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Preparation and thermal properties of microencapsulated stearyl alcohol with silicon dioxide shell as thermal energy storage materials
Applied Thermal Engineering 169 (2020) 114943
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Preparation and thermal properties of microencapsulated stearyl alcohol with silicon dioxide shell as thermal energy storage materials Chuqiao Zhu, Yaxue Lin, Guiyin Fang
T
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School of Physics, Nanjing University, Nanjing 210093, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal energy storage Microencapsulated phase change material Thermal properties Stearyl alcohol Silica
Microencapsulation of stearyl alcohol (SAL) with silica shell using sol-gel method has been attempted to obtain microencapsulated phase change materials (MPCM). Tetraethoxysilane (TEOS) was used as silica precursor to form silica shell. In order to inveatigate thermal properties of the MPCM, some tests were carried out. The results of Fourier transformation infrared spectroscope (FT–IR) indicated that the combination of the SAL with silica by sol-gel method is a physical action. X–ray diffractometer (XRD) was used to confirm PCM maintains stable crystal structure. Scanning electronic microscope (SEM) was employed to determine the MPCM has relatively stable physical structure. Thermal properties of MPCM were measured by Differential scanning calorimeter (DSC), where melting temperature and latent heat of MPCM1 (made from 10 g SAL, 10 g TEOS) is 55.89 °C and 229.73 J/g, respectively. Liquid–solid phase change has latent heat of 112.05 J/g at 56.74 °C while solid-solid phase change has latent heat of 72.01 J/g at 49.75 °C. Thermogravimetric analyzer (TGA) analysis confirms that MPCM has good thermal stability. After 100 thermal cycles of the MPCM1, there was no change in phase transition temperature and latent heat, which confirms that the MPCM has good reliability. The thermal conductivity of MPCM1 was measured to be 0.1508 W/m·K over melting point of MPCM1 and 0.1412 W/m·K at room temperature. Therefore, MPCM1 is found to be a promising candidate for thermal energy storage applications.
1. Introduction With the development and progress of human society, energy consumption is increasing. Fossil fuels such as coal, oil, and natural gas are facing an imminent situation in which they will be used out [1]. At the same time, excessive use of some energy sources has caused irreversible environmental pollution [2]. Therefore, finding suitable alternative energy sources, improving energy efficiency and optimizing energy utilization structure are the next things people should pay attention to. Some renewable energy sources such as solar energy are slowly gaining popularity. However, these energy sources have more or less problems of uneven geographical and temporal distribution. For example, solar energy only exists during the day or on a sunny day and can not be collected continuously [3]. Thermal energy storage systems can solve this problem. It can store excess heat or waste heat, etc., and take it out when needed [4]. In other words, the diversification of the structure of the thermal energy storage system is a subject worth studying. Thermal energy storage system mainly has three energy storage modes, namely sensible heat storage, latent heat storage and chemical reaction heat storage. Compared with sensible heat storage, latent heat
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storage can store more energy and has better controllability [5]. Chemical reaction heat storage has high storage capacity than other thermal energy storage system due to chemical reaction and high operating temperature. However, chemical reaction heat storage reaction’s conditions are demanding and difficult to be controlled [6]. Phase change material (PCM) is a typical thermal storage material that utilizes latent heat storage, which has high latent heat and stable phase transition temperature [7]. Phase change material has the ability to change their physical state within a certain temperature range. Take solid-liquid phase change as an example, when heated to the melting temperature, a phase change from solid to liquid is generated. During the melting process, a large amount of latent heat is absorbed and stored by the phase change material. When a phase-change material is cooled, the stored heat is released into the environment within a certain temperature range for the reverse phase transition from liquid to solid. The energy stored or released during these two phase transitions is called latent heat of phase transition. When the physical state changes, the temperature of the material itself remains almost unchanged before the phase change is completed, forming a wide temperature platform. Although the temperature remains unchanged, the latent heat absorbed
Corresponding author. E-mail address: [email protected] (G. Fang).
https://doi.org/10.1016/j.applthermaleng.2020.114943 Received 18 July 2019; Received in revised form 10 December 2019; Accepted 12 January 2020 Available online 14 January 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.
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stability and weaker compatibility with building materials [29-31]. So inorganic shells have also been developed and studied, such as silica, titania [32,33], calcium carbonate [34] etc., and the study of the silica shells is relatively extensive. Lin et al. [35] used methyl triethoxysilane (MTES) as a raw material to synthesize silica shell by sol-gel method. Chen et al. [36] used sol-gel method to prepare MPCM with paraffin core and silica shell, and the TEOS was selected as silica precursor. The mass ratio of the paraffin is 87.5% and the MPCM melts at 58.37 °C with a latent heat of 165.68 J/g. He et al. [37] used sodium silicate precursor to form silica shell with the core material of n-octadecane. Sami et al. [38] carried out microencapsulation of lauric acid with polystyrene shell, and the results indicated that microencapsulated PCM with a melting point of 43.77 °C and latent heat of 167.26 kJ/kg can be used in solar energy systems. Fang et al. [39] performed an experimental study about the thermal characteristics of microencapsulated phase change composite cylinders, the results showed that the higher PCM core fraction leads to a lower temperature difference between the internal surface and the external surface. Song et al. [40] prepared microencapsulated capric-stearic acid with silica shell as a novel phase change material, and the phase change temperature and latent heat are 21.4 °C and 91.48 kJ/kg. However, such information on the microencapsulation of saturated polyols PCM has been limited. Saturated polyols are rarely tried as core materials because polyols are hydrophilic. It seems that there have been no fundamental studies which related to the silica-shell microcapsules containing saturated polyols PCM. There still exists a knowledge gap about the effective combination between silica shell and stearyl alcohol. Based on the above facts, the aim of this study is to develop high thermal conductivity MPCM. The silica shell caters the requirements of this study. Besides, silicon dioxide as the shell material is non-toxic and flame retardant, that is not easy to degrade at high temperature. In previous work, core materials are usually alkanes, fatty acids, etc. For example, the SA/SiO2 nanocapsules fabricated through the sol-gel process were spherical with diameters in the 62–464 nm range. The satisfactory sample melted at 63.9 °C with the latent heats of 169.4 J/g [48]. In this work, stearyl alcohol was used as core material, which has higher latent heat than paraffin or other organics phase change material. The results showed that the MPCM can retain the high latent heat of stearyl alcohol which can attain 229.73 J/g at the transition temperature of 55.9 °C. Meanwhile, the experimental results revealed there is no chemical reaction between the silica shell and stearyl alcohol. Another highlight of this work is that the working efficiency of the MPCM can reach up to 90.6%. This can greatly improve the conversion efficiency of practical applications such as MPCM systems in green building walls which cater to this phase change temperature. In addition, thermal conductivity is enhanced obviously with a little latent heat loss because the results showed that the heat storage and release rates are significantly increased by preparing silica-shell microcapsules. In this work, microencapsulated stearyl alcohol with silica shell by using sol–gel method was performed. Thermal properties and thermal stability of MPCM were measured and analyzed. The microcapsules will be a potential candidate for thermal energy storage.
