TWI820611B - Zno-based varistor material, the manufacturing method of the same, and the zno-based varistor applying the same - Google Patents

Abstract

The present invention provides a ZnO-based varistor material, the manufacturing method of the same, and the ZnO-based varistor applying the same. The reactants of the manufacturing method include transition metal oxides, ZnO which serves as a base material, and SrCO ₃and Co ₃O ₄which serve as additives. The SrCO ₃and Co ₃O ₄are first milled, calcined, and again milled. A first additive is therefore gotten. The ZnO and the transition metal oxides are calcined, and then sintered with the first additive. A ZnO-based varistor material is therefore prepared. The ZnO-based varistor material can significantly improve the varistor properties.

Description

Zinc oxide varistor material, its preparation method and zinc oxide varistor using the same

The present invention relates to a zinc oxide varistor material, its preparation method and a zinc oxide varistor using the same, particularly to a zinc oxide varistor material prepared by a solid-state reaction method.

With the booming development of semiconductors and the advent of the 5G era, the demand for protective components is increasing day by day, and electronic equipment is becoming increasingly thinner, lighter, smaller, and more high-performance. However, the integrated circuits used in the equipment are very susceptible to damage by electrostatic discharge (ESD). , so ESD protection element varistors are widely used in electronic equipment. There are many kinds of materials for varistor: ZnO, TiO ₂ , SrTiO ₃ , CaCu ₃ Ti ₄ O ₁₂ , BaTiO ₃ , SnO ₂ , WO _3, etc. At present, zinc oxide varistor is still the most common.

In the research on zinc oxide (ZnO) varistor, transition metal oxides with a higher valence than zinc ions and similar ionic radii have been added and solid dissolved into ZnO to replace zinc (Zn) to increase the donor concentration of the crystal grains. Previous technology pointed out that the varistor properties of ZnO varistor can be improved in an oxygen-rich environment, and the 3d energy level of the transition metal interacts with the π energy level of oxygen to form adsorbed oxygen on the grain boundaries, increasing the interface state concentration and thus improving the varistor resistance. nature. Take cobalt (Co) as an example. During the cooling process, the transition metal is oxidized and loses electrons, which promotes the chemical adsorption of oxygen on the ZnO grain boundaries. Therefore, when the valence of Co ions is high, high-valent metals can be produced when ZnO is dissolved into the solid solution to replace Zn. The replaced donor and the accompanying formation of donor zinc vacancies.

However. Traditional zinc oxide varistors are mainly based on bismuth (Bismuth, Bi) and praseodymium (Pr) series. Bi-based varistors tend to produce insulating spinel secondary phases during the sintering process, resulting in a decrease in varistor properties. Although multilayer Pr-based varistors have excellent electrostatic discharge characteristics, the sintering temperature is too high, resulting in the need to use high Pd content. The silver-palladium (Ag-Pd) internal electrode greatly increases the cost of component manufacturing. In addition, vanadium-based varistors are not conducive to industrial production due to the high toxicity of vanadium pentoxide (V ₂ O ₅ ). Due to rising environmental awareness, there have been restrictions on adding chromium to components in recent years. [Prior art documents] [Non-patent documents]

[Non-patent document 1] Y. Yano, Y. Takai, and H. Morooka, "Interface states in ZnO varistor with Mn, Co, and Cu impurities," J. Mater. Res., vol. 9, no. 1, pp. 112-118, 1994.

[Technical problem to be solved by the invention]

In view of the shortcomings and shortcomings of the existing technology, the present invention develops a composition and preparation method of a high-voltage zinc oxide varistor material without bismuth, chromium, vanadium, and chromium additives. The technical focus of the present invention is to adopt a new reactant combination and calcination and sintering process to solve the doubts about these common reactants in the past and achieve excellent performance. [Technical means]

Accordingly, the present invention provides a zinc oxide varistor material. The reactants of the preparation method include transition metal oxides, zinc oxide as a base material, strontium carbonate and cobalt oxide as additive materials. The strontium carbonate and the cobalt oxide are first The first additive is obtained by ball milling, calcining and ball milling for the second time. After calcining the zinc oxide and the transition metal oxide, they are sintered with the first additive to obtain the zinc oxide varistor material.

Further, the transition metal oxide may include cobalt oxide and manganese oxide.

Further, the strontium carbonate and the cobalt oxide can be calcined at 1000 ^° C for 8 hours.

Further, after the zinc oxide and the transition metal oxide are calcined, they can be sintered with the first additive at 1100 ^° C.

Further, the additive material may further include niobium pentoxide and tantalum pentoxide. The strontium carbonate, the cobalt oxide, the niobium pentoxide and the tantalum pentoxide may first be ball milled, calcined, and The second additive is obtained by secondary ball milling. After the zinc oxide and the transition metal oxide are calcined, they are sintered with the second additive.

