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expanded graphite composite phase-change material for thermal energy storage
Materials Chemistry and Physics 240 (2020) 122178
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Preparation and thermal properties of n-eicosane/nano-SiO2/expanded graphite composite phase-change material for thermal energy storage Xinyi Zhang, Chuqiao Zhu, Guiyin Fang * School of Physics, Nanjing University, Nanjing, 210093, China
H I G H L I G H T S
� N-eicosane/nano–SiO2/expanded graphite CPCM were prepared for thermal energy storage. � The CPCM3 melting and solidifying latent heat is 135.80 J/g and 125.93 J/g, respectively. � Thermal conductivity of the CPCM with 7 wt% EG is 2.37 times that of the CPCM without EG. � The CPCM with EG has good thermal stability and reliability. A R T I C L E I N F O
A B S T R A C T
Keywords: Thermal energy storage Composite phase-change material Nano-SiO2 n-eicosane Thermal properties
Composite phase-change materials (CPCM) were prepared using n-eicosane as phase-change material, nano-SiO2 as supporting matrix, and expanded graphite (EG) as thermal conductivity promoter. Nano-SiO2 with strong adsorption performance can prevent leakage of n-eicosane in CPCM. Adding expanded graphite significantly enhanced thermal conductivity of CPCM. Leakage tests showed that the maximum mass proportion of n-eicosane in CPCM was 70 wt%. Fourier transformation infrared spectroscopy (FT-IR), an X-ray diffractometer (XRD), and a scanning electronic microscope (SEM) were used to determine chemical structures, crystal phases, and mor phologies of CPCM. The DSC results indicated that the melting latent heat of CPCM was 135.80 J/g with a melting temperature of 35.35 � C, and that the solidifying latent heat of CPCM was 125.93 J/g with a solidifying temperature of 36.23 � C. Thermal conductivity meter (TCM) test results showed that CPCM had high thermal conductivity, representing a 15%–137% increase compared to CPCM without EG. Thermal properties and chemical structures of CPCM were investigated after 100 thermal cycles. The results indicated that CPCM3 (with 7 wt% EG) had good structure and thermal stability. Thermal conductivity of CPCM3 is 2.37 times that of CPCM without EG. Therefore, CPCM3 has potential application prospects for thermal energy storage.
1. Introduction Global energy demand has surged, particularly in 2017, when it rose by 2.2% compared to its 10-year average of 1.7% [1]. Nowadays, fossil fuels are still the main source of energy supply, resulting in a series of problems such as resource shortages and the greenhouse effect [2]. Therefore, improving energy efficiency and storing heat energy are effective ways to alleviate energy and environmental problems. All countries are stepping up efforts to develop new energy sources. Ther mal energy, which is widely distributed and used, is a kind of conven tional energy that develops continuously [3]. However, heat-storage materials must meet a series of requirements for thermal energy use, such as good thermal stability, structural stability, and high latent heat
to achieve temperature regulation, waste heat recovery, and solar thermal storage [4]. Phase-change materials (PCM) can be used as latent heat-storage materials due to their varieties, wide working temperature range, and high latent heat [5]. Thermal energy storage uses PCM state changes to absorb and release heat. When the ambient temperature is higher than the PCM phasechange temperature, the PCM melts and absorbs heat [6]. When the ambient temperature is lower than the PCM phase-change temperature, the PCM solidifies and releases heat. To control the ambient temperature and energy storage, thermal energy storage is realized in the phase-change process of PCM [7]. According to different phase change forms, phase change materials can be divided into four types: solid-solid, solid-liquid, liquid-gas, and solid-gas phase change materials [8].
* Corresponding author. E-mail address: [email protected] (G. Fang). https://doi.org/10.1016/j.matchemphys.2019.122178 Received 16 June 2019; Received in revised form 23 August 2019; Accepted 12 September 2019 Available online 12 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Among these, solid-solid and solid-liquid phase-change materials are widely used in practical applications, but solid-gas and liquid-gas phase change are difficult to apply in practical engineering due to their huge volume change. In addition, PCM may leak during the solid-liquid phase-change process [9]. Inorganic PCMs have excellent latent heat and high thermal conductivity, but their structure is unstable, and phase separation may occur [10]. Organic PCMs, including polyols [11], fatty acids [12], paraffins [13], and polymer materials [14] (mainly poly ethylene glycol [15]) have stable structure, are non-corrosive, and offer low toxicity, low cost, and low thermal conductivity [16], which can be improved by adding materials [17] with high thermal conductivity. Therefore, organic PCMs are widely used for thermal energy storage [18]. Yuan et al. [12] investigated fatty acids, including stearic acid (SA), lauric acid (LA), myristic acid (MA), and palmitic acid (PA) as phase-change materials. The results showed that fatty acids have broad prospects as energy storage materials for low-temperature heating. N-octadecane, which has a stable structure in application, was prepared as a PCM by Su et al. [19]. N-eicosane, which has a stable structure, is chemically inert, and does not pollute water, has a phase change-point of 36� C–38 � C. To reduce PCM leakage in the molten state, materials with good adsorption inclusions such as diatomite [20], expanded graphite [21], nano-silica [22], metal foam [23], and organic polymer [24] are added to PCM as supporting materials. Huang et al.’s work indicates that metal foam not only adsorbs phase-change materials, but also greatly improves PCM thermal conductivity [25]. Tang et al. [26] used high density polyethylene as the support material and stearyl alcohol as the PCM to prepare CPCM with a phase-change temperature of 56 � C and a latent heat of 200 kJ/kg. Wang et al. [27] prepared morphologically stable PCM with paraffin and nano-SiO2 and investigated the effect of pore size on paraffin adsorption. The experimental results showed that the adsorption capacity of nano-SiO2 with different pore sizes on paraffin varies and that the PCM with the smallest pore size and the most stable shape of nano-SiO2 has the best thermal stability and reliability. As mentioned above, organic PCM has low thermal conductivity [28], and therefore it is necessary to modify the PCM material. The common way to do this is to add a thermal conductivity promoter to the PCM [29]. Metal oxides, some nanomaterials, and carbon materials are often added to enhance PCM thermal conductivity [30]. In this work, nano-silica is used as the supporting material. Nanosilica has the characteristics of stable, porous, and loose structure, strong adsorption capacity, and very large surface area [31]. In addition, it is a good supporting material because it is nontoxic and non-polluting [32]. N-eicosane also has non-toxic and non-polluting characteristics, with a melting temperature of 36.9 � C and a melting latent heat of 243.28 J/g. The latent heat of n-eicosane is higher than that of alcohol phase-change materials. Compared with other phase-change materials, n-eicosane with the same mass has higher latent heat. N-eicosane is structurally stable and can maintain structural integrity during repeated phase transitions without chemical change. The addition of EG to the PCM can improve CPCM thermal conductivity, making CPCM more suitable for daily use. In this work, the properties of CPCM are investi gated, including chemical structure, crystal phase, morphology, thermal properties, thermal stability and reliability, and thermal conductivity. N-eicosane is insoluble in water and soluble in organic solvents, which can be used to regulate the temperature of buildings to save energy. In addition, in the process of preparing the composite, melting and dispersion-mixing methods, which are normally used to make metalfoam PCM, are used to make alkane samples. Compared with previous methods of making alkane phase-change materials, the supporting ma terials of the composites made by this process are evenly mixed and structurally stable. EG is similar to nanometer silica in having a porous, loose structure and good adsorption. When EG is added to the sample, the specific surface area of the sample is further increased, and EG and silica form a larger structural support, making the sample structure more stable. With
