Preparation method of high-performance FeNi alloy graphene foam composite wave-absorbing material

Abstract

The application discloses a preparation method of high-performance FeNi alloy graphene foam composite wave-absorbing material and belongs to the technical field of wave-absorbing material preparation; the method comprises the following steps: S1, a certain amount of graphene oxide slurry and ethylene glycol are measured and uniformly mixed, naphthalene is added into the mixed solution of the graphene oxide slurry and the ethylene glycol under stirring to obtain a suspension; S2, a certain mass of FeNi alloy powder is taken and added into the suspension obtained in the step S1, then the mixed solution is transferred into a polytetrafluoroethylene lining, and the solution is cooled to room temperature after reaction for a period of time to obtain FeNi alloy / graphene gel; and S3, the gel obtained in the step S2 is placed into a drying box and dried for a period of time to obtain FeNi alloy / graphene foam. The FeNi alloy / graphene foam composite material is prepared by using a one-step hydrothermal method.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of wave-absorbing material preparation, in particular to a preparation method of a high-performance FeNi alloy graphene foam composite wave-absorbing material. BACKGROUND

[0002] The advent of the 5G era has ushered in another round of rapid development of electronic information technology. Correspondingly, the application of electromagnetic waves in industrial applications, aerospace, medical fields, military fields and wireless communication fields is also more in-depth. However, the widespread application of electromagnetic waves inevitably causes electromagnetic pollution, which undoubtedly brings many potential hazards to human health and living environment. Long-term exposure to high doses of electromagnetic fields increases the risk of cancer, and can also cause damage to the reproductive system, immune system and nervous system. In addition, improper management of electromagnetic radiation can interfere with communication systems, causing communication damage or even serious accidents. In the natural environment, electromagnetic pollution can also interfere with the behavior and migration patterns of wild animals. In the military field, the use of advanced reconnaissance, surveillance and multi-dimensional strike means poses a great threat to the survival and safety of various military targets. As an effective means of effectively absorbing electromagnetic waves and reducing electromagnetic radiation, wave-absorbing materials have been widely studied. Designing and preparing high-performance microwave absorbing materials to reduce electromagnetic pollution and improve the survivability of major weapons has been a hot issue for many years.

[0003] The research on wave-absorbing materials has a long history. So far, a variety of materials including ceramic materials, ferrite materials, alloy materials, carbon materials and conductive polymers have been reported. Among them, carbon materials are favored for their excellent chemical stability, excellent electrical conductivity and good mechanical properties. Carbon materials commonly used for wave absorption mainly include carbon nanotubes, graphene, carbon fibers, graphite and various modified materials thereof. Among them, graphene has attracted widespread attention due to its low density, high specific surface area and excellent electrical and dielectric properties. At the same time, graphene oxide, reduced graphene oxide and three-dimensional graphene modified by graphene have become a research hotspot in recent years due to their better impedance matching performance and stronger electromagnetic attenuation performance.

[0004] However, compared with the other two, three-dimensional graphene has lower density and stronger dielectric properties, ultimately achieving stronger electromagnetic wave attenuation capability. In addition, the reflection, scattering and interference effects of the porous network structure inside the three-dimensional graphene on electromagnetic waves are also not possessed by the former two.

[0005] Therefore, the application of three-dimensional graphene is expected to overcome the shortcomings of traditional wave-absorbing materials such as large density, difficult to balance wave-absorbing strength and wave-absorbing bandwidth, and better meet the requirements of wave-absorbing materials being thin, light, wide and strong. Our research group previously prepared graphene foam by using a hydrothermal method combined with naphthalene sublimation principles.

[0006] Research has shown that although the prepared graphene foam has relatively excellent wave absorption properties, its impedance matching performance needs to be improved. We then compounded it with magnetic materials and used a two-step hydrothermal method to prepare FeCo alloy / graphene foam composite materials, which showed higher absorption strength and wider effective absorption bandwidth. Compounding FeNi alloy with graphene foam can improve impedance matching performance, allowing more electromagnetic waves to enter the interior of the material. In addition, the electromagnetic wave loss mechanism of synergistic magnetic loss and electrical loss can effectively improve the material's ability to attenuate electromagnetic waves and enhance wave absorption performance. However, the two-step hydrothermal method is relatively cumbersome, which reduces the preparation efficiency of the composite material.

