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SiO2 composite phase change materials with high thermal conductivity
Solar Energy Materials and Solar Cells 174 (2018) 538–544
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Solar Energy Materials and Solar Cells journal homepage: www.elsevier.com/locate/solmat
Novel light–driven CF/PEG/SiO2 composite phase change materials with high thermal conductivity
MARK
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Zilu Liua,b, Huipeng Weib, Bingtao Tangb, , Shimei Xuc, Zhang Shufenb a b c
Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education, Xinjiang University, Urumqi 830046, China State Key Laboratory of Fine Chemicals, Dalian University of Technology, West Campus, Dalian 116024, China College of Chemistry, Sichuan University, Chengdu 610064, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Light–driven Composite phase change materials Thermal conductivity
Light-driven carbon fiber (CF)/polyethylene glycol (PEG)/silica (SiO2) composite phase change materials (PCMs) with high thermal conductivity, and at low cost were successfully prepared through a sol–gel process. Structural and thermal property were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT–IR) analysis, X–ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetry analysis (TGA), and differential thermogravimetry (DTG). The CF/PEG/ SiO2 composite PCMs showed amazing fusion and crystallization enthalpies of 142.6 and 154.4 J/g at CF content of 3%, and the thermal conductivity coefficient reached 0.45 W/(m·K). The prepared PCMs also showed excellent light–thermal energy conversion capability, and a potential for wide application in improving the utilization efficiency of solar energy.
1. Introduction The current energy crisis has become more critical because of the decreasing in fossil fuel and the increasing in greenhouse emissions [1–9]. Under these circumstances, renewable energy has been presented due to its cleanliness and renewability. Solar energy as an inexhaustible energy shows a wide application prospect because of the outstanding advantage of universal, innocuousness, generous quantity and permanency. Consequently, conversion, storage, and application of solar energy has caused considerable attention [4,5,10]. However, the intermittence of time and space of solar irradiation seriously limits the application of solar energy. Moreover, almost 44% of visible light cannot be applied effectively and directly due to low thermal efficiency at the present stage [11]. Therefore, development of light–driven PCMs for storing solar energy has become a popular research topic [6]. Our research group has introduced a new kind of organic dye which could absorb and convert it to heat energy to improve the utilization efficiency of solar energy [12,13]. The utilization efficiency of visible light is more than 0.90 under single–band irradiation. However, the thermal conductivity of phase change materials (PCMs) still not satisfied the requirements of application. Therefore, Our and other research groups introduced carbon nanotubes and graphene into the phase change system to improve the thermal conductivity [14–24]. A new form-stable PCMs with the latent heats of 63.76 J/g and 64.89 J/g is prepared by vacuum impregnation of paraffin within grapheme oxide
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(GO) sheets. The GO as a modifier is highly improved the thermal conductivity from 0.305-0.985 W/(m∙K) [25]. The expanded perlite (Exp)/ n-eicosane (C20)/Carbon nano tubes (CNTs) composite PCMs has been prepared by doping the C20 into the structure of Exp/CNTs matrix. The result PCMs showed latent heat of 157.43 J/g, and in this case, the thermal conductivity has enhanced considerably as 113.3% cause by the addition of CNTs [26]. Comparing with the unmodified PCMs, the modified PCMs showed more excellent absorption of visible light, light–thermal energy conversion, phase change thermal storage, and thermal conductivity. The rate of thermal loading was significantly improved with the introduction of carbon nanotubes and graphene, but the high cost still severely limits the application of such materials. Carbon fiber (CF) is a graphite–like material which prepared by carbonation and graphitization with organic fibers. Each CF is composed of thousands of tiny fibers which is stacked with carbon atoms arranged in a hexagonal form at the atomic level. CF showed almost the same thermal conductivity, corrosion resistance, durability, and light–thermal energy conversion capability with carbon nanotubes and grapheme, but at a more cheap cost [27,28]. Therefore, CF as an additive for improving the property of composite materials has attracted much attention [29–32]. In this work, CF was introduced to a polyethylene glycol (PEG)/ silica (SiO2) phase change system through in situ doping, and prepared novel CF/PEG/SiO2 composite PCMs by a sol–gel process. The prepared PCMs showed outstanding phase change enthalpy (fusion enthalpy of
Corresponding author. E-mail address: [email protected] (B. Tang).
http://dx.doi.org/10.1016/j.solmat.2017.09.045 Received 31 May 2017; Received in revised form 4 August 2017; Accepted 25 September 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
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The synthetic route of the CF/PEG/SiO2 composite PCMs is showed in Fig. 1. It was worth to mention that heterogeneous distribution of unmodified CF was observed due to its low water-solubility and large specific surface. Therefore, the BDSNH2 diazonium was used to graft the sulphinyl groups on the surface of CF via free radical reaction (Fig. 1b) to improve the water-solubility. Silica sol was produced by hydrolysis of TEOS with addition of HCl (Fig. 1c). Then added Na2CO3 to the silica sol including melted PEG and modified CF to adjust pH to 7 to obtain silica hydrogel (Fig. 1d). Finally, the gel was dried to prepare the CF/PEG/SiO2 composite PCMs.
142.6 J/g and crystallization enthalpy of 154.4 J/g), superior thermal conductivity coefficient (0.45 W/(m∙K)), and excellent light–thermal energy conversion. The utilization of solar energy was significantly improved caused by the enhancement of thermal conductivity. Therefore, the CF/PEG/SiO2 composite PCMs could be used for thermal energy and solar storage, exterior thermal insulation materials and solar water heater, etc. 2. Experimental section 2.1. Materials and chemicals
2.3. Characterization Tetraethyl orthosilicate (TEOS, density in terms of silica is 28.4%, 208.33 g/mol, 0.932–0.936 g/mL (20 °C)) and polyethylene glycol6000 (PEG–6000, 6000 g/mol, 60 ± 3 °C) was purchased from Xilong Chemical Co., Ltd and Sinopharm Chemical Reagent Co., Ltd, respectively. CF (7 µm) was produced by Hangzhou Hi–Tech Composite Material Co., Ltd. Hydrochloric acid (HCl) was received from Beijing Chemical Works. Sodium carbonate (Na2CO3) and Aniline–2,5–disulfonic acid monosodium salt (C6H5NO6S2Na) was purchased from Tianjin Bodi Chemical Co., Ltd and Luoyang Tengyi Chemical Company. Deionized water was used in the whole experiment. All reagents were used without further purification.
