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The electromagnetic characteristics and absorbing properties of multi-walled carbon nanotubes filled with Er2O3 nanoparticles as microwave absorbers
Materials Science and Engineering B 153 (2008) 78–82
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
The electromagnetic characteristics and absorbing properties of multi-walled carbon nanotubes filled with Er2 O3 nanoparticles as microwave absorbers Lan Zhang a , Hong Zhu b,∗ , Yuan Song a , Yongming Zhang a , Yi Huang a 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 10 July 2008 Received in revised form 15 October 2008 Accepted 24 October 2008 Keywords: Carbon nanotubes Rare earth oxide Electromagnetic parameters Absorbers
a b s t r a c t Multi-walled carbon nanotubes (MWCNTs) filled with Er2 O3 were synthesized by wet chemical method. Their electromagnetic characteristics and microwave absorbing properties were investigated in the frequency range of 2–18 GHz. The complex permittivity and electric loss tangent of Er2 O3 -filled MWCNTs demonstrate a decreasing trend while the complex permeability and magnetic loss tangent are larger than those of the unfilled MWCNTs. The Er2 O3 nanoparticles encapsulated in the cavities modulate the electromagnetic parameters of MWCNTs, and thus affect the microwave absorbing properties. The modified MWCNTs possess much broader absorbing bandwidth and larger reflectivity than those of unfilled MWCNTs. With the increase of thickness, the peak value of reflectivity shifts to lower frequencies and multiple absorbing peaks appear. The result of calculation indicates that Er2 O3 -filled MWCNTs have potential applications in thin thickness and light-weight microwave absorbers. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Radar absorbing materials (RAMs) have gained much interest due to their civil and military applications. RAMs can effectively reduce electromagnetic backscatter so that they are expected to have promising applications in the stealth technology of aircrafts and microwave dark-room and shielding [1]. Multi-walled carbon nanotubes (MWCNTs) as a new allotrope of carbon [2], have been the focus of a number of scientific studies [3] since they were discovered by Iijima in 1991 [4], because they exhibit many fascinating properties such as one-dimensional quantum effects and high flexibility, and have various potential applications in hydrogen storage, solid-state secondary batteries, filtering media and nanoscale electronic devices [5]. The microwave absorbing effect of MWCNTs has also attracted considerable interest in recent years for theoretical and practical importance in fundamental science and application [6–11]. To optimize the use of MWCNTs in various applications, it is necessary to modify MWCNTs by coating or filling [12] with other nanomaterials, such as transition metal elements Fe, Ni, Co, etc. [13–15]. The hybrid materials of carbon nanotubes and other nanocrystals are expected to be applied to catalysts, sensors, nanoelectronic devices, data storage/processing devices, field emission
∗ Corresponding author. Tel.: +86 10 51684001; fax: +86 10 82161887. E-mail address: [email protected] (H. Zhu). 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.10.029
displays, and polymer or ceramic reinforcement [16]. It is well known that rare earth (RE) materials often possess some special properties [17], and RE elements are of great importance in magnetic, electronic, and optical materials because of the number of unpaired electrons in their 4f shells. The novel properties of RE compounds make them rather appealing in, for instance, luminescence [18], catalysis [19], florescence imaging [20], and biological fields [21]. The synthesis of nanomaterials of RE-related compounds encapsulated in MWCNTs as microwave absorbers is just beginning to emerge [22], which leads us to expect that the combination of MWCNTs with RE oxides may have some interesting microwave absorbing effects. We introduce RE oxides into the cavities of MWCNTs through capillary action to modify their electromagnetic properties. The results show that this material possesses special microwave absorbing properties. In this paper, we report the investigation of the microwave absorbing properties of Er2 O3 -filled MWCNTs synthesized by wet chemical method. 2. Experimental MWCNTs were generously supplied by Tsinghua University. Er2 O3 was obtained from Sinopharm Chemical Reagent Co. Ltd., China. Typical synthesis of Er2 O3 -filled MWCNTs is as follows: the calculated amount of raw MWCNTs (i.e. prior to encapsulation) and Er2 O3 were first added in a round-bottomed flask containing 60 ml of nitric acid solution and dispersed sufficiently by ultrasonication for 1 h. The mixture was refluxed at 80 ◦ C for 24 h. The resultant
L. Zhang et al. / Materials Science and Engineering B 153 (2008) 78–82
sample particles were collected by centrifugation, then they were dried at 80 ◦ C in an oven for 2 days and subsequently heated in a stream of nitrogen (N2 ) at 450 ◦ C for 3 h to convert the metal nitrate within the MWCNTs into the corresponding metal oxide (Er2 O3 ). After cooling naturally to ambient temperature, the final products were obtained. The morphology of the final products was characterized by transmission electron microscopy (TEM, Hitachi H-700) with an accelerating voltage of 150 kV, high resolution TEM (HRTEM, JEOL
79
JEM-2010) with an accelerating voltage of 200 kV. XRD analysis was carried out on a Rigaku D/max2500 diffractometer at a voltage of 40 kV and a current of 200 mA with Cu K␣ radiation ( = 0.15406 nm), employing a scanning rate of 0.02◦ s−1 in the 2 ranging from 10◦ to 100◦ . For the studies of microwave absorbing properties, a coaxial line method was used to determine the complex permittivity (ε , ε ) and permeability ( , ) of the Er2 O3 filled MWNTs/paraffin composite with a HP8722ES vector network analyzer in the frequency range of 2–18 GHz. Er2 O3 -filled MWC-
Fig. 1. Typical TEM images of raw MWCNTs (a) and Er2 O3 -filled MWCNTs (b). (c–f) are HRTEM images of the modified MWCNTs. (c and d) are taken from the area marked with black arrows in (b), respectively.
