A three-layer structure pressure-sensitive ceramic containing only a single double schottky grain boundary barrier and a sol-gel preparation method and application thereof

Abstract

The application discloses a three-layer structure pressure-sensitive ceramic containing only a single double-Schottky grain boundary barrier and a sol-gel preparation method and application thereof, and belongs to the technical field of pressure-sensitive materials. A solid-phase sintering method is adopted, ZnO is used as raw material, oxide doped ZnO ceramic green bodies are obtained by doping oxide and pressing, a sol-gel solution containing Bi elements is coated in the middle of the ZnO ceramic green bodies doped with metal oxide, and ZnO green body-rich Bi layer-ZnO green body three-layer structure bodies are obtained by pressing. After sintering, the three-layer structure pressure-sensitive ceramic containing only a single double-Schottky grain boundary barrier with simple structure is obtained, which comprises two ZnO ceramic layers and a middle Bi-rich layer. Based on the structure, the structure, performance and conductive mechanism of the single double-Schottky grain boundary barrier in the ZnO pressure-sensitive ceramic have obvious advantages, the single ZnO grain-grain boundary-ZnO grain structure formed in the complex structure of the conventional ZnO pressure-sensitive ceramic can be simulated, and this provides strong evidence for exploring the internal characteristics of the polycrystal.

Description

Technical Field

[0001] This invention belongs to the field of pressure-sensitive materials technology, specifically relating to a three-layer structure pressure-sensitive ceramic containing only a single double Schottky grain boundary barrier, its sol-gel preparation method, and its application. Background Technology

[0002] ZnO varistors exhibit a characteristic where the current changes drastically with the voltage after it reaches a certain value, exhibiting excellent nonlinear characteristics. It is precisely because of this characteristic that ZnO varistor ceramics have become the most widely used and extensively studied oxide varistor ceramic material to date. In the zinc oxide varistor ceramic system, the nonlinear characteristics of the device are due to the incorporation of metal oxides with larger ionic radii. The transition metal oxide phase segregated at the grain boundaries forms Schottky barriers with the zinc oxide main crystalline phases on both sides; these back-to-back Schottky barriers are called double Schottky barriers (DSB). Due to the existence of such barriers, when the voltage across the device suddenly increases, the corresponding current also surges, thus protecting the load circuit.

[0003] However, in previous studies, ZnO varistors produced by traditional solid-state sintering are polycrystalline ceramic materials with extremely complex grain boundary types, most of which are distributed in a highly complex manner between the grains. Our theoretical research methods for their voltage nonlinearity are based on traditional semiconductor physics. However, studying nonlinear materials like ZnO varistors requires utilizing a fundamental yet difficult-to-determine structural parameter—the structural size of the grain boundary potential barrier within the material. For polycrystalline oxides, regardless of the method used, it is impossible to accurately determine the precise shape and area of ​​the effective grain boundaries within the material under study. Therefore, testing the electrical performance of the entire resistor cannot accurately reveal the characteristics of individual grain boundaries. Some researchers have prepared three-layer (ZnO layer-Bi₂O₃ layer-ZnO layer) thin film samples using ZnO and bismuth oxide targets. However, too low a power result in small and uneven grain sizes, hindering barrier formation; too high a power causes numerous defects, leading to a decline in thin film performance. Furthermore, this differs significantly from the preparation methods and conditions of traditional zinc oxide varistors. Therefore, the preparation of a three-layer varistor ceramic with a single double Schottky grain boundary barrier by solid-state sintering is of great significance to the study of ZnO varistor ceramics. At the same time, considering that if the oxide is directly dissolved in polyvinyl alcohol (PVA) aqueous solution to make Bi-rich layer, the success rate of the experiment may be reduced due to the large molecular radius and increased porosity. If it is encapsulated and sintered, the subsequent experimental process is more complicated.

[0004] To address the technical challenges of constructing ZnO varistor ceramics with a single double Schottky grain boundary barrier from polycrystalline ZnO ceramics, and the reduced experimental success rate due to the large gaps between the Bi-rich and ZnO layers preventing tight adhesion, it is urgently necessary to construct a ZnO varistor ceramic with a single grain boundary, based on the double Schottky barrier model. This would simulate a single double Schottky grain boundary barrier, allowing for the testing of macroscopic electrical performance parameters of this type of ceramic. This would characterize the electrical properties of the single grain boundary barrier within the ZnO varistor ceramic material, thus preparing for the exploration of the internal characteristics of polycrystalline materials. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention aims to provide a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, its sol-gel preparation method, and its application. This addresses the problems of existing conventional ZnO varistor ceramics being unable to construct ZnO varistor ceramics containing only a single double Schottky grain boundary barrier from polycrystalline ZnO ceramics, and the reduced experimental success rate due to the large gaps between Bi-rich and ZnO layers, which prevent tight adhesion.

[0006] To achieve the above objectives, the present invention employs the following technical solution:

[0007] This invention discloses a sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, comprising:

[0008] 1) Mix ZnO, doped oxide, polyvinyl alcohol aqueous solution, tributyl phosphate and deionized water evenly, dry, ball mill to obtain powder, and then granulate and press to obtain oxide-doped ZnO green body.

