U.S. patent number 3,602,595 [Application Number 04/737,252] was granted by the patent office on 1971-08-31 for method of and apparatus for generating aerosols by electric arc.
This patent grant is currently assigned to Applied Research Laboratories, Inc.. Invention is credited to Ralph Leon Dahlquist, James Latimer Jones, Kenneth William Paschen.
United States Patent |
3,602,595 |
Dahlquist , et al. |
August 31, 1971 |
METHOD OF AND APPARATUS FOR GENERATING AEROSOLS BY ELECTRIC ARC
Abstract
An electric arc struck between a counterelectrode connected to
the anode of a source of current, and a material to be sampled
connected to the cathode, causes the ejection of very small
droplets of the material. The droplets solidify and are carried
away as an aerosol by the gas used to sustain the arc. The droplets
are representative in their composition of the entire region of the
material struck by the arc.
Inventors: |
Dahlquist; Ralph Leon (Santa
Barbara, CA), Jones; James Latimer (Santa Barbara, CA),
Paschen; Kenneth William (Goleta, CA) |
Assignee: |
Applied Research Laboratories,
Inc. (Sunland, CA)
|
Family
ID: |
24963181 |
Appl.
No.: |
04/737,252 |
Filed: |
May 20, 1968 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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644987 |
Jun 9, 1967 |
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Current U.S.
Class: |
356/36;
73/863.11; 356/313 |
Current CPC
Class: |
G01N
21/67 (20130101); G01N 1/22 (20130101) |
Current International
Class: |
G01N
1/00 (20060101); G01N 21/62 (20060101); G01N
21/67 (20060101); G01n 001/00 (); G01j
003/00 () |
Field of
Search: |
;356/85,87,74,36
;313/231 ;219/121,271,192 ;204/249 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wibert; Ronald L.
Assistant Examiner: McGraw; V. P.
Parent Case Text
BRIEF SUMMARY
This application is a continuation-in-part of the pending
application of the same inventors, Ser. No. 644,987, filed June 9,
1967, entitled "Method of and Apparatus for Generating Aerosols by
Electric Arc to Obtain Samples for Chemical Analysis," and assigned
to the present assignee now abandoned .
Claims
WHAT IS CLAIMED IS:
1. Method of producing an aerosol comprising passing an electric
arc between a counter electrode and a source material with the net
current flow being in the direction from the counter electrode to
the source material, providing enough energy in the arc to cause
droplets of the source material to be ejected from it to form an
aerosol, simultaneously flowing a selected gas through the region
of the arc to carry the aerosol composed of the gas and droplets
ejected from the material away from the source material, the
counter electrode being arranged to avoid sputtering from it.
2. Method according to claim 1 wherein the selected gas is selected
from the group consisting of argon, helium, and nitrogen.
3. Method of producing an aerosol comprising passing an electric
arc between a counter electrode and a source material with the net
current flow being in the direction from the counter electrode to
the source material, providing enough energy in the arc to cause
droplets of the source material to be ejected from it to form an
aerosol, simultaneously flowing a selected gas through the region
of the arc to carry the aerosol so formed away from the material,
and to solidify the droplets, the counter electrode being arranged
to avoid sputtering from it.
4. Method of producing an aerosol from a molten source material
comprising passing a direct current electric arc between a counter
electrode an an anode and the source material as a cathode with
sufficient energy to cause droplets of the source material to be
ejected from its surface to form an aerosol, simultaneously flowing
a selected gas through the region of the arc to carry the aerosol
so formed away from the material, the counter electrode being
arranged to avoid sputtering from it.
5. Method of producing a specimen the composition of which is
representative on a microscopic scale of the composition of a
fairly large region of a solid body comprising passing an electric
arc between a counter electrode an an anode and the solid body as a
cathode with sufficient energy to cause droplets of the material of
the body to be ejected from its surface to form an aerosol,
simultaneously flowing a selected gas through the region of the arc
to carry the aerosol so formed away from the surface of the body,
the counter electrode being arranged to avoid sputtering from
it.
