Low-frequency wave-absorbing material with gradient structure and preparation method thereof

Abstract

The application discloses a kind of low-frequency wave-absorbing materials with gradient structure and preparation method thereof, low-frequency wave-absorbing material includes by outside to inside sequentially layering arrangement impedance matching surface layer, transition intermediate layer and bottom layer;The impedance matching surface layer, transition intermediate layer and bottom layer all include silicon rubber matrix and the core-shell structure iron-nickel alloy wave-absorbing agent dispersed therein;Wherein, from the surface layer to the bottom layer, the mass fraction of the core-shell structure iron-nickel alloy wave-absorbing agent increases in steps, the impedance matching surface layer is 90-95 parts, the transition intermediate layer is 70-80 parts, and the bottom layer is 60-70 parts.The application forms the stepwise distribution of wave-absorbing agent content from high to low from outside to inside by gradient structure design, the high content wave-absorbing agent of surface layer strongly absorbs electromagnetic wave, and the impedance gradient of gradually transition is formed in intermediate layer and bottom layer, so as to improve overall impedance matching effect, effectively improve low-frequency band wave-absorbing performance.

Description

Technical Field

[0001] This invention relates to the field of microwave absorbing materials, and in particular to a low-frequency microwave absorbing material with a gradient structure and its preparation method. Background Technology

[0002] Existing microwave absorbing materials can be broadly classified into two categories: magnetic microwave absorbing materials and dielectric microwave absorbing materials. Iron-nickel alloys (FeNi alloys) are among the most widely used magnetic absorbing agents in the low-frequency microwave absorption field due to their high saturation magnetization, high permeability, and good magnetic loss characteristics. Dispersing iron-nickel alloy powder in flexible matrices such as rubber and resin to create flexible microwave absorbing composite materials retains the microwave absorbing properties of magnetic fillers while imparting flexibility and processability to the material, thus leading to its widespread application in practical engineering.

[0003] However, existing iron-nickel alloy microwave absorbing materials suffer from poor compatibility between the absorbing agent and the matrix, leading to uneven filler distribution. Iron-nickel alloy powder, being a metallic powder with high surface energy, is prone to agglomeration in silicone rubber matrices, making uniform dispersion difficult. Filler agglomeration not only reduces the material's microwave absorption uniformity but also creates stress concentrations in the agglomerated areas, affecting the material's mechanical properties and service life.

[0004] Therefore, existing technologies still need further development and improvement. Summary of the Invention To address the aforementioned technical problems, this invention proposes a low-frequency absorbing material with a gradient structure and its preparation method, thereby solving the problem of poor compatibility between the absorbing agent and the matrix in existing technologies, leading to uneven filler distribution. Specifically: In a first aspect, a low-frequency absorbing material with a gradient structure includes an impedance matching surface layer, a transition intermediate layer, and a bottom layer stacked sequentially from the outside to the inside. The impedance matching surface layer, transition intermediate layer and bottom layer all include a silicone rubber matrix and a core-shell structured iron-nickel alloy microwave absorber dispersed therein. The mass fraction of the core-shell iron-nickel alloy absorbing agent decreases in a stepwise manner from the impedance matching surface layer to the bottom layer, with 90-95 parts in the impedance matching surface layer, 70-80 parts in the transition intermediate layer, and 60-70 parts in the bottom layer.

[0005] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The purpose and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0006] As a preferred technical solution, the low-frequency absorbing material further comprises, in part, the impedance matching surface layer, the transition intermediate layer, and the bottom layer, the following components: The mixture contains 5-8 parts silicone rubber, 1-3 parts catalyst, 0.3-0.8 parts inhibitor, and 0.5-1 parts silane coupling agent.

[0007] As a preferred technical solution, the low-frequency absorbing material, wherein the core-shell structured iron-nickel alloy absorbing agent comprises an iron-nickel alloy powder core and a silicon oxide coating layer covering the surface of the core, the thickness of the silicon oxide coating layer being 5-15 nm.

[0008] As a preferred technical solution, in the low-frequency absorbing material, the absorbing agent has a differentiated physical morphology in each layer. Specifically, the iron-nickel alloy powder in the impedance matching surface layer is a flake-shaped powder with a particle size of 270 mesh; the iron-nickel alloy powder in the transition intermediate layer and the bottom layer is a spherical powder with a particle size of 200 mesh.

[0009] As a preferred technical solution, in the low-frequency absorbing material, the thickness of the impedance matching surface layer is 0.1-0.3mm, the thickness of the transition intermediate layer is 0.1-0.2mm, and the thickness of the bottom layer is 0.1-0.3mm.

[0010] As a preferred technical solution, in the low-frequency absorbing material, the iron-nickel alloy powder is selected from one or more of FeNi50 alloy powder and FeNi80 alloy powder; The silicone rubber is selected from one or more of methyl vinyl silicone rubber, methyl silicone rubber, fluorosilicone rubber, and phenyl silicone rubber.

[0011] As a preferred technical solution, in the low-frequency absorbing material, the catalyst is selected from one or more of the following: Kalster catalyst and coated platinum catalyst; The inhibitor is selected from one or more of 1-ethyn-1-cyclohexanol, 3-methyl-1-butyn-3-ol, 3,5-dimethyl-1-hexyn-3-ol, and 3-phenyl-1-butyn-ol.

