Commercial Varistors Research Articles

The effect of processing conditions on varistors prepared from nanocrystalline ZnO

Nanoparticles of ZnO were prepared by the reaction of ethanolic solutions of zinc acetate and oxalic acid followed by drying (80 °C) and calcination (500 °C). Subsequently varistor materials were fabricated from this nanoparticular ZnO via two separate routes:- a) from a “core shell” material using metal salts as additives; b) by using a conventional solid state mixing of metal oxides. Sintering (1050 °C) and subsequent electrical studies were carried out for each of these samples and they were compared with commercial varistor samples prepared under similar conditions. “Core shell” type varistor material showed considerably higher breakdown voltage (Vc = 850 ± 30 V mm−1) as compared to a sample prepared by mixing with metal oxides (Vc = 683 ± 30 V mm−1) or commercial varistor discs (Vc = 507 ± 30 V mm−1). The high breakdown voltage obtained is attributed to the formation of more varistor-active grain boundaries per unit area.

Temperature behaviour of ZnO varistors before and after post sintering heat treatment

The temperature behaviour of ZnO varistor discs has been examined. Criteria have been found that predominantly determine the thermal behaviour of a ZnO varistor disc. In addition, it has been found that a post sintering heat treatment can change the temperature behaviour. The majority of the tempered samples are in agreement with established models. However, one varistor disc, consisting of a commercial varistor mixture, appeared to be deviant. The tempered samples of this mixture showed an improved thermal behaviour, i.e., a significantly lower increase of the leakage current with increasing temperature.

Effect of trap density on the dielectric response of varistor ceramics

A careful analysis of the dielectric response of ZnO-based commercial varistors in the frequency range between 100 Hz and 15 MHz and temperature range from 290 to 400 K enables a complete characterization of deep level electronic transitions in space-charge regions. In the investigated systems, oxygen vacancies are the origin of trap states situated ∼0.30 eV below the conduction band. However, samples exhibit a contrasting dielectric response concerning the power law exponent of the loss peak. First we present the case of near-Debye behaviour modeled by a Cole-Cole response with slight deviations α∼0.1 (it is ascribed to a distribution of activation energies) and next the case of very broad loss peaks (nearly constant-loss dielectric response). In this last case, the relative increment in the value of the real part of the capacitance ( C 0- C ∞)/ C ∞ (which is commonly used as a measure of the density of trapping states in semiconductor devices) reaches higher values than those obtained for the former. This suggests that dielectric loss broadness can be regarded as an indication of high density or a disordered distribution of traps.

Processing and Characterization of Cofired Capacitor and Varistor Ceramics

AbstractThe aim of the present work was to prepare a monolithic varistor-capacitor element from a commercial ZnO-based varistor and a relaxor based on lead-magnesium niobate. The materials were chosen on the basis of similar sintering temperatures, in the range from 900 to 1000°C. “Sandwich”-structured composite-pellets were uniaxially dry-pressed and sintered at the conditions that lead to shrinkages observed on the pellets of separately sintered materials. The microstructural analysis of the ‘sandwich’ interface revealed a good contact between the two parts without a noticeable interaction layer. Selected functional properties of both varistor and relaxor parts were adequate

HREM and STEM of intergranular films at zinc oxide varistor grain boundaries.

Grain boundaries in model ZnO-Bi2O3 and ZnO-Bi2O3-CoO varistors and a commercial multicomponent varistor have been characterized by high-resolution electron microscopy (HREM) and scanning transmission electron microscopy (STEM), in order to determine the relationship between Bi grain boundary segregation and formation of thin intergranular films. By controlling Bi2O3 content, applied pressure and temperature, the grain boundary Bi excess has been systematically varied from nearly zero to GammaBi = 1 x 1015 cm-2 ( approximately 1 monolayer), as measured by HB 603 STEM using an area-scan method. HREM shows that intergranular amorphous films are clearly distinguishable in samples with GammaBi > 8 x 1014 cm-2. These films range in thickness, depending on the Bi excess, from 0.6 to 1.5 nm. Similar films of approximately 1 nm thickness are widely observed in the commercial varistor. The composition of the films is a ZnO-Bi2O3 solid solution, which is in all cases more enriched in ZnO than the bulk eutectic liquid. The Bi-doped grain boundaries in ZnO varistors therefore contain an intergranular amorphous film which has not only an equilibrium thickness, but also a distinct equilibrium composition.

Dependence of non-linearity coefficients on transition metal oxide concentration in simplified compositions of ZnO+Bi2O3+MO varistor ceramics (M=Co or Mn)

Ceramics with simplified compositions of ZnO + Bi2O3 + CoO or MnO show non-linearity coefficients (α) of 40–65 provided the concentration of transition metal ions is > 1.5mol.... Samples doped with Co have higher non-linearity coefficients than those with Mn. This is attributed to the stability of multiple oxidation states of Co(III) + Co(II) in contrast to Mn(II) as the stable species in sintered ZnO ceramics. Electron paramagnetic resonance and diffuse reflectance spectral studies establish this fact. Low-signal capacitance-voltage measurements show that donor density (Nd) in these ceramics ranges from 0.3 to 1.8 × 1019 cm−3, which is comparatively larger than those of commercial varistors. The barrier height (Φb) reaches up to 0.66 eV and the breakdown voltage is around 3.4–3.7 eV. Admittance spectroscopy and isothermal capacitance transient spectroscopy (ICTS) are used for characterizing the bulk traps originating from the transition metal dopants. Capacitance-voltage analyses above the breakdown voltages show negative capacitance, indicative of oscillatory charge redistribution involving multivalent states of Co and the shallower interface states. Multiple trapping relaxations are evident from the complex-plane capacitance studies.

