The 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 NaCt containing 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. In light of available models of phonon scattering, this suggests that the grain boundaries are sessile. 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 2-20 K. The measurements revealed no measurable scattering from the boundary and, within the resolution of our measurement, the grain boundary reflection coefficient was determined to be < 2%. The thermal conductivities of LiF crystals containing 5xl06_3xl07 dislocations per square cm were measured over the temperature range 0.1-10 K. The crystals were deformed either by shearing or bending then measured in the freshly deformed condition and after a series of irradiations with 0.66 MeV gamma rays. The measurements of the sheared crystal indicated that the slow transverse phonon mode was strongly scattered by a dynamic phonondislocation interaction at T ~ 2 K. while the remaining modes were scattered primarily by the boundaries. The measurements of the bent crystals indicated that, for T S 2 K. the slow transverse and possibly the longitudinal phonons were strongly scattered by a dynamic phonon-dislocation interaction while the remainder of the phonons underwent scattering of theoretical magnitude by the boundaries and the dislocations. For T > 2 K, some fraction of the phonons (at least the slow transverse mode) were still strongly scattered, even after long exposure to irradiation, while the remaining phonons were scattered primarily by the boundaries. At present, no theoretical model can explain, in LiF, the observed strong scattering of thermal phonons by dislocations.U of I OnlyThesi