Microstructure evolution and electrical properties of Ba ₂NaNb₅O₁₅ and BaTiO₃ composites

Phase development and sintering behavior and dielectric characteristics of the composites of tungsten bronze structured Ba₂NaNb ₅O₁₅ (BNN) and perovskite structured BaTiO₃ (BT) were examined as a function of molar ratio x in (1-x)Ba ₂NaNb₅O_15- -xBaTiO₃. A wide range of solid solution of El in BNN was observedup tox=0.4-0.6, and the lattice constants increased with BT contents. Since the solid solution of BT expelled Na ions from BNN, liquid phase with excess Na might be produced during sintering and resulted in accelerated densification and abnormal grain growth. More BT addition in BNN over the solubility limit produced a second phase of Ba ₆Ti₇Nb₉O₄₀together with BT, and those phases played a role as grain growth inhibitors. The Curie temperature (TC) and the maximum dielectric constant of BNN decreased as the content of BT increased. Besides, when BNN is added to BT, the solubility limit of BNN in BT was very narrow compared to the case of BT addition to BNN, which resulted in the generation of Ba₆Ti₇Nb₉O₄₀ second phase. The addition of BT to BNN also shifted the TC to lower temperature. In the case of x=0.6, two dielectric relaxation peaks were observed which is believed due to the coexistence of BNN and BT in the sample.