increasing EG content, the thermal conductivity of the composite in creases, but the mass fraction of n-eicosane in the composite decreases, and the latent heat of the composite decreases accordingly. Therefore, an appropriate EG content should be selected to prepare the composite. 2. Experimental 2.1. Materials N-eicosane (C20H42, melting point: 36.0–38.0 � C, analytical reagent) was obtained from Sinopharm Chemical Reagent Co., Ltd., and was used as a thermal-energy storage medium. Nano-silica (nano-SiO2, particle size: 20 nm, purity: 99 wt%, treated by KH550) as a supporting matrix to prevent PCM leakage was bought from Nanjing XFNANO Materials Tech Co., Ltd. Expanded graphite (EG, thickness: 5–20 nm, flake diameter: 5–10 μm, specific surface area: >30 m2/g, purity: 99.5%) was used as a thermal conductivity promoter and was purchased from Nanjing XFNANO Materials Co., Ltd. Anhydrous ethanol (analytical reagent, Sinopharm Chemical Reagent Co., Ltd.) was used as the solvent. 2.2. Preparation of n-eicosane/nano-SiO2 composites N-eicosane was adsorbed onto nano-SiO2 to fabricate n-eicosane/ nano-SiO2 composites. The specific steps were as follows: (1) nano-SiO2 was dispersed in absolute ethanol and stirred at 1,000 rpm for 15 min by a magnetic stirrer, after which the temperature was stabilized at 50 � C. (2) N-eicosane was melted into the liquid state at 50 � C for 15 min (3) The liquid n-eicosane was added to the ethanol solution and mixed with nano-SiO2. (4) The composite was heated to 70 � C and continuously stirred by a magnetic stirrer at 1300 rpm until the absolute ethanol was completely evaporated. After completing this preparation process, the composite was cooled at room temperature for 2 h (5) The composite was dried at 35 � C for 10 h in a vacuum oven to obtain the CPCM without water. A series of CPCMs with different n-eicosane contents were tested to maximize the n-eicosane content in the CPCM without leakage. As shown in Table 1, CPCMs with 65 wt%, 67 wt%, 70 wt%, 73 wt%, and 75 wt% n-eicosane were called NPCM1, NPCM2, NPCM3, NPCM4, and NPCM5. The next step was to perform the leakage tests on the samples obtained. At first, an appropriate amount of the sample was placed on a filter paper, and then the filter paper containing the sample was heated at 50 � C for 30 min. After sample heating, the size of the oil stain on the filter paper indicated the degree of leakage of the sample. The leakage test results are shown in Fig. 1. It is apparent that when the n-eicosane content was 65 wt%, 67 wt%, and 70 wt%, there were no grease spots on the filter paper, which means that no leakage occurred. When the neicosane content was 73 wt%, there was a small circular stain, and when the n-eicosane content was 75 wt%, there was a significant circular stain on the filter paper, which showed substantial leakage. In other words, the maximum load capacity for n-eicosane in CPCM was 70 wt%, and therefore the remaining experiments were carried out on the basis of an n-eicosane content of 70 wt%.
Table 1 Composition of the n-eicosane/nano-SiO2 composite PCM.
2
Samples
n-Eicosane (g)
Nano–SiO2 (g)
Mass fraction of neicosane (wt %)
Leakage in molten state
NPCM1 NPCM2 NPCM3 NPCM4 NPCM5
19.5 20.1 21.0 21.6 22.5
10.5 9.9 9.0 8.4 7.5
65 67 70 72 75
no leakage no leakage no leakage slight leakage leakage
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measured by performing 100 thermal cycles from 20 � C to 50 � C, which simulated the working conditions of CPCM. 3. Results and discussion 3.1. FT-IR analysis The chemical structure and composition of CPCM was measured and analyzed by FT-IR. Fig. 2 presents the FT-IR spectra of n-eicosane, silica, NPCM3, CPCM1, CPCM2, and CPCM3. There are four absorption peaks in Fig. 2a. The peak at 2916 cm 1 represents symmetrical stretching vibration of the –CH3 group, and the symmetrical stretching vibration of the CH2 group is at 2849 cm 1. The peak at 1471 cm 1 signifies the deformation vibration of the CH2 and CH3 groups. The peak at 717 cm 1 represents the rocking vibration of the CH2 chain [33]. As for the spectrum of SiO2 in Fig. 2b, there are three sharp peaks. The peak at 1101 cm 1 signifies the Si–O–Si bond stretching vibration [34]. The peaks at 804 cm 1 and 470 cm 1 are the results of stretching and bending vibration of the Si–O bond [35]. Fig. 2c shows the spectrum of the EG, which is a smooth line without absorption because EG is an elementary substance and does not have a dipole moment [36]. The spectra of NPCM3, CPCM1, CPCM2, and CPCM3 are shown in Fig. 2d–g. All of these have the same absorption peaks from n-eicosane and nano-SiO2. The peaks at 2916, 2849, 1474, and 717 cm 1 are attributed to n-eicosane. The peaks at 1101 and 800 cm 1 belong to SiO2. More over, there are no new peaks, providing evidence that no chemical re action occurred in the CPCM [37].
Fig. 1. Leakage tests of the n-eicosane/nano–SiO2 composites with different neicosane content.
2.3. Preparation of the n-eicosane/nano-SiO2/EG composites EG was used as an additive to enhance the thermal conductivity of neicosane/nano-SiO2 CPCM. Three CPCMs with 3, 5, and 7 wt% EG, called CPCM1, CPCM2, and CPCM3, were prepared. Table 2 shows the specific values. The preparation process was similar to that of n-eico sane/nano-SiO2 composite PCM except that anhydrous ethanol was mixed with the nano-SiO2 for 30 min before adding EG. During prepa ration, the heating temperature and stirring speed were kept constant until the anhydrous ethanol was completely evaporated. As EG content in composite increases, the thermal conductivity of the composite also increases. Meanwhile, as n-eicosane content in compos ite decreases, the latent heat of the composite also decreases. Therefore, the values of thermal conductivity and latent heat need to be balanced, meaning that the EG content in the composite cannot be greatly increased.
3.2. XRD analysis Fig. 3 shows the diffraction patterns of n-eicosane, nano-SiO2, EG, NPCM3, and CPCM1–CPCM3 to determine the crystal structure of CPCM. According to the literature [28], characteristic diffraction peaks at 19.48, 23.59, and 24.91 index as the (010), (100) and (111) planes. As shown in curves d–g, CPCM characteristic peaks appear at 19.48, 23.57, and 24.90. As compared with curves a, b, and c, the CPCM characteristic diffraction peaks are the same as the peaks for n-eicosane. This means that CPCM does not involve any new structure occurrence. CPCM and NPCM were similar, but with the addition of EG, there were no additional diffraction peaks in the image, and the existing peaks became more and more obvious as the EG content increased. This is due to the fact that EG can act as nucleating agent, resulting in strong crystal
2.4. Characterization techniques Fourier transform infrared spectroscopy (FT-IR, Nicolet Nexus 870, resolution using KBr pellets: 2 cm 1, spectral range: 400–4000 cm 1) and X-ray diffractometry (XRD, D/MAX-Ultima III, working voltage: 40 kV, working current: 40 mA, scanning rate: 5� (2θ)/min, 2θ range: 10� to 60� , Rigaku Corporation, Japan) were used to determine the chemical structures and the crystal phase of CPCM. The morphology and microstructure of CPCM were observed and analyzed using a scanning electron microscope (SEM, S–3400NII, operating voltage: 3 kV, Hitachi, Inc., Japan). Thermal properties, including latent heat and phase-change tem perature, of CPCM were measured by a differential scanning calorimeter (DSC, Pyris 1 DSC, PerkinElmer, temperature accuracy: �0.2 � C, enthalpy accuracy: �5%, temperature range: 10–90 � C, rate of temper ature change: 5 � C/min). The thermal conductivity of CPCM was tested by a thermal conductivity meter (TCM 3020, Xiatech Electronic Tech nology Co., Ltd., accuracy: �2%) at room temperature. In addition, the thermal stability of CPCM was tested by thermogravimetry (TGA, Pyris 1 TGA, PerkinElmer, temperature range: 0–700 � C, heating rate: 20 � C/ min). The thermal conductivity of each sample was measured five times, and the average thermal conductivity value was taken as the final thermal conductivity of the sample. The thermal reliability of CPCM was Table 2 Composition of the n-eicosane/nano-SiO2/EG composite PCM. Samples
NPCM3 (g)
EG (g)
Mass fraction of EG (%)
CPCM1 CPCM2 CPCM3
30 30 30
0.93 1.58 2.27
3 5 7
Fig. 2. FT–IR spectra of the (a) n-eicosane (b) nano–SiO2, (c) EG, (d) NPCM3, (e) CPCM1, (f) CPCM2 and (g) CPCM3. 3
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orientation. This has a beneficial effect on crystallization of the CPCM. The higher crystallinity of the CPCM, the stronger diffraction peaks of the CPCM. Therefore, it can be concluded that the n-eicosane remains intact in CPCM without chemical reaction and is not affected by silica. All combinations are formed by physical action. 3.3. Morphological analysis The morphologies of EG, nano-SiO2, NPCM3, and CPCM1–CPCM3 were observed by SEM and are shown in Fig. 4. The picture shows that nano-silica has a porous and loose particle structure, but the structure of the EG is layered and multi-pore. In the NPCM3, the n-eicosane is well coated with silica, and the particles are stacked together. The CPCM1 structure, with a mass fraction of 3% EG, is still not different from that of NPCM3. The n-eicosane and EG are still well coated by nano-silica. When the mass fraction of the EG is 7 wt%, CPCM3 shows an obvious layered structure. It can be clearly seen that the n-eicosane is evenly dispersed and attached to the structure formed by nano-silica and EG. Both nano-silica and the EG multilayer structure can avoid n-eicosane leakage in the molten state. According to the leakage test and observa tion of the morphology, the maximum weight of n-eicosane in CPCM is 70 wt%, beyond which n-eicosane cannot be completely adsorbed, resulting in leakage. In addition, EG addition leads to the lamellar state of the stacked CPCM, further increasing CPCM surface area, but the decrease in n-eicosane content affects the latent heat.
