Varistor Behavior Research Articles

Microstructure and electric properties of a SnO 2 based varistor

High non-linear J× E electrical characteristic (α=41) were obtained in the Nb 2O 5 and Cr 2O 3 doped CoO highly densified SnO 2 ceramics. X-ray diffraction analysis showed that these ceramics are apparently single phase. Electrical properties and microstructure are highly dependent on the Cr 2O 3 concentration and on the sintering temperature. Excess of Cr 2O 3 leads to porous ceramics destroying the material’s electrical characteristics probably due to precipitation of second phase of CoCr 2O 4. Dopant segregation and/or solid solution formation at the grain boundaries can be responsible for the formation of the electrical barriers which originate the varistor behaviour.

Depth resolved conductive mode imaging of varistor grain boundaries

An SEM conductive mode method is described for estimating the angle a grain boundary makes with the specimen surface, by varying the beam energy. Application of this technique to sloping grain boundaries in zinc oxide eliminates grain boundary geometry as a cause for a certain type of conductive mode contrast that is incompatible with existing models for varistor behaviour. Factors responsible for this contrast are discussed. Estimation of the grain boundary declination is also of use in crystallographic studies of interfaces in polycrystalline zinc oxide, enabling the grain boundary plane to be estimated and hence providing the necessary information for calculating the grain boundary structure without recourse to (destructive) sectioning techniques.

Electron Spectroscopy and Holography of Electrically Active Electroceramic Interfaces

Many electroceramics derive their technologically useful and scientifically appealing properties through electrically active grain boundaries (GBs). Examples of such properties include traditional nonlinear current-voltage relationship (i.e. varistor behavior) and positive temperature of coefficient of resistance (PTCR effect), to recently “rediscovered” phenomena of space-charge formation across functional interfaces, including ferroeletric thin films.In many such cases, the transport of charge across these interfaces is mediated through potential barriers which form at the core of these interfaces. For example, varistor behavior is attributed to formation of Schottky barriers at GBs which modify the transport of electrons, leading to the nonlinearity in I-V characteristics. The formation of Schottky barrier at the GB core, in turn, is attributed to the formation of space-charge across GBs. The formation of space-charge, further, is clearly attributable to decoration of GB core with charged defects with compensating opposite charge across the GB. The presence, sign, magnitude and spatial distribution of space-charge form the key to understand the Schottky barriers, thus the transport properties of interfacial systems.

Electronic structure of grain boundaries in SrTiO3

The formation of Schottky potential barriers and their complex interplay with current transport across electrically active grain boundaries (GBs) give rise to many novel properties of electroceramics such as the varistor behavior. The origin of the GB electrical activity lies in the defect chemistry, and associated variations in geometry, chemistry and electronic structure at GBs. Motivated by the recent experimental results of dopant identification and spatially resolved quantification of GB charge and associated space-charge across GBs in SrTiO3, we have performed atomic-level electronic structure calculations on GBs in SrTiO3. The first-principles density-functional embedded-cluster Discrete-Variational (DV) method is used to determine charge densities and densities of states (DOS) for several idealized models of symmetrical tilt GBs in SrTiO3 which have been derived from a combination of atomistic latticestatic simulations and experimental electron microscopy analysis.

Effect of Bi2O3 addition on the microstructure and electrical properties of the SnO2.CoO.Nb2O5 varistor system

Tin dioxide (SnO2) is an n type semiconductor with crystalline structure of rutile type and has low densification rate due to its high surface diffusion at low temperatures and high SnO2 partial pressure at high temperatures [1]. Dense SnO2 based ceramics can be achieved by introducing dopants [2–5] or by hot isostatic pressure processing [6, 7]. Dopants with valence ‡2 can promote densification of SnO2 ceramics due to the formation of solid solution with the creation of oxygen vacancies. In our previous work [8, 9] we have shown that the CoO and Nb2O5 doped SnO2 ceramic is single phase and presents varistor behaviour with AE ˆ 8.0. By adding 0.05 mol % Cr2O3 the system is still a single phase with the non-linear coefficient increasing to AE ˆ 41. The electrical behaviour of varistors is governed by the presence of voltage barriers at the grain boundaries [10, 11]. For a given varistor system each voltage barrier is characterized by a specific value vb. The reference voltage barrier for a varistor is determined by the average number of barriers n in series multiplied by vb, that is: Vr ˆ n:vb (1)

An ab initio study of oxygen vacancies and doping process of Nb and Cr atoms on TiO2 (110) surface models

