PVA@alkali lignin-based carbon aerogel and preparation method and application thereof

Abstract

The application discloses PVA@alkali lignin-based carbon aerogel as well as a preparation method and application thereof. 2+ Chelation is formed with the hydroxyl groups of PVA (polyvinyl alcohol) and AL (alkali lignin), the pore structure of the carbon aerogel is adjusted by adjusting the addition amount of the soluble nickel salt, the prepared carbon aerogel has more developed and richer pore structure, the electromagnetic shielding efficiency of the carbon aerogel reaches the best state, in addition, the soluble nickel salt is converted into nickel nanoparticles after high-temperature carbonization, the nickel nanoparticles are uniformly dispersed in the interior of the carbon aerogel, and the nickel nanoparticles also help to improve the electric conduction and magnetic conduction performance of the carbon aerogel, thereby further improving the electromagnetic shielding efficiency of the carbon aerogel.

Description

Technical Field

[0001] This invention relates to the field of aerogel materials technology, specifically to a PVA@alkali lignin-based carbon aerogel, its preparation method, and its applications. Background Technology

[0002] With the rapid development of communication technology and the upgrading of electronic devices, while bringing convenience to our lives and work, it has also caused electromagnetic radiation pollution. This not only interferes with the normal operation of electronic devices but also affects human health, causing discomfort such as decreased sleep quality and electromagnetic hypersensitivity, and even inducing cancer. Therefore, the development of high-performance electromagnetic shielding materials has become a current research hotspot.

[0003] Currently, common electromagnetic shielding materials mainly include metallic materials and polymer conductive materials. Among them, metallic materials, as the most common electromagnetic shielding materials, possess excellent electromagnetic shielding performance due to their superior conductivity, but they also suffer from problems such as heavy weight, susceptibility to corrosion, and the potential to cause secondary electromagnetic pollution. Polymer conductive materials, by adding conductive fillers such as metal particles or carbon-based materials to synergistically construct a conductive network, are a promising new type of electromagnetic shielding composite material to replace metallic materials. Common structures include membrane structures, foam structures, and aerogel structures. To improve the conductivity of polymer conductive materials, the amount of conductive filler added is usually increased; however, excessive filler content can severely affect the processability, mechanical properties, and mixing uniformity of the composite material.

[0004] Based on this, researchers have prepared carbon aerogels by carbonizing aerogels at high temperatures. This process enables the aerogels to achieve conductivity during high-temperature pyrolysis without the need for any conductive fillers. Carbon aerogels, with their unique three-dimensional controllable structure, excellent conductivity, and good chemical stability, are considered a highly promising and efficient electromagnetic shielding material. In existing carbon aerogel preparation processes, the pore size structure can usually be effectively controlled by adjusting the raw material ratios and molding methods. The well-developed pore structure facilitates multiple reflections, scattering, and absorption of electromagnetic waves within the carbon aerogel. However, the aforementioned methods of controlling the internal pore structure of carbon aerogels have limited effect on adjusting their electromagnetic shielding effectiveness, and the controllability of these control processes is poor. Summary of the Invention

[0005] To address the problems existing in the background art, this invention provides a PVA@alkali lignin-based carbon aerogel, its preparation method, and its application. A soluble nickel salt is added during the aerogel preparation process. 2+The chelation reaction occurs with the hydroxyl groups of PVA (polyvinyl alcohol) and AL (alkali lignin). By adjusting the amount of soluble nickel salt added, the pore structure of the carbon aerogel can be modified, resulting in a more developed and abundant pore structure. This optimizes the electromagnetic shielding performance of the carbon aerogel. In addition, the soluble nickel salt is transformed into nickel nanoparticles after high-temperature carbonization. These nanoparticles are uniformly dispersed inside the carbon aerogel, which also helps to improve the electrical and magnetic conductivity of the carbon aerogel, thereby further enhancing its electromagnetic shielding performance.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a PVA@alkali lignin-based carbon aerogel, comprising the following raw material components: Alkali lignin, polyvinyl alcohol, soluble nickel salt, water; The mass ratio of alkali lignin to polyvinyl alcohol is (1-2):1, the mass ratio of alkali lignin to water is 1:(40-60), and the mass ratio of soluble nickel salt to water is (0-4):300.

[0007] Furthermore, the mass ratio of the soluble nickel salt to water is (0-2):300.

[0008] Further, the mass ratio of the soluble nickel salt to water is (0-1):300. More preferably, the mass ratio of the soluble nickel salt to water is 0.5:300.

[0009] Furthermore, the soluble nickel salt is NiCl2·6H2O or NiSO4·7H2O.

[0010] Furthermore, the PVA@alkali lignin-based carbon aerogel has a tubular structure inside.

[0011] A second aspect of the present invention provides a method for preparing the above-mentioned PVA@alkali lignin-based carbon aerogel, comprising the following steps: S1. Add alkali lignin to water and stir to mix evenly; S2. Add polyvinyl alcohol and soluble nickel salt to the solution obtained in step S1, stir thoroughly and mix evenly, then pour it into a mold; S3. Freeze-dry the mold and contents from step S2, and obtain aerogel after demolding; S4. The aerogel obtained in step S3 is placed in an inert gas atmosphere for carbonization to obtain PVA@alkali lignin-based carbon aerogel.

