U.S. patent application number 10/513305 was filed with the patent office on 2006-03-16 for plasma-assisted sintering.
Invention is credited to Devendra Kumar, Satyendra Kumar.
Application Number | 20060057016 10/513305 |
Document ID | / |
Family ID | 36034184 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060057016 |
Kind Code |
A1 |
Kumar; Devendra ; et
al. |
March 16, 2006 |
Plasma-assisted sintering
Abstract
Methods and systems for plasma-assisted sintering are provided.
The method can include initiating a sintering plasma with a cavity
(12) by subjecting a gas to radiation in the presence of a plasma
catalyst and exposing at least a portion of an object which can be
a powdered material component to the plasma for a period of time
sufficient to sinter at least a portion of the object.
Inventors: |
Kumar; Devendra; (Rochester
Hills, MI) ; Kumar; Satyendra; (Troy, MI) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
36034184 |
Appl. No.: |
10/513305 |
Filed: |
May 7, 2003 |
PCT Filed: |
May 7, 2003 |
PCT NO: |
PCT/US03/14054 |
371 Date: |
August 3, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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60378693 |
May 8, 2002 |
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60430677 |
Dec 4, 2002 |
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60435278 |
Dec 23, 2002 |
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Current U.S.
Class: |
419/56 |
Current CPC
Class: |
B22F 2999/00 20130101;
B82Y 30/00 20130101; C23C 4/134 20160101; H05H 1/24 20130101; C04B
35/64 20130101; H05H 1/46 20130101; C04B 2235/666 20130101; C23C
26/00 20130101; H05H 1/461 20210501; B22F 3/105 20130101; B22F
2999/00 20130101; B22F 3/105 20130101; B22F 2202/13 20130101 |
Class at
Publication: |
419/056 |
International
Class: |
B22F 3/105 20060101
B22F003/105 |
Claims
1. A method of plasma-assisted sintering of an object including at
least one powdered material component, the method comprising:
initiating a plasma in a first cavity by subjecting a gas in the
first cavity to electromagnetic radiation having a frequency less
than about 333 GHz in the presence of a plasma catalyst; and
exposing at least a portion of the object to the plasma for a
period of time sufficient to sinter at least a portion of the at
least one powdered material component.
2. The method of claim 1, wherein the plasma catalyst includes at
least one of a passive plasma catalyst and an active plasma
catalyst.
3. The method of claim 1, wherein the plasma catalyst includes at
least one of powdered carbon, carbon nanotubes, carbon
nanoparticles, carbon fibers, graphite, solid carbon, and any
combination thereof.
4. The method of claim 1, wherein the plasma catalyst includes at
least one of x-rays, gamma radiation, alpha particles, beta
particles, neutrons, protons, and any combination thereof.
5. The method of claim 1, wherein the plasma catalyst includes at
least one of electrons and ions.
6. The method of claim 1, wherein the plasma catalyst includes at
least one of a metal, carbon, a carbon-based alloy, a carbon-based
composite, an electrically conductive polymer, a conductive
silicone elastomer, a polymer nanocomposite, an organic-inorganic
composite, and any combination thereof.
7. The method of claim 1, further comprising placing the portion of
the object in a location selected from within the first cavity and
adjacent an aperture in the first cavity.
8. The method of claim 7, wherein the initiating occurs in the
first cavity in a gaseous environment having an initial pressure
level of at least about 760 Torr.
9. The method of claim 1, wherein the exposing causes heating of
the at least a portion of the object that proceeds at a rate of at
least 400 degrees Celsius per minute until the portion of the
object reaches a temperature no greater than about a melting
temperature of the at least one powdered material component.
10. The method of claim 1, wherein the object includes multiple
powder material components, and wherein the exposing causes heating
of the at least a portion of the object that proceeds at a rate of
at least 400.degree. C. per minute until the portion of the object
reaches a temperature up to a melting temperature for any one of
the multiple powder material components.
11. The method of claim 1, further comprising flowing gas through
the first cavity.
12. The method of claim 1, further comprising sustaining the plasma
by directing additional radiation into the first cavity.
13. The method of claim 12, further comprising mode-mixing the
additional radiation.
14. The method of claim 1, further comprising moving the object
with respect to the plasma during the exposing.
15. The method of claim 1, wherein the powdered material component
comprises a material selected from a group consisting of a metal, a
ceramic, an ore, a salt, an alloy, silicon, aluminum, tungsten,
carbon, iron, an oxygen-containing compound, a nitrogen containing
compound, and any combination thereof.
16. The method of claim 1, wherein the first cavity has an interior
surface with at least one surface feature, wherein the exposing
comprises forming a sintering pattern on the object based on the at
least one surface feature.
17. The method of claim 1, wherein the first cavity is connected to
a second cavity through a conduit, the method further comprising:
placing the object in the second cavity; sustaining the plasma in
the first cavity during the exposing; and forming a plasma jet in
the second cavity at the conduit, thereby permitting the exposing
to occur in the second cavity.
18. The method of claim 1, wherein the first cavity is formed in a
vessel that has an aperture, the method further comprising: placing
the object outside the first cavity near the aperture; sustaining
the plasma in the first cavity during the exposing; and forming a
plasma jet at the aperture, thereby permitting the exposing to
occur outside the first cavity.
19. The method of claim 1, further comprising: supplying a source
of a processing material to the plasma, and subjecting the object
to a treatment using the processing material.
20. The method of claim 19, wherein the processing material
includes carbon and the treatment comprises carbunizing.
21. The method of claim 19, wherein the processing material
includes nitrogen and the treatment comprises nitriding.
22. The method of claim 19, further comprising: supplying a coating
material to the plasma, and depositing a coating on the object.
23. The method of claim 22, wherein the coating includes at least
one of tungsten carbide, tungsten nitride, tungsten oxide, tantalum
nitride, tantalum oxide, titanium oxide, titanium nitride, silicon
oxide, silicon carbide, silicon nitride, aluminum oxide, aluminum
nitride, aluminum carbide, boron nitride, boron carbide, boron
oxide, gallium phosphide, aluminum phosphide, chromium oxide, tin
oxide, yttria, zirconia, silicon-germanium, indium tin oxide,
indium gallium arsenide, aluminum gallium arsenide, boron,
chromium, gallium, germanium, indium, phosphorus, magnesium,
silicon, tantalum, tin, titanium, tungsten, yttrium, and
zirconium.
24. The method of claim 22, wherein at least one of the steps of
subjecting the object to a treatment and depositing a coating on
the object is performed in a location where the exposing at least a
portion of the object to the plasma occurs.
25. A system for plasma-assisted sintering of an object including
at least one powdered material component, the system comprising: a
plasma catalyst; a vessel in which a first cavity is formed and in
which a plasma can be initiated by subjecting a gas to an amount of
electromagnetic radiation having a frequency less than about 333
GHz in the presence of the plasma catalyst, wherein the vessel has
a shape that permits at least a portion of the object to be exposed
to the plasma; a radiation source coupled to the cavity such that
the radiation source can direct radiation into the cavity; and a
gas source coupled to the cavity such that a gas can flow into the
cavity during sintering.
26. The system of claim 25, wherein the cavity has an aperture at
which a plasma jet can form.
27. The system of claim 25, further comprising: a temperature
sensor for monitoring a temperature of the object; and a controller
that adjusts a power level of the radiation source in response to
the temperature of the object.
28. The system of claim 27, wherein the controller is programmed to
control the power level of the radiation source such that the
temperature of the object substantially conforms to a predetermined
temperature profile.
29. The system of claim 27, further comprising an applicator that
contains the vessel, where the applicator is a multi-mode
applicator.
30. The system of claim 29, further comprising a mode mixer that
can move relative to the applicator to make a time-averaged
radiation density in a treatment zone of the applicator
substantially uniform.
31. The system of claim 27, further comprising an electrical bias
source configured to be connected to the object during
sintering.
32. The system of claim 31, wherein the electrical bias-source
generates an AC bias.
33. The system of claim 31, wherein the electrical bias source
generates a DC bias.
34. The system of claim 31, wherein the electrical bias source
generates a pulsed DC bias.
35. The system of claim 27, further comprising a magnetic field
source positioned to apply a magnetic field to the portion of the
object during sintering.
