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1858
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Leakage-proof phase change composites
supported by biomass carbon aerogels from
succulents†
Yanhong Wei,a Juanjuan Li,a Furong Sun,a Jinrong Wu
*b and Lijuan Zhao*a
The practical applications of organic phase change materials (PCM) are greatly limited, due to their leakage in
the melted state and unacceptably low thermal conductivity. To address such a challenge, we use a succulent-based carbon aerogel (SCA), which consists of the epidermis, palisade tissue and spongy tissue, as an
encapsulation scaffold for paraffin to fabricate PCM composites. The spongy tissue consisting of rich closed
spherical cells allows a high loading efficiency (up to 95 wt%) for organic PCM, while the tightly-arranged
palisade tissue and dense epidermis cells can act as two protective layers to prevent the leakage of liquid,
enabling a mass loss as low as 1.3 wt% upon phase change. The PCM composites also show a high latent heat
approaching that of pure paraffin and an excellent thermal cycling performance with 100% retention after
Received 29th November 2017,
Accepted 6th March 2018
DOI: 10.1039/c7gc03595k
rsc.li/greenchem
being tested by using a differential scanning calorimeter 20 times. Moreover, the SCA not only serves as
thermal conductive paths within the organic matrix, thereby remarkably enhancing the thermal conductivity
of the PCM composites, but also acts as an effective photon captor and molecular heater, thus significantly
increasing the light-to-thermal energy conversion efficiency of the PCM composites. As such, the SCA is an
ideal multifunctional scaffold for PCM, which can advance the practical applications of PCM composites.
Introduction
The world is facing an energy crisis due to the growing energy
demand and the reduction of traditional resources.1–3 To
handle such a crisis, most countries have emphasized the
investigation of thermal energy, which is renewable and
expected to replace fossil fuels. Phase change materials (PCM)
are considered as one of the most effective ways to tackle the
energy crisis, because they can store and release thermal
energy during the process of melting or solidification.4–6
Among various PCM, organic PCM are particularly attractive
owing to their large latent heat, chemical stability, and
absence of super-cooling or phase segregation.7,8 However,
one of the main drawbacks of organic PCM is their unacceptably low thermal conductivity, which decreases the latent heat
and actual efficiency.9 In addition, the leakage in the melted
a
College of Chemistry and Materials Science, Sichuan Normal University,
Chengdu 610068, China. E-mail: lijuan_zhao@sicnu.edu.cn
b
State Key Laboratory of Polymer Materials Engineering, College of Polymer Science
and Engineering, Sichuan University, Chengdu 610065, China.
E-mail: wujinrong@scu.edu.cn
† Electronic supplementary information (ESI) available: SEM images of SCP-63
and SCP-95; XRD patterns of SCP-78, pure paraffin, SCA and succulent aerogel;
TGA curves of different SCP-n and the data for TGA curves; comparison of the
measured and predicted latent heat of melting of SCP with various loadings of
the SCA; light-to-thermal energy conversion and storage tests in an insulated
environment. See DOI: 10.1039/c7gc03595k
1858 | Green Chem., 2018, 20, 1858–1865
state is another challenge that hinders the practical application of the organic PCM. Accordingly, efficient energy
storage and conversion of organic PCM require not only a high
thermal conductivity but also a proper encapsulation structure
without compromising the material enthalpy.
In order to improve the thermal conductivity of organic
PCM, thermal conductive fillers are introduced to constitute
effective heat transfer paths. For example, an enhancement of
40% to 140% in thermal conductivity can be achieved through
the addition of graphene sheets, carbon nanotubes, expanded
graphite flakes, and silver or copper nanowires.10–16 However,
the thermal conductive fillers are unstable and easily aggregated in the cyclic process, thus separating the thermal conductive paths. To overcome this drawback, three-dimensional
(3D) networks with high thermal conductivity have been used
to serve as scaffolds for organic PCM to fabricate phase change
composites, including metal foams, carbon nanotube
sponges, graphene aerogels, etc.17–22 These phase change composites show improved thermal conductivity, high photo-heat
efficiency, large latent heat of fusion, and magnificent durability and strength.
