Study Of Niobate-based Glass-ceramic Composite Dielectrics

With the progress in the high-tech field, realization of miniaturized portable high energy density device has become the orientation of long-time development and achieved rapid development, leading to very urgent demand for dielectric materials with both high dielectric constant and high breakdown strength. Recently, zero-porosity glass-ceramic dielectrics prepared by the controlled-crystallization technology play an important role in the field of the high energy density dielectric materials. In contrast with the conventional sintering ceramics, these materials exhibit the advantages of having both high dielectric constant and high breakdown strength. It can be predicted that the glass-ceramic dielectric composites have higher dielectric energy storage density. Thus, these materials are strong candidates for the applications in many areas, such as high energy density ceramic capacitors, high power pulse technology, et al. However, the design and preparation of glass-ceramic dielectrics with comprehensive excellent properties still face some critical engineering problems.In the field of dielectric glass-ceramic materials, a large number of studies focus on niobate-based glass-ceramic systems whose typical phases are tungsten-bronze phases (A1)4(A2)2Nb10O30, perovskite phases MNbO3and glass phase SiO2. In these glass-ceramic dielectrics with dual dielectric ceramic phases, in-depth systemic study of the precipitation process of the ceramic phases will contribute to the adjustment of the microstructure and dielectric properties in final glass-ceramics. This study can provide basic guidance of the engineering practical application of these materials, which has great practical significance. Based on this, the central theme of this paper will focus on the improvement in energy density of these materials. Taking the element replacement and composite in A-site of tungsten bronze phases in these glass-ceramics as a starting point, we mainly analyze the influence of the composition triangle variation caused by the replacement and composite of Pb, Ba and Sr in A-site of tungsten bronze ceramic phases on the crystallization process, microstructure and dielectric behaviors of these glass-ceramics. The purpose of this systemic study is to achieve the adjustment of the key dielectric properties of these materials. In this paper, the crystallization, microstructure and dielectric performance of glass-ceramics in ANb2Ob-NaNbO3-SiO2,A=((1-x)Pb, xSr)(PSNNS), ANb2O6-NaNbO3SiO2.A(1-y)Pb,yBa)(PBNNS),ANb2O6-NaNbO3-SiO2.A=((1-z)Ba, zSr)(BSNNS) systems were discussed in detail. Furthermore, we analyzed the solution behavior between tungsten-bronze phase and perovskite phase and the structures of the ceramic phases precipitated in the glass-ceramics were discussed through XRD full profile analysis method.For PSNNS system, the element substitution effects of Sr for Pb on the crystallization behavior, microstructure and dielectric performance have been investigated. X-ray diffraction (XRD) analysis revealed a major crystal phase transition in glass matrix as the crystallization temperature increased. At low temperatures (700～750℃), the major crystal phases precipitating in the glass matrix are identified as Pb2Nb2()7(x=0),(Pb, Sr)2Nb2O7(0.2≤x≤0.8) and Sr2Nb2O7(x=1.0); while at higher temperatures (≥850℃), heat treatment produces different crystalline phases, PbNb2O6and NaNbO3for x=0, SrNb2O6and NaNbO3for x=1, and the solid solution of these three phases for0.2≤x≤0.8. TEM results show that the shape of the crystallized ceramic particles gradually varies from sphere to dendrite, leading to the reduction of breakdown strength. At different crystallization temperatures, the dielectric properties of the glass-ceramics show a strong dependence on the chemical composition x. At low temperatures (700～750℃), the dielectric constants of all samples (0≤x≤1) exhibit monotonic variation trend with increasing x and excellent electric-field stability; while at higher crystallization temperatures (≥850℃), owing to the MPB-effect, a maximum of the dielectric constant is found for the composition x=0.6. The substitution effect of Pb and Sr at A-site in tungsten-bronze phase ANb2O6was systematically studied for the purpose of energy storage density optimization. Within the studied substitution range for x=0to x=1.0, the highest energy density of2.27J/cm3is found at x=0.6, which is twice higher than those of the end-products (i.e.0.872J/cm3for x=0and0.988J/cm3for x=1.0).Analogically, for PBNNS system, X-ray diffraction (XRD) analysis also revealed a major crystal phase transition in PBNNS glass matrix as the crystallization temperature increased. The glass-ceramics treated at low temperatures (700℃～750℃) contain dielectric phases with tungsten-bronze (BaNb2O6) and pyrochlore (Pb2Nb2O7) structures, while the dielectric phases are transformed to tungsten-bronze ((Ba, Pb)Nb2O6) and perovskite (NaNbO3) structures at higher crystallization temperatures (≥850℃). Corresponding to the result of phase transition, microstructural observation proves increasing crystallite sizes with increasing crystallization temperature. At low temperatures (700～750℃), a maximum of the dielectric constant of the PBNNS glass-ceramic is found for the composition y=0.6; while at higher crystallization temperatures (≥850℃), both of the dielectric constants and the electric-field dependence of dielectric constant exhibit a decreasing trend with increasing;/for all samples (0≤y≤1).Furthermore, for lead-free nanostructured BSNNS glass-ceramics, the element substitution influences of Sr for Ba on crystallization process, microstructure and dielectric performance have been investigated. X-ray diffraction (XRD) analysis revealed a major crystal phase transition in the glass matrix as the crystallization temperature increased. TEM bright field observation shows that spherical nanometer-sized particles appear in the samples with z=0and z=0.6, while typical dendritic grains are found in the glass-ceramics with z=1.0. All the glass-ceramics present an increasing trend of dielectric constant, dielectric loss and electric-field dependence of dielectric constant with increasing crystallization temperature. In particular, the increasing z leads to a decrease of these three dielectric properties for the glass-ceramics heat-treated at750℃, while dielectric constant and electric-field dependence of dielectric constant both exhibit an increasing trend with the increasing z except z=0.6and an abnormal increase at z=0.6for the samples heat-treated at above850℃due to MPB performance.Finally, the solution behavior between tungsten-bronze (TB) phases and perovskite (P) phases in niobate based glass-ceramics has been analyzed and discussed. For glass-ceramics in PbNb2O6-NaNbO3-SiO2, BaNb2O6-NaNbO3-SiO2and SrNb2O6-NaNbO3-SiO2systems, the decreasing relative volumetric ratios of TB/P (VolTB/Volp) lead to the increasing dielectric constant with the increasing crystallization temperature. From the analysis of the A-site occupation behavior in the ANb2O6-ANbO3solution process, as the A-site ionic radiuses are closer, the solid solution degree of these two phases is higher. The presence of A2Nb2O7phase with low dielectric constant increases the crystallization temperature of ANb2O6phase with high permittivity.A2Nb2O7is a stable phase at low crystallization temperatures. Nb2O5content plays an important role on the formation and transition of A2Nb2O7phase. At low crystallization temperatures,A2Nb2O7phase is formed and the excess Nb2O5is stored in glass phase. When the crystallization temperature increases,ANb2O6is produced by the reaction of A2Nb2O7and the excess Nb2O5existing in glass phase.