Low-Temperature Spark Plasma Sintering of Zirconium Oxide through In-situ Conversion from Zirconium Hydroxide

Abstract

Zirconia (ZrO2) has emerged as a promising technical ceramic, electrolyte for solid fuel cells, and as a topcoat for thermal barrier coatings at high temperatures, etc. in the past decades. The traditional synthesis of ZrO2 usually necessitates a sintering temperature as high as 1200°C. General interest in lowering the sintering temperature to reduce energy consumption and thermal stresses has led to many research works. In this thesis, a novel route of sintering ceramics at lower sinter temperatures named ‘Cold sintering’ was adopted to sinter Zirconia bulks. Sintering is performed with the Spark Plasma Sintering (SPS) technique through in-situ chemical conversion of Zirconium hydroxide [Zr(OH)4] precursors into Zirconium oxide. The sintering process is accelerated by the water vapor emission from the conversion reaction. For the commercial applications of Zirconia, its density, hardness, and the stabilization of favourable tetragonal phases Is necessary. Many methods were adopted to increase the densification of the powders. The applied mechanical pressure is increased, and sintering aids are used to densify the sintered bulks. Mechanical properties like hardness and thermal conductivity values are measured. The dependence of relative densities, hardness, and thermal conductivity on the sinter conditions such as sintering dwell pressure (range: 50 MPa – 300 MPa) and sintering dwell temperature (range 400 °C – 1200 °C) is mapped out as guidance for further material property design. In addition to the improved sintered density, the phase stabilization of tetragonal ZrO2 phases is also enhanced at sintering dwell temperature of 900°C and dwell pressure of 50 MPa using the aliovalent phase stabilizing compounds Yttrium oxide (Y2O3) and Bismuth oxide (Bi2O3).