Zinc Oxide Varistor Ceramics Research Articles

Nonlinearity Characteristics of Low-Voltage Barium Titanate Based Zinc Oxide Varistor Ceramics Modified by Cobalt Dopants

Application of ZnO varistor at low voltage has increased significantly due to the high demands of low-voltage electronics with high nonlinearity characteristics and low leakage current. The varistor ceramics were developed via solid-state reaction method and the resultant sample was analyzed by means of SEM, EDS and XRD. The nonlinearity characteristics of ZnO varistor ceramics for different contents of cobalt oxide (Co3O4) at a given barium titanate (BaTiO3) amount were analyzed based on the J-E characteristics measurement. The increased value of nonlinear coefficient (α) equal to 4.8 was exhibited by the sample made with 12 wt.% BaTiO3 additive. As the concentration of dopant (Co3O4) incorporated was increased from 0.5 to 1.5 wt.%, the varistor voltage limit decreased from 8.9 V/mm to 7.0 V/mm, respectively. Additionally, the barrier height increased from 0.88 to 0.98 eV for 0.0 wt.% to 1.0 wt.% of Co3O4 concentration, respectively. The highest α of 7.2 was obtained at 0.5 wt.% Co3O4 and decreased with further doping content due to to the reduction of barrier height caused by the variation of electronic state at the grain boundaries.

Multiscale defect responses in understanding degradation in zinc oxide varistor ceramics

Dielectric responses of multiscale defects, including electron trapping behaviours of intrinsic point defects in grains, heterogeneous interface of depletion/intergranular layers, and dynamic responses of interface states at grain boundaries, were investigated in ZnO varistor ceramics via the DC ageing process. The dielectric relaxation peaks corresponding to zinc interstitials and oxygen vacancies increased, revealing that the donor densities gradually increase with ageing time. Dynamic response of the interface states, which were observed to undergo significant dc conductance, using traditional dielectric spectroscopy, is unveiled based on an optimised (∂ε'/∂lnω)/ε′ spectroscopy. The energy level of interface states monotonously decreased while the relaxation peak value increased, corresponding to decreased interface states with degradation time. Collectively, the degradation of Schottky barrier during DC ageing process is discussed based on multiscale defect responses with the help of the (∂ε'/∂lnω)/ε′ spectroscopy.

Microstructure and Electrical Properties of Low-Voltage Barium Titanate Doped Zinc Oxide Varistor Ceramics

In the present, varistor ceramics through the combination of zinc oxide (ZnO) with a perovskite material have become widespread because of their unique properties for a wide range of applications in electronic protection devices. Low-voltage zinc oxide (ZnO) varistors with fast response and highly nonlinear electrical properties for overvoltage protection in an integrated circuit are increasingly significant in the application of low-voltage electronics. The present study highlights the interaction between barium titanate (BaTiO3) and ZnO varistors through the employment of solid-state reaction method in the production of low-voltage varistors. The effects of BaTiO3 on the microstructure of ZnO varistors were analyzed through scanning electron microscopy (SEM), energy dispersive X-ray analysis spectroscopy (EDS) and X-ray diffraction (XRD). The EDS analysis and XRD measurements suggest the presence of ZnO and BaTiO3 phases. The electrical properties of BaTiO3-doped ZnO varistors were examined based on the current density-electric field (J-E) characteristics measurement. The varistor properties showed the nonlinear coefficient (α) from 1.8 to 4.8 with the barrier height (φB) ranged from 0.70 to 0.88 eV. The used of BaTiO3 additive in ZnO varistors produced varistor voltages of 4.7 to 14.1 V/mm with the voltage per grain boundary (Vgb) was measured in the ranges 0.03 to 0.05 V. The lowest leakage current density was 348 μA/cm2, obtained at the samples containing 12 wt.% BaTiO3 with high barrier height. The reduction in barrier height with increasing BaTiO3 content was associated with the excessive amount of BaTiO3 phase, hence cause the deterioration of active grain boundary due to the variation of oxygen (O) vacancies in the grain boundary.

Boundary breakdown time of ZnO varistors under nanosecond pulse current

Conductive properties of zinc oxide (ZnO) varistor ceramics depend on the boundary barrier structure. By using the photoconductivity switches (PCSS) technology, the boundary breakdown characteristics of ZnO varistors for nanosecond electric pulse response was measured under bias voltage 700 - 1600 V and the relationship between conductance and material structure was analyzed. Charging time of grain boundary capacitor was ∼ 100 ns and shortened with the increase of the bias electric field. The barrier height of ZnO grain boundary was influenced by bias electric field and the grain boundary capacitance was changed accordingly. The calculating numerical results are in agreement with the measured values.

