Thermal stability of hollow YSZ microspheres and processing of their porous materials

Hollow yttria-stabilized zirconia (YSZ) microspheres with a diameter of 1.2–5.1 μm and a wall thickness of about 125 nm were synthesized, and their thermal stability was revealed for a hold time of 1 h at temperatures of 800°C, 1000°C and 1200°C. The microspheres were then used as raw materials to prepare bulk porous ceramics by gelcasting processing. The effects of the solid loading contents and sintering temperatures of the hollow YSZ microspheres on the microstructures, porosity, compressive strength and room-temperature thermal conductivity of the porous ceramic materials were investigated. Increases in the solid contents and sintering temperatures were found to decrease the bulk porosity and number of larger pores. Porous materials made with hollow spherical powders exhibited porosities ranging from 61.50% to 90.17%, with a peak value of 90.17% obtained under a solid loading of 5 vol% at a sintering temperature of 1100°C. The experimental thermal conductivity as a function of porosity was within the ranges predicted by one form of the Maxwell-Eucken model (Maxwell-Eucken 1) and the effective medium theory (EMT), reaching the lowest value of 0.052 W/(m·K) at a porosity 90.17%.