DSpace at VNU: Effects of Carbon Nanotube Dispersion Methods on the Radar Absorbing Properties of MWCNT Epoxy Nanocomposites

Macromolecular Research, Vol 22, No 11, pp 1221-1228 (2014) DOI 10.1007/s13233-014-2169-8 www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673 Effects of Carbon Nanotube Dispersion Methods on the Radar Absorbing Properties of MWCNT/Epoxy Nanocomposites Bien Dong Che1, Le-Thu T Nguyen*,2, Bao Quoc Nguyen1, Ha Tran Nguyen2, Thang Van Le2, and Nieu Huu Nguyen*,1 National Key Laboratory of Polymer and Composite Materials- Ho Chi Minh City University of Technology, Vietnam National University, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam Faculty of Materials Technology and Materials Technology Key Laboratory (Mtlab), Ho Chi Minh City University of Technology, Vietnam National University, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam Received April 16, 2014; Revised July 29, 2014; Accepted July 31, 2014 Abstract: Radar absorbing materials (RAMs) for practical applications are expected not only to have strong microwave absorption and a wide absorption bandwidth, but also to be lightweight, to have a fine thickness and acceptable structural performance, as well as being cost-effective Although the dispersion of carbon-nanofillers in polymer matrices is a key factor determining the microwave absorbing properties of the composites, there have few studies on these effects To our knowledge, to date, the realization of pristine multi-walled carbon nanotube (MWCNT)/polymer composites as RAMs in industrial production has been restricted, due to high CNT contents or large composite thicknesses Thus, in this work, two MWCNT dispersion processing methods, a solution process with surfactant-aid and a ball-milling dispersion, were investigated to fabricate pristine MWCNT/epoxy nanocomposites The effects of the different dispersion processes, CNT loading, and composite thickness on CNT dispersion in the matrix, were observed by TEM, and the electrical conductivity and X-band absorbing performance of the composites were assessed The use of an ionic surfactant to aid the dispersion of CNTs in solution resulted in the best RAMs, with a good compromise among effective X-band absorption, small composite thickness, and very low CNT content The ball-milling method also resulted in materials with a low CNT content and microwave absorbing performance acceptable for industrial applications Moreover, it offers a very simple and efficient route suitable for low-cost, mass production of RAMs The results showed that by facile approaches of dispersing pristine commercial MWCNTs in an epoxy resin matrix, composites of only 2-3 mm thickness and as little as 0.25-0.5 wt% CNT loading could be obtained, with a relatively wide X-band operating bandwidth and maximum absorptions exceeding 18-25 dB Keywords: radar absorbing materials (RAMs), carbon nanotubes, polymer composites, nanocomposites Introduction prompted extensive studies in the last decade Numerous composites based on carbon black, graphenes, fullerences, graphites, carbon nanotubes and nanofibers as radar absorbing materials (RAMs) have been reported.1,2 In this scenario, carbon nanotubes (CNTs) have demonstrated a potential as great conductive nanofillers with outstanding electrical properties, such as ultra-low percolation thresholds for both electrical conductivity and microwave absorbance,3,4 ascribed to the high aspect ratio between 100 and 1000 An effective RAM needs to achieve a reflection loss value in the X-band frequency region above 10 dB (more than 90% microwave energy absorbed) Thus, a wealth of experimental efforts has been devoted to enhance the microwave absorption efficiency of CNT/polymer composites through tailoring the geometry and composition of CNT fillers and host polymers, as well as CNT content, composite thickness, processing The research in the area of carbon nanostructure-filled polymer nanocomposites as microwave absorbers both in civil and military applications has gained remarkable attention, owing to their ability to tailor the electrical and magnetic properties at relatively low nanofiller concentrations, as well as their light weight, excellent thermal stability and high mechanical properties In