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The electrochemical preparation and microwave absorption properties of magnetic carbon fibers coated with Fe3O4 films
Applied Surface Science 257 (2011) 10808–10814
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The electrochemical preparation and microwave absorption properties of magnetic carbon fibers coated with Fe3 O4 films Xianguang Meng, Yizao Wan, Qunying Li, Jing Wang, Honglin Luo ∗ School of Materials Science and Engineering, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 9 May 2011 Accepted 22 July 2011 Available online 30 July 2011 Keywords: Carbon fibers Fe3 O4 films Electrodeposition Surface treatment Microwave absorption
a b s t r a c t Electrodeposition was employed to fabricate magnetite (Fe3 O4 ) coated carbon fibers (MCCFs). Temperature and fiber surface pretreatment had a significant influence on the composition and morphology of Fe3 O4 films. Uniform and compact Fe3 O4 films were fabricated at 75 ◦ C on both nitric acid treated and untreated carbon fibers, while the films prepared at 60 ◦ C were continuous and rough. Microwave measurements of MCCF/paraffin composites (50 wt.% of MCCFs, pretreated carbon fibers as deposition substrates) were carried out in the 2–18 GHz frequency range. MCCFs prepared at 60 ◦ C obtained a much higher loss factor than that prepared at 75 ◦ C. However, the calculation results of reflection loss were very abnormal that MCCFs prepared at 60 ◦ C almost had no absorption property. While MCCFs prepared at 75 ◦ C exhibited a good absorption property and obtained −10 dB and −20 dB refection loss in wide matching thickness ranges (1.0–6.0 mm and 1.7–6.0 mm range, respectively). A secondary attenuation peak could also be observed when the thickness of MCCF/paraffin composite exceeded 4.0 mm. The minimum reflection loss was lower. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Electromagnetic (EM) wave absorption materials have attracted great attention in recent years due to the expanded EM interference problems and their potential applications in wireless data communication, local area network systems, satellite televisions, etc. [1]. Carbon fiber (CF) radar absorption materials are multifunctional composites with high strength and modulus, good carrying capacity, excellent electrical property and low radar cross-section, which are increasingly recognized as practical structural absorption composites [2]. Dielectric and magnetic loss are required for a desired microwave absorption material. As an important magnetic material, Fe3 O4 has been reported extensively in view of its excellent microwave absorption property [3–6]. Considering these unique properties of CFs and Fe3 O4 , we tried to prepare Fe3 O4 films on CFs, which are expected to exhibit good radar-absorbing property. Fe3 O4 films have been successfully fabricated on many different substrates by hydrothermal growth [7], chemical bath technique [8], sputtering [9], molecular beam epitaxy [10,11], electrodeposition [12–15], etc. So far, to the best of our knowledge, the preparation of Fe3 O4 films on CF substrates can be only achieved by sol–gel [16] and wet chemical [17] methods. However, both methods are time consuming and need a series of reaction steps.
∗ Corresponding author. Tel.: +86 22 8371 9504; fax: +86 22 8371 9504. E-mail addresses: [email protected], [email protected] (H. Luo). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.108
Moreover, the Fe3 O4 films are not uniform and dense, the surface quality is unsatisfactory. Electrodeposition (cathodic deposition) was developed by Kothari and co-workers [15] for preparing Fe3 O4 films on stainless steel substrates, by the electrochemical reduction of Fe(III) complexed with triethanolamine. In view of its easy operation, high efficient, low cost and the controllability of the morphologies of Fe3 O4 films, electrodeposition opens up a new way to fabricate magnetite (Fe3 O4 ) coated carbon fibers (MCCFs). Being different from the substrates used in the previous studies [12–15], CFs do not have perfect crystalline atom-ordered surfaces, whether the electrochemically prepared Fe3 O4 films on CFs can be obtained with good uniformity and compactness is also an important concern. Not just for the preparation of MCCFs, the electrodeposition may also provide a feasibility of producing other functional ceramic films, such as ZnO, CuO and Cu2 O films on CFs. These oxide films have been successfully prepared on certain substrates by electrodeposition [18–27]. It is believed that functional ceramics coated CFs produced by electrochemical deposition method will realize the integration of structural strength and functionality of composites and expand the applications of CFs in the future. In this paper, cathodic deposition was applied to fabricate MCCFs. The structure, morphology, magnetic and microwave absorption properties of MCCFs were investigated. As have been found in our experiment, temperature has the most important influence on the composition, morphology, magnetic and microwave absorption properties, comparison was made between
