Varistor Behavior Research Articles

Borosilicate Frit Addition Impact on the Structure and Electric Behavior of ZnO Nanoparticle-Based Varistors

In this study, ZnO-Bi2O3-Mn2O3 varistors fabricated from the nanoparticle size of ZnO powder and doped with borosilicate frit were prepared via the conventional ceramic processing method. The influence of different borosilicate frit concentrations (0-3.0 mol%) on the sintering, microstructure enhancement, and nonlinear behavior of the ZnO-Bi2O3-Mn2O3 system was investigated. Results show that the borosilicate frit was liquefied to improve the density of the ceramic during sintering and found to have significant effects on the ZnO varistors, especially on enhancing grain growth even at a low doping concentration of only 0.5% mol. The strong solid-state reaction in the varistor made from 20 nm ZnO powder during sintering may be attributed to the high surface area of the 20 nm ZnO nanoparticles. X-ray diffraction analysis indicated that the addition of borosilicate frit to the ZnO-Bi2O3-Mn2O3 varistor system results in the formation of Zn4O(BO2)6 during sintering if too much borosilicate frit was added (over 0.5% mol). Borosilicate frit doping also significantly influenced the electrical properties of the varistor with a marked drop in the breakdown voltage from 545 V to 188 V with increase of borosilicate frit doping concentration. The resistivity also experienced a dramatic drop from 535.7 kΩ.cm to 133.5 kΩ.cm with increase of borosilicate frit doping contents. Therefore, borosilicate frit doping can be used to control the structural properties and breakdown voltage of ZnO-Bi2O3-Mn2O3 varistor system fabricated from 20 nm ZnO powder.

Varistores à base de WO3 - revisão

Varistores são elementos que fazem parte do sistema de transmissão e distribuição de energia elétrica ou de instalações elétricas especiais. Estes dispositivos são amplamente produzidos variando a escala, dependendo de sua aplicação, como dispositivos de baixa voltagem, com poucos grãos, exibindo ruptura de alguns volts, a vários kilovolts como usado em para-raios em rede de distribuição de energia. A física de operação dos varistores tem sido amplamente estudada e tem elementos comuns como barreiras Schottky formado na junção metal/semicondutor. Vários estudos têm sido desenvolvidos a fim de obter cerâmicas varistoras para aplicação em alta e baixa tensão. Dentre esses estudos encontram-se os varistores a base de ZnO, SnO2, TiO2 e mais recentemente os de WO3. Diferente das três composições citadas, os a base de WO3 apresentam comportamento varistor intrínseco devido a presença das fases monoclínica e triclínica. A adição de dopantes doadores e aceitadores de elétrons e o tratamento térmico em diferentes atmosferas também alteram as propiedades não lineares desses sistemas, uma vez que influencia na formação da barreira Schottky. Neste estudo foi realizada uma revisão sobre trabalhos relacionados a nova composição de cerâmica varistora a base de óxido de tungstênio (WO3).

Low voltage disk varistors under non-standard high current derivative impulse environment

In this study, the behavior of low voltage disk varistors was analyzed under oscillatory type non-standard high current derivative impulses. The experiment was performed using 20mm disk varistors with different nominal operating voltages of 14, 30, 40, 75, 150, 250, and 275V. Non-standard current impulses were generated using a Van De Graaff generator. The current impulse is in the shape of double exponential oscillatory type wave which has the oscillation frequency of 14.53MHz. The rise time of the impulse was found to be 8ns with the maximum peak current rate-of-rise of 2.41×1011A/s. According to the results, under very fast non-standard current derivative impulse environment, clamping voltage could be depend on both nominal voltage of varistors and the peak current rate-of-rise of the injected impulse. Interestingly, the clamping voltages under non-standard high current derivative impulse environment for all the tested varistors were below the order of values reported for 8/20μs standard current impulses. It is believed that the varistor response is mainly governed by the lead's inductance whereas the influence of the varistor nonlinear resistance becomes negligible. It was found that the maximum impulse impedance values of the varistors are caused due to the increasing of its nominal voltage. Calculated current integrals were lying between 80 and 94A2s whereas the maximum peak current values were lying in the range of 2070–2496A. Another important feature observed in this study was the DC offset voltage appears between terminals of varistor immediately after it is triggered due to its first oscillatory pulse of the current impulse.

