Low-dielectric microwave composite ceramics (1-y)Ca0.97Sr0.03B2O4-yTiO2 with near-zero τf and superior silver compatibility

(1- y )Ca 0.97 Sr 0.03 B 2 O 4 - y TiO 2 composite ceramics were produced through a solid-phase reaction procedure, ensuring that all samples had a uniform and dense microstructure, where each constituent phase exhibited complete crystallinity. Their crystal structures, microwave dielectric properties, microstructures, and co-sintering performance at low temperatures were thoroughly investigated. To enhance the thermal stability of the resonant frequency, rutile TiO 2 ( τ f  = +450 ppm/°C) was introduced as an additional phase during the preparation of the composite ceramics. According to the XRD analysis, the characteristic peak signals became more pronounced as the content of rutile TiO 2 in the composite gradually rose, and the peak intensity was found to vary linearly with the relative content of TiO 2 . SEM analysis revealed that the TiO 2 additions ( y  = 0.05, 0.10, and 0.15) resulted in tightly arranged grains, the absence of visible pores, and a high degree of densification. However, as the TiO 2 content increased ( y  = 0.20, 0.25), pores appeared, grain boundaries became more pronounced, and densification decreased. At y  = 0.15 and a sintering temperature of 900 °C, the best dielectric properties were achieved for the (1- y )Ca 0.97 Sr 0.03 B 2 O 4 - y TiO 2 composite ceramics: ε r  = 7.61, Q×f  = 33,983 GHz, and τ f  = +3.51 ppm/°C. By introducing TiO 2 , the initially negative τ f was effectively adjusted toward zero, thereby achieving greater thermal reliability. Additionally, the absence of interfacial chemical reactions between the composite ceramic and the silver after co-firing indicates that the material possesses favorable chemical compatibility with silver, demonstrating strong applicability in LTCC technology.