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nano SiO2 composite aerogels with enhanced electromagnetic absorption
Accepted Manuscript In Situ Preparation of Ultralight Three-dimensional Polypyrrole /Nano SiO2 Composite Aerogels with Enhanced Electromagnetic Absorption Aming Xie, Fan Wu, Zhuanghu Xu, Mingyang Wang PII:
S0266-3538(15)00197-9
DOI:
10.1016/j.compscitech.2015.05.010
Reference:
CSTE 6108
To appear in:
Composites Science and Technology
Received Date: 5 February 2015 Revised Date:
29 April 2015
Accepted Date: 11 May 2015
Please cite this article as: Xie A, Wu F, Xu Z, Wang M, In Situ Preparation of Ultralight Threedimensional Polypyrrole /Nano SiO2 Composite Aerogels with Enhanced Electromagnetic Absorption, Composites Science and Technology (2015), doi: 10.1016/j.compscitech.2015.05.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT In Situ Preparation of Ultralight Three-dimensional Polypyrrole /Nano SiO2 Composite Aerogels with Enhanced Electromagnetic Absorption Aming Xie,*ab Fan Wu,*b Zhuanghu Xu,b Mingyang Wang*ab School of Mechanical Engineering, Nanjing University of Science & Technology, Nanjing 210094, P. R. China.
State Key Laboratory for Disaster Prevention & Mitigation of Explosion & Impact,
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b
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a
PLA University of Science and Technology, Nanjing 210007, P. R. China.
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*Corresponding author: E-mail addresses: [email protected] (Aming Xie)， [email protected] (Fan Wu), [email protected] (Mingyang Wang), Tel.: +86 25 80825361. ABSTRACT.
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Ultralight three-dimensional polypyrrole/nano
SiO2 aerogels have been
synthesized through an in situ gelation process. These composites showed enhanced
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properties of electromagnetic absorption and can be easily to scale up. Only with a low filler loading, the maximum effective EMA bandwidth could reach 6.0 GHz when the
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thickness of absorber is 2.5 mm. This work provides a facile method to fabricate a potential and excellent electromagnetic absorption material with low loading ratio and wide absorption bandwidth. KEYWORDS: A. Nano composites; A. Polymers; B. Electrical properties. Introduction During the past decades, electromagnetic absorption (EMA) was playing an
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ACCEPTED MANUSCRIPT increasing role in the protection of electronic devices as well as human beings from the damage of electromagnetic radiation [1]. To effectively absorb electromagnetic wave, several materials with specific structures have been used for EMA, such as inorganic
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nanoparticles including Bi2S3 [2], CuS [3,4], α-MnO2 [5], and ZnO nanorods [6], conducting polymers (CPs) including poly(3,4-ethylenedioxythiophene) (PEDOT) [7,8],
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carbonaceous materials including intercalated graphite [9], carbon nanotubes (CNTs) [10,11], and lattices [12-14]. However, these above mentioned materials were not
reflection performances or densities.
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beneficial for the practical applications on account of the high filler loading ratios,
For the next generation EMA materials, lightweight is a key factor because it can cut down the amount of used materials and energy, as well as simplify the manipulation.
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The general method to reduce the weight or the density of bulk materials is through the formation of pores during synthesis [15]. As a highly porous solid nano-material,
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aerogel has been anticipated for extensive applications because of its unique characteristics such as low densities, large pores and high surface areas.
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Three-dimensional reduced graphene oxide (3D-RGO) is an ultralight aerogel that prepared through a one-step hydrothermal process [16] and later used for energy storage [17], environmental disposition [18], etc. By virtue of low densities, electrical conductivities and high specific surface areas, 3D-RGO nano-composites have been significantly applied to the field of electromagnetic absorption, such as Fe2O3 [19], Fe3O4 [20], Co3O4 [21], hematite [22], MnFe2O4 [23], NiFe2O4 [24], polyaniline [25],
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ACCEPTED MANUSCRIPT PEDOT [26], and CNTs [27]. However, there are difficulties for the large-scaled preparation of 3D-RGO based carbonaceous materials including the troublesome purification and functionalization. In contrast, conducting polymer aerogels (CPAs), a
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kind of three-dimensional (3D) polymeric networks made from CPs, can be easily produced in large scale and combine the excellent electrical properties of
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semiconductors and lightweight of aerogels [28-32].