[0007] Therefore, it is necessary to provide a method for preparing a high-performance FeNi alloy graphene foam composite absorbing material. Summary of the Invention

[0008] The purpose of the present invention is to provide a method for preparing a high-performance FeNi alloy graphene foam composite absorbing material, so as to solve the problem that the existing two-step hydrothermal method is relatively cumbersome and reduces the preparation efficiency of the composite material.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] A method for preparing a high-performance FeNi alloy graphene foam composite absorbing material comprises the following steps:

[0011] S1, measuring a certain amount of graphene oxide slurry and ethylene glycol and mixing them uniformly, adding naphthalene to the mixed solution of the graphene oxide slurry and ethylene glycol under stirring to obtain a suspension;

[0012] S2, taking a certain mass of FeNi alloy powder, adding it to the suspension obtained in step S1, and then transferring the mixed solution to a polytetrafluoroethylene liner, reacting for a period of time and cooling it to room temperature to obtain FeNi alloy / graphene gel;

[0013] S3, placing the gel obtained in step S2 into a drying oven and drying for a period of time to obtain FeNi alloy / graphene foam.

[0014] Furthermore, in step S1, the graphene oxide slurry and ethylene glycol were both measured to be 170 ml, and 80 g of naphthalene was added to the mixed solution of the graphene oxide slurry and ethylene glycol under mechanical stirring.

[0015] Furthermore, the reaction in step S2 is specifically a hydrothermal reaction performed at a temperature of 200° C. for 12 hours.

[0016] Further, the step S3 is specifically: placing the gel obtained in the step S2 into a vacuum drying oven, drying at 70 DEG C for 36h, adjusting the drying temperature to 120 DEG C and drying for another 24h, to obtain the FeNi alloy / graphene foam.

[0017] Further, the residual oxygen-containing functional groups on the surface of the graphene foam can be used as active adsorption sites of the FeNi alloy, so that the combination of the FeNi alloy and the graphene foam forms a composite material connected by ionic bonds.

[0018] Further, the saturation magnetization of the FeNi alloy / graphene foam increases with the increase of the content of the FeNi alloy.

[0019] Further, the electrical loss capability of the FeNi alloy / graphene foam decreases with the increase of the content of the FeNi alloy.

[0020] Further, the electrical conductivity of the FeNi alloy / graphene foam decreases with the increase of the content of the FeNi alloy.

[0021] The present application has the following beneficial effects:

[0022] The present application uses one-step hydrothermal method to prepare the FeNi alloy / graphene foam composite material, and by controlling the addition amount of the FeNi alloy, five groups of samples are successfully prepared.

[0023] The FeNi alloy / graphene foam composite material prepared by the present application has excellent wave absorption performance when the filling amount is only 5%. When the matching thickness of the FeNi alloy / graphene foam composite material is 2.09mm, the effective wave absorption frequency covers the entire Ku band (11.76-18GHz); the minimum reflection loss value is obtained at the matching thickness of 5.38mm, and the minimum reflection loss value is-55.01dB. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The preparation flow chart of the FeNi alloy / graphene foam composite material provided for the embodiments of the present application is shown in the figure;

[0025] Figure 2 The SEM and EDS graphs of the FeNi alloy / graphene foam composite material provided for the embodiments of the present application are shown in the figure: (a)-(e) are the SEM graphs of 1#-5# samples, (f) is the SEM graph of 3# sample, and (g)-(i) are the EDS graphs of 3# sample;

[0026] Figure 3(a) XRD spectra and (b) Raman spectrum of FeNialloy / GF.FeNi / graphene foam composite material XPS spectra provided in the embodiments of the present invention: (c) total spectrum, (d) C1s spectrum, (e) Ni 2p spectrum, (f) Fe 2p spectrum;

[0027] Figure 4 The hysteresis loop of the FeNi alloy / graphene foam composite material provided in an embodiment of the present invention;

[0028] Figure 5 Electromagnetic parameters of the FeNi alloy / graphene foam composite material provided by the embodiment of the present invention: (a) real part of the complex dielectric constant, (b) imaginary part of the complex dielectric constant, (c) dielectric constant loss tangent, (d) real part of the complex magnetic permeability, (e) imaginary part of the complex magnetic permeability, (f) magnetic loss tangent;

[0029] Figure 6 A diagram showing the microwave absorption performance of the FeNi alloy / graphene foam composite material provided in an embodiment of the present invention;

[0030] Figure 7 The Cole-Cole plot of the FeNi alloy / graphene foam composite material provided by an embodiment of the present invention;

[0031] Figure 8 Schematic diagram of the performance of the FeNi alloy / graphene foam composite material provided by an embodiment of the present invention: (a) conductivity, (b) C0 value, (c) attenuation coefficient, and (d) characteristic impedance;

[0032] Figure 9 Schematic diagram of the wave absorption mechanism of the FeNi alloy / graphene foam composite material provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0033] The technical solutions in the embodiments of the present invention will be described clearly and completely below with reference to the accompanying drawings in the embodiments of the present invention.