or released is quite large [8–10]. However, microencapsulated phase change materials have emerged as phase change materials are prone to leakage during solid-liquid phase transition and their thermal conductivity is not very high. Microencapsulated phase change material refers to a structure in which a phase change material is wrapped with another material to form a capsule-like structure. It not only reduces the leakage of the phase change material, but also acts as a physical protector to adapt to the volume change during the phase change. Some thermal properties such as thermal conductivity and thermal stability have also been improved [11,12]. Microencapsulation describes a process in which PCM particles or droplets are surrounded or embedded in a homogeneous matrix at the size of below 1000 µm [13]. MPCM can be encapsulated by physical method like spray drying, physical–chemical methods like coacervation and sol–gel process, and chemical methods like interfacial polymerization, suspension polymerization and emulsion polymerization. The spray drying method has the advantages of being economical and easy to scale up to produce MPCM, but agglomeration is prone to occur by spray drying, and some particles will be missed [14,15]. The process of interfacial polymerization is simple, where the purity of the reaction monomer is low but the activity of the monomer must be enough high to carry out the polycondensation reaction. The ratio of core and wall materials is not strict, and the reaction rate is fast. The encapsulation efficiency is high [16]. Suspension polymerization and coacervation are suitable for producing larger particles with high core material content and high encapsulation efficiency [17]. Emulsion polymerization can be used to prepare microcapsules with smaller size [18]. The sol-gel method is suitable for the synthesis of inorganic shells [19]. The agglomeration of spray drying is unacceptable for this experiment. The reaction rate of interfacial polymerization is so fast that the time is not easy to be controlled. Suspension and emulsion polymerization methods are not easy to be scaled up to large scale production. In this study, inorganic silica was used as the shell, so the sol-gel method was adopted. In addition, MPCM has a great ability to increase the thermal conductivity of the phase change material, due to the increased specific surface area of the PCM. What’s more, thermal conductivity of MPCM encapsulated with inorganic materials is higher than that of MPCM encapsulated with organic materials. Thermal stability and heat capacity of MPCM are also proved to be better than that of PCM. The MPCM offers a reliable way for the mixing solid and liquid PCM with polymer and other structural materials, which expand the application fields of the PCM. They have the function of protecting core material from environmental influence and reducing toxicity. Due to the large storage capacity and isothermal nature of thermal storage process, microencapsulation techniques of PCM have attracted great attention. The microencapsulation technology has extended to buildings, textiles, MPCM slurry, etc. Generally, the solid-liquid phase change material consists of organic PCM, inorganic PCM and eutectic [20]. Compared with the inorganic PCM, the organic PCM behaves non-corrosives, low subcooling, chemical and thermal stability [21,22]. In addition, the inorganic PCM has serious disadvantages of phase separation and poor thermal stability. Stearyl alcohol (SAL) would be employed as the PCM in this work, which phase change enthalpy is above 200 J/g [23]. The shell for encapsulating PCM may be organic materials, inorganic materials or mixed materials, but all of them possess the common characteristics: good thermal and chemical stability, no reactions between shell and PCM and the higher melting point than PCM [12]. Currently, the extensively used organic shells are polymers containing poly (methyl methacrylate) (PMMA) [24–26], melamine–formaldehyde [27], poly (melamine–urea–formaldehyde) [28] etc., which possess good sealing properties and thermal and chemical stability [29]. Nevertheless, organic shells usually suffer from toxicity and flammability. As compared with inorganic shells, they have weaker mechanical strength, lower thermal conductivity, poorer thermal
2. Synthesis and characterization 2.1. Materials Stearyl alcohol (SAL, C18H38O, melting point: 59.4–59.8 °C, analytical reagent) was used as PCM to store latent heat, which was supplied by Sinopharm Chemical Reagent Co., Ltd.. Tetraethoxysilane (TEOS, C8H20O4Si, reagent grade) was provided by Sinopharm Chemical Reagent Co., Ltd. for producing silica. Sodium dodecyl sulfate (SDS, C12H25SO4Na, reagent grade) was used as emulsifier to form stable oilwater emulsion. Anhydrous ethanol (Reagent grade, Sinopharm Chemical Reagent Co., Ltd) was employed as the solvent and water was distilled and deionized using a Millipore Milli–Q Purification System. 2
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The raw materials for the preparation of the SAL oil-water emulsion were 10 g of SAL, 0.5 g of SDS and 150 mL of deionized water. The specific operation was as follows: the weighed SAL and SDS were added to deionized water in a 250 mL beaker. Then, the suspension was stirred by a magnetic stirrer with the speed rate of 800 rpm for 120 min at a constant temperature of 70 °C. Finally, stop stirring the emulsion and keep heating it at the temperature until the SAL oil-water emulsion was formed. In the process of heating and stirring, the hydrophilic group of the surfactant SDS was directed to the water molecule, and the lipophilic group was combined with the SAL oil droplet, so that a layer which surface is active was formed on the outer layer of the SAL oil droplet. After a period of time, a stable oil-water emulsion was formed, in which the SAL oil droplets were uniformly dispersed in the deionized water [41].
water solvent in the solution evaporated, and the microcapsules were formed. The reaction mechanism of hydrolysis and polymerization process and the final shell-forming process can be seen in Fig. 1. The prepared microencapsulated phase change materials were washed and filtered. The filter paper was folded into a funnel shape and pressed against the funnel wall. The funnel was inserted into a large round-bottomed flask. At room temperature, the samples were put on the filter paper. Appropriate amount of deionized water was poured to clean and filter. Then they were dried in a vacuum oven at 50 °C for 24 h. Three microencapsulated phase change materials were prepared, which are called MPCM1, MPCM2 and MPCM3 respectively. To confirm the stable physical structure of the three MPCM samples, a leakage test was performed. The MPCM on filter paper was put on the heater and heated for 10 min at a constant temperature of 70 °C, which is higher than phase change temperature of the SAL. Then, the MPCM on filter paper was removed and cooled in the air. After repeating the above steps for five times, leakage test results were observed. As shown in Fig. 2, there was no oily liquid on the filter paper. The conclusion is that the PCM were well encapsulated.
2.3. Preparation of the MPCM
2.4. Characterization of the MPCM
The next procedure was to prepare the precursor solution. Here, three different doses of raw materials were used to get three MPCM samples with different core-shell ratios. They are called MPCM1, MPCM2 and MPCM3, and the mass of raw materials used for preparation process are listed in Table 1. The first step was the hydrolysis of the TEOS, and the process was as follows: (1) Take MPCM1 as an example, 10 g absolute ethanol and 20 g deionized water were mixed as a solvent in a beaker, and then add the solution into another beaker which contained 10 g TEOS. (2) The pH value of the TEOS solution was adjusted to 2–3 by adding hydrochloric acid resulting in a steady solution. The acid region pH value was to inhibit the TEOS hydrolysis rate, so that the hydrolysis reaction did not go too fast. It can be seen from the reaction mechanism that TEOS can also participate in the polymerization process, and partial hydrolysis is what we need. If the hydrolysis is complete, the formation of intermediate products wound be affected, and the coating may be incomplete. The pH value was measured by pH test paper. (3) The TEOS precursor solution was stirred by a magnetic stirrer with a speed rate of 500 rpm for 30 min at a constant temperature of 70 °C. The hydrolysis reaction of the TEOS occurred and orthosilicic acid sol solution was obtained with colloidal substance. The second step was the polymerization of orthosilicic acid, and the process was as follows: (1) The prepared SAL oil-water emulsion was stirred with a speed of 600 rpm and the temperature was maintained at 70 °C. (2) The orthosilicic acid sol solution was added to the emulsion drop by drop, and then the mixed solution was stirred for 3 h at the same stirring speed and temperature. At this stage, the polymerization reaction began. Due to the electrostatic interaction between the orthosilicic acid produced by the hydrolysis and the hydrophilic segment of the surfactant, the orthosilicic acid was attracted to the surface of the SAL micelle. Then, silanol which contains the Si-O-Si bond formed a backbone structure. As a result, silica shell was formed on the surface of the SAL droplet [35,42]. After 3 h of agitation, most of the ethanol and
Fourier transformation infrared spectroscope (FT–IR, Nicolet Nexus 870, spectra from 400 to 4000 cm−1, resolution using KBr pellets: 2 cm−1) was used to analyze the chemical structures of the MPCM. X–ray diffractometer (XRD, D/MAX–Ultima III, working voltage: 40 kV, working current: 40 mA, scanning rate: 5° (2θ)/min, Rigaku Corporation, Japan) was employed for revealing crystal structure of the MPCM. A scanning electron microscope (SEM, S–3400NII, operating voltage: 3 kV, Hitachi Inc., Japan) was used to examine the morphology of the MPCM. The thermal properties of the MPCM were measured by differential scanning calorimeter (DSC, Pyris 1 DSC, Perkin–Elmer, temperature accuracy: ± 0.2 °C, enthalpy accuracy: ± 5%) under a sustained stream of argon at a rate of 5 °C/min, where the range of temperature was 10 to 100 °C. The thermal stability of the MPCM was measured by a thermogravimeter (TGA, Pyris 1 TGA, Perkin–Elmer) under a constant stream of nitrogen, and the temperature was increased from room temperature to 700 °C at a rate of 20 °C/min. The thermal conductivity of the MPCM was obtained by a thermal conductivity meter (TCM 3020, Xiatech Electronic Technology co., Ltd., accuracy: ± 2%) through transient hot wire technique.
