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Facilitated synthesis and thermal performances of novel SiO2 coating Na2HPO4⋅7H2O microcapsule as phase change material for thermal energy storage
Solar Energy Materials & Solar Cells 206 (2020) 110257
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Facilitated synthesis and thermal performances of novel SiO2 coating Na2HPO4⋅7H2O microcapsule as phase change material for thermal energy storage Yutang Fang *, Lihang Huang, Xianghui Liang, Shuangfeng Wang, Hao Wei, Xuenong Gao, Zhengguo Zhang Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, South China University of Technology, Guangzhou, 510640, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: SiO2 Na2HPO4⋅7H2O Microencapsulated phase change material Sol-gel Anti-solvent
Novel microencapsulated phase change material (MEPCM) with SiO2 coating Na2HPO4⋅7H2O were synthesized by one-pot method. A possible formation mechanism of Na2HPO4⋅7H2O core and SiO2 shell was preliminarily analyzed. The morphology, composition, crystalline phase and thermal performances of the synthesized MEPCM was characterized by scanning electron microscopies (SEM), Transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), dif ferential scanning calorimetry (DSC), thermogravimetry analysis (TG) and thermal constants analyzer. Pre liminary analysis indicated that the Na2HPO4⋅7H2O nanocrystals as the core was obtained by anti-solvent process with partially dehydration (loss of five crystal water molecules) from Na2HPO4⋅12H2O, and the SiO2 as the shell was formed by sol-gel method with hydrolysis and condensation of tetraethyl orthosilicate. Analysis results of FTIR and XRD illuminated that only physical interaction occurred between the SiO2 shell and the Na2HPO4⋅7H2O core in the MEPCM. The results of SEM and DSC showed that the synthesized MEPCM displayed a flat cylinder structure and the particle size of about 8 μm, and exhibited a phase change temperature of 50.12 � C, latent heat of 159.8 kJ/kg and encapsulation ratio of 82.40%. Moreover, the formation of the phase change microcapsules could reduce the supercooling of Na2HPO4⋅7H2O to some degree and the MEPCM displayed a suitable thermal conductivity (0.3838 W/m⋅K) and an excellent thermal stability. The thermal properties of the MEPCM make it suitable for solar energy storage.
1. Introduction Thermal energy storage technology has aroused considerable concern in recent years because the shortage of fuel energy and envi ronmental pollution are increasingly prominent [1]. Included in the thermal energy storage approaches, latent heat storage technology, which utilizes phase change materials (PCMs), is found to be an effective method to satisfy demand of the energy [2]. So far, latent heat storage technology has broad application prospects in the fields of solar energy storage [3], heat transfer fluid [4], intelligent air-condition [5], aero space engineering [6] and so on. As the essential of latent heat storage technology, PCMs can revers ibly store or release a large amount of energy at a constant temperature
during the phase transition [7]. There are two most common PCMs classification, namely organic PCMs and inorganic PCMs. Usually, organic PCMs have the defects of low energy storage density and less thermal conductivity [8]. Instead, the inorganic ones, particularly hy drated salts, have high phase change enthalpy, nice thermal conduc tivity and environmental-friendly characteristics [9,10]. However, direct utilization of hydrated salts presents some disadvantages such as leakage in the melting process, supercooling and phase separation [8], which reduce their thermal storage capacity. To overcome these shortcomings of hydrated salts, many attempts have been made including the composite of hydrated salts with porous materials making up so-called shape-stable PCMs [11], and the micro encapsulation with a micro-sized compact shell coating hydrated salts [12]. Between two methods, due to the weak capillary action of porous
* Corresponding author. Tel.: 13318819891. E-mail addresses: [email protected] (Y. Fang), [email protected] (L. Huang), [email protected] (X. Liang), [email protected] (S. Wang), [email protected] (H. Wei), [email protected] (X. Gao), [email protected] (Z. Zhang). https://doi.org/10.1016/j.solmat.2019.110257 Received 2 July 2019; Received in revised form 19 October 2019; Accepted 25 October 2019 Available online 8 November 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
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silica precursor and the melting enthalpy of the obtained microcapsules was found to be 123.0 kJ/kg. The thermal conductivity of the micro capsules was up to 0.6213 W/m⋅K. which was much higher than those of the microcapsules with organic shells. Wu et al. [26] reported the microencapsulation of mannitol into silica shell via an interfacial poly merization process in a water-in-oil emulsion, which obtained regular spherical capsules with uniform particle size. Lin et al. [27] micro encapsulated stearic acid in silica shell by sol-gel method to form microencapsulated PCMs, and then attached graphene oxide to surface of silica to further improve thermal performances of the microcapsules. Despite many studies on silica-shelled microcapsules, as we known, it is more difficult to microencapsulate hydrated salts owing to their hy drophilicity nature, and their tendency to change crystal water content [15], the research on feasible synthetization of inorganic hydrated salts/silica microcapsules is limited so far. For instance, Zhang et al. [28] described a microencapsulation of binary carbonate salt mixture in silica shell to enhance its effective heat capacity for its application in high temperature. Liu et al. [29] successfully microencapsulated sodium thiosulfate pentahydrate with silica shell by sol–gel method and the microcapsules showed a high encapsulation ratio of 94.65%. Liu et al. [30] prepared a sodium phosphate dodecahydrate micro-PCM with sil ica shell through interfacial polymerization combined with sol-gel pro cess. With high energy density (193.9 kJ⋅kg 1) and applicable phase change temperature (52.3 � C), disodium hydrogen phosphate heptahy drate (Na2HPO4⋅7H2O) is a potential hydrated salt as phase change material for solar energy storage. In the extreme small amounts of studies about microencapsulated Na2HPO4⋅7H2O as micro-PCMs, Huang et al. [31] prepared a Na2HPO4⋅7H2O microcapsule coated with modi fied PMMA via the suspension copolymerization-solvent volatile method, the method was relatively complicated and to our knowledge there is few literatures reported on preparation and characterization for microencapsulated Na2HPO4⋅7H2O with inorganic shell so far. Besides, on account of the developed technique in applying silica as shell in microencapsulated PCM, we attempted to employ silica to coat Na2HPO4⋅7H2O. Hence, in the present work, a novel inorganic silica (SiO2) coating Na2HPO4⋅7H2O microencapsulated phase change material (Na2H PO4⋅7H2O @SiO2 MEPCM) for thermal energy storage was successfully fabricated by one-pot method. For the application in solar energy stor age, Na2HPO4⋅7H2O have a considerable potential based on its phase change temperature of 52.3 � C and melting enthalpy of 193.9 kJ kg 1, and SiO2 can take advantage of its good thermal conductivity. In this work, a possible formation mechanism of Na2HPO4⋅7H2O core and SiO2 shell was analyzed. The morphology and phase change properties of the MEPCM were also investigated.
Nomenclature Tm ΔHm R E
phase change temperature (� C) latent heat (kJ⋅kg-1) encapsulation ratio (%) encapsulation efficiency (%)
Abbreviations PCM phase change material MEPCM microencapsulated phase change material DSP disodium hydrogen phosphate dodecahydrate TEOS tetraethyl orthosilicate NPA nanoparticle size analyzer SEM scanning electron microscope EDS energy dispersive spectroscopy TEM Transmission electron microscopy FTIR Fourier-transform infrared XRD X-ray diffraction DSC differential scanning calorimetry TGA thermogravimetric analysis
material on hydrated salt, the hydrated salt composites carry the risk of leakage and thermal unreliability when the temperature rises above the melting point of the hydrated salts [13], while the microencapsulation of hydrated salts is considered to be an effective method to solve the aforementioned problems. Intuitively, the microcapsule shell can confine the hydrated salts in a limited space, which reducing volume changes of the salts and controlling the phase separation when phase change occurs [14]. Moreover, the microcapsule shell can protect hy drated salts from outside environments, prevent leakage from their location and restrict the supercooling during cooling process [15]. Therefore, choosing a suitable shell material for the microencapsulated hydrated salts plays an important role in controlling the structure, me chanical stability and supercooling degree. For organic PCMs, the traditional shell materials selected are generally the polymers, such as polyurea [16], polystyrene [17], urea-formaldehyde resin [18], etc. But for inorganic hydrated salts, the literatures reported about their microencapsulation are rare. Fu et al. [19] used poly (ethyl-2-cyanoacrylate) (PECA) as organic shell and sodium thiosulfate pentahydrate(Na2S2O3⋅5H2O) as core to prepare a novel phase change microcapsule. After microencapsulation, it showed an improved thermal stability and a good thermal reliability. Yang et al. [20] combined sol –gel process with interfacial polymerization to synthesized a calcium chloride hexahydrate(CaCl2⋅6H2O) microcapsule with organo alkoxysilane shell, the SEM images confirmed a successful microen capsulation and displayed a nearly spherical shapes with a fine core–shell structure of the microcapsule. Although the reported hy drated salt microcapsules with polymeric shells usually present an excellent core-shell structure, their application is always restricted by the flammability, toxicity, poor mechanical strength and the low ther mal conductivity of their polymeric shells [21,22]. Crucially, high thermal conductivity of hydrated salts microcapsules is imminently required to improve the heat transfer efficiency during phase change processes. In this case, compared with polymeric shell materials, the inorganic shells are outstanding in thermal conductivity and mechanical strength [23], thus a new research trend toward microencapsulating PCMs by inorganic shells has boomed recently. Besides, the inorganic shell materials, especially the silica material, has arrested people’s attention due to their fine biocompatibility, nontoxicity, and chemical inertness [24]. Some literatures have reported the research on employing silica shells in microencapsulation of PCMs these years. Zhang et al. [25] synthesized the microencapsulated n-octadecane phase change material with silica shell through sol-gel process using TEOS as a
2. Materials and methods 2.1. Materials Disodium hydrogen phosphate dodecahydrate (Na2HPO4⋅12H2O, DSP, AR) were purchased from Shanghai TCI Limited, China. Tetraethyl orthosilicate (TEOS, CP) served as silica source were supplied by Guangzhou Chemical Reagent Factory, China. Absolute ethanol (ET, AR) were obtained from Guangzhou Donghong Chemical Plant, China. Ammonium hydroxide (NH3⋅H2O, 28 wt%) were used to serve as a catalyst. All chemicals were reagent quality and used without further purification. 2.2. Synthesis of Na2HPO4⋅7H2O@SiO2 MEPCM The Na2HPO4⋅7H2O@SiO2 MEPCM was synthesized by one-pot method, in which Na2HPO4⋅7H2O nanocrystals as the core was pro duced by anti-solvent process with partially dehydration from DSP, and the SiO2 as the shell was synthesized by sol-gel method with hydrolysis and condensation of TEOS. 2
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The typical synthesis was as follows. Under magnetic stirring, suc cessively adding 10.0 ml ET and 0.5 ml (2.2 mmol) TEOS into a 50 ml beaker, an alcoholic solution A was obtained. Similarly, a saturated Na2HPO4 aqueous solution B was formed by adding 0.53 g (1.3 mmol) DSP, 2.0 ml (111.1 mmol) deionized water and 0.1 ml (2.0 mmol) ammonia. Subsequently, the aqueous solution B was added into alco holic solution A dropwise using a syringe under the stirring rate of 1600 rpm. After that, the resulting mixture was continually encapsu lated at 500 rpm and 25 � C for 4 h, and a pinch of white powder was obtained by suction filtration. After orderly leaching the powder with 3 ml deionized water and 3 ml ethanol to remove the uncoated hydrated salt crystals and the residual water on powder surface, the Na2H PO4⋅7H2O@SiO2 MEPCM was obtained by drying at 25 � C for 12 h. The detailed schematic diagram of synthesis process for MEPCM is shown in Fig. 1. In order to obtain a satisfactory Na2HPO4⋅7H2O nanocrystals, the crystallization concentration (Cc), which refers to the volume ratio of the saturated Na2HPO4 aqueous solution B and the alcoholic solution A is proposed. A series of crystallization concentrations (0.15, 0.20, 0.25, 0.30, 0.35, 0.40) were obtained by fixing the amount of solution B and changing the amount of antisolvent ET in solution A.
