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SiO2–Al2O3 hybrid form-stable phase change materials with enhanced thermal conductivity
Materials Chemistry and Physics 144 (2014) 162e167
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
PEG/SiO2eAl2O3 hybrid form-stable phase change materials with enhanced thermal conductivity Bingtao Tang*, Cheng Wu, Meige Qiu, Xiwen Zhang, Shufen Zhang State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, PR China
h i g h l i g h t s The PEG/SiO2eAl2O3 hybrid form-stable phase change material (PCM) was obtained through the solegel method. The inexpensive aluminum nitrate and tetraethyl orthosilicate were used as sol precursors. This organiceinorganic hybrid process can effectively enhance the thermal conductivity of PCMs. The PCM exhibited high thermal stability and excellent form-stable effects.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 March 2013 Received in revised form 24 October 2013 Accepted 29 December 2013
The thermal conductivity of form-stable PEG/SiO2 phase change material (PCM) was enhanced by in situ doping of Al2O3 using an ultrasound-assisted solegel method. Fourier transform infrared spectroscopy (FT-IR) was used to characterize the structure, and the crystal performance was characterized by the Xray diffraction (XRD). Differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA) were used to determine the thermal properties. The phase change enthalpy of PEG/SiO2eAl2O3 reached 124 J g1, and thermal conductivity improved by 12.8% for 3.3 wt% Al2O3 in the PCM compared with PEG/ SiO2. The hybrid PCM has excellent thermal stability and form-stable effects. Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved.
Keywords: Composite material Solegel growth Phase transition Thermal conductivity
1. Introduction Form-stable organiceinorganic hybrid phase change materials (PCMs) are functional materials that store thermal energy in the process of phase transition [1e7]. They are widely known because of their easy preparation and direct use without additional encapsulation [8e12]. However, the chargingedischarging rate of thermal energy was strongly restricted due to low thermal conductivity [1]. Among various approaches of enhancing the thermal conductivity of organiceinorganic hybrid form-stable PCMs, doping of particles with high thermal conductivity is an effective method [13e15]. Polyethylene glycol (PEG) is one kind of important phase change material due to its good characteristics [16] such as suitable phase change temperature, high latent heat capacity, nontoxicity, low vapor pressure, and high thermal and chemical
* Corresponding author. Tel.: þ86 411 84986267; fax: þ86 411 84986264. E-mail address: [email protected] (B. Tang).
stability after long-term utility period [17,18]. However, homogeneous dispersion remains a great challenge due to the large specific surface area and high surface energy of particles. In our recent work we have demonstrated that the solegel process was a convenient way of obtaining organiceinorganic hybrid PCMs. Thermal conductivity of PCMs was significantly enhanced by in situ doping of Cu nanoparticles [19]. But Cu nanoparticle was sensitive to temperature and humidity due to the particular size and surface effect of nanomaterials. Therefore, the long-term storage property of organiceinorganic hybrid PCMs was a great challenge [20]. Al2O3 has high thermal conductivity, high chemical durability and environmental resistance [21]. In the present study, we used the inexpensive aluminum nitrate and tetraethyl orthosilicate as sol precursors and one PEG/SiO2eAl2O3 hybrid form-stable PCM with high thermal conductivity was obtained through the solegel method. In the PEG/SiO2eAl2O3 system, PEG acted as the solide liquid PCM and the inorganic SiO2eAl2O3 net was used as the supporting material. The hybrid PCM exhibited high conductive performance and excellent form-stable effect.
