1.       Introduction

With an excellent nonlinear coefficient and a low leakage current, varistor devices have been used in electronic and electrical systems, such as surge protection devices. The breakdown voltage and resistance of these varistors rely on microstructural conditions; as such, grain size and microstructural homogeneity are the most important parameters in varistor manufacturing [1]. One way to achieve these objectives is to use homogeneous ZnO nanoparticle powder for varistor fabrication. ZnO nano-scale particles have various chemical and physical properties compared with those of bulk materials. High homogeneity, enhanced sintering ability, and other unusual properties may be predicted because of large surface areas, nano-sized crystallites, and various surface properties [2]. Thus, the fabrication of varistors with ZnO nanoparticle powder should improve their properties tremendously. ZnO varistors are divided into Bi₂O₃- [3] and Pr₆O₁₁-based [4] varistors. However, V₂O₅ is added as a varistor-forming oxide that decreases the sintering temperature of varistors [5,6]. However, the addition of V₂O₅ does not improve electrical properties, and other oxide additives are necessary to obtain high performance; that is, the nonlinear characteristics of varistors can be enhanced by incorporating some oxide additives, which are used as minor oxide additives, such as MnO_X oxides (MnO, MnO₂, and Mn₂O₃) [6,7] and CoO_X oxides (CoO and Co₃O₄) [8]. The addition of different CoO_x oxides and MnO_x oxides in ZnO varistors helps detect different electrical characteristics, and the valence of the added oxides affects the nonlinear characteristics of ZnO ceramics.

To improve the crystalline quality of ZnO, we can implement thermal annealing treatment, which is a widely known effective technique. Annealing temperature plays an important role in controlling intrinsic defects in ZnO and properties of samples. At high annealing temperatures, samples can recrystallize, and the concentration of defects changes as annealing temperature and atmosphere are altered. Other important annealing parameters include ambient gas, time, and gas flow rate. Annealing is an important tool to modify the properties of varistors at high temperatures, extended exposures, and various atmospheres [9-12]. Wide-bandgap semiconductors, such as ZnO and SnO₂, tend to be sensitive to ambient atmosphere [13]. Their electrical characteristics, which are controlled by intrinsic electronic defect formation, can be considerably modified by exposure to a specific gas [14]. This influence, which depends on the surface-to-volume ratio of a material, is enhanced when a metal oxide is deposited in the form of a thin layer. In this work, thermal annealing under different conditions is used for ZnO nanoparticle-V₂O₅-Mn₂O₃varistors. The effect of heat treatments on the structural and electrical behaviors of varistor ceramics was also investigated.

2.       Experimental Details

2.1.  Sample Preparation

ZnO nanoparticle-V₂O₅-Mn₂O₃varistors were prepared via the conventional ceramic processing method involving ball milling, drying, pressing, and sintering. Oxide precursors of 99.9% purity were used. The composition consists of 99 mol% 20-nm ZnO+ 0.5 mol% V₂O₅ + 0.5 mol% Mn₂O₃ powder. The powder was blended with Poly Vinyl Alcohol (PVA) by mixing with distilled water in a ball milling jar for 6 hours. The ZnO slurry was dried at 60^°C in air for 1 hour and then was granulated by sieving through a 20-mesh sieve. The resulting granules were used to make discs by pressing at 4 ton/cm² pressure. The green ZnO discs were 26 mm in diameter and 2 mm thick. Finally, the green discs were sintered at 900^°C in air for 1 h. The as-grown ZnO varistors were annealed in O₂, N₂ and N₂+10%H₂, respectively, and the temperature was varied from 200 to 800^°C.In order to measure the electrical properties, silver pastes were coated and toasted on both sides of the sintered samples.

2.2.  Characterization

The crystalline phases were studied using a high resolution X-Ray Diffractometer (XRD) equipment (PANalytical X' Pert PRO MED PW3040) with Cu K_α radiation (ʎ=1.5406Å). Surface roughness was determined and surface images were taken by atomic force microscopy (AFM model BURKER).