particular, the demand for the operation of radar absorbing materials in the 8-12 GHz region (X-band) with enhanced shielding and microwave absorption effectiveness for applications in military communication satellites, Doppler and weather radars, television satellite transmitters and telephone microwave relay systems, has *Corresponding Author E-mail: huunieu@vnn.vn The Polymer Society of Korea 1221 B D Che et al technique and the dispersion of CNTs in the polymer matrix.1 To increase the reflection loss, which was less than dB in the range of 2-18 GHz, of a multi-walled CNT (MWCNT)/ epoxy nanocomposite with 20 wt% CNT loading and 1.2 mm thickness, Che et al.5 investigated the use of Fe-filled CNTs By filling crystalline -Fe into the carbon shells of CNTs, the reflection loss of Fe-filled CNT/epoxy composites was enhanced substantially up to 17-25 dB Other effective RAMs with high CNT loadings of 15 to 30 wt% have also been obtained by filling MWCNTs with Fe,6,7 Fe3C,7 cobalt,8 Er2O3,9 and Sm2O3,10 or by coating the MWCNT structure with TiO2 or Nickel.11-13 On the other hand, without modifying CNTs, numerous studies employing different polymer hosts and CNT-resin mixing methods have been conducted to gain desirable microwave absorption performance of pristine CNT/polymer nanocomposites via the use of either high CNT contents or large matching thicknesses Fan et al.14 applied twin-screw extrusion and sand-milling to prepare nanocomposites of MWCNTs and several polymer matrices such as PET, PP, PE and varnish CNT/PET and CNT/varnish composites with and wt% of CNTs and thicknesses of and mm were obtained, showing reflection loss peaks at 7.6 and 15.3 GHz with maximum values of 17.61 and 24.27 dB, respectively Liu et al.15 prepared mm thick CNT/polyurethane nanocomposites with 0-25 wt% of single-walled CNTs (SWCNTs) through solution mixing in dimethylformamide followed by slow drying wt% was the optimal CNT loading 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.6,8 did not reach the acceptable limit above 10 dB, whereas those by Zhang et al.9,10 achieved maximum peaks of 22 dB in the X-band region Despite the use of the same source of MWCNTs in these works, such different absorbing performance might originate from the difference in CNT dispersion quality Helical and worm-like MWCNT/paraffin composites with 30 wt% CNTs and 2.8-3 mm thicknesses have also been reported, exhibiting maximum reflection loss values of about 26 dB at 7-8 GHz.16 Twin carbon nanocoils were synthesized and their nanocomposites in paraffin were prepared, obtaining 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.17 Lately, Bhattacharya et al.12 prepared a 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 Using ultrasonication and ultraturax mixing to disperse CNTs in epoxy resins, MWCNT/epoxy nanocomposites with CNT loadings, matching thicknesses and maximum reflection loss of 0.5 wt%, mm, 25 dB at 11 GHz as well as wt%, mm, 18 dB at GHz, respectively, have been reported.18,19 1222 However, either the high CNT loadings of 4-30 wt% or large composite thicknesses reported so far for pristine CNT/ polymer composite RAMs can be drawbacks limiting their commercial applications In this sense, light weight, thin composites are preferred Moreover, it is known by the literature that the elastic modulus of MWCNT/epoxy increases with CNT content.20,21 However, the fracture strength decreases with increasing CNT concentration.20,22 The strain-at-break of MWCNT/polypropylene composites has been reported to decrease significantly at and wt% CNT loadings.21 Hence, addition of more than several weight percents of CNTs may not maintain the structural mechanical integrity of composites A good dispersion of CNTs in composites is a crucial factor for optimization of their performance Various processing techniques to enhance the dispersion of CNTs have been suggested, such as melt mixing using extruders and solvent processing by means of centrifugation, ultrasonication and surfactant treatment, as well as chemical modification of CNTs.23-28 While a majority of works have studied the effects of dispersion methods on the mechanical, thermal and electrical properties of carbon filler/polymer composites,21,23,24,28-37 literature on the dependence