1. No category

International Journal of Heat and Mass Transfer 62 (2013) 711–717
Contents lists available at SciVerse ScienceDirect
International Journal of Heat and Mass Transfer
journal homepage: www.elsevier.com/locate/ijhmt
Optimal preparation of PCM/diatomite composites for enhancing
thermal properties
Su-Gwang Jeong, Jisoo Jeon, Jeong-Hun Lee, Sumin Kim ⇑
Building Environment & Materials Lab, School of Architecture, Soongsil University, Seoul 156-743, Republic of Korea
a r t i c l e
i n f o
Article history:
Received 28 September 2012
Received in revised form 18 February 2013
Accepted 15 March 2013
Keywords:
PCM
Diatomite
Latent heat
Impregnation
Thermal properties
a b s t r a c t
This paper deals with the thermal performances of PCM/diatomite composites for energy saving. The
PCM/diatomite composites were prepared by incorporating PCMs in the pores of diatomite to increase
the form stability of PCMs. In experiment, we used n-hexadecane, n-octadecane and paraffin wax as
PCMs, which have latent heat capacities of 254.7 J/g, 247.6 J/g and 144.6 J/g, respectively; and melting
points of 20.84 °C, 30.4 °C and 57.09 °C, respectively. The PCMs could be retained at 50 wt% in the pores
of the diatomite without leakage. The thermal effect of vacuum impregnation was also analyzed through
vacuum treatment during the preparation process of samples. An optimal preparing method for 50 wt% of
PCM impregnation is proposed. Thermal properties of samples were determined using DSC and TGA. And
SEM and FTIR analyses were carried out to analyze microstructure and chemical properties of samples.
SEM results showed that the PCMs are well-infiltrated into the structure of diatomite. DSC analysis
results showed that the latent heat capacities of PCM/diatomite composites were 50% the value of pure
PCMs, and TGA analysis results showed that PCM/diatomite composites have greater thermal durability
compared with pure PCM.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, the energy demand to provide a comfortable environment for humans in buildings has continuously increased worldwide. However, the energy used for heating, cooling and lighting
increases the level of greenhouse gas emissions and decreases fossil fuel resources [1,2]. Thermal energy storage systems are essential for reducing dependency on fossil fuels and contributing to a
more efficient, environmentally friendly energy use. As the demand for thermal comfort in buildings rises, the energy consumption increases correspondingly [3]. Phase change materials (PCMs)
have been widely used in many applications, such as passive cooling for electronic devices, protection systems in aircrafts, food processing, and energy conservation in buildings, because of their high
latent heat, chemical stability, suitable phase-change temperature,
and reasonable price. Experimental and analytical/numerical studies in published literature have focused on moving boundary problems [4–9].
PCMs can be integrated with different buildings’ structures such
as in gypsum board, plaster, concrete, clay minerals or other wallcovering materials. However, they also have some inherent limitations, such as low thermal conductivity, and the need for a container to prevent leaking and issues with flammability. To solve
⇑ Corresponding author. Tel.: +82 2 820 0665; fax: +82 2 816 3354.
E-mail address: [email protected] (S. Kim).
0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.03.043
these problems, some investigators have studied the possibility
of a container that can prevent the leaking of liquid PCMs by using
Shape-Stabilized PCM (SSPCM) and Microencapsulated PCM techniques [10–14]. Shape-stabilized PCM and Microencapsulated
PCM can maintain their shape even when the PCM changes from
solid to liquid, by a physical combination with a polymer or mineral (HDPE, diatomite, etc.) [15,16]. In recent years, shape-stabilized PCM (SSPCM) has attracted the interests of researchers. Liu
et al. proposed a re-coating method using an inorganic polymer
material for the SSPCM, and silica gel microcapsules containing
SSPCM were prepared by in situ polymerization, in which the content of paraffin wax was up to 69.12 wt% and its enthalpy was
153.46 J/g [17]. Wang et al. prepared a kind of macro-capsule
through in situ polymerization by using silica gel as the shell material and shape-stabilized phase change materials containing
50 wt% each of n-octadecane and of high density polyethylene as
the core [18]. Karaman et al. determined thermal energy storage
properties of polyethylene glycol/diatomite composite as a novel
form-stable composite phase change material. The composite
PCM was prepared by incorporating PEG in the pores of diatomite
[17].
