The lowered dielectric loss tangent and grain boundary effects in fluorine-doped calcium copper titanate ceramics

The effects of F− concentration on the dielectric properties and electric modulus of CaCu3Ti4O12−xFx ceramics (0 ≦ x ≦ 0.8) were investigated in detail. Proper amount of F− substitution in CaCu3Ti4O12−xFx ceramics reduced the dielectric loss tangent. When x = 0.8, the dielectric loss tangent of CaCu3Ti4O12−xFx ceramic was about 0.05 (at 5 kHz), which was the lowest value and exhibited the highest grain boundary resistance (1.18 MΩ); meanwhile, the samples retained a relatively high dielectric constant above 7000 over a wide frequency range of 100 Hz to 1 MHz. The grain size was reduced as the F− doping concentration increased, which enhanced grain boundary resistance, and the decreased dielectric loss tangent was attributed to the increased grain boundary resistance. With the increase of F− concentration, the dielectric relaxation behaviors correlated with the grain boundary effects were significantly enhanced. The thermal-activated Schottky-type barrier was formed in the grain boundaries, and CaCu3Ti4O12−xFx ceramics had a nonlinear-Ohmic property.