4.3.  Electrical Testing

The I - V characteristics of the samples were measured using a high voltage source measure unit (KEITHLEY instruments 246 high voltage supply). Impedance measurements were taken using a frequency response analyzer (HP 4294A) at frequencies ranging from 40 Hz to 15 MHz, with an amplitude voltage of 0.5 V. The carrier concentration, carrier mobility and electrical resistivity of the were determined by Hall effect measurement (HEM-2000). Sheet resistance was performed in this work by using a Sheet Resistance/Resistivity Measurement system, model: Changmin Tech CMT-SR2000N.

3.       Results and Discussion

3.1.  Microstructural Analysis

Figure 1 shows the X-Ray Diffraction (XRD) spectra of the ZnO-V₂O₅-Mn₂O₃varistors made from ZnO nanoparticle powder and annealed in O₂, N₂, and N₂+10%H₂ at 700^°C. The varistor samples were characterized by the strongest major peaks of (101), (100), (002), and (110), which were from the ZnO layer and emphasized through the polycrystalline nature of the varistors. Other peaks, such as (Zn₃ (VO₄)₂) and (Mn₃O₄) phases, are secondary phases, and ZnO varistors are multiphase materials. ZnO varistor materials contain ZnO as a major phase. Spinal, pyrochlore, and several other phases are present in the specimen. The presence of these phases is dependent on their processing parameters and the amount and nature of oxide additives to ZnO. The incorporation of these oxide additives forms atomic defects at the ZnO grain and grain boundary; donor or donor-like defects dominate the depletion layer, and acceptor and acceptor-like defects dominate the grain boundary states [15].

Although several peaks of the disc annealed in an oxygen atmosphere are slightly higher than those of the discs annealed in other environments, the diffraction peaks of all of the samples are higher and narrower than those of the as-grown varistor sample, and the FWHM of the former during annealing is smaller than that of the later. This result indicates that the crystallinity and structural ordering of ZnO in the grain and grain boundaries are enhanced, and the sample annealed in O₂ atmosphere is of high quality. The XRD pattern (101) major peak corresponds to a peak shift of 0.15^° toward a higher diffraction angle for the samples annealed in N₂ and to a peak shift of 0.16^° for the samples annealed in N₂+10%H₂ compared with that of the as-grown sample possibly because of the chemisorption of N₂ or H₂ on the surface of the varistor after annealing, resulting in the distortion of crystallites [16,17]. ZnO nanoparticles exhibit a thermodynamically stable crystallographic phase, and the intensity of the peaks reflects a high degree of crystallinity. However, the width of the peaks increases because of the effect of quantum size on ZnO nanoparticles.

During annealing in an O₂ atmosphere, oxygen vacancies are complemented dominantly by chemisorption, and they combine with sufficient Zn atoms to produce new ZnO, thereby leading to an increase in XRD peaks during annealing in an O₂ atmosphere. However, annealing in a N₂ ambient can easily lead to perversion from the stoichiometric state, forming a significant oxygen deficiency. Oxygen complemented via annealing in an O₂ atmosphere reduces the amount of oxygen defects, suggesting that the structure and stoichiometry of the ZnO varistor annealed in an O₂ atmosphere are better than those annealed in N₂ or N₂+10%H₂ environments. The crystal size of the varistors was calculated using Scherrer’s formula [18,19]:

D = 0.9 λ / β Cosθ

Where λ = 1.54 Å, β= (B2 − b2) 1/2, B is the observed FWHM, and b is the instrument function determined from the broadening of the monocrystalline zinc diffraction line. The (101) diffraction peak is considered to calculate the crystal size. The crystal sizes are nearly the same as those of the varistors before and after they are annealed in O₂ or N₂+10%H₂. However, the crystal size of the samples annealed in N₂ slightly increases. The increase in the size of the grain can be attributed to the generation of oxygen vacancies in a N₂ atmosphere as vacancy concentration exponentially increases with an increase in temperature. This atmosphere may affect the kinetics of grain growth by changing the diffusion flux of oxygen vacancy types.

