CoFe@C / RC composite material based on MOF, preparation and application thereof

Abstract

The application discloses a CoFe@C / RC composite material based on MOF derivation, preparation and application, and steps of a preparation method of the composite material are as follows: S1: preparation of coal gasification fine slag residual carbon RC; S2: preparation of a CoFe-MOF-74 / RC precursor: Co(CH3COO)2.4H2O, FeCl3.6H2O, 2,5-dihydroxy terephthalic acid and the RC prepared in S1 are dissolved in deionized water, stirring, centrifugation, ethanol flushing and vacuum drying are carried out to obtain the CoFe-MOF-74 / RC precursor; S3: preparation of the CoFe@C / RC composite material: the CoFe-MOF-74 / RC precursor prepared in S2 is pyrolyzed under a protective gas atmosphere to obtain the CoFe@C / RC composite material. The composite material prepared in the application has excellent EMW absorption capacity, and the ultra-wide fe reaches 10.0 GHz (8.0-18.0 GHz), and the Ku and X wave bands are included.

Description

Technical Field

[0001] This invention relates to the field of microwave absorbing materials technology, and in particular to MOF-derived CoFe@C / RC composite materials, their preparation and application. Background Technology

[0002] The rapid development of electronic technology, while bringing convenience to life, has also brought about significant problems such as electromagnetic radiation and interference. For example, sophisticated electronic devices are susceptible to electromagnetic interference, which can affect their performance. Electromagnetic waves radiated by computer equipment can lead to the leakage of critical information. Weather radar, communication base stations, substations, and even mobile phones all radiate electromagnetic waves into the environment. These complex electromagnetic radiations and their cumulative effects can negatively impact human health. In the field of radar detection and the competition among nations for stealth fighters, the upgrading of radar detection technology has spurred the military's demand for more powerful stealth technologies. Therefore, developing electromagnetic wave absorbing materials that meet functional requirements (matching thin thickness, low density, wide absorption bandwidth, and strong absorption capacity) and cost constraints has become an important research direction for solving electromagnetic pollution and meeting national defense stealth requirements.

[0003] Carbon materials possess excellent dielectric properties, tunable morphology, light weight, and chemical stability, showing great potential in fields such as EMW absorption. However, the development of high-performance carbon-based EMW absorbing materials remains challenging, particularly in impedance matching tuning and low-cost fabrication. Metal-organic frameworks (MOFs) are porous crystalline materials formed by the coordination of metals and organic compounds. Their unique physical and chemical properties have attracted widespread attention in many scientific fields. In the field of EMW absorption, MOFs / metal oxides / metal carbon derivatives, as a novel type of absorbing material, have advantages such as simple preparation methods, good stability, high porosity, and large specific surface area. Furthermore, the magnetic materials in MOF derivatives exhibit uniform dispersion, thus attracting significant attention. However, simple MOF derivatives as EMW absorbing materials still suffer from limitations such as thick absorption layers, large addition amounts in the matrix, and insufficient absorption performance, making it difficult to meet the requirements of high-efficiency absorbing materials. Therefore, the preparation of composite absorbing materials based on MOFs and their derivatives is of great significance. Yang et al. successfully obtained a NiCo / C / CNT / rGO aerogel-modified MOF derivative with an effective absorption bandwidth of 7.6 GHz at 1.8 mm. Shan et al. prepared three two-dimensional MOFs, M3(HHTP)2 (M = Cu, Zn, Ni), using a hydrothermal method, achieving a strong reflection loss of -56.45 dB. Shen et al. synthesized two multi-metal MOFs: CoMn-MOF and FeCoMn-MOF, finding that the introduction of Co and FeCo could cleverly improve permeability and impedance matching, achieving an effective absorption bandwidth of 7.72 GHz at 1.63 mm and a reflection loss of -54.07 dB at 2.37 mm. Coal gasification technology has developed rapidly as a clean utilization technology, but it also generates a large amount of coal gasification slag, causing serious environmental and health problems. How to prepare composite materials with good microwave absorption performance using coal gasification slag as raw material is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0004] Based on the technical problems existing in the background technology, this invention proposes a CoFe@C / RC composite material derived from MOF, its preparation and application. This composite material has excellent EMW absorption capacity, covering the entire Ku and X bands.

