Facile and low energy consumption synthesis of microencapsulated phase change materials with hybrid shell for thermal energy storage

Accepted Manuscript Facile and low energy consumption synthesis of microencapsulated phase change materials with hybrid shell for thermal energy storage Hao Wang, Liang Zhao, Lijie Chen, Guolin Song, Guoyi Tang PII:

S0022-3697(17)30394-3

DOI:

10.1016/j.jpcs.2017.08.002

Reference:

PCS 8156

To appear in:

Journal of Physics and Chemistry of Solids

Received Date: 1 March 2017 Revised Date:

11 June 2017

Accepted Date: 1 August 2017

Please cite this article as: H. Wang, L. Zhao, L. Chen, G. Song, G. Tang, Facile and low energy consumption synthesis of microencapsulated phase change materials with hybrid shell for thermal energy storage, Journal of Physics and Chemistry of Solids (2017), doi: 10.1016/j.jpcs.2017.08.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Facile and low energy consumption synthesis of microencapsulated phase change materials with hybrid shell for thermal energy storage

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Hao Wang a,b, Liang Zhao a,b, Lijie Chen a, Guolin Song a*, Guoyi Tang a,b∗∗ a Institute of Advanced Materials, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China b Key Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Haidian District, Beijing 100084, China ABSTRACT: We designed a photocurable pickering emulsion polymerization to create microencapsulated phase change materials (MicroPCM) with polymer-silica hybrid shell. The emulsion was stabilized by modified SiO2 particles without any surfactant or dispersant. The polymerization process can be carried out at ambient temperature only for 5 min ultraviolet radiation, which is a low-energy procedure. The resultant capsules were shown a good core-shell structure and uniform in size. The surface of the microcapsules was covered by SiO2 particles. According to the DSC and TGA examinations, the microcapsules has good thermal energy storage-release performance, enhanced thermal reliability and thermal stability. When ratio of MMA/ n-octadecane was 1.5/1.5. The encapsulation efficiency of the microcapsules reached 62.55%, accompanied with 122.31 J/g melting enthalpy. The work is virtually applicable to the construction of a wide variety of organic-inorganic hybrid shell MicroPCM. Furthermore, with the application of this method, exciting opportunities may arise for realizing rapid, continuous and large-scale industrial preparation of MicroPCM. Keywords: Energy storage and coversion; Phase transformation; Thermal analysis; Microcapsulated PCM; Photocurable; pickering emulsion

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1. Introduction In recent years, the researches on the developing and utilizing new green energy sources has been gaining more and more attention [1]. Worth mentioning in these resources is a technique of latent heat storage employing phase change materials (PCM), which have become a hotspot in the study of thermal energy storage materials due to their high energy storage density, isothermal operating characteristics, and extremely small temperature variation during charging and discharging processes [2]. Therefore, phase change energy storage technology have been applied to many fields such as heat storage fibers [3], regulation of building temperature [4, 5], solar heating systems and heat recovery [6, 7]. Encapsulated phase change materials (MicroPCM) is a key issue for the application of phase change materials [8, 9]. Microencapsulation can prevent leakage of the melted PCM during the phase change process, reduce PCM reactivity with the outside environment, enlarge heat transfer area and increase the heat transfer rate [10, 11]. These features make them more functional than pristine PCM in their application [12]. Nowadays, various methods have been developed for the encapsulation of PCM, such as interfacial polycondensation [13], suspension polycondensation [14], in situ polycondensation [15], and complex coacervation [16]. However, all these conventional methods are employed surfactant or dispersant to stable emulsion, which would ∗

Corresponding author at: Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China. Tel.: +86 75526036752; fax: +86 75526036752. E-mail addresses: [email protected] (G. Song), [email protected] (G. Tang).

