Revisiting transient heat transfer in coated transparent media

Measurements of transparent materials are required to assess their conductive properties for industrial applications. This is typically done with a laser flash analysis (LFA) instrument, where a transient temperature response is analysed. Due to the multi-modality of heat transfer, the conductive contribution must be separated from e.g. the radiative transfer. Depending on the required complexity, this can be done either with a multi-parametric model or, in the diathermic case, with a model based on just one extra parameter, η. Unfortunately, this parameter has not been quantified based on any valid physical description. Furthermore, the simplified model solved analytically occasionally produced unexpected results, suspected to originate from the specific implementation. A correct treatment for the diathermic model is given in this paper, both in the analytical and numerical cases. Tests with polycrystalline alumina (PCA) and sapphire on two LFA setups were made to delineate the limits of applicability of the diathermic model. Finally, a revised version is proposed for both the diathermic and the coupled model to include convective losses and rear-face heating due to flashthroughs or circumferential heat fluxes