Recent technological developments in fields relating to various electronic and mechanical devices and components utilized in automotive, aviation and renewable energy industries, have had their power and temperature requirements considerably increased. Above all, capacitors have been identified as the most critical materials that must meet the new demands and also fulfill such domineering requirements. Recent studies have already established ceramics as the most auspicious materials for high temperature capacitors, however, as of now, no ceramic or ceramic based derivative material has been able to meet the obligatory electrical properties. Following a series of studies, Na1/2Bi1/2TiO3– (NBT)-based materials have shown outstanding attributes that could enable them to be used in the fabrication of high temperature capacitors. Nonetheless, NBT-based materials possess a daunting shortcoming in that they have a large sensitivity towards changes in defect chemistry of NBT solid solutions, where a control of this defect chemistry would lead to highly optimized electrical properties.
Recently, Technische Universität Darmstadt researchers: Marion Höfling, Sebastian Steiner, An-Phuc Hoang, In-Tae Seo and Till Frömling from the Institute of Materials Science investigated NBT solutions in a bid to optimize their respective electrical properties. They hoped that their investigation would provide vital information on how to adjust the A-site defect chemistry for optimizing the electrical properties of NBT-based capacitor materials. Their work was reported in Journal of Material Chemistry C.
Briefly, the research method using by the scientists commenced with preparation of ceramic powders using the conventional solid-state method. Next, various powders used were ball-milled, dried and ground. They then proceeded to undertake impedance measurements in the frequency range of 0.1Hz to 1MHz. Lastly, nonlinear behavior was investigated at 1Hz and an electric field of 40kV per centimeter, so as to determine the energy density and the degree of efficiency for the selected compositions from room temperature to 150°C.
The authors observed that a reduction of bismuth vacancy and oxygen vacancy concentration by increasing the initial bismuth content led to a considerable decrease in dielectric loss. Moreover, they noted that very high energy efficiencies were achieved for the composition Na1/2Bi1/2O3– BaTiO3–CaZrO3 (NBT–BT–CZ) and the temperature range of stable high permittivity together with low dielectric loss (tan δ ≥ 0.02) was extended from -67°C to 362°C.
In conclusion, the Technische Universität Darmstadt study successfully presented the optimization of electrical properties through the reduction of oxygen and bismuth vacancy concentration with the increase of bismuth content which led to the satisfaction of the harsh requirements outlined for high temperature capacitors in automotive and aviation applications. In general, they observed that the increase of bismuth-content led to higher resistivity, lower dielectric loss and higher energy efficiency for the solid NBT–BT–CZ over a large temperature range. Altogether, the material under investigation obtained extraordinarily stable properties in a large temperature and electric field range by control of vacancy concentration, which makes it a top contender for use as the best high temperature capacitor material.
Marion Höfling, Sebastian Steiner, An-Phuc Hoang, In-Tae Seo and Till Frömling. Optimizing the defect chemistry of Na1/2Bi1/2TiO3– based materials paving the way for excellent high temperature capacitors. Journal of Material Chemistry C, 2018, volume 6, page 4769.