Nevertheless, most reported phase change composites still
have the leakage problem in spite of good encapsulation
efficiency.23–26 To address this issue, significant research
efforts have been devoted to developing various methods to
microencapsulate PCM; examples include the suspension
polymerization method, hyperbranched method, hydrothermal
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reaction, simple or complex coacervation and interfacial polycondensation reaction.27–31 For example, the suspension
polymerization and hydrothermal reaction methods usually
use polymers, such as polystyrene (PS), melamine-formaldehyde resin, and urea-formaldehyde resin, to encapsulate
PCM.32–35 The resulting microcapsules can realize the protection of the core materials from leakage, because they have
tough shells. The hyperbranched method intercalates organic
PCM molecules between their branches, which is also an
effective way to realize the leakage-proof performance.28
However, a common drawback of these methods is the
decreased thermal conductivity of the microencapsulated PCM
due to the low thermal conductivity of polymers (about 0.15
W m−1 K−1).36,37 In addition, chemical grafting is another
effective approach to improve the leakage-proof property. The
solid–solid PCM, which are obtained by grafting long chains of
crystalline polymers on the framework of other polymers
through chemical reactions, usually decrease the phase
change enthalpy and show high transition temperatures, thus
limiting their practical application.38,39 Therefore, it is a huge
challenge to make PCM composites with enhanced thermal
conductivity, good leakage-proof performance, and high phase
change enthalpy.
Succulents, also known as water storage plants, have some
parts that are very thick and fleshy, so that they can retain
water in dry soil conditions or arid climates. Generally, leaves
of most succulents are thick and sappy, and their moisture
content is high up to 98 wt%. If the water in the leaves is
replaced with organic PCM, a composite material with a high
loading of organic PCM can be formed. Moreover, the leaves
consist of epidermis, mesophyll, and vein. Epidermis is composed of a layer of dense cells, acting as a protective layer to
prevent the leakage of fluid. Mesophyll cells are divided into
palisade tissue and spongy tissue. Palisade tissue consists of
one or two layers of tightly-arranged palisade cells, serving as
another liquid impermeable layer. Spongy tissue has a high
capacity to store a high loading of organic PCM. In addition,
succulents can be freeze dried and pyrolyzed into 3D porous
carbon material, which could maintain the tissue structure of
succulents with enhanced thermal conductivity. In this work,
we use succulents to fabricate phase change composites by
infiltrating paraffin into the succulent-based carbon network.
These phase change composites show good leakage-proof
properties, high loading efficiency and high thermal
conductivity. Therefore, 3D porous carbon materials based on
succulents can be an ideal scaffold to fabricate high-performance phase change composites, which have a wide potential
application in energy storage and release.
Results and discussion
Fabrication and morphology of the phase change composites
To obtain the phase change composites with closed-cell structure, we use a two-step strategy to fabricate the composites
(Fig. 1). In the first step, we adopt the leaves of succulents
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Fig. 1 Schematic illustration of the synthesis of SCP. The fresh leaves of
Adromischus cooperi (①) are freeze dried and then pyrolyzed so that a
SCA (②) is formed. The SCA is infused with melted paraffin (③) and is
further cooled to room temperature so that a SCP is formed (④).
(Adromischus cooperi) as the raw material. The leaves of succulents are composed of epidermis, palisade tissue, and spongy
tissue (Fig. 2a–c). The epidermis has a dense and uniform
structure, acting as a protective layer to prevent the leakage of
fluid. Palisade tissue, under the epithelial tissue, consists of
one or two layers of tightly-arranged palisade cells, whose
thickness is about 900 micrometers. Spongy tissue, as the
major component of the leaves, consists of many closed
spherical cells, whose size ranges from 200 to 250 micrometers.
The leaves of the succulent are freeze-dried to dehydrate them
with the maintenance of their original shape, and then pyrolyzed in a vacuum tube furnace to obtain the succulent-based
carbon aerogel (SCA). We find that the SCA retains the
compact framework and organizational structure of succulent
leaves, even though the volume shows a little shrinkage after
pyrolyzation. The epidermis, palisade tissue and spongy tissue
can be observed clearly in the SEM images of SCA (Fig. 2d–f),
and their structures show no damage through pyrolysis. Here,
the epidermis in SCA consists of one or two layers of keratinized
cell wall, and its cells are very densely packed. Palisade tissue is
next to the epithelial tissue and consists of palisade cells with a
thickness of 700 micrometers. Spongy tissue is composed of
many closed spherical cells, whose size ranges from 100 to
150 micrometers. These epidermis and palisade tissues could
act as two protective layers that prevent fluid leakage, while the
spongy tissue enables a high loading efficiency for PCM.
