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graphene nanoplatelets composites as form-stable phase change materials
Solar Energy Materials & Solar Cells 155 (2016) 421–429
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Synthesis, characterization and properties of palmitic acid/high density polyethylene/graphene nanoplatelets composites as form-stable phase change materials Yaojie Tang, Yuting Jia, Guruprasad Alva, Xiang Huang, Guiyin Fang n School of Physics, Nanjing University, Nanjing 210093, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 May 2016 Received in revised form 5 June 2016 Accepted 25 June 2016
In this work, form-stable phase change materials (FSPCM) consisting of palmitic acid (PA) and high density polyethylene (HDPE) were modified by graphene nanoplatelets (GNP). In the FSPCM, PA was used as a solid–liquid phase change material (PCM) for thermal energy storage, HDPE was a supporting material to prevent the leakage of the melted PA, and GNP were added for improving thermal conductivity and shape stabilized of the FSPCM. Thermal properties and shape stability of the composites vary with their different mixture ratios. According to the results of Fourier transformation infrared spectroscope (FT-IR) and X-ray diffractometer (XRD), the composites have advantages like stabilized chemical structure and crystalline phase. The differential scanning calorimeter (DSC) results show that the FSPCM has a constant melting temperature of around 62 °C and a high latent heat of at least 155.8 J/g. With the help of scanning electronic microscope (SEM), a layer structure and uniform dispersion of the PA is observed in the modified FSPCM. The thermos-gravimetric analyzer (TGA) and thermal cycling test results indicate that the modified FSPCM has a good thermal reliability, and the leakage of the PA drops significantly with the assistance of the GNP. The thermal conductivity of the FSPCM was measured by thermal conductivity meter and it increased to 0.8219 W (m K) 1 which is nearly 2.5 times as high as that of the pure FSPCM, when the mass fraction of the GNP is 4%. It is anticipated that the modified FSPCM possess a potential application in solar energy and building heating systems. & 2016 Elsevier B.V. All rights reserved.
Keywords: Form-stable phase change materials Graphene nanoplatelets Thermal properties Thermal conductivity Thermal reliability
1. Introduction Improving energy efficiency and developing renewable energy are two crucial solutions to mitigate the green-house effect and solve the energy crisis. In recent years, thermal energy storage system (TESS) in forms of latent heat, sensible heat and reversible thermochemical reactions has been put into various applications for realizing energy redistribution and energy efficiency on shortterm or long-term basis [1]. The ordinary applications include building-energy management [2,3] and solar-thermal conversion [4,5], in which storing and releasing energy is time dependent. Among the different forms of TESS, latent heat storage method has a big role to play in optimizing energy efficiency and energy management because the materials used in latent heat storage system, especially phase change materials (PCM), have the advantages of high enthalpy change, constant operating temperature, stable chemical structure, low cost, etc. [6]. n
Corresponding author. E-mail address: [email protected] (G. Fang).
http://dx.doi.org/10.1016/j.solmat.2016.06.049 0927-0248/& 2016 Elsevier B.V. All rights reserved.
Solid–liquid PCM are the most common materials for thermal energy storage. They store thermal energy during melting process and release thermal energy during solidifying process. The solid– liquid PCM are usually divided into three types: organic PCM (OPCM), inorganic PCM and their eutectics [7]. In addition to the normal benefits of the PCM, OPCMs, such as fatty acid, n-alkanes and esters etc, are economically available, non-toxic, and have characteristics like low degree of subcooling and appropriate phase change temperature [8]. However, pure OPCM has major shortcomings like liquid leakage in melting state, low thermal conductivity and high volume variation which limit their application in TESS [9]. Previously, encapsulating the OPCM into special storage devices was an alternative approach to preventing the leakage of the OPCM, but this way of resolution increases cost and the thermal resistance between storage devices and the OPCM [10]. Nowadays, this defect can be overcome by absorbing OPCM into supporting materials to form shape-stabilized PCM which are called formstable phase change materials (FSPCM) [11]. Supporting materials involve polymers, such as high density polyethylene (HDPE) [12–
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16], low density polyethylene (LDPE) [17], polypyrrole [18,19], poly (methyl methacrylate) [20–22], polyurethane [23], eudragit [24], cross-linked poly (acrylonitrile-co-itaconate) [25], poly (vinyl chloride) [26], styrene maleic anhydride copolymer [27], and inorganic frameworks, such as TiO2 foam [28], gypsum [29,30], expanded vermiculite and perlite [31,32]. Among them, the HDPE is an arresting matrix for synthesis of the FSPCM owning to the merits of high insulation resistance, acid and alkali resistance, organic solvent resistance, strong intermolecular forces, etc. Mu et al. [33] prepared shape stabilized phase change materials based on HDPE/paraffin waxes using twin-screw extrusion and the results indicate that the waxes are uniformly dispersed and distributed in the HDPE and have a strong plasticizing effect on the HDPE. Chen and Wolcott [34] developed the stabilized paraffin waxes with three different polyethylene (HDPE, LDPE and linear low density polyethylene (LLDPE)) and the result shows the paraffin leakage of paraffin/HDPE performs best among the three polyethylene and the blends develop a co-continuous structure which is responsible for the control of leakage behavior. Low thermal conductivity is still the limit for the FSPCM based on the polymer matrixes, since high thermal conductivity is able to accelerate the speed of thermal energy storage and release, which can improve the energy efficiency. Many outstanding works have been done to improve thermal conductivity of the FSPCM [35,36]. A natural method against this defect is to add materials with high thermal conductivity into the FSPCM. Nanometer materials with extremely high thermal conductivity and high specific surface area are the best suited additives for thermal conductivity enhancement of the FSPCM. Deng et al. [37] embedded silver nanowire into polyethylene glycol/expanded vermiculite FSPCM, and the result showed that the thermal conductivity increased to 0.68 W (m K) 1 with 19.3 wt% of silver nanowire and a theoretical model was introduced for describing the relationship between the thermal conductivity of the doped FSPCM and the loading of silver nanowire. However, few nano-additives are reported to be added into polymer-based FSPCM for thermal conductivity enhancement. Zeng et al. [38] found that the thermal conductivity of PA/polyaniline PCM doped with 7.87 wt% of exfoliated graphite nanoplatelets could reach 1.08 W (m K) 1 which was 2.375 times higher than that of pure PA/polyaniline PCM. Silakhori et al. [19] prepared FSPCM consisting of PA, polypyrrole and graphene nanoplatelets (GNP) and discovered that the thermal conductivity and latent heat of modified FSPCM reached up to 0.43 W (m K) 1 and151 J/g by addition of 1.6 wt% GNP. Therefore, the carbon nanoplatelets are good nano-additives for thermal conductivity enhancement of polymer-based FSPCM. In addition, since the structure of the GNP is similar with that of expanded graphite (EP) and a few studies have confirmed that the GNP can prevent the leakage of the PCM, the GNP may be provided with the same characteristic [36]. In previous literatures, the effects of the carbon nanoplatelets on the thermal conductivity enhancement and leakage prevention are hardly investigated together, and this work will focus on the two effects. In this paper, the PA and HDPE were selected as PCM and supporting materials, respectively. The GNP as nano-additives were added into PA/HDPE form-stable phase change materials for thermal conductivity enhancement. In order to prevent the leakage of the melted PCM, the loading of the HDPE in FSPCM should reach a certain level, which will limit the thermal enthalpy of the FSPCM as PCM is the only material to provide latent heat. If the GNP can significantly ameliorate the leakage behavior of the HDPE-based FSPCM, the loading of the PCM will increase thus the thermal enthalpy of the FSPCM will also increase. Therefore, the main purpose of this work is to investigate the influences of the GNP on the thermal conductivity and the leakage behavior of the PA/HDPE FSPCM. The FSPCM with different ratios of the GNP and
HDPE will be evaluated in terms of thermal properties, thermal conductivity, chemical stability, microstructure and thermal reliability. According to the results, the FSPCM modified by the GNP will have capacities of high thermal conductivity and thermal enthalpy, which ensure promising applications in solar energy and building heating systems.