36. The system of claim 29, wherein the applicator includes an
outer housing comprising a material that is substantially opaque to
the radiation.
37. The system of claim 36, wherein the applicator includes the
vessel, which comprises a material that is substantially
transmissive to the radiation.
38. The system of claim 25, wherein the plasma catalyst includes at
least one of a passive plasma catalyst and an active plasma
catalyst.
39. The system of claim 25, wherein the plasma catalyst includes at
least one of powdered carbon, carbon nanotubes, carbon
nanoparticles, carbon fibers, graphite, solid carbon, and any
combination thereof.
40. The system of claim 25, wherein the plasma catalyst includes at
least one of x-rays, gamma radiation, alpha particles, beta
particles, neutrons, protons, and any combination thereof.
41. The system of claim 25, wherein the plasma catalyst includes at
least one of electrons and ions.
42. The system of claim 25, wherein the plasma catalyst includes at
least one of a metal, carbon, a carbon-based alloy, a carbon-based
composite, an electrically conductive polymer, a conductive
silicone elastomer, a polymer nanocomposite, an organic-inorganic
composite, and any combination thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Priority is claimed to U.S. Provisional Patent Application
No. 60/378,693, filed May 8, 2002, No. 60/430,677, filed Dec. 4,
2002, and No. 60/435,278, filed Dec. 23, 2002, all of which are
fully incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates to sintering systems and methods.
More specifically, the invention relates to systems and methods for
igniting, modulating, and sustaining plasmas from gases using
electromagnetic radiation in the presence of plasma catalysts and
for using the plasmas in sintering processes.
BACKGROUND
[0003] Various sintering methods are known. These methods can
involve the thermal treatment of a powder at a temperature below
its melting point. This thermal treatment can bond the powder
particles together to increase the strength of the resulting
sintered material.
[0004] Prior to some sintering processes, for example, the powder
(e.g., a metal, ceramic, or other) can be compressed in a die under
a large pressure to form a desired shape. Compaction of the
powdered material may produce an object known as a compact. Prior
to sintering, this compact is often referred to as a green part,
and its density can depend on factors such as compaction pressure,
dimensions of the compact, and powder hardness. Compacts generally
have a low strength and a high porosity compared to their sintered
counterparts. Sintering of the compacts may promote grain growth by
solid-state diffusion and bonding between the powder particles of
the compact.
[0005] While some sintering methods have reported acceptable
results, some of these methods include several disadvantages. For
example, some reported methods employ traditional furnaces for
heating the materials to be sintered. It may difficult, however, to
precisely control the temperature of the material using these
furnaces. For example, for a particular rate of increase in
temperature within the furnace, there may be a corresponding lag in
the temperature of the material. This lag may be significant, and
in certain sintering processes, not all of the material to be
sintered may achieve a desired processing temperature or satisfy a
desired time-temperature profile. This can lead to incomplete
sintering of the material, and as a result, the sintered material
may be less dense than predicted or desired.
[0006] Further, some sintering methods using conventional furnaces
may not be suited for sintering objects with non-standard profiles
or shapes, such as, for example, reentrant features, multiple
thicknesses, thin or small features, and variable cross sections.
For example, small or thin features may heat faster than the bulk
of the object. As a result, these features may exhibit physical
properties (e.g., porosity, density, etc.) upon sintering that are
different from the bulk of the object. Moreover, atmospheric
sintering furnaces may be slow in heating and may lack the ability
to precisely control the temperature of the object.
[0007] Plasma-assisted sintering has also been reported. While
plasma sintering methods may offer potential increases in heating
rates over traditional furnace sintering methods, these plasma
sintering methods normally involve the use of costly vacuum
equipment. Further, generation of the sintering plasma may also
depend upon the use of large electrical potentials of several
hundred volts.
SUMMARY OF THE INVENTION
[0008] One aspect of the invention may provide a plasma-assisted
method of sintering an object that includes at least one powdered
material component. The method can include initiating a sintering
plasma by subjecting a gas to electromagnetic radiation (e.g.,
microwave radiation) in the presence of a plasma catalyst. The
method may further include exposing at least a portion of the
object to the plasma for a period of time sufficient to sinter at
least a portion of the at least one powdered material
component.
[0009] Another aspect of the invention provides a system for
plasma-assisted sintering of an object. The system can include a
plasma catalyst, a vessel in which a cavity is formed and in which
a plasma can be initiated by subjecting a gas to radiation in the
presence of the plasma catalyst, a radiation source connected to
the cavity for supplying radiation into the cavity, a gas source
coupled to the cavity such that a gas can flow into the cavity
during sintering.
[0010] A number of plasma catalysts are also provided for
plasma-assisted sintering consistent with this invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] Further aspects of the invention will be apparent upon
consideration of the following detailed description, taken in
conjunction with the accompanying drawings, in which like reference
characters refer to like parts throughout, and in which:
[0012] FIG. 1 shows a schematic diagram of an illustrative
plasma-assisted sintering system consistent with this
invention;
[0013] FIG. 1A shows an illustrative embodiment of a portion of a
plasma-assisted sintering system for adding a powder plasma
catalyst to a plasma cavity for igniting, modulating, or sustaining
a plasma in a cavity consistent with this invention;
[0014] FIG. 2 shows an illustrative plasma catalyst fiber with at
least one component having a concentration gradient along its
length consistent with this invention;
[0015] FIG. 3 shows an illustrative plasma catalyst fiber with
multiple components at a ratio that varies along its length
consistent with this invention;
[0016] FIG. 4 shows another illustrative plasma catalyst fiber that
includes a core underlayer and a coating consistent with this
invention;
[0017] FIG. 5 shows a cross-sectional view of the plasma catalyst
fiber of FIG. 4, taken from line 5-5 of FIG. 4, consistent with
this invention;
[0018] FIG. 6 shows an illustrative embodiment of another portion
of a plasma system including an elongated plasma catalyst that
extends through ignition port consistent with this invention;
[0019] FIG. 7 shows an illustrative embodiment of an elongated
plasma catalyst that can be used in the system of FIG. 6 consistent
with this invention;
[0020] FIG. 8 shows another illustrative embodiment of an elongated
plasma catalyst that can be used in the system of FIG. 6 consistent
with this invention;
[0021] FIG. 9 shows an illustrative embodiment of a portion of a
plasma sintering system for directing radiation into a plasma
chamber consistent with this invention; and
[0022] FIG. 10 shows an illustrative plasma-jet apparatus
consistent with this invention.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Methods and apparatus for plasma-assisted sintering may be
provided consistent with this invention. The plasmas can be
ignited, as well as modulated and sustained, with a plasma catalyst
consistent with this invention.
[0024] The following commonly owned, concurrently filed U.S. patent
applications are hereby incorporated by reference in their
entireties: U.S. patent application Ser. No. 10/______ (Atty.
Docket No. 1837.0008), Ser. No. 10/______ (Atty. Docket No.
1837.0009), Ser. No. 10/______ (Atty. Docket No. 1837.0010), Ser.
No. 10/______ (Atty. Docket No. 1837.0011), Ser. No. 10/______
(Atty. Docket No. 1837.0013), Ser. No. 10/______ (Atty. Docket No.
1837.0015), Ser. No. 10/______ (Atty. Docket No. 1837.0016), Ser.
No. 10/______ (Atty. Docket No. 1837.0017), Ser. No. 10/______
(Atty. Docket No. 1837.0018), Ser. No. 10/______ (Atty. Docket No.
1837.0020), Ser. No. 10/______ (Atty. Docket No. 1837.0021), Ser.
No. 10/______ (Atty. Docket No. 1837.0023), Ser. No. 10/______
(Atty. Docket No. 1837.0024), Ser. No. 10/______ (Atty. Docket No.
1837.0025), Ser. No. 10/______ (Atty. Docket No. 1837.0026), Ser.
No. 10/______ (Atty. Docket No. 1837.0027), Ser. No. 10/______
(Atty. Docket No. 1837.0028), Ser. No. 10/______ (Atty. Docket No.
1837.0029), Ser. No. 10/______ (Atty. Docket No. 1837.0030), Ser.
