A coated electromagnetic wave absorber and its preparation method

Abstract

This invention belongs to the field of microwave absorbing materials technology, specifically relating to a coated electromagnetic wave absorber and its preparation method. Through a coating process, this invention forms a silica film coating layer on the surface of flake-shaped carbonyl iron powder. The silica coating layer, as an excellent insulator, effectively prevents the formation of conductive circuits between the flake-shaped carbonyl iron powder particles, hindering the formation of a conductive network in the flake structure. This effectively reduces the dielectric constant while maintaining high permeability, improving the impedance mismatch problem in the design of the electromagnetic wave absorber coating for flake-shaped carbonyl iron powder, achieving better impedance matching, and enhancing microwave absorption performance.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic wave absorbing materials technology, specifically relating to a coated electromagnetic wave absorber and its preparation method. Background Technology

[0002] Electromagnetic absorbing materials dissipate electromagnetic waves by converting them into other forms of energy, such as heat, through dielectric loss or magnetic loss, thereby absorbing and attenuating incident electromagnetic waves. They are core functional materials in fields such as aerospace stealth and electronic information anti-interference, and are widely used in current military and civilian applications.

[0003] Currently, the most commonly used absorber in microwave absorbing materials is flake-shaped carbonyl iron powder, which possesses high magnetic permeability and high saturation magnetization, exhibiting excellent microwave absorption potential and becoming a research focus and mainstream choice in the field of microwave absorbing materials. However, ordinary flake-shaped carbonyl iron powder has a high dielectric constant due to its high conductivity, making impedance matching difficult and affecting the realization of microwave absorption capabilities.

[0004] To address the aforementioned challenges, it is typically necessary to coat the surface of the flake carbonyl iron powder to modify its surface. However, existing coating methods generally suffer from poor process adaptability and difficulty in mass production, making it difficult to meet the demands of large-scale industrial production. Furthermore, existing coating methods are prone to defects such as uneven coating layer distribution and weak adhesion, making it difficult to effectively reduce its dielectric constant and further optimize its impedance matching performance. Summary of the Invention

[0005] In view of this, the present invention provides a coated electromagnetic wave absorber and its preparation method. The coated electromagnetic wave absorber provided by the present invention solves the problem of excessively high conductivity and high dielectric constant of flake carbonyl iron powder, optimizes impedance matching, and improves wave absorption performance.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing a coated electromagnetic wave absorber, comprising the following steps: (1) The flake carbonyl iron powder and the ethanol aqueous solution are mixed and heated to obtain a dispersion of flake carbonyl iron powder; the volume ratio of anhydrous ethanol to deionized water in the ethanol aqueous solution is (8~12):(1~4). (2) The dispersion of flake carbonyl iron powder is mixed with ammonia and tetraethyl orthosilicate and coated to obtain a flake carbonyl iron powder slurry with a coating layer. (3) The flake carbonyl iron powder slurry is subjected to solid-liquid separation, and the resulting solid phase is dried to obtain a coated electromagnetic wave absorber.

[0007] Preferably, the mass-to-volume ratio of the flake carbonyl iron powder to the ethanol aqueous solution is (1~3) kg: (5~8) L.

[0008] Preferably, the final heating temperature can be 40~60℃.

[0009] Preferably, the first mixing is stirring; the stirring speed is 150~300 r / min.

[0010] Preferably, the mass concentration of the ammonia water is 20%; the volume ratio of the ethanol aqueous solution to the ammonia water and the tetraethyl orthosilicate is (10~15):(4~7):(3~5).

[0011] Preferably, the second mixing method is stirring; the stirring speed is 150~300 r / min, and the stirring time is 4~6 h.

[0012] Preferably, the drying is vacuum drying; the vacuum drying temperature is 45~70℃ and the time is 15~20h.

[0013] The present invention also provides a coated electromagnetic wave absorber prepared by the above preparation method, comprising flake carbonyl iron powder and a silica coating layer coating the flake carbonyl iron powder.

[0014] Preferably, the thickness of the silicon dioxide coating layer is 80~200nm.

