Synthesis of (1 − x)ZnAl2O4–xTiO2 microwave dielectric ceramics by molten-salt process

Sintering characteristic, phase compositions, microstructures and microwave dielectric properties of (1 − x)ZnAl2O4–xTiO2 ceramics synthesized by LiCl and ZnCl2 molten-salt were investigated. Molten-salt process can prepare smaller and more homogenous grains than the solid-state reaction method, and can lower effectively its densification temperature. Besides ZnAl2O4 spinel and rutile main phase, sometimes Zn2Ti3O8 phase could exist in the ceramic system, which is closely related to TiO2 content or sintering temperature. When x value is equal to 0.25, a stable-temperature (1 − x)ZnAl2O4–xTiO2 ceramics calcined at 900 °C in LiCl molten-salt and sintered at 1300 °C in air can be obtained, and exhibits microwave dielectric properties with an ɛr value of 9.8, a Q·f value of 27,000 GHz, and a τf value of −7.1 ppm/°C. ZnCl2 molten-salt can improve significantly the quality factor (Q·f = 56,440 GHz), however, deteriorate the temperature coefficient of resonant frequency (τf = −25.4 ppm/°C) of the ceramics.

Research highlights▶ ZnAl2O4–TiO2 spinel-based ceramics have outstanding electrical insulating properties, thermochemical stability properties, thermal-expansion properties, and they can be proposed as potential microwave substrate and antenna materials substituting for the commonly used alumina substrate. ▶ However, the sintering temperature of the ZnAl2O4–TiO2 ceramics is too high and needs to be decreased. Molten-salt process is a well established low temperature synthesis technique that has recently attracted increasing interest. It has been used to synthesize low melting electroceramic powders and high temperature complex oxide powders. Until now there is no report dealing with the effects of molten-salt process on the microwave dielectric properties of the ZnAl2O4–TiO2 ceramics. In this study, the effects of molten-salt process on sintering characteristic, phase compositions, microstructures and microwave dielectric properties of (1 − x)ZnAl2O4–xTiO2 ceramics were investigated. ▶ It is found that LiCl and ZnCl2 molten-salt process can prepare smaller and more homogenous grains than the solid-state reaction method, and can lower effectively its densification temperature. Li2xZn(2−x)Ti3O8 (or Zn2Ti3O8) low temperature phase exists easily in the calcined powder synthesized by molten-salt, and it reduces gradually until disappears with the increase of TiO2 content or sintering temperature. Stable-temperature ceramics can be obtained by controlling the phase composition in the ZnAl2O4–TiO2 system. The above preparing method, experimental phenomena and results might be useful to investigate other microwave dielectric ceramic system.