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graphene nanoplatelets
Accepted Manuscript Title: Preparation and thermal properties of form-stable phase change materials composed of palmitic acid/polypyrrole/graphene nanoplatelets Author: Mahyar Silakhori Hadi Fauzi Mohammad R. Mahmoudian Hendrik Simon Cornelis Metselaar Teuku Meurah Indra Mahlia Hossein Mohammad Khanlou PII: DOI: Reference:
S0378-7788(15)00341-2 http://dx.doi.org/doi:10.1016/j.enbuild.2015.04.042 ENB 5834
To appear in:
ENB
Received date: Revised date: Accepted date:
10-1-2015 16-4-2015 21-4-2015
Please cite this article as: M. Silakhori, H. Fauzi, M.R. Mahmoudian, H.S.C. Metselaar, T.M.I. Mahlia, H.M. Khanlou, Preparation and thermal properties of form-stable phase change materials composed of palmitic acid/polypyrrole/graphene nanoplatelets, Energy and Buildings (2015), http://dx.doi.org/10.1016/j.enbuild.2015.04.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Preparation and thermal properties of form-stable phase change materials composed of palmitic acid/polypyrrole/graphene nanoplatelets
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Mahyar Silakhori1*, Hadi Fauzi 1, Mohammad R Mahmoudian1, Hendrik Simon Cornelis Metselaar 1*, Teuku Meurah Indra Mahlia2, Hossein Mohammad Khanlou 1
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Department of Mechanical Engineering and Advanced Material Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia 2
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Department of Mechanical Engineering, Universiti Tenaga Nasional, 43000 Kajang, Selangor, Malaysia
Highlights
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The thermal conductivity of PA/PPy improved with adding GNPs for 1.6% by 34.3%.
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The lowest latent heat storage capacity of prepared composite is 151J/g.
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Prepared PA/PPy/GNPs composite PCMs have more favorable properties in solar systems.
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Abstract In this study, the thermal conductivity of form-stable phase change materials (PCMs) is improved by the addition of graphene nanoplatelets (GNPs). In the first step, the dispersed GNPs were mixed with palmitic acid (PA) particles using an ultra-sonication method. Then, the prepared material was added to form-stable phase change materials (PA/PPy) obtained by a polymerisation technique. The amount of PA particles remained constant in the prepared form-stable phase change materials. FT-IR and SEM analyses were used to investigate the structure and surface morphology of form-stable PCMs, respectively. The thermal properties, stability and conductivity of the materials were also characterised using DSC, TGA and thermal conductivity analysis, respectively. The results indicated that GNPs had a significant effect on improving the thermal conductivity of form-stable PCMs (PA/PPy). By doping 1.6% of GNPs, the thermal conductivity and thermal capacity of PA/PPy/GNPs form-stable PCMs could reach up to 0.43 W/m K and 151 J/g, respectively. In other words, thermal conductivity increased by 34.3% in comparison with PA/PPy form-stable PCMs. The prepared PA/PPy/GNPs form-stable PCMs are anticipated to show improved performance for solar thermal energy storage applications.
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Keywords: Phase Change Materials (PCMs), Thermal energy storage, Thermal properties, Graphene nanoplatelets.
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*Corresponding authors. Tel: +603-7967 4451 (H.S.C.M.); Fax: +603-7967 44 (H.S.C.M.). E-Mails: [email protected] (H.S.C.M.), [email protected] (M.S.).
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1. Introduction
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Renewable energy plays an important role in the world due to environmental concerns and the considerable price of fossil fuel. Solar energy, which is inexhaustible, abundant and clean, is the most promising form of renewable energy. There are different forms of energy, such as solar-thermal, electrical and chemical; of these, the conversion of solar energy to thermal and electrical energy has been applied successfully, but solar-chemical conversion is still in the research stage [1, 2]. The significant thermal conversion efficiency, low price and small scale of equipment are the main advantages of solar thermal applications [3]. A significant
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drawback of solar energy is the diurnal fluctuation of solar radiation, which makes the use of thermal energy storage indispensable.
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In this regard, latent thermal energy storage in the field of phase change materials (PCMs) has significant merits, such as a high storage density at small temperature intervals and low storage media costs [4, 5]. Materials that are used as solid-liquid PCMs therefore have a capacity for thermal energy storage. A form-stable PCM is defined as a composite structure with solid-liquid PCMs and supporting materials [6]. A significant part of the available literature has addressed form-stable PCMs. Kenisarin and Kenisarina published a paper in which they reviewed the latest developments of form-stable PCMs [7]. In general, latent heat thermal storage materials such as paraffin [8-10], fatty acids [11] and their mixtures [12, 13], are mostly used as solid–liquid PCMs in a stable form. When the temperature exceeds the melting temperature of the solid–liquid PCM, the polymers and inorganic materials act as supporting material [14-21].
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One of the main factors in thermal energy storage is high thermal conductivity of PCMs, which improves the speed of thermal storage/release and the efficiency of the solar thermal device. However, one major drawback of PCMs combined with polymer structures is their low thermal conductivity [22]. Therefore, it is worthwhile to improve the thermal conductivity of form-stable PCMs to consequently improve performance. Studies have been published on the role of copper [23], nickel foam [24], carbon nanotubes (CNTs) [25], carbon nanofibers (CNFs) [26], carbon foam [27] graphene oxide [28], and graphene platelets (xGnP) [29, 30] in improving the thermal conductivity of phase change materials. Furthermore, previous research has indicated that copper nanowire [31], Ag nanowire [32] and expanded graphite [33] have a positive impact on the thermal conductivity of form-stable phase change materials. However, the significant particle size of worm-like expanded graphite and its ability to absorb organic material make the dispersion of expanded graphite in form-stable PCMs a challenging issue [22]. This problem is more obvious when considering a polymer structure as a supporting material. Graphene nanoplatelets (GNPs) are particles that consist of several layers of graphene sheets and possess a high aspect ratio and high thermal conductivity [29]. Because the size of GNPs is much smaller than that of wormlike expanded graphite, GNPs are more conductive and can thus be incorporated with matrix materials to improve the electrical, thermal and mechanical properties of composite materials [34].
