C A R B O N 4 9 ( 2 0 1 1 ) 3 6 2 2 – 3 6 3 0

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Development of mesophase pitch derived high thermalconductivity graphite foam using a template method

Abhay Yadav a, Rajeev Kumar b, Gopal Bhatia b,*, G.L. Verma a

a Delhi College of Engineering, Bawana Road, New Delhi 110 042, Indiab National Physical Laboratory, New Delhi 110 012, India

A R T I C L E I N F O

Article history:

Received 15 December 2010

Accepted 27 April 2011

Available online 5 May 2011

0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.04.065

* Corresponding author: Fax: +91 11 25726938E-mail address: gbhatia@mail.nplindia.er

A B S T R A C T

A simple and inexpensive method is described for preparing high thermal conductivity

graphite foam by impregnating a coal tar pitch based mesophase pitch into a substrate

polyurethane foam template. Mesophase pitch impregnated polyurethane foam was con-

verted into graphite foam by several heat treatments in air as well as in an inert atmo-

sphere. Scanning electron microscope images show the retention of an excellent open

pore structure despite volume shrinkage of over 50%. The graphite foam prepared by this

sacrificial template method is found to possess a thermal conductivity of 60 W/m K with

a compressive strength in the range of 3.0–5.0 MPa. The X-ray diffraction pattern shows

an interlayer spacing (d002) of 0.3388 nm at a heat treatment temperature of 2400 �C. Differ-

ent concentrations of slurries of mesophase pitch in water were used in combination with

substrate foams of different densities to prepare graphite foams of density in the range

0.23–0.58 g cm�3. The specific thermal conductivity of the carbon foam with a low density

of 0.58 g cm�3 is found to be higher than that of copper metal traditionally used in thermal

management applications.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon foams have existed for many years. Research and

development related to them started few decades ago with

the synthesis of what was known as reticulated vitreous car-

bon foam. It was prepared by Ford [1] through carbonization

of the thermosetting organic polymer foam via simple heat

treatment in 1960s. Control on structure and material proper-

ties of carbon and graphitic foams were achieved by Googin

et al. [2] by varying the precursor material of partially cured

urethane polymer. Since then, various carbon foams have

been developed rapidly and various applications ranging from

electrodes to insulating liner (for temperature up to 3000 �C in

inert atmospheres) have been explored simultaneously with

the development of these foams. Though some structural

er Ltd. All rights reserved

.net.in (G. Bhatia).

applications have also been mentioned in the literature,

earlier applications of majority of carbon foams were pre-

dominantly limited to thermal insulation purposes [3–8].

In an attempt to develop lightweight and highly structural

material with highest specific strength, researchers at the

Wright Patterson Air Force Base Materials Lab succeeded in

synthesizing first mesophase pitch derived graphitic foam

to replace expensive 3-D woven fiber performs in polymer

composites [9–10]. Later in 1997, Klett [11] reported the suc-

cessful synthesis of first graphitic foams with bulk thermal

conductivities greater than 40 W/m K. Recently, there has

been a great interest on the development of graphite foams

which possess several specific properties such as low density

(q = 0.2–0.8 g/cc), high bulk thermal conductivities (>100 W/

m K prepared at 2800 �C), high temperature tolerance (up to

.

C A R B O N 4 9 ( 2 0 1 1 ) 3 6 2 2 – 3 6 3 0 3623

3000 �C under inert atmosphere), large specific surface area

with open cell structures (20,000 m2/m3), adjustable thermal

and electrical conductivities, low coefficient of thermal

expansion, and isotropic thermal conductivities.

High thermal conductivity, low weight, relatively large

surface area with open cell structure and low coefficient of

thermal expansion of graphite foams recognize their poten-

tial applications in thermal management systems. Therefore,

graphite foams have emerged as material of great importance

and are now being developed as a new class of thermal man-

agement material and are fabricated with a wide variety of

thermo-mechanical properties which can replace bulky heat

exchangers and heat sinks to dump the waste heat to the

environment.