2. Experimental 2.1. Materials PA (C16H32O2, hexadecanoic acid, Chemically Pure), which was used for thermal energy storage, was purchased from Jiangsu Huakang Chemical Reagent Company. HDPE (melt flow index: 20 g/10 min, density: 0.953 g/cm3, Vicat softening temperature: 126 °C) was obtained from Dongguan Huangjiang Co., Ltd. GNP (Thickness: 3–20 nm, Flake diameter: 5–10 mm, Specific surface area: 430 m2/g, purity: 99.5%) were provided by Nanjing XFNANO Materials Co., Ltd. 2.2. Preparation of the PA/HDPE/GNP composite In this work, in order to improve the thermal conductivity and leakage behavior of the PCM, a series of the PA/HDPE FSPCM modified by the GNP were prepared and the compositions of the PA/HDPE/GNP composites are listed in Table 1. HPCM1 and CPCM1 with same ratio of the PA to HDPE (9∶1) are set for comparison to investigate the effect of the GNP on the leakage behavior of the PA, and so do HPCM2/CPCM2 ( the ratio of PA to HDPE is 8∶2) and HPCM3/CPCM3 (the ratio of PA to HDPE is 7∶3). The pure PA/HDPE composites were blended by twin-screw agitator at a screw speed of 1000 rpm for 30 min, and the operating temperature remained at 160 °C which was below the PA decomposition temperature of 285.07 °C. The GNP doped FSPCM were prepared in different mass ratio of PA/HDPE/GNP. Firstly, the GNP were blended with melted PA by water bath at 65°C and the blends were stirred by a magnetic stirrer at the rate of 1500 rpm for 30 min. Meanwhile, the HDPE was heated at 160 °C until the HDPE is reduced to a pulpy consistency in a stainless steel vessel. Finally, the preheated HDPE was mixed with the melted blends at 160 °C by twin-screw agitator at the speed of 1000 rpm for 30 min. Fig. 1 shows the photographs of the HPCM1–3 and CPCM1–6 at room temperature. The white samples were the pure PA/HDPE composites while the black samples were the GNP doped FSPCM. The effects of the GNP on thermal conductivity are analyzed by comparison of the CPCM2 and CPCM4–6 in which the mass ratios of the PA to HDPE are 8∶2. 2.3. Characterization techniques The chemical structures and crystalloid phase of the FSPCM modified by the GNP were analyzed by a fourier transformation Table 1 The compositions of the PA/HDPE/GNP composites. Samples
HDPE (g)
PA (g)
GNP (g)
Mass fraction of GNP (%)
HPCM1 HPCM2 HPCM3 CPCM1 CPCM2 CPCM3 CPCM4 CPCM5 CPCM6
5.00 10.00 15.00 4.95 9.90 14.85 9.80 9.70 9.60
45.00 40.00 35.00 44.55 39.60 34.65 39.20 38.80 38.40
0 0 0 0.5 0.5 0.5 1 1.5 2
0 0 0 1 1 1 2 3 4
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Fig. 1. Photographs of the HPCM1–HPCM3 and CPCM1–CPCM6.
infrared spectroscope (FT-IR, Nicolet Nexus 870, spectra range: 400–4000 cm 1) and X-ray diffractometer (XRD, D/MAX-Ultima III, scanning rate: 5° (2θ) min 1), respectively. A field emission scanning electron microscope (FESEM, ZEISS Ultra 55, Carl Zeiss, Germany) was used to observe microstructure of the samples. The thermal energy storage properties of the samples were characterized by differential scanning calorimeter (DSC, Pyris 1 DSC, Perkin-Elmer, temperature accuracy: 70.2 °C, enthalpy accuracy: 75%, heating rate: 10 °C/min, operating temperature range: 20– 700 °C) under a constant stream of argon. The thermal reliability was tested by a thermogravimetric analyzer (TGA, Pyris 1 TGA, Perkin-Elmer, operating temperature range: 20–700 °C, heating rate: 20 °C/min). The leakage behavior was observed by thermal cycling test of 150 melting and solidifying cycles for about 50 h, and the temperature range was from 50 °C to 70 °C. The setup is composed of three thermostatic water baths and a temperature logger. The sample wrapped by wire entanglement was put into a special conical flask immersed in a water bath and the temperature was changed by other two water baths at 50 71 °C and 7071 °C. The special conical flask was also filled with distilled water for
separation of the PA leakage as PA is insoluble in water. The FSPCM was melt by the water bath of 7071 °C and cooled by the water bath of 50 71 °C, and each thermal cycling spent about 20 min. After thermal cycling, the oven-dried samples would be weighed. A thermal conductivity meter (TC 3020, Xiatech Electronic Technology co., Ltd, accuracy: 7 2.0%) was used to determine the thermal conductivity of the doped FSPCM by the hot-wire method. The setup for the measurement of thermal conductivity is shown in Fig. 2. Two parts are involved in the setup: thermal conductivity meter and constant temperature water-bath. The accuracy of constant temperature water-bathing is 70.5 °C. At first, to determine the accuracy and reliability of the TC 3020, the pyrex glass (Pyrex 7740) and stainless steel (304 L) were measured as referenced materials to calibrate the accuracy of the thermal conductivity meter. Then, the samples melted at 180 °C were poured into two watch-glasses equally and were divided into two similar pieces (diameter: 6 cm, thickness: 1 cm). When the samples cooled down to room temperature, the surfaces contacted with the hot-wire probe were polished by fine sandpaper to a glowing evenness, which can reduce the thermal contact resistance between the samples and the hot-wire probe. Finally, the hot-wire
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Fig. 2. Setup for the measurement of thermal conductivity.
probe was inserted into the two smooth surfaces. The samples were placed in the constant temperature water-bathing and pressed with the weight to ensure close contact with the probe, as seen in Fig. 2. Each value of thermal conductivity was determined by the average of five measurements.
3. Results and discussion 3.1. FT-IR analysis The chemical structures of the PA/HDPE/GNP composites are characterized by the FT-IR spectrum which is recorded from 400 cm 1 to 4000 cm 1 and their FT-IR spectrums are shown in Fig. 3. Fig. 3a identifies six major absorption peaks in the FT-IR spectrum of the PA. The absorption peak at 2921 cm 1 is attributed to the antisymmetric stretching vibration of –CH2 group while the peak at 2846 cm 1 corresponds to symmetric stretching
vibration of –CH2 group. The obtained peaks at 1701 cm 1 and 947 cm 1 ¼ signify the C ¼O stretching vibration and –OH out-ofplane wagging vibration, respectively. The –CH2 scissoring bending vibration and the rocking vibration of –(CH2)4 group chain are the reason behind the absorption peaks at 1471 and 717 cm 1. A seen in Fig. 3b, the four major peaks at 2921 cm 1, 2846 cm 1, 1471 cm 1 and 717 cm 1 are observed in the FT-IR spectrums of the HDPE, and the sharp peaks were cause by the different vibration mode of –CH2 group, which is the same in the FT-IR spectrum of the PA. Fig. 3c appears a flat line because GNP is a kind of elementary substance and does not possess dipole moment. Fig. 3d–f and Fig. 3g–l characterize the FT-IR spectrums of the pure PA/HDPE composites and the GNP doped FSPCM. Obviously, the absorption peaks in the spectra of pure and doped composites are consistent with the spectra of the PA, which reveals that the characteristic peaks of the PA remain unchanged in the composites. In addition, there is no new peak in the spectra of the mixtures. From the results, it can be confirmed that the doped FSPCM have stable chemical structures and there is no chemical reaction among the PA, HDPE and GNP. 3.2. XRD analysis
Fig. 3. FT-IR spectra of the (a) PA, (b) HDPE, (c) GNP, (d)HPCM1, (e) HPCM2, (f) HPCM3, (g) CPCM1, (h) CPCM2, (i) CPCM3, (j) CPCM4, (k) CPCM5 and (l) CPCM6.
