Varistor System Research Articles

Structure modification and electrical properties by Mn2O3 dopant addition to SnO2 varistor system

This work aims to study the effect of Mn2O3 additions on the electrical properties and microstructure characteristics of a SnO2-based ceramic varistor system doped additionally with Sb2O5, Cr2O3, and CoO. The specimens were prepared using conventional ceramic processing and homogenized by a high-energy ball milling system using the following composition: (98.99-X) % SnO2 – 0.05% Cr2O3 – 0.05% Sb2O5 – 1.00% CoO – X% Mn2O3 where X = 0.00, 0.05, 0.10, 0.20, 0.50 and 1% mol. Characterization by TG-DSC/DTA, XRD, XPS, and SEM/EDS, and the proposed chemical and defect-formation reactions allowed the conclusion that Mn2O3 additions produce alterations of the microstructure consisting of the in situ formation of the spinel Co2MnO4 secondary phase and modification of the potential barriers in the intergranular regions, where, in addition, oxygen vacancies are formed. With the 0.05 mol % Mn2O3, the grain size (average in the range of 2–3 μm) drops by 20 %, thus augmenting the grain boundaries. Altogether, this leads to a decrease in the nonlinearity coefficient (α) and an advantageous displacement of the breakdown electric field (EB). The 0.05 mol % Mn2O3 specimen compares favorably with the reference material by surpassing the EB value by a factor of 8. Elucidation of Mn2+ and Co2+ in 1 mol % Mn2O3 specimens by XPS suggests the role of MnO and CoO as intermediate phases, essential for Co2MnO4 formation. The pathway for in situ formation of Co2MnO4 is set forth based on the above characterization techniques in conjunction with proposed chemical and defect-formation reactions.

SnO2 Doping Effect on the Microstructural and Electrical Behavior of ZnO nanoparticles-Bi2O3 Based Varistor Ceramics

In the current study, ZnO-Bi2O3 varistors manufactured from ZnO nanoparticle size and doped with SnO2 were processed by the ceramic industrial technique. The impact of the various SnO2 concentration upon the sintering, microstructural improvement and nonlinearity characteristic of the varistor system was investigated. Different SnO2 concentration was found to possess considerable influences on the varistors, particularly on improving ZnO grains growth at a lower SnO2 amount. But, the adding of greater contents of SnO2 leads to inhibit the ZnO grain growth via the raising percentage of Zn2SnO4 type spinel phase. In discs with a Sn/Bi less than 1.5 mol%, the pyrochlore phase forms and bounds all the SnO2, whereas excess Bi2O3 leads to the formation of a liquid phase at 740 °C which promotes sintering process. For Sn/Bi more than 1.5 mol%, all the Bi2O3 is bounded in Bi2Sn2O7 pyrochlore phase and sintering process is prevented. X-ray diffraction investigated that additive of SnO2 to the ZnO-Bi2O3 varistor sample caused the structure of the Bi2Sn2O7 type pyrochlore phase and the Zn2SnO4 type spinel phase during the sintering step. The SnO2 adding furthermore noticeably affected the electrical behaviors of the sample with a significant rise in the breakdown voltage with increasing the SnO2 doping more than 1.5 mol%. The resistivity also achieved an obvious increase with increase the SnO2 doping contents. Moderate SnO2 doping enhanced the nonlinear properties of the ZnO-Bi2O3 varistor system, while a significant amount of SnO2 causing its deterioration.

Sensitivity of electrical behaviours of ZnO nanoparticle–Bi2O3–Mn2O3 varistor system to various La2O3 doping compositions

