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Energy 39 (2012) 294e302

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Energy

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Thermal and electrical conductivity enhancement of graphite nanoplateletson form-stable polyethylene glycol/polymethyl methacrylate composite phasechange materials

Lei Zhang*, Jiaoqun Zhu, Weibing Zhou, Jun Wang, Yan WangKey Laboratory of Ministry of Education for Silicate Materials Science and Engineering, Wuhan University of Technology, 122 Luoshi Road, Wuhan, Hubei 430070, PR China

a r t i c l e i n f o

Article history:Received 6 September 2011Received in revised form27 December 2011Accepted 4 January 2012Available online 14 February 2012

Keywords:Graphite nanoplateletsForm-stable phase change materialSelf-supportingThermal conductivityElectrical conductivity

* Corresponding author. Tel.: þ86 15527705826; faE-mail address: zhanglei209@yahoo.cn (L. Zhang)

0360-5442/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.energy.2012.01.011

a b s t r a c t

Graphite nanoplatelets (GnPs), obtained by sonicating the expanded graphite, were employed tosimultaneously enhance the thermal (k) and electrical (s) conductivity of organic form-stable phasechange materials (FSPCMs). Using the method of in situ polymerization upon ultrasonic irradiation, GnPsserving as the conductive fillers and polyethylene glycol (PEG) acting as the phase change material (PCM)were uniformly dispersed and embedded inside the network structure of polymethyl methacrylate(PMMA), which contributed to the well package and self-supporting properties of composite FSPCMs.X-ray diffraction and Fourier transform infrared spectroscopy results indicated that the GnPs werephysically combined with PEG/PMMA matrix and did not participate in the polymerization. The GnPsadditives were able to effectively enhance the k and s of organic FSPCM. When the mass ratio of GnP was8%, the k and s of FSPCM changed up to 9 times and 8 orders of magnitude over that of PEG/PMMAmatrix, respectively. The improvements in both k and swere mainly attributed to the well dispersion andlarge aspect ratio of GnPs, which were endowed with benefit of forming conducting network in polymermatrix. It was also confirmed that all the prepared specimens possessed available thermal storagedensity and thermal stability.

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1. Introduction

Latent heat thermal energy storage (LHTES) technique canreduce the imbalance between thermal energy supply and demandby using phase change material (PCM) to store and release thermalenergy [1]. The LHTES method is usually efficient and reliableowing to the large heat storage capacity and nearly isothermalphase change behavior of PCM [2]. Because of its superior advan-tages, the LHTES has been widely applied in many fields such assolar thermal application [3], LHTES packed bed [4], buildingenergy conservation [5] and thermal management of automotiveengine [6]. Based on the present literature, a large number ofmaterials are suitable candidates for PCMs [7e10]. Among thesematerials, organic form-stable phase change materials (FSPCMs),a group of composites consisting of organic solideliquid PCMs andsupporting materials (commonly using polymer matrix), areexcellent due to their innocuity, high energy storage density, slightsupercooling, excellent machinability and direct usage advantage

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(i.e. not requiring special packaging to prevent the leakage of PCMsduring their solideliquid phase transition processes) [11e13].

In spite of their desirable properties, the overwhelmingmajorityof organic FSPCMs suffer the disadvantage of low k (0.2e0.3 Wm�1 K�1) which severely reduces the heat transfer efficiency ofLHTES systems [14], and thereby resulting in great attention beingpaid on the heat transfer enhancement of organic FSPCM [15,16].On the other hand, numerous studies have demonstrated that theelectrical conduct polymeric materials, obtained by the dispersalof conductive fillers such as carbon or metal in polymer matrix,have great potential in many applications such as electromagneticinterference (EMI) shielding materials [17], anti-electrostaticmaterials [18] and bipolar plates in proton exchange membranefuel cells [19]. So, considering these two aspects above, the simul-taneous enhancement of the k and s would effectively broaden theapplication scope of organic FSPCMs. For instance, the FSPCMswithexcellent heat transfer property could effectively control thetemperature variation rate of electronic devices [20]. When beingequipped with proper electroconductibility, these FSPCMs couldalso simultaneously act as EMI shielding materials [17] whichprotect the electronic components from the interference of EMwaves generated by electronic systems. Therefore, it might besignificant for us to make the FSPCMs more versatile.

