Electromagnetic Interference Shielding Behavior of Magnetic Carbon Fibers Prepared by Electroless FeCoNi-Plating

In this study, soft magnetic metal was coated on carbon fibers (CFs) using an electroless FeCoNi-plating method to enhance the electromagnetic interference (EMI) shielding properties of CFs. Scanning electron microscopy, X-ray diffraction, and a vibrating sample magnetometer were employed to determine the morphologies, structural properties, and magnetic properties of the FeCoNi-CFs, respectively. The EMI shielding behavior of the FeCoNi-CFs was investigated in the frequency range of 300 kHz to 3 GHz through vector network analysis. The EMI shielding properties of the FeCoNi-CFs were significantly enhanced compared with those of the as-received CFs. The highest EMI shielding effectiveness of the 60-FeCoNi-CFs was approximately 69.4 dB at 1.5 GHz. The saturation magnetization and coercivity of the 60-FeCoNi-CFs were approximately 103.2 emu/g and 46.3 Oe, respectively. This indicates that the presence of FeCoNi layers on CFs can lead to good EMI shielding due to the EMI adsorption behavior of the magnetic metal layers.

Carbon materials such as carbon fibers (CFs), carbon nanotubes (CNTs), carbon black, and graphene are continually being researched and developed for use in a variety of industrial applications including EMI shielding [18][19][20][21][22][23][24][25][26][27][28][29]. As one of the most developed and critical reinforcements, CFs have excellent properties such as high strength, high modulus, low density, good chemical stability, and outstanding electrical conductivity. These properties make CFs suitable for use in materials such as aircrafts, automotive parts, and electrical equipment as electromagnetic interference (EMI) shielding materials. However, as EMI shielding materials, CFs have weaknesses (e.g., poor magnetic properties) which must be improved in order to enable their wide application and performance improvement in the area of EMI shielding [30,31]. Soft magnetic materials such as Fe, Co, Ni, and alloys thereof have attracted considerable attention as EMI shielding materials due to their high magnetic properties. However, since these alloys have a high density, they not only increase the weight of the EM wave absorbing materials but are also low in dispersibility for use as reinforcements [32][33][34][35].

Characterization
The morphologies of the FeCoNi-CFs were measured using field emission SEM (FE SEM, JSM-7100F, JEOL Ltd., Tokyo, Japan). The structure properties of these samples were examined by XRD (D2 PHASER, Bruker AXS, Karlsruhe, Germany) with Cu Kα radiation. The magnetic properties of the FeCoNi-CFs were measured using a Vibrating Sample Magnetometer (VSM, 7404, Lake Shore Cryotronics Inc., Westerville, OH, USA). The volume resistivities of the FeCoNi-CFs were measured according to JIS K 7194 at room temperature. The measuring instrument used a four-probe electrical resistivity tester (MCP-T700, Mitsubishi Chemical Co., Chigasaki, Japan) with an ASP type probe, and the power source condition was AC85-24V/47-63Hz/40VA. The EMI shielding properties of the FeCoNi-CFs were measured using a vector network analyzer (E5071C, Keysight Technologies, Santa Rosa, CA, USA) based on the ASTM ES-7 in the frequency range of 300 kHz to 3 GHz. The test set-up and sample dimension information are schematically shown in Figure 2. The test set-up consists of network analyzers, coaxial cable, and coaxial transverse electromagnetic (TEM) cells as a sample holder. The network analyzer was able to measure the incident, transmitted, and reflected powers of a sample. The EMI shielding effectiveness (EMI SE) of the materials was evaluated by measuring the attenuation or reduction of electromagnetic waves and was calculated using the following equation [41]: where P 1 and P 2 are the incident and transmitted powers, respectively.

