A high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic and a preparation method and application of the same

Abstract

The application discloses a high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic and a preparation method and application of defect regulation, and belongs to the technical field of pressure-sensitive materials. The method comprises the following steps: ZnO, doped oxides, polyvinyl alcohol aqueous solution, tributyl phosphate and deionized water are ball-mixed to obtain mixed powder; after drying, sieving, granulating and compression molding, oxide-doped ZnO green bodies are obtained; after glue removal and sintering, high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic is obtained. The bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic is obtained by adopting a solid-phase sintering method, taking ZnO as main raw material, and doping multiple oxides for compression and sintering. Immovable acceptor defects and stable double Schottky barrier structures are formed in the oxides enriched at the ZnO grain boundaries, which provides an important technical thought for the electrical performance optimization of different system pressure-sensitive ceramics. The high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic has high nonlinearity coefficient and breakdown field strength and small leakage current density, and has a wide application prospect in miniaturized electronic circuit systems.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of pressure-sensitive materials, and particularly relates to a high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic and a defect regulation preparation method and application thereof. BACKGROUND

[0002] ZnO varistors are widely used in power electronics as protection circuits due to their excellent nonlinear voltage-current characteristics and surge absorption capacity. The main parameters of their electrical performance include nonlinear coefficient, breakdown field strength, and leakage current density. The higher the nonlinear coefficient, the shorter the response time of the varistor to the protection of electronic components, and the more accurate the overvoltage clamping. The smaller the leakage current density, the smaller the joule heat loss of the varistor when the circuit is working normally. Varistors with different breakdown field strengths can be used for overvoltage protection of different power electronic devices. With the development of device miniaturization, there is an urgent need to prepare varistor ceramics with high breakdown field strength and excellent comprehensive electrical performance. Therefore, it is an important goal for researchers to prepare ZnO varistors with high nonlinear coefficient and breakdown field strength and small leakage current density. ZnO varistors are packaged by ZnO pressure-sensitive ceramics, and their electrical performance mainly depends on the performance of ZnO pressure-sensitive ceramics. ZnO pressure-sensitive ceramics are mainly made of ZnO, and by doping other oxides, they have excellent electrical performance.

[0003] At present, ZnO-Bi2O3-based varistor ceramics are most widely used. They are prepared by solid phase sintering method using ZnO as main raw material, doping a small amount of Bi2O3 and a small amount of other oxides. In the sintering process, Bi2O3 forms a liquid phase, and at the same time, a plurality of oxides are dissolved, fully infiltrating around the ZnO crystal grains. In the cooling process, a grid-shaped bismuth-rich phase rich in acceptor defect free oxygen is formed, which is distributed at the ZnO grain boundary. At the same time, the doped oxides are solid-solved into the ZnO crystal grains, forming a large number of donor defects, that is, N-type ZnO crystal grains are formed. Therefore, the N-type ZnO crystal grains and the bismuth-rich phase at the grain boundary together form a grain boundary double Schottky barrier. The grain boundary double Schottky barrier is the main structural basis for determining the electrical properties of ZnO varistor ceramics. Doping or improving the sintering process is the main means to improve the characteristics of the grain boundary double Schottky barrier, and thus to improve the electrical properties of ZnO varistor ceramics. At present, the mainstream view is that by doping, the growth of ZnO grains can be controlled, and the uniformity of the microstructure can be improved; at the same time, the donor concentration in the ZnO grains and the free oxygen concentration at the grain boundary as an acceptor can be controlled; and then the characteristics of the grain boundary double Schottky barrier and the electrical properties can be controlled. Based on this view, researchers have done a lot of work to improve the electrical properties of ZnO-Bi2O3-based varistor ceramics, and have obtained varistor ceramics with excellent electrical properties. However, due to the volatility of Bi2O3 in the sintering process of ZnO-Bi2O3-based varistor ceramics, and the large number of doped oxides, the structure uniformity and sample preparation repeatability are poor, which seriously restricts its application. In recent years, researchers have developed bismuth-free ZnO-Cr2O3-based varistor ceramics, which have the advantages of no volatile doped oxides in the sintering process, fewer types of doped oxides, and simple structure. People have studied the influence of doping on the microstructure and electrical properties of ZnO-Cr2O3-based varistor ceramics, and at the same time, referring to the research on ZnO-Bi2O3-based varistor ceramics, it is pointed out that by doping, the donor concentration in the ZnO grains and the adsorbed oxygen concentration at the ZnO grain boundary as an acceptor defect can be increased, and then the double Schottky barrier height can be increased, and the nonlinear electrical properties can be improved. However, the grain boundary adsorbed oxygen has good fluidity, which will inevitably lead to unstable grain boundary double Schottky barrier structure and poor electrical property stability of the varistor ceramics.

[0004] In view of the problems existing in ZnO-Bi2O3-based varistor ceramics and bismuth-free ZnO-Cr2O3-based varistor ceramics, it is urgent to develop a preparation method of bismuth-free ZnO-Cr2O3-based varistor ceramics, so as to form immovable acceptor defects at the ZnO grain boundary, control the characteristics of the grain boundary double Schottky barrier, and then prepare bismuth-free ZnO-Cr2O3-based varistor ceramics with high nonlinear coefficient and breakdown field strength and small leakage current density, so as to be applied in small-sized electronic circuits. SUMMARY

[0005] In order to overcome the above-mentioned defects of the prior art, the purpose of the present application is to provide a high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic and a defect regulation preparation method and application, so as to solve the technical problems that the existing ZnO-Bi2O3-based pressure-sensitive ceramic is easy to volatilize Bi2O3 in the sintering process, and the types of doped oxides are various, resulting in poor structural uniformity and sample preparation repeatability, and the grain boundary of the ZnO-Cr2O3-based pressure-sensitive ceramic has strong adsorbed oxygen flowability, resulting in unstable grain boundary double Schottky barrier structure.