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MCCFs prepared at 60 ◦ C and 75 ◦ C on CFs with nitric acid pretreatment. In order to investigate the effects of surface pretreatment on the composition and morphology of final products, the untreated CFs were also used as substrates to deposit Fe3 O4 films at 75 ◦ C as reference. 2. Materials and methods 2.1. Sample preparation In this experiment, the deposition solution was prepared as the description of Kothari and co-workers [15]. The unsized PAN-based CFs used in this experiment contained 12,000 filaments each bundle and the diameter of a single filament was about 7 m. Before the cathodic deposition, CFs were pretreated with nitric acid (65 wt.%) for 2.5 h at room temperature in order to enhance the interfacial adhesion between the films and CFs, and then washed thoroughly and cut into 10 cm in length to serve as cathode. Pure iron (99.99%) sheets were polished and served as anode. The deposition reaction was conducted for 2 min under a constant electric current of 1 A at 60 ◦ C (MCCFs-1) and 75 ◦ C (MCCFs-2), respectively. The untreated CFs were also used as substrates to deposit Fe3 O4 films at 75 ◦ C (MCCFs-0). All the final products were dried in the heating oven at 80 ◦ C for 1 h, then washed with deionized water thoroughly and dried at 80 ◦ C again. 2.2. Sample characterization The products were ground into powder samples and characterized by X-ray diffraction (XRD) for phase identification using Cu K␣ radiation. XRD measurements were performed on a Rigaku D/Max 2500 v/pc X-ray diffractometer from 10◦ to 90◦ . The morphology and thickness of Fe3 O4 films were analyzed on an FEI NANOSEM 430 field emission scanning electron microscope (FE-SEM) operated at an acceleration voltage of 15.0 kV. The magnetic properties of MCCFs were measured at room temperature in a vibrating sample magnetometer (VSM) option of physical properties measurement system (PPMS-9) from Quantum Design. For microwave measurement, the MCCFs were cut into short fragments (2–3 mm in length) and mixed with paraffin through ultrasonic agitation. The mixtures were pressed into a ring form of 7 mm outer and 3 mm inner diameter, 2 mm in thickness. The relative complex permittivity (εr = εr − jεr ) and permeability (r = r − jr ) of the MCCF/paraffin composites were measured in the frequency range of 2–18 GHz over an HP8722ES vector network analyzer. The reflection loss (RL) of a microwave absorption layer backed by a perfect conductor was calculated by means of the transmit-line theory using the measured relative complex permittivity and permeability:
Fig. 1. XRD patterns of (a) MCCFs-1 and (b) MCCFs-2.
3. Results and discussion
of three phases. The broadened peak around 26◦ could be ascribed to the diffraction of graphite from the CF substrates. The other sharp peaks marked with (1 1 1), (2 2 0), (3 1 1), (4 0 0) and the like in the pictures could be indexed and assigned to face-centered cubic (fcc) Fe3 O4 (JCPDS Card No. 19-0629). The Fe3 O4 prepared by electrodeposition in this experiment is same as that reported by Kothari and co-workers [15]. The strong and sharp reflection peaks suggest that the as-prepared Fe3 O4 films are well crystallized. Besides, Fe (JCPDS Card No. 06-0696) was also found in both patterns. The ratios of the first-strong peak intensities of Fe3 O4 (3 1 1) and Fe (1 1 0) reflections (IM(3 1 1) /IFe(1 1 0) ) are 0.33 and 1.72 corresponding to MCCFs-1 and MCCFs-2, respectively, which indicate that Fe is inclined to be generated at lower temperature. The XRD patterns suggest that temperature has an obvious influence on the composition of MCCFs. A two-step electrochemical reduction and precipitation process is proposed by Kothari and co-workers [15]. When the cathodic reduction happens, Fe3 O4 is generated on the surface of CFs following the reactions outlined in Eqs. (3) and (4):
3.1. Composition of MCCFs
Fe(TEA)3+ + e− → Fe2+ + TEA
(3)
The XRD patterns of MCCFs-1 and MCCFs-2 are shown in Fig. 1. The results indicate that both MCCFs-1 and MCCFs-2 are composed
Fe2+ + 2Fe(TEA)3+ + 8OH− → Fe3 O4 + 2TEA + 4H2 O
(4)
Zin − 1 Z +1
RL (dB) = 20 log10
Zin =
r εr
(1)
in
2fd √
tanh j
c
r εr
(2)
where RL is a ratio of reflected power to incident power in dB, Zin is the input impedance of absorber, d is the thickness of the absorber, and c and f are the velocity of light and the frequency of microwave, respectively.
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Fig. 2. FE-SEM images of (a, c, and e) MCCFs-1; and (b, d, f and g) MCCFs-2.
3.2. Morphologies of Fe3 O4 films The surface morphologies of MCCFs-1 and MCCFs-2 are shown in Fig. 2. It can be seen from Fig. 2b, d and f that the CFs are well coated by Fe3 O4 films. The thickness of the Fe3 O4 films is approximately 820 nm. Fig. 2g shows that the Fe3 O4 films of MCCFs-2 have a faceted, dense morphology, which is similar to that obtained by
Kothari and co-workers [15]. Compared with the Fe3 O4 films fabricated by sol–gel [16] and wet chemical method [17], the uniformity and compactness of the films prepared in this paper are obviously improved. Fig. 2a, c and e shows that the Fe3 O4 films of MCCFs-1 are continuous, rough, and no longer composed of numerous Fe3 O4 grains. The FE-SEM results suggest that temperature has an obvious influence on the morphology of MCCFs.
