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Fabrication and microwave absorption properties of carbon-coated cementite nanocapsules
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 035704 (http://iopscience.iop.org/0957-4484/25/3/035704) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.203.227.61 This content was downloaded on 26/06/2014 at 20:12
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 035704 (5pp)
doi:10.1088/0957-4484/25/3/035704
Fabrication and microwave absorption properties of carbon-coated cementite nanocapsules Y Tang1 , Y Shao1 , K F Yao1 and Y X Zhong2 1
School of Material Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China 2 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China E-mail: [email protected] Received 21 September 2013, revised 4 November 2013 Accepted for publication 18 November 2013 Published 20 December 2013 Abstract
By utilizing a simple and low-cost arc-discharge method in either liquid nitrogen or ethanol at ambient temperature and pressure, carbon-coated cementite (Fe3 C) nanocapsules, with size ranges of 10–60 nm and 10–20 nm, respectively, have been synthesized on a large scale. The Fe3 C/C nanocapsules synthesized in different media possess similar permeability but different permittivity, which results from the different defect amounts within the carbon shell. It has been found that the as-prepared products exhibit different electromagnetic wave absorption abilities: for the ones prepared in liquid nitrogen, the optimal reflection loss is above −10 dB in the range of 1–18 GHz with the thickness ranging from 1 to 10 mm; meanwhile, for those fabricated in ethanol, the reflection loss could be below −20 dB within the thickness range of 1.5–2.4 mm in the frequency range of 10–15 GHz, and reach −38 dB at a thickness of 1.9 mm with a matching frequency of 12.9 GHz. This indicates that the nanocapsules prepared in ethanol exhibit good electromagnetic wave absorption properties. These results provide a new way to fabricate carbon-coated Fe3 C nanocapsules with the ability of electromagnetic wave absorption. Keywords: nanocapsules, microwave absorption, carbon shell (Some figures may appear in colour only in the online journal)
1. Introduction
a result of eddy current loss, the complex permeability of metallic magnets will decrease in the high frequency range [3, 4]. Thus, composites with metallic magnets with diameters of less than skin depth are widely studied, and show promising prospects, such as α-Fe/SmO, α-Fe/Y2 O3 , and (Fe, Ni)/C [6–9]. Among them, Fe(C)/C nanocapsules with a core–shell structure attract extensive interest as the onion-like graphite shell, similarly to carbon nanotubes (CNTs), possesses good dielectric properties and an enhanced cooperative effect with the ferromagnetic cores [3, 4, 10]. Moreover, attributed to the higher ratio of iron content, Fe(C)/C nanocapsules exhibit better wave absorption ability than Fe/CNT composites: the optimal reflection loss (RL) of
Nowadays, with an increasing number of devices using electromagnetic (EM) waves in the range of GHz, much attention has been focused on EM absorbing materials for their applications in stealth technology, anti-electromagnetic interference, and microwave dark rooms [1, 2]. It is reported that, in comparison with the traditionally used ferrite, with metallic magnets, such as iron and nickel, it is possible to design thinner microwave absorbers in the GHz range, due to their high complex permeability values in a wide frequency range, which is ascribed to their large saturation magnetization and high Snoek’s limit [3–5]. However, as 0957-4484/14/035704+05$33.00
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c 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 035704
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Fe(C)/C can reach as low as −40 dB but that of Fe/CNTs is always above −25 dB [3, 4, 9, 10]. However, although Fe(C)/C nanocapsules possess excellent wave absorption ability, their application is limited by the cost and low yield ratio of their fabrication method: arc-discharge in gas [3, 9, 11, 12]. To bypass the intricacy of the vacuum system and make large-scale synthesis possible, in this work, arc-discharge in liquid phase was applied to fabricate carbon-coated cementite nanocapsules. The microstructure, magnetic properties and microwave absorption abilities of the as-prepared carbon-coated cementites were studied. The results showed that carbon-coated cementites prepared in ethanol with diameter of around 10–20 nm demonstrate an optimal RL of −38 dB at a thickness of 1.9 mm with a matching frequency of 12.9 GHz, possessing a better high-frequency wave absorption ability with a smaller thickness than nanocapsules prepared by arc-discharge in methane [3]. By studying the differences in Fe3 C/C nanocapsules prepared in different media, we found that the defects in the onion-like graphite layer played an important role in the microwave absorption ability, which means that Fe3 C/C nanocapsules with different wave absorption properties can be obtained by adjusting the medium and process conditions. This may provide a new way to fabricate promising EM wave absorbers.