Stable electrical properties of ZnO varistor ceramics with multiple additives against the AC accelerated aging process

The aging performance of zinc oxide (ZnO) varistors doped with aluminum and indium was investigated. Alternating current (AC) accelerated aging voltages of 0.85U1mA, 0.90U1mA and 0.95U1mA were applied to the doped varistors at a temperature of 115 °C for more than 1000 h. The electrical properties of the varistors before and after the aging process were evaluated by measuring their current–voltage, capacitance–voltage, and dielectric characteristics. The results showed that the ZnO varistor samples exhibited stable electrical properties under the applied aging conditions. Degradation rate coefficients (Kt) of 0.03, 0.05 and 0.06 were obtained at AC accelerated aging voltages of 0.85U1mA, 0.90U1mA and 0.95U1mA, respectively. The results indicated that the varistor ceramics should be able to operate safely and continuously under the given conditions for more than 110 years, which meets the requirements of surge arresters in power systems. These findings are important for improving the aging performance of ZnO varistors and the general security and stability of power systems.

Influence of Sintering Temperature and ZrO2 Dopants on the Microstructure and Electrical Properties of Zinc Oxide Varistors

This paper investigated the effects of sintering temperature on the microstructure and electrical properties of ZrO 2 -doped zinc oxide (ZnO) varistor ceramics. The results show that, the additive ZrO 2 exists as independent second phase between ZnO grains, which can limit the growth of ZnO grains and improve the voltage gradient. With the increasing of ZrO 2 , the content of extrinsic elements (Mn, Sb, Co, Cr) in the grain boundary layers tends to increase first and then decease. When ZrO 2 content is more than 1.0mol%, the electrical performance of ZnO varistors decreases sharply. With the increasing of sintering temperature, the ZnO grain size increases and the voltage gradient decreases. When the sintering temperature is larger than 1200°C, more monoclinic ZrO 2 phase transformed into cubic phase, and more micropores are generated, causing the non-linear coefficient to decrease and the residual voltage ratio and leakage current to increase. With a sintering temperature of 1150°C and a ZrO 2 content of 1.0mol%, the ZnO varistors can reach the overall optimum electric performance, exhibiting a breakdown voltage of E 1mA = 420V/mm, a nonlinear coefficient of α = 58, a residual voltage ratio of CR = 1.87, and a leakage current of I L = 4μA. The studies in this paper can give reference for the development of high quality ZnO arresters.

Effect of Sintering Temperature on Zinc Oxide Varistor Ceramics

Zinc oxide (ZnO) varistors is a solid state electronic ceramic component whose function to protect electronic devices against over voltage surges due to their highly nonlinear electrical properties and ability in controlling the energy. In this study, ZnO varistor is prepared via solid state reaction method. The ZnO based varistor ceramics undergoes sintering temperature at 900 °C, 1100 °C and 1300 °C for 45 minutes. The used of sintering temperature from 900 to 1300 °C, has increased the grain size of ZnO and improved their crystallinity as shown from the peaks intensity of X-ray diffraction (XRD). Current-Voltage properties exhibit the value of nonlinear coefficient in the range of 1.44 to 1.76. Different sintering temperature is used to adjust the microstructure and improve the varistor ceramic characteristics where it shows a potential difference in various application of electronic devices. Increasing in sintering temperature would lead to improved grain growth for better electrical properties.

High-frequency capacitance-voltage characteristics of the heterogeneous structure based on the model of spherical semiconductor particles in a dielectric

The dependence of the parameters of the capacitance effect in heterogeneous dispersed two-component structures based on semiconductors from the bulk fraction of the semiconductor component is modeled. The used method for determining the changes of the energy bands bending on the surface of the spherical semiconductor particle by applying dc electric field allowed to calculate the changes of the dipole moment and effective (taking into account the polarization of the free charge) dielectric constant of this semiconductor particle. This result allowed to use the known models of the dielectric constant of two-component structures for the description of the capacitance field effect in the heterogeneous structures. The relations allowing to estimate the value of the bulk donor concentration in the semiconductor component of the matrix of the heterogeneous system and the statistical mixture have been obtained. The approbation of the obtained calculation relations to evaluate the donor concentration in the ZnO grains of zinc oxide varistor ceramics leads to the correct values that are consistent with estimates of other methods and models. It is established that the sensitivity of the relative dielectric constant to the applied dc electric field is dependent on the bulk fraction of the semiconductor particles in the heterogeneous structures. The bulk fraction of the semiconductor particles significantly affects on the dielectric constant beginning with the values from [Formula: see text] for matrix systems and [Formula: see text] for statistical mixtures.