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more or less cause environmental and health problems and increase the difficulties of the post-processing. Recently, some research groups using pickering emulsion template to encapsulate PCM [17, 18]. Pickering emulsion is considered to be a simple and robust template for MicroPCM preparation. The pickering emulsion possesses many advantages compared with the surfactant-stabilized emulsions: lower toxicity and lower cost because of surfactant-free, excellent droplet stability, less foam and combination of the properties of both organic and inorganic components [19, 20]. Yin et al. [21] reported the synthesis of MicroPCM with polymer-silica hybrid shell via pickering emulsion polymerization. The obtained MicroPCM presents good sealing tightness and high endurance. Nevertheless, almost all of these methods fabricate polymer shell material using thermal-initiation polymerization, which need to react at high temperature for several hours. It’s a high energy input and low efficency procedure. Unlike thermal-initiation polymerization, ultraviolet (UV) light can be initiated polymerization at ambient temperature for short time. This photopolymerization method has been widely applied to emulsion polymerization. Kim et al. [22] prepared polymeric microparticles by UV irradiation within a few seconds. Ma et al. [23] successfully prepared spherical-like PMMA/paraffin microcapsules by introducing UV irradiation to the emulsion polymerization. The polymerization time was only about 30 min. For the above reasons, we employed a photocurable pickering emulsion route to fabricate microencapsulated phase change materials with polymer-silica hybrid shell. Compared with other emulsion polymerization methods, photocurable pickering emulsion is combination of pickering emulsion and UV irradiation-initiated. One can be inherited the advantage of two techniques. In our work, Modified SiO2 particles, having amphiphilic property, are used as a pickering stabilizer to encapsulate and stabilize the oil phase in water. After polymerization, the SiO2 particles are covered at the surface of the microcapsules forming part of the shell material. In addition, UV irradiation-initiated MMA polymerization can be realized at ambient temperature for only 5 min. Our research indicated that photocurable pickering emulsion is an eco-friendly, high efficiency and low-energy and efficient approach for the preparation of MicroPCM with polymer-inorganic hybrid shell. More importantly, this study could also provide a feasible way to fabricate functional microPCMs with the polymer shell through the introduction of characteristic inorganic component such as ZrO2, TiO2, Fe3O4, etc. With the application of this method, exciting opportunities may arise for realizing rapid, continuous and large-scale industrial preparation of MicroPCM. 2. Experimental 2.1 Materials Methylmethacrylate (MMA) was obtained from Tianjin Damao Chemistry Reagent Co., LTD. Chemical grade n-octadecane was supplied by Tianjin Chemical Reagents Co., LTD. (Tianjin, China). SiO2 particles with an average diameter of 50 nm were offered by Shenzhen Guanxinyuan Chemical Co., LTD. (Shenzhen, China). γ-methacryloxypropyltrimethoxysilane (MPTMS) was provided by Sinopharm Chemical Reagent Co., LTD. (Shanghai, China). Photoinitiator 819 was bought from Shenzhen Youwei Chemical Technology Co., LTD. (Shenzhen, China). Pentaerythritol tetraacrylate (PETRA) was offered by Nanjing Shoulashou Co., LTD. (Nanjing, China). 2.2 Preparation of SiO2/PMMA-MicroPCM In order to obtain a suitable surface hydrophilicity to stabilize W/O pickering emulsion.

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n-octadecane (g)

1g MMA-MicroPCM 1.5g MMA -MicroPCM 2g MMA -MicroPCM 2.5g MMA -MicroPCM

MMA (g)

Modified SiO2 (g)

PETRA (g)

Photoinitiator 819 (g)

2

1

0.5

0.2

0.050

1.5

1.5

0.5

0.3

0.075

1

2

0.5

0.5

2.5

0.5

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SiO2 particles were firstly modified by MPTMS according to procedure described in literature [24]. For SiO2/PMMA-MicroPCM, the preparation was carried out as follows (seen in Fig. 1(a)): Initially, modified SiO2 was added to 50 ml deionized water and sonicated (Frequency: 35 KHz) for 10 min. An organic solution of MMA monomer, n-octadecane, PETRA and Photoinitiator 819 were prepared and then poured into the above silica suspension to obtain W/O pickering emulsion using an emulsifier at 15000 rpm for 5 min. Subsequently, the emulsion was exposed directly to a UV light (405 nm) for 5 min at ambient temperature (25 T 40 ℃) to start the photocurable polymerization. The SiO2/PMMA-MicroPCM was obtained after filtered, washed and dried. Table 1 summarizes the formulations for the preparation of microcapsules. Table 1 The formulations for the preparation of SiO2/PMMA-MicroPCM.