Figure 1. XRD patterns of products prepared in ethanol ((a), (b))
and liquid nitrogen ((c), (d)) before ((a) and (c)) and after ((b), (d)) acid washing.
where f is the frequency and d is the thickness of the absorber, c is the velocity of light, Z0 is the impedance of air, and Zin is the input impedance of the absorber. 3. Results and discussion
The XRD spectra of the as-prepared and acid-washed samples prepared by arc-discharge in ethanol and liquid nitrogen are shown in figure 1. The diffraction peaks of the four samples are indexed as cementite, graphite (26◦ , corresponding to (002) layer spacing) and α-Fe. These confirm that the major product is cementite. After acid washing, the intensity of the α-Fe peak is decreased or barely detected for samples prepared in both media. This phenomenon may be ascribed to the fact that most α-Fe nanoparticles are not wrapped with a carbon layer. Besides, the graphite peak for samples prepared in the medium of liquid nitrogen almost vanished after acid washing but the intensity of the graphite peak for samples prepared in ethanol increased after acid washing. The carbon source may be the key to explaining this phenomenon. In liquid nitrogen, the only carbon source is the graphite cathode and the nanocapsules could only be fabricated around the cathode graphite rod, which results in a high content of residual graphite and a low ratio of carbon to cementite in the nanocapsules. Therefore, after acid washing, the residual graphite will be removed and the intensity of the carbon peak will decrease to be barely noticeable, which is consistent with the report by Liu [9]. In ethanol, besides the graphite cathode, carbon could come from the pyrolysis of ethanol, which augments the number of nanocapsules and the ratio of carbon to cementite. Therefore, the intensity of the carbon peak increases after eliminating residual iron and nanoparticles without integral carbon layers. High-resolution TEM was employed to further characterize the microstructures of the products after acid washing. The TEM results for the products are shown in figure 2. Through the TEM examination, it was found that the size of nanoparticles prepared in ethanol is around 10–20 nm (figures 2(a) and (b)), while the size of those prepared in liquid nitrogen is about 10–60 nm (figures 2(c) and (d)).
2. Experimental details
The Fe3 C/C nanocapsules were prepared using a handmade arc-discharge apparatus [13]. The detailed synthesis process has been described in previous work [14]. A graphite rod of 12 mm in diameter and an industrial pure iron bar (99.4%) of 6 mm in diameter were used as cathode and anode, respectively. During the arc-discharge process, the electrodes were submerged in a medium of liquid nitrogen or ethanol. The as-prepared particles were washed with hydrochloric acid (HCl) to eliminate residual iron particles without the carbon shell, then cleansed with deionized water and ethanol with magnets to eliminate carbon shell without cementite and amorphous carbon. The resultant powders were characterized by x-ray diffraction (XRD, Rigaku D/maxRB) and high-resolution transmission electron microscopy (HRTEM, Tecnai F20). The magnetic property measurements were carried out using a vibrating sample magnetometer (VSM, LakeShore 7307). The Fe3 C/C nanocapsule/paraffin composite sample used for EM property measurement was prepared by homogeneously mixing paraffin with 40 wt% resultant powder and pressing the mixture into a pellet with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. The EM properties (complex permeability and permittivity) of the composites with 40 wt% Fe3 C/C nanocapsules were measured in the range of 1–18 GHz using an Agilent HP-8722ES network analyzer. The reflection loss (RL) curves were calculated from the complex permeability (εr ) and permittivity (µr ) at the given frequency and absorber thickness according to the following equations [1, 4]: Zin = Z0 (µr /εr )1/2 tanh[j(2π fd/c)(µr εr )1/2 ], RL = 20 lg |(Zin − Z0 )/(Zin + Z0 )|, 2
Nanotechnology 25 (2014) 035704
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Figure 3. Magnetization hysteresis loops of the Fe3 C/C nanocapsules prepared in liquid nitrogen and ethanol.