As one of the most important CPs, polypyrrole (PPy) can be widely used in the
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applications of supercapacitor [33], electromagnetic pollution [34], bionic technology [35], etc. due to its good conductivity, facile synthesis, low cost and stability. Recently, because of the advantages, some lightweight PPy aerogels has been prepared from hydrogels or organogels through a drying process. For example, Lu et al. synthesized an
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elastic and smart PPy aerogel with conductively polymeric 3D micro-structures [36]. Shi et al. prepared a 3D porous nanostructured conductive PPy aerogel with good
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mechanical properties and high performance acting as flexible supercapacitor electrodes [37]. Nevertheless, the sole PPy aerogel generally cannot always meet all the
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requirements of practical applications. The preparation of aerogel composites may be a feasible strategy to avoid some inadequacies. Nano SiO2, a common nanoparticle, is often used as seeds to grow
one-dimensional nanomaterials [38] or as core materials to prepare core-shell nanostructures [39]. In many cases, the import of SiO2 can greatly improve the impedance matching of the as-prepared material toward the atmosphere, and thus
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ACCEPTED MANUSCRIPT effectively enhance the EMA performance. For microwave absorption, Cao et al. showed that CNTs/SiO2 composites possess excellent performances in different temperatures [40-42]. After coating with SiO2 nanoshell, Fe/SiO2 composites showed
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better microwave absorption property, as well as enhanced antioxidant capacity [43]. Herein, to combine the merits of nano SiO2 and lightweight PPy aerogel, we
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synthesized ultralight 3D-PPy/Nano SiO2 aerogels through an in situ preparation process and exploited the EMA properties of the as-prepared composites.
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Experimental section 2.1 Materials
The pyrrole monomer, nano-SiO2 (average 15 nm), anhydrous FeCl3 and ethanol were purchased from GENERAL-REAGENT, Titan Scientific Co., Ltd, Shanghai, China.
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Distilled water was obtained from Direct-Q3 UV, Millipore. 2.2 The preparation of 3D-PPy/nano SiO2 (3D-PPy/SiO2) composites
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Pyrrole (Py) monomer (5 mmol) and nano SiO2 power were added to 3 mL H2O/ethanol (1:1) and dispersed by sonication. Then a solvent of anhydrous FeCl3 (11.5 mmol) in 3
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mL H2O/ethanol (1:1) was quickly poured into the dispersion. A black gel formed within several minutes and aged for 24 hours. The obtained gel was washed with large amount of water and alcohol and dried at 50 oC under vacuum. Several compositions of 3D-PPy/SiO2 composites with different weight ratios of SiO2 and Py have been synthesized and abbreviated as 3D-PPy where Py and SiO2 are taken in a 1:0 wt. ratio, 3D-PPy/20%SiO2 where Py and SiO2 are taken in a 1:0.2 wt. ratio, 3D-PPy/50%SiO2
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ACCEPTED MANUSCRIPT where Py and SiO2 are taken in a 1:0.5 wt. ratio. 2.3 Characterization and measurement The scanning electron microscope (SEM) images were obtained on a field
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emission scanning electron microscope (FE-SEM, S4800, Hitachi) sputtering with gold. The transmission electron microscopy (TEM) images were obtained on a field emission
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high resolution transmission electron microscope (FE-HRTEM, Tecnai G2 F20, FEI). X-ray photoelectron spectra (XPS) was carried out in a Thermo Scientific ESCALAB
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250Xi X-ray photoelectron spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV). Fourier transfor infrared (FTIR) spectra were recorded on a Nicolet iS10 FTIR instrument (Thermo Fisher Scientific, USA). The crystal structure of the as-prepared samples was measured by X-ray diffractometer (XRD, X’ Pert Pro, Philips),
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using Cu Kα (λ = 1.54 Å) radiation.
2.4 Dielectric and EMA Measurement.
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The 3D-PPy/SiO2 composites used for microwave absorption measurement were prepared by uniformly mixing the samples with paraffin in different mass percentages.
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The mixtures were then pressed into toroidal shaped samples with an outer diameter of 7.00 mm and inner diameter of 3.04 mm. The relative complex permittivity and permeability values were measured with coaxial wire method in the frequency range of 2-18 GHz with the vector network analyzer (VNA, N5242A PNA-X, Agilent). 3. Results and discussions 3.1 The synthesis of 3D-PPy/SiO2 composites
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ACCEPTED MANUSCRIPT Figure 1 gives the synthesis strategy for ultralight 3D-PPy/SiO2 composites. Pyrrole monomer and nano SiO2 powder mixture was dispersed under stirring and polymerized to form a 3D structure in several minutes after FeCl3 solution was added.
We can
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clearly see that 3D-PPy hydrogel composites were formed in the bottom of a bottle, even if the bottle was inverted (See ESI Fugure S1). After washing and drying, the
3.2 The characterization of 3D-PPy/SiO2 composites
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composites were obtained with low densities (See ESI S1).
