effective improvement of the properties of light weight carbon foam by decoration with multi-wall...

Journal ofMaterials Chemistry A

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View Article OnlineView Journal | View Issue

Division of Materials Physics and Engineerin

K. S. Krishnan Marg, New Delhi-110012, In

Cite this: J. Mater. Chem. A, 2013, 1,5727

Received 7th February 2013Accepted 6th March 2013

DOI: 10.1039/c3ta10604g

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

Effective improvement of the properties of light weightcarbon foam by decoration with multi-wall carbonnanotubes

Rajeev Kumar, Sanjay R. Dhakate,* Tejendra Gupta, Parveen Saini, Bhanu P. Singhand Rakesh B. Mathur

In the present investigation, carbon foam (CF) has been decorated with multi-wall carbon nanotubes

(MWCNTs) by two different routes to improve its electromagnetic interference (EMI) shielding

effectiveness (SE) and mechanical properties. In the first case, a MWCNTs incorporated carbon precursor

was used for the development of CF whereas in the other case, MWCNTs were directly grown over CF

by a chemical vapor deposition (CVD) technique. These foams were characterized by scanning electron

microscopy, Raman spectroscopy, X-ray diffraction and vector network analyzer for its EMI-SE. It was

observed that, EMI-SE was dominated by reflection phenomena and it increased with an increasing

MWCNTs content. The MWCNTs incorporated CF demonstrated a maximum EMI-SE value of �72 dB at

1 wt% MWCNTs. In comparison, MWCNTs directly grown on CF gives a maximum EMI-SE of �85 dB at

only 0.5 wt% MWCNTs. The higher extent of improvement in EMI-SE in MWCNTs decorated CF was due

to the increase in surface area and surface conductivity. The specific shielding effectiveness was 163 dB

cm3 g�1 for MWCNTs decorated carbon foam of thickness 2.75 mm. This is the highest value reported in

the open literature for CF in the X-band (8.2 to 12.4 GHz) frequency region. Moreover, besides EMI-SE

improvement, compressive strength and thermal conductivity were increased by 66 and 75% respectively.

1 Introduction

The functioning of aerospace and aircra power systemssignicantly depends upon electronic systems, which not onlyrequire shielding against electromagnetic interference (EMI),but also proper thermal interfacing to avoid the overheating ofelectronic systems. EMI may come in the form of lightningstrikes, interference from radio emitters, nuclear electromag-netic pulses or even high power microwave threats. Light weightshielding materials and structure technologies are needed tomitigate EMI to protect humans from the hazards of spaceradiation in aerospace vehicles in the frequency range 8.2 to12.4 GHz.1,2 Metals are the most widely used shielding materialsbut they suffer from problems such as high density, corrosion,difficult/uneconomic processing and a low specic shieldingcapability.3 Among other shielding alternatives, carbon basedmaterials (e.g. graphite, carbon black, carbon bers, carbonnanotubes, graphene etc.) have gained popularity because oftheir high electrical/thermal conductivity, low density, goodcorrosion resistance, thermal/environmental stability and pro-cessing advantages.4 Generally, technologists and scientists arelooking for highly efficient, thermally conducting and lightweight EMI shielding materials, particularly for aerospace

g, CSIR-National Physical Laboratory, Dr.

dia. E-mail: [email protected]

Chemistry 2013

transportation vehicles and space structures application.Recently, light weight carbon foam (CF) has emerged as apromising candidate for EMI shielding owing to its outstandingproperties such as low density, large surface area with an opencell wall structure, good thermal/electrical transport propertiesand mechanical stability.5,6 Light weight CF is a sponge-like,rigid and high performance engineering material in whichcarbon ligaments are interconnected to each other. The elec-trical conductivity of CF derived from different organic andinorganic precursors can be tailored by controlling the pro-cessing parameters. The early CF was prepared from thermo-setting polymeric material by heat treatment under a controlledatmosphere.7 Later on, coal tar and petroleum pitches wereused for CF synthesis.8 The foam derived from organic polymerand pitches gives low thermal conductivity, and these werepredominantly used as a thermal insulation material.9–11 Tomake highly crystalline CF of high electrical and thermalconductivity, generally mesophase pitch is used as the startingmaterial12,13 and it is prepared by a high temperature andpressure foaming process. It is an expensive process, thereforein the present study, using the simple sacricial templatetechnique14 CF is developed from the coal tar pitch.15 But coaltar pitch based CF does not gives the high electrical conductivityand hence the poor shielding effectiveness. To overcome thepoor SE of coal tar pitch based CF, a new approach of nano-structuring is adopted to improve the overall properties of CF by

