Enhanced microwave-absorbing properties of FeCo magnetic film-functionalized silicon carbide fibers fabricated by a radio frequency magnetron method

Author’s Accepted Manuscript Enhanced microwave-absorbing properties of FeCo magnetic film-functionalized silicon carbide fibers fabricated by a radio frequency magnetron method Yongshan Wei, Jianling Yue, Xiuzhi Tang, Xiaozhong Huang www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)31934-X http://dx.doi.org/10.1016/j.ceramint.2017.09.011 CERI16182

To appear in: Ceramics International Received date: 7 August 2017 Accepted date: 3 September 2017 Cite this article as: Yongshan Wei, Jianling Yue, Xiuzhi Tang and Xiaozhong Huang, Enhanced microwave-absorbing properties of FeCo magnetic filmfunctionalized silicon carbide fibers fabricated by a radio frequency magnetron m e t h o d , Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.09.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enhanced microwave-absorbing properties of FeCo magnetic film-functionalized silicon carbide fibers fabricated by a radio frequency magnetron method Yongshan Wei 1, Jianling Yue 2*, Xiuzhi Tang 2, Xiaozhong Huang 2* 1 School of Physics and Electronics, Central South University, Key laboratory for Advanced Fibers and Composites of Hunan Province, Changsha 410083, China 2 School of Aeronautics and Astronautics, Central South University, Key laboratory for Advanced Fibers and Composites of Hunan Province, Changsha 410083, China Abstract Silicon carbide (SiC) fibers have potential application in microwave absorption materials in recent years. In this study, we provide a new method for improving the microwave-absorbing properties of SiC fibers. Magnetic FeCo films were fabricated on SiC fibers at low temperature and high vacuum by a radio frequency magnetron sputtering method. The properties of FeCo film/SiC fiber (FeCo/SiCf) composites were investigated. When compared with SiC fiber, the FeCo/SiCf composites exhibit excellent

microwave-absorbing

properties

in

the

microwave

range,

with

enhancements in the optimal reflectivity loss from -5.03 to -25.51 dB. This excellent performance may be because of the magnetic loss due to ferromagnetic resonance and interfacial polarization, thus inducing dielectric relaxation. In addition, the magnetic properties of FeCo/SiCf composites are significantly improved: the value of saturation magnetization reaches up to 41.45 emu/g and the coercivity is 116.27 Oe. In addition, the strength of SiC fiber remains at 99.17% after the fabrication process. The method provided in this study for enhancing the microwave-absorbing properties of FeCo/SiCf composites will pave a new way for the development of SiC microwave-absorbing materials.

Keywords: Silicon carbide fiber; Microwave absorption; Magnetic materials; PVD.

1 Introduction In recent years, microwave-absorbing materials have become increasingly important, not only for military purposes but also in communication technology[1-5]. Silicon carbide (SiC) materials have been chosen as promising candidates for microwave absorption materials because of their many attractive properties, such as semiconductivity, high thermal conductivity, high saturated carrier drift velocity, and high breakdown field strength [6-10]. SiC fiber, which plays an important role in SiC materials, has been used as wave absorber and reinforced filler in structural microwave-absorbing materials because of its excellent dielectric and mechanical properties[11]. SiC fiber is a dielectric lossy material, and the complex permittivity (  r   '  j " ) is an important parameter that determines its wave-absorbing properties. Efforts have been made to investigate the dielectric and electromagnetic wave-absorbing properties of SiC fiber-reinforced composites[12-13]. For example, Wang et al.[14] studied the effect of SiC fiber type on the electromagnetic microwave-absorbing properties of SiCf/epoxy composites in the frequency range of 8.2–12.4 GHz (X band), and the minimum reflection loss was about -2 dB for KD-ISiCf/epoxy composite with a thickness of 2.8–3.2 mm. Ye et al.[15] also reported the dielectric and microwave-absorbing properties of two types of SiC fibers with different compositions in the frequency range of 8–18 GHz, and the minimum reflection loss was about -10.13 dB for SiC fiber-L. In the case of SiC fiber-woven