The effects of La2O3 on the behaviours of ZnO nanoparticle–Bi2O3–Mn2O3 varistors were determined. The discs with various La2O3 compositions (0.0–3.0 mol%) were prepared via traditional ceramic techniques and their characteristics were investigated. The results demonstrate that La2O3 remarkably affects the varistor samples, in which the size of grain diminished from 29.3 to 8.5 µm according to increasing La2O3 content from 0.0 to 3.0 mol%. The significant response within the varistor sample fabricated from ZnO nanoparticles through sintering process at 1200oC perhaps caused by the surface properties of the powdered ZnO nanoparticles. X-ray diffraction results show that the addition of La2O3 to the varistor resulted in additional peaks, which can be attributed to the construction of La-based phases through the sintering process at high temperature. The spinal phases of the (Bi48ZnO73) and (Mn3O4) diminish with the 3.0 mol% content of La2O3. The varistor samples with 2.0 mol% La2O3 show a superior spinal phase construction of (Bi48ZnO73) and (Mn3O4) and La-based phases, resulting in homogeneous grain size. La2O3 doping substantially enhanced the electrical behaviours of the disc sample with a notable increase in breakdown voltage from 2614.8 to 5730.7 V/mm caused by the increase in La2O3 doping concentration from 0.0% to 3.0 mol%. The improvement was associated with the potential lanthanide ion segregation caused by its greater ionic radius to the boundaries between ZnO grains, thus enhancing its electrical properties. Moderate La2O3 doping concentration at 1.0 mol% improved the non-linear characteristics of the sample (α = 66) and 4400 V/mm breakdown voltage was introduced, while a significant amount of La2O3 doping concentration (more than 1.0 mol%) caused its deterioration (α = 35). The increase in donor concentration Nd to 2.1 × 1020 cm−3 for the varistor doped with 1.0 mol% La2O3 is accompanied by a decrease of interfacial states density NIS to 2.2 × 1012 cm−2 eV−1, in addition to decrease in depletion layer ω to 3.9 nm. The leakage currents of the sample decreased from 0.473 to 0.310 mA/cm2 with the increase of La2O3 doping contents from 0.0% to 2.0 mol% and then increased to 0.596 mA/cm2 at 3.0 mol% La2O3 doping concentration. Therefore, La2O3 may be added to dominate the microstructural characteristics and electrical behaviours of ZnO–Bi2O3–Mn2O3 varistor manufactured using powdered ZnO nanoparticles.

Electrical and microstructural evaluation of ZnO varistor ceramics with different CaSiO3 contents

In the present study, the impact of CaSiO3 addition on microstructure and electrical characteristics of a ZnO-based varistor system was investigated. The ceramic materials were prepared by solid-state reaction route by varying the CaSiO3 concentrations (0, 3, 6, 9, 12, and 15 wt%) and sintered at 1300 °C. Characterization of the pelletized samples was carried out by density measurement, scanning electron microscopy, energy dispersive X-ray, X-ray diffraction, and current density-electric field characteristics measurement. The growth of ZnO grain was restrained with the introduction of CaSiO3; the average grain size decreases from 18.09 to 10.85 μm, whereas their relative density increases up to 99.79%. Structural results indicated the existence of the CaSiO3-rich phase without any other secondary phase formation. It is also revealed that different CaSiO3 contents significantly affect the electrical properties, prominently in the sample with 12 wt% CaSiO3 addition. The maximal value of the nonlinear coefficient, breakdown voltage, and barrier height acquired in this work were 4.41, 0.80 V/mm, and 0.691 eV, respectively, along with a sharp decline in leakage current density to 373.93 μA/cm2. Further increase in CaSiO3 content (15 wt%) eventually caused a deterioration in varistor behavior.

Structural, nonlinear electrical characteristics, and stability against DC-accelerated aging stress behavior of Er-doped 20 nm ZnO-Bi2 O3 -Mn2 O3 -based varistors.

In this research, ZnO-Bi2 O3 -Mn2 O3 varistors manufactured from 20 nm ZnO powder and doped with different amounts of Er2 O3 were fabricated via the conventional ceramic processing method. The effect of various Er2 O3 concentrations (0.5-2.0mol%) on the sintered density, structural improvement, and nonlinear behavior of the 20 nm ZnO-Bi2 O3 -Mn2 O3 varistors was discussed. Different Er2 O3 concentrations exerted considerable influence on the varistors. The addition of large amounts of Er2 O3 led to the inhibition of grain growth by increasing the amount of the Er2 O3 -rich spinel phase. X-ray diffraction analysis showed that the addition of Er2 O3 to the varistor systems led to the development of Er2 O3 -rich phases during sintering. During sintering, the considerable surface area of the nanoparticle powder resulted in numerous interactions in the surfaces of the ceramics manufactured from 20 nm ZnO powder. The electrical behaviors of the varistors were substantially influenced by Er2 O3 . E1 mA evidently increased from 2,144.7 to 5,482.2V/mm with the increment in doping amount. The nonlinear coefficient also dramatically increased with the increase in Er2 O3 content. The ZnO nanoparticle-Bi2 O3 -Mn2 O3 varistors doped with 1.0-2.0mol% Er2 O3 possessed high voltage and nonlinearity and were stable against DC-accelerated aging stress.

