A bismuth-free ZnO voltage-sensitive ceramic, a preparation method and application thereof

Abstract

The application discloses a kind of bismuth-free ZnO pressure-sensitive ceramic and its preparation method and application, belong to functional pressure-sensitive ceramic and its preparation technical field.The application solves the problem of low nonlinear coefficient and large leakage current of existing bismuth-free ZnO ceramic.The application is compounded with ZnO by using SrCO3, Cr2O3, Co3O4 and MnO2 as additives, and the synergistic effect of the above additives can optimize the pressure-sensitive performance of bismuth-free ZnO pressure-sensitive ceramic, with high voltage gradient, high nonlinear coefficient and low leakage current density, and the voltage gradient and nonlinear coefficient can be adjusted, and the stability is good.

Description

Technical Field

[0001] This invention relates to a bismuth-free ZnO varistor ceramic, its preparation method, and its application, belonging to the field of functional varistor ceramics and their preparation technology. Background Technology

[0002] As power systems develop towards higher voltage, larger capacity, longer distances, and higher reliability, higher requirements are placed on the electrical strength, response speed, and operational reliability of their overvoltage protection devices. ZnO varistors, due to their excellent nonlinear volt-ampere characteristics, strong current-carrying capacity, and fast response, are widely used in surge protectors, lightning arresters, electronic circuit protection components, and overvoltage suppression devices. Currently, widely used ZnO varistors are made by sintering ZnO as the matrix with various oxides such as Bi2O3, Sb2O3, Co3O4, Cr2O3, and MnO2. Although Bi2O3 can form a low-melting-point liquid phase during sintering, giving ZnO-based ceramics good varistor performance, Bi2O3 is prone to volatilization during high-temperature sintering, easily leading to porosity defects in the material. Furthermore, the segregation of Bi2O3 at grain boundaries makes the formulation composition control more complex, resulting in large performance dispersion in industrially produced ceramics. Furthermore, in multilayer chip varistors, Bi₂O₃ readily reacts with the Ag-Pd alloy of the internal electrode during high-temperature sintering, degrading the material's insulation performance. Therefore, developing bismuth-free ZnO varistors and optimizing their performance is one of the research hotspots in this field.

[0003] Bismuth-free ZnO ceramics avoid Bi₂O₃ volatilization and bismuth-rich phase segregation, which is beneficial to improving the performance stability and long-term service reliability of ceramics. Since the varistor performance of ZnO-based ceramics strongly depends on the liquid-phase amorphous film formed during Bi₂O₃ sintering, bismuth-free ZnO ceramics lack the auxiliary effect of Bi₂O₃, typically exhibiting problems such as difficulty in effectively constructing grain boundary barriers, low nonlinear coefficients, and large leakage currents. Therefore, determining the appropriate additive to effectively replace Bi₂O₃ and effectively construct grain boundary barriers in bismuth-free ZnO-based ceramics is crucial for developing high-performance bismuth-free ZnO varistor ceramics. Summary of the Invention

[0004] To address the aforementioned shortcomings and problems of existing technologies, this invention provides a bismuth-free ZnO varistor ceramic, its preparation method, and its applications.

[0005] The technical solution of this invention: One objective of this invention is to provide a bismuth-free ZnO varistor ceramic, which is composed of zinc oxide and modifying additives, wherein the modifying additives include SrCO3, Cr2O3, Co3O4 and MnO2.

[0006] Further specified, the bismuth-free ZnO varistor ceramic has the following molar fractions: SrCO3 is 2 mol%, Cr2O3 is 0.1 mol%, Co3O4 is 0.5 mol%, and the molar ratio of MnO2 to ZnO is (0.1~0.7):(97.4~96.7).

[0007] The second objective of this invention is to provide a method for preparing the above-mentioned bismuth-free ZnO varistor ceramic, the method comprising the following steps: Step 1: Disperse ZnO and modified additives in ethanol solvent, obtain a uniform mixed powder by ball milling, and dry the mixed powder. Step 2: After the mixed powder obtained in Step 1 is mixed evenly with the binder, it is pressed into a preform block and sintered at high temperature to obtain bismuth-free ZnO varistor ceramic.

