A low-firing melilite ceramic Ba₂CuGe₂O₇ and compositional modulation on microwave dielectric properties through Mg substitution

A melilite Ba₂CuGe₂O₇ ceramic was characterized by low sintering temperature and moderate microwave dielectric properties. Sintered at 960 ℃, the Ba₂CuGe₂O₇ ceramic had a high relative density 97%, a low relative permittivity (ε_r) 9.43, a quality factor (Q×f) of 20,000 GHz, and a temperature coefficient of resonance frequency (τ_f) -76 ppm/℃. To get a deep understanding of the relationship between composition, structure, and dielectric performances, magnesium substitution for copper in Ba₂CuGe₂O₇ was conducted. Influences of magnesium doping on the sintering behavior, crystal structure, and microwave dielectric properties were studied. Mg doping in Ba₂CuGe₂O₇ caused negligible changes in the macroscopic crystal structure, grain morphology, and size distribution, while induced visible variation in the local structure as revealed by Raman analysis. Microwave dielectric properties exhibit a remarkable dependence on composition. On increasing the magnesium content, the relative permittivity featured a continuous decrease, while both the quality factor and the temperature coefficient of resonance frequency increased monotonously. Such variations in dielectric performances were clarified in terms of the polarizability, packing fraction, and band valence theory.