Fig. 4 presents the XRD patterns of the PA, HDPE, GNP, HPCM1– 3 and CPCM1–6. In Fig. 4a, the two sharp diffraction peaks at 21.8° and 24.1° are attributed to the crystallization of the PA. Fig. 4b also appears the same peaks as the PA in the XRD diffractogram of the HDPE. The peak at 26.6° in Fig. 4c represents the regular crystallization of the GNP according to the previous literature [19]. Fig. 4d–l show the XRD patterns of the PA/HDPE composites and doped FSPCM. As seen in Fig. 4d–f, the sharp peaks at 21.7° and 24.1° are in agreement with the diffraction peaks of the PA and HDPE. However, from Fig. 4g–l, the PA/HDPE/GNP composites have not inherited the characteristic peak of the GNP at 26.6°. The dispersion of the GNP in the network of the PA/HDPE composites may be responsible for the disappearance of the GNP diffraction peak in the GNP doped FSPCM. The results indicate that there is no effect of the GNP on the crystal structure of the PA/HDPE composites.
Y. Tang et al. / Solar Energy Materials & Solar Cells 155 (2016) 421–429
Fig. 4. XRD patterns of the (a) PA, (b) HDPE, (c) GNP, (d)HPCM1, (e) HPCM2, (f) HPCM3, (g) CPCM1, (h) CPCM2, (i) CPCM3, (j) CPCM4, (k) CPCM5 and (l) CPCM6.
3.3. Microstructure analysis The microstructures of the GNP, HDPE, pure PA/HDPE composites and doped PA/HDPE composite with 4 wt% GNP are shown in Fig. 5. The morphology of the GNP and HDPE were displayed in Fig. 5a and Fig. 5b, respectively. The GNP is flaky particle with smooth surface and high specific surface area as the thickness of the GNP is less than 20 nm. The microcosmic surface of the HDPE is also flat and smooth. The SEM photograph of the HPCM2 is used to describe the microstructure of PA/HDPE composite which is
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Fig. 6. Melting DSC curves of the PA, HDPE and HPCM1–HPCM3.
presented in Fig. 5c. The dark and gray parts in Fig. 5c represent the PA and HDPE, respectively. It is clear that the PA is uniformly wrapped by the network structure of the HDPE which can prevent the PA form leakage. The microcosmic surface of the GNP doped PA/HDPE composite (CPCM6) is observed in Fig. 5d. The microsurface of the CPCM6 is irregular and the PA/HDPE composite is attached to the broad surface of the GNP. It seem that a layer structure was developed in the GNP doped FSPCM. These features of the CPCM6 microstructure indicate the GNP can help HDPE in preventing the leakage of the PCM.
Fig. 5. SEM images of the (a) GNP, (b) HDPE, (c) HPCM2 (the ratio of PA to HDPE is 8∶2) and (d) CPCM6 with 4 wt% of the GNP.
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Fig. 9. Solidifying DSC curves of the CPCM1–CPCM6.
Fig. 7. Solidifying DSC curves of the PA, HDPE and HPCM1–HPCM3.
3.4. Thermal energy storage properties analysis The thermal energy storage properties of the PA, HDPE, pure PA/HDPE composites and GNP doped PA/HDPE composites were characterized by DSC experiments. Figs. 6 and 7 show the melting and solidifying processes of the PA, HDPE and pure/HDPE composites while Figs. 8 and 9 present the DSC curve of the GNP doped FSPCM. The detailed data are listed in Table 2. Two peaks appear in the melting and solidifying DSC curve of the composite samples. The first peak at low temperature is caused by the melt/solidification of the PA and the second peak at high temperature results from the melt/solidification of the HDPE, which indicates that the PA is insoluble in HDPE. From Table 2, the onset melting temperatures of the PA and HDPE are 62.71 °C and 124.79 °C, respectively. The PA onset melting temperatures of the HPCM1–3 are 61.13 °C, 61.60 °C, and 61.48 °C while the HDPE onset melting temperatures is 115.48 °C, 116.73 °C and 117.38 °C, which is similar to the corresponding DSC data of the CPCM1–6. The PA onset meting temperatures of the composites is around 1 °C lower than that of pure PA, but for the HDPE, the descending temperature is around 8 °C. As seen in Table 2, the melting and solidifying latent heat of the composites drop with the decrease of the PA content because only
Table 2 DSC data of the PA, HDPE, HPCM1–HPCM3 and CPCM1–CPCM6. Samples Melting
Solidifying
Mass fraction of PA (%)
Temperature (°C) Latent heat (J/ g)
Temperature (°C) Latent heat (J/ g)
PA HDPE HPCM1
62.71 124.79 61.13a 115.48b
211.99 179.90 182.37a 17.86b
59.59 118.60 57.13a 111.60b
164.47 195.18 172.63a 18.423b
100 0 90
HPCM2
61.60a 116.73b
161.10a 37.95b
58.42a 112.21b
157.94a 37.34b
80
HPCM3
61.48a 117.38b
143.46a 58.04b
59.34a 112.87b
140.25a 54.50b
70
CPCM1
61.44a 116.16b
182.14a 17.89b
58.49a 113.32b
169.77a 18.49b
89.1
CPCM2
61.31a 116.70b
169.57a 36.52b
58.94a 113.68b
162.24a 37.22b
79.2
CPCM3
61.22a 117.49b
141.22a 58.82b
59.05a 115.15b
138.02a 52.90b
69.3
CPCM4
61.60a 117.59b
164.57a 35.22b
58.99a 114.54b
157.72a 34.41b
78.4
CPCM5
61.84a 117.58b
155.80a 41.32b
58.20a 114.91b
153.64a 36.07b
77.6
CPCM6
61.77a 117.30b
157.82a 35.33b
58.27a 115.68b
154.55a 35.31b
76.8
a b
The DSC data of the PA during melting and solidifying processes. The DSC data of the HDPE during melting and solidifying processes.
the PA plays role in thermal energy storage at operating temperature. Therefore, the melting/solidifying latent heat of the composite has linear relationship with the mass fraction of the PA, and the relationship expression is presented by following:
ΔHFSPCM = ηΔHPA
Fig. 8. Melting DSC curves of the CPCM1–CPCM6.