The 3D AFM images of the ZnO-V₂O₅-Mn₂O₃varistors annealed in different atmospheres at various annealing temperatures reveal rough and non-uniform structures. The surface roughness of the structure of the varistor annealed in O₂ is lower than that of the structure in other atmospheres. The Root Mean Square (RMS) roughness of the varistors annealed in diverse atmospheres is improved (Figure 2) with the variation of annealing temperatures. The RMS roughness of the varistors annealed in an O₂ atmosphere at 200^°C decreases slightly. As the annealing temperatures further increase, the roughness increases. The surface roughness and morphological characteristics can affect carrier mobility [20]. Surface roughness can be attributed to the increased grain size and secondary growth during annealing. Lin et al. [21] reported that high temperatures can stimulate the migration of grain boundaries and thus result in the coalescence of more grains during annealing. Fang et al. [22] further indicated that high activation energy should be available for atoms at high temperatures so that they may diffuse and occupy the correct site in a crystal lattice. Consequently, grains with low surface energies grow and enlarge. Grain growth also contributes to the increased surface roughness and enlarged microcracks [23,24].

Thermal annealing at different temperatures significantly affect the surface roughness of the varistors fabricated from the ZnO nanoparticle powder because of the large surface-to-volume ratio of nanoparticles, thereby promoting oxygen or nitrogen absorption during annealing and forming an increased grain size.

3.2.  Electrical Properties

The V-I curves of the as-grown ZnO nanoparticle-V₂O₅-Mn₂O₃ varistors and annealed in different annealing environments are plotted in Figure 3. Some electrical parameters, such as E_b and α, are listed in Table 1. These results show that the breakdown voltage reduces when ZnO nanoparticles are used to fabricate the varistors. The reduction in breakdown voltage can be clarified by the increase in the average grain size after sintering is completed, thereby reducing the number of grain boundaries between electrodes and decreasing "P-N Junctions". Decreased “P-N Junctions” result in reduced breakdown voltages. The grain size within the varistor sample increases noticeably after sintering because of the significant surface area of ZnO nanoparticles. The addition of Mn₂O₃ to ZnO promotes grain growth and allows ZnO grains to enlarge, thereby reducing the breakdown voltage in the varistor. The V-I nonlinear behavior of the ZnO disc is a phenomenon of the grain boundaries between semiconducting ZnO grains. The breakdown voltage of the varistors is directly proportional to the number of grain boundaries per unit of thickness and inversely proportional to the size of ZnO grain. Decreased “P-N Junctions” result in reduced breakdown voltages. The grain size within the varistor sample increases noticeably after sintering because of the significant surface area of ZnO nanoparticles. The addition of Mn₂O₃ to ZnO promotes grain growth and allows ZnO grains to enlarge, thereby reducing the breakdown voltage in the varistor. The V-I nonlinear behavior of the ZnO disc is a phenomenon of the grain boundaries between semiconducting ZnO grains. The breakdown voltage of the varistors is directly proportional to the number of grain boundaries per unit of thickness and inversely proportional to the size of ZnO grain.

The as-grown sample is nonlinear with a nonlinear coefficient α of 43, which is consistent with the value reported by Makarov et al. [25]. Its nonlinearity disappears after it is subjected to heat treatment in N₂+10%H₂ atmosphere. However, the nonlinear properties appear again by repeating the thermal treatment in O₂ atmospheres. The influence of atmospheric treatment at 700^°C on E_b and α is significant (Table 1). Annealing in N₂ atmosphere decreases the nonlinear properties of the varistors. However, thermal annealing in the O₂ atmosphere significantly improves the non-ohmic value. The different behaviors of varistors in various annealing atmospheres are attributed to the potential barrier formation and to the O₂ species at the ZnO grain boundary that causes a Schottky-like barrier and trapping states. This improvement in the nonlinear properties of these varistors is associated with the degree of oxidation (when a varistor is annealed in an O₂ atmosphere) or reduction (when a varistor is annealed in an N₂ atmosphere) of metal oxide additives precipitated at the ZnO grain boundary. Sonderet al. [26] fabricated traditional ZnO-based varistors from ZnO microparticle powder and obtained similar results regarding these varistors annealed in oxidizing and reducing atmospheres. Santos et al. [27] reported that electrical behaviors are strongly affected by the atmosphere because of the oxidizing mechanism at the ZnO grain boundary. Bueno et al. [28] showed that thermal annealing in N₂ atmosphere decreases the surface states (N_IS) of a double (back-to-back) Schottky barrier and potential barrier height values.