[0012] Secondly, a method for preparing the aforementioned low-frequency absorbing material includes the following steps: S10: A core-shell structured iron-nickel alloy microwave absorber was prepared by coating silicon oxide onto the surface of iron-nickel alloy powder using the sol-gel method. S20: Surface modification pretreatment of the core-shell structured iron-nickel alloy microwave absorber using a silane coupling agent; S30: Based on the physical morphology distribution of the absorbing agent in each layer, prepare a surface layer adhesive containing sheet-like absorbing agent, and an intermediate layer adhesive and a bottom layer adhesive containing spherical absorbing agent respectively. S40: The surface rubber compound, the intermediate rubber compound and the bottom rubber compound are stacked in sequence, and a sheet blank with a gradient structure is formed by calendering process; S50: The sheet blank is pre-cured and subjected to secondary vulcanization to obtain the low-frequency absorbing material.

[0013] As a preferred technical solution, in the preparation method, in step S20, the mass ratio of the core-shell structured iron-nickel alloy microwave absorber, silane coupling agent, ethanol and water is 100:(0.5-1):(10-20):(1-5); In step S40, the gradient structure is formed by a three-layer co-extrusion calendering process or a multiple calendering and lamination process.

[0014] As a preferred technical solution, in the preparation method, in step S50, the temperature of the pre-curing treatment is 120-140℃ and the time is 10-20 min; the temperature of the secondary vulcanization treatment is 180-200℃ and the time is 2-4 h.

[0015] Beneficial effects: Compared with existing technologies, this invention, through gradient structural design, creates a stepped distribution of absorber content from high to low from the outside to the inside of the material. The high absorber content on the surface layer strongly absorbs electromagnetic waves, while the middle and bottom layers form a gradually transitioning impedance gradient, thereby improving the overall impedance matching effect and effectively enhancing the low-frequency absorption performance. By coating the iron-nickel alloy powder with a silica shell, a core-shell structure is formed. This effectively blocks direct contact between the alloy powder and the external humid and corrosive environment, significantly improving the weather resistance of the absorber. Furthermore, the introduction of the silica shell improves the modifiability of the absorber surface, providing favorable conditions for subsequent surface modification of the silane coupling agent, thus improving the uniformity of absorber dispersion in the silicone rubber matrix. The differentiated morphology design, using sheet-like iron-nickel alloy powder for the surface layer and spherical iron-nickel alloy powder for the middle and bottom layers, allows different layers to perform different electromagnetic functions. The sheet-like powder is beneficial for enhancing magnetic loss and improving high-frequency absorption performance, while the spherical powder is beneficial for maintaining isotropic absorption characteristics. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or related technologies, the drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 This is a schematic cross-sectional view of the low-frequency absorbing material with a gradient structure provided in an embodiment of the present invention.

[0018] Figure 2This is a flowchart of the preparation method of a low-frequency absorbing material with a gradient structure provided in the embodiments of the present invention. Detailed Implementation

[0019] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0020] In the description of the embodiments of the present invention, it should be noted that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of the present invention. In addition, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0021] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0022] See Figure 1 The low-frequency absorbing material with a gradient structure provided by this invention comprises, from the outside to the inside, an impedance matching surface layer 10, a transition intermediate layer 20, and a bottom layer 30, stacked sequentially. All three layers use silicone rubber as the continuous phase matrix and a core-shell structured iron-nickel alloy absorbing agent as the main functional filler. The three layers are integrally formed through a co-calendering process, without relying on additional adhesives. Interfacial bonding is achieved through interfacial interdiffusion and co-crosslinking of the silicone rubber matrix during the curing process, resulting in high interfacial bonding strength and minimizing delamination.

[0023] The core-shell structured iron-nickel alloy microwave absorber comprises an iron-nickel alloy powder core and a silica coating layer covering the surface of the core. The silica coating layer is uniformly and densely coated on the surface of the iron-nickel alloy powder core, with a thickness of 5–15 nm. This silica coating layer serves two purposes: firstly, it acts as a physical barrier, preventing surface oxidation of the iron-nickel alloy powder in humid or corrosive environments, thereby maintaining the magnetic properties of the iron-nickel alloy; secondly, the silica surface is rich in silanol groups (Si-OH), providing effective reaction sites for subsequent chemical grafting of silane coupling agents, significantly improving the interfacial compatibility between the microwave absorber and the silicone rubber matrix.

[0024] In this embodiment, regarding the gradient structure design, the mass fraction of the core-shell iron-nickel alloy microwave absorber decreases in a stepwise manner from the surface layer to the bottom layer: the surface layer contains 90-95 parts of absorber (based on 100 parts of silicone rubber, the same below), the intermediate layer contains 70-80 parts, and the bottom layer contains 60-70 parts. The physical significance of this design is that electromagnetic waves first irradiate the surface layer, and the higher absorber content in the surface layer gives it strong magnetic loss capability, enabling it to convert most of the electromagnetic energy entering the material into heat energy at the surface. The absorber content in the intermediate layer and the bottom layer decreases sequentially, and the permeability and dielectric constant also gradually decrease accordingly. This results in a gradient change in the overall electromagnetic parameters of the material from the bottom layer to the surface layer, thereby forming a gradient transition in impedance from near-free space impedance (bottom layer side) to high absorber loss impedance (surface layer side), improving the overall impedance matching characteristics of the material and suppressing the reflection of electromagnetic waves on the material surface.