J. Mater. Chem. A, 2013, 1, 5727–5735 | 5727

http://dx.doi.org/10.1039/c3ta10604g

http://pubs.rsc.org/en/journals/journal/TA

http://pubs.rsc.org/en/journals/journal/TA?issueid=TA001018

Journal of Materials Chemistry A Paper

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taking the advantages of the outstanding properties of themicro and nano forms of carbon e.g. coal tar pitch and carbonnanotubes (CNTs). Since their rst observation by Iijima,16

CNTs have attracted considerable attention because of theirexcellent electrical conductivity (103 to 106 S cm�1), ultrahighstrength and large aspect ratio, which have made them an idealreinforcing additive for the development of high performanceadvanced materials.17–19 In the past, many efforts have beenmade to improve the electrical conductivity and hence the EMIshielding effectiveness of CNTs–polymer composites by themeticulous incorporation and dispersion of CNTs.20 However,to the best of our knowledge, no attempt has been made toutilize the high electrical conductive and high surface areaCNTs in improving the EMI shielding effectiveness of CFs.

In the present investigation, an effort has been made todevelop multi-wall carbon nanotubes (MWCNTs) decorated CFswith improved electrical/thermal conductivity, compressivestrength, EMI shielding effectiveness and thermal stability. TheMWCNTs decorated CFs were characterized by various tech-niques and their EMI shielding capability was demonstratedindicating potential applications in EMI shielding control foraerospace transportation vehicles and space explorationstructures.

2 Experimental2.1 Material production

The CF was developed by a sacricial template technique frommodied coal tar pitch. The starting coal tar pitch possesses asoening point of 86.6 �C, quinoline insoluble (QI) 0.2%,toluene insoluble (TI) 15.9% and coking value 47.6%. The coaltar pitch was modied by heat treating at 400 �C for 5 hours.The modied coal tar pitch possess a soening point of 236 �C,QI content 23.6%, TI content 63.0% and coking value 78.5%respectively. The water slurry of modied coal tar pitch with 3%polyvinyl alcohol (PVA) was impregnated into a polyurethanefoam (density 0.030 g cm�3 and average pore size 0.45 mm)template under vacuum. The modied coal tar pitch impreg-nated polyurethane foam was converted into CF by several heattreatments in air, as well as in an inert atmosphere up to2500 �C.15 Initially, the modied coal tar pitch impregnatedfoams were heat treated @1 �C min�1 up to 275 �C in nitrogenatmosphere for 1 h followed by oxidation and stabilization in airatmosphere at a temperature of 300 �C. The stabilized foam wascarbonized in a tubular high temperature furnace at 1000 �Cwith heating rate 10 �C h�1 in an inert atmosphere. Thecarbonized foam was later on heat treated at a temperature of2500 �C at the heating rate of 15 �C min�1 in an inert atmo-sphere in the resistive heating high temperature furnaceprocured from Thermal Technology INC, USA.

In one case, MWCNTs were dispersed in an organic solvent,such as ethanol and mixed with the modied coal tar pitch indifferent weight fractions (0.5, 1.0 and 2.0 wt%). The MWCNTsincorporated CF was then developed by a sacricial templatetechnique from the mixture of modied coal tar pitch andMWCNTs. However, in the other case MWCNTs were directlygrown over 2500 �C heat treated CF by CVD.21,22 Initially, the CF

5728 | J. Mater. Chem. A, 2013, 1, 5727–5735

was kept inside a quartz reactor of the CVD furnace and thetemperature of the reaction zone was maintained at 750 �C.Once the desired temperature was reached, the solution offerrocene and toluene was injected in the reactor @20 ml h�1.The argon gas was also fed along with a solution of ferroceneand toluene, as a carrier gas and its ow rate 2 l min�1 wasadjusted so that the maximum amount of precursor must havebeen consumed inside the desired zone. The reaction parame-ters were controlled to grow different amounts of MWCNTs onCF. The MWCNTs incorporated and directly grown on CF weredesignated as; without (CF-A), 0.5 wt% MWCNTs incorporated(CF-B), 1.0 wt% MWCNTs incorporated (CF-C), 2.0 wt%MWCNTs incorporated (CF-D), 0.5 wt% MWCNTs decorated CF(CF-E) and 1.0 wt% MWCNTs decorated CF (CF-F).

2.2 Characterization

The bulk density of the foam was measured by ASTM standard(ASTM C559), the bulk density was a ratio of the weight of CF inair and the volume of the CF. The weight of the CF wasmeasured by a digital balance (model ME40290) and the volumeof foam by measuring the dimensions with the help of digitalvernier calipers.

