Double-layer coated carbonyl iron powder, preparation method thereof and wave-absorbing material

Abstract

The invention provides double-layer coated carbonyl iron powder and a preparation method thereof and a wave-absorbing material, and belongs to the technical field of wave-absorbing materials, the double-layer coated carbonyl iron powder comprises carbonyl iron powder particles, and a silane coupling agent layer and a silicon dioxide layer which sequentially coat the carbonyl iron powder particles from inside to outside. The surfaces of carbonyl iron powder particles are coated with the silane coupling agent layer and the silicon dioxide layer, two functional groups of an inorganic material and an organic material exist in silane coupling agent molecules at the same time, and the organic material and the inorganic material can be combined, so that the silane coupling agent can be used as a bridge for connecting the flaky iron powder and the silicon dioxide; the silane coupling agent is connected with the surface of the carbonyl iron powder and the silicon dioxide, so that the bonding strength of the silicon dioxide and the carbonyl iron powder is improved, the interface performance of the material is further improved, and the double-layer coated carbonyl iron powder successfully has the 250 DEG C oxidation resistance.

Description

Technical Field

[0001] The present invention relates to the technical field of microwave absorbing materials, and particularly relates to a double-layer coated carbonyl iron powder, a preparation method thereof, and a microwave absorbing material. Background Art

[0002] In recent years, with the further upgrading of weaponry, the speed requirements during the flight of aircraft, missiles, etc. have become increasingly high, resulting in a large amount of heat generated by the friction between the aircraft skin and missile casing and the air. In order to achieve the required stealth effect, stealth materials not only need to meet the requirements of "thin, light, strong, and wide", but also need to have properties such as high temperature resistance and oxidation resistance.

[0003] Carbonyl iron powder is a traditional magnetic loss type microwave absorbing matrix, which is an onion-like spherical ultrafine powder obtained by the pyrolysis of Fe(CO) 5 Among them, the flaky carbonyl iron powder overcomes the Snoek limit, and has the characteristics of low density, high dielectric loss, higher magnetic loss, broadband absorption, and better impedance matching, and has good application prospects and development potential in the field of radar stealth microwave absorption. However, in practical applications, it is found that flaky carbonyl iron powder has defects such as poor oxidation resistance at high temperatures and poor corrosion resistance, and the bonding strength between the prepared coating and the matrix is not high.

[0004] The invention patent "A low-reflectivity microwave absorbing material and a preparation method thereof" with the application number 202010702675.6 proposes to oxidize carbonyl iron powder by bluing treatment and then coat it with a silica layer. Although this method can improve the corrosion resistance, high temperature resistance, and insulation performance of carbonyl iron powder, the bluing oxidation causes a large amount of iron on the surface of carbonyl iron powder to be oxidized, and it is difficult to control the thickness of the oxide layer, significantly affecting the magnetic properties of carbonyl iron powder. Summary of the Invention

[0005] In view of the technical problems existing in the background art, the present application provides a double-layer coated carbonyl iron powder, a preparation method thereof, and a microwave absorbing material, aiming to solve the technical problems that carbonyl iron powder has poor oxidation resistance at high temperatures, poor corrosion resistance, and the bonding strength between the prepared coating and the matrix is not high.

[0006] In a first aspect, an embodiment of the present application provides a double-layer coated carbonyl iron powder, including carbonyl iron powder particles, and a silane coupling agent layer and a silica layer sequentially coated on the carbonyl iron powder particles from the inside to the outside.

[0007] In some embodiments, the double-layer coated carbonyl iron powder is flaky, the diameter of the double-layer coated carbonyl iron powder is 5 - 10 μm, and the thickness is 0.6 - 1.5 μm.

[0008] In some embodiments, the thickness ratio of the carbonyl iron powder particles to the silica layer is (60 - 150):(2 - 9).

[0009] In some embodiments, the thickness of the silica layer is 20 - 90 nm.

[0010] In some embodiments, the silane coupling agent is at least one of γ - aminopropyltriethoxysilane and γ - glycidoxypropyltrimethoxysilane.

[0011] In some embodiments, the silica layer is prepared by hydrolysis of tetraethyl orthosilicate.

