CN111453715A - Ultra-light efficient electromagnetic shielding composite material and preparation method thereof - Google Patents
Ultra-light efficient electromagnetic shielding composite material and preparation method thereof Download PDFInfo
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Abstract
The invention relates to an ultra-light high-efficiency electromagnetic shielding composite material and a preparation method thereof. The aerogel composite material is prepared from cellulose and Co2+The cobalt salt and the imidazole derivative are used as raw materials to prepare aerogel, and the aerogel composite material is obtained through carbonization treatment; the mass of the cellulose accounts for the mass of the cellulose and contains Co2+20 to 45 percent of the total mass of the cobalt salt and the imidazole derivative. The aerogel composite has a density as low as 0.023gcm‑3The shielding effectiveness under unit density is high, the average absorption coefficient of the electromagnetic wave exceeds 0.77, the requirements of light weight, excellent electromagnetic interference shielding performance and internal absorption as a main shielding mechanism are met, and electricity caused by reflection can be effectively avoidedThe magnetic wave secondary reflection pollution solves the problem that most electromagnetic shielding materials in the prior art can cause secondary reflection pollution, and has very wide application prospect as the electromagnetic shielding material in the fields of military equipment, aerospace and aviation and civil electronic equipment.
Description
Technical Field
The invention belongs to the field of electromagnetic shielding, and particularly relates to an ultra-light efficient electromagnetic shielding composite material and a preparation method thereof.
Background
Due to the rapid development of wireless communication and high frequency electronic devices, Electromagnetic (EM) radiation not only generates non-negligible interference to high sensitivity electromagnetic devices, but also seriously endangers human health, and thus, electromagnetic interference (EMI) shielding materials (electromagnetic shielding materials for short) have gained more and more attention.
Generally, the EMI shielding mechanism includes three parts: surface reflection (SER), internal absorption (SEA) and multiple reflection (SEM). Most of the EMI shielding mechanisms of the existing EMI shielding materials are mainly surface reflection, but the reflection is likely to cause secondary reflection pollution of electromagnetic waves. Therefore, it is important to develop an electromagnetic interference shielding material having internal absorption as a dominant shielding mechanism, and the electromagnetic wave absorption coefficient (a) is an important index for evaluating such an electromagnetic interference shielding material. Meanwhile, considering the complex practical application environment in the fields of military equipment, aerospace, civil electronic equipment and the like, the excellent electromagnetic interference shielding performance and the portability of the material are two other important requirements of the electromagnetic interference shielding material.
The electromagnetic shielding materials reported in the prior art include traditional metals and metal composite materials, three-dimensional aerogels, porous carbon nanowires, fibers and related composite materials, and the like. They are superior and inferior, but it is difficult to satisfy both lightness, excellent electromagnetic interference shielding performance, and a dominant shielding mechanism by internal absorption.
The research group of y, L u (ACS appl. mater. interfaces,2015,7, 13604-.
Compared with metal fillers, carbon fibers have the advantage of light weight; in addition, the carbon fiber also has the advantages of good conductivity and other properties, high modulus, high strength and the like. Accordingly, carbon fiber-containing composite materials have received a great deal of attention in the field of electromagnetic shielding.
The document "electromagnetic shielding effectiveness of carbonized bacterial cellulose, hippeas" describes a composite obtained by blending carbon nanofibers obtained by carbonizing bacterial cellulose with paraffin, wherein the composite has good electromagnetic shielding effectiveness, and the electromagnetic shielding effectiveness is increased along with the increase of the content of the carbon fibers in the composite.
Y. L i et al (adv. funct. mater, 2019,29,1807624) constructed a composite conductive network material having excellent EMI shielding ability by using Carbon Nanotubes (CNTs) as a conductive filler and cellulose aerogel as a porous carrier, however, these electromagnetic shielding materials having high conductive properties have high reflectivity to electromagnetic waves and are liable to cause secondary pollution of the electromagnetic waves.
Therefore, there is a need to develop an electromagnetic interference shielding material that satisfies both of light weight, excellent electromagnetic interference shielding performance, and a dominant shielding mechanism by internal absorption.
Disclosure of Invention
The invention aims to provide an ultra-light high-efficiency electromagnetic shielding composite material which is low in cellulose addition amount, light in weight, excellent in electromagnetic interference shielding performance and mainly takes internal absorption as a shielding mechanism, and a preparation method thereof.