[0008] Further specifying, the ethanol solvent in step 1 is not anhydrous ethanol, and the amount used is 5-5.2 times the total mass of ZnO and modified additives.

[0009] Further specifying the process, the ball milling process in step 1 is as follows: rotation speed is 300 rad / min, the forward and reverse rotation cycle is 1 hour, the forward and reverse rotation interval is 10 minutes, the total ball milling time is 12 hours, and the temperature is room temperature.

[0010] Further specifying, the drying temperature in step 1 is 60~90℃.

[0011] Further specifying, the binder in step 2 is a 5 wt% PVA aqueous solution with a molecular weight of 10,000, and its amount is 50% of the mass of the mixed powder.

[0012] Further specifying the compression conditions in step 2: pressure of 15 MPa and holding time of 10 min.

[0013] Further specifying, the sintering temperature in step 2 is 1190℃, the time is 1~3h, and the heating rate is 3℃ / min.

[0014] The third objective of this invention is to provide an application of the above-mentioned bismuth-free ZnO varistor ceramic, characterized in that it is used to prepare high-voltage varistors, surge protectors, lightning arresters, and overvoltage protection devices in ultra-high voltage power transmission and distribution systems.

[0015] Beneficial effects: This invention uses SrCO3, Cr2O3, Co3O4, and MnO2 as additives to composite with ZnO. Taking advantage of the lower ionization energies of Sr and Cr compared to Zn, when SrCO3 and Cr2O3 dissolve into the ZnO lattice, they readily precipitate at grain boundaries, forming acceptor defects and promoting the construction of interfacial barriers. Furthermore, Sr and Cr readily react during high-temperature sintering to form a second phase, which pins to the grain boundaries, allowing for the control of ZnO grain size and grain boundary content. Co3O4 influences the defect states and charge distribution in the grain and grain boundary regions, thereby improving grain boundary barrier characteristics. Mn readily segregates at grain boundaries during sintering, enhancing the density of interfacial defect states. The introduction of MnO2 further regulates grain size and grain boundary barriers, improving varistor performance. Through the synergistic effect of these additives, the varistor performance of bismuth-free ZnO varistor ceramics is optimized, exhibiting high voltage gradient, high nonlinear coefficient, and low leakage current density. Simultaneously, the voltage gradient and nonlinear coefficient are controllable, and the stability is excellent. Attached Figure Description

[0016] Figure 1 The figures show the phase analysis results of the bismuth-free ZnO varistors prepared in Examples 1-5 and the control group; Figure 2 The images show surface SEM images and elemental energy dispersive spectroscopy (EDS) distribution maps of the bismuth-free ZnO varistors prepared in Examples 1-5 and the control group. Figure 3 The dielectric constant of the bismuth-free ZnO varistors prepared in Examples 1-5 and the control group is shown as a function of frequency. Figure 4 The dielectric loss curves of the bismuth-free ZnO varistors prepared in Examples 1-5 and the control group are as follows: Figure 5 The current density-electric field strength characteristic curves of the bismuth-free ZnO varistors prepared in Examples 1-5 and the control group are shown. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0018] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0019] Example 1 The preparation of bismuth-free ZnO varistor ceramics in this embodiment includes the following steps: Step 1: Disperse 97.3 mol% ZnO (9.4743 g), 2 mol% SrCO3 (0.3533 g), 0.1 mol% Cr2O3 (0.0182 g), 0.5 mol% Co3O4 (0.1440 g), and 0.1 mol% MnO2 (0.0104 g) in anhydrous ethanol. The amount of anhydrous ethanol is 5 times the total mass of the inorganic substances. Mix the materials at room temperature using a planetary ball mill at a speed of 300 rad / min and a forward and reverse rotation cycle of 1 h for 12 h until the mixture is homogeneous. Then, keep the mixture at 80℃ for 4 h and dry the mixed powder to obtain a mixed powder.

