Photothermal Mirror Method for the Study of Thermal Diffusivity and Thermo-Elastic Properties of Opaque Solid Materials

We have carried out the theoretical and experimental time evolution and amplitude study of the photothermal mirror signal generated by focusing a laser beam on the surface of a suite of solid samples. Based on a theoretical model that resolves the thermal diffusivity equation and the equation for thermo-elastic deformations simultaneously, we have calculated the transient time evolution and amplitude of the signal. We observe the same time evolution pattern for samples as diverse as glass, quartz, metals, and synthetic ceramic oxides. The data have yielded a linear dependence between the time build-up of the thermal mirror and the inverse of the thermal diffusivity for all the samples. For moderate power levels, we also observe a linear behavior between the stationary value of the signal and the thermally induced phase shift value. From the calibration curves, we have determined the thermally induced phase and the thermal diffusivity coefficients of two prospective nuclear reactor control rod materials, dysprosium titanate (Dy2TiO5) and dysprosium dititanate (Dy2Ti2O7) to be D = (7.0 +/- 0.4) x 10(-7) m(2) . s(-1).