Influence of bismuth oxide as a sintering aid on the densification of cold sintering of zirconia

In the past decades, Zirconia (ZrO₂) has emerged as a promising technical ceramic, both as high temperature structural material and electrolyte for fuel cells, etc. The traditional synthesis of ZrO₂ with spark plasma sintering (SPS) usually requires 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 research on two promising routes – cold sintering via temperature-dependent chemical reactivity and sintering aids, which facilitates mass transport and improves densification. Here we combine both by developing a single-step sintering process benefitting from both water vapor through the in-situ conversion of Zr(OH)₄ to ZrO₂ and liquid phase Bi₂O₃ as a sintering aid. The resultant ZrO₂ has a relative density above 80% with a sintering temperature as low as 900 °C, significantly higher than that of ZrO₂ without sintering aids, which had a relative density of 54%, both sintered at 50 MPa. The dependence of porosity of sintered samples as a function of sintering pressure (range: 50 MPa–300 MPa) and temperature (range 400 °C–1200 °C) is mapped out as guidance for further material property design. A linear relationship between hardness and relative density was found, with a maximal hardness of 6.6 GPa achieved in samples with 30% porosity. In addition to sintered density, phase stabilization of tetragonal ZrO₂ is enhanced at sintering temperature of 900 °C with water vapor and Bi₂O₃, respectively.