and a means for directing the
molten particles from the source of molten particles to the means
for collecting the molten particles.
[0090] The molten particles can be metallic or non-metallic. The
term "metallic" includes metals and alloys, including, for example,
iron, cobalt, nickel, aluminum, hafnium, zinc, titanium, niobium,
zirconium, tin, copper, tungsten, molybdenum, tantalum, magnesium,
stainless steels, bronze, brass, lithium alloys and nickel/cobalt
based superalloys.
[0091] The source of molten particles may also be referred to
herein as a "dispensing means." The dispensing means can be any
known in the art including, for example, a container, an atomizer,
a grinder, or other means of producing and/or dispensing the molten
particles. The dispensing means is generally electrically
insulated. Preferably, the dispensing means is a gas atomizing
means. Any gas atomizing means known in the art may be used as the
dispensing means in the present invention.
[0092] The acceleration, speed and/or direction of the molten
particles can be manipulated and controlled by an electrostatic
field and/or an electromagnetic field. The term "electrostatic
field" can refer to a single electrostatic field or a plurality of
electrostatic fields. The term "electromagnetic field" can refer to
a single electromagnetic field or a plurality of electromagnetic
fields.
[0093] The means for collecting the molten particles may be
referred to herein as a "collecting means." Generally, the
collecting means is electrically insulated. For spray forming
powders, the collecting means can be a hopper or other container.
The container may comprise a lid and a mechanism for closing the
lid. The collecting means may have a geometric shape, including,
for example, a near net shape. Preferably, the distance between the
dispensing means and the collecting means is from about 10 cm to
about 250 cm, more preferably, from about 20 cm to about 100 cm,
and even more preferably, from about 25 cm to about 75 cm.
[0094] The invention may further comprise means for charging the
molten particles before and/or after they leave the dispensing
means. The means for charging the molten particles may comprise,
for example, a thermionic emission source, a tribocharging device,
or the like.
[0095] In one embodiment, an electrostatic field is produced
between the dispensing means and the collecting means by connecting
the collecting means to the positive or negative polarity of a
high-voltage DC power supply and by grounding the dispensing means.
Preferably a positive polarity is used. Generally, the high volt DC
power supply is between about 4 kV and about 250 kV; more
preferably, between about 8 kV and about 125 kV; and even more
preferably between about 12 kV and about 100 kV. The molten
particles may be induction charged by the electric field. The
induction charge causes the molten particles to move along the
electrostatic field lines, thereby controlling the speed and
direction of the molten particles and directing the molten
particles from the dispensing means to the collecting means.
[0096] In another embodiment, an electrostatic field is produced
between the dispensing means and the collecting means by connecting
the dispensing means to the positive or negative polarity of a high
voltage DC power supply and by grounding the collecting means.
Preferably, a positive polarity is used. By connecting the
dispensing means to the positive or negative polarity of a high
voltage DC power supply, the molten particles become electrically
charged. The electrostatic field causes the electrically charged
molten particles to move along the electrostatic field lines,
thereby controlling the speed and direction of the molten particles
and directing the molten particles from the dispensing means to the
collecting means.
[0097] The apparatus can further comprise a high voltage DC power
supply and one or more electrodes that are placed between the
dispensing means and the collecting means to shape the
electrostatic field between the dispensing means and the collecting
means. The electrostatic field then directs the molten particles to
the collecting means.
[0098] The apparatus can also comprise a plurality of high voltage
DC power supplies each attached to one or more electrodes that are
placed between the dispensing means and the collecting means that
change the shape of the electrostatic field between the dispensing
means and the collecting means in a time dependant manner to direct
the molten particles to specific areas or points on the collecting
means. This embodiment can produce near net shapes.
[0099] In another embodiment, an electromagnetic field is produced
between the dispensing means and the collecting means by placing a
magnetic coil between the dispensing means and the collecting
means. The magnetic coil is connected to a power supply. The molten
particles leaving the dispensing means are directed by the
electromagnetic field to the collecting means. Preferably, the
magnetic coil is capable of moving so that it can direct the molten
particles to specific areas or points on the collecting means. The
molten particles can be directed to produce, for example, near net
shapes.
[0100] In another embodiment, a plurality of magnetic coils can be
placed between the dispensing means and the collecting means. The
electromagnetic fields that are produced by the plurality of
magnetic coils, which are singly or multiply energized to different
magnetic field intensities, direct the molten particles to specific
areas or points on the collecting means. The molten particles can
be directed to produce, for example, near net shapes.
[0101] The embodiments of the invention presented in the following
figures are for purposes of illustration only, and are not intended
to limit the scope of the invention or the appended claims.
[0102] In FIG. 1, a dispensing means 201 produces molten particles
202, and an electrostatic field 203 is produced between the
dispensing means 201 and the collecting means 204. The
electrostatic field 203 charges the molten particles 202, which
then causes the molten particles 202 to accelerate toward the
collecting means 204. The acceleration causes the solid workpiece
(preform) 205 to build up on the collecting means 204 with a
minimum of over-spray and bounce-off, thereby enhancing the yield
of the process. The process can also enhance the density of the
resulting solid workpiece (preform) 205. As shown in FIG. 1, the
electric field is preferably intensified in the area where the
molten particles 202 leave the dispensing means 201. The inventors
have unexpectedly discovered that the electrostatic field is most
intense and compressed at the point just after the molten droplet
leaves the nozzle. Surprisingly, pulling a droplet apart works for
water, but does not work for liquid metal. To atomize a liquid
metal, the inventors have discovered compressing the liquid or
molten metal droplets.
[0103] In FIG. 2, a dispensing means 201 produces charged molten
particles 202, and an electrostatic field 203 is produced between
the dispensing means 201 and the shaped collecting means 206 to
accelerate the charged molten particles 202 toward the shaped
collecting means 206. The acceleration and directional control of
the charged molten particles 202 enhances the density of the solid
workpiece, and produces a near net shape solid workpiece 207.
Optionally, a non-equilibrium plasma 24 is created in the path of
the molten particles 202 between two heat sink electrodes 209 which
are connected to an outside thermal mass 210 by a dielectric liquid
which flows through pipes 211 by the motive force provided by pumps
212. The arrangement between the heat sink electrodes 209 and the
outside thermal mass 210 allows heat to be removed from the molten
particles 202. The non-equilibrium plasma 24 between the heat sinks
209 is produced, for example, by means of an AC glow discharge or a
corona discharge. The non-equilibrium plasma 24 transfers heat from
the molten particles 202 to the two heat sink electrodes 209 which
transfer the heat to the outside thermal mass 210.
[0104] In FIG. 3, a dispensing means 201 produces charged molten
particles 202, and an electromagnetic field 213 is produced by a
magnetic coil 214 which directs the molten particles 202 towards
the collecting means 204. This directional control of the molten
particles 202 can reduce over-spray, thereby enhancing the yield of
the spray forming process. The invention can also enhance the
density of the solid workpiece 205. Optionally, a non-equilibrium
plasma 24 is created in the path of the molten particles 202
between two heat sink electrodes 209 which are connected to an
outside thermal mass 210 by a dielectric liquid that flows through
pipes 211 by the motive force provided by pumps 212. The
arrangement between the heat sink electrodes 209 and the outside
thermal mass 210 allows heat to be removed from the molten
particles 202. The non-equilibrium plasma 24 between the heat sink
electrodes 209 is produced, for example, by means of an AC glow
discharge or a corona discharge. The non-equilibrium plasma 24
transfers heat to the outside thermal mass 210. The non-equilibrium
plasma 24 extends from the heat sink electrodes 209 into the path
of the molten particles 202 and to the electrically grounded solid
workpiece 205 and the collecting means 204. In this embodiment heat
is transferred from the molten particles 202, the solid workpiece
205 and the collecting means 204 by the non-equilibrium plasma 24
which allows heat to be transferred to the heat sink electrodes 209
which transfer the heat to the outside thermal mass 210.
[0105] In FIG. 4, a dispensing means 201 produces charged molten
particles 202, and an electromagnetic field 213 produced by a
magnetic coil 214 directs the molten particles 202 to spread out,
thereby reducing the probability of their collision, and hence the
formation of larger molten particles and larger powder particles.
Optionally, a non-equilibrium plasma 24 is created in the path of
the molten particles 202 between two heat sink electrodes 209 that
are connected to the outside thermal mass 210 by a dielectric fluid
which flows through pipes 211 by the motive force provided by pumps
212. The arrangement between the heat sink electrodes 209 and the
outside thermal mass 210 allows heat to be removed from the molten
particles 202. A second electromagnetic field 216 produced by a
magnetic coil 217 directs the cooled powder 218 to facilitate
collection in the container 219 which is automatically closed by a
mechanism 220 that attaches a lid 221 The entire powder
manufacturing process can be carried out in a full or partial
vacuum to reduce or eliminate contamination of the powder by
chemical interaction with gases.
[0106] The present invention may also optionally comprise a heat
sink placed between the dispensing means and the collecting means;
a means for transferring heat from the molten particles to the heat
sink to control the temperature of the molten particles once they
have been ejected from the dispensing means; and a means for
removing heat from the collecting means. The apparatus can comprise
a means for transferring heat from the molten particles to the heat
sink to control the temperature of the molten particles once they
have been ejected from the dispensing means. The means for
transferring heat can be gas conduction and/or convection. In
addition to or in place of gas conduction and/or convection,
another means for transferring heat can be a non-equilibrium
plasma.
[0107] The present invention also provides non-equilibrium plasmas
for transferring heat between a heat-transfer device and a
workpiece. In preferred embodiments, the non-equilibrium plasma is
used for removing heat from molten particles after they are
dispensed and/or electrostatically atomized, but before they are
collected either as a solid workpiece or as a powder.
[0108] A class of plasmas known as non-equilibrium (NE) plasmas is
produced when the temperature of the electrons in the gas exceeds
the temperature of the neutral particles and large ions in the gas
by at least 100%. Since the thermal conductivity of non-equilibrium
plasmas depends on the electron temperature, the non-equilibrium
plasmas will exhibit a high thermal conductivity. Since the
temperature of neutral particles and large ions, which account for
more than 99.9% of the mass present, is low, the overall heat
content of the non-equilibrium plasma is low. Non-equilibrium
plasmas used for heat transfer can be generated under very high and
very low pressure conditions using gases which are inert or benign
to the material(s) (e.g., the molten particles of the present
invention) involved in the heat transfer. Thus, non-equilibrium
plasmas can be used to add or remove heat from a workpiece without
the undesirable mechanical, thermal, or chemical effects associated
with plasmas in local thermal equilibrium.
[0109] The present invention also describes methods and apparatus
for heat transfer between a heat-transfer device and a workpiece
(e.g., molten particles and preforms) using non-equilibrium
plasmas. Non-equilibrium plasmas eliminate the need for mechanical
contact between the workpiece and the heat-transfer device. There
are many applications in which mechanical contact between the heat
sink and the workpiece is not physically possible without
undesirable damage to or chemical contamination of the workpiece,
including, for example, spray forming, casting and other processes
which use molten or non-solid substrate states.
[0110] In a preferred embodiment of the present invention, heat
transfer is accomplished using non-equilibrium plasmas wherein the
neutral and heavy ions have a temperature less than about 1000 K,
preferably less than about 800 K, and more preferably less than
about 600 K. Since non-equilibrium plasmas are produced when the
temperature of the electrons exceeds the temperature of the neutral
particles and large ions by at least 100%, the electrons preferably
have a corresponding temperature of at least about 100 K, more
preferably in excess of about 2000 K.
[0111] "Heat-transfer device," as used herein, refers to a heat
sink or a heat source. "Heat source" refers to the object that is
becoming colder, i.e., supplying the heat. "Heat sink" refers to
the object that is becoming warmer, i.e., accepting the heat. It
will be appreciated that the same object can function as a heat
source and as a heat sink, depending upon the temperature variation
in the other object, e.g., the workpiece, during the spray forming
process. Accordingly, by means of the invention it is possible to
closely control the cooling (and heating) rate of the workpiece as
a whole, as well as individual parts or sub-parts of the workpiece,
and thereby to control those properties of the workpiece or parts
thereof which are known to be affected by cooling or heating
rate.
[0112] In the present invention, the heat-transfer device or
heat-transfer device electrode can be electrically charged or held
at a potential. "Heat sink electrode" refers to the electrical
potential source and the heat sink when they are integrated into a
single object. "Heat source electrode" refers to the electrical
potential source and the heat source when they are integrated into
a single object. "Heat-transfer device electrode" is used to refer
to either a "heat sink electrode" or a "heat source electrode."
"Being held at a potential" refers to a DC offset voltage upon
which an AC waveform may be superimposed.
[0113] The distance between the heat-transfer device or
heat-transfer device electrode and the workpiece and the voltage
applied to the heat-transfer device or heat-transfer device
electrode and/or the workpiece is selected to create a
non-equilibrium plasma between the workpiece and the heat-transfer
device or heat-transfer device electrode to provide heat transfer.
The non-equilibrium plasma is in contact with the heat-transfer
device or heat-transfer device electrode and the workpiece, while
the workpiece is not in mechanical contact with the heat-transfer
device or heat-transfer device electrode. The heat-transfer device
or heat-transfer device electrode and the workpiece may be
electrically connected, preferably through wires and a high voltage
power supply.
[0114] Preferably, the heat-transfer device and the electrode
producing the non-equilibrium plasma are a single element, e.g., "a
heat-transfer device electrode." An alternative embodiment uses a
charged electrode to produce the non-equilibrium plasma and a
mechanically separate heat-transfer device, which is grounded or
charged to about half the opposite potential of the electrode
producing the non-equilibrium plasma. For example, the electrode
may have a voltage of about 25,000 to about 150,000 volts to
produce the non-equilibrium plasma, while the heat-transfer device
has a voltage about half the voltage of the electrode, such as from
greater than about 0 to less than about 75,000 volts. It will be
appreciated in this regard that the minimum generally desirable
voltage of the heat transfer device will be that voltage which is
required to be applied to effect in the workpiece the desired
temperature, and so may approach 0, while the maximum generally
desirable voltage will be about one-half that of the electrode.
Preferably, the electrode and the heat-transfer device are not
electrically connected.
[0115] Generally, the workpiece is electrically grounded or held at
a potential opposite to the potential of the heat-transfer device
or heat-transfer device electrode by a high voltage power supply.
An object held at the opposite potential is one with a positive DC
voltage applied to it when the other electrode is negative or vice
versa. Opposite potentials are used to create the field strength
required to produce a plasma. The distance between the workpiece
and the heat-transfer device or heat-transfer device electrode is
from about 10 cm to about 250 cm, more preferably, from about 20 cm
to about 100 cm, and even more preferably, from about 25 cm to
about 75 cm. Generally, the electrical potential or voltage between
the workpiece and the heat-transfer device or heat-transfer device
electrode is from about 25,000 to about 150,000 volts DC, or from
about 25,000 to about 150,000 volts AC.
[0116] The electrical potential applied between the workpiece and
the heat-transfer device or heat-transfer device electrode produces
a non-equilibrium plasma having a desired thermal conductivity. The
non-equilibrium plasma is preferably a glow discharge or a cold
corona discharge. Alternatively, radio frequency signals, microwave
signals or radiation can be used to produce the non-equilibrium
plasmas. The thermal conductivity of the non-equilibrium plasma is
generally about 2-10 times greater than the thermal conductivity of
helium, preferably, about 5-10 times greater, and more preferably,
about 8-10 times greater, and may exceed 10 times greater.
[0117] The workpiece can be any workpiece known in the art,
including metals and non-metals. As used herein, "workpiece" refers
to and includes a single workpiece or a plurality of workpieces.
Nonlimiting examples of workpieces according to the invention
include powders and/or preforms produced by spray forming. The
workpiece can be a plurality of workpieces having an average
diameter of about 0.1 to about 10 cm. The workpiece can be a
material or a section or portion of a material that requires a high
rate of cooling to control solidification, thereby controlling
grain structure and other metallurgical properties, such as, but
not limited to, articles for gas turbine engines, including, for
example, airfoils, blades, discs and blisks. Preferably, the
workpiece is a molten particle or preform, as described herein.
[0118] The workpiece can be stationary or can move or pass through
the non-equilibrium plasma. A dispensing means, as described
herein, can be used to move or pass the workpiece through the
non-equilibrium plasma. After the workpiece moves or passes through
the non-equilibrium plasma, it can be captured or accumulated in
any collecting means known in the art, as described herein.
[0119] The heat-transfer device or heat-transfer device electrode
is connected to a thermal mass which allows heat to be added or
removed from the workpiece by the non-equilibrium plasma. Heat can
be transferred from the heat-transfer device to the thermal mass by
any method known in the art. Preferably, the thermal mass will be a
large thermal mass. A large thermal mass is one which can accept or
donate a significant amount of thermal energy with only a small
change in temperature. Heat can be transferred from the
heat-transfer device to the large thermal mass by heat transfer
means including, for example, a dielectric fluid, a heat pipe, a
thermally conductive metal, a thermally conductive ceramic and the
like. Dielectric fluids include, for example, silicon, mineral oil
and the like. Conductive metals include, for example, copper,
aluminum, brass, silver, gold and the like. Conductive ceramics
include, for example, mullites, steatites and other ceramic forms.
For example, a dielectric liquid can be circulated through the
heat-transfer device or beat-transfer device electrode through
pipes by a pump that is used to move heat between the heat-transfer
device or heat-transfer device electrode and the large thermal mass
to keep the temperature of the heat-transfer device or
heat-transfer device electrode constant during the heat transfer
process. In another embodiment, the heat-transfer device or
heat-transfer device electrode can comprise a heat pipe to transfer
heat between the heat-transfer device or heat-transfer device
electrode and the large thermal mass to keep the temperature of the
heat-transfer device or heat-transfer device electrode constant
during the heat transfer process.
