C A R B O N 4 9 ( 2 0 1 1 ) 4 2 6 9 – 4 2 7 9

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Characterisation of graphite nanoplatelets and the physicalproperties of graphite nanoplatelet/silicone compositesfor thermal interface applications

Mohsin Ali Raza a,*, Aidan Westwood a, Andy Brown a, Nicole Hondow a, Chris Stirling b

a Institute for Materials Research, University of Leeds, Leeds LS2 9JT, UKb Morgan AM&T, Swansea SA6 8PP, UK

A R T I C L E I N F O

Article history:

Received 8 April 2011

Accepted 1 June 2011

Available online 6 June 2011

0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.06.002

* Corresponding author:E-mail address: pmmar@leeds.ac.uk (M.A

A B S T R A C T

Thermally conducting and highly compliant composites were developed by dispersing

graphite nanoplatelets (GNPs) into a silicone matrix by mechanical mixing. X-ray diffrac-

tion (XRD) indicates that the average thickness of the GNPs decreased from 60 to 35 nm dur-

ing mechanical mixing. XRD-texture analysis demonstrated that GNP/silicone composites

at 8 wt.% GNPs have a higher degree of basal plane alignment than at 20 wt.%. Differential

scanning calorimetry showed that GNPs raised the curing temperature of silicone with no

significant effect on the glass transition temperature. The thermal conductivity of the

20 wt.% composites reached 1.909 W/m.K, an 11-fold increase over silicone suggesting an

improved dispersion compared to similar composites prepared by dual asymmetric centri-

fuge mixing. The percolation threshold for electrical conductivity of the composites was at

�15 wt.%. The compressive modulus of the composite increased to twice that of silicone at

20 wt.%. The corresponding strength decreased by a factor of two compared to silicone and

this can be attributed to the weak bonding at the GNP-silicone interface. Overall, these

GNP/silicone composites, with a high thermal conductivity, low electrical conductivity

and compliant nature are promising materials for use as thermal pads for thick gap filling

thermal interface applications.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Graphite nanoplatelets (GNPs) are an emerging class of

nanomaterials that are gaining favour for many technological

applications such as conducting composites [1–4], electronics

[5], batteries, sensors, transparent conducting films, hydrogen

storage and supercapacitors [6–8]. GNPs offer an alternative to

carbon nanotubes due to their excellent physical and chemi-

cal properties and low cost [9,10]. GNPs have a platelet thick-

ness in the range of 0.35–100 nm [11]. ‘‘GNP’’ in the literature

refers to various forms of graphite such as exfoliated graphite,

expanded graphite or graphene [12]. Graphene is a single

er Ltd. All rights reserved. Raza).

layered GNP in which the carbon atoms are close-packed into

a two-dimensional hexagonal lattice [13].

GNP/polymer conducting composites have been devel-

oped by dispersing GNPs in both thermoplastics and in

thermosetting polymers for potential applications such as

electromagnetic interference shielding [14], barrier layers

[10], electrodes [15] and thermal interface applications

[3,16,17]. In particular, these composites are attractive for

thermal interface applications in electronics due to the high

thermal conductivity of GNPs. Several authors report high

thermal conductivities of GNP/polymer composites. Debalak

and Lafdi [2] report that a GNP/epoxy composite containing

.

4270 C A R B O N 4 9 ( 2 0 1 1 ) 4 2 6 9 – 4 2 7 9

20 wt.% GNPs has a thermal conductivity of 4.3 W/m.K

which is a 19-fold increase over that of epoxy alone. Ganguli

et al. [3] report that GNPs increase the thermal conductivity

of GNP/epoxy composites by 19-fold (for GNPs with average

particle size of 3.9 lm and 100 nm thickness) from 0.2 W/

m.K (for the epoxy alone) to 4.2 W/m.K. In our previous work

[17], we proposed the use of GNP/silicone composites for

thermal interface applications due to their high thermal

conductivity, high electrical resistivity and highly compliant

nature. We reported that the thermal conductivity of a GNP/

silicone composite produced by a Flaktek dual asymmetric

centrifuge ‘‘Speed Mixer’’ (SM) reached 1.43 W/m.K at

20 wt.% loading of GNPs (of 15 lm particle size). This mea-

sured conductivity is comparable to commercially available

thermal interface materials but is achieved at 40 wt.% lower

loading of the filler compared to when more commonly used

commercial fillers such as BN, ZnO or AlN are used. Re-

cently, Kujawski et al. [15] were the first to suggest the use

of GNP/silicone composites for compliant electrodes due to

their high electrical conductivity and mechanical compli-

ance. However, similarly compliant composites with low

electrical conductivity would still be useful for thermal inter-

face applications particularly for thermal pads (for example,

they can be used between the heat spreader and heat sink).

