US3720598A

US3720598A - Cryogenic arc furnace and method of forming materials

Info

Publication number: US3720598A
Application number: US00103086A
Authority: US
Inventors: W Thompson
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1970-12-31
Filing date: 1970-12-31
Publication date: 1973-03-13
Anticipated expiration: 1990-03-13
Also published as: DE2164762A1; JPS547966B1; FR2120783A5; GB1368972A

Abstract

This disclosure provides apparatus for achieving rapidly a high temperature arc discharge in the region of a material to be vaporized. Surrounding the region of the arc discharge is a cryogenic fluid against which both the arc and the vaporized produces exert pressure. The effect of the presence of the cryogenic fluid adjacent to the high temperature region is to constrain the arc discharge strongly and to quench rapidly the material in the vapor state to the solid state. As a consequence of the localized heating and rapid quenching in the cryogenic arc furnace, special materials and physical states thereof are achieved. Illustratively, chemical products and amorphous conditions of materials are achieved for the practice of this disclosure not heretofore contemplated in the practice of the prior art. For an embodiment of this disclosure, the material to be vaporized is ab initio established in location for a capacitive arc discharge and the capacitor plates are caused by mechanical shock to approach each other so that the discharge occurs preferentially at a preselected path on the material. Practice of this invention is readily extrapolated to the very high temperatures required for fusion experiments in liquid deuterium, e.g., greater than 100,000*C.

Description

United States Patent [191 Thompson 1March 13, 1973 CRYOGENIC ARC FURNACE AND METHOD OF FORMING MATERIALS [75] Inventor: William A. Thompson, Yorktown Heights, NY.

[73] Assignee: International Business Machines Corporation, Armonk, N.Y.

[22] Filed: Dec. 31, 1970 [21] Appl.N0.: 103,086

Primary Examiner F. C. Edmundson Attorney-Hanifm & Jancin and Bernard N. Wiener [57] ABSTRACT This disclosure provides apparatus for achieving rapidly a high temperature are discharge in the region of a material to be vaporized. Surrounding the region of the arc discharge is a cryogenic fluid against which both the arc and the vaporized produces exert pressure. The effect of the presence of the cryogenic fluid adjacent to the high temperature region is to constrain the arc discharge strongly and to quench rapidly the material in the vapor state to the solid state. As a consequence of the localized heating and rapid quenching in the cryogenic arc furnace, special materials and physical states thereof are achieved. Illustratively, chemical products and amorphous conditions of materials are achieved for the practice of this disclosure not heretofore contemplated in the practice of the prior art.

For an embodiment of this disclosure, the material to be vaporized is ab initio established in location for a capacitive arc discharge and the capacitor plates are caused by mechanical shock to approach each other so that the discharge occurs preferentially at a preselected path on the material.

Practice of this invention is readily extrapolated to the very high temperatures required for fusion experiments in liquid deuterium, e.g., greater than 100,000PC.

13 Claims, 3 Drawing Figures PATENTEDHARISISYS 3,720,598

SHEET 1 [IF 2 i l as 34 W 24 18 I I c INVENTOR VOLTAGE L1 22 I WILLIAM A. THOMPSON SOURCE 20 ATTORNEY CRYOGENIC ARC FURNACE AND METHOD OF FORMING MATERIALS BACKGROUND OF THE INVENTION Arc discharges are known in the prior art for vaporizing materials at high temperatures. Cryogenic fluids have been utilized for quenching high temperature environments. However, the simultaneous utilization of a plasma generating arc discharge in the intimate presence of a cryogenic fluid has not been utilized for establishing chemical and physical states in materials although arc discharges have been used to study the character of non-cryogenic liquids.

In co-pending application Ser. No. 15,788 filed Mar. 2, 1970 and commonly assigned, there is provided superconducting oscillators and method for making the same wherein a very small Josephson oscillator is fabricated by spark erosion between capacitor elec trodes comprising the materials to be vaporized. Spark erosion occurs in a liquid helium environment which causes the formation ofa Josephson junction having extremely small dimensions between the electrodes. Accordingly, the explicit distinction of provisions of this invention involve the controlled plasma formation of material and its simultaneous quenching through use of an apparatus ancillary to the material itself.