[0120] As used herein, the term "heat-transfer device" or
"heat-transfer device electrode" can include a single heat-transfer
device or heat-transfer device electrode or a plurality of
heat-transfer devices or heat-transfer device electrodes that may
or may not be mechanically and/or electrically separate. For
example, a plurality of heat-transfer devices can be used, wherein
each individual heat-transfer device is electrically connected to a
high voltage power supply, such that the potential between the
plurality of heat-transfer devices produces a non-equilibrium
plasma. The electrode, in conjunction with the voltage applied by
the power supply and the field gradient within the geometry,
produces the non-equilibrium plasma. When a plurality of
heat-transfer devices is used, the distance between the individual
heat-transfer devices can be any desired distance, such as about 1
to about 2,500 mm, preferably about 1 to about 1,500 mm, and the
voltage between the individual heat-transfer devices can be any
desired voltage, such as about 25,000 to about 150,000 volts DC or
about 25,000 to about 150,000 volts AC.
[0121] When a plurality of heat-transfer devices is used, some of
the heat-transfer devices can produce a potential equal to about
half the potential that is being used to produce the
non-equilibrium plasma, but having the opposite polarity. For
example, if two heat-transfer devices are used, the voltage applied
to the first heat-transfer device producing the non-equilibrium
plasma can be AC, and the second heat-transfer device can be
connected to a separate high voltage power supply that produces a
potential equal to about half the potential that is being used by
the first heat-transfer device to produce the non-equilibrium
plasma, but having a negative or positive polarity. In another
embodiment, if two heat-transfer devices are used, the voltage
applied to the first heat-transfer device producing the
non-equilibrium plasma can be AC, and the second heat-transfer
device can be connected to a separate high voltage power supply to
produce a potential equal to about half the AC potential being used
by the first heat-transfer device to produce the non-equilibrium
plasma, but having a positive or negative DC polarity. In still
other embodiments, when two heat-transfer devices are used, the
voltage applied to the first heat-transfer device producing the
non-equilibrium plasma can be AC, and the second heat-transfer
device can be connected to a separate high voltage power supply
producing an AC potential equal to about half the potential that is
being used by the first heat-transfer device to produce the
non-equilibrium plasma, but being out of phase with the potential
of the first heat-transfer device that is producing the
non-equilibrium plasmas. Thus, for example, the phase difference
between the AC potential in the first heat-transfer device and the
AC potential in the second heat-transfer device can be adjusted
between about 1 degree and about 180 degrees, and is preferably
about 180 degrees. In these embodiments, the voltages are
preferably between about 5 kV and about 75 kV, more preferably
between about 10 kV and about 50 kV, most preferably between about
15 kV and about 25 kV. Although two heat-transfer devices have been
exemplified, it will be appreciated by one skilled in the art that
these principles may readily be applied to more than two
heat-transfer devices in view of the teachings herein.
[0122] In some cases, a chamber can be used to enclose or contain
the workpiece, the dispensing means, the collecting means, the
means for directing the molten particle, the heat-transfer device
and the electrode or the heat-transfer device electrode, and the
non-equilibrium plasma. Such a chamber can be used to regulate the
gas species present and/or the pressure. For example, the chamber
may be evacuated and completely or partially filled with an inert
gas (e.g., argon or nitrogen), or vice versa, to achieve the
desired final metallurgy, to control the oxidation of other
non-metal materials being processed, and/or to and nitridation. In
a preferred embodiment, the pressure in such an enclosed chamber is
less than atmospheric pressure, preferably from about 0.1 to about
0.0001 torr, more preferably from about 0.01 to about 0.001
torr.
[0123] In some cases, the voltage between the heat-transfer device
electrode and the workpiece may not be sufficient to initiate
and/or maintain the non-equilibrium plasma. In such cases, an
external means for generating and/or maintaining the
non-equilibrium plasma can be used. Alternatively, an external
means for generating and/or maintaining the non-equilibrium plasma
can be used instead of using the electrodes and/or heat-transfer
device electrodes. The external means can maintain and/or elevate
the temperature difference between the electrons and the neutral
and heavy ions in the non-equilibrium plasma by supplying energy to
the electrons. The external means can be any known in the art,
including, for example, electron beams, thermionic emissions, RF
electromagnetic radiation, electromagnetic radiation in the range
of frequencies from soft ultraviolet to hard x-rays, or magnetic
fields.
[0124] The embodiments of the invention in FIGS. 6-9 are for
purposes of illustration only, and are not intended to limit the
scope of the invention or claims. Although FIGS. 6-9 refer to a
heat sink, one skilled in the art will appreciate from the
teachings herein that the heat sink can be replaced with a heat
source. In FIGS. 6-9, the workpiece 101 is preferably a molten
particle or preform, as described herein.
[0125] In FIG. 6, the workpiece 101 is electrically grounded or
held at a potential opposite to the potential of the heat sink or
heat sink electrode 102 by a high voltage power supply 103
connected by wires 104. An electrical potential is applied between
the workpiece 101 and the heat sink or heat sink electrode 102 to
produce a non-equilibrium plasma 24 having a desired thermal
conductivity. In a preferred embodiment, a dielectric liquid 106 is
circulated through the heat sink or heat sink electrode 102 through
pipes 107 by a pump 108 that moves heat between the heat sink or
heat sink electrode 102 and a large thermal mass 109 to keep the
temperature of the heat sink or heat sink electrode 102 constant
during the heat transfer process.
[0126] In FIG. 7, the workpiece 101 is electrically grounded or
held at a potential opposite to the potential of the heat sink or
heat sink electrode 102 by a high voltage power supply 103
connected by wires 104. An electrical potential is applied between
the workpiece 101 and the heat sink or heat sink electrode 102 to
produce a non-equilibrium plasma 24 having the desired thermal
conductivity. In a preferred embodiment, the heat sink electrode
102 comprises a heat pipe 110 which transfers heat between the heat
source or sink electrode 102 and a large thermal mass 109 to keep
the temperature of the heat sink or heat sink electrode 102
constant during the heat transfer process.
[0127] In FIG. 8, an AC electrical potential is applied between a
first and second heat sink or heat sink electrode 102 by a high
voltage power supply 103 connected by wires 104 to produce a
non-equilibrium plasma 24 through which the workpieces 101 are
passed. The source of the workpieces 101 is the dispensing means
111 which may comprise a container, atomizer, grinder or other
means of producing or dispensing the workpieces 101. A means for
collecting the heated or cooled workpieces 101 is provided by the
hopper 112. The dispensing means 111 is contained within a chamber
113 and a vacuum pump 114 connected to the chamber 113 by a pipe
107 which serves to reduce the pressure within the chamber 113.
This pressure reduction within the chamber 113 is often desirable
to reduce or eliminate contamination by unwanted gasses and also
serves to reduce the voltages required to produce the
non-equilibrium plasma 24. In this embodiment, a dielectric liquid
106 is circulated through the heat sink or heat sink electrode 102
and through pipes 107 by pumps 108 that move heat between the heat
sink or heat sink electrode 102 and a large thermal mass 109 to
keep the temperature of the heat sinks or heat sink electrodes 102
constant during the heat transfer process. In this embodiment, a
plurality of electrically charged heat sinks or heat sink
electrodes may also be used and they may be oriented perpendicular
to the direction of movement of the workpiece.
[0128] In FIG. 9, the workpiece 101 is electrically grounded or
held at a potential opposite to the potential of the electrode 115
by a high voltage power supply 103 connected by wires 104. An
electrical DC potential is applied between the workpiece 101 and
the electrode 115 to produce a non-equilibrium plasma 24 having the
desired thermal conductivity, and which impinges on the surfaces of
the workpiece 101, the electrode 115 producing the non-equilibrium
plasma 24 and the heat sink or heat sink electrode 102. In this
embodiment, a dielectric liquid 106 is circulated through the heat
sink or heat sink electrode 102 and through pipes 107 by a pump 108
that is used to move heat between the heat sink or heat sink
electrode 102 and a large thermal mass 109 to keep the temperature
of the heat source or sink electrode 102 constant during the heat
transfer process. In this embodiment, the heat sink or heat sink
electrode 102 is either grounded or held at a potential opposite to
that of electrode 115 producing the non-equilibrium plasma 24 and
having approximately 50% of the potential applied to the electrode
115. The potential of the heat sink or heat sink electrode 102 is
controlled by a high voltage power supply 103 which is connected to
the heat sink or heat sink electrode 102 by a wire 104. In this
case, the electrode 115 producing the non-equilibrium plasma 24 and
the heat sink or heat sink electrode 102, which adds or removes
heat, are two separate elements. Typically voltages in the range of
25,000 to 150,000 volts are applied to electrode 115 to produce the
non-equilibrium plasma 24, while the potential of the heat sink or
heat sink electrode 102 has a voltage about half the voltage of
electrode 115, such as from greater than about 0 to less than about
75,000 volts. The minimum generally desirable voltage of the heat
sink or heat sink electrode 102 will be that voltage which is
required to be applied to effect in the workpiece the desired
temperature, and so may approach 0, while the maximum generally
desirable voltage will be about one-half that of electrode 115.
[0129] Heat transfer using non-equilibrium plasmas has a wide range
of applications, including, for example, arc welding, Mig welding,
Tig welding, laser welding, metal spraying of preforms and powders,
powder manufacture, and other metal fabrication and manufacturing
processes which require a high rate of cooling, such as
solidification and grain structure control in the cooling of
alloys, superalloy casts and welds. A surprising and unexpected
aspect of the present invention, then, is the use of electron flow
within a non-equilibrium plasma to transfer heat, which in aspects
of the invention may be accomplished in a vacuum.
[0130] Preferably, the dispensing means of the present invention is
an atomizing means. Atomization of molten particles using rapid
electrostatic charging results in the rapid breakup of particles
into smaller particles due to electrostatic repulsion forces. The
production of small particles has a wide range of commercial and
industrial applications, including, for example, powder production,
spray forming and metal coating processes.
[0131] Advantages of the present atomization methods and apparatus
over conventional gas atomization include, for example, that the
present invention can be carried out in a vacuum so that chemical
interactions with the molten material can be controlled or
eliminated, and any voids in the solid workpiece (e.g., preform)
produced by the present invention would collapse during subsequent
working of the workpiece (e.g., preform) so that no defects would
exist in the final product.
[0132] In one embodiment of the present invention, a high voltage
DC power supply is used to rapidly electrostatically charge molten
particles beyond the Rayleigh limit, such that the electrostatic
forces within the particles exceed the surface tension of the
material and the particles break up into smaller particles. The
"Rayleigh limit" is the maximum charge a droplet can sustain before
the electrostatic repulsion forces overcome the surface tension.
This rapid electrostatic charge can also be used to further break
up the particles resulting from the first rapid electrostatic
charge. Thus, several size refinements using rapid electrostatic
charging are possible. Preferably, electrostatic charging is
applied one, two, three, four or more times to refine the particles
to a desired size. The final size to which droplets can be atomized
is based on the applied voltage, the starting diameter of the
particle, the rate of charging of the particle, and the geometry of
the electrostatic or electromagnetic field present.
[0133] In exemplary processes of the present invention, a material
is placed in a container and liquified. The material can be
metallic or non-metallic. The container can have one or more
nozzles or orifices through which the molten material can flow. The
container may also be referred to herein as a "dispensing means" or
"melt chamber." The inside diameter of the orifice is preferably
about 0.1 mm to about 10 mm, more preferably, about 0.15 mm to
about 2 mm, yet more preferably, about 0.15 mm to about 0.3 mm,
most preferably, about 0.15 mm. When the inside diameter of the
orifice is less than about 0.1 mm, it is difficult to achieve a
consistent flow of the liquid metal The size of the primary
droplet(s) need not be minimized since the goal of the invention is
not to achieve a liquid metal spray at the tip of the nozzle.
[0134] In one embodiment, the dispensing means is sealed so that a
vacuum and/or pressure can be created. The molten material is
forced or expelled through the orifice(s) by a positive pressure
that is created in the head space above the molten material. The
pressure in the head space can be increased and decreased (e.g.,
pulsed or oscillated) in a time dependant manner to cause molten
particles to be formed at the orifice(s) due to the periodic
interruption of flow of the molten material. When the particles are
ejected from the orifice(s), they enter the particle formation and
collection chamber. The particle formation and collection chamber
is preferably sealed so that a vacuum or pressure can be created in
the chamber and so that gases cannot contaminate the molten
particles or final product.
[0135] The pressure in the head space above the molten material in
the dispensing means is preferably equal to or less than pressure
in the particle formation and collection chamber to prevent molten
material from discharging from the orifice. The pressure in the
head space above the molten material in the dispensing means is
preferably increased by about 1 to about 1,500 mm of mercury at a
frequency of about 1 to about 500 Hz, more preferably about 2 to
about 200 Hz to cause interrupted flow (e.g., pulsed flow or
oscillated flow) of the molten material through the orifice(s). Any
method of interrupting flow by, for example, creating a positive or
negative pressure differential between the head space and the
dispensing means, or by electrical or mechanical means, may be
used. This interrupted flow causes the molten particles to form.
The molten particles formed at this point may be referred to herein
as "primary molten particles" because they are the first particles
formed in the process.
[0136] The primary molten particles can be charged in several ways.
The molten particles can be rapidly charged by conduction charging
in the orifice(s) (e.g., before being expelled from the orifice) or
by an electrostatic discharge into the molten particles as the
molten particles are expelled from the orifice(s), and/or after the
molten particles are expelled from the orifice(s). Preferably, the
primary molten particles are rapidly electrostatically charged. The
rapid electrostatic charge can be created by, for example, an arc
discharge or an electron beam. As used herein, "rapid" is from
about 1 to about 500 microseconds, preferably about 1 to about 100
microseconds, most preferably about 1 to about 50 microseconds. The
rapid charging of the primary molten particles creates a plurality
of secondary molten particles that have a uniform diameter of about
5 to about 2,500 microns, preferably about 5 to about 250 microns.
The secondary molten particles can be used to produce solid
preforms or powders, or to coat a substrate(s), as described
herein.
[0137] In an alternative embodiment, a nozzle and a dispensing
means are arranged so that a flow control rod is moved by a
mechanical or electromechanical actuator to allow the molten
material to flow out of the nozzle through an orifice(s).
Preferably, the flow control rod is moved vertically by the
mechanical or electromechanical actuator. Optionally, pressure or a
vacuum can be applied in the dispensing means. The container can
comprise one or a plurality of nozzles and flow control rods. A
high voltage power supply, capable of providing a voltage rise rate
of at least 3 million volts per second, is connected to the nozzle
by a conductor. Preferably, the voltage rise rate is about 100 to
about 100 million volts/second, more preferably from about 500 kV
to about 50 million volts/second, even more preferably from about 1
million to about 30 million volts/second. The rise rate is the
slope of the waveform where the x axis is time and the y axis is
voltage. The high voltage is applied to the nozzle at a high rise
rate by the power supply and conductor and is synchronized with the
momentary retraction of the flow control rod by the mechanical or
electromechanical actuator which causes a primary molten particle
to form. The high voltage applied at a high rise rate causes the
rapid electrostatic charging of the primary molten particle which
causes the primary molten particle to break up or atomize into
smaller secondary molten particles due to electrostatic forces.
[0138] The embodiment in FIG. 10 describes an apparatus for
producing small molten particles that can be collected as a solid
or used to coat a substrate. The apparatus comprises a vacuum and
pressure vessel 1 which serves as the dispensing means. A vacuum
source 2 is connected by a pipe 4a to a valve 3a which is in turn
connected to the vacuum and pressure vessel 1 by a tube 5a. A
pressure source 6 is connected by a pipe 4b to a valve 3b which is
in turn connected to vacuum and pressure vessel 1 by a tube 5b. A
computer 7 reads the temperature of the molten material 8 by a
temperature sensor 9 which is connected to the computer 7 by wire
10a. The computer 7 reads the pressure in the vacuum and pressure
vessel 1 by a pressure sensor 11a which is connected to the
computer by wire 10b. The induction heat sources 12 are connected
to the computer 7 by wires 10c. The positive side of the high
voltage power supply 13 is connected to an electrode 14a by an
insulated wire 10d and the negative side of the high voltage power
supply 13 is connected to electrodes 14b by an insulated wire 10e
which passes through a vacuum tight insulated connector 18 and wire
10f. A second vacuum source 21 is connected by a pipe 4e to a valve
3c which is connected to the particle formation chamber 20 by a
tube 5c and is connected to the computer 7 by wire 10g. A pressure
sensor 11b is connected to the computer 7 by wire 10h. The high
voltage power supply 13 is connected to the computer for control by
wire 10i.
[0139] In use, when the system initially starts, the computer 7
senses the pressure in vacuum and pressure vessel 1 and in the
particle formation chamber 20 by the pressure sensors 11a and 11b,
respectively. The computer 7 then controls the evacuation of the
particle formation chamber 20 by the valve 3c controlling the
second vacuum source 21 to produce a pre-set partial pressure level
specific to the material to be atomized, and controls the first
vacuum source 2 and the pressure source 6 by valves 3a and 3b,
respectively, to maintain a partial pressure in the vacuum and
pressure vessel 1 equal to that in the particle formation chamber
20. The pressure is varied from atmospheric pressure down to a
lower pressure until the desired flow rate and resulting particle
size is achieved.
[0140] The computer 7 then senses the temperature of the material 8
by the temperature sensor 9 and provides power to the induction
heaters 12 by wires 10c until the material achieves the desired
pre-set melt temperature which causes the material to liquefy. At
this point, a normal atomization cycle begins.
