Thermal Conductivity of Powder

The thermal conductivity of powder was studied by measuring the temperature rise of a linear heat source burned in samples. The materials tested were several kinds of sand for casting and five kinds of glass beads having slightly different diameters.The results obtained are as follows.(1) The thermal conductivity of powder becomes smaller as the particle size becomes smaller and as the porosity of the sample becomes larger.(2) The thermal conductivity of powder decreases with decreasing air pressure. The thermal conductivity vs. air pressure curve can be divided into three parts. In the pressure ranges lower than a few Torr and in the neighbour of atmospheric pressure, the thermal conductivity of powder doesn't vary so sensitively with the change of air pressure. But in the intermediate region of the above two regions, the thermal conductivity k varies with air pressure p as given byk=Alogp+Bwhere A and B are the constants depending on a particle diameter, powder materials and porosity.(3) When the packing density is constant, the smaller the particle diameter is, the smaller the thermal conductivity is. If the particle diameter is extremely small, it seems that the thermal conductivity becomes independent of the powder materials.(4) The smaller the pore spaces in powder are, the larger the thermal insulation effect becomes. Therefore, as the powders for vacuum thermal insulation, it is necessary that their particle diameters and pore spaces are small.(5) To explain the reason why the thermal conductivity of porous materials varies with pressure, the following model was proposed. Solid parts and pore spaces are arranged layer by layer and the intervals of pore spaces are distributed in a wide range. In the pore spaces where their intervals are smaller than the mean free path of gas molecules, the behavior of gas molecules is controlled by the molecular flow, but in the other pore spaces it is by the viscous flow. These two regions controlled by the molecular and viscous flows, change depending on the environmental pressure, and thus the thermal conductivity of porous materials varies with pressure.