US3254955A

US3254955A - Method of preparing a tantalum carbide crystal

Info

Publication number: US3254955A
Application number: US219996A
Authority: US
Inventors: George R Bird; Brewer Leo; Jr Willis E Gray
Original assignee: Individual
Current assignee: Individual
Priority date: 1962-08-28
Filing date: 1962-08-28
Publication date: 1966-06-07
Anticipated expiration: 1983-06-07

Description

' June 7, 1966 G. R. BIRD ET AL 3,254,955

METHOD OF PREPARING A TANTALUM CARBIDE CRYSTAL Filed Aug. 28, 1962 2 Sheets-Sheet 1 CARBON ATOMIC PERCENT IO 4000' 1 i I I +TOC T C TaC 5 L+TQ L'- 2800-v T C B? 2 -5000 5 D To ToC+C I CC CC LLI E I 4500 g 2 2 m E 2400- T00 o 0.6 2 3.2 4 s 6.2 8 l0 l2 CARBON,WEIGHT PERCENT To-C SYSTEM FIG.I

Am zw i M INVENTORS ATTOR N EYS June 7, 1966 G. R. BIRD ET AL 3,254,955

METHOD OF PREPARING A TANTALUM CARBIDE CRYSTAL Filed Aug. 28, 1962 2 Sheets-Sheet 2 FIG. 2

RF POWER 30 SOURCE INVENTORS ATTORNEYS 3,254,955 METHOD OF PREPARING A TANTALUM CARBIDE CRYSTAL George R. Bird, 6 Lowell Road, Concord, Mass; Leo

Brewer, Vista Del Orinda, Orinda, Calif.; and Willis E. Gray, Jr., Irving St., Boston, Mass.

Filed Aug. 28, 1962, Ser. No. 219,996 6 Claims. (Cl. 23-208) The present invention relates to a method of growing single refractory metal carbide crystals.

It is a primary object of the present invention to provide a method for growing refractory metal carbide single crystals by zone melting a carbide-carbon eutectic.

It is an additional object of the present invention to provide such a method wherein a carbon crucible makes up the second phase of the eutectic and contains a first phase metal carbide charge therein.

It is a further object of the present invention to provide such a method wherein the carbide-carbon eutectic is zone melted by induction heating in an inert atmosphere which acts to retard evaporation of the carbon crucible.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

.The invention accordingly comprises the several steps and the relation and order of one or more of such steps with respect to each of the others, and the product possessing the features, properties and the relation of elements which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

FIGURE 1 is a diagram of a refractory metal carbidecarbon system, which in this case is the tantalum-carbon system;

FIG. 2 is a diagrammatic illustration of the apparatus and the relationship of the several parts thereof used in the process of the present invention;

FIG. 3 is a cutaway perspective view of the carbon crucible during the process of the present invention; and

FIG. 4 is an enlarged view of a single crystal lamp filament obtained by the method of the present invention.

Previous efforts to grow large single crystals of refractory metal carbides, particularly of TaC, have been largely unsuccessful due to the fact that such carbides have an extremely high melting point and tend to disassociate to gaseous carbon and either subcarbides or metal during attempted single crystal growth. The present invention proposes a process wherein the single crystal growth is accomplished in the eutectic composition region of the Ta-C system whereby a much lower temperature is required to melt the carbide. Philip McKenna in Metal Progress, volume 36, 1939, described a method whereby minute single crystals of TaC were obtained. The method used in that case was the dissolving of tantalum in molten aluminum with carbon present and then heating the mass to approximately 2000 C. in a graphite crucible. The TaC crystals thus formed were isolated by treating the mass with acid to remove th aluminum and aluminum carbide. A problem with this method is its inability to grow a single crystal of T aC having any significant size, the crystals .obtained thereby being tiny flakes having a small surface area with negligible depth. The present invention uses a zone heating technique to obtain single crystal growth from a system comprising polycrystalline TaC contained in a bored carbon rod crucible which is heated in an inert gas atmosphere by an induction heating method. While the present invention uses an induction heating method, other methods Patented June 7, 1966 known in the art, such as are imaging, which create a narrow zone of heating, could also be used. The practice of zone melting as a metallurgical technique is also known in the art. However, the present invention is distinguished in that it includes a grouping of elements containing a first and second phase or component of the tantalum carbon system comprising a compound which controls the composition of the system by.means of the carbon crucible, which crucible not only provides a means for holding the first component but also functions as the second component in the Ta-C system.