We theoretically investigated how the formation of oxygen vacancies and the addition of niobium and chromium atoms as dopants modify the varistor properties of TiO2. The calculations were carried out at the HF level using a contracted basis set, developed by Huzinaga et al., to represent the atomic centers on the (110) surface for the large (TiO2)15 cluster model. The change of the values for the net atomic charges and band gap after oxygen vacancy formation and the presence of dopants in the lattice are analyzed and discussed. It is shown that the formation of oxygen vacancies decreases the band gap while an opposite effect is found when dopants are located in the reduced surface. The theoretical results are compared with available experimental data. A plausible explanation of the varistor behavior of this system is proposed. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 65: 625–631, 1997

Atomic Structure of the (310) Symmetrical TILT Grain Boundary in Strontium Titanate - A Comparison among Experimental Methods and Atomistic Simulations

Strontium Titanate is a prototypical perovskite oxide whose macroscopic electroceramic properties, like varistor and grain boundary barrier layer capacitor (GBBLC) behavior, are controlled by grain boundary (GB) phenomena. In this paper, we present a comparative analysis of the results of both experimental and theoretical investigations of the atomic structure of a (310) symmetrical tilt GB (a Σ=5 (310) [001] GB in the CSL notation) in SrTiO3. Bicrystals of SrTiO3 were procured from Shinkosha Ltd., and electron backscatter kikuchi pattern analysis as well as transmission electron diffraction studies confirm that the GB conforms to the Σ =5 misorientation. The atomic structure of this GB has been investigated using high resolution transmission electron microscopy (HRTEM) and associated image simulations, as well as by atomistic simulations using static iterative energy minimization schemes (“lattice-statics”).2

An investigation of acceptor-doped grain boundaries in

Grain boundary (GB) doped exhibits interesting electroceramic phenomena including varistor and barrier layer capacitor behaviour. We present here our investigation of GB acceptor-doped using analytical electron microscopy including electron holography. Mn was diffused into sintered polycrystalline to attain GBs which are rich in Mn. The presence and spatial extent of Mn at the GBs were analysed using x-ray emission spectroscopy (XES) and parallel electron energy loss spectroscopy (PEELS). The valence state of Mn was determined using PEELS to be predominantly +2. Finally, transmission high-energy electron holography was utilized to directly image and quantify the electrostatic potential and associated space-charge across the GBs directly. The holography results reveal a negatively charged GB with positive space-charge, indicating that Mn with a valence of +2 resides as an acceptor dopant on the Ti site at the GB core. The barrier height and local charge density distribution, including the Debye length, of the double Schottky barrier at the GB are derived from these holography results. This investigation demonstrates the usefulness of electron holography as a bulk-sensitive technique to probe the statics and dynamics of electrostatic field distribution and electrical charge across interfaces in technologically useful materials, and the need to employ diverse analytical techniques for such an investigation.

Influence of microstructural disorder on the current transport behavior of varistor ceramics

Microstructural disorder in the form of variations in grain size, grain boundary area between adjacent grains and grain boundary barrier heights are characteristic features of semiconducting ceramics exhibiting varistor behavior. We investigate the effect of such disorder by computing the current flow through a network of grains whose boundaries are described by nonlinear electrical characteristics. Disorder is found to decrease the maximum attainable nonlinearity of the varistor current-voltage characteristics and also cause a rounding of the breakdown portion of the curve. Disorder also has the effect of introducing high conductance percolative paths into the microstructure which can cause localization in current flow. We show that current localization can be quantitatively described by two statistical parameters: an effective number of current paths through the network and a participation ratio. The former is related to the infinite moment of the current distribution whereas the latter is related to the second moment.

Varistor Performance of ZnO‐Based Ceramics Related to Their Densification and Structural Development

Electrical potential barriers are often observed in ZnO‐based ceramics. Earlier studies on ZnO photoconduction have shown that the narrow regions, where the sintered grains have grown together, control the resistance of the entire sample. In those regions, the surface/volume ratio is sufficiently high for the acceptor concentration (which occurs because of adsorbed oxygen) to exceed the donor concentration inside the ZnO grains. More recent works have shown that Schottky barriers result from interface states because of the chemisorbed oxygen ion at the ZnO‐ceramic grain boundaries. The work reported in this paper involves the relationship between the densification of the microstructure and the varistor performance of ZnO ceramics. The emphasis of densification percentage as an indicator of the degree of sintering shows the desirability of continuity across ZnO grain boundaries, without the presence of voids or films of second phases, in optimizing varistor behavior. The effect of oxygen partial pressure on the development of varistor microstructure and electrical properties, as well the kinetics of grain growth, during the sintering process have been determined and are discussed.