The effect of negative capacitance in varistor structure on the basis of its models with voltage drop on the intergranular interlayer

Purpose – The purpose of this paper is modeling the effect of negative capacitance in the capacitance-voltage characteristic of the intergranular potential barrier of varistor structure. Design/methodology/approach – The modeling of the capacitance-voltage characteristic of the intergranular barrier in metal oxide varistor ceramics is based on the development of the algorithm. It includes all the known mechanisms of electrotransfer in a wide range of voltages and currents, and also takes into account the voltage drop on the intergranular interlayer of intergranular potential barrier. Findings – The models and algorithms for calculating the capacitance-voltage characteristics of a single intergranular potential barrier with the use of the most established understanding used at the interpretation of the nonlinear conductivity intergranular barrier are developed. The results of the capacitance-voltage characteristics modeling correspond to the existing understanding of the electrical properties on the ac current varistor ceramics are based on zinc oxide. The model allows to predict the behavior of varistors on the alternating current (voltage). Originality/value – It is established that the recharge of the surface localized states occurs when a voltage is applied to the varistor structure, it can lead to a relaxation decrease in the width of the potential barrier overcome by tunneling electrons in the field emission from the conduction band of the one crystallite in the conduction band of the other crystallite and thus to the current backlog of applied voltage on the phase (i.e. the expression of the negative capacitance effect).

Effect of Sintering Temperature on the Microstructure and Electrical Characteristics of Low Clamping Voltage Zinc Oxide- Based Ceramic Varistor

Pelletized samples of polycrystalline zinc oxide ceramics doped with 2mol% MnO 2 and 3mol% PbO were prepared by the conventional method of ceramic processing. The requisite composition was obtained by the direct mixing of constituent phases (DMCP) technique. Sintering was carried out at various temperatures ranging from 650 o C to 850 o C. The effects of sintering on the non-linear characteristics of the doped ceramic samples were investigated. The threshold or breakdown voltages were found to decrease as sintering temperature increased. Microstructural investigation revealed improved homogeneity and increased grain sizes with increasing sintering temperature. The random distribution of secondary phases also featured prominently. These findings were observed to correlate with the evolution of electrical characteristics while the sample sintered at 850 o C exhibited the best electrical response suitable for varistor behaviour.

Origin of the High Permittivity and Varistor Behavior in SnO2–Zn2SnO4 Composite Ceramics

SnO2–Zn2SnO4 composite ceramics with a colossal dielectric permittivity and varistor behavior are prepared by traditional ceramic processing. By increasing bias voltage from 0 to 10 V at a low frequency (~103 Hz) and at room temperature, the relative permittivity decreased rapidly from about 20 000 to several thousand, whereas the radius of the semicircle in the complex impedance decreased and the tail gradually disappeared. However, the peak height and the position of the imaginary part of the complex modulus in the spectra were independent of the applied DC voltage. The slope deduced from the bias voltage‐dependent straight lines of the double‐logarithmic imaginary permittivity spectra were constant with a value of −0.63 at high frequencies and they decreased to −1 at low frequencies. The results strongly indicate that a number of weekly trapped charges existed in the ceramic bulk. From the temperature‐dependent dielectric and electric modulus spectra, the trapped charge activation energy was about 0.32 eV, which may be associated with the oxygen vacancies. Based on the results, a modified equivalent circuit related to the colossal dielectric permittivity and varistor properties was proposed, in which a Warburg impedance was added in parallel with the resistance and capacitance.

High Permittivity and Varistor Properties of SnO<sub>2</sub>-Zn<sub>2</sub>SnO<sub>4</sub> Composite Ceramics

(1-x)SnO2-xZn2SnO4composite ceramics were prepared by traditional ceramic processing and the varistor, dielectric properties were investigated. With increasing Zn2SnO4content, the breakdown electrical fieldEBand nonlinear coefficientαreaches the minimum of 6.9 V/mm and 2.5 at x=0.206, respectively. In the dielectric spectra, the relative dielectric constantεrexhibits strong frequency dependent character and at 40 Hz,εrfor the sample of x=0.206 reaches a maximum as high as 3×104. In the frequency region lower than 1 kHz, accompanied by the sharp increase of dielectric loss at 40 Hz,εris depressed and a dielectric peak is presented in the spectra with increasing bias voltage. In the low electrical current range of 1.37-20 μA, The barrier heightφBabout 1.0 eV are obtained and it is found thatφBdecreases with increasing measuring current for each sample. Based on the results, the varistor behavior with high dielectric constant is explained by the Schottky barriers at grain boundaries.