thickness of the outer carbon layer. From figure 2, it is clear that the ratio of Fe3 C over C for nanocapsules prepared in liquid nitrogen is much higher than for the ones prepared in ethanol. This also suggests that during the synthesis process the ethanol plays an important role as a carbon supply. Figure 4 shows the frequency dependence of the permeability and the permittivity (real and imaginary) of the Fe3 C/C nanocapsule–paraffin composite. As shown in figure 4(a), the products prepared in different media have complex permeabilities in the same ranges. The real part (µ0r ) lies in the range of 0.9–1.1, while the imaginary part (µ00r ) lies in the range of 0.0–0.2. It is reported that the complex permeability is affected by the inner magnetic core’s composition and shape (the anisotropy field) [10]. According to the XRD pattern and TEM images, the inner cores of the nanoparticles prepared in different media are both spherical Fe3 C and of a similar size. Figure 4(b) shows that there is a significant difference in the complex permittivity, in both the real and imaginary parts, for the two kinds of samples. For the nanocapsules prepared in liquid nitrogen, both the real and imaginary parts (εr0 and εr00 ) stay almost constant, with values of 3 and 0, respectively, in the frequency range of 1–18 GHz. However, for the ones prepared in ethanol, the real part εr0 decreases from 13 to 9 while the imaginary part εr00 fluctuates between 2 and 4 with increase of frequency. It is reported that a higher real permittivity presents a higher polarization and a higher imaginary permittivity presents a higher absorption or electric loss [15]. From the XRD and TEM results, the major difference of the products prepared in different media is the number of defects within the carbon shell. According to the study of Watts et al [15], the introduction of lattice defects will act as polarized centers which would account for the higher real permittivity for the nanocapsules prepared in ethanol. Besides, the defects will create localized states near the Fermi level and lead to absorption by contiguous states to the Fermi level, which relates to the higher imaginary part under ethanol medium. Therefore, the huge difference in the complex permittivity of the nanocapsules prepared in different
Figure 2. TEM and HRTEM images of the products: (a) and (b)
were prepared in ethanol and (c) and (d) were prepared in liquid nitrogen.
Based on the HRTEM images and XRD patterns, it was confirmed that all particles are covered by a layered structure and form a core–shell structure with an Fe3 C core. As shown in figure 2(d), the lattice distance of the outer layer structure is measured as 0.34 nm, corresponding to the (002) plane of graphite, in agreement with the XRD analysis. This indicates that the as-prepared nanocapsule is composed of an outer layer of graphite carbon with an onion-like structure and an Fe3 C core. However, for the particles prepared in ethanol medium (figure 2(b)), the outer layer exhibits distorted lattice fringes, which are confirmed as carbon too. This indicates that, in comparison with those prepared in liquid nitrogen, the nanoparticles prepared in ethanol possess the same capsule structure but more lattice defects in the carbon outer layer and smaller Fe3 C cores. Since the large saturation magnetization plays a major role in the microwave absorption ability of the metallic magnet composite [5], the magnetic hysteresis loops of the samples were measured and are shown in figure 3. The saturation magnetization of the Fe3 C/C nanocapsules reaches 110 emu g−1 for the products prepared in ethanol and 150 emu g−1 for those prepared in the liquid nitrogen medium; these values are much higher than those of Fe/C (104.6 emu g−1 ) and (Fe, Ni)/C nanocapsules (106.1 emu g−1 ) reported before [3, 9]. Because the magnetic properties are mainly contributed by the Fe3 C particles, the higher ratio of Fe3 C over C for the nanocapsules prepared in liquid nitrogen may explain their larger saturation magnetization. From the XRD spectra, for the nanocapsules prepared in liquid nitrogen, after acid washing, the intensity of the graphite peak is reduced significantly. On the other hand, based on the TEM results, the ratio of Fe3 C over C could be roughly estimated from the diameter of the Fe3 C core and the 3
Nanotechnology 25 (2014) 035704
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Figure 4. The relative permittivity (εr ) (a) and permeability (µr ) (b) plotted against frequency for the Fe3 C/C nanocapsule–paraffin
composite.
Figure 5. The frequency dependence of the RL of Fe3 C/C nanocapsule–paraffin composite of different thicknesses (mm): (a) nanocapsules
prepared in liquid nitrogen, (b) and (c) nanocapsules prepared in ethanol.