In the present study, we report the curing behaviour, glass

transition temperature, transport and mechanical properties

of composites produced by conventional mechanical mixing

and compare these with previous results from composites

prepared using the SM process [17]. The results are also corre-

lated to the characterisation of the microstructures of the

GNPs and composites using electron microscopy and X-ray

diffraction.

2. Experimental

2.1. Materials used

Graphite nanoplatelets (GNPs) were purchased from XG Sci-

ences, Ltd. These were thin particles having a platelet mor-

phology with reported thicknesses in the range of 5–10 nm.

Platelets with a reported average width of 15 lm were used

in this study. The manufacturer also reported that the GNPs

contain up to 3 wt.% crystalline silica as an impurity.

Sylgard 184 Silicone Elastomer, purchased from Dow Corn-

ing, was employed as the silicone matrix material. The elasto-

mer is supplied as a two-part liquid component kit. These

parts are mixed at 10:1 by weight respectively to make a sili-

cone polymer matrix for the composites. Part A is comprised

of primarily vinyl end capped oligomeric dimethyl siloxane

and 10 units of this (by weight) are reacted, by catalytic

hydrosilation, with one unit of part B, which is comprised of

methylhydrosiloxane as the crosslinking agent and a plati-

num complex as a catalyst [18]. Further details regarding

the curing reaction can be found in [19].

2.2. Fabrication of composites

GNP/silicone composites were prepared by conventional

mechanical mixing (MM). ‘Bulk’ samples were prepared with

minimum dimensions of 40 · 25 · 10 mm3 from 40–50 g

batches by mixing the appropriate weight proportions of the

GNPs and silicone. All of the composite dispersions were pre-

pared at room temperature. GNPs were dried in an oven at

80 �C for 12 h to remove any moisture adsorbed on their sur-

face. The dried GNPs were then mixed in the appropriate

weights with the silicone base by using a conventional

mechanical mixer with a high-speed motor attached to a

shaft with a propeller (25 mm radius steep pitch with three

blades), rotated at a speed of 2500 rpm for 25 min. After mix-

ing the GNPs in the silicone base, the curing agent was added

and mixed for an additional 2 min at 1000 rpm. The batch was

then degassed under vacuum and poured into a custom made

aluminium mould. The filled mould was again degassed for

15 min to completely remove any trapped air. Composites

were prepared with a loading of 8, 15 and 20 wt.% GNP. It

was not feasible to load GNPs above 20 wt.% because the high

viscosity of the dispersion inhibited the movement of the stir-

rer. The temperature of the dispersions reached 75 �C during

mixing. Before addition of the curing agent the GNP/silicone

base dispersion was cooled down to about 5 �C. The GNP/sili-

cone was then cured at 95 �C for 60 min. A sample of unadul-

terated silicone was also produced by this mechanical mixing

(MM) method.

2.3. Characterisation

The curing behaviour and glass transition temperature (Tg) of

the GNP/silicone composites were investigated using a Perkin

Elmer DSC7 differential scanning calorimeter (DSC). A sample

of 8–10 mg was placed in aluminium crucibles for DSC mea-

surements. A dynamic scan was carried out from 40 to

200 �C at a heating rate of 10 �C/min in a nitrogen flow of

10 ml/min. To measure Tg, a �10 mg sample was placed in

an aluminium crucible and a dynamic scan was carried out

from 30 to �120 �C at a cooling rate of 10 �C/min in a nitrogen

flow of 10 ml/min.

The thermal conductivity of the silicone and GNP/silicone

composites was measured (in the direction parallel to direc-

tion of gravity in the original curing mould) by a hot disk ther-

mal constant analyser (Hot Disk� AB), which is a transient

plane source technique [20]. The sensor, which acts as both

a heat source and a temperature recorder, has a radius of

3.18 mm and was sandwiched between two pieces of sample.

For measurement, each piece of sample was cut to 8–10 mm

thickness, 20 mm · 20 mm area and left with flat surfaces so

that a good thermal contact could be made across the thick-

ness of the two pieces and the sandwiched sensor. The con-

ductivity measurements were made by applying a power of

0.1–0.2 W for between 10 and 80 s, depending on the thermal

conductivity of the sample.

For electrical conductivity measurement, cuboidal pieces

of the composites (�6 · 6 · 2 mm3) were placed between two

copper electrodes having dimensions slightly greater than

those of the sample. The electrodes were connected to a mul-

timeter (Agilent 34401A) which measured the resistance of

the sample using the two probe method.

To observe the effect of orientation of the GNPs in the

composites, the electrical conductivity was measured in three

orthogonal directions across the sample: rz is the measure-

Fig. 1 – Schematic illustration of the electrical conductivity

measurement system.

C A R B O N 4 9 ( 2 0 1 1 ) 4 2 6 9 – 4 2 7 9 4271

ment in the same direction as the thermal conductivity mea-

surement and parallel to the direction of gravity in the origi-

nal curing moulds; rx and ry are the measurements in the

horizontal plane of the original curing mould as shown in

Fig. 1.