[0012] Furthermore, in step S3, the freeze-drying process includes directional freezing and drying.

[0013] Freeze-drying is a drying method that involves freezing water-containing materials below their freezing point, turning the water into ice, and then removing the ice by converting it into vapor under a high vacuum.

[0014] In this invention, directional freezing is preferred. During directional freezing, ice crystals grow slowly in a specific direction (such as from bottom to top), resulting in a distinct tubular structure inside the aerogel.

[0015] Furthermore, in step S4, the inert gas flow rate is 200-400 mL / min, and the carbonization process is as follows: the temperature is increased to 900-1100℃ at a heating rate of 1-5℃ / min, and held for 1-2 hours.

[0016] During the carbonization process, soluble nickel salts are reduced at high temperatures to obtain Ni nanoparticles, which are uniformly dispersed within the carbon aerogel. Furthermore, by controlling the carbonization conditions (heating rate, carbonization temperature, and holding time), the carbon aerogel retains its original tubular structure and exhibits good graphitization effect.

[0017] Furthermore, in step S2, the solution obtained in step S1 is first heated and controlled at 80-95°C, and then polyvinyl alcohol and soluble nickel salt are added to it.

[0018] Under the above temperature conditions, polyvinyl alcohol and alkali lignin undergo a cross-linking reaction to form a cross-linked network structure.

[0019] A third aspect of the present invention provides an application of the above-mentioned PVA@alkali lignin-based carbon aerogel in electromagnetic shielding materials and electrothermal conversion materials.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) In this invention, alkali lignin (AL) is used as a carbon precursor and polyvinyl alcohol (PVA) is used as a long chain structure. Alkali lignin and polyvinyl alcohol have a large number of hydroxyl groups, which makes different types of intermolecular hydrogen bonding and van der Waals forces form between alkali lignin and polyvinyl alcohol, and between polyvinyl alcohol and polyvinyl alcohol. Polyvinyl alcohol connects alkali lignin in series through hydrogen bonding. Through directional freeze drying and high-temperature carbonization, the polyvinyl alcohol molecular chain is carbonized and polycondensed, which shortens the distance between alkali lignin carbon particles, increases the contact area between alkali lignin carbon particles, and improves the conductivity of carbon aerogel, thereby preparing carbon aerogel with high electromagnetic shielding performance. (2) To adjust the pore structure of carbon aerogel, soluble nickel salt is added during the aerogel preparation process. The soluble nickel salt contains Ni 2+ It forms chelate bonds (coordinate bonds) with the hydroxyl groups of alkali lignin and polyvinyl alcohol, playing a cross-linking role in the entire system. Meanwhile, Ni... 2+Effective and uniform anchoring on polyvinyl alcohol and alkali lignin, the pore structure of carbon aerogel is adjusted by regulating the amount of soluble nickel salt added, so that the electromagnetic shielding performance of carbon aerogel reaches the optimal state. In addition, the transformation of soluble nickel salt into nickel nanoparticles after high-temperature carbonization also helps the aerogel to improve its electrical and magnetic conductivity, thereby enhancing the electromagnetic shielding performance of carbon aerogel. (3) Aerogels were prepared by directional freezing and drying. During the directional freezing process, ice crystals grew slowly along a specific direction, which made the aerogel have obvious tubular structure and alkali lignin was uniformly embedded in the tubular structure. (4) During the carbonization process, by controlling the carbonization conditions (heating rate, carbonization temperature and holding time), the carbon aerogel retains the original tubular structure of the aerogel and has a better graphitization effect. Attached Figure Description

[0021] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0022] Figure 1 This is a schematic diagram illustrating the formation mechanism of the carbon aerogel CAPNix in this invention.

[0023] Figure 2 This is a scanning electron microscope (SEM) image (ai) of the carbon aerogel sample CAPNix from this invention. 0.5 EDS energy spectra of C, O, and Ni (i1-i3).

[0024] Figure 3 The following are the phase characterization and identification results of the carbon aerogel sample CAPNix in this invention: a) Fourier transform infrared spectrum; b) XPS spectrum; c) C and O content in the sample; d) CAPNix content in the sample. 0.5 The spectrum of C 1s; e is CAPNi 0.5 Medium to high resolution Ni 2p spectrum; f is Raman spectrum; g is hysteresis loop; h is N2 adsorption-desorption isotherm; i is pore size distribution.

[0025] Figure 4 This is a test diagram of the electromagnetic interference shielding effect of the carbon aerogel sample CAPNix in this invention.

[0026] Figure 5 This is a schematic diagram of the electromagnetic shielding mechanism of the carbon aerogel sample CAPNix in this invention.

[0027] Figure 6 This is a graph showing the Joule heating performance of the carbon aerogel sample CAPNix in this invention.

[0028] Figure 7 A test diagram demonstrating electromagnetic shielding for a Tesla coil. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0030] Main experimental materials in each embodiment: NiCl2·6H2O, NiSO4·7H2O, and polyvinyl alcohol were purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0031] Alkali lignin was purchased from Shanghai Yixing Technology Co., Ltd.

