Rapid synthesis of dielectric tantalum-based oxynitrides

Perovskite-type oxynitride with a general formula of ATa(O,N)3 (A = Sr, Ba) is a class of promising dielectric material due to their very high permittivity. Conventional synthesis routes for these materials always require multiple processing steps, long durations and elevated temperatures. In this study, thermodynamic calculations were employed to predict the feasible synthesis reactions, possible impurities and reasonable processing parameters. Then, ceramic powders of SrTaO2N and BaTaO2N were fabricated through direct calcination of SrCO3/BaCO3 and Ta2O5 with urea as nitrogen source by using a modified pressureless spark plasma sintering set-up. High-purity oxynitrides can be obtained within 10 min. Thermal stability, corrosion resistance and dielectric property were evaluated. The results showed that SrTaO2N was thermally stable up to 475 °C in air while that temperature for BaTaO2N was 605 °C. The oxynitrides possessed a good resistance to hot water and strong acid/alkali. BaTaO2N had a very high room-temperature relative permittivity up to 9550 with a dielectric loss down to 0.001 at 100 Hz, while the values for SrTaO2N were 3141 and 0.017 respectively. The temperature dependence of permittivity for BaTaO2N was weak at −10–200 °C. The efficient synthesis method enabled the fast preparation of the tantalum-based oxynitride materials for energy storage applications. Keywords: Perovskite, Oxynitride, Urea, Spark plasma sintering, Permittivity