3. Results and discussion 3.1. Possible formation mechanism of Na2HPO4⋅7H2O@SiO2 MEPCM Generally, the smaller the size of the hydrated salt core material, the easier it is coated to form PCM microcapsule. On the other hand, since there are few sources and high price of the commercial Na2HPO4⋅7H2O as core, an attempt was made to prepare Na2HPO4⋅7H2O nanocrystals from DSP with wide sources and low price. 3.1.1. Formation of Na2HPO4⋅7H2O nanocrystals as core In this experiment, ethanol served as an anti-solvent (corresponding to solution A) to separate out Na2HPO4⋅7H2O crystals from the saturated Na2HPO4 solution (corresponding to solution B). Notably, the crystal lization concentration (Cc) plays an important role in the formation of Na2HPO4⋅7H2O crystals and the control of its size [32]. Table 1 listed the phase change temperature (Tm), enthalpy (ΔHm) and particle size (D) of the Na2HPO4⋅7H2O crystals synthesized with different crystallization concentrations. It can be seen from Table 1 that when the Cc was small, such as 0.15, 0.20, the obtained crystal sizes were 227, 272 nm, respectively, which were small and had no evident changes. When the Cc was above 0.20, such as 0.25, 0.30, 0.35, the obtained crystal sizes obviously increased, which were 892, 1234, 760 nm, respectively. However, when the Cc was increased to 0.40, the crystal size showed a significant reverse reduction and was only 107 nm. According to the crystallization theory [33], generally, in antisolvent crystallization, the crystals size is determined by supersaturation of system. In the Na2H PO4/ethanol system, the existence of ethanol as antisolvent would cause an increase in supersaturation of Na2HPO4 solution due to the anti solvent effect, thus it separated out DSP crystals. Meanwhile, ethanol would remove part of the crystal water of DSP into the system, which reducing the supersaturation (dehydration effect). In a small Cc such as 0.15, the large amount of ethanol made the antisolvent effect play a leading role in affecting the supersaturation and making the supersat uration increase. As a result, the increased supersaturation accelerated the nucleation and growth rate to get small crystals. When Cc was increased to 0.3, it caused a gradual reduction in supersaturation, and directly, brought a reduction in the nucleation and growth rate, thus a
2.3. Characterization of Na2HPO4⋅7H2O@SiO2 MEPCM The chemical composition of the microcapsules was tested by Fourier transform infrared spectroscopy (FTIR, TENSOR 27, Bruker Corpora tion, Germany) within a range from 400 to 4000 cm-1. The crystalline structure was analyzed by X-ray diffraction (XRD, D/ max-A, Rigaku Corporation, Japan) at a scanning rate of 2 deg⋅min-1. The morphology and elemental analysis of the MEPCM were observed by scanning electron microscope with energy dispersive spectrometer (SEM-EDS, S-3700 N, Hitachi, Japan) at an operating voltage of 10 kV. The microstructure of the MEPCM was detected by Transmission electron microscopy (TEM, JEM-2100, JEOL, Japan). The phase change properties were measured by differential scanning calorimeter (DSC, Q20, TA Instrument Company, USA) under N2 at a heating or cooling rate of 5 � C⋅min-1. The thermal stability was characterized using thermogravimetric analysis (TG, TG209F3, Netzsch company, Germany) under N2 at a heating rate of 10 � C⋅min-1 in the temperature range from 30 � C to 500 � C. The particle size of Na2HPO4⋅7H2O crystals as core in the solution A was measured with nanoparticle size analyzer (NPA150, Microtrac Co. Ltd., USA). The thermal conductivity of the sample was tested by thermal con stants analyzer (Hot Disk, Hot Disk Company Limited, Sweden) at about 25 � C. 3 g MEPCM sample was added into a cylindrical mold and com pressed it into two flat tablets with 2 cm in diameter and 0.5 cm in height, and a 7577 probe was put between them. The tests were repeated for three times and the results were averaged.
Table 1 Tested data of phase change temperature (Tm), enthalpy(ΔHm) and particle size (D) of the Na2HPO4⋅7H2O crystals obtained at different crystallization concentrations. Crystallization concentration
0.15
0.20
0.25
0.30
0.35
0.40
Tm/� C
51.70
52.30
51.08
190.1
193.9
195.5
D/nm
227
272
892
37.28, 50.21 181.6, 47.9 760
34.68
ΔH/kJ/kg
39.18, 53.24 125.7, 73.80 1234
Fig. 1. Process flow diagram of Na2HPO4⋅7H2O@SiO2 MEPCM synthesized by one-pot method. 3
239.9 107
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rise in crystals size came out. However, when Cc was further increased, such as 0.35, the dehydration effect would be weakened due to the reduction of ethanol, thus the supersaturation of the system was reversely increased, making the particle smaller. Finally, as seen from Table 1, at Cc of 0.4, the dehydration of DSP did not occur, which indicated that the supersaturation reached the maximum, and a mini mum particle size appeared. More importantly, the effects of the Cc on the thermal performances of the Na2HPO4⋅7H2O crystals were more distinct. When the Cc was below 0.25, the corresponding Tm and ΔHm of the products obtained were about 51 � C and 190 kJ/kg, respectively, which assigned the product to Na2HPO4⋅7H2O crystals. When the Cc was 0.30 and 0.35, there were two melting points about 37 � C and 51 � C and two melting enthalpies for the crystals, which corresponding to the mixed products of Na2HPO4⋅7H2O and DSP crystals. As the Cc was 0.4, only DSP crystals appeared in the product. A possible reason was that, when the Cc was larger, such as 0.4, it meant the smaller amount of antisolvent ET was used in the solution A, only tiny DSP crystals containing 12 crystal water molecules could be generated and evenly dispersed in solution A. With a fewer Cc, such as 0.30, 0.35, it meant an increase in the amount of ET, some of DSP crystals lost five crystal water molecules, and became Na2HPO4⋅7H2O crystals, so the mixed and big crystal particles con taining DSP and Na2HPO4⋅7H2O were obtained, and they couldn’t be evenly dispersed in solution A. When Further reduced Cc, such as below 0.25, it meant a further increase of ET solvent, all DSP crystals became Na2HPO4⋅7H2O crystals by dehydration and evenly dispersed Na2H PO4⋅7H2O crystals appeared in solution A. Considering the phase tran sition temperature, enthalpy and particle size of the crystals obtained by anti-solvent method, Cc of 0.20 was appropriate. Fig. 2 shows the particle size distribution of the Na2HPO4⋅7H2O crystals obtained at Cc of 2.0. The results indicated that the obtained Na2HPO4⋅7H2O crystals had a small and narrow size distribution.
These monomeric ortho-silicic acids have great activity and can be easily adsorbed on the surface of the Na2HPO4⋅7H2O nanocrystals. Later, condensation occurs between ortho-silicic acid or between ortho-silicic acid and TEOS to form silica-sol particles on the surface of crystals, it can be described by the following equation.
Finally, the silica oligomer formed in last step continues to conden sate to form silica gel with solid three-dimensional network structures. And the fine Na2HPO4⋅7H2O nanocrystals are coated by the formed silica gel to finish the microencapsulation.