The morphology of the CF, MWCNTs incorporated CF andMWCNTs grown CF was observed by scanning electron micro-scope (SEM model LEO 440). The morphology of the CF wasobserved aer the ultra-sonication in deionized water andacetone to remove the lose MWCNTs on the surface and insidethe pores of the foam in the case of MWCNTs grown CF.

Electrical conductivity of the CF was measured using thefour-probe technique. Kiethley 224 programmable currentsource was used for providing a constant current (I). The voltagedrop (V) in between two pinpoints with a span of 1.2 cm wasmeasured by Keithley 197A auto rangingmicrovolt digital multi-meter. The compressive strength of all CF was measured on auniversal INSTRON testing machine model 4411 as per theASTM standard. The thermal conductivity of CF was measuredby a laser ash method having a xenon laser as a source in athermo ash line 2003 instrument (Anter Corporation, USA). Bythe laser ash method, the thermal diffusivity and specic heatof each sample was measured at 25 �C. The thermal conduc-tivity was then calculated from the equation, a ¼ k/rCp, where ais the thermal diffusivity, k is the thermal conductivity, Cp is thespecic heat and r is the density of the foam.

A Raman spectrum of CF was recorded using a RenishawRaman spectrometer, UK, with an excitation wavelength514 nm. All the CF was also characterized by X-ray diffractom-eter (XRD, RIGAKU Tokyo) to understand the structural changesthat take place aer the MWCNTs growth on the CF. The surfacearea of CF and MWCNTs decorated CF was measured by Auto-sorb 3B, Quantacrome Instruments by the gas sorptiontechnique.

Electromagnetic interference (EMI) shielding effectivenesswas measured by a waveguide using a vector network analyzer(VNA, E8263BAgilent Technologies). The rectangular samples ofthickness 2.75 � 0.05 mm were placed inside the cavity of asample holder which matches the internal dimensions of the

This journal is ª The Royal Society of Chemistry 2013


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X-band (8.2–12.4 GHz) waveguide. The sample holder wasplaced between the anges of the waveguide connected betweenthe two ports of the VNA. A full two-port calibration was per-formed using quarter wavelength offset and terminations andkeeping the input power level at �5.0 dBm.

3 Results and discussion3.1 Microstructure of the CFs

Fig. 1 shows the SEM micrographs of the CF (CF-A), MWCNTsincorporated CF (CF-C) and MWCNTs decorated CF (CF-E). Inthe CF (Fig. 1a), the pores are uniformly distributed and somepore walls (i.e. ligaments) are broken during the machining ofsamples for SEM characterization because of the brittle natureof the material. In CF-C, encapsulation of MWCNTs in theligament leads to wider and thicker cell walls as compared toCF. Further, the number of cracks got reduced in the CF-C dueto the presence of MWCNTs that can act as a barrier for prop-agating the cracks. The MWCNTs also act as nucleation sites ina modied coal tar pitch derived carbon for the alignment of

Fig. 1 SEM micrographs of (a) CF, (b) 1 wt% MWCNTs incorporated CF(c) MWCNTS decorated CF, (d) uniform growth of MWCNTs in ligaments, (e)MWCNTs grown on the surface at higher magnification, (f) micrograph ofMWCNTs grown in the cracks of CF, (g) 1 wt%MWCNTs directly grown on CF withcarbon particles and, (h) a region of (g) carbon particles at higher magnification.


carbon atoms or graphene layers along the MWCNTs axis(Fig. 1b, MWCNTs incorporated CF with 1 wt%). Fig. 1c showsthe SEM image of CF-E which reveals the extensive growth ofMWCNTs over ligaments, as well as pores which inuence theligament thickness and pore size. Further, homogeneous anduniform distribution of MWCNTs over the entire surface ofligaments of CF is also observed (Fig. 1d). Fig. 1e is a magniedview of CNTs directly grown on the surface of CF-E.