[0032] Deionized water is collected from a water purification system.

[0033] Experimental characterization: The morphology of the samples was observed using a field emission scanning electron microscope (SEM, Hitachi SU 8010, Japan).

[0034] The carbon aerogel was characterized by Raman spectroscopy (Raman, Horiba LabRAM HR Evolution, Japan).

[0035] The chemical structure and elements of the samples were analyzed using X-ray electron spectroscopy (XPS, Thermo Scientific K-Alpha, USA).

[0036] Chemical groups were observed using an FT-IR spectrometer (Nicolet iS20, USA), with wavenumbers ranging from 4000 to 400 cm⁻¹. -1 .

[0037] The conductivity of the sample was obtained by the four-point probe method (Suzhou Crystallography, ST2258C, China).

[0038] The specific surface area (SSA), pore volume, and size distribution were determined using an automated adsorption system (ASAP 2420, Micromeritics, USA) and the Brunauer-Emmet-Teller (BET) equation for the physical adsorption of nitrogen at 77 K.

[0039] The magnetic properties of the samples were characterized using a vibrating sample magnetometer (VSM, Lakeshore 8604, USA).

[0040] The samples were analyzed using a vector network analyzer (Agilent Technologies E5071C, USA) to determine the electromagnetic interference shielding effect. The waveguide rectangular region method was followed, and the samples were pre-cut into strips of 22.86 mm × 10.16 mm to match the support (8.2–12.4 GHz).

[0041] Based on the law of conservation of energy, the reflection coefficient R, absorption coefficient A, and transmission coefficient T can be expressed as formula (1):

[0042]

[0043] .

[0044] In the formula, S 11 S is the reflection coefficient. 21 is the transmission coefficient. The total electromagnetic interference shielding effectiveness (SET) generally consists of three parts: the reflection effectiveness (SER) of the material surface, the absorption effectiveness (SEA) of the magnetic permeability loss of the material surface, and the multiple reflection effectiveness (SEMR) of the material interior. When SET is higher than 15 dB, SEMR is technically negligible. SER and SEA can be calculated by equation (2):

[0045]

[0046] .

[0047] Examples 1-5

[0048] The preparation method of PVA@alkali lignin-based carbon aerogel includes the following steps: S1. Slowly add 6g of alkali lignin to 300g of deionized water and stir and mix for 3 hours under magnetic stirring. S2. Place the solution obtained in step S1 in an oil bath at 90°C and heat it. Slowly add 4.5g of polyvinyl alcohol and a measured amount of NiCl2·6H2O to it. After it is fully stirred and dissolved, pour it into a circular mold. S3. The mold in step S2 is subjected to liquid nitrogen directional freezing. First, the shelf is placed in liquid nitrogen, and then the circular mold is placed on the shelf after being frozen by liquid nitrogen. The ice crystals formed in the contents of the circular mold grow slowly from bottom to top. After vacuum drying, the ice crystals are directly converted into vapor and removed to obtain aerogel. The aerogel has obvious tubular structure, and alkali lignin is uniformly embedded in the tubular structure. In Example 1, NiCl2·6H2O was not added. In step S3, the aerogel obtained is denoted as AAP. In Example 2, 0.5g of NiCl2·6H2O was added. In step S3, the resulting aerogel was denoted as AAPNi. 0.5 ; In Example 3, 1g of NiCl2·6H2O was added. In step S3, the aerogel obtained was denoted as AAPNi1. In Example 4, 1.5g of NiCl2·6H2O was added. In step S3, the resulting aerogel was denoted as AAPNi. 1.5 ; In Example 5, 2g of NiCl2·6H2O was added. In step S3, the aerogel obtained was denoted as AAPNi2. S4. Place the aerogel obtained in step S3 in a tube furnace, introduce nitrogen gas at a flow rate of 300 mL / min, heat to 1100 °C at a rate of 2.5 °C / min, hold at this temperature for 1 hour, and then allow to cool naturally to room temperature. The carbon aerogels prepared in Examples 1-5 are designated as CAP and CAPNi, respectively. 0.5 CAPNi1, CAPNi 1.5 ,CAPNi2. Example 6

[0049] The preparation method of PVA@alkali lignin-based carbon aerogel includes the following steps: S1. Slowly add 4.5g of alkali lignin to 180g of deionized water and stir and mix for 3 hours under magnetic stirring. S2. Place the solution obtained in step S1 in an oil bath at 95°C and heat it. Slowly add 4.5g of polyvinyl alcohol and 1.8g of NiCl2·6H2O to it. After stirring and dissolving thoroughly, pour it into a round mold. S3. The mold in step S2 is subjected to liquid nitrogen directional freezing. First, the shelf is placed in liquid nitrogen, and then the circular mold is placed on the shelf after being frozen by liquid nitrogen. The ice crystals formed in the contents of the circular mold grow slowly from bottom to top. After vacuum drying, the ice crystals are directly converted into vapor and removed to obtain aerogel. The aerogel has obvious tubular structure, and alkali lignin is uniformly embedded in the tubular structure. S4. Place the aerogel obtained in step S3 in a tube furnace, introduce nitrogen gas at a flow rate of 200 mL / min, heat to 1100℃ at a rate of 3℃ / min, hold for 1 hour, and then cool naturally to room temperature to obtain carbon aerogel. Example 7