Table 1The components of the prepared FSPCMs in detail.

FSPCM no. Composition (wt.%)

1 70 PEG þ 30 PMMA2 69.3 PEG þ 29.7 PMMA þ 1 GnP3 68.6 PEG þ 29.4 PMMA þ 2 GnP4 67.2 PEG þ 28.8 PMMA þ 4 GnP5 65.8 PEG þ 28.2 PMMA þ 6 GnP6 64.4 PEG þ 27.6 PMMA þ 8 GnP

L. Zhang et al. / Energy 39 (2012) 294e302 295

In order to improve the thermal and electrical performance ofPCMs, a considerable amount of research has been carried out on thepreparation of carbon-based composites. Carbon materials, such asgraphite powder [21], carbon nanotubes [22] and graphite nano-platelets (GnPs) [23] are suitable candidates for thermal and elec-trical conductive fillers due to their excellent thermal and electricalproperties, prominent chemical stability as well as larger specificsurface area and lower density than those of metals. Among variouscarbon fillers, GnPs, endowed with the layered structure and lowprice of nanoclays as well as the superior electrical and thermalproperties of carbonnanotubes [24] have attracted more and moreinterest all over the world. With nanometer in thickness andmicrometer in diameter, the GnPs have a large aspect ratio andpossess advantage in formingconductingnetwork inpolymermatrixcompared with traditional fillers [25]. By means of melt blending,Kim and Drzal [23] have prepared the paraffin/GnPs nanocompositewith high k, s and latent heat enthalpy. It was concluded that, thepercolation threshold for the s of paraffin/GnPs composite wasbetween 1 and 2 wt.%. However, the paraffin/GnPs compositewithout supporting materials (polymer matrix) also suffers thedisadvantages of the PCM leakage during solideliquid phase transi-tion and poor self-supporting property. And as far as we know, thereare few reports about the effects of GnPs on thermal and electricperformance of organic FSPCMs that have been published.

In this study, the GnPs, obtained by sonicating the expandedgraphite, were employed to simultaneously enhance the k and s oforganic FSPCMs for the first time. The nanodispersion polyethyleneglycol (PEG)/polymethyl methacrylate (PMMA)/GnPs compositeswere achieved via an in situ polymerization of monomer uponultrasonic irradiation in the presence of GnPs. Then, the effects ofGnPs additives on the morphology, structure and form-stableperformance, together with the thermal and electrical propertiesof the composite FSPCMs, were experimentally investigated.

2. Experimental

2.1. Materials

Concentrated sulfuric acid and fuming nitric acid intercalatedexpandable graphite (80 mesh) was supplied from QingdaoTianhe Graphite Co. Ltd. PEG (AR) with an average molecularweight of 2000 (melting point, Tm ¼ 50.9 �C; latent heat ofmelting, DHm ¼ 178.3 kJ kg�1; freezing point, Tf ¼ 35.7 �C;latent heat of freezing, DHf ¼ 160.6 kJ kg�1) was purchased fromSinopharm Chemical Reagent Co. Ltd., and used without furtherpurification. Methylmethacrylate (MMA, AR), obtained fromShanghai Jingchun Reagent Co. Ltd., was distilled thrice before use.Azobisisobutyronitrile (AIBN, AR), serving as the initiator, waspurchased from Shanghai Experiment Reagent Co. Ltd., andrecrystallized before use.

2.2. Preparation

2.2.1. Preparation of GnPsThe dried expandable graphite particles were exfoliated by

rapid heating in the muffle furnace at 900 �C for 30 s. Then, theexpanded graphite (EG), expanded up to 250 times in their initialvolume, was mixed and immersed in an alcohol solution consistingof alcohol and distilled water with a volume ratio of 7:3 for 12 h.Following, the mixture was subjected to ultrasonic irradiation witha power of 100 W for 8 h to obtain GnPs.

