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indium tin oxide composites
Accepted Manuscript Effects of calcination temperature on the electromagnetic properties of carbon nanotubes/indium tin oxide composites Chaoqun Ge, Liuying Wang, Gu Liu, Renbing Wu PII:
S0925-8388(18)33764-2
DOI:
10.1016/j.jallcom.2018.10.098
Reference:
JALCOM 47916
To appear in:
Journal of Alloys and Compounds
Received Date: 11 October 2017 Revised Date:
25 August 2018
Accepted Date: 9 October 2018
Please cite this article as: C. Ge, L. Wang, G. Liu, R. Wu, Effects of calcination temperature on the electromagnetic properties of carbon nanotubes/indium tin oxide composites, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.10.098. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
Effects of calcination temperature on the electromagnetic
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properties of carbon nanotubes/indium tin oxide composites
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Abstract
Carbon nanotubes (CNTs)/Indium tin oxide (ITO) composites have
been successfully synthesized via coprecipitation and calcination processes. The effects
6
of calcination temperature on the electromagnetic properties of the CNTs/ITO
7
composites were investigated. The real part of the permittivity and the dielectric loss
8
increase with increasing calcination temperature, which can be attributed to the
9
enhanced dielectric relaxation and space charge polarization. The CNTs/ITO
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composites synthesized at 600 °C can achieve lower reflection loss (RL) value at the
11
thickness of 2–4 mm, while the composites synthesized at 850 °C exhibit broader
12
bandwidth, corresponding to an RL value below −10 dB at a thin coating thickness of 1–
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1.5 mm. The microwave absorption properties of the CNTs/ITO composites at different
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frequencies can be tuned by controlling the calcination temperature and the thickness of
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the absorbing coating. The dielectric loss, quarter-wavelength cancellation, and
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well-matched characteristic impedance in the air-absorber interface are believed to
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contribute to the superior microwave absorption performance of the composites. The
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effects of the secondary phase ITO on the electromagnetic properties of the CNTs/ITO
19
composites were also discussed in this study.
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temperature, Microwave absorption.
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Keywords: Carbon nanotubes, Indium tin oxide, Coprecipitation, Calcination
1. Introduction
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Microwave absorption materials (MAMs) have become effective means to mitigate
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civil electromagnetic pollution and enhance the survival and attack capability of
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military weapons nowadays. Research on MAMs has attracted considerable attention in
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recent years. Ideal MAMs should be lightweight, thin thickness, and exhibit wide band
28
absorption as well as strong absorption properties [1]. Therefore, the microwave
29
absorption properties of various absorbers have been widely investigated [2-20].
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Carbon-based materials, such as graphite, graphene, carbon fibers, carbon black, and
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carbon nanotubes (CNTs), are typical lightweight MAMs which present high complex
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ACCEPTED MANUSCRIPT permittivity values due to their superior electric conductive loss and relaxation loss [21,
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22]. Among them, CNTs are of particular interest because of their excellent mechanical
34
[23], electrical [24], thermal [25], and optical [26] properties, as well as their
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outstanding dielectric/electrical loss and low percolation threshold [8]. Nevertheless,
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their poor microwave impedance matching, low absorption capabilities, and narrow
37
absorption bandwidth limited their practical applications. The strategy of introducing a
38
second phase, such as magnetic metals or metal oxides, into CNTs has been employed
39
to improve the absorption performance for both dielectric and magnetic loss [8-20]. In
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addition, combinations of CNTs with semiconductors, such as ZnO [11, 12], SnO2
41
[27-29], and TiO2 [30, 31], have also been studied as efficient MAMs, which has
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revealed that the use of composites resulted in increasing of the impedance matching
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and dielectric loss, and greatly enhanced their microwave absorption performance.
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As an n-type semiconductor, indium tin dioxide (ITO) has attracted significant
45
attention duo to its good optical performance, high infrared reflectivity, low resistivity,
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and a favorable thermal stability [32-36]. Meng et al. [37] prepared an Ni/ITO
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nanocomposite via an in situ powder-separating reductive process, demonstrating
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enhanced microwave absorption properties due to the proper electromagnetic
49
impedance matching and facilitation of extra interfacial polarization. Fu et al. [38]
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synthesized FeNi3/ITO composite via a self-catalyzed reduction method and a sol–gel
51
process, and revealed that the microwave absorption properties are significantly
52
dependent on the content of the ITO phase. Duong et al. [39] introduced 30~40 nm
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thick ITO thin films at the interface between TiO2 and single-walled CNTs using
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nanocluster deposition. Kim et al. [40] fabricated a ternary nanomaterial based on
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sulfur-impregnated multi-walled CNTs filled with ordered SnO nanoparticles using a
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dry plasma reduction method. However, to the best of our knowledge, the combination
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of CNTs and ITO as MAMs has rarely been explored. The only known research was the
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investigation of the electrical conductivity of a 1D CNTs/ITO nanocomposite, which
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was fabricated using a typical coprecipitation process [41].
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Herein, we synthesized CNTs/ITO core–shell nanostructured composites as MAMs.
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Coating CNTs with ITO is beneficial for the enhancement of the uniformity to maintain
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fine dispersity on the composites, as well as the interface of the heterostructure. It is
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known that the presence of multi-interfaces in composites results in significant
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ACCEPTED MANUSCRIPT interfacial polarization, which would have an important effect on the increase in
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dielectric loss [42]. Moreover, the impedance matching of CNTs can be modulated by
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doping ITO at the surface of CNTs, which would reduce the reflection of microwaves.
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Furthermore, as a type of dielectric MAMs, the CNTs/ITO composites have better
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thermal stability compared to magnetic or dielectric/magnetic MAMs. Therefore, we
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assume that the CNTs/ITO composites should be high-performance MAMs.
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Compared to the methods for preparing CNTs/ITO composites, including the
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reduction process [37, 38], nanocluster deposition [39], and plasma reduction [40, 43],
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the coprecipitation and calcination process has the unique advantages of simplicity in
73
equipment, facilitation in processing and suitability for applications. In this work, we
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have successfully prepared core-shell structured CNTs/ITO composites through
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coprecipitation and calcination processes. The influence of the calcination temperature
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on the electromagnetic properties of the composites was studied. The CNTs/ITO
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composites synthesized at 600 and 850 °C exhibited excellent microwave absorption
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properties. Moreover, the electromagnetic properties of the composites can be tuned by
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varying the calcination temperature and thickness of the absorber coating. The
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microwave absorption mechanism and effects of the secondary ITO phase on the
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electromagnetic properties of the composites were discussed.
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2. Experimental
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2.1. Materials
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Multi-walled CNTs were supplied by Chengdu Organic Chemical Co., Ltd, China.
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The diameters of the CNTs ranged from 10 to 30 nm, their lengths were in the 10–200
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µm range, and their purity was 95%. All of the chemicals and reagents were purchased
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from Sinopharm Chemical Reagent Co., Ltd, China and used without further
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purification.
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2.2. Synthesis of CNTs/ITO composites
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The raw CNTs were pretreated using a typical method. First, 5 g of CNTs and 250
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mL of nitric acid (68 wt%) were dispersed in a flask and maintained at 90 °C under
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refluxing and constant stirring for 18 h. The suspension was then rinsed with deionized
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water until a neutral pH was achieved and dried in a vacuum oven at 80 °C for 24 h.
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The starting materials, 5.864 g of InCl3·4H2O, 0.738 g of SnCl4·5H2O, and 1.542
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g of pretreated CNTs, were dissolved in 100 mL of deionized water, followed by
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ACCEPTED MANUSCRIPT ultrasonication for 1 h. Subsequently, ammonia solution (10 wt%) was added to the
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continuously stirred bath until a pH 6.5 was achieved, and then it was dropped slowly to
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a pH of 8.5. The bath temperature was maintained at 60 °C. After aging for 4 h, the
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black suspension was centrifuged at 4000 rpm for 8 min, thus forming the CNTs/ITO
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precursor. The precursor was washed with deionized water and absolute ethyl alcohol
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until no chloride ions were detected in the solution and then dried at 80 °C in a vacuum
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oven for 8 h. Finally, the dried precursor was calcined in vacuum in a tube furnace at
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various temperatures (350, 600, and 850 °C) for 3 h to obtain the CNTs/ITO
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composites.
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2.3. Characterization
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The products were characterized by X-ray diffraction (XRD,▼Rigaku D/max-2400)
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with Cu Kα radiation at 40 kV and 40 mA. The morphology and structure of the
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products were investigated by transmission electron microscopy (TEM) and
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selected-area electron diffraction (SAED) using a JEOL JEM3010 electron microscope.
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The frequency dependence of the complex permittivity ( εr = ε ′ − jε ′′ ) and permeability
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( µr = µ′ − jµ′′ ) of the CNTs/ITO composites were measured by the transmission and
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reflection (T/R) coaxial line method using an HP 8720ES vector network analyzer in a
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frequency range of 2–18 GHz at room temperature. The samples were prepared by
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uniformly mixing paraffin with the as-prepared CNTs/ITO composites. The mixture
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was pressed into a coaxial clapper with an outer diameter of 7.0 mm and an inner
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diameter of 3.0 mm. The CNTs/ITO composites were used as control. Throughout the
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measurements, the composites were dispersed in the paraffin matrix at a concentration
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of 15 wt%.
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3. Results and discussion
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3.1 Structure and morphology
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The XRD patterns of the CNTs/ITO composites prepared at different temperatures
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are presented in Fig. 1. The characteristic peak of CNTs is not obviously present,
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possibly due to the low CNTs content and enhanced diffraction peak intensity of ITO.