[0009] 2) Mix Bi salt, precursor solution and solvent, heat in a water bath and stir until homogeneous to obtain a sol-gel solution rich in Bi element; mix Bi salt with other metal salts, precursor solution and solvent, heat in a water bath and stir until homogeneous to obtain a sol-gel solution rich in Bi element and other metal elements.

[0010] 3) First, uniformly coat the surface of the oxide-doped ZnO green blank obtained in step 1) with a sol-gel solution rich in Bi element or a sol-gel solution rich in Bi element and other metal elements prepared in step 2); then coat another oxide-doped ZnO green blank obtained in step 1) with a polyvinyl alcohol aqueous solution, and place it on the surface of the oxide-doped ZnO green blank coated with a sol-gel solution rich in Bi element and other metal elements; apply pressure to fully bond them together to obtain a three-layer structure of ZnO green blank-Bi-rich layer-ZnO green blank;

[0011] 4) The three-layer structure of ZnO green body-Bi-rich layer-ZnO green body obtained in step 3) is debinded and sintered to obtain a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier.

[0012] Preferably, in step 1), the molar percentage of ZnO:doped metal oxide is (96.00-100.00):(0.00-4.00);

[0013] The amount of polyvinyl alcohol aqueous solution added is 10%-20% of the total mass of ZnO and doped metal oxides;

[0014] The amount of tributyl phosphate added is 1%-5% of the total mass of ZnO and doped metal oxides;

[0015] The amount of deionized water added is 80%-180% of the total mass of ZnO and doped metal oxides.

[0016] Preferably, in step 1), the mass percentage of the polyvinyl alcohol aqueous solution is 1%-5%; the doped metal oxide is Co2O3, MnO2, or SiO2; when the powder is pressed, 0.1-0.15g of powder is pressed into a green blank with a diameter of 8.0mm and a thickness of 0.6-0.8mm.

[0017] Preferably, in step 2), the mass ratio of Bi salt: precursor solution: solvent is (0.5%-2.0%):(0.0%-49.75%):(49.0%-99.5%); the mass ratio of the mixture of Bi salt and other metal salts: precursor solution: solvent is (0.5%-2.0%):(0.0%-49.75%):(49.0%-99.5%); the Bi salt is Bi(NO3)3·5H2O; the other metal salts are MnSO4·H2O or Mn(NO3)2·4H2O; the precursor solution is anhydrous ethanol; the solvent is glycerol or deionized water; the mass ratio of Bi salt: other metal salts is (1-3):(1-3).

[0018] Preferably, in step 2), the water bath heating temperature is 40-80℃, and the stirring time of the water bath heating is 1-4h.

[0019] Preferably, in step 3), the mass percentage of the polyvinyl alcohol aqueous solution is 1%-5%, and the applied pressure is 0.1-0.5 MPa.

[0020] Preferably, in step 4), the glue discharge temperature is 500-600℃ and the glue discharge time is 100-300min.

[0021] Preferably, in step 4), the sintering temperature is 600-1000℃ and the sintering time is 2-3h.

[0022] This invention also discloses a three-layer varistor ceramic containing only a single bis-Schottky grain boundary barrier, prepared by the above method. This three-layer varistor ceramic, containing only a single bis-Schottky grain boundary barrier, is a three-layer preform consisting of a ZnO layer, a Bi-rich layer, and a ZnO layer. Its nonlinear coefficient ranges from 2.02 to 18.73, its breakdown voltage from 12.38 to 1072.40 V, and its leakage current density from 0.03 to 2.13 mA / cm². 2 The thickness of the Bi-rich layer is ≤10.23μm.

[0023] The present invention also discloses the application of the above-mentioned three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier in power systems and electronic circuits.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] This invention discloses a sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier. The method employs solid-state sintering, using ZnO as the raw material, and pressing doped oxide ZnO green blanks. This oxide-doped ZnO green blank improves both conductivity and density, and reduces the requirements for the uniformity of ZnO and the doped oxide additives during ball milling, making the operation simple and easy to implement. A sol-gel solution rich in Bi and other metal elements is then coated onto the surface of the oxide-doped ZnO green blank, followed by a coating suspension. A ZnO green body coated with a polyvinyl alcohol aqueous solution is placed on the surface of a ZnO green body. Under pressure without damaging it, the green body is fully bonded, resulting in a three-layer structure: ZnO green body-Bi-rich layer-ZnO green body. After sintering, a three-layer varistor ceramic is obtained. By improving the conventional ZnO varistor ceramic preparation process, and modifying the experimental process based on the traditional solid-state sintering method, a novel three-layer varistor ceramic containing only a single bismuth-Schottky grain boundary barrier is prepared by brush-coating a doped layer onto the ZnO ceramic. This facilitates the analysis of the single bismuth-Schottky grain boundary barrier. The three-layer structure of this varistor ceramic includes two outer ZnO ceramic layers and an intermediate bismuth-rich layer. Based on this structure, the study of the structure, performance, and conductivity mechanism of the single bismuth-Schottky grain boundary barrier in ZnO varistor ceramics has significant advantages, and it can simulate the single ZnO grain-grain boundary-ZnO grain structure formed in the complex structure of conventional ZnO varistor ceramics. Compared to methods that directly prepare Bi-rich layers by dissolving Bi₂O₃ and metal oxides in PVA solution, the sol-gel method disclosed in this invention primarily uses alcohols or inorganic salts as raw materials, which are easy to purify, resulting in high-purity materials. Furthermore, since it utilizes solution-based chemical reactions, the raw materials can be mixed at the molecular (or atomic) level, leading to a thinner, more uniform Bi-rich layer. It also facilitates micro-doping, which is the main reason for the significantly higher success rate in preparing three-layer ZnO varistors using the sol-gel method. Moreover, the reaction process is easy to control, allowing adjustment of the gel density and specific surface area, and the process is simple.