The invention provides an aerogel composite material which is prepared from cellulose and Co2+The cobalt salt and the imidazole derivative are used as raw materials to prepare aerogel, and the aerogel composite material is obtained through carbonization treatment; the mass of the cellulose accounts for the mass of the cellulose and contains Co 2+20 to 45 percent of the total mass of the cobalt salt and the imidazole derivative.
Further, the mass of the cellulose accounts for the mass of the cellulose and the mass of the Co2+35-45% of the total mass of the cobalt salt and the imidazole derivative, preferably 41-42%;
and/or, said Co-containing2+The mass ratio of the cobalt salt to the imidazole derivative is 1: (2-3), preferably 1: 2.82.
Further, the cellulose is nano-cellulose, preferably bacterial cellulose;
and/or, said Co-containing2+The cobalt salt of (a) is selected from cobalt hydroxide or hydrate thereof, cobalt oxide or hydrate thereof, cobalt sulfate or hydrate thereof, cobalt nitrate or hydrate thereof, cobalt carbonate or hydrate thereof, cobalt silicate or hydrate thereof, cobalt halide or hydrate thereof, preferably cobalt nitrate hexahydrate;
and/or, the imidazole derivative is a compound of the following structure:
r is selected from H or C1～6An alkyl group; preferably, the imidazole derivative is 2-methylimidazole.
Further, the preparation method of the aerogel comprises the following steps:
(1) mixing cellulose with Co2+Adding the cobalt salt into a solvent for reaction;
(2) then adding imidazole derivative, stirring and standing; and then removing the liquid, washing the residual system with a solvent, and freeze-drying to obtain the product.
Further, the solvent is water;
and/or, in step (1), the Co-containing2+The mass volume ratio of the cobalt salt to the solvent is 3-20 mg/m L, preferably 5-15 mg/m L;
and/or in the step (1), the reaction temperature is 20-30 ℃, preferably 25 ℃, and the reaction time is 4-8 hours, preferably 6 hours;
and/or in the step (2), the stirring time is 1-3 hours, preferably 2 hours; the standing temperature is 20-30 ℃, the preferred temperature is 25 ℃, and the standing time is 16-20 hours, the preferred time is 18 hours.
Further, the cellulose is the cellulose soaked by alkaline aqueous solution;
preferably, the alkaline aqueous solution is a KOH aqueous solution of 1% mg/m L, and/or the soaking time is 1-8 hours, preferably 5 hours.
Further, the thickness of the aerogel composite material is more than 1mm, preferably 2-4 mm.
The invention also provides a method for preparing the aerogel composite material, which comprises the following steps: carbonizing the aerogel to obtain the product.
Further, the carbonization is performed under an inert gas atmosphere; and/or the carbonization temperature is 500-1000 ℃, preferably 900 ℃;
and/or the carbonization time is 1-6 hours, preferably 3 hours.
The invention also provides application of the aerogel composite material in preparing electromagnetic shielding materials.
According to the invention, through a self-assembly method, aerogel obtained by reacting bacterial cellulose, cobalt nitrate hexahydrate and 2-methylimidazole is carbonized to obtain a CNF @ Co/C aerogel composite material with a bead chain structure, and experimental results show that the density of the CNF @ Co/C aerogel composite material is as low as 0.023g cm-3The shielding efficiency under unit density is high, the average absorption coefficient of the electromagnetic wave exceeds 0.77, the requirements of light weight, excellent electromagnetic interference shielding performance and internal absorption as a main shielding mechanism are met, secondary reflection pollution of the electromagnetic wave caused by reflection can be effectively avoided, the safety of a user is guaranteed, the problem that most electromagnetic shielding materials in the prior art possibly cause secondary reflection pollution is solved, and the electromagnetic shielding material has a very wide application prospect as the electromagnetic shielding material in the fields of military equipment, aerospace and civil electronic equipment.
The CNF @ Co/C aerogel composite material with the bead chain structure prepared by the self-assembly method disclosed by the invention is low in cellulose addition amount, and the prepared material is still high in electromagnetic shielding efficiency and average absorption coefficient, so that the composite material prepared by combining the raw materials and the process disclosed by the invention achieves unexpected technical effects.
The aerogel composite material has the advantages of simple preparation method, easily obtained raw materials, small cellulose addition amount and low cost, and is suitable for expanded production.
Obviously, many modifications, substitutions, and variations are possible in light of the above teachings of the invention, without departing from the basic technical spirit of the invention, as defined by the following claims.
