The impact of different multi-walled carbon nanotubes on the X-band microwave absorption of their epoxy nanocomposites
© Che et al.; licensee Springer. 2015
Received: 24 July 2014
Accepted: 6 February 2015
Published: 4 March 2015
Carbon nanotube (CNT) characteristics, besides the processing conditions, can change significantly the microwave absorption behavior of CNT/polymer composites. In this study, we investigated the influence of three commercial multi-walled CNT materials with various diameters and length-to-diameter aspect ratios on the X-band microwave absorption of epoxy nanocomposites with CNT contents from 0.125 to 2 wt%, prepared by two dispersion methods, i.e. in solution with surfactant-aiding and via ball-milling.
The laser diffraction particle size and TEM analysis showed that both methods produced good dispersions at the microscopic level of CNTs. Both a high aspect ratio resulting in nanotube alignment trend and good infiltration of the matrix in the individual nanotubes, which was indicated by high Brookfield viscosities at low CNT contents of CNT/epoxy dispersions, are important factors to achieve composites with high microwave absorption characteristics. The multi-walled carbon nanotube (MWCNT) with the largest aspect ratio resulted in composites with the best X-band microwave absorption performance, which is considerably better than that of reported pristine CNT/polymer composites with similar or lower thicknesses and CNT loadings below 4 wt%.
A high aspect ratio of CNTs resulting in microscopic alignment trend of nanotubes as well as a good level of micro-scale CNT dispersion resulting from good CNT-matrix interactions are crucial to obtain effective microwave absorption performance. This study demonstrated that effective radar absorbing MWCNT/epoxy nanocomposites having small matching thicknesses of 2–3 mm and very low filler contents of 0.25-0.5 wt%, with microwave energy absorption in the X-band region above 90% and maximum absorption peak values above 97%, could be obtained via simple processing methods, which is promising for mass production in industrial applications.
Carbon nanotubes (CNTs) as nano-fillers in polymer matrix composites have captivated much interest from many industries and research groups, owing to the impressive physical properties of CNTs such as high elastic modulus as well as high thermal and electrical conductivities. CNT-filled composites have proven great potential for commercial applications for aerospace, transportation, automotive and electronic industries. CNTs as fillers offering a good conductive network in polymer matrices can also result in enhanced dielectric loss, which causes attenuation of microwave energy. Thus, there have been abundant studies on CNT-filled polymer nanocomposites as microwave absorbers and electromagnetic shielding materials gaining remarkable attention in both civil and military applications [1-7].
Due to strong van der Waals forces, CNTs tend to agglomerate. The ability to effectively minimize the amount of CNT entangled bundles and disperse the nanotubes in polymer matrices influences nearly all relevant properties of the composites. The effects of CNT dispersibility via different dispersion methods, such as melt mixing using extruders, solvent processing by means of centrifugation, ultrasonication, surfactant treatment and chemical modification of CNTs, on the mechanical, thermal and electrical properties of CNT composites have been well-addressed [8-21]. While an excellent dispersion is essential for effectively reinforcing polymer matrices , a good conductivity requires both a good distribution of dis-entangled CNT agglomerates and conglomeration of CNTs in an anisotropic morphology necessary for constitution of a conductive network . The shape anisotropy and spatial orientation of nano-fillers in nanocomposites could have a crucial influence on the electrical conductivity . It has been reported that strong CNT-polymer interactions or increased compatibility of CNTs to the polymer matrix, which enhance polymer-wrapping around CNTs, could decrease the electrical conductivity [15,23]. It has also been found that multi-walled carbon nanotube (MWCNT)/polymer composite films with CNT agglomerations at the micro-scale have higher electrical conductivity than those with uniformly dispersed CNTs . Depending on the synthesis and processing conditions, the properties of MWCNTs from different producers can vary enormously. Several works have compared the mechanical and thermal properties and electrical conductivity of polymer composites of various commercial CNTs. For example, Pötschke and coworkers [16,26] compared the nanotube dispersity via light microscopy, the mechanical and electrical characteristics, associated with the extrusion feeding conditions, of twin-screw extruded polypropylene composites of two types of MWCNTs, namely Baytubes C150P and Nanocyl NC7000 having different mean length-to-diameter ratios, bulk densities and agglomerate strength. Three-roll mill processed epoxy composites of Baytubes C150P and Nanocyl NC7000 with equal filler contents showed different electrical resistivities . Castillo et al.  compared five MWCNT materials from different suppliers with various aspect ratios on the electrical, mechanical and glass transition behavior of polycarbonate-based nanocomposites. Rahaman et al.  reported the different electrical properties of polyethylene nanocomposites of three types of commercial MWCNTs with different aspect ratios. Ball-milling treatment of the as-synthesized Nanocyl NC700 MWCNTs to alter the CNT length and bulk density resulting in a change in the electrical conductivity of their melt-mixed polypropylene-based nanocomposites has been observed by Menzer et al. . Gojny et al.  investigated the different thermal and electrical conductivities of epoxy composites of different single-walled, double-walled and multi-walled CNTs as well as amino-functionalized CNTs from various producers. The effects of MWCNTs with different properties on mechanical reinforcement as well as on the electrical percolation threshold of composites based on other types of polymers, such as high density polyethylene and polyamide, have also been shown in other works [22,31,32].