Referring now to FIGURE 1, there is shown the tantalum carbon system according to Finley H. Ellinger, which was first proposed in Transactions of the ASM, volume 31, March 1943. It is apparent from this figure that tantalum forms two carbides; Ta C with a 3.2 weight percent of carbon and TaC with a 6.2 weight percent carbon. The system also has two eutectic composition regions present, a Ta-Ta C eutectic which occurs at 0.6 weight percent carbon and 2800 C. and a TaC-graphite eutectic which is formed at 10 weight percent carbon and a temperature of 3300 C. The melting point of TaC has been variously proposed between the regions of 3540 C. (Geach and Jones, Plansee Proc., -90, 1955) and 3880 i C. (Agte and Alterhum, Z. Tech Physik, 11, 1930). It is believed that the correct melting point is in the range of 3880 C. or higher in view of experimental results obtained in conjunction with the development of the present invention. Because of the fact that the melting point of the refractory metal carbide, TaC, is so high, the present invention proposes that the process of growing single TaC crystals be carried out within the second eutectic composition region or the TaC-graphite eutectic. This eutectic melts at a temperature of 3300 C. which, while high, is still substantially lower than the estimated melting point of the TaC phase.

As seen in this system diagram, the pure tantalum phase, which has a melting point of 2996 C., goes into a eutectic phase with the carbon deficient carbide Ta C with the addition of a small carbon weight percent. The addition of further increasing amounts of carbon will cause a movement of the mass over into the TaC and then the TaC-C eutectic region. Thus by use-of a graphite crucible as the second phase of the mass used in the present process, it is conceivable that a first phase charge of solid or powdered tantalum might be used, with conversion by carburization to TaC and then formation of the TaC-C eutectic. Similarly tantalum and carbon might be used as the first phase charge, with carburization of the tantalum taking place. A preferred method for the present process, however, uses powdered TaC as a first phase charge. This reduces the requirement of carburizing a tantalum or tantalum deficient phase as would be necesary in the other suggested first phases.

FIG. 2 illustrates the apparatus and physical relationship of the several parts of the present invention. The graphite or second phase of the eutectic region in'which the processing is made to take place is supplied by the use of a carbon crucible 10. This crucible is in the form of a bored carbon rod. The hollow core 12 formed within the rod is preferably filled with powdered polycrystalline TaC as a first phase charge 14 or may alternatively be filled with powdered tantalum or powdered tantalum and carbon in combination as described above. Zone melting is accomplished by the use of an induction heater 16 whose field may be moved relative to the longitudinal axis of the crucible. The crucible is provided with an advancing means 18 by which it is moved through a water cooled induction coil 20, in the direction indicated by arrows 28, in the preferred method of the present invention. A power input sufiicient to raise the mass to the TaC-C eutectic melting point is supplied by means of an RF power source 30. The power requirement for effective melting will vary with the physical diameter of the mass. Because of the fact that the crucible is evaporating while the process of growing the single crystal is taking place, an inert gas atmosphere 22 is provided about the crucible to retard evaporation. A preferred inert gas for this atmosphere is argon. Other members of the inert gas family such as helium and krypton might also be used. In one form of the present invention, successful results have been obtained using a bored carbon rod crucible having an outside diameter of approximately 7 inch and an inside diameter of 4; inch to provide a hollow cylinder in which the charge comprising the first phase is inserted. The crucible in this case had a height of approximately 2 inches. It will be realized that the largest crystal which can be formed is primarily dependent upon the capacity of the power heating device. Additions to available space within the gaseous chamber'and the apparatus used for advancing the crucibble through the zone heating means may be made with a relatively small additional expense. The crucible may be openended at the base thereof as shown in FIG. 3, or it may be solidly formed at the end thereof. In the former case the advancing support means 18 shown in FIG. 2 would form a base for the crucible to prevent the charge 14 from falling out of the core 12 of the crucible.

A cutaway view of the interior of the crucible during the growth process is shown in FIG. 3. The gaseous atmosphere is not shown in this figure but is assumed to be present to accomplish the desired results. As shown,

the carbon crucible decomposes as the zone melting progresses along the longitudinal axis of the crucible in the direction shown by arrow 32. This is due in part to combination with the TaC phase and in part due to the escape of carbon atoms into the atmosphere. The induction heating device, which is not shown in this figure, is held at the top region of the crucible until a temperature of 3300 C. is reached wherein the first (14) and second (10) phases combine to form a eutectic composition 24 comprising the TaC phase and carbon. The eutectic composition at the temperature of 3300 C. with a 10 weight percent of carbon becomes molten. Continued heating at this temperature will cause excess carbon to be driven off from the molten mass, causing the weight percent of carbon within the mass to decrease. A slight decrease of the weight percent of carbon will shift the mass into the liquid +TaC phase region as shown in FIGURE 1. As the mass moves into this region a solid crystal of tantalum carbide will begin to form. By careful control of the temperature, and movement of the heating device as described below, a single crystal of tantalum carbide is initially formed from the mass and is subsequently grown by extending its lattice structure down through the core charge as the core is progressively zone melted. As the heated area progresses, that portion which has been molten, when cooled, will form a single crystal of tantalum carbide. The heating device is progressively moved along the crucible at a predetermined rate to stimulate growth of the single crystal. Melting is accomplished by the resistance of the mass to passage of secondary current induced in the mass charge by electromagnetic induction, the coil being supplied with current from the RF power supply 30. The induction method of heating also acts to ensure homogeneity of composition by a stirring action caused by the induced lines of force. The molten region containing the eutectic composition 24 is progressively moved down the length of the crucible. A single crystal 26 of tantalum carbide is thus grown and is constantly solidifying above the molten region, which latter region is held in place by the crucible walls as the heating element is moved downward. Decomposition of the crucible is not so rapid as to allow this molten region to escape from within the confines of the crucible core and does not affect the solidified single crystal which is self-supporting.