Fig. 3. XRD patterns of (a) n-eicosane, (b) nano–SiO2, (c) EG, (d) NPCM3, (e) CPCM1, (f) CPCM2 and (g) CPCM3.
Fig. 4. SEM photographs of (a) EG, (b) nano–SiO2, (c) NPCM3, (d) CPCM1, (e) CPCM2 and (f) CPCM3. 4
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5
241.00 � 12.05 153.70 � 7.69 153.86 � 7.70 138.09 � 6.90 125.93 � 6.30 33.31 � 0.20 34.10 � 0.20 34.08 � 0.20 34.47 � 0.20 34.30 � 0.20 33.60 � 0.20 35.90 � 0.20 36.18 � 0.20 36.19 � 0.20 36.23 � 0.20 243.28 � 12.16 163.41 � 8.15 163.91 � 8.20 148.78 � 7.44 135.80 � 6.79 38.78 � 0.20 38.10 � 0.20 38.89 � 0.20 37.87 � 0.20 37.71 � 0.20 36.90 � 0.20 35.00 � 0.20 35.67 � 0.20 35.21 � 0.20 35.35 � 0.20
Onset temperature (� C)
Samples
Table 3 DSC data for n-eicosane, NPCM3, and CPCM1–CPCM3.
The thermal stability of CPCM was measured by TGA and DTG (de rivative thermogravimetric analysis), as shown in Figs. 6 and 7. The specific data are listed in Table 5, including the temperature (Tp) and the weight loss percentage (△W) at the maximum thermal degradation rate and the residual percentage at 700 � C. As shown in Fig. 6, n-eicosane had one decomposition stage. In the phase from 25 � C to 230 � C, n-eicosane decomposition occurred, with a decomposition weight greater than 95%. The maximum decomposition rate of n-eicosane was greater than 50% at a temperature of 220.9 � C, as shown in Fig. 7. When the tem perature was below 100 � C, n-eicosane shows almost no weight loss, which proved the thermal stability of n-eicosane as a phase-change material at room temperature. For nano-SiO2, there was almost no loss except for the weight loss of water, which could be attributed to evap oration of water adsorbed into nano-SiO2. NPCM3, CPCM1, CPCM2, and CPCM3 are all dry samples, and they had only one stage of decompo sition. Most of the n-eicosane decomposes, and after 230 � C, the remaining phase-change materials decomposed. However, it could be clearly seen that, as the weight fraction of EG increased from 3% to 7%, the decomposition degree of n-eicosane decreased, the residual CPCM increased. This indicated that adding EG can improved the CPCM coating. This is due to that the EG can form charred layers building up on the surface. The charred layers may create a physical protective barrier on the surface of the CPCM. This can effectively reduce the thermal decomposition of n-eicosane, thus improving the thermal stability of the
Peak temperature (� C)
3.5. Thermal stability and reliability analysis
N-eicosane NPCM3 CPCM1 CPCM2 CPCM3
Peak temperature (� C) Solidifying
Latent heat (J/g)
where △HCPCM and △HPCM refer to the latent heats of CPCM and of pure n-eicosane and η is the n-eicosane mass fraction in CPCM. It can be seen from Eq. (1) that the CPCM latent heat and the n-eicosane mass fraction had a proportional relationship, which could be verified using the data in Table 3. Besides, the latent heat of CPCM was lower than that of the NPCM3 because of the addition of EG. In other words, if the mass fraction of n-eicosane in CPCM was less than 70 wt%, the mass fraction of n-eicosane in CPCM3 was 66.5 wt%. Table 4 shows some thermal properties of CPCM using fatty acids and alcohol as heat-storage media in previous research, including melting temperature, latent heat of melting, and mass fraction of PCM. It can be determined that the CPCM proposed in this study is a potential thermalenergy storage material.
Onset temperature (� C)
(1)
Melting
ΔHCPCM ¼ η⋅ΔHPCM
Latent heat (J/g)
Thermal properties include phase-change temperature and latent heat during melting and solidification, which are measured by DSC. The thermal properties of n-eicosane, NPCM3, CPCM1, CPCM2, and CPCM3, were tested. The relevant data are presented in Table 3, and the phasechange processes are illustrated in Fig. 5, where Fig. 5a shows an endothermic process and Fig. 5b an exothermic process. The melting/ solidification temperatures of n-eicosane are 36.90/33.60 � C, and those of NPCM3, CPCM1, CPCM2, and CPCM3 are 35.00/35.90 � C, 35.67/ 36.18 � C, 35.21/36.19 � C, and 35.35/36.23 � C respectively. The CPCM phase-change temperature was found to be slightly different from that of pure n-eicosane, as shown in Fig. 5. CPCM, with its phase-change tem perature around 36 � C, can be used as a building material, as indoor thermal insulation, and for industrial waste heat recovery. The latent heats of melting and solidification of n-eicosane are 243.28/241.00 J/g, and those of NPCM3, CPCM1, CPCM2, and CPCM3 are 163.41/153.70 J/g, 163.91/153.86 J/g, 148.78/138.09 J/g, and 135.80/125.93 J/g respectively. Obviously, CPCM latent heat decreases as EG content increases because n-eicosane is the only phase-change material in CPCM, and therefore the latent heat of CPCM is related to the proportion of n-eicosane. The relationship between the latent heat of CPCM and the proportion of n-eicosane can be expressed as:
– 169.50 164.40 161.06 157.57
Theoretical latent heat (J/g)
3.4. Thermal properties analysis
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Fig. 6. TGA curves of n-eicosane, nano–SiO2, NPCM3, CPCM1, CPCM2 and CPCM3.
Fig. 5. (a) Melting DSC curves and (b) solidifying DSC curves of n-eicosane, NPCM3, CPCM1, CPCM2 and CPCM3. Fig. 7. DTG curves of n-eicosane, nano–SiO2, NPCM3, CPCM1, CPCM2 and CPCM3.
