Preparation method and microwave absorption application of layered gradient polyimide foam material

Abstract

The invention relates to a composite material, in particular to a layered gradient polyimide foam material applied to microwave absorption. According to the invention, ionic liquid is introduced into polyimide, a heterogeneous layered structure comprising a porous layer and a compact layer is constructed, a function differentiation design is innovatively introduced into the PI-based composite material, and a CNT-coated FeO nano filler in the porous layer realizes electromagnetic wave energy dissipation by using a dual mechanism of dielectric loss (epsilon ''> 15) and magnetic loss (mu'' > 8); the high-conductivity CNT (the conductivity is greater than 10S / cm) in the compact layer inhibits and reduces secondary pollution through strong reflection.

Description

Technical Field

[0001] This invention relates to a composite material, specifically a microwave-absorbing layered gradient polyimide foam material. Background Technology

[0002] With the rapid development of 5G communications, the Internet of Things, and aerospace, electronic devices are evolving towards higher frequencies and higher integration, making electromagnetic interference (EMI) a growing concern. Traditional electromagnetic shielding materials mainly rely on metal-based composite materials, achieving shielding through reflection mechanisms. However, these materials suffer from high density, poor corrosion resistance, and insufficient shielding effectiveness at high frequencies. In recent years, polymer-based electromagnetic shielding materials have attracted attention due to their lightweight, ease of processing, and high designability. However, their intrinsic conductivity is relatively low, requiring filler doping or structural modulation to improve performance. Furthermore, single-structure materials cannot simultaneously meet the requirements for high shielding effectiveness and wideband absorption, especially in applications in the 8-12 GHz band (such as the X-band). Therefore, it is necessary to develop multifunctional composite materials that combine dielectric and magnetic losses to achieve electromagnetic wave energy dissipation and impedance matching.

[0003] Polyimide (PI), as a class of high-performance engineering plastics, has shown significant application potential in the field of electromagnetic shielding due to its excellent thermal stability (long-term operating temperature up to 250-400℃), mechanical strength and chemical corrosion resistance. However, traditional PI-based shielding materials face multiple technical challenges in the functionalization process: First, the rigid structure and low gas solubility of PI molecular chains lead to high nucleation resistance and limited cell growth during supercritical CO2 foaming, making it difficult to prepare porous structures that simultaneously meet the requirements of high porosity (>60%) and uniform pore size (10-50 μm), thus affecting the multi-reflection attenuation effect of electromagnetic waves within the material; Second, chemical foaming methods require the introduction of a large amount of crosslinking agents to improve foaming capacity, but the crosslinked structure reduces the material's recyclability and thermal processing performance, while physical foaming methods (such as freeze-drying) are prone to problems such as excessively large pore size (>100 μm) or insufficient closed-pore ratio, resulting in enhanced frequency selectivity and decreased broadband adaptability of electromagnetic shielding effectiveness; Third, homogeneous PI composite materials are difficult to achieve synergistic regulation of electromagnetic wave absorption and reflection through a single structure, especially in ultra-low reflectivity (A / R>60) applications, where traditional materials require high filler loading (>30 wt%) to achieve the target performance, but this significantly sacrifices the material's mechanical flexibility and processing fluidity. Therefore, developing layered gradient polyimide foam materials that combine high foaming quality, structural designability, and excellent electromagnetic wave absorption performance has become a key direction for breaking through existing technological bottlenecks. Summary of the Invention

[0004] To address the aforementioned problems, this invention designs a layered gradient polyimide foam material, constructing a layered gradient polyimide foam material with alternating porous and dense regions. CNT@Fe3O4 nanofillers, which have both dielectric and magnetic losses, are introduced into the porous layer, while conductive CNTs are added to the dense layer. The resulting layered gradient polyimide foam material exhibits excellent electromagnetic wave absorption performance.

[0005] The present invention provides a layered gradient polyimide foam material comprising alternating porous PI layers and dense PI layers, wherein the porous PI layers contain ionic liquid and CNT@Fe3O4 nanofiller, and the dense PI layers contain ionic liquid and CNT nanofiller.

[0006] Preferably, the ionic liquid in the porous PI layer and the ionic liquid in the dense PI layer are selected from bis(trifluoromethanesulfonyl)imide ionic liquids or their derivatives.

