- Email: [email protected]
epoxy resin composite plates
Accepted Manuscript Microwave Absorption and Flexural Properties of Fe Nanoparticle/Carbon Fiber/Epoxy Resin Composite Plates Asif Shah, Yonghui Wang, Hao Huang, Li Zhang, Fanghong Xue, Yuping Duan, Xinglong Dong, Zhidong Zhang PII: DOI: Reference:
S0263-8223(15)00428-6 http://dx.doi.org/10.1016/j.compstruct.2015.05.054 COST 6472
To appear in:
Composite Structures
Please cite this article as: Shah, A., Wang, Y., Huang, H., Zhang, L., Xue, F., Duan, Y., Dong, X., Zhang, Z., Microwave Absorption and Flexural Properties of Fe Nanoparticle/Carbon Fiber/Epoxy Resin Composite Plates, Composite Structures (2015), doi: http://dx.doi.org/10.1016/j.compstruct.2015.05.054
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.
Microwave Absorption and Flexural Properties of Fe Nanoparticle/Carbon Fiber/Epoxy Resin Composite Plates Asif Shah1, Yonghui Wang1, Hao Huang1, Li Zhang1, Fanghong Xue1, Yuping Duan1, Xinglong Dong1* and Zhidong Zhang2 1
Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Ministry of
Education),and School of Materials Science and Engineering, Dalian University of Technology, Liaoning, 116024, People's Republic of China 2
Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning, 110015, People's Republic of China
*corresponding author: Prof. X.L. Dong Email: [email protected] Tel:
+86-411-84706130
Fax:
+86-411-84709284
Abstract: A flat nanocomposite plate was fabricated by using the surface-modified Fe nanoparticles (NPs) as the microwave absorbent, carbon fibers (CFs) as the reinforced phase and epoxy resin (ER) as the matrix. Fe NPs were synthesized by an arc discharge plasma method and subsequently modified by silane coupling agent (KH-550) to improve its dispersion in the organic matrix of ER. To measure the realistic microwave absorption properties of such a flat nanocomposite plate, a series of square plates (20´20 cm2) was made from recombining the modified Fe NPs (20 wt.%, 30 wt.% and 40 wt.%) into the ER matrix with/without orientated
1
CFs inside. It was observed that the orientation of CFs plays an important role in the microwave absorption, in particular through a strong reflection of microwave inwardly as the CFs’ array is vertical to the direction of incident microwave. The inner strong reflection of microwave by CFs can bring great probabilities to further consume it by Fe NPs absorbent and result in improved microwave absorption performance of the nanocomposite plate. It is indicated that the plate containing 30 wt.% of Fe NPs with a perpendicular manner between the directions of CFs array and incident microwave exhibits higher reflection loss (RL) of -16.2 dB at 6.1 GHz frequency, and this plate has 77.78 MPa flexural strength at 3.74% deformation. Excellent RL property is ascribed to an optimum structure of nanocomposite plate with favorable multi-reflection of microwave inside, structural resonance, appropriate conductivity, impedance match, interface polarization. Keywords: Carbon Fiber, Mechanical Properties, Nanocomposite, Nanoparticles, Epoxy Resin, Reflection Loss 1. Introduction In recent years, electromagnetic wave absorbing materials have aroused a great deal of interest because of increasing civil, commercial and military applications in the electromagnetic interference shielding and radar cross-section reduction in the gigahertz (GHz) band range. The microwave absorbing materials that are structurally fantabulous light in weight and flexible have a high demand in the commercial applications like, cellular systems, computers, wireless LAN devices, wireless antenna systems [1, 2]. As compared to bulk magnetic and ferrite materials, nanomaterials have more pros like their being light in weight, durable, having smaller particles size, increased surface’s anisotropies and lower eddy current phenomena. Such qualities make them good electromagnetic wave absorbers
2
[3-5]. The ferrite nanocomposite material is a promising candidate as an excellent microwave absorbing material at high frequency because of its light weight, high dielectric and magnetic loss [6, 7]. So far, much research has been devoted to the making of nanocomposite such as (Fe)/amorphous carbon [8], α-Fe/Y2O3 [9] and α-Fe/SmO [10] and to investigate their microwave absorption characteristics. Fe NPs have an excellent microwave absorbing property and have been extensively used in different forms as an absorbent in composites [11-13]. A ternary alloy based on Fe NPs prepared by self-catalyzed reduction method exhibited a calculated minimum RL of -59 dB at 9.5 GHz [14]. α-Fe(N) NPs prepared by chemical vapor condensation method indicated the optimum RL of -37.5 dB at 10.4 GHz frequency [15]. In our previous work as per theoretical calculations, γ-Fe2.6Ni1.4N NPs derived from γ-Fe2.6Ni1.4 prepared by an arc discharge method showed the optimal RL of -39.9 dB at 5.2 GHz, the core/shell type Fe-Ni NPs possessed minimum RL of -34.9 dB at 10.60 GHz and metallic Fe NPs exhibited a minimum RL of -47.3 dB at 9.6 GHz [7, 13, 16]. The structures with coating and multi-components are of two main types concerning their application to the microwave absorbing materials. As compared to the microwave absorbing coatings, the composite structures possess both microwave absorption and loading characteristics, which avoid the disadvantages of microwave absorbing coating, such as an increase in structural weights, poor mechanical and environment resistant properties, as well as high cost due to hard maintenance and mending. Through multifunctional design, the sandwich structure may have multi-abilities of microwave absorption and load carrying to meet the requirements for the applications in naval and aeronautical engineering [17-19]. CFs are one of the most advanced and important candidates due to their having high modulus, high strength, low density and low coefficient of thermal expansion [20-22]. These properties make CFs a potential
3
material for aircraft, automotive parts, electrical equipment, as well as to some extend to the field of electromagnetic material [23, 24]. However, in the field of electromagnetic materials, the properties of CFs are needed to be improved by coating the surface with a layer of metal or other oxide [25, 26]. Kim and Lee [27] manufactured composites, which were composed of carbon nanotubes, carbon fabric/epoxy and PVC foam. It was observed that, when the thickness and weight fraction of carbon nanotubes in nanocomposite were 2.52 mm and 3 wt.%, the absorbing band width of -10 dB was 3.3 GHz (at 8.2–11.5 GHz). Park et al. [17] composed the sandwich structures made up of two face sheets and foam core. Glass fabric/epoxy composites containing conductive carbon black and carbon fabric/epoxy composites were used for the face sheets. Polyurethane foams containing multiwall nanotubes were used as the core material. The core of sandwich composite was conventionally made up of the foams. This research has also shown that such type of lattice structures have higher weight efficiency than that of foams [28, 29]. The lattice cores reinforced by CFs of the Kagome-grid were manufactured by Fan et al [30], which was filled with microwave absorbing foams. Such lattice composites show excellent microwave absorption within 4-18 GHz. In this work a flat nanocomposite plate, i.e. a multilayered structure filled by the microwave absorber of Fe NPs, has been designed and fabricated. A series of such plates was prepared using surface-modified Fe NPs as the absorber filling into the matrix of ER, meanwhile CFs were paved to improve both properties of electromagnetism and mechanics. The freestanding Fe NPs were prepared by the arc discharge plasma method; while its surface modification was carried out by chemical treatment using the silane coupling agent (KH-550). Such a multifunctional structure combines the microwave absorbing and mechanical performances. The relationships
4
between composite structures and the actual microwave absorption properties were investigated and emphasized in this research. 2. Experimental details 2.1 Preparation and characterization of Fe NPs Fe NPs were synthesized by an arc discharge method, and the experimental details had been given in our previous work [31]. During the synthesis of Fe NPs, bulk Fe target was evaporated, which was laid on a water-cooled copper stage that is serving as the anode, while an upper carbon rod which served as the cathode was supported by a copper arm. After the chamber evacuation, a gas, mixture of hydrogen and argon was introduced into the chamber to a certain pressure. The distance between the two electrodes could be automatically adjusted from outside the chamber, so that the arc can be started and controlled during the continuous operation. Surface modification of Fe NPs was carried out by using 5% silane coupling agents (KH-550), which was purchased from Sahn Chemical Technology (Shanghai) Co. LTD. Its mechanism is shown in Fig. 1. Fe NPs precursor was dispersed into distilled water with 5 wt.% of silane coupling agent for 10 minutes. Then, hydrochloric acid was added to reduce the pH level of suspension to 4. After that, suspension was further mixed at ultrasonic oscillation for 45 minutes at 70oC, and then the dispersed Fe NPs were separated from solvent by centrifugation (10 min. at 7000 rpm) followed by washing with ethanol for four times to remove the excessive silane. Once the process was finished, the surface-modified Fe NPs were dried in an oven at 80oC for 12h.
5
(a)
(b)
Figure 1: Mechanism for the surface modification of Fe NPs. (a) Hydrolysis of γ-aminopropyl triethoxysilane and (b) Condensation of silanol on the surface of Fe NPs. The morphologies of as-prepared Fe NPs and surface-modified Fe NPs were analyzed with a High Resolution Transmission Electron Microscopy (HRTEM, Tecnai G220S). Its crystal structure was identified by X-ray diffractometer (XRD, Dutch EMPYREAN). FTIR test was conducted on FT/IR-430 made by Jasco, Japan. Scanning electron microscopy (FEI Company, NOVA nano SEM 450) was used to observe microstructures of the nanocomposite plate. The distribution of Fe NPs in ER matrix was analyzed by electron probe X-ray microanalyzer (EPMA, Shimadzu EPMA-1600). 2.2 Preparation of Fe NPs/CFs/ER nanocomposite plate CFs used in this work were poly-acrylonitrile (PAN)-based fibers (12K), the average diameter of the filaments is 6.9 µm, tensile modulus is 240 GPa, tensile strength is 4200 MPa, strain (% ) is 1.8 and density is 1.76 g/cm3, which were manufactured by AKSA, Turkey. Bisphenol-A ER (E51) was purchased from Shenyang Zheng Tai Corrosion Material Co., Ltd. China, and was
6
used to prepare a series of square plates (20´20 cm2) for electromagnetic testing. The plate samples were fabricated with the surface-modified Fe NPs (20 wt.%, 30 wt.% and 40 wt.%), with or without CFs in ER matrix, respectively. A pure ER plate and a CF-paved ER plate were prepared for comparison. In the preparation of nanocomposite, the surface-modified Fe NPs, ER, coupling agent (KH-550) and catalyst [(C6H5)3P] were used. Mixing of the surface-modified Fe NPs, coupling agent and ER were carried out by ultrasonic oscillation for 40 minutes at 70oC, followed by an addition of the catalyst and further mixed for 5 minutes. The suspension was poured into a pre-set mould by hand lay-up method. CFs were paved between the two layers of suspension, then the plate was dried in the furnace at 124oC for 12 hours. The schematic diagram of the preparation process is shown in Fig. 2.