The potential applications of high thermal conductivity

carbon foams are not limited to thermal management alone

but also to several other fields such as power electronic heat

sink, compact light weight radiators, anode electrode mate-

rial for lithium ion rechargeable batteries etc. [12–14]. Because

of orientation of liquid crystal domains in it, mesophase pitch

derived graphite foams display superior anode performance

with higher stable capacitance and therefore offer a potential

replacement of anode material in lithium ion rechargeable

batteries [14]. High thermal conductivity and porosity helps

graphite core (radiator) rejecting more heat than conventional

metal (aluminum) based high performance radiators with les-

ser surface area.1 The other significant potential applications

of graphite foams include catalyst support, filters and fuel cell

radiators.

Several methods are reported for the development of

graphite foams and most of them are based on foaming

techniques employed on AR mesophase pitches followed by

oxidation stabilization, carbonization and graphitization

[11,15–20]. Foaming has been achieved by either using blow-

ing agent or pressure release employing the expensive high

temperature-high pressure autoclave systems. Further, it is

also known that it is difficult to obtain carbon foam with large

and uniform cells by the foaming methods. Therefore, a need

of relatively simpler and inexpensive technique for meso-

phase pitch derived graphite foam was felt and the same

has been attempted in this paper by using the sacrificial

template method. Search of the literature reveals that a few

references are available for the preparation of carbon foam

by this sacrificial template method by impregnating polyim-

ide resin [21] or petroleum pitch [22] into the polyurethane

(PU) foam as a template. Further, Chen et al. [22] used the heat

treated petroleum pitch in place of mesophase pitch during

the preparation of carbon foam and did not determine the

thermal conductivity as well as its compressive strength.

In this paper, the present authors have attempted to devel-

op the high thermal conductivity graphite foams of different

densities by impregnating mesophase pitch (its water slurries

in the presence of suitable organic adhesive as thickening

agent) into the PU foams of different densities as a template

material and then carbonizing and graphitizing the same up

to 2400 �C to obtain the porous graphite foams. The bulk

1 Klett J. High thermal conductivity graphite foams for compactor%20Graphite-Foam.pdf.

densities of the graphite foams so prepared have been studied

as a function of slurry concentrations, densities of the

substrate foams at heat treatment temperatures (HTT) up to

2400 �C. Further, the variation of compressive strength as well

as that of thermal conductivity as a function of bulk density

of the graphite foam has also been investigated in detail in

the present study which has not been so far reported. It

may be worth mentioning that this method of making graph-

ite foams from mesophase pitch using the template method

would be inexpensive, economical and time saving in

comparison to the existing methods of using expensive high

temperature-high pressure autoclaves reported in literature

by Klett et al. and other researchers [11,15–20]. Moreover, this

method does not involve complex foaming method and

therefore easier to fabricate the graphite foam from easily

available cheaper PU foams. This paper describes and

discusses the preparation of graphite foam using the

template method.

2. Experimental

2.1. Mesophase pitch characterization

Mesophase pitch was prepared in-house from a suitable coal

tar pitch by thermal treatment at 430 �C in inert atmosphere

and was characterized with respect to various parameters

such as mettler softening points, quinoline insoluble content,

toluene insoluble content, coking value, specific gravity, ash

content, thermogravimetric analysis (TGA). The mesophase

content was determined using optical microscopy. For optical

microscopy, 2–3 pieces of the mesophase pitch sample were

mounted in cup of 2.5 cm diameter using epoxy resin and

hardener in the ratio 10:1. Mounting was followed by grinding

of the specimens with water proof grit SiC paper in the order

of 200, 400, 600, 800, 1000 and 1200 grit sizes to remove the

layer of resin from the surface. The specimens were then pol-

ished with alumina powders of varying sizes in the order of

0.5 lm, 0.1 lm and finally 0.05 lm on clothes of lapping

machine for 10 h. The enhanced images were captured by film

camera mounted on Zeiss Polarized Optical Microscope MC 80

DX model. The mesophase content was estimated by visual

observation of mesophase pitch, under polarized optical

microscope by comparing the area occupied by coalesced

liquid crystals (as percentage) of total area. The TGA of meso-

phase pitch was done in nitrogen atmosphere at two different

heating rates 120 and 600 �C/h. TGA of one sample was also

carried out partially in air atmosphere and then continued

in nitrogen atmosphere for better correlation of weight losses

of pitch impregnated foams observed during various stages of

heat treatment.

