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Flexible polymer composites with enhanced wave absorption properties based on beta-manganese dioxide nanorods and PVDF
Inorganic Chemistry Communications 55 (2015) 25–29
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Flexible polymer composites with enhanced wave absorption properties based on beta-manganese dioxide nanorods and PVDF Yan Niu a, Xiang-Ping Li b,⁎ a b
Faculty of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, People's Republic of China School of Chemical and Engineer, Guangzhou University, Guangzhou 510006, People's Republic of China
a r t i c l e
i n f o
Article history: Received 10 September 2014 Received in revised form 15 October 2014 Accepted 16 October 2014 Available online 18 October 2014 Keywords: β-MnO2 nanorods PVDF Nanocomposites Wave absorption properties
a b s t r a c t Beta-manganese dioxide (β-MnO2) nanorods have been fabricated on a large scale by a simple hydrothermal process in a wild condition. Several characterizations such as XRD, SEM, TEM and FESEM have been employed. The wave absorption properties of β-MnO2/PVDF nanocomposites have been investigated. The results indicated that the β-MnO2/PVDF nanocomposites exhibit enhanced wave absorption properties. The minimum reflection loss of the β-MnO2/PVDF nanocomposite reaches −30.1 dB (N 99.9% attenuation) at 8.16 GHz with a filler loading of 40 wt.%, and the frequency bandwidth less than –10 dB is from 7.12 to 9.20 GHz. The main microwave absorbing mechanism has been also discussed. © 2014 Published by Elsevier B.V.
As a potential environmental pollution, electromagnetic (EM) wave radiation is of a great harm to electromagnetic compatibility, information safety and human health. Thus, in order to decrease the EM pollution, much attention has been devoted to microwave absorbing materials in civil and military fields [1–3]. Nanomaterial, as a new type of wave absorbing material, has attracted more and more attention. Up to now, many inorganic nanomaterials, such as CuS nanostructures [4,5], CeO2 [6], MnFe2O4 nanoparticles [7], Co3O4 [8,9], ZnO nanostructures [10,11], NiSx microspheres [12], 3D α-MnO2 [13], spongy porous Fe3O4 polyhedra [14] and urchin-like α-Fe2O3 and Fe3O4 nanostructures [15] have been studied as microwave absorbing materials. In addition, much effort has been put into studying one-dimensional (1D) nanostructures due to their novel and unexpected properties. 1D nanostructures include nanowires, nanorods, nanotubes and nanoribbons/nanobelts [16,17]. There are many researchers who have paid attention to 1D nanostructures used as microwave absorbing materials. For example, Zhu et al. [18] synthesized BaTiO3 nanotubes and investigated their microwave absorption properties. The minimum reflection loss of the BaTiO3 nanotubes/wax composite reached − 21.8 dB at 15 GHz. Silicon carbide (SiC) nanowires were synthesized by Wu et al. [19] via the moltensalt-mediated method. The results indicated that SiC nanowires possessed excellent wave absorption property, showing a minimum
⁎ Corresponding author. E-mail address: [email protected] (X.-P. Li).
http://dx.doi.org/10.1016/j.inoche.2014.10.010 1387-7003/© 2014 Published by Elsevier B.V.
reflection loss of −17.4 dB at 11.2 GHz with 30 wt.% SiC nanowires filled in the silicone matrix. Recently, to further investigate the wave absorption property of 1D nanostructures, considerable attention has been focused on 1D MnO2 nanostructures in EM wave absorption field due to manganese dioxide which is one of the most attractive inorganic materials possessing low cost, abundant resources, innocuity and other excellent performance [17,20]. Guan et al. [21] fabricated one-dimensional α-MnO2 nanorods at low temperature and studied their microwave absorption properties. An absorbing peak value of −25 dB was achieved for α-MnO2 nanorods with a thickness of 3 mm. They also investigated the microwave absorption characteristics of α-MnO2 nanowires and β-MnO2 nanorods [20]. Similarly, Song et al. [22] demonstrated that β-MnO2 nanorods could sever as a high-performance microwave absorbing filler as well. However, a single homogeneous material is difficult to simultaneously satisfy low density, tiny thickness, strong wave absorption and broad bandwidth that an “ideal” electromagnetic wave absorbing material should exhibit. Therefore, it is necessary to synthesize the composite material for EM wave absorbing material. In this paper, we used polyvinylidene fluoride (PVDF) as matrix and firstly synthesized β-MnO2 nanorods/PVDF nanocomposites by a simple blending method and investigated their microwave absorption properties. PVDF is a typical dielectric material and its simple chemical structure (-CH2-CF2-) gives the molecular chain high flexibility which will increase the practical application of the β-MnO2 nanorods/PVDF nanocomposites immensely. And from the previous research [5,7,11], a synergetic effect between nanomaterials and PVDF is benefit to improve
26
Y. Niu, X.-P. Li / Inorganic Chemistry Communications 55 (2015) 25–29
° Fig. 1. XRD pattern of the synthesized β-MnO2 nanorods.