7
Figure 2: Preparation process of Fe NPs/CFs/ER nanocomposite plate. (a) Ultrasonic mixing of modified Fe NPs and ER, (b) Pouring of 1st layer into mould, (c) Paving CFs, (d) Pouring of 2nd layer into mould, (e) Schematic illustration of nanocomposite plate with dimension and internal geometry (f) Drying in furnace and (g) As prepared flat nanocomposite plate. 2.3 RL measurement with respect to orientation of CFs Microwave RL of the nanocomposite plates was measured using Agilent 8729B Vector Network Analyzer. The surface-modified Fe NPs were added into an ER matrix at different concentration (20 wt.%, 30 wt.% and 40 wt.%) with laying of CFs. Taking into account the orientation of CFs, flat nanocomposite plates were tested concerning a parallel and vertical direction to the transmission direction of electromagnetic wave. First RL measurement was taken
8
in such a way that the direction of CFs was parallel to propagation direction of microwave. Later, second measurement was taken as the direction of CFs was vertical to propagation direction of microwave, which is shown in the schematic diagram in Fig. 3.
(1) Network Analyzer, (2) Transmitter, (3) Receiver, (4) Nanocomposite flat plate, (5) CFs in ERs matrix (parallel to propagation direction of microwave), (6) CFs in ERs matrix (vertical to propagation direction of microwave), (7,8) Pyramidal absorber, (9) Transmission line.
Figure 3: Schematic diagram of RL test by Vector Network Analyzer according to orientation of CFs in the nanocomposite plates. 2.4 Flexural tests of Fe NPs/CFs/ER nanocomposite plate The three-point flexural test of unidirectional CFs reinforced in ER matrix with/without Fe NPs was performed in accordance to ASTM D7264. The tests were carried out on Changchun Research Institute for Mechanical Science Co. Ltd. (CSS-2205). The specimens were designed as 153.6 mm in length, 13 mm in width and 4 mm in thickness. The span-to-depth ratio was set as 32:1 and the tests were carried out at constant cross-head speed of 1 mm/min, at least five specimens were tested from each nanocomposite plate. The flexural strength (σf) and flexural strain (Ɛf) were calculated according to ASTM standard as follows.
and
Ɛ =
Where, P is maximum force, L is supporting span, b is the width, h is thickness and span deflection.
9
is mid-
3. Results and discussion 3.1 Fe NPs and surface modification Fig. 4(a) shows XRD profile of Fe NPs. It indicates that three diffraction peaks in accordance to α-Fe (BCC) structures were detected. The grain size of Fe NPs was calculated as 65.7 nm using the Scherrer formula [32]. During the preparation of Fe NPs by the arc discharge method, the passivation process was incorporated to form a thin oxide layer on the fresh Fe nanoparticle, which is not observed in the XRD profile. HRTEM image shows these thin oxide layers on the Fe NPs as in Fig. 4(b), which are spherical with mutual adhesion between the particles due to high surface energy and magnetic interaction. Fig. 4(c) displays HRTEM photo of the surface-modified Fe NPs by γ-aminopropyl triethoxysilane (KH-550) (see Fig. 1 for its mechanism). During the surface modification, the two reactions take place simultaneously with respect to silane, i.e. the hydrolysis of silane ethoxy groups to generate KH-550-Si(OH)3 (hydroxyl silane) and the condensation of the resultant silanols with free -OH groups with the surface of Fe NPs to render the stable bond. These two reactions are followed by amino ionization, which improves the dispersion stability of Fe NPs by the steric hindrance while maintaining the electrostatic repulsion between the particles.
10
(110)
b
Intensity (arb.units)
a
c
(200)
(211)
Fe NPs
20
30
40
50
60
70
80
90
100
2q (degree)
Figure 4: (a) XRD spectrum and (b) HRTEM image of the as-prepared Fe NPs, and (c) HRTEM image of the surface-modified Fe NPs. In order to confirm the surface modification of Fe NPs, FTIR spectra was measured as shown in Fig. 5. IR spectra of pure KH-550, Fe NPs before/after the surface modifications are presented for comparison. It can also be observed that the surface-modified Fe NPs have some new vibration peaks in comparison to pure KH-550 and Fe spectra. A peak at 2980 cm-1 corresponds to the asymmetric stretching vibration of CH3, while the peak at 1390 cm-1 is corresponding to C-Si-O in-plane bending vibration [33]. 1053 cm-1 peak is related to Si-O-Fe, which is the result of the –OH (on the surface of Fe NPs) reacted with KH-550 [34]. 1080 cm-1 peak is related to the symmetric stretching vibration of Si-O-C [35]. After the surface-modification, as compared to pure KH-550 and Fe IR spectra, new peaks appear at 1053 cm1, 1390 cm-1 and 2980 cm-1, which indicate that KH-550 has been successfully grafted and made bond onto the surface of Fe NPs.
11
Fe
KH550 modified Fe
1053 KH550
1390 2980
1080 500
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumbers(cm )
Figure 5: FTIR of KH-550, Fe and modified Fe NPs. 3.2 Distribution of Fe NPs and CFs in nanocomposite To study the distribution of Fe NPs and CFs in the flat nanocomposite plates, electron probe micro-analyzer (EPMA) test was conducted on 20 wt.% Fe NPs plate, in cross section of the plate to linearly analyze Fe and carbon elements from top to bottom. Fig. 6 shows the distributions of Fe and carbon elements, and SEM image of the nanocomposite plate without paved CFs. A distinctly gradient concentration of Fe NPs is found from top to bottom of the plate as shown in Fig. 6(a). It is reasonable that the gravity can force the majority of Fe NPs settled down to the bottom of the plate during the curing process. Fig. 6(b) indicates the presence of carbon, which is attributed to ER matrix, exhibiting a fluctuate distribution due to the heterogeneous density of ER. While, Fig. 6(c) displays the microstructure of Fe NPs/ER composite, in which the uniform mixing and strong linking between NPs and matrix can be observed.
12
100
Top side
Bottom side
Counts
80
Distribution of Fe
60 40 20
(a) 0 5000
Distribution of Carbon
Counts
4000
3000
(c)
2000
(b) 1000 -0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Positions (mm)
Figure 6: Fe NPs/ER nanocomposite without paved CFs. EPMA results linearly scanned from top to bottom side of the plate. (a) Carbon element analysis, (b) Fe element analysis and (c) SEM image of cross section of the plate. Fig. 7 presents the analysis of elements (Fe, C) and the microstructure of the nanocomposite plate, in which CFs was paved inside Fe NPs/ER matrix. CFs can also be detected from high counts of carbon at the bottom of plate (Fig. 7b). It can be observed from Fig. 7(a-b) that CFs are surrounded by the majority of Fe NPs, which may be caused by the gravity and weak interactions between NPs and CFs. As one knows, CFs/epoxy composites have been regarded as a new generation of high performance materials because of their high strength-to-weight ratio which makes them more popular in aerospace and automotive industries. In this work, a new component, i.e. Fe NPs, was added into such CFs/epoxy composites, the enhanced mechanical
13
properties and extra electromagnetic performances can be anticipated with a view to the interfacial interactions in the Fe NPs/CFs/ER interfaces. To study the microstructure of this Fe NPs/CFs/ER nanocomposite, SEM image was taken as shown in Fig. 7(c). SEM image verifies the EPMA results and shows that the symmetry of CFs with Fe NPs/ER matrix. The existence of CFs and its orientation favor to increase mechanical strength and it can greatly influence the microwave absorption properties which will be discussed in next sections.
400
Top side
Bottom side
Counts
300
Distribution of Fe 200
CFs 100
(a)
0
CFs
4000
Distribution of Carbon Counts
3000
2000
(c)
1000
(b)
0 -0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Position (mm)
Figure 7: Fe NPs/CFs/ER nanocomposite with paved CFs inside. EPMA results linearly scanned (c)
(c) from top to bottom side of the plate. (a) Carbon element analysis, (b) Fe element analysis and (c) SEM image of cross section of the plate.
14
3.3 RL measurements on flat nanocomposite plates (c) (a) Fe NPs/ER nanocomposite without paved CFs Pure ER is an excellent wave transparent material and has no electromagnetic wave absorption as shown in Fig. 8(a). After filling the modified Fe NPs (20 wt.%, 30 wt.%, and 40 wt.%) into the ER matrix, the nanocomposite plates become microwave absorbent at higher frequency. At 20 wt.% of Fe NPs, the plate shows minimum RL of -2 dB at 17.4 GHz (Fig. 8b), which greatly increases to -12.4 dB at 17.4 GHz by adding 30 wt.% of Fe NPs (Fig. 8c), and slightly decreases to -10.4 dB at 16.8 GHz after adding Fe NPs to 40 wt.% as shown in Fig. 8(d). Without doubt, adding magnetic Fe NPs into ER can cause the nanocomposite microwave absorbent at high frequency to a certain RL value, but an optimum concentration of the absorbent is necessary. Increasing the content of Fe NPs built up a strong cross linked network and increases the permittivity and permeability of nanocomposite plate. The parameters such as thickness, minimum RL and peak frequency are always associated with the unitary absorption performance. In an extreme case, the incident and reflected wave can be out of phase to cancel each other and result in a minimum RL value, if the thickness of plate equals to a quarter of wave length. When incident wave strikes at absorbing material, the specific content of absorber, matching thickness and resonance frequency are essential to the loss of microwave. In this study, the thickness of pure epoxy plate, and the nanocomposite plates with 20 wt.%, 30 wt.% and 40 wt.% Fe NPs mixed in ER matrix were measured as 0.93 mm, 1.18 mm, 1.48 mm and 1.77 mm, respectively. These experimental results reveal the optimum thickness of nanocomposite plate is 1.48 mm for the concentration of 30 wt.% Fe NPs.
15
1.0
RL (dB)
0.5 0.0
(a) Pure ERs
-0.5 -1.0
RL (dB)
0
-1
(b) 20 wt% Fe NPs -2
-2 dB (17.4 GHz)
RL (dB)
0 -5
(c) 30 wt% Fe NPs
-10
-12.4 dB (17.4 GHz) -15
RL (dB)
0
-5
(d) 40 wt% Fe NPs -10
-10.4 dB (16.8 GHz) 0
5
10
15
20
Frequency (GHz)
Figure 8: RL measurements of nanocomposite plates without CFs. (a) Pure ER, (b) 20 wt.% of Fe NPs in ER matrix, (c) 30 wt.% of Fe NPs in ER matrix and (d) 40 wt.% of Fe NPs in ER matrix. Core-shell type Fe NPs are used as absorbent due to their excellent microwave absorption ability because of high magnetic and dielectric losses as shown in our previous work [6, 7, 13, 31]. Either the only magnetic loss or only dielectric loss may cause a weak microwave wave absorption property due to dissymmetry of the electromagnetic match. Fe NPs can show up better absorption in the existence of dielectric graphite and oxide shell with ferromagnetic cores as shown in Fig. 4(b-c). The graphite shells give rise to a high density of defects, which leads to an asymmetry of charge density distribution. The polarizations of defective sites are due to the
16
action of electromagnetic wave, which results in multi-polarization phenomenon in core/shell interface resulting in enhancement of microwave absorption.
