Thermal response of two-layer systems:

Heat transfer through a two-layer system constituted of different materials has been studied. We focus on the overall apparent thermal conductivity in the direction perpendicular to the planes of the layers and consider the case of two materials, alumina and glass, which exhibit a strong difference in individual thermal conductivity. Numerical simulations using finite element analysis have been performed for two situations: transient response corresponding to characterisation of the thermal conductivity by the laser-flash experiment and steady-state response corresponding to the behaviour of two resistors in series. The calculations show that the two methods give similar results up to a volume fraction equal to 0.5 of the alumina phase. However, for alumina volume fractions above 0.5 the laser-flash experiment overestimates the overall thermal conductivity predicted by the steady-state series model. This is explained by a limit to the validity of a homogeneous approximation to the two-layer heterogeneous system, typically used in the analysis of laser-flash data. Experimental results, obtained on alumina/glass two-layer samples of varying relative thicknesses, support these deductions, though the thermal resistance of the contact between the layers should also be taken into account.