International Journal of Heat and Mass Transfer 71 (2014) 245–250
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer
journal homepage: www.elsevier.com/locate/ijhmt
Thermal performance evaluation of Bio-based shape stabilized PCM
with boron nitride for energy saving
Su-Gwang Jeong, Jeong-Hun Lee, Jungki Seo, 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 1 August 2013
Received in revised form 8 December 2013
Accepted 9 December 2013
Keywords:
Bio-based PCM
Boron nitride
Thermal properties
Impregnation
a b s t r a c t
Among the PCMs, Bio-based PCMs are considered one of the most promising candidates, due to their large
latent heat, low vapor pressure in the melt, good chemical stability, self-nucleating behavior, safety, and
commercial availability at low cost. However, the leakage problem and low thermal conductivity of Biobased PCM limit its application, to some extent. Therefore, porous materials with a high thermal conductivity, such as boron nitride, are promising candidates for simultaneously solving these two problems. In
this study, we prepared Bio-based PCM with boron nitride, by using the vacuum impregnation process.
We analyzed the microstructure, chemical bonding, heat capacity, thermal resistance and Thermal conductivity of Bio-based PCM with boron nitride, by SEM, FTIR, DSC, TGA and TCi analyses. From the analyses, we expect Bio-based PCM with boron nitride to be useful in applications in various fields, due to its
high thermal properties.
Ó 2013 Published by Elsevier Ltd.
1. Introduction
Energy is becoming one of the most important issues in our
society, due to the depletion of fossil fuels and primary energy resources. So, many researchers have developed efficient energy systems. These systems are capable of enhancing the energy efficiency
of buildings, which in turn reduce the environmental impact related to energy use [1]. Thermal energy storage (TES) can be
accomplished either by using sensible heat storage, latent heat
storage, or reversible chemical reaction heat storage [2]. The use
of phase change material (PCM) for thermal energy storage has
gained importance in recent years [3–7], since it can absorb or release large amounts of latent heat, working in narrow margins of
temperature [8]. The PCMs have been used for various hot/cold energy storage applications in the last four decades; however, they
have also been used as a storage media for space cooling application [9]. So far, the PCMs for cool storage have been mainly inorganic salt hydrates, organic paraffin waxes, and mixtures of
these. The integration of a few phase change materials into building fabrics has been discussed and reported, as a potential method
of reducing energy consumption in passive buildings [10–13].
PCMs with a phase change temperature in the human comfort
range have been investigated for several decades in building envelopes, as a way to reduce HVAC (heating, ventilation, and air-conditioning system) energy usage by maintaining comfortable
temperatures, and are increasingly being incorporated in commer⇑ 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 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.12.017
cially available products [14,1,15]. Among the PCMs, Bio-based
PCMs are considered one of the most promising candidates, due
to their large latent heat, low vapor pressure in the melt, good
chemical stability, self-nucleating behavior, safety, and commercial availability at low cost [16–18]. For this reason, we decided
on the Bio-based PCMs, which contained various fatty acids. However, the leakage problem of Bio-based PCM during the solid–liquid
change process limits its application, to some extent [19]. To overcome this problem, shape stabilization supports are introduced, to
fabricate form-stable composite PCM. Stabilization supports of
PCM usually include microencapsulation containers, e.g. polymer
microencapsulation shells, and porous materials, such as expanded
graphite, carbon foam, metallic foam, perlite, diatomite, vermiculite, attapulgite and clay minerals [20–31]. However, Bio-based
PCMs also have a low thermal conductivity, which restricts their
application to energy storage [32]. Therefore, using Bio-based
PCM filled with high thermal conductivity materials is a good strategy to enhance the thermal properties of Bio-based PCM, for their
wide application. Therefore, porous materials with a high thermal
conductivity are promising candidates for simultaneously solving
these two problems. It is considered that porous materials can assume a stable shape that prevents the leakage of liquid Bio-based
PCM, due to the capillary and surface tension forces of a porous
structure [33]. High performance polymeric composites with high
thermal conductivity and low dielectric loss have attracted great
attention worldwide, owing to their great importance in many cutting-field industries [34–36]. A simple and effective method for
preparing thermally conductive materials is adding thermally conductive inorganic particles into a polymer [37,38]. Boron nitride
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(BN) is isoelectronic with carbon, and is also known as white
graphite. Whereas graphite has metallic conductivity, boron nitride is an electrical insulator. This is due to the covalent interlayer
bonding of the boron and nitrogen atoms, which localize the free
electrons. This also explains the different colors of graphite and
BN. Other important properties of BN are its high temperature
resistance, thermal shock resistance, high-thermal conductivity,
chemical inertness, non-toxicity and environmental safety [39].
