Characterization and thermal performance of microencapsulated sodium thiosulfate pentahydrate as phase change material for thermal energy storage

Solar Energy Materials and Solar Cells 193 (2019) 149–156

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Characterization and thermal performance of microencapsulated sodium thiosulfate pentahydrate as phase change material for thermal energy storage

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Wanwan Fu, Ting Zou, Xianghui Liang, Shuangfeng Wang, Xuenong Gao, Zhengguo Zhang, ⁎ Yutang Fang Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, South China University of Technology, Guangzhou 510640, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Microencapsulated phase change material Sodium thiosulfate pentahydrate Poly(ethyl-2-cyanoacrylate) Interfacial polymerization Shell protection Thermal reliability

In this work, a novel microencapsulated phase change material based on sodium thiosulfate pentahydrate as core and poly(ethyl-2-cyanoacrylate) as shell was successfully synthesized by interfacial polymerization in a water-inoil emulsion system. The morphology, microstructure, surface elemental distribution, chemical composition and crystalline structure of the resultant microcapsules were determined by scanning and transmission electron microscopies, energy dispersive spectroscopy, Fourier-transform infrared spectroscopy and X-ray diffraction. Besides, their thermal properties were also investigated systematically by differential scanning calorimetry and thermogravimetry analysis. The results showed that the microcapsules presented almost spherical profiles with a diameter of about 1.0 µm and a well-defined core-shell structure. Meanwhile, the microcapsules possessed phase change temperature of 46.44 °C and latent heat of 107.0 kJ·kg−1 at the core material/monomer mass ratio of 4/ 2. Due to the protective effect of shell material, the thermal stability of the microcapsules was improved. In addition, the thermal cycling test revealed that the microcapsules had good thermal reliability. Considering the above results, this synthetic technique can be considered as a feasible way to prepare microencapsulated salt hydrates and is expected to extend to the encapsulation of other hydrophilic substances. And the obtained microcapsules have great potential as a solar energy storage material.

1. Introduction In recent years, driven by the urgent need for reconciling the contradiction between shortage of traditional fossil energy sources and increasing energy demand, thermal energy storage has attracted considerable attentions [1,2]. Particularly, latent heat storage technology embedded with phase change materials (PCMs) is considered to be one of the most preferred forms of energy storage since it can store/release large amounts of thermal energy quasi-isothermally when phase change occurs [3–6]. Numerous organic substances (such as paraffins, carboxylic acids and esters) and inorganic substances (like salt hydrates, molten salts and eutectic salts) can be used as PCMs [7,8]. Organic PCMs, in spite of their availability as commercial materials for latent heat energy storage, have low thermal conductivity and are flammable and expensive [9] which extremely restrict their further application. By contrast, inorganic PCMs, especially salt hydrates, possess charming features including nonflammability, wide range of melting temperatures, low cost, high volumetric energy density and relatively high

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thermal conductivity [10], exhibiting great potential in latent heat energy storage. Nevertheless, bulk salt hydrates own the inherent disadvantages of phase separation and supercooling [11], which reduces energy utilization efficiency during charge and discharge process. In addition, the application of salt hydrates in traditional manner would be confronted with corrosion and leakage during solid-liquid phase transition [12]. To handle the unfavorable issues about salt hydrates, many researchers have suggested to introduce microencapsulation technology which is a process of wrapping PCMs in a micro-sized container. With microencapsulation, the salt hydrates can be isolated from the surroundings, making their leakage and corrosion avoidable. Moreover, the shell materials of microencapsulated phase change materials (micro-PCMs) can confine salt hydrates to a designed volume and serve as ideal heterogeneous nucleation sites, thus preventing water loss of the melted salt hydrates and enhancing nucleation ability during cooling [13]. In addition, with enhanced surface area-to-volume ratio, the resultant micro-sized capsules are conductive to improving heat

Corresponding author. E-mail address: [email protected] (Y. Fang).

https://doi.org/10.1016/j.solmat.2019.01.007 Received 21 October 2018; Received in revised form 27 December 2018; Accepted 3 January 2019 0927-0248/ © 2019 Elsevier B.V. All rights reserved.

