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SiO2 composite as novel PCM for cold energy storage
Journal of Energy Storage 28 (2020) 101276
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Thermal characterization of net-like and form-stable ML/SiO2 composite as novel PCM for cold energy storage
T
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Xian Wana, , Cong Chena, Songyun Tiana, Baohua Guob a b
School of Materials Science and Mechanical Engineering, Beijing Technology and Business University, Beijing, 100048, People's Republic of China Advanced Materials Laboratory, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Methyl laurate Net-like structure Form-stable PCMs composite Cold energy storage Sol-gel method
A novel phase change materials (PCMs) composite was fabricated by sol-gel method for cold energy storage. Methyl laurate (ML) as cold energy medium was preferred due to its appropriate phase change temperature (3.96 ℃) and high latent heat (210.1 J/g), and silica provided a complete external protection for the ML. Various techniques were used to characterize the as-prepared ML/SiO2 PCMs composite so as to investigate its structure and thermal properties, including scanning electron microscopy (SEM), fourier transforms infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA). During the PCMs composite forming process, hydrogen bonds generated by the interaction of carboxyl group of ML, the silicon hydroxyl group of silica, and the hydroxyl groups of emulsifier PVA promoted the morphology change of the PCMs composite from sphere-like to net-like structure. The DSC results showed that the net-like PCMs melted at 6.71 ℃ with a latent heat of 151.3 J/g and solidified at -5.43 ℃ with a latent heat of 148.9 J/g. Besides, the PCMs composite exhibited outstanding chemical compatibility, thermal reliability and complete micromorphology. Thus, the prepared ML/SiO2 PCMs composite with novel net-like structure had great potential for cold energy storage.
1. Introduction Cold energy storage systems for urban cities is considered as an effective solution to meet the increasing demand of cooling in a sustainable manner [1]. The main applications of cold energy storage systems are in air-conditioning and thermal comfort (4 to 20 ℃), medical and food cold chain logistics (−20 to 10 ℃) [2,3], cold source for power generation (−50 ℃) and high grade cold energy recovery from cryogenic energy storage (< −100 ℃) [4]. Among cold storage strategies, phase change materials (PCMs) are preferred due to the inherent superiorities of high energy storage density and nearly isothermal phase change behavior by absorbing and releasing a large amount of latent heat during phase change process [5–7]. However, the phase change process of PCMs not only brings cold storage capacity, but also brings difficulties in application because of the leakage of its liquid phase [8,9]. In order to prevent molten PCMs from leakage, the development of form-stable PCMs is conducted by the incorporation of PCMs into suitable support material through microencapsulation, impregnation or blending [10–13]. The shape-stabilized PCMs, with oleic acid-polyethylene glycol as the core material and SiO2/SnO2 as the shell, could be used as a cooling functional material
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for electronic chips and an electrode material for electrochemical energy storage [14]. The n-octadecane embedded within the melamine urea-formaldehyde copolymer could provide a low temperature protection in a short time [15]. The PCMs composite formed by zinc oxide enwrapping n-eicosane, possessed the photocatalytic and antibacterial functions in the application for cold energy storage [16]. In order to solve the low-temperature disaster of asphalt pavement, the melamineurea-formaldehyde resin was used to encapsulate the two-component organic low-temperature polyvinyl chloride [17]. The n-alkane PCMs composite also played an effective role in the field of human cryogenic protection [18]. In recent years, phase change material microcapsules with different shells has been widely explored, such as polymethylmethacrylate (PMMA) [19–22], polyurea [23], calcium carbonate [24], polystyrene (PS) [25–27], and so on. Especially, silica is an excellent candidate as matrix materials for PCMs composite because of its excellent mechanical strength, good thermal conductivity and chemical compatibility [28–32]. As PCMs for cold energy storage, methyl laurate (ML) has the advantages of high energy storage density, low volume change, cheap price, low vapor pressure and no toxicity [33,34], which could be fabricated as PCMs microcapsules for improving the energy efficiency
Corresponding author. E-mail address: [email protected] (X. Wan).
https://doi.org/10.1016/j.est.2020.101276 Received 15 October 2019; Received in revised form 29 January 2020; Accepted 7 February 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Energy Storage 28 (2020) 101276
X. Wan, et al.
of buildings or for the deicing of pavements [35]. Its phase change temperature meets the temperature requirement of medical and food cold chain logistics (−20 to 10 ℃), which make ML a promising candidate. However, at present, there are few researches on ML as PCMs for cold energy storage. In this study, the form-stable net-like PCMs composite which selected ML as PCM and silica as protection materials was fabricated by sol-gel method, aiming at applying it in medical and food cold chain logistics to enhance the effect of temperature retention. Firstly, the mass ratios of ML/SiO2 were optimized in order to obtain desirable phase change behavior of ML. Secondly, during the formation of PCMs composite, the influence of the preparation temperature on the composite's thermal storage capacity was investigated. Finally, the formstable net-like ML/SiO2 PCMs composite was prepared, and its morphology, chemical compatibility, thermal properties as well as thermal reliability were systematically investigated. Unlike PCMs microcapsules prepared in the previous reports, the novel net-like form-stable ML/ SiO2 composite with high enthalpy was successfully obtained by the simple sol-gel method in our study. The special structure presented a novel form of PCMs composite and had a broad application prospect in the cold chain logistics, agricultural product preservation, medical refrigeration transportation and building energy efficiency et al.
Table 1 Components of the prepared PCMs composite.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. FTIR analysis of the PCMs composite
Vinyltriethoxysilane (VTES, Analytical Reagent) was purchased from qufu yishun chemical co., LTD. Methyl laurate (ML, 98%) was purchased from Beijing enokai technology co., LTD. Polyvinyl alcohol1788 (PVA-1788, Analytical Reagent) was purchased from Shanghai Aladdin biochemical technology co., LTD. Ammonia water (NH3•H2O, Analytical Reagent, 24–28 wt%) was purchased from sinopharm chemical reagent co., LTD. Deionized water was used as the aqueous medium. The chemicals were all used as received.
