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The form-stable phase change materials based on polyethylene glycol and functionalized carbon nanotubes for heat storage
Accepted Manuscript The form-stable phase change materials based on polyethylene glycol and functionalized carbon nanotubes for heat storage Lili Feng, Chongyun Wang, Ping Song, Haibo Wang, Xiaoran Zhang PII:
S1359-4311(15)00778-4
DOI:
10.1016/j.applthermaleng.2015.07.080
Reference:
ATE 6880
To appear in:
Applied Thermal Engineering
Received Date: 10 March 2015 Revised Date:
30 June 2015
Accepted Date: 27 July 2015
Please cite this article as: L. Feng, C. Wang, P. Song, H. Wang, X. Zhang, The form-stable phase change materials based on polyethylene glycol and functionalized carbon nanotubes for heat storage, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.07.080. 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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Graphical abstract The form-stable phase change materials based on polyethylene glycol and functionalized carbon nanotubes for heat storage
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Lili Feng a, *, Chongyun Wang b, Ping Song c, Haibo Wang a, Xiaoran Zhang a a Key Laboratory of Urban Stormwater System and Water Environment (Ministry of Education), Beijing Climate Change Response Research and Education Center, Beijing University of Civil Engineering and Architecture, Beijing 100044, PR China b Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China c National Research Center for Geoanalysis, Beijing 100037, PR China
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The form-stable phase change materials based on polyethylene glycol and functionalized carbon nanotubes for heat storage
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Lili Feng a, *, Chongyun Wang b, Ping Song c, Haibo Wang a, Xiaoran Zhang a a Key Laboratory of Urban Stormwater System and Water Environment (Ministry of Education), Beijing Climate Change Response Research and Education Center, Beijing University of Civil Engineering and Architecture, Beijing 100044, PR China b Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China c National Research Center for Geoanalysis, Beijing 100037, PR China
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* Corresponding author. Tel./ fax: +86 10 68322124. E-mail address: [email protected] (L. Feng).
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ABSTRACT Form-stable phase change materials (PCMs) for heat storage based on polyethylene glycol (PEG) and functionalized multi-wall carbon nanotubes (MWNTs) were prepared via a blending and impregnating method. The structural and thermal properties of the as-prepared PCMs were analyzed by various techniques. The phase change temperatures and enthalpies of the composite PCMs with hydroxylated (-OH), carboxylated (-COOH) and aminoated (-NH2) MWNTs (PEG/MWNT-x PCMs) are lower than those with pristine MWNTs (PEG/MWNT PCMs). The influence of functional groups on the phase change behavior of the composite PCMs follows the order of: MWNT-COOH> MWNT-NH2> MWNT-OH> MWNT. Capillary forces, surface areas and hydrogen bonding are mainly influential on the phase change behavior of PEG/MWNT-x PCMs.
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Keywords: Form-stable phase change materials; Polyethylene glycol; Multi-wall carbon nanotubes; Functional group modification; Microstructure; Thermal properties
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1. Introduction The thermal energy sources do not always match the demands. The energy storage devices and systems could improve the energy efficiency and reduce the gap between energy sources and demands of energy [1]. One of prospective techniques of storing thermal energy is the application of phase change materials (PCMs), which store thermal energy when changing from solid to liquid state and release the energy later when changing from liquid to solid state [2]. PCMs have received much attention as energy storage mediums because of their high energy density, heat recovery with small temperature drop, constant heat source temperature and repeatable utilization [3-7]. Polyethylene glycol (PEG) is a very promising solid–liquid organic PCM because of its high phase change enthalpy, chemical stability and suitable melting temperature ranging from 3.2 to 68.7 °C, which can be tuned by its molecular weight [8-12]. However, it is the main drawback of PEG as PCM that there is some leakage 1