The microwave absorption properties are determined by the relative permeability ( "
), the relative permittivity (
=
−
"
=
−
), the EM impedance match and the microstructure
of the absorber [36]. As the microwave strikes the surface of an absorber, a good matching
condition of EM impedance is enabled to have almost zero reflectivity of the incident microwave, the transmitted microwave can be dissipated by dielectric loss and magnetic loss into absorbing materials. The absorbers of core-shell type are excellent to achieve microwave absorption properties [7, 8, 37]. The magnetic Fe NPs are one rather core-shell nanostructures with the ‘‘shell’’ of ferrimagnetic oxide (Fe3O4) consisting of Fe3+ and Fe2+ ions and the “core” of metal Fe. The electron transfer between Fe3+ and Fe2+ creates to ion jumps and relaxation in the Fe3O4 shells. The dipole polarization is the dominant at higher frequency and the weak space charge polarization mainly works at lower frequency, such Fe NPs can act as the dipoles [7]. The “core” of metallic Fe can also cause magnetic loss by the natural resonance [7, 13]. At low content of Fe NPs, the nanocomposite would be less in absorber’s quantity and there would be larger spacing between NPs, as a result, the vast majority of incoming electromagnetic waves would not strike the Fe NPs and lower effective absorption is inevitable. At higher particle’s content, the permittivity and permeability, as well as the attenuation constant for microwave transmission of such nanocomposite will be increased [38]. The NPs contact to form a cross linked network in the nanocomposite plate, the electromagnetic wave can quickly be dissipated resulting in strong absorption. It can be assumed that the higher RL loss of this nanocomposite plate is also associated with the high relative permittivity and permeability, high attenuation
17
constant and with EM impedance match, which was measured according to transmission line theory [26], (
tan ℎ
= Where,
−1
) = 20log ( 2
+ 1) (
)
is the input impedance of absorber, d is the thickness of absorber, c is the velocity of
light, is the frequency and
and
are the permeability and permittivity respectively.
(b) Fe NPs/CFs/ER nanocomposite with paved CFs inside To make an ideal microwave absorbing structure, the reflection coefficient has been considered as low as possible so that the microwave could enter the structure at the maximum extent, which generally requires high input impedance. Optimally, the input impedance should be quite close to the free space intrinsic impedance of air, and only then can the microwave entirely enter the structure to be absorbed at the maximum extent. Considering this factor a flat nanocomposite plate has been designed by filling different concentration of Fe NPs (20 wt.%, 30 wt.%, and 40 wt.%) into the ER matrix with paved CFs 8.72 g by weight, as shown in Figs. 2 and 7. For comparison, RL values of the nanocomposite plates were measured regarding orientations of CFs (parallel and vertical to the incident direction of microwave) and the results are illustrated in Fig. 9. In comparison to pure ER, paving of CFs into the ER matrix endows the microwave absorbent at higher frequency and shows a similar tendency with the frequency at both paved directions (perpendicular and parallel to microwave incidence) as shown in Fig. 9(c). These results indicate that CFs can cause microwave loss solely in transparent matrix of ER, regardless of their orientations. This state can be greatly affected by adding Fe NPs with different contents,
18
especially as the microwave incidence is vertical to the direction of CFs as shown in Fig. 9(d-f). For the parallel direction, it shows a monotonous increase in microwave absorption with increasing frequency at any Fe NPs contents, but for the vertical direction it shows a single peak with minimum RL of -10 dB at 7.5 GHz (20 wt.% of Fe NPs, Fig. 9d), two peaks with minimum RL of -16.2 dB at 6.1 GHz and -13.1 dB at 15.6 GHz (30 wt.% of Fe NPs, Fig. 9e) and likewise, two peaks with minimum RL of -16.2 dB at 4.9 GHz and -18.3 dB at 16.8 GHz (40 wt.% of Fe NPs, Fig. 9f). It is well indicated that the orientations of CFs and collective interaction with Fe NPs play an influential role to enhance the microwave absorption, which is ascribed to the multi-reflection, scattering and absorption of waves inside Fe NPs/CFs/ER composite structure. Concerning the parallel case between the directions of CFs and incident wave, more electromagnetic energy is dissipated and absorbed in the vertical case within the nanocomposite structure, resulting in excellent microwave absorbing properties.
19
(a)
CFs parallel to incident way
0
CFs vertical to incident way
(b)
(d)
(c)
-2 -4 -6
RL (dB)
-8 -10
-10 dB (7.5 GHz)
-12 -14 -16 -18 -20
20 wt% Fe NPs with CFs (Parallel) 20 wt% Fe NPs with CFs (Vertical)
Pure ERs with CFs (Parallel) Pure ERs with CFs (Vertical)
-22 -24 -26 0
(e)
-2
(f)
-4 -6
RL (dB)
-8 -10 -12 -14
-13.1 dB (15.6 GHz)
-16 -18
-18.3 dB (16.8 GHz)
-16.2 dB (6.1 GHz)
-16.2 dB (4.9 GHz)
-20
40 wt% Fe NPs with CFs (Parallel) 40 wt% Fe NPs with CFs (Vertical)
30 wt% Fe NPs with CFs (Parallel) 30 wt% Fe NPs with CFs (Vertical)
-22 -24 -26 0
2
4
6
8
10
12
14
16
18
20 0
2
4
6
8
10
12
14
16
18
20
Frequency (GHZ)
Frequency (GHZ)
Figure 9: RL measurements of nanocomposite plates according to orientation of CFs. (a) Schematic illustration of parallel arranged CFs to incident wave, (b) Schematic illustration of
20
vertical arranged CFs to incident wave, (c) Pure ER with CFs, (d) 20 wt.% Fe NPs with CFs in ER matrix, (e) 30 wt.% Fe NPs with CFs in ER matrix and (f) 40 wt.% Fe NPs with CFs in ER matrix.
3.4 RL Mechanism of nanocomposite plate with or without paved CFs As one kind of absorption type, the nanocomposite plate consumes the incident electromagnetic wave by dissipating electromagnetic energy into heat through the mechanisms of the dielectric and magnetic losses. To achieve higher microwave absorption, the electromagnetic wave entering into the material should be entirely attenuated, according to the following equation [38] = √2
× ′ ′ (tan
tan
− 1) + (tan
tan
Where, is the attenuation constant, c is the speed of light,
− 1) + (tan
+ tan
is the frequency, ′, ′,
)
and
represent the real permeability, real permittivity, dielectric and magnetic loss,
respectively. This equation shows that the higher values of ′ and
as well as f, results in higher
. As shown in Fig. 8, adding Fe NPs absorbent into the ER matrix from 10 wt.% up to 40 wt.%
contents can improve the electromagnetic parameters of nanocomposites, which cause the enhanced losses at high frequency indeed. CFs are conductive material and consumes microwave to a certain extent by dielectric loss at higher frequency as shown in the results of Fig. 9(c), in particular, such loss is retained in all “parallel” cases (Fig. 9d-f). The plates of CFs/ER, Fe NPs/ER and Fe NPs/CFs/ER (parallel cases) composites exhibit an emblematical type in which the microwave is attenuated at higher frequency without distinguishable resonance peaks. Besides to attenuate microwave by CFs (dielectric loss) or Fe NPs (both dielectric and magnetic losses), the nanocomposite actually has constructed an effective structure with the
21
orientated CFs, gradual distribution of Fe NPs and a definite thickness of plate. For such a structure, it is in practice an absorption entity to dissipate microwave by structure related resonance. To achieve the best performance, the thickness of nanocomposite structure should quarter of wavelength, the best adequate matching condition [39]. In Fig. 9(d-f), visible absorption peaks can be found in “vertical” cases in which the array of CFs is vertical to incident wave. These peaks usually occur in a typical structure consuming microwave by a resonance. As one knows, electromagnetic wave has two vectors of electric and magnetic components, both vertical to the direction of wave propagation. In the “vertical” case, the electric vector component of the wave is actually parallel to CFs’ orientation (Fig. 9-b), which can cause a strong reflection by the conductive CFs and supply great opportunity to be absorbed by Fe NPs surrounded. In such a condition, the nanocomposite plate practically acts as an effective structure to consume microwave by resonance and absorption. Otherwise, mere absorption of microwave without any resonances would occur in the nanocomposite plate, as shown for the “parallel” cases in Fig. 9(d-f), here into no strong reflections of wave had happened due to perpendicular between the electric vector component and CFs (Fig. 9-a) . Therefore, it can be concluded that sole existence of oriented CFs or distributed Fe NPs can just cause an absorption type with a raised loss happened at high frequency, but co-existence and reciprocity between CFs and Fe NPs can play a radical role in microwave consumption by the structural resonance. Consequently, the relative directions of CFs’ array and the incident microwave should be exceptionally considered. According to the equation below, it is well described that the matching (resonance) frequency is determined by the thickness of structural entity and the electromagnetic parameters [39]. f
nc/4t
| || |
(n=1,3,5,7,9,…)
22
Where,
is matching frequency,
is the matching thickness,
and
represent the relative
permittivity and the relative permeability respectively. As indicated by Bruggeman equation [40], the electromagnetic parameters of Fe NPs/CFs/ER nanocomposite can be improved by increasing the Fe NPs contents in ER matrix. For a certain thickness, the matching frequency of nanocomposite plate will vary to lower ones with the increase of absorbent contents, e.g. from 7.5 GHz (20 wt.% of Fe NPs) to 6.1 GHz (30 wt.% of Fe NPs), to 4.9 GHz (40 wt.% of Fe NPs). Table 1 summarizes the minimum RL values of Fe NPs/CFs/ER nanocomposite plates with the orientation of CFs perpendicular to the incident microwave, average densities and their thicknesses. It can be observed in “vertical” cases that all RL values fall in range of C band (4-8 GHz) and Ku band (12-18 GHz). Table 1: Minima of RL measurements with the orientation of CFs perpendicular to the incident microwave Fe NPs/CFs/ER
S band
C band
plates
(2-4 GHz)
(4-8 GHz)
X band
Ku band
Plate
Density
(8-12 GHz)
(12-18 GHz)
thickness
(g/cm3)
--------
--------
4.10 mm
1.49
4.22 mm
1.71
4.48 mm
1.97
-10 dB 20 wt.% of Fe NPs
-------(7.5 GHz) -16.2 dB
30 wt.% of Fe NPs
-13.1 dB
--------
-------(6.1 GHz)
(15.6 GHz)
-16.2 dB 40 wt.% of Fe NPs
18.3 dB
--------
-------(4.9 GHz)
(16.8 GHz)
3.5 Effect of Fe NPs on flexural properties
23
The three-point flexural test was carried out to measure the flexural properties of nanocomposite plate with/without addition of Fe NPs into the ER matrix with CFs, whose results are given in Fig. 10. Pure ER and CFs plates exhibit 82.58 MPa flexural strength with 3.81% deformation, while ER, CFs and 30 wt.% Fe NPs plates show flexural strength 77.78 MPa at 3.74% deformation. Apparently, the additions of Fe NPs are not effective on the flexural strength and flexural strain of nanocomposite plates.