Fig. 1(a–c) showed the FTIR spectra of ML, silica as well as PCMs composite prepared with different ML/SiO2 ratios, at 25 ℃, 40 ℃ and 50 ℃, respectively. The characteristic peaks of silica were at 3424 cm−1 (-OH stretching vibration), 1165 cm−1 and 728 cm−1 (Si-O-Si asymmetric stretching vibration and symmetric stretching vibration), 965 cm−1 (Si-OH bending vibration). For ML, the typical characteristic peaks were at 923 and 2854 cm−1 (CeH stretching vibration of methyl and methylene), 1745 cm−1 (C]O stretching vibration), 1198 cm−1 (CeO stretching vibration). Furthermore, the intensity of the characteristic peaks of ML was amplified along with the increase in the loading of ML. All the characteristic peaks of silica and ML could be observed in PCMs composite (M1-M15), which revealing that the prepared PCMs composite were composed of ML and silica without any remarkable chemical interaction.
Samples
ML (g)
VTES (g)
PVA1788 (g)
Theoretical ML/ SiO2 ratio
Temperature (℃)
Silica M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15
0 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
6 6 5 4 3 2 6 5 4 3 2 6 5 4 3 2
0.15 0.15 0.24 0.33 0.42 0.51 0.15 0.24 0.33 0.42 0.51 0.15 0.24 0.33 0.42 0.51
0/6 1/6 2/5 3/4 4/3 5/2 1/6 2/5 3/4 4/3 5/2 1/6 2/5 3/4 4/3 5/2
25 25 25 25 25 25 40 40 40 40 40 50 50 50 50 50
TA, Q50) at a heating rate of 20 ℃/min from room temperature to 700 ℃.
2.2. Preparation of PCMs composite The sol-gel method was adopted to prepare PCMs composite through the hydrolysis of VTES. A typical synthesis process was as follows: 60 mL of deionized water and a given amount of PVA-1788 were mixed and stirred until clear. A certain amount of ML was dispersed in the above solution with a homogenizer at a stirring speed of 4000 rpm to form an oil-in-water emulsion. Then, at a certain temperature, the emulsion was transferred to a three-neck round bottom flask with an overhead stirrer. Then, different amounts of VTES were added (with ML/SiO2 mass ratios of 1/6, 2/5, 3/4, 4/3 and 5/2, respectively) into the solution, and the mixtures were stirred for 1 h. Then, PCMs composite was obtained after 3 mL of ammonia added with stirring for another 2 h. The PCMs composite prepared with different mass ratios of ML/SiO2 was described as MX (X = 0, 1, 2, 3, 4, 5, 6 … and 15) as shown in Table 1.
3.2. Morphology of PCMs composite Fig. 2 showed SEM images of PCMs composite with different mass ratios of ML/SiO2 from M1-M15. To examine the internal structure of the PCMs composite, the SEM images of broken PCMs composite, which were frozen in liquid nitrogen and triturated by a mortar, were shown as the inset in Fig. 2. As seen from Fig. 2, when the PCMs composite’ preparation temperature was 25 ℃, the morphology of PCMs composite changed from sphere-like (M1-M3) into net-like structure (M4-M5) along with the increase of ML content. From the inset in Fig. 2, for M1M3, a clear core-shell structure of broken PCMs composite was confirmed by the SEM images. For M4-M5, a honeycomb-like porous structure was observed inside their net-like surface by the SEM image of the broken composite. And the same phenomenon was observed when the preparation temperature of PCMs composite was 40 ℃ and 50 ℃. The net-like structure of PCMs composite, where the ML was engulfed in, was smooth and complete. It was noticeable, however, the net-like structure appeared earlier with the higher temperature. At the same time, almost all of the sphere-like PCMs composite agglomerated together due to the strong particle-particle interactions [36]. The mechanism, which the morphology of PCMs composite transformed from sphere-like to net-like structure, is reasonable to assume as follows. Fig. 3(a–d) described the transition of the prepared PCMs composite
2.3. Characterization of the PCMs composite The thermal analysis of PCMs composite was done using a differential scanning calorimeter (DSC, TA, Q20) with a heating rate of 5 ℃/min ranging from −40 to 20 ℃, in a nitrogen atmosphere. The morphology of PCMs composite was studied with scanning electronic microscope (TESCAN, XFlash Detector 410-M, Bruker Nano, Germany) at an acceleration voltage of 20 KV under low vacuum. Before SEM examination, all samples were gold-coated. The chemical compatibility of PCMs composite was investigated by Fourier transform infrared spectroscopy (FTIR, Nicolet iN10MX-type spectrometer) with the wavenumbers ranging from 400 cm−1 to 4000 cm−1. The thermal stability of composite was investigated by thermo-gravimetric analysis (TGA, 2
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Fig. 1. FTIR spectra of silica, ML and PCMs composite prepared with different ML/SiO2 ratios at (a) 25 ℃ (M1-M5); (b) 40 ℃ (M6-M10); (c) 50 ℃ (M11-M15).
from an obvious core-shell structure to net-like structure as the temperature and ML/SiO2 mass ratio increased. And the results were consistent with the SEM images (Fig. 3(e) and (f)), respectively. The reason speculated for the structure change was that the hydrogen bonds, which could be strengthened along with the increase of the temperature and ML/SiO2 mass ratios during the formation of PCMs composite, were formed by the carboxyl group of ML, the hydroxyl groups of PVA and the silicon hydroxyl group of silica [37,38]. The strengthened hydrogen bonds promoted the interaction between ML core and silica shell, which led to the transformation of PCMs composite from sphere-like to netlike structure. 3.3. Thermal energy storage of PCMs composite Based on the mass fraction of the pristine ML in PCMs composite, the theoretical melting and solidifying enthalpy of PCMs composite were calculated. The corresponding data were summarized in Table 2. The theoretical solidifying and melting enthalpy of PCMs composite was calculated by:
Hs1 = ωML × HML − s
(1)
Hm1 = ωML × HML − m
(2)
Here, Hm1 and Hs1 respectively represented the theoretical melting and solidifying enthalpy for the PCMs composite. ωML was the mass fraction of the ML in the PCMs composite; HML − m and HML − s was respectively the measured melting and solidifying enthalpy for pure ML. The resulted thermograms of PCMs composite, prepared at 25 ℃, 40 ℃ and 50 ℃, were shown in Fig. 4, and some significant parameters obtained from DSC evaluation were summarized in Table 2. It was
Fig. 3. Illustration of morphology change of the prepared PCMs composite and the structural formula of silica, ML and PVA.
