A rare earth-based PMI composite microwave absorbing foam and its preparation method

Abstract

This invention provides a method for preparing rare-earth-based PMI composite microwave absorbing foam, comprising the following steps: mixing methacrylonitrile and methacrylic acid, adding a foaming agent, an initiator, and a crosslinking agent, stirring and mixing thoroughly, pouring the mixture into a polymerization mold, sealing the polymerization mold and placing it in a constant-temperature water bath for heat preservation, demolding to obtain a PMI prepolymer, and then pulverizing the PMI prepolymer to obtain PMI prepolymer particles; pretreating a composite microwave absorbing agent using a surfactant, wherein the composite microwave absorbing agent includes yttrium-doped cobalt powder and carbon nanotubes; adding the pretreated composite microwave absorbing agent to epoxy resin, mixing thoroughly to obtain an impregnating material, uniformly mixing the PMI prepolymer particles and the impregnating material and injecting it into a mold, placing the mold in a foaming furnace for heating, cooling to room temperature, and then demolding to obtain the composite microwave absorbing foam. The foam of this invention can fully utilize the advantages of rare-earth-based and carbon-based microwave absorbing agents, possessing good and stable mechanical properties and microwave absorption performance, and has good military application value.

Description

Technical Field

[0001] This invention relates to the field of microwave absorbing materials technology, and in particular to a rare earth-based PMI composite microwave absorbing foam and its preparation method. Background Technology

[0002] Polymethacrylimide (PMI) foam possesses low density, excellent mechanical properties, and high-temperature resistance, making it an ideal core material for structural microwave absorbing materials. However, PMI foam itself has a low dielectric constant, which prevents it from providing electromagnetic loss function. Researchers have attempted to introduce low-cost metallic microwave absorbing agents (such as ferrites and iron carbonyl) and lightweight carbon-based microwave absorbing agents (such as carbon black, carbon nanotubes, graphene, and chopped carbon fibers) into PMI foam. However, metallic microwave absorbing agents have a high density, and carbon-based microwave absorbing agents are prone to agglomeration, resulting in sedimentation and poor dispersibility during the preparation process, thus leading to unstable microwave absorption performance.

[0003] In view of this, the present invention is proposed. Summary of the Invention

[0004] The purpose of this invention is to provide a rare earth-based PMI composite microwave absorbing foam and its preparation method. This foam can fully utilize the advantages of rare earth-based microwave absorbing agents and carbon-based microwave absorbing agents, and has good and stable mechanical properties and microwave absorption properties.

[0005] In a first aspect, the present invention provides a method for preparing rare-earth-based PMI composite microwave absorbing foam, comprising the following steps:

[0006] S1. Mix methacrylonitrile and methacrylic acid, add foaming agent, initiator and crosslinking agent, stir and mix well, pour into a polymerization mold, seal the polymerization mold and place it in a constant temperature water bath environment for heat preservation, demold to obtain PMI prepolymer, and then crush the PMI prepolymer to obtain PMI prepolymer particles.

[0007] S2. The composite microwave absorbing agent is pretreated with a surfactant, wherein the composite microwave absorbing agent comprises yttrium-doped cobalt powder and carbon nanotubes;

[0008] S3. Add the pretreated composite microwave absorbing agent to the epoxy resin, mix well to obtain the impregnating material, mix the PMI prepolymer particles and the impregnating material evenly and inject them into the mold, place the mold in the foaming oven for heating, cool to room temperature and demold to obtain composite microwave absorbing foam.

[0009] Preferably, in step S1, the mass ratio of methacrylonitrile to methacrylic acid is (4-6):(4-6).

[0010] Preferably, in step S1, the amount of foaming agent added is 4-8% wt, the amount of initiator added is 0.2-0.4% wt, and the amount of crosslinking agent added is 0.01-0.1% wt.

[0011] Preferably, in step S1, the foaming agent is formamide, the initiator is azobisisobutyronitrile, and the crosslinking agent is bismaleimide.

[0012] Preferably, in step S1, after sealing the polymerization mold, it is placed in a constant temperature water bath environment of 60-80℃ for 12-24 hours.

[0013] Preferably, in step S2, the mass ratio of the yttrium-doped cobalt powder to the carbon nanotubes is (50-70):(1-3).