[0050] The preparation method of PVA@alkali lignin-based carbon aerogel includes the following steps: S1. Slowly add 9g of alkali lignin to 540g of deionized water and stir and mix for 3 hours under magnetic stirring. S2. Place the solution obtained in step S1 in an oil bath at 80°C and heat it. Slowly add 4.5g of polyvinyl alcohol and 7.2g of NiCl2·6H2O to it. After stirring and dissolving thoroughly, pour it into a round mold. S3. The mold in step S2 is subjected to liquid nitrogen directional freezing. First, the shelf is placed in liquid nitrogen, and then the circular mold is placed on the shelf after being frozen by liquid nitrogen. The ice crystals formed in the contents of the circular mold grow slowly from bottom to top. After vacuum drying, the ice crystals are directly converted into vapor and removed to obtain aerogel. The aerogel has obvious tubular structure, and alkali lignin is uniformly embedded in the tubular structure. S4. Place the aerogel obtained in step S3 in a tube furnace, introduce nitrogen gas at a flow rate of 400 mL / min, heat to 900℃ at a rate of 1℃ / min, hold for 2 hours, and then cool naturally to room temperature to obtain carbon aerogel. Example 8

[0051] The preparation method of PVA@alkali lignin-based carbon aerogel includes the following steps: S1. Slowly add 6g of alkali lignin to 300g of deionized water and stir and mix for 3 hours under magnetic stirring. S2. Place the solution obtained in step S1 in an oil bath at 80°C and heat it. Slowly add 4.5g of polyvinyl alcohol and 0.5g of NiSO4·7H2O to it. After stirring and dissolving thoroughly, pour it into a round mold. S3. The mold in step S2 is subjected to liquid nitrogen directional freezing. First, the shelf is placed in liquid nitrogen, and then the circular mold is placed on the shelf after being frozen by liquid nitrogen. The ice crystals formed in the contents of the circular mold grow slowly from bottom to top. After vacuum drying, the ice crystals are directly converted into vapor and removed to obtain aerogel. The aerogel has obvious tubular structure, and alkali lignin is uniformly embedded in the tubular structure. S4. Place the aerogel obtained in step S3 in a tube furnace, introduce nitrogen gas at a flow rate of 300 mL / min, heat it to 1100℃ at a rate of 5℃ / min, hold it at this temperature for 1.5 h, and then allow it to cool naturally to room temperature to obtain carbon aerogel. Example 9

[0052] The preparation method of PVA@alkali lignin-based carbon aerogel includes the following steps: S1. Slowly add 6g of alkali lignin to 300g of deionized water and stir and mix for 3 hours under magnetic stirring. S2. Place the solution obtained in step S1 in an oil bath at 90°C and heat it. Slowly add 4.5g of polyvinyl alcohol and 0.5g of NiCl2·6H2O to it. After stirring and dissolving thoroughly, pour it into a round mold. S3. The mold in step S2 is subjected to liquid nitrogen directional freezing. First, the shelf is placed in liquid nitrogen, and then the circular mold is placed on the shelf after being frozen by liquid nitrogen. The ice crystals formed in the contents of the circular mold grow slowly from bottom to top. After vacuum drying, the ice crystals are directly converted into vapor and removed to obtain aerogel. The aerogel has obvious tubular structure, and alkali lignin is uniformly embedded in the tubular structure. S4. Place the aerogel obtained in step S3 in a tube furnace, introduce nitrogen gas at a flow rate of 300 mL / min, heat to 1100℃ at a rate of 2.5℃ / min, hold for 1 hour, and then cool naturally to room temperature to obtain carbon aerogel.

[0053] Experimental test results: The preparation of carbon aerogels involves two steps: freeze-drying and carbonization pyrolysis. The formation mechanism is as follows: Figure 1 As shown.

[0054] Polyvinyl alcohol (PVA), alkali lignin, and soluble nickel salts were dissolved in deionized water to form a homogeneous and stable mixture. PVA and alkali lignin contain numerous hydroxyl groups, resulting in different types of intermolecular hydrogen bonding and van der Waals forces between PVA and alkali lignin, and between PVAs themselves. Furthermore, Ni… 2+ It can form coordination bonds with the hydroxyl groups of polyvinyl alcohol and alkali lignin, playing a cross-linking role in the entire system, while Ni 2+ It is effectively and uniformly anchored to polyvinyl alcohol and alkali lignin.

[0055] Subsequently, the mixture was poured into a circular mold and directionally frozen and dried with liquid nitrogen to obtain aerogel AAPNIx. During the directional freezing process, ice crystals grew slowly along a specific direction, giving the aerogel a distinct tubular structure, with alkali lignin uniformly embedded in the tubular structure.

