Low-dielectric microwave ceramics Ca_1-xSr_xB₂O₄ with excellent compatibility with silver

Ca_1-xSr_xB₂O₄(x = 0–0.5) ceramics are prepared using the conventional solid-state method, and their crystal structure, microstructure, microwave dielectric properties, and low-temperature co-firing characteristics are systematically studied. To enhance the crystalline structure order, this study aims to improve the sintering performance of CaB₂O₄ by forming a solid solution with Sr²⁺ substituting Ca²⁺ sites. X-ray diffraction results indicate that with the doping of Sr²⁺, the unit cell volume of Ca_1-xSr_xB₂O₄ ceramics increases, and the diffraction peaks shift to lower angles. EDS results confirm that Sr²⁺ is uniformly distributed in the Ca_1-xSr_xB₂O₄ ceramics, successfully forming a solid solution. SEM analysis reveals that an appropriate amount of Sr²⁺ improves grain uniformity and reduces porosity between grains. Raman spectroscopy shows that moderate Sr²⁺ doping enhances the order of the crystalline structure, leading to an increase in the Q × f value of the ceramics. When x = 0.03 and the sintering temperature is 900 °C, Ca_1-xSr_xB₂O₄ ceramics exhibit optimal microwave dielectric properties: ε_r = 5.53, Q × f = 33,135 GHz, τ_f = −27.37 ppm/°C. During co-sintering with Ag electrodes, Ca_1-xSr_xB₂O₄ ceramics show excellent chemical compatibility with Ag powder, with no silver diffusion observed. This demonstrates the great potential of Ca_1-xSr_xB₂O₄ ceramics for LTCC applications.