Fabrication and electrical characteristics of flash-sintered SiO2-doped ZnO-Bi2O3-MnO2 varistors

The dense ZnO-Bi2O3-MnO2-xSiO2 (ZBMS) varistors for x = 0, 1, 2, 3 wt% were fabricated by flash sintering method under the low temperature of 850 °C within 2 min. The sample temperature was estimated by a black body radiation model in the flash sintering process. The crystalline phase assemblage, density, microstructure, and electrical characteristics of the flash-sintered ZBMS varistors with different SiO2-doped content were investigated. According to the XRD analysis, many secondary phases were detected due to the SiO2 doping. Meanwhile, the average grain size decrease with increasing SiO2-doped content. The improved nonlinear characteristics were obtained in SiO2-doped samples, which can be attributed to the ion migration and oxygen absorption induced by the doped SiO2. The flash-sintered ZBMS varistor ceramics for x = 2 wt% exhibited excellent comprehensive electrical properties, with the nonlinear coefficient of 24.5, the threshold voltage and leakage current of 385 V·mm−1 and 11.8 µA, respectively.


Introduction 
ZnO varistors have been successfully applied to the surge protector in electrical transmission and transient overvoltage or circuits to prevent lightning strikes due to their excellent nonlinearly electrical characteristics where T 0 is the onset flash furnace temperature (K), W is the power dissipation in the unit of Wm −3 , A is the surface area of the sample (1.429×10 -4 m 2 ),  is the emissivity, and  = 5.67 × 10 −8 Wm −2 K −4 is the Stefan-Boltzmann constant. Based on the above discussion, ZnO varistors can be prepared by the flash sintering method. For example, Zhang and Luo [26] prepared ZnO specimen by flash sintering method at low furnace temperature (< 120 ℃). In fact, a lot of reports have been investigated on the flash sintering for the preparation of ZnO ceramics [33,34]. However, the research mainly focuses on the flash sintering of pure zinc oxide, and there are few investigations on the ZnO-based varistor. Recently, the ZnO-Bi 2 O 3 -MnO 2 varistors have been fabricated by the flash sintering at a very low sintering temperature of 768-702 ℃ under an electric field of 200-400 Vcm -1 , which showed an excellent nonlinear property [35]. The dense ZnO-Bi 2 O 3 -M (M = Cr 2 O 3 , MnO 2 , or Co 2 O 3 ) ceramics were obtained by flash sintering at a low temperature (< 750 ℃) with the highest nonlinear coefficient of 39.5 [36]. In ZnO-Bi 2 O 3 -MnO 2 varistors, Bi 2 O 3 acts as Double Schottky Barrier (DSB) to form at the grain boundary layers, which provides the medium for the liquid phase to wet the grain boundary regions [37]. Meanwhile, the doping of transition metal oxide (MnO 2 ) increases the grain boundary barrier and affects the migration of carriers. The potential barrier is not only related to the ion migration, but also to the oxygen adsorption. It is demonstrated that oxygen can be generated by a chemical defect reaction of silica and zinc oxide [38]. In addition, the Zn ions in the ZnO crystal are replaced by Si ions, forming two positively charged centers 2+ Zn Si and improving the ability to bind electrons. The ionic radius of Si is 0.26 Å. Si 4+ enters into the continuous path more easily than other transition metal ions. Besides, the density of traps can be significantly enhanced by SiO 2 , which may promote the varistor characteristics and flash sintering process.
According to the above discussions, it is very meaningful to explore the effect of different SiO 2 content on the structure and electrical properties of ZnO-Bi 2 O 3 -MnO 2 -based varistor prepared by flash sintering. In present work, the dense ZnO-Bi 2 O 3 -MnO 2 -xSiO 2 varistors were fabricated by the flash sintering method at a temperature of 850 ℃ within 2 min. The flash parameters on sintering were investigated, including current, voltage, power density, and conductivity. The specimen temperature was estimated by the black body radiation model during the flash event.

