Novel Ultralow-Permittivity BaMg2Al6Si9O30-based Microwave Dielectric Ceramics with Self Near-Zero Temperature Coefficient of Resonant Frequency and Thermal Expansion

Novel BaMg2Al6Si9O30-based ceramics were prepared using the traditional solid-phase reaction method, and this paper presents the fabrication procedure, crystal structure, microstructure, microwave dielectric properties and temperature coefficient of thermal expansion of BaMg2Al6Si9O30-based ceramics. BaMg2Al6Si9O30-based ceramics were found to possess a hexagonal crystal structure with the P6/mcc space group. The BaAl2Si2O8 and BaMg2Al6Si9O30 phases coexisted in BaMg2Al6Si9O30 ceramic given that raw MgO material reacted easily with water vapour in the air, and the BaMg2Al6Si9O30 single-phase ceramic could be synthesized with calcined raw MgO material. The BaMg2Al6Si9O30 single-phase ceramic exhibited ultralow Cr (5.68 at 14.05 GHz), moderate Qxf value (28534 GHz at 14.05 GHz), a self near-zero & tau;fvalue (-7.35 ppm/degrees C), a near-zero average temperature coefficient of thermal expansion (+1.89 ppm/degrees C) at 30 degrees C-1000 degrees C and a rare negative temperature coefficient of thermal expansion (-9.43 ppm/degrees C) at 655 degrees C-710 degrees C. The rare near-zero and negative thermal expansions of the BaMg2Al6Si9O30 single-phase ceramic were mainly related to the strong Si/Al-O bonds with high relative covalency and 2 coordinate O(1) allowing longitudinal vibrations.