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silicon dioxide composites as form-stable phase change materials for thermal energy storage
Materials Chemistry and Physics 122 (2010) 533–536
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation and properties of lauric acid/silicon dioxide composites as form-stable phase change materials for thermal energy storage Guiyin Fang a,∗ , Hui Li b , Xu Liu a a b
Department of Physics, Nanjing University, Hankou Road 22, Nanjing, Jiangsu 210093, China Department of Material Science and Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e
i n f o
Article history: Received 10 September 2009 Received in revised form 13 March 2010 Accepted 16 March 2010 Keywords: Composite materials Sol–gel growth DSC Thermal properties
a b s t r a c t Form-stable lauric acid (LA)/silicon dioxide (SiO2 ) composite phase change materials were prepared using sol–gel methods. The LA was used as the phase change material for thermal energy storage, with the SiO2 acting as the supporting material. The structural analysis of these form-stable LA/SiO2 composite phase change materials was carried out using Fourier transformation infrared spectroscope (FT-IR). The microstructure of the form-stable composite phase change materials was observed by a scanning electronic microscope (SEM). The thermal properties and thermal stability were investigated by a differential scanning calorimeter (DSC) and a thermogravimetric analysis apparatus (TGA), respectively. The SEM results showed that the LA was well dispersed in the porous network of SiO2 . The DSC results indicated that the melting latent heat of the form-stable composite phase change material is 117.21 kJ kg−1 when the mass percentage of the LA in the SiO2 is 64.8%. The results of the TGA showed that these materials have good thermal stability. The form-stable composite phase change materials can be used for thermal energy storage in waste heat recovery and solar heating systems. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, phase change materials (PCMs) have received attention for various applications in solar heating systems [1–3], building energy conservation [4] and air-conditioning systems [5]. Unlike traditional heat storage methods, latent heat storage provides much greater energy storage density with a smaller temperature difference between storing and releasing heat. Many inorganic and organic PCMs (salt hydrates, fatty acids/esters, paraffins, etc.) and PCM mixtures have been studied for latent heat storage applications [6,7]. Among the PCMs investigated, fatty acids have been widely used due to their high latent heat storage capacity and appropriate thermal properties, such as little or no supercooling, low vapor pressure, good thermal and chemical stability, and self-nucleating behavior [8,9]. However, they have low thermal conductivity, and need encapsulation in order to prevent leakage of the melted PCM during the phase change process [10]. However, these problems can be solved by using form-stable PCM composites. These composites can be prepared by encapsulation of the PCM in a polymeric structure, such as high density polyethylene (HDPE) [11,12] and the styrene–butadiene–styrene
∗ Corresponding author. Tel.: +86 25 51788228; fax: +86 25 83593707. E-mail address: [email protected] (G. Fang). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.03.042
(SBS) copolymer [13]. In addition, the expanded graphite can also act as the supporting material in the composites [14,15]. However, due to the chemical constitution of organic PCMs (fatty acids/esters, paraffins, etc.) and organic polymer supporting materials (HDPE, SBS and so on), the form-stable PCMs are easily flammable, and their application is therefore severely restricted [16]. In this paper, the preparation and properties of the form-stable LA/SiO2 composite PCMs are reported. In the composite materials, the LA was used as the latent heat storage material, and SiO2 served as the inorganic supporting material. The LA is a favorable organic PCM for thermal energy storage, melting at 44.43 ◦ C with a latent heat of 180.82 kJ kg−1 and solidifying at 42.42 ◦ C with a latent heat of 180.57 kJ kg−1 . SiO2 is an inorganic multiple porous material that is fire resistant. Therefore, the latent heat of such form-stable composite PCMs can be utilized for thermal energy storage in waste heat recovery and solar heating systems. 2. Experimental 2.1. Materials Tetraethyl silicate (Reagent grade, Sinopharm Chemical Reagent Company) was used as the precursor. LA (Reagent grade, Jiangsu Huakang Chemical Reagent Company) acted as the latent heat storage material. Anhydrous ethanol (Reagent grade, Nanjing Chemical Reagent Company) and distilled water served as solvent. Hydrochloric acid (Reagent grade, Nanjing Chemical Reagent Company) was used as the activator.
