High-efficiency microwave absorption performance of cobalt ferrite microspheres/multi-walled carbon nanotube composites

In this article, spinel ferrite CoFe2O4 and multi-walled carbon nanotubes (MWCNTs) composites are constructed by a facile one-step solvothermal method. The pure phase of CoFe2O4 particles is confirmed by X-ray diffraction patterns. Microstructure analysis demonstrates that monodisperse CoFe2O4 microspheres are wound by MWCNTs. With the introduction of CNTs, there is a significant enhancement in the imaginary part of permittivity (ε″) with the composites. The champion microwave absorption performance can be achieved in the composites by the balance of complex permittivity and permeability. When the mass fraction of CNTs is 3%, a minimum reflection loss (RLmin) of the composites is as high as − 46.65 dB at 14.4 GHz at a thin thickness of 1.5 mm, and the corresponding effective absorption bandwidth below − 10 dB reaches 4.91 GHz ranging from 12.41 to 17.32 GHz which covers almost the whole Ku band (12.0–18.0 GHz). In other words, these as-synthesized composites show the most outstanding specific RLmin of − 31.1 dB mm−1. Such superior microwave absorption behaviors of CoFe2O4/CNTs originate mainly from multiple dielectric relaxation processes, enhanced impedance matching and magnetic loss, as well as the considerable interface between mesoporous CoFe2O4 hollow microspheres and CNTs, thereby promoting microwave reflection and scattering within the samples. Our results indicate that as-fabricated CoFe2O4/CNTs composites can be a promising microwave absorbent integrating with a thin thickness, strong absorption ability, and broad bandwidth absorption.


Introduction
With the widespread use of electronic devices and wireless communication, electromagnetic interference has become a serious problem in polluting the communication environment and harming human health [1][2][3]. To tackle the adverse problems, we are facing that high-performance microwave absorbers are urgently demanded, which can convert most of the electromagnetic energy onto the surface into thermal or other forms of energy [4,5]. By and large, high-efficiency microwave absorbers are required to fulfill the four characteristics of wide bandwidth, strong absorption, low thickness, as well as light mass. If applied in the field of high temperature, it often also needs to possess high-temperature stability. As we now know, the outstanding microwave absorption properties of materials are highly associated with two aspects of impedance matching and attenuation characteristics, which can actually be achieved through modulating the electromagnetic parameters of the absorbers.
Over the past several decades, enormous efforts have been devoted to the investigation of several magnetic absorbers with high magnetic loss, especial for transition metal oxides. Among these transition metal oxides, the spinel ferrite of CoFe 2 O 4 has been extensively carried out as microwave absorbers in terms of its moderate saturation magnetization, strong anisotropy, high chemical stability, excellent oxidation resistance, and corrosion resistance, as well as low cost [6][7][8][9]. Nevertheless, as the microwave absorbers, the disadvantage is that the specific gravity and narrow frequency bandwidth along with bulk CoFe 2 O 4 always limit its actual application. As previously reported [10][11][12], a large amount of research work has been done to optimize microwave absorption performances by regulating the morphology of CoFe 2 O 4 to fiber, rugby, particle, or some other shapes, whereas it is extremely difficult to achieve outstanding microwave absorption performances using only CoFe 2 O 4 as a sole component material. In contrast, it is noteworthy that the most researchers tend to pay close attention to such composites composed of CoFe 2 O 4 -based magnetic/dielectric materials, like carbon materials, MoS 