However, a good conductivity does not necessarily correspond to an effective microwave absorbing performance, which needs to satisfy not only dielectric loss requirements, but also importantly the impedance matching condition [33,34].
The formation of a dense interconnected CNT network can give rise to enhanced dielectric loss but should not make the material substantially reflective . The microwave absorption properties of CNT-filled nanocomposites depend on not only the intrinsic electrical conductivity of CNTs, the interactions among CNTs, matrix-CNT interactions but also CNT clustering, which results in polarization phenomena and hence frequency dependence of effective permittivity . In this aspect, CNT properties like nanotube type, length, diameter, bulk density, surface quality, purity, the size and strength of agglomerates, which are dependent on the CNT synthesis conditions, affect significantly the dispersity of CNTs throughout the polymer, the tendency of CNT re-clustering, and thereby the microwave absorption performance.
Numerous studies researched the dependence of polymer composite performance on the grade of MWCNT filler as mentioned above, while fewer investigations on the influence of CNTs on the microwave absorbing efficiency of CNT-polymer composites were reported [36-39].
On the other hand, for practical applications, 0.5-0.6 wt% CNT loadings are normally the optimal CNT contents for not compromising the composite fracture strength [16,40], and a thin composite thickness of a few milimeters is often preferred for radar absorbing composite coatings on metal or textile substrates. Thin composites also give the advantages of lightweight and cost-effectiveness. It has been shown in the literature that pristine CNT/polymer nanocomposites satisfying both a low CNT content below 0.6 wt% and a small composite thickness below 4 mm have not achieved a reflection loss below −10 dB desirable for radar absorbing applications. Thus, either high CNT loadings of 4–30 wt%, large composite thicknesses or the synthesis of CNT-metallic magnetic particle hybrids have been employed in order to enhance the microwave absorption efficiency of CNT/polymer composites [33,35,41-54]. However, CNT characteristics, a crucial factor besides the processing conditions that can change significantly the microwave absorption behavior, have not been addressed.
Therefore, in this article, the microwave absorbing properties in the X-band (8–12 GHz) region of epoxy-based nanocomposites of three different commercial MWCNT materials from diverse producers, i.e. Baytubes C150P (Bayer Material-Science AG, Germany), Nanocyl NC7000 (Nanocyl S.A., Belgium) and MWCNT-VAST (VAST, Vietnam) are compared. The two methods of processing in solution with surfactant-aiding and via ball-milling were employed, and composites having different MWCNT contents were fabricated. An investigation of the dispersibility of the different MWCNTs in solution and in the epoxy matrix via transmission electron microscopy (TEM), particle sizing and Brookfield viscosity measurements was performed, and was correlated to the electrical conductivity and microwave absorption behavior of their composites.
