Thermal diffusivity of metals determined by lock-in thermography

Lock-in infrared thermography is a well-established technique for measuring the thermal diffusivity of materials. A commonly-used method is to analyse the spread of a thermal wave along the surface of a thin sample or a filament, known as the slope method. In this work, an alternative technique is proposed, in which the phase delay of the thermal wave passing through sheets made of different metals is analysed using periodical laser heating. Different lock-in frequencies are used to obtain the frequency-dependent phase behaviour of thermal waves generated by the laser pulses. A subsequent curve fitting provides the coefficients required to determine the thermal diffusivity. To achieve a comparison with ambient conditions, the measurements are also performed in vacuum. A comparison with literature data substantiates that the proposed method is a robust approach for the determination of the thermal diffusivity in applications where the sample thickness exceeds the limits of the slope method.