their wave absorption properties. For example, moreover, the possible wave absorption mechanism was also discussed. Fig. 1 shows the typical XRD pattern of β-MnO2 nanorods. All of the diffraction peaks of the product are readily indexed to a pure tetragonal phase of β-MnO2 (JCPDS No: 24-0735). The narrow sharp peaks indicate that the β-MnO2 nanorods are highly crystallized and no other characteristic peaks for impurities are observed. Fig. 2a shows a general overview of β-MnO2 nanorods. The highly magnified SEM image in Fig. 2b indicates that the β-MnO2 nanorods have an average dimension of 50–100 nm in diameter and 1–1.5 μm
in length. Additionally, the TEM image in Fig. 2c also confirms it. And from the inserted HRTEM image and SAED image of a single nanorod, it can be seen that the β-MnO2 nanorods possess a single crystalline structure. Fig. 2d displays the FESEM image of the fractured structure of the β-MnO2/PVDF membrane. It shows that the β-MnO2 nanorods disperse well in PVDF. The FESEM characterization and elemental maps of the β-MnO2/ PVDF nanocomposite are also displayed in Fig. S1. From the FESEM image of the fractured surface of β-MnO2/PVDF membrane, it can be seen that the β-MnO2 nanorods disperse in PVDF uniformly. The elemental maps of Mn, O and C in the rectangular region also confirm the good dispersion of the β-MnO2 nanorods in PVDF. To investigate the electromagnetic wave absorption properties of the β-MnO2/PVDF nanocomposites, various contents of the products were mixed with PVDF to form composites via a hot-press process. Fig. 3 shows the frequency dependence of relative permittivity and dielectric loss (a ratio of the imaginary permittivity to real permittivity) for several samples. The real permittivity is an expression of the material's polarizability, whereas the imaginary permittivity is connected with the energy dissipation [23,24]. The polarizability is mainly caused by electronic dipole polarization which arose from PVDF, a typical dielectric material due to the existence of electronegative fluorine in its molecular structure. As shown in Fig. 3a and b, the ε′ and ε″ values of pure PVDF are much lower than that of the β-MnO2 nanorods. Thus, after combining with PVDF, the ε′ and ε″ values of the β-MnO2/PVDF nanocomposites will decline. Both the values of ε′ and ε″ for the βMnO2/PVDF nanocomposites are found to a remarkable increase with the increasing mass ratios. Another reason for the variation tendency is the orientation polarization. The increasing mass ratio of β-MnO2 nanorods generates a larger shape anisotropy, which will reinforce the orientation polarization [25–27]. Both the higher storage capacity and
Fig. 2. (a) and (b) SEM images of β-MnO2 nanorods; (c) TEM image of β-MnO2 nanorods. Inset shows a lattice resolved HRTEM image and SAED image of a single nanorod; (d) FESEM image of the fractured surface of β-MnO2/PVDF membrane.
