Zinc Oxide Varistors Research Articles

Origin of the pulse response characteristics of zinc oxide varistors

Pulse response characteristics are reported for simple varistor materials in the ZnO-Bi2O3-MnO2 system. It is shown that small additions of Li2O or In2O3 can enhance or eliminate the materials inductive behavior, which is responsible for voltage overshoot and current undershoot. A possible connection of the inductive behavior with deep electron traps, which is suggested by the equal time constants at room temperature of the inductive current rise and the electron emission from the traps, is examined. Measurements of the pulse response characteristics at low temperature reveal a weak temperature dependence that fails to support such a connection. A phenomenological model is given that predicts the varistor pulse response characteristics. Both the experimental results and the model predictions are discussed in relation to theories that connect the inductive response to electron holes that become trapped at varistor barriers.

Zinc oxide varistors

An overview is given of zinc oxide varistors, which are made of semiconducting ceramics with highly nonohmic current-voltage characteristics, which originate at the grain boundaries. These varistors are widely utilized to protect electric power lines and electronic components against dangerous voltage surges. This overview covers electrical characteristics, fabrication methods, effects of additives, microstructures, the conduction mechanism, degradation, and applications.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Intercrystallite boundaries in zinc oxide varistor ceramics

Intercrystallite boundaries in zinc oxide varistor ceramics

ChemInform Abstract: Zinc Oxide Varistors Made from Powders Prepared by Amine Processing.

AbstractIt is demonstrated that the amine process is an excellent method to produce active and uniform powders for varistor production.

Non-linearity in rare earth doped zinc oxide varistor prepared by flash combustion method

A novel processing technique involving flash combustion of nitrate-urea mixtures has been reported for praseodymium doped zinc oxide containing K 2O in the range 0·0–0·5 mol%. The starting average powder particle size was 2 μm with a tap density of 1250 kg/m 3. The non-linear behaviour of the prepared zinc oxide varistor was examined in the field range 200–650 V/mm. The breakdown voltage was seen to attain a maximum value of about 600 V/mm in compositions containing 0·3 mol% K 2O. The variation of breakdown voltage is explained on the basis of the microstructure and K 2O content.

Synthesis and microstructure of gel-derived varistor precursor powders

A gel pyrolysis method for making zinc oxide varistor precursor powders is described. The gels are prepared from metal acetates, citric acid and ethylene glycol. Air pyrolysis of these gels yields mixed oxides. Transmission electron microscopy and X-ray diffraction show the resultant powders to comprise 0·04 micron ZnO particles and 0·5 micron aggregates of ZnO, Bi 2O 3, and other metal oxides. These powders densify at lower temperatures than conventionally prepared oxides, presumably due to increased capillary pressure. High-field varistors (switching stress > 10 kV/cm) with good nonlinear characteristics result from sintering at only 900°C.

Admittance—Frequency Response in Zinc Oxide Varistor Ceramics

The lumped parameter/complex plane analysis technique revealed several contributions to the terminal admittance of the ZnO—Bi2O3 based varistor grain‐boundary ac response. The terminal capacitance has been elucidated via the multiple trapping phenomena, a barrier layer polarization, and a resonance effect in the frequency range 10−2≤f≤ 109 Hz. The characterization of the trapping relaxation behavior near ∼ 105 Hz (∼ 10−6 s) provided a better understanding of a previously reported loss‐peak. The possible nonuniformity in this trapping activity associated with its conductance term observed via the depression angle of a semicircular relaxation in the complex capacitance (C*) plane has been postulated.

Microstructure and Crystal Structure of Bismuth Oxide Phases in Zinc Oxide Varistor Ceramics

The microstructure and crystal structure of bismuth oxide phases of as‐sintered and additionally heat‐treated ZnO varistor ceramics was examined by means of transmission electron microscopy and X‐ray powder diffraction. Grains of β‐Bi2O3 present in as‐sintered samples were found to consist of two types of domains which have the same basic tetragonal lattice. They can, however, be distinguished by the contrast between local strain centers due to microdefects present in one domain. Some multiple‐grain junctions contained both α‐ and β‐Bi2O3 grains. Heat treatment induces a transformation of the α‐ and β‐Bi2O3 phases into γ‐Bi2O3. A clear crystallographic relationship between the transforming phases β to γ is shown. The β‐Bi2O3 phase was also found to transform into the non‐stoichiometric crystalline Bi2O2.33 phase.