Gas Sensing Behavior of Porous ZnO Varistors Densified via Rate Controlled Sintering

AbstractA hydrogen gas sensor based on shift in breakdown voltage of a ZnO varistor was developed. Changes in rate controlled sintering schedules resulted in significant variations in microstructure. Microstructures showing more intergranular porosity and finer grain size demonstrated a comparatively more linear and broad response to H2 concentration. A mechanism for the shift in breakdown voltage based on adsorbed oxygen is suggested for this unique and potentially very useful sensor.

Production of doped ZnO powders for varistor applications using sol-gel techniques

The electrical behaviour of ZnO varistors is controlled by the characteristics of the ceramic microstructure which depends strongly on the properties of the initial powder. This paper describes a study of the effect of different chemical methods to synthesise doped ZnO powders. The ceramic powders were synthesised by the following routes: a) classical mixing of oxides (for comparison purposes), b) aqueous solution of inorganic salts and c) hydrolysis and polycondensation of metalorganic compounds in an organic solvent. The physicochemical characteristics of these powders were evaluated using a scanning electron microscope and thermoanalytical instrumentation. Standard spray drying technology was used to pelletise the powders to obtain an agglomerated powder suitable for uniaxial pressing. The discs were sintered in an electric furnace under air atmosphere using several temperature programmes. The ceramic microstructure was characterised using SEM and X-ray diffraction. The effects of powder processing route on sintering and microstructural development are discussed. Powders prepared by the metalorganic route exhibited somewhat lower sintering temperatures than conventional powders. However, the rate of sintering was slower for metalorganic and aqueous solution powders. These observations were related to powder morphology. The ZnO−Bi2O3−CoO system exhibited the best varistor characteristics as it was expected, whereas the binary systems supported much lower voltages at low currents than ternary systems.

Effect of Cr2O3 in the varistor behaviour of TiO2

Laboratorio Interdisciplinar de Eletroquimica e Cerâmica Departamento de Quimica UFSCar, C.P. 676, 13565-905 Sao Carlos, SP

Effects of Nb2O5 in (Ba,Bi,Nb)-added TiO2 ceramic varistors

Novel (Ba,Bi,Nb)-added TiO2 ceramic was previously proved to be potentially useful as a varistor material, and by varying atmosphere compensation the roles of barium and bismuth have been explored. The present work attempted to investigate how the third additive, niobium, operates on varistor characteristics. The content of added niobium is in the range from 0.125 to 1.0 cat. %, while that of the other additives is always maintained at a constant value. The results show that an adequate addition of niobium, 0.25 cat. % Nb, is beneficial to the improvement of varistor characteristics. The variations of varistor behavior are discussed in terms of a point defect model. Two kinds of charge compensation, for the substitutional incorporation of niobium into the titanium rutile lattice, are also used to illustrate the variation. The detailed results and discussions are shown in this paper.

Current-limiting property of n-BaTiO 3 ceramics

The current-voltage ( I–V) relations of donor-doped BaTiO 3 ceramics have four distinct regions in increasing order of applied potentials: (i) a linear portion at lower voltages which includes a current maximum, (ii) a negative differential conductivity (NDC) region, (iii) a nearly constant current segment and (iv) a slow upturn behaviour. For the bulk compositions having the isovalent lattice substituents, the NDC region is broadened out and the body temperatures T b attained vary with the shift in Curie point T c . In the case of ceramics with mixed-phase character (tetragonal and cubic), the linear portion directly gives way to current-limiting behaviour without any maximum. The T b values attained during the current-limiting process are much lower than T C . When T a > T C , the I–V characteristics change from current limiting to voltage limiting (varistor). Under these thermal conditions, T b is always greater than T C . The varistor behaviour could be achieved at room temperature by using isovalent substituents. The present results show that the current-limiting behaviour cannot be treated as a mere consequence of the positive temperature coefficient of resistance (PTCR). This is because the limiting currents I lim calculated from the resistivity-temperature relations in conjunction with body temperatures are three to five orders of magnitude lower than the measured values. Furthermore, current-limiting behaviour has been noted for certain titanate ceramics having no PTCR. The present observations indicate that current-limiting behaviour arises from the combined influence of Joule heating and field effect. Joule heating increases the cubic phase content much below T C owing to diffuse phase transformation behaviour in semiconducting BaTiO 3, a fact which has been experimentally demonstrated in the present investigation. The charge-trapping behaviours of the midband gap states, particularly of the acceptor type, are different for the tetragonal and the cubic phases. This imparts a heterojunction character to the tetragonal-cubic interface regions. Tunnelling across such an asymmetric barrier, generated under Joule heating, will account for the current-limiting characteristics, whereas tunnelling across a more symmetric barrier, at T a > T C , leads to voltage-limiting behaviour.

