Influence of Y2O3 on the structure of Y2O3-doped BaTiO3 powder and ceramics

Barium titanate (BaTiO3) doped with rare-earth elements (REE) is used as dielectric in the manufacture of multilayer ceramic capacitors (MLCCs). The most common REE oxide employed as dopant for this application is Y2O3. The behavior of the Y3+ in the BaTiO3 structure depends on its concentration and the sintering conditions, among other factors, which can induce the formation of secondary phases that are a potential cause a detriment to the electrical properties of BaTiO3. The purpose of this work is to perform a phase characterization of BaTiO3 doped with different concentrations of Y2O3, validating its possible contribution to the formation of secondary phases. The role of Y2O3 was evaluated on two kinds of raw materials. The first one is pure BaTiO3 (< 100 ppm Y) and the second kind is a commercial formulation designed for MLCCs known as X7R (-55°C and 125°C, 15% tolerance), which among other elements, already contained 1 wt% of Y2O3. High concentrations of Y2O3 (1% up to 20 wt%) were used aiming to promote structural changes, and even the formation of secondary phases in amounts suitable to be detected by X-ray diffraction. Heat treatment of powder and sintering of ceramics (powder compacted at 2 MPa) were conducted in air (1310°C in air for 3 h, two steps: 1350°C then 1150°C 15 h). A phase transition from tetragonal to a mixture of tetragonal and cubic was observed as Y2O3 concentration increases in the thermally treated powder and in the corresponding ceramics. Commercially formulated powder showed higher densification than pure BaTiO3, and produced cubic structure at higher Y2O3concentrations. The phase Ba6Ti17O40is detected in the 20 wt% Y2O3-doped sample.