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        Stearic acid (SA) is used as a phase change material (PCM) in energy storage devices. In this study, the sol-gel method was used to microencapsulate SiO2 shell surfactant. Various amounts of SA (5, 10, 15, 20, 30, and 50 g) were encapsulated in 10 mL of tetraethyl orthosilicate (TEOS). The synthesized microencapsulated phase change material (MEPCM) was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The characterization results showed that SA was successfully encapsulated by SiO2. Thermogravimetric analysis (TGA) showed that MEPCM has better thermal stability than CA. Using differential scanning calorimetry (DSC), it was found that the enthalpy value of MEPCM did not change even after 30 heating-cooling cycles. Among all microencapsulated samples, 50 g of SA containing MEPCM had the highest latent heat of melting and solidification, which were 182.53 J/g and 160.12 J/g, respectively. The package efficiency value was calculated using thermal data and the highest efficiency was found for the same sample which was 86.68%.
        Approximately 58% of the energy used in the construction industry is used to heat and cool buildings1. Therefore, the most necessary thing is to create efficient energy systems that take into account environmental pollution2. Latent heat technology using phase change materials (PCM) can store high energy at low temperature fluctuations3,4,5,6 and can be widely used in fields such as heat transfer, solar energy storage, aerospace and air conditioning7,8,9 . PCM absorbs thermal energy from building exteriors during the day and releases energy at night10. Therefore, phase change materials are recommended as thermal energy storage materials. In addition, there are different types of PCMs such as solid-solid, solid-liquid, liquid-gas and solid-gas11. Among them, the most popular and frequently used phase change materials are solid-solid phase change materials and solid-liquid phase change materials. However, their application is very difficult due to the enormous volumetric changes of liquid-gas and solid-gas phase transition materials.
        PCM has various applications due to its properties: those that melt at temperatures below 15°C can be used in air conditioning systems to maintain cold temperatures, and those that melt at temperatures above 90°C can be used in heating systems to prevent fires12. Depending on the application and melting point range, various phase change materials have been synthesized from different organic and inorganic chemicals13,14,15. Paraffin is the most commonly used phase change material with high latent heat, non-corrosiveness, safety and a wide melting point range16,17,18,19,20,21.
        However, due to the low thermal conductivity of phase change materials, they need to be encapsulated in a shell (outer layer) to prevent leakage of the base material during the phase change process22. In addition, operational errors or external pressure can damage the outer layer (cladding), and the molten phase change material can react with building materials, causing corrosion of embedded steel bars, thereby reducing the serviceability of the building23. Therefore, it is important to synthesize encapsulated phase change materials with sufficient shell material, which can solve the above problems24.
        Microencapsulation of phase change materials can effectively increase heat transfer and reduce environmental reactivity, and control volume changes. Various methods have been developed for PCM encapsulation, namely interfacial polymerization25,26,27,28, in situ polymerization29,30,31,32, coacervation33,34,35 and sol-gel processes36,37,38,39. Formaldehyde resin can be used for microencapsulation40,41,42,43. Melamine-formaldehyde and urea-formaldehyde resins are used as shell materials, which often emit toxic formaldehyde during operation. Therefore, these materials are prohibited from being used in packaging processes. However, environmentally friendly phase change materials for scalable thermal energy storage can be synthesized using hybrid nanocapsules based on fatty acids and lignin 44 .
        Zhang et al 45 et al synthesized lauric acid from tetraethyl orthosilicate and concluded that as the volume ratio of methyltriethoxysilane to tetraethyl orthosilicate increases, the latent heat decreases and the surface hydrophobicity increases. Lauric acid may be a potential and effective core material for kapok fibers46. In addition, Latibari et al. 47 synthesized stearic acid-based PCMs using TiO2 as the shell material. Zhu et al. prepared n -octadecane and silicone nanocapsules as potential PCMs 48 . From a review of the literature, it is difficult to understand the recommended dosage for the formation of effective and stable microencapsulated phase change materials.
