A broadband wave absorbing plate and its preparation method

Abstract

The present application relates to the field of magnetic material technology, and provides a broadband wave absorbing plate, including a first wave absorbing material layer, a second wave absorbing material layer, a third wave absorbing material layer, and a fourth wave absorbing material layer stacked sequentially from bottom to top, wherein the upper surface of each layer of the first wave absorbing material layer, the second wave absorbing material layer, and the third wave absorbing material layer is provided with a plurality of micro-prisms, and the upper surface of each layer of the first wave absorbing material layer, the second wave absorbing material layer, the third wave absorbing material layer, and the fourth wave absorbing material layer is provided with a plurality of micro-holes; wherein the magnetic permeability of the first wave absorbing material layer, the second wave absorbing material layer, the third wave absorbing material layer, and the fourth wave absorbing material layer decreases sequentially. The present application adopts a multi-layer wave absorbing material composite design, and its electromagnetic parameters increase sequentially from the outermost absorption interface to the inner side, which can enhance the absorption of electromagnetic waves and reduce the reflection ratio. The micro-prisms and micro-holes designed on the upper surface of the wave absorbing material can also enhance scattering and reduce reflection, thereby achieving ‑20dB absorption under low thickness conditions.

Description

Technical Field

[0001] The present application belongs to the technical field of magnetic materials, and in particular relates to a broadband wave absorbing plate and a preparation method thereof. Background Art

[0002] In recent years, with the rapid development of the electronic information industry, a large number of man-made electromagnetic waves have flooded the environment, such as radio, television, communication equipment, transportation equipment, household appliances, etc., as well as high-frequency equipment in industry, scientific research, and medical treatment, power systems, etc. Electromagnetic radiation has affected people's daily lives, not only causing varying degrees of damage to human health, but also causing varying degrees of information confidentiality and location leakage, affecting national information security. In order to reduce the adverse effects of electromagnetic waves, a large number of scholars have focused their research on electromagnetic wave shielding and electromagnetic absorption. Because electromagnetic absorption materials convert the energy of electromagnetic waves into other forms of energy, they can fundamentally eliminate the hazards caused by electromagnetic radiation and have gradually become the focus of research.

[0003] At present, the application scenarios of electromagnetic absorption materials are complex and varied. For example, under harsh environmental conditions such as high temperature, acidic or alkaline environment, strong light, etc., the absorbing plate is not only required to have absorbing characteristics such as broadband, but also required to have special physical properties such as high temperature resistance and corrosion resistance. In order to meet the demand for absorbing materials in these application scenarios, the design of composite of organic matter such as rubber and resin with metal powder, or the design of composite of multilayer ferrite materials is often adopted. However, metal powder is easy to oxidize, organic matter is easy to decompose under high temperature or light conditions, and due to the low resistivity and poor insulation of metal, it is easy to cause corrosion, resulting in the design of composite of organic matter and metal powder not suitable for use in harsh environments. Although the ferrite absorbing material has stable chemical properties, the ferrite material is dense and heavy, which not only makes it difficult to achieve broadband absorption, but also makes it difficult to achieve broadband absorption under thin thickness conditions. Summary of the invention

[0004] The purpose of the present application is to provide a broadband absorbing plate and a preparation method thereof, aiming to solve the problem that the existing absorbing plates are difficult to achieve broadband absorption under harsh environmental conditions.

[0005] In order to achieve the above application purpose, the technical solution adopted in this application is as follows:

[0006] In a first aspect, the present application provides a broadband absorbing plate, comprising a first absorbing material layer, a second absorbing material layer, a third absorbing material layer and a fourth absorbing material layer stacked in sequence from bottom to top, wherein the upper surface of each of the first absorbing material layer, the second absorbing material layer and the third absorbing material layer is provided with a plurality of micro-prisms, and the upper surface of each of the first absorbing material layer, the second absorbing material layer, the third absorbing material layer and the fourth absorbing material layer is provided with a plurality of micro-holes; wherein the magnetic permeabilities of the first absorbing material layer, the second absorbing material layer, the third absorbing material layer and the fourth absorbing material layer decrease in sequence.

[0007] In a second aspect, the present application provides a method for preparing a broadband absorbing plate, comprising the following steps:

[0008] preparing the first absorbing material layer;

[0009] Preparing the second absorbing material layer on the upper surface of the first absorbing material layer;

[0010] Prepare the third absorbing material layer on the upper surface of the second absorbing material layer;

[0011] The fourth absorbing material layer is prepared on the upper surface of the third absorbing material layer.

[0012] The broadband absorbing plate provided in the first aspect of the present application adopts a composite design of multi-layer absorbing materials, and its electromagnetic parameters increase successively from the outermost absorption interface to the inner side, which is conducive to enhancing the absorption of electromagnetic waves and reducing the reflection ratio; and the design of the multi-layer absorbing material can also further reduce the reflection by utilizing the interference and destructive interaction of electromagnetic waves between multiple interfaces, thereby widening the absorption bandwidth; at the same time, a plurality of micro-prisms provided on the upper surfaces of the first absorbing material layer, the second absorbing material layer, and the third absorbing material layer can further enhance scattering and reduce reflection. In addition, a plurality of micro-holes provided on the upper surfaces of the first absorbing material layer, the second absorbing material layer, the third absorbing material layer, and the fourth absorbing material layer are equivalent to smaller prisms, which can further enhance scattering and reduce reflection, thereby achieving -20dB absorption under the condition of low thickness.

