US3752665A - Synthesis of superconducting compounds by explosive compaction of powders - Google Patents

Abstract

SUPERCONDUCTING INTERMETALLIC COMPOUNDS, SUCH AS NB3SN ARE PREPARED BY DISPOSING A FINELY DIVIDED, STOICHIOMETRIC MIXTURE OF ELEMENTAL METAL POWDERS IN A CONTAINER, SURROUNDING THE CONTAINER WITH AN EXPLOSIVE AND DETONATING THE EXPLOSIVE. THE RESULTING EXPLOSIVE SHOCK WAVE PROVIDES THE NECESSARY REACTION CONDITIONS FOR COMPOUND FORMATION.

Description

' Aug. 14, 1973 u. ROY ETAL SYNTHESIS OF SUPERCONDUCTING COMPQUNDS BY EXPLOSIVE commoner: 0F POWDERS Filed July 8. 1971 UPENDRA ROY TEMPERATURE K FIG.

0 OOO GUENTHER OTTO ORVIL. Y. REECE IN VE N TORS United States Patent O SYNTHESIS F SUPERCONDUCTIN G COM- POUNDS BY EXPLOSIVE COMPACTION OF POWDERS' Upendra Roy, Guenther H. Otto, and Orvil Y. Reece,

Huntsville, Ala., assignors to the United'States of America as represented by the Administrator of the National Aeronautics and Space Administration Filed July 8, 1971, Ser. No. 160,860

. Int. Cl. B22f 3 /08 US. Cl. 75-'-135 7 Claims ABSTRACT DISCLOSURE Superconducting intermetallic compounds such asNb Sn are prepared by disposing a finely divided, stoichiometric mixture of elemental metal powders in a container, surrounding the container'with an explosive and detonating the explosive. The resulting explosive shock wave provides the necessary reaction conditions for compound formation. I I

ORIGIN OF THE INVENTION The invention described herein was made in part by an employee of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates to preparation of intermetallic compounds and more particularly to synthesis of superconducting intermetallic compounds.

Superconductors, that is, substances which undergo a transition at very low temperatures to a state of zero electrical resistance, have found numerous applications in electromagnetic devices, and many research programs directed to further applications are currently in progress.

One of the poblems presented in connection with the most common type of superconductors, namely, intermetallic compounds such as Nb sn, Nb Al, Nb Ge and the like, is their difiiculty of synthesis. Prior art methods have generally involved isostatic or direct pressure compaction of a reactant powder mixture, followed by sintering, melting or casting. Such methods involve large capital outlays for high-temperature furnaces, hydraulic presses, furnace atmosphere controls and related equipment. Furthermore, in some cases the incongruent melting characteristic of the powder mixture presents a strong obstacle to formation of the desired compound; for example, niobium and tin have widely dilfering melting points, and tin may be largely evaporated from a mixture with niobium before the temperature required for intermetallic compound formation is attained. Another disadvantage of previous methods is the tendency of the product to pick up impurities during high-temperature processing.

SUMMARY OF THE INVENTION In the present invention superconducting intermetallic compounds are synthesized by explosive compaction of a. stoichiometric powder mixture of the constituent metallic elements. The powder mixture is disposed in a container and surrounded with an explosive and the explosive is detonated. The resulting shock wave brings about an instantaneous reaction between the metals in the powder mixture, producing an intermetallic compound in consolidated form. This method is readily carried out, using inexpensive equipment, and the need for high-temperature furnaces is avoided. Incongruent melting mixtures can be reacted by explosive compaction without loss of the lowmelting component by evaporation.

It is therefore an object of this invention to provide a method of preparing such compounds without prolonged heating of a reactant mixture.

Still another object is to provide a method of reactlng an incongruent-melting metallic powder mixture.

Other objects and advantages of the invention will be apparent from the following detailed description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a sectional view of apparatus for carrying out the method of the present invention; and

FIG. 2 is a graph showing superconductivity transition curves for Nb Sn samples prepared according to the in vention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 of the drawing, a finely divided mixture 10 of metal powders in proportions corresponding to the stoichiometry of the desired intermetallic compound is disposed in a tubular metal casing 11 having crimpedover end portions 12. The joint formed by the crimped end portions 12 is left unsealed so as to allow escape of any gases generated during explosive compaction. The powder container 11 is surrounded by an explosive 13 such as nitroguanidine which in turn is disposed in a cylindrical container 14. A conventional detonator 15 is embedded in the explosive 13 at the top of container 14. Electrical leads 16 and 17 extend from the detonator 15 to a power source and switching means (not shown) for ignition of the explosive charge.

