Effect of copper-rich secondary phase at the grain boundaries on the varistor properties of CaCu₃Ti₄O₁₂ ceramics

In this study, the relationship between the grain-boundary composition and varistor properties were investigated using scanning electron microscopy, transmission electron microscopy, energy dispersion spectroscopy, and X-ray photoelectric spectroscopy (XPS). It was found that the CaCu₃Ti ₄O₁₂ (CCTO) sample did not exhibit varistor behavior when the compositions of grain boundary and grain were nearly the same before discontinuous grain growth. The CCTO sample exhibited varistor behavior after the occurrence of discontinuous grain growth. After discontinuous grain growth, copper (Cu)-rich secondary phase appeared at the grain boundaries. The XPS results reveal that Cu⁺ is the major content of the second phases. The formation of negatively charged species, or, existing at the grain boundaries due to the oxidation of the secondary phase during cooling after sintering can act as acceptors, forming double Schottky barriers with the n-type semiconductor grains.