0.4

0.100

0.5

0.125

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2.3 Characterization The morphology of emulsion was characterized by MINGMEI polarizing microscope ME41. The chemical structure of the samples was determined with a spectrophotometer (Vertex 70, Bruker, Germany). Scanning electron microscopy (SEM) images were obtained on a field emission scanning electron microscope (FESEM, S4800, HITACHI). Elemental analysis was accomplished by energy dispersive spectroscope (EDS) attached to SEM. The particle sizes were measured by laser scattering particle size distribution analyzer (LA950V2, HORIBA). X-ray diffraction (XRD) patterns were recorded by Rigaku D/max 2500/PC diffractometer. The thermal properties and thermal stability were investigated by differential scanning calorimeter (DSC, 823E METTLER TOLEDO) in the range of 0-50 °C at a heating/cooling rate of ±5 °C/min under the atmosphere of argon, and by a thermal gravimetric analyzer (TGA, TGA/DSC1 METTLER TOLEDO) at a heating rate of 10 °C/min from room temperature to 600 °C under the atmosphere of argon. The micro-PCMs were subjected to repeated cycles of melting and crystallizing using heating-cooling cyclic oven (BPH-060A, Bluepard Experimental Equipment Co. Ltd., China). Microcapsules was tested through 600th cycles of alternative heating and cooling in the temperature range of 0-50 ℃. The heating and cooling rates were both 1 °C min-1. Each sample was kept for 5 min at 0 °C or 50 °C. 3. Results and discussion 3.1 Fabrication of microcapsules Formation of SiO2/PMMA-MicroPCM was illustrated schematically in Fig. 1(a). For each step, digital photograph was shown correspondingly in Fig. 1(b). As demonstrated extensively, the absorption of nanoparticles at oil/water interface is a crucial factor for preparing stable pickering emulsion [19]. Herein, the SiO2 nanoparticles were first modified by MPTMS ensured the stabilization of oil-in-water pickering emulsion. Monomer (MMA), cross-linking agent (PETRA) and photoinitiator were dissolved in the n-octadecane. Then, photocurable polymerization of MMA occurred in oil droplet. Owning to PMMA does not dissolve in oil, it was gradually precipitated and separated towards the surface of pickering droplets. Resultantly, microcapsules with n-octadecane core and polymer/SiO2 hybrid shell were formed. 3

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The optical micrograph of pickering emulsion are shown in Fig. 1(c). It is interesting to observe that the modified SiO2 particles were absorbed at the interface of emulsion droplet. The emulsion was only stabilized by modified SiO2 nanoparticles without any surfactant or dispersant. Fig. 1(d) shows the particle size distribution plots of the pickering emulsion. It can be found that the mean particle sizes (dm) of pickering emulsion is about 15 µm and the size distribution of emulsion presents a single distribution peak. Thus, it implied that modified SiO2 particles used as a pickering stabilizer can stabilize an oil phase dispersed in water. The results obtained are consistent with those published by zhang et al [24].

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Fig. 1. (a) Schematic illustration of pickering emulsion photocurable polymerization process. (b) Digital photograph: b1 Dual phase system; b2 pickering emulsion; b3 SiO2/PMMA-MicroPCM. (c) and (d) Optical micrograph and particle size distribution plot of pickering emulsion. 3.2 Chemical characterization FT-IR spectra of modified SiO2, PMMA, n-octadecane and SiO2/PMMA-MicroPCM are presented in Fig. 2. The spectrum of modified SiO2 exhibits the characteristic stretching vibration peak of Si−O−Si at 1100 cm-1 and bending vibration peak of Si−O at 471 cm-1. The spectrum of PMMA exhibits the characteristic C=O stretching vibration peak at 1734 cm-1. In the spectrum of n-octadecane, the peak at around 722 cm-1 belongs to −CH2 bending vibration. Obviously, the spectrum of SiO2/PMMA-MicroPCM is characterized with all the characteristic peaks mentioned above. This suggested that successful encapsulation of n-octadecane with polymer-silica hybrid shell.