Table 4 Comparison of the present work with results for other PCMs in the literature. Composite PCM
Melting temperature (� C)
Melting latent heat (J/g)
References
Capric acid/expanded perlite PA/high density polyethylene PA/polypyrrole Myristic acid/high density polyethylene Octadecane/titania N-eicosane/Fe3O4 & SiO2 Octadecane/calcium carbonate N-eicosane/nano-silca
30.70
74.08
[38]
61.60
161.10
[39]
59.80 53.80
166.30 119.38
[40] [41]
25.68 33.42 29.19
42.57 69.60 84.37
[42] [43] [44]
35.00
163.41
present study
Table 5 TGA data for n-eicosane, nano-SiO2, NPCM3, and CPCM1–CPCM3. Samples
Tp (� C)
△W (%)
Residue (%) (700 � C)
N-eicosane Nano–SiO2 NPCM3 CPCM1 CPCM2 CPCM3
222.90 � 0.20 – 225.23 � 0.20 226.53 � 0.20 209.16 � 0.20 202.85 � 0.20
100 4.52 68.19 69.22 63.92 60.10
0 95.48 31.81 30.78 36.08 39.90
FT-IR spectral analysis on the obtained sample and the unprocessed sample and measure their thermal conductivity. After 100 thermal cy cles, measure the thermal performance of the CPCM3 using DSC. Fig. 8 shows the DSC curves of CPCM3 before and after 100 thermal cycles, and Table 6 shows the specific data. The melting and solidifying temperatures of CPCM3 without thermal cycle are 35.35 � C and 36.23 � C, and the melting and solidifying temperatures of CPCM3 after 100 thermal cycles are 35.12 � C and 36.54 � C. It is found that the melting temperature and solidifying temperature of CPCM3 after ther mal cycles are almost the same as that of uncycled CPCM3. In addition, compared with the uncycled CPCM3, the melting latent heat of CPCM3 after 100 thermal cycles is decreased by 1.8%, and the solidifying latent
CPCM. Thermal reliability is also crucial for the use of CPCM, which has been verified by 100 thermal cycle tests at 50 � C. The steps of the thermal cycle test were as follows: first, take an appropriate mass of CPCM3 solid powder and place it onto filter paper. Then place the sample in a thermostatic chamber at 50 � C and heat it for 20 min. After complete cooling, heat the sample again in the incubator at 50 � C. Then heat and cool the sample through 100 thermal cycles. Finally, perform 6
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second derivative of x was greater than 0. In other words, as the EG mass fraction increased, thermal conductivity increased faster and faster, and the thermal conductivity promotion effect became significantly better. According to Eq. (2), the thermal conductivity values of CPCM for 1% EG
Fig. 8. DSC curves of CPCM3 before and after 100 thermal cycles. Fig. 9. FT–IR spectra of CPCM3 before and after 100 thermal cycles.
heat of CPCM3 after 100 thermal cycles is decreased by 2.5%. It is known that thermal cycles have almost no effect on thermal properties of the CPCM3. Therefore, the CPCM3 has excellent thermal reliability. Fig. 9 shows the FT-IR spectra of CPCM3. The black curve indicates the FT-IR spectrum of the CPCM3 without thermal cycle. The peaks at 2916, 2849, 1474, and 717 cm 1 were attributed to n-eicosane, and the peaks at 1101 and 800 cm 1 belong to SiO2. The blue curve shows the spec trum of the CPCM3 after 100 thermal cycles. The peaks were not different from those of the uncycled CPCM3, which meant that the CPCM3 did not decompose or undergo chemical change. Therefore, CPCM3 has good thermal reliability. 3.6. Thermal conductivity analysis The thermal conductivity of a PCM usually cannot fully meet prac tical requirements. Therefore, EG was added to CPCM to improve thermal conductivity. Thermal conductivity with EG was greatly improved. The thermal conductivities of CPCM1, CPCM2, CPCM3, and NPCM3 were measured by TCM. Each sample was measured five times. Thermal conductivity results were obtained by taking the average value. The results are shown in Fig. 10 and Table 7. Evidently, adding EG significantly improved CPCM thermal conductivity, and the higher the EG content, the higher was the CPCM thermal conductivity. To explore in more depth the specific relationship between EG con tent and CPCM thermal conductivity, the relationship between EG mass fraction and CPCM thermal conductivity was obtained by nonlinear fitting of experimental data. In Fig. 11, black dots represent experi mental data, and solid lines represent fitted curves. The formula can be expressed as follows: λ¼
3
0:00011x þ 0:0122x
2
Fig. 10. Experimental thermal conductivity of the CPCM with different content of EG when the mass fraction of n-eicosane is 70%. Table 7 Thermal conductivity of NPCM3 and CPCM1–CPCM3.
(2)
0:0059x þ 0:1448
where x is the mass fraction of EG, λ is thermal conductivity, and the correlation coefficient of this formula is one. According to the fitted result, the curve became steeper and steeper, which meant that the
Samples
Thermal conductivity at room temperature (W/m⋅K)
NPCM3 (70% n-eicosane) CPCM1 (3% EG) CPCM2 (5% EG) CPCM3 (7% EG) CPCM4 (1% EG) CPCM5 (9% EG)
0.145 � 0.002 0.208 � 0.002 0.286 � 0.003 0.334 � 0.005 0.151 (predicted value) 1.000 (predicted value)
Table 6 DSC data for CPCM3 before and after 100 thermal cycles. Samples Uncycled After 100 thermal cycles
Melting
Solidifying
Onset temperature (� C)
Peak temperature (� C)
Latent heat (J/g)
Onset temperature (� C)
Peak temperature (� C)
Latent heat (J/g)
35.35 � 0.20 35.12 � 0.20
37.71 � 0.20 47.68 � 0.20
135.80 � 6.79 133.23 � 6.66
36.23 � 0.20 36.54 � 0.20
34.30 � 0.20 34.43 � 0.20
125.93 � 6.30 122.59 � 6.13
7
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Fig. 11. Fitting thermal conductivities of the CPCM with different content of EG when the mass fraction of n-eicosane is 70%.