[0014] Preferably, in step S2, the yttrium-doped cobalt is an easy-faceted yttrium-doped cobalt, such as Y₂Co. 17 .

[0015] Easy-to-surface yttrium-doped cobalt (Y2Co) 17 Y₂Co is a rare-earth soft magnetic material (REM) with strong in-plane magnetocrystalline anisotropy. Compared to ferrites, Y₂Co… 17 With its low density, high magnetic permeability, and excellent temperature resistance, it is suitable for introduction into PMI foam as a magnetic loss absorbing agent. Furthermore, carbon nanotubes (CNTs), as an electrically loss absorbing agent, can help improve impedance matching.

[0016] In this invention, the size of the yttrium-doped cobalt powder and carbon nanotubes used is not strictly limited. Preferably, the average particle size of the yttrium-doped cobalt powder is 2-10 μm and the average length of the carbon nanotubes is 10-50 μm.

[0017] Preferably, in step S2, the amount of surfactant added is 0.4-1.6%wt.

[0018] Preferably, in step S2, the surfactant includes one or more of sodium dodecyl sulfate (SDS), hexadecyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), and polyethylene glycol octylphenyl ether (Triton).

[0019] Preferably, step S2 includes: adding surfactant to deionized water, adding yttrium-doped cobalt powder and carbon nanotubes, stirring at a constant temperature of 40-80℃ for 1-2 hours, filtering, rinsing with anhydrous ethanol to remove excess surfactant, drying at 60-80℃ for 6-18 hours, and grinding to obtain composite microwave absorbing agent powder.

[0020] Preferably, in step S3, the mass ratio of the PMI prepolymer particles, epoxy resin and composite microwave absorber is (55-85):(5-30):(10-40).

[0021] Preferably, in step S3, the particle size of the PMI prepolymer particles is 3 mm or more.

[0022] In a second aspect, the present invention provides a rare-earth PMI composite microwave absorbing foam, which is prepared by the method described above for preparing rare-earth PMI composite microwave absorbing foam.

[0023] The present invention has at least the following beneficial effects:

[0024] This invention introduces yttrium-doped cobalt powder, which has low density, high magnetic permeability, and excellent temperature resistance, as a magnetic loss absorbing agent in PMI foam. Carbon nanotubes are then introduced as an electrical loss absorbing agent to further improve impedance matching. Together, they leverage the magnetic and electrical loss mechanisms, fully utilizing the low density advantages of rare-earth and carbon-based absorbing agents. Furthermore, pretreatment of the composite absorbing agent enhances its dispersibility and stability during preparation, resulting in PMI absorbing foam with excellent mechanical properties and stable absorption performance. It can be used as a core material in structural absorbing materials, possessing significant military application value. In addition, the mechanical mixing and co-foaming technology simplifies the preparation process, allowing for mass production and demonstrating broad prospects for industrial production and application. Attached Figure Description

[0025] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0026] Figure 1 A schematic diagram of the preparation process of PMI composite microwave absorbing foam provided by the present invention;

[0027] Figure 2 The images show the microstructure of REM-CNTs prepared in Example 1 of this invention; where (a) is 10 μm and (b) is 2 μm.

[0028] Figure 3 The XRD patterns of REM and REM-CNT prepared in Example 1 of this invention;

[0029] Figure 4 The images show the appearance of the PMI microwave absorbing foam prepared in Examples 1-3 and Comparative Examples 1-3 of this invention.

[0030] Figure 5 The images shown are scanning electron microscope (SEM) images of the PMI absorbing foam prepared in Comparative Examples 1-3 of this invention; where (a) is Comparative Example 1, (b) is Comparative Example 2, (c) is Comparative Example 3, (d) is a magnified view of a portion of Comparative Example 1, and (d1) and (d2) are EDS energy dispersive spectra.

[0031] Figure 6 The mechanical properties of PMI absorbing foam prepared in Examples 1-3 and Comparative Examples 1-3 of this invention are shown in the diagrams; where (a) represents compressive strength and compressive modulus, (b) represents tensile strength and tensile modulus, and (c) represents shear strength and shear modulus.

