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

Abstract

The invention discloses a preparation method of a high-performance FeNi alloy graphene foam composite wave-absorbing material, and belongs to the technical field of preparation of wave-absorbing materials. The preparation method comprises the following steps: S1, weighing a certain amount of graphene oxide slurry and ethylene glycol, uniformly mixing, and adding naphthalene into a mixed solution of the graphene oxide slurry and ethylene glycol under a stirring condition to obtain a turbid liquid; s2, taking a certain mass of FeNi alloy powder, adding the FeNi alloy powder into the turbid liquid obtained in the step S1, then transferring the mixed solution into a polytetrafluoroethylene lining, reacting for a period of time, and then cooling to room temperature to obtain FeNi alloy / graphene gel; and S3, the gel obtained in the step S2 is put into a drying box to be dried for a period of time, and the FeNi alloy / graphene foam is obtained. The FeNi alloy / graphene foam composite material is prepared by utilizing a one-step hydrothermal method.

Description

Technical Field

[0001] The invention relates to the technical field of preparing absorbing materials, and in particular to a method for preparing a high-performance FeNi alloy graphene foam composite absorbing material. Background Art

[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 communications has become increasingly in-depth. However, the widespread application of electromagnetic waves has inevitably caused electromagnetic pollution, which undoubtedly brings many potential hazards to human health and living environment. Long-term exposure to high-dose electromagnetic fields increases the risk of cancer and may also cause damage to the reproductive system, immune system, and nervous system. In addition, poor management of electromagnetic radiation is very likely to interfere with communication systems, resulting in communication damage and even serious accidents. In the natural environment, electromagnetic pollution may 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 methods poses a huge threat to the survival and safety of various military targets. Absorbing materials have been widely studied as an effective means to effectively absorb electromagnetic waves and thus reduce electromagnetic radiation. The design and preparation of high-performance microwave absorbing materials to reduce electromagnetic pollution and improve the survivability of main combat equipment has been a hot topic for many years.

[0003] The research on absorbing materials has a long history. So far, many 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, outstanding conductivity and good mechanical properties. The carbon materials commonly used for absorbing waves are mainly carbon nanotubes, graphene, carbon fiber, graphite and their various modified materials. Among them, graphene has attracted widespread attention due to its low density, high specific surface area and excellent conductivity and dielectric properties. At the same time, graphene oxide, reduced graphene oxide and three-dimensional graphene modified by graphene have become research hotspots in recent years due to their better impedance matching performance and stronger electromagnetic attenuation performance.

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

[0005] It can be seen that the application of three-dimensional graphene is expected to overcome the shortcomings of traditional absorbing materials such as high density and difficulty in balancing absorbing strength and absorbing bandwidth, and better meet the requirements of "thin, light, wide and strong" for absorbing materials. Our research group previously used the hydrothermal method combined with the principle of naphthalene sublimation to prepare graphene foam.

[0006] Research shows that although the prepared graphene foam has excellent wave absorption performance, its impedance matching performance needs to be improved. Subsequently, we compounded it with magnetic materials and prepared FeCo alloy / graphene foam composites by a two-step hydrothermal method. This material exhibits higher wave absorption intensity and a wider effective wave absorption bandwidth. Compounding FeNi alloy with graphene foam can improve the impedance matching performance, allowing more electromagnetic waves to enter the material interior. In addition, the electromagnetic wave loss mechanism synergistically combining magnetic loss and electrical loss can effectively improve the material's ability to attenuate electromagnetic waves and enhance the wave absorption performance. However, the two-step hydrothermal method is relatively cumbersome, reducing the preparation efficiency of the composite material.

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

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

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] A preparation method for a high-performance FeNi alloy / graphene foam composite wave-absorbing material includes the following steps:

[0011] S1, measure a certain amount of graphene oxide slurry and ethylene glycol and mix them evenly. Add naphthalene to the mixed solution of graphene oxide slurry and ethylene glycol under stirring to obtain a suspension;

[0012] S2, take a certain mass of FeNi alloy powder, add it to the suspension obtained in step S1, then transfer the mixed solution to a polytetrafluoroethylene inner liner, react for a period of time and then cool to room temperature to obtain FeNi alloy / graphene gel;

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

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

[0015] Further, the reaction in step S2 is specifically a hydrothermal reaction carried out at a temperature of 200 °C for 12 h.

