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Thermochimica Acta 638 (2016) 35–43

Contents lists available at ScienceDirect

Thermochimica Acta

j ourna l h om epage: www.elsev ier .com/ locate / tca

reparation and performance of novel form-stable composite phasehange materials based on polyethylene glycol/White Carbon Blackssisted by super-ultrasound-assisted

iaoguang Zhanga, Zhaohui Huanga,∗, Bin Maa, Ruilong Wena, Xin Mina, Yaoting Huangb

Zhaoyu Yina, Yangai Liua, Minghao Fanga,∗, Xiaowen Wua

School of Materials Science and Technology, Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, Nationalaboratory of Mineral Materials, China University of Geosciences, Beijing 100083, PR ChinaSchool of Science, Beijing University of Posts and Telecommunications, Beijing 100876, PR China

r t i c l e i n f o

rticle history:eceived 9 January 2016eceived in revised form 26 May 2016ccepted 17 June 2016vailable online 23 June 2016

eywords:

a b s t r a c t

In this study, novel polyethylene glycol (PEG)/White Carbon Black (WCB) form-stable composite phasechanges materials (FS-CPCMs) were prepared by super- ultrasound-assisted, which obviously decreasesthe reaction time. Test results showed that PEG does not easily leak from the fluffy network structureof WCB during solid-liquid phase transition. Results obtained from XRD and FTIR demonstrated that nonew chemical bond is formed between PEG4000 and WCB. Results obtained from DSC and TGA analysesshowed that FS-CPCMs exhibit excellent thermal stability and good form-stable performance. The phase

hase change materialsolyethylene glycolhite Carbon Black

uper-ultrasound-assisted

change enthalpy of FS-CPCMs reached up to 101.1J/g, and the melting and solidifying times of FS-CPCMswere 34.43% and 30.51% less than that of pure PEG, respectively. The thermal conductivity data showedthat WCB acted as the support material is very effective for enhancing the thermal conductivity of theFS-CPCMs. The FS-CPCMs thus prepared were safe, environmentally friendly, and cost-effective; hence,they can be used as potential building materials for the applications of thermal energy storage.

. Introduction

With rapid development in economy, science and technol-gy, non-renewable resources are continuously being exhausted,nergy issues have become a “bottleneck” in the economic andndustrial development [1]; for these reasons, the utilization ofenewable energy sources is receiving increasing attention amongesearchers, worldwide [2,3]. It is imperative to closely monitorhase change materials (PCMs) used for storing thermal energy4]. In recent years, PCMs have become extremely important in theelds of solar engineering, solar heating systems, building energyonservation, heat pumps, air-conditioning systems, thermal insu-ation and regulation, and waste heat recovery [5–7], because of

heir advantages such as isothermal behavior, small temperatureifferences between heat storage and release, and high storageensity [8–13].

∗ Corresponding authors at: School of Materials Science and Technology, Chinaniversity of Geosciences (Beijing), No. 29 Xueyuan Road, Haidian Distric,t Haidian0008, PR China.

E-mail addresses: huang118@cugb.edu.cn (Z. Huang), fmh@cugb.edu.cnM. Fang).

ttp://dx.doi.org/10.1016/j.tca.2016.06.012040-6031/© 2016 Elsevier B.V. All rights reserved.

© 2016 Elsevier B.V. All rights reserved.

Typically, PCMs can be divided into two principal types basedon chemical composition: inorganic and organic [14,15]. As com-pared to inorganic PCMs, such as salt hydrates and their mixtures,organic PCMs have been extensively investigated because of theircost-effectiveness, chemical and thermal stability, wide meltingtemperatures for convenience, as well as their moderate phasechange enthalpies [16,17]. Among the various organic PCMs cur-rently available, PEG is undoubtedly an ideal choice, attributedto its suitable phase change temperature and high latent heatstorage capacity, which can be simply tuned by changing its molec-ular weight [18]. Moreover, PEG molecules can also be directlyincorporated into inorganic and organic polymer matrixes [19,20].However, PEG mainly suffers from drawbacks, such as phase insta-bility in the melting state, low thermal conductivity, and weakinterfacial combination with the supporting materials, which limitits further applications [11,20].

For overcoming these issues, PEG can be feasibly combined intoporous materials. As compared to the organic supporting materi-als, inorganic materials exhibit better thermal conductivity, flame

retardancy, and chemical and thermal stabilities, indicating thatinorganic supporting matrices can be used as promising PCMs withenhanced thermal properties [21,22]. On the other hand, silicahas been extensively investigated in the fields of PCMs and heat

3 himica

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supplier) of quartz sand; it essentially consisted of SiO2, with smallquantities of metal oxides, but without any radioactive element.Based on the comprehensive analysis of the above mentioned ele-

Table 1The source and purity of each compound.

Chemical Name Source Purity

Quartz sand Beijing Tongda RefractoryTechnologies Co., Ltd

SiO2 ≥ 94%

PEG Sinopharm Chemical ReagentCo., Ltd

Analytically pure

Distilled water Sinopharm Chemical ReagentCo., Ltd

Analytically pure

6 X. Zhang et al. / Thermoc

torage. Berthou [23] has formed a new translucent passive solarall by combining silica aerogels and PCMs; Min [2] has inves-

igated PEG/mesoporous-silica-shape-stabilized PCMs; Chen [24]as utilized hydrophobic fumed silica for preparing form-stableomposite PCMs; Qian [5] and He [25] have prepared shape-tabilized PEG/SiO2 composites by sol-gel methods. Jeong [26] hasrepared a silica fume/organic PCMs.