[0015] The coated electromagnetic wave absorber prepared in this invention modifies the surface of flake carbonyl iron powder through a coating process, forming a silica coating layer on its surface. As an excellent insulator, the silica coating layer effectively prevents the formation of conductive circuits between flake carbonyl iron powder particles and hinders the formation of conductive networks in the flake structure, thereby effectively reducing the dielectric constant and maintaining a high magnetic permeability. This improves the impedance mismatch problem in the design of the electromagnetic wave absorber of flake carbonyl iron powder in the microwave absorbing coating, better achieves impedance matching, and improves the microwave absorption performance. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the embodiments will be briefly described below.

[0017] Figure 1 A schematic diagram of the process flow for preparing the coated electromagnetic wave absorber provided by the present invention; Figure 2 This is a scanning electron microscope (SEM) image of the flake carbonyl iron powder used as the raw material in Example 1. Figure 3 SEM image of the coated electromagnetic wave absorber prepared in Example 1; Figure 4 This is a comparison diagram of the real part of the dielectric constant of the coated electromagnetic wave absorber prepared in Example 1 and the original material; Figure 5 This is a comparison diagram of the imaginary part of the magnetic permeability of the coated electromagnetic wave absorber prepared in Example 1 and the original material; Figure 6 This is a comparison diagram of the real part of the dielectric constant of the coated electromagnetic wave absorber prepared in Example 2 and the original material; Figure 7 This is a comparison diagram of the imaginary part of the magnetic permeability of the coated electromagnetic wave absorber prepared in Example 2 and the original material; Figure 8 This is a comparison of the reflectivity curves of the coated electromagnetic wave absorber prepared in Example 1 and the original material. Figure 9 The graph shows a comparison of the real part of the dielectric constant between the original flake carbonyl iron powder and Comparative Examples 1 and 2. Figure 10 The graph shows a comparison of the real part of the dielectric constant between the original flake carbonyl iron powder and Comparative Examples 3-4. Figure 11 The diagram shows a comparison of the real part of the dielectric constant of the coated electromagnetic wave absorber prepared in Examples 1 and Comparative Examples 5-7 with that of the original material. Detailed Implementation

[0018] This invention provides a method for preparing a coated electromagnetic wave absorber, comprising the following steps: (1) The flake carbonyl iron powder is first mixed with an ethanol aqueous solution and heated to obtain a dispersion of flake carbonyl iron powder; (2) The dispersion of flake carbonyl iron powder is mixed with ammonia and tetraethyl orthosilicate and coated to obtain a flake carbonyl iron powder slurry with a coating layer. (3) The flake carbonyl iron powder slurry is subjected to solid-liquid separation, and the resulting solid phase is dried to obtain a coated electromagnetic wave absorber.

[0019] Unless otherwise specified, all raw materials used in this invention are commercially available products in the art.

[0020] In this invention, flake-shaped carbonyl iron powder is first mixed with an aqueous ethanol solution and then heated to obtain a dispersion of flake-shaped carbonyl iron powder.

[0021] In this invention, the mass-to-volume ratio of the flake-shaped carbonyl iron powder to the ethanol-water solution is (1~3) kg:(5~8) L, specifically 1 kg:5 L, 2 kg:7 L, or 3 kg:8 L; in this invention, the ethanol-water solution can be obtained by mixing anhydrous ethanol and deionized water in a volume ratio of (8~12):(1~4), wherein the volume ratio of anhydrous ethanol to deionized water can specifically be 8:1, 9:2, 10:3, or 11:4. In this invention, the first mixing can be stirring; the stirring speed can be 150~300 r / min, specifically 180 r / min, 200 r / min, 240 r / min, or 270 r / min; the final heating temperature can be 40~60℃, specifically 40℃, 50℃, 55℃, or 60℃.

[0022] The present invention involves mixing a dispersion of flake carbonyl iron powder with ammonia and tetraethyl orthosilicate, and then performing a coating treatment to obtain a slurry of flake carbonyl iron powder containing a coating layer.

[0023] In this invention, the mass concentration of the ammonia water can be 20%.