        Therefore, to the authors’ knowledge, the amount of phase change material used for microencapsulation is an important parameter for the production of efficient and stable microencapsulated phase change materials. Using different amounts of phase change materials will allow us to elucidate the different properties and stability of microencapsulated phase change materials. Stearic acid (fatty acid) is an environmentally friendly, medically important and economical substance that can be used to store thermal energy because it has a high enthalpy value (~200 J/g) and can withstand temperatures up to 72 °C. In addition, SiO2 is non-flammable, provides higher mechanical strength, thermal conductivity and better chemical resistance to core materials, and acts as a pozzolanic material in construction. When cement is mixed with water, poorly encapsulated PCMs can crack due to mechanical wear and high temperatures (heat of hydration) generated in massive concrete structures. Therefore, the use of microencapsulated CA with a SiO2 shell can solve this problem. Therefore, the aim of this study was to investigate the performance and efficiency of PCMs synthesized by sol–gel process in construction applications. In this work, we systematically studied different amounts of SA (as base material) of 5, 10, 15, 20, 30 and 50 g encapsulated in SiO2 shells. A fixed amount of tetraethylorthosilicate (TEOS) in a volume of 10 ml was used as a precursor solution for the formation of the SiO2 shell.
        Reactive grade stearic acid (SA, C18H36O2, melting point: 72°C) as the core material was purchased from Daejung Chemical & Metals Co., Ltd., Gyeonggi-do, South Korea. Tetraethylorthosilicate (TEOS, C8H20O4Si) as a precursor solution was purchased from Acros Organics, Geel, Belgium. In addition, absolute ethanol (EA, C2H5OH) and sodium lauryl sulfate (SLS, C12H25NaO4S) were purchased from Daejung Chemical & Metals Co., Ltd, Gyeonggi-do, South Korea, and were used as solvents and surfactants, respectively. Distilled water is also used as a solvent.
        Different amounts of SA were mixed with different proportions of sodium lauryl sulfate (SLS) in 100 mL of distilled water using a magnetic stirrer at 800 rpm and 75 °C for 1 h (Table 1). SA emulsions were divided into two groups: (1) 5, 10 and 15 g of SA were mixed with 0.10 g of SLS in 100 ml of distilled water (SATEOS1, SATEOS2 and SATEOS3), (2) 20, 30 and 50 g of SA were mixed with 0.15, 0.20 and 0.25 g of SLS were mixed with 100 ml of distilled water (SATEOS4, SATEOS5 and SATEOS6). 0.10 g SLS was used with 5, 10 and 15 g SA to form the respective emulsions. Subsequently, it was proposed to increase the number of SLS for SATEOS4, SATEOS5 and SATEOS6. Table 1 shows the ratios of CA and SLS used to obtain stable emulsion solutions.
        Place 10 ml TEOS, 10 ml ethanol (EA) and 20 ml distilled water in a 100 ml beaker. To study the encapsulation efficiency of different ratios of SA and SiO2 shells, the synthesis coefficient of all samples was recorded. The mixture was stirred with a magnetic stirrer at 400 rpm and 60°C for 1 hour. The precursor solution was then added dropwise to the prepared SA emulsion, stirred vigorously at 800 rpm and 75 °C for 2 hours, and filtered to obtain a white powder. The white powder was washed with distilled water to remove residual SA and dried in a vacuum oven at 45°C for 24 hours. As a result, a microencapsulated SC with a shell of SiO2 was obtained. The entire process of synthesis and preparation of microencapsulated SA is shown in Figure 1.
        SA microcapsules with a SiO2 shell were prepared by the sol-gel method, and their encapsulation mechanism is shown in Figure 2. The first step involves preparing an SA emulsion in an aqueous solution with SLS as a surfactant. In this case, the hydrophobic end of the SA molecule binds to SLS, and the hydrophilic end to water molecules, forming a stable emulsion. Thus, the hydrophobic moieties of SLS are protected and cover the surface of the SA droplet. On the other hand, hydrolysis of TEOS solutions occurs slowly by water molecules, leading to the formation of hydrolyzed TEOS in the presence of ethanol (Fig. 2a) 49,50,51. Hydrolyzed TEOS undergoes a condensation reaction, during which n-hydrolyzed TEOS forms silica clusters (Fig. 2b). The silica clusters were encapsulated by SA52 in the presence of SLS (Fig. 2c), which is called the microencapsulation process.
       Schematic diagram of microencapsulation of CA with a shell of SiO2 (a) hydrolysis of TEOS (b) condensation of the hydrolyzate and (c) encapsulation of CA with a shell of SiO2.
