US3541379A

US3541379A - Method for initiating gaseous plasmas

Info

Publication number: US3541379A
Application number: US666909A
Authority: US
Inventors: Calvin B Holden
Original assignee: PPG Industries Inc
Current assignee: PPG Industries Inc
Priority date: 1967-09-11
Filing date: 1967-09-11
Publication date: 1970-11-17
Anticipated expiration: 1987-11-17

Description

Nov. 17, 1970 c. B. HOLDEN METHOD FOR INITIATING GASEOUS PLASMAS Filed Sept. 11. 1967 a AUXIILIARY GA INVENTOR CALVIN B HOLDEN ATTORNEYS United States Patent Ofifice 3,541,379 Patented Nov. 17, 1970 US. Cl. 313-231 13 Claims ABSTRACT OF THE DISCLOSURE Gaseous plasma is initiated by introducing auxiliary gas having excitation potential greater than ionization potential of plasma gas into heating zone of plasma generator along same gas flow path taken by plasma gas, applying voltage which excites but does not ionize auxiliary gas, and displacing auxiliary gas with plasma gas.

CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part application of application Ser. No. 486,838, filed Sept. 13, 1965, now abandoned and application Ser. No. 400,644, filed Oct. 1, 1964, now abandoned.

BACKGROUND OF THE INVENTION Gaseous plasmas are utilized in present day research and industrial technology in areas such as welding and cutting metals, spraying metallic and non-metallic powders, high temperature chemical cracking, and synthesis reactions, materials testing, space re-entry simulations, and other processes where high temperatures and swift gas flow rates are utilized. Gaseous plasmas are formed usually by subjecting a gas to electrical energy so as to ionize the gas. Hereinafter, the plasma-forming gas is referred to as the work gas. The transfer of electrical energy into heat in the work gas results in the formation of a luminous, electrically conduct-ing gaseous stream containing electrons, ions, and atoms. The over-all charge of this gas is neutral since the negative charges existing within the gaseous stream are balanced by an equal number of positive charges. A gas existing in this physical state is defined as a plasma or is said to be in the plasma state.

The electrons and positive ions produced in the gas as a result of ionization of the gas have a tendency to recombine into neutral molecules or atoms and, at equilib rium, this recombination occurs as often as ionization occurs. When ionization starts, the ionization rate exceeds the recombination rate and the number of ions increases until the gas stream becomes electrically conductive. The final level of ionization, i.e., the equilibrium level, depends on the impedence in the external circuit.

One difiiculty connected with the use of gaseous plasmas is their initiation. Gases at ordinary pressures and temperatures are not electrically conducting because these gases do not contain enough ions. Although it is possible to expose the work gas to a voltage high enough to overcome the natural resistance of the work gas to conduct electricity, i.e., force the gas to ionize, and thereby initiate the plasma, such methods are uneconomical and unsafe because (a) the amount of voltage required to operate a gaseous plasma once it is formed is less than the voltage required to initiate the plasma, (b) such methods can result in destruction of the power supply and/or plasma generator due to a breakdown in the insulation of the equipment under such a high voltage, and (c) designing the power supply for such a high voltage is uneconomical when the high voltage is for starting alone.

As a result of this difficulty, several methods have been developed for initiating a plasma. In the case of gaseous plasmas formed by contacting a gas with an electric are conducted between electrodes, the electric arc has been initiated by high frequency starters, by evacuation of the heating zone to reduce the recombination rate, by movable carbon rod strikers, etc. In the case of gaseous plasmas formed by inductively heating a gas with high frequency electrical energy, such devices as a pilot arc discharge, a low pressure breakdown, or the temporary introduction of a piece of metal or graphite, etc.., have been used.

It has also been suggested that a mono-atomic gas, such as argon or neon, can be used to form an initial plasma utilizing the initiation techniques described hereinabove, and then, once the plasma is formed and stabilized, the mono-atomic gas can be gradually replaced with other gases to provide a variety of gaseous plasmas. In connection with this last expedient, see, for example, U.S. Pat. 3,324,334.

Initiation of gaseous plasmas with the above-described methods has not been adequate for all purposes in which such plasmas are used. For example, initiating a plasma in a gas which is at high pressures and/ or in a gas moving at high velocities is difficult because the plasma tends to extinguish before it can be stabilized. In order for a plasma to be self-perpetuating, sufficient enthalpy must continue to exist in the heating zone, i.e., the zone in which the gas contacts the electrical energy, to maintain a level of ionization which is suificient to conduct the electricity. At high gas velocities, the plasma tends to extinguish because the focal point of high enthalpy and, therefore, ionization, is removed by the flowing gas from the heating zone. At high pressures, the rate of recombination is increased due to closer packing of the ions and electrons, thus making initiation more difficult.