Compression testing of the silicone and its composites was

carried out on an Instron universal testing system (Model No.

3382 with a 100 kN load cell). Rectangular shaped samples

(�8 · 8 · 10 mm3) were compressed at a strain rate of

0.5 mm min�1. The compression tests were performed on

the samples so that compression occurred parallel to the

direction of gravity in the original curing moulds (z-direction

in Fig. 1). A typical compression test was carried out until the

sample fractured. In addition, samples of 20 wt.% GNP/sili-

cone produced previously by SM and EPM 2490 (commercial

silicone based adhesive, marketed by Nusil technology which

is filled with boron nitride particles at 65 wt.%) were tested for

comparison [17]. Hardness testing of the samples was done

using a Shore hardness tester (Zwick) and values measured

on scale A.

The as-received GNPs were prepared for powder X-ray dif-

fraction (XRD) by evaporating acetone-graphite suspensions

onto a single crystal silicon off-cut sample holder. The diffrac-

tion patterns were acquired by a Philips PW 1830 diffractom-

eter (Cu–Ka, 40 kV, 40 mA, 10–90� 2h, 0.02� step, 20 s step�1).

Samples of GNP/silicone composites were prepared by cutting

approximately cubic shapes from the bulk and fixing them to

a glass slide such that the sample axis parallel to the direction

of gravity in the original curing moulds (rz above) was normal

to the plane of the glass slide, which itself is at an angle h to

the incident X-rays during the experiment. Diffraction pat-

tern analysis such as background determination and peak

positions were carried out using Xpert high score plus soft-

ware. The instrumental broadening was measured from the

line profile of a standard silicon specimen using the same

experimental set-up. The average out of plane crystallite

thickness of the GNPs (T) was estimated using the (0 0 0 2)

FWHM 2h values and the Scherrer equation: T = Kk/bCosh,

where b is the line breadth (FWHM) in radians with the instru-

mental broadening subtracted, k is the X-ray wavelength, and

the coefficient K taken to be 0.89 and h is the diffraction angle

of the peak of interest [21]. The X-ray texture scans were

obtained in a Philips texture goniometer (PW 3040/60

diffractometer) using CuKa radiation with a Schulz reflec-

tion-specimen holder, in which the source and detector were

positioned at the fixed 2h Bragg angle for reflection from the

(0 0 0 2) planes. The 2h values for the different samples were

determined in separate Bragg scans. The sample was rotated

around the A axis at a speed of 72�/min. The sample was tilted

by 5� with respect to the w axis for every revolution around A.

The cycle of rotation around the A axis was continued up to a

w value of 85�. The data are plotted as pole figures and pre-

sented in terms of contours of intensity of reflections from

the (0 0 0 2) graphite planes.

Transmission electron microscopy (TEM) was conducted to

assess the range of thicknesses of the GNPs on two micro-

scopes; an FEI CM200 field emission gun (FEG-)TEM operating

at 197 keV, equipped with an Oxford Instruments energy dis-

persive X-ray (EDX) spectrometer and a Gatan Imaging Filter

and an FEI Tecnai TF20 microscope operating at 200 keV,

equipped with a Gatan Orius SC1000 digital camera running

Digital MicrographTM software including the tomography

package. The tilt series were collected through a tilt range

of approximately �45� to +45� with an image recorded every

degree. Samples were prepared by dispersing in methanol

and then drop-casting onto a holey carbon coated copper

TEM grid (Agar Scientific Ltd).

Fracture surfaces of the composites were observed using a

LEO 1530 field emission gun scanning electron microscope

(FEG-SEM). The secondary electron images were obtained

using the in-lens detector at an operating voltage of 3 kV with

a working distance of 3 mm. Samples were prepared by cool-

ing strips of the composites in liquid nitrogen and then brit-

tle-fracturing them. The fractured surface of the sample

was sputter coated with a 5 nm layer of Pt/Pd alloy prior to

SEM analysis. All of the samples studied by SEM were frac-

tured so that a surface roughly parallel to the direction of

gravity during moulding was exposed for the analysis.

3. Results and discussion

3.1. Characterisation of the as-received graphitenanoplatelets (GNPs)

3.1.1. Particle sizeA representative SEM image of the as-received GNPs and the

histogram showing the platelet size distribution are pre-

sented in Fig. 2. On the basis of SEM analysis, it was found

that the GNPs have average lateral width of 15 lm (with a

standard deviation of 6 lm). The histogram clearly shows that

the majority of platelets have widths between 10 and 20 lm.