absorption, thus RL < −20 dB is believed to be an adequate EM absorption level. Therefore, in comparison with the nanocapsules prepared in liquid nitrogen, the ones prepared in ethanol with 2 mm thickness are effective absorbers in the range of 10–15 GHz. In addition, as shown in figure 5(c), for a thickness of around 2 mm, the optimal RL of nanoparticles prepared in ethanol could reach −38 dB at a thickness of 1.9 mm with a matching frequency of 12.9 GHz, which is higher than previously reported iron–carbon composite results. For example, the α-Fe powders prepared by Liu showed an optimal RL of around −10 dB at a thickness of 5 mm in the range of frequency from 2 to 18 GHz [4],
media in this work would be mainly ascribed to the lattice defects within the carbon shell (figure 2). In addition, owing to the higher ratio of carbon shells, the nanocapsules prepared in ethanol possess far more lattice defects, which would further contribute to the difference of permittivity. Figure 5 shows the frequency dependence of the RL for the nanocapsule–paraffin composite in the range of 1–18 GHz. As shown in figures 5(a) and (b), with a thickness of 1–10 mm, the optimal RL of the nanocapsules prepared in liquid nitrogen is −5.66 dB, and the ones prepared in ethanol reach up to −24.5 dB. As illustrated by Liu et al [4], the case when RL = −20 dB is comparable to the 99% EM wave 4
Nanotechnology 25 (2014) 035704
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References
the (Fe, Ni)/C nanocapsules, prepared by arc-discharging in methane, exhibited an optimal RL of −26.9 dB at a thickness of 2.0 mm [9], and the optimal RL of the α-Fe/CNT composite prepared by Che was around −25 dB [10]. According to the results of Liu et al [4], Fe3 C/C composites (the size of the particles was distributed from 100 to several hundred nanometers, obtained by a mechanical alloying method) show excellent wave absorption in the 18–26.5 GHz range, reaching −46 dB at a frequency of 20.2 GHz with a thickness of 1.2 mm [4]. Due to the limitations of our apparatus, EM wave absorption in the frequency range higher than 18 GHz was not measured in this experiment. However, in comparison with the maximum wave absorption peak of about −31 dB in the frequency range from 0 to 18 GHz in Liu’s study [4], the nanocapsules prepared in ethanol in the present work show a better wave absorption ability. Therefore, it is expected that the nanocapsules prepared in ethanol might possess promising EM wave absorption ability in the frequency range higher than 18 GHz. The present results indicate that nanocapsules prepared in ethanol may exhibit excellent EM wave absorption ability in a wide frequency range. They also suggest that the number of defects in the carbon shells of the nanocapsules plays an important role in the EM wave absorption property. The present results provide a new way of preparing EM wave absorption materials.
[1] Wang Y M, Li T X, Zhao L F, Hu Z W and Gu Y 2011 Research progress on nanostructured radar absorbing materials Energy Power Eng. 3 580–4 [2] Terada M, Itoh M, Liu J R and Machida K 2009 Electromagnetic wave absorption properties of Fe3 C/carbon nanocomposites prepared by a CVD method J. Magn. Magn. Mater. 321 1209–13 [3] Zhang X F, Dong X L, Huang H, Lv B, Lei J P and Choi C J 2007 Microstructure and microwave absorption properties of carbon-coated iron nanocapsules J. Phys. D: Appl. Phys. 40 5383–7 [4] Liu J R, Itoh M, Horikawa T, Taguchi E, Mori H and Machida K 2006 Iron based carbon nanocomposites for electromagnetic wave absorber with wide bandwidth in GHz range Appl. Phys. A 82 509–13 [5] Che R C, Zhi C Y, Liang C Y and Zhou X G 2006 Fabrication and microwave absorption of carbon nanotubes/CoFe2 O4 spinel nanocomposite Appl. Phys. Lett. 88 033105 [6] Sugimoto S et al 2002 GHz microwave absorption of a fine α-Fe structure produced by the disproportionation of Sm2 Fe17 in the hydrogen J. Alloys Compounds 330–332 301–6 [7] Liu J R, Itoh M and Machida K 2003 GHz range absorption properties of α-Fe/Y2 O3 nanocomposites prepared by melt-spun technique Chem. Lett. 32 394–5 [8] Liu J R, Itoh M and Machida K 2003 Electromagnetic wave absorption properties of alpha-Fe/Fe3 B/Y2 O3 nanocomposites in gigahertz range Appl. Phys. Lett. 83 4017–20 [9] Liu X G, Geng D Y, Cui W B, Yang F, Xie Z G, Kang D J and Zhang Z D 2009 (Fe, Ni)/C nanocapsules for electromagnetic-wave-absorber in the whole Ku-band Carbon 47 470–4 [10] Che R C, Peng L M, Duan X F, Chen Q and Liang X L 2004 Microwave absorption enhancement and complex permittivity and permeability of Fe encapsulated within carbon nanotubes Adv. Mater. 