Synchronously Improved Voltage Gradient and Mechanical Properties of ZnO Based Varistor by Doping Ga 2 O 3

: ZnO based varistors with high voltage gradient and excellent mechanical and thermal properties were fabricated by Ga 2 O 3 doping and using nanoparticles. The compositions and microstructure of the varistors, as well as their electrical, mechanical and thermal properties were investigated by XRD, XPS, SEM, E-J, C-V, mechanical and thermal expansion measurements. Also, the mechanism of Ga 2 O 3 addition on electrical and mechanical properties of the varistors was discussed detailedly. Results showed that the added Ga 2 O 3 preferentially occupied the lattice position of ZnO crystal through the formation of a substitutional solid solution (Donor doping), they then occupied the void position through the formation of an interstitial solid solution (Acceptor doping), in which residual Ga 2 O 3 existed in the grain boundary and served as inversion boundaries. The formation of the substitutional and interstitial solid solutions helped to improve the electrical properties, when the Ga 2 O 3 content was 0.40 mol%, E 1mA ,  and K were 1235.00 V·mm –1 , 46.0 and 1.37, respectively, being due to the small particle size and the relative content of donor, acceptor and grain boundary in ZnO grain; The increased content of inversion boundaries stimulated the abnormal growth of ZnO grain, and the formed plate-like grain helped to improve the mechanical properties and thermal expansion coefficient of the varistors, values of  f , E f , and K IC reached 147.43 MPa, 213.61 GPa and 2.05 MPa·m 1/2 , showing improvements of 25.29%, 47.67%, and 38.51%, respectively, compared with those of ZnO varistors without Ga 2 O 3 .

Multiple donor such as Al 3+ , Ga 3+ , In 3+ , or Y 3+ trivalent ion metallic oxide have been alternatively employed to enhance the comprehensive performance of ZnO varistor for their special electronic donor structure as well as similar ion radius between these ions and Zn 2+ [4,5,15] , while the multiple dopants may result in a lower voltage gradient, 517 V·mm -1 [4] , 479 V·mm -1 [15] and 475 V·mm -1 [16] , although other electrical properties of the obtained ZnO varistor are excellent, such as lower residual voltage(1.60) and leakage current(0.96 A·cm -2 ) as well as the higher nonlinear coefficient(87) [4] , it still hinders the application of these ZnO varistor in high voltage field.
Several researchers focus on the effect of single Ga2O3 addition on the varistor characteristics of ZnO varistor [17,18] , and the associated mechanism has been reported [4] , while the disadvantage of lower voltage gradient has not been solved. Liu [6] and Vukovic [19] report that the voltage gradient of ZnO varistors can be improved by using nanoparticles as main raw materials. In addition, according to Danzer's opinion, the reason for mechanical and electrical failure of ZnO varistors is correlated. Therefore, a high strength of varistor material are both beneficial for its electrical strength and reliability [20,21] ; on the other hand, Raidl thinks that the electrical barriers at the grain boundaries of ZnO varistors are altered by mechanical stresses, which is derived from the externally applied stress as well as micro stresses that evolve naturally during fabrication of materials with anisotropic thermal expansion coefficients [22] , namely, the coefficient of thermal expansion (CTE) could also affect the electrical strength and reliability of ZnO varistors.
In summary, the electrical properties of the trivalent ion metallic oxide doped ZnO varistors have been extensively studied [23,24] , but it still exists some deficiencies(such as lower voltage gradient), also the effect of dopants on the mechanical and thermal properties of ZnO varistors is rarely reported. In this work, we introduced various Ga additives into a ZnO based varistor and selected nano-ZnO particles as the main raw materials to try to improve the voltage gradient and mechanical properties, modify the thermal properties of ZnO based varistors at same time. Furthermore, the mechanism of improving the electrical, thermal and mechanical properties of ZnO based varistors was studied system and some interesting results were obtained.