Admittance spectroscopy and electrical properties of Co3O4-doped ZnO

The varistor characteristics of 0.1–3.0 at. % Co-doped ZnO have been investigated. Three kinds of deep bulk trap levels of 0.16, 0.25, and ~0.31 eV were identified as donor levels by admittance spectroscopy. Oxygen vacancy (Vo·) appeared preferentially in 0.5–3.0 at. % Co-doped ZnO. From J-E characteristics nonohmic behavior was seen in this binary system while the nonlinear coefficient α changed between 3 and 35 depending on the composition. For 1.0 at. % Co-doped ZnO, two distinguishable activation energies of 0.65 eV and 1.04 eV related with grain boundary phenomena were confirmed above and below ~520 K by impedance and modulus spectroscopy. It is believed that the varistor behavior of Co-doped ZnO stems from the formation of double Schottky barrier by the valence change of Co ions in ZnO grains as well as oxygen chemisorption at the grain boundaries on heating and subsequent cooling.

Effect of sintering temperature on varistor and dielectric properties of Si–polymer composite films

Silicon–polyaniline–polyethylene composite films were prepared through hot pressing at a pressure of 60MPa and different temperatures and their current–voltage characteristics were investigated. Results show that these films have varistor behavior and can be used to protect circuits from 64V up to 80V overvoltages. In addition, it is found that the varistor breakdown voltage decreases by increasing sintering temperature while the corresponding nonlinear coefficient increases. Dielectric properties of the films vary as a function of frequency in such a way that, at low frequencies, capacitance, relative permittivity and loss coefficient of the samples, as well as their fluctuations, are high. By increasing frequency, both dielectric properties of the films and their reduction rate decrease. On the other hand, by increasing sintering temperature, capacitance and relative permittivity of the varistors tend to increase at a constant frequency whereas their loss coefficient decreases.

Effect of Si content on electrophysical properties of Si-polymer composite varistors

Electrophysical properties of Si-polymer composite varistors, which were prepared using hot press method, have been investigated. Research on (I–V) characteristics of samples shows that by Si content exceeding 65% of the whole, samples have varistor behavior with nonlinear coefficient of about 10. Varistor breakdown voltage tends to become low in reaction to increase in Si content. Potential barriers of samples have also been investigated. The results show that increasing Si content decreases their potential barrier. These samples show a low hysteresis compared to other semiconductor-polymer composite varistors.

Varistor and dielectric properties of Cr2O3 doped SnO2–Zn2SnO4 composite ceramics

Cr2O3 doped SnO2–Zn2SnO4 composite ceramics were prepared by traditional ceramic processing and the varistor, dielectric properties were investigated. With increasing Cr2O3 content, the breakdown electrical field EB increases from 11 to 92 V/mm and the relative dielectric constant εr measured at 1 kHz, 50 °C decreases from 11,028 to 3412, respectively. The barrier height ϕB about 0.8–0.84 eV and the decreasing of SnO2 grain size suggest that the varistor behavior with high εr is originated from SnO2–SnO2 or SnO2–Zn2SnO4 grain boundary. In the dielectric spectra lower than 1 kHz, a dielectric peak is presented and depressed with increasing bias voltage. Similarly, at high temperature, the dielectric constant also presents a dielectric peak in the temperature spectra and the peak becomes faint with increasing frequency. The exhibition of the dielectric peak is thought to be attributed to the conduction of grain boundary since it is accompanied by the sharp increase of dielectric loss. In addition, a dielectric relaxation with the activation energy about 0.4–0.5 eV was observed in the temperature range of 20–100 °C. Based on the results, the formation mechanism of Schottky barriers at grain boundaries and the varistor behavior with high dielectric constant are well understood.

Quasi-ideal Nonlinear Electrical Behavior of Polycrystalline SnO2 Ceramic Varistors Doped with SiO2

The influence of SiO2 doping on the microstructure and electrical behavior of SnO2 varistors has been studied. The varistor effect was studied over a wide range from 10−9 A to 104 A. It is shown that the J(E) characteristic of SnO2 ceramics exhibits a nonlinear coefficient >100. The SiO2 doping also resulted in a sharp-abrupt upturn region in the I–V characteristic, indicating a single semiconductor junction behavior. The leakage current of the varistors is rather low, on the order of 10−10 S m−1. In the upturn region of operation where the curve departs from the nonlinear relation and approaches the value of the bulk resistivity of the material, the ceramic is characterized by a current density almost independent of the applied voltage. A very small amount of SiO2 causes large perturbations of the conventional thermionic emission observed in varistor ceramic materials.

