A high-performance piezoelectric ceramic material based on lead magnesium niobate and lead zirconate titanate and its preparation method

Abstract

This invention discloses a high-performance piezoelectric ceramic material based on lead magnesium niobate and lead zirconate titanate, and its preparation method. The piezoelectric ceramic is composed of PbO, TiO2, ZrO2, Nb2O5, MgCO3, and trace elements such as La, Sm, Sr, Cr, and Ba, with small amounts of SiO2 and Al2O3. This invention modifies the piezoelectric ceramic based on its structure-property relationship through a combination of solid solution and ion doping, and optimizes its piezoelectric properties, high-temperature stability, and mechanical properties by combining advanced processes such as repeated sintering and vacuum evaporation. This piezoelectric ceramic exhibits a high piezoelectric coefficient dp. 33 >850 PC / N, dielectric constant ε r >5100 and electromechanical coupling coefficient k p The temperature is >0.71, while also achieving a high Curie temperature of 210℃ and good mechanical properties. The piezoelectric ceramic of this invention exhibits excellent overall performance and has promising application prospects, making it widely applicable in the fabrication of high-performance piezoelectric devices.

Description

Technical Field

[0001] This invention relates to the field of piezoelectric ceramics in piezoelectric materials, and more specifically to a high-performance piezoelectric ceramic material based on lead magnesium niobate-lead zirconate titanate and its preparation method, particularly to a high-performance piezoelectric ceramic material based on lead magnesium niobate-lead zirconate titanate and its preparation method. Background Technology

[0002] Piezoelectric ceramics possess the piezoelectric effect, enabling the conversion between mechanical and electrical energy. In recent years, piezoelectric ceramic devices have seen increasingly widespread applications in military, automotive, oil exploration, aerospace, and specialized technologies. As application scenarios expand, the performance requirements for piezoelectric ceramics are also increasing; many application areas involve increasingly harsh operating environments for piezoelectric devices. Ensuring the continuous and stable operation of piezoelectric devices under harsh conditions, including maintaining their electrical and mechanical properties and reliability in demanding environments, is crucial. Therefore, developing piezoelectric ceramics with ultra-high piezoelectric coefficients, high Curie temperatures, and high mechanical properties is a critical issue that urgently needs to be addressed.

[0003] High performance requires piezoelectric ceramics to have a large dielectric constant (ε). r ), electromechanical coupling coefficient (k) p ) and piezoelectric coefficient (d 33 High stability requires piezoelectric ceramics to have a high Curie temperature (T). C High stability is essential to avoid Curie phase transitions or mechanical depolarization under high temperatures and pressures, which would degrade the piezoelectric properties of the material. However, there is a limiting relationship between the piezoelectric properties and the Curie temperature of piezoelectric ceramics; traditionally, it has been difficult to simultaneously achieve high piezoelectric properties and high Curie temperatures in the same piezoelectric material. Therefore, the limiting relationship between the piezoelectric properties and the Curie temperature of piezoelectric ceramics has always been a bottleneck that has been difficult to overcome in the field of piezoelectric ceramics.

[0004] The influencing factors and control methods of piezoelectric ceramic performance mainly fall into three categories: preparation process improvement, solid solution modification, and element doping. Preparation process improvement mainly includes improvements in powder preparation, ceramic preparation, ceramic sintering process, and electrode preparation. Current piezoelectric ceramic preparation processes mainly suffer from problems such as lead volatilization due to high sintering temperatures, large lead loss, relatively low ceramic density, poor electrode layer performance leading to low conductivity, high insertion loss, low driving efficiency, and premature device failure. Summary of the Invention

[0005] To address the shortcomings of the prior art, this invention provides a high-performance piezoelectric ceramic material based on lead magnesium niobate and lead zirconate titanate, along with its preparation method. This system effectively avoids the poor performance of piezoelectric ceramics caused by the large-scale volatilization and loss of lead during high-temperature sintering, thereby improving ceramic density and optimizing electrode layer performance. Simultaneously, it yields piezoelectric ceramics possessing high piezoelectric coefficient, high Curie temperature, and high mechanical properties.