In this study, White Carbon Black (WCB) was used as the inor-anic supporting materials. Typically, WCB is prepared industriallyy two methods: gas-phase method and precipitation. Cost-ffective and easily available materials are used in precipitation;urthermore, production and equipment are simple, with a lowroduct price. In this study, quartz sand was used as raw materialor producing WCB, which acts as the carrier matrix for provid-ng structural strength and preventing the leakage of melted PCMs.he chemical formula of WCB (or amorphous silica) is SiO2·nH2O,hich has a large BET specific area and nanoparticles with a bet-

er sphericity; it is a white, non-toxic, as well as ecofriendly,ith good chemical stability; furthermore, it does not burn, and

t has a low density, non-photodegradable inorganic framework,esirable thermal conductivity, while simultaneously exhibitingxcellent performance. [24,26–28]. A fluffy network structure wasynthesized by the interaction between WCB nanoparticles, whichxhibits a good capacity for strong physical absorption. Moreover,he interaction force between the hydroxyl groups of PEG and thei-OH of WCB serving as the cross-linker could strengthen the inter-acial combination with WCB. WCB serving as the carrier matrix cantrengthen interfacial combination with PEG for providing struc-ural strength and preventing the leakage of molten phase change

aterials. Meanwhile, WCB with high thermal conductivity couldmprove the thermal conductivity of composite PCMs. Hence, WCBerving as the supporting materials can effectively solve the mainrawbacks of PEG, demonstrating its use for potential of compositeCMs.

Fang [1] has reported that a composite PCMs synthesizedssisted by ultrasonically initiated polymerization. In this study,n ultrasonic cell disruptor (UCD) was employed for preparing FS-PCMs. As compared to the normal ultrasonic dispersion, UCD canmit super, steady, and continuous ultrasound, which significantlymproves production efficiency under the condition of retaining the

CB nanoparticles intact.The resulting FS-CPCMs can be potential candidates for appli-

ation in fields of cooling/heating of buildings [29–31], such asCM walls and wallboard [32], Trombe wall, shutter, tiles, buildinglocks, and air-based heating systems [33,34], the temperatures ofhich are often as high as 40–60 ◦C.

. Experimental

.1. Materials

Quartz sand, PEG with an average molecular weight of 4000, dis-illed water, a hydrochloric acid solution, and absolute ethanol weresed in this experiment. The source and purity of each compoundere shown in Table 1.

.2. Method

Fig. 1 shows the schematic of the method for synthesis of FS-PCMs, which primarily involves the preparation of WCB and FS-PCMs.

.2.1. Preparation of White Carbon BlackWCB was prepared by calcination and acid treatments. First, 40 g

uartz sand was mixed with an appropriate amount of sodium car-onate for obtaining a uniform mixture. Second, this mixture was

Acta 638 (2016) 35–43

claimed at 1400 ◦C for 2 h. After cooling and polishing, sinteredpowders were obtained, followed by acid treatment. Third, 20 g ofsintered powder and 40 mL of 36 wt.% hydrochloric acid were addedin a flask, equipped with a mechanical stirrer and twin-blade pro-peller; this flask was then placed in a heated thermostatic waterbath, and the mixture was heated at 80 ◦C for 30 min. Next, theslurry was filtered, and the residue was washed with deionizedwater several times until the pH value of the washing liquid was 7.Finally, WCB was obtained after drying and grinding.

2.2.2. Preparation of FS-CPCMsIn this study, the mass ratios of PEG in the FS-CPCMs were

40% (FS-CPCMs-1), 50% (FS-CPCMs-2), and 60% (FS-CPCMs-3). WCB,PEG, and an appropriate amount of absolute ethanol were addedinto a flask and treated in the UCD (Model JY99 II DN) operating ata power output of 60% at 70 ◦C for 1 min. Then, the products weredried in a drying oven at 60 ◦C for 30 min for evaporating absoluteethanol molecules absorbed in the fluffy network structure of WCB,finally forming FS-CPCM as the product.

2.3. Analytical methods

X-ray fluorescence spectroscopy (XRF, ARL ADVANT′ XP+)was employed for investigating the chemical elements. X-raydiffraction (XRD, Model XD-3) and Fourier transform infraredspectroscopy (FT-IR, Model Frontier) were employed for charac-terizing the chemical compatibilities in the FS-PCMs. Scanningelectronic microscopy (SEM, JSM-5610LV, JEOL) was employedfor investigating the morphology of WCB and FS-CPCMs sam-ples thus synthesized. Besides these, melting points and heatsof fusion of FS-CPCMs were determined by differential scanningcalorimetry (DSC, Q2000). The variation in quality with tempera-ture was determined by thermo-gravimetric analysis (TGA, Q50).N2 adsorption-desorption isotherms of the samples were measuredat 77 K using a BELSORP 28 instrument (Bel Japan, Inc.) and thesurface area was determined using the Brunauer–Emmett–Teller(BET) method. The thermal conductivity (K) was evaluated by K = �* Cp * � (where Cp, �, and � represent thermal diffusivity, specificheat capacity, and density, respectively). The thermal conductivityof cylindrical die specimens with a diameter of 10 mm ± 0.02 mmand a thickness of 1–2 mm was measured at 35 ◦C ± 0.01 ◦C by alaser flash apparatus (TC-7000H, Sinku-Riko, Inc., Japan).