However, thermal annealing in an O₂ atmosphere results in a considerable increase in (N_d) and (N_IS) states in a thin region of the grain boundary. Many approaches have suggested that thermal annealing fundamentally changes the electronic states of the grain boundary region [29]. Thus, the physical origin of the interfacial states is an extrinsic effect obtained from metal atoms precipitated at ZnO grain boundaries, not an intrinsic effect caused by a lattice mismatch at grain boundaries [29,30]. Electrical characteristics, such as nonlinear coefficient and barrier voltage, increase, and breakdown voltage decreases when the varistors are annealed in an O₂ atmosphere (Table 1). These results confirm that the electrical behaviors of the varistors are logical for oxygen species that exist on the grain boundary. Thermal annealing treatment in O₂ atmosphere is used in manufacturing, thereby prolonging the life of varistors. Another result is the re-establishment of the electrical behavior of the degraded varistors because of the adsorption of oxygen species at grain boundaries after these varistors are annealed in O₂ atmosphere. Oxygen in the grain boundary region is essential for the property of various varistor ceramics.

Impedance spectroscopy is an efficient technique to characterize grain boundaries in varistor ceramics [31]. In our study, complex impedance plots are derived from the as-grown sample and the sample annealed in the O₂ atmosphere Figure 4(a), displaying a grain-boundary semicircle. This finding indicates that grain boundaries are highly resistive regions. The spectra obtained from the varistors treated in N₂ and N₂+10%H₂ atmospheres Figure 4(b) are devoid of any grain boundary semicircle. Therefore, resistive grain boundary layers disappear, and the grain interior is the only contribution to the frequency response of the varistors. However, the spectral shape implies the existence of inductive impacts, which are similar among various varistor samples [32]. The resistance of grain boundary is completely recovered after varistors are thermally annealed in O₂ atmosphere. Thus, the adsorption of O₂ in grain boundaries, which produce a resistive surface layer, is the origin of the electrical properties of the ZnO nanoparticle-V₂O₅-Mn₂O₃varistors. Santos et al. [33] revealed that atmosphere strongly influences the electrical properties because of the oxidizing mechanism at the grain boundary.

The electrical resistivity, the carrier concentration, and the mobility of ZnO nanoparticle-V₂O₅-Mn₂O₃varistors annealed in different atmospheres as a function of annealing temperature are shown in Figure 5. The electrical resistivity of ZnO nanoparticle-V₂O₅-Mn₂O₃varistors is significantly reduced by annealing in N₂+10%H₂ atmosphere (Figure 5). Furthermore, annealing in a N₂ atmosphere decreases resistivity, although it is not as effective as thermal annealing in N₂+10%H₂. However, resistivity increases as annealing temperature increases from 200^°C to 800^°C in N₂+10%H₂ and from 400^°C to 800^°C in N₂. Resistivity is increased by annealing in an O₂ atmosphere. Doped ZnO varistors present the characteristics of n-type semiconductors because of the existence of zinc interstitials, oxygen vacancies, and hydrogen impurities, whereas Mn-doped varistors exhibit strong n-type conductor or semiconductor characteristics. Resistivity also increases rapidly as annealing temperature increases in the case of O₂ annealing possibly because zinc interstitial and oxygen vacancy concentrations decrease as expressed in the following reactions [34]:

                            Zn_Zn + V_o + ½ O₂                                                           ZnO                                                                        (1)

                            Zn_i + ½ O₂                                                                           ZnO                                                                         (2)

Reducing zinc interstitial and oxygen vacancy concentrations leads to a decrease in mobility and carrier concentrations because they act as donors and lead to ionized impurity scattering or impurity scattering [34]. As annealing temperature increases, reactions (1) and (2) accelerate remarkably.

In the case of N₂ or N₂+10%H₂ annealing, resistivity initially decreases and subsequently increases as thermal annealing temperature increases possibly because of the desorption of negatively charged oxygen species from the grain boundary region, which acts as a trapping site during annealing and forms potential barriers [35]. Negatively charged species create depletion regions near grain boundaries, thereby reducing mobility and carrier concentration. Hydrogen atoms passivate the surface of the grain boundary during annealing in a N₂+10%H₂ atmosphere, and this passivation of hydrogen atoms leads to the removal of depleted regions near grain boundaries. Thus, the removal of these region causes an increase in mobility and carrier concentration [36]. As well as the effect of the boundary passivation of hydrogen atoms, hydrogen impurity atoms also passivate Zn grain ions, which also contribute to an increase in carrier concentrations [36]. Crystallinity is improved by annealing in O₂ as revealed by XRD analysis (Figure 1), and this improvement may contribute to the increase in mobility and carrier concentration, but this contribution appears negligible because the mobility and carrier concentration of the varistors annealed in an O₂ atmosphere tend to reduce as the annealing temperature increases from 200^°C to 800^°C (Figure 5).

