Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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JIEC-1759; No. of Pages 4

A study on thermal conductivity of electroless Ni–B platedmulti-walled carbon nanotubes-reinforced composites

Jeong-Ran Choi a, Young Sil Lee b, Soo-Jin Park a,*a Department of Chemistry, Inha University, Incheon 402-751, Republic of Koreab Cheil Industries Inc., Uiwang-si 437-711, Republic of Korea

A R T I C L E I N F O

Article history:

Received 28 October 2013

Accepted 10 December 2013

Available online xxx

Keywords:

Multi-walled carbon nanotubes (MWCNTs)

Ni–B plating

Epoxy resins

Thermal conductivity

Fracture toughness

A B S T R A C T

In this work, the effects of electroless Ni–B plating on thermal conductivity and fracture toughness of Ni–

B multi-walled carbon nanotubes (MWCNTs)/epoxy matrix composites were examined. From the

results, it was found that the thermal conductivity increases with increasing contents of electroless Ni–B

MWCNTs. The critical stress intensity factor (KIC) also increased with increasing contents of electroless

Ni–B MWCNTs. However, it rather decreased at over 10 phr. It is considered that this is because non-

uniform distribution and partial entanglement occurred in case the contents of electroless Ni–B

MWCNTs were high.

� 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

With the rapid development of the electronics industry,increasingly compact devices have emerged in recent years;nevertheless, there has been a notable increase in the heat densityof these devices. As a result, countermeasures against heatdissipation are required, and the development of next-generationhigh-heat-dissipative electronic packaging technology that cansolve this problem has emerged as an important issue. The heatgenerated from device packaging not only lowers the reliability ofthe devices but also shortens their life. Therefore, researchers havefocused on the development of highly heat-dissipative materialsthat can solve this problem [1–3].

Research on enhancing the thermal conductivity of polymermatrix composites currently focuses on the use of carbonnanotubes (CNTs) with high thermal conductivity as fillers. CNTswere discovered by Iijima [4] in 1991, and since then, they havebeen studied for use in a variety of applications. Because CNTs withexcellent flexibility, low degree of bulk, and very high aspect ratios(300–1000) also have high electrical conductivity and thermalconductivity, they are in the limelight as fillers for thermallyconductive polymer composites [5,6].

These useful properties of CNTs make them suitable forapplication in various devices such as semiconductors, flat panel

* Corresponding author. Tel.: +82 32 876 7234; fax: +82 32 860 8438.

E-mail address: sjpark@inha.ac.kr (S.-J. Park).

Please cite this article in press as: J.-R. Choi, et al., J. Ind. Eng. Chem

1226-086X/$ – see front matter � 2013 The Korean Society of Industrial and Engineer

http://dx.doi.org/10.1016/j.jiec.2013.12.029

displays, batteries, ultra-high-strength fibers, and biosensors [7,8].However, as CNTs tend to agglomerate because of intermolecularvan der Waals forces, they form pores when applied in composites,resulting in a significant decrease in the strength of the composite.Recent studies have focused on surface treatment to improve theproperties of CNTs. Surface treatment methods for CNTs include alength control method by ball milling, a dispersion method usingsurface-active agents, and a metal layer forming method throughelectroless plating [9–11].

Copper and nickel are usually used in electroless nickel plating.Copper tends to be easily oxidized in air to form CuxOy and lose itsproperties. It is also highly reactive with other chemicals used inprocessing CNT’s. In contrast to copper, nickel shows excellentresistance to oxidation and chemicals [12].

Electroless Ni-plating is a popular surface treatment technique.Electroless nickel–P plating has been widely investigated becauseof its low cost. However, Ni–B plating is rarely reported.Nevertheless, many features of Ni–B plating have been recognized,such as good conductivity, superior bonding, low porosity, andexceptional electromagnetic performance [13,14]. In addition tothe general coating properties, Ni–B plating normally has also beenfound to have a higher plating efficiency than a nickel–phosphate(Ni–P) plating technique because of the highly efficient reducingagent dimethylamine boran used in the bath [15,16].

In this study, nickel nanoparticles were introduced onto thesurface of multi-walled carbon nanotubes (MWCNTs) in order toinvestigate the effect of the presence and composition of Ni–BMWCNTs on the thermal conductivity and fracture toughness

. (2014), http://dx.doi.org/10.1016/j.jiec.2013.12.029

ing Chemistry. Published by Elsevier B.V. All rights reserved.

Fig. 1. Electroless Ni–B plating processes for MWCNTs.

Table 1Composition and conditions of the Ni plating bath.

Compositions NiSO4�6H2O 0.2 mol/L

Na3C6O7�2H2O 0.15 mol/L

(CH3)2NHBH3 0.125 mol/L

Conditions pH 6.5

Temperature (8C) 60

Time (min) 5

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behavior of epoxy matrix composites reinforced with Ni–BMWCNTs.