Hydrochloric acid (Analytical reagent, Nanjing Chemical Reagent Co., Ltd.) was used to adjust the pH value. 2.2. Preparation of the SAL oil-water emulsion
3. Results and discussion 3.1. FT–IR analysis of the MPCM The changes in the functional groups of the components of the samples due to a chemical interaction can be extracted from by FT–IR spectra. Fig. 3 shows the FT–IR spectra of SAL, silica and MPCM, respectively, which were used to analyze the material composition and possible chemical interactions in the microcapsules. As seen in Fig. 3a, the spectrum of SAL has six major absorption peaks. The broad absorption band in the range of 3234–3321 cm−1 belongs to the stretching vibration of eOH group and its normal range is 3500 cm−1 to 3200 cm−1. Due to hydrogen bonding, the broad absorption peak of eOH group is a very characteristic group frequency. The antisymmetric and symmetric stretching vibration of eCH2 group corresponds to the two sharp absorption peak at 2918 cm−1 and 2848 cm−1. The eCH2 scissoring bending vibration causes an absorption peak to appear at 1464 cm−1. The characteristic peak is observed at 1063 cm−1, corresponding to the eCeO stretching vibration antisymmetrically coupled to the CeC stretch vibration. The peak at 719 cm−1 reveals that there are four or more eCH2 groups in a chain. Fig. 3b shows the FT–IR spectrum of the silica. The broad absorption band at 3000–3600 cm−1 and 1600–1700 cm−1 agree with the bending and stretching vibrations of the eOH functional group of H2O, which indicates that a small
Table 1 The compositions of the MPCM. Samples
MPCM1 MPCM2 MPCM3
SAL emulsion
TEOS solution
SAL (g)
SDS (g)
Deionized water (mL)
TEOS (g)
Anhydrous ethanol (g)
Deionized water (mL)
10 10 10
0.5 0.5 0.5
150 150 150
10 15 20
10 15 20
20 30 40
3
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Fig. 1. The reaction mechanism of hydrolysis and polymerization process of the TEOS. (a) The hydrolysis process, (b) The polymerization process and (c) The process of forming silica shell.
Fig. 3. FT–IR spectra of (a) SAL, (b) silica, (c) – (e) MPCM1–MPCM3.
that the combination of the SAL with silica is a physical combination without chemical reaction.
3.2. XRD analysis of the MPCM The crystalloid phase and crystallinity of the SAL, silica and MPCM are presented using the XRD patterns which are shown in Fig. 4. The diffraction pattern of SAL is exhibited in Fig. 4a, which includes one strong and sharp diffraction peak at 21.79°. It results from diffraction of the (−8 0 2) crystal planes of SAL. The other diffraction peaks are relatively weak, such as the peaks at 20.71°, 24.7°and 36.28°, which correspond to the diffraction of the (2 1 1), (−17 1 0) and (3 2 1) crystal planes of the SAL, respectively [43]. Fig. 4b represents the diffraction of silica, which has no sharp peaks. This result indicates that the silica is non-crystalline with amorphous structure so that the silica shell can be completely formed. As seen in Fig. 4c–e, the set of characteristic peaks of SAL exists in the MPCM. Simultaneously, when the mass ratio of the SAL decreases, the peak becomes more flat. Combined with the result of the FT–IR analysis, it can be determined that the PCM maintain original and stable crystal structure and exist no chemical reaction with silica.
Fig. 2. Leakage test of the (a) MPCM1, (b) MPCM2 and (c) MPCM3.
amount of water molecules were adsorbed in silica. The bending vibration of the SieO functional group leads to occurrence of three peaks, which locates at 1063 cm−1, 790 cm−1 and 482 cm−1, respectively. The peak at 960 cm−1 is assigned to the SieOH functional group. Comparing Fig. 3a and b with c–e, it can be confirmed that there is no new peak occurrence in the spectrum of the MPCM, which indicates 4
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Fig. 6. Elemental composition images of MPCM1.
3.3. Morphology of the MPCM The morphology of MPCM at different magnifications is shown in Fig. 5. Photograph (b), (d) and (f) are single microscopic appearance of the MPCM. It can be seen that silica forms a shell on the surface of SAL
Fig. 4. XRD patterns of (a) SAL, (b) silica, (c) – (e) MPCM1–MPCM3.
Fig. 5. SEM images of (a) MPCM1 (1.50 k×), (b) MPCM1 (6.00 k×), (c) MPCM2 (1.50 k×), (d) MPCM2 (10.0 k×), (e) MPCM3 (1.5 k×) and (f) MPCM3 (10.0 k×). 5
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and a spherical structure is formed with particle sizes ranging from 5 µm to 12 µm. Some small spheres have something stuck on them, and it may be incompletely reacted silicic acid or excess of silica. Subsequent EDAX tests proved that the shell material was silica. Photograph (a), (c) and (e) are whole appearance of three MPCM. It can be seen that they are relatively uniform and clustered together. From the perspective of SEM, the particle size of MPCM is uniform. The results confirm that the MPCM has a relatively uniform physical structure and the silica shell is effective protection for the SAL. To further demonstrate the presence of silica in MPCM, we continued to perform elemental analysis tests using EDAX. Since the elemental analysis results from MPCM1 to MPCM3 are similar, only the results of MPCM1 are shown. As can be seen from Fig. 6, MPCM1 contains the element silicon, namely the presence of silica. The mass fraction of silicon is 1.21% in the EDAX data. The value is relatively small due to the measurement of EDAX is semi-quantitative. In addition, the thickness of the silica shell is thin and the mass of PCM in MPCM is much higher than that of silica shell.
Fig. 8. Solidifying DSC curves of the SAL and MPCM1–MPCM3.
amount of the SAL can improve the thermal performance of the MPCM. The encapsulation rate and encapsulation efficiency of silica microencapsulated stearyl alcohol were used to characterize its phase transition properties. The encapsulation rate and efficiency can be calculated respectively according to the following Eqs. (1) and (2) [48].
3.4. Thermal energy storage properties of the MPCM The phase change enthalpy and transition temperature are two vital factors for the latent storage in practical applications. DSC curves which shows the whole melting (endothermic transition) and solidifying (exothermic transition) process of the MPCM were obtained to study the thermal properties. Figs. 7 and 8 presents the melting and solidifying process DSC curves of the SAL and MPCM, respectively. The DSC data like onset temperature, peak temperature and latent heat are listed in Table 2. From the DSC melting curves, each MPCM has only one endothermic peak, which corresponds to the solid-liquid phase change of the SAL. However, two exothermic peaks occur in the solidifying process curves. Previous studies have shown that this is a nature of an alkanol. When the temperature drops to the freezing point, the PCM changes from liquid to solid and releases latent heat during this process. This process is called liquid-solid (L-S) solidification process. At this point, the molecular orientation of alkanols is disordered. With the further decrease of temperature, the arrangement of molecular orientation becomes orderly. The latent heat release of this process is relatively small, which is called solid-solid (S-S) phase transition [44]. As can be seen from the Figs. 7 and 8, the melting temperature and solidifying temperature of the MPCM are lower than SAL. This is due to the fact that silica shell has an effect on the melting temperature and solidifying temperature. As the mass of SAL in the MPCM decreases, the latent heat values of the MPCM also decrease. This is due to the SAL is the only thermal energy storage material to absorb and release heat during phase change process. As known from Table 2, the latent heat value of MPCM1 is the largest. This result indicates that suitable
R=
ΔHm, MPCM × 100% ΔHm, PCM
(1)
E=
ΔHm, MPCM + ΔHc, MPCM × 100% ΔHm, PCM + ΔHc, PCM
(2)
where ΔHm,MPCM and ΔHm,PCM note the fusion heat enthalpy of the MPCM and SAL, respectively. ΔHc,MPCM and ΔHc,PCM represent the crystallization enthalpy of the MPCM and SAL, respectively. R and E are the encapsulation rate and encapsulation efficiency. The encapsulation rate describes the effective encapsulation of SAL within the microcapsules while the loading content is considered as dry weight percent of the core material. The encapsulation efficiency results from both melting and crystallization processes. Thus, it is more accurate to evaluate the working efficiency of the MPCM than the encapsulation rate. According to the experiment data, the R value of MPCM1 can reach up to 91.1%, the E value of MPCM1 can reach up to 90.6%. It can also be understood that the conversion rate of PCM is 90.6%. Table 3 presents the comparison of the MPCM with other MPCM in literature. The MPCM1 in this work has achieved a high latent heat value. In addition, the phase change temperature range is also suitable for living and production purposes. For example, the SA/SiO2 nanocapsules fabricated through the sol-gel process were spherical with diameters in the 62–464 nm range. The satisfactory sample melted at 63.9 °C with the latent heats of 169.4 J/g [48]. PEG was used as phase change material (PCM) to store and release thermal energy and SiO2 acted as the supporting matrix. The phase change enthalpy of PEG@ SiO2 is 164.9 J/g in the melting process and 160.1 J/g in the solidifying process with the mass fraction of 97 wt% [49]. The MPCM1 in the present work melted at 55.89 °C with the latent heats of 229.73 J/g. There is a significant increase in latent heat. Thermal conductivity of PCM and MPCM were tested by a thermal conductivity meter. The data can be seen in Table 4 and the differences of thermal conductivity between PCM and MPCM in both solid and melted states are shown in Fig. 9. The thermal conductivity of each sample in the melted state is higher than that in the solid state. This is due to the fact that the natural convection of the liquid PCM in the melted state improves the thermal conductivity. In addition, the thermal conductivity of each MPCM is slightly higher than that of PCM. The thermal conductivity values of three MPCM are similar. Zhang et al. [45] prepared silica-shell microcapsules and the microcapsule had a higher thermal conductivity than that in our work. One reason is that