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The peak at 26.1° is assigned to the (002) plane of CNTs. Fig. 1a illustrates that the
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diffraction peaks of the precursor are very disordered, and many amorphous scattering
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peaks are observed. The diffraction peaks at 22.3°, 45.6°, and 69.5° correspond to the
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ACCEPTED MANUSCRIPT cubic In(OH)3 (JCPDS card No. 16-0161). Furthermore, orthorhombic InOOH (JCPDS
128
card No. 17-0549) is also detected in the precursor. Fig. 1b shows the XRD patterns of
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CNTs/ITO composites fabricated at different temperatures. The distinct peaks at 21.5°,
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30.6°, 35.5°, 51.0°, and 60.7° are assigned to the (211), (222), (400), (440), and (622)
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planes of cubic phase In2O3 (JCPDS card No. 06-0416), respectively. In addition, the
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absence of characteristic peaks associated with SnO2 suggests that the Sn4+ ions are
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homogeneously doped into the crystal lattice of cubic-phase In2O3 under the current
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conditions. The Sn4+ ion replaces and occupies the position of the In3+ ion in the ITO
135
crystal to generate a free electron, which can enhance the electrical conductivity of ITO.
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Notably, only the cubic-phase ITO is detected in our case because the cubic-phase is
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thermodynamically preferred at temperatures higher than 550 °C and the rhombohedral
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phase is favored at lower temperatures [41].
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(a)
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Intensity (a.u.)
CNTs Cubic In(OH)3 Orthorhombic InOOH
20
30
40
50
60
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2θ/(°)
(b)
Intensity (a.u.)
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CNTs Cubic In2O3
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600 350
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30
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50
60
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2θ/(°)
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Fig. 1. XRD patterns of (a) the precursor of CNTs/ITO composites and (b) the
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CNTs/ITO composites synthesized at different temperatures.
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The morphology and microstructure of the CNTs/ITO composites synthesized at 5
ACCEPTED MANUSCRIPT 600 °C were investigated by TEM. Fig. 2a shows the pretreated CNTs, which present
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clean walls and smooth surfaces without visible residual metal catalysts. Fig. 2b and c
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show the TEM images of the CNTs/ITO composites. A large quantity of ITO particles
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with relatively uniform size are distributed on the surfaces of CNTs and are clearly
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observed. The good distribution of the components in the composite is believed to be
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beneficial for impedance matching to achieve excellent microwave absorption
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properties [44]. Fig. 2d demonstrates the SAED pattern of the circled area in Fig. 2c. A
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series of labeled diffraction rings are indexed to the (222) plane of CNTs and the (211),
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(222), (400), (440), and (622) planes of cubic-phase ITO, which are consistent with the
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XRD patterns shown in Fig. 1b.
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Fig. 2. TEM images of (a) pretreated CNTs, (b) (c) CNTs/ITO composites synthesized
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at 600 °C, and (d) the corresponding SAED patterns.
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3.2 Electromagnetic and absorbing properties
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The microwave electrical conductivity of a dielectric material can be evaluated by
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using the equation σ =2π f ε 0ε ′′ , where σ is the electric conductivity (S/m), ε0 is the free 6
ACCEPTED MANUSCRIPT space permittivity (8.854×10−12 F/m), and f is the frequency of the electromagnetic
160
wave [45]. According to the calculation results shown in Fig. S1, the average
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microwave electrical conductivity values of the CNTs/ITO composites prepared at 350,
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600, and 850 °C across the test band are 0.539, 1.134, and 5.337 S/m, respectively. The
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calcination temperatures increase the electrical conductivity of the composites at
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microwave frequencies, which can be translated into the increasing dielectric loss
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( tanδε =ε′ ε′ ). (a)
350 600 850
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(b)
350 600 850
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Imaginary part of permittivity
Frequency /GHz
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0
2
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Frequency /GHz
Fig. 3. (a) Real and (b) imaginary parts of the complex permittivity of the CNTs/ITO composites.
Fig. 3 shows the complex permittivity of the CNTs/ITO composites synthesized at
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different temperatures in the frequency range of 2–18 GHz. The real and imaginary
171
parts of the complex permittivity demonstrate similar variation trends, which decreases
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slightly with increasing frequency. With the increasing calcination temperature, the real
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and imaginary parts of the complex permittivity increase gradually. The higher
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calcination temperature can improve the crystallization and electrical conductivity of the
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CNTs/ITO composites, leading to a gradual increase of the imaginary part of the 7
ACCEPTED MANUSCRIPT complex permittivity. Moreover, the increase in the real part of the complex permittivity
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can be attributed to the enhanced dielectric relaxation and space charge polarization.
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The real and imaginary parts of the relative complex permittivity represent the storage
179
and loss capability in response to the microwave energy, respectively, and a rise in the
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imaginary part is attributed to the microwave absorption [46]. In addition, the complex
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permeability of the CNTs/ITO composites is illustrated in Fig. S2. Due to the absence
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of magnetic constituents, the real and imaginary parts of the complex permeability are
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approximately 1 and 0, respectively. 1.0
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(a)
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Frequency /GHz
Fig. 4. (a) Dielectric loss and (b) impedance matching ratio of the CNTs/ITO
The
tan δε
composites.
values of the CNTs/ITO composites were calculated, as shown in Fig. tan δε
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4a. The calcination temperature has a great effect on the
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composites. The
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similar fluctuations. For the composites synthesized at 850 °C, the
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relatively high in the full test frequency. The
tan δε
of the CNTs/ITO
values of the composites synthesized at 350 and 600 °C exhibit
tan δε
8
tan δε
value is
increases with increasing calcination
ACCEPTED MANUSCRIPT 192
temperature, indicating that the polarization and leakage current will take place when
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exposed to electromagnetic wave radiation, and thus the microwave absorption
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properties of the composites can be improved.
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The increase in the complex permittivity and
tan δε
cause the improvement in tan δε
microwave absorption. Nevertheless, a larger permittivity and
are not always
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favorable for microwave absorption. Single-layer absorbing coating should meet two
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criteria to achieve good microwave absorption properties. First, the incident
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electromagnetic wave can enter the interior of the absorbing coating as much as possible
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without reflection, suggesting that the impedances of the absorbing materials and free
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space should match well. Second, the absorbing materials have the ability to absorb and
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lose the incident electromagnetic wave, suggesting that the dielectric loss and magnetic
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loss of the absorbing materials should be as high as possible [47, 48]. Consequently, the
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dielectric loss and microwave impedance of the dielectric MAMs play dual roles and
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will ultimately affect the microwave absorption properties. The microwave impedance η
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is expressed using the equation η = Z 0 µ r ( ε ′ − jε ′′ ) , where Z0 is the free-space wave
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impedance (∼377 Ω) [1]. To effectively absorb microwaves, the impedance matching
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ratio (η/Z0) should exceed 0.3 to allow the incident microwaves to efficiently enter the
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absorber with little reflection at the air–absorber interface [49].
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Fig. 4b shows the impedance matching ratio of the CNTs/ITO composites
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synthesized at different calcination temperatures. As shown in the figure, the impedance
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matching ratio of the CNTs/ITO composites declines gradually with increasing
213
calcination temperature. The composites obtained at 350 and 600 °C exhibit relatively
214
high impedance matching ratios, while the impedance matching ratio of the composite
215
obtained at 850 °C is smaller than 0.3 (~0.2) in the frequency range of 2–18 GHz. This
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is mainly due to the sharp increase in the electrical conductivity of the composites,
217
which results in the reduction of the skin depth (the skin depth of the composites is
218
presented in SI 1 and Fig. S3 in the Supplementary material file) [45], leading to the
219
relatively poor impedance matching. As a result, electromagnetic wave is able to
220
propagate into the CNTs/ITO composites obtained at 350 and 600 °C, while only a
221
small amount of the microwaves is reflected back into the air.
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Increasing the calcination temperature can effectively enhance the dielectric loss of
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ACCEPTED MANUSCRIPT 223
the CNTs/ITO composites. However, the impedance matching ratio also reduces
224
significantly. Therefore, neither
225
describe the microwave absorption properties alone. Thus, estimation of the reflection
226
loss (RL) requires further in-depth considerations. To clarify the microwave absorption
227
properties, the RL values for a single-layer absorber were evaluated according to the
228
transmission line theory using the following equations: [50, 51]
229
RL = 20 log ( Z in − Z 0 )
230
Z in = Z 0 µ r ε r tanh j ( 2π fd c ) µ r ε r
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where Zin is the input characteristic impedance of the absorber, c is the velocity of the
232
electromagnetic wave in vacuum, and d is the thickness of the absorber. Considering the
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weak magnetic properties of the composites, the complex permeability is considered to
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be 1.
nor the impedance matching ratio can directly
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( Z in + Z 0 )
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Fig. 5 shows the contour plots of the calculated RL of the CNTs/ITO composites
236
synthesized at different temperatures for various thicknesses in the frequency range of
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2–18 GHz. An RL value below −10 dB, corresponding to 90% attenuating of the
238
incident electromagnetic wave, is considered to be an ideal absorption value. As shown
239
in Fig. 5a, RL of the CNTs/ITO composites synthesized at 350 °C reaches the minimum
240
value of −7.8 dB at 14.24 GHz at a thickness of 2.2 mm. The value can hardly reach
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−10 dB within the thickness range of 0.5–4.0 mm, which will limit its practical
242
applications. As the calcination temperature increased to 600 °C (Fig. 5b), RL values
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exceeding −10 dB are reached in the range of 6.16–8.88 GHz, 9.36–11.74 GHz, and
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12.64–14.48 GHz, with absorber thicknesses of 1.7–4.0 mm. The minimum RL of
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−38.29 dB is obtained at 13.28 GHz with a thickness of 1.95 mm. For the CNTs/ITO
246
composite synthesized at 850 °C (Fig. 5c), RL values smaller than −10 dB are found in
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the wide frequency range of 8.4–18 GHz with thicknesses of 0.94–2.0 mm. The
248
minimum RL of −38.64 dB is achieved at 17.76 GHz with the matching thickness of
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1.03 mm. Although the impedance matching condition is relatively poor, the results
250
indicate that the CNTs/ITO composite synthesized at 850 °C demonstrates excellent
251
microwave absorption properties in a high-frequency range, when the thickness of the
252
absorber is smaller than 2 mm (Fig. S4). These results can be explained by the
253
relationship between the frequency dependent skin depth, attenuation constant, and RL
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ACCEPTED MANUSCRIPT of the CNTs/ITO composite synthesized at 850 °C, as presented in SI 2 [1] and Fig. S5
255
in the Supplementary materials. First, the attenuation constant of the CNTs/ITO
256
composite synthesized at 850 °C increased significantly, especially in the frequency
257
range of 9–18 GHz (Fig. S6), indicating a strong electromagnetic wave attenuation
258
ability in this frequency range. Second, as shown in Fig. S5, the skin depth of the
259
composite ranges from 1.5 to 2.35 mm in the frequency range of 9–18 GHz, which
260
means that the electromagnetic waves can propagate into composite at least 1.5 mm
261
thick in this frequency range. These two factors attribute the CNTs/ITO composite
262
synthesized at 850 °C with excellent microwave absorption properties in the frequency
263
range of 9–18 GHz with thicknesses of 1–1.5 mm.