2.2. Impregnation of mesophase pitch into PU foam

The prepared mesophase pitch was crushed, ground and ball

milled in a tungsten carbide (WC) jar for about 5–6 h to make

the particle size less than 30 lm. The particle size was

t lightweight radiators. www.acm-nevada.com/Technical/Radia-

Cutting of PU Foam Slabs into 50 mm x 30 mm x 18 mm

Mesophase Pitch + Water + PVA

Washing (with water) and drying (for 6 h at 100°C)

Slurry concentrations (25%, 30%, 35% & 40%)

Dipping of PU foam slabs in mesophase pitch slurry

Oxidation stabilization(at 300°C for 1 h in air) &

Heat treatment (at 350°C for 1 h in air)

Graphitization (at 2400°C for 1 h in nitrogen)

Carbonization (at 1000°C and 1400°C for

1 h each in nitrogen)

Drying of pitch foams (at 100°C for 12 h in air)

Heat treatment (at 300°C for 1 h in nitrogen)

Fig. 1 – Flowchart of the process developed for graphite

foam.

3624 C A R B O N 4 9 ( 2 0 1 1 ) 3 6 2 2 – 3 6 3 0

reduced so as to facilitate the penetration of mesophase pitch

particles into the small pores of PU foam. Water slurries of

different percentages of mesophase pitch (25%, 30%, 35%

and 40%) were prepared. For thickening the slurry (aqueous

suspension of pitch) and to enhance the adhesion of pitch

onto the foam, the organic additive of polyvinyl alcohol

(PVA) was added in small amounts during slurry preparation.

The different PU foams used were characterized for their pore

structure, pore sizes, cell numbers (in pores per inch, PPI) and

bulk density.

The porous PU foam slabs with dimensions of

50 · 30 · 18 mm were first cleaned with distilled water and

dried in air at 100 �C for 6 h. The dried foams were then

impregnated with mesophase pitch by dipping the substrate

into the homogenous slurry. Impregnation was carried out

under vacuum (360 mm of Hg) for 15 min. For uniform distri-

bution and removal of excess slurry, the foam slabs after

impregnation were pressed carefully by rolling glass rod on

both the sides. Mesophase pitch impregnated foams were

then dried at 100 �C for 12 h in order to evaporate the water

completely.

The dried pitch impregnated foams were then heat treated

at the rate of 1 �C/min to 300 �C in the atmosphere of nitrogen

for 1 h followed by oxidation in air atmosphere to 300 �C at

the rate of 10 �C/h and held at 300 �C for 1 h. This heat treat-

ment was carried out for oxidation stabilization of the inter-

connected pitch network by introducing cross-links in the

structure so that the mesophase pitch does not melt during

subsequent heat treatments at higher temperatures. Slow

oxidation step also results in decomposition of PU foam

material. This stabilized foam was further heated at 350 �Cin air to burn off the remaining organic substances (if any)

in the pitch network of foam slabs. The stabilized mesophase

pitch foam was then carbonized first at 1000 �C for 1 h and

then at 1400 �C for 1 h with a heating rate of 50 �C/h in an in-

ert atmosphere of nitrogen. The carbonized mesophase pitch

impregnated foam was then graphitized at 2400 �C with a

heating rate of 600 �C/h. Fig. 1 shows the flowchart for the

process used for preparing graphite foam.

