Intelligent composite aerogel for realizing shielding function switching based on pressure regulation and control and preparation method thereof

Abstract

The invention discloses intelligent composite aerogel capable of realizing shielding function switching based on pressure regulation and control, which is prepared by the following steps: uniformly mixing a polyamide acid solution and an aramid nanofiber dispersion liquid, adding into a silica gel mold, and performing directional freeze drying and high-temperature annealing on the silica gel mold to obtain PI / ANF composite aerogel; dipping the PI / ANF composite aerogel into the mercaptan modified LM dispersion liquid, and carrying out vacuum drying to obtain LM / PI / ANF composite aerogel; the aerogel shows significantly changing dielectric properties and wave absorbing / shielding behaviors under different compression strains, shows excellent electromagnetic wave absorbing ability under a low strain condition, shows enhanced electromagnetic shielding performance under a high strain condition, can realize reversible switching of electromagnetic functions by adjusting the strain state, and has good application prospects. And the material is endowed with good intelligent response characteristics. The invention provides a new thought for the design of an electromagnetic function adjustable material under pressure driving, and has a wide application prospect in the field of intelligent electromagnetic protection.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of wave-absorbing materials and wave-shielding materials, and particularly relates to an intelligent composite aerogel capable of realizing shielding function switching based on pressure regulation and a preparation method thereof. BACKGROUND

[0002] In the military field, electronic devices need to be flexibly switched between electromagnetic wave-absorbing and electromagnetic wave-shielding functions to adapt to complex battlefield electromagnetic environments. Existing electromagnetic protection materials mostly use carbon materials, magnetic particles or conductive polymers, which have good wave-absorbing or shielding performance, but their functions are fixed after the materials are prepared, and it is difficult to realize dynamic regulation or reversible switching during service. At present, some researches realize electromagnetic response regulation by adjusting the proportion of fillers or external field stimulation, such as temperature-sensitive, magnetic-sensitive or pressure-sensitive composite materials, which have made certain progress, but generally have problems such as narrow regulation range, response lag and poor structural stability, which are difficult to meet the actual application requirements.

[0003] In recent years, liquid metal has been widely used in wearable devices, intelligent sensing and flexible circuits due to its high electrical conductivity, good flexibility and structural reconfigurability. By introducing liquid metal into aerogels with compressibility and porous structure, it is expected to realize the rapid and reversible switching of materials between wave-absorbing and shielding functions by regulating the internal conductive network structure of the liquid metal under external force, and to have the advantages of light weight, flexibility and multi-functional response. Therefore, it has important research value and wide engineering application prospect to develop an intelligent composite aerogel material based on external stress driving and having reversible electromagnetic function switching capability. SUMMARY

[0004] The present application provides an intelligent composite aerogel capable of realizing shielding function switching based on pressure regulation to solve the problem of reversible switching of electromagnetic wave-absorbing and electromagnetic wave-shielding functions of traditional wave-absorbing materials which is difficult to dynamically regulate.

[0005] To achieve the above technical purposes, the present application adopts the following technical scheme: a preparation method of an intelligent composite aerogel capable of realizing shielding function switching based on pressure regulation, characterized in that it comprises: mixing a polyamide acid (PAA) solution and an aramid nanofiber (ANF) dispersion liquid uniformly to form a PAA / ANF water dispersion liquid, adding the PAA / ANF water dispersion liquid into a mold, and then performing directional freeze-drying and high-temperature annealing on the mold to obtain a PI / ANF composite aerogel; immersing the PI / ANF composite aerogel into a thiol-modified LM dispersion liquid, and vacuum drying to obtain an LM / PI / ANF composite aerogel; The thiol-modified LM dispersion liquid is prepared by adding gallium-based liquid metal and 3-mercaptopropyl triethoxysilane (MPTES) into anhydrous ethanol for ultrasonic dispersion. Further preferably, the ratio of LM, MPTES and anhydrous ethanol is 4g:40μL:100mL; and the ultrasonic dispersion time is 30min-4h.

[0006] Further, the content of LM in the LM / PI / ANF composite aerogel is 50-60wt%.

[0007] Further, the polyamide acid (PAA) solution is synthesized by polycondensation reaction using 4,4'-oxydianiline (ODA) and 3,3',4,4'-biphenyl tetracarboxylic dianhydride (BPDA) as monomers. The molar ratio of 4,4'-oxydianiline (ODA) to N-methyl-2-pyrrolidone (NMP) is 1:1.

