RSC Advances

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aMaterial Physics and Engineering Divisi

Dr K. S. Krishnan Road, New Delhi-110012

org; Tel: +91-11-45608285bChemistry Department, MN National Inst

India

Cite this: RSC Adv., 2014, 4, 23202

Received 20th January 2014Accepted 10th April 2014

DOI: 10.1039/c4ra00567h

www.rsc.org/advances

23202 | RSC Adv., 2014, 4, 23202–232

Cu–Ni alloy decorated graphite layers for EMIsuppression

Saroj Kumari,*a Anil Kumar,ab Avanish Pratap Singh,a Manjari Garg,a P. K. Dutta,b

S. K. Dhawana and Rakesh B. Mathura

Copper–nickel alloy nanoparticle decorated natural graphite based hybrid composites have been

synthesised by simple reduction chemistry at low temperature (50–90 �C) to evaluate their

electromagnetic impedance (EMI) shielding effectiveness (SE). The prepared composites were

characterized for morphology and structural analysis by scanning electron microscopy, transmission

electron microscopy and X-ray diffraction and the obtained results show the uniform covering of Cu

nanoparticles on graphite and face centred cubic structure of the alloy. EMI studies were carried out on

these composites and they were found to possess SE due to absorption (SEabs) in the range of 10.27 to

25.10 dB, while the reflection, SEref varies from 5.62 to 18.69 dB. Therefore, these hybrid composites can

be used for techno-commercial applications and absorbing mobile signal interference because they can

exhibit SE in the required range of 30 to 40 dB.

1. Introduction

Electromagnetic interference (EMI) has become a subject ofworldwide consideration due to the huge increase in the use ofelectronic gadgets and far reaching utilization of transientpower. The most common occurrence of EMI is experienced insatellite communication, radar surveillance systems, electronicequipment, digital devices and remote engineering.1–4 Thus,EMI can be considered as a side effect of modern engineeringand is found to be hazardous to health as it can cause sleepingdisorders, trepidation and headaches.5,6 Thus, essential activi-ties to handle this novel kind of pollution have triggered a questfor procient counter-measures to ensure unperturbed execu-tion of electronic devices even in the vicinity of outside EMnoise.7,8 These problems can be overcome by using a specialmaterial which can effectively reect and/or absorb the elec-tromagnetic radiation by reducing the signals so that theycannot affect the performance of electronic equipment.

In the present scenario, weight and exibility are signicantparameters for designing an effective EMI shieldingmaterial foraircra, aerospace and automobile applications and currentlythe synthesis of these materials requires great effort for indus-trial scale production.9,10 Numerous carbon-based materialsincorporating carbon in the form of graphene,11–13 graphite,14,15

carbon ber,1 carbon nanotubes,16,17 etc. have been utilized asconductive llers to fabricate composites for EMI shielding due

on, CSIR-National Physical Laboratory,

, India. E-mail: kumaris@mail.nplindia.

itute of Technology, Allahabad-211 004,

09

to their high electrical conductivity, amazing mechanicalstrength, light weight and extensive exibility. Solid lubricant(graphite) has a capacity to absorb electromagnetic waves andcopper nanoparticle reinforced graphite based composites forma separate category of superior composite material that providesunique advantages over bulk copper. Thus, they exhibit thecombined properties of both components, i.e., excellent elec-trical and thermal conductivities of copper with low thermalexpansion coefficient, lubricating and corrosion-resistanceproperties of graphite. Therefore, these hybrid composites canfull the essential criteria required for EMI shielding and can beused for EMI shielding applications. These composites can alsobe extensively used as materials for developing electric brushesfor engines and generators in electrical applications and assha materials, bearings for mechanical engineering applica-tions18 and microwave travelling wave tubes as multistagedepressed collectors.19