The Thermally Stimulated Current (TSC) Technique on Evaluating Degradation of ZnO Varistors

Thermally stimulated current (TSC) is a simple and effective test technique to study the thermal activated charge, electron trap and activation energy of dielectric and semiconducting materials. It is well known that the addition of glass frits can improve the degradation property of ZnO varistors. An activation energy of 0.45 eV has been found on the ZnO varistors with/without glass frits after the DC degradation by using TSC method. The tested quantity of thermal activated charges, QTSC, may represent the numbers of migrated zinc interstitials. ZnO visitors doped with glass frits have less QTSC. Experiment indicates that TSC test is a convenient method to evaluate the degradation behavior of ZnO varistors.

Electrical Properties at Grain Boundaries Influenced by Cr3+Diffusion in SnO2.ZnO.Nb2O5-Films Varistor Prepared by Electrophoresis Deposition

ABSTRACTSnO2-based varistors are strong candidates to replace the ZnO-based varistors due to ordering fewer additives to improve its electrical behavior as well as by showing similar nonlinear characteristics of ZnO varistors. In this work, SnO2-nanoparticles based-varistors with addition of 1.0 %mol of ZnO and 0.05 %mol of Nb2O5were synthesized by chemical route. SnO2.ZnO.Nb2O5-films with 5 μm of thickness were obtained by electrophoretic deposition (EPD) of the nanoparticles on Si/Pt substrate from alcoholic suspension of SnO2-based powder. The sintering step was carried out in a microwave oven at 1000 °C for 40 minutes. Then, Cr3+ions were deposited on the films surface by EPD after the sintering step. Each sample was submitted to different thermal treatments to improve the varistor behavior by diffusion of ions in the samples. The films showed a nonlinear coefficient (α) greater than 9, breakdown voltage (VR) around 60 V, low leakage current (IF≈ 10-6A), height potential barrier above 0.5 eV and grain boundary resistivity upward of 107Ω.cm.

Sintering behavior and non-linear properties of ZnO varistors processed in microwave electric and magnetic fields at 2.45 GHz

A study of the densification behavior and grain growth mechanisms of ZnO-based varistors composed of 98mol.% ZnO–2mol.% (Bi2O3, Sb2O3, Co3O4, MnO2) has been carried out. The pressed samples were sintered in microwave electric (E) and magnetic (H) fields using a single-mode cavity of 2.45GHz. The effect of the sintering temperature (900–1200°C), holding time (5–120min) and sintering mode (E, H) on the microstructure and electrical properties of the sintered varistor samples were investigated. The grain growth kinetics was studied using the simplified phenomenological equation Gn=kte(−Q/RT). The grain growth exponent (n) and apparent activation energy (Q) values were estimated for both electric and magnetic heating modes and were found to be n=3.06–3.27, Q=206–214kJmol−1, respectively. The lower value of n estimated in the E field was attributed to a volume diffusion mechanism, whereas the higher n value in the H field sintering was correlated mainly to a combined effect of volume and surface diffusion processes. Samples sintered in the H and E fields showed high final densities. Moreover, the ones sintered in the H field presented slightly higher density values and bigger grains for all sintering temperatures than E field heated ones. The optimal sintering conditions were achieved at 1100°C for a 5min soaking time for both H and E field processed samples, where respectively densities of 99.2±0.5% theoretical density (TD) and 98.3±0.5% TD along with grain size values of G=7.2±0.36μm and G=6.6±0.33μm were obtained. Regarding the electrical properties, breakdown voltage values as high as 500–570Vmm−1 were obtained, together with high non-linear coefficients α=29–39 and low leakage currents (Jl≈5×10−3mAcm−2), respectively, for E and H field sintered varistor samples. Moreover, samples sintered in an H field systematically exhibited higher breakdown voltage values compared to the ones sintered in the E field. This was attributed to an improved coupling between the H field and the present dopants within the ZnO matrix, this latter being mostly semiconductive, thus leading to an enhanced reactivity and improved properties of the electrostatic barrier.

