Effect of Powder Processing and Sintering Conditions on the Microstructural, Thermal, and Mechanical Properties of Reticulated Zinc Oxide Ceramic Foams

Abstract

Ceramic foams are made of zinc oxide using different amounts of Sb2O3 and Bi2O3 as sintering aids. The effect of a ball milling processing of the starting powders and the sintering temperature on the microstructure and the properties of the ZnO foams is investigated. The focus is set on the evolution of the secondary phases formed within the microstructure of ZnO. A determining effect is identified in the amount of an Al2O3 impurity which is introduced by abrasion of the milling vessels during ball milling. Alumina is partially dissolved in a spinel α–Zn7Sb2O12 secondary phase which is stabilized by a reduction of the unit cell volume. Remaining Al2O3 is incorporated into zinc oxide under formation of a defect wurtzite phase. The phase evolution is a complex function of the content of sintering aids, the Al2O3 impurity level and the sintering temperature. The shrinkage during sintering and the porosity evolution are correlated to the phase composition within the ZnO material. The thermal conductivity and the compressive strength of the foams are determined, normalized with respect to their porosity, and correlated to the microstructure and phase composition of the ZnO strut material.