Effect of Ga3+ Doping on Crystal Structure Evolution and Microwave Dielectric Properties of SrAl2Si2O8 Ceramic

The feldspar-based microwave dielectric ceramic with low relative permittivity ( 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-x Ga x Si 2 O 8 (0.1x2.0) was synthesized using the traditional solid-state method.X-ray diffraction pattern indicates that Ga 3+ can be dissolved into Al 3+ , forming a solid solution.Meanwhile, substitution of Ga 3+ for Al 3+ can promote the space group transition from I2/c (0.1x 1.4) to P21/a (1.6x2.0)with coefficient of thermal expansion (CTE) increasing from 2.910 -6 -1 to 5.210 -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 Ga 3+ averagely occupied four Al 3+ compositions to form solid solution.All ceramics have a dense microstructure and high relative density above 95%.An ultralow r of 5.8 was obtained at x=1.6 composition with high quality factor (Qf) of 50700 GHz and negative temperature coefficients of resonant frequency ( f ) of approximately -3510 -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 f can be tuned to near-zero (+3.710 -6 -1 ) by adding CaTiO 3 ceramic.