[0032] Figure 7 The diagrams show the reflection loss of PMI absorbing foam prepared in Examples 1-3 and Comparative Examples 1-3 of the present invention; where (a)-(f) correspond to Examples 1-3 and Examples 1-3, respectively. Detailed Implementation

[0033] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0034] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments according to this application. As used herein, the singular form includes the plural form unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0035] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Example 1

[0037] like Figure 1 As shown in the figure, this embodiment provides a method for preparing rare earth-based PMI composite microwave absorbing foam, using the following raw materials:

[0038] Yttrium-doped cobalt powder: average particle size 3.7 μm, purchased from Ningbo Magnetic Materials Technology Research Institute;

[0039] Carbon nanotubes: average length 10-50μm, purchased from Keruicai Industrial Products JD.com self-operated store;

[0040] Epoxy resin: E44 type resin liquid, curing agent is polyamide, purchased from Liang En Industrial Products Specialty Store;

[0041] Methacrylonitrile, methacrylic acid, azobisisobutyronitrile (initiator), formamide (foaming agent), bismaleimide (crosslinking agent), and polyethylene glycol octylphenyl ether (surfactant, Triton) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. and Sinopharm Chemical Reagent Co., Ltd., and were all of analytical grade.

[0042] Specifically, it includes the following steps:

[0043] The feed ratio of S1, methacrylonitrile (MAN), and methacrylic acid (MAA) was set to 1:1. 6% wt formamide, 0.3% wt azobisisobutyronitrile, and 0.05% wt bismaleimide were added and mixed evenly. After stirring for 2 hours, the mixture was poured into a polymerization mold.

[0044] After sealing the mold, it was placed in a constant temperature water bath at 65℃ for 12 hours. After demolding, PMI prepolymer A was obtained. The PMI prepolymer A was crushed into particles using a crusher at a speed of 600 r / min. Particles with a diameter of 3 mm or larger were separated using a sieve for subsequent foaming processes.

[0045] S2. Add 16g of polyethylene glycol octylphenyl ether (Triton) to 1000ml of deionized water, followed by 300g of yttrium-doped cobalt powder (REM) and 10g of carbon nanotubes (CNT). Stir at 60℃ for 2h and then filter. Rinse once with anhydrous ethanol to remove excess surfactant, dry at 70℃ for 12h, and grind to obtain REM-CNT composite microwave absorber.

[0046] Figure 2 The images show the microstructure of the composite REM-CNTs. As can be clearly seen from (a) and (b), the well-dispersed CNTs adhere and adsorb onto the surface of the REM particles. This work can effectively prevent CNTs from entanglement and agglomeration during mixing with PMI prepolymer particles, thereby improving the stability of the microwave absorption performance of PMI foam.

[0047] Figure 3 XRD patterns of REM and REM-CNT, with characteristic diffraction peaks and intensities of the REM sample compared to Y2Co. 17 The standard spectrum (PDF#18-0434) was consistent with the standard spectrum, with no obvious impurity peaks, indicating that the REM sample had high purity. After CNT composite, the XRD pattern showed the characteristic peaks of carbon nanotubes (002) and (100), indicating that although the amount of CNT added was relatively small, it was well dispersed on the surface of the REM particles.

[0048] S3. Add 310g of pretreated REM-CNT composite microwave absorber powder to 216g of E44 type resin liquid, mix well to obtain epoxy resin impregnating material B. Mix 850g of PMI prepolymer particles and epoxy resin impregnating material B at a speed of 1200r / min for 0.5h to obtain epoxy resin impregnating material B. Pour the mixed slurry into a 300×300×50mm slurry. 3 The cube mold is placed in a foaming oven and heated at 120°C for 1 hour. The temperature is then adjusted to 225°C and held for 3 hours. After cooling to room temperature, the foam is demolded to obtain REM-CNT / PMI foam, denoted as C1R30.

[0049] Example 2

[0050] This embodiment provides a method for preparing rare earth-based PMI composite microwave absorbing foam, which is basically the same as the steps in Example 1, except that in step S3, the following ratio is used: 282g E44 type resin liquid, 850g PMI prepolymer particles, and 320g REM-CNT composite microwave absorbing agent powder (containing 300g REM and 20g CNT), denoted as C2R30.

[0051] Example 3

[0052] This embodiment provides a method for preparing rare earth-based PMI composite microwave absorbing foam, which is basically the same as the steps in Example 1, except that in step S3, the following ratio is used: 348g of E44 type resin liquid, 850g of PMI prepolymer particles, and 330g of REM-CNT composite microwave absorbing agent powder (containing 300g of REM and 20g of CNT), denoted as C3R30.

