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polymethyl methacrylate composite phase change materials
Energy 39 (2012) 294e302
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Thermal and electrical conductivity enhancement of graphite nanoplatelets on form-stable polyethylene glycol/polymethyl methacrylate composite phase change materials Lei Zhang*, Jiaoqun Zhu, Weibing Zhou, Jun Wang, Yan Wang Key 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
a b s t r a c t
Article history: Received 6 September 2011 Received in revised form 27 December 2011 Accepted 4 January 2012 Available online 14 February 2012
Graphite nanoplatelets (GnPs), obtained by sonicating the expanded graphite, were employed to simultaneously enhance the thermal (k) and electrical (s) conductivity of organic form-stable phase change materials (FSPCMs). Using the method of in situ polymerization upon ultrasonic irradiation, GnPs serving 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 were physically combined with PEG/PMMA matrix and did not participate in the polymerization. The GnPs additives were able to effectively enhance the k and s of organic FSPCM. When the mass ratio of GnP was 8%, the k and s of FSPCM changed up to 9 times and 8 orders of magnitude over that of PEG/PMMA matrix, respectively. The improvements in both k and s were mainly attributed to the well dispersion and large aspect ratio of GnPs, which were endowed with benefit of forming conducting network in polymer matrix. It was also confirmed that all the prepared specimens possessed available thermal storage density and thermal stability. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Graphite nanoplatelets Form-stable phase change material Self-supporting Thermal conductivity Electrical conductivity
1. Introduction Latent heat thermal energy storage (LHTES) technique can reduce the imbalance between thermal energy supply and demand by using phase change material (PCM) to store and release thermal energy [1]. The LHTES method is usually efficient and reliable owing to the large heat storage capacity and nearly isothermal phase change behavior of PCM [2]. Because of its superior advantages, the LHTES has been widely applied in many fields such as solar thermal application [3], LHTES packed bed [4], building energy conservation [5] and thermal management of automotive engine [6]. Based on the present literature, a large number of materials are suitable candidates for PCMs [7e10]. Among these materials, organic form-stable phase change materials (FSPCMs), a group of composites consisting of organic solideliquid PCMs and supporting materials (commonly using polymer matrix), are excellent due to their innocuity, high energy storage density, slight supercooling, excellent machinability and direct usage advantage
* Corresponding author. Tel.: þ86 15527705826; fax: þ86 27 87883743. E-mail address: [email protected] (L. Zhang). 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.01.011
(i.e. not requiring special packaging to prevent the leakage of PCMs during their solideliquid phase transition processes) [11e13]. In spite of their desirable properties, the overwhelming majority of organic FSPCMs suffer the disadvantage of low k (0.2e0.3 W m1 K1) which severely reduces the heat transfer efficiency of LHTES systems [14], and thereby resulting in great attention being paid on the heat transfer enhancement of organic FSPCM [15,16]. On the other hand, numerous studies have demonstrated that the electrical conduct polymeric materials, obtained by the dispersal of conductive fillers such as carbon or metal in polymer matrix, have great potential in many applications such as electromagnetic interference (EMI) shielding materials [17], anti-electrostatic materials [18] and bipolar plates in proton exchange membrane fuel cells [19]. So, considering these two aspects above, the simultaneous enhancement of the k and s would effectively broaden the application scope of organic FSPCMs. For instance, the FSPCMs with excellent heat transfer property could effectively control the temperature variation rate of electronic devices [20]. When being equipped with proper electroconductibility, these FSPCMs could also simultaneously act as EMI shielding materials [17] which protect the electronic components from the interference of EM waves generated by electronic systems. Therefore, it might be significant for us to make the FSPCMs more versatile.
L. Zhang et al. / Energy 39 (2012) 294e302
In order to improve the thermal and electrical performance of PCMs, a considerable amount of research has been carried out on the preparation of carbon-based composites. Carbon materials, such as graphite powder [21], carbon nanotubes [22] and graphite nanoplatelets (GnPs) [23] are suitable candidates for thermal and electrical conductive fillers due to their excellent thermal and electrical properties, prominent chemical stability as well as larger specific surface area and lower density than those of metals. Among various carbon fillers, GnPs, endowed with the layered structure and low price of nanoclays as well as the superior electrical and thermal properties of carbonnanotubes [24] have attracted more and more interest all over the world. With nanometer in thickness and micrometer in diameter, the GnPs have a large aspect ratio and possess advantage in forming conducting network in polymer matrix compared with traditional fillers [25]. By means of melt blending, Kim and Drzal [23] have prepared the paraffin/GnPs nanocomposite with high k, s and latent heat enthalpy. It was concluded that, the percolation threshold for the s of paraffin/GnPs composite was between 1 and 2 wt.%. However, the paraffin/GnPs composite without supporting materials (polymer matrix) also suffers the disadvantages of the PCM leakage during solideliquid phase transition and poor self-supporting property. And as far as we know, there are few reports about the effects of GnPs on thermal and electric performance of organic FSPCMs that have been published. In this study, the GnPs, obtained by sonicating the expanded graphite, were employed to simultaneously enhance the k and s of organic FSPCMs for the first time. The nanodispersion polyethylene glycol (PEG)/polymethyl methacrylate (PMMA)/GnPs composites were achieved via an in situ polymerization of monomer upon ultrasonic irradiation in the presence of GnPs. Then, the effects of GnPs additives on the morphology, structure and form-stable performance, together with the thermal and electrical properties of the composite FSPCMs, were experimentally investigated. 2. Experimental 2.1. Materials Concentrated sulfuric acid and fuming nitric acid intercalated expandable graphite (80 mesh) was supplied from Qingdao Tianhe Graphite Co. Ltd. PEG (AR) with an average molecular weight of 2000 (melting point, Tm ¼ 50.9 C; latent heat of melting, DHm ¼ 178.3 kJ kg1; freezing point, Tf ¼ 35.7 C; latent heat of freezing, DHf ¼ 160.6 kJ kg1) was purchased from Sinopharm Chemical Reagent Co. Ltd., and used without further purification. Methylmethacrylate (MMA, AR), obtained from Shanghai Jingchun Reagent Co. Ltd., was distilled thrice before use. Azobisisobutyronitrile (AIBN, AR), serving as the initiator, was purchased from Shanghai Experiment Reagent Co. Ltd., and recrystallized before use. 2.2. Preparation 2.2.1. Preparation of GnPs The dried expandable graphite particles were exfoliated by rapid heating in the muffle furnace at 900 C for 30 s. Then, the expanded graphite (EG), expanded up to 250 times in their initial volume, was mixed and immersed in an alcohol solution consisting of alcohol and distilled water with a volume ratio of 7:3 for 12 h. Following, the mixture was subjected to ultrasonic irradiation with a power of 100 W for 8 h to obtain GnPs. 2.2.2. Preparation of composite FSPCMs In this work, the PMMA serving as the supporting material was contributed to the well package and self-supporting properties of
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Table 1 The components of the prepared FSPCMs in detail. FSPCM no.