[0013] The second aspect of the present application provides a method for preparing a wide-band absorbing plate, in which a first absorbing material layer, a second absorbing material layer, a third absorbing material layer and a fourth absorbing material layer are prepared in order from bottom to top to form a stacked design structure, and the interference destructive electromagnetic waves between the multiple interfaces formed between the absorbing material layers can be used to further reduce reflection, thereby widening the absorption bandwidth. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0015] Figure 1 is a cross-sectional view of a broadband absorbing plate provided in an embodiment of the present application;

[0016] Figure 2 is a cross-sectional view of another broadband absorbing plate provided in an embodiment of the present application;

[0017] Figure 3 yes Figure 1 A local enlarged schematic diagram of the middle A;

[0018] Figure 4 is a top view of a mask provided in an embodiment of the present application;

[0019] Figure 5 is a cross-sectional view of a mask provided by an embodiment of the present invention;

[0020] Among them, the reference numerals in the figure are:

[0021] 10—first absorbing material layer, 11—second absorbing material layer, 12—third absorbing material layer, 13—fourth absorbing material layer, 14—micro-prisms; 15—micro-holes;

[0022] La—micro-prism height, Lb—bottom platform width, Lc—top platform width, Ld—interval width. DETAILED DESCRIPTION

[0023] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0024] In this application, the term "and / or" describes the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0025] In this application, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can all mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, c can be single or multiple, respectively.

[0026] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0027] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "said" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0028] The weight of the relevant components mentioned in the embodiment description of the present application can not only refer to the specific content of each component, but also represent the proportional relationship between the weights of the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the embodiment description of the present application, it is within the scope disclosed in the embodiment description of the present application. Specifically, the mass described in the embodiment description of the present application can be a mass unit known in the chemical industry such as μg, mg, g, kg, etc.

[0029] The terms "first" and "second" are used only for descriptive purposes to distinguish objects such as substances from each other, and should not be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX may also be referred to as the second XX, and similarly, the second XX may also be referred to as the first XX. Thus, features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0030] A first aspect of the present application provides a broadband absorbing plate, such as Figure 1-3As shown, the broadband absorbing plate comprises a first absorbing material layer 10, a second absorbing material layer 11, a third absorbing material layer 12 and a fourth absorbing material layer 13 which are stacked in sequence from bottom to top, wherein the upper surface of each of the first absorbing material layer 10, the second absorbing material layer 11 and the third absorbing material layer 12 is provided with a plurality of micro-prisms 14, and the upper surface of each of the first absorbing material layer 10, the second absorbing material layer 11, the third absorbing material layer 12 and the fourth absorbing material layer 13 is provided with a plurality of micro-holes 15; wherein the magnetic permeabilities of the first absorbing material layer 10, the second absorbing material layer 11, the third absorbing material layer 12 and the fourth absorbing material layer 13 decrease in sequence.

[0031] The broadband absorbing plate provided in the first aspect of the present application adopts a composite design of multi-layer absorbing materials, and its electromagnetic parameters increase successively from the outermost absorption interface to the inner side, which is conducive to enhancing the absorption of electromagnetic waves and reducing the reflection ratio; and the design of the multi-layer absorbing material can also further reduce the reflection by utilizing the interference and destructive interaction of electromagnetic waves between multiple interfaces, thereby widening the absorption bandwidth; at the same time, a plurality of micro-prisms designed on the upper surfaces of the first absorbing material layer, the second absorbing material layer, and the third absorbing material layer can further enhance scattering and reduce reflection. In addition, a plurality of micro-holes are provided on the upper surfaces of the first absorbing material layer, the second absorbing material layer, the third absorbing material layer, and the fourth absorbing material layer, which are equivalent to smaller prisms, can further enhance scattering and reduce reflection, thereby achieving -20dB absorption under the condition of low thickness.

[0032] In some embodiments, the height of each micro-prism on the upper surface of each layer of the first absorbing material layer, the second absorbing material layer and the third absorbing material layer is 20 μm to 50 μm, the width of the bottom platform is 30 μm to 100 μm, and the width of the top platform is 10 μm to 20 μm; the spacing width between adjacent micro-prisms is 8 μm to 12 μm. The height of the micro-prism can be 20 μm, 30 μm, 40 μm or 50 μm, the width of the bottom platform is 30 μm, 50 μm, 70 μm, 90 μm or 100 μm, the width of the top platform is 10 μm, 12 μm, 14 μm, 16 μm or 20 μm, and the spacing width is 8 μm, 10 μm or 12 μm, but it is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable. See Appendix. Figure 4-5 , the height of the microprism is Figure 5 La in, the width of the bottom platform is Figure 5 Lb in the figure, the width of the top platform is as follows Figure 4 and Figure 5 Lc in, the interval width is as follows Figure 5 Ld in.

[0033] In the embodiment of the present application, both the top platform and the bottom platform are square. The structural parameters of the micro-prism provided in the embodiment of the present application can realize the multiple oscillation attenuation and scattering of the incident electromagnetic wave by the absorbing material, thereby reducing the reflection ability of the absorbing material to microwaves, and then improving the attenuation performance of the absorbing material to microwaves, and achieving the purpose of having better absorbing performance.

[0034] In some embodiments, the micro-prisms on the upper surface of the first absorbing material layer, the micro-prisms on the upper surface of the second absorbing material layer, and the micro-prisms on the upper surface of the third absorbing material layer are aligned or staggered.

[0035] See attached Figure 1 The micro-prisms on the upper surface of the first absorbing material layer, the micro-prisms on the upper surface of the second absorbing material layer, and the micro-prisms on the upper surface of the third absorbing material layer are aligned.

[0036] See attached Figure 2 The micro-prisms on the upper surface of the first absorbing material layer, the micro-prisms on the upper surface of the second absorbing material layer, and the micro-prisms on the upper surface of the third absorbing material layer are staggered.

[0037] In some embodiments, the magnetic permeability of the first absorbing material layer is 1600 H / m to 2000 H / m. The magnetic permeability may be 1600 H / m, 1700 H / m, 1800 H / m, 1900 H / m or 2000 H / m, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0038] In some embodiments, the magnetic permeability of the second absorbing material layer is 300H / m to 400H / m. The magnetic permeability may be 300H / m, 330H / m, 350H / m, 380H / m or 400H / m, etc., but is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable.

[0039] In some embodiments, the magnetic permeability of the third absorbing material layer is 50H / m to 70H / m. The magnetic permeability may be 50H / m, 60H / m, 65H / m or 70H / m, etc., but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0040] In some embodiments, the magnetic permeability of the fourth absorbing material layer is 9H / m to 12H / m. The magnetic permeability may be 9H / m, 10H / m, 11H / m or 12H / m, etc., but is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable.

