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.

2. Experimental Program

2.1 Materials

In this study, N-tetradecane, which is a paraffin-based organic PCM (purity: 99%, phase transition temperature: 6℃), was acquired from ISU CHEMICAL Co., Ltd., South Korea and was used for the PCM. Fig. 1 shows the DSC results of pure tetradecane, which has a heat storage capacity of 223.8 J/g and 226 J/g during the solidification and melting processes, respectively. Commercially available granular AC with mesh size of 20-40 was used as the supporting material, which has a specific gravity of 1.41 and an absorption capacity of 48.9% according to ASTM C128. Compared with tetradecane, AC has a high thermal conductivity of 1.01 W/m K ^{(Lee et al., 2017)}. TEOS or tetraethoxysilane, with chemical formula Si(OC₂H₅)4, was acquired from Daejung Chemicals, South Korea (purity: 98%) and used as the silica source. It is the most commonly used silica precursor in silica-based preparation methods and the type of metal alkoxide that produces the gel, which can encapsulate PCM particles after chemical reactions during the SOL-GEL process ^{(Ren et al., 2014; Zhang et al., 2010)}. The properties of the TEOS are presented in Table 2. Hydrochloric Acid (HCL) and ammonium hydroxide (NH₄OH) work as catalyzer. A 3-(trimethoxysilyl)propyl methacrylate silane coupling agent was used to improve the stability of the SOL. Ethanol (EtOH) and distilled water were used as the solvents for TEOS. The physical properties of the AC are presented in Table 1, and the AC particles and their verified porous structures are shown in Fig. 2(a) and (b), respectively.

Fig. 1. Bulk Phase Change Material (PCM) Differential Scanning Calorimetry DSC Results: (a) Endothermic Phase, (b) Exothermic Phase

Fig. 2. Activated Carbon (AC) Used in this Study: (a) Raw AC, (b) SEM Image of AC

Table 1. Physical Properties of the Activated Carbon (AC) according to ASTM C128

Physical Properties	Obtained Values
Relative Density (Specific Gravity) (OD)	0.95
Relative Density (Specific Gravity) (SSD)	1.41
Apparent Relative Density	1.76
Water Absorption	48.9%
Bulk Density	~0.5 kg/L

Table 2. The Properties of Tetraethyl Orthosilicate TEOS

	Color and Odor	Molecular Formula	Molecular Weight	Density
TEOS	Colorless Liquid, Alcohol-Like Odor	C8H20O4Si or (C2H5O)4Si	208.33 g·mol-1	0.933 g/ml at 20℃

2.2 Preparation of the Composite Phase Change Material by SOL-GEL

2.2.1 Vacuum Impregnation Process

The vacuum impregnation technique was used to ensure that the PCM filled the AC pores. First, AC particles were placed in a tray and oven dried for 24 h at 110℃, aimed at eliminating any moisture content within the particles. Then, the AC (~400 mL was filled in a 1 L beaker) and tetradecane were poured until AC was fully submerged and mixed together for 3 min. The AC and tetradecane were then placed inside a vacuum desiccator, and vacuum pressure was applied, until it reached -0.1 MPa, for 4 h. After the vacuum was created and no air bubbles appeared on the PCM surface, the vacuum process was stopped, and air was allowed to reenter and push the tetradecane into the pores of the AC. Fig. 3 illustrates the vacuum-impregnation process.

Fig. 3. Vacuum Impregnation Setup

2.2.2 SOL-GEL Process

The obtained composite of AC adsorbed PCM (AC-PCM) was coated using SOL-GEL method. Fig. 4 shows the complete procedure of CPCM preparations using the SOL-GEL process. First, TEOS and ethanol were mixed together in 1:4 molar ratio. 100 g of TEOS and 88.45 g of ethanol were poured in 500 ml beaker and mixed using magnetic stirrer for 10 min under controlled temperature water bath of 60℃. Second, 69.19 g of distilled water (8:1 distilled water and TEOS molar ratio) and 10 g of (3-(trimethoxysilyl) propyl methacrylate) silane coupling agent (10% of the mass of TEOS) were added to the solution while stirring; the temperature of the water bath was increased and maintained at 80℃ and the mixture was stirred for 20 min ^{(Ren et al., 2014)}. After 20 min of mixing, to bring the pH value of the solution between 3-4, a drop of the diluted HCL was added and the solution was stirred for 5 min. Using a pH meter, the pH value of approximately 3.5 was obtained. Finally, drops of ammonium hydroxide and 100 g of the AC-PCM composite were added to the mixture and mixed until silica gel was formed. Ammonium hydroxide was added to increase the pH value of the solution in the range of 7-8. HCL was added to increase the gelation rate; ammonium hydroxide can reduce the gelation time ^{(Ren et al., 2014)}. During the SOL-GEL transition, the following chemical reactions occur ^{(Jitianu et al., 2003)}:

Fig. 4. SOL-GEL Synthesis Process of CPCM

I. Hydrolysis Reaction

During the hydrolysis for TEOS, the SOL solution, which functions as an encapsulation precursor, is obtained.

where, Si represents a Si atom.

O: oxygen atom.

R: alkyl C₂H₅ in the TEOS case.

OH: the hydroxide ion.

(—): represent the chemical bond.

II. Condensation Reaction

Condensation reactions start once the hydroxide is formed, and two types of condensation reactions take place, which are:

(1) The reaction of alcohol losing condensation can be presented as follows:

After the formation of the silica gel, the magnetic stirrer was stopped and the beaker was taken out from the water bath and placed in an electrical oven at a constant temperature of 80℃ for 24 h. Subsequently, the substance was triturated and silicon dioxide was formed. The final product was a CPCM produced by the SOL-GEL technique, and it consists of AC-adsorbed PCM with a thin layer of silica shell and silica particles containing tetradecane (white particles).

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:

(1)

$\triangle H = Hx P$

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.