| Barium titanate (BaTiO3)-based dielectrics have been attracted growing interest as promising candidate materials for energy storage capacitors because of their high dielectric constant, low dielectric loss and good insulation properties. BaTiO3-based dielectrics with high density and fine grains are expected to achieve satisfactory energy storage performance. Moreover, getting a deep insight into the mechanisms of polarization responses under external electric fields for BaTiO3-based dielectrics would be helpful for designing new compositions with the desired energy storage properties. In addition, reducing sintering temperature is believed to be favorable for the application of BaTiO3-based dielectrics in energy storage capacitors. This work was performed with respect to these considerations.In this work, Ba0.95Cao.05Zro.3Ti0.7O3(BCZT) powder and glass powder with the nominal composition of7wt.%CaO-5wt.%MgO-26wt.%Al2O3-62wt.%SiO2(CMAS) were synthesized by a citrate method. BCZT ceramics and composite ceramics with the nominal composition of BCZT+x wt.%CMAS (x=3-8) were prepared from the synthesized powders. The sintering, structure and dielectric properties of the ceramic specimens were examined. The effect of CMAS on the sinterability, structure and dielectric characteristics of the composite ceramic specimens was studied. The dielectric behavior and nonlinear dielectric properties under bias electric field was investigated. Moreover, the energy storage properties of the ceramics specimen were inspected.The morphology of BZCT and CMAS powders synthesized by the citrate method were evaluated. The results indicated that the synthetic route is advantageous in producing BZCT and CMAS powders with satisfactory morphology. The powders of BZCT and CMAS were derived at a low calcination temperature of650℃. The two powders showed superfine and uniform particles with similar sizes of about60nm.The sinterability, structure and dielectric properties of BCZT ceramic specimens were investigated. The superfine morphology of BCZT powder is efficient in improving the sintering of the ceramic specimens. The ceramic specimens sintered at1280℃attained a relative density of95%. The ceramic specimens sintered at1280℃had a cubic perovskite structure and a fine-grained (-890nm) microstructure. A typical ferroelectric relaxor behavior and a strong dielectric nonlinearity under bias electric field were confirmed for the ceramic specimens. The existence of polar nano-regions (PNRs) embedded in the paraelectric background of BCZT ceramic specimens was detected by dielectric inspection. The characteristic parameters of the PNRs in the ceramic specimens were determined by fitting the dielectric constants under the bias field to a multipolarization-mechanism model. The size and polarization of the PNRs in the ceramic specimens were determined to be-6um and-0.7μC/cm2, respectively. It was found that the polarization reorientation of the PNRs in the ceramic specimens under the bias electric field appreciably contributed to the dielectric nonlinearity while the relaxor behavior was related to the small size of the PNRs on a nano-scale.The effects of CMAS content on the sinterability, structure and dielectric properties of the composite ceramic specimens were investigated. While no obvious change in the crystalline structure of BCZT grains in the composite specimens was found, the addition of CMAS greatly promoted the sintering of the composite specimens and considerably reduced the size of BCZT grains. The composite specimens sintered at1140℃showed the relative densities exceeding95%and impressively fine grains of170-230nm. Similar to the case of BCZT ceramic specimens, the composite specimens displayed a ferroelectric relaxor behavior. Significant effects of CMAS on the dielectric properties were detected. The addition of CMAS led to an obviously decayed dielectric nonlinearity, a moderately degraded energy storage density, a weakened dependence of the dielectric properties on history of applied bias field together with enhanced insulation properties at high field levels and much improved energy storage efficiency. At the bias fields of130-150kV/cm, the composite specimens showed energy storage densities of0.30-0.53J/cm3and energy storage efficiencies of76.1%-84.1%, respectively. |
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