[0024] In this invention, the volume ratio of the ethanol aqueous solution to ammonia and tetraethyl orthosilicate can be (10~15):(4~7):(3~5), specifically 10:4:3, 11:4:4, 12:6:5 or 13:7:5. In this invention, when the proportion of tetraethyl orthosilicate in the volume ratio of ethanol aqueous solution to ammonia and tetraethyl orthosilicate is too high, the reactant concentration is too high, causing the reactants to form a film on their own instead of at the required film-forming point, i.e., on the surface of the carbonyl iron powder, and thus precipitate, making it difficult to form a coating layer and resulting in no reduction in the dielectric constant. When the proportion of tetraethyl orthosilicate is too low, the reactant concentration is too low and the coating layer is too thin to cover the surface of the carbonyl iron powder, also making it difficult to form a coating layer and resulting in no reduction in the dielectric constant. The content of tetraethyl orthosilicate can affect the thickness and density of the coating layer.

[0025] In this invention, the second mixing method is stirring; the stirring speed can be 150~300 r / min, and the stirring time can be 4~6 h, specifically 4 h, 4.5 h, 5 h, or 6 h. When the stirring time is too short, the coating reaction time is insufficient, the coating material is relatively loose, it is difficult to form an effective coating layer, and it is impossible to effectively reduce the dielectric constant; when the stirring time is too long, the coating layer on the surface of the flake carbonyl iron powder gradually thickens, the coating material gradually accumulates, the coating layer adheres the carbonyl iron powder together, causing a certain degree of agglomeration, promoting the contact between particles and thus greatly improving conductivity, which in turn increases the dielectric constant; and the greater the coating layer thickness, the lower the proportion of magnetic absorber, and the lower the magnetic permeability; the reaction time directly determines the amount of coating material and the thickness of the coating layer.

[0026] In this invention, the coating process employs a sol-gel method. During the coating process, tetraethyl orthosilicate undergoes a hydrolysis reaction with deionized water, where the ethoxy group is replaced by a carbonyl group, generating silicic acid and ethanol. The addition of ammonia provides an alkaline environment, under which the silicic acid rapidly condenses, undergoing a polycondensation reaction to generate a Si-O-Si network structure, thus producing silicon dioxide. Simultaneously, a uniform silicon dioxide film is formed on the surface of the flake-shaped carbonyl iron powder, becoming the coating layer, effectively reducing the dielectric constant and improving the microwave absorption performance.

[0027] The present invention involves solid-liquid separation of the flake-shaped carbonyl iron powder slurry, followed by drying of the resulting solid phase to obtain a coated electromagnetic wave absorber.

[0028] In this invention, the solid-liquid separation is not specifically limited and can be carried out in a pneumatic filter press, as long as most of the liquid medium can be removed to obtain the solid components.

[0029] In this invention, the drying can be vacuum drying; the temperature of the vacuum drying can be 45~70℃, specifically 45℃, 50℃, 55℃, 60℃, 65℃ or 70℃; the time can be 15~20h, specifically 15h, 16h, 17h, 18h, 19h or 20h.

[0030] In this invention, placing the moistened powder into a vacuum drying device can effectively prevent the powder from oxidizing and deteriorating during the drying process, and can also lower the boiling point of the solvent. Setting the drying temperature to 45~70℃ and continuously maintaining the temperature for 15~20h can ensure that the anhydrous ethanol is fully evaporated and removed. Using this process method will not change the microscopic particle size and surface morphology of the coated powder, effectively improving the uniformity of powder dispersion, inhibiting particle agglomeration and clumping, and thus optimizing the overall microwave absorption performance of the powder.

[0031] In this invention, an aqueous ethanol solution is used as a solvent, ammonia is used as a catalyst to provide an alkaline environment, and tetraethyl orthosilicate is used as the silicon source material. The sol-gel method is used to make tetraethyl orthosilicate undergo hydrolysis-condensation reaction to generate a silicon dioxide film, i.e., the coating layer.

