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Dielectric, magnetic, and microwave absorbing properties of multi-walled carbon nanotubes filled with Sm2O3 nanoparticles
Materials Letters 63 (2009) 272–274
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Dielectric, magnetic, and microwave absorbing properties of multi-walled carbon nanotubes filled with Sm2O3 nanoparticles Lan Zhang a, Hong Zhu b,⁎ a b
Department of Chemistry, School of Science, Beijing Jiaotong University, Beijing 100044, China Department of Chemistry, School of Science, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 22 July 2008 Accepted 7 October 2008 Available online 17 October 2008 Keywords: Nanocomposites Aerospace materials Carbon nanotubes Rare earth oxide Absorbing properties
a b s t r a c t This study investigates the dielectric, magnetic, and microwave absorbing properties of Sm2O3-filled multiwalled carbon nanotubes (MWCNTs) synthesized by wet chemical method. The complex permittivity and permeability were measured at a microwave frequency range of 2–18 GHz. Sm2O3 nanoparticles encapsulated in the cavities enhance the magnetic loss of MWCNTs. The calculated results indicate that the bandwith of absorbing peak of the modified MWCNTs is much broader than that of unfilled MWCNTs. The maximum reflectivity (R) is about −12.22 dB at 13.40 GHz and corresponding bandwidth below −5 dB is more than 5.11 GHz. With the increase of thickness, the peak of R shifts to lower frequency, and multiple absorbing peaks appear, which helps to broaden microwave absorbing bandwidth. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Radar absorbing materials (RAMs) have attracted much attention for the increasing environmental pollution of microwave irradiation and being the essential part of a stealth defense system for all military platforms. Multi-walled carbon nanotubes (MWCNTs) have attracted a great deal of attention in recent years due to their unique structures, remarkable electromagnetic and mechanical properties [1,2]. Their high conductivity makes MWCNTs capable of dissipating electrostatic charges [3] or even of shielding devices from electromagnetic radiation [4]. These interesting merits of MWCNTs promise great potentials applications in microwave absorbing technologies in the near future. As conventional functional materials, rare earth (RE) compounds have been widely used in various fields, such as highperformance luminescent devices, magnets and catalysts. Popularity of these RE compounds in the application mainly originates from their special optical, electronic and magnetic properties that result from the electrons of the 4f shell [5]. To optimize the performances of MWCNTs as microwave absorbing materials, it is necessary to modify MWCNTs by coating or filling [6] with RE oxides. In this point of view we introduce Sm2O3 into nanotubes through capillary action to modulate the electromagnetic
⁎ Corresponding author. Department of Chemistry, School of Science, Beijing University of Chemical Technology, Beijing 100029, China. Tel.: +86 10 51684001; fax: +86 10 82161887. E-mail address: [email protected] (H. Zhu). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.10.015
parameters of MWCNTs. The results show this material possesses excellent microwave absorbing property. In this paper, we report the investigation of the electromagnetic and microwave absorbing properties of Sm2O3-filled MWCNTs synthesized by wet chemical method. 2. Experimental MWCNTs were supplied by Tsinghua University. Typical synthesis of Sm2O3-filled MWCNTs is as follows: the calculated amount of raw MWCNTs (i.e. prior to encapsulation) and Sm2O3 were first added in a round-bottomed flask containing 60 ml nitric acid solution and dispersed sufficiently by ultrasonication for 1 h. The mixture was refluxed at 80 °C for 24 h. The resulted sample particles were collected by centrifugation, then they were dried at 80 °C in oven for two days and subsequently heated in a stream of nitrogen (N2) at 500 °C for 3 h to convert the metal nitrate within the MWCNTs into the corresponding metal oxide (Sm2O 3). After cooling naturally to ambient temperature, the final products were obtained. The morphology of final products was characterized by high resolution transmission electron microscopy (HRTEM, JEM-2010). For the studies of microwave absorbing properties, coaxial line method was used to determine the electromagnetic parameters of Sm2O3-filled MWCNTs/paraffin composite with a HP8722ES vector network analyzer in the frequency range of 2–18 GHz. 20 wt.% Sm2O3-filled MWCNTs were mixed with paraffin and prepared as the toroidal shape with an outer diameter of 7.0 mm, an inner diameter of 3.04 mm and a thickness of 2.0 mm. The reflectivity (R)
L. Zhang, H. Zhu / Materials Letters 63 (2009) 272–274
273
Fig. 1. TEM images of raw MWCNTs (a) and Sm2O3-filled MWCNTs (b). (c), (d) are HRTEM images of the modified MWCNTs taken from the area marked with black arrows in (b), respectively.