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The morphologies and structures of 3D-PPy/SiO2 composites were first investigated by a scanning electron microscopy (SEM). As shown in Figure 2, 3D-PPy with or without incorporation of nano SiO2 is a network constructed from a great deal of PPy nanosheets which are polymerized from pyrrole monomers. We also investigate the
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morphology of the as-prepared 3D-PPy/20%SiO2 composite by TEM technique (Figure 3a and b). The TEM images imply that the three dimensional network of 3D-PPy is
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formed through the π-π stacking between PPy nanosheets. Nano SiO2 are much smaller irregular particles than the PPy sheets (Figure 3a and b). It can also be found from the
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TEM images that SiO2 nano particles are dispersed in the pores of the 3D-PPy other than uniformly absorbed on the PPy nanosheets. The elemental components of the as-prepared composite were identified by XPS
technique. Figure 3c shows a general XPS profile for 3D-PPy/20%SiO2. The figure demonstrates that the 3D-PPy/20%SiO2 composite is mainly composed of Si, Cl, C, N and O elements. The strong C 1s and N 1s peaks belong to the 3D-PPy in the sample
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ACCEPTED MANUSCRIPT and Si 2p and O 1s peaks mainly arise from the nano SiO2. Cl 2p peak may be related to chlorine anion which is used to neutralize the positive charge of 3D-PPy. In addition, trace amounts of iron can be found in the composite (Figure 3d).
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FT-IR spectroscopy was used to examine the compositions of 3D-PPy/SiO2 by the in situ process. Figure 4 shows that most of the characteristic peaks in 3D-PPy can be
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remained in the composites. This may demonstrate that PPy is the primary ingredient. Aside from these peaks, the band at 772 cm-1 is corresponded to symmetrical stretching
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vibration of the Si-O-Si. This indicates the existence of SiO2 in 3D-PPy/SiO2 material. Figure 5 shows XRD patterns of Nano SiO2, 3D-PPy and 3D-PPy/20%SiO2. The main eight peaks in 3D-PPy/20%SiO2 composite centered at 24.15°, 33.16°, 35.64°, 40.87°, 49.47°, 54.08°, 62.45°, and 64.01° match well with the standard XRD data for
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the (012), (104), (110), (113), (024), (116), (214), and (300) planes of Fe2O3 (JCPDS NO. 84-0306). This Fe2O3 comes from 3D-PPy and may form from the excess oxidant
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FeCl3 under drying process. It can also be inferred that the three samples were all amorphous solids because of the broad XRD patterns. On account of the bands close to
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each other, we cannot distinguish the patterns of 3D-PPy and Nano SiO2. Nevertheless, the well combination of the two components can be confirmed through all the analysis above.
3.3 Electromagnetic Absorption Properties. The measurements are considered under far field because the source-to-shield distance is greater than the free-space wavelength in the frequency range of 2-18 GHz
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ACCEPTED MANUSCRIPT [44]. According to the transmission line theory [45], the input impedance (Zin) on the interface can be expressed as
= ℎ
√
(1)
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Where Z0 is the impedance of free space, µr is the complex permeability, µr = µ′ − jµ″, εr is the complex permittivity, εr = ε′ − jε″, f is the frequency, d is the thickness of material,
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c is the speed of light.
Based on the model of metal backplane, the reflection loss (RL) of a sample is
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determined from Z0 and Zin according to the following equation & )&
RL dB = 20lg %&'( +&* % *
'(
(2)
When the RL is lower than – 10 dB, over 90 % of the electromagnetic energy is absorbed.
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The permittivities and permeabilities of each sample were measured using coaxial wire method.[44] Despite of containing iron element, electromagnetic absorption of all the
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composites here is due to dielectric losses because the permeabilities are close to 1. (See ESI Figure S2) Figure 6a and b show the permittivity real part (ε′) and permittivity
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imaginary part (ε″) measured in the frequency range of 2-18 GHz of filler loading with 20 wt% of samples in paraffin matrix. It is observed that the addition of nano SiO2 obviously lowers both the values of ε′ and ε″. This can be attributed to the low electrical conductivity of amorphous SiO2. Moreover, different from 3D-PPy, the values of ε′ and ε″ slightly decrease with the increase of the frequency. Dielectric loss tangent (tanδ) was defined as the specific value of ε″ and ε′. As shown in Figure 6c, the tanδ of the
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ACCEPTED MANUSCRIPT composites keep constant with increasing frequency. It was apparently found that the value of tanδ greatly decrease after combining with nano SiO2. Generally, energy loss in a material functioned by electromagnetic waves originates from damping forces acting
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on polarized atoms and molecules. In this interaction process, the electrical conductivity of a material plays an important role. However, it doesn't necessarily mean that larger ε′
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or ε″ will get better EMA. In 3D-PPy/SiO2 composites, a relatively small ε′, ε″ and tanδ are beneficial to EMA. Figure 6d shows the Cole-Cole plots of the composites and no
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obvious conspicuous semicircles are observed. This demonstrates that the Debye relaxation process in 3D-PPy/SiO2 is hidden. The main dielectric loss may come from other polarization relaxation processes such as electronic polarization and interfacial polarization.