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The major objective of this study was to investigate the effect of GNPs on PA-PPy formstable PCMs, which were prepared for a previous study [35]. Different percentages of GNPs were integrated in form-stable-PCMs containing PA and PPy as the solid–liquid PCM and supporting material, respectively. Form-stable PCMs doped with GNPs (PA/PPy/GNPs formstable PCMs) were studied in terms of structure, thermal properties and thermal conductivity. The palmitic acid/polypyrrole/graphene nanoplatelets as form-stable PCM prepared in this study is suitable for heating and cooling applications in building such as water heating and floor heating.
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2. Experimental
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2.1. Materials
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Palmitic acid (PA) was used as a latent heat storage material with a melting point of 61–62ºC. Pyrrole (Py) (C4H5N), ammonium persulphate (APS) ((NH4)2S2O8) and sodium dodecylsulphonate (SDS) operated as supporting material, oxidant and surfactant, respectively. GNPs with a specific surface area of 300m2/g were purchased from XG
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Sciences (USA). Table 1 shows the properties of these graphene nanoplatelets. All commercial chemicals were used as received without further purification. Water purification was performed using distillation followed by deionisation with the aid of ion-exchange resins.
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Table 1. Properties of Graphene Nanoplatelets (GNPs)
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2.2. Preparation of PA/PPy/GNPs form-stable phase change materials
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Material preparation begins with the dispersion of GNPs in 20 ml of ethanol by ultrasonication for half an hour at 70˚C. Then, a constant amount of PA (2.4 g) is added to the GNPs/ethanol followed by ultrasonication for half an hour under the same conditions to achieve a homogenous mixture. Next, 0.5 g of sodium dodecylsulfonate (SDS) was added to 100 ml of water and heated under stirring until reaching a temperature of 70˚C. Then, the PA/GNPs/ethanol mixture was added to this solution and stirred for 1 hour at 70 ˚C to prepare a stable emulsion. Pyrrole monomer was added to the solution and stirred for half an hour under the same conditions. Then, the temperature of the solution was reduced to 5˚C with the help of an ice bath. To initiate polymerisation, 20 ml of water containing ammonium persulphate (APS) was added drop-wise to the solution over the course of 15 min. The molar ratio of pyrrole and APS was kept constant at 1:1. The mixture was stirred for 12 h, then washed and filtered with water until the filter became clear. The products were dried at 70˚C in a vacuum oven to obtain PA/PPy/GNPs form-stable phase change materials. Table 2 indicates the amount of PA, PPy and GNPs in each sample, designated as S1, S2, S3 and S4.
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2.3. Characterisation of form-stable PCMs
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The thermal properties and chemical structure of form-stable PCMs were investigated using a differential scanning calorimeter (DSC) (model: METTLER TOLEDO 820C) and Fourier transform infrared spectrophotometer (FTIR) (model: Bruker tensor 27), respectively. Moreover, thermogravimetric analysis (TGA) (model: METTLER TOLEDO SDTA 851) was used to measure the thermal stability and weight loss of the form-stable PCMs. The resulting degradation rate was measured as a function of temperature. The microstructure of the samples was investigated by scanning electron microscopy (SEM) (model: SU8000 HITACHI). X-ray diffractometry (XRD) (model: EMPYREAN, PANALYTICAL) was employed for the analysis of the crystalloid phase of the form-stable PCMs. A Hot Disk Thermal Constants Analyzer (TPS 2500S, Hot Disk AB, Sweden), based on design principles ideal for screening homogeneous and isotropic products, was used to measure the thermal conductivity of the form-stable phase change materials. . In addition, the compact hydraulic press powder pelletiser with the pressure of 2 ton was used for compacting the samples into pellets in a steel mold with a diameter of 12mm.
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3. Results and discussion
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3.1. Form stability of prepared form-stable PCMs
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In our previous study, form-stable PA/PPy was prepared successfully [35]. It was found that the highest percentage of PA in the form-stable PCMs was approximately 79.9%. Because the purpose of this study is to explore the effect of GNPs on the thermal conductivity of form-stable PCMs, the PCM content was kept constant at 79.9% while the content of PPy and
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GNPs was changed. For uniform dispersion of GNPs and PA, ethanol was used as the solvent. The form-stable PA/PPy/GNPs were prepared based on the method described in our previous work [35]. For investigating the form-stability of the samples, the samples were stored in an oven at 80ºC (Fig. 1a-b). It is interesting to note that the shape of the prepared form-stable PCM disks remained unchanged (Fig. 1-d). However, the PA disc was completely melted and deformed in the testing process (Fig. 1-c). These findings further support the favourable stability of the form-stable PCMs (PA/PPy/GNPs).
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Fig. 1. PA and S4 cakes before (a and b) and after (c and d) being heated at 80ºC
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3.2 FT-IR analysis
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Fig. 2 depicts the FTIR spectra of PA, PPy, GNPs and form-stable PCMs. The spectrum of PA at 1698.1 cm-1 corresponds to the C=O stretching vibration. Moreover, the peaks at 2914.2 cm-1 and 2848.6 cm-1 illustrate the symmetric stretching vibration of PA. The in-plane bending vibration on the -OH group of palmitic acid, the out-of-plane bending vibration of the -OH functional group and the in-plane swinging vibration of the -OH functional group are at 1328.8 cm-1, 941 cm-1 and 730 cm-1, respectively. Furthermore, the ring vibration and the C-H band in-plane vibration of PPy are shown at 1558, 1467 cm-1 and 1295 cm-1, respectively. In addition, the peaks at 1184 cm-1 and 789, 963 cm-1 correspond to the C-N stretching vibration and the presence of polymerised pyrrole. The C–H out-of-plane vibration at 901 cm-1 is due to the polymerisation of pyrrole [36]. The IR spectra of the form-stable PCMs include PA and PPy along with GNPs. This means that the absorption peaks in the spectra of form-stable PCMs are matched with corresponding spectra of the PA, PPy and GNPs. However, the characteristic peaks of GNPs are not present on the spectrum of PA/PPy/GNPs form-stable PCM due to its weak absorption and the overlapping of absorption peaks in the range of 2800-3000 cm-1. According to the FTIR spectra, the previously mentioned key absorption peaks in the composite PCM remained constant. Based on the results, it can be deduced that the form-stable PCMs contain PA, PPy and GNPs with no significant chemical interaction.
