Synthesis and thermal properties of novel solid-solid phase change materials with comb-polyurethane block copolymer structure for thermal energy storage

Synthesis and thermal properties of novel solid-solid phase change materials with comb-polyurethane block copolymer structure for thermal energy storage

Thermochimica Acta 651 (2017) 58–64 Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Synt...

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Thermochimica Acta 651 (2017) 58–64

Contents lists available at ScienceDirect

Thermochimica Acta journal homepage: www.elsevier.com/locate/tca

Synthesis and thermal properties of novel solid-solid phase change materials with comb-polyurethane block copolymer structure for thermal energy storage Du Xiaosheng, Wang Haibo, Du Zongliang, Cheng Xu ∗ Textile Institute, College of Light Industry, Textile and Food Science Engineering, Sichuan University, Chengdu, 610065, China

a r t i c l e

i n f o

Article history: Received 11 October 2016 Received in revised form 10 February 2017 Accepted 18 February 2017 Available online 21 February 2017 Keywords: Phase change material Comb-polyurethane Methoxypolyethylene glycol Thermal energy storage

a b s t r a c t A series of novel comb-polyurethane phase change materials (CP-PCMs) with poly(ethylene glycol) (PEG) as side chain were synthesized through the reaction between diethanolamine-modified methoxypolyethylene glycol (DMPEG) with diisocyanate and 1,4-butanediol (BDO). The structure and phase change property of CP-PCM were characterized by gel permeation chromatography (GPC), 1 H nuclear magnetic resonance (1 H NMR), Fourier transform infrared (FTIR) spectroscopy, and polarization optical microscopy (POM). The 1 H NMR and FTIR spectra confirmed the success synthesis of CP-PCM. The POM images showed that CP-PCM possessed a completely crystalline structure and smaller spherulites compared with the pristine methoxypolyethylene glycol (MPEG). The influence of phase change property of CP-PCMs was also investigated using differential scanning calorimetry (DSC). The results showed that the CP-PCM synthesized using isophorone diisocyanate (IPDI) or 1,6-hexamethylene diisocyanate (HDI) as diisocyanate and MPEG with molecular weight of 5000 as soft segment possessed optimal phase change property. In addition, the phase change enthalpy and temperature of CP-PCMs decreased as the weight percentage of MPEG decreased. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Efficient utilization of energy resources is necessary given the serious shortage of mineral resources and increasing deterioration of ecological environments. One prospective technique to improve the energy efficiency is the application of phase change materials (PCMs), which store and release large amount of heat energy during the phase change process [1–4]. PCMs have been widely used for energy storage in various fields such as, waste heat recovery, smart air-conditioning buildings, telecommunications and microprocessor equipment, agricultural greenhouse, and solar energy storage [5–7]. Poly(ethylene glycol) based polyurethane (PEG-PU), where PEG (as a phase change ingredient) is covalently bonded to polyisocyanate (as a skeleton) to keep the material in solid state during its phase change process, is considered as a potential PCM for energy storage and temperature control because of its relatively high phase change enthalpy, good chemical and thermal stability, nontoxicity and convenient melting temperature range [8–12]. Research

∗ Corresponding author. E-mail address: [email protected] (X. Cheng). http://dx.doi.org/10.1016/j.tca.2017.02.012 0040-6031/© 2017 Elsevier B.V. All rights reserved.

about the relationship between structure and thermal energy storage capability of PEG-PU has been studied extensively [13–16]. Su et al. [17] synthesized a linear polyurethane PCM (with phase change enthalpy of about 100 J/g) by using PEG as soft segment, and 4,4-diphenylmethane diisocyanate (MDI) and BDO as hard segment via bulk polymerization. Chen et al. [18] prepared three novel polyurethane PCMs with different crosslinking structures, in which inositol, dipentaerythritol and sorbitol were individually employed as molecular skeleton and PEG was used as phase change functional chain. The study found that spatial hindrance of the skeleton influenced the movement of PEG in PCMs. Peng et al. [19] prepared a hyperbranched polyurethane solid–solid PCM by using PEG, ␤cyclodextrin and MDI via a two-step condensation reaction. The phase change enthalpy observed was more than 100 J/g. Overall, convenient phase change temperature and high phase change enthalpy are of prime importance in PEG-PU fabrication. However, the covalent bonds between polyisocyanate and PEG restrict the movement and crystallization of PEG, thereby decreasing the phase change enthalpy of PEG-PU [20–23]. In our previous research, a novel comb-polyurethane solid–solid PCM (CP-PCM) of high thermal energy storage capability was synthesized through the reaction between diethanolamine-modified methoxypolyethylene glycol (DMPEG) with IPDI and BDO [24].

