Synthesis and characterization of microencapsulated sodium sulfate decahydrate as phase change energy storage materials

Synthesis and characterization of microencapsulated sodium sulfate decahydrate as phase change energy storage materials

Applied Energy 255 (2019) 113830 Contents lists available at ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy Synthe...

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Applied Energy 255 (2019) 113830

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Synthesis and characterization of microencapsulated sodium sulfate decahydrate as phase change energy storage materials ⁎

T



Zhishan Zhang, Yadong Lian, Xibin Xu, Xiaonong Xu, Guiyin Fang , Min Gu

National Laboratory of Solid State Microstructures and School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

H I GH L IG H T S

of Na SO ·10H O were fabricated by emulsion polymerization. • Microencapsulation size of microcapsules was tunable by reducing amount of Triton X-100. • The and chemical structure of microcapsules were presented and analyzed. • Microstructure segregation of Na SO ·10H O has been inhibited by microencapsulation. • Phase • Microcapsule by using 1.0 mL of Triton X-100 has a high latent heat of 125.57 J/g. 2

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A R T I C LE I N FO

A B S T R A C T

Keywords: Sodium sulfate decahydrate Silica shell Reverse micelle and emulsion polymerization Microcapsule Thermal energy storage

Sodium sulfate decahydrate has been microencapsulated within a silica shell through a novel method of reverse micellization and emulsion polymerization. Tetraethoxysilane and 3-aminopropyl-triethoxysilane were used in conjunction as silicon precursors to form the silica shell, which encapsulated sodium sulfate decahydrate as a phase change material for thermal energy storage. The melting and solidifying temperatures of the microcapsules were measured as 33.6 °C and 6.0 °C, respectively, with associated latent heats of 125.6 kJ/kg and 74.0 kJ/kg. The phase segregation of various hydrate salts was inhibited by the confining effect of the silica mesopores. The size of the microcapsules could be regulated from 500 nm to 28 μm simply by reducing the amount of surfactant (Triton X-100) deployed as a stabilizer. Confined by SiO2 matrix, heat storage properties of the hydrate salts were greatly improved. Sodium sulfate decahydrate microencapsulated within a silica shell is shown to be suitable for application in thermal energy storage.

1. Introduction In recent years, materials for the storage of the latent heats of phase changes, termed phase-change materials (PCMs), have attracted extensive attention in the fields of energy storage [1,2] and drug release [3,4]. PCMs undergo phase changes upon heating or cooling, and can store or release a lot of the associated latent heat if they have sufficient energy storage capacity [5,6]. Generally, inorganic solid–liquid PCMs are suitable for energy storage due to their high latent heats and the small volume changes that accompany their phase transitions, as well as their high thermal conductivity, non-toxic and non-flammable nature, and low cost [7]. As typical inorganic PCMs, salt hydrates have been widely applied in construction [8,9], thermal energy storage [10,11], solar water heating systems [12,13], and electronic cooling devices [14,15].



Sodium sulfate decahydrate (Na2SO4·10H2O), as an inorganic hydrate salt with a moderate phase-change temperature of 32.4 °C, a high enthalpy of 251.0 kJ/kg, and a non-hazardous nature, is appropriate for the storage of energy or drug release [16]. It has disadvantages of supercooling and phase segregation [17], but these can be overcome by adding nucleating [18] or thickening agents [19,20]. A microencapsulated phase-change material (MPCM) is composed of a core and a shell, whereby the PCM constitutes core and a polymer or an inorganic compound constitutes the shell [21,22]. Recently, MPCMs have been produced for energy storage, in which the shells prevent leakage of the PCM while also inhibiting phase segregation and supercooling [23]. The core–shell structure of the microencapsulation provides a large specific surface area for heat transfer, thereby enhancing thermal conductivity [24]. In order to produce MPCMs, many technologies have been applied, including emulsification, in situ

Corresponding authors. E-mail addresses: [email protected] (G. Fang), [email protected] (M. Gu).

https://doi.org/10.1016/j.apenergy.2019.113830 Received 6 June 2019; Received in revised form 25 August 2019; Accepted 1 September 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

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Table 1 Synthetic components for Na2SO4·10H2O@SiO2 microencapsulation. Samples