fabrics, Ding et al.[16] illustrated the complex permittivity of microwave-absorbing properties in X band, and the minimum reflection loss was about -5.1 dB for Nicalon-202 fabrics. To meet the high requirements of microwave-absorbing materials, the absorbing ability (e.g., wide absorption bandwidth and thin absorption layers) of SiC-based wave-absorbing materials must be improved. To achieve this goal, Ye et al.[17] improved the microwave-absorbing ability of SiC fibers by depositing boron nitride on SiC fibers by chemical vapor infiltration method, and it was found that the microwave-absorbing properties of SiC fibers with BN coating was greatly improved in the X band with a minimum reflection loss of -16.41 dB. It is viable to improve the microwave-absorbing performances of SiC fibers by surface modification. Furthermore, some other reports also illustrated that the microwave-absorbing properties of carbon fibers could be enhanced by magnetic coatings [18-19]. However, there are only few reports on the functionalization of SiC fibers with magnetic coatings. The modification of SiC fibers by magnetic coating can be proposed to achieve excellent microwave-absorbing properties. Unlike SiC fibers that exhibit only dielectric properties, magnetic coating-modified SiC fibers have both dielectric and magnetic properties. It is difficult to deposit films onto fibers while retaining the high fiber strength because the mechanical properties of the fibers could be easily damaged during the fabrication process. The physical vapor deposition (PVD) process, which is performed under low-temperature and high-vacuum conditions that are beneficial to protect the

mechanical properties of the fibers, can be proposed to deposit films on fibers. However, there are only few reports on magnetic coating-modified SiC fibers by the PVD process. In this study, we combine SiC fibers with magnetic lossy materials (FeCo films) to form new composites (FeCo/SiCf) that acquire both magnetic and dielectric properties. FeCo films are deposited onto SiC fibers at low temperature and high vacuum (5×10-4 Pa) by a radio frequency magnetron sputtering method. The complex permittivity and complex permeability of the prepared FeCo/SiCf composites are investigated in the frequency range of 2–18 GHz. We find that the FeCo/SiCf composites exhibit excellent microwave-absorbing properties and inherit the high strength of SiC fibers.

2 Experimental 2.1 Preparation of FeCo/SiCf composites The high-strength SiC fibers used in this work are commercially available from China, and the average diameter of the fibers is about 11 µm. The SiC fibers need to be pretreated prior to the preparation of FeCo/SiCf composites. The fibers were treated at 500 oC for 1 h in a tube furnace under atmospheric pressure. After the heat treatment, these SiC fibers were soaked in concentrated HNO3 solution at room temperature for 24 h. The fibers were then removed and washed in distilled water several times to remove all the remaining nitrates on its surface. Finally, the clean fibers were dried at 100 oC in an oven for 2 h. A series of FeCo films were deposited on the pretreated SiC fibers in a radio

frequency magnetron sputtering system. The base pressure was less than 5×10-4 Pa. The Ar gas pressure was 0.35 Pa during sputtering. The sputtering power was maintained at 200 W. A Fe50Co50 alloy target (99.99% purity) was used for sputtering. The thickness of the films was controlled by adjusting the sputtering time and the sputtering power.

2.2 Characterization X-ray diffraction (XRD, D8 Discover 2500, Bruker) was used to confirm the structure of the samples. The surface topography and structure were observed using a field emission scanning electron microscope (TESCAN MIRA3 LMU) equipped with an energy-dispersive X-ray spectrometer (EDS). The sample strength was evaluated by the fracture test, and each sample was tested 30 times. The monofilament tensile strength was calculated according to the British Standard Institute ISO-11566 using the following equation:  t 

4F , where  t , F, and d represent the tensile strength, 2 d

the force at rupture, and the diameter of a single fiber, respectively. The field-dependent magnetization curves were measured at room temperature using a vibrating sample magnetometer (Lakeshore 7307, USA). To measure the complex permeability and complex permittivity, the samples were cut into lengths of 2 mm, homogeneously dispersed in paraffin, and then shaped into a toroidal ring (internal diameter 3 mm, external diameter 7 mm). The weight fraction of FeCo/SiC f composites was 50%. The complex permeability and complex permittivity were measured in the frequency range of 2–18 GHz using an Agilent 8720 ET vector

network analyzer. The reflection loss of FeCo/SiCf composites was calculated according to the transmission line theory, by using the following equations:

RL  20log

Z in  Z 0

Z0 

Zin  Z0 Zin  Z0

r   2 fd   tanh  j   r r  r   c  

0 0

(1)

(2)

(3)

where Z0 is the characteristic impedance of vacuum; Zin is the input impedance of the materials; ƒ is the frequency; c is the speed of light;  0 and  0 are the complex permeability and complex permittivity, respectively, in vacuum; and  r and  r are the relative complex permeability and complex permittivity, respectively.