The present invention will be described in further detail with reference to the following examples. This should not be understood as limiting the scope of the above-described subject matter of the present invention to the following examples. All the technologies realized based on the above contents of the present invention belong to the scope of the present invention.
Drawings
FIG. 1: schematic of CNF @ Co/C aerogel synthesis.
FIG. 2: (A) a photograph of CNF @ Co/C aerogel placed on the petals; SEM image of each sample: (B) CNF aerogel, (C, G) BC @ ZIF-67 aerogel, (D-F) CNF @ Co/C aerogel.
FIG. 3: (A) TEM picture of CNF @ Co/C aerogel; (B-D) HRTEM pictures of CNF @ Co/C aerogel; (E) TEM-EDS pictures of CNF @ Co/C aerogel; (F) FTIR spectra for BC, CNF @ Co/C aerogel, BC @ ZIF-67 aerogel, CNF aerogel.
FIG. 4: XRD patterns of CNF aerogel, CNF @ Co/C aerogel, BC @ ZIF-67 aerogel, BC and ZIF-67.
FIG. 5: TG curves for CNF aerogel, CNF @ Co/C aerogel.
FIG. 6: (A) density and conductivity of CNF aerogel and CNF @ Co/C aerogel; (B) hysteresis loop of CNF @ Co/C aerogel.
FIG. 7: (A) EMI SE of CNF aerogel and CNF @ Co/C aerogel in X wave band; (B) EMI SE of CNF @ Co/C aerogel with different thicknesses in an X wave band; (C) average SET, SEA and SER values of CNF @ Co/C aerogel with different thicknesses and CNF aerogel with the thickness of 1mm in an X wave band; (D) average A, R and T values of CNF @ Co/C aerogels with different thicknesses and CNF aerogels with the thickness of 1mm in an X waveband.
Detailed Description
The raw materials and equipment used in the invention are known products and are obtained by purchasing commercial products.
Example 1 preparation of electromagnetic shielding aerogel according to the present invention
Raw material Bacterial Cellulose (BC) was purchased from Guilin Qi HongTech Co., Ltd; 2-methylimidazole (2-MI, 98%), cobalt nitrate hexahydrate (Co (NO)3)2·6H2O, 98%) and potassium hydroxide (KOH),>85%) was purchased from alatin.
1. Synthesis of BC @ ZIF-67 aerogel
Pretreatment of Bacterial Cellulose (BC) BC (1.601g) was soaked in 1% (mg/m L) KOH aqueous solution at 70 ℃ for 5 hours, followed by washing with deionized water.
self-Assembly of pretreated BC was added to 40m L containing 2mmol (0.582g) Co (NO)3)2·6H2Stirring the O aqueous solution at 25 ℃ for 6h, then dropwise adding 60m L aqueous solution containing 1.64g of 2-MI into the O aqueous solution under stirring, uniformly mixing, continuously stirring for 2 h, standing at 25 ℃ for 18 h, carrying out interference-free aging, then removing liquid, washing the rest system with deionized water for 3 times, and carrying out freeze drying to obtain the purple BC @ ZIF-67 aerogel.
2. Synthesis of CNF @ Co/C aerogel
Carbonizing the BC @ ZIF-67 aerogel to obtain a CNF @ Co/C aerogel, wherein the specific process comprises the following steps: and calcining the BC @ ZIF-67 aerogel for 3h at 900 ℃ in an Ar atmosphere. After calcining, naturally cooling to room temperature to obtain the black CNF @ Co/C aerogel. A schematic of CNF @ Co/C aerogel formation is shown in FIG. 1.
Comparative example 1 preparation of carbon nanofiber aerogel
BC was calcined under Ar atmosphere at 900 ℃ for 3 h. And after calcining, naturally cooling to room temperature to obtain the Carbon Nanofiber (CNF) aerogel.
Comparative example 2 preparation of Metal organic framework Material ZIF-67
Taking a sample containing 2mmol of Co (NO)3)2·6H2And dropwise adding 60m of L aqueous solution containing 1.64g of 2-MI into the O aqueous solution under stirring, uniformly mixing, continuously stirring for 2 hours, standing for 18 hours at 25 ℃, carrying out interference-free aging, removing liquid, washing the rest system with deionized water for 3 times, drying and freezing to obtain the metal organic framework material ZIF-67.
The beneficial effects of the present invention are demonstrated by the following experimental examples.