1 Preparation of green specimens
ZnO, Bi 2 O 3 , MnO 2 , and SiO 2 analytical-grade chemicals were purchased from Sinopharm (Beijing, China) and the materials were used as received. The chemical formula was 98 mol% ZnO + 1.0 mol% Bi 2 O 3 + 1.0 mol% MnO 2 + xSiO 2 (x = 0-3 wt%). Firstly, the starting materials were mixed with the right amount of ethanol in a zirconia ball mill. The oxide mixture was subsequently milled for 5 h and then dried at 70 ℃ for www.springer.com/journal/40145 24 h. Secondly, all powders were mixed with 5 wt% polyvinyl alcohol (PVA) as a binder and after uniaxially pressed in a pellet at about 310 MPa for 3 min. Thereafter, the powders shaped green samples with a dimension of about 7 mm diameter and about 3 mm thickness. Finally, the green sample was calcined at 500 ℃ for 2 h to remove the binder. The densities of the green samples were in a range of 55%-57%.

2 Flash process
The specimen was placed in a retrofitted tube furnace (OTF-1200X-S, KJ Group, China) with two platinum wires, which were also connected to the electrodes sandwiched between the two sections of the specimen. Subsequently, the specimen was added electric field by a direct-current (DC) (Sorenson DLM-300, Sorensen, CA) and the furnace heating rate at 10 ℃min -1 heated to 850 ℃. Specifically, due to local sintering in the flash process, a gradient increased current was used in the experiment. Firstly, the applied electrical field (E) was 300 Vcm -1 , and the initial limit current (I 0 ) was set to be 1.0 A. Thereafter, when the limit current was reached, the current was maintained for 10 s and then increased to 1.5 A. Similarly until the maximum current (I max ) was reached to 3.0 A, the flash sintering process continued for 10 s. Finally, the power supply system and the tube furnace were turned off, and then the specimen was cooled within the furnace to room temperature. The experimental parameters during the flash sintering process were shown in Table 1.

3 Characterization
The structure of the flash-sintered specimens was studied by X-ray diffraction (XRD, D8 Advance, Germany). The electron probe microanalyzer (SEM, NANO SEM430, USA) was used to observe the sample microstructure. The density of flash-sintered samples was measured and calculated by the Archimedes method. The silver paste was applied as an electrode to the sample surface. The I-V characteristics of the flash-sintered sample were measured by a digital electrometer (CJ 1001, China) with a DC source. The varistor voltage (U 1mA ) and the leakage current (I L ) at 75% U 1mA of ZBMS varistors were measured at room temperature. More generally, the nonlinear coefficient (α) can be calculated where U 1mA and U 0.1mA are the varistor voltage at 1 and 0.1 mA, respectively. The frequency-dependent dielectric properties were characterized via a capacitance meter (Agilent 4294A, USA) in the frequency range of 10 3 -10 6 Hz at room temperature.

1 Variation of flash parameters on sintering
The curves of (a) current, (b) voltage, (c) power density, and (d) conductivity versus time during the flash sintering process were shown in Fig. 1. Zero-point was the time when the current reached a preset value. The current increased from 1.0 to 3.0 A with a gradient way ( Fig.  1(a)). As shown in Fig. 1(b), the voltage dropped sharply from 90 V to about 20 V when the current surged to 1.0 A. Thereafter, the voltage tended to be a stable state gradually and then increased with the increasing current. When the current reached to the maximum value, the voltage was kept at about 63 V. Figure 1(c) highlights the power density as functions of time during the FS process. The power density was calculated according to P = IU/V, where U is the voltage, I is the current, and V is the sample's volume. Subsequently, the power density curve had the same trend as the variations of current and voltage because the power density is the combination of the voltage and current. As shown in Fig. 1(d), the specimen conductivity increased sharply when the flash was triggered. Here, the conductivity can be calculated by the equation  = IH/US, where I is the current, H is the height of the sample, U is the voltage, and S is the sample's cross-sectional area. When the current was small (1.0-2.0 A), the conductivity increased with the increase of current. On the other hand, the conductivity decreased again because the rate of voltage increase was greater than that of the current when the current was large (2.0-3.0 A). Besides, the reduction of conductivity may be attributed to the high resistance formed by the grain boundary barrier during the FS process.  (1)). The specimen temperature was the same as the furnace temperature before the electric field was applied. When the flash was triggered, the peak temperature can reach around 1600 ℃. When the voltage control was converted to the current control model, the specimen temperature dropped to about 1200 ℃ [32,39]. The result shows that the heating rate of temperature was about 10 4 ℃min -1 , and the sample temperature was extremely higher than the furnace temperature. Furthermore, the temperature increased with the increasing current. After the current reached the pre-set value, the current increased in a gradient way. In particular, when the current reached the maximum value, the estimated specimen temperature reached to higher than 1950 ℃. At this stage, the average heating rate was about 2×10 3 ℃min -1 . The specimen temperature heating rate greatly decreased after the flash, which made the temperature of the specimen more uniform during the FS process and solved the problem of local sintering. Figure 2(b) depicts the estimated sample temperature and power dissipation as a function of current at x = 1 wt%. Apparently, the estimated specimen temperature and the power dissipation increased linearly with the increasing current, and a positive correlation between the estimated specimen temperature and the power dissipation was observed, which indicated that the sintering was strongly dependent on the Joule heating generated during the flash process. Besides, the estimated temperature was much higher than the required furnace temperature for conventional sintering, which indicated that high temperature was also an important factor for the rapid densification of ceramics.