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2.0
-30
2.2 mm, 14.24 GHz, -7.8 dB
1.5
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Reflection Loss /dB
-10
3.0
-40
1.0
-45
0.5 4
6
8
10
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2
12
14
16
18
Frequency /GHz
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4.0
0
(b)
-5
3.5
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-20
2.5
-25
2.0
-30 -35
1.5
AC C
Thickness /mm
-15
1.95 mm, 13.28 GHz, -38.29 dB
-40
1.0 -45
0.5
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2
4
6
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10
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Frequency /GHz
11
14
16
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Reflection Loss /dB
-10
3.0
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0
(c)
-5
3.5
-20
2.5
-25
2.0
-30 -35
1.5
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Thickness /mm
-15
Reflection Loss /dB
-10
3.0
-40
1.0
-45
1.03 mm, 17.76 GHz, -38.64 dB
0.5 2
4
6
8
10
12
14
16
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Frequency /GHz
Fig. 5. Contour plots of the calculated RL for the CNTs/ITO composites synthesized at
268
(a) 350 °C, (b) 600 °C, and (c) 850 °C. (a)
Reflection loss /-dB
30 25 20 15 10 5
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350℃ 600℃ 850℃
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1.0
1.5
2.0
2.5
3.0
3.5
4.0
Thickness /mm
3.0
(b)
600℃ 850℃
2.0
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Bandwidth /GHz
2.5
1.5 1.0 0.5 0.0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Thickness /mm
270
Fig. 6 Comparison of the (a) RL values and (b) bandwidths (RL<−10 dB) of the three
271
samples at different thicknesses.
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Fig. 6a demonstrates the comparison of the minimum RL values at various
273
matching thicknesses for the CNTs/ITO composites synthesized at different
274
temperatures. The composites obtained at 850 °C exhibit excellent microwave
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ACCEPTED MANUSCRIPT absorption properties at thin thicknesses (1.0 and 1.5 mm), while those obtained at 600
276
°C present lower RL values in the thickness range of 2–4 mm compared to the other two
277
samples. Fig. 6b shows the bandwidths of the CNTs/ITO composites corresponding to
278
RL values smaller than −10 dB. As can be seen, the bandwidth of the composites
279
obtained at 850 °C only appears at thin thicknesses, whereas the composite obtained at
280
600 °C exhibits broad bandwidth in the thicknesses range of 2–4 mm. In addition,
281
considering that the coating thickness is always less than 2.0 mm in practical
282
application, the CNTs/ITO composite synthesized at 850 °C shows better microwave
283
absorption properties at thin coating thickness compared to the composite obtained at
284
600 °C. The above results indicate that the microwave absorption properties of the
285
CNTs/ITO composites at different frequencies can be tuned by controlling the
286
calcination temperature and the thickness of the absorbing coating.
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3.3 Microwave absorption mechanism
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0
RL /dB
-10 -20
1mm 1.95mm 3mm 4mm
-30
10 8 6 4 2
tcal −λ/4 m
•tsim −λ/4 m
t −3λ/4
•tsim −3λ/4 m
cal m
2
EP
1
(d)
-20 -30
AC C
RLm /dB
-10
-40
2
288 289
(b)
(c)
Ζcal −λ/4 Ζcal−3λ/4
3
|Ζin/Ζ0|
(a)
9mm 10mm 11mm
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tm /mm
-40
4.79mm 6mm 7mm 8mm
4
6
8
10
12
14
16
18
Frequency /GHz
Fig. 7 (a) Frequency dependence of RL values for the CNTs/ITO composites
290
synthesized at 600 °C with different thicknesses; (b) frequency dependence of λ/4 and
291
3λ/4 thickness of the composite; (c) frequency dependence of the impedance matching
292
characteristics (Z=|Zin/Z0|) of the composites; (d) frequency dependence of the minimum
293
RL (RLm) with thicknesses in the 0.5–4.0 mm range.
294
Clearly, the RL peaks of the CNTs/ITO composites synthesized at different
295
calcination temperatures shift towards the lower frequency region as the thickness of 13
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297
thickness is greater than the critical value, as shown in Fig. 5. To illustrate the possible
298
mechanism of microwave absorption properties of CNTs/ITO composites, the
299
experimental results are analyzed using the quarter-wavelength matching model.
300
According to the model, the minimum RL can be obtained at a given frequency if the
301
thickness of the absorber (tm) satisfies the following: [3]
302
tm = nc 4 f m ε r µr
303
where fm is the peak frequency of RL, tm is the matching thickness, and
304
the complex permittivity and permeability at fm, respectively. When the matching
305
thickness of the CNTs/ITO composites satisfies Eq. (3), the reflected electromagnetic
306
wave at the air–absorber interface is equal in magnitude, but 180° out of phase with that
307
of the electromagnetic wave reflected from the absorber–metal interface, resulting in the
308
extinction of each other at the air–absorber interface. The quarter-wavelength matching
309
model has been successfully used to explain the relationship between the peak
310
frequency of RL and absorber thickness for CNTs/Fe, CNTs/Co, and CNTs/Ni [16].
) ( n = 1,3,5,L)
(3)
ε r and µr are
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The variations in the RL values versus frequency for the CNTs/ITO composites
312
synthesized at 600 °C with different thicknesses are presented in Fig. 7a. A simulation
313
of the thickness of the absorber ( tm ) versus the peak frequency (fm) for the CNTs/ITO
314
composites synthesized at 600 °C according to Eq. (3) is shown in Fig. 7b. The red and
315
sim blue dots on the λ/4 and 3λ/4 curves present the matching thicknesses (denoted as t m )
316
versus the frequencies of the absorption peaks, determined directly from the RL curves
317
sim in Fig. 7a. The simulated results t m are in good agreement with the calculated values
318
tmcal , suggesting that the relationship between the matching thickness and peak frequency
319
of CNTs/ITO composites obeys the quarter-wavelength matching model. The
320
impedance matching characteristic (Z) is an important parameter for reduction of the
321
reflection of electromagnetic waves at the air-absorber interface and can be expressed
322
using the following equation: [52]
323
Z = Zin Z0 = µr εr tanh j ( 2π fd c) µrεr
324
When the calculated value of Z is equal or close to 1, almost all electromagnetic waves
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(4)
ACCEPTED MANUSCRIPT are able to enter the absorber, resulting in zero reflection at the air-absorber interface.
326
The frequency dependence of Z for the CNTs/ITO composites synthesized at 600 °C is
327
calculated using Eq. (4), as demonstrated in Fig. 7c. The relationship between the
328
frequency and minimum RL at the matching thickness (RLm) is shown in Fig. 7d. It can
329
be observed that the RLm of −43.1 dB, corresponding to a thickness of 4.79 mm, exactly
330
matches the peak of the impedance match curve. The minimum RLm is obtained at a
331
frequency of 16.0 GHz, and Z equals approximately 1. Moreover, the matching
332
thickness of 4.79 mm is on the 3λ/4 curve. This represents the perfect matching point
333
under such conditions, and the corresponding thickness and frequency are called the
334
perfect matching thickness and frequency, respectively [53]. Moreover, the other RLm of
335
−38.29 dB is achieved at a frequency of 13.28 GHz with a matching thickness of 1.95
336
mm on the λ/4 curve, and the relevant Z is approximately 1.
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These results indicate that the microwave absorption mechanism of the CNTs/ITO
338
composites can be well illustrated using the quarter-wavelength matching model. The
339
well-matched characteristic impedance is the other factor that contributes to the
340
excellent electromagnetic wave absorbing performance of the composites at certain
341
matching frequencies and thicknesses. Therefore, it can be concluded that the dielectric
342
loss, quarter-wavelength cancellation, and well-matched characteristic impedance in the
343
air-absorber interface conspire to contribute the superior microwave absorption
344
performance of the CNTs/ITO composites.
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3.4 Effects of secondary phase particles on electromagnetic properties
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It has been proved that the dielectric properties of conductive composites can be
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enhanced by adding secondary phase particles [54]. Investigation the effects of the
348
secondary phase BaTiO3 particles on the dielectric properties of MWCNTs/poly
349
(vinylidene
350
BaTiO3-MWCNTs/PVDF exhibits a greater dielectric constant after adding light
351
BaTiO3 to the MWCNTs/PVDF [55]. To investigate the effect of the secondary phase
352
ITO particles on the electromagnetic properties of CNTs/ITO composites, we compared
353
the complex permittivity, dielectric loss, and impedance matching ratio for the
354
single-phase pretreated CNTs, ITO, and CNTs/ITO composites prepared at 600 °C, as
355
shown in Fig. 8. Pretreated CNTs, ITO, and CNTs/ITO composites were dispersed in a
356
paraffin matrix with concentrations of 5 and 15 wt%, 10 and 15 wt%, and 15 wt%,
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fluoride)
(PVDF)
shows
that,
15
compared
to
MWCNTs/PVDF,
ACCEPTED MANUSCRIPT 357
respectively. As shown in Fig. 8a and b, the permittivity of the 15 wt% CNTs/ITO composites is
359
not the simple sum of the permittivity of 5 wt% CNTs and 10 wt% ITO. The real part of
360
the permittivity slightly increases, while the imaginary part improved significantly,
361
indicating that the entire composite system has formed a better conductive network.