Recently, research has been focused on carbon–coppercomposites and the random dispersion of copper nanoparticlesin a carbon matrix acts as a highly conductive interconnectingnetwork which enhances electrical conductivity and further EMIshielding of composites.20–22 But these composites suffer fromfacing interfacial mismatch with each other, which can cause animmense problem for EMI shielding.23 Wetting agents can beused to improve the interfacing bonding between carbon (C)and copper24 and they can be added in various forms, forexample, a thin layer can be deposited in the form of an overlayer on the surface of the carbon material by electroless platingor chemical vapor deposition and can also be mixed with thecopper by the powder method. Alloying copper with a smallamount of carbide-forming active metals such as chromium,

This journal is © The Royal Society of Chemistry 2014

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nickel, titanium, iron, etc. also improves the bonding betweenthe graphite/carbon and copper.25,26

Here, we are presenting a study on the effect of an inter-mediate wetting layer of nickel (Ni) nanoparticles between thegraphite (Gr) and copper (Cu) in different concentrations by achemical reductionmethod and the application of thematerialsproduced in EMI shielding. In this study, prior to coating withCu nanoparticles, Ni nanoparticles were deposited on the Grpowder by chemical reduction of nickel chloride hexahydrate atdifferent concentrations to obtain Nix–Gr powder. Aer that Cunanoparticles were coated on the surface of Nix–Gr powders bythe same method to obtain duplex Cu–Ni coated Gr material.This type of duplex coating of Cu and Ni nanoparticles on Grmaterial by a chemical reduction method for EMI shieldingapplication is not reported in the literature. The effect of Nicontent on structural and electrical properties and EMIshielding of Cu–Ni–Gr composites has been studied.

2. Experimental2.1 Materials used

Natural graphite powder used in the present study was procuredfrom Pune Carbons Pvt. Ltd., Maharashtra, India. Palladiumchloride (99.99%), nickel chloride hexahydrate (NiCl2$6H2O,99%), cupric sulphate pentahydrate (CuSO4$5H2O, 99%),potassium sodium tartrate (99%), sodium hydroxide (99.99%)and hydrazine hydrate were procured from Merck, India.

2.2 Decoration of Ni nanoparticles on the surface of graphitepowder

2.2.1 Sensitization and activation. Puried naturalgraphite (Gr) powder, aer ball milling for 15 h at 250 rpm in aplanetary ball mill, was used as a carbon source. A chemicalreduction (electroless) method was adopted for decorating thesurface of Gr powder by nickel (Ni) nanoparticles. As thismethod requires proper surface treatment for achievinguniform deposition and adherence, surface etching of Grpowder (2 g) was carried out by stannous chloride (SnCl2)solution. SnCl2 solution used for surface treatment wasprepared by dissolving 2.5 g L�1 SnCl2 with 5 mL hydrochloricacid in deionized (DI) water. This phenomenon is calledsensitization which enables the material to act as a catalyst forthe deposition of nickel and the sensitized Gr powder was thenwashed with DI water several times and dried at 50 �C until dry.Aer that palladium chloride (PdCl2) was used for creatingactive sites on the sensitized Gr powder surface by immersing itin palladium chloride solution prepared at a concentration of0.01 g L�1 with 5 mL hydrochloric acid in 500 mL DI water andthe material was dried at 50 to 100 �C in an oven until dry.

2.2.2 Bath preparation. The bath for decorating Gr powderwith Ni nanoparticles at a concentration of 1 to 50 wt% (0, 1, 5,10, 25 and 50 wt%) was prepared by adding nickel chloridehexahydrate (NiCl2$6H2O) in DI water. To decorate the Grpowder with Ni, the sensitized and activated Gr powder (2 g) wasimmersed in the bath solution at the elevated temperature of50 �C. Hydrazine hydrate (20 mL), used as reducing agent, was

This journal is © The Royal Society of Chemistry 2014

added to the bath solution containing nickel chloride and Grpowder and stirred for 45 min at a temperature of 80–85 �C. Theprocess was continued for an hour aer which nickel decoratedGr was ltered out and then washed with DI water. Finally, itwas dried in an oven at 100 �C. The samples obtained have beennamed Nix–Gr, where x ¼ 1, 5, 10, 25 and 50 wt%.