X. Meng et al. / Applied Surface Science 257 (2011) 10808–10814
Fig. 3. The magnetic hysteretic loops of (a) MCCFs-1 and (b) MCCFs-2.
3.3. Magnetic properties of MCCFs Fig. 3 shows the magnetic hysteresis loops of MCCFs-1 and MCCFs-2. The results show that the saturation magnetizations in both patterns are reached rapidly. The saturation magnetization, residual magnetization and coercivity are 5.3 emu/g, 0.6 emu/g and 89.5 Oe for MCCFs-1, and 7.0 emu/g, 1.2 emu/g and 50.2 Oe for MCCFs-2, respectively. All the corresponding values are lower than the results reported in the previous study [17], confirming that MCCFs have a weak magnetic property. 3.4. Microwave absorption properties of MCCFs It is well known that the microwave absorption mechanism of the absorption materials can be explained by dielectric loss and magnetic loss. In order to investigate the intrinsic reasons for microwave absorption property, the real (ε , ) and imaginary (ε , ) parts of relative complex permittivity and permeability of MCCF/paraffin composites were measured in the range of 2–18 GHz. The EM parameters and microwave absorption behaviors of MCCFs-1 and MCCFs-2 are shown in Figs. 4 and 5, respectively. As shown in Fig. 4a, with the increase of frequency from 2 to 18 GHz, the real part of complex permittivity ε keeps constant below 4.4 GHz and then decreases to 26.1 in the end, the imaginary part ε decreases to 59.0 after a fluctuation. The real part of complex permeability decreases to about 0.5 at 7.2 GHz and then
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increases slightly, the imaginary part increases slightly at first and then decreases closed to 0 (Fig. 4b). The dielectric loss factor (tan ıE = ε /ε ) and the magnetic loss factor (tan ıM = / ) versus frequency are plotted in Fig. 4c. The magnetic loss factor increases slightly at first and then decreases closed to 0, the maximum of tan ıM is less than 0.4. The dielectric loss factor is very high. The tan ıE obtains its minimum more than 0.5 at about 3.2 GHz and then increases quickly to 2.3. To the best knowledge of us, this value is much higher even compared with ceramic ferroelectrics, such as barium titanate and lead zirconate titanate (tan ıE = 0.1–0.6 in microwave region) [28]. The fact that tan ıE is much higher than tan ıM reveals that the RL of MCCFs-1 is mainly attributed to the dielectric loss. As shown in Fig. 5a and b, all the four parameters (ε ; ε ; ; ) of MCCFs vary nearly at the same frequency around 13 GHz. Therefore, the dielectric loss factor and the magnetic loss factor also obtained their inflection points around 13 GHz (Fig. 5c). The fact that tan ıE is higher than tan ıM suggests that the RL of MCCFs-2 is also mainly attributed to the dielectric loss. As can be seen from Figs. 4c and 5c, the magnetic loss factors of both products are very small, confirming that the contribution of magnetic loss to the RL of MCCFs is negligible. Additionally, it is obvious that the tan ıE of MCCFs-1 is much larger than that of MCCFs-2 over the whole frequency range. In general, a higher loss factor means a better attenuation characteristic. Thus, MCCFs-1 will be more likely to have a good microwave absorption property. However, the calculation results have denied this speculation. According to Eqs. (1) and (2) and using the specific EM parameters of MCCFs, the relationships of RL versus frequency for MCCFs-1 and MCCFs-2 are plotted in Figs. 4d and 5d, respectively. The calculation results confirm that the MCCFs prepared at different temperatures have totally different absorption properties. As shown in Fig. 4d, with increasing the thickness of MCCF/paraffin composite, the attenuation peak moves to the low frequency. The minimum RL is higher than −2.2 dB at the thickness 3.0 mm, which indicates that MCCFs-1 almost has no absorption property. Fig. 5d shows that MCCFs-2 has a good absorption property among all the thicknesses we have investigated. The peak values are lower than −10 dB (90% microwave absorption) within the whole range of thickness between 1.0 mm and 6.0 mm and lower than −20 dB (99% microwave absorption) within 1.7–6.0 mm. Compared with the similar MCCFs reported in the previous study [17], MCCFs-2 can obtained −10 dB and −20 dB RL in a much wider thickness range. It is commendable that MCCFs-2 can obtain −20 dB RL in a so wide matching thickness range since a RL value of −20 dB can be considered as effective for practical applications [29]. The minimum RL of composite containing MCCFs-2 is lower than −30 dB (99.9% microwave absorption) at 3.4 GHz and the corresponding thickness is 4.0 mm. It is interesting to note that as the thickness exceeds 4.0 mm in Fig. 5d, a second RL peak (the minimum RL is lower than −5 dB) appears after the main peak, which is beneficial to improve the bandwidth characteristic of microwave absorption materials. When microwave is incident on an absorber layer backed by a perfect conductor, the relation between the matching frequency f and the matching thickness d is expressed by f = nc/(4d |ε|||) (n = 1, 3, 5, 7 and 9) [30]. Hence, the attenuation peaks shift to low frequency and more attenuation peaks might appear as increasing the thickness of MCCF/paraffin composite. An excellent absorber must meet two requirements: the first is good impedance matching characteristic which allows EM waves to propagate into the absorber sufficiently and avoids the strong reflection, this is also the precondition of microwave absorption; the second is good attenuation characteristic which ensures that the incident EM waves will be attenuated rapidly through the absorber layer, thus reducing the emerging wave to an acceptable
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Fig. 4. Microwave measurement of MCCFs-1: (a) permittivity, (b) permeability, (c) loss factor, and (d) reflection loss versus frequency.