Fast pulse response of zinc-oxide varistors

The fast pulse response of zinc-oxide varistors is studied for both high-impedance and low-impedance pulse sources. The effects of varistor capacitance and source impedance on the pulse response are examined, and quantitative descriptions of the pulse response are presented. Evidence that the varistor capacitance does not increase near breakdown voltages is found, and the implications of this result with regard to models of varistor conduction are discussed. The relationships between the properties of a varistor and its ability to protect against fast pulses are considered.

Electrical properties of ZnO-Bi2O3 thin-film varistors

A ZnO-Bi2O3 two layer composite has been studied as a model of a single grain boundary in a ceramic zinc oxide varistor whose excellent non-linearity of V-I characteristics has been believed to arise from the ZnO grain boundary. The two-layer specimen formed by sputtering shows highly non-linear V-I characteristics whose coefficient of non-linearity for reverse-bias equals or exceeds those of actual ceramic varistors. This specimen also shows a decrease in resistance after voltage application which corresponds to the degradation of ZnO varistors. This system, therefore, can be used not only as a model system for studying mechanisms of conduction and degradation of ZnO varistors but also as a thin-film varistor. The authors find that a negative charge and associated potential barrier at the ZnO-Bi2O3 interface control the non-linear V-I characteristics. A decrease of the negative charge causes the decrease of the barrier height and thus the current increase in the low-voltage region, i.e. the degradation. They suggest that the high non-linearity under reverse-bias is due to electron tunnelling through the barrier at the reverse-bias ZnO-Bi2O3 interface. This governs the non-linearity in an actual ZnO varistor whereas the forward-bias interface acts as an electron injector into an inter-granular layer.

Radiation hardness of zinc oxide varistors

The current-voltage characteristics of zinc oxide varistors were examined prior to and after exposure of up to 107 rad (Si) of 60Co gamma radiation. Nominal voltage, current-voltage nonlinearity, and leakage currents remain relatively unaffected by large doses of ionizing radiation.

Modélisation des mécanismes de conduction non linéaires dans les varistances ZnO

A calculation method has been developed for the current-voltage characteristic of zinc-oxide varistors. This method calls for the standard models of double Shottky barrier and thermionic emission in the low current range where the non-linearity coefficient α is below 30. The novel part of the model lies in the treatment of the intermediate current range which corresponds to the highest values of α. An impact ionization mechanism generated by high-energy electrons is put forward to explain the appearance of a hole current directed to the interface between two zinc-oxide grains. The ionization probability is calculated by a Monte-Carlo method and compared with analytic relations which form both a lower bound and an upper bound for the result. Hole recombination is taken into account using a number of assumptions on the energy distribution and the capture cross-section of the localized states at the interface. The positive charge resulting from hole trapping at the interface brings about a sudden lowering of the potential barriers between zinc-oxide grains. The model provides nonlinearity coefficients higher than 60, in agreement with the experiments Calcul des caracteristiques I-V en utilisant le modele de double barriere de Schottky et d'emission thermoionique, dans la zone des faibles courants ou le coefficient de non-linearite α est inferieur a 30 et un mecanisme d'ionisation d'impact initie par des electrons d'energie elevee pour expliquer l'apparition d'un courant de trous dirige vers l'interface entre deux grains de ZnO, pour la zone des courants intermediaires

Bulk electron traps in zinc oxide varistors

Three types of commercial zinc oxide varistor materials were examined using admittance spectroscopy between 30 and 350 K. Maxima in the ac conductance at frequencies from 1 to 100 kHz are observed. The maxima are interpreted as arising from electron traps located within the depletion regions of double Schottky barriers at ZnO-ZnO grain boundaries. Two traps are observed in each material and are likely to be from common origin. The trap energies are found to be 0.17 and 0.33 eV below the conduction-band edge. The traps are likely to be associated with native defects in ZnO and may influence device characteristics such as voltage overshoot.

