High-Q, temperature-stable, and medium-permittivity composite ceramics of tungstenbronze-type and perovskite titanates through layer-stacking process

Abstract

High-Q and temperature-stable ceramics are indispensable in microwave engineering. However, medium-permittivity (εr ∼ 60, named as K60) microwave dielectric ceramics that meet the above demands are yet to be synthesized. High-Q Ba4Sm9.33Ti16.5Al2O54 (BSTA) and Ca0.6Sm0.27Ti0.8Al0.27O3 (CSTA) ceramics have large resonant frequency temperature coefficients (τf) with opposite signs. The composites of the two are expected to have good dielectric properties, but there will be obvious diffusion when they are compounded with each other by sintering after directly mixing the raw powder, which would lead to severe deterioration in dielectric properties. In this work, the undesired diffusion can be effectively avoided by stacking two or three ceramic layers together, which are achieved by two methods named adhesive bonding and spark-plasma-sintering (SPS) cofiring. The finite element method (FEM) analysis with COMSOL Multiphysics proposed in this work can precisely predict the microwave dielectric properties of layered ceramics, helping to determine the ratio of different layers to reach near-zero τf. The adhesive-bonded layered ceramics exhibit excellent dielectric properties: εr = 62.2, Q×f = 21,400 GHz, and τf = –0.7 ppm/°C. The SPS-cofired layered ceramics can be fabricated in one time, partially solving the problem of sintering mismatch between different layers, together with superior properties of εr = 59.8, Q×f = 19,780 GHz, and τf = –0.6 ppm/°C. These layered ceramics could fill the gap of high-Q and temperature-stable K60 microwave dielectric ceramics. The layer-stacking process and related FEM simulations can be used to design more high-Q and temperature-stable microwave dielectric ceramics and devices.