The oxidation and carbonization behavior of precursor

material of PU foam as such and mesophase pitch were

separately studied by TGA in open air furnaces (in air atmo-

sphere) and closed furnaces (in nitrogen atmosphere) with

moderate heating rate of 2 �C/min to correlate the changes

in weight and dimensions of the foam slabs during various

heat treatments. In addition to TGA, pyrolysis, decomposition

of PU foam, PVA, carbonization and graphitization behavior of

mesophase impregnated foams were further confirmed by

Fourier Transform Infra Red (FT-IR) analysis of the specimens

after every stage of processing, using FT-IR spectrometer

Thermonicolet model 380 having resolution of 4 cm�1 in

transmittance mode in the spectral range of 4000–400 wave

number (cm�1). The surface morphologies and microstruc-

ture of the prepared graphitized carbon foams were observed

under scanning electron microscope (SEM) of Hitachi S3700

model. The crystal structure of foam graphitized at 2400 �Cwas determined by analyzing the powdered specimen by

X-ray diffraction (XRD) technique employing D-8 Advanced

Bruker powder X-ray diffractometer using CuKa radiation

(k = 1.5418 A) spectrometer. For the determination of

compressive strength, few foam pieces of dimension 12 ·6 · 5 mm were cut and subjected to compressive test on

Instron universal testing machine (UTM) Model 4411. Thermal

conductivity of few samples of the dimensions 12.0 · 12.0 ·4.0 mm were measured by flash diffusivity technique using

thermal diffusivity system Flashline 3000 K. The porosity of

the graphitized foam was determined by water porosity

method (ASTM C830-79).

3. Results and discussion

3.1. Mesophase pitch preparation and characterization

The properties of precursor QI free coal tar pitch as well as

those of developed mesophase pitch are listed in Table 1.

The optical micrograph of mesophase pitch shows the meso-

phase content of 85–90% (Fig. 2).

In TGA in nitrogen atmosphere, heating rate has been

found to affect the yield (coking value being higher in case

of heating rate of 120 �C/h). TGA curves of mesophase pitch

in different atmospheres are shown in Figs. 3 and 4.

The observation of TGA curves reveals that up to 350 �C,

there is no significant weight loss. The first major weight loss

is observed at around 500 �C after which curve for heating

rate 600 �C/h falls steeply to give a residual weight of around

55% at 950 �C however curve for heating rate 120 �C/h

becomes sluggish almost remaining constant at around 80%

at 950 �C. It may be due to volatilization or decomposition of

Table 2 – PU foams used for mesophase pitch impregnation.

Foam No. Averagecell

numbers (PPI)

Bulkdensity(g cm�3)

Averagepore size

(mm)

Color

Foam 1 30 0.018 0.75 GrayFoam 2 40 0.024 0.57 YellowFoam 3 50 0.030 0.45 Black

Table 1 – Properties of mesophase pitch.

Properties Units Determined by Coal tar pitch Mesophase pitch

Softening point �C Mettler toledo 86.6 232Quinoline insoluble content % – 0.3 30.2Toluene insoluble content % – 15.9 64.5Specific gravity . . . Water immersion density (WID) 1.28 1.32Coking value % At 950 �C (inert atmosphere) 47.6 80.1Ash content % At 950 �C (in air) 0.001 0.05Optical texture – Optical microscopy Isotropic AnisotropicMesophase content % Optical micrograph – 85–90

60

70

80

90

100

0 200 400 600 800 1000Temperature( °C)

Wei

ght

(%)

In Air

In Nitrogen

Air atmosphere

Nitrogen atmosphere

Fig. 4 – TGA curve of mesophase pitch in air and nitrogen

atmosphere.

40

50

60

70

80

90

100

0 200 400 600 800 1000Temperature (°C)

Wei

ght

(%)

Heat@120°C/hr

Heat@600°C/hr

Fig. 3 – TGA curve of mesophase pitch in nitrogen at two

heating rates.

Fig. 2 – Optical micrograph of mesophase pitch showing

coalesced liquid crystals.