In the second step, we infuse the SCA with melted paraffin
liquid. After paraffin liquid is cooled to room temperature, we
obtain a SCA/paraffin composite (SCP), which is named SCP-n,
where n is the paraffin content in the phase change composites. The SCP displays little shape change in comparison with
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Fig. 2 The SEM images of succulents, SCA and SCP. (a–c) A fresh leaf of Adromischus cooperi is freeze dried so that a succulent aerogel is formed;
(a) spongy cell, (b) palisade cell, and (c) epidermis. (d–f ) Different parts of SCA; (d) spongy cell, (e) palisade cell, and (f ) epidermis. (g–i) Different
parts of SCP-78; (g) spongy cell, (h) palisade cell, and (i) epidermis.
SCA. In addition, the SCP has nearly the same 3D carbon
network structure and cell size as the SCA, which is shown by the
SEM images in Fig. 2g–i. It indicates that the SCA is stable
enough to host the paraffin and the closed-cell structure does
not break during the absorption process. Moreover, the SEM
images further show that the paraffin molecules are absorbed
onto the cell walls first and then fill the whole cells of SCA,
which is demonstrated in SCP-63 and SCP-95 (ESI Fig. S1†).
Here, for SCP-63, most paraffin molecules are coated on the cell
walls and the absorption increases the thickness of the cell walls.
But for SCP-95, the pores are full of paraffin molecules, which
produce a composite material with a high loading of paraffin.
The crystallization of SCP is investigated by X-ray diffraction
(XRD), and the results are shown in Fig. S2.† It is clear that
there are no obvious sharp peaks for the succulent aerogel.
But after pyrolyzation, the SCA manifests several sharp and
intensive diffraction peaks, suggesting the formation of a graphitization structure. This structure has negligible influence
on the crystallization of the infused paraffin, as both pure
paraffin and SCP show two main sharp peaks at 2θ = 21.5° and
23.8°, corresponding to (110) and (200) crystal planes of the
monoclinic paraffin, respectively.40
Leakage-proof performance of SCP
Practical thermal-storage applications require organic PCM to
possess leakage-proof performance. To demonstrate the
leakage-proof property of SCP, the samples of SCP are heated
1860 | Green Chem., 2018, 20, 1858–1865
at 80 °C for 5 hours, and this process is monitored by using a
digital camera, as shown in Fig. 3a. When the samples are
heated for 10 minutes, pure paraffin starts to melt with
obvious liquid leakage on the qualitative filter paper. CPM-85,
which is fabricated through mechanically dispersing 15 wt%
of the carbon particles from succulents in paraffin, also shows
evident leakage of paraffin liquid. The SCP-n, however, retain
the original shape with little leakage. After being heated at
80 °C for 30 minutes, the pure paraffin completely melts into
liquid. In CPM-85, most of the absorbed paraffin melts into
liquid and spreads out on the filter paper, leaving the carbon
particles on the filter paper. However, there is little liquid
leakage for SCP-n, even after having been heated for 5 hours
(Fig. 3b), which suggests that SCP-n have an excellent leakageproof property.
The excellent leakage-proof property can be attributed to
the special structure of SCA. On one hand, the pores with a
closed-cell structure have enough capillary force to hold the
PCM during the phase change process.41 On the other hand,
the tightly-arranged palisade cells and dense epidermis cells
can act as two protective layers to prevent the leakage of
paraffin liquid. Therefore, the SCA is an effective host for fabricating PCM composites with little leakage during the phase
change process. For SCP-n, the lower the content of paraffin,
the better the leakage-proof property of SCP. When the
paraffin content of SCP is 95 wt%, the value of the mass loss
(Mr) is only 5.0 wt% (see Methods for the calculation),
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Fig. 3 The leakage-proof performance of the PCM composites. (a) Photographs of the SCP-n samples at 80 °C for 10 minutes and 30 minutes. (b)
The mass loss of the SCP-n samples after having been heated at 80 °C for 5 hours.
suggesting that SCA has an encapsulation efficiency for
paraffin without compromising the leakage-proof performance. As the content of paraffin further decreases from 95 wt%
to 63 wt%, the Mr is further reduced to 1.3 wt% (Fig. 3b). This
is because there are empty holes when the paraffin does not
fully fill the pores of SCA; thus the resulting composites have a
sufficient capacity to allow paraffin volume changes during the
phase transition process, bringing about excellent leakageproof performance for paraffin.