[0016] Further, step S3 is specifically as follows: The gel obtained in step S2 is placed in a vacuum drying oven and dried at 70°C for 36 hours. Then, the drying temperature is adjusted to 120°C and dried for another 24 hours to obtain FeNi alloy / graphene foam.

[0017] Further, the residual oxygen-containing functional groups on the surface of the graphene foam can serve as active adsorption sites for the FeNi alloy, enabling the composite of the FeNi alloy and the graphene foam to form a composite material connected by ionic bonds.

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

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

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

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

[0022] The present invention prepares a FeNi alloy / graphene foam composite material by a one-step hydrothermal method, and successfully prepares five groups of samples by controlling the addition amount of the FeNi alloy. Moreover, the FeNi alloy content has a great influence on the microscopic morphology, phase structure, and electromagnetic parameters of the product.

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

[0024] Figure 1 It is a preparation flow chart of the FeNi alloy / graphene foam composite material provided by the embodiment of the present invention;

[0025] Figure 2 It is the SEM and EDS diagrams of the FeNi alloy / graphene foam composite material provided by the embodiment of the present invention: (a)-(e) SEM diagrams of 1# - 5# samples, (f) SEM diagram of 3# sample, (g)-(i) EDS diagrams of 3# sample;

[0026] Figure 3(a)XRD spectra and (b) Raman spectrum of FeNi alloy / graphene foam composite material; XPS spectra of FeNi / graphene foam composite: (c) survey spectrum, (d) C 1s spectrum, (e) Ni 2p spectrum, (f) Fe 2p spectrum;

[0027] Figure 4 Hysteresis loop of FeNi alloy / graphene foam composite material provided by the embodiment of the present invention;

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

[0029] Figure 6 Absorbing performance diagram of FeNi alloy / graphene foam composite material provided by the embodiment of the present invention;

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

[0031] Figure 8 Schematic diagram of the performance of FeNi alloy / graphene foam composite material provided by the embodiment of the present invention: (a) conductivity, (b) C 0 value, (c) attenuation coefficient, (d) characteristic impedance;

[0032] Figure 9 Schematic diagram of the wave absorption mechanism of FeNi alloy / graphene foam composite material provided by the embodiment of the present invention. Detailed implementation manners

[0033] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention.

[0034] The present invention provides a preparation method of a high-performance FeNi alloy graphene foam composite wave-absorbing material, including the following steps:

[0035] S1, measure a certain amount of graphene oxide slurry and ethylene glycol and mix them evenly, and add naphthalene to the mixed solution of graphene oxide slurry and ethylene glycol under stirring to obtain a suspension;

[0036] S2. Weigh a certain mass of FeNi alloy powder, add it to the suspension obtained in step S1, then transfer the mixed solution to a polytetrafluoroethylene liner, react for a period of time, and then cool it to room temperature to obtain FeNi alloy / graphene gel.

[0037] S3. Put the gel obtained in step S2 into an oven and dry it for a period of time to obtain FeNi alloy / graphene foam.

[0038] This embodiment provides a preparation method of a high-performance FeNi alloy / graphene foam composite wave-absorbing material, which is as follows:

[0039] Measure 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), mix them evenly, and slowly add 80 g of naphthalene to the above mixed solution under mechanical stirring to obtain a suspension. Respectively take 2 g, 4 g, 6 g, 8 g, and 10 g of FeNi alloy powder (purchased from Changsha Tianjiu Metal Co., Ltd., China), add them to the above suspension under mechanical stirring, and then transfer the mixed solution to a polytetrafluoroethylene liner with a volume of 500 ml. Carry out a hydrothermal reaction at a temperature of 200 °C for 12 h, and after cooling to room temperature, obtain 5 kinds of FeNi alloy / graphene gels with different component ratios; put the gel into a vacuum drying oven, dry it at 70 °C for 36 h, adjust the drying temperature to 120 °C and dry it for another 24 h to obtain FeNi alloy / graphene foam. According to the amount of FeNi alloy powder added, from less to more, they are marked as 1#, 2#, 3#, 4#, and 5# respectively. The overall preparation process is as Figure 1 shown.