The reactant powder mixture is made up as required for formation of the desired intermetallic compound. For example, in the case of Nb Sn, three moles of niobium powder are mixed with one mole of tin powder. Other superconducting intermetallic compounds which can be synthesized by the method of this invention include Nb In, NbgAl, V3811, T21 511, and Nba (A10 751eo 25)- In order to undergo a complete reaction within the very short period of elevated temperature and pressure provided by the explosive shock wave, the reactant metals must be in finely divided form, and best results are obtained by using powder having a particle size less than 325 mesh (43 microns). Thorough mixing of the reactant metal powders by means such as tumbling is also required. The mixed powder is placed in a container, preferably a tubular metal casing with closed but unsealed ends as depicted in FIG. 1, and is compacted by means of hand tamping or the like. High-pressure compaction is not necessary. Although the invention is not to be understood as so limited, successful results are obtained by using a tubular container .3175 to 1.27 cm. in internal diameter and metal powder charge sufficient to fill the container.

The explosive 13, which is disposed around the powder container, is selected to provide the desired shock wave characteristics, in particular a converging shock front and a high energy density in the center. Explosives which can be used include nitroguanidine, dynamite and ammonium nitrate. Nitroguanidine provides advantages in its ease of handling and in that its packing density can be adjusted to allow variations in shock-wave velocity and explosive density without recourse to other explosives.

The invention is further illustrated by the following example.

EXAMPLE Fine grain (325 mesh) powders of niobium (99.99%) and tin (99.99%) were mixed in stoichiometric 3Nbz1Sn proportion and placed in a copper tube 6.2 mm. in diameter and having a 0.8 mm. thick wall. The tube ends were pinched and crimped, and the tube was inserted centrally into a cylindrical container packed with nitroguanidme explosive as shown in FIG. 1 of the drawings.

The explosive charge was then detonated by means of an electrical detonator and a tetryl booster cap positioned on top of the container. Several samples were prepared using explosive densities which varied from 0.2 to 0.65 gram per cubic centimeters. The product in consolidated rod form was examined by X-ray diffraction and photomicrographs were taken. At the lower explosive densities only a good compaction of the powders was obtained, but at densities of 0.3 gram per cubic centimeter and higher, a distinct zone of compound formation was observed in the center of the same, the compound shown by X-ray diffraction to be Nb Sn. Transitions to superconductivity were measured on copper jacketed samples using resistive and A.-C. inductance methodsTypical curves are shown in FIG. 2 of the drawing. The maximum transition temperature, 163 K. (half-point of the transition curve) was obtained at an explosive density of 0.54 gram per cubic centimeter.

It is to be understood that the above example is merely illustrative and that various changes and modifications in the procedures and apparatus described above can be employcd by one skilled in the art without departing from the invention. It is also to be understood that the principles of the invention can be applied to the synthesis of refractory compounds other than superconducting intermetallics. Examples of such other compounds include TaC, VC, and NbC.

What is claimed is:

1. The method of preparing a superconducting intermetallic compound which comprises thoroughly mixing finely divided elemental metal constituents of said compound at a predetermined molar ratio corresponding to the molar ratio of metal constituents of said compound,

'4 disposing the resulting mixture in a container, surrounding the container with an explosive charge sufiicient to produce upon detonation effective temperature and pressure conditions for compound formation and detonating said charge whereby a compound-forming chemical reaction between said elemental constituents is effected.

2. The method of claim 1 wherein said superconducting intermetallic compound is a member of the group consisting of Nb Sn, Nb In, Nb Al, V Sn, Ta Sn, V Al, Nb Si and s o.'15 0.25)-

3. The method of claim 2 wherein said superconducting intermetallic compound is Nb Sn.

4. The method of claim 3 wherein said explosive is nitroguanidine.

5. The method of claim 4 wherein the density of said charge is 0.3 to 0.65 gram per cubic centimeter.

6. The method of claim 5 wherein the density of said charge is approximately 0.54 gram per cubic centimeter.

7. The method of claim 1 wherein the particle size of said elemental metal constituents is less than -325 mesh.

US. Cl. X.R.

-l34 V, 174; 29-4205, 421 E, 599; 26484

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