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Nomenclature Tm ΔHm R

STP PECA SEM TEM EDS FTIR XRD DSC TGA

phase change temperature (°C) latent heat (kJ·kg−1) encapsulation ratio (%)

Abbreviations PCM phase change material Micro-PCM microencapsulated phase change material

sodium thiosulfate pentahydrate poly(ethyl-2-cyanoacrylate) scanning electron microscope transmission electron microscopy energy dispersive spectroscopy Fourier-transform infrared X-ray diffraction differential scanning calorimetry thermogravimetric analysis

surprisingly found that poly(ethyl-2-cyanoacrylate) (PECA) has the potential to be an ideal shell material for microencapsulating salt hydrates. On one hand, PECA occupies the advantages of non-toxicity, biocompatibility, biodegradability and simplicity of the polymerization process [48,49], which caters to sustainable development and human safety. Moreover, its monomer (ECA), as a class of highly reactive cyanoacrylate, is also non-toxic, thus the ECA residues in the shells wouldn’t be a threat to human health or the environment. On the other hand, it is a well-known fact that due to excellent chemical compatibility with soluble substances, PECA has been widely used to encapsulate water soluble drugs [50–53]. Because of the similarity in hydrophily between water soluble drugs and salt hydrates, it is expected that PECA could apply to encapsulate the salt hydrates. Besides, PECA has been proven to nanoencapsulate magnesium nitrate hexahydrate and crystallohydrate mixtures [13,54]. Based on the abovediscussed, it is believed that PECA can be regarded as a promising candidate for wrapping salt hydrates. Hence, in the present study, we aimed at developing a type of microPCM with salt hydrate core by using PECA as shell material. Sodium thiosulfate pentahydrate (STP) as a representative of salt hydrates possesses high latent heat (about 209.0 J g−1) and density (about 1676 kg m−3) [55] which are the key to realizing high volumetric energy density, and enjoys the advantages of nontoxicity, low cost and wide source [56]. Moreover, it has phase change temperature of 48.5 °C, making it especially suitable for thermal energy storage in solar energy applications. For example, the STP-PCM-based solar waterheating [56] and solar desalination [57] systems exhibited an enhancement in thermal energy storage performance compared with the ones without STP. Given the above, STP was chosen as a core material. Up to now, the research of the microencapsulated STP with PECA shell has not been reported in the published literatures. More significantly, the study on the microencapsulation of salt hydrates PCMs is still in the infant stage, which makes relevant researches valuable. In this work, by interfacial polymerization in a water-in-oil emulsion system, a novel micro-PCM based on the STP core and PECA shell was synthesized. The possible synthetic mechanism of the fabricated micro-PCM was discussed, and its morphology and thermal properties were investigated. It is expected that the results obtained herein can provide valuable information for the future encapsulation of hydrophilic substances.

transfer efficiency [14,15]. Consequently, microencapsulation technology facilitates salt hydrates a wide variety of applications in many fields such as smart fibers [16], thermal-regulating solar energy [17], functionally thermal fluid [18,19], air-conditioning [20,21], and energy-saving building materials [22,23]. As for micro-PCMs, shell materials play a crucial role in regulating their surface morphologies and thermal properties [24,25]. Various substances can be used as shell materials, which are majorly classified as inorganic and organic compounds. Although inorganic shells such as silica [26,27], titanium dioxide [28], calcium carbonate [21,29], and zirconium oxide [30] exhibit superiority in thermal conductivity and mechanical strength, there are still ubiquitous disadvantages including friable property [31] and low encapsulation efficiency [32], restricting their long-term and largescale applications. Comparatively, organic shells are more common for the reasons that they avoid the above shortcomings inherent in inorganic ones and share good sealing tightness [33], reasonable cost, wide availability [34] and excellent endurance [35]. A large number of organic polymers, like melamine formaldehyde resin [36,37], ureaformaldehyde resin [38,39], polyurea/polyurethane [40], poly-(methyl methacrylate) [41,42] were employed as shell materials. A literature survey indicates that these typical polymeric shells have achieved great success in the microencapsulation of organic PCMs over the past decades. Even though in comparison with organic PCMs, salt hydrates are more difficult to microencapsulate owing to their hydrophilicity, weak chemical compatibility with organic polymers and trend to alter their crystal water content, the promising results derived from microencapsulated organic PCMs still motivate some researchers to conduct explorations about microencapsulating salt hydrates by using these typical polymeric shells. Huang et al. [43] adopted modified polymethylmethacrylate shell to microencapsulate sodium phosphate heptahydrate via suspension copolymerization-solvent volatile method. Nevertheless, the fabricated microcapsules displayed an irregular morphology and a remarkably increased melting temperature since the shell was lack of chemical compatibility with salt hydrates. Moreover, the thermal-initiation polymerization process of the shell was timeconsuming and costly [2]. Yang et al. [44] successfully microencapsulated calcium chloride hexahydrate with partial polysiloxane and polyurea shell by combining sol–gel process with interfacial polymerization, and the obtained microcapsules presented almost-spherical shapes with a well-defined core–shell structure. Similarly, Schoth [45] and Salaün et al. [46] respectively investigated the microencapsulation of salt hydrates by using polyurethane shells. However, these polyurea/ polyurethane shells were synthesized by highly toxic and strongly irritant aromatic isocyanates [47] (as monomers) such as diphenylmethane diisocyanate and toluene diisocyanate, so any remaining monomers residues in the shells would bring about environmental and health issues in the application. Considering the above problems, for the encapsulation of salt hydrates, choosing a new desirable polymer as shell material with both favorable chemical compatibility with salt hydrates and environmental friendliness, is becoming an urgent challenge to be handled. To meet such a challenge, we carried out a literature survey and