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of the liquid phase above its melting point. Form-stable PCMs composed of working substance and supporting materials can effectively overcome the leak, which can remain in the solid state even when the working materials change from solid to liquid [11]. Building materials [6], expanded graphite (EG) [13, 14], activated carbon (AC) [15], graphene oxide [16] and ordered meso-porous carbon (CMK-5) [17], silica [18, 19] and some polymers [20, 21] have been employed as form stabilization matrixes. Multi-walled carbon nanotubes (MWNTs) are nano-materials that have light weight and high thermal conductivity. Therefore, MWNTs are candidates for applications as supporters in composite materials to enhance thermal transport [22]. Several studies have been reported wherein CNTs were embedded in the base fluids and PCMs to enhance their thermal conductivity [23-25]. Moreover, the thermal conductivities are related to the distribution of MWNTs in PCMs. It was reported that there was an enhancement of the thermal conductivity of PCMs and an improved heat transfer of PCMs with the functionalized MWNTs, which could be attributed that the functionalized MWNTs are better dispersed in the PCMs [26, 27]. However, the influence of the MWNTs and their functional groups on the phase change behavior of PCMs still remains unexplored. In this paper, hydroxylated (-OH), carboxylated (-COOH) and aminoated (-NH2) MWNTs were functionalized by plasma treatment, and then form-stable PCMs based on PEG and functionalized MWNTs were prepared via a blending and impregnating method. The structural and thermal properties of the as-prepared PCMs were analyzed by various techniques. The difference of the intermolecular interactions between PEG and functionalized MWNTs resulted in different phase change behaviors of PEG in the composites. 2. Experimental 2.1. Materials Chemically pure PEG with the average molecular weights of 2000, 4000, 6000 and 10000 were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. All multi-wall carbon nanotubes (MWNTs) from the supplier (Cheaptubes, Brattleboro, USA) were synthesized by chemical vapour deposition, which have purities of > 99%. Functionalized MWNTs were obtained by plasma treatment and contained 7±1.5 wt% functional groups according to X-ray photoelectron spectroscopy (XPS) and titration results provided by the supplier. 2.2. Preparation of the PEG/ MWNTs PCMs PEG/MWNTs PCMs were prepared by a physical blending and impregnating process. x g of PEG (x = 0.03-0.09) was melted and dissolved in 10 ml of absolute ethanol (AR, > 99.7% purity) to form a homogeneous solution. After that, (0.1-x) g of MWNTs was added to the PEG solution while stirring. The resulting solution was stirred vigorously for 4 h. Finally, the mixture was dried at 80 oC (above the melting point of PEG) for 72 h in order to make the ethanol solvent completely evaporate and explore the form stabilization of the composites above PEG’s melting point. There were no PEG leakages at 80 oC from 90 wt% composite PCMs. 2.3. Property analysis The BET surface area, total pore volume and average pore size of the MWNT 2
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matrixes were measured by the nitrogen adsorption method at liquid nitrogen temperature on a Quantachrome Autosorb-iQ-MP gas adsorption analyzer. The Fourier transform infrared (FTIR) spectra of the samples (KBr pellets) were recorded in the transmission mode on a Bruker VERTEX 70 FTIR spectrometer. Scanning electron microscopy (SEM) observations were carried out on a Hitachi S-5500 field emission SEM (Japan). The phase change temperature and enthalpy of the samples were determined using a Q100 Differential scanning calorimetry (DSC, Thermal Analysis Corporation, USA). The samples were heated and cooled between 0 oC and 100 oC at a rate of 10 oC·min-1 in a nitrogen atmosphere. 3. Results and discussion 3.1. Chemical properties Fig. 1 shows the FTIR spectra of pure PEG, MWNT matrixes and the composite PCMs with 90 wt% of PEG10000. In the spectra of various MWNT matrixes, the peak at 1383 cm-1 is caused by C–C bonds, and this peak can also be found for the composite PCMs. For the MWNT-OH matrix, the peaks at 3437 cm-1 and 1629 cm-1 are assigned to the stretching vibration of hydroxyl. However, for the MWNT-COOH matrix, the peaks at 3437 cm-1 and 1629 cm-1 can be assigned to the stretching vibration of OH and C=O; for the MWNT-NH2 matrix, the peaks at 3437 cm-1 and 1629 cm-1 can be assigned to the NH stretching vibration and the NH2 bending vibration. The absorption peaks of various MWNT marixes at 3437 cm-1 and 1629 cm-1 overlapped with the peaks of PEG at these wave numbers. There is also a peak at 1122 cm-1 in the spectra of various MWNT matrixes. For the MWNT-OH and MWNT-COOH matrixes, the peak at 1122 cm-1 is caused by C-O bonds; for the MWNT-NH2 matrix, the peak at 1122 cm-1 represents the stretching vibration of C-N functional group. However, for the composite PCMs, the C–O or C-N absorption peak (1122 cm-1) has shifted to lower wave number of 1108 cm-1, meaning that there are hydrogen bonds between bridging oxygen/nitrogen atoms of carbon and the end hydroxyl group of PEG. The adsorption peaks of PEG can be observed in the spectra of all the composite PCMs. Compared the spectra of the composite PCMs with those of the MWNT matrixes and pure PEG, no obvious new peaks were found, proving that the interaction between PEG and MWNT stabilizers was physical.
