⇦Chinese Physics Letters, 2023, Vol. 40, No. 11, Article code 118101 Magneto-Orientated Graphite Double-Layer Homo-Structure with Broadband Microwave Absorption Jun-Song Wang (王俊松)1,2, Wei Ding (丁伟)3*, Cheng-Hong Zhang (张成宏)1,2, Kang Qiu (邱康)1,3, You-Lin Gao (高佑琳)1,3, Mian-Ke Chen (陈棉科)1,3, Muhammad Adnan Aslam4, Mahmoud A. Khalifa1, Jia-Liang Luo (罗家亮)1, Jun Fang (方军)1*, and Zhi-Gao Sheng (盛志高)1* Affiliations 1High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China 2University of Science and Technology of China, Hefei 230026, China 3Institutes of Physical Science and Information Technology, Anhui University, Hefei 230601, China 4Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China Received 27 August 2023; accepted manuscript online 17 October 2023; published online 13 November 2023 *Corresponding authors. Email: weiding@mail.ustc.edu.cn; jfang@ipp.ac.cn; zhigaosheng@hmfl.ac.cn Citation Text: Wang J S, Ding W, Zhang C H et al. 2023 Chin. Phys. Lett. 40 118101    Abstract We utilized magnetic fields as an efficient tool to manipulate the orientation and electromagnetic properties of graphite micro-flakes (GMFs). As a result, we successfully developed a GMF double-layer homo-structure, which shows excellent electromagnetic absorption properties. By tuning the direction of a small magnetic field (850 G), vertical and horizontal aligned GMFs are produced. Their electromagnetic parameters are effectively tailored by this magneto-orientation effect, and the vertical and horizontal aligned GMFs achieve good results in terms of impedance matching and microwave absorption. With the combination of these two magneto-orientated layers, vertically oriented as the surficial impedance matching layer and horizontally oriented as the inner loss layer, we design a GMF-based double-layer homo-structure. After thickness optimization, $-38.2$ dB minimum reflection loss and 6.4 GHz (11.6–18.0 GHz) absorption bandwidth are achieved. Our findings further emphasize the importance of material orientation freedom and provide a magneto-strategy to design multiple-layer structures and to produce high-performance microwave devices.
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 DOI:10.1088/0256-307X/40/11/118101 © 2023 Chinese Physics Society Article Text With the rapid development of electronic devices and wireless communication, electromagnetic (EM) wave pollution has become a serious concern. Thus, microwave absorption and EM shielding have gradually become a research focus.[1-3] To achieve materials with excellent EM absorption, both surficial impedance matching and inner EM loss are required.[4-6] In this regard, double-layer microwave absorbers, which are formed by an incorporating impedance matching layer and an EM wave loss layer, have been widely investigated recently.[7-11] For instance, by assembling a two-layer absorber in which Fe$_{3}$O$_{4}$/reduced graphene oxide composite is placed at the upper layer and Fe$_{3}$O$_{4}$/carbon fiber composite is placed at the bottom layer, Gang et al.[12] observed a significant enhancement of microwave absorption properties. Such good absorption performance benefits from the synergistic effect between the matching layer and the absorbing layer. Different functional layers have different requirements.[13,14] Compared with the EM loss layer, the impedance matching layer needs low permittivity ($\sim$ 1), which allows the EM wave to transmit with minimal reflection. In general, the performance of the material is tuned by changing the composition of the material. Recently, it was found that, in addition to the composition, the morphological orientation could also tailor the EM parameters of materials.[15] For instance, Wang et al.