| With the progress of science and technology and speed of engineering demand, to make the energy storage devices smaller and lighter has become the industrial consensus target. Glass-ceramic, which is produced through melt and fast cooling followed by controlled crystallization techniques and has both the high dielectric constant and high breakdown strength, plays an important role in the miniaturization and lightweight of the devices. According to the relationship that the energy storage density of an ideal dielectric grows linearly with the dielectric constant and quadratically with the breakdown strength, it is easy to predict that dielectric glass-ceramic composite has the very good potential to acquire high energy storage density, thus it is favorable for the miniaturization and lightweight of the energy storage devices. However, because the dielectric glass-ceramic is known for a short period of time, the related theoretical and experimental research have not yet been in full swing, the comprehensive performances shown in the engineering are not completely ideal, it is of great importance to improve the performances of the dielectric glass-ceramics.Glass-ceramic composites constituted of nano-ceramics of (A1)4(A2)2Nb10O30with tungsten bronze structure and NaNbO3with perovskite structure and amorphous SiO2glass matrix were investigated systematically by our research group before. Based on the substitution among elements of Ba, Sr and Pb in the A-site of the tungsten bronze structured crystals, the composition triangle of niobate glass-ceramics was formed. The effect of the substitution in the A-site of the tungsten bronze structured crystals on the precipitation process of nano-ceramics, the microstructure of glass-ceramics and various dielectric properties was studied. Both the structural and dielectric properties of the niobate glass-ceramics were improved through regulating the precipitation of nano-ceramic phases.For the present investigation, five key compositions of BaO-Na2O-Nb2O5-SiO2, SrO-Na2O-Nb2O5-SiO2. PbO-Na2O-Nb2O5-SiO2.(Pb0.6, Ba0.4)O-Na20-Nb2O5-SiO2and (Pb0.4, Sr0.6)O-Na2O-Nb2O5-SiO2were selected as the basic glass-ceramic composition. Different types and contents of rare earth oxides were added to them and the consequent study were conducted on the effect of rare earth addition on the precipitation process of nano-ceramics, the microstructure and various dielectric properties of the niobate glass-ceramics. The purpose of this study is to further improve the crystallization properties and dielectric properties of niobate glass-ceramics by adding trace rare earth.Differential thermal analysis (DTA) showed that the glass transition temperature and the crystallization temperature were increased by rare earth addition, and the processing performance of glass was improved. The crystallization behaviors of the niobate glass-ceramics added with rare earth oxides were studied. As the temperature of heat-treatment increased, the nano-scale ceramics in the glass-ceramic in BaO-Na2O-Nb2O5-SiO2system underwent a phase evolution from the single tungsten bronze structure to the coexistence of both tungsten bronze and perovskite structures. The whole profile Rietveld analysis displayed that the rare earth ions doped into the nano-ceramic phases in the glass-ceramic in BaO-Na2O-Nb2O5-SiO2system crystallized under high temperature. When the amount of Lu2O3was larger than1mol%. a new crystalline phase appeared in the glass-ceramic in BaO-Na2O-Nb2O5-SiO2-Lu2O3system (crystallized under1000℃). In the glass-ceramics with the compositions of PbO-Na2O-Nb2O5-SiO2-R2O3(R=La. Gd) and (Pb0.4, Sr0.6)O-Na2O-Nb2O5-SiO2-La2O3, another crystalline phase with pyrochlore structure was formed.Research on the basic dielectric properties showed that the dielectric constant of the niobate glass-ceramics increased with the addition of rare earth oxides. Especially in the glass-ceramics in BaO-Na2O-Nb2O5-SiO2-La2O3system, as the addition of La2O3rose from0to3mol%, the dielectric constant of glass-ceramic increased from328to754, which is about130%higher. The loss of glass-ceramic samples could remain relatively low. In several glass-ceramic systems, rare earth addition plays an important role in improving both the frequency and the temperature stability of the basic dielectric properties including dielectric constant and loss.For glass-ceramics in systems of SrO-Na2O-Nb2O5-SiO2-R2O3(R=La, Gd), PbO-Na2O-Nb2O5-SiO2-R2O3(R=La, Gd),(Pb0.6, Ba0.4)O-Na2O-Nb2O5-SiO2-R2O3(R=La, Gd) and (Pb0.4, Sr0.6)O-Na2O-Nb2O5-SiO2-R2O3(R=La, Gd), the electric field stability of dielectric constant was improved by rare earth addition. In glass-ceramics in systems of SrO-Na2O-Nb2O5-SiO2-R2O3(R=La, Gd) and PbO-Na2O-Nb2O5-SiO2-R2O3(R=La, Gd), the addition of rare earth decreased the leakage current density. Adding rare earth and increasing heat-treatment temperature enlarged the surrounded area of the P-E curve and increased the maximum polarization Pmax. However, in the glass-ceramics in systems of SrO-Na2O-Nb2O5-SiO2-R2O3(R=La, Gd), PbO-Na2O-Nb2O5-SiO2-R2O3(R=La, Gd) and (Pbo.4, Sr0.6)O-Na2O-Nb2O5-SiO2-R2O3(R=La, Gd), the area surrounded by the P-E curve and the maximum polarization Pmax were decreased by rare earth addition.The dc breakdown strength of niobate glass-ceramics crystallized at high temperature followed the two-parameter Weibull distribution well and all the plots showed a relatively good linearity. The average breakdown strength of glass-ceramic in basic composition was increased by rare earth addition. By means of polarization curve measured under the electric field in close to breakdown, the improvement of stored energy density and discharged energy density by rare earth addition were observed.Microscopic analysis showed that the nano-ceramic particles uniformly distributed in the glass matrix and the glass ceramics were of a compact structure. As the crystallization temperature increased, the size and content of nano-ceramic particles increased gradually. The addition of rare earth optimized the microstructure of the niobate glass-ceramics. A new crystalline phase containing Lu could be clearly seen in the glass-ceramic in BaO-Na2O-Nb2O5-SiO2-Lu2O3system with the amount of Lu2O3higher than1mol%. Observation of HAADF-STEM and EDS mapping demonstrated the Gd doped into the nano-ceramics. It is the very doping effect of rare earth ions that influenced the dielectric properties of niobate glass-ceramics. |