The aforesaid difficulties have been solved, in part, by initiating the plasma at low gas flow rates and pressures. Once the plasma is established, the gas flow and current are increased by increments until the desired levels of both are attained. This procedure, however, has not solved the problem that exists for material testing in space reentry simulations wherein it is necessary to expose a material to high temperatures and high gas velocities for predetermined time periods. Thus, if the plasma is not started at test conditions, the test piece is exposed to conditions other than desired and, if the plasma is first brought to test conditions before moving the test piece into the center of the plasma, such procedure does not produce accurate test results because of uncertainties in positioning the test piece when it must be done quickly. The latter problem is particularly acute in ablation testing where the test period is very short, e.g., 10 seconds.

BRIEF SUMMARY OF THE INVENTION It has now been discovered that gaseous plasmas can be readily initiated by a technique which is applicable to high and low pressure gas, as well as high and low gas flow rates. In accordance with the present invention, it has been discovered that a gaseous plasma can be initiated while introducing work gas into the plasma generator through conventional gas injection inlet means by momentarily halting the introduction of work gas, introducing an auxiliary gas having an excitation potential greater than the ionization potential of the Work gas into the heating zone of the plasma generator along the same path taken by the work gas into the heating zone so that the auxiliary gas occupies at least a portion of the heating zone wherein ionization can take place, and meanwhile imposing within the heating zone a voltage suflicient to excite the auxiliary gas and then restoring the flow of work gas. In another embodiment of the present invention, the auxiliary gas is first introduced into the heating zone and then the flow of work gas through the plasma generator commenced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of one embodiment of the pres ent invention illustrating an electric arc plasma generator utilizing two co-axial cylindrical electrodes.

FIG. 2 is a schematic of a variation of the embodiment illustrated in FIG. 1, which 'was not effective over the same wide range of conditions as the apparatus depicted in FIG. 1.

FIG. 3 is a schematic of an embodiment of the present invention wherein the gaseous plasma is formed by induction heating.

DETAIIJED DESCRIPTION Plasma devices are variously referred to in the literature as plasma generators, plasma jets, plasma torches, plasma heaters, constricted-arc heaters, and arc-jets. Commercial plasma devices have been called plasma-flame spray guns, plasma-flame cutters, and plasma-arc welders, depending upon the particular use intended for the specific piece of equipment. Regardless of the name given or the commercial application, the first function of a plasma device, or plasma generator as referred to hereinafter, is the generation of an energy state in a gas which involves some degree of ionization of such gas. One definition of plasma generators is a heat source capable of producing temperatures in excess of 3000 E, which are attained without exothermic reactions within the gas itself.

Ionization of the plasma-forming gas, herein referred to as the work gas, takes place within the heating zone, e.g., in the area or zone Where the electric arc contacts the gas or where the high frequency field couples to the gas. In this zone, the gas exists in the plasma state. The gas leaving the heating zone, however, may be at temperatures below the ionization temperature of the gas because of heat loss to the surroundings. Although some residual ionization may briefly remain in the gas leaving the plasma generator, plasma temperatures may not persist. Nevertheless, for simplicity, and for purposes of the present description, the term gaseous plasma will be used hereinafter to refer to a gas in the heating zone where plasma temperatures are present, and t the gas exiting from the heating zone and the plasma generator.

Plasma generators, of the type hereinabove described, typically use an electric arc or an induction coil as the means of utilizing electrical energy to heat the work gas. In a typical non-transferred arc unit, the work gas flows through the are passing between the anode and cathode and emerges from the plasma generator as a highly heated gaseous stream or flame (referred to herein as a gaseous plasma). In an induction-type plasma generator, a highfrequency induction coil, such as used in conventional induction heating of metals, surrounds an insulated cylinder of suitable construction through which the work gas is flowed.

Initiation of a gaseous plasma is difficult to obtain at high 7 gas pressure and/or high gas velocities, i.e., pressures and velocities that prevent, e.g., by turbulence, the transfer of the initial point of ionization to the main body of the heating zone of the plasma generator or prevent the maintenance of the required ionization level within the heating zone. It has, therefore, been necessary to strike or initiate the gaseous plasma at low gas pressures and velocities; stabilize the plasma thus initiated; and thereafter, increase the current, gas pressure and/ or gas flow incrementally to the desired gas velocities and power rating.