3.1.2. Formation of SiO2 as shell The MEPCM was coated by the silica shell assembled on the surface of Na2HPO4⋅7H2O nanocrystals via an in-situ condensation of the generated silica precursors from the hydrolysis of TEOS. The hydrolysis and condensation of TEOS to form gel around the crystals can be described in three steps. Firstly, the hydrolysis of TEOS is initiated with the addition water and ammonia and creates hydroxylated silica pre cursors. Under the alkaline condition, the hydrolysis rate of TEOS is signally higher than its condensation rate, and the obtained main product is ortho-silicic acid (Si(OH)4). The reaction can be described by the following equation.
3.2. Phase change properties of Na2HPO4⋅7H2O@SiO2 MEPCM 3.2.1. Optimization of microencapsulation condition There are many experimental factors to the synthesis of micro encapsulated PCM, such as reaction temperature, time and reactant ratio. Thereinto, the mass ratio of core to shell directly affected the encapsulation efficiency. The phase change enthalpy of the MEPCM prepared with different Na2HPO4⋅7H2O/SiO2 mass ratio are displayed in Fig. 3a. By drawing a comparison among the enthalpy of the MEPCM synthesized in different mass ratio, we found the MEPCM with mass ratio of 3.468 had the highest enthalpy, indicating a higher microen capsulation efficiency the MEPCM achieved. Enthalpy was lower in low mass ratio such as 1.486 and 2.311, possible reason was that the silica precursors generated by hydrolysis process of TEOS were relatively excessive, these excessive precursors could bring about a selfcondensation of silica precursors or largely absorb on the Na2H PO4⋅7H2O crystals to make a thick shell for the MEPCM, resulting in the reduction of the phase change enthalpy of the MEPCM. Nevertheless, in a much higher mass ratio such as 5.021 and 8.086, the phase change enthalpy decreased inversely. It could be explained that the produced silica precursors were insufficient to build a compact shell for the core, and caused the MEPCM damaged during the agitation or washing pro cess, thus resulting in a low core loading within the MEPCM. Thus, the core to shell mass ratio of 3.468 was suitable. In addition, the amount of catalyst was a direct factor affecting the hydrolysis rate of TEOS. Fig. 3b shows the phase change enthalpy of the MEPCM prepared with different amounts of catalyst. As seen from Fig. 3b, at amount of 2.0 mmol the MEPCM reached the highest enthalpy when the amount increased from 1.2 to 2.8 mmol. Theoretically, the hydrolysis rate of TEOS increased with the increase of the amount. Low amount of catalyst, such as 1.2 mmol, meant a comparatively low
Fig. 2. Particle size distribution of the Na2HPO4⋅7H2O crystals obtained at crystallization concentrations of 2.0. 4
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Fig. 3. Effects of mass ratio of core and shell(a) and amount of catalyst(b) on the enthalpy of Na2HPO4⋅7H2O@SiO2 MEPCM.
hydrolysis rate of TEOS, which caused a lack of silica precursors generated by hydrolysis, and led to an incompact coating for Na2H PO4⋅7H2O crystals. However, higher amounts, such as 2.4 and 2.8 mmol, caused an excess of produced silica precursors, which resulted in the excessive adsorption on the crystals surface and the increased thickness of shell. Moreover, silica micronuclei came into being under high hy drolysis rate and caused problems such as uneven adsorption and silica aggregates. The results suggested that the MEPCM synthesized at 2.0 mmol catalyst had the highest enthalpy. After improving the reaction conditions such as mass ratio of core to shell and amount of catalyst, an optimized MEPCM was synthesized, and a series of characterization of the MEPCM were carried out.
properties, Encapsulation ratio (R) and encapsulation efficiency (E), and they are acquired by following equations [34]. R¼
ΔHm;micro PCM � 100% ΔHm;PCM
E¼
ΔHm;micro PCM þ ΔHc;micro ΔHm;PCM þ ΔHc;PCM
PCM
� 100%
The encapsulation ratio and encapsulation efficiency of MEPCM were calculated to be 82.40% and 74.42%, respectively, inferring that the MEPCM has high heat storage capacity. In addition, compared with the cooling curve of Na2HPO4⋅7H2O, it could be noticed that the MEPCM crystallized prior to Na2HPO4⋅7H2O during the cooling process, and the difference between the melting temperature and the crystallization temperature of MEPCM is obviously smaller than that of Na2H PO4⋅7H2O, which meant a reduction on supercooling degree emerged in the MEPCM. This reduction of supercooling degree was attributed to the space-confined silica shell, which confirms a fabrication of silica shell onto the Na2HPO4⋅7H2O core to some extent.
3.2.2. Phase change behavior Fig. 4 shows DSC curves of Na2HPO4⋅7H2O and Na2H PO4⋅7H2O@SiO2 MEPCM, and Table 2 displays the corresponding data. As shown in Fig. 4 and Table 2, the MEPCM had a melting enthalpy of 159.8 kJ/kg and a melting point of 50.12 � C, while the melting latent heat and melting point of Na2HPO4⋅7H2O were 193.9 kJ/kg and 52.30 � C. Each curve appeared to show only one melting point at almost the same temperature. Based on the analysis from the DSC results, it could be suggested that the core material encapsulated in the MEPCM was Na2HPO4⋅7H2O. The MEPCM’s melting enthalpy of 159.8 was less than 193.9 kJ/kg of Na2HPO4⋅7H2O, this change in melting enthalpy could be ascribed to the existence of the silica shell. There are two important parameters to describe the phase change
3.2.3. Thermal stability and reliability Fig. 5 shows the TG and DTG curves of Na2HPO4⋅7H2O and the MEPCM. It could be found that the two samples performed the same thermal decomposition in the temperature range of 30–400 � C, which is credited to a large number of evaporation of crystal water for core material and a small amounts of thermal decomposition for Na2HPO4 [35]. As seen from the TG curves, the MEPCM showed a higher initial evaporation temperature than the pure Na2HPO4⋅7H2O, and the MEPCM achieved a final weight loss of 34.4% while the Na2HPO4⋅7H2O achieved 40%, these results can be the evident that the silica shell was success fully formed to protect the Na2HPO4⋅7H2O core in some way. On the other hand, from the two DTG curves it could be noticed that the peak temperature (138.8 � C) at maximum weight loss rate of the MEPCM was higher than that of Na2HPO4⋅7H2O (76.2 � C), it proved that the shell can provide protection for the Na2HPO4⋅7H2O core after microencapsulation, thus improved the thermal stability of the core material. The thermal reliability of the MEPCM was evaluated by the heatingcooling cycling test. DSC curves and corresponding data of the Na2H PO4⋅7H2O@SiO2 MEPCM after different heating-cooling cycles was shown in Fig. 6. According to Fig. 6, the Tm of Na2HPO4⋅7H2O@SiO2 MEPCM after experiencing 1,15,30 heating-cooling cycles was 50.26, 50.20 and 50.11 � C, respectively, showing little difference between that of the MEPCM before cycling test (50.12 � C). As regards the ΔHm of the MEPCM, a similar result could also be learnt. The ΔHm of the MEPCM after 1, 15 and 30 cycles were 160.2, 151.5 and 149.4 kJ kg 1, respec tively. Numerically, the ΔHm of MEPCM after 30 cycles presented a 6.5%
Fig. 4. DSC curves of Na2HPO4⋅7H2O and Na2HPO4⋅7H2O@SiO2 MEPCM. 5
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Table 2 DSC data of Na2HPO4⋅7H2O and Na2HPO4⋅7H2O@SiO2 MEPCM. MEPCM Na2HPO4⋅7H2O
Fig. 5. TG and DTG PO4⋅7H2O@SiO2 MEPCM.
Process
T(� C)
ΔH (kJ⋅kg
Heating Cooling Heating Cooling
50.12 18.24 52.30 14.68
159.8 61.10 193.9 102.9
curves
of
Na2HPO4⋅7H2O
and
1
)
Encapsulation ratio (%)
Encapsulation efficiency (%)
82.40
74.42
-
-
Na2H
Fig. 7. SEM image of Na2HPO4⋅7H2O@SiO2 MEPCM after experiencing 30 heating-cooling cycles. Table 3 Phase change enthalpies and thermal conductivities of Na2HPO4⋅7H2O@SiO2 MEPCMs synthesized at different mass ratios of core to shell. Mass ratio of core to shell
ΔHm (kJ⋅kg
Na2HPO4⋅7H2O 1.486 2.311 3.468 5.021 8.080 SiO2
193.9 89.19 109.8 159.8 100.8 32.96 -
1
)
Thermal conductivity (W⋅m-1⋅K-1) 0.4951 0.3266 0.3519 0.3838 0.3234 0.3164 0.2849
with the enthalpy of 159.8 kJ kg 1. The thermal conductivity of Na2HPO4⋅7H2O (0.4951 W⋅m-1⋅K-1) is larger than that of pure SiO2(0.2849 W m-1 K-1), the thermal conduc tivity of the MEPCM would be improved with the increasing composi tion of Na2HPO4⋅7H2O in MEPCM. So, it seemed the thermal conductivity would rise with the mass ratio of core to shell increased, theoretically. However, the conductivity reached a maximum at mass ratio of 3.468 and then fell as the mass ratio increased, the probable reason was that the excessive Na2HPO4⋅7H2O resulted in incomplete microencapsulation and a loss of Na2HPO4⋅7H2O core, it meant the composition of Na2HPO4⋅7H2O was reversely reduced, thus brought a reduction in thermal conductivity. Therefore, the MEPCM synthesized at mass ratio of 3.468 possessed a fine thermal conductivity of 0.3838 W m1 -1 K , which was higher than those of some common MEPCMs with organic shell (0.10–0.20 W m-1 K-1). As can be seen, the inorganic-coated MEPCMs presented a nice capability for heat conduction.