[0041] Magnetic permeability is a physical quantity that indicates the magnetic properties of a magnetic medium, and the magnetic permeability μ is equal to the ratio of the differential of the magnetic induction intensity B in the magnetic medium to the differential of the magnetic field intensity H. Generally, the higher the magnetic permeability, the more outstanding the absorbing performance of the absorbing material. In the embodiment of the present application, a multi-layer absorbing material design is selected, and the electromagnetic parameters increase from the outermost absorption interface (the fourth absorbing material layer) to the innermost. According to the design of the magnetic permeability of the absorbing material, the absorption of electromagnetic waves can be enhanced and the reflection ratio can be reduced, thereby achieving broadband absorption.

[0042] In some embodiments, the thickness of the broadband absorbing plate is 4 mm to 7 mm. Due to the high density and thickness of the ferrite itself, it is not only difficult to meet the requirements of "thin and light" for absorbing materials, but also difficult to apply in some special application scenarios, such as the fuselage of an aircraft. If it is prepared into a thin sheet with a smaller thickness, it is difficult to achieve broadband microwave absorption due to its unsatisfactory absorbing performance. The present application achieves broadband absorption of the absorbing plate under the condition of a thickness of 4 mm to 7 mm through a composite design of multi-layer absorbing materials and a magnetic permeability design in which electromagnetic parameters increase successively from the outermost absorption interface (the fourth absorbing material layer) to the innermost.

[0043] In some embodiments, the depth of the micropores is ≤ 0.15 mm. The depth of the micropores may be 0.15 mm, 0.14 mm, 0.13 mm, 0.12 mm, 0.08 mm, or 0.06 mm, etc., but is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable. The micropores on the surface of the absorbing material are equivalent to smaller prisms, which can further enhance scattering and reduce reflection, thereby achieving -20 dB absorption under low thickness conditions.

[0044] In some embodiments, the first absorbing material layer, the second absorbing material layer, the third absorbing material layer and the fourth absorbing material layer are provided with a plurality of micropores on their upper surfaces. Figure 3 The micro-holes can be arranged on the top planes, inclined surfaces or between adjacent micro-prisms of the first absorbing material layer, the second absorbing material layer and the third absorbing material layer; and for the fourth absorbing material layer, a plurality of micro-holes can be arranged on the entire surface, and the distribution density of the micro-holes can be adjusted according to the absorbing performance of the prepared broadband absorbing plate. These micro-holes can be understood as grooves with a depth of ≤0.15mm.

[0045] In some embodiments, the first absorbing material layer includes a first ferrite, and the first ferrite includes the following components, measured by molar percentage of each component in the first ferrite: Fe2O3: 50mol% to 51mol%, NiO: 11mol% to 14mol%, and ZnO: 35mol% to 38mol%. The content of each component of the ferrite in the bottom layer of the absorbing plate provided in the embodiment of the present application enables the first absorbing material layer to achieve a magnetic permeability in the range of 1600H / m to 2000H / m during the layer-by-layer calcination process of the absorbing plate, thereby realizing multiple absorption interfaces.

[0046] In some embodiments, the second absorbing material layer includes a second ferrite, and the second ferrite includes the following components in terms of molar percentage of each component in the second ferrite: Fe2O3: 48mol% to 49mol%, NiO: 13mol% to 14mol%, CuO: 3mol% to 5mol%, and ZnO: 30mol% to 36mol%. The content of each component of the ferrite in the second layer of the absorbing plate provided in the embodiment of the present application enables the second absorbing material layer to achieve a magnetic permeability in the range of 300H / m to 400H / m during the layer-by-layer calcination process of the absorbing plate, thereby achieving multiple absorption interfaces.

[0047] In some embodiments, the third absorbing material layer includes a third ferrite, and the third ferrite includes the following components in terms of molar percentage of each component in the third ferrite: Fe2O3: 46mol% to 48mol%, NiO: 18mol% to 23mol%, CuO: 7mol% to 9mol%, and ZnO: 20mol% to 26mol%. The content of each component of the ferrite in the third layer of the absorbing plate provided in the embodiment of the present application enables the third absorbing material layer to achieve a magnetic permeability in the range of 50H / m to 70H / m during the two calcination processes of the absorbing plate, thereby achieving multiple absorption interfaces.

[0048] In some embodiments, the fourth absorbing material includes a third ferrite layer, and the fourth ferrite includes the following components, based on the molar percentage of each component in the fourth ferrite: Fe2O3: 47mol% to 50mol%, NiO: 36mol% to 39mol%, CuO: 4mol% to 7mol%, and ZnO: 4mol% to 7mol%. The content of each component of the ferrite in the fourth layer of the absorbing plate provided in the embodiment of the present application enables the fourth absorbing material layer to achieve a magnetic permeability within the range of 9H / m to 12H / m after calcination, thereby realizing multiple absorption interfaces.

[0049] In a second aspect, the present application provides a method for preparing a broadband absorbing plate, comprising the following steps:

[0050] preparing a first absorbing material layer;

[0051] Preparing a second absorbing material layer on the upper surface of the first absorbing material layer;

[0052] Preparing a third absorbing material layer on the upper surface of the second absorbing material layer;

[0053] A fourth absorbing material layer is prepared on the upper surface of the third absorbing material layer.

[0054] The second aspect of the present application provides a method for preparing a wide-band absorbing plate, in which a first absorbing material layer, a second absorbing material layer, a third absorbing material layer and a fourth absorbing material layer are prepared in order from bottom to top to form a stacked design structure, and the interference destructive electromagnetic waves between the multiple interfaces formed between the absorbing material layers can be used to further reduce reflection, thereby widening the absorption bandwidth.

[0055] In some embodiments, a first ferrite is mixed with a first dispersant, a first adhesive, a first plasticizer, and a first organic solvent, and then subjected to a first ball milling treatment to obtain a first slurry; after the first slurry is cast through a mask, a silicon nitride particle layer is spread on the formed first cast layer, and then a first calcination treatment is performed to obtain a first absorbing material layer.

