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Preparation of nitrogen-doped graphene/palmitic acid shape stabilized

composite phase change material with remarkable thermal properties

for thermal energy storage

Mohammad Mehrali a,⇑, Sara Tahan Latibari a, Mehdi Mehrali a, Teuku Meurah Indra Mahlia b,Emad Sadeghinezhad a, Hendrik Simon Cornelis Metselaar a,⇑

a Advanced Material Research Center, Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Department of Mechanical Engineering, Universiti Tenaga Nasional, 43009 Kajang, Selangor, Malaysia

h i g h l i g h t s

Nitrogen-doped graphene (NDG)/palmitic acid (PA) composite shape stabilized PCMs were prepared.

 The loading of PA in the composite PCMs with good form-stability could attain 97 wt%.

 The phase change enthalpy of the composite PCMs could attain 199.48 J/g.

 The thermal conductivity of the PCM containing 3 wt% NDG is 3.5 times higher than that of PA.

a r t i c l e i n f o

 Article history:

Received 22 March 2014

Received in revised form 8 August 2014

Accepted 26 August 2014

Available online 16 September 2014

Keywords:

Graphene

Phase change materials

Thermal energy storage

Thermal conductivity

Latent heat

a b s t r a c t

Palmitic acid (PA) is one of the main phase change materials (PCMs) for medium temperature thermal

energy storage systems. In order to stabilize the shape and enhance the thermal conductivity of PA,

the effects of adding nitrogen-doped graphene (NDG) as a carbon nanofiller were examined experimen-

tally. NDG was dispersed in liquid PA at various mass fractions (1–5 wt%) using high power ultrasonica-

tion. The dropping point test shows that there was clearly no liquid leakage through the phase change

process at the operating temperature range of the composite PCMs. The thermal stability and thermal

properties of composite PCM were investigated with a thermogravimetric analyzer (TGA) and differential

scanning calorimeter (DSC), respectively. The thermal conductivity of the PA/NDG composite was deter-

mined by the laser flash method. The thermal conductivity at 35 C increased by more than 500% for the

highest loading of NDG (5 wt%). The electrical conductivity of composite PCMs was increased signifi-

cantly by using NDG. The thermal cycling test proved that the PA/NDG composites PCMs had good ther-

mal reliability and chemical durability after 1000 cycles of melting and freezing. The thermal effusivity of 

the PA/NDG composite PCMs was larger than that of pure PA, which is advantageous for latent heat ther-

mal energy storage (LHTES).

 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The energy crisis and global warming have become serious con-

cerns, thus, outstanding attempts have been made for the efficient

utilization of alternative energy, like solar energy. Nevertheless,

the fluctuation of solar radiation makes latent heat thermal energy

storage (LHTES) indispensable within the solar thermal energy

applications. Phase change materials (PCMs) are recognized to

become critical for LHTES since they can store and release consid-

erable amounts of latent heat during their phase transition for effi-

cient utilization of thermal energy   [1]. PCMs are usually

categorized into organic and inorganic materials based on their

components. Organic PCMs have obtained extensive attention

because of their higher latent heat density, suitable phase-transi-

tion temperature and stable physical and chemical properties [2–

5]. The pure organic PCMs possess some shortcomings that hinder

their application in practice, including low thermal conductivity,

high volume change and liquid seepage during phase transition.

There are several strategies to eliminate these particular difficul-

ties such as encapsulation and shape stabilization of PCMs  [6,7].

http://dx.doi.org/10.1016/j.apenergy.2014.08.100

0306-2619/ 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. Tel.: +60 3 79674451; fax: +60 3 79675317.

E-mail addresses:   mohamad.mehrali@siswa.um.edu.my,   mehrali.gary@gmail.

com (M. Mehrali),  h.metselaar@um.edu.my (H.S.C. Metselaar).

Applied Energy 135 (2014) 339–349

Contents lists available at   ScienceDirect

Applied Energy

j o u r n a l h o m e p a g e :   w w w . e l s e v i e r . c o m / l o c a t e / a p e n e r g y

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Nowadays, form-stable PCM composites with good thermal con-

ductive supporting materials which could maintain the solid shape

even if the temperature is more than the melting point of PCMs

have become the main focus of scientific studies   [8–12]. This

method is among the most effective ways to resolve the leakage

and low thermal conductivity problems of PCMs. There have been

a lots of studies on the preparation and characterization of the

shape stabilized PCMs using porous materials, including expanded

perlite   [13], graphite   [14], expanded graphite   [15–17], carbon

nanofibers  [18,19], carbon nanotubes [20], graphite nanoplatelets

[19], graphene   [10,21,22], graphene oxide   [23,12,24], graphene

nanoplatelets [8,25]. The accessible research outcomes reveal that

the shape stabilized composite PCMs based on porous materials

possess a great application possibility within the thermal storage

field.