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Table 1 Provenance and mass fraction purity of the starting materials.

Scheme 1. Schematic illustration showing the crystallizing and decrystallizing of comb-polyurethane PCM.

Chemical

Source

Purity

Purification method

MPEGa IPDI TDI HDI MDI acryloyl chloride diethanolamine 1,4-butanediol

Aladdin Sigma Aldrich Sigma Aldrich Sigma Aldrich Sigma Aldrich Aladdin Chengdu Kelong Co. Ltd., China Chengdu Kelong Co. Ltd., China

0.99 0.99 0.99 0.99 0.99 0.98 0.99 0.99

none none none none none none none none

a

While both ends of PEG were linked with IPDI in the common linear PEG-PU, only one end of DMPEG was linked with IPDI in CP-PCM. Therefore, the arrangement and orientation of PEG in CP-PCM were more active, which made CP-PCM an efficient PCM with high phase change enthalpy (Scheme 1). In this study, a series of DMPEG with different molecular weight was synthesized, and then the novel comb-polyurethane phase change materials based on DMPEG, BDO, and diisocyanate were prepared via a two-step condensation reaction (Scheme 2). The structure and composition of synthesized CP-PCM were researched by gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, 1 H nuclear magnetic resonance (1 H NMR), and polarization optical microscopy (POM). In addition, the effects of the molecular weight of MPEG, the weight percentage of MPEG in CP-PCMs, and the types of diisocyanate on energy storage of CP-PCMs were investigated by differential scanning calorimetry (DSC) respectively. 2. Experimental 2.1. Materials and instruments MPEG with the average molecular weights of 1000, 2000, 4000, 5000, 10,000 and 20,000 were purchased from Aladdin Reagent Co. Inc., America. Acryloyl chloride was purchased from Shang-

MPEG with molecular weights of 1000, 2000, 4000, 5000, 10,000 and 20,000.

hai Chemical Reagent Co. Inc., China. 2,4-tolylene diisocyanate (TDI), 1,6-hexamethylene diisocyanate (HDI), MDI and IPDI were purchased from Sigma–Aldrich Reagent Co. Inc., America. Triethylamine, diethanolamine, BDO, and other common reagents were purchased from Chengdu Kelong Co. Ltd., China. Table 1 summarizes relevant information on the provenance and purity of the starting materials. 2.2. Synthesis of diethanolamine modified MPEG MPEG (Mn = 5000, 10.00 g, 2 mmol), triethylamine (0.20 g, 2 mmol), and dichloromethane (DCM, 150 mL) were added to a round bottomed flask. Acryloyl chloride (0.18 g, 2 mmol) dissolved in DCM (30 mL) was added dropwise at 0 ◦ C. The reaction mixture was stirred at 25 ◦ C for 24 h. Then, the solid salt was filtered and the filtrate was concentrated under reduced pressure. The crude product was purified by recrystallization in ethanol at −20 ◦ C three times. At last, methoxypolyethylene glycol ether acrylate (MPEGAC) was obtained by drying the precipitate under vacuum. Then, DMPEG was synthesized by a Michael addition between MPEGAC and diethanolamine. MPEGAC (10.18 g, 2 mmol), diethanolamine (0.84 g, 2 mmol), and ethanol (150 mL) were added to a flask at 25 ◦ C. After stirring 24 h, the reaction solution was crystallized at −20 ◦ C and a crude product was obtained

Scheme 2. Synthetic route of CP-PCM.

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Fig. 1.

1

H NMR spectrum of CP-PCM in CDCl3 .

