Preparation And Electrical Properties Of TiO 2 -based Giant Dielectric Ceramics

Colossal permittivity (CP) (> 103) materials with sufficiently low dielectric loss, and insignificant temperature dependence have been attracting continually increasing interest driven by the need for device miniaturization and high-energy-density storage applications. In 2013, Nb5++In3+ co-doped rutile TiO2 ceramics have been discovered as a high-potential dielectric material, which possess colossal dielectric permittivity (4x104) as well as low dielectric loss (<0.05)over a broad temperature range from 80 to 450 K. Since (Nb0.5In0.5)xTi1-xO2 ceramics were found to show giant dielectric constant, it has attracted particular attention. However, according toprevious research work on this material system, we could conclude that it is really difficult to reproduce such good performance whether by changing the dopants or employing some new technology to prepare the ceramics, moreover, the mechanism of colossal permittivity have yet to further clarify. Therefore, it becomes very important to optimize the preparation technology of TiO2 based ceramics, explore the relationship between structure and electric properties as well as the related mechanisms of the CP phenomenon. In this paper, the NbxTi1-xO2 (NTO), (Nb0.5In0.5)xTi1-xO2 (NITO), (NbxIny)0.06Ti0.94O2 ceramics were sintered under flowing N2 atmosphere and the evolution law of phase structure, microstructure and electric properties of the samples were revealed. Furthermore, the physical mechanism of colossal permittivity was also discussed in detail. The main contents were as follow:1. NTO ceramics with giant dielectric, low dielectric loss and good frequency-independence were prepared by solid-state method in N2 atmosphere. The Effects of sintering conditions on electrical properties were investigated systematically. The samples with the highest density were obtained from raw material powder calcined at 1060℃, sintered at 1400℃ in N2 atmosphere about 10 h, and all of NTO ceramics presented giant dielectric constant. The NbxTi1-xO2 ceramics with x=4% possess a giant permittivity (εr=1.3×104) accompanied with a low dielectric loss 4.3% at 10 kHz. According to the analysis about element valence state, with increasing the content of Nb5+, the content of Ti3+ in NTO ceramic increased first and then decreased, while the oxygen vacancies in materials decreased gradually, which suggested that the giant dielectric response in high frequency range origins from electron-pinned defect-dipoles effect.2. A series of NITO ceramics with different doping level were synthesized by solid-state method in N2 atmosphere. All of the ceramics gained giant dielectric(> 15000), meanwhile, with increasing the doping level, the dielectric constant of materials increased in low frequency rang, the dielectric loss gradually decreased and the frequency independence obviously intensified. When the doping level was about 6%, the ceramic exhibited a giant dielectric constant (ε’= 18690) and a relatively low dielectric loss (tanδ=0.047) around room temperature at 10 kHz. Furthermore, the samples showed good temperature stability (△CT/C25℃=-11.4%to 7.4%) in the temperature range from 100 to 350 K at 10 kHz.In terms of the analysis to the complex impedance spectroscopy, electric modulus and the element valence state, it can be indicated that electron-pinned defect-dipoles effect had an great contribution to giant dielectric response in high frequency range.3. (NbxIn1-x)0.06Ti0..94O2 and (NbxIn(4-5x)/3)0.06Ti0.94O2 ceramics were designed by changing the ratio of Nb5+/In3+ based on maintaining the mole ratio of atoms and charge balance. The components of (Nb0.6In0.4)0.06Ti0.94O2 are donor doping and (Nb0.4In0.6)0.06Ti0.94O2 are acceptor doping; The components of (Nb0.6In1/3)0.06Ti0.9402 introduced cation vacancies while (Nb0.4I1n2/3)0.06Ti0.94O2 introduced cation interstitial in the system. Moreover, all components showed giant dielectric characteristic and all of them obtained relatively small dielectric loss (-0.03) and better temperature dependence except for (Nb0.6In0.4)0.06Ti0.94O2 ceramic. In addition, the resistances of materials were enhanced enormously. According to the electric modulus and corresponding Arrhenius relation, it can be concluded that different doping methods will introduced various kinds of points defects and the activity of free electrons and oxygen vacancies will be significantly influenced so as to appear diverse dielectric responses.