Zhonghua Yao

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.

High-temperature relaxor cobalt-doped (1−x)BiScO3-xPbTiO3 piezoelectric ceramics

Cobalt-doped (1−x)BiScO3-xPbTiO3 piezoelectric ceramics were fabricated by conventional solid state synthesis, in which a morphotropic phase boundary (MPB) region was observed in the compositional range of 0.48⩽x⩽0.53 from the phase structure measurement. Co doping in the 0.50BiScO3–0.50PbTiO3 ceramics resulted in a relaxor ferroelectric behavior due to cation disorder. The ferroelectric phase transition of the composition x=0.50 near the MPB was found to be a displacive and first-order transition with excellent piezoelectric properties, Tm=470°C, d33=282pC∕N, Kp=0.49, indicating a future high-temperature piezoelectric application.

Structure and electrical properties of lead-free Bi0.5Na0.5TiO3-based ceramics for energy-storage applications

A new energy-storage ceramic system based on (1 − x)(Bi0.5Na0.5TiO3–BaTiO3)–xNaTaO3 ((1 − x)(BNT–BT)–xNT) is reported in this study. XRD refinement indicated a composition induced rhombohedral to tetragonal phase transition. All the samples exhibited a dense microstructure with an average grain size of 1.2–1.9 μm. The introduction of NT greatly improved the temperature stability of the dielectric properties for the BNT–BT system. For compositions x = 0.03–0.15, the working temperature range spanned over 260 °C satisfying TCC150 °C ≤ ±15%. The electric conductivity as a function of frequency followed the double power law. In the temperature region of 325–500 °C, the activation energy of DC conduction ranged from 1.47 eV to 1.71 eV, indicating intrinsic band-type electronic conduction. The optimum energy-storage properties were obtained in 0.90(BNT–BT)–0.10NT with an energy-storage density of 1.2 J cm−3 and energy-storage efficiency of 74.8% at 10 kV mm−1. The results demonstrate that (1 − x)(BNT–BT)–xNT ceramics are promising candidates for high-temperature energy-storage applications.

Structure, electrical properties, and depoling mechanism of BiScO3–PbTiO3–Pb(Zn1/3Nb2/3)O3 high-temperature piezoelectric ceramics

The structure, electrical properties, and depoling mechanism of the (0.95−x)BiScO3−xPbTiO3−0.05Pb(Zn1/3Nb2/3)O3 (BS-xPT-PZN, x=0.54–0.70) compositions close to the morphotropic phase boundary (MPB) have been systematically investigated as a function of PbTiO3 content (x). The phase approached from the rhombohedral toward the tetragonal phase when the PbTiO3 contents increased. The composition with high PT content exhibited normal ferroelectric behavior while it showed a diffused phase transition characteristic as PT decreased. In the vicinity of the MPB, the ceramics showed enhanced piezoelectric, electromechanical, and ferroelectric properties with piezoelectric constant d33=490 pC/N, planar electromechanical coupling factors kp=57.4%, remnant polarization Pr=40.1 μC/cm2, and coercive field Ec=28.5 kV/cm with a high transition temperature Tm∼417 °C, respectively. The thermal depoling experiments of the polarization for samples with different phase structures were investigated and the possible depoling me...

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.

Design, fabrication and dielectric properties in core–double shell BaTiO3-based ceramics for MLCC application

BaTiO3-based ceramics with a core–double shell structure were fabricated by precipitation and the sol–gel method. Bi(Zn1/2Ti1/2)O3-BaTiO3 (BZT-BT) or Nb oxide was chosen to be the shell-I (inner layer) or shell-II (outer layer) composition. The structure and dielectric properties were investigated by X-ray diffraction (XRD), HRTEM (High Resolution Transmission Electron Microscopy) and electron probe micro analysis (EPMA), with different core to shell ratio (nc/ns). Compared with the designed composition BT–Nb-(0.2BZT-0.8BT), BT-(0.2BZT-0.8BT)–Nb was found to possess improved dielectric temperature stability, where the capacitance variation ΔC/C ≦ ±15% was achieved over a temperature range of −60–155 °C, with the dielectric constant and dielectric loss being in the order of 1860 and 0.011 at room temperature.

Effect of Glass Additive on Microstructure and Dielectric Properties of SrTiO3 Ceramics

The compositions of 0.9SrTiO 3 -0.1TiO2 with 0–8wt% Borosilicate glass additive were prepared. The microstructures and dielectric properties of the sintered bodies were studied. It was found a tiny amount of glass additive could lower the sintering temperature of the ceramics, and a very dense microstructure with fine grains was obtained, which resulted in higher dielectric constant and higher breakdown strength (BDS). However, As Borosilicate glass content increased, SrTiO3 had a tendency to react with the glass phase resulting in the productions of SrAl2Si2O8 and TiO2, which had a negative influence on the microstructures of ceramics, consequently degraded the dielectric properties.

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.

The Effect of ZnO–B2O3–SiO2 Additive on Sintering and Dielectric Properties of Ba0.3Sr0.7TiO3 Ceramics

ZnO–B2O3–SiO2 additive with 0.55: 0.35: 0.10 molar ratio was added to Ba0.3Sr0.7TiO3 ceramics in order to lower the sintering temperature. The densification behavior, crystalline structure, microstructure, dielectric properties and breakdown strength of ceramics were studied. When 9 wt.% additive was mixed with pure Ba0.3Sr0.7TiO3 powder, the densification temperature of samples decreased from 1350°C to 1100°C compared with no additive system, and the sintered ceramics revealed dielectric properties at room temperature as follows: dielectric constant ϵr = 413, dielectric loss tanδ = 0.00231 under 1 kHz and the breakdown strength Eb = 9.2 kV/mm.

Structure, dielectric properties and energy storage performance of barium strontium titanate thin films prepared by spin-coating technique

Ba_0.4Sr_0.6TiO₃ (BST) thin films were prepared by spin-coating technique and deposited on either Pt/TiO₂/SiO₂/Si (Pt/Si) substrate or silicon buffered by a lanthanum nickel oxide buffer layer (LNO/Si). X-ray diffraction and scanning electron microscopy showed that the BST films were crack free, compact and crystallized with a single perovskite structure. The effects of bottom electrodes on dielectric properties and energy storage performance of BST films were investigated. The results shows that the dielectric constant of BST/Pt film is a little higher than that of BST/LNO film, and the dielectric loss at 1 kHz of both films is very low, <;5⁰/₀₀. A butterfly loops with good dielectric tunability was observed in the capacitance-voltage (C-V) curve of BST/LNO film. The DC leakage current densities at 100kV/cm are ~9.3×10^-9A/cm² and ~1.0×10^-8A/cm² for BST/LNO and BST/Pt films, respectively. The P-E loops and energy storage performance of both films are similar. It was indicated that BST/LNO thin film can be applied as a cost-effective and environment-friendly capacitor for high-power energy storage.