[0056] The prepared aerogel was carbonized at high temperature in a tubular furnace to obtain carbon aerogel CAPNix. Soluble nickel salts were reduced at high temperature to obtain Ni nanoparticles. The Ni nanoparticles were uniformly dispersed inside the carbon aerogel, which helped to improve the electrical and magnetic conductivity of the carbon aerogel, thereby further improving the electromagnetic shielding effectiveness of the carbon aerogel.

[0057] I. Morphological analysis of the prepared samples Figure 2 The image shows a scanning electron microscope (SEM) image of the sample (ai), CAPNi 0.5 EDS energy spectra of C, O, and Ni (i1-i3).

[0058] Figure 2a is a frontal cross-sectional view of aerogel AAP. As can be seen from the figure, the aerogel has a distinct tubular structure. This is because during the formation and directional growth of ice crystals, polyvinyl alcohol and alkali lignin are repelled by the ice crystals and form a directional structure along the ice crystal growth direction. Figure 2 b is a magnified view of 2a. Due to the excellent film-forming properties of polyvinyl alcohol, the vessel wall exhibits a distinct transverse septal structure with only a small number of pores. Furthermore, the vessel wall surface is relatively smooth, indicating the presence of alkali lignin and Ni. 2+ The reason why PVA, AL, and NiCl2 are evenly distributed inside PVA is that PVA, AL, and NiCl2 can form a homogeneous solution after dissolving in water.

[0059] Figure 2 c is carbon aerogel CAP obtained by carbonizing aerogel AAP at 1100℃. Compared with 2a and 2b, it was found that the carbon aerogel retained the original tubular structure of the aerogel. However, the tubular structure showed obvious shrinkage, and the surface of the pore wall became more dense. The main reason is that the long chain molecules of polyvinyl alcohol underwent obvious wrinkling after high-temperature carbonization, which made the alkali lignin carbon particles and Ni nanoparticles more closely contacted under the action of polyvinyl alcohol.

[0060] Figure 2 c to 2g are front cross-sectional views of CAPNix carbon aerogels with different nickel additions. As the Ni content increases, the pore diameter of the carbon aerogel gradually increases. This is due to the presence of polyvinyl alcohol, alkali lignin, and Ni. 2+ The organic-inorganic cross-linked network weakens the aggregation phenomenon after aerogel carbonization.

[0061] Figure 2 i1-i3 are carbon aerogel CAPNi 0.5 The EDS spectra of C, O, and Ni show that C constitutes the largest proportion in the carbon aerogel, with only a small amount of O remaining after high-temperature carbonization. Furthermore, Ni is uniformly distributed, which helps improve the electrical and magnetic conductivity of the carbon aerogel, thereby enhancing its electromagnetic shielding effectiveness.

[0062] II. Phase Characterization and Identification of the Prepared Samples Figure 3 The results of phase characterization and identification of the sample. Figure 3 a represents the Fourier transform infrared (FTIR) spectra of aerogel AAP and carbon aerogel CAPNix, respectively. AAP aerogel has the highest FTIR spectrum at 3415 cm⁻¹. -1 There is a broad band to the left and right, which is caused by the stretching vibration of -OH. These characteristic peaks are attributed to the phenolic hydroxyl groups of alkali lignin, lignin, and aliphatic hydroxyl groups of polyvinyl alcohol in the aerogel. 2939cm -1 and 1456cm -1The peaks at 1600 cm⁻¹ represent the stretching and deformation vibrations of the -CH groups in the methoxy, methyl, and methylene groups of the aerogel. -1 1512cm -1 The peak at 2939 cm⁻¹ represents the skeletal vibration of the aromatic ring. After high-temperature carbonization, the -OH stretching vibration peak of each carbon aerogel significantly weakens, reaching 2939 cm⁻¹. -1 and 1456cm -1 The stretching and deformation vibration peaks at 1634 cm⁻¹ have largely disappeared. -1 The C=C value indicates that a small amount of aromatic ring structure is still retained in its carbon aerogel.

[0063] Figure 3 Figure b shows the XPS spectrum of carbon aerogel CAPNix. Except for sample CAP, the other four samples all have four characteristic peaks, namely C 1s, O 1s, Ni 2p, and Na 1s. Among them, the C 1s characteristic peak is significantly higher than the other characteristic peaks, and the atomic content of each is not less than 90%. Figure 3 c). Test results revealed the presence of Na 1s in the carbon aerogel, which is attributed to the presence of sodium salt in the alkali lignin raw material. Furthermore, a weak Ni 2p characteristic peak was observed in the CAPNix carbon aerogel, indicating that nickel nanoparticles were successfully loaded into the carbon aerogel.

[0064] Figure 3 d is the high-resolution C 1s spectrum. The peak of CC / C=C (284 eV) is strong, while the peaks of CO (285.2 eV) and OC=O (289.3 eV) are relatively weak, indicating that most carbon species in the carbon aerogel belong to the sp2 conjugated carbon skeleton. Figure 3 Figure e shows the high-resolution Ni 2p spectrum. Peak fitting reveals that nickel mainly exists in two states: Ni (852.8 eV) and Ni2P. 2+ (855.6 eV), where Ni 2+ The characteristic peak is larger than that of Ni, indicating that the content of nickel compounds is greater than that of metallic Ni.