Fig. 5. Microwave measurement of MCCFs-2: (a) permittivity, (b) permeability, (c) loss factor, and (d) reflection loss versus frequency.
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generated after the fiber pretreatment. The grain shape of MCCFs0 is also different from that of MCCFs-2. Fig. 6b shows that the films of MCCFs-0 are composed of rectangle shape grains. As far as we know, rectangle shape Fe3 O4 grains have never been prepared by electrodeposition in the previous studies [12–15]. Many studies have shown that the surface chemical properties of PANbased CFs will be changed by introducing acidic oxygen-containing groups onto CFs after oxidation by nitric acid [32–34]. These acidic groups, such as carboxyl and phenolic groups [34] will adsorb Fe(III) and Fe(II) on CFs and act as nucleation sites to promote the formation of Fe3 O4 films. Therefore, the crystal nucleuses are inclined to be created on the surface of oxidized CFs during the electrodeposition process. The difference between grain shapes of MCCFs-0 and MCCFs-2 may be related to the diversity of nucleation rates on untreated and pretreated CFs. It has also been reported that the adsorption and reduction processes of metal ions on activated CFs are closely related to the surface chemical properties of CFs [35,36]. Therefore, the surface chemical properties of CFs play an important role in the composition and morphology of Fe3 O4 films. Generally, the chemical adsorption effect between surface groups and Fe(III) and/or Fe(II) will enhance the interfacial adhesion between CFs and films. 4. Conclusions
Fig. 6. XRD pattern (a) and FE-SEM image (b) of MCCFs-0.
low magnitude [31]. As summarized and elaborated by Petrov and Gagulin [28], a high ε will bring about a high reflection coefficient at the interface of absorber and air. In this work, the bad absorption property of MCCFs-1 is owing to its high ε which brings about a strong reflection and cannot meet the first condition, while MCCFs2 satisfies both conditions properly at the same time and thus has a good microwave absorption property. As the composition and morphology usually act as the fundamentally factor to influence the microwave properties of materials, which can explain why the MCCFs prepared at different temperature exhibit totally different microwave absorption properties. Therefore, one can change the preparation temperature to tune the EM parameters and optimize the performance of MCCF reinforced microwave absorption composite materials in the future applications. 3.5. Effects of fiber surface pretreatment on Fe3 O4 films To explore the influence of fiber surface pretreatment on Fe3 O4 films, comparison is made between MCCFs-0 and MCCFs-2. The composition and grain shape of MCCFs-0 are shown in Fig. 6a and b (like MCCFs-2, low magnification FE-SEM images of MCCFs-0 show that CFs are also well coated by Fe3 O4 films, the similar data are not shown again). The IM(3 1 1) /IFe(1 1 0) of MCCFs-0 (4.67, in Fig. 6a) is higher than that of MCCFs-2, which indicates that Fe tends to be
In this paper, electrodeposition was employed to fabricate Fe3 O4 films on the surface of CFs. Our results confirm that temperature and fiber surface pretreatment have a significant influence on the composition and morphology of MCCFs. Fe3 O4 films prepared at 75 ◦ C (MCCFs-0 and MCCFs-2) are uniform, dense and composed of numerous rectangle shape or faceted grains. While the films prepared at 60 ◦ C are continuous and rough. As a component contained in the films, Fe is inclined to be generated at lower temperature or on the pretreated fibers. MCCFs-1 and MCCFs-2 have a low magnetic property and exhibit totally different microwave absorption behaviors in the 2–18 GHz frequency range. MCCFs-1 almost has no absorption property owning to the overlarge permittivity. MCCFs2 exhibits a good microwave absorption property within a wide range of thickness from 1.0 mm to 6.0 mm. When the thickness of MCCF/paraffin composite exceeded 4.0 mm, there would be two attenuation peaks in the RL curves. The minimum RL of composite containing MCCFs-2 was lower than −30 dB in the thickness of 4.0 mm. Acknowledgments The authors are grateful to Litao Liu (MS) and professor Haiyan Du respectively for their courtesies of providing conveniences for the FE-SEM and XRD tests in the Analysis Center of Tianjin University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The electrochemical preparation and microwave absorption properties of magnetic carbon fibers coated with Fe3 O4 films Xianguang Meng, Yizao Wan, Qunying Li, Jing Wang, Honglin Luo ∗ School of Materials Science and Engineering, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, PR China
a r t i c l e
i n f o
Article history: Received 9 May 2011 Accepted 22 July 2011 Available online 30 July 2011 Keywords: Carbon fibers Fe3 O4 films Electrodeposition Surface treatment Microwave absorption
