Thermal conductivity of pressed powder compacts: tin oxide and alumina

Three aspects, which significantly reduce heat transfer through a polycrystalline material, are considered in this paper: porosity, grain boundary thermal resistance and the state of the grain–grain contacts. Tin oxide and alumina were chosen as model systems. Tin oxide, without a sintering additive, does not densify during thermal treatment but grain growth is not inhibited and consequently the microstructure can be varied. In alumina, variation of the thermal treatment conditions varies both grain size and porosity. Thermal conductivity measurements, using the laser-flash technique, reveal that the thermal resistance of a pressed powder compact is almost independent of temperature and at least a factor of 2.5 greater than a consolidated material with similar pore volume fraction and grain size. The reduced contact area of the grain–grain interfaces in the green body can explain this as demonstrated by numerical simulation. We also show that larger grain size increases the thermal conductivity of the porous ceramic.