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NTs of 20 wt.% were mixed with paraffin and prepared in a toroidal shape with an external diameter of 7.0 mm, an internal diameter of 3.04 mm and a thickness of 2.0 mm. The absorbing properties with different thicknesses were calculated from equations shown below [23]:
Zin − 1 Z +1
R(dB) = 20 log10
Zin =
1/2 r
εr
(1)
in
2fd
tanh j
c
(r εr )1/2
(2)
where Zin is the normalized input impedance at the free space and material interface, εr = ε − jε and r = − j the complex permittivity and permeability of the material, d the thickness of the absorber, and c and f are the velocity of light and the frequency of the microwaves in free space, respectively. The impedance matching condition is given by Zin = 1 to represent the perfect absorbing properties. The impedance matching condition is determined by the combinations of the six parameters: ε , ε , , , f and d. Also, knowing εr and r , the R value versus frequency can be evaluated at a specified thickness.
3. Results and discussion 3.1. Morphology observation The TEM images of raw MWCNTs and Er2 O3 -filled MWCNTs are shown in Fig. 1. The MWCNTs have an average outer diameter and inner diameter of 8.6 and 3.5 nm, respectively, and range from hundreds of nanometers to several micrometers in length. Fig. 1b is the morphology of Er2 O3 -filled MWCNTs in comparison with raw MWCNTs (Fig. 1a). It shows that the Er2 O3 nanoparticles are filled into most of cavities of the MWCNTs. The HRTEM image (Fig. 1c) shows that Er2 O3 nanocrystals array continuously and orientate randomly along the nanotube capillary. Individual crystals tend to be either spherical or ellipsoidal in shape with diameters ranging from 4 to 20 nm, which are approximately equal to the diameters of the cross-sections of MWCNTs. TEM studies also indicate that there are several Er2 O3 nanocrystals present outside the MWCNTs (Fig. 1d). By measuring the spacing of lattice fringes of individual crystallites relative to the d-spacing of the carbon nanotube wall (corresponding to 0.34 nm), it is possible to attribute these lattice fringes to specific lattice planes anticipated for Er2 O3 . In this way, the lattice fringes with an observed fringe separation of 0.30 and 0.19 nm in Fig. 1e and Fig. 1f are attributed to the (2 2 2) and (4 0 0) planes of Er2 O3 , respectively.
Fig. 2. XRD pattern of Er2 O3 -filled MWCNTs.