Results and discussion
Characterization of dry MWCNT powders
The XRD interlayer spacing d and width of the (002) peak, and the Raman band characteristics of the MWCNT powders
d (002) (Å)
FWHM (002) ( o )
I D /I G
FWHM G (cm −1 )
FWHM D (cm −1 )
MWCNT/epoxy nanocomposites prepared via the solution dispersion method
Particle size distribution of MWCNTs in ethanol dispersions
In the solution dispersion method, composites of MWCNTs and epoxy resin were fabricated by mixing the epoxy resin with nanotubes pre-dispersed in ethanol, followed by solvent evaporation afterward. The dispersion of MWCNTs in ethanol was conducted under ultrasonication, with the addition of 0.05 wt% of sodium dodecyl benzene sulfonate (NaDDBS), which is one of ionic surfactants commonly used to reduce the aggregative tendency of CNTs in water .
The initial swelling of CNT agglomerates by solvent infiltration and interaction has to be considered as a crucial precondition to obtain a good dispersion of CNTs inside the polymer matrix, which is a critical aspect for achieving good absorbing materials. Thus, investigations of the dispersability of different MWCNT materials in ethanol, via assessment of their average aggregated size and size distribution, were performed by laser diffraction particle size analysis. It has been reported that Nanocyl NC7000 and Baytubes C150P particles in ultrasonicated aqueous surfactant dispersions had rod-like shapes, as indicated by dynamic light scattering . It should be noted that the mean particle diameter obtained by this method does not refer directly to nanotube size, but to their agglomerate size, which is an average between tube bundle length and diameter.
Mean diameters (μm) of the MWCNTs obtained by laser diffraction particle size analysis with ethanol as dispersant
Dispersion in ethanol
Dispersion in ethanol with 0.05 wt% of NaDDBS
Microwave absorption of MWCNT/epoxy nanocomposites via the solution dispersion method
To study the microwave absorption performance of the MWCNT/epoxy composites, the reflection loss of the prepared metal-backed single-layered composites was measured in the X-band.
MWCNT/epoxy nanocomposites prepared via the ball-milling dispersion method
The influence of the MWCNT materials on the microwave absorption properties of their epoxy composites prepared via ball-milling dispersion of nanotubes in the resin matrix was further investigated. From a practical point of view, this dispersion method is advantageous especially for mass production, since it requires no addition of a solvent and thereby no solvent evaporation as well as ultrasonication and mechanical stirring. For all the MWCNT materials used, CNT loadings in the matrix for radar-absorbing study were limited to maximum 2 wt%, in order to ensure the composite structural integrity and mechanical properties.
Brookfield viscosity values measured for the epoxy resin and different ball-milled MWCNT/epoxy dispersions
CNT content (wt%)
Microwave absorption properties
Regarding the microwave absorption mechanism, the MWCNTs in the epoxy composites can absorb the microwave energy and attenuate the radiation via the interaction between interior electrons and exterior microwave radiation. On the other hand, the defects in MWCNTs can also act as polarization centers and contribute to strong microwave absorption, attributed mainly to the dielectric relaxation [33,34].
Besides the dielectric loss requirements, the impedance matching condition (where Z in is close to Z 0) is important to obtain a good microwave absorption.
As to be shown below, the prepared MWCNT-epoxy composites exhibited CNT content and frequency dependence of the microwave absorbing characteristics, which is attributed mainly to dielectric loss of the composites [50,52].