100 90
5.0
Flexural Strength Flexural Strain
4.5
82.56 3.81
3.74
77.76
4.0
70
3.5
60
3.0
50
2.5
40
2.0
30
1.5
20
1.0
10
0.5
0
Flexural Strain (%)
Flexural Strength (MPa)
80
0.0 Pure ER with CFs
ER and CFs with 30 wt.% Fe NPs
Figure 10: Flexural strength and strain of nanocomposite plates. Pure ER with CFs and 30 wt.% Fe NPs with CFs into ER matrix. 4. Conclusion Microwave absorbent of Fe NPs was synthesized by an arc discharge method and subsequently surface-modified by using silane coupling agent (KH-550) to improve its dispersion in ER matrix. A series of Fe NPs/CFs/ER nanocomposite flat plates were made of the standard size (20´20 cm2) with different contents of Fe NPs (20 wt.%, 30 wt.% and 40 wt.%) and orientated CFs array side. “Parallel” state between CFs array and incident microwave cannot
24
cause an observable reflection of microwave inside, nevertheless, a strong reflection happened in “vertical” case, which resulted in strong absorption of microwave by the structural resonance. In the “vertical” case, the increase of Fe NPs’ content at a certain thickness of the nanocomposite plate gives rise to a slight move of matching frequency towards lower one, indicating that a resonance structure has been successfully constructed to consume the incident microwave. Coalesce and reciprocity between Fe NPs and the oriented CFs play a radical role in microwave absorption. Optimal RL for the nanocomposite plate with 30 wt.% of Fe NPs and vertical between CFs array and incident microwave can reach -16.2 dB at 6.1 GHz with flexural strength 77.78 MPa at 3.74% deformation. This work indicates that such magnetic NPs/CFs/ER nanocomposite plates are significant in the integration of excellent microwave absorption and mechanical properties with the lightweight merit. Acknowledgement This work was financially supported by National Natural Science Foundations of China (No. 51331006, 51271044 and 51171033). References: [1] Petrov VM, Gagulin VV. Microwave absorbing materials. Inorg Mater 2001;37(2):93-98. [2] Yan D, Cheng S, Zhuo RF, Chen JT, Feng JJ, Feng HT, et al. Nanoparticles and 3D spongelike porous networks of manganese oxides and their microwave absorption properties. Nanotechnol 2009;20(10):105706. [3] Peng DL, Hihara T, Sumiyama K, Morikawa H. Structural and magnetic characteristics of monodispersed Fe and oxide-coated Fe cluster assemblies. J Appl Phys 2002;92(6):3075-83.
25
[4] Respaud M, Broto JM, Rakoto H, Fert AR, Thomas L, Barbara B, et al. Surface effects on the magnetic properties of ultrafine cobalt particles. Phys Rev B 1998;57(5):2925-35. [5] Bødker F, Mørup S, Linderoth S. Surface effects in metallic iron nanoparticles. Phys Rev Lett 1994;72(2):282-85. [6] Dong XL, Zhang ZD, Jin SR, Kim BK. Characterization and magnetic properties of Fe–Co ultrafine particles. J Magn Magn Mater 2000;210(1):143-49. [7] Lu B, Dong XL, Huang H, Zhang XF, Zhu XG, Lei JP, et al. Microwave absorption properties of the core/shell-type iron and nickel nanoparticles. J Magn Magn Mater 2008;320(6):1106-11. [8] Liu JR, Itoh M, Horikawa T, Machida KI, Sugimoto S, Maeda T. Gigahertz range electromagnetic wave absorbers made of amorphous-carbon-based magnetic nanocomposites. J Appl Phys 2005;98(5):054305-05-7. [9] Liu JR, Itoh M, Machida KI. Frequency dispersion of complex permeability and permittivity on iron-based nanocomposites derived from rare earth-iron intermetallic compounds. J Alloys Compd 2006;408:1396-99. [10] Sugimoto S, Maeda T, Book D, Kagotani T, Inomata K, Homma M, et al. GHz microwave absorption of a fine α-Fe structure produced by the disproportionation of Sm2 Fe17 in hydrogen. J Alloys Compd 2002;330:301-06. [11] Zou Z, Xuan A, Yan Z, Wu Y, Li N. Preparation of Fe3O4 particles from copper/iron ore cinder and their microwave absorption properties. Chem Eng Sci 2010;65(1):160-64.
26
[12] Ni S, Wang X, Zhou G, Yang F, Wang J, He D. Designed synthesis of wide range microwave absorption Fe3O4 carbon sphere composite. J Alloys Compd 2010;489(1):252-56. [13] Lu B, Huang H, Dong XL, Zhang XF, Lei JP, Sun JP, et al. Influence of alloy components on electromagnetic characteristics of core/shell-type Fe–Ni nanoparticles. J Appl Phys 2008;104(11):114313. [14] Yan SJ, Zhen L, Xu CY, Jiang JT, Shao WZ, Lu L, et al. The influence of Fe content on the magnetic and electromagnetic characteristics for Fex(CoNi)1− x ternary alloy nanoparticles. J Appl Phys 2011;109(7):07A320. [15] Li D, Choi CJ, Han Z, Liu XG, Hu WJ, Li J, et al. Magnetic and electromagnetic wave absorption properties of α-Fe(N) nanoparticles. J Magn Magn Mater 2009;321(24):4081-85. [16] Huang H, Wang F, Lv B, Xue FH, Guo DY, Park WJ, et al. Microwave Absorption of γ′Fe2. 6Ni1. 4N Nanoparticles Derived from Nitriding Counterpart Precursor. J Nanosci Nanotechnol 2012;12(4):3040-47. [17] Park KY, Lee SE, Kim CG, Han JH. Fabrication and electromagnetic characteristics of electromagnetic wave absorbing sandwich structures. Compos Sci Technol 2006;66(3):576-84. [18] Choi I, Kim JG, Seo IS, Lee DG. Radar absorbing sandwich construction composed of CNT, PMI foam and carbon/epoxy composite. Compos Struct 2012;94(9):3002-08. [19] Kim PC, Lee DG, Seo IS, Kim GH. Low-observable radomes composed of composite sandwich constructions and frequency selective surfaces. Compos Sci Technol 2008;68(9):216370.
27
[20] Li W, Liu L, Zhong C, Shen B, Hu W. Effect of carbon fiber surface treatment on Cu electrodeposition: The electrochemical behavior and the morphology of Cu deposits. J Alloys Compd 2011;509(8):3532-36. [21] Fan Y, Yang H, Liu X, Zhu H, Zou G. Preparation and study on radar absorbing materials of nickel-coated carbon fiber and flake graphite. J Alloys Compd 2008;461(1):490-94. [22] Guo H, Huang YD, Meng LH, Liu Ll, Fan DP, Liu DX. Interface property of carbon fibers/epoxy resin composite improved by hydrogen peroxide in supercritical water. Mater Lett 2009;63(17):1531-34. [23] Xu X, Weber SG. Carbon fiber/epoxy composite ring–disk electrode: Fabrication, characterization and application to electrochemical detection in capillary high performance liquid chromatography. J Electroanal Chem 2009;630(1):75-80. [24] Pierozynski B, Smoczynski L. Electrochemical corrosion behavior of nickel-coated carbon fiber materials in various electrolytic media. J Electrochem Soc 2008;155(8):C427-C36. [25] Zeng J, Xu J. Microwave absorption properties of CuO/Co/carbon fiber composites synthesized by thermal oxidation. J Alloys Compd 2010;493(1):L39-L41. [26] Zeng J, Xu J, Tao P, Hua W. Ferromagnetic and microwave absorption properties of copper oxide-carbon fiber composites. J Alloys Compd 2009;487(1):304-08. [27] Kim PC, Lee DG. Composite sandwich constructions for absorbing the electromagnetic waves. Compos Struct 2009;87(2):161-67.
28
[28] Sun Y, Gao L. Structural responses of all-composite improved-pyramidal truss sandwich cores. Mater Des 2013;43:50-58. [29] Fan HL, Meng FH, Yang W. Sandwich panels with Kagome lattice cores reinforced by carbon fibers. Compos Struct 2007;81(4):533-39. [30] Fan HL, Yang W, Chao ZM. Microwave absorbing composite lattice grids. Compos Sci Technol 2007;67(15):3472-79. [31] Dong XL, Zhang ZD, Zhao XG, Chuang YC, Jin SR, Sun WM. The preparation and characterization of ultrafine Fe–Ni particles. J Mater Res 1999;14(02):398-406. [32] Patterson AL. The Scherrer formula for X-ray particle size determination. Phys Rev 1939;56(10):978-82. [33] Osaka T, Matsunaga T, Nakanishi T, Arakaki A, Niwa D, Iida H. Synthesis of magnetic nanoparticles and their application to bioassays. Anal Bioanal Chem 2006;384(3):593-600. [34] Bruce IJ, Sen T. Surface modification of magnetic nanoparticles with alkoxysilanes and their application in magnetic bioseparations. Langmuir 2005;21(15):7029-35. [35] Mikhailik OM, Fedorenko OM, Mikhailova SS, Povstugar VI, Lyakhovich AM, Kurbatova GT, et al. Surface structure of finely dispersed iron powders III. Structure of a γaminopropyltriethoxysilane-modified coating. Colloids Surf 1991;52:331-38. [36] Liu XG, Geng DY, Meng H, Shang PJ, Zhang ZD. Microwave-absorption properties of ZnO-coated iron nanocapsules. Appl Phys Lett 2008;92(17):173117.
29
[37] Li D, Choi C, Han Z, Liu X, Hu W, Li J, et al. Magnetic and electromagnetic wave absorption properties of α-Fe(N) nanoparticles. J Magn Magn Mater 2009;321(24):4081-85. [38] Zhang B, Feng Y, Xiong J, Yang Y, Lu H. Microwave-absorbing properties of deaggregated flake-shaped carbonyl-iron particle composites at 2-18 GHz. IEEE Trans Magn 2006;42(7):1778-81. [39] Yusoff AN, Abdullah MH, Ahmad SH, Jusoh SF, Mansor AA, Hamid SAA. Electromagnetic and absorption properties of some microwave absorbers. J Appl Phys 2002;92(2):876-82. [40] Berthault A, Rousselle D, Zerah G. Magnetic properties of permalloy microparticles. J Magn Magn Mater 1992;112(1):477-80.