Fig. 7. Melting DSC curves of the SAL and MPCM1–MPCM3. 6
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Table 2 DSC data of the SAL and MPCM. Samples
SAL MPCM1 MPCM2 MPCM3
Melting
Solidifying
Onset temperature (°C)
Latent heat (kJ/kg)
Onset temperature for L-S/S-S (°C)
Latent heat for L-S/S-S (J/g)
57.84 55.89 56.35 56.65
243.03 229.73 224.98 218.30
57.06/51.10 56.74/49.75 56.46/50.43 56.43/50.06
143.39 112.05 111.64 112.83
± ± ± ±
0.2 0.2 0.2 0.2
± ± ± ±
12.15 11.84 11.25 10.91
± ± ± ±
0.2 0.2 0.2 0.2
± ± ± ±
7.17/81.18 5.60/72.01 5.58/75.39 5.64/71.95
± ± ± ±
4.06 3.60 3.77 3.60
Table 3 Comparison of the present work with results of other MPCM in literature. Samples
Melting point (°C)
Melting latent heat (J/g)
Reference
GO1@SA + silica Paraffin + silica LA + silica n-OD + silica n-docosane + silica Stearic acid + silica Polyethylene glycol + silica Stearyl alcohol + silica
65.12 58.37 41.5 28.12 46.5 63.9 60.4 55.89
146.72 165.68 93.8 115.7 141.2 169.4 164.9 229.73
[35] [36] [45] [46] [47] [48] [49] Present work
Table 4 Thermal conductivity of the PCM and MPCM. Samples
Thermal conductivity in the solid state (W/m⋅K)
Thermal conductivity in the melted state (W/m⋅K)
PCM MPCM1 MPCM2 MPCM3
0.1297 ± 0.00454 0.14124 ± 0.00122 0.13032 ± 0.0012 0.13968 ± 0.00231
0.14602 0.15078 0.15332 0.15434
Fig. 10. The temperature curves of the PCM and MPCM1 in melting process. ± ± ± ±
0.00672 0.00587 0.00453 0.00411
Fig. 11. The temperature curves of the PCM and MPCM1 in solidifying process.
temperature to simulate the solidifying process. The current temperature of the samples in the beaker was recorded every 15 s by a thermocouple thermometer. In Fig. 10, the charging time initiates from the same temperature (23 °C) and ends at the same melting temperature (71.4 °C). The charging time of the PCM and MPCM1 are 36.5 min and 23 min respectively, which means the melting time of the MPCM1 reduces 37% as compared to that of PCM. In Fig. 11, the discharging time starts from the same temperature (71.4 °C) and finishes at the same solidifying temperature (23.8 °C). The discharging time of the PCM and MPCM1 are respectively 75 min and 63 min, indicating that the solidifying time of the MPCM1 diminished about 16% in contrasting with that of PCM. The results obviously showed that by preparing silica-shell microcapsules, the heat storage and release rates are significantly increased. The silica with good thermal conductivity acted as heat conductive bridge, which is beneficial in accelerating thermal energy storage and release.
Fig. 9. Thermal conductivity of the PCM and MPCMs in both solid and melted state.
core materials are different, which results in different connection with silica shell. Another reason is that our samples were powdery in thermal conductivity test, so the porous structure also affected thermal conductivity of the MPCM.
3.5. The heat storage and heat release properties of the MPCM PCM and MPCM1 were evaluated by comparing their charging and discharging time, the temperature curves are displayed in Figs. 10 and 11. 50 g of powdered PCM and 50 g of powdered MPCM1 were placed in two separate beakers. The heater was used to simulate the actual melting process. PCM and MPCM1 were cooled naturally at room 7
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pure SAL, and the thermal stability can be improved. The reason that weight loss for MPCM is not much less is that the amount of silicon precursor is relatively small, resulting in a small amount of silica shell. 3.7. Thermal reliability of the MPCM Thermal cycling test was carried out to determine the thermal reliability of the MPCM. The thermal cycling started from 20 °C and ended at 70 °C. Among the three MPCM, the MPCM1 has the largest latent heat. Therefore, it was measured in this experiment. The melting DSC data of uncycled and cycled MPCM1 are listed in Table 6. After 100 thermal cycles, the melting temperature has almost no change. Thus, the MPCM1 has good thermal reliability in terms of the change in melting temperature. Moreover, the melting enthalpy of MPCM1 is reduced by 8.88% after thermal cycles, but still is 204.39 J/g. The latent heat loss of the MPCM1 after thermal cycles is within a reasonable range in the practical applications.
Fig. 12. TGA curves of the SAL, silica and MPCM1–MPCM3.
4. Conclusions Table 5 TGA data of the SAL, MPCMs and silica. Samples
Tpeak (°C)
△W (%)
Residue (%) (600 °C)
SAL Silica MPCM1 MPCM2 MPCM3
241.72 81.05 237.11 246.06 236.53
99.72 22.11 98.95 97.92 97.58
0.28 77.89 1.05 2.08 2.42
The investigation on microencapsulated phase change materials has greatly promoted thermal energy storage development. In this work, microencapsulated phase change materials with SAL as the core material and silica as the shell material were successfully prepared by sol–gel method. The chemical structure and crystal phase of SAL, silica and MPCM were measured by FT–IR and XRD. These results indicated that only physical interactions have taken place between SAL and silica. The images of the MPCM observed by SEM demonstrated that the MPCM has a complete spherical and compact shell. The DSC results showed that the best microencapsulated phase change materials is MPCM1. Its melting temperature and latent heat are 55.89 °C and 229.73 kJ/kg, respectively. The encapsulation rate reaches up to 90%. The latent heat becomes larger when the proportion of SAL in the MPCM increases. The results of TGA and DTG confirmed that MPCM has good thermal stability over their working range. After 100 thermal cycles, the latent heat and melting temperature of the MPCM1 hardly changed. The thermal conductivity of the MPCM1 during melting state was measured to be 0.1508 W/m·K. Therefore, MPCM1 is found to be a promising candidate for applying in thermal energy storage.
3.6. Thermal stability of the MPCM The thermal stability of the SAL, silica and MPCM were measured by TGA. The curves of the TGA are showed in Fig. 12. The data of the TGA are listed in Table 5, which contains the temperature Tpeak corresponding to the maximum decomposition rate, the percent weight loss △W in the decomposition stage corresponding to Tpeak, and the residual amount at 600 °C. Zhang et al. [50] prepared similar silica-shell microcapsules. The results of the TGA curves showed two-step degradation. The second step degradation corresponding to the silica condensation occurs at around 280 °C. However, according to our experimental results, the MPCM1 has only one step degradation, which starts at 147 °C, reaches a maximum decomposition rate at 237 °C. The SAL was almost completely decomposed during this process. In this work, the reaction was well carried out and there was no residual silanols. Therefore, there was no the second step degradation corresponding to silanols degradation. The weight loss of the silica is 22.11% at 81 °C, which can be attributed to the evaporation of water molecules remaining in the silica. When the temperature is over 100 °C, the decomposition rate of silica becomes close to zero, which indicates that silica has almost no mass loss. The working temperature of the MPCM is about 58 °C. At this temperature, the maximum weight loss percentage of the MPCM1 is 0.116%. From another point of view, the MPCM possess decomposition temperature over 242 °C, which is much higher than their working temperature. Therefore, it can be confirmed that the MPCM has good thermal stability at the working temperature range. It can be seen from Table 5 that weight loss for MPCM is less than that for
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This project is supported by the National Natural Science Foundation of China (Grant nos. 51676095, 51376087). The authors also wish to thank the reviewers and editor for kindly giving revising suggestions. Appendix A. Supplementary material Supplementary data to this article can be found online at https://
Table 6 DSC data of the MPCM1 before and after 100 thermal cycles. Samples
Before 100 thermal cycles After 100 thermal cycles
Melting
Solidifying
Onset temperature (°C)
Latent heat (J/g)
Onset temperature for L-S/S-S (°C)
Latent heat for L-S/S-S (J/g)
55.89 57.09
229.73 204.39
56.74/49.75 57.14/49.23
112.05/72.01 105.58/60.46
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doi.org/10.1016/j.applthermaleng.2020.114943.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Preparation and thermal properties of microencapsulated stearyl alcohol with silicon dioxide shell as thermal energy storage materials Chuqiao Zhu, Yaxue Lin, Guiyin Fang
T
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School of Physics, Nanjing University, Nanjing 210093, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal energy storage Microencapsulated phase change material Thermal properties Stearyl alcohol Silica
Microencapsulation of stearyl alcohol (SAL) with silica shell using sol-gel method has been attempted to obtain microencapsulated phase change materials (MPCM). Tetraethoxysilane (TEOS) was used as silica precursor to form silica shell. In order to inveatigate thermal properties of the MPCM, some tests were carried out. The results of Fourier transformation infrared spectroscope (FT–IR) indicated that the combination of the SAL with silica by sol-gel method is a physical action. X–ray diffractometer (XRD) was used to confirm PCM maintains stable crystal structure. Scanning electronic microscope (SEM) was employed to determine the MPCM has relatively stable physical structure. Thermal properties of MPCM were measured by Differential scanning calorimeter (DSC), where melting temperature and latent heat of MPCM1 (made from 10 g SAL, 10 g TEOS) is 55.89 °C and 229.73 J/g, respectively. Liquid–solid phase change has latent heat of 112.05 J/g at 56.74 °C while solid-solid phase change has latent heat of 72.01 J/g at 49.75 °C. Thermogravimetric analyzer (TGA) analysis confirms that MPCM has good thermal stability. After 100 thermal cycles of the MPCM1, there was no change in phase transition temperature and latent heat, which confirms that the MPCM has good reliability. The thermal conductivity of MPCM1 was measured to be 0.1508 W/m·K over melting point of MPCM1 and 0.1412 W/m·K at room temperature. Therefore, MPCM1 is found to be a promising candidate for thermal energy storage applications.