ADVANTAGES OF THE INVENTION In order to achieve relatively high temperatures for establishing new chemical and physical states in materials, it has been discovered for the practice of this invention that use of superconducting electrodes in a cryogenic environment permit concentration of sufficient energy in a localized region to effect sufficiently high temperature and pressure in a vaporized material which upon the rapid quenching through cryogenic fluid within which the vaporization occurs obtains materials in states thereof not known in the prior art.

The prior art has known how to produce small circuit elements through use of photoresist techniques together with diffusion of ancillary materials into a semi-conductor material. Through the practice of this invention very small circuit elements of amorphous material or superconducting material are produced in a cryogenic environment. Thus, high temperatures states are frozen into the material which do not alter sufficiently at operational temperatures of interest to change either chemical or physical state.

Among the advantages of this invention obtained by the practice thereof, are provision for small circuit elements on substrates, chemistry of new high tempera ture compounds and intermetallics, chemistry of superconductor and amorphous materials, and small grain size metallurgical materials.

There will now be provided additional discussion of advantages of this invention for providing small circuit elements of wafers in semiconductor technology. The circuit elements dimension is of the order of micron size regions. Although the precise initial portion of electrical discharge from the capacitor probe point may not be known, it is readily ascertained relative to the geometry of the substrate wafer and thereafter is essentially fixed relative to the probe for any given fabrication procedure. The localization of the arc discharge is determined both by the geometry of the capacitor probe and the constraint on the discharge by the proximatecryogenic fluid.

SUMMARY OF THE INVENTION The practice of this invention is based on the discovery that if superconducting leads are used throughout the discharge circuit to a point discharge in a cryogenic fluid, a high temperature plasma of material in the arc discharge is formed and rapidly quenched by the proximate cryogenic fluid. The rapid quench of the plasma provides new chemical and physical states for the material established in the arc discharge and homogenized distribution of the components of the material throughout the body of the resultant solid state material. A device for the practice of this invention incorporates a capacitor with superconducting plates and superconducting electrical energy transfer leads thereto, a hearth in the vicinity of the resultant arc discharge for holding initially the material to be vaporized to a plasma, and a cryogenic fluid environment within which the arc discharge is established. Since the cryogenic fluid is non-conducting, it effectively constrains the arc but does not participate therein. The device includes mechanical or electrical means for causing the capacitor probe temporally to approach the hearth material thereby initiating the arc discharge and effecting the release of the energy stored in the capacitor into a small spatial volume.

In greater detail, the practice of the invention provides a cryogenic arc discharge device which is capable of producing very high local temperatures by the use of a fast superconducting discharge circuit and a fast recondensation of the vapor products by use of a cryogenic fluid in intimate contact therewith. The superconducting capacitor is charged up and discharged through high current superconducting leads to a superconducting point electrode which is moved vertically by a mechanical shock. The use of superconductivity allows high current density for discharge over a few microseconds. The use of the cryogenic fluid gives the quick recondensation of the reactant material. The pointed discharge electrode is caused to move either over all the material or to lay out a predetermined circuit on the hearth substrate.

The movement of the hearth substrate under the discharge probe may be any type of mechanical manipulator providing X-Y motion, rotary motion or linear motion. However, it is required that the hearth surface be connected electrically to the capacitor with superconducting leads.

It is an object of this invention to provide apparatus and method for obtaining rapidly relatively high temperatures in the presence of a cryogenic fluid for rapidly quenching the high temperature state induced in the material.

It is another object of this invention to utilize a capacitive arc discharge in the presence of a material together with a cryogenic fluid adjacent thereto for rapidly vaporizing and quenching the material.

It is another object of this invention to dump a relatively large amount of energy rapidly into a small region of space and thereafter quench immediately the heated products by a proximate cryogenic fluid.

It is another object of this invention to provide a cryogenic arc furnace comprising a capacitive arc discharge device having one electrode movable by electrical or mechanical shock so that a periodic application of shocks causes sequential discharges in a given region or along a given path.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a functional diagram showing some structural components of a cryogenic arc furnace for the practice of this invention wherein an arc discharge is established between a probe mounted on a movable capacitor plate and the discharge is repetitively established by the synchronous cooperation of electrical means for charging the capacitor" and" electromechanical means for discharging it.