Aqueous phase

Oil phase

Silicon precursor

Triton X-100 added

MPCM1 MPCM2 MPCM3 MPCM4 MPCM5

8 g Na2SO4·10H2O + 8 mL distilled water

100 mL cyclohexane + 2 mL ethanol + 2 mL n-pentanol

8 mL TEOS + 2 mL APTS 8 mL TEOS + 2 mL APTS 8 mL TEOS + 2 mL APTS 4 mL TEOS + 2 mL APTS 12 mL TEOS + 2 mL APTS

0.8 mL 0.9 mL 1.0 mL 1.0 mL 1.0 mL

Fig. 1. FT-IR spectra of (a) Na2SO4·10H2O, (b) silica, and (c–g) MPCM1–MPCM5.

Fig. 2. XRD patterns for (a) silica and (b–f) MPCM1–MPCM5. 2

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microcapsules were prepared [28]. Microcapsules with a silica shell have advantages of high thermal and chemical stability, low cost, and low toxicity [29,30]. Zhang et al. employed sodium dodecyl sulfate (SDS) as an emulsifier with tetraethyl orthosilicate (TEOS) and 3-aminopropyl-triethoxysilane (APTS) as silicon precursors to enwrap Na2SO4·10H2O in a reverse micelle system. The microcapsules had a nano-bowl morphology and an enthalpy of 180.7 kJ/kg [31]. However, such information on the microencapsulation of inorganic PCMs has been limited. Salt hydrates are technically difficult to be encapsulated due to their hydrophilicity and tendency to alter their water content [32]. It seems that there have been no fundamental studies concerning the preparation of microcapsules containing inorganic PCMs. There is still a knowledge gap with respect to microencapsulation process of inorganic hydrated salt. Based on the great application potential of microencapsulated inorganic PCMs, the aim of this study is to develop high-performance, low-temperature microencapsulated PCMs with rigid inorganic shells. A novel method of emulsion polymerization is proposed for the encapsulation of sodium sulfate decahydrate within silica, denoted as Na2SO4·10H2O@SiO2. In the polymerization process, cyclohexane serves as the oil phase and sodium sulfate decahydrate solution is the aqueous phase. Triton X-100 is used as emulsifier to synthesize MPCMs for the first time. A certain amount of Triton X-100 is added to the cyclohexane as a stabilizer and emulsifier. A monomer prepared from TEOS and APTS is added to the oil phase to create the shell. Regular spherical microcapsules are obtained, the size of which is tunable from 500 nm to 28 μm depending on the amount of Triton X-100 used. The capsule with remarkable sealing property can be obtained under the circumstance of applying bicomponent silicon precursors mixed with TEOS and APTS in a certain proportion. Supercooling and phase separation of the MPCMs are significantly inhibited by the constraint of the shell. Moreover, SiO2 modified by the amine group is used as encapsulation material for Na2SO4·10H2O can keep moisture inside the shell, which is necessary for hydration of Na2SO4. Consequently, this is a promising method for the production of microcapsules containing inorganic or hydrophilic organic PCMs. The Na2SO4·10H2O@SiO2 microcapsules will be a potential candidate for the applications in thermal energy storage. 2. Synthesis and characterization 2.1. Materials Sodium sulfate decahydrate (Na2SO4·10H2O), 3-aminopropyl-triethoxysilane (APTS), and Triton X-100 were obtained from Aladdin Industrial Corporation. Tetraethoxysilane (TEOS), cyclohexane, and absolute ethanol were supplied by Sinopharm Chemical Reagent Co., Ltd. n-Pentanol was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Water was distilled and deionized using a Millipore Milli-Q purification system to a resistivity of not less than 18.2 MΩ. All reagents were used without further purification.

