The effects of the milling process on the characteristics of Ba2Ti9O20 materials were investigated. The chemical analyses using transmission electron microscopy (EDAX in TEM) revealed that the SiO2 species incorporated into the Ba2Ti9O20 materials were expelled by the Ba2Ti9O20 grains. It induced the dissociation of the Ba2Ti9O20 materials near the grain boundaries and degraded the microwave dielectric properties of the materials. The same phenomenon was assumed to be the procedure by which the high-energy-milling (HEM) process using Si3N4 grinding media (Si3N4-HEM) deleteriously influenced the microwave dielectric properties for the Ba2Ti9O20 materials. Utilizing the three-dimensional-milling (3DM) process in place of the Si3N4-HEM one markedly improved the characteristics of the Ba2Ti9O20 materials. The 3DM-processed samples own the same crystallinity as the HEM-processed ones but possess a pronouncedly more uniform microstructure and, therefore, exhibit a superior quality factor [(Q × f)3DM = 28 500 GHz and (Q × f)HEM = 21,900 GHz] with the same large dielectric constant (K = 38–39), when sintered at the same conditions (1350 °C/4 h). Such a phenomenon is ascribed to the fact that the 3DM process can pulverize the powders efficiently but induce no SiO2-contamination.
Journal of Physics D: Applied Physics 39(20), pp.4457–4465