16 401–5 [11] Dong X L et al 1998 Characterization of ultrafine γ -Fe(C), α-Fe(C) and Fe3 C particles synthesized by arc-discharge in methane J. Mater. Sci. 33 1915–9 [12] Sun X C and Nava N 2002 Microstructure and magnetic properties of Fe(C) and Fe(O) nanoparticles Nano Lett. 2 765–9 [13] Liu X M and Yao K F 2005 Large-scale synthesis and photoluminescence properties of SiC/SiOx nanocable nanocables Nanotechnology 16 2932–5 [14] Fan X L and Yao K F 2007 Structural and magnetic properties of Fe3 O4 nanoparticles prepared by arc-discharge in water Chin. Sci. Bull. 52 2866–70 [15] Watts P, Hsu W K, Barns A and Chambers B 2003 High permittivity from defective multiwalled carbon nanotubes in the X-band Adv. Mater. 15 600–3
4. Conclusion
Fe3 C/C nanocapsules, with diameter distributions of 10–60 and 10–20 nm, were prepared by applying arc-discharge submerged in liquid nitrogen or ethanol under ambient pressure. In comparison with the limited EM wave absorption ability for the ones prepared in liquid nitrogen, the RL values for the nanocapsules prepared in ethanol could exceed −20 dB in the frequency range of 10–15 GHz at a thickness of around 2 mm. The significant difference in EM wave absorption property of these two kinds of nanoparticles prepared in different media results from the number of defect within the outer carbon layer by changing the dielectric properties of the nanocapsules. These results indicate that the outer carbon layer plays an important role in EM absorption properties rather than just separating the inner magnetic particles to suppress the eddy current. This result offers a new way to design EM wave absorption materials.
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My IOPscience
Fabrication and microwave absorption properties of carbon-coated cementite nanocapsules
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 035704 (http://iopscience.iop.org/0957-4484/25/3/035704) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.203.227.61 This content was downloaded on 26/06/2014 at 20:12
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 035704 (5pp)
doi:10.1088/0957-4484/25/3/035704
Fabrication and microwave absorption properties of carbon-coated cementite nanocapsules Y Tang1 , Y Shao1 , K F Yao1 and Y X Zhong2 1
School of Material Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China 2 Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China E-mail: [email protected] Received 21 September 2013, revised 4 November 2013 Accepted for publication 18 November 2013 Published 20 December 2013 Abstract
By utilizing a simple and low-cost arc-discharge method in either liquid nitrogen or ethanol at ambient temperature and pressure, carbon-coated cementite (Fe3 C) nanocapsules, with size ranges of 10–60 nm and 10–20 nm, respectively, have been synthesized on a large scale. The Fe3 C/C nanocapsules synthesized in different media possess similar permeability but different permittivity, which results from the different defect amounts within the carbon shell. It has been found that the as-prepared products exhibit different electromagnetic wave absorption abilities: for the ones prepared in liquid nitrogen, the optimal reflection loss is above −10 dB in the range of 1–18 GHz with the thickness ranging from 1 to 10 mm; meanwhile, for those fabricated in ethanol, the reflection loss could be below −20 dB within the thickness range of 1.5–2.4 mm in the frequency range of 10–15 GHz, and reach −38 dB at a thickness of 1.9 mm with a matching frequency of 12.9 GHz. This indicates that the nanocapsules prepared in ethanol exhibit good electromagnetic wave absorption properties. These results provide a new way to fabricate carbon-coated Fe3 C nanocapsules with the ability of electromagnetic wave absorption. Keywords: nanocapsules, microwave absorption, carbon shell (Some figures may appear in colour only in the online journal)
1. Introduction
a result of eddy current loss, the complex permeability of metallic magnets will decrease in the high frequency range [3, 4]. Thus, composites with metallic magnets with diameters of less than skin depth are widely studied, and show promising prospects, such as α-Fe/SmO, α-Fe/Y2 O3 , and (Fe, Ni)/C [6–9]. Among them, Fe(C)/C nanocapsules with a core–shell structure attract extensive interest as the onion-like graphite shell, similarly to carbon nanotubes (CNTs), possesses good dielectric properties and an enhanced cooperative effect with the ferromagnetic cores [3, 4, 10]. Moreover, attributed to the higher ratio of iron content, Fe(C)/C nanocapsules exhibit better wave absorption ability than Fe/CNT composites: the optimal reflection loss (RL) of