Fig. 2. SEM images of PCMs composite with different mass ratios of ML/SiO2 from M1-M15. The inserts show the SEM images of broken PCMs composite, respectively. 3
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Table 2 Phase change characteristics of ML and PCMs composite Sample code
Silica ML M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15
ML/SiO2
– – 1/6 2/5 3/4 4/3 5/2 1/6 2/5 3/4 4/3 5/2 1/6 2/5 3/4 4/3 5/2
a
with different mass ratios of ML/SiO2 at three preparation temperature.
Melting process Tm (℃)
Hm (J/g)
Hm1(J/g)
Solidification process Ts (℃) Hs (J/g)
Hs1 (J/g)
Temperature (℃)
– 3.96 −2.18 3.22 4.12 5.66 5.97 −1.88 2.75 7.14 7.74 6.71 −11.35 −2.63 3.94 5.97 5.50
– 210.1 23.22 56.24 96.28 109.8 131.5 24.19 56.73 132.6 135.8 151.3 10.92 54.54 64.66 131.7 122.4
– 210.1 30.01 60.02 90.04 120.0 150.1 30.01 60.02 90.04 120.0 150.1 30.01 60.02 90.04 120.0 150.1
– −2.43 −13.74 −6.01 −4.85 −5.70 −8.42 −16.67 −9.63 −9.20 −6.24 −5.43 −23.03 −7.83 −14.57 −3.60 −5.49
– 208.5 29.78 59.57 89.35 119.1 148.9 29.78 59.57 89.35 119.1 148.9 29.78 59.57 89.35 119.1 148.9
– – 25 25 25 25 25 40 40 40 40 40 50 50 50 50 50
– 208.5 22.62 54.83 96.02 109.4 130.3 23.42 56.35 128.5 134.2 149.9 10.39 52.15 62.70 128.0 121.0
a Here, Tm, Hm and Hm1 represent the phase change temperature, measured enthalpy values and theoretical enthalpy values in melting process, respectively; and the Ts, Hs and Hs1 respectively represents the solidification process.
M12 were higher than its theoretical enthalpies, and the rest were the opposite. However, in the previous reports [39,40], the theoretical enthalpy of the PCMs composite were mostly higher than the corresponding measured enthalpy. The phenomenon was supposed to be the result of the strong confinement effect of the narrow space of the PCMs composite. Similarly, for the PCMs composite in our study, only with ML/ SiO2 mass ratio from 3/4 to 5/2, did the composite possibly possess higher measured enthalpy than the theoretical one. The reason for this phenomenon might be the special confinement effect of silica shell's honeycomb-like porous structure on ML. The details of experimental and mechanism will be discussed in the following research. Furthermore, the melting temperatures for the PCMs composite with a high ML/SiO2 ratio became higher than the pure ML, and the solidification temperatures for the composites became lower than the pure ML. As noted in previous paper [41], the silica could generate a confinement effect on the crystallization of ML engulfed, which restricted the motion of ML molecules during crystallization process and led to a wide temperature range for the phase transition. However, the silica shell with a net-like structure had, presumably, effectively promoted the integrity of ML's crystal structure.
noted that only the ML could absorb and release the heat during the PCMs composite melting and solidification processes, and the ML and PCMs composite had similar phase change behavior. This proved that the ML had been successfully inserted into the network structure of silica. At the same time, the higher mass percent the ML possessed in the PCMs composite, the higher enthalpy of phase transition the PCMs composite had. Fig. 5 were the enthalpies comparison diagrams of PCMs composite, with fixed mass ratios of ML/SiO2 from 1/6 to 5/2, at three preparation temperatures of 25 ℃, 40 ℃ and 50 ℃, respectively. It was surprising that the phase change enthalpies of the PCMs composite were strongly dependent on not only the loading of the ML inside the composite, but also the preparation temperature of PCMs composite. With the increase of mass ratios of ML/SiO2 from 1/6 to 5/2, enthalpies of composite increased. However, for each fixed mass ratios of ML/SiO2, enthalpy first increased then decreased with the preparation temperature increasing, while enthalpies of composite should theoretically remain the same. As could be seen in Fig. 5, when the preparation temperature of PCMs composite was increased from 25 ℃ to 50 ℃, the latent heat of PCMs composite increased first and decreased afterwards. That meant 40 ℃ may be the best preparation temperature of PCMs composite in terms of phase change enthalpy in our work. The reason might lie in the different confinement of silica shell with different morphology on ML. The optimal PCMs composite, with net-like structure and the ML/ SiO2 mass ratio of 5/2, melted at 6.71 ℃ with a latent heat of 151.3 J/g and solidified at −5.43 ℃ with a latent heat of 149.9 J/g. Significantly, the measured enthalpies of M3, M8, M9, M10 and
3.4. Thermal stability of PCMs composite Fig. 6 were the TG curves of ML, silica and PCMs composite at three preparation temperature with different mass ratios of ML/SiO2 from M1 to M15, and Table 3 showed the results obtained from TG. Obviously, the ML exhibited a typical one-step thermal degradation, which indicated the ML undergoing a simple evaporation. By comparison, the
Fig. 4. DSC curves of ML, silica and PCMs composite with different mass ratios of ML/SiO2. 4
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Fig. 5. The comparison diagrams of PCMs composite's enthalpies under different reaction temperature with the different ML/SiO2 ratios: 1/6, 2/5, 3/4, 4/3, 5/2.