Fig. 2. GPC graph of CP-PCM.

through filtration. The crude product was purified by recrystallization in ethanol at −20 ◦ C three times. Then, DMPEG was obtained by drying the precipitate under vacuum. 2.3. Synthesis of CP-PCMs CP-PCM (with 80% weight percentage of MPEG) was synthesized through a two-step polymerization in a round bottomed flask under nitrogen atmosphere. A predetermined amount of DMPEG (10.32 g, 2 mmol) and IPDI (1.97 g, 8.86 mmol) in freshly distilled butanone were mixed and stirred at 75 ◦ C for 2 h. Then, a predetermined amount of BDO (0.62 g, 6.86 mmol) dissolved in butanone was slowly dropped into the stirring reaction mixture. After stirring for another 2.5 h at 80 ◦ C, the reaction solution was casted into a glass pan and dried at 60 ◦ C for 48 h. The product was stored in vacuum at 25 ◦ C for 2 weeks before testing. A series of CP-PCMs using different diisocyanate (TDI, MDI, IPDI, HDI) and different molecular weight of MPEG (Mn = 1000, 2000, 4000, 5000, 10,000, 20,000) were prepared according to the similar procedure as mentioned earlier. The weight percentage of MPEG in CP-PCMs alters from 70 wt% to 90 wt%. 2.4. Characterization To reveal the structure and composition of synthesized CP-PCM, GPC, 1 H NMR and FTIR spectra test were performed. The molecular weight and molecular weight distribution were determined by GPC, using Waters HPLC system equipped with a Water 2414 differential refractive index detector and a 2690D separation module. Tetrahydrofuran was employed as the mobile phase at a flow rate of 1.0 mL/min−1 at 30 ◦ C. The 1 H NMR spectra of the samples were recorded at 400 MHz on a JEOL EX-400 spectrometer using CDCl3 as the solvent. FTIR spectra of the samples were recorded in the wave number range of 400–4000 cm−1 by using a Nicollet NEXUS-670 spectrometer. POM analysis was determined by Leitz Laborlux 12POL microscope equipped with a video camera. The samples were placed between a coverslip and a microscope glass and then heated with a Leitz 350 hot stage. Phase change properties of the PCMs were measured by using differential scanning calorimeter (Perkin–Elmer DSC 8500) in nitrogen atmosphere. About 5 g sample was sealed in an aluminium pan. First, the sample was heated to 120 ◦ C for 5 min and then cooled to −20 ◦ C to eliminate the thermal history. Then the sample was heated and cooled at a rate of 10 ◦ C/ min between −20 ◦ C and 120 ◦ C.

Fig. 3. FTIR spectrum of (a) DMPEG, (b) CP-PCM and (c) IPDI.

The second scanning was recorded. The degree of crystallinity (XC ) of the CP-PCM was calculated according to the following equation:

XC =

Hm 0 (1 − w) × Hm

× 100%,

where Hm is the measured phase change enthalpy of CP-PCM sample, H0 m is the phase change enthalpy of 100% crystalline PEG (196.8 J/g) [25], and w is the weight fraction of hard segments in CP-PCM. 3. Results and discussion 3.1. Molecular structure of CP-PCM The detailed procedure for synthesis of DMPEG and CP-PCM is shown in Scheme 2. To reveal the structure and composition of synthesized CP-PCM (using 80% weight percentage of MPEG with molecular weight of 5000 as soft segment and using IPDI as diisocyanate), GPC, 1 H NMR and FTIR spectra were measured. The attribution of chemical shift was labelled in the 1 H NMR spectra of CP-PCM, as shown in Fig. 1. The characteristic peaks at 3.62 ppm (c) was attributed to −CH2 –CH2 –O− belonging to MPEG, and the characteristic peak at 0.89 ppm (d) was attributed to −CH3 of IPDI.

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Fig. 4. POM images of (a) MPEG at room temperature, (b) CP-PCM at room temperature, (c) MPEG at 80 ◦ C, and CP-PCM at 80 ◦ C.

Fig. 5. DSC curves of CP-PCMs using different molecular weight of MPEG: (a) cooling curves and (b) heating curves.