with different thickness were calculated from equations shown below [7]: RðdBÞ = 20 log10 j
Zin −1 j Zin + 1
1=2 μr 2πfd Zin = tanh j ðμ r er Þ1=2 er c
lattice planes anticipated for Sm2O3 [8]. In this way, the lattice fringes with an observed fringe separation of 0.34, 0.32 and 0.31 nm in Fig. 1d are attributed to the (202), (111) and (222) planes of Sm2O3, respectively.
ð1Þ
3.2. Dielectric and magnetic properties
ð2Þ
Figs. 2 and 3 show the variations of the real and the imaginary parts of εr (ε′ and ε″) and μr (μ′ and μ″) of raw and Sm2O3-filled MWCNTs/paraffin composites with frequency.
where Zin is the normalized input impedance at free space and material interface, εr = ε′ − jε″ and μr = μ′ − jμ″ is the complex permittivity and permeability of the material, d is the thickness of the absorber, and c and f are the velocity of light and the frequency of microwave in free space, respectively. 3. Results and discussion 3.1. Morphology observation The TEM images of raw MWCNTs and Sm2O3-filled MWCNTs are shown in Fig. 1. The MWCNTs have an average outer diameter and inner diameter of 8.6 nm and 3.5 nm, respectively, and range from hundreds of nanometers to several micrometers in length. Fig. 1b is the morphology of Sm2O3-filled MWCNTs in comparison with the raw MWCNTs (Fig. 1a). HRTEM images (Fig. 1b and c) show that Sm2O3 nanoparticles presented in MWCNTs consist either of ellipsoidal crystallites with diameters 3–7 nm or of more elongated nanowires with similar diameters in cross section but which range from 30–200 nm in length. In addition, TEM observations indicate that there are few Sm2O3 crystallines outside MWCNTs. The smaller crystallites tend to pack evenly along the sides of the nanotube walls and adopt apparently random orientations. Fig. 1d shows a typical packing of Sm2O3 crystallites inside MWCNTs. By measuring the spacing of lattice fringes of individual crystallites relative to the d-spacing of the nanotube wall (corresponding to 0.34 nm), it is possible to attribute these lattice fringes to specific
Fig. 2. Permittivity spectra of raw and Sm2O3-filled MWCNTs composites.