The above described step-wise procedure for establishing a gaseous plasma in a high pressure gas stream and in a high velocity gas stream is unacceptable for many purposes. As mentioned hereinabove, in testing materials for aero-space applications, re-entry conditions cannot be simulated accurately because it is difiicult to subject the test material to a gaseous plasma stream functioning at full operating conditions. Similarly, in chemical synthesis, such as in the production of titanium dioxide by vapor phase oxidation of titanium tetrahalide with at least a stoichiometric amount of oxygen-containing gas, Wherein the gaseous plasma furnishes the enthalpy required to heat reactants to reaction temperature or above, a large amount of raw material is wasted during start-up procedures. Likewise, if the gaseous plasma should extinguish, as it occasionally will during prolonged runs because of its inherent instability, in order to reinitiate the gaseous plasma, it is necessary to shut off or cut back reactant streams until conditions are such that the gaseous plasma can be re-established. Such procedure results in the production of off-grade product and plant down time, both of which affect the economics of the process.

In accordance with the present invention, it has been discovered that gaseous plasmas can be initiated over a broader range of operating conditions than was previously possible. This invention, therefore, permits start up at full operating conditions to provide more accurate testing of materials in re-entry simulations, and less down time in commercial chemical operations.

In accordance with the method of the present invention, an auxiliary gas that has an excitation potential greater than the ionization potential of the work gas is introduced into the heating zone along the same path of flow as the work gas, e.g., through the same gas inlet means, and thereafter the work gas is introduced into the heating zone. Simultaneously with the introduction of both gaseous streams, voltage sufficient to excite the auxiliary gas is applied to the heating zone. The plasma is thereby readily established.

The work gas can be any gas, gaseous compound, or mixture of gases and gaseous compounds. For example, the work gas can be either polyatomic or mono-atomic can can be selected to provide a neutral, reducing, or oxidizing atmosphere. Of the more conventional gases employed, examples include the inert gases, i.e., helium, neon, argon, krypton, and xenon, nitrogen, hydrogen, air, and oxygen. Other gases which can be employed include chlorine, carbon monoxide, carbon dioxide, ammonia, hydrogen chloride, nitrous oxide, sulfur dioxide, methane, and carbon disulfide. The aforementioned gases are only recited to illlustrate examples of gases that typically can be employed as the work gas since any gas, or gaseous compound can be employed by varying the materials of construction of the plasma generator.

The ionization potential of a work gas is defined as the energy, in electron volts (ev), required to remove the most loosely bound electron from the normal atom or compound. There are also second, third, fourth, etc., ionization potentials which are, respectively, the energy required to remove the second, third, fourth, etc., most loosely bound electrons from the normal atom or compound. The ionization potentials for the elements in the atomic state, as well as for certain compounds and diatomic gases, can be found on page 13-41 of the Handbook of Chemistry and Physics, 45th Edition, published by the Chemical Rubber Company. A further compilation for the elements can be found on pages 582-583 (Table 623) of the Smithionian Physical Tables, (9th Revised Edition), Smithsonian Institution, Washington, DC. (1954). Both of the aforementioned compilations of ionization potentials are incorporated herein by reference.

The auxiliary gas employed in the present method for initiating a gaseous plasma can be any gas or mixture of gases which has an excitation potential greater than the ionization potential of the work gas. Advantageously, the excitation potential of the auxiliary gas is at least 15 percent greater than the ionization potential of the work gas. The greater the difference between the excitation potential and ionization potential referred to, the more easily the gaseous plasma is established.

The excitation potential of a gas is defined as the energy, in electron volts, necessary to change a system from its ground state to an excited state. Such a state is First excitation potential,

First ionozation potential,

Atom or molecule ev.

1 Calculated.

Values not given in the above table were not recited in the referenced text.

The auxiliary gas can be either polyatomic or monoatomic but is preferably a mono-atomic gas because such gas does not absorb dissociation energy. Preferably, the auxiliary gas is one of the inert gases, e.g., helium, neon, argon, or mixtures thereof, since such gases have high excitation potentials. However, the selection of the auxiliary gas can be determined by the ionization potential of the selected work gas since the auxiliary gas need only have an excitation potential greater than the ionization potential of the work gas. Thus, once the work gas has been selected, and depending on the requirements of the plasma generator and other equipment, an auxiliary gas can be selected from a table such as the one contained in the text, Electrical Breakdown of Gases. In utilizing the method of the present invention, care should be exercised in selecting a work gas which has an ionization potential lower than the excitation potential of available gases or gaseous compounds.

