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expanded graphite composite form-stable phase change material for thermal energy storage
Solar Energy Materials & Solar Cells 127 (2014) 122–128
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Tetradecanol/expanded graphite composite form-stable phase change material for thermal energy storage Ju-Lan Zeng a,n, Juan Gan a, Fu-Rong Zhu a, Sai-Bo Yu b, Zhong-Liang Xiao a, Wen-Pei Yan a, Ling Zhu a, Zhen-Qiang Liu a, Li-Xian Sun a, Zhong Cao a a Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, School of Chemistry and Biological Engineering, Changsha University of Science and Technology, Changsha 410114, People's Republic of China b China Tobacco Hunan Industrial Corporation, Changsha 410007, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 October 2013 Received in revised form 3 April 2014 Accepted 15 April 2014
Natural flake graphite was chemically intercalated to prepare expandable graphite. The expandable graphite was then expanded by means of microwave irradiation to obtain expanded graphite (EG). Tetradecanol (TD)/EG composite form-stable phase change materials (PCMs) were prepared by mixing TD with EG through an autoclave method. The highest loading of TD in the composite form-stable PCMs with good form-stability was 93 wt%. The composite form-stable PCMs exhibited excellent thermal energy storage capacity. The melting enthalpy (ΔHm) and crystallization enthalpy (ΔHc) of the composite form-stable PCM containing 93 wt% TD were 202.6 and 201.2 J/g, respectively. However, the solid–solid phase transition of TD in the composite form-stable PCMs was hindered as TD was strongly absorbed by EG. As a result, the ΔHm and ΔHc of the composite form-stable PCMs were slightly lower than the theoretical value. The thermal conductivity of the composite form-stable PCMs was greatly enhanced by EG and was increased with the increasing of EG loading. The thermal conductivity of the composite form-stable PCM containing 7 wt% EG attained 2.76 W/m K. Besides, effects of the prepared EG on properties of the composite form-stable PCMs were compared with those of EG derived from commercially obtained expandable graphite. & 2014 Elsevier B.V. All rights reserved.
Keywords: Form-stable phase change materials Expanded graphite Tetradecanol Thermal energy storage Thermal conductivity
1. Introduction Thermal energy storage technology, which can solve the problem in time and spatial mismatch between thermal energy supply and demand, is very important in solar thermal energy applications [1,2]. Thermal energy storage materials are the core of thermal energy storage technology. Phase change materials (PCMs) can absorb or release large amounts of thermal energy by changing their phase from one to another [3] and thus possess merits of high energy storage density in small temperature intervals [4]. Hence, PCMs are the most important latent heat energy storage materials in solar thermal energy applications [1, 2]. Solid–solid PCMs and solid–liquid PCMs are the two common kinds of PCMs when the changing of volume is concerned. However, the solid–liquid PCMs posses the advantage of better phase change dynamics properties than those of solid–solid PCMs [1]. Solid–liquid PCMs can be divided into organic and inorganic PCMs. Organic solid–liquid PCMs possess advantages of high thermal energy storage density, low or no undercooling, chemical and thermal
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Corresponding author. Tel./fax: þ 86 731 85258733. E-mail address: [email protected] (J.-L. Zeng).
http://dx.doi.org/10.1016/j.solmat.2014.04.015 0927-0248/& 2014 Elsevier B.V. All rights reserved.
stability and no corrosives [1]. However, the main drawback that hinders the application of organic solid–liquid PCMs is their unacceptably low thermal conductivity [1]. Lower thermal conductivity would result in slower thermal energy storing or releasing speed and lower thermal energy utilization efficiency. The thermal conductivity of PCMs is important in the field of solar thermal energy applications, since solar radiation is varied seasonally and daily. Hence, high thermal conductivity is appreciated when PCMs are applied to store thermal energy that comes from solar radiation during sun shining period. As a result, the thermal conductivity of organic solid–liquid PCMs has to be enhanced [5]. The most popular method to enhance the thermal conductivity of solid–liquid PCMs is adding high conductivity fillers such as powder [6], metal foam [7], carbon fibers [8], carbon nanofibers [9], carbon nanotubes [10], exfoliated graphite nanoplatelets [11], expanded graphite (EG) [12], etc. Among them, EG exhibits various effects on the improvement of the thermal conductivity of PCMs. It has been reported that the apparent thermal conductivity of nitrates/10 wt% EG mixture PCM was increased by about 30–40% [13]. The thermal conductivity of SiO2/paraffin/EG composite was 94.7% higher than that of paraffin [14] and the thermal conductivity of palmitic acid was improved to 0.60 W/m K by 20 wt% of EG [15]. On the other hand, the thermal conductivity
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of stearic acid (SA) (0.30 W/m K) could be increased by 266.6% by adding 10 wt% of EG [16]. The thermal diffusivity of SA/EG composites containing 20 wt% EG was 10 times higher than that of pure SA [12]. The thermal conductivity of CaCl2∙6H2O containing 50 wt% EG (8.796 W/m K) was 14 times higher than that of pure CaCl2∙6H2O (0.596 W/m K) [17]. Furthermore, the thermal conductivity of graphite matrix/paraffin composite PCM was 20–130 times greater than that of pure paraffin ( 0.2 W/m K) [18]. Based on the reported results, we believe that the thermal conductivity enhancement of EG-doped composite PCMs is affected by properties of EG, the preparation procedure of the composite PCMs and the interactions between pure PCMs and EG. As a result, the investigation of the thermal conductivity enhancement of EG on solid–liquid PCMs is a hot topic. Furthermore, to the best of our knowledge, the thermal conductivity of fatty alcohol has not been enhanced by EG. Fatty alcohol is a kind of important organic solid– liquid PCM with high thermal energy storage density. The polarity of fatty alcohol is weaker than that of fatty acid. Furthermore, fatty alcohols exhibit a solid–solid phase transition which is close to the solid–liquid phase transition [19]. The solid–solid phase transition is also beneficial to thermal energy storage but its transition properties may be affected by fillers with high specific area. Hence, it is worthwhile to investigate the phase change properties and the thermal conductivity of fatty alcohol/EG composite PCMs. In this paper, EG was prepared from natural flake graphite and effects of the preparation procedure on the expanded ratio were investigated. Tetradecanol (TD) was selected as solid–liquid PCM and TD/EG composite form-stable PCMs were prepared via an autoclave method. The structure and the thermal properties of the prepared composite form-stable PCMs were investigated and results are reported here. Besides, effects of the prepared EG on the thermal energy storage properties and thermal conductivity of form-stable PCMs were compared with those of EG derived from commercially obtained expandable graphite.