Latent heat of SCP
Latent heat is an important factor for the practical application
of PCM, and it is determined by thermal stability, phase
change temperature and specific phase change enthalpy. The
thermal stability of SCP is measured by TGA, and the results
are shown in ESI Fig. S3 and Table S1.† For all the samples,
while the single-step degradation of paraffin takes place in the
temperature range from 150 to 400 °C, there is no obvious
mass loss at temperatures lower than 150 °C, suggesting that
SCP composites are stable in the temperature range of phase
transition for energy storage application. For pure paraffin, the
outset degradation temperature is about 326 °C, and the presence of SCA in SCP composites affects the decomposition of
paraffin due to the strong absorption affinity of SCA to paraffin
wax, leading to the delay of the outset degradation temperature.42 From the TGA curves, the paraffin mass content of SCP
is calculated, which is 63, 78, 87 and 95 wt%. These results are
consistent with the encapsulation efficiency of the SCA (62.6,
77.4, 87.2 and 94.8 wt%) measured by the weight of SCA
before and after infusion. Moreover, the different encapsulation efficiencies can be regulated by adjusting the absorption
time of paraffin liquid. It is worth noting that the paraffin
content in SCAs can be as high as 95 wt%, which is comparable to the maximum loading amount reported for other PCM
composites based on graphene aerogels or carbon
sponges.20,22,43
For organic PCM, the low thermal conductivity reduces the
rate of energy storage and release, thereby restricting their
application. As we know, carbon materials have a high thermal
conductivity (k) due to efficient heat transfer by lattice
vibrations.44,45 Therefore, introducing the 3D carbon network
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in organic PCM is an effective way to improve the k value.
Here, we measure the thermal conductivity by a transient
plane source method, and the results are shown in Fig. 4a.
The k value of SCP-87 is 0.427 W m−1 K−1, which is about 72%
higher than that of pure paraffin of 0.248 W m−1 K−1. The
thermal conductivity shows a slight decrease as the content of
paraffin increases; yet it is still higher than that of pure
paraffin (Fig. 4a), due to the existence of interconnected and
conductive carbon networks in the organic matrix.
The energy storage properties of the PCM composites,
including the phase transition temperatures and phase change
enthalpy, were characterized by differential scanning calorimetry (DSC) tests. According to the results of DSC (Fig. 4b and
Table 1), both pure paraffin and SCP basically have the same
onset point in the melting and freezing process. Moreover,
both pure paraffin and SCP have two phase change peaks. The
first peak represents a solid–solid phase change and the
Fig. 4 Thermal properties of SCP. (a) Thermal conductivity of the SCPn samples. (b) DSC curves of the melting and freezing process of pure
paraffin and SCP-n. (c) The measured latent heat of the SCP-n samples.
(d) The measured latent heats of SCP-78 during 20 melting–freezing
cycles; the inset part shows the DSC curves of SCP-78 tested for 20
cycles.
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Table 1
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Data for DSC curves for the SCP-n samples
Melting process
Freezing process
Sample
Onset (°C)
Peak (°C)
Outset (°C)
ΔHm (J g−1)
Onset (°C)
Peak (°C)
Outset (°C)
ΔHf (J g−1)
Paraffin
SCP-95
SCP-87
SCP-78
SCP-63
40.8
40.8
40.9
41.1
41.0
61.7
61.9
63.0
63.9
63.5
66.6
69.2
69.2
70.9
70.1
135.6
133.1
110.9
98.7
83.6
60.5
60.4
60.5
60.4
60.4
57.4
56.5
56.1
54.3
54.5
35.5
35.1
34.6
34.5
34.3
150.4
147.7
120.8
108.6
98.8
According to the DCS curves of SCP-n (Fig. 4b), the data are summarized in this table, including the onset point, peak temperature, outset point
and phase change enthalpy during the melting and freezing process.