In this study initially the CF is placed in the quartz reactor.Once the temperature in the quartz reactor at the zone (on CF)reached 750 �C, a solution of ferrocene and toluene is ejected.The ferrocene decomposes at a temperature >500 �C and thisresults in the formation of elemental nanosize iron particles.These nanosize iron particles are inltrated in the cracks,deposited on the surface of ligaments, inside and on the surfaceof pores. The iron nanoparticles are inltrated in the cracks ofCF, this resulted in the MWCNTs also growing in the valley ofcracks (Fig. 1f). The formation of MWCNTs is highly dependenton the catalyst. It is known that the catalyst particle has to be ofa certain size in order for the MWCNTs to form, beyond whichthe MWCNTs formation is hindered.23,24 Therefore, withincreasing the reaction time, there is a possibility of a cluster ofiron particles forming on the surface of CF, which can possiblyrestrain the uniform growth of MWCNTs and formation ofcarbon particles (Fig. 1g).25 The MWCNTs diameters vary from75–100 nm, which depends upon the reaction time, ow rate ofthe hydrocarbon and catalyst particle size. On the ejection of theferrocene and toluene solution mixture, the ferrocene decom-poses to release free iron catalyst nanoparticles which eitherdeposit or diffuse into the CF. The growth of MWCNTs isprobably initiated normal to the CF substrate due to the carbonconcentration gradient present at the interface and as a resultcatalytic decomposition of toluene. The carbon is rst depositedon the exposed upper surface of the catalyst particle anddiffuses through, over the metal to precipitate in the foam andnanotubes anchored on the CF surface. Once the MWCNTsgrowth is recognized, the same is also surrounded by the othertubes growing from the nearby catalyst particles forming aMWCNTs forest covering most of the CF surface with increasingthe deposition time.22

3.2 Properties of different CF

Table 1 shows the properties of CF, MWCNTs incorporated CFand MWCNTs decorated CF. Initially, the bulk density of CF-Aheat treated at 2500 �C was 0.50 g cm�3 and on addition ofMWCNTs, the bulk density of CF increases. The extent ofincrease is higher in the case of MWCNTs incorporated CF(0.57 g cm�3), as compared to the MWCNTs directly grown onCF (0.54 g cm�3). The increase is related to ordering of thegraphene layer parallel to the MWCNTs axis. The MWCNTsprovide a template for the graphitization of carbon derived frommodied coal tar pitches in the CF, this is due to the anisotropicthermal expansion, during pyrolysis the mechanical stressesexerting at the MWCNT–carbon interface and acceleratesordering of the graphene layer. The increase in the bulk densityof CF results in the decrease in porosity as shown in the table.

J. Mater. Chem. A, 2013, 1, 5727–5735 | 5729


Table 1 Properties of different carbon foams (CF)

PropertiesCF HTT 2500 �CCF-A

MWCNTs incorporated in CFMWCNTs decoratedon CF

CF-B CF-C CF-D CF-E CF-F

Bulk density (g cm�3) 0.50 0.54 0.57 0.59 0.52 0.54Compressive strength (MPa) 5.3 6.4 7.6 5.9 9.3 7.5Porosity (%) 75 72 68 60 70 65Electrical conductivity (S cm�1) 80 120 135 110 150 130Thermal conductivity (W m�1 K�1) 48 59 70 63 80 68I(D)/I(G) ratio from Raman shi 0.542 0.486 0.382 0.793 0.938 0.995Specic thermal conductivity (W m�1

cm3 g�1 K�1)96 109 122 106 154 126

SSE (dB cm3 g�1) 90 122 126 85 163 115


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Mechanical strength. Strength is one of the essentialrequirements of CF because compressive forces are oenencountered during service life. Therefore, the compressivestrength of CF should be sufficient to avoid any form of struc-tural damage. The compressive strength of CF depends mainlyon two factors namely the microstructure and bulk density. Themicrostructure mainly includes the width of the ligaments andquantity of micro cracks. The compressive strength of the2500 �C heat treated CF is found to be 5.3 MPa, which is quitelow as compared to other pitch based carbon products. On theother hand, the compressive strength of CF incorporatedMWCNTs increases to 6.4 and 7.6 MPa for MWCNTs content of0.5 and 1.0 wt% respectively. In the case of CF, the load istransferred through ligaments; therefore, cracks in the liga-ments can be responsible for the low load bearing capacity ofCF. On the other hand, in the case of MWCNTs incorporatedfoam, the surface of MWCNTs are covered by the carbon matrixthat forms a reinforced ligament conguration. These MWCNTsencapsulated in ligaments can deect and carried most of theapplied load which results in the improvement of compressivestrength of CF by 46%. However, at higher MWCNTs content,even though the bulk density of the CF increases the simulta-neous aggregation of MWCNTs may restrain the enhancementof the compressive strength of CF.