PEG morphology in PCMs network was investigated by scanning electron micrograph (SEM, Phenom G1, FEI). The infrared spectra of PCMs was performed by Fourier transform infrared spectroscopy (FT/ IR–430, JASCO) in the range of 400–4000 cm−1 with 2 cm−1 spectral resolution using KBr pellets. X–ray diffraction (XRD) was carried out using a Rigaku D/Max 2400 Advance X–ray diffractometer and a curved graphite crystal monochromator Cu Kα in the range of 5–80° with the scanning step of 0.02°. The phase change temperature and latent heat of the products was measured using differential scanning calorimeter (DSC, DSC 240, NETZSCH) in the range of 0–80 °C at a heating rate of 5 °C/min under nitrogen stream. Thermal gravimetric analysis (TGA) of samples was on a Mettler–Toledo TGA/SDTA851 thermal analyzer at a heating rate of 10 °C/min from room temperature to 700 °C in a nitrogen stream. All the UV–vis spectra of the composite PCMs were obtained by using Hitachi U-4100 spectrophotometer, which at a scan speed of 300 nm/min and with a slit width of 8.00 nm.
2.2. Synthesis of CF/PEG/SiO2 composite PCMs by sol–gel and in situ doping method The in-situ generated diazonium cation medication was conducted by the diazotization of C6H5NO6S2Na. 10 mmol of C6H5NO6S2Na was first dispersed in 25 mL of deionized water, followed by the addition of Na2CO3 solution (100 g/L) to adjust pH to 7. Afterwards, added excess sodium nitrite (1.03 times than C6H5NO6S2Na) into above mixture under stirring. Finally, added diluted HCl solution (5 mol/L) to the mixture and stirred in an ice–bath for 15 min to prepare aniline–2, 5–disulfonic acid (BDSNH2) diazonium solution. Unmodified CF (1 g) was dispersed in 25 mL of deionized water under stirring at 60 °C, followed by the addition of 25 mL of prepared BDSNH2 diazonium solution and stirred for 1 h. Subsequently, the resulting CF were centrifuged, washed and steamed, the product was surface-modified CF powder with high water-dispersity [14]. TEOS (7.8 g) was first mixed with water (the molar ratio of TEOS to water of 0.1) and stirred at room temperature for 5 min. Then, the HCl solution (0.5 mol/L) was added to adjust the pH to 2. Afterwards, stirred above mixture for 20–30 min for hydrolysis and obtained translucent silica sol. Then dispersed modified CF in deionized water with an ultrasonic oscillator. Afterward, added melted PEG–6000 (85 wt%) and above silica sol to the modified CF solution. Na2CO3 solution was then added to adjust the pH to 7–8 to transform the sol to hydrogels. The hydrogels were dry at 50 °C to prepare the CF/PEG/SiO2 composite PCMs. The resultant composite PCMs were denoted as CF (X)/PEG/SiO2, where X was the content of CF.
3. Results and discussion 3.1. The structure characterization of CF/PEG/SiO2 composite PCMs Characteristic bands of PEG were observed at 3430 cm−1 (ν(–OH)), 2917 cm−1 (ν(–CH3)), 2889 cm−1 (ν(–CH2–)) and 1106 cm−1 (νs(C–O–C)) (Fig. 2a). The ν(–OH) of CF was observed at 3430 cm−1 (Fig. 2c). The FT-IR spectrum of SiO2 was added to compare to that of the prepared composite PCM, characteristic bands of SiO2 were observed at 3430 cm−1 (ν(–OH)), 1105 cm−1 v (νas(Si–O–Si)), 968 cm−1 (Si–OH), and 798 cm−1 (νs(Si–O–Si)) (Fig. 2b). These results suggest that the SiO2 framework was successfully prepared by hydrolytic condensation (Fig. 1c). Characteristic bands of CF/PEG/SiO2 composite PCMs were obtained at 3430 cm−1 (ν(–OH)), 2917 cm−1 (ν(–CH3)), 2889 cm−1 (ν(–CH2–)), 1106 cm−1 (νs(C–O–C)), 1105 cm−1 v (νas(Si–O–Si)), 968 cm−1 (Si–OH), and 798 cm−1 (νs(Si–O–Si)). Except for the same peaks as PEG (Fig. 2a), SiO2 (Fig. 2b), and CF (Fig. 2c), no new characteristic peaks and peak shift were observed in the FT–IR curve of the CF/PEG/SiO2 composite PCMs (Fig. 2d). Therefore, PEG was fixed on the porous structure of SiO2 by physical adsorption because no new chemical groups were prepared during the entire Fig. 1. Synthetic route of CF/PEG/SiO2 composite PCMs.
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Fig. 4. DSC curves of different materials.
Fig. 2. FT–IR spectra of the composites PCMs.
