- Email: [email protected]
RGO layered composite
Accepted Manuscript Synthesis and enhanced microwave absorption properties of PVB/Co2Z/RGO layered composite Haibo Yang, Jingjing Dai, Xiao Liu, Ying Lin, Fen Wang, Peng Liu PII:
S0925-8388(17)31459-7
DOI:
10.1016/j.jallcom.2017.04.249
Reference:
JALCOM 41653
To appear in:
Journal of Alloys and Compounds
Received Date: 5 March 2017 Revised Date:
19 April 2017
Accepted Date: 22 April 2017
Please cite this article as: H. Yang, J. Dai, X. Liu, Y. Lin, F. Wang, P. Liu, Synthesis and enhanced microwave absorption properties of PVB/Co2Z/RGO layered composite, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.249. 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.
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Synthesis and Enhanced Microwave Absorption Properties of PVB/Co2Z/RGO Layered Composite
a
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Haibo Yang a,∗, Jingjing Dai a, Xiao Liu a, Ying Lin a, Fen Wang a, Peng Liu b
School of Materials Science and Engineering, Shaanxi University of Science and
DFH Statellite Co. LTD, Beijing, 100094, PR China
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b
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Technology, Xi’an, 710021, PR China
∗
Corresponding author. Tel: +86-29-86168688; Fax: +86-29-86168688; Email: [email protected]
ACCEPTED MANUSCRIPT ABSTRACT
Polyvinyl Butyral (PVB)/Co2Z/reduced graphene oxide (RGO) layered composite was successfully prepared by a simple casting process in order to enhance impedance
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match. PVB acts as a matrix in this composite. The layered structure makes electromagnetic wave pass through the two layers successively, thereby resulting in the attenuation of electromagnetic wave. The results show that the hybrid
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PVB/Co2Z/RGO composite exhibits a highly efficient microwave attenuation property
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compared with the PVB/Co2Z and PVB/RGO composite in a wide frequency range due to the dielectric loss caused by the introduction of RGO. For the as-prepared PVB/Co2Z/RGO composite, an optimal reflection loss (RL) as low as -35.2 dB can be observed at 14.7 GHz with an absorber thickness of only 2 mm and the effective
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absorption bandwidth corresponding to the reflection loss below -10 dB is 4.24 GHz. Findings indicate that the layered PVB/Co2Z/RGO composite might be a promising
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candidate for a lightweight electromagnetic wave absorbing material.
ACCEPTED MANUSCRIPT INTRODUCTION
With the rapid development of modern electronic information industry, electromagnetic (EM) wave absorbing materials with thin thickness, lightweight, and
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strong absorption in a wide frequency range have aroused growing attention1, since EM interference pollution arises from the rapidly expanding use of communication
devices working in the gigahertz range, such as mobile telephones, local area network
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systems, and radar systems2-5. Traditional microwave absorbing materials such as
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ferrite, magnetic metal powder, conductive polymer and carbon-based materials, etc., are difficult to satisfy the above demands6-10. Hence, exploiting new types of materials with high-efficiency EM wave absorption properties has become more and more urgent. To date, composites of the above materials are the ideal candidates in EM
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wave absorbing and shielding fields, such as (100-x)La1.5Sr0.5NiO4/xNiFe2O4 composite11, SiO2/flaky metal magnetic composite powder12, ZnO/ZrSiO4 composite ceramic13, PANI/CIP/Fe3O4 composite14, CNTs/Fe3O4 composite15,
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graphene/Fe3O4/SiO2/NiO nanosheet composite2, etc.
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Hexaferrites, classified into M-, W-, Y-, Z-, X- and U-type according to their chemical formulas and structures, have been widely used in microwave and millimeter wave devices as permanent magnets or gyromagnetic materials, e.g., in circulators, filters, isolators, inductors, and phase shifters16-19. Z-type hexagonal barium ferrites (Co2Z) with hexagonal planar structure can be applied in the gigahertz region due to their significant permeability, relatively high frequency response, strong magnetocrystalline anisotropy field and high cut-off frequency20-22. Some work
ACCEPTED MANUSCRIPT concerning the preparation of single-phase Co2Z, and its crystal structure, thermal stability and ferromagnetic resonance with cobalt as metallic ion has been widely reported23-25. However, there is little work regarding its EM wave absorption
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properties in a wide frequency range. High performance Co2Z particles have large permeability, which can be used as ferromagenetic phase in EM wave absorbing
composites. Moreover, two-dimensional (2D) materials have been proven to be of
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high performance, with regard to the EM wave absorbing and shielding capability
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over the years26-29. Graphene, with a typical 2D carbon nanostructure, has attracted much attention for its unique physical, chemical, and mechanical properites30. It possesses not only a stable structure but also a high specific surface area, and it is a prospective dielectric material owing to its higher electrical conductivity and
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permittivity2. The multilayered structure is also favorable to the scattering of EM waves and thereby can contribute to the EM wave absorption properties. With the intent of optimizing the impedance match, the multilayered
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PVB/Co2Z/RGO composite was fabricated by a novel tape casting process. The
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structure and EM wave absorption properties of the as-prepared composite were investigated. The results indicate that the synergistic effect of the two phases plays an essential role in the PVB/Co2Z/RGO composite, which exhibits an excellent impedance match and enhanced EM absorption properties in terms of both the maximum reflection loss value and the absorption bandwidth compared with the PVB/Co2Z composite and the PVB/RGO composite. EXPERIMENTAL
ACCEPTED MANUSCRIPT All the chemicals used in this work were purchased from Sinopharm Chemical Reagent Co., Ltd. China. They are of analytical reagent grade (purity 99%). Deionized water was used in all the experiments.
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Synthesis of Co2Z powder. The synthesis of Co2Z random powder was carried out by the conventional
solid-state ceramic process. BaCO3, CoO and Fe2O3 powders were initially weighed
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according to the stoichiometry of Ba3Co2Fe24O41 (Co2Z), After being mixed by ball
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milling thoroughly, the mixture were calcined at 1250 oC in air for 4 h to prepare single-phase random Co2Z powder. And the synthesis of plate-like Co2Z powder was mainly divided into the following stages. The first stage is to synthesize Ba2Co2Fe12O27 (Co2Y) powders. BaCO3, Fe2O3 and CoO powders were weighed
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according to the stoichiometry of Co2Y. Powders were first mixed by ball milling and then calcined at 1250°C in air for 2 h. Secondly, plate-like BaFe12O19 (BaM) precursors were prepared by the molten salt synthesis method (MSS)31. Plate-like
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BaM and Co2Y powders were then weighed in accordance to the following formula: Ba3Co2Fe24O41
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BaFe12O19+ Ba2Co2Fe12O27
(1)
NaCl salt was added to the mixture with a mass ratio of 1:1.5 and then mixing
was carried out in an ethanol solution with an electric stirrer for 5 h. After drying, the above mixture was heated at 1250 oC for 8 h in a tightly covered Al2O3 crucible. Finally, hot deionized water was used to remove NaCl from the product. Synthesis of PVB/Co2Z/RGO composite.
ACCEPTED MANUSCRIPT The random Co2Z powder were mixed thoroughly with a solvent (60 vol.% ethanol and 40 vol.% methyl-ethyl-ketone) and a dispersant (triethyl phosphate) in a ball mill for 4 h. Some binder (PVB) and plasticizer (polyethylene
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glycol/diethylophthalate) were added before ball milling the mixture again for 4 h. With respect to the mass of random Co2Z powders, 10 wt% plate-like Co2Z powder
was then added into the above mixture, which was ball milled with a slow speed for 2
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h to form a slurry for tape casting. The slurry including Co2Z was tape cast to form a
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sheet with a thickness of ~200 µm on a polyethylene film by a doctor blade apparatus. After drying, the first layer sheet was formed.