[0040] Characterization method

[0041] Use a high-power rotating anode polycrystalline Bruck-D8 Advance X-ray diffractometer (XRD) to perform phase analysis on the material (experimental conditions: Cu target, scanning range 10° - 100°, scanning speed 8° / min); use a ZEISS sigma500 field emission scanning electron microscope (SEM) to observe the microstructure, structure, and size of the material; use a HORIBA HR Evolution Raman spectrometer to measure the Raman spectrum of the material under 532 nm laser excitation, and the scanning range is 1000 - 2500 cm -1; The chemical state changes of the structure were analyzed using a Thermo Scientific-ESCALAB Xi+ type X-ray photoelectron spectrometer (XPS); the static magnetic properties of the materials, including saturation magnetization and coercivity, were tested using a Lakeshore-7404 type vibrating sample magnetometer (VSM). The electromagnetic parameters of the materials at 2 - 18 GHz were tested using an Agilent N5234A type vector network analyzer (VNA) (when testing, the sample and paraffin were mixed at a mass ratio of 5:95); finally, according to the transmission line theory, the reflection loss characteristics of the materials under different thickness conditions were simulated and calculated.

[0042] To study the effect of the change in the FeNi alloy content in Samples 1# - 5# on the micro-morphology of the composite materials, SEM analysis as shown in Figure 2 (a)-(e) was carried out. It can be seen from the figure that the alloy distribution in Sample 1# and Sample 2# is relatively uniform and can be loaded onto the mesh sheets of the graphene foam relatively evenly. However, with the increase in the FeNi alloy content, the loading density of the graphene foam on it also increases significantly, and obvious aggregation phenomena start to appear in Sample 3#. The aggregation in Sample 4# is further aggravated, and obvious irregular lumps can be seen in the figure. This may be because the number of active adsorption sites inside the graphene foam is insufficient, and when too many FeNi alloy particles compete for the limited adsorption sites, some particles cannot be effectively adsorbed on the three-dimensional graphene and can only aggregate with each other. However, there are still pores in the graphene foam network structure in Sample 4# at this time, although the pores have deformed severely compared with Samples 1# - 3#. With the further increase in the amount of FeNi alloy powder, the pores in Sample 5# are basically blocked, and in the case of the existence of irregular lumps in this sample, the porous structure of the graphene foam almost disappears. Thus, it can be seen that the content of the FeNi alloy has a very significant impact on the micro-morphology of the composite materials. To further clarify the element distribution of the composite materials, we carried out EDS mapping detection and analysis on Sample 3# (as shown in Figure 2 (f)), and obtained the element distribution of the FeNi alloy / graphene foam composite materials. Since the graphene foam is evenly distributed throughout the region, the experiment mainly tested the distribution of three elements, Fe, Ni, and O. The EDS results (as shown in Figure 2 (g)-(i)) show that the Fe, Ni, and O elements are concentrated in some regions, and the three elements basically coincide in these regions. However, relatively speaking, the density of the O element in the concentrated region is higher, and the Ni element is the lowest. The appearance of this situation may be because the graphene oxide was not completely reduced by ethylene glycol, resulting in some residual oxygen-containing functional groups, and these residual oxygen-containing functional groups provided active adsorption sites for the loading of the FeNi alloy. The situation where the density of the Fe element is higher than that of the Ni element may be due to the oxygen-containing functional groups oxidizing a part of the FeNi alloy to NiFe2 O 4 。