[0006] To achieve the above objectives, the first aspect of the present invention provides a high-performance piezoelectric ceramic material based on lead magnesium niobate-lead zirconate titanate, which is achieved by appropriate composite doping, adding trace elements such as the third principal element La, Sm, Sr, Cr, and Ba, and simultaneously doping with SiO2 and Al2O3, with the following general chemical formula:

[0007] (1-x)Pb(Mg 1 / 3 Nb 2 / 3 )O3-xPb(Zr y Ti 1-y )O3-awt.%La d Sm e Sr f Cr g Ba h O3-bwt.%SiO2-cwt.%Al2O3

[0008] Among them: 0.5<x<0.8, 0.3<y<0.8, 0<a<5, 0<b<0.5, 0<c<0.5, 0<d<1, 0<e<1, 0<f<1, 0<g<1, 0<h<1.

[0009] The second aspect of this embodiment provides a method for preparing a high-performance piezoelectric ceramic material based on lead magnesium niobate-lead zirconate titanate, comprising the following steps:

[0010] S1: First ball milling, weigh each raw material according to the stoichiometric ratio, optimize the grinding process of the pre-synthesized powder, establish parameters such as the size distribution of the grinding media balls, stirring speed and time, to ensure that the ultrafine powder has a certain particle size distribution, and obtain the first ball milling material;

[0011] S2: Pre-calcination: The ball milling material obtained in step S1 is dried and then placed in a crucible for pre-calcination to obtain pre-calcined material;

[0012] S3: Secondary ball milling: The pre-burned material from step S2 is crushed, ground, and sieved, then ball milled with deionized water to obtain secondary ball milled material;

[0013] S4: Pressing and molding. After drying the secondary ball milling material, a plasticizer is added and a controllable ultrafine powder spray granulation technology is used to control the microstructure and particle size of the granulated powder particles, improve the uniformity of the formed ceramic body, and improve the density of the ceramic body by a combination of dry pressing or isostatic pressing to obtain a green body.

[0014] S5: Sintering into ceramic. First, the green body obtained in step S4 is subjected to medium-temperature plastic removal. The plasticized green body is then subjected to high-temperature repeated sintering technology. By controlling the sintering temperature and time repeatedly and optimizing the sintering parameters, the piezoelectric ceramic material's electrical properties, such as piezoelectric constant and dielectric constant, are improved, while also possessing advantages such as high structural stability, strong fatigue resistance, and good repeatability. During the high-temperature repeated sintering process, the internal microstructure of the piezoelectric ceramic is further optimized by controlling the temperature and time at each sintering point. The volatilization of low-melting-point substances is strictly controlled during the sintering process, effectively ensuring the stoichiometric composition of the ceramic material and obtaining a ceramic green body with excellent performance.

[0015] S6: Electrode fabrication and polarization: The ceramic blank obtained in step S5 is machined to flatten the two large surfaces. An electrode layer is then fabricated using a vacuum evaporation method to deposit a composite electrode layer. After the electrode is applied, it is first polarized at high temperature in silicone oil, then aged in high and low temperature environments, and finally left to stand at room temperature for 24 hours to obtain a piezoelectric ceramic that meets the performance requirements.

[0016] S7: Piezoelectric and dielectric property testing. The piezoelectric ceramic prepared in step S6 is used for testing piezoelectric and dielectric property parameters.

[0017] S8: Mechanical performance test. The piezoelectric ceramic prepared in step S6 is used for mechanical performance parameter testing.

[0018] In step S1, the raw materials include PbO, TiO2, ZrO2, Nb2O5, MgCO3, SiO2, Al2O3, and trace elements such as La, Sr, Sm, Cr, and Ba. The grinding balls are zirconia balls with a large, medium, and small diameter ratio of 1:2:1, 1:3:1, or 1:4:1. The dispersion medium is deionized water. The mass ratio of raw material:zirconia balls:deionized water is 1:3:3, 1:2:3, 1:2:2, or 1:2:1.5. The rotation speed is 150-300 r / min, and the ball milling time is 18-30 hours.

[0019] In step S2, the initial ball milling material is dried at a temperature of 120-130℃ for 18-24 hours, the pre-calcination process is heated at a rate of 3-5℃ / min, and the material is held at 800-900℃ for 2-3 hours.