of microwave absorption characteristics on the dispersion conditions is sparse.38 Nanni et al.38 observed the influence of the organic solvent removal conditions, i.e via evaporation or filtration after dispersion of carbon nanofibers (CNFs) in the epoxy matrix, on the aggregation of CNFs This led to different microwave absorbing performance Optimization of the filler content and matching thickness resulted in mm thick CNT/epoxy composites with 3-4 wt% CNT contents and maximum absorption peak values of 20-25 dB in the region above X-band, of 14-20 GHz In consideration of the key role of CNT dispersion in the aggregation and agglomeration of CNTs and hence material electromagnetic characteristics, in this work we studied the effects of two dispersion methods on the X-band microwave performance of MWCNT/epoxy nanocomposites The dispersion processes in solution with the aid of a surfactant and in bulk via ball-milling were employed for the fabrication of composites with various CNT loading contents Epoxy resin was chosen as the matrix because of its wide practical applications owing to the low cost, resistance to oxidative photodegradation and stability against UV light An overall investigation of the influence of fabrication conditions on the microwave absorption behavior of the composites was conducted, considering two practical important issues, i.e weight reduction and optimization of the operating bandwidth and absorption A good compromise between the microwave absorption performance, composite mechanical properties and especially costeffectiveness can be a challenge in realizing RAMs in real applications Alternatively, studies on hybrid composites of CNTs and metallic magnetic particles requiring specific inhouse particle synthesis and CNT treatment to enhance microwave absorption properties have been ongoing.1 Despite this, it Macromol Res., Vol 22, No 11, 2014 Effects of Carbon Nanotube Dispersion Methods on the Radar Absorbing Properties of MWCNT/Epoxy Nanocomposites is without question that a simple fabrication procedure of industrial grade MWCNT/polymer composites meeting the essential criterion of cost-versus-performance can be of particular attraction Through this work, we demonstrate for the first time to the best of our knowledge, that by processing design, effective radar absorbing MWCNT/epoxy nanocomposites with microwave energy absorption in the X-band region above 90% and maximum absorption peak values above 99%, could be obtained This pathway shows many advantages such as simple and easily upscalable production, very low CNT loadings (0.25-0.5 wt%), and a small matching thickness (2 mm) Experimental Materials NanocylTM NC7000 multi-walled carbon nanotube (MWCNT) material was purchased from Nanocyl S.A., Sambreville, Belgium According to the manufacturer, they have an average diameter of 9.5 nm, average length of 1.5 m and surface area of 250-300 m2/g Ethanol (99.5%, Chemsol), sodium dodecyl-benzene sulfonate (NaDDBS, Sigma-Aldrich), D.E.R.TM 331 epoxy resin (Dow) and triethylenetetramine (TETA, Dow) were used as purchased Preparation of Nanocomposites MWCNT/Epoxy Composites via the Solution Dispersion Method: MWCNT/epoxy nanocomposites containing different CNT contents (0.25, 0.5, 0.75, 1, 1.25, 2, and wt%) and with thicknesses of and mm were prepared MWCNTs were dispersed in ethanol and the mixture was sonicated at 55 oC for 60 Then, the epoxy resin was added and the mixture was subjected to continuous simultaneous mechanical stirring and ultrasonication at 55 oC for 120 min, followed by solvent evaporation while maintain mechanical stirring at 80 oC Finally, the hardener (TETA) was added and the matrix was cured under ambient conditions for 24 h before characterization MWCNT/Epoxy Composites via the Ball-Milling Method: MWCNT/epoxy nanocomposites containing different CNT contents (0.25, 0.5, 0.75, 1, and 1.25 wt%) and with a thickness of mm were prepared MWCNTs were mixed with the epoxy resin and the mixture was subjected to ball-milling using a porcelain vertical style ball mill jar (capacity of 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 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 Characterization The dispersion of MWCNTs in the cured epoxy matrix was observed