2. Experimental

2.1. Materials

The epoxy resins used in this study were bisphenol-A, suppliedby Kukdo Chem. of Korea (KFR-120), which had an epoxideequivalent weight (EEW) of 170–180 g/eq. and a density of 1.0–1.2 g/mL at 25 8C. KFH-150 was used as a curing agent for the epoxyresins. Multi-walled carbon nanotubes (MWCNTs) synthesized bythe chemical vapor deposition were supplied from NanosolutionCo., Ltd. (Korea). The properties of MWCNTs were: purity >95 wt.%,average diameter 10 nm, and average length 20 mm.

The electroless Ni–B plating processes for MWCNTs illustratedin Figure 1. Before being plated, MWCNTs were stirred into 10 wt.%nitric acid for 30 min in order to enhance the interfacial adhesionbetween the nickel coating and MWCNTs. The electroless Ni–Bplating was processed by 2 steps (activation process and metaldeposition process). First step, activation process, it was activatedin 0.1 M tin chloride (SnCl2, Aldrich, USA) for 30 min, then theproducts were activated in 0.0014 M palladium chloride (PdCl2,Aldrich, USA) for 30 min. In this activation process, Sn/Pd nucleiwere formed on the surface of MWCNTs, and the Sn/Pd nucleiformed on the surface of MWCNTs promoted metal plating at thetime of electroless Ni–B plating [17–19]. Ni–B MWCNTs wereobtained by immersing the MWCNTs in a nickel bath for 5 min. InTable 1, the chemical compositions and plating conditions of theelectroless nickel plating bath were listed in detail.

Ni–B MWCNTs of 5, 10, 15, and 20 phr were added to epoxyresins and mixed with the 3-roll mill for 1 h for even dispersion.KFH-150 was then added to the Ni–B MWCNTs/epoxy mixtures inan equivalent ratio of 1:1 as the mixing ratio and stirred. Themixture was dried in a vacuum oven to remove the residual organicsolvent and bubbles in the epoxy, and samples for analysis werefabricated in a hot-press at 150 8C and 20 MPa for 30 min using thevacuum bagging method.

The surface properties of the Ni–B MWCNTs were characterizedby X-ray photoelectron spectroscopy (XPS). The XPS experiment

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was performed using a Ka spectrometer equipped with an AlKa X-ray source. The base pressure in the sample chamber wascontrolled in the range from 10�8 to 10�9 torr. The morphologiesof the Ni–B MWCNTs were measured by a scanning electronmicroscope (SEM, JSM-6701F, JEOL, Japan).

The thermal conductivity of Ni–B MWCNTs composites wasmeasured with a ThermoCon Tester M100 (Metrotech Co., Ltd.,Korea). Measurements were performed two times on the samesamples with different thicknesses to obtain the thermalconductivity. The thermal conductivity was measured accordingto ASTM D5470. In this method, copper bars are connected to theheating part and the cooling part of the meter. The thermalconductivity of the samples measures at the moment when theheat quantities of the upper part and the lower part equilibrate.Therefore, the total heat quantity at the status when heat flowoccurs, Q, can be calculated using the following equation.

Q ¼ lA

dADT (1)

where l is thermal conductivity, A is the area of the copper bars, dA

is the distance of copper bars, T is the temperature differencebetween upper part and lower part.

The critical stress intensity factor (KIC) of the composites wasmeasured via a single-edge-notched (SEN) test as part of a three-point flexural test conducted using a universal test machine (LR5KPlus, Lloyd, UK) according to ASTM D 5045. The span-to-depth ratioand crosshead speed were 4:1 and 0.8 mm/min, respectively[20,21]. The KIC value was obtained by averaging five experimentalvalues. The fracture surfaces after the assessment of KIC wereinvestigated using an SEM.

. (2014), http://dx.doi.org/10.1016/j.jiec.2013.12.029

Fig. 2. SEM images of (a) as-received and (b) Ni–B MWCNTs.

0

20

40

60

80

100

Ni-B MWCNTs

C1s

O1s

Ni2p

O1s/C

1s

Ele

men

tal

com

posi

tion (

%)

as-received

0.00

0.15

0.30

0.45

0.60

0.75

O1s /C

1s ratio

Fig. 4. Elemental compositions of as-received and Ni–B MWCNTs.

0 5 10 15 200.000.10

0.15

0.20

0.25

0.30

0.35

Ther

mal

conduct

ivit

y (

W/m

K)

Ni-B MWCNTs con tents (phr)

Fig. 5. Thermal conductivity of Ni–B MWCNTs/epoxy composites as a function of

Ni–B MWCNTs content.

J.-R. Choi et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx 3

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3. Results and discussion

In Figure 2(a), the surface of as-received is smooth and clean. Bycontrast, the micrograph of Ni–B MWCNTs (Figure. 2(b)) showsthat nickel particles have been deposited on the surface of theMWCNTs.