Graphene is a two-dimensional all-sp2-hybridized low density

carbon with fascinating thermal, electrical and mechanical proper-

ties and has generated a rapidly growing research interest. Graph-

ene sheets have high thermal conductivity, high specific surface

area, and good electrical properties compared to other carbon

based materials [26]. The potential of using graphene-based prod-

ucts for PCMs has drawn much interest recently because it can be

simply obtained by basic chemical processing of graphite [27]. The

thermal conductivity of composite PCMs depends not merely on

the innate thermal conductivity of the additives, but it is also

strongly related to the compatibility between additives and pure

PCMs   [28]. In our earlier works, graphene nanoplatelets (GNPs)

and graphene oxide (GO) were used as supporting materials for

shape stabilization to enhance the thermal conductivity of the

composite PCMs. Since graphene properties are directly linked to

the nano structure of the graphene sheets, quite a few attempts

have been carried out to improve the electrical and thermal prop-

erties of graphene flakes by modifying their structures, as well as

using novel synthesis methods, chemical functionalization and

chemical doping with external atoms [29]. On the whole, chemical

doping with sulfur(S), boron (B) or nitrogen (N) is regarded as a

highly effective technique to modify the inherent properties of graphene-based materials. Particularly N-doping performs an

important role in regulating the electronic, thermal and chemical

properties of carbon materials because of the similar atomic size

and the availability of five valence electrons to form strong valence

bonds with carbon atoms [30]. There is a fundamental interest to

examine how nitrogen doping influences the structure of graphene

and to see whether the chemical properties can be modified

accordingly [31]. A few works have been carried out to investigate

the structural difference between graphene and N-doped graphene

(NDG) but there have been no reports related to the effect of N-

doping on the thermal conductivity of graphene. The NDG exhibits

an excellent adsorption capacity for various classes of organic liq-

uids including organic solvents. In particular, NDG can adsorb

amounts of the liquids up to 200–600 times its own weight, whichis much higher than other typical carbonaceous sorbents such as

graphene foam.

In this work, we report the preparation of PA/NDG composite

PCM as a novel form-stable composite PCM by using NDG as

supporting material. The great compatibility between NDG and

PA leads to a large improvement in the thermal conductivity at

low filler loading ratio along with a small reduction in latent

heat. Furthermore, capillary forces and surface tensions were

improved due to the larger specific surface area (793 m2/g) of 

NDG that is effective for shape-stabilization over the phase

change process of PCMs mainly because it helps to hold the mol-

ten PCM. This high thermal conductivity as well as the minimal

loss in the phase transition enthalpy makes the PA/NDG compos-

ite a promising candidate for thermal energy storageapplications.

2. Experimental description

 2.1. Preparation of NDG

Graphene oxide was synthesized from natural graphite powder

(99.99%, <45lm, Sigma Aldrich) employing a simplified Hummers’

method   [32]. NDG was synthesized by a hydrothermal process

with GO as raw material. In a typical method for preparing NDG,the pH of a dispersion of 50 mg GO in 100 ml H2O after 1 h ultra-

sonication was adjusted to 11 by using ammonia. The dispersion

was treated hydrothermally in a Teflon-lined autoclave at a tem-

perature of 160 C for 12 h. A black precipitate was washed with

deionized water and dried at 50 C.

 2.2. Preparation of PA/NDG composite PCMs

The preparation procedure was started by dissolving 2 g of pal-

mitic acid (PA) in 50 ml toluene at 80 C, after which NDG with dif-

ferent mass percentages (1–5 wt%) was added to the solution. The

solution was sonicated for 30 min at power of 150 W to break

down the NDG aggregates and obtain a homogeneous dispersion

of the nano particles. The mixture was then left inside a fume hoodat 130 C to evaporate the toluene and then poured into a mold.