[0006] In order to achieve the above-mentioned purpose, the technical scheme is adopted in the present application:

[0007] The present application discloses a method for preparing a high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic by regulating defects, comprising the following steps:

[0008] ZnO, doped oxide, polyvinyl alcohol aqueous solution, tributyl phosphate and deionized water are ball milled and mixed to obtain a mixed powder; after drying, sieving, granulating and compression molding, an oxide-doped ZnO green body is obtained; and after glue removal and sintering, a high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic is obtained.

[0009] The doped oxide includes several oxides composed of divalent and trivalent small-radius ions, and one oxide composed of trivalent or tetravalent large-radius ions;

[0010] The divalent and trivalent small-radius ion oxides are Cr2O3, Co3O4 and CaCO3;

[0011] The trivalent or tetravalent large-radius ion oxide is any one of Eu2O3, Dy2O3 and CeO2.

[0012] Preferably, the molar percentage of ZnO and doped oxide is (95.00-98.00):(2.00-5.00);

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

[0014] The mass of the tributyl phosphate is 1%-3% of the total mass of ZnO and doped oxide;

[0015] The mass of the deionized water is 150%-200% of the total mass of ZnO and doped oxide.

[0016] Preferably, the mass percentage of the polyvinyl alcohol aqueous solution is 3%-6%;

[0017] The doped oxide does not include oxides that are volatile in the sintering process.

[0018] Preferably, the ball milling time is 6-8h; and the sieving and granulating conditions are: selecting 40-120 mesh uniform powder.

[0019] Preferably, the pressing conditions are: pressing 0.15-0.25 g of the mixed powder into an oxide-doped ZnO green body with a diameter of 8.0 mm and a thickness of 0.75-1.25 mm.

[0020] Preferably, the degumming temperature is 600 ℃, and the degumming time is 2-5 h.

[0021] Preferably, the sintering temperature is 1100 ℃, and the sintering time is 1-3 h.

[0022] The application further discloses a high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic prepared by the method. 3+ , Co 3+ and Ca 2+ ; the oxide includes Eu2O3, Dy2O3 or CeO2; and the immovable acceptor defects Ca × Eu , Ca × Dy or Cr × Ce are formed in the Eu2O3, Dy2O3 or CeO2 enriched at the ZnO grain boundaries.

[0023] Preferably, the high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic has a nonlinear coefficient of 66-97, a breakdown field strength of 1441-1830 V / mm and a leakage current density of 0.12-0.76 mu A / cm 2 .

[0024] The application further discloses application of the high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic prepared by the method in an electric power electronic system.

[0025] Compared with the prior art, the application has the following beneficial effects:

[0026] The present invention discloses a method for preparing high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics by controlling defects. The method adopts a solid-phase sintering method, uses ZnO as the main raw material, and dopes a variety of oxides to obtain a green body by pressing, and then sintering to obtain bismuth-free ZnO-Cr2O3-based varistor ceramics. The core of the sample preparation in the present invention lies in the selection of doped oxides. The doped oxides do not contain oxides such as Bi2O3 that are easily volatile during the sintering process, and the types of doped oxides are relatively small, thereby avoiding the problem that Bi2O3 is easily volatile during the sintering process of existing ZnO-Bi2O3-based varistor ceramics, and avoiding the problem that the types of doped oxides are too many, resulting in poor structural uniformity and sample preparation repeatability. In the present invention, by doping several oxides composed of divalent and trivalent ions with smaller radius (such as Cr2O3, Co3O4 and CaCO3), and at the same time doping an oxide composed of trivalent or tetravalent ions with larger radius (such as Eu2O3, Dy2O3 or CeO2), the ZnO grain boundaries are enriched with solid solution such as Cr 3+ 、Co 3+ and Ca 2+ Oxides such as Eu2O3, Dy2O3 or CeO2, etc., and Ca2+ is formed in the oxides such as Eu2O3, Dy2O3 or CeO2 enriched at the ZnO grain boundaries. × Eu , Ca × Dy Cr × Ce The immovable acceptor defects such as those in ZnO-Bi2O3-based or ZnO-Cr2O3-based varistor ceramics can be removed to further form a stable double Schottky barrier structure. This avoids the problem of strong mobility of oxygen adsorbed at the grain boundaries as acceptor defects in ZnO-Bi2O3-based or ZnO-Cr2O3-based varistor ceramics reported in the literature, which leads to the instability of the double Schottky barrier structure at the grain boundaries. This also provides a very important technical idea for optimizing the electrical properties of varistor ceramics of different systems.