Diatomite is a sedimentary rock formed from the siliceous fossilized skeletons of diatoms (SiO2 nH2O and crystallized silica).
The material is a unicellular alga that existed during tertiary and
quaternary periods. It is composed of rigid cell walls, called frustules. Depending on their species, frustule dimensions from less
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optimal preparation of PCM/diatomite composites to get high latent heat capacity.
2. Experimental
2.1. Materials
Fig. 1. The image of vacuum impregnation system.
We used three types of liquid organic PCMs with different melting points. In this experiment, n-hexadecane, n-octadecane and
paraffin wax were used as organic PCMs. The n-hexadecane,
n-octadecane and paraffin wax have molecular formulae of
C16H34, C18H38 and C25H52 as alkane series and they have latent
heat capacities of 254.7 J/g, 247.6 J/g and 144.6 J/g and melting
points of 20.84 °C, 30.4 °C and 57.09 °C, respectively. The PCMs
were obtained from Celsius Korea, South Korea. Diatomite samples
were supplied by Samyoung Global Corporation in South Korea.
The diatomite samples were under 10 lm in diameter and were
dried at 105 °C for 24 h before use.
2.2. Preparation
than 1 to more than 100 lm can be found with features like protuberances and pores close to 100 nm [19]. Diatomite has light
weight, high porosity, high absorptivity, high purity, multi-shape,
rigidity, and inertness. Both the chemical composition and the
physical structure of diatomite make it suitable for many scientific
and industrial purposes. Diatomite is used in many fields as a filtering agent; building material; heat, cold, and sound insulator;
catalyst carrier; filler absorbent; abrasive; and ingredient in medicines [20]. Considering this, diatomite is a feasible candidate for
an economical and light-weight building material for incorporating
PCM for thermal energy storage in buildings.
The most important characteristic to consider is the heat capacity and melting point of the PCM in its application to building
materials. In this paper, we used three types of PCMs which have
different melting points, for application to various building sections such as radiant floor heating systems, insulation and ceiling
panels. Each building section needs a different temperature control. Therefore this study aims to prepare the PCM/diatomite composites to obtain high thermal performance. Also we suggest an
The PCM/diatomite composite was prepared using vacuum
impregnation [21,22]. The image of the vacuum impregnation system is shown in Fig. 1. 50 g of diatomite sample was placed inside a
filtering flask, which was connected to a water tromp apparatus to
evacuate air from its porous surface. Then, the valve between the
flask and a container with 80 g of liquid PCM was opened to allow
flow into the flask to cover the diatomite sample. The vacuum process was continued for 90 min, then air was allowed to enter the
flask again to force the liquid PCM to penetrate into the pore space
of diatomite. Before the impregnating process, we analyzed the
thermal properties of the PCM/diatomite composite through comparison between the vacuum treatment and no-vacuum treatment
inside a flask by analyzing TGA analysis. The TGA analysis result of
this experiment is shown in Fig. 2. The vacuum treated n-hexadecane/diatomite composite showed a 50% decrease in weight at
200 °C, but the no-vacuum sample decreased 30% in weight. This
result shows that the vacuum treatment process leads to a higher
impregnation rate of PCM in diatomite. And keeping a vacuum
Fig. 2. The TGA curves according to vacuum treatment of n-hexadecane/diatomite composite.
S.-G. Jeong et al. / International Journal of Heat and Mass Transfer 62 (2013) 711–717
713
Fig. 3. The DSC graph of n-hexadecane/diatomite composite (no-vacuum drying).
(a)
(b)
(c)
(d)
Fig. 4. Microstructure of the diatomite and PCM/diatomite composites: (a) diatomite, (b) n-hexadecane/diatomite composite, (c) n-octadecane/diatomite composite and (d)
paraffin wax/diatomite composite.
state in the preparation flasks is more favorable to obtain better
thermal properties.