Y. Niu, X.-P. Li / Inorganic Chemistry Communications 55 (2015) 25–29
ε'
(a)
25
Pure PVDF 40 wt% β-MnO2+wax
20
20 wt% β -MnO2+PVDF
10 wt% β -MnO2+PVDF 40 wt% β -MnO2+PVDF
15 10
4
6
(b) 12
8 10 12 14 Frequency (GHz)
16
18
10 wt% β -MnO2+PVDF 20 wt% β -MnO2+PVDF 40 wt% β -MnO2+PVDF
6 3 0 2
4
(c)
6
8 10 12 14 Frequency (GHz)
16
18
16
18
Pure PVDF 40 wt% β -MnO2+wax
0.9
10 wt% β -MnO2+PVDF 20 wt% β -MnO2+PVDF
0.6
40 wt% β -MnO2+PVDF
0.3 0.0 2
4
6
8 10 12 14 Frequency (GHz)
where Zin is the input characteristic impedance, which can be expressed as: [10] Z in ¼
Pure PVDF 40 wt% β -MnO2+wax
9
ε ''
ð1Þ
in
2
Dielectric Loss
[31]. As shown in Fig. 2d, it can be clearly seen that the interface exist between β-MnO2 nanorods and PVDF. In addition, the synergetic effect between these two substances could also enhance the wave absorption abilities, as our previous research studied [5,13,32]. To study the microwave absorption properties, the reflection loss (RL) of the electromagnetic radiation under the normal incidence of the electromagnetic field was calculated. According to the measured data of permittivity and permeability, reflection loss (RL) usually can be calculated by following equation: [17] Z −1 RL ¼ 20 log in Z þ 1
5
27
Fig. 3. Frequency dependence of (a) real, (b) imaginary parts of relative complex permittivity and (c) dielectric loss of samples.
rffiffiffiffiffi μr 2f πd pffiffiffiffiffiffiffiffiffi tanh j μ r εr c εr
ð2Þ
where, εr and μr (for β-MnO2/PVDF, μr is thought as 1) are the complex permittivity and permeability of the composite absorber, respectively; ƒ is the frequency; d is the thickness of the absorber, and c is the velocity of light in free space. As observed in Fig. 4, the β-MnO2/PVDF nanocomposites show enhanced wave absorption properties. Fig. 4a shows the theoretical reflection loss (RLs) of pure PVDF, β-MnO2 + wax and β-MnO2 + PVDF with various contents at a thickness of 2.5 mm in the frequency range of 2– 18 GHz. It can be clearly seen that the reflection loss of 40 wt.% βMnO2/PVDF composite is much stronger than those of others. The minimum reflection loss reaches −30.1 dB at 8.16 GHz, and the frequency bandwidth less than −10 dB is from 7.12 to 9.20 GHz; which is higher than that of the NiSx microspheres [12], 3D α-MnO2 [13] and lower than that of CuS/RGO. [33] Fig. 4b, c and d shows the threedimensional presentations of calculated theoretical RLs of the βMnO2/PVDF composites with different thicknesses (2–5 mm) in the range of 2–18 GHz with the loading of 10 wt.%, 20 wt.% and 40 wt.%, respectively. This indicates the microwave absorbing ability of the βMnO2/PVDF nanocomposites at different frequencies can be tuned by controlling the thickness of the absorbers. Except for the enhanced wave absorption properties, the β-MnO2/PVDF membrane is still as flexible as the pure PVDF and can be cut into different shapes as you want, as shown in Fig. S2. (See Scheme 1.) In summary, β-MnO2 nanorods have been synthesized via a simple hydrothermal process on a large scale. After combining with PVDF, the results indicate that the β-MnO2/PVDF nanocomposites possess excellent wave absorption properties. With a filler loading of 40 wt.%, the minimum reflection loss of β-MnO2/PVDF composite reaches − 30.1 dB at 8.16 GHz, and the frequency bandwidth less than −10 dB is from 7.12 to 9.20 GHz. The main microwave absorbing mechanism includes electronic polarization, orientation polarization, interface polarization and the synergetic effect between β-MnO2 nanorods and PVDF. Acknowledgments
more electrical energy loss are beneficial to enhance the wave absorbing ability [28–30]. From the values of dielectric loss (Fig. 3c), the maximum value of β-MnO2 nanorods is 0.428 at 7.84 GHz. For every mass ratio, the dielectric loss values of the β-MnO2/PVDF nanocomposites initially decrease and then increase after reaching a minimum with the increasing frequency. In this study, apart from electronic polarization and orientation polarization, the third wave absorption mechanism is interface polarization, which arises when the neighboring phases differ from each other in a dielectric constant, conductivity, or both, at testing frequencies
This project was financially supported by the National Natural Science Foundation of China (51472012). Appendix A. Supplementary material The preparation and characterization of β-MnO2 nanorods and βMnO2/PVDF composites are provided in the Supporting information. This section also contains the FESEM image and photograph of the βMnO2/PVDF membrane. Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2014.10.010.
