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A method to determine anisotropic thermal diffusivities parallel to the surface of a solid sample is presented. The measurement method is based on the mirage effect and the data are analyzed using multiparameter least-squares regression fitting. The specimens studied were of medium and low diffusivity: rare earths, ceramics, and polymers. Before this study, the low-diffusivity limit for the applicability of the mirage method was about 0.02 cm2/s, and therefore this technique has not been suitable for polymeric materials. In this work the limit has been reduced to 5×10−4 cm2/s by improving the measurement apparatus and the data analysis method. The thermal diffusivities obtained agree with the values obtained using the flash method within ±20%. The agreement with published values is good, taking into account that for ceramics and polymers the literature gives only the correct order of magnitude due to the differences in the manufacturing process. Even though the sensitivity of the measurement decreases with the decreasing diffusivity, the thermal diffusivity of bulk polymers can still be determined within an accuracy of 10%. For polymer foils the method has been found to be reliable when the foil thickness exceeds 100 μm.
Rantala et al. (Mon,) studied this question.