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RL values of composites with different mass percentages of nano SiO2 can be obtained through formula (1) and (2). Figure 7a and d show the RL curves of samples
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with the fillers loading of 20 wt% of 3D-PPy/20%SiO2 and 3D-PPy/50%SiO2 at different thicknesses (2.0-6.0 mm) in the frequency range 2-18 GHz. It is obviously
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found that EA performance has not been enhanced with the increase content of SiO2. As shown in Table 1, there is an effective EA bandwidth of 6.0 GHz with the thickness of 2.5 mm. The maximal RL value of 3D-PPy/20%SiO2 can reach -35.68 dB at 5.5 mm. For 3D-PPy without SiO2, the capacity of EA is not as good (See ESI Figure S3). We also give the three dimensional and contour plot of the RL versus frequency and thickness of samples with the fillers loading of 20 wt% of composites (Figure 7b, c, e
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ACCEPTED MANUSCRIPT and f). The effective EA regions (RL below -10 dB) can be clearly showed in the projection drawings and it will be beneficial for the design of electromagnetic absorber. These results imply that this material meets the main requirements for dielectric-based
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electromagnetic absorbers.
Based on formula (1) and (2), RL values of wax with different mass percentages of
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3D-PPy/20%SiO2 have been obtained and the results are showed in Figure 8. For 10 wt% of the composite, the maximum absorption peak can research -29.80 dB. But the
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corresponding frequency shift to 15.12 GHz at an incremental thickness of 6 mm. When the filler content increase to 30 wt%, the absorption of electromagnetic wave decreases severely and scarcely any RL bandwidths below -10 dB. In a word, the EA performance with an increased or reduced content is not as good as that of 20 wt%.
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4. Conclusion
In summary, through an in situ preparation process, ultralight three-dimensional
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polypyrrole/nano SiO2 aerogels have been synthesized with enhanced properties of electromagnetic absorption and can be easily to scale up. For 3D-PPy/20%SiO2, the
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maximum effective EMA bandwidth could reach 6.0 GHz when filler loading is 20 wt% in wax and thickness of absorber is 2.5 mm. This work provides a facile method to fabricate a potential and excellent EMA material with low loading ratio and wide absorption bandwidth. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of
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ACCEPTED MANUSCRIPT China (51403236, 51021001) and the Opening Project of State Key Laboratory of Disaster Prevention & Mitigation of Explosion & Impact (DPMEIKF201310). REFERENCES
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Figure 1 The preparation of PPy/SiO2 composites.
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Figure 2 SEM images of (a) 3D-PPy aerogel, (b, c) 3D-PPy/20%SiO2 composite and (d) 3D-PPy/50%SiO2 composite.
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Figure 3 (a, b) TEM images, (c) the general XPS and (d) Fe 2p region of
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3D-PPy/20%SiO2 composite.
Figure 4 The FT-IR spectra of pure nano SiO2, 3D-PPy/20%SiO2 composite, 3D-PPy/50%SiO2 composite and 3D-PPy aerogel. 19
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Figure 5 The XRD patterns of pure nano SiO2, 3D-PPy aerogel and 3D-PPy/20%SiO2
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composite.
Figure 6 (a) The permittivities, (b) permeabilities, (c) tanδ, and (d) Cole-Cole plots of the composites.
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Figure 7 (a,d) RL values, (b,e) 3D plots and (c,f) contour plots of 3D-PPy/20%SiO2
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composite and 3D-PPy/50%SiO2 composite of loading with 20 wt% in wax.
Figure 8 RL values of (a) 10 wt% and (b) 30 wt% of 3D-PPy/20%SiO2 composite in wax.
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Table 1 EA performance with different thicknesses of filler loading with 20 wt%
Maximum
Maximun RL
Thickness
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3D-PPy/20%SiO2 composite in wax.
Effective EA bandwidth RL
position
(mm)
(GHz)
(GHz)
2.0
− 16.89
17.56
2.5
− 21.65
14.56
3.0
− 22.69
3.5
− 27.11
4.0
− 32.13
4.5
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(dB)
2.44
4.96
9.68
4.04
8.24
3.52
− 29.52
7.16
3.04
5.0
− 39.31
6.36
2.64
5.5
− 35.68
5.68
2.36
6.0
5.08
2.16
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11.56
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6.00
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− 30.80
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ACCEPTED MANUSCRIPT Electronic Supplementary Information S1. The method to calculate the densities of 3D-PPy/SiO2 composites. The densities of 3D-PPy/SiO2 composites were calculated through the following process. Firstly, 3D-PPy/SiO2 composite was cut into a cube by a knife; then the
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weight of the obtained cubic 3D-PPy/SiO2 composite was measured though an analytical balance and the volume was calculated through the mathematics formula;
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finally, the density was equal to the mass per unit volume. The got densities were 47.6
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mg/cm3 for 3D-PPy/20%SiO2 and 69.9 mg/cm3 for 3D-PPy/50%SiO2.