Fig. 3. SEM images of samples prepared by addition of (a, b) 0.6 mL, (c, d) 0.8 mL (MPCM1), (e, f) 0.9 mL (MPCM2), (g, h) 1.0 mL (MPCM3), and (i, j) 1.5 mL of Triton X-100.

polymerization, electroplating, interfacial adhesion, and sol–gel synthesis [25]. Graham et al. employed an in situ inverse mini-emulsion polymerization method to encapsulate magnesium nitrate hexahydrate. The average size of the nanocapsules was in the range from 100 to 200 nm, and the latent heat of the nanoencapsulated salt hydrate of 83.2 kJ/kg remained unchanged after 100 thermal cycles [26]. Huang et al. employed an interfacial polymerization method to fabricate microcapsules composed of a Na2HPO4·7H2O core and a modified poly (methyl methacrylate) (PMMA) coating shell. The microcapsules were spherical, with a smooth and compact surface, and an average diameter of around 6.8 μm. The phase-change temperature was around 51 °C, with an associated enthalpy of 150 kJ/kg. Moreover, supercooling of the hydrated salt was significantly reduced [27]. Schoth et al. prepared a water/oil emulsion by a reversed-phase Pickering emulsion method, using cyclohexane as the oil phase and modified nano-SiO2 as a stabilizer and emulsifier, from which Na2SO4@PU (polyurethane)

2.2. Preparation of the MPCMs In a typical synthesis, ethanol (2 mL) and n-pentanol (2 mL) were added to cyclohexane (100 mL) to form an oil phase, which was placed in a spherical flask. The requisite amount of Triton X-100 (0.6, 0.8, 0.9, 1.0, or 1.5 mL) was then added, and the mixture was magnetically stirred at 70 °C for 60 min. To avoid evaporation of the solvents, the flask was sealed with a ground-glass stopper during the whole reaction process, except when adding the reagents. Na2SO4·10H2O (8 g) was dissolved in water (8 mL) at 70 °C to form the aqueous phase. The aqueous solution was then added dropwise to the stirred oil phase over a period of 1 h at 70 °C to form a water/oil (W/O) emulsion (inverse emulsion). The mixture was adjusted to pH 8–9 by adding APTS (2 mL). A certain amount (4, 8, or 12 mL) of TEOS was then added dropwise to 3

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Fig. 4. SEM images of (a, b) MPCM4, (c, d) MPCM3, and (e, f) MPCM5.

the W/O emulsion, and the mixture was stirred at 600 rpm for 16 h at 70 °C. Finally, the obtained microcapsules were washed three times with ethanol and dried at 20 °C for 24 h. Five microcapsule samples, designated as MPCM1–MPCM5, were obtained from the compositions listed in Table 1.

accuracy: ± 0.2 °C, enthalpy accuracy: ± 5%), heating at 5 °C min−1 under a constant stream of nitrogen. The thermal stability of the MPCMs was measured with a thermogravimeter (Pyris 1 TGA, Perkin–Elmer), heating from 25 °C to 700 °C at a rate of 20 °C min−1 under a constant stream of nitrogen.

2.3. Characterization of the MPCMs

3. Results and discussion

The structures of the MPCMs were analyzed by means of a Fouriertransform infrared spectrophotometer (FTIR, Nicolet Nexus 870), recording spectra from 400 to 4000 cm−1 at a resolution of 2 cm−1 from samples in KBr pellets). The crystal structures of the MPCMs were determined by means of an X-ray diffractometer (XRD, D/max-Ultima III, Rigaku Corporation, Japan) operated at 40 kV and 40 mA, scanning the 2θ range at 5° min−1. The morphologies of the MPCMs were examined by means of a scanning electron microscope (SEM, FEI Helios 600i, operating voltage: 2 kV, operating current: 34 pA). The thermal properties of the MPCMs were measured by means of a differential scanning calorimeter (DSC, Pyris 1 DSC, Perkin–Elmer, temperature

3.1. FT-IR analysis of the MPCMs FT-IR spectra of Na2SO4·10H2O, silica, and MPCMs were recorded, as shown in Fig. 1. The spectrum of Na2SO4·10H2O features four vibration modes. The absorption band at around 996 cm−1 relates to the ν1(SO42−) band, the band at 467 cm−1 can be ascribed to ν2(SO42−), three bands at ν = 1113, 1131, 1145 cm−1 can be assigned to ν3(SO42−) and the ν4(SO42−) band is split into two peaks at 616 and 634 cm−1 [33,34]. The infrared bands in the range ν = 3414–3550 cm−1 can be ascribed to the stretching vibrations of water of crystallization. The bands at ν = 1600–1700 cm−1 can be 4