Fig. 6. DSC curves of Na2HPO4⋅7H2O@SiO2 MEPCM after heating-cooling cycling test.
fall when compared with that of the MEPCM before the test (159.8 kJ kg 1), which meant a slight decline in melting enthalpy of the MEPCM occurred during 30 heating-cooling cycles. All these results suggested that the synthesized MEPCM in present work had relatively good thermal reliability. In addition, the MEPCM after 30 heating-cooling cycles was observed by SEM, the SEM image was put in Fig. 7. Fig. 7 showed the MEPCM remained intact and no breakage could be found in the morphology of the MEPCM after 30 heating-cooling cycles. The result supplemented the evidence of good thermal reliability the MEPCM possessed. 3.2.4. Thermal conductivity The thermal conductivities of MEPCM synthesized at different mass ratio of core to shell are displayed in Table 3. It showed the thermal conductivity reached a peak of 0.3838 W m-1 K-1 at mass ratio of 3.468, 6
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3.3. Composition and structure of Na2HPO4⋅7H2O@SiO2 MEPCM 3.3.1. Composition analysis The FTIR spectra of Na2HPO4⋅7H2O, SiO2 and Na2HPO4⋅7H2O@SiO2 MEPCM are demonstrated in Fig. 8. From the spectrum of Na2H PO4⋅7H2O(a), two absorption peaks of phosphate radical (PO34 ) appeared at 1069 cm-1 and 1261 cm-1, which referred to the symmetrical stretching vibration of O–P–O and the asymmetric stretching vibration – O, respectively. The peaks at 1660 cm-1 and 517 cm 1 resulted of P– – O and P–O–P. The O–H stretching from the bending vibrations of P– bands emerged near 3450 cm 1 could be attributed to the bound water on Na2HPO4⋅7H2O. As shown in the spectrum of SiO2(b), bands observed at 1096 cm-1 could be resulted from the antisymmetric stretching vibration of Si–O–Si, and the symmetrical stretching vibration of Si–OH gave rise to the bands around 952 cm-1. The bands at 461 cm-1 could be related to the bending vibration of Si–O–Si. In addition, the small peaks appeared around 3451 cm-1 and 1631 cm-1 referred to the hydroxyl stretching and bending vibration of Si–OH from the surface of SiO2. Spectrum (c) presents the characteristic absorption peaks of the MEPCM. It was easy to notice that the above characteristic peaks in spectrum (a) and (b) could be found in the spectrum of the MEPCM, and it could hardly find out new characteristic peaks. The results indicated that the SiO2 shell was successfully formed by sol-gel method in the preparation of the MEPCM, and no chemical reaction occurred between the shell (SiO2) and core (Na2HPO4⋅7H2O).
Fig. 9. XRD patterns of Na2HPO4⋅7H2O(a), SiO2(b) and Na2HPO4⋅7H2O@SiO2 MEPCM(c).
are fainter and marked at 18.88� ,20.90� ,26.49� ,30.55� and 31.61� . The above analysis of peaks suggested that the silica shell achieved a microencapsulation for Na2HPO4⋅7H2O core. These results also indi cated that there was no chemical interaction between the Na2H PO4⋅7H2O core and the silica shell, which is in coincidence with the FTIR analysis. Moreover, the reduction of the Na2HPO4⋅7H2O intensities partly demonstrated a successful microencapsulation.
3.3.2. Crystalline phase analysis Fig. 9 illustrates the XRD patterns of Na2HPO4⋅7H2O(a), SiO2(b) and Na2HPO4⋅7H2O @SiO2 MEPCM(c). As displayed in pattern (a), five distinct diffraction peaks of the Na2HPO4⋅7H2O appeared at 18.91� , 20.89� , 26.56� , 30.61� and 31.69� . By comparison with Standard XRD pattern card 10–0191, it proved the existence of crystal phase of Na2HPO4⋅7H2O. Likewise, a broad diffuse peak could be found around 20� in pattern (b), which represented the amorphous state of the SiO2 synthesized in this work. It was noteworthy that all characteristic peaks of Na2HPO4⋅7H2O can be found in the pattern of the MEPCM, but they
3.3.3. Morphology, microstructure observation and elemental analysis The SEM and TEM images of Na2HPO4⋅7H2O@SiO2 MEPCM are displayed in Fig. 10. It can be seen from Fig. 10a that the MEPCM showed a flat and nearly cylindrical appearance. As presented in Fig. 10b, the MEPCM had a rough surface with a particle size of about 8.0 μm. It could also be found that there were numbers of small particles sticking to the surface of the MEPCM. These particles are silica aggre gates, which caused by the self-condensation of silica due to a rapid
Fig. 8. FTIR spectra of Na2HPO4⋅7H2O(a), SiO2(b) and Na2HPO4⋅7H2O@SiO2 MEPCM (c). 7
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Fig. 10. SEM (a and b) and TEM (c) images of Na2HPO4⋅7H2O@SiO2 MEPCM.
condensation rate in the reaction. In addition, a handful of fragments of shell found in Fig. 10a could be the result of the broken MEPCM in the preparation or the preparative spraying process of SEM. In order to confirm a typical core-shell structure of the synthesized MEPCM, the MEPCM was observed by TEM, the result was illustrated at Fig. 10c. Fig. 10c showed a nearly oblate MEPCM with a distinct core-shell structure in a great contrast between the light core and dark shell. Several tiny silica particles could be viewed to adhered to the surface of the MEPCM, which were the result of the self-condensation of silica and also could be found in Fig. 10b. The qualitative and semi-quantitative element analysis of the MEPCM on the sample microregion were detected by EDS and are shown at Fig. 11 and Table 4. In Fig. 11, the element carbon(C) was caused by the use of conductive adhesive during the preparation of sample. Since the EDS technology can only analyze the surface of the MEPCM, Table 4
Table 4 EDS data of Na2HPO4⋅7H2O@SiO2 MEPCM. Element
Weight/%
Atomic/%
O Si Na P
52.58 32.15 7.14 8.13
65.67 22.87 6.21 5.25
indicated the MEPCM was mainly consisted of a large amount O, Si el ements which came from SiO2 shell and a small amount Na, P elements which were from Na2HPO4⋅7H2O. Table 4 also displayed the percent of different elements. The atomic percent of Si and O element was 22.87% and 65.67%, respectively, and it was notable that they weren’t equiv alent to the basic chemical composition of SiO2 shell, which may be due to semi-quantitative analysis of EDS. 4. Conclusions In this paper, a novel Na2HPO4⋅7H2O@SiO2 microencapsulated phase change material (MEPCM) for thermal energy storage was easily synthesized by one-pot method. The possible formation mechanism of Na2HPO4⋅7H2O core and SiO2 shell was preliminary discussion. The composition, structure and thermal performances of the synthesized Na2HPO4⋅7H2O@SiO2 MEPCM were performed a comprehensive anal ysis. The conclusions are as follows. (1) The complete Na2HPO4⋅7H2O nanocrystals as the core at the crystallization concentration (Cc) of 0.2 was formed by antisolvent process with partially dehydration (loss of five crystal water molecules) from Na2HPO4⋅12H2O. The SiO2 as the shell was formed by the hydrolysis and condensation of TEOS. (2) FT-IR and XRD results suggested that Na2HPO4⋅7H2O was suc cessfully wrapped in SiO2 shell, which was also confirmed by
Fig. 11. EDS spectrum of Na2HPO4⋅7H2O@SiO2 MEPCM. 8
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SEM-EDS and TEM characterization. The MEPCM presented a nearly 8 μm flat cylindrical morphology in SEM image. (3) DSC results showed that the synthesized Na2HPO4⋅7H2O@SiO2 MEPCM had a preferable melting enthalpy of 159.8 kJ/kg and melting point of 50.12 � C, and reached an encapsulation ratio of 82.40%, Due to the existence of SiO2 shell, the MEPCM showed a fine thermal conductivity of 0.3838 W/m⋅K and possessed an excellent thermal stability and reliability. According to the phase change and heat transfer performance the synthesized MEPCM showed, in some way it can be said that there exists an attractive prospect of applying this MEPCM in solar energy storage.