However, in the case of MWCNTs decorated CF (CF-E andCF-F), the compressive strength increases to 9.3 and 7.5 MPa.Thus there is maximum 75% increase in the compressivestrength over pure CF. It is notable that, during the growth ofMWCNTs on CF, iron catalyst particles can ll inside the cracksleading to subsequent growth of MWCNTs in the crack's valley(Fig. 1f). Such a growth causes a decrease in the fraction ofstress concentration centers (mechanical defects) which arequite common in the CFs. Further, in the case of CF, load istransferred only through interconnected ligaments whereas inCF-E, the load carrying capacity increases due to the MWCNTsgrown on the surface of ligaments as well as inside the poresand cracks. This overall increase in the fracture stress also givesindirect evidence of the good anchoring between CF andMWCNTs. With increasing the reaction time of synthesis ofCNTs, there is a higher uptake of MWCNTs in CF (i.e. 1 wt%),but as explained in Section 3.1, there is formation of carbon

5730 | J. Mater. Chem. A, 2013, 1, 5727–5735

particles due to the cluster of iron particles formed withincreasing the feed time of the ferrocene and hydrocarbonmixture. This resulted in to decreases in the compressivestrength at 1 wt% uptake of MWCNTs.

Electrical conductivity. The electrical conductivity of the CFis reported in Table 1. Initially the electrical conductivity of CF-Awas 80 S cm�1. On incorporation of MWCNTs (CF-B to CF-D),electrical conductivity increases with increasing content ofMWCNTs up to 1 wt% i.e. from 80 S cm�1 to 135 S cm�1. Theincrease in conductivity is due to the increase in the conductionpath of electrons which is directly related to the structure of thereinforcing material. The higher content of MWCNTs incorpo-ration has a negative effect on the electrical conductivity due toagglomeration of nanotubes and the formation of MWCNTs–MWCNTs interfaces. On the other hand, for the MWCNTsdecorated foam, the extent of increase in the electricalconductivity (from 80 S cm�1 to 150 S cm�1) is higher ascompared to MWCNTs incorporated in CF, and this is due tothe degradation of properties of the MWCNTs in the case ofMWCNTs embedded in the coal tar pitch derived carbon.However, in the case of 1 wt% of MWCNTs uptake in directgrowth based CF, electrical conductivity also decreases.

Thermal conductivity. The thermal conductivity is also oneof the important criteria for quick heat dissipation in CF used incivil and military aerospace vehicles to protect them from thethermal heating of electronic power systems. The thermalconductivity of a material is governed by phonons whichtransport via lattice vibrations. The phonon transport in anymaterial is facilitated by aligned structures i.e., crystallineparameters. The thermal conductivity of the different CF ismeasured by a laser ash method and reported in Table 1. Thethermal conductivity of CF depends upon the structure of thefoam and most of the heat is transferred by the ligaments andcell wall. The thermal conductivity of CF-A is 48 Wm�1 K�1 andit increases from 48 to 70 W m�1 K�1 of MWCNTs incorporatedCF-C. The increase in thermal conductivity might be attributedto the improved properties of ligaments i.e. a wider ligamentsize as compared to the CF without MWCNTs and a reduction inthe number of cracks or defects. This structure allows the heattransfer throughout the foam efficiently because of the intrinsicproperties of nanotubes. The thermal conductivity of the




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graphite material is mainly inuenced by phonon transport inthe crystallite lattice. In graphite material thermal conductivityis represented by the formula;

l ¼ cnL/3 (1)

where, c is the specic heat per unit volume of the crystal, n isthe velocity of the heat transporting wave, which depends on thelattice vibration; and L is the mean free path of phonon scat-tering, which is closely related to the size of the crystals. Thelarger crystallite size (La), increases in the mean free path of thephonon which attributed to the higher conductivity.26

Fig. 2 shows Raman spectra of the samples which show thatpure CF (CF-A) consist of mainly three prominent bands at�1340 cm�1 (D band), �1566 cm�1 (G band), and �2700 cm�1

(G0 or 2D). The G0 or 2D is the second most prominent band,which is always observed in graphite material. It is important tonote that the intensity ratio of D and G bands [i.e. I(D)/I(G) ratio]is commonly used to measure imperfection in the highly crys-talline graphite material as it corresponds to the relative pop-ulation of sp3-hybridized carbon atoms.27 It is also used toestimate the density of defects in the carbon structure and thefraction of in-plane crystallites in the graphite structure.28–30