2. Experimental 2.1. Materials Natural flake graphite with average size of 300 μm was obtained from Qingdao Runba Graphite Co., Ltd. China. Expandable graphite with average particle size of 300 μm and expansion ratio of 200 mL/g was supplied by Qingdao Graphite Co., Ltd., China. All other reagents were of analytical grade and were used without further purification. Deionized water was used throughout the experimental process. 2.2. Preparation of EG The EG was prepared according to reference [20] with some modification. In general, a certain amount of natural flake graphite was added to a solution of glacial acetic acid and concentrated nitric acid (65%) under stirring in a 35 1C water bath to form a welldistributed mixture. A certain amount of potassium permanganate was added to the mixture while the stirring was continued. The mixture was stirred at this temperature for a certain time. During this time, nitric acid/acetic acid/graphite intercalation compound (NA–GIC) was formed. Then the NA–GIC was filtered, washed thoroughly with water, and dried at 50 1C under vacuum to obtain dry expandable graphite. The obtained expandable graphite was then radiated for 60 s in a domestic microwave oven (WP750, GALANZ) to obtain EG. The expanded volume was applied as an evaluation standard to modify the ratio of raw materials and reaction time. The expansion ratio of EG was measured as follows: 0.20 g expandable graphite in a 50 mL beaker was irradiated for 60 s, and then the volume of the obtained EG was precisely read. The procedure was
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repeated three times for each sample and the relative deviation was less than 6%. The obtained mean volume value divided by 0.20 g resulted in the expansion ratio of EG as mL/g. 2.3. Preparation of TD/EG composite form-stable PCMs A certain amount of the EG, TD and anhydrous ethanol were mixed in an autoclave. The autoclave was sealed and heated at 120 1C for 24 h and then cooled to ambient temperature. The mixture was then taken out from the autoclave and heated at 50 1C under reduced pressure to evaporate the ethanol and was then dried under vacuum for 24 h to obtain TD/EG composite form-stable PCMs. The samples were designated as TD/ EG-1–5, corresponding to TD/EG composite form-stable PCMs containing 93 wt%, 90 wt%, 85 wt%, 80 wt% and 60 wt% of TD, respectively. In order to compare the effects of the prepared EG on the thermal energy storage properties and thermal conductivity of the formstable PCMs with EG obtained from a known method (namely, k-EG), another series of TD/k-EG composite form-stable PCMs were prepared as follows. Expandable graphite was dried at 80 1C for 24 h and was then radiated for 60 s in the microwave oven to obtain k-EG. TD/k-EG composite form-stable PCMs were then obtained according to the preparation procedure of TD/EG composite form-stable PCMs. The samples were designated as TD/k-EG-1–4, corresponding to the TD/k-EG composite form-stable PCMs containing 90 wt%, 80 wt%, 70 wt% and 60 wt% of TD, respectively. 2.4. Characterization FT-IR spectra were recorded on a FT-IR spectrometer (AVATAR360) using KBr pellet (4000–400 cm 1). The surface morphology investigation was carried out on a scanning electron microscope (SEM, JEOL JSM-6380). Before the SEM investigation, samples were sputtered with gold. Powder X-ray diffraction (XRD) experiments were performed on a Rigaku D/max-gb X-ray diffractometer with a monochromatic detector. Copper Kα radiation was used, with a power setting of 30 kV and 30 mA, and a scanning rate of 51/min. The thermal stability of samples was characterized by means of thermogravimetry (TG)/differential thermal analysis (DTA) on a thermogravimetric analyzer (NETZSCH STA 409 PG/PC) from room temperature to 800 1C at the heating rate of 10 1C/min and with N2 as carrier gas. The analyzer was calibrated using CaC2O4∙H2O (99.9%) prior to the experiment. Differential scanning calorimetry (DSC, TA, Q2000) was used to investigate the thermal energy storage properties of the prepared composite form-stable PCMs over the temperature of 10–80 1C with the heating and cooling rate of 10 1C/min in nitrogen atmosphere. Prior to the DSC experiments, the instrument was calibrated using indium (99.999%) as the standard material. The thermal conductivity of samples at room temperature was measured by means of steady-state heat flow method using a thermal conductivity tester (DRX-II-RW, Xiangtan Huafeng Instrument Manufacturing Co., Ltd, China). Before the measurement, samples were grounded and pressed under 10 MPa to obtain disks (Φ ¼30 mm). The hot plate of the tester was set at 35 1C and the cold plate was cooled by water of 15 1C. A disk was mounted between the two plates. The thermal conductivity of the disk was measured when the temperature of the two plates reached stable values for more than 1 h.
3. Results and discussions 3.1. Preparation of EG The preparation of EG was conducted according to reported procedure [20] with some modifications. The modifications were
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that, in the present work, glacial acetic acid was used instead of acetic anhydride and the expansion of the expandable graphite was completed by microwave irradiation in a domestic microwave oven rather than by heating in a muffle furnace. When acetic anhydride was applied, it should be mixed with concentrated nitric acid very carefully. Otherwise, the release of N2O5, which is the product of the side reaction between acetic anhydride and concentrated nitric acid, would occur violently [20]. In the present work, acetic anhydride was replaced by glacial acetic acid as acetic acid is the actual intercalator [20] and could be mildly mixed with concentrated nitric acid without the release of N2O5. The formation of NA–GIC was confirmed by XRD patterns obtained before and after the intercalation of natural flake graphite (Fig. 1). It could be seen from the figure that the pattern of NA–GIC exhibited two weak shoulder peaks at 2θ ¼27.51 and 53.51 (labeled with arrows in the figure), which were originated from NA–GIC [20]. Microwave assisted expansion of expandable graphite possesses the merits of quick heating process and high energy input density [21]. Our experiment results showed that 60 s of high energy irradiation was enough to obtain maximum expansion ratio.
Fig. 1. XRD patterns of natural flake graphite and NA-GIC.
Expandable graphite with high expansion ratio is preferred in the presented work. As a result, it is worthwhile to investigate the effects of raw materials ratio and reaction time on the expansion ratio. The results are shown in Table 1. It is clear that the best raw materials ratio was: flake graphite/acetic acid/nitric acid/ KMnO4 ¼ 1/4/4/0.25 (g/mL/mL/g), and the best reaction time was 90 min. The maximum expansion ratio could attain 250 mL/g. The SEM images of the natural flake graphite and the prepared EG are shown in Fig. 2. The laminated structure of the natural flake graphite can be clearly seen in Fig. 2a and b. After it was expanded, the laminated structure was replaced by worm-like structure (Fig. 2c and d). The surface morphology of the prepared EG exhibited well expanded structure that permits full and regular expansion in thedirection of c-axis. 3.2. Preparation and form-stability of the TD/EG composite form-stable PCMs The TD/EG composite form-stable PCMs were prepared according to an autoclave method [22]. The principle of the method is that the porous supporting materials absorb PCMs slowly under liquid–vapor equilibrium and hence PCMs could be uniformly absorbed in the porous materials. We tried at first mixing TD with EG in an autoclave without ethanol. However, results showed that the TD could not be uniformly mixed with EG. This phenomenon might be ascribed to the low vapor pressure of TD at 120 1C and the strong adsorption on EG for TD. In order to improve the uniformity of the composite form-stable PCMs, a certain amount of ethanol was added into the autoclave. The results showed that the TD could be mixed well with EG. The uniformity was further confirmed by a series of DSC experiments in which samples were taken from the upper, middle and bottom of the autoclave. The existence of ethanol could decrease the adsorption on EG of TD since EG could also adsorb ethanol, avoiding the adsorption of too much TD in too less EG, and hence promoted the uniformity of TD in EG. The TD/EG composite form-stable PCMs were pressed under 2 MPa to obtain discs. The discs were heated at 50 1C for 24 h to examine the form-stability. The results showed that all TD/EG composite form-stable PCMs exhibited good form stability without leakage of TD.
