Compacted 1-2-3 ceramic powders treated as superconducting composites

Magnetization studies of closely spaced fine-filamentary Cu/NbTi composites have shown that, at very low applied field strengths, proximity-effect coupling (across the Cu matrix) between the filaments causes the assembly to exclude flux in the manner of a solid block of superconductor of the same outside diameter as the filamentary bundle. Then at somewhat higher applied field strengths (but still with H <H_c1) the conductor behaves as a cluster of independent filaments. Next, in this regime and provided the filaments are sufficiently fine, field penetration reduces the volume of excluded flux significantly below the geometrically calculated value. Since the field-penetration depth increases with temperature according to 1/√(1-T⁴), where t=T/t _c, the effect becomes more and more pronounced as T approaches T_c. Comparable proximity- and field-penetration effects are seen in high-T_c ceramic-powder compacts. It is suggested that granularity of some kind, with its associated field-penetration effect, could be responsible for most of the departures from almost complex flux exclusion that have been observed in almost fully single-phase ceramic samples