362
Compared to the 15 wt% CNTs/ITO composites, the permittivity of the 15 wt% ITO is
363
relatively poor, while the 15 wt% CNTs exhibits a relatively high permittivity. Adding
364
ITO to CNTs enhances the dispersion of CNTs and reduces the percolation threshold of
365
the CNTs/ITO-paraffin composites. The combination of the two components improves
366
the interfacial polarization and effectively promotes the imaginary part of the complex
367
permittivity of the CNTs/ITO composites. The dielectric loss and impedance matching
368
ratio of the samples are demonstrated in Fig. 8c and d, respectively. The values of the
369
dielectric loss imply that the 15 wt% CNTs/ITO and 15 wt% CNTs samples exhibit
370
strong attenuation performance. However, the impedance matching ratio of the 15 wt%
371
CNTs is relatively low, indicating that it is difficult for microwaves to propagate into
372
the CNTs/paraffin composites. Considering the compromise between the dielectric loss
373
and impedance matching ratio, it is predictable that the 15 wt% CNTs/ITO composites
374
may exhibit the most excellent microwave absorption properties.
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50
15wt% CNTs 15wt% ITO
40 30 12 8 4 0
2
4
6
8
10
12
14
16
40
(b)
5wt% CNTs 10wt% ITO 15wt% CNTs/ITO
30 20
4 2 0
18
2
4
6
5wt% CNTs 10wt% ITO 15wt% CNTs/ITO
15wt% CNTs 15wt% ITO
0.5 0.4 0.3 0.2 0.1 0.0
377 378
4
6
8
10
12
Frequency /GHz
12
14
16
18
14
5wt% CNTs 10wt% ITO 15wt% CNTs/ITO
0.7
16
15wt% CNTs 15wt% ITO
0.6 0.5 0.4 0.3 0.2 0.1
18
2
4
6
8
10
12
14
16
18
Frequency /GHz
Fig. 8. Comparison of the (a) real and (b) imaginary part of the permittivity, (c) dielectric loss, and (d) impedance matching ratio for the pretreated CNTs, ITO and
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(d)
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Frequency /GHz Impedance matching ratio
Frequency /GHz 0.7
15wt% CNTs 15wt% ITO
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(a)
Imaginary part of permittivity
Real part of permittivity
60
CNTs/ITO composites.
Fig. 9 shows a comparison of RL for 5 and 15 wt% CNTs, 10 and 15 wt% ITO, and
380
15 wt% CNTs/ITO composites with a thickness of 2.0 mm. It can be observed that the
381
microwave absorption performance of the CNTs/ITO composites is greatly improved
382
compared to those of single CNTs and ITO. The RL values of the 10 and 15 wt% ITO in
383
the test band are both higher than −5 dB, while the 15 wt% CNTs has an RL peak of
384
only −6.78 dB at a frequency of 6 GHz. Nevertheless, the RL peak of the CNTs/ITO
385
composites is significantly reduced to −30.68 dB at a frequency of 13.6 GHz, with a
386
bandwidth of 5.04 GHz (smaller than −5dB).
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-5 -10 -15 -20 5wt% CNTs 10wt% ITO 15wt% CNTs 15wt% ITO 15wt% CNTs/ITO
-25 -30 -35
2
4
6
8
10
12
14
Frequency /GHz
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Reflection loss /dB
0
18
Fig. 9. Comparison of RL for the pretreated CNTs, ITO, and CNTs/ITO composites
389
with a thickness of 2.0 mm.
390
Table 1 Comparison of the microwave absorption properties of typical relative
391
absorbers. Absorber
Matrix Epoxy
Fe3O4/MWCNTs
Paraffin
CNT/Ni0.5Zn0.5Fe2O4
SiO2
Optimal absorption
Filler (wt %)
f (GHz)
20
10.4
−75
3
[8]
20
3.9
−32.5
6
[9]
10
12.4
−20.7
2.5
[11]
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ZnO@MWCNTs
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387 388
MWCNTs/ZnO MWCNTs/Fe
Ni-doped SnO2@MWCNTs
RL (dB)
Reference d (mm)
Paraffin
40
5.9
−35.5
5
[12]
Epoxy
60
2.68
−39
4.27
[16]
Paraffin
25
8.2
−39.2
2.5
[27]
Paraffin
25
15.44
−44.5
1.5
[28]
TiO2@MWCNTs
Paraffin
25
6.6
−25
3
[30]
TPU
30
12.05
−36.44
2
[31]
FeNi3/ITO
Paraffin
70
6.56
−64.2
3.04
[38]
CNTs/ITO (600 ℃)
Paraffin
15
13.28
−38.29
1.95
This work
CNTs/ITO (850 ℃)
Paraffin
15
17.76
−38.64
1.03
This work
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Fe-doped SnO2/MWCNTs
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TiO2@MWCNTs
392
Table 1 lists the typical relative absorbers and their corresponding microwave
393
absorption properties reported in recently published studies. It can be seen that the
394
minimum RL values of the CNTs/ITO composites are lower than those of
395
CNT/Ni0.5Zn0.5Fe2O4 (−32.5 dB) [9], ZnO@MWCNTs (−20.7 dB) [11], MWCNTs/ZnO
396
(−35.5 dB) [12], TiO2@MWCNTs (−25 dB) [30], and TiO2@MWCNTs (−36.44 dB) 18
ACCEPTED MANUSCRIPT [31]. Though the minimum RL of the CNTs/ITO composites is slightly higher than
398
those of MWCNTs/Fe (−39 dB) [16], Ni-doped SnO2@MWCNTs (−39.2 dB) [27] and
399
Fe-doped SnO2@MWCNTs (−44.5 dB) [28], the matching thickness d and loading
400
concentration are relatively lower. Moreover, Fe3O4/MWCNTs [8] and FeNi3/ITO [38]
401
show relatively higher RL values, but their matching thickness d and loading
402
concentration values are also higher than those of the CNTs/ITO composites. Compared
403
to most of the absorbers listed in Table 1, the CNTs/ITO composites exhibit lower
404
matching thickness and loading concentration, and relatively high microwave
405
absorption efficiency, thus meeting the demands of lightweight, thin thickness, and
406
highly efficient MAMs. Table 1 illustrates that the CNTs/ITO composites demonstrate
407
excellent microwave absorption properties compared to their counterparts.
408
4. Conclusions
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We successfully fabricated CNTs/ITO composites using coprecipitation and
410
calcination processes. The ITO particles were evenly distributed at the surface of CNTs.
411
The calcination temperature had a profound effect on the electromagnetic properties of
412
the as-prepared composites. The experimental data indicate that the permittivity and
413
dielectric loss increase with increasing calcination temperature, whereas the impedance
414
matching ratio decreases gradually. The CNTs/ITO composites synthesized at 600 °C
415
and 850 °C demonstrate excellent microwave absorption properties. The composites
416
obtained at 600 °C present lower RL values at a thickness of 2–4 mm, while the
417
composites obtained at 850 °C show broader absorption bandwidth at a thin coating
418
thickness of 1–1.5 mm. The frequency-dependent microwave absorption properties of
419
the CNTs/ITO composites can be adjusted by controlling the calcination temperature
420
and coating thickness. Moreover, the microwave absorption mechanism of the
421
CNTs/ITO composites can be well explained using the quarter-wavelength matching
422
model, thus facilitating the design of a CNTs/ITO composite coating for practical
423
applications.
424
Acknowledgments
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This work was supported by the Program for New Century Excellent Talents of
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China (No. NCET-11-0868), the Shaanxi provincial innovation team of China (No.
427
2014KCT-03) and the Natural Science Foundation of Shaanxi Province, China (No.
428
2014JM2-5084). 19
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Figure and Table captions
630 Fig. 1. XRD patterns of (a) the precursor of CNTs/ITO composites and (b) the
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CNTs/ITO composites synthesized at different temperatures.
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Fig. 2. TEM images of (a) pretreated CNTs, (b) (c) CNTs/ITO composites synthesized
634
at 600 °C, and (d) the corresponding SAED patterns.
635
Fig. 3. (a) Real and (b) imaginary parts of the complex permittivity of the CNTs/ITO
636
composites.
637
Fig. 4. (a) Dielectric loss and (b) impedance matching ratio of the CNTs/ITO
638
composites.
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Fig. 5. Contour plots of the calculated RL for the CNTs/ITO composites synthesized at
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(a) 350 °C, (b) 600 °C, and (c) 850 °C.
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Fig. 6 Comparison of the (a) RL values and (b) bandwidths (RL<−10 dB) of the three
642
samples at different thicknesses.
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Fig. 7 (a) Frequency dependence of RL values for the CNTs/ITO composites
644
synthesized at 600 °C with different thicknesses; (b) frequency dependence of λ/4 and
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3λ/4 thickness of the composite; (c) frequency dependence of the impedance matching
646
characteristics (Z=|Zin/Z0|) of the composites; (d) frequency dependence of the minimum
647
RL (RLm) with thicknesses in the 0.5–4.0 mm range.
648
Fig. 8. Comparison of the (a) real and (b) imaginary part of the permittivity, (c)
649
dielectric loss, and (d) impedance matching ratio for the pretreated CNTs, ITO and
650
CNTs/ITO composites.