(1)
where ΔHFSPCM and ΔHPA represent the melting/solidifying latent heat of the PA/HDPE/GNP composites and pure PA. η is the mass fraction of the PA in the composites. The experimental values are slightly below the theoretical values, which result from the evaporation of the PA when the composites were blended at 160 °C. In general, the experimental and theoretical values are basically consistent. As the loading of the GNP in the doped FSPCM is very
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Table 3 TGA data of the PA, HDPE, HPCM1–HPCM3 and CPCM1–CPCM6. Samples
Tpeak1 (°C)
ΔW1 (%)
PA HDPE HPCM1 HPCM2 HPCM3 CPCM1 CPCM2 CPCM3 CPCM4 CPCM5 CPCM6
285.07
100.0
271.99 266.43 266.43 264.58 271.37 267.05 277.99 275.08 286.19
91.2 80.6 71.5 88.5 80.2 70.1 78.5 78.3 77.6
Tpeak2 (°C)
ΔW2 (%)
Residue (%) (700 °C)
510.97 497.42 499.97 502.98 497.42 498.66 502.36 504.83 502.36 501.74
100.0 8.8 19.4 28.5 10.6 19.0 29.1 20.1 19.0 18.5
0 0 0 0 0 0.9 0.8 0.8 1.4 2.7 3.9
Table 4 Thermal cycling data of the HPCM1–HPCM3 and CPCM1–HPCM6. Samples Weight before cycling (g) Weight after cycling (g) Leakage rate (%) Fig. 10. TGA curves of the PA, HDPE and HPCM1–HPCM3.
small, the melting latent heat of the GNP doped FSPCM is slightly lower than that of corresponding PA/HDPE composite. 3.5. Thermal reliability and leakage analysis The TGA curves in Figs. 10–11 reveal the thermal reliability of the PA, HDPE, pure PA/HDPE composites and GNP doped PA/HDPE composites. The temperatures (Tpeak) of maximum thermal degradation rate, the percentage (ΔW) of the weight loss and the amount of residue are listed in Table 3, which are obtained from the TGA experiments. The Tpeak1 and ΔW1 represent the decomposition temperature and weight loss of the PA while the Tpeak2 and ΔW2 represent decomposition temperature and weight loss of the HDPE. The decomposition of the PA begins at around 180 °C and completes at around 320 °C, and the weight loss of the HDPE occurs from 450 °C to 600 °C. As seen in Table 3, the decomposition temperatures of the PA in the composites are close to that of the pure PA, which indicates that the modified FSPCM have good thermal reliability. In addition, the ΔW1 and ΔW2 correspond to the content of the PA and HDPE in the composites. As the maximum operating temperature in TGA experiments is below 700 °C, the residue can be considered as the GNP, and the content of the residue is also close to the loading of the GNP. These results demonstrate that the PA, HDPE and GNP are mixed successfully.
HPCM1 HPCM2 HPCM3 CPCM1 CPCM2 CPCM3 CPCM4 CPCM5 CPCM6
45.82 48.05 47.69 46.34 48.69 48.57 48.66 48.90 49.81
31.17 45.58 46.53 41.58 47.64 47.67 47.80 48.16 48.99
31.97 5.14 2.43 10.27 2.16 1.85 1.77 1.51 1.65
The leakage behavior is an important evaluation item for the FSPCM. This property is investigated by thermal cycling test. In this work, 150 times thermal cycling were conducted on the composite samples and the weight data before/after thermal cycling are listed in Table 4. The leakage of the PA/HDPE ( the mass ratio is 8∶2) composite was decreased significantly by the addition of the GNP. The broad surface of the GNP and the layer structure in the GNP doped FSPCM are possibly responsible for the little weight losses of the composite PCMs after thermal cycling. The result shows that the GNP can effectively prevent the leakage of the PA from modified FSPCM. Therefore, for the same leakage behavior of the FSPCM, the loading of the PA can increase in the GNP doped FSPCM thus the thermal enthalpy of the FSPCM can be increased. 3.6. Thermal conductivity analysis The thermal conductivities of the HCPM2, CPCM2 (1% GNP), CPCM4 (2% GNP), CPCM5 (3% GNP) and CPCM6 (4% GNP) with the same PA/HDPE ratio (8∶2) are shown in Fig. 12, and the relevant data are listed in Table 5. The thermal conductivity of the pure PA/ HDPE composite are 0.3280 W (m K) 1, and the thermal conductivities of the GNP doped FSPCM are 0.4174 W (m K) 1 (CPCM2), 0.5257 W (m K) 1 (CPCM4), 0.6570 W (m K) 1 (CPCM5) and 0.8219 W (m K) 1 (CPCM6). It is clearly found that the GNP can improve the thermal conductivity of the FSPCM significantly and the thermal conductivity of the GNP doped FSPCM will rise with the increase of the GNP mass fraction. In this work, the polynomial fitting is used to describe the relationship between thermal conductivity of the FSPCM and the mass fraction of the GNP. The solid line in Fig. 12 represents the polynomial fitting curve, and the fitting formula is given as following:
λ = 0.3289 + 7.53x + 118.1x2
Fig. 11. TGA curves of the CPCM1–CPCM6.
(2)
where x is mass fractions of the GNP. λ represents the thermal conductivity of the GNP doped FSPCM. The correlation coefficient
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of the PA in the FSPCM. The result of thermal cycling test shows that the GNP can help preventing the leakage of the PA from modified FSPCM effectively after 150 thermal cycles. The DSC result shows that the PA/HDPE composite with 4 wt% of the GNP (CPCM6) has high thermal enthalpy of 157.82 J/g and constant melting temperature of around 62 °C. The thermal conductivity of the FSPCM was measured by thermal conductivity meter and it increased to 0.8219 W (m K) 1 which is nearly 2.5 times as high as that of the pure FSPCM, when the mass fraction of the GNP is 4%. In general, thanks to the high specific surface area and thermal conductivity of the GNP, the satisfied FSPCM with 4 wt% of the GNP has capacities of high thermal enthalpy and thermal conductivity, which guarantees the promising application in solar energy and building heating systems.
Acknowledgments Fig. 12. Experimental and fitting thermal conductivities of the modified FSPCM (HPCM2) with different content of the GNP. Table 5 Thermal conductivity of the HPCM2, CPCM2 and CPCM4–CPCM6. Samples
Thermal conductivity at 30 °C (W (m K) 1)
HPCM2 (80% PA) CPCM2 (1% GNP) CPCM4 (2% GNP) CPCM5 (3% GNP) CPCM6 (4% GNP)
0.3280 7 0.0115 0.41747 0.0098 0.52577 0.0157 0.65707 0.0144 0.82197 0.0131
This work was supported by the National Natural Science Foundation of China (Grant no. 51376087) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors also wish to thank the reviewers and editor for kindly giving revising suggestions.
References
Table 6 Comparison of the thermal conductivity of the modified FSPCM with results of the previous works. Samples
Thermal conductivity (W (m K) 1)
Reference
Paraffin þ 7.5% nano graphite PA/polypyrroleþ1.6% GNP PA/polyaniline þ7.87% exfoliated graphite nanoplatelets PA/HDPE þ 4% GNP
0.5685 0.43 1.08
[39] [19] [38]
0.8219
Present study
of the Eq. (2) is at least 0.9997. The Eq. (2) can't be simplified to the first order-linear equation because the quadratic coefficient in Eq. (2) is positive and large, which indicates that the effect of the GNP on the thermal conductivity enhancement of the GNP doped FSPCM will be stronger with the increasing content of the GNP. The thermal conductivities of the PCM modified by carbon nanoplatelets in previous literatures are listed in Table 6 for comparison with that in this work. From Table 6, it is confirmed that in this work the modified FSPCM has obtained a good thermal conductivity enhancement.