[0032] This invention, through adjusting various process ratios such as solution proportions, silicon source concentration, and reaction time, produces electromagnetic wave absorbers with controllable particle size and morphology, good monodispersity, and pure, impurity-free particle surfaces, making them suitable for high-end applications. Furthermore, the resulting coating layer has uniform thickness, solving the problem of uneven coating in existing processes and improving process stability. In addition, the equipment used in the preparation method described in this invention is all conventional industrial equipment, simple to operate, and with low equipment requirements; the preparation method is controllable, easy to scale up, and suitable for industrial promotion and large-scale production.

[0033] The present invention also provides an electromagnetic wave absorber with a coating layer prepared by the above preparation method, comprising flake carbonyl iron powder and a silica coating layer coating the flake carbonyl iron powder.

[0034] In this invention, the thickness of the silicon dioxide coating layer can be 80~200nm, specifically 100~120nm.

[0035] In this invention, through a coating process, a silica film coating layer is formed on the surface of the flake carbonyl iron powder. This silica coating layer, as an excellent insulator, can effectively prevent the formation of conductive circuits between the flake carbonyl iron powder particles, hinder the formation of conductive networks in the flake structure, thereby effectively reducing the dielectric constant and maintaining a high magnetic permeability, achieving better impedance matching, and improving the wave absorption performance.

[0036] To further illustrate the present invention, the technical solutions provided by the present invention will be described in detail below with reference to the accompanying drawings and embodiments, but these should not be construed as limiting the scope of protection of the present invention.

[0037] Example 1 (1) Add the flake carbonyl iron powder and the ethanol aqueous solution to a glass reactor. The volume ratio of anhydrous ethanol to deionized water in the ethanol aqueous solution is 9:2, and the mass-volume ratio of the flake carbonyl iron powder to the ethanol aqueous solution is 2kg:7L. Stir evenly at a speed of 180r / min and heat to 50℃ and keep constant temperature. (2) Add 20wt.% ammonia water and tetraethyl orthosilicate to a glass reactor, so that the volume ratio of ethanol aqueous solution, ammonia water and tetraethyl orthosilicate is 10:4:3; keep the rotation speed at 180r / min and continue to stir evenly for 4h at a constant temperature of 50℃ to carry out coating treatment and obtain a sheet-like carbonyl iron powder slurry with a coating layer. (3) The slurry is separated into solid and liquid components by a filter press. The solid components are then put into a vacuum dryer and dried at a drying temperature of 60°C for 15 hours to remove residual liquid. The dried carbonyl iron powder with a coating layer on the surface is obtained, which is the coated electromagnetic wave absorber.

[0038] Figure 2 This is a scanning electron microscope (SEM) image of the original material, flake-shaped carbonyl iron powder, from Example 1. Figure 3 This is a scanning electron microscope (SEM) image of the coated electromagnetic wave absorber prepared in Example 1. Figures 2-3 It can be seen that the powders before and after coating are both micron-sized flakes; the uncoated flake carbonyl iron powder has a smooth and clean surface, and after coating modification, a coating layer with a thickness of 100~120nm is uniformly attached to the surface of the particles, the outline of the flake matrix remains unchanged, and the coating layer covers the surface of the powder.

[0039] Example 2 (1) Add the flake carbonyl iron powder and the ethanol aqueous solution to a glass reactor. The volume ratio of anhydrous ethanol to deionized water in the ethanol aqueous solution is 8:1. The mass-volume ratio of the flake carbonyl iron powder to the ethanol aqueous solution is 3kg:8L. Stir evenly at a speed of 250r / min and heat to 55℃ and keep constant temperature. (2) Add 20wt.% ammonia water and tetraethyl orthosilicate to a glass reactor, so that the volume ratio of ethanol aqueous solution, ammonia water and tetraethyl orthosilicate is 13:7:5; keep the rotation speed at 250r / min and continue to stir evenly for 6h at a constant temperature of 55℃ to carry out coating treatment and obtain a sheet-like carbonyl iron powder slurry with a silica coating layer. (3) The slurry is separated into solid and liquid components by a filter press, and the solid components are put into a vacuum dryer and dried at a drying temperature of 50°C for 18 hours to remove residual liquid, thereby obtaining dried flake carbonyl iron powder with a silica coating layer (thickness of 100~120nm) on the surface, which is the coated electromagnetic wave absorber.