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Fig. 2 FTIR spectra of PPy, GNPs, PA, form-stable PA/PPy and S4
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3.3. Morphology investigation of prepared form-stable PCMs
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Fig. 3 depicts SEM images of GNPs, form-stable PA/PPy and PA/PPy/GNPs. This figure shows the surface area of raw GNPs, which is a considerable factor in form-stable phase change materials (Fig. 3a). The high surface area of GNPs leads to mechanically interconnected composites and significant mechanical reinforcement. Moreover, it has been shown that form-stable PCMs consist of PA and PPy as latent thermal energy storage and supporting materials, respectively (Fig. 3b). This implies that the PA particles were wrapped by PPy particles that exert loads on the surface and absorb the melting PCMs into their pores [35]. Form-stable PA/PPy samples were characterised by a granular particle structure; however, this material underwent a considerable change into flake-like particles when GNPs were added (Fig, 3c-f). GNPs were mixed with PA with the help of ethanol and ultrasonication and then added to water containing a surfactant (SDS) to prepare the emulsion solution. The GNP particles were covered by PA particles due to the favourable tendency of GNP particles to absorb organic materials on their surface [37]. PPy films then began to cover the surface of PA/GNPs when polymerisation was initiated by dropping APS. Notably, 4 Page 4 of 20
the pristine GNPs had a smooth surface area in comparison with the rough surface area of PA/PPy/GNPs form-stable PCMs (Fig. 3c-f), suggesting that the surface of the GNPs was covered by something. IR and XRD analysis show that the form-stable PCM is made of PA, PPy and GNPs. It can be inferred that the GNP particles were covered by PA particles due to the observed properties of the former material.
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In addition, significant absorption effects among organic materials caused the PPy particles to be absorbed by GNPs/PA in the process of polymerisation. It was also noted that the GNPs/PA mixture was fluidised when the composite reached its melting temperature and above (70ºC). Therefore, the mixture of GNPs/PA is not form-stable. In contrast, only the mixture decorated by PPy yielded form-stable PCM. In this case, the GNPs/PA particles are covered by a film of PPy particles, which creates stable composite PCMs.
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Fig. 3. SEM images of GNPs (a), PA/PPy (b), S1 (c), S2 (d), S3 (e), S4 (f)
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3.4. XRD characterisation
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Fig. 4 shows the XRD patterns of PPy, PA, GNPs and selected form-stable PCMs. The broad amorphous diffraction peak observed in the range of 2Ө = 15–35̊ in the XRD diffractogram of the PPy can be attributed to scattering of the bare polymer chains in interplanar spacing [36]. The appearance of peaks at approximately 13̊, 24̊, 25̊, 27̊ confirmed the formation of highly crystalline PA (JCPDS#008-0787O) with a monoclinic system, space group of P21/a and space group number of 14. In the case of PA/PPy, similar diffraction peaks were observed; however, the broad peak of PPy in form-stable PCMs causes the PA peaks at 24º and 25º to grow slightly wider. This suggests that a fraction of the crystalline phase is affected by the ratio of PA to PPy. The peak at 2Ө =26 ̊ is the main peak in GNPs, which is in good agreement with the results of previous studies [38]. This particular GNP peak was also observed in the XRD patterns of PA/PPy/GNPs form-stable PCMs (S4) without any significant difference. PA-specific peaks were also revealed in the XRD patterns of formstable PA/PPy and S4. It is clear that the PA/PPy/GNPs form-stable PCMs are simply a combination of PA, PPy and GNPs, with no production of new phases. However, it is clear that the volume fraction of crystalline phases increased when the GNPs were added to the PA/PPy. In a similar manner, Helmut Münstedt showed that the addition of carbon nanostructures increased the crystallinity and rheological properties of polycarbonate (PC) as a particular group of thermoplastic polymers [39].
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Fig. 4. XRD pattern of PA, PPy, GNPs, form-stable PA/PPy and S4
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3.5. Thermal stability investigation
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The thermal stability results of PA, PPy, and form-stable PCMs are shown in Fig. 5. It is obvious that the pure PA particles decompose in one step, while PPy particles degrade in three stages. The thermal decomposition of PA molecular chains occurs from 230 ºC to 320 ºC due to its evaporation. The first, second and third stage of PPy thermal degradation correspond to the loss of water, the loss of dopant and the decomposition of the polymer backbone, respectively [35]. The main degradation of form-stable PCMs occurs in the second step. However, the TG curves of form-stable PCMs depict the loss of water and the decomposition of the polymer backbone. It is noticeable that the moisture of water in the 5 Page 5 of 20
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form-stable PCMs does not have any significant effect on the backbone of the polymer. Therefore, form-stable PCMs have favourable thermal stability in regard to their use as a phase change material with melting temperature of approximately 62ºC.
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Fig. 5. TG curves of PA, PPy, and form-stable PCMs
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3.6. Thermal energy storage properties
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The thermal energy storage properties of PA, PA/PPy, and form-stable PA/PPy/GNPs, which were analysed by DSC, are shown in Fig. 6 and Table 2. To improve accuracy, each sample was measured three times. Fig. 6 depicts the DSC curve of the samples. The DSC curve significantly indicates one major endothermic and exothermic peak in each sample with a peak temperature between 60ºC and 63ºC. Furthermore, the samples’ average enthalpy, onset and peak temperatures for melting and solidification were determined and are listed in Table 2. These data show that the melting and solidifying temperatures of form-stable PA/PPy/GNPs were altered to between 60ºC and 62ºC, compared with those of form-stable PA/PPy, which changed to the range of 59ºC to 60ºC. This change is not considerable for thermal energy storage applications.
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In addition, Table. 2 indicates that the enthalpy of form-stable PA/PPy/GNPs decreased from 163 to 151 J/g, which is higher than that of the previous form-stable PCMs indicated by other researchers [33, 40-43]. It is clear that the form-stable PCMs show a favourable thermal storage density. On the other hand, it is obvious that PA plays a main role in storing thermal energy [44-45]. There is a minor difference between the enthalpy of form-stable PA/PPy PCMs and those of PA/PPy/GNPs (approximately 9%). This means that the surface of GNPs was covered by PA particles, which lead to a decrease in the energy storage ability of PA. This is due to the increase in the absorption of PA with increased GNP loading. Therefore, it may be beneficial to add a small amount of GNPs to keep the energy storage almost constant, as well as to improve the thermal conductivity of form-stable PCMs.