and 9% EG could be predicted. In addition, EG was also an adsorbent material, which could reduce n-eicosane leakage and improve the thermal stability of CPCM. However, as the EG content increased and the mass fraction of n-eicosane decreased, the CPCM latent heat may be affected. 4. Conclusions In this work, a composite PCM was prepared for thermal energy storage. N-eicosane, with its melting latent heat of 243.28 J/g and melting temperature of 36.90 � C, was chosen as a heat-storage medium. Nano-silica, with its good adsorption performance, was used as the supporting material. The thermal conductivity of CPCM was improved by EG with its high thermal conductivity. The leakage test results showed that the maximum adsorption mass fraction of n-eicosane in CPCM occurred at 70% wt%, and 3, 5 and 7 wt% EG were added to the composites. According to the results of DSC, the CPCM3 melting and solidifying temperatures are 35.35 � C and 36.23 � C, and the CPCM3 melting and solidifying latent heats are 135.80 J/g and 125.93 J/g. TGA and thermal cycle test results indicated that CPCM has good thermal stability and reliability. EG improved the thermal conductivity of CPCM, and the thermal conductivity of CPCM3 was 2.37 times that of CPCM without EG. As the specific surface areas of EG and nano-silica increased, the content of n-eicosane in CPCM also increased, improving the thermal properties of CPCM. In addition, the smaller the pore diameter of EG and nano-silica, the larger was the specific surface area, and the higher was the content of n-eicosane in the composite per unit volume. Pore diameter and specific surface area can significantly affect CPCM thermal properties. So, the thermal properties of CPCM could be improved from these two aspects. Meanwhile, their chemical and crystal structures remain unchanged. Therefore, the CPCM had broad application pros pects for thermal energy storage. Acknowledgements This project is supported by the National Natural Science Foundation of China (Grant no. 51676095). The authors also wish to thank the re viewers and editor for kindly providing revision suggestions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. 8
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Preparation and thermal properties of n-eicosane/nano-SiO2/expanded graphite composite phase-change material for thermal energy storage Xinyi Zhang, Chuqiao Zhu, Guiyin Fang * School of Physics, Nanjing University, Nanjing, 210093, China
H I G H L I G H T S
� N-eicosane/nano–SiO2/expanded graphite CPCM were prepared for thermal energy storage. � The CPCM3 melting and solidifying latent heat is 135.80 J/g and 125.93 J/g, respectively. � Thermal conductivity of the CPCM with 7 wt% EG is 2.37 times that of the CPCM without EG. � The CPCM with EG has good thermal stability and reliability. A R T I C L E I N F O
A B S T R A C T
Keywords: Thermal energy storage Composite phase-change material Nano-SiO2 n-eicosane Thermal properties
Composite phase-change materials (CPCM) were prepared using n-eicosane as phase-change material, nano-SiO2 as supporting matrix, and expanded graphite (EG) as thermal conductivity promoter. Nano-SiO2 with strong adsorption performance can prevent leakage of n-eicosane in CPCM. Adding expanded graphite significantly enhanced thermal conductivity of CPCM. Leakage tests showed that the maximum mass proportion of n-eicosane in CPCM was 70 wt%. Fourier transformation infrared spectroscopy (FT-IR), an X-ray diffractometer (XRD), and a scanning electronic microscope (SEM) were used to determine chemical structures, crystal phases, and mor phologies of CPCM. The DSC results indicated that the melting latent heat of CPCM was 135.80 J/g with a melting temperature of 35.35 � C, and that the solidifying latent heat of CPCM was 125.93 J/g with a solidifying temperature of 36.23 � C. Thermal conductivity meter (TCM) test results showed that CPCM had high thermal conductivity, representing a 15%–137% increase compared to CPCM without EG. Thermal properties and chemical structures of CPCM were investigated after 100 thermal cycles. The results indicated that CPCM3 (with 7 wt% EG) had good structure and thermal stability. Thermal conductivity of CPCM3 is 2.37 times that of CPCM without EG. Therefore, CPCM3 has potential application prospects for thermal energy storage.
1. Introduction Global energy demand has surged, particularly in 2017, when it rose by 2.2% compared to its 10-year average of 1.7% [1]. Nowadays, fossil fuels are still the main source of energy supply, resulting in a series of problems such as resource shortages and the greenhouse effect [2]. Therefore, improving energy efficiency and storing heat energy are effective ways to alleviate energy and environmental problems. All countries are stepping up efforts to develop new energy sources. Ther mal energy, which is widely distributed and used, is a kind of conven tional energy that develops continuously [3]. However, heat-storage materials must meet a series of requirements for thermal energy use, such as good thermal stability, structural stability, and high latent heat
to achieve temperature regulation, waste heat recovery, and solar thermal storage [4]. Phase-change materials (PCM) can be used as latent heat-storage materials due to their varieties, wide working temperature range, and high latent heat [5]. Thermal energy storage uses PCM state changes to absorb and release heat. When the ambient temperature is higher than the PCM phasechange temperature, the PCM melts and absorbs heat [6]. When the ambient temperature is lower than the PCM phase-change temperature, the PCM solidifies and releases heat. To control the ambient temperature and energy storage, thermal energy storage is realized in the phase-change process of PCM [7]. According to different phase change forms, phase change materials can be divided into four types: solid-solid, solid-liquid, liquid-gas, and solid-gas phase change materials [8].
* Corresponding author. E-mail address: [email protected] (G. Fang). https://doi.org/10.1016/j.matchemphys.2019.122178 Received 16 June 2019; Received in revised form 23 August 2019; Accepted 12 September 2019 Available online 12 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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Among these, solid-solid and solid-liquid phase-change materials are widely used in practical applications, but solid-gas and liquid-gas phase change are difficult to apply in practical engineering due to their huge volume change. In addition, PCM may leak during the solid-liquid phase-change process [9]. Inorganic PCMs have excellent latent heat and high thermal conductivity, but their structure is unstable, and phase separation may occur [10]. Organic PCMs, including polyols [11], fatty acids [12], paraffins [13], and polymer materials [14] (mainly poly ethylene glycol [15]) have stable structure, are non-corrosive, and offer low toxicity, low cost, and low thermal conductivity [16], which can be improved by adding materials [17] with high thermal conductivity. Therefore, organic PCMs are widely used for thermal energy storage [18]. Yuan et al. [12] investigated fatty acids, including stearic acid (SA), lauric acid (LA), myristic acid (MA), and palmitic acid (PA) as phase-change materials. The results showed that fatty acids have broad prospects as energy storage materials for low-temperature heating. N-octadecane, which has a stable structure in application, was prepared as a PCM by Su et al. [19]. N-eicosane, which has a stable structure, is chemically inert, and does not pollute water, has a phase change-point of 36� C–38 � C. To reduce PCM leakage in the molten state, materials with good adsorption inclusions such as diatomite [20], expanded graphite [21], nano-silica [22], metal foam [23], and organic polymer [24] are added to PCM as supporting materials. Huang et al.’s work indicates that metal foam not only adsorbs phase-change materials, but also greatly improves PCM thermal conductivity [25]. Tang et al. [26] used high density polyethylene as the support material and stearyl alcohol as the PCM to prepare CPCM with a phase-change temperature of 56 � C and a latent heat of 200 kJ/kg. Wang et al. [27] prepared morphologically stable PCM with paraffin and nano-SiO2 and investigated the effect of pore size on paraffin adsorption. The experimental results showed that the adsorption capacity of nano-SiO2 with different pore sizes on paraffin varies and that the PCM with the smallest pore size and the most stable shape of nano-SiO2 has the best thermal stability and reliability. As mentioned above, organic PCM has low thermal conductivity [28], and therefore it is necessary to modify the PCM material. The common way to do this is to add a thermal conductivity promoter to the PCM [29]. Metal oxides, some nanomaterials, and carbon materials are often added to enhance PCM thermal conductivity [30]. In this work, nano-silica is used as the supporting material. Nanosilica has the characteristics of stable, porous, and loose structure, strong adsorption capacity, and very large surface area [31]. In addition, it is a good supporting material because it is nontoxic and non-polluting [32]. N-eicosane also has non-toxic and non-polluting characteristics, with a melting temperature of 36.9 � C and a melting latent heat of 243.28 J/g. The latent heat of n-eicosane is higher than that of alcohol phase-change materials. Compared with other phase-change materials, n-eicosane with the same mass has higher latent heat. N-eicosane is structurally stable and can maintain structural integrity during repeated phase transitions without chemical change. The addition of EG to the PCM can improve CPCM thermal conductivity, making CPCM more suitable for daily use. In this work, the properties of CPCM are investi gated, including chemical structure, crystal phase, morphology, thermal properties, thermal stability and reliability, and thermal conductivity. N-eicosane is insoluble in water and soluble in organic solvents, which can be used to regulate the temperature of buildings to save energy. In addition, in the process of preparing the composite, melting and dispersion-mixing methods, which are normally used to make metalfoam PCM, are used to make alkane samples. Compared with previous methods of making alkane phase-change materials, the supporting ma terials of the composites made by this process are evenly mixed and structurally stable. EG is similar to nanometer silica in having a porous, loose structure and good adsorption. When EG is added to the sample, the specific surface area of the sample is further increased, and EG and silica form a larger structural support, making the sample structure more stable. With