Further, after the zinc oxide and the transition metal oxide are calcined, they can be sintered with the second additive at 1100 ^° C.

The invention also provides a method for preparing a zinc oxide varistor material, which includes the following steps: (a) After ball milling the additive material, calcining it at 1000 ^° C for 8 hours, and then ball milling to obtain the first additive; (b) Calcining zinc oxide and transition metal oxide; (c) ball milling the zinc oxide and transition metal oxide calcined in step (b) with the first additive to obtain powder; (d) performing step (c) ) The powder after ball milling is press-formed and a green embryo is prepared; (e) The green embryo in step (d) is sintered at 1100 ^° C for 2 hours; wherein the additive material includes strontium carbonate and cobalt oxide.

Further, the transition metal oxide may include cobalt oxide and manganese oxide.

Further, the additive material in step (a) may include niobium pentoxide and tantalum pentoxide.

The present invention also provides a zinc oxide varistor, which includes the zinc oxide varistor material as mentioned above. [Effects of the invention]

Accordingly, the present invention provides a brand-new zinc oxide varistor material, and novel materials are prepared from unprecedented combinations of reactants. Compared with many prior technologies, the reactants of the zinc oxide varistor material of the present invention do not contain any bismuth, chromium, vanadium, and chromium additives; in contrast, the present invention includes strontium carbonate and cobalt oxide as additive materials. If transition metal oxides are used as reactants, the semiconducting degree of the crystal grains can be improved; further, if niobium pentoxide and tantalum pentoxide with good oxygen ion conductivity are added, niobium pentoxide and tantalum pentoxide can be formed between ZnO grain boundaries. The rapid diffusion channel of oxygen ions can also improve the varistor properties of ZnO, achieving high nonlinearity index, high breakdown voltage and low leakage current in the I-V curve.

The preparation steps, identification methods and analysis results of the present invention are described below through exemplary embodiments. It should be noted that the following exemplary embodiments are only used to illustrate the present invention, but not to limit the scope of the present invention. [Preparation of zinc oxide varistor material]

Table 1 shows the experimental raw materials used:

Table 1 Drug name chemical formula Company name zinc oxide ZnO Jiabang Technology cobalt oxide Co ₃ O ₄ Jiabang Technology Manganese oxide Mn ₃ O ₄ Jiabang Technology Strontium carbonate SrCO ₃ Ferak Niobium pentoxide Nb ₂ O ₅ Alfa Aesar Tantalum pentoxide Ta ₂ O ₅ Alfa Aesar ethanol C ₂ H ₅ OH Youhe Toluene C ₆ H ₅ CH ₃ JT Baker dibutyl phthalate C ₁₆ H ₂₂ O ₄ Aldrich

In some embodiments, the reactants of the zinc oxide varistor material include ZnO as the base material and SrCO ₃ and Co ₃ O ₄ as the additive materials.

In some embodiments, the reactants of the zinc oxide varistor material include transition metal oxides, ZnO as the base material, SrCO ₃ and Co ₃ O ₄ as the additive materials; the principle of adding the transition metal oxide is that it can solid dissolve into ZnO is substituted for Zn to increase the donor concentration of crystal grains. Preferably, in the ZnO varistor, a transition metal oxide with a higher valence than zinc ions and a similar ionic radius can be added. The following uses cobalt oxide or manganese oxide as an exemplary embodiment, and when the transition metal oxide is cobalt oxide When manganese oxide is used, ZnO, Co ₃ O ₄ and Mn ₃ O ₄ can be mixed in a weight ratio of 100-xy: x: y; (0<x≤1; 0≤y≤1).

In some embodiments, the reactants of the zinc oxide varistor material include transition metal oxides, ZnO as the base material, and SrCO ₃ , Co ₃ O ₄ , Nb ₂ O ₅ , and Ta ₂ O ₅ as the additive materials; by The synergistic effect of Nb and Ta can promote oxygen vacancies, ion mobility, surface electron transfer, and greatly improve the performance of fuel cells.

Accordingly, the present invention provides novel zinc oxide varistor materials prepared with new reactant combinations, and these materials can achieve excellent varistor properties. The following is an illustrative description using several examples.

[Example 1] Preparation of ZnO-SCO: 1. Weigh SrCO ₃ and Co ₃ O ₄ according to the measurement ratio of SrCoO ₃ (SCO), put them into a ball mill barrel containing zirconia balls, and add 99.5% ethanol Perform wet ball milling for 18 hours; 2. Dry in an oven at 100°C; 3. Calculate the dried powder at 1000°C for 8 hours; 4. Then perform planetary ball milling with 99.5% ethanol for 8 hours to make the powder Refine and dry in a 100°C oven. The dried powder is the grain boundary additive and is named SCO. 5. Add 6wt% SCO powder to ZnO, perform wet ball milling with 99.5% ethanol for 18 hours, and then dry it in a 100°C oven. 6. Shape the dried powder in step 5 by uniaxial pressing to prepare a green embryo of 8×8×0.6mm; 7. Sinter at 5°C/min to 1100°C for 2 hours and wait until it naturally After the furnace is cooled to room temperature, the prepared zinc oxide varistor material is completed. This material was named ZnO-SCO.