6. Method according to claim 5 including also the step of
maintaining a sufficient current in the arc to melt at least a
macroscopic portion of the body.
7. Method according to claim 6 including the step of heating the
body by electromagnetic induction and thereby stirring the molten
portion thereof.
8. Method of chemical analysis comprising the steps of producing an
aerosol in accordance with the method of claim 1, and analyzing the
solids portion of the aerosol so produced.
9. Method of monitoring the composition of a bath of a molten,
electrically conductive material comprising producing an aerosol in
accordance with the method of claim 1, conducting it to a
spectrometric analytical device, and analyzing the solid particles
of the aerosol spectrometrically.
10. Apparatus for producing an aerosol from a source material
comprising:
a. an electrode,
b. enclosure means for enclosing a region adjacent to a selected
surface of the source material, said enclosure means enabling said
electrode to be positioned within said region and adjacent to the
surface of the source material, said enclosure means also including
inlet means and outlet means,
d. a unipotential source of electric current,
e. means for connecting the source material to the cathode of said
current source and said electrode to the anode of said current
source, and
f. means for flowing a selected gas through said enclosure means
via said inlet and outlet means while said connecting means is
operative, to facilitate the striking and maintenance of an arc
between said electrode and the source material and to carry
particles of the source material dislodged by the arc out of said
region through said outlet means.
11. A lance for producing an aerosol from a bath of a molten
material comprising:
a. a tube having an open end for withdrawing an aerosol from a
region adjacent to the surface of the molten material, said tube
being electrically conductive,
b. means defining an open-ended enclosure around a terminal portion
of said tube and extending beyond the end of said tube,
c. means for flowing a gas into the enclosure defined by said
enclosure means from a point spaced from the open end thereof and
outside of said tube, and thence out of the enclosure through said
tube;
d. means for making electrical contact with a molten material at a
point spaced from said tube when said tube with said enclosure
means is placed open and first into the molten material, whereby a
source of electric current may be connected between said tube and
the source material to produce an electric arc between them strong
enough to dislodge small particles of the source material from its
surface, and
e. said gas means being operative to carry particles dislodged from
the source material by an arc out of the enclosure through said
tube.
12. Apparatus in accordance with claim 11 including dynamic cooling
means to enable the lance to withstand continued exposure to
elevated temperatures.
13. Apparatus in accordance with claim 11 including a cap of an
insulating, refractory material covering the open end of said tube
for limiting an arc struck therefrom to the inner wall surface
thereof, said cap having an aperture coaxially aligned with said
tube to permit striking the arc and passage of aerosols into said
tube.
14. Apparatus in accordance with claim 11, wherein said electrical
contact means is a cylindrical conductive member and constitutes
the outer wall portion of said enclosure means.
15. Apparatus for producing an aerosol from a source material
comprising:
a. a tubular electrode open at one end, and having an exhaust
opening spaced from said one end,
b. means for supporting said electrode with said open end adjacent
to and spaced from a source material,
c. enclosure means for enclosing a region adjacent to the source
material and including said open end of said electrode,
d. gas flow means for introducing a carrier gas into the region
enclosed by said enclosure means and withdrawing it from the region
through the open end of said electrode,
e. an unipotential source of electric current,
f. means for connecting the source material to the cathode of said
current source and said electrode to the anode thereof, and
g. said gas flow means being effective to sweep small particles
such as may be produced by an arc energized by said current source
out of said enclosure means through said electrode.
16. Apparatus according to claim 15 including an annular insulating
shield fixed to the open end of said electrode for confining an arc
struck from said electrode to the internal surface thereof.
Description
This invention relates to a novel method of and apparatus for
nebulizing a material to obtain a sample for chemical analysis or
the like, or for any other purpose when it is desired that the
composition of the nebulized material be closely representative of
the average composition of a reasonably large region of the body
from which it is taken.
