Thermal conductivity measurements for evaluation of crystal perfection at low temperatures

A thermal conductivity measurement system using a steady heat flow method has been used to study crystal perfection at low temperatures. The measurement range was from 1.5 to 300 K with a magnetic field of up to 8 T. The overall absolute measurement error was estimated to be within 4% without the magnetic field and 8% with a magnetic field of 5 T. Rectangular samples of minimum size 2×2×10 mm3 can be measured. The phonon scattering mechanism was analysed using the Callaway model for isotopically enriched β-rhombohedral single crystals of boron and transparent polycrystals of yttrium aluminium garnet (YAG). Experimental results for the boron samples showed that isotopic enrichment made the thermal conductivity higher and the theoretical curve fitting showed that there was less phonon scattering in the isotopically enriched sample. For the YAG samples, a transparent polycrystal had a higher thermal conductivity than did an opaque one. Analysis of the results indicates that the grain size of the transparent sample is about 10 times larger than that of the opaque one.