[26] The peaks related to the presence of protons of urethane bond of CP-PCM at 7.11 ppm (a) and 6.80 ppm (b), proved that CP-PCM was successfully synthesized through bulk polymerization between DMPEG and IPDI. The GPC chromatograph of CP-PCM is shown in Fig. 2. It can be seen that the GPC curve has a unimodal outflow peak, indicating that the polymerization reaction was complete. The peak molecular weight of CP-PCM is 88500, and the molecular weight distribution is 1.9 according to the outflow time. Fig. 3 shows the FTIR spectra of DMPEG, CP-PCM, and IPDI. In the FTIR spectra of CP-PCM, peak separation was carried out in the region 3680–3180 cm−1 . It was observed that this stretching region include two absorption peaks. The main absorption peak at 3375 cm−1 was attributed to N H stretching vibration, and the small absorption peak at 3520 cm−1 was attributed to O H stretching vibration. The absorption peak of O H at 3400 cm−1 in CP-PCM was very small because majority hydroxyl groups

from MPEG reacted with the isocyanate groups provided by the IPDI. Besides, the characteristic absorption peaks at 2887 cm−1 , 1468 cm−1 , 1346 cm−1 , 1281 cm−1 , 958 cm−1 , and 843 cm−1 were caused by the C H bonds. Moreover, the characteristic absorptions at 1111 cm−1 and 1244 cm−1 were attributed to the stretching vibration of C O C. The characteristic absorption peak of isocyanate from IPDI at 2260 cm−1 disappeared completely in the spectra of CP-PCM, indicating that the isocyanate groups provided by IPDI and the hydroxyl groups from DMPEG were involved in the reaction. In addition, the newly formed absorption peaks at 1540 cm−1 and 1713 cm−1 were related to the carbonyl and amide vibrations of NHCOO groups of CP-PCM as indicated in Fig. 3. Based on the newly observed characteristic absorption peaks attributed to NHCOO and the disappearance of peak of isocyanate, it can be concluded that the CP-PCM with combpolyurethane structure was successfully synthesized.

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Table 2 Phase change enthalpy (Hheating , Hcooling ), phase change temperature (Theating , Tcooling ), and degree of crystallinity (XC ) of CP-PCMsa (using 80% weight percentage of MPEG as soft segment and using IPDI as diisocyanate) using different molecular weight of MPEG. Mn of MPEG

Theating (◦ C)b

Hheating (J/g)b

Tcooling (◦ C)b

Hcooling (J/g)b

XC (%)

1000 2000 4000 5000 10000 20000

39.3 49.8 55.4 58.1 60.1 60.5

72.9 90.5 106.7 113.3 112.4 109.7

13.1 20.1 26.5 29.1 31.2 31.8

70.5 88.7 103.1 110.6 109.9 107.1

46.3 57.5 67.8 72.0 71.4 69.7

a

DSC investigation was carried out in nitrogen atmosphere of P = 101325 Pa. Standard uncertainty is u(P) = 5000 Pa. The combined uncertainty (0.95 level of confidence) for phase change temperatures is u(Theating ) = u(Tcooling ) = 0.2 ◦ C and the relative combined expanded uncertainty (0.95 level of confidence) for phase change enthalpy is u(Hheating ) = u(Hcooling ) = 0.06. b

Table 3 The phase change enthalpy (Hheating ) of CP-PCMsa using different molecular weight of MPEG and different weight percentage of MPEG. Mn of MPEG

1000 2000 4000 5000 10000 20000 a b

Hheating (J/g)b 70 wt% MPEG

75 wt% MPEG

80 wt% MPEG

85 wt% MPEG

90 wt% MPEG

40.2 66.1 79.7 87.0 82.8 81.3

58.9 79.8 94.2 103.3 100.2 96.1

72.9 90.5 106.7 113.3 112.4 109.7

84.5 100.3 113.4 122.6 119.8 117.1

93.1 107.9 117.5 128.4 125.9 120.8

DSC investigation was carried out in nitrogen atmosphere of P = 101325 Pa. Standard uncertainty is u(P) = 5000 Pa. The relative combined expanded uncertainty (0.95 level of confidence) for phase change enthalpy is u(Hheating ) = u(Hcooling ) = 0.06.

Fig. 6. Relationship curves between the phase change enthalpy and the weight percentage of MPEG.

Fig. 7. Relationship curves between the phase change temperature and the weight percentage of MPEG.