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Figure S1 The comparison of before and after gelation.
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Figure S2 Real permeabilities (u’) and imaginary permeabilities (u’’) of 3D-PPy/20%SiO2 and 3D-PPy/50%SiO2
Figure S3 RL value of 20 wt% of 3D-PPy with wax.
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S0266-3538(15)00197-9
DOI:
10.1016/j.compscitech.2015.05.010
Reference:
CSTE 6108
To appear in:
Composites Science and Technology
Received Date: 5 February 2015 Revised Date:
29 April 2015
Accepted Date: 11 May 2015
Please cite this article as: Xie A, Wu F, Xu Z, Wang M, In Situ Preparation of Ultralight Threedimensional Polypyrrole /Nano SiO2 Composite Aerogels with Enhanced Electromagnetic Absorption, Composites Science and Technology (2015), doi: 10.1016/j.compscitech.2015.05.010. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT In Situ Preparation of Ultralight Three-dimensional Polypyrrole /Nano SiO2 Composite Aerogels with Enhanced Electromagnetic Absorption Aming Xie,*ab Fan Wu,*b Zhuanghu Xu,b Mingyang Wang*ab School of Mechanical Engineering, Nanjing University of Science & Technology, Nanjing 210094, P. R. China.
State Key Laboratory for Disaster Prevention & Mitigation of Explosion & Impact,
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b
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a
PLA University of Science and Technology, Nanjing 210007, P. R. China.
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*Corresponding author: E-mail addresses: [email protected] (Aming Xie)， [email protected] (Fan Wu), [email protected] (Mingyang Wang), Tel.: +86 25 80825361. ABSTRACT.
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Ultralight three-dimensional polypyrrole/nano
SiO2 aerogels have been
synthesized through an in situ gelation process. These composites showed enhanced
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properties of electromagnetic absorption and can be easily to scale up. Only with a low filler loading, the maximum effective EMA bandwidth could reach 6.0 GHz when the
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thickness of absorber is 2.5 mm. This work provides a facile method to fabricate a potential and excellent electromagnetic absorption material with low loading ratio and wide absorption bandwidth. KEYWORDS: A. Nano composites; A. Polymers; B. Electrical properties. Introduction During the past decades, electromagnetic absorption (EMA) was playing an
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ACCEPTED MANUSCRIPT increasing role in the protection of electronic devices as well as human beings from the damage of electromagnetic radiation [1]. To effectively absorb electromagnetic wave, several materials with specific structures have been used for EMA, such as inorganic
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nanoparticles including Bi2S3 [2], CuS [3,4], α-MnO2 [5], and ZnO nanorods [6], conducting polymers (CPs) including poly(3,4-ethylenedioxythiophene) (PEDOT) [7,8],
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carbonaceous materials including intercalated graphite [9], carbon nanotubes (CNTs) [10,11], and lattices [12-14]. However, these above mentioned materials were not
reflection performances or densities.
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beneficial for the practical applications on account of the high filler loading ratios,
For the next generation EMA materials, lightweight is a key factor because it can cut down the amount of used materials and energy, as well as simplify the manipulation.
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The general method to reduce the weight or the density of bulk materials is through the formation of pores during synthesis [15]. As a highly porous solid nano-material,
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aerogel has been anticipated for extensive applications because of its unique characteristics such as low densities, large pores and high surface areas.
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Three-dimensional reduced graphene oxide (3D-RGO) is an ultralight aerogel that prepared through a one-step hydrothermal process [16] and later used for energy storage [17], environmental disposition [18], etc. By virtue of low densities, electrical conductivities and high specific surface areas, 3D-RGO nano-composites have been significantly applied to the field of electromagnetic absorption, such as Fe2O3 [19], Fe3O4 [20], Co3O4 [21], hematite [22], MnFe2O4 [23], NiFe2O4 [24], polyaniline [25],
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ACCEPTED MANUSCRIPT PEDOT [26], and CNTs [27]. However, there are difficulties for the large-scaled preparation of 3D-RGO based carbonaceous materials including the troublesome purification and functionalization. In contrast, conducting polymer aerogels (CPAs), a
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kind of three-dimensional (3D) polymeric networks made from CPs, can be easily produced in large scale and combine the excellent electrical properties of
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semiconductors and lightweight of aerogels [28-32].