3.3. Dielectric and magnetic properties Figs. 3 and 5 show the real and the imaginary parts of εr (ε and ε ) and r ( and ) of raw and Er2 O3 -filled MWCNTs/paraffin composites dependent on the frequency. Fig. 4 and Fig. 6 show the curves of the electric loss tangent (tan ıe = ε /ε ) and magnetic loss tangent (tan ım = / ) of raw and Er2 O3 -filled MWCNTs/paraffin composites versus the frequency. From the impedance matching principle, the lowest reflectivity can be obtained when the impedance matching condition is Zin = 1, and Zin depends on εr and r for a given thickness and frequency. As shown in Fig. 3, the ε of the Er2 O3 -filled MWCNTs composites (curve (c)) is lower than that of unfilled MWCNTs for frequency below 17 GHz (curve (a)) and exhibits a decreasing trend with increasing frequency. The ε of Er2 O3 -filled MWCNTs first descends to the lowest point at 12.0 GHz, after which a rise occurs although the rise is lower than that of unfilled MWCNTs. The characteristic of ε indicates the modified sample has excellent absorbing property in the low and middle frequency region for the frequency range studied. From Fig. 4, it can be seen that, the tan ıe of the Er2 O3 -filled MWCNTs gradually decreases to a minimum at about 12.0 GHz and
3.2. XRD analysis Fig. 2 shows X-ray powder diffraction pattern of the Er2 O3 -filled MWCNTs samples. The diffraction peak at 26.2◦ is assigned to the (0 0 2) planes of hexagonal graphite structure with an interlayer spacing of 0.34 nm, which corresponds to the fringe separation of the nanotube wall. Comparing with standard JCPDS 08-0050 pattern, the diffraction peaks at 29.54◦ , 33.86◦ , 48.80◦ and 57.84◦ are attributed to the (2 2 2), (4 0 0), (4 4 0) and (6 2 2) planes of cubic Er2 O3 phase, respectively. According to the Scherrer formula, the average diameter of the crystals encapsulated in MWCNTs is 16.3 nm calculated from the half-width of the (2 2 2) diffraction peak of Er2 O3 . These results are in agreement with the TEM results for the particle diameters.
Fig. 3. Permittivity spectra of raw and Er2 O3 -filled MWCNTs composites.
L. Zhang et al. / Materials Science and Engineering B 153 (2008) 78–82
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Fig. 4. The curves of tan ıe of materials dependent on the frequency.
Fig. 6. The curves of tan ım of materials dependent on the frequency.
then increases with the increasing of frequency. The variation of the tan ıe spectrum is similar to the spectrum of ε , and the inflection points of the two curves are the same at 12.0 GHz. The reducing trend of εr and tan ıe result in the change of Zin , which helps to make a better impedance matching condition. From Fig. 5, it can be seen that the and of the Er2 O3 filled MWCNTs composite are larger than those of the raw MWCNTs composite. According to transmission-line theory [24], the represents the capacity of storing energy and the stands for the loss of energy, thus the increase of suggests that the Er2 O3 nanoparticles encapsulated in MWCNTs enhance the magnetic loss of the MWCNTs. From the results shown in Fig. 6, it can be seen that the tan ım of Er2 O3 -filled MWCNTs has been changed dramatically, which increases from −0.32 to 0.01 in the frequency range of 2–18 GHz. Both r and tan ım exhibit a trend of degression with the increase of frequency. This indicates that reducing the layer thickness and broadens the bandwidth of the absorbers.
Thus, if the six parameters of the material are known, the absorbing properties of the material can be calculated. The values of reflectivity calculated by using Eqs. (1) and (2) from measured values of ε , ε , , are shown in Fig. 7. In Fig. 7, curve (a) is the variation of the reflectivity for raw MWCNTs in the frequency range of 2–18 GHz. The maximum absorbing peak of raw MWCNTs is about −21.58 dB at 9.4 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 reflectivity below −5 dB is 3.50 GHz and the bandwidth of the reflectivity below −10 dB is 1.58 GHz. In contrast, under the same matching thickness (dm = 2.0 mm), the absorbing peak of Er2 O3 -filled MWCNTs becomes broader (curve (b)), the bandwidth of the reflectivity below −5 dB is 4.65 GHz and the bandwidth of the reflectivity below −10 dB is 2.30 GHz. The maximum absorbing peak increases to −27.96 dB and shifts to 10.0 GHz for the change of impedance. From the results shown in Fig. 7, the matching thickness (dm ) influences the reflection loss significantly. For the Er2 O3 -filled MWCNTs composite, with the increase of thickness, the maximum absorbing peaks shift towards a lower frequency (curves (b–d)).
3.4. Microwave absorbing properties According to Eqs. (1) and (2), surface reflectivity of an absorber is a function of six parameters of the material: ε , ε , , , f and d.
Fig. 5. Permeability spectra of raw and Er2 O3 -filled MWCNTs composites.
Fig. 7. Reflectivity of raw and Er2 O3 -filled MWCNTs composites: (a) unfilled MWCNTs (2.0 mm); (b) re-filled MWCNTs (2.0 mm); (c) re-filled MWCNTs (4.0 mm); (d) re-filled MWCNTs (6.0 mm).