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Y. Niu, X.-P. Li / Inorganic Chemistry Communications 55 (2015) 25–29
Reflection Loss (dB)
(a)
0 -6 -12 -18
Pure PVDF 40 wt% β-MnO2+wax
-24
10 wt% β-MnO2+PVDF 20 wt% β-MnO2+PVDF
-30
40 wt% β-MnO2+PVDF
2
4
6
8 10 12 14 Frequency (GHz)
16
18
Fig. 4. (a) Reflection coefficient of the products with a thickness of 2.5 mm in the range of 2 − 18 GHz. Three-dimensional presentations of the reflection loss of β-MnO2/PVDF composites with filler loading of 10 wt.% (b); 20 wt.% (c) and 40 wt.% (d).
Scheme 1. Scheme of preparation process of β-MnO2/PVDF membrane and their EM absorption measurement.
Y. Niu, X.-P. Li / Inorganic Chemistry Communications 55 (2015) 25–29
References [1] L. Kong, X. Yin, Y. Zhang, X. Yuan, Q. Li, F. Ye, L. Cheng, L. Zhang, J. Phys. Chem. C 117 (2013) 19701. [2] J. Guo, H. Wu, X. Liao, B. Shi, J. Phys. Chem. C 115 (2011) 23688. [3] F. Ren, H. Yu, L. Wang, M. Saleem, Z. Tian, P. Ren, RSC Adv. 4 (2014) 14419. [4] S. He, G.S. Wang, C. Lu, X. Luo, B. Wen, L. Guo, M.S. Cao, ChemPlusChem 78 (2013) 250. [5] S. He, G.S. Wang, C. Lu, J. Liu, B. Wen, H. Liu, L. Guo, M.S. Cao, J. Mater. Chem. A 1 (2013) 4685. [6] H. Wu, L. Wang, Y. Wang, S. Guo, Appl. Surf. Sci. 258 (2012) 10047. [7] X.J. Zhang, G.S. Wang, W.Q. Cao, Y.Z. Wei, J.F. Liang, L. Guo, M.S. Cao, ACS Appl. Mater. Interfaces 6 (2014) 7471. [8] P. Liu, Y. Huang, L. Wang, M. Zong, W. Zhang, Mater. Lett. 107 (2013) 166. [9] G.S. Wang, Y. Wu, Y.Z. Wei, X.J. Zhang, Y. Li, L.D. Li, B. Wen, P.G. Yin, L. Guo, M.S. Cao, ChemPlusChem 79 (2014) 375. [10] M.S. Cao, X.L. Shi, X.Y. Fang, H.B. Jin, Z.L. Hou, W. Zhou, Y.J. Chen, Appl. Phys. Lett. 91 (2007) 203110. [11] G.S. Wang, Y.Y. Wu, X.J. Zhang, Y. Li, L. Guo, M.S. Cao, J. Mater. Chem. A 2 (2014) 8644. [12] S. He, C. Lu, G.S. Wang, J.W. Wang, H.Y. Guo, L. Guo, ChemPlusChem 79 (2014) 569. [13] G.S. Wang, S. He, X. Luo, B. Wen, M.M. Lu, L. Guo, M.S. Cao, RSC Adv. 3 (2013) 18009. [14] G. Tong, F. Du, L. Xiang, F. Liu, L. Mao, J. Guan, Nanoscale 6 (2014) 778. [15] G. Tong, W. Wu, J. Guan, H. Qian, J. Yuan, W. Li, J. Alloys Compd. 509 (2011) 4320. [16] A.L. Tiano, C. Koenigsmann, A.C. Santulli, S.S. Wong, Chem. Commun. 46 (2010) 8093.
