ZnO Grain Growth Research Articles

Influence of TiO2 doping on the grain growth and electrical properties of ZnO–Cr2O3-based varistor ceramics

The influence of TiO2 doping in the range 0–1 mol% on the microstructure and electrical characteristics of ZnO–Cr2O3-based varistor ceramics was studied. Exaggerated grain growth is observed owing to the presence of TiO2, which triggers the formation of inversion boundaries (IBs) in only a limited number of grains. TiO2 acts as the controller of ZnO grain growth, adjusting the grain size of the ZnO from 5.6 to 9.3 μm, and thus controlling the breakdown voltage. Meanwhile, the TiO2 doping decreased the height of the Schottky barrier, resulting in a lower nonlinear coefficient and a higher leakage current. The results are important for the control of the ZnO grain size with ZnO–Cr2O3-based varistors (having different nominal breakdown voltages) to obtain a suitable thickness for applications.

Effect of Dy2O3 doping on microstructure and electrical characteristics of ZnO linear resistors

ZnO linear resistors, due to its excellent performance, are widely used in numerous industries and gradually replacing traditional conductive resistors, presenting a broad prospect for application. To study the effects of lanthanide oxides on the microstructure and electrical properties of ZnO-based linear resistors, this paper prepared Dy2O3-doped ZnO-Al2O3-TiO2-NiO based linear resistors using a solid-state sintering method. The results showed that proper doping of Dy2O3 could inhibit the growth of ZnO grains, leading to more uniform grain growth. Additionally, doped with appropriate amount of Dy2O3 could enhance the linear performance of ZnO linear resistors, reduce the grain boundary barrier height (φb), and decrease dielectric loss. Linear resistors with nonlinearity coefficient (α) of 1.07, grain boundary barrier height (φb) of 0.0934 eV and resistance-temperature coefficient (αT) of −5.39 × 10−3/°C were obtained. The doping of Dy2O3 could improve the comprehensive performance of ZnO linear resistors, making the fabricated ZnO linear resistors are more suitable for working in high frequency electric fields. The study and analysis of Dy2O3 doped ZnO linear resistors are of significant importance for the preparation of high performance ZnO linear resistors.

Er2O3 doped ZnO–Cr2O3-based varistors with high voltage gradient

ZnO varistors are widely used in modern intelligent electrical equipment, and the preparation of miniaturized varistors with high voltage gradient has become a top priority. This work investigated the preparation of high voltage gradient ZnO–Cr2O3-based varistors doped with Er2O3 and analyzed the relationship between their microstructure and electrical properties. As the doping amount of Er2O3 increases from 0.00 to 0.75 mol%, Er2O3 phase obviously dissolving element Co distributed at the trigeminal grain boundaries increases, leading to the grain size d decreases from 3.16 to 1.70 μm, the nonlinear coefficient α increases from 6.22 to 50.48, the voltage gradient E1mA increases from 428.37 to 1616.76 V/mm, and the leakage current density JL decreases from 375.75 to 0.79 μA/cm2, achieving optimal comprehensive electrical performance of the sample with Er2O3 doping of 0.75 mol%, for which, the residual voltage ratios K reaches the minimum value of 1.14 under the impulse current density Jp of 63.7 A/cm2. When the doping amount of Er2O3 further increases to 1.00 mol%, the excessive Er2O3 phase aggregates at the trigeminal grain boundaries and distributes unevenly, leading to the growth of ZnO grains and performance deterioration of the samples. The ZnO–Cr2O3-based varistors have the advantages of much easier doping, and there is no dopant volatilization during the sintering process. This research provides an important reference for the development of high-performance, miniaturized ZnO–Cr2O3-based varistors.

Effect of Mn on the ZnBiMnO varistor ceramics sintered at a low-temperature of 875 °C