[0043] Figure 3 (a) is the XRD pattern of the FeNi alloy particles themselves and the FeNi alloy / graphene foam composite. As can be seen from the figure, the original FeNi alloy particles are composed of two phases, Fe 0.4 Ni 0.6 and FeNi alloy. After the hydrothermal reaction with graphene oxide slurry and ethylene glycol, obvious diffraction peaks appeared near 30.3°, 35.7°, 57.4° and 62.9° in samples 1#-5#. By comparing with the standard card (PDF#10-0325), they were determined to be the characteristic diffraction peaks of the (120), (311), (511) and (440) crystal planes, confirming the existence of NiFe 2 O 4 in the composite material, which is also consistent with the speculation during the above EDS analysis that the oxygen-containing functional groups oxidized the FeNi alloy to NiFe 2 O 4 . It is not difficult to find from the figure that the characteristic diffraction peaks of the (200) and (211) crystal planes of the FeNi alloy near 64.9° and 82.3° disappeared after the hydrothermal experiment, and the intensity of the characteristic diffraction peak of the FeNi alloy on the (110) crystal plane at 44.6° also decreased significantly after the hydrothermal experiment. This may be because the FeNi alloy phase is more easily oxidized to NiFe 0.4 Ni 0.6 than Fe 2 O 4 . The diffraction peak of graphene foam near 25.5° in the figure is difficult to observe. This may be because the FeNi alloy particles themselves produce additional scattering or X-ray absorption phenomena, which will be superimposed or interfere with the diffraction signal of graphene, masking or weakening the diffraction peak of graphene foam near 25.5°.

[0044] Figure 3 (b) is the Raman spectrum of the FeNi alloy / graphene foam composite. As can be seen from the figure, the D peaks of the 5 samples all appear near 1343 cm -1 , and the G peaks all appear near 1588 cm -1 . Among them, the D peak is related to the disorder degree of carbon materials. It is generated by the symmetry breaking of the lattice vibration of sp 2 atoms in the carbon ring. 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 graphitization degree of carbon materials and is mainly caused by the stretching vibration of carbon six-membered rings. The ratio of the D peak to the G peak, I D / I GIt 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. The I D / I G value of samples 1# to 5# decreases with the increase of the FeNi alloy content, which means that the increase of FeNi alloy powder repairs and improves the structure of the graphene foam to a certain extent. This may be because during the composite process of the FeNi alloy and the graphene foam, some defects on the graphene surface are filled, reducing the disordered regions of the graphene structure. In addition, the increase of the FeNi alloy content will affect the stress distribution inside the graphene. The alloy particles have a certain tensile or compressive effect on the graphene, causing changes in the lattice structure of the graphene, and then affecting its vibration mode, thus resulting in the decrease of the I D / I G value.

[0045] To further verify the surface chemical composition and valence state of the FeNi alloy / graphene foam composite material, X-ray photoelectron spectroscopy analysis was carried out on sample 3#, and the results are as Figure 3 (c-f) shown. Figure 3 (c) is the full XPS scan spectrum of the FeNi alloy / graphene foam composite material. It can be seen that the FeNi alloy / graphene foam composite material is composed of four elements: C, O, Fe, and Ni. Graphene and the FeNi alloy contribute three elements: C, Fe, and Ni respectively, and the O element comes from the residual oxygen-containing functional groups of the graphene foam. Figure 3 (d) shows the characteristic peaks of C1s of the FeNi alloy / graphene foam composite material. After peak fitting, the binding energies of 287.7 eV, 285.63 eV, 284.8 eV, and 283.89 eV correspond to C=O, C-O, C-C, 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 the graphene foam. The peak fitting results of the Ni 2p spectrum are as Figure 3 (d) shown. It consists of two spin-orbit doublets, Ni 2p 1 / 2 and Ni 2p 3 / 2 and their satellite peaks, and these peaks show the +2 and +3 valence states of Ni. Figure 3 (e) shows the results after peak fitting of the Fe 2p spectrum. The Fe 2p spectrum is composed of two peaks with different binding energies and related satellite peaks. The Fe 2p 3 / 2 and Fe 2p 1 / 2 peaks are respectively related to the oxidation states of Fe 2+ and Fe 3+ . Among them, Fe 2+ , Fe 3+ and Ni 2+ can all combine with O 2-Combined to form Fe-O bonds and Ni-O bonds, it 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 a composite material connected by ion bonds.