The composite materials made by coating/mixing/incorporating
BN have a lot of applications, such as raw material for super abrasives, cosmetic applications, in thermal management, functional
coating in the automotive industry and releasing agent in the glass
industry [40,41]. Therefore, this paper has used boron nitride PT
160 (BN PT 160) as a container of Bio-based PCM. In this study,
we prepared thermal enhanced PCM, by using the vacuum impregnation process with boron nitride. The vacuum impregnation
method guarantees the heat storage of Bio-based PCM, after the
incorporation process. Therefore, we studied thermal enhanced
Bio-based PCM, to improve the thermal conductivity and fire retardant properties. In particular, this paper analyzed the microstructure, chemical bonding, heat capacity, thermal resistance and
Thermal conductivity of thermal enhanced Bio-based PCM, from
the results of SEM, FTIR, DSC, TGA and TCi analyses.
2. Experimental
2.1. Materials
In the experiment, we used a Bio-based PCM, which has a latent
heat capacity of 149.2 J/g and melting point of 29.38 °C. The Biobased PCM was obtained from Korea C&S Corporation in South Korea. Boron nitride PT 160 was purchased from Momentive Performance Materials Inc. The boron nitride PT 160 has 7–10 lm of
mean particle size, and 13 m2/g of surface area. The physical properties of boron nitride PT 160 are shown in Table 1.
maximum Bio-based amount in the boron nitride structure. Finally,
the Bio-based composite PCM was dried in a vacuum drier at 80 °C
for 48 h.
2.3. Characterization technics
The microstructure of Bio-based composite PCM was observed
by means of scanning electron microscopy (SEM) at room temperature. A SEM with an accelerating voltage of 12 kV and working
distance of 12 mm was used to collect the SEM images. The samples were coated with a gold coating of a few nanometers in thickness. Fourier transform infrared spectroscopy (FTIR: 300E Jasco)
was also utilized, to monitor the change of chemical groups upon
curing. Clear potassium bromide (KBr) discs 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.
Thermal properties of the composite PCM, such as the melting and
freezing temperature and latent heat capacity, were measured
using differential scanning calorimetry (DSC: Q 1000). The latent
heat capacity of the composite was determined by numerical integration of the area under the peaks that represent the solid–solid
and solid–liquid phase transitions [42]. 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. Thermal durability of the composite PCM was carried out using 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 atmosphere
(99.5% nitrogen, 0.5% oxygen content), to prevent unwanted oxidation [28]. The thermal conductivity of composite PCM was measured at room temperature condition using a TCi thermal
conductivity analyzer. The TCi developed by C-Therm Technologies
Ltd. is a device for conveniently measuring the thermal conductivity of a small sample, by using the Modified Transient Plane Source
(MTPS) method [43].
2.2. Preparation
3. Results and discussion
The Bio-based composite PCM was prepared using vacuum
impregnation. 100 g of the boron nitride was dried in vacuum
oven, before the impregnation process. This boron nitride was
put inside a filtering flask, which was connected to a water tromp
apparatus, to evacuate air from the porous structure of boron nitride. Then, the valve between the flask and a container with
200 g of liquid Bio-based PCM was opened, to allow flow into the
flask, to cover the microstructure of boron nitride. 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 porous
structure of boron nitride. After the penetrating process, excess
PCM remained in the flask, and needed to be removed through filtering process. To find the maximum Bio-based PCM amount in
boron nitride, we impregnated 200 g of Bio-based PCM into 100 g
of boron nitride. In this case, excess Bio-based PCM remained in
the flask and needed to be removed through filtering. The Biobased composite PCM in a colloidal state was filtered by using vacuum pump, and filtering process was worked by 1 lm filter paper
until granular sample appeared. The remained granular sample has
3.1. Morphology and microstructure
The morphology of Bio-based PCM and its composite PCM are
shown in Fig. 1. Fig. 1 shows that Bio-based PCM and Bio-based
composite PCM have similar surfaces. This means boron nitride
particles are uniformly dispersed into the microstructure of Biobased PCM. Also, when we touched the samples, Bio-based composite PCM had a hard surface, compared with Bio-based PCM.