weight loss curves of all the PCMs composite were quite similar and exhibited a two-step degradation process. The first sharp weight loss happened at around 95 ℃ was mainly attributable to the evaporation of ML, and at around 180 ℃, ML completely leaked out from the silica supporting material. Additionally, the weight loss of the PCMs composite strongly depended on the mass ratio of ML and silica, because silica did not decompose at 600 ℃ yet. As could be seen from Table 3 and Fig. 6, at the same preparation temperature, the amount of residues decreased along with the increase of ML/SiO2 ratio. However, residual weight of PCMs composite with the same mass ratios of ML/SiO2 increased along with the rising of the preparation temperature. The phenomena could be explained through the formation mechanism of the PCMs composite's silica shell synthesized by VTES precursor. A VTES molecule had three ethoxyl groups, and they were easily changed to hydroxyl groups on proper condition [42]. In the presence of ammonia, the polycondensation was generated and silica shell could be obtained. However, when PVA existed as the emulsifier, the silica hydroxyl group formed by the hydrolysis of VTES would be occupied by
Table 3 Residual weight of ML, silica and PCMs composite (M1-M15). Samples
ML
silica
M1
M2
M3
M4
M5
Residue content (%) Samples Residue content (%) Samples Residue content (%)
0.63 ML 0.63 ML 0.63
83.70 silica 83.70 silica 83.70
69.55 M6 71.63 M11 73.57
61.39 M7 71.25 M12 72.98
39.56 M8 66.04 M13 66.87
20.29 M9 54.81 M14 63.70
16.97 M10 50.23 M15 62.85
the hydroxyl group of polyvinyl alcohol to form Type II hybrid [43,44]. The hybrids could hinder the silicon groups’ condensation with each other from forming the Si-O-Si bond. With the increase of preparation temperature, the hybrids reduced and more Si-O-Si structure formed because of the weakened interaction between hydroxyl of PVA and silica hydroxyl of hydrolyzed VTES [45,46]. Based on these results, the ML/SiO2 PCMs composite was fairly stable in the temperature range of phase transition for energy storage application.
Fig. 6. TG curves of silica, ML and PCMs composite prepared with different ML/SiO2 ratios at (a) 25 ℃ (M1-M5); (b) 40 ℃ (M6-M10); (c) 50 ℃ (M11-M15). 5
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4. Conclusions
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A novel net-like form-stable ML/SiO2 PCMs composite was successfully prepared by sol-gel method using PVA as emulsifier for the first time, and their phase change behaviors were studied. In this ML/ SiO2 PCMs composite, ML served as the PCMs for cold energy storage, while silica played the role of the external protection to prevent the leakage of the melted ML and improve the thermal stability of ML. The FTIR results confirmed the excellent chemical compatibility of the ML/ SiO2 PCMs composite. The observation of SEM revealed that ML could be well engulfed into the external silica, and the morphology of PCMs composite could transform from sphere-like structure to net-like structure along with the increase of ML content as well as the preparation temperature during the PCMs composite forming process. The DSC results indicated that the prepared ML/SiO2 PCMs composite melted at 6.71 ℃ with a latent heat of 151.3 J/g and solidified at −5.43 ℃ with a latent heat of 149.9 J/g. The TGA results showed that the ML engulfed into the silica presented a good thermal stability durable up to 95 ℃. All the prominent properties indicated that it would be a potential material used to store cold energy in practical applications, especially in cold chain logistics. CRediT authorship contribution statement Xian Wan: Conceptualization, Methodology, Writing - review & editing. Cong Chen: Data curation, Writing - original draft. Songyun Tian: Visualization, Investigation. Baohua Guo: Supervision. Declaration of Competing Interests 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. Acknowledgments This work was supported by National Natural Science foundation of China (grant no. 51503006) References [1] G. Comodi, F. Carducci, B. Nagarajan, et al., Application of cold thermal energy storage (CTES) for building demand management in hot climates, Appl. Therm. Eng. 103 (2016) 1186–1195. [2] S. Singh, K. Gaikwad K, S. Lee Y, Phase change materials for advanced cooling packaging, Environ. Chem. Lett. 16 (3) (2018) 845–859. [3] J.K. Carson, A.R. East, The cold chain in New Zealand: a review, Int. J. Refrig. 87 (2018) 185–192. [4] Y. Li, W. Xiang, Y. Ding, A cryogen-based peak-shaving technology: systematic approach and techno-economic analysis, Int. J. Energy Res. 37 (6) (2013) 547–557. [5] F. Rodríguez, L. Lopez B, Phenolic resin/PEO-PPO block copolymer composite materials as phase change materials (PCM) for latent heat thermal energy storage (LHTES), J. Energy Storage 6 (2016) 173–177. [6] F. Rodríguez-Cumplido, E. Pabón-Gelves, F. Chejne-Jana, Recent developments in the synthesis of microencapsulated and nanoencapsulated phase change materials, J. Energy Storage 24 (2019) 100821. [7] Q. Yu, F. Tchuenbou-Magaia, B. Al-Duri, et al., Thermo-mechanical analysis of microcapsules containing phase change materials for cold storage, Appl. Energy 211 (2018) 1190–1202. [8] M. Tamaru, H. Suzuki, R. Hidema, et al., Fabrication of hard-shell microcapsules containing inorganic materials, Int. J. Refrig. 82 (2017) 97–105. [9] H. Suzuki, R. Hidema, S. Usa, et al., Flow and sedimentation characteristics of silica hard-shell microcapsule slurries treated with additives, Int. J. Refrig. 106 (2019) 18–23. [10] M. Delgado, A. Lázaro, C. Peñalosa, et al., Analysis of the physical stability of PCM slurries, Int. J. Refrig. 36 (6) (2013) 1648–1656. [11] F. Souayfane, F. Fardoun, P. Biwole, Phase change materials (PCM) for cooling applications in buildings: a review, Energy Build. 129 (2016) 396–431. [12] Y. Li, X. Zhang, M. Munyalo J, et al., Preparation and thermophysical properties of low temperature composite phase change material octanoic-lauric acid/expanded graphite, J. Mol. Liq. 277 (2019) 577–583. [13] C. Veerakumar, Sreekumar A. Phase change material based cold thermal energy storage: materials, techniques and applications: a review, Int. J. Refrig. 67 (2016)
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[45] S. Bai, R. Yonker C, Pressure and temperature effects on the hydrogen-bond structures of liquid and supercritical fluid methanol, J. Phys. Chem. A 102 (45) (1998) 8641–8647. [46] O. Sneh, M. George S, Thermal stability of hydroxyl groups on a well-defined silica surface, J. Phys. Chem. 99 (13) (1995) 4639–4647.
hybrid silica spheres in aqueous solution, J. Colloid Interface Sci. 329 (2) (2009) 292–299. [43] T. Pirzada, A. Arvidson S, D. Saquing C, et al., Hybrid silica–PVA nanofibers via sol–gel electrospinning, Langmuir 28 (13) (2012) 5834–5844. [44] H. Zou, S. Wu, J. Shen, Polymer/silica nanocomposites: preparation, characterization, properties, and applications, Chem. Rev. 108 (9) (2008) 3893–3957.