[0141] Once the computer 7 senses that the pre-set melt temperature
has been reached, a positive pressure burst is applied to the
vacuum and pressure vessel 1 by the computer 7 opening the valve 3b
to the pressure source 6 thereby forcing some of the molten
material 8 through the orifices 15 to form the primary molten
particles 16. The computer 7 then closes the valve 3b and
momentarily opens valve 3c and/or valve 3a to equalize the pressure
between the vacuum and pressure vessel 1 and the particle formation
chamber 20 which stops the molten material 8 from flowing. The high
voltage power supply 13 is then turned on by the computer 7 and a
rapid charging of the primary molten material particles 16 by the
electrical arcs 19 causes the electrostatic forces within the
primary molten particles 16 to exceed the surface tension energy
resulting in the formation of smaller secondary molten particles
17.
[0142] The secondary molten particles 17 will then pass through a
non-equilibrium plasma 24 created by second electrodes 25 which
each transfer the heat to the outside of the particle formation
chamber 20 to heat exchangers 26. The resulting cooled atomized
particles are then collected by the collecting means 22 either as a
solid preform or as powder, depending on the amount of cooling
provided by the non-equilibrium plasma 24.
[0143] The cycle will then begin again at the point where normal
atomization begins. Throughout the process, the computer 7 senses
the temperature of the material 8 by the temperature sensor 9 and
provides power to the induction heater 12 by wires 10c to maintain
the desired pre-set melt temperature to maintain the material as a
liquid. At the end of the atomization cycle, the computer 7, via a
wire 10j, opens a vent 23 which is connected to the particle
formation chamber 20 by a pipe 4d and is connected to the air
outside the particle formation chamber 20, which causes the
pressure within the particle formation chamber 20 to equalize with
the outside air pressure. Thereafter, the particle formation
chamber 20 can be opened to remove the product.
[0144] In FIG. 11, a nozzle 30 and a dispensing means 1 are
arranged so that a flow control rod 27 is moved by a mechanical or
electromechanical actuator 28 to allow the molten material 8 to
flow out of the nozzle 30 through an orifice 15. A high voltage
power supply 13, capable of providing a high voltage rise rate, is
connected to nozzle 30 by a conductor 31. The high voltage is
applied to the nozzle 30 at a high rise rate by the power supply 13
and conductor 31 and is synchronized with the momentary retraction
of the flow control rod 27 by the mechanical or electromechanical
actuator 28 which causes a primary molten particle 16 to form. The
high voltage applied at a high rise rate causes the rapid
electrostatic charging of the primary molten particle 16 which
causes the primary molten particle 16 to break up or atomize into
smaller secondary molten particles 17 due to electrostatic
forces.
[0145] In FIG. 12, a nozzle 30 and dispensing means 1 are arranged
so that the primary molten particle 16 exits the orifice 15.
Thereafter, the electrode 14a releases an electrical arc 19 that
causes the electrostatic forces within the primary molten particle
16 to exceed the surface tension energy, resulting in the formation
of smaller secondary molten particles 17. Subsequently, another
electrode 14b releases an electrical arc 19 that causes the
electrostatic forces within the secondary molten particles 17 to
exceed the surface tension energy, resulting in the formation of
smaller tertiary molten particles 40. Thereafter, another electrode
14c releases an electrical arc 19 that causes the electrostatic
forces within the tertiary molten particles 40 to exceed the
surface tension energy, resulting in the formation of smaller
quaternary molten particles 41.
[0146] The electrodes 14a, 14b, 14c are rings of varying diameters,
according to the electric potentials applied. Generally, they have
diameters of about 1 to about 20 centimeters, preferably about 5 to
about 15 centimeters. The electrodes 14a, 14b, 14c can be
extractor, expansion or compression rings, preferably they are an
expansion or compression ring. An expansion ring is generally a
bare metal wire ring that is at an electric potential such that an
attractive or expansive force is exerted on the charged droplet(s).
A compression ring is generally a metal wire ring coated with a
dielectric material of varying thickness. When an electric
potential is applied to the compression ring, an opposite charge is
induced upon the surface of the dielectric material, forming a
squeezing (or compressive) force upon the droplet(s).
[0147] Preferably, an extractor ring 80 is also used in the
apparatus of the present invention. The extractor ring 80 is
generally the ring closest to the nozzle 15 that encourages
extraction of the primary drop 16 from the nozzle 15.
[0148] According to the invention, the atomization process is
manipulated using the methods and apparatus of the invention to
effect the production of smaller droplets. It has been found that
the extractor ring, when used in accordance with the invention as
described herein plays a significant role in controlling and/or
maximizing the division process. While not intending to be bound by
any particular theory, the wire rings seem to permit some expansive
(sucking) force to be applied upon the droplet as it passes the
ring plane, while PVC rings seem to permit a compressive force to
be applied upon the droplets. The importance of maintaining the
environment in the vicinity of the electrostatic field at a
temperature above the melting point of the liquid metal cannot be
over-emphasized. As the droplets become smaller their surface area
to mass ratio increases and they cool more rapidly.
[0149] Referring to FIGS. 46A-B, the distance between the nozzle
803 and the extractor ring 804 is generally about 1 to about 50
millimeters. The diameter of the extractor ring 804 varies
according to the voltages that are applied. The extractor ring 804
will be at an electric potential less than the nozzle 803 to cause
liquid to be pulled from or extracted from the nozzle 803.
[0150] A positive high voltage DC source connected to the liquid
metal reservoir produces an electric field between the nozzle and
the grounded collector cup. The force of the field produced, acting
together with gravity, causes atomized droplets of similar size to
be collected. This phenomenon is called primary atomization.
[0151] The placement of an extractor ring between the nozzle and
collector cup and concentric to the droplet path causes lateral
forces to be applied to the droplet, which can produce successively
smaller droplets. This Phenomenon is called secondary and tertiary
atomization, as shown in FIGS. 12 and 14. It is preferred to
maximize the number of tertiary droplets produced. FIG. 13 shows
the weight of the droplets produced versus the gap between the
nozzle and the extractor. This figure clearly illustrates that once
a critical value is reached, primary atomization is not sensitive
to the high voltage potential applied, or to the distance between
the nozzle and the extractor. Table 2 and Table 3 show the results
of experiments using bare copper wire extractors with different
ring diameters.
[0152] FIG. 14 shows evidence abstracted from the same experimental
sample. All droplets were produced by the same experiment. The
choice of solidified droplets P.sub.0 to P.sub.4 demonstrates the
way in which the primary droplets are subdivided. The choice of
solidified droplets S.sub.0 to S.sub.4 shows this phenomenon then
repeats upon the secondary droplets to produce tertiary droplets
such as solidified droplet T.sub.0. Droplets P.sub.4 and S.sub.4
appear severed, but they are whole, and droplet T.sub.0 seems large
for its weight. Unfortunately, these apparent anomalies arise from
lens distortion due to the scanning and copying processes involved
in producing FIG. 14.
[0153] While not intending to be bound by any particular theory,
the division of an initial liquid metal drop into smaller droplets
seems to be the result of three separate processes. Consider a case
in which a liquid metal drop is emitted in the direction of gravity
from the capillary nozzle of a positively-charged reservoir towards
a grounded baseplate. Assume that the surrounding environment is
sufficiently warm for the drop (or atomized droplets) to remain
liquid. The size of the drop emitted depends directly upon the
electric field applied to the reservoir. The drop will form at the
nozzle in a manner shown in FIG. 47, the size of the drop being
governed by the field applied up to a critical value. Thereafter,
only the rate at which the drop leaves the capillary nozzle is
affected by the field.
[0154] The possibility of liquid metal spray occurring directly
from the nozzle tip can be eliminated because the aperture of the
nozzle (preferably about 0.15 mm inside diameter) is too large to
permit formation of a Taylor-cone upon the free surface. The basic
phenomenon of a liquid film deformation is that the outward
electric stress (.sigma..sub.E) has to overcome the stress
(.sigma..sub.S) due to surface tension (i.e.,
.sigma..sub.E.gtoreq..sigma..sub.S). A charged drop begins to
atomize when the applied force is in excess of the Rayleigh limit
q.sub.r=8.pi.(.di-elect cons..sub.0.T.r.sup.3).sup.1/2 where
.di-elect cons..sub.0, T, and r are permissivity of free space,
surface tension of a liquid, and drop radius, respectively. The
downward force of gravity combines with the electrostatic force to
cause the drop to be ejected from the nozzle before sufficient
charge can accumulate to constitute a force that can overcome the
liquid metal surface tension forces. Liquid metals have high
inter-molecular binding energies and thus have high values of
surface tension so that drops are not readily torn off from the
apex, even at reasonable field strengths. Hence, sufficient charge
can never be created upon the liquid metal surface at the nozzle
aperture by a DC field to satisfy the Rayleigh Criterion.
[0155] Now consider a case in which the liquid metal drop is
emitted from the reservoir in a manner similar to that described in
the paragraph above, but in this case the drop passes through an
extractor ring connected to ground, or to a negative potential with
respect to the droplet collector cup, and is positioned concentric
to the nozzle center and slightly below the nozzle tip (e.g.,
within about 1 to about 2 centimeters). As the drop falls toward a
position that is coplanar with the ring plane, the field intensity
between the drop and the ring increases.
[0156] If the potential difference between the drop and the ring is
large enough to impart sufficient charge upon the drop, then the
Rayleigh Limit may be reached. As the drop continues to fall toward
the ring plane, the Rayleigh limit is surpassed and highly charged
particles with small mass are ejected, since the electrostatic
forces have exceeded the surface tension forces. These particles
should be ejected at the surface of the drop where the
electrostatic field is densest, and it can be estimated that up to
25% of the charge is carried away by the particles, leaving the
majority of the mass of the drop remaining with a lesser charge.
Since the charge remaining on this residual drop is less than that
required to satisfy the Rayleigh Criterion, no further atomization
will occur unless the drop is recharged by induction along its
flight path (see FIG. 48).
[0157] Finally, a case similar to that described in the paragraph
above can be considered, where some charge remains upon the
residual drop. This drop can be equated with the primary drop
described throughout the present invention (see FIGS. 12 and 14).
If this drop is permitted to pass through a ring shaped electrode
of a type similar to the extractor ring situated at some distance
between the capillary nozzle and the collector, and connected to
ground or to a negative potential, then some electrostatic force is
exerted upon the drop as it passes through the vicinity of the ring
plane. If this force is sufficient to cause distortion of the drop,
then the middle of the drop may be constricted sufficiently that
surface tension forces will act along a path of least resistance at
the neck by forming two (or possibly more) secondary drops.
[0158] Thus, this process can be repeated as the droplets pass
through other strategically placed rings. However, as the mass of
the initial drop is subdivided, so the charge on each droplet is
reduced also (i.e., the charge to mass ratio is reduced) and some
charge is lost to the immediate environment due to leakage. The
drops may regain some positive charge by induction as they pass
down the electrostatic force field, but the charge effect will
reduce with distance from the capillary nozzle, and the effect will
be further reduced as the rings through which the drops have
already passed act to nullify the electrostatic field. Any droplets
produced in this manner would be third stage, or tertiary
drops.
[0159] Thus, the possibility exists for successive atomization of a
drop of liquid metal by a DC. electric field provided that certain
criteria are met.
[0160] One criterion is reducing the flow rate of the liquid metal
by controlling the high voltage that is applied to the reservoir.
This permits the drop a slower passage through the ring system and
so maximizes the atomization effect.
[0161] Another criterion is positioning the ring electrodes with
successively negative and positive potentials in sandwich fashion
such that the drops are alternately atomized, and then recharged by
induction. For example, a ring that is funnel-shaped may be used to
investigate whether exposing the liquid metal drop to an intense
field for a longer period can increase the charge imparted by
induction, and also whether such a funnel-shaped electrode, when
connected to a polarity opposite to that induced upon the drop will
permit greater atomization to occur. However, in this situation
there has to be some trade-off between the drop's downward velocity
and the potential applied to the funnel electrodes. Too high a
charging voltage on the electrode will retard the drop's ability to
leave the nozzle with minimum mass, while too little voltage will
produce a shorter flight time in the shearing field. To some extent
these effects can be minimized by varying the positions of the ring
electrodes along the drop's flight path and it is these effects
that we are currently studying.
[0162] Another criterion is maintaining a heated flight path that
is longer than previously employed in order to facilitate the
layering of rings. This flight path should be maintained at a
temperature which ensures that all constituents of the eutectic
alloy remain liquid. Though the eutectic isotherm for pure Wood's
Metal is 70.degree. C., that for a recycled alloy, where the
relative constituency may have changed, may be considerably
higher.
[0163] At some stage successive atomization will no longer occur.
Intuitively, there may be a limiting size of drop which cannot be
further subdivided by electrostatic shearing. As the drop becomes
smaller, there is less opportunity for the electrostatic forces to
form a distorted shape which can then usurp the surface tension
forces to help form smaller droplets because as the drop's mass
becomes smaller there will be a tendency for the drop to `float` on
the field rather than be severed by it.
[0164] Tables 2 and 3 in Example 3 indicate that a finite number of
groups (ostensibly 3 or 4) containing drops of similar sizes
resulted from each experimental sample. While examining FIG. 14, we
can understand why this situation occurs. As the partially charged
drop falls through the ring electrodes, electrostatic shearing
forces act upon the drop. However, imperfections in construction
mean that the ring is not exactly concentric to the drop's flight
path, therefore the electrostatic field intensity acting upon the
drop is not totally uniform. The drop is not divided exactly in a
manner of binary division. Instead, two or more droplets of unequal
mass are produced from each drop that is emitted from the nozzle.
This produces samples containing the grouping that we have
witnessed so far. Our aim therefore is to provide an opportunity
for sufficient successive atomizations to occur until the requisite
particle size distribution is achieved.
[0165] In preferred embodiments, the atomization methods and
apparatus further comprise non-equilibrium plasmas for removing
heat from the molten particles after they are electrostatically
atomized but before they are collected either as a solid workpiece
or as a powder. Alternatively, non-equilibrium plasmas can be used
to remove heat from the molten particles after they are applied to
a substrate. FIGS. 26-34 show various preferred embodiments. In
particular, FIGS. 26-29 provide examples of methods of making a
molten metal stream or droplets in a vacuum for atomizing, while
FIGS. 30-34 provide examples of methods used to collect the
atomized liquid metal in a vacuum. The atomization methods,
electrostatic methods, and non-equilibrium plasmas described in
FIGS. 26-34 are preferably those of the present invention, as
described herein.
[0166] FIG. 26 shows twin electrode melting as the source for the
molten metal for electrostatic atomizing. The vacuum chamber 501
surrounds the electrodes 503 and the atomizing source 505. Molten
metal 504, either as droplets or a stream, falls from the
electrodes 503 to the electrostatic atomizer 505. The atomized
material 506 flows out of the atomizer 505 and into a collecting
means (not shown), examples of which are described in FIGS.
30-34.
[0167] FIG. 27 shows electron beam melting as the source for the
molten metal for electrostatic atomizing in vacuum. The vacuum
chamber 501 surrounds the electron beam source 502, the electrode
503, the atomizing source 505 and the collector (not shown). Molten
metal 504, either as a stream or droplets, falls from the electrode
503 to the electrostatic atomizer 505. The atomized material 506
flows from the electrostatic atomizer 505 into a collection means
(not shown), examples of which are described in FIGS. 30-34.
[0168] FIG. 28 shows electron beam cold hearth melting as the
source for molten metal for electrostatic atomizing in vacuum. The
vacuum chamber 501 surrounds the electron beam source 502, the
electrode 503, the water-cooled copper cold hearth 507, the
atomizing source 505, and the collection device (not shown). Molten
metal 504, either as a stream or droplets, falls from the
water-cooled copper cold hearth 507 to the electrostatic atomizer
505. The atomized material 506 flows from the electrostatic
atomizer 505 into a collection means (not shown), examples of which
are described in FIGS. 30-34.
[0169] FIG. 29 shows ESR/CIG melting as the source for the molten
metal for electrostatic atomizing in vacuum. Alternatively, a
VAR/CIG melt source may be used in place of the ESRICIG melt
source. The vacuum chamber 501 surrounds the melt source, the
electrostatic atomizer 505 and the collection device (not shown).
The ESR/CIG melt source includes an electrode 503 and a
water-cooled copper crucible 507. A molten slag 508 acts to melt
the electrode 503 to form a molten metal pool 509. The molten metal
204, either as a stream or droplets, flows through the CIG nozzle
510, and falls into the electrostatic atomizer 505. The atomized
material 506 flows from the electrostatic atomizer 505 into a
collection means (not shown), examples of which are described in
FIGS. 30-34.
[0170] Throughout the description of FIGS. 26-34, the molten metal
504 is preferably atomized using the methods described herein.
[0171] FIG. 30 shows the atomized powder being collected in the
bottom of the atomizing chamber. The vacuum chamber 501 contains a
melting and atomizing means described in FIGS. 26-29. The stream or
droplets of molten metal 504 from the melt sources described in
FIGS. 26-29 passes through the atomizing zone 511. The atomized
material 506 is collected at the bottom of the chamber 512.
[0172] FIG. 31 shows electrostatically atomized powder being
collected as a solid preform after the powder is cooled via a
non-equilibrium plasma. The vacuum chamber 501 contains a melting
and atomizing means described in FIGS. 26-29. The stream or
droplets of molten metal 504 from the melt sources described in
FIGS. 26-29 passes through the atomizing zone 511. The atomized
powder 514 passes through a non-equilibrium plasma 515 and is
collected as a solid preform 516. The non-equilibrium plasma 515 is
generated by producing a potential difference between two
electrodes 503 from a power source 517. The heat from the atomized
powder 514 is conducted through the non-equilibrium plasma 515 and
the electrode 503 into a dielectric heat transfer medium to a heat
exchanger 518.
[0173] FIG. 32 shows electrostatically atomized powder being
collected in a can, where the can is transferred into a smaller
chamber without breaking the vacuum. In the smaller chamber, the
lid may welded to the can prior to hot working to a final product.