2 , BaTiO 3 , and so on, rather than bulk CoFe 2 O 4 alone [6][7][8][13][14][15]. As the representative dielectric absorbers, carbon materials with high dielectric loss have attracted growing attention in terms of their high electric conductivity, considerable specific surface, and low density. Unfortunately, since the real and imaginary parts of complex permittivity of carbon materials are too large to match with the impedance of free space owing to the pretty strong conductivity and low magnetic loss, arousing electromagnetic waves cannot enter into the absorber interior effectively and reflect on its surface, ultimately weakening the microwave absorption performance. Therefore, it is absolutely essential to incorporate magnetic materials into the carbon materials to balance the real and imaginary parts of the complex permittivity and permeability, and optimize their impedance matching and meanwhile improve the attenuation ability. In light of this, considerable investigations have been carried out to fill carbon materials into CoFe 2 O 4 magnetic particles. For instance, Wang et al. have obtained the CoFe 2 O 4 @graphene by a spray-drying method. The flower-like structure endows it outstanding microwave absorption characteristics with a minimum reflection loss (RL min ) of -42 dB at 12.9 GHz with a layer thickness of 2 mm, and the corresponding effective absorption frequency less than -10 dB of 4.59 GHz [6]. A previous study reported that CoFe 2 O 4 could be filled into porous C, in which carbon material is obtained origin from the eggshell membrane [8]. The enhancement of microwave absorption properties is ascribed to the strong synergetic effect between CoFe 2 O 4 with high magnetic loss and porous carbon with large dielectric loss. Zhang et al. have constructed CoFe 2 O 4 /CNTs composites with core/shell structure as absorbers, from which the RL min is -32.8 dB at 11.7 GHz with 2 mm [16]. However, so far as we know, CoFe 2 O 4 -based composites still cannot achieve remarkable reflection loss values under a relatively thin thickness, namely large specific RL min , as wide bandwidth as possible because of their limited dielectric loss factor. Inspired by these thoughts mentioned above, and meanwhile considering carbon nanotubes with unique structure, light weight, and large dielectric loss, in this work, the composites of CoFe 2 O 4 / MWCNTs with different loading ratios of CNTs were fabricated through a facile solvothermal method. The effect of the loading concentration of CNTs on microstructure, electromagnetic parameters, and microwave absorption properties of the composites was investigated systematically. The probable mechanism of the enhancing microwave absorption performance of the composite was discussed in detail.

Characterization
The phase analysis of all as-prepared samples was characterized by powder X-ray diffraction (XRD) of Cu Ka radiation source. The microstructure of asobtained materials was examined by field emission scanning electron microscopy (FESEM, SU8220) and transmission electron microscope (TEM, JEOL JEM-2100). N 2 adsorption-desorption isotherm and Barrett-Joyner-Halenda (BJH) methods were conducted on a Micromeritics ASAP 2460 analyzer at 77 K. The vector network analyzer (VNA, AV3629D) was carried out to record the room-temperature electromagnetic parameters of samples in the measured frequency range of 2.0-18.0 GHz, where the composites were uniformly mixed with 50 wt% paraffin and then compressed into a cylindrical toroid with an inner diameter of 3.04 mm, an outer diameter of 7.00 mm and a thickness of 2.00 mm. In order to reduce or eliminate the errors caused by source matching, load matching, directivity, isolation, and frequency response in microwave measurement, the full dualport calibration was carried out before measurement, and the reflection loss of samples was simulated by the transmission line theory.