Graphite oxide was synthesized from natural graphite powder using a modified Hummers’ method, according to the literatures32, 33. The slurry including RGO was
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prepared by the same method and was tape cast to form the second sheet with a thickness of ~230 µm on the first layer by a doctor blade apparatus. After drying, the PVB/Co2Z/RGO composite was obtained, in which the mass ratio of PVB is 70 wt.%.
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The formation process of the PVB/Co2Z/RGO composite is shown in Scheme 1.
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Characterization.
The phase composition of the samples was analyzed by X-ray diffractometer
(XRD, D/max-2200, Rigaku, Japan) using Cu Kα radiation (λ=0.15418 nm). The IR spectrum of the samples was observed by Fourier transform infrared spectrometer (FTIR, Bruker VEC-TOR-22, Germany). The morphology of the samples was observed by field emission scanning electron microscopy (FE-SEM, Quanta 250FEG, FEI, USA). The magnetic hysteresis loops of the powders and composites were
ACCEPTED MANUSCRIPT measured by vibrating sample magnetometer (VSM) (Lake Shore 7410, USA). The electromagnetic parameters were analyzed using a HP8720ES vector network
RESULTS AND DISCUSSION Characterization of PVB/Co2Z/RGO composite.
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analyzer in the frequency range of 2-18 GHz.
The phase purity and crystal structure of the as-synthesized single-phase powder
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without PVB were characterized by XRD. The diffraction peak of the as-prepared
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RGO powder at 25.6° (Figure 1a) could be attributed to the graphite structure of short-range order in stacked grapheme sheets34. The XRD patterns over the range of 15° ≤ 2θ ≤ 70° also suggest that the as-synthesized Co2Z powders, including Co2Z random particles (Figure 1b) and plate-like Co2Z particles (Figure 1c), exhibit obvious
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diffraction peaks of Co2Z, and the detected diffraction peaks can be well indexed as hexagonal Ba3Co2Fe24O41 phase (JCPDS 19-0097). The pattern of plate-like Co2Z
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powder shows the formation of single-phase Co2Z without any traces of unwanted or parasite phases. The peaks of (0014) and (0018) have higher intensities than other
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peaks, indicating that the surface of plate-like Co2Z particles are parallel to the (00l) plane and the plate-like Co2Z powder have a high degree of grain orientation. Figure 2 presents the FT-IR spectra of PVB/Co2Z composite, PVB/RGO
composite and pure PVB, respectively. Several intense absorption peaks at around 3100-3700 cm-1 and 1600-1700 cm-1 are originated from bending and stretching vibration of O-H in PVB matrix. The absorption peak at 2942 cm-1 is characterized by the stretching vibrations of C-H and CH2 groups, and peak at 1420 cm-1 corresponds
ACCEPTED MANUSCRIPT to the vibration of CH2 bond in PVB molecule. The spectrum of pure PVB shows many absorption peaks, which is similar with the previous results35, 36. These spectra are also similar with each other, which may be attributed to the low contents and weak
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IR absorptions of Co2Z and RGO phase. In addition, an apparent absorption peak at 582 cm-1 appears in the PVB/Co2Z composite, due to the stretching vibration of Fe-O in the PVB/Co2Z composite37.
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In order to further validate the phase composition, Raman spectra of the
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PVB/Co2Z composite, the PVB/RGO composite and pure PVB are shown in Figure 3. The characteristic peaks of C-H stretching vibration and CH2 bond of the PVB hydrocarbon backbone are clearly visible at 2958 cm-1 and 1451 cm-1, respectively. Two obvious peaks at 480 cm-1and 654 cm-1 can be detected in the PVB/Co2Z
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composite, which is consistent with the peak locations of the above FT-IR spectra due to the existence of asymmetrical centers molecule in PVB molecules38. Compared with pure PVB, the typical Raman peaks of RGO are observed at 1316 cm-1 (D-band)
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and 1596 cm-1 (G-band) in the PVB/RGO composite. The ID/IG of PVB/RGO
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composite (1.506) is higher than that of the common RGO in the relative reports39, 40, which may be attributed to the inner defects in the composite. The above results confirm the coexistence of PVB, RGO and Co2Z in the as-prepared PVB/RGO/Co2Z composite.
Figure 4 shows the SEM images to study the morphology of Co2Z powder and RGO powder before and after composting. Co2Z belongs to the anisotropic hexaferrites and the growth along the a or b axis is more prominent than that along the
ACCEPTED MANUSCRIPT c-axis. Therefore, it is sensible to assume a plate-like morphology. As shown in Figure 3a, the textured Co2Z particles have a plate-like morphology with the size of 200 µm~250 µm and the size of the random Co2Z powders ranges from 20 µm~100
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µm. Compared with the random Co2Z particles, most of the plate-like Co2Z particles are larger than the random Co2Z particles and have plate-like morphology.
Figure 3b shows that the as-synthesized RGO sample possesses a multilayered
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morphology, where the nanosheets are peeled from natural graphite powder, which is
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corresponded with the results of some literatures on RGO39, 40. As shown in Figure 3c, it is observed that the two phases coexist in the as-prepared PVB/Co2Z/RGO composite and the interface is very clear. Magnetic properties analysis.
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The field-dependent magnetization for PVB/Co2Z/RGO composite has been measured at room temperature, as shown in Figure 5. In this work, the composite is composed of a magnetic Co2Z layer and a nonmagnetic RGO layer. It can be easily
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seen that the M-H curves show a similar S-type shape and are both saturated under an
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applied magnetic field of 3 kOe. Table 1 summarizes the magnetic properties of the PVB/Co2Z composite and PVB/Co2Z/RGO composite. The value of saturation manetization (Ms) prompts a decrease from 30.63 to 20.45 emu·g-1, and coercivity (Hc) keeps roughly at the same value, which could be ascribed to the fact that the introduction of nonmagnetic RGO phase dilutes the magnetic properties and Co2Z ferrite is a soft magnetic material. The decrease of magnetic properties is beneficial to the decrease of magnetic loss, thereby achieving a preferable impedance match.
ACCEPTED MANUSCRIPT Dielectric and magnetic parameters. It is well known that complex permittivity ( εr =ε′- jε″) and permeability ( µr =µ′jµ″) of materials are the fundamental physical quantities for determining the
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microwave properties41, 42. In addition, the EM wave absorption property of materials is generally determined by the complex relative permittivity and permeability as well as both dielectric loss tangent (tan δε = ε″/ε′) and magnetic loss tangent (tan δµ =
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µ″/µ′)2. Figure 6 shows the electromagnetic parameters of the PVB/Co2Z and the
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PVB/Co2Z/RGO composite.