Further, I(D)/I(G) intensities are inversely proportional to thecrystalline size La.31 The calculated results (Table 1) clearlydemonstrate that the I(D)/I(G) ratio decreases with an increasein the concentration of MWCNTs up to 1.0 wt% in the CF. Thedecreases in I(D)/I(G) ratio, lead to increase in the La, which isresponsible for the increase in thermal conductivity of CF-B andCF-C. However, the higher content of MWCNTs restricts thecrystallite growth and as a result, in an increase in the I(D)/I(G)ratio. With increasing MWCNTs content, conductivity does notincrease which may be due to the agglomerated form ofMWCNTs and MWCNTs–MWCNTs interfaces limiting thermaltransport in the CF because of there being no medium totransfer heat between MWCNTs.32

Fig. 2 Raman spectra of CF without MWCNTs (CF-A), 0.5 wt% MWCNTs incor-porated (CF-B), 1.0 wt% MWCNTs incorporated (CF-C), 2.0 wt% MWCNTs incor-porated (CF-D), 0.5 wt% MWCNTs decorated CF (CF-E) and 1.0 wt% MWCNTsdecorated CF (CF-F).


On the other hand, the thermal conductivity of MWCNTsgrown CF (CF-E and CF-F) is higher than that of MWCNTsincorporated CF (CF-B, CF-C and CF-D), while the I(D)/I(G)ratios are in contradiction to each other. The improvement inconductivity is interpreted in terms of a positive synergisticeffect of MWCNTs and heat transfer throughMWCNTs. Becausein the case of MWCNTs incorporated CF, the MWCNTsembedded in the foam. Therefore, there is a possibility ofdegradation in the properties of MWCNTs, as compared toMWCNTs directly grown on the CF. This is due to the interac-tions of carbon derived from the coal tar pitch during foamprocessing,

Fig. 3 shows the XRD spectra of the CF. The diffraction peaksare observed at 2q � 26.3�, 43�, 45�, 54� and 77� correspondingto different diffraction planes 002, 100, 101, 004 and 110respectively. The incorporation of MWCNTs inuences thestructure of CF and as a consequence peaks registered atdiffraction angles and the intensity of each peak also changes.The interlayer spacing of CF-A is 0.3387 nm and that ofMWCNTs incorporated foam is 0.3381, 0.3374, 0.3394 nm forMWCNTs loadings 0.5, 1.0 and 2.0 wt% respectively. Thissuggests that up to 1 wt% MWCNTs can help in improving thestructure of CF i.e., increases in Lc value. However, a highercontent of MWCNTs has a negative effect on the crystallineparameters.

In the case of MWCNTs decorated foam, peaks wereobserved at a comparatively higher diffraction angle, ascompared to MWCNTs incorporated foam because it experi-ences high temperatures during the processing of CF, while theMWCNTs are directly grown on CF at a temperature between800 and 1000 �C. It is a known fact that the MWCNTs treated athigh temperature possess improved crystalline parameters, ascompared to the as grown MWCNTs.33 This is due to the goodanchoring of MWCNTs with the CF surface, i.e. on ligamentsand the MWCNTs grown in the valley of cracks. This facilitatesthe pathway for conduction and heat transfer along theMWCNTs oriented direction due to the high intrinsic thermalconductivity of MWCNTs. It is interesting to note that thespecic thermal conductivity (154 W cm3 g�1 m�1 K�1) of CF-Eis three times higher than that of the specic thermal conduc-tivity of copper (45 W cm3 g�1 m�1 K�1). On increasing theuptake of MWCNTs in CF, there is a slight decrease in thethermal conductivity. As explained in Section 3.1, the formationof carbon particles due to the agglomeration of iron nano-particles with increasing the reaction time act as a barrier forthermal conduction.

EMI shielding property. The EMI shielding effectiveness (SE)of a material is the ability to attenuate EM radiation that can beexpressed in terms of a ratio of incoming (incident) andoutgoing (transmitted) power.34 The EMI attenuation offered bya shield may depend on three mechanisms: reection of thewave from the front face of the shield, absorption of the wave asit passes through the shield's thickness andmultiple reectionsof the waves at various interfaces.35 Therefore, the SE of EMIshielding materials is determined by three losses viz. reectionloss (SER), absorption loss (SEA) and multiple reection losses(SEM) and can be expressed as:

J. Mater. Chem. A, 2013, 1, 5727–5735 | 5731


Fig. 3 XRD patterns of CF without MWCNTs (CF-A), 0.5 wt% MWCNTs incorporated (CF-B), 1.0 wt% MWCNTs incorporated (CF-C), 2.0 wt% MWCNTs incorporated(CF-D), 0.5 wt% MWCNTs decorated CF (CF-E) and 1.0 wt% MWCNTs decorated CF (CF-F).