[Example 2] Preparation of ZCM-SCO: 1. Weigh SrCO ₃ and Co ₃ O ₄ according to the measurement ratio of SrCoO ₃ (SCO), put them into a ball mill barrel containing zirconia balls, and add 99.5% ethanol Perform wet ball milling for 18 hours; 2. Dry in an oven at 100°C; 3. Calculate the dried powder at 1000°C for 8 hours; 4. Then perform planetary ball milling with 99.5% ethanol for 8 hours to make the powder Refine and dry in a 100°C oven. The dried powder is the grain boundary additive and is named SCO. 5. Mix ZnO, Co ₃ O ₄ and Mn ₃ O ₄ in a weight ratio of 97.96:1.72:0.32 and calcine at 600°C. Add 6wt% SCO powder and perform wet ball milling with 99.5% ethanol. After 18 hours, place it in a 100°C oven to dry. 6. Shape the dried powder in step 5 by uniaxial pressing to prepare a green embryo of 8×8×0.6mm; 7. Sinter at 5°C/min to 1100°C for 2 hours and wait until it naturally After the furnace is cooled to room temperature, the prepared zinc oxide varistor material is completed. Name this material ZCM-SCO.

[Example 3] Preparation of ZCM-SCNT: 1. Weigh SrCO ₃ , Co ₃ O ₄ , Nb ₂ O ₅ and Ta ₂ O ₅ according to the stoichiometric ratio of SrCo _0.8 Nb _0.1 Ta _0.1 O ₃ and put them into a solution containing oxide Put the zirconium balls into the ball milling barrel, and add 99.5% ethanol for 18 hours of wet ball milling; 2. Place it in a 100°C oven to dry; 3. Calculate the dried powder at 1000°C for 8 hours; 4. Then The powder was refined by planetary ball milling with 99.5% ethanol for 8 hours, and then dried in a 100°C oven. The dried powder is the grain boundary additive and is named SCNT. 5. Mix ZnO, Co ₃ O ₄ and Mn ₃ O ₄ in a weight ratio of 97.96:1.72:0.32 and calcine at 600°C. Add 6wt% SCNT powder and perform wet ball milling with 99.5% ethanol. After 18 hours, place it in a 100°C oven to dry. 6. Shape the dried powder in step 5 by uniaxial pressing to prepare a green embryo of 8×8×0.6mm; 7. Sinter at 5°C/min to 1100°C for 2 hours and wait until it naturally After the furnace is cooled to room temperature, the prepared zinc oxide varistor material is completed. This material was named ZCM-SCNT. [Material identification]

Below, the material identification of the above three materials is carried out to discuss the composition and performance analysis of each material.