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Table 2. DSC data analysis for PA, PPy and for PA/PPy/GNPs form-stable PCMs
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Fig. 6. DSC results for PA and prepared form-stable PCMs
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3.7. Thermal conductivity investigation
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The thermal conductivity of the prepared form-stable PCMs is shown in Fig. 7. The figure indicates that the thermal conductivity of the PA/PPy/GNPs form-stable PCMs rose linearly with increased GNP particles. The thermal conductivity of PA and PA/PPy form-stable PCMs was approximately 0.31 and 0.32 W/(m K),respectively, while the thermal conductivity of the form-stable PCM doped with 1.6 wt% GNPs (S4) was measured to be 0.43 W/(m K). That is, the thermal conductivity of PA/PPy/GNPs form-stable PCM was 38.7% higher than that of PA particles and 34.3% better than that of PA/PPy form-stable PCM. Therefore, the thermal performance of form-stable PCMs that were doped with GNPs was greater than that of PA/PPy form-stable PCMs and PA. Moreover, the figure shows that 6 Page 6 of 20
the thermal conductivity tends to increase, which means that GNP particles are a candidate for improving the thermal conductivity of polymer-based form-stable PCMs.
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In solar thermal applications, where storing thermal energy is essential due to the fluctuation of solar radiation, the thermal energy storage density and the speed of thermal energy storage and release play a significant role. For real applications of PCMs, form-stable PCMs are more reliable. However, the main drawback of the PA used for latent thermal energy storage is its low thermal conductivity. Therefore, form-stable PA/PPy/GNPs materials are expected to be suitable for solar energy applications due to their relatively higher latent thermal energy storage (151 J/g), thermal conductivity (0.43 W/(mK)) and thermal stability.
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Fig. 7. Thermal conductivity of PA/PPy and PA/PPy/GNPs form-stable PCMs
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4. Conclusion
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In this study, novel PA/PPy/GNPs form-stable PCMs were prepared by an in-situ polymerisation method. PA and PPy were used as solid-liquid PCMs and supporting materials, while GNP particles were applied as filler to improve the thermal conductivity. The percentage of PA remained constant for all samples at 79.9 wt%. However, the amount of GNPs increased from 0.4% to 1.6 wt%. In this investigation, the aim was to assess the chemical structure, thermal properties, thermal stability and thermal conductivity of PA/PPy/GNPs form-stable PCMs. The results indicate that PA/PPy/GNPs form-stable phase change materials have an appropriate latent heat thermal energy storage (151 J/g) and enhanced thermal stability. Moreover, the thermal conductivity of PA/PPy/GNPs form-stable PCMs was improved by 38.7% and 34.3% in comparison with that of PA particles and PA/PPy form-stable PCMs, respectively. Overall, form-stable PA/PPy/GNPs could be considered applicable candidates for low-temperature solar thermal energy storage applications.
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Acknowledgement
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The authors would like to acknowledge the University of Malaya for financial support. This research was carried out under the high impact research grant No. UM.C/HIR/MOHE/ENG/21-(D000021-16001) and University of Malaya research grant No. UMRG RP021-2012A.
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[32] J.L. Zeng, Z. Cao, D.W. Yang, L.X. Sun, L. Zhang, Thermal conductivity enhancement of Ag nanowires on an organic phase change material, J Therm Anal Calorim, 101 (1) (2010) 385-389.
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[34] E.V. Kuvardina, L.A. Novokshonova, S.M. Lomakin, S.A. Timan, I.A. Tchmutin, Effect of the graphite nanoplatelet size on the mechanical, thermal, and electrical properties of polypropylene/exfoliated graphite nanocomposites, Journal of Applied Polymer Science, 128 (3) (2013) 1417-1424.
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[35] M. Silakhori, H.S.C. Metselaar, T.M.I. Mahlia, H. Fauzi, S. Baradaran, M.S. Naghavi, Palmitic acid/polypyrrole composites as form-stable phase change materials for thermal energy storage, Energy Conversion and Management, 80 (2014) 491-497.
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[38] Y. Li, J. Zhu, S. Wei, J. Ryu, L. Sun, Z. Guo, Poly (propylene)/graphene nanoplatelet nanocomposites: melt rheological behavior and thermal, electrical, and electronic properties, Macromolecular Chemistry and Physics, 212 (18) (2011) 1951-1959.
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[40] A. Sarı, C. Alkan, A. Biçer, A. Karaipekli, Synthesis and thermal energy storage characteristics of polystyrene-graft-palmitic acid copolymers as solid–solid phase change materials, Solar Energy Materials and Solar Cells, 95 (12) (2011) 3195-3201.
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[42] A. Sarı, A. Biçer, Thermal energy storage properties and thermal reliability of some fatty acid esters/building material composites as novel form-stable PCMs, Solar Energy Materials and Solar Cells, 101 (0) (2012) 114-122.
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[43] W. Wang, X. Yang, Y. Fang, J. Ding, Preparation and performance of form-stable polyethylene glycol/silicon dioxide composites as solid–liquid phase change materials, Applied Energy, 86 (2) (2009) 170-174.
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[44] L. Xia, P. Zhang, R. Wang, Preparation and thermal characterization of expanded graphite/paraffin composite phase change material, Carbon, 48 (9) (2010) 2538-2548.
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[45] A. Eddhahak, S. Drissi, J. Colin, S. Caré, J. Neji, Effect of phase change materials on the hydration reaction and kinetic of PCM-mortars, J Therm Anal Calorim, 117 (2) (2014) 537545.
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Figure and Table captions
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Fig. 1. PA and S4 cakes before (a and b) and after (c and d) being heated at 80ºC
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Fig. 2. FTIR spectra of PPy, GNPs, PA, form-stable PA/PPy and S4
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Fig. 3. SEM images of GNPs(a), PA/PPy(b), S1(c), S2(d), S3(e), S4(f)
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Fig. 4. XRD pattern of PA, PPy, GNPs, form-stable PA/PPy and S4
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Fig. 5. TG curves of PA, PPy, and form-stable PCMs
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Fig. 6. DSC results for PA and prepared form-stable PCMs
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Fig. 7. Thermal conductivity of PA/PPy and PA/PPy/GNPs form-stable PCMs
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Table 1. Properties of Graphene Nanoplatelets (GNPs)
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Table 2. DSC data analysis for PA, PPy and PA/PPy/GNPs form-stable PCMs
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Table 1. Specific surface area (m2/g) Width (µm) Thickness (nm) Thermal conductivity (W/mK)
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Table 2. PA/PPy 2.4 0.6 79.9 59.8 63.9 58.8 58.5 166.3 170.7
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a. Onset temperature for melting.
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b. Peak temperature for melting.