To find the maximum PCM amount in diatomite, we impregnated 80 g of PCM into 50 g of diatomite. In this case, excess
PCM remained in the flask and needed to be removed through fil-
tering. PCM/diatomite composite in a colloidal state was filtered by
1 lm filter paper until granular sample appeared, which was dried
in a vacuum drier at 80 °C for 24 h, 48 h and 72 h for n-hexadecane,
n-octadecane, and paraffin wax, respectively. If the PCM/diatomite
composite was dried in general drier, the characteristic of the la-
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S.-G. Jeong et al. / International Journal of Heat and Mass Transfer 62 (2013) 711–717
tent heat capacity would disappear from the sample. Fig. 3 shows
the DSC analysis results from when PCM/diatomite composite was
dried in the no-vacuum drier. It can be seen that no peaks are
shown in the graph. This indicates that vacuum drying is advantageous over general drying for retaining thermal properties.
(a)
2.3. Characterization techniques
Microstructure of samples was determined by scanning electron
microscopy (SEM) at room temperature. SEM with an accelerating
voltage of 12 kV and a working distance of 12 mm was used to collect SEM images. The samples were coated with a gold coating of a
few nanometers in thickness [23]. Fourier transform infrared spectroscopy (FTIR: 300E Jasco) was also utilized to monitor the change
of chemical groups upon curing. Clear potassium bromide (KBr)
disks were molded from powder and used as backgrounds. The
samples were analyzed over the range of 525–4000 cm 1 with a
spectrum resolution of 4 cm 1. All spectra were averaged over 32
scans. This analysis of the composites was performed at a pointto-point contact with a pressure device [24]. Thermal properties
such as the melting temperature and latent heat capacity of pure
PCMs and composite PCMs were measured using a differential
scanning calorimetry (DSC: Q 1000). DSC measurements were performed at a 5 °C/min heating and cooling rate and temperature
ranges of 0 to 80 °C and 80 to 0 °C. The melting temperature was
measured by drawing a line at the point of maximum slope of the
leading edge of the peak and extrapolating to the base line [10].
The latent heat of the PCM/diatomite was determined by numerical
integration of the area under the peaks that represent the solid–solid and solid–liquid phase transitions. Thermo gravimetric analysis
measurements of the PCM/diatomite composites were carried out
using a thermo gravimetric analysis (TGA: TA Instruments, TGA
Q5000) on approximately 2–4 mg samples over the temperature
range 25–600 °C, at a heating rate of 10 °C/min under a nitrogen
flow of 20 ml/min. TGA was measured with the composites placed
in a high quality nitrogen (99.5% nitrogen, 0.5% oxygen content)
atmosphere to prevent unwanted oxidation [25].
(b)
(c)
3. Results and discussion
3.1. Morphology and microstructure of composite PCMs
Scanning electron microscopy observations were performed for
the pure PCMs and the composite PCMs. The pure PCMs and composite PCMs were broken up in liquid nitrogen and the fractured
surfaces were coated with gold before SEM investigations. Fig. 4
shows SEM micrographs of diatomite and after PCM impregnation.
It can be seen that each PCM sample is incorporated into the pores
of the diatomite. Comparison between the diatomite and the PCM/
diatomite composite images is shown in Fig. 4. Porous diatomite is
shown in Fig. 4(a). It was found that the diatomite has a good
structure for incorporating liquefied materials such as PCM. In
the case of the n-hexadecane/diatomite composite, we determined
that the n-hexadecane fully filled the diatomite pores. The n-octadecane almost fully filled the diatomite pores, but less efficiently
than n-hexadecane. The Paraffin wax/diatomite composite appears
to follow the same trend. Through SEM analysis, we confirmed that
each PCM incorporated well into the structure of diatomite and
expect the characteristic of heat storage properties of the PCMs
to reflect in the composites.
Fig. 5. FTIR spectra of PCM/diatomite composites: (a) n-hexadecane/diatomite
composite, (b) n-octadecane/diatomite composite and (c) paraffin wax/diatomite
composite.
3.2. FTIR analysis
The FTIR absorption spectra of the PCMs and PCM/diatomite
composites are shown in Fig. 5. The n-hexadecane, n-octadecane
and paraffin wax have molecular formulae of C16H34, C18H38 and
C25H52, respectively, and are composed of –CH2 bonding
and –CH3 bonding. Because all the PCMs have the same group of
bonding, these FTIR absorption spectra are similar. These FTIR
S.-G. Jeong et al. / International Journal of Heat and Mass Transfer 62 (2013) 711–717
715
(a)
Table 1
The FTIR spectra of the PCMs and diatomite.
