ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 93 (2009) 136–142
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells
journal homepage: www.elsevier.com/locate/solmat
High latent heat storage and high thermal conductive phase change materials
using exfoliated graphite nanoplatelets
Sumin Kim a,, Lawrence T. Drzal b
a
b
Department of Architecture, College of Engineering, Soongsil University, Seoul 156-743, Republic of Korea
Composite Materials and Structures Center, College of Engineering, Michigan State University, East Lansing, MI 48824-1226, USA
a r t i c l e in f o
a b s t r a c t
Article history:
Received 24 April 2008
Received in revised form
8 September 2008
Accepted 16 September 2008
Available online 1 November 2008
Using exfoliated graphite nanoplatelets (xGnP), parafﬁn/xGnP composite phase change materials
(PCMs) were prepared by the stirring of xGnP in liquid parafﬁn for high electric conductivity, thermal
conductivity and latent heat storage. xGnP of 1, 2, 3, 5 and 7 wt% was added to pure parafﬁn at 75 1C.
Scanning electron microscopy (SEM) morphology showed uniform dispersion of xGnP in the parafﬁn
wax. Good dispersion of xGnP in parafﬁn/xGnP composite PCMs led to high electric conductivity. The
percolation threshold of parafﬁn/xGnP composite PCMs was between 1 and 2 wt% in resistivity
measurement. The thermal conductivity of parafﬁn/xGnP composite PCMs was increased as xGnP
loading contents. Also, reproducibility of parafﬁn/xGnP composite PCMs as continuous PCMs was
manifested in results of electric and thermal conductivity. Parafﬁn/xGnP composite PCMs showed two
peaks in the heating curve by differential scanning calorimeter (DSC) measurement. The ﬁrst phase
change peak at around 35 1C is lower and corresponds to the solid–solid phase transition of the parafﬁn,
and the second peak is high at around 55 1C, corresponding to the solid–liquid phase change. The latent
heat of parafﬁn/xGnP composite PCMs did not decrease as loading xGnP contents to parafﬁn. xGnP can
be considered as an effective heat-diffusion promoter to improve thermal conductivity of PCMs without
reducing its latent heat storage capacity in parafﬁn wax.
& 2008 Elsevier B.V. All rights reserved.
Keywords:
Exfoliated graphite nanoplatelets (xGnP)
Phase change material (PCM)
Parafﬁn wax
Latent heat storage
Thermal conductivity
1. Introduction
Solid–liquid phase change materials (PCMs) are often used for
heat-storage applications. Examples include water, salt hydrates,
parafﬁns, certain hydrocarbons and metal alloys. Salt hydrate
PCMs used for thermal storage in space heating and cooling
applications have low material costs, but high packaging costs. A
more economic installed storage may be possible with medium
priced, high latent heat organic materials suitable for low-cost
packaging, i.e. those that are insoluble in water and un-reactive
with air and some of the common packaging ﬁlms [1,2].
In recent times, several candidate inorganic and organic PCMs
and their mixtures have been studied as PCMs for latent heat
thermal energy storage (LHTES) applications [3–6]. PCMs that are
used as storage media in latent thermal energy storage can be
classiﬁed into two major categories: inorganic and organic
compounds. Inorganic PCMs include salt hydrates, salts, metals
and alloys, whereas organic PCMs are comprised of parafﬁn, fatty
acids/esters and polyalcohols. Parafﬁn is taken as the most
Corresponding author. Tel.: +82 2 820 0665; fax: +82 2 816 3354.
E-mail address: [email protected] (S. Kim).
0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.solmat.2008.09.010
promising PCM because it has a large latent heat and low cost,
and is stable, non-toxic and not corrosive [7,8]. Among the
investigated PCMs, parafﬁns have been widely used for LHTES
applications due to their large latent heat and proper thermal
characteristics such as little or no super cooling, varied phase
change temperature, low vapor pressure in the melt, good thermal
and chemical stability, and self-nucleating behavior [5,9–11].
