Superconductivity in binary eutectic alloys

A brief description of the fundamentals of superconductivity is followed by a description of the preparation and microstructures of the alloys concerned in this work. The experimental apparatus used for the superconducting measurements is described. The effect of the presence of a precipitate on several superconducting parameters has been investigated by making magnetisation, critical current and resistance transition measurements on ten eutectic alloys and many of their component solid solutions in isolation. All the eutectics after suitable heat treatment were, with one exception, type II superconductors. The presence of a precipitate, whether superconducting or normal, was found to cause flux pinning which varied in some cases according to the precipitate orientation with respect to the external magnetic field. The pinning effects can be eliminated by consideration of very thin specimens. The properties of a eutectic alloy were found to be not necessarily the sum of the properties of the conjugate solid solutions in isolation; a proximity effect occurs in three eutectic systems examined, i.e. a phase that on its own would be normal is superconducting in the presence of another superconducting phase. In the absence of precipitates, the magnetic hysteresis is thought to be due to surface currents. It is suggested that the socalled 'major loop' of a superconductor is a measure of the relative effects of surface currents and bulk currents. The effect of edges on the magnetisation behaviour has been examined by considering slit and unslit hollow cylinders and hollow ellipsoids. Surface conditions were also examined. Roul)hening the surface was found to decrease specimen magnetisation (particularly in the region of Hcl) and also to reduce the critical transport current in the mixed state and the surface sheath current above Hc2. Plating with Cu and Cd reduces sheath currents above Hc2 but plating with ferromagnetics Ni and Fe actually increases them for the case of field not parallel to the specimen surface. The model of St.James and DeGennes' surface sheath above Hx2 is endorsed and the mechanism confirmed as the same for type I and type II superconductors. The suggestion is made that the sheath may also exist below Hc2 to account for the initial part of the resistance transition curve. The presence of a precipitate increases the current carrying capacity of the sheath and may also increase HN, the field at which the specimen becomes completely normal. A model of internal surface superconductivity is proposed to account for these results. Finally, applications of the result of this work are discussed and future lines of research are suggested