No. 10/______ (Atty. Docket No. 1837.0032), and Ser. No. 10/______
(Atty. Docket No. 1837.0033).
[0025] Illustrative Plasma-Assisted Sintering System
[0026] FIG. 1 illustrates exemplary plasma sintering system 10
consistent with one aspect of this invention. In this embodiment,
cavity 12 can be formed in a vessel that is positioned inside
radiation chamber (i.e., applicator) 14. In another embodiment (not
shown), vessel 12 and radiation chamber 14 are the same, thereby
eliminating the need for two separate components. The vessel in
which cavity 12 is formed can include one or more
radiation-transmissive (e.g., microwave-transmissive) insulating
layers to improve its thermal insulation properties without
significantly shielding cavity 12 from the radiation.
[0027] In one embodiment, cavity 12 can be formed in a vessel made
of ceramic. Due to the extremely high temperatures that can be
achieved with plasmas consistent with this invention, a ceramic
capable of operating at a temperature greater than about 2,000
degrees Fahrenheit, such as about 3,000 degrees Fahrenheit, can be
used. The ceramic material can include, by weight, 29.8% silica,
68.2% alumina, 0.4% ferric oxide, 1% titania, 0.1% lime, 0.1%
magnesia, 0.4% alkalies, which is sold under Model No. LW-30 by New
Castle Refractories Company, of New Castle, Pa. It will be
appreciated by those of ordinary skill in the art, however, that
other materials, such as quartz, and those different from the one
described above, can also be used consistent with the invention. It
will be appreciated that other embodiments of the invention may
include materials intended to operate at temperatures below about
2,000 degrees Fahrenheit.
[0028] In one successful experiment, a plasma was formed in a
partially open cavity inside a first brick and topped with a second
brick. The cavity had dimensions of about 2 inches by about 2
inches by about 1.5 inches. At least two holes were also provided
in the brick in communication with the cavity: one for viewing the
plasma and at least one hole for providing a gas from which the
plasma can be formed. The size and shape of the cavity can depend
on the sintering process being performed. Also, the cavity may be
configured to discourage or prevent the plasma from rising/floating
away from the primary processing region.
[0029] Cavity 12 can be connected to one or more gas sources 24
(e.g., a source of argon, nitrogen, hydrogen, xenon, krypton) by
line 20 and control valve 22, which may be powered by power supply
28. In certain embodiments, a plasma can be formed from one or more
gases supplied by gas source 24. Line 20 may be any channel capable
of conveying the gas but can be narrow enough to prevent
significant radiation leakage. For example, line 20 may be tubing
(e.g., having a diameter between about 1/16 inch and about 1/4
inch, such as about 1/8''). Also, if desired, a vacuum pump can be
connected to the chamber to remove any undesirable fumes that may
be generated during plasma processing.
[0030] A radiation leak detector (not shown) was installed near
source 26 and waveguide 30 and connected to a safety interlock
system to automatically turn off the radiation (e.g., microwave)
power supply assisted if a leak above a predefined safety limit,
such as one specified by the FCC and/or OSHA (e.g., 5 mW/cm.sup.2),
was detected.
[0031] Radiation source 26, which may be powered by electrical
power supply 28, can direct radiation energy into chamber 14
through one or more waveguides 30. It will be appreciated by those
of ordinary skill in the art that source 26 can be connected
directly to chamber 14, thereby eliminating waveguide 30. The
radiation energy entering cavity 12 can be used to ignite a plasma
within the cavity. This plasma can be modulated or substantially
sustained and confined to the cavity by coupling additional
radiation, such as microwave radiation, with the catalyst.
[0032] Radiation energy can be supplied through circulator 32 and
tuner 34 (e.g., 3-stub tuner). Tuner 34 can be used to minimize the
reflected power as a function of changing ignition or processing
conditions, especially before the plasma has formed because
microwave power, for example, will be strongly absorbed by the
plasma.
[0033] As explained more fully below, the location of
radiation-transmissive cavity 12 in chamber 14 may not be critical
if chamber 14 supports multiple modes, and especially when the
modes are continually or periodically mixed. For example, motor 36
can be connected to mode-mixer 38 for making the time-averaged
radiation energy distribution substantially uniform throughout
chamber 14. Furthermore, window 40 (e.g., a quartz window) can be
disposed in one wall of chamber 14 adjacent to cavity 12,
permitting temperature sensor 42 (e.g., an optical pyrometer) to be
used to view a process inside cavity 12. In one embodiment, the
optical pyrometer has a voltage output that can vary with
temperature to within a certain tracking range.
[0034] Sensor 42 can develop output signals as a function of the
temperature or any other monitorable condition associated with a
work piece (not shown) within cavity 12 and provide the signals to
controller 44. Dual temperature sensing and heating, as well as
automated cooling rate and gas flow controls can also be used.
Controller 44 in turn can be used to control operation of power
supply 28, which can have one output connected to radiation source
26 as described above and another output connected to valve 22 to
control gas flow into radiation cavity 12.
[0035] The invention has been practiced with equal success
employing microwave sources at both 915 MHz and 2.45 GHz provided
by Communications and Power Industries (CPI), although radiation
having any frequency less than about 333 GHz can be used. The 2.45
GHz system provided continuously variable microwave power from
about 0.5 kilowatts to about 5.0 kilowatts. A 3-stub tuner allowed
impedance matching for maximum power transfer and a dual
directional coupler (not shown in FIG. 1) was used to measure
forward and reflected powers.
[0036] As mentioned above, radiation having any frequency less than
about 333 GHz can be used consistent with this invention. For
example, frequencies, such as power line frequencies (about 50 Hz
to about 60 Hz), can be used, although the pressure of the gas from
which the plasma is formed may be lowered to assist with plasma
ignition. Also, any radio frequency or microwave frequency can be
used consistent with this invention, including frequencies greater
than about 100 kHz. In most cases, the gas pressure for such
relatively high frequencies need not be lowered to ignite,
modulate, or sustain a plasma, thereby enabling many
plasma-processes to occur at atmospheric pressures and above.
[0037] The equipment was computer controlled using LabView 6i
software, which provided real-time temperature monitoring and
microwave power control. Noise was reduced by using sliding
averages of suitable number of data points. Also, to improve speed
and computational efficiency, the number of stored data points in
the buffer array were limited by using shift-registers and
buffer-sizing. The pyrometer measured the temperature of a
sensitive area of about 1 cm.sup.2, which was used to calculate an
average temperature. The pyrometer sensed radiant intensities at
two wavelengths and fit those intensities using Planck's law to
determine the temperature.
[0038] It will be appreciated, however, that other devices and
methods for monitoring and controlling temperature are also
available and can be used consistent with this invention. Control
software that can be used consistent with this invention is
described, for example, in commonly owned, concurrently filed U.S.
patent application Ser. No. 10/______ (Attorney Docket No.
1837.0033), which is hereby incorporated by reference in its
entirety.
[0039] Chamber 14 may include several glass-covered viewing ports
with microwave shields and a quartz window for pyrometer access.
Several ports for connection to a vacuum pump and a gas source may
also be provided, although not necessarily used.
[0040] System 10 may also include an optional closed-loop deionized
water cooling system (not shown) with an external heat exchanger
cooled by tap water. During operation, the deionized water may cool
the magnetron, then the load-dump in the circulator (used to
protect the magnetron), and finally the radiation chamber through
water channels welded on the outer surface of the chamber.
[0041] Plasma Catalysts
[0042] A plasma catalyst consistent with this invention can include
one or more different materials and may be either passive or
active. A plasma catalyst can be used, among other things, to
ignite, modulate, and/or sustain a plasma at a gas pressure that is
less than, equal to, or greater than atmospheric pressure.
[0043] One method of forming a plasma consistent with this
invention can include subjecting a gas in a cavity to
electromagnetic radiation having a frequency of less than about 333
GHz in the presence of a passive plasma catalyst. A passive plasma
catalyst consistent with this invention can include any object
capable inducing a plasma by deforming a local electric field
(e.g., an electromagnetic field) consistent with this invention,
without necessarily adding additional energy through the catalyst,
such as by applying an electric voltage to create a spark.