[16] demonstrated that horizontally oriented Ni/SiO$_{2}$/polyaniline hexagonal nanoflakes display a $\sim$ 25% increment of the real part of complex permittivity $\varepsilon'$, while the vertically oriented one has a decrement of $\sim$ 10%. Taking this orientation effect into account, it becomes possible to design and fabricate a double-layer EM absorber with a homo-structure, in which the impedance matching layer and the absorbing layer are prepared from the same material with different orientations. To the best of our knowledge, such a double-layer EM absorber with a homostructure has not yet been realized before. In this work, by utilizing the magnetic field as an efficient tool, we successfully manipulated the orientation of graphite micro-flakes (GMFs) and realized a GMF double-layer homo-structure absorber for the first time, which shows excellent EM absorption properties. By changing the orientation of an 850 G magnetic field, the GMFs are aligned into desired directions. At the same time, the permittivity of GMFs with different orientations is regulated, i.e., the vertically oriented GMFs have a lower $\varepsilon'$ while the horizontal ones have a larger $\varepsilon'$. Accordingly, a double-layer homo-structure is fabricated with vertically oriented GMFs as the impedance matching layer and horizontally oriented GMFs as the EM loss layer. It is found that the designed double-layer homo-structure shows a remarkable enhancement of microwave absorption. A minimal reflection loss (RL) of $-38.2$ dB is achieved and the effective bandwidth is broadened up to 6.4 GHz (11.6–18.0 GHz), covering the Ku band. The GMFs studied here were fabricated by the N-methyl pyrrolidone (NMP)-assisted ball-milling method from the commercial graphite flakes (GFs).[17] Figures 1(a) and 1(b) depict the size and shape of samples before and after mechanical milling. They show that ball milling can exfoliate commercial GF with a diameter of 300 µm into uniform GMFs with a diameter of 20–30 µm and a thickness of less than 1 µm. The exfoliation results were verified by the Raman spectra (Horiba Jobin Yvon T64000). As shown in Fig. 1(c), the intensity ratio ($I_{\rm D}$/$I_{\rm G})$ of the D (peak at 1360 cm$^{-1})$ and G (peak at 1580 cm$^{-1})$ bands decreased from 0.207 to 0.115.[18,19]
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Fig. 1. [(a), (b)] SEM image. (c) Raman spectra of the commercial GF and GMFs. Schematic images of the GMF/EP composites treated in (d) zero magnetic field, (e) vertical magnetic field, (f) horizontal rotating magnetic field, and their SEM images from vertical section (g)–(i).
As a typical diamagnetic material (see Fig. S1 in the Supplementary Information), the GMFs are possibly manipulated by the external magnetic field due to the large magnetic anisotropic energy.[20,21] In order to obtain GMF thin films with desired orientations, we conducted a magnetic field orientation experiment via home-made magnetic field treatment equipment (Fig. S2). The GMFs (10 wt%) were stirred at first with resin/hardener (mass ratio of 1 : 1) into a slurry and then cured in silicone molds in magnetic fields for 6 h. Then the composites were placed in an oven at 60 ℃ for 10 h to produce GMF/epoxy (EP) magneto-orientated composites (Fig. S3). The obtained three single-layer samples were denoted as SM$_{0}$ (sample treated in the absence of a field), SM$_{\rm V}$ (sample treated in a vertical magnetic field) and SM$_{\rm R}$ (sample treated in a horizontal rotating magnetic field). The scanning electron microscopy (SEM) images taken from the vertical section (side view) of these three samples are shown in Figs. 1(g)–1(i), respectively, and the orientation statistical diagram of differently oriented samples is provided in Fig. S5. It is obvious that the GMFs in SM$_{0}$ are randomly distributed in the matrix, while most of the GMFs in SM$_{\rm V}$ and SM$_{\rm R}$ are aligned in vertical and horizontal directions, respectively. In comparison, the magneto-alignment effect is more significant for the GMFs in the SM$_{\rm R}$ aligned by the horizontal rotating magnetic field (Fig. S6). Schematic images of the magnetic orientations are shown in Figs. 1(d)–1(f). Resin viscosity is an important parameter for the orientation of graphite flakes under a magnetic field. During the experiment, we optimized the viscosity of the samples by adjusting the ratio between the EP resin and the curing agent and found that a ratio of EP resin and EP hardener (1 : 1) is the best. The observed magneto-orientated effect in GMFs can be understood through the Landau diamagnetism torque.