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Fig. 1. FTIR spectra of pure PEG, MWNTs and the composite PCMs with 90 wt% of PEG10000. 3.2. Morphological properties Fig. 2 depicts SEM images of the pristine MWNT and the PEG6000/MWNT PCMs with various PEG contents. Only the MWNT morphology can be seen when the PEG content was 30 wt% (Fig. 2(b)). However, PEG blocks can be mainly observed when the PEG content was 90 wt% (Fig. 2(d)). Additionally, the diameter of MWNT in the composite PCMs was distinctly larger than that of pristine MWNT, indicating that PEG segments might be adsorbed on the outer walls of MWNT and confined within the tubes of MWNT, which hindered PEG to crystallize and agglomerate. (b)
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(d)
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Fig. 2. SEM images of the pristine MWNT and the PEG6000/MWNT PCMs with various PEG6000 contents. (a) pristine MWNT; (b) 30wt% PEG; (c) 60wt% PEG; (d) 90wt% PEG. 3.3. Thermal properties Fig. 3 shows DSC curves of the PEG6000/MWNT PCMs with different PEG contents. The melting temperatures and enthalpies of the composite PCMs reduced with the decrease of PEG contents. 30 wt% PEG6000/MWNT PCM exhibits neither endothermic nor exothermic peak. This mainly resulted from the interference of MWNT matrix with the crystallization of PEG. In the composites with low PEG content, most PEG segments were confined within the tubes or adsorbed on the surface of MWNT; thus the movement of the polymer chains was restricted. When the PEG content decreased to a certain degree (e.g. 30% PEG), PEG in PEG/MWNT PCMs was thoroughly confined by the excess MWNT and could not form crystals. As a result, no endothermic and exothermic peaks can be seen on its DSC curves. In contrast, considering the composites with high PEG content, a part of the polymer was in the free state (Fig. 2(d)) and could crystallize. Higher PEG content in the composites resulted in higher fractions of the material in a crystalline phase and thus larger enthalpy.
Fig. 3. DSC curves of the PEG6000/MWNT PCMs with various PEG contents. DSC curves of 90wt% PEG/MWNT PCMs with various PEG molecular weights are shown in Fig. 4. Higher phase change temperatures were observed for 90wt% PEG/MWNT PCMs with larger PEG molecular weights. The phase change enthalpy of the PEG10000/MWNT PCMs was larger than that of other three composite PCMs. Thus, we mainly discuss the influence of the MWNTs and their functional groups on the phase change behavior of the composites with PEG10000.
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Fig. 4. DSC curves of the 90wt% PEG/MWNT PCMs with various PEG molecular weights. Fig. 5 shows DSC curves of the 90wt% PEG10000/MWNTs PCMs with various functional group modified MWNTs. The detailed results of peak melting/crystallization temperature (Tm/Tc) and melting/crystallization enthalpy (∆Hm/∆Hc) obtained from Fig. 5 are presented in Table 1. As shown in both Fig. 5 and Table 1, the phase change temperatures and enthalpies of the MWNT-x/PEG PCMs are lower than those of pristine MWNT/PEG PCM, which suggests stronger interaction between the MWNT-x matrixes and PEG. The influence of functional groups on the phase change behavior of the composite PCMs follows the order of: MWNT-COOH> MWNT-NH2> MWNT-OH> MWNT. The phase change temperatures of the MWNT-COOH/PEG PCM, Tm: 60.1 oC and Tc: 33.7 oC, respectively reduced by 3 oC and 9.9 oC compared to the MWNT/PEG PCM, and its phase change enthalpy, ∆Hm: 111.0 J·g-1 and ∆Hc: 103.3 J·g-1, decreased by ~40 J·g-1.