[0056] In some embodiments, the step of preparing the second absorbing material layer includes: mixing the second ferrite with the second dispersant, the second adhesive, the second plasticizer and the second organic solvent, and performing a second ball milling process to obtain a second slurry; casting the second slurry on the surface of the first absorbing material layer through a mask, laying a silicon nitride particle layer on the formed second casting layer, and then performing a second calcination process to obtain the second absorbing material layer.

[0057] In some embodiments, the step of preparing the third absorbing material layer includes: mixing a third ferrite with a third dispersant, a third adhesive, a third plasticizer and a third organic solvent, and then performing a third ball milling process to obtain a third slurry; casting the third slurry on the surface of the second absorbing material layer through a mask, laying a silicon nitride particle layer on the formed third casting layer, and then performing a third calcination process to obtain the third absorbing material layer.

[0058] In some embodiments, the step of preparing the fourth absorbing material layer includes: mixing a fourth ferrite with a fourth dispersant, a fourth adhesive, a fourth plasticizer and a fourth organic solvent, and then performing a fourth ball milling process to obtain a fourth slurry; casting the fourth slurry on the surface of the third absorbing material layer through a mask, laying a silicon nitride particle layer on the formed fourth cast layer, and then performing a fourth calcination process to obtain the fourth absorbing material layer.

[0059] Ferrite materials are highly active, have small particle sizes, and are easy to agglomerate. Therefore, the present application obtains a uniform slurry by ball-milling the ferrite material with a dispersant, a plasticizer, an adhesive, and an organic solvent. The ferrite material can be evenly dispersed in the slurry, and a smooth cast film can be obtained in the subsequent casting process.

[0060] In some embodiments, the conditions of the first calcination treatment are: calcination at a temperature of 1240°C to 1260°C for 1.5h to 2.5h, and a heating rate of 3 to 5°C / min. The temperature may be 1210°C, 1250°C or 1260°C, and the calcination time may be 1.5h, 2h or 2.5h, but are not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0061] In some embodiments, the second calcination treatment is performed under the conditions of 1040°C to 1060°C for 1.5 to 2.5 hours, with a heating rate of 3 to 5°C / min. The temperature may be 1040°C, 1050°C or 1060°C, and the calcination time may be 1.5 hours, 2 hours or 2.5 hours, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0062] In some embodiments, the conditions of the third calcination treatment are: calcination at a temperature of 910°C to 930°C for 1.5h to 2.5h, and a heating rate of 3 to 5°C / min. The temperature may be 910°C, 920°C or 930°C, and the calcination time may be 1.5h, 2h or 2.5h, but are not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0063] In some embodiments, the fourth calcination treatment is performed under the conditions of 850°C to 870°C for 1.5 to 2.5 hours, with a heating rate of 3 to 5°C / min. The temperature may be 850°C, 860°C or 870°C, and the calcination time may be 1.5 hours, 2 hours or 2.5 hours, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0064] The design of multi-layer absorbing materials is to cast one layer and then sinter it in the order from bottom to top during the casting process, and then continue casting on the previous casting layer and sinter it again, and so on, until a multi-layer composite absorbing material is formed. In this process, the first absorbing material layer will be sintered 4 times, the second absorbing material layer will be sintered 3 times, and the third absorbing material layer will be sintered 2 times. The difficulty of designing in this process is that after the first absorbing material layer, the second absorbing material layer or the third absorbing material layer is sintered multiple times, the properties of its ferrite, such as magnetic permeability, will be affected, making it difficult for the entire multi-layer composite material to achieve broadband absorption. In the present application, the components of each layer of ferrite material are limited, and a method is designed in which the sintering temperature is sequentially decreased. Since the sintering temperature of the ferrite of the upper layer is relatively low, when the ferrite of the lower layer is repeatedly sintered together with the upper layer, the characteristics of the ferrite of the lower layer are slightly affected, and the absorbing material can realize multiple oscillation attenuation and scattering of the incident electromagnetic wave, thereby reducing the reflection ability of the absorbing material to microwaves, and then improving the attenuation performance of the absorbing material to microwaves, thereby achieving the purpose of having better absorbing performance.

[0065] In some embodiments, the first dispersant is Triton, the first adhesive is polyvinyl butyral, the first plasticizer is dibutyl phthalate, and the first organic solvent is anhydrous ethanol; and the added amount of the first dispersant is 1wt% to 3wt% of the total mass of the first ferrite, the added amount of the first adhesive is 15wt% to 20wt% of the total mass of the first ferrite, the added amount of the first plasticizer is 1wt% to 2wt% of the total mass of the first ferrite, and the added amount of the first organic solvent is 70wt% to 100wt% of the total mass of the first ferrite.

[0066] In some embodiments, the second dispersant is Triton, the second adhesive is polyvinyl butyral, the second plasticizer is dibutyl phthalate, and the second organic solvent is anhydrous ethanol; and the added amount of the second dispersant is 1wt% to 3wt% of the total mass of the second ferrite, the added amount of the second adhesive is 15wt% to 20wt% of the total mass of the second ferrite, the added amount of the second plasticizer is 1wt% to 2wt% of the total mass of the second ferrite, and the added amount of the second organic solvent is 70wt% to 100wt% of the total mass of the second ferrite.

[0067] In some embodiments, the third dispersant is Triton, the third adhesive is polyvinyl butyral, the third plasticizer is dibutyl phthalate, and the third organic solvent is anhydrous ethanol; and the added amount of the third dispersant is 1wt% to 3wt% of the total mass of the third ferrite, the added amount of the third adhesive is 15wt% to 20wt% of the total mass of the third ferrite, the added amount of the third plasticizer is 1wt% to 2wt% of the total mass of the third ferrite, and the added amount of the third organic solvent is 70wt% to 100wt% of the total mass of the third ferrite.