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L. Zhang et al. / Materials Science and Engineering B 153 (2008) 78–82
The bandwidths of the reflectivity below −10 dB are 2.30 GHz (dm = 2.0 mm), 1.16 GHz (dm = 4.0 mm) and 0.73 GHz (dm = 6.0 mm), respectively. It can be seen from Fig. 7 that two absorbing peaks appear on curve (c) with the matching thickness of 4.0 mm, which are −25.70 dB at 4.6 GHz and −7.32 dB at 14.8 GHz. Furthermore, there are three absorbing peaks on curve (d) when the matching thickness reaches 6.0 mm. The three absorbing peaks distributed in S, X, and Ku wave bands help to broaden the frequency range of absorption. Thus, Er2 O3 -filled MWCNTs may be applicable in the fields of RAMs and electromagnetic compatibility. As it is shown in Fig. 7, the widths at half-power maximum are: curve (a) 1.66 GHz (dm = 2.0 mm); curve (b) 1.79 GHz (dm = 2.0 mm); curve (c) 0.87 and 2.98 GHz (dm = 4.0 mm); curve (d) 0.53 GHz, 2.44 and 5.49 GHz (dm = 6.0 mm), respectively. The widths at half-power maximum of the Er2 O3 -filled composites are larger than that of unfilled samples. In the figure, the widths at half-power maximum become larger and the maximum absorbing peaks decrease with the increase of dm . In curve (d), the widths at half-power maximum increase and the peak values decrease as the frequency increases. The number of absorbing peaks increases with the increase of dm . The propagating wavelength in a material (m ) is expressed by m = o /(|εr ||r |)1/2 where o is the free space wavelength and |εr | and |r | are the moduli of εr and r , respectively. Maximal absorbing of microwave energy occurs when the dm of absorber equals an odd number multiple of m /4, and the absorbing peaks shift to lower frequencies for the increase of dm . Thus, the three peaks of curve (d) meet the interference condition that the dm is corresponding to an successive odd number multiple of m /4 at 3.0, 9.8 and 16.4 GHz, respectively. Further, calculated by using Eqs. (2), the peak values of Zin are 0.91, 0.42 and 0.29 at 3.0, 9.8 and 16.4 GHz, respectively, which are in line with the three absorbing peaks in curve (d). The impedance matching condition is in favor of maximal absorbing of microwave energy. Compared with the unfilled sample, the Er2 O3 -filled MWCNTs composite exhibits much broader absorbing bandwidth and larger reflectivity. This phenomenon, coupled with the results presented in the complex permittivity and permeability in Fig. 3 and Fig. 5, suggests that the Er2 O3 nanoparticles encapsulated in the MWCNTs modify the electromagnetic characteristics of materials and thus affect the microwave absorbing properties. Therefore, we speculate that the specific location of the Er ion in MWCNTs could generate a charge effect [25] and induce the improvement of absorbing performance. Moreover, these absorbing performance differences show that MWCNTs could play the role of resonators in the electromagnetic field, and RE oxide modified MWCNTs could improve absorbing performance in that RE oxide located in MWCNTs cavities could change the microenvironment of the resonators. The second possible reason is that the energy levels of the nanosized Er2 O3 crystals encapsulated in one-dimensional MWCNTs are not continuous but discrete because of quantum confinement effect according to Kubo theory [26,27]. This might produce a new absorbing mechanism. In addition, the Er3+ ion has the [Xe] 4f11 configuration, the 5d shell is empty and there are three unpaired 4f electrons interacting with the crystalline environment. The electron magnetic moment may cause a large magnetic loss in the composite.