Composition (wt.%)
1 2 3 4 5 6
70 PEG þ 30 PMMA 69.3 PEG þ 29.7 PMMA 68.6 PEG þ 29.4 PMMA 67.2 PEG þ 28.8 PMMA 65.8 PEG þ 28.2 PMMA 64.4 PEG þ 27.6 PMMA
þ þ þ þ þ
1 2 4 6 8
GnP GnP GnP GnP GnP
composite FSPCMs. Although the addition of supporting material was beneficial to the shape stable performance of the composite, it degraded the thermal storage density of the FSPCM. Consequently, on the condition that the PCM could be encapsulated in the PMMA network without any leakage, the mass fraction of PMMA in composite should be minimized as much as possible. Our previous study 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. In order to satisfy the requirement of testing, each sample with a total weight of 300 g was prepared, and the concrete amount of raw materials could be calculated according to the detailed components of 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 were able to be prepared as follows. Firstly, the MMA and AIBN with a mass ratio of 99:1 were added into a necked flask at a constant temperature in water bath, and the mixture was pre-polymerized under intense agitation for 30 min at 50 C. Then the quantified amount of PEG (melted) and GnPs were slowly poured into the flask for the subsequent pre-polymerization upon ultrasonic irradiation under agitation at 60 C for 2 h to obtain the uniform mixture. Finally, the mixture was poured into the mold and molded in the vacuum drying oven at 90 C for 1 h till the MMA polymerized completely. After the above operations were finished, the nanodispersion FSPCMs were obtained. 2.3. Characterization The morphologies of prepared EG, GnPs and FSPCMs were characterized by field emission-scanning electron microscopy (FESEM, S-4800, Hitachi). Utilizing the techniques of X-ray diffraction (XRD, D/MAX-RB, RIGAKU) and Fourier transform infrared spectroscopy (FTIR, Nexus-670, Thermo Nicolet), the structure and chemical properties of prepared samples were characterized, respectively. The compressive strength of cubic FSPCMs with the side length of 40 mm was characterized by electronic universal testing machine (810, MTS) at different temperatures (24 C, heat release; 55 C, heat storage). A series of samples with the dimension of F 50 mm 25 mm were prepared for the k and s testing. The k of prepared FSPCMs at 24 C was determined by the thermal constant analyzer (2500S, Hot Disk) using the transient plane source method. The s of prepared specimens was characterized by the semiconductor characterization test system (4200, Keithley) using the four probe method. These measurements were repeated three times for each sample to obtain the average value with standard deviation. 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 with indium standard, and the phase change temperature and latent heat values were reproduced within 1% and 2%, respectively. The analyses were performed in the range of 0e100 C at a heating
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and cooling rate of 10 C min1 under a static nitrogen atmosphere. And each sample weighed up to 8 mg was sealed in an aluminum pan. The Tm and Tf of FSPCMs were obtained by drawing a line at the point of maximum slope of the leading edge of the DSC curve and extrapolating the base line on the same side of the curve. Moreover, the thermal stabilities of prepared FSPCMs were characterized by thermogravimetry (TG, STA449c/3/G, NETZSCH). The TG instrument was calibrated with zinc from 25 to 600 C, and the precisions of temperature and thermogravimetry were 0.1 C and 0.1%, respectively. TG measurements were performed from room temperature to 600 C at a heating rate of 10 C min1 in a static nitrogen atmosphere. Sample with a mass of about 10 mg was sealed 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 from Fig. 1a that the EG has a worm-like appearance of its particles. Clearly, the multi-pores structure is observed from a high magnification (5000) of EG shown in Fig. 1b. As shown in Fig. 1c and d, after the EG worm has been striped by ultrasonic irradiation for 8 h, the lamellar individual GnP of about 35 mm in diameter and 80 nm in thickness has been obtained. These morphologies are matched with 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 of prepared FSPCMs with various GnPs contents. The surface of PEG/ PMMA matrix (FSPCM 1) is presented in Fig. 2a, and our previous study showed that the brighter network structure and smooth dark gray area on the SEM image correspond to PMMA and PEG,
respectively [26]. Clearly, the PEG was uniformly encapsulated and embedded inside the PMMA network, and this dispersion provided a mechanical strength to the composite FSPCM. Therefore, the composite FSPCM could maintain its shape in the solid state even when the sample was heated above the melting point of the PEG. Fig. 2bef presents the morphologies of composite FSPCMs with various GnPs contents. The figures reveal that GnPs covered with PEG are well dispersed and enwrapped inside the PEG/PMMA matrix. Moreover, we can easily recognize the existence of GnP by its uniform shape and particle size, even though the GnP loading content is relatively low (1 and 2 wt.%). These morphologies indicated that the method of in situ polymerization upon ultrasonic irradiation was an effective route to disperse GnPs into the polymer matrix. In order to verify the encapsulating properties of the prepared FSPCMs, cycling heating tests were carried out on the samples and the results showed that there was nearly no melted PEG that escaped from the composites at 55 C. The compressive strength of prepared FSPCMs at different temperatures was also investigated to indicate their self-supporting properties and the values of which are shown in Fig. 3. As can be seen, the compressive strengths of the composites decrease with the increase of the GnP content. The lamellate GnP added into the composite might bring out its function of lubrication, which speeded up the collapse of the sample during the test. Consequently, the compressive strengths of the samples would be partly sacrificed by the addition of GnPs. Although the compressive strength of the samples remarkably reduced during the heat storage stage (55 C), all the samples could still provide available self-supporting properties (with compressive strength greater than 3.7 MPa). Previous reports revealed that GnPs, obtained from the acid intercalated flake graphite, possessed large amounts of oxygen-containing groups such as eCeOH, eCeOeC and eCOOH [27], entitling GnPs the capability of absorbing and reacting with polar molecules and polar polymers
Fig. 1. SEM photographs of: (a) EG (70), (b) EG (5000), (c) GnP (5000) and (d) GnP (50,000).
