Microstructure and electrical properties of rare earth doped ZnO-based varistor ceramics

ZnO-based varistor ceramics doped with Nd₂O₃ and Y₂O₃ have been prepared by the conventional ceramics method. The phase composition, microstructure and electrical properties of the ceramics have been investigated by XRD, SEM and a V–I source/measure unit. The XRD and EDS analyses show the presence of ZnO, Bi₂O₃, Zn₇Sb₂O₁₂, Y₂O₃, Nd-rich phase and Y-containing Bi-rich phase. The electrical properties analyzed show that the nonlinear coefficient of the varistor ceramics is in the range of 4.4–70.2, the threshold voltage is in the range of 247.1–1288.8 V/mm, and the leakage current is in the range of 1.51–214.6 μA/cm². The 0.25 mol% Nd₂O₃ added varistor ceramics with 0.10 mol%Y₂O₃ sintered at 1050 °C exhibits excellent electrical properties with the high threshold voltage of 556.4 V/mm, the nonlinear coefficient of 61 and the leakage current of 1.55 μA/cm². The results illustrate that doping Nd₂O₃ and Y₂O₃ in ZnO-based varistor ceramics may be a very promising route for the production of the higher threshold voltage and the nonlinear coefficient of ZnO-based varistor ceramics.     

Effect of Ta2O5 in (Ca,Si, Ta)-doped TiO2 ceramic varistors

TiO₂ varistors doped with 0.2 mol% Ca, 0.4 mol% Si and different concentrations of Ta were obtained by ceramic sintering processing at 1350 °C. The effect of Ta on the microstructures, nonlinear electrical behavior and dielectric properties of the (Ca, Si, Ta)-doped TiO₂ ceramics were investigated. The ceramics have nonlinear coefficients of α = 3.0–5.0 and ultrahigh relative dielectric constants which is up to 10⁴. Experimental evidence shows that small quantities of Ta₂O₅ improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 0.8 mol% Ta₂O₅ leads to a low breakdown voltage of 14.7 V/mm, a high nonlinear constant of 4.8 and an ultrahigh electrical permittivity of 5.0 × 10⁴ and tg δ = 0.66 (measured at 1 kHz), which is consistent with the highest and narrowest grain boundary barriers of the ceramics. In view of these electrical characteristics, the TiO₂–0.8 mol% Ta₂O₅ ceramic is a viable candidate for capacitor–varistor functional devices. The characteristics of the ceramics can be explained by the effect and the maximum of the substitution of Ta⁵⁺ for Ti⁴⁺.     

Densification and grain growth of CuO-doped Pr6O11 varistors

The effect of CuO doping on the microstructure and electrical properties of Pr₆O₁₁ varistors was investigated. Samples were prepared by conventional ceramic techniques, and were sintered at 1150 °C in air for 2 h. The microstructure was investigated by scanning electron microscopy (SEM). The phases and chemical composition were analyzed by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The results indicated that CuO can promote the densification of the Pr₆O₁₁-based varistors to 95.8% of the theoretical density. CuO forms a solid solution with Pr₆O₁₁ up to 0.5 mol%, above which Pr₂CuO₄ precipitates in the grain boundary. From the I–V measurements, minor CuO doping can improve the nonlinear electrical properties. A further increase in CuO content induces a reduction in the nonlinear electrical properties due to the consumption of absorbed oxygen on the grain surfaces.     

Microstructure and electrical properties of Sc2O3-doped ZnO-Bi2O3-based varistor ceramics

The microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics doped with different Sc₂O₃ content sintered at 1100 °C were investigated. The results showed that the nonlinear coefficient of the varistor ceramics with Sc₂O₃ were in the range of 18-54, the threshold voltage in the range of 250-332 V/mm, the leakage current in the range of 0.1-23.0 μA, with addition of 0-1.00 mol% Sc₂O₃. The ZnO-Bi₂O₃-based varistor ceramics doped with Sc₂O₃ content of 0.12 mol% exhibited the highest nonlinearity, in which the nonlinear coefficient is 54, the threshold voltage and the leakage current is 278 V/mm and 2.9 μA, respectively. The results confirmed that doping with Sc₂O₃ was a very promising route for the production of the higher nonlinear coefficient of ZnO-Bi₂O₃-based varistor ceramics, and determining the proper amounts of addition of Sc₂O₃ was of great importance.     