[0020] In step S3, the sieve mesh size is 80-100 mesh, the grinding balls are zirconia balls with a large, medium, and small diameter ratio of 1:2:1, 1:3:1, or 1:4:1, the dispersion medium is deionized water, the mass ratio of raw material:zirconia balls:deionized water is 1:3:3, 1:2:3, 1:2:2, or 1:2:1.5, the rotation speed is 150-300 r / min, and the ball milling time is 18-30 hours.

[0021] In step S4, the secondary ball milling material is dried at 120-130℃ for 18-24 hours, and then 5-10% plasticizer is added for spray granulation. Finally, it is pressed into shape under a pressure of 80-150MPa using a combination of dry pressing and isostatic pressing.

[0022] In step S5, the heating rate of the debinding furnace is 1-2℃ / min, and it is held at 600-750℃ for 1-2 hours. Multiple sintering processes are used in the ceramic sintering process. A sealed alumina crucible is used, with zirconia powder placed inside. The green body is then buried in the zirconia and the crucible is covered to prevent the loss of low-melting-point lead and ensure the high performance of the piezoelectric ceramic. Multiple sintering processes are employed, controlling the time and temperature repeatedly to guarantee the high performance of the resulting piezoelectric ceramic. Nine temperature zones are set up for multiple sintering processes, with each zone heating at a rate of 3-8℃ / min. The high-temperature zone is held at 1200-1350℃ for 1-3 hours.

[0023] In step S6, the electrode layer is fabricated using a vacuum evaporation composite electrode layer method. The electrode layer is deposited sequentially from bottom to top with Cr-Ni, Au, and Ag metal layers, and the total thickness of the electrode layer is 150-170 nm. Polarization is performed under the protection of highly insulating silicone oil, with a polarization time of 10-30 min, a polarization temperature of 25-120 °C, and a polarization voltage of 10-30 kV / cm.

[0024] In step S7, the piezoelectric and dielectric parameters include the piezoelectric coefficient (d). 33 Curie temperature (T) C ), electromechanical coupling coefficient (k) p ), dielectric constant (ε) r ).

[0025] In step S8, the mechanical performance parameters include elastic compliance constant, Poisson's ratio, and density.

[0026] The beneficial effects of this invention are:

[0027] 1) This invention overcomes the key technologies of doping modification with trace elements such as La, Sm, and Sr, and SiO2 and Al2O3, successfully solving the technical problem that traditional piezoelectric ceramics cannot simultaneously achieve piezoelectric and mechanical properties. While ensuring mechanical strength, it significantly improves piezoelectric performance. By forming a quasi-isomorphic phase boundary structure similar to the trigonal and tetragonal phases of traditional piezoelectric ceramics, the doping of multiple ions and components causes compositional inhomogeneity in the local heterogeneous structure, altering the perovskite crystal structure, regulating the movement of electric domains and domain walls, resulting in the formation of an internal microscopic glass lattice structure, and ensuring a certain level of mechanical strength, thus enabling the piezoelectric ceramic performance to reach the international advanced level.

[0028] 2) This invention has developed a complete set of technologies for controllable granulation-grinding-forming-synthesis-sintering of nanopowders, which effectively controls the microstructure and particle size of sintered powders, ensuring the grain size, distribution, orientation, grain boundary characteristics and material density of piezoelectric ceramics, and improving the uniformity of the formed ceramic structure.

[0029] a. Controllable nanopowder granulation technology: Utilizing orthogonal experiments, the proportions of chemically pure PbO, MgCO3, Nb2O5, ZrO2, TiO2, Sm2O3, Cr2O3, SiO2, and Al2O3 in the raw materials are optimized, and wet ball milling is used for mixing to optimize the synthesis process parameters; the structural characteristics and grain size of the generated sintered powder are controlled, and the microstructure and particle size of the sintered powder are further controlled through crushing, grinding, and spray granulation to improve the uniformity and consistency of the formed ceramic structure.

[0030] b. Optimize the powder grinding process, refine the granulation and forming process of sintered powder, and establish parameters such as the size distribution of the grinding media balls, stirring speed and time to ensure the grain size, orientation, grain distribution, grain strength and grain boundary size, thereby improving the uniformity and consistency of the chemical composition and microstructure of piezoelectric ceramic materials.