[0020] Step 2: The mixed powder obtained in Step 1 is premixed with a binder, which is a 5 wt% PVA aqueous solution with a molecular weight of 10,000, and its amount is 50% of the total mass of the mixed powder. The premix is ​​cold-pressed at room temperature for 10 minutes at a pressure of 15 MPa to obtain a preform. The preform is placed in a high-temperature furnace and heated from 50°C to 1190°C, held at that temperature for 3 hours at a heating rate of 3°C / min. After the holding period, the preform is allowed to cool naturally to room temperature with the furnace to obtain a bismuth-free ZnO varistor ceramic.

[0021] Example 2 The difference between this embodiment and Example 1 is that the mixed powder prepared in step 1 consists of 97.1 mol% ZnO (9.4536 g), 2 mol% SrCO3 (0.3532 g), 0.1 mol% Cr2O3 (0.0182 g), 0.5 mol% Co3O4 (0.1440 g), and 0.3 mol% MnO2 (0.0312 g). The remaining process steps and parameter settings are the same as in Example 1, resulting in a bismuth-free ZnO varistor ceramic.

[0022] Example 3 The difference between this embodiment and Example 1 is that the mixed powder prepared in step 1 consists of 96.9 mol% ZnO (9.4329 g), 2 mol% SrCO3 (0.3532 g), 0.1 mol% Cr2O3 (0.0182 g), 0.5 mol% Co3O4 (0.1440 g), and 0.5 mol% MnO2 (0.0520 g). The remaining process steps and parameter settings are the same as in Example 1, resulting in bismuth-free ZnO varistors.

[0023] Example 4 The difference between this embodiment and Example 1 is that the mixed powder prepared in step 1 consists of 97 mol% ZnO (9.450 g), 2 mol% SrCO3 (0.353 g), 0.5 mol% Co3O4 (0.144 g), and 0.5 mol% MnO2 (0.0520 g). The remaining process steps and parameter settings are the same as in Example 1, resulting in bismuth-free ZnO varistors.

[0024] Example 5 The difference between this embodiment and Example 1 is that the mixed powder prepared in step 1 is 98.9 mol% ZnO (9.7825 g), 0.1 mol% Cr2O3 (0.0185 g), 0.5 mol% Co3O4 (0.1463 g) and 0.5 mol% MnO2 (0.0528 g). The remaining process steps and parameter settings are the same as in Example 1, and bismuth-free ZnO varistors are obtained.

[0025] control group The difference between this comparative example and Example 1 is that the mixed powder prepared in step 1 consists of 97.4 mol% ZnO (9.484 g), 2 mol% SrCO3 (0.353 g), 0.1 mol% Cr2O3 (0.018 g), and 0.5 mol% Co3O4 (0.144 g). The remaining process steps and parameter settings are the same as in Example 1, resulting in bismuth-free ZnO varistors.

[0026] Example of effect (1) X-ray diffraction was used to analyze the crystal phase of the varistor ceramics obtained in Examples 1-5 and the control group. The results are as follows: Figure 1 As shown. The control group is shown below. Figure 1 As shown in Figure a, by referring to PCPDF card #79-0206, the main diffraction peaks of the sample correspond to the standard diffraction peaks of wurtzite ZnO, indicating that ZnO is the main crystalline phase in the sample. Simultaneously, Sr₂CrO₄-related diffraction peaks can be observed, indicating that Sr₂CrO₄ has a relatively large ionic radius. 2+ and Cr 3+ If it fails to completely dissolve into the ZnO lattice, it will form Sr-containing crystals. 2+ and Cr 3+ The second phase. This result indicates that the ceramic is mainly composed of ZnO as the main crystalline phase, with a small amount of Sr2CrO4 as the second phase.

