1. Introduction
Nowadays, the demand for energy is constantly increasing. All the universal developments
require increasingly more sources of energy, and the building sector consumes approximately
40% of the global energy (Aridi and Yehya, 2022). Consequently, the building industry directly impacts CO2, greenhouse emissions, and global warming (Esabati et al., 2020; Urgessa et al., 2019). Because of the environmental aspect, elevated energy consumption, and depletion
of natural resources, there have been attempts of developing new technologies that
can reduce the usage of conventional energy resources and change the orientation towards
sustainable and renewable energy resources (Aridi and Yehya, 2022). Thermal energy storage (TES) is a key technology as a new environmental energy source
that can reduce the dependence on conventional resources such as fossil fuels and
is thus considered environmental friendly (Anupam et al., 2020). Latent heat storage (LHS) is the main technology used in TES. In LHS, the PCM works
as a thermal energy storage medium (Tao and He, 2018). Compared with other heat storage media, PCMs have a high thermal energy storage
density, and they can store and release a large amount of heat energy when they undergo
a phase change from solid to liquid or liquid to solid (Urgessa et al., 2019). Owing to this ability, PCM materials have been widely used in many sectors, such
as buildings and construction, food packaging, air conditioning systems, and underfloor
heating systems (Guo et al., 2020).
PCM can be incorporated into construction materials to obtain thermal response structures.
For high-temperature applications, it can be mixed with an asphalt binder to extend
the service life of pavements and reduce maintenance costs by reducing the extreme
temperature fluctuations of the asphalt mixture. PCM can be considered a thermoregulation
medium for asphalt pavements (Guo et al., 2020; Si et al., 2015).
PCM has attracted attention of many researchers, and the effects of incorporating
PCM on the thermal and mechanical properties of concrete have been studied. For PCM
with a low transition temperature, incorporating microencapsulated PCM prevents the
concrete slab subjected to cold wither in the field from excessive temperature drop
during winter, and the number of freezing/thawing cycles can be reduced, resulting
in probable extension in the service life by up to 5.2-35.9% (Urgessa et al., 2019). In another study by Y. Farnam, PCM was embedded in concrete slabs in two ways, with
lightweight aggregate and metallic pipes to study its ability to melt snow and ice
for concrete pavement, which showed good potential to melt snow during winter (Farnam et al., 2017). Yeon and Kim (2018) investigated the effect of PCM in concrete pavements, in reducing the freeze-thaw
deterioration using organic paraffine (N-Tetradecane) micro-PCM (tiny melamine- formaldehyde
resin shells coat the PCM) in slurry form with a phase transition temperature of 4.5℃.
Use of micro-PCM can reduce freeze-thaw deterioration and amplitude of temperature
fluctuations, which can increase the service life of concrete pavements. However,
the use of PCM negatively impacts the mechanical properties of mortars as it reduces
the compressive and flexural strengths of cement mortars (Yeon and Kim, 2018).
Different techniques can be used to incorporate PCM into the matrix of the construction
materials. The direct incorporation method is simply the incorporation of liquid or
powdered PCM directly into concrete, plaster, gypsum, or any building material, without
using any tools or special equipment (Lu et al., 2017; [1]Al-Yasiri and Szabó, 2021). Even though this method is economical and does not
require experience, it has a leakage problem during the melting process of PCM, that
is, the incompatibility with construction materials increases the fire risk, especially
for flammable PCMs ([1]Al-Yasiri and Szabó, 2021). In addition, it can negatively
affect the mechanical performance of the construction elements, where the liquid PCM,
when added to the mixture, reduces the water content ratio (Pereira da Cunha and Eames, 2016). In addition, the leaked PCM can inhibit the hydration process by coating the unreacted
cement particles, thus preventing contact with water (Sakulich and Bentz, 2012).
Generally, the composite phase change material (CPCM) consists of two main parts:
a PCM which works as a latent heat storage core material and a carrier material that
encapsulates the PCM in a fixed shape for machinability and preventing leakage (Guo et al., 2020).