0254-0584/$ e see front matter Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.12.036
B. Tang et al. / Materials Chemistry and Physics 144 (2014) 162e167
2. Experimental 2.1. Materials All chemicals were of analytical grade and used without future purification. Al(NO3)3$9H2O was supplied by Sinopharm Chemical Agent Company (PR China). Tetraethyl orthosilicate (TEOS) was purchased from Tianjin Damao Chemical Agent Company (Tianjin, PR China). PEG6000 was supplied by Sinopharm Chemical Agent Company (Shanghai, PR China). 2.2. Preparation of PEG/SiO2eAl2O3 with high thermal conductivity In total, 8.32 g TEOS and 7.2 ml deionized water were mixed in a 100 ml flask. An ultrasound probe (XH-2008D, Beijing Xianghu Science and Technology Development, PR China) was then dipped into the above mixture with the temperature controlling at 50 C. The pH value was regulated to 2 by adding a certain amount of 2 mol L1 HNO3, and translucent silica sol was achieved under the nominal ultrasonic power of 300 W. Al(NO3)3 solution (0.5 mol L1) was prepared by mixed Al(NO3)3$9H2O with a specific volume of deionized water. The pH value of the solution was regulated to 9e10 by adding a certain amount of 26%e28% ammonia water. The suspension obtained was filtered and collected after washing with deionized water. The filter cake was added to deionized water under the nominal ultrasonic power of 300 W. Then adjusting pH to 4 by adding a certain amount of 2 mol L1 HNO3. Al sol was obtained after 1 h. The silica sol was mixed with Al sol at 50 C and treated with ultrasonic treatment for 20 min. PEG6000 was then added into the silicaeAl sol. Another 20 min later, 10% Na2CO3 solution was slowly added into the mixture, and the gel was developed. The product was vacuum dried at 50 C for 24 h. Finally, the hybrid form-stable PEG/SiO2eAl2O3 PCM was obtained. 2.3. Characterization of the hybrid composite PCM Fourier transform infrared spectroscopy (Nicolet Avatar 320, KBr pellets) was used to analyze Structural analysis of the PEG/SiO2e Al2O3 PCM. Powder X-ray diffraction patterns were measured on a Rigaku D/MAX-2400 system with CueKa radiation. The scanning step size was 0.02 , 2q from 5 to 80 . Differential scanning
163
calorimetry (TA 2010) was used to research the thermal properties of PEG/SiO2eAl2O3. Differential scanning calorimetry was conducted under dry nitrogen at a heating rate of 5 C min1 in the range from 20 to 80 C. Thermogravimetric analysis was conducted using a Perkin Elmer Diamond TG/DTG system from 30 to 700 C at 10 C min1 under nitrogen. Thermal conductivities were measured using a DRLeZ tester (Measurement range: 0.05e45 W (m K)1; Xiangyi Instrument, PR China). The tests were performed by the steady-state heat flux method technique. Specimens of 3 cm in diameter and 0.5 cm in thickness were prepared with stainless mold by the pressure machine. The samples were tested under 50 C of thermal stage and 30 C of cold stage. Reported results represented the average of three measurements. The freezing performance tests of the PEG, PEG/SiO2 and PEG/SiO2eAl2O3 were conducted using the procedure in reported study [22]. Specifically, the PCMs were added in one glass bottle. Subsequently, the thermocouple (PteRhPt) was inserted into the bottle. And the bottle was placed in a water bath at 60 C for being heated. When the temperature reached 60 C, the bottle was transferred to another water bath at 20 C. The amount of each sample was 5 g. The temperature of the system was recorded during the freezing process. 3. Results and discussion 3.1. Synthesis and FTIR characterization of PEG/SiO2eAl2O3 Synthesis of the PEG/SiO2eAl2O3 PCM is illustrated in Fig. 1. First, silica sol was easily obtained under HNO3 catalysis and ultrasound-assisted conditions. Al sol was then obtained under the same conditions based on Al(NO3)3$9H2O. SilicaeAl sol was developed after silica sol was added slowly to Al sol under ultrasound-assisted conditions. After PEG was added to the silicae Al sol, Na2CO3 was used to adjust the pH to 7e8, and PEG/SiO2e Al2O3 was then obtained. The FT-IR spectrum can provide much valuable information about the structure of the material, enabling it to clearly reveal the combined mode of SiO2eAl2O3 and PEG. The FTIR spectra of SiO2e Al2O3, PEG6000, and PEG/SiO2eAl2O3 composite are shown in Fig. 2. In the spectrum of SiO2eAl2O3 (Fig. 2a), the band in the 1077 cm1 is the stretching peak of SieO bond, and 482 cm1 is that
Fig. 1. Schematic synthesis of the PCM.