[22] Within an external nonzero-gradient magnetic field, the GMFs possessed a potential energy $U \sim B^{2}\sin^{2}\theta/\mu_{\rm eff}$, in which $\mu_{\rm eff}$ is the effective mass of the $\pi$ electron in graphene, $B$ is the strength of external magnetic fields, and $\theta$ is the angle formed between the GMF basal plane and the external magnetic field. When $\theta=0$, this potential energy reaches its minimum, so that GMFs tend to align with the external magnetic field and stabilize according to the principle of minimum potential energy (Fig. S7). At the same time, the GMFs would feel a torque as ${\boldsymbol L}\sim{\boldsymbol B}\times \mathrm{\nabla}B/\mu_{\rm eff}$. This indicates that the larger the $\nabla B$ becomes, the greater the $L$ will be. In our experiments, the rotation of the magnetic field can further generate a two-dimensional $\nabla B$ distribution in the $x$–$y$ plane, which will help us to align the GMFs. Then, the SM$_{\rm R}$ sample should have the most significant orientation degree, which will be further verified in the following. For the characterize orientation order of GMFs, polarized Raman spectroscopy has been proven to be a powerful tool.[4] Considering a single GMF, the Raman scattering allows for in-plane polarization of the incoming and scattered light but it is forbidden in the perpendicular polarization direction.[23] In our Raman experiment results, as shown in Fig. 2(a), the intensity of the G-peak has a maximum when the polarization of the light is along the in-plane axis of the GMFs (SM$_{\rm R}$ sample) and a minimum when the polarization is normal to the GMFs plane (SM$_{\rm V}$ sample). This qualitatively verifies the preferred orientation direction of the GMFs in the EP matrix as observed in SEM. That is, SM$_{\rm R}$ is well horizontally aligned and SM$_{\rm V}$ is vertically aligned. When most GMFs are aligned, the oriented samples' x-ray diffraction (XRD) would show single crystal-like behavior. For horizontally oriented GMFs, its XRD pattern would be dominated by (002) and (004) peaks, while vertically oriented GMFs would mainly possess (100) and (110) peaks, as schematically illustrated in Fig. 2(b). To confirm the magneto-orientated effect on GMFs, XRD analysis in horizontal section and vertical section was carried out [Fig. 2(c) and Fig. S8].[11] It shows that SM$_{0}$ has four diffraction peaks, SM$_{\rm V}$ has two significant peaks [(100) and (110)], while SM$_{\rm R}$ has two significant peaks [(002) and (004)], which are consistent with the predictions. The degree of horizontal alignment of the GMFs was quantified by comparing the relative intensities ($I$) of the (002) and (004) peaks in the horizontal section to the sum of the relative intensities of the (002), (004), (100) and (110) peaks as $\delta=[(I_{002}+I_{004})/(I_{100}+I_{110}+I_{002}+I_{004})]\times 100{\%}$. The $\delta$ values of SM$_{\rm R}$, SM$_{0}$ and SM$_{\rm V}$ are 94.99%, 69.17% and 19.16%, respectively. Compared with the SM$_{0}$ sample, the 1.4 times larger value of $\delta$ indicates that most of the GMFs have been magneto-orientated into the horizontal direction. Similarly, the small $\delta$ of SM$_{\rm V}$ confirmed further that the magnetic field efficiently aligned GMFs into the vertical direction.
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Fig. 2. (a) Polarization Raman spectra of SM$_{\rm V}$, SM$_{0}$ and SM$_{\rm R}$. (b) Illustration of the orientation effect of GMFs on XRD pattern. (c) XRD patterns of pure EP, SM$_{\rm V}$, SM$_{0}$ and SM$_{\rm R}$ in horizontal-section. (d)–(f) Two-dimensional SAXS patterns and azimuthal dependence of scattering intensity of SM$_{0}$ and SM$_{\rm R}$.