Experimental example 1 structural characterization
1. Experimental methods
The CNF @ Co/C aerogel prepared in example 1 was used for testing; the raw material Bacterial Cellulose (BC), the CNF aerogel prepared in the comparison example 1, the metal organic framework material ZIF-67 prepared in the comparison example 2 and the intermediate product BC @ ZIF-67 aerogel prepared in the example 1 are used as the comparison.
And respectively carrying out Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), high-resolution transmission electron microscope (HRTEM), transmission electron microscope-energy spectrum (TEM-EDS), Fourier transform infrared spectrometer (FTIR) and X-ray diffraction (XRD) tests on the sample to characterize the structure of the sample.
2. Results of the experiment
SEM results are shown in FIGS. 2B to G, TEM results are shown in FIG. 3A, HRTEM results are shown in FIGS. 3B to 3D, and TEM-EDS results are shown in FIG. 3E. It can be seen that in the BC @ ZIF-67 aerogel prepared by the self-assembly method, the cellulose penetrates through a metal-organic framework material formed by 2-methylimidazole and cobalt nitrate hexahydrate, and the metal-organic framework material particles are strung together to form a 'bead chain structure'; the CNF @ Co/C aerogel formed by carbonizing the BC @ ZIF-67 aerogel is expressed by being embedded into polyhedral porous carbon containing Co metal and has a 3D interconnected network structure.
The FTIR results are shown in fig. 3F, and it can be seen that, compared with the CNF aerogel, the CNF @ Co/C aerogel prepared by the present invention has a characteristic peak of the precursor BC @ ZIF-67 (C ═ N stretching vibration (1570 cm)-1) And out-of-plane bending (748 and 692 cm)-1) Disappears, and proves that the metal organic framework material ZIF-67 formed by 2-methylimidazole and cobalt nitrate hexahydrate is successfully compounded with the carbon fiber aerogel.
The XRD results are shown in FIG. 4, and it can be seen that the BC @ ZIF-67 aerogel retains the diffraction peak of ZIF-67 during self-assembly, indicating that ZIF-67 has successfully inserted BC. After carbonization, diffraction peaks were observed at 44.2 ° and 51.6 ° of the obtained CNF @ Co/C aerogel spectra, and the diffraction peaks were assigned to (111) and (200) planes of metallic Co (JCPDS No.15-0806), indicating that there are high purity Co crystals in the CNF @ Co/C aerogel samples; in addition, a weak and sharp diffraction peak appears at 26.1 ° of the CNF @ Co/C aerogel spectrum, which is attributed to the (002) plane of graphitic carbon. Therefore, from the XRD results, the CNF @ Co/C aerogel is successfully prepared by the invention.
Experimental example 2 thermal stability characterization
1. Experimental methods
The CNF @ Co/C aerogel prepared in example 1 was used for testing; the CNF aerogel prepared in comparative example 1 was used as a control.
The samples were subjected to thermogravimetric analysis (TG) with a thermogravimetric analyzer (TG 209F1, NETZSCH, germany), respectively, with the test conditions: and (3) nitrogen atmosphere, wherein the test temperature range is 30-800 ℃, and the heating speed is 10 ℃/min.
2. Results of the experiment
As shown in FIG. 5, it can be seen that the CNF @ Co/C aerogel prepared by the present invention has excellent thermal stability.
Experimental example 3 Density and conductivity characterization
1. Experimental methods
The density and conductivity of the samples were measured for the CNF @ Co/C aerogel prepared in example 1 and the CNF aerogel prepared in comparative example 1, respectively.
2. Results of the experiment
As can be seen in FIG. 6A, the density of CNF @ Co/C aerogel is as low as 23mg cm-3(ii) a The photo of the CNF @ Co/C aerogel placed on the petals is shown in figure 2A, and the CNF @ Co/C aerogel does not collapse the petals, so that the CNF @ Co/C aerogel meets the characteristics of a light material. In addition, the electrical conductivity of the CNF @ Co/C aerogel prepared by the present invention was significantly improved over the CNF aerogel (fig. 6A).
Experimental example 4 hysteresis Loop characterization
1. Experimental methods
The hysteresis loop of the CNF @ Co/C aerogel prepared in example 1 was measured at room temperature by vibrating a sample magnetometer (VSM).