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G. Fang et al. / Materials Chemistry and Physics 122 (2010) 533–536
2.2. Preparation of the form-stable LA/SiO2 composite PCMs 20 g tetraethyl silicate, 20 g anhydrous ethanol and 35 g distilled water were added to a 250 ml flask. The pH of the mixture was adjusted to 2–3 by adding a little hydrochloric acid, the mixture was stirred at a rate of 500 rpm for 60 min with a magnetic stirrer while the temperature of the mixture was controlled at 60 ◦ C using a constant temperature bath. After the hydrolysis and polycondensation reactions of the tetraethyl silicate had taken place, the sol mixture was obtained. To prepare the composite PCMs, 3 g, 5 g or 10 g melted LA was added to the sol mixture. This was then stirred at 800 rpm for 70 min whilst the temperature of the mixture was maintained at 60 ◦ C. The composite PCM sol was then dried in an oven at 70 ◦ C for 2 h. In order to prepare form-stable LA/SiO2 composite PCM, the composite PCM gel was dried again in a vacuum oven at 70 ◦ C for 10 h. Finally, three kinds of form-stable LA/SiO2 composite PCMs were obtained, denoted FS-PCM1, FS-PCM2 and FS-PCM3. 2.3. Characterization of the form-stable LA/SiO2 composite PCMs The morphology and microstructure of the form-stable LA/SiO2 composite PCMs were observed using a scanning electronic microscope (SEM, S-3400NII, Hitachi Inc., Japan). The structural analysis of the composite PCM was carried out using a FT-IR spectrophotometer. The FT-IR spectra were recorded on a Nicolet Nexus 870 from 400 to 4000 cm−1 with a resolution of 2 cm−1 using KBr pellets. The thermal properties of the PCMs were obtained using a differential scanning calorimeter (Pyris 1 DSC, PerkinElmer) at 5 ◦ C min−1 under a constant stream of argon at a flow rate of 20 ml min−1 . The accuracy of enthalpy measurements was ±5% and the temperature accuracy was ±0.2 ◦ C. The thermal stability of the PCMs was determined by thermogravimetric analysis (Pyris 1 TGA, Perkin–Elmer) under a constant stream of nitrogen at a flow rate of 20 ml min−1 . The working temperature of the composite PCMs for thermal energy storage is usually between 40 and 50 ◦ C, which is higher than the melting temperature of the LA. Therefore, in the TGA experiments, the samples are heated to 50 ◦ C to determine their thermal stability.
3. Results and discussion 3.1. FT-IR analysis of the form-stable LA/SiO2 composite PCMs The FT-IR spectra of LA, SiO2 and FS-PCM3 are shown in Fig. 1. Fig. 1a shows the spectrum of LA in which the peak at 2500–3000 cm−1 represents the stretching vibration of the –OH
Fig. 1. FT-IR spectra of the (a) LA, (b) SiO2 and (c) FS-PCM3.
groups of LA. The peaks at 2953 cm−1 and 2917 cm−1 signify the asymmetrical stretching vibration and symmetrical stretching vibration of its –CH3 group and the peaks at 2871 cm−1 and 2849 cm−1 represent the asymmetrical stretching vibration and symmetrical stretching vibration of its –CH2 group. The absorption peak at 1701 cm−1 is assigned to the C O stretching vibration. The peak at 1303 cm−1 corresponds to the in-plane bending vibration of the functional group of –OH in LA, the peak at 938 cm−1 corresponds to the out-of-plane bending vibration of the –OH functional group and the peak at 721 cm−1 represents the in-plane swinging vibration of the –OH functional group. Fig. 1b shows the spectrum of SiO2 . The peaks at 1083 cm−1 , 797 cm−1 and 455 cm−1 signify the bending vibration of the Si–O functional group and the peak at 967 cm−1 is assigned to the Si–OH functional group. The absorption bands at 3200–3600 cm−1 and 1650–1700 cm−1 represent the
Fig. 2. SEM photographs of the (a) SiO2 , (b) FS-PCM1, (c) FS-PCM2 and (d) FS-PCM3.