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*wt % = (ΔH Melting (form-stable PCM) / ΔH Melting (PCM) )*100
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Samples PA (g) Pyrrole (g) GNPs (g) Loading of GNPs (wt %) Loading of PA (wt %)* TMo (ºC)a TMp (ºC)b TSo (ºC)c TSp (ºC)d ΔHMelting (J/g) ΔHSolidification (J/g)
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S0378-7788(15)00341-2 http://dx.doi.org/doi:10.1016/j.enbuild.2015.04.042 ENB 5834
To appear in:
ENB
Received date: Revised date: Accepted date:
10-1-2015 16-4-2015 21-4-2015
Please cite this article as: M. Silakhori, H. Fauzi, M.R. Mahmoudian, H.S.C. Metselaar, T.M.I. Mahlia, H.M. Khanlou, Preparation and thermal properties of form-stable phase change materials composed of palmitic acid/polypyrrole/graphene nanoplatelets, Energy and Buildings (2015), http://dx.doi.org/10.1016/j.enbuild.2015.04.042 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Preparation and thermal properties of form-stable phase change materials composed of palmitic acid/polypyrrole/graphene nanoplatelets
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Mahyar Silakhori1*, Hadi Fauzi 1, Mohammad R Mahmoudian1, Hendrik Simon Cornelis Metselaar 1*, Teuku Meurah Indra Mahlia2, Hossein Mohammad Khanlou 1
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Department of Mechanical Engineering and Advanced Material Research Center, University of Malaya, 50603 Kuala Lumpur, Malaysia 2
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Department of Mechanical Engineering, Universiti Tenaga Nasional, 43000 Kajang, Selangor, Malaysia
Highlights
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The thermal conductivity of PA/PPy improved with adding GNPs for 1.6% by 34.3%.
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The lowest latent heat storage capacity of prepared composite is 151J/g.
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Prepared PA/PPy/GNPs composite PCMs have more favorable properties in solar systems.
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Abstract In this study, the thermal conductivity of form-stable phase change materials (PCMs) is improved by the addition of graphene nanoplatelets (GNPs). In the first step, the dispersed GNPs were mixed with palmitic acid (PA) particles using an ultra-sonication method. Then, the prepared material was added to form-stable phase change materials (PA/PPy) obtained by a polymerisation technique. The amount of PA particles remained constant in the prepared form-stable phase change materials. FT-IR and SEM analyses were used to investigate the structure and surface morphology of form-stable PCMs, respectively. The thermal properties, stability and conductivity of the materials were also characterised using DSC, TGA and thermal conductivity analysis, respectively. The results indicated that GNPs had a significant effect on improving the thermal conductivity of form-stable PCMs (PA/PPy). By doping 1.6% of GNPs, the thermal conductivity and thermal capacity of PA/PPy/GNPs form-stable PCMs could reach up to 0.43 W/m K and 151 J/g, respectively. In other words, thermal conductivity increased by 34.3% in comparison with PA/PPy form-stable PCMs. The prepared PA/PPy/GNPs form-stable PCMs are anticipated to show improved performance for solar thermal energy storage applications.
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Keywords: Phase Change Materials (PCMs), Thermal energy storage, Thermal properties, Graphene nanoplatelets.
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*Corresponding authors. Tel: +603-7967 4451 (H.S.C.M.); Fax: +603-7967 44 (H.S.C.M.). E-Mails: [email protected] (H.S.C.M.), [email protected] (M.S.).
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1. Introduction
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Renewable energy plays an important role in the world due to environmental concerns and the considerable price of fossil fuel. Solar energy, which is inexhaustible, abundant and clean, is the most promising form of renewable energy. There are different forms of energy, such as solar-thermal, electrical and chemical; of these, the conversion of solar energy to thermal and electrical energy has been applied successfully, but solar-chemical conversion is still in the research stage [1, 2]. The significant thermal conversion efficiency, low price and small scale of equipment are the main advantages of solar thermal applications [3]. A significant
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drawback of solar energy is the diurnal fluctuation of solar radiation, which makes the use of thermal energy storage indispensable.
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In this regard, latent thermal energy storage in the field of phase change materials (PCMs) has significant merits, such as a high storage density at small temperature intervals and low storage media costs [4, 5]. Materials that are used as solid-liquid PCMs therefore have a capacity for thermal energy storage. A form-stable PCM is defined as a composite structure with solid-liquid PCMs and supporting materials [6]. A significant part of the available literature has addressed form-stable PCMs. Kenisarin and Kenisarina published a paper in which they reviewed the latest developments of form-stable PCMs [7]. In general, latent heat thermal storage materials such as paraffin [8-10], fatty acids [11] and their mixtures [12, 13], are mostly used as solid–liquid PCMs in a stable form. When the temperature exceeds the melting temperature of the solid–liquid PCM, the polymers and inorganic materials act as supporting material [14-21].
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One of the main factors in thermal energy storage is high thermal conductivity of PCMs, which improves the speed of thermal storage/release and the efficiency of the solar thermal device. However, one major drawback of PCMs combined with polymer structures is their low thermal conductivity [22]. Therefore, it is worthwhile to improve the thermal conductivity of form-stable PCMs to consequently improve performance. Studies have been published on the role of copper [23], nickel foam [24], carbon nanotubes (CNTs) [25], carbon nanofibers (CNFs) [26], carbon foam [27] graphene oxide [28], and graphene platelets (xGnP) [29, 30] in improving the thermal conductivity of phase change materials. Furthermore, previous research has indicated that copper nanowire [31], Ag nanowire [32] and expanded graphite [33] have a positive impact on the thermal conductivity of form-stable phase change materials. However, the significant particle size of worm-like expanded graphite and its ability to absorb organic material make the dispersion of expanded graphite in form-stable PCMs a challenging issue [22]. This problem is more obvious when considering a polymer structure as a supporting material. Graphene nanoplatelets (GNPs) are particles that consist of several layers of graphene sheets and possess a high aspect ratio and high thermal conductivity [29]. Because the size of GNPs is much smaller than that of wormlike expanded graphite, GNPs are more conductive and can thus be incorporated with matrix materials to improve the electrical, thermal and mechanical properties of composite materials [34].