Dependence of the nonlinear behaviour stability of a ZnO nanoparticle-Bi2 O3 -Mn2 O3 -based varistor system on cooling rates during ceramic processing.

A conventional ceramic processing method was applied to manufacture high-density 20-nm ZnO-Bi2 O3 -Mn2 O3 varistor ceramics. Different cooling rates in the range of 135-540°C/h led to a relatively slight influence on the microstructure, varistor voltage, and leakage current. In contrast, these rates strongly affected the nonlinear exponent. The specific surface area of the 20-nm ZnO nanoparticle may have led to an intense solid-state reaction even at a low cooling rate. Superior nonlinearity, with 59.7 μA nonlinear current leakage and 273.5 μA leakage current, was achieved at the 135°C/h cooling rate. The differences in the cooling rates led to a remarkable change in the material's stability under direct current (DC)-accelerated aging stress in the following order: 135°C/hr ˃ 270°C/hr˃405°C/hr ˃ 540°C/hr.

Influence of MCO3 (M=Ca, Sr, Ba)-doping on the non-ohmic properties of the ceramic varistor system SnO2–Co3O4–Cr2O3–Nb2O5

In this study, the influence of the addition of calcium, strontium and barium carbonates on the microstructure and the electrical properties of the SnO2–Co3O4–Cr2O3–Nb2O5 ceramic varistor system was investigated. The powder mixtures were previously studied by thermogravimetric analysis and differential scanning calorimetry showing the decomposition of carbonates into CaO, SrO, and BaO below 1000 °C. Then, green tablets were obtained and sintered at 1350 °C for 1 h under an ambient atmosphere. Later characterization by X-ray diffraction demonstrates a systematic increase on the lattice parameters as CaCO3 SrCO3, and BaCO3 were added. Scanning Electron Microscopy reveals a decreasing tendency for the average grain size. The electrical performance was evaluated in terms of electrical breakdown field (Eb) and non-linear coefficient (α). Although all the samples exhibited non-ohmic electrical behavior, the SrCO3-doped ceramic showed the highest non-linear coefficient (α = 10) compared to those doped with CaCO3 or BaCO3. A defect model is proposed to explain the way that Ca2+, Sr2+, or Ba2+ incorporates into SnO2 lattice.

CaO doped ZnO–Bi2O3 varistors: Grain growth mechanism, structure and electrical properties

In this study, the impact of CaO doping on the microstructure, phase and electrical properties of the ZnO–Bi2O3 system was studied. The powder was synthesized by the solution combustion route. Reduction in the crystallite size of ZnO from 23 nm to 17 nm of the powder sample with CaO doping was noted. Conventional sintering was used to consolidate the powders. Solid state reacted compositions revealed, from X-ray diffraction, formation of two phases (a) Ca4Bi6O13 at 750 °C and (b) Ca0.89Bi3.11O5.56 at 900 °C. An increase in density from 4.21 to 5.45 gcc−1 and a decrease in grain size from 4.50 to 1.54 μm with increasing CaO doping were also observed. A grain growth mechanism in the ZnO–Bi2O3–CaO system was proposed. The breakdown field increased up to 1 wt % CaO (27 kV cm−1) and after that decreased. Impedance spectroscopic investigation, in the temperature range of 100300 °C and frequency range of 0.1 Hz -1M Hz, exhibited two relaxations. These were attributed to defects formed. The two-stage activation energy of conductivity was noticed, which was similar to that observed in complex ZnO varistor systems.

(Sb2O3/Sb2O5)-doped SnO2–Co3O4–Cr2O3 varistors: The Sb2O4 in-situ formation and its influence over the electrical and microstructural properties

The present study aimed to systematically study the transitions and consequential effects of antimony oxide (Sb2O3 or Sb2O5) additions over the properties of a SnO2-based varistor system. High energy ball-milling and conventional sintering were used to obtain the samples with the following molar composition: (98.95-X)% SnO2 - 1% Co3O4 - 0.05% Cr2O3 - X% Sb2O3/Sb2O5 where X = 0, 0.05, 0.1, 0.2 and 0.4 mol%. The thermal analysis suggested the in-situ formation of Sb2O4 at ~450 °C from Sb2O3 or Sb2O5 during the sintering of mixed oxides. SEM, XRD, and electrical analysis revealed similar results by using Sb2O3 or Sb2O5; the addition of 0.05 mol% antimony oxide provides the foremost properties. The transition equations from Sb2O3 or Sb2O5 to Sb2O4 demonstrate equivalency in the amount of Sb2O4 formed. That fact, besides the results obtained, were used to discuss a reasonable route for Sb3+ and Sb5+ incorporation within the SnO2 lattice.