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B. Tang et al. / Materials Chemistry and Physics 144 (2014) 162e167
The form-stable property of the hybrid PCM was evaluated using hot stageedigital camera technology [32]. The materials were put on one hot-stage and heated at 5 C min1 from 15 to 85 C. Shape changes were observed via tracking photographs using the digital camera. The corresponding results are shown in Fig. 4. The pure PEG6000 began to flow when the temperature reached their melting point. But there were no leakage of PEG6000 from the surfaces of the composite even when the temperature was higher than the melting point of PEG6000. PEG is a kind of polymer containing long-chains, and the SiO2eAl2O3 gel has an intercross linked network. The long chains of PEG can totally or partially interpenetrate the network. The capillary force and surface tension induced by the SiO2eAl2O3 gel network limited the leakage of liquid PEG [33,34]. Accordingly, the PEG/SiO2eAl2O3 can retain its original shape after phase transition in spite of no chemical interaction between SiO2eAl2O3 and PEG. 3.3. Thermal properties of the PEG/SiO2eAl2O3 Fig. 2. FTIR spectra of PEG600 and composite PCMs.
of AleO bond [23]. The wide band at 1100e1200 cm1 is the overlap between the absorption peaks of SieO and AleO. The peaks at approximately 956 and 700 cm1 are distributed to the AleO in AlO4 and AlO6 units, respectively [24], indicating the development of SieOeAl chemical bond or the disturbance of Al on SieO vibration [25]. Typical pure PEG6000 (Fig. 2b) bands are visible at 3440 cm1 (OeH stretching), 2917 cm1 (CH2 stretching), and 1106 cm1 (Ce OeC symmetrical stretching) [26e29]. In the spectrum of the PEG/SiO2eAl2O3 composite PCM (Fig. 2c), the characteristic FTIR bands for both SiO2eAl2O3 and PEG6000 are present but new peaks and shifts in the absorption peaks are not. These above results show that no chemical bond was formatted between PEG6000 and SiO2eAl2O3. 3.2. Crystalline and form-stable properties of the PEG/SiO2eAl2O3 Fig. 3 shows the powder XRD patterns of the prepared samples from PEG, SiO2eAl2O3, and PEG/SiO2eAl2O3. In the hybrid form-stable PCM, PEG6000 accounts for the XRD peaks at 19.3 , 19.8 , 23.4 , and 24.8 , as well as a broad peak at approximately 2q ¼ 23 , which resulted from the amorphous aluminosilicate [30,31], indicating that the PEG on the support was in the crystalline state.
The thermal characteristics of the PEG6000/SiO2eAl2O3 hybrid form-stable PCM with heating and subsequent cooling in the range of 20e80 C at the rate of 5 C min1 are illustrated in Fig. 5. Table 1 shows the phase change temperature as well as endothermic and exothermic enthalpy values based on the DSC curves of PEG6000/ SiO2eAl2O3. The data in Table 1 show that the melting point of the PEG6000/ SiO2eAl2O3 PCM is nearly the same as that of pure PEG6000. The crystallization temperatures were higher than those used for pure PEG6000. Therefore, the extent of supercooling in PEG6000/SiO2e Al2O3 was lower compared with that in PEG, which is important in practical applications [35]. Although the phase change enthalpies of the hybrid PCMs are lower than those of pure PEG6000, the phase change enthalpies of the PEG6000/SiO2eAl2O3 PCMs are higher than those of PEG6000/ SiO2, due to the microphase separation of PEG6000 from SiO2e Al2O3 when Al2O3 was introduced into the system. The microphase separation leaded to a more continuous PEG domain. The crystallization region of the PEG became large, and the phase change enthalpy was high. Similar phenomenon has been published for the modification of polyurethane [36]. Meanwhile, the transformation enthalpies of the PEG6000/SiO2eAl2O3 PCM could exceed 120 J g1, which is a high phase change value [37e40]. 3.4. Thermal stability of the PEG/SiO2eAl2O3 Thermal stability of the PEG6000/SiO2eAl2O3 composite was determined by TGA and differential thermogravimetry (DTG). The corresponding results are presented in Fig. 6. Table 2 lists the data corresponding to the maximum weight loss temperature (Tmax) from the DTG curves, and the char residue at 700 C. Approximately 83% of weight loss from 290 to 700 C indicated the mass content of PEG6000 in the PEG/SiO2eAl2O (Table 2). Therefore, the PCM has good thermal stability when the temperature is below 290 C, which meets the demands of heat storage. 3.5. Thermal conductivity improvement and freezing curves of the PEG/SiO2eAl2O3
Fig. 3. XRD spectra of PEG600 and composite PCMs.
