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Highly stable hierarchical porous nanosheet composite phase change materials for thermal energy storage
Applied Thermal Engineering 163 (2019) 114417
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Highly stable hierarchical porous nanosheet composite phase change materials for thermal energy storage Xiaochao Zuoa,b,c, Zhaoli Yana, Kai Houa, Huaming Yanga,d,e, , Yunfei Xib,c, ⁎
T
⁎
a
Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, QLD 4001, Australia c Institute for Future Environments, Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia d Hunan International Joint Lab of Mineral Materials, Central South University, Changsha 410083, China e Key Lab of Clay Mineral Functional Materials in China Building Materials Industry, Central South University, Changsha 410083, China b
HIGHLIGHTS
GRAPHICAL ABSTRACT
porous nanosheets • Hierarchical (HPNTs) were synthesized from kaolinite.
HPNTs were modified by grafting • The with (3-aminopropyl)triethoxysilane (APTES)
HPNT (NH -HPNT) • APTES-modified was used a substrate for phase-change 2
materials.
stearic acid-based (SA-based) • Novel phase-change materials were prepared.
-HPNT is a promising supporting • NH material for thermal energy-storage 2
applications.
ARTICLE INFO
ABSTRACT
Keywords: Hierarchical porous nanosheet Kaolinite Stearic acid Phase change material Thermal energy storage
Thermal energy storage is considered as an effective strategy for improving energy efficiency, and phase change materials (PCMs) are promising in that regard. However, the leakage in the melting process and low thermal conductivity of phase change materials are a big challenge for their practical application. Herein we prepared an amino-functional hierarchical porous nanosheet (NH2-HPNT) by the template-free structural reorganisation of natural clay mineral (kaolinite) and subsequent modification with (3-aminopropyl) triethoxysilane (APTES). NH2-HPNT was hybridized with stearic acid (SA) to produce novel form-stable composite phase change material (PCM). The effect of the NH2-HPNT microstructure on the thermal properties of the composite PCM was investigated. The samples were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and differential thermogravimetry (DTG). After serious leakage testing, the maximum amount of SA loaded onto NH2-HPNT without leakage was determined to be 63.5 wt%. NH2-HPNT/SA composite PCM melts at 68.3 °C with a phase change enthalpy of 118.6 J/g, and solidifies at 63.7 °C with that of 111.8 J/g. The as-prepared composite PCM is
⁎ Corresponding authors at: Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China (H. Yang). School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, QLD 4001, Australia (Y. Xi). E-mail addresses: [email protected] (H. Yang), [email protected] (Y. Xi).
https://doi.org/10.1016/j.applthermaleng.2019.114417 Received 26 July 2019; Received in revised form 18 September 2019; Accepted 19 September 2019 Available online 19 September 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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thermally stable. In comparison with SA, the thermal conductivity of NH2-HPNT/SA increase by up to 60%. Additionally, thermal cycling experiments showed that NH2-HPNT/SA is significantly more reliable than HPNT/ SA after 200 heating cycles. The mechanism responsible for improving the thermal properties of the composite PCMs is proposed. The results indicate that NH2-HPNT could be a promising supporting material for PCMs in thermal energy storage applications.
1. Introduction
[36]. The proportion of capric acid, PEG600, and heptadecane impregnated in Kaol was determined by seepage to 17.5, 21, and 16.5 wt %, respectively. In conclusion, Kaol as a supporting material for PCMs has received significant attention. However, the loading of PCMs in raw Kaol leads the Kaol-based form-stable composite PCM to exhibit low thermal energy-storage efficiency, which is due to the insufficient space to adsorb the more PCMs. Therefore, it would be interesting for improving the pore property of low-cost Kaol via preparation of hierarchical porous material. Hierarchical porous nanosheet synthesised using low-cost Kaol as a starting material, to the best of our knowledge, has not been studied in thermal energy storage systems. Herein this work firstly reported the preparation of hierarchical porous nanosheet and further fabricated an amino-functional hierarchical porous nanosheet (NH2-HPNT) using (3aminopropyl) triethoxysilane (APTES) as a surface modifier to improve the thermal properties of form-stable composite PCM. Finally, an NH2HPNT/SA form-stable composite PCM was prepared by incorporating stearic acid into the pores of NH2-HPNT using the vacuum-impregnation method. The preparation procedure and sample designations are shown schematically in Fig. 1. X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were used to detailedly characterise the prepared materials. Also, thermal cycling experiments were used to determine the thermal reliabilities of the composite PCMs.