651
Fig. 9. Comparison of RL for the pretreated CNTs, ITO, and CNTs/ITO composites
652
with a thickness of 2.0 mm.
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Table 1 Comparison of the microwave absorption properties of typical relative absorbers.
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·Core-shell structured CNTs/ITO composites are successfully fabricated. ·Calcination temperature have great effects on the electromagnetic properties. ·Composites synthesized at 600 and 850℃ exhibit excellent electromagnetic properties. ·Varying calcination temperature and coating thickness can adjust the electromagnetic properties. ·Microwave absorption mechanism and effects of ITO on electromagnetic properties are proposed.
S0925-8388(18)33764-2
DOI:
10.1016/j.jallcom.2018.10.098
Reference:
JALCOM 47916
To appear in:
Journal of Alloys and Compounds
Received Date: 11 October 2017 Revised Date:
25 August 2018
Accepted Date: 9 October 2018
Please cite this article as: C. Ge, L. Wang, G. Liu, R. Wu, Effects of calcination temperature on the electromagnetic properties of carbon nanotubes/indium tin oxide composites, Journal of Alloys and Compounds (2018), doi: https://doi.org/10.1016/j.jallcom.2018.10.098. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
Effects of calcination temperature on the electromagnetic
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properties of carbon nanotubes/indium tin oxide composites
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Abstract
Carbon nanotubes (CNTs)/Indium tin oxide (ITO) composites have
been successfully synthesized via coprecipitation and calcination processes. The effects
6
of calcination temperature on the electromagnetic properties of the CNTs/ITO
7
composites were investigated. The real part of the permittivity and the dielectric loss
8
increase with increasing calcination temperature, which can be attributed to the
9
enhanced dielectric relaxation and space charge polarization. The CNTs/ITO
10
composites synthesized at 600 °C can achieve lower reflection loss (RL) value at the
11
thickness of 2–4 mm, while the composites synthesized at 850 °C exhibit broader
12
bandwidth, corresponding to an RL value below −10 dB at a thin coating thickness of 1–
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1.5 mm. The microwave absorption properties of the CNTs/ITO composites at different
14
frequencies can be tuned by controlling the calcination temperature and the thickness of
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the absorbing coating. The dielectric loss, quarter-wavelength cancellation, and
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well-matched characteristic impedance in the air-absorber interface are believed to
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contribute to the superior microwave absorption performance of the composites. The
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effects of the secondary phase ITO on the electromagnetic properties of the CNTs/ITO
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composites were also discussed in this study.
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Keywords: Carbon nanotubes, Indium tin oxide, Coprecipitation, Calcination
1. Introduction
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Microwave absorption materials (MAMs) have become effective means to mitigate
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civil electromagnetic pollution and enhance the survival and attack capability of
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military weapons nowadays. Research on MAMs has attracted considerable attention in
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recent years. Ideal MAMs should be lightweight, thin thickness, and exhibit wide band
28
absorption as well as strong absorption properties [1]. Therefore, the microwave
29
absorption properties of various absorbers have been widely investigated [2-20].
30
Carbon-based materials, such as graphite, graphene, carbon fibers, carbon black, and
31
carbon nanotubes (CNTs), are typical lightweight MAMs which present high complex
1
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22]. Among them, CNTs are of particular interest because of their excellent mechanical
34
[23], electrical [24], thermal [25], and optical [26] properties, as well as their
35
outstanding dielectric/electrical loss and low percolation threshold [8]. Nevertheless,
36
their poor microwave impedance matching, low absorption capabilities, and narrow
37
absorption bandwidth limited their practical applications. The strategy of introducing a
38
second phase, such as magnetic metals or metal oxides, into CNTs has been employed
39
to improve the absorption performance for both dielectric and magnetic loss [8-20]. In
40
addition, combinations of CNTs with semiconductors, such as ZnO [11, 12], SnO2
41
[27-29], and TiO2 [30, 31], have also been studied as efficient MAMs, which has
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revealed that the use of composites resulted in increasing of the impedance matching
43
and dielectric loss, and greatly enhanced their microwave absorption performance.
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As an n-type semiconductor, indium tin dioxide (ITO) has attracted significant
45
attention duo to its good optical performance, high infrared reflectivity, low resistivity,
46
and a favorable thermal stability [32-36]. Meng et al. [37] prepared an Ni/ITO
47
nanocomposite via an in situ powder-separating reductive process, demonstrating
48
enhanced microwave absorption properties due to the proper electromagnetic
49
impedance matching and facilitation of extra interfacial polarization. Fu et al. [38]
50
synthesized FeNi3/ITO composite via a self-catalyzed reduction method and a sol–gel
51
process, and revealed that the microwave absorption properties are significantly
52
dependent on the content of the ITO phase. Duong et al. [39] introduced 30~40 nm
53
thick ITO thin films at the interface between TiO2 and single-walled CNTs using
54
nanocluster deposition. Kim et al. [40] fabricated a ternary nanomaterial based on
55
sulfur-impregnated multi-walled CNTs filled with ordered SnO nanoparticles using a
56
dry plasma reduction method. However, to the best of our knowledge, the combination
57
of CNTs and ITO as MAMs has rarely been explored. The only known research was the
58
investigation of the electrical conductivity of a 1D CNTs/ITO nanocomposite, which
59
was fabricated using a typical coprecipitation process [41].
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Herein, we synthesized CNTs/ITO core–shell nanostructured composites as MAMs.
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Coating CNTs with ITO is beneficial for the enhancement of the uniformity to maintain
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fine dispersity on the composites, as well as the interface of the heterostructure. It is
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known that the presence of multi-interfaces in composites results in significant
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65
dielectric loss [42]. Moreover, the impedance matching of CNTs can be modulated by
66
doping ITO at the surface of CNTs, which would reduce the reflection of microwaves.
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Furthermore, as a type of dielectric MAMs, the CNTs/ITO composites have better
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thermal stability compared to magnetic or dielectric/magnetic MAMs. Therefore, we
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assume that the CNTs/ITO composites should be high-performance MAMs.
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Compared to the methods for preparing CNTs/ITO composites, including the
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reduction process [37, 38], nanocluster deposition [39], and plasma reduction [40, 43],
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the coprecipitation and calcination process has the unique advantages of simplicity in
73
equipment, facilitation in processing and suitability for applications. In this work, we
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have successfully prepared core-shell structured CNTs/ITO composites through
75
coprecipitation and calcination processes. The influence of the calcination temperature
76
on the electromagnetic properties of the composites was studied. The CNTs/ITO
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composites synthesized at 600 and 850 °C exhibited excellent microwave absorption
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properties. Moreover, the electromagnetic properties of the composites can be tuned by
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varying the calcination temperature and thickness of the absorber coating. The
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microwave absorption mechanism and effects of the secondary ITO phase on the
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electromagnetic properties of the composites were discussed.
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2. Experimental
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2.1. Materials
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Multi-walled CNTs were supplied by Chengdu Organic Chemical Co., Ltd, China.
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The diameters of the CNTs ranged from 10 to 30 nm, their lengths were in the 10–200
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µm range, and their purity was 95%. All of the chemicals and reagents were purchased
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from Sinopharm Chemical Reagent Co., Ltd, China and used without further
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purification.
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2.2. Synthesis of CNTs/ITO composites
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The raw CNTs were pretreated using a typical method. First, 5 g of CNTs and 250
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mL of nitric acid (68 wt%) were dispersed in a flask and maintained at 90 °C under
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refluxing and constant stirring for 18 h. The suspension was then rinsed with deionized
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water until a neutral pH was achieved and dried in a vacuum oven at 80 °C for 24 h.
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The starting materials, 5.864 g of InCl3·4H2O, 0.738 g of SnCl4·5H2O, and 1.542
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g of pretreated CNTs, were dissolved in 100 mL of deionized water, followed by
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continuously stirred bath until a pH 6.5 was achieved, and then it was dropped slowly to
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a pH of 8.5. The bath temperature was maintained at 60 °C. After aging for 4 h, the
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black suspension was centrifuged at 4000 rpm for 8 min, thus forming the CNTs/ITO
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precursor. The precursor was washed with deionized water and absolute ethyl alcohol
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until no chloride ions were detected in the solution and then dried at 80 °C in a vacuum
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oven for 8 h. Finally, the dried precursor was calcined in vacuum in a tube furnace at
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various temperatures (350, 600, and 850 °C) for 3 h to obtain the CNTs/ITO
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composites.
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2.3. Characterization
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The products were characterized by X-ray diffraction (XRD,▼Rigaku D/max-2400)
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with Cu Kα radiation at 40 kV and 40 mA. The morphology and structure of the
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products were investigated by transmission electron microscopy (TEM) and
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selected-area electron diffraction (SAED) using a JEOL JEM3010 electron microscope.
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The frequency dependence of the complex permittivity ( εr = ε ′ − jε ′′ ) and permeability
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( µr = µ′ − jµ′′ ) of the CNTs/ITO composites were measured by the transmission and
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reflection (T/R) coaxial line method using an HP 8720ES vector network analyzer in a
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frequency range of 2–18 GHz at room temperature. The samples were prepared by
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uniformly mixing paraffin with the as-prepared CNTs/ITO composites. The mixture
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was pressed into a coaxial clapper with an outer diameter of 7.0 mm and an inner
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diameter of 3.0 mm. The CNTs/ITO composites were used as control. Throughout the
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measurements, the composites were dispersed in the paraffin matrix at a concentration
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of 15 wt%.
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3. Results and discussion
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3.1 Structure and morphology
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The XRD patterns of the CNTs/ITO composites prepared at different temperatures
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are presented in Fig. 1. The characteristic peak of CNTs is not obviously present,
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possibly due to the low CNTs content and enhanced diffraction peak intensity of ITO.