The invention arose in connection with efforts to improve
spectrochemical analytical methods, and its background and
advantages will be described herein primarily with respect to
spectrochemical work. It is expected, however, that the invention
will also be of significant value for other purposes such as for
use in preparing samples for other methods of chemical analysis
including wet processes, and for obtaining fine powders for any
purpose where it is desired that the powders be composed of
extremely small particles, or that the composition of the powders
be closely representative of the composition of a fairly large
region of the material from which they are taken.
Spectrochemical methods of analysis are widely used and have been
found to be especially advantageous for process control, largely
because of the high speed with which analyses can be made by these
methods. For example, the manufacture of steel can be more closely
controlled if the composition of the heat is known at the time it
is poured. Previously to the adoption of spectrochemical methods,
it was regarded as impracticable to hold a heat pending completion
by wet chemical methods of an analysis of a sample taken from it.
Not only were the fuel, equipment and labor utilization costs
regarded as intolerable, but also the composition was apt to change
significantly during the time required. With spectrochemical
methods as heretofore practiced, it has been possible in most cases
to obtain fully adequate analyses of samples in less than ten
minutes, thus making highly accurate composition control fully
feasible.
The present invention arose out of efforts both to reduce the time
needed for analysis still further and to improve the quality of the
samples so that they would be more truly representative of the
actual average composition of the heat than samples heretofore
obtainable. The advantages of increased speed are obvious and need
not be discussed herein. The problem of compositional
representativeness, however, is more subtle.
It is generally recognized that for samples prepared by methods
that do not include the step of preparing a liquid solution, the
accuracy of spectrochemical analysis is limited by the homogeneity
and representativeness of the portion of the sample that is
actually excited to produce radiation during the analysis. In
optical emission analysis, for example, a spark discharge to the
solid metal sample often vaporizes less than a milligram of metal,
and even though the composition of the sample taken as a whole may
be accurately representative of the composition of the entire heat,
segregation of constituents during freezing creates substantial
variations in the compositions of even fairly closely spaced
regions of the sample. By adequate stirring and mixing in the melt,
it is easily possible to achieve a sample body, which, on the
whole, is representative in composition of the entire melt.
Homogeneity of the sample, however, is a much more difficult
condition to achieve. Various different constituents of the sample
tend to segregate as the sample freezes, so that casting
procedures, for example, cannot be expected to provide homogeneity
down to the microscopic scale desirable for optical emission
analysis.
In X-ray fluorescence analysis, much larger areas of the surface of
a solid sample may be excited than are utilized in optical
emission. Typically, 4 or 5 square centimeters may be irradiated,
but the fluorescent X-rays are produced from surface layers of only
one to a few hundred microns in thickness depending upon the
particular analytes selected and the matrix in which they are held.
Again, the analysis is based on amounts of materials of but 1 to
100 milligrams at the most, each element is determined from a layer
of different thickness, and the accuracy of the analysis depends
upon the accuracy with which the composition of a relatively thin
surface layer of the sample corresponds to the average composition
of the entire melt.
In analysis by electron microprobe, an electron beam, usually less
than 1 micron in diameter, is directed upon the sample to generate
X-rays, which are then spectrometrically analyzed. The volume of
the material involved in analysis of this type is limited to the
volume required to stop the electrons of the beam. This is
typically a few cubic microns. With the electron microprobe it has
been shown that in the solid samples normally used for analysis by
optical emission or by X-ray fluorescence, regions spaced only a
few microns from each other are of significantly different
compositions.
The practice of the invention not only enables a reduction in the
time needed for preparing samples and presenting them to the
spectrometric apparatus, but also overcomes difficulties of
obtaining samples that are homogeneous throughout on a microscopic
scale. Moreover, due to the way in which the samples are formed,
they are more readily dissolved than samples made by casting, and
thus enable a reduction in time required for wet chemical
analysis.
Briefly, the practice of the invention contemplates the use of an
electric arc together with a stream of gas to produce an aerosol
from a material to be sampled and to carry the aerosol away from
the surface where it is produced. The arc is struck directly to the
surface of the material, and causes the ejection of very fine
particles. The current of the arc may be controllably varied, and
the nature of the gas selected to achieve optimum particle emission
from the material being sampled, both as to quantity and as to the
sizes of the individual particles. In addition, so long as the
material is electrically conductive, there is no effective
limitation as to its temperature, and samples may readily be
obtained from molten materials.