       Chemical analysis of bulk SA and microencapsulated SA was carried out using a Fourier transform infrared spectrometer (FT-IR, Perkin Elmer UATR Two, USA) and spectra were recorded in the range from 500 to 4000 cm-1.
        An X-ray diffractometer (XRD, D/MAX-2500, Rigaku, Japan) was used to analyze the bulk SA phases and microcapsule materials. X-ray structural scanning was carried out in the range 2θ = 5°–95° with a scanning speed of 4°/min, using Cu-Kα radiation (λ = 1.541 Å), operating conditions of 25 kV and 100 mA, in continuous scanning mode. X-ray images were constructed in the range 2θ = 5–50°, since no peak was observed after 50° in all samples.
        X-ray photoelectron spectroscopy (XPS, Scienta Omicron R3000, USA) was performed using Al Kα (1486.6 eV) as the X-ray source to understand the chemical state of bulk SA as well as the elements present in the encapsulation material. The collected XPS spectra were calibrated to the C 1s peak using exotic carbon (binding energy 284.6 eV). After background correction using the Shirley method, the high-resolution peaks of each element were deconvoluted and fitted to Gaussian/Lorentzian functions using CASA XPS software.
        The morphology of bulk SC and microencapsulated SC was examined using scanning electron microscopy (SEM, MIRA3, TESCAN, Brno, Czech Republic) equipped with energy-dispersive X-ray spectroscopy (EDS) at 15 kV. Before SEM imaging, the samples were coated with platinum (Pt) to avoid charging effects.
        Thermal properties (melting/solidification point and latent heat) and reliability (thermal cycling) were determined by differential scanning calorimetry (DSC, TA Instrument, Discovery DSC, Newcastle, USA) at a heating/cooling rate of 10 °C/min at 40 °C. and 90°C with continuous nitrogen purge. Weight loss analysis was carried out using a TGA analyzer (TA Instrument, Discovery TGA, New Castle, USA) in a continuous flow of nitrogen starting at a temperature of 40–600 °C, with a heating rate of 10 °C/min.
        Figure 3 shows the FTIR spectra of bulk SC as well as microencapsulated SC (SATEOS1, SATEOS2, SATEOS3, SATEOS4, SATEOS5 and SATEOS6). The absorption peaks at 2910 cm-1 and 2850 cm-1 in all samples (SA as well as microencapsulated SA) are attributed to the symmetrical stretching vibrations of the –CH3 and –CH2 groups, respectively10,50. The peak at 1705 cm–1 corresponds to vibrational stretching of the C=O bond. The peaks at 1470 cm-1 and 1295 cm-1 are attributed to the in-plane bending vibration of the –OH functional group, while the peaks at 940 cm-1 and 719 cm-1 correspond to the in-plane vibration and yield. -plane deformation vibration, respectively – OH group. Absorption peaks of SA at 2910, 2850, 1705, 1470, 1295, 940 and 719 cm-1 were also observed in all microencapsulated SA. In addition, a newly discovered peak at 1103 cm-1 corresponding to the antisymmetric stretching vibration of the Si-O-Si band was observed in the SA microcapsule. The FT-IR results are consistent with Yuan et al. 50 They successfully prepared microencapsulated SA in ammonia/ethanol ratio and found that no chemical interaction occurred between SA and SiO2. The results of the current FT-IR study show that the SiO2 shell successfully encapsulated SA (core) through the condensation process and polymerization of hydrolyzed TEOS. At lower SA content, the peak intensity of the Si-O-Si band is higher (Fig. 3b-d). As the amount of SA increases to more than 15 g, the intensity of the peak and the broadening of the Si-O-Si band gradually decrease, indicating the formation of a thin layer of SiO2 on the surface of SA.
       FTIR spectra of (a) SA, (b) SATEOS1, (c) SATEOS2, (d) SATEOS3, (e) SATEOS4, (f) SATEOS5 and (g) SATEOS6.
        The XRD patterns of bulk SA and microencapsulated SA are shown in Figure 4. XRD peaks are located at 2θ = 6.50° (300), 10.94° (500), 15.46° (700), 20.26° \((\overline {5}according to JCPDS No. 0381923, 02)\), 21.42° in all samples (311), 24.04° (602) and 39.98° (913) are assigned to SA. Distortion and hybridity with bulk CA due to uncertain factors such as surfactant (SLS), other residual substances and microencapsulation of SiO250. After encapsulation occurs, the intensity of the main peaks (300), (500), (311), and (602) gradually decreases compared to bulk CA, indicating a decrease in the crystallinity of the sample.