We confirmed that the composite maintained its form stable morphology at above melting point of Bio-based PCM. Fig. 2 shows
microstructures of the Bio-based PCM, boron nitride and Bio-based
composite PCM. From the SEM analysis, Bio-based PCM shows the
typical microstructure of organic PCM, such as paraffinic PCMs.
Also micro particles of BN are seen in Fig. 2(b). The Bio-based composite PCM is shown in Fig. 2(c). In Fig. 2(c), we found that the
composite PCM has formed a kind of fiber network. In this fiber
network structure, we determined that the composite PCM is prevented from leakage of Bio-based PCM at above its melting point,
Table 1
Physical properties of boron nitride PT 160.
Crystal (type)
Mean particle size (lm)
Crystal size (lm)
Surface area (m2/g)
Hexagonal
Tap density, g/cm3
0.4
7–10
Oxygen %
0.4
4
Soluble borates %
0.2
13
Carbon %
0.03
S.-G. Jeong et al. / International Journal of Heat and Mass Transfer 71 (2014) 245–250
(a) Bio-based PCM
247
(b) Bio-based composite PCM
Fig. 1. Morphology of (a) Bio-based PCM, and (b) Bio-based composite PCM.
due to capillary action and surface tension force from the impregnation process. Also, we confirmed this micro network affected the
solid surface of composite. In total, the boron nitride micro particles are well dispersed into the microstructure of Bio-based PCM.
(a) Bio-based PCM (x1000)
3.2. FTIR analysis of Bio-based PCM/BN composite PCM
Fig. 3 shows the FTIR peaks of the Bio-based PCM, composite
PCM and Boron nitride PT 160. Its chemical formula is BN. The FTIR
spectrum of Boron nitride shows two distinct absorption bands at
1375 and 760 cm 1, which represent B–N stretching and B–N
bending, respectively. Also, the weak adsorption peak at
1450 cm 1 by the B–OH band shows the existence of strong
absorption of BN. Bio-based PCMs are made from fatty acids, such
as soybean oils, coconut oils, palm oils and beef tallow. Soybean
oils, coconut oils and palm oil, etc. are composed of fatty acids,
such as a-linolenic acid. a-linolenic acid is an organic compound
found in many common vegetable oils. Its molecular formula is
C18H30O2. Like a-linolenic acid, fatty acids, which are contained
in Bio-based PCM, have –CH2 bonding and –CH3 bonding, because
all PCMs are composed of oily components. It is mainly composed
of –CH2 and –CH3 bonding, because all PCMs are composed of oily
components. In the following graphs, we determined the FTIR
spectrum of Bio-based PCM was from 2855 to 2962, 1379 and
725 cm 1, caused by stretching vibration of the functional groups
of –CH2 and –CH3. These FTIR spectrum peaks are shown in Table 2.
The FTIR peaks of Bio-based composite PCM have all of the peaks in
accordance with Bio-based PCM and boron nitride. The FTIR peaks
of Bio-based composite PCM showed the adsorption peak at 1375,
760, 1450 cm 1 which caused by the boron nitride and the adsorption peak at 2855 to 2962, 1379 and 725 cm 1 caused by Bio-based
PCM as shown Fig. 3. This means that the chemical properties of
Bio-based PCM and boron nitride have not changed. Therefore,
we expected that the composite PCM would have high latent heat
capacity and high thermal conductivity. As a result, in this experiment, we found that there is no chemical interaction between Biobased PCM and boron nitride during the incorporating process. In
other words, the PCM molecules were retained easily in the porous
structure of boron nitride 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.