[0026] This invention also discloses a three-layer varistor ceramic containing only a single bis-Schottky grain boundary barrier, prepared by the above-mentioned sol-gel method. This three-layer varistor ceramic with only a single bis-Schottky grain boundary barrier is a polycrystalline structure with high density and controllable uniformity. Its electrical properties can be improved by doping with metal oxides. The doping layer is simple and easy to fabricate, the metal salt content in the sol-gel solution is controllable, and the oxide-doped ZnO ceramic and the oxide-doped Bi-rich layer have good contact. Its nonlinear coefficient is 2.02-18.73, breakdown voltage is 12.38-1072.40V, and leakage current density is 0.03-2.13mA / cm². 2The average thickness of the Bi-rich layer is ≤10.23μm. The relatively small thickness of the Bi-rich layer is consistent with the grain boundary structure size ratio in conventional ZnO varistors. At the same time, its breakdown voltage range is relatively large, so it can be applied to different circuits according to the required voltage.

[0027] This invention also discloses the application of the aforementioned three-layer varistor ceramic containing only a single bis-Schottky grain boundary barrier in power systems and electronic circuits. The three-layer varistor ceramic with only a single bis-Schottky grain boundary barrier disclosed in this invention provides overvoltage protection and surge absorption for power systems and electronic circuits, effectively protecting them from damage caused by overcurrent surges. Meanwhile, the current trend in electronic devices is miniaturization, requiring smaller materials, which leads to a reduction in the number of grain boundaries. Therefore, it is necessary to improve the characteristics of individual grain boundaries. This invention is the first to prepare single-grain-boundary ZnO varistor ceramics using the sol-gel method. Based on its breakdown voltage and nonlinearity, it can be applied to power systems as a miniaturized electronic device. Attached Figure Description

[0028] Figure 1 This is a SEM image of the three-layer varistor ceramic sample Mn1, which contains only a single double Schottky grain boundary barrier, prepared in Example 1 of the present invention.

[0029] Figure 2 EJ characteristic curves of some samples of the three-layer varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 1-5 of the invention.

[0030] Figure 3 This is a SEM image of the three-layer varistor ceramic sample Mn4 containing only a single double Schottky grain boundary barrier, prepared in Example 4 of the present invention.

[0031] Figure 4 EJ characteristic curves of some samples of the three-layer varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 6-10 of the invention.

[0032] Figure 5 This is a SEM image of the three-layer varistor ceramic sample Si4 containing only a single double Schottky grain boundary barrier, prepared in Example 9 of the present invention.

[0033] Figure 6 The EJ characteristic curves are of some samples of the three-layer varistor ceramics containing only a single double Schottky grain boundary barrier obtained in Examples 11-13 of the invention.

[0034] Figure 7 This is a SEM image of the Co3 three-layer varistor ceramic sample containing only a single double Schottky grain boundary barrier obtained in Example 13 of this invention.

[0035] Figure 8 EJ characteristic curves of some samples of the three-layer varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 14 and 15 of the invention.

[0036] Figure 9 This is a SEM image of the Zn2 three-layer varistor ceramic sample containing only a single double Schottky grain boundary barrier, obtained in Example 15 of this invention. Detailed Implementation

[0037] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0038] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0039] The present invention will now be described in further detail with reference to the accompanying drawings:

[0040] This invention provides a sol-gel preparation method for a three-layer structure varistor ceramic with a ZnO grain-grain boundary-ZnO grain structure containing only a single double Schottky grain boundary barrier, which is beneficial for in-depth research on its grain boundary characteristics.

[0041] A sol-gel preparation method for a three-layer varistor ceramic containing only a single double Schottky grain boundary barrier includes the following steps:

[0042] 1) Mix ZnO, doped oxide, polyvinyl alcohol aqueous solution, tributyl phosphate and deionized water evenly, dry, ball mill to obtain powder, and then granulate and press to obtain oxide-doped ZnO green body.

[0043] 2) Mix Bi salt, precursor solution and solvent, heat in a water bath and stir until homogeneous to obtain a sol-gel solution rich in Bi element; mix Bi salt with other metal salts, precursor solution and solvent, heat in a water bath and stir until homogeneous to obtain a sol-gel solution rich in Bi element and other metal elements.