[0025] In this embodiment, regarding the differentiated morphology design, the surface layer uses flake-shaped iron-nickel alloy powder with a particle size of 250–290 mesh; the intermediate and bottom layers use spherical iron-nickel alloy powder with a particle size of 180–220 mesh. The flake-shaped powder has a large aspect ratio and exhibits anisotropy in the in-plane direction, which is beneficial for enhancing eddy current loss and magnetic loss, thereby improving the absorption capacity for low-frequency electromagnetic waves. Simultaneously, after rolling and orientation, the flake-shaped powder facilitates the formation of an oblique incidence scattering effect similar to that in "radar absorbing coatings," further increasing the propagation path of electromagnetic waves within the surface layer and improving energy loss efficiency. The spherical powder, on the other hand, is isotropic and uniformly dispersed, which helps ensure the consistency of the absorption performance of the intermediate and bottom layers in all directions.

[0026] Based on the same inventive concept, the present invention also provides a method for preparing a low-frequency absorbing material, characterized by comprising the following steps: Step S10: Silicon oxide is coated onto the surface of iron-nickel alloy powder using the sol-gel method to prepare a core-shell structured iron-nickel alloy microwave absorber.

[0027] Specifically, tetraethyl orthosilicate is mixed with anhydrous ethanol and stirred at room temperature to obtain a silicon source solution. Deionized water and anhydrous ethanol are mixed at a volume ratio of 1:4 to 1:6, and concentrated hydrochloric acid is added dropwise to adjust the pH value to 2 to 3 to obtain a catalytic solution.

[0028] Iron-nickel alloy powder was ultrasonically dispersed in anhydrous ethanol. Under continuous stirring, a silicon source solution was slowly added dropwise to the iron-nickel alloy dispersion. Subsequently, a catalytic solution was slowly added to the mixture, and the reaction temperature was controlled at 25–40°C. Stirring was continued for 2–4 hours to allow TEOS to hydrolyze and condense, forming a SiO2 gel layer that coats the iron-nickel alloy surface. The product was then suspended, aged, and centrifuged to obtain a solid precipitate. This precipitate was dried and subjected to high-temperature treatment to obtain a core-shell structured iron-nickel alloy microwave absorber.

[0029] Step S20: Surface modification pretreatment of the core-shell structured iron-nickel alloy microwave absorber is performed using a silane coupling agent.

[0030] Specifically, γ-aminopropyltriethoxysilane (KH-550) or γ-glycidoxypropyltrimethoxysilane (KH-560) were selected as modifiers, mixed with anhydrous ethanol, stirred at room temperature, and the pH was adjusted to 4-5 to hydrolyze and obtain a silane coupling agent hydrolysate. The prepared core-shell structure microwave absorber was added to the silane coupling agent hydrolysate, ultrasonically dispersed, and stirred in a water bath at 40-60℃ for 2-4 hours. After the reaction was completed, the mixture was centrifuged and washed with anhydrous ethanol until neutral, and then vacuum dried at 60-80℃ for 4-8 hours to obtain the surface-modified core-shell structure microwave absorber.

[0031] Step S30: Based on the physical morphology distribution of the absorbing agent in each layer, prepare a surface layer adhesive containing sheet-like absorbing agent, and an intermediate layer adhesive and a bottom layer adhesive containing spherical absorbing agent.

[0032] Specifically, according to the proportions of each layer, the surface-modified core-shell structure microwave absorber prepared in step S20 is mixed with natural rubber or nitrile rubber on a two-roll mill, and then a catalyst and an inhibitor are added to prepare the surface rubber compound, the intermediate rubber compound and the bottom rubber compound respectively.

[0033] Step S40: The surface rubber, intermediate rubber and bottom rubber are stacked in sequence and formed into a sheet blank with a gradient structure by calendering process.

[0034] Specifically, the open mill rollers are preheated. The surface layer, intermediate layer, and bottom layer of rubber compound are sequentially pressed onto the rollers in a thickness ratio of 1:2:1 (or adjusted according to design) to form a thin sheet. These layers are then stacked in the order of surface / intermediate / bottom layer, with each layer controlled to a thickness of 0.5–2 mm. The stacked rubber compound is then calendered using a three-roll or four-roll calender at a roller temperature of 50–70°C, gradually compressing it to the target thickness (total thickness 2–6 mm). During calendering, the interfaces between layers are kept tightly bonded to avoid air bubbles and delamination defects. The calendered material is then cut into sheet blanks of the required size.

[0035] The calendering process enables the molecular chains of each layer of rubber to be oriented and densified under pressure. The interlayer interfaces form an interpenetrating network structure under shear force and pressure, ensuring the mechanical bonding and electromagnetic continuity between the gradient layers.

[0036] Step S50: The sheet blank is pre-cured and subjected to secondary vulcanization to obtain the low-frequency absorbing material.

[0037] Specifically, the sheet material is placed in a flat vulcanizing machine and pre-vulcanized under certain pressure to initially shape the rubber compound and prevent deformation during subsequent processing. The pre-cured sheet is then placed in an oven and subjected to secondary vulcanization at 150–170°C for 30–60 minutes to fully cross-link the rubber and form a stable three-dimensional network structure. After natural cooling to room temperature, the finished low-frequency absorbing material is obtained.

[0038] Pre-curing gives the material initial shape stability, while secondary vulcanization ensures the cross-linking reaction proceeds fully, improving the material's heat resistance, mechanical strength, and long-term stability. During vulcanization, the rubber molecular chains react with the coupling agent functional groups on the surface of the microwave absorber, forming a strong interfacial bond.

[0039] The following specific preparation examples will further explain the above-mentioned technical solutions provided by the present invention.