The real part (ε′, µ′) and imaginary part (ε″, µ″) of the relative complex permittivity and permeability for the Co2Z/RGO composites respectively present a similar tendency from 2 to 18 GHz, as shown in Figure 6a-d. Both the real part
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permittivity (ε′) and the imaginary part permittivity (ε″) of εr show a fluctuation in the 12-18 GHz range, where the real part permeability (µ′) and the imaginary part permeability (µ″) emerge likewise. The values of ε′ and ε″ for the PVB/Co2Z/RGO
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composite are in the ranges of 5.2-8.1 GHz and 1.0-2.9 GHz (Figure 6a-b),
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respectively. Compared with those of PVB/Co2Z composite ( ε′= 6.8-15.1, ε″= 0-1.7 ), the ε″ value for the PVB/Co2Z/RGO composite exhibits a strong peak in the 12~18 GHz range, indicating a resonance behavior, which is due to the fact that for the PVB/Co2Z/RGO composite the interface effect becomes significant43. In addition, RGO also possesses the chemically active surfaces, coupled with native defects and metallic character44. The variation of the complex permeability spectra in the range of 2-18 GHz is shown in Figure 6c-d. The curve of µ′ versus frequency exhibits a distinct
ACCEPTED MANUSCRIPT plateaus over 2-8 GHz and the plateaus of µ″ is over 12-18 GHz. And the µ′ and µ″ values of the PVB/Co2Z/RGO composite are lower than those of the PVB/Co2Z composite in a wide frequency range, which could be ascribed to the introduction of
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non-magnetic RGO, which is consistent with the above MS results. Figure 6e-f shows the dielectric loss tangent (tanδε) and magnetic loss tangent (tanδµ) of the PVB/Co2Z composite and the PVB/CO2Z/RGO composite. It is clear
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that the PVB/CO2Z/RGO composite possesses a far higher dielectric loss tangent than
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the PVB/Co2Z composite and the dielectric loss tangent exhibits a distinct plateaus over 12-18 GHz. The enhanced dielectric loss could be originated from the introduction of RGO and the enhanced interfacial polarization relaxation in the PVB/CO2Z/RGO composite. Compared with the PVB/Co2Z composite, the value of
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magnetic loss of the PVB/CO2Z/RGO composite exhibits an increase in the range of 12-18 GHz.
From Figure 7a, it is obvious that the dielectric loss tangent and magnetic loss
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tangent of the PVB/CO2Z/RGO composite have a very similar tendency in the 2~18
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GHz frequency range than those of the PVB/Co2Z composite and the PVB/RGO composite, indicating that the combination of Co2Z and RGO achieves an improved impedance match.
To clarify the polarization mode of the PVB/Co2Z/RGO composite, Cole-Cole
semicircles (ε′ versus ε″) of the PVB/Co2Z/RGO composite were employed. According to Debye relaxation law41, the relationship between ε′ and ε″ can be deduced that:
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+ " =
(1)
Where εs and ε∞ are the stationary permittivity and optical permittivity, respectively. And the plot of ε′ versus ε″ would be a single semicircle, which is usually denoted as
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the Cole-Cole semicircle, and each semicircle corresponds to one Debye relaxation process44. As shown in Figure 7b, four semicircles can be found in the as-synthesized PVB/Co2Z/RGO composite, corresponding to their relative complex permittivity and
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dielectric loss tangent. It has been reported that localized states near to the Fermi level
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could be created via introducing lattice defects45. The existence of defects in FLG (as also indicated by Raman analysis in this work) favors absorption of electromagnetic energy by the transition from contiguous states to Fermi level when the absorbing surface is irradiated with electromagnetic waves. The existence of residual
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oxygen-containing chemical species such as C-O in the composite can generate electric dipole polarization due to their ability to trap electrons between C and O atoms. Here, under an alternating electromagnetic field, electron motion hysteresis in
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these dipoles can induce additional polarization relaxation processes, which are
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favorable in enhancing microwave absorption46. Furthermore, due to the layered structure, surface charges accumulate at the interfaces between the two phases in the PVB/Co2Z/RGO composite and the space charge polarization arises in the inhomogeneous structure dielectric materials47. The contributions of magnetic hysteresis, domain-wall displacement, and eddy current loss to magnetic loss can be excluded in the present PVB/Co2Z/RGO composite35. In this case, magnetic hysteresis loss from irreversible magnetization is
ACCEPTED MANUSCRIPT negligible under a weak applied magnetic field. And the domain wall motion loss could occur only in the MHz frequency range rather than GHz frequency range, and thus the contribution of domain wall resonance could also be excluded43. Besides,
” ’ = 2 ⁄3
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eddy current loss could also be evaluated by the following equation40, 48: (2)
Where is the vaccum permeability and d is the sample thickness. According
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to the skin-effect criterion49, if magnetic loss exclusively arises from the eddy current loss effect, the values of remain constant as the frequency are varied. As can be seen
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from Figure 7c, the ” ’ plot of the PVB/Co2Z/RGO composite shows a strong resonance peak over 2-6 GHz and a weak peak over 6-18 GHz, indicating that eddy current effect contributes to magnetic loss50. Herein, the natural resonance loss
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may be the main contributor to result in strong magnetic loss abilities, implying enhanced EM wave absorption in the GHz frequency range51-53. 3.4 EM wave absorption properties.
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To evaluate the EM wave absorption properties of the PVB/Co2Z composite, the
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PVB/RGO composite and the PVB/Co2Z/RGO composite, based on the generalized transmission line theory52 and metal back-panel model53, their reflection losses (RL) can be calculated from the measured complex relative permittivity using the following equation48:
(%) RL dB = 20 log $%%&' *% $ ) &'
(3)
The normalized input impedance (Zin) was calculated by /
Z,- = Z . 0 tan h 56 0
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√: : ;
(4)
ACCEPTED MANUSCRIPT Where Z0 is the impedance of free space, c is the velocity of light in free space, f is the microwave frequency, d is the thickness of the absorber, εr and µr are the relative permittivity and permeability of the materials, respectively. Figure 8 shows
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the plots of the RL versus frequency of samples with various thicknesses in a wide frequency range. As a matrix, the mass ratio of PVB in the above composites is
unchanged and thereby may be negligible. The PVB/Co2Z/RGO composite exhibits a
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greatly enhanced EM wave absorption compared with the PVB/Co2Z composite and
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the PVB/RGO composite, and it can be observed that the maximum reflection losses of the PVB/Co2Z composite and the PVB/Co2Z/RGO composite shift towards lower frequencies with increasing the sample thickness. As shown in Figure 8a, the maximum of RL (RLmax) value of the PVB/Co2Z composite is -32.1 dB at 8.2 GHz
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with an optimal sample thickness of 3 mm. The RL value of the PVB/RGO composite increases with increasing the thickness at the same frequency, and reaches the maximum value of -26.8 dB at 7.7 GHz (Figure 8b). For the PVB/Co2Z/RGO
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composite, the RLmax value achieves -35.2 dB at 14.7 GHz with a thickness of only 2.0
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mm and a bandwidth corresponding to the reflection loss below -10 dB is 4.24 GHz (from 12 GHz to 12.64 GHz) (Figure 8c). Besides, It can be observed that the attenuation peaks would shift towards lower frequencies with increasing the thickness. It could be attributed to the phenomena of quarter-wavelength attenuation that the incident and reflected waves in the absorber are out of phase 180°, which leads to the reflected waves in the air-absorber interface totally cancelled54, 55. Herein, the above
ACCEPTED MANUSCRIPT results indicate that the PVB/Co2Z/RGO composite possesses an excellent EM wave absorption properties. The EM wave absorption properties of the related composites in literatures are
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summarized in Table 256-58. It can be seen that the pristine graphene, Co2Z and their composites were extensively studied in recent years. According to the comparison,
PVB/Co2Z/RGO composite presents great advantages in terms of low thickness and
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strong absorption and PVB may be a more suitable matrix in the preparation of wave
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absorbing composites. The current researches mainly focus on EM wave absorbing materials with thin thickness, lightweight, strong absorption and application in a wide frequency range. Hence, the as-prepared PVB/Co2Z/RGO composite is very promising to be used as an EM wave absorbing material with thin thickness in a wide
4. Conclusions
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frequency range.