30
S0263-8223(15)00428-6 http://dx.doi.org/10.1016/j.compstruct.2015.05.054 COST 6472
To appear in:
Composite Structures
Please cite this article as: Shah, A., Wang, Y., Huang, H., Zhang, L., Xue, F., Duan, Y., Dong, X., Zhang, Z., Microwave Absorption and Flexural Properties of Fe Nanoparticle/Carbon Fiber/Epoxy Resin Composite Plates, Composite Structures (2015), doi: http://dx.doi.org/10.1016/j.compstruct.2015.05.054
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.
Microwave Absorption and Flexural Properties of Fe Nanoparticle/Carbon Fiber/Epoxy Resin Composite Plates Asif Shah1, Yonghui Wang1, Hao Huang1, Li Zhang1, Fanghong Xue1, Yuping Duan1, Xinglong Dong1* and Zhidong Zhang2 1
Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Ministry of
Education),and School of Materials Science and Engineering, Dalian University of Technology, Liaoning, 116024, People's Republic of China 2
Institute of Metal Research, Chinese Academy of Sciences, Shenyang, Liaoning, 110015, People's Republic of China
*corresponding author: Prof. X.L. Dong Email: [email protected] Tel:
+86-411-84706130
Fax:
+86-411-84709284
Abstract: A flat nanocomposite plate was fabricated by using the surface-modified Fe nanoparticles (NPs) as the microwave absorbent, carbon fibers (CFs) as the reinforced phase and epoxy resin (ER) as the matrix. Fe NPs were synthesized by an arc discharge plasma method and subsequently modified by silane coupling agent (KH-550) to improve its dispersion in the organic matrix of ER. To measure the realistic microwave absorption properties of such a flat nanocomposite plate, a series of square plates (20´20 cm2) was made from recombining the modified Fe NPs (20 wt.%, 30 wt.% and 40 wt.%) into the ER matrix with/without orientated
1
CFs inside. It was observed that the orientation of CFs plays an important role in the microwave absorption, in particular through a strong reflection of microwave inwardly as the CFs’ array is vertical to the direction of incident microwave. The inner strong reflection of microwave by CFs can bring great probabilities to further consume it by Fe NPs absorbent and result in improved microwave absorption performance of the nanocomposite plate. It is indicated that the plate containing 30 wt.% of Fe NPs with a perpendicular manner between the directions of CFs array and incident microwave exhibits higher reflection loss (RL) of -16.2 dB at 6.1 GHz frequency, and this plate has 77.78 MPa flexural strength at 3.74% deformation. Excellent RL property is ascribed to an optimum structure of nanocomposite plate with favorable multi-reflection of microwave inside, structural resonance, appropriate conductivity, impedance match, interface polarization. Keywords: Carbon Fiber, Mechanical Properties, Nanocomposite, Nanoparticles, Epoxy Resin, Reflection Loss 1. Introduction In recent years, electromagnetic wave absorbing materials have aroused a great deal of interest because of increasing civil, commercial and military applications in the electromagnetic interference shielding and radar cross-section reduction in the gigahertz (GHz) band range. The microwave absorbing materials that are structurally fantabulous light in weight and flexible have a high demand in the commercial applications like, cellular systems, computers, wireless LAN devices, wireless antenna systems [1, 2]. As compared to bulk magnetic and ferrite materials, nanomaterials have more pros like their being light in weight, durable, having smaller particles size, increased surface’s anisotropies and lower eddy current phenomena. Such qualities make them good electromagnetic wave absorbers
2
[3-5]. The ferrite nanocomposite material is a promising candidate as an excellent microwave absorbing material at high frequency because of its light weight, high dielectric and magnetic loss [6, 7]. So far, much research has been devoted to the making of nanocomposite such as (Fe)/amorphous carbon [8], α-Fe/Y2O3 [9] and α-Fe/SmO [10] and to investigate their microwave absorption characteristics. Fe NPs have an excellent microwave absorbing property and have been extensively used in different forms as an absorbent in composites [11-13]. A ternary alloy based on Fe NPs prepared by self-catalyzed reduction method exhibited a calculated minimum RL of -59 dB at 9.5 GHz [14]. α-Fe(N) NPs prepared by chemical vapor condensation method indicated the optimum RL of -37.5 dB at 10.4 GHz frequency [15]. In our previous work as per theoretical calculations, γ-Fe2.6Ni1.4N NPs derived from γ-Fe2.6Ni1.4 prepared by an arc discharge method showed the optimal RL of -39.9 dB at 5.2 GHz, the core/shell type Fe-Ni NPs possessed minimum RL of -34.9 dB at 10.60 GHz and metallic Fe NPs exhibited a minimum RL of -47.3 dB at 9.6 GHz [7, 13, 16]. The structures with coating and multi-components are of two main types concerning their application to the microwave absorbing materials. As compared to the microwave absorbing coatings, the composite structures possess both microwave absorption and loading characteristics, which avoid the disadvantages of microwave absorbing coating, such as an increase in structural weights, poor mechanical and environment resistant properties, as well as high cost due to hard maintenance and mending. Through multifunctional design, the sandwich structure may have multi-abilities of microwave absorption and load carrying to meet the requirements for the applications in naval and aeronautical engineering [17-19]. CFs are one of the most advanced and important candidates due to their having high modulus, high strength, low density and low coefficient of thermal expansion [20-22]. These properties make CFs a potential
3
material for aircraft, automotive parts, electrical equipment, as well as to some extend to the field of electromagnetic material [23, 24]. However, in the field of electromagnetic materials, the properties of CFs are needed to be improved by coating the surface with a layer of metal or other oxide [25, 26]. Kim and Lee [27] manufactured composites, which were composed of carbon nanotubes, carbon fabric/epoxy and PVC foam. It was observed that, when the thickness and weight fraction of carbon nanotubes in nanocomposite were 2.52 mm and 3 wt.%, the absorbing band width of -10 dB was 3.3 GHz (at 8.2–11.5 GHz). Park et al. [17] composed the sandwich structures made up of two face sheets and foam core. Glass fabric/epoxy composites containing conductive carbon black and carbon fabric/epoxy composites were used for the face sheets. Polyurethane foams containing multiwall nanotubes were used as the core material. The core of sandwich composite was conventionally made up of the foams. This research has also shown that such type of lattice structures have higher weight efficiency than that of foams [28, 29]. The lattice cores reinforced by CFs of the Kagome-grid were manufactured by Fan et al [30], which was filled with microwave absorbing foams. Such lattice composites show excellent microwave absorption within 4-18 GHz. In this work a flat nanocomposite plate, i.e. a multilayered structure filled by the microwave absorber of Fe NPs, has been designed and fabricated. A series of such plates was prepared using surface-modified Fe NPs as the absorber filling into the matrix of ER, meanwhile CFs were paved to improve both properties of electromagnetism and mechanics. The freestanding Fe NPs were prepared by the arc discharge plasma method; while its surface modification was carried out by chemical treatment using the silane coupling agent (KH-550). Such a multifunctional structure combines the microwave absorbing and mechanical performances. The relationships
4
between composite structures and the actual microwave absorption properties were investigated and emphasized in this research. 2. Experimental details 2.1 Preparation and characterization of Fe NPs Fe NPs were synthesized by an arc discharge method, and the experimental details had been given in our previous work [31]. During the synthesis of Fe NPs, bulk Fe target was evaporated, which was laid on a water-cooled copper stage that is serving as the anode, while an upper carbon rod which served as the cathode was supported by a copper arm. After the chamber evacuation, a gas, mixture of hydrogen and argon was introduced into the chamber to a certain pressure. The distance between the two electrodes could be automatically adjusted from outside the chamber, so that the arc can be started and controlled during the continuous operation. Surface modification of Fe NPs was carried out by using 5% silane coupling agents (KH-550), which was purchased from Sahn Chemical Technology (Shanghai) Co. LTD. Its mechanism is shown in Fig. 1. Fe NPs precursor was dispersed into distilled water with 5 wt.% of silane coupling agent for 10 minutes. Then, hydrochloric acid was added to reduce the pH level of suspension to 4. After that, suspension was further mixed at ultrasonic oscillation for 45 minutes at 70oC, and then the dispersed Fe NPs were separated from solvent by centrifugation (10 min. at 7000 rpm) followed by washing with ethanol for four times to remove the excessive silane. Once the process was finished, the surface-modified Fe NPs were dried in an oven at 80oC for 12h.
5
(a)
(b)
Figure 1: Mechanism for the surface modification of Fe NPs. (a) Hydrolysis of γ-aminopropyl triethoxysilane and (b) Condensation of silanol on the surface of Fe NPs. The morphologies of as-prepared Fe NPs and surface-modified Fe NPs were analyzed with a High Resolution Transmission Electron Microscopy (HRTEM, Tecnai G220S). Its crystal structure was identified by X-ray diffractometer (XRD, Dutch EMPYREAN). FTIR test was conducted on FT/IR-430 made by Jasco, Japan. Scanning electron microscopy (FEI Company, NOVA nano SEM 450) was used to observe microstructures of the nanocomposite plate. The distribution of Fe NPs in ER matrix was analyzed by electron probe X-ray microanalyzer (EPMA, Shimadzu EPMA-1600). 2.2 Preparation of Fe NPs/CFs/ER nanocomposite plate CFs used in this work were poly-acrylonitrile (PAN)-based fibers (12K), the average diameter of the filaments is 6.9 µm, tensile modulus is 240 GPa, tensile strength is 4200 MPa, strain (% ) is 1.8 and density is 1.76 g/cm3, which were manufactured by AKSA, Turkey. Bisphenol-A ER (E51) was purchased from Shenyang Zheng Tai Corrosion Material Co., Ltd. China, and was
6
used to prepare a series of square plates (20´20 cm2) for electromagnetic testing. The plate samples were fabricated with the surface-modified Fe NPs (20 wt.%, 30 wt.% and 40 wt.%), with or without CFs in ER matrix, respectively. A pure ER plate and a CF-paved ER plate were prepared for comparison. In the preparation of nanocomposite, the surface-modified Fe NPs, ER, coupling agent (KH-550) and catalyst [(C6H5)3P] were used. Mixing of the surface-modified Fe NPs, coupling agent and ER were carried out by ultrasonic oscillation for 40 minutes at 70oC, followed by an addition of the catalyst and further mixed for 5 minutes. The suspension was poured into a pre-set mould by hand lay-up method. CFs were paved between the two layers of suspension, then the plate was dried in the furnace at 124oC for 12 hours. The schematic diagram of the preparation process is shown in Fig. 2.