[0027] Example 1 Figure 1As shown in Figure b, the main diffraction peaks of the sample from Example 1 still correspond to the standard diffraction peaks of wurtzite ZnO, indicating that the ceramic is still dominated by ZnO as the main crystalline phase. Simultaneously, a small amount of Sr₂CrO₄-related diffraction peaks can be observed in the spectrum, indicating that Sr and Cr elements did not completely dissolve into the ZnO lattice, but rather formed a small amount of Sr–Cr-containing second phase during sintering. Compared with the control group, the introduction of a small amount of MnO₂ did not change the ZnO main crystalline phase structure of the ceramic.

[0028] Example 2 Figure 1 As shown in Figure b, the sample of Example 2 still exhibits a wurtzite-structured ZnO as the main crystalline phase. Besides the main ZnO peak, a small number of Sr₂CrO₄-related diffraction peaks can still be observed in the spectrum. These results indicate that the introduction of MnO₂ did not disrupt the main crystal structure of ZnO.

[0029] Example 3 Figure 1 As shown in Figure b, the main diffraction peaks of the sample in Example 3 still correspond to the wurtzite ZnO structure, indicating that ZnO remains the main crystalline phase of the sample. Simultaneously, a small amount of Sr₂CrO₄ and Sr₂Mn₂O₅ related diffraction peaks can be observed in the spectrum, indicating that with further increases in the amount of MnO₂ added, some Mn elements react with Sr elements to form a second phase containing Sr–Mn. This result demonstrates that the sample in Example 3 consists of a ZnO main crystalline phase and a small amount of Sr–Cr and Sr–Mn composite second phase.

[0030] Example 4 Figure 1 As shown in Figure c, the main diffraction peaks of the sample in Example 4 still correspond to the wurtzite structure ZnO, indicating that ZnO remains the main crystalline phase of the ceramic. Simultaneously, a small number of second-phase diffraction peaks containing Sr or Mn can be observed in the spectrum. This indicates that Sr, Mn, and other components have not completely dissolved into the ZnO lattice, forming precipitates such as Sr₂CoO at the grain boundaries. 2.29 And SrZnO2.

[0031] Example 5 Figure 1 As shown in Figure c, the main diffraction peaks of the sample in Example 5 correspond to the wurtzite structure of ZnO, indicating that ZnO remains the main crystalline phase of the ceramic, and no diffraction peaks of the second phase were observed. This result demonstrates that under the combined addition of Cr2O3, Co3O4, and MnO2, the ceramic remains dominated by the ZnO main crystalline phase, and no clearly identifiable second phase is formed.

[0032] (2) The surface morphology and elemental surface scan distribution of the pressure-sensitive ceramics obtained in Examples 1-5 and the control group were characterized, and the SEM images are shown below. Figure 2As shown in the image, the control group samples exhibited a clearer grain and grain boundary structure, better sintering density, and no obvious pores were observed, with an average grain size of approximately 8.25 μm. Simultaneously, a small amount of strip-shaped precipitates were observed on the sample surface and at the grain boundaries. Elemental energy dispersive spectroscopy (EDS) results showed that Zn was uniformly distributed throughout the ceramic matrix, while Sr and Cr were enriched at the grain boundaries, corresponding to the precipitate positions in the surface morphology images. This indicates that Sr and Cr exhibited a certain degree of segregation in the samples, rather than being uniformly dissolved within the ZnO grains. These results correspond to the small amount of Sr₂CrO₄ second phase detected by XRD.

[0033] The sample from Example 1 exhibited a clear grain / grain boundary structure, with well-developed grains and a relatively dense overall structure. No obvious large-sized pores were observed. The average grain size of the sample from Example 1 was approximately 12.5 μm, larger than the 8.25 μm of the control group, as shown in Table 1. This indicates that the addition of a small amount of MnO2 promoted the growth of ZnO-based ceramic grains to some extent. EDS surface scan results showed that Zn was mainly distributed in the ceramic matrix, while Sr and Cr were enriched in local areas, corresponding to the positions of rod-shaped or strip-shaped precipitates. Mn was generally dispersed, with no obvious large-area segregation observed. These results indicate that ZnO in Example 1 remained a continuous matrix phase, while Sr and Cr components tended to form local second phases or accumulate near grain boundaries.