Table 1 The dose of raw materials used to prepare expandable graphite and the corresponding expansion ratio.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Flake graphite (g)
HNO3 (mL)
HAC (mL)
KMnO4 (g)
Flake graphite/mixed acid (g/mL)
V HNO3 /VHAc (mL/mL)
Reaction time (min)
Flake graphite /KMnO4 (g/g)
Expansion ratio (mL/g)
1.0029 0.9998 1.0056 1.007 1.001 1.0042 1.0046 1.0047 1.0074 1.0103 1.0021 1.003 1.0007 1.0011 1.0004 1.0002 1.0018 1.0019 1.0026 1.0013 1.0013
1 2 3 4 5 6 6 5.3 4 2.7 2 4 4 4 4 4 4 4 4 4 4
1 2 3 4 5 6 2 2.7 4 5.3 6 4 4 4 4 4 4 4 4 4 4
0.2044 0.1997 0.2017 0.2057 0.2042 0.2043 0.2018 0.2045 0.2099 0.2008 0.2047 0.2018 0.2 0.202 0.2004 0.2018 0.1014 0.1504 0.202 0.2505 0.3001
1/2 1/4 1/6 1/8 1/10 1/12 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8
1 1 1 1 1 1 3/1 2/1 1/1 1/2 1/3 1 1 1 1 1 1 1 1 1 1
90 90 90 90 90 90 90 90 90 90 90 15 30 60 90 120 90 90 90 90 90
1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.1 1/0.15 1/0.2 1/0.25 1/0.3
160 185 233 237 227 232 82 163 234 210 187 135 146 176 234 204 182 199 233 257 210
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Fig. 2. SEM images of natural flake graphite (a and b), EG (c and d) and TD/EG-2 (e and f).
3.3. Morphology and structure investigation of the TD/EG composite form-stable PCMs The morphology of the TD/EG composite form-stable PCMs was characterized by SEM. The SEM images of TD/EG-2 are shown in Fig. 2e and f. It can be clearly observed that a layer of TD was adsorbed on the surface of the worm-like EG. The graphite layer became thicker compared to pure EG (Fig. 2c and d). The pores of the EG were not fully occupied by TD. The form-stability of the TD/EG composite form-stable PCMs under certain pressure comes from these partially occupied pores. The IR spectra of TD and the TD/EG composite form-stable PCMs are shown in Fig. 3. The broad band centered at 3395 cm 1 in the spectrum of TD was caused by the stretching vibration of hydroxyl group of alcohol. The vCH and δCH absorption bands of TD were exhibited at 2917, 2846 and 1465, 721 cm 1, respectively. The C–O stretching vibration of primary alcohol was exhibited at 1062 cm 1. The absorption peaks in the spectrum of the composite form-stable PCM are similar to that of TD. The results indicated that TD was just physically mixed with EG and no chemical interaction occured. The phase structure of the composite form-stable PCMs was investigated by XRD and the obtained XRD patterns are drawn in Fig. 4. A strong diffraction peak located at 2θ ¼26.391 was exhibited in the XRD pattern of EG, which could be attributed to the feature peak (002) of graphite [23]. The diffraction peak of EG
was clearly exhibited in the XRD patterns of the TD/EG composite form-stable PCMs. However, the peak was slightly shifted to 2θ ¼26.471, indicating that the lattice spacing of graphite layer was slightly squeezed by TD. The diffraction patterns of two TD/EG composite form-stable PCMs, TD/EG-2 and TD/EG-3, were similar. However, closer inspection of the XRD patterns of TD and the TD/EG composite form-stable PCMs reveals that two peaks located at 2θ ¼ 21.341 and 21.751 were exhibited in the XRD pattern of TD. However, these two peaks were merged into a single peak that located at 2θ ¼ 21.751 in the XRD patterns of the composite formstable PCMs. It seems that in the XRD patterns of TD/EG the composite form-stable PCMs, the peak located at 2θ ¼21.341 was hidden by the peak located at 2θ ¼21.751. Furthermore, the stronger diffraction peak located at 2θ ¼24.151 in the XRD pattern of TD was weakened in the XRD patterns of the TD/EG composite form-stable PCMs, while the weaker diffraction peak located at 2θ ¼24.711 in the XRD pattern of TD was strengthened in the XRD patterns of the TD/EG composite form-stable PCMs. It is known that solid fatty alcohols exhibit two phases [19]. Liquid fatty alcohols are crystallized at its' melting point to form hexagonal solid phase (SHEX). The SHEX is metastable and would turn to orthorhombic solid phase (SORT) at a slightly lower temperature. The TD in the composite TD/EG form-stable PCMs was strongly absorbed by EG and was constrained in the pores of EG. As a result, the transition between SHEX and SORT was hindered. The pure TD was preferred in SORT while the TD in the TD/EG composite
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Fig. 3. IR spectra of TD and TD/EG-3.
Fig. 5. TG–DTA curves of TD and the composite form-stable PCMs.
Fig. 4. XRD patterns of TD, EG and the composite form-stable PCMs.
form-stable PCMs was constrained and was mainly existed as SHEX [24]. As a result, the XRD patterns of pure TD and the TD/EG composite form-stable PCMs showed the differences mentioned above. 3.4. Thermal stability characteristics of the TD/EG composite form-stable PCMs The thermal stability of the TD/EG composite form-stable PCMs was investigated by TG/DTA and the TG and DTA curves are shown in Fig. 5a and b, respectively. It is clear that both TD and the TD/EG composite form-stable PCMs exhibited similar thermal stability characteristics. The weight loss that started at 180 1C was caused by the evaporation of TD. The residue of the TD/EG composite form-stable PCMs agreed well with the loading of EG in the TD/EG composite form-stable PCMs. There were two endothermic peaks exhibited in the DTA curves. The peak that appeared at lower temperature was corresponded to the melting of TD while the peak appeared at higher temperature was corresponded to the evaporation of TD. The results implied that the thermal stability of TD was not affected by EG, and the thermal stability of the TD/EG composite form-stable PCMs was satisfactory as far as the solid– liquid phase change temperature of TD was concerned. 3.5. Thermal energy storage properties of the TD/EG composite formstable PCMs The thermal energy storage properties of the TD/EG and TD/k-EG composite form-stable PCMs were investigated by means
of DSC. For each sample, three DSC experiments were performed with individual samples taken from different sites of the autoclave. The results exhibited good repeatability and the homogeneity of samples could be confirmed. The DSC curves are shown in Fig. 6. There was one endothermic peak appeared on the heating DSC curve of TD. On cooling run, two exothermic peaks were exhibited on the DSC curve. The peak located at higher temperature was the transition from liquid to SHEX, while the peak located at lower temperature was corresponded to the transition from SHEX to SORT [19]. The DSC curve of TD showed slight undercooling for the liquid–solid transition while the undercooling of transition from SHEX to SORT was apparent. It seems that the peak located at lower temperature was due to disorder as the temperature of the sample increases when the SHEX–SORT transition starts. Actually, the increasing of the temperature was caused by the undercooling of SHEX–SORT transition. The differential scanning calorimeter (TA, Q2000) used in this work is a heat flux DSC. During the cooling period, the temperature of TD in SHEX state would drops to a temperature lower than the SHEX–SORT transition point due to undercooling. When the transition started, a large amount of heat as released and resulted in a temperature rise. A normal DSC curve, which is inserted in Fig. 6, could be obtained when the x-axis of the DSC curve of TD was set as time rather than temperature. The DSC curves of the TD/EG composite formstable PCMs were similar to that of TD. One endothermic peak on the heating DSC curve and two exothermic peaks on the cooling DSC curve can be found, indicating that the thermal energy storage property of the composite form-stable PCMs comes from TD. However, the SHEX–SORT transition of the TD/EG composite form-stable PCMs changed considerably and the undercooling was disappeared on the DSC curves. The exothermic peak corresponding to the SHEX–SORT transition of TD was shifted to a lower temperature and the shift became larger as the loading of EG was increased. The results further showed that the SHEX–SORT transition
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Fig. 7. Phase change enthalpy and thermal conductivity of the composite form-stable PCMs.
Fig. 6. DSC curves of TD and the composite form-stable PCMs.