2.2.3 Cu nanoparticle coating on the surface of Ni–Grpowder. A sufficient amount of Nix–Gr powder was sensitizedand activated again by the same process as discussed above. Thebath for copper coating was prepared by adding cupric sulphatepentahydrate (CuSO4$5H2O) in a Cu/C ratio of 0.57 in DI wateraer that potassium sodium tartrate, used as a chelating agent,was added to this solution. The pH of the bath solution wasmaintained by adding sodium hydroxide. The sensitized andactivated Nix–Gr powder was added into the bath solution at atemperature of 75 �C on a hot plate to coat of copper on theNixGr. Finally hydrazine hydrate was added to reduce the cupricsulphate in the copper nanoparticles and further stirred for about30 min to obtain a duplex/bimetallic coating of copper on NixGr,i.e. Cu–Nix–Gr powder. The temperature and pH of the solutionwere maintained at 75–80 �C and 9–12, respectively.27 The Cucoated material was ltered, washed with DI water and dried inan oven at 100 �C. In this process copper may be converted intocopper oxide (CuO) due to the presence of dissolved oxygen in thebath solution, which was reduced with hydrogen at a tempera-ture of 400 �C in a furnace. A nitrogen and hydrogen (08 : 92)mixture was used for the reduction process.28 These Cu coatedNix–Gr powders are described as Cu–Nix–Gr powders. Thesehydrogenated Cu–Nix–Gr powders were consolidated withoutusing any extra binder into 45� 15 � 5 mm3 plates at a pressureof 200 MPa using a hydraulic press and nally sintered forsolidication at a temperature of 750 �C in nitrogen atmospherein order to obtain Cu–Nix–Gr composites. These consolidatedand heat treated bimetallic Cu–Nix–Gr (x ¼ 0, 1, 5, 10, 25, 50wt%), composites are also referred to as S1, S2, S3, S4, S5 and S6. Aschematic diagram showing the Cu–Ni coating on the graphitesurface via reduction chemistry is shown in Fig. 1.

2.2.4 Characterization. The Cu–Nix–Gr powder and consol-idated samples aer sintering were characterized for phaseformation by powder X-ray diffractometer (Bruker D-8 Advanced)using CuKa radiation (l ¼ 1.5418 A). Raman spectra with laserwavelength of 514 nm for composites were obtained using aRenishaw Raman Spectrometer, UK. The surface morphology ofmetal coated graphite and the composites was studied by scan-ning electron microscopy (SEM, ZEISS-EVO MA10). A trans-mission electron microscope (TEM) (FEI, Tecnai T30) wasemployed to investigate the microstructural characteristics withcrystallographic orientation of the samples. The electrical resis-tivity was determined using a laboratory developed four-probeapparatus.29 EMI shielding was studied by a Vector Networkanalyzer aer making a rectangular pellet (15.8� 7.9� 2.0 mm3).

3. Results and discussion3.1 X-ray analysis

The X-ray diffraction (XRD) patterns of S6 (Cu–Ni50–Grcomposite) before and aer hydrogenation are shown in Fig. 2A.

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Fig. 1 Schematic of Cu–Ni coating on the graphite surface via reduction chemistry.

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The XRD pattern exhibits the face centered cubic structure ofnickel with hkl values of (111), (200), (220) and (311) (JCPDF no.04-0850). The pure crystalline form of nickel repudiates theformation of nickel oxide in the process of deposition. However,before hydrogenation, prominent characteristic peaks of CuO(JCPDF no. 80-1916) can be seen along with peaks due to Cu, Niand graphite as shown in Fig. 2A(a). This may be attributed tothe fact that the electroless process is a wet chemical methodresulting in the formation of nano-dimension copper which ismore reactive and therefore converts partially into copperoxide.30 Fig. 2A(b) shows the XRD analysis of the hydrogenatedS6 sample and there is no signature of CuO which conrmed thereduction of CuO into Cu.

