Strategies for low-temperature sintering of BST ceramics with attractive dielectric properties

In this study, the powders of the Ba0.75Sr0.25TiO3 (BST) nanoparticles were directly synthesized by milling of Ba(OH)2·8H2O, Sr(OH)2·8H2O, and Ti(BuO)4 in ethanol at room temperature. They have homogenous grains of ~ 15 nm and high sintering activity. The dense ceramics with the density > 90% can be obtained at a sintering temperature of ≤ 950 °C by adding 3 wt% sintering aids of Bi2O3 and Li2CO3. Several Bi-related intermediate compounds act as perovskite-structured templates to sintering the ceramics at a different temperature. They enhance the mass transfer and promote the sintering densification. These compounds such as Ba2BiO4 and SrBiO4 appear at 800 °C, LiBa4Bi3O11 and Sr1.2Bi0.8O3 appear over 830 °C, and Bi8.11Ba0.89O13.05 appears at 950 °C. The cation Bi in them can have mixture valences of 3+ and 5+. It makes the ceramics as semiconducting state with the dark gray color and decreases the ceramic resistivities. With the sintering temperature increase, especially at 950 °C, the cation Bi tends back to single valence of +3 in the ceramics. The most of alkaline earth cations in Bi-related compounds will release and resorb into the lattice of BST and drive the sintering densification. The BST ceramics can have a peak dielectric constant > 6500 (at 53 °C) with loss < 0.025 at 10 kHz, and resistivity > 1012 Ω cm when sintered at a temperature of ≥ 900 °C with 3 wt% sintering aids. They have a potential application for multiple layer ceramic capacitors (MLCC) with silver inner electrodes.