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Solar Energy Materials and Solar Cells

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Using silicagel industrial wastes to synthesize polyethylene glycol/silica-hydroxyl form-stable phase change materials for thermal energy storageapplications

Keyan Suna,b, Yan Koua, Hui Zhenga, Xin Liua, Zhicheng Tana, Quan Shia,⁎

a Thermochemistry Laboratory, Liaoning Province Key Laboratory of Thermochemistry for Energy and Materials, Dalian National Laboratory for Clean Energy, DalianInstitute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR ChinabUniversity of Chinese Academy of Sciences, Beijing 100049, PR China

A R T I C L E I N F O

Keywords:Sol-gel methodSilica-hydroxyl compoundPolyethylene glycolForm-stable phase change materialsThermal energy storage

A B S T R A C T

Polyethylene glycol form-stable phase change materials (PEG FSPCMs) have received much attention in recentyears for thermal energy storage applications due to their remarkable thermal properties. However, the con-ventional synthesis of PEG FSPCMs usually employed chemical grade regents as starting materials, which isunlikely suitable for large-scale industrial preparation of PCMs. In the present work, silicagel industrial wasteswere employed as starting materials for the first time to synthesize a polyethylene glycol/silica-hydroxylcompound (PEG/SHC) form-stable phase change material using a facile sol-gel method. The morphology andchemical compatibility were characterized using scanning electron microscopy (SEM), fourier transform infraredspectroscopy (FT-IR) and X-ray diffraction (XRD). The thermal energy storage performance was evaluated usingdifferential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermal constants analysis, respec-tively. The results indicated that the PEG was encapsulated in the SHC matrix through a physical interaction, andthe weight fraction of PEG in the FSPCM could be as high as 80% with no significant leaking liquid observed. Thethermal energy storage capacity in this FSPCM was found to be (59.38–132.4) J/g and (63.56–133.4) J/g in themelting and crystallization process, respectively, as the loaded PEG weight fraction ranging from 50% to 80%.The thermal conductivity of the FSPCM enhanced by the SHC matrix was determined to be as high as 30%compared with that of the pure PEG. Additionally, the FSPCM synthesized in this method could maintain a stablethermal property during the heating/cooling cycles. On the basis of these results, it was demonstrated that thesol-gel method developed in this work could not only obtain PEG based FSPCMs with good performance forthermal energy storage, but also propose an effective way of producing economic benefits by reusing silicagelindustrial wastes.

1. Introduction

With the depletion of fossil resources in the earth, the effectiveutilization and rational management of nonrenewable resources havebecome the urgent demand of the development of human society. Onthe other hand, the continuous increase of industrial waste has alsobeen a serious threat currently to human living environment, andtherefore developing techniques for the industrial waste recycling isvital to the construction of social ecological balances. Most importantly,the exploiture and application of new techniques or methods for theenergy resources utilization and management on the basis of industrialwaste would be more significant and impressive for constructing greenand energy-saving societies.

Thermal energy storage using phase change materials (PCMs) hasintrigued a great deal of interests in recent years due to its potentialapplications in the fields of intelligent temperature control design [1] ,indoor thermal fluctuation reduction [2], solar cooling technology [3],heat management on electronic devices [4], thermal energy collectionin thermosolar industry [5] and so on. The PCMs involved in this en-ergy-saving technology are capable of storing/releasing thermal energyduring the phase transition process at almost constant temperatureswith the involved latent heats absorbed/released, which are generallyseveral times larger than those commonly used in sensible heat storagematerials [6,7].

As for the PCMs currently studied, polyethylene glycol (PEG) basedmaterials have received much attention because of their large heat

https://doi.org/10.1016/j.solmat.2018.01.016Received 27 September 2017; Received in revised form 30 December 2017; Accepted 9 January 2018

⁎ Corresponding author.E-mail address: shiquan@dicp.ac.cn (Q. Shi).

Solar Energy Materials and Solar Cells 178 (2018) 139–145

0927-0248/ © 2018 Elsevier B.V. All rights reserved.