As one of the most important CPs, polypyrrole (PPy) can be widely used in the
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applications of supercapacitor [33], electromagnetic pollution [34], bionic technology [35], etc. due to its good conductivity, facile synthesis, low cost and stability. Recently, because of the advantages, some lightweight PPy aerogels has been prepared from hydrogels or organogels through a drying process. For example, Lu et al. synthesized an
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elastic and smart PPy aerogel with conductively polymeric 3D micro-structures [36]. Shi et al. prepared a 3D porous nanostructured conductive PPy aerogel with good
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mechanical properties and high performance acting as flexible supercapacitor electrodes [37]. Nevertheless, the sole PPy aerogel generally cannot always meet all the
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requirements of practical applications. The preparation of aerogel composites may be a feasible strategy to avoid some inadequacies. Nano SiO2, a common nanoparticle, is often used as seeds to grow
one-dimensional nanomaterials [38] or as core materials to prepare core-shell nanostructures [39]. In many cases, the import of SiO2 can greatly improve the impedance matching of the as-prepared material toward the atmosphere, and thus
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ACCEPTED MANUSCRIPT effectively enhance the EMA performance. For microwave absorption, Cao et al. showed that CNTs/SiO2 composites possess excellent performances in different temperatures [40-42]. After coating with SiO2 nanoshell, Fe/SiO2 composites showed
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better microwave absorption property, as well as enhanced antioxidant capacity [43]. Herein, to combine the merits of nano SiO2 and lightweight PPy aerogel, we
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synthesized ultralight 3D-PPy/Nano SiO2 aerogels through an in situ preparation process and exploited the EMA properties of the as-prepared composites.
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Experimental section 2.1 Materials
The pyrrole monomer, nano-SiO2 (average 15 nm), anhydrous FeCl3 and ethanol were purchased from GENERAL-REAGENT, Titan Scientific Co., Ltd, Shanghai, China.
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Distilled water was obtained from Direct-Q3 UV, Millipore. 2.2 The preparation of 3D-PPy/nano SiO2 (3D-PPy/SiO2) composites
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Pyrrole (Py) monomer (5 mmol) and nano SiO2 power were added to 3 mL H2O/ethanol (1:1) and dispersed by sonication. Then a solvent of anhydrous FeCl3 (11.5 mmol) in 3
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mL H2O/ethanol (1:1) was quickly poured into the dispersion. A black gel formed within several minutes and aged for 24 hours. The obtained gel was washed with large amount of water and alcohol and dried at 50 oC under vacuum. Several compositions of 3D-PPy/SiO2 composites with different weight ratios of SiO2 and Py have been synthesized and abbreviated as 3D-PPy where Py and SiO2 are taken in a 1:0 wt. ratio, 3D-PPy/20%SiO2 where Py and SiO2 are taken in a 1:0.2 wt. ratio, 3D-PPy/50%SiO2
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ACCEPTED MANUSCRIPT where Py and SiO2 are taken in a 1:0.5 wt. ratio. 2.3 Characterization and measurement The scanning electron microscope (SEM) images were obtained on a field
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emission scanning electron microscope (FE-SEM, S4800, Hitachi) sputtering with gold. The transmission electron microscopy (TEM) images were obtained on a field emission
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high resolution transmission electron microscope (FE-HRTEM, Tecnai G2 F20, FEI). X-ray photoelectron spectra (XPS) was carried out in a Thermo Scientific ESCALAB
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250Xi X-ray photoelectron spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV). Fourier transfor infrared (FTIR) spectra were recorded on a Nicolet iS10 FTIR instrument (Thermo Fisher Scientific, USA). The crystal structure of the as-prepared samples was measured by X-ray diffractometer (XRD, X’ Pert Pro, Philips),
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using Cu Kα (λ = 1.54 Å) radiation.
2.4 Dielectric and EMA Measurement.
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The 3D-PPy/SiO2 composites used for microwave absorption measurement were prepared by uniformly mixing the samples with paraffin in different mass percentages.
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The mixtures were then pressed into toroidal shaped samples with an outer diameter of 7.00 mm and inner diameter of 3.04 mm. The relative complex permittivity and permeability values were measured with coaxial wire method in the frequency range of 2-18 GHz with the vector network analyzer (VNA, N5242A PNA-X, Agilent). 3. Results and discussions 3.1 The synthesis of 3D-PPy/SiO2 composites
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ACCEPTED MANUSCRIPT Figure 1 gives the synthesis strategy for ultralight 3D-PPy/SiO2 composites. Pyrrole monomer and nano SiO2 powder mixture was dispersed under stirring and polymerized to form a 3D structure in several minutes after FeCl3 solution was added.
We can
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clearly see that 3D-PPy hydrogel composites were formed in the bottom of a bottle, even if the bottle was inverted (See ESI Fugure S1). After washing and drying, the
3.2 The characterization of 3D-PPy/SiO2 composites
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composites were obtained with low densities (See ESI S1).