C A R B O N 4 9 ( 2 0 1 1 ) 3 6 2 2 – 3 6 3 0 3625

low molecular weight pitch components and decomposition

of the heavy pitch molecular weights component.

The low pyrolysis yield of 55% at 600 �C/h shows the rela-

tively faster evolution of volatile decomposition products due

to higher rate of heating compared to 120 �C/h.

3.2. Impregnation of mesophase pitch into PU foam

The properties are PU foams used for impregnation are listed

in Table 2. The PU foams of three different densities (0.018,

0.024 and 0.030 g/cc) of different cell numbers per unit length

(30, 40 and 50 pores per inch, PPI) were used to correlate the

cell numbers of the precursor foams and slurry concentra-

tions with the bulk densities of resulting carbon foams.

The variation of densities of foams graphitized at 2400 �Cwith mesophase pitch slurry concentrations is plotted in

Fig. 5. After graphitization of mesophase pitch impregnated

foams, the foams shrank by almost 50% retaining around

70% weight, however, the appearance remained porous. In

the case of foam of 30 PPI, template foam with very low den-

sity 0.018 g/cc (high porosity) the amount of impregnated

mesophase pitch is quite low resulting in relatively lower bulk

densities (0.23–0.46 g/cc) after graphitization. On the other

hand, in case of template foam of 50 PPI with higher bulk

density 0.030 g/cc, the higher amount of pitch could be

impregnated resulting in higher bulk densities (0.31–0.58 g/

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

15 20 25 30 35 40

Conc. of Slurry (%)

Bul

k D

ensi

ty o

f G

raph

itis

ed F

oam

(gc

m-3

)

Foam 30 PPI

Foam 40 PPI

Foam 50 PPI

Fig. 5 – Effect of slurry concentration on bulk densities of

graphitized foam.

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 0.028 0.03

Density of Substrate PU Foam (gcm-3 )

Bul

k D

ensi

ty o

f G

raph

itis

ed F

oam

(gc

m-3

) 40%

35%

30%

25%

Fig. 6 – Effect of PU substrate foam densities on resultant

bulk densities.

3626 C A R B O N 4 9 ( 2 0 1 1 ) 3 6 2 2 – 3 6 3 0

cc). For a particular slurry concentration, density increases

rapidly on going from foam of 30 PPI to foam of 40 PPI whereas

the increase is relatively smaller on going further from 40 PPI

foam to 50 PPI foam. This trend is also evident from the plot of

variation of bulk densities of graphitized foam with the

densities of precursor PU foams.

From Table 3 and Fig. 5, it can be inferred that density of

the graphitized foam can be increased in two ways, either

by increasing the slurry concentrations keeping the pore sizes

same or by decreasing the pore sizes for the same mesophase

pitch slurry concentration. For a fixed slurry concentration,

variation of bulk densities of graphitized foam with the

densities of precursor PU foam is plotted in Fig. 6. From this

plot, it is also evident that for a foam of particular cell sizes,

the increase in resultant bulk densities is more pronounced

while increasing slurry concentration from 30% to 35% and

from 35% to 40% in comparison to increase of slurry concen-

trations from 25% to 30%.

For a fixed slurry concentration of mesophase pitch, density

marks smaller incremental effect with the reduction of pore

sizes. The incremental effect is highest (1.34 times) for slurry

concentration of 25%, however, the incremental factor is

almost same (around 1.25 times) for other three slurry concen-

trations. From the trends of Figs. 5 and 6, it follows that differ-

ent pore sizes of substrate foam in combination with different

mesophase pitch slurry concentrations can be used effectively

to develop graphite foam of desired bulk density.

Table 3 – Weight and volume residues of mesophase pitch imp

Heat treatment

Impregnated foam dried at 100 �C for 12 h (air)Impregnated foam at 300 �C for 1 h (inert)Oxidation stabilization at 300 �C for 1 h and heated at 350 �C (aiCarbonization at 1000 �C for 1 h (inert)Carbonization at 1400 �C for 1 h (inert)Graphitization at 2400 �C (inert)

3.3. Formation of foam

The TGA curves of PU substrate foam and PVA in air as well as

nitrogen are shown in Figs. 7 and 8 respectively.