[0044] A passive plasma catalyst consistent with this invention can
be, for example, a nano-particle or a nano-tube. As used herein,
the term "nano-particle" can include any particle having a maximum
physical dimension less than about 100 nm that is at least
electrically semi-conductive. Also, both single-walled and
multi-walled carbon nanotubes, doped and undoped, can be
particularly effective for igniting plasmas consistent with this
invention because of their exceptional electrical conductivity and
elongated shape. The nanotubes can have any convenient length and
can be a powder fixed to a substrate. If fixed, the nanotubes can
be oriented randomly on the surface of the substrate or fixed to
the substrate (e.g., at some predetermined orientation) while the
plasma is ignited or sustained.
[0045] A passive plasma catalyst consistent with this invention can
also be, for example, a powder and need not comprise nano-particles
or nano-tubes. It can be formed, for example, from fibers, dust
particles, flakes, sheets, etc. When in powder form, the catalyst
can be suspended, at least temporarily, in a gas. By suspending the
powder in the gas, the powder can be quickly dispersed throughout
the cavity and more easily and uniformly consumed, if desired.
[0046] In one embodiment, the powder catalyst can be carried into
the sintering cavity and at least temporarily suspended with a
carrier gas. The carrier gas can be the same or different from the
gas that forms the plasma. Also, the powder can be added to the gas
prior to being introduced to the cavity. For example, as shown in
FIG. 1A, radiation source 52 can supply radiation to cavity 55,
which includes plasma cavity 60 (e.g, where sintering may occur).
Powder source 65 can provide catalytic powder 70 into gas stream
75. In an alternative embodiment, powder 70 can be first added to
cavity 60 in bulk (e.g., in a pile) and then distributed in the
cavity in any number of ways, including flowing a gas through or
over the bulk powder. In addition, the powder can be added to the
gas for igniting, modulating, or sustaining a plasma by moving,
conveying, drizzling, sprinkling, blowing, or otherwise feeding the
powder into or within the cavity.
[0047] In one experiment, a plasma was ignited in a cavity by
placing a pile of carbon fiber powder in a copper pipe that
extended into the cavity. Although sufficient radiation was
directed into the cavity, the copper pipe shielded the powder from
the radiation and no plasma ignition took place. However, once a
carrier gas began flowing through the pipe, forcing the powder out
of the pipe and into the cavity, and thereby subjecting the powder
to the radiation, a plasma was nearly instantaneously ignited in
the cavity at about atmospheric pressure.
[0048] A powder plasma catalyst consistent with this invention can
be substantially non-combustible, thus it need not contain oxygen
or burn in the presence of oxygen. Thus, as mentioned above, the
catalyst can include a metal, carbon, a carbon-based alloy, a
carbon-based composite, an electrically conductive polymer, a
conductive silicone elastomer, a polymer nanocomposite, an
organic-inorganic composite, and any combination thereof.
[0049] Also, powder catalysts can be substantially uniformly
distributed in the plasma cavity (e.g., when suspended in a gas),
and plasma ignition can be precisely controlled within the cavity.
Uniform ignition can be important in certain applications,
including those applications requiring brief plasma exposures, such
as in the form of one or more bursts. Still, a certain amount of
time can be required for a powder catalyst to distribute itself
throughout a cavity, especially in complicated, multi-chamber
cavities. Therefore, consistent with another aspect of this
invention, a powder catalyst can be introduced into the cavity
through a plurality of ignition ports to more rapidly obtain a more
uniform catalyst distribution therein (see below).
[0050] In addition to powder, a passive plasma catalyst consistent
with this invention can include, for example, one or more
microscopic or macroscopic fibers, sheets, needles, threads,
strands, filaments, yarns, twines, shavings, slivers, chips, woven
fabrics, tape, whiskers, or any combination thereof. In these
cases, the plasma catalyst can have at least one portion with one
physical dimension substantially larger than another physical
dimension. For example, the ratio between at least two orthogonal
dimensions can be at least about 1:2, but can be greater than about
1:5, or even greater than about 1:10.
[0051] Thus, a passive plasma catalyst can include at least one
portion of material that is relatively thin compared to its length.
A bundle of catalysts (e.g., fibers) may also be used and can
include, for example, a section of graphite tape. In one
experiment, a section of tape having approximately thirty thousand
strands of graphite fiber, each about 2-3 microns in diameter, was
successfully used. The number of fibers in and the length of a
bundle are not critical to igniting, modulating, or sustaining the
plasma. For example, satisfactory results have been obtained using
a section of graphite tape about one-quarter inch long. One type of
carbon fiber that has been successfully used consistent wit this
invention is sold under the trademark Magnamite.RTM. Model No.
AS4C-GP3K, by the Hexcel Corporation, of Anderson, S.C. Also,
silicon-carbide fibers have been successfully used.
[0052] A passive plasma catalyst consistent with another aspect of
this invention can include one or more portions that are, for
example, substantially spherical, annular, pyramidal, cubic,
planar, cylindrical, rectangular or elongated.
[0053] The passive plasma catalysts discussed above can include at
least one material that is at least electrically semi-conductive.
In one embodiment, the material can be highly conductive. For
example, a passive plasma catalyst consistent with this invention
can include a metal, an inorganic material, carbon, a carbon-based
alloy, a carbon-based composite, an electrically conductive
polymer, a conductive silicone elastomer, a polymer nanocomposite,
an organic-inorganic composite, or any combination thereof. Some of
the possible inorganic materials that can be included in the plasma
catalyst include carbon, silicon carbide, molybdenum, platinum,
tantalum, tungsten, and aluminum, although other electrically
conductive inorganic materials are believed to work just as
well.
[0054] In addition to one or more electrically conductive
materials, a passive plasma catalyst consistent with this invention
can include one or more additives (which need not be electrically
conductive). As used herein, the additive can include any material
that a user wishes to add to the plasma. For example, in sintering
semiconductors and other materials, one or more dopants can be
added to the plasma through the catalyst. See, e.g., commonly
owned, concurrently filed U.S. patent application Ser. No.
10/______ (Atty. Docket No. 1837.0026), which is hereby
incorporated by reference in its entirety. The catalyst can include
the dopant itself, or it can include a precursor material that,
upon decomposition, can form the dopant. Thus, the plasma catalyst
can include one or more additives and one or more electrically
conductive materials in any desirable ratio, depending on the
ultimate desired composition of the plasma and the process using
the plasma.
[0055] The ratio of the electrically conductive components to the
additives in a passive plasma catalyst can vary over time while
being consumed. For example, during ignition, the plasma catalyst
could desirably include a relatively large percentage of
electrically conductive components to improve the ignition
conditions. On the other hand, if used while sustaining the plasma,
the catalyst could include a relatively large percentage of
additives. It will be appreciated by those of ordinary skill in the
art that the component ratio of the plasma catalyst used to ignite
and sustain the plasma could be the same.
[0056] In certain embodiments of the invention, a predetermined
plasma catalyst ratio profile can be used. In some conventional
plasma processes, the components within the plasma are added as
necessary, but such addition normally requires programmable
equipment to add the components according to a predetermined
schedule. However, consistent with this invention, the ratio of
components in the catalyst can be varied, and thus the ratio of
components in the plasma itself can be automatically varied. That
is, the ratio of components in the plasma at any particular time
can depend on which of the catalyst portions is currently being
consumed by the plasma. Thus, the catalyst component ratio can be
different at different locations within the catalyst. And, the
ratio of components in a plasma can depend on the portions of the
catalyst currently and/or previously consumed, especially when the
flow rate of a gas passing through the plasma chamber is relatively
slow.
[0057] A passive plasma catalyst consistent with this invention can
be homogeneous, inhomogeneous, or graded. Also, the plasma catalyst
component ratio can vary continuously or discontinuously throughout
the catalyst. For example, in FIG. 2, the component ratio can vary
smoothly forming a ratio gradient along the length of catalyst 100.
Thus, catalyst 100 can include a strand of material that includes a
relatively low concentration of one or more components at section
105 and a continuously increasing concentration toward section
110.
[0058] Alternatively, as shown in FIG. 3, the ratio can vary
discontinuously in each portion of catalyst 120, which includes,
for example, alternating sections 125 and 130 having different
concentrations. It will be appreciated that catalyst 120 can have
more than two section types. Thus, the catalytic component ratio
being consumed by the plasma can vary in any predetermined fashion.
