Reaction mechanism of BaTiO3 from powder compacts, BaCO3 and TiO2 and expansion phenomena during the formation process

Abstract

The formation process of BaTiO3 from powder compacts, equi-molar mixture of BaCO3 and TiO2, and an abnormal expansion during the formation process were investigated. Powder compacts were fired for 30 min in air atmosphere at various temperatures, up to 1200°C. The formation process of BaTiO3 is divided into three steps; firstly, BaTiO3 is formed at the contact points between BaCO3 and TiO2 particles, secondly, BaTiO4 is formed by the reaction of the prior-formed BaTiO3 with BaCO3, finally, BaTiO3 is formed by the reaction of residual TiO2 and Ba2TiO4. In the macroscopic observation, it was confirmed that a powder compact of BaCO3 on magnesia and rutile substrate begins to melt at 950°C. This liquid phase is thought to influence the formation reaction of BaTiO3 greatly. It is well known that Ba component diffuses to TiO2 one-sidedly in this reaction. As a result, the space where BaCO3 occupied in a compact specimen remains as a pore after the termination of reactions. Then, an increase in the volume happens by TiO2's changing in BaTiO3. These are considered to be the cause of an abnormal expansion caused by the reaction of BaCO3 and TiO2.