The TGA curves of foam disclosed the first (main) decom-

position temperatures of 200 and 300 �C in air and nitrogen

respectively. Pyrolysis of PU foam in air start at around

200 �C and intensifies between the temperature range of

200–300 �C where as in presence of nitrogen pyrolysis starts

at around 300 �C and intensifies between 300 and 400 �C.

The decomposition products from the foam on heat treat-

ments are diisocynates, polylols, amines, olefins and carbon

dioxide due to destruction of polymeric chains and functional

groups [23]. It is interesting to note that residue (TGA weight

%) of the pyrolized foam above 500 �C in air is same as in

nitrogen i.e. there is no severe oxidation/burn-off above

500 �C in air. This may be attributed to the fact that oxygen

(in air) enhances the char formation in the foam which may

be inhibiting the severe oxidation (burn-off) above 500 �C in

air.

Upon heating PVA above the decomposition temperature

(200 �C) the polymer begins a rapid chain-stripping elimina-

tion of H2O and it couples with melting (230 �C) resulting in

the material to foam or intumesce (bubbling up) as it decom-

poses. This and other decomposition reactions cause color

changes and cross linking to yield yellow to black rigid

foam-like residues. The residue yields, percent volume

shrinkage, percent weight loss of impregnated foam recorded

regnated foam.

Average residualweight (%)

Average residualvolume (%)

100 10096.6 75.8

r) 92.5 70.280.1 68.375.2 54.072.8 48.9

0

20

40

60

80

100

100 200 300 400 500 600 700

Temperature(°C)

Wei

ght

(%)

In Nitrogen

In air

Fig. 7 – TGA curve of PU foam in air and nitrogen.

0

20

40

60

80

100

0 100 200 300 400 500 600Temperature (°C)

Wei

ght

(%)

In Nitrogen

In air

Fig. 8 – TGA curve of PVA in air and nitrogen.

C A R B O N 4 9 ( 2 0 1 1 ) 3 6 2 2 – 3 6 3 0 3627

at every stage of heat treatment are summarized in Table 3

and plotted in Fig. 9.

The close observation of Table 3 and Fig. 9 reveals that

considerable weight loss occurs up to 350 �C and is supported

40

50

60

70

80

90

100

0 500 1000 1500 2000 2500

Heat Treatment Temperature (°C)

Res

idua

l Wei

ght/

Res

idua

l Vol

ume

(%)

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

Bul

k D

ensi

ty (

gcm

-3)

Residual Weight (%)

Residual Volume (%)

Density

Fig. 9 – Variation of residual weight, residual volume and

bulk density of impregnated foam with HTT.

by the information provided by the Figs. 3 and 4 (TGA curves

of mesophase pitch) and Fig. 7 (TGA curve of PU foam). The

weight loss up to 350 �C is due to decomposition of PU foam

in air at around 300 �C (Fig. 7) and the decomposition of resin

in this temperature range (Fig. 8). The further weight loss

after 350 �C is primarily due to loss of relatively high molecu-

lar components of the oxidation stabilized mesophase pitch

in the temperature range of 350–1000 �C. At 1400 �C, the

weight loss is very small due to escape of volatiles from

impregnated foam. Between 1400 (carbonization) and

2400 �C (graphitization) weight loss gets gradually decreased.

However, trends in volume shrinkage are somewhat different

from that of weight loss. At 300 �C in nitrogen, volume shrink-

age of 24% occurs due to formation of interconnected

network of mesophase pitch present in different well

connected pores of PU foam. At 350 �C in air further shrinkage

of 6% (total 30%) occurs mainly due to degradation of PU foam

skeleton. Between 350 and 1000 �C, the shrinkage is very

small. At 1400 �C, volume shrinkage of about 15% was found

due to completion of carbonization. After 1400 �C, further

shrinkage though small occurs due to graphitization of foam.

