Minghe Cao

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.

Grain size dependence of relaxor behavior in CaCu3Ti4O12 ceramics

The dielectric properties show a typical ferroelectric relaxor behavior in CaCu3Ti4O12 ceramics with different grain sizes in the temperature range of 400–600K. The relaxation strength and the frequency dispersion increase with grain size decreasing, which can be due to the different stresses in grains with different sizes. These variations have been confirmed by the modified Curie-Weiss law. The results fitted well by the Volgel-Fulcher relationship and the variations in the slim current-voltage loops are correlated with the microstructures closely. It can be proposed that the grain size is one of the main factors of affecting the relaxor behavior.

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.

Structure Control of NaNbO3 Template for Textured Ceramics Synthesized by Two-Step Molten Salt Method

In this paper, through control the precursors structure with layer morphology, the plate-like particles with different edge length and thickness can be syntheszed by the two step molten salt (MSS) method. The results of SEM showed that the NaNbO3 particles were characterized by platelet-like morphology with an edge length of 10∼20 μm and a thickness of 1∼4 μm. The results of XRD indicated the NaNbO3 particles synthesized had a strong orientation in {0 0 l} crystal planes. The plate-like NaNbO3 particles we synthesized are a suitable template candidate to fabricate textured ceramics by TGG or RTGG method, especially to the textured potassium sodium niobate lead-free piezoelectric ceramics.

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.

Energy Storage Characteristics in Sr(1-1.5x)BixTiO3 Ceramics

Due to their poor frequency stability and high dielectric loss compared to common energy storage ceramics, bismuth strontium titanate ceramics are rarely employed for energy storage. This paper systematically researches the energy storage characteristics of Sr(1-1.5x)BixTiO3 ceramics. The bismuth strontium titanate ceramics were prepared via the traditional solid phase sintering method. The experiment results indicate that, as x = 0.100, Sr(1-1.5x)BixTiO3 ceramics possess fine frequency stability, high dielectric constant, low dielectric loss, high resistivity, high energy storage efficiency and weak DC electric field dependence of dielectric constant. At 217.6 kV/cm electric field, the energy storage density of Sr(1-1.5x)BixTiO3 ceramic (x = 0.100) reaches 1.63 J/cm3. Therefore, it is a potential dielectric material for energy storage.