Many techniques are available to produce CPCM, such as the direct immersion method
(directly immersing PCM into porous supporting material) and microencapsulation (consisting
of PCM works as core and layer of other material works as carrier), where formaldehyde
resins are used to protect PCM (melamine- and urea-formaldehyde resin shell materials,
for example), which can release poison formaldehyde; the microencapsulated PCMs are
easily flammable, therefore their applications are restricted (Cai et al., 2009; Fang et al., 2010). In the SOL-GEL method, the PCM is absorbed in a gel during the preparation process
by capillary force and surface tension (Guo et al., 2020). Shape-stabilised PCMs are also considered effective in preventing leakage problems
in addition to overcoming the low thermal conductivity of PCM (especially organic
PCM) by loading it into porous materials or nanomaterials (Rathore and Shukla, 2021). The SOL-GEL technique can be used to obtain CPCM. It is a simple method in which
inorganic materials such as silica shells are formed and coat the PCM drops to obtain
CPCM. This method starts with SOL (colloidal suspension), which is dissolving in ethanol
or alcohol and then hydrolysing to silicic acid, which produces a silica gel network
during the condensation process (Ren et al., 2014; Zhang et al., 2010). The SOL-to-GEL transition is driven by covalent crosslinking or van der Waals forces.
SOL-GEL can be applied under specific experimental conditions of reaction time, temperature,
PH value, and solution concentrations.
Ren et al. (2014) prepared different types of CPCM through SOL-GEL using three absorbent materials
(silica powder, floating beads, and activated carbon) to adsorb PCM. The coating effectiveness
was studied using scanning electron microscopy (SEM), and the results showed that
the CPCM prepared using activated carbon-adsorbed PCM (AC-PCM) had better coating
effectiveness. In addition, using a silane coupling agent has a positive effect on
the particle distribution of the composite when it is used in a specific ratio with
respect to tetraethyl orthosilicate (TEOS) (Ren et al., 2014).
Another study by Li prepared paraffin/silicon dioxide/ expanded graphite (EG) CPCM
using SOL-GEL. EG was used to increase the thermal conductivity, and silica gel worked
as the supporting material. These results revealed that the composite was chemically
stable. The latent heat of the composite with and without EG were 112.8 J/g and 104.4
J/g, respectively. Both silica gel and EG can increase the thermal conductivity of
the PCM, and the SiO2/paraffin composite has a 28.2% higher thermal conductivity than pure paraffin, while
EG can increase the thermal conductivity to 94.7% (Li et al., 2012).
Fang et al.(2010) prepared a microcapsulated paraffin composite with a silica shell using the SOL-GEL
method, in which TEOS was used as the precursor. The encapsulation ratio reached 87.5%
with latent heat of 107.05 kJ/kg at solidification temperature of 58.27℃ obtained
via differential scanning calorimetry (DSC) test. The thermogravimetric analysis (TGA)
results show that the thermal stability and flammability of the composite can be improved
by using a silica shell (Fang et al., 2010). The silica (SiO2) shell formed through the SOL-GEL method has desirable properties, such as it is
non-inflammable, can provide higher mechanical strength, thermal conductivity, and
better chemical resistance to the PCM, and can act as a pozzolanic material in building
applications (Ishak et al., 2020).
In this study, the characteristics of CPCM produced by the SOL-GEL method were studied,
where activated carbon (AC) was used as the carrier and TEOS was used as the source
of silica gel, which can coat and cover the tetradecane PCM. The thermal performance
and stability were analysed using DSC and TGA, respectively. In addition, its chemical
stability was studied using Fourier-transform infrared (FT-IR) spectroscopy, and its
surface morphology was analysed using SEM.
3. Results and Discussion
3.1 DSC and TGA Results
The DSC results of the AC-PCM are presented in Fig. 5. The enthalpy of the AC-PCM, as shown in Fig. 5(a and b), was 62.13 and 57.28 J/g during heating and cooling processes and the corresponding
peak temperatures were 5.6℃ and 3.2℃, respectively. Considering oversaturated condition
of AC-PCM, that is, an extra amount of free PCM, it was coated by the silica gel during
the SOL-GEL process.
Fig. 5. AC-PCM DSC Results: (a) Endothermic Phase, (b) Exothermic Phase
Fig. 6 shows the DSC curve of the CPCM prepared using the SOL-GEL method. From the Fig. 6(a) and (b), the enthalpy values of CPCM during the heating and cooling process were
observed to be 32.98 and 27.82 J/g, respectively. Where peak temperature for the melting
was 7.1℃, and two peak temperatures at 2.4℃ and -7.6℃ during the solidification process.