FIG. 2A is a detailed cross-sectional view of a cryogenic arc discharging apparatus embodying the principles illustrated in FIG. 1 and showing the cooperative environment of capacitive arc discharge region, cryogenic fluid, means for charging the capacitor and means for discharging it to modify either a chemical or physical state in the material of the arc discharge by rapidly vaporizing it and simultaneously quenching it via the cryogenic fluid.

FIG. 2B illustrates a modification of FIG. 2A wherein the support mount for the hearth is indicated to be also movable in the plane relative to the capacitor probe as well as vertically with respect to it.

EMBODIMENTS OF THE INVENTION Exemplary embodiments of this invention are illustrated in FIGS. 1A, 2A and 2B of which FIG. 1 is a functional diagram showing the correspondence of the several material elements of the apparatus, FIG. 2A is a derived cross-sectional drawing showing the actual relationship of the several aspects of the apparatus in an operational environment and FIG. 2B is an addendum to FIG. 2A for indicating means for adding planar movement of the material mounting platform relative to the capacitive point electrode.

With reference to FIG. 1, a

capacitor having plates

10 and 18 is connected to

electrodes

20 and 22, respectively, of the arc discharge furnace. The

electrodes

20 and 22 are respectively the locations for the material to be treated and the determining point for the arc discharge. A direct voltage source 34 charges the capacitor via a resistor 36. The discharge of the capacitor is achieved by shock from plunger 24 driven by magnetic coil 26 which is powered from alternating current source 32. The capacitor electrodes and the mechanical shock device are within the chamber of a cryogenic fluid dewar 30.

The embodiment presented in detailed cross-sectional view in FIG. 2A includes a capacitor having a base plate 10 and a top plate 12. Superconducting superconductors such as tantalum or niobium are used for the base plate 10 and the top plate 12. Base plate 10 and top plate 12 together comprise one electrode of the capacitor. Tantalum is especially suitable as the capacitor material because of the beneficial characteristics of the tantalum oxide, i.e., thinness and pin hole free. Further, the tantalum oxide is readily grown naturally on the tantalum surface in a high temperature oxidizing atmosphere. The other electrode of the capacitor 18 is a thin tantalum sheet having a natural oxide or anodized oxide layer thereon. lllustratively the capacitor plate 18 is a thin membrane of tantalum of about 10 mils thickness. In principle, the linear dimension of the capacitor is limited by the parameters of the cryogenic environment in which it is inserted and the amount of required capacitance. The central electrode 18 of the capacitor and the two

outer electrodes

10 and 12 comprise a transmission strip line which transmits the stored energy in the capacitor to the arc discharge at electrode 22. The hearth 20 is shown in screw form for establishing the material to be vaporized in the cryogenic arc furnace. The material 23 is established in the hearth adjacent to the point electrode 22. Although the hearth 20 need not be a superconducting material, it is beneficial so as to minimize resistive losses in the arc discharge. According to the design of the apparatus shown in FIG. 2A the screw 20 is positioned vertically in a manner which will be described hereinafter.

The discharge point electrode 22 is also a superconductor such as tantalum which is affixed to the capacitor membrane 18. The discharge electrode 22 is welded or established in good electrical contact for conventional fabrication technique to the surface of the capacitor membrane 18. The shape of the point electrode 22 is determined by the size of the discharge area to be formed on the surface of the hearth 20.

The impact slug is mounted in upper housing portion 12A established above discharge point electrode 22. Drive coil 26 surrounds impact slug 24. The drive coil 26 is connected to oscillator 22 which periodically drives the impact slug 24 against the membrane 18 causing point electrode 22 to approach the hearth 20. The timing of the alternating voltage is coordinated with the mechanical recovery of the system comprising the capacitive membrane 18, point electrode 22 and impact slug 24. The impact slug 24 is insulated from the capacitor 12 by an insulation sleeve 28 which may be, for example, of teflon or other suitable dielectric, providing insulation properties for the operation of the apparatus of FIG. 2A. The electrical circuit for charging the capacitor comprises a battery 34 and resistor 36 connected by leads 35A and 358 to capacitor base plate 10. and capacitor membrane 18, it being understood that capacitor top plate 12 is electrically connected to base plate 10 and is therefore at the same electrical potential.