[0034] The sample from Example 2 exhibited clear grain outlines, a relatively complete grain boundary structure, and good overall compactness, with no obvious large-sized pores. Compared to Example 1, the grain size of Example 2 was significantly reduced, with an average grain size of approximately 6.0 μm, indicating that the increased MnO2 content had a certain inhibitory effect on ZnO grain growth. EDS surface scan results showed that Zn was continuously distributed in the ceramic matrix, while Sr and Cr were mainly enriched at grain boundaries or in local second-phase regions. Mn was relatively uniformly distributed overall, with no obvious aggregation or segregation observed.

[0035] The grain size of the sample in Example 3 was significantly reduced, the grains were more compact, the grain boundaries were clear, and the overall density was good. Specifically, the average grain size of the sample in Example 3 was approximately 3.8 μm, significantly smaller than that of the control group and Examples 1 and 2, indicating that the introduction of 0.5 mol% MnO2 had a strong inhibitory effect on ZnO grain growth. EDS surface scan results showed that Zn was uniformly distributed in the ceramic matrix, while Sr, Cr, and Mn were enriched at grain boundaries and in local areas. Combined with XRD results, it can be inferred that a small amount of Sr-Cr and Sr-Mn second phases may have formed near the grain boundaries in Example 3.

[0036] The sample in Example 4 exhibited a relatively clear grain / grain boundary structure. The average grain size of the sample was approximately 7.1 μm. This result indicates that, with the same MnO2 addition, the inhibitory effect of grain boundary precipitation on ceramic grain growth is weakened without the addition of Cr2O3, and the grain size is significantly increased compared to Example 3. EDS surface scan results showed that Zn was mainly distributed in the ceramic matrix, while Sr and Mn were enriched in the grain boundary regions, corresponding to the composition of the second phase in the XRD results.

[0037] The sample from Example 5 exhibited a relatively clear grain / grain boundary structure, with a dense overall sintering and no obvious large-sized pores observed. The average grain size of the sample was approximately 6.4 μm. Compared to Example 3, the grain size of Example 5 was significantly increased, indicating that the system's inhibitory effect on ZnO grain growth was weakened in the absence of SrCO3. EDS surface scan results showed that Zn was continuously distributed in the ceramic matrix, while Cr and Mn were relatively uniformly distributed overall, with no obvious large-area segregation observed.

[0038] (3) The room temperature dielectric spectra of the varistor ceramics obtained in Examples 1-5 and the control group are as follows: Figure 3 and Figure 4 As shown in the figure, the dielectric constant of the control group samples is generally smaller and gradually decreases with increasing frequency, exhibiting certain frequency dispersion characteristics. Furthermore, the dielectric loss of the control group samples is higher at low frequencies, and decreases with increasing frequency.

[0039] The dielectric constant of the sample in Example 1 is generally small and gradually decreases with increasing frequency, exhibiting certain frequency dispersion characteristics. Furthermore, the dielectric loss of the sample in Example 1 is relatively high at low frequencies, and decreases with increasing frequency.

[0040] The dielectric constant of the sample in Example 2 is 10. 4 The significant decrease in dielectric constant at Hz indicates the presence of relaxation polarization in the ceramic. As the frequency increases, relaxation polarization becomes difficult to establish, leading to a decrease in the ceramic's dielectric constant. Furthermore, the dielectric loss of the sample in Example 2 is relatively low in the low-frequency region, then increases to some extent with increasing frequency, forming a loss peak in the mid-to-high frequency region, before gradually decreasing again. This indicates that some grain boundary relaxation behavior already exists in this sample.