The amount of auxiliary gas required in the process of the present invention is only that quantity required to fill a portion of the heating zone of the plasma generator for a moment so as to provide an initial point of ionization. Of course, larger quantities than indicated can be used. Preferably, the entire heating zone is filled with the auxiliary gas. Once the aforesaid is accomplished, the work gas is introduced into the heating zone. In the time required for the work gas to displace the auxiliary gas in the heating zone, the plasma is initiated.

The auxiliary gas and work gas can be supplied from appropriate sources at initial pressures of from 5 to 600 pounds per square inch gage, usually 50 to 500 p.s.i.g., and at temperatures ranging from ambient to 1000 F.

The voltage and current characteristics for the plasma generator are a function of the over-all configuration and design of the plasma generation equipment and the flow rate and enthalpy level desired in the gaseous plasma. Power requirements will increase with increases in the rate of gas flow and enthalpy levels. Relative voltage and current levels will vary depending on whether the plasma generator is designed for high voltage and low current, or low voltage and high current, and in the case of an induction plasma generator, the amount of inductance required to generate the required enthalpy in the gaseous plasma. Either direct current or alternating current may be used for operating plasma arc generators. High frequency alternating current is used for operating an induction plasma generator.

The field intensity, sometimes referred to as the electric field or electric intensity, between the electrodes of a plasma arc heater at start-up is typically at least 10 volts per millimeter, and usually is at least 250 volts per millimeter. The field intensity, as used herein, is defined as the open circuit potential, in volts, between the electrodes divided by the shortest distance or gap between the electrodes measured in millimeters.

Although not intending to be limited thereby, it is believed that when the auxiliary gas is contacted with electrical energy in the heating zone, the atoms of the auxiliary gas are excited. When the work gas enters the heating zone, the energy from the excited auxiliary gas is transferred to the work gas causing ionization thereof. Once ionization has commenced, the gaseous plasma is initiated, the gas becomes conductive, and heat released by the passage of electricity maintains the ionization of the work gas. The gaseous plasma is then transferred from its local point of initiation to the principal heating zone of the plasma generator by the flow of the work gas. Since the auxiliary gas and work gas have the same flow pattern, into and through the heating zone, e.g., by using the same gas inlet means, the same flow pattern for both gases exists and no disruption of gas flow through the plasma generator occurs.

Gas introduced into the heating zone of the plasma generator can be in laminar flow or turbulent flow and can be introduced at any angle, e.g., parallel, perpendicular, at any angle therebetween, or in a tangential direction, to the main axis of the heating zone. The type and direction of gas flow will, in most instances, depend upon the type of plasma generator used and the type of arc stabilization utilized.

In one embodiment of the process of the present invention, the desired rate of flow of work gas can be produced in the plasma generator and thereafter such flow interrupted momentarily by the introduction of the auxiliary gas. In another embodiment, the auxiliary gas can be first introduced into the heating zone of the plasma generator so that it fills at least a portion of the heating zone and, thereafter, the auxiliary gas flow halted by the introduction of the work gas. In the first described embodiment, the auxiliary gas is introduced at a pressure greater than that of the work gas so that it can interrupt the flow of such gas. In such case, the pressure ratio of auxiliary gas to work gas is greater than 1, and preferably is greater than 1.5. Most preferably, said ratio ranges from 2 to 5. In the second described embodiment, the auxiliary gas is introduced at a pressure adequate to cause flow into the plasma generator, i.e., above the pressure of the plasma generator. In the event the plasma is not immediately initiated, another attempt can be made by simply re-introducing auxiliary gas. Since the auxiliary gas is at a higher pressure than the plasma generator, it can interrupt momentarily the flow of work gas and thus repeat the starting operation. In this latter described event, the first and second described embodiments may overlap.