[0034] The present invention provides a method for preparing a high-performance FeNi alloy graphene foam composite absorbing material, comprising the following steps:

[0035] S1, measuring a certain amount of graphene oxide slurry and ethylene glycol and mixing them uniformly, adding naphthalene to the mixed solution of the graphene oxide slurry and ethylene glycol under stirring to obtain a suspension;

[0036] S2, taking a certain mass of FeNi alloy powder, adding it to the suspension obtained in step S1, and then transferring the mixed solution to a polytetrafluoroethylene liner, reacting for a period of time and cooling it to room temperature to obtain FeNi alloy / graphene gel;

[0037] S3, placing the gel obtained in step S2 into a drying oven and drying for a period of time to obtain FeNi alloy / graphene foam.

[0038] This embodiment provides a method for preparing a high-performance FeNi alloy graphene foam composite absorbing material, which is specifically as follows:

[0039] 170 ml of graphene oxide slurry (3 mg / ml, purchased from Qitaihe Baotailong Materials Co., Ltd., China) and 170 ml of ethylene glycol (purchased from Aladdin, USA) were measured and mixed evenly. 80 g of naphthalene was slowly added to the above mixed solution under mechanical stirring to obtain a suspension. 2 g, 4 g, 6 g, 8 g, and 10 g of FeNi alloy powder (purchased from Changsha Tianjiu Metal Co., Ltd., China) were taken respectively and added to the above suspension under mechanical stirring. The mixed solution was then transferred to a polytetrafluoroethylene liner with a volume of 500 ml, and a hydrothermal reaction was carried out at a temperature of 200 ° C for 12 h. After cooling to room temperature, 5 FeNi alloy / graphene gels with different component proportions were obtained; the gel was placed in a vacuum drying oven and dried at 70 ° C for 36 h. The drying temperature was adjusted to 120 ° C and dried for another 24 h to obtain FeNi alloy / graphene foams. According to the different amounts of FeNi alloy powder added, they were marked as 1#, 2#, 3#, 4#, and 5# from small to large. The overall preparation process is as follows Figure 1 shown.

[0040] Representation

[0041] Phase analysis of the materials was performed using a high-power rotating target polycrystalline Bruck-D8 Advance X-ray diffractometer (XRD) (experimental conditions: Cu target, scanning range 10°–100°, scanning speed 8° / min). The micromorphology, structure, and size of the materials were observed using a ZEISS Sigma500 field emission scanning electron microscope (SEM). The Raman spectra of the materials were measured using a HORIBA HR Evolution Raman spectrometer under 532 nm laser excitation with a scanning range of 1000–2500 cm. -1 The chemical state of the structure was analyzed using a Thermo Scientific ESCALAB Xi+ X-ray photoelectron spectrometer (XPS). A Lakeshore-7404 vibrating sample magnetometer (VSM) was used to measure the static magnetic properties of the material, including saturation magnetization and coercivity. An Agilent N5234A vector network analyzer (VNA) was used to measure the electromagnetic parameters of the material at frequencies between 2 and 18 GHz (the sample was mixed with paraffin wax at a mass ratio of 5:95 during testing). Finally, transmission line theory was used to simulate and calculate the reflection loss characteristics of the material under different thickness conditions.

[0042] In order to study the effect of the change of FeNi alloy content in 1#-5# on the micromorphology of the composite material, the following Figure 2 SEM analysis (a)-(e) shows that the alloy distribution in samples 1# and 2# is relatively uniform, allowing for relatively even loading onto the graphene foam's network sheets. However, as the FeNi alloy content increases, the loading density of the graphene foam increases significantly, with significant aggregation beginning in sample 3#. The aggregation in sample 4# becomes more pronounced, with visible irregular lumps visible in the figure. This is likely due to a shortage of active adsorption sites within the graphene foam. As the excessive FeNi alloy particles compete for limited adsorption sites, some particles are unable to effectively adsorb onto the three-dimensional graphene and instead aggregate. Sample 4# still retains pores within the graphene foam's network structure, although these pores are significantly deformed compared to samples 1#-3. As the FeNi alloy powder content increases further, the pores in sample 5# become essentially blocked, and the presence of irregular lumps in this sample almost eliminates the porous structure of the graphene foam. This demonstrates that the FeNi alloy content significantly influences the micromorphology of the composite material. In order to further clarify the element distribution of the composite material, we conducted a Figure 2 (f) was used to perform EDS analysis to obtain the element distribution of the FeNi alloy / graphene foam composite material. Since the graphene foam is evenly distributed in the entire area, the experiment mainly tested the distribution of the three elements Fe, Ni, and O. EDS results (as shown in Figure 2 Figures (g)-(i)) show that Fe, Ni, and O are highly concentrated in certain areas, and the three elements largely overlap in these regions. However, the density of O is higher in these concentrated areas, while Ni has the lowest. This may be because the graphene oxide is not completely reduced by ethylene glycol, leaving some oxygen-containing functional groups. These residual oxygen-containing functional groups provide active adsorption sites for the loading of the FeNi alloy. The higher density of Fe compared to Ni may be due to the oxygen-containing functional groups oxidizing a portion of the FeNi alloy to NiFe2O4.