a b s t r a c t Electrodeposition was employed to fabricate magnetite (Fe3 O4 ) coated carbon fibers (MCCFs). Temperature and fiber surface pretreatment had a significant influence on the composition and morphology of Fe3 O4 films. Uniform and compact Fe3 O4 films were fabricated at 75 ◦ C on both nitric acid treated and untreated carbon fibers, while the films prepared at 60 ◦ C were continuous and rough. Microwave measurements of MCCF/paraffin composites (50 wt.% of MCCFs, pretreated carbon fibers as deposition substrates) were carried out in the 2–18 GHz frequency range. MCCFs prepared at 60 ◦ C obtained a much higher loss factor than that prepared at 75 ◦ C. However, the calculation results of reflection loss were very abnormal that MCCFs prepared at 60 ◦ C almost had no absorption property. While MCCFs prepared at 75 ◦ C exhibited a good absorption property and obtained −10 dB and −20 dB refection loss in wide matching thickness ranges (1.0–6.0 mm and 1.7–6.0 mm range, respectively). A secondary attenuation peak could also be observed when the thickness of MCCF/paraffin composite exceeded 4.0 mm. The minimum reflection loss was lower. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Electromagnetic (EM) wave absorption materials have attracted great attention in recent years due to the expanded EM interference problems and their potential applications in wireless data communication, local area network systems, satellite televisions, etc. [1]. Carbon fiber (CF) radar absorption materials are multifunctional composites with high strength and modulus, good carrying capacity, excellent electrical property and low radar cross-section, which are increasingly recognized as practical structural absorption composites [2]. Dielectric and magnetic loss are required for a desired microwave absorption material. As an important magnetic material, Fe3 O4 has been reported extensively in view of its excellent microwave absorption property [3–6]. Considering these unique properties of CFs and Fe3 O4 , we tried to prepare Fe3 O4 films on CFs, which are expected to exhibit good radar-absorbing property. Fe3 O4 films have been successfully fabricated on many different substrates by hydrothermal growth [7], chemical bath technique [8], sputtering [9], molecular beam epitaxy [10,11], electrodeposition [12–15], etc. So far, to the best of our knowledge, the preparation of Fe3 O4 films on CF substrates can be only achieved by sol–gel [16] and wet chemical [17] methods. However, both methods are time consuming and need a series of reaction steps.
∗ Corresponding author. Tel.: +86 22 8371 9504; fax: +86 22 8371 9504. E-mail addresses: [email protected], [email protected] (H. Luo). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.07.108
Moreover, the Fe3 O4 films are not uniform and dense, the surface quality is unsatisfactory. Electrodeposition (cathodic deposition) was developed by Kothari and co-workers [15] for preparing Fe3 O4 films on stainless steel substrates, by the electrochemical reduction of Fe(III) complexed with triethanolamine. In view of its easy operation, high efficient, low cost and the controllability of the morphologies of Fe3 O4 films, electrodeposition opens up a new way to fabricate magnetite (Fe3 O4 ) coated carbon fibers (MCCFs). Being different from the substrates used in the previous studies [12–15], CFs do not have perfect crystalline atom-ordered surfaces, whether the electrochemically prepared Fe3 O4 films on CFs can be obtained with good uniformity and compactness is also an important concern. Not just for the preparation of MCCFs, the electrodeposition may also provide a feasibility of producing other functional ceramic films, such as ZnO, CuO and Cu2 O films on CFs. These oxide films have been successfully prepared on certain substrates by electrodeposition [18–27]. It is believed that functional ceramics coated CFs produced by electrochemical deposition method will realize the integration of structural strength and functionality of composites and expand the applications of CFs in the future. In this paper, cathodic deposition was applied to fabricate MCCFs. The structure, morphology, magnetic and microwave absorption properties of MCCFs were investigated. As have been found in our experiment, temperature has the most important influence on the composition, morphology, magnetic and microwave absorption properties, comparison was made between