The identification instruments and methods in the embodiment of the present invention include the following: Thermogravimetric curve: The starting powder of each sample is raised to 1100°C using a differential thermal/thermogravimetric analysis instrument (DTA/TG, Netzsch STA409PC) at 5°C/min. , and then perform thermogravimetric analysis using a heating and cooling curve from 2°C/min to 400°C to observe the weight changes during sintering and cooling. Microstructure: The sintered bodies of each sample were polished and acid-etched, and the surface was plated with platinum to increase conductivity. A high-resolution scanning electron microscope (SEM, Hitachi SU-1510, Tokyo, Japan) was used to observe the microstructure and calculate the crystal structure. grain size. Archimedean density: Weigh the sintered samples, boil them in deionized water for 24 hours, measure their saturated water content and suspended weight, and then calculate the apparent density of each sample. Phase identification: Grind the powder of the substance to be identified into fine powder, and use an X-ray diffraction analyzer (XRD, Dandong Fangyuan, DX-2700, Sandong, China) with a copper target (CuKα1=1.5406Å). The scanning angle 2θ is 15~80˚. (Step Angle = 0.04°, Step Time = 1 second), the operating voltage is 35kV, the current is 30mA, and combined with the powder diffraction data of the International Center for Diffraction Data-powder diffraction file (ICDD-PDF) Perform phase identification. Rheostatic properties: Coat both sides of the sintered body with silver gel electrodes, measure the IV curve with a high voltage source tester (Keithley Model 2410), and use the following formula to calculate the varistor properties. The breakdown voltage refers to the reference voltage (V _1mA ) divided by the distance (d) between the two electrodes at a current of 1mA; the leakage current ( _IL ) is the current flowing through the sample at a voltage of 0.8 times V _1mA . Chemical analysis electronic spectrum: Use a hammer to hit the sintered body to obtain the fracture surface, use a chemical analysis electron spectrometer (XPS, ESCA PHI 5000 VersaProbe) to obtain the full spectrum, correct it with the C 1s characteristic peak 284.5eV, and combine it with the software Origin85 for analysis The composition ratio of O 1s and Co 2p. Element distribution: The sintered bodies of each sample were polished and acid-etched, and a carbon layer was plated on the surface to increase conductivity. The elements were analyzed using a field emission high-resolution electron microprobe (EPMA, JEOL JXA-8530F Field Emission Electron Probe Microanalyzer). Distribution of Co, Mn, Sr. The sintered body of ZnO-SCO was polished and acid-etched, and the surface was plated with platinum to increase the conductivity. The composition of the grains and grain boundaries was analyzed using a micro-element analyzer (BRUKER, XFLASH-Detector 6/10, German). Mott Schottky: The sintered body coated with silver gel electrode was analyzed under different bias voltages (Bias, V) using an electrochemical analyzer (SI 1260, Solartron Analytical, UK) combined with the software ZPlot (Scribner Associates. Inc., USA). capacitance change and calculate the Mott Schottky result using the following formula. will Plotting with V, we can get the equation of a straight line That is rule Among them, 𝛷 _b is the Schottky barrier height; N _d is the donor concentration of the crystal grain; N _t is the concentration of the interface state (acceptor concentration); W is the width of the depletion region; C ₀ is the capacitance when V _bias = 0 value; e is the basic charge of 1eV 1.6×10 ^-19 C; ɛ ₀ is the vacuum dielectric constant 8.85×10 ^-12 F/m; ɛ _r is the dielectric constant of zinc oxide 8.5×ɛ ₀ F/m; A is The electrode area is 9×10 ^-6 m ² . Impedance analysis: The impedance value of the sintered body coated with silver gel electrodes at 250~300°C was analyzed using an electrochemical analyzer (SI 1260, Solartron Analytical, UK) combined with the software ZPlot (Scribner Associates. Inc., USA). And use the software Zview to perform circuit fitting. The resistance value obtained after fitting is used to calculate the conductive activation energy of the grain and grain boundary using the Arrhenius equation. Among them, σ is the conductivity; σ ₀ is a constant; Ea is the conductive activation energy; k is Boltzmann's constant; T is the absolute temperature.

Phase identification:

The X-ray diffraction patterns of SCO and SCNT after calcining are shown in Figure 1. SCO is mainly composed of hexagonal crystal system SrCoO _2.52 and secondary phase Sr ₆ Co ₅ O ₁₅ , while SCNT is mainly cubic crystal system SrCoO _2.29 and secondary phase. Phase SrCo _0.33 Nb _0.67 O ₃ . The starting powders are mixed according to the stoichiometric ratio, but the synthesis of fully oxidized pure phase SrCoO ₃ requires high temperature and oxygen enrichment. Under normal atmospheric conditions, it is easily thermally decomposed into Sr ₂ Co ₂ O ₅ at high temperatures and absorbs oxygen during cooling. Sr ₆ Co ₅ O ₁₅ and Co ₃ O ₄ are formed, and their reaction formula is: (high temperature) (while cooling)

The main phase SrCoO _2.29 in the SCNT phase composition has a higher oxygen vacancy concentration than SrCoO _2.52 in SCO. It can act as a rapid oxygen diffusion channel on the ZnO grain boundaries, allowing oxygen to quickly diffuse from the grain boundaries to the ZnO grain surface during the cooling process. , forming more adsorbed oxygen on the grain boundaries, thereby improving the varistor properties.

I-V curve:

Figure 2 shows the IV curve of each sample, and Table 2 shows its varistor properties. The results show that adding appropriate amounts of transition metal oxides Co ₃ O ₄ and Mn ₃ O ₄ can increase the nonlinear coefficient (α) from 14.82 to 53.02, and the breakdown voltage (Breakdown voltage, Vb) also increases from 611V/mm. to 1741V/mm; when the grain boundary additive is changed from SCO to SCNT, the α value further increases to 65.82; Vb even increases to 4173V/mm; both ZCM-SCO and ZCM-SCNT simultaneously significantly reduce the leakage current (IL).