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△Hm (J·g-1)
Tc (oC)
△Hc (J·g-1)
PEG/MWNT
63.1
150.4
43.6
143.5
PEG/MWNT-NH2
62.2
141.7
37.5
133.3
PEG/MWNT-COOH
60.1
111.0
33.7
103.3
PEG/MWNT-OH
62.9
147.0
41.0
143.3
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Samples
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Larger BET surface areas and smaller average pore size of the pristine MWNT matrix (in Table 2) indicate that MWNT has more adsorption sites and stronger capillary forces, compared to the MWNT-x matrixes. Thus, the interaction of pristine MWNT with PEG should be larger than that of MWNT-x, which is inconsistent to the DSC results from Fig. 5. According to the FTIR results, there is hydrogen bonding in the PEG/MWNT-x PCMs. Under the combined effects of hydrogen bonding, capillary forces and surface adsorption, the interaction of the MWNT-x matrix with PEG in PEG/MWNT-x PCMs is much stronger than that of MWNT with PEG in PEG/MWNT PCMs for which capillary forces and surface areas are significant influential factors of the phase change behavior. Furthermore, as shown in Fig. 6, there are more possibilities of forming hydrogen bonding for MWNT-COOH than other MWNT-x matrixes. Therefore, the influence of MWNT-COOH on the phase change behavior of the composite PCMs is the largest. Table 2 Porous characteristics of the selected MWNT matrixes. Vpore (cm3·g-1)
Dpore (nm)
224
1.86
33.2
MWNT-OH
205
2.31
45.3
MWNT-COOH
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1.97
44.2
MWNT-NH2
206
2.12
41.2
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Fig. 6. Schematic illustration of the interaction of PEG with MWNTs in the PEG/MWNTs PCMs with various functional groups. 4. Conclusions In summary, the form-stable PCMs were prepared via the direct blending of PEG with MWNTs modified by various functional groups and an impregnating process. The properties of the composites were investigated by a number of characterization methods. The melting temperatures and enthalpies of PEG in the composites 8
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increased with higher PEG content. The phase change temperatures decreased with lower molecular weights of PEG. The phase change temperatures and enthalpies of the composite PCMs with functionalized MWNTs are lower than those of the composites with pristine MWNTs. The influence of functional groups on the phase change behavior of the composite PCMs follows the order of: MWNT-COOH> MWNT-NH2> MWNT-OH> MWNT. Capillary forces and surface areas are significant factors for the phase change behavior of PEG/MWNT PCMs, and capillary forces, surface areas and hydrogen bonding are more influential in PEG/MWNT-x PCMs.
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Acknowledgements This work was funded by the National Natural Science Foundation of China (No. 51206009), China Scholarship Council (No. 201408110015) and Beijing Higher Education Young Elite Teacher Project (No. 21147514022).
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References [1] Dutil Y, Rousse DR, Salah NB, Lassue S, Zalewski L. A review on phase-change materials: Mathematical modeling and simulations. Renew Sust Energy Rev 2011; 15: 112–30. [2] Sharma A, Tyagi VV, Chen CR, Buddhi D. Review on thermal energy storage with phase change materials and applications. Renew Sust Energy Rev 2009; 13: 318–45. [3] Su J, Liu P. A novel solid–solid phase change heat storage material with polyurethane block copolymer structure. Energy Convers Manage 2006; 47: 3185–91. [4] Nomura T, Okinaka N, Akiyama T. Impregnation of porous material with phase change material for thermal energy storage. Mater Chem Phys 2009; 115: 846–50. [5] Fang G, Li H, Yang F, Liu X, Wu S. Preparation and characterization of nano-encapsulated n-tetradecane as phase change material for thermal energy storage. Chem Eng J 2009; 153: 217–21. [6] Zhang D, Tian S, Xiao D. Experimental study on the phase change behavior of phase change material confined in pores. Sol Energy 2007; 81: 653–60. [7] Lafdi K, Mesalhy O, Elgafy A. Graphite foams infiltrated with phase change materials as alternative materials for space and terrestrial thermal energy storage applications. Carbon 2008; 46: 159–68. [8] Oh C, Do Ki C, Young Chang J, Oh SG. Preparation of PEG-grafted silica particles using emulsion method. Mater Lett 2005; 59: 929–33. [9] Meng Q, Hu J. A poly(ethylene glycol)-based smart phase change material. Sol Energy Mater Sol Cells 2008; 92: 1260–68. [10] Li W, Ding E. Preparation and characterization of cross-linking PEG/MDI/PE copolymer as solid–solid phase change heat storage material. Sol Energy Mater Sol Cells 2007; 91: 764–68. [11] Jiang Y, Ding E, Li G. Study on transition characteristics of PEG CDA solid–solid phase change materials. Polymer 2002; 43: 117–22. [12] Alkan C, Sari A. Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage. Sol Energy 9