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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. Acknowledgement This work was supported by National Natural Science Foundation of China (No. 51536003, No. 21471059). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.solmat.2019.110257. References [1] T. Yan, C.Y. Wang, D. Li, Performance analysis of a solid-gas thermochemical composite sorption system for thermal energy storage and energy upgrade, Appl. Therm. Eng. 150 (2019) 512–521. [2] J.-F. Su, X.-Y. Wang, S. Han, X.-L. Zhang, Y.-D. Guo, Y.-Y. Wang, et al., Preparation and physicochemical properties of microcapsules containing phase-change material with graphene/organic hybrid structure shells, J. Mater. Chem. 5 (2017) 23937–23951. [3] B. Mu, M. Li, Synthesis of novel form-stable composite phase change materials with modified graphene aerogel for solar energy conversion and storage, Sol. Energy Mater. Sol. Cells 191 (2019) 466–475. [4] K. Tumuluri, J.L. Alvarado, H. Taherian, C. Marsh, Thermal performance of a novel heat transfer fluid containing multiwalled carbon nanotubes and microencapsulated phase change materials, Int. J. Heat Mass Transf. 54 (2011) 5554–5567. [5] T. Zou, W. Fu, X. Liang, S. Wang, X. Gao, Z. Zhang, et al., Preparation and performance of modified calcium chloride hexahydrate composite phase change material for air-conditioning cold storage, Int. J. Refrig. 95 (2018) 175–181. [6] X. Huang, G. Alva, Y. Jia, G. Fang, Morphological characterization and applications of phase change materials in thermal energy storage: a review, Renew. Sustain. Energy Rev. 72 (2017) 128–145. [7] H. Nazir, M. Batool, F.J. Bolivar Osorio, M. Isaza-Ruiz, X. Xu, K. Vignarooban, et al., Recent developments in phase change materials for energy storage applications: a review, Int. J. Heat Mass Transf. 129 (2019) 491–523. [8] Y. Lin, G. Alva, G. Fang, Review on thermal performances and applications of thermal energy storage systems with inorganic phase change materials, Energy 165 (2018) 685–708. [9] Y.E. Mili� an, A. Guti�errez, M. Gr� ageda, S. Ushak, A review on encapsulation techniques for inorganic phase change materials and the influence on their thermophysical properties, Renew. Sustain. Energy Rev. 73 (2017) 983–999. [10] A.M. Khudhair, M.M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Convers. Manag. 45 (2004) 263–275. [11] C. Liu, P. Hu, Z. Xu, X. Ma, Z. Rao, Experimental investigation on thermal properties of sodium acetate trihydrate based phase change materials for thermal energy storage, Thermochim. Acta 674 (2019) 28–35.
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Facilitated synthesis and thermal performances of novel SiO2 coating Na2HPO4⋅7H2O microcapsule as phase change material for thermal energy storage Yutang Fang *, Lihang Huang, Xianghui Liang, Shuangfeng Wang, Hao Wei, Xuenong Gao, Zhengguo Zhang Key Laboratory of Enhanced Heat Transfer and Energy Conservation of the Ministry of Education, South China University of Technology, Guangzhou, 510640, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Keywords: SiO2 Na2HPO4⋅7H2O Microencapsulated phase change material Sol-gel Anti-solvent
Novel microencapsulated phase change material (MEPCM) with SiO2 coating Na2HPO4⋅7H2O were synthesized by one-pot method. A possible formation mechanism of Na2HPO4⋅7H2O core and SiO2 shell was preliminarily analyzed. The morphology, composition, crystalline phase and thermal performances of the synthesized MEPCM was characterized by scanning electron microscopies (SEM), Transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), dif ferential scanning calorimetry (DSC), thermogravimetry analysis (TG) and thermal constants analyzer. Pre liminary analysis indicated that the Na2HPO4⋅7H2O nanocrystals as the core was obtained by anti-solvent process with partially dehydration (loss of five crystal water molecules) from Na2HPO4⋅12H2O, and the SiO2 as the shell was formed by sol-gel method with hydrolysis and condensation of tetraethyl orthosilicate. Analysis results of FTIR and XRD illuminated that only physical interaction occurred between the SiO2 shell and the Na2HPO4⋅7H2O core in the MEPCM. The results of SEM and DSC showed that the synthesized MEPCM displayed a flat cylinder structure and the particle size of about 8 μm, and exhibited a phase change temperature of 50.12 � C, latent heat of 159.8 kJ/kg and encapsulation ratio of 82.40%. Moreover, the formation of the phase change microcapsules could reduce the supercooling of Na2HPO4⋅7H2O to some degree and the MEPCM displayed a suitable thermal conductivity (0.3838 W/m⋅K) and an excellent thermal stability. The thermal properties of the MEPCM make it suitable for solar energy storage.
1. Introduction Thermal energy storage technology has aroused considerable concern in recent years because the shortage of fuel energy and envi ronmental pollution are increasingly prominent [1]. Included in the thermal energy storage approaches, latent heat storage technology, which utilizes phase change materials (PCMs), is found to be an effective method to satisfy demand of the energy [2]. So far, latent heat storage technology has broad application prospects in the fields of solar energy storage [3], heat transfer fluid [4], intelligent air-condition [5], aero space engineering [6] and so on. As the essential of latent heat storage technology, PCMs can revers ibly store or release a large amount of energy at a constant temperature
during the phase transition [7]. There are two most common PCMs classification, namely organic PCMs and inorganic PCMs. Usually, organic PCMs have the defects of low energy storage density and less thermal conductivity [8]. Instead, the inorganic ones, particularly hy drated salts, have high phase change enthalpy, nice thermal conduc tivity and environmental-friendly characteristics [9,10]. However, direct utilization of hydrated salts presents some disadvantages such as leakage in the melting process, supercooling and phase separation [8], which reduce their thermal storage capacity. To overcome these shortcomings of hydrated salts, many attempts have been made including the composite of hydrated salts with porous materials making up so-called shape-stable PCMs [11], and the micro encapsulation with a micro-sized compact shell coating hydrated salts [12]. Between two methods, due to the weak capillary action of porous
* Corresponding author. Tel.: 13318819891. E-mail addresses: [email protected] (Y. Fang), [email protected] (L. Huang), [email protected] (X. Liang), [email protected] (S. Wang), [email protected] (H. Wei), [email protected] (X. Gao), [email protected] (Z. Zhang). https://doi.org/10.1016/j.solmat.2019.110257 Received 2 July 2019; Received in revised form 19 October 2019; Accepted 25 October 2019 Available online 8 November 2019 0927-0248/© 2019 Elsevier B.V. All rights reserved.
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silica precursor and the melting enthalpy of the obtained microcapsules was found to be 123.0 kJ/kg. The thermal conductivity of the micro capsules was up to 0.6213 W/m⋅K. which was much higher than those of the microcapsules with organic shells. Wu et al. [26] reported the microencapsulation of mannitol into silica shell via an interfacial poly merization process in a water-in-oil emulsion, which obtained regular spherical capsules with uniform particle size. Lin et al. [27] micro encapsulated stearic acid in silica shell by sol-gel method to form microencapsulated PCMs, and then attached graphene oxide to surface of silica to further improve thermal performances of the microcapsules. Despite many studies on silica-shelled microcapsules, as we known, it is more difficult to microencapsulate hydrated salts owing to their hy drophilicity nature, and their tendency to change crystal water content [15], the research on feasible synthetization of inorganic hydrated salts/silica microcapsules is limited so far. For instance, Zhang et al. [28] described a microencapsulation of binary carbonate salt mixture in silica shell to enhance its effective heat capacity for its application in high temperature. Liu et al. [29] successfully microencapsulated sodium thiosulfate pentahydrate with silica shell by sol–gel method and the microcapsules showed a high encapsulation ratio of 94.65%. Liu et al. [30] prepared a sodium phosphate dodecahydrate micro-PCM with sil ica shell through interfacial polymerization combined with sol-gel pro cess. With high energy density (193.9 kJ⋅kg 1) and applicable phase change temperature (52.3 � C), disodium hydrogen phosphate heptahy drate (Na2HPO4⋅7H2O) is a potential hydrated salt as phase change material for solar energy storage. In the extreme small amounts of studies about microencapsulated Na2HPO4⋅7H2O as micro-PCMs, Huang et al. [31] prepared a Na2HPO4⋅7H2O microcapsule coated with modi fied PMMA via the suspension copolymerization-solvent volatile method, the method was relatively complicated and to our knowledge there is few literatures reported on preparation and characterization for microencapsulated Na2HPO4⋅7H2O with inorganic shell so far. Besides, on account of the developed technique in applying silica as shell in microencapsulated PCM, we attempted to employ silica to coat Na2HPO4⋅7H2O. Hence, in the present work, a novel inorganic silica (SiO2) coating Na2HPO4⋅7H2O microencapsulated phase change material (Na2H PO4⋅7H2O @SiO2 MEPCM) for thermal energy storage was successfully fabricated by one-pot method. For the application in solar energy stor age, Na2HPO4⋅7H2O have a considerable potential based on its phase change temperature of 52.3 � C and melting enthalpy of 193.9 kJ kg 1, and SiO2 can take advantage of its good thermal conductivity. In this work, a possible formation mechanism of Na2HPO4⋅7H2O core and SiO2 shell was analyzed. The morphology and phase change properties of the MEPCM were also investigated.