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L. Zhang, H. Zhu / Materials Letters 63 (2009) 272–274
As shown in Fig. 2, both the ε′ and ε″ of the Sm2O3-filled MWCNTs composite are lower than those of unfilled MWCNTs, curve (c) exhibits a decreasing trend with the increasing of frequency. A peak can be seen on the ε″ plots (curve (d)), and the ε″ first increases to the maximum at about 9.6 GHz and then decrease to a constant level with the variation of frequency. It can be found that, the electric loss tangent (tanδe = ε″/ε′) of the Sm2O3-filled MWCNTs composite decreases gradually for the reducing of εr as the frequency increases, which is favorable for a better impedance matching condition. From Fig. 3, the μ′ and μ″ of raw MWCNTs exhibit a trend of degression. On the contrary, the μ′ and μ″ of Sm2O3-filled MWCNTs rise with the increasing of frequency. Furthermore, it is obvious that the μ″ of Sm2O3-filled MWCNTs composite is larger than that of unfilled sample. The enhancement of the magnetic loss of MWCNTs is most likely due to Sm2O3 nanoparticles encapsulated in the cavities, which function as magnetic materials. The magnetic loss tangent (tanδm = μ″/μ′) of Sm2O3-filled MWCNTs is much larger than that of raw MWCNTs, which has been improved dramatically, increasing from −0.08 to 0.14 in the frequency range of 2–18 GHz. 3.3. Microwave absorbing properties R values calculated by foregoing Eqs. (1) and (2) from measured values of ε′, ε″, μ′, μ″ are shown in Fig. 4. As shown in Fig. 4, curve (a) is the variation of R for the raw MWCNTs in the frequency range of 2–18 GHz. The maximum absorbing peak of the raw MWCNTs is about −21.58 dB at 9.40 GHz which is in the range of X wave band and the corresponding value of matching thickness (dm) is 2.0 mm, the bandwidth of the R below −10 dB is 1.58 GHz. In contrast, under the same dm, the absorbing peak of Sm2O3filled MWCNTs becomes broader (curve (b)), and the maximum absorbing peak decreases to 12.22 dB at 13.4 GHz, which is in the frequency range of Ku wave band. From the results shown in Fig. 4, it can be found that dm influences the R significantly. For the Sm2O3-filled MWCNTs composite, with increasing thickness, the absorbing peaks shift towards lower frequency (curves (b), (c), (d), (e)), correspondingly, the bandwidths of the R below −10 dB are 1.70 GHz (dm = 2.0 mm), 2.44 GHz (dm = 3.0 mm), 1.24 GHz (dm = 5.0 mm) and 0.93 GHz (dm = 9.0 mm). It can be seen from Fig. 4 that double absorbing peaks appear on curve (d) with dm of 5.0 mm, which are −8.28 dB at 5.2 GHz and −11.71 dB at 15.6 GHz. Furthermore, There are three absorbing peaks on curve (e) when dm reaches 9.0 mm, and the three absorbing peaks belong to S, C, and Ku wave band, respectively. It is clear that the modified MWCNTs can be used to design wideband microwave absorbing materials. Compared with the unfilled sample, the Sm2O3-filled MWCNTs composite exhibits much broader absorbing peaks and the maximum absorbing peak shifts to higher frequency range. This phenomenon suggests that Sm2O3 nanoparticles encapsulated in nanotubes have significant effects on the electromagnetic and microwave absorbing properties of MWCNTs. On the one hand, Sm2O3 nanocrystals and nanowires encapsulated in MWCNTs, which improve the conductance and reduce tanδe of the material, help to adjust Zin. On the other hand, quantum confine effect greatly changes the absorbing properties of Sm2O3-filled MWCNTs. According to Kubo theory [9], the energy levels of Sm2O3 nanoparticles in MWCNTs are not continuous but discrete because of quantum confine effect. In addition, it is well know that RE elements are of novel magnetic properties for the number of unpaired electrons in their 4f shells, the magnetism of Sm2O3 nanoparticles may has strong coercive force and lead to large loss of magnetic hysteresis [10].
4. Conclusion The microwave absorbing properties of Sm2O3-filled MWCNTs, which were prepared by wet chemical method, were investigated in
Fig. 3. Permeability spectra of raw and Sm2O3-filled MWCNTs composites.