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[17] G.S. Wang, L.Z. Nie, S.H. Yu, RSC Adv. 2 (2012) 6216. [18] Y.F. Zhu, L. Zhang, T. Natsuki, Y.Q. Fu, Q.Q. Ni, ACS Appl. Mater. Interfaces 4 (2012) 2101. [19] R. Wu, K. Zhou, Z. Yang, X. Qian, J. Wei, L. Liu, Y. Huang, L. Kong, L. Wang, CrystEngComm 15 (2013) 570. [20] H. Guan, G. Chen, S. Zhang, Y. Wang, Mater. Chem. Phys. 124 (2010) 639. [21] H. Guan, J. Xie, G. Chen, Y. Wang, Mater. Chem. Phys. 143 (2014) 1061. [22] W.L. Song, M.S. Cao, B.B. Qiao, Z.L. Hou, M.M. Lu, C.Y. Wang, J. Yuan, D.N. Liu, L.Z. Fan, Mater. Res. Bull. 51 (2014) 277. [23] X. Yuan, L. Cheng, L. Kong, X. Yin, L. Zhang, J. Alloys Compd. 596 (2014) 132. [24] W. Zhou, X. Hu, X. Bai, S. Zhou, C. Sun, J. Yan, P. Chen, ACS Appl. Mater. Interfaces 3 (2011) 3839. [25] Q. Hu, G. Tong, W. Wu, F. Liu, H. Qian, D. Hong, CrystEngComm 15 (2013) 1314. [26] G. Tong, Q. Hu, W. Wu, W. Li, H. Qian, Y. Liang, J. Mater. Chem. 22 (2012) 17494. [27] G. Tong, J. Yuan, W. Wu, Q. Hu, H. Qian, L. Li, J. Shen, CrystEngComm 14 (2012) 2071. [28] G. Tong, W. Wu, Q. Hua, Y. Miao, J. Guan, H. Qian, J. Alloys Compd. 509 (2011) 451. [29] G.X. Tong, J.H. Yuan, J. Ma, J.G. Guan, W.H. Wu, L.C. Li, R. Qiao, Mater. Chem. Phys. 129 (2011) 1189. [30] G. Tong, W. Wu, R. Qiao, J. Yuan, J. Guan, H. Qian, J. Mater. Res. 26 (2011) 1639. [31] T. Liu, P.H. Zhou, J.L. Xie, L.J. Deng, J. Appl. Phys. 110 (2011) 033918. [32] X.J. Zhang, G.S. Wang, Y.Z. Wei, L. Guo, M.S. Cao, J. Mater. Chem. A 1 (2013) 12115. [33] G.S. Wang, X.J. Zhang, Y.Z. Wei, S. He, L. Guo, M.S. Cao, J. Mater. Chem. A 1 (2013) 7031.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Flexible polymer composites with enhanced wave absorption properties based on beta-manganese dioxide nanorods and PVDF Yan Niu a, Xiang-Ping Li b,⁎ a b
Faculty of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, People's Republic of China School of Chemical and Engineer, Guangzhou University, Guangzhou 510006, People's Republic of China
a r t i c l e
i n f o
Article history: Received 10 September 2014 Received in revised form 15 October 2014 Accepted 16 October 2014 Available online 18 October 2014 Keywords: β-MnO2 nanorods PVDF Nanocomposites Wave absorption properties
a b s t r a c t Beta-manganese dioxide (β-MnO2) nanorods have been fabricated on a large scale by a simple hydrothermal process in a wild condition. Several characterizations such as XRD, SEM, TEM and FESEM have been employed. The wave absorption properties of β-MnO2/PVDF nanocomposites have been investigated. The results indicated that the β-MnO2/PVDF nanocomposites exhibit enhanced wave absorption properties. The minimum reflection loss of the β-MnO2/PVDF nanocomposite reaches −30.1 dB (N 99.9% attenuation) at 8.16 GHz with a filler loading of 40 wt.%, and the frequency bandwidth less than –10 dB is from 7.12 to 9.20 GHz. The main microwave absorbing mechanism has been also discussed. © 2014 Published by Elsevier B.V.