Fig. 2B illustrates the XRD diffractograms of samples S4, S5and S6. In this gure the two peaks at 2q ¼ 26.50 and 54.61�

Fig. 2 (A) XRD patterns of Cu–Ni50–Gr: (a) after hydrogenation and (b) afNix–Gr composites for x¼ 10, 25 and 50wt% after heat treatment at 750 �

graphite and (D) Raman spectra of Cu–Nix–Gr composites for x ¼ 10, 2

23204 | RSC Adv., 2014, 4, 23202–23209

correspond to the graphitic peak of carbon and the three peaksat 2q ¼ 43.65, 50.85 and 74.79� are in an intermediate positionin relation to the peaks corresponding to the pure Cu and Nimetals. It is also observed that intermediate peaks betweencopper and nickel gradual shi towards higher 2q values asshown in Fig. 2B (inset) which proves an increase in solubilitywith increasing Ni content.31 This indicates a contraction in theCu lattice as a result of the substitution of Cu by Ni. Theseresults suggest that a Cu–Ni alloy was formed only for x ¼50 wt% by means of the substitution of Cu by Ni. Cu–Ni alloysare basically a solid solution formed by the substitution of Cu byNi because both metals have the same ionic radius, crystallinestructure, electronegativity and identical valence, which help infullling the Hume–Rothery rules for a substitutional solidsolution.32 The shi in the graphitic peak (Fig. 2B, inset) of

ter consolidation and heat treatment at 750 �C, (B) XRD patterns of Cu–C denoted as (a), (b) and (c) respectively, (C) Raman spectrum of natural5 and 50 wt% after heat treatment at 750 �C.

This journal is © The Royal Society of Chemistry 2014

Fig. 3 SEM images of (A) natural graphite, (B) Ni nanoparticles coatedon graphite (Ni50–Gr), (C) Cu nanoparticles coated on Ni coatedgraphite (Cu–Ni50–Gr) and (D) Cu–Ni50–Gr composite after heattreatment at 750 �C.

Fig. 4 TEM images of (A) natural graphite and (B) Cu–Ni50–Grcomposite. Insets: (B) left shows zoomed image of highlighted regiondepicting the size of Cu–Ni nanoparticles; right top shows fringepatterns marking the interlayer spacing of graphite and right bottomEDS of composite.

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carbon towards a higher value and the narrowing of the peakson increasing the amount of Ni decorated on Gr clearly indi-cates an improvement in the crystallinity and an increase inparticle size. Apart from these results, no peak of nickel carbide(Ni3C) is seen in the diffractogram although nickel is a carbideforming agent and it easily reacts with graphite and forms Ni3C.This is because, this carbide is stable below 420 �C but at highertemperature, i.e., above 500 �C it decomposes into Ni and C.33

3.2 Raman analysis

Fig. 2C and D demonstrate structural changes in graphite andS4, S5 and S6 samples. In which graphite is active at 1351, 1580and 2720 cm�1 Raman shis,34 which are characteristic vibra-tional peaks of carbon atoms. The Raman shi at 1351 cm�1 isfor the A1g vibration mode (D band), which appears from thedisordered graphite. The E2g vibration mode (G band) corre-sponds to the Raman shi at 1580 cm�1 which originates fromgraphite sheets and sp2 hybridised unlocalised electrons. Inaddition, the third Raman shi at 2720 cm�1 (2D band) isattributed to the high ordering of graphite. It is also observedthat the intensity ratio (ID/IG) of the D band (ID) to the G band(IG) continuously decreases with an increasing amount of Ninanoparticles on graphite and it decreased from 0.429 for S4 to0.225 for S5 and for S6 this ratio is very low�0.104. However, theintensity ratio (ID/IG) for Gr powder is about 0.291, which is lessthan sample S4. It may be that at low concentration, Ni on thegraphite surface creates some defects between the interlayersheets of graphite. However, increasing the loading of Ninanoparticles in the composite may contribute as an unlo-calised electron (sp2 hybridization characteristic), which isresponsible for the reduction in the ID/IG ratio. Another factormay be reduction in grain boundaries, defects and coalescenceof dangling bonds of graphite on incorporation of metalnanoparticles.