[0012] In a second aspect, an embodiment of the present application provides a method for preparing double - coated carbonyl iron powder, including the following steps: Disperse carbonyl iron powder particles in deionized water to obtain a carbonyl iron powder dispersion; Disperse the silane coupling agent in a solvent to obtain a silane coupling agent dispersion; Mix the carbonyl iron powder dispersion and the silane coupling agent dispersion evenly, then heat under reflux. After the reaction is completed, dry to obtain carbonyl iron powder coated with the silane coupling agent; Disperse the carbonyl iron powder coated with the silane coupling agent in tetraethyl orthosilicate, add ammonia water and deionized water thereto, and obtain double - coated carbonyl iron powder after hydrothermal reaction, washing, and drying.

[0013] In some embodiments, the ratio of carbonyl iron powder to the silane coupling agent is (1 - 8) g: 5 mL.

[0014] In some embodiments, the ratio of the carbonyl iron powder coated with the silane coupling agent to the silicate is (1 - 8) g: 25 mL.

[0015] In some embodiments, the conditions for heating under reflux are: heating temperature is 40 - 60 °C, stirring speed is 300 - 500 r / min, and heating time is 5 - 10 h; The conditions for hydrothermal reaction are: heating temperature is 60 - 80 °C, stirring speed is 250 - 300 r / min, and heating time is 5 - 8 h.

[0016] In a second aspect, an embodiment of the present application provides an absorbing material, including double - coated carbonyl iron powder.

[0017] Different from the prior art solutions, the beneficial effects of the present application include: 1. In the present application, a silane coupling agent layer and a silica layer are coated on the surface of carbonyl iron powder particles. The silane coupling agent molecule has two functional groups, namely, an inorganic - material - loving functional group and an organic - material - loving functional group, which can combine organic materials and inorganic materials. Therefore, it can be used as a bridge connecting flaky iron powder and silica. The silane coupling agent connects the surface of carbonyl iron powder and silica, improves the bonding strength between silica and carbonyl iron powder, and further improves the interfacial properties of the material, successfully enabling the double - coated carbonyl iron powder to have the performance of antioxidant at 250 °C.

[0018] 2. The present application uniformly coats the silica material on the surface of flaky carbonyl iron powder by a hydrothermal method, enabling it to have good antioxidant performance and corrosion resistance, while still retaining the original magnetic properties. The coating method of the present application is simple and easy to operate, and the equipment used is inexpensive, with high production efficiency and market prospects.

[0019] 3. The present application uses silica as a wave-transparent agent and antioxidant, which can improve the antioxidant performance of flaky carbonyl iron powder without affecting the wave-absorbing performance of the material.

[0020] The above description is only an overview of the technical solution of the present application. In order to better understand the technical means of the present application, it can be implemented according to the content of the specification. And in order to make the above and other purposes, features and advantages of the present application more obvious and understandable, the following specifically gives the specific implementation manners of the present application. Brief Description of the Drawings

[0021] In order to more clearly illustrate the technical solution of the present application, the drawings used in the present application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0022] Figure 1 It is the EDS energy spectrum scanning diagram of the surface of the double-layer coated carbonyl iron powder prepared in Example 2 of the present application.

[0023] Figure 2 It is the cross-sectional EDS energy spectrum scanning diagram of the double-layer coated carbonyl iron powder prepared in Example 2 of the present application.

[0024] Figure 3 It is the XRD test diagram of the double-layer coated carbonyl iron powder prepared in Examples 1 - 4 of the present application before and after sintering.

[0025] Figure 4 It is the oxidation weight gain detection diagram of the double-layer coated carbonyl iron powder prepared in Example 2 of the present application.

[0026] Figure 5 It is the electromagnetic loss performance detection diagram of the double-layer coated carbonyl iron powder and the uncoated carbonyl iron powder prepared in Example 2 of the present application. Detailed Description of the Specific Embodiments

[0027] The following will describe in detail the embodiments of the technical solution of the present application with reference to the drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, so they are only examples and cannot be used to limit the protection scope of the present application.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the technical field to which this application belongs; the terms used herein are for the purpose of describing specific embodiments only and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above description of the drawings are intended to cover non-exclusive inclusion.

[0029] In the description of the embodiments of this application, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity, specific order or primary-secondary relationship of the indicated technical features. In the description of the embodiments of this application, the meaning of "a plurality" is more than two, unless otherwise specifically defined.

[0030] Referring to "embodiments" herein means that the specific features, structures or characteristics described in connection with the embodiments can be included in at least one embodiment of this application. The phrase appearing at various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described herein can be combined with other embodiments.