[0044] 3) First, uniformly coat the surface of the oxide-doped ZnO green blank obtained in step 1) with a sol-gel solution rich in Bi element or a sol-gel solution rich in Bi element and other metal elements prepared in step 2); then coat another oxide-doped ZnO green blank obtained in step 1) with a polyvinyl alcohol aqueous solution, and place it on the surface of the oxide-doped ZnO green blank coated with a sol-gel solution rich in Bi element and other metal elements; apply pressure to fully bond them together to obtain a three-layer structure of ZnO green blank-Bi-rich layer-ZnO green blank;

[0045] 4) The three-layer structure of ZnO green body-Bi-rich layer-ZnO green body obtained in step 3) is debinded and sintered to obtain a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier.

[0046] In step 1), the molar percentage of ZnO and doped metal oxide is (96.00-100.00):(0.00-4.00);

[0047] The amount of polyvinyl alcohol aqueous solution added is 10%-20% of the total mass of ZnO and doped metal oxides;

[0048] The amount of tributyl phosphate added is 1%-5% of the total mass of ZnO and doped metal oxides;

[0049] The amount of deionized water added is 80%-180% of the total mass of ZnO and doped metal oxides;

[0050] The doped oxides are Co2O3, MnO2, and SiO2.

[0051] Take 0.1-0.15g of powder and press it into a green body with a diameter of 8.0mm and a thickness of 0.6-0.8mm.

[0052] In step 2), the mass ratio of Bi salt, Bi salt and other metal salt mixture: precursor solution: solvent is (0.5%-2.0%):(0.0%-49.75%):(49.0%-99.5%).

[0053] The mass ratio of Bi salt to other metal salts is (1-3):(1-3);

[0054] Bi salt is Bi(NO3)3·5H2O; other metal salts are MnSO4·H2O or Mn(NO3)2·4H2O; the precursor solution is anhydrous ethanol; the solvent is glycerol or deionized water.

[0055] In step 3), the mass percentage of the polyvinyl alcohol aqueous solution is 1%-5%;

[0056] The applied pressure is 0.1-0.5 MPa;

[0057] The water bath heating temperature is 40-80℃, and the stirring time is 1-4 hours.

[0058] In step 4), the glue discharge temperature is 500-600℃ and the glue discharge time is 100-300min.

[0059] In step 4), the sintering temperature is 600-1000℃ and the sintering time is 2-3h.

[0060] Example 1

[0061] ZnO ceramic green body doped with 0.01 mol% MnO2 was used.

[0062] A method for preparing a three-layer pressure-sensitive ceramic based on the sol-gel method includes the following steps:

[0063] 1) First, mix 9.9988g of ZnO and 0.0012g of MnO2 with 18g of deionized water, 2g of polyvinyl alcohol (PVA) aqueous solution, and 0.1g of tributyl phosphate. Then, use a planetary ball mill to ball mill the mixture at 500r / min for 5h. Dry the resulting slurry at 90℃ for 12h to obtain a mixed powder.

[0064] 2) Grind and sieve the mixed powder obtained in step 1), select uniform powder between 40 mesh and 120 mesh, weigh 0.1g of powder and press it into a green body with a diameter of 8.0mm and a thickness of 0.6mm.

[0065] 3) Prepare the following sol-gel solution: a. Take 0.1g Bi(NO3)3·5H2O and place it in 19.9g glycerol. Stir for 1h while heating in a water bath at 40℃ to form a colorless and transparent gel solution with a mass percentage of 0.5%.

[0066] 4) Take 2.5g of polyvinyl alcohol and place it in 47.5g of deionized water. Stir for 3 hours while heating in a water bath at 40℃ to form a 5% polyvinyl alcohol aqueous solution.

[0067] 5) Take the sol-gel solution from step 3) and uniformly coat it onto the surface of the oxide-doped ZnO green body obtained in step 2). Then, place another oxide-doped ZnO green body coated with the polyvinyl alcohol solution from step 4) on the surface coated with the suspension. Apply a pressure of 0.1 MPa to it without damaging it, so that it can fully adhere to each other and obtain a three-layer structure of ZnO green body-Bi-rich layer-ZnO green body with a "sandwich" structure.

[0068] 6) The ZnO green-Bi-rich layer-ZnO green three-layer structure green body obtained in step 5) is heated to 600℃ at 3℃ / min and held for 5h to remove the binder. Then, it is heated to 1000℃ at a heating rate of 3℃ / min and held for 2h to sinter. After that, it is cooled to 500℃ at 3℃ / min and then naturally cooled to room temperature to obtain a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier.

[0069] Example 2

[0070] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 1 in that, in step 3), the sol-gel solution is prepared as follows: b. Take 0.1g Bi(NO3)3·5H2O and 0.1g MnSO4·H2O and place them in a mixed solution of 9.8g deionized water and 9.8g anhydrous ethanol, and stir for 2h while heating in a water bath at 50℃ to form a uniform milky white gel solution with a mass percentage of 1.0%.

[0071] Example 3

[0072] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 1 in that, in step 3), the sol-gel solution is prepared as follows: c. Take 0.05g of Bi(NO3)3·5H2O and the same mass of Mn(NO3)2·4H2O respectively and place them in a mixed solution of 9.95g of deionized water and 9.95g of anhydrous ethanol, and stir for 3h during heating in a water bath at 60℃ to form a uniform milky white gel solution with a mass percentage of 0.5%; in step 6), the sintering temperature is 900℃.