2.2.2. Preparation of composite FSPCMsIn this work, the PMMA serving as the supporting material was

contributed to the well package and self-supporting properties of

composite FSPCMs. Although the addition of supporting materialwas beneficial to the shape stable performance of the composite, itdegraded the thermal storage density of the FSPCM. Consequently,on the condition that the PCM could be encapsulated in the PMMAnetwork without any leakage, the mass fraction of PMMA incomposite should be minimized as much as possible. Our previousstudy indicated that the optimal mass fraction of PEG in the PEG/PMMA matrix was 70% [26]. Accordingly, the mass ratios of PEG/PMMA in all the prepared FSPCMs were determined to be 7:3. Inorder to satisfy the requirement of testing, each sample with a totalweight of 300 g was prepared, and the concrete amount of rawmaterials could be calculated according to the detailed componentsof various samples, as is shown in Table 1.

By means of in situ polymerization upon ultrasonic irradiation,FSPCMs with various mass fraction of PEG, PMMA and GnPs wereable to be prepared as follows. Firstly, the MMA and AIBN witha mass ratio of 99:1 were added into a necked flask at a constanttemperature in water bath, and the mixture was pre-polymerizedunder intense agitation for 30 min at 50 �C. Then the quantifiedamount of PEG (melted) and GnPswere slowly poured into the flaskfor the subsequent pre-polymerization upon ultrasonic irradiationunder agitation at 60 �C for 2 h to obtain the uniform mixture.Finally, the mixture was poured into the mold and molded in thevacuum drying oven at 90 �C for 1 h till the MMA polymerizedcompletely. After the above operations were finished, the nano-dispersion FSPCMs were obtained.

2.3. Characterization

The morphologies of prepared EG, GnPs and FSPCMs werecharacterized by field emission-scanning electron microscopy (FE-SEM, S-4800, Hitachi). Utilizing the techniques of X-ray diffraction(XRD, D/MAX-RB, RIGAKU) and Fourier transform infrared spec-troscopy (FTIR, Nexus-670, Thermo Nicolet), the structure andchemical properties of prepared samples were characterized,respectively.

The compressive strength of cubic FSPCMs with the side lengthof 40 mm was characterized by electronic universal testingmachine (810, MTS) at different temperatures (24 �C, heat release;55 �C, heat storage). A series of samples with the dimension of F50 mm � 25 mm were prepared for the k and s testing. The k ofprepared FSPCMs at 24 �C was determined by the thermal constantanalyzer (2500S, Hot Disk) using the transient plane sourcemethod. The s of prepared specimens was characterized by thesemiconductor characterization test system (4200, Keithley) usingthe four probe method. These measurements were repeated threetimes for each sample to obtain the average value with standarddeviation.

Thermal storage properties of prepared FSPCMs, such as Tm, Tf,DHm and DHf, were measured by differential scanning calorimeter(DSC, Pyris-1, PE). Prior to use, the calorimeter was calibrated withindium standard, and the phase change temperature and latentheat values were reproduced within �1% and �2%, respectively.The analyses were performed in the range of 0e100 �C at a heating

L. Zhang et al. / Energy 39 (2012) 294e302296

and cooling rate of 10 �C min�1 under a static nitrogen atmosphere.And each sample weighed up to 8 mg was sealed in an aluminumpan. The Tm and Tf of FSPCMs were obtained by drawing a line atthe point of maximum slope of the leading edge of the DSC curveand extrapolating the base line on the same side of the curve. More-over, the thermal stabilities of prepared FSPCMs were character-ized by thermogravimetry (TG, STA449c/3/G, NETZSCH). The TGinstrument was calibrated with zinc from 25 to 600 �C, and theprecisions of temperature and thermogravimetry were �0.1 �C and�0.1%, respectively. TG measurements were performed from roomtemperature to 600 �C at a heating rate of 10 �C min�1 in a staticnitrogen atmosphere. Sample with a mass of about 10 mg wassealed in an alumina pan.

3. Results and discussion

3.1. Morphology of expanded graphite and GnP

Fig. 1 shows the SEM images of EG and GnP. It can be seen fromFig. 1a that the EG has a worm-like appearance of its particles.Clearly, the multi-pores structure is observed from a high magni-fication (�5000) of EG shown in Fig. 1b. As shown in Fig. 1c and d,after the EGworm has been striped by ultrasonic irradiation for 8 h,the lamellar individual GnP of about 35 mm in diameter and 80 nmin thickness has been obtained. These morphologies are matchedwith the results in the previous report [25].