As a potential environmental pollution, electromagnetic (EM) wave radiation is of a great harm to electromagnetic compatibility, information safety and human health. Thus, in order to decrease the EM pollution, much attention has been devoted to microwave absorbing materials in civil and military fields [1–3]. Nanomaterial, as a new type of wave absorbing material, has attracted more and more attention. Up to now, many inorganic nanomaterials, such as CuS nanostructures [4,5], CeO2 [6], MnFe2O4 nanoparticles [7], Co3O4 [8,9], ZnO nanostructures [10,11], NiSx microspheres [12], 3D α-MnO2 [13], spongy porous Fe3O4 polyhedra [14] and urchin-like α-Fe2O3 and Fe3O4 nanostructures [15] have been studied as microwave absorbing materials. In addition, much effort has been put into studying one-dimensional (1D) nanostructures due to their novel and unexpected properties. 1D nanostructures include nanowires, nanorods, nanotubes and nanoribbons/nanobelts [16,17]. There are many researchers who have paid attention to 1D nanostructures used as microwave absorbing materials. For example, Zhu et al. [18] synthesized BaTiO3 nanotubes and investigated their microwave absorption properties. The minimum reflection loss of the BaTiO3 nanotubes/wax composite reached − 21.8 dB at 15 GHz. Silicon carbide (SiC) nanowires were synthesized by Wu et al. [19] via the moltensalt-mediated method. The results indicated that SiC nanowires possessed excellent wave absorption property, showing a minimum
⁎ Corresponding author. E-mail address: [email protected] (X.-P. Li).
http://dx.doi.org/10.1016/j.inoche.2014.10.010 1387-7003/© 2014 Published by Elsevier B.V.
reflection loss of −17.4 dB at 11.2 GHz with 30 wt.% SiC nanowires filled in the silicone matrix. Recently, to further investigate the wave absorption property of 1D nanostructures, considerable attention has been focused on 1D MnO2 nanostructures in EM wave absorption field due to manganese dioxide which is one of the most attractive inorganic materials possessing low cost, abundant resources, innocuity and other excellent performance [17,20]. Guan et al. [21] fabricated one-dimensional α-MnO2 nanorods at low temperature and studied their microwave absorption properties. An absorbing peak value of −25 dB was achieved for α-MnO2 nanorods with a thickness of 3 mm. They also investigated the microwave absorption characteristics of α-MnO2 nanowires and β-MnO2 nanorods [20]. Similarly, Song et al. [22] demonstrated that β-MnO2 nanorods could sever as a high-performance microwave absorbing filler as well. However, a single homogeneous material is difficult to simultaneously satisfy low density, tiny thickness, strong wave absorption and broad bandwidth that an “ideal” electromagnetic wave absorbing material should exhibit. Therefore, it is necessary to synthesize the composite material for EM wave absorbing material. In this paper, we used polyvinylidene fluoride (PVDF) as matrix and firstly synthesized β-MnO2 nanorods/PVDF nanocomposites by a simple blending method and investigated their microwave absorption properties. PVDF is a typical dielectric material and its simple chemical structure (-CH2-CF2-) gives the molecular chain high flexibility which will increase the practical application of the β-MnO2 nanorods/PVDF nanocomposites immensely. And from the previous research [5,7,11], a synergetic effect between nanomaterials and PVDF is benefit to improve
26
Y. Niu, X.-P. Li / Inorganic Chemistry Communications 55 (2015) 25–29
° Fig. 1. XRD pattern of the synthesized β-MnO2 nanorods.