increasing EG content, the thermal conductivity of the composite in creases, but the mass fraction of n-eicosane in the composite decreases, and the latent heat of the composite decreases accordingly. Therefore, an appropriate EG content should be selected to prepare the composite. 2. Experimental 2.1. Materials N-eicosane (C20H42, melting point: 36.0–38.0 � C, analytical reagent) was obtained from Sinopharm Chemical Reagent Co., Ltd., and was used as a thermal-energy storage medium. Nano-silica (nano-SiO2, particle size: 20 nm, purity: 99 wt%, treated by KH550) as a supporting matrix to prevent PCM leakage was bought from Nanjing XFNANO Materials Tech Co., Ltd. Expanded graphite (EG, thickness: 5–20 nm, flake diameter: 5–10 μm, specific surface area: >30 m2/g, purity: 99.5%) was used as a thermal conductivity promoter and was purchased from Nanjing XFNANO Materials Co., Ltd. Anhydrous ethanol (analytical reagent, Sinopharm Chemical Reagent Co., Ltd.) was used as the solvent. 2.2. Preparation of n-eicosane/nano-SiO2 composites N-eicosane was adsorbed onto nano-SiO2 to fabricate n-eicosane/ nano-SiO2 composites. The specific steps were as follows: (1) nano-SiO2 was dispersed in absolute ethanol and stirred at 1,000 rpm for 15 min by a magnetic stirrer, after which the temperature was stabilized at 50 � C. (2) N-eicosane was melted into the liquid state at 50 � C for 15 min (3) The liquid n-eicosane was added to the ethanol solution and mixed with nano-SiO2. (4) The composite was heated to 70 � C and continuously stirred by a magnetic stirrer at 1300 rpm until the absolute ethanol was completely evaporated. After completing this preparation process, the composite was cooled at room temperature for 2 h (5) The composite was dried at 35 � C for 10 h in a vacuum oven to obtain the CPCM without water. A series of CPCMs with different n-eicosane contents were tested to maximize the n-eicosane content in the CPCM without leakage. As shown in Table 1, CPCMs with 65 wt%, 67 wt%, 70 wt%, 73 wt%, and 75 wt% n-eicosane were called NPCM1, NPCM2, NPCM3, NPCM4, and NPCM5. The next step was to perform the leakage tests on the samples obtained. At first, an appropriate amount of the sample was placed on a filter paper, and then the filter paper containing the sample was heated at 50 � C for 30 min. After sample heating, the size of the oil stain on the filter paper indicated the degree of leakage of the sample. The leakage test results are shown in Fig. 1. It is apparent that when the n-eicosane content was 65 wt%, 67 wt%, and 70 wt%, there were no grease spots on the filter paper, which means that no leakage occurred. When the neicosane content was 73 wt%, there was a small circular stain, and when the n-eicosane content was 75 wt%, there was a significant circular stain on the filter paper, which showed substantial leakage. In other words, the maximum load capacity for n-eicosane in CPCM was 70 wt%, and therefore the remaining experiments were carried out on the basis of an n-eicosane content of 70 wt%.
Table 1 Composition of the n-eicosane/nano-SiO2 composite PCM.
2
Samples
n-Eicosane (g)
Nano–SiO2 (g)
Mass fraction of neicosane (wt %)
Leakage in molten state
NPCM1 NPCM2 NPCM3 NPCM4 NPCM5
19.5 20.1 21.0 21.6 22.5
10.5 9.9 9.0 8.4 7.5
65 67 70 72 75
no leakage no leakage no leakage slight leakage leakage
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measured by performing 100 thermal cycles from 20 � C to 50 � C, which simulated the working conditions of CPCM. 3. Results and discussion 3.1. FT-IR analysis The chemical structure and composition of CPCM was measured and analyzed by FT-IR. Fig. 2 presents the FT-IR spectra of n-eicosane, silica, NPCM3, CPCM1, CPCM2, and CPCM3. There are four absorption peaks in Fig. 2a. The peak at 2916 cm 1 represents symmetrical stretching vibration of the –CH3 group, and the symmetrical stretching vibration of the CH2 group is at 2849 cm 1. The peak at 1471 cm 1 signifies the deformation vibration of the CH2 and CH3 groups. The peak at 717 cm 1 represents the rocking vibration of the CH2 chain [33]. As for the spectrum of SiO2 in Fig. 2b, there are three sharp peaks. The peak at 1101 cm 1 signifies the Si–O–Si bond stretching vibration [34]. The peaks at 804 cm 1 and 470 cm 1 are the results of stretching and bending vibration of the Si–O bond [35]. Fig. 2c shows the spectrum of the EG, which is a smooth line without absorption because EG is an elementary substance and does not have a dipole moment [36]. The spectra of NPCM3, CPCM1, CPCM2, and CPCM3 are shown in Fig. 2d–g. All of these have the same absorption peaks from n-eicosane and nano-SiO2. The peaks at 2916, 2849, 1474, and 717 cm 1 are attributed to n-eicosane. The peaks at 1101 and 800 cm 1 belong to SiO2. More over, there are no new peaks, providing evidence that no chemical re action occurred in the CPCM [37].
Fig. 1. Leakage tests of the n-eicosane/nano–SiO2 composites with different neicosane content.
2.3. Preparation of the n-eicosane/nano-SiO2/EG composites EG was used as an additive to enhance the thermal conductivity of neicosane/nano-SiO2 CPCM. Three CPCMs with 3, 5, and 7 wt% EG, called CPCM1, CPCM2, and CPCM3, were prepared. Table 2 shows the specific values. The preparation process was similar to that of n-eico sane/nano-SiO2 composite PCM except that anhydrous ethanol was mixed with the nano-SiO2 for 30 min before adding EG. During prepa ration, the heating temperature and stirring speed were kept constant until the anhydrous ethanol was completely evaporated. As EG content in composite increases, the thermal conductivity of the composite also increases. Meanwhile, as n-eicosane content in compos ite decreases, the latent heat of the composite also decreases. Therefore, the values of thermal conductivity and latent heat need to be balanced, meaning that the EG content in the composite cannot be greatly increased.
3.2. XRD analysis Fig. 3 shows the diffraction patterns of n-eicosane, nano-SiO2, EG, NPCM3, and CPCM1–CPCM3 to determine the crystal structure of CPCM. According to the literature [28], characteristic diffraction peaks at 19.48, 23.59, and 24.91 index as the (010), (100) and (111) planes. As shown in curves d–g, CPCM characteristic peaks appear at 19.48, 23.57, and 24.90. As compared with curves a, b, and c, the CPCM characteristic diffraction peaks are the same as the peaks for n-eicosane. This means that CPCM does not involve any new structure occurrence. CPCM and NPCM were similar, but with the addition of EG, there were no additional diffraction peaks in the image, and the existing peaks became more and more obvious as the EG content increased. This is due to the fact that EG can act as nucleating agent, resulting in strong crystal
2.4. Characterization techniques Fourier transform infrared spectroscopy (FT-IR, Nicolet Nexus 870, resolution using KBr pellets: 2 cm 1, spectral range: 400–4000 cm 1) and X-ray diffractometry (XRD, D/MAX-Ultima III, working voltage: 40 kV, working current: 40 mA, scanning rate: 5� (2θ)/min, 2θ range: 10� to 60� , Rigaku Corporation, Japan) were used to determine the chemical structures and the crystal phase of CPCM. The morphology and microstructure of CPCM were observed and analyzed using a scanning electron microscope (SEM, S–3400NII, operating voltage: 3 kV, Hitachi, Inc., Japan). Thermal properties, including latent heat and phase-change tem perature, of CPCM were measured by a differential scanning calorimeter (DSC, Pyris 1 DSC, PerkinElmer, temperature accuracy: �0.2 � C, enthalpy accuracy: �5%, temperature range: 10–90 � C, rate of temper ature change: 5 � C/min). The thermal conductivity of CPCM was tested by a thermal conductivity meter (TCM 3020, Xiatech Electronic Tech nology Co., Ltd., accuracy: �2%) at room temperature. In addition, the thermal stability of CPCM was tested by thermogravimetry (TGA, Pyris 1 TGA, PerkinElmer, temperature range: 0–700 � C, heating rate: 20 � C/ min). The thermal conductivity of each sample was measured five times, and the average thermal conductivity value was taken as the final thermal conductivity of the sample. The thermal reliability of CPCM was Table 2 Composition of the n-eicosane/nano-SiO2/EG composite PCM. Samples
NPCM3 (g)
EG (g)
Mass fraction of EG (%)
CPCM1 CPCM2 CPCM3
30 30 30
0.93 1.58 2.27
3 5 7
Fig. 2. FT–IR spectra of the (a) n-eicosane (b) nano–SiO2, (c) EG, (d) NPCM3, (e) CPCM1, (f) CPCM2 and (g) CPCM3. 3
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orientation. This has a beneficial effect on crystallization of the CPCM. The higher crystallinity of the CPCM, the stronger diffraction peaks of the CPCM. Therefore, it can be concluded that the n-eicosane remains intact in CPCM without chemical reaction and is not affected by silica. All combinations are formed by physical action. 3.3. Morphological analysis The morphologies of EG, nano-SiO2, NPCM3, and CPCM1–CPCM3 were observed by SEM and are shown in Fig. 4. The picture shows that nano-silica has a porous and loose particle structure, but the structure of the EG is layered and multi-pore. In the NPCM3, the n-eicosane is well coated with silica, and the particles are stacked together. The CPCM1 structure, with a mass fraction of 3% EG, is still not different from that of NPCM3. The n-eicosane and EG are still well coated by nano-silica. When the mass fraction of the EG is 7 wt%, CPCM3 shows an obvious layered structure. It can be clearly seen that the n-eicosane is evenly dispersed and attached to the structure formed by nano-silica and EG. Both nano-silica and the EG multilayer structure can avoid n-eicosane leakage in the molten state. According to the leakage test and observa tion of the morphology, the maximum weight of n-eicosane in CPCM is 70 wt%, beyond which n-eicosane cannot be completely adsorbed, resulting in leakage. In addition, EG addition leads to the lamellar state of the stacked CPCM, further increasing CPCM surface area, but the decrease in n-eicosane content affects the latent heat.