Table 3 summarizes the materials and varistor properties used in each literature and compares them with the ZCM-SCNT in this case (document source: E. Koga, N. Sawada, and M. Amisawa, "NON-LINEAR PROPERTIES OF ZnO+ ACoO ₃ CERAMICS(A = Ca, Sr AND Ba) AND THEIR APPLICATION TO MULTILAYER CERAMIC VARISTOR FOR ESD-SUPPRESSION AROUND HIGH FREQUENCY," Journal of the Australian Ceramic Society, vol. 48, no. 2, pp. 232-235, 2012; J. He, S. Li, J. Lin, L. Zhang, K. Feng, L. Zhang, W. Liu, and J. Li, "Reverse manipulation of intrinsic point defects in ZnO-based varistor ceramics through Zr-stabilized high ionic conducting βIII -Bi ₂ O ₃ intergranular phase," J. Eur. Ceram. Soc., vol. 38, no. 4, pp. 1614-1620, 2018. W. Cao, X. Xie, Y. Wang, M. Chen, Y. Qiao, P. Wang, Y. Zhang, and J. Liu, "Effect of Pr ₆ O ₁₁ doping on the microstructure and electrical properties of ZnO varistors," Ceram. Int., vol. 45, no. 18, pp . 24777-24783, 2019; W. Liu, L. Zhang, F. Kong, K. Wu, S. Li, and J. Li, "Enhanced voltage gradient and energy absorption capability in ZnO varistor ceramics by using nano-sized ZnO powders," J. Alloys Compd., vol. 828, p. 154252, 2020.), where Material represents the added material, showing that the varistor properties obtained by the present invention (the first column of Table 3) are far better than those reported in the literature. It has also been confirmed that adding an appropriate amount of transition metal oxide and Nb- and Ta-doped SrCoO ₃ at the same time can indeed significantly improve the varistor properties of ZnO varistor materials.

Table 2

table 3

Microstructure and Density:

Figure 3 and Table 4 show the microstructure, grain size, thickness and apparent density of ZnO-SCO, ZCM-SCO and ZCM-SCNT after sintering at 1100°C. The microstructure shows that no secondary phase is produced, the hole distribution and apparent density results are similar, and the grain size is ZnO-SCO>>ZCM-SCO>ZCM-SCNT. When the grain size is larger and the sample thickness (D) is the same, the number of grain boundaries is smaller, and the collapse voltage is proportional to the number of grain boundaries. The results in Table 4 show that the collapse voltage of ZCM-SCNT is larger, that is, because of its crystal There are more boundaries.

Table 4 sample Apparent density (g/cm ³ ) Thickness(𝜇m) Grain size (𝜇m) Number of grain boundaries ZnO-SCO 5.22 560 5.26 106.46 ZCM-SCO 5.23 3.07 182.41 ZCM-SCNT 5.25 2.38 235.29

Element distribution:

Each sample was subjected to area scanning for the element Co, and the distribution results are shown in Figure 4(d) to (f). The element Co is evenly distributed inside the crystal grains. Since the ion radius of Co is slightly smaller than that of Zn, an appropriate amount of Co will be solid dissolved into the ZnO crystal grains.

In the ZnO-SCO sample, some Co ₃ O ₄ is formed during the cooling process of the grain boundary additive SCO during the synthesis, so Co is still solid dissolved into the ZnO during the sintering process. For ZCM-SCO and ZCM-SCNT, appropriate amounts of transition metal oxides Co ₃ O ₄ and Mn ₃ O ₄ are first added to ZnO in the starting powder and then calcined at 600°C. The purpose is to allow Co ions to replace ZnO mainly in the trivalent form. Zn in it to produce a higher donor concentration. Because the valence state of transition metal elements changes at different temperatures, taking cobalt oxide as an example, oxidation occurs at 600°C, and reduction occurs at 800 to 1000°C.

The element Co is significantly enriched at certain grain boundaries. This is mainly because when the transition metal heats up, some Co ³⁺ will be reduced to Co ²⁺ . The increase in the ionic radius causes the crystal lattice to become larger, so it is easy to move to the grain boundaries. , and the Co-enriched area has multiple interlaced grains with relatively many grain boundaries, so the color intensity in the regional scan is more obvious (Figure 4(e) and (f)).

The distribution of the element Mn in ZCM-SCO and ZCM-SCNT is shown in Figure 4(j) and (k). The results are the same as the element Co. They are evenly distributed in the grains and enriched at some grain boundaries.

The distribution of the element Sr in each sample is shown in Figure 4(g) to (i). The color intensity results show that Sr is mainly located at the grain boundaries. Figure 5 shows the image of ZnO-SCO under a high-resolution scanning electron microscope. The ZnO-SCO was analyzed at the points between grains and grain boundaries. The results are shown in Table 5, which are the points covered by the circles in Figure 5 between grains and grain boundaries. The composition analysis results of the grain boundaries (O and C are not calculated) show that the Sr content is higher at the grain boundaries.

table 5

Thermogravimetric curve:

Figure 6 shows the thermogravimetric loss curves of ZnO-SCO, ZCM-SCO and ZCM-SCNT under (a) heating (rate 5°C/min) and (b) cooling (rate 2°C/min). During the sintering process, it can be found that ZCM-SCO has obvious weight loss. This is due to the reduction reaction of the added transition metal oxides Co ₃ O ₄ and Mn ₃ O ₄ at high temperatures, forming CoO and MnO and accompanied by the loss of oxygen. ZCM-SCNT is slightly heavier because the grain boundary additive SCNT itself has more oxygen vacancies, making it less likely for oxygen to diffuse into the atmosphere during the sintering process.