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Fig. 5. Melting DSC curves of (a) Na2SO4·10H2O and (b–f) MPCM1–MPCM5. Table 2 DSC data for Na2SO4·10H2O and MPCMs. Samples

Na2SO4·10H2O MPCM1 MPCM2 MPCM3 MPCM4 MPCM5

Melting

Solidifying

Onset temperature (°C)

Peak temperature (°C)

Latent heat (kJ/kg)

Onset temperature (°C)

Peak temperature (°C)

Latent heat (kJ/kg)

33.7 33.8 33.8 33.6 33.3 33.5

37.4 36.1 36.2 36.2 35.4 35.9

211.9 110.9 115.9 125.6 56.0 99.8

8.8/−12.4/−18.2 1.4 7.2 6.0 1.2 3.7

6.7/−13.1/−18.1 −3.4 4.7 0.1 1.1 2.4

140.3 51.8 46.9 74.0 18.4 58.1

ascribed to eOH bending vibrations. In Fig. 1b, the peaks at ν = 1058, 785, and 455 cm−1 can be attributed to bending vibrations of SieO bonds. The APTS-modified silica nanoparticles also displayed characteristic peaks due to amino groups in the range ν = 1400–1460 cm−1. Compared with the spectra in Fig. 1a, b, no new peaks are observed in Fig. 1c–g, all FT-IR signals match the core/shell components of the corresponding microcapsule sample [35].

the Na2SO4·10H2O was dehydrated to anhydrous Na2SO4 during the silica coating and surface amino-functionalization processes [36,37].

3.3. Formation of the MPCMs The surfactant Triton X-100 played a key role in preparing the miniemulsion and the microcapsules. When 0.6 mL of Triton X-100 was added, the mean size of the microcapsules was around 28 μm, but most of them were broken, as shown in Fig. 3b. This amount of surfactant was evidently insufficient for the formation of a stable microemulsion, and demulsification occurred during the synthetic process. Upon increasing the amount of Triton X-100 to 0.8 mL, the mean size of the microcapsules (MPCM1) was around 1.43 μm, as shown in Fig. 3c, d. The shell surfaces were slightly rough and the number of intact microcapsules increased. On increasing the amount of Triton X-100 to 0.9 mL, the obtained microcapsules (MPCM2) were almost homogeneous and spherical, of size about 900 nm, as shown in Fig. 3e, f. On increasing the amount of Triton X-100 to 1.0 mL, the microcapsules (MPCM3) showed good morphology and a uniform size of about 500 nm, as shown in Fig. 3g, h. With 1.5 mL of Triton X-100, the size of the microcapsules was still about 500 nm, but they appeared less symmetrical due to aggregation or adhesion, as shown in Fig. 3i, j. These results illustrated that the size and morphology of the MPCMs could be systematically controlled by varying the amount of Triton X-

3.2. XRD analysis of the MPCMs The XRD patterns of silica and the MPCMs are shown in Fig. 2, with the standard XRD patterns for Na2SO4 (JCPDS card no. 75-0914) and Na2SO4·10H2O (JCPDS card no. 74-0937) are shown at the bottom. Diffraction peaks at 19.09, 28.10, 32.32, 33.91° correspond to the lattice planes (1 1 1), (1 3 1), (1 1 3), and (2 2 0) of Na2SO4, respectively. Diffraction peaks at 16.16, 18.56, 27.32, and 27.82° correspond to the lattice planes (2 0 0), (0 2 1), (−1 3 1), and (−1 0 4) of Na2SO4·10H2O, respectively. A weak, broad diffraction peak at 22° in Fig. 2b indicated that the crystallinity of the silica shell was very low. As can be seen in Fig. 4c–g, the peaks of both Na2SO4 and Na2SO4·10H2O appeared simultaneously in the diffraction profiles of the MPCMs, and the intensity of the Na2SO4·10H2O peaks was much stronger than that of the Na2SO4 peaks, implying that the content of Na2SO4·10H2O was much greater than that of Na2SO4 in the MPCMs. This is due to the fact that some of 5

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Fig. 6. Solidifying DSC curves of (a) Na2SO4·10H2O and (b–f) MPCM1–MPCM5. Table 3 Comparison of the present work with results for other MPCMs reported in the literature. MPCMs

Melting point (°C)

Melting latent heat (kJ/kg)