[0046] Figure 4 Table 1 shows the hysteresis loops of FeNi alloy / graphene foam composite materials with different FeNi alloy contents at room temperature. The saturation magnetization and coercive force are shown in Table 1. It can be seen from the chart 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 just starts to increase, 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. Generally, 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 material composition, 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, thus 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 absorbing performance

[0052] Generally speaking, the complex dielectric constant (ε r =ε′-jε") and complex magnetic permeability (μr =(μ′ - jμ″) is an important parameter describing the interaction between the wave - absorbing material and electromagnetic waves. Among them, ε r and μ r The real parts of (ε′ and μ′) respectively represent the ability of the material to store electrical energy and magnetic energy, while the imaginary parts of the two (ε″ and μ″) represent the ability of the material to dissipate electrical energy and magnetic energy. This indicates that the larger the real part, the stronger the ability of the material to store energy, and the larger the imaginary part, the stronger the ability of the material to dissipate energy, and the stronger the ability to absorb electromagnetic waves. In addition, the strength of the electrical and magnetic loss capabilities of the wave - absorbing material is generally reflected by the dielectric loss tangent value (tanδ ε ) and the magnetic loss tangent value (tanδ μ ), and their expressions are as follows:

[0053]

[0054] As Figure 5 (a) shows, the real parts of the complex permittivity of the 1# - 5# samples exhibit typical dispersion phenomena, that is, they all show a decreasing trend with the increase of frequency. However, relatively speaking, the real part of the complex permittivity of the samples decreases with the increase of the FeNi alloy content. Among them, the real part of the complex permittivity of the 1# sample is significantly higher than the other 4 groups of samples. This is because the content of graphene foam in the 1# sample is the highest. On the one hand, the conductivity of graphene foam is stronger than that of FeNi alloy, and on the other hand, the high content of graphene foam improves the conductive network of the composite material, so the real part of the complex permittivity of the 1# sample is significantly stronger than the other samples. From Figure 5 (b), it can be seen that different from the real part of the complex permittivity, the imaginary parts of the complex permittivity of the 1# - 5# samples do not show obvious frequency response characteristics. This may be because of the mutual 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, making the imaginary part of the complex permittivity have no obvious upward or downward trend. Although the imaginary parts of the complex permittivity of the 5 groups of samples are relatively flat in the 2 - 18 GHz frequency range, there are still multiple peaks appearing, which means that multiple polarization relaxation phenomena occur in the composite material. Combining Figure 5 (a) and Figure 5 (b), it is not difficult to judge that the electrical loss ability of the composite material will decrease with the increase of the FeNi alloy content, Figure 5 (c) also shows this result. The curves of the real part and the imaginary part of the complex permeability of the FeNi alloy / graphene foam composite material changing with frequency are as Figure 5 (d) and Figure 5As shown in (e). It can be seen from the figure that 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 ability. The remaining 4 groups of samples show different magnetic storage abilities in different frequency bands, and it is difficult to distinguish their advantages and disadvantages. The imaginary part of the complex permeability of the 2#-5# samples shows an increasing trend with the increase of the FeNi alloy content in the frequency range of 2-18 GHz, but the 1# sample shows a stronger result than the 3# sample. This may be because the aggregation of FeNi alloy particles leads to the distortion of the local magnetic field, and the interaction between the aggregates will also change the overall magnetism of the material. This non-uniform microstructure makes the magnetic loss performance unable to improve with the simple increase of the 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 the 1#-4# groups of samples. Obvious peaks appear at 6, 12, and 16 GHz for the 5 groups of samples, indicating that obvious magnetic loss phenomena occur in the above frequency bands. This may be because the frequency of the externally applied alternating magnetic field is close to or equal to the precession natural frequency of the FeNi alloy magnetic moment, resulting in a natural resonance phenomenon. And when the frequency of the externally applied alternating magnetic field matches the exchange interaction strength between Fe and Ni atoms, the occurrence of the exchange resonance phenomenon will also greatly improve the magnetic loss performance, resulting in the appearance of peaks. As shown in (f), the curve of the magnetic loss tangent value of the 5 groups of samples is almost the same as the curve of the imaginary part of the complex permeability, that is, the 5# sample has the strongest magnetic loss ability, and the 2# sample has the weakest magnetic loss ability, which is consistent with the results of the above static magnetic performance analysis.