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SE (dB) ¼ SER + SEA + SEM ¼ 10log(Pt/Pi)

where Pi and Pt are the power of incident and transmitted EMwaves respectively. As, the Pt is always less than Pi, SE is there-fore a negative quantity such that a shi towards a morenegative value means an increase in the magnitude of SE. It isimportant to note that the loss associated with multiplereections can be ignored (SEM � 0) when the SE of an EMIshielding material is more than �10 dB (i.e.more negative than�10)36 so that the SE can be expressed as SE (dB) ¼ SER + SEA.

Fig. 4 shows the EMI-SE of nano-structured CF in thefrequency range of 8.2 to 12.4 GHz. The SE for CF heat treated at2500 �C (CF-A) was �45 dB. However, on the incorporation ofMWCNTs in CF (CF-B) EMI-SE increased to �66 dB on 0.5 wt%of MWCNTs. Further, increase of the MWCNTs content of EMI-SE increases to�72 dB but beyond 1.0 wt% of MWCNTs contentin CF, the EMI SE decreases to �50 dB (CF-D). On the otherhand, in the case of MWCNTs directly grown on CF, the extentof increase in EMI-SE was higher even at 0.5 wt% of MWCNTs.The EMI-SE increases to�85 dB (CF-E) and decreases to�62 dBfor 1 wt% of MWCNTs grown on the CF (CF-F).

The electromagnetic radiation at high frequencies pene-trates only near the surface region of an electrically conductingmaterial. This is known as the skin effect. The depth at whichthe eld drops to 1/e of the incident value is called the skindepth (d) which is represented by3

d ¼ (2/fms)1/2,

where f is frequency, s is electrical conductivity and m ispermeability. Hence, the skin depth decreases with increasingfrequency, permeability or conductivity. Therefore, it is expec-ted that the use of CNTs with a larger aspect ratio and higher

5732 | J. Mater. Chem. A, 2013, 1, 5727–5735

conductivities can be possible to achieve a higher SE. In the caseof the MWCNTs incorporated foam, MWCNTs are embedded inthe carbon derived from the pitch. During the processing,MWCNTs can make chemical interactions with the pitch. Sothere is the possibility of degradation in the properties ofMWCNTs. With the direct growth of CNTs on the CF surface,the large aspect ratio of CNTs helps to create extensivelycontinuous networks that facilitates electron transport in theCF more quickly at low loading. The incident radiation directlyinteracts with CNTs on the surface which leads to the higher SEbecause of incident radiation scattering. The higher content ofMWCNTs in CF has a negative effect on EMI shielding effec-tiveness due to the multiple interfaces between CNTs. Also fromthe surface area data of CF measured by the sorption ofnitrogen, it is observed that with increasing the MWCNTscontent, the surface area increases continually from 2.439 m2

g�1 to 7.60 m2 g�1 with 2 wt% of MWCNTs content in CF.However, in the case of MWCNTs decorated CF at a 0.5 wt%

uptake of nanotubes the surface area is 6.7 m2 g�1 and there isfurther increase in the uptake of MWCNTs content, the surfacearea decreases to 5.2 m2 g�1. This conrms the phenomena ofcarbon particle formation with increasing reaction time whichnegatively affects the EMI-SE value. Therefore in 1 wt%MWCNTs decorated CF, the EMI-SE signicantly decreases.

Rather than comparing the EMI shielding effectiveness (SE)of light weight CF with different materials used in aircra andspacecra applications, it will be more suitable to compare thespecic shielding effectiveness (SSE) with other materials. TheSSE of different CNTs based composite materials in the X-bandregion (8.2 to 12.4 GHz) are compared in this work. The SSE ofsolid copper is 10 dB cm3 g�1.37 On the other hand, the CNT-PEEK nanocomposite with 12 wt% puried MWCNTs has a SSEvalue of 37.3 dB cm3 g�1.38 Yang et al.39 reported the SSE of a



Fig. 4 EMI shielding effectiveness (A-absorption, R-reflection and T ¼A + R) of (a) carbon foam (CF-A), (b) 0.5 wt%MWCNTs incorporated carbon foam (CF-B), (c) 1.0wt%MWCNTs incorporated carbon foam (CF-C), (d) 2.0 wt%MWCNTs incorporated carbon foam (CF-D), (e) 0.5 wt%MWCNTs decorated carbon foam and (f) 1.0 wt%MWCNTs decorated carbon foam.