Reference

Na2SO4·10H2O@SiO2 microparticles Mg(NO3)2·6H2O@PECA (poly(ethyl-2-cyanoacrylate)) Na2SO4@PU (polyurethane) Na2SO4·10H2O@SiO2 solid nanobowls Na2SO4·10H2O@PECA (poly(ethyl-2-cyanoacrylate)) Na2SO4@SiO2 microcapsules Na2SO4·10H2O@SiO2 microcapsules

– 91.0 – – 32.0 – 33.6

90.7 83.2 58.0 180.7 138.6 52.3 125.6

[16] [26] [28] [31] [35] [36] Present study

further small exothermic peaks were observed at around −13.0 and −18.0 °C. These three peaks could be attributed to crystallizations of low-grade hydrates of Na2SO4 with different numbers of H2O molecules, which formed during the cooling process depending on the conditions of relative humidity and temperature [34]. The formation reaction is represented by Eq. (2), where subscripts A and B refer to different hydrated and anhydrous forms of sodium sulfate [35,41]. Detailed DSC results are presented in Table 2.

100. As the number of droplets forming the mini-emulsion increased with increasing amount of Triton X-100, the particle size became smaller [38,39]. The mean diameters of the microcapsules decreased from 28 μm to 500 nm when the amount of Triton X-100 was increased from 0.6 to 1.0 mL. When 1.0 mL of Triton X-100 was added, the shape of the microcapsules was most favorable, as can be seen in Fig. 3e. In order to determine the optimal ratio of TEOS to Triton X-100, the amount of Triton X-100 was fixed at 1.0 mL, and the amount of TEOS applied was either 4, 8, or 12 mL. As can be seen from Fig. 4a, b, when 4 mL of TEOS was added, the obtained microcapsules were inhomogeneous compared to those obtained with 8 or 12 mL of TEOS. The silica shell produced by hydrolysis of 4 mL of TEOS may not have been adequate to enwrap the salt hydrate core. The silica shell formation reaction is represented by Eq. (1) [40].

Si(OC2 H5)4 + 2H2 O= SiO2 + 4C2 H5 OH

Na2SO4 ·v0, A H2 O (cr ) + (vO, B − vO, A) H2 O (g ) ⇌ Na2SO4 ·vO, B H2 O (cr )

(2)

DSC curves of the MPCMs were also measured under the same conditions, and are also shown in Figs. 5 and 6. The peak temperatures for the endothermic processes in curves (b)–(f) in Fig. 5 were depressed by about 1.0 °C, and the number of exothermic peaks was reduced to one or two, as shown in Fig. 6. These results indicated that phase segregation of the hydrate salt had been inhibited due to the confinement effect of the silica mesopores [31]. The SiO2 shell can also serve as a nucleation agent to prevent the formation of low-grade hydrates [42], and the SiO2 with a hydrophilic surface can keep moisture inside the shell, which is necessary for the hydration of Na2SO4. Under different conditions of relative humidity (RH) and temperature, Na2SO4 can exist as an anhydrous or hydrated crystal with varying degrees of water content [34]. The salt will absorb moisture from the air if the relative humidity exceeds the equilibrium humidity of hydration-dehydration reaction or dehydration humidity (DRH) at a given temperature. Under

(1)

3.4. Thermal energy storage capacities of the MPCMs DSC curves for Na2SO4·10H2O for heating and cooling between −45.0 and 45.0 °C at a rate of 5 °C min−1 are shown in Fig. 5a and Fig. 6a. During heating, an endothermic process started at Ton = 33.7 °C and reached a maximum at Tpeak = 37.4 °C due to melting. Upon cooling, an exothermic peak appeared at around 7.0 °C and then two 6

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Fig. 7. TGA curves for Na2SO4·10H2O and MPCM1–MPCM5.