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Journal of Energy Storage journal homepage: www.elsevier.com/locate/est
Thermal characterization of net-like and form-stable ML/SiO2 composite as novel PCM for cold energy storage
T
⁎
Xian Wana, , Cong Chena, Songyun Tiana, Baohua Guob a b
School of Materials Science and Mechanical Engineering, Beijing Technology and Business University, Beijing, 100048, People's Republic of China Advanced Materials Laboratory, Department of Chemical Engineering, Tsinghua University, Beijing, 100084, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Methyl laurate Net-like structure Form-stable PCMs composite Cold energy storage Sol-gel method
A novel phase change materials (PCMs) composite was fabricated by sol-gel method for cold energy storage. Methyl laurate (ML) as cold energy medium was preferred due to its appropriate phase change temperature (3.96 ℃) and high latent heat (210.1 J/g), and silica provided a complete external protection for the ML. Various techniques were used to characterize the as-prepared ML/SiO2 PCMs composite so as to investigate its structure and thermal properties, including scanning electron microscopy (SEM), fourier transforms infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA). During the PCMs composite forming process, hydrogen bonds generated by the interaction of carboxyl group of ML, the silicon hydroxyl group of silica, and the hydroxyl groups of emulsifier PVA promoted the morphology change of the PCMs composite from sphere-like to net-like structure. The DSC results showed that the net-like PCMs melted at 6.71 ℃ with a latent heat of 151.3 J/g and solidified at -5.43 ℃ with a latent heat of 148.9 J/g. Besides, the PCMs composite exhibited outstanding chemical compatibility, thermal reliability and complete micromorphology. Thus, the prepared ML/SiO2 PCMs composite with novel net-like structure had great potential for cold energy storage.
1. Introduction Cold energy storage systems for urban cities is considered as an effective solution to meet the increasing demand of cooling in a sustainable manner [1]. The main applications of cold energy storage systems are in air-conditioning and thermal comfort (4 to 20 ℃), medical and food cold chain logistics (−20 to 10 ℃) [2,3], cold source for power generation (−50 ℃) and high grade cold energy recovery from cryogenic energy storage (< −100 ℃) [4]. Among cold storage strategies, phase change materials (PCMs) are preferred due to the inherent superiorities of high energy storage density and nearly isothermal phase change behavior by absorbing and releasing a large amount of latent heat during phase change process [5–7]. However, the phase change process of PCMs not only brings cold storage capacity, but also brings difficulties in application because of the leakage of its liquid phase [8,9]. In order to prevent molten PCMs from leakage, the development of form-stable PCMs is conducted by the incorporation of PCMs into suitable support material through microencapsulation, impregnation or blending [10–13]. The shape-stabilized PCMs, with oleic acid-polyethylene glycol as the core material and SiO2/SnO2 as the shell, could be used as a cooling functional material
⁎
for electronic chips and an electrode material for electrochemical energy storage [14]. The n-octadecane embedded within the melamine urea-formaldehyde copolymer could provide a low temperature protection in a short time [15]. The PCMs composite formed by zinc oxide enwrapping n-eicosane, possessed the photocatalytic and antibacterial functions in the application for cold energy storage [16]. In order to solve the low-temperature disaster of asphalt pavement, the melamineurea-formaldehyde resin was used to encapsulate the two-component organic low-temperature polyvinyl chloride [17]. The n-alkane PCMs composite also played an effective role in the field of human cryogenic protection [18]. In recent years, phase change material microcapsules with different shells has been widely explored, such as polymethylmethacrylate (PMMA) [19–22], polyurea [23], calcium carbonate [24], polystyrene (PS) [25–27], and so on. Especially, silica is an excellent candidate as matrix materials for PCMs composite because of its excellent mechanical strength, good thermal conductivity and chemical compatibility [28–32]. As PCMs for cold energy storage, methyl laurate (ML) has the advantages of high energy storage density, low volume change, cheap price, low vapor pressure and no toxicity [33,34], which could be fabricated as PCMs microcapsules for improving the energy efficiency
Corresponding author. E-mail address: [email protected] (X. Wan).
https://doi.org/10.1016/j.est.2020.101276 Received 15 October 2019; Received in revised form 29 January 2020; Accepted 7 February 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.
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of buildings or for the deicing of pavements [35]. Its phase change temperature meets the temperature requirement of medical and food cold chain logistics (−20 to 10 ℃), which make ML a promising candidate. However, at present, there are few researches on ML as PCMs for cold energy storage. In this study, the form-stable net-like PCMs composite which selected ML as PCM and silica as protection materials was fabricated by sol-gel method, aiming at applying it in medical and food cold chain logistics to enhance the effect of temperature retention. Firstly, the mass ratios of ML/SiO2 were optimized in order to obtain desirable phase change behavior of ML. Secondly, during the formation of PCMs composite, the influence of the preparation temperature on the composite's thermal storage capacity was investigated. Finally, the formstable net-like ML/SiO2 PCMs composite was prepared, and its morphology, chemical compatibility, thermal properties as well as thermal reliability were systematically investigated. Unlike PCMs microcapsules prepared in the previous reports, the novel net-like form-stable ML/ SiO2 composite with high enthalpy was successfully obtained by the simple sol-gel method in our study. The special structure presented a novel form of PCMs composite and had a broad application prospect in the cold chain logistics, agricultural product preservation, medical refrigeration transportation and building energy efficiency et al.
Table 1 Components of the prepared PCMs composite.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. FTIR analysis of the PCMs composite
Vinyltriethoxysilane (VTES, Analytical Reagent) was purchased from qufu yishun chemical co., LTD. Methyl laurate (ML, 98%) was purchased from Beijing enokai technology co., LTD. Polyvinyl alcohol1788 (PVA-1788, Analytical Reagent) was purchased from Shanghai Aladdin biochemical technology co., LTD. Ammonia water (NH3•H2O, Analytical Reagent, 24–28 wt%) was purchased from sinopharm chemical reagent co., LTD. Deionized water was used as the aqueous medium. The chemicals were all used as received.