Voltage polarity dependent current paths through polycrystalline ZnO varistors

Due to the scatter of properties of individual grain boundaries, a scatter of the behavior of low-voltage varistor components and asymmetric current–voltage characteristics with respect to the electrical field direction is observed. Using complementary methods (thermography, scanning surface probe microscopy, electron back-scattered diffraction), we investigated the grain boundary behavior of Praseodymium type varistors. Measurements on the relevant grain boundaries revealed a newly observed effect: dependent on the field direction, the main current flows via different spatial paths.

Preparation, microstructure, and improved dielectric and nonlinear electrical properties of Na1/2La1/2Cu3Ti4O12 ceramics by sol–gel method

Na1/2La1/2Cu3Ti4O12 (NLCTO) ceramics were prepared by the sol–gel method (SG) and solid-state method (SS), and the effect of sintered temperature on microstructure and dielectric and nonlinear electrical properties has investigated. The experimental results indicated that NLCTO-SG ceramics sintered at 1090°C for 10h showed larger grain size, higher density and more homogeneous microstructure, especially higher dielectric constant up to ca 1.3–1.8×104, lower dielectric loss of about 0.057 and better temperature stability than NLCTO-SS counterparts. However, the good low-voltage varistor behavior and the third kind of dielectric relaxation behavior could not be observed from the NLCTO-SS ceramics. The higher dielectric constant of the NLCTO-SG ceramics may be due to the stronger internal barrier layer capacitor (IBLC) effect. Two values of grain activation energy were obtained above room temperature for the first time. The transition temperature of two values was basically coincident with that of sudden changes of the dielectric properties.

Effect of changing Gallium arsenide content on Gallium arsenide–polymer composite varistors

GaAs–polymer composites have been prepared using the hot pressing method and their current–voltage characteristics have been studied. The results show that samples with GaAs content exceeding 50% of the whole composition have varistor behavior. They can be used to protect circuits from 60V up to over 90V voltages. In addition, it is found that the higher the content of GaAs in the varistor, the lower the breakdown voltage. Samples with higher polymer content have less nonlinearity. Each composite varistor has hysteresis which increases through increasing GaAs content. Main results have been analyzed with respect to SEM micrographs of the samples and compared with reported results for ZnO–polymer composite varistors. Energy gaps of varistors are evaluated using both (ln(σ)–1/T) diagram and their UV spectra analysis regarding Tauc relation. UV spectra show that reducing the GaAs content in the varistors causes their absorptions and corresponding impurity band gap to decrease.

Effects of Y<sub>2</sub>O<sub>3</sub> and In<sub>2</sub>O<sub>3</sub> on the Electrical Properties of SnO<sub>2</sub>-Based Varistors

The effects of Y2O3 and In2O3 on the microstructure and electrical properties of SnO2-based varistors were investigated. It was observed that the grain size of the samples decreased with doping Y2O3 and In2O3 and accordingly, the breakdown electrical field EB increased greatly. The measurements of barrier height and grain size reveal that the variation of grain size was not the only reason for the change of electrical properties of the sample doped with In2O3 and, the improvement of nonlinear coefficient α may mainly attribute to the increase of barrier height. Some energy levels of different state defects on the grain boundary were obtained and the energy about 0.15 eV detected here of all the samples may be attributed to the activation of . The different effects of doping Y2O3 and In2O3 indicate that In2O3 is more effective to improve nonlinear electrical behavior and breakdown electrical field of SnO2-based varistors.

Temperature dependent electrophysical characteristics of GaAs-polymer composite varistors

A composition of GaAs-polymer tiny particles was pressed at a temperature of 130 °C and a pressure of 60 MPa and its current–voltage characteristic was studied. The result shows that the prepared disk has varistor behavior and can therefore be used to protect circuits from low overvoltage transients higher than 62 V. Temperature dependence of current-voltage characteristic and the electrical conductivity of the sample were investigated in the temperature range of 25–100 °C. It has been found that increase in temperature results in lower breakdown voltage and nonlinearity coefficient. At high temperatures, nonlinearity in the I–V characteristic of sample disappears and the conductivity becomes Ohmic in nature. The sample has hysteresis which decreases through increase in temperature. Annealing effect on leakage current and breakdown voltage was both investigated and analyzed using SEM micrographs. Finally, the electrical bandgap of the sample was measured.