T

storage capacity, adjustable phase change temperature varied withdifferent molecular weights, stable chemical property, no toxicity andrelatively reasonable price [8–11]. However, the liquid phase leakageand low thermal conductivity of PEG have greatly hindered its practicalapplications in thermal energy storage fields [12–14]. To overcomethese obstacles, the synthesis technique called form-stable PCMs(FSPCMs) preparation has been proposed by embedding PCMs intosome supporting materials, including expanded graphite [15], diato-mite [16], cellulose [17], bentonite [18]，graphite [19] and so on. Thesupporting materials could not only prevent the liquid phase leakage byencapsulating PCMs in materials, but also significantly improve thethermal conductivities by providing a three-dimensional inorganicstructure in the entire FSPCMs [20]. Among the numerous supportingmaterials, silica-hydroxyl compounds (SHCs) are of interest due to theirexcellent encapsulation capacity and relatively high thermal con-ductivity compared to the pure PEG [21,22]. Consequently, the designand synthesis of PEG/SHCs based FSPCMs have become one of thefrontier research topics in developing new PCMs for thermal energystorage applications.

Tang et al. synthesized PEG/SiO2/MWCNT and PEG/SiO2 doped Cuusing tetraethoxyl silane (TEOS) as starting materials to obtainedFSPCMs [23,24]. Qian et al. prepared co-crystallized poly (ethyleneglycol) composites utilizing TEOS as supporting matrix precursor [1].Yang et al. prepared PEG/SiO2 composites with TEOS as the silicaframework precursor [25]. Fang et al. prepared microencapsulatedoctadecane with silica shell by using sol-gel method with methyl trie-thoxysilane [26]. In these synthesis strategies for PEG based FSPCMs,the chemical grade TEOS or methyl triethoxysilane were generallyemployed as starting materials to obtain the prerequisite silica pre-cursor. However, these conventional regents used as the resources forSHC is unlikely suitable for large-scale industrial preparation ofFSPCMs due to their high cost, chemical toxicity and complicated re-action conditions involved in the synthetic process [27]. It is worthnoting that Qian et al. developed a temperature-assisted so-gel methodfor preparation of a green shape-stabilized composite PCM of PEG/SiO2

with enhanced thermal performance using the “hazardous waste” oilshale ash produced in the oil shale processing [21]. Although the

synthesis process employed in their work seems a little complicated andthe silica extraction efficiency is relatively low, the waste oil shale ashused in this work instead of the chemical grade regents provides aninnovative technique of low cost synthesis of PEG based FSPCMs usingindustrial wastes.

In this work, we have presented a facile sol-gel method of synthe-sizing PEG/SHC FSPCMs using silicagel industrial wastes provided by asilicagel production plant for the first time. This sol-gel method in-volved only sodium hydroxide and acetic acid as coreagents withoutany other surfactants or coagulants needed in the conventional sol-gelpreparation process. Moreover, the sample morphology, chemicalstructure, crystal phase and thermal stability of the as prepared PEG/SHC FSPCMs have been characterized using scanning electron micro-scopy (SEM), fourier transform infrared spectroscopy (FTIR), X-raydiffraction (XRD) and thermogravimetric analysis (TGA) techniques,respectively. The thermal property and conductivity of the PEG/SHCFSPCM have been determined using a differential scanning calorimeter(DSC) and thermal constants analyzer, respectively. The results indicatethat the PEG/SHC FSPCMs prepared based on the silicagel industrialwastes behave a good thermal property, enhanced thermal conductivityand stable cycle performance comparable to those synthesized using theconventional sol-gel method.

2. Experimental

2.1. Materials

The silicagel industrial wastes, a kind of amorphous byproduct froman enterprise of silica gel producer in Tangshan city, China, mainlycontains silica, denoted by SIW. Polyethylene glycol with an averagemolecular weight of 1500 (PEG1500), was purchased from HaianPetroleum Chemical Factory, Jiangsu province, China. Sodium hydro-xide (NaOH, AR,) and glacial acetic acid (HAc, AR,) were provided bySinopharm Chemical Reagent Co., Ltd., China and Tianjing Fuyu FineChemical Co., Ltd., China, respectively. All of these starting materialswere used as received in the flowing PCM synthesis process.

Fig. 1. Synthesis process of PEG/SHC FSPCMs.