[0007] Preferably, the ionic liquid content in the porous PI layer is 50-70 wt%, and the CNT@Fe3O4 nanofiller content is 1-10 wt%; the ionic liquid content in the dense PI layer is 10-30 wt%, and the CNT nanofiller content is 5-20 wt%. In the dense PI layer, due to the low ionic liquid content, the composite material formed by blending PI, ionic liquid, and CNT nanofiller does not have foaming properties.

[0008] Preferably, the CNT@Fe3O4 nanofiller is prepared by a one-step coprecipitation method, the specific steps of which are as follows: carbon nanotubes are placed in water and ultrasonically stirred, then FeCl2·4H2O and FeCl3·6H2O are added, stirring is continued, and finally ammonia is added dropwise. The reaction is continued at 30-70℃ for 0.2-5 hours. The mass ratio of carbon nanotubes, FeCl2·4H2O, and FeCl3·6H2O is 0.1-0.3:0.3-0.6:1, and the molar ratio of ammonia to FeCl2·4H2O is 4-8:1.

[0009] The present invention also provides a method for preparing a layered gradient polyimide foam material, which includes the following steps: (1) Under the protection of an inert gas, aromatic tetracarboxylic dianhydride is dissolved in a polar aprotic solvent to form a homogeneous solution, and an aromatic diamine is added to the homogeneous solution to carry out a polycondensation reaction to generate a polyamic acid solution; (2) A certain amount of ionic liquid and CNT@Fe3O4 nanofiller were added to a portion of the polyamic acid solution, stirred and mixed evenly at room temperature, and the mixed solution was subjected to thermal imidization treatment and then foamed with supercritical carbon dioxide to obtain PI@ILs / CNT@Fe3O4 composite material. (3) A certain amount of ionic liquid and CNT nanofiller were added to a portion of the polyamic acid solution, stirred and mixed evenly at room temperature, and the mixed solution was subjected to thermal imidization treatment, and then foamed with supercritical carbon dioxide to obtain PI@ILs / CNT composite material. (4) The PI@ILs / CNT@Fe3O4 composite material and the PI@ILs / CNT composite material are combined by hot pressing to form the layered gradient polyimide foam material.

[0010] Preferably, the aromatic tetracarboxylic dianhydride is selected from fluorinated or non-fluorinated aromatic tetracarboxylic dianhydrides and their derivatives; the aromatic diamine is selected from fluorinated or non-fluorinated aromatic diamines and their derivatives; the ionic liquid is a bis(trifluoromethanesulfonyl)imide ionic liquid or its derivative; and the polar aprotic solvent is at least one of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), or dimethyl sulfoxide (DMSO).

[0011] Preferably, the conditions for the polycondensation reaction are: temperature 0-50℃, reaction time 4-24 hours; and the molar ratio of the ionic liquid to polyamic acid is 0.5-2:1. Preferably, the thermal imidization treatment adopts a gradient heating program: first, treat at 150-180℃ for 1-5 hours, and then heat to 190-220℃ for 0.5-3 hours.

[0012] Preferably, the supercritical carbon dioxide foaming process is a single-stage or two-stage foaming process, wherein the single-stage foaming process includes: (a) The polyimide / ionic liquid composite material was placed in a high-pressure reactor and supercritical carbon dioxide was introduced; (b) Maintain at a temperature of 120-180℃ and a pressure of 10-20 MPa for 30-120 minutes; (c) Quickly depressurize to normal pressure to induce foaming and form composite foam material; the secondary foaming is to repeat the process of primary foaming twice.

[0013] The layered gradient polyimide foam material prepared by this invention can be used in microwave absorption, wherein the porous PI layer is an absorbing layer and the dense PI layer is a shielding layer.