[0008] Further, the preparation method of the polyamide acid (PAA) solution is as follows: 4,4'-oxydianiline (ODA) and N-methyl-2-pyrrolidone (NMP) are mixed in an ice water bath and continuously stirred until completely dissolved, then 3,3',4,4'-biphenyl tetracarboxylic dianhydride (BPDA) is added, and after stirring for 8-12h, triethylamine (TEA) is added and continues to be stirred, then washed in ice water, dried to obtain water-soluble PAA, and then add appropriate amount of deionized water to obtain the polyamide acid (PAA) solution.

[0009] Further, the preparation method of the aramid nanofiber (ANF) dispersion liquid is as follows: Kevlar fiber is added into a three-necked flask, dimethyl sulfoxide and potassium hydroxide are sequentially added, and under mechanical stirring, the Kevlar fiber is gradually depolymerized to form aramid nanofiber. After the reaction is completed, deionized water is used for displacement filtration to remove dimethyl sulfoxide, and finally a uniform aramid nanofiber dispersion liquid is obtained. Further preferably, the mass concentration of potassium hydroxide is 3-4mg / ml; and the stirring time is 4-6 days.

[0010] Further, the directional freeze-drying is to immerse the mold into liquid nitrogen at-196℃ for rapid directional freezing, and then put it into a freeze dryer for freeze-drying for 5 days after complete freezing.

[0011] The directional freeze-drying is to place the mold containing PAA / ANF water dispersion liquid on a metal platform with good heat conduction performance, and the heat of-196℃ liquid nitrogen is transferred to the PAA / ANF water dispersion liquid through the metal plate to realize rapid directional freezing. After complete freezing, it is put into a freeze dryer for freeze-drying for 5 days to form PAA / ANF water dispersion liquid.

[0012] Further, the high-temperature annealing is as follows: the aerogel after directional freeze-drying is heated from room temperature to 100℃ and held for 1 hour in a tube furnace under nitrogen atmosphere at a heating rate of 3℃ / min, then heated from 100℃ to 200℃ and held for 1 hour, then heated from 200℃ to 300℃ and held for 1 hour, and then naturally cooled to obtain PI / ANF aerogel.

[0013] A silicone mold containing a PAA / ANF aqueous dispersion is placed on a metal platform. A metal rod connected to the bottom of the metal platform is inserted into liquid nitrogen at -196°C. The cold energy of the liquid nitrogen is transferred to the mold through the metal platform, causing the PAA / ANF aqueous dispersion inside the mold to cool down. Since the direction of cold energy transfer is from bottom to top, the ice crystals inside grow directionally from bottom to top, forming a PAA / ANF freeze-dried body after freezing.

[0014] After high-temperature annealing, PAA / ANF lyophilized form undergoes a thermal imidization reaction to transform into polyimide (PI). Meanwhile, the internal ice crystals sublimate to form a regular porous structure, thus forming a PI / ANF composite aerogel.

[0015] The present invention also provides an aerogel with reversible shielding function prepared by the above preparation method. Beneficial effects

[0016] This invention provides an LPA composite aerogel material with switchable electromagnetic functions. Through structural design and component control, it achieves reversible switching between electromagnetic absorption and electromagnetic shielding under different compression states, exhibiting excellent intelligent response characteristics. On a macroscopic scale, the material has a porous structure containing a large amount of air, forming a good impedance match with free space, which is beneficial for electromagnetic wave incidence. Under compression, the porosity decreases, impedance mismatch increases, and more electromagnetic waves are reflected, achieving a functional transformation from absorption to shielding. On a mesoscopic scale, the internal porous structure induces multiple reflections and scattering of electromagnetic waves, extending their propagation path and enhancing energy loss. On a microscopic scale, the introduced liquid metal (LM) exhibits conductivity loss due to electron hopping in the uncompressed state. With increasing compressive strain, the LM gradually evolves from a dispersed state to a connected network, enhancing conductivity and significantly improving the ohmic loss effect. Simultaneously, the composite of LM, PI, and ANF forms numerous heterogeneous interfaces. The difference in carrier concentration and polarity at the interfaces induces strong interfacial polarization, which is further enhanced under compression. Furthermore, the Si–O–C polar groups generated on the LM surface after MPTES modification undergo dipole orientation polarization under the action of an alternating electromagnetic field, enhancing dielectric loss. The compression process also induces the structural evolution of LM in the aerogel pores, gradually transforming from an initial dispersed state into a continuous conductive network, realizing the transformation from an insulator to a conductor, and providing a structural basis for aerogels to achieve dynamic electromagnetic function switching. Attached Figure Description