It can be seen from the cooling process that both ZCM-SCO and ZCM-SCNT have significantly increased in weight. Therefore, the oxygen in the atmosphere diffuses into the interior of the embryo. The SCO and SCNT located at the grain boundary have an oxygen vacancy defect structure. It acts as a good oxygen channel, allowing oxygen to quickly diffuse from the grain boundary to the ZnO grain surface, and interacts with the 3d energy level of the transition metal to form an interface state of adsorbed oxygen on the grain boundary, resulting in ZCM-SCO and ZCM- SCNT gained significant weight.

Table 6 shows the weight percentage changes of each starting powder during the sintering and cooling processes.

Table 6

Chemical Analysis Electronic Spectroscopy:

In order to further confirm that adding transition metal oxides and/or grain boundary additives can effectively increase the adsorbed oxygen in the interface state, the fractured surface of the sintered body was analyzed by X-ray electron spectrometer. The results are shown in Figure 7 and Table 7; among them , is the lattice oxygen, For oxygen vacancy, To adsorb oxygen. Figure 7(b)(e)(h) are the Co 2p spectra of ZnO-SCO, ZCM-SCO, and ZCM-SCNT respectively. After fitting, there are two main peaks at 780.0eV and 796.0eV, which are Co 2p _3/2 respectively. With Co 2p _1/2 , the peaks at 779.4eV and 794.8eV correspond to Co ³⁺ , the peaks at 781.1eV and 796.5eV are Co ²⁺ , and the accompanying peaks at 786.5eV and 803.7eV are also Co ²⁺ . An appropriate amount of transition metal oxides ZCM-SCO and ZCM-SCNT are first added to ZnO, and more Co ³⁺ is present after calcination at 600°C. ZCM-SCNT has more oxygen vacancies due to the grain boundary additive SCNT, which acts as a good oxygen channel on the grain boundary, allowing oxygen to quickly diffuse from the grain boundary to the surface of ZnO grains during the cooling process, providing more oxygen for transition metals to Oxidation occurs during cooling, so the Co ³⁺ ratio is significantly higher.

Figure 7(c)(f)(i) are the O 1s spectra of ZnO-SCO, ZCM-SCO, and ZCM-SCNT respectively. After fitting, there are three peaks at 530.2, 531.8, and 532.6eV, corresponding to lattice oxygen, Oxygen vacancies and adsorbed oxygen. The adsorbed oxygen ratio ZCM-SCNT>ZCM-SCO>>ZnO-SCO is mainly because when the transition metal cools down, some Co ²⁺ and Mn ²⁺ will be oxidized into Co ³⁺ and Mn ³⁺ , losing electrons and being absorbed by oxygen at the same time. Capture, forming more adsorbed oxygen, thereby increasing the adsorbed oxygen concentration in the interface state and improving the rheostat properties. The α value and collapse voltage appear to increase as the proportion of adsorbed oxygen increases. The oxygen vacancy ratio is ZnO-SCO>ZCM-SCO>>ZCM-SCNT. The grain boundary additive SCNT has more oxygen vacancies, but the XPS results show that it is the least. This may be because the adsorbed oxygen is negatively charged and the oxygen vacancies are positive. It is easy to combine and turn into lattice oxygen, so ZCM-SCNT has the smallest oxygen vacancy ratio, the same as ZCM-SCO.

Based on the ratio of Co ³⁺ , oxygen vacancies and adsorbed oxygen in ZCM-SCO and ZCM-SCNT, the grain boundary additive SCNT provides a path for rapid diffusion of oxygen, allowing the transition metal to oxidize during the cooling process. Therefore, the ratio of Co ³⁺ in ZCM-SCNT It is about 18% higher than ZCM-SCO, but the adsorbed oxygen is only slightly higher by 3%. This can be echoed in the above description. Because the adsorbed oxygen and oxygen vacancies combine to form lattice oxygen, ZCM-SCNT has the lowest oxygen vacancy content, while the adsorbed oxygen is only Slightly more ZCM-SCO.