2. Experiment 2.1. Materials Sodium thiosulfate pentahydrate (STP, Na2S2O3·5H2O) served as core material and Tween 80 (polysorbate 80) and Span 20 (sorbitan monolaurate) as emulsifiers were supplied by Tianjin Kemiou Chemical Reagent Co., Ltd., China. Ethyl-2-cyanoacrylate (ECA) employed as shell-forming monomer was kindly provided by Hubei Chengtianheng Biological Technology Co., Ltd., China. Cyclohexane was purchased from Jiangsu Qiangsheng Chemical Co., Ltd., China and used as oil phase of emulsion. Chloroform was solvent for the monomer and obtained commercially from Guangzhou Chemical Reagent Factory, 150

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microscope (JEOL, Japan) working at 120 kV. Prior to the measurement, the sample was dispersed in cyclohexane with the help of an ultrasonicator. Then one drop of the resulting dispersion was pipetted onto a carbon-coated copper grid (200 mesh) on which some microcapsules were collected for the TEM observation. Fourier-transform infrared (FTIR) spectroscopy was used to characterize chemical composition of sample on a Vertex 70 spectrophotometer (Bruker, Germany) in the range 4000–400 cm−1 with KBr pellets at a scanning number of 32. The powder X-ray diffraction (XRD) measurement was carried out to confirm crystalline structure of sample by a D8 Advance X-ray diffractometer (Bruker, Germany) with Cu Ka radiation in the range 5–80°. Phase change behaviors including phase change temperature and latent heat were measured with assistance of a Q20 differential scanning calorimetry (DSC, TA, USA) and the accuracy of the calorimeter in DSC was within ± 1%. The test was carried out under a nitrogen atmosphere at a heating rate of 5 °C·min−1 in the range of −50–100 °C. Thermal stability of sample was analyzed by thermogravimetric analysis (TGA) on a TG 209 F3 Tarsus thermal gravimetric analyzer (Nietzsche, Germany) at a heating rate of 10 °C·min−1 from 30° to 800°C in a nitrogen atmosphere. Thermal reliability of sample was investigated by performing an accelerated thermal cycling test in a WHTH-080 constant temperature chamber (Dongguan Weihuang Testing Equipment Co., Ltd., China). After experiencing 25 and 50 cycles, the changes in phase change behaviors of the sample were evaluated by DSC.

China. Deionized water was used throughout the present work. All materials were analytical reagent and used without further purification. 2.2. Synthesis of STP@PECA microcapsules The microencapsulated sodium thiosulfate pentahydrate with PECA shell was synthetized through interfacial polymerization in a water-inoil (W/O) emulsion system. A typical synthesis procedure was that, an aqueous solution containing 0.6 g STP and 0.4 g deionized water was added into oil phase obtained by mixing 8.55 g cyclohexane and 0.45 g emulsifiers composed of span-20 and tween-80 with mass ratio of 2:3. Then, the mixture was magnetically stirred for 1 h at 700 rpm in a water bath of 25 °C to create an initial macroemulsion. Subsequently, the resulting macroemulsion was sonicated for 15 min (1 s on, 2 s off pulse regime, 70% amplitude) in an ice-water bath using a Scientz-II D sonicator (Ningbo Scientz Biotechnology Co., Ltd., China). After a stable W/O emulsion was formed, 6 ml chloroform solution containing 0.3 g ECA was added dropwise to the emulsion under a constant agitation of 200 rpm for several minutes. The reaction was carried out at 15 °C for 3 h under the stirring speed of 200 rpm. After reaction, the resultant microcapsule suspension was centrifuged and washed with cyclohexane three times repeatedly to purify the microcapsules. Finally, the microcapsules, recorded as STP@PECA, were dried at 50 °C. In addition, by same procedure, pure PECA particles were synthesized without adding STP. 2.3. Characterization of STP@PECA microcapsules