Finally, the PCM samples have been dried inside a vacuum oven

overnight at 120 C to eliminate PA which had not been absorbed

by NDGs.

The PA/NDG samples that were prepared by this process are

seems to be naturally isotropic by taking into consideration that

the NDG nano sheets had no preferential direction and haphaz-

ardly oriented. The prepared composite PCMs with 1–5 wt% NDG

were named S1–S5, respectively.

 2.3. Analysis methods

Field emission scanning electron microscopy (FESEM- CARL 

ZEISS- AURIGA 60) was used to visually characterize the morphol-

ogy of the NDG and composite PCMs. The Brunauer–Emmett–

Teller method (BET-Autosorb-iQ2) was used to measure specific

surface area and pore distribution of the NDG sample. Transmis-

sion electron microscopy (TEM) measurements were conducted

on a CARL ZEISS-LIBRA120 microscope. An X-Ray photoemission

spectrometer (XPS-PHI 5400 ESCA) with an Al Ka (hm = 1486.69 eV)

X-ray source was used to identify the elements’ states in the NDG.

X-ray diffraction (XRD) patterns were measured on an Empyrean

PANALYTICAL diffractometer. The melting and freezing tempera-

tures, latent heats and specific heat capacity of composite PCMs

were obtained by differential scanning calorimeter (METTLER 

TOLEDO 820C-Error ± 0.25–1 C) at a heating rate of 5 C/min.

The weight loss and thermal stability of PCMs are obtained by ther-

mogravimetric analysis (METTLER TOLEDO SDTA 851-Error ± 5 lg)

at a heating rate of 10 C/min and a temperature of 50–500 C in

purified nitrogen atmosphere. The temperature distribution photos

were taken by an infrared camera (FLIR i5) with a 0.1 C thermal

sensitivity. The shape stability of composite PCMs was analyzed

by using Mettler Toledo FP83HT Dropping Point Cell at a heating

rate of 2 C/min.

 2.4. Thermal cycling test 

The thermal reliability of PA/NDG composite PCMs was investi-

gated after 1000 thermal cycles by using an accelerated thermal

cycling system (Fig. 1). The heat storage/retrieval performance of 

the samples was investigated by recording temperatures duringthe whole cycling process.

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 2.5. Thermal conductivity and electrical resistivity measurement 

The laser flash technique (Netzsch LFA 447 NanoFlash) was

used to measure the thermal diffusivity of prepared samples at

35 C. The thermal conductivity may also be derived from the ther-

mal diffusivity when the specific heat and the sample bulk density

are known. The electrical resistance of PA/NDG composites was

measured with a LCR HiTESTER 3532 meter. The resistance of thesample was measured in the frequency range from 0.6 Hz to

100 kHz. Electrical resistivity was calculated from the resistance

values at room temperature.

3. Results and discussion

 3.1. Characterization of NDG

Fig. 2(a) and (b) shows the FESEM images of the sample, dis-

playing a uniform structure like crumpled silk veil waves with a

porous architecture while NDG nanosheets are randomly stacked

together. The FESEM images of the NDG sample reveal that the

two dimensional graphene structures with high specific surface

area and volume ratio are well retained. Three major peaks located

at 284.4, 399.5 and 532 eV was determined from the XPS spectrum

of the NDG sample (Fig. 2(c)) that was linked to C1s, N1s and O1s,

respectively, showing that nitrogen was incorporated within the

graphene structure  [33]. The nitrogen was doped by atomic per-

centage of 2.6% within the graphene network by pyridinic and pyr-

rolic nitrogen species, respectively. They refer to the nitrogen

atoms which might be positioned in a  p  conjugated system andcontribute to the  p  system with one or two  p-electrons, respec-

tively [34]. The specific surface area of the NDG sample was mea-

sured and the graph is shown in Fig. 2(d). The unique mesoporous

structure of NDG contributes to the high specific surface area

(793 m2/g) which is higher than our prepared GO (684 m2/g) with

a uniform pore size distribution around 3–5 nm. Over the synthesis

of NDG sheets, beside the carbon atoms that was replaced by nitro-

gen atoms (most likely located on the reactive edge) ammonia can

also react with graphene to form hydrogen cyanide and hydrogen

(C + NH3 = HCN + H2)   [35]. This reaction takes some carbon that

will make the NDG sheets more porous, which in turn resulted in

the higher specific surface area, pore volume, and pore size. The

large surface area can enhance both the heat transfer rate and

the adsorption quantity of the PA.