[0031] In the description of the embodiments of this application, the term "and / or" is merely a description of the association relationship of associated objects, indicating that there can be three relationships, for example, A and / or B, which can mean: A exists alone, A and B exist simultaneously, and B exists alone. In addition, the character " / " in this article generally represents an "or" relationship between the associated objects before and after.

[0032] In the description of the embodiments of this application, the term "a plurality" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0033] Carbonyl iron powder is a traditional magnetic loss type microwave absorbing matrix, which is an onion-like spherical ultrafine powder obtained by the pyrolysis of Fe(CO) 5 The flaky carbonyl iron powder overcomes the Snoek limit, has the characteristics of low density, high dielectric loss, higher magnetic loss, broadband absorption and better impedance matching, and has good application prospects and development potential in the field of radar stealth microwave absorption. However, in practical applications, it is found that the flaky carbonyl iron powder has defects such as poor antioxidant performance and corrosion resistance at high temperatures, and the bonding strength between the prepared coating and the matrix is not high.

[0034] To solve the technical problems of poor antioxidant performance, poor corrosion resistance at high temperatures, and low bonding strength between the prepared coating and the substrate of carbonyl iron powder, the present application provides a double-coated carbonyl iron powder, its preparation method, and a microwave absorbing material. Among them, a silane coupling agent layer and a silica layer are coated on the surface of the carbonyl iron powder particles. There are two functional groups, namely, an inorganic material-philic functional group and an organic material-philic functional group, in the silane coupling agent molecule, which can combine organic materials and inorganic materials. Therefore, it can be used as a bridge connecting two materials, namely, flaky iron powder and silica. The silane coupling agent connects the surface of the carbonyl iron powder and the silica, improving the bonding strength between the silica and the carbonyl iron powder, and further improving the interfacial properties of the material. Successfully, the double-coated carbonyl iron powder has the performance of antioxidant at 250 °C.

[0035] In a first aspect, an embodiment of the present application provides a double-coated carbonyl iron powder, including carbonyl iron powder particles, and a silane coupling agent layer and a silica layer coated on the carbonyl iron powder particles in sequence from the inside to the outside.

[0036] In the technical solution of the embodiment of the present application, the currently common method for surface coating with SiO 2 adopts the hydrothermal method. The hydrothermal method refers to adding an appropriate amount of tetraethyl orthosilicate (TEOS) or other silicates to a mixture of water, ammonia water, and ethanol, heating it to a certain temperature to cause hydrolysis to synthesize SiO 2 nano-particles, and making them evenly coated on the surface of the carbonyl iron powder by stirring. Since TEOS does not contain strong polar groups such as carbonyl (-OH), amino (-NH2), and carboxyl (-COOH), it is not easy to combine with the surface of metal particles by polar adsorption in the reaction system, and can only form a loose SiO 2 coating layer on the surface of the metal particles, resulting in a weak bonding force between the coating layer and the metal powder, and a large difference in the physical properties of the two materials. Furthermore, the silica layer cannot be deposited thickly, and the insulating and corrosion-resistant functions of the silica layer cannot be fully exerted.

[0037] The present application first coats a layer of silane coupling agent layer on the surface of the carbonyl iron powder particles. On the one hand, the hydrolysis of the silane coupling agent can form a relatively dense silica coating layer on the surface of the flaky carbonyl iron powder, providing an attachment site for double coating. On the other hand, there are two functional groups, namely, an inorganic material-philic functional group and an organic material-philic functional group, in the silane coupling agent molecule, which can combine with organic materials and inorganic materials at the same time. Therefore, it can be used as a bridge connecting two materials, namely, flaky iron powder and hydrolyzed silica, improving the bonding strength between the silica and the carbonyl iron powder, and further obtaining a thicker and denser silica layer, improving the insulating and corrosion-resistant properties of the carbonyl iron powder.

[0038] Compared with the method of preparing the coating layer by mixing silane coupling agent and silica, the coating method adopted in this application is easy to control. After the silane coupling agent-coated flaky carbonyl iron powder is surface-modified, the hydrolysis of silicate will not be affected by the hydrolysis of the free silane coupling agent in the solution during the second-layer coating. If the silane coupling agent and silica are mixed and coated by a one-step method, the process control is difficult. The hydrolysis of tetraethyl orthosilicate is easily affected by the free silane coupling agent. At the same time, the two compete for the active sites on the surface of the iron powder, resulting in uneven coating of silica and affecting the antioxidant performance of the modified iron powder.

