Effect of grain-boundary behavior on the dc electric conduction in Rb-doped CaCu3Ti4O12

Ca_1?x Rb _x Cu₃Ti₄O₁₂ (x = 0, 0.03, 0.05, 0.07 and 0.09) ceramics have been synthesized by the sol–gel method. The results of X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis indicate that a small amount of CuO and the grain boundary layers with Cu-rich and Ti-poor compositions are formed in the Rb-doped ceramic samples. The curves of dc electrical current density versus electric field strength have been collected at different temperatures, and the electrical conduction behaviors at grain boundary have been studied in detail based on the grain-boundary Schottky model. The results show that the formation of the Cu-rich and Ti-poor grain boundaries greatly affects the properties of grain-boundary Schottky potential barrier. For example, the barrier width increases with the increase on doping concentration x, and the barrier height (Ф_B) is also greatly enhanced as the concentration of dopants is very small, but as x is larger than 0.05, the Ф_B values reduce. These results should be ascribed to the second phases at grain boundary and the change of electronic structure.