their wave absorption properties. For example, moreover, the possible wave absorption mechanism was also discussed. Fig. 1 shows the typical XRD pattern of β-MnO2 nanorods. All of the diffraction peaks of the product are readily indexed to a pure tetragonal phase of β-MnO2 (JCPDS No: 24-0735). The narrow sharp peaks indicate that the β-MnO2 nanorods are highly crystallized and no other characteristic peaks for impurities are observed. Fig. 2a shows a general overview of β-MnO2 nanorods. The highly magnified SEM image in Fig. 2b indicates that the β-MnO2 nanorods have an average dimension of 50–100 nm in diameter and 1–1.5 μm
in length. Additionally, the TEM image in Fig. 2c also confirms it. And from the inserted HRTEM image and SAED image of a single nanorod, it can be seen that the β-MnO2 nanorods possess a single crystalline structure. Fig. 2d displays the FESEM image of the fractured structure of the β-MnO2/PVDF membrane. It shows that the β-MnO2 nanorods disperse well in PVDF. The FESEM characterization and elemental maps of the β-MnO2/ PVDF nanocomposite are also displayed in Fig. S1. From the FESEM image of the fractured surface of β-MnO2/PVDF membrane, it can be seen that the β-MnO2 nanorods disperse in PVDF uniformly. The elemental maps of Mn, O and C in the rectangular region also confirm the good dispersion of the β-MnO2 nanorods in PVDF. To investigate the electromagnetic wave absorption properties of the β-MnO2/PVDF nanocomposites, various contents of the products were mixed with PVDF to form composites via a hot-press process. Fig. 3 shows the frequency dependence of relative permittivity and dielectric loss (a ratio of the imaginary permittivity to real permittivity) for several samples. The real permittivity is an expression of the material's polarizability, whereas the imaginary permittivity is connected with the energy dissipation [23,24]. The polarizability is mainly caused by electronic dipole polarization which arose from PVDF, a typical dielectric material due to the existence of electronegative fluorine in its molecular structure. As shown in Fig. 3a and b, the ε′ and ε″ values of pure PVDF are much lower than that of the β-MnO2 nanorods. Thus, after combining with PVDF, the ε′ and ε″ values of the β-MnO2/PVDF nanocomposites will decline. Both the values of ε′ and ε″ for the βMnO2/PVDF nanocomposites are found to a remarkable increase with the increasing mass ratios. Another reason for the variation tendency is the orientation polarization. The increasing mass ratio of β-MnO2 nanorods generates a larger shape anisotropy, which will reinforce the orientation polarization [25–27]. Both the higher storage capacity and
Fig. 2. (a) and (b) SEM images of β-MnO2 nanorods; (c) TEM image of β-MnO2 nanorods. Inset shows a lattice resolved HRTEM image and SAED image of a single nanorod; (d) FESEM image of the fractured surface of β-MnO2/PVDF membrane.
Y. Niu, X.-P. Li / Inorganic Chemistry Communications 55 (2015) 25–29
ε'
(a)
25
Pure PVDF 40 wt% β-MnO2+wax
20
20 wt% β -MnO2+PVDF
10 wt% β -MnO2+PVDF 40 wt% β -MnO2+PVDF
15 10
4
6
(b) 12
8 10 12 14 Frequency (GHz)
16
18
10 wt% β -MnO2+PVDF 20 wt% β -MnO2+PVDF 40 wt% β -MnO2+PVDF
6 3 0 2
4
(c)
6
8 10 12 14 Frequency (GHz)
16
18
16
18
Pure PVDF 40 wt% β -MnO2+wax
0.9
10 wt% β -MnO2+PVDF 20 wt% β -MnO2+PVDF
0.6
40 wt% β -MnO2+PVDF
0.3 0.0 2
4
6
8 10 12 14 Frequency (GHz)
where Zin is the input characteristic impedance, which can be expressed as: [10] Z in ¼
Pure PVDF 40 wt% β -MnO2+wax
9
ε ''
ð1Þ
in
2
Dielectric Loss
[31]. As shown in Fig. 2d, it can be clearly seen that the interface exist between β-MnO2 nanorods and PVDF. In addition, the synergetic effect between these two substances could also enhance the wave absorption abilities, as our previous research studied [5,13,32]. To study the microwave absorption properties, the reflection loss (RL) of the electromagnetic radiation under the normal incidence of the electromagnetic field was calculated. According to the measured data of permittivity and permeability, reflection loss (RL) usually can be calculated by following equation: [17] Z −1 RL ¼ 20 log in Z þ 1
5
27
Fig. 3. Frequency dependence of (a) real, (b) imaginary parts of relative complex permittivity and (c) dielectric loss of samples.