3.2. Morphology and form-stable performance of prepared FSPCM

Fig. 2 shows the morphology of the fractured surfaces ofprepared FSPCMs with various GnPs contents. The surface of PEG/PMMA matrix (FSPCM 1) is presented in Fig. 2a, and our previousstudy showed that the brighter network structure and smooth darkgray area on the SEM image correspond to PMMA and PEG,

Fig. 1. SEM photographs of: (a) EG (�70), (b) EG (�

respectively [26]. Clearly, the PEG was uniformly encapsulated andembedded inside the PMMA network, and this dispersion provideda mechanical strength to the composite FSPCM. Therefore, thecomposite FSPCM could maintain its shape in the solid state evenwhen the sample was heated above the melting point of the PEG.Fig. 2bef presents the morphologies of composite FSPCMs withvarious GnPs contents. The figures reveal that GnPs covered withPEG are well dispersed and enwrapped inside the PEG/PMMAmatrix. Moreover, we can easily recognize the existence of GnP byits uniform shape and particle size, even though the GnP loadingcontent is relatively low (1 and 2 wt.%). These morphologies indi-cated that the method of in situ polymerization upon ultrasonicirradiationwas an effective route to disperse GnPs into the polymermatrix.

In order to verify the encapsulating properties of the preparedFSPCMs, cycling heating tests were carried out on the samples andthe results showed that there was nearly no melted PEG thatescaped from the composites at 55 �C. The compressive strength ofprepared FSPCMs at different temperatures was also investigated toindicate their self-supporting properties and the values of whichare shown in Fig. 3. As can be seen, the compressive strengths of thecomposites decrease with the increase of the GnP content. Thelamellate GnP added into the composite might bring out its func-tion of lubrication, which speeded up the collapse of the sampleduring the test. Consequently, the compressive strengths of thesamples would be partly sacrificed by the addition of GnPs.Although the compressive strength of the samples remarkablyreduced during the heat storage stage (55 �C), all the samples couldstill provide available self-supporting properties (with compressivestrength greater than 3.7 MPa). Previous reports revealed thatGnPs, obtained from the acid intercalated flake graphite, possessedlarge amounts of oxygen-containing groups such as eCeOH,eCeOeC and eCOOH [27], entitling GnPs the capability ofabsorbing and reacting with polar molecules and polar polymers

5000), (c) GnP (�5000) and (d) GnP (�50,000).

Fig. 2. SEM photographs of composite FSPCMs with various mass fractions of GnPs: (a) 0, (b) 1%, (c) 2%, (d) 4%, (e) 6% and (f) 8%. (The arrows in the figure denoted the GnPsdispersed in the composites.)

Fig. 3. Dependence of the compressive strength of FSPCM on the GnPs contents atdifferent temperatures.

L. Zhang et al. / Energy 39 (2012) 294e302 297

(including PEG) to form graphite nanocomposites [28]. Further-more, the GnPs also possessed large specific surface area (approx-imately 17.55 m2 g�1) and a reasonable amount of macropores(50e300 nm) which would facilitate and promote the absorption ofpolymer (or monomer) [25]. These evidences indicated that theGnPs might also take positive part in the absorption and im-mobilization of liquid PEG, which contributed to the form-stableperformance of the samples. Accordingly, along with the increaseof GnP loading content, the decrease amplitude of compressivestrength at 55 �C was significantly slowing down.

3.3. Structure and chemical properties of composite FSPCM

The structures of PEG, GnPs, FSPCM 1 and FSPCM 6 were carriedout by XRD, and the results are presented in Fig. 4. As can be seenfrom the patterns of PEG/PMMA and PEG/PMMA/GnPs composites,the sharp peaks appeared at 19.1�, 23.2� and 26.9�, which indicatedthat the structure of PEG was well-preserved during the polymer-ization process. The weak hump around 14.1� could be related tothe poorly crystallized PMMA, which was obtained by free radical

Fig. 4. XRD spectra of PEG, FSPCM 1, GnPs and FSPCM 6.

