US 3602595 A
Description (OCR text may contain errors)
United States Patent  Inventors Ralph Leon Dahlqulst 50 Field 6: Search 356/85, 87, Sam Barbara; 74, 36; 313/231;219/121, 271, 192; 204/249 James Latimer Jones, Santa Barbara; Kenneth William Paschen, 061m, all of, 1 References CI'ed m, UNITED STATES PATENTS Appl. No. 737,252 3,188,180 6/1965 Holler 356/86 Filed May 0,1968 3,217,162 11/1965 Wehner... 356/85 at s- 3 1971 3,304,402 2/1967 Thorpe... 313/231 x Assignee pp Research Laboratories, 3,325,976 6/1967 West 356/85 Sun and, C 3,402,31 1 9 1968 Fitzgerald 356/86 C n -p of avplimfion 3,467,471 9/1969 Greenfield et al. 356/85 644,987, June 9, 1967, now abandoned.
PrimaryExaminer-Ronald L. Wibert Assistant ExaminerV. P. McGraw Att0rney Hoffman Stone METHOD OF AND APPARATUS FOR AB T C l t t GENE A I AEROSOLS BY ELECTRIC ARC S RA T An e ec r1c arc struck between a coun erelec trode connected to the anode of a source of' current, and a 16 Claims 8 Drawing material to be sampled connected to the cathode, causes the US. Cl 356/36, ejection of very small droplets of the material. The droplets 356/74, 356/86, 313/231 solidify and are carried away as an aerosol by the gas used to Int. Cl G0ln 1/00, sustain the arc. The droplets are representative in their composition of the entire region of the material struck by the arc.
PATENTED M1831 I97l SHEET 1 UF 2 ow /Ayn" 4 INVENTORS JAMES L. JONES KENNETH W. PASCHEN RALPH L. DAHLQUIST ATTORNEY PATENTED AUG31 I97! 3302595 SHEET 2 OF 2 SOURCE SOURCE I MAT MAT'L. POTENTlAL POTENTIAL --4oo,usEc.
300,44 SEC. H6 7 FIG. 4
' -40AMPERES L-30-4O M SEC.
F IO M SEC.
SOURCE MAT'L. POTENTIAL U CE [1 K- t A A A A A -soo,q SEC. POTENTIAL V UV HG 8 INVENTORS 21k; mi mm K N woo/a SEC. RALPH L. HLQUIST FIG. 6 BY ATTORNEY METHOD OF AND APPARATUS FOR GENERATING AEROSOLS BY ELECTRIC ARC 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.
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. Y
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 averagecomposition 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 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 onlyenables 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 are 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 the solved 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. I 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 showingthe 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 are 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 effeet 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. Altemately, 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 l4.'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 are is then struck by passing a momentary high voltage discharge between the anode l4 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 are 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 mate 2'? ill 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 are 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 are 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 20in 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 are. 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 are 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 are 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.
WHAT IS CLAIMED 1S:
1. Method of producing an aerosol comprising passing an electric are 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 are 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 are 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 are 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 materi ls t pri i e 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 are between said electrode and the source material and to carry particles of the source material dislodged by the are 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 are 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 parti cles 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.