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The peak at 26.1° is assigned to the (002) plane of CNTs. Fig. 1a illustrates that the
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diffraction peaks of the precursor are very disordered, and many amorphous scattering
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peaks are observed. The diffraction peaks at 22.3°, 45.6°, and 69.5° correspond to the
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ACCEPTED MANUSCRIPT cubic In(OH)3 (JCPDS card No. 16-0161). Furthermore, orthorhombic InOOH (JCPDS
128
card No. 17-0549) is also detected in the precursor. Fig. 1b shows the XRD patterns of
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CNTs/ITO composites fabricated at different temperatures. The distinct peaks at 21.5°,
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30.6°, 35.5°, 51.0°, and 60.7° are assigned to the (211), (222), (400), (440), and (622)
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planes of cubic phase In2O3 (JCPDS card No. 06-0416), respectively. In addition, the
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absence of characteristic peaks associated with SnO2 suggests that the Sn4+ ions are
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homogeneously doped into the crystal lattice of cubic-phase In2O3 under the current
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conditions. The Sn4+ ion replaces and occupies the position of the In3+ ion in the ITO
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crystal to generate a free electron, which can enhance the electrical conductivity of ITO.
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Notably, only the cubic-phase ITO is detected in our case because the cubic-phase is
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thermodynamically preferred at temperatures higher than 550 °C and the rhombohedral
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phase is favored at lower temperatures [41].
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(a)
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Intensity (a.u.)
CNTs Cubic In(OH)3 Orthorhombic InOOH
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30
40
50
60
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2θ/(°)
(b)
Intensity (a.u.)
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50
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2θ/(°)
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Fig. 1. XRD patterns of (a) the precursor of CNTs/ITO composites and (b) the
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CNTs/ITO composites synthesized at different temperatures.
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The morphology and microstructure of the CNTs/ITO composites synthesized at 5
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clean walls and smooth surfaces without visible residual metal catalysts. Fig. 2b and c
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show the TEM images of the CNTs/ITO composites. A large quantity of ITO particles
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with relatively uniform size are distributed on the surfaces of CNTs and are clearly
147
observed. The good distribution of the components in the composite is believed to be
148
beneficial for impedance matching to achieve excellent microwave absorption
149
properties [44]. Fig. 2d demonstrates the SAED pattern of the circled area in Fig. 2c. A
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series of labeled diffraction rings are indexed to the (222) plane of CNTs and the (211),
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(222), (400), (440), and (622) planes of cubic-phase ITO, which are consistent with the
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XRD patterns shown in Fig. 1b.
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Fig. 2. TEM images of (a) pretreated CNTs, (b) (c) CNTs/ITO composites synthesized
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at 600 °C, and (d) the corresponding SAED patterns.
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3.2 Electromagnetic and absorbing properties
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The microwave electrical conductivity of a dielectric material can be evaluated by
158
using the equation σ =2π f ε 0ε ′′ , where σ is the electric conductivity (S/m), ε0 is the free 6
ACCEPTED MANUSCRIPT space permittivity (8.854×10−12 F/m), and f is the frequency of the electromagnetic
160
wave [45]. According to the calculation results shown in Fig. S1, the average
161
microwave electrical conductivity values of the CNTs/ITO composites prepared at 350,
162
600, and 850 °C across the test band are 0.539, 1.134, and 5.337 S/m, respectively. The
163
calcination temperatures increase the electrical conductivity of the composites at
164
microwave frequencies, which can be translated into the increasing dielectric loss
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( tanδε =ε′ ε′ ). (a)
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Frequency /GHz
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Frequency /GHz
Fig. 3. (a) Real and (b) imaginary parts of the complex permittivity of the CNTs/ITO composites.
Fig. 3 shows the complex permittivity of the CNTs/ITO composites synthesized at
170
different temperatures in the frequency range of 2–18 GHz. The real and imaginary
171
parts of the complex permittivity demonstrate similar variation trends, which decreases
172
slightly with increasing frequency. With the increasing calcination temperature, the real
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and imaginary parts of the complex permittivity increase gradually. The higher
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calcination temperature can improve the crystallization and electrical conductivity of the
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CNTs/ITO composites, leading to a gradual increase of the imaginary part of the 7
ACCEPTED MANUSCRIPT complex permittivity. Moreover, the increase in the real part of the complex permittivity
177
can be attributed to the enhanced dielectric relaxation and space charge polarization.
178
The real and imaginary parts of the relative complex permittivity represent the storage
179
and loss capability in response to the microwave energy, respectively, and a rise in the
180
imaginary part is attributed to the microwave absorption [46]. In addition, the complex
181
permeability of the CNTs/ITO composites is illustrated in Fig. S2. Due to the absence
182
of magnetic constituents, the real and imaginary parts of the complex permeability are
183
approximately 1 and 0, respectively. 1.0
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Fig. 4. (a) Dielectric loss and (b) impedance matching ratio of the CNTs/ITO
The
tan δε
composites.
values of the CNTs/ITO composites were calculated, as shown in Fig. tan δε
188
4a. The calcination temperature has a great effect on the
189
composites. The
190
similar fluctuations. For the composites synthesized at 850 °C, the
191
relatively high in the full test frequency. The
tan δε
of the CNTs/ITO
values of the composites synthesized at 350 and 600 °C exhibit
tan δε
8
tan δε
value is
increases with increasing calcination
ACCEPTED MANUSCRIPT 192
temperature, indicating that the polarization and leakage current will take place when
193
exposed to electromagnetic wave radiation, and thus the microwave absorption
194
properties of the composites can be improved.
195
The increase in the complex permittivity and
tan δε
cause the improvement in tan δε
microwave absorption. Nevertheless, a larger permittivity and
are not always
197
favorable for microwave absorption. Single-layer absorbing coating should meet two
198
criteria to achieve good microwave absorption properties. First, the incident
199
electromagnetic wave can enter the interior of the absorbing coating as much as possible
200
without reflection, suggesting that the impedances of the absorbing materials and free
201
space should match well. Second, the absorbing materials have the ability to absorb and
202
lose the incident electromagnetic wave, suggesting that the dielectric loss and magnetic
203
loss of the absorbing materials should be as high as possible [47, 48]. Consequently, the
204
dielectric loss and microwave impedance of the dielectric MAMs play dual roles and
205
will ultimately affect the microwave absorption properties. The microwave impedance η
206
is expressed using the equation η = Z 0 µ r ( ε ′ − jε ′′ ) , where Z0 is the free-space wave
207
impedance (∼377 Ω) [1]. To effectively absorb microwaves, the impedance matching
208
ratio (η/Z0) should exceed 0.3 to allow the incident microwaves to efficiently enter the
209
absorber with little reflection at the air–absorber interface [49].
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Fig. 4b shows the impedance matching ratio of the CNTs/ITO composites
211
synthesized at different calcination temperatures. As shown in the figure, the impedance
212
matching ratio of the CNTs/ITO composites declines gradually with increasing
213
calcination temperature. The composites obtained at 350 and 600 °C exhibit relatively
214
high impedance matching ratios, while the impedance matching ratio of the composite
215
obtained at 850 °C is smaller than 0.3 (~0.2) in the frequency range of 2–18 GHz. This
216
is mainly due to the sharp increase in the electrical conductivity of the composites,
217
which results in the reduction of the skin depth (the skin depth of the composites is
218
presented in SI 1 and Fig. S3 in the Supplementary material file) [45], leading to the
219
relatively poor impedance matching. As a result, electromagnetic wave is able to
220
propagate into the CNTs/ITO composites obtained at 350 and 600 °C, while only a
221
small amount of the microwaves is reflected back into the air.
222
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9
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the CNTs/ITO composites. However, the impedance matching ratio also reduces
224
significantly. Therefore, neither
225
describe the microwave absorption properties alone. Thus, estimation of the reflection
226
loss (RL) requires further in-depth considerations. To clarify the microwave absorption
227
properties, the RL values for a single-layer absorber were evaluated according to the
228
transmission line theory using the following equations: [50, 51]
229
RL = 20 log ( Z in − Z 0 )
230
Z in = Z 0 µ r ε r tanh j ( 2π fd c ) µ r ε r
231
where Zin is the input characteristic impedance of the absorber, c is the velocity of the
232
electromagnetic wave in vacuum, and d is the thickness of the absorber. Considering the
233
weak magnetic properties of the composites, the complex permeability is considered to
234
be 1.
nor the impedance matching ratio can directly
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tan δε
( Z in + Z 0 )
)
(2)
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Fig. 5 shows the contour plots of the calculated RL of the CNTs/ITO composites
236
synthesized at different temperatures for various thicknesses in the frequency range of
237
2–18 GHz. An RL value below −10 dB, corresponding to 90% attenuating of the
238
incident electromagnetic wave, is considered to be an ideal absorption value. As shown
239
in Fig. 5a, RL of the CNTs/ITO composites synthesized at 350 °C reaches the minimum
240
value of −7.8 dB at 14.24 GHz at a thickness of 2.2 mm. The value can hardly reach
241
−10 dB within the thickness range of 0.5–4.0 mm, which will limit its practical
242
applications. As the calcination temperature increased to 600 °C (Fig. 5b), RL values
243
exceeding −10 dB are reached in the range of 6.16–8.88 GHz, 9.36–11.74 GHz, and
244
12.64–14.48 GHz, with absorber thicknesses of 1.7–4.0 mm. The minimum RL of
245
−38.29 dB is obtained at 13.28 GHz with a thickness of 1.95 mm. For the CNTs/ITO
246
composite synthesized at 850 °C (Fig. 5c), RL values smaller than −10 dB are found in
247
the wide frequency range of 8.4–18 GHz with thicknesses of 0.94–2.0 mm. The
248
minimum RL of −38.64 dB is achieved at 17.76 GHz with the matching thickness of
249
1.03 mm. Although the impedance matching condition is relatively poor, the results
250
indicate that the CNTs/ITO composite synthesized at 850 °C demonstrates excellent
251
microwave absorption properties in a high-frequency range, when the thickness of the
252
absorber is smaller than 2 mm (Fig. S4). These results can be explained by the
253
relationship between the frequency dependent skin depth, attenuation constant, and RL
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255
in the Supplementary materials. First, the attenuation constant of the CNTs/ITO
256
composite synthesized at 850 °C increased significantly, especially in the frequency
257
range of 9–18 GHz (Fig. S6), indicating a strong electromagnetic wave attenuation
258
ability in this frequency range. Second, as shown in Fig. S5, the skin depth of the
259
composite ranges from 1.5 to 2.35 mm in the frequency range of 9–18 GHz, which
260
means that the electromagnetic waves can propagate into composite at least 1.5 mm
261
thick in this frequency range. These two factors attribute the CNTs/ITO composite
262
synthesized at 850 °C with excellent microwave absorption properties in the frequency
263
range of 9–18 GHz with thicknesses of 1–1.5 mm.
