Zhe Song

Homogeneous/Inhomogeneous-Structured Dielectrics and their Energy-Storage Performances

The demand for dielectric capacitors with higher energy-storage capability is increasing for power electronic devices due to the rapid development of electronic industry. Existing dielectrics for high-energy-storage capacitors and potential new capacitor technologies are reviewed toward realizing these goals. Various dielectric materials with desirable permittivity and dielectric breakdown strength potentially meeting the device requirements are discussed. However, some significant limitations for current dielectrics can be ascribed to their low permittivity, low breakdown strength, and high hysteresis loss, which will decrease their energy density and efficiency. Thus, the implementation of dielectric materials for high-energy-density applications requires the comprehensive understanding of both the materials design and processing. The optimization of high-energy-storage dielectrics will have far-reaching impacts on the sustainable energy and will be an important research topic in the near future.

The effect of grain boundary on the energy storage properties of (Ba 0.4 Sr 0.6M )TiO 3 paraelectric ceramics by varying grain sizes

(Ba0.4Sr0.6)TiO3 (BST) ceramics with various grain sizes (0.3-3.4 μm) were synthesized by the oxalate coprecipitation method and prepared by plasma activated sintering and conventional solid-state sintering process. The effect of grain boundary on the energy storage properties and the dielectric relaxation characteristics of BST paraelectric ceramics (Curie point ≈ -67°C) with various grain sizes were investigated. The dielectric breakdown strength (simplified as BDS) is obviously improved and then deteriorated with decreasing grain size, accounting for the energy density variation. The enhancement of interfacial polarization at grain boundary layers has a negative effect on the BDS, leading to the decreased values for samples with grain size smaller than 0.7 μm. In addition, the insulation effect of grain boundary barriers was discussed based on the complex impedance spectroscopy analysis, which was found to play a dominant role in controlling the BDS with coarser grain size. Among them, the sharply decreased BDS for BST with grain size of 1.8 μm was believed to be attributed to the combination of lower grain boundary density and higher interfacial polarization, due to the significant increase of oxygen vacancies at higher sintering temperature.

Effect of the grain boundary on the dielectric breakdown strength of (Ba 0.4 Sr 0.6 )TiO 3 paraelectric ceramics with various grain sizes

(Ba<inf>0.4</inf>Sr<inf>0.6</inf>)TiO<inf>3</inf> (BST) paraelectric ceramics (T<inf>c</inf> ≈ −67 °C) with various grain sizes (0.6–1.8 μm) were synthesized by oxalate co-precipitation method and prepared by conventional solid-state sintering process. With decreasing grain sizes, the dielectric breakdown strength increases gradually. Based on the conductivity activation energy analysis, it was found that the larger grain boundary density plays a dominant role in controlling dielectric breakdown strength for samples with smaller grain sizes (1.0 to 0.6 μm). While with the growth of grain sizes above 1.0 μm, the sharply decreasing dielectric breakdown strength is induced by a combined effect of lower grain boundary density and more intensive interface polarization.

Structure and electrical properties of ternary BiFeO 3 -BaTiO 3 -PbTiO 3 high-temperature piezoceramics

In the current work, the bulk ternary (0.85-x) BiFeO3-xBaTiO3-0.15PbTiO3 (BF-BTx-PT, x=0.08–0.35) system has been studied as a potential high-temperature piezoceramics. Samples with various content of BT were prepared via solid-state route, and pure perovskite phase was confirmed by X-ray diffraction. The temperature dependence of dielectric constants confirmed the decrease of Curie temperature with increasing BT content. It was found that the morphotropic phase boundary (MPB) composition of BF-BTx-PT ceramics was in the vicinity of x=0.15, which exhibits optimal properties with piezoelectric constant d33 of 60 pC/N, high Curie temperature of 550 °C, and low sintering temperature of 920 °C. Measurements also showed that the depoling temperature was 300 °C, about 150 °C higher than that of commercialized PZT ceramics, which indicated good temperature stability. BF-BTx-PT ceramics are promising candidates for high temperature applications.

Investigation of Dielectric Properties for Ba0.4Sr0.6TiO3 Ceramics with Various Grain Sizes

The effect of grain size on the dielectric properties of Ba0.4Sr0.6TiO3 ceramics with various grain sizes in the range of 405–1635 nm was investigated. Diffusion phase transition and a tendency to lower dielectric nonlinearity were found when the grain size decreased to 405 nm. The dielectric tunability of sample with grain size of 405 nm was 2.83% at the electric field of 20 kV/cm, exhibiting the weakest dielectric nonlinearity. The dielectric breakdown strength as well as the energy storage density were enhanced obviously with the refinement of grain sizes in the range of 660–1635 nm, due to the insulation effect of increasing grain boundary density, exactly as the highest energy storage density with 1.7 J/cm3 for the sample with grain size of 660 nm.