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2.2. Synthesis of PEG/SHC FSPCMs

The synthesis process of the PEG/SHC FSPCMs was presented inFig. 1. The general description was given as follows. SIW was addedinto a NaOH solution with magnetic stirring until a clear solutioncontaining some impurities in SIW was formed. The SIW impuritieswere removed from the solution using a vacuum filter method to collecta tetrasodium orthosilicate (Na4SiO4) sol. And then, a diluted HAc wascontinuously added into the Na4SiO4 sol until the pH value of the solwas reached at about 4. The magnetic stirring was continued for about1 h until a white orthosilicic acid (H4SiO4) gel was produced in thesolution. The H4SiO4 gel was finally obtained by filtering and washingout the excess of HAc remained in the sol using the deionized water.The PEG/SHC composites was consequently synthesized by adding1–2 mL deionized water and different amounts of PEG1500 into theH4SiO4 gel with weight ratios of (wPEG: wSIW = 0: 10, 8: 2, 7: 3, 6: 4 and5: 5). The final FSPCM products were obtained by stirring the mixturesfor about 1 h and then drying it in an oven at 60 °C for 12 h, and thesefinal products were designated as SHC, PEG/SHC-80%, PEG/SHC-70%,PEG/SHC-60% and PEG/SHC-50% according to the PEG-SIW weightratios, respectively.

2.3. Sample characterization

The compositions of SIW materials were evaluated by an energydispersive X-Ray spectroscopy (EDX) using a VarioEL III elementalanalyzer with xylenol orange as the indicator. The sample morphologieswere examined using a field-emission SEM (JEOL JSM-6360LV) with20 kV acceleration voltages, the phase compositions were detectedusing a PANalytical X′Pert-Pro powder X-ray diffractometer with Cu Karadiation (λ = 0.1514 nm), and the chemical compatibility betweenPEG and SHC were investigated using a fourier transform infrared (FT-IR) spectrometer (Bruker Equinox 55) in the form of KBr discs over therange of 400–4000 cm−1. The thermal stability was determined using athermogravimetric analyzer (TGA, SETSYS 16/18, SETARAM, France)from room temperature to 600 °C with a heating rate of 10 °C/minunder a nitrogen atmosphere. The phase transition properties wereinvestigated by means of a differential scanning calorimeter (DSC 204HP NETZSCH, German) under a nitrogen atmosphere with a flow rate of50 mL min−1. The measurement uncertainties are within±0.1 °C forthe temperature and within±5% for the enthalpy. In the DSC mea-surements, the samples were heated from 20 °C to 80 °C and held at80 °C for 3 min to erase the thermal history of the sample, following bycooling to 0 °C and then heated to 80 °C at the scanning rate of 10 ℃/min, and the second run was employed to analyze the phase changeproperties. The thermal conductivity measurement with an uncertaintyof± (2–5)% was performed on a thermal constants analyzer (Hot DiskTPS2500S) at room temperature for three times, and the averaged va-lues were used as the samples’ thermal conductivity.

3. Results and discussion

3.1. EDX, SEM, XRD and FT-IR analysis

The silicagel industrial wastes of SIW used in this work are whitesolid powders and insoluble in water. The element compositions of SIWdetermined using EDX are listed in Table 1. It can be seen that SIWmainly contains silicon and oxygen, as well as a little amount of carbonand zinc, which could be removed out in the PEG/SHC FSPCMs

synthesis process. The SEM image of SIW is shown in Fig. 2(a), fromwhich it can be seen that SIW are irregular lumps with sizes as large asseveral microns. After the treatment in the sol-gel process, the SIW largelumps became relatively small and uniform spherical particles (SHC)with the size of dozens to hundreds of nanometers (Fig. 2(b) and (c)).These SHC spherical particles clustered together and formed a porousstructure that could provide an ideal matrix for supporting PEG PCMs.Fig. 2(d) shows the SEM image of PEG/SHC-60%, and it can be seenthat the PEG was absorbed and dispersed into the porous SHC since theSHC particle sizes were significantly enhanced due to the PEG absorp-tion.

FT-IR and XRD were employed to determine the chemical compat-ibility between SHC and PEG, and the corresponding spectra are shownin Fig. 3. In the FT-IR spectrum of PEG, the typical adsorption peaks of-C-H2 group are located at 964 cm−1 and 2889 cm−1, respectively. Thepeaks at 3404 cm−1 and 1116 cm−1 can be assigned to C-O and O-Hgroup, respectively. For SHC, the peaks at 470 cm−1 and 1095 cm−1

represent the symmetric and asymmetric stretching vibration of -Si-O-Si- group, respectively. The stretching vibration of SiO-H group is foundat 802 cm−1, and the peak of 3403 cm−1 represents -OH group. It isclear to see that all of the characteristic peaks attributed to PEG andSHC can be found in the spectrum of PEG/SHC without any new peaks,suggesting that there is only physical interaction formed and no che-mical bonds could be found in the PEG/SHC composites. This physicalinteraction was further confirmed by XRD results. As can be seen in theXRD patterns of PEG/SHC-60%, the broad peak from 20° to 30° is at-tributed to the amorphous silica in SHC, and the most typical peaksappearing at 19° and 23° is corresponding to crystal PEG. Also, it can beseen that no any other XRD peaks can be found, indicating that PEG andSHC are physically combined in the PEG/SHC-60%, where the PEGcould maintain a good crystal property for phase transition.