In summary, the layered PVB/Co2Z/RGO composite has been successfully
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prepared by a simple tape casting process. The as-prepared composite exhibits a
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highly efficient EM wave attenuation in a wide frequency range, especially at higher frequencies. Due to its layered structure, EM wave pass successively through the two layer absorber, thereby resulting in the attenuation of EM wave. The RLmax value of PVB/Co2Z/RGO composite reaches -35.2 dB at 14.7 GHz and the effective absorption bandwidth corresponding to the reflection loss below -10 dB is 4.24 GHz with a thickness of only 2.0 mm. Our results reveal that the introduction of two-dimensional material RGO in this work achieve a preferable impedance match and thus the
ACCEPTED MANUSCRIPT PVB/Co2Z/RGO composite has been obtained with the features of thin thickness, strong absorption and wide absorption bandwidth as a new type of EM wave absorbing materials.
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Acknowledgements This work is supported by the National Natural Science Foundation of China
(Grant No.51402178, 51572159), the Science and Technology Foundation of Shaanxi
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Province (Grant no.2016JM5066), the Chinese Postdoctoral Science Foundation
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(Grant No.2015M582595) and State Education Ministry and the Academic Backbone Cultivation Program of Shaanxi University of Science and Technology (Grant No. XSG(4)003). References
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ACCEPTED MANUSCRIPT [48] M. Han, X. Yin, L. Kong, et al. Graphene-wrapped ZnO hollow spheres with enhanced electromagnetic wave absorption properties. J. Mater. Chem. A, 2 (2014) 16403-16409. [49] N.D. Wu, X.G. Liu, C.Y. Zhao, C.Y. Cui, A.L. Xia, Effects of particle size on
nanocapsules. J. Alloys Compd. 656 (2016) 628-634.
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the magnetic and microwave absorption properties of carbon-coated nickel
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CoxNi100-x nanoparticles encapsulated by curved graphite layers: Controlled in-situ metal-catalytic preparation and broadband microwave absorption. Nanoscale, 7 (2015) 17312-17319.
[52] R. Shang, Y. Zhang, L. Yan, et al. Static magnetic and microwave absorption
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properties of FeCo/Al2O3 composites synthesized by high-energy ball milling method. J. Phys. D Appl. Phys. 47 (2014) 162-172.
[53] R.S. Alam, M. Moradi, H. Nikmanesh, Influence of multi-walled carbon
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[54] R. Li, T. Wang, G. Tan, et al. Microwave absorption properties of oriented Pr2Fe17N3−δ, particles/paraffin composite with planar anisotropy. J. Alloys Compds. 586 (2014) 239-243. [55] C. Wang, X. Han, X. Zhang, et al. Controlled synthesis and morphology-dependent electromagnetic properties of hierarchical cobalt assemblies. J. Phys. Chem. C, 114 (2010) 3196-3203.
ACCEPTED MANUSCRIPT [56] H.F. Li, R.Z. Gong, L.R. Fan, et al. Synthesis, characterization and electromagnetic wave absorption properties of Z-Type hexaferrites prepared by molten salt method. Adv. Mater. Resear. 66 (2009) 69-72. [57] X.J. Zhang, G.S. Wang, W.Q. Cao, et al. Fabrication of multi-functional
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PVDF/RGO composites via a simple thermal reduction process and their enhanced
electromagnetic wave absorption and dielectric properties, Rsc Adv. 4 (2014) 19594. [58] J. Shen, K. Chen, L. Li, et al. Fabrication and microwave absorbing properties of
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(Z-type barium ferrite/silica)@polypyrrole composites. J. Alloys Compds. 615 (2014)
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488-495.
ACCEPTED MANUSCRIPT Figure Captions Scheme 1. Formation process of the PVB/Co2Z/RGO composite Fig.1 XRD patterns of the as-synthesized RGO, the as-synthesized Co2Z random
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powders and (001) plate-like Co2Z powders
Fig. 2 FTIR spectra of pure PVB, PVB/Co2Z composite and PVB/RGO composite
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Fig. 3 Raman spectra of pure PVB, PVB/Co2Z composite and PVB/RGO composite. Fig. 4 SEM images of (a) the mixture of Co2Z random powder and (001) plate-like
PVB/Co2Z/RGO composite
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oriented Co2Z powder, (b) the as-synthesized RGO nanosheets and (c)
Fig. 5 Magnetic hysteresis loops (M-H) of Co2Z powder and PVB/Co2Z/RGO composite
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Fig. 6 EM parameters of PVB/Co2Z composite, PVB/RGO composite and PVB/Co2Z/RGO composite (The real ε’ (a) and imaginary ε” (b) parts of the complex permittivity, (c) dielectric loss tan δε’real µ’ (d) and imaginary µ” (e) parts of the
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complex permeability, and (f) magnetic loss tan δ µ’) Fig. 7 (a) Dielectric loss factor and magnetic loss factor as a function of frequency for
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PVB/Co2Z/RGO composite. (b) µ” (µ’)-2 f-1 (representing eddy current loss) versus frequency of PVB/Co2Z composite, PVB/RGO composite and PVB/Co2Z/RGO composite.
Fig. 8 Reflection loss (RL) of (a) PVB/Co2Z composite; (b) PVB/RGO composite and (c) PVB/Co2Z/RGO composite
ACCEPTED MANUSCRIPT Table Caption Table 1 Magnetic parameters of the PVB/Co2Z composite and PVB/Co2Z/RGO composite.
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absorption properties in the recent literature
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Table 2 Co2Z, RGO and their composites, and their corresponding EM wave
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Scheme 1. Formation process of the PVB/Co2Z/RGO composite
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Fig.1 XRD patterns of the as-synthesized RGO, the as-synthesized Co2Z random
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powders and (001) plate-like Co2Z powders.
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Fig. 2 FTIR spectra of pure PVB, PVB/Co2Z composite and PVB/RGO composite.
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Fig. 3 Raman spectra of pure PVB, PVB/Co2Z composite and PVB/RGO composite.
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Fig. 4 SEM images of (a) the mixture of Co2Z random powder and plate-like oriented
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Co2Z powder, (b) the as-synthesized RGO nanosheets and (c) PVB/Co2Z/RGO
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composite.
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Fig. 5 Magnetic hysteresis loops (M-H) of (a) Co2Z powder and (b) PVB/Co2Z/RGO
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composite.
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Fig. 6 EM parameters of PVB/Co2Z composite, PVB/RGO composite and PVB/Co2Z/RGO composite (The real ε’ (a) and imaginary ε” (b) parts of the complex permittivity, (c) dielectric loss tan δε’real µ’ (d) and imaginary µ” (e) parts of the
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complex permeability, and (f) magnetic loss tan δ µ’).
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Fig. 7 (a) Dielectric loss factor and magnetic loss factor as a function of frequency for
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PVB/Co2Z/RGO composite; (b) Cole-Cole semicircles of PVB/Co2Z/RGO composite (ε’ versus ε”);(c) µ” (µ’)-2 f-1 (representing eddy current loss) versus frequency of
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PVB/Co2Z composite, PVB/RGO composite and PVB/Co2Z/RGO composite.
