Doped ZnO Varistors Research Articles

Structural, morphological, and electrical characterization of heteroepitaxial ZnO thin films deposited on Si (100) by pulsed laser deposition: Effect of annealing (800°C) in air

Pulsed laser deposition technique was used for growing thin films of ZnO on Si (100) substrate held at different temperatures (Ts). All the as-deposited films have shown a preferential c-axis orientation associated with varying grain size as a function of Ts ranging from 100to600°C. Current-voltage (I-V) characteristics of these films show Ohmic behavior over the entire Ts range studied. These films were subjected to annealing at 800°C in air ambient for 4h. The grain size was observed to increase after the annealing process for all the films deposited at different Ts. Interestingly, these annealed films show nonlinear variation of current with applied voltage, very similar to the one observed in doped ZnO varistors. The nonlinear parameters such as the asymmetric behavior of change in current on the polarity reversal of voltage, the plateau region, and the break down voltage are observed to depend on Ts. This nonlinear behavior can perhaps be explained on the basis of electronic conduction model proposed for the bulk, doped ZnO varistors. The role of insulating intergranular layers between the disoriented microcrystallites is expected to be similar to that played by insulating intergranular layers in the doped ZnO varistors.

Microstructure and Electrical Properties of Doped ZnO Varistor Nanomaterials

A sol-gel method of preparation doped ZnO varistor nanomaterials is described, The influences of doped ZnO nanomaterials for varistor microstructure and electrical properties (nonlinear coefficient α, breakdown voltage V1mA , dielectric constant ε, and dielectric loss tan δ) are investigated. Compared with the conventional mixed oxide technique, varistor ceramic of prepared by nanometer materials showed a more homogeneous microstructure, smaller grain sizes, higher densities and excellent electrical properties.

Fully Dense, Fine‐Grained, Doped Zinc Oxide Varistors with Improved Nonlinear Properties by Thermal Processing Optimization

Fully dense, doped ZnO varistors were prepared using an easy two‐stage pressureless‐sintering method at temperatures as low as 825°C, with a grain size of ∼0.5 μm. After the highly nonohmic ZnO varistors were sintered, their fine microstructure consisted of uniformly sized grains, small spinel grains with partially dissolved manganese and cobalt oxides discontinuously distributed in the fairly wide grain boundaries, and an intergranular layer of bismuth‐rich crystalline phase mainly detected at three or four ZnO grain junctions. There were twins near the middle of almost all the ZnO grains. The abnormally high nonlinear properties of the almost nanostructured varistors (FB≈ 6–8 kV/mm and α= 270) were attributed to a uniform and very fine microstructure, a high ZnO–ZnO grain direct contacts concentration, and a uniform hybrid layer substructure (grain boundaries and twin boundaries) with different (but probably accumulative) potential barriers.

Photoacoustic Spectra on the Bi or Pr Doped ZnO Varistors.

PA spectra with several sintering temperatures on the Bi doped and Pr doped ZnO varistors were measured using the gas microphone method and compared, where the grain structure was observed by the laser microscope. The PA signals in the long wavelength excitaion light decreased as the sintering temperature increased, although the PA signals were large at the short wavelength region on the all samples. The PA signal intensity seems not to depend on the surface area caused by the grain structure change, but it might also relate to the quality of the sintered samples such as the deep levels or defects in the grain. The Pr doped samples need the higher sintering temperature to get the high quality samples than the Bi doped samples.

Coprecipitation preparation and voltage sensitive characteristics of doped ZnO varistor

A process of preparing ZnO voltage-sensitive ceramics doped with some oxides by coprecipitation was described in the paper. The thermal properties of ZnO nanometer powders and the current-voltage characteristics of the ceramics have been investigated. The results showed that the six-additive ZnO powders with hexagonal system were homogeneous grain size distribution in microstructure, the optimal reaction pH is 6.90±0.05, the temperature for calcining and sintering was at about 500 °C and 1100 °C, respectively. The powders were also examined by SEM, IR and XRD etc, and the effect of doping La2O3 on the electrical properties of ZnO varistor was investigated too.