1. Introduction With the development and progress of human society, energy consumption is increasing. Fossil fuels such as coal, oil, and natural gas are facing an imminent situation in which they will be used out [1]. At the same time, excessive use of some energy sources has caused irreversible environmental pollution [2]. Therefore, finding suitable alternative energy sources, improving energy efficiency and optimizing energy utilization structure are the next things people should pay attention to. Some renewable energy sources such as solar energy are slowly gaining popularity. However, these energy sources have more or less problems of uneven geographical and temporal distribution. For example, solar energy only exists during the day or on a sunny day and can not be collected continuously [3]. Thermal energy storage systems can solve this problem. It can store excess heat or waste heat, etc., and take it out when needed [4]. In other words, the diversification of the structure of the thermal energy storage system is a subject worth studying. Thermal energy storage system mainly has three energy storage modes, namely sensible heat storage, latent heat storage and chemical reaction heat storage. Compared with sensible heat storage, latent heat
⁎
storage can store more energy and has better controllability [5]. Chemical reaction heat storage has high storage capacity than other thermal energy storage system due to chemical reaction and high operating temperature. However, chemical reaction heat storage reaction’s conditions are demanding and difficult to be controlled [6]. Phase change material (PCM) is a typical thermal storage material that utilizes latent heat storage, which has high latent heat and stable phase transition temperature [7]. Phase change material has the ability to change their physical state within a certain temperature range. Take solid-liquid phase change as an example, when heated to the melting temperature, a phase change from solid to liquid is generated. During the melting process, a large amount of latent heat is absorbed and stored by the phase change material. When a phase-change material is cooled, the stored heat is released into the environment within a certain temperature range for the reverse phase transition from liquid to solid. The energy stored or released during these two phase transitions is called latent heat of phase transition. When the physical state changes, the temperature of the material itself remains almost unchanged before the phase change is completed, forming a wide temperature platform. Although the temperature remains unchanged, the latent heat absorbed
Corresponding author. E-mail address: [email protected] (G. Fang).
https://doi.org/10.1016/j.applthermaleng.2020.114943 Received 18 July 2019; Received in revised form 10 December 2019; Accepted 12 January 2020 Available online 14 January 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.
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stability and weaker compatibility with building materials [29-31]. So inorganic shells have also been developed and studied, such as silica, titania [32,33], calcium carbonate [34] etc., and the study of the silica shells is relatively extensive. Lin et al. [35] used methyl triethoxysilane (MTES) as a raw material to synthesize silica shell by sol-gel method. Chen et al. [36] used sol-gel method to prepare MPCM with paraffin core and silica shell, and the TEOS was selected as silica precursor. The mass ratio of the paraffin is 87.5% and the MPCM melts at 58.37 °C with a latent heat of 165.68 J/g. He et al. [37] used sodium silicate precursor to form silica shell with the core material of n-octadecane. Sami et al. [38] carried out microencapsulation of lauric acid with polystyrene shell, and the results indicated that microencapsulated PCM with a melting point of 43.77 °C and latent heat of 167.26 kJ/kg can be used in solar energy systems. Fang et al. [39] performed an experimental study about the thermal characteristics of microencapsulated phase change composite cylinders, the results showed that the higher PCM core fraction leads to a lower temperature difference between the internal surface and the external surface. Song et al. [40] prepared microencapsulated capric-stearic acid with silica shell as a novel phase change material, and the phase change temperature and latent heat are 21.4 °C and 91.48 kJ/kg. However, such information on the microencapsulation of saturated polyols PCM has been limited. Saturated polyols are rarely tried as core materials because polyols are hydrophilic. It seems that there have been no fundamental studies which related to the silica-shell microcapsules containing saturated polyols PCM. There still exists a knowledge gap about the effective combination between silica shell and stearyl alcohol. Based on the above facts, the aim of this study is to develop high thermal conductivity MPCM. The silica shell caters the requirements of this study. Besides, silicon dioxide as the shell material is non-toxic and flame retardant, that is not easy to degrade at high temperature. In previous work, core materials are usually alkanes, fatty acids, etc. For example, the SA/SiO2 nanocapsules fabricated through the sol-gel process were spherical with diameters in the 62–464 nm range. The satisfactory sample melted at 63.9 °C with the latent heats of 169.4 J/g [48]. In this work, stearyl alcohol was used as core material, which has higher latent heat than paraffin or other organics phase change material. The results showed that the MPCM can retain the high latent heat of stearyl alcohol which can attain 229.73 J/g at the transition temperature of 55.9 °C. Meanwhile, the experimental results revealed there is no chemical reaction between the silica shell and stearyl alcohol. Another highlight of this work is that the working efficiency of the MPCM can reach up to 90.6%. This can greatly improve the conversion efficiency of practical applications such as MPCM systems in green building walls which cater to this phase change temperature. In addition, thermal conductivity is enhanced obviously with a little latent heat loss because the results showed that the heat storage and release rates are significantly increased by preparing silica-shell microcapsules. In this work, microencapsulated stearyl alcohol with silica shell by using sol–gel method was performed. Thermal properties and thermal stability of MPCM were measured and analyzed. The microcapsules will be a potential candidate for thermal energy storage.