4. Conclusion The microwave absorbing properties of Er2 O3 -filled MWCNTs, which were prepared by wet chemical method, were investigated in the frequency range of 2–18 GHz. The Er2 O3 nanocrystals encapsulated in MWCNTs are considered to have an impact on microwave absorbing behavior of MWCNTs. The εr and tan ıe exhibit a decreasing trend up to 18 GHz, which helps to improve impedance matching condition. Both the r and tan ım are larger than those of unfilled MWCNTs. The improved absorbing properties are proposed to be the result of the modification of the electromagnetic parameters. The maximum reflectivity is about −27.96 dB at 10.0 GHz and the bandwidth of the reflectivity below −10 dB is 2.30 GHz with a matching thickness of 2.0 mm. With the increase of thickness, the maximum reflectivity shifts to a lower frequency and multiple absorbing peaks appear. The calculated absorbing results indicate that Er2 O3 -filled MWCNTs are good candidates for preparing efficient microwave absorbers which are of low thickness and light in weight. Acknowledgement This work was supported by the National Natural Science Foundation of China (grant no. 50674006). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
The electromagnetic characteristics and absorbing properties of multi-walled carbon nanotubes filled with Er2 O3 nanoparticles as microwave absorbers Lan Zhang a , Hong Zhu b,∗ , Yuan Song a , Yongming Zhang a , Yi Huang a 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 10 July 2008 Received in revised form 15 October 2008 Accepted 24 October 2008 Keywords: Carbon nanotubes Rare earth oxide Electromagnetic parameters Absorbers
a b s t r a c t Multi-walled carbon nanotubes (MWCNTs) filled with Er2 O3 were synthesized by wet chemical method. Their electromagnetic characteristics and microwave absorbing properties were investigated in the frequency range of 2–18 GHz. The complex permittivity and electric loss tangent of Er2 O3 -filled MWCNTs demonstrate a decreasing trend while the complex permeability and magnetic loss tangent are larger than those of the unfilled MWCNTs. The Er2 O3 nanoparticles encapsulated in the cavities modulate the electromagnetic parameters of MWCNTs, and thus affect the microwave absorbing properties. The modified MWCNTs possess much broader absorbing bandwidth and larger reflectivity than those of unfilled MWCNTs. With the increase of thickness, the peak value of reflectivity shifts to lower frequencies and multiple absorbing peaks appear. The result of calculation indicates that Er2 O3 -filled MWCNTs have potential applications in thin thickness and light-weight microwave absorbers. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Radar absorbing materials (RAMs) have gained much interest due to their civil and military applications. RAMs can effectively reduce electromagnetic backscatter so that they are expected to have promising applications in the stealth technology of aircrafts and microwave dark-room and shielding [1]. Multi-walled carbon nanotubes (MWCNTs) as a new allotrope of carbon [2], have been the focus of a number of scientific studies [3] since they were discovered by Iijima in 1991 [4], because they exhibit many fascinating properties such as one-dimensional quantum effects and high flexibility, and have various potential applications in hydrogen storage, solid-state secondary batteries, filtering media and nanoscale electronic devices [5]. The microwave absorbing effect of MWCNTs has also attracted considerable interest in recent years for theoretical and practical importance in fundamental science and application [6–11]. To optimize the use of MWCNTs in various applications, it is necessary to modify MWCNTs by coating or filling [12] with other nanomaterials, such as transition metal elements Fe, Ni, Co, etc. [13–15]. The hybrid materials of carbon nanotubes and other nanocrystals are expected to be applied to catalysts, sensors, nanoelectronic devices, data storage/processing devices, field emission
∗ Corresponding author. Tel.: +86 10 51684001; fax: +86 10 82161887. E-mail address: [email protected] (H. Zhu). 0921-5107/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2008.10.029
displays, and polymer or ceramic reinforcement [16]. It is well known that rare earth (RE) materials often possess some special properties [17], and RE elements are of great importance in magnetic, electronic, and optical materials because of the number of unpaired electrons in their 4f shells. The novel properties of RE compounds make them rather appealing in, for instance, luminescence [18], catalysis [19], florescence imaging [20], and biological fields [21]. The synthesis of nanomaterials of RE-related compounds encapsulated in MWCNTs as microwave absorbers is just beginning to emerge [22], which leads us to expect that the combination of MWCNTs with RE oxides may have some interesting microwave absorbing effects. We introduce RE oxides into the cavities of MWCNTs through capillary action to modify their electromagnetic properties. The results show that this material possesses special microwave absorbing properties. In this paper, we report the investigation of the microwave absorbing properties of Er2 O3 -filled MWCNTs synthesized by wet chemical method. 2. Experimental MWCNTs were generously supplied by Tsinghua University. Er2 O3 was obtained from Sinopharm Chemical Reagent Co. Ltd., China. Typical synthesis of Er2 O3 -filled MWCNTs is as follows: the calculated amount of raw MWCNTs (i.e. prior to encapsulation) and Er2 O3 were first added in a round-bottomed flask containing 60 ml of nitric acid solution and dispersed sufficiently by ultrasonication for 1 h. The mixture was refluxed at 80 ◦ C for 24 h. The resultant
L. Zhang et al. / Materials Science and Engineering B 153 (2008) 78–82
sample particles were collected by centrifugation, then they were dried at 80 ◦ C in an oven for 2 days and subsequently heated in a stream of nitrogen (N2 ) at 450 ◦ C for 3 h to convert the metal nitrate within the MWCNTs into the corresponding metal oxide (Er2 O3 ). After cooling naturally to ambient temperature, the final products were obtained. The morphology of the final products was characterized by transmission electron microscopy (TEM, Hitachi H-700) with an accelerating voltage of 150 kV, high resolution TEM (HRTEM, JEOL
79
JEM-2010) with an accelerating voltage of 200 kV. XRD analysis was carried out on a Rigaku D/max2500 diffractometer at a voltage of 40 kV and a current of 200 mA with Cu K␣ radiation ( = 0.15406 nm), employing a scanning rate of 0.02◦ s−1 in the 2 ranging from 10◦ to 100◦ . For the studies of microwave absorbing properties, a coaxial line method was used to determine the complex permittivity (ε , ε ) and permeability ( , ) of the Er2 O3 filled MWNTs/paraffin composite with a HP8722ES vector network analyzer in the frequency range of 2–18 GHz. Er2 O3 -filled MWC-
Fig. 1. Typical TEM images of raw MWCNTs (a) and Er2 O3 -filled MWCNTs (b). (c–f) are HRTEM images of the modified MWCNTs. (c and d) are taken from the area marked with black arrows in (b), respectively.