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The major objective of this study was to investigate the effect of GNPs on PA-PPy formstable PCMs, which were prepared for a previous study [35]. Different percentages of GNPs were integrated in form-stable-PCMs containing PA and PPy as the solid–liquid PCM and supporting material, respectively. Form-stable PCMs doped with GNPs (PA/PPy/GNPs formstable PCMs) were studied in terms of structure, thermal properties and thermal conductivity. The palmitic acid/polypyrrole/graphene nanoplatelets as form-stable PCM prepared in this study is suitable for heating and cooling applications in building such as water heating and floor heating.
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2. Experimental
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2.1. Materials
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Palmitic acid (PA) was used as a latent heat storage material with a melting point of 61–62ºC. Pyrrole (Py) (C4H5N), ammonium persulphate (APS) ((NH4)2S2O8) and sodium dodecylsulphonate (SDS) operated as supporting material, oxidant and surfactant, respectively. GNPs with a specific surface area of 300m2/g were purchased from XG
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Sciences (USA). Table 1 shows the properties of these graphene nanoplatelets. All commercial chemicals were used as received without further purification. Water purification was performed using distillation followed by deionisation with the aid of ion-exchange resins.
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Table 1. Properties of Graphene Nanoplatelets (GNPs)
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2.2. Preparation of PA/PPy/GNPs form-stable phase change materials
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Material preparation begins with the dispersion of GNPs in 20 ml of ethanol by ultrasonication for half an hour at 70˚C. Then, a constant amount of PA (2.4 g) is added to the GNPs/ethanol followed by ultrasonication for half an hour under the same conditions to achieve a homogenous mixture. Next, 0.5 g of sodium dodecylsulfonate (SDS) was added to 100 ml of water and heated under stirring until reaching a temperature of 70˚C. Then, the PA/GNPs/ethanol mixture was added to this solution and stirred for 1 hour at 70 ˚C to prepare a stable emulsion. Pyrrole monomer was added to the solution and stirred for half an hour under the same conditions. Then, the temperature of the solution was reduced to 5˚C with the help of an ice bath. To initiate polymerisation, 20 ml of water containing ammonium persulphate (APS) was added drop-wise to the solution over the course of 15 min. The molar ratio of pyrrole and APS was kept constant at 1:1. The mixture was stirred for 12 h, then washed and filtered with water until the filter became clear. The products were dried at 70˚C in a vacuum oven to obtain PA/PPy/GNPs form-stable phase change materials. Table 2 indicates the amount of PA, PPy and GNPs in each sample, designated as S1, S2, S3 and S4.
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2.3. Characterisation of form-stable PCMs
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The thermal properties and chemical structure of form-stable PCMs were investigated using a differential scanning calorimeter (DSC) (model: METTLER TOLEDO 820C) and Fourier transform infrared spectrophotometer (FTIR) (model: Bruker tensor 27), respectively. Moreover, thermogravimetric analysis (TGA) (model: METTLER TOLEDO SDTA 851) was used to measure the thermal stability and weight loss of the form-stable PCMs. The resulting degradation rate was measured as a function of temperature. The microstructure of the samples was investigated by scanning electron microscopy (SEM) (model: SU8000 HITACHI). X-ray diffractometry (XRD) (model: EMPYREAN, PANALYTICAL) was employed for the analysis of the crystalloid phase of the form-stable PCMs. A Hot Disk Thermal Constants Analyzer (TPS 2500S, Hot Disk AB, Sweden), based on design principles ideal for screening homogeneous and isotropic products, was used to measure the thermal conductivity of the form-stable phase change materials. . In addition, the compact hydraulic press powder pelletiser with the pressure of 2 ton was used for compacting the samples into pellets in a steel mold with a diameter of 12mm.
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3. Results and discussion
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3.1. Form stability of prepared form-stable PCMs
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In our previous study, form-stable PA/PPy was prepared successfully [35]. It was found that the highest percentage of PA in the form-stable PCMs was approximately 79.9%. Because the purpose of this study is to explore the effect of GNPs on the thermal conductivity of form-stable PCMs, the PCM content was kept constant at 79.9% while the content of PPy and
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GNPs was changed. For uniform dispersion of GNPs and PA, ethanol was used as the solvent. The form-stable PA/PPy/GNPs were prepared based on the method described in our previous work [35]. For investigating the form-stability of the samples, the samples were stored in an oven at 80ºC (Fig. 1a-b). It is interesting to note that the shape of the prepared form-stable PCM disks remained unchanged (Fig. 1-d). However, the PA disc was completely melted and deformed in the testing process (Fig. 1-c). These findings further support the favourable stability of the form-stable PCMs (PA/PPy/GNPs).
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Fig. 1. PA and S4 cakes before (a and b) and after (c and d) being heated at 80ºC
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3.2 FT-IR analysis
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Fig. 2 depicts the FTIR spectra of PA, PPy, GNPs and form-stable PCMs. The spectrum of PA at 1698.1 cm-1 corresponds to the C=O stretching vibration. Moreover, the peaks at 2914.2 cm-1 and 2848.6 cm-1 illustrate the symmetric stretching vibration of PA. The in-plane bending vibration on the -OH group of palmitic acid, the out-of-plane bending vibration of the -OH functional group and the in-plane swinging vibration of the -OH functional group are at 1328.8 cm-1, 941 cm-1 and 730 cm-1, respectively. Furthermore, the ring vibration and the C-H band in-plane vibration of PPy are shown at 1558, 1467 cm-1 and 1295 cm-1, respectively. In addition, the peaks at 1184 cm-1 and 789, 963 cm-1 correspond to the C-N stretching vibration and the presence of polymerised pyrrole. The C–H out-of-plane vibration at 901 cm-1 is due to the polymerisation of pyrrole [36]. The IR spectra of the form-stable PCMs include PA and PPy along with GNPs. This means that the absorption peaks in the spectra of form-stable PCMs are matched with corresponding spectra of the PA, PPy and GNPs. However, the characteristic peaks of GNPs are not present on the spectrum of PA/PPy/GNPs form-stable PCM due to its weak absorption and the overlapping of absorption peaks in the range of 2800-3000 cm-1. According to the FTIR spectra, the previously mentioned key absorption peaks in the composite PCM remained constant. Based on the results, it can be deduced that the form-stable PCMs contain PA, PPy and GNPs with no significant chemical interaction.