3. Result and discussion Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were performed to observe morphology and surface elemental distribution of sample using a SU8220 scanning electron microscope (Hitachi, Japan) coupled with a X-Max energy-dispersive Xray spectrometer (Horiba, Japan). The SEM images were acquired in a high vacuum mode at an acceleration voltage of 10 kV. The sample was dispersed in cyclohexane using bath ultrasonication and then a drop of the resulting dispersion was dripped on silicon cover slide attached to aluminium SEM stub. After that, the sample was air-dried, followed by being coated with a thin layer of platinum for the SEM observation. Transmission electron microscopy (TEM) was conducted to investigate microstructure of sample on a JEM-1400plus transmission electron

3.1. Formation of STP@PECA microcapsules According to the open literatures, the emulsion method has been widely used to fabricate core-shell structural microcapsules [58,59] because of its simple procedure. For such a method, perfect encapsulation of core materials can be realized by preparing an emulsion with core materials in the dispersed phase. The reason for this is that encapsulation operations are generally carried out at the interface of the dispersed phase and then the core materials in the dispersed phase can be wrapped. Usually, the method is categorized into two major groups: oil-in water (O/W) and W/O emulsion methods. Considering

Fig. 1. Scheme of synthetic mechanism for STP@PECA microcapsules. 151

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3.2. Morphology, microstructure and surface elemental distribution of microcapsules

the traits of the method, it is deduced that the O/W and W/O emulsion methods are suitable for encapsulating lipophilic PCM cores (eg. paraffins) and hydrophilic ones (eg. hydrated salts), respectively. This has been proven by many successful cases as reported [58,60,61]. In the present work, STP as a PCM core is hydrophilic in nature, thus it is evident that the W/O emulsion method should be adopted to prepare microcapsules with PECA shell and STP core via interface polymerization. The possible synthetic mechanism for the microcapsules is schematically depicted in Fig. 1. As seen from Fig. 1, the aqueous solution of STP was added into cyclohexane solution containing mixed nonionic surfactants (i.e. Span-20 and Tween-80) to create a W/O emulsion. In this case, the mixed nonionic surfactants molecules trimly cover the surfaces of STP aqueous solution droplets with hydrophilic segments (i.e. hydroxyl groups) associated with the hydrophilic droplets. On the other hand, the hydrophobic chains of surfactants are oriented toward the cyclohexane solution by arranging along the hydroxyl groups alternatively. It is well known that α-carbon of ECA monomer contains the strong electron withdrawing groups (i.e. –COOC2H5 and –CN groups), which reduce the electron density on the β-carbon, thereby making β-carbon susceptible to attack by nucleophilic species such as water. Hence, with addition of ECA/chloroform solution into the emulsion, the presence of hydroxyl ions in the hydrophilic droplets could initiate the anionic polymerization of ECA at the W/O surface and thus produced a reactive anion at the α-carbon. During this stage, ECA molecules migrated from the continuous phase to the W/O surface at first and then their polymerization was initiated, thus producing reactive anions at the α-carbon. Moreover, the resulting reactive anions were inclined to attract onto the surfaces of the inverse micelles due to an electrostatic interaction between the reactive anions and the hydrophilic segments of surfactants. Meanwhile, the reactive anions performed further polymerization on the surfaces of the inverse micelles. Consequently, a PECA shell was constructed surrounding the inverse micelle to form core-shell structural microcapsules.