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L. Zhang et al. / Materials Science and Engineering B 153 (2008) 78–82
NTs of 20 wt.% were mixed with paraffin and prepared in a toroidal shape with an external diameter of 7.0 mm, an internal diameter of 3.04 mm and a thickness of 2.0 mm. The absorbing properties with different thicknesses were calculated from equations shown below [23]:
Zin − 1 Z +1
R(dB) = 20 log10
Zin =
1/2 r
εr
(1)
in
2fd
tanh j
c
(r εr )1/2
(2)
where Zin is the normalized input impedance at the free space and material interface, εr = ε − jε and r = − j the complex permittivity and permeability of the material, d the thickness of the absorber, and c and f are the velocity of light and the frequency of the microwaves in free space, respectively. The impedance matching condition is given by Zin = 1 to represent the perfect absorbing properties. The impedance matching condition is determined by the combinations of the six parameters: ε , ε , , , f and d. Also, knowing εr and r , the R value versus frequency can be evaluated at a specified thickness.
3. Results and discussion 3.1. Morphology observation The TEM images of raw MWCNTs and Er2 O3 -filled MWCNTs are shown in Fig. 1. The MWCNTs have an average outer diameter and inner diameter of 8.6 and 3.5 nm, respectively, and range from hundreds of nanometers to several micrometers in length. Fig. 1b is the morphology of Er2 O3 -filled MWCNTs in comparison with raw MWCNTs (Fig. 1a). It shows that the Er2 O3 nanoparticles are filled into most of cavities of the MWCNTs. The HRTEM image (Fig. 1c) shows that Er2 O3 nanocrystals array continuously and orientate randomly along the nanotube capillary. Individual crystals tend to be either spherical or ellipsoidal in shape with diameters ranging from 4 to 20 nm, which are approximately equal to the diameters of the cross-sections of MWCNTs. TEM studies also indicate that there are several Er2 O3 nanocrystals present outside the MWCNTs (Fig. 1d). By measuring the spacing of lattice fringes of individual crystallites relative to the d-spacing of the carbon nanotube wall (corresponding to 0.34 nm), it is possible to attribute these lattice fringes to specific lattice planes anticipated for Er2 O3 . In this way, the lattice fringes with an observed fringe separation of 0.30 and 0.19 nm in Fig. 1e and Fig. 1f are attributed to the (2 2 2) and (4 0 0) planes of Er2 O3 , respectively.
Fig. 2. XRD pattern of Er2 O3 -filled MWCNTs.