The aerosol particles produced in accordance with the present
invention may readily be made predominantly 1 micron and smaller in
diameter. They may be conducted directly to a plasma flame chamber
for immediate analysis by optical emission, or otherwise analyzed
as desired. The aerosol may be passed through a filter to collect
its solid particles. Which may then be analyzed by X-ray
spectrometric techniques or otherwise as desired. The collected
solid particles, being very finely divided may also be very quickly
dissolved for analysis by wet processes, so that even for wet
analyses, the practice of the invention enables a significant
improvement in speed as well as in homogeneity and
representativeness.
The practice of the invention enables the production of a fine
metal powder of a very high degree of homogeneity, which is truly
representative composition-wise of the material from which the
aerosol droplets are ejected.
The material to be nebulized is made the cathode for a DC arc,
which appears to produce cavitation, or some generally similar
effect on the surface of molten materials accompanied by the
ejection of fine particles. Similar action is obtained when the arc
is applied to solid surfaces, apparently accompanied by highly
localized melting of the material on the surface. The action is
presently thought to be caused by a very steep potential gradient
immediately adjacent to the cathode. Even when the arc is of low
voltage, the potential gradient appears to be very high at the
cathode.
The quantity of particles ejected from the surface has been found
to depend upon the selection of the gas used to sustain the arc,
which, if a large flow of aerosol is desired, should be one capable
of producing a large number of positive ions in the arc. The
quantity of aerosol produced also depends upon the flow of gas,
which determines the rate at which the particles are removed from
the region adjacent to the surface of the material.
When small bodies of a molten material are to be analyzed of the
kind where constituents of the material tend to separate in the
melt, it is desirable to utilize supplemental heating of the type
that causes adequate stirring, such as, for example, induction
heating, or to provide for stirring in some other way before
starting to draw the aerosol from the material.
DETAILED DESCRIPTION
Representative embodiments of the invention will now be described
in connection with the accompanying drawing, wherein;
FIG. 1 is a schematic, cross-sectional view of an aerosol generator
in accordance with the invention arranged for nebulizing a solid
material;
FIG. 2 is a schematic, cross-sectional view showing apparatus
according to a modified form of the invention, as arranged for
producing an aerosol from a small body of molten material or from a
flowing stream of molten material;
FIG. 3 is a schematic, cross-sectional view of a lance in
accordance with the invention for obtaining an aerosol from a large
body of molten material, such as, for example, a heat of steel in
an open hearth furnace;
FIG. 4 is a chart showing the arc current produced by a periodic
high voltage discharge;
FIG. 5 is a chart showing the arc current produced by application
of a constant direct current source of fairly low internal
impedance;
FIG. 6 is a chart showing the arc current produced by a periodic
low voltage discharge, with certain reactors in series between the
arc and the discharge source to damp oscillations to a small
extent;
FIG. 7 is a chart showing the arc current produced as in the case
of FIG. 6, but with the reactors selected to achieve critical
damping; and
FIG. 8 is a chart showing the arc current produced as in the cases
of FIGS. 6 and 7, but with the reactors chosen to produce greater
than critical damping.
According to a first illustrative embodiment of the invention as
shown in FIG. 1, an aerosol 10 is produced from a solid,
electrically conductive body 12, and withdrawn through an exhaust
tube 14 for any desired use. The open tip 16 of the exhaust tube is
placed closely adjacent to the surface of the body 12 to be
analyzed, and serves as an anode for striking an arc between the
tube 14 and the body 12. The tube 14 is preferably of copper, and
water cooled, as shown, so that it does not become heated by the
arc sufficiently to eject its own constituent materials. Its open
end 16 is preferably additionally protected, as shown, by a
centrally apertured cap 17 of highly refractory and corrosion
resistant, insulating material, which operates to restrict the arc
to the inner wall surface of the tube 14, to stabilize the arc, to
distribute its upper end around the inner circumference of the tube
14, and to concentrate its lower end toward a region on the surface
of the body 12 near the central axis of the tube 14.