Microstructure, electrical properties, and aging behavior of ZnO-Pr6O11-CoO-Cr2O3-Y2O3-Er2O3 varistor ceramics

The microstructure, electrical properties, and aging behavior of the ZnO-Pr₆O₁₁-CoO-Cr₂O₃-Y₂O₃-Er₂O₃ varistor ceramics were investigated for different contents of Er₂O₃. The microstructure consisted of ZnO grain and an intergranular layer (Pr, Y, and Er-rich phases) as a secondary phase. The increase of Er₂O₃ content decreased the average grain size and increased the sintered density. As the Er₂O₃ content increased, the breakdown field increased from 4206 V/cm to 5857 V/cm and the nonlinear coefficient increased from 32.6 to 48.6. The varistor ceramics added with 1.0 mol% Er₂O₃ exhibited excellent stability by exhibiting −0.2% in the variation rate of the breakdown field and −2.7% in the variation rate of the nonlinear coefficient for aging stress of 0.95 E_1 mA/150 °C/24 h.     

Effects of MgO doping in ZnO–0.5 mol% V2O5 varistors

The effects of MgO (0–40 mol%) on the microstructure and the electrical properties have been studied in a binary ZnO–0.5 mol% V₂O₅ system. The microstructure of the samples consists mainly of ZnO grains with MgO and γ-Zn₃(VO₄)₂ as the minority secondary phases. MgO is found to be effective as a grain growth inhibitor in controlling the ZnO grain growth, and a more uniform microstructure can be obtained. The non-linear coefficient α value is found to increase with the amount of MgO, and a highest value of 8.7 is obtained for the sample doped with 10 mol% MgO. Further addition of ≥20 mol% MgO decreases the α value.     

Cr2O3 doping in ZnO–0.5 mol% V2O5 varistor ceramics

The effects of the amount of Cr₂O₃ (0.5–4 mol%) on the microstructure and the electrical properties have been studied in a binary ZnO–0.5 mol% V₂O₅ system. The microstructure of the samples consists mainly of ZnO grains with ZnCr₂O₄ and α-Zn₃(VO₄)₂ as the minority secondary phases. The addition of Cr₂O₃ is found to be effective in controlling the abnormal ZnO grain growth often found in V₂O₅-doped ZnO ceramic system, and a more uniform microstructure can be obtained. The varistor performance is also improved as observed from the increase in the non-linear coefficient α of the Cr₂O₃-doped ZnO–V₂O₅ samples. The α value is found to increase with the amount of Cr₂O₃ for up to 3 mol% Cr₂O₃ content. Further increase in Cr₂O₃ is found to cause a decrease in the α value. The highest α value of 28.9 is obtained for the ZnO–0.5 mol% V₂O₅–3 mol% Cr₂O₃ sample.     

Sintering of glass-added BaZn1/3Nb2/3O3 ceramics

The complex perovskite oxide Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been studied for its attractive dielectric properties which place this material interesting for applications as multilayer ceramics capacitors or hyperfrequency resonators. This material is sinterable at low temperature with combined glass phase–lithium salt additions, and exhibits, at 1 MHz very low dielectric losses combined with relatively high dielectric constant and a good stability of this later versus temperature. The 2 wt.% of ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of LiF-added BZN sample sintered at 900 °C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant ?_r of 39, low dielectrics losses (tan(δ) < 10⁻³) and a temperature coefficient of permittivity τ_? of 45 ppm/°C⁻¹. The 2 wt.% ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of B₂O₃-added BZN sintered at 930 °C exhibits also attractive dielectric properties (?_r = 38, tan(δ) < 10⁻³) and it is more interesting in terms of temperature coefficient of the permittivity (τ_? = −5 ppm/°C). Their good dielectric properties and their compatibility with Ag electrodes, make these ceramics suitable for L.T.C.C applications.     