Referring now to FIG. 1, there is shown a schematic of two spaced co-axial cylindrical electrodes, E and E across which an are (A) can be struck when voltage from a suitable power source, not shown, is impressed upon the electrodes. Either electrode can be the cathode or anode, depending upon the polarity of the leads from the power source. In the schematic shown, electrode E is the cathode, direct current is used to power the plasma generator, and the gaseous plasma exits from the lower portion of electrode E Electrode E, as shown, has a larger internal diameter than electrode E Surrounding electrodes E and E respectively, are cooling jackets 2 and 7, through which water or any suitable cooling fluid can be circulated. The inlets and outlets for jackets 2 and 7 are not illustrated. Surrounding the electrode cooling jacket 2 is a magnetic field coil 33 which serves to stabilize the upper end of are A at electrode E and extend the life of electrode E by keeping the upper termination of the are moving by means of the rotational vector produced by interaction of the magnetic field with the arc current.

Surrounding the lower portion of electrode E and the upper portion of electrode E is cylindrical swirl chamber 5, which has a diameter greater than either that of electrode E or electrode E Provided in cylindrical chamber 5 are

gaseous inlet ports

6 and 6, positioned at opposite ends of a diameter of said chamber, which contain jet nozzles 8 and 8'. Preferably,

inlet ports

6 and 6 are connected by a manifold. Alternatively, auxiliary gas can be introduced into chamber 5 through inlet 6' and nozzle 8' and work gas introduced through inlet 6 and nozzle 8. In this case, both gases still trace the same flow path into the heating zone. Although only two gas inlets and jet nozzles are illustrated, a multiplicity of jet inlets, e.g., 4, .6, 8, or more, can be provided. Alternatively, only one gas inlet and jet need be used. The jet nozzles are usually designed to produce a large pressure drop across the nozzle. Jet nozzles 8 and 8' can provide for radial, tangential, or some other directional introduction of the gas into chamber 5 and in the schematic shown in FIG. 1 preferably provides for tangential introduction of the work gas. Pressure gage 28 is placed in line 16 and measures the pressure of gas flow at about the intersection of

lines

16 and 14.

Work gas from a source, not shown, is delivered through line 12 through check valve 24, which permits the flow of gas only in the direction of the plasma generator, through line 15 into gas inlet 6 and jet nozzle 8. As indicated, a manifold, not shown, can connect

inlet ports

6 and 6 and thereby provide for symmetrical addition of work gas into chamber 5. Auxiliary gas from a source, not shown, is delivered through line 13, solenoid valve 26, and line 14. When the auxiliary gas enters line 16, it follows the same path described for the work gas. Pressure gage 29 measures the auxiliary gas static pressure in line 13.

When work gas is introduced tangentially into chamber 5, most of it takes an ever-decreasing concentric path within chamber 5 until it attains the diameter of elec trode E At this point, the work gas travels helically up the walls of electrode E until it approaches the conical section 2A which aids in turning the continually swirling arc gas back down and through electrodes E and E and in contact with arc (A) maintained between said electrodes. The diameter of the pathway of the work gas passing down through electrodes E and E is essentially that of the internal diameter of electrode E Insulation 4 and 9 are provided at each end, respectively, of electrodes E and E when alternating current is employed to power the arc. When direct current is used, insulation may be removed from one of the electrodes and that electrode grounded, e.g., insulation 9 can be removed from the bottom of electrode E and that electrode grounded. Also shown in FIG. 1 is the top of reactor chamber 40 surrounded by insulating material 41.

In performing one embodiment of the present invention, after the power to the electrodes is turned on, work gas is delivered through check valve 24 at the desired pressure and rate of flow. Under these conditions, no arc is formed and the work gas is not heated. In response to a manual or an automatic signal, solenoid valve 26 is opened permitting auxiliary gas, which is at a higher pressure than the work gas, to flow through line 14 into line 16 where it follows the path previously described by the work gas into the heating zone of the plasma generator. Check valve 24 blocks the flow of auxiliary gas into line 12. After a short time, which is selected so as to introduce auxiliary gas into at least a portion of the heating zone,

preferably so as to fill the heating zone, solenoid valve 26 is closed. The closing of valve 26 permits the resumption of work gas flow through check valve 24 and into the heating zone of the plasma generator. As the work gas displaces the auxiliary gas within the heating zone, the arc is initiated and the gaseous plasma is formed.

In FIG. 2, wherein like numbers and letters refer to the same items as described in FIG. 1, there is illustrated a schematic of a variation of the embodiment of the present invention described in FIG. 1. In this embodiment, auxiliary gas is introduced into chamber 5 through gas inlet 6A and nozzle 8A. Note, when auxiliary gas is introduced through inlet 6A, it does not follow the same path into the heating zone as the work gas. This method has been found to be successful for plasma initiation only at reduced rates of gas flow.