[0017] Figure 1 is a preparation flow chart of the method of the present application; Figure 2 is a photograph of aerogel prepared in Example 1; Figure 3 is a photograph of aerogel prepared in Comparative Example 1; Figure 4 is a SEM and EDS graph of aerogel prepared in Example 1 under non-compressed and compressed strain of 80%; Figure 5 is a graph of electromagnetic parameters of Example 1; Figure 6 is a graph of electromagnetic parameters of Comparative Example 1; Figure 7 is a graph of electromagnetic performance of Example 1; Figure 8 is a graph of electromagnetic performance of Comparative Example 1. DETAILED DESCRIPTION

[0018] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0019] In the following examples, the experimental methods used are conventional methods unless otherwise specified, and the reagents, methods and equipment used are conventional reagents, methods and equipment in the technical field unless otherwise specified. EMBODIMENT

[0020] The present embodiment provides an intelligent composite aerogel for realizing switching of shielding function based on pressure regulation, and a preparation method thereof comprises the following steps: (1) Modified liquid metal gallium: 4g of LM and 40μL of MPTES were added into a beaker containing 100mL of anhydrous ethanol, and ultrasonic dispersion was performed for 1h to obtain a thiol-modified LM dispersion liquid, (2) Preparation of soluble polyamide acid solution: Into a three-neck flask containing 50 mL of N-methyl-2-pyrrolidone (NMP), 4 g of 4,4'-diaminodiphenyl ether (ODA) was added and continuously stirred at an ice water bath (0°C) until completely dissolved. Then 5.938 g of 3,3', 4,4'-biphenyltetracarboxylic dianhydride (BPDA) was added into the above solution in several portions, and stirred for 12 h until the solution was viscous. Then 2.8 mL of triethylamine (TEA) was added and continuously stirred for 2 h. The obtained PAA solution was washed in ice water and dried in an oven at 60°C overnight to obtain water-soluble PAA. Then a proper amount of deionized water was added to obtain a PAA solution with a concentration of 40 mg / g.

[0021] (3) Preparation of aramid nanofiber solution: Into a three-neck flask, 2 g of ANF, 3 g of KOH and 1000 mL of DMSO were sequentially added and mechanically stirred at room temperature for 5 days. The solution changed from orange red to red brown. Then the DMSO in the solution was replaced by suction filtration, and a proper amount of deionized water was added and stirred uniformly to obtain an ANF colloidal solution with a concentration of 10 mg / g.

[0022] (4) Preparation of LM / PI / ANF composite aerogel: The PAA solution with a concentration of 40 mg / g and the ANF colloidal solution with a concentration of 10 mg / g were mixed at a mass ratio of PAA:ANF = 7:3 to form a PAA / ANF dispersion liquid by ultrasonic dispersion. The dispersion liquid was added into a mold with a size of 2.5 mm x 2.5 mm x 2.5 mm (the bottom of the mold was sealed with tin paper), and then the mold was placed on a metal platform almost immersed in liquid nitrogen at -196°C (a metal rod connected to the bottom of the metal platform was inserted into liquid nitrogen at -196°C). The mold was quickly directionally frozen, and then was placed in a freeze dryer for freeze drying for 5 days. Then the temperature was increased to 100°C at a rate of 3°C / min in a tube furnace under nitrogen atmosphere, and was kept at 100°C for 1 h. The temperature was increased to 200°C at a rate of 3°C / min, and was kept at 200°C for 1 h. The temperature was increased to 300°C at a rate of 3°C / min, and was kept at 300°C for 1 h. After natural cooling, a PI / ANF aerogel was obtained. Then the PI / ANF aerogel was immersed in a modified LM dispersion liquid, and was vacuum dried at 50°C for 12 h. The above steps were repeated to obtain LM / PI / ANF composite aerogels with different LM contents. In this example, the immersion and drying operations were repeated for 3 times, and a LM / PI / ANF composite aerogel with a LM content of 50-60 wt% was obtained.

[0023] This comparative example provides the preparation of a polyimide / aramid nanofiber aerogel, including the following steps: (1) Preparation of soluble polyamide acid solution: Into a three-necked flask containing 50 mL N-methyl-2-pyrrolidone (NMP), 4 g 4,4'-diaminodiphenyl ether (ODA) was added and continuously stirred at ice water bath (0°C) until completely dissolved. Then 5.938 g 3,3', 4,4'-biphenyltetracarboxylic dianhydride (BPDA) was added into the above solution in several portions, and stirred for 12 h until the solution was viscous. Then 2.8 mL triethylamine (TEA) was added and stirred for another 2 h. The obtained PAA solution was washed in ice water and dried in an oven at 60°C overnight to obtain water-soluble PAA. Then a proper amount of deionized water was added to obtain a PAA solution with a concentration of 40 mg / g.