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2008; 82: 118–24. [13] Zhang ZG, Shao G, Fang XM. Study on paraffin/expanded graphite composite phase change thermal energy storage material. Acta Energiae Solaris Sinica 2005; 26: 698–02. [14] Ahmet S, Ali K. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Appl Therm Eng 2007; 27: 1271–77. [15] Feng L, Zheng J, Yang H, Guo Y, Li W, Li X. Preparation and characterization of polyethylene glycol/Active carbon composites as shape-stabilized phase change materials. Sol Energy Mater Sol Cells 2011; 95: 644–50. [16] Wang C, Feng L, Yang H, Xin G, Li W, Zheng J, Tian W, Li X. Graphene oxide stabilized polyethylene glycol for heat storage. Phys Chem Chem Phys 2012; 14 : 13233–38. [17] Wang C, Feng L, Li W, Zheng J, Tian W, Li X. Shape-stabilized phase change materials based on polyethylene glycol/porous carbon composite: The influence of the pore structure of the carbon materials. Sol Energy Mater Sol Cells 2012; 105: 21–26. [18] Feng L, Zhao W, Zheng J, Frisco S, Song P, Li X. The shape-stabilized phase change materials composed of polyethylene glycol and various mesoporous matrixes (AC, SBA–15 and MCM–41). Sol Energy Mater Sol Cells 2011; 95: 3550–56. [19] Yang H, Feng L, Wang C, Zhao W, Li X. Confinement effect of SiO2 framework on crystallization of PEG in the shape-stabilized PEG/SiO2 composites. Eur Polym J 2012; 48: 803–10. [20] Ye H, Ge XS. Preparation of polyethylene–paraffin compound as a form–stable solid–liquid phase change material. Sol Energy Mater Sol Cells 2000; 64: 37–44. [21] Ding EY, Jiang Y, Li GK. Comparative studies of the structures and transition characteristics of cellulose diacetate modified with polyethylene glycol prepared by chemical bonding and physical blending methods. J Macromol Sci B 2001; 40: 1053–68. [22] Han Z, Fina A. Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review. Prog Polym Sci 2011; 36: 914–44. [23] Cui Y, Liu C, Hu S, Yu X. The experimental exploration of carbon nanofiber and carbon nanotube additives on thermal behavior of phase change materials. Sol Energy Mater Sol Cells 2011; 95: 1208–12. [24] Teng T, Cheng C, Cheng C. Performance assessment of heat storage by phase change materials containing MWNTs and graphite. Appl Therm Eng 2013; 50: 637–44. [25] Wang J, Xie H, Xin Z. Thermal properties of paraffin based composites containing multi-walled carbon nanotubes. Thermochim Acta 2009; 488: 39–42. [26] Ji P, Sun H, Zhong Y, Feng W. Improvement of the thermal conductivity of a phase change material by the functionalized carbon nanotubes. Chem Eng Sci 2012; 81: 140–45. [27] Wang J, Xie H, Xin Z, Li Y, Chen L. Enhancing thermal conductivity of palmitic acid based phase change materials with carbon nanotubes as fillers. Sol Energy 2010; 84: 339–44. 10
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Highlights * New form-stable PCMs were prepared using functionalized carbon nanotubes as matrices. * Phase change temperature and enthalpy of PEG/MWNT-x are lower than those of
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PEG/MWNT. * The influence of MWNT-COOH on the phase change behavior of the PCMs is the largest.
* Capillary forces, surface areas and hydrogen bonding are influential on PEG/
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MWNT-x.
S1359-4311(15)00778-4
DOI:
10.1016/j.applthermaleng.2015.07.080
Reference:
ATE 6880
To appear in:
Applied Thermal Engineering
Received Date: 10 March 2015 Revised Date:
30 June 2015
Accepted Date: 27 July 2015
Please cite this article as: L. Feng, C. Wang, P. Song, H. Wang, X. Zhang, The form-stable phase change materials based on polyethylene glycol and functionalized carbon nanotubes for heat storage, Applied Thermal Engineering (2015), doi: 10.1016/j.applthermaleng.2015.07.080. 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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Graphical abstract The form-stable phase change materials based on polyethylene glycol and functionalized carbon nanotubes for heat storage
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Lili Feng a, *, Chongyun Wang b, Ping Song c, Haibo Wang a, Xiaoran Zhang a a Key Laboratory of Urban Stormwater System and Water Environment (Ministry of Education), Beijing Climate Change Response Research and Education Center, Beijing University of Civil Engineering and Architecture, Beijing 100044, PR China b Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China c National Research Center for Geoanalysis, Beijing 100037, PR China
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The form-stable phase change materials based on polyethylene glycol and functionalized carbon nanotubes for heat storage
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Lili Feng a, *, Chongyun Wang b, Ping Song c, Haibo Wang a, Xiaoran Zhang a a Key Laboratory of Urban Stormwater System and Water Environment (Ministry of Education), Beijing Climate Change Response Research and Education Center, Beijing University of Civil Engineering and Architecture, Beijing 100044, PR China b Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China c National Research Center for Geoanalysis, Beijing 100037, PR China
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* Corresponding author. Tel./ fax: +86 10 68322124. E-mail address: [email protected] (L. Feng).