[0005] The method for preparing MOF-derived CoFe@C / RC composite materials proposed in this invention comprises the following steps:

[0006] S1: Preparation of residual carbon (RC) from coal gasification fine slag;

[0007] S2: Preparation of CoFe-MOF-74 / RC precursor

[0008] The RC prepared by Co(CH3COO)2·4H2O, FeCl3·6H2O, 2,5-dihydroxyterephthalic acid and S1 was dissolved in deionized water, and the CoFe-MOF-74 / RC precursor was obtained by stirring, centrifugation, ethanol washing and vacuum drying.

[0009] S3: Preparation of CoFe@C / RC composite material

[0010] The CoFe-MOF-74 / RC precursor prepared by S2 was pyrolyzed under a protective gas atmosphere to obtain the CoFe@C / RC composite material.

[0011] Preferably, the molar ratio of Co(CH3COO)2·4H2O, FeCl3·6H2O, 2,5-dihydroxyterephthalic acid and RC in S2 is 1mmol:0.1-10mmol:0.5-2mmol:0.1-0.5g.

[0012] Preferably, the stirring conditions in S2 are: temperature 90-110℃, time 40-60min, and stirring speed 200-400rpm.

[0013] Preferably, the centrifugation temperature in S2 is 75-85℃, and the centrifugation speed is 8000-12000 rpm.

[0014] Preferably, the protective gas in S3 is one of nitrogen, argon, and helium.

[0015] Preferably, the pyrolysis temperature in S3 is 760-840℃, and the pyrolysis time is 1.5-2.5h.

[0016] Preferably, the heating rate of pyrolysis is 1.8-2.2℃ / min.

[0017] The method proposed in this invention prepares a MOF-derived CoFe@C / RC composite material.

[0018] The present invention proposes the application of the above-mentioned MOF-derived CoFe@C / RC composite material in electromagnetic wave absorbing materials.

[0019] Mechanism of action

[0020] The synergistic effect of dielectric loss, conductive loss, and magnetic loss endows the CoFe@C / RC hybrid with excellent microwave absorption performance. First, the abundant oxygen-containing functional groups and defects on the RC surface act as polarization centers, inducing dipole polarization and enhancing dielectric loss, which is beneficial for attenuation of incident EMW. Second, the introduction of CoFe nanoparticles provides magnetic loss to the CoFe@C / RC hybrid. Third, the CoFe / C and CoFe / RC interfaces lead to interfacial polarization loss, significantly increasing dielectric loss. Fourth, the large distribution of CoFe nanoparticles in the carbon matrix results in dipole polarization loss. Finally, electrons can absorb EMW energy and then transition to the conductive carbon layer, where conduction losses readily occur, converting the EMW energy into heat.

[0021] Beneficial technical effects

[0022] The CoFe@C / RC composite material prepared in this invention exhibits excellent EMW absorption capacity. With a filler content of 20 wt.%, sample S4 showed an RL (reduction) at 1.7 mm. min -20.0dB, f e The frequency is 3.95 GHz. By adjusting the thickness from 1.5 mm to 3.0 mm, the ultra-wide f / 1.00 GHz band... e Reaching 10.0 GHz encompasses the entire Ku and X bands. Attached Figure Description

[0023] Figure 1 The synthesis steps of the CoFe@C / RC composite material proposed in this invention are as follows;

[0024] Figure 2 The XRD diffraction pattern (a), infrared spectrum (b), Raman spectrum (c), and hysteresis loop diagram (d, e) of the RC and CoFe@C / RC composite materials proposed in this invention are shown.

[0025] Figure 3 The SEM images of S1(ac), S2(df), S3(gi) and S4(jl) and the EDX mapping image (m) of S2 are provided in this invention.

[0026] Figure 4 XPS full spectrum of S4 and RC proposed in this invention (a), XPS spectrum of C1s, Co1s and Fe2p of S4 (bd);

[0027] Figure 5 The graphs show the ε′(a), ε″(b), μ′(c), μ″(d), tanδε(e), and tanδμ(f) of the CoFe@C / RC composite material proposed in this invention.

[0028] Figure 6The RL-f curves and 3D diagrams of S1(a,b), S2(c,d), S3(e,f) and S4(g,h) proposed in this invention are shown.

[0029] Figure 7 The Cole-Cole curves for S1(a), S2(b), S3(c), and S4(d) proposed in this invention are shown.

[0030] Figure 8 The figures show the CO(a) and α(b) curves of the CoFe@C / RC composite material proposed in this invention.