Results and discussion
The crystalline phase of as-prepared CoFe 2 O 4 , CoFe 2 O 4 /CNTs and acid-treated MWCNTs samples is examined by XRD patterns, as shown in Fig. 1.   [17]. Figure 2 exhibits the surface morphology and microstructure of CFO/CNTs with different loading ratios of CNTs. By observing some magnified CFO spheres from Fig. 2a, it is found that these spheres with rough surfaces are formed by the accumulation of some nanoparticles, and no obvious disassociation of CFO is observed. The size distribution of CFO/ CNTs particles is estimated by taking the average of 200 particles and fitting the resultant histogram by a Gaussian function. As observed by the insets of Fig. 2a-d, the average particle size of the composites is decreased significantly from 0.25 to 0.17 lm with increasing CNTs content. Some significant disassociation of CFO is recorded in the composites, possibly because acid-treated MWCNTs provide adequate active sites for the deposition of Co 2? and Fe 3? ions, leading to their redistribution during the reaction. Furthermore, some occasionally broken microspheres have been observed towards all as-obtained samples, reflecting the formation of the mesoporous hollow structure. The construction of mesoporous hollow spheres can be understood by the typical Ostwald ripening process [18]. Undoubtedly, a kind of material with a mesoporous hollow structure can possess a larger surface, which can be regarded as the active sites promoting interfacial polarization. Moreover, the large surface can prolong the propagation path of electromagnetic waves, which is beneficial for electromagnetic energy loss because of microwave reflection and scattering. Through observing these composites, CNTs are uniformly wound on the surface of CFO microspheres accompanied with well dispersibility, indicating that CNTs and CFO are perfectly assembled, and meanwhile the agglomeration of CNTs is retarded effectively. To further confirm the mesoporous hollow structure of CFO, a few representative TEM images of composites (CFO/3% CNTs) are displayed in Fig. 2e and f. It can be examined that CFO microspheres assembled by the smaller nanocrystals are connected to the reticular CNTs, in which the transparent area in the middle is inconsistent with the surrounding color, confirming the hollow interior structure in accordance with the above SEM analysis. It is found that the size of the hollow interior chamber in these microspheres is about 65 nm. As marked in Fig. 2f, the clear lattice fringes of grains determined by the high-resolution TEM (HRTEM) images indicate that CFO is of high crystallinity, where the interplanar distance is 0.252 nm matched with (113) crystallographic plane of spinel ferrite CoFe 2 O 4 . Figure 3 exhibits the N 2 adsorption-desorption isotherms and pore size distribution of as-prepared CFO and CFO/3% CNTs specimens. These isotherms are identified as IV type, which is features of mesoporous structure materials according to the IUPAC classification [19]. These isotherms have a drastic increase at the high P/P 0 of 0.8-1.0, indicating the considerable formation of mesopores due to their hollow interior [20]. As illustrated by the inset of Fig. 3, the pore size distribution of these samples displays typical mesoporous characteristics, where the pore diameter is concentrated at about 60 nm close to the size of hollow chamber observed by TEM images. The BET surface area is 9.0 m 2 g -1 and 15.4 m 2 g -1 for CFO and CFO/ 3% CNTs, respectively. As-prepared samples fail to exhibit a large surface area as expected, which can be explained by the fact that the shells of CoFe 2 O 4 spheres are so dense that N 2 molecules cannot enter the inner chamber effectively. The single-point adsorption total pore volume of pores is enhanced due to the introduction of CNTs from 0.45 to 0.67 cm 3 g -1 . The high void volume can contribute significantly to repeated scattering and reflection of microwave, promoting the propagation path within the absorbers to gain further attenuation.
It is well known that electromagnetic absorption performances of materials are highly related to two crucial factors of impedance matching and attenuation ability. For as-obtained magnetic/dielectric materials, electromagnetic parameters of the composites can be adjusted effectively by varying the loading ratio of CNTs to achieve better impedance matching and meanwhile arouse properly large attenuation constant. Figure 4 shows the electromagnetic parameters of the complex permittivity (e r = e 0 -je 00 ) and permeability (l r = l 0 -jl 00 ) of all as-synthesized samples, where e 0 and l 0 stand for the storage capability of electromagnetic wave energy, and e 00 and l 00 represent the energy loss capability. As revealed by Fig. 4a, the as-obtained samples, especially composites with a high additive amount of CNTs, have a downward trend within the entire frequency range of 2-18 GHz. This can be explained as the fact that the polarization rotation does not have