Novel reaction mechanism for the synthesis ofBa2Ti9O20 materials prepared from nano-sized oxides

Abstract

In this work, the phase evolution during calcinations of aBaTiO3 and TiO2 mixture was investigated. Based on the observations, a model for the reaction sequence ofthe phases was proposed and a modified process was suggested, namely to synthesizeBa2Ti9O20 materialsutilizing a BaTi4O9 and BaTi5O11 mixture,instead of a BaTiO3 and TiO2 mixture, as starting materials. The phase transformation kinetics isthus greatly enhanced by this process. While the reaction of the2BaTiO3+7TiO2 mixture requires1100 °C/4 h to transform completelyinto the Ba2Ti9O20 Hollandite-likephase, it needs only 1000 °C/4 hto fulfil the phase transformation process in theBaTi5O11+BaTi4O9 mixture.The BaTi5O11+BaTi4O9 mixture also leads to higher sinterability and more uniform granular structure, as compared to the2BaTiO3+7TiO2 mixture, when these powder mixtures were densified by a direct sinteringprocess. Furthermore, local microwave dielectric properties measuredby an evanescent microwave probe (EMP) reveals that the grains ofBa2Ti9O20 materialsprepared from the BaTi5O11+BaTi4O9 mixture possess larger dielectric constant than2BaTiO3+7TiO2 derived ones,indicating that the BaTi5O11+BaTi4O9 mixture has higher reactivity for the formation of theBa2Ti9O20 Hollandite-like phase.