[0040] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that the volume ratio of ethanol aqueous solution, ammonia and tetraethyl orthosilicate is 12:5:1, and the thickness of the silica coating layer in the final coated electromagnetic wave absorber is 30~40nm.

[0041] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that the volume ratio of ethanol aqueous solution, ammonia and tetraethyl orthosilicate is 13:7:8, and the thickness of the silica coating layer in the final coated electromagnetic wave absorber is 30~40nm.

[0042] Comparative Example 3 The only difference between Comparative Example 3 and Example 2 is that the stirring time for coating is 2 hours, and the thickness of the silica coating layer in the final coated electromagnetic wave absorber is 30~40nm.

[0043] Comparative Example 4 The only difference between Comparative Example 4 and Example 2 is that the stirring time for coating is 8 hours, and the thickness of the silica coating layer in the final coated electromagnetic wave absorber is 280~350nm.

[0044] Comparative Example 5 The only difference from Example 1 is that the mass ratio of anhydrous ethanol to deionized water in the ethanol aqueous solution is 15:1, and the thickness of the silica coating layer in the final coated electromagnetic wave absorber is 10~20nm.

[0045] Comparative Example 6 The only difference from Example 1 is that the mass ratio of anhydrous ethanol to deionized water in the ethanol aqueous solution is 14:1, and the thickness of the silica coating layer in the final coated electromagnetic wave absorber is 10~20nm.

[0046] Comparative Example 7 The only difference from Example 1 is that the mass ratio of anhydrous ethanol to deionized water in the ethanol aqueous solution is 16:1, and the thickness of the silica coating layer in the final coated electromagnetic wave absorber is 10~20nm.

[0047] The coated electromagnetic wave absorbers obtained in Examples 1-2 and Comparative Examples 1-7 were used as samples to test their electromagnetic parameters and reflectivity. Specifically, the coated electromagnetic wave absorbers obtained in the examples and comparative examples were mixed with paraffin wax at a mass ratio of 2:8. Coaxial ring samples with a thickness of 2 mm were prepared according to the standard of 3.04 mm inner diameter and 7 mm outer diameter. Their permeability and dielectric constant were tested in the range of 2-18 GHz, and their reflectivity was calculated. The results are shown in Table 1.

[0048] Table 1 Test results of coated electromagnetic wave absorbers

[0049] From Table 1 and Figures 4-5 as well as Figure 9 The data shows that, based on Example 1 and Comparative Examples 1-2, when the tetraethyl orthosilicate content is too high in the ratio of ethanol aqueous solution, ammonia water, and tetraethyl orthosilicate, the reactant concentration is too high, causing the reactants to form a film on their own instead of at the required film-forming point (i.e., the surface of carbonyl iron powder), resulting in precipitation and difficulty in forming a coating layer, thus failing to reduce the dielectric constant. Conversely, when the tetraethyl orthosilicate content is too low, the reactant concentration is too low, resulting in a coating layer that is too thin to cover the surface of the carbonyl iron powder, again making it difficult to form a coating layer and failing to reduce the dielectric constant. The tetraethyl orthosilicate content affects the thickness and density of the coating layer, see [reference needed]. Figure 9 When the volume ratio of aqueous ethanol, ammonia, and tetraethyl orthosilicate is (10~15):(4~7):(3~5), the electromagnetic wave absorber obtained after coating exhibits a relatively unchanged imaginary part of permeability compared to the original material, while the real part of the dielectric constant is effectively reduced, with its maximum value decreasing from 15 to 11. (See...) Figures 4-5 .

[0050] Based on Examples 2 and Comparative Examples 3 and 4, it can be observed that when the stirring time is too short, the coating reaction time is insufficient, the coating material is relatively loose, making it difficult to form an effective coating layer and thus failing to effectively reduce the dielectric constant. When the stirring time is too long, the coating layer on the surface of the flake carbonyl iron powder gradually thickens, and the coating material gradually accumulates. The coating layer binds the carbonyl iron powder together, causing a certain degree of agglomeration, which promotes the contact between particles and thus significantly improves conductivity, thereby increasing the dielectric constant. Furthermore, the greater the coating layer thickness, the lower the proportion of magnetic absorber and the lower the magnetic permeability. The reaction time directly determines the amount of coating material and the thickness of the coating layer, see... Figure 10 When the stirring time for coating is 4-6 hours, the electromagnetic wave absorber obtained after coating maintains essentially the same imaginary part of permeability compared to the original material, while the real part of dielectric constant is effectively reduced, with its maximum value decreasing from 15 to 12.5. (See...) Figures 6-7 .