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Fig. 8 Reflection loss (RL) of (a) PVB/Co2Z composite; (b) PVB/RGO composite and
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(c) PVB/Co2Z/RGO composite.
ACCEPTED MANUSCRIPT Table 1 Magnetic parameters of the PVB/Co2Z composite and PVB/Co2Z/RGO composite. Ms (emu/g)
Mr (emu/g)
Hc (Oe)
PVB/Co2Z
30.63
4.61
158.74
PVB/Co2Z/RGO
20.45
3.12
118.24
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Sample
ACCEPTED MANUSCRIPT Table 2 Co2Z, RGO and their composites, and their corresponding EM wave absorption properties in the recent literature Matrix
Maximum
Optimum
Optimum
Bandwidth
RL value
thickness
frequency
(RL<-10 dB)
(dB)
(mm)
(GHz)
(GHz)
WAX
-24
3
9.5
PVB
-32.1
3
8.2
RGO
PVDF
-25.6
4
10.8
4.32
57
RGO
PVB
-26.8
4
7.7
3.1
This work
Co Z/SiO
PPY
-19.7
2
16.16
5.56
58
Co2Z/RGO
PVB
-35.2
2
14. 7
4.24
This work
Co Z 2
(001) flake texture Co Z
2
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2
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Layers
2
1.9
56
5.6
this work
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2
filled random Co Z
Ref.
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Sample
ACCEPTED MANUSCRIPT Highlights 1. Layered PVB/Co2Z/RGO composite was fabricated via tape casting process. 2. Traversing two layer absorber successively benefits to attenuation of EM wave. 3. An optimal RL as low as -35.2 dB is observed with the thickness of 2.0 mm.
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4. The introduction of RGO in this work achieve a preferable impedance match.
S0925-8388(17)31459-7
DOI:
10.1016/j.jallcom.2017.04.249
Reference:
JALCOM 41653
To appear in:
Journal of Alloys and Compounds
Received Date: 5 March 2017 Revised Date:
19 April 2017
Accepted Date: 22 April 2017
Please cite this article as: H. Yang, J. Dai, X. Liu, Y. Lin, F. Wang, P. Liu, Synthesis and enhanced microwave absorption properties of PVB/Co2Z/RGO layered composite, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.249. 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.
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Synthesis and Enhanced Microwave Absorption Properties of PVB/Co2Z/RGO Layered Composite
a
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Haibo Yang a,∗, Jingjing Dai a, Xiao Liu a, Ying Lin a, Fen Wang a, Peng Liu b
School of Materials Science and Engineering, Shaanxi University of Science and
DFH Statellite Co. LTD, Beijing, 100094, PR China
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b
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Technology, Xi’an, 710021, PR China
∗
Corresponding author. Tel: +86-29-86168688; Fax: +86-29-86168688; Email: [email protected]
ACCEPTED MANUSCRIPT ABSTRACT
Polyvinyl Butyral (PVB)/Co2Z/reduced graphene oxide (RGO) layered composite was successfully prepared by a simple casting process in order to enhance impedance
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match. PVB acts as a matrix in this composite. The layered structure makes electromagnetic wave pass through the two layers successively, thereby resulting in the attenuation of electromagnetic wave. The results show that the hybrid
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PVB/Co2Z/RGO composite exhibits a highly efficient microwave attenuation property
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compared with the PVB/Co2Z and PVB/RGO composite in a wide frequency range due to the dielectric loss caused by the introduction of RGO. For the as-prepared PVB/Co2Z/RGO composite, an optimal reflection loss (RL) as low as -35.2 dB can be observed at 14.7 GHz with an absorber thickness of only 2 mm and the effective
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absorption bandwidth corresponding to the reflection loss below -10 dB is 4.24 GHz. Findings indicate that the layered PVB/Co2Z/RGO composite might be a promising
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candidate for a lightweight electromagnetic wave absorbing material.
ACCEPTED MANUSCRIPT INTRODUCTION
With the rapid development of modern electronic information industry, electromagnetic (EM) wave absorbing materials with thin thickness, lightweight, and
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strong absorption in a wide frequency range have aroused growing attention1, since EM interference pollution arises from the rapidly expanding use of communication
devices working in the gigahertz range, such as mobile telephones, local area network
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systems, and radar systems2-5. Traditional microwave absorbing materials such as
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ferrite, magnetic metal powder, conductive polymer and carbon-based materials, etc., are difficult to satisfy the above demands6-10. Hence, exploiting new types of materials with high-efficiency EM wave absorption properties has become more and more urgent. To date, composites of the above materials are the ideal candidates in EM
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wave absorbing and shielding fields, such as (100-x)La1.5Sr0.5NiO4/xNiFe2O4 composite11, SiO2/flaky metal magnetic composite powder12, ZnO/ZrSiO4 composite ceramic13, PANI/CIP/Fe3O4 composite14, CNTs/Fe3O4 composite15,
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graphene/Fe3O4/SiO2/NiO nanosheet composite2, etc.
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Hexaferrites, classified into M-, W-, Y-, Z-, X- and U-type according to their chemical formulas and structures, have been widely used in microwave and millimeter wave devices as permanent magnets or gyromagnetic materials, e.g., in circulators, filters, isolators, inductors, and phase shifters16-19. Z-type hexagonal barium ferrites (Co2Z) with hexagonal planar structure can be applied in the gigahertz region due to their significant permeability, relatively high frequency response, strong magnetocrystalline anisotropy field and high cut-off frequency20-22. Some work
ACCEPTED MANUSCRIPT concerning the preparation of single-phase Co2Z, and its crystal structure, thermal stability and ferromagnetic resonance with cobalt as metallic ion has been widely reported23-25. However, there is little work regarding its EM wave absorption
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properties in a wide frequency range. High performance Co2Z particles have large permeability, which can be used as ferromagenetic phase in EM wave absorbing
composites. Moreover, two-dimensional (2D) materials have been proven to be of
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high performance, with regard to the EM wave absorbing and shielding capability
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over the years26-29. Graphene, with a typical 2D carbon nanostructure, has attracted much attention for its unique physical, chemical, and mechanical properites30. It possesses not only a stable structure but also a high specific surface area, and it is a prospective dielectric material owing to its higher electrical conductivity and
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permittivity2. The multilayered structure is also favorable to the scattering of EM waves and thereby can contribute to the EM wave absorption properties. With the intent of optimizing the impedance match, the multilayered
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PVB/Co2Z/RGO composite was fabricated by a novel tape casting process. The
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structure and EM wave absorption properties of the as-prepared composite were investigated. The results indicate that the synergistic effect of the two phases plays an essential role in the PVB/Co2Z/RGO composite, which exhibits an excellent impedance match and enhanced EM absorption properties in terms of both the maximum reflection loss value and the absorption bandwidth compared with the PVB/Co2Z composite and the PVB/RGO composite. EXPERIMENTAL
ACCEPTED MANUSCRIPT All the chemicals used in this work were purchased from Sinopharm Chemical Reagent Co., Ltd. China. They are of analytical reagent grade (purity 99%). Deionized water was used in all the experiments.