Comparison between the use of Co3O4 or CoO on microstructure and electrical properties in a varistor system based on SnO2

This comparative study pursues to clarify the differences between the addition of CoO or Co3O4 as a dopant on the non-linear electrical behavior and microstructure characteristics of a varistor system based on SnO2. The samples were fabricated according to the conventional ceramic processing using the nominal composition: (99.99-X)%SnO2 – 0.05%Cr2O3 – 0.05%Sb2O5 – X%CoO/Co3O4 where X = 0, 1, 3 and 5 mol%. The TGA-DTA analysis revealed essential differences in the thermal stability of CoO and Co3O4 during the sintering process, being CoO the oxide that induced the most visible changes. SEM micrographs, supported by EDS and XRD, showed remarkable grain growth even with a slight addition of cobalt oxide in comparison with the reference sample. The use of CoO holds the average grain size (6.7 μm), composition (SnO2), and shape homogeneity. In the case of Co3O4, a decreasing tendency in the average grain size and an increase in the residual porosity due to the in-situ formation of Co2SnO4 phase is observed. The electrical analysis revealed similarities on the alpha value for all molar percentages of both cobalt oxides: ∼9.6 and ∼8.1 units for CoO and Co3O4, respectively. In contrast, the breakdown electrical field shows notable differences: 1.08–1.16 kV cm−1 for CoO and 1.08–2.46 kV cm−1 for Co3O4. The evidence of this study confirms previous findings and suggests the existence of an essential role for the phase transitions of cobalt oxide on the final performance of SnO2-based varistors.

A comparative study between the addition of nano and micro-particles of Co3O4 on the electrical and microstructural properties of a ceramic system based on SnO2

A comparative study between the addition of Co3O4 micro-particles and nano-particles as densifying dopant of a SnO2 based varistor system was conducted. The ceramic composition was (99.9-X) %SnO2–X %Co3O4–0.05 %Cr2O3–0.05 %Nb2O5 where X = 0, 0.5, 1.0, 2.0 and 4.0 mol%. Two particle sizes of Co3O4 were used (~5 µm and ~50 nm). The addition of 0.5 mol% of Co3O4 nano-particles promoted an increase of grain size of sintered samples up to 7.9 µm, that is, the maximum value among all variations. Characterization techniques such as TGA, DTA, XRD, and Rietveld analysis revealed a decrease of 16 ºC in the formation temperature of Co2SnO4 as well as an increase of 2.6 wt% in the amount of said phase with the use of 4.0 mol% of Co3O4 nano-particles in comparison with micro-particles. Statistical analysis indicated that the addition of nano-particles of Co3O4 yield better repeatability on densification of ceramic samples. Residual porosity also was decreased. Electrical breakdown and non-linear coefficient values correspond to a non-ohmic behavior with potential application on manufacture of high voltage varistors. The findings of this work can be used as a reference for conducting a later study to improve the electrical properties or even to lower the sintering temperature.

Effects of Ta2O5 Addition on Electrical Properties of ZnO-V2O5 Based Varistor Ceramics

ZnO varistors are widely used for the protection of electronic and electrical equipment against transient surges. ZnO–V2O5 based varistor system is a potential candidate which can co-fire with Ag, and avoids the use of expensive Pa and Pt as the inner electrode in making multilayer chip varistors. However, the study of ZnO–V2O5-based ceramics is still in the initial stage for practical applications. The current work reports the effects of Ta2O5 on the electrical properties of ZnO-V2O5 based varistor ceramics. It shows that within 850-925°C experimental sintering temperature, the addition of Ta2O5 (0.05-0.20 mol%) may not improve the nonlinear coefficient but reduces the breakdown field of ZnO–V2O5 varistor ceramics.