3.3. Thermal properties analysis
Thermal properties of the Bio-based PCM and Bio-based composite PCM were carried out by using the DSC analyzer. DSC analysis was carried out by using 5 samples. And the data showed 2–
3 J/g of errors, it means that this DSC results are very accurate.
The results are shown in Fig. 4. Fig. 4(a) shows that the Bio-based
PCM has 29.38 °C melting temperature and 21.26 °C freezing temperature, and its latent heat capacities are 149.2 and 133.5 J/g,
(b) Boron nitride (x1000)
(c) Bio-based composite PCM (x1000)
Fig. 2. SEM images of (a) Bio-based PCM, (b) boron nitride, and (c) Bio-based
composite PCM.
when the bio PCM was changed from solid to liquid, and from liquid to solid. The phase transition of Bio-based composite PCM occurred at 29.98 °C and 19.39 °C during heating and freezing, and
the corresponding heat capacity was 117.6 J/g and 117.3 J/g,
respectively. The values for latent heat capacity of the Bio-based
composite PCM were nearly 80% of the pure Bio-based PCM. This
means that the incorporated rate of Bio-based PCM is 80%. Its
incorporated rate presented very high value, compared to other
researches. From this result, we confirmed that a great deal of
Bio-based PCM was well incorporated into the micro structure of
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(a) Bio-based PCM
Fig. 3. FTIR spectra of Bio-based PCM, boron nitride and Bio-based composite PCM.
Table 2
FTIR spectra of Bio-based PCM with boron nitride.
Vibration
Wave number range (cm
B–N stretching
B–N bending
B–OH band
C–H3 asymmetric stretch
C–H3 symmetric stretch
C–H2 asymmetric stretch
C–H2 symmetric stretch
C–H3 umbrella bending mode
C–H2 rocking vibration
1375 ± 10
760 ± 10
1450 ± 10
2962 ± 10
2872 ± 10
2926 ± 10
2855 ± 10
1377, 1379
720, 725, 729
1
)
(b) Bio-based composite PCM
boron nitride, because boron nitride PT 160 has high surface area,
and very small particle size. Also, the melting temperature and
freezing temperature of both Bio-based PCM and Bio-based composite PCM show a similar trend. However, the freezing curve of
Fig. 4(b) shows a small peak, in contrast with the freezing curve
of Bio-based PCM. However we determined that its effect is not
big because its temperature difference is similar to the pure Biobased PCM. Sharma et al. indicate in literature that super-cooling
of more than a few degrees affects the heat extraction from the
store, and 5–10 °C super-cooling can prevent it entirely [44].
Therefore we determined that the boron nitride has not much effect on the super-cooling prevention of Bio-based PCM. And we
carried out the thermal cycling test of Bio-based composite PCM.
Thermal cycling test of the Bio-based composite PCM was measured by 30 thermal cycling in DSC analyzer and the temperature
range for thermal cycling test was 0 to 80 °C. The Bio-based PCM
has 29.98 °C and 19.38 °C during heating and freezing after thermal cycling test. Its heat capacity showed 115.3 J/g and 115.0 J/g,
respectively. These results are similar to the heat capacity of Biobased composite PCM which was tested before thermal cycling
test. It means that thermal properties of Bio-based PCM remained
after thermal cycling test. Also it means that leakage of PCM was not
occurred after thermal cycling. We expect that this high latent heat
capacity of Bio-based PCM composite with boron nitride will have
potential for heating and cooling applications in various fields.
3.4. Thermo gravimetric analysis
Fig. 5 and Table 3 show thermo gravimetric analysis of the Biobased PCM and Bio-based composite PCM. In the case of the Biobased PCM, the first peak of derivative weight presented at
(c) Bio-based composite PCM (After thermal cycling test)
Fig. 4. DSC graph of (a) Bio-based PCM, (b) Bio-based composite PCM with boron
nitride and (c) Bio-based composite PCM (after thermal cycling test).