Effect of Liquid Phase and Vaporization on the Formation of Microstructure of Pr Doped ZnO Varistor

Our previous works and our recent data were summarized to discuss the effect of liquid phase formation and vaporization of the components on the densification, grain growth and change of the microstructure of Pr doped ZnO ceramics in air. In the ZnO-Pr2O3 binary system, eutectic liquid forms at 1382± 5°C° and significant vaporization of the components occurred above the eutectic temperature. Below eutectic temperature (1350°C), only 0.1 mol % of Pr doping into ZnO brought about the grain growth of ZnO, however further addition of Pr brought about the suppression of the grain growth. Comparing the grain size distribution at the surface and inside part of Pr doped ZnO ceramics, it was clarified that wider grain size distribution was observed at the surface than inside part. At the temperature just below the eutectic (1370°C), abnormal grain growth was also observed. Depth profile of Pr content indicated that no concentration gradient was observed below eutectic temperature (1350°C), however above eutectic temperature (1500°C), condensation of Pr was observed at the surface. Grain growth rate as well as weight loss rate were drastically increased between 1350 and 1370°C, which suggested that formation of liquid phase accelerate the grain growth rate and weight loss rate. Consequently the microstructure of Pr doped ZnO ceramics was formed by both effects on the formation of liquid phase and on the vaporization of the components.

Current Change Method of Reducing Gas Sensing Using ZnO Varistors

The conductivity of partially sintered doped ZnO varistor pellets was shown to increase via two mechanisms with introduction of or CO/Ar atmospheres. Reducing gas extracted chemisorbed oxygen, which in turn reduced current‐opposing potentials which had formed at grain boundaries. This mechanism was gas concentration independent. After more extended times, the conductivity showed a second increase as extraction of lattice oxygen near the grain boundaries further lowered the grain boundary Schottky barriers. During this time period, the time rates of change of current through the sensor were shown to increase with concentration (1 to 20%), when the compact sustained continuous porosity, and was heated between 300 and 500°C. This resulted from the rate of extraction of lattice oxygen being limited by the availability of gas at grain boundary neck surfaces. CO/Ar atmospheres were less effective at removing chemisorbed and lattice oxygen, and current slope sensitivity to CO concentration was not demonstrated.

Quantitative Icts Measurement of Interface States at Grain Boundaries in ZnO Varistors

ABSTRACTIsothermal capacitance transient spectroscopy(ICTS) measurement is applied to ZnO:Pr varistors. A simple peak corresponding to the interface states at grain boundaries was obtained and the energy level of the interface states revealed to be monoenergetic and located around 0.9eV below the conduction band. The cross section was calculated as around 10−14 cm2. Quantitative treatment of the ICTS intensity in relation to the density of interface states at grain boundaries was established. The density of interface states was obtained from the linear relation between ICTS intensity and reciprocal carrier density(ND). According to experiments on series of rare-earth doped ZnO varistors, the interface states of Pr, Tb or Nd doped ZnO varistors had higher density of states than La or Er doped varistors. Moreover, application of ICTS measurement to single grain boundary using microelectrodes revealed that higher density of interface states gave higher nonlinearity in I-V characteristics.

New low-voltage varistor composites

New varistor-type polymer composites for low-voltage application have been developed. The filler is made of commercially available doped ZnO-varistor powder. The polycrystalline filler particles act as varistors due to their typical grain-boundary structure. The presented varistor composite materials show very low values for the breakdown field strength down to 200 V mm−1, as compared with already existing varistor-type composites, and fairly highα-values in the range of 10.

The electrical properties and d.c. degradation characteristics of silver doped ZnO varistors

ZnO-Bi2O3-based ceramic varistors containing up to 2500 p.p.m. Ag2O were prepared by the mixed oxide route. The products were characterized in terms of current-voltage (I–V) behaviour, capacitance-voltage (C-V) behaviour and d.c. degradation behaviour. Additions of silver did not significantly affect ZnO grain growth or sintered density, but decreased the non-linearity exponent, a, and increased the breakdown voltage. Silver-doped specimens exhibited an unusual d.c. degradation behaviour, with the leakage current initially decreasing before reaching a stable value. For doping levels ≥ 500 p.p.m. Ag2O, the change in breakdown voltage was positive for both forward and reverse directions. Silver doping appeared to increase the Schottky barrier height under conditions of continuous d.c. stress and improve the degradation behaviour. The optimum levels of Ag2O addition were in the range 500–1000 p.p.m.

Role and effect of Co2O3 additive on the upturn characteristics of ZnO varistors

The effect of Co2O3 additions on the characteristics of electric field-current density was investigated at high current densities. With the increase of mole % of Co2O3, the onset of voltage upturn descends to lower current density due to the decrease in the conductivity in ZnO grains. This is confirmed by impedance measurements at high frequencies. Analysis on the capacitance-voltage characteristics shows that the decreased grain conductivity is attributed to the lower carrier density caused by the reduced concentration of interstitial Zni or oxygen vacancy, VO. In the case of the highly doped ZnO varistors containing more than 0.5 mole % Co2O3, the lowered carrier mobility improves the decrease in grain conductivity.