3.3. Dielectric and magnetic properties Figs. 3 and 5 show the real and the imaginary parts of εr (ε and ε ) and r ( and ) of raw and Er2 O3 -filled MWCNTs/paraffin composites dependent on the frequency. Fig. 4 and Fig. 6 show the curves of the electric loss tangent (tan ıe = ε /ε ) and magnetic loss tangent (tan ım = / ) of raw and Er2 O3 -filled MWCNTs/paraffin composites versus the frequency. From the impedance matching principle, the lowest reflectivity can be obtained when the impedance matching condition is Zin = 1, and Zin depends on εr and r for a given thickness and frequency. As shown in Fig. 3, the ε of the Er2 O3 -filled MWCNTs composites (curve (c)) is lower than that of unfilled MWCNTs for frequency below 17 GHz (curve (a)) and exhibits a decreasing trend with increasing frequency. The ε of Er2 O3 -filled MWCNTs first descends to the lowest point at 12.0 GHz, after which a rise occurs although the rise is lower than that of unfilled MWCNTs. The characteristic of ε indicates the modified sample has excellent absorbing property in the low and middle frequency region for the frequency range studied. From Fig. 4, it can be seen that, the tan ıe of the Er2 O3 -filled MWCNTs gradually decreases to a minimum at about 12.0 GHz and
3.2. XRD analysis Fig. 2 shows X-ray powder diffraction pattern of the Er2 O3 -filled MWCNTs samples. The diffraction peak at 26.2◦ is assigned to the (0 0 2) planes of hexagonal graphite structure with an interlayer spacing of 0.34 nm, which corresponds to the fringe separation of the nanotube wall. Comparing with standard JCPDS 08-0050 pattern, the diffraction peaks at 29.54◦ , 33.86◦ , 48.80◦ and 57.84◦ are attributed to the (2 2 2), (4 0 0), (4 4 0) and (6 2 2) planes of cubic Er2 O3 phase, respectively. According to the Scherrer formula, the average diameter of the crystals encapsulated in MWCNTs is 16.3 nm calculated from the half-width of the (2 2 2) diffraction peak of Er2 O3 . These results are in agreement with the TEM results for the particle diameters.
Fig. 3. Permittivity spectra of raw and Er2 O3 -filled MWCNTs composites.
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Fig. 4. The curves of tan ıe of materials dependent on the frequency.
Fig. 6. The curves of tan ım of materials dependent on the frequency.
then increases with the increasing of frequency. The variation of the tan ıe spectrum is similar to the spectrum of ε , and the inflection points of the two curves are the same at 12.0 GHz. The reducing trend of εr and tan ıe result in the change of Zin , which helps to make a better impedance matching condition. From Fig. 5, it can be seen that the and of the Er2 O3 filled MWCNTs composite are larger than those of the raw MWCNTs composite. According to transmission-line theory [24], the represents the capacity of storing energy and the stands for the loss of energy, thus the increase of suggests that the Er2 O3 nanoparticles encapsulated in MWCNTs enhance the magnetic loss of the MWCNTs. From the results shown in Fig. 6, it can be seen that the tan ım of Er2 O3 -filled MWCNTs has been changed dramatically, which increases from −0.32 to 0.01 in the frequency range of 2–18 GHz. Both r and tan ım exhibit a trend of degression with the increase of frequency. This indicates that reducing the layer thickness and broadens the bandwidth of the absorbers.
Thus, if the six parameters of the material are known, the absorbing properties of the material can be calculated. The values of reflectivity calculated by using Eqs. (1) and (2) from measured values of ε , ε , , are shown in Fig. 7. In Fig. 7, curve (a) is the variation of the reflectivity for raw MWCNTs in the frequency range of 2–18 GHz. The maximum absorbing peak of raw MWCNTs is about −21.58 dB at 9.4 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 reflectivity below −5 dB is 3.50 GHz and the bandwidth of the reflectivity below −10 dB is 1.58 GHz. In contrast, under the same matching thickness (dm = 2.0 mm), the absorbing peak of Er2 O3 -filled MWCNTs becomes broader (curve (b)), the bandwidth of the reflectivity below −5 dB is 4.65 GHz and the bandwidth of the reflectivity below −10 dB is 2.30 GHz. The maximum absorbing peak increases to −27.96 dB and shifts to 10.0 GHz for the change of impedance. From the results shown in Fig. 7, the matching thickness (dm ) influences the reflection loss significantly. For the Er2 O3 -filled MWCNTs composite, with the increase of thickness, the maximum absorbing peaks shift towards a lower frequency (curves (b–d)).
3.4. Microwave absorbing properties According to Eqs. (1) and (2), surface reflectivity of an absorber is a function of six parameters of the material: ε , ε , , , f and d.
Fig. 5. Permeability spectra of raw and Er2 O3 -filled MWCNTs composites.
Fig. 7. Reflectivity of raw and Er2 O3 -filled MWCNTs composites: (a) unfilled MWCNTs (2.0 mm); (b) re-filled MWCNTs (2.0 mm); (c) re-filled MWCNTs (4.0 mm); (d) re-filled MWCNTs (6.0 mm).