[0073] Example 4

[0074] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 1 in that, in step 3), the sol-gel solution is prepared as follows: d, 0.1g of Bi(NO3)3·5H2O and the same mass of Mn(NO3)2·4H2O are placed in a mixed solution of 9.9g of deionized water and 9.9g of anhydrous ethanol, and stirred for 3h in a water bath at 70℃ to form a uniform milky white gel solution with a mass percentage of 1.0%; in step 6), the sintering temperature is 900℃.

[0075] Example 5

[0076] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 1 in that, in step 3), the sol-gel solution is prepared as follows: e. Take 0.2g of Bi(NO3)3·5H2O and the same mass of Mn(NO3)2·4H2O respectively and place them in a mixed solution of 9.8g of deionized water and 9.8g of anhydrous ethanol, and stir for 4h during heating in an 80℃ water bath to form a uniform milky white gel solution with a mass percentage of 2.0%; in step 6), the sintering temperature is 900℃.

[0077] Finally, the three-layer varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 1-5 were polished, silver-coated, and dried before being sintered with silver electrodes. This reduced silver oxide to elemental silver, which then adhered tightly to the sample surface as electrodes, facilitating the testing of their electrical properties. Table 1 below shows the electrical properties of some of the prepared samples.

[0078] Table 1. Electrical properties of the three-layer varistor ceramic samples containing only a single double Schottky grain boundary barrier prepared in Examples 1-5

[0079]

[0080]

[0081] See Figure 1 The image shows a SEM image of Mn1 obtained in Example 1 of this invention. As can be seen from the image, in the three-layer varistor ceramic sample prepared by the sol-gel method, which contains only a single double Schottky grain boundary barrier, the Bi-rich layer and the ZnO layer are in close contact, and the sample surface has good uniformity and flatness.

[0082] See Figure 2The figures show the EJ characteristic curves of some samples of the three-layer structure varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 1-5 of the invention. As can be seen from the figures, the Mn-doped ZnO varistor ceramics have a significant impact on its nonlinearity, with the nonlinear coefficient reaching a maximum of 18.73. At the same time, its breakdown voltage is relatively large, and the trend of leakage current density is opposite to the trend of nonlinear coefficient, which is the same as the characteristics of polycrystalline ZnO varistor ceramics.

[0083] See Figure 3 This is a SEM image of the three-layer varistor ceramic sample Mn4, which contains only a single double Schottky grain boundary barrier, prepared in Example 4 of the present invention. As can be seen from the image, the ZnO ceramic has high density, and the doped layer is in close contact with the ceramic with very small gaps. At the same time, the calculated thickness of the Bi-rich layer is 8.75-10.37 μm, with an average thickness of 9.45 μm.

[0084] Example 6

[0085] ZnO ceramic green body doped with 0.01 mol% SiO2 was used.

[0086] A method for preparing a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier differs from Example 1 in that, in step 1), 9.9993 g of ZnO and 0.0007 g of SiO2 are first mixed with 15 g of deionized water, 1.5 g of polyvinyl alcohol (PVA) aqueous solution, and 0.3 g of tributyl phosphate. The mixture is then ball-milled at 550 r / min for 5 h using a planetary ball mill. The resulting slurry is then dried at 90 °C for 12 h to obtain a mixed powder.

[0087] In step 2), 0.12g of powder is weighed and uniaxially pressed to obtain a green body with a diameter of 8.0mm and a thickness of 0.7mm;

[0088] In step 5), apply a pressure of 0.3 MPa to ensure thorough adhesion;

[0089] In step 6), the temperature is increased to 550℃ at 3℃ / min and held for 100min for debinding; the sintering time is 2.5h.

[0090] Example 7

[0091] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 6 in that, in step 3), the sol-gel solution is prepared as follows: b. Take 0.1g Bi(NO3)3·5H2O and 0.1g MnSO4·H2O and place them in a mixed solution of 9.8g deionized water and 9.8g anhydrous ethanol, and stir for 2h while heating in a water bath at 50°C to form a uniform milky white gel solution with a mass percentage of 1.0%.

[0092] Example 8

[0093] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 6 in that, in step 3), the sol-gel solution is prepared as follows: c. Take 0.05g of Bi(NO3)3·5H2O and the same mass of Mn(NO3)2·4H2O respectively and place them in a mixed solution of 9.95g of deionized water and 9.95g of anhydrous ethanol, and stir for 3h during heating in a water bath at 60℃ to form a uniform milky white gel solution with a mass percentage of 0.5%; in step 6), the sintering temperature is 900℃.

[0094] Example 9

[0095] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 6 in that, in step 3), the sol-gel solution is prepared as follows: d, 0.1g of Bi(NO3)3·5H2O and the same mass of Mn(NO3)2·4H2O are placed in a mixed solution of 9.9g of deionized water and 9.9g of anhydrous ethanol, and stirred for 3h in a water bath at 70℃ to form a uniform milky white gel solution with a mass percentage of 1.0%; in step 6), the sintering temperature is 900℃.