[0065] Figure 3 f represents the Raman spectrum of the carbon aerogel CAPNix; the D band (~1350 cm⁻¹) can be observed in all of the figures. −1 ) and G-band (~1580cm) −1 The D-band is related to structural defects or disorder caused by the presence of sp3 hybrid carbon atoms, reflecting the degree of disorder, while the G-band is formed due to the vibration of sp2 hybrid bonds in the hexagonal lattice of graphite carbon, reflecting the degree of order. The ratio of ID to IG is used to indicate the degree of graphitization of the sample; the smaller the ID / IG ratio, the better the degree of graphitization of the carbon aerogel.

[0066] When the nickel content is zero, the carbon aerogel CAP has the lowest ID / IG ratio, which is 0.933. As the nickel content increases, the ID / IG ratio of the carbon aerogel CAPNix gradually increases, indicating that the degree of graphitization of the carbon aerogel gradually decreases with increasing nickel content. This may be because the carbon aerogel skeleton gradually increases with increasing nickel content, which is conducive to the release of CO and CO2 gases during high-temperature processes. Gas release promotes the formation of porous structures, leading to structural defects and thus reducing the degree of graphitization. This is consistent with the changes in oxygen element measured in XPS. Figure 3 c) As the nickel content increases, the oxygen content of the carbon aerogel gradually decreases while the carbon content gradually increases. Although the ID / IG ratio of the carbon aerogel CAPNix decreases slightly with increasing nickel content, it still exhibits better graphitization performance compared to other materials.

[0067] Figure 3 g represents the hysteresis loops of the carbon aerogel CAPNix prepared with different nickel addition amounts. Analysis of the hysteresis loops revealed that the saturation magnetization of CAPNix increased with increasing NiCl2·6H2O concentration, reaching -0.05 emu / g, 0.31 emu / g, 0.93 emu / g, 1.59 emu / g, and 2.48 emu / g, respectively. This indicates that the magnetism of the carbon aerogel CAPNix increases with increasing nickel ion concentration. This is because the presence of more nickel nanoparticles on the carbon aerogel framework helps improve the magnetic loss resistance of the carbon aerogel CAPNix to electromagnetic waves.

[0068] Figure 3 As shown in h, the N2 adsorption-desorption isotherm of the carbon aerogel CAPNix conforms to the type IV isotherm adsorption curve (IUPAC standard), indicating that the carbon aerogel CAPNix is ​​a mesoporous material with a distinct mesoporous structure. Without nickel, the carbon aerogel CAPNix has the smallest specific surface area and total pore volume. The specific surface area of ​​the carbon aerogel CAPNix gradually increases with increasing nickel content, from 16.2 m² / h. 2 / g increased to 235.5 m 2 / g, and the total pore volume also increased from 0.009 cm³. 3 / g increased to 0.237 cm 3 / g indicates that the aggregation of the carbon aerogel framework gradually weakens with increasing nickel content, resulting in a more developed porous structure, consistent with CAPNix electron microscopy images ( Figure 2 The results were consistent with those of d-2g. Figure 3 Figure i shows the pore size distribution curve of carbon aerogel. The pore size distribution range of carbon aerogel widens significantly with increasing nickel content, and the number of pores of different sizes also increases substantially. High specific surface area and well-developed pore structure help improve the electromagnetic shielding effectiveness of the material.

[0069] III. Electromagnetic shielding performance of the prepared samples Figure 4 This is a test diagram showing the electromagnetic interference shielding effect of the sample. (Example) Figure 4 As shown in Figure a, the carbon aerogel CAPNix exhibits good electrical conductivity, primarily due to the high degree of graphitization achieved after high-temperature carbonization. Nickel nanoparticles also contribute to the conductivity of the carbon aerogel. With increasing nickel content, the conductivity initially increases, then decreases, and then increases again. Without Ni addition, the carbonization and aggregation of polyvinyl alcohol in the aerogel AAP under high-temperature pyrolysis results in close contact between the encapsulated alkali lignin carbon particles, forming a good conductive network and contributing to the excellent conductivity of the carbon aerogel CAP (365.8 S / m). With increasing nickel content, the conductivity of the carbon aerogel CAPNi... 0.5 The electrical conductivity further increased (437.9 S / m), mainly due to the close contact between the lignin carbon particles and nickel nanoparticles, which jointly promoted the improvement in electrical conductivity. With further increases in nickel content, the carbon aerogels CAPNi1 and CAPNi... 1.5 The conductivity of the carbon aerogel decreased significantly, mainly because the pore structure of the carbon aerogel expanded significantly with increasing nickel content, greatly weakening the aggregation phenomenon after PVA carbonization and reducing the contact between lignin char, thus weakening the conductivity of the carbon aerogel. When the amount of NiCl2·H2O added was 2g, the conductivity of the carbon aerogel CAPNi2 showed an upward trend, because the high nickel content promoted the formation of the conductive network of the carbon aerogel. However, considering the raw material cost and the impact on the overall performance of the finished product, it is necessary to control the amount of soluble nickel salt added.

