Two-step heat treatments have been carried on ceramic-route precursor wires and lead to the optimization of critical current density. J(c) curves as a function of heat treatment time show a maximum value at any temperature, corresponding to a compromise between the fabrication and the degradation of the superconducting Chevrel phase.

Actual critical current densities (Jc) obtained by most of the groups working on superconducting PbMo6S8 wires are about 2 × 108A/m2 at 4.2 K and 20 T. But Jc fluctuations along the wire length are still large. We present here a two-step heat treatment technique that greatly improves the Jc homogeneity and also a new and promising route for producing PbMo6S8 below 500°C.

Monofilamentary PbMo6S8 wires have been made with a central core of a non-reacted Chevrel phase, i.e. a mixed phase of PbS, MoS2, Mo constituents in good proportions. Critical current densities Jc of these wires have been measured after different heat treatment conditions. The Jc results are quite comparable to ones of prereacted P-M-S wires.

Metallic silver, or silver oxide, is more and more used to improve the properties of YBa2Cu3O7 ceramics. Increases of the current carrying capacity are thus commonly obtained, but the role of this silver addition has not yet been clearly explained. As it is generally considered that silver improves the connectivity between grains, we assumed that this improvement resulted of a chemical interaction between silver and the superconducting material. In order to check this asumption, we have studied the chemical reactions of silver with the phases possibly existing in a YBaCuO ceramic : starting reagents, secondary phases and the superconducting oxide itself. Results obtained show that silver reacts with barium based compounds (BaCuO2, BaCO3) and melts. This melting probably “cleans” the grain surfaces and increase their reactivity.

A.c. susceptibility measurements have been carried out on Nb3.25(Al0.8Ge0.2) and V3Si in a wire, powder or compact form and on (RE)Ba2Cu3O7−δ powdeers and sintered compact materials. They show the dependence of the complex susceptibilityχ = χ′ + iχ″ on the surface volume ratio of the grains and on the contact resistance between the grains (i.e. the ability of carrying supercurrent).

In the process of elaboration of composite superconducting wires bases on the Chevrel phases compound SnMo6S8, a niobium barrier should be introduced between the stabilizer (Cu) and the superconductor to prevent Cu diffusion which degrades the critical properties.

Cet article rassemble les principaux resultats du programme de developpement francais de fils supraconducteurs appeles «Phases de Chevrel». L'elaboration de brins supraconducteurs a base de poudres de phases de Chevrel ou de precurseurs dans une matrice de cuivre avec barriere de niobium a ete realisee grâce au processus d'etirage des poudres a froid. Les J c des meilleurs filaments, constitues de phases reagies in situ dopees a l'etain (Pb ou SnMo 6 S 8 ) sont superieurs a 10 8 A/m 2 a 10 T et de l'ordre de 10 8 A/M 2 a 20 T

We show that the current carrying capacity of powdered superconductors is controlled by three main parameters: the chemical homogeneity of the starting material, the contact area between grains, and the surface state of the grains. These different cases are illustrated by three powders, a Chevrel phase compound, a NbAlGe alloy and a high Tc copper oxide.

Cold-powder metallurgical processed prereacted PbMo6S8 monofilamentary wires have been fabricated with a niobium sheath material into a copper tube. Critical current densities are 1.13× 108A/m2 at 3 T and 0.45× 108A/m2 at 9.2 T and 4.2 K. These results are the best ones obtained on wires containing prereacted PbMo6S8 powder as a starting material.

For eighteen months, we have manufactured long lengths of multifilamentary superconducting wires, which comprise from several ten thousand to several hundred thousand 0.08 to 1 micron-diameter NbTi filaments. In these wires, the matrix between the filaments is made of pure CuNi and their structure provides a good magnetic stability and leads to strongly reduced 50 Hertz losses. Obtained results op...

Rdsume : Des brins supraconducteurs multifilamentaires, comportant plusleurs milh e r s de filaments d e Nb-Ti de 0 , 5 a I micron de diametre, o n t kte' elaborks e n grandes longueurs.C e s brins multifilamentaires, h matrice mixte Cu-Cu Ni ou a m a t r i c e e n t i e r e m e n t e n C u Ni dans la zone multifilamentaire, ont d e s densltks de courant 5 2 critique elevees, atteignant 1,95 1 0 A/cm a 5 Teslas.Leur s t r u c t u r e leur conf6re une bonne stabilite magne'tique e t perrnet une t r e s f o r t e rdduction d e s pertes i 5 0 Hz C e s resultats p e r m e t t e n t d'envisager d e s a l t e r n a t e u r s dont le s t a t o r et le rotor seraient I'un e t I'autre supraconducteurs e t , de f a ~o n plus gknkrale, ils ouvrent d e s perspectives e n t i e r e m e n t nouvelles d'applications industrielles d e l a supraconductivite e n r6gimes t r e s rapidement variables e t e n alternatif aux frequences industrielles.

Description des diverses etapes d'une fabrication de type industriel des conducteurs supraconducteurs a base de Nb−Ti, ainsi que des differents problemes rencontres. Exemple des caracteristiques typiques des conducteurs industrialises par Alsthom-Atlantique