Thermal conductivity of porous Alumina-20 wt% zirconia ceramic composites

Effect of porosity and temperature on thermal conductivity of the porous Alumina-20 wt% Zirconia (3 mol.% Y₂O₃) ceramic composites with and without niobia were investigated. The ceramic powders were synthesized by the sol-gel route using alkoxide precursors. The porosity in the composites was maintained in the range of 9.5–65 vol% using starch as a space holder material. After processing, samples were compacted uniaxially and sintered at 1873 K for 3 h. The thermal conductivity of porous ceramic composites with and without niobia dopant was measured at three different temperatures of 300, 473, and 673 K using laser-flash technique. The thermal conductivity of the samples was reduced with increasing temperature and porosity. At temperature of 300 K, the thermal conductivity value of 11 W/m.K was obtained for the undoped sample S0 with 17 vol% residual porosity, dropped to 2 W/mK for the sample S40 containing 65 vol% porosity, and for the same sample it was further reduced to the lowest value of 0.68 W/m.K at 673 K. The measured conductivity values were used to determine the grain boundary thermal resistance value (R) of the samples which exhibited an ascending trend with the porosity. The obtained thermal conductivity for the different porous composites was verified and formulated with the Maxwell-Eucken and Ticha models. The results showed that the experimentally measured conductivity values follow a descending order with the models while at the higher-porosity level (57–65 vol%), it fits well with the Ticha equation with only 9% and 4.6% deviation for undoped and doped samples, respectively. Results also revealed that the addition of niobia significantly reduced thermal conductivity at the lower porosity levels, but at higher porosity level the effect of porosity was more dominant.