[0039] In some embodiments, the double-layer coated carbonyl iron powder is flaky, with a diameter of 5-10 μm and a thickness of 0.6-1.5 μm.

[0040] In the technical solution of the embodiment of the present application, the flaky carbonyl iron powder overcomes the Snoek limit and has the characteristics of low density, high dielectric loss, higher magnetic loss, broadband absorption, and better impedance matching, showing good application prospects and development potential in the field of radar stealth wave absorption. The double-layer coated carbonyl iron powder prepared from flaky carbonyl iron powder particles as raw materials is also flaky.

[0041] In some embodiments, the thickness ratio of the carbonyl iron powder particles to the silica layer is (60-150):(2-9), and the thickness of the silane coupling agent layer is the coating thickness at the atomic level.

[0042] In some embodiments, the thickness of the silica layer is 20-90 nm.

[0043] In some embodiments, the silane coupling agent is at least one of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane.

[0044] In some embodiments, the silica layer is prepared by hydrolyzing tetraethyl orthosilicate.

[0045] In a second aspect, the embodiment of the present application provides a method for preparing double-layer coated carbonyl iron powder, including the following steps: Disperse the carbonyl iron powder particles in deionized water to obtain a carbonyl iron powder dispersion; Disperse the silane coupling agent in a solvent to obtain a silane coupling agent dispersion; Mix the carbonyl iron powder dispersion and the silane coupling agent dispersion evenly, heat under reflux, and after the reaction is completed, dry to obtain silane coupling agent-coated carbonyl iron powder; Disperse the silane coupling agent-coated carbonyl iron powder in tetraethyl orthosilicate, add ammonia water and deionized water thereto, and obtain double-layer coated carbonyl iron powder after hydrothermal reaction, washing, and drying.

[0046] In some embodiments, the ratio of carbonyl iron powder to silane coupling agent is (1 - 8) g: 5 mL.

[0047] In some embodiments, the ratio of silane coupling agent-coated carbonyl iron powder to silicate is (1 - 8) g: 25 mL.

[0048] In some embodiments, the conditions for heating under reflux are: the heating temperature is 40 - 60 °C, the stirring speed is 300 - 500 r / min, and heating is carried out for 5 - 10 h; The conditions for the hydrothermal reaction are: the heating temperature is 60 - 80 °C, the stirring speed is 250 - 300 r / min, and heating is carried out for 5 - 8 h.

[0049] In a second aspect, an embodiment of the present application provides a wave-absorbing material, including double-layer coated carbonyl iron powder.

[0050] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application. For those where specific technologies or conditions are not indicated in the embodiments, the technologies or conditions described in the literature in the relevant field or according to the product specifications are followed. For reagents or instruments whose manufacturers are not indicated, they are all conventional products that can be obtained through commercial purchase.

[0051] I. Preparation method Example 1 A preparation method for double-layer coated carbonyl iron powder includes the following steps: 1. Coating with KH550 (γ-aminopropyltriethoxysilane) 1) Mix 4 g of carbonyl iron powder and deionized water at a mass ratio of 1:15, and disperse them by ultrasonic oscillation for 30 min to obtain a suspension; 2) Add 20 mL of KH550 to 200 mL of absolute ethanol and stir to dissolve evenly; 3) Add the carbonyl iron powder suspension in 1) and the mixed solution in 2) into a four-necked flask and mix evenly; 4) Heat the four-necked flask in a 60 °C constant temperature water bath, install a reflux pipe, and use a mechanical stirrer to stir and heat in the water bath at 400 r / min for 5 h; 5) Dry in an oven at 70 °C for 8 h to obtain a dried sample of KH550-coated carbonyl iron powder.

[0052] 2. Coating with tetraethyl orthosilicate 1) Uniformly disperse 8 g of the coated sample in 1 in 100 mL of tetraethyl orthosilicate and ultrasonic oscillate for 30 min; 2) Uniformly mix 7 mL of ammonia water and 200 mL of deionized water; 3) Stir the turbid solution in 1) at a speed of 250 r / min in a constant temperature water bath at 80 °C, and slowly add the mixed solution in 2) into the stirred turbid solution in 1). The slower the dropping rate, the better. Stir and react for 5 h; 4) After the reaction is completed, let the system stand still, pour off the upper layer solution, wash the lower layer powder with absolute ethanol three times, and dry it in a drying oven at 60 °C for 8 h to obtain a double-layer coated sample.