Portable electronic devices such as notebook computers and
wearable electronic devices possess unique characteristics that
nearly eliminate the use of traditional methods of thermal
management [12]. Cooling by heat transfer to PCMs is one of
the promising directions. This cooling technology has been widely
regarded overseas in recent years, and it also has had certain
applications in some high-tech systems, such as aviation, microelectronics and military electronic systems [13]. Therefore,
electric conductivity of PCMs is one of the important factors for
electric device application.
In spite of these desirable properties of parafﬁns, the low thermal
conductivity (0.21–0.24 W/m K) is its major drawback decreasing the
rates of heat stored and released during melting and crystallization
processes which in turn limits their utility areas [14]. These drawbacks
reduce the rate of heat storage and extraction during the melting and
solidiﬁcation cycles and restrict their wide applications, respectively
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[8]. To overcome the low thermal conductivity problem of parafﬁn as
PCMs, studies have been carried out with the purpose of developing
LHTES systems with unﬁnned and ﬁnned conﬁgurations, dispersing
high conductivity particles and inserting a metal matrix into parafﬁn
wax [14–16].
Expanded graphite (EG) is generally produced by using
H2SO4–graphite intercalation compounds (GICs). H2SO4–GICs are
widely used for the exfoliation process, because they can give a
high expansion volume during the thermal treatment. The
electrochemical intercalation of H2SO4 as well as the chemical
one were described in the Tryba’s works [17,18]. The EG maintains
the layered structures similar to natural graphite ﬂake but
produces tremendously different sizes of pores and nanosheets
with very high aspect ratio [19,20].
Research in the Drzal group has shown that exfoliated graphite
nanoplatelets (xGnPTM), which combine the layered structure and
low price of nanoclays with the superior mechanical, electrical
and thermal properties of carbon nanotubes, are very cost
effective and can simultaneously provide a multitude of physical
and chemical property enhancements [21–23]. Nanocomposites
prepared with xGnP in thermosetting and thermoplastic polymer
systems showed excellent mechanical properties and electrical
conductivity [24–26].
To increase thermal conductivity, EG has been used to insert
into the parafﬁn wax [3,8,12,27,28]. However, Zhang and Fang
studied the effect of the EG addition on the thermal properties of
the parafﬁn (m.p.: 48–50 1C)/EG composite prepared as formstable PCM, and they reported that the latent heat capacity of the
PCM decreased with increase of the mass fraction of the graphite
137
[8,14]. This study aimed to prepare the composites of parafﬁn (ndocosane, m.p.: 42–44 1C)/xGnPTM with low mass fraction of xGnP
to obtain a form-stable composite PCM and to investigate the
effect of xGnP addition on thermal conductivity and melting time,
melting temperature, and latent heat capacity of the parafﬁn,
especially to keep latent heat of the parafﬁn as xGnP contents is
the main purpose of this research.
2. Experimental
2.1. Materials
xGnPTM are prepared from sulfuric acid-intercalated expandable graphite (3772) obtained from Asbury Graphite Mills, Inc., NJ,
USA by applying a cost- and time-effective exfoliation process
initially proposed by Drzal’s group [21,26]. The sulfuric acid-based
GIC was fabricated from natural graphite through chemical
oxidation in the presence of concentrated sulfuric acid. It is
composed of layered, but compactly fastened nanoplates of
graphite shown in Fig. 1a. Fig. 1b shows a worm- or accordionlike expanded structure of GIC which was exfoliated up to
300–500 times in their initial volume by rapid heating in a
microwave environment. Multi-pores structure is observed from
high magniﬁcation ( 350) of EG shown in Fig. 1c. Pulverization
using an ultrasonic processor is employed to break down the
worm-like structure and to reduce its size, resulting in individual
graphite nanoplatelets that are o10 nm thick and have an average
diameter of 15 mm as shown in Fig. 1d. This xGnP is denoted as
Fig. 1. Scanning electron microphotographs of (a) acid-intercalated graphite, (b) expended graphite by microwave EG ( 50), (c) expended graphite by microwave EG
( 350), and (d) exfoliated graphite; xGnP15.