or released is quite large [8–10]. However, microencapsulated phase change materials have emerged as phase change materials are prone to leakage during solid-liquid phase transition and their thermal conductivity is not very high. Microencapsulated phase change material refers to a structure in which a phase change material is wrapped with another material to form a capsule-like structure. It not only reduces the leakage of the phase change material, but also acts as a physical protector to adapt to the volume change during the phase change. Some thermal properties such as thermal conductivity and thermal stability have also been improved [11,12]. Microencapsulation describes a process in which PCM particles or droplets are surrounded or embedded in a homogeneous matrix at the size of below 1000 µm [13]. MPCM can be encapsulated by physical method like spray drying, physical–chemical methods like coacervation and sol–gel process, and chemical methods like interfacial polymerization, suspension polymerization and emulsion polymerization. The spray drying method has the advantages of being economical and easy to scale up to produce MPCM, but agglomeration is prone to occur by spray drying, and some particles will be missed [14,15]. The process of interfacial polymerization is simple, where the purity of the reaction monomer is low but the activity of the monomer must be enough high to carry out the polycondensation reaction. The ratio of core and wall materials is not strict, and the reaction rate is fast. The encapsulation efficiency is high [16]. Suspension polymerization and coacervation are suitable for producing larger particles with high core material content and high encapsulation efficiency [17]. Emulsion polymerization can be used to prepare microcapsules with smaller size [18]. The sol-gel method is suitable for the synthesis of inorganic shells [19]. The agglomeration of spray drying is unacceptable for this experiment. The reaction rate of interfacial polymerization is so fast that the time is not easy to be controlled. Suspension and emulsion polymerization methods are not easy to be scaled up to large scale production. In this study, inorganic silica was used as the shell, so the sol-gel method was adopted. In addition, MPCM has a great ability to increase the thermal conductivity of the phase change material, due to the increased specific surface area of the PCM. What’s more, thermal conductivity of MPCM encapsulated with inorganic materials is higher than that of MPCM encapsulated with organic materials. Thermal stability and heat capacity of MPCM are also proved to be better than that of PCM. The MPCM offers a reliable way for the mixing solid and liquid PCM with polymer and other structural materials, which expand the application fields of the PCM. They have the function of protecting core material from environmental influence and reducing toxicity. Due to the large storage capacity and isothermal nature of thermal storage process, microencapsulation techniques of PCM have attracted great attention. The microencapsulation technology has extended to buildings, textiles, MPCM slurry, etc. Generally, the solid-liquid phase change material consists of organic PCM, inorganic PCM and eutectic [20]. Compared with the inorganic PCM, the organic PCM behaves non-corrosives, low subcooling, chemical and thermal stability [21,22]. In addition, the inorganic PCM has serious disadvantages of phase separation and poor thermal stability. Stearyl alcohol (SAL) would be employed as the PCM in this work, which phase change enthalpy is above 200 J/g [23]. The shell for encapsulating PCM may be organic materials, inorganic materials or mixed materials, but all of them possess the common characteristics: good thermal and chemical stability, no reactions between shell and PCM and the higher melting point than PCM [12]. Currently, the extensively used organic shells are polymers containing poly (methyl methacrylate) (PMMA) [24–26], melamine–formaldehyde [27], poly (melamine–urea–formaldehyde) [28] etc., which possess good sealing properties and thermal and chemical stability [29]. Nevertheless, organic shells usually suffer from toxicity and flammability. As compared with inorganic shells, they have weaker mechanical strength, lower thermal conductivity, poorer thermal
2. Synthesis and characterization 2.1. Materials Stearyl alcohol (SAL, C18H38O, melting point: 59.4–59.8 °C, analytical reagent) was used as PCM to store latent heat, which was supplied by Sinopharm Chemical Reagent Co., Ltd.. Tetraethoxysilane (TEOS, C8H20O4Si, reagent grade) was provided by Sinopharm Chemical Reagent Co., Ltd. for producing silica. Sodium dodecyl sulfate (SDS, C12H25SO4Na, reagent grade) was used as emulsifier to form stable oilwater emulsion. Anhydrous ethanol (Reagent grade, Sinopharm Chemical Reagent Co., Ltd) was employed as the solvent and water was distilled and deionized using a Millipore Milli–Q Purification System. 2
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The raw materials for the preparation of the SAL oil-water emulsion were 10 g of SAL, 0.5 g of SDS and 150 mL of deionized water. The specific operation was as follows: the weighed SAL and SDS were added to deionized water in a 250 mL beaker. Then, the suspension was stirred by a magnetic stirrer with the speed rate of 800 rpm for 120 min at a constant temperature of 70 °C. Finally, stop stirring the emulsion and keep heating it at the temperature until the SAL oil-water emulsion was formed. In the process of heating and stirring, the hydrophilic group of the surfactant SDS was directed to the water molecule, and the lipophilic group was combined with the SAL oil droplet, so that a layer which surface is active was formed on the outer layer of the SAL oil droplet. After a period of time, a stable oil-water emulsion was formed, in which the SAL oil droplets were uniformly dispersed in the deionized water [41].
water solvent in the solution evaporated, and the microcapsules were formed. The reaction mechanism of hydrolysis and polymerization process and the final shell-forming process can be seen in Fig. 1. The prepared microencapsulated phase change materials were washed and filtered. The filter paper was folded into a funnel shape and pressed against the funnel wall. The funnel was inserted into a large round-bottomed flask. At room temperature, the samples were put on the filter paper. Appropriate amount of deionized water was poured to clean and filter. Then they were dried in a vacuum oven at 50 °C for 24 h. Three microencapsulated phase change materials were prepared, which are called MPCM1, MPCM2 and MPCM3 respectively. To confirm the stable physical structure of the three MPCM samples, a leakage test was performed. The MPCM on filter paper was put on the heater and heated for 10 min at a constant temperature of 70 °C, which is higher than phase change temperature of the SAL. Then, the MPCM on filter paper was removed and cooled in the air. After repeating the above steps for five times, leakage test results were observed. As shown in Fig. 2, there was no oily liquid on the filter paper. The conclusion is that the PCM were well encapsulated.
2.3. Preparation of the MPCM
2.4. Characterization of the MPCM
The next procedure was to prepare the precursor solution. Here, three different doses of raw materials were used to get three MPCM samples with different core-shell ratios. They are called MPCM1, MPCM2 and MPCM3, and the mass of raw materials used for preparation process are listed in Table 1. The first step was the hydrolysis of the TEOS, and the process was as follows: (1) Take MPCM1 as an example, 10 g absolute ethanol and 20 g deionized water were mixed as a solvent in a beaker, and then add the solution into another beaker which contained 10 g TEOS. (2) The pH value of the TEOS solution was adjusted to 2–3 by adding hydrochloric acid resulting in a steady solution. The acid region pH value was to inhibit the TEOS hydrolysis rate, so that the hydrolysis reaction did not go too fast. It can be seen from the reaction mechanism that TEOS can also participate in the polymerization process, and partial hydrolysis is what we need. If the hydrolysis is complete, the formation of intermediate products wound be affected, and the coating may be incomplete. The pH value was measured by pH test paper. (3) The TEOS precursor solution was stirred by a magnetic stirrer with a speed rate of 500 rpm for 30 min at a constant temperature of 70 °C. The hydrolysis reaction of the TEOS occurred and orthosilicic acid sol solution was obtained with colloidal substance. The second step was the polymerization of orthosilicic acid, and the process was as follows: (1) The prepared SAL oil-water emulsion was stirred with a speed of 600 rpm and the temperature was maintained at 70 °C. (2) The orthosilicic acid sol solution was added to the emulsion drop by drop, and then the mixed solution was stirred for 3 h at the same stirring speed and temperature. At this stage, the polymerization reaction began. Due to the electrostatic interaction between the orthosilicic acid produced by the hydrolysis and the hydrophilic segment of the surfactant, the orthosilicic acid was attracted to the surface of the SAL micelle. Then, silanol which contains the Si-O-Si bond formed a backbone structure. As a result, silica shell was formed on the surface of the SAL droplet [35,42]. After 3 h of agitation, most of the ethanol and
Fourier transformation infrared spectroscope (FT–IR, Nicolet Nexus 870, spectra from 400 to 4000 cm−1, resolution using KBr pellets: 2 cm−1) was used to analyze the chemical structures of the MPCM. X–ray diffractometer (XRD, D/MAX–Ultima III, working voltage: 40 kV, working current: 40 mA, scanning rate: 5° (2θ)/min, Rigaku Corporation, Japan) was employed for revealing crystal structure of the MPCM. A scanning electron microscope (SEM, S–3400NII, operating voltage: 3 kV, Hitachi Inc., Japan) was used to examine the morphology of the MPCM. The thermal properties of the MPCM were measured by differential scanning calorimeter (DSC, Pyris 1 DSC, Perkin–Elmer, temperature accuracy: ± 0.2 °C, enthalpy accuracy: ± 5%) under a sustained stream of argon at a rate of 5 °C/min, where the range of temperature was 10 to 100 °C. The thermal stability of the MPCM was measured by a thermogravimeter (TGA, Pyris 1 TGA, Perkin–Elmer) under a constant stream of nitrogen, and the temperature was increased from room temperature to 700 °C at a rate of 20 °C/min. The thermal conductivity of the MPCM was obtained by a thermal conductivity meter (TCM 3020, Xiatech Electronic Technology co., Ltd., accuracy: ± 2%) through transient hot wire technique.
