A double–inverse microemulsion (IME) process is used for synthesizing nano-sized Ba2Ti9O20 powders. The crystallization of powders thus obtained and the microwave dielectric properties of the sintered materials were examined. The IME-derived powders are of nano-size (∼21.5 nm) and possess high activity. The BaTi5O11, intermediate phase resulted when the IME-derived powders were calcined at 800°C (4 h) in air. However, high-density Ba2Ti9O20 materials with a pure triclinic phase (Hollandite like) can still be obtained by sintering such a BaTi5O11 dominated powders at 1250°C/4 h. The phase transformation kinetics for the IME-derived powders were markedly enhanced when air was replaced by O2 during the calcinations and sintering processes. Both the calcination and densification temperatures were reduced by around 50°C compared with the process undertaken in air. The microwave dielectric properties of sintered materials increase with the density of the samples, resulting in a large dielectric constant (K≅39) and high-quality factor (Q×f≅28 000 GHz) for samples possessing a density higher than 95% theoretical density, regardless of the sintering atmosphere. Overfiring dissociates Ba2Ti9O20 materials and results in a poor-quality factor.
Journal of the American Ceramic Society 88(12), pp.3405-3411