Varistor Grain Boundary Electrical Properties Of Tio <sub> 2 </ Sub>

In recent years with the development of automatic control circuits and semiconductor electronics, the design requirements of varistors have been turned into multi-purpose and low-voltage applications. As a capacitor-varistor multifunctional ceramics, TiO₂ varistors have been attracting more and more attention. The electrical properties of TiO₂ varistors, similar to ZnO varistors, are controlled by electro-static potential barriers, formed around the grain boundaries. ZnO varistors consist of multi-phases, such as spinel (Zn₇Sb₂O₁₂), pyrochlore, and bismuth-rich phase at grain boundaries. However, TiO₂ varistors comprise of single rutile phase. Because of the differences in phase-structures, ZnO and TiO₂ varistors exhibit different forming mechanism of barriers. The barrier model for ZnO varistors would not be suitable, if it was adopted in TiO₂ varistors without any correction. Therefore, it becomes necessary to find out a suitable barrier model for TiO₂ varistors. The primary objective of this dissertation is to propose a new barrier model for TiO₂ varistors by investigating the electrical characteristics of the grain-boundary barriers in TiO₂ varistors, including the forming mechanism of barriers, the temperature dependence, and the voltage dependence of barriers, etc.In consideration of the simple compositions, the distinct microstructures, and the effortless analysis of electronic defect states, the one-oxide-doped TiO₂ varistors were selected as the beginning of our research. It was found that TiO₂ + 0.25mol%WO₃ presented an optimal nonlinear coefficient of 9.6, a breakdown electrical field of 44.5V/mm, and TiO₂+0.25mol%Ta₂O₅ presented an optimal nonlinear coefficient of 5.6, a breakdown electrical field of 7.6V/mm.At first the I-V behaviors of TiO₂WO₃ and TiO₂Ta₂O₅ varistors were investigated in the prebreakdown region (1-10uA). The mechanism, by which the weak current flows across the boundary, is generally consistent with a thermionic emission process. The values of barriers were obtained through fitting the experimental results. It was found that TiO₂WO₃ and TiO₂Ta₂O₅ varistors exhibited a similar value and same temperature dependence.And then, the capacitance-vs-voltage (C-V) characteristics of these two varistor systems were studied to prove the results obtained by I-V measurement. It was found that there were differences between the values of barriers obtained by I-V and C-V measurement, especially in high temperature range. A correction for the values obtainedby I-Vmeasurement was made. It was found that the barrier height after corrected, 0.6eV, was consistent with the result obtained by C-V experiment. Moreover, it was found the barrier was constant in the temperature range lower than 150°C.Contrasted with the forming mechanism of barriers in ZnO varistors, a barrier model for TiCh varistors was proposed. According to the model, the molecular oxygen was absorbed at the T1O2 grain boundary regions, and, accordingly, the interface states must be formed by electrons trapped in the molecular oxygen. To maintain local electrical neutrality, a depletion layer was formed to compensate the charges. As a result, a double potential barrier was formed, which was responsible for the varistor behavior. From the results of C-V experiment, the barrier was determined as Schottky type. For TiO^WOs varistor system, the barrier parameters were obtained by C-V experiment; I =0.5leV, N*^l^XlO^/m'.tf^.OlXlo'Vm2, 2 surface, a new tentative explanation Was proposed. As on TiC>2 surface, there are three species for adsorbed oxygen: o\^, O2? and O" in the TiCh grain boundary regions. With the temperature increasing, O2" converted toOI? and Oi dissociated to O ? The transformation of adsorbed oxygen species gave rise to thechange of interfacial density of states. The change of interfacial density of states induced a movement of interface Fermi level. As a result, the barrier varied with temperature. Therefore, three activation energies were found through In pT — 1/T line.In this dissertation, Isothermal Capacitance Transient Spectroscopy (ICTS) was performed for TiC*2 varistors. Through ICTS, a value of interfacial density of states, 0.62eV below conduction band, was obtained. According to the analysis of barrier model,it was concluded that this value was identical with the position of O2" â– The dielectric properties of Er2O3-doped TiC?2 varistors were investigated by dielectric spectroscopy (DS). A Debye type dielectric relaxation with a thermally activated relaxation time was obtained. According to the electron energy level diagram of Schottky barrier in TiCb varistor, the Debye type dielectric relaxation is suggested to be associated with an ionized oxygen vacancy, Vq+ , in depletion layer at grain boundary. Moreover, the energy level of deep donor Vq+ shifted toward the conduction band at high temperature range. The origin of the energy shift was supposed to be related to the degradation of the barrier at grain boundary.Furthermore, the Impedance Spectroscopy (IS) for TiC?2 varistors doped with P^On was investigated. Compared the results with DS measurement, a similar Debye type relaxation was found in impedance spectroscopy. However, this relaxation is suggested to be associated with interface trap states, rather than Vq+ . Through the Electric modulus, M, the value of this interface trap states, 0.42eV below conduction band, was obtained.In the impedance measurement, a new Impedance Spectroscopy induced by the applied voltage, not the high temperature, was found. There are few reports about this impedance spectroscopy for varistors. For Sc2(>?-doped TiC>2 varistors, the relaxation behavior presented in impedance spectroscopy induced by voltage was investigated byElectric modulus, it/ and Complex permittivity, C*. ft was found that in the high frequency range, the impedance spectroscopies will deviate from the track of the semicircle, which is a type diagram for the impedance spectroscopy induced by temperature. So the difference between these two impedance spectroscopies suggested different mechanism. The thorough investigation and study on the impedance spectroscopies induced by voltage is underway.