Hydrochloric acid (Analytical reagent, Nanjing Chemical Reagent Co., Ltd.) was used to adjust the pH value. 2.2. Preparation of the SAL oil-water emulsion
3. Results and discussion 3.1. FT–IR analysis of the MPCM The changes in the functional groups of the components of the samples due to a chemical interaction can be extracted from by FT–IR spectra. Fig. 3 shows the FT–IR spectra of SAL, silica and MPCM, respectively, which were used to analyze the material composition and possible chemical interactions in the microcapsules. As seen in Fig. 3a, the spectrum of SAL has six major absorption peaks. The broad absorption band in the range of 3234–3321 cm−1 belongs to the stretching vibration of eOH group and its normal range is 3500 cm−1 to 3200 cm−1. Due to hydrogen bonding, the broad absorption peak of eOH group is a very characteristic group frequency. The antisymmetric and symmetric stretching vibration of eCH2 group corresponds to the two sharp absorption peak at 2918 cm−1 and 2848 cm−1. The eCH2 scissoring bending vibration causes an absorption peak to appear at 1464 cm−1. The characteristic peak is observed at 1063 cm−1, corresponding to the eCeO stretching vibration antisymmetrically coupled to the CeC stretch vibration. The peak at 719 cm−1 reveals that there are four or more eCH2 groups in a chain. Fig. 3b shows the FT–IR spectrum of the silica. The broad absorption band at 3000–3600 cm−1 and 1600–1700 cm−1 agree with the bending and stretching vibrations of the eOH functional group of H2O, which indicates that a small
Table 1 The compositions of the MPCM. Samples
MPCM1 MPCM2 MPCM3
SAL emulsion
TEOS solution
SAL (g)
SDS (g)
Deionized water (mL)
TEOS (g)
Anhydrous ethanol (g)
Deionized water (mL)
10 10 10
0.5 0.5 0.5
150 150 150
10 15 20
10 15 20
20 30 40
3
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Fig. 1. The reaction mechanism of hydrolysis and polymerization process of the TEOS. (a) The hydrolysis process, (b) The polymerization process and (c) The process of forming silica shell.
Fig. 3. FT–IR spectra of (a) SAL, (b) silica, (c) – (e) MPCM1–MPCM3.
that the combination of the SAL with silica is a physical combination without chemical reaction.
3.2. XRD analysis of the MPCM The crystalloid phase and crystallinity of the SAL, silica and MPCM are presented using the XRD patterns which are shown in Fig. 4. The diffraction pattern of SAL is exhibited in Fig. 4a, which includes one strong and sharp diffraction peak at 21.79°. It results from diffraction of the (−8 0 2) crystal planes of SAL. The other diffraction peaks are relatively weak, such as the peaks at 20.71°, 24.7°and 36.28°, which correspond to the diffraction of the (2 1 1), (−17 1 0) and (3 2 1) crystal planes of the SAL, respectively [43]. Fig. 4b represents the diffraction of silica, which has no sharp peaks. This result indicates that the silica is non-crystalline with amorphous structure so that the silica shell can be completely formed. As seen in Fig. 4c–e, the set of characteristic peaks of SAL exists in the MPCM. Simultaneously, when the mass ratio of the SAL decreases, the peak becomes more flat. Combined with the result of the FT–IR analysis, it can be determined that the PCM maintain original and stable crystal structure and exist no chemical reaction with silica.
Fig. 2. Leakage test of the (a) MPCM1, (b) MPCM2 and (c) MPCM3.
amount of water molecules were adsorbed in silica. The bending vibration of the SieO functional group leads to occurrence of three peaks, which locates at 1063 cm−1, 790 cm−1 and 482 cm−1, respectively. The peak at 960 cm−1 is assigned to the SieOH functional group. Comparing Fig. 3a and b with c–e, it can be confirmed that there is no new peak occurrence in the spectrum of the MPCM, which indicates 4
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Fig. 6. Elemental composition images of MPCM1.
3.3. Morphology of the MPCM The morphology of MPCM at different magnifications is shown in Fig. 5. Photograph (b), (d) and (f) are single microscopic appearance of the MPCM. It can be seen that silica forms a shell on the surface of SAL
Fig. 4. XRD patterns of (a) SAL, (b) silica, (c) – (e) MPCM1–MPCM3.
Fig. 5. SEM images of (a) MPCM1 (1.50 k×), (b) MPCM1 (6.00 k×), (c) MPCM2 (1.50 k×), (d) MPCM2 (10.0 k×), (e) MPCM3 (1.5 k×) and (f) MPCM3 (10.0 k×). 5
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and a spherical structure is formed with particle sizes ranging from 5 µm to 12 µm. Some small spheres have something stuck on them, and it may be incompletely reacted silicic acid or excess of silica. Subsequent EDAX tests proved that the shell material was silica. Photograph (a), (c) and (e) are whole appearance of three MPCM. It can be seen that they are relatively uniform and clustered together. From the perspective of SEM, the particle size of MPCM is uniform. The results confirm that the MPCM has a relatively uniform physical structure and the silica shell is effective protection for the SAL. To further demonstrate the presence of silica in MPCM, we continued to perform elemental analysis tests using EDAX. Since the elemental analysis results from MPCM1 to MPCM3 are similar, only the results of MPCM1 are shown. As can be seen from Fig. 6, MPCM1 contains the element silicon, namely the presence of silica. The mass fraction of silicon is 1.21% in the EDAX data. The value is relatively small due to the measurement of EDAX is semi-quantitative. In addition, the thickness of the silica shell is thin and the mass of PCM in MPCM is much higher than that of silica shell.
Fig. 8. Solidifying DSC curves of the SAL and MPCM1–MPCM3.
amount of the SAL can improve the thermal performance of the MPCM. The encapsulation rate and encapsulation efficiency of silica microencapsulated stearyl alcohol were used to characterize its phase transition properties. The encapsulation rate and efficiency can be calculated respectively according to the following Eqs. (1) and (2) [48].
3.4. Thermal energy storage properties of the MPCM The phase change enthalpy and transition temperature are two vital factors for the latent storage in practical applications. DSC curves which shows the whole melting (endothermic transition) and solidifying (exothermic transition) process of the MPCM were obtained to study the thermal properties. Figs. 7 and 8 presents the melting and solidifying process DSC curves of the SAL and MPCM, respectively. The DSC data like onset temperature, peak temperature and latent heat are listed in Table 2. From the DSC melting curves, each MPCM has only one endothermic peak, which corresponds to the solid-liquid phase change of the SAL. However, two exothermic peaks occur in the solidifying process curves. Previous studies have shown that this is a nature of an alkanol. When the temperature drops to the freezing point, the PCM changes from liquid to solid and releases latent heat during this process. This process is called liquid-solid (L-S) solidification process. At this point, the molecular orientation of alkanols is disordered. With the further decrease of temperature, the arrangement of molecular orientation becomes orderly. The latent heat release of this process is relatively small, which is called solid-solid (S-S) phase transition [44]. As can be seen from the Figs. 7 and 8, the melting temperature and solidifying temperature of the MPCM are lower than SAL. This is due to the fact that silica shell has an effect on the melting temperature and solidifying temperature. As the mass of SAL in the MPCM decreases, the latent heat values of the MPCM also decrease. This is due to the SAL is the only thermal energy storage material to absorb and release heat during phase change process. As known from Table 2, the latent heat value of MPCM1 is the largest. This result indicates that suitable
R=
ΔHm, MPCM × 100% ΔHm, PCM
(1)
E=
ΔHm, MPCM + ΔHc, MPCM × 100% ΔHm, PCM + ΔHc, PCM
(2)
where ΔHm,MPCM and ΔHm,PCM note the fusion heat enthalpy of the MPCM and SAL, respectively. ΔHc,MPCM and ΔHc,PCM represent the crystallization enthalpy of the MPCM and SAL, respectively. R and E are the encapsulation rate and encapsulation efficiency. The encapsulation rate describes the effective encapsulation of SAL within the microcapsules while the loading content is considered as dry weight percent of the core material. The encapsulation efficiency results from both melting and crystallization processes. Thus, it is more accurate to evaluate the working efficiency of the MPCM than the encapsulation rate. According to the experiment data, the R value of MPCM1 can reach up to 91.1%, the E value of MPCM1 can reach up to 90.6%. It can also be understood that the conversion rate of PCM is 90.6%. Table 3 presents the comparison of the MPCM with other MPCM in literature. The MPCM1 in this work has achieved a high latent heat value. In addition, the phase change temperature range is also suitable for living and production purposes. For example, the SA/SiO2 nanocapsules fabricated through the sol-gel process were spherical with diameters in the 62–464 nm range. The satisfactory sample melted at 63.9 °C with the latent heats of 169.4 J/g [48]. PEG was used as phase change material (PCM) to store and release thermal energy and SiO2 acted as the supporting matrix. The phase change enthalpy of PEG@ SiO2 is 164.9 J/g in the melting process and 160.1 J/g in the solidifying process with the mass fraction of 97 wt% [49]. The MPCM1 in the present work melted at 55.89 °C with the latent heats of 229.73 J/g. There is a significant increase in latent heat. Thermal conductivity of PCM and MPCM were tested by a thermal conductivity meter. The data can be seen in Table 4 and the differences of thermal conductivity between PCM and MPCM in both solid and melted states are shown in Fig. 9. The thermal conductivity of each sample in the melted state is higher than that in the solid state. This is due to the fact that the natural convection of the liquid PCM in the melted state improves the thermal conductivity. In addition, the thermal conductivity of each MPCM is slightly higher than that of PCM. The thermal conductivity values of three MPCM are similar. Zhang et al. [45] prepared silica-shell microcapsules and the microcapsule had a higher thermal conductivity than that in our work. One reason is that
Fig. 7. Melting DSC curves of the SAL and MPCM1–MPCM3. 6
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Table 2 DSC data of the SAL and MPCM. Samples
SAL MPCM1 MPCM2 MPCM3
Melting
Solidifying
Onset temperature (°C)
Latent heat (kJ/kg)
Onset temperature for L-S/S-S (°C)
Latent heat for L-S/S-S (J/g)
57.84 55.89 56.35 56.65
243.03 229.73 224.98 218.30
57.06/51.10 56.74/49.75 56.46/50.43 56.43/50.06
143.39 112.05 111.64 112.83
± ± ± ±
0.2 0.2 0.2 0.2
± ± ± ±
12.15 11.84 11.25 10.91
± ± ± ±
0.2 0.2 0.2 0.2
± ± ± ±
7.17/81.18 5.60/72.01 5.58/75.39 5.64/71.95
± ± ± ±
4.06 3.60 3.77 3.60
Table 3 Comparison of the present work with results of other MPCM in literature. Samples
Melting point (°C)
Melting latent heat (J/g)
Reference
GO1@SA + silica Paraffin + silica LA + silica n-OD + silica n-docosane + silica Stearic acid + silica Polyethylene glycol + silica Stearyl alcohol + silica
65.12 58.37 41.5 28.12 46.5 63.9 60.4 55.89
146.72 165.68 93.8 115.7 141.2 169.4 164.9 229.73
[35] [36] [45] [46] [47] [48] [49] Present work
Table 4 Thermal conductivity of the PCM and MPCM. Samples
Thermal conductivity in the solid state (W/m⋅K)
Thermal conductivity in the melted state (W/m⋅K)
PCM MPCM1 MPCM2 MPCM3
0.1297 ± 0.00454 0.14124 ± 0.00122 0.13032 ± 0.0012 0.13968 ± 0.00231
0.14602 0.15078 0.15332 0.15434
Fig. 10. The temperature curves of the PCM and MPCM1 in melting process. ± ± ± ±
0.00672 0.00587 0.00453 0.00411
Fig. 11. The temperature curves of the PCM and MPCM1 in solidifying process.