To further quantify the orientation degree of the magneto-orientated GMFs, 2D small-angle x-ray scattering (SAXS) was utilized (BL16B1 stations of Shanghai Synchrotron Radiation Facility). Figures 2(d)–2(f) show 2D SAXS patterns, as well as the azimuthal SAXS intensity of SM$_{0}$ and SM$_{\rm R}$. In a horizontal rotating magnetic field, a large eccentricity elliptical diffusive pattern can be seen in the 2D SAXS pattern [Fig. 2(e)], qualitatively showing a higher degree of alignment in SM$_{\rm R}$. This effect can also be clearly seen in the narrowing of the peaks of the azimuthal intensity [Fig. 2(f)]. The degree of alignment of the GMFs was quantified by correlating the azimuthal dependence of the scattered intensity to an orientational distribution coefficient named the orientation orders ($f$).[24] A perfect orientation corresponds to $f=1$, whereas a completely random orientation has $f=0.29$. The orientation order of SM$_{\rm R}$ is 0.83 ($f=0.83$), which further proves that SM$_{\rm R}$ has very good horizontal alignment. After obtaining magneto-orientated GMFs, it is expected that the materials with orientational order will have completely different EM properties than those with a disordered arrangement.[19] The complex permittivity and permeability of the prepared GMF/EP composite samples were measured by a vector network analyzer (VNA, MS4647B) employing the coaxial method in the range of 2–18 GHz. The measurement setups and basic calculations of permittivity can be found in Fig. S9. Figures 3(a) and 3(b) illustrate the frequency dependences of the real part ($\varepsilon '$) and the imaginary part ($\varepsilon ''$) of the complex permittivity for the three samples, respectively. It shows clearly that both $\varepsilon '$ and $\varepsilon ''$ are closely related to the orientation of the samples and their values follow the relationship SM$_{\rm R} > {\rm SM_{0}} >{\rm SM}_{\rm V}$. Specifically, the $\varepsilon '$ value of SM$_{\rm R}$ is about 2 times that of SM$_{\rm V}$ and the $\varepsilon ''$ value of SM$_{\rm R}$ is about 2.5 times that of SM$_{\rm V}$. Figure S10 in the Supplementary Information illustrates the frequency-dependent complex permeability of the SM$_{0}$, SM$_{\rm V}$ and SM$_{\rm R}$ samples in 2–18 GHz, and the $\mu '$ and $\mu ''$ values were $\sim $ 1 and $\sim $ 0, respectively. The results indicate that, in our material, the magnetic loss is very weak and the dielectric loss serves as the key factor to determine the electromagnetic losses.[25-30]
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Fig. 3. (a) Real part of complex permittivity ($\varepsilon '$). (b) Imaginary part of complex permittivity ($\varepsilon''$). (c) Three-dimensional RL plots. (d) Dependence of $\mid$$Z_{\rm in}$/$Z_{0}$$\mid$ values on the frequency for samples of SM$_{0}$, SM$_{\rm V}$, and SM$_{\rm R}$, respectively.
It is worth noting at this point that the magnetic field intensity effect on the observed magneto-alignment GMFs has been examined (Fig. S11). The optimized magnetic field intensity of 850 G is found. Figure 3(c) shows the RL of the GMF/EP composites with a 10 wt% filling ratio in the frequency range of 2–18 GHz. Similar to the complex permittivity, the microwave absorption properties of SM$_{0}$ and SM$_{\rm R}$ are similar, and both have a poor absorption coefficient (RL$_{\min} \sim -7.5$ dB). Otherwise, the microwave absorption performance of the SM$_{\rm R}$ sample made with a slow horizontal rotating magnetic field [850 G, 0.5 r/h] is significantly improved. Its RL$_{\min}$ reaches $-23.2$ dB at 4.8 GHz and the effective absorption bandwidth is 2.5 GHz (14–16.5 GHz). An interesting phenomenon related to the effect of magneto-alignment on EM parameters occurs in terms of input impedance ($Z_{\rm in})$. Figure 3(d) shows the frequency dependence of the three samples' relative input impedance $\mid$$Z_{\rm in}$/$Z_{0}$$\mid$. It is found that the $\mid$$Z_{\rm in}$/$Z_{0}$$\mid$ value of SM$_{\rm R}$ composites is quite large and it reaches a peak value $\sim$ 3.6 at 14.1 GHz, while the $\mid$$Z_{\rm in}$/$Z_{0}$$\mid$ of SM$_{\rm V}$ composites shows a slight fluctuation around 1. It is known that such $\mid$$Z_{\rm in}$/$Z_{0}$$\mid\,\sim$ 1 indicates that the propagation of EM waves at the air/SM$_{\rm V}$ interface has perfect impedance matching.