[0043] Figure 3 (a) is the XRD spectrum of FeNi alloy particles and FeNi alloy / graphene foam composite materials. As can be seen from the figure, the original FeNi alloy particles are composed of Fe 0.4 Ni 0.6and FeNi alloy. After the hydrothermal reaction with graphene oxide slurry and ethylene glycol, samples 1# to 5# all showed obvious diffraction peaks at 30.3°, 35.7°, 57.4° and 62.9°. By comparing with the standard card (PDF#10-0325), the characteristic diffraction peaks of (120), (311), (511) and (440) crystal planes were determined, confirming the presence of NiFe2O4 in the composite material. This is also consistent with the above-mentioned EDS analysis, which speculated that the oxygen-containing functional groups oxidized the FeNi alloy to NiFe2O4. It is not difficult to find from the figure that the characteristic diffraction peaks of the FeNi alloy (200) and (211) crystal planes that appeared near 64.9° and 82.3° in the original FeNi alloy disappeared after the hydrothermal experiment, and the intensity of the characteristic diffraction peak of the FeNi alloy (110) crystal plane that appeared at 44.6° also decreased significantly after the hydrothermal experiment. This may be because, FeNi alloy phase is different from Fe 0.4 Ni 0.6 It is more easily oxidized to NiFe2O4. The diffraction peak of graphene foam near 25.5° is difficult to observe in the figure. This may be because the FeNi alloy particles themselves produce additional scattering or absorption of X-rays, which will overlap or interfere with the diffraction signal of graphene, causing the original diffraction peak of graphene foam near 25.5° to be obscured or weakened.

[0044] Figure 3 (b) is the Raman spectrum of FeNi alloy / graphene foam composite material. It can be seen from the figure that the D peak of the five samples appears at 1343 cm -1 Nearby, G peaks appear at 1588cm -1 Among them, the D peak is related to the disorder degree of carbon materials. It is composed of carbon ring sp 2 The D peak is caused by the symmetry breaking of the atomic lattice vibration. The appearance of the D peak indicates that the structure of the carbon material is not a perfect ideal crystal structure. The G peak is related to the degree of graphitization of the carbon material and is mainly caused by the stretching vibration of the carbon hexagonal ring. The ratio of the D peak to the G peak is I D / I G It is often used to judge the structural characteristics of carbon materials. A higher ratio means a higher lattice defect density and a lower degree of graphitization, which also means more disorder. D / I GThe value decreases with the increase of FeNi alloy content, which means that the increase of FeNi alloy powder makes the structure of graphene foam repaired and improved to a certain extent. This may be because in the process of compounding FeNi alloy with graphene foam, some defects on the graphene surface are filled, reducing the disordered area of ​​the graphene structure. In addition, the increase of FeNi alloy content will affect the stress distribution inside the graphene. The alloy particles exert a certain stretching or compressing effect on the graphene, causing the lattice structure of the graphene to change, thereby affecting its vibration mode, thereby making I D / I G The decrease in value.

[0045] In order to further verify the surface chemical composition and valence state of FeNi alloy / graphene foam composite materials, X-ray photoelectron spectroscopy analysis was performed on sample 3. The results are as follows: Figure 3 (cf). Figure 3 (c) is the full XPS scan spectrum of the FeNi alloy / graphene foam composite. It can be seen that the FeNi alloy / graphene foam composite is composed of four elements: C, O, Fe, and Ni. Graphene and the FeNi alloy contribute C, Fe, and Ni, respectively, while the O element comes from the residual oxygen-containing functional groups in the graphene foam. Figure 3 (d) shows the characteristic peaks of C1s of FeNi alloy / graphene foam composite material. After peak fitting, the peaks with binding energies of 287.7eV, 285.63eV, 284.8eV and 283.89eV correspond to C=O, CO, CC and C=C respectively. It can be seen that there are indeed residual oxygen-containing functional groups such as hydroxyl and carboxyl groups on the surface of graphene foam. The peak fitting results of Ni 2p spectrum are shown in Figure 2. Figure 3 As shown in (d), there are two spin-orbit doublets, Ni 2p 1 / 2 and Ni 2p 3 / 2 and their satellite peaks, which show the +2 and +3 valence states of Ni. Figure 3 (e) shows the result of peak fitting of Fe 2p spectrum. The Fe 2p spectrum is composed of two peaks with different binding energies and related satellite peaks. 3 / 2 and Fe 2p 1 / 2 Fe 2+ and Fe 3+ The oxidation state of Fe 2+ , Fe 3+ and Ni 2+ Both can be used with O 2-The Fe-O bonds and Ni-O bonds are formed, which shows that the residual oxygen-containing functional groups on the surface of graphene foam can indeed serve as active adsorption sites for FeNi alloys, making the composite of FeNi alloy and graphene foam not just a simple load, but forming an ionically bonded composite material.