The cryogenic dewar 30 surrounds the capacitor and the drive mechanism and chamber 31 thereof is filled with cryogenic fluid, e.g., He, introduced into the dewar chamber 31 via entrance orifice 31A and removed therefrom via exit orifice 318. Base plate 10 has orifices 40 from dewar chamber 31 into chamber 41 wherein .are located the point electrode 22 and hearth 20 with the material 23.

The capacitor leads must remain superconducting at the current load, e.g., 10 to 10 amps/cm. Probe superconductors and alloys may illustratively be Nb, Ta, Pb, NbTa, NbZr.

The reactants are placed in the electrode hearth 20 either as small grains or as evaporated layers. A mechanical adjustment is made to set the electrodeelectrode distance both before and after impact by slug 24. The'discharge spacing is set by varying the coil 26 drive power and the predischarge spacing is set by the hearth adjustment to be low current with field emission insufficient to cause heating or capacitor discharge, e.g., to 100,000A. The capacitor is then charged to test voltage and the impact slug 24 is driven down onto the drumhead 18 surface giving sharp vertical motion to the probe 22 and discharging the capacitor into the arc plasma formed immediately above the surface of hearth 20. The probe is tantalum with an oxide coating. An exemplary rig using a slow (millisecond heating) 0.01 joule discharge obtained temperature greater than 1,300C.

Further details of the nature of the materials and the operation of the cryogenic arc furnace illustrated in FIGS. 1, 2A and 2B will now be presented. In practice the superconducting capacitor is charged to several hundred volts which is a few joules and then discharged through the high current leads, i.e., the positive leads 10 and 12 and the negative lead 18 of FIG. 2A. Thus, a high current density is achieved in the discharge for a few microseconds. The cryogenic fluid, e.g., liquid He, in the chamber 31 is introduced into the chamber 41 in the presence of the hearth material and the capacitor discharge electrodes via orifices 40. The liquid helium in the chamber 41 causes the reactive products to recondense quickly, thereby freezing in the high temperature phases. Additionally, because of the mechanical vacuum properties of liquid helium, contamination of the reacted product is minimized.

The discharge time of the device is determined by the electrical circuit characteristics in the cryogenic environment. Important parameters are the superconducting electrode path between the capacitive region which is on the outside of the diaphragm to the central discharge point which is at the center. This is a superconducting waveguide with low losses up to discharge times being of the order of the energy gap frequency. The discharge may be increased to the gap frequency which is in the high microwave frequency region. Other than that the discharge time is determined solely by the geometrical waveguide properties of the capacitor. The charging time of the capacitor is determined by the RC time constant of the electrical circuit consisting of the external battery 34 and resistor 36 combination in series with the capacitor which must be fast enough to recharge the capacitor before the next discharge. Since the discharge time (frequency) is in the order of milliseconds, the external charging time must be faster than this. Under high frequency discharges and large energy discharge of certain operational conditions the pressure in the discharge chamber may be sufficient to require relief of the pressure by additional vent holes or use ofa pump liquid system.

FIG. 28 illustrates a modification of the device as shown in FIG. 2A whereby the hearth screw 20, hearth 20A and material therein 23 is readily moved via seal 21 forpositioning any point thereof both vertically and horizontally relative to the probe point electrode 22. The hearth screw 20 is shown expanded in size so that it may conveniently support a conventional X-Y adjustment table relative to the probe point electrode 22. The X-Y table is indicated generally by the arrow 50. The X-Y table and the controls therefor consist of X movement station 52 and Y movement station 54. The hearth 20A and material 23 therein is supported on Y movement stage 54. Control cable 56 is connected to the X stage 52 and via the hearth shaft 58 is conveyed to the knob 60 whose rotation controls the X stage movement. The Y stage 54 is controlled by cable 62 which communicates via hearth shaft 58 to knob 64 whose rotation controls the Y movement. The Z movement is controlled by rotation of the hearth screw 20 as is accomplished in the design of FIG. 2A.

