The pre-reaction step, which is the holding time at 1000°C in reaction-sintering process, was observed to influence the characteristics of the Ba2Ti9O20 materials profoundly. For the materials possessing high sintered density and of pure Hollandite-like structure, the uniformity of granular structure is the prime factor, altering the microwave dielectric properties of the samples. Utilizing the BaTi4O9 + BaTi5O11 mixtures to replace for the 2BaTiO3 + 7TiO2 ones pronouncedly improved the uniformity of the microstructure and thus markedly increased the Q × f-value for the Ba2Ti9O20 materials. The possible explanation for such a phenomenon is that the B-series mixture forms the Ba2Ti9O20 Hollandite-like phase in a simpler way and results in more uniform microstructure then the A-series mixture. The microwave properties of B-series materials are superior to those of A-series ones, achieving K = 38.2 and Q × f-value = 36,000 GHz for the samples, which were pre-reacted at 1000°C for 6 h and were sintered at 1410°C for 4 h by reaction-sintering processes.
Journal of the European Ceramic Society 30(2), pp.159–163