These results are also conrmed from X-ray analysis, as givenin Fig. 2B and discussed above. Another fact is reported in theliterature on the use of nickel as a catalyst for graphitization atan early stage or lower temperature,35 which also enhances theintensity of the G band with increasing crystallinity of thecomposite.

3.3 SEM analysis

The morphology and microstructure of samples have beeninvestigated by SEM and are presented in Fig. 3. The micro-structure of the graphite powder shows a ake like structurewith �1 to 5.0 mm particle size (Fig. 3A). Fig. 3B shows amicrograph of Nix–Gr (x ¼ 50 wt%). It is observed that nickelnanoparticles are successfully decorated on the graphiteexhibiting a uniform decoration on the graphite akes whichcontributes to the improvement of the interfacial wetting abilitybetween Cu and C.36 The duplex coating of Cu on Nix–Gr (x ¼50 wt%) powder through chemical reduction presented inFig. 3C, shows a homogeneous distribution of Cu nanoparticleson Nix–Gr. The size of the Cu particles coated on Nix–Gr is�200 nm which is conrmed by the inset of Fig. 3C. Fig. 3D is aSEM micrograph of sample S6. Bead like spherical particles of

This journal is © The Royal Society of Chemistry 2014

size <200 nm of Cu–Ni alloy on graphite can be seen from thismicrograph (inset Fig. 3D).

3.4 TEM analysis

Fig. 4 shows the TEM images of graphite and sample S6. Fig. 4Ais a TEM image of Gr powder indicating a well arranged layeredstructure of graphite having thickness varying from a few tens tohundreds of nm. There are some regions where graphite layersare randomly oriented and in some places overlapping ofgraphite sheets can be seen. The TEM image of S6 proves thedecoration of Cu–Ni alloy nanoparticles on Gr. The particle sizeof the Cu–Ni alloy is in the rage of 10 to 40 nm as shown in themiddle le inset of Fig. 4B. The upper right inset of Fig. 4Bshows the atomic scale image of the alloy decorated Gr withinter-planer spacing of �3.34 A for graphite. This value is closeto the d-spacing of natural graphite �3.35 A calculated fromXRD data. The presence of metallic Cu and Ni and carbon isconrmed by their EDS analysis presented in lower right inset ofFig. 4B which is well consistent with XRD results.

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3.5 Dielectric and EMI shielding measurements

The room temperature dc electrical conductivity for allcomposites is shown in Table 1. Graphite sheets possess verygood conductivity,37 therefore, since S1 has the highest wt% ofgraphite the conductivity of S1 is highest among all thecomposites. We have observed a gradual decay in conductivity(from 1724.14–313.48 S cm�1) in S1 to S3 and a continuoussharp increase in conductivity (from 416.67–1298.70 S cm�1) inS4 to S6, as shown in Fig. 5a.

The decrease in electrical conductivity from 1724.14 Scm�1 for x ¼ 0 to 313.48 S cm�1 as the amount of Ni increasedfrom x ¼ 1 to 5 wt% can be explained in terms of the higherelectrical resistivity of Ni (69.90 nUm) and the amount of itcoated on graphite particles as compared to Cu (16.78 nUm)which is kept constant. A further increase in the amount of Ninanoparticles (x ¼ 10 to 50 wt%) decorated on the graphite,increases electrical conductivity of the composites from416.67 to 1298.70 S cm�1. The improvement in electricalconductivity of composite samples having higher Niloading is due to the formation of Cu–Ni alloy and thereduction in defect sites in graphite as conrmed by theRaman spectra.