Giant dielectric permittivity and non-linear electrical behavior in CaCu3Ti4O12 varistors from the molten-salt synthesized powder

CaCu₃Ti₄O₁₂ (CCTO) powder has been prepared by a molten salt method using the NaCl–KCl mixture. Crystal structure and microstructure of the powder and the resulting ceramics have been characterized by using X-ray diffraction (XRD) and scanning electron microcopy (SEM). Impedance analyzer and current–voltage meter were employed to analyze dielectric and nonlinear (I–V) properties of the CCTO ceramics with different sintering durations and subsequent cooling rates. The values of dielectric permittivity and nonlinear coefficient of the quenched sample were found to be higher than those of the slowly cooled sample. More specifically, the cooling methods (quenching and furnace-cooling) have allowed to adjust; (?) the breakdown voltage within a rather low range of 0.3–4.4 kV cm⁻¹; (??) the nonlinear coefficient between 2 and 6 and (???) the giant dielectric permittivity for the ceramics within a range from 5000 to 20000. A double Schottky barrier can be evidenced from the linear behavior between the ln J and E^1/2 in grain boundary regions. The relationship between the electrical current density and the applied electrical field indicates that the potential barrier height Φ_B is holding time dependent.     

Electrical properties of Pr6O11-doped WO3 capacitor–varistor ceramics

The microstructure and electrical properties of Pr₆O₁₁-doped WO₃ ceramics were investigated. Results showed that the breakdown voltage of doped samples was lower than that of the undoped. The dielectric constant of doped samples was higher than that of the undoped, and the high dielectric constant made Pr₆O₁₁-doped WO₃ ceramics to be applicable as a kind of capacitor–varistor materials. A small content of Pr₆O₁₁ could significantly improve nonlinear properties of the samples. The WO₃–0.03 mol% Pr₆O₁₁ obtained a large nonlinear coefficient of 3.8, a low breakdown voltage of 8.8 V/mm, and a high dielectric constant of 7.69 × 10⁴ at 1 kHz. The defects theory was introduced to explain the nonlinear electrical behavior of Pr₆O₁₁-doped WO₃ ceramics.     

Electrochemical performance of the carbon coated Li3V2(PO4)3 cathode material synthesized by a sol-gel method

A carbon coated Li₃V₂(PO₄)₃ cathode material for lithium ion batteries was synthesized by a sol-gel method using V₂O₅, H₂O₂, NH₄H₂PO₄, LiOH and citric acid as starting materials, and its physicochemical properties were investigated using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) spectroscopy, scanning electron microscopy (SEM), energy dispersive analysis of X-ray (EDAX), transmission electron microscope (TEM), and electrochemical methods. The sample prepared displays a monoclinic structure with a space group of P2₁/n, and its surface is covered with a rough and porous carbon layer. In the voltage range of 3.0-4.3 V, the Li₃V₂(PO₄)₃ electrode displays a large reversible capacity, good rate capability and excellent cyclic stability at both 25 and 55 °C. The largest reversible capacity of 130 mAh g⁻¹ was obtained at 0.1C and 55 °C, nearly equivalent to the reversible cycling of two lithium ions per Li₃V₂(PO₄)₃ formula unit (133 mAh g⁻¹). It was found that the increase in total carbon content can improve the discharge performance of the Li₃V₂(PO₄)₃ electrode. In the voltage range of 3.0-4.8 V, the extraction and reinsertion of the third lithium ion in the carbon coated Li₃V₂(PO₄)₃ host are almost reversible, exhibiting a reversible capacity of 177 mAh g⁻¹ and good cyclic performance. The reasons for the excellent electrochemical performance of the carbon coated Li₃V₂(PO₄)₃ cathode material were also discussed.     

Sintering of surfactant modified ZnO–Bi2O3 based varistor nanopowders

Surfactant modified nano-origin ZnO–Bi₂O₃ varistor powder was prepared in presence of cetyltrimethyl ammonium bromide (CTAB) surfactant through an aqueous reflux reaction at 100 °C. The compacted varistor discs made from the nano-origin powders were subjected to step-sintering, microwave sintering and solid-state sintering. The influences of CTAB in different sintering methods were analyzed from the densification characteristics, evolution of sintered microstructures and associated varistor properties (I–V). The conventional solid-state sintering produced 96% theoretical sintered dense samples at 1100 °C. The step and microwave sintered samples showed 93% and 99% sintered densities, respectively, with controlled microstructures having grain sizes in the range of 2–6 μm at the given conditions. The CTAB advantages were clearly seen in grain structuring and grain boundary properties, in addition to the enhanced densification and homogenous microstructures for obtaining high breakdown voltage and non-linearity coefficient.     

