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expanded graphite composites for thermal storage
Accepted Manuscript Title: Shape-stabilized phase change materials based on fatty acid eutectics/expanded graphite composites for thermal storage Author: Xuehui Tang Bei Zhu Minghan Xu Wei Zhang Zhi Yang Yafei Zhang Guilin Yin Dannong He Hao Wei Xiaoqiang Zhai PII: DOI: Reference:
S0378-7788(15)30314-5 http://dx.doi.org/doi:10.1016/j.enbuild.2015.09.074 ENB 6194
To appear in:
ENB
Received date: Revised date: Accepted date:
24-6-2015 23-9-2015 30-9-2015
Please cite this article as: X. Tang, B. Zhu, M. Xu, W. Zhang, Z. Yang, Y. Zhang, G. Yin, D. He, H. Wei, X. Zhai, Shape-stabilized phase change materials based on fatty acid eutectics/expanded graphite composites for thermal storage, Energy and Buildings (2015), http://dx.doi.org/10.1016/j.enbuild.2015.09.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Shape-stabilized phase change materials based on fatty acid eutectics/expanded graphite composites for thermal storage
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Xuehui Tang a, Bei Zhu b, Minghan Xu a, Wei Zhang a, Zhi Yang a,c *[email protected], Yafei Zhang a, Guilin Yin c, Dannong He c, Hao Wei a *[email protected], Xiaoqiang Zhai b*[email protected] a
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Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China b Institute of Refrigeration and Cryogenics, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China c National Engineering Research Center for Nanotechnology, Shanghai 200241, People’s Republic of China
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*Corresponding authors. Tel.: +86-21-34206398. Fax: +86-21-34205665.
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Abstract Despite the well-known properties of organic phase change materials (PCMs), drawbacks including high phase transformation point, fluid leakage, and low thermal conductivity limit their practical applications. In this study, a kind of PCMs with low melting point (below 25 °C) was prepared using capric and lauric acid as substrates with additive oleic acid (C-L-O acid). C-L-O acid was then impregnated into worm-like expanded graphite (EG) to form the shape-stabilized PCMs (SPCMs) without any chemical reaction. EG was used to be not only a heat transfer intensifier, but also a shape-stabilized container for C-L-O acid. The melting enthalpy (∆Hm) of the SPCMs (mass ratio of EG to C-L-O acid was 1:35) was calculated to be 114.65 J/g, which was extremely close to the value of original C-L-O acid (115.91 J/g). The thermal conductivity of the SPCMs (1:5) was measured to be 3.15 W/m·K, which was 22.5 times higher than that of original acid (0.14 W/m·K). The SPCMs also showed good thermal reliability after thermal treatment cycles measurements. With consideration of latent heat and thermal conductivity, the SPCMs (1:15) was eventually selected as the optimal SPCMs with its ∆Hm of 109.18 J/g and thermal conductivity of 1.95 W/m·K. These composite SPCMs can be easily mass prepared and further used as potential materials for practical radiant cooling system.
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Keywords Shape-stabilized phase change materials; Expanded graphite; Low melting point; Thermal energy storage; Thermal conductivity
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Nomenclature C-L acid C-L-O acid
capric and lauric acid mixture capric and lauric acid as substrates with additive oleic acid typical interlayer distance of (002) crystal phase expanded graphite phase change enthalpy [J/g] freezing enthalpy [J/g] melting enthalpy [J/g] heating, ventilation and air conditioning shape-stabilized phase change materials melting point [°C] freezing point [°C]
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d(002)
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EG ∆H ∆Hf ∆Hm HVAC SPCMs Tm Tf
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Introduction Thermal energy storage technology is now widely used in various applications such as solar energy utilization, waste heat recovery, and air-condition in buildings [1]. Heating, ventilation and air conditioning (HVAC) system in construction industry has attracted more and more attention due to its outstanding features like thermal comfort, high efficiency, and energy conservation [2]. For the purpose of large and efficient energy storage in HVAC system, phase change materials (PCMs) as excellent thermal energy storage materials are urgently needed, which can absorb/release abundant quantities of heat during their phase change process and thus reserve amounts of energy with extremely high thermal energy storage capability over narrow temperature intervals [3-5]. Materials selected and investigated as candidate PCMs can be simply divided into three categories: inorganic, organic and eutectic [4,6,7]. The practical applications of inorganic PCMs including fuse salts, hydrous salts, metals and alloys are commonly restricted by incongruent melting, which will cause the corrosion and instability during the heat transfer process in the air conditioning [8]. On the contrary, organic PCMs, especially fatty acids [9] are more extensively used in thermal energy storage because of their advantages of high latent heat, thermal stability, chemical durability, non-corrosiveness, non-toxicity and easy availability [8]. In spite of these excellent properties, low thermal conductivity of organic PCMs definitely hinders their further applications in thermal energy storage [9,10]. A great number of researches had been done in previous studies [11,12] to increase the thermal energy absorbing/releasing speed and improve the heat transfer efficiency of organic PCMs. The common method to enhance thermal conductivity of organic PCMs is to add high thermal conductivity fillers [3,8-10] into the substrate, such as metal or metal oxide [13-15], carbon fiber [16,17], graphene [18], graphene oxide [19], exfoliated graphite [20], expanded graphite (EG) [21,22] and expanded perlite [23,24]. Among various materials mentioned above, expanded graphite with a porous network structure is preferred 3 Page 3 of 18
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because of its high thermal conductivity, low density, and good compatibility with organic PCMs. Li et al. [25] reported that the thermal conductivity of paraffin/6 wt% EG composites was improved from 0.32 to 3.66 W/m⋅K compared with pristine paraffin. Xia et al. [26] mixed acetamide with 10 wt% EG thus increasing the thermal conductivity from 0.43 to 2.61 W/m⋅K. Karaipekli et al. [27] reported that the thermal conductivity of stearic acid (0.3 W/m⋅K) was increased to 0.79 W/m⋅K by 10 wt% of EG. Zhang et al. [28] reported that the thermal conductivity of lauric acid, myristic acid, and palmitic acid was enhanced by EG from 0.21 to 1.67 W/m⋅K. Previous works have shown that the thermal conductivity of PCMs could be obviously improved by impregnating them into EG. Therefore, the thermal conductivity enhancement of EG-contained organic PCMs has attracted many research interests. However, the thermal conductivity enhancements of organic PCMs with low melting point, especially below room temperature, have not been studied yet. Low melting point PCMs are extensively used in thermal energy storage applications, especially in HVAC system [29]. In the past decades, radiant cooling system has been regarded as a solution to the reduction of energy consumption while keeping the indoor environment comfortable [30,31]. Previous studies show that 16-18 °C is usually designed as the optimal water temperature of a radiant cooling terminal [32], and the target indoor design temperature of an air-conditioning system is commonly adjusted to 25 °C [5,33]. Therefore, the suitable phase transition temperature of the PCMs for these applications should be about 15~20 °C. In this context, low melting point organic PCMs with high latent heat are required. Capric and lauric acid mixture (C-L acid) is identified as a promising PCM for HVAC system [32,34]. Moreover, Wang et al. had reported that the phase transition temperature of C-L acid could be changed to about 15 °C by organic additives such as oleic acid [32], which made this mixture to be one of the most appropriate PCMs for further investigation. On the other hand, different from the above-mentioned PCMs such as paraffin (melting point is around 50 °C), acetamide (81 °C), and stearic acid (56 °C), low melting point PCMs (around 20 °C) would show liquid state at room temperature. Therefore, the flow ability restricts their practical application in HVAC system. EG with a well-developed pore network structure can reduce the seepage of liquid PCMs through capillary force [35,36]. Moreover, the pore network can prevent the relative large volume change of the PCMs based on fatty acid eutectic during their phase change process. In general, EG is selected to be not only a heat transfer intensifier, but also a shape-stabilized container of low melting point organic PCMs. In this paper, capric acid, lauric acid, and oleic acid were mixed in a fixed proportion to form a eutectic mixture, followed by absorbing into the EG to prepare the composite shape-stabilized PCMs (SPCMs). Morphological and chemical properties of the raw materials and the composite SPCMs were observed. Their thermal stability, thermal energy properties, and thermal conductivity were also characterized. Experimental 2.1. Materials Expanded graphite was obtained from Qingdao Chenyang Graphite Co., Ltd., 4
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pre-dried at 70 °C in a drying oven to ensure its desiccation. Capric acid, lauric acid, and oleic acid were purchased from Sinopharm Group Co., Ltd.. All the acids were analytical grade without further purification.