L. Zhang et al. / Energy 39 (2012) 294e302
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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 GnPs dispersed in the composites.)
(including PEG) to form graphite nanocomposites [28]. Furthermore, the GnPs also possessed large specific surface area (approximately 17.55 m2 g1) and a reasonable amount of macropores (50e300 nm) which would facilitate and promote the absorption of polymer (or monomer) [25]. These evidences indicated that the GnPs might also take positive part in the absorption and immobilization of liquid PEG, which contributed to the form-stable performance of the samples. Accordingly, along with the increase of GnP loading content, the decrease amplitude of compressive strength at 55 C was significantly slowing down. 3.3. Structure and chemical properties of composite FSPCM
Fig. 3. Dependence of the compressive strength of FSPCM on the GnPs contents at different temperatures.
The structures of PEG, GnPs, FSPCM 1 and FSPCM 6 were carried out by XRD, and the results are presented in Fig. 4. As can be seen from the patterns of PEG/PMMA and PEG/PMMA/GnPs composites, the sharp peaks appeared at 19.1, 23.2 and 26.9 , which indicated that the structure of PEG was well-preserved during the polymerization process. The weak hump around 14.1 could be related to the poorly crystallized PMMA, which was obtained by free radical
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L. Zhang et al. / Energy 39 (2012) 294e302 Table 2 The detailed compositions of the FT-IR spectrum for composite FSPCMs.a Characteristic vibration GnPs CeO stretching C]O stretching CeN stretching OeH stretching PEG Interim eCH2e group Crystallization band CeO stretching CeH bending CeH stretch of CH2 OeH stretching PMMA CeO stretching CeH bending C]O stretching CeH stretching OeH stretching a
Fig. 4. XRD spectra of PEG, FSPCM 1, GnPs and FSPCM 6.
polymerization of MMA. In the pattern of PEG/PMMA/GnPs composite, the sharp peaks at 26.4 and 54.5 indicated that the structure of GnP was also well-preserved during the polymerization. Moreover, no other impurities were detected from XRD analysis of PEG/PMMA/GnPs composite. It can be confirmed that the GnPs and PEG preserved their respective structural integrity on the whole during polymerization process, and exhibited well compatibility with PMMA. Then the GnPs, PEG and PMMA were able to be physically combined with each other through the in situ polymerization process. FTIR spectra of pure GnPs, PEG, PMMA and their composite are shown in Fig. 5, and the detailed compositions of the spectra are given in Table 2. As can be seen, there are three oxygen-containing groups presented on the spectrum of pure GnPs at wave numbers of 1098, 1633 and 3458 cm1, corresponding to CeO, C]O and OeH
Fig. 5. FTIR spectra of GnPs, PEG, PMMA and FSPCM 6.
Characteristic peaks (cm1) 1115 (1098) 1642 (1633) 2926, 2843 (2924, 2850) 3429 (3458) 842 (843) 963 (962) 1062, 1115, 1243 (1060, 1114, 1239) 1453, 1468 (1454, 1467) 2887 (2883) 3429 (3418) 988, 1062, 1147, 1243 (989, 1064, 1145, 1244) 1280, 1386, 1468 (1279, 1385, 1467) 1642, 1730 (1638, 1731) 2948 (2945) 3429 (3437)
Here, the data in brackets are the corresponding peaks in the pure components.
stretching vibration, respectively. This result substantiated the statement reported in the literature that acid intercalation could result in the oxidization of carbon bonds in the surface of graphite. And the presence of the oxygen-containing groups were supposed to be beneficial to the interaction between the polymer and GnPs. Comparing the spectra of pure GnPs, PEG, PMMA with their composite, it is clearly seen that the spectrum of the composite was mainly organized by all the peaks of its individual component. For instance, the peaks at 1098, 1114 and 1145 cm1 present CeO stretching 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 around 3418e3458 cm1 in the spectra of pure GnPs, PEG and PMMA, the spectrum of the composite formed an overlapping OeH stretching peaks of all three as shown in the spectrum. In addition, the peak positions in the spectrum of the composite FSPCM slightly deviated from the original positions in their pure spectra. For instance, the carbonyl peaks (C]O) at 1633 and 1638 cm1 in pure GnPs and PEG shifted to 1642 cm1 in the composite FSPCM. These changes might be caused by the interactions between OeH group in PEG and the carbonyl group in GnPs and PEG, which could form intermolecular hydrogen 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
GnPs, PEG and PMMA, the spectrum of PEG/PMMA/GnPs composite had no significant new functional groups, which indicated that no chemical reaction occurred among the components of composite FSPCMs during the polymerization process. This observation was in well agreement with the above discussion of XRD results. According to the XRD and FTIR results, the PEG, PMMA and GnPs were physically combined with each other during the polymerization process. Therefore, the thermalephysical properties of individual components in PEG/PMMA/GnPs composite, such as high k and s, high latent heat enthalpy and suitable phase change temperature, were well maintained without much degeneration.
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3.4. Thermal and electrical conducting properties of 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 establish the relationship between the k and s of composite FSPCMs and GnPs loading contents, and the results are shown in Fig. 6. It can be found that the k of composite FSPCMs obviously increased with the increase of GnPs loading contents. When the mass fraction of GnP was up to 8%, the k of the composite FSPCM changed from 0.253 W m1 K1 to 2.339 W m1 K1, more than 9 times of that
Fig. 7. DSC thermograms of composite FSPCMs.
