PHOTOTHERMAL MEASUREMENT OF THERMAL ANISOTROPY IN PYROLYTIC-GRAPHITE

We investigated the anisotropic thermal conductivity in pyrolytic grap hite by thermoreflectance. A laser-heated circular spot on a surface p erpendicular to the planes developed into an elliptical temperature di stribution which was recorded by a raster scanning technique at modula tion frequencies ranging from 600 Hz to 100 kHz. The ratio of in-plane and perpendicular thermal conductivity was determined by fitting the phase of the temperature data with an analytical model, and was found to decrease with increasing modulation frequency. Highest conductivity values were considerably smaller than previously published data based on steady-state measurements. The frequency dependence and additional features in the phase profiles at high frequencies are discussed in v iew of sample surface preparation and the local nature of the thermore flectance measurement.