Li Shengtao

Physical meaning of conductivity spectra for ZnO ceramics

With the help of broadband dielectric spectroscopy in a wide temperature and frequency range, the conductivity spectra of ZnO polycrystalline ceramics are measured and the direct-current-like (DC-like) conductivity and relaxation polarization conductivity are observed successively along the frequency axis. According to the classical Debye theory and Cole—Cole equation, the physical meanings of the two conductivities are discussed. It is found that the DC-like conductivity corresponds to electron transportation over the Schottky barrier at the grainboundary. The relaxation polarization conductivity corresponds to electronic trap relaxation of intrinsic point defects (zinc interstitial and oxygen vacancy). When in the high frequency region, the relaxation conductivity obeys the universal law with the index n equal to the index α in the Cole—Cole equation as an indictor of disorder degree.

Influence of Pulse Steepness on Vacuum Flashover of Casting Epoxy Resin

Research on the pulsed flashover characteristics of vacuum insulation material is significant for the design and manufacture of pulse power devices. In view of the voltage increase rate of the fast pulse (pulse steepness), the vacuum flashover characteristics of pure epoxy under different steepnesses is investigated by using a pulse generator with parameters of a rise time of 40 ns and a full width at half maximum of 2.5 μs. Pulses with six levels of steepness were achieved by changing the charging voltage of the generator. Based on the linear equation and electron emission equation, the relationship between the flashover voltage and pulse steepness was fitted. By virtue of the fitted formula, it was possible to predict the flashover voltage under near DC or higher steepness conditions. Based on the electron emission equation, the relationship between the time delay and flashover field was also fitted. Result shows that F-N electron emission dominates the flashover process.

Investigation on the dielectric properties of nano-titanium dioxide — low density polyethylene composites

Dielectric properties of nano-titanium dioxide (TiO2) with diameter about 25nm — low density polyethylene (LDPE) composites were investigated at the temperature ranging from 293K to 343K with the frequency ranging from 0.1Hz to 100kHz in the present paper. Experimentally, we observed two peaks in the spectrums; one appears in both pure and incorporated LDPE (abbreviated as peak A) while the other appears only in the samples with nano-TiO2 content exceeding 1wt% (abbreviated as peak B). The activation energies calculated from dielectric spectrum for peak A and peak B are 1.09eV and 0.8eV respectively; they are independent of the concentration of TiO2 nanofiller. Furthermore, the position of peak B is also independent of the nano-TiO2 content in the frequency spectrums. Thus, it is reasonable to conclude that peak B is intrinsic, corresponding to the a response of LDPE while peak A may originate from the traps at the “Interaction Zone” around the nanoparticles. The dielectric properties of nanocomposite were determined by such “Interaction Zone”.

Effect of Tunneling Current on Schottky Barrier Height in ZnO Varistors at Low Temperature

On the basis of the Schottky barrier and thermionic emission models, the temperature dependence of barrier height in ZnO varistors is investigated by the I – V characteristics in a wide temperature range from 93 K to 373 K. The obtained barrier height decreases with reducing temperature, which is ascribed to the contribution of tunneling current in measured current. From the proposed equivalent circuit, it is suggested that two current components coexist. One is thermionic emission current, which reflects the thermionic emission barrier height. The other is tunneling current, which appears even at low voltage, especially in low temperature ranges, and thus makes the barrier height obtained from measured current vary with temperature.

Dielectric properites of Al-doped CaCu 3 Ti 4 O 12 ceramics by coprecipitation method

Undoped and Al-doped CaCu3Ti4O12 Ceramics (CCTO) ceramics were prepared by the coprecipitation method, and the electric and dielectric properties were investigated. The concentration of Al-doped in CCTO influenced the electric and dielectric properties. Through adjusting the concentration of Al-doped, we obtained polycrystalline ceramics samples with the improved properties of dielectric loss and high dielectric permittivity over the wide frequency range. The results indicated that a high dielectric constant and low dielectric loss can be achieved by Al3+ substitution for Ti4+, because the Al-doped CCTO could enhanced the grain boundary resistivity, and the optimum doping concentration is about x=0.3. With x=0.3 Al doping, the dielectric constant was 16000 at 1 kHz and the loss value was below 0.1 over the frequency range of 102 Hz to 104 Hz.

Residual voltage caused by grain boundary in ZnO varistor ceramics

The residual voltage of ZnO varistor ceramics is not only determined by grain resistance but also by the process of grain boundary breakdown. Al-doped ZnO varistor ceramics were prepared on the basis of traditional composition by the solid stated method. The current-voltage characteristics (I–V), dielectric spectroscopy of the samples were measured and the grain boundary micro-parameters were calculated, including the height and width of the Schottky barrier, the carrier concentration and so on. It was indicated that the barrier height plays a major role in determining grain boundary residual voltage. From this point, the grain boundary residual voltage can be decreased by controlling the grain boundary barrier through adjusting the concentration of intrinsic defects with doping method.

SrTiO/sub 3/ capacitor-varistor ceramics exhibiting surface effect

The electrical properties and microstructure of SrTiO/sub 3/ capacitor-varistor ceramics sintered in different atmospheres are investigated. The results show that samples sintered in a strong reducing atmosphere of 25%N/sub 2/+75%H/sub 2/ and heat-treated in air have a low-resistive core with a high-resistive surface layer around it, which we name the surface effect. XPS results reveal that the surface effect results from an oxygen diffusion process during heat treatment.

An automatic measuring device based on PC for MOVs' aging test

A new kind of system based on a PC for the metal oxide varistor (MOV) aging test has been developed, which can detect and analyze the leakage current signal with the aid of the computer. The principle of the system is presented in brief.

Polarization absorption currents and conduction response characteristics of zinc oxide varistors

The absorption currents and conduction response characteristics of zinc oxide varistors were measured with step voltage pulses of different amplitude, and a series resistance was used to stimulate the ZnO grain resistance of zinc oxide varistors. Based on an analysis of the experimental results, three kinds of absorption current were found, and an equivalent circuit describing the electrical properties of zinc oxide varistors was developed. It is deduced that conduction response time results from the equivalent capacitance of the grain boundary, and there is a close relation between the conduction delay phenomenon and the polarization absorption current. The larger the ZnO grain resistivity the shorter is the conduction response time.<<ETX>>