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The morphologies and structures of 3D-PPy/SiO2 composites were first investigated by a scanning electron microscopy (SEM). As shown in Figure 2, 3D-PPy with or without incorporation of nano SiO2 is a network constructed from a great deal of PPy nanosheets which are polymerized from pyrrole monomers. We also investigate the
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morphology of the as-prepared 3D-PPy/20%SiO2 composite by TEM technique (Figure 3a and b). The TEM images imply that the three dimensional network of 3D-PPy is
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formed through the π-π stacking between PPy nanosheets. Nano SiO2 are much smaller irregular particles than the PPy sheets (Figure 3a and b). It can also be found from the
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TEM images that SiO2 nano particles are dispersed in the pores of the 3D-PPy other than uniformly absorbed on the PPy nanosheets. The elemental components of the as-prepared composite were identified by XPS
technique. Figure 3c shows a general XPS profile for 3D-PPy/20%SiO2. The figure demonstrates that the 3D-PPy/20%SiO2 composite is mainly composed of Si, Cl, C, N and O elements. The strong C 1s and N 1s peaks belong to the 3D-PPy in the sample
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ACCEPTED MANUSCRIPT and Si 2p and O 1s peaks mainly arise from the nano SiO2. Cl 2p peak may be related to chlorine anion which is used to neutralize the positive charge of 3D-PPy. In addition, trace amounts of iron can be found in the composite (Figure 3d).
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FT-IR spectroscopy was used to examine the compositions of 3D-PPy/SiO2 by the in situ process. Figure 4 shows that most of the characteristic peaks in 3D-PPy can be
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remained in the composites. This may demonstrate that PPy is the primary ingredient. Aside from these peaks, the band at 772 cm-1 is corresponded to symmetrical stretching
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vibration of the Si-O-Si. This indicates the existence of SiO2 in 3D-PPy/SiO2 material. Figure 5 shows XRD patterns of Nano SiO2, 3D-PPy and 3D-PPy/20%SiO2. The main eight peaks in 3D-PPy/20%SiO2 composite centered at 24.15°, 33.16°, 35.64°, 40.87°, 49.47°, 54.08°, 62.45°, and 64.01° match well with the standard XRD data for
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the (012), (104), (110), (113), (024), (116), (214), and (300) planes of Fe2O3 (JCPDS NO. 84-0306). This Fe2O3 comes from 3D-PPy and may form from the excess oxidant
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FeCl3 under drying process. It can also be inferred that the three samples were all amorphous solids because of the broad XRD patterns. On account of the bands close to
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each other, we cannot distinguish the patterns of 3D-PPy and Nano SiO2. Nevertheless, the well combination of the two components can be confirmed through all the analysis above.
3.3 Electromagnetic Absorption Properties. The measurements are considered under far field because the source-to-shield distance is greater than the free-space wavelength in the frequency range of 2-18 GHz
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ACCEPTED MANUSCRIPT [44]. According to the transmission line theory [45], the input impedance (Zin) on the interface can be expressed as
= ℎ
√
(1)
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Where Z0 is the impedance of free space, µr is the complex permeability, µr = µ′ − jµ″, εr is the complex permittivity, εr = ε′ − jε″, f is the frequency, d is the thickness of material,
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c is the speed of light.
Based on the model of metal backplane, the reflection loss (RL) of a sample is
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determined from Z0 and Zin according to the following equation & )&
RL dB = 20lg %&'( +&* % *
'(
(2)
When the RL is lower than – 10 dB, over 90 % of the electromagnetic energy is absorbed.
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The permittivities and permeabilities of each sample were measured using coaxial wire method.[44] Despite of containing iron element, electromagnetic absorption of all the
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composites here is due to dielectric losses because the permeabilities are close to 1. (See ESI Figure S2) Figure 6a and b show the permittivity real part (ε′) and permittivity
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imaginary part (ε″) measured in the frequency range of 2-18 GHz of filler loading with 20 wt% of samples in paraffin matrix. It is observed that the addition of nano SiO2 obviously lowers both the values of ε′ and ε″. This can be attributed to the low electrical conductivity of amorphous SiO2. Moreover, different from 3D-PPy, the values of ε′ and ε″ slightly decrease with the increase of the frequency. Dielectric loss tangent (tanδ) was defined as the specific value of ε″ and ε′. As shown in Figure 6c, the tanδ of the
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on polarized atoms and molecules. In this interaction process, the electrical conductivity of a material plays an important role. However, it doesn't necessarily mean that larger ε′
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or ε″ will get better EMA. In 3D-PPy/SiO2 composites, a relatively small ε′, ε″ and tanδ are beneficial to EMA. Figure 6d shows the Cole-Cole plots of the composites and no
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obvious conspicuous semicircles are observed. This demonstrates that the Debye relaxation process in 3D-PPy/SiO2 is hidden. The main dielectric loss may come from other polarization relaxation processes such as electronic polarization and interfacial polarization.