second peak represents a solid–liquid phase change. The
incorporation of the SCA network delays the melting and freezing peak temperatures of the solid–liquid phase change
process, which shift from 61.7 and 57.4 °C to 63.9 and 54.3 °C
for SCP-78, respectively. This change is possibly due to the
strong absorption affinity of SCA to paraffin wax, which affects
the surrounding organic molecules near the SCA network and
delays the phase change of paraffin.42
For good PCM, the high phase change enthalpy is a crucial
factor. The phase change enthalpy of SCP is investigated by
DSC, and the results are shown in Fig. 4b and c. For the pure
paraffin, the melting enthalpy (ΔHm) is found to be 135.6 J g−1.
When the paraffin content of SCP is 95 wt%, the melting
enthalpy is 130.1 J g−1, which is only 1.8% lower than that of
pure paraffin. As the content of paraffin further decreases from
95 wt% to 63 wt%, the melting enthalpy shows a slight decrease
(Fig. 4c). For the freezing enthalpy (ΔHf ) of SCP-n samples, it
increases as the paraffin content increases, showing the same
change trend as that of melting enthalpy. It suggests that the
addition of the SCA reduces the energy storage capacity of the
PCM composites, due to the fact that SCA do not contribute to
latent heat storage. However, the decreased energy storage
capacity of SCP seems to be acceptable. According to the
mixture theory, the melting enthalpy of the SCP composites can
be predicted by a simple linear function of the loading, as
follows:
are shown in Fig. 4d. It is clear that the 20 cyclic DSC curves of
SCP-78 are virtually identical. Moreover, the value of the phase
change enthalpy basically remains the same, only exhibiting a
loss of ≤0.6% after the 20th cycle in comparison with the first
cycle. It suggests that the SCP has excellent cyclic stability.
Light-to-thermal energy conversion and storage
In order to directly visualize the thermal conversion and
storage of the SCP, we put a sample of SCP-63 on a paperboard, and then place a 125 W heater over the sample with a
distance of 20 centimeters. The heater, the generator of
thermal energy, is used to simulate heating and cooling processes by controlling the switch (Fig. 5a). The temperature
change of the sample is recorded by using an infrared camera
at an environmental temperature of 30 °C and the temperature-distribution images taken at different times are shown in
Fig. 5b and c. The SCP-63 sample has obviously lower temperature than that of the paperboard during the heating process,
and its color changes faster than that of pure paraffin. It indicates that the SCA can improve the thermal transmission of
SCP-63. But during the cooling process, the surface tempera-
ΔH eff ¼ ΔH m ð1 ϕwt Þ:
Here, ΔHeff and ΔHm are the predicted effective melting
enthalpy of the SCP-n samples and the melting enthalpy of the
pure paraffin, respectively; ϕwt is the mass fraction of the SCA.
We find that the measured melting enthalpy of SCP-95 is
higher than the predicted value (Fig. S4†), which along with
the improved thermal conductivity indicates that SCP-95 is a
promising high-performance PCM composite. However, as the
content of SCA further increases from 13 wt% to 37 wt%, the
measured melting enthalpy is lower than the predicted value.
Such a phenomenon can be understood in terms of the stronger absorption affinity of heavily loaded SCA to paraffin wax,
making the molecules of the matrix PCM difficult to
rearrange.46
In addition, to prove the thermal cyclic stability of the composites, 20 cycling tests are measured by DSC, and the results
1862 | Green Chem., 2018, 20, 1858–1865
Fig. 5 The light-to-thermal energy conversion and storage. (a) A schematic of the measuring system. Thermal transmission evolution of
SCP-63 and pure paraffin during the (b) heating process and (c) cooling
process. (d) Light-to-thermal energy conversion curves for pure
paraffin, SCP-63, and the paperboard.