In operation, the cap 17 also produces a jetlike, restrictive
effect upon the arc, confining it to a fairly small region on the
surface of the material being sampled directly opposite the central
aperture 19. The effect is thought to be due, at least in part, to
the reduction in gas pressure caused by the flow of gas through the
aperture 19, which is accelerated by heating of the gas by the arc
itself. The effect may be enhanced by imparting a tangential motion
to the gas to create a swirling effect as it enters the aperture
19, further to reduce the pressure along the central axis of the
aperture.
An enclosure 18, which may be of insulating material, as shown, is
fitted around the lower end of the exhaust tube 14 to confine the
working gas and prevent its escape except through the exhaust tube
14. Alternately, if desired, the enclosure 18 may be of a
conductive material, in which case it should be insulated from the
exhaust tube 14 and of adequate internal diameter to insure against
striking of an arc between it and the exhaust tube 14. The working
gas, which may, typically, be helium, argon, or nitrogen, is
introduced through an inlet 20 in the enclosure 18.
In operation, the enclosure 18 and the exhaust tube 14 are first
flushed by flowing the working gas through them to remove air and
to provide the desired working atmosphere. The arc is then struck
by passing a momentary high voltage discharge between the anode 14
and the body 12, and may thereafter be maintained at a low voltage
sufficient to maintain an average current of at least about 3
amperes, and preferably less than about 50 amperes. The arc strikes
the surface of the body 12 at a very small point and tends to move
rapidly over the surface in what may be called a random scanning
pattern. After a few minutes, the whole surface of the body 12
beneath the open end of the exhaust tube 14 presents an etched
appearance. Local melting and sputtering occur wherever the arc
strikes the body 12. The arc has a natural tendency to avoid molten
portions of the body and to anchor itself to a solid surface. It is
seen to be constantly moving over the surface, thereby providing
successive very small samples from successive different portions of
the body 12, thus insuring that the aerosol is highly
representative in composition of a fairly large region of the body
12.
The working gas continues to flow through the enclosure picking up
droplets of the material that are ejected from the surface of the
body 12, and carrying the droplets through the exhaust tube in the
form of an aerosol 10. The droplets freeze rapidly, without
coalescing, to form an aerosol of minute solid particles, typically
smaller than 1 micron in diameter. The flow of the working gas
tends to concentrate the arc and to stabilize it, depending upon
the nature of the gas and its flow rate in relation to the physical
dimensions of the exhaust tube 14 and its spacing from the surface
of the body 12 under analysis. Maximum concentration of the arc and
the most satisfactory results have been achieved in the work done
thus far by the use of helium, which has been found to be effective
at much lower rates than, for example, argon.
The embodiment of the invention illustrated in FIG. 2 is intended
primarily for obtaining aerosols from molten materials. It includes
a cover 24 of an insulating material enclosing a crucible 26, which
has insulating sidewalls 27, and a conductive bottom wall 30 to
provide electrical contact with the material 28 in the crucible.
The bottom wall 30 is preferably water cooled, as shown, for use
with materials that melt at high temperatures. The crucible 26 may
be of any desired configuration. It may, for example, be in the
form of an elongated trough for conducting a continuous stream of
molten material through the sampling zone. As shown, an induction
coil 32 is mounted around the crucible 26 for electromagnetically
heating and stirring the molten material 28. The combination
exhaust tube and arcing anode 14 extends through one wall of the
cover 24 and terminates adjacent to the upper surface of the molten
specimen material 28.
Operation of this embodiment of the invention is identical in
principle to the operation of the first described embodiment
herein. There is less rapid motion of the arc over the surface of
the material 28, but compositional representativeness is assured by
reason of convection currents and agitation in the melt. Droplets
of the material 28 of microscopic size are ejected by the arc from
the surface of the material 28 and are swept into the exhaust tube
14 by the flow of working gas, which enters through an inlet 20 in
the cover. In this case also, the droplets freeze rapidly to form
an aerosol of solids.