The POM images of MPEG and synthesized CP-PCM (using 80% weight percentage of MPEG with molecular weight of 5000 as soft segment and using IPDI as diisocyanate) at room temperature and at 80 ◦ C are shown in Fig. 4 to observe the crystallization morphology. Fig. 4(a) and (b) showed obvious cross-extinction patterns, which suggested that MPEG and CP-PCM are crystalline at room temperature and their crystalline morphologies are spherulites. However, spherulite size in CP-PCM is smaller than that in MPEG, indicating that the arrangement and orientation of MPEG was limited by the hard segments in CP-PCM. As shown in Fig. 4(c) and (d), the spherulite structures were destroyed completely at 80 ◦ C (above the phase transition temperature). As a typical solid–liquid PCM, MPEG transformed from a white crystal solid to a transparent liquid when the material was heated to about 61.5 ◦ C. By contrast, CP-PCM remained in solid state even when the temperature increased to 100 ◦ C. The result showed that the synthesized CP-PCM is a solid–solid PCM with good crystallization property.

3.2. Effect of molecular weight of MPEG on phase change property of CP-PCM A series of CP-PCMs using different molecular weight of MPEG (Mn = 1000, 2000, 4000, 5000, 10,000, 20,000) was prepared using IPDI as hard segment and MPEG for the remaining 80% weight percentage. Fig. 5 shows the DSC curves of CP-PCMs using different molecular weights of MPEG, and Table 2 summarizes the data of their phase change properties. As shown in Table 2, the phase change temperature and enthalpy of CP-PCMs are associated with the molecular weight of MPEG. When the molecular weight of MPEG was less than 5000, the phase change temperature and enthalpy of CP-PCMs increased rapidly as the molecular weight of MPEG increased; and after the molecular weight of MPEG exceeded 5000, the phase change temperature became more stable and the phase change enthalpy and degree of crystallinity of CP-PCMs decreased slowly as the molecular weight increased. This

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Fig. 8. DSC curves of CP-PCMs using different diisocyanates.

Table 4 The phase change temperature (Theating ) of CP-PCMsa using different molecular weight of MPEG and different weight percentage of MPEG. Mn of MPEG

1000 2000 4000 5000 10000 20000 a b

Theating (◦ C)b 70 wt% MPEG

75 wt% MPEG

80 wt% MPEG

85 wt% MPEG

90 wt% MPEG

35.9 46.7 52.3 55.0 57.3 57.5

37.7 48.3 54.0 56.7 58.8 59.1

39.3 49.8 55.4 58.1 60.1 60.5

40.7 51.1 56.6 59.1 61.2 61.5

41.8 52.4 57.7 59.8 62.0 62.3

DSC investigation was carried out in nitrogen atmosphere of P = 101325 Pa. Standard uncertainty is u(P) = 5000 Pa. The combined uncertainty (0.95 level of confidence) for phase change temperatures is u(Theating ) = u(Tcooling ) = 0.2 ◦ C.

result was because the entanglement between molecular chains of MPEG reduced the movement and crystallization of the molecular chain segment after the molecular weight exceeded 5000. Summarizing the results above, the CP-PCM using MPEG with a molecular weight of 5000 possessed optimal phase change property. 3.3. Effect of weight percentage of MPEG on phase change property of CP-PCM To further study the influence of MPEG on phase change property of CP-PCMs, the effect of weight percentage of MPEG on the phase change enthalpy and temperature were investigated. Fig. 6 shows the relationship curves between the weight percentage of MPEG and the phase change enthalpy of CP-PCMs in the heating cycle, and Table 3 summarizes the data of their phase change enthalpy. As shown in Fig. 6, the phase change enthalpy of CP-PCMs decreased rapidly as the weight percentage of MPEG decreased. This result was because MPEG was used as the active ingredient for CP-PCMs, and IPDI did not contribute to the phase change enthalpy. Once the weight percentage of MPEG in CP-PCM decreased, the

active ingredient for phase change enthalpy in the unit mass of CP-PCM decreased as well. Therefore, the phase change enthalpy of CP-PCMs decreased rapidly as the weight percentage of MPEG decreased. However, the CP-PCM with a lower weight content MPEG demonstrated a better thermal reliability and stability. Under the comprehensive consideration, the CP-PCM with 80 wt% MPEG was seleted for further investigations. Fig. 7 shows the relationship curves between the weight percentage of MPEG and the phase change temperature of CP-PCMs in the heating cycle, and Table 4 summarizes the data of their phase change temperature. As shown in Fig. 7, the phase change temperature of CP-PCMs decreased slowly as the weight percentage of MPEG decreased. For CP-PCM, the phase change temperature was determined using the PEG in CP-PCM. It is important to note that the crystallization of PEG was restrained by the hard segments in CPPCM. The percentage of hard segments in CP-PCM increased with the reduction of weight percentage of MPEG in CP-PCM, destroying the integrity of crystallization of PEG. Therefore, the phase change temperature of CP-PCMs decreased as the weight percentage of MPEG decreased.