7
Figure 2: Preparation process of Fe NPs/CFs/ER nanocomposite plate. (a) Ultrasonic mixing of modified Fe NPs and ER, (b) Pouring of 1st layer into mould, (c) Paving CFs, (d) Pouring of 2nd layer into mould, (e) Schematic illustration of nanocomposite plate with dimension and internal geometry (f) Drying in furnace and (g) As prepared flat nanocomposite plate. 2.3 RL measurement with respect to orientation of CFs Microwave RL of the nanocomposite plates was measured using Agilent 8729B Vector Network Analyzer. The surface-modified Fe NPs were added into an ER matrix at different concentration (20 wt.%, 30 wt.% and 40 wt.%) with laying of CFs. Taking into account the orientation of CFs, flat nanocomposite plates were tested concerning a parallel and vertical direction to the transmission direction of electromagnetic wave. First RL measurement was taken
8
in such a way that the direction of CFs was parallel to propagation direction of microwave. Later, second measurement was taken as the direction of CFs was vertical to propagation direction of microwave, which is shown in the schematic diagram in Fig. 3.
(1) Network Analyzer, (2) Transmitter, (3) Receiver, (4) Nanocomposite flat plate, (5) CFs in ERs matrix (parallel to propagation direction of microwave), (6) CFs in ERs matrix (vertical to propagation direction of microwave), (7,8) Pyramidal absorber, (9) Transmission line.
Figure 3: Schematic diagram of RL test by Vector Network Analyzer according to orientation of CFs in the nanocomposite plates. 2.4 Flexural tests of Fe NPs/CFs/ER nanocomposite plate The three-point flexural test of unidirectional CFs reinforced in ER matrix with/without Fe NPs was performed in accordance to ASTM D7264. The tests were carried out on Changchun Research Institute for Mechanical Science Co. Ltd. (CSS-2205). The specimens were designed as 153.6 mm in length, 13 mm in width and 4 mm in thickness. The span-to-depth ratio was set as 32:1 and the tests were carried out at constant cross-head speed of 1 mm/min, at least five specimens were tested from each nanocomposite plate. The flexural strength (σf) and flexural strain (Ɛf) were calculated according to ASTM standard as follows.
and
Ɛ =
Where, P is maximum force, L is supporting span, b is the width, h is thickness and span deflection.
9
is mid-
3. Results and discussion 3.1 Fe NPs and surface modification Fig. 4(a) shows XRD profile of Fe NPs. It indicates that three diffraction peaks in accordance to α-Fe (BCC) structures were detected. The grain size of Fe NPs was calculated as 65.7 nm using the Scherrer formula [32]. During the preparation of Fe NPs by the arc discharge method, the passivation process was incorporated to form a thin oxide layer on the fresh Fe nanoparticle, which is not observed in the XRD profile. HRTEM image shows these thin oxide layers on the Fe NPs as in Fig. 4(b), which are spherical with mutual adhesion between the particles due to high surface energy and magnetic interaction. Fig. 4(c) displays HRTEM photo of the surface-modified Fe NPs by γ-aminopropyl triethoxysilane (KH-550) (see Fig. 1 for its mechanism). During the surface modification, the two reactions take place simultaneously with respect to silane, i.e. the hydrolysis of silane ethoxy groups to generate KH-550-Si(OH)3 (hydroxyl silane) and the condensation of the resultant silanols with free -OH groups with the surface of Fe NPs to render the stable bond. These two reactions are followed by amino ionization, which improves the dispersion stability of Fe NPs by the steric hindrance while maintaining the electrostatic repulsion between the particles.
10
(110)
b
Intensity (arb.units)
a
c
(200)
(211)
Fe NPs
20
30
40
50
60
70
80
90
100
2q (degree)
Figure 4: (a) XRD spectrum and (b) HRTEM image of the as-prepared Fe NPs, and (c) HRTEM image of the surface-modified Fe NPs. In order to confirm the surface modification of Fe NPs, FTIR spectra was measured as shown in Fig. 5. IR spectra of pure KH-550, Fe NPs before/after the surface modifications are presented for comparison. It can also be observed that the surface-modified Fe NPs have some new vibration peaks in comparison to pure KH-550 and Fe spectra. A peak at 2980 cm-1 corresponds to the asymmetric stretching vibration of CH3, while the peak at 1390 cm-1 is corresponding to C-Si-O in-plane bending vibration [33]. 1053 cm-1 peak is related to Si-O-Fe, which is the result of the –OH (on the surface of Fe NPs) reacted with KH-550 [34]. 1080 cm-1 peak is related to the symmetric stretching vibration of Si-O-C [35]. After the surface-modification, as compared to pure KH-550 and Fe IR spectra, new peaks appear at 1053 cm1, 1390 cm-1 and 2980 cm-1, which indicate that KH-550 has been successfully grafted and made bond onto the surface of Fe NPs.
11
Fe
KH550 modified Fe
1053 KH550
1390 2980
1080 500
1000
1500
2000
2500
3000
3500
4000
-1
Wavenumbers(cm )
Figure 5: FTIR of KH-550, Fe and modified Fe NPs. 3.2 Distribution of Fe NPs and CFs in nanocomposite To study the distribution of Fe NPs and CFs in the flat nanocomposite plates, electron probe micro-analyzer (EPMA) test was conducted on 20 wt.% Fe NPs plate, in cross section of the plate to linearly analyze Fe and carbon elements from top to bottom. Fig. 6 shows the distributions of Fe and carbon elements, and SEM image of the nanocomposite plate without paved CFs. A distinctly gradient concentration of Fe NPs is found from top to bottom of the plate as shown in Fig. 6(a). It is reasonable that the gravity can force the majority of Fe NPs settled down to the bottom of the plate during the curing process. Fig. 6(b) indicates the presence of carbon, which is attributed to ER matrix, exhibiting a fluctuate distribution due to the heterogeneous density of ER. While, Fig. 6(c) displays the microstructure of Fe NPs/ER composite, in which the uniform mixing and strong linking between NPs and matrix can be observed.
12
100
Top side
Bottom side
Counts
80
Distribution of Fe
60 40 20
(a) 0 5000
Distribution of Carbon
Counts
4000
3000
(c)
2000
(b) 1000 -0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Positions (mm)
Figure 6: Fe NPs/ER nanocomposite without paved CFs. EPMA results linearly scanned from top to bottom side of the plate. (a) Carbon element analysis, (b) Fe element analysis and (c) SEM image of cross section of the plate. Fig. 7 presents the analysis of elements (Fe, C) and the microstructure of the nanocomposite plate, in which CFs was paved inside Fe NPs/ER matrix. CFs can also be detected from high counts of carbon at the bottom of plate (Fig. 7b). It can be observed from Fig. 7(a-b) that CFs are surrounded by the majority of Fe NPs, which may be caused by the gravity and weak interactions between NPs and CFs. As one knows, CFs/epoxy composites have been regarded as a new generation of high performance materials because of their high strength-to-weight ratio which makes them more popular in aerospace and automotive industries. In this work, a new component, i.e. Fe NPs, was added into such CFs/epoxy composites, the enhanced mechanical
13
properties and extra electromagnetic performances can be anticipated with a view to the interfacial interactions in the Fe NPs/CFs/ER interfaces. To study the microstructure of this Fe NPs/CFs/ER nanocomposite, SEM image was taken as shown in Fig. 7(c). SEM image verifies the EPMA results and shows that the symmetry of CFs with Fe NPs/ER matrix. The existence of CFs and its orientation favor to increase mechanical strength and it can greatly influence the microwave absorption properties which will be discussed in next sections.
400
Top side
Bottom side
Counts
300
Distribution of Fe 200
CFs 100
(a)
0
CFs
4000
Distribution of Carbon Counts
3000
2000
(c)
1000
(b)
0 -0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Position (mm)
Figure 7: Fe NPs/CFs/ER nanocomposite with paved CFs inside. EPMA results linearly scanned (c)
(c) from top to bottom side of the plate. (a) Carbon element analysis, (b) Fe element analysis and (c) SEM image of cross section of the plate.
14
3.3 RL measurements on flat nanocomposite plates (c) (a) Fe NPs/ER nanocomposite without paved CFs Pure ER is an excellent wave transparent material and has no electromagnetic wave absorption as shown in Fig. 8(a). After filling the modified Fe NPs (20 wt.%, 30 wt.%, and 40 wt.%) into the ER matrix, the nanocomposite plates become microwave absorbent at higher frequency. At 20 wt.% of Fe NPs, the plate shows minimum RL of -2 dB at 17.4 GHz (Fig. 8b), which greatly increases to -12.4 dB at 17.4 GHz by adding 30 wt.% of Fe NPs (Fig. 8c), and slightly decreases to -10.4 dB at 16.8 GHz after adding Fe NPs to 40 wt.% as shown in Fig. 8(d). Without doubt, adding magnetic Fe NPs into ER can cause the nanocomposite microwave absorbent at high frequency to a certain RL value, but an optimum concentration of the absorbent is necessary. Increasing the content of Fe NPs built up a strong cross linked network and increases the permittivity and permeability of nanocomposite plate. The parameters such as thickness, minimum RL and peak frequency are always associated with the unitary absorption performance. In an extreme case, the incident and reflected wave can be out of phase to cancel each other and result in a minimum RL value, if the thickness of plate equals to a quarter of wave length. When incident wave strikes at absorbing material, the specific content of absorber, matching thickness and resonance frequency are essential to the loss of microwave. In this study, the thickness of pure epoxy plate, and the nanocomposite plates with 20 wt.%, 30 wt.% and 40 wt.% Fe NPs mixed in ER matrix were measured as 0.93 mm, 1.18 mm, 1.48 mm and 1.77 mm, respectively. These experimental results reveal the optimum thickness of nanocomposite plate is 1.48 mm for the concentration of 30 wt.% Fe NPs.
15
1.0
RL (dB)
0.5 0.0
(a) Pure ERs
-0.5 -1.0
RL (dB)
0
-1
(b) 20 wt% Fe NPs -2
-2 dB (17.4 GHz)
RL (dB)
0 -5
(c) 30 wt% Fe NPs
-10
-12.4 dB (17.4 GHz) -15
RL (dB)
0
-5
(d) 40 wt% Fe NPs -10
-10.4 dB (16.8 GHz) 0
5
10
15
20
Frequency (GHz)
Figure 8: RL measurements of nanocomposite plates without CFs. (a) Pure ER, (b) 20 wt.% of Fe NPs in ER matrix, (c) 30 wt.% of Fe NPs in ER matrix and (d) 40 wt.% of Fe NPs in ER matrix. Core-shell type Fe NPs are used as absorbent due to their excellent microwave absorption ability because of high magnetic and dielectric losses as shown in our previous work [6, 7, 13, 31]. Either the only magnetic loss or only dielectric loss may cause a weak microwave wave absorption property due to dissymmetry of the electromagnetic match. Fe NPs can show up better absorption in the existence of dielectric graphite and oxide shell with ferromagnetic cores as shown in Fig. 4(b-c). The graphite shells give rise to a high density of defects, which leads to an asymmetry of charge density distribution. The polarizations of defective sites are due to the
16
action of electromagnetic wave, which results in multi-polarization phenomenon in core/shell interface resulting in enhancement of microwave absorption.