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without any additives. The improvements in the k of composite FSPCMs were attributed to the formation of thermal conductive networks in the composites. As can be seen from Fig. 2, with the increase of mass fraction of GnPs, a number of particles gradually connected with each other to form the conductive network in the composite. The GnPs as conductive fillers could also greatly improve the s of composite FSPCMs with a sharp transition from an electrical insulator to an electrical conductor. The s of 1 wt.% of GnP loaded was quite low around 109 S cm1. However, when the content of GnP reached 2 wt.%, the s was immediately enhanced up to 104 S cm1. Hence, the percolation threshold for the s of PEG/PMMA/GnPs composite, obtained by in situ polymerization upon ultrasonic irradiation, could be firmly determined between 1 and 2 wt.%, which was comparable to the result in the literature [23]. In addition, it is known to all that the geometry and dispersion of conductive filler are extremely critical to the conductive behavior of the composites. As can be deduced from the morphologies in Fig. 2, the well-dispersed GnPs with higher aspect ratios could offer great advantage in forming conducting network in polymer matrix, thus resulting in a much lower percolation threshold. When the mass ratio of GnP was 8%, the s of composite FSPCM changed up to 8 orders of magnitude over that of PEG/PMMA matrix. 3.5. Thermal storage properties and thermal stabilities of prepared FSPCMs The DSC thermograms of prepared FSPCMs during the melting and freezing processes are shown in Fig. 7. These almost similar curves indicated that the composite FSPCMs with various mass fractions of GnPs exhibited similar thermal characteristics. The phase change temperature and latent heat obtained from DSC are shown in Figs. 8 and 9, respectively. The theoretical latent heat of composite FSPCMs was obtained by multiplying the latent heat of the pure PEG with its mass fractions in the composites according to the theory of mixtures [21]. As can be seen in Fig. 8, the phase change temperatures of PEG in the composite FSPCMs were significantly influenced by the addition of GnPs. With the increase of the GnPs content, the Tm of composite FSPCM gradually decreased, but on the contrary, the Tf had the opposite tendency, which minimized the degree of supercooling (the difference between Tm and Tf) of composite FSPCM to
Fig. 8. The phase change temperature and degree of supercooling of composite FSPCMs with respect to the GnPs contents.
Fig. 9. The latent heat of composite FSPCMs with respect to the GnPs contents.
a great extent. These phenomena are in accordance with author’s results about acetamide/expanded graphite composite [29]. It is believed that the inorganic GnPs additives with large aspect ratios and specific surface areas could provide extra surfaces for the crystallization of PEG [29]. The surfaces of GnPs might act as a kind of heterogeneous nucleation centers, which could be in great favor for promoting the crystallization of PEG dispersed in the composite. The heterogeneous nucleation effect of GnPs would cause the lower grain size of PEG [30], which is responsible for the decrease of Tm. The crystallization-promoting effect of GnPs, however, would cause the increase of Tf. Fig. 9 shows that all the prepared samples could provide available thermal storage density (larger than 114.7 kJ kg1 and 97.0 kJ kg1 in DHm and DHf, respectively). However, the latent heats (including DHm and Hf) of PEG/PMMA/GnPs composites were partly decreased with the increase of GnPs contents. This is because the addition of GnPs reduced the content of PEG, and the GnPs did not undergo a phase change within the test temperature range of 0e100 C. Although the addition of GnP was beneficial to thermal and electrical conducting properties of composite FSPCMs, it was also subjected to degrade the thermal storage density of the composite. Therefore, much attention should be paid to the content of GnP added in composite FSPCMs in order to meet the demand of the practical application. Moreover, it can be observed in Fig. 9 that the experimental latent heats of PEG/PMMA/GnPs composites were more close to the theoretical values, when compared with that of PEG/PMMA composites. This phenomenon might attribute to the crystallization-promoting effect of GnPs which could be in great favor of enhancing the crystallinity of PEG dispersed in the composite FSPCM. By means of TG, the thermal stability of prepared FSPCMs was evaluated, and the results are shown in Fig. 10. According to the TG curves, all the samples mainly degraded in two steps (Fig. 10). The detailed degradation data for the first step of weight loss processes is presented in Table 3. As can be seen from Table 3, the degradations occurred roughly between 175 and 220 C and ended at the range of 300e312 C, and all the samples showed small weight losses (lower than 8.2%) at the first stage. These degradations corresponded to the monomer evolution initiated at the unstable terminal double bonds present in some of the macromolecules [31]. Moreover, the first step weight loss of the composite FSPCM decreased with the increase of the mass fraction of GnP. This indicated that the GnPs had positive effects on the thermal stability of the nanocomposites. Similar results could also be found in the
L. Zhang et al. / Energy 39 (2012) 294e302
301
Fig. 10. TG curves of composite FSPCMs.
Table 3 The detailed degradation data for the first step of weight loss processes. FSPCM no.
Degradation interval ( C)
Mass loss (%)
1 2 3 4 5 6
175e302 180e301 190e300 198e308 220e304 215e312
8.2 7.9 7.8 7.1 6.2 5.8
research about the GnPs/thermoplastic polyurethane nanocomposites [32]. As discussed above, the presence of the oxygencontaining groups was supposed to be beneficial to the adsorption and interaction (form intermolecular hydrogen bonds) between the polymer (or monomer) and GnPs. Thus, it could be concluded that, the GnPs could restrain the volatilization of organic matrix and enhance the thermal stability of composite FSPCMs. The second degradation step, mainly as a consequence of the thermal decompositions of PEG and PMMA, mostly took place at the range of 300e312 C and terminated around 450 C. After the second degradation step, the GnP content of each composite could be checked with weight percent of remaining materials. It is obvious that GnPs with mass fractions of 1%, 2%, 4%, 6% and 8% were exactly loaded in the PEG/PMMA/GnPs composite. Because the designed working temperature of FSPCMs in the present paper was usually below 80 C, which was far less than the degradation temperature at the first step, the prepared composites exhibit available thermal stability.