Referring now to FIG. 3, there is shown a nonconductive refractory tube through which work gas is passed. Surrounding tube 60 is cooling chamber 62 through which water or any other suitable cooling fluid is circulated. Cooling fluid inlet and outlet for chamber 62 are not shown. Surrounding tube 60 is inductance coil 72 which can be composed of many turns of water-cooled copper tubing. Inductance coil 72 is connected to a high frequency power source 70 by leads 74 and 75. Work gas from a source, not shown, is supplied through check valve 67 and thence through conduit 63 to tube 60. Auxiliary gas is supplied from a source, not shown, through solenoid valve 68 in line 65. In operating the apparatus of FIG. 3, work gas is introduced into tube 60 at the desired rate. Auxiliary gas, at a higher pressure than the work gas in line 63, is introduced through solenoid valve 68 in line and thence into tube 60 until auxiliary gas fills at least a portion of the heating zone therein. The flow of auxiliary gas is then terminated and the flow of work gas automatically resumes. Simultaneously with the flow of gas through tube 60, the high frequency power to inductance coil 72 is supplied from power source to provide a strong magnetic field in tube 60.

The present process is more particularly described in the following examples which are intended to be illustrative only, since numerous modifications and variations therein will be apparent to those skilled in the art.

Example I Apparatus similar to FIG. 2 was employed. Work gas was introduced through gas inlet 6 and nozzle 8 and auxiliary gas was introduced through gas inlet 6A and nozzle 8A. Electrodes E and E were fabricated from a silver-copper alloy. The diameter of chamber 5 was about thre times larger than the diameter of electrode E The gap between electrodes E and E was about eight millimeters and the open circuit potential between the electrodes was about 3500 volts. Helium gas (excitation potential2'0.9 ev.) at ambient temperature was passed through nozzle 8A in a quantity sufiicient to fill the gap between the electrodes. The helium was supplied to nozzle 8A through about 75 feet of quarter inch diameter tubing from a gas cylinder at about 60 p.s.i.g.

Simultaneously, oxygen, at about 20 C. (ionization potentiall2.2 ev.), was introduced tangentially into chamber 5 through four jet inlets similar to nozzle 8 and arranged around the circumference of chamber 5, at about 20 p.s.i.g. and at a rate of about 10 gram-moles per minme. After about two seconds of helium flow, the helium flow was stopped and an arc was observed to occur. Initial are conditions were observed to be about 500 volts and 70 amperes. The voltage, amperage, and oxygen flow rate were gradually increased in step-wise fashion until the arc was operating at about 1000 volts, about amperes, and oxygen was flowing at a rate of about 44 gram-moles per minute.

Example II The procedure of Example I was repeated, except that oxygen was introduced into chamber 5 at the full rate of 44 gram-moles per minute. Under such conditions, initiation of the arc failed to occur.

Example III Apparatus similar to FIG. 1 was employed. Electrodes E and E were of the same silver-copper alloy composition as that of Example I. The gap between the electrodes was about eight millimeters. Oxygen was employed as the work gas and was introduced into chamber 5 through four jet nozzles similar to 8 and -8. The nozzles were arranged around chamber 5 and were manifoldly connected. Helium, as the auxiliary gas, was introduced into chamber 5 through solenoid valve 26 and

lines

14 and 16. Since the helium was introduced after the fiow of oxygen was commenced, it was necessary that the helium be at a greater pressure than the oxygen. Eight arc initiation runs were made with various combinations of oxygen and helium pressures. The open circuit voltage impressed on rear electrode E varied in these runs from about 2600 to 3500 volts. In all of the cases enumerated in Table I, the arc started smoothly and with no difiiculty.

TABLE I Open circuit 02 pressure He pressure voltage Run No (p.s.i.g., gage 28) (p.s.i.g., gage 29) (volts) Example IV The procedure of Example III, Run 5 is repeated, except that neon (excitation potential 16.58 ev.) is employed as the auxiliary gas. Arc initiation is observed to occur readily.

Example V Utilizing apparatus similar to FIG. 3, and the procedure described with respect to FIG. 3, an ionized state is initiated in tube 60 utilizing oxygen as the work gas and neon as the auxiliary gas.

Example VI Employing apparatus similar to that schematically illustrated in FIG. 1, titanium dioxide is prepared by vapor phase oxidation of titanium tetrachloride with oxygen.