In one embodiment, when the plasma is monitored and a particular
additive is detected, further processing can be automatically
commenced or terminated.
[0059] Another way to vary the ratio of components in a modulated
or sustained plasma is by introducing multiple catalysts having
different component ratios at different times or different rates.
For example, multiple catalysts can be introduced at approximately
the same location or at different locations within the cavity. When
introduced at different locations, the plasma formed in the cavity
can have a component concentration gradient determined by the
locations of the various catalysts. Thus, an automated system can
include a device by which a consumable plasma catalyst is
mechanically inserted before and/or during plasma igniting,
modulating, and/or sustaining a plasma.
[0060] A passive plasma catalyst consistent with this invention can
also be coated. In one embodiment, a catalyst can include a
substantially non-electrically conductive coating deposited on the
surface of a substantially electrically conductive material.
Alternatively, the catalyst can include a substantially
electrically conductive coating deposited on the surface of a
substantially electrically non-conductive material. FIGS. 4 and 5,
for example, show fiber 140, which includes underlayer 145 and
coating 150. In one embodiment, a plasma catalyst including a
carbon core is coated with nickel to prevent oxidation of the
carbon.
[0061] A single plasma catalyst can also include multiple coatings.
If the coatings are consumed during contact with the plasma, the
coatings could be introduced into the plasma sequentially, from the
outer coating to the innermost coating, thereby creating a
time-release mechanism. Thus, a coated plasma catalyst can include
any number of materials, as long as a portion of the catalyst is at
least electrically semi-conductive.
[0062] Consistent with another embodiment of this invention, a
plasma catalyst can be located entirely within a radiation cavity
to substantially reduce or prevent radiation energy leakage via the
catalyst. In this way, the plasma catalyst does not electrically or
magnetically couple with the vessel containing the cavity or to any
electrically conductive object outside the cavity. This prevents
sparking at the ignition port and prevents radiation from leaking
outside the cavity during the ignition and possibly later if the
plasma is sustained. In one embodiment, the catalyst can be located
at a tip of a substantially electrically nonconductive extender
that extends through an ignition port.
[0063] FIG. 6, for example, shows radiation chamber 160 in which
plasma cavity 165 is placed. Plasma catalyst 170 can be elongated
and can extend through ignition port 175. As shown in FIG. 7, and
consistent with this invention, catalyst 170 can include
electrically conductive distal portion 180 (which is placed in
chamber 160 but can extend into chamber 160) and electrically
non-conductive portion 185 (which is placed substantially outside
chamber 160). This configuration prevents an electrical connection
(e.g., sparking) between distal portion 180 and chamber 160.
[0064] In another embodiment, shown in FIG. 8, the catalyst can be
formed from a plurality of electrically conductive segments 190
separated by and mechanically connected to a plurality of
electrically non-conductive segments 195. In this embodiment, the
catalyst can extend through the ignition port between a point
inside the cavity ad another point outside the cavity, but the
electrically discontinuous profile significantly prevents sparking
and energy leakage.
[0065] As an alternative to the passive plasma catalysts described
above, active plasma catalysts can be used consistent with this
invention. A method of forming a sintering plasma using an active
catalyst consistent with this invention can include subjecting a
gas in a cavity to electromagnetic radiation having a frequency
less than about 333 GHz in the presence of the active plasma
catalyst, which generates or includes at least one ionizing
particle or ionizing radiation. It will be appreciated that both
passive and active plasma catalysts can be used in the same
sintering process.
[0066] An active plasma catalyst consistent with this invention can
be any particle or high energy wave packet capable of transferring
a sufficient amount of energy to a gaseous atom or molecule to
remove at least one electron from the gaseous atom or molecule in
the presence of electromagnetic radiation. Depending on the source,
the ionizing radiation and/or particles can be directed into the
cavity in the form of a focused or collimated beam, or they may be
sprayed, spewed, sputtered, or otherwise introduced.
[0067] For example, FIG. 9 shows radiation source 200 directing
radiation into chamber 205. Plasma cavity 210 can be positioned
inside of chamber 205 and may permit a gas to flow therethrough via
ports 215 and 216. Source 220 directs ionizing particles and/or
radiation 225 into cavity 210. Source 220 can be protected from the
radiation provided by source 200 and the plasma formed thereform,
for example, by a metallic screen that allows the ionizing
particles to pass through but shields source 220 from the
radiation. If necessary, source 220 can be water-cooled.
[0068] Examples of ionizing radiation and/or particles consistent
with this invention can include x-rays, gamma radiation, alpha
particles, beta particles, neutrons, protons, and any combination
thereof. Thus, an ionizing particle catalyst can be charged (e.g.,
an ion from an ion source) or uncharged and can be the product of a
radioactive fission process. In one embodiment, the vessel in which
the plasma cavity is formed could be entirely or partially
transmissive to the ionizing particle catalyst. Thus, when a
radioactive fission source is located outside the cavity, the
source can direct the fission products through the vessel to ignite
the plasma. The radioactive fission source can be located inside
the radiation chamber to substantially prevent the fission products
(i.e., the ionizing particle catalyst) from creating a safety
hazard.
[0069] In another embodiment, the ionizing particle can be a free
electron, but it need not be emitted in a radioactive decay
process. For example, the electron can be introduced into the
cavity by energizing an electron source (such as a metal), such
that the electrons have sufficient energy to escape from the
source. The electron source can be located inside the cavity,
adjacent the cavity, or even in the cavity wall. It will be
appreciated by those of ordinary skill in the art that the any
combination of electron sources is possible. A common way to
produce electrons is to heat a metal, and these electrons can be
further accelerated by applying an electric field.
[0070] In addition to electrons, free energetic protons can also be
used to catalyze a plasma. In one embodiment, a free proton can be
generated by-ionizing hydrogen and, optionally, accelerated with an
electric field.
[0071] Multi-Mode Radiation Cavities
[0072] A radiation waveguide, cavity, or chamber can be designed to
support or facilitate propagation of at least one electromagnetic
radiation mode. As used herein, the term "mode" refers to a
particular pattern of any standing or propagating electromagnetic
wave that satisfies Maxwell's equations and the applicable boundary
conditions (e.g., of the cavity). In a waveguide or cavity, the
mode can be any one of the various possible patterns of propagating
or standing electromagnetic fields. Each mode is characterized by
its frequency and polarization of the electric field and/or the
magnetic field vectors. The electromagnetic field pattern of a mode
depends on the frequency, refractive indices or dielectric
constants, and waveguide or cavity geometry.
[0073] A transverse electric (TE) mode is one whose electric field
vector is normal to the direction of propagation. Similarly, a
transverse magnetic (TM) mode is one whose magnetic field vector is
normal to the direction of propagation. A transverse electric and
magnetic (TEM) mode is one whose electric and magnetic field
vectors are both normal to the direction of propagation. A hollow
metallic waveguide does not typically support a normal TEM mode of
radiation propagation. Even though radiation appears to travel
along the length of a waveguide, it may do so only by reflecting
off the inner walls of the waveguide at some angle. Hence,
depending upon the propagation mode, the radiation (e.g., microwave
radiation) may have either some electric field component or some
magnetic field component along the axis of the waveguide (often
referred to as the z-axis).
[0074] The actual field distribution inside a cavity or waveguide
is a superposition of the modes therein. Each of the modes can be
identified with one or more subscripts (e.g., TE.sub.10 ("tee ee
one zero")). The subscripts normally specify how many "half waves"
at the guide wavelength are contained in the x and y directions. It
will be appreciated by those skilled in the art that the guide
wavelength can be different from the free space wavelength because
radiation propagates inside the waveguide by reflecting at some
angle from the inner walls of the waveguide. In some cases, a third
subscript can be added to define the number of half waves in the
standing wave pattern along the z-axis.
[0075] For a given radiation frequency, the size of the waveguide
can be selected to be small enough so that it can support a single
propagation mode. In such a case, the system is called a
single-mode system (i.e., a single-mode applicator). The TE.sub.10
mode is usually dominant in a rectangular single-mode
waveguide.