Vibration
Wavenumber range
Stretching vibration of functional group CH2
Stretching vibration of functional group CH3
Si–O–Si asymmetric stretch
Silanol Si–O stretch
Si–O–Si symmetric stretch
Si–O–Si bend
2918, 2850 cm 1
1468 and 720 cm
1085 cm 1
944 cm 1
802 cm 1
464 cm 1
1
absorption spectra were almost the same, with absorption peaks of
2918, 2850, 1468 and 720 cm 1 caused by stretching vibration of
functional groups of –CH2 and –CH3. We confirmed that these bondings were not broken or changed during the incorporating process
as seen in the PCM/diatomite composite peaks. The pure diatomite
shows main absorption bands at 1085, 944, 802, 464 cm 1. The
band at 1085 cm 1 is due to the Si–O–Si asymmetric stretch, and
the band at 944 cm 1 reflects the silanol Si–O stretch. The peak
at 802 cm 1 represents Si–O–Si symmetric stretch, and the band
at 464 cm 1 reflects the Si–O–Si bend. The FTIR spectrum of
PCM/diatomite composite has new absorption peaks at 2918,
2850, 1468, 1085, 944, 802, 720 and 464 cm 1. These FTIR peaks
are shown as in Table 1. In this experiment, we found that the FTIR
adsorption spectra of diatomite did not change for each PCM. This
means that there is no chemical interaction between PCM and diatomite. However, the shifts in characteristic absorption peaks of
pure PCM indicated that the interactions between the functional
groups of PCM and diatomite were physical in nature. In other
words, the PCM molecules were retained easily in the pores of diatomite by these physical interactions such as capillary and surface
tension forces, and leakage of the melted PCM from the composite
was prevented by these forces. Consequently, we determined that
the heat storage characteristics of PCMs could integrate into the
structure of diatomite due to its physical bonding, without a
change in its chemical properties.
(b)
3.3. Thermal properties of PCM/diatomite composites
We carried out DSC analysis more than three times to confirm
thermal properties of the samples. And these DSC peaks according
to each PCM show nearly the same value of latent heat storage like
in other literature. The heating and freezing curves from the DSC
measurements of the pure PCMs and PCM/diatomite composites
are presented in Fig. 6. The values for thermal performances of
the PCM/diatomite composites were nearly 50% those of the pure
PCMs. The latent heats are obtained by numerical integration of
the total area under the peaks of the solid–liquid transition curves
of the PCMs in the composite. It can be seen that the latent heats of
the PCM/diatomite composites approach half of heat storage performance of the pure PCMs in Fig. 6(a). The phase transition of nhexadecane occurred between 17 °C and 25 °C during heating,
and the corresponding heat capacity was 254.7 J/g. While freezing
from 80 °C to 0 °C, the heat released for n-hexadecane was 250.6 J/
g. However, the temperature range of solidification was lowered to
between 15 °C and 10 °C. There are no strong interactions between
the n-hexadecane molecules and the pore walls of diatomite. This
leads to a depression of the phase change temperatures of n-hexadecane in the composite. And the latent heats of melting and freezing were found to be 120.1 J/g and 118.0 J/g, respectively, for nhexadecane/diatomite composite. These latent heat values were
slightly lower than the 50% of the value for pure n-hexadecane.
The phase transition of n-octadecane occurred between 28 °C and
32 °C during heating, and the corresponding heat capacity was
247.6 J/g. The heat released for n-octadecane was 245.8 J/g during
freezing. The latent heat storage of n-octadecane/diatomite composite was 116.8 J/g and 112.9 J/g during heating and freezing,
(c)
Fig. 6. The heating and freezing curves by DSC of (a) n-hexadecane/diatomite
composite, (b) n-octadecane/diatomite composite and (c) paraffin wax/diatomite
composite.
respectively. These were also nearly 50% of the heat storage characteristic of pure n-octadecane. It was found that n-octadecane and
n-hexadecane has one phase change peak, while the paraffin wax
has two phase change peaks. The one phase change peak for nhexadecane corresponds to the solid–liquid phase change. The first
phase change peak of paraffin wax is lower and corresponds to the
solid–solid phase transition of the paraffin wax, and the second
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Table 2
Melting point and latent heat of PCMs and PCM/diatomite composites.