[0027] The bismuth-free ZnO-Cr2O3-based varistor ceramic prepared by the present invention has better comprehensive electrical properties than the ZnO-Cr2O3-based varistor ceramics reported in the literature. Its nonlinear coefficient is 66-97, the breakdown field strength is 1441-1830 V / mm, and the leakage current density is 0.12-0.76 μA / cm 2 , which has broad application prospects in miniaturized electronic circuit systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is the XRD spectrum of the Eu2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic sample (sample number: Zn-Eu) prepared in Example 1 of the present invention;

[0029] Figure 2is a SEM photo of a Eu2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic sample (sample No. Zn-Eu) prepared in Example 1 of the present application and EDS element distribution images of Cr, Co, Ca and Eu elements corresponding thereto; wherein a is the SEM photo, b~e are respectively the EDS element distribution images of Cr, Co, Ca and Eu elements;

[0030] Figure 3 is an XRD spectrum of a Dy2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic sample (sample No. Zn-Dy) prepared in Example 2 of the present application;

[0031] Figure 4 is a SEM photo of a Dy2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic sample (sample No. Zn-Dy) prepared in Example 2 of the present application and EDS element distribution images of Cr, Co, Ca and Dy elements corresponding thereto; wherein a is the SEM photo, b~e are respectively the EDS element distribution images of Cr, Co, Ca and Dy elements;

[0032] Figure 5 is a SEM photo of a Dy2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic sample (sample No. Zn-Dy) prepared in Example 2 of the present application and EDS element distribution images of Cr, Co, Ca and Dy elements corresponding thereto; wherein a is the SEM photo, b~e are respectively the EDS element distribution images of Cr, Co, Ca and Dy elements;

[0033] Figure 6 is an XRD spectrum of a CeO2-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic sample (sample No. Zn-Ce) prepared in Example 3 of the present application;

[0034] Figure 7 is a SEM photo of a CeO2-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic sample (sample No. Zn-Ce) prepared in Example 3 of the present application and EDS element distribution images of Cr, Co, Ca and Ce elements corresponding thereto; wherein a is the SEM photo, b~e are respectively the EDS element distribution images of Cr, Co, Ca and Ce elements;

[0035] Figure 8 is an XRD spectrum of an Er2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic sample (sample No. Zn-Er) prepared in Comparative Example 1;

[0036] Figure 9SEM image of the Er2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic sample (sample No. Zn-Er) prepared in Comparative Example 1 and EDS element distribution images of Cr, Co, Ca and Er elements corresponding thereto; wherein a is the SEM image, b~e are the EDS element distribution images of Cr, Co, Ca and Er elements, respectively.

[0037] Figure 10 XRD spectrum of the Tm2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic sample (sample No. Zn-Tm) prepared in Comparative Example 2.

[0038] Figure 11 SEM image of the Tm2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic sample (sample No. Zn-Tm) prepared in Comparative Example 2 and EDS element distribution images of Cr, Co, Ca and Tm elements corresponding thereto; wherein a is the SEM image, b~e are the EDS element distribution images of Cr, Co, Ca and Tm elements, respectively. DETAILED DESCRIPTION

[0039] The technical solutions of the present application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0040] In the present application, all the embodiments and preferred embodiments mentioned in the present application can be combined to form new technical solutions, if not otherwise specified.

[0041] In the present application, all the technical features and preferred features mentioned in the present application can be combined to form new technical solutions, if not otherwise specified.

[0042] In the present application, percentage (%) or part refers to the weight percentage or weight part of the composition, if not otherwise specified.

[0043] In the present application, all the components or preferred components mentioned in the present application can be combined to form new technical solutions, if not otherwise specified.

[0044] In the present application, unless otherwise specified, the numerical range "a~b" represents a shorthand notation for any real number combination between a and b, wherein a and b are both real numbers. For example, the numerical range "6~22" represents that all the real numbers between "6~22" have been listed herein, and "6~22" is only a shorthand notation for these numerical combinations.

[0045] The ranges disclosed herein are inclusive of the lower and upper limits, separately.

[0046] In the present application, the term "and / or" as used herein refers to any combination of one or more of the associated listed terms and all possible combinations, and includes these combinations.

[0047] In the present application, unless otherwise specified, each reaction or operation step can be carried out sequentially or in accordance with the sequence. Preferably, the reaction method herein is carried out sequentially.

[0048] Unless otherwise specified, the professional and scientific terms used herein have the same meaning as those familiar to those skilled in the art. In addition, any method or material similar or equivalent to that described can also be applied in the present application.

[0049] The present application provides a method for preparing high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic by regulating defects, which provides a very important technical idea for optimizing the electrical performance of different system pressure-sensitive ceramics, and finally obtains high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic with high nonlinear coefficient and breakdown field strength and small leakage current density, which has broad application prospects in miniaturized electronic circuit systems.

[0050] A method for preparing high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic by regulating defects, comprising the following steps:

[0051] 1) ZnO, doped oxide, polyvinyl alcohol aqueous solution, tributyl phosphate and deionized water are ball milled and mixed, dried, sieved, granulated and pressed to form an oxide-doped ZnO green body;

[0052] 2) The oxide-doped ZnO green body prepared in step 1) is calcined to remove glue and further sintered to obtain a bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic.