[0031] Figure 9 The |Z| of S1(a), S2(b), S3(c), and S4(d) proposed in this invention in / Z0|Graph;

[0032] Figure 10 This is a diagram illustrating the electromagnetic wave absorption mechanism of the CoFe@C / RC composite material proposed in this invention. Detailed Implementation

[0033] The present invention will be further explained below with reference to specific embodiments.

[0034] The coal gasification fine slag of this invention is collected from the gasifier and the particle size of the coal gasification fine slag is ground to 2 μm using a ball milling method. Similar to previous studies, the RC is prepared by a two-step acidification method using coal gasification fine slag. The coal gasification fine slag is used as raw material, mixed, stirred, and filtered with hydrofluoric acid solution in a container. The filtered solid is washed with water until neutral and then dried. The dried solid is mixed, stirred, and filtered with hydrochloric acid solution in a container, and the filtered solid is washed with water until neutral and then dried to obtain the coal gasification fine slag residue RC.

[0035] The cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O, 99.5%), ferric chloride hexahydrate (FeCl3·6H2O, 99%), and 2,5-dihydroxyterephthalic acid (DHTA) of this invention were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China).

[0036] Example

[0037] First, X mmol Co(CH3COO)2·4H2O, (4-X) mmol FeCl3·6H2O (X was 4, 3, 2, and 1 respectively), 2 mmol DHTA, and 0.4 g RC were dissolved in 100 mL of deionized water and stirred at 100 °C for 1 h. Then, the mixture was centrifuged at 80 °C, washed with ethanol, and vacuum dried to obtain the CoFe-MOF-74 / RC precursor. Finally, the dried precursor was pyrolyzed at 800 °C for 2 h under a nitrogen atmosphere at a heating rate of 2 °C / min. The resulting CoFe@C / RC hybrids with different Co / Fe molar ratios were labeled as S1 (Co4Fe0@C / RC), S2 (Co3Fe1@C / RC), S3 (Co2Fe2@C / RC), and S4 (Co1Fe3@C / RC), respectively.

[0038] This invention characterized the structure, chemical composition, and microstructure of the CoFe@C / RC composite material using Fourier transform infrared spectroscopy (FT-IR, Nicolet iS5, Seymour, USA), X-ray diffraction (XRD, LabX XRD-6000, Shimadzu, Japan), X-ray photoelectron spectroscopy (XPS, ESCALABMK, ThermoFisher Scientific Inc, USA), Raman spectroscopy (Renishaw-2000, Renishaw, UK), and field emission scanning electron microscopy (SEM, FEI Sirion 2000, FEI Co., Netherlands). Magnetic properties were measured using a vibrating sample magnetometer (VSM, PPMS-9, QuantumDesign, USA). Electromagnetic parameters were measured using a vector network analyzer (VNA, AV 3629D, 41st Research Institute of China Electronics Technology Group Corporation), with a frequency range of 8-18 GHz. CoFe@C / RC composite material was mixed with paraffin wax at a mass ratio of 1:4 (outer diameter / inner diameter of 7 mm / 3.04 mm) and pulverized into a coaxial ring.

[0039] In the synthesis of CoFe@C / RC composite materials, Co 2+ and Fe 3+ Ions are attracted to the negatively charged functional groups and defects of RC through strong electrostatic interactions. As the oil bath reaction proceeds, CoFe-MOF-74 is controllably synthesized by simply adjusting the molar ratio of Co / Fe, and then adsorbed onto the RC surface to form CoFe-MOF-74 / RC. Finally, the CoFe@C / RC composite material is obtained through a pyrolysis carbonization process in a tube furnace. The synthesis steps of the CoFe@C / RC composite material are as follows: Figure 1 As shown.