[0040] Example 1 Surface layer components (parts by weight): Methyl vinyl silicone rubber: 100 parts (standard amount, same below), core-shell structure FeNi50 sheet-like microwave absorber (particle size 250-270 mesh, silica coating thickness approximately 8 nm): 92 parts, Kalster catalyst: 2 parts, 1-acetylene-1-cyclohexanol: 0.5 parts, silane coupling agent (KH-570): 0.8 parts, transition intermediate layer components (parts by weight): Methyl vinyl silicone rubber: 100 parts; core-shell structure FeNi50 spherical microwave absorber (particle size 190-210 mesh, silica coating thickness approximately 8 nm): 75 parts; Kalster catalyst: 2 parts; 1-acetylene-1-cyclohexanol: 0.5 parts; silane coupling agent (KH-570): 0.8 parts; bottom layer components (parts by weight): methyl vinyl silicone rubber: 100 parts; core-shell structure FeNi50 spherical microwave absorber (particle size 190-210 mesh, silica coating thickness approximately 8 nm): 63 parts; Kalster catalyst: 2 parts; 1-acetylene-1-cyclohexanol: 0.5 parts; silane coupling agent (KH-570): 0.8 parts.

[0041] Thickness of each layer: top layer 0.2mm, intermediate layer 0.15mm, bottom layer 0.2mm, total thickness 0.55mm.

[0042] 100g of FeNi50 flake alloy powder (particle size 250-270 mesh) was added to a beaker containing 150mL of anhydrous ethanol and ultrasonically dispersed for 15min in an ultrasonic cleaner to ensure thorough dispersion. Then, TEOS was dissolved in anhydrous ethanol at a mass ratio of 100:4:150:15 and slowly added dropwise to the ethanol dispersion containing the FeNi50 alloy powder. Simultaneously, approximately 3mL of ammonia (25% by volume) was added as a catalyst, and the pH of the solution was adjusted to 9-10. The hydrolysis and polycondensation reaction was carried out under continuous stirring at room temperature for 6h. After the reaction was complete, the mixture was filtered, washed three times with anhydrous ethanol, and vacuum dried at 60℃ for 4h to obtain a core-shell structured FeNi50 flake microwave absorber with a silicon oxide layer (approximately 8nm thick).

[0043] The preparation process of the core-shell structure of spherical FeNi50 powder is the same as the above process, except that the flake powder is replaced with spherical powder (particle size 190-210 mesh), and the other steps and parameters remain unchanged.

[0044] In the above preparation, tetraethyl orthosilicate (TEOS) forms a uniform and dense SiO2 shell on the surface of iron-nickel alloy powder through hydrolysis and polycondensation reaction. The thickness of the shell can be controlled by adjusting the amount of TEOS.

[0045] 100g of the core-shell structured microwave absorber (prepared in flake and spherical forms respectively) was added to 150mL of a mixed solvent of ethanol and deionized water at a volume ratio of 90:10, and ultrasonically dispersed for 10min. 0.8g of silane coupling agent KH-570 (γ-methacryloyloxypropyltrimethoxysilane) was added, and after stirring until homogeneous, the mixture was refluxed in a 55℃ water bath for 2h. After treatment, the mixture was filtered, washed twice with anhydrous ethanol, and dried at 60℃ for 3h to obtain the surface-modified core-shell structured iron-nickel alloy microwave absorber.

[0046] After hydrolysis, the methoxy group in the silane coupling agent KH-570 undergoes a condensation reaction with the silanol group on the silica shell of the microwave absorber to form a covalent bond. The methacryloyloxy group at the other end undergoes a grafting reaction with the vinyl group in the silicone rubber during the curing process, thereby establishing a chemically bonded interface layer between the filler and the matrix, which significantly improves the interfacial compatibility and bonding strength of the two.

[0047] According to the above-mentioned layer composition ratios, the modified core-shell structure microwave absorber (sheet or spherical) and methyl vinyl silicone rubber raw rubber were weighed out and mixed on a two-roll mill. First, the silicone rubber raw rubber was passed through the rollers until it was uniformly wrapped around the rollers, and then the microwave absorber was added in batches and mixed until the microwave absorber was uniformly dispersed. After the rubber compound temperature dropped to room temperature, silane coupling agent (0.8 parts added) was added and mixing was continued for 5 minutes. Finally, inhibitor (0.5 parts of 1-acetylene-1-cyclohexanol) and Kalster catalyst (2 parts) were added in sequence, and the mixture was quickly mixed evenly. The mixture was then sheeted out for later use, resulting in surface compound, intermediate compound, and bottom compound.

[0048] It should be noted that the catalyst and inhibitor must be added last, and the process must be rapid after adding the catalyst to prevent premature cross-linking of the rubber compound due to excessive catalytic activity at room temperature. The role of the inhibitor is to suppress catalyst activity at room temperature, extend the workability time (i.e., scorch time) of the rubber compound, and ensure the operability of subsequent calendering processes.

[0049] The surface, intermediate, and bottom layers of rubber compound are fed into the corresponding feeding systems of a three-roll calender. A three-layer co-extrusion calendering process is employed, with the calender rolls at a temperature of 50–60°C and a calendering speed of 3 m / min. The three layers are simultaneously extruded and compounded under the pressure of the rolls to obtain a sheet-like preform with the surface, intermediate, and bottom layers stacked sequentially from the outside in. The thickness of each layer is controlled by the roll spacing: 0.2 mm for the surface layer, 0.15 mm for the intermediate layer, and 0.2 mm for the bottom layer.

[0050] During the rolling process, the sheet-like iron-nickel alloy powder is oriented and aligned along the rolling direction under the action of shear force, which further increases the anisotropy of the magnetic permeability of the powder in the in-plane direction, which is beneficial to improving the magnetic loss capability of the surface layer.