Nomenclature Tm ΔHm R E
phase change temperature (� C) latent heat (kJ⋅kg-1) encapsulation ratio (%) encapsulation efficiency (%)
Abbreviations PCM phase change material MEPCM microencapsulated phase change material DSP disodium hydrogen phosphate dodecahydrate TEOS tetraethyl orthosilicate NPA nanoparticle size analyzer SEM scanning electron microscope EDS energy dispersive spectroscopy TEM Transmission electron microscopy FTIR Fourier-transform infrared XRD X-ray diffraction DSC differential scanning calorimetry TGA thermogravimetric analysis
material on hydrated salt, the hydrated salt composites carry the risk of leakage and thermal unreliability when the temperature rises above the melting point of the hydrated salts [13], while the microencapsulation of hydrated salts is considered to be an effective method to solve the aforementioned problems. Intuitively, the microcapsule shell can confine the hydrated salts in a limited space, which reducing volume changes of the salts and controlling the phase separation when phase change occurs [14]. Moreover, the microcapsule shell can protect hy drated salts from outside environments, prevent leakage from their location and restrict the supercooling during cooling process [15]. Therefore, choosing a suitable shell material for the microencapsulated hydrated salts plays an important role in controlling the structure, me chanical stability and supercooling degree. For organic PCMs, the traditional shell materials selected are generally the polymers, such as polyurea [16], polystyrene [17], urea-formaldehyde resin [18], etc. But for inorganic hydrated salts, the literatures reported about their microencapsulation are rare. Fu et al. [19] used poly (ethyl-2-cyanoacrylate) (PECA) as organic shell and sodium thiosulfate pentahydrate(Na2S2O3⋅5H2O) as core to prepare a novel phase change microcapsule. After microencapsulation, it showed an improved thermal stability and a good thermal reliability. Yang et al. [20] combined sol –gel process with interfacial polymerization to synthesized a calcium chloride hexahydrate(CaCl2⋅6H2O) microcapsule with organo alkoxysilane shell, the SEM images confirmed a successful microen capsulation and displayed a nearly spherical shapes with a fine core–shell structure of the microcapsule. Although the reported hy drated salt microcapsules with polymeric shells usually present an excellent core-shell structure, their application is always restricted by the flammability, toxicity, poor mechanical strength and the low ther mal conductivity of their polymeric shells [21,22]. Crucially, high thermal conductivity of hydrated salts microcapsules is imminently required to improve the heat transfer efficiency during phase change processes. In this case, compared with polymeric shell materials, the inorganic shells are outstanding in thermal conductivity and mechanical strength [23], thus a new research trend toward microencapsulating PCMs by inorganic shells has boomed recently. Besides, the inorganic shell materials, especially the silica material, has arrested people’s attention due to their fine biocompatibility, nontoxicity, and chemical inertness [24]. Some literatures have reported the research on employing silica shells in microencapsulation of PCMs these years. Zhang et al. [25] synthesized the microencapsulated n-octadecane phase change material with silica shell through sol-gel process using TEOS as a
2. Materials and methods 2.1. Materials Disodium hydrogen phosphate dodecahydrate (Na2HPO4⋅12H2O, DSP, AR) were purchased from Shanghai TCI Limited, China. Tetraethyl orthosilicate (TEOS, CP) served as silica source were supplied by Guangzhou Chemical Reagent Factory, China. Absolute ethanol (ET, AR) were obtained from Guangzhou Donghong Chemical Plant, China. Ammonium hydroxide (NH3⋅H2O, 28 wt%) were used to serve as a catalyst. All chemicals were reagent quality and used without further purification. 2.2. Synthesis of Na2HPO4⋅7H2O@SiO2 MEPCM The Na2HPO4⋅7H2O@SiO2 MEPCM was synthesized by one-pot method, in which Na2HPO4⋅7H2O nanocrystals as the core was pro duced by anti-solvent process with partially dehydration from DSP, and the SiO2 as the shell was synthesized by sol-gel method with hydrolysis and condensation of TEOS. 2
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The typical synthesis was as follows. Under magnetic stirring, suc cessively adding 10.0 ml ET and 0.5 ml (2.2 mmol) TEOS into a 50 ml beaker, an alcoholic solution A was obtained. Similarly, a saturated Na2HPO4 aqueous solution B was formed by adding 0.53 g (1.3 mmol) DSP, 2.0 ml (111.1 mmol) deionized water and 0.1 ml (2.0 mmol) ammonia. Subsequently, the aqueous solution B was added into alco holic solution A dropwise using a syringe under the stirring rate of 1600 rpm. After that, the resulting mixture was continually encapsu lated at 500 rpm and 25 � C for 4 h, and a pinch of white powder was obtained by suction filtration. After orderly leaching the powder with 3 ml deionized water and 3 ml ethanol to remove the uncoated hydrated salt crystals and the residual water on powder surface, the Na2H PO4⋅7H2O@SiO2 MEPCM was obtained by drying at 25 � C for 12 h. The detailed schematic diagram of synthesis process for MEPCM is shown in Fig. 1. In order to obtain a satisfactory Na2HPO4⋅7H2O nanocrystals, the crystallization concentration (Cc), which refers to the volume ratio of the saturated Na2HPO4 aqueous solution B and the alcoholic solution A is proposed. A series of crystallization concentrations (0.15, 0.20, 0.25, 0.30, 0.35, 0.40) were obtained by fixing the amount of solution B and changing the amount of antisolvent ET in solution A.
3. Results and discussion 3.1. Possible formation mechanism of Na2HPO4⋅7H2O@SiO2 MEPCM Generally, the smaller the size of the hydrated salt core material, the easier it is coated to form PCM microcapsule. On the other hand, since there are few sources and high price of the commercial Na2HPO4⋅7H2O as core, an attempt was made to prepare Na2HPO4⋅7H2O nanocrystals from DSP with wide sources and low price. 3.1.1. Formation of Na2HPO4⋅7H2O nanocrystals as core In this experiment, ethanol served as an anti-solvent (corresponding to solution A) to separate out Na2HPO4⋅7H2O crystals from the saturated Na2HPO4 solution (corresponding to solution B). Notably, the crystal lization concentration (Cc) plays an important role in the formation of Na2HPO4⋅7H2O crystals and the control of its size [32]. Table 1 listed the phase change temperature (Tm), enthalpy (ΔHm) and particle size (D) of the Na2HPO4⋅7H2O crystals synthesized with different crystallization concentrations. It can be seen from Table 1 that when the Cc was small, such as 0.15, 0.20, the obtained crystal sizes were 227, 272 nm, respectively, which were small and had no evident changes. When the Cc was above 0.20, such as 0.25, 0.30, 0.35, the obtained crystal sizes obviously increased, which were 892, 1234, 760 nm, respectively. However, when the Cc was increased to 0.40, the crystal size showed a significant reverse reduction and was only 107 nm. According to the crystallization theory [33], generally, in antisolvent crystallization, the crystals size is determined by supersaturation of system. In the Na2H PO4/ethanol system, the existence of ethanol as antisolvent would cause an increase in supersaturation of Na2HPO4 solution due to the anti solvent effect, thus it separated out DSP crystals. Meanwhile, ethanol would remove part of the crystal water of DSP into the system, which reducing the supersaturation (dehydration effect). In a small Cc such as 0.15, the large amount of ethanol made the antisolvent effect play a leading role in affecting the supersaturation and making the supersat uration increase. As a result, the increased supersaturation accelerated the nucleation and growth rate to get small crystals. When Cc was increased to 0.3, it caused a gradual reduction in supersaturation, and directly, brought a reduction in the nucleation and growth rate, thus a
2.3. Characterization of Na2HPO4⋅7H2O@SiO2 MEPCM The chemical composition of the microcapsules was tested by Fourier transform infrared spectroscopy (FTIR, TENSOR 27, Bruker Corpora tion, Germany) within a range from 400 to 4000 cm-1. The crystalline structure was analyzed by X-ray diffraction (XRD, D/ max-A, Rigaku Corporation, Japan) at a scanning rate of 2 deg⋅min-1. The morphology and elemental analysis of the MEPCM were observed by scanning electron microscope with energy dispersive spectrometer (SEM-EDS, S-3700 N, Hitachi, Japan) at an operating voltage of 10 kV. The microstructure of the MEPCM was detected by Transmission electron microscopy (TEM, JEM-2100, JEOL, Japan). The phase change properties were measured by differential scanning calorimeter (DSC, Q20, TA Instrument Company, USA) under N2 at a heating or cooling rate of 5 � C⋅min-1. The thermal stability was characterized using thermogravimetric analysis (TG, TG209F3, Netzsch company, Germany) under N2 at a heating rate of 10 � C⋅min-1 in the temperature range from 30 � C to 500 � C. The particle size of Na2HPO4⋅7H2O crystals as core in the solution A was measured with nanoparticle size analyzer (NPA150, Microtrac Co. Ltd., USA). The thermal conductivity of the sample was tested by thermal con stants analyzer (Hot Disk, Hot Disk Company Limited, Sweden) at about 25 � C. 3 g MEPCM sample was added into a cylindrical mold and com pressed it into two flat tablets with 2 cm in diameter and 0.5 cm in height, and a 7577 probe was put between them. The tests were repeated for three times and the results were averaged.
Table 1 Tested data of phase change temperature (Tm), enthalpy(ΔHm) and particle size (D) of the Na2HPO4⋅7H2O crystals obtained at different crystallization concentrations. Crystallization concentration
0.15
0.20
0.25
0.30
0.35
0.40
Tm/� C
51.70
52.30
51.08
190.1
193.9
195.5
D/nm
227
272
892
37.28, 50.21 181.6, 47.9 760
34.68
ΔH/kJ/kg
39.18, 53.24 125.7, 73.80 1234
Fig. 1. Process flow diagram of Na2HPO4⋅7H2O@SiO2 MEPCM synthesized by one-pot method. 3
239.9 107
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rise in crystals size came out. However, when Cc was further increased, such as 0.35, the dehydration effect would be weakened due to the reduction of ethanol, thus the supersaturation of the system was reversely increased, making the particle smaller. Finally, as seen from Table 1, at Cc of 0.4, the dehydration of DSP did not occur, which indicated that the supersaturation reached the maximum, and a mini mum particle size appeared. More importantly, the effects of the Cc on the thermal performances of the Na2HPO4⋅7H2O crystals were more distinct. When the Cc was below 0.25, the corresponding Tm and ΔHm of the products obtained were about 51 � C and 190 kJ/kg, respectively, which assigned the product to Na2HPO4⋅7H2O crystals. When the Cc was 0.30 and 0.35, there were two melting points about 37 � C and 51 � C and two melting enthalpies for the crystals, which corresponding to the mixed products of Na2HPO4⋅7H2O and DSP crystals. As the Cc was 0.4, only DSP crystals appeared in the product. A possible reason was that, when the Cc was larger, such as 0.4, it meant the smaller amount of antisolvent ET was used in the solution A, only tiny DSP crystals containing 12 crystal water molecules could be generated and evenly dispersed in solution A. With a fewer Cc, such as 0.30, 0.35, it meant an increase in the amount of ET, some of DSP crystals lost five crystal water molecules, and became Na2HPO4⋅7H2O crystals, so the mixed and big crystal particles con taining DSP and Na2HPO4⋅7H2O were obtained, and they couldn’t be evenly dispersed in solution A. When Further reduced Cc, such as below 0.25, it meant a further increase of ET solvent, all DSP crystals became Na2HPO4⋅7H2O crystals by dehydration and evenly dispersed Na2H PO4⋅7H2O crystals appeared in solution A. Considering the phase tran sition temperature, enthalpy and particle size of the crystals obtained by anti-solvent method, Cc of 0.20 was appropriate. Fig. 2 shows the particle size distribution of the Na2HPO4⋅7H2O crystals obtained at Cc of 2.0. The results indicated that the obtained Na2HPO4⋅7H2O crystals had a small and narrow size distribution.