Sintering Effect on Clamping Characteristics and Pulse Aging Behavior of ESD-Sensitive V2O5/Mn3O4/Nb2O5Codoped Zinc Oxide Varistors

V₂O₅/Mn₃O₄/Nb₂O₅ codoped zinc oxide varistor ceramics were sintered at a temperature range as low as 875~950℃. The voltage clamping characteristics of V₂O₅/Mn₃O₄/Nb₂O₅ codoped zinc oxide varistor ceramics were investigated at a pulse current range of 1~50 A. The sintering temperature had a significant effect on clamp voltage ratio, which exhibits surge protection capabilities. The varistor ceramics sintered at 875℃ exhibited the best clamping characteristics, in which the clamp voltage ratio was 2.69 at a pulse current of 50 A. The varistor ceramics sintered at 900℃ exhibited the highest electrical stability, where = 3,824 V/cm (initial 3,909 V/cm), and E_{1 mA/cm²} = 27 (initial 39) after application of a pulse current of 100 A.

Gum Tragacanth-Mediated Synthesis of Nanocrystalline ZnO Powder for Use in Varistors

Zinc oxide nanopowders were synthesized by a sol–gel method with gum tragacanth and zinc nitrate as raw materials. Gum tragacanth was used as stabilizer to control the mobility of zinc cations and the growth of the nanopowders. Thermo-gravimetric analysis, x-ray diffraction, Fourier-transform infrared spectroscopy, transmission electron microscopy, energy dispersive x-ray spectroscopy, and scanning electron microscopy were used to characterize the as-prepared samples. Zinc oxide (ZnO) nanoparticles calcined at different temperatures had a hexagonal wurtzite structure with average particle size ranging from 32.29 nm to 42.83 nm. The crystallinity of ZnO nanoparticles was improved by increasing the calcination temperature. The density of ZnO varistor ceramics sintered at 1150°C for 2 h in air was 5.46 g/cm3, which was 97.5% of the theoretical density, their breakdown voltage was 4572 V/cm, and their nonlinear coefficient was ~16.8. This method can be used as an excellent alternative method for synthesis of ZnO nanoparticles with a plant extract as a raw material. Our experimental results show our method had the advantage of improving the electrical performance of ZnO varistors.

Microhardness, microstructure and electrical properties of ZVM ceramics

The effect of Mn3O4 addition on microhardness, microstructure and electrical properties of vanadium oxide doped zinc oxide varistor ceramics is systematically investigated. The Vicker’s microhardness HV has decreased with increasing the amount of Mn3O4. Also, the average grain size has decreased from 27.51 μm to 19.55 μm with increasing the amount of Mn3O4 up to 0.50 mol%, whereas an increase in Mn3O4 up to 0.75 mol% has caused the average grain size to increase and then it decreases with increasing Mn3O4 from 0.75 mol% to 1.00 mol%. The sintered density has decreased from 5.38 g/cm3 to 5.31 g/cm3 with increasing the amount of Mn3O4. The varistor ceramic modified with 0.50 mol% Mn3O4 has exhibited excellent nonlinear properties, with 16.29 for the nonlinear coefficient and 441.9 μA/cm2 for the leakage current density. Furthermore, the sample doped with 0.50 mol% Mn3O4 has been found to possess donor density as 0.77×1018 cm−3 and 0.916 eV barrier height.

Grain resistivity in zinc oxide and tin dioxide varistor ceramics

In this paper the grain resistivity was measured for commercial ZnO-based and laboratory-made SnO2-based varistor ceramics exhibiting highly nonlinear current–voltage characteristics at low current (1mAcm−2). The high-current study was performed in the current range 2–500Acm−2 using recently developed technique based on a concept of the differential resistance and application of a single exponential voltage pulse. At 295K commercial ZnO varistors exhibit some lower values of the grain resistivity 0.59–0.61Ωсm in comparison with higher values 2.4–28Ωсm for SnO2 varistors. Respectively, the high-current nonlinearity coefficients for ZnO varistors are higher than for SnO2 varistors. The capacitive currents did not affect the obtained results. The activation energy of electrical conduction at low fields (in the range of Ohm’s law) for SnO2 varistors (0.82–0.98eV) is larger than for ZnO varistors (0.69–0.76eV). The observed correlation between the high-current and the low-current properties of ZnO-based and SnO2-based varistors is discussed.