Table 1 Phase change temperature and enthalpy of composite PCMs. Sample
Tc (°C)
△Hc (J/g)
Tm (°C)
△Hm (J/g)
PEG–6000 PEG/SiO2 CF(1%)/PEG/SiO2 CF(2%)/PEG/SiO2 CF(3%)/PEG/SiO2 CF(4%)/PEG/SiO2 CF(5%)/PEG/SiO2
37.8 36.6 41.9 34.8 34.8 35.3 41.6
222.7 167.2 154.1 169.3 154.4 160.5 150.6
61.4 61.0 62.3 58.3 57.5 57.7 61.4
212.0 145.2 133.2 159.1 142.6 147.1 132.4
almost similar to those of pure PEG. The fusion and crystallization enthalpies of PEG/SiO2 PCMs were almost the same as CF/PEG/SiO2 PCMs, but lower than that of pure PEG. That confirmed the decreasing of enthalpy was caused by the downgrade of PEG crystallinity which was limited by SiO2. It was noteworthy that with the increasing of CF content, obvious change was not observed in phase change enthalpies of the prepared composite PCMs. Therefore, the addition of CF exerted no effect on the enthalpy of CF/PEG/SiO2 composite PCMs. The thermostability of the composite PCMs was investigated through TGA (Fig. 5a) and DTG (Fig. 5b) measurements. The residual masses of PEG and CF(2%)/PEG/SiO2 composite PCMs were 1.24 wt% and 20.7 wt%, respectively. The maximum weight loss temperature of the composite PCMs (456.2 °C) was higher than that of PEG (430.2 °C). The weight loss of PEG and CF(2%)/PEG/SiO2 composite PCMs was within the temperature range of 354–435 °C and 345–456 °C, respectively. Therefore, the thermostability of the CF/PEG/SiO2 composite PCMs was significantly improved by CF, and the thermostability of the PCMs was high at 340 °C.
Fig. 3. XRD patterns of the composite PCMs.
synthesis process of composite PCMs. The amorphous peaks of CF and SiO2 were observed at 2θ = 25.5° (Fig. 3a) and 22° (Fig. 3b), respectively, whereas the peaks of PEG were observed at 19.24° and 23.42° (Fig. 3c). All the characteristic peaks of CF, SiO2, and PEG were observed in the curve of the CF/PEG/SiO2 composite PCMs (Fig. 3d) at the same position. However, the intensity of the crystallization peaks was lower than that of pure PEG (Fig. 3c). This result confirms that the crystallinity of PEG was limited by the 3D porous network structure of the supporting material (SiO2) [4,6,22].
3.2. The thermal property of CF/PEG/SiO2 composite PCMs 3.3. The protective role of SiO2 played in CF/PEG/SiO2 composite PCMs The phase change temperature and enthalpy of the composite PCMs were measured through DSC (Fig. 4). The fusion temperature, fusion enthalpy, crystallizing point and crystallization enthalpy of the composite PCMs are listed in Table 1. The fusion enthalpy of PEG, PEG/SiO2 PCMs, CF(1%)/PEG/SiO2, CF (2%)/PEG/SiO2, CF(3%)/PEG/SiO2, CF(4%)/PEG/SiO2, and CF(5%)/ PEG/SiO2 reached 222.7, 167.2, 154.1, 169.3, 154.4, 160.5, and 150.6 J/g, respectively, and the crystallization enthalpy was 212.0, 145.2, 133.2, 159.1, 142.6, 147.1, and 132.4 J/g. The melting temperatures of PEG, PEG/SiO2 PCMs, CF(1%)/PEG/SiO2, CF(2%)/PEG/ SiO2, CF(3%)/PEG/SiO2, CF(4%)/PEG/SiO2, and CF(5%)/PEG/SiO2 reached 61.4, 61.0, 62.3, 58.3, 57.5, 57.7, and 61.4 °C, respectively, and the crystallization temperatures was 37.8, 36.6, 41.9, 34.8, 34.8, 35.3, and 41.6 °C. These results indicate that with the addition of SiO2, the melting and crystallization temperatures of PEG/SiO2 PCMs were
PEG, CF (2%)/PEG/SiO2, CF (3%)/PEG/SiO2 and CF (4%)/PEG/ SiO2 powders were pressed into slices and heated on a heating platform to the phase change temperature to investigate the confinement effect of PCMs (Fig. 6). All of the PCMs remained solid, and no PEG leaked out even above the phase change temperature of 30 °C. The result confirmed that the high confinement effect of PCMs was attributed to the protection provided by SiO2. To confirm the protective role of SiO2 in PEG in composite PCMs, SEM was used to observe the morphology of SiO2, PEG/SiO2 PCMs and CF (2%)/PEG/SiO2 composite PCMs (Fig. 7). The phase change component, PEG, was filled with the porous structure of the support material SiO2. The images confirmed the large contract area and strong physisorption between the SiO2 framework and PEG. The powerful adsorption could prevent the leakage of PEG and limit the crystallinity 540
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Fig. 5. (a) TG curves of CF(2%)/PEG/SiO2 composite PCMs and PEG–6000. (b) DTG curves of CF(2%)/PEG/SiO2 composite PCMs and PEG–6000.
thermal energy. To observe the light–thermal energy conversion in a practical application, PEG/SiO2, CF (1%)/PEG/SiO2 and CF (3%)/PEG/ SiO2 were measured under sunlight (Fig. 10). The temperatures of PEG/ SiO2, CF(1%)/PEG/SiO2 and CF(3%)/PEG/SiO2 composite PCMs exposed under sunlight for 2000 s reached 45.2 °C, 50.4 °C, and 52.5 °C, respectively, and it was almost similar to the measured values under simulative light. Therefore, the CF/PEG/SiO2 composite PCMs have a favorable application background due to the high light–thermal energy conversion in practical application. In summary, CF as a light absorber provided light–thermal energy conversion to the composite PCMs, improved the utilization of solar energy, and enhanced thermal conductivity.