[0053] Example 2 The difference between Example 2 and Example 1 is that the addition amounts of each raw material are different, and other steps are the same as those in Example 1. The addition amounts of each raw material in Example 2 are shown in Table 1 below.

[0054] Example 3 The difference between Example 3 and Example 1 is that the addition amounts of each raw material are different, and other steps are the same as those in Example 1. The addition amounts of each raw material in Example 3 are shown in Table 1 below.

[0055] Example 4 The difference between Example 4 and Example 1 is that the addition amounts of each raw material are different, and other steps are the same as those in Example 1. The addition amounts of each raw material in Example 4 are shown in Table 1 below.

[0056] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the addition amounts of each raw material are different, and other steps are the same as those in Example 1. The addition amounts of each raw material in Comparative Example 1 are shown in Table 1 below.

[0057] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the addition amounts of each raw material are different, and other steps are the same as those in Example 1. The addition amounts of each raw material in Comparative Example 2 are shown in Table 1 below.

[0058] Comparative Example 3 A preparation method of double-layer coated carbonyl iron powder, which is different from Example 1 in that only a silane coupling agent layer is coated on the carbonyl iron powder, and the method includes the following steps: 1. Coating with KH550 (γ-aminopropyltriethoxysilane) 1) Mix 4 g of carbonyl iron powder and 60 mL of deionized water, and ultrasonically disperse for 30 min to obtain a suspension; 2) Add 20 mL of KH550 to 200 mL of absolute ethanol and stir to dissolve evenly; 3) Add the carbonyl iron powder suspension in 1) and the mixed solution in 2) into a four-necked flask and mix evenly; 4) Heat the four-necked flask in a constant temperature water bath at 60 °C, install a reflux pipe, and use a mechanical stirrer to stir and heat in the water bath at 400 r / min for 5 h; 5) The dried KH550-coated carbonyl iron powder sample was obtained by drying in an oven at 70 °C for 8 h.

[0059] Comparative Example 4 A preparation method of double-layer coated carbonyl iron powder, which is different from Example 1 in that only a silica layer is coated on the carbonyl iron powder, and includes the following steps: 1) 8 g of flaky carbonyl iron powder was evenly dispersed in 100 mL of tetraethyl orthosilicate (the amounts of the powder and tetraethyl orthosilicate can be increased or decreased in multiples), and ultrasonic oscillation was carried out for 30 min; 2) 7 mL of ammonia water and 200 mL of deionized water were evenly mixed; 3) The turbid liquid in 1) was stirred at a speed of 250 r / min in a constant temperature water bath at 80 °C, and the mixed solution in 2) was slowly added dropwise into the stirred turbid liquid in 1). The slower the dropping rate, the better. Stirring reaction was carried out for 5 h; After the reaction ended, the system was allowed to stand, the upper layer solution was poured out, the lower layer powder was washed 3 times with absolute ethanol, and dried in a drying oven at 60 °C for 8 h to obtain a silica-coated sample obtained by hydrolyzing tetraethyl orthosilicate.

[0060] Comparative Example 5 A preparation method of double-layer coated carbonyl iron powder, which is different from Example 1 in that a silane coupling agent is added when coating the silica layer, and includes the following steps: 1) 8 g of flaky carbonyl iron powder was evenly dispersed in 100 mL of tetraethyl orthosilicate (the amounts of the powder and tetraethyl orthosilicate can be increased or decreased in multiples), and ultrasonic oscillation was carried out for 30 min; 2) 20 mL of KH550, 7 mL of ammonia water and 200 mL of deionized water were evenly mixed; 3) The turbid liquid in 1) was stirred at a speed of 250 r / min in a constant temperature water bath at 80 °C, and the mixed solution in 2) was slowly added dropwise into the stirred turbid liquid in 1). The slower the dropping rate, the better. Stirring reaction was carried out for 5 h; After the reaction ended, the system was allowed to stand, the upper layer solution was poured out, the lower layer powder was washed 3 times with absolute ethanol, and dried in a drying oven at 60 °C for 8 h to obtain a silica-coated sample obtained by hydrolyzing tetraethyl orthosilicate.