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ture of the sample is higher than that of the paperboard. We
randomly select one point on SCP-63 as point A, one point on
the paperboard as point B, and another point on the pure
paraffin as point C, and then follow the temperature change of
these points as a function of time during the heating and
cooling process, as shown in Fig. 5d. It can be seen that the
temperature of both point A and B rises rapidly first and then
levels off during the heating process, while the temperature of
pure paraffin increases slowly at first and then rapidly after a
certain time. Point A takes 200 seconds to level off in its temperature, which is longer than 150 seconds of point
B. Moreover, the temperature of point A is 5–18 °C lower than
that of point B. This is mainly caused by the melting of the
paraffin, which absorbs much heat, thus preventing the rise of
the surface temperature. After turning off the heater, the temperature of all points drops drastically first and then gradually
levels to room temperature. But the temperature of the paperboard changes faster than that of point A or point C. Point B
takes 200 seconds to level off in its temperature, while point A
takes 320 seconds to do so. And the temperature of point A is
6–12 °C higher than that of point B. It is mainly due to the
freezing of PCM, which can release heat, thus preventing the
reduction of the surface temperature. These phenomena
suggest that the SCP indeed shows good thermal storage
property.
Moreover, we can define the light-to-thermal energy conversion efficiency (η) of SCP as the ratio between heat stored in
the composites and the light radiation energy received during
the phase-change period.38,42,47 The value can be calculated by
the following equation.
η ¼ m ΔH=ðP S ΔtÞ
ð1Þ
where m is the mass of the sample, ΔH is the phase-change
enthalpy obtained by DSC, P is the light irradiation intensity of
300 mW cm−2, S is the surface area, and Δt is the phasechange time.
According to eqn (1), the η values of pure paraffin and
SCP-63 are calculated to be about 63% and 82%, respectively.
This efficiency improvement of SCP is ascribed to the function
of SCA as an effective photon captor and molecular heater by
absorbing light radiation and converting it to thermal energy,
and storing the thermal energy in the SCP.
To further confirm the light-to-thermal energy conversion
efficiency of SCP, an illumination experiment in an insulated
environment is necessary. We perform this test in a vacuum
oven with a pressure of about 20 Pa; this low pressure is used
to approximate the insulated environment, as the measurement in a strictly insulated environment is extremely time consuming. Here, the samples were also placed on a paperboard,
and then directly illuminated by a heater of 40 W, which was
placed over the samples with a distance of about 10 centimeters. The temperature change was recorded by using a
digital thermometer (TES-1310). The schematic of the measuring system and the light-to-thermal energy conversion curves
for pure paraffin, SCP-63, and the paperboard in a vacuum
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environment are shown in Fig. S5.† The results are basically
similar to those in Fig. 5. According to eqn (1), the η values of
pure paraffin and SCP-63 in the insulated environment are calculated to be about 65% and 73%, respectively. This again confirms the higher light-to-thermal energy conversion efficiency
of SCP.
Conclusion
We present that succulent-based carbon aerogels with a
unique tissue structure can serve as a practical host for fabricating PCM with enhanced thermal conductivity and good
leakage-proof performance. As a proof of concept, we infiltrate
paraffin into SCA to acquire SCP with the preservation of the
tissue structure of succulent leaves. In the composites, the
carbon network from succulents is the key factor, which not
only enables an enhanced thermal conductivity as high as
0.427 W m−1 K−1, but also results in excellent leakage-proof
performance. Furthermore, such PCM composites show outstanding thermal stability with almost 100% retention after
being tested 20 times by DSC, and large phase change
enthalpy with a ΔHm of 133.1 J g−1 and a ΔHf of 147.7 J g−1
comparable to pure paraffin. Our method represents an important step to develop leakage-proof PCM with high performance
and enhanced thermal conductivity. Thus we believe our
materials can be widely applied to energy conversion and
storage.
Methods
Materials
Paraffin wax obtained from Chengdu Kelong Chemical
Reagent Co. Ltd (China) was chosen as the PCM, which is a
mixture of purified paraffin and microcrystalline waxes. The
mass fraction of paraffin wax is 97–98 wt%. The density is
0.91 g cm−1 at 13 °C, and the kinematic viscosity is
3.94 mm2 s−1 at 100 °C. Succulents (Adromischus cooperi) were
purchased from a flower market.