of PEG. 3.4. Light–thermal conversion of CF/PEG/SiO2 composite PCMs In the light–thermal energy conversion process, the materials absorbed light energy and converted it into thermal energy. The absorption values and UV–vis wavelengths of the CF/PEG/SiO2 composite PCMs and CF were observed with a solid UV–vis absorption spectrometer (Fig. 8). CF demonstrated high absorbability at almost all UV–vis wavebands (Fig. 8f). Moreover, the absorbance of the composite PCMs (Figs. 8c, 8d, and 8e) was improved with the introduction of CF (Fig. 8a), and this was extremely advantageous to the absorption of sun rays proportional to the CF content. The composite PCMs were placed under a simulative light source with the power of 0.93 W (Fig. 9) and sunlight (Fig. 10) for same time. The numerical control thermometer (SK–130RD, Sukow) was used to measure the light–thermal energy conversion of prepared PCMs. The temperature date of PCMs were collected by Pt electrode and recorded per 4 s. The temperatures of PEG, PEG/SiO2, CF(1%)/PEG/SiO2, CF (3%)/PEG/SiO2 and CF(5%)/PEG/SiO2 composite PCMs under the simulative light source for 2000 s were 49.1 °C, 50.5 °C, 62.2 °C, 64.4 °C and 68.5 °C, respectively (Fig. 9). The light–thermal energy conversion of the composite PCMs was significantly improved by the introduction of CF (Fig. 9). In addition, the conversion was proportional to the content of CF and caused the material to transform light into more
3.5. Thermal conductivity of CF/PEG/SiO2 composite PCMs The composite PCMs were pressed into slices with the same mass and diameter (30 mm), the thermal conductivity of PCMs was measured by thermal conductivity meter (DRE-III, Xiang Yi) at room temperature. Comparing the thermal conductivity to research the enhancement of CF. The measurement result showed that the thermal conductivity coefficient of PEG, PEG/SiO2, CF(1%)/PEG/SiO2, CF(2%)/PEG/SiO2 and CF(3%)/PEG/SiO2 was 0.17, 0.26, 0.39, 0.44 and 0.45 W/(m∙K), respectively. The thermal conductivity of the composite PCMs was significantly enhanced with the increasing of CF content. The enhancement ratio of CF (2%)/PEG/SiO2 composite PCMs to PEG/SiO2
Fig. 6. Digital photos of the composite PCMs under different temperature.
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Fig. 7. SEM images of samples: (a) SiO2, (b) PEG/SiO2, (c) CF (2%)/PEG/SiO2.
Fig. 8. Solid ultraviolet–visible absorption curves of CF/PEG/SiO2 composite PCMs with different content of CF.
Fig. 10. Light–thermal conversion curves of different samples under sunlight measured at late May.
Fig. 9. Light–thermal conversion curves of different samples under simulative light source.
Fig. 11. Heating–rate curves of composite PCMs.
much more shorter than that of PEG/SiO2 (568 s) and PEG (840 s) under the same heating rate and was proportional to the content of CF (Fig. 11). In other words, the heat storage and release heat rate of CF/ PEG/SiO2 composite PCMs were much higher than those of PEG/SiO2 or PEG. Therefore, the introduction of CF significantly improved the heat storage and release rates of the composite PCMs and the latter improved the utilization of solar energy by a large margin with an excellent heat storage rate. In general, the light–thermal energy conversion efficiency and the
was 69.2%. The addition of CF contributed to the release and storage of energy due to the significant improvement in thermal conductivity. The energy efficiency of the composite PCMs was decided by the rate of storage and release energy [5,8]. The heat storage and release rates of the PCMs were measured by the curves of the heating–rate (Fig. 11) and cooling–rate (Fig. 12). The enhancement in the thermal conductivity of PCMs by CF was confirmed because the heating time of CF (3%)/PEG/SiO2 composite PCMs from 0 °C to 65 °C (508 s) was 542
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[2]
[3] [4]
[5]
[6]
[7]
[8]
[9] Fig. 12. Cooling–rate curves of composite PCMs. [10]
thermal conductivity of the composite PCMs were significantly enhanced by the addition of CF and were proportional to the content of CF due to the high thermal conductivity and light absorption. Therefore, the CF/PEG/SiO2 composite PCMs could transform light energy into thermal energy and store more energy at the same time compared with traditional PCMs. The composite PCMs achieved energy storage and release effectively and rapidly. The eicosane/ graphene nanoplatelets (GNPs) composites with the remarkable latent heat of fusion of 220 J/g were synthesized. In this case, the thermal conductivity of PCMs was significantly improved from 0.4148 W/(m∙K) to over 3 W/(m∙K) [33]. The PEG/GO composite PCMs were prepared by physical blending and impregnation method, the resulted product showed high latent heat of 142.8 J/g [34]. It is confirmed that the enhancement effect of CF was comparable with carbon nanotubes and grapheme during comparing with above cases, but at a much lower cost.
[11] [12]
[13]
[14]
[15]
[16] [17]
4. Conclusion
[18]
The CF/PEG/SiO2 composite PCMs with a excellent confinement effect, phase change property, thermal conductivity and light–thermal energy conversion were synthesized successfully by a sol–gel process and in situ doping. With 3% CF content, the composites PCMs showed high fusion enthalpy of 142.6 J/g, crystallization enthalpy of 154.4 J/g, and thermal conductivity coefficient of 0.45 W/(m∙K). The composite PCMs also showed strong thermostability under 340 °C and high light–thermal energy conversion at all UV–vis wavebands. The thermal conductivity of CF/PEG/SiO2 composite PCMs was significantly improved above 50% than PEG/SiO2 and the energy utilization ratio was also enhanced due to its high thermal conductivity. The prepared CF/ PEG/SiO2 composite PCMs provide an important method of solving social environment and energy crisis given their excellent utilization of solar energy.
[19]
[20]
[21]
[22]
[23]
Acknowledgments
[24]
This work was financially supported by the National Natural Science Foundation of China (21576039, 21536002, 21421005, 21276042 and U1608223). Program for Innovative Research Team in University (IRT–13R06). Program for New Century Excellent Talents in University (NCET130080). Fundamental Research Funds for the Central Universities (DUT13LK35, DUT14YQ209, DUT2013TB07, DUT14QY13).