Electrodes E and B are constructed out of a silvercopper alloy of about 80 percent silver and about 20 percent copper. An electric arc is initiated between electrodes E and E by introducing oxygen, at room temperature, about 20 C., and at about 45 gram-moles per minute tangentially into chamber 5 through manifoldly

connected jet nozzle

8 and 8. The oxygen line pressure at gage 28 is about 117 p.s.i.g. Its speed at

jet nozzles

8 and 8 is about the speed of sound. Helium at about 500 p.s.i.g., at pressure gage 29, is then permitted to flow into line 14 and chamber 5 by opening valve 26. The flow of helium interrupts the flow of oxygen at check valve 24. Simultaneously with the introduction of oxygen to chamber 5, an open circuit voltage of about 3100 volts is impressed on electrode E from a direct current power source. When helium flowing into chamber 5 is suflicient to fill the heating zone of the plasma arc generator, the helium flow is terminated. This action automatically resumes the flow of oxygen through check valve 24. Almost immediately after the flow of helium is terminated, are A appears, as seen on monitoring instruments. Power requirements to the are after initiation are about 1100 volts and about 100 amperes. Power to field coil 33 in an amount of about 10 25 volts and 350 amperes is applied to establish a magnetic field about electrode E The temperature of the oxygen exiting from the bot tom of electrode E is about 4500 F. Its speed is about 0.4 mach. As the heated oxygen passes down through tube 40, it is first surrounded by chlorine gas at a temperature of about F. Vaporous titanium tetrachloride, in an amount of about 36 gram-moles per minute is then introduced adjacent to the chlorine gas stream surrounding the oxygen stream. The titanium tetrachloride stream contains aluminum chloride in an amount sufiicient to provide about 2 weight percent A1 0 based on TiO produced, and silicon tetrachloride in an amount sufiicient to provide about 0.6 Weight percent SiO based on Ti0 produced. Inlet tubes for the chlorine and vaporous titanium. tetrachloride are not shown.

The heated oxygen stream, chlorine gas and vaporous titanium tetrachloride admix and the oxygen and titanium tetrachloride react at a point beyond the introduction point of the vaporous titanium tetrachloride in a reaction zone co-axial with tube 40. The average reaction zone temperature in the upper portion of the recation zone is about 2300 F.

Finely-divided pigmentary titanium dioxide is withdrawn from the reactor and has a typical analysis of:

Tinting strength-l810 (Reynolds Scale) Tint toneNeutral Rutile content98.6 percent Particle size-0.28 micron (weight median) The tinting strength and tone is determined in accordance with A.S.T.M. Method D-332-26, 1949 Book of A.S.T.M. Standards, part 4, page 31, published by the American Society for Testing Material, Philadelphia, Pa.

Example VII The process of Example VI is continued for a period of about 72 hours. After this period, arc A, for an undeterminable reason, goes out. In response to a signal generated by a current monitor, solenoid valve 26 opens long enough to fill the heating zone of the plasma generator with helium gas. Almost immediately after restoration of the oxygen flow due to the closing of solenoid valve 26, are A is reignited. The entire reignition process is performed in a very short space of time and, therefore, results in an insignificant period of down time.

Examples I-VII demonstrate that a gaseous plasma can be initiated by utilizing an auxiliary gas having an excitation potential greater than the ionization potential of the work gas. Comparison of Examples I and II with Example III illustrate that when the auxiliary gas is introduced into the heating zone along the same path taken by the work gas, plasma initiation can be performed at high rates of gas flow whereas if the same flow path is not taken, reduced rates of gas flow are required for plasma initiation. Examples VI and VII illustrate the usefulness of the present invention when applied to chemical synthesis techniques such as. in the production of titanium dioxide.

Although the present invention has been described specifically with respect to the production of titanium dioxide in connection with chemical syntheses, it should be noted that other pigmentary metal oxides can be produced from their corresponding halides, i.e., chlorides, fluorides, bromides, and iodides, by the aforementioned described process or a process analogous thereto. Examples, not by way of limitation, of such metal oxides, include the oxides of aluminum, arsenic, boron, iron, phosphorous, silicon, strontium, tin, zinc, zirconium, antimony, lead and mercury. When titanium dioxide is the metal oxide, titanium tetrachloride, titanium tetrabromide, and titanium tetraiodide are advantageously used. A more detailed description of the preparation of titanium dioxide can be found in copending Application Ser. No. 400,644, filed Oct. 1, 1964, now abandoned,

. 11 which is incorporated herein by reference in toto. In addition, the present process for plasma initiation can also be employed in other chemical synthesis utilizing gaseous plasmas in the production of nitrides (aluminum and boron nitrides), acetylene, cyanogen, hydrogen cyanide, cracking of hydrocarbons, etc.

