Micro-Structural Design of CoFe 2 O 4 /SWCNTs Composites for Enhanced Electromagnetic Properties

: In order to prepare microwave-absorbing materials with low density and high wave absorption performance, CoFe 2 O 4 /SWCNTs composites with well-designed necklace-like structures were successfully prepared in this paper by a simple solvothermal method. CoFe 2 O 4 /SWCNTs composites with different cobalt salt contents were synthesized by adjusting the experimental parameters. The results of the relative complex permeability and relative permittivity of the samples, which were investigated by vector network analysis in the frequency range of 2 to 18 GHz, are collected to support the study of the microwave absorption characteristics of the samples. Different microsphere densities and different cobalt salt contents have obvious differences in the electromagnetic absorption properties of the composites. When the additions of FeCl 3 · 6H 2 O, Co(Ac) 2 · 4H 2 O, and NH 4 Ac were 0.432, 0.200, and 0.400 g, respectively, the best reﬂection loss reached − 42.07 dB, and the effective absorption frequency (RL < − 10 dB) ranges from 3.2 to 18 GHz. Therefore, this is a preparation strategy of CoFe 2 O 4 /SWCNTs composites with necklace structure, which has the advantages of simple process, environmental friendliness, low cost, and high stability. The unique necklace-like structure design makes the carbon nanotubes partially exposed, which is more beneﬁcial to achieve good impedance matching and giving the CoFe 2 O 4 /SWCNTs composite excellent electromagnetic loss capability.


Introduction
Nowadays, with the rapid development and broad application of electronic devices and wireless communication technologies, electromagnetic radiation poses a serious threat to human health and the regular operation of electronic devices and has become another major problem after air, water, and noise pollution [1][2][3][4]. To solve the problem of electromagnetic radiation pollution, absorbing materials that can absorb and attenuate incident electromagnetic waves and convert the incident electromagnetic energy into heat-based energy to be consumed are attracting much attention [5]. There are many materials for radiation protection, such as glasses based on TeO 2 -WO 3 -Bi 2 O 3 -MoO 3 -SiO [6], while this paper focuses on carbon nanotube-based microwave-absorbing composites. Therefore, the research and development of high-performance absorbing materials to solve the electromagnetic pollution problem are urgent [7]. According to the research progress of absorbing materials, using the excellent properties of composite absorbing materials with different components to make materials with both magnetic loss and electrical loss will become the future requirements and development direction of absorbing materials. grain development, and controllable size. Because of these advantages of the solvothermal method, it is better to control the microstructure of the sample and thus adjust the electromagnetic parameters of the microwave absorber.
In order to meet the requirements of lightweight and thin thickness, a simple solvothermal method is chosen to prepare CoFe 2 O 4 /SWCNTs composites in this paper, and necklacelike CoFe 2 O 4 /SWCNTs composites with dual dielectric and magnetic losses are successfully prepared. Due to the synergistic effect of the magnetic loss capacity of CoFe 2 O 4 and the dielectric loss capacity of SWCNTs, as well as the multiple scattering attenuation effects of the 3D lattice necklace-like structure on electromagnetic waves, the composites exhibit good wave absorption performance. The minimum RL value reaches −42.07 dB, the EABD reaches 3.8 GHz, and the thickness range is 1-5 mm. This method is simple and easy to implement and provides an effective way to develop high-performance MAM.

Functionalization of Single-Walled Carbon Nanotubes
After acid treatment, the number of functional groups on the surface of carbon nanotubes increases, making them more suitable for accepting other nanoparticles on their surface growth. A typical acidification process is as follows, 200 mg of carbon nanotubes are added to a three-necked flask containing a mixture of concentrated nitric acid and concentrated sulfuric acid (volume ratio: 3:1). The above mixture was dispersed by continuous strong sonication for 3 h. When the carbon nanotubes were completely dispersed, the three-neck flask was placed in an oil bath, heated to 80 • C, and maintained at that constant temperature for 1 h. Then the acidified carbon nanotubes were cooled to room temperature and left to stand in deionized water for 12 h. The rested carbon nanotubes were rinsed to neutral (PH ≈ 7) by filtration with a large amount of deionized water and dried at 60 • C in an oven for 8 h. The functionalized carbon nanotubes are ready for subsequent use.