[0041] The dielectric constant of the sample in Example 3 remained high over a wide frequency range, exhibiting good frequency stability. Compared with the control group, Example 1, and Example 2, Example 3 had a higher dielectric constant, indicating that a more pronounced electrically inhomogeneous structure was formed between the grains and grain boundaries in this sample, and the contribution of interface polarization was enhanced. Furthermore, the dielectric loss of the sample in Example 3 was generally low, with smaller losses in the low-frequency region. The loss increased slightly with increasing frequency and then gradually decreased, indicating that its conductivity loss was effectively suppressed. The grain boundaries had a good blocking effect on carrier migration, and the dielectric constant remained relatively high at 10 Hz. 5 The appearance of a polarization relaxation peak near Hz indicates the presence of interfacial relaxation polarization in the ceramic.

[0042] The sample in Example 4 exhibits a high dielectric constant in the low-frequency region, but the dielectric constant decreases significantly with increasing frequency. Furthermore, the dielectric loss of the sample in Example 4 increases significantly in the mid-to-high frequency region, with a large loss at the high-frequency end, indicating the possible presence of interfacial polarization relaxation in this sample. Compared to Example 3, although Example 4 has a higher dielectric constant at low frequencies, its dielectric loss is significantly higher, resulting in poorer frequency stability.

[0043] The dielectric constant of the sample in Example 5 was generally low and gradually decreased with increasing frequency. Furthermore, the dielectric loss of the sample in Example 5 generally decreased with increasing frequency, but was higher in the low-frequency region.

[0044] (4) Room temperature of the pressure-sensitive ceramics obtained in Examples 1-5 and the control group I - V Characteristic curves as follows Figure 5 As shown in the figure, within the tested electric field range, the current density of the control group sample remained at a low level, without any obvious current surge or the typical nonlinear conductivity characteristics of ZnO varistor ceramics. This indicates that the sample does not possess varistor properties. In other words, although the control group sample has a relatively clear grain / grain boundary structure and a small amount of Sr2CrO4 precipitates at the grain boundaries, it does not exhibit significant varistor nonlinear characteristics.

[0045] Within the tested electric field range, the current density of the sample in Example 1 increased slowly with increasing electric field strength, but no obvious current surge region appeared, nor did it exhibit the obvious nonlinear conductivity characteristics typical of ZnO varistors. This result indicates that under 0.1 mol% MnO2 doping conditions, the ceramic grain boundary barrier has not been effectively established, and the electrical inhomogeneity between grains / grain boundaries is insufficient, thus failing to produce a significant varistor effect.

[0046] The current density of the sample in Example 2 exhibited a significant nonlinear characteristic with increasing electric field strength, indicating that the sample had developed certain varistor characteristics. Tables 1 and 2 were obtained through calculation. Table 1 compares the average grain size and varistor performance parameters of the bismuth-free ZnO varistor ceramics prepared in Examples 1-5 and the control group; it includes sample name, average grain size, voltage gradient, single grain boundary breakdown voltage, nonlinear coefficient, and leakage current. Table 2 shows the calculated grain boundary barrier parameters of the bismuth-free ZnO varistor ceramic samples prepared in Examples 2 and 3; the barrier height is shown in Table 2. b Used to characterize the strength of grain boundary barriers, donor state density N d interface state density N s and exhaustion layer width t Used to analyze the grain boundary barrier structure and its impact on varistor performance.

[0047] Table 1 Table 2 The control group samples did not exhibit varistor characteristics; therefore, characteristic parameters used to characterize varistor performance, such as voltage gradient, nonlinear coefficient, and leakage current, are all represented by "—" in Table 1. Furthermore, parameters such as grain boundary barrier height, donor state density, interface state density, and depletion layer width lack a reliable calculation basis; therefore, the grain boundary barrier parameters of the control group are not listed in Table 2. Since Example 1 did not exhibit clear nonlinear breakdown behavior, it was impossible to accurately extract varistor performance parameters such as voltage gradient, single grain boundary breakdown voltage, nonlinear coefficient, and leakage current; therefore, the relevant varistor performance parameters of Example 1 in Table 1 are represented by "—". In addition, grain boundary barrier parameters usually need to be calculated based on samples with obvious nonlinear conductivity characteristics. Since the Example 1 sample did not form effective varistor nonlinear characteristics, its barrier height, donor state density, interface state density, and depletion layer width lack a reliable calculation basis; therefore, the grain boundary barrier parameters of Example 1 are not listed in Table 2. Overall, although the sample in Example 1 has a large average grain size and a relatively clear grain-grain boundary structure, it failed to exhibit effective varistor nonlinear performance due to the low MnO2 doping content and the imperfect grain boundary barrier structure.