The microwave absorption properties are determined by the relative permeability ( "
), the relative permittivity (
=
−
"
=
−
), the EM impedance match and the microstructure
of the absorber [36]. As the microwave strikes the surface of an absorber, a good matching
condition of EM impedance is enabled to have almost zero reflectivity of the incident microwave, the transmitted microwave can be dissipated by dielectric loss and magnetic loss into absorbing materials. The absorbers of core-shell type are excellent to achieve microwave absorption properties [7, 8, 37]. The magnetic Fe NPs are one rather core-shell nanostructures with the ‘‘shell’’ of ferrimagnetic oxide (Fe3O4) consisting of Fe3+ and Fe2+ ions and the “core” of metal Fe. The electron transfer between Fe3+ and Fe2+ creates to ion jumps and relaxation in the Fe3O4 shells. The dipole polarization is the dominant at higher frequency and the weak space charge polarization mainly works at lower frequency, such Fe NPs can act as the dipoles [7]. The “core” of metallic Fe can also cause magnetic loss by the natural resonance [7, 13]. At low content of Fe NPs, the nanocomposite would be less in absorber’s quantity and there would be larger spacing between NPs, as a result, the vast majority of incoming electromagnetic waves would not strike the Fe NPs and lower effective absorption is inevitable. At higher particle’s content, the permittivity and permeability, as well as the attenuation constant for microwave transmission of such nanocomposite will be increased [38]. The NPs contact to form a cross linked network in the nanocomposite plate, the electromagnetic wave can quickly be dissipated resulting in strong absorption. It can be assumed that the higher RL loss of this nanocomposite plate is also associated with the high relative permittivity and permeability, high attenuation
17
constant and with EM impedance match, which was measured according to transmission line theory [26], (
tan ℎ
= Where,
−1
) = 20log ( 2
+ 1) (
)
is the input impedance of absorber, d is the thickness of absorber, c is the velocity of
light, is the frequency and
and
are the permeability and permittivity respectively.
(b) Fe NPs/CFs/ER nanocomposite with paved CFs inside To make an ideal microwave absorbing structure, the reflection coefficient has been considered as low as possible so that the microwave could enter the structure at the maximum extent, which generally requires high input impedance. Optimally, the input impedance should be quite close to the free space intrinsic impedance of air, and only then can the microwave entirely enter the structure to be absorbed at the maximum extent. Considering this factor a flat nanocomposite plate has been designed by filling different concentration of Fe NPs (20 wt.%, 30 wt.%, and 40 wt.%) into the ER matrix with paved CFs 8.72 g by weight, as shown in Figs. 2 and 7. For comparison, RL values of the nanocomposite plates were measured regarding orientations of CFs (parallel and vertical to the incident direction of microwave) and the results are illustrated in Fig. 9. In comparison to pure ER, paving of CFs into the ER matrix endows the microwave absorbent at higher frequency and shows a similar tendency with the frequency at both paved directions (perpendicular and parallel to microwave incidence) as shown in Fig. 9(c). These results indicate that CFs can cause microwave loss solely in transparent matrix of ER, regardless of their orientations. This state can be greatly affected by adding Fe NPs with different contents,
18
especially as the microwave incidence is vertical to the direction of CFs as shown in Fig. 9(d-f). For the parallel direction, it shows a monotonous increase in microwave absorption with increasing frequency at any Fe NPs contents, but for the vertical direction it shows a single peak with minimum RL of -10 dB at 7.5 GHz (20 wt.% of Fe NPs, Fig. 9d), two peaks with minimum RL of -16.2 dB at 6.1 GHz and -13.1 dB at 15.6 GHz (30 wt.% of Fe NPs, Fig. 9e) and likewise, two peaks with minimum RL of -16.2 dB at 4.9 GHz and -18.3 dB at 16.8 GHz (40 wt.% of Fe NPs, Fig. 9f). It is well indicated that the orientations of CFs and collective interaction with Fe NPs play an influential role to enhance the microwave absorption, which is ascribed to the multi-reflection, scattering and absorption of waves inside Fe NPs/CFs/ER composite structure. Concerning the parallel case between the directions of CFs and incident wave, more electromagnetic energy is dissipated and absorbed in the vertical case within the nanocomposite structure, resulting in excellent microwave absorbing properties.
19
(a)
CFs parallel to incident way
0
CFs vertical to incident way
(b)
(d)
(c)
-2 -4 -6
RL (dB)
-8 -10
-10 dB (7.5 GHz)
-12 -14 -16 -18 -20
20 wt% Fe NPs with CFs (Parallel) 20 wt% Fe NPs with CFs (Vertical)
Pure ERs with CFs (Parallel) Pure ERs with CFs (Vertical)
-22 -24 -26 0
(e)
-2
(f)
-4 -6
RL (dB)
-8 -10 -12 -14
-13.1 dB (15.6 GHz)
-16 -18
-18.3 dB (16.8 GHz)
-16.2 dB (6.1 GHz)
-16.2 dB (4.9 GHz)
-20
40 wt% Fe NPs with CFs (Parallel) 40 wt% Fe NPs with CFs (Vertical)
30 wt% Fe NPs with CFs (Parallel) 30 wt% Fe NPs with CFs (Vertical)
-22 -24 -26 0
2
4
6
8
10
12
14
16
18
20 0
2
4
6
8
10
12
14
16
18
20
Frequency (GHZ)
Frequency (GHZ)
Figure 9: RL measurements of nanocomposite plates according to orientation of CFs. (a) Schematic illustration of parallel arranged CFs to incident wave, (b) Schematic illustration of
20
vertical arranged CFs to incident wave, (c) Pure ER with CFs, (d) 20 wt.% Fe NPs with CFs in ER matrix, (e) 30 wt.% Fe NPs with CFs in ER matrix and (f) 40 wt.% Fe NPs with CFs in ER matrix.
3.4 RL Mechanism of nanocomposite plate with or without paved CFs As one kind of absorption type, the nanocomposite plate consumes the incident electromagnetic wave by dissipating electromagnetic energy into heat through the mechanisms of the dielectric and magnetic losses. To achieve higher microwave absorption, the electromagnetic wave entering into the material should be entirely attenuated, according to the following equation [38] = √2
× ′ ′ (tan
tan
− 1) + (tan
tan
Where, is the attenuation constant, c is the speed of light,
− 1) + (tan
+ tan
is the frequency, ′, ′,
)
and
represent the real permeability, real permittivity, dielectric and magnetic loss,
respectively. This equation shows that the higher values of ′ and
as well as f, results in higher
. As shown in Fig. 8, adding Fe NPs absorbent into the ER matrix from 10 wt.% up to 40 wt.%
contents can improve the electromagnetic parameters of nanocomposites, which cause the enhanced losses at high frequency indeed. CFs are conductive material and consumes microwave to a certain extent by dielectric loss at higher frequency as shown in the results of Fig. 9(c), in particular, such loss is retained in all “parallel” cases (Fig. 9d-f). The plates of CFs/ER, Fe NPs/ER and Fe NPs/CFs/ER (parallel cases) composites exhibit an emblematical type in which the microwave is attenuated at higher frequency without distinguishable resonance peaks. Besides to attenuate microwave by CFs (dielectric loss) or Fe NPs (both dielectric and magnetic losses), the nanocomposite actually has constructed an effective structure with the
21
orientated CFs, gradual distribution of Fe NPs and a definite thickness of plate. For such a structure, it is in practice an absorption entity to dissipate microwave by structure related resonance. To achieve the best performance, the thickness of nanocomposite structure should quarter of wavelength, the best adequate matching condition [39]. In Fig. 9(d-f), visible absorption peaks can be found in “vertical” cases in which the array of CFs is vertical to incident wave. These peaks usually occur in a typical structure consuming microwave by a resonance. As one knows, electromagnetic wave has two vectors of electric and magnetic components, both vertical to the direction of wave propagation. In the “vertical” case, the electric vector component of the wave is actually parallel to CFs’ orientation (Fig. 9-b), which can cause a strong reflection by the conductive CFs and supply great opportunity to be absorbed by Fe NPs surrounded. In such a condition, the nanocomposite plate practically acts as an effective structure to consume microwave by resonance and absorption. Otherwise, mere absorption of microwave without any resonances would occur in the nanocomposite plate, as shown for the “parallel” cases in Fig. 9(d-f), here into no strong reflections of wave had happened due to perpendicular between the electric vector component and CFs (Fig. 9-a) . Therefore, it can be concluded that sole existence of oriented CFs or distributed Fe NPs can just cause an absorption type with a raised loss happened at high frequency, but co-existence and reciprocity between CFs and Fe NPs can play a radical role in microwave consumption by the structural resonance. Consequently, the relative directions of CFs’ array and the incident microwave should be exceptionally considered. According to the equation below, it is well described that the matching (resonance) frequency is determined by the thickness of structural entity and the electromagnetic parameters [39]. f
nc/4t
| || |
(n=1,3,5,7,9,…)
22
Where,
is matching frequency,
is the matching thickness,
and
represent the relative
permittivity and the relative permeability respectively. As indicated by Bruggeman equation [40], the electromagnetic parameters of Fe NPs/CFs/ER nanocomposite can be improved by increasing the Fe NPs contents in ER matrix. For a certain thickness, the matching frequency of nanocomposite plate will vary to lower ones with the increase of absorbent contents, e.g. from 7.5 GHz (20 wt.% of Fe NPs) to 6.1 GHz (30 wt.% of Fe NPs), to 4.9 GHz (40 wt.% of Fe NPs). Table 1 summarizes the minimum RL values of Fe NPs/CFs/ER nanocomposite plates with the orientation of CFs perpendicular to the incident microwave, average densities and their thicknesses. It can be observed in “vertical” cases that all RL values fall in range of C band (4-8 GHz) and Ku band (12-18 GHz). Table 1: Minima of RL measurements with the orientation of CFs perpendicular to the incident microwave Fe NPs/CFs/ER
S band
C band
plates
(2-4 GHz)
(4-8 GHz)
X band
Ku band
Plate
Density
(8-12 GHz)
(12-18 GHz)
thickness
(g/cm3)
--------
--------
4.10 mm
1.49
4.22 mm
1.71
4.48 mm
1.97
-10 dB 20 wt.% of Fe NPs
-------(7.5 GHz) -16.2 dB
30 wt.% of Fe NPs
-13.1 dB
--------
-------(6.1 GHz)
(15.6 GHz)
-16.2 dB 40 wt.% of Fe NPs
18.3 dB
--------
-------(4.9 GHz)
(16.8 GHz)
3.5 Effect of Fe NPs on flexural properties
23
The three-point flexural test was carried out to measure the flexural properties of nanocomposite plate with/without addition of Fe NPs into the ER matrix with CFs, whose results are given in Fig. 10. Pure ER and CFs plates exhibit 82.58 MPa flexural strength with 3.81% deformation, while ER, CFs and 30 wt.% Fe NPs plates show flexural strength 77.78 MPa at 3.74% deformation. Apparently, the additions of Fe NPs are not effective on the flexural strength and flexural strain of nanocomposite plates.