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Table 5 Phase change enthalpy (Hheating , Hcooling ), phase change temperature (Theating , Tcooling ), and degree of crystallinity (XC ) of CP-PCMsa (using 80% weight percentage of MPEG with molecular weight of 5000 as soft segment) using different diisocyanates. Sample

Theating (◦ C)b

Hheating (J/g)b

Tcooling (◦ C)b

Hcooling (J/g)b

XC (%)

IPDI TDI MDI HDI

58.1 56.5 57.7 58.9

113.3 102.1 105.4 114.5

29.1 27.6 28.7 28.5

110.6 99.7 102.4 111.6

72.0 64.9 66.9 72.3

a

DSC investigation was carried out in nitrogen atmosphere of P = 101325 Pa. Standard uncertainty is u(P) = 5000 Pa. The combined uncertainty (0.95 level of confidence) for phase change temperatures is u(Theating ) = u(Tcooling ) = 0.2 ◦ C and the relative combined expanded uncertainty (0.95 level of confidence) for phase change enthalpy is u(Hheating ) = u(Hcooling ) = 0.06. b

3.4. Effect of diisocyanate on phase change property of CP-PCM The hard segment in CP-PCM, serving as a skeleton to keep CPPCM in solid state during phase change process, was composed of diisocyanates and BDO. To research the influence of diisocyanates on the phase change property of CP-PCM, a series of CP-PCMs using different diisocyanates (TDI, MDI, IPDI, and HDI) with 80% weight percentage of MPEG (Mn = 5000) was prepared. Fig. 8 shows the DSC curves of CP-PCMs using different diisocyanates, and Table 5 summarizes the data of their phase change properties. As shown in Table 5, the phase change enthalpy, phase change temperature, and the degree of crystallinity of CP-PCMs using IPDI and HDI were higher than that of CP-PCMs using MDI and TDI. This result was because both MDI and TDI were aromatic diisocyanates. The rigid benzene ring groups of MDI and TDI restricted the movement and crystallization of PEG in CP-PCMs, which resulted to the low phase change enthalpy and temperature. Summarizing the result above, the CP-PCMs using IPDI and HDI possessed better phase change property. 4. Conclusion In this study, a series of novel comb-polyurethane CP-PCMs with PEG as side chains were successfully synthesized as new solid–solid PCMs through bulk polymerization. The analysis results of DSC indicated that CP-PCMs were typical solid–solid PCMs with suitable phase transition temperature and high thermal energy storage capability. Further research about the influence of phase change property of CP-PCMs showed that the CP-PCM using IPDI or HDI as diisocyanate and MPEG with molecular weight of 5000 as soft segment possessed optimal phase change property. In addition, the phase change enthalpy and temperature of CP-PCMs decreased as the weight percentage of MPEG decreased. Altogether, the CPPCM possesses great potential in the application for thermal energy storage and temperature control. Acknowledgements This study was supported by the Natural Science Foundation of China (NSFC 51503130), support plan of Science and Technology Department of Sichuan Province, China (No. 2012GZ0096) and Xiamen Southern Ocean Research Center Program (14GZP004NF04, 14GQT61HJ31). References [1] T.T. Qian, J.H. Li, X. Min, Y. Deng, W.M. Guan, L. Ning, Diatomite: a promising natural candidate as carrier material for low, middle and high temperature phase change material, Energy Convers. Manag. 98 (2015) 34–45. [2] A. Pasupathy, R. Velraj, R.V. Seeniraj, Phase change material-based building architecture for thermal management in residential and commercial establishments, Renew. Sustain. Energy Rev. 12 (2008) 39–64. [3] D. Feldman, D. Banu, D. Hawes, Low chain esters of stearic acid as phase change materials for thermal energy storage, Sol. Energy Mater. Sol. C 36 (1995) 311–322.

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