2.5 Thermal properties of HTS bulk Ë 41Fig. 2.9:(a) Comparison of the thermal conductivity measured within a YBCO grain and across a
grain boundary in zero applied magnetic field, measured using: a physical property measurement
system with the thermal transport option (TTO) (system 1) and a home-made high-sensitivity
system with three thermal radiation screens (system 2). (b) Intra- and (c) inter-granular thermal
conductivities at 0 and 8 T [130].
thermal expansion is closely related to the specific heat. Thermal expansion is also
very useful for predicting how solid state properties respond to volume changes in the
limit of zero applied pressure.
The thermal expansion of sintered HTS bulk is similar to that of non-
superconducting ceramics. The HTS single crystals are necessary for study of struc-
tural anisotropy. Since HTS bulk has a layered orthorhombic structure, one needs to
measure the linear coefficients of thermal expansion along all three crystallographic
axes, i.e. thea- andb-axes ina-bplane and in thec-direction perpendicular to the
CuO 2 planes. The linear thermal expansion coefficient of HTS bulk is in the range