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2.2. Preparation of C-L-O acid and the acid/EG composites Capric acid and lauric acid were used as major PCMs. Meanwhile, oleic acid was dripped into the mixture as an additive to increase the latent heat, it could also decline the phase transition temperature comparing with the original C-L acid substrate [32]. Capric acid and lauric acid were compounded with a mole proportion of 7:3. The mixture was vigorously stirred at 50 °C for 30 min. Then oleic acid was added into the system with a mole fraction of 8.0% with the stir continued. The eutectic fluid was stirred at 50 °C to form a well-distributed mixture. The C-L acid with oleic acid as an additive (C-L-O acid) was obtained after a natural cooling process. Next, a certain amount of raw EG and C-L-O acid were successively added into a 50 mL beaker, stirred with continuously mechanical mixing for about 30 min so that the C-L-O acid was completely absorbed into the EG. Lastly, the solid-phase composite SPCMs were obtained and collected for further characterization. In the experiments, C-L-O acid exhibited a liquid phase, while EG and SPCMs showed solid state. For convenience, the samples were named from SPCMs 1 to SPCMs 7, corresponding to the mass ratio of EG to C-L-O acid that were 1:5, 1:10, 1:15, 1:20, 1:25, 1:30 and 1:35, respectively.
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2.3. Characterization The surface morphologies and microstructures of raw EG and SPCMs were analyzed by field emission scanning electron microscope (SEM, Ultra Plus, Carl Zeiss, Germany) at 5 kV. Fourier transformation infrared (FT-IR) spectra of C-L-O acid and SPCMs were recorded on a FT-IR spectrometer (Vertex 70, Bruker Optics, Germany) and scanned from 4,000 to 400 cm−1 using KBr pellet. X-ray diffraction (XRD) patterns of raw EG and SPCMs were obtained by X-ray diffractometer (D8 Advance, Bruker, Germany), Cu Kα radiation was used with a scanning rate of 10 °/min. Thermal stability of C-L-O acid and SPCMs were characterized by means of thermogravimetry analysis (TGA) on a thermal gravimetric analyzer (Pyris 1, PerkinElmer, USA) from room temperature to 900 °C at a heating rate of 10 °C/min in nitrogen atmosphere. Differential scanning calorimetry (DSC, DSC 8000, PerkinElmer, USA) was used to investigate the thermal energy storage properties of the C-L-O acid and SPCMs. For DSC measurements, samples were all heated and cooled repeatedly between 35 to -20 °C at a rate of 10 °C/min under a constant stream of nitrogen at a flow rate of 10 mL/min. The thermal conductivities of the raw materials and SPCMs at room temperature were tested by means of transient plane heat source method using a thermal constants analyzer (Hot Disk, TPS 2500, Hot Disk, Sweden). Results and discussions Morphologies and microstructures of EG and the composite SPCMs 5 Page 5 of 18
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SEM images shown in Fig. 1 are used to investigate the surface morphologies and microstructures of the raw EG and the as-prepared composite SPCMs. It can be observed from Fig. 1a that the raw EG shows worm-like structure. Moreover, a large amount of well-developed network pores at micro scale in worm-like EG can be clearly observed (Fig. 1b), indicating that the raw EG has a large surface area thereby presenting excellent absorption ability. On the other hand, due to the capillary force of the porous microstructure in raw EG, the loaded liquid C-L-O acid can be prevented from exuding, the influence of the volume change of C-L-O acid during its solid-liquid transformation process can also be avoided. Fig. 1b also exhibits the smooth surface and layer edge of the raw EG. In contrast, after C-L-O acid is impregnated and absorbed into the micro pores of the raw EG (Fig. 1c, d, e and f), the surface and the layer edges of the samples become rougher, along with the layers turn thicker. Moreover, with the increase of the mass fraction of C-L-O acid, the micro pore structure of EG was gradually filled up. The growth trend of the thickness of SPCM layers can be observed from SPCMs 1 to SPCMs 7, corresponding to more C-L-O acid being absorbed into the micro pores of EG. It can be discovered that there are still some pore structures in the SPCMs, indicating that C-L-O acid in both SPCMs 1 (Fig. 1c) and SPCMs 3 (Fig. 1d) is not fully absorbed. When C-L-O acid is fully impregnated into EG, the SPCMs become saturated with the micro pores being filled up, as shown in Fig. 1e and f. Furthermore, compared with the well-developed network in raw EG, the pore structures in SPCMs are slightly damaged because of the mechanical stirring during the mixing process of EG and C-L-O acid.
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Chemical characterizations of C-L-O acid and the composite SPCMs Functional groups of C-L-O acid and the composite SPCMs, shown in Fig. 2, are characterized by FT-IR spectroscopy. In the spectrum of the C-L-O acid, the absorption peaks center at 2926, 2856 and 1466, 723 cm-1 signify the stretching vibrations of –CH3 and –CH2 groups in the acid molecules, respectively. The C=O stretching vibration of the acid is exhibited at 1711 cm-1. The peak at 1413 cm-1 may be attributed to C–O vibrations. The peaks at 1284, 936 and 722 cm-1 correspond to the in-plane bending vibration, the out-of-plane bending vibration, and the in-plane swinging vibration of the –OH group in the carboxyl group of the C-L-O acid, respectively. These absorption peaks are also observed in the spectrum of the composite SPCMs. Moreover, there is no obvious shift in the above main absorption peaks between the two spectra. The results demonstrate that C-L-O acid is just physically impregnated into pores of EG without any chemical reaction occurred during the whole process.
The phase structure of raw EG and the composite SPCMs are investigated by XRD. The obtained XRD patterns are shown in Fig. 3. The peaks at 26.5° and 54.6° are exhibited in the XRD patterns of EG, which are often taken as the feature peaks of 6 Page 6 of 18
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graphite (002) and (004) crystal phase [21], respectively. The diffraction peaks of EG are also clearly observed in the XRD patterns of the composite SPCMs. However, the (002) peak is slightly shifted to 26.7° as shown in the inset of Fig. 3, indicating that the typical interlayer distance (d002) is slightly squeezed from 0.3363 to 0.3338 nm by C-L-O acid. The broad peak presented at 20.0° can be attributed to the impregnation of C-L-O acid.
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Thermal reliability test of the composite SPCMs The leakage tests of the as-prepared composite SPCMs are carried out to characterize the thermal reliability by their shape-stable performance after thermal cycles above the melting temperature. Photographs of the composite SPCMs before and after the 30 thermal treatment cycles in 50 °C atmosphere are exhibited in Fig. 4. As seen from the photographs, the composite SPCMs trend to adhere with each other with the increase of C-L-O acid usage. In addition, after the 30 thermal treatment cycles in 50 °C atmosphere, C-L-O acid impregnated into the SPCMs 7 partly leak out, while other composites SPCMs from 1 to 6 show no obvious disclose. The results show that it is not necessary to continue increasing the mass ratio of C-L-O acid and EG, according to the detectable leakage of the composite SPCMs 7 shown in Fig. 4.