It should be emphasized that with a thickness of only 3 mm and low CNT contents, i.e. 2 wt% for Baytubes C150P and 0.25 wt% for Nanocyl NC7000, these composites showed reflection loss values much better than other pristine CNT/polymer composites with similar or lower thicknesses and CNT loadings below 4 wt% reported in the literature. For instance, the MWCNT/epoxy nanocomposite with 20 wt% CNT loading and 1.2 mm thickness reported by Che et al.  had a reflection loss of less than 2 dB. Thus, to gain desirable microwave absorption performance of pristine CNT/polymer nanocomposites, high CNT contents were utilized in many other studies. Fan et al.  applied twin-screw extrusion and sand-milling to prepare CNT/PET and CNT/varnish composites with 4 and 8 wt% of CNTs and thicknesses of 2 and 1 mm, showing reflection loss peaks at 7.6 and 15.3 GHz with maximum values of 17.61 dB and 24.27 dB, respectively. Liu et al.  prepared 2 mm thick CNT/polyurethane nanocomposites with 5 wt% of single-walled CNTs through solution mixing in dimethylformamide followed by slow drying, giving a maximum absorbing value of 22 dB at 8.8 GHz. In other studies on MWCNT/paraffin composites at a substantially high CNT loading of 20 wt%, the maximum absorbing values of the pristine CNT composites reported by Lin et al. [42,44] did not reach the acceptable limit above 10 dB, whereas those by Zhang et al. [45,46] achieved maximum peaks of 22 dB in the X-band region. Helical and worm-like MWCNT/paraffin composites with 30 wt% CNTs and 2.8-3 mm thicknesses have been reported to exhibit maximum reflection loss values of about 26 dB at 7–8 GHz . The nanocomposites of synthesized twin carbon nanocoils in paraffin were prepared obtained maximum reflection loss values above 10 dB in the X-band region at carbon nanocoil contents of 15–22 wt% and matching thicknesses of 3–3.5 mm . Bhattacharya et al.  prepared a 2 mm thick unmodified MWCNT/polyurethane nanocomposite at a 30 wt% CNT loading through solution blending using mechanical stirring, with the maximum reflection loss of 16.03 dB at 10.99 GHz. MWCNT/epoxy nanocomposites with CNT loadings, matching thicknesses and maximum reflection loss of 0.5 wt%, 9 mm, 25 dB at 11 GHz as well as 5 wt%, 3 mm, 18 dB at 8 GHz, respectively, have also been reported [53,54].
Electrical conductivities of 3 mm thick MWCNT/epoxy composites prepared via the ball-milling method with 2 wt% of MWCNT-VAST, 2 wt% of Baytubes C150P and 0.25 wt% of Nanocyl NC7000
MWCNT content (wt%)
Electrical conductivity (10 5 S/cm)
Three different commercially available carbon nanotube materials were studied with regard to the microwave absorption properties of their epoxy composites prepared using the solution mixing and ball-milling dispersion methods. The correlation of the microwave absorption performance of the composites with the CNT dispersability in the matrix and CNT characteristics could indirectly be indicated, to a certain extent, by the CNT agglomerate size in ethanol surfactant solutions, as well as the viscosity of the ball-milled CNT/epoxy dispersions. For all the CNT materials used, the spectra of the reflection loss versus frequency showed the presence of two minima. This phenomenon has been observed for the epoxy composites filled with porous carbon fibers, and was ascribed to the combination of absorption and interference of the microwaves .
The difference in microwave absorption of the composites of the different MWCNT materials did not correspond to the trend in the difference of the electrical conductivities. The best microwave absorption behavior was found for the composite of Nanocyl NC7000, even at a much lower CNT content as compared to Baytubes C150P and MWCNT-VAST. It was found that a high aspect ratio of CNTs resulting in microscopic alignment trend of nanotubes as well as a good level of micro-scale CNT dispersion resulting from good CNT-matrix interactions are crucial to obtain effective microwave absorption performance. Especially, Nanocyl NC7000, with a small mean tube diameter, thin tube wall, high length-to-diameter aspect ratio and uniform size distribution, proved to be the most suitable MWCNT material for the fabrication of effective MWCNT/polymer composite RAMs at very low CNT contents and small composite thicknesses. For instance, up to 2 wt% of Baytubes C150P was required to give a relatively effective 3 mm thick RAM with reflection loss above 10 dB. It is noted that the radar absorbing performance of the epoxy composites of Nanocyl NC7000 obtained in this work is considerably better than that of pristine CNT/polymer composites with similar or lower thicknesses and CNT loadings below 5 wt% reported so far .
Through this study, we demonstrate for the first time to the best of our knowledge, that by suitable selection of the MWCNT material, effective radar absorbing MWCNT/epoxy nanocomposites having small matching thicknesses of 2–3 mm and very low filler contents of 0.25-0.5 wt%, with microwave energy absorption in the X-band region above 90% and maximum absorption peak values above 97%, could be obtained via simple processing methods, which is promising for mass production in industrial applications.