Sintering effect on electrical properties and pulse aging behavior of (V2O5-Mn3O4-Er2O3)-doped zinc oxide varistor ceramics

The effect of sintering temperature on microstructure, electrical properties, and pulse aging behavior of (V2O5-Mn3O4- Er2O3)-doped zinc oxide varistor ceramics was systematically studied. When the sintering temperature increased, the average grain size increased from 6.1 to 8.7 μm and the sintered density decreased from 5.52 to 5.43 g/cm3. The breakdown field decreased from 3856 to 922 V/cm with an increase in the sintering temperature up to 900 °C, whereas a further increase to 2352 V/cm at 925 °C. The nonlinear coefficient increased pronouncedly from 4.6 to 30.0 with an increase in the sintering temperature. The varistor ceramics sintered at 850 °C exhibited the best clamping characteristics, with the clamp voltage ratio of the range of 2.22–2.88 for pulse current of 1–25 A. The varistor ceramics sintered at 925 °C exhibited the strongest stability, with %ΔE1 mA/cm2−—8.8% after applying the multi-pulse current of 25 A.

Effect of gadolinia addition on varistor characteristics of vanadium oxide–doped zinc oxide ceramics

The effect of gadolinia addition on microstructure, electrical and dielectric characteristics, and aging behavior of vanadium oxide–doped zinc oxide varistor ceramics was systematically investigated. The average grain size decreased from 5.6 to 5.2 μm with an increase in the amount of Gd2O3 up to 0.1 mol%, whereas a further increase caused it to increase to 5.7 μm at 0.25 mol%. The sintered densities decreased from 5.51 to 5.44 g/cm3 with an increase in the amount of Gd2O3. With increasing the amount of Gd2O3, the breakdown field increased from 4,800 to 5,365 V/cm up to 0.05 mol%, whereas a further increase decreased it to 4,781 V/cm at 0.25 mol%. The varistor ceramics modified with 0.05 mol% Gd2O3 exhibited excellent nonlinear properties, with 66.1 in the nonlinear coefficient, whereas a further increase caused it to decrease to 17.6 at 0.25 mol%. The gadolinium acted like a donor, based on the electron concentration increasing from 4.20 × 1017/cm3 to 7.38 × 1017/cm3 with an increase in the amount of Gd2O3.

Sintering Time Influences on Microstructure and Electrical Properties of Scandium Doped Zinc Oxide Varistor Ceramics

Sc2O3-doped zinc oxide varistor ceramics were prepared by a solid reaction route. The microstructure and the electrical properties was analyzed by scanning electron microscope and X-ray diffraction. Sintering time influences on the microstructure and the electrical properties of Sc2O3-doped zinc oxide varistor ceramics are studied. The results show that the electrical properties of Sc2O3-doped zinc oxide varistor ceramics sintered at 1000 and sintered for 1 h was better: the voltage gradient was 653 V/mm; the leakage current was 0.18 μA and the nonlinear coefficient was 57.1.

High-current measurement of the grain resistivity in zinc oxide varistor ceramics

Abstract A new pulse technique for grain resistivity measurement in varistor ceramics is suggested. Such technique allows obtaining more precise value of the grain resistivity due to the use of the concept of differential electrical resistance. This technique can be used in the current density range where the overheating of varistor sample is insignificant. The technique was verified using commercial ZnO varistors. Grain resistivities of 0.60±0.02 Ω cm at 293 K and of 3.40±0.13 Ω cm at 77 K were obtained. This result indicates the negative temperature coefficient of grain resistance in ZnO varistor in the range (77–293) K. The contribution of the grain boundaries to the current–voltage characteristic of ZnO varistor is estimated on the basis of the measured grain resistivity and the current–voltage data. It is shown that the electrical conduction in ZnO varistor is controlled by grains if the current density exceeds approximately 1000 А сm −2 .

Varistor characteristics of vanadium oxide-doped zinc oxide ceramics modified with bismuth oxide

The effect of Bi2O3 on microstructure, electrical properties, dielectric characteristics, and aging behavior vanadium oxide-doped zinc oxide varistor ceramics was systematically investigated. Analysis of the phase indicated that the ceramics modified with Bi2O3 consisted of ZnO grain as a main phase and a few secondary phases such as Zn3(VO4)2, ZnV2O4, BiVO4, V2O5, and Mn-rich phase. The average grain size increased from 5.6 to 7.2 μm and the sintered density decreased in the range of 5.51–5.37 g/cm3 up to 0.05 mol%, whereas a further addition increased it to 5.40 g/cm3 at 0.25 mol%. The breakdown field decreased from 4,874 to 2,205 V/cm with an increase in the amount of Bi2O3. The ceramics added with 0.025 mol% Bi2O3 were characterized by a surprisingly high nonlinear coefficient (60) and very low leakage current density (20 μA/cm2). Bi atoms in the bulk acted as a donor to increase the electron concentration with an increase in the amount of Bi2O3.

