Preparation of dense and phase-pure Ba2Ti9O20 is generally difficult to achieve using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition. This study investigated the effects of B2O3 on the densification, microstructural evolution, and phase stability of Ba2Ti9O20. Samples from the host material (2BaO · 9TiO2) with and without the addition of 5 wt% B2O3 were prepared through different processing routes. For the pure host material sintered at temperatures ranging from 800 to 1100 °C, the reaction products followed the sequence of BaTi2O5 → BaTi4O9 → BaTi5O11 → Ba2Ti9O20. The phase transformation proceeded faster in the bulk compared to the free surface of the sample. BaTi5O11 and BaTi4O9 with a minor amount of Ba2Ti9O20 were found in the ground powder of ceramics sintered at 1100 °C. For the samples prepared from host material with the addition of 5 wt% B2O3, Ba2Ti9O20 started to form at temperatures as low as 800 °C. The sequence of reaction products followed Ba4Ti13O30 → BaTi4O9 → BaTi5O11 → Ba2Ti9O20. Sintering at above 1000 °C yielded pure Ba2Ti9O20 phase, suggesting the effective role of B2O3 on the phase stability of Ba2Ti9O20. It was found that precalcination of host material before the addition of B2O3 gives an additional benefit to the Ba2Ti9O20 formation. Crystallization of pure Ba2Ti9O20 phase was completed at a sintering temperature as low as 900 °C without any solid solution additive such as SnO2 or ZrO2, due to the fact that the phase transformation of the samples began with BaTi4O9 and BaTi5O11 during sintering. Also, B2O3 was found to be unstable during the high-temperature sintering at 1200 °C, and the results are discussed.

Preparation of dense and phase‐pure Ba2Ti9O20 is generally difficult using solid‐state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba2Ti9O20 equilibrium phase boundary in the BaO–TiO2 system at high temperatures. In this study, the effects of B2O3 on the densification, microstructural evolution, and phase stability of Ba2Ti9O20 were investigated. It was found that the densification of Ba2Ti9O20 sintered with B2O3 was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid‐state sintering became dominant because of the evaporation of B2O3. With the addition of 5 wt% B2O3, the ceramic yielded a pure Ba2Ti9O20 phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO2 or ZrO2. The facilities of B2O3 addition to the stability of Ba2Ti9O20 are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.

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