3.1.2. Thickness measurement of the edges of GNPs usingTEMThe exfoliation of intercalated graphite produces GNPs with a

wide range of thicknesses [22]. It is of value to measure the

range of thicknesses of the GNPs since for a given wt.% load-

ing in a composite thinner GNPs would give better dispersion

and so better transport properties. Hence, the quality of GNPs

as fillers might to some extent be evaluated from the average

and distribution of thicknesses of the GNPs.

Previously researchers have mainly used atomic force

microscopy (AFM) to measure the thicknesses of GNPs by

Fig. 2 – (a) SEM image of as-received GNPs. (b) Histogram showing the platelet size distribution to be centred around 15 lm.

4272 C A R B O N 4 9 ( 2 0 1 1 ) 4 2 6 9 – 4 2 7 9

measuring GNP height relative to that of the substrate [23].

This requires the uniform deposition of individual GNPs onto

the substrate (mica or silicon wafer), which is not always pos-

sible due to overlapping sheets that are physically adhered to

one another via Van der Waals forces.

In the present work a series of TEM images were recorded

at different tilts on the as-received GNPs in order to measure

thicknesses at the platelet edge. In the TEM, under normal

imaging conditions most of the drop-cast GNPs appear to be

lying flat (Fig. 3a). As the stage tilt angle is varied from �40�to +40� (Fig. 3a–e) the folded edge of a GNP is revealed

(Fig. 3d and e). Such tilting was used to find a GNP that could

be oriented precisely edge-onto the electron beam: the ‘‘opti-

mum tilt’’ at which the edge-thickness is maximised gives the

true thickness of the edge of the sheet. For the GNP in Fig. 3

Fig. 3 – Bright field TEM images of the as-received GNPs. (a–e) Ex

(b) �20� (c) 0� (d) +20� and (e) +40�. (f) Image of the platelet edge e

platelet indicated. (g) Image of another platelet, with thicknesse

the thickest edge cross-section was seen at a tilt angle of

�2� and the edge is folded up with respect to the bulk of the

platelet (Fig. 3f). The thickness of the edge of this platelet var-

ied from �5 to �10 nm. A cross-sectional image of the edge of

another GNP in Fig. 3g shows it to be folded and its thickness

varies from �13 to �25 nm. The variation in the diffraction

contrast across the GNP as the tilt angle changes (visible in

the tilt series movies, see supplementary information,

Appendix A.) suggests that the GNP is not a perfectly flat

plate. Thus, tilt series imaging of GNPs in the TEM reveals that

the GNPs used in the present work have wrinkled topography

and some have folded edges.

Optimum tilt TEM images of the unfolded edges of several

GNPs show these to have thicknesses in the range 15–34 nm

(Fig. 4). Direct confirmation that the (0 0 0 2) plane stacking

cerpts of a platelet tilt series at specific tilt angles of: (a) �40�ndon to the beam (�2� tilt), with thickness of the edge of the

s of the edge of the platelet indicated.

Fig. 4 – (a–f) Optimum tilt angle TEM images of several as-received GNPs revealing the thicknesses at the edge of the platelets

to be in the range of 19–34 nm. (g) TEM image of silica particle (right) next to GNP (left), with EDX spectrum from silica particle

inset. (h) TEM image of contaminant nano-particles on the surface of a GNP. An EDX spectrum of the area is shown inset and

indicates the contaminant to be an iron oxide, probably remnant catalyst particles. The copper signal is due to the TEM

support grid.

Fig. 5 – XRD patterns of the as-received GNPs and the GNP/

silicone composites with 8–20 wt.% GNPs produced by the

mechanical mixing (MM) process. For comparison the XRD

pattern of GNP/silicone composite produced by speed

mixing (SM) is also presented. (*Corresponds to SiO2

impurity).

C A R B O N 4 9 ( 2 0 1 1 ) 4 2 6 9 – 4 2 7 9 4273

edges of the GNPs are parallel to the edges of the platelets is

seen in the TEM lattice image of the edges (Fig. 4c and d in

particular). The crystalline silica particles, present as impuri-

ties in the GNPs, are observed to be large (lm in width) and

distinct from the GNPs i.e., are not attached to the surface

of the GNPs (Fig. 4g). Iron oxide particles (remnants of the

intercalation process used to manufacture GNPs) can be iden-

tified still attached to the surface of the GNPs (Figs. 3 and 4)

and their presence is confirmed by the point EDX spectrum

from the particles on the GNP shown in Fig. 4h.

Specimen tilting in the TEM thus proves to be a quick

method for estimating the thicknesses of GNPs. This method

is also useful to observe the morphology of GNPs.

3.2. X-ray diffraction (XRD)

XRD patterns of the as received GNPs and the composites are

presented in Fig. 5. All peaks can be indexed to graphite and

the dominance of the 0 0 0 2 peak indicates the basal plane

alignment of the GNPs during the curing/settling process.