Fig. 8. DTG curves for Na2SO4·10H2O and MPCM1–MPCM5. Table 4 TGA data for Na2SO4·10H2O and MPCMs. Samples

Tpeak1 (°C)

ΔW1 (%)

Tpeak2 (°C)

ΔW2 (%)

Tpeak3 (°C)

ΔW3 (%)

Residue (%) (700 °C)

Na2SO4·10H2O MPCM1 MPCM2 MPCM3 MPCM4 MPCM5

61.5 48.1 46.3 53.1 38.1 46.3

52.11 23.42 22.10 28.70 15.23 18.83

– 308.9 338.8 337.7 345.9 332.7

– 4.67 9.27 7.86 5.06 11.86

– 590.7 585.4 577.9 560.9 604.2

– 3.61 5.01 4.57 8.33 5.23

47.89 68.30 63.62 58.87 71.38 64.08

7

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Fig. 9. The temperature curves of the Na2SO4·10H2O and MPCM3 in melting process.

Fig. 10. The temperature curves of the Na2SO4·10H2O and MPCM3 in solidifying process.

the relative humidity between the equilibrium humidity of the hydration–dehydration reaction and DRH, the salt absorbs water vapor and forms a higher hydration state [43]. As shown in Table 2, the latent heat of MPCM3 is 125.6 kJ/kg, higher than those of the other MPCMs. The magnitude of the latent heat varied linearly with the mass fraction of Na2SO4·10H2O in the MPCMs, which can be expressed as:

Na2SO4·10H2O, respectively, and η represents the mass fraction of Na2SO4·10H2O in the microcapsules. According to DSC data, the highest latent heat was attained by using 1 mL of Triton X-100 and 8 mL of TEOS. In Table 3, the latent heat of MPCM3 is compared with those of other MPCMs reported in the literature. The MPCM3 prepared in this work clearly has a competitively high value of latent heat.

ΔHm = η ·ΔHp

3.5. Thermal stability analysis of the MPCMs

(3)

The thermal stabilities of Na2SO4·10H2O and MPCMs were analyzed

where ΔHm and ΔHp represent the latent heats of the MPCMs and bulk 8

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Fig. 11. DSC curves for MPCM3 before and after 100 thermal cycles. Table 5 DSC data for MPCM3 before and after 100 thermal cycles. Samples

Uncycled After 100 thermal cycles

Melting

Solidifying

Onset temperature (°C)

Peak temperature (°C)

Latent heat (kJ/kg)

Onset temperature (°C)

Peak temperature (°C)

Latent heat (kJ/kg)

33.6 33.7

36.2 36.1

125.6 100.9

6.0 −5.0

0.1 −5.0/−7.4/−8.6

74.0 58.5

Fig. 12. FTIR spectra of MPCM3 before and after 100 thermal cycles.

9

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DSC curves. Therefore, the change in thermal properties can be mainly attributed to chemical degradation of Na2SO4·10H2O.

Table 6 Application parameters of the optimal MPCM. Application parameter

Unit of measurement

Value

Charging temperature Charging capacity Discharging temperature Discharging capacity Energy storage efficiency

°C kJ/kg °C kJ/kg %

33.6 125.6 6.0 74.0 58.9

Remarks

3.8. Application parameters of the MPCM Per unit mass

The application parameters of the MPCM3 were listed in Table 6, including phase change temperatures, phase change enthalpy, energy storage efficiency etc. It is known from Table 6 that the solidifying enthalpy of MPCM3 is 58.92% of the melting enthalpy. It is due to the fact that most salt hydrates, especially those proposed for energy storage applications, melt incongruently. For Na2SO4·10H2O, the solidifying enthalpy is never more than 60% of the enthalpy of fusion [46]. Compared to other MPCMs, the microcapsules with a Na2SO4·10H2O core have a moderate phase-change temperature of 33.6 °C and a high enthalpy of 125.6 kJ/kg. The microcapsules with a silica shell also have advantages of high thermal stability and nonflammability. However, the microcapsules present some limitations to be overcome, including super cooling and weak discharging capacity. Through analysis, the MPCM3 with mild charging temperature and high phase change enthalpy has potential in the development of advanced materials for fields such as energy storage, solar heat, and healthcare.