Nowadays, with an increasing number of devices using electromagnetic (EM) waves in the range of GHz, much attention has been focused on EM absorbing materials for their applications in stealth technology, anti-electromagnetic interference, and microwave dark rooms [1, 2]. It is reported that, in comparison with the traditionally used ferrite, with metallic magnets, such as iron and nickel, it is possible to design thinner microwave absorbers in the GHz range, due to their high complex permeability values in a wide frequency range, which is ascribed to their large saturation magnetization and high Snoek’s limit [3–5]. However, as 0957-4484/14/035704+05$33.00
1
c 2014 IOP Publishing Ltd Printed in the UK
Nanotechnology 25 (2014) 035704
Y Tang et al
Fe(C)/C can reach as low as −40 dB but that of Fe/CNTs is always above −25 dB [3, 4, 9, 10]. However, although Fe(C)/C nanocapsules possess excellent wave absorption ability, their application is limited by the cost and low yield ratio of their fabrication method: arc-discharge in gas [3, 9, 11, 12]. To bypass the intricacy of the vacuum system and make large-scale synthesis possible, in this work, arc-discharge in liquid phase was applied to fabricate carbon-coated cementite nanocapsules. The microstructure, magnetic properties and microwave absorption abilities of the as-prepared carbon-coated cementites were studied. The results showed that carbon-coated cementites prepared in ethanol with diameter of around 10–20 nm demonstrate an optimal RL of −38 dB at a thickness of 1.9 mm with a matching frequency of 12.9 GHz, possessing a better high-frequency wave absorption ability with a smaller thickness than nanocapsules prepared by arc-discharge in methane [3]. By studying the differences in Fe3 C/C nanocapsules prepared in different media, we found that the defects in the onion-like graphite layer played an important role in the microwave absorption ability, which means that Fe3 C/C nanocapsules with different wave absorption properties can be obtained by adjusting the medium and process conditions. This may provide a new way to fabricate promising EM wave absorbers.
Figure 1. XRD patterns of products prepared in ethanol ((a), (b))
and liquid nitrogen ((c), (d)) before ((a) and (c)) and after ((b), (d)) acid washing.
where f is the frequency and d is the thickness of the absorber, c is the velocity of light, Z0 is the impedance of air, and Zin is the input impedance of the absorber. 3. Results and discussion
The XRD spectra of the as-prepared and acid-washed samples prepared by arc-discharge in ethanol and liquid nitrogen are shown in figure 1. The diffraction peaks of the four samples are indexed as cementite, graphite (26◦ , corresponding to (002) layer spacing) and α-Fe. These confirm that the major product is cementite. After acid washing, the intensity of the α-Fe peak is decreased or barely detected for samples prepared in both media. This phenomenon may be ascribed to the fact that most α-Fe nanoparticles are not wrapped with a carbon layer. Besides, the graphite peak for samples prepared in the medium of liquid nitrogen almost vanished after acid washing but the intensity of the graphite peak for samples prepared in ethanol increased after acid washing. The carbon source may be the key to explaining this phenomenon. In liquid nitrogen, the only carbon source is the graphite cathode and the nanocapsules could only be fabricated around the cathode graphite rod, which results in a high content of residual graphite and a low ratio of carbon to cementite in the nanocapsules. Therefore, after acid washing, the residual graphite will be removed and the intensity of the carbon peak will decrease to be barely noticeable, which is consistent with the report by Liu [9]. In ethanol, besides the graphite cathode, carbon could come from the pyrolysis of ethanol, which augments the number of nanocapsules and the ratio of carbon to cementite. Therefore, the intensity of the carbon peak increases after eliminating residual iron and nanoparticles without integral carbon layers. High-resolution TEM was employed to further characterize the microstructures of the products after acid washing. The TEM results for the products are shown in figure 2. Through the TEM examination, it was found that the size of nanoparticles prepared in ethanol is around 10–20 nm (figures 2(a) and (b)), while the size of those prepared in liquid nitrogen is about 10–60 nm (figures 2(c) and (d)).