[0040] The composition and crystal structure of CoFe@C / RC and RC were characterized by XRD. Figure 2 As shown in (a), RC exhibits two distinct diffraction peaks at 2θ = 43.5° and 25.6°, corresponding to the (100) and (002) planes of graphitic carbon, respectively. The S1 sample shows peaks at 2θ = 44.2°, 51.5°, and 75.9°, which are characteristic peaks of the (111), (200), and (220) planes of Co (JPDS No. 15-0806), indicating the successful preparation of Co metal nanoparticles on the RC surface. With the increase of Fe... 3+ With the addition of Co, the characteristic peaks of Co metal in S2, S3, and S4 gradually disappeared, mainly because the content of metallic Co decreased. In S2, the three peaks at 2θ = 82.3°, 65.0°, and 44.6° correspond to the (211), (200), and (110) crystal planes of metallic Fe. These three peaks also appeared in S3. In addition to the characteristic peaks of FeCo alloy, two peaks at 25.9° and 35.1° were also found in S3 and S4, corresponding to the (020) and (200) crystal planes of Fe3C (JCPDS 35-0772). This indicates that when the molar ratio of Co / Fe exceeds 0.25, the unit cell of metallic Co cannot contain too many Fe atoms, thus forming carbides. Furthermore, seven peaks corresponding to the (111), (220), (311), (400), (422), (511), and (440) crystal planes of CoFe2O4 (JCPDS79-1744) were found in S4 at 2θ = 18.3°, 30.1°, 35.5°, 43.2°, 53.5°, 57.0°, and 62.6°. XRD results indicate that the CoFe@C / RC composite material was successfully prepared.

[0041] The functional groups of RC and CoFe@C / RC composites were characterized using FT-IR. Figure 2 (b) It can be seen that at 1100, 1615, 2930 and 3440 cm -1 The characteristic absorption peaks at 590 cm⁻¹ belong to CO, C=C, CH, and -OH, respectively. Furthermore, at 590 cm⁻¹... -1 The tensile vibration peak at the point corresponds to Co-O / Fe-O, which proves that Co / Fe nanoparticles were successfully synthesized on the RC surface. Compared with RC, the characteristic peak position of CO bond in CoFe@C / RC composite material shifts to a higher wavenumber range. This is because the metal nanoparticles in the composite material react chemically with the CO bonds in the RC matrix.

[0042] The microwave absorption properties of carbon-based materials are closely related to the defect structure of carbon. Therefore, we used Raman spectroscopy to study the defect structure of RC and CoFe@C / RC hybrids. In Raman spectroscopy, the intensity ratio (ID) of the D band and G band is typically used. D / I G To evaluate the degree of graphitization. For example... Figure 2 As shown in (c), the RC band and the D and G bands of the four samples are located at 1350 cm⁻¹. -1 and 1585cm -1 I of RC, S1, S2, S3 and S4 D / I G The values ​​were 1.03, 1.04, 1.06, 1.08, and 1.09, respectively, indicating that their combined use with CoFe nanoparticles can enhance the defects in RC. Furthermore, the I values ​​of S3 and S4... D / I G The highest value indicates that the formation of Fe3C caused more defect structures and lattice distortions.

[0043] Besides the defective structure, the magnetic permeability of a material affects its magnetic loss capability, which is another important factor affecting the microwave absorption performance of a material. Figure 2 (d,e) shows the hysteresis loops of the CoFe@C / RC hybrid. The saturation magnetization and coercivity of the CoFe@C / RC hybrid vary with the Co / Fe molar ratio, reflecting the tunability of its intrinsic magnetic properties and providing an effective means of controlling the magnetic loss of the material. From Figure 2 (d) It can be seen that the Ms values ​​of S1, S2, S3, and S4 are 2.0, 58.2, 30.9, and 49.3 emu / g, respectively. When the molar ratio of Co / Fe is 3:1, S2 has the highest Ms value. The Hc value is a key factor determining the matching thickness that produces the optimal RL value. The Hc value of S4 (135.8 Oe) is lower than that of S1 (410.2 Oe), S2 (366.7 Oe), and S3 (372.8 Oe). Figure 2 As shown in (e). Therefore, S4 exhibits better absorption performance at thin matching thicknesses.

[0044] The microstructure of the prepared CoFe@C / RC hybrid was characterized using scanning electron microscopy (SEM), such as... Figure 3 As shown, the samples obtained exhibit different morphologies and particle sizes depending on the Co / Fe molar ratio. The Co metal obtained on the S1 surface shows a small number of cotton-like flocculent particles on the RC surface, with particle sizes ranging from 50 to 100 nm. Figure 3 (ac)). For example Figure 3As shown in (df), sample S2 contains fine CoFe alloy nanoparticles with a particle size range of less than 20 nm. The nanoparticles exhibit an agglomeration effect. The microstructure of S3 is similar to that of S2, but its particle size is significantly larger than that of S2, and the agglomeration phenomenon is more obvious. Figure 3 (gi)). With further increases in Fe content, the CoFe alloy on the RC surface no longer exhibits a nanoparticle structure at S4, but instead presents a lamellar structure of different sizes. Figure 3 (jl)). S4 corresponds to the EDX mapping image ( Figure 3 (m) shows the presence of Co, Fe, O and C elements, and these images are consistent with the results of the features mentioned above.