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novel CNT-polystyrene foam composite as 33.1 dB cm3 g�1,which is much higher than solid copper. Recently Yan et al.40

reported the light weight porous graphene–polystyrenecomposite as having a specic shielding effectiveness of 64.4 dBcm3 g�1 with 30% concentration of functionalized graphenesheet. The specic shielding effectiveness of carbon foamderived foam coal tar pitch is 90 dB cm3 g�1 and that of 1 wt%MWCNTs incorporated in carbon is 126 dB cm3 g�1. However,in 0.5 wt% MWCNTs decorated carbon foam the SSE increasesto 163 dB cm3 g�1.

In the case of CF the ligaments are in a continuous networkinterconnected to each other and they are more conductive thanbulk carbon due to the specic conductivity. With the incor-poration of MWCNTs in the CF, they aligned with ligaments andthis contributed to the enhancement of the continuous con-ducting network throughout the CF structure. However, in thecase of the CNTs decorated CF the EMI-SE dominated thesurface conductivity i.e. skin effect, as compared to CNTs


incorporated CF. The EMI-SE and SSE of ��85 dB and 163 dBcm3 g�1 were obtained with CNTs decorated CF of a thickness of2.75 mm. This is the highest value reported in the open litera-ture at such low density and thickness in the X-band frequencyregion of CF.

Thermal stability. The thermal stability of the CF wasinvestigated by thermogravimetric analyzer (TGA) in an oxida-tive (air) atmosphere. The thermal stability of CF depends uponthe structure of carbon, the higher the extent of graphiticstructure of the CF the higher the thermal stability.41,42 It isobserved that the thermal stability of sample b (1 wt%MWCNTsincorporated foam) is superior than sample a (such as CF) andsample c (MWCNTs decorated CF) as shown in Fig. 5.

The thermal stability is higher by �50 �C. The sample a, isthermally stable up to 550 �C, aer that weight loss was initiateddue to an oxidation of carbon (reaction product carbonmonoxide and carbon dioxide). The major weight loss takesplace between temperatures 700 and 950 �C in both cases. The

J. Mater. Chem. A, 2013, 1, 5727–5735 | 5733



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increase in the thermal stability of MWCNTs incorporated CF isdue to the increase in the degree of graphitizability, which canreduce the number of edge carbon atoms that are responsiblefor the oxidation reaction in an oxidizing environment.However, for theMWCNTs decorated CF, the thermal stability isalso mostly the same as CF. This is because the MWCNTs grownon the CF surface have the ability to be oxidized throughnanotubes caps (pentagon–heptagon pair defects) which aremore reactive due to the presence of dangling bonds.

4 Conclusions

In the present work CF with improved EMI-SE was developed bydecorating with MWCNTs. The two approaches of MWCNTsdecoration was adapted, it was found that, direct growth ofMWCNTs on CF was more striking in improving the overallproperties of CF. The direct growth was able to improve thesurface area and surface conductivity of the CF. In the case ofMWCNTs incorporated carbon foam, MWCNTs were encapsu-lated in the ligaments. However, in the case of MWCNTsdecorated carbon foam, MWCNTs were grown on the outersurface, in pores, ligaments and in the valley of cracks. Theencapsulation can cause degradation in the properties ofMWCNTs due to the chemical bonding. The MWCNTs incor-porated CF demonstrated a maximum EMI-SE value of �72 dBat 1 wt% MWCNTs. In comparison, MWCNTs directly grown onCF gives a maximum EMI-SE of �85 dB at only 0.5 wt%MWCNTs. The higher extent of improvement in EMI-SE inMWCNTs decorated CF was due to the increase in surface areaand surface conductivity i.e., dominated by the skin effect. TheSSE was 163 dB cm3 g�1 for MWCNTs decorated CF of thickness2.75 mm. This is the highest value reported in the open litera-ture for CF in the X-band (8.2 to 12.4 GHz) frequency region.Moreover, EMI-SE improvement, compressive strength andthermal conductivity were increased by 66 and 75% respectively.The studies showed that instead of developing the CF from anexpensive mesophase pitch, it is possible by incorporating ordecorating CNTs on CF that can help in improving the overallproperties, which is useful for aerospace structural materials.

Fig. 5 TGA of carbon foams (a) CF-A, (b) CF-C and (c) CF-E in oxidizingenvironments.

5734 | J. Mater. Chem. A, 2013, 1, 5727–5735

Acknowledgements

Authors are highly grateful to Director, NPL, for his kindpermission to publish the results. Also thanks to Mr Sood forproviding SEM characterization facility. One of the authors(Rajeev Kumar) would like to thanks CSIR for SRF fellowship.

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effective improvement of the properties of light weight carbon foam by decoration with multi-wall...

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