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xGnP-15 and was used in this study as a reinforcement in parafﬁn
matrix for PCM. The Brunauer–Emmet–Teller (BET) surface area of
the xGnP was measured using an auto N2 absorption instrument
(ASAP 2010, Micrometrics, USA). The measurement results
showed that the BET surface area of the EG was around 30 m2/g.
Parafﬁn wax (n-docosane) with melting temperature of 53–57 1C
was purchased from Sigma-Aldrich company.
frequency range of 0.1–1,00,000 Hz and converted to conductivity
by taking into account the sample dimensions. Eq. (1) can be used
to calculate the resistivity of the sample.
R ¼ I S=T
(1)
where I is the impedance value at 1 Hz, R is the resistivity, S is the
intercept surface area, T is the thickness of the sample.
2.2. Preparation of parafﬁn/xGnP composite PCM
To establish the relationship between thermal conductivity of
the composite PCM and the mass fraction of xGnP and determine
the minimum mass fraction of xGnP that is adequate to obtain
parafﬁn/xGnP composite as form-stable PCM, the composite PCMs
were prepared by stirring of xGnP in liquid parafﬁn with mass
fraction of 1%, 2%, 3%, 5% and 7%. The parafﬁn was melted by
heating it at 75 1C, and then, the xGnP was mixed into the liquid
parafﬁn. After being ﬁltered and dried, the parafﬁn/EG composite
PCM was obtained. To check the availability of PCMs as
continuous PCMs, the samples were remelted for measuring
electrical and thermal conductivity.
2.3. Characterization techniques
2.3.1. Scanning electron microscopy (SEM)
The morphology of intercalated graphite, exfoliated graphite,
xGnP and parafﬁn/xGnP composite PCMs were observed by SEM
at room temperature. A JEOL (model JSM-6400) SEM with
accelerating voltage of 12 kV and working distance of 15 mm
was used to collect SEM images. To compare images by gold
coating, non-coating and gold coating samples were prepared. A
gold coating of a few nanometers in thickness was coated on
samples.
2.3.2. Electrical property measurement
The resistivity of parafﬁn/xGnP composite PCMs was measured, with a Gamry instrument under FAS2TM Femtostat plug
system and potentiostatic mode, along the ﬂow direction, in case
of the injection-molded samples, using impedance spectroscopy
by applying the two-probe method at room temperature. Samples
with dimensions of 5 3 12 mm3 were cut from the middle
portion of ﬂexural bars, and the resistivity was measured along
the thickness direction (5 mm). The two surfaces that were
connected to the electrodes were ﬁrst treated with O2 plasma
(14 min, 550 W) in order to remove the top surface layers which
are rich in polymer, to ensure good contact of the sample surface
with the electrodes. The resistance of sample was measured in the
2.3.3. Thermal conductivity measurement
The thermal conductivity of parafﬁn wax and parafﬁn/xGnP
composite PCMs were measured using a UNITHERMTM machineUNITHERMTM Model 2022 (Anter Corporation, Pittsburgh, PA).
The tests were performed according to ASTM E1530 (Standard test
method for evaluating the resistance to thermal transmission of
materials by the guarded heat ﬂow meter method technique).
Specimens of 1 in diameter were prepared with stainless mold as
shown in Fig. 2. Hot liquid sample was put into the mold and
cooled down by liquid nitrogen. In order to ensure that the sample
thickness was within the recommended range, 3–5 discs were
stacked-up for the composites with higher xGnP loading. The
samples were tested at 20 1C under an applied load of 30 psi.
Reported results represent the average of three measurements for
each xGnP loading.