Variation of density of foam of 50 PPI impregnated by meso-

phase pitch slurry (35%) is plotted against temperature of

various heat treatment stages (Fig. 9). This variation is in

accordance with the weight loss and volume shrinkage occur-

ring simultaneously at various heat treatment stages. Initial

increase in density of dried impregnated foam at 300 �C in

nitrogen is due to high shrinkage (24%) accompanied by

negligible weight loss of 4%. Between 300 and 350 �C, density

remains almost constant due to small weight loss of 4%

against small volume shrinkage of around 5%. It decreases

appreciably at 1000 �C as additional weight loss of 12% occurs

with a small shrinkage of 2%. After reaching to a minimum

density shoots up at 1400 �C as small weight loss of 5% is out-

weighed by larger shrinkage of about 15%. Further negligible

increase in density at 2400 �C occurs due to further shrinkage

of 5% accompanied by around 3% weight loss.

The Fig. 10 shows the FT-IR spectrum of PU foam as such,

mesophase pitch having aromatic constituents, PVA powder;

oxidation stabilized impregnated foam, carbonized at 1000

and 1400 �C and graphitized foam. The gradual disappearance

of all functional groups with increasing temperatures is ob-

served. The specimen carbonized at 1400 �C and graphitized

at 2400 �C does not show the presence of any functional

group. The absence of any functional group in the residue

indicates that the graphite foam structure is entirely

consisting of carbon which established the effectiveness of

carbonization at 1000 and 1400 �C.

The microstructure of graphitized foam in SEM images

(Fig. 11) shows that pore structure is retained despite volume

shrinkage of over 50%. Also the pores are uniformly distrib-

uted although some pores in the image are seen broken

during machining and sample preparation.

The XRD pattern of graphitized foam after HTT of 2400 �Cis shown in Fig. 12. The presence of a sharp diffraction peak

at 26.28� suggests that mesophase pitch derived graphitized

foam has high degree of crystallinity and graphitic structure.

The interlayer spacing d002 is found to be 0.3388 nm which is

higher than the value of 0.3362 nm for those prepared at

2800 �C by foaming methods [11]. The La and Lc values of the

Fig. 10 – FT-IR spectra of PU foam, PVA powder, mesophase pitch, and foams at 300, 350, 1000, 1400 and 2400 �C.

Fig. 11 – SEM images of: (a) graphitized foam at 2400 �C and (b) its pore wall.

3628 C A R B O N 4 9 ( 2 0 1 1 ) 3 6 2 2 – 3 6 3 0

20 30 40 50 60 70 800

200

400

600

800

1000

1200

1400

1600

1800

Inte

nsity

(ar

b. u

nits

)

2θ

002 peak

110 peak

d002

=0.3388nm

Lc(002)=21nm

La(110)=36nm

Fig. 12 – XRD pattern of graphitized foam (HTT of 2400 �C).

0

10

20

30

40

50

60

70

0.1 0.2 0.3 0.4 0.5 0.6 0.7

Density (gcm-3)

The

rmal

Con

duct

ivit

y (W

/m.K

)

Fig. 14 – Variation of thermal conductivity with density.

C A R B O N 4 9 ( 2 0 1 1 ) 3 6 2 2 – 3 6 3 0 3629

graphite foam as calculated from Scherrer equation are found

to be 36 and 21 nm, respectively.

3.4. Mechanical and thermal properties of the foam

The results of compressive strength are plotted against the

densities in Fig. 13. As expected the compressive strength

increases with bulk density of the graphitized foam. The

maximum compressive strength is found to be 5.0 MPa for a

density of 0.58 g cm�3.

The maximum thermal conductivity of 59.74 W/m K was

achieved for foam with density of 0.58 g cm�3 graphitized at

2400 �C and the specific thermal conductance (thermal con-

ductivity to density ratio) is 103 which is more than twice that

of copper (45). The results are plotted against the respective

densities in Fig. 14.