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ABSTRACT Form-stable phase change materials (PCMs) for heat storage based on polyethylene glycol (PEG) and functionalized multi-wall carbon nanotubes (MWNTs) were prepared via a blending and impregnating method. The structural and thermal properties of the as-prepared PCMs were analyzed by various techniques. The phase change temperatures and enthalpies of the composite PCMs with hydroxylated (-OH), carboxylated (-COOH) and aminoated (-NH2) MWNTs (PEG/MWNT-x PCMs) are lower than those with pristine MWNTs (PEG/MWNT PCMs). The influence of functional groups on the phase change behavior of the composite PCMs follows the order of: MWNT-COOH> MWNT-NH2> MWNT-OH> MWNT. Capillary forces, surface areas and hydrogen bonding are mainly influential on the phase change behavior of PEG/MWNT-x PCMs.
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Keywords: Form-stable phase change materials; Polyethylene glycol; Multi-wall carbon nanotubes; Functional group modification; Microstructure; Thermal properties
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1. Introduction The thermal energy sources do not always match the demands. The energy storage devices and systems could improve the energy efficiency and reduce the gap between energy sources and demands of energy [1]. One of prospective techniques of storing thermal energy is the application of phase change materials (PCMs), which store thermal energy when changing from solid to liquid state and release the energy later when changing from liquid to solid state [2]. PCMs have received much attention as energy storage mediums because of their high energy density, heat recovery with small temperature drop, constant heat source temperature and repeatable utilization [3-7]. Polyethylene glycol (PEG) is a very promising solid–liquid organic PCM because of its high phase change enthalpy, chemical stability and suitable melting temperature ranging from 3.2 to 68.7 °C, which can be tuned by its molecular weight [8-12]. However, it is the main drawback of PEG as PCM that there is some leakage 1
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of the liquid phase above its melting point. Form-stable PCMs composed of working substance and supporting materials can effectively overcome the leak, which can remain in the solid state even when the working materials change from solid to liquid [11]. Building materials [6], expanded graphite (EG) [13, 14], activated carbon (AC) [15], graphene oxide [16] and ordered meso-porous carbon (CMK-5) [17], silica [18, 19] and some polymers [20, 21] have been employed as form stabilization matrixes. Multi-walled carbon nanotubes (MWNTs) are nano-materials that have light weight and high thermal conductivity. Therefore, MWNTs are candidates for applications as supporters in composite materials to enhance thermal transport [22]. Several studies have been reported wherein CNTs were embedded in the base fluids and PCMs to enhance their thermal conductivity [23-25]. Moreover, the thermal conductivities are related to the distribution of MWNTs in PCMs. It was reported that there was an enhancement of the thermal conductivity of PCMs and an improved heat transfer of PCMs with the functionalized MWNTs, which could be attributed that the functionalized MWNTs are better dispersed in the PCMs [26, 27]. However, the influence of the MWNTs and their functional groups on the phase change behavior of PCMs still remains unexplored. In this paper, hydroxylated (-OH), carboxylated (-COOH) and aminoated (-NH2) MWNTs were functionalized by plasma treatment, and then form-stable PCMs based on PEG and functionalized MWNTs were prepared via a blending and impregnating method. The structural and thermal properties of the as-prepared PCMs were analyzed by various techniques. The difference of the intermolecular interactions between PEG and functionalized MWNTs resulted in different phase change behaviors of PEG in the composites. 2. Experimental 2.1. Materials Chemically pure PEG with the average molecular weights of 2000, 4000, 6000 and 10000 were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. All multi-wall carbon nanotubes (MWNTs) from the supplier (Cheaptubes, Brattleboro, USA) were synthesized by chemical vapour deposition, which have purities of > 99%. Functionalized MWNTs were obtained by plasma treatment and contained 7±1.5 wt% functional groups according to X-ray photoelectron spectroscopy (XPS) and titration results provided by the supplier. 2.2. Preparation of the PEG/ MWNTs PCMs PEG/MWNTs PCMs were prepared by a physical blending and impregnating process. x g of PEG (x = 0.03-0.09) was melted and dissolved in 10 ml of absolute ethanol (AR, > 99.7% purity) to form a homogeneous solution. After that, (0.1-x) g of MWNTs was added to the PEG solution while stirring. The resulting solution was stirred vigorously for 4 h. Finally, the mixture was dried at 80 oC (above the melting point of PEG) for 72 h in order to make the ethanol solvent completely evaporate and explore the form stabilization of the composites above PEG’s melting point. There were no PEG leakages at 80 oC from 90 wt% composite PCMs. 2.3. Property analysis The BET surface area, total pore volume and average pore size of the MWNT 2
