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 Si 3N 4 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 W m −1 K −1). The thermal anisotropy inside individual Si 3N 4 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.