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Thermal stability characteristics of C-L-O acid and the composite SPCMs The thermal stability and weight loss percentage of C-L-O acid and the composite SPCMs from sample 1 to 7 are measured by TGA. The TGA curves are shown in Fig. 5. It is clearly shown that C-L-O acid and the composite SPCMs exhibit similar thermal stability characteristics. When the temperature increases up to about 120 °C in nitrogen atmosphere, liquid C-L-O acid begins to evaporate and the final weight loss percentage is almost 100% at about 300 °C. The curves of SPCMs from 1 to 7 are nearly the same as that of C-L-O acid. The weight loss from 120 to 300 °C is attributed to the removal of the C-L-O acid impregnated in the SPCMs. The residues of SPCMs from 1 to 7 also agree well with the loading ratios of raw EG in the composites, respectively. The results prove that the thermal stability of C-L-O acid is not influenced by raw EG. The thermal stability of SPCMs is satisfactory in the low temperature range, especially near room temperature, which can avoid the metamorphism of the composite SPCMs in the heat transfer process of practical radiant cooling system at about 15~20 °C.
Flammability of the composite SPCMs According to literature, EG is one of the most promising halogen-free flame retardants because of its low cost [37]. Generally, EG limits the mass transfer from the substrate to the flame and the heat transfer from the flame to the substrate. In order to further characterize the flammability of the composite SPCMs, the combustion tests [38] of the composite SPCMs are carried out in a muffle furnace from room 7 Page 7 of 18
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temperature to 600 °C at a heating rate of 2 °C/min in air atmosphere. The residues of the SPCMs after combustion are investigated by SEM. As shown in Fig. 6, the residues of the composite SPCMs are homogeneous and continuous, the structures of the residues become tight and dense compared with the raw EG shown in Fig. 1b. These structures may increase the trapping of the decomposed products, hence decrease flammable molecules during combustion. On the other hand, C-L-O acid impregnated in the SPCMs will absorbs heat, thus the temperature of the composite SPCMs’ surface may reduce.
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Thermal energy properties of C-L-O acid and the composite SPCMs DSC is used to investigate the phase transition temperatures and latent heat of C-L-O acid and the composite SPCMs from 1 to 7. Fig. 7 exhibits the melting-freezing DSC curves of the samples. For each sample, three DSC experiments were conducted with individual specimens randomly sampling from the beaker. The results show good repeatability, and the uniformity of the samples can be certified as well. The DSC curve of C-L-O acid presents an endothermic peak centered at 18.10 °C and an exothermic peak centered at 11.13 °C, corresponding to the melting point (Tm) and the freezing point (Tf) of C-L-O acid, respectively. The melting enthalpy (∆Hm) and the freezing enthalpy (∆Hf) of C-L-O acid are calculated to be 115.91 and 117.04 J/g, respectively.
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The DSC curves of SPCMs from 1 to 7 are similar to that of C-L-O acid. One endothermic peak and one exothermic peak can be observed, indicating that the thermal energy properties of SPCMs almost come from the C-L-O acid. However, when C-L-O acid is absorbed into EG, the melting and freezing point of the composites are increased by about 1 °C, owing to the melting/freezing process limitation of C-L-O acid by the micro pores existing in raw EG. Moreover, all the melting temperatures of SPCMs from 1 to 7 fluctuate near 19 °C, which further proves that the usage of EG in the experiments make no difference to the thermal energy properties of SPCMs. As is mentioned in the introduction section, the suitable phase transition temperature of the PCMs for practical applications should be around 20 °C, it is obvious that the SPCMs we prepared are acceptable with their melting point around 19 °C. Meanwhile, according to the DSC results, the mean value of the ∆Hm and ∆Hf can also be calculated, which are given in Table 1, as well as the phase transition temperatures of C-L-O acid and the composite SPCMs from 1 to 7. The ∆Hm and ∆Hf (115.91 and 117.04 J/g) of C-L-O acid reflect the optimal latent heat of all the samples. As shown in Table 1, when the mass ratio of EG to C-L-O acid is 1:5 (SPCMs 1), The ∆Hm and ∆Hf (93.12 and 109.05 J/g) are at low level. Thereafter, with more C-L-O acid being impregnated into EG, the latent heat of SPCMs gradually improved, which can be judged from the increase of the ∆Hm and ∆Hf. The ∆Hm and ∆Hf (113.55, 115.34 and 116.52, 116.82 J/g) of SPCMs 5 and 6 are both very close to 8 Page 8 of 18
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the value of the original C-L-O acid (115.91 and 117.04 J/g), indicating that C-L-O acid is enough absorbed into EG in these two SPCMs to ensure high latent heat of the composites, which further prove the results from the SEM images in Fig. 1. By unceasingly increasing the loading ratio of C-L-O acid to 1:35 (SPCMs 7), it can be found that the ∆Hm and ∆Hf (114.65 and 116.56 J/g) slightly drop, possibly because SPCMs 6 has already reached saturation, the further absorbed C-L-O acid just deposit on the surface of EG sheets or fill into the large space between EG stacks. The redundant C-L-O acid cannot be loaded stably on the composites, so that the latent heat of SPCMs 7 is found slightly decreased. Moreover, the DSC results also indicate that there is no chemical reaction occurred between C-L-O acid and EG in the preparation of SPCMs, which is consistent with the result of FT-IR spectra.
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Thermal conductivity characteristics of EG and the composite SPCMs The thermal conductivity of C-L-O acid, raw EG, and the composite SPCMs from 1 to 7 are analyzed using Hot Disk by a transient plane heat source method. The results are shown in Fig. 8. All the Hot Disk data presented in our work are average values over triple measurements of each sample. The thermal conductivity of original C-L-O acid is 0.14 W/m⋅K. However, with the addition of EG, significant improvements of thermal conductivity of SPCMs can be clearly observed. The thermal conductivity of SPCMs 7 reaches up to 1.37 W/m⋅K. Moreover, with the increasing usage of EG, the thermal conductivity of SPCMs is also obviously enhanced. When more EG is added and the mass ratio of EG to C-L-O acid reaches 1:5, the thermal conductivity of SPCMs 1 is observed to be further enhanced to 3.15 W/m⋅K, which is 22.5 times higher than that of original C-L-O acid (0.14 W/m⋅K) and closes to that of raw EG (3.43 W/m⋅K). All results indicate that the thermal conductivity of SPCMs can be highly enhanced by EG. A large amount of well-developed pore network structure in raw worm-like EG exhibited in Fig. 1a act as the carrier during the heat transfer process. Therefore, the solid-phase SPCMs enhance the thermal transmission of the liquid C-L-O acid.