3. Test results and discussions

3.1. Chemical composition of quartz sand

Table 2 summarizes the chemical composition (provided by the

Hydrochloric acidsolution

Sinopharm Chemical ReagentCo., Ltd

11.74 mol/L

Absolute ethanol Sinopharm Chemical ReagentCo., Ltd

Analytically pure

X. Zhang et al. / Thermochimica Acta 638 (2016) 35–43 37

Fig. 1. Schematic of the preparation of FS-PCMs.

Table 2Chemical composition of quartz sand.

Quartz sand SiO K O Al O Fe O MgO TiO Others

mu

3

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3

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3

birPCcnop

Table 3Parameters obtained from DSC of pristine PEG and FS-CPCMs, include onset meltingtemperature Tm, latent heat during the heating process Hm, onset freezing tempera-ture Ts and latent heat during the cooling process Hs. (‘T’ is temperature, ‘H’ specificenthalpy, ‘m’ melting, and ‘s’ cooling).

Samples Tm (◦C) Hm (J/g) Ts (◦C) Hs (J/g)

FS-CPCMs-1 54.39 70.32 37.54 61.23FS-CPCMs-2 51.42 83.44 38.94 73.42FS-CPCMs-3 51.42 101.10 40.74 89.15PEG 58.27 183.40 39.04 158.50

2 2 2 3 2 3 2

Components (wt.%) 94.1520 2.8654 2.3106 0.3236 0.1396 0.1322 0.0766

ents, the synthesized FS-CPCMs based on quartz sand could besed in buildings as they are safe and environment friendly.

.2. Performance analysis of White Carbon Black

As shown in Fig. 2a, WCB was a white powder with a typicalon-crystalline structure, without any crystallization peak. Fig. 2bhowed that WCB resembled a typical hysteresis loop in the P/P0ange of 0.4–1.0, which is typical for microporous solids. More-ver, the BET surface area and average pore diameter of WCB were08.519 m2/ g and 23.37 nm, respectively, with a wide distributionf pore sizes.

.3. Exudation stability of FS-CPCMs

Form-stable properties are crucial aspects for organic PCMs.he shape-stabilized properties of pure PEG, and FS-CPCMs haveeen characterized by hot stage-digital camera technology [35,36].s shown in Fig. 3a, FS-CPCMs and pure PEG were pressed intoafers, each with a diameter of 10 mm, and a height of 2 mm. As

hown in Fig. 3b, 2 g each of FS-CPCMs powder and pure PEG waslaced on the papers. These wafers and powder were heated at0 ◦C for 4 h for exploring their exudation stabilities. The wafers ofhe FS-CPCMs were stable, but pristine PEG completely melted. TheEG powder completely melted, while slight leakage was observedor FS-CPCMs-3, and no leakage was observed for FS-CPCMs-1 andS-CPCMs-2. Hence, because of the capillary sorption and surfaceension of the fluffy network structure and the hydrogen bondsormed between the hydroxyl groups of PEG and the Si-OH of WCB,he FS-CPCMs are stable, and the mass ratio of PEG in FS-CPCMs-3s less than 60%.

.4. Chemical compatibility of FS-CPCMs

As shown in Fig. 4a, the curve for WCB exhibited a broad peaketween 18◦- 32◦. The curves for PEG and FS-CPCMs mainly exhib-

ted two similar diffraction peaks at approximately 19◦ and 23◦,espectively. However, the diffraction peak heights and widths ofEG are higher and narrower, respectively, than that those of FS-PCMs, indicating that the PEG segments of FS-CPCMs can still

rystallize, albeit the degree of crystalline decreases [25]. The fluffyetwork structure and the hydroxyl groups of PEG and the Si-OHf WCB are thought to interact and form hydrogen bonds, whichrevent PEG from forming stable crystals.

Parameters were obtained at pressure p = 0.1 ± 0.001 MPa in N2 gas atmosphere.

As shown in Fig. 4b, the spectrum for WCB exhibited a peakat 461 cm−1, attributed to the bending vibration of the Si O group.Another peak was observed at approximately 1084 cm−1 attributedto the stretching vibration of the Si O Si group. Moreover, anotherpeak was observed at 796 cm−1, attributed to the vibration ofO H group. The spectrum of pristine PEG exhibited peaks at 3423and 1090 cm−1, attributed to the stretching vibrations of O H andCOC, respectively. Furthermore, peaks were also observed 950 and2881 cm−1, attributed to the bending vibration of C H [5,37]. Fromthe spectra of FS-PCMs with different PEG mass ratios, all themain absorption peaks of both pristine PEG and WCB appeared asexpected, except for some slight shifts.