[0096] Example 10

[0097] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 6 in that, in step 3), the sol-gel solution is prepared as follows: e. Take 0.2g of Bi(NO3)3·5H2O and the same mass of Mn(NO3)2·4H2O respectively and place them in a mixed solution of 9.8g of deionized water and 9.8g of anhydrous ethanol, and stir for 4h during heating in an 80°C water bath to form a uniform milky white gel solution with a mass percentage of 2%; in step 6), the sintering temperature is 900°C.

[0098] After polishing, silver coating, drying and other steps, the three-layer structure varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 6-10 were sintered with silver electrodes, thereby reducing silver oxide to elemental silver and using it as an electrode to be closely attached to the sample surface for easy testing of its electrical properties. Table 2 below shows the electrical properties of some of the prepared samples.

[0099] Table 2. Electrical properties of the three-layer varistor ceramic samples containing only a single double Schottky grain boundary barrier prepared in Examples 6-10

[0100]

[0101] See Figure 4The figures show the EJ characteristic curves of some samples of the three-layer structure varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 6-10 of the invention. As can be seen from the figures, the Si-doped ZnO varistor ceramics have a small impact on its nonlinearity, with the highest nonlinear coefficient being 12.15. However, the Si doping reduces its breakdown voltage, and the trend of the leakage current density is opposite to the trend of the nonlinear coefficient, which is the same as the characteristics of polycrystalline ZnO varistor ceramics.

[0102] See Figure 5 This is a SEM image of the three-layer varistor ceramic sample Si4 containing only a single double Schottky grain boundary barrier obtained in Example 9 of the present invention. As can be seen from the image, the ZnO ceramic has a high density, the doped layer is in close contact with the ceramic with very small gaps, and the calculated thickness of the Bi-rich layer is 3.92-6.91 μm, with an average thickness of 6.16 μm.

[0103] Example 11

[0104] ZnO ceramic green bodies doped with 4.00 mol% Co2O3 were used.

[0105] A method for preparing a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier differs from Example 1 in that, in step 1), 9.3100g of ZnO and 0.7900g of Co2O3 are first mixed with 10g of deionized water, 1g of polyvinyl alcohol (PVA) aqueous solution, and 0.4g of tributyl phosphate. The mixture is then ball-milled at 600r / min for 5h using a planetary ball mill. The resulting slurry is then dried at 90℃ for 12h to obtain a mixed powder.

[0106] In step 2), 0.15g of powder is weighed and pressed uniaxially to obtain a green body with a diameter of 8.0mm and a thickness of 0.8mm;

[0107] In step 5), apply a pressure of 0.5 MPa to ensure thorough adhesion;

[0108] In step 6), the temperature is increased to 600℃ at 3℃ / min and held for 250 minutes for debinding; the sintering time is 3 hours.

[0109] Example 12

[0110] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 11 in that, in step 3), the sol-gel solution is prepared as follows: d, 0.1g of Bi(NO3)3·5H2O and the same mass of Mn(NO3)2·4H2O are placed in a mixed solution of 9.9g of deionized water and 9.9g of anhydrous ethanol, and stirred for 3h in a water bath at 70℃ to form a uniform milky white gel solution with a mass percentage of 1.0%; in step 6), the sintering temperature is 900℃.

[0111] Example 13

[0112] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 12 in that the sintering temperature in step 6) is 900°C.

[0113] After polishing, silver coating, drying and other steps, the three-layer structure varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 11-13 were sintered with silver electrodes, thereby reducing silver oxide to elemental silver and using it as an electrode to be closely attached to the sample surface for easy testing of its electrical properties. Table 3 below shows the electrical properties of some of the prepared samples.

[0114] Table 3. Electrical properties of the three-layer varistor ceramic samples containing only a single double Schottky grain boundary barrier prepared in Examples 11-13

[0115]

[0116]

[0117] See Figure 6 The figures show the EJ characteristic curves of some samples of the three-layer structure varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 11-13 of the invention. As can be seen from the figures, the Co-doped ZnO varistor ceramic has a small influence on its nonlinearity, with the highest nonlinear coefficient being 6.70. However, the Co doping contributes significantly to the increase in its breakdown voltage. The trend of leakage current density is opposite to the trend of nonlinear coefficient, which is the same as the characteristics of polycrystalline ZnO varistor ceramics.

[0118] See Figure 7 This is a SEM image of the Co3 three-layer structure varistor ceramic sample containing only a single double Schottky grain boundary barrier obtained in Example 13 of the present invention. As can be seen from the image, the ZnO ceramic has a high density and the doped layer is in close contact with the ceramic. At the same time, the calculated thickness of the Bi-rich layer is 6.51-10.23 μm, with an average thickness of 8.72 μm.

[0119] Example 14

[0120] Pure ZnO ceramic green body

[0121] A method for preparing a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier differs from Example 1 in that, in step 1), 10.0 g of ZnO is first mixed with 8 g of deionized water, 2 g of polyvinyl alcohol (PVA) aqueous solution, and 0.5 g of tributyl phosphate. The mixture is then ball-milled at 600 r / min for 5 h using a ball mill. The resulting slurry is then dried at 90 °C for 12 h to obtain a mixed powder.

[0122] In step 6), the sintering temperature is 600℃.