Some of the droplets ejected from the surface of the material 28
may become completely vaporized as they pass through the arc, but
the resulting vapors recondense very rapidly as they enter the
exhaust tube 14 because of the cooling effect of the working gas.
Insofar as is presently known, vaporization is not significant in
the practice of the invention, and its occurrence or absence may be
ignored.
Generally similar, but usually less satisfactory results may be
achieved in this embodiment of the invention without using the
induction coil 32 to heat the material 28. In cases where it is
desired to melt an initially solid material in the crucible 26, the
arc itself may provide sufficient heat to melt the entire body, or
a portion of it to form a puddle on its surface. In cases where the
material 28 is already molten when it is fed into the crucible 26,
additional heating may not be needed. The use of induction heating,
however, is preferred, because it permits better control, and
especially because of its stirring effect, which enhances the
representativeness of the composition of the aerosol produced.
The lance shown in FIG. 3 is proposed for use in monitoring on a
continuous or intermittent basis, as desired, the composition of a
large mass of molten material such as, for example, a heat in an
open hearth furnace. The lance includes an exhaust tube 40
generally similar to the exhaust tubes 14 shown in the embodiments
of FIGS. 1 and 2, but preferably of more rigid and rugged
construction to enable it better to withstand the buffeting it may
be subjected to in this type of environment. A lower end portion of
the tube 40 of any desired length is surrounded by an enclosure
arrangement, which as shown is constituted by a water cooled,
cylindrical contact electrode 41. The contact electrode 41 extends
beyond the open end of the tube 40 and is insulated from the tube
40 by any desired means such as the cup-shaped mounting element 36
shown.
Several alternative arrangements are contemplated. For example, the
contact electrode 41 may be in the form of a rod, in which case the
insulating mounting element would constitute the enclosure
arrangement and would extend beyond the end of the tube 40, In
another construction, the mounting element 36 may be a simple
annulus, in which case insulation in the form either of a coating
or of additional spacing would be provided between the exhaust tube
40 and the contact electrode 41.
The contact electrode 41 extends beyond the lower end of the
exhaust tube 40 sufficiently far so that when the lance is lowered
into the molten bath, only moderate pressure of the working gas
will be required to keep the surface of the melt 44 spaced away
from the open end of the exhaust tube 40. The contact electrode 41
makes a relatively large area electrical contact with the melt 44
in a region reasonably close to the exhaust tube 40, thereby
minimizing the loss of energy by joulean heating of the melt.
In operation, the lance is simply lowered into the melt 44 to any
desired depth short of immersing the entire length of the contact
electrode 41, and the working gas is introduced into the annular
space 42 within the electrode 41. If the surface of the melt 44 is
contaminated as, for example, by slag or an oxide coating, the
contaminants may be swept away and a clean surface provided by
increasing the pressure of the working gas to cause it to escape
radially outwardly from the lower end of the electrode 41.
The pressure is reduced before starting the arc to a value that
holds the melt below the end of the exhaust tube 40 but does not
cause the gas to escape outwardly from the contact electrode
41.
In actual operation with devices of this type, using arc currents
of from 5 to 50 amperes at relatively low voltages, it has been
found that aerosols may readily be generated containing at least
about 100 milligrams of solids per minute. When collected in the
form of a film such as, for example, by passing the aerosol through
a filter membrane, the solid particles make an excellent sample for
X-ray fluorescence analysis, and also may be rapidly dissolved for
use in wet chemical processes.
It is expected that the process will be found advantageous also for
the production of powders such as powdered iron in cases where it
is desired that the particles of the powders be of very small size
or of very uniform composition, or both. The particles produced in
the practice of the invention are highly uniform in composition,
and may easily be made smaller than 1 micron in diameter, on the
average, by suitable control of arcing current and gas flow
rate.
