The development of ferroelectric ceramics is driven by a large number of applications such as capacitors. However, the standard X8R ceramic capacitors are limited to a maximal operating temperature of ~150 °C, whereas the maximum working temperature of commercially available high-temperature capacitors is still restricted to 200 °C. Nevertheless, cutting-edge technology requires electronic systems to work properly at temperatures above 200 °C for various industrial applications such as in automobiles, aircrafts and oil/gas exploration.
In this work, relaxor ferroelectrics based high-temperature dielectrics have been studied, covering basic theories, new materials development as well as application-oriented research. BNT-6BT system shares similarities with canonical relaxors, although peculiar dielectric properties are found in BNT-6BT due to its diffuse phase transition between rhombohedral PNRs and tetragonal PNRs in the temperature range from ~150 °C to ~300 °C. Dielectric properties of BNT-BT-KNN have been investigated in order to clarify the improvement in the thermal stability of permittivity in these materials. Thermal evolution of PNRs in BNT-6BT is tuned by KNN-modification, leading to a wide operational range. Based on the understanding of BNT-BT and BNT-BT-KNN systems, novel high-temperature dielectrics have been developed by modifying these two materials with CZ, which is believed to act similarly as KNN in flattening the temperature-dependent permittivity profile. Performance of thick-film capacitors with compositions BNT-BT-KNN has been assessed. Despite the additional processing steps, the dielectric properties of the thick films are not significantly impaired.
As a perspective on the future development of high-temperature dielectrics, this work suggests that an effective way is to tune the thermal behavior of PNRs in relaxor ferroelectrics. By doing this, the anomaly caused by dielectric relaxation of PNRs can be suppressed and shifted to low temperatures. In the mean time, the overall polarizability of the material should be modified in a way that a balance between suppressed anomaly and reasonable magnitude of permittivity can be reached. The processing conditions during the device fabrication must be optimized in order to maintain high density of the films.