The vacuum chamber 501 contains a melting and atomizing means
described in FIGS. 26-29. The stream or droplets of molten metal
504 from the melt sources described in FIGS. 26-29 passes through
the atomizing zone 511. The atomized powder 514 is directed into a
can 519 via the process described in FIG. 34. When the can 519 is
sufficiently full of atomized powder 514, it is transferred in the
chamber 520 and the chamber 520 is sealed by a vacuum lock 521. A
lid can then be applied to the filled atomized powder can and the
can released to the atmosphere via a second lock 521B for
thermomechanical processing.
[0174] FIG. 33 shows the production of a solid ingot in a mold from
a powder produced by electrostatic atomization. The vacuum chamber
501 contains a melting and atomizing means described in FIGS.
26-29. The stream or droplets of molten metal 504 from the melt
sources described in FIGS. 26-29 passes through the atomizing zone
511. The atomized powder 514 is collected in a mold 522 and the
solid ingot 524 withdrawn from the mold 522. Power supplies 517
provide the potential difference to form a non-equilibrium plasma
515 emanating from the electrodes 503. Heat is conducted from the
surface of the solidifying ingots 524 to the electrodes 503 which
are cooled with a dielectric liquid. The liquid is passed through
heat exchangers 518 and returned to the electrodes 503.
[0175] FIG. 34 shows three stages of electrostatic atomizing using
plasma and one stage of electrostatic steering of the atomized
powder. The vacuum chamber 501 contains a melting and atomizing
means described in FIGS. 26-29. The stream or droplets of molten
metal 504 from the melt sources described in FIGS. 26-29 passes
through the atomizing zone 511. The non-equilibrium plasma 515 for
imparting the atomizing conditions is provided by the potential
difference between the electrodes 503. The potential difference is
supplied by a high-voltage power supply 517. The atomized material
from the first stage 525 passes to the second atomizing stage, and
atomized materials of smaller size from the second stage 526 pass
to the third stage. Atomized materials from the third stage 527
pass through the steering stage to be steered in a direction which
depends on the potential between the electrodes 503. Power for
these electrodes is supplied by power supply 517.
[0176] Using various features described above, it would be readily
apparent to one of ordinary skill in the art that the following
exemplary embodiments can be implemented. Of instance, in one
embodiment, the present invention describes apparatus comprising
dispensing means, collecting means, and means for directing molten
particles from the dispensing means to the collecting means
comprising an electrostatic field and/or an electromagnetic field.
Optionally, the apparatus may further comprise atomization
apparatus and/or non-equilibrium heat transfer apparatus.
[0177] In another embodiment, the present invention describes spray
forming methods comprising directing molten particles from
dispensing means to collecting means by producing an electrostatic
field and/or electromagnetic field between the dispensing means and
the collecting means. Optionally, the apparatus may further
comprise atomization apparatus and/or non-equilibrium heat transfer
apparatus.
[0178] In another embodiment, the present invention is directed to
apparatus comprising a melt chamber that comprises at least one
orifice; a means for expelling a molten material through the at
least one orifice in the melt chamber; and a means for applying a
rapid electrostatic charge to the molten material. Preferably, the
means for forcing the molten material through the at least one
orifice in the melt chamber is a mechanical or electromechanical
actuator or a pressure means. In a preferred embodiment, the
apparatus further comprises a means for cooling the molten
particle. Preferably, the means for cooling the molten particle
comprises a means for generating a non-equilibrium plasma.
[0179] In another embodiment, the present invention describes
methods for forming particles comprising producing a first molten
particle; and applying a rapid electrostatic charge to the first
molten particle, wherein the rapid electrostatic charge causes the
first molten particle to form at least one smaller second particle.
Preferably, the first molten particle is expelled through at least
one orifice in the melt chamber via mechanical means or by a
pressure means. In a preferred embodiment, the at least one smaller
second molten particle is cooled, preferably by a non-equilibrium
plasma.
[0180] In another embodiment, the present invention is directed to
apparatus for transferring heat between a heat-transfer device and
a workpiece comprising the heat-transfer device, wherein the
heat-transfer device is electrically charged or held at a
potential; the workpiece, wherein the workpiece is mechanically
separate from the heat-transfer device; and means for transferring
heat between the workpiece and the heat-transfer device comprising
a means for generating a non-equilibrium plasma. The heat-transfer
device can be either a heat sink or a heat source.
[0181] In yet another embodiment, the present invention is directed
to methods of transferring heat between a heat-transfer device and
a workpiece comprising producing a non-equilibrium plasma capable
of transferring heat between the heat-transfer device and the
workpiece, wherein the heat-transfer device is electrically charged
or held at a potential, and wherein the heat-transfer device is
mechanically separate from the workpiece. The heat-transfer device
can be either a heat sink or a heat source.
[0182] Accordingly, in various embodiments, non-equilibrium plasmas
are advantageously employed to effect optimal heat transfer, and
the non-equilibrium plasma must act with a heat sink/source that
has a thermal conductivity capable of removing the desired quantity
of heat. While two or more electrodes have been used in the past to
produce a plasma in a region of high heat, such as a weld zone, so
that the plasma would serve to conduct heat outward from the weld
zone, thereby increasing the surface area for heat, embodiments of
the present invention are directed to the discovery that a
non-equilibrium plasma may be used to introduce heat into a
workpiece as well as from a workpiece. It has further been
surprisingly discovered that under the correct conditions a
non-equilibrium plasma can be used to efficiently transfer heat in
a vacuum.
[0183] The novel methods of the present invention are particularly
useful in preparing any metal article, such as articles for gas
turbine engines, including, for example, airfoils, blades, discs
and blisks.
[0184] Accordingly, in one aspect, there is provided according to
the present invention an apparatus comprising: a dispensing means;
a collecting means; and a means for directing a molten particle
from the dispensing means to the collecting means comprising at
least one of an electrostatic field or an electromagnetic field. In
another aspect is provided the apparatus described above, wherein
the means for directing the molten particles from the dispensing
means to the collecting means comprises an electrostatic field or
an electromagnetic field. The apparatus may further comprise at
least one magnetic coil, and may also further comprise a means for
charging the molten particles. In one embodiment, the means for
charging the molten particles may comprise a thermionic emission
source or a tribocharging device. The dispensing means of the
apparatus may be a gas atomizer, and may further comprise a means
for transferring heat from the molten particles. The means for
transferring heat from the molten particles may comprise gas
conduction and/or convection and/or a non-equilibrium plasma.
[0185] In another aspect, there is provided according to the
present invention an apparatus comprising: a dispensing means; a
collecting means; and a means for directing a molten particle from
the dispensing means to the collecting means comprising at least
one of an electrostatic field or an electromagnetic field, and
further comprising a means for transferring heat from the
collecting means. The means for transferring heat from the
collecting means may comprise a means for generating a
non-equilibrium plasma. In a particular aspect, the means for
transferring heat from the molten particles comprises a first heat
sink, wherein the first heat sink is electrically charged or held
at a potential; and a means for transferring heat from the molten
particles to the first heat sink comprising a means for generating
a non-equilibrium plasma. The non-equilibrium plasma may be a glow
discharge or a cold corona discharge.
[0186] In another aspect, there is provided according to the
present invention an apparatus comprising: a dispensing means; a
collecting means; and a means for directing a molten particle from
the dispensing means to the collecting means comprising at least
one of an electrostatic field or an electromagnetic field, and
further comprising a means for expelling the molten particle
through at least one orifice in the dispensing means; and a means
for applying a rapid electrostatic charge to the molten material.
The means for expelling the molten particle through the at least
one orifice may comprise a mechanical or electromechanical
actuator. In one aspect, the means for expelling the molten
particle through the at least one orifice may be a pressure means
that produces a pressure in the dispensing means that is greater
than the pressure on the outside of the dispensing means. The
pressure means may cause interrupted flow of the molten particle
from the dispensing means. The rapid electrostatic charge may be an
arc discharge or an electron beam.
[0187] In another aspect, the present invention provides for a
spray forming method comprising directing molten particles from a
dispensing means to a collecting means by producing at least one of
an electrostatic field or an electromagnetic field between the
dispensing means and the collecting means. The electromagnetic
field may be produced by, for example, means comprising at least
one magnetic coil. The method according to this aspect of the
invention may further comprise charging the molten particles.
Charging the molten particles may be accomplished, for example,
using a thermionic emission source or a tribocharging device. In
one aspect, the dispensing means may be a gas atomizer. According
to this aspect of the invention, the method may further comprise
transferring heat from the molten particle. Transferring heat from
the molten particles may be accomplished, for example, by gas
conduction and/or convection and/or non-equilibrium plasma. In
another aspect, the method of the invention further comprises
producing a second electromagnetic field. According to the
invention, the method may further comprise transferring heat from
the collecting means, which may be by a non-equilibrium plasma.
[0188] In another aspect, the present invention provides for a
spray forming method comprising directing molten particles from a
dispensing means to a collecting means by producing at least one of
an electrostatic field or an electromagnetic field between the
dispensing means and the collecting means, further comprising
applying a rapid electrostatic charge to the molten particle,
wherein the rapid electrostatic charge causes the molten particle
to form at least one smaller molten particle. In a particular
aspect, the rapid electrostatic charge may be an arc discharge or
an electron beam. In another aspect, the method of the invention
may further comprise transferring heat from the molten particle
comprising producing a non-equilibrium plasma that transfers heat
from the molten particle to a first heat sink, wherein the first
heat sink is electrically charged or held at a potential. The
non-equilibrium plasma may be a glow discharge or a cold corona
discharge.
[0189] In another aspect, the invention is directed to an apparatus
comprising a melt chamber comprising at least one orifice; a means
for forcing a molten material through the at least one orifice in
the melt chamber; and a means for applying a rapid electrostatic
charge to the molten material. The rapid electrostatic charge may
be an arc discharge or en electron beam. The apparatus of the
invention may further comprise a means for cooling the molten
material. In a particular aspect, the means for cooling the molten
material may comprise a first heat sink, wherein the first heat
sink is electrically charged or held at a potential; and a means
for transferring heat from the molten material to the first heat
sink comprising a means for generating a non-equilibrium plasma.
The non-equilibrium plasma may be a glow discharge or a cold corona
discharge.
[0190] In another aspect, there is provided a method for atomizing
a particle comprising producing a first molten particle; applying a
rapid electrostatic charge to the first molten particle, wherein
the rapid electrostatic charge causes the first molten particle to
form at least one smaller second molten particle. According to the
method of the invention, the first molten particle may be produced
by melting a material in a melt chamber, and expelling the first
molten particle through at least one orifice in the melt chamber.
The rapid electrostatic charge may be an arc discharge or en
electron beam. The method of the invention may further comprise
cooling the second molten particle by producing a non-equilibrium
plasma that transfers heat from the second molten particle to a
first heat sink, wherein the first heat sink is electrically
charged or held at a potential. The non-equilibrium plasma may be a
glow discharge or a cold corona discharge.
[0191] In another aspect, the invention provides for an apparatus
for transferring heat between a first heat-transfer device and a
workpiece comprising a first heat-transfer device, wherein the
first heat-transfer device is electrically charged or held at a
potential, and wherein the first heat-transfer device is a heat
sink or a heat source; a workpiece, wherein the workpiece is
mechanically separate from the first heat-transfer device; and
means for transferring heat between the workpiece and the first
heat-transfer device comprising a means for generating a
non-equilibrium plasma. The non-equilibrium plasma may be a glow
discharge or a cold corona discharge. The apparatus of the
invention may further comprise an external means for generating or
maintaining the non-equilibrium plasma. The external means for
generating or maintaining the non-equilibrium plasma may be a
thermionic emission, an RF electromagnetic radiation, an
electromagnetic radiation, a magnetic field or an electron beam.
The first heat-transfer device of the apparatus of the invention
may comprise a plurality of heat-transfer devices. In a particular
aspect, the apparatus of the invention may further comprise a
second heat-transfer device that may be mechanically and
electrically separate from the first heat-transfer device, wherein
the second heat-transfer device is a heat sink or a heat source,
and wherein the potential between the first heat-transfer device
and the second heat-transfer device produces a non-equilibrium
plasma.
[0192] In another aspect is provided a method for transferring heat
between a first heat-transfer device and a workpiece comprising
producing a non-equilibrium plasma that transfers heat between the
first heat-transfer device and the workpiece, wherein the first
heat-transfer device is electrically charged or held at a
potential, wherein the first heat-transfer device is mechanically
separate from the workpiece, and wherein the first heat-transfer
device is a heat sink or a heat source. The non-equilibrium plasma
may be a glow discharge or a cold corona discharge. The method may
further comprise generating or maintaining the non-equilibrium
plasma via an external means. In an aspect, the external means for
generating or maintaining the non-equilibrium plasma comprises a
thermionic emission, an RF electromagnetic radiation, an
electromagnetic radiation, a magnetic field or an electron
beam.
[0193] In another aspect, the invention provides for a preform
produced by the methods of the invention. The preform of the
invention may be a near net preform. There is also provided an
article of manufacture produced by the method of the invention.
[0194] In another embodiment, present invention includes an
apparatus comprising, a dispenser, a collector, a means for
directing a molten particle from the dispenser to the collector
comprising at least one of an electrostatic field or an
electromagnetic field, and a heat transferring device from the
collector comprising a non-equilibrium plasma generator.
[0195] In yet another embodiment, the present invention includes an
apparatus comprising, a dispenser, a collector, a means for
directing a molten particle from the dispenser to the collector
comprising at least one of an electrostatic field or an
electromagnetic field, and heat transferring device from the molten
particle comprising a non-equilibrium plasma generator.
EXAMPLES
[0196] The following examples are for purposes of illustration
only, are not indented to bind the scope of the present invention
to any particular theories or embodiments described therein, and
are not intended to limit the scope of the invention or the
appended claims.
Example 1
[0197] Clean metal spraying experiments revealed that Wood's metal
could easily be charged positively or negatively. It was observed
that as the melting point of the metal increased, the ability to
positively charge the metal did not change or improved slightly,
while the ability to negatively charge the metal decreased. It was
determined that the metals were positively charged to about 78% of
the Rayleigh limit.
[0198] This example demonstrates that as the temperature of the
metal increases, the electron emission rate increases. Thus, the
ability to positively charge the metal is unaltered or improved,
while the ability to negatively charge the metal is reduced.
Example 2
[0199] In this example, the feasibility of deflecting charged metal
particles in a controlled and repeatable manner using an
electrostatic field was investigated. To this end, molten metal
particles comprising tin were positively charged and then passed
within 2 cm of an electrostatically charged plate. The particle
sizes were about 0.050 inches to about 0.250 inches. The polarity
and magnitude of the charge on the plate was varied in different
trials.
[0200] The results indicated that the positively charged plate
repelled the positively charged particles and the negatively
charged plate attracted the metal particles. The deflection
characteristics versus the applied voltage are shown in FIG. 5 for
the molten particles comprising tin.
Example 3
[0201] In this example, a video tape data analysis was developed to
analyze the results obtained in Example 2 to provide a
statistically viable comparison between the video of the spray
forming process with and without an electrostatic field
applied.
[0202] An 8 mm video tape was digitized and replayed frame by frame
on a high contrast NTSC video monitor. Each frame was judged as
demonstrating good or poor collection efficiency based on three
criteria. (1) Attenuation: If less than 80% of the particles
directly targeted the preform then the frame was judged as
demonstrating poor collection efficiency. (2) Bounce: If the
particles appeared to bounce off the preform then the frame was
judged as demonstrating poor collection efficiency. (3) Glow: If
the particles produced a glow over the preform, it was indicative
of a combination of poor attenuation and bounce off, and was judged
as demonstrating poor collection efficiency.
[0203] After the frames were categorized as demonstrating good or
poor collection efficiency, groups of frames were selected for
analysis. The frames for analysis were chosen based on a review of
the strip chart of the voltage and current. An electrostatic event
zone was defined as a period of time wherein high voltage was being
applied and current was being drawn. Thus, any momentary changes in
the collection efficiency could be eliminated from consideration in
the experiment by comparing the number of frames demonstrating good
collection efficiency versus the total number of frames in the
electrostatic event zone as compared to the neutral zones. It was
estimated that the synchronization between the video tape of the
clean metal spray run and the strip chart were accurate within
.+-.0.3 seconds. Thus, 0.3 seconds were added to the beginning and
end of each electrostatic event to ensure that the event was fully
captured. Three electrostatic event zones were found and selected
for analysis. Three neutral zone periods, where no current was
drawn, were selected for comparison controls.
[0204] The results of the analysis of the experimental data are
shown in Table 1 below. The results in Table 1 indicate that in
neutral Zones 1, 2 and 5, an average of 13.7% of the frames
demonstrated good collection efficiency. In Zone 6, where an
average of 198 watts of electrostatic power was applied, 31.6% of
the frames demonstrated good collection efficiency. In Zone 4,
where an average of 245 watts of electrostatic power was applied,
39.3% of the frames demonstrated good collection efficiency. In
Zone 3, where an average of 2000 watts of electrostatic power was
applied, 59.3% of the frames demonstrated good collection
efficiency.
[0205] This example shows that a significant improvement in the
number of frames demonstrating good collection efficiency
corresponded with the electrostatic events, that the improvement in
the number of frames demonstrating good collection is proportional
to the power drawn, and that the significant improvement observed
is not merely additive.
Example 4
[0206] This experiment demonstrated successful atomization of
liquids (colored water, molten Wood's metal) using an apparatus
according to the invention comprising a high voltage DC source. The
apparatus permitted limited atomization of liquid metal when the
flow was in an upward direction against gravity, or in a downward
direction with gravity. A number of controlling parameters such as
nozzle size, ambient conditions, spacing between electrodes,
dielectric medium between electrodes, and shape and size of
extractor electrode were varied such that atomization of liquid
metal was substantially effected using only a DC source.
Atomization involving DC alone according to the invention was
exemplified by demonstrating (i) liquid metal flow against the
direction of gravity; and (ii) liquid metal flow in the direction
of gravity.