[0024] (2) Preparation of aramid nanofiber solution: Into a three-necked flask, 2 g Kevlar fiber, 3 g KOH and 1000 mL DMSO were sequentially added and mechanically stirred at room temperature for 5 days. The solution changed from orange red to red brown. Then the DMSO in the solution was replaced by suction filtration, and a proper amount of deionized water was added to obtain an ANF colloidal solution with a concentration of 10 mg / g.

[0025] (3) Preparation of PI / ANF composite aerogel: After 40 mg / g PAA solution and 10 mg / g ANF colloidal solution were mixed at a mass ratio of PAA:ANF = 7:3 to form a PAA / ANF dispersion, the dispersion was added to a mold (the bottom of the mold was sealed with tin paper) with a size of 2.5 mm x 2.5 mm x 2.5 mm. Then the mold was placed on a metal plate almost immersed in -196°C liquid nitrogen (the metal rod connected below the metal platform was inserted into -196°C liquid nitrogen). The mold was quickly directional frozen, and then placed in a freeze dryer for freeze drying for 5 days. Then the temperature was raised to 100°C at a rate of 3°C / min in a nitrogen environment in a tube furnace, and kept for 1 h. The temperature was raised to 200°C from 100°C, and kept for 1 h. The temperature was raised to 300°C from 200°C, and kept for 1 h. After natural cooling, a PI / ANF aerogel was obtained.

[0026] The electromagnetic wave absorbing performance and EMI shielding performance of the sample of Example 1 and Comparative Example 1 were measured by a coaxial method in the frequency range of 2-18 GHz using a vector network analyzer (VNA, Ceyear 3672B). The test sample was obtained by vacuum impregnating paraffin into the aerogel ring (outer diameter 7.0 mm, inner diameter 3.04 mm, height 1-3 mm).

[0027] Test results: As can be seen from 5-6, in the tested 2-18 GHz frequency range, the dielectric real part, imaginary part and loss tangent of the sample in Example 1 of the present application change significantly compared with Comparative Example 1, and increase with the increase of compressive strain. The electromagnetic parameters of the comparative example also do not increase significantly when the compressive strain is 80%.

[0028] In Figure 7 , it is found that the wave absorption performance obtained by transmission line theory that Example 1 exhibits significant electromagnetic response regulation ability under different compressive strains. Under 20%-40% compressive strain, the material exhibits excellent electromagnetic wave absorption performance, and the reflection loss is far lower than-10 dB, and the effective absorption bandwidth is significantly widened. With the increase of compressive strain, the conductive path and the interface polarization effect are enhanced, realizing the dynamic adjustment of the absorption frequency and strength. Under higher compressive strain (50%-80%), the LPA aerogel presents a functional transition from wave absorption to shielding. The total shielding effectiveness gradually increases, and the shielding mechanism dominated by absorption is established, and the transmission rate decreases significantly, reflecting the strong blocking and dissipating ability of the material to the incident electromagnetic wave.

[0029] The present application provides a preparation method of intelligent composite aerogel capable of realizing shielding function switching based on pressure regulation. First, the surface of liquid metal gallium is functionally modified to enhance the interface compatibility with the aerogel matrix, and then it is introduced into the precursor liquid composed of polyamide acid and aramid nanofiber, and through directional freeze-drying process and high temperature annealing, the LM / PI / ANF composite aerogel with electromagnetic switching function is prepared. The coaxial line vector network analysis of Example 1 obtained by the present application shows that after introducing the liquid metal, the real part and imaginary part of the dielectric constant of the aerogel are significantly improved, and the dielectric loss tangent is also greatly increased, and it presents an obvious regulation trend with the change of compressive strain, while Figure 8 Comparative Example 1 (without liquid metal) shown in the figure changes little, indicating that the liquid metal effectively endows the material with adjustable electromagnetic response ability. In addition, the electromagnetic function switching behavior exhibited by Example 1 benefits from its high porosity skeleton structure, adjustable conductive network and multi-scale synergistic electromagnetic loss mechanism. Especially when driven by compressive strain, the liquid metal gradually restructures into a continuous conductive path in the porous structure, while strong polarization effect and dipole response are induced at the heterojunction interface, promoting the material to change from wave absorption state to shielding state, thereby realizing reversible regulation of electromagnetic function.