3.2. Thermal stability

The thermal stability of PEG, SHC and PEG/SHC composites wereinvestigated using the TGA measurements, and the results and corre-sponding thermal parameters are presented in Fig. 4 and Table 2, re-spectively. As can be seen in the TGA curve, the SHC loses about 7%weights before 183 °C due to the removal of surface water and im-purities from the sample; and with the temperature continuing up to600 °C, the SHC sample behaves a relatively stable thermal propertywith only about 3% weights gradually lost, which is likely attributed tothe further removal of water absorbed deeply into the SHC structures.The TGA result for PEG indicates that the PEG sample is thermal stablefrom room temperature to about 393 °C, and then it begins to loseweight at temperatures above 393 °C due to the decomposition of PEGchains. The PEG can be detected to be completely decomposed at about426 °C with total weight loss of 92.5%. As for the PEG/SCH composites(taking PEG/SHC-60% as an example), it behaves a combined thermalstability of SCH and PEG in the related temperature region. PEG/SHC-60% is found to be thermal stable before about 357 °C with only a smallamount of surface water or impurities removal from the sample. ThePEG in PEG/SHC-60% begins to decompose at about 357 °C, which islower than the pure PEG decomposition temperature of 393 °C. Inreality, this discrepancy of the degradation temperature between thepure PEG against the corresponding shape stabilized composites hasalso been observed in other previous work reported from Kong [28] andZhang et al. [29,30]. This may be attributed to the physical interactionbetween PEG and SHC matrix [31]. The total weight loss of PEG/SHC-60% is found to be 60% up to 600 °C, which is roughly agreement withthe combined weight loss of SHC and PEG. This result indicates that thePEG/SHC-60% composite synthesized in the present work can maintaina good thermal stability in the temperature region below about 350 °C.

Table 1Element compositions of SIW determined using EDX.

Element Si O C Zn Totals

Weight (%) 37.81 48.41 11.42 2.36 100

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3.3. Form stable behavior

In order to evaluate the form stable behavior, the PEG/SHC com-posites synthesized in this work were heated up to through the PEGsolid-liquid phase transition region to check the PEG leakage from thecomposites. The PEG/SHC samples were placed on a filter paper andheated to 70 °C in an oven for about 2 h, and the PEG leakage could bedetected by the liquid trace appearing in the paper and the increasedpaper weight due to the PEG absorption. The photographs of PEG/SHC-60%, PEG/SHC-70%, PEG/SHC-80% and PEG for comparison in thisleaking test are shown in Fig. 5, and the corresponding filter paperweights before and after PEG melting are listed in Table 3. It can beseen that all the samples have a solid state in room temperature; and as

the temperature increasing to the PEG liquid region, PEG/SHC-60% andPEG/SHC-70% are still in a solid region and no leaking traces could bedetected in the paper. As for the PEG/SHC-80%, there is a slight amountof PEG liquid traces can be found in the paper with a notable weightenhancement, but it still could maintain a solid state. Also, the PEGsample was completely melted and absorbed into the paper with asignificant increased weight. These results indicate that PEG/SHCcomposites could exhibit a good form stable property with the PEGencapsulated amounts up to 80%. As illuminated in the above FT-IRand XRD results, there is no any chemical bonds formed in the PEG/SHC composites, and the PEG and SHC are combined together only withphysical interactions, such as hydrogen bonding, capillary forces andsurface adsorption. It is suggested that these physical interactions are

Fig. 2. SEM images of SIW before (a) and after sol-gel treatment (b), (c), and PEG/SHC-60% FSPCMs (d).

Fig. 3. FT-IR(left) and XRD(right) patterns of SHC, PEG/SHC-60% and PEG.

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strong enough to prevent the PEG leaking and make the PEG/SHCcomposites maintaining a stable form in the PEG phase transition re-gion [32,33]. Also, it is demonstrated that the sol-gel method based onthe silicagel industrial wastes developed in this work may provide aneffective way to construct PEG-based FSPCMs.