[0061] II. Test methods 1. Test method for antioxidant performance The improvement of the antioxidant performance of the powder before and after coating was determined by carrying out an oxidation weight gain test in a box furnace at 250 °C on the prepared powders with different coatings.

[0062] 2. Test method for wave absorption performance Using a vector network analyzer, a flaky mixed sample of paraffin and flaky powder was pressed by the coaxial method to test the microwave absorption performance of the sample before and after coating.

[0063] III. Analysis of Test Results of Each Example and Comparative Example (1) An oxidation weight gain test was carried out on the double-layer coated carbonyl iron powder prepared in Examples 1-4 and Comparative Examples 1-5, and the uncoated carbonyl iron powder at 250 °C. The test results are shown in Table 1 below.

[0064] Table 1 Addition amounts of each raw material and oxidation performance data in Examples 1-4 and Comparative Examples 1-4

[0065] As can be seen from Table 1, for the double-layer coated carbonyl iron powder prepared in Examples 1-4, the oxidation weight gain of the sample did not change after 10 h of oxidation, indicating that the double-layer coated carbonyl iron powder prepared according to the methods in Examples 1-4 has good antioxidant performance.

[0066] Although the oxidation weight gain of the double-layer coated carbonyl iron powder prepared in Comparative Example 1 did not change, it was found during the test that due to the large mass of the carbonyl iron powder, uneven stirring and uneven ultrasonic oscillation occurred during the stirring process and ultrasonic oscillation process, further resulting in that the silicon dioxide generated by the hydrolysis of tetraethyl orthosilicate could not be completely coated on the surface of the carbonyl iron powder; in Comparative Example 2, when KH550 was in excess, the surface of the carbonyl iron powder was completely coated with silane coupling agent, but the excessive KH550 had no obvious effect on improving the antioxidant performance of the material; therefore, the ratio of carbonyl iron powder to silane coupling agent of (1-8 g):5 mL is more appropriate.

[0067] As can be seen from Table 1, the antioxidant performance of the powder coated with KH550 in Comparative Example 3 increased, but the single-layer coating produced by coating KH550 was only a very thin layer and could not completely resist oxidation at 250 °C. The oxidation weight gain percentage of the carbonyl iron powder coated with a silicon dioxide layer obtained in Comparative Example 4 at 250 °C was 15.3%, and the oxidation weight gain percentage of the single carbonyl iron powder at 250 °C was 24%. The improvement of the antioxidant performance of the single-layer silicon dioxide layer coating on the carbonyl iron powder at 250 °C was not obvious. Mainly because the silicon dioxide generated by the hydrolysis of tetraethyl orthosilicate in an alkaline environment was in a fluffy shape and was relatively dispersed and not concentrated on the surface of the carbonyl iron powder, and could not completely achieve the coating effect. Therefore, not coating KH550 would result in the coated silicon dioxide not being tight, affecting its antioxidant effect.

[0068] In Comparative Example 5, KH550 was added during the hydrothermal reaction for coating the silica layer. During the experiment, it was found that there was a large amount of free silica in the mixed solution, which was not coated on the carbonyl iron powder. This was because part of the silica generated by the hydrolysis of tetraethyl orthosilicate did not combine with the carbonyl iron powder, but combined with the adjacent silica under the action of KH550. When KH550 and silica were used to coat the carbonyl iron powder simultaneously, the reaction process was difficult to control, and the generated silica coating layer was not as tight as the double-layer coating layer in the present application, and the antioxidant effect was limited.

[0069] (2) The surface and cross-section of the double-layer coated carbonyl iron powder prepared in Example 2 were respectively scanned by EDS energy spectrum to obtain Figure 1 and Figure 2 .

[0070] It can be seen from Figure 1 that the silica layer is relatively evenly coated on the surface of the carbonyl iron powder, and the double-layer coating has no effect on the surface morphology of the carbonyl iron powder.

[0071] It can be seen from Figure 2 that nano-scale silica is coated on the surface of the flaky carbonyl iron powder.

[0072] (3) The double-layer coated carbonyl iron powder prepared in Examples 1-4 was sintered at 250 °C for 10 h. XRD test analysis was performed on the samples before and after sintering respectively to obtain Figure 3 , and it can be seen from Figure 3 that before and after the treatment at 250 °C, the phase of the double-layer coated carbonyl iron powder prepared in Examples 1-4 did not change significantly. It can be obtained that after the treatment at 250 °C, the double-layer coated carbonyl iron powder was not oxidized.