[0207] The apparatus used for liquid metal flow against the
direction of gravity is shown in FIGS. 15 and 15A. FIG. 15A shows
the liquid metal reservoir 801, a connecting tube 802, a nozzle 803
and an extractor ring 804. A copper reservoir was connected to the
nozzle assembly by means of a copper tube. Solid pieces of eutectic
alloy (Wood's metal) were placed in the reservoir and heated to
approximately 100.degree. C. by radiation from two 150 watt halogen
bulbs. A pressure difference between the wider reservoir and the
nozzle caused the liquid to flow to the tip of the nozzle, and the
flow rate from the nozzle was regulated by the amount of alloy
placed in the reservoir. The reservoir and nozzle assembly was
connected directly to the high voltage terminal. A copper spheroid
which was placed within 1/16th of an inch from the nozzle tip
surrounded the replaceable nozzle, and a copper tube thermally
connected this spheroid to the reservoir assembly. In this way, not
only heat was imparted to the eutectic alloy by conduction along
the majority of its path, but also a charge buildup area was
extended. The whole arrangement was painted black to behave as a
black body absorber. A brass extractor ring was suspended at
varying distances directly above the nozzle opening and was
connected to a grounded electrical terminal by means of an
adjustable mounting assembly.
[0208] For liquid metal flow in the direction of gravity, a similar
copper reservoir was connected to a nozzle assembly by means of a
short downward tube of thinner internal diameter than the
reservoir, and this arrangement was attached directly to the
positive high voltage terminal. An extractor ring was placed at
varying distances below, or to the side of, the nozzle tip, and a
collector cup, placed directly below the nozzle and partially
filled with water, collected atomized samples.
[0209] A sensitive electronic balance was used to weigh drops and
droplets from atomization experiments. Control experiments were
performed when drops were produced by gravitational force and free
hydrostatic pressure (for downward liquid) alone. Repeat
experiments were then conducted using a similar head of molten
metal, and a high voltage (max 36 keV, max 0.2 mA) was applied to
the fluid. Equal numbers of drops and droplets (nominally 100) were
weighed, and their weights compared. FIGS. 16 and 17 show the drops
and droplets collected from one such series of experiments. For
each figure, the larger drops (upper portion of the figure) are
those collected during the control experiments, and the smaller
droplets (lower portion of the figure) are those collected during
experiments using electrostatic field.
[0210] It was observed that atomization of a liquid metal utilizing
an electric field between two electrodes alone was hampered by two
over-riding factors: (a) in air, at atmospheric pressure, arcing
occurred at a voltage lower than that required to atomize the
liquid metal by electrostatic means; and (b) at pressures slightly
less than atmospheric, there was an inability to create potential
differences between the electrodes sufficiently high enough to
enable atomization to occur. The reason for this is thought to be
that the formation of plasma permitted an easy path for current
flow.
[0211] In order to address these factors and the resultant
uncontrolled voltage breakdown, a preferred embodiment of the
invention employed a piece of CPVC pipe or other dielectric
material 825, preferably 1/8th of an inch thick, placed in such a
way as to surround the extractor ring and its supporting arm 804
(see FIGS. 18A and 18-25). By incorporating this CPVC 825 into the
assembly, substantially higher potential differences between nozzle
803 and extractor 804 could be achieved before arcing occurred.
This meant that more electrical energy was available for
atomization, though the distance between the electrodes could only
be increased to a point where leaking would occur to neighboring
components.
[0212] For liquid metal flow against the direction of gravity, the
nozzle aperture that permitted the best atomization was on the
order of 0.3 to 0.4 mm, which is accordingly a preferred nozzle
aperture size for this purpose. For apertures smaller than this,
difficulty was encountered in securing a liquid metal flow.
Apertures larger than this permitted too great a surface area of
liquid metal at the nozzle tip, which could not be satisfactorily
atomized using the particular experimental apparatus. It will be
appreciated that larger apertures may be achieved by varying the
design of the apparatus, with the exercise of no more than routine
skill once the present teachings are understood. Liquid metal
atomization was captured on video camera, but could only continue
for as long as the head of pressure in the reservoir permitted flow
of fluid to the nozzle tip. A prelude to atomization could be
witnessed as high voltage was increased by either an increase in
fluid flow rate, or as a vertex (Taylor Cone) forming on the
surface of the liquid metal at the tip of the nozzle. Although not
intending to be bound by any particular theory, it may be that if
too long a time elapses between achieving a complete flow of fluid
from the reservoir to the nozzle tip and increasing the high
voltage from zero to a level sufficient to permit atomization, then
satisfactory atomization would not occur. Instead, cooling and
oxidation of the free liquid surface would be apparent and the
liquid metal would flow downward. The potential difference between
the nozzle and the grounded steel base plate was sufficient to
increase the liquid flow. Thus, a "pipe" of solidified metal would
slowly form down the side of the nozzle assembly until the pressure
difference at the reservoir decreased to zero.
[0213] Following this approach, the inventors successfully achieved
atomization by means of a combination of mechanical and electrical
forces. In an exemplary experiment, a plunger was placed over the
open end of the reservoir to increase the pressure difference at
the nozzle tip. It was found that production of droplets by
electrostatic atomization required less potential difference
between the electrodes, and droplet sizes were smaller.
[0214] In another experiment, the eutectic alloy was removed from
the reservoir assembly and was replaced by water. The water was
then drained from the assembly, leaving the inner walls wet. The
eutectic alloy was then returned to the reservoir and reheated.
When liquid metal flow from the nozzle occurred, drops were ejected
by the force of steam that was trapped within the nozzle assembly.
When a high voltage was applied, the drops became atomized into
smaller droplets, some of which adhered to the underside of the
CPVC.
[0215] FIGS. 18, 19 and 20 show consecutive frames of the
atomization of liquid metal against gravity without any applied
mechanical force other than that due to the head of liquid in the
reservoir. It was found that liquid metal flow in this direction
was controlled more easily. Conditions could be created such that a
continuous flow of molten metal was produced and nozzle apertures
could be much smaller. Nozzles having preferred aperture sizes in
the range of about 0.1 mm to about 11 mm, more preferably, about
0.15 mm to about 2 mm, yet more preferably, about 0.15 mm to about
0.4 mm, and even more preferably, about 0.15 mm to about 0.3 mm,
most preferably about 0.15 mm, are employed. It was observed in use
that gravitational forces alone could not overcome the adhesive
forces of surface tension upon the inner walls of the nozzle, and
the cohesive forces within the surface of the metal. In such
instances, it was found that electrostatic atomization could
produce droplets with diameters of substantially smaller
magnitude.
[0216] The atomized droplets produced without applied external
force were substantially larger than the aperture of the nozzle.
For example, using a nozzle with an aperture diameter of 0.5 mm and
an outer diameter of 1.2 mm, the observed droplet diameter was 1.4
mm. Generally, smaller nozzle sizes permitted the production of
smaller drops to be emitted from the nozzle.
[0217] Drops produced by gravitational forces alone were irregular
in size and are not completely spherical. Droplets produced when
electrostatic atomization occurs were found to demonstrate a more
regular size distribution and were more nearly spherical. It is
believed that gravity acts upon the mass of liquid within the
reservoir and nozzle assembly to produce a tendency for liquid
flow. Application of high voltage produced a force field which
combined with gravity to produce a greater tendency for liquid
flow.
[0218] The liquid metal drops emitted from the nozzle were charged
by electrostatic means. Positioning the extractor electrode in a
coplanar position with the tip of the nozzle appeared to produce
the strongest field for electrostatic atomization to occur. The
application of DC high voltage to a liquid metal source was found
to produce atomized droplets. The size distribution of droplets
emitted from the nozzle was determined by nozzle size, hydrodynamic
and electrostatic forces. If the charge upon the drops was
sufficiently high, there was evidence to suggest that they may be
subject to binary division by purely electrostatic means. For a
nozzle with an outer diameter of 0.64 mm, the measured weight ratio
of drops produced without high voltage to those produced with high
voltage was approximately 2:1. For a nozzle with an outer diameter
of 0.40 mm, the measured weight ratio increased to approximately
4:1.
[0219] As the aperture of the nozzle was decreased, the size of the
droplets produced decreased rapidly, indicating that the fluid
emitted was severed before surface tension forces caused it to
adhere to the nozzle tip. These observations suggest that
electrostatic forces begin to predominate over mechanical forces.
The plume of spray evident in FIG. 24 indicates that liquid metal
ionization may have occurred as a result of a Taylor Cone being
produced on the surface of the fluid that is being emitted from the
nozzle. As electrostatic forces play a more dominant role in the
atomization process, a high rise-rate AC signal may be
advantageously incorporated into the system to create an extremely
efficient method of producing high quality pure metallic grains of
similar size.
Example 5
[0220] Practical difficulties arise when cooling molten or
near-molten metals under low pressures. Heat transfer from a metal
surface into a gas medium happens via convection, radiation, and
conduction. For many low-pressure and near-vacuum applications,
heat transfer by convection is negligible, while radiative heat
transfer alone may be insufficient. According to the present
invention, a solution is provided to this problem by increasing the
effective thermal conductivity of the gas medium by introducing
non-equilibrium ionization in the gas medium. This may be carried
out at low pressure and under near-vacuum conditions. The
non-equilibrium plasma thermal conductivity gain achieved by the
present invention was demonstrated by a series of experiments
performed in a vacuum chamber which provided comprehensive
temperature decay characteristics data under a variety of
conditions.
[0221] With reference to FIGS. 35-36, a stainless steel disk, Block
A 601, was heated using a 1 kW Calrod element 614. A large
stainless steel cylinder, Block B 602, was located at a distance of
4 inches from Block A 601 and functioned primarily as an electrode,
and secondarily as a heat sink. FIG. 35 shows a schematic diagram
of the experimental setup, and FIG. 36 shows the dimensions of the
blocks. FIG. 35 shows the vacuum chamber 603, the high voltage
connection to Block A 604, the HDPE insulators 606, Block A with
Calrod element and ceramic cover 607, the steel bases for Blocks A
and B 608, the thermocouple terminals 609, the ceramic tube
supports 610, the terminals for the thermocouple and Calrod element
611, the thermocouples 612, the ground connection to Block B 605,
and the viewing port 613. FIG. 36 shows the relationship between
Block A 601 and Block B 602, the holes for the thermocouples 616,
the Calrod element 614, the ceramic cover 615, the ceramic tube
supports 610, and the steel bases 608.
[0222] This example presents the results of heat transfer gain in a
non-equilibrium plasma. In this experiment, an ORAM high voltage
power supply (model number DSR 100-100-JTTF, input: 240V AC single
phase, 60 Hz, output: 0 to -100 kV rectified) provided the
non-equilibrium plasma between the blocks. Throughout all
experiments, the Calrod element 614 remained attached to the
smaller block A 601, while the larger block B 602 continued to
absorb a portion of the heat conducted from block A. Generally,
block A 601 was connected to the high voltage negative terminal 604
and block B 602 was grounded 605, although a few experiments were
performed to examine the effect of polarity reversal on heat
decay.
[0223] In order to obtain comprehensive information for study,
several variables were introduced into the experimental plan. These
included different gases)(air, Ar, He), different vacuum pressures
(atmosphere, 10.sup.-1 and 10.sup.-2 Torr), different plasma
currents (up to 25 mA and/or voltages up to 50 kV), and different
spacings between the block faces (11/2 inches and 4 inches). For
comparison of data, attempts were made to ensure that two
parameters were held reasonably constant (1) the initial
temperature of block A at the start of the decay process, and (2)
the duration of the heat decay measurement process.
[0224] A non-intrusive instrument for continuously measuring the
temperature of the high-voltage block from outside the cathode was
not available. An infrared optical pyrometer obtained for this
purpose proved unsatisfactory, hence the necessity of using
thermocouples. Even though the voltage control was set low, the
high voltage source could unexpectedly generate a short pulse up to
60 kV when first switched on due to the type of circuit used within
the control unit, and to the inherent cyclical nature of the high
voltage generator.
[0225] Thermocouples were successfully employed to measure the
temperatures of the two blocks. However, the temperature of the
high voltage block (cathode) could not be monitored continuously.
Instead, the high voltage was switched off at five minute intervals
during which temperatures were recorded from digital multimeters
with thermocouple module attachments. The meter arrangement was
then disconnected from access terminals protruding from the side of
the vacuum chamber, and the high voltage was again switched on.
This measuring procedure meant that the high voltage was
disconnected once for a period of approximately 35 to 40 seconds
during each five minute interval. The Calrod heating element
remained disconnected during all heat decay measurements.
Therefore, it presented no obstacle to high voltage plasma
production.
[0226] The complete experimental procedure in air contained the
following stages. The vacuum chamber was pumped down to
approximately 10.sup.-4 Torr. The Calrod heater for block A was
switched on while the pump cycle continued. The heater was switched
off close to the required initial temperature for start of decay
measurements. Pumping down of the vacuum chamber was continued
until the desired pressure was reached. The foregoing steps were
repeated until the desired pressure and initial temperature were
reached.
[0227] Thereafter, the heater was switched off and the initial t=0
details of block A temperature, block B temperature, vacuum
chamber's top skin temperature, and vacuum chamber pressure were
immediately recorded. The high voltage block (cathode) thermocouple
meter arrangement was disconnected. The exposed terminals were
covered with an insulator cap. The high voltage controller was
switched on and smoothly brought it to the required voltage or
current. Each minute thereafter, the grounded block temperature,
vacuum chamber top skin temperature, vacuum pressure, applied
voltage (in kV) and current (mA) displayed on the high voltage
control panel were monitored. At five minute intervals, switched
off the high voltage supply was switched off, the insulating cap
was removed, all terminals exposed from the cap area were briefly
grounded, and the thermocouple meter arrangement was connected as
described above. This was continued for an elapsed time of 131
minutes.
[0228] For experiments in insert gas, one skilled in the art would
recognize that the pumping and heating steps would have to be
modified slightly to ensure the chamber was filled with gas, and
that pressures higher than required were not attained.
[0229] In order to test all instruments, plasma tests were carried
out in air at atmospheric pressure. This served several purposes:
(i) the vacuum chamber door could be left open for observation of
plasma or arcing, (ii) the high voltage equipment could be used
without the background noise of vacuum pumps, etc., for detection
of voltage breakdown, and (iii) the lighting could be dimmed for
better observation of the experimental area.
[0230] The non-equilibrium plasma was first witnessed when the gap
between the block faces was 11/2 inches and the voltage was in the
41 kV to 47 kV range. The non-equilibrium plasma was poorly
visible, being blue-green in color, and was localized and faint.
Attempts to increase the current to a steady flow above 1 mA to
achieve more brightness resulted in arcing problems and voltage
breakdown. 51 kV was found to be the maximum voltage that could be
applied for steady experimental results, but current flow was less
than 0.5 mA, i.e., lower than the multimeter's detection threshold.
The non-equilibrium plasma was being produced and was accompanied
by a sizzling "boiling oil" sound which began at approximately 27
kV. FIG. 37 demonstrates the results when 51 kV was applied in air
at atmospheric pressure.
[0231] Attempts to place two point sources (spike) in block B
(grounded block) in an effort to improve the visible
non-equilibrium plasma volume were ineffective and were not
continued. Non-equilibrium plasma was created at the point tips
from 34 kV, being faint blue-green color. The two spikes were
removed after these initial tests and not used again.
[0232] The first tests in partial vacuum were conducted with all
other conditions being the same. A CCD camera was fixed to the
window of the vacuum chamber so that the non-equilibrium plasma
could be monitored throughout the experiment, and video recorded
when desired. The chamber was pumped down and kept at a steady
pressure of 10.sup.-1 Torr. At this pressure, extensive
non-equilibrium plasma was easily created with voltages less than 1
kV. A steady purple color was visible and an initial experiment was
conducted with approximately 20 mA current. FIG. 38 demonstrates
the results achieved during this trial. In order to investigate how
temperature decay changes, the polarity of the blocks are reversed,
the wiring of the block was changed. Block A was grounded and the
larger block B was connected to the negative high voltage terminal.
All other conditions remained the same, except that the
thermocouples were interchanged for safety reasons. The
experimental run was then repeated. The non-equilibrium plasma that
was formed was extensive, purple, and was captured on videotape.
FIG. 39 compares the temperature decay data when block A acted as
cathode, against the result when polarity was reversed and block B
acted as cathode.
[0233] For the remainder of the experimental study, the heated
block A was kept at negative potential while the passive block B
remained grounded. The gap between the block faces was next
increased to 4 inches, and the resulting effect was examined. This
proved to be a more convenient gap to work with because the larger
spacing meant that greater plasma currents, hence greater electron
densities, could be achieved without arching, and the plasma region
was more visible through the viewing window. This became more
important at lower pressures, when the optically bright part of the
plasma occupied only a portion of the space between the blocks.
[0234] The majority of the remaining tests were therefore carried
out with the 4 inch gap. By reasoning similar to the above, a
larger gap (e.g., 8 inches) may have provided even better results,
but internal constraints within the vacuum chamber prevented this.
Given the limitations of the equipment and geometric constraints,
the 4 inch gap proved most practical. FIGS. 40-42 show the
temperature decay curves in non-equilibrium plasma air, argon, and
helium using the 4 inch gap, placed along with the coated decay
curves.
[0235] Since the non-equilibrium plasma heat decay in air at
10.sup.-1 Torr demonstrated results comparable with the best of the
other gas media tested, air was chosen as the medium to examine the
effect of varying current on the heat decay process. Tests were
conducted at 10, 15, 20, and 25 mA non-equilibrium plasma currents.
Unfortunately, limitations of the high voltage power supply
controller resulted in frequent arcing and voltage breakdowns,
affecting data acquisition process. As a result, some of the
experimental trials were not run to completion. FIG. 43 compares
the heat decay curves at 10.sup.-1 Torr for various non-equilibrium
plasma currents.
