Effect of milling process on the microwave dielectric properties of Ba ₂Ti₉O₂₀ materials

The effects of the milling process on the characteristics of Ba ₂Ti₉O₂₀ materials were investigated. The chemical analyses using transmission electron microscopy (EDAX in TEM) revealed that the SiO₂ species incorporated into the Ba₂Ti ₉O₂₀ materials were expelled by the Ba₂Ti ₉O₂₀ grains. It induced the dissociation of the Ba ₂Ti₉O₂₀ 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 (Si ₃N₄-HEM) deleteriously influenced the microwave dielectric properties for the Ba₂Ti₉O₂₀ materials. Utilizing the three-dimensional-milling (3DM) process in place of the Si ₃N₄-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.