[0068] In some embodiments, the fourth dispersant is Triton, the fourth adhesive is polyvinyl butyral, the fourth plasticizer is dibutyl phthalate, and the fourth organic solvent is anhydrous ethanol; and the added amount of the fourth dispersant is 1wt% to 3wt% of the total mass of the fourth ferrite, the added amount of the fourth adhesive is 15wt% to 20wt% of the total mass of the fourth ferrite, the added amount of the fourth plasticizer is 1wt% to 2wt% of the total mass of the fourth ferrite, and the added amount of the fourth organic solvent is 70wt% to 100wt% of the total mass of the fourth ferrite.

[0069] Since the ferrite material has high activity and small particle size after ball milling, it is easy to form agglomeration in the slurry. Therefore, in the embodiment of the present application, Triton is used as the first dispersant, the second dispersant, the third dispersant or the fourth dispersant, so that the ferrite material can be evenly dispersed in the slurry, and the ferrite powder is prevented from agglomerating to cause defects such as particles and wrinkles on the surface of the product during casting. Polyvinyl butyral is used as the first adhesive, the second adhesive, the third adhesive or the fourth adhesive, which can be completely decomposed and volatilized during the calcination process, which is conducive to the ferrite material forming a flat absorbent during the calcination process. material layer; using dibutyl phthalate as the first plasticizer, the second plasticizer, the third plasticizer or the fourth plasticizer can ensure that the ferrite will not shrink significantly or shrink unevenly during the sintering process, thereby avoiding cracking or deformation of the cast layer; using anhydrous ethanol as the first organic solvent, the second organic solvent, the third organic solvent or the fourth organic solvent can improve the volatilization rate of the slurry during the casting process, provide sufficient time for forming a silicon carbide particle layer on the surface of the casting layer, and improve the dispersibility of the ferrite to obtain a uniform slurry.

[0070] In some embodiments, the conditions of the first ball milling treatment, the second ball milling treatment, the third ball milling treatment and the fourth ball milling treatment are: the time is 4h to 6h, and the rotation speed is 250rpm to 350rpm. Since the ferrite material has high activity, small particle size and is easy to agglomerate, the ferrite material and each component are subjected to ball milling treatment respectively, so that the ferrite material can be evenly dispersed in the slurry, which is conducive to obtaining a flat cast film.

[0071] In some embodiments, the shape of the mask is a tic-tac-toe structure, and the middle part of the tic-tac-toe structure is a square, and the width of the square is the width of the top platform. Figure 4 As shown, the cross-sectional diagram is Figure 5 As shown, the prepared slurry is cast through a mask, and the slurry flows into the "mouth" structure in the middle of the "well" structure to form a prism of a specific shape, and the micro-prism structure can also be adjusted by changing the size and thickness of the mask.

[0072] In some embodiments, the thickness of the silicon nitride particle layer is 100 μm to 150 μm. The thickness of the silicon nitride particle layer can be 100 μm, 110 μm, 120 μm, 130 μm, 140 μm or 150 μm, but is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable.

[0073] In some embodiments, the particle size of the silicon nitride particles in the silicon nitride particle layer is ≤ 0.15 mm. The particle size of the silicon nitride particles can be 0.15 mm, 0.12 mm, 0.10 mm, 0.08 mm or 0.06 mm, but is not limited to the listed values, and other values ​​not listed in the numerical range are also applicable.

[0074] In this embodiment, before the cast film is completely dried, a layer of silicon nitride particles with a thickness of 100μm to 150μm is embedded in the ferrite green body, and the particle size of the silicon nitride particles is ≤0.15mm. After calcination, a number of microporous structures can be formed on the surface of the absorbing material. The specific size of the micropores can be adjusted according to the silicon carbide particle size to form a rough interface of the prism surface, which can further enhance scattering and reduce reflection, thereby achieving -20dB broadband absorption.

[0075] In some specific embodiments, silicon nitride particles are evenly spread on the surface of the ferrite green body, lightly pressed with a mask and a flat plate (the pressure of the flat plate is 0.1Mpa to 0.2Mpa), and then an air flow is used to remove the unembedded silicon carbide particles, thereby achieving a layer of silicon nitride particles evenly spread on the surface of the cast layer.

[0076] In some embodiments, the preparation method of the first ferrite, the second ferrite, the third ferrite or the fourth ferrite is as follows:

[0077] Step S01, independently providing raw material components of the first ferrite, the second ferrite, the third ferrite or the fourth ferrite;

[0078] Step S02, the powders formed by mixing the raw material components are subjected to a fifth ball milling treatment with water, and then dried and sieved. After the fifth calcination treatment, the powders are subjected to a sixth ball milling treatment, dried and sieved to obtain the first ferrite, the second ferrite, the third ferrite or the fourth ferrite respectively.

[0079] In the present application, each component in the ferrite is subjected to a fifth ball milling treatment, a calcination treatment, and a sixth ball milling treatment, so that atoms such as Ni, Cu, and Zn enter the lattice of Fe atoms to form ferrite materials with different magnetic permeabilities.

[0080] In step S01, according to the components of the ferrite described above, chemically pure raw materials including Fe2O3, NiO, CuO, and ZnO are weighed respectively, which will not be described in detail here.

[0081] In some embodiments, the conditions of the fifth calcination treatment are: temperature of 900°C to 1000°C, and time of 1.5h to 2.5h. Specifically, the calcination treatment temperature may be 900°C, 930°C, 950°C or 1000°C, and the time may be 1.5h, 2h or 2.5h, but it is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable. After calcination of the components in the ferrite, atoms such as Ni, Cu, and Zn enter the lattice of Fe atoms to form ferrite materials with different magnetic permeabilities, and it can also prevent the ferrite material from being deformed due to layer-by-layer calcination during the casting process.

[0082] In some embodiments, the fifth ball milling conditions are: time of 50 min to 70 min, rotation speed of 250 rpm to 350 rpm. After the fifth ball milling treatment, the components in the ferrite not only promote uniform mixing between the components, but also improve the activity of the components, which is conducive to the formation of ferrite materials with specific magnetic permeability in the later calcination.

