Phase structures, microstructures, and dielectric characteristics of high εr (1-x-y)Bi0.5Na0.5TiO3-xLi0.5Sm0.5TiO3-yNa0.5La0.5TiO3 microwave ceramic systems

Abstract

Ultrahigh dielectric constant (εr) (1-x-y)Bi0.5Na0.5TiO3-xLi0.5Sm0.5TiO3-yNa0.5La0.5TiO3 (BNT-LST-NLT, 0.3 ≤ x ≤ 0.7, 0.0 ≤ y ≤ 0.4) microwave ceramic systems were fabricated using a conventional solid-state reaction method, and their phase structures, microstructures, and microwave dielectric characteristics were systematically investigated. From XRD pattern analyses and refinement results, (1-x-y)BNT-xLST-yNLT (x = 0.3–0.6 and y = 0.4–0.1) samples were crystallized into a cubic perovskite structure, and XRD peaks corresponding to the (031), (131), and (230) planes were identified as super-reflections belonging to the tilt system of orthorhombic a−a−b+ with the Pnma space group. In particular, on the basis of the Raman spectra and microwave dielectric properties analysis results, the crystal structure of the sample (x = 0.6 and y = 0.1) sintered at 1200 °C for 4 h was considered to be closer to orthorhombic symmetry. In addition, solid orthorhombic perovskite-type solutions were formed together with a small amount of secondary phase Sm2Ti2O7 as x = 0.6 and y = 0.2, 0.3. However, an increase in the Sm2Ti2O7 phase content was also detected for x = 0.7 and y = 0.0, accompanied by the perovskite phase. For the microwave dielectric properties, the εr and Q×f values strongly depended on the apparent density, ionic polarizability, phase structure, and phase composition, and also, the τf value was predicted by adjusting the tolerance factor (t). The εr and τf values basically agreed with the existing models, such as the mixing rule. As a result, the (1-x-y)BNT-xLST-yNLT (x = 0.6 and y = 0.1) ceramic sample sintered at 1200 °C for 4 h exhibited superior microwave dielectric properties of εr ~ 149.1, Q×f ~ 3270 GHz, and τf ~ 39.1 ppm/°C.