Preparation of CoFe 2 O 4 /SWCNTs Necklace-like Structure
0.54 g FeCl 3 ·6H 2 O, 0.25 g Co(Ac) 2 ·4H 2 O, and 0.50 g NH 4 Ac were dissolved in 60 mL EG. After the solution was completely dissolved, different contents of functionalized SWCNTs were weighed and added to the light yellow solution, then sonicated continuously for 3 h. After the SWCNTs were well dispersed, small magnets were added and stirred for 30 min. Then the solution was poured into a 100 mL Teflon-lined stainless autoclave and held in an oven at 200 • C for 24 h. The samples were removed and washed with deionized water and ethanol 3 times, respectively. Vacuum drying was conducted at 60 • C for 8 h.
The experimental parameters (cobalt salt gradient (0.2 g, 0.25 g, 0.3 g, 0.35 g as Co-1, Co-2, Co-3, Co-4) where carbon nanotubes were 4 mg) were adjusted to achieve the modulation of the microstructure of spinel ferrites, to investigate the effect of micro-structural changes on the absorbing properties of the samples. The cobalt and iron salts were varied together, and they were added in multiples of 0.8, 1, 1.2, and 1.4 based on 0.25 g of cobalt salt and 0.54 g of iron salt as a reference. Finally, the stoichiometry of the cobalt salt was chosen as the standard to differentiate the four samples. The ratio of cobalt salt to iron salt did not change; what changed was the amount added. By adjusting the ratio of cobalt salts, it is the amount of cobalt and iron salts added that adjusts the microstructure of the material

Characterization
The crystal structure and phase composition of CoFe 2 O 4 /SWCNTs composites were characterized by an X-ray diffractometer (XRD, Model Smartlab 9, Tokyo, Japan) with a Cu Kα source (40 KV,200 mA). Scanning electron microscopy (SEM, JSM-7001F, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-2100F, Tokyo, Japan) were used to analyze the morphology and structure of the CoFe 2 O 4 /SWCNTs nanomaterials. The composition and distribution of chemical elements on the surface of the samples were investigated by X-ray photoelectron spectroscopy (XPS, Kratos, Manchester, U.K.). The magnetic hysteresis loops were measured by a vibrating sample magnetometer (VSM, Lake Shore Cryotronics, Westerville, OH, USA) at room temperature.

Characterization of Absorption Properties
The relative complex permeability and relative permittivity of the samples were tested by vector network analysis (VNA, Agilent N5234A, California, CA, USA) in the frequency range of 2 to 18 GHz. The sample powder was miscible with paraffin wax in the mass ratio of 3:7 at 80 • C. The miscible material is quickly poured into a hollow cylindrical mold with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness was about 3 mm.

