Thermal conductivity and thermoelectric power of melt processed (Nd/Y)BCO intergrowth crystals

Abstract

In a previous paper [C. Cipagauta Mino, J.W. Cochrane, E.H. Volckmann, G.J. Russell, J. Electron. Mater. 26 (1997) 915.], we described a cryogenic thermoelectric cooler with a superconducting passive branch. The efficiency of this device depends on selecting an optimal cross-sectional area for the superconducting element based on its thermal conductivity in a magnetic field. (Nd/Y)BCO intergrowth crystals make an ideal superconducting element due to their relativity low thermal conductivity, high critical current, and large size. In this paper, we describe the thermal conductivity and thermoelectric power over the temperature range 20–300 K in applied magnetic fields up to 5000 G, for a specimen cut from a large high quality melt processed (Nd/Y)BCO intergrowth crystal that has almost optimum oxygen content, estimated to be 6.92±0.02. The shape of the κ ab and κ c curves, without applied field, are similar to those reported for Y123 single crystals, but the absolute values are significantly smaller. This result is discussed in terms of the presence of dispersed particles of the (Nd/Y)211 phase and increased phonon and carrier scattering. However, in the normal state, the anisotropic ratio κ ab n/ κ c n is almost identical in shape and magnitude to that of the electrical conductivity ratio σ ab n/ σ c n. The application of magnetic fields either parallel or perpendicular to the heat flow direction always decreases κ ab and κ c for temperatures below T c and 120 K, respectively. Superconducting fluctuation phenomena was observed about T c for both the thermoelectric power and κ ab data.