Effect of <scp> <scp>Ca</scp> </scp> ²⁺ Substitution on the Structure, Microstructure, and Microwave Dielectric Properties of <scp> <scp>Sr</scp> </scp> ₂ <scp> <scp>Al</scp> </scp> ₂ <scp> <scp>SiO</scp> </scp> ₇ Ceramic

Abstract

The effect of Ca 2+ substitution on the structure, microstructure, and microwave dielectric properties of Sr–gehlenite ( Sr 2 Al 2 SiO 7 ) ceramic has been investigated. The structure and microstructure of Sr 2− x Ca x Al 2 SiO 7 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 P 2 1 m (no. 113) with tetragonal crystal symmetry. The porosity‐corrected relative permittivity at microwave frequencies showed a gradual increase with Ca 2+ content; however, Ca 2+ 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.