temperature to simulate the solidifying process. The current temperature of the samples in the beaker was recorded every 15 s by a thermocouple thermometer. In Fig. 10, the charging time initiates from the same temperature (23 °C) and ends at the same melting temperature (71.4 °C). The charging time of the PCM and MPCM1 are 36.5 min and 23 min respectively, which means the melting time of the MPCM1 reduces 37% as compared to that of PCM. In Fig. 11, the discharging time starts from the same temperature (71.4 °C) and finishes at the same solidifying temperature (23.8 °C). The discharging time of the PCM and MPCM1 are respectively 75 min and 63 min, indicating that the solidifying time of the MPCM1 diminished about 16% in contrasting with that of PCM. The results obviously showed that by preparing silica-shell microcapsules, the heat storage and release rates are significantly increased. The silica with good thermal conductivity acted as heat conductive bridge, which is beneficial in accelerating thermal energy storage and release.
Fig. 9. Thermal conductivity of the PCM and MPCMs in both solid and melted state.
core materials are different, which results in different connection with silica shell. Another reason is that our samples were powdery in thermal conductivity test, so the porous structure also affected thermal conductivity of the MPCM.
3.5. The heat storage and heat release properties of the MPCM PCM and MPCM1 were evaluated by comparing their charging and discharging time, the temperature curves are displayed in Figs. 10 and 11. 50 g of powdered PCM and 50 g of powdered MPCM1 were placed in two separate beakers. The heater was used to simulate the actual melting process. PCM and MPCM1 were cooled naturally at room 7
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pure SAL, and the thermal stability can be improved. The reason that weight loss for MPCM is not much less is that the amount of silicon precursor is relatively small, resulting in a small amount of silica shell. 3.7. Thermal reliability of the MPCM Thermal cycling test was carried out to determine the thermal reliability of the MPCM. The thermal cycling started from 20 °C and ended at 70 °C. Among the three MPCM, the MPCM1 has the largest latent heat. Therefore, it was measured in this experiment. The melting DSC data of uncycled and cycled MPCM1 are listed in Table 6. After 100 thermal cycles, the melting temperature has almost no change. Thus, the MPCM1 has good thermal reliability in terms of the change in melting temperature. Moreover, the melting enthalpy of MPCM1 is reduced by 8.88% after thermal cycles, but still is 204.39 J/g. The latent heat loss of the MPCM1 after thermal cycles is within a reasonable range in the practical applications.
Fig. 12. TGA curves of the SAL, silica and MPCM1–MPCM3.
4. Conclusions Table 5 TGA data of the SAL, MPCMs and silica. Samples
Tpeak (°C)
△W (%)
Residue (%) (600 °C)
SAL Silica MPCM1 MPCM2 MPCM3
241.72 81.05 237.11 246.06 236.53
99.72 22.11 98.95 97.92 97.58
0.28 77.89 1.05 2.08 2.42
The investigation on microencapsulated phase change materials has greatly promoted thermal energy storage development. In this work, microencapsulated phase change materials with SAL as the core material and silica as the shell material were successfully prepared by sol–gel method. The chemical structure and crystal phase of SAL, silica and MPCM were measured by FT–IR and XRD. These results indicated that only physical interactions have taken place between SAL and silica. The images of the MPCM observed by SEM demonstrated that the MPCM has a complete spherical and compact shell. The DSC results showed that the best microencapsulated phase change materials is MPCM1. Its melting temperature and latent heat are 55.89 °C and 229.73 kJ/kg, respectively. The encapsulation rate reaches up to 90%. The latent heat becomes larger when the proportion of SAL in the MPCM increases. The results of TGA and DTG confirmed that MPCM has good thermal stability over their working range. After 100 thermal cycles, the latent heat and melting temperature of the MPCM1 hardly changed. The thermal conductivity of the MPCM1 during melting state was measured to be 0.1508 W/m·K. Therefore, MPCM1 is found to be a promising candidate for applying in thermal energy storage.
3.6. Thermal stability of the MPCM The thermal stability of the SAL, silica and MPCM were measured by TGA. The curves of the TGA are showed in Fig. 12. The data of the TGA are listed in Table 5, which contains the temperature Tpeak corresponding to the maximum decomposition rate, the percent weight loss △W in the decomposition stage corresponding to Tpeak, and the residual amount at 600 °C. Zhang et al. [50] prepared similar silica-shell microcapsules. The results of the TGA curves showed two-step degradation. The second step degradation corresponding to the silica condensation occurs at around 280 °C. However, according to our experimental results, the MPCM1 has only one step degradation, which starts at 147 °C, reaches a maximum decomposition rate at 237 °C. The SAL was almost completely decomposed during this process. In this work, the reaction was well carried out and there was no residual silanols. Therefore, there was no the second step degradation corresponding to silanols degradation. The weight loss of the silica is 22.11% at 81 °C, which can be attributed to the evaporation of water molecules remaining in the silica. When the temperature is over 100 °C, the decomposition rate of silica becomes close to zero, which indicates that silica has almost no mass loss. The working temperature of the MPCM is about 58 °C. At this temperature, the maximum weight loss percentage of the MPCM1 is 0.116%. From another point of view, the MPCM possess decomposition temperature over 242 °C, which is much higher than their working temperature. Therefore, it can be confirmed that the MPCM has good thermal stability at the working temperature range. It can be seen from Table 5 that weight loss for MPCM is less than that for
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This project is supported by the National Natural Science Foundation of China (Grant nos. 51676095, 51376087). The authors also wish to thank the reviewers and editor for kindly giving revising suggestions. Appendix A. Supplementary material Supplementary data to this article can be found online at https://
Table 6 DSC data of the MPCM1 before and after 100 thermal cycles. Samples
Before 100 thermal cycles After 100 thermal cycles
Melting
Solidifying
Onset temperature (°C)
Latent heat (J/g)
Onset temperature for L-S/S-S (°C)
Latent heat for L-S/S-S (J/g)
55.89 57.09
229.73 204.39
56.74/49.75 57.14/49.23
112.05/72.01 105.58/60.46
8
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doi.org/10.1016/j.applthermaleng.2020.114943.
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