Properties of the as-received MWCNTs according to the suppliers and literature
From the suppliers
Estimated by TEM/SEM (according to ref. [ 28 ] )
Carbon purity (%)
Bulk density (kg/m 3 )
Surface area (m 2 /g)
Average diameter (nm)
Average length (μm)
Average aspect ratio
5-20 (average 11 nm )
10-50 (average diameter 25 nm)
The polymer matrix used was an epoxy resin based on Bisphenol A epichlorohydrin cured by TETA, with a vitrification temperature of around 120°C .
Preparation of MWCNT/epoxy composites via the solution dispersion method
MWCNTs were dispersed in ethanol and the mixture was sonicated at 55°C for 60 min. Then, the epoxy resin (containing 20 wt% of RD 108) was added and the mixture was subjected to continuous simultaneous mechanical stirring and ultrasonication (50 Hz, 300 W) at 55°C for 120 min, followed by solvent evaporation while maintaining mechanical stirring at 80°C. Finally, the hardener (TETA) was added and the matrix was cured under ambient conditions for 24 h before characterization.
Preparation of MWCNT/epoxy composites via the ball-milling method
MWCNTs were mixed with the epoxy resin (containing 20 wt% of RD 108) and the mixture was subjected to ball-milling using a porcelain vertical style ball mill jar (capacity of 1 L) containing one pivot and 0.5 kg of porcelain balls of 10–20 mm diameters. The milling intensity was 300 rpm, the optimal milling time was 60 min and the weight of each batch was 300 g. After ball-milling, the hardener (TETA) was added and the matrix was cured under ambient conditions for 24 h before characterization.
Transmission electron microscopy
The morphology of MWCNT powders and the dispersion of MWCNTs in the cured epoxy matrix was observed by transmission electron microscopy (TEM, JEM 1400, JEOL, Japan) of 70 nm thick microtomed layers of the composites.
Raman spectra were recorded with a Horiba Jobin Yvon HR800 UV spectrometer using an excitation wavelength of 633 nm.
Wide-angle powder X-ray diffraction
Wide-angle powder X-ray diffraction (XRD) patterns were recorded at room temperature on a Bruker AXS D8 Advance diffractometer using Cu-Kα radiation (k = 0.15406 nm), at a scanning rate of 0.05 degrees per second. The data were analyzed using DIFRAC plus Evaluation Package (EVA) software. The d-spacing was calculated from peak positions using Cu-Kα radiation and Bragg’s law.
Laser diffraction particle size analysis
Laser diffraction particle size analysis was performed on a Horiba LA 920 analyzer, using ethanol as the dispersant. The CNT dispersions in ethanol were prepared at a concentration of 0.5 g/L. Approximately 5–10 mL of the CNT dispersions or 5–10 mg of the CNT powder were introduced into the 100 mL dispersion unit device of the laser particle analyzer for measurements, corresponding to a laser light transmission level between 85-95%. To maintain random orientation of particles in suspension, in-stream 30 watt-ultrasonication (power setting number 3, 1 min) and circulation (level 5) was applied during the measurements.
Electrical conductivity measurements
Reflection loss measurements
The composite samples for microwave absorption study were fabricated in a single-layered sheet form with dimensions of 150 × 150 × 2–3 mm.
Microwave absorption study at the 8–12 GHz band was performed on a two port vector network analyzer (Anritsu MS2028B; accuracy ± 0.05%, temperature stability ± 1.5 ppm), using a reflection/transmission method. The incident and transmitted waves in the two port vector network analyzer can be mathematically represented by complex scattering parameters (or S-parameters) i.e. S11 and S21, respectively, which in-turn can be conveniently correlated with reflectance (R) and transmittance (T), i.e. T = |ET/EI|2 = |S21|2, R = |ER/EI|2 = |S11|2, giving absorbance (A) as: A = (1-R-T), where EI, ER and ET are the power of incident, reflected and transmitted electromagnetic waves respectively. Practically, the reflection was measured at an incident angle of 90°. The electromagnetic wave was incident on the sample backed by metal plate resulting in T ≈ 0. Thus, the reflection loss can be measured as: RL = 10log10 (1- R).
The measurement uncertainties of the S-parameters and thickness (standard deviations calculated from measurements made on three nominally identical samples) in the frequency range of 8–12 GHz were about 4-5%.
The authors thank the Vietnam Ministry of Science and Technology for funding this research.
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