[0123] Example 15

[0124] A sol-gel preparation method for a three-layer structure varistor ceramic containing only a single double Schottky grain boundary barrier, differing from Example 14 in that, in step 3), the sol-gel solution is prepared as follows: d, 0.1g of Bi(NO3)3·5H2O and the same mass of Mn(NO3)2·4H2O are placed in a mixed solution of 9.9g of deionized water and 9.9g of anhydrous ethanol, and stirred for 3h in a water bath at 70℃ to form a uniform milky white gel solution with a mass percentage of 1.0%; in step 6), the sintering temperature is 1000℃.

[0125] After polishing, silver coating, drying and other steps, the three-layer structure varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 14 and 15 were sintered with silver electrodes, thereby reducing silver oxide to elemental silver and using it as an electrode to be closely attached to the sample surface for easy testing of its electrical properties. Table 4 below shows the electrical properties of some of the prepared samples.

[0126] Table 4. Electrical properties of the three-layer varistor ceramic samples containing only a single double Schottky grain boundary barrier prepared in Examples 14 and 15

[0127]

[0128] See Figure 8 The figures show the EJ characteristic curves of some samples of the three-layer varistor ceramics containing only a single double Schottky grain boundary barrier prepared in Examples 14 and 15 of the invention. As can be seen from the figures, the three-layer varistor ceramic prepared using pure ZnO ceramic green body has a smaller breakdown voltage, and the Bi-rich layer prepared using different sol-gel solutions has a smaller nonlinear contribution to the varistor ceramic.

[0129] See Figure 9 This is a SEM image of the Zn2 three-layer varistor ceramic sample containing only a single double Schottky grain boundary barrier, prepared in Example 15 of this invention. As can be seen from the image, the ZnO ceramic has a high density, but due to the high brittleness of pure ZnO, the edges are easily broken and the flatness is low. However, the internal doped layer is in close contact with the ceramic with very small gaps. At the same time, the calculated thickness of the Bi-rich layer is 8.67-12.19 μm, with an average thickness of 10.23 μm.

[0130] For the three-layer varistor ceramic with only a single double Schottky grain boundary barrier obtained by the sol-gel method in the embodiments, the nonlinear coefficient α and the varistor voltage U were tested respectively. 1mA Leakage current density J LTable 1 shows that the maximum nonlinear coefficient α measured in Examples 1-5 is 18.73, and the varistor voltage U... 1mA 975.84V, leakage current density J L 0.03 mA / cm 2 Meanwhile, the average thickness of the Bi-rich layer was calculated to be 9.45 μm; in Table 2, the maximum nonlinear coefficient α measured in Examples 6-10 was 12.15, and the varistor voltage U... 1mA 766.44V, leakage current density J L 0.06 mA / cm 2 Meanwhile, the average thickness of the Bi-rich layer was calculated to be 6.16 μm; in Table 3, the maximum nonlinear coefficient α measured in Examples 11-13 was 6.70, and the varistor voltage U... 1mA The voltage is 948.41V and the leakage current density is J. L 0.39 mA / cm 2 Meanwhile, the average thickness of the Bi-rich layer was calculated to be 8.72 μm; in Table 4, the maximum nonlinear coefficient α measured in Examples 14 and 15 was 4.17, and the varistor voltage U... 1mA 366.77V, leakage current density J L 0.58 mA / cm 2 The average thickness of the Bi-rich layer was calculated to be 10.23 μm. The results showed that when using the same sol-gel solution to prepare the Bi-rich layer, the MnO2-doped ZnO ceramics prepared in Examples 1-5 significantly improved its nonlinear coefficient, achieving a varistor voltage as high as 975.84 V with low leakage current. The SiO2-doped ZnO ceramics prepared in Examples 6-10 and the Co2O3-doped ZnO ceramics prepared in Examples 11-13 contributed less to the improvement of the nonlinear coefficient, but the SiO2-doped ZnO ceramics significantly reduced its breakdown voltage. The pure ZnO ceramics prepared in Examples 14 and 15 contributed the least to the improvement of the nonlinear coefficient. When using the same ceramic green body, the sol-gel solution of Bi(NO3)3·5H2O and Mn(NO3)2·4H2O showed the most significant effect in preparing the Bi-rich layer, with the effect being most pronounced when 0.1 g was used.

[0131] As can be seen from the SEM images of the implementation cases, in the three-layer structure of ZnO ceramic-Bi-rich layer-ZnO ceramic varistor ceramic prepared by the sol-gel method, the gaps between the Bi-rich layer and the ZnO ceramic are small and the contact is tight. This proves that this invention can effectively improve the stability and uniformity of the sample. Furthermore, the thickness of the Bi-rich layer is basically controlled within 10 μm, which is similar to the size ratio of the conventional single ZnO grain-grain boundary-ZnO grain structure. Therefore, this invention is of great significance for the study of the structure, performance, and conductivity mechanism of the single double Schottky grain boundary barrier in ZnO varistor ceramics. As shown in Examples 1-15, MnO2 doping of ZnO ceramics has a significant effect on improving its nonlinear coefficient. It can be considered that MnO2 doping can effectively increase the height of the double Schottky barrier, thereby improving the nonlinear coefficient.