[0070] Figure 4 Figures b and 4d show the EMI SE of carbon aerogel CAPNix in the 8.2-12.4 GHz band. Without Ni, the EMI SE of carbon aerogel CAP reaches 77 dB, mainly due to the high conductivity and dense tubular structure of CAP, which provides excellent electromagnetic shielding performance. With increasing Ni content, the EMI SE of carbon aerogel changes similarly to that of conductivity, showing a trend of first increasing, then decreasing, and then increasing again. 0.5 It boasts a maximum EMI SE of 108dB, achieving military-grade shielding standards. Its main component is carbon aerogel CAPNi. 0.5 It exhibits the highest electrical conductivity; furthermore, the addition of nickel promotes both the magnetic permeability and the formation of the pore structure in carbon aerogels. Compared to CAP, carbon aerogel CAPNi... 0.5 With a higher specific surface area and total pore volume, the reflection and scattering paths of electromagnetic waves within it are increased, making carbon aerogel CAPNi... 0.5 The electromagnetic shielding performance reaches its optimal state. With further increases in Ni content, carbon aerogels CAPNi1 and CAPNi...1.5 The EMI SE showed a gradual decreasing trend, which was due to a significant decrease in the conductivity of carbon aerogel, especially CAPNi. 1.5 Although the well-developed pore structure and good magnetic permeability contribute to electromagnetic loss, electrical conductivity still plays a dominant role, resulting in the degradation of carbon aerogels CAPNi1 and CAPNi. 1.5 The electromagnetic shielding effectiveness is weaker than that of CAPNi. 0.5 Compared to CAPNi1, CAPNi 1.5 The conductivity decreased by approximately 49%, from 353.2 S / m to 180 S / m, while the specific surface area increased by approximately 150%. Despite the combined effect of high specific surface area and magnetic permeability, the EMI shielding effectiveness decreased by only about 8%, indicating that both high specific surface area and magnetic permeability contribute to improved electromagnetic shielding performance. Compared to CAPNi... 1.5 The electrical conductivity of carbon aerogel CAPNi2 increased slightly. Furthermore, compared to other samples, carbon aerogel CAPNi2 exhibited a more developed pore structure and higher magnetic permeability, resulting in superior electromagnetic shielding performance compared to CAPNi. 1.5 The carbon aerogel CAPNix prepared in this invention exhibits EMI SE superior to most previously reported materials, including naturally derived materials and conductive polymer composites containing fillers such as carbon nanotubes, AgNS (silver nanowires), rGO (reduced graphene oxide), and Mxene (two-dimensional transition metal carbide or nitride materials).

[0071] Reflectance coefficient (R), absorption coefficient (A), and transmission coefficient (T) are commonly used to evaluate the shielding mechanism of electromagnetic shielding materials, representing the sample's shielding ability against electromagnetic wave reflection, absorption, and transmission, respectively. For example... Figure 4 As shown in c, the T values ​​of the carbon aerogel CAPNix all tend to be 0, indicating that most electromagnetic waves are effectively shielded. The R coefficients of the carbon aerogels are all higher than A, indicating that each carbon aerogel is a highly reflective material. Figure 4 As shown in d, regardless of the Ni content, the SEA value of the carbon aerogel CAPNix is ​​always higher than the SER value, so the absorption mechanism plays an important role in this process.

[0072] Figure 5The electromagnetic shielding mechanism of the carbon aerogel CAPNix prepared in this invention is illustrated in the figure, showing the shielding process of electromagnetic waves passing through the carbon aerogel: When the electromagnetic wave encounters the surface of the carbon aerogel with high conductivity, a small portion of the electromagnetic wave is directly reflected due to impedance mismatch, resulting in reflection loss. When the remaining majority of the electromagnetic wave enters the interior of the carbon aerogel, it undergoes multiple reflections and scattering within the well-developed porous structure, and interacts with high-density mobile charge carriers, resulting in conduction loss and the conversion of electromagnetic wave energy into heat energy. The heterogeneous interface between the graphitized portion and the defect structure within the carbon aerogel also promotes electromagnetic wave attenuation due to interface polarization. Furthermore, the uniformly distributed magnetic Ni nanoparticles can also serve as effective electromagnetic wave loss points, converting some electromagnetic waves into heat energy through magnetic loss, thereby increasing absorption loss. Therefore, the synergistic effect of high conductivity, well-developed porous structure, heterogeneous interface, and good magnetic permeability gives the carbon aerogel CAPNix prepared in this invention excellent electromagnetic shielding performance.