Fig. 3. XRD patterns of (a) n-eicosane, (b) nano–SiO2, (c) EG, (d) NPCM3, (e) CPCM1, (f) CPCM2 and (g) CPCM3.
Fig. 4. SEM photographs of (a) EG, (b) nano–SiO2, (c) NPCM3, (d) CPCM1, (e) CPCM2 and (f) CPCM3. 4
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5
241.00 � 12.05 153.70 � 7.69 153.86 � 7.70 138.09 � 6.90 125.93 � 6.30 33.31 � 0.20 34.10 � 0.20 34.08 � 0.20 34.47 � 0.20 34.30 � 0.20 33.60 � 0.20 35.90 � 0.20 36.18 � 0.20 36.19 � 0.20 36.23 � 0.20 243.28 � 12.16 163.41 � 8.15 163.91 � 8.20 148.78 � 7.44 135.80 � 6.79 38.78 � 0.20 38.10 � 0.20 38.89 � 0.20 37.87 � 0.20 37.71 � 0.20 36.90 � 0.20 35.00 � 0.20 35.67 � 0.20 35.21 � 0.20 35.35 � 0.20
Onset temperature (� C)
Samples
Table 3 DSC data for n-eicosane, NPCM3, and CPCM1–CPCM3.
The thermal stability of CPCM was measured by TGA and DTG (de rivative thermogravimetric analysis), as shown in Figs. 6 and 7. The specific data are listed in Table 5, including the temperature (Tp) and the weight loss percentage (△W) at the maximum thermal degradation rate and the residual percentage at 700 � C. As shown in Fig. 6, n-eicosane had one decomposition stage. In the phase from 25 � C to 230 � C, n-eicosane decomposition occurred, with a decomposition weight greater than 95%. The maximum decomposition rate of n-eicosane was greater than 50% at a temperature of 220.9 � C, as shown in Fig. 7. When the tem perature was below 100 � C, n-eicosane shows almost no weight loss, which proved the thermal stability of n-eicosane as a phase-change material at room temperature. For nano-SiO2, there was almost no loss except for the weight loss of water, which could be attributed to evap oration of water adsorbed into nano-SiO2. NPCM3, CPCM1, CPCM2, and CPCM3 are all dry samples, and they had only one stage of decompo sition. Most of the n-eicosane decomposes, and after 230 � C, the remaining phase-change materials decomposed. However, it could be clearly seen that, as the weight fraction of EG increased from 3% to 7%, the decomposition degree of n-eicosane decreased, the residual CPCM increased. This indicated that adding EG can improved the CPCM coating. This is due to that the EG can form charred layers building up on the surface. The charred layers may create a physical protective barrier on the surface of the CPCM. This can effectively reduce the thermal decomposition of n-eicosane, thus improving the thermal stability of the
Peak temperature (� C)
3.5. Thermal stability and reliability analysis
N-eicosane NPCM3 CPCM1 CPCM2 CPCM3
Peak temperature (� C) Solidifying
Latent heat (J/g)
where △HCPCM and △HPCM refer to the latent heats of CPCM and of pure n-eicosane and η is the n-eicosane mass fraction in CPCM. It can be seen from Eq. (1) that the CPCM latent heat and the n-eicosane mass fraction had a proportional relationship, which could be verified using the data in Table 3. Besides, the latent heat of CPCM was lower than that of the NPCM3 because of the addition of EG. In other words, if the mass fraction of n-eicosane in CPCM was less than 70 wt%, the mass fraction of n-eicosane in CPCM3 was 66.5 wt%. Table 4 shows some thermal properties of CPCM using fatty acids and alcohol as heat-storage media in previous research, including melting temperature, latent heat of melting, and mass fraction of PCM. It can be determined that the CPCM proposed in this study is a potential thermalenergy storage material.
Onset temperature (� C)
(1)
Melting
ΔHCPCM ¼ η⋅ΔHPCM
Latent heat (J/g)
Thermal properties include phase-change temperature and latent heat during melting and solidification, which are measured by DSC. The thermal properties of n-eicosane, NPCM3, CPCM1, CPCM2, and CPCM3, were tested. The relevant data are presented in Table 3, and the phasechange processes are illustrated in Fig. 5, where Fig. 5a shows an endothermic process and Fig. 5b an exothermic process. The melting/ solidification temperatures of n-eicosane are 36.90/33.60 � C, and those of NPCM3, CPCM1, CPCM2, and CPCM3 are 35.00/35.90 � C, 35.67/ 36.18 � C, 35.21/36.19 � C, and 35.35/36.23 � C respectively. The CPCM phase-change temperature was found to be slightly different from that of pure n-eicosane, as shown in Fig. 5. CPCM, with its phase-change tem perature around 36 � C, can be used as a building material, as indoor thermal insulation, and for industrial waste heat recovery. The latent heats of melting and solidification of n-eicosane are 243.28/241.00 J/g, and those of NPCM3, CPCM1, CPCM2, and CPCM3 are 163.41/153.70 J/g, 163.91/153.86 J/g, 148.78/138.09 J/g, and 135.80/125.93 J/g respectively. Obviously, CPCM latent heat decreases as EG content increases because n-eicosane is the only phase-change material in CPCM, and therefore the latent heat of CPCM is related to the proportion of n-eicosane. The relationship between the latent heat of CPCM and the proportion of n-eicosane can be expressed as:
– 169.50 164.40 161.06 157.57
Theoretical latent heat (J/g)
3.4. Thermal properties analysis
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Materials Chemistry and Physics 240 (2020) 122178
Fig. 6. TGA curves of n-eicosane, nano–SiO2, NPCM3, CPCM1, CPCM2 and CPCM3.
Fig. 5. (a) Melting DSC curves and (b) solidifying DSC curves of n-eicosane, NPCM3, CPCM1, CPCM2 and CPCM3. Fig. 7. DTG curves of n-eicosane, nano–SiO2, NPCM3, CPCM1, CPCM2 and CPCM3.