[0053] In step 1), the molar percentage of ZnO to doped oxide is (95.00-98.00):(2.00-5.00);

[0054] The amount of polyvinyl alcohol aqueous solution added is 10%-20% of the total mass of ZnO and doped oxide;

[0055] The amount of tributyl phosphate added is 1%-3% of the total mass of ZnO and doped oxide;

[0056] The amount of deionized water added is 150%-200% of the total mass of ZnO and doped oxide;

[0057] The mass percentage of the polyvinyl alcohol aqueous solution is 3-6 %;

[0058] The doped oxide includes several oxides (such as Cr2O3, Co3O4 and CaCO3) composed of divalent and trivalent small radius ions, and an oxide (such as Eu2O3, Dy2O3 or CeO2) composed of trivalent or tetravalent large radius ions, and does not include Bi2O3 and the like which are volatile in the sintering process;

[0059] The ball milling time is 6-8 h;

[0060] The sieving and granulating condition is that 40-mesh to 120-mesh uniform powder is selected.

[0061] When the powder is pressed into a shape, 0.15-0.25 g of powder is taken to press a green body with a diameter of 8.0 mm and a thickness of 0.75-1.25 mm.

[0062] In the step 2), the degassing temperature is 600 DEG C, and the degassing time is 2-5 h;

[0063] The sintering temperature is 1100 DEG C, and the sintering time is 1-3 h.

[0064] The application further discloses the bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic prepared by the preparation method, wherein the ZnO grain boundary is enriched with oxides such as Cr 3+ , Co 3+ and Ca 2+ , and the oxides such as Eu2O3, Dy2O3 or CeO2 enriched in the ZnO grain boundary form immobile acceptor defects such as Ca × Eu , Ca × Dy , Cr × Ce .

[0065] The application further discloses the bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic prepared by the preparation method, wherein the non-linear coefficient is 66-97, the breakdown field strength is 1441-1830 V / mm, and the leakage current density is 0.12-0.76 mu A / cm 2 .

[0066] The application further discloses the application of the bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic prepared by the preparation method in an electric power electronic system.

[0067] The application discloses a high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramic, a preparation method and application of the pressure-sensitive ceramic, and solves the problems that Bi2O3 in the existing ZnO-Bi2O3-based pressure-sensitive ceramic is easy to volatilize in a sintering process, and a large number of doped oxides result in poor structure uniformity and sample preparation repeatability, and the problems that the grain boundary of the ZnO-Cr2O3-based pressure-sensitive ceramic has strong oxygen flowability, and results in an unstable double Schottky barrier structure of the grain boundary, and provides a very important technical thought for optimizing the electrical properties of different system pressure-sensitive ceramics, and high-performance bismuth-free ZnO-Cr2O3-based pressure-sensitive ceramics with high nonlinearity coefficient and breakdown field strength and small leakage current density are prepared and can be applied to miniaturized electronic circuits.

[0068] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the following will be combined with the drawings in the embodiments of the present application to make a clear and complete description of the technical solutions in the embodiments of the present application. Obviously, the described embodiments are some of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0069] Embodiment 1

[0070] The preparation method of the Eu2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic comprises the following steps:

[0071] 1) According to the optimal performance formula 96.65 mol% ZnO + 0.10 mol% Cr2O3 + 0.50 mol% Co3O4 + 2.00 mol% CaCO3 + 0.75 mol% Eu2O3, 10 g of oxide powders are weighed, and the oxide powders, 18 g of deionized water, 2 g of a 4% polyvinyl alcohol (PVA) aqueous solution and 0.1 g of tributyl phosphate are placed in a nylon ball mill jar, a planetary ball mill is used to mill the mixture at a speed of 500 r / min for 7 h, and the obtained slurry is dried at 90 DEG C for 12 h to obtain mixed powders;

[0072] 2) The mixed powders obtained in step 1) are ground and sieved, and uniform powders between 40 meshes and 120 meshes are selected, 0.15 g of the powders is uniaxially pressed to obtain a green body with a diameter of 8.0 mm and a thickness of 0.75 mm;

[0073] 3) The green body prepared in step 2) is raised to 600 ℃ at a rate of 3 ℃ / min and held for 5 h for degumming, then raised to 1100 ℃ at a rate of 3 ℃ / min and held for 3 h, then reduced to 500 ℃ at a rate of 3 ℃ / min, and then naturally cooled to room temperature to obtain Eu2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-dependent ceramic, sample No. Zn-Eu.

[0074] Referring to Figure 1 The XRD spectrum of the Eu2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-dependent ceramic sample (sample No. Zn-Eu) prepared in Example 1 of the present application; it can be seen from the spectrum that, in addition to the ZnO phase, the sample mainly contains the Eu2O3 phase.

[0075] Referring to Figure 2 The SEM photo of the Eu2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-dependent ceramic sample (sample No. Zn-Eu) prepared in Example 1 of the present application and the EDS element distribution images of Cr, Co, Ca and Eu elements corresponding thereto; wherein a is the SEM photo, and b-e are the EDS element distribution images of Cr, Co, Ca and Eu elements, respectively; it can be seen from the figure that the enrichment degree of Eu and Ca is the highest, and a small amount of Cr and Co is distributed coincidentally with Eu. Combined with Figure 1 It can be known that part of Ca is solid-solved in the Eu2O3 phase, and a small amount of Cr and Co is also solid-solved in the Eu2O3 phase and distributed at the ZnO grain boundary. Ca is solid-solved in the Eu2O3 phase to form immobile acceptor defects Ca × Eu , which leads to the formation of a stable double Schottky barrier structure at the ZnO grain boundary, further improving the electrical properties of the sample.

