Co-precipitation process was observed to be able to synthesize the Hollandite Ba2Ti9O20 phase without the necessity of doping the SnO2 additives. The BaTi5O11 and BaTi4O9 phases, which still form preferentially during the calcinations of the co-precipitants, can be converted into Hollandite phase by heat treatment at 1250 °C and higher temperature. The reactivity of the powders can be further improved by using ultrasonic spraying process to improve the co-precipitation process. The heat treatment temperature necessary to fully transform the co-precipitants into Hollandite phase decreased from 1250 °C for conventional–coprecipitation-derived powders to 1150 °C for spraying–coprecipitation-derived ones. The sinterability of the powders and microwave dielectric properties of the sintered Ba2Ti9O20 materials were also pronouncedly improved. High density Ba2Ti9O20 ceramic materials possessing good microwave dielectric properties can be obtained by sintering the 1000 °C-calcined powders at 1300 °C (4 h) or higher temperature. The best properties obtained are K=39 and Q×F=21,450, for a 1300 °C sintered samples.
Journal of the European Ceramic Society 24(6), pp.1781-1785