Thermal conductivity of β-Si3N4:: I, effects of various microstructural factors

Calculations based on a simple modified Wiener's model for thermal conductivity of a composite material predict that the thermal conductivity of beta-Si3N4 decreases quickly as the grain-boundary film thickness increases within a range of a few tenths of a nanometer and also that it initially increases steeply with increased grain size, then reaches almost constant values. Because of the faceted nature of the beta-Si3N4 crystal, the "average" grain-boundary film thickness is much greater than that in equilibrium. The present study demonstrates both theoretically and experimentally that grain growth alone cannot improve the thermal conductivity of beta-Si3N4.