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MCCFs prepared at 60 ◦ C and 75 ◦ C on CFs with nitric acid pretreatment. In order to investigate the effects of surface pretreatment on the composition and morphology of final products, the untreated CFs were also used as substrates to deposit Fe3 O4 films at 75 ◦ C as reference. 2. Materials and methods 2.1. Sample preparation In this experiment, the deposition solution was prepared as the description of Kothari and co-workers [15]. The unsized PAN-based CFs used in this experiment contained 12,000 filaments each bundle and the diameter of a single filament was about 7 m. Before the cathodic deposition, CFs were pretreated with nitric acid (65 wt.%) for 2.5 h at room temperature in order to enhance the interfacial adhesion between the films and CFs, and then washed thoroughly and cut into 10 cm in length to serve as cathode. Pure iron (99.99%) sheets were polished and served as anode. The deposition reaction was conducted for 2 min under a constant electric current of 1 A at 60 ◦ C (MCCFs-1) and 75 ◦ C (MCCFs-2), respectively. The untreated CFs were also used as substrates to deposit Fe3 O4 films at 75 ◦ C (MCCFs-0). All the final products were dried in the heating oven at 80 ◦ C for 1 h, then washed with deionized water thoroughly and dried at 80 ◦ C again. 2.2. Sample characterization The products were ground into powder samples and characterized by X-ray diffraction (XRD) for phase identification using Cu K␣ radiation. XRD measurements were performed on a Rigaku D/Max 2500 v/pc X-ray diffractometer from 10◦ to 90◦ . The morphology and thickness of Fe3 O4 films were analyzed on an FEI NANOSEM 430 field emission scanning electron microscope (FE-SEM) operated at an acceleration voltage of 15.0 kV. The magnetic properties of MCCFs were measured at room temperature in a vibrating sample magnetometer (VSM) option of physical properties measurement system (PPMS-9) from Quantum Design. For microwave measurement, the MCCFs were cut into short fragments (2–3 mm in length) and mixed with paraffin through ultrasonic agitation. The mixtures were pressed into a ring form of 7 mm outer and 3 mm inner diameter, 2 mm in thickness. The relative complex permittivity (εr = εr − jεr ) and permeability (r = r − jr ) of the MCCF/paraffin composites were measured in the frequency range of 2–18 GHz over an HP8722ES vector network analyzer. The reflection loss (RL) of a microwave absorption layer backed by a perfect conductor was calculated by means of the transmit-line theory using the measured relative complex permittivity and permeability:
Fig. 1. XRD patterns of (a) MCCFs-1 and (b) MCCFs-2.
3. Results and discussion
of three phases. The broadened peak around 26◦ could be ascribed to the diffraction of graphite from the CF substrates. The other sharp peaks marked with (1 1 1), (2 2 0), (3 1 1), (4 0 0) and the like in the pictures could be indexed and assigned to face-centered cubic (fcc) Fe3 O4 (JCPDS Card No. 19-0629). The Fe3 O4 prepared by electrodeposition in this experiment is same as that reported by Kothari and co-workers [15]. The strong and sharp reflection peaks suggest that the as-prepared Fe3 O4 films are well crystallized. Besides, Fe (JCPDS Card No. 06-0696) was also found in both patterns. The ratios of the first-strong peak intensities of Fe3 O4 (3 1 1) and Fe (1 1 0) reflections (IM(3 1 1) /IFe(1 1 0) ) are 0.33 and 1.72 corresponding to MCCFs-1 and MCCFs-2, respectively, which indicate that Fe is inclined to be generated at lower temperature. The XRD patterns suggest that temperature has an obvious influence on the composition of MCCFs. A two-step electrochemical reduction and precipitation process is proposed by Kothari and co-workers [15]. When the cathodic reduction happens, Fe3 O4 is generated on the surface of CFs following the reactions outlined in Eqs. (3) and (4):
3.1. Composition of MCCFs
Fe(TEA)3+ + e− → Fe2+ + TEA
(3)
The XRD patterns of MCCFs-1 and MCCFs-2 are shown in Fig. 1. The results indicate that both MCCFs-1 and MCCFs-2 are composed
Fe2+ + 2Fe(TEA)3+ + 8OH− → Fe3 O4 + 2TEA + 4H2 O
(4)
Zin − 1 Z +1
RL (dB) = 20 log10
Zin =
r εr
(1)
in
2fd √
tanh j
c
r εr
(2)
where RL is a ratio of reflected power to incident power in dB, Zin is the input impedance of absorber, d is the thickness of the absorber, and c and f are the velocity of light and the frequency of microwave, respectively.
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Fig. 2. FE-SEM images of (a, c, and e) MCCFs-1; and (b, d, f and g) MCCFs-2.
3.2. Morphologies of Fe3 O4 films The surface morphologies of MCCFs-1 and MCCFs-2 are shown in Fig. 2. It can be seen from Fig. 2b, d and f that the CFs are well coated by Fe3 O4 films. The thickness of the Fe3 O4 films is approximately 820 nm. Fig. 2g shows that the Fe3 O4 films of MCCFs-2 have a faceted, dense morphology, which is similar to that obtained by
Kothari and co-workers [15]. Compared with the Fe3 O4 films fabricated by sol–gel [16] and wet chemical method [17], the uniformity and compactness of the films prepared in this paper are obviously improved. Fig. 2a, c and e shows that the Fe3 O4 films of MCCFs-1 are continuous, rough, and no longer composed of numerous Fe3 O4 grains. The FE-SEM results suggest that temperature has an obvious influence on the morphology of MCCFs.