Table 2 Phase change parameters of TD and the composite form-stable PCMs. Sample
EG/TD (wt%/ wt%)
Tonset-m (1C)
ΔHm (J/g)
Tonset-c (1C)
ΔHc (J/g)n
TD TD/EG-1 TD/EG-2 TD/EG-3 TD/EG-4 TD/EG-5 TD/k-EG-1 TD/k-EG-2 TD/k-EG-3 TD/k-EG-4
0/100 7/93 10/90 15/85 20/80 40/60 10/90 20/80 30/70 40/60
35.87 35.35 35.13 36.27 34.96 34.57 36.30 35.90 35.87 35.41
236.1 202.6 196.9 193.1 174.1 131.1 198.1 169.6 151.2 132.2
32.3 34.93 34.73 35.57 34.77 34.66 34.79 35.33 35.44 35.57
234.5 201.2 196.4 191.6 172.8 129.2 196.1 163.0 147.2 127.85
n The total phase change enthalpy of the liquid–SHEX and SHEX–SORT transition of the cooling run.
of TD in the composite form-stable PCMs was greatly restrained since the TD was constrained in the pores of EG, which was in accordance with the results of XRD investigation. Form the DSC experiments, the mean value of the melting enthalpy (ΔHm), crystallization enthalpy (ΔHc, the total phase change enthalpy of the liquid- SHEX and SHEX–SORT transition of the cooling run) and the extrapolated peak onset temperatures during melting (Tonset-m) and solidifying (Tonset-c) were obtained. These data are shown in Table 2. It could be seen that the Tonset-m was fluctuated within the temperature range of 34–36 1C and there was no regularity that could be observed. The Tonset-c of TD was about 3.5 1C lower than the Tonset-m of TD, indicating that there existed a certain degree of undercooling. However, the undercooling was diminished as EG was added. Furthermore, there was a regularity that could be found. That is, the higher the loading of
EG, the lower the undercooling. When the loading of EG attained 40%, the undercooling was vanished. On the other hand, ΔHm and ΔHc were decreased as the loading of TD was decreased. This is understandable since ΔHm and ΔHc were originated from the phase change of TD. However, from Fig. 7, it could be seen that ΔHm and ΔHc of the composite form-stable PCMs were lower than the theoretical value, which was calculated by multiplying ΔHm or ΔHc of TD by the loading of TD in the TD/EG composite form-stable PCMs. The results of XRD investigation have implied that the phase change enthalpy of the SHEX–SORT transition would be decreased, though it was very small comparing with that of the SHEX–liquid transition. Furthermore, TD was adsorbed on the surface on EG due to the strong adsorption on EG by TD. The absorption was so tight that a thin layer of TD could not change its phase. As a result, ΔHm and ΔHc of the TD/EG composite form-stable PCMs were slightly lower than the theoretical value. The adsorbed TD layer could also act as seeds of solid phase. Hence, the undercooling of the TD/EG composite formstable PCMs could be diminished by the addition of EG. The DSC curves of the TD/k-EG composite form-stable PCMs are also shown in Fig. 6. The thermodynamic parameters such as ΔHm, ΔHc, Tonset-m and Tonset-c derived from DSC experiments are shown in Table 2. It could be seen that the DSC curves of TD/k-EG composite form-stable PCMs were similar to that of TD/EG composite form-stable PCMs. The exothermic peak corresponding to the SHEX–SORT transition of TD in TD/k-EG composite form-stable PCMs showed the same trend as that of TD/EG composite formstable PCMs. Furthermore, ΔHm and ΔHc of the TD/k-EG composite form-stable PCMs were also equal to that of TD/EG composite form-stable PCMs. These results indicated that the effect of the prepared EG on the phase change properties of the form-stable PCMs was similar to EG obtained from the known method. 3.6. Thermal conductivity of the TD/EG composite form-stable PCMs The thermal conductivity of the composite form-stable PCMs is drawn in Fig. 7. The results showed that the thermal conductivity of the TD/EG composite form-stable PCMs could be greatly enhanced by EG. The thermal conductivity of pure TD was 0.433 W/m K. However, the thermal conductivity of the TD/EG composite form-stable PCM containing 7 wt% of EG attained 2.76 W/m K, which was 5.37 times higher than that of pure TD. The thermal conductivity of the TD/EG composite form-stable PCMs could be further enhanced to 5.71 W/m K when 40 wt% of EG was added. The figure shows that the line of ΔH and the line of thermal conductivity were intersected at where the loading of TD
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was about 85 wt% (corresponding to 15 wt% of EG). The figure also showed that, with the increasing of EG loading, the thermal conductivity of the TD/EG composite form-stable PCMs was accelerated much rapid at lower EG loading (o 20 wt%) than at higher EG loading (420 wt%). Besides, the thermal energy storage density is also a key factor for the application of PCMs. As a result, it is recommended that the optimum weight ration of TD in the TD/EG composite form-stable PCMs should be 80 wt% (corresponding to 20 wt% of EG) as far as the phase change enthalpy, thermal conductivity and form-stability are concerned. The thermal conductivity of TD/k-EG composite form-stable PCMs is also shown in Fig. 7. Obviously, the thermal conductivity of the composite form-stable PCMs was also greatly enhanced by k-EG. From the figure we could find that the thermal conductivity enhancement ability of k-EG was almost equal to that of the prepared EG and the variation trend in the thermal conductivity of both kinds of form-stable PCMs was similar. Hence, we could conclude that the thermal conductivity enhancement ability of the prepared EG was as much as that of EG obtained from known methods. 4. Conclusions In general, we have prepared expandable graphite from natural flake graphite by chemical intercalation reaction and the expandable graphite was expanded to worm-like EG by microwave irradiation. The EG was then mixed with TD through an autoclave method to obtain TD/EG composite form-stable PCMs with the assistance of ethanol. TD was absorbed into the pores of the worm-like EG and the loading of TD in the composite form-stable PCMs with good form-stability could be as higher as 93 wt%. The solid phase of TD in the composite form-stable PCMs was mainly SHEX as it was strongly absorbed by EG and was constrained in the pores of EG. The composite form-stable PCMs exhibited excellent thermal energy storage capacity. The ΔHm and ΔHc of the composite form-stable PCM containing 93 wt% TD could attain 202.6 and 201.2 J/g, respectively. However, ΔHm and ΔHc of the composite form-stable PCMs were slightly lower than the theoretical value since the SHEX–SORT transition was hindered and a thin layer of TD was strongly absorbed on the surface of EG. The absorbed TD layer also acted as seeds of crystallization and hence the undercooling of the composite form-stable PCMs was diminished. The thermal conductivity of the composite form-stable PCMs was greatly enhanced by EG. The thermal conductivity of the composite form-stable PCM containing 7 wt% of EG could attain 2.76 W/m K, which is 5.37 times higher than that of pure TD. Furthermore, the thermal conductivity of the composite formstable PCMs was accelerated much rapidly in lower EG loading range. The optimum weight ration of PCM in the composite should be 80 wt% (corresponding to 20 wt% of EG) as far as the practical utilization of the composite form-stable PCMs in solar thermal energy applications was concerned. Besides, the effects of the prepared EG on the thermal energy storage properties and thermal conductivity of composite form-stable PCMs were compared with EG derived from commercially obtained expandable graphite. The results indicated that both kinds of EG exhibited similar effects on these properties of the composite PCMs. Acknowledgment The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21003014,
21001017 and 21275022), the Natural Science Foundation of Hunan Province, China (13JJ3068 and 10JJ5002), the Scientific Research Fund of Hunan Provincial Education Department (12C0007) and Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation (Changsha University of Science & Technology) (2014CL05).