2. Experimental details
The Fe3 C/C nanocapsules were prepared using a handmade arc-discharge apparatus [13]. The detailed synthesis process has been described in previous work [14]. A graphite rod of 12 mm in diameter and an industrial pure iron bar (99.4%) of 6 mm in diameter were used as cathode and anode, respectively. During the arc-discharge process, the electrodes were submerged in a medium of liquid nitrogen or ethanol. The as-prepared particles were washed with hydrochloric acid (HCl) to eliminate residual iron particles without the carbon shell, then cleansed with deionized water and ethanol with magnets to eliminate carbon shell without cementite and amorphous carbon. The resultant powders were characterized by x-ray diffraction (XRD, Rigaku D/maxRB) and high-resolution transmission electron microscopy (HRTEM, Tecnai F20). The magnetic property measurements were carried out using a vibrating sample magnetometer (VSM, LakeShore 7307). The Fe3 C/C nanocapsule/paraffin composite sample used for EM property measurement was prepared by homogeneously mixing paraffin with 40 wt% resultant powder and pressing the mixture into a pellet with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. The EM properties (complex permeability and permittivity) of the composites with 40 wt% Fe3 C/C nanocapsules were measured in the range of 1–18 GHz using an Agilent HP-8722ES network analyzer. The reflection loss (RL) curves were calculated from the complex permeability (εr ) and permittivity (µr ) at the given frequency and absorber thickness according to the following equations [1, 4]: Zin = Z0 (µr /εr )1/2 tanh[j(2π fd/c)(µr εr )1/2 ], RL = 20 lg |(Zin − Z0 )/(Zin + Z0 )|, 2
Nanotechnology 25 (2014) 035704
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Figure 3. Magnetization hysteresis loops of the Fe3 C/C nanocapsules prepared in liquid nitrogen and ethanol.
thickness of the outer carbon layer. From figure 2, it is clear that the ratio of Fe3 C over C for nanocapsules prepared in liquid nitrogen is much higher than for the ones prepared in ethanol. This also suggests that during the synthesis process the ethanol plays an important role as a carbon supply. Figure 4 shows the frequency dependence of the permeability and the permittivity (real and imaginary) of the Fe3 C/C nanocapsule–paraffin composite. As shown in figure 4(a), the products prepared in different media have complex permeabilities in the same ranges. The real part (µ0r ) lies in the range of 0.9–1.1, while the imaginary part (µ00r ) lies in the range of 0.0–0.2. It is reported that the complex permeability is affected by the inner magnetic core’s composition and shape (the anisotropy field) [10]. According to the XRD pattern and TEM images, the inner cores of the nanoparticles prepared in different media are both spherical Fe3 C and of a similar size. Figure 4(b) shows that there is a significant difference in the complex permittivity, in both the real and imaginary parts, for the two kinds of samples. For the nanocapsules prepared in liquid nitrogen, both the real and imaginary parts (εr0 and εr00 ) stay almost constant, with values of 3 and 0, respectively, in the frequency range of 1–18 GHz. However, for the ones prepared in ethanol, the real part εr0 decreases from 13 to 9 while the imaginary part εr00 fluctuates between 2 and 4 with increase of frequency. It is reported that a higher real permittivity presents a higher polarization and a higher imaginary permittivity presents a higher absorption or electric loss [15]. From the XRD and TEM results, the major difference of the products prepared in different media is the number of defects within the carbon shell. According to the study of Watts et al [15], the introduction of lattice defects will act as polarized centers which would account for the higher real permittivity for the nanocapsules prepared in ethanol. Besides, the defects will create localized states near the Fermi level and lead to absorption by contiguous states to the Fermi level, which relates to the higher imaginary part under ethanol medium. Therefore, the huge difference in the complex permittivity of the nanocapsules prepared in different
Figure 2. TEM and HRTEM images of the products: (a) and (b)
were prepared in ethanol and (c) and (d) were prepared in liquid nitrogen.
Based on the HRTEM images and XRD patterns, it was confirmed that all particles are covered by a layered structure and form a core–shell structure with an Fe3 C core. As shown in figure 2(d), the lattice distance of the outer layer structure is measured as 0.34 nm, corresponding to the (002) plane of graphite, in agreement with the XRD analysis. This indicates that the as-prepared nanocapsule is composed of an outer layer of graphite carbon with an onion-like structure and an Fe3 C core. However, for the particles prepared in ethanol medium (figure 2(b)), the outer layer exhibits distorted lattice fringes, which are confirmed as carbon too. This indicates that, in comparison with those prepared in liquid nitrogen, the nanoparticles prepared in ethanol possess the same capsule structure but more lattice defects in the carbon outer layer and smaller Fe3 C cores. Since the large saturation magnetization plays a major role in the microwave absorption ability of the metallic magnet composite [5], the magnetic hysteresis loops of the samples were measured and are shown in figure 3. The saturation magnetization of the Fe3 C/C nanocapsules reaches 110 emu g−1 for the products prepared in ethanol and 150 emu g−1 for those prepared in the liquid nitrogen medium; these values are much higher than those of Fe/C (104.6 emu g−1 ) and (Fe, Ni)/C nanocapsules (106.1 emu g−1 ) reported before [3, 9]. Because the magnetic properties are mainly contributed by the Fe3 C particles, the higher ratio of Fe3 C over C for the nanocapsules prepared in liquid nitrogen may explain their larger saturation magnetization. From the XRD spectra, for the nanocapsules prepared in liquid nitrogen, after acid washing, the intensity of the graphite peak is reduced significantly. On the other hand, based on the TEM results, the ratio of Fe3 C over C could be roughly estimated from the diameter of the Fe3 C core and the 3
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Figure 4. The relative permittivity (εr ) (a) and permeability (µr ) (b) plotted against frequency for the Fe3 C/C nanocapsule–paraffin
composite.