Synthesis of LiNi0.9Co0.1O2 from Li2CO3, NiO or NiCO3, and CoCO3 or Co3O4 and their electrochemical properties

Cathode active materials with a composition of LiNi_0.9Co_0.1O₂ were synthesized by a solid-state reaction method at 850 °C using Li₂CO₃, NiO or NiCO₃, and CoCO₃ or Co₃O₄, as the sources of Li, Ni, and Co, respectively. Electrochemical properties, structure, and microstructure of the synthesized LiNi_0.9Co_0.1O₂ samples were analyzed. The curves of voltage vs. x in Li_xNi_0.9Co_0.1O₂ for the first charge–discharge and the intercalated and deintercalated Li quantity Δx were studied. The destruction of unstable 3b sites and phase transitions were discussed from the first and second charge–discharge curves of voltage vs. x in Li_xNi_0.9Co_0.1O₂. The LiNi_0.9Co_0.1O₂ sample synthesized from Li₂CO₃, NiO, and Co₃O₄ had the largest first discharge capacity (151 mA h/g), with a discharge capacity deterioration rate of −0.8 mA h/g/cycle (that is, a discharge capacity increasing 0.8 mA h/g per cycle).     

Fabrication of tridoped p-ZnO thin film and homojunction by RF magnetron sputtering

Tridoping (Al–As–N) into ZnO has been proposed to realize low resistive and stable p-ZnO thin film for the fabrication of ZnO homojunction by RF magnetron sputtering. The tridoped films have been grown by sputtering the AlN mixed ZnO ceramic targets (0, 0.5, 1 and 2 mol%) on GaAs substrate at 450 °C. Here, Al and N from the target, and As from the GaAs substrate (back diffusion) takes part into tridoping. The grown films have been characterized by Hall measurement, X-ray diffraction, photoluminescence, time-of-flight secondary ion mass spectroscopy and X-ray photoelectron spectroscopy. It has been found that all the films showed p-conductivity except for 2 mol% AlN doped film. The obtained resistivity (8.6×10⁻² Ω cm) and hole concentration (4.7×10²⁰ cm⁻³) for the best tridoped film (1 mol% AlN) is much better than that of monodoped and codoped ZnO films. It has been predicted that [(As_Zn−2V_Zn)+N_O] acceptor complex is responsible for the p-conduction. The homojunction fabricated using the best tridoped ZnO film showed typical rectifying characteristics of a diode. The junction parameters have been determined for the fabricated homojunction by Norde's and Cheung's method.     

Indium tailors the leakage current and voltage gradient of multiple dopant-based ZnO varistors

In the present study, the effect of indium doping on the micro-characteristics and electrical properties of ZnO varistors co-doped with Al₂O₃ and Y₂O₃ were determined. Scanning electron microscopy, current-voltage testing in a range from small to large current, capacitance-voltage testing, and X-ray diffraction pattern testing were conducted. The results show that both the residual voltage ratio and the leakage current of sintered ZnO varistors decrease and then increase as the indium dopant increases at a given aluminum and yttrium content. The nonlinear coefficient shows an inverse relationship. In addition, the voltage gradient of the samples increases as the indium dopant increases. The sintered ZnO varistor samples with 0.02 mol% indium, 0.2 mol% aluminum, and 0.9 mol% yttrium show the optimal performance, exhibiting a 1-mA residual voltage of 448 V/mm, a leakage current of 0.69 µA/cm², a nonlinear coefficient of 76, and a residual voltage ratio of 1.58. This study has great significance for improving the protective effects of surge protection devices assembled with ZnO varistors and the stability of power systems.     