3 Results and Discussion 3.1 Morphology and monofilament mechanical properties The SEM image of the surface of FeCo/SiCf composites was obtained to examine their morphology. The EDS measurement showed that the films on the surface of SiC fibers mainly consisted of FeCo, and the ratio of Co to Fe was about 48:52, which is close to that of the Fe50Co50 target. The surface and cross-sectional images of FeCo/SiCf composites are shown in Fig. 1a and Fig. 1b, respectively. It can be found that small FeCo particles grew on the surface as shown in Fig. 1a, and the diameter of a FeCo/SiCf composite was about 12 µm. The thickness of FeCo films on SiC fiber surface was in a range of 500–600 nm, as shown in Fig. 1b.

Fig. 1 Typical SEM images of the surface morphologies of FeCo/SiCf composites (a) and the cross-section of FeCo/SiCf composites (b). Fig. 2 shows the X-ray diffraction patterns of the surface of SiC fibers and FeCo/SiC f composites. As shown in Fig. 2, typical (111), (220), and (311) diffraction peaks of

β  SiC were observed for the SiC fibers, indicating that the SiC fibers mainly consisted of   SiC . However, for the FeCo/SiCf composites, only a weak (111) diffraction peak of   SiC was observed, but a strong (110) diffraction peak of FeCo appeared, thus indicating that the films deposited on SiC fibers were mainly composed of FeCo.

Fig. 2 XRD spectra of SiC fiber and FeCo/SiCf composites. Fig. 3 shows the single-fiber tensile strengths of SiC fibers and FeCo/SiCf composites. The mean tensile strength of SiC fibers was 2.40 GPa with an error bar of 0.45 GPa and that of FeCo/SiCf composites was 2.38 GPa with an error bar of 0.41 GPa. The strength of SiC fiber remains at 99.17% after the fabrication process. It can be observed that the strengths of FeCo/SiCf composites were almost the same as those of the raw SiC fibers after the preparation process.

Fig. 3 Single-fiber tensile strengths of SiC fibers and FeCo/SiCf composites

3.2 Magnetic and microwave-absorbing properties of FeCo/SiCf composites

Fig. 4 shows the typical hysteresis loops of FeCo/SiCf composites. The saturation magnetization and the coercivity of FeCo/SiCf composites were 41.45 emu/g and 116.27 Oe, respectively. Because the SiC fibers are nonmagnetic materials, it is obvious that the achievement of magnetic properties of FeCo/SiCf composites was attributed to the deposition of FeCo films on the SiC fibers.

Fig. 4 Magnetic hysteresis loops of FeCo/SiCf composites The complex permeability ( r  '  j " ) and complex permittivity (  r   '  j " ) of SiC fibers and FeCo/SiCf composites were measured in the frequency range of 2– 18 GHz. Fig. 5 and Fig. 6 represent the frequency dependence of the complex permeability and complex permittivity of SiC fibers and FeCo/SiCf composites, respectively. Because the SiC fibers are nonmagnetic, it is reasonable that the real part

 ' and imaginary part  " of the permeability are close to 1 and 0, respectively, in the frequency range of 2–18 GHz, as shown in Fig. 5a. Fig. 5b shows that the real part

 ' of the permittivity is distributed in the range of 2.28–2.75, while the imaginary part

 " of permittivity is distributed in the range of 0.08–0.81. It should be noted that the wave absorption mechanism of SiC fibers depends on the dielectric loss.

Fig. 5 Frequency dependence of the complex permeability and complex permittivity spectra of SiC fibers. Fig. 6 shows the frequency dependence of the complex permeability and complex permittivity of FeCo/SiCf composites. Compared with those of SiC fibers, the

complex permeability and complex permittivity of FeCo/SiCf composites were remarkably enhanced. As shown in Fig. 6a, the  ' of the permeability changed slightly with increase in frequency up to 13.52 GHz, and then, it increased rapidly up to 1.53 with further increase in the frequency. The  " of the permeability exhibited two peaks at around 5.00 and 14.00 GHz, respectively, which can be due to the ferromagnetic resonance of thin FeCo films[21,24]. Unlike bulk FeCo alloy that has a small anisotropy field H A , the thin FeCo films on the surface of SiC fibers exhibited a large anisotropy field H A , which had resulted from the surface anisotropy field induced by the small size effect[24]. Therefore, the resonance frequencies f r of FeCo/SiCf composites appeared at high frequencies (around 5.00 and 14.00 GHz), according to the ferromagnetic resonance equation: 2f r  H A , where  represents the gyromagnetic ratio. The complex permittivity of FeCo/SiCf composites is shown in Fig. 6b. The  ' of the permittivity declined from 6.68 to 3.64 with the increase in frequency from 2 to 18 GHz. The  " of the permittivity presented two broad peaks at about 9.60 and 14.48 GHz, respectively, which can be due to the interfacial polarization [20-24]. The induced interfacial polarization was associated with the entrapment of free charges at the interfaces between the constituent phases (FeCo and SiC). Dielectric relaxation resulting from the interfacial polarization can be achieved in the FeCo/SiCf composites at microwave frequencies.