The ability to produce single crystals of refractory metal carbides having an appreciable size is of particular interest in the manufacture of lamp filaments. Single crystals having a diameter of inch and a length greater than V inch have been produced utilizing the method of the present invention. Of course, for lamp filament use, such crystal rods would normally have to be drawn down to a diameter of less than 10 mils. A typical filament shape 34 of such a single crystal refractory metal carbide wire is shown in FIG. 4. Filament structures of this nature and the details of their supporting structure and operating environment are more fully described in US. Patent No. 3,022,437, issued Feb. 20, 1962 in the name of Dexter P. Cooper, Jr. and in copending US. application Serial No. 5,525, filed Jan. 29, 1960, now Patent No. 3,022,437, in the name of Dexter P. Cooper, Jr.

The particular advantages of the single crystal structure in a wire filament derives from the fact that in polycrystalline structured metal carbide filaments, oxides such as thoria or alumina must often be added to lengthen the individual grains whereby the grain boundaries make smaller angles with the wire axis and the probability of failure by offset is reduced. A single crystal grain wire obviates this problem and also gives a much stronger material having a higher brittle failure.

The process of the present invention is thus seen to provide a practical method of growing single tantalumcarbide crystals. The process herein proposed involves the combination of three primary techniques: the use of a carbon crucible as the second phase of the composition; the use of a zone melting technique; and finally, the use of an induction heater. While the illustration used in the above description has primarily been the tantalum-carbon system, it should be understood that any of the refractory metal carbidesystems are similarly compatible for the growth of single crystals as described in the process of the present invention.

Since certain changes may be made in the above process and product without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A method of producing single crystals of tantalum carbide, comprising the steps of:

preparing a two-component composition in a Ta-C system, a first component of which contains a substantial weight percent of tantalum, and a second component of which is carbon in the form of a crucible in which said first component is placed; and

zone melting said composition in the Tac-C eutectic composition region of the Ta-C system.

2. The method of claim 1 wherein the said first component is selected from the group consisting of solid tantalum and powdered tantalum.

3. The method of claim 1 wherein said first component is a mixture of powdered tantalum and carbon.

4. The method of claim 1 wherein the said first component is powdered tantalum carbide.

5. The method of claim 1 wherein the zone melting is by induction heating in an argon atmosphere.

6. A method of producing single crystals of tantalum carbide comprising the steps of:

preparing a two-component composition in a Ta-C system, a first component of which contains a substantial weight percent of tantalum and a second component of which is carbon in the form of a crucible into which said first component is placed;

applying heat to a selective portion of said composition by means of a heating device of suificient power to raise the said composition to the Tac-C eutectic composition melting point; and

advancing said heating device along the longitudinal axis of said crucible at a predetermined rate.

(References on following page) References Cited by the Examiner UNITED STATES PATENTS Kieffer 23208 X Pfann 23223.5 X 5 Sloan 23208 X FOREIGN PATENTS 6 OTHER REFERENCES Ellinger: The Tantalum-Carbon System, Transactions of American Society for Metals, vol. 31, pp. 89- 104 (1943), page 98 in particular.

BENJAMIN HENKIN, Primary Examiner. MAURICE A. BRINDISI, Examiner.

A. STEWART, G. T. OZAKI, Assistant Examiners.

Claims

1. A METHOD FOR PRODUCING SINGLE CRYSTALS OF TANTALUM CARBIDE, COMPRISING THE STEPS OF: PREPARING A TWO-COMPONENT COMPOSITION IN A TA-C SYSTEM, A FIRST COMPONENT OF WHICH CONTAINS A SUBSTANTIAL WEIGHT PERCENT OF TANTALUM, AND A SECOND COMPONENT OF WHICH IS CARBON IN THE FORM OF A CRUCIBLE IN WHICH SAID FIRST COMPONENT IS PLACED; AND ZONE MELTING SAID COMPOSITION IN THE TAC-C EUTECTIC COMPOSITION REGION OF THE TA-C SYSTEM.