Table 2The detailed compositions of the FT-IR spectrum for composite FSPCMs.a

Characteristic vibration Characteristic peaks (cm�1)

GnPsCeO stretching 1115 (1098)C]O stretching 1642 (1633)CeN stretching 2926, 2843 (2924, 2850)OeH stretching 3429 (3458)

PEGInterim eCH2e group 842 (843)Crystallization band 963 (962)CeO stretching 1062, 1115, 1243 (1060, 1114, 1239)CeH bending 1453, 1468 (1454, 1467)CeH stretch of CH2 2887 (2883)OeH stretching 3429 (3418)

PMMACeO stretching 988, 1062, 1147, 1243 (989, 1064, 1145, 1244)CeH bending 1280, 1386, 1468 (1279, 1385, 1467)C]O stretching 1642, 1730 (1638, 1731)CeH stretching 2948 (2945)OeH stretching 3429 (3437)

a Here, the data in brackets are the corresponding peaks in the pure components.

L. Zhang et al. / Energy 39 (2012) 294e302298

polymerization of MMA. In the pattern of PEG/PMMA/GnPscomposite, the sharp peaks at 26.4� and 54.5� indicated that thestructure of GnP was also well-preserved during the polymeriza-tion. Moreover, no other impurities were detected from XRDanalysis of PEG/PMMA/GnPs composite. It can be confirmed thatthe GnPs and PEG preserved their respective structural integrity onthe whole during polymerization process, and exhibited wellcompatibility with PMMA. Then the GnPs, PEG and PMMA wereable to be physically combined with each other through the in situpolymerization process.

FTIR spectra of pure GnPs, PEG, PMMA and their composite areshown in Fig. 5, and the detailed compositions of the spectra aregiven in Table 2. As can be seen, there are three oxygen-containinggroups presented on the spectrum of pure GnPs at wave numbers of1098, 1633 and 3458 cm�1, corresponding to CeO, C]O and OeH

Fig. 5. FTIR spectra of GnPs, PEG, PMMA and FSPCM 6.

stretching vibration, respectively. This result substantiated thestatement reported in the literature that acid intercalation couldresult in the oxidization of carbon bonds in the surface of graphite.And the presence of the oxygen-containing groups were supposedto be beneficial to the interaction between the polymer and GnPs.Comparing the spectra of pure GnPs, PEG, PMMA with theircomposite, it is clearly seen that the spectrum of the composite wasmainly organized by all the peaks of its individual component. Forinstance, the peaks at 1098, 1114 and 1145 cm�1 present CeOstretching vibration in GnPs, PEG and PMMA spectra, respectively,which could also be clearly seen in the spectrum of PEG/PMMA/GnPs composite. In the case of OeH stretching peaks at around3418e3458 cm�1 in the spectra of pure GnPs, PEG and PMMA, thespectrum of the composite formed an overlapping OeH stretchingpeaks of all three as shown in the spectrum. In addition, the peakpositions in the spectrum of the composite FSPCM slightly deviatedfrom the original positions in their pure spectra. For instance, thecarbonyl peaks (C]O) at 1633 and 1638 cm�1 in pure GnPs and PEGshifted to 1642 cm�1 in the composite FSPCM. These changes mightbe caused by the interactions between OeH group in PEG and thecarbonyl group in GnPs and PEG, which could form intermolecularhydrogen bonds. Moreover, compared with the spectra of pure

Fig. 6. Dependence of the k and s of composite FSPCMs on the GnPs contents.

L. Zhang et al. / Energy 39 (2012) 294e302 299

GnPs, PEG and PMMA, the spectrum of PEG/PMMA/GnPs compositehad no significant new functional groups, which indicated that nochemical reaction occurred among the components of compositeFSPCMs during the polymerization process. This observationwas inwell agreement with the above discussion of XRD results.

According to the XRD and FTIR results, the PEG, PMMA and GnPswere physically combined with each other during the polymeri-zation process. Therefore, the thermalephysical properties ofindividual components in PEG/PMMA/GnPs composite, such ashigh k and s, high latent heat enthalpy and suitable phase changetemperature, were well maintained without much degeneration.