[0045] The elemental composition and chemical valence state of RC and Co1Fe3@C / RC hybrids were determined by XPS analysis. Figure 4 In (a), the total spectrum of RC shows two main peaks at 284.6 eV and 531.1 eV, corresponding to O1s and C1s, respectively. The total spectrum of S4 has four main peaks, namely Co2p, Fe2p, O1s, and C1s. The absence of other elemental peaks indicates that the synthesized product has high purity. Figure 4 (b) shows the C1s spectrum of sample S4. The four peaks at 284.5, 285.2, and 287.3 eV represent CC / C=C, CO, and C=O, respectively. The abundant oxygen-containing functional groups on the RC surface are important adsorption sites for cobalt and iron ions. The occurrence of π-π* excitation oscillations in aromatic molecules leads to an additional peak at 290.7 eV. Figure 4 As shown in (c), the deconvolution integral of the Co 2p spectrum indicates that three types of Co are present in sample S4, namely Co... 3+ (780.4 and 795.7 eV), Co 2+ (782.6 and 798.2 eV) and Co 0 (778.4 and 793.4 eV), this is likely due to oxidation of the Co metal surface. Similarly, Figure 4 In (d), Fe2p is decomposed into 5 peaks, which are Fe... 3+ (713.4 and 727.3 eV), Fe 2+ (710.9 and 723.7 eV) and Fe 0 (718.9eV).

[0046] Based on transmission line theory, the absorption performance of an electromagnetic wave (EMW) is related to the complex permittivity (εr = ε′ - jε″) and complex permeability (μr = μ′ - jμ″), while the complex permeability and complex permeability depend on the composition and structure of the absorbent. Electromagnetic parameters of S1, S2, S3, and S4 were measured in the 8-18 GHz range (Ku and X bands) with a filler mass of 20 wt.%. Figure 5 As shown. From Figure 5 (a) It can be seen that with increasing frequency, the ε′ values ​​of S1, S3, and S4 gradually decrease between 5.5–5.1, 7.4–7.0, and 10.4–9.3, respectively. However, the ε′ value of S2 increases from 9.4 to 9.8 in the 8.0–9.1 GHz range, decreases to 5.3 at 14.5 GHz, and increases to 6.5 at 18.0 GHz. The ε″ values ​​of S3 and S4 decrease from 0.97 to 1.4 and from 2.2 to 2.7, respectively. The ε″ value of S1 decreases slowly with increasing frequency, from 0.93 to 0.84. The ε″ value of S2 increases from 5.0 to 7.9 in the 8.0–11.8 GHz range and decreases to 4.6 in the 18.0 GHz range. This result indicates that the dielectric constant can be effectively adjusted by changing the molar ratio of Co / Fe. Figure 5 (cd) depicts the μ′ and μ″ ~ f curves of the CoFe@C / RC hybrid. The μ′ and μ″ values ​​show a decreasing trend with slight fluctuations. From Figure 5 As can be seen from (c) and (d), the trends and values ​​of μ′ and μ″ in S1, S3, and S4 are similar, while the trends and values ​​of S2 still show significant differences, indicating that S2's ability to dissipate and store magnetic energy differs from other samples. In the range of 11.2–15.0 GHz, the μ″ value of S2 becomes negative, indicating the presence of radiation. Figure 5 (ef) shows the magnetic loss tangent (tanδμ=μ″ / μ′) and dielectric loss tangent (tanδε=ε″ / ε′) based on measured EM parameters to better elucidate the principle of microwave absorption. Figure 5 (ef) shows that the tanδε~f and tanδμ~f curves of the synthesized hybrids have similar trends to ε″ and μ″, respectively. The tanδε of the CoFe@C / RC hybrids with different Co / Fe molar ratios is significantly greater than tanδμ, indicating that dielectric loss is the main mechanism of electromagnetic energy attenuation.