[0014] Compared with the prior art, the present invention has the following beneficial effects: This invention introduces ionic liquids into polyimide and utilizes different ionic liquid contents to construct a heterogeneous layered structure containing porous and dense layers. It innovatively introduces functional differentiation design into layered gradient polyimide foam materials. In the porous layer, CNT@Fe3O4 nanofillers utilize a dual mechanism of dielectric loss (ε'' > 15) and magnetic loss (μ'' > 8) to dissipate electromagnetic wave energy. In the dense layer, highly conductive CNTs (conductivity > 10)... 4 The S / cm) reduces secondary pollution by suppressing strong reflection. The heterogeneous layered structure provided by this invention innovatively introduces functional differentiation design into PI-based composite materials. The CNT@Fe3O4 nanofiller in the porous layer has an average absorption coefficient of 0.92 in the X-band (8.2-12.4 GHz), a reflection coefficient as low as 0.18, and an absorption / reflection ratio as high as 67.1. Attached Figure Description

[0016] Figure 1 (a) Schematic diagram of CNT@Fe3O4 nanofiller synthesis; (b) SEM image of CNT@Fe3O4 filler, (b') surface scan distribution of Fe element, (b'') surface scan distribution of O element; (c) XPS characterization and hysteresis loop comparison of original CNT and CNT@Fe3O4 filler; (e) Dielectric / magnetic loss properties of ILs@PI / CNT@Fe3O4 nanocomposite foam; Electromagnetic wave absorption characteristics of foamed nanocomposites with different porosities (VF) ((f) VF=38 vol.%; (g) VF=50 vol.%; (h) VF=59 vol.%), filler content: 3 wt.%; Figure 2 (a) Schematic diagram of the double-layer electromagnetic shielding material structure: the top layer is a porous layer of ILs@PI / CNT@Fe3O4, and the bottom layer is a dense layer of ILs@PI / CNT; (b) SEM micrograph of heterogeneous composite foam; (c)-(e) Schematic diagram of double-layer samples with different foam layer thicknesses and their (c')-(e') shielding effectiveness (SE) curves; (c'')-(e'') corresponding reflection coefficient (R) / transmission coefficient (T) / absorption coefficient (A) and (c''')-(e''') absorption reflectance ratio (A / R) data. Foam layer thickness parameters: [(c), (c'), (c''), (c''')]: 0 mm (no foam layer); [(d), (d'), (d''), (d''')]: 2.6 mm foam layer; [(e), (e'), (e''), (e''')]: 3.45 mm foam layer. Detailed Implementation Example

[0018] The method for preparing layered gradient polyimide foam material in this embodiment is as follows: (1) First, 0.2 g of carbon nanotubes (CNTs) were placed in 100 mL of distilled water and ultrasonically stirred for 2 hours. Then, 100 mL of a solution containing 0.5 g of FeCl2·4H2O and 1.0 g of FeCl3·6H2O was added to the CNT mixture, and the mixture was stirred for 30 minutes. Finally, ammonia water (NH3·H2O) was added dropwise (the molar ratio of ammonia water to FeCl3·6H2O was 6), and the reaction was continued at 50 °C for 1 hour. After the reaction was completed, the precipitate was collected, washed with water-ethanol, and dried to obtain CNT@Fe3O4 nanofiller.

[0019] (2) Dissolve 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane (6FDA) in a three-necked round-bottom flask equipped with a nitrogen inlet and a condenser (using DMAc as solvent). After 6FDA is completely dissolved, add 2,2'-bis(trifluoromethyl)-[1,1'-biphenyl]-4,4'-diamine (TFMB) in equimolar amounts and react overnight to obtain a polyamic acid solution.

[0020] (3) Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (content as noted above, 64wt%) and 3% CNT@Fe3O4 filler were added to the above polyamic acid solution. The mixture was stirred at room temperature for two hours, then sonicated for half an hour, and repeated three times to achieve a relatively uniform distribution. After the reaction was completed, the resulting paste-like reactant was treated on a hot plate at 160℃ for three hours, and then heated to 200℃ for one and a half hours to simultaneously remove the DMAc solvent and imide the PI to obtain the PI@ILs@CNT@Fe3O4 nanocomposite material. The composite material was hot-pressed for subsequent foaming and microwave absorption tests at a temperature of 315℃. The hot-pressed nanocomposite material was placed in a high-pressure reactor to dissolve scCO2. Before injecting CO2, the reactor needed to be preheated to the predetermined temperature. After reaching the target pressure (15 MPa), the nanocomposite material sample was kept in a high-pressure CO2 environment for 60 minutes to ensure that the material fully absorbed CO2. Subsequently, a rapid depressurization-induced foaming process was employed to obtain the PI@ILs / CNT@Fe3O4 composite foam material. By adjusting the foaming temperature, the porosity can be modified, thereby altering the volume content and distribution of the filler, ultimately achieving the goal of regulating the microwave absorption performance of the composite foam material.