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Novel light–driven CF/PEG/SiO2 composite phase change materials with high thermal conductivity
MARK
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Zilu Liua,b, Huipeng Weib, Bingtao Tangb, , Shimei Xuc, Zhang Shufenb a b c
Key Laboratory of Oil & Gas Fine Chemicals, Ministry of Education, Xinjiang University, Urumqi 830046, China State Key Laboratory of Fine Chemicals, Dalian University of Technology, West Campus, Dalian 116024, China College of Chemistry, Sichuan University, Chengdu 610064, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Light–driven Composite phase change materials Thermal conductivity
Light-driven carbon fiber (CF)/polyethylene glycol (PEG)/silica (SiO2) composite phase change materials (PCMs) with high thermal conductivity, and at low cost were successfully prepared through a sol–gel process. Structural and thermal property were characterized by scanning electron microscopy (SEM), Fourier transform infrared (FT–IR) analysis, X–ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetry analysis (TGA), and differential thermogravimetry (DTG). The CF/PEG/ SiO2 composite PCMs showed amazing fusion and crystallization enthalpies of 142.6 and 154.4 J/g at CF content of 3%, and the thermal conductivity coefficient reached 0.45 W/(m·K). The prepared PCMs also showed excellent light–thermal energy conversion capability, and a potential for wide application in improving the utilization efficiency of solar energy.
1. Introduction The current energy crisis has become more critical because of the decreasing in fossil fuel and the increasing in greenhouse emissions [1–9]. Under these circumstances, renewable energy has been presented due to its cleanliness and renewability. Solar energy as an inexhaustible energy shows a wide application prospect because of the outstanding advantage of universal, innocuousness, generous quantity and permanency. Consequently, conversion, storage, and application of solar energy has caused considerable attention [4,5,10]. However, the intermittence of time and space of solar irradiation seriously limits the application of solar energy. Moreover, almost 44% of visible light cannot be applied effectively and directly due to low thermal efficiency at the present stage [11]. Therefore, development of light–driven PCMs for storing solar energy has become a popular research topic [6]. Our research group has introduced a new kind of organic dye which could absorb and convert it to heat energy to improve the utilization efficiency of solar energy [12,13]. The utilization efficiency of visible light is more than 0.90 under single–band irradiation. However, the thermal conductivity of phase change materials (PCMs) still not satisfied the requirements of application. Therefore, Our and other research groups introduced carbon nanotubes and graphene into the phase change system to improve the thermal conductivity [14–24]. A new form-stable PCMs with the latent heats of 63.76 J/g and 64.89 J/g is prepared by vacuum impregnation of paraffin within grapheme oxide
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(GO) sheets. The GO as a modifier is highly improved the thermal conductivity from 0.305-0.985 W/(m∙K) [25]. The expanded perlite (Exp)/ n-eicosane (C20)/Carbon nano tubes (CNTs) composite PCMs has been prepared by doping the C20 into the structure of Exp/CNTs matrix. The result PCMs showed latent heat of 157.43 J/g, and in this case, the thermal conductivity has enhanced considerably as 113.3% cause by the addition of CNTs [26]. Comparing with the unmodified PCMs, the modified PCMs showed more excellent absorption of visible light, light–thermal energy conversion, phase change thermal storage, and thermal conductivity. The rate of thermal loading was significantly improved with the introduction of carbon nanotubes and graphene, but the high cost still severely limits the application of such materials. Carbon fiber (CF) is a graphite–like material which prepared by carbonation and graphitization with organic fibers. Each CF is composed of thousands of tiny fibers which is stacked with carbon atoms arranged in a hexagonal form at the atomic level. CF showed almost the same thermal conductivity, corrosion resistance, durability, and light–thermal energy conversion capability with carbon nanotubes and grapheme, but at a more cheap cost [27,28]. Therefore, CF as an additive for improving the property of composite materials has attracted much attention [29–32]. In this work, CF was introduced to a polyethylene glycol (PEG)/ silica (SiO2) phase change system through in situ doping, and prepared novel CF/PEG/SiO2 composite PCMs by a sol–gel process. The prepared PCMs showed outstanding phase change enthalpy (fusion enthalpy of
Corresponding author. E-mail address: [email protected] (B. Tang).
http://dx.doi.org/10.1016/j.solmat.2017.09.045 Received 31 May 2017; Received in revised form 4 August 2017; Accepted 25 September 2017 0927-0248/ © 2017 Elsevier B.V. All rights reserved.
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The synthetic route of the CF/PEG/SiO2 composite PCMs is showed in Fig. 1. It was worth to mention that heterogeneous distribution of unmodified CF was observed due to its low water-solubility and large specific surface. Therefore, the BDSNH2 diazonium was used to graft the sulphinyl groups on the surface of CF via free radical reaction (Fig. 1b) to improve the water-solubility. Silica sol was produced by hydrolysis of TEOS with addition of HCl (Fig. 1c). Then added Na2CO3 to the silica sol including melted PEG and modified CF to adjust pH to 7 to obtain silica hydrogel (Fig. 1d). Finally, the gel was dried to prepare the CF/PEG/SiO2 composite PCMs.
142.6 J/g and crystallization enthalpy of 154.4 J/g), superior thermal conductivity coefficient (0.45 W/(m∙K)), and excellent light–thermal energy conversion. The utilization of solar energy was significantly improved caused by the enhancement of thermal conductivity. Therefore, the CF/PEG/SiO2 composite PCMs could be used for thermal energy and solar storage, exterior thermal insulation materials and solar water heater, etc. 2. Experimental section 2.1. Materials and chemicals
2.3. Characterization Tetraethyl orthosilicate (TEOS, density in terms of silica is 28.4%, 208.33 g/mol, 0.932–0.936 g/mL (20 °C)) and polyethylene glycol6000 (PEG–6000, 6000 g/mol, 60 ± 3 °C) was purchased from Xilong Chemical Co., Ltd and Sinopharm Chemical Reagent Co., Ltd, respectively. CF (7 µm) was produced by Hangzhou Hi–Tech Composite Material Co., Ltd. Hydrochloric acid (HCl) was received from Beijing Chemical Works. Sodium carbonate (Na2CO3) and Aniline–2,5–disulfonic acid monosodium salt (C6H5NO6S2Na) was purchased from Tianjin Bodi Chemical Co., Ltd and Luoyang Tengyi Chemical Company. Deionized water was used in the whole experiment. All reagents were used without further purification.
