The influence of grain boundaries on heat transfer through polycrystalline oxides has been investigated. The approach is based on comparison of macroscopic thermal conductivity values for materials which have different grain sizes. Samples of alumina, magnesia, and tin oxide were made in the form of discs by standard ceramic processing of pressed powders. By control of the firing conditions, strong variations in the grain size and pore content were achieved in all three materials. The thermal conductivity was measured via the thermal diffusivity by the laser-flash technique. The average grain-boundary thermal resistances at room temperature in dense ceramics were deduced to be 0.9 * 10~(-8) m~2 K W~(-1) for alumina, 0.7 * 10~(-8) m~2 K W~(-1) for magnesia, and 1.2 * 10~(-1) m~2 KW~(-1) for tin oxide. Moreover, the grain-boundary thermal resistance in tin oxide is shown to be virtually constant in the temperature range 20-400 ℃. An alternative method was used to confirm these values, based on the measurement of the thermal conductivity as a function of temperature. The role of the effective heat-carrying cross section for a grain-boundary plane in a porous ceramic is also discussed.
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