3.5. Micro-morphological analysis

Fig. 5a shows the distribution of nanoparticles in WCB; uniform-sized spherical nanoparticles were observed. Fig. 5b shows the TEMimages of WCB. The size of nanoparticles ranged from 16 nm to30 nm. As shown in Fig. 5(c, d), as compared with the size WCBnanoparticles, that of FS-CPCMs-3 particles significantly increasedwith severe agglomeration. Under the assistance of super ultra-sound, PEG and the agglomerated nanoparticles of WCB wereuniformly dispersed. With the evaporation of anhydrous ethanol,most of the PEG was adsorbed and dispersed into the fluffy net-work structure, thus promoting the growth of FS-CPCMs particles.Moreover, free hydroxyl groups on the WCB surface draw theneighboring particles together via hydrogen bonds, thereby form-ing big particles.

3.6. Phase change behavior of FS-CPCMs

DSC was employed for determining the thermal energy stor-age capacity and phase changes temperature. Table 3 summarizesthe phase change parameters, such as onset melting temperature

(Tm), latent heat during the heating process (Hm), onset freezingtemperature (Ts), and the latent heat during the cooling process(Hs), from the evaluation of DSC results. According to the literature

38 X. Zhang et al. / Thermochimica Acta 638 (2016) 35–43

Fig. 2. (a) XRD pattern and photograph of White Carbon Black and (b) N2 adsorption-desorption isotherm and pore size distribution curve for WCB.

Fig. 3. Macroscopic photographs of (a) wafers of FS-CPCMs and pristine PEG at 80 ◦C; (b) Powder of FS-CPCMs and pristine PEG at room temperature; (c) Leakage of FS-CPCMsand pristine PEG at 80 ◦C.

Fig. 4. (a) XRD patterns; (b) FTIR spectra of pristine PEG, WCB and FS-CPCMs.

X. Zhang et al. / Thermochimica Acta 638 (2016) 35–43 39

Fig. 5. SEM (a) and TEM (b) photographs of WCB and SEM (c and d) photographs of FS-CPCMs-3.

nd so

[d

H

td

Fig. 6. PEG and FS-CPCMs: (a) DSC curves; (b) Enthalpies of melting a

20], the theoretical latent heat value (HTheo) of FS-CPCMs can beetermined by Eq. (1):

Theo = �·HPCM (1)

Here HTheo was represents the theoretical latent heat value ofhe FS-PCMs. HPCM denotes the latent heat of pure PEG, and �enotes the mass ratio of PEG in FS-CPCMs.

lidifying; (c) Phase change temperatures; (d) Percentage of heat loss.

As shown in Table 3 and Fig. 6a, FS-CPCMs exhibited optimumphase change temperatures and excellently high phase changeenthalpies. Because of the incorporation of PEG in FS-CPCMs, thelatent heat of FS-CPCMs increased from 70.32 J/g to 101.10 J/g. How-ever, in accordance with Eq. (1), the measured value was less than

the theoretical value, attributed to the fluffy network structureof WCB and interaction between the hydroxyl groups of PEG andthe Si-OH of WCB, forming hydrogen bonds that restrict the spa-

40 X. Zhang et al. / Thermochimica Acta 638 (2016) 35–43

ves of pristine PEG and the FS-PCMs.

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Fig. 7. (a) TGA and (b) DTG cur

ial movement of PEG in FS-CPCMs [38]. For FS-CPCMs-3, anothereason is that the mass ratio in FS-CPCMs-3 was less than 60%.

As shown in Fig. 6b, the melting enthalpy is greater than theolidifying value of FS-CPCMs, attributed to mass loss during melt-ng by DSC. Then, the solidifying process test of FS-CPCMs waserformed [2,5]. With the mass loss of FS-CPCMs during melting,he melting enthalpy was greater than the solidifying value of FS-PCMs.

The degree of supercooling can be determined by Eq. (2):

T = Tm−Ts (2)

The degree of supercooling for PCMs is an important factor toe considered for practical applications [39]. According to Eq. (2),s compared to pure PEG, the degree of supercooling for FS-CPCMsere reduced to 12.38%, 35.10%, and 43.06%, respectively, (Fig. 6c).

his result indicated that WCB can significantly decrease the super-ooling degree of PEG though encapsulation.

According to a previous study [2], the percentage of heat loss�) can be calculated by Eq. (3):

= (Hm−Hs)/Hm × 100% (3)

In accordance with Eq. (3), the percentage of heat loss andhase change enthalpy for pure PEG and FS-CPCMs are shown inig. 6d. From Fig. 6d, the heat loss percentage of pure PEG betweenndothermic and exothermic cycles was greater than that of FS-PCMs.

As shown in Table 4, previous comparison studies have reported

ifferent phase change temperatures [12,19,30,40,41]. Comparedith those of other PCMs, the phase change latent heat of therepared composite PCM in this study exhibited a relativelyigher enthalpy. Hence, the prepared composite PCM demonstrates

able 4omparison of the thermal performance of composite PCMs prepared in this study anerformance include onset melting temperature Tm, latent heat during the heating proce‘T’ is temperature, ‘H’ specific enthalpy, ‘m’ melting, and ‘s’ cooling).