Table 7: Composition ratio of Co 2p and O 1s in each sample

Mott Schottky:

Figure 9 and Table 8 show the Mott Schottky analysis results of each sample. Energy barrier height (ψ _b ) ZCM-SCNT>ZCM-SCO>ZnO-SCO, because the energy barrier is mainly affected by the amount of adsorbed oxygen on the grain boundary, because the adsorbed oxygen can effectively act as an electron trap, thereby improving the varistor properties, combined with the above The thermogravimetric curve and O 1s spectrum echo that the energy barrier of ZCM-SCO and ZCM-SCNT is higher than that of ZnO-SCO, and the changing trend of the α value is also the same as the energy barrier height. The results show that the donor concentration is ZCM-SCO>ZnO-SCO>ZCM-SCNT. This is because there will be a re-oxidation reaction during the cooling process. At this time, only the oxygen vacancies will be filled, so that the oxygen vacancy concentration decreases. Therefore, although the grain boundary additive SCNT has more oxygen vacancies, it combines with adsorbed oxygen to form lattice oxygen, which reduces the oxygen vacancy concentration and results in the smallest ZCM-SCNT donor concentration. The energy barrier height and the width of the depletion layer will increase as the annealing time increases. According to Mott Schottky's relationship, they are related to the decrease in donor concentration. It is known that the oxygen vacancy is the donor concentration. The oxygen vacancy concentration will decrease as the annealing time increases, and the valence of Mn increases. The grain boundary additive SCNT acts as a good oxygen channel during the cooling process, allowing oxygen to quickly diffuse from the grain boundary to the ZnO grain surface and causing a decrease in the oxygen vacancy concentration. The XPS analysis result shows that the proportion of Co ³⁺ is significantly higher, so the grain boundary additive is The SCNT donor concentration is the smallest and can effectively increase the energy barrier height and depletion layer width.

Table 8: Average values of physical quantities obtained by Mott Schottky analysis of each sample

Impedance analysis:

The AC impedance and conductive activation energy of ZCM-SCO and ZCM-SCNT with better varistor properties were analyzed at 250~300°C. The impedance fitting diagram in Figure 10 shows that the total resistance of each sample decreases as the temperature increases, with a negative temperature coefficient. Therefore, the conductive activation energy can be calculated using the Arrhenius equation. The results are shown in Figure 9 and Table 9, where Grain represents the grain and Grain boundary represents the grain boundary. It can be seen from the impedance fitting diagram that the total resistance of ZCM-SCMT is larger than that of ZCM-SCO, and the total resistance values at 270°C are 1.12×10 ⁷ and 2.52×10 ⁶ Ω respectively. The total resistance is affected by the number of grain boundaries. When the number of grain boundaries is small, the total resistance of the varistor is relatively small. Therefore, under the same sintering state, the total resistance of ZCM-SCO per unit volume is relatively low due to the relatively small number of grain boundaries.

Table 9: Crystal grain and grain boundary conductive activation energies of ZCM-SCO and ZCM-SCNT

Figure 11 further analyzes the impedance and fitting spectra of ZCM-SCO and ZCM-SCNT at different temperatures; Table 10 provides the equivalent resistance and capacitance of ZCM-SCO and ZCM-SCNT after fitting at different temperatures, where, R represents the equivalent resistance and CPE is a constant phase element: an equivalent circuit used to fit the data. CPE-T is a pseudocapacitor, and CPE-P is related to the semicircular shape in the Nyquist diagram. By using CPE-P and CPE-T and resistors, the true capacitance of the electrodes can be calculated. In addition, the subscript 1 indicates a grain boundary, and the subscript 2 indicates a crystal grain.

Table 10

The above identification methods and performance analysis prove that the zinc oxide varistor material of the present invention has extremely excellent performance. Below, cobalt oxide is further used as an example to illustrate the mechanism and principle of the material and preparation method of the present invention:

Figure 12 is a diagram of the reaction mechanism of the varistor material, illustrating the impact of each step in the preparation process on the material itself.

(a) is the reaction mechanism diagram of ZCM calcination. Add appropriate amounts of transition metal oxides Co ₃ O ₄ and Mn ₃ O ₄ to ZnO, and calcine at 600°C for 6 hours, so that the transition elements become trivalent. Form replaces Zn in ZnO to produce higher donor concentration and , accompanied by zinc vacancies its formation.

(b) is the reaction mechanism diagram of sintering. SCO and SCNT are added to ZCM and sintered at 1100°C for 2 hours. During the sintering process, transition metal elements will undergo reduction reactions accompanied by oxygen loss, and the reduction reaction of the green body at high temperatures will also cause oxygen loss, accompanied by the formation of oxygen vacancies, and Co ³⁺ is reduced to Co ²⁺ , the ionic radius As they become larger, they tend to move toward grain boundaries.

(c) is the reaction mechanism of the cooling process. During the cooling process, the transition metal elements undergo an oxidation reaction, and at this time, the oxygen in the atmosphere will diffuse to the body. The SCO and SCNT located at the grain boundaries have an oxygen vacancy defect structure. Acts as a good oxygen channel, allowing oxygen to quickly diffuse to the ZnO surface, and interacts with the 3d energy level of the transition metal to form an interface state of adsorbed oxygen on the grain boundary. .