From the above, it is concluded that the threshold limit ofNi–Cu alloy formation is 10 wt%. Moreover, the electricalconductivity of these samples lies in the optimum value ofconductivity which is desired for exhibiting good microwaveshielding response.17,38,39 Hence, these materials can bepotentially used in several applications such as in electro-static charge dissipation (ESD) and in EMI shielding. Here,we have focused mainly on EMI shielding application, ratherthan its use in ESD. Highly conducting materials such asalloys with exotic carbons have been primarily proved to workfor EMI shielding applications. To date, to the best of ourknowledge, no work on the incorporation of Cu–Ni alloys ingraphite has been reported for enhancing the EMI shieldingeffectiveness.40–42

EMI SE is measured in decibels (dB), �1, �3, �10, �20, �30dB. When an EM wave is incident on a slab or material, threephenomena happen, absorption, reection and transmissionmeaning that a part of the incident wave is absorbed and somepart is reected from the surface and the rest is transmittedthrough the shield. Therefore, the total EMI SE (SEtot) is the sumof contributions from absorption (SEabs), reection (SEref), andtransmission or multiple reections (SEM), which can be simplyquoted as follows.43,44

Table 1 Electrical conductivity and shielding effectiveness of composite

Sample nameNi concentration (wt%)Cu NixGr where x ¼

ElectricaS cm�1

S1 0 1724.14S2 1 571.43S3 5 313.48S4 10 416.67S5 25 467.29S6 50 1298.70

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SEtotðdBÞ ¼ 10 logPT

PI

¼ SEref þ SEabs þ SEM (1)

where, PI and PT are the power of incident and transmitted EMwaves, respectively. SEM can be neglected when SE > 10 dB.45

Then SEtot z SEabs + SEref.Therefore, the effective absorbance (Aeff) can be described

as Aeff ¼ (1 � R � T)/(1 � R) with respect to the power ofthe effectively incident EM wave inside the shieldingmaterial.

For a material, the skin depth (d) is the distance up to whichthe intensity of the EM wave decreases to 1/e of its originalstrength. The d value is related to angular frequency, relativepermeability and total conductivity sT¼ (sdc + sac). According toEM theory, for electrically thick samples (t > d), frequency (u)dependence of far eld losses can be expressed in the terms oftotal conductivity (sT), real permeability (m0), skin depth (d) andthickness (t) of the shield material as:45

SERðdBÞ ¼ 10 log

�sac

16u3om0

�(2)

SEabsðdBÞ ¼ 20�td

�log e ¼ 20d

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffimusac

2 log e¼ 8:68

�td

�r(3)

The sac and d values can be related to the imaginarypermittivity (300) and real permeability m0 as sac ¼ u3o3

0 0 andd ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

2=sum0pwhich gives absorption loss as:

SEabsðdBÞ ¼ 8:68t

ffiffiffiffiffiffiffiffiffiffisum0

2

r(4)

From eqn (2) and (3), it is revealed that for compositeshaving moderate conductivity and non-magnetic materials(m0 � 1 and m0 0 � 0), electrical conductivity and complexpermittivity are two governing parameters for suppression ofreection or improvement of microwave absorption. Therefore,incorporation of Cu–Ni alloy (as a highly stable interconnectnetwork) in an appropriate amount within graphite (as con-ducting matrix) is expected to improve shielding effectiveness.

Fig. 5a shows the total SE while Fig. 5b illustrates the SEabsand SEref of Cu–Nix–Gr composites. SE values of 39.48, 26.46,21.08, 20.24, 34.07 and 35.34 dB are achieved for S1, S2, S3, S4, S5,and S6, respectively.

In order to probe the relationship between observedshielding response and electromagnetic attributes, dielectric

s

l conductivitySEabs SEref SEtot

25.10 14.38 39.4820.84 5.62 26.4610.97 10.10 21.0810.27 9.97 20.2418.06 16.01 34.0716.65 18.69 35.34

This journal is © The Royal Society of Chemistry 2014

Fig. 5 (a) Variation of EMI SE of Cu Ni coated graphite powdercomposite as a function of Ni concentration (b) behaviour of SEAbs andSERef of graphite and its composites as a function of Ni concentrationand (c) variation of EMI SE of Cu Ni coated graphite powder compositeas a function of frequency.