The peak at 28.27� in the XRD pattern of the as-received GNPs

is due to the presence of some SiO2 impurity. No iron oxide

impurity was detected by XRD and therefore we assume its

content (as identified by TEM) to be less than a few percent.

The mean thicknesses of the as-received and processed

GNPs can be measured from the broadening of the (0 0 0 2)

peak and are presented in Table 1.

The average thickness of the as-received GNPs was found

to be �62 nm. This is significantly higher than that deter-

mined by TEM analysis. This may be because XRD gives an

average estimation of thickness across the full width of many

Table 1 – Thicknesses of GNPs determined from XRD peak/line-broadening analysis.

Samples 0 0 0 2 dspacing (�A)

AveragethicknessGNPs (nm)

Standarddeviation*

(nm)

GNPs (as received) 3.37 62 18 wt.% GNP/silicone by MM 3.35 49 615 wt.% GNP/silicone by MM 3.35 39 1420 wt.% GNP/silicone by MM 3.35 36 320 wt.% GNP/silicone by SM 3.35 47 2

* Standard deviation is obtained by testing at least three specimens of each sample.

4274 C A R B O N 4 9 ( 2 0 1 1 ) 4 2 6 9 – 4 2 7 9

platelets, whereas TEM analysis was carried out on only a few

platelets (�10 in the present work) and only at the edge of the

platelets. Also the GNPs observed by TEM were smaller than

5 lm across and it may be that these smaller plates are thin-

ner than the larger platelets which comprise a large volume

proportion of the GNPs measured by XRD (Fig. 2b).

The average thicknesses of GNPs measured from XRD

analysis of the 15 and 20 wt.% GNP/silicone composites

had decreased to 39 and 36 nm, respectively. This is signifi-

cantly lower than the as-received GNPs and this result

showed that the MM caused a shearing of GNPs resulting

in the dispersion of thinner GNPs. It might be that the ob-

served high viscosity of the 20 wt.% GNP composite also

made the shearing process more intense and this effect con-

tributed to the decrease in the average platelet thickness

with increased GNPs loadings.

The average thickness of GNPs measured from the XRD

analysis of a 20 wt.% GNP/silicone composite produced by

SM (our previous work; [17]) was 47 nm. This is significantly

higher than the average thickness measured for the 20 wt.%

GNPs composite produced by MM, indicating that SM pro-

duces less shearing of GNPs than MM.

The pole figures obtained from XRD texture analysis of the

GNP/silicone composites prepared by MM are presented in

Fig. 6. The composite consisting of 8 wt.% GNPs has highly ori-

ented basal planes with a high abundance of (0 0 0 2) planes

oriented at low w values (0–5�) i.e., the GNPs have oriented in

the mould so that the plates lie flat and the [0 0 0 2] is parallel

to the direction of gravity (Fig. 6a). The high degree of orienta-

tion of this composite is attributed to the low viscosity of the

Fig. 6 – Pole figures of the: (a) 8 wt.% GNP/silicone and (b) 20 wt

preferred [0 0 0 2] texture normal to the specimen surface.

GNP/silicone dispersion at 8 wt.%, which allows the GNPs to

easily settle parallel to the bottom surface of the mould during

curing. In the case of the 20 wt.% GNPs, the orientation of

(0 0 0 2) planes is spread over w values between 0–40� (Fig. 6b).

This suggests that mutual hindrance by the GNPs and the high-

er viscosity of the 20 wt.% composite pre-mix also limits orien-

tation (as well as increasing the amount of GNP shearing). The

texture measurements highlight how readily GNP/silicone

composites form with a preferred orientation of the GNPs.

3.3. Differential scanning calorimetry (DSC)

DSC scans of silicone and GNP/silicone dispersions at 20 wt.%

loading are presented in Fig. 7. The increase in temperature of

the curing exotherm and the more gradual curing of the sili-

cone in the case of the composite show that the GNPs hinder

the process of curing. The curing temperature of the silicone

rises from 92 to 132 �C. The degree of hindrance to the curing

reaction depends on the filler dispersion and it is possible to

predict a filler’s dispersion quality from DSC scans [24]. The

better the dispersion, the more severe is the hindrance since

increased interface area between the GNPs and the silicone

decreases the mobility of the silicone polymer chains. In our

previous work on the GNP/silicone composites produced by

SM, we reported that 20 wt.% loading increased the curing

temperature of silicone from 92 to 116 �C and so the current

DSC scans suggest that the dispersion quality of the GNPs

in the composite produced by MM is better.

For MM, there was no significant change observed in Tg of

pure silicone and silicone with 15 and 20 wt.% loading of

.% GNP/silicone composites prepared by MM both, showing

Fig. 8 – DSC cooling profiles for silicone and the GNP/silicone

composites for determination of Tg (Tg was determined from

the inflection point of the cooling curves and does not

change with loading of GNPs, Table 2).