properties of composite FSPCMs. Besides, GnPs were physically combined with PEG/PMMA matrix and did not participate in the polymerization. The k and s of composite FSPCMs obviously enlarged with the increase of GnPs loading contents. When the mass fraction of GnP was up to 8%, the k of the composite FSPCM changed from 0.253 W m1 K1 to 2.339 W m1 K1, more than 9 times of that without any additives. Meanwhile, as the filler loading content changed from 0 to 8 wt.%, the s of composite FSPCM approximately increased 8 orders of magnitude, and the percolation threshold of transition in electroconductibility could be achieved as long as adding merely 1e2 wt.% filler. The improvements in both k and s were mainly attributed to the well dispersion and large aspect ratio of GnPs, which were endowed with the benefit of forming conducting network in polymer matrix. It was also proved that all the prepared samples possessed available thermal storage density and thermal stability, and the GnPs could decrease the supercooling and enhance the thermal stability of organic FSPCM. Based on the results above, it can be concluded that the prepared PEG/PMMA/GnPs composites have great potentialities in specific fields (such as EMI shielding material, anti-electrostatic materials and bipolar plates in proton exchange membrane fuel cells) requiring thermal management, due to their desirable thermal and electric performance. Acknowledgments The authors gratefully acknowledge the financial support from the Major State Basic Research and Development Program of China (973 Program) (No. 2010CB227100). References
4. Conclusions In summary, PEG/PMMA/GnPs composite FSPCMs were successfully prepared by in situ polymerization upon ultrasonic irradiation. Results show that the GnPs and PEG were uniformly dispersed and embedded inside the network structure of PMMA, which contributed to the well package and self-supporting
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Thermal and electrical conductivity enhancement of graphite nanoplatelets on form-stable polyethylene glycol/polymethyl methacrylate composite phase change materials Lei Zhang*, Jiaoqun Zhu, Weibing Zhou, Jun Wang, Yan Wang Key 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
a b s t r a c t
Article history: Received 6 September 2011 Received in revised form 27 December 2011 Accepted 4 January 2012 Available online 14 February 2012
Graphite nanoplatelets (GnPs), obtained by sonicating the expanded graphite, were employed to simultaneously enhance the thermal (k) and electrical (s) conductivity of organic form-stable phase change materials (FSPCMs). Using the method of in situ polymerization upon ultrasonic irradiation, GnPs serving 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 were physically combined with PEG/PMMA matrix and did not participate in the polymerization. The GnPs additives were able to effectively enhance the k and s of organic FSPCM. When the mass ratio of GnP was 8%, the k and s of FSPCM changed up to 9 times and 8 orders of magnitude over that of PEG/PMMA matrix, respectively. The improvements in both k and s were mainly attributed to the well dispersion and large aspect ratio of GnPs, which were endowed with benefit of forming conducting network in polymer matrix. It was also confirmed that all the prepared specimens possessed available thermal storage density and thermal stability. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Graphite nanoplatelets Form-stable phase change material Self-supporting Thermal conductivity Electrical conductivity
1. Introduction Latent heat thermal energy storage (LHTES) technique can reduce the imbalance between thermal energy supply and demand by using phase change material (PCM) to store and release thermal energy [1]. The LHTES method is usually efficient and reliable owing to the large heat storage capacity and nearly isothermal phase change behavior of PCM [2]. Because of its superior advantages, the LHTES has been widely applied in many fields such as solar thermal application [3], LHTES packed bed [4], building energy conservation [5] and thermal management of automotive engine [6]. Based on the present literature, a large number of materials are suitable candidates for PCMs [7e10]. Among these materials, organic form-stable phase change materials (FSPCMs), a group of composites consisting of organic solideliquid PCMs and supporting materials (commonly using polymer matrix), are excellent due to their innocuity, high energy storage density, slight supercooling, excellent machinability and direct usage advantage
* Corresponding author. Tel.: þ86 15527705826; fax: þ86 27 87883743. E-mail address: [email protected] (L. Zhang). 0360-5442/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2012.01.011
(i.e. not requiring special packaging to prevent the leakage of PCMs during their solideliquid phase transition processes) [11e13]. In spite of their desirable properties, the overwhelming majority of organic FSPCMs suffer the disadvantage of low k (0.2e0.3 W m1 K1) which severely reduces the heat transfer efficiency of LHTES systems [14], and thereby resulting in great attention being paid on the heat transfer enhancement of organic FSPCM [15,16]. On the other hand, numerous studies have demonstrated that the electrical conduct polymeric materials, obtained by the dispersal of conductive fillers such as carbon or metal in polymer matrix, have great potential in many applications such as electromagnetic interference (EMI) shielding materials [17], anti-electrostatic materials [18] and bipolar plates in proton exchange membrane fuel cells [19]. So, considering these two aspects above, the simultaneous enhancement of the k and s would effectively broaden the application scope of organic FSPCMs. For instance, the FSPCMs with excellent heat transfer property could effectively control the temperature variation rate of electronic devices [20]. When being equipped with proper electroconductibility, these FSPCMs could also simultaneously act as EMI shielding materials [17] which protect the electronic components from the interference of EM waves generated by electronic systems. Therefore, it might be significant for us to make the FSPCMs more versatile.
L. Zhang et al. / Energy 39 (2012) 294e302
In order to improve the thermal and electrical performance of PCMs, a considerable amount of research has been carried out on the preparation of carbon-based composites. Carbon materials, such as graphite powder [21], carbon nanotubes [22] and graphite nanoplatelets (GnPs) [23] are suitable candidates for thermal and electrical conductive fillers due to their excellent thermal and electrical properties, prominent chemical stability as well as larger specific surface area and lower density than those of metals. Among various carbon fillers, GnPs, endowed with the layered structure and low price of nanoclays as well as the superior electrical and thermal properties of carbonnanotubes [24] have attracted more and more interest all over the world. With nanometer in thickness and micrometer in diameter, the GnPs have a large aspect ratio and possess advantage in forming conducting network in polymer matrix compared with traditional fillers [25]. By means of melt blending, Kim and Drzal [23] have prepared the paraffin/GnPs nanocomposite with high k, s and latent heat enthalpy. It was concluded that, the percolation threshold for the s of paraffin/GnPs composite was between 1 and 2 wt.%. However, the paraffin/GnPs composite without supporting materials (polymer matrix) also suffers the disadvantages of the PCM leakage during solideliquid phase transition and poor self-supporting property. And as far as we know, there are few reports about the effects of GnPs on thermal and electric performance of organic FSPCMs that have been published. In this study, the GnPs, obtained by sonicating the expanded graphite, were employed to simultaneously enhance the k and s of organic FSPCMs for the first time. The nanodispersion polyethylene glycol (PEG)/polymethyl methacrylate (PMMA)/GnPs composites were achieved via an in situ polymerization of monomer upon ultrasonic irradiation in the presence of GnPs. Then, the effects of GnPs additives on the morphology, structure and form-stable performance, together with the thermal and electrical properties of the composite FSPCMs, were experimentally investigated. 2. Experimental 2.1. Materials Concentrated sulfuric acid and fuming nitric acid intercalated expandable graphite (80 mesh) was supplied from Qingdao Tianhe Graphite Co. Ltd. PEG (AR) with an average molecular weight of 2000 (melting point, Tm ¼ 50.9 C; latent heat of melting, DHm ¼ 178.3 kJ kg1; freezing point, Tf ¼ 35.7 C; latent heat of freezing, DHf ¼ 160.6 kJ kg1) was purchased from Sinopharm Chemical Reagent Co. Ltd., and used without further purification. Methylmethacrylate (MMA, AR), obtained from Shanghai Jingchun Reagent Co. Ltd., was distilled thrice before use. Azobisisobutyronitrile (AIBN, AR), serving as the initiator, was purchased from Shanghai Experiment Reagent Co. Ltd., and recrystallized before use. 2.2. Preparation 2.2.1. Preparation of GnPs The dried expandable graphite particles were exfoliated by rapid heating in the muffle furnace at 900 C for 30 s. Then, the expanded graphite (EG), expanded up to 250 times in their initial volume, was mixed and immersed in an alcohol solution consisting of alcohol and distilled water with a volume ratio of 7:3 for 12 h. Following, the mixture was subjected to ultrasonic irradiation with a power of 100 W for 8 h to obtain GnPs. 2.2.2. Preparation of composite FSPCMs In this work, the PMMA serving as the supporting material was contributed to the well package and self-supporting properties of
295
Table 1 The components of the prepared FSPCMs in detail. FSPCM no.