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Fig. 3. The magnetic hysteretic loops of (a) MCCFs-1 and (b) MCCFs-2.
3.3. Magnetic properties of MCCFs Fig. 3 shows the magnetic hysteresis loops of MCCFs-1 and MCCFs-2. The results show that the saturation magnetizations in both patterns are reached rapidly. The saturation magnetization, residual magnetization and coercivity are 5.3 emu/g, 0.6 emu/g and 89.5 Oe for MCCFs-1, and 7.0 emu/g, 1.2 emu/g and 50.2 Oe for MCCFs-2, respectively. All the corresponding values are lower than the results reported in the previous study [17], confirming that MCCFs have a weak magnetic property. 3.4. Microwave absorption properties of MCCFs It is well known that the microwave absorption mechanism of the absorption materials can be explained by dielectric loss and magnetic loss. In order to investigate the intrinsic reasons for microwave absorption property, the real (ε , ) and imaginary (ε , ) parts of relative complex permittivity and permeability of MCCF/paraffin composites were measured in the range of 2–18 GHz. The EM parameters and microwave absorption behaviors of MCCFs-1 and MCCFs-2 are shown in Figs. 4 and 5, respectively. As shown in Fig. 4a, with the increase of frequency from 2 to 18 GHz, the real part of complex permittivity ε keeps constant below 4.4 GHz and then decreases to 26.1 in the end, the imaginary part ε decreases to 59.0 after a fluctuation. The real part of complex permeability decreases to about 0.5 at 7.2 GHz and then
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increases slightly, the imaginary part increases slightly at first and then decreases closed to 0 (Fig. 4b). The dielectric loss factor (tan ıE = ε /ε ) and the magnetic loss factor (tan ıM = / ) versus frequency are plotted in Fig. 4c. The magnetic loss factor increases slightly at first and then decreases closed to 0, the maximum of tan ıM is less than 0.4. The dielectric loss factor is very high. The tan ıE obtains its minimum more than 0.5 at about 3.2 GHz and then increases quickly to 2.3. To the best knowledge of us, this value is much higher even compared with ceramic ferroelectrics, such as barium titanate and lead zirconate titanate (tan ıE = 0.1–0.6 in microwave region) [28]. The fact that tan ıE is much higher than tan ıM reveals that the RL of MCCFs-1 is mainly attributed to the dielectric loss. As shown in Fig. 5a and b, all the four parameters (ε ; ε ; ; ) of MCCFs vary nearly at the same frequency around 13 GHz. Therefore, the dielectric loss factor and the magnetic loss factor also obtained their inflection points around 13 GHz (Fig. 5c). The fact that tan ıE is higher than tan ıM suggests that the RL of MCCFs-2 is also mainly attributed to the dielectric loss. As can be seen from Figs. 4c and 5c, the magnetic loss factors of both products are very small, confirming that the contribution of magnetic loss to the RL of MCCFs is negligible. Additionally, it is obvious that the tan ıE of MCCFs-1 is much larger than that of MCCFs-2 over the whole frequency range. In general, a higher loss factor means a better attenuation characteristic. Thus, MCCFs-1 will be more likely to have a good microwave absorption property. However, the calculation results have denied this speculation. According to Eqs. (1) and (2) and using the specific EM parameters of MCCFs, the relationships of RL versus frequency for MCCFs-1 and MCCFs-2 are plotted in Figs. 4d and 5d, respectively. The calculation results confirm that the MCCFs prepared at different temperatures have totally different absorption properties. As shown in Fig. 4d, with increasing the thickness of MCCF/paraffin composite, the attenuation peak moves to the low frequency. The minimum RL is higher than −2.2 dB at the thickness 3.0 mm, which indicates that MCCFs-1 almost has no absorption property. Fig. 5d shows that MCCFs-2 has a good absorption property among all the thicknesses we have investigated. The peak values are lower than −10 dB (90% microwave absorption) within the whole range of thickness between 1.0 mm and 6.0 mm and lower than −20 dB (99% microwave absorption) within 1.7–6.0 mm. Compared with the similar MCCFs reported in the previous study [17], MCCFs-2 can obtained −10 dB and −20 dB RL in a much wider thickness range. It is commendable that MCCFs-2 can obtain −20 dB RL in a so wide matching thickness range since a RL value of −20 dB can be considered as effective for practical applications [29]. The minimum RL of composite containing MCCFs-2 is lower than −30 dB (99.9% microwave absorption) at 3.4 GHz and the corresponding thickness is 4.0 mm. It is interesting to note that as the thickness exceeds 4.0 mm in Fig. 5d, a second RL peak (the minimum RL is lower than −5 dB) appears after the main peak, which is beneficial to improve the bandwidth characteristic of microwave absorption materials. When microwave is incident on an absorber layer backed by a perfect conductor, the relation between the matching frequency f and the matching thickness d is expressed by f = nc/(4d |ε|||) (n = 1, 3, 5, 7 and 9) [30]. Hence, the attenuation peaks shift to low frequency and more attenuation peaks might appear as increasing the thickness of MCCF/paraffin composite. An excellent absorber must meet two requirements: the first is good impedance matching characteristic which allows EM waves to propagate into the absorber sufficiently and avoids the strong reflection, this is also the precondition of microwave absorption; the second is good attenuation characteristic which ensures that the incident EM waves will be attenuated rapidly through the absorber layer, thus reducing the emerging wave to an acceptable
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Fig. 4. Microwave measurement of MCCFs-1: (a) permittivity, (b) permeability, (c) loss factor, and (d) reflection loss versus frequency.