Table 4 Comparison of the present work with results for other PCMs in the literature. Composite PCM
Melting temperature (� C)
Melting latent heat (J/g)
References
Capric acid/expanded perlite PA/high density polyethylene PA/polypyrrole Myristic acid/high density polyethylene Octadecane/titania N-eicosane/Fe3O4 & SiO2 Octadecane/calcium carbonate N-eicosane/nano-silca
30.70
74.08
[38]
61.60
161.10
[39]
59.80 53.80
166.30 119.38
[40] [41]
25.68 33.42 29.19
42.57 69.60 84.37
[42] [43] [44]
35.00
163.41
present study
Table 5 TGA data for n-eicosane, nano-SiO2, NPCM3, and CPCM1–CPCM3. Samples
Tp (� C)
△W (%)
Residue (%) (700 � C)
N-eicosane Nano–SiO2 NPCM3 CPCM1 CPCM2 CPCM3
222.90 � 0.20 – 225.23 � 0.20 226.53 � 0.20 209.16 � 0.20 202.85 � 0.20
100 4.52 68.19 69.22 63.92 60.10
0 95.48 31.81 30.78 36.08 39.90
FT-IR spectral analysis on the obtained sample and the unprocessed sample and measure their thermal conductivity. After 100 thermal cy cles, measure the thermal performance of the CPCM3 using DSC. Fig. 8 shows the DSC curves of CPCM3 before and after 100 thermal cycles, and Table 6 shows the specific data. The melting and solidifying temperatures of CPCM3 without thermal cycle are 35.35 � C and 36.23 � C, and the melting and solidifying temperatures of CPCM3 after 100 thermal cycles are 35.12 � C and 36.54 � C. It is found that the melting temperature and solidifying temperature of CPCM3 after ther mal cycles are almost the same as that of uncycled CPCM3. In addition, compared with the uncycled CPCM3, the melting latent heat of CPCM3 after 100 thermal cycles is decreased by 1.8%, and the solidifying latent
CPCM. Thermal reliability is also crucial for the use of CPCM, which has been verified by 100 thermal cycle tests at 50 � C. The steps of the thermal cycle test were as follows: first, take an appropriate mass of CPCM3 solid powder and place it onto filter paper. Then place the sample in a thermostatic chamber at 50 � C and heat it for 20 min. After complete cooling, heat the sample again in the incubator at 50 � C. Then heat and cool the sample through 100 thermal cycles. Finally, perform 6
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Materials Chemistry and Physics 240 (2020) 122178
second derivative of x was greater than 0. In other words, as the EG mass fraction increased, thermal conductivity increased faster and faster, and the thermal conductivity promotion effect became significantly better. According to Eq. (2), the thermal conductivity values of CPCM for 1% EG
Fig. 8. DSC curves of CPCM3 before and after 100 thermal cycles. Fig. 9. FT–IR spectra of CPCM3 before and after 100 thermal cycles.
heat of CPCM3 after 100 thermal cycles is decreased by 2.5%. It is known that thermal cycles have almost no effect on thermal properties of the CPCM3. Therefore, the CPCM3 has excellent thermal reliability. Fig. 9 shows the FT-IR spectra of CPCM3. The black curve indicates the FT-IR spectrum of the CPCM3 without thermal cycle. The peaks at 2916, 2849, 1474, and 717 cm 1 were attributed to n-eicosane, and the peaks at 1101 and 800 cm 1 belong to SiO2. The blue curve shows the spec trum of the CPCM3 after 100 thermal cycles. The peaks were not different from those of the uncycled CPCM3, which meant that the CPCM3 did not decompose or undergo chemical change. Therefore, CPCM3 has good thermal reliability. 3.6. Thermal conductivity analysis The thermal conductivity of a PCM usually cannot fully meet prac tical requirements. Therefore, EG was added to CPCM to improve thermal conductivity. Thermal conductivity with EG was greatly improved. The thermal conductivities of CPCM1, CPCM2, CPCM3, and NPCM3 were measured by TCM. Each sample was measured five times. Thermal conductivity results were obtained by taking the average value. The results are shown in Fig. 10 and Table 7. Evidently, adding EG significantly improved CPCM thermal conductivity, and the higher the EG content, the higher was the CPCM thermal conductivity. To explore in more depth the specific relationship between EG con tent and CPCM thermal conductivity, the relationship between EG mass fraction and CPCM thermal conductivity was obtained by nonlinear fitting of experimental data. In Fig. 11, black dots represent experi mental data, and solid lines represent fitted curves. The formula can be expressed as follows: λ¼
3
0:00011x þ 0:0122x
2
Fig. 10. Experimental thermal conductivity of the CPCM with different content of EG when the mass fraction of n-eicosane is 70%. Table 7 Thermal conductivity of NPCM3 and CPCM1–CPCM3.
(2)
0:0059x þ 0:1448
where x is the mass fraction of EG, λ is thermal conductivity, and the correlation coefficient of this formula is one. According to the fitted result, the curve became steeper and steeper, which meant that the
Samples
Thermal conductivity at room temperature (W/m⋅K)
NPCM3 (70% n-eicosane) CPCM1 (3% EG) CPCM2 (5% EG) CPCM3 (7% EG) CPCM4 (1% EG) CPCM5 (9% EG)
0.145 � 0.002 0.208 � 0.002 0.286 � 0.003 0.334 � 0.005 0.151 (predicted value) 1.000 (predicted value)
Table 6 DSC data for CPCM3 before and after 100 thermal cycles. Samples Uncycled After 100 thermal cycles
Melting
Solidifying
Onset temperature (� C)
Peak temperature (� C)
Latent heat (J/g)
Onset temperature (� C)
Peak temperature (� C)
Latent heat (J/g)
35.35 � 0.20 35.12 � 0.20
37.71 � 0.20 47.68 � 0.20
135.80 � 6.79 133.23 � 6.66
36.23 � 0.20 36.54 � 0.20
34.30 � 0.20 34.43 � 0.20
125.93 � 6.30 122.59 � 6.13
7
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Fig. 11. Fitting thermal conductivities of the CPCM with different content of EG when the mass fraction of n-eicosane is 70%.
and 9% EG could be predicted. In addition, EG was also an adsorbent material, which could reduce n-eicosane leakage and improve the thermal stability of CPCM. However, as the EG content increased and the mass fraction of n-eicosane decreased, the CPCM latent heat may be affected. 4. Conclusions In this work, a composite PCM was prepared for thermal energy storage. N-eicosane, with its melting latent heat of 243.28 J/g and melting temperature of 36.90 � C, was chosen as a heat-storage medium. Nano-silica, with its good adsorption performance, was used as the supporting material. The thermal conductivity of CPCM was improved by EG with its high thermal conductivity. The leakage test results showed that the maximum adsorption mass fraction of n-eicosane in CPCM occurred at 70% wt%, and 3, 5 and 7 wt% EG were added to the composites. According to the results of DSC, the CPCM3 melting and solidifying temperatures are 35.35 � C and 36.23 � C, and the CPCM3 melting and solidifying latent heats are 135.80 J/g and 125.93 J/g. TGA and thermal cycle test results indicated that CPCM has good thermal stability and reliability. EG improved the thermal conductivity of CPCM, and the thermal conductivity of CPCM3 was 2.37 times that of CPCM without EG. As the specific surface areas of EG and nano-silica increased, the content of n-eicosane in CPCM also increased, improving the thermal properties of CPCM. In addition, the smaller the pore diameter of EG and nano-silica, the larger was the specific surface area, and the higher was the content of n-eicosane in the composite per unit volume. Pore diameter and specific surface area can significantly affect CPCM thermal properties. So, the thermal properties of CPCM could be improved from these two aspects. Meanwhile, their chemical and crystal structures remain unchanged. Therefore, the CPCM had broad application pros pects for thermal energy storage. Acknowledgements This project is supported by the National Natural Science Foundation of China (Grant no. 51676095). The authors also wish to thank the re viewers and editor for kindly providing revision suggestions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. 8
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