[0236] Feasibility tests in air at pressure less than 10.sup.-1
Torr (e.g., 10.sup.-2, 10.sup.-3 Torr) suggest that higher voltages
are required to produce similar non-equilibrium plasma currents.
The low pressure tests also demonstrated that the non-equilibrium
plasma structure within the block gap resembled the classical
discharge model, with glow column and Faraday dark space becoming
more apparent as pressure was decreased. Due to the limitations of
the equipment, maintaining a constant current at the lower
pressures proved to be more difficult, and the arcing problem
became more pronounced.
[0237] FIG. 44 shows a graph of the data collected for air at
10.sup.-2 Torr. Although arcing was again a persistent problem, and
the data collection run was cut short to 40 minutes, it does
provide further evidence of the trend of increased heat transfer
with increased current, even at these low pressures.
[0238] The experimental results are summarized in the graphs FIGS.
37-44. The following is a discussion of some of the features of
these graphs. A parameter called the heat transfer gain coefficient
(.gamma.) is introduced to explain the graphs. In considering a
time dependent heat transfer problem where a metal block is giving
up heat through a thermally conductive neutral gas medium, the heat
decay curve will be determined by the thermal diffusivity
(.alpha.), where .alpha.=.lamda./(C.rho.). C is the heat capacity
of the medium at constant pressure, .lamda. is the thermal
conductivity, and .rho. is the density of the medium.
[0239] In considering a different gas medium, with conductivity
.lamda.', heat capacity C' and density .rho.', then
.alpha.'=.lamda.'/(C'.rho.'). The heat transfer gain coefficient
between these two systems is defined as
.gamma.=.alpha.'/.alpha..
[0240] In considering the case of a neutral gas versus a
non-equilibrium plasma, for the case of a non-equilibrium plasma
with a small ionization fraction .chi.<0.1, the presence of
electrons hardly affects the density of the gas, so
.rho.'.apprxeq..rho..
[0241] In considering heat capacity, the molar heat capacity of
electrons is extremely small with calculated via Fermi Dirac
statistics (Sears, An Introduction to Thermodynamics, the Kinetic
Theory of Gases and Statistical Mechanics, 2nd Ed., Addison-Wesley,
pages 335-337 (1959)). So the contribution of the electrons to C
.rho. can be ignored as a first approximation. Likewise the
contribution of the ionic heat capacity can also be ignored, again
owing to the small ionization fraction. Essentially, this reveals
C'.apprxeq.C. The gain coefficient becomes
.gamma.=.lamda.'/.lamda..
[0242] Since all other system parameters are the same between the
two systems (e.g., pressure, metal composition, radiative heat
flux, etc.), there really is no other mechanism by which heat decay
time can increase or decrease other than by gain change
.gamma.>1 or .gamma.<1 between the two systems. In other
words, .gamma.>1 implies an increase in the thermal conductivity
of the gas medium. The gain coefficient is a convenient way of
comparing systems with exactly same geometries and similar gas
chemistry, only difference being the introduction of
non-equilibrium ionization. If .gamma.>1 between two systems
then there is a gain in thermal conductivity due to non-equilibrium
ionization. If .gamma.<1 then there is a net decrease in thermal
conductivity due to non-equilibrium effects.
[0243] FIGS. 37, 38, 40, 41 and 44 all clearly exhibit (>1, such
that the non-equilibrium plasma effect has been clearly
demonstrated and verified by these data sets. The effect is most
obvious in FIGS. 38, 40 and 41.
[0244] FIG. 42, which is a graph of the data for helium at
10.sup.-1 Torr is very interesting. At first one may be tempted to
infer that the effect is practically nonexistent for He, but that
would be a hasty judgment. In fact, this data set is direct
evidence for the theoretical mechanism of nonequilibrium plasma
conductivity gain. Consider the data presented in FIG. 41 for
argon, and FIG. 42 for helium, both of which are under comparable
pressures and applied power. Ignoring the ionic contribution to 8
as negligible, the gains for these data sets can be written
respectively as
.gamma..sub.41=(.lamda..sub.Ar+.lamda..sub.e)/.lamda..sub.Ar and
.gamma..sub.42=(.lamda..sub.He+.lamda..sub.e)/.lamda..sub.He,
respectively. The primary factors that affect the gain in thermal
conductivity .lamda. are the density of the electrons n.sub.e, and
the electrons' temperature T.sub.e. For midrange and large
ionization fractions .chi., .lamda..sub.e starts to dominate over
the thermal conductivity of heavy ions and heavy neutrals. In view
thereof, a gain in the overall gas thermal conductivity is expected
when electrons are supplied at an elevated temperature via a
non-equilibrium plasma or corona discharge.
[0245] The clear gain visible in FIG. 41 suggests that
.lamda..sub.e is not negligible for the argon case. Now, the first
ionization potential for argon is about 15.3 electron volts, but
for helium it is 35 electron volts (Cobine, Gaseous Conductors,
Dover Publications (1958)). Considering that ionization kinetics
are exponentially dependent on ionization potential, it is entirely
reasonable to assume that applied power to argon will produce more
ionization than the same power applied to helium under similar
conditions. Therefore, .lamda..sub.e for helium should be less than
.lamda..sub.e for argon. Also .lamda..sub.He is an order of
magnitude larger than .lamda..sub.Ar to begin with, so the combined
effects give .lamda..sub.42<<<<.lamda..sub.41, which is
clearly the case when the data in FIG. 42 is compared to FIG. 41.
The practical implications of this are as follows: if it is
necessary to use helium as the working medium, the power supply
should be modified to compensate for helium's high ionization
potential.
[0246] Concerning FIG. 39, this data set exhibits another
interesting phenomenon. Considering FIG. 39, it is clear that the
gain y is actually a function of plasma current. That .gamma.
increases with current is not surprising, since higher currents
lead to larger probabilities of collisional and cascade ionization,
finally leading to higher values of electron density n.sub.e, and
ionization fraction .chi.. What is surprising is the reversal of
.gamma., i.e., .gamma.<1 when the polarity of blocks A and B are
reversed, as can be seen in FIG. 39. In order to gain insight into
this peculiar polarity preference of .gamma., the full 3.times.3
thermal conductivity tensor must be considered and it must be
determined how the components of this tensor depend on the
direction of the applied external electric field vector (Hasse,
Thermodynamics of Irreversible Processes, Dover Publications). This
polarity anisotropy may have significant value in applications
where one would like to control the anisotropy of the thermal
conductivity.
TABLE-US-00001 TABLE 1 Zone 1 Zone 2 Zone 3 Zone 4 Zone 5 Zone 6
Description neutral neutral alpha beta neutral gamma Voltage none
none 20-30 kV 35-40 kV 10-2 kV 12-25 kV decaying Current none none
45-100 mA 4-7 mA 0 mA 6-16 mA Estimated none none 2000 W 245 W none
198 W Wattage Start Time 5:00 11:00 22:11 25:02 30:00 42:17 Stamp
Stop Time 8:00 14:00 24:10 28:01 33:00 45:06 Stamp Elapsed 90 90 59
89 90 79 Frames Positive 7 17 35 35 13 25 Frames % Positive 7.8%
18.9% 59.3% 39.3% 14.4% 31.6% Frames
TABLE-US-00002 TABLE 2 A set of experimental data for liquid wood's
metal atomization FIRST STATE OF SECOND STAGE OF ATOMIZATION
ATOMIZATION PARENT DROPS weight of Weight weight of one Of one
Extractor No. of weight one drop no. of weight of droplet no. of
weight of droplet to source Applied parent of drops (x3) daughter
droplets (y3) daughter droplets (z3) Ratio of Ratio of distance
Voltage current drops (x2) (gm) droplets (y2) (dm) droplets (z2)
(gm) weight weight (mm) (keV) (mA) (x1) (gm) (x2/x1) (y1) (gm)
(y2/y1) (z1) (gm) (z2/z1) x3/y3 x3/z3 0 22-25 10 0.255 0.025 86
0.608 0.0069 4 0 20 10 0.191 0.018 50 0.224 0.0048 4 5 20 30 0.289
0.009 60 0.279 0.0046 2 5 25 8 0.163 0.021 60 0.293 0.0048 4 10 25
7 0.186 0.019 80 0.366 0.0045 4 15 20 4 0.081 0.021 80 0.384 0.0048
4 0.005 0.0012 4 16 15 20 4 0.081 0.021 80 0.384 0.0048 4 0.005
0.0012 4 16 20 25 0.035 17 0.304 0.018 109 0.522 0.0047 4 0.008
0.0012 4 15 30 28 0.048 15 0.299 0.021 146 0.746 0.0051 4 35 29.5
0.048 11 0.211 0.019 100 0.489 0.0048 5 0.003 0.0006 4 32 68 38 3
0.061 0.021 67 0.334 0.0049 7 0.007 0.0011 4 20 Notes: (1) Parent
drop means mechanical drop without electrostatic field. (2)
Extractor Type: bare copper wire and 5.2 cm ring diameter.
TABLE-US-00003 TABLE 3 A set of experimental data for liquid wood's
metal atomization FIRST STATE OF SECOND STAGE OF ATOMIZATION
ATOMIZATION PARENT DROPS weight of Weight weight of one Of one
Extractor No. of weight one drop no. of weight of droplet no. of
weight of droplet to source Applied parent of drops (x3) daughter
droplets (y3) daughter droplets (z3) Ratio of Ratio of distance
Voltage current drops (x2) (gm) droplets (y2) (dm) droplets (z2)
(gm) weight weight (mm) (keV) (mA) (x1) (gm) (x2/x1) (y1) (gm)
(y2/y1) (z1) (gm) (z2/z1) x3/y3 x3/z3 15 16 0.014 7 0.136 0.019 139
0.66 0.0047 6 0.013 0.0021 4 8 15 16 0.014 20 0.398 0.021 129 0.647
0.0051 11 0.023 0.0021 4 9 15 16 0.017 14 0.254 0.025 113 0.572
0.0051 4 0.007 0.0017 4 10 15 17 0.018 0 0.195 0.021 224 1.111
0.0049 5 0.006 0.0012 4 16 15 -- -- 73 1.454 0.021 58 0.267 0.0049
51 0.107 0.0021 4 10 15 17 0.014 4 0.084 0.021 161 0.905 0.0051 4
0.007 0.0017 4 12 Notes: (1) Parent drop means mechanical drop
without electrostatic field. (2) Extractor Type: bare copper wire
and 9 cm ring diameter.
[0247] Each patent application and publication referenced herein is
hereby incorporated by reference herein in its entirety.
[0248] Various modifications of the invention, in addition to those
described herein, will be apparent to one skilled in the art from
the foregoing description. Such modifications are also intended to
fall within the scope of the appended claims.
Example 6
[0249] Practical difficulties arise when cooling molten or
near-molten metals under low pressures. Heat transfer from a metal
surface into a gas medium happens via convection, radiation, and
conduction. For many low pressure and near-vacuum applications,
heat transfer by convection is negligible, while radioactive heat
transfer alone may be insufficient. One solution to this problem is
to increase the thermal conductivity of the gas medium in a
non-intrusive manner by introducing nonequilibrium ionization in
the gas medium. In order to study this nonequilibrium plasma
thermal conductivity gain, a series of control experiments were
performed in a vacuum chamber to establish comprehensive background
data for temperature decay characteristics under a variety of
conditions. Collisions between species is the mechanism by which
transport processes occur in gases. As such the fundamental
quantities are the collision cross sections. Consider ionized Argon
with three species: i, a, e, signifying heavy ions, heavy neutrals,
and electrons, respectively. Furthermore consider the gas to be in
thermal nonequilibrium between the heavy species and the electrons:
i.e., the heavy particles are thermalized at a temperature
T.sub.a=T.sub.i=T.sub.r, while the electrons are thermalized at
T.sub.e>T. The following are collision cross section models for
the case of low pressure and low electron density. Q.sub.xy denotes
the collision cross section between species x and species y. These
expressions for cross section can be adapted to cover higher
n.sub.g regimes by working with tabulated data in:
Q pn = 3.6 .times. 10 - 4 T e - 0.1 ) .times. 10 - 20 ( 1 ) Q aa =
1.7 .times. 10 - 16 T - 0.25 ( 2 ) Q ia = 2.45 .times. 10 - 18 T -
0.9 ( 3 ) Q ii = 2 .pi. ( 2 12 .pi. 0 kT ) 2 ln ( 1 12 .pi. n i (
12 .pi. o kT 2 ) 3 ) ( 4 ) Q ei = .pi. ( 2 12 .pi. 0 kT ) 2 ln ( 1
4 .pi. n e ( 4 .pi. o kT 2 ( 1 + Ta / T ) ) 3 ) ( 5 )
##EQU00001##
where e is the electron charge, eo is the permittivity of vacuum, k
is Boltzmann's constant, and n.sub.i, n.sub.e are the ion and
electron number densities. All units are MKS. Electron-electron
collision are ignored in the small .PSI. (ionization fraction)
approximation. Collision frequencies can be obtained by combining
the relations (1), (6) with the Maxwellian velocities of each
species:
v e = u e ( n i Q at + n a Q aa ) ( 6 ) v i = u i ( n e Q et + n a
Q ia + n a Q ii ) ( 7 ) v a = u a ( n i Q ia + n e Q ea + n a Q aa
) ( 8 ) u k = 8 kt k .pi. mk ; k = i , a , e . ( 9 )
##EQU00002##
Thermal conductivity in terms of the collision frequencies is the
sum of the contributions from all three species:
.lamda.=.lamda..sub.a+.lamda..sub.i+.lamda..sub.e (10)
where
.lamda. o = 75 2 k 2 T 32 m a n a v a ( 11 ) .lamda. i = 75 2 k 2 T
32 m i n i v i ( 12 ) .lamda. e = 2 k 1 k 2 k - 2 T e m e n e v e (
13 ) ##EQU00003##
and K.sub.1, K.sub.2 are kinetic correction factors typically in
the range of 1.16.ltoreq.K.sub.i<2.8 and
0.3.ltoreq.K.sub.2.ltoreq.1.5. When the ionization fraction is
zero, the model gives the usual values of thermal conductivity for
neutral Argon. From equations (10)-(13) the primary factors that
affect the gain in thermal conductivity .lamda. are the density of
the electrons n.sub.e, and their temperature T.sub.e. For midrange
and large .chi..lamda..sub.e starts to dominate over the other
terms in (10). Based on this a gain is expected in the overall gas
thermal conductivity when electrons are supplied at an elevated
temperature via a nonequilibrium glow or corona discharge. In this
example, a stainless steel disk, block A, is heated using a 1 kW
Calrod element. A large stainless steel cylinder, block B, is
located at a distance of 4'' from block A and functions primarily
as an electrode, and secondarily as a heat sink. FIG. 49 shows a
schematic diagram of the experimental setup, and FIG. 50 shows the
dimensions of the blocks. For the temperature decay process,
measurements were taken in preset pressure stages. Measurements
were repeated using various inert gases as described. This example
reports the experimental results of heat transfer gain in
nonequilibrium plasmas. In this experiment, an ORAM high voltage
power supply, Model number DSR 100-100-JTTF input 240 VAC single
phase 50 H2, output, 0 to -200 kV rectified provided the
nonequilibrium plasma between the blocks. Throughout all
experiments, the Calrod element remained attached to the smaller
block A, while the larger block B continued to absorb a portion of
the heat conducted from block A. Generally, block A was connected
to the high voltage negative terminal and block B was grounded,
although a few experiments were performed to examine the effect of
polarity reversal on heat decay. In order to obtain comprehensive
information for study, several variables were introduced into the
experimental plan. These included: a) different gases (air, Ar,
He); b) different vacuum pressures (atmospheric, 10.sup.-1 and
10.sup.-2 Torr); c) different plasma currents (up to 25 mA and/or
voltages up to 50 kV); d) different spacings between the block
faces (11/2'' and 4''). However, for comparison of data, attempts
were made to ensure that two parameters were held reasonably
constant: a) the initial temperature of block A at the start of the
decay process; b) the duration of the heat decay measurement
process. The equipment available presented the following
limitations upon the experimental procedure: [0250] i) A
non-intrusive instrument for continuously measuring the temperature
of the high-voltage block from outside the cathode was not
available. An infrared optical pyrometer obtained for this purpose
proved unsatisfactory, hence the necessity of using thermocouples.
[0251] ii) Even though the voltage control was set low, the high
voltage source could unexpectedly generate a short pulse up to 60
kV when first switched on. This was due to the type of circuit used
within the control unit, and to the inherent cyclical nature of the
high voltage generator. The following measuring procedure was
therefore adopted: [0252] a) Thermocouples were successfully
employed to measure the temperatures of the two blocks. However,
the temperature of the high voltage block cathode could not be
monitored continuously. Instead, the high voltage was switched off
at five minute intervals during which temperatures were recorded
from digital multimeters with thermocouple module attachments.
[0253] b) The meter arrangement was then disconnected from access
terminals protruding from the side of the vacuum chamber, and the
high voltage was again switched on. This measuring procedure meant
that the high voltage was disconnected once for a period of
approximately 35 to 40 seconds during each five minute interval.