Figure 5. The frequency dependence of the RL of Fe3 C/C nanocapsule–paraffin composite of different thicknesses (mm): (a) nanocapsules
prepared in liquid nitrogen, (b) and (c) nanocapsules prepared in ethanol.
absorption, thus RL < −20 dB is believed to be an adequate EM absorption level. Therefore, in comparison with the nanocapsules prepared in liquid nitrogen, the ones prepared in ethanol with 2 mm thickness are effective absorbers in the range of 10–15 GHz. In addition, as shown in figure 5(c), for a thickness of around 2 mm, the optimal RL of nanoparticles prepared in ethanol could reach −38 dB at a thickness of 1.9 mm with a matching frequency of 12.9 GHz, which is higher than previously reported iron–carbon composite results. For example, the α-Fe powders prepared by Liu showed an optimal RL of around −10 dB at a thickness of 5 mm in the range of frequency from 2 to 18 GHz [4],
media in this work would be mainly ascribed to the lattice defects within the carbon shell (figure 2). In addition, owing to the higher ratio of carbon shells, the nanocapsules prepared in ethanol possess far more lattice defects, which would further contribute to the difference of permittivity. Figure 5 shows the frequency dependence of the RL for the nanocapsule–paraffin composite in the range of 1–18 GHz. As shown in figures 5(a) and (b), with a thickness of 1–10 mm, the optimal RL of the nanocapsules prepared in liquid nitrogen is −5.66 dB, and the ones prepared in ethanol reach up to −24.5 dB. As illustrated by Liu et al [4], the case when RL = −20 dB is comparable to the 99% EM wave 4
Nanotechnology 25 (2014) 035704
Y Tang et al
References
the (Fe, Ni)/C nanocapsules, prepared by arc-discharging in methane, exhibited an optimal RL of −26.9 dB at a thickness of 2.0 mm [9], and the optimal RL of the α-Fe/CNT composite prepared by Che was around −25 dB [10]. According to the results of Liu et al [4], Fe3 C/C composites (the size of the particles was distributed from 100 to several hundred nanometers, obtained by a mechanical alloying method) show excellent wave absorption in the 18–26.5 GHz range, reaching −46 dB at a frequency of 20.2 GHz with a thickness of 1.2 mm [4]. Due to the limitations of our apparatus, EM wave absorption in the frequency range higher than 18 GHz was not measured in this experiment. However, in comparison with the maximum wave absorption peak of about −31 dB in the frequency range from 0 to 18 GHz in Liu’s study [4], the nanocapsules prepared in ethanol in the present work show a better wave absorption ability. Therefore, it is expected that the nanocapsules prepared in ethanol might possess promising EM wave absorption ability in the frequency range higher than 18 GHz. The present results indicate that nanocapsules prepared in ethanol may exhibit excellent EM wave absorption ability in a wide frequency range. They also suggest that the number of defects in the carbon shells of the nanocapsules plays an important role in the EM wave absorption property. The present results provide a new way of preparing EM wave absorption materials.
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4. Conclusion
Fe3 C/C nanocapsules, with diameter distributions of 10–60 and 10–20 nm, were prepared by applying arc-discharge submerged in liquid nitrogen or ethanol under ambient pressure. In comparison with the limited EM wave absorption ability for the ones prepared in liquid nitrogen, the RL values for the nanocapsules prepared in ethanol could exceed −20 dB in the frequency range of 10–15 GHz at a thickness of around 2 mm. The significant difference in EM wave absorption property of these two kinds of nanoparticles prepared in different media results from the number of defect within the outer carbon layer by changing the dielectric properties of the nanocapsules. These results indicate that the outer carbon layer plays an important role in EM absorption properties rather than just separating the inner magnetic particles to suppress the eddy current. This result offers a new way to design EM wave absorption materials.
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