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1.5
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Reflection Loss /dB
-10
3.0
-40
1.0
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4.0
0
(b)
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2.0
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1.5
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-15
1.95 mm, 13.28 GHz, -38.29 dB
-40
1.0 -45
0.5
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2
4
6
8
10
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Frequency /GHz
11
14
16
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Reflection Loss /dB
-10
3.0
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-5
3.5
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2.5
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2.0
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1.5
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Reflection Loss /dB
-10
3.0
-40
1.0
-45
1.03 mm, 17.76 GHz, -38.64 dB
0.5 2
4
6
8
10
12
14
16
18
Frequency /GHz
Fig. 5. Contour plots of the calculated RL for the CNTs/ITO composites synthesized at
268
(a) 350 °C, (b) 600 °C, and (c) 850 °C. (a)
Reflection loss /-dB
30 25 20 15 10 5
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1.0
1.5
2.0
2.5
3.0
3.5
4.0
Thickness /mm
3.0
(b)
600℃ 850℃
2.0
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2.5
1.5 1.0 0.5 0.0
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Thickness /mm
270
Fig. 6 Comparison of the (a) RL values and (b) bandwidths (RL<−10 dB) of the three
271
samples at different thicknesses.
272
Fig. 6a demonstrates the comparison of the minimum RL values at various
273
matching thicknesses for the CNTs/ITO composites synthesized at different
274
temperatures. The composites obtained at 850 °C exhibit excellent microwave
12
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276
°C present lower RL values in the thickness range of 2–4 mm compared to the other two
277
samples. Fig. 6b shows the bandwidths of the CNTs/ITO composites corresponding to
278
RL values smaller than −10 dB. As can be seen, the bandwidth of the composites
279
obtained at 850 °C only appears at thin thicknesses, whereas the composite obtained at
280
600 °C exhibits broad bandwidth in the thicknesses range of 2–4 mm. In addition,
281
considering that the coating thickness is always less than 2.0 mm in practical
282
application, the CNTs/ITO composite synthesized at 850 °C shows better microwave
283
absorption properties at thin coating thickness compared to the composite obtained at
284
600 °C. The above results indicate that the microwave absorption properties of the
285
CNTs/ITO composites at different frequencies can be tuned by controlling the
286
calcination temperature and the thickness of the absorbing coating.
287
3.3 Microwave absorption mechanism
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0
RL /dB
-10 -20
1mm 1.95mm 3mm 4mm
-30
10 8 6 4 2
tcal −λ/4 m
•tsim −λ/4 m
t −3λ/4
•tsim −3λ/4 m
cal m
2
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-10
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2
288 289
(b)
(c)
Ζcal −λ/4 Ζcal−3λ/4
3
|Ζin/Ζ0|
(a)
9mm 10mm 11mm
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-40
4.79mm 6mm 7mm 8mm
4
6
8
10
12
14
16
18
Frequency /GHz
Fig. 7 (a) Frequency dependence of RL values for the CNTs/ITO composites
290
synthesized at 600 °C with different thicknesses; (b) frequency dependence of λ/4 and
291
3λ/4 thickness of the composite; (c) frequency dependence of the impedance matching
292
characteristics (Z=|Zin/Z0|) of the composites; (d) frequency dependence of the minimum
293
RL (RLm) with thicknesses in the 0.5–4.0 mm range.
294
Clearly, the RL peaks of the CNTs/ITO composites synthesized at different
295
calcination temperatures shift towards the lower frequency region as the thickness of 13
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297
thickness is greater than the critical value, as shown in Fig. 5. To illustrate the possible
298
mechanism of microwave absorption properties of CNTs/ITO composites, the
299
experimental results are analyzed using the quarter-wavelength matching model.
300
According to the model, the minimum RL can be obtained at a given frequency if the
301
thickness of the absorber (tm) satisfies the following: [3]
302
tm = nc 4 f m ε r µr
303
where fm is the peak frequency of RL, tm is the matching thickness, and
304
the complex permittivity and permeability at fm, respectively. When the matching
305
thickness of the CNTs/ITO composites satisfies Eq. (3), the reflected electromagnetic
306
wave at the air–absorber interface is equal in magnitude, but 180° out of phase with that
307
of the electromagnetic wave reflected from the absorber–metal interface, resulting in the
308
extinction of each other at the air–absorber interface. The quarter-wavelength matching
309
model has been successfully used to explain the relationship between the peak
310
frequency of RL and absorber thickness for CNTs/Fe, CNTs/Co, and CNTs/Ni [16].
) ( n = 1,3,5,L)
(3)
ε r and µr are
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The variations in the RL values versus frequency for the CNTs/ITO composites
312
synthesized at 600 °C with different thicknesses are presented in Fig. 7a. A simulation
313
of the thickness of the absorber ( tm ) versus the peak frequency (fm) for the CNTs/ITO
314
composites synthesized at 600 °C according to Eq. (3) is shown in Fig. 7b. The red and
315
sim blue dots on the λ/4 and 3λ/4 curves present the matching thicknesses (denoted as t m )
316
versus the frequencies of the absorption peaks, determined directly from the RL curves
317
sim in Fig. 7a. The simulated results t m are in good agreement with the calculated values
318
tmcal , suggesting that the relationship between the matching thickness and peak frequency
319
of CNTs/ITO composites obeys the quarter-wavelength matching model. The
320
impedance matching characteristic (Z) is an important parameter for reduction of the
321
reflection of electromagnetic waves at the air-absorber interface and can be expressed
322
using the following equation: [52]
323
Z = Zin Z0 = µr εr tanh j ( 2π fd c) µrεr
324
When the calculated value of Z is equal or close to 1, almost all electromagnetic waves
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14
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(4)
ACCEPTED MANUSCRIPT are able to enter the absorber, resulting in zero reflection at the air-absorber interface.
326
The frequency dependence of Z for the CNTs/ITO composites synthesized at 600 °C is
327
calculated using Eq. (4), as demonstrated in Fig. 7c. The relationship between the
328
frequency and minimum RL at the matching thickness (RLm) is shown in Fig. 7d. It can
329
be observed that the RLm of −43.1 dB, corresponding to a thickness of 4.79 mm, exactly
330
matches the peak of the impedance match curve. The minimum RLm is obtained at a
331
frequency of 16.0 GHz, and Z equals approximately 1. Moreover, the matching
332
thickness of 4.79 mm is on the 3λ/4 curve. This represents the perfect matching point
333
under such conditions, and the corresponding thickness and frequency are called the
334
perfect matching thickness and frequency, respectively [53]. Moreover, the other RLm of
335
−38.29 dB is achieved at a frequency of 13.28 GHz with a matching thickness of 1.95
336
mm on the λ/4 curve, and the relevant Z is approximately 1.
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These results indicate that the microwave absorption mechanism of the CNTs/ITO
338
composites can be well illustrated using the quarter-wavelength matching model. The
339
well-matched characteristic impedance is the other factor that contributes to the
340
excellent electromagnetic wave absorbing performance of the composites at certain
341
matching frequencies and thicknesses. Therefore, it can be concluded that the dielectric
342
loss, quarter-wavelength cancellation, and well-matched characteristic impedance in the
343
air-absorber interface conspire to contribute the superior microwave absorption
344
performance of the CNTs/ITO composites.
345
3.4 Effects of secondary phase particles on electromagnetic properties
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It has been proved that the dielectric properties of conductive composites can be
347
enhanced by adding secondary phase particles [54]. Investigation the effects of the
348
secondary phase BaTiO3 particles on the dielectric properties of MWCNTs/poly
349
(vinylidene
350
BaTiO3-MWCNTs/PVDF exhibits a greater dielectric constant after adding light
351
BaTiO3 to the MWCNTs/PVDF [55]. To investigate the effect of the secondary phase
352
ITO particles on the electromagnetic properties of CNTs/ITO composites, we compared
353
the complex permittivity, dielectric loss, and impedance matching ratio for the
354
single-phase pretreated CNTs, ITO, and CNTs/ITO composites prepared at 600 °C, as
355
shown in Fig. 8. Pretreated CNTs, ITO, and CNTs/ITO composites were dispersed in a
356
paraffin matrix with concentrations of 5 and 15 wt%, 10 and 15 wt%, and 15 wt%,
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fluoride)
(PVDF)
shows
that,
15
compared
to
MWCNTs/PVDF,
ACCEPTED MANUSCRIPT 357
respectively. As shown in Fig. 8a and b, the permittivity of the 15 wt% CNTs/ITO composites is
359
not the simple sum of the permittivity of 5 wt% CNTs and 10 wt% ITO. The real part of
360
the permittivity slightly increases, while the imaginary part improved significantly,
361
indicating that the entire composite system has formed a better conductive network.