[0055] To deeply explore the microwave absorption performance of the FeNi alloy / graphene foam composite, based on the transmission line theory, a simulation study was carried out on the reflectivity of the samples in the frequency range of 2-18 GHz and under different absorber layer thickness conditions. Among them, the expression of the reflectivity is shown in the following formula:

[0056]

[0057] In this formula, Zin refers to 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. When RL ≤ -10 dB, it can be considered that at least 90% of the electromagnetic waves are absorbed, and this situation is also called effective absorption. In this case, the width of the frequency range covered by the electromagnetic wave frequency is defined as the effective absorption bandwidth.

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

[0059] Analysis of Absorbing Mechanism

[0060] To explore the absorbing mechanism of the FeNi alloy / graphene foam composite material, the Cole-Cole plot and conductivity are used to analyze the electrical loss characteristics of the composite material, and the magnetic loss characteristics of the composite material are studied by means of the curve of the C 0 value corresponding to the frequency. At the same time, considering the impedance matching performance and attenuation coefficient of the composite material comprehensively, the absorbing mechanism of the composite material is elaborated from the perspective of the synergistic effect of electrical and magnetic losses.

[0061] In the Cole-Cole plot, the Debye semicircle characterizes the spectral characteristics of dielectric polarization and the degree of deviation of the dielectric from Debye relaxation, and it plays a very crucial role in the interpretation of the polarization relaxation characteristics of the dielectric constant. The Cole-Cole plot is obtained from the Debye equation derived from the relationship between the real part and the imaginary part 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. As can be seen from the above equation, when the material exhibits dielectric polarization relaxation in a specific frequency range, the real part ε′ and the 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 of 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 diagrams of samples 1# to 5# are as shown in Figure 7 shown, and each group of samples presents at least 3 obvious rings, indicating that multiple polarization relaxations have occurred in the composite material.

[0064] Combined with the analysis of the microstructure, composition and interfacial properties of the composite material, the multiple polarization relaxation phenomenon may include interfacial polarization and dipole polarization. Among them, interfacial polarization is caused by the obstruction of electrons when passing through the complex interfacial 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 an alternating electromagnetic field, electrons will be trapped by the defects and vacancies to form local electric dipoles. The orientation and intensity of these electric dipoles will change with time to respond to the change of the external electric field, thus leading to the occurrence of dipole polarization. At the same time, it is found that the ordinate of the center of the semicircle of the sample decreases continuously with the increase of the FeNi alloy content, and it can be inferred that the increase of the FeNi alloy reduces the conductivity of the composite material.

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

[0066] σ = 2πfε 0 ε″ (6)

[0067] where ε 0 is the vacuum permittivity (8.854×10-12 F / m). As can be seen from Figure 8 (a), the conductivity of samples 1# to 5# decreases continuously with the increase of the FeNi alloy content, which also confirms the above inference of the results shown in the Cole-Cole diagram. Since the conductivity of the FeNi alloy is worse than that of the graphene foam, and with the increase of the alloy particle content, the damage degree of the conductive network of the graphene foam itself is getting higher and higher, and the conductivity naturally decreases. The conductivity value of sample 1# far exceeds that of the other 4 groups of samples. Most likely, 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 - 18 GHz, the magnetic loss mechanisms of the wave-absorbing material mainly include eddy current loss, natural resonance, and exchange resonance. Both 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 precession of the magnetic moments and then generate natural resonance at an appropriate frequency. In addition, the FeNi alloy used in this experiment has a high saturation magnetization and a low coercivity, showing typical soft magnetic properties, that is, the movement of the magnetic domain walls of the FeNi alloy has a certain flexibility, which enables it to respond to an external magnetic field at a suitable frequency. When this frequency matches the natural frequency of the magnetic domain wall movement, exchange resonance can be triggered. 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 electromagnetic wave frequency, and μ 0 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 wave-absorbing material, then the corresponding C 0 value should be a constant with a very small variation range. However, as can be seen from Figure 8 (b), the C 0 values of samples 1# - 5# all change with the frequency, and peaks appear near the frequency ranges of 6, 12, and 16 GHz, corresponding to natural resonance and exchange resonance, which also indicates that eddy current loss is not the main magnetic loss mode of the composite material.