In the practice of the invention, the material from which the
aerosol is to be generated is always connected to the negative
terminal of the source of electricity used to sustain the arc. In
this sense, the term cathodic DC arcing may be used to characterize
the invention.
The currents in the arcs, however, have been found to include
substantial alternating components under all conditions so far
investigated, and actual current reversals occur, so that with
reference to the currents in the arcs, the terms DC and
unidirectional may be very misleading.
The chart of FIG. 4, for example, shows the damped current
oscillations that occur in an arc in the practice of the invention
during one discharge of a high voltage spark generator of a
conventional type. The generator was set for a repetition rate of
240 sparks per second at 18,000 volts. As may be seen, the arc
current rapidly builds up at the beginning of the discharge to a
very high value in the direction of the initially applied voltage.
It thereafter "rings" for the balance of about 300 microseconds at
a rate of about 1 megahertz.
The chart of FIG. 5 shows schematically the current an arc produced
in the practice of the invention in a case wherein the arc was
energized by a DC power supply of fairly low internal impedance,
set to indicate a nominal average output of about 3 amperes. The
arc current fluctuated widely, and included substantial AC
components at various frequencies up to at least about 1 megahertz.
Accurate measurements of frequency were difficult to make because
of the apparently random variations observed, but from observations
of the oscilloscope, it appeared that the high frequency
oscillatory currents, or "hash" 50, were interrupted from time to
time by trains of unidirectional pulses 52 of current of about 40
amperes, each pulse persisting for from 10 to about 40
milliseconds.
The charts of FIGS. 6, 7, and 8 show the arc currents produced by
the discharge of a capacitor through the arc in series with
selected reactors. In each case, the arc was initiated by a very
brief high voltage pulse, and then sustained by current from the
capacitor, which was charged to 1000 volts at the beginning of each
discharge. The repetition rate was 60 per second.
In the first case, FIG. 6, the capacitor was 5 microfarads in
value, and a 360 microhenry inductor was connected in series with
it. The discharge was underdamped, and the current in the arc
oscillated, as shown, at about 3500 hertz for about 1500
microseconds following the initiation of the arc.
In the second case, FIG. 7, the circuit was arranged to produce
critical damping. The capacitor was 10 microfarads in value. A
resistor of 5 ohms, and an inductor of 50 microhenries were
connected in series between the capacitor and the arc electrodes.
After arc initiation by the high voltage pulse, the current in the
arc responded quickly to the voltage impressed by the capacitor,
and decayed within about 400 microseconds without oscillation.
In the third case, FIG. 8, overdamping was provided. The capacitor
was of 30 microfarads, and connected to the arc electrodes through
a 3 ohm resistor and a 360 microhenry inductor. In this case also,
there was no detectable reversal of current once the capacitive
discharge assumed control after arc initiation.
It is not clearly understood why the cathodic arc produces the
improved results that have been noted. The currents in the arc seem
to be seldom purely unidirectional. It appears that at the cathode,
the arc is much less stable in position than at the anode. It moves
across a fairly large surface area of the cathode, sputtering
material from different successive incremental areas of it, and
producing less intense localized heating. Both of these effects are
believed to contribute to the compositional representativeness of
the aerosols produced.
Movement of the arc insures sampling over a macroscopic portion of
the cathode. Lack of intense local heating tends to avoid excessive
volatization and the effects of preferential volatization of the
various different components of the cathode.
The term unipotential source seems to be the most apt one to
describe the principal limiting feature of the invention. As used
herein it is intended to include not only conventional batteries
and direct current power supplies, but also high voltage spark
generators in which the output voltage at the time of spark
initiation is always of a predetermined polarity, and repetitive
capacitive discharge sources in which the output capacitor is
always charged in a predetermined polarity at the start of each
discharge. In the practice of the invention, the material to be
nebulized is connected to the nominally negative terminal of the
source, and serves as the cathode for the arc current on a time
average basis. Although the arc current may reverse momentarily,
the net current taken over the arcing period flows from the counter
electrode to the material to be nebulized.
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