PCM samples
Melting point (°C)
Freezing point (°C)
Pure n-hexadecane
n-Hexadecane/diatomite
Pure n-octadecane
n-Octadecane/diatomite
Pure paraffin wax
Paraffin wax/diatomite
20.84
23.68
30.40
31.29
57.09
54.24
16.78
13.17
26.23
23.65
53.76
50.23
peak is very high, corresponding to the solid–liquid phase change
[23,26]. The DSC curves of the paraffin wax and paraffin wax/diatomite composite PCM are shown in Fig. 6(c). The phase transition
of paraffin wax occurred between 52 °C and 60 °C during heating,
and the corresponding heat capacity was 144.6 J/g. The heat
released for paraffin wax was 144.4 J/g during freezing. The latent
heat storage of paraffin wax is lower than n-hexadecane and
n-octadecane, but could be used in applications that require high
temperature control, since the temperature change range of paraffin wax is higher than that of the other materials. The latent heat
storage of paraffin wax/diatomite composite was 61.96 J/g and
59.74 J/g, respectively, during heating and freezing. These latent
heat values were 42% of the value for pure paraffin wax. Table 2
showed melting points and the latent heats of the PCMs and
PCM/diatomite composites. Consequently, the phase transition
temperature range of the PCM/diatomite composites significantly
overlapped with that of each respective PCM incorporated in the
porous structure of diatomite. This high latent heat storage of
PCM/diatomite composites has an important potential for heating
and cooling applications in buildings.
3.4. Thermal gravimetric analysis of PCM/diatomite composites
Since diatomite is known for its chemical inertness and resistance to thermal degradation, thermal decomposition of the PCM
and PCM/diatomite composites was analyzed by TGA in nitrogen
atmosphere shown in Fig. 7. The n-hexadecane and n-hexadecane/diatomite composite has one curve of thermal oxidation degradation unlike diatomite. The weight loss of the diatomite was
very small in spite of a high temperature. It means that diatomite
has thermal durability properties. As shown in Fig. 7, one step occurs between 130 and 200 °C for the n-hexadecane and n-hexadec-
Latent heat (J/g)
Incorporated rate (%)
Solid–liquid
Liquid–solid
254.7
120.1
247.6
116.8
144.6
61.96
250.6
118.0
245.8
112.9
144.4
59.74
47
47
42
ane/diatomite composite, corresponding to the thermal-oxidation–
degradation of n-hexadecane molecular chains. 50% thermal
degradation was found. And we found the pure n-hexadecane
has a higher temperature of oxidation compared to the n-hexadecane/diatomite. It shows that impregnation of PCM into the diatomite negatively affects the thermal durability of PCM. However,
in the case of the n-hexadecane/diatomite composite, the diatomite still remained in the samples although PCM was fully oxidized. Therefore it shows that n-hexadecane/diatomite composite
has a higher thermal durability compared to the pure n-hexadecane. The TGA curves of the n-octadecane/diatomite composite
and paraffin wax/diatomite composite appear to follow the same
trend.
4. Conclusion
We prepared PCM/diatomite composites for the reduction of
energy saving by improving thermal efficiency through latent heat
storage. PCM/diatomite composites designed to produce high thermal performance were prepared through vacuum impregnation.
We suggested an optimal preparation of PCM/diatomite composite
to produce high latent heat capacity. Vacuum treatment was effective in conserving the thermal properties of pure PCM. SEM, FTIR,
DSC and TGA analyses were performed to confirm the characteristics of the PCM/diatomite composites. SEM analysis revealed that
each PCM was incorporated into the pores of the diatomite. FTIR
analysis confirmed that the peaks of pure PCM are still shown in
the PCM/diatomite composites peaks, because PCM/diatomite
composites involve physical bonding between the PCMs and the
diatomite. DSC analysis confirmed that the PCM/diatomite
composites have high latent heat storages since each PCM was
well-incorporated into the diatomite pores. And we found the nhexadecane/diatomite composite has a higher thermal durability
compared to the pure n-hexadecane. Consequently, we expect
the PCM/diatomite composites to be useful in applications in various fields due to its shape stability characteristics and high thermal performance.
Acknowledgments
This work was supported by a National Research Foundation of
Korea (NRF) grant, funded by the Korea government (MEST) (No.
2012-0005188).
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Fig. 7. The TGA graph of the diatomite, n-hexadecane and n-hexadecane/diatomite
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