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C. S. STOKES and W. W. KNIPE’ e
Research Institute of Temple University, Philadelphia, Pa.
The Plasma Jet in Chemical Synthesis Use of the plasma iet to synthesize chemical compounds opens a new field for investigation
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HIGH TEMPERATURES attainable with plasma jets (7-3, 6) can be used to make highly endothermic compounds. The jet may comprise a heat source alone-e.g., when argon is the working gas-or the working gas such as nitrogen may participate in the reaction. I n the work described here, a nitrogen-plasma jet is used to prepare endothermic nitrogen-containing compounds.
Apparatus The plasma generator, powered by a 600-ampere d.c. welder, has a ‘/S-inch, 2% thoriated-tungsten cathode and a Present address, Pennsalt Chemicals Gorp., Philadelphia, Pa.
copper anode with a pure-tungsten insert. T h e actual arc stream strikes between the two tungsten surfaces which are essentially nonconsumable. Recently, a distilled-water-cooled copper anode has been successfully operated for periods of 15 minutes or longer. Previous to use, the water-cooling cavities were cleaned with nitric acid, giving a heat transfer surface free of contaminants. Copper anodes erode rapidly when cooled by “commercial” water and mineral deposits build u p and lower the effective heat transfer area. The voltage-amperage characteristics of the nitrogen arc produced by the apparatus used are shown in Figure 1. T h e anode and cathode, which are separated with a ‘/a-inch Teflon gasket, a i d the chamber are water-cooled.
c ANNULAR GAS MANIFOLD 8 - I / 16”HOLES
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The flow of nitrogen, fed through an annular gas manifold, is measured by a Fischer and Porter flowmeter; gases or powders are injected into the plasma from a ring attached to the bottom of the plasma generator. T h e solidsinjection system uses the fluidization principle. The chamber (Figure 2) has a fitting through which variously shaped “cold fingers” may be inserted. For collecting solids, the cold finger is set close to the feed ring to quench the products. Such devices are also used to quench the reaction when gaseous compounds are prepared. When gaseous compounds are prepared, freeze-out traps are used with either dry ice or liquid nitrogen, but these are not required when solids are prepared. I n all cases, a positive pressure is maintained on the system to prevent the entry of air.
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The plasma generator, powered by a 600-ampere d.c. welder, has a l/g-inch, 2% thoriated-tungsten cathode and a copper anode with a pure-tungsten insert
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Figure 1. The apparatus produces a nitrogen arc having these voltage and amperage characteristics VOL. 52, NO. 4
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Figure 2. The chamber has a fitting through which variously shaped cold fingers may be inserted
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Figure 3. At plasma iet temperatures all the compounds have a positive free energy of formation. However, various cold finger devices can freeze the reaction and allow the compound to b e collected
Preparation of Compounds
T h e method and values used in plotting free energy of formation us. temperature (Figure 3) have been previously reported (5). At the plasma jet temperatures, all of the compounds have a positive free energy of formation. However, various cold finger devices can freeze the reaction and allow the compound to be collected. An empirical method can be set u p for determining whether a compound can be made in this apparatus. T h e temperature at which AF equals zero is a n indication of the ease of formation. Titanium nitride and magnesium nitride were prepared with little difficulty; but under exactly identical conditions, molybdenum nitride and tungsten nitride could not be prepared. T h e temperature a t which AF equals zero is much higher for titanium and magnesium nitrides than for molybdenum and tungsten nitrides (Figure 3). Thus, for this system it can be concluded that the higher the temperature at which A F = 0, the more likely it is that the compound can be made. Tantalum nitride and zirconium nitride should also be produced in the arc. The two nitrides were prepared by fluidizing the metal powder in nitrogen and injecting it into the nitrogen plasma. T h e power to the plasma was 9.3 to 10.5 kw. and the average nitrogen flow was 2.5 liters per minute. T h e titanium feed was 1.72 grams per minute. T h e nitride obtained is a lustrous golden crystalline compound. Purification is a simple operation; the product is ground and leached with 10% hydrochloric acid to remove titanium. T h e average yield was 30% with the cold finger l/z-inch below the point at which the titanium was fed. The magnesium was also fluidized
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and injected into the plasma. Power to the plasma was 12 to 15 kw., and nitrogen flow 2.5 liters per minute. T h e nitride is dull yellow. Conversion was 40%. I t is difficult to purify the magnesium nitride, since it decomposes rapidly, yielding ammonia in the presence of water. Decomposition occurs even in contact with air, as is proved by strong odor of ammonia. Cyanogen was prepared by injecting carbon powder fluidized in nitrogen into the plasma by means of the feed ring. T h e plasma is a t a sufficiently high temperature to vaporize the carbon. A cold finger was used to quench the gases. Unreacted carbon is condensed in the chamber. T h e cyanogen was condensed in the traps with dry ice. T h e molecular weight of the material, as measured with a gas-density balance, was 50 (actual molecular weight 52). T h e vapor pressure us. temperature relationship was determined experimentally and corresponded to that of pure cyanogen. T h e power to the plasma was 10 to 14 kw. I n all cases the nitrogen flow was 9.5 liters per minute. ‘The conversion based on carbon was 2%. Nitrogen dioxide was prepared by injecting oxygen into the nitrogen plasma through the feed ring. A cold finger was used to quench the gases and the nitrogen dioxide formed was collected in the traps cooled with dry ice. T h e solid material was sky-blue in color. This would indicate the presence of nitric oxide (4). This follows since there was an excess of nitrogen in the reaction chamber. The solid was allowed to melt and a green liquid was obtained. The plasma jet was operated at 10 to 13 kw. with a nitrogen flow of 9.5 liters per minute. T h e oxygen was 8.25 liter per minute. A portion of the material was added to water. If the
INDUSTRIAL AND ENGINEERING CHEMISTRY
material were nitrogen tetroxide, the reaction would yield nitric and nitrous acids. Tests for both nitrate and nitrite were positive. T h e average conversion based on the oxygen was 276. Discussion
The experiments conducted to explore the possibilities of using the plasma jet as a preparative device, indicate that further study is warranted. The yields should not be considered as the maximum obtainable, but rather as proof that the reactions are possible. Considerably higher yields probably can be obtained by careful control of the parameters involved in the apparatus. Acknowledgment
T h e authors thank A . V. Grosse for his valuable discussions in carrying out this work. Literature Cited (1) Fisher, H., Mansur. W., “Conferences on Extremely High Temperatures,” John Wiley, h-ew York, 1958. (2) Kuhn, SV. E.,“Symposium on Arc.: in Inert Atmospheres and Vacuum,” John Wiley, S e w York, 1956. ( 3 ) Mark, S. D., Jr., “Hi711 Intensity .\rc Symposium,” The Carborundum Co.. Kiagara Falls, N. Y . . 1958. (4) Moeller, T.: “Inorganic Chemistry,” pp, 591-612, John Wiley, New York, 1952. (5) Quill, L. L., “Chemistry and Metallurgy of Miscellaneous Materials, Thermodynamics,” McGraw-Hill, S e w York, 1950. (6) Stokes, C. S., Knipe, 147. W., Streng, L. .4., ,J. Rlectrochem. Soc., 107, 35 1960.
RECEIVED for review May 25, 1959 ACCEPTEDJai?uary 6: 1960 Division of Industrial and Engineering Chemistry, Symposium on High Temperature, 135th Meeting, ACS, Boston, Mass., April 5-10. Work supported by a grant from t h e National Science Foundation.