The effect of Mn on ZnO–Bi2O3 (ZnBiO) based varistor ceramics sintered below 900 °C has not been fully understood due to the conventional ZnBiO-based varistor ceramics usually being sintered at temperatures higher than 900 °C. Therefore, (99.5-x) ZnO+0.5Bi2O3+xMnCO3 (x = 0, 1, 2, and 3; all in mole ratio) ceramics were prepared by solid-state sintering at 875 °C for 3 h to study the effect of Mn on the microstructure, electrical nonlinear properties and their stability within the range of 25–100 °C. The results show that doping MnCO3 results in the formation of Mn3O4 and Bi12MnO20 secondary phases within the sintered samples. Mn3O4 mainly exists at grain boundary as particle phase, restricting the growth of ZnO grains during sintering. The presence of Bi12MnO20 indicates MnCO3 may enhance the sintering of ZnBiMnO ceramics by increasing the amount of Bi-rich liquid phase. MnCO3 effectively improves the electrical nonlinear properties of ZnBiMnO varistor ceramic and their stability against the variations in ambient temperature. The 2 mol% MnCO3 doped ZnBiMnO varistor ceramic shows the best electrical nonlinearity, with a nonlinear coefficient, a breakdown voltage and a leakage current density of 53.61, 503.11 V/mm, and 2.85 μA/cm2 respectively. This study may improve our understanding of the effect of Mn on the low-temperature sintered ZnBiO based varistor ceramics, and provide a promising candidate material for manufacturing low-cost ZnO based varistor.

Effect of Sc2O3 doping on the microstructure and electrical characteristics of ZnO linear resistors

Herein, to improve the electrical characteristics, ZnO linear resistors mixed with MgO, TiO2, Al2O3, and various concentrations of Sc2O3 (with 0, 0.35, 0.7, and 1.05 wt %) were obtained using the conventional solid-state sintering technique, while the influences of Sc2O3 doping on the micro-morphology and electrical characteristics was investigated in detail. The crystalline phases and microstructure were investigated by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), and scanning electron microscopy (SEM). The dielectric properties and electrical characteristics were evaluated using a precision inductance-capacitance-resistance meter and a high-voltage source meter. Results from SEM micrographs, XRD patterns, and EDS mappings illustrated that the doping of Sc2O3 hinders the growth of ZnO grains. Electric field-current density and resistivity-frequency measurements illustrated that the doping of Sc2O3 improves the linear performance of ZnO linear resistors and increases its grain resistivity and grain boundary resistivity. With Sc2O3 doping, the dielectric properties of the samples are significantly affected, while the resistance-temperature features reveal the negative temperature constant function. By appropriate Sc2O3 doping, the ZnO linear resistors exhibited excellent performance with grain boundary barrier height φb of 0.0106 eV, nonlinear coefficient α of 1.03, and resistance temperature constant αT of −2.88 × 10−3. It is beneficial to investigate the influence of Sc2O3 doping on the micro-morphology and electrical performances in detail for the preparation of zinc oxide linear resistors with superior performance.

Grain size and sintering temperatures effects on the mechanical properties of ZnO nanoparticle-based varistor ceramics

The grain size impact on the behavior of ZnO–Bi2O3–Mn2O3 varistors was determined in this study. Samples with different sintering temperatures and times were prepared via traditional ceramic methodology, and their properties were described. Results exhibited that various temperatures and times of the sintering process had considerable effects on the varistors. In particular, the growth of ZnO grains was improved at a minimal-temperature sintering process of 900 °C for 1 h. Significant solid-state reaction even at lower sintering temperatures perhaps caused the surface characteristics of ZnO nanoparticles. Vickers indentation analyses were applied to define the fracture toughness and hardness of the samples. Hardness diminished with the growth of ZnO grain. Fracture toughness did not exhibit any change with the increase in grain size (~ 5.1 MPam0.5) up to 1.6 μm for WZ-V and 3.5 μm 20Z-V. Above these grain sizes, a spontaneous conversion from hexagonal to monoclinic phase occurred. WZ-V and 20Z-V samples were used to investigate the analogous bending strength at 1152 and 1148 MPa, respectively. The structural coarsening desired for realizing significant grain size led to increased scattering strength values, as evidenced by lesser Weibull modulus (9.23 vs. 5.10). Variations in the mechanical behavior of the investigated varistors with different grain sizes were demonstrated, and their probable reasons were discussed in brief.