PEG morphology in PCMs network was investigated by scanning electron micrograph (SEM, Phenom G1, FEI). The infrared spectra of PCMs was performed by Fourier transform infrared spectroscopy (FT/ IR–430, JASCO) in the range of 400–4000 cm−1 with 2 cm−1 spectral resolution using KBr pellets. X–ray diffraction (XRD) was carried out using a Rigaku D/Max 2400 Advance X–ray diffractometer and a curved graphite crystal monochromator Cu Kα in the range of 5–80° with the scanning step of 0.02°. The phase change temperature and latent heat of the products was measured using differential scanning calorimeter (DSC, DSC 240, NETZSCH) in the range of 0–80 °C at a heating rate of 5 °C/min under nitrogen stream. Thermal gravimetric analysis (TGA) of samples was on a Mettler–Toledo TGA/SDTA851 thermal analyzer at a heating rate of 10 °C/min from room temperature to 700 °C in a nitrogen stream. All the UV–vis spectra of the composite PCMs were obtained by using Hitachi U-4100 spectrophotometer, which at a scan speed of 300 nm/min and with a slit width of 8.00 nm.
2.2. Synthesis of CF/PEG/SiO2 composite PCMs by sol–gel and in situ doping method The in-situ generated diazonium cation medication was conducted by the diazotization of C6H5NO6S2Na. 10 mmol of C6H5NO6S2Na was first dispersed in 25 mL of deionized water, followed by the addition of Na2CO3 solution (100 g/L) to adjust pH to 7. Afterwards, added excess sodium nitrite (1.03 times than C6H5NO6S2Na) into above mixture under stirring. Finally, added diluted HCl solution (5 mol/L) to the mixture and stirred in an ice–bath for 15 min to prepare aniline–2, 5–disulfonic acid (BDSNH2) diazonium solution. Unmodified CF (1 g) was dispersed in 25 mL of deionized water under stirring at 60 °C, followed by the addition of 25 mL of prepared BDSNH2 diazonium solution and stirred for 1 h. Subsequently, the resulting CF were centrifuged, washed and steamed, the product was surface-modified CF powder with high water-dispersity [14]. TEOS (7.8 g) was first mixed with water (the molar ratio of TEOS to water of 0.1) and stirred at room temperature for 5 min. Then, the HCl solution (0.5 mol/L) was added to adjust the pH to 2. Afterwards, stirred above mixture for 20–30 min for hydrolysis and obtained translucent silica sol. Then dispersed modified CF in deionized water with an ultrasonic oscillator. Afterward, added melted PEG–6000 (85 wt%) and above silica sol to the modified CF solution. Na2CO3 solution was then added to adjust the pH to 7–8 to transform the sol to hydrogels. The hydrogels were dry at 50 °C to prepare the CF/PEG/SiO2 composite PCMs. The resultant composite PCMs were denoted as CF (X)/PEG/SiO2, where X was the content of CF.
3. Results and discussion 3.1. The structure characterization of CF/PEG/SiO2 composite PCMs Characteristic bands of PEG were observed at 3430 cm−1 (ν(–OH)), 2917 cm−1 (ν(–CH3)), 2889 cm−1 (ν(–CH2–)) and 1106 cm−1 (νs(C–O–C)) (Fig. 2a). The ν(–OH) of CF was observed at 3430 cm−1 (Fig. 2c). The FT-IR spectrum of SiO2 was added to compare to that of the prepared composite PCM, characteristic bands of SiO2 were observed at 3430 cm−1 (ν(–OH)), 1105 cm−1 v (νas(Si–O–Si)), 968 cm−1 (Si–OH), and 798 cm−1 (νs(Si–O–Si)) (Fig. 2b). These results suggest that the SiO2 framework was successfully prepared by hydrolytic condensation (Fig. 1c). Characteristic bands of CF/PEG/SiO2 composite PCMs were obtained at 3430 cm−1 (ν(–OH)), 2917 cm−1 (ν(–CH3)), 2889 cm−1 (ν(–CH2–)), 1106 cm−1 (νs(C–O–C)), 1105 cm−1 v (νas(Si–O–Si)), 968 cm−1 (Si–OH), and 798 cm−1 (νs(Si–O–Si)). Except for the same peaks as PEG (Fig. 2a), SiO2 (Fig. 2b), and CF (Fig. 2c), no new characteristic peaks and peak shift were observed in the FT–IR curve of the CF/PEG/SiO2 composite PCMs (Fig. 2d). Therefore, PEG was fixed on the porous structure of SiO2 by physical adsorption because no new chemical groups were prepared during the entire Fig. 1. Synthetic route of CF/PEG/SiO2 composite PCMs.
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Fig. 4. DSC curves of different materials.
Fig. 2. FT–IR spectra of the composites PCMs.