The morphologies of the STP@PECA microcapsules were investigated by SEM, and the obtained images are shown in Fig. 2. As observed in Fig. 2(a) and (b), the resultant microcapsules presented almost spherical profiles with a diameter of about 1.0 µm. Additionally, it should be noted from the magnified SEM image (Fig. 2(b)) that the surfaces of the microcapsules were compact but mildly rough, in which some dimples could be found. Interestingly, the similar phenomenon was also found in the published literatures on the microcapsules with polymer shells [62–64]. The phenomenon was caused by the shrinkage of shells, stemming from the difference in density between the polymer and the monomer as well as the decrease in core volume resulting from the consumption of water as the reactant in inverse micelles during polymerization process. To confirm a typical core-shell structure of the microcapsules synthesized in this study, the microcapsules were detected with TEM, and the corresponding image is illustrated in Fig. 2(c). As seen from Fig. 2(c), a well-defined core-shell structure of the microcapsules could be distinctly observed through the great contrast between the dark core and light shell. Moreover, the shell thickness could also be determined and was estimated to be around 0.1 µm. The surface elemental distribution of the obtained microcapsules was analyzed by EDS, and the corresponding spectrum with the data of atomic percentages from EDS analysis is shown in Fig. 2(d). From Fig. 2(d), a set of signals corresponding to C, N and O elements were clearly observed, suggesting that the shell of microcapsules was mainly composed of C, N and O elements. It was notable that the atomic ratios of elements weren’t in accord with theoretical ones of PECA shell, which may be due to semi-quantitative analysis of EDS. Surprisingly, a weak signal associated with S element was found in the EDS spectrum, which was resulted from STP within the microcapsules. Considering the fact that the EDS analysis could probe to subsurface depth of hundreds

Fig. 2. SEM images (a and b), TEM image (c) and EDS spectrum (d) of STP@PECA microcapsules. 152

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suffered a confinement in crystallization. This caused the formation of the imperfect STP crystals, thus resulting in the decline of Tm. On the other hand, the heterogeneous nucleation effect of the inner PECA shell on the microencapsulated STP caused the excessive and much smaller crystalline grains. This had negative effect on the integrality of STP crystals, thus leading to the reduction in Tm. Generally, latent heat (ΔHm) is regarded as a crucial parameter to reflect latent heat storage capacity of a PCM. From data listed in Table 1, the latent heat of bulk STP was measured to be 209.3 kJ·kg−1, revealing its high thermal energy storage capability. Nevertheless, it is clearly seen that the latent heats of the microcapsules were significantly reduced in comparison with that of bulk STP. This was due to the PECA shell without any solid-liquid phase transition under the testing condition, while only the STP within PECA shell can experience phase change transition to contribute latent heat. Hence, the latent heats of the microcapsules were predominantly determined by the core loading in the microcapsules. Meanwhile, it was observed from Table 1 that the ΔHm of the microcapsules increased from 74.0 to 107.0 kJ·kg−1 as the STP/ECA mass ratio increased from 4/3–4/2. It implied that by properly increasing the STP/ECA mass ratio used in the synthesis process, the latent heats of the microcapsules could be increased. However, when the STP/ECA mass ratio increased to 4/1, the ΔHm of the microcapsules reduced to 77.9 kJ·kg−1. It was well-known that such STP/ ECA mass ratio meant the low ECA loading during synthesis process, making the synthesized microcapsules with a thin shell. With such a thin shell, the microcapsules were fragile and then the STP core could leak out from the microcapsules easily during synthesis process, thus resulting in a low core loading within the microcapsules. The encapsulation ratio (R) of STP@PECA, which indicates effective encapsulation of the STP within the PECA shell, can be deduced by the following equation:

of nanometers, EDS scanning might reach the core material of the microcapsules, accordingly, certain elements of STP such as S element were detected. 3.3. Chemical composition of STP@PECA microcapsules The chemical compositions of STP, pure PECA and the resulting microcapsules STP@PECA were evaluated by FT-IR, as displayed in Fig. 3. As observed in the spectrum of STP, the absorption peaks at 3442.7 and 1652.1 cm−1 were attributed to O-H stretching and bending vibrations of crystal water. Meanwhile, the peaks at 1122.6 and 1000.1 cm−1 were corresponded to S=O symmetrical stretching vibration, and those at 1384.4, 674.4 and 541.4 cm−1 were assigned to S=O asymmetric stretching vibration, S-O and S-S stretching vibrations, respectively. It was notable that the above characteristic peaks can be found in the spectrum of STP@PECA microcapsules, suggesting the presence of STP within the microcapsules. On the other hand, the STP@PECA microcapsules also exhibited the characteristic peaks at 2990.7, 2925.3, 2874.1, 1470.8 and 1444.6 cm−1, which were ascribed to C-H stretching and bending vibrations of -CH2- and -CH3. Moreover, the peaks were observed at 2249.2 cm−1, 1751.3 cm−1 and 1255.8 cm−1 due to C≡N stretching vibration, C=O and C-O-C stretching vibration, respectively. These characteristic peaks indicated the existence of PECA in the prepared microcapsules. It was worth mentioning that the characteristic peak representing the C=C at 1630 cm−1 disappeared, implying that the polymerization of the monomers was complete. Based on the results gained from SEM, TEM, EDS and FT-IR, it was concluded that STP was successfully encapsulated by PECA shell. 3.4. Crystalline structure of STP@PECA microcapsules