[0046] Figure 4 Table 1 shows the hysteresis loops of FeNi alloy / graphene foam composite materials with different FeNi alloy contents at room temperature. Their saturation magnetization and coercive force are shown in Table 1. From the graph, it can be seen that the saturation magnetization of samples 1# to 5# increases with the increase of FeNi alloy content. This is because the increase of FeNi alloy will increase the number of magnetic atoms, and these atomic magnetic moments will be arranged along the direction of the magnetic field. As the number of atomic magnetic moments increases, the total magnetic moment after their superposition will increase, resulting in an increase in saturation magnetization. At the same time, the coercive force of samples 1# to 5# shows a trend of first increasing and then decreasing with the increase of FeNi alloy powder. This is because when the alloy powder is first increased, the static magnetic interaction between the magnetic particles increases, the movement of the magnetic domain wall becomes more difficult, and the coercive force increases. When the alloy powder continues to increase, agglomeration and irregular agglomerates appear, which destroys the magnetic domain structure and becomes relatively disordered, making it easier to move and reducing the coercive force. Under normal circumstances, the saturation magnetization (M S ) and coercivity (H c ) can determine the initial magnetic permeability (μ i ), the specific expression is as follows:

[0047]

[0048] Among them, a and b are two constants determined by the composition of the material, λ represents the magnetostriction coefficient, and ξ is the elastic strain parameter of the crystal. Generally, there is a positive relationship between the initial magnetic permeability and the magnetic loss capacity, which helps to predict the magnetic loss characteristics in magnetic absorbing materials. From the above formula, it can be inferred that high M S and low Hc are conducive to achieving high μi, thereby enhancing magnetic loss performance. Based on these magnetic parameters, we can speculate that sample 5# may have the strongest magnetic loss capability, while sample 2# may have the weakest magnetic loss capability.

[0049] Table 1 Saturation magnetization and coercivity of FeNi alloy / graphene foam composites

[0050]

[0051] Analysis of electromagnetic performance and wave absorption performance

[0052] Generally speaking, the complex dielectric constant (ε r =ε′-jε") and complex magnetic permeability (μr =μ′-jμ″) is an important parameter to describe the interaction between absorbing materials and electromagnetic waves. Among them, ε r and μ r The real part (ε′ and μ′) represents the material's ability to store electrical energy and magnetic energy, respectively, while the imaginary parts (ε″ and μ″) represent the material's ability to dissipate electrical energy and magnetic energy. This shows that the larger the real part, the stronger the material's ability to store electrical energy and the larger the imaginary part, the stronger the material's ability to dissipate energy, and the stronger the electromagnetic wave absorption capacity. In addition, the strength of the absorbing material's electrical and magnetic loss capacity is generally determined by the dielectric loss tangent (tanδ ε ) and the magnetic loss tangent (tanδ μ ) reflects, and its expression is as follows:

[0053]

[0054] like Figure 5 As shown in (a), the real part of the complex dielectric constant of samples 1# to 5# shows a typical dispersion phenomenon, that is, they all show a decreasing trend with the increase of frequency, but in comparison, the real part of the complex dielectric constant of the samples decreases with the increase of FeNi alloy content. Among them, the real part of the complex dielectric constant of sample 1# is significantly higher than that of the other four groups of samples. This is because the graphene foam content of sample 1# is the highest. On the one hand, the conductivity of graphene foam is stronger than that of FeNi alloy. On the other hand, the high content of graphene foam makes the conductive network of the composite material perfect, so the real part of the complex dielectric constant of sample 1# is significantly stronger than that of the other samples. Figure 5 (b) It can be seen that, unlike the real part of the complex dielectric constant, the imaginary part of the complex dielectric constant of samples 1# to 5# does not show obvious frequency response characteristics. This may be due to the competition and balance of multiple polarization mechanisms, that is, at a certain frequency, the enhancement of dipole polarization just compensates for the weakening of electronic polarization, so that the imaginary part of the complex dielectric constant has no obvious upward or downward trend. Although the imaginary part of the complex dielectric constant of the five groups of samples is relatively flat in the frequency range of 2 to 18 GHz, there are still multiple peaks, which means that multiple polarization relaxation phenomena have occurred in the composite material. Combined with Figure 5 (a) and Figure 5 (b) It is not difficult to judge that the electrical loss capacity of the composite material will decrease with the increase of FeNi alloy content. Figure 5 (c) also shows this result. The curves of the real and imaginary parts of the complex magnetic permeability of FeNi alloy / graphene foam composite material changing with frequency are shown in Figure 2. Figure 5 (d) and Figure 5(e) shown. As can be seen from the figure, the average value of the real part of the complex permeability of the 5# sample is the largest in the frequency range of 2-18 GHz, indicating that it has the strongest magnetic energy storage capacity. The remaining four groups of samples exhibit different magnetic storage capacities in different wave bands, and it is difficult to distinguish the advantages and disadvantages. The imaginary part of the complex permeability of samples 2-5 shows a trend of increasing with increasing FeNi alloy content in the frequency range of 2-18 GHz, but sample 1 shows a result stronger than sample 3. This may be because the agglomeration of FeNi alloy particles leads to the distortion of the local magnetic field, and the interaction between the agglomerates also changes the overall magnetism of the material. This inhomogeneous microstructure makes it impossible for the magnetic loss performance to increase simply with the increase of alloy content, which also confirms the phenomenon that it is difficult to distinguish the advantages and disadvantages of the real part of the complex permeability of samples 1-4. The 5# sample has a relatively obvious wave peak at 6, 12 and 16 GHz, which indicates that the composite material has obvious magnetic loss phenomenon in the above wave bands. This may be due to the fact that the frequency of the applied alternating magnetic field is close to or equal to the inherent frequency of the precession of the FeNi alloy magnetic moment, resulting in natural resonance phenomenon. And when the frequency of the applied alternating magnetic field matches the exchange interaction strength between Fe and Ni atoms, the occurrence of exchange resonance phenomenon will also greatly improve the magnetic loss performance, resulting in the appearance of wave peak. As shown in figure (f), the curve of the tangent value of the magnetic loss of the 5# sample is almost the same as the curve of the imaginary part of the complex permeability, that is, the magnetic loss capacity of the 5# sample is the strongest, and the magnetic loss capacity of the 2# sample is the weakest, which is consistent with the results of the above static magnetic performance analysis.