Table 1 Phase change temperature and enthalpy of composite PCMs. Sample
Tc (°C)
△Hc (J/g)
Tm (°C)
△Hm (J/g)
PEG–6000 PEG/SiO2 CF(1%)/PEG/SiO2 CF(2%)/PEG/SiO2 CF(3%)/PEG/SiO2 CF(4%)/PEG/SiO2 CF(5%)/PEG/SiO2
37.8 36.6 41.9 34.8 34.8 35.3 41.6
222.7 167.2 154.1 169.3 154.4 160.5 150.6
61.4 61.0 62.3 58.3 57.5 57.7 61.4
212.0 145.2 133.2 159.1 142.6 147.1 132.4
almost similar to those of pure PEG. The fusion and crystallization enthalpies of PEG/SiO2 PCMs were almost the same as CF/PEG/SiO2 PCMs, but lower than that of pure PEG. That confirmed the decreasing of enthalpy was caused by the downgrade of PEG crystallinity which was limited by SiO2. It was noteworthy that with the increasing of CF content, obvious change was not observed in phase change enthalpies of the prepared composite PCMs. Therefore, the addition of CF exerted no effect on the enthalpy of CF/PEG/SiO2 composite PCMs. The thermostability of the composite PCMs was investigated through TGA (Fig. 5a) and DTG (Fig. 5b) measurements. The residual masses of PEG and CF(2%)/PEG/SiO2 composite PCMs were 1.24 wt% and 20.7 wt%, respectively. The maximum weight loss temperature of the composite PCMs (456.2 °C) was higher than that of PEG (430.2 °C). The weight loss of PEG and CF(2%)/PEG/SiO2 composite PCMs was within the temperature range of 354–435 °C and 345–456 °C, respectively. Therefore, the thermostability of the CF/PEG/SiO2 composite PCMs was significantly improved by CF, and the thermostability of the PCMs was high at 340 °C.
Fig. 3. XRD patterns of the composite PCMs.
synthesis process of composite PCMs. The amorphous peaks of CF and SiO2 were observed at 2θ = 25.5° (Fig. 3a) and 22° (Fig. 3b), respectively, whereas the peaks of PEG were observed at 19.24° and 23.42° (Fig. 3c). All the characteristic peaks of CF, SiO2, and PEG were observed in the curve of the CF/PEG/SiO2 composite PCMs (Fig. 3d) at the same position. However, the intensity of the crystallization peaks was lower than that of pure PEG (Fig. 3c). This result confirms that the crystallinity of PEG was limited by the 3D porous network structure of the supporting material (SiO2) [4,6,22].
3.2. The thermal property of CF/PEG/SiO2 composite PCMs 3.3. The protective role of SiO2 played in CF/PEG/SiO2 composite PCMs The phase change temperature and enthalpy of the composite PCMs were measured through DSC (Fig. 4). The fusion temperature, fusion enthalpy, crystallizing point and crystallization enthalpy of the composite PCMs are listed in Table 1. The fusion enthalpy of PEG, PEG/SiO2 PCMs, CF(1%)/PEG/SiO2, CF (2%)/PEG/SiO2, CF(3%)/PEG/SiO2, CF(4%)/PEG/SiO2, and CF(5%)/ PEG/SiO2 reached 222.7, 167.2, 154.1, 169.3, 154.4, 160.5, and 150.6 J/g, respectively, and the crystallization enthalpy was 212.0, 145.2, 133.2, 159.1, 142.6, 147.1, and 132.4 J/g. The melting temperatures of PEG, PEG/SiO2 PCMs, CF(1%)/PEG/SiO2, CF(2%)/PEG/ SiO2, CF(3%)/PEG/SiO2, CF(4%)/PEG/SiO2, and CF(5%)/PEG/SiO2 reached 61.4, 61.0, 62.3, 58.3, 57.5, 57.7, and 61.4 °C, respectively, and the crystallization temperatures was 37.8, 36.6, 41.9, 34.8, 34.8, 35.3, and 41.6 °C. These results indicate that with the addition of SiO2, the melting and crystallization temperatures of PEG/SiO2 PCMs were
PEG, CF (2%)/PEG/SiO2, CF (3%)/PEG/SiO2 and CF (4%)/PEG/ SiO2 powders were pressed into slices and heated on a heating platform to the phase change temperature to investigate the confinement effect of PCMs (Fig. 6). All of the PCMs remained solid, and no PEG leaked out even above the phase change temperature of 30 °C. The result confirmed that the high confinement effect of PCMs was attributed to the protection provided by SiO2. To confirm the protective role of SiO2 in PEG in composite PCMs, SEM was used to observe the morphology of SiO2, PEG/SiO2 PCMs and CF (2%)/PEG/SiO2 composite PCMs (Fig. 7). The phase change component, PEG, was filled with the porous structure of the support material SiO2. The images confirmed the large contract area and strong physisorption between the SiO2 framework and PEG. The powerful adsorption could prevent the leakage of PEG and limit the crystallinity 540
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Fig. 5. (a) TG curves of CF(2%)/PEG/SiO2 composite PCMs and PEG–6000. (b) DTG curves of CF(2%)/PEG/SiO2 composite PCMs and PEG–6000.
thermal energy. To observe the light–thermal energy conversion in a practical application, PEG/SiO2, CF (1%)/PEG/SiO2 and CF (3%)/PEG/ SiO2 were measured under sunlight (Fig. 10). The temperatures of PEG/ SiO2, CF(1%)/PEG/SiO2 and CF(3%)/PEG/SiO2 composite PCMs exposed under sunlight for 2000 s reached 45.2 °C, 50.4 °C, and 52.5 °C, respectively, and it was almost similar to the measured values under simulative light. Therefore, the CF/PEG/SiO2 composite PCMs have a favorable application background due to the high light–thermal energy conversion in practical application. In summary, CF as a light absorber provided light–thermal energy conversion to the composite PCMs, improved the utilization of solar energy, and enhanced thermal conductivity.