[0048] The voltage gradient of the sample in Example 2 was 1166.26 V / mm, the single grain boundary breakdown voltage was 7.0 V, the nonlinear coefficient was 5.58, and the leakage current was 192.52 μA / cm. 2The results indicate that as the MnO2 content increases, a grain boundary barrier is established in the bismuth-free ZnO ceramic, and the sample exhibits a certain degree of nonlinearity in its conductivity, displaying varistor characteristics. However, compared to the ZnO-Bi2O3 system varistor ceramic, its nonlinear coefficient is still relatively low, and the leakage current is relatively large. Further analysis reveals that the barrier height in Example 2... b The donor state density is 0.712 eV. N d 1.37×10 15 cm -3 interface state density N s It is 9.57×10 10 cm -2 Exhausting layer width t The value is 699 nm, as shown in Table 2. Overall, the addition of MnO2 to the sample of Example 2 effectively changed the grain size, second-phase composition, and grain boundary electrical structure of the ZnO-based ceramic, causing the sample to begin exhibiting varistor nonlinear characteristics. Compared with the control group and Example 1, the varistor performance of the sample of Example 2 has been significantly improved; however, due to the insufficient optimization of the grain boundary barrier, its nonlinear coefficient is still low, and the leakage current is relatively large, indicating that this component is not the optimal formulation in this system.

[0049] The sample in Example 3 exhibits a low current density in the low electric field region. However, as the electric field strength increases, the current density rises rapidly after reaching a certain level, demonstrating significant nonlinear conductivity characteristics. The calculated voltage gradient for Example 3 is 614.31 V / mm, the single grain boundary breakdown voltage is 2.33 V, the nonlinear coefficient is 33.21, and the leakage current is 3.66 μA / cm. 2 Compared to Example 2, Example 3 showed a significantly improved nonlinear coefficient and voltage gradient, and a substantial decrease in leakage current, indicating that appropriate MnO2 doping can significantly increase the grain boundary barrier height of ZnO ceramics and improve their varistor performance. Further analysis revealed that the barrier height of Example 3... b The value is 0.842 eV, higher than 0.712 eV in Example 2; the donor state density Nd is 2.30 × 10⁻⁶. 18 cm -3 The interface state density Ns is 4.27 × 10⁻⁶. 12 cm -2The depletion layer width t was 18.53 nm. Compared with Example 2, Example 3 exhibited a higher barrier height, greater donor state density and interface state density, while the depletion layer width was significantly reduced, indicating that its grain boundary barrier structure was more effective in optimizing the varistor performance of the bismuth-free ZnO ceramic. A higher interface state density facilitates carrier capture and the formation of a stable barrier at grain boundaries, thereby suppressing low-field leakage current. A higher barrier height also helps enhance nonlinear conductivity. Therefore, Example 3 exhibited a higher nonlinear coefficient, voltage gradient, and lower leakage current. Overall, the addition of 0.5 mol% MnO2 in Example 3 effectively controlled the grain size, second-phase composition, and grain boundary barrier structure of the ZnO ceramic. Compared with the control group and Example 1, Example 3 changed from having no significant varistor performance to exhibiting significant nonlinear conductivity characteristics; compared with Example 2, its nonlinear coefficient was significantly improved, and its leakage current was significantly reduced. This demonstrates that appropriate MnO2 doping can promote the formation of an effective grain boundary barrier, which is one of the key factors in improving the varistor performance of this bismuth-free ZnO ceramic.