[0083] In some embodiments, the D50 of the powder after the sixth ball milling treatment is 1 μm to 2 μm, and the rotation speed is 250 rpm to 350 rpm. The activity of the ferrite material can be improved by performing the sixth ball milling treatment after calcination.

[0084] In some embodiments, the aperture of the sieve is 30-50 mesh. The aperture of the sieve can be 30 mesh, 40 mesh or 50 mesh, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0085] The following describes the invention in conjunction with specific embodiments.

[0086] Example 1

[0087] The embodiment of the present invention provides a broadband wave absorbing plate and a method for preparing the same.

[0088] A broadband absorbing plate, the cross-section of which is shown in FIG. Figure 1 As shown, the broadband absorbing plate comprises a first absorbing material layer 10, a second absorbing material layer 11, a third absorbing material layer 12 and a fourth absorbing material layer 13 stacked in sequence from bottom to top; wherein the magnetic permeabilities of the first absorbing material layer 10, the second absorbing material layer 11, the third absorbing material layer 12 and the fourth absorbing material layer 13 are 2000H / m, 400H / m, 70H / m and 12H / m respectively. A plurality of micro-prisms 14 are provided on the upper surface of each of the first absorbing material layer 10, the second absorbing material layer 11, the third absorbing material layer 12 and the fourth absorbing material layer 13, and a plurality of micro-holes 15 are provided on the upper surface of each of the first absorbing material layer 10, the second absorbing material layer 11, the third absorbing material layer 12 and the fourth absorbing material layer 13.

[0089] The height of each micro-prism 14 is 40 μm, the width of the bottom platform is 60 μm, and the width of the top platform is 10 μm; the interval width between adjacent micro-prisms 14 is 10 μm. The upper surface of each layer of the first absorbing material layer 10, the second absorbing material layer 11, the third absorbing material layer 12 and the fourth absorbing material layer 13 is provided with a plurality of micro-holes 15, and the depth of the micro-holes 15 is 0.15 mm. The thickness of the broadband absorbing plate is 5 mm, and the thickness of each layer of absorbing material is the same.

[0090] The first absorbing material layer 10 includes a first ferrite, and in terms of molar percentage of each component in the first ferrite, the first ferrite includes the following components: iron oxide: 51 mol %, nickel oxide: 11 mol %, and zinc oxide: 38 mol %.

[0091] The second absorbing material layer 11 includes a second ferrite, and in terms of molar percentage of each component in the second ferrite, the second ferrite includes the following components: iron oxide: 49 mol %, nickel oxide: 13 mol %, copper oxide: 3 mol %, and zinc oxide: 35 mol %.

[0092] The third absorbing material layer 12 includes a third ferrite, and in terms of molar percentage of each component in the third ferrite, the third ferrite includes the following components: iron oxide: 47 mol %, nickel oxide: 18 mol %, copper oxide: 9 mol %, and zinc oxide: 26 mol %.

[0093] The fourth absorbing material layer 13 includes the third ferrite, and the fourth ferrite includes the following components in terms of molar percentage: iron oxide: 50 mol%, nickel oxide: 39 mol%, copper oxide: 7 mol%, and zinc oxide: 4 mol%.

[0094] The first ferrite preparation method comprises the following steps:

[0095] Step S01, providing the raw material components of the first ferrite;

[0096] Step S02: Add the same amount of water as the powder to the powder formed by mixing the components, perform the fifth ball milling treatment at 300 rpm for 1 hour, dry the material and pass it through a 40-mesh sieve, calcine it at 950°C for 2 hours, and then perform the sixth ball milling treatment at 300 rpm until D50 is 2 μm, and then dry the material and pass it through a 10-mesh sieve to obtain the first ferrite.

[0097] The preparation methods of the second ferrite, the third ferrite and the fourth ferrite are all consistent with the preparation method of the first ferrite, and will not be repeated here.

[0098] A method for preparing a broadband wave absorbing plate comprises the following steps:

[0099] Step S10, mixing the first ferrite provided by the first aspect with Triton, polyvinyl butyral, dibutyl phthalate and anhydrous ethanol respectively, and performing a first ball milling treatment at 350rpm for 5h to obtain a first slurry; wherein the amount of Triton added is 2wt% of the total mass of the first ferrite, the amount of polyvinyl butyral added is 15wt% of the total mass of the first ferrite, the amount of dibutyl phthalate added is 1wt% of the total mass of the first ferrite, and the amount of anhydrous ethanol added is 80wt% of the total mass of the first ferrite;

[0100] The second ferrite, the third ferrite and the fourth ferrite provided in the first aspect are also prepared into the second slurry, the third slurry and the fourth slurry respectively; the preparation process and the components and addition amounts of the dispersant, adhesive, plasticizer and organic solvent are consistent with the preparation process of the first slurry and will not be repeated here.

[0101] Step S20, after casting the first slurry through a mask, evenly spread a layer of silicon nitride particles with a thickness of 100 μm on the formed first casting layer, and then calcine at 1250° C. for 2 hours to obtain a first absorbing material layer, wherein the particle size of the silicon nitride particles is 0.15 mm;

[0102] Step S30, after casting the second slurry on the surface of the first absorbing material layer through a mask, evenly spread a layer of silicon nitride particles with a thickness of 100 μm on the formed second casting layer, the particle size of the silicon nitride particles is 0.15 mm, and then calcined at 1050° C. for 2 hours to obtain a second absorbing material layer;

[0103] Step S40, after casting the third slurry on the surface of the second absorbing material layer through a mask, evenly spread a layer of silicon nitride particles with a thickness of 100 μm on the formed third casting layer, the particle size of the silicon nitride particles is 0.15 mm, and then calcined at 920° C. for 2 hours to obtain the third absorbing material layer;

[0104] Step S50, after casting the fourth slurry on the surface of the third absorbing material layer, evenly spread a layer of silicon nitride particles with a thickness of 100 μm on the formed fourth casting layer, the particle size of the silicon nitride particles is 0.15 mm, and then calcined at 860° C. for 2 hours to obtain a broadband absorbing plate.

