Novel reaction mechanism for the synthesis of Ba₂Ti ₉O₂₀ materials prepared from nano-sized oxides

In this work, the phase evolution during calcinations of a BaTiO ₃ and TiO₂ mixture was investigated. Based on the observations, a model for the reaction sequence of the phases was proposed and a modified process was suggested, namely to synthesize Ba₂Ti ₉O₂₀ materials utilizing a BaTi₄O₉ and BaTi₅O₁₁ mixture, instead of a BaTiO₃ and TiO₂ mixture, as starting materials. The phase transformation kinetics is thus greatly enhanced by this process. While the reaction of the 2BaTiO₃+7TiO₂ mixture requires 1100 °C/4 h to transform completely into the Ba₂Ti₉O₂₀ Hollandite-like phase, it needs only 1000 °C/4 h to fulfil the phase transformation process in the BaTi₅O₁₁+BaTi ₄O₉ mixture. The BaTi₅O₁₁+BaTi ₄O₉ mixture also leads to higher sinterability and more uniform granular structure, as compared to the 2BaTiO₃+7TiO ₂ mixture, when these powder mixtures were densified by a direct sintering process. Furthermore, local microwave dielectric properties measured by an evanescent microwave probe (EMP) reveals that the grains of Ba ₂Ti₉O₂₀ materials prepared from the BaTi ₅O₁₁+BaTi₄O₉ mixture possess larger dielectric constant than 2BaTiO₃+7TiO₂ derived ones, indicating that the BaTi₅O₁₁+BaTi₄O ₉ mixture has higher reactivity for the formation of the Ba ₂Ti₉O₂₀ Hollandite-like phase.