by transmission electron microscopy (TEM, JEM 1400, JEOL, Japan) Measurements of electrical conductivities of the samples were performed by a two-probe method using the Keithley Model 2750 multimeter (Keithley Instruments Inc., USA) Microwave absorption Macromol Res., Vol 22, No 11, 2014 study at the 8-12 GHz band was performed on a two port vector network analyzer (Anritsu MS2028B), using a reflection/transmission method The reflection loss (RL) of a single-layered electromagnetic absorber is defined as:39 Zin – Z0 RL = 20log10 -Zin + Z0 (1) j2 r  r r fd Zin = Z0 r c (2) where Zin is the normalized input impedance at free space and material interface, Z0 is the characteristic impedance of free space, r and r are respectively the complex relative permeability and permittivity of the material, c is the velocity of light, f is the frequency and d is the sample thickness The relationship between frequency and thickness can be interpreted as:1 c f = r d (3) where ''r is the imaginary part of relative permeability Results and Discussion MWCNT/Epoxy Nanocomposites via the Solution Dispersion Method Influence of CNT Content and Composite Thickness: Via the solution mixing method for dispersion of CNTs in the epoxy matrix, combining ultrasonication, mechanical stirring and the use of ethanol as the dispersed solvent which was evaporated afterward, composites of NanocylTM NC 7000 MWCNTs and epoxy resin were fabricated The microwave absorption, operating frequencies and frequency bandwidth are known to be dependent on the CNT filler content and matching thickness.1 Thus, these parameters were varied in order to optimize the X-band microwave absorption performance Figure shows the variation of electrical conductivity versus CNT weight content for composite samples with a thickness of mm A low electrical percolation threshold, defined as a critical filler concentration to achieve a conductivity of 10-8 S/ cm, less than 0.25 wt% is observed Above the percolation threshold, increasing CNT content led to increases in electrical conductivity, as a result of the increase of conductive inclusions making electrical paths inside the matrix Further increasing CNT content to wt% significantly enhanced the conductivity However, such a high CNT fraction compromised the composite structural integrity We observed that the wt% CNT composite was brittle and appeared to crack upon curing Thus, to ensure the mechanical properties of the composites, the addition of MWCNTs in the matrix for radar-absorbing study was limited to maximum wt% The frequency dependence of the microwave absorbing 1223 B D Che et al Figure Electrical conductivities versus CNT content of MWCNT/ epoxy composites (of mm thickness) prepared via the solution dispersion method characteristics in the X band region of the MWCNT/epoxy composites prepared via the solution dispersion method is presented in Figure The effects of variation in both CNT content and matching thickness were evaluated It is seen that, irrespective of the composite thickness, the maximum absorption increases with CNT content up to 0.75 wt%, above which it drops significantly Such phenomenon was explained by the fact that the material needs to satisfy not only dielectric loss requirements but also importantly the impedance matching condition (where Zin is close to Z0, eqs (1) and (2)) The observation of an optimal CNT content for optimal absorbing ability has been found for other composites of SWCNTs and MWCNTs.14,15,38 Below the CNT loading of 0.75 wt%, the increase in microwave absorption with CNT content could be attributed to the enhancement of dielectric loss tangent, the factor mainly contributing to the attenuation of microwave energy of carbon nanofiller composites.15,17 At CNT contents above 0.75 wt%, it was likely that conductivity prevailed, as a result of short-range electric multipole interactions,14 making the material mostly reflective Nevertheless, at mm matching thickness, reflection loss peaks did not lie in the desired frequency range of 8-12 GHz As also shown in Figure 2, the microwave absorbing properties were influenced greatly by tuning the nanocomposite thickness Below 0.75 wt% CNT loading, increasing the thickness to mm resulted in considerable enhancement of absorbing