2. Results of the experiment
As can be seen from FIG. 6B, the CNF @ Co/C aerogel prepared by the invention shows typical ferromagnetic behavior and has a clear hysteresis loop in the hysteresis (MH) curve, which is helpful for enhancing the magnetic loss capability of the CNF @ Co/C aerogel. In addition, the saturation magnetization (Ms) value of the CNF @ Co/C aerogel prepared by the invention is 55.9emu g-1Lower than the saturation magnetization of bulk Co (164.8 emu g at 300K ═ T--1)。
Experimental example 5 electromagnetic interference shielding Performance characterization
1. Experimental methods
The CNF @ Co/C aerogel prepared in example 1 was subjected to characterization of electromagnetic interference shielding property (EMI SE for short) in comparison with the CNF aerogel prepared in comparative example 1. an Agilent vector network analyzer was used in the X band to prepare an aerogel sample having a cross-section of 22.8 × 10mm2The thickness of the sample is 1mm, 2mm and 4mm respectively, and then the sample is put into a network analyzer for detection.
2. Test results
The electromagnetic interference shielding effectiveness results of the CNF @ Co/C aerogel and the CNF aerogel with the thickness of 1mm are shown in FIG. 7A, and it can be seen that in the measurement frequency range, the EMI SE maximum value of the CNF aerogel is 14dB, and the EMISE maximum value of the CNF @ Co/C aerogel is up to 31.05dB, which indicates that the CNF @ Co/C aerogel prepared by the invention is a material with good electromagnetic shielding effect.
In addition, by measuring the electromagnetic interference shielding effectiveness of the CNF @ Co/C aerogel with different thicknesses, it can be seen that the EMI SE value of the material gradually increases as the thickness of the CNF @ Co/C aerogel sample increases (FIG. 7B), and at the thickness of 4mm, the EMI SE value of the material is as high as 56.07dB, and 99.999% of the incident electromagnetic wave can be shielded. The above results indicate that the electromagnetic shielding effect of the CNF @ Co/C aerogel increases with increasing thickness.
For emi shielding materials, the overall Shielding Efficiency (SET) depends mainly on surface reflection (SER), internal absorption (SEA) and multiple reflection (SEM). SEM is generally ignored when SET is greater than 15dB, so this relationship can be expressed as: SET ═ SER + SEA + SEM ≈ SER + SEA. As can be seen from fig. 7C, the CNF @ Co/C aerogel had a higher SEA value, and a lower SER value, than the CNF aerogel of the same thickness (1 mm); for CNF @ Co/C aerogels with different thicknesses, the SEA values of the materials are far higher than the SER values, and the SEA of the aerogels is obviously increased along with the increase of the thicknesses, which shows that the shielding of the CNF @ Co/C aerogels on electromagnetic interference is based on internal absorption as a leading mechanism, and the electromagnetic interference shielding mechanism is favorable for avoiding electromagnetic wave secondary reflection pollution caused by reflection.
In order to further study the EMI shielding mechanism of CNF @ Co/C aerogel, the power coefficients of absorption coefficient (a), reflection coefficient (R) and transmission coefficient (T) were used to characterize the absorption, reflection and transmission ability of the shielding material to electromagnetic waves, and the power balance of the interaction between electromagnetic waves and aerogel was evaluated separately and calculated from S parameter, and the result is shown in fig. 7D. It can be seen that as the thickness of the sample increases, a and R show increasing and decreasing trends, respectively. Moreover, regardless of the CNF @ Co/C aerogel thickness, A is always greater than R, consistent with the EMI SE results (SEA > SER). Further verifies that the shielding of the CNF @ Co/C aerogel prepared by the invention on electromagnetic interference is based on internal absorption as a leading mechanism.
To further highlight the excellent electromagnetic interference shielding performance of the CNF @ Co/C aerogel of the present invention, the densities and parameters representing the electromagnetic shielding performance of the CNF @ Co/C aerogel of the present invention and the aerogel/foam electromagnetic interference shielding materials reported in the prior art are summarized, and specific values are listed in table 1. Among them, SSE ((dB cm)3g-1) EMI SE (dB)/density (g cm)-3) The shielding effectiveness per unit density of the electromagnetic shielding material is shown.
TABLE 1 comparison of EMI Shielding Performance of electromagnetic interference Shielding materials
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Watch with watch1, the following steps: the CNF @ Co/C aerogel prepared by the invention is light (the density is as low as 0.023g cm)-3) The shielding effectiveness under unit density is high, and the average absorption coefficient (A) of the electromagnetic wave exceeds 0.77, which shows that more than 77% of the electromagnetic wave can be absorbed by the CNF @ Co/C aerogel of the invention in the shielding process. Compared with other light electromagnetic interference shielding materials reported in the prior art, the CNF @ Co/C aerogel disclosed by the invention has the advantages that the average absorption coefficient is remarkably improved, the secondary reflection pollution of electromagnetic waves caused by reflection can be effectively avoided, and the application potential is huge.