Fig. 7. DSC thermograms o

3.4. Thermal and electrical conducting propertiesof prepared FSPCMs

In this study, GnPs with mass fractions of 1%, 2%, 4%, 6% and 8%were dispersed in the PEG/PMMA matrix respectively to establishthe relationship between the k and s of composite FSPCMs andGnPs loading contents, and the results are shown in Fig. 6. It can befound that the k of composite FSPCMs obviously increased with theincrease of GnPs loading contents. When the mass fraction of GnPwas up to 8%, the k of the composite FSPCM changed from0.253 W m�1 K�1 to 2.339 W m�1 K�1, more than 9 times of that

f composite FSPCMs.

Fig. 9. The latent heat of composite FSPCMs with respect to the GnPs contents.

L. Zhang et al. / Energy 39 (2012) 294e302300

without any additives. The improvements in the k of compositeFSPCMs were attributed to the formation of thermal conductivenetworks in the composites. As can be seen from Fig. 2, with theincrease of mass fraction of GnPs, a number of particles graduallyconnected with each other to form the conductive network in thecomposite.

The GnPs as conductive fillers could also greatly improve the s ofcomposite FSPCMs with a sharp transition from an electrical insu-lator to an electrical conductor. The s of 1 wt.% of GnP loaded wasquite low around 10�9 S cm�1. However, when the content of GnPreached 2 wt.%, the swas immediately enhanced up to 10�4 S cm�1.Hence, the percolation threshold for the s of PEG/PMMA/GnPscomposite, obtained by in situ polymerization upon ultrasonicirradiation, could be firmly determined between 1 and 2 wt.%,which was comparable to the result in the literature [23]. In addi-tion, it is known to all that the geometry and dispersion ofconductive filler are extremely critical to the conductive behavior ofthe composites. As can be deduced from the morphologies in Fig. 2,the well-dispersed GnPs with higher aspect ratios could offer greatadvantage in forming conducting network in polymer matrix, thusresulting in a much lower percolation threshold. When the massratio of GnP was 8%, the s of composite FSPCM changed up to 8orders of magnitude over that of PEG/PMMA matrix.

3.5. Thermal storage properties and thermal stabilitiesof prepared FSPCMs

The DSC thermograms of prepared FSPCMs during the meltingand freezing processes are shown in Fig. 7. These almost similarcurves indicated that the composite FSPCMs with various massfractions of GnPs exhibited similar thermal characteristics. Thephase change temperature and latent heat obtained from DSC areshown in Figs. 8 and 9, respectively. The theoretical latent heat ofcomposite FSPCMs was obtained by multiplying the latent heat ofthe pure PEG with its mass fractions in the composites according tothe theory of mixtures [21].

As can be seen in Fig. 8, the phase change temperatures of PEG inthe composite FSPCMs were significantly influenced by the addi-tion of GnPs. With the increase of the GnPs content, the Tm ofcomposite FSPCM gradually decreased, but on the contrary, the Tfhad the opposite tendency, which minimized the degree of super-cooling (the difference between Tm and Tf) of composite FSPCM to

Fig. 8. The phase change temperature and degree of supercooling of compositeFSPCMs with respect to the GnPs contents.

a great extent. These phenomena are in accordance with author’sresults about acetamide/expanded graphite composite [29]. It isbelieved that the inorganic GnPs additives with large aspect ratiosand specific surface areas could provide extra surfaces for thecrystallization of PEG [29]. The surfaces of GnPs might act as a kindof heterogeneous nucleation centers, which could be in great favorfor promoting the crystallization of PEG dispersed in the composite.The heterogeneous nucleation effect of GnPs would cause the lowergrain size of PEG [30], which is responsible for the decrease of Tm.The crystallization-promoting effect of GnPs, however, would causethe increase of Tf.

Fig. 9 shows that all the prepared samples could provide avail-able thermal storage density (larger than 114.7 kJ kg�1 and97.0 kJ kg�1 in DHm and DHf, respectively). However, the latentheats (including DHm and Hf) of PEG/PMMA/GnPs composites werepartly decreased with the increase of GnPs contents. This is becausethe addition of GnPs reduced the content of PEG, and the GnPs didnot undergo a phase change within the test temperature range of0e100 �C. Although the addition of GnP was beneficial to thermaland electrical conducting properties of composite FSPCMs, it wasalso subjected to degrade the thermal storage density of thecomposite. Therefore, much attention should be paid to the contentof GnP added in composite FSPCMs in order to meet the demand ofthe practical application. Moreover, it can be observed in Fig. 9 thatthe experimental latent heats of PEG/PMMA/GnPs composites weremore close to the theoretical values, when compared with that ofPEG/PMMA composites. This phenomenon might attribute to thecrystallization-promoting effect of GnPs which could be in greatfavor of enhancing the crystallinity of PEG dispersed in thecomposite FSPCM.