Fabrication of high voltage gradient ZnO nanoparticle-Bi2O3-Mn2O3-based thick film varistors at various sintering temperature

In this research, 20 nm zinc oxide nanoparticles were utilized to fabricate significant voltage gradient zinc oxide nanoparticle-Bi2O3-Mn2O3-based thick film varistors (TFVs) by applying screen printing method. Different low temperatures during the sintering process had a considerable effect on the zinc oxide nanoparticle-based TFVs. In particular, the low temperatures improved the growth of ZnO grains, which was obvious even at 550 °C. The huge surface area of the 20 nm zinc oxide encouraged an intense surface interaction even at minimal sintering temperatures. The sintering technique also enhanced the internal structure of the crystal. Minimizing the fundamental compressive stress depended on the XRD lattice constant and the FWHM analysis. The variable temperatures of the sintering process considerably impacted the electrical behaviours of the samples. A remarkable increase in the voltage gradient of the varistor sintered up to 5732.5 V/mm at 700 °C was observed. The electrical resistivity dropped dramatically from 621.8 kΩ.cm (varistor at 550 °C) to 147.3 kΩ.cm at 800 °C sintering temperature. The small size of grains with perfect boundaries was the main reason for the enhancement of the voltage gradient. The optimal electrical characteristics with a 63 nonlinearity coefficient and 103 μA leakage current were achieved in the varistor sintered at 700 °C. Moreover, maximum permittivity and minimum dissipation factor were attained through the minimal frequency range. Therefore, the sintering process could be applied as a novel technical approach for the dominant voltage gradient of zinc oxide nanoparticle-based TFVs with enhanced microstructural and electrical behavior and good nonlinearity.

Low temperature sintering and characterization of 0.25–1 mol% Bi2O3 doped ZnBiMnNbO based varistor ceramics

The effect of Bi2O3 on the microstructure and properties of ZnO-based varistor ceramics sintered around 900 °C has not been fully understood. Therefore, ZnO (balanced)-x Bi2O3–2%MnCO3-0.1%Nb2O5 (x = 0.25, 0.5, 0.75 and 1%, all in molar ratio) varistor ceramics were prepared by solid-state sintering at 850–900 °C. Then, the effect of varying Bi2O3 concentration on the microstructure and nonlinear electrical properties of the as fabricated ceramic was studied by using XRD, SEM, EDS and conventional electric field-current density (E-J) test. The Results show that the Mn and Nb doping is the main reason for making Bi2O3 derived liquid phase capable of densifying the ZnBiMnNbO varistor ceramic at 850–900 °C. The sintered ceramics consist of ZnO and other three secondary phases:Bi2O3, Mn3O4 and Bi5Nb3O15. The increase of Bi2O3 can improve the grain growth of the ceramic up to 0.75mol%. The further addition of Bi2O3 slightly inhibits the growth of ZnO grains. As a result, the breakdown voltage first decreases from 633.83 to 403.63 V/mm with the increase of Bi2O3 up to 0.75mol%, then increases slightly to 439.71 V/mm with the further increase of Bi2O3 to 1mol%. The nonlinear coefficient reaches its maximum value (58.68) at 0.5 mol% Bi2O3 in the ceramic sintered at 900 °C. Meanwhile, the leakage current density is as low as 3.16 μA/cm2. This study provides a good reference for producing the ZnO-based varistors at low costs.

Cold sintering ZnO based varistor ceramics with controlled grain growth to realize superior breakdown electric field

Controlling the grain growth and grain boundary morphology is of great importance in the manipulation of electrical properties of electro-ceramics. However, it has been a challenge to achieve dense varistor ceramics with grain sizes in submicrons and nanometers using conventional thermal sintering at high temperatures. Here we present a strategy to fabricate dense ZnO based ceramics with controlled grain growth and thin grain boundaries using cold sintering process (CSP). With CSP, the sintering temperature of ZnO based ceramics dramatically drops from 1100 °C to 300 °C. The Bi2O3, Mn2O3, and CoO dopants suppress the grain growth of ZnO under CSP conditions, and Bi-rich intergranular films (2−5 nm) can be observed along grain boundaries. The cold sintered ZnO-Bi2O3-Mn2O3-CoO ceramic shows a non-linear coefficient of 33.5, and a superior breakdown electric field of 3550 V/mm. This work thus demonstrates that CSP is a promising technique for designing new submicron-/nano-ceramics with superior performances.