4. Conclusions In this work, the preparation, characterization and thermal properties of the PA/HDPE and PA/HDPE/GNP composites as modified FSPCM were presented. The SEM result shows that the PA is uniformly dispersed into the network structure of the HDPE and the PA/HDPE composite is attached to the broad surface of the GNP. According to the results of the FT-IR and XRD, the modified FSPCM has stabilized chemical structure and crystalline phase, and no chemical reaction takes place among components. The TGA data reveal the high decomposition temperatures (around 275 °C)
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Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Synthesis, characterization and properties of palmitic acid/high density polyethylene/graphene nanoplatelets composites as form-stable phase change materials Yaojie Tang, Yuting Jia, Guruprasad Alva, Xiang Huang, Guiyin Fang n School of Physics, Nanjing University, Nanjing 210093, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 May 2016 Received in revised form 5 June 2016 Accepted 25 June 2016
In this work, form-stable phase change materials (FSPCM) consisting of palmitic acid (PA) and high density polyethylene (HDPE) were modified by graphene nanoplatelets (GNP). In the FSPCM, PA was used as a solid–liquid phase change material (PCM) for thermal energy storage, HDPE was a supporting material to prevent the leakage of the melted PA, and GNP were added for improving thermal conductivity and shape stabilized of the FSPCM. Thermal properties and shape stability of the composites vary with their different mixture ratios. According to the results of Fourier transformation infrared spectroscope (FT-IR) and X-ray diffractometer (XRD), the composites have advantages like stabilized chemical structure and crystalline phase. The differential scanning calorimeter (DSC) results show that the FSPCM has a constant melting temperature of around 62 °C and a high latent heat of at least 155.8 J/g. With the help of scanning electronic microscope (SEM), a layer structure and uniform dispersion of the PA is observed in the modified FSPCM. The thermos-gravimetric analyzer (TGA) and thermal cycling test results indicate that the modified FSPCM has a good thermal reliability, and the leakage of the PA drops significantly with the assistance of the GNP. The thermal conductivity of the FSPCM was measured by thermal conductivity meter and it increased to 0.8219 W (m K) 1 which is nearly 2.5 times as high as that of the pure FSPCM, when the mass fraction of the GNP is 4%. It is anticipated that the modified FSPCM possess a potential application in solar energy and building heating systems. & 2016 Elsevier B.V. All rights reserved.
Keywords: Form-stable phase change materials Graphene nanoplatelets Thermal properties Thermal conductivity Thermal reliability
1. Introduction Improving energy efficiency and developing renewable energy are two crucial solutions to mitigate the green-house effect and solve the energy crisis. In recent years, thermal energy storage system (TESS) in forms of latent heat, sensible heat and reversible thermochemical reactions has been put into various applications for realizing energy redistribution and energy efficiency on shortterm or long-term basis [1]. The ordinary applications include building-energy management [2,3] and solar-thermal conversion [4,5], in which storing and releasing energy is time dependent. Among the different forms of TESS, latent heat storage method has a big role to play in optimizing energy efficiency and energy management because the materials used in latent heat storage system, especially phase change materials (PCM), have the advantages of high enthalpy change, constant operating temperature, stable chemical structure, low cost, etc. [6]. n
Corresponding author. E-mail address: [email protected] (G. Fang).
http://dx.doi.org/10.1016/j.solmat.2016.06.049 0927-0248/& 2016 Elsevier B.V. All rights reserved.
Solid–liquid PCM are the most common materials for thermal energy storage. They store thermal energy during melting process and release thermal energy during solidifying process. The solid– liquid PCM are usually divided into three types: organic PCM (OPCM), inorganic PCM and their eutectics [7]. In addition to the normal benefits of the PCM, OPCMs, such as fatty acid, n-alkanes and esters etc, are economically available, non-toxic, and have characteristics like low degree of subcooling and appropriate phase change temperature [8]. However, pure OPCM has major shortcomings like liquid leakage in melting state, low thermal conductivity and high volume variation which limit their application in TESS [9]. Previously, encapsulating the OPCM into special storage devices was an alternative approach to preventing the leakage of the OPCM, but this way of resolution increases cost and the thermal resistance between storage devices and the OPCM [10]. Nowadays, this defect can be overcome by absorbing OPCM into supporting materials to form shape-stabilized PCM which are called formstable phase change materials (FSPCM) [11]. Supporting materials involve polymers, such as high density polyethylene (HDPE) [12–
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16], low density polyethylene (LDPE) [17], polypyrrole [18,19], poly (methyl methacrylate) [20–22], polyurethane [23], eudragit [24], cross-linked poly (acrylonitrile-co-itaconate) [25], poly (vinyl chloride) [26], styrene maleic anhydride copolymer [27], and inorganic frameworks, such as TiO2 foam [28], gypsum [29,30], expanded vermiculite and perlite [31,32]. Among them, the HDPE is an arresting matrix for synthesis of the FSPCM owning to the merits of high insulation resistance, acid and alkali resistance, organic solvent resistance, strong intermolecular forces, etc. Mu et al. [33] prepared shape stabilized phase change materials based on HDPE/paraffin waxes using twin-screw extrusion and the results indicate that the waxes are uniformly dispersed and distributed in the HDPE and have a strong plasticizing effect on the HDPE. Chen and Wolcott [34] developed the stabilized paraffin waxes with three different polyethylene (HDPE, LDPE and linear low density polyethylene (LLDPE)) and the result shows the paraffin leakage of paraffin/HDPE performs best among the three polyethylene and the blends develop a co-continuous structure which is responsible for the control of leakage behavior. Low thermal conductivity is still the limit for the FSPCM based on the polymer matrixes, since high thermal conductivity is able to accelerate the speed of thermal energy storage and release, which can improve the energy efficiency. Many outstanding works have been done to improve thermal conductivity of the FSPCM [35,36]. A natural method against this defect is to add materials with high thermal conductivity into the FSPCM. Nanometer materials with extremely high thermal conductivity and high specific surface area are the best suited additives for thermal conductivity enhancement of the FSPCM. Deng et al. [37] embedded silver nanowire into polyethylene glycol/expanded vermiculite FSPCM, and the result showed that the thermal conductivity increased to 0.68 W (m K) 1 with 19.3 wt% of silver nanowire and a theoretical model was introduced for describing the relationship between the thermal conductivity of the doped FSPCM and the loading of silver nanowire. However, few nano-additives are reported to be added into polymer-based FSPCM for thermal conductivity enhancement. Zeng et al. [38] found that the thermal conductivity of PA/polyaniline PCM doped with 7.87 wt% of exfoliated graphite nanoplatelets could reach 1.08 W (m K) 1 which was 2.375 times higher than that of pure PA/polyaniline PCM. Silakhori et al. [19] prepared FSPCM consisting of PA, polypyrrole and graphene nanoplatelets (GNP) and discovered that the thermal conductivity and latent heat of modified FSPCM reached up to 0.43 W (m K) 1 and151 J/g by addition of 1.6 wt% GNP. Therefore, the carbon nanoplatelets are good nano-additives for thermal conductivity enhancement of polymer-based FSPCM. In addition, since the structure of the GNP is similar with that of expanded graphite (EP) and a few studies have confirmed that the GNP can prevent the leakage of the PCM, the GNP may be provided with the same characteristic [36]. In previous literatures, the effects of the carbon nanoplatelets on the thermal conductivity enhancement and leakage prevention are hardly investigated together, and this work will focus on the two effects. In this paper, the PA and HDPE were selected as PCM and supporting materials, respectively. The GNP as nano-additives were added into PA/HDPE form-stable phase change materials for thermal conductivity enhancement. In order to prevent the leakage of the melted PCM, the loading of the HDPE in FSPCM should reach a certain level, which will limit the thermal enthalpy of the FSPCM as PCM is the only material to provide latent heat. If the GNP can significantly ameliorate the leakage behavior of the HDPE-based FSPCM, the loading of the PCM will increase thus the thermal enthalpy of the FSPCM will also increase. Therefore, the main purpose of this work is to investigate the influences of the GNP on the thermal conductivity and the leakage behavior of the PA/HDPE FSPCM. The FSPCM with different ratios of the GNP and
HDPE will be evaluated in terms of thermal properties, thermal conductivity, chemical stability, microstructure and thermal reliability. According to the results, the FSPCM modified by the GNP will have capacities of high thermal conductivity and thermal enthalpy, which ensure promising applications in solar energy and building heating systems.
