Two low-permittivity melilite ceramics in the SrO-MO-GeO2 (M = Mg, Zn) system and their temperature stability through compositional modifications

Two melilite ceramics Sr 2 AGe 2 O 7 (A = Mg, Zn) with low permittivity were prepared by a solid-state reaction method. The crystal structure and microstructure of the ceramics were characterized by X-ray diffraction, transmission electron microscopy , Raman spectroscopy , and scanning electron microscopy. Both ceramics crystallized in a single melilite structure with a tetragonal space group P-42 m (113) and exhibited homogeneous microstructures. Optimum microwave dielectric properties with relative permittivity ( ε r ) of 8.56, quality factor ( Q × f ) of 28,800 GHz, and temperature coefficient of resonance frequency ( τ f ) of −70.5 ppm/°C were obtained in Sr 2 MgGe 2 O 7 sintered at 1330 °C. Sr 2 ZnGe 2 O 7 possessed ε r of 8.81, Q × f of 35,700 GHz, and τ f of −84.4 ppm/°C when sintered at 1290 °C. Thermal stability of resonance frequency was accessible when the negative τ f values of Sr 2 AGe 2 O 7 were adjusted after the formation of composite ceramics with CaTiO 3 .