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Synthesis of Co2Z powder. The synthesis of Co2Z random powder was carried out by the conventional
solid-state ceramic process. BaCO3, CoO and Fe2O3 powders were initially weighed
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according to the stoichiometry of Ba3Co2Fe24O41 (Co2Z), After being mixed by ball
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milling thoroughly, the mixture were calcined at 1250 oC in air for 4 h to prepare single-phase random Co2Z powder. And the synthesis of plate-like Co2Z powder was mainly divided into the following stages. The first stage is to synthesize Ba2Co2Fe12O27 (Co2Y) powders. BaCO3, Fe2O3 and CoO powders were weighed
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according to the stoichiometry of Co2Y. Powders were first mixed by ball milling and then calcined at 1250°C in air for 2 h. Secondly, plate-like BaFe12O19 (BaM) precursors were prepared by the molten salt synthesis method (MSS)31. Plate-like
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BaM and Co2Y powders were then weighed in accordance to the following formula: Ba3Co2Fe24O41
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BaFe12O19+ Ba2Co2Fe12O27
(1)
NaCl salt was added to the mixture with a mass ratio of 1:1.5 and then mixing
was carried out in an ethanol solution with an electric stirrer for 5 h. After drying, the above mixture was heated at 1250 oC for 8 h in a tightly covered Al2O3 crucible. Finally, hot deionized water was used to remove NaCl from the product. Synthesis of PVB/Co2Z/RGO composite.
ACCEPTED MANUSCRIPT The random Co2Z powder were mixed thoroughly with a solvent (60 vol.% ethanol and 40 vol.% methyl-ethyl-ketone) and a dispersant (triethyl phosphate) in a ball mill for 4 h. Some binder (PVB) and plasticizer (polyethylene
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glycol/diethylophthalate) were added before ball milling the mixture again for 4 h. With respect to the mass of random Co2Z powders, 10 wt% plate-like Co2Z powder
was then added into the above mixture, which was ball milled with a slow speed for 2
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h to form a slurry for tape casting. The slurry including Co2Z was tape cast to form a
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sheet with a thickness of ~200 µm on a polyethylene film by a doctor blade apparatus. After drying, the first layer sheet was formed.
Graphite oxide was synthesized from natural graphite powder using a modified Hummers’ method, according to the literatures32, 33. The slurry including RGO was
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prepared by the same method and was tape cast to form the second sheet with a thickness of ~230 µm on the first layer by a doctor blade apparatus. After drying, the PVB/Co2Z/RGO composite was obtained, in which the mass ratio of PVB is 70 wt.%.
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The formation process of the PVB/Co2Z/RGO composite is shown in Scheme 1.
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Characterization.
The phase composition of the samples was analyzed by X-ray diffractometer
(XRD, D/max-2200, Rigaku, Japan) using Cu Kα radiation (λ=0.15418 nm). The IR spectrum of the samples was observed by Fourier transform infrared spectrometer (FTIR, Bruker VEC-TOR-22, Germany). The morphology of the samples was observed by field emission scanning electron microscopy (FE-SEM, Quanta 250FEG, FEI, USA). The magnetic hysteresis loops of the powders and composites were
ACCEPTED MANUSCRIPT measured by vibrating sample magnetometer (VSM) (Lake Shore 7410, USA). The electromagnetic parameters were analyzed using a HP8720ES vector network
RESULTS AND DISCUSSION Characterization of PVB/Co2Z/RGO composite.
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analyzer in the frequency range of 2-18 GHz.
The phase purity and crystal structure of the as-synthesized single-phase powder
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without PVB were characterized by XRD. The diffraction peak of the as-prepared
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RGO powder at 25.6° (Figure 1a) could be attributed to the graphite structure of short-range order in stacked grapheme sheets34. The XRD patterns over the range of 15° ≤ 2θ ≤ 70° also suggest that the as-synthesized Co2Z powders, including Co2Z random particles (Figure 1b) and plate-like Co2Z particles (Figure 1c), exhibit obvious
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diffraction peaks of Co2Z, and the detected diffraction peaks can be well indexed as hexagonal Ba3Co2Fe24O41 phase (JCPDS 19-0097). The pattern of plate-like Co2Z
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powder shows the formation of single-phase Co2Z without any traces of unwanted or parasite phases. The peaks of (0014) and (0018) have higher intensities than other
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peaks, indicating that the surface of plate-like Co2Z particles are parallel to the (00l) plane and the plate-like Co2Z powder have a high degree of grain orientation. Figure 2 presents the FT-IR spectra of PVB/Co2Z composite, PVB/RGO
composite and pure PVB, respectively. Several intense absorption peaks at around 3100-3700 cm-1 and 1600-1700 cm-1 are originated from bending and stretching vibration of O-H in PVB matrix. The absorption peak at 2942 cm-1 is characterized by the stretching vibrations of C-H and CH2 groups, and peak at 1420 cm-1 corresponds
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IR absorptions of Co2Z and RGO phase. In addition, an apparent absorption peak at 582 cm-1 appears in the PVB/Co2Z composite, due to the stretching vibration of Fe-O in the PVB/Co2Z composite37.
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In order to further validate the phase composition, Raman spectra of the
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PVB/Co2Z composite, the PVB/RGO composite and pure PVB are shown in Figure 3. The characteristic peaks of C-H stretching vibration and CH2 bond of the PVB hydrocarbon backbone are clearly visible at 2958 cm-1 and 1451 cm-1, respectively. Two obvious peaks at 480 cm-1and 654 cm-1 can be detected in the PVB/Co2Z
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composite, which is consistent with the peak locations of the above FT-IR spectra due to the existence of asymmetrical centers molecule in PVB molecules38. Compared with pure PVB, the typical Raman peaks of RGO are observed at 1316 cm-1 (D-band)
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and 1596 cm-1 (G-band) in the PVB/RGO composite. The ID/IG of PVB/RGO
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composite (1.506) is higher than that of the common RGO in the relative reports39, 40, which may be attributed to the inner defects in the composite. The above results confirm the coexistence of PVB, RGO and Co2Z in the as-prepared PVB/RGO/Co2Z composite.
Figure 4 shows the SEM images to study the morphology of Co2Z powder and RGO powder before and after composting. Co2Z belongs to the anisotropic hexaferrites and the growth along the a or b axis is more prominent than that along the
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µm. Compared with the random Co2Z particles, most of the plate-like Co2Z particles are larger than the random Co2Z particles and have plate-like morphology.
Figure 3b shows that the as-synthesized RGO sample possesses a multilayered
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morphology, where the nanosheets are peeled from natural graphite powder, which is
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corresponded with the results of some literatures on RGO39, 40. As shown in Figure 3c, it is observed that the two phases coexist in the as-prepared PVB/Co2Z/RGO composite and the interface is very clear. Magnetic properties analysis.
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The field-dependent magnetization for PVB/Co2Z/RGO composite has been measured at room temperature, as shown in Figure 5. In this work, the composite is composed of a magnetic Co2Z layer and a nonmagnetic RGO layer. It can be easily
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seen that the M-H curves show a similar S-type shape and are both saturated under an
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applied magnetic field of 3 kOe. Table 1 summarizes the magnetic properties of the PVB/Co2Z composite and PVB/Co2Z/RGO composite. The value of saturation manetization (Ms) prompts a decrease from 30.63 to 20.45 emu·g-1, and coercivity (Hc) keeps roughly at the same value, which could be ascribed to the fact that the introduction of nonmagnetic RGO phase dilutes the magnetic properties and Co2Z ferrite is a soft magnetic material. The decrease of magnetic properties is beneficial to the decrease of magnetic loss, thereby achieving a preferable impedance match.
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microwave properties41, 42. In addition, the EM wave absorption property of materials is generally determined by the complex relative permittivity and permeability as well as both dielectric loss tangent (tan δε = ε″/ε′) and magnetic loss tangent (tan δµ =
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µ″/µ′)2. Figure 6 shows the electromagnetic parameters of the PVB/Co2Z and the
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PVB/Co2Z/RGO composite.