[0047] RL = 20Log|(Z) in -z0) / (z in +Z0)| (1)

[0048]

[0049] Based on transmission line theory, the RL values ​​of the four CoFe@C / RC composite materials are calculated using equations (1) and (2), where Z in Let Z0 be the characteristic input impedance of the material, Z0 be the characteristic impedance of free space, Z0≈377Ω, f be the electromagnetic wave frequency, c be the electromagnetic wave velocity in vacuum, and d be the thickness of the sample. The calculation results are as follows: Figure 6 As shown in (ah):

[0050] When the fill loading is 20 wt.%, the RL of S1, S2, S3 and S4 min The values ​​are -6.2, -7.4, -13.1, and -20.0 dB, respectively. From Figure 6 (ad) shows that, due to the poor impedance matching mechanism, S1 and S2 almost never exhibit absorption performance exceeding -10dB, which cannot meet the requirements of practical applications. From Figure 6 (ef) It can be seen that S3 exhibits dual absorption characteristics, which may be due to resonant absorption. The actual reason for this needs further investigation, but the dual-peak absorption intensity is not obvious. S4 shows better microwave absorption capability than S1, S2, and S3. When the molar ratio of Co / Fe is 1:3, S4 exhibits better microwave absorption performance at a lower matching thickness. Figure 6 (gh)), RL at 1.5, 1.7, 2.0, 2.5 and 3.0 mm min The corresponding values ​​are 17.0, 20.0, 15.4, 13.9, and 12.8 dB. Meanwhile, the S4 exhibits a wide absorption bandwidth of 3.95 GHz when matched with a thickness of 2.0 mm. Furthermore, by easily adjusting the thickness from 1.5 mm to 3.0 mm, f e Reaching 10.0 GHz (8.0-18.0 GHz), encompassing the entire Ku and X bands.

[0051] To study the Debye relaxation process, a Cole-Cole semicircle is drawn according to the following formula:

[0052]

[0053] Where ε ∞ and ε s These are the dielectric constant values ​​at the high-frequency limit and the static dielectric constant values, respectively. According to equation (3), each semicircle represents the Debye relaxation process. That is, when the ε'-ε” curve is semicircular, it indicates that the material model has undergone the Debye relaxation process. The Cole-Cole curves of S1, S2, S3 and S4 all have at least two semicircles ( Figure 7 This confirms the existence of the Debye relaxation process. Furthermore, the irregular semi-circular shape of the composite material indicates the presence of other forms of microwave loss, such as interfacial polarization loss or conduction loss. Based on the above FT-IR, XPS, and Raman characterization results, the CoFe@C / RC hybrid possesses abundant defects and oxygen-containing functional groups as polarization centers, which can further attenuate the incident EMW. Under the action of an alternating electromagnetic field, the heterogeneous interface of RC-C-Co / Fe exhibits strong interfacial polarization due to the accumulation of charge at the heterogeneous interface.

[0054] In addition to dielectric loss, the presence of Co / Fe alloy magnetic nanoparticles endows the CoFe@C / RC hybrid with magnetic loss ability. Eddy current loss and magnetic resonance (exchange resonance and natural resonance) constitute the magnetic loss capacity of the material within the studied frequency range. SEM results show that the diameter of the CoFe alloy nanomaterials synthesized on the surface of RC ranges from a few nanometers to 100 nanometers. Especially on the surfaces of S2, S3, and S4, there are a large number of CoFe alloy nanomaterials with diameters less than 10 nm, indicating the presence of exchange resonance. Eddy current loss is usually evaluated by the eddy current coefficient (C0), and its expression is:

[0055] C0 = μ″(μ′) -2 f -1 (4)

[0056] μ″ = 2πμ(μ′) 2 σd 2 f / 3 (5)

[0057] As can be seen from Figure 8 (a), for S1, S3, and S4 within the range of 8.0 - 10.0 GHz and for all samples within the range of 16.0 - 18.0 GHz, the C0 - f curves show almost no change, indicating the presence of eddy current loss. Therefore, natural resonance and eddy current loss are the main forms of magnetic loss in the CoFe@C / RC hybrid.

[0058] The microwave absorption ability of the material is mainly determined by two factors. One of the factors is the attenuation constant, which is expressed by the following formula:

[0059]

[0060] As can be seen from Figure 8 (b), the α values of the four samples increase in the order of S1 < S3 < S4 < S2. Therefore, S2 exhibits the strongest attenuation behavior. However, the microwave absorption performance of S2 is poor, which is related to another factor, impedance matching. The matching ability is evaluated by |Z in / Z0|, the modulus of the normalized input impedance, and the matching ability is described by Equation (2).