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Tetradecanol/expanded graphite composite form-stable phase change material for thermal energy storage Ju-Lan Zeng a,n, Juan Gan a, Fu-Rong Zhu a, Sai-Bo Yu b, Zhong-Liang Xiao a, Wen-Pei Yan a, Ling Zhu a, Zhen-Qiang Liu a, Li-Xian Sun a, Zhong Cao a a Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, School of Chemistry and Biological Engineering, Changsha University of Science and Technology, Changsha 410114, People's Republic of China b China Tobacco Hunan Industrial Corporation, Changsha 410007, People's Republic of China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 October 2013 Received in revised form 3 April 2014 Accepted 15 April 2014
Natural flake graphite was chemically intercalated to prepare expandable graphite. The expandable graphite was then expanded by means of microwave irradiation to obtain expanded graphite (EG). Tetradecanol (TD)/EG composite form-stable phase change materials (PCMs) were prepared by mixing TD with EG through an autoclave method. The highest loading of TD in the composite form-stable PCMs with good form-stability was 93 wt%. The composite form-stable PCMs exhibited excellent thermal energy storage capacity. The melting enthalpy (ΔHm) and crystallization enthalpy (ΔHc) of the composite form-stable PCM containing 93 wt% TD were 202.6 and 201.2 J/g, respectively. However, the solid–solid phase transition of TD in the composite form-stable PCMs was hindered as TD was strongly absorbed by EG. As a result, the ΔHm and ΔHc of the composite form-stable PCMs were slightly lower than the theoretical value. The thermal conductivity of the composite form-stable PCMs was greatly enhanced by EG and was increased with the increasing of EG loading. The thermal conductivity of the composite form-stable PCM containing 7 wt% EG attained 2.76 W/m K. Besides, effects of the prepared EG on properties of the composite form-stable PCMs were compared with those of EG derived from commercially obtained expandable graphite. & 2014 Elsevier B.V. All rights reserved.
Keywords: Form-stable phase change materials Expanded graphite Tetradecanol Thermal energy storage Thermal conductivity
1. Introduction Thermal energy storage technology, which can solve the problem in time and spatial mismatch between thermal energy supply and demand, is very important in solar thermal energy applications [1,2]. Thermal energy storage materials are the core of thermal energy storage technology. Phase change materials (PCMs) can absorb or release large amounts of thermal energy by changing their phase from one to another [3] and thus possess merits of high energy storage density in small temperature intervals [4]. Hence, PCMs are the most important latent heat energy storage materials in solar thermal energy applications [1, 2]. Solid–solid PCMs and solid–liquid PCMs are the two common kinds of PCMs when the changing of volume is concerned. However, the solid–liquid PCMs posses the advantage of better phase change dynamics properties than those of solid–solid PCMs [1]. Solid–liquid PCMs can be divided into organic and inorganic PCMs. Organic solid–liquid PCMs possess advantages of high thermal energy storage density, low or no undercooling, chemical and thermal
n
Corresponding author. Tel./fax: þ 86 731 85258733. E-mail address: [email protected] (J.-L. Zeng).
http://dx.doi.org/10.1016/j.solmat.2014.04.015 0927-0248/& 2014 Elsevier B.V. All rights reserved.
stability and no corrosives [1]. However, the main drawback that hinders the application of organic solid–liquid PCMs is their unacceptably low thermal conductivity [1]. Lower thermal conductivity would result in slower thermal energy storing or releasing speed and lower thermal energy utilization efficiency. The thermal conductivity of PCMs is important in the field of solar thermal energy applications, since solar radiation is varied seasonally and daily. Hence, high thermal conductivity is appreciated when PCMs are applied to store thermal energy that comes from solar radiation during sun shining period. As a result, the thermal conductivity of organic solid–liquid PCMs has to be enhanced [5]. The most popular method to enhance the thermal conductivity of solid–liquid PCMs is adding high conductivity fillers such as powder [6], metal foam [7], carbon fibers [8], carbon nanofibers [9], carbon nanotubes [10], exfoliated graphite nanoplatelets [11], expanded graphite (EG) [12], etc. Among them, EG exhibits various effects on the improvement of the thermal conductivity of PCMs. It has been reported that the apparent thermal conductivity of nitrates/10 wt% EG mixture PCM was increased by about 30–40% [13]. The thermal conductivity of SiO2/paraffin/EG composite was 94.7% higher than that of paraffin [14] and the thermal conductivity of palmitic acid was improved to 0.60 W/m K by 20 wt% of EG [15]. On the other hand, the thermal conductivity
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of stearic acid (SA) (0.30 W/m K) could be increased by 266.6% by adding 10 wt% of EG [16]. The thermal diffusivity of SA/EG composites containing 20 wt% EG was 10 times higher than that of pure SA [12]. The thermal conductivity of CaCl2∙6H2O containing 50 wt% EG (8.796 W/m K) was 14 times higher than that of pure CaCl2∙6H2O (0.596 W/m K) [17]. Furthermore, the thermal conductivity of graphite matrix/paraffin composite PCM was 20–130 times greater than that of pure paraffin ( 0.2 W/m K) [18]. Based on the reported results, we believe that the thermal conductivity enhancement of EG-doped composite PCMs is affected by properties of EG, the preparation procedure of the composite PCMs and the interactions between pure PCMs and EG. As a result, the investigation of the thermal conductivity enhancement of EG on solid–liquid PCMs is a hot topic. Furthermore, to the best of our knowledge, the thermal conductivity of fatty alcohol has not been enhanced by EG. Fatty alcohol is a kind of important organic solid– liquid PCM with high thermal energy storage density. The polarity of fatty alcohol is weaker than that of fatty acid. Furthermore, fatty alcohols exhibit a solid–solid phase transition which is close to the solid–liquid phase transition [19]. The solid–solid phase transition is also beneficial to thermal energy storage but its transition properties may be affected by fillers with high specific area. Hence, it is worthwhile to investigate the phase change properties and the thermal conductivity of fatty alcohol/EG composite PCMs. In this paper, EG was prepared from natural flake graphite and effects of the preparation procedure on the expanded ratio were investigated. Tetradecanol (TD) was selected as solid–liquid PCM and TD/EG composite form-stable PCMs were prepared via an autoclave method. The structure and the thermal properties of the prepared composite form-stable PCMs were investigated and results are reported here. Besides, effects of the prepared EG on the thermal energy storage properties and thermal conductivity of form-stable PCMs were compared with those of EG derived from commercially obtained expandable graphite.