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matrixes were measured by the nitrogen adsorption method at liquid nitrogen temperature on a Quantachrome Autosorb-iQ-MP gas adsorption analyzer. The Fourier transform infrared (FTIR) spectra of the samples (KBr pellets) were recorded in the transmission mode on a Bruker VERTEX 70 FTIR spectrometer. Scanning electron microscopy (SEM) observations were carried out on a Hitachi S-5500 field emission SEM (Japan). The phase change temperature and enthalpy of the samples were determined using a Q100 Differential scanning calorimetry (DSC, Thermal Analysis Corporation, USA). The samples were heated and cooled between 0 oC and 100 oC at a rate of 10 oC·min-1 in a nitrogen atmosphere. 3. Results and discussion 3.1. Chemical properties Fig. 1 shows the FTIR spectra of pure PEG, MWNT matrixes and the composite PCMs with 90 wt% of PEG10000. In the spectra of various MWNT matrixes, the peak at 1383 cm-1 is caused by C–C bonds, and this peak can also be found for the composite PCMs. For the MWNT-OH matrix, the peaks at 3437 cm-1 and 1629 cm-1 are assigned to the stretching vibration of hydroxyl. However, for the MWNT-COOH matrix, the peaks at 3437 cm-1 and 1629 cm-1 can be assigned to the stretching vibration of OH and C=O; for the MWNT-NH2 matrix, the peaks at 3437 cm-1 and 1629 cm-1 can be assigned to the NH stretching vibration and the NH2 bending vibration. The absorption peaks of various MWNT marixes at 3437 cm-1 and 1629 cm-1 overlapped with the peaks of PEG at these wave numbers. There is also a peak at 1122 cm-1 in the spectra of various MWNT matrixes. For the MWNT-OH and MWNT-COOH matrixes, the peak at 1122 cm-1 is caused by C-O bonds; for the MWNT-NH2 matrix, the peak at 1122 cm-1 represents the stretching vibration of C-N functional group. However, for the composite PCMs, the C–O or C-N absorption peak (1122 cm-1) has shifted to lower wave number of 1108 cm-1, meaning that there are hydrogen bonds between bridging oxygen/nitrogen atoms of carbon and the end hydroxyl group of PEG. The adsorption peaks of PEG can be observed in the spectra of all the composite PCMs. Compared the spectra of the composite PCMs with those of the MWNT matrixes and pure PEG, no obvious new peaks were found, proving that the interaction between PEG and MWNT stabilizers was physical.
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Fig. 1. FTIR spectra of pure PEG, MWNTs and the composite PCMs with 90 wt% of PEG10000. 3.2. Morphological properties Fig. 2 depicts SEM images of the pristine MWNT and the PEG6000/MWNT PCMs with various PEG contents. Only the MWNT morphology can be seen when the PEG content was 30 wt% (Fig. 2(b)). However, PEG blocks can be mainly observed when the PEG content was 90 wt% (Fig. 2(d)). Additionally, the diameter of MWNT in the composite PCMs was distinctly larger than that of pristine MWNT, indicating that PEG segments might be adsorbed on the outer walls of MWNT and confined within the tubes of MWNT, which hindered PEG to crystallize and agglomerate. (b)
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Fig. 2. SEM images of the pristine MWNT and the PEG6000/MWNT PCMs with various PEG6000 contents. (a) pristine MWNT; (b) 30wt% PEG; (c) 60wt% PEG; (d) 90wt% PEG. 3.3. Thermal properties Fig. 3 shows DSC curves of the PEG6000/MWNT PCMs with different PEG contents. The melting temperatures and enthalpies of the composite PCMs reduced with the decrease of PEG contents. 30 wt% PEG6000/MWNT PCM exhibits neither endothermic nor exothermic peak. This mainly resulted from the interference of MWNT matrix with the crystallization of PEG. In the composites with low PEG content, most PEG segments were confined within the tubes or adsorbed on the surface of MWNT; thus the movement of the polymer chains was restricted. When the PEG content decreased to a certain degree (e.g. 30% PEG), PEG in PEG/MWNT PCMs was thoroughly confined by the excess MWNT and could not form crystals. As a result, no endothermic and exothermic peaks can be seen on its DSC curves. In contrast, considering the composites with high PEG content, a part of the polymer was in the free state (Fig. 2(d)) and could crystallize. Higher PEG content in the composites resulted in higher fractions of the material in a crystalline phase and thus larger enthalpy.
Fig. 3. DSC curves of the PEG6000/MWNT PCMs with various PEG contents. DSC curves of 90wt% PEG/MWNT PCMs with various PEG molecular weights are shown in Fig. 4. Higher phase change temperatures were observed for 90wt% PEG/MWNT PCMs with larger PEG molecular weights. The phase change enthalpy of the PEG10000/MWNT PCMs was larger than that of other three composite PCMs. Thus, we mainly discuss the influence of the MWNTs and their functional groups on the phase change behavior of the composites with PEG10000.