The actual (effective) PCM content in CPCM can be calculated using the following formula:
where ∆H is the phase-change enthalpy of the CPCM, H is the phase-change enthalpy
of the tetradecane, and P is the mass content (%) in the sample. The effective mass
content of tetradecane in the composite was calculated as 14.74% using Eq. (1).
By analysing the test results, a change in the peak temperatures and enthalpy values
was observed for the pure PCM, AC-PCM, and CPCM. The peak temperatures for the pure
PCM were 1.3℃ and 6.4℃ for the freezing and melting processes, respectively, while
for AC-PCM they were 3.2℃ and 5.6℃, respectively. The same change was observed in
the CPCM, where its solidification temperature was 2.4℃, which was higher than that
for pure tetradecane. The change in peak temperatures that occurred for both the AC-PCM
and CPCM can be attributed to the effect of pore size due to the PCM impregnation,
which reduces the freezing temperature (Farnam et al., 2017), and the change in enthalpy values is due to the change in the PCM content in the
composite.
The thermal stability of the AC-PCM substance, pure tetradecane, silica gel, and CPCM
were studied using TGA. Fig. 7 shows the thermal stability of the components and the weight loss in the temperature
range from room temperature up to 950℃. TGA results showed that the AC exhibited good
thermal stability up to 500℃, where no decomposition was observed. The curve showed
a one-step mass loss, where the AC mass loss sharply dropped at approximately 500℃;
it was totally decomposed approximately above 625℃, where the residue mass was approximately
1.365% of the initial weight. The pure tetradecane curve showed a one-step mass loss,
which started at slow rate at approximately 120-160℃, then rapidly decreased until
240℃, where it was totally decomposed. According to the results, AC is a suitable
supporting material for tetradecane PCM because of its higher decomposition temperature
than that of pure tetradecane.
The AC-PCM TGA curve showed a two-step mass loss due to the decomposition of tetradecane
and AC. From Fig. 7, the PCM content in the AC-PCM composite can be estimated to be approximately 50%,
which is equal to the maximum weight within the first step of mass loss. However,
this value of PCM content represents the tetradecane inside, on the surface, and between
the AC particles, because of its oversaturated condition. There was mass loss in temperature
range of 80-360℃.
The TGA curve of the silica gel produced by the SOL-GEL method exhibited a two-step
mass loss. The first degradation step started at approximately 100℃, which was mainly
due to the incomplete removal of water and ethanol in the samples after oven drying,
it contributed to approximately 10% of the sample weight. The second step of degradation
started at approximately 200-400℃, which represented more condensation of silanol.
The total mass loss in the silica gel sample was approximately 40%, and the remaining
60% of the sample weight represented silica, which did not decompose with an increase
in the temperature.
In the TGA curve of the CPCM obtained using SOL-GEL technique, the first degradation
step started at approximately 100-340℃, which was mainly due to the decomposition
of PCM. Also, the second mass loss was at approximately 500℃ due to decomposition
of AC. The residual mass was approximately 13.14% of the total sample weight, which
represented the mass of silica that did not decompose.
However, the mass loss behaviours of the pure PCM, AC-PCM, and CPCM were almost the
same, except for AC-PCM and CPCM, where the mass loss started earlier than the pure
tetradecane, which can be attributed to the different physical behaviours of free
and confined PCMs in the pores of the AC (Memon et al., 2015). In the comparison between the AC-PCM and CPCM cases, the performance of the CPCM
was better owing to the coating effect after the SOL-GEL process.
Fig. 6. CPCM DSC Results: (a) Endothermic Phase, (b) Exothermic Phase
Fig. 7. Thermal Stability of Bulk PCM, AC, Silica Shell (SiO2), AC-PCM and CPCM
3.2 FT-IR Spectroscopy Results
FT-IR spectroscopy was used to study the chemical stability and compatibility of the
system components and conducted on pure tetradecane, AC-PCM, silica gel, and CPCM
produced by the SOL-GEL method.
Fig. 8(a) shows the FT-IR spectrum of pure tetradecane, where the peaks at 1466 cm-1 and 1375 cm-1 represent the C-H bonding vibrations as a result of the methylene bridges, and the
band peaks at 2927 and 2852 cm-1 were attributed to C-H asymmetric and symmetric stretching, respectively. In addition,
the peak at 718 cm-1 was due to the in-plane rocking vibration of the methylene group.