[0105] Comparative Example 1

[0106] This comparative example provides a broadband absorbing plate, which differs from Example 1 in that only the second absorbing material layer is used to prepare a 5 mm thick absorbing plate, and no microprisms or micropores are provided, and other slurry components and preparation methods, as well as the preparation method of the broadband absorbing plate, are the same.

[0107] Comparative Example 2

[0108] This comparative example provides a broadband absorbing plate, which is different from Example 1 in that: the upper surfaces of the first absorbing material layer, the second absorbing material layer and the third absorbing material layer are not provided with micro-prisms, the surfaces of the first absorbing material layer, the second absorbing material layer, the third absorbing material layer and the fourth absorbing material layer are not provided with micropores, and the components and preparation methods of each other layer of absorbing material, the components and preparation methods of each layer of slurry, and the preparation method of the broadband absorbing plate are all consistent.

[0109] Comparative Example 3

[0110] This comparative example provides a broadband absorbing plate, which is different from Example 1 in that the upper surfaces of the first absorbing material layer, the second absorbing material layer and the third absorbing material layer are not provided with micro-prisms, and the components and preparation methods of each other layer of absorbing material, the components and preparation methods of each layer of slurry, and the preparation method of the broadband absorbing plate are all consistent.

[0111] Comparative Example 4

[0112] This comparative example provides a broadband absorbing plate, which is different from Example 1 in that the upper surfaces of the first absorbing material layer, the second absorbing material layer, the third absorbing material layer and the fourth absorbing material layer are not provided with micropores, and the components and preparation methods of each other layer of absorbing material, the components and preparation methods of each layer of slurry, and the preparation method of the broadband absorbing plate are all consistent.

[0113] The broadband absorbing plates prepared in Example 1 and Comparative Examples 1 to 4 were respectively prepared into absorbing bricks with a specification of 180mm×180mm, and the absorbing performance was tested respectively. The specific testing process is: the prepared absorbing plates with a length and width of 180mm×180mm are assembled into samples of 900mm×900mm, and a pair of horn antennas are used to test the reflection loss test frequency range (1MHz-1GHz) of the incident electromagnetic wave in a microwave darkroom using the return loss method. The absorption characteristics are usually characterized by the decibel ratio of the intensity of the electromagnetic wave reflected from the surface of the absorbing plate to the intensity of the incident wave. Generally, when it reaches -20dB, it is basically considered that there is no reflection. The wider the -20dB bandwidth, the better the absorption performance. The test results of the absorbing plates prepared in Example 1 and Comparative Examples 1 to 4 are shown in Table 1 below. It can be seen from Table 1 that the broadband absorbing plate prepared in the embodiment of the present application not only has good absorbing performance, but is also suitable for harsh application scenarios and has good absorbing performance in harsh environments, such as cold, high temperature, acid and alkali resistance, etc. It can work for a long time in the range of -55°C to 220°C, and can also work for a long time in different acid and alkali ranges of pH 1 to 14.

[0114] Table 1

[0115]

[0116]

[0117] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A broadband absorbing plate, characterized in that: The broadband wave absorbing plate comprises: a first wave absorbing material layer, a second wave absorbing material layer, a third wave absorbing material layer and a fourth wave absorbing material layer stacked in sequence from bottom to top, wherein the upper surface of each of the first wave absorbing material layer, the second wave absorbing material layer and the third wave absorbing material layer is provided with a plurality of micro-prisms, and the upper surface of each of the first wave absorbing material layer, the second wave absorbing material layer, the third wave absorbing material layer and the fourth wave absorbing material layer is provided with a plurality of micro-holes; wherein the magnetic permeabilities of the first wave absorbing material layer, the second wave absorbing material layer, the third wave absorbing material layer and the fourth wave absorbing material layer are sequentially decreased; The height of each micro-prism on the upper surface of each layer of the first absorbing material layer, the second absorbing material layer and the third absorbing material layer is 20 μm to 50 μm, the width of the bottom platform is 30 μm to 100 μm, the width of the top platform is 10 μm to 20 μm, and the interval width between adjacent micro-prisms is 8 μm to 12 μm; The magnetic permeability of the first absorbing material layer is 1600H / m~2000H / m, the magnetic permeability of the second absorbing material layer is 300H / m~400H / m, the magnetic permeability of the third absorbing material layer is 50H / m~70H / m, and the magnetic permeability of the fourth absorbing material layer is 9H / m~12H / m; The thickness of the broadband wave absorbing plate is 4 mm to 7 mm, and the depth of the microholes is ≤0.15 mm.

2. The broadband absorbing plate according to claim 1, wherein: The micro-prisms on the upper surface of the first absorbing material layer, the micro-prisms on the upper surface of the second absorbing material layer, and the micro-prisms on the upper surface of the third absorbing material layer are aligned or staggered.

3. The broadband absorbing plate according to any one of claims 1 to 2, characterized in that: The first absorbing material layer includes a first ferrite, and the first ferrite includes the following components, based on the molar percentage of each component in the first ferrite: iron oxide: 50mol%~51mol%, nickel oxide: 11mol%~14mol%, zinc oxide: 35mol%~38mol%; and / or The second absorbing material layer includes a second ferrite, and the second ferrite includes the following components, based on the molar percentage of each component in the second ferrite: iron oxide: 48mol%~49mol%, nickel oxide: 13mol%~14mol%, copper oxide: 3mol%~5mol%, zinc oxide: 30mol%~36mol%; and / or The third absorbing material layer includes a third ferrite, and the third ferrite includes the following components, measured by molar percentage of each component in the third ferrite: iron oxide: 46mol%~48mol%, nickel oxide: 18mol%~23mol%, copper oxide: 7mol%~9mol%, zinc oxide: 20mol%~26mol%; and / or The fourth absorbing material layer includes a fourth ferrite, and based on the molar percentage of each component in the fourth ferrite, the fourth ferrite includes the following components: iron oxide: 47mol%~50mol%, nickel oxide: 36mol%~39mol%, copper oxide: 4mol%~7mol%, and zinc oxide: 4mol%~7mol%.