Raw materials
All oxides used were 99.9% pure and commercially obtained from Aladdin company. Nano-ZnO powders with particle sizes of 30 ± 10 nm were used as the main materials. Other oxides such as Bi2O3, Sb2O3, Co2O3, SiO2, MnO2, Cr2O3, and Ga2O3 with particle sizes of 1.0 ± 0.5 m served as additives.
Oxides at appropriate ratios were mixed until homogenization by using a planetary mill, where alcohol was used as the medium and ZrO2 ball was the abrasive. The homogenized mixture was dried at 80 °C to remove alcohol, and then a small amount of PVA binder solution was added into the dried mixture. After subjecting to an aging process, the mixture was pressed at 100 MPa into disks with 30 mm diameter and 3.0 mm thickness. These disks were sintered at different temperatures and holding time at a heating rate of 4 °C·min -1 and a cooling rate of 2 °C·min -1 in air. After sintering, the upper and lower surfaces of ZnO varistor samples were polished and then covered with silver paste, and then these samples were heated at 600 °C for 10 min in air to form electrodes [1] .
The corresponding ZnO based varistors were labeled as GaX-T-t, where index GaX represented the amount of Ga2O3, T was the sintered temperature in degrees Celsius, and t was the holding time at the sintered temperature.

Characterization of the Ga2O3 doped ZnO based varistors
Bulk density of ZnO based varistors were measured using Archimedes method [25] .
The phase constitution of ZnO based varistor was analyzed via an in-situ X-ray diffraction analysis (XRD, Smartlab, Rigaku, Japan) when Cu Kα worked as radiation and surface of the varistor was pre-polished. 2θ ranged between 10° and 70° and the scan speed was 1 o ·min -1 .
Microstructure of ZnO based varistor was examined via scanning electron microscopy (SEM, Regulus 8100, Hitachi, Japan) when the surface of specimen was pre-polished and glass phase was removed via etching method [1] .
The electric field-current density (E-J) characteristic of ZnO based varistor was obtained via a source measurement unit (Keithley 2410, USA). The voltage gradient (E1mA) is the breakdown voltage at a unit height, and the leakage current (JL) is determined at 0.75 U1mA, the nonlinear coefficient (α) is defined by Eq(1) [1] : Where E2 is the corresponding electric field at J2 = 1 mA·cm -2 and E1 is the corresponding electric field at J1 = 0.1 mA·cm -2 . The residual voltage ratio K is calculated according to Eq(2): Where Un is the voltage under a current density of 63.7 A·cm -2 , which is the standard requirement for the surge arresters applied in ultrahigh voltage (1000 kV AC) system [23] .
The capacitance-voltage (C-V) measurement of ZnO based varistor was determined under DC bias voltages using a broadband dielectric device (Novocontrol Concept 80, Germany). The most important parameters related to the Schottky barrier, including barrier height (φb), donor density (Nd) and interface state density (Ns), these values can be calculated according to Eq(3) [4] : In which C is the capacitance at a unit area for a single grain boundary when biased voltage for a single grain boundary is Vgb, C0 is the capacitance when Vgb = 0, q is the electronic charge, and εs is the permittivity of ZnO grain. φb and Nd are calculated via the intercept and the slope of the curve of ( 1 − 1 2 ) 2 versus Vgb, respectively.
Besides, after getting φb and Nd, Ns is obtained by equations (4) [4] : (4) An Instron 3382 universal electronic experimental machine was conducted the mechanical properties measurement of ZnO based varistor. All specimens were grind and the tensile surfaces were polished better than 1 m in advance, the edge of specimen was beveled to avoid stress concentration during testing. The size of the specimen were 3 mm (height)  2 mm (width)  30 mm (length), a batch of five specimens was tested and the obtained values were averaged. Threepoint flexural strength (f) of the varistor was measured in a span of 20 mm and a loading rate of 0.5 mm min -1 , the slope of stress-strain curve can be used to calculate the modulus of elasticity (Ef). The fracture toughness (KIC) of the specimen was measured via the single-edge-notched-beam method (SENB), with a span of 20 mm and a loading velocity of 0.5 mm min -1 , notches were incised in the middle of the specimen along the height with a width of 0.3 mm to the depth of about 0.2 mm [26] .
The coefficient of thermal expansion (CTE) of ZnO based varistor was measured via a high-temperature dilatometer (Model DIL 402E, Netzsch, Selb, Germany) under an air atmosphere with a heating rate of 10 o C min -1 .