The thermal conductivity of PEG6000/SiO2 was improved when Al2O3 was introduced into the system. The results are shown in Table 3. Thermal conductivity of the composite PCMs with 0, 3.3, 9.2, and 12.6 wt% Al2O3 reached 0.36 0.398, 0.419 and 0.435 W (m K)1, respectively (thermal conductivity for PEG is 0.297 [41]). Comparing with pure PEG6000 and PEG6000/SiO2, thermal
B. Tang et al. / Materials Chemistry and Physics 144 (2014) 162e167
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Fig. 4. PEG6000/SiO2eAl2O3 composite with varying Al2O3 contents.
conductivity of the composite PCM with 3.3 wt% Al2O3 increased 34% and 10.2%. The PEG6000/SiO2eAl2O3 hybrid form-stable PCM exhibits the same high thermal conductivity as other PCMs (PEG6000/SiO2/AlN [18], Ag NWs/1-tetradecanol [39], and PANI/
tetradecanol/MWNTs [40]), but it has a larger enthalpy and involves minimal cost due to the inexpensive aluminum nitrate. The enhancement of thermal transfer was also investigated by comparing the freezing process of the PEG6000/SiO2eAl2O3 with that of PEG6000/SiO2, and the results are presented in Fig. 7. The results demonstrate that to achieve the same temperature 41 C, the freezing times are 780 s for PEG6000/SiO2, 680 s for PEG6000/ SiO2eAl2O3 (3.3 wt% Al2O3), 560 s for PEG6000/SiO2eAl2O3 (9.2 wt % Al2O3), and 420 s for PEG6000/SiO2eAl2O3 (12.6 wt% Al2O3). The freezing time of 3.3 wt% Al2O3 in PEG/SiO2 was reduced by 12.8% by comparing with that of PEG6000/SiO2.
Table 1 Phase transition temperature and enthalpy of composite PCMs with varying Al2O3 contents.
Fig. 5. DSC curves of the composite PCMs with varying Al2O3 contents.
Sample
wt% Al2O3
Tc ( C)
DHc (J g1)
Tm ( C)
DHm (J g1)
PEG6000 PEG6000/SiO2 PEG6000/SiO2eAl2O3-1 PEG6000/SiO2eAl2O3-2 PEG6000/SiO2eAl2O3-3
e 0 3.3 9.2 12.6
39.6 43.6 43.8 43.8 42.0
177.9 103.8 126.2 112.3 126.4
59.6 56.5 57.2 56.3 57.1
181.5 102.8 122.0 107.8 123.8
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B. Tang et al. / Materials Chemistry and Physics 144 (2014) 162e167
Fig. 6. TGA and differential thermogravimetric (DTG) analysis curves of the composite PCMs with and without Al2O3.
Table 2 TGA results of different composite PCMs. Sample
wt% Al2O3
Tmax ( C)
Char residue at 700 C (wt%)
PEG6000 PEG6000/SiO2 PEG6000/SiO2eAl2O3
e 0 9.2
403.9 415.5 404.7
2.1 24.2 17.2
Acknowledgments
Table 3 Thermal conductivities of the composite PCMs. Sample
PEG6000 PEG6000/SiO2 PEG6000/SiO2eAl2O3-1 PEG6000/SiO2eAl2O3-2 PEG6000/SiO2eAl2O3-3
wt% Al2O3
e 0 3.3 9.2 12.6
freezing time was reduced by 12.8% for 3.3 wt% Al2O3 in PEG6000/ SiO2 PCM compared with PEG6000/SiO2. Analysis revealed that PEG6000/SiO2eAl2O3 is a form-stable PCM with good thermal stability and large enthalpy value (124 J g1).
Thermal conductivity (W (m K)1)
Thermal conductivity enhancement (%)a E1
E2
0.297 0.36 0.398 0.419 0.435
e 21.2% 34.0% 41.1% 46.5%
e e 10.60% 16.40% 20.80%
a
E1: Thermal conductivity enhancement comparing with PEG6000. E2: Thermal conductivity enhancement comparing with PEG6000/SiO2.
This work was supported by the National Natural Science Foundation of China (21276042), the National Science and Technology Pillar Program (2013BAF08B06), Program for New Century Excellent Talents in University (NCET130080), Fundamental Research Funds for the Central Universities of China (DUT13LK35), and Program for Liaoning Excellent Talents in University (LJQ2013006). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Fig. 7. Freezing curves of the composite PCMs with varying Al2O3 contents.