2.3.4. Differential scanning calorimeter (DSC)
The melting and heat storage behaviors of the parafﬁn/xGnP
composite PCMs were studied using a TA Instruments 2920 DSC
equipped with a cooling attachment, under a nitrogen atmosphere. The data were collected with a scan rate of 10 1C min–1
over a temperature range of 50–110 1C. The measurement was
made using a 5–10 mg sample on a DSC sample cell after the
sample was quickly cooled to 50 1C from the melt of the ﬁrst
scan.
2.3.5. Thermogravimetric analysis (TGA)
TGA was conducted with a TA Instruments TGA 2950 that was
ﬁtted to a nitrogen purge gas from 30 to 600 1C. This unit has the
ability to decrease the ramp rate when an increased weight loss is
detected in order to obtain better temperature resolution of a
decomposition event. The general ramp rate was 4 1C/min with a
weight loss detection sensitivity set to 4.0 in the furnace control
software. Approximately 5–15 mg of cut samples was used to
determine the decomposition temperatures.
Fig. 2. Mold for parafﬁn/xGnP composite PCM and thermal conductivity test sample: (a) mold for parafﬁn/xGNP composite PCM and (b) mold and sample for thermal
conductivity.
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139
3. Results and discussion
3.2. Resistivity of parafﬁn/xGnP composite PCMs
3.1. Morphology of parafﬁn/xGnP composite PCMs
This high electrical conductivity was detected by the resistivity of
parafﬁn/xGnP composite PCMs as xGnP loading content as shown in
Fig. 5. The incorporation of xGnP can greatly decrease the resistivity of
composites with a sharp transition from an electrical insulator to an
electrical conductor. For the purpose of ﬁnding a percolation threshold
for the resistivity 1, 2, 3, 5 and 7 wt% xGnP loaded samples were
measured. The percolation threshold of xGnP-LLDPE nanocomposite
by solution mixing and injection molded was between 1 and 2 wt%.
This percolation threshold is extremely low compared to the Author’s
result [26] of xGnP dispersed into low linear density poly propylene. It
was between 12 and 15 wt%. Resistivity of 1 wt% of xGnP loaded was
high around 109 O cm, even second melted sample. However,
resistivity was down to 104 O cm from 2 wt% of xGnP. The percolation
threshold for the resistivity depends very much on the geometry of
the conducting ﬁllers. Fillers with elongated geometry such as sheets
can be used to achieve very low percolation threshold value, due to
the fact that sheets with higher aspect ratios have great advantage
over spherical or elliptical ﬁllers in forming conducting networks in
polymer matrix. As we check the morphology in Figs. 3 and 4, xGnP
were connected to each other to make electric conductivity. As
continuous PCMs, resistivity of second melted sample was measured.
Resistivity of second melted samples showed similar behavior with
ﬁrst melted samples. Reproducibility of parafﬁn/xGnP composite
PCMs as continuous PCMs was manifested (Fig. 5).
The cryogenically fractured surface of the parafﬁn/xGnP
composite PCMs was studied by SEM. Fig. 3 shows the
SEM photographs of the parafﬁn/xGnP composite PCMs of
2% and 5% xGnP loading contents with gold coating and
non-coating (magniﬁcation of 2000). It is observed from
Fig. 3a and c that the dispersions of the xGnP in the parafﬁn
wax are uniform. xGnP was well-dispersed in parafﬁn. Actually, it
looks like parafﬁn covered slightly on the xGnP surface. It is a
different morphology compared to the Author’s result in which
xGnP is dispersed in LLDPE polymer matrix [26]. We can easily
recognize the existence of xGnP by its uniform shape. As shown in
Fig. 3b and d the dispersion of xGnP in parafﬁn is indicated by
the clear white plate phase even it was not coated by gold. From
this morphology of non-coating samples, it can be expected that
these 2% and 5% xGnP-loaded PCM composites are electrically
conductive because SEM can detect only electrically conductive
materials by the electric beam. The dispersion of xGnPs covered
by parafﬁn is more signiﬁcantly indicated in high magniﬁcation
( 5000) as shown in Fig. 7. Furthermore, the uniform xGnP
particle size is indicated with these ﬁgures. Although xGnP
loading contents were low, 2 and 5 wt%, xGnP are well embedded
and dispersed enough to show their existence.