Fig. 6 Frequency dependence of complex permeability and complex permittivity spectra of FeCo/SiCf composites.

Fig. 7 shows the frequency dependence of reflection loss for the raw SiC fibers in the frequency range of 2–18 GHz. The reflectivity was close to zero in the entire range when the thickness of the samples was less than 2 mm, and it reached a minimum value of 5.03 dB when the thickness was 4 mm. Furthermore, it is obvious that the raw SiC fibers exhibited inferior microwave-absorbing properties in the above frequency range. The reason for the poor absorption ability may be because the dielectric loss was low to contribute to the absorption of microwaves.

Fig. 7 Reflectivity curves for different thicknesses of SiC fibers. The frequency dependence of reflection loss for FeCo/SiCf composites in the frequency range of 2–18 GHz is presented in Fig. 8. As shown in the figure, it is obvious that the minimum value of reflectivity shifted to lower frequency range with the increase in thickness and exceeded -10 dB when the thickness was no less than 2 mm. The FeCo/SiCf composites with a thickness of 3 mm exhibited an optimal reflectivity loss of -25.51 dB at 10.73 GHz and had an absorption bandwidth less than -20 dB over the frequency range of 10.08–11.45 GHz. It can be seen that the FeCo/SiCf composites had achieved excellent microwave-absorbing properties , which are better than those reported in previous works [13-17]. These excellent microwave absorption performances can be attributed to the magnetic loss resulting from magnetic FeCo films and FeCo/SiCf coaxial structure (SiC fibers as the inner core and FeCo as the outer films), in which the interfacial polarization that improves dielectric loss was induced.

Fig. 8 Reflectivity curves for different thicknesses of FeCo/SiCf composites. The magnetic loss factor ( tan        ) and dielectric loss factor ( tan        ) were calculated from the complex permittivity and complex permeability of FeCo/SiCf composites, respectively, as shown in Fig. 9. It can be seen that both magnetic loss and dielectric loss contribute to microwave absorption in the entire frequency range, i.e., 2–18 GHz. The value of dielectric loss factor increased from 0.08 to 0.42 in the frequency range from 2 to 15.12 GHz, with a slight fluctuation at about 12 GHz, and then decreased to 0.26. This indicates that the dielectric loss significantly contributes to microwave absorption in the high frequency range. However, small changes in the value of magnetic loss factor were observed in the entire frequency range, i.e., 2–18 GHz, and the minimum and maximum values were 0.06 and 0.23, respectively. This indicates that the magnetic loss can contribute more evenly to microwave absorption in the entire frequency range, i.e., 2–18 GHz.

Fig. 9 Dielectric loss factor and magnetic loss factor of FeCo/SiCf composites versus frequency

Conclusion Magnetic FeCo films were successfully fabricated on SiC fiber surface at low temperature and high vacuum by a radio frequency magnetron sputtering method. The magnetic properties of FeCo/SiCf composites improved markedly, with a coercivity

value of 116.27 Oe and a saturation magnetization value of 41.45 emu/g. The FeCo/SiCf composites exhibited excellent microwave-absorbing performances. In particular, the composites with a thickness of 3 mm exhibited an optimal reflectivity loss of -25.51 dB at 10.73 GHz and had an absorption bandwidth less than -20 dB over the frequency range of 10.08–11.45 GHz. In addition, after the fabrication process, the tensile strength of FeCo/SiCf composites was almost the same as that of the raw SiC fibers, with a value of 2.38 GPa. In conclusion, it is believed that the FeCo/SiCf composites are promising candidates for application in structural microwave-absorbing materials because of their light weight, high strength, and absorption of stronger and wider frequency microwaves.

Acknowledgments This work was supported by the National Natural Science Foundation of China (General Program) (51074193) and the Natural Science Foundation of China (Grant No. 51201187).

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