Invited paper: Dielectric properties of CaCu3Ti4O12 polycrystalline ceramics

Abstract

We investigated the relationship between the microstructures and dielectric properties of various CaCu3Ti4O12 (CCTO) polycrystalline ceramics sintered in air. An abrupt increase in the dielectric constant (ɛ r) from ∼3,000 to ∼170,000 at 1 kHz occurred with increasing the sintering temperature from 980 to 1000°C for 12 h, respectively, which was accompanied by a very large increase in the average grain size from 5 to 300 µm, respectively, due to an abnormal grain growth. With further increasing the sintering temperature, the ɛ r value at 1 kHz was slightly decreased to ∼150,000 at 1020°C with no variation in the average grain size, significantly decreased to ∼77,000 at 1040°C with a large decrease in the average grain size (∼150 µm), and then maintained the values of ∼76,000 and ∼69,000 at 1060 and 1080°C, respectively, without noticeable variation in the average grain size. While no abnormal grain growth occurred in the CCTO samples sintered at 980°C for the holding time to 24 h and thus their ɛ r values showed relatively lower ɛ r values (< ∼4,000 at 1 kHz), the abnormal grain growth occurred in the samples after a certain holding time at a given sintering temperature of higher than 1000°C and thus their ɛ r values abruptly increased. Analyses by the complex impedance (Z*) and modulus (M*) spectroscopy revealed that the ɛ r values of the CCTO samples were dominantly affected by the electrical properties of grain boundary so that high ɛ r values over 10,000 at 1 kHz were attributable to the high capacitance (C) of grain boundary, which is in good agreement with grain boundary internal barrier layer capacitor (IBLC) model.