3 Phase structure, density, and microstructure
The crystal structures of flash-sintered ZBM-xSiO 2 ceramics were characterized via XRD, and the results were shown in Fig. 3  Si ions entered into the lattice of ZnO grains when x was lower than 1 wt%. In the case of x ≥ 2 wt%, Si ions may segregate at the grain boundary and lead to the formation of Zn 2 SiO 4 phase. The relative density of all flash-sintered ZBM-xSiO 2 varistors was measured and listed in Table 2, and the values of the relative density for x = 0, 1, 2, and 3 wt% samples were 97.8%, 98.4%, 97.1%, and 94.3%, respectively. Clearly, the relative density increased first and then decreased with the increase SiO 2 in doping content. It indicated that the proper addition of SiO 2 can form glass phase at the grain boundary, thereby improving the wet ability of the grain boundary, enhancing the surface tension of the Bi-rich liquid phase, accelerating the particle flow rate in the sample, promoting the rearrangement process, and increasing the accumulation density and densification degree of particles in the sample. However, owing to that the density of SiO 2 (2.63 gcm -3 ) is much lower than that of ZnO (5.67 gcm -3 ), excessive SiO 2 will lead to the decreasing of the density [40]. On the other hand, the Zn 2 SiO 4 phase may reduce the growth speed of ZnO grains and eventually decrease the density. More details were found in the following discussion.
The microscopic morphologies of the flash-sintered ZBM-xSiO 2 ceramics were shown in Fig. 4. Clearly, the grain size decreased with increasing SiO 2 doping content. The decrease in grain size led to an increase in the grain boundary, which eventually increased the threshold voltage. This is consistent with the results in Table 2. The decreased grain size can be attributed to the doping with SiO 2 , which promoted the formation of the glass phase with other additives at high temperatures. Then, the Zn 2 SiO 4 spinel phase was condensed in the cooling process, and mainly distributed at the grain boundary. Similar to the mechanism of the Zn 7 Sb 2 O 12 phase, the Zn 2 SiO 4 phase acted as a nail at the grain boundary, thus hindering grain movement and inhibiting grain growth [40]. In addition, taking the 3 wt% sample as an example, the effect of the electrode on the  microstructure was investigated. The microstructures on both sides of the flash-sintered sample electrode were shown in Fig. 5. Apparently, the grain size on both sides of the electrode did not change obviously, which is consistent with the previous reports [35].  Fig. 6(b). Meanwhile, the electrical properties of flash-sintered samples were evaluated and listed in Table 2 . As shown in Fig. 6(b), the nonlinear coefficient and a threshold voltage of ZBMS varistors were enhanced by doping with SiO 2 . The optimal nonlinear characteristics were obtained in 2 wt% sample with a value of 24.5. The threshold voltage increased significantly with increasing SiO 2 content, and the largest threshold voltage was obtained at x = 3 wt% sample with a value of 498 Vmm -1 . Moreover, the varistor voltage depended on grain size according to Eq. (3) [41]:

