SURFACE DECONTAMINATION TREATMENTS OF UNDOPED BATIO3 .2. INFLUENCE ON SINTERING

Natural sintering and microstructure of undoped BaTiO3 ceramics obtained by slip casting from five commercial powders subjected to various surface decarbonation treatments (calcination and cleaning in pure water or acetate buffer solution) were investigated. Two densification steps followed in certain instances by a desintering step were observed for as-received powders. At maximum density the microstructure is either coarse-grained or bimodal, with dense large grains between 20 and 100 mu m in a fine-grained matrix (<1 mu m). At the end of swelling a coarse intergranular and fine intragranular porosity is present and total conversion into large grains has occurred their size remaining unchanged up to 1400 degrees C. Calcination and water cleaning had no noticeable influence on sintering behaviour. Acid cleaning appeared to substantially modify the shape of dilatometric curves: shrinkage starts later, the onset of abnormal grain growth is delayed, the maximum fired densities ave slightly increased, but the subsequent swelling of specimens above 1300 degrees C is also enhanced. It is suggested that abnormal grain growth in undoped barium titanate results from the presence of a liquid phase at grain boundaries. Due to the presence of trace impurities, it can form locally at a temperature below the eutectic temperature of 1317 degrees C of the system BaO-TiO2. Decontamination by acid cleaning raises the eutectic point, but also increase-the amount of liquid phase above 1300 degrees C, by lowering the Ba content at the surface of the particles. The plasticity of the material is then increased, promoting swelling due to internal CO2 pressure, originating from the decomposition of BaCO3 which still subsists after acid cleaning.