Results and Discussion
We successfully synthesized necklace-like CoFe 2 O 4 /SWCNTs composite heterostructures by a simple solvothermal method using functionalized carbon nanotubes as raw materials, and the schematic illustration of the synthesis strategy of CoFe 2 O 4 /SWCNTs composites is shown in Figure 1. The phase composition and structure of the necklace-like CoFe2O4/SWCNTs composites were investigated by XRD, as shown in Figure 2. All the diffraction peaks obtained were in accordance with the CoFe2O4 standard card (JCPDS No. 22-1086). The diffraction peaks were clear and intense, and no obvious impurity peaks were observed, which indicated that the CoFe2O4/SWCNTs composite has good crystallization and high purity. In addition, the diffraction peaks of CoFe2O4/SWCNTs composites are almost consistent with the CoFe2O4 standard card, which indicates that it is an amorphous structure with little SWCNTs content. First, when CNTs were treated with acid, some negatively charged oxygen-containing functional groups were produced, which provide sites for the growth of CoFe 2 O 4 grains. Then, the oxygen-containing functional groups were used as nucleation centers, which are firmly bonded with carbon nanotubes to form a necklace-like composite structure. The specific process in the solvent heat is that when the carbon nanotubes are completely dispersed in the EG solution, Co 2+ and Fe 3+ in the solution can be fixed on the surface of single-walled carbon nanotubes by electrostatic adsorption. During the solvothermal process, EG can be dehydrated to generate H 2 O in a sealed autoclave under high temperature and pressure. Then an alkaline environment is created by hydrolysis of NH 4 AC. Finally, cobalt and iron salts react with OH − to form cobalt ferrite (Co 2+ + 2Fe 3+ + 8OH − → CoFe 2 O 4 + 4H 2 O) [37,38]. Thus, necklace-like CoFe 2 O 4 /SWCNTs composites are formed, with CoFe 2 O 4 nanoparticles passing through and immobilizing on the CNTs surface.
The phase composition and structure of the necklace-like CoFe 2 O 4 /SWCNTs composites were investigated by XRD, as shown in Figure 2. All the diffraction peaks obtained were in accordance with the CoFe 2 O 4 standard card (JCPDS No. . The diffraction peaks were clear and intense, and no obvious impurity peaks were observed, which indicated that the CoFe 2 O 4 /SWCNTs composite has good crystallization and high purity. In addition, the diffraction peaks of CoFe 2 O 4 /SWCNTs composites are almost consistent with the CoFe 2 O 4 standard card, which indicates that it is an amorphous structure with little SWCNTs content. The phase composition and structure of the necklace-like CoFe2O4/SWCNTs composites were investigated by XRD, as shown in Figure 2. All the diffraction peaks obtained were in accordance with the CoFe2O4 standard card (JCPDS No. . The diffraction peaks were clear and intense, and no obvious impurity peaks were observed, which indicated that the CoFe2O4/SWCNTs composite has good crystallization and high purity. In addition, the diffraction peaks of CoFe2O4/SWCNTs composites are almost consistent with the CoFe2O4 standard card, which indicates that it is an amorphous structure with little SWCNTs content. The microscopic morphology of the CoFe2O4/SWCNTs composites was characterized using SEM, and the morphology of the sample under the cobalt salt gradient is shown in Figure 3. The sample is composed of spherical nanoclusters and cylindrical carbon nanotubes, and the nanospheres wrap around the carbon nanotubes to form a necklace-like structure. SEM of Figure 3a-d clearly shows that the number and distribution density of spherical CoFe2O4 clusters obtained by adding different amounts of cobalt salts are significantly different at a carbon nanotube content of 4 mg. With the gradual increase in the cobalt salt content, the number of CoFe2O4 microspheres increases, and the spheres become more uniform in size but with little change in size. It is not difficult to understand The microscopic morphology of the CoFe 2 O 4 /SWCNTs composites was characterized using SEM, and the morphology of the sample under the cobalt salt gradient is shown in Figure 3. The sample is composed of spherical nanoclusters and cylindrical carbon nanotubes, and the nanospheres wrap around the carbon nanotubes to form a necklace-like structure. SEM of Figure 3a-d clearly shows that the number and