[0076] Example 2

[0077] A preparation method of Dy2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-dependent ceramic, comprising the following steps:

[0078] 1) 10 g of oxide powders are weighed according to the best performance formula 96.40 mol% ZnO + 0.10 mol% Cr2O3 + 0.50 mol% Co3O4 + 2.00 mol% CaCO3 + 1.00 mol% Dy2O3, and the oxide powders are placed in a nylon ball mill jar together with 18 g of deionized water, 2 g of polyvinyl alcohol (PVA) aqueous solution with a mass percentage of 4%, and 0.1 g of tributyl phosphate, and a planetary ball mill is used to mill at a speed of 500 r / min for 8 h, and the obtained slurry is dried at 90 ℃ for 12 h to obtain a mixed powder;

[0079] 2) The mixed powder obtained in step 1) is ground and sieved, and a uniform powder between 40 mesh and 120 mesh is selected. 0.15 g of the powder is uniaxially pressed to obtain a green body with a diameter of 8.0 mm and a thickness of 0.75 mm;

[0080] 3) The green body prepared in step 2) is heated to 600 ℃ at a rate of 3 ℃ / min and held for 5 h for degassing. Then, the temperature is increased to 1100 ℃ at a rate of 3 ℃ / min and held for 3 h. Then, the temperature is decreased to 500 ℃ at a rate of 3 ℃ / min, and then the sample is naturally cooled to room temperature to obtain a Dy2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic, which is sample No. Zn-Dy.

[0081] Referring to Figure 3 The XRD spectrum of the Dy2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic sample (sample No. Zn-Dy) prepared in Example 2 of the present application is shown in the figure. As can be seen from the figure, the sample mainly contains Dy2O3 phase in addition to ZnO phase.

[0082] Referring to Figure 4 The SEM photo of the Dy2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic sample (sample No. Zn-Dy) prepared in Example 2 of the present application and the EDS element distribution images of Cr, Co, Ca and Dy elements corresponding thereto are shown in the figure. Wherein, a is the SEM photo, and b-e are the EDS element distribution images of Cr, Co, Ca and Dy elements, respectively. As can be seen from the figure, Dy and Ca are rich and coincide, and Cr and Co are uniformly distributed in the whole sample.

[0083] Due to Figure 4 The phenomenon of coincidence of the distribution of Dy and Ca is not intuitive enough, referring to Figure 5 The SEM photo of the Dy2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 pressure-sensitive ceramic sample (sample No. Zn-Dy) prepared in Example 2 of the present application and the EDS line scanning spectrum of Zn, Cr, Co, Ca and Dy elements corresponding thereto are shown in the figure. As can be seen from the figure, Dy and Ca are rich and coincide, and Cr and Co are uniformly distributed. Combined with Figure 3 and 4 It can be known that part of Ca is solid-solved in Dy2O3 phase and distributed at the ZnO grain boundary. Dy is solid-solved in Eu2O3 phase to form immobile acceptor defects Ca × Dy , which leads to the formation of a stable double Schottky barrier structure at the ZnO grain boundary, further improving the electrical properties of the sample.

[0084] Example 3

[0085] A preparation method of CeO2-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-sensitive ceramic, comprising the following steps:

[0086] 1) According to the best performance formula 96.40 mol% ZnO + 0.10 mol% Cr2O3+ 0.50 mol% Co3O4+ 2.00 mol% CaCO3+ 1.00 mol% CeO2, 10 g of oxide powder is weighed, and is placed in a nylon ball mill tank together with 18 g of deionized water, 2 g of polyvinyl alcohol (PVA) aqueous solution with a mass percentage of 4%, and 0.1 g of tributyl phosphate, a planetary ball mill is used to mill at a speed of 500 r / min for 6 h, the obtained slurry is dried at 90 ℃ for 12 h to obtain a mixed powder;

[0087] 2) The mixed powder obtained in step 1) is ground and sieved, and uniform powder between 40 meshes and 120 meshes is selected, 0.15 g of the powder is uniaxially pressed to obtain a green body with a diameter of 8.0 mm and a thickness of 0.75 mm;

[0088] 3) The green body prepared in step 2) is subjected to degassing at 600 ℃ with a rising rate of 3 ℃ / min for 5 h, then is raised to 1100 ℃ with a rising rate of 3 ℃ / min for 3 h, and then is lowered to 500 ℃ with a lowering rate of 3 ℃ / min, and then is naturally cooled to room temperature, to obtain the CeO2-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-sensitive ceramic, and the sample number is Zn-Ce.

[0089] Referring to Figure 6 The XRD spectrum of the CeO2-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-sensitive ceramic sample (sample number: Zn-Ce) prepared in Example 3 of the present application; it can be seen from the spectrum that, in addition to the ZnO phase, the sample mainly contains the CeO2 phase.

[0090] Referring to Figure 7 The SEM photo of the CeO2-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-sensitive ceramic sample (sample number: Zn-Ce) prepared in Example 3 of the present application and the EDS element distribution images of Cr, Co, Ca and Ce elements corresponding thereto; wherein a is the SEM photo, b~e are the EDS element distribution images of Cr, Co, Ca and Ce elements, respectively; it can be seen from the figure that the enrichment degree of Ce and Cr is relatively high. Figure 6 It can be known that part of Cr is solid-solved in the CeO2 phase and is distributed at the ZnO grain boundary. Cr is solid-solved in the CeO2 phase to form immobile acceptor defects Cr × Cewhich leads to the formation of stable double Schottky barrier structure at the grain boundary of ZnO, further improving the electrical properties of the sample.