[0053] Comparative Example 1

[0054] This embodiment provides a method for preparing rare earth-based PMI composite microwave absorbing foam, including the following steps:

[0055] The feed ratio of S1, methacrylonitrile (MAN), and methacrylic acid (MAA) is set to 1:1. Add 4-8% wt formamide, 0.2-0.4% wt azobisisoheptanenitrile, and 0.01-0.1% wt bismaleimide and mix evenly. After stirring for 2 hours, pour into a polymerization mold.

[0056] After sealing the mold, it was placed in a constant temperature water bath at 65℃ for 12 hours. After demolding, PMI prepolymer was obtained. The PMI prepolymer was crushed into particles using a crusher at a speed of 600 r / min. Particles with a diameter of 3 mm or larger were separated using a sieve for subsequent foaming processes.

[0057] S2. Add 16g of polyethylene glycol octylphenyl ether (Triton) to 1000ml of deionized water, then add 300g of yttrium-doped cobalt powder (REM), stir at 60℃ for 2h, and filter. Then wash once with anhydrous ethanol to remove excess surfactant, dry at 70℃ for 12h, and grind to obtain REM microwave absorber.

[0058] S3. Add 200g of pretreated REM microwave absorber powder to 100g of E44 type resin liquid, mix well to obtain epoxy resin impregnating material B. Mix 850g of PMI prepolymer particles and epoxy resin impregnating material B at a speed of 1200r / min for 0.5h to obtain epoxy resin impregnating material B. Pour the mixed slurry into a 300×300×50mm slurry. 3 The cube mold is placed in a foaming oven and heated at 120°C for 1 hour. The temperature is then adjusted to 225°C and held for 3 hours. After cooling to room temperature, the mold is demolded to obtain PMI composite microwave absorbing foam, denoted as R20.

[0059] Comparative Example 2

[0060] This comparative example provides a method for preparing rare earth-based PMI composite microwave absorbing foam, which is basically the same as the steps in Comparative Example 1, except that in step S3, the following ratio is used: 150g E44 type resin liquid, 850g PMI prepolymer particles, and 300g REM microwave absorbing agent powder, denoted as R30.

[0061] Comparative Example 3

[0062] This comparative example provides a method for preparing rare earth-based PMI composite microwave absorbing foam, which is basically the same as the steps in Comparative Example 1, except that in step S3, the following ratio is used: 200g E44 type resin liquid, 850g PMI prepolymer particles, and 400g REM microwave absorbing agent powder, denoted as R40.

[0063] Experimental Example 1: Optical Photographs and Microscopic Morphology Analysis

[0064] The PMI foams in Examples 1-3 and Comparative Examples 1-3 have the following appearance: Figure 4 As shown in the figure, the black absorber is uniformly distributed inside the white PMI foam, forming an interconnected network pattern. REM absorber is not enriched on the upper or lower surface of the PMI foam, and the sedimentation effect of surface REM during co-foaming is not significant, which is attributed to the relatively low density of REM. Comparing different PMI foams, it can be seen that as the amount of REM added increases, the aggregation of black REM particles at the patterns becomes more pronounced, which is clearly detrimental to the uniform dispersion of the absorber. The microstructure of the PMI foams in Comparative Examples 1-3 after cutting was observed using SEM (see...). Figure 5 ).from Figure 5As can be seen in (a) of Comparative Example 1, the pore size of the C20-PMI foam cells is relatively uniform, while the pore size is relatively smaller around the REM enrichment region. From the enlarged EDS energy dispersive spectroscopy (EDS) scan of the region... Figure 5 As can be seen from (d1) and (d2), cobalt and yttrium are mainly concentrated inside the epoxy resin and are evenly distributed. This reflects good REM dispersion and no obvious aggregation, which will be beneficial to the electromagnetic loss. As the amount of REM added continues to increase, it can be seen from... Figure 5 Figures (b) and (c) show a sharp decrease in the pore size of the C30-PMI and C40-PMI foam cells in Comparative Examples 2 and 3. This may be due to excessive REM particles inhibiting the foaming behavior of the PMI prepolymer, or excessive epoxy resin binder being added.