2. Experimental 2.1. Materials PA (C16H32O2, hexadecanoic acid, Chemically Pure), which was used for thermal energy storage, was purchased from Jiangsu Huakang Chemical Reagent Company. HDPE (melt flow index: 20 g/10 min, density: 0.953 g/cm3, Vicat softening temperature: 126 °C) was obtained from Dongguan Huangjiang Co., Ltd. GNP (Thickness: 3–20 nm, Flake diameter: 5–10 mm, Specific surface area: 430 m2/g, purity: 99.5%) were provided by Nanjing XFNANO Materials Co., Ltd. 2.2. Preparation of the PA/HDPE/GNP composite In this work, in order to improve the thermal conductivity and leakage behavior of the PCM, a series of the PA/HDPE FSPCM modified by the GNP were prepared and the compositions of the PA/HDPE/GNP composites are listed in Table 1. HPCM1 and CPCM1 with same ratio of the PA to HDPE (9∶1) are set for comparison to investigate the effect of the GNP on the leakage behavior of the PA, and so do HPCM2/CPCM2 ( the ratio of PA to HDPE is 8∶2) and HPCM3/CPCM3 (the ratio of PA to HDPE is 7∶3). The pure PA/HDPE composites were blended by twin-screw agitator at a screw speed of 1000 rpm for 30 min, and the operating temperature remained at 160 °C which was below the PA decomposition temperature of 285.07 °C. The GNP doped FSPCM were prepared in different mass ratio of PA/HDPE/GNP. Firstly, the GNP were blended with melted PA by water bath at 65°C and the blends were stirred by a magnetic stirrer at the rate of 1500 rpm for 30 min. Meanwhile, the HDPE was heated at 160 °C until the HDPE is reduced to a pulpy consistency in a stainless steel vessel. Finally, the preheated HDPE was mixed with the melted blends at 160 °C by twin-screw agitator at the speed of 1000 rpm for 30 min. Fig. 1 shows the photographs of the HPCM1–3 and CPCM1–6 at room temperature. The white samples were the pure PA/HDPE composites while the black samples were the GNP doped FSPCM. The effects of the GNP on thermal conductivity are analyzed by comparison of the CPCM2 and CPCM4–6 in which the mass ratios of the PA to HDPE are 8∶2. 2.3. Characterization techniques The chemical structures and crystalloid phase of the FSPCM modified by the GNP were analyzed by a fourier transformation Table 1 The compositions of the PA/HDPE/GNP composites. Samples
HDPE (g)
PA (g)
GNP (g)
Mass fraction of GNP (%)
HPCM1 HPCM2 HPCM3 CPCM1 CPCM2 CPCM3 CPCM4 CPCM5 CPCM6
5.00 10.00 15.00 4.95 9.90 14.85 9.80 9.70 9.60
45.00 40.00 35.00 44.55 39.60 34.65 39.20 38.80 38.40
0 0 0 0.5 0.5 0.5 1 1.5 2
0 0 0 1 1 1 2 3 4
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Fig. 1. Photographs of the HPCM1–HPCM3 and CPCM1–CPCM6.
infrared spectroscope (FT-IR, Nicolet Nexus 870, spectra range: 400–4000 cm 1) and X-ray diffractometer (XRD, D/MAX-Ultima III, scanning rate: 5° (2θ) min 1), respectively. A field emission scanning electron microscope (FESEM, ZEISS Ultra 55, Carl Zeiss, Germany) was used to observe microstructure of the samples. The thermal energy storage properties of the samples were characterized by differential scanning calorimeter (DSC, Pyris 1 DSC, Perkin-Elmer, temperature accuracy: 70.2 °C, enthalpy accuracy: 75%, heating rate: 10 °C/min, operating temperature range: 20– 700 °C) under a constant stream of argon. The thermal reliability was tested by a thermogravimetric analyzer (TGA, Pyris 1 TGA, Perkin-Elmer, operating temperature range: 20–700 °C, heating rate: 20 °C/min). The leakage behavior was observed by thermal cycling test of 150 melting and solidifying cycles for about 50 h, and the temperature range was from 50 °C to 70 °C. The setup is composed of three thermostatic water baths and a temperature logger. The sample wrapped by wire entanglement was put into a special conical flask immersed in a water bath and the temperature was changed by other two water baths at 50 71 °C and 7071 °C. The special conical flask was also filled with distilled water for
separation of the PA leakage as PA is insoluble in water. The FSPCM was melt by the water bath of 7071 °C and cooled by the water bath of 50 71 °C, and each thermal cycling spent about 20 min. After thermal cycling, the oven-dried samples would be weighed. A thermal conductivity meter (TC 3020, Xiatech Electronic Technology co., Ltd, accuracy: 7 2.0%) was used to determine the thermal conductivity of the doped FSPCM by the hot-wire method. The setup for the measurement of thermal conductivity is shown in Fig. 2. Two parts are involved in the setup: thermal conductivity meter and constant temperature water-bath. The accuracy of constant temperature water-bathing is 70.5 °C. At first, to determine the accuracy and reliability of the TC 3020, the pyrex glass (Pyrex 7740) and stainless steel (304 L) were measured as referenced materials to calibrate the accuracy of the thermal conductivity meter. Then, the samples melted at 180 °C were poured into two watch-glasses equally and were divided into two similar pieces (diameter: 6 cm, thickness: 1 cm). When the samples cooled down to room temperature, the surfaces contacted with the hot-wire probe were polished by fine sandpaper to a glowing evenness, which can reduce the thermal contact resistance between the samples and the hot-wire probe. Finally, the hot-wire
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Fig. 2. Setup for the measurement of thermal conductivity.
probe was inserted into the two smooth surfaces. The samples were placed in the constant temperature water-bathing and pressed with the weight to ensure close contact with the probe, as seen in Fig. 2. Each value of thermal conductivity was determined by the average of five measurements.
3. Results and discussion 3.1. FT-IR analysis The chemical structures of the PA/HDPE/GNP composites are characterized by the FT-IR spectrum which is recorded from 400 cm 1 to 4000 cm 1 and their FT-IR spectrums are shown in Fig. 3. Fig. 3a identifies six major absorption peaks in the FT-IR spectrum of the PA. The absorption peak at 2921 cm 1 is attributed to the antisymmetric stretching vibration of –CH2 group while the peak at 2846 cm 1 corresponds to symmetric stretching
vibration of –CH2 group. The obtained peaks at 1701 cm 1 and 947 cm 1 ¼ signify the C ¼O stretching vibration and –OH out-ofplane wagging vibration, respectively. The –CH2 scissoring bending vibration and the rocking vibration of –(CH2)4 group chain are the reason behind the absorption peaks at 1471 and 717 cm 1. A seen in Fig. 3b, the four major peaks at 2921 cm 1, 2846 cm 1, 1471 cm 1 and 717 cm 1 are observed in the FT-IR spectrums of the HDPE, and the sharp peaks were cause by the different vibration mode of –CH2 group, which is the same in the FT-IR spectrum of the PA. Fig. 3c appears a flat line because GNP is a kind of elementary substance and does not possess dipole moment. Fig. 3d–f and Fig. 3g–l characterize the FT-IR spectrums of the pure PA/HDPE composites and the GNP doped FSPCM. Obviously, the absorption peaks in the spectra of pure and doped composites are consistent with the spectra of the PA, which reveals that the characteristic peaks of the PA remain unchanged in the composites. In addition, there is no new peak in the spectra of the mixtures. From the results, it can be confirmed that the doped FSPCM have stable chemical structures and there is no chemical reaction among the PA, HDPE and GNP. 3.2. XRD analysis
Fig. 3. FT-IR spectra of the (a) PA, (b) HDPE, (c) GNP, (d)HPCM1, (e) HPCM2, (f) HPCM3, (g) CPCM1, (h) CPCM2, (i) CPCM3, (j) CPCM4, (k) CPCM5 and (l) CPCM6.