[0021] (4) The preparation method of the dense layer (reflective layer) is similar to that of the porous layer (absorbing layer). The two monomers are reacted to obtain a polyamic acid solution, and then 25.6 wt% ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide and 12 wt% CNT are added. The remaining steps are the same as those of the porous layer.

[0022] (5) The PI@ILs / CNT@Fe3O4 composite material and the PI@ILs / CNT composite material, which were respectively hot-pressed, were stacked in a mold and hot-pressed at 250°C to obtain a double-layer structure. This double-layer composite material was then foamed using the above-mentioned foaming process at a holding temperature of 140°C to finally obtain the layered gradient polyimide foam material.

[0023] Characterization results To enhance the dielectric and magnetic loss capabilities of ILs@PI composite materials, such as... Figure 1 As shown in (a), CNT@Fe3O4 nanofillers with both dielectric and magnetic double loss properties were prepared by anchoring nano-Fe3O4 particles on the surface of carbon nanotubes. Figure 1 (b) In-situ TEM-EDS surface scanning analysis confirmed the presence of Fe3O4 particles (Fe / O atoms). XPS quantitative elemental analysis ( Figure 1 (c) indicates that the Fe and O atom concentrations are approximately 3.6 at.% and 7 at.%, respectively. The magnetic properties of the filler were evaluated using room temperature hysteresis loop testing (magnetic field range -12 to 12 kOe). Figure 1 (d) The magnetization curve of CNT@Fe3O4 filler exhibits an S-shaped change, and its hysteresis loop displays typical ferromagnetic characteristics, which contrasts sharply with the original carbon nanotubes (whose magnetic response is negligible). This ferromagnetic behavior is beneficial for enhancing magnetic loss and electromagnetic wave dissipation.

[0024] Subsequently, CNT@Fe3O4 nanofillers, which possess both dielectric and magnetic double loss capabilities, were introduced into the ILs@PI matrix to prepare ILs@PI / CNT@Fe3O4 composite foams. Figure 1 As shown in (e), the complex permittivity (ε'-ε'') and complex permeability (μ'-μ'') spectra of the composite foam exhibit arc / semi-circular arc characteristics, confirming its significantly enhanced dielectric and magnetic loss properties. By further controlling the porous structure of the composite material to optimize the electromagnetic wave loss mechanism and impedance matching, this composite foam achieves excellent electromagnetic wave absorption performance across the entire X-band (8.2-12.4 GHz). Figure 1 (f), (g), (h)).

[0025] Figure 2 (a) SEM images of this heterostructure are also shown, clearly revealing the alternating structure of regularly spaced foamed porous layers and unfoamed dense layers. This heterogeneous foamed structure provides an ideal carrier for achieving gradient electromagnetic wave attenuation. Figure 2 As shown in (b), this design functionally integrates a foamed porous layer filled with CNT@Fe3O4 nanofiller (to achieve electromagnetic wave absorption) with a dense solid layer containing pure CNT (to achieve strong electromagnetic wave reflection).

[0026] Figure 2 As shown in (c)-(c'''), the original solid-state ILs@PI / CNT nanocomposite thin film layer exhibits ultra-high electrical conductivity, resulting in most incident electromagnetic waves being reflected at the air / film interface (average reflection coefficient R = 0.80). However, for Figure 2 As shown in (d), the double-layer electromagnetic shielding material exhibits a significantly reduced reflective shielding effectiveness (SER), and the total shielding effectiveness (SET) almost entirely originates from the absorbed shielding effectiveness (SEA). Figure 2 As shown in (e)-(e'''), the 3.45mm double-layer absorbing foam with a reflective film exhibits optimal electromagnetic shielding performance: an average absorption coefficient of 0.92 in the X-band, a maximum absorption / reflection ratio of 301.05, and an average absorption / reflection ratio of 67.12 in the X-band. Since the SET is almost entirely contributed by the SEA, this heterogeneous foam shows great application potential in the field of ultra-low reflection electromagnetic shielding.