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Fig. 2 FTIR spectra of PPy, GNPs, PA, form-stable PA/PPy and S4
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3.3. Morphology investigation of prepared form-stable PCMs
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Fig. 3 depicts SEM images of GNPs, form-stable PA/PPy and PA/PPy/GNPs. This figure shows the surface area of raw GNPs, which is a considerable factor in form-stable phase change materials (Fig. 3a). The high surface area of GNPs leads to mechanically interconnected composites and significant mechanical reinforcement. Moreover, it has been shown that form-stable PCMs consist of PA and PPy as latent thermal energy storage and supporting materials, respectively (Fig. 3b). This implies that the PA particles were wrapped by PPy particles that exert loads on the surface and absorb the melting PCMs into their pores [35]. Form-stable PA/PPy samples were characterised by a granular particle structure; however, this material underwent a considerable change into flake-like particles when GNPs were added (Fig, 3c-f). GNPs were mixed with PA with the help of ethanol and ultrasonication and then added to water containing a surfactant (SDS) to prepare the emulsion solution. The GNP particles were covered by PA particles due to the favourable tendency of GNP particles to absorb organic materials on their surface [37]. PPy films then began to cover the surface of PA/GNPs when polymerisation was initiated by dropping APS. Notably, 4 Page 4 of 20
the pristine GNPs had a smooth surface area in comparison with the rough surface area of PA/PPy/GNPs form-stable PCMs (Fig. 3c-f), suggesting that the surface of the GNPs was covered by something. IR and XRD analysis show that the form-stable PCM is made of PA, PPy and GNPs. It can be inferred that the GNP particles were covered by PA particles due to the observed properties of the former material.
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In addition, significant absorption effects among organic materials caused the PPy particles to be absorbed by GNPs/PA in the process of polymerisation. It was also noted that the GNPs/PA mixture was fluidised when the composite reached its melting temperature and above (70ºC). Therefore, the mixture of GNPs/PA is not form-stable. In contrast, only the mixture decorated by PPy yielded form-stable PCM. In this case, the GNPs/PA particles are covered by a film of PPy particles, which creates stable composite PCMs.
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Fig. 3. SEM images of GNPs (a), PA/PPy (b), S1 (c), S2 (d), S3 (e), S4 (f)
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3.4. XRD characterisation
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Fig. 4 shows the XRD patterns of PPy, PA, GNPs and selected form-stable PCMs. The broad amorphous diffraction peak observed in the range of 2Ө = 15–35̊ in the XRD diffractogram of the PPy can be attributed to scattering of the bare polymer chains in interplanar spacing [36]. The appearance of peaks at approximately 13̊, 24̊, 25̊, 27̊ confirmed the formation of highly crystalline PA (JCPDS#008-0787O) with a monoclinic system, space group of P21/a and space group number of 14. In the case of PA/PPy, similar diffraction peaks were observed; however, the broad peak of PPy in form-stable PCMs causes the PA peaks at 24º and 25º to grow slightly wider. This suggests that a fraction of the crystalline phase is affected by the ratio of PA to PPy. The peak at 2Ө =26 ̊ is the main peak in GNPs, which is in good agreement with the results of previous studies [38]. This particular GNP peak was also observed in the XRD patterns of PA/PPy/GNPs form-stable PCMs (S4) without any significant difference. PA-specific peaks were also revealed in the XRD patterns of formstable PA/PPy and S4. It is clear that the PA/PPy/GNPs form-stable PCMs are simply a combination of PA, PPy and GNPs, with no production of new phases. However, it is clear that the volume fraction of crystalline phases increased when the GNPs were added to the PA/PPy. In a similar manner, Helmut Münstedt showed that the addition of carbon nanostructures increased the crystallinity and rheological properties of polycarbonate (PC) as a particular group of thermoplastic polymers [39].
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Fig. 4. XRD pattern of PA, PPy, GNPs, form-stable PA/PPy and S4
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The thermal stability results of PA, PPy, and form-stable PCMs are shown in Fig. 5. It is obvious that the pure PA particles decompose in one step, while PPy particles degrade in three stages. The thermal decomposition of PA molecular chains occurs from 230 ºC to 320 ºC due to its evaporation. The first, second and third stage of PPy thermal degradation correspond to the loss of water, the loss of dopant and the decomposition of the polymer backbone, respectively [35]. The main degradation of form-stable PCMs occurs in the second step. However, the TG curves of form-stable PCMs depict the loss of water and the decomposition of the polymer backbone. It is noticeable that the moisture of water in the 5 Page 5 of 20
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form-stable PCMs does not have any significant effect on the backbone of the polymer. Therefore, form-stable PCMs have favourable thermal stability in regard to their use as a phase change material with melting temperature of approximately 62ºC.
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Fig. 5. TG curves of PA, PPy, and form-stable PCMs
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3.6. Thermal energy storage properties
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The thermal energy storage properties of PA, PA/PPy, and form-stable PA/PPy/GNPs, which were analysed by DSC, are shown in Fig. 6 and Table 2. To improve accuracy, each sample was measured three times. Fig. 6 depicts the DSC curve of the samples. The DSC curve significantly indicates one major endothermic and exothermic peak in each sample with a peak temperature between 60ºC and 63ºC. Furthermore, the samples’ average enthalpy, onset and peak temperatures for melting and solidification were determined and are listed in Table 2. These data show that the melting and solidifying temperatures of form-stable PA/PPy/GNPs were altered to between 60ºC and 62ºC, compared with those of form-stable PA/PPy, which changed to the range of 59ºC to 60ºC. This change is not considerable for thermal energy storage applications.
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In addition, Table. 2 indicates that the enthalpy of form-stable PA/PPy/GNPs decreased from 163 to 151 J/g, which is higher than that of the previous form-stable PCMs indicated by other researchers [33, 40-43]. It is clear that the form-stable PCMs show a favourable thermal storage density. On the other hand, it is obvious that PA plays a main role in storing thermal energy [44-45]. There is a minor difference between the enthalpy of form-stable PA/PPy PCMs and those of PA/PPy/GNPs (approximately 9%). This means that the surface of GNPs was covered by PA particles, which lead to a decrease in the energy storage ability of PA. This is due to the increase in the absorption of PA with increased GNP loading. Therefore, it may be beneficial to add a small amount of GNPs to keep the energy storage almost constant, as well as to improve the thermal conductivity of form-stable PCMs.