Fig. 1(a–c) showed the FTIR spectra of ML, silica as well as PCMs composite prepared with different ML/SiO2 ratios, at 25 ℃, 40 ℃ and 50 ℃, respectively. The characteristic peaks of silica were at 3424 cm−1 (-OH stretching vibration), 1165 cm−1 and 728 cm−1 (Si-O-Si asymmetric stretching vibration and symmetric stretching vibration), 965 cm−1 (Si-OH bending vibration). For ML, the typical characteristic peaks were at 923 and 2854 cm−1 (CeH stretching vibration of methyl and methylene), 1745 cm−1 (C]O stretching vibration), 1198 cm−1 (CeO stretching vibration). Furthermore, the intensity of the characteristic peaks of ML was amplified along with the increase in the loading of ML. All the characteristic peaks of silica and ML could be observed in PCMs composite (M1-M15), which revealing that the prepared PCMs composite were composed of ML and silica without any remarkable chemical interaction.
Samples
ML (g)
VTES (g)
PVA1788 (g)
Theoretical ML/ SiO2 ratio
Temperature (℃)
Silica M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15
0 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5
6 6 5 4 3 2 6 5 4 3 2 6 5 4 3 2
0.15 0.15 0.24 0.33 0.42 0.51 0.15 0.24 0.33 0.42 0.51 0.15 0.24 0.33 0.42 0.51
0/6 1/6 2/5 3/4 4/3 5/2 1/6 2/5 3/4 4/3 5/2 1/6 2/5 3/4 4/3 5/2
25 25 25 25 25 25 40 40 40 40 40 50 50 50 50 50
TA, Q50) at a heating rate of 20 ℃/min from room temperature to 700 ℃.
2.2. Preparation of PCMs composite The sol-gel method was adopted to prepare PCMs composite through the hydrolysis of VTES. A typical synthesis process was as follows: 60 mL of deionized water and a given amount of PVA-1788 were mixed and stirred until clear. A certain amount of ML was dispersed in the above solution with a homogenizer at a stirring speed of 4000 rpm to form an oil-in-water emulsion. Then, at a certain temperature, the emulsion was transferred to a three-neck round bottom flask with an overhead stirrer. Then, different amounts of VTES were added (with ML/SiO2 mass ratios of 1/6, 2/5, 3/4, 4/3 and 5/2, respectively) into the solution, and the mixtures were stirred for 1 h. Then, PCMs composite was obtained after 3 mL of ammonia added with stirring for another 2 h. The PCMs composite prepared with different mass ratios of ML/SiO2 was described as MX (X = 0, 1, 2, 3, 4, 5, 6 … and 15) as shown in Table 1.
3.2. Morphology of PCMs composite Fig. 2 showed SEM images of PCMs composite with different mass ratios of ML/SiO2 from M1-M15. To examine the internal structure of the PCMs composite, the SEM images of broken PCMs composite, which were frozen in liquid nitrogen and triturated by a mortar, were shown as the inset in Fig. 2. As seen from Fig. 2, when the PCMs composite’ preparation temperature was 25 ℃, the morphology of PCMs composite changed from sphere-like (M1-M3) into net-like structure (M4-M5) along with the increase of ML content. From the inset in Fig. 2, for M1M3, a clear core-shell structure of broken PCMs composite was confirmed by the SEM images. For M4-M5, a honeycomb-like porous structure was observed inside their net-like surface by the SEM image of the broken composite. And the same phenomenon was observed when the preparation temperature of PCMs composite was 40 ℃ and 50 ℃. The net-like structure of PCMs composite, where the ML was engulfed in, was smooth and complete. It was noticeable, however, the net-like structure appeared earlier with the higher temperature. At the same time, almost all of the sphere-like PCMs composite agglomerated together due to the strong particle-particle interactions [36]. The mechanism, which the morphology of PCMs composite transformed from sphere-like to net-like structure, is reasonable to assume as follows. Fig. 3(a–d) described the transition of the prepared PCMs composite
2.3. Characterization of the PCMs composite The thermal analysis of PCMs composite was done using a differential scanning calorimeter (DSC, TA, Q20) with a heating rate of 5 ℃/min ranging from −40 to 20 ℃, in a nitrogen atmosphere. The morphology of PCMs composite was studied with scanning electronic microscope (TESCAN, XFlash Detector 410-M, Bruker Nano, Germany) at an acceleration voltage of 20 KV under low vacuum. Before SEM examination, all samples were gold-coated. The chemical compatibility of PCMs composite was investigated by Fourier transform infrared spectroscopy (FTIR, Nicolet iN10MX-type spectrometer) with the wavenumbers ranging from 400 cm−1 to 4000 cm−1. The thermal stability of composite was investigated by thermo-gravimetric analysis (TGA, 2
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Fig. 1. FTIR spectra of silica, ML and PCMs composite prepared with different ML/SiO2 ratios at (a) 25 ℃ (M1-M5); (b) 40 ℃ (M6-M10); (c) 50 ℃ (M11-M15).
from an obvious core-shell structure to net-like structure as the temperature and ML/SiO2 mass ratio increased. And the results were consistent with the SEM images (Fig. 3(e) and (f)), respectively. The reason speculated for the structure change was that the hydrogen bonds, which could be strengthened along with the increase of the temperature and ML/SiO2 mass ratios during the formation of PCMs composite, were formed by the carboxyl group of ML, the hydroxyl groups of PVA and the silicon hydroxyl group of silica [37,38]. The strengthened hydrogen bonds promoted the interaction between ML core and silica shell, which led to the transformation of PCMs composite from sphere-like to netlike structure. 3.3. Thermal energy storage of PCMs composite Based on the mass fraction of the pristine ML in PCMs composite, the theoretical melting and solidifying enthalpy of PCMs composite were calculated. The corresponding data were summarized in Table 2. The theoretical solidifying and melting enthalpy of PCMs composite was calculated by:
Hs1 = ωML × HML − s
(1)
Hm1 = ωML × HML − m
(2)
Here, Hm1 and Hs1 respectively represented the theoretical melting and solidifying enthalpy for the PCMs composite. ωML was the mass fraction of the ML in the PCMs composite; HML − m and HML − s was respectively the measured melting and solidifying enthalpy for pure ML. The resulted thermograms of PCMs composite, prepared at 25 ℃, 40 ℃ and 50 ℃, were shown in Fig. 4, and some significant parameters obtained from DSC evaluation were summarized in Table 2. It was
Fig. 3. Illustration of morphology change of the prepared PCMs composite and the structural formula of silica, ML and PVA.