performance as well as shifts of maximum reflection loss to lower frequencies, which can be explained by the relationship between frequency and thickness described in eq (3) In addition, it was clearly observed that at this matching thickness, increasing CNT concentration led to shifts of reflectivity peaks toward lower frequencies The peak values reached maxima of -22.2 dB at 11.2 GHz at 0.25 wt% CNT content, and -32.4 dB at 8.2 GHz at 0.5 wt% CNT Thus, to achieve the operating frequency at 8-12 GHz, the optimal CNT content should be below 0.5 wt% at a matching thickness of mm Influence of the Use of an Ionic Surfactant: Although the microwave absorption could be optimized by adjusting both CNT content and matching thickness, as usually performed in the literature,1 we found that the aid of an ionic surfactant in the dispersion process had a significant effect on maximizing absorbing properties of the MWCNT/epoxy nanocomposites Apparently, a good dispersion of CNTs inside the matrix is a critical aspect for achieving good absorbing materials Chemical modification methods (for example to provide amine-functionalized CNTs) for enhancing the dispersion and compatibility of CNTs to the epoxy matrix, have shown to be detrimental for the overall electrical conductivity.40 Unlike chemical functionalization pathways, the surfactant treatment has been reported to exhibit little adverse effect on the electrical properties of CNT/epoxy nanocomposites.31 Thus, sodium dodecyl benzene sulfonate (NaDDBS), one of ionic surfactants commonly used to reduce the aggregative tendency of CNTs in water,25 was used for the preparation of NanocylTM NC 7000 MWCNT/epoxy composites Figure Reflection loss versus frequency at different CNT contents of MWCNT/epoxy composites prepared via the solution dispersion method, with thicknesses of (a) and (b) mm 1224 Macromol Res., Vol 22, No 11, 2014 Effects of Carbon Nanotube Dispersion Methods on the Radar Absorbing Properties of MWCNT/Epoxy Nanocomposites Table I Electrical Conductivities of mm Thick MWCNT/ Epoxy Composites with Various CNT and NaDDBS Contents MWCNT Content (wt%) NaDDBS Content Electrical Conductivity (wt%) (S/cm) 0.25 1.196×10-5 0.25 0.05 12.275×10-5 0.5 1.787×10-5 0.5 0.05 16.979×10-5 Their X-band absorbing performance was assessed while maintaining low CNT contents of 0.25 and 0.5 wt% and a matching thickness of mm Despite the enhancement of thermal and mechanical properties of CNT nanocomposites by a good dispersion of CNTs in the epoxy matrix, the best dispersion conditions resulting in an insulating resin layer between CNTs may reduce the conductivity.32 Hence, the content of added NaDDBS was optimized preferably as low as possible, being 0.05 wt% As shown in Table I, with the use of NaDDBS, the electrical conductivities of the nanocomposites increased considerably, approximately by ten times From TEM analysis (Figure 3), it appeared that without using NaDDBS in the dispersion process, CNTs were entangled in ropes In this case, it seemed that their intrinsic properties such as aspect ratio and surface area were less effective in creating conductive pathways The addition of NaDDBS allowed the disassembly of ropes into individual CNT tubes, with a tendency of being aligned in the same directions as a result of their conglomeration trend This arises from the adsorption of surfactants around the nanotubes because of the strong hydrophobic interactions of NaDDBS alkyl chains as well as π-stacking interactions of the surfactant benzene groups with CNTs, besides the hydrogen bonding between sulfonate groups and the epoxy matrix It is likely that a good dispersion of CNTs exhibiting an anisotropic morphology, with a certain aspect ratio, of CNT bundles is a control parameter to constitute a conductive network inside the matrix In accordance to the enhanced dispersion and electrical conductivity of the composites, the use of NaDDBS as a pro- Figure TEM micrographs of mm thick, 0.25 wt% CNT MWCNT/ epoxy composites prepared via the solution dispersion method without (a) and with the use of 0.05 wt% NaDDBS (b) (scalebar: 100 nm) Macromol Res., Vol 22, No 11, 2014 Figure Reflection loss versus frequency of mm thick MWCNT/ epoxy composites prepared via the solution dispersion