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and permeability parameters have been calculated frommeasured scattering parameters (S11, S22, S12, S21) using theNicolson Ross weir algorithm. For the Cu–Ni50–Gr composite(S6) the value of 30 lies in the range of 44–23, while the value of30 0 lies in the range of 16–13 as shown in Fig. 6. The observed

This journal is © The Royal Society of Chemistry 2014

m0 and m0 0 are found in the range of 0.28–0.30, 0.14–0.26respectively. The small values of m0 and m0 0 are attributed tothe non-magnetic character of the composite. Generally,permeability and its loss are found to be higher in polymerferrite composites46 but in non-ferromagnetic compositestheir values are very small and similar to the results observedfor Cu–Nix–Gr composites.11,37,46

In Cu–Nix–Gr composites, the existence of interfacesbetween alloy particles and graphite sheets are responsiblefor interfacial polarization which contributes to dielectriclosses.4,6 Interfacial polarization occurs in heterogeneousmedia due to accumulation of charges at the interfaces.Furthermore, at microwave frequency, due to the differencein the conductivity of Cu (1.70 S cm�1), Ni (�10�12 S cm�1)and graphite (1612.12 S cm�1),37 some charges have beentrapped, and as a result, space charge is developed on thesurface of the graphite sheets. The presence of space chargeat the heterogeneous interface results in eld distortion. Theconductivity of the Cu–Nix–Gr composite is improved bygraphite. The natural resonances in the Ku-band can beattributed to the small size of the Cu–Ni alloy. The anisotropyenergy of the small sized materials,47 especially at the nano-scale, would be higher due to the surface anisotropic elddue to the small size effect.48 The high values of 30 and 300 arethe combined effect of the above mentioned effects andassociated loss mechanisms respectively.

An SE value of around 30 dB is required for commercialEMI shielding application49 and more than 80 dB SE value isdesired for high-tech defence applications. Therefore, theCu–Ni composites can potentially be applied for makingfuturistic shielding solutions in daily life. Besides the EMIshielding application, we rmly believe that Cu–Nix–Grcomposite with optimum conductivity along with highdielectric constant may be an ultimate choice in manyapplications such as ESD, battery, thermoelectrics andenergy applications etc.

Fig. 6 Dielectric attributes of the Cu–Ni50–Gr composite (S6).

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4. Conclusion

In the present work a simple straightforward and commercialmethod has been used to develop Cu–Ni alloy nanoparticledecorated graphite based hybrid composites for EMI shieldingapplication. Nanoparticles of Cu–Ni alloy of size 10 to 40 nmhave been successfully decorated on commercial naturalgraphite powder by chemical reduction process. The electricalconductivity threshold limit is also observed aer 10 wt% of Niloading on graphite and conductivity sharply increases (from416.67–1298.70 S cm�1) continuously. EMI shielding ofcomposites varies from 39.48 to 35.34 dB (S1¼ 39.48, S2¼ 26.46,S3 ¼ 21.08, S4 ¼ 20.24, S5 ¼ 34.07 and S6 ¼ 35.34) and dependson the conductivity of the composites. Although sample S1 hashigher EMI shielding effectiveness than S6, from the stability forlong life point of view S6 is more stable. The intermediate layerof Ni nanoparticles in S6 enhances the conductivity and wetta-bility/solubility between carbon and copper which helps infurther improvement in mechanical properties such as bendingand compressive strength of the composites. Also, Cu–Ni alloynanoparticles are chemically stable therefore this material canbe used in EMI shielding application for a long time and caneven be used in a wet atmosphere.

Acknowledgements

The authors are thankful to Prof. R. C. Budhani, Director, CSIR-National Physical Laboratory, New Delhi for his keen interest inthis article. Thanks are also due to Mr Naval Kishor and Mr JayTavale for their valuable help in X-ray and SEM studies,respectively. The authors express their sincere thanks to DrChandra Shekher, Director, CSIR-CEERI, Pilani for giving us anopportunity to co-operate in the network project sponsored bythe Council of Scientic and Industrial Research (CSIR).

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