R=

The XRD patterns of pure PECA and STP@PECA microcapsules are illustrated in Fig. 4, and the standard XRD pattern of STP is also given as a control. It was clearly seen that no diffraction peak appeared in the pattern of pure PECA, indicating that the PECA shell of the microcapsules obtained in the present work was amorphous. Therefore, it is available to confirm the crystalline structure of STP in the microcapsules from the XRD pattern of STP@PECA. As shown in Fig. 4, the STP@PECA microcapsules displayed a set of distinct diffraction peaks at 15.27°, 16.40°, 19.70°, 21.13°, 23.23°, 26.77°, 28.35°, 30.22°, 31.99° and 36.77°, which were assigned to the (001), (040), (031), (-131) (121), (-221), (-231), (-161), (250) and (-311) planes, respectively, according to the standard PDF card No. 31–1325. It should be emphasized that the characteristic peaks of STP@PECA microcapsules were in good agreement with those in the standard XRD pattern of STP. The results not only provided another evidence for the successful encapsulation of STP within PECA shell, but also identified the good crystallinity of STP inside the prepared microcapsules.

∆Hm, micro − PCM ∆Hm, PCM

Where ΔHm, micro-PCM and ΔHm, PCM are latent heats of the STP@PECA microcapsules and bulk STP, respectively. The encapsulation ratios of STP@PECA microcapsules are also shown in Table 1. From Table 1, R of M1, M2 and M3 were 37.2%, 51.1% and 35.4%, respectively. As expected, the R increased firstly and then dropped with the increase of the STP/ECA mass ratio. It should be mentioned that compared with other microcapsules in literatures, the R of M2 obtained in the present work was on the comparable level. 3.6. Thermal stability and reliability of STP@PECA microcapsules The thermal stabilities of bulk STP, pure PECA and STP@PECA microcapsules were evaluated by TGA and the measured results are

3.5. Phase change behavior of STP@PECA microcapsules The phase change behaviors of bulk STP and the microcapsules synthesized at different STP/ECA mass ratios, denoted as M1, M2 and M3, were analyzed using DSC. The resulting DSC curves are presented in Fig. 5 and the obtained data are also summarized in Table 1. It could be observed from Fig. 5 that the STP@PECA microcapsules showed the similar phase change behaviors as bulk STP. Despite this, a slight difference between the STP@PECA microcapsules and the bulk STP should be noted that the endothermic peaks of the STP@PECA microcapsules shifted toward a lower temperature compared with that of the bulk STP. The phenomenon also can be reflected by their phase change temperatures (Tm), as illustrated in Table 1. This can be explained by the confined crystallization of STP and the heterogeneous nucleation effect of PECA shell. On one hand, because of microencapsulation, the STP was confined within a micrometric space of microcapsules and thus