(d) is the reaction mechanism between oxygen vacancies and adsorbed oxygen after the cooling process. There will be a re-oxidation reaction during the cooling process. At this time, only the oxygen vacancies will be filled, and electric holes will be generated at the same time. In addition, since the adsorbed oxygen is negatively charged and the oxygen vacancy is positive, the two easily combine to become lattice oxygen.

Based on this, it is proved that the novel reactant combination used in the new zinc oxide varistor material of the present invention, that is, adding transition metal oxides to the ZnO base material, and SCO or SCNT as grain boundary additives, can effectively achieve high non-linearity in the I-V curve. Linear index, high breakdown voltage and low leakage current; and applying the zinc oxide varistor material to a varistor can improve the overall varistor properties.

[Figure 1] X-ray diffraction patterns of (a) SCO and (b) SCNT. [Figure 2] I-V curves of each sample. [Figure 3] Microstructure and (d) grain size distribution of (a) ZnO-SCO (b) ZCM-SCO (c) ZCM-SCNT after sintering at 1100°C. [Figure 4] Color intensity distribution of elements Co, Mn, and Sr in different samples. [Figure 5] High-resolution scanning electron microscope image of ZnO-SCO. [Figure 6] Thermogravimetric curves of different starting samples: (a) heating rate 5°C/min (b) cooling rate 2°C/min. [Figure 7] XPS analysis of different samples (a) (d) (g) full spectrum (b) (e) (h) Co 2p and (e) (f) (i) O 1s spectra. [Figure 8] Scatter diagram of physical quantities obtained by Mott Schottky analysis of each sample. [Figure 9] (a) Crystal grain (b) grain boundary conductivity of ZCM-SCO and ZCM-SCNT. [Figure 10] Impedance fitting spectra of (a) ZCM-SCO (b) ZCM-SCNT at 250~300°C. [Figure 11] Impedance and fitting spectra of ZCM-SCO and ZCM-SCNT at different temperatures. [Figure 12] Reaction mechanism diagram: (a) Substitution reaction during ZCM calcination process (b) Oxygen vacancy formation during varistor sintering process (c) Interfacial chemical adsorption during cooling process (d) Formation of lattice oxygen after cooling.

Claims (10)

A zinc oxide varistor material, characterized in that the reactants of its preparation method include transition metal oxides, zinc oxide as a base material, strontium carbonate and cobalt oxide as additive materials, and the strontium carbonate and the cobalt oxide are first processed for the first time Ball milling, calcination, and second ball milling are performed to obtain the first additive. After calcining the zinc oxide and the transition metal oxide, the calcined zinc oxide and transition metal oxide are ball milled with the first additive to obtain powder. The powder is press-molded and a green embryo is prepared, and the green embryo is sintered to obtain the zinc oxide varistor material. The zinc oxide varistor material of claim 1, wherein the transition metal oxide includes cobalt oxide and manganese oxide. The zinc oxide varistor material as claimed in claim 1, wherein the strontium carbonate and the cobalt oxide are calcined at 1000°C for 8 hours. The zinc oxide varistor material according to claim 1, wherein the green embryo is sintered at 1100°C. The zinc oxide varistor material as described in claim 1, wherein the additive material further includes niobium pentoxide and tantalum pentoxide, and the strontium carbonate, the cobalt oxide, the niobium pentoxide and the tantalum pentoxide are first The first ball milling, calcining, and second ball milling are performed to obtain the second additive. After calcining the zinc oxide and the transition metal oxide, the calcined zinc oxide, the transition metal oxide, and the second additive are ball milled. A powder is obtained, which is press-molded and a green embryo is prepared, and the green embryo is sintered. The zinc oxide varistor material according to claim 5, wherein the green embryo is sintered at 1100°C. A method for preparing a zinc oxide varistor material, which is characterized by including the following steps: (a) After ball milling the additive material, calcining it at 1000°C for 8 hours, and then ball milling to obtain the first additive material agent; (b) calcining zinc oxide and transition metal oxide; (c) ball milling the calcined zinc oxide and transition metal oxide in step (b) with the first additive to obtain powder; ( d) pressurize the powder after ball milling in step (c) and prepare a green embryo; (e) sinter the green embryo in step (d) at 1100°C for 2 hours; wherein the additive material includes strontium carbonate and Cobalt oxide. The method for preparing a zinc oxide varistor material as claimed in claim 7, wherein the transition metal oxide includes cobalt oxide and manganese oxide. The method for preparing a zinc oxide varistor material as described in claim 7, wherein the additive material in step (a) further includes niobium pentoxide and tantalum pentoxide. A zinc oxide varistor, characterized by comprising the zinc oxide varistor material described in any one of claims 1 to 6.