While there are above described a number of specific embodiments of the present invention, it is obviously possible to produce other embodiments and various equivalent modifications thereof without departing from the spirit of the invention.

Having set forth the general nature and specific embodiments of the present invention, the true scope is now particularly pointed out in the appended claims.

I claim:

1. In the process for establishing a gaseous plasma wherein work gas is introduced into a plasma generator and heated by electrical energy, the improvement which comprises initiating said gaseous plasma by introducing auxiliary gas into the heating zone of the plasma generator in the same gas flow path described by the work gas, applying a voltage sufiicient to excite but insufficient to ionize said auxiliary gas and displacing said auxiliary gas with work gas, said auxiliary gas having an excitation potential greater than the ionization potential of the work gas.

2. A process according to claim 1 wherein said auxiliary gas is an inert gas.

3. A process according to claim 2 wherein said inert gas is selected from the group consisting of helium, neon, argon, or mixtures thereof.

'4. A process according to claim 1 wherein said gaseous plasma is initiated by establishing the flow of work gas through the heating zone of the plasma generator, interrupting the flow of work gas with auxiliary gas, and then resuming the flow of work gas.

5. A process according to claim 1 wherein the plasma generator is an electric arc plasma generator.

6. A method for initiating an electric arc between the electrodes of a plasma arc generator, which comprises establishing a voltage differential between the electrodes, introducing auxiliary gas into the space between the electrodes, and displacing the auxiliary gas with work gas, said auxiliary gas being introduced along the same gas ll'lOW path described by the work gas and having an excitation potential greater than the ionization potential of the work gas, and said voltage differential being sufficient to excite but insufficient to ionize said auxiliary gas.

7. A method for initiating an oxygen plasma with oxygen-containing gas in a plasma generator, which comprises introducing auxiliary gas having an excitation potential greater than the ionization potential of oxygen into the heating zone of the plasma generator along the same gas flow path described by the oxygen-containing gas, applying a voltage sufficient to excite but insutficient to ionize said auxiliary gas to the heating zone of the plasma 12 generator and displacing said auxiliary gas with oxygencontaining gas.

8. A method according to claim 7 wherein the flow of oxygen-containing gas through the heating zone of the plasma generator is established and then momentarily interrupted by introducing auxiliary gas into the heating zone in an amount sufiicient to fill at least a portion of the heating zone before stopping the flow of auxiliary gas.

9. A method according to claim 7 wherein the auxiliary gas is selected from the group consisting of helium, neon and mixtures thereof.

10. In a process for preparing pigmentary metal oxide by vapor phase oxidation of metal halide in a reaction zone With oxygen-containing gas at elevated temperatures wherein a plasma generator is used to produce a gaseous plasma that furnishes heat for said oxidation reaction, the improvement which comprises initiating said gaseous plasma by introducing auxiliary gas into the heating zone of the plasma generator along the same gas flow path described by the plasma-forming gas, applying voltage sufficient to excite but insufficient to ionize said auxiliary gas to the heating zone of the plasma generator and displacing said auxiliary gas with plasma-forming gas, said auxiliary gas having an excitation potential greater than the ionization potential of the plasma-forming gas.

11. A process according to claim 10 wherein said metal halide is titanium tetrachloride.

12. A process according to claim 10 wherein said plasma generator is a plasma arc generator.

13. A process according to claim 10 wherein said auxiliary gas is selected from the group consisting of helium, neon, and mixtures thereof.

References Cited UNITED STATES PATENTS 3,042,830 7/1962 Orbach 315111 X 3,121,641 2/1964 Wikswo et a1. 231 X 3,253,113 5/1966 Breymeier 21974 3,275,412 9/1966 Skrivan 231 X 3,296,410 1/1967 Hedger 315-111 X 3,311,735 3/1967 Winzeler et al. 313231 X 3,324,334 6/1967 Reed 315-111 X 3,357,794 12/1967 Mas et a1. 106300 X 3,264,508 8/1966 Lai et al 315111 X 3,3 16,082 4/1967 Barloga et al 139 X 3,448,333 6/1969 Arkless et al 315-111 FOREIGN PATENTS 1,049,536 11/1966 Great Britain.

RAYMOND F. HOSSFELD, Primary Examiner US. Cl. X.R.