Effect of Ca²⁺ Substitution on the Structure, Microstructure, and Microwave Dielectric Properties of Sr₂Al₂SiO₇ Ceramic

The effect of Ca²⁺ substitution on the structure, microstructure, and microwave dielectric properties of Sr-gehlenite (Sr ₂Al₂SiO₇) ceramic has been investigated. The structure and microstructure of Sr_2-xCa_xAl ₂SiO₇ ceramics were analyzed via X-ray diffraction (XRD) as well as scanning and transmission electron microscopic techniques. While the end-members (x = 0 and 2) form isostructural compounds, a highly defective, nonstoichiometric, Ca-rich secondary phase was observed via bright-field transmission electron microscopy and energy dispersive X-ray spectroscopy in compositions corresponding to x = 0.75 and 1.5. The concentration of secondary phase in x = 0.75 is too low to be detected via XRD or scanning electron microscopy. Identical selected-area electron-diffraction patterns of the compounds (x = 0, 1, and 2) confirmed that they belong to the space group P4̄2₁m (no. 113) with tetragonal crystal symmetry. The porosity-corrected relative permittivity at microwave frequencies showed a gradual increase with Ca²⁺ content; however, Ca²⁺ substitution made only marginal changes to the microwave dielectric properties except in the case of x = 1.5, in which the secondary phase reduced the quality factor considerably. Thermal conductivity decreased with increasing Ca ²⁺ content, and the compounds with defective structures showed the lowest thermal conductivity. All the compounds exhibited low coefficients of linear thermal expansion, with values varying in the range 2.3-3.6 ppm/°C.