Samples Tm (◦C) Hm (J

Paraffin @ PMMA SiO2 nanocapsules 26.80 69.90PEG6000/graphitic carbon nitride 32.90 45.80PEG600/gypsum 10.55 ± 0.07 24.18PEG600/natural clay 10.85 ± 0.05 28.79n-Heptadecane/silica 10.45 60.25n-Octadecane/silica – 73.52n-Nonadecane/silica 25.80 74.78Fatty acid/expanded vermiculite 25.64 71.53PEG4000/WCB 51.42 101.1

n this paper, the experimental data were obtained at pressure p = 0.1 ± 0.001 MPa in N2 g

Fig. 8. Melting curves of FS-CPCMs-3 before and after thermal cycle experiment.

potential as a candidate for thermal energy storage applications inthe building field.

3.7. Thermal stability of FS-CPCMs

Thermal stability is a very important parameter for FS-CPCMs,which is needed as a material for heat storage applications [38].Table 5 shows the onset temperatures (◦C), peak temperatures (◦C),end temperatures (◦C) and residual masses (%) of pure PEG and

FS-CPCMs. As shown in Fig. 7, FS-CPCMs exhibited good thermalstability at temperatures below 316.61 ◦C, and the weight loss forboth pure PEG and FS-CPCMs was observed in only one step. Thesharp weight loss is attributed to the decomposition of the PEG

d some composite PCMs previously prepared in related references. The thermalss Hm, onset freezing temperature Ts and latent heat during the cooling process Hs.

/g) Ts (◦C) Hs (J/g) References

19.80 71.00 [12] 23.00 42.70 [19]

± 1.00 – – [30] ± 1.07 – – [30]

16.15 61.37 [40] 24.27 72.18 [40] 26.24 80.79 [40] 24.90 69.64 [41]0 40.74 89.15 This work

as atmosphere.

himica Acta 638 (2016) 35–43 41

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Table 5TGA data of pristine PEG and FS-PCMs, include onset temperature To, peak temper-ature Tp, and end temperature Te. (‘T’ is temperature).

Samples To (◦C) Tp (◦C) Te (◦C) Residual mass (%)

FS-CPCMs-1 316.61 374.53 486.13 55.60FS-CPCMs-2 317.52 374.86 485.32 45.83FS-CPCMs-3 318.23 378.04 487.46 38.10PEG 336.37 399.71 488.52 1.22

TGA data was obtained at pressure p = 0.1 ± 0.001 MPa in N2 gas atmosphere.

Fp

X. Zhang et al. / Thermoc

hains. As shown in Table 6, compared with previous compari-on studies [12,30,42,43], the prepared FS-CPCMs-3 exhibited goodhermal stability at temperature below 316.61 ◦C. In this study,he residual masses of PEG and FS-PCMs-1, FS-CPCMs-2, and FS-PCMs-3 were 1.22 wt.%, 55.60 wt.%, 45.83 wt.% and 38.10 wt.%,espectively, indicating that FS-CPCMs are homogeneous and aren good agreement with the value obtained in the experiment.

For determining the excellent thermal reliability of the FS-PCMs, a 200-cycle experiment was carried out. As shown in Fig. 8,o obvious change was observed in the endothermic curve afterhe cycles, indicating that FS-CPCMs exhibit excellent thermal reli-bility, implying a longer life cycle.

.8. Thermal conductivity improvement

Fig. 9 shows the thermal conductivity obtained for WCB, PEG,nd FS-CPCMs. The thermal diffusivity, specific heat capacity,ensity, thermal conductivity and corresponding uncertainties ofhermal diffusivity/conductivity were shown in Table 7. As shownn Fig. 9, the thermal conductivity of the prepared WCB, PEG, FS-PCMs-1, FS-CPCMs-2, and FS-CPCMs-3 were 0.87, 0.22, 0.29, 0.31,nd 0.33 W/(mK), respectively. As compared with that of pureEG, the thermal conductivities of FS-CPCMs-1, FS-CPCMs-2, andS-CPCMs-3 were enhanced by 31.82%, 40.91%, and 50.00%, respec-ively, indicating that WCB serving as a support material is veryffective for enhancing the thermal conductivity of FS-CPCMs.

.9. Melting and solidifying characteristics of FS-CPCMs

As shown in Fig. 10(a), an experimental facility was employedor characterizing the thermal property of the prepared FS-CPCMs

5,44]. First, 20 g of pristine PEG and 20 g of FS-CPCMs-3 were addednto two glass tubes, respectively. Second, thermometers with aemperature accuracy of ±0.1 ◦C were placed in the center of thewo tubes. Third, two water baths were employed for the complete

ig. 10. (a) An experimental facility for characterizing thermal properties, (b) Melting prristine PEG and FS-CPCMs-3.