Effect of erbium on varistor characteristics of vanadium oxide-doped zinc oxide ceramics

The effect of erbium addition on microstructure, electrical properties, and ageing behavior of vanadium oxide-doped zinc oxide varistor ceramics was systematically investigated. Analysis of the microstructure indicated that the ceramics added with erbium consisted of ZnO grain as a main phase and secondary phases such as Zn3(VO4)2, ZnV2O4, ErVO4, V2O5, and Mn-rich. The average grain size decreased from 5.5 to 5.2 μm up to 0.05 mol%, whereas a further addition gradually increased it to 5.7 μm at 0.25 mol%. The sintered density increased from 5.51 to 5.61 g/cm3 with an increase in the amount of Er2O3. With increasing the amount of Er2O3, the breakdown field increased from 4,800 to 5,444 V/cm up to 0.05 mol%, whereas a further addition decreased it to 4,061 V/cm at 0.25 mol%. The varistor ceramics added with 0.05 mol% Er2O3 additives induced excellent nonlinear properties, with nonlinear coefficient of 63.4 by properly adding the amount of Er2O3 (0.05 mol%). The study indicated that the erbium acted as a donor to increase the donor concentration with an increase in the amount of Er2O3.

Synthesis Mechanism of Low-Voltage Praseodymium Oxide Doped Zinc Oxide Varistor Ceramics Prepared Through Modified Citrate Gel Coating

High demands on low-voltage electronics have increased the need for zinc oxide (ZnO) varistors with fast response, highly non-linear current-voltage characteristics and energy absorption capabilities at low breakdown voltage. However, trade-off between breakdown voltage and grain size poses a critical bottle-neck in the production of low-voltage varistors. The present study highlights the synthesis mechanism for obtaining praseodymium oxide (Pr6O11) based ZnO varistor ceramics having breakdown voltages of 2.8 to 13.3 V/mm through employment of direct modified citrate gel coating technique. Precursor powder and its ceramics were examined by means of TG/DTG, FTIR, XRD and FESEM analyses. The electrical properties as a function of Pr6O11 addition were analyzed on the basis of I-V characteristic measurement. The breakdown voltage could be adjusted from 0.01 to 0.06 V per grain boundary by controlling the amount of Pr6O11 from 0.2 to 0.8 mol%, without alteration of the grain size. The non-linearity coefficient, α, varied from 3.0 to 3.5 and the barrier height ranged from 0.56 to 0.64 eV. Breakdown voltage and α lowering with increasing Pr6O11 content were associated to reduction in the barrier height caused by variation in O vacancies at grain boundary.

Microstructure and electrical properties of Sm2O3 doped Bi2O3-based ZnO varistor ceramics

Abstract The dependence of the bulk density, microstructure and dc electrical properties of bismuth oxide (Bi 2 O 3 )-based zinc oxide (ZnO) varistor ceramics for various samarium oxide (Sm 2 O 3 ) contents was investigated. The value of bulk density was found to 5.43–5.50 g cm −3 with Sm 2 O 3 (mol%) content. The maximum value of bulk density is observed to be 5.50 for 0.30 mol% Sm 2 O 3 containing varistor ceramics. The grain sizes for all the samples calculated from the scanning electron micrographs were found to decrease as Sm 2 O 3 increases. The presence of ZnO phases, Bi-rich phases, spinel phases and Sm 2 O 3 phases were observed in the samples by the energy dispersive X-ray analysis and X-ray diffraction analysis. As the Sm 2 O 3 amount increased in the Bi 2 O 3 -based ZnO varistor ceramics, the nonlinear coefficient, α increased up to 0.10 mol%, reaching a maximum value of 58 and then decreased. The breakdown electric field, E b , increased with the increase of Sm 2 O 3 content with a maximum value of 3220 V cm −1 for the 0.75 mol% Sm 2 O 3 doped ZnO varistor ceramics. The leakage current, I L , showed a minimum value of 1.10 μA for the composition of 0.30 mol% Sm 2 O 3 doped Bi 2 O 3 -based ZnO varistor ceramics. The 0.30 mol% Sm 2 O 3 -doped Bi 2 O 3 -based ZnO varistor ceramics sintered at 1200 °C exhibited a good stability for dc accelerated aging stress of 0.90 V 1 mA /90 °C/12 h.