method, with 0.25 and 0.5 wt% of CNT contents, with and 0.05 wt% of NaDDBS cessing aid led to drastic increases in microwave absorbing properties Without the use of the surfactant, the composites satisfying both a small matching thickness not more than mm and a low CNT content not more than 0.5 wt% exhibited little microwave absorption Similar results have often been observed for pristine CNT/polymer composites in the literature As shown in Figure 4, with the use of 0.05 wt% of NaDDBS and at a matching thickness of mm, the 0.25 wt% CNT composite shows maximum reflection loss peaks of 17.9 dB at 9.2 GHz and 21.5 dB at 10.6 GHz, while the 0.5 wt% CNT composite exhibits a reflection loss peak with the maximum value of 26.1 dB at 11.2 GHz In the contrary to the case of mm matching thickness and without using a surfactant, here it appeared that the maximum reflection loss peak shifted to higher frequency with increasing the CNT content from 0.25 to 0.5 wt% Interestingly, the bandwidth also increased Especially, the 0.25 wt% CNT composite exhibited a wide X-band operating bandwidth (corresponding to reflection loss values above 10 dB) from 8.8 to 11.4 GHz MWCNT/Epoxy Nanocomposites via the Dry State Dispersion Method Using Ball-Milling Ball-milling in the dry state to disperse CNTs in the epoxy matrix was employed to prepare MWCNT/epoxy nanocomposites with different CNT contents Such a method requires no addition of a solvent and thereby no solvent evaporation as well as ultrasonication and mechanical stirring, which is advantageous for mass production in industrial applications The ball-milling time was limited to 60 min, which was optimal for the dispersion of CNTs without any noticeable decrease of CNT lengths As the reflection loss performance generally increases with matching thickness, the composite thickness was optimized, in terms of giving both reasonably good reflection loss and relative thinness, being mm From the morphological observation by TEM, as shown 1225 B D Che et al Figure TEM micrographs of mm thick MWCNT/epoxy nanocomposites prepared using the ball-milling method with CNT contents of 0.125 (a), 0.25 (b), 0.5 (c), 0.75 (d), (e), and 1.25 (f) wt% in Figure 5, it is observed that at CNT contents below wt%, the nanotubes were mostly dis-entangled and dispersed relatively homogeneously in the matrix There was the co-existence of a small fraction of CNT aggregates as randomly dispersed entangled clusters of a few hundreds of nanometer size at CNT contents of 0.5 and 0.75 wt%, which was more visible at the higher CNT content At wt% CNT loading, the CNTs were still quite homogeneously dispersed, despite that they tended to conglomerate as a result of their dense coverage in the matrix Further increasing CNT content to 1.25 wt% led to non-uniform distribution of entangled CNT bundles As shown in Figure 6, according to the increased coverage of CNTs in the matrix, the electrical conductivity increases greatly with CNT content owing to the formation of denser conductive networks Above wt% CNT loading, the electrical conductivity increased inconsiderably due to the uneven spreading of CNTs across the matrix A high CNT content giving a high electrical conductivity is not necessary to be optimal for microwave absorbing properties As shown in Figure 7, the X-band absorption of the composites prepared via the ball-milling method shows a similar trend to that of the composites prepared via the solution dispersion method For CNT contents between 1226 Figure Electrical conductivities versus CNT content of mm thick MWCNT/epoxy composites prepared via the ball-milling method 0.25 and 0.75 wt%, microwave absorption above 10 dB was obtained, with the maximum reflection loss peaks in the Xband region shifting to lower frequencies with increasing CNT content The 0.25 wt% CNT composite showed maximum reflection loss peaks of 16.5 dB at 10.3 GHz and 18.4 Macromol Res., Vol 22, No 11, 2014 Effects of Carbon Nanotube Dispersion Methods on the Radar Absorbing Properties of MWCNT/Epoxy Nanocomposites frequency and CNT content In summary, composites of thicknesses of 2-3 mm prepared using both dispersion methods, i.e in solution with the aid of a surfactant and through ballmilling, exhibited optimal X-band microwave