Fig. 5. Microwave measurement of MCCFs-2: (a) permittivity, (b) permeability, (c) loss factor, and (d) reflection loss versus frequency.
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generated after the fiber pretreatment. The grain shape of MCCFs0 is also different from that of MCCFs-2. Fig. 6b shows that the films of MCCFs-0 are composed of rectangle shape grains. As far as we know, rectangle shape Fe3 O4 grains have never been prepared by electrodeposition in the previous studies [12–15]. Many studies have shown that the surface chemical properties of PANbased CFs will be changed by introducing acidic oxygen-containing groups onto CFs after oxidation by nitric acid [32–34]. These acidic groups, such as carboxyl and phenolic groups [34] will adsorb Fe(III) and Fe(II) on CFs and act as nucleation sites to promote the formation of Fe3 O4 films. Therefore, the crystal nucleuses are inclined to be created on the surface of oxidized CFs during the electrodeposition process. The difference between grain shapes of MCCFs-0 and MCCFs-2 may be related to the diversity of nucleation rates on untreated and pretreated CFs. It has also been reported that the adsorption and reduction processes of metal ions on activated CFs are closely related to the surface chemical properties of CFs [35,36]. Therefore, the surface chemical properties of CFs play an important role in the composition and morphology of Fe3 O4 films. Generally, the chemical adsorption effect between surface groups and Fe(III) and/or Fe(II) will enhance the interfacial adhesion between CFs and films. 4. Conclusions
Fig. 6. XRD pattern (a) and FE-SEM image (b) of MCCFs-0.
low magnitude [31]. As summarized and elaborated by Petrov and Gagulin [28], a high ε will bring about a high reflection coefficient at the interface of absorber and air. In this work, the bad absorption property of MCCFs-1 is owing to its high ε which brings about a strong reflection and cannot meet the first condition, while MCCFs2 satisfies both conditions properly at the same time and thus has a good microwave absorption property. As the composition and morphology usually act as the fundamentally factor to influence the microwave properties of materials, which can explain why the MCCFs prepared at different temperature exhibit totally different microwave absorption properties. Therefore, one can change the preparation temperature to tune the EM parameters and optimize the performance of MCCF reinforced microwave absorption composite materials in the future applications. 3.5. Effects of fiber surface pretreatment on Fe3 O4 films To explore the influence of fiber surface pretreatment on Fe3 O4 films, comparison is made between MCCFs-0 and MCCFs-2. The composition and grain shape of MCCFs-0 are shown in Fig. 6a and b (like MCCFs-2, low magnification FE-SEM images of MCCFs-0 show that CFs are also well coated by Fe3 O4 films, the similar data are not shown again). The IM(3 1 1) /IFe(1 1 0) of MCCFs-0 (4.67, in Fig. 6a) is higher than that of MCCFs-2, which indicates that Fe tends to be
In this paper, electrodeposition was employed to fabricate Fe3 O4 films on the surface of CFs. Our results confirm that temperature and fiber surface pretreatment have a significant influence on the composition and morphology of MCCFs. Fe3 O4 films prepared at 75 ◦ C (MCCFs-0 and MCCFs-2) are uniform, dense and composed of numerous rectangle shape or faceted grains. While the films prepared at 60 ◦ C are continuous and rough. As a component contained in the films, Fe is inclined to be generated at lower temperature or on the pretreated fibers. MCCFs-1 and MCCFs-2 have a low magnetic property and exhibit totally different microwave absorption behaviors in the 2–18 GHz frequency range. MCCFs-1 almost has no absorption property owning to the overlarge permittivity. MCCFs2 exhibits a good microwave absorption property within a wide range of thickness from 1.0 mm to 6.0 mm. When the thickness of MCCF/paraffin composite exceeded 4.0 mm, there would be two attenuation peaks in the RL curves. The minimum RL of composite containing MCCFs-2 was lower than −30 dB in the thickness of 4.0 mm. Acknowledgments The authors are grateful to Litao Liu (MS) and professor Haiyan Du respectively for their courtesies of providing conveniences for the FE-SEM and XRD tests in the Analysis Center of Tianjin University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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