       XRD patterns of (a) SA, (b) SATEOS1, (c) SATEOS2, (d) SATEOS3, (e) SATEOS4, (f) SATEOS5 and (g) SATEOS6.
        The intensity of SATEOS1 decreases sharply compared to other samples. No other peaks were observed in all microencapsulated samples (Fig. 4b–g), which confirms that physical adsorption of SiO252 rather than chemical interaction occurs on the SA surface. In addition, it was also concluded that microencapsulation of SA did not lead to the appearance of any new structures. SiO2 remains intact on the SA surface without any chemical reaction, and as the amount of SA decreases, the existing peaks become more obvious (SATEOS1). This result indicates that SiO2 mainly encapsulates the SA surface. The peak at (700) completely disappears, and the peak at \((\overline{5}02)\) becomes a hump in SATEOS 1 (Fig. 4b), which is associated with reduced crystallinity and increased amorphism. SiO2 is amorphous in nature, so the peaks observed from 2θ = 19° to 25° have a hump and broadening53 (Fig. 4b–g), which confirms the existence of amorphous SiO252. The lower diffraction peak intensity of microencapsulated SA is due to the nucleation effect of the silica inner wall and the limiting crystallization behavior49. It is believed that with a lower SA content, a thicker silica shell is formed due to the presence of a large amount of TEOS, which is largely adsorbed on the outer surface of the SA. However, as the amount of SA increases, the surface area of ​​SA droplets in the emulsion solution increases and more TEOS is required for proper encapsulation. Therefore, with a higher SA content, the SiO2 peak in FT-IR is suppressed (Fig. 3), and the intensity of the diffraction peak near 2θ = 19–25° in XRF (Fig. 4) decreases and the expansion also decreases. Not visible. However, as can be seen in Figure 4, as soon as the amount of SA is increased from 5 g (SATEOS1) to 50 g (SATEOS6), the peaks become very close to bulk SA, and the peak at (700) appears with all peak intensities identified. This result correlates with the FT-IR results, where the intensity of the SiO2 SATEOS6 peak decreases at 1103 cm-1 (Fig. 3g).
        The chemical states of the elements present in SA, SATEOS1 and SATEOS6 are shown in Figures 1 and 2. Figures 5, 6, 7 and 8 and Table 2. Measurement scans for bulk SA, SATEOS1 and SATEOS6 are shown in Figure 5 and high resolution scans for C 1s, O 1s and Si 2p are shown in Figures 5, 6, 7 and 8 and Table 2. 6, 7 and 8 respectively. The binding energy values ​​obtained by XPS are summarized in Table 2. As can be seen from Figure 5, obvious Si 2s and Si 2p peaks were observed in SATEOS1 and SATEOS6, where microencapsulation of the SiO2 shell occurred. Previous researchers have reported a similar Si 2s peak at 155.1 eV54. The presence of Si peaks in SATEOS1 (Fig. 5b) and SATEOS6 (Fig. 5c) confirms the FT-IR (Fig. 3) and XRD (Fig. 4) data.
        As shown in Figure 6 a, the C 1s of bulk SA has three different peaks of CC, caliphatic, and O=C=O at the binding energy, which are 284.5 eV, 285.2 eV, and 289.5 eV, respectively. C–C, caliphatic and O=C=O peaks were also observed in SATEOS1 (Fig. 6b) and SATEOS6 (Fig. 6c) and are summarized in Table 2. In addition to this, the C 1s peak also corresponds to an additional Si-C peak at 283 .1 eV (SATEOS1) and 283.5 eV (SATEOS6). Our observed binding energies for C–C, caliphatic, O=C=O and Si–C correlate well with other sources55,56.
        The XPS spectra of O 1 SA, SATEOS1 and SATEOS6 are shown in Figures 7a–c, respectively. The O 1s peak of bulk SA is deconvoluted and has two peaks, namely C=O/C–O (531.9 eV) and C–O–H (533.0 eV), while the O 1 of SATEOS1 and SATEOS6 are consistent. there are only three peaks: C=O/C–O, C–O–H and Si–OH55,57,58. The O 1s binding energy in SATEOS1 and SATEOS6 changes slightly compared to bulk SA, which is associated with a change in the chemical fragment due to the presence of SiO2 and Si-OH in the shell material.
