Nb-doped BaTiO3 Ceramics Research Articles

Space-charge-induced colossal dielectric constant and large flexoelectricity in Nb-doped BaTiO3 ceramics

Donor doping can increase the dielectric constant of a material by several orders of magnitude due to induced space charge causing interfacial polarization. Giant dielectric properties and interface polarization are also both expected to greatly enhance the flexoelectric behavior of a material. In this work, a typical flexoelectric ceramic material, BaTiO3, was selected and donor doped using elemental Nb. Compared with the nominal BaTiO3 ceramic, the dielectric constant and flexoelectric coefficient of the Nb-doped BaTiO3 ceramics were significantly improved. The transverse flexoelectric coefficient of 0.3 mol. % Nb-BaTiO3 was found to increase to nearly 40 times the nominal value, reaching 387 μC/m. The results indicate that the giant dielectric response, and therefore the giant flexoelectric response, is the result of the combined effects of internal barrier-layer capacitance and surface barrier-layer capacitance. This study not only deepens the understanding of the semiconductor macro-dielectric effect and the flexoelectric mechanism caused by doping, but it also provides a feasible strategy for the design of giant dielectric/flexoelectric response materials and related devices with high dielectric constants and flexoelectric coefficients.

An effective optimization strategy and analysis for BaTiO3–Na0.5Bi0.5TiO3 system with colossal permittivity

Colossal permittivity (CP, ε＞104) behavior in BaTiO3–Na0.5Bi0.5TiO3 (BT-NBT) ceramics has been studied, which showed extremely high permittivity up to ~105. Dielectric properties of samples showed Debye-like relaxations in the frequency range 20Hz–30MHz. Two different polarizations located in grain boundaries and grains respectively are responsible for the CP behavior and the models of defect charge compensation achieved by niobium doping are proposed to explain the phenomenon of abnormal variation of dielectric constant.By using defect engineering, a Nb-doped BaTiO3 ceramics with stable colossal permittivity (εr =1.3×104 at 1kHz and room temperature)，high bulk resistivity (>1010Ω·cm) as well as relative low dielectric loss (tanδ~0.06) has been obtained over a wide temperature range of −55–150°C, satisfying IEA X8R specification, which has a potential application prospect in high capacity solid supercapacitor.

Effects of ultrasonication and conventional mechanical homogenization processes on the structures and dielectric properties of BaTiO3 ceramics

The effects of the homogenization process on the structures and dielectric properties of pure and Nb-doped BaTiO3 ceramics have been investigated using an ultrasonic homogenization and conventional mechanical methods. The reagents were homogenized using an ultrasonic processor with high-intensity ultrasonic waves and using a compact mixer-shaker. The components and crystal types of the powders were determined by Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analyses. The complex permittivity (ε′, ε″) and AC conductivity (σ′) of the samples were analyzed in a wide frequency range of 20Hz to 2MHz at room temperature. The structures and dielectric properties of pure and Nb-doped BaTiO3 ceramics strongly depend on the homogenization process in a solid-state reaction method. Using an ultrasonic processor with high-intensity ultrasonic waves based on acoustic cavitation phenomena can make a significant improvement in producing high-purity BaTiO3 ceramics without carbonate impurities with a small dielectric loss.

Effects of Nb2O5 doping on the microstructure and the dielectric temperature characteristics of barium titanate ceramics

In this work, the effects of Nb2O5 addition on the dielectric properties and phase formation of BaTiO3 were investigated. A core–shell structure was formed for Nb-doped BaTiO3 resulted from a low diffusivity of Nb5+ ions into BaTiO3 when grain growth was inhibited. In the case of 0.3–4.8 mol% Nb2O5 additions, two dielectric constant peaks were observed. The Curie dielectric peak was determined by the ferroelectric-paraelectric transition of grain core, whereas the secondary broad peak at lower temperature was due to strong chemical inhomogeneity in Nb-doped BaTiO3 ceramics. The dielectric constant peak at Curie temperature was markedly depressed with the addition of Nb2O5. On the other hand, the secondary dielectric constant peak was enhanced when sintered above 1280 °C for higher Nb2O5 concentrations (≥1.2 mol%). The Curie temperature was shifted to higher temperatures, whereas the transition temperature corresponding to the secondary peak moved to lower temperatures as increasing the amount of Nb2O5 more than 1.2 mol%. The decrease of this lower transition temperature was assumed to be closely related with the secondary phase formation when Nb concentration greater than 1.2 mol%. From XRD analyses, a large amount of secondary phases was observed when Nb2O5 amount exceeded 1.2 mol%. The coefficients of thermal expansion of Nb-doped BaTiO3 were increased with increasing Nb2O5 contents, resulting in large internal stress between cores and shells. Therefore, the shift of Curie temperature to higher temperatures was attributed to internal stress resulting from the formation of a core–shell structure and a large amount of secondary phase grains.

Sintering Behavior of Pure and Nb-Doped BaTiO<sub>3</sub> Powder Prepared by High-Gravity Reactive Precipitation

Sintering behavior of pure and Nb-doped barium titanate powders prepared by high-gravity reactive precipitation (HGRP) process was investigated. A thermal dilatometer was employed to monitor the sintering shrinkage continuously during sintering. TEM and SEM were used to characterize the microstructures of the samples. The experimental results confirmed that Nb-doped BaTiO3 ceramics sintered at 1220°C for 2 h reached a larger densification of 97.0% relative density compared with 94.6% of pure BaTiO3 ceramics sintered at 1265°C for 2 h, and Nb-doped BaTiO3 ceramics had a smaller grain size (1.2μm) than that of pure BaTiO3 ceramics (2μm).

PTC effect in doped BaTiO3 ceramics: evidence for a bulk contribution

Using photolithographic techniques, four-probe electrical resistivity measurements of Nb-doped BaTiO3 ceramics, at the microscopic scale, are presented from room temperature up to 180 degrees C. The PTC effect displayed by several grains exhibits an abrupt jump at the Curie temperature, followed by a monotonic quasi linear increase of the resistance. This behaviour looks quite different from the usual PTC effect observed on macroscopic ceramics. The jump is clearly due to a change of resistance within the bulk. It appears, from the microscopical results, that the bulk itself must be partially involved in the increase of resistance above Tc contrary to the generally accepted theory.

About the interpretation of the PTC effect in Nb-Doped BaTiO3ceramics

In order to check the consistency of the well known Heywang model for explaining the PTC effect, DC resistivity, AC impedance and thermoelectric power measurements have been performed on Nb-doped BaTiO3 ceramics. It is found that the Heywang model can fit the shape of the experimental DC resistivity data, but it appears that the donor density in the grain is much lower than the nominal one, while impedance measurements emphasize the role of the grain boundary resistance. The thermoelectric power exhibits an anomalous behavior in contradiction with the usual semiconductivity mechanism of electrical transport.

Dielectric and piezoelectric aging of pure and Nb-doped BaTiO3 ceramics

Abstract Pure and 0.8 at % Nb-doped BaTiO3 were sintered between 1175 and 1250°C. Structural, microstructural and electrical properties were determined. A. c. conductivity and electromechanical coefficients were measured in as-prepared samples and at different time intervals until a year after. In pure BaTiO3 the aging process increases the bulk and grain boundary resistivity. Samples, poled near the Curie temperature, showed an anomalous aging effect on kt. In Nb-doped samples, a decrease in the grain boundary resistivity was observed after one year aging. An exceptionally high thickness coupling factor of 0.62 was found in as-poled samples, which strongly decreased with time.