According to the summarization of ∆Hm and ∆Hf data from Table 1 and thermal conductivity results from Fig. 8, the trends of the thermal properties of the composite SPCMs can be observed. With the increase of the mass fraction of C-L-O acid (SPCMs from 1 to 7), the ∆Hm and ∆Hf of the composite SPCMs gradually increase from 93.12 to 114.65 J/g and from 109.05 to 116.56 J/g, respectively, while their corresponding thermal conductivity continually declines from 3.15 to 1.37 W/m⋅K. The increase trend of ∆Hm is more sharply than that of ∆Hf. Fig. 8 also shows that the line of ∆Hm and the line of thermal conductivity are intersected where the mass ratio of EG to C-L-O acid approaching 1:15 (SPCM 3). In the application of thermal energy storage, high latent heat is necessary as well as excellent thermal conductivity. Therefore, SPCMs 3 is finally selected as the optimal composite SPCMs with its ∆Hm of 109.18 J/g, ∆Hf of 115.28 J/g, and thermal conductivity of 1.95 W/m⋅K. 9 Page 9 of 18
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Conclusions We have prepared C-L-O acid with a low melting point at around 20 °C. The C-L-O acid is then mechanically mixed with pre-dried worm-like EG to obtain the composite shape-stabilized PCMs. The worm-like EG is used as both a heat transfer intensifier and a shape-stabilized container of C-L-O acid. Morphological and chemical characterizations are carried out by SEM, XRD and FT-IR. Thermal energy properties are characterized by TGA, DSC and Hot Disk. The pore size of the EG network is at micro scale. C-L-O acid is impregnated into raw EG only by capillary forces to make SPCMs maintain solid state without any chemical reaction occurred. The relative large volume change of C-L-O acid can also be prevented by the pore network structure of EG. TGA results prove that C-L-O acid is stably absorbed into EG. The latent heat of SPCMs increases with the addition of C-L-O acid from SPCMs 1 to 6 and slightly drops in SPCMs 7. When the mass ratio of EG to C-L-O acid is 1:30, ∆Hm of SPCMs 6 is measured to be 115.34 J/g, which is close to the value of the original C-L-O acid (115.91 J/g). The latent heat of SPCMs is greatly improved by the adding of C-L-O acid. On the contrary, the thermal conductivity of SPCMs declines with the mass fraction increase of C-L-O acid. However, thermal conductivity of SPCMs 7 is still 1.37 W/m⋅K, which is 9.8 times higher than that of the original C-L-O acid (0.14 W/m⋅K). With comprehensive consideration of latent heat and thermal conductivity, SPCMs 3 is deemed to be the optimal mass ratio of EG to C-L-O acid among all the SPCMs, whose ∆Hm is 109.18 J/g, and thermal conductivity is 1.95 W/m⋅K. The microstructures of the residues after combustion indicate that the homogeneous and dense residue decreased the flammability of the composite SPCMs. These composite SPCMs can be easily, inexpensively mass prepared (the price of EG, C-L-O acid are both at low-level) and further used as potential materials for practical radiant cooling system due to their satisfactory thermal properties, including favorable thermal conductivity, uninflammable property, high latent heat, good thermal reliability and stability. Acknowledgements The authors gratefully acknowledge financial supports by the National Basic Research Program of China (2013CB932500), the National Natural Science Foundation of China (No. 51276114, 21171117 and 61376003), Program for New Century Excellent Talents in University (NCET-12-0356), the Program of Shanghai Academic/Technology Research Leader (15XD1525200), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. We also acknowledge analysis support from the Instrumental Analysis Center of Shanghai Jiao Tong University and the Center for Advanced Electronic Materials and Devices of Shanghai Jiao Tong University.
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phase change materials, Carbon 43 (2005) 3067-3074. [17] J. F. Wang, H. Q. Xie, Z. Xin, Y. Li, C. Yin, Investigation on thermal properties of heat storage composites containing carbon fibers, Journal of Applied Physics 110 (2011) 094302. [18] F. Yavari, H. R. Fard, K. Pashayi, M. A. Rafiee, A. Zamiri, Z. Z. Yu, R. Ozisik, T. Borca–Tasciuc, N. Koratkar, Enhanced thermal conductivity in a nanostructured phase change composite due to low concentration graphene additives, Journal of Physical Chemistry C 115 (2011) 8753–8758. [19] B. X. Li, T. X. Liu, L. Y. Hu, Y. F. Wang, S. B. Nie, Facile preparation and adjustable thermal property of stearic acid-graphene oxide composite as shape-stabilized phase change material, Chemical Engineering Journal 215 (2013) 819–826. [20] S. Kim, L. T. Drzal, High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets, Solar Energy Materials & Solar Cells 93 (2009) 136–142. [21] S. P. Wang, P. Qin, X. M. Fang, Z. G. Zhang, S. F. Wang, X. H. Liu, A novel sebacic acid/expanded graphite composite phase change material for solar thermal medium-temperature applications, Solar Energy 99 (2014) 283–290. [22] X. Huang, Y. D. Cui, B. N. Zhang, G. Q. Yin, G. Z. Feng, Preparation and properties of capric–lauric–palmitic acid eutectic mixtures/expanded graphite composite as phase change materials for energy storage, Advances in Materials Research 1028 (2014) 40–45. [23] T. Wei, B. C. Zheng, J. Liu, Y. F. Gao, W. H. Guo, Structures and thermal properties of fatty acid/expanded perlite composites as form-stable phase change materials, Energy and Buildings 68 (2014) 587–592. [24] N. Zhang, Y. P. Yuan, Y. G. Yuan, T. Y. Li, X. L. Cao, Lauric–palmitic–stearic acid/expanded perlite composite asform-stable phase change material: Preparation and thermal properties, Energy and Buildings 82 (2014) 505–511. [25] Z. Li, W. G. Sun, G. Wang, Z. G. Wu, Experimental and numerical study on the effective thermal conductivity of paraffin/expanded graphite composite, Solar Energy Materials & Solar Cells 128 (2014) 447–455. [26] L. Xia, P. Zhang, Thermal property measurement and heat transfer analysis of acetamide and acetamide/expanded graphite composite phase change material for solar heat storage, Solar Energy Materials & Solar Cells 95 (2011) 2246–2254. [27] A. Karaipekli, A. Sari, K. Kaygusuz, Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications, Renewable Energy 32 (2007) 2201–2210. [28] N. Zhang, Y. P. Yuan, X. Wang, X. L. Cao, X. J. Yang, S. C. Hu, Preparation and characterization of lauric-myristic-palmitic acid ternary eutectic mixtures/expanded graphite composite phase change material for thermal energy storage, Chemical Engineering Journal 231 (2013) 214–219. [29] H. S. Ge, H. Y. Li, S. F. Mei, J. Liu, Low melting point liquid metal as a new class of phase change material: An emerging frontier in energy area, Renewable & Sustainable Energy Reviews 21 (2013) 331–346. 12
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[30] R. Hu, J. L. Niu, A review of the application of radiant cooling & heating systems in Mainland China, Energy and Buildings 52 (2012) 11–19. [31] S. Kamali, Review of free cooling system using phase change material for building, Energy and Buildings 80 (2014) 131–136. [32] X. L. Wang, X. Q. Zhai, T. Wang, H. X. Wang, Y. L. Yin, Performance of the capric and lauric acid mixture with additives as cold storage materials for high temperature cooling application, Applied Thermal Engineering 58 (2013) 252–260. [33] S. Kamali, Review of free cooling system using phase change material for building, Energy and Buildings 80 (2014) 131–136. [34] M. N. R. Dimaano, T. Watanabe, The capric-lauric acid and pentadecane combination as phase change material for cooling applications, Applied Thermal Engineering 22 (2002) 365–377. [35] J. L. Zeng, J. Gan, F. R. Zhu, S. B. Yu, Z. L. Xiao, W. P. Yan, L. Zhu, Z. Q. Liu, L. X. Sun, Z. Cao, Tetradecanol/expanded graphite composite form-stable phase change material for thermal energy storage, Solar Energy Materials & Solar Cells 127 (2014) 122–128. [36] F. Tang, L. Cao, G. Y. Fang, Preparation and thermal properties of stearic acid/titanium dioxide composites as shape-stabilized phase change materials for building thermal energy storage, Energy and Buildings 80 (2014) 352–357. [37] M. J. Mochane, A. S. Luyt, The effect of expanded graphite on the flammability and thermal conductivity properties of phase change material based on PP/wax blends, Polymer Bulletin 72 (2015) 2263–2283. [38] Y. B. Cai, Y. Hu, L. Song, H. D. Lu, Z. Y. Chen, W. C. Fan, Preparation and characterizations of HDPE–EVA alloy/OMT nanocomposites/paraffin compounds as a shape stabilized phase change thermal energy storage material, Thermochimica Acta 451 (2006) 44–51.
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Fig. 1 SEM images of (a, b) raw EG, (c) SPCMs 1, (d) SPCMs 3, (e) SPCMs 5, and (f)
SPCMs 7.