Influence of sintering temperature on microstructure and electrical properties of Er2O3 added ZnO-V2O5-MnO2-Nb2O5 varistor ceramics

The effect of sintering temperature (950–1100 °C) on the densification, microstructure and nonlinear electrical properties of 0.5 mol. % Er2O3 added ZnO–V2O5–MnO2–Nb2O5 ceramics powder mixture was investigated. Microstructural characterizations by XRD and SEM coupled with EDS microanalyses confirm the development of equiaxed ZnO grains surrounded by different secondary phases, mainly Er-rich spinel phases in the intergranular layers and triple points. Image analyses reveal that the average size of ZnO grain increases from 1.8 to 7.2 μm with increasing sintering temperature from 950 to 1100 °C. Coarsening of grain structure is found to reduce the breakdown field (10286–2584 V cm−1), and nonlinear exponent (150–26). The donor concentration, ND increases from 2.2 × 1017 to 4.9 × 1017 cm−3 with increasing sintering temperature from 950 to 1100 °C, whereas, the depletion layer width, ω decreases from 56.4 nm to 37.8 nm. Typically, a fine-grained (1.8 μm) and dense (95% TD) varistor sample possessing a high value of nonlinear exponent (α = 150); enhanced breakdown field (10286 V cm− 1) and leakage current density of 0.18 mA cm−2 could be made by conventional sintering technique at a reasonably lower temperature of 950 °C from the chosen multicomponent ZnO varistor system.

Controlling the breakdown electric field in SnO2 based varistors by the insertion of SnO2 nanobelts

Semiconducting metal oxides have many practical applications, including varistors. Varistors based on SnO2 exhibit both high nonlinear coefficients and high breakdown electric fields. In this work we present a facile means for controlling the breakdown electric field in the SnO2-CoO-Cr2O3-Nb2O5 varistor system by the introduction of 1D SnO2 nanobelts. The materials were prepared by a solid state reaction method with Nb2O5 doping levels at 0.10 and 0.20, mole percent. The materials were studied in detail by dual beam microscopy, direct current and impedance measurements. Exaggerated three-dimensional growth of the tin dioxide belts was observed, which was attributed to Ostwald ripening. The breakdown electric field was observed to decrease from 2510V/cm to 2280V/cm and from 1700V/cm to 804V/cm after nanobelt insertion, into the systems with 0.10 and 0.20, mole percent Nb2O5 respectively. A model for the observed results was proposed based on the percolation of electrons through the belts, decreasing the number of effective potential barriers at the grain boundaries. The simulations of the impedance data showed one order of magnitude decrease in the grain boundary resistance due to the nanobelt insertion for both studied systems.

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.

Microstructure and nonohmic properties of SnO2–Ta2O5–TiO2 varistor system doped with CuO

The microstructure and nonohmic properties of SnO 2– Ta 2 O 5– TiO 2- CuO varistor system were investigated. The proposed samples were doped with different contents of CuO (0–6 mol%) and sintered at 1400°C for 2 h with conventional ceramic processing method. In all the samples, the commonly identified phase was SnO 2 (rutile); however, with increasing doping amount of CuO , the peaks of CuO phase emerged in the X-ray diffraction (XRD) patterns. Scanning electron microscopy (SEM) examination on the fractured surfaces of the samples revealed that a minor amount of CuO dopant can facilitate the sintering of the varistor ceramics, but excessive CuO would mainly segregate at grain-boundaries. The doped CuO may also act as a modifier in the SnO 2 based varistors. The measured electric-field versus current-density characteristics of the samples indicated that both nonlinear exponent and varistor voltage increased with increasing doping amount of CuO up to 3 mol% and then decreased with excessive CuO .

<b>Effect of Pr<sub>6</sub>O<sub>11</sub> doping in electrical and microstructural properties of SnO<sub>2</sub>-based varistors<b>