[0061] Figure 9 (a - c), the |Z in / Z0| values of S1, S2, and S3 are generally far from 1.0, indicating poor impedance matching, which makes it difficult for electromagnetic waves to enter the interior of the material, resulting in poor wave absorption performance. For S4, the |Z in / Z0| value is closest to 1 at a thickness of 1.5 - 3.0 mm( Figure 9(d) At a thickness of 1.7 mm, the impedance matching value at 15.2 GHz is 1.1, closest to 1. Therefore, the S4 exhibits the best absorption performance and a wide effective absorption bandwidth at 1.7 mm. Thus, due to its high attenuation capability and good impedance matching, the S4 can be easily adjusted from a thickness of 1.5 mm to 3.0 mm, exhibiting an ultra-wide fe band, thereby covering the entire Ku and X bands.

[0062] To further demonstrate the excellent absorption performance of CoFe@C / RC hybrid materials, Table 1 lists the filler content, RL, and f of Fe or Co-based composites at different matching thicknesses. e Comparison of values. The obtained S3 composite material sample, with a filling amount of only 20 wt.%, not only exhibits considerable reflection loss intensity, but also demonstrates a relatively high f-value at a matching thickness of only 1.7 mm. e .

[0063] Table 1 Comparison of the performance of Fe or Co-based microwave absorbing materials

[0064]

[0065] Figure 10 The potential microwave absorption mechanism of CoFe@C / RC hybrid materials was demonstrated. The synergistic effect of dielectric loss, conductive loss, and magnetic loss endows the CoFe@C / RC hybrid with excellent microwave absorption performance. First, the abundant oxygen-containing functional groups and numerous defects on the RC surface can act as polarization centers, inducing dipole polarization and enhancing dielectric loss, which is beneficial for the attenuation of incident microwave energy (EMW). Second, the introduction of CoFe nanoparticles can provide magnetic loss to the CoFe@C / RC hybrid. Third, the CoFe / C and CoFe / RC interfaces lead to interfacial polarization loss, significantly increasing dielectric loss. Fourth, the large distribution of CoFe nanoparticles in the carbon matrix leads to dipole polarization loss. Finally, electrons can absorb EMW energy and then jump to the conductive carbon layer, where conduction losses easily occur, converting the EMW energy into heat energy.

Claims

1. A method for preparing a CoFe@C / RC composite material based on MOF derivatization, characterized in that, The method steps are as follows: S1: preparation of coal gasification fine slag residual carbon RC; S2: preparation of CoFe-MOF-74 / RC precursor Co(CH3COO)2·4H2O, FeCl3·6H2O, 2,5-dihydroxyterephthalic acid and RC prepared in S1 are dissolved in deionized water, and CoFe-MOF-74 / RC precursor is obtained by stirring, centrifugation, ethanol washing and vacuum drying; S3: preparation of CoFe@C / RC composite material CoFe-MOF-74 / RC precursor prepared in S2 is pyrolyzed under a protective gas atmosphere to obtain CoFe@C / RC composite material; In S2, the molar mass ratio of Co(CH3COO)2·4H2O, FeCl3·6H2O, 2,5-dihydroxyterephthalic acid and RC is 1 mmol:0.1-10 mmol:0.5-2 mmol:0.1-0.5 g.

2. The method for preparing MOF-derived CoFe@C / RC composite material according to claim 1, characterized in that, The stirring conditions in S2 are: temperature 90-110℃, time 40-60min, stirring rate 200-400rpm. 3.The method for preparing MOF-derived CoFe@C / RC composite material according to claim 1, characterized in that, The temperature of centrifugation in S2 is 75-85℃, and the centrifugation speed is 8000-12000rpm. 4.The method for preparing MOF-derived CoFe@C / RC composite material according to claim 1, characterized in that, The protective gas in S3 is one of nitrogen, argon and helium. 5.The method for preparing MOF-derived CoFe@C / RC composite material according to claim 1, characterized in that, The pyrolysis temperature in S3 is 760-840℃, and the pyrolysis time is 1.5-2.5h.

6. The method for preparing MOF-derived CoFe@C / RC composite material according to claim 5, characterized in that, The heating rate of pyrolysis is 1.8-2.2℃ / min.

7. MOF-derived CoFe@C / RC composite material prepared by the method of any one of claims 1-6.

8. Application of the MOF-derived CoFe@C / RC composite material of claim 7 in electromagnetic wave absorbing materials.

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