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Fig. 2. FTIR spectra of modified SiO2, PMMA, n-octadecane and SiO2/PMMA-MicroPCM 3.3 Microstructure of microcapsules SEM images in Fig. 3 show the morphology of SiO2/PMMA-MicroPCM with different MMA / n-octadecane ratios. The total amount of oil phase has not been changed. The diameter of the microcapsules was mainly dominated by the emulsion preparation process [25]. Since the experiment condition was the same for all samples, the obtained microcapsules had similar diameters in the range of 5-15 µm. As seen from Fig. 3, owning to the excellent isolation effect of nanoparticles on the interface of pickering emulsion droplet, the regularly spherical and dispersive microcapsules were obtained. Meanwhile, the SEM photographs showed a phenomena: with low amount of monomer, serious collapse appeared on some microcapsule. This is plausible, as smaller amounts of monomer, and therefore lower monomer concentrations, would result in a poor strength of thinner shell being formed (Fig. 3(a)). Under a high ratios of MMA/n-octadecane ( 1.5:1.5), the robust hybrid spheres can be achieved. It is clearly observed that the mechanical stability is greatly enhanced, because no broken spheres were found under the high-vacuum SEM analysis conditions (Fig. 3(b)-(d)).

Fig. 3. SEM images of SiO2/PMMA-MicroPCM prepared by a photocurable pickering emulsion polymerization method. (a) Image of SiO2/PMMA-MicroPCM obtained from a MMA/n-octadecane = 1:2. (b) MMA/n-octadecane = 1.5:1.5. (c) MMA/n-octadecane = 2:1. (d) MMA/n-octadecane = 2.5:0.5.

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Detailed observations have been carried out for 1.5g MMA-MicroPCM in order to understand its structure in depth. From Fig. 4(a)-(c), it can be clearly seen that the SiO2 nanoparticles are approximately 50 nm. The surface of the microcapsules is coarse with many pimples. SiO2 building blocks are randomly distributed over the surface of the sphere. To probe the inner structure, microcapsules were pressed gently to fracture mechanically the shell. The broken morphology images are shown in Fig. 4(d) and (f), which verified that the microcapsules have typical core-shell structure. In addition, from an image showing a magnification of the shell, the thickness of the shell is found to be ca. 500nm, and the SiO2 nanoparticles are mainly attached to the surface of the shell. Formation mechanism of these structures was illustrated in Fig. 4(g). The modification of SiO2 nanoparticles by MPTMS was grafted C=C groups on the surface of nanoparticles. The lipophilicity of SiO2 nanoparticles was improved. The modified nanoparticles can be dispersed and self-assembled at the oil-water interface. During polymerization, these C=C groups were copolymerize with monomer [21, 24]. Through these reactions, SiO2 nanoparticles could be bonded Polymer shells. Finally, a thin layer of polymer-silica hybrid shell was formed.

Fig. 4. SEM micrographs: (a) a magnified 1.5g MMA-MicroPCM. (b) SiO2 nanoparticles. (c) 6

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observed that SiO2 presents a broad diffraction peak at 2θ of around 22°. For SiO2/PMMA-MicroPCM, in addition to this bread peak, four highly intensive diffraction peaks at 2θ of 19.2°, 19.7°, 23.3°, and 24.7° were obtained, which are ascribed to crystal of n-octadecane [26]. This result would seem to imply that the encapsulation of n-octadecane within the polymer-silica hybrid shell microcapsules. EDS spectrum (Fig. 5(b)) shows that besides C and O, Si atoms also existed. Al is derived from aluminum foil substrate. It further proves the microcapsules with a polymer-silica hybrid shell. Elemental mappings of SiO2/PMMA-MicroPCM (Fig. 3(c)) illustrate that SO2 particles are covered at the surface of the microcapsules.