Fig. 4. Reflectivity of raw and Sm2O3-filled MWCNTs composites. (a) unfilled MWCNTs (2.0 mm); (b) Re-filled MWCNTs (2.0 mm); (c) Re-filled MWCNTs (3.0 mm); (d) Re-filled MWCNTs (5.0 mm); (e) Re-filled MWCNTs (9.0 mm).
the frequency range of 2–18 GHz. Sm2O3 nanoparticles encapsulated in the nanotubes are considered to play an important role in improving the magnetic loss of MWCNTs. The electromagnetic parameters result in the variation of Zin and thus affects the absorbing properties. Compared to unfilled MWCNTs, the maximum absorbing peak of Sm2O3-filled MWCNTs shifts to higher frequency range, and the absorbing bandwidth becomes much broader. With increasing thickness, the maximum R of Sm2O3-filled MWCNTs shifts to lower frequency and multiple absorbing peaks appear. The peaks distributed in different wave bands help to broaden the frequency range of absorption. Thus, Sm2O3-filled MWCNTs may have potential applications in the fields of microwave absorbing materials. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 50674006). References [1] Cabria I, López MJ, Alonso JA. Comput Mater Sci 2006;35:238–42. [2] Wilson M. Chem Phys Lett 2004;397:340–3. [3] Sandler J, Shaffer MSP, Prasse T, Bauhofer W, Schulte K, Windle AH. Polymer 1999;40:5967–71. [4] Martin CA, Sandler JKW, Shaffer MSP, Schwarz MK, Bauhofer W, Schulte K, et al. Compos Sci Technol 2004;64:2309–16. [5] Kuang Q, Lin ZW, Lian W, Jiang ZY, Xie ZX, Huang RB, et al. J Solid State Chem 2007;180:1236–42. [6] Zhu H, Lin HY, Guo HF, Yu LF. Mater Sci Eng B 2007;138:101–4. [7] Michielssen Y, Sager JM, Ranjithan S, Mittra R. IEEE Trans Microwave Theor Tech 1993;41:1024–31. [8] Sloan J, Cook J, Green MLH, Hutchisonb JL, Tennec R. J Mater Chem 1997;7(7):1089–95. [9] Kawabata A, Kubo R. J Phys Soc Jpn 1966;21:1765–72. [10] Singh P, Babbar VK, Razdan A, Puri RK, Goel TC. J Appl Phys 2000;87(91):4362–6.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Dielectric, magnetic, and microwave absorbing properties of multi-walled carbon nanotubes filled with Sm2O3 nanoparticles Lan Zhang a, Hong Zhu b,⁎ a b
Department of Chemistry, School of Science, Beijing Jiaotong University, Beijing 100044, China Department of Chemistry, School of Science, Beijing University of Chemical Technology, Beijing 100029, China
a r t i c l e
i n f o
Article history: Received 22 July 2008 Accepted 7 October 2008 Available online 17 October 2008 Keywords: Nanocomposites Aerospace materials Carbon nanotubes Rare earth oxide Absorbing properties
a b s t r a c t This study investigates the dielectric, magnetic, and microwave absorbing properties of Sm2O3-filled multiwalled carbon nanotubes (MWCNTs) synthesized by wet chemical method. The complex permittivity and permeability were measured at a microwave frequency range of 2–18 GHz. Sm2O3 nanoparticles encapsulated in the cavities enhance the magnetic loss of MWCNTs. The calculated results indicate that the bandwith of absorbing peak of the modified MWCNTs is much broader than that of unfilled MWCNTs. The maximum reflectivity (R) is about −12.22 dB at 13.40 GHz and corresponding bandwidth below −5 dB is more than 5.11 GHz. With the increase of thickness, the peak of R shifts to lower frequency, and multiple absorbing peaks appear, which helps to broaden microwave absorbing bandwidth. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Radar absorbing materials (RAMs) have attracted much attention for the increasing environmental pollution of microwave irradiation and being the essential part of a stealth defense system for all military platforms. Multi-walled carbon nanotubes (MWCNTs) have attracted a great deal of attention in recent years due to their unique structures, remarkable electromagnetic and mechanical properties [1,2]. Their high conductivity makes MWCNTs capable of dissipating electrostatic charges [3] or even of shielding devices from electromagnetic radiation [4]. These interesting merits of MWCNTs promise great potentials applications in microwave absorbing technologies in the near future. As conventional functional materials, rare earth (RE) compounds have been widely used in various fields, such as highperformance luminescent devices, magnets and catalysts. Popularity of these RE compounds in the application mainly originates from their special optical, electronic and magnetic properties that result from the electrons of the 4f shell [5]. To optimize the performances of MWCNTs as microwave absorbing materials, it is necessary to modify MWCNTs by coating or filling [6] with RE oxides. In this point of view we introduce Sm2O3 into nanotubes through capillary action to modulate the electromagnetic
⁎ Corresponding author. Department of Chemistry, School of Science, Beijing University of Chemical Technology, Beijing 100029, China. Tel.: +86 10 51684001; fax: +86 10 82161887. E-mail address: [email protected] (H. Zhu). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.10.015
parameters of MWCNTs. The results show this material possesses excellent microwave absorbing property. In this paper, we report the investigation of the electromagnetic and microwave absorbing properties of Sm2O3-filled MWCNTs synthesized by wet chemical method. 2. Experimental MWCNTs were supplied by Tsinghua University. Typical synthesis of Sm2O3-filled MWCNTs is as follows: the calculated amount of raw MWCNTs (i.e. prior to encapsulation) and Sm2O3 were first added in a round-bottomed flask containing 60 ml nitric acid solution and dispersed sufficiently by ultrasonication for 1 h. The mixture was refluxed at 80 °C for 24 h. The resulted sample particles were collected by centrifugation, then they were dried at 80 °C in oven for two days and subsequently heated in a stream of nitrogen (N2) at 500 °C for 3 h to convert the metal nitrate within the MWCNTs into the corresponding metal oxide (Sm2O 3). After cooling naturally to ambient temperature, the final products were obtained. The morphology of final products was characterized by high resolution transmission electron microscopy (HRTEM, JEM-2010). For the studies of microwave absorbing properties, coaxial line method was used to determine the electromagnetic parameters of Sm2O3-filled MWCNTs/paraffin composite with a HP8722ES vector network analyzer in the frequency range of 2–18 GHz. 20 wt.% Sm2O3-filled MWCNTs were mixed with paraffin and prepared as the toroidal shape with an outer diameter of 7.0 mm, an inner diameter of 3.04 mm and a thickness of 2.0 mm. The reflectivity (R)
L. Zhang, H. Zhu / Materials Letters 63 (2009) 272–274
273
Fig. 1. TEM images of raw MWCNTs (a) and Sm2O3-filled MWCNTs (b). (c), (d) are HRTEM images of the modified MWCNTs taken from the area marked with black arrows in (b), respectively.
with different thickness were calculated from equations shown below [7]: RðdBÞ = 20 log10 j
Zin −1 j Zin + 1
1=2 μr 2πfd Zin = tanh j ðμ r er Þ1=2 er c
lattice planes anticipated for Sm2O3 [8]. In this way, the lattice fringes with an observed fringe separation of 0.34, 0.32 and 0.31 nm in Fig. 1d are attributed to the (202), (111) and (222) planes of Sm2O3, respectively.
ð1Þ
3.2. Dielectric and magnetic properties
ð2Þ
Figs. 2 and 3 show the variations of the real and the imaginary parts of εr (ε′ and ε″) and μr (μ′ and μ″) of raw and Sm2O3-filled MWCNTs/paraffin composites with frequency.