4. Conclusions A PEG6000/SiO2eAl2O3 hybrid form-stable PCM was successfully obtained through an ultrasound-assisted solegel method and using aluminum nitrate and TEOS as sol precursors. The thermal conductivity of PEG6000/SiO2 increased significantly by in situ doping of Al2O3, which formed a SiO2eAl2O3 net to serve as the supporting material. The freezing rate tests showed that the
[23] [24] [25] [26] [27] [28] [29] [30] [31]
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
PEG/SiO2eAl2O3 hybrid form-stable phase change materials with enhanced thermal conductivity Bingtao Tang*, Cheng Wu, Meige Qiu, Xiwen Zhang, Shufen Zhang State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, PR China
h i g h l i g h t s The PEG/SiO2eAl2O3 hybrid form-stable phase change material (PCM) was obtained through the solegel method. The inexpensive aluminum nitrate and tetraethyl orthosilicate were used as sol precursors. This organiceinorganic hybrid process can effectively enhance the thermal conductivity of PCMs. The PCM exhibited high thermal stability and excellent form-stable effects.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 March 2013 Received in revised form 24 October 2013 Accepted 29 December 2013
The thermal conductivity of form-stable PEG/SiO2 phase change material (PCM) was enhanced by in situ doping of Al2O3 using an ultrasound-assisted solegel method. Fourier transform infrared spectroscopy (FT-IR) was used to characterize the structure, and the crystal performance was characterized by the Xray diffraction (XRD). Differential scanning calorimetry (DSC) and thermogravimetric analyzer (TGA) were used to determine the thermal properties. The phase change enthalpy of PEG/SiO2eAl2O3 reached 124 J g1, and thermal conductivity improved by 12.8% for 3.3 wt% Al2O3 in the PCM compared with PEG/ SiO2. The hybrid PCM has excellent thermal stability and form-stable effects. Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved.
Keywords: Composite material Solegel growth Phase transition Thermal conductivity
1. Introduction Form-stable organiceinorganic hybrid phase change materials (PCMs) are functional materials that store thermal energy in the process of phase transition [1e7]. They are widely known because of their easy preparation and direct use without additional encapsulation [8e12]. However, the chargingedischarging rate of thermal energy was strongly restricted due to low thermal conductivity [1]. Among various approaches of enhancing the thermal conductivity of organiceinorganic hybrid form-stable PCMs, doping of particles with high thermal conductivity is an effective method [13e15]. Polyethylene glycol (PEG) is one kind of important phase change material due to its good characteristics [16] such as suitable phase change temperature, high latent heat capacity, nontoxicity, low vapor pressure, and high thermal and chemical
* Corresponding author. Tel.: þ86 411 84986267; fax: þ86 411 84986264. E-mail address: [email protected] (B. Tang).
stability after long-term utility period [17,18]. However, homogeneous dispersion remains a great challenge due to the large specific surface area and high surface energy of particles. In our recent work we have demonstrated that the solegel process was a convenient way of obtaining organiceinorganic hybrid PCMs. Thermal conductivity of PCMs was significantly enhanced by in situ doping of Cu nanoparticles [19]. But Cu nanoparticle was sensitive to temperature and humidity due to the particular size and surface effect of nanomaterials. Therefore, the long-term storage property of organiceinorganic hybrid PCMs was a great challenge [20]. Al2O3 has high thermal conductivity, high chemical durability and environmental resistance [21]. In the present study, we used the inexpensive aluminum nitrate and tetraethyl orthosilicate as sol precursors and one PEG/SiO2eAl2O3 hybrid form-stable PCM with high thermal conductivity was obtained through the solegel method. In the PEG/SiO2eAl2O3 system, PEG acted as the solide liquid PCM and the inorganic SiO2eAl2O3 net was used as the supporting material. The hybrid PCM exhibited high conductive performance and excellent form-stable effect.