G. Fang et al. / Materials Chemistry and Physics 122 (2010) 533–536
Fig. 3. The melting DSC curves of the LA, FS-PCM1, FS-PCM2 and FS-PCM3.
stretching and bending vibrations of the –OH functional group of H2 O. The peaks at 2917 cm−1 and 2849 cm−1 signify the stretching vibration of the –CH3 and –CH2 groups, respectively. As can be seen in Fig. 1c, the absorption peaks of LA at 2953, 2917, 2871, 2849, 1701, 1303, 938 and 721 cm−1 also appear in the FS-PCM3 spectra. This result shows that the LA was dispersed in the porous network of the SiO2 . It is also found that there is no shift in the absorption peaks of the LA/SiO2 composites when compared with the spectrum of SiO2 . This result indicates that there is no chemical interaction between the functional groups of the LA and SiO2 . The LA was retained easily in the pores of the SiO2 by capillary and surface tension forces [17], and hence leakage of the melted LA from the composites was prevented. 3.2. Microstructure of the form-stable LA/SiO2 composite PCMs Fig. 2 shows SEM photographs of SiO2 , FS-PCM1, FS-PCM2 and FS-PCM3. In the photographs, the white and black parts represent the LA and SiO2 , respectively. As seen in Fig. 2a, the SiO2 has a rough and angular surface. Fig. 2b–d shows that the LA was embedded and dispersed in the porous SiO2 network. The multiple porous structure of the SiO2 provided the mechanical strength for the whole composite and prevented the seepage of the melted LA due to the effect of capillary and surface tension forces between the LA and the porous network of the SiO2 . The maximum mass percentage of LA in the composites was found to be 64.8%. There was no leakage of LA from the composites with %LA up to this value even when it melts. 3.3. Thermal properties of the form-stable LA/SiO2 composite PCMs The DSC results of LA, FS-PCM1, FS-PCM2 and FS-PCM3 are presented in Figs. 3 and 4 and Table 1. Comparing the latent heat data of the form-stable composites with that of pure LA, the PCM mass percentage in the composites can be determined from Eq. (1). The
535
Fig. 4. The solidifying DSC curves of the LA, FS-PCM1, FS-PCM2 and FS-PCM3.
value of is the mass percentage of the PCM in the composites, HFSPCM represents the melting latent heat of the form-stable composites, and HPCM represents the melting latent heat of the LA as measured by the DSC. =
HFSPCM × 100% HPCM
(1)
As shown in Table 1, when the PCM mass in the composites increases from 3 g to 5 g to 10 g, the mass percentage of the PCM in the composites increases from 26.1% to 28.5% to 64.8%. In the composites, only the PCMs absorb/release thermal energy during the melting/solidifying process, hence a high PCM content will result in a high latent heat storage capacity. It is known that the melting latent heat of FS-PCM3 is 117.21 kJ kg−1 , and is larger than that of FS-PCM1 and FS-PCM2. As seen in Figs. 3 and 4, the heat flows from the FS-PCM1 and FS-PCM2 are higher than that of FS-PCM3 since a greater percentage of water molecules from the air were adsorbed in the pores of the SiO2 in the FS-PCM1 and FS-PCM2 samples. The DSC results in Table 1 show that the melting and solidifying temperatures of the form-stable composite PCMs decrease by 1–3 ◦ C when compared with the melting and solidifying temperatures of pure LA. This is due to the fact that there are no strong interactions between the LA molecules and the pore walls of the SiO2 . This leads to a depression of the phase change temperatures of the LA in the composites [18]. 3.4. Thermal stability of the form-stable LA/SiO2 composite PCMs Fig. 5 shows the TG curves of FS-PCM1, FS-PCM2 and FS-PCM3. As can be seen, the variations of weight loss rate in FS-PCM1, FSPCM2 and FS-PCM3 are faster during the initial 10 min due to the release of water molecules adsorbed in the pores of the SiO2 , and then the weight loss rates are nearly constant. It is also known that the weight loss rates of FS-PCM1, FS-PCM2 and FS-PCM3 are 28.94%, 32.78% and 23.39%. The weight loss rates of FS-PCM1 and FS-PCM2 are larger than that of FS-PCM3 because the PCM contents in FSPCM1 and FS-PCM2 are smaller than in FS-PCM3, so more water