enough time to respond to the variation at the highfrequency electric field, resulting in the hysteresis phenomenon. Clearly, the e 0 and e 00 values of composites are obviously higher compared with those of pristine CFO. Furthermore, the e 0 and e 00 values of composites go up with increasing CNTs, which may be due to the good electrical conductivity of carbon materials. When the CNTs is 3 wt%, the e 0 reaches a moderate value of about 12.5. As verified previously, the sample is most likely to exhibit the excellent microwave absorption capacity with the magnitude of e 0 in the range of 10-20 [21]. However, there has been a remarkable increase in the e 0 and e 00 as CNT content further increases to 5 wt%, which is not conducive to the balance of complex permittivity  [7]. Since CFO/CNTs composites with a high content of CNTs can present a high electrical conductivity, which means its resistivity is low corresponding to a high e 00 . Therefore, it can be inferred that a suitable permittivity can be achieved which is favorable for obtaining the outstanding microwave absorption performances by adjusting the amount of CNTs. But too high permittivity can cause the electromagnetic wave to reflect on the surface at the air-absorber interface, which is not conducive to electromagnetic wave absorption. Additionally, the introduction of CNTs can increase interfaces and polarization charges on the composites. The increasing interfacial polarization and associated relaxation are beneficial for the increase of dielectric loss. The dielectric loss tangent tand e of composites is exhibited in Fig. 4e, keeping an upward tendency in 2-18 GHz, which can be caused by varying polarization relaxation mechanisms due to the presence of carbon materials. The Cole-Cole semicircles derived from the plots of e 0 versus e 00 represent Debye relaxation process to confirm the polarization relaxation behaviors. On the basis of classical Debye theory, the relationship between e 0 and e 00 may be described below [22]: where e s and e ? represent the static dielectric constant and dielectric constant at the infinite frequency, respectively. Generally, a single semicircle denoted as Cole-Cole symbolizes one Debye polarization relaxation process. Fig. 5 shows relation curves of e 0 versus e 00 of pure CFO and CFO/CNTs composites in 2-18 GHz. From Fig. 5a, the Cole-Cole plot of pure CoFe 2 O 4 exhibits a completely disordered behavior, i.e., no obvious dielectric relaxation process, indicating that interface polarization contributes little to dielectric loss. These similar results have been also observed in Fe 3 O 4 microspheres [23]. Therefore, with the gradual increase in CNTs, some relative arcs from Cole-Cole curves appear, confirming the existence of multiple dielectric relaxation processes along with the three samples [7]. The multiple relaxation processes are basically caused by the defect polarization, functional groups, special structure of porous CoFe 2 O 4 hollow spheres, and multiple interfaces between CoFe 2 O 4 and CNTs where a considerable amount of space charges accumulates at the heterogeneous interface, improving the dielectric loss of composites.
As observed by Fig. 4c and d, l 0 and l 00 plots of the four samples are partially overlapped and accompanied by several formants which suggest that the addition of CNTs with a small amount does not contribute much to the magnetic loss of CFO. In particular, l 0 value of all samples slightly goes down from 1.6 to 1.0 between 2.0 and 10.5 GHz and then fluctuates around approximately 1.2 with the increasing frequency and meanwhile l 00 varies with some slight fluctuations within the entire frequency range. Interestingly enough, the l 00 exhibits negative values at the high frequency, which be explained by the fact that EM energy is radiated out from CFO/ CNTs composites due to the motion of charges, and this similar phenomena have been also reported in other work [24]. The magnetic loss tangent tand m value of pure CoFe 2 O 4 is mostly higher than that of the other three composites, as exhibited by Fig. 4f, indicating its strong magnetic loss ability. From the composites, the decrease in tand m value is ascribed to the non-magnetism of CNTs. As a rule, the magnetic loss is generally associated with the exchange resonance, natural resonance as well as eddy current loss over the microwave frequency range. Some obvious resonance peaks are found in the curves of tand m , which can be ascribed to the exchange resonance and natural resonance. In addition, the eddy current effect is defined as follows: where l 0 and r are the permeability of vacuum and the electric conductivity of the composites, respectively. When the magnetic loss is only brought about by the eddy current loss, C 0 should maintain a constant within the measured frequency range. As depicted in Fig. 6, C 0 is almost constant at the high frequency range of 12-18 GHz, indicating eddy current effect plays a dominant role in the magnetic loss.
