In-situ formed composites of V3Si

Superconducting properties of high-Tsub(c) A-15 compound V3Si are investigated. The wire samples of three ternary alloys viz., Cusub(90)Vsub(7.5)Sisub(2.5), Cusub(88)Vsub(9)Sisub(3) and Cusub(84)Vsub(12)Sisub(4) show a two stage superconducting resistive transition, the first occuring around 17K and the second around 6K. The sample resistivity, however, does not drop to zero down to 2.5K. A simple calculation, based upon the phenomenological expressions o.iven by Davidson and Tinkham (1976), shows that the effective superconducting volume fraction in these alloys is well below the threshold value qiven by either the effective medium theory or the site percolation theory. This volume fraction is further reduced consequent upon the reduction of the wire diameter and also upon annealing. Experimental evidence confirms the formation of the brittle phase, V3Si, during the melting of the alloys which does not chan-oe to fine structure with the matrix during rolling or drawing. The effective V3Si volume is thus further reduced which in turn reduces the resistivity drop. Annealing also has a similar effect as it causes the superconducting particles to coalesce and grow in size. The proximity effect which operates well in V3Ga and Nb3Sn composites in copper is not observed in these alloys. Possible explanations for the occurrence of the second transition are discussed. (author)