Influence of Ge and GeO2 on the microstructure and varistor properties of TiO2-Ta2O5-CaCO3 ceramics

This study investigated the effect of elemental crystal Ge or/and GeO2 doping on the microstructure and varistor properties of TiO2-Ta2O5-CaCO3 varistor ceramics, which were prepared via the traditional ball milling molding-sintering process. X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, scanning electron microscopy-energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy demonstrated that co-doping with Ge and GeO2 changed the microstructure of TiO2-Ta2O5-CaCO3 ceramics, thereby increasing the nonlinear coefficient and decreasing the breakdown voltage. The optimum doping concentrations of Ta2O5, CaCO3, Ge, and GeO2 exhibited the highest nonlinear coefficient (alpha=14.6), a lower breakdown voltage (E-B=18.7 V mm(-1)), the least leakage current (J(L)=10.5 yA cm(-2)), and the highest grain boundary barrier (Phi(B)=1.05 eV). In addition, Ge and GeO2 function as sintering aids, which reduce the sintering temperature because of their low melting points.