[0031] c. The repeated synthesis and sintering technology, by controlling the sintering temperature and time of nine temperature zones and protecting low-melting-point lead from loss, optimizes the sintering parameters of the powder, thereby improving the electrical properties of piezoelectric ceramic materials such as piezoelectric constant and dielectric constant, while also giving them advantages such as high structural stability, strong fatigue resistance, and good repeatability.

[0032] 3) A novel electrode fabrication process using vacuum evaporation composite metal films solves the problems of reduced insulation caused by silver paste penetration into piezoelectric ceramics during the silver infiltration electrode process. This process also prevents silver ion migration under high humidity conditions, effectively improving the weather resistance of piezoelectric devices. It ensures that the ceramic sheet remains highly insulated and leak-proof during long-term high-potential power supply, allowing for normal operation. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of the present invention clearer, further detailed description is provided in conjunction with the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0034] The embodiments of this application will be described in detail below. It should be noted that the following embodiments are only used to explain this application and do not constitute a limitation on this application.

[0035] This application achieves high-performance piezoelectric ceramics by combining advanced and specialized preparation processes. Solid solution modification primarily involves forming multiphase solid solutions with other perovskite phases through solid solution. By adjusting the solid solution amount, quasi-isomorphic phase boundaries (MPBs) are formed, giving the piezoelectric ceramic domains more polarization orientations and forming a polar nanodomain structure. Simultaneously, the potential energy curve between the two phases becomes gentler, which is conducive to polarization rotation, resulting in piezoelectric ceramics with high voltage and dielectric properties.

[0036] Elemental doping mainly alters the perovskite crystal structure through high-valence donor doping, low-valence acceptor doping, and equivalent doping, thereby regulating the movement of electric domains and domain walls by forming oxygen vacancies and lead vacancies. At the same time, the different charges, radii, and interactions with surrounding atoms of the doped heteroatoms disrupt the original ferroelectric ordered state, enhance the disorder of the local structure, and further optimize the piezoelectric properties, high-temperature stability, and mechanical properties of piezoelectric ceramics.

[0037] By using solid solution modification and element doping, high-performance piezoelectric ceramic components were developed. Combined with advanced and special preparation processes, these methods address issues such as lead volatilization and large lead loss due to high sintering temperatures, low ceramic density, and poor electrode layer performance. Simultaneously, piezoelectric ceramics with high piezoelectricity, high stability, and high mechanical properties were obtained, which has significant research value and enhances the core competitiveness of high-performance smart materials and devices.

[0038] Example 1

[0039] This embodiment provides a high-performance piezoelectric ceramic material based on lead magnesium niobate-lead zirconate titanate and its preparation method, the chemical formula of which is:

[0040] 0.35Pb(Mg 1 / 3 Nb 2 / 3 O3-0.65Pb(Zr) 0.43 Ti 0.57 O3-2.5wt.%La 0.8 Sm 0.8 Sr 0.1 Cr 0.2 Ba 0.2 O3-0.1wt.%SiO2-0.05wt.%Al2O3

[0041] The specific method is as follows:

[0042] S1: The raw materials are PbO, TiO2, ZrO2, Nb2O5, MgCO3, SiO2, Al2O3, and trace elements such as La, Sr, Sm, Cr, and Ba. The grinding balls are zirconium oxide, with a ball diameter ratio of large, medium, and small of 1:3:1. The dispersion medium is deionized water, and the mass ratio of raw material:zirconia balls:deionized water is 1:2:2. The rotation speed is 200 r / min, and the ball milling time is 24 hours.

[0043] S2: The initial ball milling material drying temperature is 120℃, the drying time is 24 hours, the pre-calcination process heating rate is 4℃ / min, and it is held at 800℃ for 2 hours.

[0044] S3: The sieve mesh size is 80 mesh, the grinding balls are zirconia balls with a diameter ratio of large, medium and small of 1:3:1, the dispersion medium is deionized water, the mass ratio of raw material:zirconia balls:deionized water is 1:2:2, the rotation speed is 200 r / min, and the ball milling time is 24 hours.

[0045] S4: The secondary ball milling material is dried at 120℃ for 24 hours, then 6% plasticizer is added for spray granulation. Finally, it is pressed into shape using a combination of dry pressing and isostatic pressing at 120MPa pressure.