Fig. 9. thermal conductivities of the prepared WCB, PEG, and FS-CPCMs.

melting and solidification process. During melting process, the twotesting tubes were placed into a water bath at 100 ◦C, and ther-mometers were used to measure and record once every 30s. Next,the two testing tubes were immediately placed into another water

◦

bath at 20 C. As shown in Fig. 10(b), the times required for the tem-perature increase from 20 ◦C to 80 ◦C for PEG and FS-CPCMs-3 were1830s and 1200s, respectively. The melting time of FS-CPCMs-3 was34.43% less than that of pure PEG. As can be observed from Fig. 10(c),

ocess curves of pristine PEG and FS-CPCMs-3, and (c) Solidifying process curves of

42 X. Zhang et al. / Thermochimica Acta 638 (2016) 35–43

Table 6Comparison of the TGA data between composite PCMs prepared in this study and some composite PCMs previously prepared in related references. TGA data include onsettemperature To, peak temperature TP, and end temperature Te. (‘T’ is temperature).

Samples To (◦C) TP (◦C) Teg (◦C) References

Paraffin @ PMMA SiO2 nanocapsules 140.00 – – [12]PEG600/gypsum 210.00 390.00 – [30]Paraffin/expanded vermiculite 169.70 262.20 400.30 [42]paraffin@SiO2 222.87 – 465.48 [43]PEG4000/WCB 318.23 378.04 487.46 This work

In this paper, TGA data were obtained at pressure p = 0.1 ± 0.001 MPa in N2 gas atmosphere.

Table 7Parameters of specific heat capacity Cp, density �, thermal diffusivity �, and thermal conductivity K of samples.

Samples Cp (103 J Kg−1 K−1) � (103 Kg m−3) � (10−7 m2 s−1) K (W m−1 K−1) ua (�) (m2 s−1) Ucb (K) (W m−1 K−1)

WCB 0.98 4.63 1.91 0.87 0.0071 0.012PEG 2.23 1.15 0.86 0.22 0.0089 0.0081FS-CPCMs-1 0.63 3.11 1.47 0.29 0.0063 0.014FS-CPCMs-2 0.82 2.74 1.38 0.31 0.0084 0.0099FS-CPCMs-3 1.09 2.50 1.21 0.33 0.0084 0.010

ters wh

tfsTCp

4

unteith3f8ciFpwst

A

eZYX

C

[

[

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a Standard uncertainties u are u (�).b combined expanded uncertainty Uc is Uc(K) (0.95 level of confidence). Parameelium gas atmosphere.

he times required for the temperature decrease from 80 ◦C to 25 ◦Cor PEG and FS-CPCMs-3 were 1770s and 1230s, respectively. Theolidifying time of FS-CPCMs-3 is 30.51% less than that of pure PEG.he test results indicated that the heat storage/release rate of FS-PCMs-3 is significantly more rapid than that of pristine PEG forractical applications.

. Conclusions

In this study, FS-CPCMs were synthesized by super ultrasoundnder the condition of retaining the WCB nanoparticles intact. Thisovel method exhibits high-efficiency, which obviously decreaseshe reaction time. The test results show that PEG is successfullyncapsulated by the fluffy network structure of WCB and the bond-ng force between the surface silanols of WCB and the hydrogen oferminal hydroxyl groups of PEG. DSC results indicated that latenteat for melting and the melting temperature of the FS-CPCMs-

are 101.10 J/g and 51.42 ◦C, respectively, while, the latent heator freezing, and the freezing temperature of the composite are,9.15 J/g and 40.74 ◦C, respectively. The TGA results and thermalycling test indicated that the composite PCMs exhibit good chem-cal stability and thermal reliability. The thermal conductivity ofS-CPCMs-3 (0.33 W m−1 K−1) was enhanced by 50.00% as com-ared with that of pure PEG (0.22 W m−1 K−1). Hence, FS-CPCMsith good thermal properties, thermal reliability, and chemical

tability can be considered as a promising candidate material forhermal energy storage applications.

uthor contributions

G. Zhang, Z.H. Huang and M.H. Fang conceived and designed thexperiments. X.G. Zhang carried out the experiments. X.G. Zhang,.H. Huang, M.H Fang, X. Min., R.L. Wen, B. Ma, Z.Y. Yin, Y.T. Huang,.G. Liu, and X.W. Wu analyzed the data and discussed the results..G. Zhang wrote the paper.

ompeting financial interests

The authors declare no competing financial interests.

[

ere obtained at pressure p = 0.1 ± 0.001 MPa and temperature T = 35 ◦C ± 0.01 ◦C in

Acknowledgements

This present work was supported by the National NaturalScience Foundations of China (Grant Nos.51472222, 51372232).Xiaowen Wu also thanks the Beijing Higher Education Young EliteTeacher Project (Grant No.YETP0636).

References

[1] Y.T. Fang, X. Liu, X.H. Liang, H. Liu, X.N. Gao, Z.G. Zhang, Ultrasonic synthesisand characterization of polystyrene/n-dotriacontane compositenanoencapsulated phase change material for thermal energy storage, Appl.Energy 132 (2014) 551–556.

[2] X. Min, M.H. Fang, Z.H. Huang, Y.G. Liu, Y.T. Huang, R.L. Wen, T.T. Qian, X.W.Wu, Enhanced thermal properties of novel shape-stabilized PEG compositephase change materials with radial mesoporous silica sphere for thermalenergy storage, Sci. Rep. 5 (2015) 12964.