2. Experimental 2.1. Materials Natural flake graphite with average size of 300 μm was obtained from Qingdao Runba Graphite Co., Ltd. China. Expandable graphite with average particle size of 300 μm and expansion ratio of 200 mL/g was supplied by Qingdao Graphite Co., Ltd., China. All other reagents were of analytical grade and were used without further purification. Deionized water was used throughout the experimental process. 2.2. Preparation of EG The EG was prepared according to reference [20] with some modification. In general, a certain amount of natural flake graphite was added to a solution of glacial acetic acid and concentrated nitric acid (65%) under stirring in a 35 1C water bath to form a welldistributed mixture. A certain amount of potassium permanganate was added to the mixture while the stirring was continued. The mixture was stirred at this temperature for a certain time. During this time, nitric acid/acetic acid/graphite intercalation compound (NA–GIC) was formed. Then the NA–GIC was filtered, washed thoroughly with water, and dried at 50 1C under vacuum to obtain dry expandable graphite. The obtained expandable graphite was then radiated for 60 s in a domestic microwave oven (WP750, GALANZ) to obtain EG. The expanded volume was applied as an evaluation standard to modify the ratio of raw materials and reaction time. The expansion ratio of EG was measured as follows: 0.20 g expandable graphite in a 50 mL beaker was irradiated for 60 s, and then the volume of the obtained EG was precisely read. The procedure was
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repeated three times for each sample and the relative deviation was less than 6%. The obtained mean volume value divided by 0.20 g resulted in the expansion ratio of EG as mL/g. 2.3. Preparation of TD/EG composite form-stable PCMs A certain amount of the EG, TD and anhydrous ethanol were mixed in an autoclave. The autoclave was sealed and heated at 120 1C for 24 h and then cooled to ambient temperature. The mixture was then taken out from the autoclave and heated at 50 1C under reduced pressure to evaporate the ethanol and was then dried under vacuum for 24 h to obtain TD/EG composite form-stable PCMs. The samples were designated as TD/ EG-1–5, corresponding to TD/EG composite form-stable PCMs containing 93 wt%, 90 wt%, 85 wt%, 80 wt% and 60 wt% of TD, respectively. In order to compare the effects of the prepared EG on the thermal energy storage properties and thermal conductivity of the formstable PCMs with EG obtained from a known method (namely, k-EG), another series of TD/k-EG composite form-stable PCMs were prepared as follows. Expandable graphite was dried at 80 1C for 24 h and was then radiated for 60 s in the microwave oven to obtain k-EG. TD/k-EG composite form-stable PCMs were then obtained according to the preparation procedure of TD/EG composite form-stable PCMs. The samples were designated as TD/k-EG-1–4, corresponding to the TD/k-EG composite form-stable PCMs containing 90 wt%, 80 wt%, 70 wt% and 60 wt% of TD, respectively. 2.4. Characterization FT-IR spectra were recorded on a FT-IR spectrometer (AVATAR360) using KBr pellet (4000–400 cm 1). The surface morphology investigation was carried out on a scanning electron microscope (SEM, JEOL JSM-6380). Before the SEM investigation, samples were sputtered with gold. Powder X-ray diffraction (XRD) experiments were performed on a Rigaku D/max-gb X-ray diffractometer with a monochromatic detector. Copper Kα radiation was used, with a power setting of 30 kV and 30 mA, and a scanning rate of 51/min. The thermal stability of samples was characterized by means of thermogravimetry (TG)/differential thermal analysis (DTA) on a thermogravimetric analyzer (NETZSCH STA 409 PG/PC) from room temperature to 800 1C at the heating rate of 10 1C/min and with N2 as carrier gas. The analyzer was calibrated using CaC2O4∙H2O (99.9%) prior to the experiment. Differential scanning calorimetry (DSC, TA, Q2000) was used to investigate the thermal energy storage properties of the prepared composite form-stable PCMs over the temperature of 10–80 1C with the heating and cooling rate of 10 1C/min in nitrogen atmosphere. Prior to the DSC experiments, the instrument was calibrated using indium (99.999%) as the standard material. The thermal conductivity of samples at room temperature was measured by means of steady-state heat flow method using a thermal conductivity tester (DRX-II-RW, Xiangtan Huafeng Instrument Manufacturing Co., Ltd, China). Before the measurement, samples were grounded and pressed under 10 MPa to obtain disks (Φ ¼30 mm). The hot plate of the tester was set at 35 1C and the cold plate was cooled by water of 15 1C. A disk was mounted between the two plates. The thermal conductivity of the disk was measured when the temperature of the two plates reached stable values for more than 1 h.
3. Results and discussions 3.1. Preparation of EG The preparation of EG was conducted according to reported procedure [20] with some modifications. The modifications were
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that, in the present work, glacial acetic acid was used instead of acetic anhydride and the expansion of the expandable graphite was completed by microwave irradiation in a domestic microwave oven rather than by heating in a muffle furnace. When acetic anhydride was applied, it should be mixed with concentrated nitric acid very carefully. Otherwise, the release of N2O5, which is the product of the side reaction between acetic anhydride and concentrated nitric acid, would occur violently [20]. In the present work, acetic anhydride was replaced by glacial acetic acid as acetic acid is the actual intercalator [20] and could be mildly mixed with concentrated nitric acid without the release of N2O5. The formation of NA–GIC was confirmed by XRD patterns obtained before and after the intercalation of natural flake graphite (Fig. 1). It could be seen from the figure that the pattern of NA–GIC exhibited two weak shoulder peaks at 2θ ¼27.51 and 53.51 (labeled with arrows in the figure), which were originated from NA–GIC [20]. Microwave assisted expansion of expandable graphite possesses the merits of quick heating process and high energy input density [21]. Our experiment results showed that 60 s of high energy irradiation was enough to obtain maximum expansion ratio.
Fig. 1. XRD patterns of natural flake graphite and NA-GIC.
Expandable graphite with high expansion ratio is preferred in the presented work. As a result, it is worthwhile to investigate the effects of raw materials ratio and reaction time on the expansion ratio. The results are shown in Table 1. It is clear that the best raw materials ratio was: flake graphite/acetic acid/nitric acid/ KMnO4 ¼ 1/4/4/0.25 (g/mL/mL/g), and the best reaction time was 90 min. The maximum expansion ratio could attain 250 mL/g. The SEM images of the natural flake graphite and the prepared EG are shown in Fig. 2. The laminated structure of the natural flake graphite can be clearly seen in Fig. 2a and b. After it was expanded, the laminated structure was replaced by worm-like structure (Fig. 2c and d). The surface morphology of the prepared EG exhibited well expanded structure that permits full and regular expansion in thedirection of c-axis. 3.2. Preparation and form-stability of the TD/EG composite form-stable PCMs The TD/EG composite form-stable PCMs were prepared according to an autoclave method [22]. The principle of the method is that the porous supporting materials absorb PCMs slowly under liquid–vapor equilibrium and hence PCMs could be uniformly absorbed in the porous materials. We tried at first mixing TD with EG in an autoclave without ethanol. However, results showed that the TD could not be uniformly mixed with EG. This phenomenon might be ascribed to the low vapor pressure of TD at 120 1C and the strong adsorption on EG for TD. In order to improve the uniformity of the composite form-stable PCMs, a certain amount of ethanol was added into the autoclave. The results showed that the TD could be mixed well with EG. The uniformity was further confirmed by a series of DSC experiments in which samples were taken from the upper, middle and bottom of the autoclave. The existence of ethanol could decrease the adsorption on EG of TD since EG could also adsorb ethanol, avoiding the adsorption of too much TD in too less EG, and hence promoted the uniformity of TD in EG. The TD/EG composite form-stable PCMs were pressed under 2 MPa to obtain discs. The discs were heated at 50 1C for 24 h to examine the form-stability. The results showed that all TD/EG composite form-stable PCMs exhibited good form stability without leakage of TD.