[0254] c) The Calrod heating element remained disconnected during
all heat decay measurements. Therefore, it presented no obstacle to
high voltage plasma production. A complete experimental procedure
in air would consist of the following stages: 1. Pump down vacuum
chamber to approximately 10.sup.-4 Torr. 2. Switch on Calrod heater
for block A while pump cycle continues. 3. Switch off heater at
close to required initial temperature for start of decay
measurements. 4. Continue pumping down from vacuum chamber to
desired pressure. 5. Repeat steps 2 through 4 until desired
pressure and initial temperature are reached. [0255] 6. Switch off
heater and immediately record the initial t=0 details of block A
temperature, block B temperature, vacuum chamber's top skin
temperature, and vacuum chamber pressure. 7. Disconnect high
voltage block (cathode) thermocouple meter arrangement. 8. Cover
exposed terminal with an insulator cap. 9. Switch on high voltage
controller and smoothly bring it to the required voltage or
current. [0256] 10. Each minute thereafter, monitoring grounded
block temperature, vacuum chamber top skin temperature, vacuum
pressure, applied voltage (in kV) and current (mA) displayed on the
high voltage control panel. [0257] 11. At five minute intervals,
switch off the high voltage supply, remove the insulating cap,
briefly ground all terminals exposed from the cap area and connect
the thermocouple meter arrangement in the manner described in a)
through c) above. 12. Continue for an elapsed time of 131 minutes.
For experiments in inert gas, steps 4 and 5 would be modified
slightly to ensure the chamber was filled with gas, and that
pressures higher than required were not attained. In this example,
the following observations can be made: 1. In order to test all
instrumentation, the initial plasma tests were carried out in air
at atmospheric pressure. This served several purposes; [0258] a)
the vacuum chamber door could be left open for observation of
plasma or arcing. [0259] b) the high voltage equipment could be
used without the background noise of the vacuum pumps, etc., for
detection of voltage breakdown. [0260] c) the lighting could be
dimmed for better observation of the experimental area. 2. Plasma
was first witnessed when the gap between the block faces was 11/2''
and the voltage was in the 41 kV to 47 kV range. The plasma was
poorly visible, being blue-green in colour, and was localized and
faint. Attempts to increase the current to a steady flow above 1 mA
to achieve more brightness resulted in arcing problems and voltage
breakdown. 51 kV was found to be the maximum voltage that could be
applied for steady experimental results, but current flow was less
than 0.5 mA, i.e., lower than the multimeter's detection threshold.
However, plasma was being produced and was accompanied by a
sizzling "boiling oil" sound which began at approximately 27 kV.
FIG. 51 demonstrates the results when 51 kV was applied in air at
atmospheric pressure. 3. Attempts to place two point sources
(spikes) in block B (grounded block) in an effort to improve the
visible plasma volume were ineffective and were not continued.
Plasma was created at the point tips from 34 kV, being a faint
blue-green colour. The two spikes were removed after these initial
tests and not used again. 4. The first test in partial vacuum was
conducted with all other conditions same as in 2 above. A CCD
camera was fixed to the window of the vacuum chamber so that the
plasma could be monitored throughout the experiment, and video
recorded when desired. The chamber was pumped down and kept at a
steady pressure of 10.sup.-1 Torr. At this pressure, extensive
plasma was easily created with voltages less than 1 kV. A steady
purple color was visible and an initial experiment was conducted
with approximately 20 mA current. FIG. 52 demonstrates the results
achieved during this trial. In order to investigate how temperature
decay changes when the polarity of the blocks are reversed, the
wiring of the block was changed. Block A was grounded and the block
B was connected to the negative high voltage terminal. All other
conditions remained the same, except that the thermocouples were
interchanged for safety reasons. The experimental run was then
repeated. The plasma that was formed was extensively purple. FIG.
53 compares the temperature decay data when block A acted as
cathode, against the result when polarity was reversed and block B
acted as cathode.
[0261] 5. For the remainder of the experimental study, the heated
block A was kept at negative potential while the passive block B
remained grounded.
[0262] 6. The gap between the block faces was next increased to
4'', and the resulting effect was examined. This proved to be a
more convenient gap to work with because the larger spacing meant
that: [0263] a) greater plasma currents, hence greater electron
densities, could be achieved without arcing. [0264] b) the plasma
region was more visible through the viewing window. This became
more important at lower pressures, when the optically bright part
of the plasma occupied only a portion of the space between the
blocks.
[0265] The majority of the remaining tests were therefore carried
out with the 4'' gap. By reasoning similar to the above, a larger
gap, e.g., 8'' may have provided even better results, but internal
constraints within the vacuum chamber prevented this. Given the
limitations of the equipment and geometric constraints, the 4'' gap
proved most practical. FIGS. 54-56 show the temperature decay
curves in plasma air, Argon, and Helium using the 4'' gap, plotted
along with the decay curves.
7. Since the plasma heat decay in air at 10.sup.-1 Torr
demonstrated results comparable with the best of the other gas
media tested, air was chosed as the medium to examine the effect of
varying current on the heat decay process. Tests were conducted at
10, 15, 20, and 25 mA plasma currents. Unfortunately, limitations
of the high voltage power supply controller resulted in frequent
arcing and voltage breakdowns, affecting data acquisition process.
As a result, some of the experimental trials were not run to
completion. 8. Feasibility tests in air at pressures less the
10.sup.-1 Torr (say 10.sup.-2, 10.sup.-3 Torr) suggest that higher
voltages are required to produce similar plasma currents. The low
pressure tests also demonstrated that the plasma structure within
the block gap resembled the classical discharge model, with glow
column and Faraday dark space becoming more apparent as pressure
was decreased. Due to the limitations of the equipment, maintaining
a constant current at the lower pressures proved to be more
difficult, and the arching problem became more pronounced.
[0266] FIG. 57 shows a graph of the data collected for air at
10.sup.-2 Torr. Although arcing was again a persistent problem, and
the data collection run was cut short to 40 minutes, it does
provide further evidence of the trend of increased heat transfer
with increased current, even at these low pressures.
The experimental results are summarized in the graphs FIG. 51 to
FIG. 57. The following is a discussion of some of the features of
these graphs.
[0267] A heat transfer coefficient, .gamma..sub..chi., is first
introduced. Consider a time dependent heat transfer problem where a
metal block is giving up heat through a thermally conductive
neutral gas medium the heat decay curve will be determined by the
thermal diffusivity.
a = .lamda. C .rho. . ##EQU00004##
Where C is the heat capacity of the medium at constant pressure,
.lamda. is the thermal conductivity, and .rho. is the density of
the medium. Now consider a different gas medium, with conductivity
.lamda., heat capacity C.sup.1, and density .rho..sup.1. In this
case we have
a ' = .lamda. ' C ' .rho. ' ##EQU00005##
The heat transfer gain coefficient between these two systems can be
defined as
.gamma. = n ' .chi. . ##EQU00006##
Now, consider the case of neutral gas vs. plasma. For the case of
plasma with small ionization fraction X<0.1, the presence of
electrons hardly affects the density of the gas, so
.mu.'.apprxeq..mu..
Now consider heat capacity. The molar heat capacity of electrons is
extremely small when calculated via Fermi Dixac statistic. So the
contribution of the electrons to C.sub.p can be ignored as a first
approximation. Likewise the contribution of the ionic heat capacity
can also be ignored, again owing to the small ionization fraction.
Therefore,
C'.apprxeq.C.
The gain coefficient becomes
.gamma. = .lamda. ' .lamda. . ##EQU00007##
Since all other system parameters are the same between the two
systems (pressure, metal composition, radiative heat flux, etc.),
there really is no other mechanism by which heat decay time can
increase or decrease other than by gain change .gamma.>1 or
<1 between the two systems. In other words >1 implies an
increase in the thermal conductivity of the gas medium. The gain
coefficient is a convenient way of comparing systems with exactly
same geometries and similar gas chemistry, the only difference
being the introduction of nonequilibrium ionization. If
.gamma.>1 between two systems then there is a gain in thermal
conductivity due to nonequilibrium ionization. If <1 then there
is a net decrease in thermal conductivity due to nonequilibrium
effects, which is an interesting and possibly useful phenomenon in
its own right. 1. FIGS. 51, 52, 54, 55, 57 all clearly exhibit
.gamma.>1, and accordingly, the nonequilibrium plasma effect to
have been clearly demonstrated. The effect is most obvious in FIGS.
52, 54 and 55. 2. FIG. 56, which is a graph of the data for Helium
at 10.sup.-1 Torr, shows the mechanism of nonequilibrium plasma
conductivity gain discussed above. Consider the data presented in
FIG. 55 for Argon, and FIG. 57 for Helium, both of which are under
comparable pressures and applied power. Ignoring the ionic
contribution to .lamda. as negligible, the gains for these data
sets can be written respectively as
.gamma. 6 = .lamda. Ar + .lamda. e .lamda. Ar ##EQU00008## and
##EQU00008.2## .gamma. 7 = .lamda. He + .lamda. e .lamda. He .
##EQU00008.3##
The clear gain visible in FIG. 55 suggests that .lamda..sub.e is
not negligible for the Argon case. Now, the first ionization
potential for Argon is .apprxeq.15.3 electron volts, but for
Helium, it is 35 eV. When considering that ionization kinetics are
exponentially dependent on ionization potential, it is entirely
reasonable to assume that applied power to Argon will produce more
ionization than the same power applied to Helium under similar
conditions. Therefore .lamda..sub.e for Helium should be less than
.lamda..sub.e for Argon. Also, .lamda..sub.He is an order of
magnitude larger than .lamda..sub.Ar to begin with, so the combined
effects give
.gamma.<<.gamma..sub.6
which is clearly the case when the data in FIG. 56 is compared to
FIG. 55.
[0268] The practical implications of this are as follows. If it is
necessary to use Helium as the working medium, the power supply
should be modified to compensate for Helium's high ionization
potential.
3. Concerning FIG. 53. Referring to FIG. 57, it is clear that the
gain .gamma. is actually a function of plasma current, i.e.,
.gamma.=.gamma. (I). That .gamma. increases with current is not
surprising, since higher currents lead to larger probabilities of
collisional and cascade ionization, finally leading to higher
values of electron density n.sub.e and ionization fraction .chi..
What is surprising is the reversal of .gamma. i.e., .gamma.<1
when the polarity of blocks A and B are reversed, as can be seen in
FIG. 53. It now remains to assign some numerical estimates to the
actual values of .gamma.. The objective is to provide a simple
model that can provide curves similar to the data sets in FIGS.
51-57, from which .gamma. values can be inferred.
[0269] When gas temperature and electron density are actually
distributed unevenly throughout a gas/plasma medium, the terminal
conductivities of the gas .lamda. and plasma .lamda. are actually
functions of x, y, z, t. Furthermore .lamda..sup.1 depends on the
electron density n.sub.e and the electron temperature T.sub.e both
of which are functions of x, y, z, t, applied electric field, and
current. So it is unrealistic to attempt to solve for n.sub.e and
hence .lamda..sup.1 directly. However, it is possible solve for
constant effective .lamda., .lamda..sup.1 which model the net
transport effect of the neutral gas and plasma. The resulting gain
factor .gamma.=.lamda..sup.1/.lamda.0 will be a good estimate of
the true gain.
[0270] First, a model constructed by making a number of assumptions
which drastically simplify the problem numerically without losing
any essential detail of the experimentally observed physical
behavior. [0271] a) Since the experimental setup is almost
cylindrically symmetric, we map the vacuum chamber onto a
axisymmetric region in (z, r). Block A, block B, the insulator
section, and the plasma region will be considered axisymmetric
subregions of the vacuum chamber region. [0272] b) The plasma, when
present, is confined to a region surrounding both block A and block
B. The extents of the plasma are chosen to be rather large (not
just confined to the gap between blocks A and B). Choosing a large
plasma volume a priori is a conservative policy, because if the
plasma is measured by probes and found to be in fact smaller, then
the .gamma. calculated with the larger plasma volume will be an
underestimate of the true .gamma.. [0273] c) For any given .gamma.
calculation problem, all thermophysical properties are considered
constant. [0274] d) Radiation reflected from the inner gap face of
block B is ignored. [0275] e) The Calrod element is eliminated and
approximated by a thermal power density applied to block A. [0276]
f) Radiative losses of the insulator and block A are approximated
by equivalent thermal power density losses within each their
respective regions. [0277] g) The boundary of the vacuum chamber is
convectively coupled to the external ambient environment. Actually,
this boundary condition can be fixed to some temperature (310K, for
instance) without much difference to the overall model. Convective
coupling is handy in order to model the skin temperature of the
vacuum chamber. [0278] h) Radiation of block B is ignored entirely.
Since block B's temperature range is typically
300K.ltoreq.u.sub.B.ltoreq.450K, the error introduced by this
assumption is small. It is desired to find the solution of the
initial-boundary value problem for the time-dependent heat
equation.
[0278] .gradient. - ( .lamda. .gradient. u ( z , r , t ) ) - S + S
R + C p .differential. .differential. t u ( x , r , t ) = 0
##EQU00009##
Where C is heat capacity at constant pressure, .rho. is density, S
is the applied power density source term, and S.sub.R is the power
density loss due to radiation. The time dependent problem begins
when the applied power is switched off, i.e., S=0 for a pure
temperature decay problem.
[0279] The initial value is the initial temperature distribution at
t-0.sub.1
u(z,r,0)=u.sub.0(z,r).
obtained by solving the steady state problem
.gradient.(-.lamda..gradient.u(z,r))-S+R.sub.R=O.
with applied thermal power density source S and radiative power
density loss S.sub.R.
[0280] Each region has its own thermophysical properties .lamda.,
p, C, S, and S.sub.R. Radiative boundary conditions are treated in
the following manner. The total power loss P.sub.R for a region due
to radiation is
P.sub.R=A.sub.R.di-elect cons.(u.sup.4-u.sub.1.sup.4),
where A.sub.R is the exposed radiating surface area of the region
is the region's emissivity, and u.sub.1 is approximated by a
constant reference temperature in the range 300K-450K. If V.sub.R
is the volume of the radiating region, the radiative power loss
density with the region becomes
S R = P R V R = A R ( u 4 - u 1 4 ) V R . ##EQU00010##
[0281] Experimental data sets like those in FIG. 53 consist of a
control temperature decay curve, where there is no plasma, and one
or more decay curves with plasma present. By fitting the time
dependence of U(z, r, t) to curve sets like FIG. 53, we can
directly compute estimates for .gamma. can be directly
computed.
The above initial-boundary value problem would be straightforward
if it were not for the following difficulties: [0282] 1. Exact
values for the thermophysical properties are not known a priori
because the temperature distribution in the vacuum chamber is not
uniform. Furthermore, there are slight pressure variations in the
experimental data due to the changes in temperature as a function
of time. The exact values for the emissivities of the materials
under the conditions of the experiment are also unknown. [0283] 2.
Simplification of the geometry, as discussed earlier, has the side
effect of modifying some material properties in ways that cannot be
known beforehand. [0284] 3. Ideally it is desired to establish
realistic bounds for many problem parameters and allow them to vary
within these bounds. The following, however, are known:
theoretically, for a given data set (like FIG. 53) there should be
only one parameter upon which the difference in shape between
control and plasma decay curves depend: =.lamda..sup.1/.lamda.. In
other words, there should be only one unknown parameter. In light
of this, a procedure for obtaining this type of parameter
estimation is described. The technique can constructed for general
cases, where IBV models must be fit to curve sets that have
dependencies on N unknown parameters. Description is provided where
N=1.
[0285] Let .phi..sub.1, .phi..sub.2, . . . , .phi..sub.k be all
parameters which define the model, in no particular order. These
include geometric dimensions of the primary region and all
subregions, all coefficients appearing in the partial differential
equations, and all parameters appearing in the boundary conditions.
Suppose that M curves to which curve-fit parameters under the
hypothesis that these curve fits differ from each other by changes
in one parameter. For the sake of simplicity, let us consider the
case in FIG. 53 where M=2, i.e., there is an experimentally
determined control curve f, and an experimentally determined plasma
curve f'. Suppose some set of numbers
{.phi..sub.1,.phi..sub.2, . . . , .phi..sub.k}
results in a tolerable curve fit for f. Consider another set of
numerical data
{.phi.'.sub.1,.phi.'.sub.2, . . . , .phi.'.sub.k}
that gives a tolerable curve fit for f'. Now, consider the
variations o{acute over (o)}.sub.j between these parameters:
.delta..phi..sub.j=.phi..sub.j-.phi.'.sub.j
For f and f' to be related by at most one parameter, all these
variations must be zero, save for one:
.delta..phi..sub.1=0,.delta..phi..sub.2=0, . . . ,
.delta..phi..sub.k-1=0,.delta..sub.K.noteq.0.
If the curve fits to f, f' are good, and the IVB is well defined,
the resulting estimates for the uncertain fixed parameters
.phi..sub.1, .phi..sub.2 . . . , .phi..sub.k can be quite accurate,
barring pathological cases.
[0286] The following describes the application of this technique to
the f, f' curves in FIG. 53. Some preliminary observations: [0287]
1. Consider all geometric data as fixed, and not subject to the
variation minimization procedure. [0288] 2. Express all
thermophysical properties as .phi..sub.jx textbook values. Estimate
gas density from equation of state and use this as a base value.
Let textbook thermophysical properties and ideal gas density
estimates be superscripted by 0. [0289] 3. Define and organize all
model parameters using a logical notation. Superscripts for .phi.
are used as labels to identify what property the parameters
affects. Subscripts indicate materials s=steel, i=insulator, g=gas
(Argon), p=plasma. [0290] 4. It is possible to get an upper
estimate on the maximum power density applied to block A. The
Calrod heater is rated at 1000 W, and the volume of block A is
4.633.times.10.sup.-4 m.sup.3, giving a maximum power density of
S=2.158.times.10.sup.6. But since this quantity is not exactly it
can be replaced with .phi..sup.5. This is further constrained by
the requirement that the initial model temperature be close to the
initial experimental temperature in block A. Here is a complete
summary of the model, and all parameters used and their initial
estimates. All values are in MKS units Steady state problem.