Atomic structure of undoped Σ=5 symmetrical-tilt grain boundary in strontium titanate

Strontium titanate is an important electroceramic material which, under appropriate processing and dopant additions exhibits both varistor and Grain Boundary Layer Capacitor (GBLC) behavior. The presence of electrically active grain boundaries is essential for obtaining these properties. The first step towards correlating the grain boundary structure to properties is to determine the detailed atomic structure of the boundary, which includes its geometric structure, chemistry and electronic structure. We present here our TEM investigation of the atomic structure of an undoped (“pristine”) symmetrical tilt grain boundary in SrTiO3. This provides the basic reference structure, changes to which can be studied as a function of doping and/or processing parameters, and correlated to electrical and dielectric properties.Self-supported TEM samples were made from bicrystals of SrTiO3 through the conventional sample preparation route. Fig. 1 shows a high resolution electron micrograph (Hitachi H9000) of a representative grain boundary region in the sample.

Behaviour of metal-oxide-based varistors subjected to partial discharges in air

The behaviour of zinc oxide varistors subjected to partial discharges in air is studied with respect to their protective coating (glass and epoxy resin). The decomposition of the medium with residual impurities can considerably influence the varistor current-voltage characteristics. Degradation is due to a conducting deposit around the varistors and a physical corrosion of the coating. The conducting deposit is analysed by physico-chemical methods. The authors show the influence of the partial discharge energy, humidity and surrounding gases on the varistor degradation. Moreover they point out a significant modification of the l(U) characteristic in the ohmic region, with a large increase of leakage current following degradation.

Impedance Spectroscopic Study of Zinc Oxide Varistors

ABSTRACTPolycrystalline zinc oxide containing oxides of bismuth, cobalt, manganese and antimony is found to exhibit highly non-ohmic I–V characteristic arising due to the presence of schottky barriers at the grain boundaries. Defect states present at the grain–grain boundary interfaces store excess negative charge and give rise to such potential barriers. Any improvement in the I–V characteristics therefore demands the understanding of grain boundary properties of the material. Impedance spectroscopy is found to separate grain and grain boundary contributions in a polycrystalline material and is used to determine grain boundary contributions towards the electrical behavior of the zinc oxide varistors. It also helps in identifying stored charges and defect states present and elucidates different trapping phenomena occurring at the interface behavior of zinc oxide varistors.

Influence of the particle size and addition of K 2O on the non-ohmic behaviour of ZnO praseodymium oxide varistors

The role of minor additions of K 2O in praseodymium oxide-doped ZnO varistors was investigated by changing the K 2O concentration on the one hand and the starting particle size of ZnO on the other. The results of this investigation show that up to 0·3 mol% K 2O effectively controls ZnO grain growth, especially with submicron powders, resulting in a considerable increase in the breakdown field. However, when the starting ZnO particle size is larger, higher K 2O additions are required in order to have an effective increase of the breakdown field. Hence it is possible to arrive at higher breakdown fields for praseodymium-doped zinc oxide, either by selecting zinc oxide particles in the submicron range or by the controlled use of additives such as K 2O.

Development of interfacial microstructure during cooling of a ZnO varistor material

The development of interfacial microstructure during cooling of a ZnO varistor material from the sintering temperature has been studied by a combination of analytical electron microscopy and x-ray diffractometry. It was found that pyrochlore is the first Bi-rich phase to form from the liquid-phase sintering medium during cooling from the sintering temperature. When cooling at normal rates, pyrochlore forms between 940 and 740 °C, and γ-Bi2O3 forms at about 740 °C. Quenching from higher temperatures results in the formation of δ-Bi2O3 from the Bi-rich intergranular liquid. The formation of barriers to electrical conduction at individual ZnO junctions was studied by use of microelectrodes. The barriers started to form while cooling between the temperatures of 1000 and 800 °C. Quenching from higher temperatures did not result in varistor behavior, but barriers to electrical conduction could be created in such specimens by post-sintering heat treatments.