Fig. 1.  Accelerated thermal cycler [12].

Fig. 2.   (a and b) FESEM images of NDG (5 K, 60 K); (c) XPS spectra of NDG and graphene; (d) nitrogen adsorption/desorption isotherms of NDG. Inset in (d) is the BJH poresize distribution.

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 3.2. Morphology and microstructure of the form-stable PCMs

The FESEM images of the form-stable PCMs with varying NDG

content are shown in   Fig. 3. The NDG particles are generally

embedded in the PA as indicated by the white parts protruding

from the background or lying on the surface and suggested by their

irregular but sharp edges in contrast with the smooth and soft PA.

At the lower nanofiller loading, the NDG particles in the nanocom-

posites are easier to recognize because of their particular structure

as shown in Fig. 3(a) and (b). The NDG particles in PA composites

preserved their structure while the PA was adsorbed completely

by the surface and pores of NDG.   Fig. 3(c–e) shows nearly only

white areas, suggesting that the PA was adsorbed well into the

NDG network. It can be clearly observed in Fig. 3(f) that NDG layers

possess the substantial capability to absorb the melted PA once the

temperature is higher than the melting point of PA. Besides, the

NDG particles develop a network which could avoid the composite

PCM from falling apart even when the PA is melted. Therefore, the

composite PCM still demonstrates the exact property of form-sta-

bility. In fact, in our experiments, the composite PCM has not been

collapsed even when the PA loading was 97%.

Fig. 4 displays the TEM images of NDG along with the PA/NDG

composite PCM. The TEM image shows that NDG sheets are ran-

domly compact and stacked together, demonstrating uniform lam-

inar morphology like crumpled silk veil waves. The partial lining

level of the PA/NDG composite (Fig. 4(b)) is deeper compared to

the NDG. That is attributable to the PA adsorbed by the network

structure of NDG. The outcomes from TEM images are similar to

the results from the SEM images.

 3.3. Shape-stabilization tests of PA/NDG composite PCMs

Almost certainly the majority of the PCMs which have been

employed in applications have numerous advantages such as highlatent heat and minimal costs however they might need storage

containers. The thermal efficiency of the systems will be affected

by utilizing storage containers while the leakage issues also will

appear in energy storage systems. Shape stabilization of PCMs will

resolve these issues and may even change the thermal conductivity

of the PCMs based on the additives used.   Fig. 5  shows different

structures of PA/NDG composite PCMs after drying and it can be

clearly seen that the PA was totally absorbed by NDG particles

for higher mass percentage of NDG. The continuous solid shape

of the PA was observed in mass percentage of 1% and 2%, while

in other samples this shape was totally changed due to the absorp-

tion of the PA by NDG particles.

The results of the dropping tests are presented in  Table 1.

The dropping point of pure PA was 65.03 C and was increasedsignificantly with the addition of NDG particles. The capillary and

surface tension forces between PA and NDG may result in a great

enhancement in the shape stabilization of composite PCMs. It

can be mentioned that even at a NDG content of 2 wt%, the shape

Fig. 3.  FESEM images of (a) S1, (b) S2, (c) S3, (d) S4, (e) S5, and (f) S5 (25 K).

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of the composite can be preserved up to 142.2 C, considerably

more than the normal operating temperature range of energy stor-

age systems. The dropping test was corroborated by putting the

prepared PA/NDG pellets into the oven at 150 C for 1 h.   Fig. 6

shows the effect of temperature on the shape of composite PCMs.

It can be seen that there was some leakage from the surface of 

the composites that was reduced by increasing the NDG mass per-

centage however for S1 and S2 major leakage was seen while the

shape of pellets was remained for all samples.

 3.4. XRD characterization of PA/NDG composites PCM 

Fig. 7 displays the XRD patterns of pure PA, NDG and PA/NDGcomposite PCM samples. A wide peak centered at around 24.2  is

observed for the NDG sample, confirming the recovery of a gra-

phitic crystal structure [28]. The XRD pattern of the PA/NDG com-

posite PCM mostly indicates the features of the PA due to the

strong diffraction intensity of crystalline PA. The diffraction peaks

of NDG at 2h = 24.2  was covered by PA as a result of the higher

loading of the PA. The XRD patterns from the composite PCMs gen-

erally show that the crystal formation of the PA within the com-

posite PCMs is not different from pure PA.

