Effect of Ca substitution on the microwave properties and the inhibition of Ag diffusion in Ba_1-xCa_xMoO₄ ceramics

Ba_1-xCa_xMoO₄ (0 ≤ x ≤ 0.20) ceramics were prepared from powders to form solid solutions by a solid-state reaction sintering process. The influence of the Ca²⁺ content on the microstructure, sintering, densification, microwave dielectric properties and chemical stability of BaMoO₄-based ceramics with Ag metal was discussed in detail. The sintering temperatures of the Ba_1-xCa_xMoO₄ ceramics were effectively reduced to less than 950 °C by the formation of the solid solutions. Structural analysis indicates that the Ba_1-xCa_xMoO₄ ceramics belong to the class of tetragonal scheelites. The crystal grain size begins to decrease and become more regular as x increases from 0 to 0.12. However, as x continues to increase, a liquid phase begins to appear, and the grain boundaries are no longer clear. The ε_r value increases from approximately 8.6 to 9.8 as x increases from 0 to 0.2. The Ba_0.92Ca_0.08MoO₄ sample possesses the best microwave dielectric performance, namely an ε_r = 9.3 and the maximum Q × f value of 33593 GHz. The addition of 15 wt% TiO₂ or 10 wt% CaTiO₃ can effectively change the τ_f values of the Ba_1-xCa_xMoO₄ ceramics to approximately 0. The Ba_1-xCa_xMoO₄ ceramic samples can coexist with silver during the LTCC cofiring process.