where Zin is the normalized input impedance at free space and material interface, εr = ε′ − jε″ and μr = μ′ − jμ″ is the complex permittivity and permeability of the material, d is the thickness of the absorber, and c and f are the velocity of light and the frequency of microwave in free space, respectively. 3. Results and discussion 3.1. Morphology observation The TEM images of raw MWCNTs and Sm2O3-filled MWCNTs are shown in Fig. 1. The MWCNTs have an average outer diameter and inner diameter of 8.6 nm and 3.5 nm, respectively, and range from hundreds of nanometers to several micrometers in length. Fig. 1b is the morphology of Sm2O3-filled MWCNTs in comparison with the raw MWCNTs (Fig. 1a). HRTEM images (Fig. 1b and c) show that Sm2O3 nanoparticles presented in MWCNTs consist either of ellipsoidal crystallites with diameters 3–7 nm or of more elongated nanowires with similar diameters in cross section but which range from 30–200 nm in length. In addition, TEM observations indicate that there are few Sm2O3 crystallines outside MWCNTs. The smaller crystallites tend to pack evenly along the sides of the nanotube walls and adopt apparently random orientations. Fig. 1d shows a typical packing of Sm2O3 crystallites inside MWCNTs. By measuring the spacing of lattice fringes of individual crystallites relative to the d-spacing of the nanotube wall (corresponding to 0.34 nm), it is possible to attribute these lattice fringes to specific
Fig. 2. Permittivity spectra of raw and Sm2O3-filled MWCNTs composites.
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L. Zhang, H. Zhu / Materials Letters 63 (2009) 272–274
As shown in Fig. 2, both the ε′ and ε″ of the Sm2O3-filled MWCNTs composite are lower than those of unfilled MWCNTs, curve (c) exhibits a decreasing trend with the increasing of frequency. A peak can be seen on the ε″ plots (curve (d)), and the ε″ first increases to the maximum at about 9.6 GHz and then decrease to a constant level with the variation of frequency. It can be found that, the electric loss tangent (tanδe = ε″/ε′) of the Sm2O3-filled MWCNTs composite decreases gradually for the reducing of εr as the frequency increases, which is favorable for a better impedance matching condition. From Fig. 3, the μ′ and μ″ of raw MWCNTs exhibit a trend of degression. On the contrary, the μ′ and μ″ of Sm2O3-filled MWCNTs rise with the increasing of frequency. Furthermore, it is obvious that the μ″ of Sm2O3-filled MWCNTs composite is larger than that of unfilled sample. The enhancement of the magnetic loss of MWCNTs is most likely due to Sm2O3 nanoparticles encapsulated in the cavities, which function as magnetic materials. The magnetic loss tangent (tanδm = μ″/μ′) of Sm2O3-filled MWCNTs is much larger than that of raw MWCNTs, which has been improved dramatically, increasing from −0.08 to 0.14 in the frequency range of 2–18 GHz. 3.3. Microwave absorbing properties R values calculated by foregoing Eqs. (1) and (2) from measured values of ε′, ε″, μ′, μ″ are shown in Fig. 4. As shown in Fig. 4, curve (a) is the variation of R for the raw MWCNTs in the frequency range of 2–18 GHz. The maximum absorbing peak of the raw MWCNTs is about −21.58 dB at 9.40 GHz which is in the range of X wave band and the corresponding value of matching thickness (dm) is 2.0 mm, the bandwidth of the R below −10 dB is 1.58 GHz. In contrast, under the same dm, the absorbing peak of Sm2O3filled MWCNTs becomes broader (curve (b)), and the maximum absorbing peak decreases to 12.22 dB at 13.4 GHz, which is in the frequency range of Ku wave band. From the results shown in Fig. 4, it can be found that dm influences the R significantly. For the Sm2O3-filled MWCNTs composite, with increasing thickness, the