0254-0584/$ e see front matter Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.12.036
B. Tang et al. / Materials Chemistry and Physics 144 (2014) 162e167
2. Experimental 2.1. Materials All chemicals were of analytical grade and used without future purification. Al(NO3)3$9H2O was supplied by Sinopharm Chemical Agent Company (PR China). Tetraethyl orthosilicate (TEOS) was purchased from Tianjin Damao Chemical Agent Company (Tianjin, PR China). PEG6000 was supplied by Sinopharm Chemical Agent Company (Shanghai, PR China). 2.2. Preparation of PEG/SiO2eAl2O3 with high thermal conductivity In total, 8.32 g TEOS and 7.2 ml deionized water were mixed in a 100 ml flask. An ultrasound probe (XH-2008D, Beijing Xianghu Science and Technology Development, PR China) was then dipped into the above mixture with the temperature controlling at 50 C. The pH value was regulated to 2 by adding a certain amount of 2 mol L1 HNO3, and translucent silica sol was achieved under the nominal ultrasonic power of 300 W. Al(NO3)3 solution (0.5 mol L1) was prepared by mixed Al(NO3)3$9H2O with a specific volume of deionized water. The pH value of the solution was regulated to 9e10 by adding a certain amount of 26%e28% ammonia water. The suspension obtained was filtered and collected after washing with deionized water. The filter cake was added to deionized water under the nominal ultrasonic power of 300 W. Then adjusting pH to 4 by adding a certain amount of 2 mol L1 HNO3. Al sol was obtained after 1 h. The silica sol was mixed with Al sol at 50 C and treated with ultrasonic treatment for 20 min. PEG6000 was then added into the silicaeAl sol. Another 20 min later, 10% Na2CO3 solution was slowly added into the mixture, and the gel was developed. The product was vacuum dried at 50 C for 24 h. Finally, the hybrid form-stable PEG/SiO2eAl2O3 PCM was obtained. 2.3. Characterization of the hybrid composite PCM Fourier transform infrared spectroscopy (Nicolet Avatar 320, KBr pellets) was used to analyze Structural analysis of the PEG/SiO2e Al2O3 PCM. Powder X-ray diffraction patterns were measured on a Rigaku D/MAX-2400 system with CueKa radiation. The scanning step size was 0.02 , 2q from 5 to 80 . Differential scanning
163
calorimetry (TA 2010) was used to research the thermal properties of PEG/SiO2eAl2O3. Differential scanning calorimetry was conducted under dry nitrogen at a heating rate of 5 C min1 in the range from 20 to 80 C. Thermogravimetric analysis was conducted using a Perkin Elmer Diamond TG/DTG system from 30 to 700 C at 10 C min1 under nitrogen. Thermal conductivities were measured using a DRLeZ tester (Measurement range: 0.05e45 W (m K)1; Xiangyi Instrument, PR China). The tests were performed by the steady-state heat flux method technique. Specimens of 3 cm in diameter and 0.5 cm in thickness were prepared with stainless mold by the pressure machine. The samples were tested under 50 C of thermal stage and 30 C of cold stage. Reported results represented the average of three measurements. The freezing performance tests of the PEG, PEG/SiO2 and PEG/SiO2eAl2O3 were conducted using the procedure in reported study [22]. Specifically, the PCMs were added in one glass bottle. Subsequently, the thermocouple (PteRhPt) was inserted into the bottle. And the bottle was placed in a water bath at 60 C for being heated. When the temperature reached 60 C, the bottle was transferred to another water bath at 20 C. The amount of each sample was 5 g. The temperature of the system was recorded during the freezing process. 3. Results and discussion 3.1. Synthesis and FTIR characterization of PEG/SiO2eAl2O3 Synthesis of the PEG/SiO2eAl2O3 PCM is illustrated in Fig. 1. First, silica sol was easily obtained under HNO3 catalysis and ultrasound-assisted conditions. Al sol was then obtained under the same conditions based on Al(NO3)3$9H2O. SilicaeAl sol was developed after silica sol was added slowly to Al sol under ultrasound-assisted conditions. After PEG was added to the silicae Al sol, Na2CO3 was used to adjust the pH to 7e8, and PEG/SiO2e Al2O3 was then obtained. The FT-IR spectrum can provide much valuable information about the structure of the material, enabling it to clearly reveal the combined mode of SiO2eAl2O3 and PEG. The FTIR spectra of SiO2e Al2O3, PEG6000, and PEG/SiO2eAl2O3 composite are shown in Fig. 2. In the spectrum of SiO2eAl2O3 (Fig. 2a), the band in the 1077 cm1 is the stretching peak of SieO bond, and 482 cm1 is that
Fig. 1. Schematic synthesis of the PCM.