Fig. 4.  TEM images of (a) NDG, and (b) PA/NDG composite PCM.

Fig. 5.  Dried PA/NDG composite PCMs.

 Table 1

Dropping points of PA and PA/NDG composite PCMs.

Sample Dropping point (C)

PA 65.03

S1 77.40

S2 142.20

S3 195.60

S4 >375.00a

S5 >375.00

a

a The upper limit of themeasuring range is at 375 C.

Fig. 6.  The images of PA/NDG composite PCMs (a) 30 C, and (b) 150 C.

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 3.5. Thermal energy storage properties

Thermal energy storage properties of pure PA and prepared PA/

NDG composite PCMs were studied with DSC and results are sum-

marized in Table 2. For each sample, four different parts of the pel-

let were taken to achieve accurate results and the relative standard

deviation (RSD) also was calculated. All DSC curves in Fig. 8 exhibit

one big endothermic peak with  T peak   located in the temperature

range of 64–67 C, which is ascribed to the melting of PA. In other

words, it is a PA that performs the role of storing thermal energy.

The DSC curves shown in Fig. 8 and the data listed in Table 2 indi-

cate that the thermal energy storage properties of the prepared

form-stable PCMs are not affected by using NDG as a supporting

material.

The  T m   of composite PCMs increased slightly with increasing

NDG content. The NDG can play a role in promoting the heteroge-

neously nucleated phase transition from melting to a triclinic

phase, and consequently enhances its crystallization temperature.In addition, the relative error between theoretical and experimen-

tal latent heat values of composites decreased with increasing the

mass percentage of NDG in the prepared composites. This was indi-

cated that the molten PA was adsorbed properly by the NDG nano

sheets network. In addition, the NDG nano sheets with high and

uniform porous structure was capable to minimize seepage of 

the molten PA that will improve the latent heat of the PA/NDG

composite PCMs. Here the S3 sample, which contained 3 wt% of 

NDG with the relative error of less than 1%, gave the highest latent

heat value without any leakage, which was 96.9% of that of pure

PA.

Latent heat is particularly attractive due to its ability to provide

high-energy storage density in quasi-isothermal process. The

energy storage density could be increased using PCM having a highphase change (latent heat) within the temperature range of the

storage while the total heat stored in a PCM can be calculated as

follows:

Q t  ¼Z   T mT i

mC  p  dT  þ mamDH m þZ   T  f T m

mC  p  dT    ð1Þ

where Q  (J) – quantity of heat stored, T i (C) – initial temperature, T  f 

(C) – final temperature, m  (kg) – mass of heat storage medium, C  p(J/kg K) – specific heat, am – fraction of melted PCM andDH m is heat

of melting per unit mass (J/kg). The Specific heat of PCMs and its

change with temperature are not given in most of the articles.

Researchers ignore this thermal property because of the lower ther-

mal storage density of sensible heat storage. However, it has an

additional side effect on total thermal energy storage capacity of 

the materials and it is better to investigate the change of specific

heat values with temperature to obtain more thermal data for engi-

neering approaches. The specific heat of the pure PA and PA/NDG

composite PCM was measured with the multiple curves method

in temperatures between 35 and 50 C and results are shown inFig. 9.

The results show that the specific heats of composite PCMs

were increased by adding NDG, with the increment more pro-

nounced for higher NDG loadings. The equations are generated

according to the measured data between 35 and 50 C and they

represent the temperature interval where no phase change occurs.

The quadratic function was chosen according to the literature [36].

The A,  B  and C  coefficients of the solid phase are given in Table 3.

 3.6. Thermal stability of the PA/NDG composite PCMs

Thermal stability is considered as an important parameter in

assessing the performance of newly designed composite PCMs

when used for thermal energy storage or thermal regulation.Fig. 10 shows the TG curves of PA and the prepared form-stable

Fig. 7.  XRD patterns of NDG, pure PA and PA/NDG composite PCM.

 Table 2

Thermal energy storage properties of the PA and PA/NDG composite PCMs.