The influence of the dopant Pr6O11 was investigated with regard to the electrical and microstructural properties of the system (98.95-x)%SnO2.1.0%CoO.0.05%Ta2O5.x%Pr6O11, where x = 0.05%, 0.10%, 0.30% and 0.50% in mol. Pr6O11 doping modifies the electrical behavior of the ceramics. The electrical parameters were: α = 8.0, EB = 319 V cm-1 and Vb = 0.66 V barrier-1 for the system without Pr6O11 and α = 17.0, EB = 853 V cm-1 and Vb = 1.15 V barrier-1 with the addition of 0.10% in mol Pr6O11. The system with 0.05% in mol Pr6O11 had the same non-linearity coefficient α as the system with 0.10% in mol. However, breakdown electrical field and voltage per barrier rates were lower (EB = 708 V cm-1 and Vb = 0.98 V barrier-1). The low rates in the breakdown electrical field enabled the varistor systems under study to be used in protection systems for low-voltage energy grids. In the case of Pr6O11 concentrations above 0.10% in mol, the presence of the dopant became deleterious to the varistor’s electrical characteristics. This effect was due to an increase in praseodymium stannate (Pr2Sn2O7) secondary phase. The crystalline phase coupled to the cassiterite (SnO2) phase was found with XRD and SEM/EDS and quantified by Rietveld’s refining method.

Microstructure and Nonohmic Properties of SnO2-Ta2O5-ZnO System Doped with ZrO2

The microstructure and nonohmic properties of SnO2-Ta2O5-ZnO varistor system doped with different amounts of ZrO2 (0–2.0 mol%) were investigated. The proposed samples were sintered at 1400°C for 2 h with conventional ceramic processing method. By X-ray diffraction, SnO2 cassiterite phase was found in all the samples, and no extra phases were identified in the detection limit. The doping of ZrO2 would degrade the densification of the varistor ceramics but inhibit the growth of SnO2 grains. In the designed range, varistors with 1.0 mol% ZrO2 presented the maximum nonlinear exponent of 15.9 and lowest leakage current of 110 μA/cm2, but the varistor voltage increased monotonously with the doping amount of ZrO2.

Influence of Nb2O5 on the varistor behavior of TiO2–Cr2O3 system

This work investigated the influence of Nb2O5 dopant on the varistor behavior of the ternary system (99.95 − x)%TiO2, 0.05 %Cr2O3, x%Nb2O5, where x = 0.10; 0.15; 0.20; 0.25 % in mol. The processing was carried out using the conventional oxide mixture method. The initial oxides were homogenized in alcoholic media in a ball mill, for 3 h, dried in oven and isostatically pressed at 210 MPa and sintered at 1,400 °C for 2 h in air atmosphere and cooled at 5 °C/min cooling rate, resulting in pellets with high densification. Electrical measurements in continuous current at different temperatures revealed that for the lowest Nb2O5 concentration the breakdown electric field, EB = 4.41 V/cm and non-linear coefficient, α = 4.6 were obtained, and for the highest Nb2O5 concentration the breakdown electric field, EB = 9.71 V/cm and non-linear coefficient, α = 15.3 were obtained. These low values in the breakdown electric field enable these varistor systems to be used in protection systems for low-voltage energy grids. Changes in the potential barrier present in the grain–grain boundaries could also be observed, in which the height increased and the width decreased with the increase in the dopant concentration. However, for x = 0.25 % in mol Nb2O5, a significant reduction in the potential barrier height and the voltage per barrier was obtained. Furthermore, the tendency to increase the volume of the unitary cell with the increase of dopant concentration, which was interrupted in the system with 0.25 % Nb2O5, was another evidence that suggested the occurrence of Nb2O5 segregation in the grain boundaries, or even that the reduction in the average grain size could possibly dilute Nb2O5 concentration in the grain boundaries. The average grain size was calculated through SEM micrographs and ranged from 7 to 16 μm, with larger sizes occurring for lower dopant concentrations and presenting higher porosity and lower uniformity in the grains shape.

Fabrication of Sn-Doped ZnO Varistor by Solid State Processing

In this work, systematic substitution of ZnO with SnO2 in ZnO-based varistor composition was investigated through analysis of densification, phase formation, microstructure and non-linear properties. The general formula was Zn0.96-xSnxBi0.02Co0.02Oβ with x = 0, 0.01, 0.02 and 0.04. The high rate of densification starting at 800°C is believed to be related to Bi2O3 via liquid-phase sintering. The complex nature of this multi-phasic varistor system was demonstrated through occurrence of different phases at different sintering temperatures. The low-temperature pyrochlore (Bi2Sn2O7) started to dissociate at approximately 1100°C with the spinel (Zn2SnO4) forming afterwards. The grain size variation with composition might be induced by the changes in the availability of Bi2O3 which has been reported to help facilitate abnormal grain growth. All samples sintered at 1200°C, with data extrapolation, possessed breakdown voltages of higher than 1400 V/cm.