By means of TG, the thermal stability of prepared FSPCMs wasevaluated, and the results are shown in Fig. 10. According to the TGcurves, all the samples mainly degraded in two steps (Fig. 10). Thedetailed degradation data for the first step of weight loss processesis presented in Table 3. As can be seen from Table 3, the degrada-tions occurred roughly between 175 and 220 �C and ended at therange of 300e312 �C, and all the samples showed small weightlosses (lower than 8.2%) at the first stage. These degradations cor-responded to the monomer evolution initiated at the unstableterminal double bonds present in some of themacromolecules [31].Moreover, the first step weight loss of the composite FSPCMdecreased with the increase of the mass fraction of GnP. Thisindicated that the GnPs had positive effects on the thermal stabilityof the nanocomposites. Similar results could also be found in the

Fig. 10. TG curves of composite FSPCMs.

Table 3The detailed degradation data for the first step of weight loss processes.

FSPCM no. Degradation interval (�C) Mass loss (%)

1 175e302 8.22 180e301 7.93 190e300 7.84 198e308 7.15 220e304 6.26 215e312 5.8

L. Zhang et al. / Energy 39 (2012) 294e302 301

research about the GnPs/thermoplastic polyurethane nano-composites [32]. As discussed above, the presence of the oxygen-containing groups was supposed to be beneficial to the adsorptionand interaction (form intermolecular hydrogen bonds) between thepolymer (or monomer) and GnPs. Thus, it could be concluded that,the GnPs could restrain the volatilization of organic matrix andenhance the thermal stability of composite FSPCMs. The seconddegradation step, mainly as a consequence of the thermal decom-positions of PEG and PMMA, mostly took place at the range of300e312 �C and terminated around 450 �C. After the seconddegradation step, the GnP content of each composite could bechecked with weight percent of remaining materials. It is obviousthat GnPs with mass fractions of 1%, 2%, 4%, 6% and 8% were exactlyloaded in the PEG/PMMA/GnPs composite. Because the designedworking temperature of FSPCMs in the present paper was usuallybelow 80 �C, which was far less than the degradation temperatureat the first step, the prepared composites exhibit available thermalstability.

4. Conclusions

In summary, PEG/PMMA/GnPs composite FSPCMs weresuccessfully prepared by in situ polymerization upon ultrasonicirradiation. Results show that the GnPs and PEG were uniformlydispersed and embedded inside the network structure of PMMA,which contributed to the well package and self-supporting

properties of composite FSPCMs. Besides, GnPs were physicallycombined with PEG/PMMA matrix and did not participate in thepolymerization. The k and s of composite FSPCMs obviouslyenlarged with the increase of GnPs loading contents. When themass fraction of GnP was up to 8%, the k of the composite FSPCMchanged from 0.253 W m�1 K�1 to 2.339 W m�1 K�1, more than 9times of that without any additives. Meanwhile, as the filler loadingcontent changed from 0 to 8 wt.%, the s of composite FSPCMapproximately increased 8 orders of magnitude, and the percola-tion threshold of transition in electroconductibility could beachieved as long as adding merely 1e2 wt.% filler. The improve-ments in both k and sweremainly attributed to the well dispersionand large aspect ratio of GnPs, which were endowed with thebenefit of forming conducting network in polymer matrix. It wasalso proved that all the prepared samples possessed availablethermal storage density and thermal stability, and the GnPs coulddecrease the supercooling and enhance the thermal stability oforganic FSPCM. Based on the results above, it can be concluded thatthe prepared PEG/PMMA/GnPs composites have great potentialitiesin specific fields (such as EMI shielding material, anti-electrostaticmaterials and bipolar plates in proton exchange membrane fuelcells) requiring thermal management, due to their desirablethermal and electric performance.

Acknowledgments

The authors gratefully acknowledge the financial support fromthe Major State Basic Research and Development Program of China(973 Program) (No. 2010CB227100).

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