Samples Melting temperature T m(C)

Temperature of the melting peak

T  p  (C)

Phase change latent heat (DH m) Relative standard

deviation (%)Experimental value

(kJ/kg)

Calculated* value

(kJ/kg)

Relative error

(%)

Pure PA 62.40 64.12 206.32 – – 0.63

S1 63.27 65.13 199.65 204.25 2.25 0.58

S2 63.36 65.89 198.84 202.19 1.65 0.67

S3 63.78 65.99 199.48 200.13 0.32 0.73

S4 63.85 66.44 197.53 198.06 0.26 0.76

S5 63.89 66.52 195.54 196.00 0.23 0.85

* Calculated by: DH composite = (1-NDG (wt%)) *DH PA.

Fig. 8.  DSC curves of the pure PA and PA/NDG composite PCMs.

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PCMs. The onset and maximum weight loss temperatures are given

in Table 4. PA exhibits a one-step weight loss starting at 201.48 C

and the maximum weight loss temperatures at 275.16 C, which is

caused by the evaporation of PA. The PA/NDG samples also show a

one-stage weight loss behavior while the temperatures at maxi-mum weight loss rate are significantly increased up to 349.21 C.

It is reasonable to believe that the PA must first break through

the NDG layers during the heating process and then can evaporate

out of the composite. The NDG layers are compact and rigid

enough to protect the absorbed PA from diffusing at the normal

boiling point, thus improving the degradation temperature of com-

posite PCMs. The derivative TG thermograms in Fig. 10(b) also con-

firm that the maximum weight loss occurs at a higher temperature

for the composite PCMs with a higher content of NDGs. This

indicates that the thicker NDG layers are more effective in enhanc-

ing the thermal stability of composite PCMs. It is also observed

from Fig. 10(a) that the composite samples exhibit different char

yields depending on the remained NDG. There is no doubt that

the composite PCMs achieved higher stability by adding NDGs as

a supporting material.

 3.7. Thermal conductivity of PA/NDG composite PCMs

The PA has a high latent heat, but a low thermal conductivity

that may delay the thermal response to the storage and release

of latent heat. Therefore, thermal conductivity enhancement is

absolutely required when designing composite PCMs. It was

expected that adding inorganic materials with a higher thermal

conductivity can enhance the thermal conductivity of the resulting

composite PCMs. The thermal diffusivity was measured by the

laser flash method and the thermal conductivity can be calculated

as follows:

k ¼  a q C  p   ð2Þ

where k is the thermal conductivity (W/(m k)),a is thermal diffusiv-

ity (m2/s), q  is density (kg/m3) and  C  p  is specific heat capacity (J/

(kg K)). The thermal conductivity of PA and PA/NDG composite

PCMs are measured at 35 C and results are shown in Fig. 11.

From the data presented in Fig. 11, it can be seen that the ther-

mal conductivities of the PA/NDG composites (S1–S5) clearly

improved compared to the pure PA. The maximum enhancement

was 517% by adding only 5 wt% of NDG and it shows that NDG

has a significant effect on the thermal conductivity of the PA. How-

ever, the figure also shows that the thermal conductivity does not

increase linearly with increasing NDG loading. It was suggested

that the arrangement of the layer structures in NDGs influences

the interaction between PA and NDGs. By comparing S2 with S3,

it can be seen that the enhancement was significant due to the vir-

tual heat transfer network made by NDG layers. At the high NDG

loading, as the content of NDG is further increased, the increasingof thermal conductivity slows down and exhibits a tendency to

approach a limit. As a result, the PA/NDG composite PCMs achieved

a much higher thermal conductivity than pure PA.

Temperature versus time graphs for the PA and PA/NDG com-

posite PCMs during heating and cooling are shown in Fig. 12. These

results was taken from thermal cycler setup and indicate that the

total charging and discharging time for the pure PA was around

1100 s and was decreased to 790 s for S5 due to the higher thermal

conductivity of the S5. By comparing the melting and freezing time

Fig. 9.  Specific heat curves of PA and composite PCMs.

 Table 3

Coefficients of the second order polynomials in solid State,  C  p (kJ/kg  C) =  AT 2 (C) +  BT 

(C) +  C .

Sample   A B C R 2

Pure PA 0.0024   0.1862 5.4358 0.94

S1 0.0012   0.0757 3.1324 0.99

S2 0.0015   0.1012 3.6725 0.99

S3 0.0016   0.103 3.6636 0.99

S4 0.0012   0.0663 2.9727 0.99

S5 0.0018   0.1078 3.8882 0.99

Fig. 10.  TG (a) and DTG (b) graphs of the PA and composite PCMs.