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The form-stable property of the hybrid PCM was evaluated using hot stageedigital camera technology [32]. The materials were put on one hot-stage and heated at 5 C min1 from 15 to 85 C. Shape changes were observed via tracking photographs using the digital camera. The corresponding results are shown in Fig. 4. The pure PEG6000 began to flow when the temperature reached their melting point. But there were no leakage of PEG6000 from the surfaces of the composite even when the temperature was higher than the melting point of PEG6000. PEG is a kind of polymer containing long-chains, and the SiO2eAl2O3 gel has an intercross linked network. The long chains of PEG can totally or partially interpenetrate the network. The capillary force and surface tension induced by the SiO2eAl2O3 gel network limited the leakage of liquid PEG [33,34]. Accordingly, the PEG/SiO2eAl2O3 can retain its original shape after phase transition in spite of no chemical interaction between SiO2eAl2O3 and PEG. 3.3. Thermal properties of the PEG/SiO2eAl2O3 Fig. 2. FTIR spectra of PEG600 and composite PCMs.
of AleO bond [23]. The wide band at 1100e1200 cm1 is the overlap between the absorption peaks of SieO and AleO. The peaks at approximately 956 and 700 cm1 are distributed to the AleO in AlO4 and AlO6 units, respectively [24], indicating the development of SieOeAl chemical bond or the disturbance of Al on SieO vibration [25]. Typical pure PEG6000 (Fig. 2b) bands are visible at 3440 cm1 (OeH stretching), 2917 cm1 (CH2 stretching), and 1106 cm1 (Ce OeC symmetrical stretching) [26e29]. In the spectrum of the PEG/SiO2eAl2O3 composite PCM (Fig. 2c), the characteristic FTIR bands for both SiO2eAl2O3 and PEG6000 are present but new peaks and shifts in the absorption peaks are not. These above results show that no chemical bond was formatted between PEG6000 and SiO2eAl2O3. 3.2. Crystalline and form-stable properties of the PEG/SiO2eAl2O3 Fig. 3 shows the powder XRD patterns of the prepared samples from PEG, SiO2eAl2O3, and PEG/SiO2eAl2O3. In the hybrid form-stable PCM, PEG6000 accounts for the XRD peaks at 19.3 , 19.8 , 23.4 , and 24.8 , as well as a broad peak at approximately 2q ¼ 23 , which resulted from the amorphous aluminosilicate [30,31], indicating that the PEG on the support was in the crystalline state.
The thermal characteristics of the PEG6000/SiO2eAl2O3 hybrid form-stable PCM with heating and subsequent cooling in the range of 20e80 C at the rate of 5 C min1 are illustrated in Fig. 5. Table 1 shows the phase change temperature as well as endothermic and exothermic enthalpy values based on the DSC curves of PEG6000/ SiO2eAl2O3. The data in Table 1 show that the melting point of the PEG6000/ SiO2eAl2O3 PCM is nearly the same as that of pure PEG6000. The crystallization temperatures were higher than those used for pure PEG6000. Therefore, the extent of supercooling in PEG6000/SiO2e Al2O3 was lower compared with that in PEG, which is important in practical applications [35]. Although the phase change enthalpies of the hybrid PCMs are lower than those of pure PEG6000, the phase change enthalpies of the PEG6000/SiO2eAl2O3 PCMs are higher than those of PEG6000/ SiO2, due to the microphase separation of PEG6000 from SiO2e Al2O3 when Al2O3 was introduced into the system. The microphase separation leaded to a more continuous PEG domain. The crystallization region of the PEG became large, and the phase change enthalpy was high. Similar phenomenon has been published for the modification of polyurethane [36]. Meanwhile, the transformation enthalpies of the PEG6000/SiO2eAl2O3 PCM could exceed 120 J g1, which is a high phase change value [37e40]. 3.4. Thermal stability of the PEG/SiO2eAl2O3 Thermal stability of the PEG6000/SiO2eAl2O3 composite was determined by TGA and differential thermogravimetry (DTG). The corresponding results are presented in Fig. 6. Table 2 lists the data corresponding to the maximum weight loss temperature (Tmax) from the DTG curves, and the char residue at 700 C. Approximately 83% of weight loss from 290 to 700 C indicated the mass content of PEG6000 in the PEG/SiO2eAl2O (Table 2). Therefore, the PCM has good thermal stability when the temperature is below 290 C, which meets the demands of heat storage. 3.5. Thermal conductivity improvement and freezing curves of the PEG/SiO2eAl2O3
Fig. 3. XRD spectra of PEG600 and composite PCMs.
