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This paper aims to fabricate and characterize ZnO-based multilayer varistors.

Tape casting technique was utilized for preparation of multilayer varistors based on ZnO doped with Pr, Bi, Sb, Co, Cr, Mn and Si oxides. Scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) methods were used to study the microstructure, elemental and phase compositions, respectively, of the varistors. Dielectric properties were investigated by impedance spectroscopy. Current–voltage (I–U) dependences were measured to characterize nonlinear behavior of the fabricated varistors.

XRD, SEM and EDS studies revealed dense microstructure of ceramic layers with ZnO grains sized 1-4 μm surrounded by nanometric Bi-rich films, submicrometer Zn₇Sb₂O₁₂ spinel grains and needle-shaped Pr₃SbO₇ crystallites. Praseodymium oxide was found to be very effective as an additive restricting the ZnO grain growth. I–U characteristics of the fabricated multilayer varistors were nonlinear, with the nonlinearity coefficients of 23-27 and 19-51 for the lower and higher Pr₂O₃ content, respectively. The breakdown voltages were 60-150 V, decreasing with increasing sintering temperature.

Low-temperature cofired ceramics technology enables attaining a significant progress in miniaturization of electronic passive components. Literature concerning application of this technology for multilayer varistors fabrication is limited. In the present work, the results of XRD, SEM and EDS studies along with the I–U and complex impedance dependences are analyzed to elucidate the origin of the observed varistor effect. The influence of sintering temperature and Pr₂O₃-doping level was investigated.

The purpose of this study was to develop fabrication procedure of multilayer varistors based on doped ZnO and to investigate their microstructure and electrical properties.

Two ceramic compositions based on ZnO doped with Bi₂O₃, Sb₂O₃, CoO, MnO, Cr₂O₃, B₂O₃, SiO₂ and Pr₂O₃ were used for tape casting of varistor tapes. Multilayer varistors were prepared by stacking of several green sheets with screen printed Pt electrodes, isostatic lamination and firing at 1,050-1,100°C. Scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) studies were carried out to examine the microstructure and elemental composition of the varistors. Current-voltage characteristics were measured in the temperature range from −20 to 100°C.

The desired compact and fine-grained microstructure of multilayer varistors and nonlinear current-voltage characteristics were attained as a result of the applied fabrication procedure. The breakdown voltage of the varistors is 33-35 V and decreases slightly in the temperature range from −20 to 100°C. The nonlinearity coefficient changes from 14 to 23 with rising measurement temperature.

New improved formulations of varistor ceramic foils based on doped ZnO were developed using tape casting method and applied for fabrication of multilayer varistors with good electrical characteristics. The influence of temperature in the range from −20 to 100°C on the varistor parameters was studied.

This paper aims to study the effects of MnO₂ on the ZnO–Bi₂O₃-based varistor prepared via flash sintering (FS)

MnO₂-doped ZnO–Bi₂O₃-based varistors were successfully prepared by the FS with a step-wise increase of the .current in 60 s at the furnace temperature <750°C under the direct current electric field of 300 V cm⁻¹. The FS process, microstructure and the electrical performance of ZnO–Bi₂O₃-based varistors were systematically investigated.

The doping of MnO₂ significantly decreased the onset temperature of FS and improved the electrical performance of FS ZnO varistor ceramic. The sample with 0.5 mol% MnO₂ doping shows the highest improvement, with the nonlinear coefficient of 18, the leakage current of 16.82 µA, the threshold voltage of 459 V/mm and the dielectric constant of 1,221 at 1 kHz.

FS is a wonderful technology to enhance ZnO varistors for its low energy consumption, and a short sintering time can reduce grain growth and inhabit Bi₂O₃ volatilize, yet few research studies work on that. In this research, the authors analyzed the FS process and improved the electrical characteristics through MnO₂ doping.