These monomeric ortho-silicic acids have great activity and can be easily adsorbed on the surface of the Na2HPO4⋅7H2O nanocrystals. Later, condensation occurs between ortho-silicic acid or between ortho-silicic acid and TEOS to form silica-sol particles on the surface of crystals, it can be described by the following equation.
Finally, the silica oligomer formed in last step continues to conden sate to form silica gel with solid three-dimensional network structures. And the fine Na2HPO4⋅7H2O nanocrystals are coated by the formed silica gel to finish the microencapsulation.
3.1.2. Formation of SiO2 as shell The MEPCM was coated by the silica shell assembled on the surface of Na2HPO4⋅7H2O nanocrystals via an in-situ condensation of the generated silica precursors from the hydrolysis of TEOS. The hydrolysis and condensation of TEOS to form gel around the crystals can be described in three steps. Firstly, the hydrolysis of TEOS is initiated with the addition water and ammonia and creates hydroxylated silica pre cursors. Under the alkaline condition, the hydrolysis rate of TEOS is signally higher than its condensation rate, and the obtained main product is ortho-silicic acid (Si(OH)4). The reaction can be described by the following equation.
3.2. Phase change properties of Na2HPO4⋅7H2O@SiO2 MEPCM 3.2.1. Optimization of microencapsulation condition There are many experimental factors to the synthesis of micro encapsulated PCM, such as reaction temperature, time and reactant ratio. Thereinto, the mass ratio of core to shell directly affected the encapsulation efficiency. The phase change enthalpy of the MEPCM prepared with different Na2HPO4⋅7H2O/SiO2 mass ratio are displayed in Fig. 3a. By drawing a comparison among the enthalpy of the MEPCM synthesized in different mass ratio, we found the MEPCM with mass ratio of 3.468 had the highest enthalpy, indicating a higher microen capsulation efficiency the MEPCM achieved. Enthalpy was lower in low mass ratio such as 1.486 and 2.311, possible reason was that the silica precursors generated by hydrolysis process of TEOS were relatively excessive, these excessive precursors could bring about a selfcondensation of silica precursors or largely absorb on the Na2H PO4⋅7H2O crystals to make a thick shell for the MEPCM, resulting in the reduction of the phase change enthalpy of the MEPCM. Nevertheless, in a much higher mass ratio such as 5.021 and 8.086, the phase change enthalpy decreased inversely. It could be explained that the produced silica precursors were insufficient to build a compact shell for the core, and caused the MEPCM damaged during the agitation or washing pro cess, thus resulting in a low core loading within the MEPCM. Thus, the core to shell mass ratio of 3.468 was suitable. In addition, the amount of catalyst was a direct factor affecting the hydrolysis rate of TEOS. Fig. 3b shows the phase change enthalpy of the MEPCM prepared with different amounts of catalyst. As seen from Fig. 3b, at amount of 2.0 mmol the MEPCM reached the highest enthalpy when the amount increased from 1.2 to 2.8 mmol. Theoretically, the hydrolysis rate of TEOS increased with the increase of the amount. Low amount of catalyst, such as 1.2 mmol, meant a comparatively low
Fig. 2. Particle size distribution of the Na2HPO4⋅7H2O crystals obtained at crystallization concentrations of 2.0. 4
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Fig. 3. Effects of mass ratio of core and shell(a) and amount of catalyst(b) on the enthalpy of Na2HPO4⋅7H2O@SiO2 MEPCM.
hydrolysis rate of TEOS, which caused a lack of silica precursors generated by hydrolysis, and led to an incompact coating for Na2H PO4⋅7H2O crystals. However, higher amounts, such as 2.4 and 2.8 mmol, caused an excess of produced silica precursors, which resulted in the excessive adsorption on the crystals surface and the increased thickness of shell. Moreover, silica micronuclei came into being under high hy drolysis rate and caused problems such as uneven adsorption and silica aggregates. The results suggested that the MEPCM synthesized at 2.0 mmol catalyst had the highest enthalpy. After improving the reaction conditions such as mass ratio of core to shell and amount of catalyst, an optimized MEPCM was synthesized, and a series of characterization of the MEPCM were carried out.
properties, Encapsulation ratio (R) and encapsulation efficiency (E), and they are acquired by following equations [34]. R¼
ΔHm;micro PCM � 100% ΔHm;PCM
E¼
ΔHm;micro PCM þ ΔHc;micro ΔHm;PCM þ ΔHc;PCM
PCM
� 100%
The encapsulation ratio and encapsulation efficiency of MEPCM were calculated to be 82.40% and 74.42%, respectively, inferring that the MEPCM has high heat storage capacity. In addition, compared with the cooling curve of Na2HPO4⋅7H2O, it could be noticed that the MEPCM crystallized prior to Na2HPO4⋅7H2O during the cooling process, and the difference between the melting temperature and the crystallization temperature of MEPCM is obviously smaller than that of Na2H PO4⋅7H2O, which meant a reduction on supercooling degree emerged in the MEPCM. This reduction of supercooling degree was attributed to the space-confined silica shell, which confirms a fabrication of silica shell onto the Na2HPO4⋅7H2O core to some extent.
3.2.2. Phase change behavior Fig. 4 shows DSC curves of Na2HPO4⋅7H2O and Na2H PO4⋅7H2O@SiO2 MEPCM, and Table 2 displays the corresponding data. As shown in Fig. 4 and Table 2, the MEPCM had a melting enthalpy of 159.8 kJ/kg and a melting point of 50.12 � C, while the melting latent heat and melting point of Na2HPO4⋅7H2O were 193.9 kJ/kg and 52.30 � C. Each curve appeared to show only one melting point at almost the same temperature. Based on the analysis from the DSC results, it could be suggested that the core material encapsulated in the MEPCM was Na2HPO4⋅7H2O. The MEPCM’s melting enthalpy of 159.8 was less than 193.9 kJ/kg of Na2HPO4⋅7H2O, this change in melting enthalpy could be ascribed to the existence of the silica shell. There are two important parameters to describe the phase change
3.2.3. Thermal stability and reliability Fig. 5 shows the TG and DTG curves of Na2HPO4⋅7H2O and the MEPCM. It could be found that the two samples performed the same thermal decomposition in the temperature range of 30–400 � C, which is credited to a large number of evaporation of crystal water for core material and a small amounts of thermal decomposition for Na2HPO4 [35]. As seen from the TG curves, the MEPCM showed a higher initial evaporation temperature than the pure Na2HPO4⋅7H2O, and the MEPCM achieved a final weight loss of 34.4% while the Na2HPO4⋅7H2O achieved 40%, these results can be the evident that the silica shell was success fully formed to protect the Na2HPO4⋅7H2O core in some way. On the other hand, from the two DTG curves it could be noticed that the peak temperature (138.8 � C) at maximum weight loss rate of the MEPCM was higher than that of Na2HPO4⋅7H2O (76.2 � C), it proved that the shell can provide protection for the Na2HPO4⋅7H2O core after microencapsulation, thus improved the thermal stability of the core material. The thermal reliability of the MEPCM was evaluated by the heatingcooling cycling test. DSC curves and corresponding data of the Na2H PO4⋅7H2O@SiO2 MEPCM after different heating-cooling cycles was shown in Fig. 6. According to Fig. 6, the Tm of Na2HPO4⋅7H2O@SiO2 MEPCM after experiencing 1,15,30 heating-cooling cycles was 50.26, 50.20 and 50.11 � C, respectively, showing little difference between that of the MEPCM before cycling test (50.12 � C). As regards the ΔHm of the MEPCM, a similar result could also be learnt. The ΔHm of the MEPCM after 1, 15 and 30 cycles were 160.2, 151.5 and 149.4 kJ kg 1, respec tively. Numerically, the ΔHm of MEPCM after 30 cycles presented a 6.5%
Fig. 4. DSC curves of Na2HPO4⋅7H2O and Na2HPO4⋅7H2O@SiO2 MEPCM. 5
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Table 2 DSC data of Na2HPO4⋅7H2O and Na2HPO4⋅7H2O@SiO2 MEPCM. MEPCM Na2HPO4⋅7H2O
Fig. 5. TG and DTG PO4⋅7H2O@SiO2 MEPCM.