Table 1 DSC data of the LA, FS-PCM1, FS-PCM2 and FS-PCM3. Sample name
LA FS-PCM1 FS-PCM2 FS-PCM3
PCM percentage (%)
100.0 26.1 28.5 64.8
Melting
Solidifying
Onset temperature (◦ C)
Peak temperature (◦ C)
Latent heat (kJ kg−1 )
Onset temperature (◦ C)
Peak temperature (◦ C)
44.43 42.76 41.24 42.46
46.80 44.28 43.84 44.78
180.82 47.30 51.47 117.21
42.42 40.12 40.83 41.30
41.05 39.84 40.09 40.33
Latent heat (kJ kg−1 ) −180.57 −10.25 −36.22 −90.00
536
G. Fang et al. / Materials Chemistry and Physics 122 (2010) 533–536
References
Fig. 5. TG curves of the FS-PCM1, FS-PCM2 and FS-PCM3.
molecules from the surrounding air were adsorbed in the pores of the SiO2 for FS-PCM1 and FS-PCM2. 4. Conclusions The preparation and characterization of the form-stable LA/SiO2 composite PCMs are reported. The LA was used as the phase change material for thermal energy storage, and SiO2 was used as the supporting material. The highest mass percentage of LA in the composites was 64.8%. The LA was well dispersed in the porous network of the SiO2 by capillary and surface tension forces, and the leakage of melted LA from the composites can be prevented even when it was heated above the melting temperature of the LA. The LA/SiO2 composite PCMs solidify at 41.30 ◦ C with a latent heat of 90.00 kJ kg−1 and melt at 42.46 ◦ C with a latent heat of 117.21 kJ kg−1 . The formstable LA/SiO2 composite PCMs have greater thermal stability and can be used repeatedly in a thermal energy storage system. This method may also be used to disperse many other organic PCMs in SiO2 for a variety of thermal energy storage applications. Acknowledgement The authors thank the National Natural Science Foundation of China (Grant No. 50776043) for financial support of this research.
[1] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renewable and Sustainable Energy Reviews 13 (2009) 318. [2] A. Shukla, D. Buddhi, R.L. Sawhney, Solar water heaters with phase change material thermal energy storage medium: a review, Renewable and Sustainable Energy Reviews 13 (2009) 2119. [3] W. Saman, F. Bruno, E. Halawa, Thermal performance of PCM thermal storage unit for a roof integrated solar heating system, Solar Energy 78 (2005) 341. [4] V.V. Tyagi, D. Buddhi, PCM thermal storage in buildings: a state of art, Renewable and Sustainable Energy Reviews 11 (2007) 1146. [5] A. Felix Regin, S.C. Solanki, J.S. Saini, Heat transfer characteristics of thermal energy storage system using PCM capsules: a review, Renewable and Sustainable Energy Reviews 12 (2008) 2438. [6] S.M. Hasnain, Review on sustainable thermal energy storage technologies. Part I. Heat storage materials and techniques, Energy Conversion and Management 39 (1998) 1127. [7] M. Kenisarin, K. Mahkamov, Solar energy storage using phase change materials, Renewable and Sustainable Energy Reviews 11 (2007) 1913. [8] A. Sari, Thermal reliability test of some fatty acids as PCMs used for solar thermal latent heat storage applications, Energy Conversion and Management 44 (2003) 2277. [9] A. Hasan, A.A.M. Sayigh, Some fatty acids as phase change thermal energy storage materials, Renewable Energy 4 (1994) 69. [10] C. Alkan, A. Sari, Fatty acid/poly(methyl methacrylate) (PMMA) blends as formstable phase change materials for latent heat thermal energy storage, Solar Energy 82 (2008) 118. [11] H. Ye, X.S. Ge, Preparation of polyethylene–paraffin compound as a form-stable solid–liquid phase change material, Solar Energy Material & Solar Cells 64 (2000) 37. [12] A. Sari, Form-stable paraffin/high density polyethylene composites as solid–liquid phase change materials for thermal energy storage: preparation and thermal properties, Energy Conversion and Management 45 (2004) 2033. [13] M. Xiao, B. Feng, K.C. Gong, Preparation and performance of shape stabilized phase change thermal storage materials with high thermal conductivity, Energy Conversion and Management 43 (2002) 103. [14] Z.G. Zhang, X.M. Fang, Study on paraffin/expanded graphite composite phase change thermal storage material, Energy Conversion and Management 47 (2006) 303. [15] A. Sari, A. Karaipekli, Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material, Applied Thermal Engineering 27 (2007) 1271. [16] Y.B. Cai, Q.F. Wei, F.L. Huang, S.L. Lin, F. Chen, W.D. Gao, Thermal stability, latent heat and flame retardant properties of the thermal energy storage phase change materials based on paraffin/high density polyethylene composites, Renewable Energy 34 (2009) 2117. [17] W.L Wang, X.X. Yang, Y.T. Fang, J. Ding, Preparation and performance of formstable polyethylene glycol/silicon dioxide composites as solid–liquid phase change materials, Applied Energy 86 (2009) 170. [18] A. Sari, A. Karaipekli, Preparation, thermal properties and thermal reliability of palmitic acid/expanded graphite composite as form-stable PCM for thermal energy storage, Solar Energy Materials and Solar Cells 93 (2009) 571.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation and properties of lauric acid/silicon dioxide composites as form-stable phase change materials for thermal energy storage Guiyin Fang a,∗ , Hui Li b , Xu Liu a a b
Department of Physics, Nanjing University, Hankou Road 22, Nanjing, Jiangsu 210093, China Department of Material Science and Engineering, Nanjing University, Nanjing 210093, China
a r t i c l e
i n f o
Article history: Received 10 September 2009 Received in revised form 13 March 2010 Accepted 16 March 2010 Keywords: Composite materials Sol–gel growth DSC Thermal properties
a b s t r a c t Form-stable lauric acid (LA)/silicon dioxide (SiO2 ) composite phase change materials were prepared using sol–gel methods. The LA was used as the phase change material for thermal energy storage, with the SiO2 acting as the supporting material. The structural analysis of these form-stable LA/SiO2 composite phase change materials was carried out using Fourier transformation infrared spectroscope (FT-IR). The microstructure of the form-stable composite phase change materials was observed by a scanning electronic microscope (SEM). The thermal properties and thermal stability were investigated by a differential scanning calorimeter (DSC) and a thermogravimetric analysis apparatus (TGA), respectively. The SEM results showed that the LA was well dispersed in the porous network of SiO2 . The DSC results indicated that the melting latent heat of the form-stable composite phase change material is 117.21 kJ kg−1 when the mass percentage of the LA in the SiO2 is 64.8%. The results of the TGA showed that these materials have good thermal stability. The form-stable composite phase change materials can be used for thermal energy storage in waste heat recovery and solar heating systems. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent years, phase change materials (PCMs) have received attention for various applications in solar heating systems [1–3], building energy conservation [4] and air-conditioning systems [5]. Unlike traditional heat storage methods, latent heat storage provides much greater energy storage density with a smaller temperature difference between storing and releasing heat. Many inorganic and organic PCMs (salt hydrates, fatty acids/esters, paraffins, etc.) and PCM mixtures have been studied for latent heat storage applications [6,7]. Among the PCMs investigated, fatty acids have been widely used due to their high latent heat storage capacity and appropriate thermal properties, such as little or no supercooling, low vapor pressure, good thermal and chemical stability, and self-nucleating behavior [8,9]. However, they have low thermal conductivity, and need encapsulation in order to prevent leakage of the melted PCM during the phase change process [10]. However, these problems can be solved by using form-stable PCM composites. These composites can be prepared by encapsulation of the PCM in a polymeric structure, such as high density polyethylene (HDPE) [11,12] and the styrene–butadiene–styrene