[0076] As the size of the waveguide (or the cavity to which the
waveguide is connected) increases, the waveguide or applicator can
sometimes support additional higher order modes forming a
multi-mode system. When many modes are capable of being supported
simultaneously, the system is often referred to as highly
moded.
[0077] A simple, single-mode system has a field distribution that
includes at least one maximum and/or minimum. The magnitude of a
maximum largely depends on the amount of radiation supplied to the
system. Thus, the field distribution of a single mode system is
strongly varying and substantially non-uniform.
[0078] Unlike a single-mode cavity, a multi-mode cavity can support
several propagation modes simultaneously, which, when superimposed,
results in a complex field distribution pattern. In such a pattern,
the fields tend to spatially smear and, thus, the field
distribution usually does not show the same types of strong minima
and maxima field values within the cavity. In addition, as
explained more fully below, a mode-mixer can be used to "stir" or
"redistribute" modes (e.g., by mechanical movement of a radiation
reflector). This redistribution desirably provides a more uniform
time-averaged field distribution within the cavity.
[0079] A multi-mode sintering processing cavity consistent with
this invention can support at least two modes, and may support many
more than two modes. Each mode has a maximum electric field vector.
Although there may be two or more modes, one mode may be dominant
and may have a maximum electric field vector magnitude that is
larger than the other modes. As used herein, a multi-mode cavity
may be any cavity in which the ratio between the first and second
mode magnitudes is less than about 1:10, or less than about 1:5, or
even less than about 1:2. It will be appreciated by those of
ordinary skill in the art that the smaller the ratio, the more
distributed the electric field energy between the modes, and hence
the more distributed the radiation energy is in the cavity.
[0080] The distribution of plasma within a sintering processing
cavity may strongly depend on the distribution of the applied
radiation. For example, in a pure single mode system, there may
only be a single location at which the electric field is a maximum.
Therefore, a strong plasma may only form at that single location.
In many applications, such a strongly localized plasma could
undesirably lead to non-uniform plasma treatment or heating (i.e.,
localized overheating and underheating).
[0081] Whether or not a single or multi-mode sintering processing
cavity is used consistent with this invention, it will be
appreciated by those of ordinary skill in the art that the cavity
in which the plasma is formed can be completely dosed or partially
open. For example, in certain applications, such as in
plasma-assisted furnaces, the cavity could be entirely closed. See,
for example, commonly owned, concurrently filed U.S. patent
application Ser. No. 10/______ (Attorney Docket No. 1837.0020),
which is fully incorporated herein by reference. In other
applications, however, it may be desirable to flow a gas through
the cavity, and therefore the cavity must be open to some degree.
In this way, the flow, type, and pressure of the flowing gas can be
varied over time. This may be desirable because certain gases that
facilitate formation of plasma, such as argon, for example, are
easier to ignite but may not be needed during subsequent plasma
processing.
[0082] Mode-Mixing
[0083] For many sintering applications, a cavity containing a
substantially uniform plasma is desirable. Therefore, consistent
with one aspect of this invention, the radiation modes in a
multi-mode cavity can be mixed, or redistributed, over a period of
time to provide a more uniform radiation field distribution.
Because the field distribution within the cavity must satisfy all
of the boundary conditions set by the inner surface of the cavity,
those field distributions can be changed by changing the position
of any portion of that inner surface.
[0084] In one embodiment consistent with this invention, a movable
reflective surface can be located inside the sintering cavity. The
shape and motion of the reflective surface can change the
reflective properties of the inner surface of the cavity, as a
whole, during motion. For example, an "L" shaped metallic object
(i.e., "mode-mixer") when rotated about any axis will change the
location or the orientation of the reflective surfaces in the
cavity and therefore change the radiation distribution therein. Any
other asymmetrically shaped object can also be used (when rotated),
but symmetrically shaped objects can also work, as long as the
relative motion (e.g., rotation, translation, or a combination of
both) causes some change in the location or orientation of the
reflective surfaces. In one embodiment, a mode-mixer can be a
cylinder that is rotatable about an axis that is not the cylinder's
longitudinal axis.
[0085] Each mode of a multi-mode sintering cavity may have at least
one maximum electric field vector, but each of these vectors could
occur periodically across the inner dimension of the cavity.
Normally, these maxima are fixed, assuming that the frequency of
the radiation does not change. However, by moving a mode-mixer such
that it interacts with the radiation, it is possible to move the
positions of the maxima. For example, mode-mixer 38 can be used to
optimize the field distribution within sintering cavity 12 such
that the plasma ignition conditions and/or the plasma sustaining
conditions are optimized. Thus, once a plasma is excited, the
position of the mode-mixer can be changed to move the position of
the maxima for a uniform time-averaged plasma process (e.g.,
sintering).
[0086] Thus, consistent with this invention, mode-mixing can be
useful during plasma ignition. For example, when an electrically
conductive fiber is used as a plasma catalyst, it is known that the
fiber's orientation can strongly affect the minimum plasma-ignition
conditions. When such a fiber is oriented at an angle that is
greater than 60.degree. to the electric field, for example, the
catalyst does little to improve, or relax, these conditions. By
moving a reflective surface either in or near the sintering cavity,
however, the electric field distribution can be significantly
changed.
[0087] Mode-mixing can also be achieved by launching the radiation
into the applicator chamber through, for example, a rotating
waveguide joint that can be mounted inside the applicator chamber.
The rotary joint can be mechanically moved (e.g., rotated) to
effectively launch the radiation in different directions in the
radiation chamber. As a result, a changing field pattern can be
generated inside the applicator chamber.
[0088] Mode-mixing can also be achieved by launching radiation in
the radiation chamber through a flexible waveguide. In one
embodiment, the waveguide can be mounted inside the chamber. In
another embodiment, the waveguide can extend into the chamber. The
position of the end portion of the flexible waveguide can be
continually or periodically moved (e.g., bent) in any suitable
manner to launch the radiation (e.g., microwave radiation) into the
chamber at different directions and/or locations. This movement can
also result in mode-mixing and facilitate more uniform plasma
processing (e.g., sintering) on a time-averaged basis.
Alternatively, this movement can be used to optimize the location
of a plasma for ignition or other plasma-assisted process.
[0089] If the flexible waveguide is rectangular, for example, a
simple twisting of the open end of the waveguide will rotate the
orientation of the electric and the magnetic field vectors in the
radiation inside the applicator chamber. Then, a periodic twisting
of the waveguide can result in mode-mixing as well as rotating the
electric field, which can be used to assist ignition, modulation,
or sustaining of a plasma.
[0090] Thus, even if the initial orientation of the catalyst is
perpendicular to the electric field, the redirection of the
electric field vectors can change the ineffective orientation to a
more effective one. Those skilled in the art will appreciate that
mode-mixing can be continuous, periodic, or preprogrammed.
[0091] In addition to plasma ignition, mode-mixing can be useful
during subsequent sintering processes and other types of plasma
processing to reduce or create (e.g., tune) "hot spots" in the
chamber. When a cavity only supports a small number of modes (e.g.,
less than 5), one or more localized electric field maxima can lead
to "hot spots" (e.g., within cavity 12). In one embodiment, these
hot spots could be configured to coincide with one or more
separate, but simultaneous, plasma ignitions or sintering events.
Thus, the plasma catalyst can be located at one or more of those
ignition or subsequent plasma processing positions.
[0092] Multi-Location Ignition
[0093] A sintering plasma can be ignited using multiple plasma
catalysts at different locations. In one embodiment, multiple
fibers can be used to ignite the plasma at different points within
the cavity. Such multi-point ignition can be especially beneficial
when a uniform plasma ignition is desired. For example, when a
plasma is modulated at a high frequency (i.e., tens of Hertz and
higher), or ignited in a relatively large volume, or both,
substantially uniform instantaneous striking and restriking of the
plasma can be improved. Alternatively, when plasma catalysts are
used at multiple points, they can be used to sequentially ignite a
sintering plasma at different locations within a plasma chamber by
selectively introducing the catalyst at those different locations.
In this way, a sintering plasma ignition gradient can be
controllably formed within the cavity, if desired.