[0051] In summary, when the volume ratio of ethanol aqueous solution, ammonia water and tetraethyl orthosilicate is (10~15):(4~7):(3~5), and the stirring time for coating is 4~6h, the prepared coated electromagnetic wave absorber can improve impedance matching and enhance wave absorption performance.

[0052] The coated electromagnetic wave absorber prepared according to the present invention has the following characteristics in the frequency range of 2~18GHz: In Example 1, the imaginary part of the permeability μ”max is 1.7 and the real part of the dielectric constant ε’max is 11; in Example 2, the imaginary part of the permeability μ”max is 1.55 and the real part of the dielectric constant ε’max is 12.5. The test results show that the preparation method of the present invention can effectively reduce the dielectric constant while maintaining the permeability of the material.

[0053] The reflectivity calculated based on the test results of Example 1 shows that the coated electromagnetic wave absorber prepared in this invention significantly improves the reflectivity of electromagnetic waves in the low-frequency band. The absorption bandwidth with reflectivity below -5dB increases from 6GHz to 10GHz, and the extreme reflectivity value increases from -8.5dB to -11.9dB. Figure 8 As shown.

[0054] Figure 11 This is a comparison diagram of the real parts of the dielectric constants of the coated electromagnetic wave absorbers prepared in Examples 1, 5, 6, and 7 and the original materials. Figure 11 It can be seen that when the relative content of deionized water is too low in the ratio of anhydrous ethanol to deionized water, it cannot provide enough water for the hydrolysis reaction of tetraethyl orthosilicate, resulting in incomplete and insufficient hydrolysis, leaving a large number of silanol groups, and failing to form a dense coating network, which directly affects the material properties and the dielectric constant cannot be effectively reduced.

[0055] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing a coated electromagnetic wave absorber, comprising the following steps: (1) The flake carbonyl iron powder and the ethanol aqueous solution are mixed and heated to obtain a dispersion of flake carbonyl iron powder; the volume ratio of anhydrous ethanol to deionized water in the ethanol aqueous solution is (8~12):(1~4). (2) The dispersion of flake carbonyl iron powder is mixed with ammonia and tetraethyl orthosilicate and coated to obtain a flake carbonyl iron powder slurry with a coating layer. (3) The flake carbonyl iron powder slurry is subjected to solid-liquid separation, and the resulting solid phase is dried to obtain a coated electromagnetic wave absorber.

2. The preparation method according to claim 1, characterized in that, The mass-to-volume ratio of the flake carbonyl iron powder to the ethanol aqueous solution is (1~3) kg: (5~8) L.

3. The preparation method according to claim 1, characterized in that, The final heating temperature can be 40~60℃.

4. The preparation method according to claim 1, characterized in that, The first mixing is stirring; the stirring speed is 150~300 r / min.

5. The preparation method according to claim 1, characterized in that, The mass concentration of the ammonia water is 20%; the volume ratio of the ethanol aqueous solution to the ammonia water and the tetraethyl orthosilicate is (10~15):(4~7):(3~5).

6. The preparation method according to claim 1, characterized in that, The second mixing method is stirring; the stirring speed is 150~300 r / min, and the stirring time is 4~6 h.

7. The preparation method according to claim 1, characterized in that, The drying process is vacuum drying; the vacuum drying temperature is 45~70℃, and the time is 15~20h.

8. The electromagnetic wave absorber with a coating layer prepared by any one of claims 1 to 7, characterized in that, It includes flake carbonyl iron powder and a silica coating layer that coats the flake carbonyl iron powder.

9. The electromagnetic wave absorber with a coating layer as described in claim 8, characterized in that, The thickness of the silica coating layer is 80~200nm.

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