[0132] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A sol-gel process for the preparation of a three-layer structure pressure sensitive ceramic containing only a single double Schottky grain boundary barrier, characterized in that, The application relates to a method for preparing a three-layer structure pressure-sensitive ceramic containing a single double Schottky grain boundary barrier. 1) ZnO, doped oxide, polyvinyl alcohol aqueous solution, tributyl phosphate and deionized water are uniformly mixed, dried, ball-milled to obtain a powder, and then granulated and pressed to obtain an oxide-doped ZnO green body; the doped oxide is Co2O3, MnO2 or SiO2; when the powder is pressed to form the green body, 0.1-0.15 g of the powder is taken to press a green body with a diameter of 8.0 mm and a thickness of 0.6-0.8 mm; 2) Bi salt, precursor solution and solvent are mixed and uniformly stirred in a water bath to obtain a sol-gel solution rich in Bi elements; a mixture of Bi salt and other metal salts, precursor solution and solvent are mixed and uniformly stirred in a water bath to obtain a sol-gel solution rich in Bi elements and other metal elements; the mass ratio of the mixture of Bi salt and other metal salts, the precursor solution and the solvent is (0.5%-2.0%):(0.0%-49.75%):(49.0%-99.5%); the Bi salt is Bi(NO3)3.5H2O; the other metal salt is MnSO4.H2O or Mn(NO3)2.4H2O; 3) the surface of the oxide-doped ZnO green body obtained in step 1) is uniformly coated with the sol-gel solution rich in Bi elements or the sol-gel solution rich in Bi elements and other metal elements prepared in step 2); the surface of another piece of the oxide-doped ZnO green body obtained in step 1) is coated with polyvinyl alcohol aqueous solution, and is placed on the surface of the oxide-doped ZnO green body coated with the sol-gel solution rich in Bi elements and other metal elements; pressure is applied to sufficiently bond the three layers to obtain a ZnO green body-rich Bi layer-ZnO green body three-layer structure body; the applied pressure is 0.1-0.5 MPa; 4) the ZnO green body-rich Bi layer-ZnO green body three-layer structure body prepared in step 3) is degassed and sintered to obtain a three-layer structure pressure-sensitive ceramic containing a single double Schottky grain boundary barrier.

2. A sol-gel process for the preparation of a three-layer structure pressure sensitive ceramic containing only a single double Schottky grain boundary barrier according to claim 1, characterized in that, In step 1), the molar percentage of ZnO to the doped metal oxide is (96.00-100.00):(0.00-4.00); The added amount of the polyvinyl alcohol aqueous solution is 10%-20% of the total mass of ZnO and the doped metal oxide; The added amount of the tributyl phosphate is 1%-5% of the total mass of ZnO and the doped metal oxide; The added amount of the deionized water is 80%-180% of the total mass of ZnO and the doped metal oxide.

3. A sol-gel process for the preparation of a three-layer structure pressure sensitive ceramic containing only a single double Schottky grain boundary barrier according to claim 1, characterized in that, In step 1), the mass percentage of the polyvinyl alcohol aqueous solution is 1%-5%.

4. The sol-gel process for the preparation of a three layer structure pressure sensitive ceramic containing only a single double Schottky grain boundary barrier according to claim 1, characterized by, In step 2), the mass ratio of the Bi salt, the precursor solution and the solvent is (0.5%-2.0%):(0.0%-49.75%):(49.0%-99.5%); the precursor solution is anhydrous ethanol; the solvent is glycerol or deionized water; the mass ratio of the Bi salt to the other metal salt is (1-3):(1-3).

5. A sol-gel process for the preparation of a three-layer structure pressure sensitive ceramic containing only a single double Schottky grain boundary barrier according to claim 1, characterized in that, In step 2), the water bath heating temperature is 40-80 DEG C, and the stirring time of the water bath heating is 1-4 h.

6. The sol-gel process for the preparation of a three layer structure pressure sensitive ceramic containing only a single double Schottky grain boundary barrier as recited in claim 1, wherein, The mass percentage of the polyvinyl alcohol aqueous solution in step 3) is 1%-5%.

7. A sol-gel process for the preparation of a three-layer structure pressure sensitive ceramic containing only a single double Schottky grain boundary barrier according to claim 1, characterized in that, In step 4), the degassing temperature is 500-600 ℃, and the degassing time is 100-300 min.

8. The sol-gel process for the preparation of a three layer structure pressure sensitive ceramic containing only a single double Schottky grain boundary barrier according to claim 1, characterized by, In step 4), the sintering temperature is 600-1000 ℃, and the sintering time is 2-3 h.

9. A three-layer structure pressure sensitive ceramic containing only a single double Schottky grain boundary barrier prepared by the method of any one of claims 1 to 8, characterized in that, The three-layer structure pressure-sensitive ceramic containing only a single double Schottky grain boundary barrier is a three-layer structure blank of ZnO layer-rich Bi layer-ZnO layer, the nonlinear coefficient of which is 2.02-18.73, the breakdown voltage is 12.38-1072.40 V, and the leakage current density is 0.03-2.13 mA / cm 2 , and the thickness of the rich Bi layer is ≤10.23 μm.

10. Use of the three-layer structure pressure-sensitive ceramic containing only a single double Schottky grain boundary barrier according to claim 9 in power systems and electronic circuits.

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