The fabrication of biomass carbon aerogels
Biomass carbon aerogels from succulents (SCA) were fabricated by pyrolyzing succulents. The fresh leaves were frozen at
−10 °C for 6 hours and then freeze-dried for 36 hours to be dehydrated with a freeze-dryer (the temperature of the cold trap
is −55 °C, and the sample temperature is about 2 °C, <20 Pa).
The resulting samples were pyrolyzed at a rate of 5 °C min−1 to
1000 °C under vacuum conditions, and were made to maintain
this temperature for 1 hour. Then, they were cooled down at a
rate of 8 °C min−1 to room temperature and finally SCA were
formed from them.
The fabrication of PCM composites
The PCM composites were fabricated by absorbing molten
paraffin. The paraffin was first heated at 80 °C (above the
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melting point of paraffin, 61 °C, measured by using a differential scanning calorimeter in Fig. 4a) in a vacuum oven. Then
the prefabricated SCA were immersed in melted paraffin.
Having been soaked for 10–60 minutes, the samples were
taken out from the liquid and cooled to room temperature so
that biomass succulent carbon/paraffin PCM composites (SCP)
were formed. And the extra paraffin coated on the composites
surface was scraped. The paraffin content of the composites
was tested by measuring the weight difference of SCA before
and after immersion in melted paraffin.
Leakage test of PCM composites
The leakage-proof performance of SCP was monitored by
leakage tests. In this case, the PCM composites were first
wrapped up in qualitative filter paper. Then, the samples were
placed into an 80 °C vacuum oven and kept for 5 hours. The
mass loss (Mr) of the SCP was calculated according to the
following equation
W1 W2
100%
ð2Þ
Mr ð%Þ ¼
W1
Here, W1 and W2 are the weight of the composites before
and after calefaction in the vacuum oven, respectively.
Light-to-thermal energy conversion and storage
To directly visualize the thermal conversion and storage property of the phase change composites, an illumination experiment was performed. Here, the samples were placed on a
paperboard, and then directly illuminated by a heater of 125
W, which was placed over the samples with a distance of about
20 centimeters. The temperature change was recorded by
using an infrared camera (FLIR T420, FLIR Systems AB,
Sweden).
Characterization
The morphology of SCA and SCP was examined by using a
scanning electron microscope (SEM, Zeiss Ultra 55, USA) at an
acceleration voltage of 1 to 15 kV. Before the test, the samples
were coated with Au nano-powder under a current of 20 milliamperes for 2 minutes. X-ray diffraction (XRD) patterns were
recorded on a diffractometer (Smartlab, Rigaku) with Ni-filtered CuKα radiation (k = 0.154 nm) at a tube current of 30 mA
and a generator voltage of 40 kV. Scanning was performed at a
speed of 8 °C min−1, from 0 to 80° of 2θ. The thermal conductivity of the samples was measured by using a thermal conductivity meter (Hot Disk 2500-OT, Sweden), based on the transient plane source method. All the thermal conductivity
measurements were taken when the samples were in the solid
state at a temperature of 30 °C. For this test, sensor 7577 with
a radius of 2.001 millimeters was installed between the two
samples, which were cut into cylinders with a height of about
0.5 centimeters and a diameter of about 1.2 centimeters. The
thermal stability and the content of paraffin in SCP-n were
measured by using a thermal gravimetric analyzer (TGA,
Discovery, USA). The SCP was heated from room temperature
1864 | Green Chem., 2018, 20, 1858–1865
Green Chemistry
to 700 °C at a heating rate of 10 °C min−1 under a nitrogen
atmosphere, and their paraffin contents were measured by
TGA. The phase change temperature and specific phase
change enthalpy of the PCM composites were tested on a
differential scanning calorimeter (DSC, Discovery, USA) under
a nitrogen atmosphere. Each sample weighed about 5 mg and
was heated to 90 °C at a rate of 10 °C min−1 to erase the previous thermal history. Then the data were collected when the
sample was cooled to −10 °C and later heated to 90 °C at a rate
of 10 °C min−1. The experiments of SCP on their thermal stability were conducted 20 times by following the same heating–
cooling steps.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 51303116). The
authors thank Mr Siwen Feng from Sichuan Normal University
for polishing the writing.
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