Abstract
The heat conduction coefficient of hexagonal ice single crystals is measured in the temperature range between 0.5 and 20K on samples with the c axis oriented respectively parallel and perpendicular to the heat flux. The anisotropy reaches out 20% near the maximum of heat conduction at 7K, the crystal oriented perpendicular to the heat flux being the better heat conductor. The anisotropy is negligible at temperatures higher than 10K and lower than 2K. The results are fitted using a simplified Callaway model. It appears that the predominant defect relaxation rate is of the type tau -1=B omega 3. This kind of relaxation rate is characteristic of phonon interactions with dislocation cores or similar obstacles of one dimension. Specular reflection seems to affect the phonon scattering on the boundaries of the sample.