3.1.1Volume density and linear shrinkage of ZnO based varistors
Bulk density plays an important role in the electrical properties of ZnO varistors [25] . Figure 1 (a) shows the measured bulk density (ρ) of ZnO varistors sintered at different temperatures for 2 h. Clearly, with increased sintered temperatures, value of ρ initially increased and then decreased. All reached the maximum at 1100 °C due to particle growth and densification behavior [8] ; Moreover, with increased Ga2O3 addition, ρ initially increased and then decreased, and when the addition of Ga2O3 was 0.4 mol% and the sintering temperature was 1100 °C, ρ reached the maximum (5.59 g·cm -3 ). According to Wang opinion, when the addition of Ga2O3 was lower, Ga 3+ preferentially occupied the lattice position of ZnO crystal via the formation of a substitutional solid solution, further increased Ga2O3 addition resulted in a partially occupied void position and partially occupied lattice position of ZnO crystal because of the smaller ion radii (0.062 nm for Ga 3+ and 0.075 nm for Zn 2+ ) [27] . The formation of a Gd 3+ solid solution in ZnO crystal could reduce the lattice energy [28] , and the small particle size of raw ZnO (30 nm) could reduce the sintering activation energy [6] , both factors helped improve the densification of ZnO varistors [6] .
When the addition of Ga2O3 exceeded 0.04 mol%, some additives may existed in the grain boundary of ZnO crystal to inhibit ZnO grain growth and reduce the density of the ZnO varistors [4] .

Phase identification of ZnO based varistors
The surface of ZnO based varistors was polished to analyze their composition with a slow scan speed of 1°·min -1 , and results are shown in Figure 2. As shown in Fig.2 Ga2O3 amount, similar to the findings of Fu [1] and Zhao [4] . The Ga2O3 phase was not observed in the specimens, which can be attributed to the small amount [4] .
An interesting appearance can be observed in Fig.2-b that with increased Ga2O3 addition, diffraction peak of ZnO firstly moved toward a higher degree and then moved back, as we know that the ion radius of Ga 3+ is slightly smaller than that of Zn 2 + , so when the amount of Ga 3+ added was small (e.g., x = 0.2, Ga0.2-1100-2h), it prioritized occupying the lattice position of ZnO in the form of a substitutional solid solution, causing the lattice constant of ZnO crystal to decrease and the diffraction peak to move toward a higher degree; When the addition of Ga2O3 was further increased, most Ga 3+ ions still occupied the lattice position, whereas some Ga 3+ ions begun to occupy the pores of ZnO crystal, which could cause the lattice constant to increase and the diffraction peak to move toward a lower degree, similar to the findings of Qiu [29] , and the residual Ga2O3 could exist in the grain boundary but did not affect the lattice constant of ZnO crystal(e.g., x 0.4). Thus, the shift of the ZnO diffraction peak resulted from the interaction of these two effects, as shown in Figure 2 (b).

Please insert Figure 2 here
To increase understanding of the composition and chemical states, XPS spectra of Zn2p, Bi4f, and Ga2p of ZnO varistors with different Ga2O3 contents were obtained and were displayed in Figure 3. The collected spectra were analyzed with XPSPEAK software, and a Shirley type background subtraction was used to fit the curve [30] . The binding energy (B.E.) and relative ratio (R.R. calculated from the relative area) under deconvoluted XPS peaks are listed in Table 1.
As shown in Fig. 3(d), the B.E. peaks of Ga2p spectra were at 1116.22, 1116.60, and 1117.02 eV, corresponding to the substitutional solid solution, interstitial solid solution, and boundary phase of Ga2O3 in ZnO crystal, respectively [27,34,35] . It can be seen that with the increase of Ga2O3 content, the abundance ratio of a/b/c calculated from the area under the peaks exhibited that Ga 3+ preferentially formed a substitutional solid solution in ZnO crystal (as a /b/c= 0.63:0.18:0.19 in Ga0.20-1100-2h); with increased Ga 3+ content, the amounts of Ga 3+ for the formation of substitutional solid solution was still higher than that for the formation of interstitial solid solution, while the amount of Ga2O3 in the grain boundary increased quickly, and acted as the main phase when content of Ga2O3 equaled to 0.8 (as a /b/c= 0.32 : 0.20 : 0.48 in Ga0.80-1100-2h), similar to Wang's result [27] . High content of Ga2O3 in the grain boundary might affect the grain growth behavior of ZnO varistors. Figure 3 here Please insert Table 1 here Figure 4 shows the absolute amount of Ga 3+ for different distribution positions with the increased Ga2O3 content. It can be seen that with the increase of Ga2O3 content, the amount of Ga 3+ in three forms monotonically increased, and that of substitutional solid solution was always higher than the interstitial solid solution, the amount of grain boundary phase increased steeply, it exceeded the amount of substitutional solid solution when content of Ga2O3 exceeded 0.60 mol%; the difference between the amount of substitutional solid solution and interstitial solid solution reached maximum when content of Ga2O3 equaled to 0.40 mol%.