distribution density of spherical CoFe 2 O 4 clusters obtained by adding different amounts of cobalt salts are significantly different at a carbon nanotube content of 4 mg. With the gradual increase in the cobalt salt content, the number of CoFe 2 O 4 microspheres increases, and the spheres become more uniform in size but with little change in size. It is not difficult to understand that the increase in cobalt salt concentration will produce more CoFe 2 O 4 microsphere nanocrystals and increase the loading capacity of CoFe 2 O 4 microspheres on the surface of single-walled carbon nanotubes, the interface in the material increases, and the necklace-like structures pile up with each other to form a network. It provides the conditions for improving wave absorption performance. The morphology and structure of the CoFe 2 O 4 /SWCNTs composites were further characterized by TEM, and the results are shown in Figure 4. As can be observed from that the increase in cobalt salt concentration will produce more CoFe2O4 microsphere nanocrystals and increase the loading capacity of CoFe2O4 microspheres on the surface of single-walled carbon nanotubes, the interface in the material increases, and the necklacelike structures pile up with each other to form a network. It provides the conditions for improving wave absorption performance.  To further investigate the chemical composition and bonding of the CoFe2O4/SWCNTs composites, XPS characterization was performed, as shown in Figure  5. All binding energies were normalized concerning C 1s at 284.8 eV. It can be clearly observed from the full spectrum in Figure 5a that there are four elements present, Co, Fe, O, and C, respectively. Figure 5b demonstrates the high-resolution Co 2p spectra, where the peaks at 781.2 eV and 796.8 eV are attributed to the Co 2+ 2p3/2 and Co 2+ 2p1/2 spectra,   To further investigate the chemical composition and bonding of the CoFe2O4/SWCNTs composites, XPS characterization was performed, as shown in Figure  5. All binding energies were normalized concerning C 1s at 284.8 eV. It can be clearly observed from the full spectrum in Figure 5a that there are four elements present, Co, Fe, O, and C, respectively. Figure 5b demonstrates the high-resolution Co 2p spectra, where the peaks at 781.2 eV and 796.8 eV are attributed to the Co 2+ 2p3/2 and Co 2+ 2p1/2 spectra, To further investigate the chemical composition and bonding of the CoFe 2 O 4 /SWCNTs composites, XPS characterization was performed, as shown in Figure 5. All binding energies were normalized concerning C 1s at 284.8 eV. It can be clearly observed from the full spectrum in Figure 5a that there are four elements present, Co, Fe, O, and C, respectively. Figure 5b demonstrates the high-resolution Co 2p spectra, where the peaks at 781.2 eV and 796.8 eV are attributed to the Co 2+ 2p 3/2 and Co 2+ 2p 1/2 spectra, respectively [39,40]. The double-peaked signals at about 786.3 eV and 803.2 eV are assigned to the two oscillating satellites. Thus, the high-resolution XPS spectra of Co 2p indicate the presence of Co 2+ ions. The Fe 2p spectra of the CoFe 2 O 4 /SWCNTs composites are shown in Figure 5c. The peaks at 724.4 and 711.2 eV correspond to the binding energies of Fe 3+ 2p 1/2 and Fe 3+ 2p 3/2 , respectively, with the oscillating satellites located at 714.8 eV [41]. The detected Co 2p and Fe 2p photoelectron peaks are consistent with the reported peaks of Co 2+ and Fe 3+ in CoFe 2 O 4 . In Figure 5d, the O 1s spectrum is divided into three parts. The peak at 533.0 eV can be attributed to the adsorbed water. The two peaks located at 531.5 and 530.4 eV are consistent with oxygen in the defect and metal-oxygen bonds, respectively [42]. are shown in Figure 5c. The peaks at 724.4 and 711.2 eV correspond to the binding energies of Fe 3+ 2p1/2 and Fe 3+ 2p3/2, respectively, with the oscillating satellites located at 714.8 eV [41]. The detected Co 2p and Fe 2p photoelectron peaks are consistent with the reported peaks of Co 2+ and Fe 3+ in CoFe2O4. In Figure 5d, the O 1s spectrum is divided into three parts. The peak at 533.0 eV can be attributed to the adsorbed water. The two peaks located at 531.5 and 530.4 eV are consistent with oxygen in the defect and metal-oxygen bonds, respectively [42]. With the above analysis, we believe that the necklace-like CoFe2O4/SWCNTs nanocomposites have been successfully