[0091] Example 4

[0092] The preparation method of the Eu2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-sensitive ceramic comprises the following steps:

[0093] 1) According to the best performance formula 95.00 mol% ZnO + 0.25 mol% Cr2O3+ 1.00 mol% Co3O4+ 2.50 mol% CaCO3+ 1.25 mol% Eu2O3, 10 g of oxide powder is weighed, and is placed in a nylon ball mill tank together with 20 g of deionized water, 1 g of polyvinyl alcohol (PVA) aqueous solution with a mass percentage of 6%, and 0.3 g of tributyl phosphate, and a planetary ball mill is used to mill at a speed of 500 r / min for 6 h. The obtained slurry is dried at 90 ℃ for 12 h to obtain a mixed powder;

[0094] 2) The mixed powder obtained in step 1) is ground and sieved, and uniform powder between 40 meshes and 120 meshes is selected. 0.25 g of the powder is uniaxially pressed to obtain a green body with a diameter of 8.0 mm and a thickness of 1.25 mm;

[0095] 3) The green body prepared in step 2) is degassed at 600 ℃ for 2 h with a temperature rising rate of 3 ℃ / min, and then is heated to 1100 ℃ for 1 h with a temperature rising rate of 3 ℃ / min, and then is cooled to 500 ℃ with a temperature falling rate of 3 ℃ / min, and then is naturally cooled to room temperature to obtain the Eu2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-sensitive ceramic, which is sample No. Zn-Eu-1.

[0096] Example 5

[0097] The preparation method of the CeO2-doped ZnO-Cr2O3-Co3O4-CaCO3 voltage-sensitive ceramic comprises the following steps:

[0098] 1) According to the best performance formula 98.00 mol% ZnO + 0.10 mol% Cr2O3+ 0.15 mol% Co3O4+ 1.00 mol% CaCO3+ 0.75 mol% CeO2, 10 g of oxide powder is weighed, and is placed in a nylon ball mill tank together with 15 g of deionized water, 2 g of polyvinyl alcohol (PVA) aqueous solution with a mass percentage of 3%, and 0.2 g of tributyl phosphate, and a planetary ball mill is used to mill at a speed of 500 r / min for 6 h. The obtained slurry is dried at 90 ℃ for 12 h to obtain a mixed powder;

[0099] 2) Grind and sieve the mixed powder obtained in step 1) to select a uniform powder with a mesh size of 40 to 120. Weigh 0.2 g of the powder and uniaxially press it to obtain a green body with a diameter of 8.0 mm and a thickness of 1 mm.

[0100] 3) The green body prepared in step 2) was heated to 600 °C at a rate of 3 °C / min and kept at that temperature for 3 h for debinding. Thereafter, the green body was heated to 1100 °C at a rate of 3 °C / min and kept at that temperature for 2 h. The green body was then cooled to 500 °C at a rate of 3 °C / min and then naturally cooled to room temperature to obtain CeO2-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramics, sample numbered Zn-Ce-1.

[0101] Finally, the oxide-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramics prepared in Examples 1-5 were polished, silver-coated, dried, and then silver electrodes were sintered to reduce the silver oxide to elemental silver and tightly adhere to the sample surface as an electrode to facilitate testing of its electrical properties.

[0102] See Table 1 for the electrical properties of high performance bismuth-free ZnO-Cr2O3 based varistor ceramics obtained in Examples 1-5. It can be seen from the table that the nonlinear coefficient of the sample obtained in Example 1 is α 79, breakdown field strength E 1 mA 1830 V / mm, leakage current density J L 0.76 μA / cm 2 ; The nonlinear coefficient of the sample obtained in Example 2 α 66, breakdown field strength E 1 mA 1507 V / mm, leakage current density J L 0.16 μA / cm 2 ; The nonlinear coefficient of the sample obtained in Example 3 α 97, breakdown field strength E 1 mA 1441 V / mm, leakage current density J L 0.12 μA / cm 2 ; The nonlinear coefficient of the sample obtained in Example 4 α 67, breakdown field strength E 1 mA 1688V / mm, leakage current density J L 0.65 μA / cm 2 ; The nonlinear coefficient of the sample obtained in Example 5 α 73, breakdown field strengthE 1 mA was 1523 V / mm, and the leakage current density J L was 0.45 μA / cm 2 In summary, the bismuth-free ZnO-Cr2O3-based varistor ceramics prepared in Examples 1-5 have excellent electrical properties such as higher nonlinearity coefficient (66-97), higher breakdown field strength (1441-1830 V / mm), and smaller leakage current density (0.12-0.76 μA / cm 2 ), and have broad application prospects in miniaturized electronic circuit systems. In combination with Figures 1-7 It can be seen that, for the above bismuth-free ZnO-Cr2O3-based varistor ceramics, the Ca × Eu , Ca × Dy , Cr × Ce and other immobile acceptor defects are formed in the Eu2O3, Dy2O3 or CeO2 oxides enriched at the ZnO grain boundaries, resulting in the formation of stable double Schottky barrier structure at the ZnO grain boundaries, and further resulting in the excellent electrical properties of the samples.

