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SIDNEY KATZ, EDWIN J. LATOS, and ELLIOTT RAISEN Armour Research Foundation of Illinois Institute of Techno Chicago 16, 111.
The Plasma Jet in High Temperature Research
The glass chamber in the Armour Research Foundation’s plasma jet permits unobstructed observation of the complete arc process
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jets can maintain temperatures up to 50,000° C. for long periods of time (2). They are now being used to simulate conditions associated with hypersonic flight, especially missile re-entry. Despite several inherent defects, they provide the only method for producing a sustained flow of supersonic, high-temperature gas. Beck ( 7 ) described a high-intensity arc which formed plasma by vaporizing anodic material. This arc attained a temperature of 8000’ C. and was similar to conventional carbon arcs, except that a “tail” of incandescent anodic material was produced by high current densities. The prototype (Figure 1) of modern fluid-stabilized arcs was devised in 1922 ( 4 ) . The arc, struck between the anode and the cathode, was constricted by two doughnut-shaped ceramic nozzles whose inner surfaces were cooled by a thin layer of water maintained by centrifugal force. The water stabilized the arc by cooling its perimeter and causing the current to flow through a smaller cross section. This confined the energy and produced temperatures as high as 50,000° C. Many variations of fluid-stabilized arcs have been reported since. By appropriate changes in design, the boundaries of each property have been extended (Table I). This large range of variability provides great versatility in the application of plasma jets to chemical research. I n the recently developed Weiss arc ( 7 ) , the cathode is an annular disk with an orifice in the center, and the anode is a cylindrical rod (Figure 2). The stabilizing fluid produces a vortex in the upper chamber and drains into the lower chamber. The arc, in traversing
Because it can provide extremely high temperatures for long periods, the plasma iet is a versatile tool for chemical research
the distance between the anode and the cathode in a path concentric with the fluid vortex, extracts molecules from the vortex and produces a flamelike plasma as it flows out through the orifice. Although the plasma, unlike that in the Beck arc, consists primarily of molecules from the stabilizing fluid and their decomposition products, it is usually contaminated with carbon from the electrodes. The plasma jet built for this study by Armour Research Foundation (Figure 3) contains additional refinements based on a design by Grosse (5). The annular anode is a hollow copper chamber and is cooled by rapidly flowing water. The tungsten cylindrical cathode is in contact with a water-cooled copper chamber. The entire arc process is observable through the glass vortex chamber. The arc is struck between the anode and the cathode. As the stabilizing gas swirls around the cathode and passes through the arc, it abstracts energy from it and emerges through the orifice. The power supply is a standard 900ampere Lincoln converter. This jet has been operated at 10 to 30 volts and 100 to 700 amperes with gaseous argon or nitrogen as the stabilizing fluid.
Table I. Properties of Plasma Jets Currently M a d e in the United States Vary Temp., O C. 8000 to 50,00OU Power, watts 500 to 3,000,000 Orifice dia., in. 0 . 0 4 to 3 Thermal flux, 40,000* cal./sq. cm.-min. Plasma velocity Subsonic to Mach 20 a Surface of the sun is 5600O C. Solar radiation at the surface of the earth is 2 cal./sq. cm.-min.
In this report, the distinction between arc and plasma is arbitrary. The arc is considered to be the portion between the anode and the cathode because this part carries the electric current-i.e., excess electrons supplied by the generator. I t is therefore analogous to conventional arcs. The plasma does not carry electrons from the external circuit and is thus analogous to the plasma in the Beck arc. Examination
of the Plasma
The plasma resembles a chemical flame in appearance, and, in fact, there are many similarities. Both are hot gases containing neutral and charged particles which are atomic or molecular. The term plasma, as used in this report, refers to a gas which has enough ionized particles to affect its properties significantly. The temperature of a flame or a plasma depends on energy input and energy absorption. The energy input is by chemical reaction for a flame and by electronic means for the plasma. Energy absorption by translational, vibrational, and rotational modes increases the temperature. Energy absorption by changes from the condensed state to other phases maintain an equilibrium temperature, and energy absorption by dissociation and ionization cause lower temperatures. Dissociation and ionization are extremely energetic processes. In many cases they determine the maximum temperature of a flame, because, even with highly energetic reactants, the chemical energy is insufficient to overcome their effect. The energy input to the plasma, however, is determined only by the amount of electrical energy that VOL. 52, NO. 4
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Figure 1. The Gerdian arc demonstrates the two basic characteristics of the plasma jet: stabilization b y fluid and confinement of energy
can be transferred to it, and this amount is limited only by the properties of the electrodes. At present, plasma jets can attain temperatures an order of magnitude higher than flames. As techniques of cooling electrodes are perfected and energy losses are reduced, this temperature difference will be even greater. The plasma jet itself can be compared to a flame. The arc is analogous to the heat of reaction, since it supplies energy to the gas, and the plasma is analogous to the flame, since it consists of highly energetic products. The processes in the arc, where the electrical energy is transferred, are extremely complex and are not discussed in this report. The processes in the plasma depend on the nature of the stabilizing fluid. For a monatomic gas, such as argon, the energy input excites atomic argon and heats it to 10,OOOo C., at which temperature ionization to the singly charged species commences. More energy is absorbed, until all the argon is ionized. At 20,000" C. formation of the doubly ionized species begins. and by 30,000° C. most of the gas is doubly ionized. For a diatomic gas such as nitrogen, the process is the same except for its initial dissociation. The energy required for dissociation limits the temperature of a chemical reaction involving nitrogen. Although argon has no corresponding limitation, its inertness precludes the possibility of producing a chemical flame. In the plasma jet, however, both argon and nitrogen can be raised to high temperatures because these energy sinks are relatively unimportant compared with the electrical energy available.