∗ Corresponding author. Tel.: +86 25 51788228; fax: +86 25 83593707. E-mail address: [email protected] (G. Fang). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.03.042
(SBS) copolymer [13]. In addition, the expanded graphite can also act as the supporting material in the composites [14,15]. However, due to the chemical constitution of organic PCMs (fatty acids/esters, paraffins, etc.) and organic polymer supporting materials (HDPE, SBS and so on), the form-stable PCMs are easily flammable, and their application is therefore severely restricted [16]. In this paper, the preparation and properties of the form-stable LA/SiO2 composite PCMs are reported. In the composite materials, the LA was used as the latent heat storage material, and SiO2 served as the inorganic supporting material. The LA is a favorable organic PCM for thermal energy storage, melting at 44.43 ◦ C with a latent heat of 180.82 kJ kg−1 and solidifying at 42.42 ◦ C with a latent heat of 180.57 kJ kg−1 . SiO2 is an inorganic multiple porous material that is fire resistant. Therefore, the latent heat of such form-stable composite PCMs can be utilized for thermal energy storage in waste heat recovery and solar heating systems. 2. Experimental 2.1. Materials Tetraethyl silicate (Reagent grade, Sinopharm Chemical Reagent Company) was used as the precursor. LA (Reagent grade, Jiangsu Huakang Chemical Reagent Company) acted as the latent heat storage material. Anhydrous ethanol (Reagent grade, Nanjing Chemical Reagent Company) and distilled water served as solvent. Hydrochloric acid (Reagent grade, Nanjing Chemical Reagent Company) was used as the activator.
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G. Fang et al. / Materials Chemistry and Physics 122 (2010) 533–536
2.2. Preparation of the form-stable LA/SiO2 composite PCMs 20 g tetraethyl silicate, 20 g anhydrous ethanol and 35 g distilled water were added to a 250 ml flask. The pH of the mixture was adjusted to 2–3 by adding a little hydrochloric acid, the mixture was stirred at a rate of 500 rpm for 60 min with a magnetic stirrer while the temperature of the mixture was controlled at 60 ◦ C using a constant temperature bath. After the hydrolysis and polycondensation reactions of the tetraethyl silicate had taken place, the sol mixture was obtained. To prepare the composite PCMs, 3 g, 5 g or 10 g melted LA was added to the sol mixture. This was then stirred at 800 rpm for 70 min whilst the temperature of the mixture was maintained at 60 ◦ C. The composite PCM sol was then dried in an oven at 70 ◦ C for 2 h. In order to prepare form-stable LA/SiO2 composite PCM, the composite PCM gel was dried again in a vacuum oven at 70 ◦ C for 10 h. Finally, three kinds of form-stable LA/SiO2 composite PCMs were obtained, denoted FS-PCM1, FS-PCM2 and FS-PCM3. 2.3. Characterization of the form-stable LA/SiO2 composite PCMs The morphology and microstructure of the form-stable LA/SiO2 composite PCMs were observed using a scanning electronic microscope (SEM, S-3400NII, Hitachi Inc., Japan). The structural analysis of the composite PCM was carried out using a FT-IR spectrophotometer. The FT-IR spectra were recorded on a Nicolet Nexus 870 from 400 to 4000 cm−1 with a resolution of 2 cm−1 using KBr pellets. The thermal properties of the PCMs were obtained using a differential scanning calorimeter (Pyris 1 DSC, PerkinElmer) at 5 ◦ C min−1 under a constant stream of argon at a flow rate of 20 ml min−1 . The accuracy of enthalpy measurements was ±5% and the temperature accuracy was ±0.2 ◦ C. The thermal stability of the PCMs was determined by thermogravimetric analysis (Pyris 1 TGA, Perkin–Elmer) under a constant stream of nitrogen at a flow rate of 20 ml min−1 . The working temperature of the composite PCMs for thermal energy storage is usually between 40 and 50 ◦ C, which is higher than the melting temperature of the LA. Therefore, in the TGA experiments, the samples are heated to 50 ◦ C to determine their thermal stability.