[0103] Table 1 Electrical properties of high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics prepared in Examples 1-3

[0104]

[0105] Comparative Example 1

[0106] The preparation method of the Er2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic is different from that of Example 1 in that, in step 1), the best performance formula is 96.65 mol% ZnO + 0.10 mol% Cr2O3+ 0.50 mol% Co3O4+ 2.00 mol% CaCO3+ 0.75 mol% Er2O3; and in step 3), the Er2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic is obtained, and the sample number is Zn-Er.

[0107] Referring to Figure 8 is the XRD spectrum of the Er2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic sample (sample number: Zn-Er) prepared in Comparative Example 1; it can be seen from the spectrum that, in addition to the ZnO phase, the sample mainly contains the Er2O3 phase.

[0108] Referring to Figure 9The SEM photos of the Er2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic sample (sample No.: Zn-Er) prepared in Comparative Example 1 and the EDS element distribution images of Cr, Co, Ca and Er elements corresponding thereto; wherein a is the SEM photo, and b-e are the EDS element distribution images of Cr, Co, Ca and Er elements, respectively; it can be seen from the figure that the enrichment degree of Er and Co is relatively high. In combination with the XRD spectrum of the sample, it can be known that part of Co is dissolved in the Eu2O3 phase and distributed at the ZnO grain boundaries. However, the defects Co Figure 8 , Cr formed by the dissolution of Co and Cr in the Tm2O3 phase are not donors or acceptors, and cannot improve the double Schottky barrier characteristics at the ZnO grain boundaries. The improvement of the double Schottky barrier characteristics at the ZnO grain boundaries in the sample is mainly due to the increase of the grain boundary adsorbed oxygen content, and the improvement of the electrical properties is limited. × Eu The double Schottky barrier characteristics at the ZnO grain boundaries cannot be improved, and the electrical properties cannot be improved.

[0109] Comparative Example 2

[0110] The preparation method of the Tm2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic is different from that of Example 1. In step 1), the best performance formula is 96.90 mol% ZnO + 0.10 mol% Cr2O3+ 0.50 mol% Co3O4+ 2.00 mol% CaCO3+ 0.50 mol% Tm2O3; in step 3), the Tm2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic is obtained, and the sample No. is Zn-Tm.

[0111] See Figure 10 The XRD spectrum of the Tm2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic sample (sample No.: Zn-Tm) prepared in Comparative Example 2; it can be seen from the spectrum that, in addition to the ZnO phase, the sample mainly contains the Tm2O3 phase.

[0112] See Figure 11 The SEM photos of the Tm2O3-doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic sample (sample No.: Zn-Tm) prepared in Comparative Example 2 and the EDS element distribution images of Cr, Co, Ca and Tm elements corresponding thereto; wherein a is the SEM photo, and b-e are the EDS element distribution images of Cr, Co, Ca and Tm elements, respectively; it can be seen from the figure that the enrichment of Tm and Co, Cr exists to a certain extent. In combination with the XRD spectrum of the sample, it can be known that part of Co and Cr is dissolved in the Tm2O3 phase and distributed at the ZnO grain boundaries. However, the defects Co Figure 10 , Cr formed by the dissolution of Co and Cr in the Tm2O3 phase are not donors or acceptors, and cannot improve the double Schottky barrier characteristics at the ZnO grain boundaries. The improvement of the double Schottky barrier characteristics at the ZnO grain boundaries in the sample is mainly due to the increase of the grain boundary adsorbed oxygen content, and the improvement of the electrical properties is limited. × Tm , Cr× Tm It is neither a donor nor an acceptor and cannot improve the double Schottky barrier characteristics at the ZnO grain boundary. The improvement of the double Schottky barrier characteristics at the ZnO grain boundary in the sample is mainly attributed to the increase in the adsorbed oxygen content at the grain boundary, which has limited improvement on the electrical properties.

[0113] The varistor ceramic samples obtained in Comparative Example 1 and Comparative Example 2 (sample numbers are Zn-Er and Zn-Tm) are polished, silver-coated, and dried, and then silver electrodes are infiltrated to reduce the silver oxide to elemental silver. The silver electrodes are then tightly attached to the sample surface as electrodes to facilitate testing of their electrical properties.

[0114] See Table 2 for the electrical properties of the varistor ceramic samples obtained under the best performance formula in Comparative Example 1 and Comparative Example 2. It can be seen from the table that the nonlinear coefficient of the sample obtained in Comparative Example 1 is α 50, breakdown field strength E 1 mA 1617 V / mm, leakage current density J L 0.79 μA / cm 2 ; The nonlinear coefficient of the sample obtained in Comparative Example 2 α 54, breakdown field strength E 1 mA 1778 V / mm, leakage current density J L 0.97 μA / cm 2 Compared with the electrical properties of the bismuth-free ZnO-Cr2O3-based varistors obtained in Examples 1-5, the varistors obtained in Comparative Examples 1 and 2 have lower nonlinear coefficients, higher leakage current densities, and poorer overall electrical properties. Figures 8-11 It can be seen that for the bismuth-free ZnO-Cr2O3-based varistor ceramics obtained in Comparative Examples 1 and 2, the defects Co formed at the ZnO grain boundaries × Eu / Co × Tm / Cr × Tm Neither a donor nor an acceptor, it can't improve the double Schottky barrier properties at the ZnO grain boundaries. The improvement in the double Schottky barrier properties at the ZnO grain boundaries in the sample is primarily attributed to the increased adsorbed oxygen content at the grain boundaries, which has limited impact on electrical properties. Furthermore, the high mobility of adsorbed oxygen at the grain boundaries leads to structural instability of the double Schottky barrier at the grain boundaries.