In conclusion, the invention carbonizes aerogel obtained by reacting bacterial cellulose, cobalt nitrate hexahydrate and 2-methylimidazole through a self-assembly method to obtain CNF @ Co/C aerogel with a bead chain structure, wherein the CNF @ Co/C aerogel is an ultra-light high-efficiency electromagnetic shielding composite material, and the density of the CNF @ Co/C aerogel is as low as 0.023g cm-3The shielding efficiency under unit density is high, the average absorption coefficient of the electromagnetic wave exceeds 0.77, the requirements of light weight, excellent electromagnetic interference shielding performance and internal absorption as a main shielding mechanism are met, secondary reflection pollution of the electromagnetic wave caused by reflection can be effectively avoided, the safety of a user is guaranteed, the problem that most electromagnetic shielding materials in the prior art possibly cause secondary reflection pollution is solved, and the electromagnetic shielding material has a very wide application prospect as the electromagnetic shielding material in the fields of military equipment, aerospace and civil electronic equipment.
Claims (10)
1. An aerogel composite, characterized by: it is made of cellulose and contains Co2+The cobalt salt and the imidazole derivative are used as raw materials to prepare aerogel, and the aerogel composite material is obtained through carbonization treatment; the mass of the cellulose accounts for the mass of the cellulose and contains Co2+20 to 45 percent of the total mass of the cobalt salt and the imidazole derivative.
2. The aerogel composite of claim 1, wherein: the mass of the cellulose accounts for the mass of the cellulose and contains Co2+35-45% of the total mass of the cobalt salt and the imidazole derivative, preferably 41-42%;
and/or, said Co-containing2+The mass ratio of the cobalt salt to the imidazole derivative is 1: (2-3), preferably 1: 2.82.
3. An aerogel composite as claimed in claim 2, wherein: the cellulose is nano-cellulose, preferably bacterial cellulose;
and/or, said Co-containing2+The cobalt salt of (a) is selected from cobalt hydroxide or hydrate thereof, cobalt oxide or hydrate thereof, cobalt sulfate or hydrate thereof, cobalt nitrate or hydrate thereof, cobalt carbonate or hydrate thereof, cobalt silicate or hydrate thereof, cobalt halide or hydrate thereof, preferably cobalt nitrate hexahydrate;
and/or, the imidazole derivative is a compound of the following structure:
r is selected from H or C1～6An alkyl group; preferably, the imidazole derivative is 2-methylimidazole.
4. An aerogel composite as claimed in any of claims 1 to 3, wherein: the preparation method of the aerogel comprises the following steps:
(1) mixing cellulose with Co2+Adding the cobalt salt into a solvent for reaction;
(2) then adding imidazole derivative, stirring and standing; and then removing the liquid, washing the residual system with a solvent, and freeze-drying to obtain the product.
5. An aerogel composite as claimed in claim 4, wherein: the solvent is water;
and/or, in step (1), the Co-containing2+The mass volume ratio of the cobalt salt to the solvent is 3-20 mg/m L, preferably 5-15 mg/m L;
and/or in the step (1), the reaction temperature is 20-30 ℃, preferably 25 ℃, and the reaction time is 4-8 hours, preferably 6 hours;
and/or in the step (2), the stirring time is 1-3 hours, preferably 2 hours; the standing temperature is 20-30 ℃, the preferred temperature is 25 ℃, and the standing time is 16-20 hours, the preferred time is 18 hours.
6. An aerogel composite as claimed in claim 5, wherein: the cellulose is the cellulose soaked by alkaline aqueous solution;
preferably, the alkaline aqueous solution is a KOH aqueous solution of 1% mg/m L, and/or the soaking time is 1-8 hours, preferably 5 hours.
7. An aerogel composite as claimed in claim 6, wherein: the thickness of the aerogel composite material is more than 1mm, preferably 2-4 mm.
8. A method of making an aerogel composite as described in any of claims 1 to 7, wherein: the method comprises the following steps: carbonizing the aerogel of any one of claims 1 to 7.
9. The method of claim 8, wherein: the carbonization is carried out under an inert gas atmosphere; and/or the carbonization temperature is 500-1000 ℃, preferably 900 ℃;
and/or the carbonization time is 1-6 hours, preferably 3 hours.
10. Use of the aerogel composite of any of claims 1 to 7 in the preparation of electromagnetic shielding materials.
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