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of PA with that of composite, it is obvious that the total charging

and discharging time was decreased by NDG addition due to the

thermal conductivity enhancement.

During cooling, temperature distribution images after 10 and

150 s are shown in Fig. 13(a–f) for PA/NDG composites. The tem-

perature distribution around the PA/NDG composites after 10 s is

almost the same for all samples while after 150 s, the lowest tem-

perature was found in S5 and the highest was in S1. From the

results it can be seen that the differences between temperatures

for S2, S3 and S4 are higher than can be explained by the thermal

conductivity graph in Fig. 11. The difference between thermal con-

ductivity of these samples are higher than others so we couldexpect the results from the temperature distributions. These

results also show that the thermal conductivity of composite sam-

ples was improved by using a higher mass fraction of the NDG.

Moreover, the comparison of thermal properties of the prepared

form-stable PCM with those of different composite PCMs in the lit-

erature is shown in Table 5. It can be seen that the prepared PA/

NDG form-stable composite PCM enhanced the thermal properties.

These results indicate that the PA/NDG composites have a potential

use in the thermal management materials of an electronic devices

or a solar energy storage systems.

 3.8. Thermal effusivity of PA/NDG composite PCMs

The thermal effusivity is a measure of a material’s ability to

exchange thermal energy with its surroundings. The thermal effu-

sivity is considered to be a critical physical quantity to depict the

unsteady state heat transfer in a LHTES system. The effusivity of 

a material is the square root of the product of the thermal conduc-

tivity, density and heat capacity that can be calculated as follows:

e ¼ ffiffiffiffiffiffiffiffiffiffiffiffiqKC  p

q   ð3Þ

where e  (kJ/(K m2 s(0.5)) is thermal effusivity,  K  is thermal conduc-

tivity (W/(m K)), q is density (kg/m3) and C  p is specific heat capacity(kJ/(kg K)). The thermal effusivity of the PA and PA/NDG composite

PCMs were calculated at 35 C and listed in Table 6.

The thermal effusivity of all composite PCMs was larger than

that of pure PA, which is beneficial for the LTES systems. The ther-

mal effusivity of the PA was increased about 173% by adding only

5 wt% of NDG.

 3.9. Electrical resistivity

The electrical resistances of the composite PCMs were mea-

sured and resistivity of samples was calculated by taking the

geometry of the sample into account using the following equation:

qðX  mÞ ¼  Rð A=LÞ ð4Þ

where R  is the electrical resistance of a uniform specimen at 1 Hz, L

is the length and  A  is the cross-sectional area of the specimen. The

electrical resistivity of different composite PCMs with a different

mass fraction of NDGs are shown in  Fig. 14.

The results demonstrate that NDG additives can enhance the

electrical conductivity of the PA considerably. A percolation

threshold between 2 and 3 wt% is observed for PA/NDG composite

PCM, the place where a significant reduction in resistivity from

18,500 to 4310Xm occurs. At 5 wt% loading, the electrical resis-

tivity of the PA/NDG composite is around 1910Xm. The extremely

high aspect ratio of NDGs considerably improves the probability of 

contacts between neighboring flakes and thus raises the possibility

of developing percolating networks at a lower loading content. The

main increment in thermal conductivity also happened between 2and 3 wt%, which shows that a percolating network forms between

these two mass fractions.

 3.10. Thermal reliability of the PA/NDG composite PCMs

Thermal reliability of the composite PCMs is critical to evaluate

the potency of TES systems for long period-utility. Hence, a PCM

needs to maintain its TES attributes even when it was subjected

to a prolonged thermal cycling process. Thermal reliability is also

one of the essential criteria employed for the selection of the PCMs.

For this specific purpose, in this research the thermal reliability of 

the composite PCMs was examined after the exposure to 1000

melting–freezing cycles. The changes of latent heats in composite

PCMs after thermal cycling were considered to determine whetherthey are thermally reliable.

 Table 4

Decomposition temperatures of the PA and composite PCMs.

Sample Onset decomposition

temperature (C)

Maximum weight loss

temperature (C)

Pure

PA

201.48 275.16

S1 212.93 326.31

S2 227.17 332.26

S3 230.08 339.53S4 233.53 343.36

S5 237.69 349.21

Fig. 11.  Thermal conductivity of PA and PA/NDG composite PCMs at 35 C.