Fig. 7 – DSC heating profiles of neat silicone and 20 wt.%

GNP/silicone dispersions produced by MM showing that the

exothermal curing process occurs at higher temperature

with addition of GNPs.

C A R B O N 4 9 ( 2 0 1 1 ) 4 2 6 9 – 4 2 7 9 4275

GNPs (Fig. 8 and Table 2). The glass transition temperature of

polymer composites depends on the free volume of the poly-

mer, which is affected by the amount of interaction between

the polymer and filler particles [24]. Several studies on GNP/

polymer composites [3,25] report an increase of Tg with

increasing content of GNPs. This suggests that in this case

Table 2 – Glass transition temperatures of silicone and GNP/silicone composites (the Tg of silicone is reported to be at114.6 �C by Xu et al. [24]).

Material Tg (�C) *Standarddeviation (�C)

Silicone �114 115 GNP/silicone �112 120 GNP/silicone �113 1

* Standard deviation is obtained by testing 2–3 specimens of each

sample.

however, the GNPs do not chemically interact with the

silicone.

3.4. Morphology of the composites

SEM images of the GNP/silicone composite at 8 and 20 wt.%

loading of GNPs are shown in Fig. 9a and b and c–f, respec-

tively. The SEM images of the 8 wt.% GNPs composite

(Fig. 9a and b) show that the GNPs are quite well dispersed

and the fracture surface is reasonably smooth. By contrast,

the low magnification images of the 20 wt.% GNPs composite

(Fig. 9c and d) shows a much rougher fracture surface indicat-

ing pull-out of GNPs upon loading. The edges of some GNPs

can be clearly identified in the images. The high magnifica-

tion images of 20 wt.% GNPs composite (Fig. 9e and f) show

that the GNPs are well dispersed in the silicone matrix with

individual GNPs separated by some silicone matrix. However,

there are many GNPs interconnecting with one another,

which is essential for the formation of conducting networks.

Crude estimates from SEM images (Fig. 9e and f) suggest that

the GNPs have thicknesses �20–40 nm in these composites.

It was observed in the SEM images of GNP/silicone pro-

duced by SM (our previous work [17]) that the gaps between

the GNPs were larger than in the composites produced here

by MM. This suggests again that the higher-shear mixing by

MM (cf. SM) results in thinner GNPs and that this should lead

to more efficient conducting networks at equivalent loadings.

3.5. Thermal conductivity

A plot of the thermal conductivity of the GNP/silicone com-

posites produced by MM as a function of wt.% of GNPs (and

measured in the direction parallel to gravity during the curing

in the mould i.e., z direction as shown in Fig. 1) is presented in

Fig. 10. For comparison, the thermal conductivity data from

the previous work for the GNP/silicone composites produced

by speed mixer (SM) are also presented in Fig. 10 [17]. The

thermal conductivities of the GNP/silicone composites in-

crease with increasing content of GNPs. The thermal conduc-

tivities of the composites produced by MM are higher with

smaller standard deviations than the composites produced

by SM at large loadings. This can be attributed to the more

uniform dispersion of the GNPs in the silicone by MM than

by SM.

The thermal conductivity of GNP/silicone composite at

20 wt.% loading of GNPs reached 1.909 W/m.K, which repre-

sents an �11-fold increase over pure silicone (0.175 W/m.K).

This is 35% higher than the equivalent composite produced

by SM. This significant improvement is attributed to the

thinner GNPs in the composite produced by MM compared

to the composite produced by SM as observed by the XRD

analysis (Fig. 5 and Table 1). Thinner GNPs means more parti-

cles for a given wt.% loading and this results in more efficient

conducting networks within the matrix, leading to the im-

proved thermal conductivity of the composites.

3.6. Electrical conductivity

Unlike thermal conductivity, electrical conductivity is gener-

ally an undesirable property in thermal interface materials

Fig. 9 – SEM images of the fracture surfaces of: (a and b) 8 wt.% GNP/silicone composite and (c–f) 20 wt.% GNP/silicone

composite showing the GNPs in the silicone matrix. Increased GNP pull-out is evident in the 20 wt.% composite. Arrows point

towards some of the GNPs in the matrix.

Fig. 10 – Thermal conductivities (measured in the settling

direction) of GNP/silicone composites produced by MM and

SM rise as a function of wt.% of GNPs. *Refers to our previous

work [17].

Fig. 11 – Electrical conductivities of the GNP/silicone

composites measured in different directions on the

specimen as a function of wt.% of GNPs showing significant

increase in electrical conductivity as GNPs content increases

from 15 to 20 wt.%.

