Process for increasing the density and improving the homogeneity

Details Process for increasing the density and improving the homogeneity Process for increasing the density and improving the homogeneity

1990-09-03

1994-05-24

Wyszomierski, George

Coating processes

Electrical product produced

Superconductor

148 96, 419 10, 419 47, 505823, B05D 512, B22F 702

Patent

active

053147146

DESCRIPTION:

BRIEF SUMMARY
The present invention concerns a process for increasing the density and improving the homogeneity of Chevrel phase powders.
This invention also concerns Chevrel phase wire obtained by this process.
At present, commercially available superconducting wires of NbTi and Nb.sub.3 Sn cannot produce magnetic fields in excess of about 18 Tesla. This limitation is due to superconducting properties such as the critical temperature T.sub.c and the upper critical field H.sub.c2, which are specific for the materials and cannot be improved significantly. It can be confirmed that these materials reached their physical limits.
New materials named Chevrel phases are ternary molybdenum chalcogenides based on the formula M.sub.6 X.sub.8 where M is a metal, in particular lead, tin or lanthanum, and X is a chalcogenide. Some of these compounds PbMo.sub.6 S.sub.8, SnMo.sub.6 S.sub.8 or LaMo.sub.6 Se.sub.8 have remarkably high upper critical fields which make them good candidates for use in superconducting coils that can produce steady state magnetic fields above about 18 Tesla. For instance, PbMo.sub.6 S.sub.8 (PMS) with a critical temperature of 15.degree. K. and its critical field of 52 Tesla at 4.2.degree. K. is a particularly interesting compound for practical use. This is two and a half time higher than the best commercially available superconductor Nb.sub.3 Sn. This opens, at least theoretically, the possibility of achieving steady state magnetic fields in the range of 45 Tesla, representing a considerable scientific and technical interest.
For the production of very high magnetic fields, the superconductors must be in the form of a wire which can be wound to a coil. Different attempts have been performed in order to manufacture Chevrel phase wires. Two different techniques may be distinguished: in the first, the Chevrel phase is deposited out of the gas phase onto a substrate in the form of a wire or tape and the second technique is based on a powder metallurgical method which has been described in "Ternary Supraconductors", Eds. G. K. Shenoy, B. D. Dunlap and F. Y. Fradin, Elsevier North Holland, 1981, page 119. For the manufacturing of large lengths and quantities the powder metallurgical approach is the most economic.
Corresponding to the known techniques for manufacturing Chevrel phase wires, a very fine powder of Chevrel phase with a mean particle size inferior to 1 um is obtained by grinding after the last heat treatment. The grinding can be done for instance in a planetary mill. Then the Chevrel phase powder is pressed isostatically at ambient or elevated temperatures into the form of a cylinder, machined, passed and sealed under vacuum into one or more metallic cans (matrix material). The most commonly used materials for the matrix are, for instance, stainless steel with a diffusion barrier of molybdenum (see also Proceedings of the International Conference on Magnet Technology MT9, Zurich, 1985, page 560). Other materials such as niobium, tantalum or silver may also be used for the diffusion barrier.
It is known that an interesting critical current density for the Chevrel phase supraconductors can be reached only if the residual porosity of the Chevrel phase powders is as small as possible. In other words the more the effective density of the Chevrel phase powders approaches the theoretical density, the more the current quantity capable of being transferred by these superconductors is important.
After many investigations of hot pressed Chevrel phase powders it has been seen that, after other physical parameters, the specific mass influences the critical current density in a not inconsiderable manner. The highest specific mass and critical current density of Chevrel phase samples has been obtained after hot pressing at about 1100.degree. C. and under pressure of the order of 2,5 kbar. During the manufacturing of Chevrel phase wires with a stainless steel matrix and a molybdenum diffusion barrier, conditions approximating the above conditions are obtained by the applied hot extrusion at 1200.degree. C. and the sub

REFERENCES:
patent: 4594218 (1986-06-01), Dubots et al.
patent: 4746373 (1988-05-01), Yamada et al.
patent: 4808488 (1989-02-01), Chevrel et al.
patent: 4917871 (1990-04-01), Dahn et al.
patent: 4966749 (1990-10-01), Kondo et al.
Tarascon, J. M. et al., "New superconducting ternary . . . TlMo.sub.6 Se.sub.8 ", Physical Review B, Jan. 1, 1984, pp. 172-180.

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