Fig. 8(b) shows the FT-IR spectra of the AC-PCM after the vacuum impregnation process, silica
gel (SiO2), and final CPCM. For the AC-PCM case, no new peaks appeared in the spectra, whereas
the same peaks were observed for pure tetradecane. This can be considered as an evidence
of only the physical interaction between AC and tetradecane. In other words, AC is
inert. For the silica gel (SiO2) produced by the SOL-GEL method, the peak at 1083 cm-1 represented the Si-O-Si asymmetric stretching vibration, and that at 794 cm-1 was the Si-O-Si symmetric stretching vibration peak. The peak at 459.03 cm-1 corresponded to the Si-O-Si bending vibration peak. The stretching vibration of Si-OH
was represented at 945.07 cm-1 peak. The peak at 3456.28 cm-1 corresponded to the stretching vibration of -OH, while the 3456.28 cm-1 corresponded to the bending vibration of -OH. As shown in the CPCM FT-IR spectrum,
no new peaks were observed, indicating occurrence of no major chemical interactions
between tetradecane, AC, and SiO2. Thus, it can be inferred that CPCM is chemically stable, and the interaction between
the components is physical.
Fig. 8. FT-IR Spectra of (a) Pure Tetradecane PCM, (b) Silica Shell (SiO2), AC-PCM, CPCM
3.3 Surface Morphology Results
The pore structure and surface morphology of AC, AC-PCM after vacuum impregnation,
and CPCM prepared by the SOL-GEL method were analysed using SEM.
Fig. 9(a) and (b) show SEM images of the raw AC. As shown, the porous structure of the AC provided
space for the liquid to be adsorbed and reduced the leakage problem due to the different
interactions of surface tension, capillary force, Van der Waals’ force, or hydrogen
bond (Huang et al., 2019). In addition, it provided mechanical strength to the composites. Fig. 9(c) and (d) show SEM images of the AC-PCM after the vacuum impregnation process. Fig. 9(c) shows that liquid tetradecane filled the AC pores and the AC-PCM was oversaturated.
Thus, a free liquid PCM was observed on the surface and between the AC particles.
Fig. 9(d) shows that the pores were empty of PCM, which may be due to the leakage of melted
PCM over time, or the pores were superficial or minute to adsorb PCM.
The surface morphology of the CPCM is illustrated in Fig. 9(e) and (f). The CPCM showed that a thin layer of silica gel formed on the surface of
the AC-PCM particles. Owing to the good SOL-GEL coating, no pores were observed on
the surface of the AC. This effectively eliminated the PCM leakage phenomena.
Fig. 9. SEM Images of AC Surface: (a, b) AC, (c, d) AC-PCM, (e, f) CPCM
4. Conclusion
In this research, we aimed to study and analyse the thermal properties of a CPCM prepared
by the SOL-GEL method. The first step was to obtain AC-PCM composite, which was prepared
by the vacuum impregnation method, where AC was used as a supporting material for
tetradecane. The AC-PCM composite was then coated with a thin shell of silica gel
through the SOL-GEL process. The thermal response, thermal stability, surface morphology,
and chemical compatibility of the final composites were evaluated using differential
scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy
(SEM), and Fourier transform infrared (FT-IR) spectroscopy. After performing the aforementioned
experiments and analysing the results, we concluded the following:
(1) DSC results for the CPCM showed that the effective PCM content in the composite
PCM was approximately 14.74%. This value is sufficient and can be used in construction
materials for low-temperature applications.
(2) The TGA results illustrated that the AC is a good supporting material for the
tetradecane; its decomposition started approximately at 500℃, which was a much higher
temperature than that of the pure PCM, approximately at 120℃. The AC-PCM and CPCM
cases undergo mass loss at lower temperatures than the pure PCM, which can be due
to the different physical behaviours between the free and confined PCMs in the structure
of the porous material.
(3) FT-IR analysis showed that the CPCM is chemically stable. No newly formed peak
and shift (only the vertical intensity was changed, reflecting the material concentration)
in the CPCM spectra confirmed that the interaction between the components was physical.
(4) SEM images showed that the AC has many pores, which can be considered a good supporting
material for the PCM. The SEM images of AC-PCM showed that the PCM was in the pores
and on the surface of the AC particles. In addition, SEM of the CPCM showed a thin
layer of silica gel formed on the AC-PCM surface, which reduced the leakage problem.