Pulse method of measuring thermal diffusivity and optical absorption depth for partially transparent materials

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Abstract
Data analysis in the laser-pulse method of measuring thermal diffusivity is based on the solution of the heat diffusion equation, assuming several boundary and initial conditions. The initial condition generally results from considering the material opaque to the laser’s radiation. This requires building special absorbing layers on the sample’s surface when the material is transparent. A more general solution of the heat diffusion equation was obtained in this paper, in order to simplify sample preparation, and in some cases (high temperatures, sample contamination, etc.) even make possible the measurement of partially transparent materials. Based on an exponential-like absorption of the laser’s radiation, the solution includes an absorption-depth parameter p, which describes the material’s degree of transparency. A computer-programmable data-reduction procedure, including a least-squares estimation algorithm of nonlinear parameters, was also developed. It permits the simultaneous determination of both the thermal diffusivity and the absorption depth from a single laser-pulse experiment. The accuracy of the method was verified by measuring a sintered glass sample, both with the usual and the new procedures. The results indicate an accuracy level compatible with the current state of the art.

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