prepared. Next, we compared and analyzed the wave absorption properties of CoFe2O4/SWCNTs composites with different experimental parameters. The wave absorption properties of electromagnetic wave-absorbing materials are mainly determined by the complex permittivity ( ) and complex permeability ( ), with the real part and are related to the ability of the material to store electric field energy and magnetic field energy, respectively, and and are related to the ability of the material to lose electric field energy and magnetic field energy, respectively [43]. Figure 6a  With the above analysis, we believe that the necklace-like CoFe 2 O 4 /SWCNTs nanocomposites have been successfully prepared. Next, we compared and analyzed the wave absorption properties of CoFe 2 O 4 /SWCNTs composites with different experimental parameters. The wave absorption properties of electromagnetic wave-absorbing materials are mainly determined by the complex permittivity (ε r = ε − jε ) and complex permeability (µ r = µ − jµ ), with the real part ε and µ are related to the ability of the material to store electric field energy and magnetic field energy, respectively, and ε and µ are related to the ability of the material to lose electric field energy and magnetic field energy, respectively [43]. Figure 6a-b shows the real and imaginary parts of the dielectric constants of the prepared CoFe 2 O 4 /SWCNTs nanocomposites at cobalt salt gradients in the range of 2-18 GHz. The ε and ε values of the four samples showed a trend of increasing and then gradually decreasing as the content of added cobalt salts increased, indicating that the ε and ε values did not keep getting larger as the content of cobalt salts increased. The higher ε of the Co-2 sample indicates that the sample has better electromagnetic wave storage and polarization ability. The dielectric tangential loss factor (tanδ ε =ε /ε ) indicates the dielectric loss capacity of MAM. The tanδ ε of Co-1 gradually increases from 0.21 to 0.6 at 10.4-12.3 GHz. The tanδ ε of Co-4 varied around 0.3. The values of Co-2 reach a maximum peak of 0.73 at 15.4 GHz, and the values of Co-3 reach a maximum peak of 0.65 at 6 GHz. It is proved that the dielectric loss capacity of Co-2 and Co-3 is stronger than that of Co-4 in the frequency range of 2-18 GHz (Figure 6c). In terms of complex permeability, the real part of Co-1 and Co-4 varies around 1.1, and the real part of Co-2 and Co-3 varies around 1.3 (Figure 6d). For the imaginary part of magnetic permeability, Co-2 and Co-3 show a decrease followed by an increase in the range of 4-8 GHz, while Co-4 shows a regular change around 0.1and Co-1 decreases from 0.02 to −0.26 at 10.4-12.3 GHz (Figure 6e). The values of the magnetic loss tangent factor (tanδ µ =µ /µ ) of all four samples in Figure 6f are below 0.25, and the small values indicate that the magnetic storage capacity and magnetic loss capacity are not strong. Taken together, it shows that the CoFe 2 O 4 /SWCNTs necklace composite has dielectric and magnetic losses, mainly dielectric losses.
indicates the dielectric loss capacity of MAM. The tan of Co-1 gradually increases from 0.21 to 0.6 at 10.4-12.3 GHz. The tan of Co-4 varied around 0.3. The values of Co-2 reach a maximum peak of 0.73 at 15.4 GHz, and the values of Co-3 reach a maximum peak of 0.65 at 6 GHz. It is proved that the dielectric loss capacity of Co-2 and Co-3 is stronger than that of Co-4 in the frequency range of 2-18 GHz (Figure 6c). In terms of complex permeability, the real part of Co-1 and Co-4 varies around 1.1, and the real part of Co-2 and Co-3 varies around 1.3 (Figure 6d). For the imaginary part of magnetic permeability, Co-2 and Co-3 show a decrease followed by an increase in the range of 4-8 GHz, while Co-4 shows a regular change around 0.1and Co-1 decreases from 0.02 to −0.26 at 10.4-12.3 GHz (Figure 6e). The values of the magnetic loss tangent factor (tan = / ) of all four samples in Figure 6f are below 0.25, and the small values indicate that the magnetic storage capacity and magnetic loss capacity are not strong. Taken together, it shows that the CoFe2O4/SWCNTs necklace composite has dielectric and magnetic losses, mainly dielectric losses. According to the Debye theory, ′ and ′′ follow Equation (1) [44], where ∞ and represent the relative permittivity and static permittivity at the high-frequency limit, respectively.