[0071] To further explore the wave-absorbing mechanism of the composite material with different contents of FeNi alloy, the attenuation coefficient and characteristic impedance are introduced, and Figure 8 (c) and (d) are plotted. The attenuation coefficient α is an important parameter for evaluating the attenuation ability of incident electromagnetic waves inside the material. The larger α is, the stronger the loss of the electromagnetic waves entering the material interior. The characteristic impedance Z can reflect the ability of the electromagnetic waves to enter the material interior. When Z is closer to 1, more electromagnetic waves enter the material interior, and the impedance matching performance of the material is better. The specific formulas are as follows:

[0072]

[0073] where f is the electromagnetic wave frequency and c is the speed of light. As shown in Figure 8 (c), the attenuation coefficient of the composite material decreases with the increase in the content of FeNi alloy. This is because the increase in FeNi alloy leads to a significant decrease in the electrical loss ability of the composite material, while the magnetic loss ability is not significantly enhanced. From this, it can be analyzed that the attenuation coefficient of the composite material is more related to the electrical loss ability. Figure 8(d) shows the characteristic impedance curves corresponding to the minimum reflection loss values of various samples. It can be seen that the frequency corresponding to the best impedance matching characteristic of the 1# sample is the smallest, around 4.64 GHz, which is consistent with the frequency corresponding to its minimum reflection loss. The corresponding frequencies of the best 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 their respective minimum reflection loss values. At this time, the impedance matching performance of the 2# sample does not reach 1, showing a certain gap compared with the other four groups of samples. Among the four groups of composite materials except the 2# sample, at the frequencies where they achieve the best impedance matching performance, the corresponding attenuation coefficients are between 50 and 70, with little difference. However, the attenuation coefficient corresponding to the 2# sample is 124.9, which is much larger than that of the other samples. This shows that an excessive attenuation coefficient will lead to poor impedance matching performance, resulting in poor wave absorption performance.

[0074] According to the above analysis, the FeNi alloy / graphene foam composite material exhibits strong electro-magnetic loss compatibility ability, and its wave absorption mechanism diagram is as Figure 9 shown. The defects and vacancies existing 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 interface polarization is likely to occur when electrons cross this region. Analyzing separately, the FeNi alloy, as a magnetic loss material, has magnetic anisotropy and relatively flexible magnetic domain walls. Coupled with a high saturation magnetization intensity and a low coercivity, it has the prerequisite for natural resonance and exchange resonance. The FeNi alloy, as a metal material, has certain electrical conductivity and can play a certain electrical loss ability. The porous structure of the 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 the multiple reflection and scattering of electromagnetic waves, which is conducive to improving the attenuation ability of the composite material to electromagnetic waves.

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

[0076] The embodiments described above are only descriptions of the preferred embodiments of the present invention and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined 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 evenly, 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 a FeNi alloy / graphene gel; S3, putting the gel obtained in step S2 into a drying oven and drying it for a period of time to obtain FeNi alloy / graphene foam.

2. The method for preparing the high-performance FeNi alloy graphene foam composite absorbing material according to claim 1, characterized in that: In step S1, the graphene oxide slurry and ethylene glycol are both measured to be 170 ml, and 80 g of naphthalene is added to the mixed solution of the graphene oxide slurry and ethylene glycol under mechanical stirring.

3. The method for preparing the high-performance FeNi alloy graphene foam composite absorbing material according to claim 1, characterized in that: The reaction in step S2 is specifically a hydrothermal reaction carried out at a temperature of 200° C. for 12 hours.

4. The method for preparing the high-performance FeNi alloy graphene foam composite absorbing material according to claim 1, characterized in that: Step S3 is specifically as follows: 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.

5. The method for preparing the high-performance FeNi alloy graphene foam composite absorbing material according to claim 1, characterized in that: 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.

6. The method for preparing the high-performance FeNi alloy graphene foam composite absorbing material according to claim 1, characterized in that: The saturation magnetization of the FeNi alloy / graphene foam increases with the increase of the FeNi alloy content.

7. The method for preparing the high-performance FeNi alloy graphene foam composite absorbing material according to claim 1, characterized in that: The electrical loss capacity of the FeNi alloy / graphene foam decreases with the increase of the FeNi alloy content.

8. The method for preparing the high-performance FeNi alloy graphene foam composite absorbing material according to claim 1, characterized in that: The electrical conductivity of the FeNi alloy / graphene foam decreases as the FeNi alloy content increases.

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