[7] In other words, EM waves can pass through the air/SM$_{\rm V}$ interface almost without loss. From the magneto-alignment effects on the EM parameters mentioned above, it is easy to find that the magneto-orientated SM$_{\rm V}$ has good impedance matching performance. In contrast, the magneto-orientated SM$_{\rm R}$ has the best microwave absorption performance. It can be imagined that if we use the SM$_{\rm V}$ as the upper impedance matching layer to efficiently introduce EM waves and use the SM$_{\rm R}$ as the bottom absorbing layer to absorb EM waves effectively, such magneto-orientated double-layer structure constructed in this way could achieve high EM waves-absorbing performance. Accordingly, we designed a homo-structure double-layer GMF absorber using the magneto-alignment technique, as shown in Fig. 4(a). In this homo-structure, SM$_{\rm V}$ is on the top and acts mainly as a matching layer, while SM$_{\rm R}$ is on the bottom and acts as an absorbing layer. Figure 4(b) shows optical micrographs of the fabricated double-layer homo-structure GMFs and the interface can be distinguished clearly. The vertical-section SEM image of the interfacial region [Fig. 4(c)] illustrates that the GMFs in the top layer are magneto-orientated vertically and magneto-orientated horizontally in the bottom layer, which exactly matches the idea of our design. According to earlier findings, the thicknesses of the impedance matching layer and the absorbing layer are significant factors that could affect the overall structure's absorption performance in the double-layer structure.[4] Here, we prepared three samples, each with a thickness of 2 mm but with different thickness ratios of the matching and absorbing layers. They are SM0.5-1.5 (0.5 mm matching layer, 1.5 mm absorbing layer), SM1-1 and SM1.5-0.5. The RL curves of the three samples are shown in Fig. 4(d). It can be found that the microwave absorption properties of the double-layer absorbers vary significantly with the thickness ratio of the matching layer and the absorbing layer. With a thickness of 0.5 mm of the matching layer and 1.5 mm of the absorbing layer, the SM0.5-1.5 double-layer structure exhibits poor EM wave dissipation performance, the RL$_{\min}$ is $-10.1$ dB at 13 GHz. Similarly, the microwave absorption performance of the SM1.5-0.5 double-layer structure is also poor and the RL$_{\min}$ value is only $-9.5$ dB at 14.1 GHz. In contrast, for the SM1-1 sample, the RL$_{\min}$ achieves $-38.2$ dB at 16.5 GHz with an effective absorption bandwidth of 6.4 GHz (11.6–18 GHz), covering the Ku band. Further analysis shows that, compared with SM0.5-1.5 and SM1.5-0.5, the SM1-1 has better relative input impedance $\mid$$Z_{\rm in}$/$Z_{0}$$\mid$ [Fig. 4(e)] and a larger microwave attenuation constant $\alpha$ [Fig. 4(f)], which may be the reason why this sample has better microwave absorption performance. Figure 4(g) collects the RL values from two single-layer absorbers (SM$_{\rm V}$ and SM$_{\rm R})$ and one double-layer homo-structure (SM1-1) with the same sample thickness. Compared with the SM$_{\rm V}$ and SM$_{\rm R}$ single-layer absorbers, the microwave absorption performance of the SM1-1 double-layer absorber has been significantly improved. Specifically, the RL$_{\min}$ of SM1-1 is 3.54 times that of SM$_{\rm R}$, and the effective absorption bandwidth is 2.56 times that of the SM$_{\rm R}$ single-layer absorber. The enhancement of the microwave absorption performance of SM1-1 is consistent with the expectation. In such a double-layer homo-structure, the vertically oriented GMFs have good impedance matching; most of the incident microwaves could pass through it and reach the bottom absorbing layer. Moreover, the horizontally aligned absorbing layer possesses conductive channels and has the largest conductivity (Fig. S12), which would help to deliver active migrating and hopping electrons. Meanwhile, the two parallel oriented GMFs are also regarded as miniature parallel-plate capacitors, leading to charge accumulation and rearrangement, as well as enhanced dielectric loss (Fig. S13). In addition, the parallel arrangement of GMFs induces multiple scattering of the EM wave, increasing its propagation path in the absorber. The interface between two layers also leads to interface scattering, further weakening the EM wave (Fig. S14).
