Measuring the anisotropic thermal diffusivity of silicon nitride grains by thermoreflectance microscopy

High-resolution thermoreflectance microscopy measurements were performed at five frequencies on rod-shaped Si3N4 grains in a ceramic. Our heat diffusion model takes account of the coating and of a coating/substrate resistance. The parameters are adjusted to fit the measurements at the five frequencies simultaneously. The principal diffusivities obtained in individual grains are 0.32 cm(2) s(-1) along the a-axis, and 0.84 cm(2) s(-1) along the c-axis (corresponding conductivities: 69 and 180 Wm(-1) K-1). The thermal anisotropy inside individual Si3N4 grains is found to be intrinsic, without direct connection with their elongated shape. 'Macroscopic' diffusivities, obtained by mirage effect, are different from the values measured inside individual grains, as a consequence of the dispersion of the grains' orientations in the ceramic and of a second-phase effect. (C) 1999 Elsevier Science Limited. All rights reserved.