| High temperature thermal diffusivity measurements were utilized in order to ascertain information about the nature of phonon scattering mechanisms in Si3N4. The process isolated the contribution from intrinsic and extrinsic phonon scattering mechanisms. The analysis was run on experiments designed to determine the effect of the amount of secondary phase, the composition of secondary phase, the effect of microstructural differences, and the effect of solid solution.; Samples were fabricated used sintering aids of Y2O3 (Y) or a 1:1 ratio of Y2O3 and Nd2O 3 (YN). Compositions were either gas pressure sintered or hot isostatic pressed for densification. Microstructural changes were imparted by ball milling a 4mol% (Y) slip for 4, 8, 16, 32, 64, and 128 hours with zirconia milling media. Zirconia was used in order to avoid alumina contamination. Other compositional ranges tested were 3.5–6.5 vol% YN, and 1–6mol%Y and 1–6mol% YN. Solid solution was examined by the use of commercial SiAlON materials with the empirical formula ranging in z value from 0 (pure Si 3N4) to 2.; The intrinsic phonon scattering was not affected by microstructure, sintering aid composition, sintering aid content, degree of secondary phase crystallization, or, within the ranges measured, crystalline phase. Assuming representative values for the phonon velocity and the Debye temperature, an intrinsic mean free path of 18.57 ± 1.0nm at 21°C was determined. This translates into a theoretical diffusivity of 0.405cm2/s, or 90W/mK, in rough agreement with the highest values quoted for isotropic, polycrystalline Si3N4.; The intrinsic scattering was affected by changes in the solid solution content of the material. Addition of Al2O3 degrades the lattice conductivity by both intrinsic and extrinsic means. The increase in intrinsic scattering is believed to arise from the increased number of phonon modes. This resulted in a mean free path of 11.12nm for the sample with z value equal to 2. The increase in extrinsic scattering is believed to arise from the mass and volume differences between the host and foreign atoms. This was a more effective scattering mechanism than the intrinsic, with a mean free path at z = 2 of 1.25nm. |