4276 C A R B O N 4 9 ( 2 0 1 1 ) 4 2 6 9 – 4 2 7 9

for electronics, although high electrical conductivity can be

tolerated in some of these applications. The electrical con-

ductivity of the GNP/silicone composites produced by MM

measured in different directions on the samples (as shown

in Fig. 1) as a function of wt.% of GNPs is presented in

Fig. 11. The GNP/silicone composite at 8 wt.% loading of GNPs

was highly insulating and its exact value was not determined

as the metre used only has a measurable range up to 100 MO.

The SEM images of this composite showed that GNPs were

separated by thick layers of silicone. The GNP/silicone

composite at 15 wt.% was also highly insulating but with

the electrical conductivity slightly higher than the percolation

threshold (defined as the filler content to achieve an electrical

conductivity P 10�6 S.m�1 [26]). The electrical conductivity

above this percolation threshold of �15 wt.% GNPs increases

abruptly reaching 0.360 S.m�1 for the composite produced

with 20 wt.% GNPs. This could be due to the presence of an

increased contact between GNPs or of gaps of only a few

nanometres between GNPs that allow the electrons to tunnel

through the insulating matrix (Fig. 9).

To see the effect of orientation of GNPs in the silicone, the

electrical conductivity of GNP/silicone composite was

Fig. 12 – Compression stress–strain curves of neat silicone,

GNP/silicone composites produced by MM and EPM 2490.*Refers to composite produced by SM [17].

C A R B O N 4 9 ( 2 0 1 1 ) 4 2 6 9 – 4 2 7 9 4277

measured in three orthogonal directions on the samples

(Fig. 11). The GNP/silicone composite at 15 wt.% loading has

an order of magnitude higher electrical conductivity in the x

and y directions compared to the z direction, confirming the

alignment of basal planes of the GNPs in the x–y plane of

the sample as determined by XRD (Fig. 6). On the other hand,

the composite with 20 wt.% loading of GNPs has only a

slightly higher electrical conductivity in the x and y directions

compared to the z direction, consistent with the spread in

textural alignment seen in XRD (Fig. 6b).

Compared to our previous work [17] in which the mea-

sured electrical conductivity for 20 wt.% GNP/silicone com-

posite made via SM was only 1.7 · 10�4 S.m�1, the much

higher corresponding electrical conductivity here (0.360

S.m�1) can be attributed to the better dispersion of GNPs in

the silicone achieved by MM. The percolation threshold of

the current GNP/silicone composites (at 15 wt.%) is however

significantly higher than the 3 wt.% reported by Kujawski

et al. [15]. These authors also reported electrical conductivity

that was five orders of magnitude higher for a 15 wt.% GNP/

silicone composite compared to the present work. The differ-

ence in results could be because Kujawski et al. used a differ-

ent fabrication technique, a �50% lower proportion of curing

agent and a different source of exfoliated graphite. They pro-

duced exfoliated GNPs by microwave irradiation using nitro-

gen and hydrogen as the forming gas. They have reported the

average platelet size to lie in the range of 5–20 lm and a

thickness in the range 20–200 nm, i.e., potentially smaller

and as thin as the GNPs used here. Since their exfoliation

process was carried out in hydrogen, this may reduce oxide

groups on the surface of the GNPs and possibly result in few-

er functional groups than on the GNPs produced by XG Sci-

ences; the presence of a large amount of hydroxyl and

epoxide on the basal planes of GNPs or carboxylic groups

on the edges of GNPs has been reported to reduce electrical

conductivity [1,27,28]. However, the most important differ-

ence between the present work and Kujawski et al. [15] might

be the composite fabrication technique. They prepared com-

posites by mixing silicone in hexane solvent followed by

ultrasonication. It is well known that the solvent mixing

method allows the fillers to develop percolating networks at

relatively low loadings, by reducing the viscosity of the poly-

mer [29,30]. High viscosity of the GNP/silicone dispersions

may be a limiting factor in achieving uniform dispersions

of GNPs by the MM or SM method and as such, the relatively

high percolation value reported here may indicate some

inadequate mixing. Ultimately though, the composite fabri-

cation method used in this work (MM) is simpler and more

environmentally friendly since no solvents are used.

3.7. Compression and hardness testing

The compression stress–strain curves of silicone and GNP/sil-

icone composites (at 15 and 20 wt.% loading) are presented in

Fig. 12. For comparison, the compression stress–strain curves

of the GNP/silicone composite at 20 wt.% loading of GNPs pro-

duced by SM [17] and commercial TIM EPM 2490 are also pre-

sented in Fig. 12. The compression properties and Shore

hardness values of silicone and the composites are presented

in Table 3.