Composition (wt.%)
1 2 3 4 5 6
70 PEG þ 30 PMMA 69.3 PEG þ 29.7 PMMA 68.6 PEG þ 29.4 PMMA 67.2 PEG þ 28.8 PMMA 65.8 PEG þ 28.2 PMMA 64.4 PEG þ 27.6 PMMA
þ þ þ þ þ
1 2 4 6 8
GnP GnP GnP GnP GnP
composite FSPCMs. Although the addition of supporting material was beneficial to the shape stable performance of the composite, it degraded the thermal storage density of the FSPCM. Consequently, on the condition that the PCM could be encapsulated in the PMMA network without any leakage, the mass fraction of PMMA in composite should be minimized as much as possible. Our previous study 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. In order to satisfy the requirement of testing, each sample with a total weight of 300 g was prepared, and the concrete amount of raw materials could be calculated according to the detailed components of 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 were able to be prepared as follows. Firstly, the MMA and AIBN with a mass ratio of 99:1 were added into a necked flask at a constant temperature in water bath, and the mixture was pre-polymerized under intense agitation for 30 min at 50 C. Then the quantified amount of PEG (melted) and GnPs were slowly poured into the flask for the subsequent pre-polymerization upon ultrasonic irradiation under agitation at 60 C for 2 h to obtain the uniform mixture. Finally, the mixture was poured into the mold and molded in the vacuum drying oven at 90 C for 1 h till the MMA polymerized completely. After the above operations were finished, the nanodispersion FSPCMs were obtained. 2.3. Characterization The morphologies of prepared EG, GnPs and FSPCMs were characterized by field emission-scanning electron microscopy (FESEM, S-4800, Hitachi). Utilizing the techniques of X-ray diffraction (XRD, D/MAX-RB, RIGAKU) and Fourier transform infrared spectroscopy (FTIR, Nexus-670, Thermo Nicolet), the structure and chemical properties of prepared samples were characterized, respectively. The compressive strength of cubic FSPCMs with the side length of 40 mm was characterized by electronic universal testing machine (810, MTS) at different temperatures (24 C, heat release; 55 C, heat storage). A series of samples with the dimension of F 50 mm 25 mm were prepared for the k and s testing. The k of prepared FSPCMs at 24 C was determined by the thermal constant analyzer (2500S, Hot Disk) using the transient plane source method. The s of prepared specimens was characterized by the semiconductor characterization test system (4200, Keithley) using the four probe method. These measurements were repeated three times for each sample to obtain the average value with standard deviation. 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 with indium standard, and the phase change temperature and latent heat values were reproduced within 1% and 2%, respectively. The analyses were performed in the range of 0e100 C at a heating
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and cooling rate of 10 C min1 under a static nitrogen atmosphere. And each sample weighed up to 8 mg was sealed in an aluminum pan. The Tm and Tf of FSPCMs were obtained by drawing a line at the point of maximum slope of the leading edge of the DSC curve and extrapolating the base line on the same side of the curve. Moreover, the thermal stabilities of prepared FSPCMs were characterized by thermogravimetry (TG, STA449c/3/G, NETZSCH). The TG instrument was calibrated with zinc from 25 to 600 C, and the precisions of temperature and thermogravimetry were 0.1 C and 0.1%, respectively. TG measurements were performed from room temperature to 600 C at a heating rate of 10 C min1 in a static nitrogen atmosphere. Sample with a mass of about 10 mg was sealed 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 from Fig. 1a that the EG has a worm-like appearance of its particles. Clearly, the multi-pores structure is observed from a high magnification (5000) of EG shown in Fig. 1b. As shown in Fig. 1c and d, after the EG worm has been striped by ultrasonic irradiation for 8 h, the lamellar individual GnP of about 35 mm in diameter and 80 nm in thickness has been obtained. These morphologies are matched with 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 of prepared FSPCMs with various GnPs contents. The surface of PEG/ PMMA matrix (FSPCM 1) is presented in Fig. 2a, and our previous study showed that the brighter network structure and smooth dark gray area on the SEM image correspond to PMMA and PEG,
respectively [26]. Clearly, the PEG was uniformly encapsulated and embedded inside the PMMA network, and this dispersion provided a mechanical strength to the composite FSPCM. Therefore, the composite FSPCM could maintain its shape in the solid state even when the sample was heated above the melting point of the PEG. Fig. 2bef presents the morphologies of composite FSPCMs with various GnPs contents. The figures reveal that GnPs covered with PEG are well dispersed and enwrapped inside the PEG/PMMA matrix. Moreover, we can easily recognize the existence of GnP by its uniform shape and particle size, even though the GnP loading content is relatively low (1 and 2 wt.%). These morphologies indicated that the method of in situ polymerization upon ultrasonic irradiation was an effective route to disperse GnPs into the polymer matrix. In order to verify the encapsulating properties of the prepared FSPCMs, cycling heating tests were carried out on the samples and the results showed that there was nearly no melted PEG that escaped from the composites at 55 C. The compressive strength of prepared FSPCMs at different temperatures was also investigated to indicate their self-supporting properties and the values of which are shown in Fig. 3. As can be seen, the compressive strengths of the composites decrease with the increase of the GnP content. The lamellate GnP added into the composite might bring out its function of lubrication, which speeded up the collapse of the sample during the test. Consequently, the compressive strengths of the samples would be partly sacrificed by the addition of GnPs. Although the compressive strength of the samples remarkably reduced during the heat storage stage (55 C), all the samples could still provide available self-supporting properties (with compressive strength greater than 3.7 MPa). Previous reports revealed that GnPs, obtained from the acid intercalated flake graphite, possessed large amounts of oxygen-containing groups such as eCeOH, eCeOeC and eCOOH [27], entitling GnPs the capability of absorbing and reacting with polar molecules and polar polymers
Fig. 1. SEM photographs of: (a) EG (70), (b) EG (5000), (c) GnP (5000) and (d) GnP (50,000).