[0051] The sheet-like preform was placed in a flat vulcanizing machine and pre-cured for 15 minutes at 130°C and 10 MPa to allow the rubber compound to initially cross-link and solidify, obtaining a semi-finished product with certain mechanical strength and interlayer bonding strength. The semi-finished product was then transferred to an oven and subjected to secondary vulcanization (post-vulcanization) at 190°C for 3 hours to fully cross-link the silicone rubber matrix, eliminate residual small molecules, and further improve the material's mechanical properties and thermal stability. After curing, it was naturally cooled to room temperature to obtain the low-frequency absorbing material with a gradient structure as described in this embodiment.

[0052] Example 2 This embodiment provides another low-frequency absorbing material with a gradient structure. The main difference from Embodiment 1 is that the iron-nickel alloy powder is FeNi80 alloy powder, the silicone rubber matrix is ​​phenyl silicone rubber, the catalyst is a coated platinum catalyst, the inhibitor is 3,5-dimethyl-1-hexyn-3-ol, the thickness of the silicon oxide coating layer in the core-shell structure absorbing agent is controlled to be about 12nm, and a multi-layer calendering process is used instead of a three-layer co-extrusion calendering process.

[0053] Components of each layer: Surface layer: 100 parts phenyl silicone rubber, 94 parts core-shell structure FeNi80 sheet-like microwave absorber (particle size 270-290 mesh, SiO2 coating layer about 12nm), 2.5 parts coated platinum catalyst, 0.6 parts 3,5-dimethyl-1-hexyn-3-ol, 0.7 parts silane coupling agent (KH-550).

[0054] Transition intermediate layer: 100 parts phenyl silicone rubber, 78 parts core-shell structure FeNi80 spherical microwave absorber (particle size 200-220 mesh, SiO2 coating layer about 12nm), 2.5 parts coated platinum catalyst, 0.6 parts 3,5-dimethyl-1-hexyn-3-ol, and 0.7 parts silane coupling agent (KH-550).

[0055] Bottom layer: 100 parts phenyl silicone rubber, 68 parts core-shell structure FeNi80 spherical microwave absorber (particle size 200-220 mesh, SiO2 coating layer about 12nm), 2.5 parts coated platinum catalyst, 0.6 parts 3,5-dimethyl-1-hexyn-3-ol, and 0.7 parts silane coupling agent (KH-550).

[0056] Thickness of each layer: surface layer 0.25mm, intermediate layer 0.15mm, bottom layer 0.25mm, total thickness 0.65mm.

[0057] Preparation of the core-shell structure microwave absorber: The preparation method is the same as in Example 1, except that the amount of TEOS is adjusted so that the thickness of the silicon oxide coating layer is approximately 12 nm. A thicker SiO2 coating layer can provide better corrosion protection for the iron-nickel alloy, making it suitable for applications with higher weather resistance requirements. However, it should be noted that an excessively thick coating layer will dilute the volume fraction of the magnetic core to some extent. Therefore, in this example, the upper limit of the silicon oxide coating layer thickness is controlled within 15 nm.

[0058] Silane coupling agent modification: 100g of core-shell structure microwave absorber was dispersed in a mixed solvent of ethanol and water (mass ratio: microwave absorber: silane coupling agent: ethanol: water = 100:0.7:15:3), and refluxed at 55℃ for 2 hours. The mixture was then filtered and dried for later use. In this example, KH-550 (γ-aminopropyltriethoxysilane) was used as the coupling agent. Its amino end groups have good compatibility with the phenyl silicone rubber system and can form effective interfacial bonding in the phenyl silicone rubber matrix.

[0059] Multiple calendering and lamination process: The bottom layer rubber compound is pressed into a 0.25mm thick sheet by a calender, the middle layer rubber compound is pressed into a 0.15mm thick sheet, and the top layer rubber compound is pressed into a 0.25mm thick sheet. Then, the three layers of sheets are stacked in the order of bottom layer, middle layer, and top layer. They are then heat-bonded by a calender under light pressure (roller temperature 50℃, roll pressure about 0.5MPa) to make the interfaces of the three layers of sheets tightly bonded, resulting in a three-layer laminated sheet blank.

[0060] Pre-curing and secondary vulcanization: Pre-curing temperature 125℃, time 15min; secondary vulcanization temperature 195℃, time 3.5h, the rest of the operation is the same as in Example 1.

[0061] Example 3 This embodiment provides a third type of low-frequency absorbing material with a gradient structure. The main difference from Embodiment 1 is that the content of the absorbing agent in each layer is different, as detailed below.

[0062] Components of each layer (parts by weight): Surface layer: 100 parts methyl silicone rubber, 90 parts core-shell structure FeNi50 sheet-like microwave absorber (particle size 260-280 mesh, SiO2 coating layer about 10nm), 1.5 parts Kalster catalyst, 0.4 parts 3-methyl-1-butyn-3-ol, 0.6 parts silane coupling agent (KH-570).

[0063] Transition intermediate layer: 100 parts methyl silicone rubber, 70 parts core-shell structure FeNi50 spherical microwave absorber (particle size 180-200 mesh, SiO2 coating layer about 10 nm), 1.5 parts Kalster catalyst, 0.4 parts 3-methyl-1-butyn-3-ol, and 0.6 parts silane coupling agent (KH-570).

[0064] Bottom layer: 100 parts methyl silicone rubber, 60 parts core-shell structure FeNi50 spherical microwave absorber (particle size 180-200 mesh, SiO2 coating layer about 10nm), 1.5 parts Kalster catalyst, 0.4 parts 3-methyl-1-butyn-3-ol, and 0.6 parts silane coupling agent (KH-570).