Fig. 3. FTIR spectra of STP, pure PECA and STP@PECA microcapsules. 153

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losses were corresponded to the dehydration of crystalline water, the thermal decompositions of PECA shell and Na2S2O3, respectively. It was notable that the weight losses of STP@PECA microcapsules were slightly slower than those of the bulk STP during the first two weight loss stages, which was attributed to the protective effect of the PECA shell on the STP. Based on the above results, it can be inferred that encapsulating STP into the PECA shell slightly improved the thermal stability of STP. It was worth mentioning that the merely slight improvement may be caused by the moderate thermostability of the PECA shell synthesized in the present work derived from its low molecular weight. Therefore, the study on how to enhance thermal stability of shell material should be expected in the future work. In addition, a thermal cycling test was conducted to evaluate the thermal reliability of the STP@PECA microcapsules. The DSC curves and the corresponding results of the STP@PECA microcapsules before and after thermal cycling test are showed in Fig. 6(b) and Table 1. It could be seen from Fig. 6(b) that the endothermic peaks of the STP@ PECA microcapsules after thermal cycling test moved toward a lower temperature region compared with that of the ones before the test. However, the endothermic peaks of the microcapsules after 25 and 50 cycles were located very close to each other. Such phenomenon could also be reflected by their Tm. As listed in Table 1, the Tm of the STP@ PECA microcapsules after 25 and 50 cycles were 43.00 and 43.83 °C, respectively. These values of Tm for the microcapsules after 25 and 50 cycles were close, but slightly lower than that of the ones before thermal cycling test. The results indicated that the Tm of the microcapsules, instead of decreasing consistently, stabilized during 50 cycles, although it was slightly lower than that of the ones before experiencing thermal cycling test. With respect to the ΔHm of the STP@PECA microcapsules, the similar results could also be obtained. The ΔHm of the microcapsules stabilized at around 91.0 kJ·kg−1 during 50 cycles. All results implied that the microcapsules synthesized in the current work had good thermal reliability for the reason that the phase change behaviors of the STP@PECA stabilized during 50 thermal cycles.

Fig. 4. XRD patterns of pure PECA and STP@PECA microcapsules.

4. Conclusion

Fig. 5. DSC curves of bulk STP and STP@PECA microcapsules.

In the present work, a novel type of STP@PECA microcapsules were fabricated successfully. The resultant microcapsules were characterized by various analysis techniques and their thermal properties were investigated. The main conclusions are as follows:

Table 1 DSC results and encapsulation ratio of bulk STP and STP@PECA microcapsules. Sample

STP/ECA mass ratio (g/ g)

Tm(°C)

ΔHm(kJ·kg−1)

Encapsulation Ratio (%)

STP M1 M2 M3 PECA M2 after 25 cycles M2 after 50 cycles

1/0 4/1 4/2 4/3 0/1 4/2

48.54 44.80 46.44 46.71 – 43.00

209.3 77.9 107.0 74.0 – 91.5

– 37.2 51.1 35.4 – 43.7

4/2

43.83

90.9

43.4

1. SEM and TEM images showed that the microcapsules exhibited almost spherical profiles with a diameter of about 1.0 µm and a welldefined core-shell structure, and their surfaces were compact but mildly rough. 2. EDS, FT-IR and XRD results confirmed that the presences of STP and PECA in the microcapsules. Combined with the results derived from SEM and TEM, it was concluded that STP core was successfully encapsulated by PECA shell. 3. DSC results revealed that compared with bulk STP, the Tm of the microcapsules decreased due to the confined crystallization of STP and the heterogeneous nucleation effect of PECA shell. The ΔHm of the microcapsules increased firstly and then decreased with the increase of STP/ECA mass ratio. The microcapsules possessed Tm of 46.44 °C, ΔHm of 107.0 kJ·kg−1 and R of 51.1% at the STP/ECA mass ratio of 4/2. 4. TGA and thermal cycling test implied that the microcapsules displayed an improved thermal stability and a good thermal reliability due to encapsulating STP within PECA shell.

displayed in Fig. 6(a). It was observed that the bulk STP mainly exhibited a two-step thermal weight loss behavior. The first weight loss occurred from 33.7 °C to 163.3 °C due to the dehydration of crystalline water, and the second one at temperature range from 346.0 °C to 600 °C resulted from the decomposition of Na2S2O3. Moreover, the first weight loss percentage was 33.8% which was almost consistent with the content of crystalline water in the STP (about 36.3%), and the second one was 11.0%. After 800 °C, the residual was primarily consisted of sodium sulfide. For pure PECA, there was a one-step thermal degradation pattern in the temperature range of 111.7–421.2 °C. It was surprisingly found from Fig. 6(a) that the STP@PECA microcapsules exhibited a three-step thermal degradation process at 35.0–112.0 °C, 112.0–421.3 °C and 421.3–600 °C. The first, second and third weight

With the above features, the microcapsules synthesized in this work show the great potential for application in solar energy storage systems such as solar water-heating system. In addition, it is expected that the synthetic technique in this work can be used for encapsulating other hydrophilic substances. 154

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Fig. 6. TGA thermograms of STP, pure PECA and STP@PECA microcapsules (a), DSC curves of STP@PECA microcapsules before and after thermal cycling test (b).

Acknowledgement This work was supported by National Natural Science Foundation of China (No. 51536003, No. 21471059).

[17]

[18]

Declarations of interest

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