4. A method for preparing a broadband absorbing plate according to any one of claims 1 to 3, characterized in that: The following steps are involved: preparing the first absorbing material layer; Preparing the second absorbing material layer on the upper surface of the first absorbing material layer; Prepare the third absorbing material layer on the upper surface of the second absorbing material layer; The fourth absorbing material layer is prepared on the upper surface of the third absorbing material layer.

5. The method for preparing the broadband absorbing plate according to claim 4, characterized in that: The steps of preparing the first absorbing material layer include: mixing the first ferrite with the first dispersant, the first adhesive, the first plasticizer and the first organic solvent, and then performing a first ball milling process to obtain a first slurry; casting the first slurry through a mask, laying a silicon nitride particle layer on the formed first casting layer, and then performing a first calcination process to obtain the first absorbing material layer; and / or The step of preparing the second absorbing material layer comprises: mixing a second ferrite with a second dispersant, a second adhesive, a second plasticizer and a second organic solvent, and then performing a second ball milling process to obtain a second slurry; casting the second slurry on the surface of the first absorbing material layer through a mask, laying a silicon nitride particle layer on the formed second casting layer, and then performing a second calcination process to obtain the second absorbing material layer; and / or The steps of preparing the third absorbing material layer include: mixing a third ferrite with a third dispersant, a third adhesive, a third plasticizer and a third organic solvent, and then performing a third ball milling process to obtain a third slurry; casting the third slurry on the surface of the second absorbing material layer through a mask, laying a silicon nitride particle layer on the formed third casting layer, and then performing a third calcination process to obtain the third absorbing material layer; and / or The step of preparing the fourth absorbing material layer includes: mixing a fourth ferrite with a fourth dispersant, a fourth adhesive, a fourth plasticizer and a fourth organic solvent, and then performing a fourth ball milling process to obtain a fourth slurry; casting the fourth slurry on the surface of the third absorbing material layer through a mask, laying a silicon nitride particle layer on the formed fourth casting layer, and then performing a fourth calcination process to obtain the fourth absorbing material layer.

6. The method for preparing the broadband absorbing plate according to claim 5, wherein: The first calcination treatment conditions are: calcination at a temperature of 1240° C. to 1260° C. for 1.5 h to 2.5 h, with a heating rate of 3 to 5° C. / min; and / or The second calcination treatment conditions are: calcination at a temperature of 1040° C. to 1060° C. for 1.5 h to 2.5 h, with a heating rate of 3 to 5° C. / min; and / or The conditions of the third calcination treatment are: calcination at a temperature of 910° C. to 930° C. for 1.5 h to 2.5 h, with a heating rate of 3 to 5° C. / min; and / or The fourth calcination treatment is carried out under the conditions of: calcining at a temperature of 850° C. to 870° C. for 1.5 h to 2.5 h, and a heating rate of 3° C. to 5° C. / min.

7. The method for preparing a broadband absorbing plate according to claim 5, wherein: The first dispersant is Triton, the first adhesive is polyvinyl butyral, the first plasticizer is dibutyl phthalate, and the first organic solvent is anhydrous ethanol; and the first dispersant is added in an amount of 1wt% to 3wt% of the total mass of the first ferrite, the first adhesive is added in an amount of 15wt% to 20wt% of the total mass of the first ferrite, the first plasticizer is added in an amount of 1wt% to 2wt% of the total mass of the first ferrite, and the first organic solvent is added in an amount of 70wt% to 100wt% of the total mass of the first ferrite; and / or The second dispersant is Triton, the second adhesive is polyvinyl butyral, the second plasticizer is dibutyl phthalate, and the second organic solvent is anhydrous ethanol; and the amount of the second dispersant added is 1wt%~3wt% of the total mass of the second ferrite, the amount of the second adhesive added is 15wt%~20wt% of the total mass of the second ferrite, the amount of the second plasticizer added is 1wt%~2wt% of the total mass of the second ferrite, and the amount of the second organic solvent added is 70wt%~100wt% of the total mass of the second ferrite; and / or The third dispersant is Triton, the third adhesive is polyvinyl butyral, the third plasticizer is dibutyl phthalate, and the third organic solvent is anhydrous ethanol; and the added amount of the third dispersant is 1wt%~3wt% of the total mass of the third ferrite, the added amount of the third adhesive is 15wt%~20wt% of the total mass of the third ferrite, the added amount of the third plasticizer is 1wt%~2wt% of the total mass of the third ferrite, and the added amount of the third organic solvent is 70wt%~100wt% of the total mass of the third ferrite; and / or The fourth dispersant is Triton, the fourth adhesive is polyvinyl butyral, the fourth plasticizer is dibutyl phthalate, and the fourth organic solvent is anhydrous ethanol; and the added amount of the fourth dispersant is 1wt%~3wt% of the total mass of the fourth ferrite, the added amount of the fourth adhesive is 15wt%~20wt% of the total mass of the fourth ferrite, the added amount of the fourth plasticizer is 1wt%~2wt% of the total mass of the fourth ferrite, and the added amount of the fourth organic solvent is 70wt%~100wt% of the total mass of the fourth ferrite.

8. The method for preparing a broadband absorbing plate according to claim 5, characterized in that: The shape of the mask is a tic-tac-toe structure, and the middle part of the tic-tac-toe structure is a square, and the width of the square is the width of the top platform of the micro-prism; and / or The thickness of the silicon nitride particle layer is 100 μm to 150 μm, and the particle size of the silicon nitride particles in the silicon nitride particle layer is ≤0.15 mm.

9. The method for preparing a broadband wave absorbing plate according to any one of claims 5 to 8, characterized in that: The preparation method of the first ferrite, the second ferrite, the third ferrite or the fourth ferrite comprises: mixing the raw material components of the first ferrite, the second ferrite, the third ferrite or the fourth ferrite with water independently, performing a fifth ball milling treatment, then drying and screening, performing a fifth calcination treatment and then performing a sixth ball milling treatment.

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