[0065] Thickness of each layer: surface layer 0.1mm, intermediate layer 0.1mm, bottom layer 0.1mm, total thickness 0.3mm.

[0066] This embodiment represents the case where the content of each layer of absorbing agent is at the lower limit of its respective range. The total amount of absorbing agent is relatively low, which helps to reduce the material density and is suitable for applications with high requirements for lightweighting. The thickness of each layer is at the lower limit of its respective range, and the total thickness is only 0.3 mm, demonstrating the feasibility of the present invention under ultra-thin conditions.

[0067] The preparation process is basically the same as in Example 1, using a three-layer co-extrusion calendering process, with a pre-curing temperature of 120℃ and a time of 10 min; and a secondary vulcanization temperature of 180℃ and a time of 2 h.

[0068] Example 4 This embodiment provides a fourth type of low-frequency absorbing material with a gradient structure to verify the material performance when the content of each layer of absorbing agent is taken to the upper limit of its respective range.

[0069] Components of each layer (parts by weight): Surface layer: 100 parts of fluorosilicone rubber, 95 parts of core-shell structure FeNi80 sheet-like microwave absorber (particle size 280-290 mesh, SiO2 coating layer about 5nm), 3 parts of coated platinum catalyst, 0.8 parts of 3-phenyl-1-butynediol, and 1 part of silane coupling agent (KH-570).

[0070] Transition intermediate layer: 100 parts of fluorosilicone rubber, 80 parts of core-shell structure FeNi80 spherical microwave absorber (particle size 210-220 mesh, SiO2 coating layer about 5nm), 3 parts of coated platinum catalyst, 0.8 parts of 3-phenyl-1-butynediol, and 1 part of silane coupling agent (KH-570).

[0071] Bottom layer: 100 parts fluorosilicone rubber, 70 parts core-shell structure FeNi80 spherical microwave absorber (particle size 210-220 mesh, SiO2 coating layer about 5nm), 3 parts coated platinum catalyst, 0.8 parts 3-phenyl-1-butynediol, and 1 part silane coupling agent (KH-570).

[0072] Thickness of each layer: surface layer 0.3mm, intermediate layer 0.2mm, bottom layer 0.3mm, total thickness 0.8mm.

[0073] In this embodiment, fluorosilicone rubber is selected as the matrix. Fluorosilicone rubber combines the high and low temperature resistance of silicone rubber with the fuel and solvent resistance of fluorinated materials, making it suitable for applications with special requirements for oil and chemical resistance, such as electromagnetic shielding protection near aerospace engine compartments. The silica coating thickness is set to a minimum of 5nm to ensure a certain level of weather protection while maximizing the retention of the magnetic permeability of the iron-nickel alloy, which is beneficial for further improving magnetic loss capability under conditions of higher absorber filling.

[0074] Pre-curing temperature 140℃, time 20min; secondary curing temperature 200℃, time 4h.

[0075] Comparative Example 1 (homogeneous single-layer structure) To verify the effectiveness of the gradient structure, a homogeneous single-layer microwave absorbing material was prepared as a comparison. The homogeneous layer composition was: 100 parts methyl vinyl silicone rubber, 80 parts FeNi50 spherical microwave absorbing agent (uncoated, unmodified), 2 parts Kalster catalyst, 0.5 parts 1-acetylene-1-cyclohexanol, and 0.8 parts silane coupling agent; the total thickness was 0.55 mm, and the curing process was the same as in Example 1.

[0076] Comparative Example 2 (without core-shell structure, without silane coupling agent modification) A comparative material with the same number of layers and gradient structure as in Example 1 was prepared, but each layer of the microwave absorber was an uncoated bare iron-nickel alloy powder, and no silane coupling agent modification was performed, in order to verify the contribution of core-shell structure and surface modification to microwave absorption performance and weather resistance.

[0077] Examples 1 to 4, as well as Comparative Examples 1 and 2, were fully tested according to the following test methods.

[0078] Test Method Description: The absorption performance test adopts the Arch Method. According to the relevant provisions of the national standard GB / T 15949, the reflection loss (RL) of each sample in the frequency band of 0.1GHz to 3GHz is measured in a microwave anechoic chamber. The peak value of reflection loss (RL peak) and the effective absorption bandwidth (the frequency range corresponding to RL≤-10dB) are used as the main performance indicators.

[0079] The weather resistance test adopts the accelerated aging test method: each sample is placed in a constant temperature and humidity chamber at 85℃ and 85% relative humidity for 500 hours. After being taken out, it is left to stand at room temperature for 24 hours, and its reflection loss is tested again. The change in the peak value of RL before and after aging (ΔRL) characterizes the weather resistance stability.

[0080] Mechanical property testing was conducted in accordance with GB / T 528. Dumbbell-shaped specimens were used to test the tensile strength and elongation at break of each sample to reflect the flexibility and interlayer bond strength of the material. At the same time, T-peel strength testing (referring to GB / T 2792) was performed on the three-layer composite material to evaluate the degree of interlayer interface bonding.

[0081] Wave absorption performance test results: In Example 1, within the 0.1 GHz to 3 GHz frequency band, the peak impedance RL (RL) occurs at approximately 0.8 GHz, with a peak RL of -28.4 dB. The effective absorption bandwidth (RL ≤ -10 dB) covers the 0.4 GHz to 1.9 GHz band, with a bandwidth of approximately 1.5 GHz. Throughout the entire test frequency band, the material exhibits good absorption performance for low-frequency electromagnetic waves, with no obvious reflection peaks, indicating that the gradient structure design effectively improves the impedance matching characteristics of the material.