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Fig. 4. DSC curves of the 90wt% PEG/MWNT PCMs with various PEG molecular weights. Fig. 5 shows DSC curves of the 90wt% PEG10000/MWNTs PCMs with various functional group modified MWNTs. The detailed results of peak melting/crystallization temperature (Tm/Tc) and melting/crystallization enthalpy (∆Hm/∆Hc) obtained from Fig. 5 are presented in Table 1. As shown in both Fig. 5 and Table 1, the phase change temperatures and enthalpies of the MWNT-x/PEG PCMs are lower than those of pristine MWNT/PEG PCM, which suggests stronger interaction between the MWNT-x matrixes and PEG. The influence of functional groups on the phase change behavior of the composite PCMs follows the order of: MWNT-COOH> MWNT-NH2> MWNT-OH> MWNT. The phase change temperatures of the MWNT-COOH/PEG PCM, Tm: 60.1 oC and Tc: 33.7 oC, respectively reduced by 3 oC and 9.9 oC compared to the MWNT/PEG PCM, and its phase change enthalpy, ∆Hm: 111.0 J·g-1 and ∆Hc: 103.3 J·g-1, decreased by ~40 J·g-1.
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△Hm (J·g-1)
Tc (oC)
△Hc (J·g-1)
PEG/MWNT
63.1
150.4
43.6
143.5
PEG/MWNT-NH2
62.2
141.7
37.5
133.3
PEG/MWNT-COOH
60.1
111.0
33.7
103.3
PEG/MWNT-OH
62.9
147.0
41.0
143.3
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Larger BET surface areas and smaller average pore size of the pristine MWNT matrix (in Table 2) indicate that MWNT has more adsorption sites and stronger capillary forces, compared to the MWNT-x matrixes. Thus, the interaction of pristine MWNT with PEG should be larger than that of MWNT-x, which is inconsistent to the DSC results from Fig. 5. According to the FTIR results, there is hydrogen bonding in the PEG/MWNT-x PCMs. Under the combined effects of hydrogen bonding, capillary forces and surface adsorption, the interaction of the MWNT-x matrix with PEG in PEG/MWNT-x PCMs is much stronger than that of MWNT with PEG in PEG/MWNT PCMs for which capillary forces and surface areas are significant influential factors of the phase change behavior. Furthermore, as shown in Fig. 6, there are more possibilities of forming hydrogen bonding for MWNT-COOH than other MWNT-x matrixes. Therefore, the influence of MWNT-COOH on the phase change behavior of the composite PCMs is the largest. Table 2 Porous characteristics of the selected MWNT matrixes. Vpore (cm3·g-1)
Dpore (nm)
224
1.86
33.2
MWNT-OH
205
2.31
45.3
MWNT-COOH
178
1.97
44.2
MWNT-NH2
206
2.12
41.2
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Fig. 6. Schematic illustration of the interaction of PEG with MWNTs in the PEG/MWNTs PCMs with various functional groups. 4. Conclusions In summary, the form-stable PCMs were prepared via the direct blending of PEG with MWNTs modified by various functional groups and an impregnating process. The properties of the composites were investigated by a number of characterization methods. The melting temperatures and enthalpies of PEG in the composites 8
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increased with higher PEG content. The phase change temperatures decreased with lower molecular weights of PEG. The phase change temperatures and enthalpies of the composite PCMs with functionalized MWNTs are lower than those of the composites with pristine MWNTs. The influence of functional groups on the phase change behavior of the composite PCMs follows the order of: MWNT-COOH> MWNT-NH2> MWNT-OH> MWNT. Capillary forces and surface areas are significant factors for the phase change behavior of PEG/MWNT PCMs, and capillary forces, surface areas and hydrogen bonding are more influential in PEG/MWNT-x PCMs.
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Acknowledgements This work was funded by the National Natural Science Foundation of China (No. 51206009), China Scholarship Council (No. 201408110015) and Beijing Higher Education Young Elite Teacher Project (No. 21147514022).
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Highlights * New form-stable PCMs were prepared using functionalized carbon nanotubes as matrices. * Phase change temperature and enthalpy of PEG/MWNT-x are lower than those of
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PEG/MWNT. * The influence of MWNT-COOH on the phase change behavior of the PCMs is the largest.
* Capillary forces, surface areas and hydrogen bonding are influential on PEG/
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MWNT-x.