[0046] S5: The debinding furnace has a heating rate of 1.5℃ / min and holds at 700℃ for 1.5 hours. Multiple sintering processes are used during the ceramic sintering process. A sealed alumina crucible is used, with zirconia powder placed inside. The green body is then buried in the zirconia and the crucible is covered to preserve lead and performance. Multiple sintering processes are employed, with controlled time and temperature to ensure the performance of the resulting piezoelectric ceramic. The heating rate for these multiple sintering processes is 4℃ / min, and the ceramic is held at 1250℃ for 2 hours.

[0047] S6: The electrode layer is fabricated using a vacuum evaporation composite electrode layer method. The electrode layer is deposited with Cr-Ni, Au, and Ag metal layers sequentially from bottom to top, with a total thickness of approximately 150 nm. Polarization is performed under the protection of insulating silicone oil for 30 min at a polarization temperature of 80 °C and a polarization voltage of 20 kV / cm.

[0048] S7: The piezoelectric and dielectric parameters tested include the piezoelectric coefficient (d). 33 Curie temperature (T) C Electromechanical coupling coefficient (kp), dielectric constant (ε) r ).

[0049] S8: The mechanical property parameters tested include elastic compliance constant, Poisson's ratio, and density.

[0050] Example 2

[0051] This embodiment provides a high-performance piezoelectric ceramic material based on lead magnesium niobate-lead zirconate titanate and its preparation method, the chemical formula of which is:

[0052] 0.35Pb(Mg 1 / 3 Nb 2 / 3 O3-0.65Pb(Zr) 0.43 Ti 0.57 O3-2.5wt.%La 0.8 Sm 0.8 Sr 0.1 Cr 0.2 Ba 0.2 O3-0.1wt.%SiO2-0.05wt.%Al2O3

[0053] The specific implementation method differs from Example 1 in that the grinding balls are zirconia balls with a diameter order of large, medium, and small of 1:2:1; the mass ratio of raw material, zirconia balls, and deionized water is 1:2:3; and the rotation speed is 300 r / min. The heating rate of the debinding furnace is 1℃ / min, and the heating rate for repeated sintering is 5℃ / min, with a holding time of 2 hours. The thickness of the composite electrode layer is approximately 170 nm.

[0054] Example 3

[0055] This embodiment provides a high-performance piezoelectric ceramic material based on lead magnesium niobate-lead zirconate titanate and its preparation method, the chemical formula of which is:

[0056] 0.4Pb(Mg 1 / 3 Nb 2 / 3 O3-0.6Pb(Zr) 0.42 Ti 0.58 O3-2.5wt.%La 0.8 Sm 0.8 Sr 0.1 Cr 0.2 Ba 0.2 O3-0.1wt.%SiO2-0.05wt.%Al2O3

[0057] The specific implementation method is the same as in Example 1.

[0058] Piezoelectric and mechanical property testing

[0059] The piezoelectric ceramics prepared in Examples 1 to 3 were tested for piezoelectric and mechanical properties after being aged for 24 hours. The test results are shown in Table 1.

[0060] Table 1. Test results of piezoelectric and mechanical properties in the examples.

[0061]

[0062] This invention modifies piezoelectric ceramics based on structure-property relationships through a combination of solid solution and ion doping, and optimizes the piezoelectric properties, high-temperature stability, and mechanical properties of the ceramics by incorporating advanced processes such as repeated sintering and vacuum evaporation. The resulting piezoelectric ceramic exhibits a high piezoelectric coefficient dp. 33 >850 PC / N, dielectric constant ε r >5100 and electromechanical coupling coefficient k p >0.71, while achieving a high Curie temperature of 210℃ and good mechanical properties (elastic compliance constant S). ij >17×10 -12 m 2 / N, Poisson's ratio μ=0.37, density ρ>7.6g / cm³ 3 The piezoelectric ceramic of this invention has excellent comprehensive performance and great application prospects, and can be widely used in the preparation of high-performance piezoelectric devices.