It can be observed from Table 3 and Fig. 12 that the com-

pressive moduli of GNP/silicone composites made via MM in-

crease with increasing content of GNPs. The compressive

modulus of the composites increased by 1.65· and 2· that

of silicone at loadings of 15 and 20 wt.% GNPs, respectively,

as would be expected. The modulus of the 20 wt.% GNP/sili-

cone composite prepared by MM is 1.5· higher than that pre-

pared by SM (Table 3) again suggesting an improved

dispersion of GNPs with the associated potential for

improved reinforcement. There is however a significant de-

crease in compressive strength (at failure) in the composites

compared to that of silicone. Debalak and Lafdi [2] have also

observed a similar effect in GNP/epoxy composites: they

attributed this decrease in strength (at loading >4 wt.%) to

poor interaction between the GNPs and the epoxy (i.e.,

debonding at the platelet-matrix interface) and to easy

shear and glide of the graphite 0 0 0 2 layers. The Shore hard-

ness values of the GNP/silicone composites are slightly

increased with the addition of GNPs as would be expected

(Table 3).

Although the GNP/silicone composites produced by MM

have slightly higher modulus than the silicone, this is still

1.5 times lower than a commercial BN/silicone composite

(EPM 2490) as shown in Fig. 12 and Table 3. These compressive

properties and Shore hardness values demonstrate that the

GNP/silicone composites are highly compliant materials.

4. Conclusions

Commercial GNPs were used to fabricate GNP/silicone com-

posites. The GNPs were measured to have a platelet width

of �15 lm (by SEM) and a thickness of �60 nm (measured

by (0 0 0 2) line-broadening in XRD). They were shown to

have a wrinkled morphology and folded edges by tilting in

the TEM and the observed thickness at the edge of the plate-

lets was 5–35 nm. XRD analysis was found useful in assess-

ing the shearing effect of the GNPs by the mechanical

mixing process technique; the average platelet thickness de-

creases from 62 to 36 nm after mixing. This is compared to a

Table 3 – Compression and hardness properties of neat silicone and GNP/silicone composites.

Material Fabricationmethod

Compressivemodulus(at 20% strain)(MPa)

Compressivestrengthat failure(MPa)

Compressive strainat failure (%)

Shore hardness(scale A)

Silicone MM 6.18 ± 0.53 41.47 ± 6 62.51 ± 1.44 53.0 ± 1.015 wt.% GNP/silicone MM 10.2 ± 0.33 19.58 ± 2.59 66.3 ± 2.26 58 ± 1.220 wt.% GNP/silicone MM 12.2 ± 1.11 17.34 ± 2.2 66.12 ± 1.47 60.8 ± 2.120 wt.% GNP/silicone [17] SM 7.98 ± 0.54 18.46 ± 1.91 69.69 ± 1.36 54.6 ± 1.2EPM 2490 – 18.72 ± 3.92 7.5 ± 1.4 51.61 ± 2.86 81.2 ± 2.14

4278 C A R B O N 4 9 ( 2 0 1 1 ) 4 2 6 9 – 4 2 7 9

drop in thickness to only 47 nm via SM [17]. The composite

produced with 8 wt.% of GNPs was shown (by XRD textural

analysis) to have strong alignment of the basal planes of

the GNPs caused by their settling parallel to the bottom

plane of the composite moulds. This alignment was less pro-

nounced at 20 wt.% loading due to the increased mutual hin-

drance between GNPs and to their effect on increasing the

viscosity of the mixture. DSC curing scans showed that at

20 wt.% GNPs the silicone cured more gradually and the cur-

ing temperature increased to 132 �C, suggesting some inter-

action of the GNPs with the silicone polymer chains.

However, addition of GNPs produced no significant effect

on the glass transition temperature of the silicone suggest-

ing no strong chemical interaction.

The thermal conductivities of GNP/silicone composites

produced by mechanical mixing reached 1.909 W/m.K at

20 wt.% loading. This represents an 11-fold increase over that

of silicone (0.1795 W/m.K) and is a 35% increase cf. similar

composites prepared by SM [17]. The high thermal conductiv-

ity reported in the present work is believed to be achieved by

the excellent dispersion and thinning of GNPs produced by

mechanical mixing. The GNP/silicone composites were elec-

trically insulating at loadings below 15 wt.%. The electrical

conductivity of the GNP/silicone composite at 20 wt.% loading

reached 0.360 S.m�1 compared to 8.33 · 10�13 S.m�1 for sili-

cone. Compression testing showed that the addition of GNPs

increased the modulus of silicone by a factor of up to two.

However, it decreased the compressive strength by a factor

of two which might indicate weak bonding between the GNPs

and silicone. Shore hardness of the silicone did not change

much with the addition of GNPs.

The GNP/silicone composite at 20 wt.% loading of GNPs

produced by MM with its high thermal conductivity, relatively

low electrical conductivity and compliant nature makes it a

promising material to be used for thick gap filling thermal

interface applications.

Acknowledgements

The authors thank Morgan AM&T and EPSRC for funding

M.A.R.’s Dorothy Hodgkin Postgraduate Award Scholarship

and A.P.B.’s Advanced Research Fellowship (EP/H008578/1).

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.carbon.2011.06.002.

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