Electrical Properties of Multilayer‐Chip ZnO Varistors in a Moist‐Air Environment

Multilayer‐chip varistors were prepared using the ZnO‐glass system by applying tape casting technology. The effect of processing conditions on the multilayer ceramic microstructure and electrical properties was studied. The location, size, and density of the pores within the multilayer structure were examined by scanning electron microscopy and the displacement method using distilled water. The experimental results showed that the electrical properties of a multiplayerchip varistor could be influenced substantially by the porosities associated with environmental moisture in the range ≅15%–95% relative humidity at 25°C. Such an effect can cause substantial current leakage as well as the separation of the I–V curves into four regions, i.e., pre‐breakdown, linear, breakdown, and upturn. A proposed pore model and equivalent circuit for the multilayer‐chip ZnO varistors. have been simulated with a commercial varistor and a serial resistance to demonstrate the observed I–V phenomena.

Frequency limitation of metal-oxide varistors

The frequency possibilities of metal-oxide varistors are investigated. An expression is deduced for the limiting frequency f o of their application. It is also expressed in terms of the catalogue data of commercial varistors or by means of the parameters or the ceramic material. An evaluation of the f o of ZnO, SnO2 and SrTiO3 varistors is made. For ZnO devices f o has a maximum value (3-5) kHz, but for SrTiO3 it is 100 Hz. The structure of the varistor's ceramic determines the limiting frequency of their application.

Ac impedance measurements and V-I characteristics for Co-, Mn-, or Bi-doped ZnO

ZnO has been doped with various oxides, mainly bismuth, cobalt, and manganese oxides. The aim of the present approach is to look at the influence of each dopant on the electrical behavior of ZnO in a system which is more simple and chemically purer in comparison with commercial varistors. It appears that cobalt or manganese additions do not substantially modify the bulk conductivity of ZnO. Also these dopants by themselves do not lead to a varistor effect when the material is subjected to a potential sweep. On the other hand, bismuth-based compositions behave like varistors. The best performances have been recorded for a simultaneous doping with manganese and bismuth.

Conduction mechanisms in ZnO varistors

The thermionic emission enhanced by barrier lowering and the Zener tunnelling in ZnO varistors are discussed. Capacitance versus voltage, reverse current versus temperature, and 1/f noise measurements have been performed on commercial varistors. A model is proposed from which follows a diffusion voltage VD which is consistent both with the 1/C2 vs V plot and with the thermal activation energy obtained from the reverse current versus 1/T. The empirical law β=d log V/dlog I= constant, is explained by Zener breakdown. The 1/f current noise is proportional to the square of the current and independent of the conduction mechanism across reversely biased barriers.

Low-temperature ac properties of metal-oxide varistors

Low-temperature (3.5<T<77 °K) measurements of the dielectric constant and dissipation of ZnO ceramic varistors are presented for frequencies f in the range 30<f<105 Hz. A loss peak and dispersion in ε observed at 4 °K are identified as resulting from the two-phase structure of the ZnO varistor (Maxwell-Wagner model). We find the zinc oxide grain resistivity ρZnO∼106 Ω cm in a typical commercial varistor at 4.2 °K. The physical origin of a second loss peak observed in measurements around room temperature is discussed. We show the latter dispersion is not interpretable using a Maxwell-Wagner model for an inhomogeneous conductor.

High-Energy Solid-State Electrical Surge Arrestor

In this paper an engineering model of a high-energy (20 ~ 40 C of charge, 15 ~ 35 kJ) solid-state electrical surge arrestor (ESA) is presented. The basic elements are commercial or custom-made metal oxide varistors (MOV). The approach to achieve a high-coulomb ESA is to use paralleled MOV's with efforts concentrated on ensuring uniform current partition by matching the MOV's conduction characteristics and using ballast resistors. The unit with custom-made large area MOV's survived consecutive 40-C surges of 5-ms exponential decay. The peak current was 6.5 kA at a clamping voltage of 1.1 kV. Design consideration, screening techniques, packaging, and test results are reported. A brief review of the MOV physics is also included.

The Characteristics and Some Applications of Varistors

Varistors, circuit elements whose resistance is a function of the voltage applied, represent one important commercial application of semiconductors. They may be divided into two classifications: nonsymmetrical and symmetrical varistors. The first classification includes both metallic rectifiers such as copper oxide, selenium, and copper sulfide, and point contact rectifiers such as silicon and germanium. The only commercial varistor of the symmetrical class is the silicon carbide varistor, although a symmetrical characteristic may be obtained by connecting two nonsymmetrical varistors in parallel with proper polarity. Each varistor has its volt-ampere characteristic and at each point on this characteristic two different values of resistance may be defined, namely the dc resistance, defined as the ratio of voltage to current, and the dynamic or ac resistance, defined as the ratio of dE to dI. The former is important in problems dealing with steady-state dc or large-signal applications, while the latter is important when dealing with small applied signals. Because of the state of the art, varistors as manufactured commercially are less uniform than many other circuit elements and required uniformity is often obtained by special selection. Economical use of these elements therefore requires the circuit engineer to recognize clearly which of the several properties are important in his application and to specify special selection for only those properties and to the extent necessary for his application.