Fig. 4 presents the XRD patterns of the PA, HDPE, GNP, HPCM1– 3 and CPCM1–6. In Fig. 4a, the two sharp diffraction peaks at 21.8° and 24.1° are attributed to the crystallization of the PA. Fig. 4b also appears the same peaks as the PA in the XRD diffractogram of the HDPE. The peak at 26.6° in Fig. 4c represents the regular crystallization of the GNP according to the previous literature [19]. Fig. 4d–l show the XRD patterns of the PA/HDPE composites and doped FSPCM. As seen in Fig. 4d–f, the sharp peaks at 21.7° and 24.1° are in agreement with the diffraction peaks of the PA and HDPE. However, from Fig. 4g–l, the PA/HDPE/GNP composites have not inherited the characteristic peak of the GNP at 26.6°. The dispersion of the GNP in the network of the PA/HDPE composites may be responsible for the disappearance of the GNP diffraction peak in the GNP doped FSPCM. The results indicate that there is no effect of the GNP on the crystal structure of the PA/HDPE composites.
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Fig. 4. XRD patterns of the (a) PA, (b) HDPE, (c) GNP, (d)HPCM1, (e) HPCM2, (f) HPCM3, (g) CPCM1, (h) CPCM2, (i) CPCM3, (j) CPCM4, (k) CPCM5 and (l) CPCM6.
3.3. Microstructure analysis The microstructures of the GNP, HDPE, pure PA/HDPE composites and doped PA/HDPE composite with 4 wt% GNP are shown in Fig. 5. The morphology of the GNP and HDPE were displayed in Fig. 5a and Fig. 5b, respectively. The GNP is flaky particle with smooth surface and high specific surface area as the thickness of the GNP is less than 20 nm. The microcosmic surface of the HDPE is also flat and smooth. The SEM photograph of the HPCM2 is used to describe the microstructure of PA/HDPE composite which is
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Fig. 6. Melting DSC curves of the PA, HDPE and HPCM1–HPCM3.
presented in Fig. 5c. The dark and gray parts in Fig. 5c represent the PA and HDPE, respectively. It is clear that the PA is uniformly wrapped by the network structure of the HDPE which can prevent the PA form leakage. The microcosmic surface of the GNP doped PA/HDPE composite (CPCM6) is observed in Fig. 5d. The microsurface of the CPCM6 is irregular and the PA/HDPE composite is attached to the broad surface of the GNP. It seem that a layer structure was developed in the GNP doped FSPCM. These features of the CPCM6 microstructure indicate the GNP can help HDPE in preventing the leakage of the PCM.
Fig. 5. SEM images of the (a) GNP, (b) HDPE, (c) HPCM2 (the ratio of PA to HDPE is 8∶2) and (d) CPCM6 with 4 wt% of the GNP.
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Fig. 9. Solidifying DSC curves of the CPCM1–CPCM6.
Fig. 7. Solidifying DSC curves of the PA, HDPE and HPCM1–HPCM3.
3.4. Thermal energy storage properties analysis The thermal energy storage properties of the PA, HDPE, pure PA/HDPE composites and GNP doped PA/HDPE composites were characterized by DSC experiments. Figs. 6 and 7 show the melting and solidifying processes of the PA, HDPE and pure/HDPE composites while Figs. 8 and 9 present the DSC curve of the GNP doped FSPCM. The detailed data are listed in Table 2. Two peaks appear in the melting and solidifying DSC curve of the composite samples. The first peak at low temperature is caused by the melt/solidification of the PA and the second peak at high temperature results from the melt/solidification of the HDPE, which indicates that the PA is insoluble in HDPE. From Table 2, the onset melting temperatures of the PA and HDPE are 62.71 °C and 124.79 °C, respectively. The PA onset melting temperatures of the HPCM1–3 are 61.13 °C, 61.60 °C, and 61.48 °C while the HDPE onset melting temperatures is 115.48 °C, 116.73 °C and 117.38 °C, which is similar to the corresponding DSC data of the CPCM1–6. The PA onset meting temperatures of the composites is around 1 °C lower than that of pure PA, but for the HDPE, the descending temperature is around 8 °C. As seen in Table 2, the melting and solidifying latent heat of the composites drop with the decrease of the PA content because only
Table 2 DSC data of the PA, HDPE, HPCM1–HPCM3 and CPCM1–CPCM6. Samples Melting
Solidifying
Mass fraction of PA (%)
Temperature (°C) Latent heat (J/ g)
Temperature (°C) Latent heat (J/ g)
PA HDPE HPCM1
62.71 124.79 61.13a 115.48b
211.99 179.90 182.37a 17.86b
59.59 118.60 57.13a 111.60b
164.47 195.18 172.63a 18.423b
100 0 90
HPCM2
61.60a 116.73b
161.10a 37.95b
58.42a 112.21b
157.94a 37.34b
80
HPCM3
61.48a 117.38b
143.46a 58.04b
59.34a 112.87b
140.25a 54.50b
70
CPCM1
61.44a 116.16b
182.14a 17.89b
58.49a 113.32b
169.77a 18.49b
89.1
CPCM2
61.31a 116.70b
169.57a 36.52b
58.94a 113.68b
162.24a 37.22b
79.2
CPCM3
61.22a 117.49b
141.22a 58.82b
59.05a 115.15b
138.02a 52.90b
69.3
CPCM4
61.60a 117.59b
164.57a 35.22b
58.99a 114.54b
157.72a 34.41b
78.4
CPCM5
61.84a 117.58b
155.80a 41.32b
58.20a 114.91b
153.64a 36.07b
77.6
CPCM6
61.77a 117.30b
157.82a 35.33b
58.27a 115.68b
154.55a 35.31b
76.8
a b
The DSC data of the PA during melting and solidifying processes. The DSC data of the HDPE during melting and solidifying processes.
the PA plays role in thermal energy storage at operating temperature. Therefore, the melting/solidifying latent heat of the composite has linear relationship with the mass fraction of the PA, and the relationship expression is presented by following:
ΔHFSPCM = ηΔHPA
Fig. 8. Melting DSC curves of the CPCM1–CPCM6.