Fig. 2. SEM images of PCMs composite with different mass ratios of ML/SiO2 from M1-M15. The inserts show the SEM images of broken PCMs composite, respectively. 3
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Table 2 Phase change characteristics of ML and PCMs composite Sample code
Silica ML M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15
ML/SiO2
– – 1/6 2/5 3/4 4/3 5/2 1/6 2/5 3/4 4/3 5/2 1/6 2/5 3/4 4/3 5/2
a
with different mass ratios of ML/SiO2 at three preparation temperature.
Melting process Tm (℃)
Hm (J/g)
Hm1(J/g)
Solidification process Ts (℃) Hs (J/g)
Hs1 (J/g)
Temperature (℃)
– 3.96 −2.18 3.22 4.12 5.66 5.97 −1.88 2.75 7.14 7.74 6.71 −11.35 −2.63 3.94 5.97 5.50
– 210.1 23.22 56.24 96.28 109.8 131.5 24.19 56.73 132.6 135.8 151.3 10.92 54.54 64.66 131.7 122.4
– 210.1 30.01 60.02 90.04 120.0 150.1 30.01 60.02 90.04 120.0 150.1 30.01 60.02 90.04 120.0 150.1
– −2.43 −13.74 −6.01 −4.85 −5.70 −8.42 −16.67 −9.63 −9.20 −6.24 −5.43 −23.03 −7.83 −14.57 −3.60 −5.49
– 208.5 29.78 59.57 89.35 119.1 148.9 29.78 59.57 89.35 119.1 148.9 29.78 59.57 89.35 119.1 148.9
– – 25 25 25 25 25 40 40 40 40 40 50 50 50 50 50
– 208.5 22.62 54.83 96.02 109.4 130.3 23.42 56.35 128.5 134.2 149.9 10.39 52.15 62.70 128.0 121.0
a Here, Tm, Hm and Hm1 represent the phase change temperature, measured enthalpy values and theoretical enthalpy values in melting process, respectively; and the Ts, Hs and Hs1 respectively represents the solidification process.
M12 were higher than its theoretical enthalpies, and the rest were the opposite. However, in the previous reports [39,40], the theoretical enthalpy of the PCMs composite were mostly higher than the corresponding measured enthalpy. The phenomenon was supposed to be the result of the strong confinement effect of the narrow space of the PCMs composite. Similarly, for the PCMs composite in our study, only with ML/ SiO2 mass ratio from 3/4 to 5/2, did the composite possibly possess higher measured enthalpy than the theoretical one. The reason for this phenomenon might be the special confinement effect of silica shell's honeycomb-like porous structure on ML. The details of experimental and mechanism will be discussed in the following research. Furthermore, the melting temperatures for the PCMs composite with a high ML/SiO2 ratio became higher than the pure ML, and the solidification temperatures for the composites became lower than the pure ML. As noted in previous paper [41], the silica could generate a confinement effect on the crystallization of ML engulfed, which restricted the motion of ML molecules during crystallization process and led to a wide temperature range for the phase transition. However, the silica shell with a net-like structure had, presumably, effectively promoted the integrity of ML's crystal structure.
noted that only the ML could absorb and release the heat during the PCMs composite melting and solidification processes, and the ML and PCMs composite had similar phase change behavior. This proved that the ML had been successfully inserted into the network structure of silica. At the same time, the higher mass percent the ML possessed in the PCMs composite, the higher enthalpy of phase transition the PCMs composite had. Fig. 5 were the enthalpies comparison diagrams of PCMs composite, with fixed mass ratios of ML/SiO2 from 1/6 to 5/2, at three preparation temperatures of 25 ℃, 40 ℃ and 50 ℃, respectively. It was surprising that the phase change enthalpies of the PCMs composite were strongly dependent on not only the loading of the ML inside the composite, but also the preparation temperature of PCMs composite. With the increase of mass ratios of ML/SiO2 from 1/6 to 5/2, enthalpies of composite increased. However, for each fixed mass ratios of ML/SiO2, enthalpy first increased then decreased with the preparation temperature increasing, while enthalpies of composite should theoretically remain the same. As could be seen in Fig. 5, when the preparation temperature of PCMs composite was increased from 25 ℃ to 50 ℃, the latent heat of PCMs composite increased first and decreased afterwards. That meant 40 ℃ may be the best preparation temperature of PCMs composite in terms of phase change enthalpy in our work. The reason might lie in the different confinement of silica shell with different morphology on ML. The optimal PCMs composite, with net-like structure and the ML/ SiO2 mass ratio of 5/2, melted at 6.71 ℃ with a latent heat of 151.3 J/g and solidified at −5.43 ℃ with a latent heat of 149.9 J/g. Significantly, the measured enthalpies of M3, M8, M9, M10 and
3.4. Thermal stability of PCMs composite Fig. 6 were the TG curves of ML, silica and PCMs composite at three preparation temperature with different mass ratios of ML/SiO2 from M1 to M15, and Table 3 showed the results obtained from TG. Obviously, the ML exhibited a typical one-step thermal degradation, which indicated the ML undergoing a simple evaporation. By comparison, the
Fig. 4. DSC curves of ML, silica and PCMs composite with different mass ratios of ML/SiO2. 4
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Fig. 5. The comparison diagrams of PCMs composite's enthalpies under different reaction temperature with the different ML/SiO2 ratios: 1/6, 2/5, 3/4, 4/3, 5/2.