3. Results and discussion 3.1. FT-IR analysis of the form-stable LA/SiO2 composite PCMs The FT-IR spectra of LA, SiO2 and FS-PCM3 are shown in Fig. 1. Fig. 1a shows the spectrum of LA in which the peak at 2500–3000 cm−1 represents the stretching vibration of the –OH
Fig. 1. FT-IR spectra of the (a) LA, (b) SiO2 and (c) FS-PCM3.
groups of LA. The peaks at 2953 cm−1 and 2917 cm−1 signify the asymmetrical stretching vibration and symmetrical stretching vibration of its –CH3 group and the peaks at 2871 cm−1 and 2849 cm−1 represent the asymmetrical stretching vibration and symmetrical stretching vibration of its –CH2 group. The absorption peak at 1701 cm−1 is assigned to the C O stretching vibration. The peak at 1303 cm−1 corresponds to the in-plane bending vibration of the functional group of –OH in LA, the peak at 938 cm−1 corresponds to the out-of-plane bending vibration of the –OH functional group and the peak at 721 cm−1 represents the in-plane swinging vibration of the –OH functional group. Fig. 1b shows the spectrum of SiO2 . The peaks at 1083 cm−1 , 797 cm−1 and 455 cm−1 signify the bending vibration of the Si–O functional group and the peak at 967 cm−1 is assigned to the Si–OH functional group. The absorption bands at 3200–3600 cm−1 and 1650–1700 cm−1 represent the
Fig. 2. SEM photographs of the (a) SiO2 , (b) FS-PCM1, (c) FS-PCM2 and (d) FS-PCM3.
G. Fang et al. / Materials Chemistry and Physics 122 (2010) 533–536
Fig. 3. The melting DSC curves of the LA, FS-PCM1, FS-PCM2 and FS-PCM3.
stretching and bending vibrations of the –OH functional group of H2 O. The peaks at 2917 cm−1 and 2849 cm−1 signify the stretching vibration of the –CH3 and –CH2 groups, respectively. As can be seen in Fig. 1c, the absorption peaks of LA at 2953, 2917, 2871, 2849, 1701, 1303, 938 and 721 cm−1 also appear in the FS-PCM3 spectra. This result shows that the LA was dispersed in the porous network of the SiO2 . It is also found that there is no shift in the absorption peaks of the LA/SiO2 composites when compared with the spectrum of SiO2 . This result indicates that there is no chemical interaction between the functional groups of the LA and SiO2 . The LA was retained easily in the pores of the SiO2 by capillary and surface tension forces [17], and hence leakage of the melted LA from the composites was prevented. 3.2. Microstructure of the form-stable LA/SiO2 composite PCMs Fig. 2 shows SEM photographs of SiO2 , FS-PCM1, FS-PCM2 and FS-PCM3. In the photographs, the white and black parts represent the LA and SiO2 , respectively. As seen in Fig. 2a, the SiO2 has a rough and angular surface. Fig. 2b–d shows that the LA was embedded and dispersed in the porous SiO2 network. The multiple porous structure of the SiO2 provided the mechanical strength for the whole composite and prevented the seepage of the melted LA due to the effect of capillary and surface tension forces between the LA and the porous network of the SiO2 . The maximum mass percentage of LA in the composites was found to be 64.8%. There was no leakage of LA from the composites with %LA up to this value even when it melts. 3.3. Thermal properties of the form-stable LA/SiO2 composite PCMs The DSC results of LA, FS-PCM1, FS-PCM2 and FS-PCM3 are presented in Figs. 3 and 4 and Table 1. Comparing the latent heat data of the form-stable composites with that of pure LA, the PCM mass percentage in the composites can be determined from Eq. (1). The
535
Fig. 4. The solidifying DSC curves of the LA, FS-PCM1, FS-PCM2 and FS-PCM3.