such as A1203 has been reported as 2250" to 3500° C. I t is likely that A l z 0 3has a decomposition range rather than a true boiling point. Also, although A120, A10, and A1202 are known to exist in the vapor phase above 2000 C., little is known about the vaporization process of A1203. The spectroscopic data available on species existing in the vapor phase at high temperatures represent a small fraction of the information required. At temperatures above 6000° C. data are obtained mostly from astronomical studies. An advantage of the plasma jet is that it enables examination of vaporization phenomena under controlled conditions. Data on thermal conductivity, electrical conductivity, strengths of materials, and specific heats are scarce. Information on the latter is needed to determine the temperature rise of a system as a function of energy input-e.g., to calculate the theoretical temperature of an exothermic reaction. Thermodynamics of systems and species is another fertile field for study. High-temperature data on free energies, enthalpies, and entropies are as useful for high temperature studies as the corresponding data are for low-temperature processes. Kinetic data at high temperatures are practically nonexistent. At plasma jet temperatures and pressures reactions are probably extremely rapid. However, high energies of activation, steric effects, and molecularity of the reaction may moderate the kinetics. Also, rapid quenching can control the kinetics and, in fact, may enable isolation of species which are stable at high temperatures and unstable or metastable at moderate temperatures. This is roughly analogous to Rice's work (6) on stabilization of free radicals, such as NH, at lower temperatures of formation. Drawin (3) has used the plasma jet to determine the electronic cross sections of atomic hydrogen, argon, fluorine, carbon, nitrogen, and oxygen. Since the plasma jet vaporizes all materials, it makes possible many vapor-
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Figure 3. This jet designed b y the Armour Research Foundation has been operated at 10 to 30 volts and 100 io 700 amperes with gaseous argon or nitrogen as the stabilizing fluid
phase processes. Crystals of refractory materials can be grown from the vapor phase, and poly- or monocrystalline refractories or ceramics can be produced from melts. Although problems of containment in such processes are formidable, they undoubtedly can be overcome. Another practical application, which is being developed, is flame spraying. In this process a powdered refractory material, such as zirconium oxide, is fed into a high-temperature flame and simultaneously sprayed onto a substrate; the refractory melts and the globules coalesce into a coherent coating. At present flame spraying is limited to materials which melt in acetylene-oxygen or hydrogen-oxygen flames, but the plasma jet extends its potential to any known refractory and even to a new field-vapor-phase Aame spraying. Other possible applications for the plasma jet are vapor-phase separation of ores and production of alloys, welding in the conventional manner, and special welding to form refractory materials. By ingenious use of the plasma jet, new approaches to high-temperature chemistry can be developed and much-needed information can be obtained. Literature Cited
(1) Beck, Ger. Patent, 262,913 (1910). (2) Burhorn, F., Maecker, H., Peters, Th., Z. Physik. 131, 28 (1951). (3) Drawin, H. W., Ibid., 146, 295-313 VORTEX C?*UBER
11,356) \-,--,. (4) Gerdian, H., Wiss. Veriyentl. Siemens-
Konzern 2, 489 (1922). (5) Grosse, A . V., personal communication. (6) Rice, F. O., .4bstract of Papers, p. 5N, 132nd Meeting, ACS (September 1957). (7) Weiss, R., Z. Physik. 138, 170 (1954).
Applications of the Plasma Jet
The plasma jet can be used to obtain fundamental data on chemical and physical properties of matter at high 200O0 to 50,000° C. temperatures-i.e., The available data are very limited and sometimes unreliable. For example, the boiling point of a common substance
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RECEIVED for review September 4. 1959 ACCEPTEDFebruary 3, 1960 Figure 2. In the Weiss arc, the stabilizing fluid produces a vortex with the anode as its center
INDUSTRIAL AND ENGINEERING CHEMISTRY
Division of Industrial and Engineering Chemistry, Symposium on High Temperature, 135th Meeting, ACS, Boston, Mass., April 5-10.