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Fig. 5. (a) XRD patterns for SiO2 and SiO2/PMMA-MicroPCM. (b) EDS spectrum of SiO2/PMMA-MicroPCM. (c) Elemental mappings of SiO2/PMMA-MicroPCM. 3.4 Thermal properties and structural stability of microcapsules Thermal properties of microcapsules are shown in Fig. 6 and summarized in Table 2. It can be seen that the latent heat values of the microcapsules with different MMA / n-octadecane ratios are 41.23 J/g, 94.63 J/g, 123.21 J/g and 131.35 J/g, respectively (Fig. 6(a)). Obviously, with the decrease of MMA / n-octadecane ratios, the latent heat value of the microcapsules increases, which is due to an increase in the phase changeable n-octadecane core. Therefore, the latent enthalpies of SiO2/PMMA-MicroPCM are largely dependent on the loading of n-octadecane within the microcapsules. The encapsulation efficiency (R) and the energy storage efficiency (E), as two important parameters, are always used to evaluate the microencapsulation ratio of n-octadecane. The encapsulation efficiency (R) (n-octadecane, wt.%) and the energy storage efficiency (E) were calculated using the following Eq. (1) and Eq. (2): R= (∆Hm, microencapsulation/∆Hm, n-octadecane)× 100% (1) E= (∆Hm, microencapsulation+∆Hc, microencapsulation )/(∆Hm, n-octadecane+∆Hc, n-octadecane)× 100% (2) Where ∆Hm, microencapsulation and ∆Hm, n-octadecane are melting enthalpy of microcapsules and 7

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n-octadecane, respectively; ∆Hc, microencapsulation and ∆Hc, n-octadecane are crystallization enthalpy of microcapsules and n-octadecane, respectively [27]. In fact, the encapsulation efficiency describes the effective encapsulation of n-octadecane within the SiO2/PMMA-MicroPCM, while the energy storage efficiency represents an effective performance of the n-octadecane inside the SiO2/PMMA-MicroPCM for latent heat storage. As shown in Fig. 6(b), the encapsulation efficiency increase as n-octadecane content increases. In addition, the values of R and E are found to be closed to each other (Table 2), which indicate that almost all the latent heat of n-octadecane in the SiO2/PMMA-MicroPCM can be stored and released. Comprehensive analysis of morphology and thermal properties, the best MMA/n-octadecane for SiO2/PMMA-MicroPCM is 1.5:1.5. The 1.5g MMA-MicroPCM has a good spherical morphology, high thermal performance and encapsulation efficiency. Whereafter, an accelerated thermal cycling test was carried out for 1.5g MMA-MicroPCM to evaluate the thermal reliability of microcapsules. As depicted in Fig. 6(c), the melting temperature and freezing temperature are not much influenced by the endothermic and exothermic cycles. After 600th cycles, the enthalpy value, instead of dropping drastically, only decrease slightly, suggesting SiO2/PMMA-MicroPCM has a good thermal reliability. Fig. 6(d), (e) show the SEM images of 1.5gMMA-MicroPCM in the initial state and after 600th endothermic and exothermic cycles. Comparing with primeval microcapsules, due to the evaporation of n-octadecane and the mechanical stability of microcapsules are reduced during repetitive melting and freezing process, some little dimples and broken spheres are inevitably existed.

Fig. 6. Heating performance of MicroPCM. (a) DSC curves of n-octadecane and SiO2/PMMA-MicroPCM. (b) Plot of encapsulation efficiency against the weight of n-octadecane for the SiO2/PMMA-MicroPCM. (c) A collection of DSC curves of the 1.5g MMA-MicroPCM tested at 200th, 400th and 600th cycles, showing the stable encapsulation during endothermic and exothermic processes. (d) and (e) SEM images of 1.5g MMA-MicroPCM in the initial state and after 600th endothermic and exothermic cycles.

