Laser flash method was succeeded in thermal diffusivity t, and with the heat capacity and density, the thermal conductivity can be calculated out. But it is difficult to apply this method in the thermal diffusivity measurement of semitransparent materials, especially for those materials which were semitransparent both in the infra-red wave length of laser beam and detector. In this paper, a new method has been made up to solve this problem. The diffusivity of polyimide （PI） film was measured in the temperature range of - 73 ℃ -290 ℃ and the thermal diffusivity of PI/SiO2 composite films with different content of SiO2 were researched also.
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