Microwave dielectric properties of SnO2-doped CaSiO3 ceramics

SnO₂-doped CaSiO₃ ceramics were successfully synthesized by a solid-state method. Effects of different SnO₂ additions on the sintering behavior, microstructure and dielectric properties of Ca(Sn_1−xSi_x)O₃ (x=0.5–1.0) ceramics have been investigated. SnO₂ improved the densification process and expanded the sintering temperature range effectively. Moreover, Sn⁴⁺ substituting for Si⁴⁺ sites leads to the emergence of Ca₃SnSi₂O₉ phase, which has a positive effect on the dielectric properties of CaO–SiO₂–SnO₂ materials, especially the Qf value. The Ca(Sn_0.1Si_0.9)O₃ ceramics sintered at 1375 °C possessed good microwave dielectric properties: ε_r =7.92, Qf =58,000 GHz and τ_f=−42 ppm/°C. The Ca(Sn_0.4Si_0.6)O₃ ceramics sintered at 1450 °C also exhibited good microwave dielectric properties of ε_r=9.27, Qf=63,000 GHz, and τ_f=−52 ppm/°C. Thus, they are promising candidate materials for millimeter-wave devices.     

Effect of Sm2O3 on the microstructure and electrical properties of SnO2-based varistors

The effect of Sm₂O₃ on the microstructure and non-linear electrical properties of (Co, Nb)-doped SnO₂-based varistors was investigated. The addition of Sm₂O₃ improved the non-linear characteristics of (Co, Nb)-doped SnO₂-based varistors. The threshold electric field (E_B) of SnO₂-based varistors increased significantly from 5340 to 12,460 V cm⁻¹ and the mean grain size decreased from 4.7 μm to 1.7 μm as Sm₂O₃ concentration increased up to 0.20 mol%. There was an optimal value (0.20 mol%) of the Sm₂O₃ concentration. The sample doped with 0.20 mol% Sm₂O₃ had the highest non-linear coefficient (α = 28). The addition of Sm₂O₃ reduced the relative density of (Co, Nb)-doped SnO₂-based varistors.     

TEM observation of liquid phase sintering in V2O5 modified Zr0.8Sn0.2TiO4 microwave ceramics

The phenomena of liquid phase sintering in the V₂O₅ modified (Zr_0.8, Sn_0.2)TiO₄ (ZST) microwave ceramics has been investigated by using transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDS). The amounts of second phase were too low to be detected by X-ray diffraction (XRD), but could be observed by TEM bright field image. However, the presence of grain boundary phases did not degrade the microwave properties of V₂O₅ modified ZST ceramics. The ?_r value of 37.2, Q × f value of 51,000 (at 7 GHz) and τ_f value of −2.1 ppm/°C were obtained for ZST ceramics with 1 wt% V₂O₅ addition sintered at 1300 °C.     

Al-doped ZnO varistors prepared by a two-step doping process

ABSTRACT

It is difficult to dope Al into main grains of ZnO varistor ceramics, especially for small doping amount. Generally, all raw materials including Al dopant are directly mixed together and sintered into ceramics. However, in this direct doping process, Al is apt to stay in grain boundaries, and almost does not enter grains. This does harm to the electrical properties of ZnO varistors. In this paper, we proposed a two-step doping process. Al₂O₃ powder was first mixed only with a part of the ZnO powder and pre-sintered. The pre-sintered powder was mixed with other additives such as Bi₂O₃ and the rest ZnO. Then ZnO varistor ceramics were prepared via solid state sintering processes. Results showed that two-step doped ZnO varistors exhibited improved electrical properties with a significant increased nonlinear coefficient and a great decreased leakage current compared to directly doped ones because more Al was incorporated into ZnO grains.     

Effects of addition of chromium and/or nickel oxides on the electrical characteristics of yttrium oxide-doped high-voltage zinc oxide varistors

The effects of Cr₂O₃ and/or NiO addition to Y₂O₃-doped Bi-based high-voltage ZnO varistors were investigated, including crystal structure and interface states analysis. Incorporating 0.35 mol% Cr₂O₃ in Y-doped varistors increased the single-grain varistor voltage V_NNGB from 2.6 to 3.3 V and decreased the leakage current density by a factor of 40, from 2 × 10^?5 to 5 × 10^-7 A/cm². The nonlinearity index α before and after degradation increased from 21 to 35. 1.2 mol% NiO increased V_NNGB to 4 V. Highest varistor voltage was 1500 V for 0.3 mol% NiO. Resistance to electrical degradation improved with optimal amounts of Cr or Ni by reduction of grain boundary oxygen vacancies. Though the donor density was changed prominently by addition of Cr and/or Ni, changes in the barrier height were suppressed by the change in interface state density. As a result, the change in V_NNGB was related mainly to the empty interface state under no bias.