The thermal conductivity of PEG6000/SiO2 was improved when Al2O3 was introduced into the system. The results are shown in Table 3. Thermal conductivity of the composite PCMs with 0, 3.3, 9.2, and 12.6 wt% Al2O3 reached 0.36 0.398, 0.419 and 0.435 W (m K)1, respectively (thermal conductivity for PEG is 0.297 [41]). Comparing with pure PEG6000 and PEG6000/SiO2, thermal
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Fig. 4. PEG6000/SiO2eAl2O3 composite with varying Al2O3 contents.
conductivity of the composite PCM with 3.3 wt% Al2O3 increased 34% and 10.2%. The PEG6000/SiO2eAl2O3 hybrid form-stable PCM exhibits the same high thermal conductivity as other PCMs (PEG6000/SiO2/AlN [18], Ag NWs/1-tetradecanol [39], and PANI/
tetradecanol/MWNTs [40]), but it has a larger enthalpy and involves minimal cost due to the inexpensive aluminum nitrate. The enhancement of thermal transfer was also investigated by comparing the freezing process of the PEG6000/SiO2eAl2O3 with that of PEG6000/SiO2, and the results are presented in Fig. 7. The results demonstrate that to achieve the same temperature 41 C, the freezing times are 780 s for PEG6000/SiO2, 680 s for PEG6000/ SiO2eAl2O3 (3.3 wt% Al2O3), 560 s for PEG6000/SiO2eAl2O3 (9.2 wt % Al2O3), and 420 s for PEG6000/SiO2eAl2O3 (12.6 wt% Al2O3). The freezing time of 3.3 wt% Al2O3 in PEG/SiO2 was reduced by 12.8% by comparing with that of PEG6000/SiO2.
Table 1 Phase transition temperature and enthalpy of composite PCMs with varying Al2O3 contents.
Fig. 5. DSC curves of the composite PCMs with varying Al2O3 contents.
Sample
wt% Al2O3
Tc ( C)
DHc (J g1)
Tm ( C)
DHm (J g1)
PEG6000 PEG6000/SiO2 PEG6000/SiO2eAl2O3-1 PEG6000/SiO2eAl2O3-2 PEG6000/SiO2eAl2O3-3
e 0 3.3 9.2 12.6
39.6 43.6 43.8 43.8 42.0
177.9 103.8 126.2 112.3 126.4
59.6 56.5 57.2 56.3 57.1
181.5 102.8 122.0 107.8 123.8
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Fig. 6. TGA and differential thermogravimetric (DTG) analysis curves of the composite PCMs with and without Al2O3.
Table 2 TGA results of different composite PCMs. Sample
wt% Al2O3
Tmax ( C)
Char residue at 700 C (wt%)
PEG6000 PEG6000/SiO2 PEG6000/SiO2eAl2O3
e 0 9.2
403.9 415.5 404.7
2.1 24.2 17.2
Acknowledgments
Table 3 Thermal conductivities of the composite PCMs. Sample
PEG6000 PEG6000/SiO2 PEG6000/SiO2eAl2O3-1 PEG6000/SiO2eAl2O3-2 PEG6000/SiO2eAl2O3-3
wt% Al2O3
e 0 3.3 9.2 12.6
freezing time was reduced by 12.8% for 3.3 wt% Al2O3 in PEG6000/ SiO2 PCM compared with PEG6000/SiO2. Analysis revealed that PEG6000/SiO2eAl2O3 is a form-stable PCM with good thermal stability and large enthalpy value (124 J g1).
Thermal conductivity (W (m K)1)
Thermal conductivity enhancement (%)a E1
E2
0.297 0.36 0.398 0.419 0.435
e 21.2% 34.0% 41.1% 46.5%
e e 10.60% 16.40% 20.80%
a
E1: Thermal conductivity enhancement comparing with PEG6000. E2: Thermal conductivity enhancement comparing with PEG6000/SiO2.
This work was supported by the National Natural Science Foundation of China (21276042), the National Science and Technology Pillar Program (2013BAF08B06), Program for New Century Excellent Talents in University (NCET130080), Fundamental Research Funds for the Central Universities of China (DUT13LK35), and Program for Liaoning Excellent Talents in University (LJQ2013006). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Fig. 7. Freezing curves of the composite PCMs with varying Al2O3 contents.
4. Conclusions A PEG6000/SiO2eAl2O3 hybrid form-stable PCM was successfully obtained through an ultrasound-assisted solegel method and using aluminum nitrate and TEOS as sol precursors. The thermal conductivity of PEG6000/SiO2 increased significantly by in situ doping of Al2O3, which formed a SiO2eAl2O3 net to serve as the supporting material. The freezing rate tests showed that the
[23] [24] [25] [26] [27] [28] [29] [30] [31]
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