4 Electrical properties
where N gb is the number of grain boundaries and V gb is the average voltage per grain boundary. When the content of SiO 2 increased from 1 to 3 wt%, the grain size gradually decreased, that is, the number of grain boundaries per unit thickness increased, and the threshold voltage gradually increased. However, the nonlinear coefficient decreased to 12.2, and the leakage current increased significantly to 58.7 µA at x = 3 wt%. The deteriorated nonlinear properties at this sample were ascribed to the excessive SiO 2 , which is because the excessive SiO 2 may form a large number of secondary phases. The increased leakage current was attributed to  the increased conductivity of the grain boundary layer, while the increased conductivity was attributed to the intergranular phase (Bi 2 O 3 , Zn 2 SiO 4 , and Bi 12 SiO 20 ) formed by a large amount of SiO 2 [42][43][44]. In addition, the leakage current of the samples may have a certain relationship with the relative density, where the larger the density, the smaller the leakage current. When the specimen was under a low electric field, electrons cross the Schottky Barrier to generate a thermal excitation current, which is related to the electric field as Eq. (4) [45]: where * A is the Richardson's constant, k is the Boltzmann constant, E is the electric field, B  is the barrier voltage, and  satisfies Eq. (5) [45]: where r is the number of grains per unit length,  is the barrier width. Based on the natural logarithm of Eq. (5), the lnJ-E 1/2 curves of flash-sintered ZBM-xSiO 2 ceramics were calculated and displayed in Fig. 6(c). It was found that lnJ changes linearly with the increase of E 1/2 , then a linear fit was performed, and the fitted line extended to E = 0. As shown in Fig. 6(c), the slope of the lnJ  E 1/2 curves was /kT, and the intercept was the difference between a constant and the barrier heights. The values of barrier voltage which were fitted by Eq. (5) were listed in Table 2. When the content of SiO 2 increased from 0 to 3 wt%, the barrier voltage was 0.76, 0.89, 0.80, and 0.78 eV, respectively. The enhanced barrier voltage can be attributed to the doping with SiO 2 , which can form the glass phase with Bi 2 O 3 at the grain boundary in the flash sintering process. The wetting effect of the liquid phase can be improved at the grain boundary. Moreover, it also affected the segregation concentration of the extrinsic surface impurities on the grain surface, which increased the grain boundary barrier. The defect reactions of SiO 2 and ZnO can be represented by Kroger-Vink symbols: Zn  . Furthermore, due to that the Si 4+ ion radius (0.26 Å) is smaller than those of Zn 2+ (0.74 Å) and Mn 4+ (0.53 Å), it will preferentially diffuse in the ZnO lattice, and may reduce the diffusion rate of Mn 4+ . As a result, the solid solution reaction on the grain surface was enhanced, the formation of grain boundary was promoted, the grain boundary barrier was increased, and the nonlinear characteristics were improved [38]. Figure 7 features the frequency-dependent dielectric characteristics of the flash-sintered ZBM-xSiO 2 varistors (x = 0-3 wt%) at room temperature. As shown in Fig.  7, the doped SiO 2 led to a decrease in dielectric constant (′) and an increase in dielectric loss (tan) at low frequency. With the increase of SiO 2 doping content, the grain size decreased and the depletion layer width increased, resulting in a decrease in dielectric constant. Meanwhile, the dielectric constant showed a certain frequency dependence, which decreased with the increasing frequency. Especially for x = 0 wt%, the dielectric constant decreased rapidly at above 10 5 Hz ( Fig. 7(a)). Interestingly, the frequency stability enhanced with increasing of SiO 2 -doped content . Moreover, the dielectric loss was mainly caused by Joule heating which generated from leakage current and eddy current heat which originated from electric dipole moment rotation [46]. The dielectric loss was high at x = 3 wt%, which may be attributed to the large leakage current (Fig. 7(b)).

Conclusions
The dense ZBM-xSiO 2 varistors for x = 0, 1, 2, 3 wt% were prepared by flash sintering method under a low temperature of 850 ℃ within 2 min. The temperature of the samples was estimated by the black body radiation model during the FS process. The effects of SiO 2 contents on the structure and properties of ZnO-based varistors were investigated. Except for ZnO main phase, some secondary phases induced by doping with SiO 2 were detected. Meanwhile, the grain size decreased, which can be attributed to the inhibition of grain growth by the secondary phase. The electrical performance analysis of the flash-sintered specimens showed that the electrical properties were improved by doping with SiO 2 . The improved nonlinear electrical properties can be ascribed to the ion migration and oxygen absorption. The flashsintered ZBMS varistor exhibited excellent comprehensive electrical properties at x = 2 wt%, with a nonlinear coefficient of 24.5, a threshold voltage of 385 Vmm -1 , and a leakage current of 11.8 µA. Therefore, the flash sintering method is a promising method for preparing ceramics, and there are still many works to be investigated in the future.