3.4. Phase transition properties

The phase transition properties of the PEG/SHC composites wereinvestigated using the DSC technique. The melting temperatures ofthese samples were determined using the extrapolated onset tempera-tures in the transition region of DSC curve, and the phase transitionenthalpy was calculated on the basis of excess heat flow due to thetransition. The DSC curves of PEG/SHC-50%, PEG/SHC-60%, PEG/SHC-70%, PEG/SHC-80% and PEG are presented in Fig. 6, and thecorresponding melting temperatures and phase transition enthalpies are

listed in Table 4. It can be seen from the DSC results that the meltingand peak temperatures for these composites in both melting and crys-tallization are slightly lower than those of the pure PEG sample. Thesecompressed phase transition temperatures could be attributed to strongphysical interaction between PEG and SHC in the limited SHC porousspace [34,35]. The onset temperatures of the PEG/SHC composites in

Fig. 4. TGA (solid line) and corresponding DTG (dashed line) curves of SCH, PEG/SHC-60% and PEG.

Table 2TGA data of SCH, PEG/SHC-60% and pure PEG.

Sample TOnset (°C) TMax (°C) TEndset (°C) Total weight loss (%)

SHC 85.96 117.63 182.59 10PEG/SHC-60% 356.78 398.06 426.76 60PEG 393.42 411.48 425.50 92.5

Fig. 5. PEG/SHC-60%, PEG/SHC-70%, PEG/SHC-80% and PEG in the leaking test.

Table 3The filter paper weight before and after the leakage test.

Sample（PEG%） Before (g) After (g)

60 0.32 0.3270 0.32 0.3280 0.32 0.44100 0.33 1.80

Fig. 6. DSC curves of PEG/SHC composites.

Table 4Phase transition properties of PEG and PEG/SHC composites.

Sample Melting progress Crystallization process

(PEG%) Onset(°C) Peak(°C) ΔHm (J/g) Onset(°C) Peak(°C) ΔHm (J/g)

50% 38.13 45.58 59.38 27.64 21.38 63.5660% 40.01 46.43 86.73 30.09 22.49 87.0170% 39.86 47.39 105.3 31.35 22.33 107.180% 38.40 46.36 132.4 30.88 25.43 133.4100% 41.06 48.83 164.6 31.57 26.87 168.1

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the melting temperature could maintain a roughly constant value, whilein the crystallization process the onset temperatures are roughly tendedto decrease with the PEG amounts decreasing from 80% to 50%. It islikely that the physical interaction between PEG and SHC are moreimpressive in the crystallization than that in the melting process.

Additionally, as can be seen from Table 4, the phase transition en-thalpies of these PEG/SHC composites increase from 59.38 J/g to132.4 J/g in the melting process and from 63.56 J/g to 133.4 J/g in thecrystallization process, with the PEG amounts increasing from 50% to80%. These transition enthalpies are comparable with the reportedvalues for the PEG based FSPCMs [23,27,36–38]. In order to evaluatethe performance of these PEG/SHC composites for thermal energystorage, we have employed the following equations as FSPCM standardsfor determining their thermal capacities [36,39],

= ×

=+

+

×

= ×

R HHm P

E Hm HHm PEG Hc PEG

ER

Δ mΔ . EG

100%

Δ Δ cΔ . Δ .

100%

η 100%

where R, E and η represent the encapsulation ratio, encapsulation ef-ficiency and thermal storage capability of the FSPCMs, respectively;and ΔHm, ΔHc, ΔHm.PEG and ΔHc.PEG are phase transition enthalpy ofPEG/SHC and pure PEG in melting and crystallization process, re-spectively. Based on the DSC results, the R, E and η of these PEG/SHCcomposites have been calculated and the results are listed in Table 5. Inreality, R can evaluate the PCM effective filling ratio in the carrier bycalculating the enthalpy in the melting process, E further indicates thePCM working efficiency involved in both the melting process andcrystallization process, and η represents the PCM thermal capacity ex-pressed using the working efficiency from the effectively loaded PCM inthe composite. It can be noted that almost all the R and E values cal-culated using the DSC results are slightly lower than the actual amountsof PEG loaded in the synthesis, indicating that not all the PEG couldprovide the transition thermal energy in the composites, or a slightamount of PEG may be washed away in the synthesis procedure. Also, itis found that the thermal storage capability of these PEG-SHC compo-sites are almost or larger than 100%, suggesting that the PEG en-capsulated in these composites could effectively perform the thermalenergy storage function during the phase transition process.