Theoretically, one Cole-Cole semicircle corresponds to one Debye relaxation polarization process. As can be seen from Figure 7, the number of Cole-Cole semicircles for the sample with the least amount of cobalt and iron salt addition in the figure is significantly According to the Debye theory, ε and ε follow Equation (1) [44], where ε ∞ and ε s represent the relative permittivity and static permittivity at the high-frequency limit, respectively.
Theoretically, one Cole-Cole semicircle corresponds to one Debye relaxation polarization process. As can be seen from Figure 7, the number of Cole-Cole semicircles for the sample with the least amount of cobalt and iron salt addition in the figure is significantly higher than the rest of the samples, probably due to the higher number of CoFe 2 O 4 clusters in both samples, which provide more heterogeneous interfaces.
To further explore the absorption performance, we calculated the reflection loss (RL) for different thicknesses in the frequency range from 2 to 18 GHz according to Equations (2) and (3), where Z in is the input impedance of the absorbing material, Z 0 is the impedance in free space, ε r is the complex permittivity, µ r is the complex permeability, f is the frequency of electromagnetic waves in free space, d is the thickness of the absorber, and c is the speed of light in free space [45].

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Coatings 2022, 12, x FOR PEER REVIEW 9 of 16 higher than the rest of the samples, probably due to the higher number of CoFe2O4 clusters in both samples, which provide more heterogeneous interfaces. To further explore the absorption performance, we calculated the reflection loss (RL) for different thicknesses in the frequency range from 2 to 18 GHz according to Eqs. (2) and (3), where is the input impedance of the absorbing material, is the impedance in free space, is the complex permittivity, is the complex permeability, f is the frequency of electromagnetic waves in free space, d is the thickness of the absorber, and c is the speed of light in free space [45].

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(3) The data of the four samples were calculated, and a 1.5 mm coating thickness was selected to carry out. The reflection loss (RL) and impedance matching change with frequency was drawn, as shown in Figure 8.
From Figure 8a, we can see that with a coating thickness of 1.5 mm, the absorption effect of the Co-1 sample is better, and the peak value of reflection loss reaches −42.07 dB at near 14 GHz. With the increase in cobalt salt addition, the value of reflection loss is −42.07 dB, −7.50 dB, −11.27 dB, and −11.81 dB, and the absorption performance appears to decrease at first and then increase. With the increase in CoFe2O4 content, the effective absorption band of the samples shifted to low frequencies, then to high frequencies, when The data of the four samples were calculated, and a 1.5 mm coating thickness was selected to carry out. The reflection loss (RL) and impedance matching change with frequency was drawn, as shown in Figure 8. The results indicate that the CoFe2O4/SWCNTs composites can exhibit excellent microwave absorption performance only when the component ratio is appropriate. The closer the impedance matching Z is to 1, the more the incident wave propagates in the absorbing medium and the better the absorption performance. As can be seen in Figure 8b, Co-1 is closest to 1, so it has the best wave absorption performance. To further visualize the comparative observation of the absorption capacity of the four samples, three-dimensional plots and contour plots of the relationship between reflection loss, frequency, and sample thickness were drawn, as shown in Figure 9. From Figure 8a, we can see that with a coating thickness of 1.5 mm, the absorption effect of the Co-1 sample is better, and the peak value of reflection loss reaches −42.07 dB at near 14 GHz. With the increase in cobalt salt addition, the value of reflection loss is −42.07 dB, −7.50 dB, −11.27 dB, and −11.81 dB, and the absorption performance appears to decrease at first and then increase. With the increase in CoFe 2 O 4 content, the effective absorption band of the samples shifted to low frequencies, then to high frequencies, when the Co-2 sample reached the lowest frequencies. The results indicate that the CoFe 2 O 4 /SWCNTs composites can exhibit excellent microwave absorption performance only when the component ratio is appropriate. The closer the impedance matching Z is to 1, the more the incident wave propagates in the absorbing medium and the better the absorption performance. As can be seen in Figure 8b, Co-1 is closest to 1, so it has the best wave absorption performance.