[3] W.L. Wang, X.X. Yang, Y.T. Fang, J. Ding, Preparation and performance ofform-stable polyethylene glycol–silicon dioxide composites as solid–liquidphase change materials, Appl. Energy 86 (2009) 170–174.

[4] K. Pielichowska, K. Pielichowski, Phase change materials for thermal energystorage, Prog. Mater. Sci. 65 (2014) 67–123.

[5] T.T. Qian, J.H. Li, H.W. Ma, J. Yang, The preparation of a green shape-stabilizedcomposite phase change material of polyethylene glycol/SiO2 with enhancedthermal performance based on oil shale ash via temperature-assisted sol–gelmethod, Sol. Energy Mater. Sol. Cells 132 (2015) 29–39.

[6] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storagewith phase change materials and applications, Renewable Sustainable EnergyRev. 13 (2009) 318–345.

[7] B.T. Tang, Y.M. Wang, M.G. Qiu, S.F. Zhang, A full-band sunlight-driven carbonnanotube/PEG/SiO2 composites for solar energy storage, Sol. Energy Mater.Sol. Cells 123 (2014) 7–12.

[8] A.A. Aydın, H. Okutan, High-chain fatty acid esters of myristyl alcohol witheven carbon number: novel organic phase change materials for thermalenergy storage—1, Sol. Energy Mater. Sol. Cells 95 (2011) 2752–2762.

[9] A. Lazaro, C. Penalosa, A. Solé, G. Diarce, T. Haussmann, M. Fois, B. Zalba, S.Gshwander, L.F. Cabeza, Intercomparative tests on phase change materialscharacterisation with differential scanning calorimeter, Appl. Energy 109(2013) 415–420.

10] M. Liu, R.Y. Wang, Phase change nanocomposites with tunable meltingtemperature and thermal energy storage density, Nanoscale 5 (2013)7234–7237.

11] G.Q. Qi, C.L. Liang, R.Y. Bao, Z.Y. Liu, W. Yang, B.H. Xie, M.B. Yang, Polyethyleneglycol based shape-stabilized phase change material for thermal energystorage with ultra-low content of graphene oxide, Sol. Energy Mater. Sol. Cells123 (2014) 171–177.

12] J. Shi, X.L. Wu, X.Z. Fu, R. Sun, Synthesis and thermal properties of a novel

nanoencapsulated phase change material with PMMA and SiO2 as hybridshell materials, Thermochim. Acta 617 (2015) 90–94.

13] X.X. Zhang, P.F. Deng, R.X. Feng, J. Song, Novel gelatinous shape-stabilizedphase change materials with high heat storage density, Sol. Energy Mater. Sol.Cells 95 (2011) 1213–1218.

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chemical precipitation method for building energy conservation, EnergBuildings 108 (2015) 373–380.

[44] F. He, X.D. Wang, D.Z. Wu, New approach for sol–gel synthesis ofmicroencapsulated n-octadecane phase change material with silica wall usingsodium silicate precursor, Energy 67 (2014) 223–233.

X. Zhang et al. / Thermoc

14] D.P. Bentz, R. Turpin, Potential applications of phase change materials inconcrete technology, Cem. Concr. Compos. 29 (2007) 527–532.

15] D. Zhang, Z.J. Li, J.M. Zhou, K.R. Wu, Development of thermal energy storageconcrete, Cem. Concr. Res. 34 (2004) 927–934.

16] F. Chen, M.P. Wolcott, Miscibility studies of paraffin/polyethylene blends asform-stable phase change materials, Eur. Polym. J. 52 (2014) 44–52.

17] G.Q. Qi, J. Yang, R.Y. Bao, Z.Y. Liu, W. Yang, B.H. Xie, M.B. Yang, Enhancedcomprehensive performance of polyethylene glycol based phase changematerial with hybrid graphene nanomaterials for thermal energy storage,Carbon 88 (2015) 196–205.

18] T.T. Qian, J.H. Li, X. Min, Y. Deng, W.M. Guan, H.W. Ma, Polyethyleneglycol/mesoporous calcium silicate shape-stabilized composite phase changematerial: preparation, characterization, and adjustable thermal property,Energy 82 (2015) 333–340.

19] L.L. Feng, P. Song, S.C. Yan, H.B. Wang, J. Wang, The shape-stabilized phasechange materials composed of polyethylene glycol and graphitic carbonnitride matrices, Thermochim. Acta 612 (2015) 19–24.

20] Y. Qian, P. Wei, P.K. Jiang, Z. Li, Y.G. Yan, J.P. Liu, Preparation of a novel PEGcomposite with halogen-free flame retardant supporting matrix for thermalenergy storage application, Appl. Energy 106 (2013) 321–327.

21] C.Y. Wang, L.L. Feng, W. Li, J. Zheng, W.H. Tian, X.G. Li, Shape-stabilized phasechange materials based on polyethylene glycol/porous carbon composite: theinfluence of the pore structure of the carbon materials, Sol. Energy Mater. Sol.Cells 105 (2012) 21–26.

22] X.L. Wang, X.Q. Zhai, C. Wang, B. Zhu, Review on phase change cool storage inair-conditioning system, Build. Sci. 6 (2013) 021.