[0082] In Example 2, the FeNi80 alloy powder exhibits higher saturation magnetization and initial permeability, further enhancing magnetic loss capability under the same structural design. Test results show that the peak RL of Example 2 occurs at approximately 0.7 GHz, with a peak RL of -31.2 dB. The effective absorption bandwidth covers 0.35 GHz to 2.1 GHz, with a bandwidth of approximately 1.75 GHz. Its absorption performance is superior to that of Example 1, demonstrating the performance advantage of FeNi80 alloy in low-frequency absorption.

[0083] Example 3 features a thin design with a total thickness of only 0.3 mm and the amount of absorbing agent used is at the lower limit of its respective range. Despite this, its RL peak value can still reach -18.6 dB, and the effective absorption bandwidth is about 0.7 GHz. It achieves low-frequency absorption function under the condition of a total thickness of less than 0.35 mm, which fully demonstrates the effectiveness of the gradient structure of the present invention under extremely thin conditions and provides a feasible solution for application scenarios with strict limitations on thickness and weight.

[0084] Example 4 has a total thickness of 0.8 mm, with the absorber dosage set to the upper limit of the range. The highest volume fraction of magnetic filler corresponds to stronger magnetic and dielectric loss capabilities. Test results show that Example 4 has a peak RL of -34.8 dB and an effective absorption bandwidth of approximately 2.1 GHz. It exhibits the most outstanding absorption effect among the four examples, but also has a relatively high material density, making it suitable for applications with extremely high absorption performance requirements and relatively relaxed weight constraints.

[0085] The test results of Comparative Example 1 (homogeneous single layer, uncoated exposed absorber) show that its peak absorption ratio (RL) is only -12.3dB, the effective absorption bandwidth is about 0.45GHz, and there is almost no effective absorption at the low frequency end (0.1GHz~0.4GHz) within the test frequency band (RL>-5dB). This fully demonstrates that the homogeneous single layer structure has obvious deficiencies in the absorption performance of the low frequency band, and impedance mismatch is one of its main limiting factors.

[0086] The newly prepared sample of Comparative Example 2 (with the same number of gradient layers, but without core-shell structure and coupling agent modification) had a peak RL of -22.1 dB, which was about 6 dB lower than that of Example 1 with the same structural parameters. The effective absorption bandwidth was also narrowed by about 0.4 GHz, indicating that the core-shell structure and surface modification have an undeniable contribution to improving the dispersion uniformity of the absorber in the matrix and thus improving the absorption performance.

[0087] Weather resistance test results: After accelerated aging treatment at 85℃ / 85%RH for 500 hours, the peak RL change (ΔRL) of Examples 1 to 4 were -1.8dB, -1.5dB, -2.1dB, and -1.2dB, respectively, with a microwave absorption performance retention rate of over 90%. This indicates that the core-shell structured silica coating effectively protects the iron-nickel alloy powder, significantly inhibits the oxidation failure of the microwave absorber under high temperature and high humidity conditions, and endows the material with good weather resistance and stability.

[0088] Comparative Example 2 (without core-shell structure) showed a decrease of -9.3 dB in peak RL after the same aging treatment, a decrease of more than 40%. The oxidation rate of exposed iron-nickel alloy powder in high temperature and high humidity environment is much higher than that of core-shell structure microwave absorber with silicon oxide protective shell. This comparative data intuitively proves the key role of core-shell structure design in improving the long-term stability of materials.

[0089] Example 4 uses a fluorosilicone rubber matrix, which has excellent oil resistance and chemical corrosion resistance. It works synergistically with the core-shell structure microwave absorber. After aging, the ΔRL is only -1.2dB. It has the best weather resistance stability among the four examples, which further confirms the effectiveness of the synergistic protection of the matrix and the core-shell structure.

[0090] Mechanical property test results: The tensile strengths of Examples 1 to 4 were 5.8 MPa, 6.2 MPa, 4.9 MPa, and 6.5 MPa, respectively, and the elongations at break were 380%, 420%, 350%, and 410%, respectively, all maintaining the good flexibility of silicone rubber materials. In the T-peel strength test, the interlayer peel strengths of Examples 1 to 4 were 4.2 N / mm, 4.5 N / mm, 3.8 N / mm, and 4.8 N / mm, respectively, indicating that the three-layer structure has good interlayer bonding force and will not delamination failure under normal use conditions.

[0091] Comparative Example 2, due to the unmodified and unevenly dispersed microwave absorber, resulted in localized filler agglomeration defects. Its tensile strength (4.1 MPa) and elongation at break (280%) were significantly lower than those of Example 1 with the same formulation. The decrease in mechanical properties also indirectly confirms the comprehensive improvement effect of interface modification on the overall performance of materials.

[0092] Based on the above test results, the low-frequency absorbing materials with gradient structures prepared in the various embodiments of the present invention can achieve the following technical effects: First, it exhibits a wide effective absorption bandwidth and outstanding low-frequency absorption capability. The gradual impedance change formed by the gradient structure allows electromagnetic waves to enter the material efficiently and be fully attenuated. The effective absorption bandwidth (RL≤-10dB) of Examples 1 to 4 is no less than 0.7GHz, with Examples 2 and 4 reaching 1.75GHz and 2.1GHz respectively. It exhibits excellent absorption performance in the low-frequency range of 0.1GHz to 3GHz, which is far superior to that of the homogeneous single-layer structure in Comparative Example 1.