[0290]
.gradient.(-.lamda..gradient.u(x,r))-.phi..sup.SS+.phi..sup.nS.su-
b.r=U,.lamda..sub.g=.phi..sub.g.sup..lamda..lamda..sub.g.sup.0,.lamda..sub-
.g.sup.0=0.01799 (14)
.lamda..sub.2-.phi..sub.3.sup..lamda..lamda..sub.3.sup.0,.lamda..sub.3.s-
up.0=23.43 (15)
.lamda..sub.i=.phi..sub.i.sup..lamda..lamda..sub.i.sup.0,.lamda..sub.i.s-
up.0=3.096 (16)
.di-elect cons..sub.S.sup.0=0.08 (17)
.di-elect cons..sub.1.sup.0=0.18 (18)
Time dependent problem.
.gradient. ( - .lamda. .gradient. u ( z , r , t ) ) + .phi. R S R +
C P .differential. .differential. t u ( z , r , t ) = 0 p a C a =
.phi. g pc p g 0 C g 0 , p g 0 = 1.784 .times. 10 - 4 , C g 0 = 518
( 19 ) p s C s = .phi. s pc p s 0 C s 0 , p s 0 = 7758 , C s 0 =
431 ( 20 ) p i C i = .phi. i pc p i 0 C i 0 , p i 0 = 1595 , C i 0
= 753.1 ( 21 ) .lamda. p = .phi. p .lamda. .lamda. g ( 22 )
##EQU00011##
.gamma.=.sub..lamda..sub.s.sup..lamda..sup.p-.delta.o.sub.p.sup..lamda.
i 1.
It is possible to retain .sub..phi..sup..lamda.p as the single
parameter relating curves f and f' while reducing the variation of
all other parameters to zero. This gives the estimate for
conductivity gain. The procedure outlined above is extremely time
consuming to implement, so the estimation of .gamma. is limited to
the case of FIG. 53. The larger gap is preferable due to the
smaller effect of reflected radiation. After consideration
iterations, the parameter set can be obtained:
TABLE-US-00004 .phi. Curve f Curve f' Variation .delta..phi.
.phi..sup.R 1.0 1.0 0.0 .phi..sup.S 0.82 0.848 -0.028
.phi..sub.g.sup.oC 5.0 3.0 0.0 .phi..sub.i.sup.pO 1.8 1.8 0.0
.phi..sub.g.sup..lamda. 1.0 1.0 0.0 .phi..sub.p.sup..lamda. 1.0
10.5 0.5
All other parameters are simply unity, with zero variation. The
conductivity gain derived from the data set in FIG. 53 is thus
.gamma..sub.4=10.5.
Curve fitting results for f, f' are displayed in FIGS. 58-59. Note
that a small variation in o.sup.5 was necessary in order to match
the slightly different experimental initial temperatures in curves
f and f'. When considering that the plasma is extinguished for at
least 10% of the time during the measurement time, the true plasma
conductivity gain would be somewhat greater than 10.5.
Example 7
[0291] Calculating .gamma. can be relied only on the temperature
decay curves f(t) (no plasma) and f'.sup.1 on block B by solving
the 3D axisymmetric boundary value problem. It can be assumed that
block B is isothermal.
[0292] This example is set up by assuming that the plasma is large
and not confined to the interblock gap, ignoring radiation and
convection, as well as the insulator section, and using same
dimensions for blocks and vacuum chamber as the example 6, and
making the plasma region larger than the gap. The temperature of
the vacuum chamber boundary is fixed at T=300K For the exact
dimensions used. Also, the following thermophysical properties are
used (all units are MKS):
.sup.pmetal=8000 .sup.pgas=1.0
.sup.cmetal=431 .sup.Cgas=518
.sup.kmetal=29 .sup.kgas=0.018
[0293] These figures give us the thermal diffusivities
.sup..alpha.metal=8.4.times.10.sup.-6
.alpha.gas=3.48.times.10.sup.-5
which can be used in the time dependent problem. Subsequently, the
steady state problem is solved to obtain the initial temperature
distribution T (z, r) and then plug this in as the initial value T
(r, z, t=0) to a time-dependent boundary value problem. The time
dependent problem is solved twice. Once for .gamma.=1 (no
"plasma"), and then again for .gamma.=10 ("plasma"). Here are the
block A temperature decay curves for this fictitious problem, as
shown in FIG. 58: Now, the lumped model for conduction is
T.sub.2=T.sub.s T(T.sub.1-T.sub.s)e.sup.-3 t2-t3
where
.beta. = aA V .DELTA. x ##EQU00012##
If all factors (other than k) remain the same between the control
run and the plasma run, then the ratio of c's gives you the gain in
conductivity:
.gamma. = ' ##EQU00013##
This yields -0.94 for the argon data. In other words, the ratio of
c's obtained from the lumped model should give us roughly the same
.gamma. I started with. We can solve for c:
.beta. = - 1 t 2 - t 2 log ( - ( T 2 - T S ) - T 2 - T s )
##EQU00014##
As long as 300K<T.sub.5<T.sub.2, T.sub.5 can be any value.
From FIG. 60, we have
T.sub.1(control)=570 (2)
T.sub.2(control)=329 (3)
T.sub.1(plasma)=570 (4)
T.sub.2(plasma)=304 (5)
t=0 (6)
t.sub.2=600 (7)
Taking the following values for isothermal T.sub.5 (again, any
value can be taken, they are just to illustrate the issues):
T.sub.S(control)=320 (8)
T.sub.S(plasma)=303 (9)
The following values for are obtained:
.beta.=5.54.times.10.sup.-3
.beta.'=9.312.times.10.sup.-3
and a conductivity gain of
.gamma. = .beta. ' .beta. = 1.681 . ##EQU00015##
This is very much different from the value=10.0 upon which this
sample problem is based. It may be worthwhile to check the Biot
modulus Bi for this problem.
Example 8
[0294] The example shows how colored water could be atomized
successfully using a high voltage DC source alone, and how
experiments employing a eutectic alloy (wood's metal) looked
promising. The apparatus used was to be substantially refined
before further liquid metal experiments continued.
[0295] An apparatus has been employed that has permitted limited
atomization of liquid metal when the flow is in an upward direction
against gravity, or in a downward direction with gravity. A number
of controlling parameters such as nozzle size, ambient conditions,
spacing between electrodes, dielectric medium between electrodes,
and shape and size of extractor electrode have been varied such
that an optimum environment for atomization of liquid metal to
occur using only a DC source can be approached.
[0296] The results are very encouraging and further substantial
improvement ins anticipated as the experimental apparatus is
further refined, and when a high rise-rate AC signal is
incorporated into the system. [0297] i) Liquid metal flow against
the direction of gravity, and [0298] ii) Liquid metal flow in the
direction of gravity.
i) Liquid Metal Flow Against the Direction of Gravity.
[0299] The apparatus used for these experiments is shown in FIG.
73. A copper reservoir is connected to the nozzle assembly by means
of a copper tube. Solid pieces of eutectic alloy (wood's metal) are
placed in the reservoir and heated to approximately 100.degree. C.
by radiation from two 150 watt halogen bulbs. A pressure difference
between the wider reservoir and the nozzle causes the liquid to
flow to the tip of the nozzle, and the flow rate from the nozzle
can be regulated by the amount of alloy placed in the reservoir.
The reservoir and nozzle assembly is connected directly to the high
voltage terminal. A copper spheroid which is placed within
1/16.sup.th of an inch from the nozzle tip surrounds the
replaceable nozzle, and a copper tube thermally connects this
spheroid to the reservoir assembly. In this way, not only heat is
imparted to the eutectic alloy by conduction along the majority of
its path, but also a charge buildup area has been extended. The
whole arrangement is painted black to behave as a black body
absorber. A brass extractor ring is suspended at varying distances
directly above the nozzle opening and is connected to a grounded
electrical terminal by means of an adjustable mounting
assembly.
ii) Liquid Metal Flow in The Direction of Gravity.
[0300] In this instance, a similarly copper reservoir is connected
to a nozzle assembly by means of a short downward tube of thinner
internal diameter than the reservoir, and this arrangement is
attached directly to the positive high voltage terminal. An
extractor ring sits at varying distances below, or to the side of
the nozzle tip, and a collector cup, a partially filled with water
sits for collection of atomized samples, directly below the
nozzle.
[0301] A sensitive electronic balance was used to weigh drops and
droplets from atomization experiments. Control experiments were
performed when drops were produced by gravitational force and free
hydrostatic pressure (for downward liquid) alone. Repeat
experiments were then conducted using a similar head of molten
metal, and a high voltage (max 36 keV, max 0.2 mA) was applied to
the fluid. Equal number of drops and droplets (nominally 100) were
weighed, and their weights compared. FIGS. 71 and 72 show the drops
and droplets collected from one such series of experiments. For
each figure the larger drops (upper portion of the figure) are
those collected during the control experiments, and the smaller
droplets (lower portion of the figure) are those collected during
experiments using electrostatic field.
[0302] Under normal conditions, atomization of a liquid metal
utilizing an electric field between two electrodes alone is
hampered by two over-riding factors: [0303] a) In air, at
atmospheric pressure, arcing occurs at a voltage lower than that
required to atomize the liquid metal by electrostatic means. [0304]
b) At pressures slightly less than atmospheric, there is an
inability to create potential differences between the electrodes
sufficiently high enough to enable atomization to occur because the
formation of plasma permits an easy path for current flow.
[0305] As a way of overcoming these voltage breakdown problems, a
piece of CPVC pipe, 1/8.sup.th of an inch thick was placed in such
a way as to surround the extractor ring and its supporting arm (see
FIGS. 74 to 76, and FIGS. 65 to 69). By incorporating this CPVC
into the assembly, substantially higher potential differences
between nozzle and extractor could be achieved before arcing
occurred. This meant that more electrical energy was available for
atomization, though the distance between the electrodes could only
be increased to a point where leaking would occur to neighboring
components.
[0306] Liquid Metal Flow Against the Direction of Gravity.
[0307] The nozzle aperture that permitted the best atomization was
of the order of 0.3 to 0.4 mm. For apertures smaller that this,
difficulty was encountered in securing a liquid metal flow.
Apertures larger than this permitted too great a surface area of
liquid metal at the nozzle tip than could be atomized by the
experimental apparatus. Liquid metal atomization was captured on
video camera, but could only continue for as long as the head of
pressure in the reservoir permitted flow of fluid to the nozzle
tip. A prelude to atomization can be witnessed as high voltage is
increased by either an increase in fluid flow rate, or as a vertex
(Taylor Cone) forming on the surface of the liquid metal at the tip
of the nozzle. If too long a time elapsed between achieving a
complete flow of fluid from the reservoir to the nozzle tip and
increasing the high voltage from zero to a level sufficient to
permit atomization, then satisfactory atomization would not occur.
Instead, cooling and oxidation of the free liquid surface would be
apparent and the liquid metal would flow downward. The potential
difference between the nozzle and the grounded steel base plate was
sufficient to increase the liquid flow. Thus, a "pipe" of
solidified metal would slowly form down the side of the nozzle
assembly until the pressure difference at the reservoir decreased
to zero.
[0308] In this manner, efforts to achieve atomization by means of a
combination of mechanical and electrical forces were successful.
One such experiment involved placing a plunger over the open end of
the reservoir to increase the pressure difference at the nozzle
tip. In this case, production of droplets by electrostatic
atomization required less potential difference between the
electrodes, and droplet sizes were smaller.
[0309] In another experiment, the eutectic alloy was removed from
the reservoir assembly and was replaced by water. The water was
then drained from the assembly, leaving the inner walls wet. The
eutectic alloy was the returned to the reservoir and reheated. When
liquid metal flow from the nozzle occurred, drops were ejected by
the force of stream that was trapped within the nozzle assembly.
When a high voltage was applied, the drops became atomized into
smaller droplets, some of which adhered to the undersided of the
CPVC.
[0310] FIGS. 74, 75 and 76 show consecutive frames of the
atomization of liquid metal against gravity without any applied
mechanical force other than that due to the head of liquid in the
reservoir.
[0311] Liquid Metal Flow in the Direction of Gravity.
[0312] Liquid metal flow in this direction can be controlled much
more satisfactorily. Conditions can be created such that a
continuous flow of molten metal is produced and nozzle apertures
can be much smaller. Currently nozzles with apertures in the region
of 0.1 to 0.15 mm are being employed and can be such that
gravitational forces alone cannot overcome the adhesive forces of
surface tension upon the inner walls of the nozzle, and the
cohesive forces within the surface of the metal. In such instances,
electrostatic atomization can produce droplets with diameters of
substantially smaller magnitude.
[0313] Rapid cooling during droplet flight provides this evidence.
If the flight path were sufficiently long enough, and the
environment in close proximity to the apparatus could be maintained
above the temperature of the liquid metal melting point then the
droplets would achieve a more nearly spherical shape.
[0314] This is true for a particular stage of electrostatic
atomization. At the time of the Interim Report, we had reached a
stage that we now dub "primary atomization". Since October
28.sup.th, we have had some partial success in achieving secondary
and (to a lesser extent) tertiary atomization. Rapid cooling during
flight continues to provide this evidence.
[0315] Evident by experimental observation. Current nozzles being
employed are stainless steel tubing with an outer diameter (O.D.)
of 0.012 inch (0.3 mm) and an inner diameter (I.D.) of 0.006 inch
(0.15 mm).
[0316] Binary division occurs in some instances, but our ongoing
studies suggest that the production of smaller droplets may not be
due simply to successive binary division. This matter is discussed
further herein.
[0317] Undoubtedly this is true. However, current research is aimed
at discovering whom, and in what manner smaller droplets are
produced.
[0318] Until we can employ a high resolution camera utilizing high
speed film, we have no further comment upon this phenomenon.
Example 9
[0319] Rapid cooling during droplet flight provides this evidence.
If the flight path were sufficiently long enough, and the
environment in close proximity to the apparatus could be maintained
above the temperature of the liquid metal melting point then the
droplets would achieve a more nearly spherical shape.
[0320] This is true for a particular stage of electrostatic
atomization. At the time of the Interim Report, we had reached a
stage that we now dub "primary atomization". Since October
28.sup.th, we have had some partial success in achieving secondary
and (to a lesser extent) tertiary atomization. Rapid cooling during
flight continues to provide this evidence.
[0321] were all evident to the client when a demonstration
experiment was provided for him on the occasion of his visit on
December 14.sup.th.
[0322] We have no further comment at this time.
[0323] We now discuss section E. Conclusions:
[0324] Evident by experimental observation. Current nozzles being
employed are stainless steel tubing with an outer diameter (O.D.)
of 0.012 inch (0.3 mm) and an inner diameter (I.D.) of 0.006 inch
(0.15 mm).
[0325] Binary division occurs in some instances, but our ongoing
studies suggest that the production of smaller droplets may not be
due simply to successive binary division. This matter is discussed
further herein.
[0326] Undoubtedly this is true. However, current research is aimed
at discovering whom, and in what manner smaller droplets are
produced.
[0327] Until we can employ a high resolution camera utilizing high
speed film, we have no further comment upon this phenomenon.
[0328] Smaller gauge stainless steel tubing has now arrived, as has
a eutectic alloy containing Indium that has a melting point of only
47.degree. C., and, delivery of high voltage control units
(currently our units have a maximum range of 50 keV, Bertan model
815-30P) supplying up to 100 keV DC is imminent.
[0329] The means by which atomization occurs is by no means clearly
understood at this time. However, we continue to review our
thinking as each new piece of evidence becomes available. In deed,
in recent days we have found it necessary to review our thinking
yet again in the light of new experimental information. The
extractor ring seems to play a significant role in maximizing the
division process. The most appropriate material for its
composition, and its best location within the electrostatic field
remain uncertain at this stage. Ongoing experiments revolve around
using i) bare copper rings, each with a different radius, and ii)
rings composed of a variety of good dielectric materials such as
PVC.
[0330] The wire rings seem to permit some expansive (sucking) force
to be applied upon the droplet as it passes the ring plane, whilst
the PVC rings seem to permit a compressive force to be applied upon
the droplets. The importance of maintaining the environment in the
vicinity of the electrostatic field at a temperature above the
melting point of the liquid metal cannot be overemphasized. As the
droplets become smaller their surface area to mass ratio increases
and they cool more rapidly. [0331] i) A positive high voltage DC
source connected to the liquid metal reservoir produces an electric
field between the nozzle and the grounded collector cup. The force
of the field produced, acting together with gravity, causes
atomized droplets of similar size to be collected. We have called
this phenomenon primary atomization. [0332] ii) The placement of an
extractor ring between the nozzle and collector cup and concentric
to the droplet path causes lateral forces to be applied to the
droplet, which can produce successively small droplets. We have
called this phenomenon secondary and tertiary atomization.
[0333] A deeper understanding of the atomization process will
enable us to improve these numbers. FIG. 61 shows the weight of the
droplets produced vs the gap between the nozzle and the extractor.
This FIG. clearly illustrates that once a critical value is
reached, primary atomization is not sensitive to the high voltage
potential applied, or to the distance between the nozzle and the
extractor. Table 1 and Table 2 in FIGS. 63 and 64, respectively
show the results of experiments using bare copper wire extractors
with different ring diameters.
[0334] FIG. 62 shows evidence abstracted from the same experimental
sample in an attempt to understand the amortization phenomenon. All
droplets were produced by the same experiment. The choice of
solidified droplets P.sub.0 to P.sub.4 is an attempt to demonstrate
the way in which the primary droplets are subdivided. The choice of
solidified droplets S.sub.0 to S.sub.4 is an attempt to show this
phenomenon then repeats upon the secondary droplets to produce
tertiary droplets such as solidified droplet T.sub.0. Droplets
P.sub.4 and S.sub.4 appear severed, but they are not. They are
whole; and droplet T.sub.0 seems large for its weight.
Unfortunately, these apparent anomalies arise from lens distortion
due to the scanning and copy processes involved in producing FIG.
62.
[0335] Each patent application and publication referenced herein is
hereby incorporated by reference herein in its entirety.
[0336] Various modifications of the invention, in addition to those
described herein, will be apparent to one skilled in the art from
the foregoing description. Such modifications are also intended to
fall within the scope of the appended claims.
* * * * *