The real part (ε′, µ′) and imaginary part (ε″, µ″) of the relative complex permittivity and permeability for the Co2Z/RGO composites respectively present a similar tendency from 2 to 18 GHz, as shown in Figure 6a-d. Both the real part
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permittivity (ε′) and the imaginary part permittivity (ε″) of εr show a fluctuation in the 12-18 GHz range, where the real part permeability (µ′) and the imaginary part permeability (µ″) emerge likewise. The values of ε′ and ε″ for the PVB/Co2Z/RGO
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composite are in the ranges of 5.2-8.1 GHz and 1.0-2.9 GHz (Figure 6a-b),
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respectively. Compared with those of PVB/Co2Z composite ( ε′= 6.8-15.1, ε″= 0-1.7 ), the ε″ value for the PVB/Co2Z/RGO composite exhibits a strong peak in the 12~18 GHz range, indicating a resonance behavior, which is due to the fact that for the PVB/Co2Z/RGO composite the interface effect becomes significant43. In addition, RGO also possesses the chemically active surfaces, coupled with native defects and metallic character44. The variation of the complex permeability spectra in the range of 2-18 GHz is shown in Figure 6c-d. The curve of µ′ versus frequency exhibits a distinct
ACCEPTED MANUSCRIPT plateaus over 2-8 GHz and the plateaus of µ″ is over 12-18 GHz. And the µ′ and µ″ values of the PVB/Co2Z/RGO composite are lower than those of the PVB/Co2Z composite in a wide frequency range, which could be ascribed to the introduction of
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non-magnetic RGO, which is consistent with the above MS results. Figure 6e-f shows the dielectric loss tangent (tanδε) and magnetic loss tangent (tanδµ) of the PVB/Co2Z composite and the PVB/CO2Z/RGO composite. It is clear
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that the PVB/CO2Z/RGO composite possesses a far higher dielectric loss tangent than
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the PVB/Co2Z composite and the dielectric loss tangent exhibits a distinct plateaus over 12-18 GHz. The enhanced dielectric loss could be originated from the introduction of RGO and the enhanced interfacial polarization relaxation in the PVB/CO2Z/RGO composite. Compared with the PVB/Co2Z composite, the value of
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magnetic loss of the PVB/CO2Z/RGO composite exhibits an increase in the range of 12-18 GHz.
From Figure 7a, it is obvious that the dielectric loss tangent and magnetic loss
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tangent of the PVB/CO2Z/RGO composite have a very similar tendency in the 2~18
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GHz frequency range than those of the PVB/Co2Z composite and the PVB/RGO composite, indicating that the combination of Co2Z and RGO achieves an improved impedance match.
To clarify the polarization mode of the PVB/Co2Z/RGO composite, Cole-Cole
semicircles (ε′ versus ε″) of the PVB/Co2Z/RGO composite were employed. According to Debye relaxation law41, the relationship between ε′ and ε″ can be deduced that:
ACCEPTED MANUSCRIPT −
+ " =
(1)
Where εs and ε∞ are the stationary permittivity and optical permittivity, respectively. And the plot of ε′ versus ε″ would be a single semicircle, which is usually denoted as
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the Cole-Cole semicircle, and each semicircle corresponds to one Debye relaxation process44. As shown in Figure 7b, four semicircles can be found in the as-synthesized PVB/Co2Z/RGO composite, corresponding to their relative complex permittivity and
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dielectric loss tangent. It has been reported that localized states near to the Fermi level
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could be created via introducing lattice defects45. The existence of defects in FLG (as also indicated by Raman analysis in this work) favors absorption of electromagnetic energy by the transition from contiguous states to Fermi level when the absorbing surface is irradiated with electromagnetic waves. The existence of residual
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oxygen-containing chemical species such as C-O in the composite can generate electric dipole polarization due to their ability to trap electrons between C and O atoms. Here, under an alternating electromagnetic field, electron motion hysteresis in
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these dipoles can induce additional polarization relaxation processes, which are
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favorable in enhancing microwave absorption46. Furthermore, due to the layered structure, surface charges accumulate at the interfaces between the two phases in the PVB/Co2Z/RGO composite and the space charge polarization arises in the inhomogeneous structure dielectric materials47. The contributions of magnetic hysteresis, domain-wall displacement, and eddy current loss to magnetic loss can be excluded in the present PVB/Co2Z/RGO composite35. In this case, magnetic hysteresis loss from irreversible magnetization is
ACCEPTED MANUSCRIPT negligible under a weak applied magnetic field. And the domain wall motion loss could occur only in the MHz frequency range rather than GHz frequency range, and thus the contribution of domain wall resonance could also be excluded43. Besides,
” ’ = 2 ⁄3
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eddy current loss could also be evaluated by the following equation40, 48: (2)
Where is the vaccum permeability and d is the sample thickness. According
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to the skin-effect criterion49, if magnetic loss exclusively arises from the eddy current loss effect, the values of remain constant as the frequency are varied. As can be seen
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from Figure 7c, the ” ’ plot of the PVB/Co2Z/RGO composite shows a strong resonance peak over 2-6 GHz and a weak peak over 6-18 GHz, indicating that eddy current effect contributes to magnetic loss50. Herein, the natural resonance loss
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may be the main contributor to result in strong magnetic loss abilities, implying enhanced EM wave absorption in the GHz frequency range51-53. 3.4 EM wave absorption properties.
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To evaluate the EM wave absorption properties of the PVB/Co2Z composite, the
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PVB/RGO composite and the PVB/Co2Z/RGO composite, based on the generalized transmission line theory52 and metal back-panel model53, their reflection losses (RL) can be calculated from the measured complex relative permittivity using the following equation48:
(%) RL dB = 20 log $%%&' *% $ ) &'
(3)
The normalized input impedance (Zin) was calculated by /
Z,- = Z . 0 tan h 56 0
7 8
√: : ;
(4)
ACCEPTED MANUSCRIPT Where Z0 is the impedance of free space, c is the velocity of light in free space, f is the microwave frequency, d is the thickness of the absorber, εr and µr are the relative permittivity and permeability of the materials, respectively. Figure 8 shows
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the plots of the RL versus frequency of samples with various thicknesses in a wide frequency range. As a matrix, the mass ratio of PVB in the above composites is
unchanged and thereby may be negligible. The PVB/Co2Z/RGO composite exhibits a
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greatly enhanced EM wave absorption compared with the PVB/Co2Z composite and
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the PVB/RGO composite, and it can be observed that the maximum reflection losses of the PVB/Co2Z composite and the PVB/Co2Z/RGO composite shift towards lower frequencies with increasing the sample thickness. As shown in Figure 8a, the maximum of RL (RLmax) value of the PVB/Co2Z composite is -32.1 dB at 8.2 GHz
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with an optimal sample thickness of 3 mm. The RL value of the PVB/RGO composite increases with increasing the thickness at the same frequency, and reaches the maximum value of -26.8 dB at 7.7 GHz (Figure 8b). For the PVB/Co2Z/RGO
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composite, the RLmax value achieves -35.2 dB at 14.7 GHz with a thickness of only 2.0
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mm and a bandwidth corresponding to the reflection loss below -10 dB is 4.24 GHz (from 12 GHz to 12.64 GHz) (Figure 8c). Besides, It can be observed that the attenuation peaks would shift towards lower frequencies with increasing the thickness. It could be attributed to the phenomena of quarter-wavelength attenuation that the incident and reflected waves in the absorber are out of phase 180°, which leads to the reflected waves in the air-absorber interface totally cancelled54, 55. Herein, the above
ACCEPTED MANUSCRIPT results indicate that the PVB/Co2Z/RGO composite possesses an excellent EM wave absorption properties. The EM wave absorption properties of the related composites in literatures are
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summarized in Table 256-58. It can be seen that the pristine graphene, Co2Z and their composites were extensively studied in recent years. According to the comparison,
PVB/Co2Z/RGO composite presents great advantages in terms of low thickness and
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strong absorption and PVB may be a more suitable matrix in the preparation of wave
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absorbing composites. The current researches mainly focus on EM wave absorbing materials with thin thickness, lightweight, strong absorption and application in a wide frequency range. Hence, the as-prepared PVB/Co2Z/RGO composite is very promising to be used as an EM wave absorbing material with thin thickness in a wide
4. Conclusions
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frequency range.