[0093] Secondly, it exhibits strong weather resistance. The silicon oxide core-shell structure effectively protects the iron-nickel alloy powder, enabling the material to retain over 90% of its microwave absorption performance under high temperature and high humidity aging conditions. This overcomes the shortcomings of existing iron-nickel alloy microwave absorbing materials, which are prone to oxidation and failure, and ensures the reliability of the material under long-term use conditions.

[0094] Third, it exhibits high interlayer bonding strength and good structural integrity. The three-layer material is integrally formed through a calendering process, achieving an interlayer peel strength of no less than 3.8 N / mm. This effectively avoids the delamination risk associated with traditional adhesive lamination processes, ensuring the stability of the gradient structure during actual use.

[0095] Fourth, the material exhibits good flexibility and can be made into thinner forms. Using silicone rubber as the matrix, the material possesses excellent flexibility and bending properties, with an elongation at break exceeding 350%, making it suitable for applications requiring curved surface bonding. Example 3 verified the effective wave absorption capability with a total thickness of only 0.3 mm, providing technical support for thinner and lighter applications.

[0096] Fifth, the manufacturing process is controllable and can be mass-produced. The calendering process enables the integrated molding of multi-layer materials, with clearly defined process parameters. The thickness and composition of each layer can be independently adjusted, facilitating continuous and mass production in industrial settings.

[0097] Finally, it should be noted that the above embodiments are only for illustrating the present invention and not for limiting the present invention. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications, or equivalent substitutions of the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention and should be covered within the scope of the claims of the present invention.

Claims

1. A low-frequency absorbing material with a gradient structure, characterized in that, It includes an impedance matching surface layer, a transition intermediate layer, and a bottom layer, which are stacked sequentially from the outside to the inside. The impedance matching surface layer, transition intermediate layer and bottom layer all include a silicone rubber matrix and a core-shell structured iron-nickel alloy microwave absorber dispersed therein. The mass fraction of the core-shell iron-nickel alloy absorbing agent decreases in a stepwise manner from the impedance matching surface layer to the bottom layer, with 90-95 parts in the impedance matching surface layer, 70-80 parts in the transition intermediate layer, and 60-70 parts in the bottom layer.

2. The low-frequency absorbing material according to claim 1, characterized in that, The impedance matching surface layer, transition intermediate layer, and bottom layer also include the following components: The ingredients are 5-8 parts silicone rubber, 1-3 parts catalyst, 0.3-0.8 parts inhibitor and 0.5-1 part silane coupling agent.

3. The low-frequency absorbing material according to claim 1, characterized in that, The core-shell structured iron-nickel alloy microwave absorber comprises an iron-nickel alloy powder core and a silicon oxide coating layer covering the surface of the core, wherein the thickness of the silicon oxide coating layer is 5-15 nm.

4. The low-frequency absorbing material according to claim 1, characterized in that, The core-shell structured iron-nickel alloy microwave absorber exhibits differentiated physical morphology in each layer. Specifically, the iron-nickel alloy powder in the impedance matching surface layer is flake-shaped powder with a particle size of 270 mesh; while the iron-nickel alloy powder in the transition intermediate layer and the bottom layer is spherical powder with a particle size of 200 mesh.

5. The low-frequency absorbing material according to claim 1, characterized in that, The thickness of the impedance matching surface layer is 0.1-0.3 mm, the thickness of the transition intermediate layer is 0.1-0.2 mm, and the thickness of the bottom layer is 0.1-0.3 mm.

6. The low-frequency absorbing material according to claim 1, characterized in that, The iron-nickel alloy powder is selected from one or more of FeNi50 alloy powder and FeNi80 alloy powder; The silicone rubber is selected from one or more of methyl vinyl silicone rubber, methyl silicone rubber, fluorosilicone rubber, and phenyl silicone rubber.

7. The low-frequency absorbing material according to claim 2, characterized in that, The catalyst is selected from one or more of the following: Kelster catalyst and coated platinum catalyst; The inhibitor is selected from one or more of 1-ethyn-1-cyclohexanol, 3-methyl-1-butyn-3-ol, 3,5-dimethyl-1-hexyn-3-ol, and 3-phenyl-1-butyn-ol.

8. A method for preparing the low-frequency absorbing material according to claim 1, characterized in that, Includes the following steps: S10: A core-shell structured iron-nickel alloy microwave absorber was prepared by coating silicon oxide onto the surface of iron-nickel alloy powder using the sol-gel method. S20: Surface modification pretreatment of the core-shell structured iron-nickel alloy microwave absorber using a silane coupling agent; S30: Based on the physical morphology distribution of the absorbing agent in each layer, prepare a surface layer adhesive containing sheet-like absorbing agent, and an intermediate layer adhesive and a bottom layer adhesive containing spherical absorbing agent respectively. S40: The surface rubber compound, the intermediate rubber compound and the bottom rubber compound are stacked in sequence, and a sheet blank with a gradient structure is formed by calendering process; S50: The sheet blank is pre-cured and subjected to secondary vulcanization to obtain the low-frequency absorbing material.

9. The preparation method according to claim 8, characterized in that, In step S20, the mass ratio of the core-shell structured iron-nickel alloy microwave absorber, silane coupling agent, ethanol and water is 100:(0.5-1):(10-20):(1-5); In step S40, a gradient structure is formed by a three-layer co-extrusion calendering process or a multiple calendering and stacking process.

10. The preparation method according to claim 8, characterized in that, In step S50, the temperature of the pre-curing treatment is 120-140℃ and the time is 10-20 min; the temperature of the secondary vulcanization treatment is 180-200℃ and the time is 2-4 h.

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