Moreover, the thermal performance of the PEG/SHC composite wasalso evaluated using a series of programed heating-cooling measure-ments on the DSC instrument. The DSC curves of the PEG/SHC-60%sample in the 1st, 5th, 10th, 20th and 30th cycle are shown in Fig. 7,and the phase transition temperatures and enthalpies for these cyclesare listed in Table 6. It can be seen from the results that the transitiontemperature and enthalpy of the PEG/SHC composite are almost un-changed during these 30 cycles, suggesting that the PEG/SHC compo-sites synthezised in this work may behave a good cycling thermalproperty in real applications.

3.5. Thermal conductivity

The thermal conductivities of the PEG/SHC composites as well asthe pure PEG sample were measured using the thermal constants

analyzer, and the results are plotted in Fig. 8. The thermal conductivityof the pure PEG sample used in this work was determined to be 0.31 W/(m K). As for the PEG/SHC composites synthesized using this PEG, theirthermal conductivities can be found to be larger than that of the purePEG sample, and increase with the SHC contents in the compositesincreasing from PEG/SHC-80% to PEG/SHC-50%, due to the largerthermal conductivities of the inorganic SHC matrix employed in thePEG/SHC composites. Moreover, the largest thermal conductivity en-hancement can be obtained to be 0.40 W/(m K) for the PEG/SHC-50%sample, which is 30% larger than that of the pure PEG sample. It isnoted that the thermal conductivity enhancement of PEG/SHC com-posites synthesized using TEOS as starting materials was only about24% [23,24], suggesting that the sol-gel method developed in this workcould be an efficient way to synthesize PEG/SHC FSPCMs with

Table 5Encapsulation ratio (R), encapsulation efficiency (E) and thermal storage capability (η) ofPEG/SHC composites.

Sample 50% 60% 70% 80%(PEG%)

R(%) 36.1 52.7 64.0 80.4E(%) 37.0 52.2 63.8 79.9η(%) 102.4 99.1 99.7 99.3

Fig. 7. DSC curves of PEG/SHC-60% in different heating-cooling cycles.

Table 6Phase transition parameters of PEG/SHC composites in the cycle test.

Cycles Melting progress Crystallization process

Onset(°C) Peak(°C) ΔHm (J/g) Onset(°C) Peak(°C) ΔHm (J/g)

1 41.04 47.63 82.53 25.67 23.90 82.595 41.92 48.05 81.04 25.07 23.33 81.7610 42.01 48.18 80.58 25.63 23.26 81.7920 42.13 48.22 82.25 24.96 23.25 82.4930 40.70 47.55 86.35 26.04 24.25 85.53

Fig. 8. Thermal conductivity SHC and PEG/SHC composites (the circle represents thevalues of thermal conductivity and the column represents the corresponding enhance-ment).

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enhanced thermal conductivity using the silicagel industrial wastes asstarting materials.

4. Conclusions

In this work, the PEG/SHC form-stable phase change materials havebeen successfully synthesized using a facile sol-gel method, in whichthe silicagel industrial wastes have been reused for the first time toobtain the SHC network for supporting the PEG. The PEG has beenfound to be bonded with the SHC by means of strong physical inter-actions, which could encapsulate as much as 80% weight percent ofPEG in the SHC with good form stable properties. The thermal energystored in the PEG/SHC composites have been found to be (59.38–132.4)J/g and (63.56–133.4) J/g in the melting and crystallization process,respectively, as the loaded PEG weight fraction ranging from 50% to80%, which are comparable with the previous results for PEG basedFSPCMs. Also, these PEG/SHC composites have been showed to havestable thermal properties after 30 heating-cooling cycles, and thethermal conductivity of the composites has be enhanced as high as 30%compared with that of the pure PEG. On the basis of these results, it wasdemonstrated that the facile sol-gel method developed in this workcould not only obtain PEG based FSPCMs with good performance forthermal energy storage, but also propose an effective way of producingeconomic benefits by reusing industrial wastes.

Acknowledgements

This work was financially supported by National Natural ScienceFoundation of China under grant 21473198, Liaoning ProvincialNatural Science Foundation of China under grant 201602741, andDalian Institute of Chemical Physics under grant DICP ZZBS201608. Q.Shi would like to thank Hundred-Talent Program founded by ChineseAcademy of Sciences.

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