23] Y. Berthou, P.H. Biwole, P. Achard, H. Sallée, M. Tantot-Neirac, F. Jay, Full scaleexperimentation on a new translucent passive solar wall combining silicaaerogels and phase change materials, Sol. Energy 115 (2015) 733–742.

24] J.J. Chen, Z.Y. Ling, X.M. Fang, Z.G. Zhang, Experimental and numericalinvestigation of form-stable dodecane/hydrophobic fumed silica compositephase change materials for cold energy storage, Energy Convers. Manage. 105(2015) 817–825.

25] L.H. He, J.R. Li, C. Zhou, H.Z. Zhu, X.J. Cao, B.M. Tang, Phase changecharacteristics of shape-stabilized PEG/SiO2 composites using calciumchloride-assisted and temperature-assisted sol gel methods, Sol. Energy 103(2014) 448–455.

26] S.G. Jeong, J. Jeon, J. Cha, J. Kim, S. Kim, Preparation and evaluation of thermalenhanced silica fume by incorporating organic PCM, for application toconcrete, Energy Buildings 62 (2013) 190–195.

27] L.Y. Zhou, L.S. Yin, K.S. Zhou, D.M. Kong, X.W. Zhao, Recent developments insilica research: preparation surface modification and application, Mater. Rev.11 (2003) 016.

28] H.Q. Li, H.S. Chen, X.Y. Li, J.G. Sanjayan, Development of thermal energystorage composites and prevention of PCM leakage, Appl. Energy 135 (2014)225–233.

29] M. Kenisarin, K. Mahkamov, Passive thermal control in residential buildings

using phase change materials, Renewable Sustainable Energy Rev. 55 (2016)371–398.

30] A. Sarı, Composites of polyethylene glycol (PEG600) with gypsum and naturalclay as new kinds of building PCMs for low temperature-thermal energystorage, Energ Buildings 69 (2014) 184–192.

Acta 638 (2016) 35–43 43

31] D. Zhou, C.Y. Zhao, Y. Tian, Review on thermal energy storage with phasechange materials (PCMs) in building applications, Appl. Energy 92 (2012)593–605.

32] T. Stovall, J. Tomlinson, What are the potential benefits of including latentstorage in common wallboard? J. Sol. Energy Eng. 117 (1995) 318–325.

33] J. Giro-Paloma, M. Martínez, L.F. Cabeza, A.I. Fernández, Types, methods,techniques, and applications for microencapsulated phase change materials(MPCM): a review, Renewable Sustainable Energy Rev. 53 (2016) 1059–1075.

34] R.K. Sharma, P. Ganesan, V.V. Tyagi, H.S.C. Metselaar, S.C. Sandaran,Developments in organic solid–liquid phase change materials and theirapplications in thermal energy storage, Energy Convers. Manage. 95 (2015)193–228.

35] B.T. Tang, L.J. Wang, Y.J. Xu, J.H. Xiu, S.F. Zhang, Hexadecanol/phase changepolyurethane composite as form-stable phase change material for thermalenergy storage, Sol. Energy Mater. Sol. Cells 144 (2016) 1–6.

36] Y.M. Wang, B.T. Tang, S.F. Zhang, Single-walled carbon nanotube/phasechange material composites: sunlight-driven reversible, form-Stable phasetransitions for solar thermal energy storage, Adv. Funct. Mater. 23 (2013)4354–4360.

37] M. Catauro, F. Bollino, M. Gallicchio, S. Pacifico, Influence of the polymeramount on bioactivity and biocompatibility of SiO2/PEG hybrid materialssynthesized by sol–gel technique, Mat. Sci. Eng. C Mater 48 (2015) 548–555.

38] L.L. Feng, W. Zhao, J. Zheng, S. Frisco, P. Song, X.G. Li, The shape-stabilizedphase change materials composed of polyethylene glycol and variousmesoporous matrices (AC, SBA-15 and MCM-41), Sol. Energy Mater. Sol. Cells95 (2011) 3550–3556.

39] B.T. Tang, M.G. Qiu, S.F. Zhang, Thermal conductivity enhancement ofPEG/SiO2 composite PCM by in situ Cu doping, Sol. Energy Mater. Sol. Cells105 (2012) 242–248.

40] F. He, X.D. Wang, D.Z. Wu, Phase-change characteristics and thermalperformance of form-stable n-alkanes/silica composite phase changematerials fabricated by sodium silicate precursor, Renewable Energy 74(2015) 689–698.

41] A. Karaipekli, A. Sarı, Preparation, thermal properties and thermal reliabilityof eutectic mixtures of fatty acids/expanded vermiculite as novel form-stablecomposites for energy storage, J. Ind. Eng. Chem. 16 (2010) 767–773.

42] W.M. Guan, J.H. Li, T.T. Qian, X. Wang, Y. Deng, Preparation ofparaffin/expanded vermiculite with enhanced thermal conductivity byimplanting network carbon in vermiculite layers, Chem. Eng. J. 277 (2015)56–63.

43] R.L. Luo, S.F. Wang, T.Y. Wang, C.Y. Zhu, T. Nomura, T. Akiyama, Fabrication ofparaffin@ SiO2 shape-stabilized composite phase change material via