237.08 °C, and second peak of derivative weight presented at
297.24 °C. However, Bio-based PCM composite with boron nitride
PT 160 shows 186.68 °C and 268.88 °C, in accordance with the first
and second peaks of derivative weight. In the result, we confirmed
that Bio-based PCM composite shows a low value of derivative
weight, compared to the pure Bio-based PCM. From the result,
we adjudged that the increased thermal conductivity of Bio-based
PCM by the effect of boron nitride had an effect on the peak temperature of derivative weight. Also, in the case of the peak temperature, that of Bio-based composite PCM is greater than that of pure
Bio-based PCM. This means that mass loss of the Bio-based
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ity of BN showed approximately 30 W/m K at 25 °C temperature as
reported from literatures. In the analysis, the thermal conductivity
of Bio-based PCM and Bio-based composite PCM show 0.153 and
0.729 W/m K, respectively. As a result, Bio-based composite PCM
has a 477% increase of thermal conductivity, compared to the pure
Bio-based PCM. This shows that the boron nitride has an effect on
an enhancement of the thermal conductivity of Bio-based PCM.
Through the thermal conductivity analysis, we confirmed that
Bio-based composite PCM is useful for application in various fields,
such as the building materials sector.
4. Conclusion
Fig. 5. Thermo gravimetric analysis of Bio-based PCM, and Bio-based composite
PCM.
Fig. 6. Thermal conductivity of Bio PCM, and Bio-based composite PCM.
composite PCM was delayed from the first peak of derivative
weight to the second peak of derivative weight. In the graph, the
oxidation rate of the samples shows 99.81% and 85.69% accordance
with Bio-based PCM and Bio-based composite PCM. Also, Boron nitride is known for its chemical inertness, and resistance to thermal
degradation. Therefore, more amounts of Bio-based composite
PCM remained over 600 °C. As a result, we confirmed that the boron nitride sample has an affect on the thermal conductivity and
thermal resistance of the Bio-based PCM.
The integration of a few phase change materials into building
fabrics has been discussed and reported, as a potential method of
reducing energy consumption in passive buildings. Among the
PCMs, Bio-based PCMs are considered one of the most promising
candidates, due to their large latent heat, low vapor pressure in
the melt, good chemical stability, self-nucleating behavior, safety,
and commercial availability at low cost. However, the leakage
problem and low thermal conductivity of Bio-based PCM, to some
extent, limits its application. Therefore, porous materials with a
high thermal conductivity, such as boron nitride, are promising
candidates for simultaneously solving these two problems. In this
study, we prepared Bio-based PCM with boron nitride, which has
many advantages, such as high temperature resistance, thermal
shock resistance, high thermal conductivity, chemical inertness,
non-toxicity and environmental safety, by using the vacuum
impregnation process. We analyzed the microstructure, chemical
bonding, heat capacity, thermal resistance and Thermal conductivity of Bio-based PCM with boron nitride, by SEM, FTIR, DSC, TGA
and TCi analyses. From the SEM analysis, we confirmed that Boron
nitride micro particles are well dispersed into the microstructure
of Bio-based PCM. The FTIR peaks of Bio-based PCM still remained
after the incorporating process. This shows the prepared composite
was composed of physical bonding between the Bio-based PCM
and boron nitride. The latent heat capacity of the Bio-based PCM
with boron nitride was nearly 80% of the pure Bio-based PCM.
From the TGA and TCi analyses, boron nitride has an effect on
the thermal resistance and thermal conductivity of Bio-based
PCM. Bio-based PCM has adequate phase change temperature
range to apply buildings. So the prepared Bio-based composite
PCM would be applied to the gypsum board and mortar etc. due
to its form stable properties. And the building materials with
Bio-based composite would help reduction of heating and cooling
loads in buildings due to its high thermal properties. Consequently,
we expect the Bio-based PCM with boron nitride to be useful to
applications in various fields.
3.5. Thermal conductivity analysis
Acknowledgment
The thermal conductivity analysis of Bio-based PCM and Biobased composite PCM is shown in Fig. 6. Prior to the analysis of
thermal conductivity of samples, we found the thermal conductiv-
This work was supported by a National Research Foundation of
Korea (NRF) grant, funded by the Korea government (MEST) (No.
2013-030588).
Table 3
Thermo gravimetric analysis of (a) Bio-based PCM, and (b) Bio-based composite PCM.
PCM samples
First peak of derivative weight
(°C)
Second peak of derivative weight
(°C)
Peak temperature difference
(°C)
Oxidation rate
(%)
Bio-based PCM
Bio-based PCM composite with boron
nitride
237.08
186.68
297.24
268.88
60.16
82.20
99.81
85.69
250
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