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Fig. 4. (a) Model of the homo-structure. (b) Optical microscope image of the homo-structure. (c) SEM of the interface between the absorbing layer and the matching layer. (d) RL curves. (e) Impedance matching, and (f) attenuation constant of SM0.5-1.5, SM1-1, and SM1.5-0.5 with thickness 2 mm. (g) RL of the SM1-1 and the single-layer absorber. (h) Comparison of the microwave absorption properties from this work and some recently reported double-layer absorbers.
To further investigate the absorption mechanism of the SM1-1 double-layer homo-structure absorber, the power flow distribution and the power loss distribution are simulated by the limited integral method (Fig. S15). It can be seen that the power flow of the EM wave declines with increasing depth and maintains the same direction, and the gradual reduction of the power flow confirms the absorbing ability of the double-layer absorber. We compared our materials with recently reported advanced composite wave-absorbing materials (Table S1)[31-42] and double-layer absorbers [Fig. 4(h)].[7,12,43-47] It can be found that our homo-structure double-layer absorber has comparable microwave absorbing performance with others. Additionally, the filler in our double-layer wave absorber is made entirely of pure carbon, which is low-cost, low-filling, lightweight, and easy to prepare. These results confirmed our expectations and indicated that both the magneto-orientation technique and matching/absorbing double-layer design work well in this field. In summary, by utilizing the alignment power of magnetic fields, we have designed and fabricated a homo-structure double-layer absorber based on GMFs that exhibits enhanced microwave absorption properties. With the application of vertical and horizontal magnetic fields, the GMFs obtained from commercial graphite are aligned in the desired directions. The horizontally oriented GMFs show a dielectric constant higher than that of the vertically oriented ones. With the combination of these two magneto-orientated layers, vertically oriented as the surficial impedance matching layer and horizontally oriented as the inner EM loss layer, we obtain a GMF-based double-layer homo-structure absorber. After optimization of the thickness of the matching and absorbing layer, the minimum absorption of $-38.2$ dB and absorption bandwidth of 6.4 GHz (11.6–18.0 GHz) are achieved. One can see the Supplementary Information for the magnetic hysteresis loop of the GMFs, the magneto-orientated equipment; SEM images and statistical chart of SM$_{\rm V}$, SM$_{0}$ and SM$_{\rm R}$, the SEM and optical microscope images of higher and lower resin viscosity sample, the optical microscope and SEM images of SM$_{\rm R}$ from vertical section, XRD patterns of SM$_{\rm V}$, SM$_{0}$ and SM$_{\rm R}$ in vertical-section, SEM image of the interface between matching layer and absorbing layer from vertical section, RL curves of SM$_{\rm V}$, SM$_{\rm R}$ and SM1-1 with different magnetic field intensities, in-plane electrical conductivity of EP resin, SM$_{\rm V}$, SM$_{0}$ and SM$_{\rm R}$, the EM parameters of SM$_{\rm V}$, SM$_{0}$ and SM$_{\rm R}$, the schematic diagram of the microwave absorption mechanism of the double-layer homo-structure absorber, and simulated results of the power flow distribution and the power loss distribution of the double-layer homo-structure absorber. Acknowledgements. This work was supported by the National Key R&D Program of China (Grant No. 2021YFA1600203), the National Natural Science Foundation of China (Grant Nos. U2032218, 11904116, and 12111530283), the Plan for Major Provincial Science & Technology Project (Grant No. 202003a05020018). A portion of this work was performed on the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS and supported by the High Magnetic Field Laboratory of Anhui Province. 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