[0094] Also, in a multi-mode sintering cavity, random distribution
of the catalyst throughout multiple locations in the cavity can
increase the likelihood that at least one of the fibers, or any
other passive plasma catalyst consistent with this invention, is
optimally oriented with the electric field lines. Still, even where
the catalyst is not optimally oriented (not substantially aligned
with the electric field lines), the ignition conditions are
improved.
[0095] Furthermore, because a catalytic powder can be suspended in
a gas, it is believed that each powder particle may have the effect
of being placed at a different physical location within the cavity,
thereby improving ignition uniformity within the sintering
cavity.
[0096] Dual-Cavity Plasma Igniting/Sustaining
[0097] A dual-cavity arrangement can be used to ignite and sustain
a plasma consistent with this invention. In one embodiment, a
system includes at least an ignition cavity and a sintering cavity
in fluid communication with the ignition cavity. To ignite a
plasma, a gas in the ignition cavity can be subjected to
electromagnetic radiation having a frequency less than about 333
GHz, optionally in the presence of a plasma catalyst. In this way,
the proximity of the ignition and sintering cavities may permit a
plasma formed in the ignition cavity to ignite a sintering plasma
in the sintering cavity, which may be modulated or sustained with
additional electromagnetic radiation.
[0098] In one embodiment of this invention, the ignition cavity can
be very small and designed primarily, or solely, for plasma
ignition. In this way, very little microwave energy may be required
to ignite the plasma, permitting easier ignition, especially when a
plasma catalyst is used consistent with this invention.
[0099] In one embodiment, the ignition cavity may be a
substantially single mode cavity and the sintering cavity may be a
multi-mode cavity. When the ignition cavity only supports a single
mode, the electric field distribution may strongly vary within the
cavity, forming one or more precisely located electric field
maxima. Such maxima are normally the first locations at which
plasmas ignite, making them ideal points for placing plasma
catalysts. It will be appreciated, however, that when a plasma
catalyst is used, it need not be placed in the electric field
maximum and, many cases, need not be oriented in any particular
direction.
[0100] Illustrative Sintering Processes
[0101] Consistent with the invention, there may be provided a
method of sintering an object (e.g., a compact or other powder
metallurgy part) that includes at least one powdered material
component. In an illustrative embodiment of the invention, a
sintering plasma may be initiated within a cavity, as described
above, by subjecting a gas (e.g., supplied by gas source 24 of FIG.
1) to radiation (e.g., supplied by radiation source 26 of FIG. 1)
in the presence of a plasma catalyst. Plasma ignition may occur
within cavity 12, which may be formed in a vessel positioned inside
chamber (i.e., applicator) 14. The plasma source gas may be
supplied to the cavity substantially simultaneously or at different
times with the radiation used to initiate the plasma.
[0102] Thus, a sintering plasma consistent with the invention may
be initiated using a plasma catalyst. While a sintering plasma may
be initiated without the use of a plasma catalyst, the presence of
a passive or active plasma catalyst consistent with this invention
may reduce the radiation energy density needed to ignite, modulate,
or sustain the sintering plasma. This reduction may allow a plasma
to be generated in a controlled manner with a relatively low amount
of radiation energy, which can be especially useful when sensitive
portions of an object are exposed to the sintering plasma. In one
embodiment, a sintering plasma may be ignited using a time-averaged
radiation energy (e.g., microwave energy) density below about 10
W/cm.sup.3, or below about 5 W/cm.sup.3. Advantageously, plasma
ignition can be achieved at these relatively low energy densities
without the use of vacuum equipment.
[0103] In addition to ignition, the use of a plasma catalyst may
facilitate control over any portion of the plasma-assisted
sintering process. Specifically, because plasma can be an efficient
absorber of electromagnetic radiation, including microwave
radiation, any radiation used to initiate the sintering plasma may
be mostly and immediately absorbed by the plasma. Therefore, the
radiation energy directed into a sintering cavity may be less
subject to reflection at the early stages of generating the plasma.
As a result, a plasma catalyst may be used to increase control over
the heating rate of an object exposed to the plasma, the
temperature of an object, or any other plasma-assisted process.
[0104] The use of a plasma catalyst may also enable initiation of a
sintering plasma over a broad range of pressures including
pressures less than, equal to, or greater than atmospheric
pressure. Thus, a sintering plasma consistent with the invention
may be ignited, modulated, and sustained not only in vacuum
environments, where the total pressure is less than atmospheric
pressure, but also at pressures at or above atmospheric
pressure.
[0105] The temperature of catalyzed plasmas may be precisely
controlled consistent with the invention. For example, the
temperature can be controlled by varying the amount of radiation
supplied to the plasma. Because heat from the plasma may be
efficiently transferred to objects, the temperature of an object to
be sintered may be accurately varied by controlling the temperature
of the plasma and the exposure level between the object and the
plasma. For example, in a sintering process consistent with the
invention, the plasma may be used to adjust the temperature of an
object to a predetermined sintering temperature, such as by varying
the position of a mode-mixer or varying the rate at which gas flows
through the sintering cavity.
[0106] Energy can be transferred from the plasma to an object at
any desirable rate. For example, the heating rate of an object may
reduced by reducing the power level of the radiation supplied to
the plasma and/or by limiting the amount of exposure between the
object and the plasma (e.g., via mode-mixing, modulating, etc.). By
increasing the radiation power level and/or the amount of plasma
exposure, however, the rate of increase of the temperature of an
object may be increased. For example, in certain embodiments, at
least a portion of an object exposed to the plasma may be heated at
a rate of at least 400 degrees Celsius per minute.
[0107] The temperature of the object also may be controlled by
adjusting the percentage of the total surface area of the object
exposed to the plasma. Exposure of the object to the plasma may be
sustained for any period of time sufficient to sinter at least a
portion of the powdered material component of the object. The
exposure time may be varied to affect the properties of the
sintered object. For example, longer exposure times may promote
more complete sintering and, therefore, more dense objects.
[0108] Plasma-assisted sintering of the present invention can also
be used to sinter an object that includes more than one powdered
material component. Such an object can be sintered by exposing the
object to the sintering plasma until its temperature approaches the
melting temperature of any one of the powdered material components.
In certain embodiments, an object may be liquid phase sintered by
heating the object to a temperature above the melting temperature
of at least one of the powdered material components of the object.
Thus, the presence of a liquid phase from the melted powdered
material component(s) may facilitate sintering in some embodiments.
It will be appreciated that powdered material components may
include metals, ceramics, ores, salts, alloys, silicon, aluminum,
tungsten, carbon, iron, oxygen-containing compounds,
nitrogen-containing compounds, and any combination thereof.
[0109] Consistent with the plasma-assisted sintering methods of the
invention, the object may be uniformly sintered or may be subjected
to a non-uniform sintering pattern. In one embodiment, the
sintering cavity may include an interior surface with one or more
surface features. During exposure to the plasma, a sintering
pattern can be formed on the sintered object based on these surface
feature(s).
[0110] For example, the surface features on the interior of the
plasma sintering cavity may affect sintering by effectively masking
certain areas of the object from the sintering plasma. As
previously discussed, the number or order of modes of the radiation
in cavity 12 may depend on the size or configuration of the cavity.
The presence of an object to be sintered within cavity 12 may also
affect the field distribution in the modes of radiation within the
cavity. The boundary conditions for normal incidence of
electromagnetic radiation on metallic objects require that the
electric field at the surface be zero and the first maxima occur at
a distance of a quarter wavelength from the surface of the object.
Consequently, if the gap between the surface of the metallic object
and the inner wall of the cavity is less than about a quarter
wavelength of the radiation, little or no sintering plasma may be
sustained in these areas, and the regions of the object satisfying
this condition may experience little or no sintering. These
"masked" surface regions may be provided through positioning of the
object within cavity 12, by configuring the walls of cavity 12, or
by any other suitable method for controlling the distance between
the surface of the object and the cavity walls.
[0111] In order to generate or maintain a substantially uniform
time-averaged radiation field distribution within cavity 12, mode
mixer 38 may be provided, as shown in FIG. 1. Alternatively, or
additionally, the object may be moved with respect to the plasma
while being exposed to the plasma. Such motion may provide more
uniform exposure of all surface regions of the object to the
plasma, which may cause more uniform heating of the object or may
assist in heating certain areas of the object more rapidly than
other areas.