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Fig. 2 FT-IR spectra of C-L-O acid and the composite SPCMs. SPCMs 6 was used for
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the measurement.
Fig. 3 XRD patterns of EG and the composite SPCMs. SPCMs 6 was used for the
measurement. The inset shows the enlarged image from 25° to 28°.
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Fig. 4 Photographs of the composite SPCMs placed on filter paper before and after the
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thermal treatment cycles.
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Fig. 5 TGA curves of C-L-O acid and the composite SPCMs.
Fig. 6 SEM images of the residues after 600 °C combustion: (a) SPCMs 1, (b) SPCMs
3, (c) SPCMs 5, and (d) SPCMs 7.
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Fig. 7 DSC curves of C-L-O acid and the composite SPCMs.
Fig. 8 Phase change enthalpy (∆H) and thermal conductivity of the composite SPCMs.
The error bar represents the standard error of three independent experiments.
Table 1 Phase change parameters of C-L-O acid and the composite SPCMs. Sample Mass ratio Tm Tf ∆Hm ∆Hf (°C) (EG wt%) (°C) (J/g) (J/g) SPCMs 1 1:5 (16.7) 18.96 ± 0.03 93.12 ± 0.18 13.64 ± 0.01 109.05 ± 1.22 SPCMs 2 1:10 (9.1) 18.50 ± 0.03 105.88 ± 0.35 12.76 ± 0.09 114.16 ± 0.51 SPCMs 3 1:15 (6.3) 19.04 ± 0.09 109.18 ± 0.20 12.19 ± 0.38 115.28 ± 0.13 SPCMs 4 1:20 (4.8) 18.78 ± 0.71 111.70 ± 0.67 11.68 ± 0.01 115.96 ± 0.62 SPCMs 5 1:25 (3.9) 19.18 ± 0.01 113.55 ± 0.47 12.07 ± 0.19 116.53 ± 0.25 SPCMs 6 1:30 (3.2) 18.78 ± 0.94 115.34 ± 0.55 11.87 ± 0.02 116.82 ± 0.06 SPCMs 7 1:35 (2.8) 19.15 ± 0.18 114.65 ± 0.03 11.78 ± 0.49 116.56 ± 0.01 C-L-O acid 0:1 (0) 18.10 ± 0.04 115.91 ± 0.11 11.13 ± 0.51 117.04 ± 0.12 17 Page 17 of 18
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Highlights Thermal conductivity improvement of low melting point organic PCMs is investigated for the first time. Expanded graphite (EG) is used to be a heat transfer and a shape-stabilized container. Thermal conductivity of the shape-stabilized PCMs is obviously enhanced. Latent heat of the shape-stabilized PCMs show little change compared with original PCM. An optimal mass ratio of EG and organic PCMs is discussed.
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S0378-7788(15)30314-5 http://dx.doi.org/doi:10.1016/j.enbuild.2015.09.074 ENB 6194
To appear in:
ENB
Received date: Revised date: Accepted date:
24-6-2015 23-9-2015 30-9-2015
Please cite this article as: X. Tang, B. Zhu, M. Xu, W. Zhang, Z. Yang, Y. Zhang, G. Yin, D. He, H. Wei, X. Zhai, Shape-stabilized phase change materials based on fatty acid eutectics/expanded graphite composites for thermal storage, Energy and Buildings (2015), http://dx.doi.org/10.1016/j.enbuild.2015.09.074 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Shape-stabilized phase change materials based on fatty acid eutectics/expanded graphite composites for thermal storage
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Xuehui Tang a, Bei Zhu b, Minghan Xu a, Wei Zhang a, Zhi Yang a,c *[email protected], Yafei Zhang a, Guilin Yin c, Dannong He c, Hao Wei a *[email protected], Xiaoqiang Zhai b*[email protected] a
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Key Laboratory for Thin Film and Microfabrication of Ministry of Education, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China b Institute of Refrigeration and Cryogenics, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China c National Engineering Research Center for Nanotechnology, Shanghai 200241, People’s Republic of China
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*Corresponding authors. Tel.: +86-21-34206398. Fax: +86-21-34205665.
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Abstract Despite the well-known properties of organic phase change materials (PCMs), drawbacks including high phase transformation point, fluid leakage, and low thermal conductivity limit their practical applications. In this study, a kind of PCMs with low melting point (below 25 °C) was prepared using capric and lauric acid as substrates with additive oleic acid (C-L-O acid). C-L-O acid was then impregnated into worm-like expanded graphite (EG) to form the shape-stabilized PCMs (SPCMs) without any chemical reaction. EG was used to be not only a heat transfer intensifier, but also a shape-stabilized container for C-L-O acid. The melting enthalpy (∆Hm) of the SPCMs (mass ratio of EG to C-L-O acid was 1:35) was calculated to be 114.65 J/g, which was extremely close to the value of original C-L-O acid (115.91 J/g). The thermal conductivity of the SPCMs (1:5) was measured to be 3.15 W/m·K, which was 22.5 times higher than that of original acid (0.14 W/m·K). The SPCMs also showed good thermal reliability after thermal treatment cycles measurements. With consideration of latent heat and thermal conductivity, the SPCMs (1:15) was eventually selected as the optimal SPCMs with its ∆Hm of 109.18 J/g and thermal conductivity of 1.95 W/m·K. These composite SPCMs can be easily mass prepared and further used as potential materials for practical radiant cooling system.
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Keywords Shape-stabilized phase change materials; Expanded graphite; Low melting point; Thermal energy storage; Thermal conductivity
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Nomenclature C-L acid C-L-O acid
capric and lauric acid mixture capric and lauric acid as substrates with additive oleic acid typical interlayer distance of (002) crystal phase expanded graphite phase change enthalpy [J/g] freezing enthalpy [J/g] melting enthalpy [J/g] heating, ventilation and air conditioning shape-stabilized phase change materials melting point [°C] freezing point [°C]
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EG ∆H ∆Hf ∆Hm HVAC SPCMs Tm Tf
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Introduction Thermal energy storage technology is now widely used in various applications such as solar energy utilization, waste heat recovery, and air-condition in buildings [1]. Heating, ventilation and air conditioning (HVAC) system in construction industry has attracted more and more attention due to its outstanding features like thermal comfort, high efficiency, and energy conservation [2]. For the purpose of large and efficient energy storage in HVAC system, phase change materials (PCMs) as excellent thermal energy storage materials are urgently needed, which can absorb/release abundant quantities of heat during their phase change process and thus reserve amounts of energy with extremely high thermal energy storage capability over narrow temperature intervals [3-5]. Materials selected and investigated as candidate PCMs can be simply divided into three categories: inorganic, organic and eutectic [4,6,7]. The practical applications of inorganic PCMs including fuse salts, hydrous salts, metals and alloys are commonly restricted by incongruent melting, which will cause the corrosion and instability during the heat transfer process in the air conditioning [8]. On the contrary, organic PCMs, especially fatty acids [9] are more extensively used in thermal energy storage because of their advantages of high latent heat, thermal stability, chemical durability, non-corrosiveness, non-toxicity and easy availability [8]. In spite of these excellent properties, low thermal conductivity of organic PCMs definitely hinders their further applications in thermal energy storage [9,10]. A great number of researches had been done in previous studies [11,12] to increase the thermal energy absorbing/releasing speed and improve the heat transfer efficiency of organic PCMs. The common method to enhance thermal conductivity of organic PCMs is to add high thermal conductivity fillers [3,8-10] into the substrate, such as metal or metal oxide [13-15], carbon fiber [16,17], graphene [18], graphene oxide [19], exfoliated graphite [20], expanded graphite (EG) [21,22] and expanded perlite [23,24]. Among various materials mentioned above, expanded graphite with a porous network structure is preferred 3 Page 3 of 18