The values of thermal conductivity, specific thermal con-

ductivity, compressive strength and degree of graphitization

of the high thermal conductivity graphite foam prepared by

sacrificial template method are compared in Table 4 with

those obtained from foaming techniques of the same density

2.5

3

3.5

4

4.5

5

5.5

6

0.3 0.35 0.4 0.45 0.5 0.55 0.6

Density of foam (gcm-3)

Com

pres

sive

Str

engt

h (M

Pa)

Fig. 13 – Variation of compressive strength with density.

and a metal copper, a commonly used standard, for thermal

management systems in industries. A high thermal conduc-

tivity above 60 W/m K is expected of the graphite foam heated

to a temperature of 2800 �C.

It is interesting to note from Table 4 that the compressive

strength of the graphite foam prepared by template method is

higher than that of prepared by foaming method for the same

density (�0.5 g/cm3), while the thermal conductivity shows

the opposite trend. The reasons for high compressive

strength and low value of thermal conductivity achieved in

our samples may be attributed to several factors like rela-

tively higher value of d002 obtained at the highest HTT of

2400 �C, nature of mesophase pitch (coal tar pitch based),

mesophase content (85–90%) used during the processing in

place of 2800 �C, Mitsubishi ARA24 pitch having 100% meso-

phase content respectively used by Klett and coworker [11].

It is expected that using a mesophase pitch with higher meso-

phase content (>85%) and HTT of 2800 �C is likely to enhance

the thermal conductivity and reduce the compressive

strength further.

4. Summary

High thermal conducting carbon foams (q = 0.23–0.58 g/cc)

with thermal conductivity up to 60 W/m K have been

prepared by impregnating from water slurries of different

concentrations of mesophase pitch precursor material into

a template of PU foam of suitable PPI followed by various heat

treatments involving oxidation stabilization, carbonization

and graphitization. The progress at every stage has been

monitored in terms of various characterization parameters

which established the effectiveness and simplicity of the

template method during the graphite foam development.

SEM images show that the graphitized carbon foams have

an excellent pore structure and the existence of a sharp peak

at 26.28� with d002 = 0.3388 nm in XRD pattern reveal that the

foams are graphitized. The compressive strength is in the

range of 3.0–5.0 MPa and is similar to those prepared by foam-

ing techniques. The graphitized foams in the present study

have a thermal conductivity as high as 60 W/m K and a

specific thermal conductivity more than twice that of copper,

an industry standard for thermal management.

Table 4 – Comparison of properties of graphite foam, standard foam and copper.

Properties Graphitized foam*(sacrificial template)

Graphitized foam#

(foaming method)Copper

Mesophase pitch Coal tar based Mitsubishi ARA 24 –Mesophase content (%) 85–90 100 –Highest HTT (�C) 2400 2800 –Density (g/cc) 0.58 0.54 8.9Porosity (%) 68 75 0Compressive strength (MPa) 5.0 3.4 –Thermal conductivity (W/m K) 60 106 400Specific thermal conductivity 103 198 45

* Present study.

# Published work [11].

3630 C A R B O N 4 9 ( 2 0 1 1 ) 3 6 2 2 – 3 6 3 0

Acknowledgements

The author Mr. Abhay Yadav is thankful to Dr. A. Subhananda

Rao, Distinguished Scientist and Director HEMRL, Pune for

granting him the permission to carry out this project work

at National Physical Laboratory (NPL), Delhi. The author Mr.

Rajeev Kumar is thankful to Council of Scientific and Indus-

trial Research (CSIR) for awarding on him the Senior Research

Fellowship (SRF) for this project. The authors are thankful to

Prof. R.C. Budhani, Director NPL for his kind permission to

publish the results. Thanks are due to Dr. Vijayan, Mr. Gopal

Arora and Mr. Vinod Kumar for their valuable help in X-ray,

FT-IR and SEM studies, respectively.

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