ZrO2 ceramics usually have nonuniform microstructures due to the agglomeration of fine ZrO2 powders. In this paper, the thermal conductivity of porous ZrO2 ceramics with different microstructures was investigated by the laser-flash method. It was found that the thermal conductivity of porous ZrO2 ceramics with uniform microstructure was clearly lower than that of ceramics with nonuniform microstructure. Scanning electron microscopy observations showed that porous ceramics with nonuniform microstructures have wider pore and grain size distributions, relative to those with uniform microstructures. Theoretical analyses were done to model the thermal conductivity of porous ceramics using an effective medium approach. These analyses revealed that matrix grain size distribution has a significant impact on the thermal conductivity of porous ceramics, while the effect of pore size istribution is negligible.

Porous ZrO2 ceramics were fabricated by adding Zr(OH)4 hard agglomerates to ZrO2 powder, followed by pressureless sintering. The mechanical properties of porous ceramics sintered from pure ZrO2 powder were poor. The addition of Zr(OH)4 increased the strength and fracture toughness of the porous ZrO2 ceramics for sintered specimens containing lower porosity. However, the Young’s modulus had little change so that the strain to failure of porous ZrO2 ceramics increased with the incorporation of Zr(OH)4. Scanning electron microscopy (SEM) observations revealed that microstructures of the green compacts prepared from pure ZrO2 powder were nonuniform due to the ZrO2 soft agglomeration, which resulted in a localized nonuniform shrinkage during densification. The localized nonuniform shrinkage led to a weaker grain bonding and degraded the mechanical properties of porous ZrO2 ceramics. In this work, we found that this microstructure nonuniformity could be eliminated by the addition of Zr(OH)4, because the bimodal particle size distribution confined the formation of ZrO2 soft agglomerates due to a space constraint and an internal friction between the Zr(OH)4 hard agglomerates during compaction. As Zr(OH)4 decomposed into ZrO2 grains during heating, the Zr(OH)4 hard agglomerates disappeared before sintering occurred. The present study indicates that Zr(OH)4 hard agglomerate is a unique agent to improve the mechanical properties of porous ZrO2 ceramics.

Bulk porous Al2O3 support was fabricated using a mixture of fine α-Al2O3 powder and Al(OH)3 particles, followed by pressureless sintering at temperatures between 1100°C and 1300°C. Al2O3 support with high surface area was obtained, due to the presence of transitional Al2O3 phases that were produced by the decomposition of Al(OH)3 even after sintering. The Al2O3 support exhibited superior mechanical properties and high strain to failure, compared with those fabricated by traditional methods. The strain to failure increased with the amount of Al(OH)3 in the mixture, but decreased with increasing sintering temperature. A conceptual model for the microstructural evolution and reinforcing mechanisms, based on transmission electron microscopy (TEM) observations, is proposed. It reveals that the interface bonding of the Al2O3 boundaries formed at the initial nucleation stage during θ- to α-Al2O3 transformation is stronger than that formed by subsequent grain growth. The interface bonding between the Al2O3 grains, which came from the decomposition of Al(OH)3, is stronger than that between the original Al2O3 grains in the starting mixture.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.