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because of its high thermal conductivity, low density, and good compatibility with organic PCMs. Li et al. [25] reported that the thermal conductivity of paraffin/6 wt% EG composites was improved from 0.32 to 3.66 W/m⋅K compared with pristine paraffin. Xia et al. [26] mixed acetamide with 10 wt% EG thus increasing the thermal conductivity from 0.43 to 2.61 W/m⋅K. Karaipekli et al. [27] reported that the thermal conductivity of stearic acid (0.3 W/m⋅K) was increased to 0.79 W/m⋅K by 10 wt% of EG. Zhang et al. [28] reported that the thermal conductivity of lauric acid, myristic acid, and palmitic acid was enhanced by EG from 0.21 to 1.67 W/m⋅K. Previous works have shown that the thermal conductivity of PCMs could be obviously improved by impregnating them into EG. Therefore, the thermal conductivity enhancement of EG-contained organic PCMs has attracted many research interests. However, the thermal conductivity enhancements of organic PCMs with low melting point, especially below room temperature, have not been studied yet. Low melting point PCMs are extensively used in thermal energy storage applications, especially in HVAC system [29]. In the past decades, radiant cooling system has been regarded as a solution to the reduction of energy consumption while keeping the indoor environment comfortable [30,31]. Previous studies show that 16-18 °C is usually designed as the optimal water temperature of a radiant cooling terminal [32], and the target indoor design temperature of an air-conditioning system is commonly adjusted to 25 °C [5,33]. Therefore, the suitable phase transition temperature of the PCMs for these applications should be about 15~20 °C. In this context, low melting point organic PCMs with high latent heat are required. Capric and lauric acid mixture (C-L acid) is identified as a promising PCM for HVAC system [32,34]. Moreover, Wang et al. had reported that the phase transition temperature of C-L acid could be changed to about 15 °C by organic additives such as oleic acid [32], which made this mixture to be one of the most appropriate PCMs for further investigation. On the other hand, different from the above-mentioned PCMs such as paraffin (melting point is around 50 °C), acetamide (81 °C), and stearic acid (56 °C), low melting point PCMs (around 20 °C) would show liquid state at room temperature. Therefore, the flow ability restricts their practical application in HVAC system. EG with a well-developed pore network structure can reduce the seepage of liquid PCMs through capillary force [35,36]. Moreover, the pore network can prevent the relative large volume change of the PCMs based on fatty acid eutectic during their phase change process. In general, EG is selected to be not only a heat transfer intensifier, but also a shape-stabilized container of low melting point organic PCMs. In this paper, capric acid, lauric acid, and oleic acid were mixed in a fixed proportion to form a eutectic mixture, followed by absorbing into the EG to prepare the composite shape-stabilized PCMs (SPCMs). Morphological and chemical properties of the raw materials and the composite SPCMs were observed. Their thermal stability, thermal energy properties, and thermal conductivity were also characterized. Experimental 2.1. Materials Expanded graphite was obtained from Qingdao Chenyang Graphite Co., Ltd., 4
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pre-dried at 70 °C in a drying oven to ensure its desiccation. Capric acid, lauric acid, and oleic acid were purchased from Sinopharm Group Co., Ltd.. All the acids were analytical grade without further purification.
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2.2. Preparation of C-L-O acid and the acid/EG composites Capric acid and lauric acid were used as major PCMs. Meanwhile, oleic acid was dripped into the mixture as an additive to increase the latent heat, it could also decline the phase transition temperature comparing with the original C-L acid substrate [32]. Capric acid and lauric acid were compounded with a mole proportion of 7:3. The mixture was vigorously stirred at 50 °C for 30 min. Then oleic acid was added into the system with a mole fraction of 8.0% with the stir continued. The eutectic fluid was stirred at 50 °C to form a well-distributed mixture. The C-L acid with oleic acid as an additive (C-L-O acid) was obtained after a natural cooling process. Next, a certain amount of raw EG and C-L-O acid were successively added into a 50 mL beaker, stirred with continuously mechanical mixing for about 30 min so that the C-L-O acid was completely absorbed into the EG. Lastly, the solid-phase composite SPCMs were obtained and collected for further characterization. In the experiments, C-L-O acid exhibited a liquid phase, while EG and SPCMs showed solid state. For convenience, the samples were named from SPCMs 1 to SPCMs 7, corresponding to the mass ratio of EG to C-L-O acid that were 1:5, 1:10, 1:15, 1:20, 1:25, 1:30 and 1:35, respectively.
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2.3. Characterization The surface morphologies and microstructures of raw EG and SPCMs were analyzed by field emission scanning electron microscope (SEM, Ultra Plus, Carl Zeiss, Germany) at 5 kV. Fourier transformation infrared (FT-IR) spectra of C-L-O acid and SPCMs were recorded on a FT-IR spectrometer (Vertex 70, Bruker Optics, Germany) and scanned from 4,000 to 400 cm−1 using KBr pellet. X-ray diffraction (XRD) patterns of raw EG and SPCMs were obtained by X-ray diffractometer (D8 Advance, Bruker, Germany), Cu Kα radiation was used with a scanning rate of 10 °/min. Thermal stability of C-L-O acid and SPCMs were characterized by means of thermogravimetry analysis (TGA) on a thermal gravimetric analyzer (Pyris 1, PerkinElmer, USA) from room temperature to 900 °C at a heating rate of 10 °C/min in nitrogen atmosphere. Differential scanning calorimetry (DSC, DSC 8000, PerkinElmer, USA) was used to investigate the thermal energy storage properties of the C-L-O acid and SPCMs. For DSC measurements, samples were all heated and cooled repeatedly between 35 to -20 °C at a rate of 10 °C/min under a constant stream of nitrogen at a flow rate of 10 mL/min. The thermal conductivities of the raw materials and SPCMs at room temperature were tested by means of transient plane heat source method using a thermal constants analyzer (Hot Disk, TPS 2500, Hot Disk, Sweden). Results and discussions Morphologies and microstructures of EG and the composite SPCMs 5 Page 5 of 18
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SEM images shown in Fig. 1 are used to investigate the surface morphologies and microstructures of the raw EG and the as-prepared composite SPCMs. It can be observed from Fig. 1a that the raw EG shows worm-like structure. Moreover, a large amount of well-developed network pores at micro scale in worm-like EG can be clearly observed (Fig. 1b), indicating that the raw EG has a large surface area thereby presenting excellent absorption ability. On the other hand, due to the capillary force of the porous microstructure in raw EG, the loaded liquid C-L-O acid can be prevented from exuding, the influence of the volume change of C-L-O acid during its solid-liquid transformation process can also be avoided. Fig. 1b also exhibits the smooth surface and layer edge of the raw EG. In contrast, after C-L-O acid is impregnated and absorbed into the micro pores of the raw EG (Fig. 1c, d, e and f), the surface and the layer edges of the samples become rougher, along with the layers turn thicker. Moreover, with the increase of the mass fraction of C-L-O acid, the micro pore structure of EG was gradually filled up. The growth trend of the thickness of SPCM layers can be observed from SPCMs 1 to SPCMs 7, corresponding to more C-L-O acid being absorbed into the micro pores of EG. It can be discovered that there are still some pore structures in the SPCMs, indicating that C-L-O acid in both SPCMs 1 (Fig. 1c) and SPCMs 3 (Fig. 1d) is not fully absorbed. When C-L-O acid is fully impregnated into EG, the SPCMs become saturated with the micro pores being filled up, as shown in Fig. 1e and f. Furthermore, compared with the well-developed network in raw EG, the pore structures in SPCMs are slightly damaged because of the mechanical stirring during the mixing process of EG and C-L-O acid.