L. Zhang et al. / Energy 39 (2012) 294e302
297
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 GnPs dispersed in the composites.)
(including PEG) to form graphite nanocomposites [28]. Furthermore, the GnPs also possessed large specific surface area (approximately 17.55 m2 g1) and a reasonable amount of macropores (50e300 nm) which would facilitate and promote the absorption of polymer (or monomer) [25]. These evidences indicated that the GnPs might also take positive part in the absorption and immobilization of liquid PEG, which contributed to the form-stable performance of the samples. Accordingly, along with the increase of GnP loading content, the decrease amplitude of compressive strength at 55 C was significantly slowing down. 3.3. Structure and chemical properties of composite FSPCM
Fig. 3. Dependence of the compressive strength of FSPCM on the GnPs contents at different temperatures.
The structures of PEG, GnPs, FSPCM 1 and FSPCM 6 were carried out by XRD, and the results are presented in Fig. 4. As can be seen from the patterns of PEG/PMMA and PEG/PMMA/GnPs composites, the sharp peaks appeared at 19.1, 23.2 and 26.9 , which indicated that the structure of PEG was well-preserved during the polymerization process. The weak hump around 14.1 could be related to the poorly crystallized PMMA, which was obtained by free radical
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L. Zhang et al. / Energy 39 (2012) 294e302 Table 2 The detailed compositions of the FT-IR spectrum for composite FSPCMs.a Characteristic vibration GnPs CeO stretching C]O stretching CeN stretching OeH stretching PEG Interim eCH2e group Crystallization band CeO stretching CeH bending CeH stretch of CH2 OeH stretching PMMA CeO stretching CeH bending C]O stretching CeH stretching OeH stretching a
Fig. 4. XRD spectra of PEG, FSPCM 1, GnPs and FSPCM 6.
polymerization of MMA. In the pattern of PEG/PMMA/GnPs composite, the sharp peaks at 26.4 and 54.5 indicated that the structure of GnP was also well-preserved during the polymerization. Moreover, no other impurities were detected from XRD analysis of PEG/PMMA/GnPs composite. It can be confirmed that the GnPs and PEG preserved their respective structural integrity on the whole during polymerization process, and exhibited well compatibility with PMMA. Then the GnPs, PEG and PMMA were able to be physically combined with each other through the in situ polymerization process. FTIR spectra of pure GnPs, PEG, PMMA and their composite are shown in Fig. 5, and the detailed compositions of the spectra are given in Table 2. As can be seen, there are three oxygen-containing groups presented on the spectrum of pure GnPs at wave numbers of 1098, 1633 and 3458 cm1, corresponding to CeO, C]O and OeH
Fig. 5. FTIR spectra of GnPs, PEG, PMMA and FSPCM 6.
Characteristic peaks (cm1) 1115 (1098) 1642 (1633) 2926, 2843 (2924, 2850) 3429 (3458) 842 (843) 963 (962) 1062, 1115, 1243 (1060, 1114, 1239) 1453, 1468 (1454, 1467) 2887 (2883) 3429 (3418) 988, 1062, 1147, 1243 (989, 1064, 1145, 1244) 1280, 1386, 1468 (1279, 1385, 1467) 1642, 1730 (1638, 1731) 2948 (2945) 3429 (3437)
Here, the data in brackets are the corresponding peaks in the pure components.
stretching vibration, respectively. This result substantiated the statement reported in the literature that acid intercalation could result in the oxidization of carbon bonds in the surface of graphite. And the presence of the oxygen-containing groups were supposed to be beneficial to the interaction between the polymer and GnPs. Comparing the spectra of pure GnPs, PEG, PMMA with their composite, it is clearly seen that the spectrum of the composite was mainly organized by all the peaks of its individual component. For instance, the peaks at 1098, 1114 and 1145 cm1 present CeO stretching 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 around 3418e3458 cm1 in the spectra of pure GnPs, PEG and PMMA, the spectrum of the composite formed an overlapping OeH stretching peaks of all three as shown in the spectrum. In addition, the peak positions in the spectrum of the composite FSPCM slightly deviated from the original positions in their pure spectra. For instance, the carbonyl peaks (C]O) at 1633 and 1638 cm1 in pure GnPs and PEG shifted to 1642 cm1 in the composite FSPCM. These changes might be caused by the interactions between OeH group in PEG and the carbonyl group in GnPs and PEG, which could form intermolecular hydrogen 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
GnPs, PEG and PMMA, the spectrum of PEG/PMMA/GnPs composite had no significant new functional groups, which indicated that no chemical reaction occurred among the components of composite FSPCMs during the polymerization process. This observation was in well agreement with the above discussion of XRD results. According to the XRD and FTIR results, the PEG, PMMA and GnPs were physically combined with each other during the polymerization process. Therefore, the thermalephysical properties of individual components in PEG/PMMA/GnPs composite, such as high k and s, high latent heat enthalpy and suitable phase change temperature, were well maintained without much degeneration.
299
3.4. Thermal and electrical conducting properties of 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 establish the relationship between the k and s of composite FSPCMs and GnPs loading contents, and the results are shown in Fig. 6. It can be found that the k of composite FSPCMs obviously increased with the increase of GnPs loading contents. When the mass fraction of GnP was up to 8%, the k of the composite FSPCM changed from 0.253 W m1 K1 to 2.339 W m1 K1, more than 9 times of that
Fig. 7. DSC thermograms of composite FSPCMs.
