New barium titanate/glass composites for high energy storage capacitor applications

Abstract

Then this study presents two material systems developed for high energy storage capacitor applications: Micron-Scale BaTiO3/Glass Composites and Nano-Scale BaTiO3/Glass Nanocomposites. Both systems involved glass composition design to diminish the dilution effect and maintain a good sintering behavior and fabrication process control to ensure the uniform distribution of two phases at either micron scale or nanometer scale.

In addition, various dielectric breakdown mechanisms of BaTiO3-related materials were reviewed and discussed regarding to their virtues and drawbacks to explain the existing experimental results.

The Nano-Scale BaTiO3/Glass Nanocomposites were developed in this study to explore the possibility of further improving energy storage density of BaTiO3/glass composites by controlling BaTiO3 grain size at the nanometer scale. In order to achieve uniform distribution, a core-shell nano-scale mixing technique was utilized to fabricate BaTiO3/glass core-shell nanocomposite. With designed glass composition (65Bi2O3-20B2O3-15SiO2), the nanocomposite was sintered to high densification (99%) under low sintering temperature at 900°C without any noticeable grain growth. Also, by the use of nano-scale BaTiO3, benefit of thin film sample preparation was realized. As a result, the nanocomposite exhibited a high dielectric breakdown strength (>1 MV/cm) and postponed polarization saturation, which are beneficial to energy storage improvement. The highest energy storage density obtained in the core-shell nanocomposite was 10 J/cm3 at 1 MV/cm, which is promising for high energy storage capacitor applications.

The Micron-Scale BaTiO3/Glass Composites were investigated to examine the trade-off effect of glass additive on the energy storage performance of BaTiO3. In spite of the dilution effect, the glass additive led to a dielectric breakdown strength improvement (up to 400 kV/cm) and a shift of the polarization saturation to higher electric field, which contributed to a higher energy storage density. It was found that 20 wt% glass-added BaTiO3 composite exhibited the highest energy storage density of 1.3 J/cm3, which is over 10 times that of pure BaTiO3.

Energy storage plays an important role in managing the fluctuations in both power generation and user demand. Among various energy storage devices, capacitors offer the highest power density (up to 107 W/kg) and the longest cycling lifetime, which are attractive in high power electronic industry. Nevertheless, the energy storage densities of conventional capacitors are relatively low (10-2 to 10-1 W*hr/kg). Therefore, improving capacitive energy storage density is of great significance to extend the usage of capacitors in future energy storage systems.

Capacitors store electrical energy through the polarization of dielectric materials under externally applied electric field. A high-energy storage density requires the dielectric material to possess a high polarization value (high dielectric constant) as well as a high dielectric breakdown strength. Barium titanate (BaTiO3) as a ferroelectric ceramic material exhibits a high polarization value (high dielectric constant), which is desirable for energy storage capacitor application. However, the dielectric breakdown strength of BaTiO3 is relatively low (~100 kV/cm) due to the remnant pores in the sintered body. In order to promote densification of ceramic materials, glass has been used as a sintering agent to introduce viscous flow sintering for reducing porosity. However, the addition of low dielectric constant glass deteriorates the dielectric constant of the composite drastically due to the dilution effect. Thus, glass composition design is very important to alleviate this dilution effect.

In this research, a crystallizable glass was first explored as the sintering agent for barium titanate ceramics. The selected glass with components of BaO and TiO2 can crystallize into BaTiO3-containing glass-ceramics upon a crystallization heat-treatment after the sintering process, which mitigates the dilution effect remarkably. The crystallizable glass is demonstrated to be a new sintering agent candidate for barium titanate ceramics in applications where high dielectric constant is essential. However, the sintering temperature resulted from this crystallizable glass is still high (≥1200°C).