To confirm the changes in the surface functional groups on theMWCNTs by Ni–B plating, XPS studies were performed; the resultsare shown in Figure 3. The C1s peak (binding energy, BE = 291.1 eV)and an O1s peak (BE = 537.5 eV) were observed for both as-received

120 0 900 600 300 0

Ni2p

O1s

Inte

nsi

ty

Binding Ene rgy (eV)

C1s

as-rece ived

Ni-B MWCNTs

Fig. 3. XPS spectra of as-received and Ni–B MWCNTs.

Please cite this article in press as: J.-R. Choi, et al., J. Ind. Eng. Chem

and Ni–B MWCNTs. In the case of the Ni–B MWCNTs, a peakcorresponding to Ni2p (BE = 862 eV) was observed, which was notobserved in the as-received MWCNTs. The intensity of the O1s peakincreased for the Ni–B MWCNTs relative to that of the as-receivedMWCNTs, while the intensity of the C1s peak decreased. The

0 5 10 15 200.03.0

3.5

4.0

4.5

5.0

KIC

(MP

a!m

1/2)

Ni-B MWC NTs con ten t (ph r)

Fig. 6. KIC results of Ni–B MWCNTs/epoxy composites as a function of Ni–B

MWCNTs content.

. (2014), http://dx.doi.org/10.1016/j.jiec.2013.12.029

Fig. 7. Cross-section images of Ni–B MWCNTs/epoxy composites after KIC tests: (a)

10 phr and (b) 20 phr.

J.-R. Choi et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx4

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changes of the elemental compositions of the MWCNTs accordingto the Ni–B plating process are shown in Figure 4. The amount ofO1s increased by Ni–B plating. From the XPS results, the increase inthe intensity of the O1s peak is attributed to the introduction ofnickel oxide during the electroless Ni–B plating, along with theformation of new oxygen-containing functional groups [22,23].

The thermal conductivity of the Ni–B MWCNTs/epoxy compo-sites as a function of Ni–B MWCNTs is shown in Figure 5. Thethermal conductivity of neat epoxy was 0.17 W/mK. All Ni–BMWCNTs/epoxy composites showed a higher thermal conductivitythan that of the neat epoxy. At 20 phr, the thermal conductivityincreased to about 0.304 W/mK, which was about 78% enhance-ment. This increased heat transfer is attributed to increase thenumber of contact points between the Ni–B MWCNTs and theepoxy matrix with increasing content of Ni–B MWCNTs [24,25].

Figure 6 shows the KIC results of Ni–B MWCNTs/epoxycomposites as a function of the Ni–B MWCNTs content. Fracturetoughness for the neat epoxy sample was measured to be3.94 MPa�m1/2. At 10 phr, the fracture toughness increased to4.76 MPa�m1/2, which was about 20% enhancement. This isattributed to the oxygen-containing functional groups and nickelintroduced to the surface of Ni–B MWCNTs, which increased theinterfacial binding strength with epoxy resins, thus enhancing theresistance of Ni–B MWCNTs against cracks. However, when the Ni–B MWCNTs content was more than 15 phr, the KIC value decreasedfrom the maximum. This was due to the non-uniform distributionof Ni–B MWCNTs within the epoxy and partial entanglement of the

Please cite this article in press as: J.-R. Choi, et al., J. Ind. Eng. Chem

Ni–B MWCNTs [26,27]. The morphologies of the fractured surfacesof the composites are shown in Figure 7(a) and (b). The SEM imagesof the 10 phr sample revealed very severe roughness as comparedto the case of the 20 phr sample. It is concluded that the interfacialadhesion between the Ni–B MWCNTs and the epoxy matrix washigher in the 10 phr sample than in the 20 phr sample.

4. Conclusions

In this study, we investigated the effects of electroless Ni–Bplating on the thermal conductivity and fracture toughness of Ni–BMWCNTs/epoxy matrix composites. Based on the experimentalresults, Ni–B MWCNTs surfaces which have higher activity areconcluded to be effective in promoting increased adhesion becauseof the presence of oxygen and nickel functional groups on theMWCNTs surface, which cause changes in the O1s/C1s ratios of theMWCNTs. The thermal conductivity increased with increasing Ni–B MWCNTs content. The heat transfer pathway in these samples isenhanced because of the larger number of contact points betweenNi–B MWCNTs. The KIC values also increased with increasingelectroless Ni–B MWCNTs content to 10 phr. At higher Ni–BMWCNTs content, non-uniform distribution and partial entangle-ment of the Ni–B MWCNTs resulted in a lower KIC value.

Acknowledgement

This research was supported by a grant from the FundamentalR&D Program for Technology of World Premier Materials fundedby the Ministry of Trade, Industry & Energy, Republic of Korea.

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