[0065] Experimental Example 2: Mechanical Property Testing

[0066] The mechanical properties of PMI foam were tested using an Instron 5982 electronic universal testing machine (USA). The loading speed of the testing machine was uniformly set to 5 mm / min, and all samples underwent five repeated tests. The control sample was PMI foam without absorbent filling, denoted as C0R0. Compression tests were conducted according to GB / T 8813-2008 standard. The sample was a cube with a side length of 30 mm. Tensile tests were conducted according to GB 9641-1988 standard. The sample was dumbbell-shaped with a total length of 150 mm, a clamping section width of 25 mm, and a thickness of 10 mm. Shear tests were conducted according to GB / T 1455-2005 standard. The shear clamp was 200 mm long and 60 mm wide. The sample was a cuboid with a length of 150 mm, a width of 60 mm, and a thickness of 14 mm. DP460 epoxy adhesive (3M, USA) was used to tightly bond the samples and clamps.

[0067] PMI absorber foam mechanical properties such as Figure 6 As shown, Figure 6 As shown in (a), the compressive strength of PMI foam filled with microwave absorber is slightly lower than that of unfilled PMI foam. This may be because the air in the closed cells of unfilled PMI foam provides additional support. With increasing REM content, the compressive strength first increases and then remains relatively stable; with increasing CNT content, the compressive strength shows a slight decreasing trend. Furthermore, with increasing absorber content, the error fluctuation in the sample's compressive strength increases, which is directly related to the degree of filler dispersion. In Example 1, the C1R30 sample exhibited a maximum compressive strength of 2.35 ± 0.15 MPa, an increase of 11.9% compared to unfilled PMI foam. Figure 6Figure (b) shows the histograms of tensile strength and tensile modulus for different PMI foams. It can be seen that the overall tensile strength of PMI foam is lower than that of unfilled PMI foam. This may be related to the fact that the large addition of microwave absorbers and binders inhibits the foaming process of the PMI prepolymer. The decreased integrity of the foam wall structure leads to a decrease in tensile strength. With the increase of microwave absorber content, the tensile strength of PMI foam shows an upward trend. In addition, the tensile modulus is generally higher than that of unfilled PMI foam, indicating that the introduction of microwave absorbers plays a certain reinforcing role. In Example 3, the C3R30 sample showed a maximum tensile strength of 1.19 ± 0.22 MPa, which is close to the tensile strength of unfilled PMI foam (2.10 ± 0.22 MPa). Figure 6 Figure (c) shows the histograms of shear strength and shear modulus for different PMI foams. As can be seen from the figure, the shear strength of PMI foam filled with absorber is greater than that of unfilled foam, and increases with increasing filler content. The maximum shear strength is 1.54 ± 0.10 MPa, and the maximum shear modulus is 96.26 ± 13.02 MPa, representing increases of 22.2% and 32.8%, respectively.

[0068] Experimental Example 3: Wave Absorption Performance Test

[0069] The electromagnetic parameters of PMI absorbing foam in Examples 1-3 and Comparative Examples 1-3 were tested using the free space method. The sample size was a cuboid of 100mm×100mm×30mm, and the test frequency band was 2-18GHz.

[0070] Figure 7 The curves show the electromagnetic wave reflection loss of PMI foam at different matching thicknesses. Figure 7 As can be seen in (a), only when the matching thickness is 50 mm, the reflection loss of the R20-PMI foam in Comparative Example 1 is less than -10 dB in the Ku segment. (Comparison) Figure 7 In (a)-(c), as the amount of REM added increases, the matching thickness of PMI foam to achieve the same absorption effect in the Ku band can be reduced to 30mm. When the matching thickness is 50mm, the maximum reflection loss of R30-PMI foam in Comparative Example 2 is -24.1dB, and the effective absorption bandwidth is 10.3GHz (7.7~18GHz); the maximum reflection loss of R40-PMI foam in Comparative Example 3 is -19.9dB, and the effective absorption bandwidth is 11.3GHz (6.7~18GHz). The electromagnetic reflection loss curve after adding CNTs is shown in Figure 1. Figure 7As shown in (d)-(f) in the figure. Compared with the R30-PMI foam in Comparative Example 2, it can be seen that the maximum electromagnetic reflection loss did not increase significantly, but the effective absorption bandwidth was greatly improved. In particular, when the matching thickness is 50mm, the C3R30-PMI foam in Example 3 can achieve effective absorption full coverage in the 2-18GHz frequency band. In addition, when the matching thickness is 30mm, the C2R30-PMI foam in Example 2 performs best, with a maximum reflection loss of -13.1dB and an effective absorption bandwidth of 11.0GHz (6.5~15.3GHz and 15.8~18GHz).