To further visualize the comparative observation of the absorption capacity of the four samples, three-dimensional plots and contour plots of the relationship between reflection loss, frequency, and sample thickness were drawn, as shown in Figure 9.
As we can see from Figure 9b,f, the Co-2 samples do not exhibit significant microwave absorption properties. None of the RL values reach −10 dB (absorbing 90% of electromagnetic waves) when the range of the thickness of the absorber is 1-5 mm, and the range of frequency of the microwave is 2-18 GHz. When d is 2.09 mm, and f is 4.6 GHz, the minimum RL value of Co-2 is −8.3 dB. As can be seen from Figure 6a-f, Co-2 and Co-3 have extremely similar variations in the parameters of dielectric constant, permeability, dielectric loss factor, and magnetic loss factor. However, since the dielectric constant of Co-3 is lower than that of Co-2, the material impedance matching is improved, and the wave absorption performance is improved. The minimum RL value of Co-3 is −10.52 dB when d is 2.08 mm, and f is 4.9 GHz. The performance of this weak absorption property is consistent with its poorer values of dielectric and magnetic loss factors. In contrast, both Co-1 and Co-4 have good absorption properties. The minimum RL value reaches −42.07 dB (at 14.3 GHz) and EAB of 3.8 GHz (from 12.8 GHz to 16.6 GHz) when the d of Co-1 is 1.50 mm, while when the d of Co-4 is 4.00 mm, the minimum RL value is −19.37 dB and EAB of 0.7 GHz (from 3 GHz to 3.7 GHz). According to Figure 11, the necklace-like CoFe 2 O 4 /SWCNTs composites have good absorption capability and low absorption thickness compared with other reported CoFe 2 O 4 /CNT composites, but the effective absorption bandwidth is narrow, which limits their practical application capability. However, considering the potential of low-density CoFe 2 O 4 /SWCNTs composites in the field of microwave-absorbing materials, this simple and controlled solvothermal method can be extended to composite carbon nanotubes with other ferrites to prepare ferrite/carbon nanotube composites with better microwave absorption properties.
In general, materials with high microwave absorption performance are required to possess low absorber thickness, light mass, wide effective absorption bandwidth, and strong absorption capability. As the requirements for absorbing materials increase, it is clear that a single material cannot meet the requirements for high efficiency, lightweight, and wide efficient absorption bandwidth. In this paper, the samples prepared by simple solvent heat consist of carbon nanotubes as dielectric material and CoFe 2 O 4 nanoparticles as magnetic material. Figure 10 shows the magnetization properties of the cobalt salt gradient samples characterized by VSM at room temperature with a magnetic field of −10,000 Oe: magnetic hysteresis loops of Co-1, Co-2, Co-3, and Co-4. The saturation magnetization (Ms) of Co-1, Co-2, Co-3, and Co-4 were 70.0, 58.1, 63.4 and 73.5 emu/g, respectively. By increasing the addition of cobalt and iron salts, differences in the degree of crystallization of cobalt ferrate particles and changes in intra-grain defects are produced, causing fluctuations in saturation magnetization. As can be seen from the Co-1 and Co-4 samples in Figure 10, Ms can be enhanced by increasing the magnetic phase. From Figure 10, it was found that the Ms of the samples appeared to decrease and then increase with the increase in the cobalt salt addition ratio, but the difference in the value change was not large. From Figure 3, it can be seen that the size of CoFe 2 O 4 microspheres does not vary much, which is in accordance with the literature expression [46], and the Ms, residual magnetization (Mr), and coercivity (Hc) of nanocrystals are only determined by the size.    