Microstructure characteristics of ZnO based varistors
The SEM images of ZnO based varistors with various Ga2O3 contents are presented in Figures 5 (a)-(e). The average grain size (d) and maximum and minimum aspect ratios of these specimens determined by the lineal intercept method are exhibited in Figure 5 (f). Interestingly, with increased Ga2O3 content, the particle shape of ZnO varistors transformed from ellipse into plate-like, similar to Daneu's report [36] , for examples, particle shapes of Ga0.00-1100-2h exhibited typical ellipse, while that of Ga0.20-1100-2h displayed obviously plate-like, a further increased Ga2O3 content resulted in an increased aspect ratios of plate-like particles for ZnO varistors , as shown in Figure 5 (f). According to Zhao's report, no plate-like grain was observed even after adding 1.44 mol% Ga2O3 and calcination at 1200 °C for 2 h in ZnO-Bi2O3 based varistors, owing to the high sintering activity of ZnO nanoparticles (30 nm) [37] . In addition, the average grain size slightly decreased with increased Ga2O3 content, suggesting that the dopant concentration minimally influenced the ZnO grain size [37] .
As shown in Figure 5(f), it can be seen clearly that with increased Ga2O3 content, the maximum and minimum aspect ratios initially increased slowly and then rapidly due to the content of inversion boundaries (IBs; where Ga2O3 existed in the grain boundary in ZnO grain) initially increasing slowly and then rapidly [36] , being consist with XPS result (as shown in Figure 4). According to Nina's opinion [36] , IBs are a major factor influencing ZnO grain growth. Under the influence of IB-forming dopants (such as SnO2, TiO2, and Sb2O3), IB nucleation occurred in ZnO grains, and these grains grew dramatically and anisotropically in the direction of the inherent IB, causing plate-like development of the grains. Given the smaller ionic radius of Ga 3+ than that of Sb 3+ (0.076 nm) [24,38] and its larger electronegativity (1.579) than that of Sb 3+ (1.476) [39] , when Ga2O3 existed in the grain boundary, Ga 3+ could work as an IB-forming dopant and affect the growth behavior of ZnO grain more efficiently. When combined with the XPS results in Figure 4, we can see that with increased Ga2O3 in ZnO varistors, more Ga2O3 existed in the grain boundary, consequently, the maximum and minimum aspect ratios of ZnO grain initially increased slowly and then rapidly. Figure 6 shows the electrical field-current density (E-J) and capacitance-voltage (C-V) plots of ZnO based varistors with various Ga2O3 amounts and sintered at 1100 °C for 2 h. The electrical parameters of the specimen deduced from these are summarized in Table 2, where E1mA, JL, α, and K represent the voltage gradient, leakage current, nonlinear coefficient, and residual voltage ratio, respectively. From Table 2, it can be seen clearly that with increased Ga2O3, values of E1mA, JL, and  initially increased and then decreased. Conversely, K exhibited the opposite trend, E1mA and  had the maximum values of 1235 V·mm -1 , and 46.0, while K possessed the lowest value of 1.37 when Ga2O3 was doped at 0.40 mol%. Figure 6 here Through the C-V curves, the parameters related to the Schottky barrier including donor density (Nd), barrier height (φb), and interface state density (Ns) were calculated, and results were also shown in Table 2.