[0055] In order to further explore the wave absorption performance of FeNi alloy / graphene foam composite materials, the reflectivity of the samples in the frequency range of 2-18 GHz and under different wave absorption layer thickness conditions was simulated according to the transmission line theory. The expression of the reflectivity is shown in the following formula:

[0056]

[0057] In the formula, Zin represents the input impedance of the material, f represents the microwave frequency, d is the thickness of the material, and c represents the speed of light. The reflectivity is usually negative, and when RL≤-10dB, it can be determined that at least 90% of the electromagnetic wave is absorbed, which is also called effective absorption. In this case, the width of the range covered by the electromagnetic wave frequency is defined as the effective wave absorption bandwidth.

[0058] The reflectance loss diagram of the 5# sample is shown in Figure 6As shown in (a) to (o). As can be seen from the figure, sample 1# has the best effective absorption bandwidth, and sample 3# has the lowest reflection loss value. Among them, when the matching thickness of sample 1# is 2.09mm, its effective absorption frequency covers the entire Ku band (11.76~18GHz); its minimum reflection loss value is obtained at a matching thickness of 5.38mm, and the minimum reflection loss value is -55.01dB. Sample 2# achieves the maximum effective absorption bandwidth of 4.56GHz (13.44~18GHz) when the matching thickness is 2.61mm. Its minimum reflection loss is obtained at a frequency of 16.88GHz, with a value of -15.65GHz. At this time, the matching thickness is 6.65mm. Sample 3# achieved the best minimum reflection loss of all samples, achieving a minimum reflection loss of -63.76 dB at 15.68 GHz with a matching thickness of 7.6 mm. Its maximum effective absorption bandwidth was 3.68 GHz (14.32 to 18 GHz) at a matching thickness of 7.49 mm. Sample 4# achieved the lowest reflection loss of -19.3 dB at 15.84 GHz with a matching thickness of 7.99 mm. When the matching thickness was 7.73 mm, the maximum effective absorption bandwidth was 2.08 GHz (15.92 to 18 GHz). Sample 5# achieved both the minimum reflection loss and maximum effective absorption bandwidth at a matching thickness of 7.98 mm, with values ​​of -14.92 dB and 1.52 GHz (15.6 to 17.12 GHz), respectively.

[0059] Wave absorption mechanism analysis

[0060] To investigate the microwave absorption mechanism of the FeNi alloy / graphene foam composite, the electrical loss characteristics of the composite were analyzed using Cole-Cole plots and electrical conductivity, and the magnetic loss characteristics of the composite were studied using a curve showing the variation of the C0 value with respect to frequency. Furthermore, the composite's impedance matching performance and attenuation coefficient were comprehensively considered, and the microwave absorption mechanism of the composite was explained from the perspective of the synergistic effect of electrical and magnetic losses.