Consequently, it turns out that magnetic loss is mainly brought out by resonance effect and eddy current loss in the corresponding frequency range of 2-12 GHz and 12-18 GHz, respectively. In comparison, the value of tand m is far lower than that of tand e in most frequency, which demonstrates that the microwave absorption ability of these composites is primarily determined by dielectric loss in most cases. Usually, the reflection loss (RL) is carried out to estimate the intensity of microwave absorption. The commercial standards require that RL value should be lower than -10 dB, defined as the effective absorption bandwidth, within the range of microwave frequency, which means that 90% of EM waves can be absorbed and attenuated. According to the transmission line theory, the RL values can be calculated as follows [17]: where Z in is the input impedance of the absorber, Z 0 is the impedance of free space, l r and e r are the relative permeability and permittivity, respectively, f is the frequency of microwaves, d is the thickness of the absorber, and c is the velocity of light. Figure 7 shows the calculated results of RL of the CFO/CNTs composites with different loading ratios of CNTs at different thicknesses within the whole frequency range. For pure CoFe 2 O 4 shown in Fig. 7a, nearly all RL values are above -10 dB by changing the thickness of 1-5 mm over the entire frequency range. This displays that the microwave absorption ability of sole CoFe 2 O 4 magnetic materials is unsatisfactory. The good news is that the microwave absorption properties are enhanced by adding a certain amount of CNTs into the CoFe 2 O 4 . By introducing a weight ratio of 1% CNTs, the composites possess an optimal absorption peak of about -28.07 dB at 8.09 GHz, and the corresponding effective absorption bandwidth reaches 4 GHz from 7.03 to 11.03 GHz with the thickness of 2.8 mm, as exhibited by Fig. 7b. When CNTs continues to increase to 3 wt%, the superior microwave absorption performances (shown in Fig. 7c) Table 1.
In contrast, CoFe 2 O 4 /CNTs composites in the present work possess distinct advantages by overall consideration of specific RL min and effective absorption bandwidth. The as-obtained CFO/3% CNTs show the highest specific RL min of -31.1 dB mm -1 and a relatively wide effective absorption bandwidth. As expected, the absorption peaks of the four samples move toward low frequency with the increasing thickness of 1-5 mm. This phenomenon can be related to the electromagnetic cancellation effect, which is described as below: where t m is the thickness of absorber, k is the wavelength of the electromagnetic wave, c is the light velocity, f is the microwave frequency, and l r and e r are complex permeability and permittivity, respectively. As shown in Fig. 8, almost all experimental points marked by the heart type at the peak frequency of RL min happen to be at the location matched with the simulated thickness on the frequency at n = 1 of CFO/3% CNTs, which indicates that the corresponding frequency with RL min peak mainly originates from the quarter-wavelength cancelation model. As analyzed by the section mentioned above, the microwave absorption performance of composites can be greatly optimized by adjusting the composition ratio of CNTs to CoFe 2 O 4 materials. Compared with pure CoFe 2 O 4 , the electromagnetic wave can enter into the composites and be dissipated effectively. The detailed mechanisms of the enhanced microwave absorption performance can be discussed based on the impedance matching and attenuation characteristics as follows. According to the propagation mechanism of the absorbers, when an electromagnetic wave is projected onto the surface of the absorbing layer, it should firstly fulfill the requirement that the electromagnetic wave can enter into the interior of the layer to the maximum extent. In view of this, it is required that the absorbing materials should possess a superior impedance matching. According to the impedance matching principle, when the value of |Z in /Z 0 | is equal to or close to 1, no reflection of incident microwaves occurs at the airabsorber interface, thereby indicating the impedance matching is better. After the incident electromagnetic wave enters the interior of the medium without reflection, electromagnetic energy will be consumed by the absorbent as much as possible. The loss modes of the waves mostly cover dielectric loss, magnetic loss, as well as physical loss caused by the structure of the materials, like multiple scattering and reflection. In light of this mentioned above, the champion microwave absorption capability of the materials can be achieved by obtaining satisfactory impedance matching and attenuation loss characteristics.