Measuring the anisotropic thermal diffusivity of silicon nitride grains bythermoreflectance microscopy

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