Results and Discussion
Figure 1b presents Energy Dispersive X-ray Spectroscopy (EDS) results and photographs of the FeCoNi-CFs as a function of plating time. The changes in elemental compositions and fabric color of the FeCoNi-CFs were confirmed after electroless plating. The metal content of FeCoNi-CFs increased with increasing FeCoNi-plating time. In the 60-FeCoNi-CFs, the total content of metal (FeCoNi) was the highest, which was nearly similar to the metal content of FeCoNi-CFs plated during 90 and 120 min. As clearly visible to the naked eye per the photographs of FeCoNi-CFs, 5-and 15-FeCoNi-CFs were not partially plated. These results revealed that a plating time of 30 min or more is required to coat the fabric in its entirety. Figure 3 shows the SEM images of 5-,-FeCoNi-CFs, 15-,-FeCoNi-CFs, 30-,-FeCoNi-CFs, and 60-FeCoNi-CFs. As the plating time increased, the FeCoNi layers on the fabric surfaces gradually formed and the FeCoNi grain size also increased. However, the surface morphologies of 5-, 15-, and 30-FeCoNi-CFs indicated that the FeCoNi layer was not yet completely formed. By contrast, the surfaces morphologies of 60-FeCoNi-CFs revealed that the FeCoNi layers were homogeneously deposited on the surfaces of CFs. Figure 4 presents EDS-mapping images of 5-and 60-FeCoNi-CFs. In the figure, C, O, Fe, Co, and Ni are denoted as blue, red, purple, green, and khaki, respectively. The EDS-mapping images reveal a more accurate distribution of the metal layer formed on the CF surface. It appears that the 60-min plating led to a significant improvement in metal coating quality, which was also likely to affect the EMI shielding properties.   Figure 5 shows the XRD patterns of the FeCoNi-CFs as a function of plating time. All Fe-CoNi-CFs clearly exhibited peaks of (110), (200), and (211) planes of the FeCoNi BCC structure under various plating time conditions. It was confirmed that the FeCoNi structure peak became more pronounced as the plating time increased, and the crystallinity increased with the increase in the thickness of the plating layer as the particles were well aligned [42,43]. In the FeCoNi-CFs plated for more than 30 min, the (002) of the carbons was not observed, indicating that the carbon did not restack due to the FeCoNi layer grown on the fiber surface [44].
ture peak became more pronounced as the plating time increased, and the crystallin increased with the increase in the thickness of the plating layer as the particles were w aligned [42,43]. In the FeCoNi-CFs plated for more than 30 min, the (002) of the carbo was not observed, indicating that the carbon did not restack due to the FeCoNi lay grown on the fiber surface [44]. Oe, respectively. These results were considered to have derived from the formation small-sized FeCoNi grains and uniform plating layers [45], as confirmed by SEM da The Ms and Hc of 30-FeCoNi-CFs were 43.1 emu/g and 57.8 Oe, respectively. The ma netic properties of 30-FeCoNi-CFs were improved as compared to 5-and 15-FeCoNi-C Of the samples, the best soft magnetic properties were obtained for 60-FeCoNi-CFs. T 60-FeCoNi-CFs had the highest Ms (103.2 emu/g) and lowest Hc (46.3 Oe) due to the f that the FeCoNi grains were sufficiently large and the FeCoNi layers were uniformly ge erated. In addition, it was observed that the magnetic properties of 90-and 120-FeCoN   Figure 6c shows the effect of FeCoNi-plating on the volume resistivity of magnetic CFs. It can be seen that the change in volume resistivity was similar to that of the magnetic properties. The volume resistivity of FeCoNi-CFs gradually decreased with the increase in plating time, indicating that uniformed FeCoNi layers can cause good surface conductivity for the FeCoNi-CFs [46]. This also means that the EMI shielding properties of FeCoNi-CFs can be enhanced by increasing the FeCoNi-plating time in this work. Figure 6d shows the EMI shielding properties over the frequency range of 300 kHz to 3 GHz for the FeCoNi-CFs as a function of plating time. As Figure 5d shows, the EMI SE of the as-received CFs was approximately 49.2 dB at 1.5 GHz. All of the FeCoNi-CFs had a higher EMI SE than the as-received CFs. In addition, the EMI SE of FeCoNi-CFs was enhanced according to FeCoNi-plating time. In particular, the EMI SE of the FeCoNi-CFs was enhanced significantly up to the 60-FeCoNi-CFs (69.4 dB at 1.5 GHz), suggesting that the presence of an FeCoNi layer on the surface of CFs can enhance the EMI SE of the magnetic CFs. In the 90-FeCoNi-CFs and 120-FeCoNi-CFs, the EMI SE was reduced to 66.9 and 66.1 dB, respectively, at 1.5 GHz. This means that a metal content threshold can exist for the EMI SE, which indicates that a further increase in plating time is unnecessary. It seems that FeCoNi was separated from the surface after excessive plating. This can be confirmed through Figure 7. As confirmed in Figures 3 and 4, the 60-FeCoNi-CF surface was once again uniformly or perfectly plated through Figure 7. On the other hand, in the case of the surface of 120-FeCoNi-CFs, it was confirmed that metal particles were either agglomerated to form a cluster, or the over-aggregated metal was separated from the fiber surface. This is probably due to the high magnetic affinity of FeCoNi. Also, EMI SE(dB) is converted to EMI Shielding efficiency (%) using Equation (2) as [47]; Shielding efficiency (%) = 100 − 1 10 SE /10 × 100 (2) As a result of the calculation through Equation (2), The 60-FeCoNi-CFs exhibited outstanding EMI shielding efficiency (99.99999%) as shielding materials. The 60-FeCoNi-CFs exhibited higher magnetic properties than the other samples, and for EMI shielding materials, a previous study showed that high permeability increases the absorption behavior and improves the EMI shielding properties [48]. Therefore, we can expect additional EMI behaviors on the magnetic layer. This means that FeCoNi plays a major role in generating the additional EMI SE.

Conclusions
In this study, we prepared magnetic CFs fibers using electroless FeCoNi plating to enhance the EMI SE of FeCoNi-CFs. We also and investigated the effects of FeCoNi plating on the EMI SE. The formation of FeCoNi layers on CF surfaces were confirmed by EDS, SEM, and XRD studies, and the CF surfaces were successfully modified by electroless plating. Experimental results indicate that the magnetic properties, electrical conductivity, and EMI SE of FeCoNi-CFs were enhanced with increased plating time. The 60-FeCoNi-CFs showed an enhancement greater than 40% for the EMI SE as compared to the as-received CFs due to the presence of FeCoNi layers and the additional shielding effects of the FeCoNi-CFs as produced in this study.

Data Availability Statement:
The data presented in this study are available on reasonable request from the corresponding author.