EXAMPLES OF THE INVENTION Examples of this invention in accordance with the principles thereof will now be presented for the systems GaAs transformed to Ga and La Se transformed to La Se The gallium arsenide wafers doped with zinc with a carrier concentration of 2 X 10 carriers per cc were used as starting wafers in the device described. The formation temperature was in the range from 1.35 to 4.2, the boiling point of liquid helium. The discharge voltage was normally in the range of l 10 volts on the capacitor providing a temperature greater than l,300C. Tunneling curves of current vs. voltage were obtained both before and after the formation of amorphous gallium (Ga) from a (GaAs) wafer. The tunneling trace before amorphous Ga formation showed the characteristic Schottky barrier type tunneling, the tunneling trace after formation of the amorphous gallium on one of the wafers showed the existence of the superconducting region. The superconducting energy gap and the transition temperature of this region was determined by the tunneling method. The superconducting transition temperature T was 8.2, in good agreement with the published data on amorphous gallium. The superconducting energy gap ratio to T was 4.2, also in agreement with published data on amorphous gallium indicating that amorphous gallium had formed in a small region on the wafer. The magnetic field characteristics indicated that the particle size of amorphous gallium created was somewhat less than 1,000A in diameter.

The lanthanum selenide system was also investigated by the technique of this invention. The selenium rich phase, La,Se was converted to the lanthanum rich phase, La Se, by means of vaporizing in situ and fast recondense according to the principles of this invention. The material properties were investigated by electron probe tunneling. The current vs. voltage curves indicated the enhancement of superconductivity. The lanthanum rich phase is a good superconductor with a transition temperature which depends on the density of vacancies in the material. The vaporization temperature was greater than 2,000C and the formation temperature was in the l.35 to 4.2"K range.

With the high temperatures possible through the practice of this invention the reactant material is made much more reactive than normally because it is ionized more and because there is more impact energy per collision. For example, the chemically inert element argon has been made in the prior art to form compounds by first heating it to high temperatures in an accelerator. It is believed that new compounds of the high melting point elements W, Os, Rh, Mo and Ta will be formed by the rapid quench technique of this invention.

The transition temperature of superconducting material is sensitive to the crystal phase and in the search for higher temperature superconductors, the high temperature crystal and amorphous phases of the binary, ternary and extended compositions of the presently known superconductors can be looked at. Namely, through using a hearth material which consists of various combinations of Nb, Si, Sn, Al, V, Ga, Ge and Zr, there may be provided unique superconductor materials.

With the pin point placement of the discharge according to the principles of this invention, a small region on a gallium arsenide wafer can be changed in its surface chemical state, e.g., 4GaAs+3O 2Ga O +A 5 or superconducting electrical elements of amorphous Ga, e.g., GaAs Ga+As, can be placed at preferred locations.

Superconductors with higher transition temperatures may be obtained by going to high temperature formation through the practice of this invention. It is known that amorphous material has lower phonon energy values than the corresponding crystalline material which enhance the superconducting correlations in the material thereby giving it a higher transition temperature. Examples are the crystalline gallium which has a transition temperature of a little over 1 and the amorphous gallium which has a transition temperature of 8.4K, an enhancement of about 7 times.

I claim:

1. Apparatus for changing a material from one state to a different state comprising:

means for establishing a cryogenic fluid in a region;

means for establishing an electrical discharge circuit in said cryogenic fluid and for defining a path for said discharge in said region;

means for supplying electrical energy to said electrical discharge circuit;

means for establishing a material to be vaporized in said path; and

means for discharging said electrical discharge circuit for establishing said discharge in said path. 2. Apparatus as set forth in claim 1 wherein said means for discharging said electrical discharge circuit includes mechanical means.

3. Apparatus according to claim 2 wherein said mechanical means discharges said electrical discharge circuit periodically.

4. Apparatus according to claim 3 wherein said periodicity of said discharging is slower than the charging time for said electrical discharge circuit.

5. Apparatus for changing a material from one state to a different state comprising:

a capacitor having a first and second plate positioned on either side of said material to be changed;

means for charging the plates of said capacitor sufficient to cause vaporization of said material upon discharge;

means for discharging said capacitor; and

means for establishing the discharge circuit of said capacitor at a cryogenic temperature to rapidly quench said vaporized material and thereby to establish said material in a different state.