Electrical Surface Breakdown of Thin-Film ZNO Varistors

Samples of zinc oxide varistor material representing a thin film are studied. An electrical discharge is consistently produced at electric field strengths and energy levels both well below those required to cause bulk material with the same composition to fail. The investigation examines each of the major issues which could be responsible for this anomalous behavior. Electrode/sample contact integrity, electrode shape, and surface considerations are studied. It is concluded that the discharge is directly related to the surface preparation. Study of the sample surface under different preparation conditions indicates that macroscopic as well as microscopic surface properties influence the behavior of the discharge.

Chemical preparation and properties of high-field zinc oxide varistors

Chemical preparation methods were developed for high-field ZnO varistors which used precipitation techniques to prepare precursor powders. Varistors were made by sintering uniaxially pressed pellets in the range of 675°–740 °C in air. Properties of these varistors included electric fields (E) in the 10–100 kV/cm range at current densities (J) of 5 A/cm2, nonlinearity coefficients (α) greater than 30 for 2.5≤J≤5.O A/cm2, and densities in the range of 65%-99% of theoretical depending both on sintering temperature and composition.

A high-resolution transmission electron microscope study of a zinc oxide varistor

Investigations were made of varistor microstructure, the morphology of Bi2O3 at multiple ZnO grain junctions, Bi2O3/ZnO grain boundaries and ZnO/ZnO grain boundaries (especially whether Bi2O3 is present or not at the ZnO/ZnO grain boundary) by means of high-resolution transmission electron microscopy and X-ray microanalysis in the scanning transmission electron microscope. Bi2O3 at multiple ZnO grain junctions consists of small particles of 0.1μm in diameter, and they are vitrified to some extent. It is suggested that bismuth ions dissolve into ZnO grains over a 30 nm range from a Bi2O3/ZnO grain boundary; however, there is no bismuth at ZnO/ZnO grain boundaries.

Grain boundaries in semiconductors

This review presents the available experimental and theoretical understanding on the structure and electronic properties of grain boundaries in semiconducting materials. High-resolution electron microscope images of interfaces are interpreted within the framework of the structural unit model of grain boundaries, and the electronic properties of the grain boundaries discussed with relation to the popular symmetric Schottky barrier model for charge trapping and potential barrier formation. It is shown that these models give some limited understanding of the physical processes that occur at grain boundaries in elemental semiconductors, but that in compound semiconducting materials the effects of non-stoichiometry at the boundary regions must also be considered. Segregation of impurity and dopant species to the boundaries can have significant influence on their electrical properties, and the question of what structural or chemical features are responsible for the observed properties is posed. Diffusion at semiconductor grain boundaries is also discussed, and finally the electrical properties of zinc oxide varistor material are presented in the light of the models of carrier interactions with grain boundaries.

Electrical characteristics of zinc oxide varistors subjected to hydrostatic pressure

This paper presents the changes of electrical characteristics in zinc oxide varistors subjected to hydrostatic pressure up to 4 kbar. Electrical measurements have been carried out with pressurized samples. Beyond 1 kbar, the leakage current increases exponentially with pressure as described by the relationship IF=P7,4. This leads to a decrease, by a factor of 2, of the nonlinear coefficient. In spite of choosing a different experimental approach and different material compounds, our results are in very good agreement with those already published.

直流しゃ断用酸化亜鉛素子の熱応力解析

直流しゃ断用酸化亜鉛素子の熱応力解析

SEM EBIC INVESTIGATIONS OF ZnO VARISTOR CERAMICS

The electron beam induced conductivity (EBIC) of zinc oxide varistor ceramics is studied in the scanning electron microscope (SEM). It is found that only particular grain boundaries give rise to an EBIC signal and that the signal strength and its linescan profile show strong variation with bias voltage. The experimental results are discussed in terms of Schottky emission of majority carriers across the grain boundary potential barrier.