Process
T(� C)
ΔH (kJ⋅kg
Heating Cooling Heating Cooling
50.12 18.24 52.30 14.68
159.8 61.10 193.9 102.9
curves
of
Na2HPO4⋅7H2O
and
1
)
Encapsulation ratio (%)
Encapsulation efficiency (%)
82.40
74.42
-
-
Na2H
Fig. 7. SEM image of Na2HPO4⋅7H2O@SiO2 MEPCM after experiencing 30 heating-cooling cycles. Table 3 Phase change enthalpies and thermal conductivities of Na2HPO4⋅7H2O@SiO2 MEPCMs synthesized at different mass ratios of core to shell. Mass ratio of core to shell
ΔHm (kJ⋅kg
Na2HPO4⋅7H2O 1.486 2.311 3.468 5.021 8.080 SiO2
193.9 89.19 109.8 159.8 100.8 32.96 -
1
)
Thermal conductivity (W⋅m-1⋅K-1) 0.4951 0.3266 0.3519 0.3838 0.3234 0.3164 0.2849
with the enthalpy of 159.8 kJ kg 1. The thermal conductivity of Na2HPO4⋅7H2O (0.4951 W⋅m-1⋅K-1) is larger than that of pure SiO2(0.2849 W m-1 K-1), the thermal conduc tivity of the MEPCM would be improved with the increasing composi tion of Na2HPO4⋅7H2O in MEPCM. So, it seemed the thermal conductivity would rise with the mass ratio of core to shell increased, theoretically. However, the conductivity reached a maximum at mass ratio of 3.468 and then fell as the mass ratio increased, the probable reason was that the excessive Na2HPO4⋅7H2O resulted in incomplete microencapsulation and a loss of Na2HPO4⋅7H2O core, it meant the composition of Na2HPO4⋅7H2O was reversely reduced, thus brought a reduction in thermal conductivity. Therefore, the MEPCM synthesized at mass ratio of 3.468 possessed a fine thermal conductivity of 0.3838 W m1 -1 K , which was higher than those of some common MEPCMs with organic shell (0.10–0.20 W m-1 K-1). As can be seen, the inorganic-coated MEPCMs presented a nice capability for heat conduction.
Fig. 6. DSC curves of Na2HPO4⋅7H2O@SiO2 MEPCM after heating-cooling cycling test.
fall when compared with that of the MEPCM before the test (159.8 kJ kg 1), which meant a slight decline in melting enthalpy of the MEPCM occurred during 30 heating-cooling cycles. All these results suggested that the synthesized MEPCM in present work had relatively good thermal reliability. In addition, the MEPCM after 30 heating-cooling cycles was observed by SEM, the SEM image was put in Fig. 7. Fig. 7 showed the MEPCM remained intact and no breakage could be found in the morphology of the MEPCM after 30 heating-cooling cycles. The result supplemented the evidence of good thermal reliability the MEPCM possessed. 3.2.4. Thermal conductivity The thermal conductivities of MEPCM synthesized at different mass ratio of core to shell are displayed in Table 3. It showed the thermal conductivity reached a peak of 0.3838 W m-1 K-1 at mass ratio of 3.468, 6
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3.3. Composition and structure of Na2HPO4⋅7H2O@SiO2 MEPCM 3.3.1. Composition analysis The FTIR spectra of Na2HPO4⋅7H2O, SiO2 and Na2HPO4⋅7H2O@SiO2 MEPCM are demonstrated in Fig. 8. From the spectrum of Na2H PO4⋅7H2O(a), two absorption peaks of phosphate radical (PO34 ) appeared at 1069 cm-1 and 1261 cm-1, which referred to the symmetrical stretching vibration of O–P–O and the asymmetric stretching vibration – O, respectively. The peaks at 1660 cm-1 and 517 cm 1 resulted of P– – O and P–O–P. The O–H stretching from the bending vibrations of P– bands emerged near 3450 cm 1 could be attributed to the bound water on Na2HPO4⋅7H2O. As shown in the spectrum of SiO2(b), bands observed at 1096 cm-1 could be resulted from the antisymmetric stretching vibration of Si–O–Si, and the symmetrical stretching vibration of Si–OH gave rise to the bands around 952 cm-1. The bands at 461 cm-1 could be related to the bending vibration of Si–O–Si. In addition, the small peaks appeared around 3451 cm-1 and 1631 cm-1 referred to the hydroxyl stretching and bending vibration of Si–OH from the surface of SiO2. Spectrum (c) presents the characteristic absorption peaks of the MEPCM. It was easy to notice that the above characteristic peaks in spectrum (a) and (b) could be found in the spectrum of the MEPCM, and it could hardly find out new characteristic peaks. The results indicated that the SiO2 shell was successfully formed by sol-gel method in the preparation of the MEPCM, and no chemical reaction occurred between the shell (SiO2) and core (Na2HPO4⋅7H2O).
Fig. 9. XRD patterns of Na2HPO4⋅7H2O(a), SiO2(b) and Na2HPO4⋅7H2O@SiO2 MEPCM(c).
are fainter and marked at 18.88� ,20.90� ,26.49� ,30.55� and 31.61� . The above analysis of peaks suggested that the silica shell achieved a microencapsulation for Na2HPO4⋅7H2O core. These results also indi cated that there was no chemical interaction between the Na2H PO4⋅7H2O core and the silica shell, which is in coincidence with the FTIR analysis. Moreover, the reduction of the Na2HPO4⋅7H2O intensities partly demonstrated a successful microencapsulation.
3.3.2. Crystalline phase analysis Fig. 9 illustrates the XRD patterns of Na2HPO4⋅7H2O(a), SiO2(b) and Na2HPO4⋅7H2O @SiO2 MEPCM(c). As displayed in pattern (a), five distinct diffraction peaks of the Na2HPO4⋅7H2O appeared at 18.91� , 20.89� , 26.56� , 30.61� and 31.69� . By comparison with Standard XRD pattern card 10–0191, it proved the existence of crystal phase of Na2HPO4⋅7H2O. Likewise, a broad diffuse peak could be found around 20� in pattern (b), which represented the amorphous state of the SiO2 synthesized in this work. It was noteworthy that all characteristic peaks of Na2HPO4⋅7H2O can be found in the pattern of the MEPCM, but they
3.3.3. Morphology, microstructure observation and elemental analysis The SEM and TEM images of Na2HPO4⋅7H2O@SiO2 MEPCM are displayed in Fig. 10. It can be seen from Fig. 10a that the MEPCM showed a flat and nearly cylindrical appearance. As presented in Fig. 10b, the MEPCM had a rough surface with a particle size of about 8.0 μm. It could also be found that there were numbers of small particles sticking to the surface of the MEPCM. These particles are silica aggre gates, which caused by the self-condensation of silica due to a rapid
Fig. 8. FTIR spectra of Na2HPO4⋅7H2O(a), SiO2(b) and Na2HPO4⋅7H2O@SiO2 MEPCM (c). 7
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Fig. 10. SEM (a and b) and TEM (c) images of Na2HPO4⋅7H2O@SiO2 MEPCM.
condensation rate in the reaction. In addition, a handful of fragments of shell found in Fig. 10a could be the result of the broken MEPCM in the preparation or the preparative spraying process of SEM. In order to confirm a typical core-shell structure of the synthesized MEPCM, the MEPCM was observed by TEM, the result was illustrated at Fig. 10c. Fig. 10c showed a nearly oblate MEPCM with a distinct core-shell structure in a great contrast between the light core and dark shell. Several tiny silica particles could be viewed to adhered to the surface of the MEPCM, which were the result of the self-condensation of silica and also could be found in Fig. 10b. The qualitative and semi-quantitative element analysis of the MEPCM on the sample microregion were detected by EDS and are shown at Fig. 11 and Table 4. In Fig. 11, the element carbon(C) was caused by the use of conductive adhesive during the preparation of sample. Since the EDS technology can only analyze the surface of the MEPCM, Table 4
Table 4 EDS data of Na2HPO4⋅7H2O@SiO2 MEPCM. Element
Weight/%
Atomic/%
O Si Na P
52.58 32.15 7.14 8.13
65.67 22.87 6.21 5.25
indicated the MEPCM was mainly consisted of a large amount O, Si el ements which came from SiO2 shell and a small amount Na, P elements which were from Na2HPO4⋅7H2O. Table 4 also displayed the percent of different elements. The atomic percent of Si and O element was 22.87% and 65.67%, respectively, and it was notable that they weren’t equiv alent to the basic chemical composition of SiO2 shell, which may be due to semi-quantitative analysis of EDS. 4. Conclusions In this paper, a novel Na2HPO4⋅7H2O@SiO2 microencapsulated phase change material (MEPCM) for thermal energy storage was easily synthesized by one-pot method. The possible formation mechanism of Na2HPO4⋅7H2O core and SiO2 shell was preliminary discussion. The composition, structure and thermal performances of the synthesized Na2HPO4⋅7H2O@SiO2 MEPCM were performed a comprehensive anal ysis. The conclusions are as follows. (1) The complete Na2HPO4⋅7H2O nanocrystals as the core at the crystallization concentration (Cc) of 0.2 was formed by antisolvent process with partially dehydration (loss of five crystal water molecules) from Na2HPO4⋅12H2O. The SiO2 as the shell was formed by the hydrolysis and condensation of TEOS. (2) FT-IR and XRD results suggested that Na2HPO4⋅7H2O was suc cessfully wrapped in SiO2 shell, which was also confirmed by
Fig. 11. EDS spectrum of Na2HPO4⋅7H2O@SiO2 MEPCM. 8
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SEM-EDS and TEM characterization. The MEPCM presented a nearly 8 μm flat cylindrical morphology in SEM image. (3) DSC results showed that the synthesized Na2HPO4⋅7H2O@SiO2 MEPCM had a preferable melting enthalpy of 159.8 kJ/kg and melting point of 50.12 � C, and reached an encapsulation ratio of 82.40%, Due to the existence of SiO2 shell, the MEPCM showed a fine thermal conductivity of 0.3838 W/m⋅K and possessed an excellent thermal stability and reliability. According to the phase change and heat transfer performance the synthesized MEPCM showed, in some way it can be said that there exists an attractive prospect of applying this MEPCM in solar energy storage.
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