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Chemical characterizations of C-L-O acid and the composite SPCMs Functional groups of C-L-O acid and the composite SPCMs, shown in Fig. 2, are characterized by FT-IR spectroscopy. In the spectrum of the C-L-O acid, the absorption peaks center at 2926, 2856 and 1466, 723 cm-1 signify the stretching vibrations of –CH3 and –CH2 groups in the acid molecules, respectively. The C=O stretching vibration of the acid is exhibited at 1711 cm-1. The peak at 1413 cm-1 may be attributed to C–O vibrations. The peaks at 1284, 936 and 722 cm-1 correspond to the in-plane bending vibration, the out-of-plane bending vibration, and the in-plane swinging vibration of the –OH group in the carboxyl group of the C-L-O acid, respectively. These absorption peaks are also observed in the spectrum of the composite SPCMs. Moreover, there is no obvious shift in the above main absorption peaks between the two spectra. The results demonstrate that C-L-O acid is just physically impregnated into pores of EG without any chemical reaction occurred during the whole process.
The phase structure of raw EG and the composite SPCMs are investigated by XRD. The obtained XRD patterns are shown in Fig. 3. The peaks at 26.5° and 54.6° are exhibited in the XRD patterns of EG, which are often taken as the feature peaks of 6 Page 6 of 18
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graphite (002) and (004) crystal phase [21], respectively. The diffraction peaks of EG are also clearly observed in the XRD patterns of the composite SPCMs. However, the (002) peak is slightly shifted to 26.7° as shown in the inset of Fig. 3, indicating that the typical interlayer distance (d002) is slightly squeezed from 0.3363 to 0.3338 nm by C-L-O acid. The broad peak presented at 20.0° can be attributed to the impregnation of C-L-O acid.
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Thermal reliability test of the composite SPCMs The leakage tests of the as-prepared composite SPCMs are carried out to characterize the thermal reliability by their shape-stable performance after thermal cycles above the melting temperature. Photographs of the composite SPCMs before and after the 30 thermal treatment cycles in 50 °C atmosphere are exhibited in Fig. 4. As seen from the photographs, the composite SPCMs trend to adhere with each other with the increase of C-L-O acid usage. In addition, after the 30 thermal treatment cycles in 50 °C atmosphere, C-L-O acid impregnated into the SPCMs 7 partly leak out, while other composites SPCMs from 1 to 6 show no obvious disclose. The results show that it is not necessary to continue increasing the mass ratio of C-L-O acid and EG, according to the detectable leakage of the composite SPCMs 7 shown in Fig. 4.
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Thermal stability characteristics of C-L-O acid and the composite SPCMs The thermal stability and weight loss percentage of C-L-O acid and the composite SPCMs from sample 1 to 7 are measured by TGA. The TGA curves are shown in Fig. 5. It is clearly shown that C-L-O acid and the composite SPCMs exhibit similar thermal stability characteristics. When the temperature increases up to about 120 °C in nitrogen atmosphere, liquid C-L-O acid begins to evaporate and the final weight loss percentage is almost 100% at about 300 °C. The curves of SPCMs from 1 to 7 are nearly the same as that of C-L-O acid. The weight loss from 120 to 300 °C is attributed to the removal of the C-L-O acid impregnated in the SPCMs. The residues of SPCMs from 1 to 7 also agree well with the loading ratios of raw EG in the composites, respectively. The results prove that the thermal stability of C-L-O acid is not influenced by raw EG. The thermal stability of SPCMs is satisfactory in the low temperature range, especially near room temperature, which can avoid the metamorphism of the composite SPCMs in the heat transfer process of practical radiant cooling system at about 15~20 °C.
Flammability of the composite SPCMs According to literature, EG is one of the most promising halogen-free flame retardants because of its low cost [37]. Generally, EG limits the mass transfer from the substrate to the flame and the heat transfer from the flame to the substrate. In order to further characterize the flammability of the composite SPCMs, the combustion tests [38] of the composite SPCMs are carried out in a muffle furnace from room 7 Page 7 of 18
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temperature to 600 °C at a heating rate of 2 °C/min in air atmosphere. The residues of the SPCMs after combustion are investigated by SEM. As shown in Fig. 6, the residues of the composite SPCMs are homogeneous and continuous, the structures of the residues become tight and dense compared with the raw EG shown in Fig. 1b. These structures may increase the trapping of the decomposed products, hence decrease flammable molecules during combustion. On the other hand, C-L-O acid impregnated in the SPCMs will absorbs heat, thus the temperature of the composite SPCMs’ surface may reduce.
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Thermal energy properties of C-L-O acid and the composite SPCMs DSC is used to investigate the phase transition temperatures and latent heat of C-L-O acid and the composite SPCMs from 1 to 7. Fig. 7 exhibits the melting-freezing DSC curves of the samples. For each sample, three DSC experiments were conducted with individual specimens randomly sampling from the beaker. The results show good repeatability, and the uniformity of the samples can be certified as well. The DSC curve of C-L-O acid presents an endothermic peak centered at 18.10 °C and an exothermic peak centered at 11.13 °C, corresponding to the melting point (Tm) and the freezing point (Tf) of C-L-O acid, respectively. The melting enthalpy (∆Hm) and the freezing enthalpy (∆Hf) of C-L-O acid are calculated to be 115.91 and 117.04 J/g, respectively.
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The DSC curves of SPCMs from 1 to 7 are similar to that of C-L-O acid. One endothermic peak and one exothermic peak can be observed, indicating that the thermal energy properties of SPCMs almost come from the C-L-O acid. However, when C-L-O acid is absorbed into EG, the melting and freezing point of the composites are increased by about 1 °C, owing to the melting/freezing process limitation of C-L-O acid by the micro pores existing in raw EG. Moreover, all the melting temperatures of SPCMs from 1 to 7 fluctuate near 19 °C, which further proves that the usage of EG in the experiments make no difference to the thermal energy properties of SPCMs. As is mentioned in the introduction section, the suitable phase transition temperature of the PCMs for practical applications should be around 20 °C, it is obvious that the SPCMs we prepared are acceptable with their melting point around 19 °C. Meanwhile, according to the DSC results, the mean value of the ∆Hm and ∆Hf can also be calculated, which are given in Table 1, as well as the phase transition temperatures of C-L-O acid and the composite SPCMs from 1 to 7. The ∆Hm and ∆Hf (115.91 and 117.04 J/g) of C-L-O acid reflect the optimal latent heat of all the samples. As shown in Table 1, when the mass ratio of EG to C-L-O acid is 1:5 (SPCMs 1), The ∆Hm and ∆Hf (93.12 and 109.05 J/g) are at low level. Thereafter, with more C-L-O acid being impregnated into EG, the latent heat of SPCMs gradually improved, which can be judged from the increase of the ∆Hm and ∆Hf. The ∆Hm and ∆Hf (113.55, 115.34 and 116.52, 116.82 J/g) of SPCMs 5 and 6 are both very close to 8 Page 8 of 18
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the value of the original C-L-O acid (115.91 and 117.04 J/g), indicating that C-L-O acid is enough absorbed into EG in these two SPCMs to ensure high latent heat of the composites, which further prove the results from the SEM images in Fig. 1. By unceasingly increasing the loading ratio of C-L-O acid to 1:35 (SPCMs 7), it can be found that the ∆Hm and ∆Hf (114.65 and 116.56 J/g) slightly drop, possibly because SPCMs 6 has already reached saturation, the further absorbed C-L-O acid just deposit on the surface of EG sheets or fill into the large space between EG stacks. The redundant C-L-O acid cannot be loaded stably on the composites, so that the latent heat of SPCMs 7 is found slightly decreased. Moreover, the DSC results also indicate that there is no chemical reaction occurred between C-L-O acid and EG in the preparation of SPCMs, which is consistent with the result of FT-IR spectra.