Fig. 3. Scanning electron microphotographs of 5 and 2 wt% of parafﬁn/xGnP composite PCMs by coating condition for SEM (low magniﬁcation, 2000): (a) parafﬁn/xGNP
5%—gold coating, (b) parafﬁn/xGNP 5%—non-coating, (c) parafﬁn/xGNP 2%—gold coating, and (d) parafﬁn/xGNP 2%—non-coating.
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Fig. 4. Scanning electron microphotographs of 5 and 2 wt% of parafﬁn/xGnP composite PCMs by coating condition for SEM (high magniﬁcation, 5000): (a) parafﬁn/xGNP
5%—gold coating, (b) parafﬁn/xGNP 5%—non-coating, (c) parafﬁn/xGNP 2%—gold coating, and (d) parafﬁn/xGNP 2%—non-coating.
1010
paraffin/xGnP PCMs
2nd melted
1st melted
109
Resistivity (ohm∗cm)
108
107
106
105
104
ductivity of the composite PCM including mass 7 wt% xGnP is
found to be 0.8 W/mK. These results are comparable to Zhang’s
report [29]. Ten percent of graphite mass fraction in the shapestabilized PCM showed 0.229 W/mK, while 20% of cases were
0.482 W/mK in this report. Theoretically, the thermal conductivities will increase continually with increasing additive quantity of
exfoliate graphite. The thermal conductivity of parafﬁn/xGnP
composite PCMs was increased as xGnP loading contents.
Reproducibility of parafﬁn/xGnP composite PCMs as continuous
PCMs for thermal conductivity also reappeared like electric
conductivity, even second samples were little higher than the
ﬁrst samples.
103
3.4. Thermal storage performance and thermal stability of parafﬁn/
xGnP composite PCMs
102
101
1
2
3
4
5
6
xGnP loading content (wt%)
7
Fig. 5. Resistivity of parafﬁn/xGnP composite PCMs by melting times.
3.3. Thermal conductivity of parafﬁn/xGnP composite PCMs
The thermal conductivity results of pure parafﬁn and the
parafﬁn/xGnP composite PCMs are shown in Fig. 6. It can be found
that the thermal conductivities of the composite PCMs improve
evidently compared to that of pure parafﬁn. When the thermal
conductivity of pure parafﬁn is 0.26 W/mK, the thermal con-
The heating and freezing curves by DSC measurements of the
parafﬁn and the parafﬁn/xGnP composite PCMs are presented in
Fig. 7. It can be seen from the heating curve in Fig. 7(a) that the
parafﬁn has two phase change peaks. The ﬁrst phase change peak
at about 35.3 1C is lower and corresponds to the solid–solid phase
transition of the parafﬁn, and the second peak is very high at
around 55.2 1C, corresponding to the solid–liquid phase change.
These peaks are matched with pure parafﬁn peaks in the previous
report [30]. The DSC curve of the parafﬁn/xGnP composite PCMs is
shown in Fig. 7b–d. There are also two peaks around 35 and 55 1C
in the DSC curve of the parafﬁn/xGnP composite PCMs, and the
thermal characteristics of the composite PCM are very close to
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0.9
Thermal Conductivity (W/mK)
0.8
2nd melted
Y=0.080X + 0.31
R2=0.99
0.7
0.6
0.5
1st melted
Y=0.085X + 0.26
R2=0.99
0.4
0.3
0.2
0
1
2
3
4
5
xGnP loading content (wt%)
6
7
Fig. 6. Thermal conductivity of parafﬁn/xGnP composite PCMs by melting times.