The purpose of this paper is to investigate the effects of the sintering temperature on the microstructural, morphological and electrical characteristics of Zinc oxide (ZnO)-based varistors.

This study used a conventional method to design and produce ZnO varistors by sintering ZnO powder with small amounts of various metal oxides. Furthermore, the effect of sintering temperature on varistor properties of (Bi, Co, Cr, Sb, Mn)-doped ZnO ceramics was investigated in the range of 1280–1350 °C.

The obtained results showed an E_B value of 2109.79 V/cm, a V_gb value of 0.831 V and a nonlinear coefficient (α) value of 19.91 for sample sintered at temperature of 1300 °C. In addition, the low value of tan δ at low frequency range confirmed that the grain boundaries created in 1300 °C sintering temperature were obviously good.

Based on the previous research on the ZnO-based varistors, a thorough study was carried out on these components to improve their electrical characteristics. Thus, it is necessary that those varistors have low leakage current and low value of dissipation factor to ensure their good quality. High breakdown fields and nonlinearity coefficients are also required in such kind of components. The effect of sintering temperature on the varistor properties of the new compositions (zinc, bismuth, manganese, chrome, cobalt, antimony and silicon oxides)-doped ZnO ceramics was studied in the range of 1280–1350 °C. Also, the microstructure and the phase evolution of the samples sintered at various temperatures (1280 °C, 1300 °C, 1320 °C and 1350 °C) were investigated according to X-ray diffraction and scanning electron microscope measurements.

The purpose of this paper is to develop a generalized model of the nonlinear conductivity of varistor ceramic suitable for solving problems of prediction and control of ceramic nonlinearity, stability of varistor properties.

The modeling of current-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 of the current-voltage characteristics, and also takes into account the deviation of the barrier form the Schottky barrier.

The models of double Schottky barrier and double barrier of arbitrary form, as well as the algorithms for calculating the current-voltage characteristics of a single intergranular potential barrier and a separate “microvaristor” with the use of the most well-established understanding of the main mechanisms of electrical are developed. The results of current-voltage characteristics modeling correspond to the existing understanding of the nonlinear electrical conductivity varistor ceramics are based on zinc oxide. The model of double barrier of arbitrary form takes into account the deviation of the barrier form the Schottky barrier which is important in predicting the deformation of the current-voltage characteristics of the varistor products in the process of degradation.

The relation between the form of the current-voltage characteristic and the distribution profile of the donor concentration in the surface regions of the semiconductor crystallites constituting the intergranular potential barrier is established. The accumulation of donors in the space charge region leads to the increase in the current on the prebreakdown region of the current-voltage characteristic and the reduction of voltage corresponding to the breakdown region beginning of the current-voltage characteristic. The significant role of the interlayer in the formation of current-voltage characteristic of the intergranular potential barrier is shown.

The purpose of this study is to model the dependences of the output voltage, temperature, current and electrical power dissipation of a voltage limiter based on a two-layer varistor–posistor structure on time and analysis the influence of operating modes and design parameters of such a limiter on these characteristics.

The behavior of the limiting voltage, temperature and other parameters of the voltage limiter when an input constant overvoltage is applied is studied by the simulation method. The voltage limiter was a two-layer construction. One layer was a zinc oxide ceramic varistor. The second layer was a posistor polymer composite with a nanocarbon filler of PolySwitch technology.

The output voltage across the varistor layer decreases and reaches some fixed value related to its breakdown voltage after applying a constant overvoltage to the structure over time. The temperature of the structure increases to some steady state value, while the current decreases significantly. The amplitude of the transient current pulse increases, its duration and energy of the transient process decrease with increasing overvoltage. An increase in the internal resistance of the overvoltage source can cause a decrease in the amplitude and an increase in the duration of transient currents.

The ranges of values for the activation energy of conduction of the varistor layer in weak electric fields, the intensity of heat exchange between the structure under study and the environment are determined to ensure the stable operation of this structure as a voltage limiter. The results obtained make it possible to select the necessary parameters of the indicated structures to ensure the required operating modes of the voltage limiter for various applications.