weight loss curves of all the PCMs composite were quite similar and exhibited a two-step degradation process. The first sharp weight loss happened at around 95 ℃ was mainly attributable to the evaporation of ML, and at around 180 ℃, ML completely leaked out from the silica supporting material. Additionally, the weight loss of the PCMs composite strongly depended on the mass ratio of ML and silica, because silica did not decompose at 600 ℃ yet. As could be seen from Table 3 and Fig. 6, at the same preparation temperature, the amount of residues decreased along with the increase of ML/SiO2 ratio. However, residual weight of PCMs composite with the same mass ratios of ML/SiO2 increased along with the rising of the preparation temperature. The phenomena could be explained through the formation mechanism of the PCMs composite's silica shell synthesized by VTES precursor. A VTES molecule had three ethoxyl groups, and they were easily changed to hydroxyl groups on proper condition [42]. In the presence of ammonia, the polycondensation was generated and silica shell could be obtained. However, when PVA existed as the emulsifier, the silica hydroxyl group formed by the hydrolysis of VTES would be occupied by
Table 3 Residual weight of ML, silica and PCMs composite (M1-M15). Samples
ML
silica
M1
M2
M3
M4
M5
Residue content (%) Samples Residue content (%) Samples Residue content (%)
0.63 ML 0.63 ML 0.63
83.70 silica 83.70 silica 83.70
69.55 M6 71.63 M11 73.57
61.39 M7 71.25 M12 72.98
39.56 M8 66.04 M13 66.87
20.29 M9 54.81 M14 63.70
16.97 M10 50.23 M15 62.85
the hydroxyl group of polyvinyl alcohol to form Type II hybrid [43,44]. The hybrids could hinder the silicon groups’ condensation with each other from forming the Si-O-Si bond. With the increase of preparation temperature, the hybrids reduced and more Si-O-Si structure formed because of the weakened interaction between hydroxyl of PVA and silica hydroxyl of hydrolyzed VTES [45,46]. Based on these results, the ML/SiO2 PCMs composite was fairly stable in the temperature range of phase transition for energy storage application.
Fig. 6. TG curves of silica, ML and PCMs composite prepared with different ML/SiO2 ratios at (a) 25 ℃ (M1-M5); (b) 40 ℃ (M6-M10); (c) 50 ℃ (M11-M15). 5
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4. Conclusions
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A novel net-like form-stable ML/SiO2 PCMs composite was successfully prepared by sol-gel method using PVA as emulsifier for the first time, and their phase change behaviors were studied. In this ML/ SiO2 PCMs composite, ML served as the PCMs for cold energy storage, while silica played the role of the external protection to prevent the leakage of the melted ML and improve the thermal stability of ML. The FTIR results confirmed the excellent chemical compatibility of the ML/ SiO2 PCMs composite. The observation of SEM revealed that ML could be well engulfed into the external silica, and the morphology of PCMs composite could transform from sphere-like structure to net-like structure along with the increase of ML content as well as the preparation temperature during the PCMs composite forming process. The DSC results indicated that the prepared ML/SiO2 PCMs composite melted at 6.71 ℃ with a latent heat of 151.3 J/g and solidified at −5.43 ℃ with a latent heat of 149.9 J/g. The TGA results showed that the ML engulfed into the silica presented a good thermal stability durable up to 95 ℃. All the prominent properties indicated that it would be a potential material used to store cold energy in practical applications, especially in cold chain logistics. CRediT authorship contribution statement Xian Wan: Conceptualization, Methodology, Writing - review & editing. Cong Chen: Data curation, Writing - original draft. Songyun Tian: Visualization, Investigation. Baohua Guo: Supervision. Declaration of Competing Interests 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. Acknowledgments This work was supported by National Natural Science foundation of China (grant no. 51503006) References [1] G. Comodi, F. Carducci, B. Nagarajan, et al., Application of cold thermal energy storage (CTES) for building demand management in hot climates, Appl. Therm. Eng. 103 (2016) 1186–1195. [2] S. Singh, K. Gaikwad K, S. Lee Y, Phase change materials for advanced cooling packaging, Environ. Chem. Lett. 16 (3) (2018) 845–859. [3] J.K. Carson, A.R. East, The cold chain in New Zealand: a review, Int. J. Refrig. 87 (2018) 185–192. [4] Y. Li, W. Xiang, Y. Ding, A cryogen-based peak-shaving technology: systematic approach and techno-economic analysis, Int. J. Energy Res. 37 (6) (2013) 547–557. [5] F. Rodríguez, L. Lopez B, Phenolic resin/PEO-PPO block copolymer composite materials as phase change materials (PCM) for latent heat thermal energy storage (LHTES), J. Energy Storage 6 (2016) 173–177. [6] F. Rodríguez-Cumplido, E. Pabón-Gelves, F. Chejne-Jana, Recent developments in the synthesis of microencapsulated and nanoencapsulated phase change materials, J. Energy Storage 24 (2019) 100821. [7] Q. Yu, F. Tchuenbou-Magaia, B. Al-Duri, et al., Thermo-mechanical analysis of microcapsules containing phase change materials for cold storage, Appl. Energy 211 (2018) 1190–1202. [8] M. Tamaru, H. Suzuki, R. Hidema, et al., Fabrication of hard-shell microcapsules containing inorganic materials, Int. J. Refrig. 82 (2017) 97–105. [9] H. Suzuki, R. Hidema, S. Usa, et al., Flow and sedimentation characteristics of silica hard-shell microcapsule slurries treated with additives, Int. J. Refrig. 106 (2019) 18–23. [10] M. Delgado, A. Lázaro, C. Peñalosa, et al., Analysis of the physical stability of PCM slurries, Int. J. Refrig. 36 (6) (2013) 1648–1656. [11] F. Souayfane, F. Fardoun, P. Biwole, Phase change materials (PCM) for cooling applications in buildings: a review, Energy Build. 129 (2016) 396–431. [12] Y. Li, X. Zhang, M. Munyalo J, et al., Preparation and thermophysical properties of low temperature composite phase change material octanoic-lauric acid/expanded graphite, J. Mol. Liq. 277 (2019) 577–583. [13] C. Veerakumar, Sreekumar A. Phase change material based cold thermal energy storage: materials, techniques and applications: a review, Int. J. Refrig. 67 (2016)
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