362
Compared to the 15 wt% CNTs/ITO composites, the permittivity of the 15 wt% ITO is
363
relatively poor, while the 15 wt% CNTs exhibits a relatively high permittivity. Adding
364
ITO to CNTs enhances the dispersion of CNTs and reduces the percolation threshold of
365
the CNTs/ITO-paraffin composites. The combination of the two components improves
366
the interfacial polarization and effectively promotes the imaginary part of the complex
367
permittivity of the CNTs/ITO composites. The dielectric loss and impedance matching
368
ratio of the samples are demonstrated in Fig. 8c and d, respectively. The values of the
369
dielectric loss imply that the 15 wt% CNTs/ITO and 15 wt% CNTs samples exhibit
370
strong attenuation performance. However, the impedance matching ratio of the 15 wt%
371
CNTs is relatively low, indicating that it is difficult for microwaves to propagate into
372
the CNTs/paraffin composites. Considering the compromise between the dielectric loss
373
and impedance matching ratio, it is predictable that the 15 wt% CNTs/ITO composites
374
may exhibit the most excellent microwave absorption properties.
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16
ACCEPTED MANUSCRIPT 5wt% CNTs 10wt% ITO 15wt% CNTs/ITO
50
15wt% CNTs 15wt% ITO
40 30 12 8 4 0
2
4
6
8
10
12
14
16
40
(b)
5wt% CNTs 10wt% ITO 15wt% CNTs/ITO
30 20
4 2 0
18
2
4
6
5wt% CNTs 10wt% ITO 15wt% CNTs/ITO
15wt% CNTs 15wt% ITO
0.5 0.4 0.3 0.2 0.1 0.0
377 378
4
6
8
10
12
Frequency /GHz
12
14
16
18
14
5wt% CNTs 10wt% ITO 15wt% CNTs/ITO
0.7
16
15wt% CNTs 15wt% ITO
0.6 0.5 0.4 0.3 0.2 0.1
18
2
4
6
8
10
12
14
16
18
Frequency /GHz
Fig. 8. Comparison of the (a) real and (b) imaginary part of the permittivity, (c) dielectric loss, and (d) impedance matching ratio for the pretreated CNTs, ITO and
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(d)
0.8
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Frequency /GHz Impedance matching ratio
Frequency /GHz 0.7
15wt% CNTs 15wt% ITO
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(a)
Imaginary part of permittivity
Real part of permittivity
60
CNTs/ITO composites.
Fig. 9 shows a comparison of RL for 5 and 15 wt% CNTs, 10 and 15 wt% ITO, and
380
15 wt% CNTs/ITO composites with a thickness of 2.0 mm. It can be observed that the
381
microwave absorption performance of the CNTs/ITO composites is greatly improved
382
compared to those of single CNTs and ITO. The RL values of the 10 and 15 wt% ITO in
383
the test band are both higher than −5 dB, while the 15 wt% CNTs has an RL peak of
384
only −6.78 dB at a frequency of 6 GHz. Nevertheless, the RL peak of the CNTs/ITO
385
composites is significantly reduced to −30.68 dB at a frequency of 13.6 GHz, with a
386
bandwidth of 5.04 GHz (smaller than −5dB).
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-5 -10 -15 -20 5wt% CNTs 10wt% ITO 15wt% CNTs 15wt% ITO 15wt% CNTs/ITO
-25 -30 -35
2
4
6
8
10
12
14
Frequency /GHz
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Reflection loss /dB
0
18
Fig. 9. Comparison of RL for the pretreated CNTs, ITO, and CNTs/ITO composites
389
with a thickness of 2.0 mm.
390
Table 1 Comparison of the microwave absorption properties of typical relative
391
absorbers. Absorber
Matrix Epoxy
Fe3O4/MWCNTs
Paraffin
CNT/Ni0.5Zn0.5Fe2O4
SiO2
Optimal absorption
Filler (wt %)
f (GHz)
20
10.4
−75
3
[8]
20
3.9
−32.5
6
[9]
10
12.4
−20.7
2.5
[11]
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ZnO@MWCNTs
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MWCNTs/ZnO MWCNTs/Fe
Ni-doped SnO2@MWCNTs
RL (dB)
Reference d (mm)
Paraffin
40
5.9
−35.5
5
[12]
Epoxy
60
2.68
−39
4.27
[16]
Paraffin
25
8.2
−39.2
2.5
[27]
Paraffin
25
15.44
−44.5
1.5
[28]
TiO2@MWCNTs
Paraffin
25
6.6
−25
3
[30]
TPU
30
12.05
−36.44
2
[31]
FeNi3/ITO
Paraffin
70
6.56
−64.2
3.04
[38]
CNTs/ITO (600 ℃)
Paraffin
15
13.28
−38.29
1.95
This work
CNTs/ITO (850 ℃)
Paraffin
15
17.76
−38.64
1.03
This work
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Fe-doped SnO2/MWCNTs
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TiO2@MWCNTs
392
Table 1 lists the typical relative absorbers and their corresponding microwave
393
absorption properties reported in recently published studies. It can be seen that the
394
minimum RL values of the CNTs/ITO composites are lower than those of
395
CNT/Ni0.5Zn0.5Fe2O4 (−32.5 dB) [9], ZnO@MWCNTs (−20.7 dB) [11], MWCNTs/ZnO
396
(−35.5 dB) [12], TiO2@MWCNTs (−25 dB) [30], and TiO2@MWCNTs (−36.44 dB) 18
ACCEPTED MANUSCRIPT [31]. Though the minimum RL of the CNTs/ITO composites is slightly higher than
398
those of MWCNTs/Fe (−39 dB) [16], Ni-doped SnO2@MWCNTs (−39.2 dB) [27] and
399
Fe-doped SnO2@MWCNTs (−44.5 dB) [28], the matching thickness d and loading
400
concentration are relatively lower. Moreover, Fe3O4/MWCNTs [8] and FeNi3/ITO [38]
401
show relatively higher RL values, but their matching thickness d and loading
402
concentration values are also higher than those of the CNTs/ITO composites. Compared
403
to most of the absorbers listed in Table 1, the CNTs/ITO composites exhibit lower
404
matching thickness and loading concentration, and relatively high microwave
405
absorption efficiency, thus meeting the demands of lightweight, thin thickness, and
406
highly efficient MAMs. Table 1 illustrates that the CNTs/ITO composites demonstrate
407
excellent microwave absorption properties compared to their counterparts.
408
4. Conclusions
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We successfully fabricated CNTs/ITO composites using coprecipitation and
410
calcination processes. The ITO particles were evenly distributed at the surface of CNTs.
411
The calcination temperature had a profound effect on the electromagnetic properties of
412
the as-prepared composites. The experimental data indicate that the permittivity and
413
dielectric loss increase with increasing calcination temperature, whereas the impedance
414
matching ratio decreases gradually. The CNTs/ITO composites synthesized at 600 °C
415
and 850 °C demonstrate excellent microwave absorption properties. The composites
416
obtained at 600 °C present lower RL values at a thickness of 2–4 mm, while the
417
composites obtained at 850 °C show broader absorption bandwidth at a thin coating
418
thickness of 1–1.5 mm. The frequency-dependent microwave absorption properties of
419
the CNTs/ITO composites can be adjusted by controlling the calcination temperature
420
and coating thickness. Moreover, the microwave absorption mechanism of the
421
CNTs/ITO composites can be well explained using the quarter-wavelength matching
422
model, thus facilitating the design of a CNTs/ITO composite coating for practical
423
applications.
424
Acknowledgments
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This work was supported by the Program for New Century Excellent Talents of
426
China (No. NCET-11-0868), the Shaanxi provincial innovation team of China (No.
427
2014KCT-03) and the Natural Science Foundation of Shaanxi Province, China (No.
428
2014JM2-5084). 19
ACCEPTED MANUSCRIPT 429
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Figure and Table captions
630 Fig. 1. XRD patterns of (a) the precursor of CNTs/ITO composites and (b) the
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CNTs/ITO composites synthesized at different temperatures.
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Fig. 2. TEM images of (a) pretreated CNTs, (b) (c) CNTs/ITO composites synthesized
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at 600 °C, and (d) the corresponding SAED patterns.
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Fig. 3. (a) Real and (b) imaginary parts of the complex permittivity of the CNTs/ITO
636
composites.
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Fig. 4. (a) Dielectric loss and (b) impedance matching ratio of the CNTs/ITO
638
composites.
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Fig. 5. Contour plots of the calculated RL for the CNTs/ITO composites synthesized at
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(a) 350 °C, (b) 600 °C, and (c) 850 °C.
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Fig. 6 Comparison of the (a) RL values and (b) bandwidths (RL<−10 dB) of the three
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samples at different thicknesses.
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Fig. 7 (a) Frequency dependence of RL values for the CNTs/ITO composites
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synthesized at 600 °C with different thicknesses; (b) frequency dependence of λ/4 and
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3λ/4 thickness of the composite; (c) frequency dependence of the impedance matching
646
characteristics (Z=|Zin/Z0|) of the composites; (d) frequency dependence of the minimum
647
RL (RLm) with thicknesses in the 0.5–4.0 mm range.
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Fig. 8. Comparison of the (a) real and (b) imaginary part of the permittivity, (c)
649
dielectric loss, and (d) impedance matching ratio for the pretreated CNTs, ITO and
650
CNTs/ITO composites.
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Fig. 9. Comparison of RL for the pretreated CNTs, ITO, and CNTs/ITO composites
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with a thickness of 2.0 mm.
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Table 1 Comparison of the microwave absorption properties of typical relative absorbers.
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·Core-shell structured CNTs/ITO composites are successfully fabricated. ·Calcination temperature have great effects on the electromagnetic properties. ·Composites synthesized at 600 and 850℃ exhibit excellent electromagnetic properties. ·Varying calcination temperature and coating thickness can adjust the electromagnetic properties. ·Microwave absorption mechanism and effects of ITO on electromagnetic properties are proposed.