[0112] An electric potential bias may be applied to the object
during the plasma-assisted sintering processes consistent with the
invention. Such a potential bias may facilitate heating of the
object by attracting the charged ions in the plasma to the object.
Such an attraction may encourage uniform coverage of the plasma
over the object and contribute to more uniform heating of the
object. The potential bias applied to the object may be, for
example, an AC bias, a DC bias, or a pulsed DC bias. The magnitude
of the bias may be selected according to a particular application.
For example, the magnitude of the voltage may range from 0.1 volts
to 100 volts, or even several hundred volts depending on the
desired rate of attraction of the ionized species. Further, the
bias may be either positive or negative. In addition to an electric
potential bias, a magnetic field source may positioned with respect
to the object to apply a magnetic field to the object during
plasma-assisted sintering.
[0113] It will be appreciated by those of ordinary skill in the art
that the plasma-assisted sintering methods consistent with this
invention need not occur within a cavity. Rather, a sintering
plasma formed in a cavity can be flowed through an aperture in the
form of a plasma jet, for example, and used outside the cavity to
heat an object located adjacent to the aperture.
[0114] FIG. 10 shows illustrative apparatus 650 for forming a
sintering plasma jet for sintering objects consistent with this
invention. Apparatus 650 can include vessel 657, in which cavity
655 can be formed, and a gas source (not shown) for directing a gas
into cavity 655. Cavity 655 can include at least one aperture 660
formed in cavity wall 665. An electromagnetic radiation source for
directing electromagnetic radiation into cavity 655 and a plasma
catalyst for relaxing the plasma ignition, modulation, and
sustaining conditions can also be included, although they are not
necessary, nor are they shown in FIG. 10 for illustrative
simplicity. Additional methods and apparatus for forming a plasma
jet are described in commonly owned, concurrently filed U.S. patent
application Ser. No. 10/______ (Attorney Docket No. 1837.0025),
which is hereby incorporated by reference in its entirety.
[0115] Consistent with this invention, cavity 655 can include
electrically conductive and substantially thermally resistant inner
surface 670, which can be proximate to aperture 660, electrically
conductive surface 675, which faces surface 670, and voltage source
680, which can apply a potential difference between surfaces 670
and 675. A magnetic field H can also be applied to the plasma by
passing an electric current through coil winding 676, which can be
external or internal to vessel 657.
[0116] A method for forming plasma jet 685 at aperture 660 can also
be provided. The method can include (1) flowing a gas into cavity
655, (2) forming plasma 690 from the gas in cavity 655, (3)
allowing at least a portion of plasma 690 to pass out of cavity 655
through aperture 660 such that plasma jet 685 is formed outside
cavity 655 proximate to aperture 660, and (4) applying an electric
potential between surfaces 670 and 675 and/or passing an electric
current through coil 676.
[0117] Application of an electric potential between surfaces 670
and 675 can cause plasma 690 to accelerate charged particles to
move toward aperture 660. Surfaces 670 and 675 can be disposed on,
or be integral with, vessel 657. Alternatively, surfaces 670 and
675 can be separate from the internal surface of vessel 657. In
this case, these surfaces can be plates or screens that are
suspended or otherwise mounted in cavity 655. Alternatively,
surfaces 670 and 675 can be discs or rings or any other part having
a convenient shape configured for use in plasma cavity 655.
[0118] Magnetic field H can be generated by passing a current
through coil 676 and applied to plasma 690. The magnetic field can
exert a deflecting force on the charged particles that try to move
perpendicular to the magnetic field. Consequently, charged
particles in the plasma will be less able to move radially outward
(i.e., perpendicular to the longitudinal axis of coil 676) and, as
a result, the inner surface of cavity 655 close to coil 676 will be
heated less. In addition, because the plasma will tend to form
along the longitudinal axis of coil 676, a hotter and more
efficient plasma jet can be formed.
[0119] The potential can be applied between surfaces 670 and 675
during any time period, including before the formation of plasma
690, during the formation of plasma 690, and after the formation of
plasma 690, although the principal benefit may result when the
potential is applied while the plasma is formed (that is, while the
plasma is being modulated or sustained) in cavity 655. Also,
magnetic field H can be applied at any time, including before,
during, or after plasma formation. As a result, one or more plasma
characteristics (e.g., physical shape, density, etc.) can be varied
by applying a potential between surfaces 670 and 675 and a current
through coil 676.
[0120] The potential difference can cause surface 670 to be more
positive or more negative than surface 675. In one embodiment,
positively charged ions of atoms and molecules within plasma 690
can be attracted toward surface 670 by applying a relatively
negative potential to surface 670. Because the positive ions, which
are attracted by negative surface 670, will transfer at least some
of the kinetic energy to surface 670, surface 670 can be made from
a material that can withstand relatively high temperatures (e.g.,
1,000 degrees Fahrenheit and above). In one embodiment, that
surface can include molybdenum, which is also electrically
conductive.
[0121] In another embodiment, surface 670 can include two or more
layers. The outer layer, which faces or contacts plasma 690 during
operation, can be selected to withstand very high temperatures
(although not necessarily electrically conductive). The under
layer, then, can be electrically conductive, but not necessarily
capable of withstanding very high temperatures. Additional layers
can be used as well to enhance its heat-resistance and/or its
electrical conductivity.
[0122] An electric potential can also be applied between vessel 657
and work piece 681 can be located outside cavity 655 to accelerate
plasma 690 through aperture 660 toward a surface of work piece 681.
When a sufficient electric current flows through the work piece,
the temperature of the work piece can be increased through a
resistive heating as well as from the kinetic energy of the charged
particles striking the work piece.
[0123] In addition to sintering, the plasma of the present
invention may be used in processes performed either prior to,
simultaneously, or subsequent to the sintering process. That is,
before, during, or after the sintering process, a source of a
processing material may be supplied to the plasma. By exposing an
object to the plasma, the object can be subjected to a treatment
using the processing material. For example, in one embodiment, the
processing material can include carbon, and the treatment may
comprise carburizing. During carburizing, some of the carbon
supplied to the plasma may diffuse into the surface of the object.
In another embodiment, the processing material can include
nitrogen, and the treatment may comprise nitriding. During
nitriding, some of the nitrogen supplied to the plasma may diffuse
into the surface of the object. Both carburizing and nitriding can
result in a hardened surface layer formed on the object.
[0124] Additionally, the plasma consistent with the invention may
be used for depositing a coating on the surface of the object
before, during, or after sintering. In one embodiment, a coating
material may be supplied to the plasma. This material may
dissociate and/or disperse within the plasma. By exposing the
object to the plasma containing the coating material, some of the
coating material may be deposited on the surface of the object.
Coatings that may be deposited on the object may include at least
one of tungsten carbide, tungsten nitride, tungsten oxide, tantalum
nitride, tantalum oxide, titanium oxide, titanium nitride, silicon
oxide, silicon carbide, silicon nitride, aluminum oxide, aluminum
nitride, aluminum carbide, boron nitride, boron carbide, boron
oxide, gallium phosphide, aluminum phosphide, chromium oxide, tin
oxide, yttria, zirconia, silicon-germanium, indium tin oxide,
indium gallium arsenide, aluminum gallium arsenide, boron,
chromium, gallium, germanium, indium, phosphorus, magnesium,
silicon, tantalum, tin, titanium, tungsten, yttrium, and
zirconium.
[0125] Still other processes may be performed in conjunction with
the sintering process of the invention. For example, after
sintering, the plasma may be used to heat treat the object. Such a
heat treatment may alter one or more properties of the sintered
part (e.g., hardness, ductility, grain size, etc.).
[0126] In the foregoing described embodiments, various features are
grouped together in a single embodiment for purposes of
streamlining the disclosure. This method of disclosure is not to be
interpreted as reflecting an intention that the claimed invention
requires more features than are expressly recited in each claim.
Rather, as the following claims reflect, inventive aspects lie in
less than all features of a single foregoing disclosed embodiment.
Thus, the following claims are hereby incorporated into this
Detailed Description of Embodiments, with each claim standing on
its own as a separate preferred embodiment of the invention.
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