[0073] (4) The double-layer coated carbonyl iron powder prepared in Example 2 was analyzed by thermogravimetry (TG). The powder was heated from room temperature to 500 °C in an air atmosphere, and its oxidation weight gain percentage was observed to obtain Figure 4 , and it can be seen from Figure 4 that the double-layer coated carbonyl iron powder can withstand a high temperature oxidation of up to 442 °C.

[0074] (5) The double-layer coated carbonyl iron powder prepared in Example 2 was mixed with paraffin to make coaxial samples. Coaxial samples of uncoated hydroxyl iron powder and paraffin were prepared, where the mass percentage of the uncoated hydroxyl iron powder in the coaxial sample was 75%, denoted as raw powder - 75%; Coaxial samples with the mass percentage of the double-layer coated carbonyl iron powder in the coaxial sample being 70%, 75% and 80% were prepared respectively, denoted as coated powder - 70%, coated powder - 75% and coated powder - 80%. The electromagnetic loss performance was tested using a vector network analyzer, and the test results are shown in Figure 5 .

[0075] from Figure 5 It can be obtained that the reflection loss value of the double-layer coated carbonyl iron powder does not change much compared with the uncoated carbonyl iron powder, and increasing the iron powder content in the coaxial sample can further optimize the material's absorbing performance. Therefore, the double-layer coating of KH550 and silica layer will not seriously affect the absorbing performance of the iron powder. The present application scheme is feasible for improving the 250°C oxidation resistance of the carbonyl iron powder without affecting its absorbing performance.

[0076] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A double-layer coated carbonyl iron powder, characterized in that: The invention comprises carbonyl iron powder particles, and a silane coupling agent layer and a silicon dioxide layer which are sequentially coated on the carbonyl iron powder particles from the inside to the outside.

2. The double-layer coated carbonyl iron powder according to claim 1, characterized in that: The double-layer coated carbonyl iron powder is in the form of flakes, and has a diameter of 5 to 10 μm and a thickness of 0.6 to 1.5 μm.

3. The double-layer coated carbonyl iron powder according to claim 1, characterized in that: The thickness ratio of the carbonyl iron powder particles to the silicon dioxide layer is (60-150):(2-9).

4. The double-layer coated carbonyl iron powder according to claim 1, characterized in that: The thickness of the silicon dioxide layer is 20-90 nm.

5. The double-layer coated carbonyl iron powder according to claim 1, characterized in that: The silane coupling agent is at least one of γ-aminopropyltriethoxysilane and γ-glycidyloxypropyltrimethoxysilane.

6. The double-layer coated carbonyl iron powder according to claim 1, characterized in that: The silicon dioxide layer is prepared by hydrolyzing ethyl orthosilicate.

7. A method for preparing the double-layer coated carbonyl iron powder as claimed in any one of claims 1 to 6, characterized in that: The steps include: dispersing carbonyl iron powder particles in deionized water to obtain a carbonyl iron powder dispersion; dispersing a silane coupling agent in a solvent to obtain a silane coupling agent dispersion; The carbonyl iron powder dispersion and the silane coupling agent dispersion are mixed evenly and then heated to reflux, and dried after the reaction is completed to obtain carbonyl iron powder coated with the silane coupling agent; The carbonyl iron powder coated with the silane coupling agent is dispersed in ethyl orthosilicate, to which ammonia water and deionized water are added, and double-layer coated carbonyl iron powder is obtained after hydrothermal reaction, washing and drying.

8. The method for preparing a double-layer coated carbonyl iron powder according to claim 7, characterized in that: The ratio of the carbonyl iron powder to the silane coupling agent is (1-8) g:5 mL; and / or, The ratio of the carbonyl iron powder coated with the silane coupling agent to the silicate is (1-8) g:25 mL.

9. The method for preparing a double-layer coated carbonyl iron powder according to claim 7, characterized in that: The heating reflux conditions are: heating temperature of 40-60°C, stirring speed of 300-500r / min, heating for 5-10h; The hydrothermal reaction conditions are: heating temperature of 60-80°C, stirring speed of 250-300 r / min, and heating for 5-8 h.

10. A wave absorbing material, characterized in that: It comprises the double-layer coated carbonyl iron powder as described in claims 1 to 6.

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