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Thermal conductivity characteristics of EG and the composite SPCMs The thermal conductivity of C-L-O acid, raw EG, and the composite SPCMs from 1 to 7 are analyzed using Hot Disk by a transient plane heat source method. The results are shown in Fig. 8. All the Hot Disk data presented in our work are average values over triple measurements of each sample. The thermal conductivity of original C-L-O acid is 0.14 W/m⋅K. However, with the addition of EG, significant improvements of thermal conductivity of SPCMs can be clearly observed. The thermal conductivity of SPCMs 7 reaches up to 1.37 W/m⋅K. Moreover, with the increasing usage of EG, the thermal conductivity of SPCMs is also obviously enhanced. When more EG is added and the mass ratio of EG to C-L-O acid reaches 1:5, the thermal conductivity of SPCMs 1 is observed to be further enhanced to 3.15 W/m⋅K, which is 22.5 times higher than that of original C-L-O acid (0.14 W/m⋅K) and closes to that of raw EG (3.43 W/m⋅K). All results indicate that the thermal conductivity of SPCMs can be highly enhanced by EG. A large amount of well-developed pore network structure in raw worm-like EG exhibited in Fig. 1a act as the carrier during the heat transfer process. Therefore, the solid-phase SPCMs enhance the thermal transmission of the liquid C-L-O acid.
According to the summarization of ∆Hm and ∆Hf data from Table 1 and thermal conductivity results from Fig. 8, the trends of the thermal properties of the composite SPCMs can be observed. With the increase of the mass fraction of C-L-O acid (SPCMs from 1 to 7), the ∆Hm and ∆Hf of the composite SPCMs gradually increase from 93.12 to 114.65 J/g and from 109.05 to 116.56 J/g, respectively, while their corresponding thermal conductivity continually declines from 3.15 to 1.37 W/m⋅K. The increase trend of ∆Hm is more sharply than that of ∆Hf. Fig. 8 also shows that the line of ∆Hm and the line of thermal conductivity are intersected where the mass ratio of EG to C-L-O acid approaching 1:15 (SPCM 3). In the application of thermal energy storage, high latent heat is necessary as well as excellent thermal conductivity. Therefore, SPCMs 3 is finally selected as the optimal composite SPCMs with its ∆Hm of 109.18 J/g, ∆Hf of 115.28 J/g, and thermal conductivity of 1.95 W/m⋅K. 9 Page 9 of 18
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Conclusions We have prepared C-L-O acid with a low melting point at around 20 °C. The C-L-O acid is then mechanically mixed with pre-dried worm-like EG to obtain the composite shape-stabilized PCMs. The worm-like EG is used as both a heat transfer intensifier and a shape-stabilized container of C-L-O acid. Morphological and chemical characterizations are carried out by SEM, XRD and FT-IR. Thermal energy properties are characterized by TGA, DSC and Hot Disk. The pore size of the EG network is at micro scale. C-L-O acid is impregnated into raw EG only by capillary forces to make SPCMs maintain solid state without any chemical reaction occurred. The relative large volume change of C-L-O acid can also be prevented by the pore network structure of EG. TGA results prove that C-L-O acid is stably absorbed into EG. The latent heat of SPCMs increases with the addition of C-L-O acid from SPCMs 1 to 6 and slightly drops in SPCMs 7. When the mass ratio of EG to C-L-O acid is 1:30, ∆Hm of SPCMs 6 is measured to be 115.34 J/g, which is close to the value of the original C-L-O acid (115.91 J/g). The latent heat of SPCMs is greatly improved by the adding of C-L-O acid. On the contrary, the thermal conductivity of SPCMs declines with the mass fraction increase of C-L-O acid. However, thermal conductivity of SPCMs 7 is still 1.37 W/m⋅K, which is 9.8 times higher than that of the original C-L-O acid (0.14 W/m⋅K). With comprehensive consideration of latent heat and thermal conductivity, SPCMs 3 is deemed to be the optimal mass ratio of EG to C-L-O acid among all the SPCMs, whose ∆Hm is 109.18 J/g, and thermal conductivity is 1.95 W/m⋅K. The microstructures of the residues after combustion indicate that the homogeneous and dense residue decreased the flammability of the composite SPCMs. These composite SPCMs can be easily, inexpensively mass prepared (the price of EG, C-L-O acid are both at low-level) and further used as potential materials for practical radiant cooling system due to their satisfactory thermal properties, including favorable thermal conductivity, uninflammable property, high latent heat, good thermal reliability and stability. Acknowledgements The authors gratefully acknowledge financial supports by the National Basic Research Program of China (2013CB932500), the National Natural Science Foundation of China (No. 51276114, 21171117 and 61376003), Program for New Century Excellent Talents in University (NCET-12-0356), the Program of Shanghai Academic/Technology Research Leader (15XD1525200), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. We also acknowledge analysis support from the Instrumental Analysis Center of Shanghai Jiao Tong University and the Center for Advanced Electronic Materials and Devices of Shanghai Jiao Tong University.
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Fig. 1 SEM images of (a, b) raw EG, (c) SPCMs 1, (d) SPCMs 3, (e) SPCMs 5, and (f)
SPCMs 7.
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Fig. 2 FT-IR spectra of C-L-O acid and the composite SPCMs. SPCMs 6 was used for
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the measurement.
Fig. 3 XRD patterns of EG and the composite SPCMs. SPCMs 6 was used for the
measurement. The inset shows the enlarged image from 25° to 28°.
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Fig. 4 Photographs of the composite SPCMs placed on filter paper before and after the
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Fig. 5 TGA curves of C-L-O acid and the composite SPCMs.
Fig. 6 SEM images of the residues after 600 °C combustion: (a) SPCMs 1, (b) SPCMs
3, (c) SPCMs 5, and (d) SPCMs 7.
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Fig. 7 DSC curves of C-L-O acid and the composite SPCMs.
Fig. 8 Phase change enthalpy (∆H) and thermal conductivity of the composite SPCMs.
The error bar represents the standard error of three independent experiments.
Table 1 Phase change parameters of C-L-O acid and the composite SPCMs. Sample Mass ratio Tm Tf ∆Hm ∆Hf (°C) (EG wt%) (°C) (J/g) (J/g) SPCMs 1 1:5 (16.7) 18.96 ± 0.03 93.12 ± 0.18 13.64 ± 0.01 109.05 ± 1.22 SPCMs 2 1:10 (9.1) 18.50 ± 0.03 105.88 ± 0.35 12.76 ± 0.09 114.16 ± 0.51 SPCMs 3 1:15 (6.3) 19.04 ± 0.09 109.18 ± 0.20 12.19 ± 0.38 115.28 ± 0.13 SPCMs 4 1:20 (4.8) 18.78 ± 0.71 111.70 ± 0.67 11.68 ± 0.01 115.96 ± 0.62 SPCMs 5 1:25 (3.9) 19.18 ± 0.01 113.55 ± 0.47 12.07 ± 0.19 116.53 ± 0.25 SPCMs 6 1:30 (3.2) 18.78 ± 0.94 115.34 ± 0.55 11.87 ± 0.02 116.82 ± 0.06 SPCMs 7 1:35 (2.8) 19.15 ± 0.18 114.65 ± 0.03 11.78 ± 0.49 116.56 ± 0.01 C-L-O acid 0:1 (0) 18.10 ± 0.04 115.91 ± 0.11 11.13 ± 0.51 117.04 ± 0.12 17 Page 17 of 18
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Highlights Thermal conductivity improvement of low melting point organic PCMs is investigated for the first time. Expanded graphite (EG) is used to be a heat transfer and a shape-stabilized container. Thermal conductivity of the shape-stabilized PCMs is obviously enhanced. Latent heat of the shape-stabilized PCMs show little change compared with original PCM. An optimal mass ratio of EG and organic PCMs is discussed.
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