of PEG. 3.4. Light–thermal conversion of CF/PEG/SiO2 composite PCMs In the light–thermal energy conversion process, the materials absorbed light energy and converted it into thermal energy. The absorption values and UV–vis wavelengths of the CF/PEG/SiO2 composite PCMs and CF were observed with a solid UV–vis absorption spectrometer (Fig. 8). CF demonstrated high absorbability at almost all UV–vis wavebands (Fig. 8f). Moreover, the absorbance of the composite PCMs (Figs. 8c, 8d, and 8e) was improved with the introduction of CF (Fig. 8a), and this was extremely advantageous to the absorption of sun rays proportional to the CF content. The composite PCMs were placed under a simulative light source with the power of 0.93 W (Fig. 9) and sunlight (Fig. 10) for same time. The numerical control thermometer (SK–130RD, Sukow) was used to measure the light–thermal energy conversion of prepared PCMs. The temperature date of PCMs were collected by Pt electrode and recorded per 4 s. The temperatures of PEG, PEG/SiO2, CF(1%)/PEG/SiO2, CF (3%)/PEG/SiO2 and CF(5%)/PEG/SiO2 composite PCMs under the simulative light source for 2000 s were 49.1 °C, 50.5 °C, 62.2 °C, 64.4 °C and 68.5 °C, respectively (Fig. 9). The light–thermal energy conversion of the composite PCMs was significantly improved by the introduction of CF (Fig. 9). In addition, the conversion was proportional to the content of CF and caused the material to transform light into more
3.5. Thermal conductivity of CF/PEG/SiO2 composite PCMs The composite PCMs were pressed into slices with the same mass and diameter (30 mm), the thermal conductivity of PCMs was measured by thermal conductivity meter (DRE-III, Xiang Yi) at room temperature. Comparing the thermal conductivity to research the enhancement of CF. The measurement result showed that the thermal conductivity coefficient of PEG, PEG/SiO2, CF(1%)/PEG/SiO2, CF(2%)/PEG/SiO2 and CF(3%)/PEG/SiO2 was 0.17, 0.26, 0.39, 0.44 and 0.45 W/(m∙K), respectively. The thermal conductivity of the composite PCMs was significantly enhanced with the increasing of CF content. The enhancement ratio of CF (2%)/PEG/SiO2 composite PCMs to PEG/SiO2
Fig. 6. Digital photos of the composite PCMs under different temperature.
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Fig. 7. SEM images of samples: (a) SiO2, (b) PEG/SiO2, (c) CF (2%)/PEG/SiO2.
Fig. 8. Solid ultraviolet–visible absorption curves of CF/PEG/SiO2 composite PCMs with different content of CF.
Fig. 10. Light–thermal conversion curves of different samples under sunlight measured at late May.
Fig. 9. Light–thermal conversion curves of different samples under simulative light source.
Fig. 11. Heating–rate curves of composite PCMs.
much more shorter than that of PEG/SiO2 (568 s) and PEG (840 s) under the same heating rate and was proportional to the content of CF (Fig. 11). In other words, the heat storage and release heat rate of CF/ PEG/SiO2 composite PCMs were much higher than those of PEG/SiO2 or PEG. Therefore, the introduction of CF significantly improved the heat storage and release rates of the composite PCMs and the latter improved the utilization of solar energy by a large margin with an excellent heat storage rate. In general, the light–thermal energy conversion efficiency and the
was 69.2%. The addition of CF contributed to the release and storage of energy due to the significant improvement in thermal conductivity. The energy efficiency of the composite PCMs was decided by the rate of storage and release energy [5,8]. The heat storage and release rates of the PCMs were measured by the curves of the heating–rate (Fig. 11) and cooling–rate (Fig. 12). The enhancement in the thermal conductivity of PCMs by CF was confirmed because the heating time of CF (3%)/PEG/SiO2 composite PCMs from 0 °C to 65 °C (508 s) was 542
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[2]
[3] [4]
[5]
[6]
[7]
[8]
[9] Fig. 12. Cooling–rate curves of composite PCMs. [10]
thermal conductivity of the composite PCMs were significantly enhanced by the addition of CF and were proportional to the content of CF due to the high thermal conductivity and light absorption. Therefore, the CF/PEG/SiO2 composite PCMs could transform light energy into thermal energy and store more energy at the same time compared with traditional PCMs. The composite PCMs achieved energy storage and release effectively and rapidly. The eicosane/ graphene nanoplatelets (GNPs) composites with the remarkable latent heat of fusion of 220 J/g were synthesized. In this case, the thermal conductivity of PCMs was significantly improved from 0.4148 W/(m∙K) to over 3 W/(m∙K) [33]. The PEG/GO composite PCMs were prepared by physical blending and impregnation method, the resulted product showed high latent heat of 142.8 J/g [34]. It is confirmed that the enhancement effect of CF was comparable with carbon nanotubes and grapheme during comparing with above cases, but at a much lower cost.
[11] [12]
[13]
[14]
[15]
[16] [17]
4. Conclusion
[18]
The CF/PEG/SiO2 composite PCMs with a excellent confinement effect, phase change property, thermal conductivity and light–thermal energy conversion were synthesized successfully by a sol–gel process and in situ doping. With 3% CF content, the composites PCMs showed high fusion enthalpy of 142.6 J/g, crystallization enthalpy of 154.4 J/g, and thermal conductivity coefficient of 0.45 W/(m∙K). The composite PCMs also showed strong thermostability under 340 °C and high light–thermal energy conversion at all UV–vis wavebands. The thermal conductivity of CF/PEG/SiO2 composite PCMs was significantly improved above 50% than PEG/SiO2 and the energy utilization ratio was also enhanced due to its high thermal conductivity. The prepared CF/ PEG/SiO2 composite PCMs provide an important method of solving social environment and energy crisis given their excellent utilization of solar energy.
[19]
[20]
[21]
[22]
[23]
Acknowledgments
[24]
This work was financially supported by the National Natural Science Foundation of China (21576039, 21536002, 21421005, 21276042 and U1608223). Program for Innovative Research Team in University (IRT–13R06). Program for New Century Excellent Talents in University (NCET130080). Fundamental Research Funds for the Central Universities (DUT13LK35, DUT14YQ209, DUT2013TB07, DUT14QY13).
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