Control of grain boundary in alumina doped CCTO showing colossal permittivity by core-shell approach
Résumé
Grain boundaries of CaCu3Ti4O12 (CCTO) materials have been shown to play leading role in colossal permittivity. Core-shell design is an attractive approach to make colossal dielectric capacitors by controlling the grain boundaries. Core-shell grains of CCTO surrounded by Al2O3 shell were synthesized by ultrasonic sol-gel reaction from alumina alkoxide precursor. The influence of alumina shell by comparison with bare CCTO grains was studied. Particularly, microstructure, dielectric and electric effects on sintered ceramics are reported. The average grain size and the density are increased compared to undoped CCTO leading to an improvement of permittivity from 58,000 to 81,000 at 1 kHz. Furthermore a decrease of dielectric loss is found in a frequency range of 10(2)-10(3) Hz. Moreover, the activation energy of grain boundaries is increased from 0.55 to 0.73 eV and the electrical properties such as breakdown voltage, non-linear coefficient and resistivity are improved with the aim of making industrial capacitors.