[0050] In Example 4, the current density of the sample remained at a low level. When the electric field strength increased to approximately 700 V / mm, the current density suddenly increased sharply. This abrupt change in the ceramic leakage current indicates that the ceramic had broken down under this electric field strength. The sample did not exhibit nonlinear conductivity characteristics, and the ceramic lacked varistor properties. Therefore, the relevant varistor performance parameters of Example 4 in Table 1 are indicated by "—". Furthermore, the grain boundary barrier parameters of Example 4 are not listed in Table 2. In summary, Example 4 demonstrates that Cr2O3 plays a crucial role in stabilizing the grain boundary barrier structure in this system. When Cr2O3 is absent, although the sample can form a dense microstructure and exhibit composite precipitates at the grain boundaries, it cannot form an effective grain boundary barrier, resulting in the sample failing to achieve stable varistor nonlinear performance.

[0051] Within the tested electric field range, the current density of the sample in Example 5 remained consistently low, with no obvious current surge region and no nonlinear conductivity characteristics. Compared to Example 3, since no SrCO3 was added in Example 5, there was no obvious precipitated phase at its grain boundaries, indicating that the Sr composition plays an important role in the formation of the effective grain boundary barrier. Because Example 5 did not exhibit clear nonlinear breakdown behavior, the relevant varistor performance parameters of Example 5 in Table 1 are indicated by "—". Furthermore, since this sample does not possess reliable nonlinear conductivity characteristics, the grain boundary barrier parameters of Example 5 are not listed in Table 2.

[0052] Overall, the above results indicate that adding only MnO2, Co3O4, and Cr2O3 is insufficient to construct an effective and stable grain boundary barrier in ZnO ceramics. Therefore, the SrCO3 composition is a crucial factor affecting the grain boundary barrier structure and ceramic varistor properties in this bismuth-free ZnO ceramic system.

[0053] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A bismuth-free ZnO varistor ceramic, characterized in that, It consists of zinc oxide and modifying additives, including SrCO3, Cr2O3, Co3O4 and MnO2.

2. The bismuth-free ZnO varistor ceramic according to claim 1, characterized in that, The bismuth-free ZnO varistor ceramic has a SrCO3 molar fraction of 2 mol%, a Cr2O3 molar fraction of 0.1 mol%, a Co3O4 molar fraction of 0.5 mol%, and a MnO2 to ZnO molar ratio of (0.1~0.7):(97.4~96.7).

3. A method for preparing bismuth-free ZnO varistor ceramic as described in claim 1 or 2, characterized in that, include: Step 1: Disperse ZnO and modified additives in ethanol solvent, obtain a uniform mixed powder by ball milling, and dry the mixed powder. Step 2: After the mixed powder obtained in Step 1 is mixed evenly with the binder, it is pressed into a preform block and sintered at high temperature to obtain bismuth-free ZnO varistor ceramic.

4. The preparation method according to claim 3, characterized in that, In step 1, the ethanol solvent is not anhydrous ethanol, and the amount used is 5-5.2 times the total mass of ZnO and the modified additives.

5. The preparation method according to claim 3, characterized in that, The ball milling process in step 1 is as follows: the rotation speed is 300 rad / min, the forward and reverse rotation cycle is 1 hour, the forward and reverse rotation interval is 10 minutes, the total ball milling time is 12 hours, and the temperature is room temperature.

6. The preparation method according to claim 3, characterized in that, The drying temperature in step 1 is 60~90℃.

7. The preparation method according to claim 3, characterized in that, In step 2, the binder is a 5 wt% PVA aqueous solution with a molecular weight of 10,000, and its amount is 50% of the mass of the mixed powder.

8. The preparation method according to claim 3, characterized in that, The pressing conditions in step 2 are: pressure of 15 MPa and holding time of 10 min.

9. The preparation method according to claim 3, characterized in that, In step 2, the sintering temperature is 1190℃, the time is 1~3h, and the heating rate is 3℃ / min.

10. An application of the bismuth-free ZnO varistor ceramic according to claim 1 or 2, characterized in that, Used in the manufacture of high voltage varistors, surge protectors, lightning arresters, and overvoltage protection devices in ultra-high voltage power transmission and distribution systems.

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