Abstract:

The feldspar-based microwave dielectric ceramic with low relative permittivity (ε_r) and excellent mechanical properties has attracted much attention in the fifth-generation wireless communication technology. In this work, a series of microwave dielectric ceramic SrAl_2-xGa_xSi₂O₈ (0.1≤x≤2.0) was synthesized using the traditional solid-state method. X-ray diffraction pattern indicates that Ga³⁺ can be dissolved into Al³⁺, forming a solid solution. Meanwhile, substitution of Ga³⁺ for Al³⁺ can promote the space group transition from I2/c (0.1≤x≤1.4) to P21/a (1.6≤x≤2.0) with coefficient of thermal expansion (CTE) increasing from 2.9×10^-6 ℃^-1 to 5.2×10^-6 ℃^-1. During this substitution, the phase transition can significantly improve the structural symmetry to enhance the dielectric properties and mechanical properties. Rietveld refinement results indicate that Ga³⁺ averagely occupied four Al³⁺ compositions to form solid solution. All ceramics have a dense microstructure and high relative density above 95%. An ultralow ε_r of 5.8 was obtained at x=1.6 composition with high quality factor (Q×f) of 50700 GHz and negative temperature coefficients of resonant frequency (τ_f) of approximately −35×10^-6 ℃^-1. The densification temperature can be reduced to 940 ℃ by adding 4% (in mass) LiF, resulting in good chemical compatibility with Ag electrode. Meanwhile, negative τ_f can be tuned to near-zero (+3.7×10^-6 ℃^-1) by adding CaTiO₃ ceramic.

Key words: ion substitution, microwave dielectric ceramic, dielectric property, low-temperature co-fired ceramic