Table 1 The dose of raw materials used to prepare expandable graphite and the corresponding expansion ratio.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
Flake graphite (g)
HNO3 (mL)
HAC (mL)
KMnO4 (g)
Flake graphite/mixed acid (g/mL)
V HNO3 /VHAc (mL/mL)
Reaction time (min)
Flake graphite /KMnO4 (g/g)
Expansion ratio (mL/g)
1.0029 0.9998 1.0056 1.007 1.001 1.0042 1.0046 1.0047 1.0074 1.0103 1.0021 1.003 1.0007 1.0011 1.0004 1.0002 1.0018 1.0019 1.0026 1.0013 1.0013
1 2 3 4 5 6 6 5.3 4 2.7 2 4 4 4 4 4 4 4 4 4 4
1 2 3 4 5 6 2 2.7 4 5.3 6 4 4 4 4 4 4 4 4 4 4
0.2044 0.1997 0.2017 0.2057 0.2042 0.2043 0.2018 0.2045 0.2099 0.2008 0.2047 0.2018 0.2 0.202 0.2004 0.2018 0.1014 0.1504 0.202 0.2505 0.3001
1/2 1/4 1/6 1/8 1/10 1/12 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8 1/8
1 1 1 1 1 1 3/1 2/1 1/1 1/2 1/3 1 1 1 1 1 1 1 1 1 1
90 90 90 90 90 90 90 90 90 90 90 15 30 60 90 120 90 90 90 90 90
1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.2 1/0.1 1/0.15 1/0.2 1/0.25 1/0.3
160 185 233 237 227 232 82 163 234 210 187 135 146 176 234 204 182 199 233 257 210
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Fig. 2. SEM images of natural flake graphite (a and b), EG (c and d) and TD/EG-2 (e and f).
3.3. Morphology and structure investigation of the TD/EG composite form-stable PCMs The morphology of the TD/EG composite form-stable PCMs was characterized by SEM. The SEM images of TD/EG-2 are shown in Fig. 2e and f. It can be clearly observed that a layer of TD was adsorbed on the surface of the worm-like EG. The graphite layer became thicker compared to pure EG (Fig. 2c and d). The pores of the EG were not fully occupied by TD. The form-stability of the TD/EG composite form-stable PCMs under certain pressure comes from these partially occupied pores. The IR spectra of TD and the TD/EG composite form-stable PCMs are shown in Fig. 3. The broad band centered at 3395 cm 1 in the spectrum of TD was caused by the stretching vibration of hydroxyl group of alcohol. The vCH and δCH absorption bands of TD were exhibited at 2917, 2846 and 1465, 721 cm 1, respectively. The C–O stretching vibration of primary alcohol was exhibited at 1062 cm 1. The absorption peaks in the spectrum of the composite form-stable PCM are similar to that of TD. The results indicated that TD was just physically mixed with EG and no chemical interaction occured. The phase structure of the composite form-stable PCMs was investigated by XRD and the obtained XRD patterns are drawn in Fig. 4. A strong diffraction peak located at 2θ ¼26.391 was exhibited in the XRD pattern of EG, which could be attributed to the feature peak (002) of graphite [23]. The diffraction peak of EG
was clearly exhibited in the XRD patterns of the TD/EG composite form-stable PCMs. However, the peak was slightly shifted to 2θ ¼26.471, indicating that the lattice spacing of graphite layer was slightly squeezed by TD. The diffraction patterns of two TD/EG composite form-stable PCMs, TD/EG-2 and TD/EG-3, were similar. However, closer inspection of the XRD patterns of TD and the TD/EG composite form-stable PCMs reveals that two peaks located at 2θ ¼ 21.341 and 21.751 were exhibited in the XRD pattern of TD. However, these two peaks were merged into a single peak that located at 2θ ¼ 21.751 in the XRD patterns of the composite formstable PCMs. It seems that in the XRD patterns of TD/EG the composite form-stable PCMs, the peak located at 2θ ¼21.341 was hidden by the peak located at 2θ ¼21.751. Furthermore, the stronger diffraction peak located at 2θ ¼24.151 in the XRD pattern of TD was weakened in the XRD patterns of the TD/EG composite form-stable PCMs, while the weaker diffraction peak located at 2θ ¼24.711 in the XRD pattern of TD was strengthened in the XRD patterns of the TD/EG composite form-stable PCMs. It is known that solid fatty alcohols exhibit two phases [19]. Liquid fatty alcohols are crystallized at its' melting point to form hexagonal solid phase (SHEX). The SHEX is metastable and would turn to orthorhombic solid phase (SORT) at a slightly lower temperature. The TD in the composite TD/EG form-stable PCMs was strongly absorbed by EG and was constrained in the pores of EG. As a result, the transition between SHEX and SORT was hindered. The pure TD was preferred in SORT while the TD in the TD/EG composite
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Fig. 3. IR spectra of TD and TD/EG-3.
Fig. 5. TG–DTA curves of TD and the composite form-stable PCMs.
Fig. 4. XRD patterns of TD, EG and the composite form-stable PCMs.
form-stable PCMs was constrained and was mainly existed as SHEX [24]. As a result, the XRD patterns of pure TD and the TD/EG composite form-stable PCMs showed the differences mentioned above. 3.4. Thermal stability characteristics of the TD/EG composite form-stable PCMs The thermal stability of the TD/EG composite form-stable PCMs was investigated by TG/DTA and the TG and DTA curves are shown in Fig. 5a and b, respectively. It is clear that both TD and the TD/EG composite form-stable PCMs exhibited similar thermal stability characteristics. The weight loss that started at 180 1C was caused by the evaporation of TD. The residue of the TD/EG composite form-stable PCMs agreed well with the loading of EG in the TD/EG composite form-stable PCMs. There were two endothermic peaks exhibited in the DTA curves. The peak that appeared at lower temperature was corresponded to the melting of TD while the peak appeared at higher temperature was corresponded to the evaporation of TD. The results implied that the thermal stability of TD was not affected by EG, and the thermal stability of the TD/EG composite form-stable PCMs was satisfactory as far as the solid– liquid phase change temperature of TD was concerned. 3.5. Thermal energy storage properties of the TD/EG composite formstable PCMs The thermal energy storage properties of the TD/EG and TD/k-EG composite form-stable PCMs were investigated by means
of DSC. For each sample, three DSC experiments were performed with individual samples taken from different sites of the autoclave. The results exhibited good repeatability and the homogeneity of samples could be confirmed. The DSC curves are shown in Fig. 6. There was one endothermic peak appeared on the heating DSC curve of TD. On cooling run, two exothermic peaks were exhibited on the DSC curve. The peak located at higher temperature was the transition from liquid to SHEX, while the peak located at lower temperature was corresponded to the transition from SHEX to SORT [19]. The DSC curve of TD showed slight undercooling for the liquid–solid transition while the undercooling of transition from SHEX to SORT was apparent. It seems that the peak located at lower temperature was due to disorder as the temperature of the sample increases when the SHEX–SORT transition starts. Actually, the increasing of the temperature was caused by the undercooling of SHEX–SORT transition. The differential scanning calorimeter (TA, Q2000) used in this work is a heat flux DSC. During the cooling period, the temperature of TD in SHEX state would drops to a temperature lower than the SHEX–SORT transition point due to undercooling. When the transition started, a large amount of heat as released and resulted in a temperature rise. A normal DSC curve, which is inserted in Fig. 6, could be obtained when the x-axis of the DSC curve of TD was set as time rather than temperature. The DSC curves of the TD/EG composite formstable PCMs were similar to that of TD. One endothermic peak on the heating DSC curve and two exothermic peaks on the cooling DSC curve can be found, indicating that the thermal energy storage property of the composite form-stable PCMs comes from TD. However, the SHEX–SORT transition of the TD/EG composite form-stable PCMs changed considerably and the undercooling was disappeared on the DSC curves. The exothermic peak corresponding to the SHEX–SORT transition of TD was shifted to a lower temperature and the shift became larger as the loading of EG was increased. The results further showed that the SHEX–SORT transition
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Fig. 7. Phase change enthalpy and thermal conductivity of the composite form-stable PCMs.
Fig. 6. DSC curves of TD and the composite form-stable PCMs.