[0061] In the Cole-Cole diagram, the Debye semicircle represents the spectral characteristics of the dielectric polarization and the degree to which the dielectric deviates from the Debye relaxation. It plays a crucial role in explaining the polarization relaxation characteristics of the dielectric constant. The Cole-Cole diagram is derived from the Debye equation derived from the relationship between the real and imaginary parts of the dielectric constant. The specific formula is as follows:

[0062]

[0063] Among them, ε S represents the static dielectric constant, ε ∞Represents the static high-frequency limit dielectric constant. From the above equation, it can be seen that when the dielectric polarization relaxation process occurs in a material within a specific frequency range, the real part ε′ and imaginary part ε″ of its dielectric constant will follow the Debye relaxation model, and then present a Cole-Cole semicircle. The radius of the Cole-Cole semicircle is positively correlated with the degree of dielectric polarization relaxation in the material, that is, the larger the radius, the higher the degree of polarization relaxation, and each polarization relaxation process corresponds to a semicircle. The Cole-Cole plots of samples 1# to 5# are shown in Figure 2. Figure 7 As shown, each group of samples exhibits at least three obvious rings, which indicates that multiple polarization relaxation occurs in the composite material.

[0064] Combined with the analysis of the microstructure, composition, and interface characteristics of the composite material, the multipolarization relaxation phenomenon may include interface polarization and dipole polarization. Among them, interface polarization is caused by the obstruction of electrons when crossing the complex interface region formed by FeNi alloy and graphene foam, resulting in charge accumulation and redistribution. The reduced graphene foam and the oxidized FeNi alloy inevitably have defects and vacancies. Under the action of the alternating electromagnetic field, electrons are captured by defects and vacancies, forming local electric dipoles. The orientation and intensity of these electric dipoles will change over time in response to changes in the external electric field, resulting in the occurrence of dipole polarization. It was also found that the vertical coordinate of the center of the sample semicircle decreased continuously with the increase of FeNi alloy content. It can be inferred that the increase of FeNi alloy reduces the conductivity of the composite material.

[0065] In order to explore the effect of FeNi alloy content on the conductive loss of the composite material, the effective conductivity of the sample in the alternating electromagnetic field was calculated based on electromagnetic parameters. The calculation formula of the effective conductivity is as follows:

[0066] σ=2πfε0ε″ (6)

[0067] Where ε0 is the dielectric constant of vacuum (8.854×10-12f / m). Figure 8 As can be seen in (a), the electrical conductivity of samples 1# to 5# decreases with the increase of FeNi alloy content, which also confirms the above inference of the results shown in the Cole-Cole diagram. Since the conductivity of FeNi alloy is poorer than that of graphene foam, and as the content of alloy particles increases, the conductive network of graphene foam itself is increasingly damaged, and the conductivity naturally decreases. Sample 1# shows a conductivity value far exceeding that of the other four groups of samples. It is very likely that the conductive network structure constructed by sample 1# is more complete, continuous and uniform, thus providing a smoother and more efficient channel for the transmission of electrons, resulting in a significant increase in conductivity.

[0068] In the frequency range of 2 to 18 GHz, the magnetic loss mechanisms of absorbing materials are mainly eddy current loss, natural resonance, and exchange resonance. The Fe and Ni in the FeNi alloy used in this experiment are elements with large magnetic moments, and there is a very strong interaction between the magnetic moments, which can drive the magnetic moment precession and thus produce natural resonance at the appropriate frequency. In addition, the FeNi alloy used in this experiment has a high saturation magnetization and a low coercive force, and has typical soft magnetic properties, that is, the movement of the magnetic domain wall of the FeNi alloy has a certain flexibility, which enables it to respond to the external magnetic field at the appropriate frequency. When this frequency matches the natural frequency of the magnetic domain wall movement, it can induce exchange resonance. For eddy current loss, its influencing factors mainly include conductivity (σ) and material thickness (d), and its expression is as follows:

[0069]

[0070] Where f is the frequency of the electromagnetic wave and μ0 is the magnetic permeability of vacuum. From this equation, it can be inferred that if eddy current loss is the main magnetic loss mechanism of the absorbing material, then the corresponding C0 value should be a constant with a very small variation range. Figure 8 (b) It can be seen that the C0 values ​​of samples 1# to 5# vary with frequency, and peaks appear near the frequency ranges of 6, 12, and 16 GHz, corresponding to natural resonance and exchange resonance. This also shows that eddy current loss is not the main magnetic loss mode of the composite material.

[0071] In order to further explore the absorption mechanism of composite materials with different contents of FeNi alloys, the attenuation coefficient and characteristic impedance were introduced and plotted. Figure 8 (c) and (d). The attenuation coefficient α is an important parameter for evaluating the attenuation of incident electromagnetic waves within a material. The larger α is, the greater the loss of electromagnetic waves entering the material. The characteristic impedance Z can reflect the ability of electromagnetic waves to enter the material. When Z is closer to 1, more electromagnetic waves enter the material, and the impedance matching performance of the material is better. The specific formula is as follows:

[0072]