        The Si 2p XPS spectra of SATEOS1 and SATEOS6 are shown in Figure 8a and b, respectively. In bulk CA, Si 2p was not observed due to the absence of SiO2. The Si 2p peak corresponds to 105.4 eV for SATEOS1 and 105.0 eV for SATEOS6, corresponding to Si-O-Si, while the SATEOS1 peak is 103.5 eV and the SATEOS6 peak is 103.3 eV, corresponding to Si-OH55 . Si-O-Si and Si-OH peak fitting in SATEOS1 and SATEOS6 revealed successful microencapsulation of SiO2 on the SA core surface.
        The morphology of the microencapsulated material is very important, affecting solubility, stability, chemical reactivity, flowability and strength59. Therefore, SEM was used to characterize the morphology of bulk SA (100×) and microencapsulated SA (500×), as shown in Figure 9. As can be seen from Figure 9a, the SA block has an elliptical shape. The particle size exceeds 500 microns. However, once the microencapsulation process continues, the morphology changes dramatically, as shown in Figures 9 b–g.
       SEM images of (a) SA (×100), (b) SATEOS1, (c) SATEOS2, (d) SATEOS3, (e) SATEOS4, (f) SATEOS5 and (g) SATEOS6 at ×500.
        In the SATEOS1 sample, smaller quasi-spherical SiO2-wrapped SA particles with a rough surface are observed (Fig. 9b), which may be due to the hydrolysis and condensation polymerization of TEOS on the SA surface, accelerating the rapid diffusion of ethanol molecules. As a result, SiO2 particles are deposited and agglomeration is observed52,60. This SiO2 shell provides mechanical strength to the microencapsulated CA particles and also prevents leakage of molten CA at higher temperatures10. This result indicates that SA microcapsules containing SiO2 can be used as potential energy storage materials61. As can be seen from Figure 9b, the SATEOS1 sample has a uniform particle distribution with a thick SiO2 layer encapsulating the SA. The particle size of microencapsulated SA (SATEOS1) is approximately 10–20 μm (Fig. 9b), which is significantly smaller compared to bulk SA due to the lower SA content. The thickness of the microcapsule layer is due to hydrolysis and condensation polymerization of the precursor solution. Agglomeration occurs at lower doses of SA, i.e. up to 15 g (Fig. 9b-d), but as soon as the dosage is increased, no agglomeration is observed, but clearly defined spherical particles are observed (Fig. 9e-g) 62 .
        In addition, when the amount of SLS surfactant is constant, the SA content (SATEOS1, SATEOS2 and SATEOS3) also affects the efficiency, shape and particle size distribution. Thus, SATEOS1 was found to exhibit smaller particle size, uniform distribution and dense surface (Fig. 9b), which was attributed to the hydrophilic nature of SA promoting secondary nucleation under constant surfactant63. It is believed that by increasing the SA content from 5 to 15 g (SATEOS1, SATEOS2 and SATEOS3) and using a constant amount of surfactant, i.e. 0.10 g SLS (Table 1), the contribution of each particle of the surfactant molecule will decrease, thereby reducing the particle size and particle size. The distribution of SATEOS2 (Fig. 9c) and SATEOS3 (Fig. 9d) differs from the distribution of SATEOS 1 (Fig. 9b).
        Compared with SATEOS1 (Fig. 9b), SATEOS2 showed a dense morphology of microencapsulated SA and the particle size increased (Fig. 9c). This is due to agglomeration 49, which reduces the coagulation rate (Fig. 2b). As the amount of SC increases with increasing SLS, the microcapsules become clearly visible, as shown in Fig. how aggregation occurs. In addition, Figures 9e–g show that all particles are clearly spherical in shape and size. It has been recognized that in the presence of large amounts of SA, an appropriate amount of silica oligomers can be obtained, causing appropriate condensation and encapsulation and hence the formation of well-defined microcapsules49. From the SEM results, it is clear that SATEOS6 formed corresponding microcapsules compared to a small amount of SA.