Please insert
It can be seen clearly that with increased Ga2O3 content, the value of donor density Nd monotonously increased, and Ga0.80-1100-2h had the highest value of 2.42 ×10 23 m -3 , being due to the highest absolute content of Ga 3+ formed substitutional solid solution, as listed in Figure 4; According to Fu's opinion, the Schottky barrier height (φb) formation was attributed to the defect structures at the grain boundary, where the intrinsic acceptor defects were located at the interface and the donor defects were located at the depletion layer [1] . In this paper, Ga 3+ existed in substitutional solid solution worked as donor and existed in interstitial solid solution worked as acceptor, the former provided an electron to the conduction band and the latter generated a vacancy in the valence band, the higher amount of the difference between these two states, the higher the value of φb [18,40] . As shown in Figure 4 Thus, the lower barrier height of samples indicated that although the lower content of IBs (with Ga2O3 existing in the grain boundary) and sintering process offered sufficient energy to maintain grain-boundary-diffusion activity while suppressing grain-boundary migration, the energy from these factors were insufficient to form a large barrier height. Increased content of IBs benefited grain growth and also led to increased barrier height at single grain boundary. In particular, with increased Ga2O3 content from 0.20 mol% to 0.40 mol%, the value of b increased from 1.24 eV to 2.25 eV. Another factor worth considering was the influence of bulk density on the electrical properties of ZnO varistors [41] . These two factors determined that the highest value of E1mA was reached when the content of Ga2O3 was 0.40 mol%. Table 2 Figures 7a and 7b, respectively. Fig. 7-a reveals that all curves displayed approximately linear behavior, regardless of Ga2O3 content, indicating brittle fracture [42] . As shown in Table 3, with increased Ga2O3 content, f , Ef and KIC initially increased and then decreased, and all reached their maximum values when the content of Ga2O3 was 0.4 mol%, these values indicated improvements of 25.29 %, 47.67 %, and 38.51 % compared with those of ZnO varistors without Ga2O3, respectively. Moreover, the improvements were 44.54 %, 90.72 %, and 61.41% compared with the results of Yoshimura [43] due to the toughness of the small grain size and plate-like shape [44,45] .

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Regarding the CTE, it increased with increased measured temperature, similar to Ni's report [46] . At a fixed temperature, with increased Ga2O3 content, CTE values initially increased and then decreased, and all reached the maximum value when Ga2O3 content was 0.40 mol%. The CTE of ZnO is reportedly anisotropic, i.e., 6.0×10 -6 °C -1 perpendicular to the c axis and 5.0 × 10 -6 °C -1 parallel to the c-axis [40] . Plate-like grains formed because the growth of the c-axis was limited in the (0001) face of ZnO crystal [29] , which helped to improve the CTE of the ZnO varistors. The bulk density of ZnO varistors also positively influenced the CTE of the materials [40] . These two factors worked together to result in an initial increase followed by a decrease in CTE with increased Ga2O3 content, and value of CTE was 6.65×10 -6°C-1 for Ga0.40-1100-2h at 800 °C. Figure 7 here Please insert Table 3  varistors. As reported that the doping of Ga 3+ ions in ZnO varistors possessed three functions [18] :

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First, Ga 3+ ions replaced Zn 2 + in ZnO lattice to form a substitutional solid solution, in this condition, Ga 3+ acted as a donor to release an electron to the conduction band of ZnO crystal, which would reduce the resistance of ZnO grains (as shown in Fig.8-a), and the defect reaction equation was as follows [4,16] : Second, when Ga 3+ filled in the vacancy of ZnO lattice, it existed a 'position competition' between Ga 3+ and Zni 2+ , which could produce a vacancy in valence band of ZnO crystal and acted as acceptor to improve the resistance of ZnO grains (Fig. 8-b), the defect reaction equation was as follows [18] : Third, when Ga 3+ accumulated in the grain boundary of ZnO, the negative free charge at the boundary of ZnO grain was increased, resulted in an increased of the barrier height and a decreased of the leakage current of ZnO varistors [15,18] . In addition, Ga2O3 gathered at the grain boundary of ZnO can be used as an IB-forming dopant to affect the growth behavior of ZnO grain (Fig. 8-c). Therefore, the addition of Ga 3+ and its location play important roles on the electrical and mechanical properties of ZnO varistors, the change of electrical properties is the result of three factors. Figure 8 here

Conclusions
We investigated the mechanism of Ga2O3 addition on electrical and mechanical properties of ZnO based varistors with nanoparticles serving as the main raw materials. The following interesting results were obtained:   Table 3 Mechanical and thermal properties of ZnO based varistors with different content of Ga2O3