[0073] Where f is the frequency of electromagnetic waves and c is the speed of light. Figure 8 As shown in (c), the composite's attenuation coefficient decreases with increasing FeNi alloy content. This is because the increase in FeNi alloy significantly decreases the composite's electrical loss capacity, while its magnetic loss capacity does not significantly increase. This suggests that the composite's attenuation coefficient is more closely related to its electrical loss capacity. Figure 8(d) shows the characteristic impedance curve corresponding to the lowest reflection loss value achieved by each sample. It can be seen that the frequency corresponding to the optimal impedance matching characteristics of sample #1 is the smallest, at around 4.64 GHz, which coincides with the frequency corresponding to the lowest reflection loss. The corresponding frequencies of the optimal impedance matching characteristics of the other four groups of samples are between 45 and 17 GHz, which are also consistent with the frequencies corresponding to the lowest reflection loss values ​​achieved by each group. At this time, the impedance matching performance of sample #2 did not reach 1, and there was a certain gap between it and the other four groups of samples. Except for sample #2, the attenuation coefficients of the four groups of composite materials at the frequencies at which they achieved the best impedance matching performance were between 50 and 70, with little difference. The attenuation coefficient corresponding to sample #2 was 124.9, which was much larger than that of the other samples. This shows that an excessively large attenuation coefficient will lead to poor impedance matching performance, which in turn leads to poor absorption performance.

[0074] According to the above analysis, the FeNi alloy / graphene foam composite material shows strong compatibility between electrical and magnetic losses. Its absorption mechanism is shown in the figure below. Figure 9 As shown. The defects and vacancies in the reduced graphene foam and the oxidized FeNi alloy lead to the occurrence of dipole polarization under the action of the alternating electromagnetic field. In addition, the FeNi alloy and graphene foam in the composite material form a complex interface region, and electrons are very likely to produce interface polarization when passing through this region. Analyzed separately, FeNi alloy, as a magnetic loss material, has magnetic anisotropy and relatively flexible magnetic domain walls, coupled with high saturation magnetization and low coercive force, which are the prerequisites for natural resonance and exchange resonance. As a metal material, FeNi alloy has certain electrical conductivity and can exert a certain electrical loss capacity. The porous structure of graphene foam itself can provide a longer loss path for the propagation of electromagnetic waves in the composite material, and at the same time provide a basis for multiple reflections and scattering of electromagnetic waves, which is beneficial to improving the composite material's attenuation ability to electromagnetic waves.

[0075] The present invention uses a one-step hydrothermal method to prepare FeNi alloy / graphene foam composite materials, and successfully prepares five groups of samples by controlling the addition amount of FeNi alloy. The results show that the FeNi alloy content has a significant influence on the micromorphology, phase structure and electromagnetic parameters of the product. Compared with many reported alloy / carbon material absorbers, the FeNi alloy / graphene foam composite material prepared in this experiment has relatively excellent absorbing performance when the filling amount is only 5%. When the matching thickness of sample 1# is 2.09mm, its effective absorbing frequency covers the entire Ku band (11.76~18GHz); its minimum reflection loss value is obtained at a matching thickness of 5.38mm, and the minimum reflection loss value is -55.01dB. Sample 3# can achieve a minimum reflection loss value of 63.76dB at a frequency of 15.68GHz.

[0076] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a high-performance FeNi alloy graphene foam composite absorbing material, characterized in that: The following steps are involved: S1, measuring a certain amount of graphene oxide slurry and ethylene glycol and mixing them uniformly, adding naphthalene to the mixed solution of the graphene oxide slurry and ethylene glycol under stirring to obtain a suspension; S2, taking a certain mass of FeNi alloy powder, adding it to the suspension obtained in step S1, and then transferring the mixed solution to a polytetrafluoroethylene liner, reacting for a period of time and cooling it to room temperature to obtain FeNi alloy / graphene gel; S3, placing the gel obtained in step S2 into a drying oven and drying for a period of time to obtain FeNi alloy / graphene foam; In step S1, the graphene oxide slurry and ethylene glycol are measured to be 170 ml each, and 80 g of naphthalene is added to the mixed solution of the graphene oxide slurry and ethylene glycol under mechanical stirring; The reaction in step S2 is specifically a hydrothermal reaction performed at a temperature of 200° C. for 12 hours; Step S3 specifically comprises: placing the gel obtained in step S2 into a vacuum drying oven, drying at 70° C. for 36 hours, adjusting the drying temperature to 120° C. and drying for another 24 hours to obtain FeNi alloy / graphene foam; The residual oxygen-containing functional groups on the surface of graphene foam can serve as active adsorption sites for FeNi alloy, so that the composite of FeNi alloy and graphene foam forms an ionically bonded composite material. The saturation magnetization of the FeNi alloy / graphene foam increases with the increase of the FeNi alloy content; The electrical loss capacity of the FeNi alloy / graphene foam decreases with increasing FeNi alloy content; The electrical conductivity of the FeNi alloy / graphene foam decreases as the FeNi alloy content increases.

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