[0030] The above is only the preferred embodiment of the present application, and the protection scope of the present application is not limited to the above-mentioned examples. Any technical solutions falling within the concept of the present application shall fall within the protection scope of the present application. It should be noted that for ordinary skilled persons in the art, some improvements and decorations without departing from the principle of the present application shall be regarded as the protection scope of the present application.

Claims

1. A method for preparing a smart composite aerogel with shielding function switching based on pressure regulation, characterized in that, include: A polyamic acid solution and an aramid nanofiber dispersion were mixed evenly to form a PAA / ANF aqueous dispersion, which was then added to a mold and subjected to directional freeze-drying with liquid nitrogen and high-temperature annealing to obtain a PI / ANF composite aerogel. The PI / ANF composite aerogel was impregnated in a thiol-modified LM dispersion and then vacuum dried to obtain the LM / PI / ANF composite aerogel. The thiol-modified LM dispersion is prepared by adding gallium-based liquid metal and 3-mercaptopropyltriethoxysilane to anhydrous ethanol and dispersing them by ultrasonication.

2. The preparation method of the intelligent composite aerogel based on pressure regulation to achieve shielding function switching according to claim 1, characterized in that, In the LM / PI / ANF composite aerogel, the LM content is 50~60wt%.

3. The preparation method of the intelligent composite aerogel based on pressure regulation to achieve shielding function switching according to claim 1, characterized in that, The polyamic acid solution was synthesized by polycondensation reaction using 4,4′-diaminodiphenyl ether and 3,3′,4,4′-biphenyltetracarboxylic dianhydride as monomers.

4. The preparation method of the intelligent composite aerogel based on pressure regulation to achieve shielding function switching according to claim 2, characterized in that, The polyamic acid solution is prepared by: 4,4′-diaminodiphenyl ether and N-methyl-2-pyrrolidone were mixed in an ice-water bath and stirred continuously until completely dissolved. Then, 3,3′,4,4′-biphenyltetracarboxylic dianhydride was added and stirred for 8-12 hours. Triethylamine was then added and stirring continued. The mixture was then washed in ice water and dried to obtain a water-soluble polyamide. An appropriate amount of deionized water was added to obtain a polyamic acid solution.

5. The intelligent composite aerogel and its preparation method based on pressure regulation to achieve shielding function switching according to claim 1, characterized in that, The preparation method of the aramid nanofiber dispersion is as follows: Kevlar fibers were added to a three-necked flask, followed by the addition of dimethyl sulfoxide and potassium hydroxide. The mixture was then reacted under mechanical stirring to gradually depolymerize and form aramid nanofibers. After the reaction was complete, deionized water was used for filtration to remove the dimethyl sulfoxide, ultimately yielding a uniform aramid nanofiber dispersion.

6. The preparation method of the intelligent composite aerogel based on pressure regulation to achieve shielding function switching according to claim 1, characterized in that, The directional freeze-drying process involves placing a mold containing a PAA / ANF aqueous dispersion on a metal plate with good thermal conductivity. The -196°C liquid nitrogen heat is transferred to the PAA / ANF aqueous dispersion through the metal plate, achieving directional freezing. After complete freezing, the mixture is placed in a freeze dryer for 5 days of freeze-drying.

7. The intelligent composite aerogel and its preparation method based on pressure regulation to achieve shielding function switching according to claim 1, characterized in that, The high-temperature annealing process involves heating the directional freeze-dried aerogel at a rate of 3-5℃ / min in a tube furnace under nitrogen atmosphere from room temperature to 100℃ and holding for 1 hour, then heating it from 100℃ to 200℃ and holding for 1 hour, then heating it from 200℃ to 300℃ and holding for 1 hour, and finally cooling it naturally to room temperature to obtain PI / ANF aerogel.

8. The preparation method of the intelligent composite aerogel based on pressure regulation to achieve shielding function switching according to claim 1, characterized in that, The mass ratio of the polyamic acid solution to the aramid nanofiber dispersion is 7:3, the concentration of polyamic acid in the polyamic acid solution is 30-50 mg / g, and the concentration of aramid nanofiber in the aramid nanofiber dispersion is 10-12 mg / g.

9. An aerogel with reversible shielding function prepared by the preparation method according to any one of claims 1 to 8.

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