128.8J/g
26.2J/g
29.2J/g
128.1J/g
35.3°C
51.0°C
paraffin/xGnP1%
32.9°C
Heat flow (w/g)
32.9°C
Heat flow (w/g)
those of the pure parafﬁn. This is because there is no chemical
reaction between the parafﬁn and the EG in the preparation of the
composite PCM [8]. The latent heat of the parafﬁn is obtained as
the total area under the peaks of the solid–solid and solid–liquid
transitions of the parafﬁn in the composite by numerical
integration. From the Fig. 8, it can be seen that the latent heat
of the parafﬁn/xGnP composite PCMs approach those of the pure
parafﬁn. The latent heat of parafﬁn/xGnP composite PCMs did not
decrease as loading xGnP contents to parafﬁn. There is no
signiﬁcant difference of the latent heat between parafﬁn and
parafﬁn/xGnP composite PCMs. Previous results showed a
decrease of latent heat as graphite loading contents increased
[8,30]. In these results, due to graphite and EG, although thermal
conductivity of parafﬁn/graphite composite PCM was increased,
the latent heat of PCM was decreased as graphite loading
contents. They explained the reason that the three-dimensional
net structure conﬁnes the molecular heat movement of the
parafﬁn in the PCM composites. However, in the case of xGnP,
there was no problem because of good dispersion of xGnP in
parafﬁn with high surface area.
51.1°C
paraffin only
141
134.0J/g
26.8J/g
27.8J/g
131.0J/g
35.1°C
55.1°C
55.2°C
20
30
40
50
Temperature (°C)
70
10
51.4°C
paraffin/xGnP3%
33.0°C
Heat flow (w/g)
60
28.1J/g
127.6J/g
34.9°C
30
40
50
Temperature (°C)
paraffin/xGnP5%
132.4J/g
27.8J/g
20
27.4J/g
20
30
40
50
Temperature (°C)
60
70
131.5J/g
27.4J/g
35.4°C
130.0J/g
54.9°C
55.1°C
10
60
50.8°C
32.9°C
Heat flow (w/g)
10
70
10
20
30
40
50
Temperature (°C)
Fig. 7. The heating and freezing curves by DSC of parafﬁn/xGnP composite PCMs.
60
70
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surface of the parafﬁn/xGnP composite PCMs, xGnP was welldispersed into parafﬁn wax, and it led to high electric conductivity
and thermal conductivity. As increasing xGnP loading contents,
electric conductivity and thermal conductivity were increased.
The results clearly indicated an almost linear relationship
between thermal conductivity and mass fraction of xGnP in
composite PCM. The percolation threshold of Parafﬁn/xGnP
composite PCMs on resistivity was between 1 and 2 wt%. This
low percolation threshold was caused by well dispersion of high
aspect ratio of xGnP. On the other hand, latent heat was not
decreased as xGnP loading contents. xGnP of uniform high surface
area showed improved thermal storage performance. As a result,
xGnP can be considered as an effective heat diffusion promoter to
improve thermal conductivity of PCMs without reducing its latent
heat storage capacity.
140
Latent heat capacity (J/g)
130
120
Heaing, Phase transition
Cooling, Phase transition
Heaing, Phase change
Cooling, Phase change
110
40
30
20
0
1
2
3
4
xGnP loading content (wt%)
5
Acknowledgement
Fig. 8. Latent heat storage performance of parafﬁn/xGnP composite PCMs at phase
transition and phase change by DSC.
100
paraffin only
paraffin/1% xGnP PCMs
paraffin/3% xGnP PCMs
paraffin/5% xGnP PCMs
Weight change (%)
80
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Temperature (°C)
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This work (Sumin Kim) was supported by the Soongsil
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600
Fig. 9. Thermal decomposition behavior of parafﬁn/xGnP composite PCMs by TGA.
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