Full Length ArticleEnhanced magnetic and microwave absorption properties of FeCo-SiO2 nanogranular film functionalized carbon fibers fabricated with the radio frequency magnetron method
Introduction
The rapid development of electronic and communication devices has led to a rise in certain problems, such as electromagnetic wave radiation pollution affecting human health and electromagnetic interference [1], [2], [3]. Microwave absorbing materials, therefore, have been widely studied to meet the urgent application requirements of anti-electromagnetic radiation [4]. Nanophase materials with magnetic properties, such as cobalt nanoplatelets [5], FeNi3/Al2O3 core-shell nanocomposites [6] and (Fe, Ni)/C nanocapsules [7], have been investigated for microwave absorption. Granular films, such as (Fe81Co19)N, Fe51.1Co48.9B-Al2O3, FeCoB-Al2O3, and Fe65Co35-B2O3, have also been intensively researched for their applications in high-frequency absorption [8], [9], [10], [11], [12], [13]. In these cases, the introduction of nonmagnetic phases such as C, Al2O3, and B2O3 can significantly improve the magnetic and microwave absorption properties of the materials owing to interfacial polarization and ferromagnetic exchange. However, because of their inferior mechanical properties, there has been little or no application prospects for these magnetic materials for use as base materials.
Carbon fibers (CFs) with high strength, low density, good knittability, and low thermal expansion have usually been chosen as reinforcing fillers in ceramic-matrix composites, metal-based composites, and polymer composites over the past few decades [14], [15], [16], [17]. It should also be noted that CFs have attracted significant interest as microwave absorbers, which are dielectric absorbers with no magnetic properties [18], [19]. Unlike CFs, whose absorbing properties are determined by the complex permittivity (εr = ε' − jε"), the absorbing performance of CFS modified with magnetic materials is determined by both the complex permittivity (εr = ε' − jε") and the complex permeability (μr = μ' − jμ"). In addition, those CFS modified with magnetic materials exhibit better impedance-matching characteristics () than those of CFs.
Recently, several strategies have been successfully utilized to endow carbon fibers with electromagnetic materials. By using chemical vapor deposition, carbonyl iron/CF composites could be prepared that exhibited strong electromagnetic-wave absorption properties with a minimum reflectivity (R) of −38 dB at 13.4 GHz [20]. In a case involving the introduction of magnetic materials, a Ni-Fe alloy was successfully coated onto CFs with an electroplating method. For these Ni0.45Fe0.55/CFs with a thickness of 3 mm, the R could reach a minimum value of −14.7 at 2.0 GHz [21]. With a typical wet chemical method, Fe3O4 nanoparticles were plated onto CFs to form a non-continuous coating layer, while the minimum R of the composites was −35 dB at 6.37 GHz for a 4.41 mm layer [22]. In addition, CFs can also be treated with an electroless plating method combined with a thermal oxidation treatment. Zeng et al. prepared Cu/Co/CFs using a two-step electroless plating first, and then a thermal oxidation in air was applied to obtain CuO/Co/CF composites. The R of the final products could be enhanced to −42.7 dB at 10.8 GHz [23], [24]. These methods, which are applied to improve the microwave absorbing properties of the carbon fibers, have been proven to be highly effective. Nevertheless, there is no guarantee that the composites can be obtained without damaging the carbon fiber strength.
All the treatments referring to wet chemistry or harsh environments would produce irreversible damage to the CFs, because of the high temperature, corrosive solutions, and so on. Because those carbon fibers modified by magnetic substances would be bound well and weaved into various shapes, the ability to maintain the mechanical properties should be carefully considered. To date, it has been difficult to deposit films onto carbon fibers without damaging the fiber strength owing to the introduction of impure atoms such as oxygen. Compared to electroplating methods or chemical vapor deposition (CVDs) methods as mentioned above, the physical vapor deposition (PVD) process, which is carried out at lower deposition temperatures and a high vacuum condition, is favorable for keeping the strength of the carbon fibers. Moreover, the compact coating layer, which is critical for the electromagnetic properties of carbon fibers, is very hard to form on carbon fiber through an electroplating method. Therefore, using the PVD method to deposit magnetic nanogranular films can provide a new and better way to prepare a high-quality microwave absorption layer on carbon fibers. However, there are few reports about the functionalization of carbon fibers using such a PVD strategy.
It is well known that high absorption properties and absorption frequency regulation are important in the practical application of microwave absorbing materials. The frequency of the maximum absorption must be tuned when the frequency of the outer microwave changes. In this work, FeCo films were deposited on carbon fibers to improve the absorption properties. To further improve and tailor the absorption properties, SiO2 as a nonmagnetic phase was introduced into FeCo to form FeCo-SiO2 nanogranular films on carbon fiber. FeCo-SiO2 nanogranular films were deposited onto carbon fibers at low-temperature and high-vacuum conditions (5 × 10−4 Pa) by a radio frequency magnetron sputtering method. The microwave absorption properties of carbon fiber/FeCo-SiO2 nanogranular film composites (CF/FeCo-SiO2) were investigated. Moreover, the influence of SiO2 upon the magnetic properties and microwave absorption properties of CF/FeCo-SiO2 composites was also investigated. We found that the introduction of SiO2 into FeCo films can tune the electromagnetic properties of the composites obtained.
Section snippets
Preparation of CF/FeCo-SiO2 nanogranular film composites
The polyacrylonitrile (PAN)-based high strength carbon fibers used in this work are commercially available from Toray Industries Inc. based in Japan., and the average diameter of the fiber is about 7 μm. The pre-treatment of the carbon fibers was performed before the preparation of the composites. The PAN fibers were treated at 700 °C for 4 h in a tube furnace in a N2 atmosphere. After the heat treatment, the carbon fibers were soaked in a concentrated HNO3 solution at room temperature for 24 h.
Morphology and monofilament mechanical properties
A series of CF/FeCo-SiO2 nanogranular film composites was prepared for further examination and the SiO2-target area ratio (SiO2%) was set as 0, 10, 20, 30, 40 or 50%. A SEM measurement was taken to examine the morphology of the samples. Fig. 1(a)–(d) shows the typical SEM images of the CF/FeCo-SiO2 nanogranular film composites with a SiO2% of 20%. The cross-sectional images of the CF/FeCo-SiO2 nanogranular film composites are shown in Fig. 1(a) and (b). It is can clearly be observed that the
Conclusion
In this work, a series of CF/FeCo-SiO2 nanogranular film composites was successfully prepared using a radio frequency magnetron sputtering method. The FeCo-SiO2 nanogranular films were deposited completely onto the carbon fibers at varying thicknesses in the range of 150–200 nm. The CF/FeCo-SiO2 nanogranular film composites still maintained a high strength after the preparation process and the mean tensile strength was about 3.54 GPa. The magnetic properties of the samples were improved with the
Acknowledgments
This work is supported by the National Natural Science Foundation of China (General Program) (Grant No. 51074193), the Innovation-Driven Project of Central South University (No. 2016CXS006) and the Natural Science Foundation of China (Grant No. 51201187).
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