[0063] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a high-performance piezoelectric ceramic material based on lead magnesium niobate-lead zirconate titanate, characterized in that, The general chemical formula of the high-performance piezoelectric ceramic material based on lead magnesium niobate and lead zirconate titanate is as follows: (1-x)Pb(Mg 1 / 3 Nb 2 / 3 )O3-xPb(Zr y Ti 1-y )O3-awt.%La d Sm e Sr f Cr g Ba h O3-bwt.%SiO2-cwt.%Al2O3 Where: 0.5 < x < 0.8, 0.3 < y < 0.8, 0 < a < 5, 0 < b < 0.5, 0 < c < 0.5, 0 < d < 1, 0 < e < 1, 0 < f < 1, 0 < g < 1, 0 < h < 1; including the following steps: S1: First ball milling: Weigh each raw material according to the stoichiometric ratio, establish the parameters of grinding media ball size distribution, stirring speed and time, and ensure that the ultrafine powder has a certain particle size distribution to obtain the first ball milling material; S2: Pre-calcination: The ball milling material obtained in step S1 is dried and then placed in a crucible for pre-calcination to obtain pre-calcined material; S3: Secondary ball milling: The pre-burned material from step S2 is crushed, ground, and sieved, then ball milled with deionized water to obtain secondary ball milled material; S4: Pressing and molding. After drying the secondary ball milling material, a plasticizer is added and a controllable ultrafine powder spray granulation technology is used to control the microstructure and particle size of the granulated powder particles. The green body is obtained by a combination of dry pressing and isostatic pressing. S5: Sintering into ceramic. First, the green body obtained in step S4 is subjected to medium-temperature plastic removal. The green body after plastic removal is sintered repeatedly at high temperature to obtain a ceramic green body with excellent performance. S6: Fabricate and polarize the electrodes. The ceramic blank prepared in step S5 is machined to flatten the two large surfaces of the ceramic. Then, the electrode layer is fabricated by vacuum evaporation of the composite electrode layer. After the electrode is attached, it is placed in silicone oil for high-temperature polarization, then placed in high and low temperature environments for aging, and finally left to stand at room temperature to obtain a piezoelectric ceramic that meets the performance requirements. In step S2, the initial ball milling material drying temperature is 120-130℃, the drying time is 18-24 hours, the pre-firing process heating rate is 3-5℃ / min, and the temperature is held at 800-900℃ for 2-3 hours. In step S3, the sieve mesh size is 80-100 mesh, the grinding balls are zirconium oxide, the ratio of large, medium, and small ball diameters is 1:2:1, 1:3:1, or 1:4:1, the dispersion medium is deionized water, the mass ratio of raw material: zirconium balls: deionized water is 1:3:3, 1:2:3, 1:2:2, or 1:2:1.5, the rotation speed is 150-300 r / min, and the ball milling time is 18-30 hours. In step S4, the secondary ball milling material is dried at a temperature of 120-130℃ for 18-24 hours, and then 5-10% plasticizer is added for spray granulation; finally, it is pressed into shape using a combination of dry pressing and isostatic pressing under a pressure of 80-150MPa. In step S5, the heating rate of the debinding furnace is 1-2℃ / min, and it is kept at 600-750℃ for 1-2 hours. During the sintering process, a sealed alumina crucible is used, zirconium oxide powder is placed in the crucible, the green body is buried in the zirconium oxide, and the crucible lid is closed. In step S6, the electrode layer is fabricated by vacuum evaporation of the composite electrode layer. The electrode layer is deposited with Cr-Ni, Au, and Ag metal layers from bottom to top, and the total thickness of the electrode layer is 150-170 nm.

2. The preparation method according to claim 1, characterized in that, In step S1, the raw materials include PbO, TiO2, ZrO2, Nb2O5, MgCO3, SiO2, Al2O3, and trace element materials of La, Sr, Sm, Cr, and Ba; the grinding balls are zirconia with a ball diameter ratio of large, medium, and small of 1:2:1, 1:3:1, or 1:4:1; the dispersion medium is deionized water; the mass ratio of raw materials:zirconia balls:deionized water is 1:3:3, 1:2:3, 1:2:2, or 1:2:1.5; the rotation speed is 150-300 r / min; and the ball milling time is 18-30 hours.

3. The preparation method according to claim 1, characterized in that, The process involves multiple sintering cycles with nine temperature zones. Each zone has a heating rate of 3-8℃ / min, and the high-temperature zone is held at 1200-1350℃ for 1-3 hours.

4. The preparation method according to claim 1, characterized in that, In step S6, polarization is carried out under the protection of highly insulating silicone oil, with a polarization time of 10-30 min, a polarization temperature of 25-120℃, and a polarization voltage of 10-30 kV / cm.

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