248
Table 2. DSC data analysis for PA, PPy and for PA/PPy/GNPs form-stable PCMs
250
us
an
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Ac ce p
249
cr
227
Fig. 6. DSC results for PA and prepared form-stable PCMs
251
3.7. Thermal conductivity investigation
252 253 254 255 256 257 258 259 260
The thermal conductivity of the prepared form-stable PCMs is shown in Fig. 7. The figure indicates that the thermal conductivity of the PA/PPy/GNPs form-stable PCMs rose linearly with increased GNP particles. The thermal conductivity of PA and PA/PPy form-stable PCMs was approximately 0.31 and 0.32 W/(m K),respectively, while the thermal conductivity of the form-stable PCM doped with 1.6 wt% GNPs (S4) was measured to be 0.43 W/(m K). That is, the thermal conductivity of PA/PPy/GNPs form-stable PCM was 38.7% higher than that of PA particles and 34.3% better than that of PA/PPy form-stable PCM. Therefore, the thermal performance of form-stable PCMs that were doped with GNPs was greater than that of PA/PPy form-stable PCMs and PA. Moreover, the figure shows that 6 Page 6 of 20
the thermal conductivity tends to increase, which means that GNP particles are a candidate for improving the thermal conductivity of polymer-based form-stable PCMs.
263 264 265 266 267 268 269
In solar thermal applications, where storing thermal energy is essential due to the fluctuation of solar radiation, the thermal energy storage density and the speed of thermal energy storage and release play a significant role. For real applications of PCMs, form-stable PCMs are more reliable. However, the main drawback of the PA used for latent thermal energy storage is its low thermal conductivity. Therefore, form-stable PA/PPy/GNPs materials are expected to be suitable for solar energy applications due to their relatively higher latent thermal energy storage (151 J/g), thermal conductivity (0.43 W/(mK)) and thermal stability.
ip t
261 262
271
cr
270
Fig. 7. Thermal conductivity of PA/PPy and PA/PPy/GNPs form-stable PCMs
us
272
4. Conclusion
274 275 276 277 278 279 280 281 282 283 284 285 286
In this study, novel PA/PPy/GNPs form-stable PCMs were prepared by an in-situ polymerisation method. PA and PPy were used as solid-liquid PCMs and supporting materials, while GNP particles were applied as filler to improve the thermal conductivity. The percentage of PA remained constant for all samples at 79.9 wt%. However, the amount of GNPs increased from 0.4% to 1.6 wt%. In this investigation, the aim was to assess the chemical structure, thermal properties, thermal stability and thermal conductivity of PA/PPy/GNPs form-stable PCMs. The results indicate that PA/PPy/GNPs form-stable phase change materials have an appropriate latent heat thermal energy storage (151 J/g) and enhanced thermal stability. Moreover, the thermal conductivity of PA/PPy/GNPs form-stable PCMs was improved by 38.7% and 34.3% in comparison with that of PA particles and PA/PPy form-stable PCMs, respectively. Overall, form-stable PA/PPy/GNPs could be considered applicable candidates for low-temperature solar thermal energy storage applications.
288
M
d
te
Ac ce p
287
an
273
289
Acknowledgement
290 291 292 293 294 295 296
The authors would like to acknowledge the University of Malaya for financial support. This research was carried out under the high impact research grant No. UM.C/HIR/MOHE/ENG/21-(D000021-16001) and University of Malaya research grant No. UMRG RP021-2012A.
297 298
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Figure and Table captions
an
430 431
Fig. 1. PA and S4 cakes before (a and b) and after (c and d) being heated at 80ºC
433
Fig. 2. FTIR spectra of PPy, GNPs, PA, form-stable PA/PPy and S4
434
Fig. 3. SEM images of GNPs(a), PA/PPy(b), S1(c), S2(d), S3(e), S4(f)
435
Fig. 4. XRD pattern of PA, PPy, GNPs, form-stable PA/PPy and S4
436
Fig. 5. TG curves of PA, PPy, and form-stable PCMs
437
Fig. 6. DSC results for PA and prepared form-stable PCMs
438
Fig. 7. Thermal conductivity of PA/PPy and PA/PPy/GNPs form-stable PCMs
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Table 1. Properties of Graphene Nanoplatelets (GNPs)
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Table 2. DSC data analysis for PA, PPy and PA/PPy/GNPs form-stable PCMs
442
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443 444 445 446 447 11 Page 11 of 20
448 449 450
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451 452
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453 454
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455 456
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457
458 459 460
Fig. 1.
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cr
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469
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471 472 473
Fig. 2.
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475 476 477
ip t
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481 482 14 Page 14 of 20
483
Fig. 3.
484 485
489 490 491 492 493
te
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Fig. 4.
Ac ce p
487
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486
494 495 496 497 498
15 Page 15 of 20
499
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500
501
504 505 506 507 508
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Fig. 5.
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502
16 Page 16 of 20
ip t cr us an
509
M
510 511
Ac ce p
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d
Fig. 6.
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ip t cr us an 515 516 517 518 519 520
d
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Fig. 7.
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521 522 523 524
18 Page 18 of 20
525 526 527
Table 1. Specific surface area (m2/g) Width (µm) Thickness (nm) Thermal conductivity (W/mK)
ip t
300 Less than 2 Less than 2 Parallel to surface: 3000 Perpendicular to surface: 6 2.2 ˃99%
cr
Density (g/cc) Carbon content
us
528 529
an
530 531
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532 533
537 538 539 540 541 542
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d
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543 544 545 546 547 19 Page 19 of 20
Table 2. PA/PPy 2.4 0.6 79.9 59.8 63.9 58.8 58.5 166.3 170.7
549
S1 2.4 0.588 0.012 0.4 78.2 60.2 62.6 61.2 61 162.8 167.5
a. Onset temperature for melting.
551
b. Peak temperature for melting.
552
c.
553
d. Peak temperature for solidification.
554
*wt % = (ΔH Melting (form-stable PCM) / ΔH Melting (PCM) )*100
557 558 559
M
d te
556
S4 2.4 0.552 0.048 1.6 72.5 61 63 61.3 60.5 151 152
Ac ce p
555
S3 2.4 0.564 0.036 1.2 75 62 64 61.2 60.7 156 157
an
550
Onset temperature for solidification.
S2 2.4 0.576 0.024 0.8 76.9 61 63 61.5 60 160 161
ip t
PA 100 61.3 63.7 58.9 58.6 208 210
cr
Samples PA (g) Pyrrole (g) GNPs (g) Loading of GNPs (wt %) Loading of PA (wt %)* TMo (ºC)a TMp (ºC)b TSo (ºC)c TSp (ºC)d ΔHMelting (J/g) ΔHSolidification (J/g)
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20 Page 20 of 20