Table 2 presents the sheet resistance of ZnO nanoparticle-V₂O₅-Mn₂O₃varistors annealed in different atmospheres at various annealing temperatures. The as-grown sample yields a sheet resistance of 12 Ω/sq. However, the sheet resistance initially decreases as the annealing temperature increases to 400^°C in a N₂ and N₂+10%H₂ atmosphere. As annealing temperature exceeds 400^°C, the sheet resistance increases significantly. The initial decrease in the sheet resistance during reduction in O₂ annealing or N₂ and N₂+10%H₂ annealing may be attributed to the increased crystallinity of the varistor [37], which is modified at high annealing temperatures. The Inter diffusion of Mn and V changes the morphological characteristics of the varistor, thereby affecting the electrical characteristics of the samples. The sheet resistance significantly increases as the annealing temperature increases to more than 400^°C, and this increase is attributed to the agglomeration of the Mn layer [37]. The surface scattering effects of electrons caused by the agglomeration of Mn and the oxidation degree on the surface of Mnvaristor increase, leading to a considerable increase in sheet resistance. The sheet resistance of the varistors annealed in an O₂ atmosphere increases remarkably as annealing temperatures increase. The O₂ chemisorbed easily acts as an acceptor on the surface of the varistor and accordingly decreases the electron concentration. Tansley and Neely [17] reported that the existence of ionized adsorbates on the surfaces of crystallines can increase the height of inter crystalline barriers, thereby minimizing the effective carrier mobilities and conductivities. Another possibility is that Mn has a high diffusion coefficient, and it can migrate rapidly into ZnO varistors during annealing treatment [4]. The diffusion of Mn and V significantly changes the properties of varistors. Mn efficiently decreases mobility and conductivity once it diffuses into the sample because Mn is a deep acceptor in ZnO [38].

4.       Conclusion

ZnO nanoparticle-V₂O₅-Mn₂O₃varistors were prepared through conventional ceramic processing, and the effects of annealing in different atmospheres and at various temperatures on their structural and electrical properties were investigated. sZnO-V₂O₅-Mn₂O₃varistors made from ZnO nanoparticles and annealed in an oxygen atmosphere exhibit optimal performance, good quality, and superior electrical behavior compared with those of other varistors treated under other conditions. The electrical resistivity of the varistors is effectively minimized by thermal annealing in a reducing atmosphere, such as N₂ and N₂+10%H₂, possibly because the passivation of zinc ions and grain boundaries by hydrogen atoms causes an increase in mobility and carrier concentration. However, thermal treatment at temperatures lower than 400^°C is less efficient than that of other temperatures. The lowest resistivity of the varistor is obtained by annealing at 200^°C in N₂+10%H₂ atmosphere and 9.2 Ω/sq sheet resistance, which further increases as annealing temperature increases. This result indicates that the non-ohmic behavior of varistors physically originates from oxygen on grain surfaces adsorbed by intrinsic defects. 

Figure 1: The X-ray diffraction patterns of the as grown ZnO nanoparticles-V₂O₅-Mn₂O₃varistorsat room temperature and annealed in O₂, N₂, N₂+10%H₂ at 700^°C.

Figure 2: The Root Mean Square roughness (RMS) of the as grown ZnO nanoparticles-V₂O₅-Mn₂O₃varistors annealed under different atmosphere with the variation of annealing temperatures.

Figure 3: Current-Voltage characteristic of the as grown ZnO nanoparticles-V₂O₅-Mn₂O₃varistorsat room temperature and annealed in O₂, N₂, N₂+10%H₂ at 700^°C.

Figures 4(a,b): Complex impedance plots of the as grown ZnO nanoparticles-V₂O₅-Mn₂O₃ varistors at room temperature and annealed in various atmosphere at 700^°C.

Figure 5: Electrical properties of the ZnO nanoparticles-V₂O₅-Mn₂O₃ varistors as function of annealing temperature in different atmospheres.

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