[0073] IV. Joule heating performance of the prepared samples Figure 6 This is a graph showing the Joule heating performance of the sample. Figure 6 Figure a shows the surface temperature change curves of carbon aerogel CAPNix under a voltage of 1.5 V. After energizing, the surface temperature of each carbon aerogel rises rapidly and reaches equilibrium within a short time. As the pore volume increases, the surface temperature of each carbon aerogel shows a decreasing trend. This may be because a larger pore volume reduces the density of the carbon aerogel, thereby reducing its thermal conductivity. Specifically, CAP and CAPNi... 0.5 All of them have high surface temperatures, with surface temperatures exceeding 180°C under a 1.5V voltage. Figure 6 b represents carbon aerogel CAPNi 0.5 Surface temperature curves under different voltages. As the applied voltage increases, the carbon aerogel CAPNi... 0.5 The surface temperature becomes higher, in which carbon aerogel CAPNi 0.5 Under a 2V voltage, the surface temperature can reach 272℃, exhibiting excellent electrothermal conversion performance. For example... Figure 6 As shown in c, carbon aerogel CAPNi 0.5 Temperature changes and U 2 The relationship is linear, with a correlation coefficient as high as 0.99, conforming to Joule's law; furthermore, carbon aerogel CAPNi 0.5 With a high slope K value (58.2±3.5), it can achieve a high Joule heating effect under a low voltage, further indicating that the carbon aerogel CAPNi 0.5 It has the potential to manufacture high-performance electric heaters. For example... Figure 6 Figure d shows the carbon aerogel CAPNi. 0.5The surface temperature change when the input voltage is gradually varied; the carbon aerogel CAPNi changes as the applied voltage increases or decreases. 0.5 It responds quickly and adjusts its surface temperature, exhibiting high sensitivity and excellent electrothermal conversion performance. To evaluate the carbon aerogel CAPNi... 0.5 The heating stability of the electric heater under constant supply voltage conditions was studied, and long-term heating was investigated, such as... Figure 6 As shown in Figure e, when the applied voltage is 1.5 V, the surface temperature remains relatively stable within 40 minutes, and the temperature remains constant at around 184℃, demonstrating excellent heating stability and reliability.

[0074] The test results above show that the carbon aerogel CAPNix prepared in this invention has excellent electrical conductivity and can be used to prepare high-performance electric heaters by utilizing the Joule heating effect.

[0075] V. Electromagnetic Interference Shielding Application of Prepared Samples Figure 7 An electromagnetic shielding experiment was conducted to demonstrate the Tesla coil, showcasing CAPNix's superior electromagnetic shielding performance. Figure 7 As shown in diagram a, when an LED light is brought close to a energized Tesla coil, the high-frequency electric field in the Tesla coil generates electromagnetic radiation, providing sufficient energy to the LED light and illuminating it. Figure 7 As shown in Figure b, inserting a piece of wood between the Tesla coil and the LED light does not significantly affect the brightness of the LED light, indicating that natural wood has poor electromagnetic shielding properties. However, as... Figure 7 As shown in Figure c, after inserting the carbon aerogel CAPNix prepared in the embodiment of the present invention at the same position, the LED light immediately turns off. This is because the excellent electromagnetic shielding performance of the carbon aerogel CAPNix prevents the Tesla coil from outputting energy to the LED light through the electromagnetic field.

[0076] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A PVA@alkali lignin-based carbon aerogel, characterized in that, It includes the following raw material components: Alkali lignin, polyvinyl alcohol, soluble nickel salt, water; The mass ratio of alkali lignin to polyvinyl alcohol is (1-2):1, the mass ratio of alkali lignin to water is 1:(40-60), and the mass ratio of soluble nickel salt to water is (0.5-4):

300. Aerogels are made by freeze-drying mixed raw materials; The freeze-drying process includes directional freezing and drying.

2. The PVA@alkali lignin-based carbon aerogel according to claim 1, characterized in that, The mass ratio of the soluble nickel salt to water is (0.5-2):

300.

3. The PVA@alkali lignin-based carbon aerogel according to claim 2, characterized in that, The mass ratio of the soluble nickel salt to water is (0.5-1):

300.

4. The PVA@alkali lignin-based carbon aerogel according to claim 1, characterized in that, The soluble nickel salt is NiCl2·6H2O or NiSO4·7H2O.

5. The PVA@alkali lignin-based carbon aerogel according to claim 1, characterized in that, The PVA@alkali lignin-based carbon aerogel has a tubular structure inside.

6. A method for preparing PVA@alkali lignin-based carbon aerogel as described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Add alkali lignin to water and stir to mix evenly; S2. Add polyvinyl alcohol and soluble nickel salt to the solution obtained in step S1, stir thoroughly and mix evenly, then pour it into a mold; S3. Freeze-dry the mold and contents from step S2, and obtain aerogel after demolding; The freeze-drying process includes directional freezing and drying; S4. The aerogel obtained in step S3 is placed in an inert gas atmosphere for carbonization to obtain PVA@alkali lignin-based carbon aerogel.

7. The method for preparing PVA@alkali lignin-based carbon aerogel according to claim 6, characterized in that, In step S4, the carbonization process is as follows: the temperature is increased to 900-1100℃ at a heating rate of 1-5℃ / min, and then held for 1-2 hours.

8. The method for preparing PVA@alkali lignin-based carbon aerogel according to claim 6, characterized in that, In step S2, the solution obtained in step S1 is first heated and controlled at 80-95°C, and then polyvinyl alcohol and soluble nickel salt are added to it.

9. The application of PVA@alkali lignin-based carbon aerogel as described in any one of claims 1 to 5 in the fields of electromagnetic shielding materials and electrothermal conversion materials.

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