In summary, the layered PVB/Co2Z/RGO composite has been successfully
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prepared by a simple tape casting process. The as-prepared composite exhibits a
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highly efficient EM wave attenuation in a wide frequency range, especially at higher frequencies. Due to its layered structure, EM wave pass successively through the two layer absorber, thereby resulting in the attenuation of EM wave. The RLmax value of PVB/Co2Z/RGO composite reaches -35.2 dB at 14.7 GHz and the effective absorption bandwidth corresponding to the reflection loss below -10 dB is 4.24 GHz with a thickness of only 2.0 mm. Our results reveal that the introduction of two-dimensional material RGO in this work achieve a preferable impedance match and thus the
ACCEPTED MANUSCRIPT PVB/Co2Z/RGO composite has been obtained with the features of thin thickness, strong absorption and wide absorption bandwidth as a new type of EM wave absorbing materials.
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Acknowledgements This work is supported by the National Natural Science Foundation of China
(Grant No.51402178, 51572159), the Science and Technology Foundation of Shaanxi
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Province (Grant no.2016JM5066), the Chinese Postdoctoral Science Foundation
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(Grant No.2015M582595) and State Education Ministry and the Academic Backbone Cultivation Program of Shaanxi University of Science and Technology (Grant No. XSG(4)003). References
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488-495.
ACCEPTED MANUSCRIPT Figure Captions Scheme 1. Formation process of the PVB/Co2Z/RGO composite Fig.1 XRD patterns of the as-synthesized RGO, the as-synthesized Co2Z random
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powders and (001) plate-like Co2Z powders
Fig. 2 FTIR spectra of pure PVB, PVB/Co2Z composite and PVB/RGO composite
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Fig. 3 Raman spectra of pure PVB, PVB/Co2Z composite and PVB/RGO composite. Fig. 4 SEM images of (a) the mixture of Co2Z random powder and (001) plate-like
PVB/Co2Z/RGO composite
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oriented Co2Z powder, (b) the as-synthesized RGO nanosheets and (c)
Fig. 5 Magnetic hysteresis loops (M-H) of Co2Z powder and PVB/Co2Z/RGO composite
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Fig. 6 EM parameters of PVB/Co2Z composite, PVB/RGO composite and PVB/Co2Z/RGO composite (The real ε’ (a) and imaginary ε” (b) parts of the complex permittivity, (c) dielectric loss tan δε’real µ’ (d) and imaginary µ” (e) parts of the
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complex permeability, and (f) magnetic loss tan δ µ’) Fig. 7 (a) Dielectric loss factor and magnetic loss factor as a function of frequency for
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PVB/Co2Z/RGO composite. (b) µ” (µ’)-2 f-1 (representing eddy current loss) versus frequency of PVB/Co2Z composite, PVB/RGO composite and PVB/Co2Z/RGO composite.
Fig. 8 Reflection loss (RL) of (a) PVB/Co2Z composite; (b) PVB/RGO composite and (c) PVB/Co2Z/RGO composite
ACCEPTED MANUSCRIPT Table Caption Table 1 Magnetic parameters of the PVB/Co2Z composite and PVB/Co2Z/RGO composite.
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absorption properties in the recent literature
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Table 2 Co2Z, RGO and their composites, and their corresponding EM wave
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Scheme 1. Formation process of the PVB/Co2Z/RGO composite
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Fig.1 XRD patterns of the as-synthesized RGO, the as-synthesized Co2Z random
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powders and (001) plate-like Co2Z powders.
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Fig. 2 FTIR spectra of pure PVB, PVB/Co2Z composite and PVB/RGO composite.
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Fig. 3 Raman spectra of pure PVB, PVB/Co2Z composite and PVB/RGO composite.
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Fig. 4 SEM images of (a) the mixture of Co2Z random powder and plate-like oriented
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Co2Z powder, (b) the as-synthesized RGO nanosheets and (c) PVB/Co2Z/RGO
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composite.
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Fig. 5 Magnetic hysteresis loops (M-H) of (a) Co2Z powder and (b) PVB/Co2Z/RGO
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composite.
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Fig. 6 EM parameters of PVB/Co2Z composite, PVB/RGO composite and PVB/Co2Z/RGO composite (The real ε’ (a) and imaginary ε” (b) parts of the complex permittivity, (c) dielectric loss tan δε’real µ’ (d) and imaginary µ” (e) parts of the
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complex permeability, and (f) magnetic loss tan δ µ’).
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Fig. 7 (a) Dielectric loss factor and magnetic loss factor as a function of frequency for
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PVB/Co2Z/RGO composite; (b) Cole-Cole semicircles of PVB/Co2Z/RGO composite (ε’ versus ε”);(c) µ” (µ’)-2 f-1 (representing eddy current loss) versus frequency of
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PVB/Co2Z composite, PVB/RGO composite and PVB/Co2Z/RGO composite.
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Fig. 8 Reflection loss (RL) of (a) PVB/Co2Z composite; (b) PVB/RGO composite and
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(c) PVB/Co2Z/RGO composite.
ACCEPTED MANUSCRIPT Table 1 Magnetic parameters of the PVB/Co2Z composite and PVB/Co2Z/RGO composite. Ms (emu/g)
Mr (emu/g)
Hc (Oe)
PVB/Co2Z
30.63
4.61
158.74
PVB/Co2Z/RGO
20.45
3.12
118.24
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Sample
ACCEPTED MANUSCRIPT Table 2 Co2Z, RGO and their composites, and their corresponding EM wave absorption properties in the recent literature Matrix
Maximum
Optimum
Optimum
Bandwidth
RL value
thickness
frequency
(RL<-10 dB)
(dB)
(mm)
(GHz)
(GHz)
WAX
-24
3
9.5
PVB
-32.1
3
8.2
RGO
PVDF
-25.6
4
10.8
4.32
57
RGO
PVB
-26.8
4
7.7
3.1
This work
Co Z/SiO
PPY
-19.7
2
16.16
5.56
58
Co2Z/RGO
PVB
-35.2
2
14. 7
4.24
This work
Co Z 2
(001) flake texture Co Z
2
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2
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2
1.9
56
5.6
this work
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ACCEPTED MANUSCRIPT Highlights 1. Layered PVB/Co2Z/RGO composite was fabricated via tape casting process. 2. Traversing two layer absorber successively benefits to attenuation of EM wave. 3. An optimal RL as low as -35.2 dB is observed with the thickness of 2.0 mm.
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4. The introduction of RGO in this work achieve a preferable impedance match.