[0071] In summary, this invention introduces yttrium-doped cobalt powder, which has low density, high magnetic permeability, and excellent temperature resistance, as a magnetic loss absorbing agent in PMI foam. Carbon nanotubes are then introduced as an electrical loss absorbing agent to assist in improving impedance matching. This collaborative approach leverages both magnetic and electrical loss mechanisms, fully utilizing the low density advantages of rare-earth and carbon-based absorbing agents. Furthermore, pretreatment of the composite absorbing agent enhances its dispersibility and stability during preparation, resulting in PMI absorbing foam with excellent mechanical properties and stable absorption performance. This foam can be used as a core material in structural absorbing materials, possessing significant military application value. In addition, the mechanical mixing and co-foaming technology simplifies the preparation process, enabling mass production and demonstrating broad prospects for industrial production and application.

[0072] This invention fully leverages the low density of rare earth-based and carbon-based microwave absorbers, focusing on optimizing the dispersibility issue in the preparation process. It has significant practical application value and importance for developing a "light, thin, wide, strong" PMI foam with stable microwave absorption performance.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing rare-earth-based PMI composite microwave absorbing foam, characterized in that, Includes the following steps: S1. Mix methacrylonitrile and methacrylic acid, add foaming agent, initiator and crosslinking agent, stir and mix well, pour into a polymerization mold, seal the polymerization mold and place it in a constant temperature water bath environment for heat preservation, demold to obtain PMI prepolymer, and then crush the PMI prepolymer to obtain PMI prepolymer particles. S2. The composite microwave absorbing agent is pretreated with a surfactant, wherein the composite microwave absorbing agent comprises yttrium-doped cobalt powder and carbon nanotubes; S3. Add the pretreated composite microwave absorbing agent to the epoxy resin, mix well to obtain the impregnating material, mix the PMI prepolymer particles and the impregnating material evenly and inject them into the mold, place the mold in the foaming oven for heating, cool to room temperature and demold to obtain composite microwave absorbing foam.

2. The method for preparing rare earth-based PMI composite microwave absorbing foam according to claim 1, characterized in that, In step S1, the mass ratio of methacrylonitrile to methacrylic acid is (4-6):(4-6).

3. The method for preparing rare earth-based PMI composite microwave absorbing foam according to claim 1, characterized in that, In step S1, the foaming agent is formamide, the initiator is azobisisobutyronitrile, and the crosslinking agent is bismaleimide.

4. The method for preparing rare earth-based PMI composite microwave absorbing foam according to claim 1, characterized in that, In step S2, the mass ratio of the yttrium-doped cobalt powder to the carbon nanotubes is (50-70):(1-3).

5. The method for preparing rare earth-based PMI composite microwave absorbing foam according to claim 1, characterized in that, In step S2, the yttrium-doped cobalt is an easy-faceted yttrium-doped cobalt.

6. The method for preparing rare earth-based PMI composite microwave absorbing foam according to claim 1, characterized in that, In step S2, the average particle size of the yttrium-doped cobalt powder is 2-10 μm, and the average length of the carbon nanotubes is 10-50 μm.

7. The method for preparing rare earth-based PMI composite microwave absorbing foam according to claim 1, characterized in that, In step S2, the surfactant includes one or more of sodium dodecyl sulfate, hexadecyltrimethylammonium bromide, polyvinylpyrrolidone, and polyethylene glycol octylphenyl ether.

8. The method for preparing rare earth-based PMI composite microwave absorbing foam according to claim 1, characterized in that, In step S3, the mass ratio of the PMI prepolymer particles, epoxy resin and composite microwave absorber is (55-85):(5-30):(10-40).

9. The method for preparing rare earth-based PMI composite microwave absorbing foam according to claim 1, characterized in that, In step S3, the particle size of the PMI prepolymer particles is greater than 3 mm.

10. A rare-earth-based PMI composite microwave absorbing foam, characterized in that, It is prepared by the method of any one of claims 1-9 for rare earth-based PMI composite microwave absorbing foam.

Images