100 90
5.0
Flexural Strength Flexural Strain
4.5
82.56 3.81
3.74
77.76
4.0
70
3.5
60
3.0
50
2.5
40
2.0
30
1.5
20
1.0
10
0.5
0
Flexural Strain (%)
Flexural Strength (MPa)
80
0.0 Pure ER with CFs
ER and CFs with 30 wt.% Fe NPs
Figure 10: Flexural strength and strain of nanocomposite plates. Pure ER with CFs and 30 wt.% Fe NPs with CFs into ER matrix. 4. Conclusion Microwave absorbent of Fe NPs was synthesized by an arc discharge method and subsequently surface-modified by using silane coupling agent (KH-550) to improve its dispersion in ER matrix. A series of Fe NPs/CFs/ER nanocomposite flat plates were made of the standard size (20´20 cm2) with different contents of Fe NPs (20 wt.%, 30 wt.% and 40 wt.%) and orientated CFs array side. “Parallel” state between CFs array and incident microwave cannot
24
cause an observable reflection of microwave inside, nevertheless, a strong reflection happened in “vertical” case, which resulted in strong absorption of microwave by the structural resonance. In the “vertical” case, the increase of Fe NPs’ content at a certain thickness of the nanocomposite plate gives rise to a slight move of matching frequency towards lower one, indicating that a resonance structure has been successfully constructed to consume the incident microwave. Coalesce and reciprocity between Fe NPs and the oriented CFs play a radical role in microwave absorption. Optimal RL for the nanocomposite plate with 30 wt.% of Fe NPs and vertical between CFs array and incident microwave can reach -16.2 dB at 6.1 GHz with flexural strength 77.78 MPa at 3.74% deformation. This work indicates that such magnetic NPs/CFs/ER nanocomposite plates are significant in the integration of excellent microwave absorption and mechanical properties with the lightweight merit. Acknowledgement This work was financially supported by National Natural Science Foundations of China (No. 51331006, 51271044 and 51171033). References: [1] Petrov VM, Gagulin VV. Microwave absorbing materials. Inorg Mater 2001;37(2):93-98. [2] Yan D, Cheng S, Zhuo RF, Chen JT, Feng JJ, Feng HT, et al. Nanoparticles and 3D spongelike porous networks of manganese oxides and their microwave absorption properties. Nanotechnol 2009;20(10):105706. [3] Peng DL, Hihara T, Sumiyama K, Morikawa H. Structural and magnetic characteristics of monodispersed Fe and oxide-coated Fe cluster assemblies. J Appl Phys 2002;92(6):3075-83.
25
[4] Respaud M, Broto JM, Rakoto H, Fert AR, Thomas L, Barbara B, et al. Surface effects on the magnetic properties of ultrafine cobalt particles. Phys Rev B 1998;57(5):2925-35. [5] Bødker F, Mørup S, Linderoth S. Surface effects in metallic iron nanoparticles. Phys Rev Lett 1994;72(2):282-85. [6] Dong XL, Zhang ZD, Jin SR, Kim BK. Characterization and magnetic properties of Fe–Co ultrafine particles. J Magn Magn Mater 2000;210(1):143-49. [7] Lu B, Dong XL, Huang H, Zhang XF, Zhu XG, Lei JP, et al. Microwave absorption properties of the core/shell-type iron and nickel nanoparticles. J Magn Magn Mater 2008;320(6):1106-11. [8] Liu JR, Itoh M, Horikawa T, Machida KI, Sugimoto S, Maeda T. Gigahertz range electromagnetic wave absorbers made of amorphous-carbon-based magnetic nanocomposites. J Appl Phys 2005;98(5):054305-05-7. [9] Liu JR, Itoh M, Machida KI. Frequency dispersion of complex permeability and permittivity on iron-based nanocomposites derived from rare earth-iron intermetallic compounds. J Alloys Compd 2006;408:1396-99. [10] Sugimoto S, Maeda T, Book D, Kagotani T, Inomata K, Homma M, et al. GHz microwave absorption of a fine α-Fe structure produced by the disproportionation of Sm2 Fe17 in hydrogen. J Alloys Compd 2002;330:301-06. [11] Zou Z, Xuan A, Yan Z, Wu Y, Li N. Preparation of Fe3O4 particles from copper/iron ore cinder and their microwave absorption properties. Chem Eng Sci 2010;65(1):160-64.
26
[12] Ni S, Wang X, Zhou G, Yang F, Wang J, He D. Designed synthesis of wide range microwave absorption Fe3O4 carbon sphere composite. J Alloys Compd 2010;489(1):252-56. [13] Lu B, Huang H, Dong XL, Zhang XF, Lei JP, Sun JP, et al. Influence of alloy components on electromagnetic characteristics of core/shell-type Fe–Ni nanoparticles. J Appl Phys 2008;104(11):114313. [14] Yan SJ, Zhen L, Xu CY, Jiang JT, Shao WZ, Lu L, et al. The influence of Fe content on the magnetic and electromagnetic characteristics for Fex(CoNi)1− x ternary alloy nanoparticles. J Appl Phys 2011;109(7):07A320. [15] Li D, Choi CJ, Han Z, Liu XG, Hu WJ, Li J, et al. Magnetic and electromagnetic wave absorption properties of α-Fe(N) nanoparticles. J Magn Magn Mater 2009;321(24):4081-85. [16] Huang H, Wang F, Lv B, Xue FH, Guo DY, Park WJ, et al. Microwave Absorption of γ′Fe2. 6Ni1. 4N Nanoparticles Derived from Nitriding Counterpart Precursor. J Nanosci Nanotechnol 2012;12(4):3040-47. [17] Park KY, Lee SE, Kim CG, Han JH. Fabrication and electromagnetic characteristics of electromagnetic wave absorbing sandwich structures. Compos Sci Technol 2006;66(3):576-84. [18] Choi I, Kim JG, Seo IS, Lee DG. Radar absorbing sandwich construction composed of CNT, PMI foam and carbon/epoxy composite. Compos Struct 2012;94(9):3002-08. [19] Kim PC, Lee DG, Seo IS, Kim GH. Low-observable radomes composed of composite sandwich constructions and frequency selective surfaces. Compos Sci Technol 2008;68(9):216370.
27
[20] Li W, Liu L, Zhong C, Shen B, Hu W. Effect of carbon fiber surface treatment on Cu electrodeposition: The electrochemical behavior and the morphology of Cu deposits. J Alloys Compd 2011;509(8):3532-36. [21] Fan Y, Yang H, Liu X, Zhu H, Zou G. Preparation and study on radar absorbing materials of nickel-coated carbon fiber and flake graphite. J Alloys Compd 2008;461(1):490-94. [22] Guo H, Huang YD, Meng LH, Liu Ll, Fan DP, Liu DX. Interface property of carbon fibers/epoxy resin composite improved by hydrogen peroxide in supercritical water. Mater Lett 2009;63(17):1531-34. [23] Xu X, Weber SG. Carbon fiber/epoxy composite ring–disk electrode: Fabrication, characterization and application to electrochemical detection in capillary high performance liquid chromatography. J Electroanal Chem 2009;630(1):75-80. [24] Pierozynski B, Smoczynski L. Electrochemical corrosion behavior of nickel-coated carbon fiber materials in various electrolytic media. J Electrochem Soc 2008;155(8):C427-C36. [25] Zeng J, Xu J. Microwave absorption properties of CuO/Co/carbon fiber composites synthesized by thermal oxidation. J Alloys Compd 2010;493(1):L39-L41. [26] Zeng J, Xu J, Tao P, Hua W. Ferromagnetic and microwave absorption properties of copper oxide-carbon fiber composites. J Alloys Compd 2009;487(1):304-08. [27] Kim PC, Lee DG. Composite sandwich constructions for absorbing the electromagnetic waves. Compos Struct 2009;87(2):161-67.
28
[28] Sun Y, Gao L. Structural responses of all-composite improved-pyramidal truss sandwich cores. Mater Des 2013;43:50-58. [29] Fan HL, Meng FH, Yang W. Sandwich panels with Kagome lattice cores reinforced by carbon fibers. Compos Struct 2007;81(4):533-39. [30] Fan HL, Yang W, Chao ZM. Microwave absorbing composite lattice grids. Compos Sci Technol 2007;67(15):3472-79. [31] Dong XL, Zhang ZD, Zhao XG, Chuang YC, Jin SR, Sun WM. The preparation and characterization of ultrafine Fe–Ni particles. J Mater Res 1999;14(02):398-406. [32] Patterson AL. The Scherrer formula for X-ray particle size determination. Phys Rev 1939;56(10):978-82. [33] Osaka T, Matsunaga T, Nakanishi T, Arakaki A, Niwa D, Iida H. Synthesis of magnetic nanoparticles and their application to bioassays. Anal Bioanal Chem 2006;384(3):593-600. [34] Bruce IJ, Sen T. Surface modification of magnetic nanoparticles with alkoxysilanes and their application in magnetic bioseparations. Langmuir 2005;21(15):7029-35. [35] Mikhailik OM, Fedorenko OM, Mikhailova SS, Povstugar VI, Lyakhovich AM, Kurbatova GT, et al. Surface structure of finely dispersed iron powders III. Structure of a γaminopropyltriethoxysilane-modified coating. Colloids Surf 1991;52:331-38. [36] Liu XG, Geng DY, Meng H, Shang PJ, Zhang ZD. Microwave-absorption properties of ZnO-coated iron nanocapsules. Appl Phys Lett 2008;92(17):173117.
29
[37] Li D, Choi C, Han Z, Liu X, Hu W, Li J, et al. Magnetic and electromagnetic wave absorption properties of α-Fe(N) nanoparticles. J Magn Magn Mater 2009;321(24):4081-85. [38] Zhang B, Feng Y, Xiong J, Yang Y, Lu H. Microwave-absorbing properties of deaggregated flake-shaped carbonyl-iron particle composites at 2-18 GHz. IEEE Trans Magn 2006;42(7):1778-81. [39] Yusoff AN, Abdullah MH, Ahmad SH, Jusoh SF, Mansor AA, Hamid SAA. Electromagnetic and absorption properties of some microwave absorbers. J Appl Phys 2002;92(2):876-82. [40] Berthault A, Rousselle D, Zerah G. Magnetic properties of permalloy microparticles. J Magn Magn Mater 1992;112(1):477-80.
30