Measurement of the thermal diffusivity by the laser-flash method with repeated pulses

The measurement of thermal diffusivity by the laser-flash method with repeated pulses is examined. The aim of the proposed experimental method is to present a new approach to overcome particular experimental difficulties associated with the measurement of some types of insulators, temperature-sensitive materials, and large-grain heterogeneous materials by the standard laser-flash method. Unlike the known approaches, based on reducing the energy intensity and simultaneously increasing the exposure time (the flash method with extended pulse), or substituting step (continuous) heating for pulse irradiation (the step-heating method), respectively, in the technique presented the pulse energy consists of several consecutive pulses periodically applied to the sample front face. The thermal diffusivity is calculated from the resulting temperature rise of the rear face, in a similar way to the standard one-pulse' laser-flash method. The mathematical basis of the flash method with repeated instantaneous pulses as well as with repeated extended square-wave-shaped pulses is presented. The simple adiabatic mathematical model as well as the more realistic non-ideal theory that considers heat loss from the sample is introduced here. The derived theory is the basis for the sensitivity analyses and the data reduction process. The rebuilt experimental apparatus is described in detail and the results of the thermal diffusivity measurements on samples made of austenitic steel and TiAl alloys are presented, compared, and discussed.