Measurements of thermal conductivities of alumina powders and Min-K in vacuum [AMTEC applications]

Thermal conductivities of two types of alumina powders, having average particle sizes of 1.0 μm and 0.1 μm and effective porosities of 91.4% and 96.2%, respectively, and of Min-K AMTEC cell insulation materials were measured in vacuum. The thermal conductivities of all three materials increased with temperature, but at varying slopes. The developed thermal conductivity correlations were within ±10% and ±5% of the data of the alumina powders and the Min-K, respectively. The Min-K thermal conductivity was the lowest and least dependent on temperature, increasing from 0.03 to 0.042 W/mK as the temperature increased from 400 K to 800 K, respectively. Analysis showed that the thermal conductance of the Min-K could be an order of magnitude lower in the axial direction, but as much as 2 to 3 orders of magnitude higher that those of 60 and 240 Molybdenum foils insulation in the perpendicular direction, respectively