absorbing performance at only 0.25 wt% CNT, with relatively wide working bandwidths The surfactant-aiding solution dispersion method produced RAMs with slightly better reflection loss values as well as higher electrical conductivities for similar CNT contents, although at a smaller matching thickness This is ascribed to better CNT dispersion in the epoxy matrix Nevertheless, the ball-milling dispersion method still offers good RAMs for industrial applications and is a facile processing route with elimination of the steps of ultrasonication and solvent evaporation Figure Reflection loss versus frequency of mm thick MWCNT/ epoxy composites with different CNT contents prepared via the ball-milling method dB at 8.8 GHz, while the 0.5 wt% CNT composite exhibited maximum reflection loss peaks of 14.5 dB at 9.8 GHz and 16.7 dB at 8.1 GHz On the other hand, CNT contents below 0.5 wt% and above 0.75 wt% led to ineffective microwave absorption performance A comparison of the X-band microwave absorption properties of MWCNT/epoxy composites prepared using the ball-milling, solution mixing and surfactant-aiding solution mixing dispersion methods, with 0.25 and 0.5 wt% CNT contents is shown in Figure It is clearly observed that the dispersion method can change drastically the reflection loss characteristics, not only the reflection peak values and bandwidth, but also the correlation between the absorption peak Conclusions In this paper, two MWCNT dispersion processes, a solution process with surfactant-aid and room-temperature ballmilling, were employed to prepare industrial grade-MWCNT/ epoxy nanocomposites as radar absorbing materials The effects of these methods as well as CNT content, matching thickness and the addition of a surfactant on the electrical conductivity and X-band absorbing performance of the composites were investigated Both methods resulted in MWCNT/epoxy composite RAMs compromising both very low CNT contents of only 0.25-0.5 wt% and small matching thicknesses of 2-3 mm as well as good absorbing properties At only 0.25 wt% CNT addition, they showed microwave absorption values above 10 dB in wide frequency ranges of 8.8-11.4 and 8.5-11 GHz, with maximum reflection loss peaks between 16.5 and 21.5 dB These values are considerably better than the absorbing performance of pristine CNT/polymer composites with similar or lower thicknesses and CNT loadings below wt% reported so far The results showed that such facile processing routes, not requiring any in-house synthesis or chemical modification of the CNT structure, are of particular attraction for industrial production of cost-effective and lightweight radar absorbers Acknowledgments The authors thank the Vietnam Ministry of Science and Technology for funding this research References Figure Comparision of the X-band microwave absorption of MWCNT/epoxy composites prepared using the ball-milling (triangles), solution mixing (squares) and surfactant-aiding solution mixing (circles) dispersion methods, with 0.25 (filled symbols) and 0.5 wt% (open symbols) CNT contents The composites prepared via the ball-milling method had thicknesses of mm, while those via the solution dispersion method had thicknesses of mm Macromol Res., Vol 22, No 11, 2014 (1) F Qin and C Brosseau, J Appl Phys., 111, 061301 (2012) (2) D Micheli, C Apollo, R Pastore, and M Marchetti, Compos Sci Technol., 70, 400 (2010) (3) J K W Sandler, J E Kirk, I A Kinloch, M S P Shaffer, and A H Windle, Polymer, 44, 5893 (2003) (4) K R Paton and A H Windle, Carbon, 46, 1935 (2008) (5) R C Che, L M Peng, X F Duan, Q Chen, and X L Liang, Adv Mater., 16, 401 (2004) (6) H Lin, H Zhu, H Guo, and L Yu, Mater Lett., 61, 3547 (2007) (7) Q Su, G Zhong, J Li, G Du, and B Xu, 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nanofibers (CNFs) in the epoxy matrix, on the aggregation of CNFs... reflection loss peaks of 16.5 dB at 10.3 GHz and 18.4 Macromol Res., Vol 22, No 11, 2014 Effects of Carbon Nanotube Dispersion Methods on the Radar Absorbing Properties of MWCNT/ Epoxy Nanocomposites. .. thicknesses of (a) and (b) mm 1224 Macromol Res., Vol 22, No 11, 2014 Effects of Carbon Nanotube Dispersion Methods on the Radar Absorbing Properties of MWCNT/ Epoxy Nanocomposites Table I Electrical Conductivities