The microwave absorption performance of CoFe2O4/SWCNTs composites can be attributed to the unique necklace-like structure and the composite loss mechanism of carbon nanotubes and CoFe2O4 particles. For pure carbon nanotubes, only dielectric losses contribute to the energy loss of electromagnetic waves, while for pure CoFe2O4 particles, the effect of magnetic losses exceeds the dielectric losses. This means that in both cases, the magnetic and dielectric losses are not balanced, which leads to poor wave absorption performance. However, for CoFe2O4/SWCNTs nanocomposites, the microwave absorption is improved due to the combination of paramagnetic CNTs and magnetic CoFe2O4 with a better match between dielectric and magnetic losses. Moreover, in addition to defect polarization, dipole polarization, and interfacial polarization, the high impedance matching, and 3D lattice structure allow more electromagnetic waves to enter the absorber ( Figure  12), and the CoFe2O4/SWCNTs composites show a large number of nanoparticles anchored on the surface of carbon nanotubes (Figure 3a-d), which provide multiple   The microwave absorption performance of CoFe 2 O 4 /SWCNTs composites can be attributed to the unique necklace-like structure and the composite loss mechanism of carbon nanotubes and CoFe 2 O 4 particles. For pure carbon nanotubes, only dielectric losses contribute to the energy loss of electromagnetic waves, while for pure CoFe 2 O 4 particles, the effect of magnetic losses exceeds the dielectric losses. This means that in both cases, the magnetic and dielectric losses are not balanced, which leads to poor wave absorption performance. However, for CoFe 2 O 4 /SWCNTs nanocomposites, the microwave absorption is improved due to the combination of paramagnetic CNTs and magnetic CoFe 2 O 4 with a better match between dielectric and magnetic losses. Moreover, in addition to defect polarization, dipole polarization, and interfacial polarization, the high impedance matching, and 3D lattice structure allow more electromagnetic waves to enter the absorber (Figure 12), and the CoFe 2 O 4 /SWCNTs composites show a large number of nanoparticles anchored on the surface of carbon nanotubes (Figure 3a-d), which provide multiple reflections in the conductive network with a structure that can effectively dissipate the incident electromagnetic waves. All these play an important role in improving electromagnetic absorption performance. reflections in the conductive network with a structure that can effectively dissipate the incident electromagnetic waves. All these play an important role in improving electromagnetic absorption performance.

Conclusions
In summary, in this work, necklace-like CoFe2O4/SWCNTs nanocomposites were prepared by the solvothermal method. This special structural design not only effectively solves the problem of poor impedance matching between a single carbon material and a single ferrite material but also can combine dielectric loss and magnetic loss, which can reflect and scatter electromagnetic waves many times to achieve electromagnetic wave energy dissipation, and as long as good impedance matching is achieved when the component ratio is appropriate. The CoFe2O4/SWCNTs nanocomposites exhibit excellent absorption performance: when the additions of FeCl3·6H2O, Co(Ac)2·4H2O, and NH4Ac are 0.432 g, 0.200 g, and 0.400 g, respectively, the samples obtained at this time, the best reflection loss reaches −42.07 dB. This simple microstructure design scheme provides a new idea for the preparation of new wave-absorbing materials.

Conclusions
In summary, in this work, necklace-like CoFe 2 O 4 /SWCNTs nanocomposites were prepared by the solvothermal method. This special structural design not only effectively solves the problem of poor impedance matching between a single carbon material and a single ferrite material but also can combine dielectric loss and magnetic loss, which can reflect and scatter electromagnetic waves many times to achieve electromagnetic wave energy dissipation, and as long as good impedance matching is achieved when the component ratio is appropriate. The CoFe 2 O 4 /SWCNTs nanocomposites exhibit excellent absorption performance: when the additions of FeCl 3 ·6H 2 O, Co(Ac) 2 ·4H 2 O, and NH 4 Ac are 0.432 g, 0.200 g, and 0.400 g, respectively, the samples obtained at this time, the best reflection loss reaches −42.07 dB. This simple microstructure design scheme provides a new idea for the preparation of new wave-absorbing materials.

Conflicts of Interest:
The authors declare no conflict of interest.