value of is the mass percentage of the PCM in the composites, HFSPCM represents the melting latent heat of the form-stable composites, and HPCM represents the melting latent heat of the LA as measured by the DSC. =
HFSPCM × 100% HPCM
(1)
As shown in Table 1, when the PCM mass in the composites increases from 3 g to 5 g to 10 g, the mass percentage of the PCM in the composites increases from 26.1% to 28.5% to 64.8%. In the composites, only the PCMs absorb/release thermal energy during the melting/solidifying process, hence a high PCM content will result in a high latent heat storage capacity. It is known that the melting latent heat of FS-PCM3 is 117.21 kJ kg−1 , and is larger than that of FS-PCM1 and FS-PCM2. As seen in Figs. 3 and 4, the heat flows from the FS-PCM1 and FS-PCM2 are higher than that of FS-PCM3 since a greater percentage of water molecules from the air were adsorbed in the pores of the SiO2 in the FS-PCM1 and FS-PCM2 samples. The DSC results in Table 1 show that the melting and solidifying temperatures of the form-stable composite PCMs decrease by 1–3 ◦ C when compared with the melting and solidifying temperatures of pure LA. This is due to the fact that there are no strong interactions between the LA molecules and the pore walls of the SiO2 . This leads to a depression of the phase change temperatures of the LA in the composites [18]. 3.4. Thermal stability of the form-stable LA/SiO2 composite PCMs Fig. 5 shows the TG curves of FS-PCM1, FS-PCM2 and FS-PCM3. As can be seen, the variations of weight loss rate in FS-PCM1, FSPCM2 and FS-PCM3 are faster during the initial 10 min due to the release of water molecules adsorbed in the pores of the SiO2 , and then the weight loss rates are nearly constant. It is also known that the weight loss rates of FS-PCM1, FS-PCM2 and FS-PCM3 are 28.94%, 32.78% and 23.39%. The weight loss rates of FS-PCM1 and FS-PCM2 are larger than that of FS-PCM3 because the PCM contents in FSPCM1 and FS-PCM2 are smaller than in FS-PCM3, so more water
Table 1 DSC data of the LA, FS-PCM1, FS-PCM2 and FS-PCM3. Sample name
LA FS-PCM1 FS-PCM2 FS-PCM3
PCM percentage (%)
100.0 26.1 28.5 64.8
Melting
Solidifying
Onset temperature (◦ C)
Peak temperature (◦ C)
Latent heat (kJ kg−1 )
Onset temperature (◦ C)
Peak temperature (◦ C)
44.43 42.76 41.24 42.46
46.80 44.28 43.84 44.78
180.82 47.30 51.47 117.21
42.42 40.12 40.83 41.30
41.05 39.84 40.09 40.33
Latent heat (kJ kg−1 ) −180.57 −10.25 −36.22 −90.00
536
G. Fang et al. / Materials Chemistry and Physics 122 (2010) 533–536
References
Fig. 5. TG curves of the FS-PCM1, FS-PCM2 and FS-PCM3.
molecules from the surrounding air were adsorbed in the pores of the SiO2 for FS-PCM1 and FS-PCM2. 4. Conclusions The preparation and characterization of the form-stable LA/SiO2 composite PCMs are reported. The LA was used as the phase change material for thermal energy storage, and SiO2 was used as the supporting material. The highest mass percentage of LA in the composites was 64.8%. The LA was well dispersed in the porous network of the SiO2 by capillary and surface tension forces, and the leakage of melted LA from the composites can be prevented even when it was heated above the melting temperature of the LA. The LA/SiO2 composite PCMs solidify at 41.30 ◦ C with a latent heat of 90.00 kJ kg−1 and melt at 42.46 ◦ C with a latent heat of 117.21 kJ kg−1 . The formstable LA/SiO2 composite PCMs have greater thermal stability and can be used repeatedly in a thermal energy storage system. This method may also be used to disperse many other organic PCMs in SiO2 for a variety of thermal energy storage applications. Acknowledgement The authors thank the National Natural Science Foundation of China (Grant No. 50776043) for financial support of this research.
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