6. Apparatus of claim 5 wherein the plates of said capacitor are made of materials capable of acting as superconductors.

7. Apparatus of claim 5 wherein the discharge circuit of said capacitor is established by surrounding such circuit with a cryogenic fluid.

8. Apparatus for changing a material from one state to another state comprising:

a capacitor structure including two capacitor plates consisting of respective superconductive materials;

a hearth for material whose state is to be changed mounted on one said plate;

a probe point on said other capacitor plate juxtaposed relative to said hearth;

mechanical means for moving temporally said probe relative to said hearth to discharge said capacitor; means for establishing a cryogenic fluid in said are discharge region;

and electrical means for charging said capacitor with energy for said are discharge.

9. Apparatus as set forth in claim 8 wherein said one capacitor plate includes two portions, one said portion mounting said hearth and said other portion mounting said mechanical means;

' said portion mounting said hearth including a means for juxtaposing said hearth relative to said probe point;

said other capacitor plate being in membrane form;

said mechanical means including:

electromagnetic coil means mounted on said other portion of said capacitor for mechanically driving said probe point to cause it to change relative position to said hearth;

an impact slug in said electromagnetic coil means;

and

alternating current electrical means for driving repetitively said impact slug on said superconductive capacitor membrane synchronously with the mechanical vibration parameter thereof.

10. Apparatus for generating a plasma comprising:

means for establishing a cryogenic fluid in a region;

means for supplying electrical energy to said electrical discharge circuit; and

means for discharging said electrical discharge circuit for establishing said discharge in said path.

11. Apparatus asset forth in claim 10 wherein said means for discharging said electrical discharge circuit includes mechanical means.

12. Apparatus according to claim 11 wherein said mechanical means discharges said electrical discharge circuit periodically.

13. Apparatus according to claim 12 wherein said periodicity of said discharging is slower than the charging time for said electrical discharge circuit.

a m a a a

Claims

1. Apparatus for changing a material from one state to a different state comprising: means for establishing a cryogenic fluid in a region; means for establishing an electrical discharge circuit in said cryogenic fluid and for defining a path for said discharge in said region; means for supplying electrical energy to said electrical discharge circuit; means for establishing a material to be vaporized in said path; and means for discharging said electrical discharge circuit for establishing Said discharge in said path.

2. Apparatus as set forth in claim 1 wherein said means for discharging said electrical discharge circuit includes mechanical means.

5. Apparatus for changing a material from one state to a different state comprising: a capacitor having a first and second plate positioned on either side of said material to be changed; means for charging the plates of said capacitor sufficient to cause vaporization of said material upon discharge; means for discharging said capacitor; and means for establishing the discharge circuit of said capacitor at a cryogenic temperature to rapidly quench said vaporized material and thereby to establish said material in a different state.

8. Apparatus for changing a material from one state to another state comprising: a capacitor structure including two capacitor plates consisting of respective superconductive materials; a hearth for material whose state is to be changed mounted on one said plate; a probe point on said other capacitor plate juxtaposed relative to said hearth; mechanical means for moving temporally said probe relative to said hearth to discharge said capacitor; means for establishing a cryogenic fluid in said arc discharge region; and electrical means for charging said capacitor with energy for said arc discharge.

9. Apparatus as set forth in claim 8 wherein said one capacitor plate includes two portions, one said portion mounting said hearth and said other portion mounting said mechanical means; said portion mounting said hearth including a means for juxtaposing said hearth relative to said probe point; said other capacitor plate being in membrane form; said mechanical means including: electromagnetic coil means mounted on said other portion of said capacitor for mechanically driving said probe point to cause it to change relative position to said hearth; an impact slug in said electromagnetic coil means; and alternating current electrical means for driving repetitively said impact slug on said superconductive capacitor membrane synchronously with the mechanical vibration parameter thereof.

10. Apparatus for generating a plasma comprising: means for establishing a cryogenic fluid in a region; means for establishing an electrical discharge circuit in said cryogenic fluid and for defining a path for said discharge in said region; means for supplying electrical energy to said electrical discharge circuit; and means for discharging said electrical discharge circuit for establishing said discharge in said path.

11. Apparatus as set forth in claim 10 wherein said means for discharging said electrical discharge circuit includes mechanical means.