Table 2 Phase change parameters of TD and the composite form-stable PCMs. Sample
EG/TD (wt%/ wt%)
Tonset-m (1C)
ΔHm (J/g)
Tonset-c (1C)
ΔHc (J/g)n
TD TD/EG-1 TD/EG-2 TD/EG-3 TD/EG-4 TD/EG-5 TD/k-EG-1 TD/k-EG-2 TD/k-EG-3 TD/k-EG-4
0/100 7/93 10/90 15/85 20/80 40/60 10/90 20/80 30/70 40/60
35.87 35.35 35.13 36.27 34.96 34.57 36.30 35.90 35.87 35.41
236.1 202.6 196.9 193.1 174.1 131.1 198.1 169.6 151.2 132.2
32.3 34.93 34.73 35.57 34.77 34.66 34.79 35.33 35.44 35.57
234.5 201.2 196.4 191.6 172.8 129.2 196.1 163.0 147.2 127.85
n The total phase change enthalpy of the liquid–SHEX and SHEX–SORT transition of the cooling run.
of TD in the composite form-stable PCMs was greatly restrained since the TD was constrained in the pores of EG, which was in accordance with the results of XRD investigation. Form the DSC experiments, the mean value of the melting enthalpy (ΔHm), crystallization enthalpy (ΔHc, the total phase change enthalpy of the liquid- SHEX and SHEX–SORT transition of the cooling run) and the extrapolated peak onset temperatures during melting (Tonset-m) and solidifying (Tonset-c) were obtained. These data are shown in Table 2. It could be seen that the Tonset-m was fluctuated within the temperature range of 34–36 1C and there was no regularity that could be observed. The Tonset-c of TD was about 3.5 1C lower than the Tonset-m of TD, indicating that there existed a certain degree of undercooling. However, the undercooling was diminished as EG was added. Furthermore, there was a regularity that could be found. That is, the higher the loading of
EG, the lower the undercooling. When the loading of EG attained 40%, the undercooling was vanished. On the other hand, ΔHm and ΔHc were decreased as the loading of TD was decreased. This is understandable since ΔHm and ΔHc were originated from the phase change of TD. However, from Fig. 7, it could be seen that ΔHm and ΔHc of the composite form-stable PCMs were lower than the theoretical value, which was calculated by multiplying ΔHm or ΔHc of TD by the loading of TD in the TD/EG composite form-stable PCMs. The results of XRD investigation have implied that the phase change enthalpy of the SHEX–SORT transition would be decreased, though it was very small comparing with that of the SHEX–liquid transition. Furthermore, TD was adsorbed on the surface on EG due to the strong adsorption on EG by TD. The absorption was so tight that a thin layer of TD could not change its phase. As a result, ΔHm and ΔHc of the TD/EG composite form-stable PCMs were slightly lower than the theoretical value. The adsorbed TD layer could also act as seeds of solid phase. Hence, the undercooling of the TD/EG composite formstable PCMs could be diminished by the addition of EG. The DSC curves of the TD/k-EG composite form-stable PCMs are also shown in Fig. 6. The thermodynamic parameters such as ΔHm, ΔHc, Tonset-m and Tonset-c derived from DSC experiments are shown in Table 2. It could be seen that the DSC curves of TD/k-EG composite form-stable PCMs were similar to that of TD/EG composite form-stable PCMs. The exothermic peak corresponding to the SHEX–SORT transition of TD in TD/k-EG composite form-stable PCMs showed the same trend as that of TD/EG composite formstable PCMs. Furthermore, ΔHm and ΔHc of the TD/k-EG composite form-stable PCMs were also equal to that of TD/EG composite form-stable PCMs. These results indicated that the effect of the prepared EG on the phase change properties of the form-stable PCMs was similar to EG obtained from the known method. 3.6. Thermal conductivity of the TD/EG composite form-stable PCMs The thermal conductivity of the composite form-stable PCMs is drawn in Fig. 7. The results showed that the thermal conductivity of the TD/EG composite form-stable PCMs could be greatly enhanced by EG. The thermal conductivity of pure TD was 0.433 W/m K. However, the thermal conductivity of the TD/EG composite form-stable PCM containing 7 wt% of EG attained 2.76 W/m K, which was 5.37 times higher than that of pure TD. The thermal conductivity of the TD/EG composite form-stable PCMs could be further enhanced to 5.71 W/m K when 40 wt% of EG was added. The figure shows that the line of ΔH and the line of thermal conductivity were intersected at where the loading of TD
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was about 85 wt% (corresponding to 15 wt% of EG). The figure also showed that, with the increasing of EG loading, the thermal conductivity of the TD/EG composite form-stable PCMs was accelerated much rapid at lower EG loading (o 20 wt%) than at higher EG loading (420 wt%). Besides, the thermal energy storage density is also a key factor for the application of PCMs. As a result, it is recommended that the optimum weight ration of TD in the TD/EG composite form-stable PCMs should be 80 wt% (corresponding to 20 wt% of EG) as far as the phase change enthalpy, thermal conductivity and form-stability are concerned. The thermal conductivity of TD/k-EG composite form-stable PCMs is also shown in Fig. 7. Obviously, the thermal conductivity of the composite form-stable PCMs was also greatly enhanced by k-EG. From the figure we could find that the thermal conductivity enhancement ability of k-EG was almost equal to that of the prepared EG and the variation trend in the thermal conductivity of both kinds of form-stable PCMs was similar. Hence, we could conclude that the thermal conductivity enhancement ability of the prepared EG was as much as that of EG obtained from known methods. 4. Conclusions In general, we have prepared expandable graphite from natural flake graphite by chemical intercalation reaction and the expandable graphite was expanded to worm-like EG by microwave irradiation. The EG was then mixed with TD through an autoclave method to obtain TD/EG composite form-stable PCMs with the assistance of ethanol. TD was absorbed into the pores of the worm-like EG and the loading of TD in the composite form-stable PCMs with good form-stability could be as higher as 93 wt%. The solid phase of TD in the composite form-stable PCMs was mainly SHEX as it was strongly absorbed by EG and was constrained in the pores of EG. The composite form-stable PCMs exhibited excellent thermal energy storage capacity. The ΔHm and ΔHc of the composite form-stable PCM containing 93 wt% TD could attain 202.6 and 201.2 J/g, respectively. However, ΔHm and ΔHc of the composite form-stable PCMs were slightly lower than the theoretical value since the SHEX–SORT transition was hindered and a thin layer of TD was strongly absorbed on the surface of EG. The absorbed TD layer also acted as seeds of crystallization and hence the undercooling of the composite form-stable PCMs was diminished. The thermal conductivity of the composite form-stable PCMs was greatly enhanced by EG. The thermal conductivity of the composite form-stable PCM containing 7 wt% of EG could attain 2.76 W/m K, which is 5.37 times higher than that of pure TD. Furthermore, the thermal conductivity of the composite formstable PCMs was accelerated much rapidly in lower EG loading range. The optimum weight ration of PCM in the composite should be 80 wt% (corresponding to 20 wt% of EG) as far as the practical utilization of the composite form-stable PCMs in solar thermal energy applications was concerned. Besides, the effects of the prepared EG on the thermal energy storage properties and thermal conductivity of composite form-stable PCMs were compared with EG derived from commercially obtained expandable graphite. The results indicated that both kinds of EG exhibited similar effects on these properties of the composite PCMs. Acknowledgment The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21003014,
21001017 and 21275022), the Natural Science Foundation of Hunan Province, China (13JJ3068 and 10JJ5002), the Scientific Research Fund of Hunan Provincial Education Department (12C0007) and Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation (Changsha University of Science & Technology) (2014CL05).
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