Various thick film varistor constructions were made and characterised: ‘sandwich’, ‘interdigitated’ and ‘segmented’ varistors. The varistor active layer thickness, the electrode surface value and shape were varied. The Ul characteristics of these varistors were compared mutually, and with the Ul characteristics of the smallest chip varistors. In accordance with the results obtained, it has been shown that thick film printed varistors composed of ZnO and with additives could be applied as discrete components or integrated into a hybrid circuit.

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.

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.

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).

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).

The purpose of this paper is to develop the models of the dielectric permittivity dispersion of heterogeneous systems based on semiconductors to a level that would allow to apply effectively the method of broadband dielectric spectroscopy for the study of electronic processes in ceramic and composite materials.

The new approach for determining the complex dielectric permittivity of heterogeneous systems with semiconductor particles is used. It includes finding the analytical expression of the effective dielectric permittivity of the separate semiconductor particle of spherical shape. This approach takes into account the polarization of the free charge carriers in this particle, including capturing to localized electron states. This enabled the authors to use the known equations for complex dielectric permittivity of two-component matrix systems and statistical mixtures.

The presented dispersion equations establish the relationship between the parameters of the dielectric spectrum and electronic processes in the structures like semiconductor particles in a dielectric matrix in a wide frequency range. Conditions of manifestation and location of the different dispersion regions of the complex dielectric heterogeneous systems based on semiconductors in the frequency axis and their features are established. The most high-frequency dispersion region corresponds to the separation of free charge carriers at polarization. After this region in the direction of reducing of the frequency, the dispersion regions caused by recharge bulk and/or surface localized states follow. The most low-frequency dispersion region is caused by recharging electron traps in the boundary layer of the dielectric matrix.

Dielectric dispersion models are developed that are associated with: electronic processes of separation of free charge carriers in the semiconductor component, recapture of free charge carriers in the localized electronic states in bulk and on the surface of the semiconductor and also boundary layers of the dielectric at the polarization. The authors have analyzed to situations that correspond applicable and promising materials: varistor ceramics and composite structure with conductive and semiconductor fillers. The modelling results correspond to the existing level of understanding of the electron phenomena in matrix systems and statistical mixtures based on semiconductors. It allows to raise efficiency of research and control properties of heterogeneous materials by dielectric spectroscopy.

The purpose of this paper on the one hand is to reduce the sintering temperature, shorten the sintering time and improve the electrical properties of the sample through the two-step flash sintering method and on the other hand is to study the effect of electric field on the phase structure, microstructure and electrical properties of the flash sintering sample.

In this paper, (Mg_1/3Ta_2/3)_0.01Ti_0.99O₂ giant dielectric ceramics were prepared by conventional sintering and two-step flash sintering, respectively. Further, the effect of electric field (600–750 V/cm) on the electrical properties of (Mg_1/3Ta_2/3)_0.01Ti_0.99O₂ giant dielectric ceramics was studied.

The results show that compared with the conventional sintering, the sintering temperature of the two-step flash sintering can be reduced by 200°C and the sintering time can be shortened by 12 times. All sintered samples were single rutile TiO₂ structure. Compared with conventional sintering, two-step flash sintering samples have finer grain size. The two-step flash sintered sample has similar dielectric properties to the conventional sintered sample. The dielectric constant of flash sintered samples decreases with the increase of electric field. When the electric field is 700 V/cm, the ceramic sample has the optimal dielectric properties, where the dielectric constant is approximately 5.5 × 10³ and the dielectric loss is about 0.18 at 1 kHz. Impedance spectroscopy analysis shows that the excellent dielectric properties are attributed to the internal barrier layer capacitance model.

This paper not only provides a new method for the preparation of co-doped TiO₂ giant dielectric ceramics but also has great potential in greatly improving efficiency and saving energy.