Investigation of the Effects of CaO Doping on the Microstructure and Electrical Properties of ZnO-Based Linear Resistance Ceramics

ZnO-Al2O3-MgO-SiO2-TiO2-CaO (ZnO-based) linear resistance ceramics were prepared by the conventional ceramics method with different amounts of CaO. The impact of CaO doping on the phase composition, microstructures and electrical properties of ZnO-based linear resistance ceramics was investigated in detail. The research showed that CaO doping not only limits the growth of ZnAl2O4 and ZnO grains—in particular, the grain size of ZnAl2O4 decreased from 4.40 μm to 1.38 μm—but also improves the electrical properties. The grain boundary barrier height and nonlinear coefficient decreased with the increase in CaO content, and the resistance temperature coefficient increased from −5.65 × 10−3/°C to 1.04 × 10−3/°C. The 4.00 mol.% CaO-doped ZnO-based linear resistance ceramics were found to possess excellent electrical properties: the resistivity is 44.87 Ω cm, the grain boundary barrier height is 0.011 eV, the nonlinear coefficient is 1.05 and the resistance temperature coefficient is −0.822 × 10−3/°C, a stable resistivity against frequency.

Sintering mechanism and properties of ZnVMnNbO varistor ceramic and their evolutions by Ce–La doping

This paper studies the sintering mechanism and property of ZnO–V2O5–MnCO3–Nb2O5 (ZnVMnNbO) varistor ceramic and their evolution by Ce–La dopant. The XRD, SEM, EBSD and TEM results show MnZn2Nb2O8 has a similar effect as ZnV2O4 and Zn3(VO4)2 on contributing to the formation of Vanadium (V)-rich liquid phase and promoting the sintering of ZnVMnNbO ceramic. Oppositely, ZnMn2O4 particle phase can only postpone sintering by pinning the growth of ZnO grains. Adding Ce–La dopant leads to forming Ce(La)VO4 intergranular particle phase that further hinders the sintering of the ceramic. This conclusion agrees with the result of the following kinetics study that shows the grain-growth exponent and apparent activation energy of ZnVMnNbO ceramic increasing from 2.39 and 186 KJ mol-1 to 2.95 and 263 KJ mol-1 with the addition of Ce–La dopant. Meanwhile, the electrical properties of the Ce–La doped ZnVMnNbO varistor ceramics are generally better than its Rare Earth (RE)-free counterparts. Sintering at 875 °C for 3 h can increase the nonlinear coefficient of Ce–La doped ZnVMnNbO varistor ceramic to its highest value of 48.4, therefore, is proposed as the optimum fabricating procedure.

Influence of Bi3Zn2Sb3O14 pre-synthesis phase on electrical properties of the ZnO-Bi2O3 based varistor ceramics

Abstract The effects of the pre-synthesis phase Bi3Zn2Sb3O14 on microstructure and non-linear characteristic of ZnO varistor ceramics are investigated. The results are discussed and compared with previous studies of ZnO varistors with Sb2O3 additions. It is shown that adding Bi3Zn2Sb3O14 to ZnO varistor ceramics can effectively inhibit the growth of ZnO grains, which results in a smaller grain size and a significantly higher breakdown voltage comparing with Sb2O3 samples. On the other hand, Bi3Zn2Sb3O14 has a significant influence on the defect structure and component distribution of grain-boundary regions. When Bi3Zn2Sb3O14 content increases from 0 to 3 wt%, the value of the interface state density (Ns) increases sharply. At the same time, the electrical properties are improved gradually and reached an optimized value with the nonlinear coefficient (α) up to 43 corresponding to the high barrier height (ϕb) of 2.04 eV.

Effects of dopants Ti and Al on microstructure, mechanical and tribological behaviors of ZnO/MoS2 coating deposited by magnetron co-sputtering

This paper investigates the underlying mechanisms of Ti and Al doped into ZnO/MoS2 coatings to develop better tribological properties for MoS2-based lubrication coatings. To this end, Ti-ZnO/MoS2 and Al-ZnO/MoS2 coatings with gradient processing were successfully prepared using magnetron co-sputtering with a subsequent annealing treatment. The effects of Ti and Al on the structures and properties of ZnO/MoS2 coatings were analyzed using various analytical methods. The results showed that Ti-ZnO/MoS2 coatings annealed at 300 °C displayed a minimum friction coefficient of 0.07, and Al-ZnO/MoS2 coatings annealed at 300 °C possessed the best oxidation resistance during wear process. Similarly, the highest nano-hardnesses of Ti and Al doped coatings were 1.9 GPa and 2.0 GPa after annealing at 300 °C. The growth of ZnO grains in Ti-ZnO/MoS2 coatings were different from the other coatings based on the Raman analysis, and Al-doped coating showed better fracture toughness and binding forces from indentation and scratch testing. These results suggest that the respective underlying mechanisms of these metals in ZnO/MoS2 coatings were different. As a result, this study shows that select metals could improve the different properties of metal-ZnO/MoS2 coating by utilizing different mechanisms. Formation of ZnS might account for the synergistic lubrication of ZnO and MoS2 co-doped coatings, and ZnO/MoS2 coatings had moisture resistance because of no sulfate radical appeared by XPS analysis. Therefore, metal-ZnO/MoS2 will be potential lubrication coatings to be widely used under natural ambient conditions.