Per unit mass

by TGA and the temperature derivatives of the TG curves (DTG). Test results are presented in Fig. 7 and Fig. 8, respectively. Experimental data are collected in Table 4, where Tpeak refers to the temperature of maximum degradation rate of the samples, and ΔW is the percentage weight loss corresponding to the Tpeak. As can be seen in Fig. 7, there was only one weight loss step for Na2SO4·10H2O, corresponding to a sharp TG dehydration curve with Tpeak = 61.5 °C in Fig. 8, attributable to the removal of lattice water molecules. Three weight loss steps for the MPCMs can be seen in Fig. 7 and Fig. 8. The weight loss in the range 32–150 °C corresponds to the removal of lattice water molecules [35]. The weight loss in the region 250–400 °C is due to the loss of coordinated water molecules. The weight loss from 450 to 650 °C relates to decomposition of the aminopropyl groups grafted on the silica surface [44]. The residue at 700 °C consisted of SiO2 and anhydrous sodium sulfate.

4. Conclusions Na2SO4·10H2O has been successfully encapsulated within SiO2 microcapsules by an in situ inverse mini-emulsion polymerization method. FTIR and XRD analyses confirmed the presence of Na2SO4·10H2O together with a small amount of Na2SO4 in the microcapsules. SEM images showed microcapsules with diameters in the range 500 nm–28 μm by reducing the amount of Triton X-100 used from 1.0 mL to 0.6 mL. The most satisfactory MPCM sample melted at 36.2 °C with a latent heat of 125.6 kJ/kg and solidified at 6.0 °C with a latent heat of 74.0 kJ/kg. Phase segregation of Na2SO4·10H2O is inhibited according to DSC results. Therefore, microcapsulated Na2SO4·10H2O with SiO2 shell has been formed. T-history method shows that the heat storage and release rates of Na2SO4·10H2O were increased by microencapsulating it into a SiO2 shell. After 100 heating–cooling cycles, MPCM3 maintained a melting enthalpy of 100.9 kJ/kg. TGA and DTG analysis have indicated that the samples have good thermal stability in the applicable temperature range. Therefore, MPCMs with a silica shell would seem to be promising candidates for application in thermal energy storage. The method might also be utilized to encapsulate other inorganic or hydrophilic organic PCMs for different applications to expand their functions.

3.6. The heat storage and heat release properties analysis A T-history method was employed to determine thermophysical properties of the Na2SO4·10H2O and MPCM3 [45]. Two samples were evaluated by comparing their charging and discharging time, the temperature curves are shown in Figs. 9 and 10 respectively. 8 g of powdered Na2SO4·10H2O and MPCM3 samples were put into beakers, respectively, which were inserted into a thermocouple. The samples were heated by a water bath and cooled by ice bath. In Fig. 9, the charging time initiated from the same temperature (26 °C) and ended at the same temperature (45 °C). In Fig. 10, the discharging time started from 45 °C and finished at 0 °C. The charging time of the Na2SO4·10H2O and MPCM3 were 25 min and 23.4 min respectively. The discharging time of the Na2SO4·10H2O and MPCM3 were 17 min and 12 min respectively. The results indicated that by microencapsulating Na2SO4·10H2O into a SiO2 shell, the heat storage and release rates may increase. 3.7. Thermal reliability analysis of MPCM3

Acknowledgements

To determine the thermal reliability of MPCM3, a sample was heated from −50 to 50 °C, and this process was repeated 100 times. The MPCM3 was then re-examined by DSC. The DSC curve of the MPCM3 after 100 cycles is shown in Fig. 11 (black line), and data are collected in Table 5. After 100 thermal cycles, the melting temperature was almost unchanged. Therefore, this MPCM showed good thermal reliability in terms of the maintained melting temperature. Three exothermic peaks appeared at around −5.0 °C, −7.4 and −8.6 °C after thermal cycling. These three sharp peaks could be attributed to crystallizations of low-grade hydrates of Na2SO4 with different numbers of H2O molecules. MPCM3 maintained a melting enthalpy of 100.9 kJ/kg and a solidifying enthalpy of 58.5 kJ/kg, which was reduced by 19.7% and 18.2% respectively, as compared with those of the uncycled MPCM3. The chemical reliability was assessed by FTIR analysis. Spectra of MPCM3 before and after thermal cycling are shown in Fig. 12. After the MPCM3 had been cycled between −50 and 50 °C for 100 times, the absorption band in the range ν = 3414–3550 cm−1 due to the eOH stretching vibrations of water of crystallization was somewhat weaker. This implies that some water of crystallization was lost during the heating–cooling cycling, which is consistent with the findings from the

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