(1)
where ΔHFSPCM and ΔHPA represent the melting/solidifying latent heat of the PA/HDPE/GNP composites and pure PA. η is the mass fraction of the PA in the composites. The experimental values are slightly below the theoretical values, which result from the evaporation of the PA when the composites were blended at 160 °C. In general, the experimental and theoretical values are basically consistent. As the loading of the GNP in the doped FSPCM is very
Y. Tang et al. / Solar Energy Materials & Solar Cells 155 (2016) 421–429
427
Table 3 TGA data of the PA, HDPE, HPCM1–HPCM3 and CPCM1–CPCM6. Samples
Tpeak1 (°C)
ΔW1 (%)
PA HDPE HPCM1 HPCM2 HPCM3 CPCM1 CPCM2 CPCM3 CPCM4 CPCM5 CPCM6
285.07
100.0
271.99 266.43 266.43 264.58 271.37 267.05 277.99 275.08 286.19
91.2 80.6 71.5 88.5 80.2 70.1 78.5 78.3 77.6
Tpeak2 (°C)
ΔW2 (%)
Residue (%) (700 °C)
510.97 497.42 499.97 502.98 497.42 498.66 502.36 504.83 502.36 501.74
100.0 8.8 19.4 28.5 10.6 19.0 29.1 20.1 19.0 18.5
0 0 0 0 0 0.9 0.8 0.8 1.4 2.7 3.9
Table 4 Thermal cycling data of the HPCM1–HPCM3 and CPCM1–HPCM6. Samples Weight before cycling (g) Weight after cycling (g) Leakage rate (%) Fig. 10. TGA curves of the PA, HDPE and HPCM1–HPCM3.
small, the melting latent heat of the GNP doped FSPCM is slightly lower than that of corresponding PA/HDPE composite. 3.5. Thermal reliability and leakage analysis The TGA curves in Figs. 10–11 reveal the thermal reliability of the PA, HDPE, pure PA/HDPE composites and GNP doped PA/HDPE composites. The temperatures (Tpeak) of maximum thermal degradation rate, the percentage (ΔW) of the weight loss and the amount of residue are listed in Table 3, which are obtained from the TGA experiments. The Tpeak1 and ΔW1 represent the decomposition temperature and weight loss of the PA while the Tpeak2 and ΔW2 represent decomposition temperature and weight loss of the HDPE. The decomposition of the PA begins at around 180 °C and completes at around 320 °C, and the weight loss of the HDPE occurs from 450 °C to 600 °C. As seen in Table 3, the decomposition temperatures of the PA in the composites are close to that of the pure PA, which indicates that the modified FSPCM have good thermal reliability. In addition, the ΔW1 and ΔW2 correspond to the content of the PA and HDPE in the composites. As the maximum operating temperature in TGA experiments is below 700 °C, the residue can be considered as the GNP, and the content of the residue is also close to the loading of the GNP. These results demonstrate that the PA, HDPE and GNP are mixed successfully.
HPCM1 HPCM2 HPCM3 CPCM1 CPCM2 CPCM3 CPCM4 CPCM5 CPCM6
45.82 48.05 47.69 46.34 48.69 48.57 48.66 48.90 49.81
31.17 45.58 46.53 41.58 47.64 47.67 47.80 48.16 48.99
31.97 5.14 2.43 10.27 2.16 1.85 1.77 1.51 1.65
The leakage behavior is an important evaluation item for the FSPCM. This property is investigated by thermal cycling test. In this work, 150 times thermal cycling were conducted on the composite samples and the weight data before/after thermal cycling are listed in Table 4. The leakage of the PA/HDPE ( the mass ratio is 8∶2) composite was decreased significantly by the addition of the GNP. The broad surface of the GNP and the layer structure in the GNP doped FSPCM are possibly responsible for the little weight losses of the composite PCMs after thermal cycling. The result shows that the GNP can effectively prevent the leakage of the PA from modified FSPCM. Therefore, for the same leakage behavior of the FSPCM, the loading of the PA can increase in the GNP doped FSPCM thus the thermal enthalpy of the FSPCM can be increased. 3.6. Thermal conductivity analysis The thermal conductivities of the HCPM2, CPCM2 (1% GNP), CPCM4 (2% GNP), CPCM5 (3% GNP) and CPCM6 (4% GNP) with the same PA/HDPE ratio (8∶2) are shown in Fig. 12, and the relevant data are listed in Table 5. The thermal conductivity of the pure PA/ HDPE composite are 0.3280 W (m K) 1, and the thermal conductivities of the GNP doped FSPCM are 0.4174 W (m K) 1 (CPCM2), 0.5257 W (m K) 1 (CPCM4), 0.6570 W (m K) 1 (CPCM5) and 0.8219 W (m K) 1 (CPCM6). It is clearly found that the GNP can improve the thermal conductivity of the FSPCM significantly and the thermal conductivity of the GNP doped FSPCM will rise with the increase of the GNP mass fraction. In this work, the polynomial fitting is used to describe the relationship between thermal conductivity of the FSPCM and the mass fraction of the GNP. The solid line in Fig. 12 represents the polynomial fitting curve, and the fitting formula is given as following:
λ = 0.3289 + 7.53x + 118.1x2
Fig. 11. TGA curves of the CPCM1–CPCM6.
(2)
where x is mass fractions of the GNP. λ represents the thermal conductivity of the GNP doped FSPCM. The correlation coefficient
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of the PA in the FSPCM. The result of thermal cycling test shows that the GNP can help preventing the leakage of the PA from modified FSPCM effectively after 150 thermal cycles. The DSC result shows that the PA/HDPE composite with 4 wt% of the GNP (CPCM6) has high thermal enthalpy of 157.82 J/g and constant melting temperature of around 62 °C. The thermal conductivity of the FSPCM was measured by thermal conductivity meter and it increased to 0.8219 W (m K) 1 which is nearly 2.5 times as high as that of the pure FSPCM, when the mass fraction of the GNP is 4%. In general, thanks to the high specific surface area and thermal conductivity of the GNP, the satisfied FSPCM with 4 wt% of the GNP has capacities of high thermal enthalpy and thermal conductivity, which guarantees the promising application in solar energy and building heating systems.
Acknowledgments Fig. 12. Experimental and fitting thermal conductivities of the modified FSPCM (HPCM2) with different content of the GNP. Table 5 Thermal conductivity of the HPCM2, CPCM2 and CPCM4–CPCM6. Samples
Thermal conductivity at 30 °C (W (m K) 1)
HPCM2 (80% PA) CPCM2 (1% GNP) CPCM4 (2% GNP) CPCM5 (3% GNP) CPCM6 (4% GNP)
0.3280 7 0.0115 0.41747 0.0098 0.52577 0.0157 0.65707 0.0144 0.82197 0.0131
This work was supported by the National Natural Science Foundation of China (Grant no. 51376087) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors also wish to thank the reviewers and editor for kindly giving revising suggestions.
References
Table 6 Comparison of the thermal conductivity of the modified FSPCM with results of the previous works. Samples
Thermal conductivity (W (m K) 1)
Reference
Paraffin þ 7.5% nano graphite PA/polypyrroleþ1.6% GNP PA/polyaniline þ7.87% exfoliated graphite nanoplatelets PA/HDPE þ 4% GNP
0.5685 0.43 1.08
[39] [19] [38]
0.8219
Present study
of the Eq. (2) is at least 0.9997. The Eq. (2) can't be simplified to the first order-linear equation because the quadratic coefficient in Eq. (2) is positive and large, which indicates that the effect of the GNP on the thermal conductivity enhancement of the GNP doped FSPCM will be stronger with the increasing content of the GNP. The thermal conductivities of the PCM modified by carbon nanoplatelets in previous literatures are listed in Table 6 for comparison with that in this work. From Table 6, it is confirmed that in this work the modified FSPCM has obtained a good thermal conductivity enhancement.
4. Conclusions In this work, the preparation, characterization and thermal properties of the PA/HDPE and PA/HDPE/GNP composites as modified FSPCM were presented. The SEM result shows that the PA is uniformly dispersed into the network structure of the HDPE and the PA/HDPE composite is attached to the broad surface of the GNP. According to the results of the FT-IR and XRD, the modified FSPCM has stabilized chemical structure and crystalline phase, and no chemical reaction takes place among components. The TGA data reveal the high decomposition temperatures (around 275 °C)
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