Effects of finite absorption depth and infrared detector nonlinearity on thermal diffusivity measurement of thin films using the flash method

The flash method including the single- and double-ended method has gained widespread acceptance for measuring thermal diffusivity of thick foils (in millimeters) as well as thin films (in microns). However when the method is employed, some basic experimental conditions are assumed. In this paper, two of the assumptions, the finite absorption depth effect and the nonlinearity of the detector, are discussed in the situation of thin film samples. For the first one, the deviation of the factor ω1/2 (=π2αt1/2/L2) from 1.37 and the corresponding errors in deriving thermal diffusivity from t1/2 are discussed for various relative absorption depth δ. The result indicates criteria for the method to be available, that is, L≳10δ and L≳14δ for the double- and the single-ended method. For the second one, by considering the errors in voltage output of a (Hg, Cd)Te IR detector, how the factor ω1/2 deviates from 1.37 and the corresponding errors in thermal diffusivity measurement under various initial temperature conditions are discussed. The results are shown graphically and tabulated.