Thermal conductivity studies of YBa(_2)Cu(_3)O(_7-δ)

Apparatus to measure the thermal conductivity of YBa(_2)Cu(_3)O(_7-δ) at temperatures between 20K and 120K has been designed and constructed. The thermal conductivity is measured using a longitudinal steady state heat flow technique. Thermal conductivity measurements have been performed upon a sample of YBa(_2)Cu(_3)O(_7-δ) which has been subjected to a series of heat treatments in order to remove oxygen from the material. The measurements show conclusively that the thermal conductivity of YBa(_2)Cu(_3)O(_7-δ) is very strongly influenced by the oxygen content of the material. A reduction of the oxygen content of the material results in a substantial lowering of the thermal conductivity. To explain this result, a quantitative model has been constructed; the model demonstrates that consideration of the changes in phonon interactions alone cannot account for the differences in the behaviour of the thermal conductivity of YBa(_2)Cu(_3)O(_6) and YBa(_2)Cu(_3)O(_7). In addition; the model, shows that there must be a significant carrier contribution to the thermal conductivity in both the normal and superconducting states. A physical process has been proposed which provides the required large carrier contribution below T(_c). Further studies have been performed on a series of samples of YBa(_2)Cu(_3)O(_7-δ) which were sintered at slightly different temperatures. Qualitative analysis of the physical properties, of these samples has been performed.