Microstructure and Current-Voltage Characteristics of Erbium Oxide Doped Multicomponent Zinc Oxide Varistors

The present work emphasizes the influence of Er2O3 addition on the microstructure and nonlinear current-voltage characteristics of ZnO based varistors prepared by mixing in a high energy ball mill followed by compaction and sintering at a temperature of 1100 °C for duration ranging from 0.5 to 8 h. Increasing sintering time is found to enhance the size of ZnO grains of the sintered pellets and thereby, degrades the electrical properties. However, Er2O3 addition retards the grain growth of ZnO due to the generation of secondary spinel phases (ErVO4 and Er-rich) at grain boundaries and triple points that restrict the grain boundary migration. Er2O3 modified ZnO varistor sintered at 1100 °C for 0.5 h exhibits considerably improved electrical property with nonlinear exponent and breakdown field of 27 and 3880 V cm-1, respectively.

Effect of mesopore-induced strain/stress on the thermoelectric properties of mesoporous ZnO thin films

In present study, induced strain and stress effect of mesoporous structure on the thermoelectric properties of ZnO thin films were systematically investigated. In this work, mesoporous ZnO thin films were synthesized by evaporation-induced self-assembly and sol-gel process. As the pore structure is formed, the grain growth of ZnO is inhibited and lattice distortion is induced. In this paper, strain/stress induction according to surfactant (Brij-S10) concentration was analyzed through Williamson-Hall analysis. And the relationship between strain induction and thermoelectric properties was studied. 0.07 M ratio of Brij-S10 to Zn induces the enhanced thermoelectric properties as compared with pristine ZnO thin films. Hence, the induced strain and stress could play an important role in enhancing the thermoelectric properties of mesoporous ZnO thin films.

The Effect of TiO2 and B2O3 Additions on the Grain Growth of ZnO

The microstructure properties and grain growth kinetics of ZnO-B2O3-TiO2 ceramics with 0.5 and 1 mol pct doping of TiO2 were investigated. The samples were sintered at 1373 K, 1473 K, and 1573 K (1100 °C, 1200 °C, and 1300 °C) for 1, 2, and 3 hours. The bulk density and grain size of ZnO samples increased with the addition of B2O3 and TiO2. The grain size of 0.5 mol pct TiO2-doped ZnO ceramics was larger than the grain size of the 1 mol pct TiO2-doped ZnO ceramics. This is due to the increase in the ZnTi2O4 phase for the 1 mol pct TiO2-doped ZnO ceramics. The value of grain growth kinetic parameters (n and Q) increased when the TiO2 content in ZnO ceramics increased from 0.5 to 1 mol pct. The lowest n and Q values were obtained for the 0.5 mol pct B2O3-doped ZnO samples which showed that the addition of B2O3 has a significant role on the grain growth of ZnO ceramics.

Improvement in the electrical performance and bias-stress stability of dual-active-layered silicon zinc oxide/zinc oxide thin-film transistor**Projected supported by the National Natural Science Foundation of China (Grant Nos. 61076113 and 61274085), the Natural Science Foundation of Guangdong Province (Grant No. 2016A030313474), and the University Development Fund (Nanotechnology Research Institute, Grant No. 00600009) of the University of Hong Kong, China.

Si-doped zinc oxide (SZO) thin films are deposited by using a co-sputtering method, and used as the channel active layers of ZnO-based TFTs with single and dual active layer structures. The effects of silicon content on the optical transmittance of the SZO thin film and electrical properties of the SZO TFT are investigated. Moreover, the electrical performances and bias-stress stabilities of the single- and dual-active-layer TFTs are investigated and compared to reveal the effects of the Si doping and dual-active-layer structure. The average transmittances of all the SZO films are about 90% in the visible light region of 400 nm–800 nm, and the optical band gap of the SZO film gradually increases with increasing Si content. The Si-doping can effectively suppress the grain growth of ZnO, revealed by atomic force microscope analysis. Compared with that of the undoped ZnO TFT, the off-state current of the SZO TFT is reduced by more than two orders of magnitude and it is 1.5 × 10−12 A, and thus the on/off current ratio is increased by more than two orders of magnitude. In summary, the SZO/ZnO TFT with dual-active-layer structure exhibits a high on/off current ratio of 4.0 × 106 and superior stability under gate-bias and drain-bias stress.