Claims

1. A layered gradient polyimide foam material, characterized in that, The layered gradient polyimide foam material comprises alternating porous PI layers and dense PI layers. The porous PI layers contain ionic liquids and CNT@Fe3O4 nanofillers, and the dense PI layers contain ionic liquids and CNT nanofillers.

2. The foamed material according to claim 1, characterized in that, The ionic liquid in the porous PI layer and the ionic liquid in the dense PI layer are selected from bis(trifluoromethanesulfonyl)imide ionic liquids or their derivatives.

3. The foamed material according to claim 1, characterized in that, The PI porous layer contains 50-70 wt% ionic liquid and 1-10 wt% CNT@Fe3O4 nanofiller; the PI dense layer contains 10-30 wt% ionic liquid and 5-20 wt% CNT nanofiller.

4. The foamed material according to claim 1, characterized in that, The CNT@Fe3O4 nanofiller was prepared by a one-step coprecipitation method, the specific steps of which are as follows: carbon nanotubes were placed in water and ultrasonically stirred, then FeCl2·4H2O and FeCl3·6H2O were added and stirred continuously, and finally ammonia was added dropwise. The reaction was carried out at 30-70℃ for 0.2-5 hours. The mass ratio of carbon nanotubes, FeCl2·4H2O and FeCl3·6H2O was 0.1-0.3:0.3-0.6:1, and the molar ratio of ammonia to FeCl2·4H2O was 4-8:

1.

5. The method for preparing the layered gradient polyimide foam material according to claim 1, characterized in that, Includes the following steps: (1) Under the protection of an inert gas, aromatic tetracarboxylic dianhydride is dissolved in a polar aprotic solvent to form a homogeneous solution, and an aromatic diamine is added to the homogeneous solution to carry out a polycondensation reaction to generate a polyamic acid solution; (2) A certain amount of ionic liquid and CNT@Fe3O4 nanofiller were added to a portion of the polyamic acid solution, stirred and mixed evenly at room temperature, and the mixed solution was subjected to thermal imidization treatment and then foamed with supercritical carbon dioxide to obtain PI@ILs / CNT@Fe3O4 composite material. (3) A certain amount of ionic liquid and CNT nanofiller were added to a portion of the polyamic acid solution, stirred and mixed evenly at room temperature, and the mixed solution was subjected to thermal imidization treatment, and then foamed with supercritical carbon dioxide to obtain PI@ILs / CNT composite material. (4) The PI@ILs / CNT@Fe3O4 composite material and the PI@ILs / CNT composite material are combined by hot pressing to form the layered gradient polyimide foam material.

6. The preparation method according to claim 5, characterized in that, The aromatic tetracarboxylic dianhydride is selected from fluorinated or non-fluorinated aromatic tetracarboxylic dianhydrides and their derivatives; the aromatic diamine is selected from fluorinated or non-fluorinated aromatic diamines and their derivatives; the ionic liquid is a bis(trifluoromethanesulfonyl)imide ionic liquid or its derivative; and the polar aprotic solvent is at least one of N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), N-methylpyrrolidone (NMP), or dimethyl sulfoxide (DMSO).

7. The preparation method according to claim 5, characterized in that, The conditions for the polycondensation reaction are: temperature 0-50℃, reaction time 4-24 hours; the molar ratio of the ionic liquid to polyamic acid is 0.5-2:

1. According to the preparation method of claim 5, the thermal imidization treatment adopts a gradient heating program: first, it is treated at 150-180℃ for 1-5 hours, and then the temperature is raised to 190-220℃ for 0.5-3 hours.

8. The preparation method according to claim 5, characterized in that, The supercritical carbon dioxide foaming process is a single-stage or two-stage foaming process, wherein the single-stage foaming process includes: (a) The polyimide / ionic liquid composite material was placed in a high-pressure reactor and supercritical carbon dioxide was introduced; (b) Maintain at a temperature of 120-180℃ and a pressure of 10-20 MPa for 30-120 minutes; (c) Quickly depressurize to normal pressure to induce foaming and form composite foam material; the secondary foaming is to repeat the process of primary foaming twice.

9. The application of the layered gradient polyimide foam material according to claim 1 in microwave absorption, characterized in that, The porous PI layer is a microwave absorbing layer, and the dense PI layer is a shielding layer.

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