        The results of energy dispersive X-ray spectroscopy (EDS) of bulk SA and microcapsule SA are presented in Table 3. As can be seen from this table, the Si content gradually decreases from SATEOS1 (12.34%) to SATEOS6 (2.68%). Increase in SA. Therefore, we can say that an increase in the amount of SA leads to a decrease in the deposition of SiO2 on the SA surface. There are no consistent values ​​for C and O contents in Table 3 due to the semi-quantitative analysis of EDS51. The Si content of microencapsulated SA was correlated with FT-IR, XRD and XPS results.
        The melting and solidification behavior of bulk SA as well as microencapsulated SA with SiO2 shell are shown in Figures 1 and 2. They are shown in Figures 10 and 11 respectively, and the thermal data are shown in Table 4. The melting and solidification temperatures of microencapsulated SA were found to be different . As the amount of SA increases, the melting and solidification temperatures increase and approach the values ​​of bulk SA. After SA microencapsulation, the silica wall increases the crystallization temperature, and its wall acts as a core to promote heterogeneity. Therefore, as the amount of SA increases, the melting (Fig. 10) and solidification (Fig. 11) temperatures also gradually increase49,51,64. Among all microencapsulated SA samples, SATEOS6 exhibited the highest melting and solidification temperatures, followed by SATEOS5, SATEOS4, SATEOS3, SATEOS2, and SATEOS1.
        SATEOS1 shows the lowest melting point (68.97 °C) and solidification temperature (60.60 °C), which is due to the smaller particle size in which the movement of SA particles inside the microcapsules is very small and the SiO2 shell forms a thick layer and hence Core Material limits stretch and movement49. This hypothesis is related to the SEM results, where SATEOS1 showed a smaller particle size (Fig. 9b), which is due to the fact that SA molecules are confined within a very small area of ​​the microcapsules. The difference in the melting and solidification temperatures of the main mass, as well as all SA microcapsules with SiO2 shells, is in the range of 6.10–8.37 °C. This result indicates that microencapsulated SA can be used as a potential energy storage material due to the good thermal conductivity of the SiO2 shell 65 .
        As can be seen from Table 4, SATEOS6 has the highest enthalpy among all microencapsulated SCs (Fig. 9g) due to the proper encapsulation observed by SEM. The SA packing rate can be calculated using equation (1). (1) By comparing the latent heat data of microencapsulated SA49.
        The R value represents the encapsulation degree (%) of microencapsulated SC, ΔHMEPCM,m represents the latent heat of fusion of microencapsulated SC, and ΔHPCM,m represents the latent heat of fusion of SC. In addition, packaging efficiency (%) is calculated as another important technical parameter, as shown in Equation (1). (2)49.
       The E value represents the encapsulation efficiency (%) of microencapsulated CA, ΔHMEPCM,s represents the latent heat of cure of microencapsulated CA, and ΔHPCM,s represents the latent heat of cure of CA.
        As shown in Table 4, the packing degree and efficiency of SATEOS1 are 71.89% and 67.68%, respectively, and the packing degree and efficiency of SATEOS6 are 90.86% and 86.68%, respectively (Table 4). Sample SATEOS6 exhibits the highest encapsulation coefficient and efficiency among all microencapsulated SAs, indicating its high thermal capacity. Therefore, the transition from solid to liquid requires large amounts of energy. In addition, the difference in the melting and solidification temperatures of all SA microcapsules and bulk SA during the cooling process indicates that the silica shell is spatially confined during microcapsule synthesis. Thus, the results show that as the amount of SC increases, the encapsulation rate and efficiency gradually increase (Table 4).
