Preparation and thermal conductivity characterisation of highly porous ceramics - Comparison between experimental results, analytical calculations and numerical simulations

Two sets of highly porous materials with different intrinsic thermal conductivity values for the solid skeleton were prepared: zirconia and alumina. Microstructural characterisation was performed, including the determination of the pore size distribution (PSD). The pore volume fraction was varied from 40 to 75%. Zirconia samples exhibit heterogeneous microstructures with a bimodal PSD. Alumina samples seems to be more organised with a monomodal PSD. The effective thermal conductivity was evaluated using the laser-flash technique. The experimental values were compared with predictions made with analytical models as a function of porosity. In particular, effective medium percolation theory (EMPT) for a two-phase system gives calculated values which are close to the experimental results for zirconia samples when the bimodal PSD was taken into account. Measurements on alumina samples have shown that EMPT is of limited use for an organised microstructure with porosity exceeding 65%. Complementary studies by computer simulation were also used to support some of the deductions which were made concerning the influence of the microstructure organisation on the effective thermal conductivity. (c) 2006 Elsevier Ltd. All rights reserved.