absorbing peaks shift towards lower frequency (curves (b), (c), (d), (e)), correspondingly, the bandwidths of the R below −10 dB are 1.70 GHz (dm = 2.0 mm), 2.44 GHz (dm = 3.0 mm), 1.24 GHz (dm = 5.0 mm) and 0.93 GHz (dm = 9.0 mm). It can be seen from Fig. 4 that double absorbing peaks appear on curve (d) with dm of 5.0 mm, which are −8.28 dB at 5.2 GHz and −11.71 dB at 15.6 GHz. Furthermore, There are three absorbing peaks on curve (e) when dm reaches 9.0 mm, and the three absorbing peaks belong to S, C, and Ku wave band, respectively. It is clear that the modified MWCNTs can be used to design wideband microwave absorbing materials. Compared with the unfilled sample, the Sm2O3-filled MWCNTs composite exhibits much broader absorbing peaks and the maximum absorbing peak shifts to higher frequency range. This phenomenon suggests that Sm2O3 nanoparticles encapsulated in nanotubes have significant effects on the electromagnetic and microwave absorbing properties of MWCNTs. On the one hand, Sm2O3 nanocrystals and nanowires encapsulated in MWCNTs, which improve the conductance and reduce tanδe of the material, help to adjust Zin. On the other hand, quantum confine effect greatly changes the absorbing properties of Sm2O3-filled MWCNTs. According to Kubo theory [9], the energy levels of Sm2O3 nanoparticles in MWCNTs are not continuous but discrete because of quantum confine effect. In addition, it is well know that RE elements are of novel magnetic properties for the number of unpaired electrons in their 4f shells, the magnetism of Sm2O3 nanoparticles may has strong coercive force and lead to large loss of magnetic hysteresis [10].
4. Conclusion The microwave absorbing properties of Sm2O3-filled MWCNTs, which were prepared by wet chemical method, were investigated in
Fig. 3. Permeability spectra of raw and Sm2O3-filled MWCNTs composites.
Fig. 4. Reflectivity of raw and Sm2O3-filled MWCNTs composites. (a) unfilled MWCNTs (2.0 mm); (b) Re-filled MWCNTs (2.0 mm); (c) Re-filled MWCNTs (3.0 mm); (d) Re-filled MWCNTs (5.0 mm); (e) Re-filled MWCNTs (9.0 mm).
the frequency range of 2–18 GHz. Sm2O3 nanoparticles encapsulated in the nanotubes are considered to play an important role in improving the magnetic loss of MWCNTs. The electromagnetic parameters result in the variation of Zin and thus affects the absorbing properties. Compared to unfilled MWCNTs, the maximum absorbing peak of Sm2O3-filled MWCNTs shifts to higher frequency range, and the absorbing bandwidth becomes much broader. With increasing thickness, the maximum R of Sm2O3-filled MWCNTs shifts to lower frequency and multiple absorbing peaks appear. The peaks distributed in different wave bands help to broaden the frequency range of absorption. Thus, Sm2O3-filled MWCNTs may have potential applications in the fields of microwave absorbing materials. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 50674006). References [1] Cabria I, López MJ, Alonso JA. Comput Mater Sci 2006;35:238–42. [2] Wilson M. Chem Phys Lett 2004;397:340–3. [3] Sandler J, Shaffer MSP, Prasse T, Bauhofer W, Schulte K, Windle AH. Polymer 1999;40:5967–71. [4] Martin CA, Sandler JKW, Shaffer MSP, Schwarz MK, Bauhofer W, Schulte K, et al. Compos Sci Technol 2004;64:2309–16. [5] Kuang Q, Lin ZW, Lian W, Jiang ZY, Xie ZX, Huang RB, et al. J Solid State Chem 2007;180:1236–42. [6] Zhu H, Lin HY, Guo HF, Yu LF. Mater Sci Eng B 2007;138:101–4. [7] Michielssen Y, Sager JM, Ranjithan S, Mittra R. IEEE Trans Microwave Theor Tech 1993;41:1024–31. [8] Sloan J, Cook J, Green MLH, Hutchisonb JL, Tennec R. J Mater Chem 1997;7(7):1089–95. [9] Kawabata A, Kubo R. J Phys Soc Jpn 1966;21:1765–72. [10] Singh P, Babbar VK, Razdan A, Puri RK, Goel TC. J Appl Phys 2000;87(91):4362–6.