Non-destructive evaluation of thermal diffusivity distributions of functionally graded materials by photothermal radiometry

We have developed a new apparatus to evaluate thermal duffusivity and thermal effusivity distributions of functionally graded materials (FGMs) by photothermal radiometry (PTR). Since this method is applicable to the low modulation frequency range down to 1 mHz, it is suitable for the measurement of thermally thick samples such as FGMs. We have solved heat conduction equations for a multi-layered model applicable to the present PTR measurement. The effects on the phase lag measurement caused by three-dimensional heat flow in the low frequency range have also been analysed. For solving the inverse problem of extracting thermophysical properties from phase lag versus frequency data, we have proposed an analytical method in which the distribution of thermal diffusivity is approximated by an exponential function. The distributions of thermal diffusivity and thermal effusivity have been determined for a four-layered Ni/ZrO₂ FGM at room temperature.