        The TGA curves of bulk SA and microcapsule SA with a SiO2 shell (SATEOS1, SATEOS3 and SATEOS6) are shown in Figure 12. The thermal stability properties of bulk SA (SATEOS1, SATEOS3 and SATEOS6) were compared with microencapsulated samples. It is clear from the TGA curve that the weight loss of bulk SA as well as microencapsulated SA shows a smooth and very slight decrease from 40°C to 190°C. At this temperature, bulk SC does not undergo thermal decomposition, whereas microencapsulated SC releases adsorbed water even after drying at 45 °C for 24 h. This resulted in slight weight loss,49 but beyond this temperature the material began to degrade. At lower SA content (i.e. SATEOS1), the adsorbed water content is higher and hence the mass loss up to 190 °C is higher (inset in Fig. 12). As soon as the temperature rises above 190 °C, the sample begins to lose mass due to decomposition processes. Bulk SA starts to decompose at 190°C and only 4% remains at 260°C, whereas SATEOS1, SATEOS3 and SATEOS6 retain 50%, 20% and 12% at this temperature, respectively. After 300 °C, the mass loss of bulk SA was approximately 97.60%, while the mass loss of SATEOS1, SATEOS3, and SATEOS6 was approximately 54.20%, 82.40%, and 90.30%, respectively. With an increase in the SA content, the SiO2 content decreases (Table 3), and a thinning of the shell is observed in the SEM (Fig. 9). Thus, the weight loss of microencapsulated SA is lower compared to bulk SA, which is explained by the favorable properties of the SiO2 shell, which promotes the formation of a carbonaceous silicate-carbonaceous layer on the surface of SA, thereby isolating the SA core and slowing down the release of the resulting volatile products10. This char layer forms a physical protective barrier during thermal decomposition, limiting the transition of flammable molecules into the gas phase66,67. In addition to this, we can also see significant weight loss results: SATEOS1 shows lower values ​​compared to SATEOS3, SATEOS6 and SA. This is because the amount of SA in SATEOS1 is less than in SATEOS3 and SATEOS6, where the SiO2 shell forms a thick layer. In contrast, the total weight loss of bulk SA reaches 99.50% at 415 °C. However, SATEOS1, SATEOS3, and SATEOS6 showed 62.50%, 85.50%, and 93.76% weight loss, respectively, at 415 °C. This result indicates that the addition of TEOS improves the degradation of SA by forming a SiO2 layer on the surface of SA. These layers can form a physical protective barrier, and therefore an improvement in the thermal stability of microencapsulated CA can be observed.
        The thermal reliability results of bulk SA and the best microencapsulated sample (i.e. SATEOS 6) after 30 heating and cooling cycles of DSC51,52 are shown in Figure 13. It can be seen that bulk SA (Figure 13a) does not show any difference in melting temperature. solidification and enthalpy value, while SATEOS6 (Fig. 13b) does not show any difference in temperature and enthalpy value even after the 30th heating cycle. and the cooling process. Bulk SA showed a melting point of 72.10 °C, a solidification temperature of 64.69 °C, and the heat of fusion and solidification after the first cycle were 201.0 J/g and 194.10 J/g, respectively. After the 30th cycle, the melting point of these values ​​​​decreased to 71.24 °C, the solidification temperature decreased to 63.53 °C, and the enthalpy value decreased by 10%. Changes in melting and solidification temperatures, as well as decreases in enthalpy values, indicate that bulk CA is unreliable for non-microencapsulation applications. However, after proper microencapsulation occurs (SATEOS6), the melting and solidification temperatures and enthalpy values ​​do not change (Fig. 13b). Once microencapsulated with SiO2 shells, SA can be used as a phase change material in thermal applications, especially in construction, due to its optimal melting and solidification temperatures and stable enthalpy.
       DSC curves obtained for samples SA (a) and SATEOS6 (b) at the 1st and 30th heating and cooling cycles.
        In this study, a systematic investigation of microencapsulation was carried out using SA as the core material and SiO2 as the shell material. TEOS is used as a precursor to form a SiO2 support layer and a protective layer on the SA surface. After successful synthesis of microencapsulated SA, FT-IR, XRD, XPS, SEM and EDS results showed the presence of SiO2. SEM analysis shows that the SATEOS6 sample exhibits well-defined spherical particles surrounded by SiO2 shells on the SA surface. However, MEPCM with lower SA content exhibits agglomeration, which reduces the performance of PCM. XPS analysis showed the presence of Si-O-Si and Si-OH in the microcapsule samples, which revealed the adsorption of SiO2 on the SA surface. According to the thermal performance analysis, SATEOS6 shows the most promising heat storage ability, with melting and solidification temperatures of 70.37°C and 64.27°C, respectively, and latent heat of melting and solidification of 182.53 J/g and 160.12 J/g. G. respectively. The maximum packaging efficiency of SATEOS6 is 86.68%. TGA and DSC thermal cycle analysis confirmed that SATEOS6 still has good thermal stability and reliability even after 30 heating and cooling processes.
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