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Melting process

Crystallization process

Tpm (°C)

∆Hm (J/g)

Tpc (°C)

Encapsulation

Energy storage

efficiency (%)

efficiency (%)

∆Hc (J/g)

25.50

195.53

15.60

200.98

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—

2.5g MMA-MicroPCM

21.47

39.09

14.92

41.23

19.99

20.26

2g MMA-MicroPCM

24.01

94.51

12.35

94.63

48.34

47.70

1.5g MMA-MicroPCM

26.27

122.31

12.95

123.21

62.55

61.92

1g MMA-MicroPCM

24.30

129.75

13.66

131.35

66.36

65.85

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n-octadecane

Note: Tpm, Peak temperature on DSC heating curve; ∆Hm, Enthalpy on DSC heating curve; Tpc, Peak temperature on DSC cooling curve; ∆Hc, Enthalpy on DSC cooling curve.

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3.5 Thermal stability of microcapsules Thermal stability is a significant parameter in evaluating the properties of microcapsules used for heat energy storage or thermal regulation [28] .The thermal degradation behaviors of n-octadecane and SiO2/PMMA-MicroPCM were investigated by TGA, and their TGA thermograms are shown in Fig. 7. The weight loss of n-octadecane starts at 128.5 °C and almost does not remain any residual at 270.0 °C, suggesting that n-octadecane experienced a simple decomposition reaction. For SiO2/PMMA-MicroPCM, the weight loss trend of microcapsules are similar, i.e., a two-step weight loss process was carried out. The decomposition starts at ~220 °C, which is mainly attributed to the leakage of n-octadecane from the microcapsules and its further decomposition. The second decomposition step starts at ~370 °C, which resulted from degradation of the polymer shell. According to the above results, it is obvious that the thermal resistant temperatures of SiO2/PMMA-MicroPCM are higher than that of n-octadecane, which is reasonable to believe that this microencapsulation can provide good protection for n-octadecane.

Fig. 7. TGA curves of n-octadecane and SiO2/PMMA-MicroPCM. 4. Conclusion We have successfully synthesized MicroPCM with polymer-silica hybrid shell by photocurable pickering emulsion polymerization without any surfactant or dispersant. This low-energy procedure can be carried out at ambient temperature only for 5 min ultraviolet radiation. The SiO2/PMMA-MicroPCM was proved to have good core-shell structure and uniform 9

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size. SiO2 particles are covered at the surface of the microcapsules. The mechanical stability of the microcapsules can be enhanced by increasing the relative amount of the monomer. In addition, the results obtained from DSC and TGA indicate that the microcapsules has good thermal energy storage-release performance, enhanced thermal reliability and thermal stability. A high utility efficiency of core material can be achieved. The encapsulation efficiency of the microcapsules reached 62.55%, accompanied with 122.31 J/g melting enthalpy. The work is virtually applicable to the construction of a wide variety of organic-inorganic hybrid shell MicroPCM. Furthermore, with the application of this method, exciting opportunities may arise for realizing rapid, continuous and large-scale industrial preparation of MicroPCM. Acknowledgements The authors gratefully for the financial supports from Shenzhen government project JCYJ20140417115840229. The authors also would like to thank National Science Foundation of China 2014M550722. References [1] C. Liu, Z. Rao, J. Zhao, Y. Huo, Y. Li, Review on nanoencapsulated phase change materials: Preparation, characterization and heat transfer enhancement, Nano Energy 13 (2015) 814-826.

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Surfactant-free pickering emulsion was employed to fabricate MicroPCM with polymer-silica hybrid shell The polymerization process can be carried out at ambient temperature only for 5 min ultraviolet radiation SiO2/PMMA-MicroPCM achieved a good thermal stability and the core content in the microcapsules reached 62.19%

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