Synthesis of Ba2Ti9O20 materials via a dissolution–precipitation mechanism in a hydrothermal process

A hydrothermal process has been successfully utilized for the preparation of Hollandite-like Ba2Ti9O20 precursors. Transmission electron microscopy in conjunction with chemical analyses of reacted powders indicate that Ti4+ species are first dissolved in the solution and then react with Ba2+ species to form perovskite-phased BaTiO3 in the hydrothermal process. An excessively large particle size for the starting TiO2 (anatase powders) results in insufficient Ti ions in the solvent and incomplete reaction with Ba2+ species, which leads to a Ba2+-deficient powder mixture. A residual TiO2 phase thus results after calcination. Only small TiO2 particles (40 nm) can result in sufficient Ti4+ species in the solution, which fully react with Ba2+ species and lead to a TiO2/BaTiO3 ratio of the correct stoichiometry to form Ba2Ti9O20. TiO2/BaTiO3 powder mixtures prepared in this way possess high activity and can be converted into pure Ba2Ti9O20 materials. After calcination and sintering processes, such materials possess high sintered density (∼93% TD) and good microwave dielectric properties (K = 36 and Q × f = 27,000).