Scattering of thermal phonons by extended defects in dielectric crystals

The scattering of thermal phonons by extended defects in dielectric crystals has been observed through measurements of thermal conductivity and ballistic heat pulse propagation. The thermal conductivities of LiF and NaCl conatining 500 low-angle grain boundaries per cm were measured in the range 0.08-5 K. The measurements gave little or no evidence for phonon scattering from the grain boundaries. Measurements of phonon scattering at a 10 deg. grain boundary in silicon using direct generation and detection of ballistically propagating heat pulses were made over an effective phonon temperature range of 2 to 20 K. The grain boundary reflection coefficient was determined to be 2 k, some fraction of the phonons (at least the slow transverse mode) were still strongly scattered, even after long exposure to ..gamma.. irradiation, while the remaining phonons were scattered primarily by the boundaries.« less