3.1 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 A 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 ~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 ~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 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 s =IH/US, where I is the current, H is the height of the sample, U is the voltage, S is the sample’s cross-sectional area. When the current is small, the conductivity increases with the increase of current. On the other hand, the conductivity decreases again because the rate of voltage increase is greater than that of the current as of the current increases when the current is large. From the microstructure, it may because the grain boundary barrier formed high resistance during the FS process, which led to the reduction of conductivity.
3.2 Measurements and estimates of the specimen temperature
Figure 2(a) reveals the estimated temperature of x=1 wt.% sample versus time during the flash sintering process, and the estimated sample temperature can be calculated by the black body radiation BBR model (Eq. (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 ~1600 ℃. When the voltage control was converted to the current control model, the specimen temperature dropped to ~1200 ℃ [32, 39]. The result shows that the heating rate of temperature was about 104 ℃×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 high than 1950 ℃. At this stage, the average heating rate was about 2×103 ℃×min-1. The specimen temperature heating rate greatly decreased after the flash, which made the specimen more uniformly 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.
3.3 Phase structure, Density, and Microstructure
The crystal structures of flash-sintered ZBM-xSiO2 ceramics were characterized via XRD, and the results were shown in Fig.3. Obviously, the main phase of all flash-sintered samples was the ZnO phase (PDF no.36.1451). Except for ZnO phase, many extra diffraction peaks were detected which can be identified as Bi2O3 phase (PDF no.41.1449), Bi12SiO20 glass phase (PDF no.37.0485), and Zn2SiO4 phase (PDF no.37.1485) (Fig. 3). Owing to the radius of Bi ions (r= 1.03 Å) was larger than that of Zn ions (r= 0.74 Å), more Bi2O3 existed in the grain boundary layer. In addition, because MnO2 and ZnO may form a solid solution, no diffraction peaks about Mn ions were found. It is worth noting that the type of secondary phase was related to the doping content of SiO2. When the doping content was 1 wt.%, the secondary phase was mainly Bi12SiO20 and Bi2O3. When the doping content increased to 2 wt.%, Bi2O3, Zn2SiO4, and Bi12SiO20 phases can be identified in these compositions. Since the radius of Si ions (r= 0.26 Å) is smaller than that of Zn ions (r= 0.74 Å), excess Si ions may segregate at grain boundary to form Zn2SiO4 phase.
The relative density of all flash-sintered ZBM-xSiO2 varistors was measured and listed in Table 2, and the values of the relative density forx=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 SiO2 in doping content. It indicated that the proper addition of SiO2 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 the density of SiO2 (2.63 g×cm-3) is much lower than that of ZnO (5.67 g×cm-3), excessive SiO2 will lead to the decreasing of the density [40].
The microscopic morphologies of the flash-sintered ZBM-xSiO2 ceramics were shown in Fig. 4. Clearly, the grain size decreased with increasing SiO2 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 SiO2, which promoted the formation of the glass phase with other additives at high temperatures. Then, the Zn2SiO4 spinel phase was condensed in the cooling process, and mainly distributed at the grain boundary. Similar to the mechanism of the Zn7Sb2O12 phase, the Zn2SiO4 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 samples 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].
3.4 Electrical properties
Figure 6(a) shows the E-J curves of flash-sintered samples doped with different SiO2 contents. Obviously, all samples exhibited nonlinear properties. The variation of nonlinear coefficient () and a threshold voltage (VT) of the ZBM-xSiO2 varistors were shown in Fig. 6(b). Meanwhile, the electrical properties of flash-sintered samples were evaluated and listed in Table 2. As shown in Figure 6(b), the nonlinear coefficient and a threshold voltage of ZBMS varistors were enhanced by doping with SiO2. The optimal nonlinear characteristics were obtained in 2 wt.% sample with a value of 24.5. The threshold voltage increased significantly with increasing SiO2 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 the Eq. (3) [41]:
See formula 3 in the supplementary files.
Where is the number of grain boundaries and is the average voltage per grain boundary. When the content of SiO2 increased from 1 wt.% 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 SiO2, which is due to the fact that the excessive SiO2 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 (Bi2O3, Zn2SiO4 and Bi12SiO20) formed by a large amount of SiO2 [42-44]. 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]:
See formula 4 in the supplementary files.
Where is the Richardson’s constant, is the Boltzmann constant, E is the electric field, 𝜑B is the barrier voltage, and satisfies the Eq. (5) [45]:
See formula 5 in the supplementary files.
Where is the number of grains per unit length, is the barrier width. Based on the natural logarithm of Eq. (5), the ln J-E1/2 curves of flash-sintered ZBM-xSiO2 ceramics were calculated and displayed in Fig. 6(c). It was found that ln J changes linearly with the increase of E1/2, then a linear fit was performed, and the fitted line extended to E= 0. As shown in Fig. 6(c), the slope of the ln Jµ E1/2 curves was b/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 SiO2 increased from 0 to 3 wt.%, the barrier voltage was 0.76 eV, 0.89 eV, 0.80 eV and 0.78 eV, respectively. The enhanced barrier voltage can be attributed to the doping with SiO2, which can form the glass phase with Bi2O3 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 SiO2 and ZnO can be represented by Kroger-Vink symbols:
See formulas in the supplementary files.
According to the Kroger-Vink symbols, the grain boundary defect structures of the undoped and doped samples were illustrated in Figure 7. When ZnO was undoped with SiO2, the defect structure was mainly composed of MnO2 related solution diffusion reaction. When the ZnO varistors were doped with SiO2, oxygen was generated due to the chemical defect reaction between SiO2, ZnO and Bi2O3, resulting in an increase in the adsorbed oxygen content. After that, a large number of free electrons migrated toward the grain boundary, resulting in a significant increase in the width of the depletion layer and a decrease of intrinsic donor defects and . Meanwhile, the impurity donor defects and can be observed as shown in Fig. 7(b), and the formation of and was attributed to the SiO2 additives. Furthermore, due to the Si4+ ion radius (0.26 Å) is smaller than that of Zn2+ (0.74 Å) and Mn4+ (0.53 Å), it will preferentially diffuse in the ZnO lattice, and may reduce the diffusion rate of Mn4+. 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 8 features the frequency-dependent dielectric characteristics of the flash-sintered ZBM-xSiO2 varistors (x=0-3 wt.%) at room temperature. As shown in Fig. 8, the doped-SiO2 led to a decrease in dielectric constant (e¢) and an increase in dielectric loss (tand) at low frequency. With the increase of SiO2 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 105 Hz (Fig. 8(a)). Interestingly, the frequency stability enhanced with increasing of SiO2-doped content. Moreover, the dielectric loss was mainly caused by Joule heating which generated from leakage current and eddies 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. 8(b)).