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RL values of composites with different mass percentages of nano SiO2 can be obtained through formula (1) and (2). Figure 7a and d show the RL curves of samples
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with the fillers loading of 20 wt% of 3D-PPy/20%SiO2 and 3D-PPy/50%SiO2 at different thicknesses (2.0-6.0 mm) in the frequency range 2-18 GHz. It is obviously
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found that EA performance has not been enhanced with the increase content of SiO2. As shown in Table 1, there is an effective EA bandwidth of 6.0 GHz with the thickness of 2.5 mm. The maximal RL value of 3D-PPy/20%SiO2 can reach -35.68 dB at 5.5 mm. For 3D-PPy without SiO2, the capacity of EA is not as good (See ESI Figure S3). We also give the three dimensional and contour plot of the RL versus frequency and thickness of samples with the fillers loading of 20 wt% of composites (Figure 7b, c, e
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ACCEPTED MANUSCRIPT and f). The effective EA regions (RL below -10 dB) can be clearly showed in the projection drawings and it will be beneficial for the design of electromagnetic absorber. These results imply that this material meets the main requirements for dielectric-based
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electromagnetic absorbers.
Based on formula (1) and (2), RL values of wax with different mass percentages of
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3D-PPy/20%SiO2 have been obtained and the results are showed in Figure 8. For 10 wt% of the composite, the maximum absorption peak can research -29.80 dB. But the
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corresponding frequency shift to 15.12 GHz at an incremental thickness of 6 mm. When the filler content increase to 30 wt%, the absorption of electromagnetic wave decreases severely and scarcely any RL bandwidths below -10 dB. In a word, the EA performance with an increased or reduced content is not as good as that of 20 wt%.
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4. Conclusion
In summary, through an in situ preparation process, ultralight three-dimensional
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polypyrrole/nano SiO2 aerogels have been synthesized with enhanced properties of electromagnetic absorption and can be easily to scale up. For 3D-PPy/20%SiO2, the
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maximum effective EMA bandwidth could reach 6.0 GHz when filler loading is 20 wt% in wax and thickness of absorber is 2.5 mm. This work provides a facile method to fabricate a potential and excellent EMA material with low loading ratio and wide absorption bandwidth. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of
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ACCEPTED MANUSCRIPT China (51403236, 51021001) and the Opening Project of State Key Laboratory of Disaster Prevention & Mitigation of Explosion & Impact (DPMEIKF201310). REFERENCES
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Figure 1 The preparation of PPy/SiO2 composites.
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Figure 2 SEM images of (a) 3D-PPy aerogel, (b, c) 3D-PPy/20%SiO2 composite and (d) 3D-PPy/50%SiO2 composite.
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Figure 3 (a, b) TEM images, (c) the general XPS and (d) Fe 2p region of
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3D-PPy/20%SiO2 composite.
Figure 4 The FT-IR spectra of pure nano SiO2, 3D-PPy/20%SiO2 composite, 3D-PPy/50%SiO2 composite and 3D-PPy aerogel. 19
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Figure 5 The XRD patterns of pure nano SiO2, 3D-PPy aerogel and 3D-PPy/20%SiO2
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composite.
Figure 6 (a) The permittivities, (b) permeabilities, (c) tanδ, and (d) Cole-Cole plots of the composites.
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Figure 7 (a,d) RL values, (b,e) 3D plots and (c,f) contour plots of 3D-PPy/20%SiO2
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composite and 3D-PPy/50%SiO2 composite of loading with 20 wt% in wax.
Figure 8 RL values of (a) 10 wt% and (b) 30 wt% of 3D-PPy/20%SiO2 composite in wax.
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Table 1 EA performance with different thicknesses of filler loading with 20 wt%
Maximum
Maximun RL
Thickness
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3D-PPy/20%SiO2 composite in wax.
Effective EA bandwidth RL
position
(mm)
(GHz)
(GHz)
2.0
− 16.89
17.56
2.5
− 21.65
14.56
3.0
− 22.69
3.5
− 27.11
4.0
− 32.13
4.5
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(dB)
2.44
4.96
9.68
4.04
8.24
3.52
− 29.52
7.16
3.04
5.0
− 39.31
6.36
2.64
5.5
− 35.68
5.68
2.36
6.0
5.08
2.16
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6.00
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ACCEPTED MANUSCRIPT Electronic Supplementary Information S1. The method to calculate the densities of 3D-PPy/SiO2 composites. The densities of 3D-PPy/SiO2 composites were calculated through the following process. Firstly, 3D-PPy/SiO2 composite was cut into a cube by a knife; then the
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weight of the obtained cubic 3D-PPy/SiO2 composite was measured though an analytical balance and the volume was calculated through the mathematics formula;
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finally, the density was equal to the mass per unit volume. The got densities were 47.6
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mg/cm3 for 3D-PPy/20%SiO2 and 69.9 mg/cm3 for 3D-PPy/50%SiO2.
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Figure S1 The comparison of before and after gelation.
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Figure S2 Real permeabilities (u’) and imaginary permeabilities (u’’) of 3D-PPy/20%SiO2 and 3D-PPy/50%SiO2
Figure S3 RL value of 20 wt% of 3D-PPy with wax.
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