Fig. 12.  Charging and discharging graph of PA and PA/NDGs composite PCM.

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The composite PCMs had only one phase transition curve withparticular onset temperatures as before the cycling process. In

other words, there were clearly no extra peaks related to chemical

decomposition or phase segregation within the composites.

Through comparing the latent heat values in Fig. 15, the maximum

change in latent heat capacity was determined as 4.3 kJ/kg for S1

and the minimum was 0.99 kJ/kg for S5. These results suggested

that the changes in the latent heat capacity of the composites after

thermal cycling were less than 1% for S3, S4 and S5 demonstrate

their good thermal reliability. The results indicate that the varia-

tions of the phase change temperatures and latent heats are negli-

gible for LHTES applications. Therefore, it canbe concluded that the

thermal properties of PA/NDG composite PCMs keep stable for at

least 1000 cycling. Thermal energy storage density and speed of 

storing and releasing are two key factors when PCMs are appliedto store thermal energy such as solar thermal energy. Besides, as

far as the practical application is concerned, it is worthwhile to

transform solid–liquid PCMs into form-stable PCMs. The prepared

PA/NDG composite form-stable PCM by applying only 3 wt% of the

NDG possesses the merits of high thermal energy storage density

(199.48 kJ/kg), high thermal conductivity 0.98 (W/m K) and good

form stability. As a result, we believe that the prepared PA/NDG

form-stable PCM certainly can find its application in low tempera-ture solar-thermal applications.

Fig. 13.   (a) Thermal images of the composite samples during the cooling time at 10 s (b–f) thermal images at 150 s for (b) S1, (c) S2, (d) S3, (e) S4, and (f) S5.

 Table 5

Thermal conductivity and latent heat of recent form-stable PCMs in literature.

Composite Thermal conductivity (W/mK) Latent heat (kJ/Kg) References

Palmitic acid/polyaniline/exfoliated graphite nanoplatelets 1.08 157.7   [37]

Nano-graphite/paraffin 0.9362 181.81   [14]

Palmitic acid/polyaniline/copper nanowires 0.455 149   [38]

Palmitic acid/graphene oxide 1.02 101.23   [23]

Palmitic acid/graphene nanoplatelets 1.84 188.98   [8]

Paraffin/hexagonal boron nitride 0.53 177   [39]

Nitrogen-doped grapheme (4 wt%)/palmitic acid 1.54 197.53 This study

 Table 6

Thermal effusivity of the PA and composite PCMs.

Sample Thermal effusivity (kJ/(K m2 s0.5))

Pure PA 0.670

S1 0.758

S2 0.895

S3 1.335

S4 1.701

S5 1.836

Fig. 14.  Electrical resistivity of the composite PCMs.

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4. Conclusions

Novel form-stable PCMs were successfully prepared using NDG

as a supporting material. The results of the dropping point tests

and thermal cycling show that the shape stabilization of the PA

was achieved using only 3 wt% of NDG. The DSC results indicate

that the prepared composite has a high thermal energy storage

density (199.48 kJ/kg) while it has good stability as proven by

TGA results. The specific heat capacity and thermal effusivity of 

the PA are significantly increased by using NDG as an additive.

XRD patterns show that NDG sheets were sufficiently exfoliated

and dispersed homogeneously. Meanwhile, the shape-stabilized

composite PCM containing had a good thermal reliability with

respect to the changes in its thermal properties after 1000 melting

and cooling cycles. The electrical resistivity of composite PCM

decreased significantly while the thermal conductivity of the com-

posite was increased up to 6 times that of PA. On the whole, such a

shape-stabilized PCM with a high PA content of 97 wt% can be con-

sidered as a promising PCM for LHTES due to its form-stable prop-

erty, very low content of NDG, satisfying latent heat capacity and

excellent reliability. This may open a way to enhance the heat stor-

age density of PA based shape-stabilized PCM.

 Acknowledgements

This work has been financially supported by Ministry of High

Education (MOHE) of Malaysia, Grants number UM.C/HIR/MOH-

ENG/21-(D000021-16001) ‘‘Phase Change Materials (PCM) for

Energy Storage System and University of Malaya research Grant

No. UMRG RP021-2012A. The author thanks to the Bright Sparks

unit (University of Malaya) for additional financial support.

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