Simultaneous two-step enhanced permittivity and reduced loss tangent in Mg/Ge-Doped CaCu3Ti4O12 ceramics

Abstract

Twice-enhanced dielectric permittivity for CaCu3Ti4O12 with a twofold reduced loss tangent was realized by doping with Mg at the Cu site. The dielectric permittivity was further enhanced by codoping with Ge at the Ti site with a further threefold reduction in the loss tangent. Theoretical calculations showed that Mg and Ge are preferentially occupied at the Cu sites in the CaCu3Ti4O12 structure. CaCu2.95Mg0.05Ti4-xGexO12 ceramics showed an enormous expansion of grain size with the segregation of Cu-rich phase along the grain boundary, which is likely attributed to the liquid phase sintering mechanism correlated to the preferential substitution of Ge ions. The enhancement of the dielectric permittivity originates from the enhanced ratio of mean grain size to grain boundary thickness and increased free charge inside the semiconducting grains, obeying a simple series-layer model of the internal barrier layer capacitor (IBLC). The dielectric properties can be well fitted by the Maxwell-Wagner polarization relaxation model based on the IBLC structure. Accordingly, a double-step reduction in the low-frequency loss tangent by only doping with Mg and further codoping with Ge ions was attributed to the double-step enhanced resistance of the insulating grain boundaries, which was due to the segregation of the Cu-rich phase.