300
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without any additives. The improvements in the k of composite FSPCMs were attributed to the formation of thermal conductive networks in the composites. As can be seen from Fig. 2, with the increase of mass fraction of GnPs, a number of particles gradually connected with each other to form the conductive network in the composite. The GnPs as conductive fillers could also greatly improve the s of composite FSPCMs with a sharp transition from an electrical insulator to an electrical conductor. The s of 1 wt.% of GnP loaded was quite low around 109 S cm1. However, when the content of GnP reached 2 wt.%, the s was immediately enhanced up to 104 S cm1. Hence, the percolation threshold for the s of PEG/PMMA/GnPs composite, obtained by in situ polymerization upon ultrasonic irradiation, could be firmly determined between 1 and 2 wt.%, which was comparable to the result in the literature [23]. In addition, it is known to all that the geometry and dispersion of conductive filler are extremely critical to the conductive behavior of the composites. As can be deduced from the morphologies in Fig. 2, the well-dispersed GnPs with higher aspect ratios could offer great advantage in forming conducting network in polymer matrix, thus resulting in a much lower percolation threshold. When the mass ratio of GnP was 8%, the s of composite FSPCM changed up to 8 orders of magnitude over that of PEG/PMMA matrix. 3.5. Thermal storage properties and thermal stabilities of prepared FSPCMs The DSC thermograms of prepared FSPCMs during the melting and freezing processes are shown in Fig. 7. These almost similar curves indicated that the composite FSPCMs with various mass fractions of GnPs exhibited similar thermal characteristics. The phase change temperature and latent heat obtained from DSC are shown in Figs. 8 and 9, respectively. The theoretical latent heat of composite FSPCMs was obtained by multiplying the latent heat of the pure PEG with its mass fractions in the composites according to the theory of mixtures [21]. As can be seen in Fig. 8, the phase change temperatures of PEG in the composite FSPCMs were significantly influenced by the addition of GnPs. With the increase of the GnPs content, the Tm of composite FSPCM gradually decreased, but on the contrary, the Tf had the opposite tendency, which minimized the degree of supercooling (the difference between Tm and Tf) of composite FSPCM to
Fig. 8. The phase change temperature and degree of supercooling of composite FSPCMs with respect to the GnPs contents.
Fig. 9. The latent heat of composite FSPCMs with respect to the GnPs contents.
a great extent. These phenomena are in accordance with author’s results about acetamide/expanded graphite composite [29]. It is believed that the inorganic GnPs additives with large aspect ratios and specific surface areas could provide extra surfaces for the crystallization of PEG [29]. The surfaces of GnPs might act as a kind of heterogeneous nucleation centers, which could be in great favor for promoting the crystallization of PEG dispersed in the composite. The heterogeneous nucleation effect of GnPs would cause the lower grain size of PEG [30], which is responsible for the decrease of Tm. The crystallization-promoting effect of GnPs, however, would cause the increase of Tf. Fig. 9 shows that all the prepared samples could provide available thermal storage density (larger than 114.7 kJ kg1 and 97.0 kJ kg1 in DHm and DHf, respectively). However, the latent heats (including DHm and Hf) of PEG/PMMA/GnPs composites were partly decreased with the increase of GnPs contents. This is because the addition of GnPs reduced the content of PEG, and the GnPs did not undergo a phase change within the test temperature range of 0e100 C. Although the addition of GnP was beneficial to thermal and electrical conducting properties of composite FSPCMs, it was also subjected to degrade the thermal storage density of the composite. Therefore, much attention should be paid to the content of GnP added in composite FSPCMs in order to meet the demand of the practical application. Moreover, it can be observed in Fig. 9 that the experimental latent heats of PEG/PMMA/GnPs composites were more close to the theoretical values, when compared with that of PEG/PMMA composites. This phenomenon might attribute to the crystallization-promoting effect of GnPs which could be in great favor of enhancing the crystallinity of PEG dispersed in the composite FSPCM. By means of TG, the thermal stability of prepared FSPCMs was evaluated, and the results are shown in Fig. 10. According to the TG curves, all the samples mainly degraded in two steps (Fig. 10). The detailed degradation data for the first step of weight loss processes is presented in Table 3. As can be seen from Table 3, the degradations occurred roughly between 175 and 220 C and ended at the range of 300e312 C, and all the samples showed small weight losses (lower than 8.2%) at the first stage. These degradations corresponded to the monomer evolution initiated at the unstable terminal double bonds present in some of the macromolecules [31]. Moreover, the first step weight loss of the composite FSPCM decreased with the increase of the mass fraction of GnP. This indicated that the GnPs had positive effects on the thermal stability of the nanocomposites. Similar results could also be found in the
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301
Fig. 10. TG curves of composite FSPCMs.
Table 3 The detailed degradation data for the first step of weight loss processes. FSPCM no.
Degradation interval ( C)
Mass loss (%)
1 2 3 4 5 6
175e302 180e301 190e300 198e308 220e304 215e312
8.2 7.9 7.8 7.1 6.2 5.8
research about the GnPs/thermoplastic polyurethane nanocomposites [32]. As discussed above, the presence of the oxygencontaining groups was supposed to be beneficial to the adsorption and interaction (form intermolecular hydrogen bonds) between the polymer (or monomer) and GnPs. Thus, it could be concluded that, the GnPs could restrain the volatilization of organic matrix and enhance the thermal stability of composite FSPCMs. The second degradation step, mainly as a consequence of the thermal decompositions of PEG and PMMA, mostly took place at the range of 300e312 C and terminated around 450 C. After the second degradation step, the GnP content of each composite could be checked with weight percent of remaining materials. It is obvious that GnPs with mass fractions of 1%, 2%, 4%, 6% and 8% were exactly loaded in the PEG/PMMA/GnPs composite. Because the designed working temperature of FSPCMs in the present paper was usually below 80 C, which was far less than the degradation temperature at the first step, the prepared composites exhibit available thermal stability.
properties of composite FSPCMs. Besides, GnPs were physically combined with PEG/PMMA matrix and did not participate in the polymerization. The k and s of composite FSPCMs obviously enlarged with the increase of GnPs loading contents. When the mass fraction of GnP was up to 8%, the k of the composite FSPCM changed from 0.253 W m1 K1 to 2.339 W m1 K1, more than 9 times of that without any additives. Meanwhile, as the filler loading content changed from 0 to 8 wt.%, the s of composite FSPCM approximately increased 8 orders of magnitude, and the percolation threshold of transition in electroconductibility could be achieved as long as adding merely 1e2 wt.% filler. The improvements in both k and s were mainly attributed to the well dispersion and large aspect ratio of GnPs, which were endowed with the benefit of forming conducting network in polymer matrix. It was also proved that all the prepared samples possessed available thermal storage density and thermal stability, and the GnPs could decrease the supercooling and enhance the thermal stability of organic FSPCM. Based on the results above, it can be concluded that the prepared PEG/PMMA/GnPs composites have great potentialities in specific fields (such as EMI shielding material, anti-electrostatic materials and bipolar plates in proton exchange membrane fuel cells) requiring thermal management, due to their desirable thermal and electric performance. Acknowledgments The authors gratefully acknowledge the financial support from the Major State Basic Research and Development Program of China (973 Program) (No. 2010CB227100). References
4. Conclusions In summary, PEG/PMMA/GnPs composite FSPCMs were successfully prepared by in situ polymerization upon ultrasonic irradiation. Results show that the GnPs and PEG were uniformly dispersed and embedded inside the network structure of PMMA, which contributed to the well package and self-supporting
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