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 degrees 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 degrees 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 degrees C-1200 degrees 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 degrees C with water vapor and Bi2O3, respectively.

Automated summary

In this study, a single-step sintering process was developed to prepare zirconia material with high relative density and low sintering temperature by combining water vapor and Bi2O3 sintering aid. The influence of sintering pressure and temperature on sample porosity was investigated, and a linear relationship between hardness and relative density was found.