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The bandwidths of the reflectivity below −10 dB are 2.30 GHz (dm = 2.0 mm), 1.16 GHz (dm = 4.0 mm) and 0.73 GHz (dm = 6.0 mm), respectively. It can be seen from Fig. 7 that two absorbing peaks appear on curve (c) with the matching thickness of 4.0 mm, which are −25.70 dB at 4.6 GHz and −7.32 dB at 14.8 GHz. Furthermore, there are three absorbing peaks on curve (d) when the matching thickness reaches 6.0 mm. The three absorbing peaks distributed in S, X, and Ku wave bands help to broaden the frequency range of absorption. Thus, Er2 O3 -filled MWCNTs may be applicable in the fields of RAMs and electromagnetic compatibility. As it is shown in Fig. 7, the widths at half-power maximum are: curve (a) 1.66 GHz (dm = 2.0 mm); curve (b) 1.79 GHz (dm = 2.0 mm); curve (c) 0.87 and 2.98 GHz (dm = 4.0 mm); curve (d) 0.53 GHz, 2.44 and 5.49 GHz (dm = 6.0 mm), respectively. The widths at half-power maximum of the Er2 O3 -filled composites are larger than that of unfilled samples. In the figure, the widths at half-power maximum become larger and the maximum absorbing peaks decrease with the increase of dm . In curve (d), the widths at half-power maximum increase and the peak values decrease as the frequency increases. The number of absorbing peaks increases with the increase of dm . The propagating wavelength in a material (m ) is expressed by m = o /(|εr ||r |)1/2 where o is the free space wavelength and |εr | and |r | are the moduli of εr and r , respectively. Maximal absorbing of microwave energy occurs when the dm of absorber equals an odd number multiple of m /4, and the absorbing peaks shift to lower frequencies for the increase of dm . Thus, the three peaks of curve (d) meet the interference condition that the dm is corresponding to an successive odd number multiple of m /4 at 3.0, 9.8 and 16.4 GHz, respectively. Further, calculated by using Eqs. (2), the peak values of Zin are 0.91, 0.42 and 0.29 at 3.0, 9.8 and 16.4 GHz, respectively, which are in line with the three absorbing peaks in curve (d). The impedance matching condition is in favor of maximal absorbing of microwave energy. Compared with the unfilled sample, the Er2 O3 -filled MWCNTs composite exhibits much broader absorbing bandwidth and larger reflectivity. This phenomenon, coupled with the results presented in the complex permittivity and permeability in Fig. 3 and Fig. 5, suggests that the Er2 O3 nanoparticles encapsulated in the MWCNTs modify the electromagnetic characteristics of materials and thus affect the microwave absorbing properties. Therefore, we speculate that the specific location of the Er ion in MWCNTs could generate a charge effect [25] and induce the improvement of absorbing performance. Moreover, these absorbing performance differences show that MWCNTs could play the role of resonators in the electromagnetic field, and RE oxide modified MWCNTs could improve absorbing performance in that RE oxide located in MWCNTs cavities could change the microenvironment of the resonators. The second possible reason is that the energy levels of the nanosized Er2 O3 crystals encapsulated in one-dimensional MWCNTs are not continuous but discrete because of quantum confinement effect according to Kubo theory [26,27]. This might produce a new absorbing mechanism. In addition, the Er3+ ion has the [Xe] 4f11 configuration, the 5d shell is empty and there are three unpaired 4f electrons interacting with the crystalline environment. The electron magnetic moment may cause a large magnetic loss in the composite.
4. Conclusion The microwave absorbing properties of Er2 O3 -filled MWCNTs, which were prepared by wet chemical method, were investigated in the frequency range of 2–18 GHz. The Er2 O3 nanocrystals encapsulated in MWCNTs are considered to have an impact on microwave absorbing behavior of MWCNTs. The εr and tan ıe exhibit a decreasing trend up to 18 GHz, which helps to improve impedance matching condition. Both the r and tan ım are larger than those of unfilled MWCNTs. The improved absorbing properties are proposed to be the result of the modification of the electromagnetic parameters. The maximum reflectivity is about −27.96 dB at 10.0 GHz and the bandwidth of the reflectivity below −10 dB is 2.30 GHz with a matching thickness of 2.0 mm. With the increase of thickness, the maximum reflectivity shifts to a lower frequency and multiple absorbing peaks appear. The calculated absorbing results indicate that Er2 O3 -filled MWCNTs are good candidates for preparing efficient microwave absorbers which are of low thickness and light in weight. Acknowledgement This work was supported by the National Natural Science Foundation of China (grant no. 50674006). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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