[0115] Table 2 Electrical properties of the varistor ceramic samples obtained under the best performance formula in Comparative Example 1 and Comparative Example 2

[0116]

[0117] In the present application, by selecting a suitable doped oxide, the resulting Eu2O3 / Dy2O3 / CeO2 doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic sample forms immovable acceptor defects Ca × Eu / Ca × Dy / Cr × Ce , resulting in the formation of a stable double Schottky barrier structure at the ZnO grain boundary, further improving the electrical properties of the sample, obtaining a varistor ceramic sample with a higher nonlinear coefficient (66-97), a higher breakdown field strength (1441-1830 V / mm), and a smaller leakage current density (0.12-0.76 μA / cm 2 × Eu / Co × Tm / Cr × Tm neither a donor nor an acceptor, and cannot improve the double Schottky barrier characteristics at the ZnO grain boundary; and the improvement of the double Schottky barrier characteristics at the ZnO grain boundary in the sample is mainly due to the increase in the amount of grain boundary adsorbed oxygen, which has limited improvement on the electrical properties. At the same time, the grain boundary adsorbed oxygen has strong mobility, leading to the problem of instability of the grain boundary double Schottky barrier structure. Compared with the present application, the Er2O3 / Tm2O3 doped ZnO-Cr2O3-Co3O4-CaCO3 varistor ceramic sample has a lower nonlinear coefficient (50-54), a larger leakage current density (0.79-0.97 μA / cm 2 ), and poor comprehensive electrical properties. Through the comparison of the examples of the present application and the comparative examples 1 and 2, it is found that the correct selection of the doped oxide is crucial and is the technical core of the present application, which also provides a very important technical idea for the optimization of the electrical properties of varistor ceramics of different systems.

[0118] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.​

Claims

1. A method for preparing high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics by controlling defects, characterized in that: The following steps are involved: ZnO, doped oxide, polyvinyl alcohol aqueous solution, tributyl phosphate and deionized water are ball-milled to obtain a mixed powder; after drying, sieving and granulating, and pressing to form, an oxide-doped ZnO green body is obtained; and after debinding and sintering, a high-performance bismuth-free ZnO-Cr2O3-based varistor ceramic is obtained. The doped oxides include several oxides composed of divalent and trivalent ions with small radius, and one oxide composed of trivalent or tetravalent ions with large radius; The divalent and trivalent small radius ion oxides are Cr2O3, Co3O4 and CaCO3; The trivalent or tetravalent large-radius ion oxide is any one of Eu2O3, Dy2O3 and CeO2.

2. The method for preparing high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics by controlling defects according to claim 1, characterized in that: The molar percentage of ZnO and doped oxide is (95.00-98.00): (2.00-5.00); The mass of the polyvinyl alcohol aqueous solution is 10%-20% of the total mass of ZnO and doped oxides; The mass of the tributyl phosphate is 1%-3% of the total mass of ZnO and the doped oxide; The mass of the deionized water is 150%-200% of the total mass of ZnO and the doped oxide.

3. The method for preparing high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics by controlling defects according to claim 1, characterized in that: The mass percentage of the polyvinyl alcohol aqueous solution is 3%-6%; The doping oxide does not include oxides that are volatile during the sintering process.

4. The method for preparing high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics by controlling defects according to claim 1, characterized in that: The ball milling time is 6-8 hours; the sieving granulation condition is: selecting uniform powder of 40 mesh to 120 mesh.

5. The method for preparing high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics by controlling defects according to claim 1, characterized in that: The pressing conditions are as follows: 0.15-0.25 g of the mixed powder is pressed into an oxide-doped ZnO green compact with a diameter of 8.0 mm and a thickness of 0.75-1.25 mm.

6. The method for preparing high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics by controlling defects according to claim 1, characterized in that: The debinding temperature is 600°C and the debinding time is 2-5 hours.

7. The method for preparing high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics by controlling defects according to claim 1, characterized in that: The sintering temperature is 1100° C., and the sintering time is 1-3 h.

8. A high-performance bismuth-free ZnO-Cr2O3-based varistor ceramic, characterized in that: The method for preparing high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics by controlling defects according to any one of claims 1 to 7 is adopted, wherein the high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics have Cr-rich solid solution at the ZnO grain boundaries. 3+ 、Co 3+ and Ca 2+ Oxide; the oxide includes Eu2O3, Dy2O3 or CeO2; and forming an immovable acceptor defect Ca in the ZnO grain boundary enriched Eu2O3, Dy2O3 or CeO2 × Eu , Ca × Dy or Cr × Ce .

9. The high-performance bismuth-free ZnO-Cr2O3-based varistor ceramic according to claim 8, characterized in that: The high-performance bismuth-free ZnO-Cr2O3-based varistor ceramic has a nonlinear coefficient of 66-97, a breakdown field strength of 1441-1830 V / mm, and a leakage current density of 0.12-0.76 μA / cm 2 .

10. Use of high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics prepared by the method for preparing high-performance bismuth-free ZnO-Cr2O3-based varistor ceramics by controlling defects according to any one of claims 1 to 7 in power electronic systems.

Images