rffiffiffiffiffi μr 2f πd pffiffiffiffiffiffiffiffiffi tanh j μ r εr c εr
ð2Þ
where, εr and μr (for β-MnO2/PVDF, μr is thought as 1) are the complex permittivity and permeability of the composite absorber, respectively; ƒ is the frequency; d is the thickness of the absorber, and c is the velocity of light in free space. As observed in Fig. 4, the β-MnO2/PVDF nanocomposites show enhanced wave absorption properties. Fig. 4a shows the theoretical reflection loss (RLs) of pure PVDF, β-MnO2 + wax and β-MnO2 + PVDF with various contents at a thickness of 2.5 mm in the frequency range of 2– 18 GHz. It can be clearly seen that the reflection loss of 40 wt.% βMnO2/PVDF composite is much stronger than those of others. The minimum reflection loss reaches −30.1 dB at 8.16 GHz, and the frequency bandwidth less than −10 dB is from 7.12 to 9.20 GHz; which is higher than that of the NiSx microspheres [12], 3D α-MnO2 [13] and lower than that of CuS/RGO. [33] Fig. 4b, c and d shows the threedimensional presentations of calculated theoretical RLs of the βMnO2/PVDF composites with different thicknesses (2–5 mm) in the range of 2–18 GHz with the loading of 10 wt.%, 20 wt.% and 40 wt.%, respectively. This indicates the microwave absorbing ability of the βMnO2/PVDF nanocomposites at different frequencies can be tuned by controlling the thickness of the absorbers. Except for the enhanced wave absorption properties, the β-MnO2/PVDF membrane is still as flexible as the pure PVDF and can be cut into different shapes as you want, as shown in Fig. S2. (See Scheme 1.) In summary, β-MnO2 nanorods have been synthesized via a simple hydrothermal process on a large scale. After combining with PVDF, the results indicate that the β-MnO2/PVDF nanocomposites possess excellent wave absorption properties. With a filler loading of 40 wt.%, the minimum reflection loss of β-MnO2/PVDF composite reaches − 30.1 dB at 8.16 GHz, and the frequency bandwidth less than −10 dB is from 7.12 to 9.20 GHz. The main microwave absorbing mechanism includes electronic polarization, orientation polarization, interface polarization and the synergetic effect between β-MnO2 nanorods and PVDF. Acknowledgments
more electrical energy loss are beneficial to enhance the wave absorbing ability [28–30]. From the values of dielectric loss (Fig. 3c), the maximum value of β-MnO2 nanorods is 0.428 at 7.84 GHz. For every mass ratio, the dielectric loss values of the β-MnO2/PVDF nanocomposites initially decrease and then increase after reaching a minimum with the increasing frequency. In this study, apart from electronic polarization and orientation polarization, the third wave absorption mechanism is interface polarization, which arises when the neighboring phases differ from each other in a dielectric constant, conductivity, or both, at testing frequencies
This project was financially supported by the National Natural Science Foundation of China (51472012). Appendix A. Supplementary material The preparation and characterization of β-MnO2 nanorods and βMnO2/PVDF composites are provided in the Supporting information. This section also contains the FESEM image and photograph of the βMnO2/PVDF membrane. Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.inoche.2014.10.010.
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Y. Niu, X.-P. Li / Inorganic Chemistry Communications 55 (2015) 25–29
Reflection Loss (dB)
(a)
0 -6 -12 -18
Pure PVDF 40 wt% β-MnO2+wax
-24
10 wt% β-MnO2+PVDF 20 wt% β-MnO2+PVDF
-30
40 wt% β-MnO2+PVDF
2
4
6
8 10 12 14 Frequency (GHz)
16
18
Fig. 4. (a) Reflection coefficient of the products with a thickness of 2.5 mm in the range of 2 − 18 GHz. Three-dimensional presentations of the reflection loss of β-MnO2/PVDF composites with filler loading of 10 wt.% (b); 20 wt.% (c) and 40 wt.% (d).
Scheme 1. Scheme of preparation process of β-MnO2/PVDF membrane and their EM absorption measurement.
Y. Niu, X.-P. Li / Inorganic Chemistry Communications 55 (2015) 25–29
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