Structural and optical properties of ZnO/TiO2 composites

This work investigates the structural and optical properties of mixed ZnO/TiO2 ceramics synthesized at 900 °C by the conventional solid state sintering method. We doped different weights of TiO2 powder (x = 1–7 wt%) to prepare the ceramic pellets. Then we studied the effect of TiO2 content on crystalline properties, grain size and residual stress using X-ray diffraction. XRD patterns revealed the alloying of hexagonal ZnTiO3 and spinel Zn2TiO4 phases as the doping rate x (wt%) increased. The growth of ZnO grains in wurtzite structure was favored along the (100) direction with coefficient texture 2.27 for a TiO2 amount x = 1wt%. Grain sizes were obtained in the range 33–73 nm. We used Williamson-Hall (W–H) analysis and size–strain plot (SSP) method to study the individual contributions of crystallite sizes and lattice strain on the peak broadening of the ZnO/TiO2 composites. Strain, stress and energy density values were calculated by assuming a uniform deformation model (UDM). We investigated the optical properties of the ceramics by Absorbance and Reflectance measurements at room temperature in the wavelength range 200–1000 nm. The results suggested that the addition of TiO2 to ZnO powder led to a high transparency in the visible range and two absorption edges in the UV region. Interestingly, it was seen slight enhancement of the band gap from 3.1 eV to 3.18 eV for the first edge while the second one was enhanced from 4.8 eV to 5.05 eV as the TiO2 doping concentration increased from 1 wt% to 7 wt% respectively.

High dielectric constant in Al-doped ZnO ceramics using high-pressure treated powders

Pure and Al-doped ZnO ceramics were prepared by conventional sintering high pressure treated nano powders at 1150 °C for 2 h, and the effect of Al-doping on the microstructure and dielectric properties were mainly investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results revealed that Al reacted with ZnO and formed ZnAl2O4 nano-precipitates on the grain boundaries. Small amount of ZnAl2O4 nano-precipitates (i.e. < 1 mol%) enhanced the grain growth of ZnO, while larger amount suppressed the grain growth. AC impedance analysis indicated that Al-doping decreases the resistivity of ZnO ceramics. Dielectric studies revealed that the incorporation of Al in ZnO can significantly enhance the dielectric constant (1.39 × 104 at 1 kHz) of ZnO ceramics. And the dielectric behavior can be well-explained by Maxwell–Wagner relaxation.

Effect of Pr&lt;sub&gt;6&lt;/sub&gt;O&lt;sub&gt;11&lt;/sub&gt; Doping on the Microstructural and Electrical Properties of ZnO-Pr&lt;sub&gt;6&lt;/sub&gt;O&lt;sub&gt;11&lt;/sub&gt;-Co&lt;sub&gt;3&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;-Cr&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-SnO&lt;sub&gt;2&lt;/sub&gt; Varistors

ZnO-Pr6O11-Co3O4-Cr2O3-SnO2 varistors with different doping levels of Pr6O11 (0.25-2 mol%) were prepared at 1300 °C with conventional ceramic processing, and the effect of Pr6O11 doping on the microstructure and electrical properties of the varistor materials were investigated. The results indicated that the doped Pr6O11 basically existed at the boundary of ZnO grains in the varistor ceramics, and SnO2 might enter into the lattice of ZnO grains or precipitate in reaction with Pr6O11 into Pr2Sn2O7 at the gain boundaries particularly where there were three or more ZnO grains. The analysis of scanning electron microscopy further revealed that Pr6O11 doping would inhibit the growth of ZnO grains, resulting in decreasing ZnO grain size with increasing doping level of Pr6O11. The measured electric-field/current-density characteristics of the samples showed that the varistor voltage increased with increasing doping level of Pr6O11 when the doping level was no more than 1.5 mol%, and the nonlinear coefficient of the varistors increased with increasing doping level of Pr6O11 up to no more than 1.0 mol% in the varistors, respectively.