With the developing economy and improving living stands, the demand for energy has increased dramatically in recent years, which has led to a variety of problems that include the exhaustion of fossil fuels, environmental pollution, and increasing levels of carbon dioxide emissions. Therefore, improving energy-use efficiency and environmental protection are important issues. Thermal energy storage (TES), which includes sensible heat storage, latent heat storage, and thermochemical storage, is considered to be one of the most effective methods for improving energy efficiency [1]. Compared with other methods, latent heat storage is a more-promising heat-storage technology by virtue of the advantages associated with phase change materials (PCMs), which have high energy-storage densities and isothermal characteristics [2,3]. PCMs are mainly divided into two types (inorganic and organic) based on their chemical compositions [1,2,4]. In addition to the abovementioned advantages of PCMs, organic PCMs have many other desirable properties, such as a variety of phase-change temperatures, negligible supercooling behavior, low vapor pressures in the melt, chemical inertness and stabilities, self-nucleation, a lack of phase segregation, and their commercial availabilities [5,6]. However, the disadvantages of their low thermal conductivities and the leakage of organic PCMs during the phase change process have limited their applications. To overcome the two problems, considerable efforts have been devoted to developing form-stable composite PCMs without leakage and with high conductivity [7–9]. Clay minerals with porous structures and considerable specific surface areas have been widely used for the synthesis of advanced functional materials [10–17], and especially integrated with organic PCMs to prepare form-stable composite PCMs [18]. Pore confinement and mutual interactions between the mineral pore surface and PCM molecules could restrict the leakage during the PCM phase-change process. On the other hand, due to the high thermal conductivity of the natural mineral, the thermal conductivity of the organic material is enhanced significantly after incorporation into the clay [19,20]. To date, clay minerals have been used to prevent organic PCMs leakage and increase the thermal conductivity, including perlite [21,22], sepiolite [23,24], diatomite [25], bentonite [26], montmorillonite [27,28], attapulgite [29], opal [30], and hydroxyapatite [31], etc. Among various clay minerals, kaolinite (Kaol), Al2Si2O5(OH)4, is a layered magnesium aluminum silicate with a 1:1 crystalline structure belonging to kaolin group of minerals. Globally, Kaol is distributed widely, especially in China, and has been extensively used to manufacture paper, ceramics, inks, and paints, as well as additives in the production of rubber and polymers [32–34]. In recent years, a new application that uses Kaol as a support matrix for PCMs has been explored. Li et al. incorporated paraffin into three different kinds of kaolin (platelet kaolin, layered kaolin, and rod kaolin) [20], and revealed that the layered kaolin had relatively good thermal properties and thermal conductivity (0.78 W/(m K)). In addition, Lv et al. studied the effect the Kaol particle size on the thermal properties of composite PCMs, showing that the highest thermal conductivity was exhibited by the Kaol-based composite PCM with the largest particle size [19]. A lauricacid/intercalated-Kaol form-stable composite PCM was reported by Song et al. [35]; the phase-change temperature and latent heat of this material were determined to be 43.7 °C and 72.5 J/g. Sari fabricated three Kaol-based PCMs using capric acid, PEG600, and heptadecane
2. Experimental 2.1. Materials Kaolinite (Kaol) was supplied by the China Kaolin Clay Co., Ltd., China. Stearic acid (SA) was obtained from the Tianjin Kemiou Chemical Reagent Co., Ltd., China. Hydrochloric acid, ammonium hydroxide, ethanol, and methylbenzene, were purchased from the Sinopharm Chemical Reagent Co., Ltd., China. Methanol was supplied by the Xilong Chemical Reagent Co., Ltd., China. APTES (98%) was purchased from the Aladdin Reagent Co., Ltd., China. All reagents were used as received. 2.2. Preparation of HPNT Kaol was first converted to anhydrous-phase metakaolinite by heating in a muffle furnace at 700 °C for 2 h (Fig. 1). An 8-g sample of metakaolinite and 400 mL of 2 M HCl were added to 500-mL flask. The mixture was placed in a water bath at 85 °C with stirring for 8 h. After standing and cooling to room temperature, the precipitate was collected by centrifugation and washed repeatedly (~5×) with distilled water. The precipitate was then dispersed in 100 mL of ethanol in distilled water (1:1 v/v) followed by sonication for 2 h (400 W, 24 kHz). The solution was then mixed with about 300 mL of a leached aluminum-ion solution, and the leached Al ions were then reprecipitated by the gradual addition of 28% ammonium hydroxide with vigorous stirring until the desired pH of 8.6 was reached. The suspension was transferred to Teflon-lined stainless steel autoclave and then placed in an oven at 180 °C for 12 h. The white product was collected by centrifugation and repeatedly washing with distilled water. The sample was then dried at 100 °C for 12 h and ground into a fine powder to obtain hierarchical porous nanosheet, referred to as “HPNT”. 2
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Fig. 1. Schematic diagram depicting the preparation of NH2-HPNT and the vacuum-impregnation method for the preparation of the NH2-HPNT/SA composite.
2.3. Modification of HPNT
2.5. Characterisation
A 0.5-g sample of HPNT powder was dispersed in 25 mL of fresh methanol by ultrasonication for 30 min, after which 50 mL of toluene was added into to the suspension with vigorous stirring. The methanol was then evaporated under a flow of argon at 75 °C for 1.5 h. A 5-mL aliquot of APTES was added dropwise to the suspension, vigorously stirred for 1 h under the same conditions. After naturally cooling to room temperature, the precipitate was collected by centrifugation and washed three times with fresh toluene and ethanol, then dried in a vacuum drying oven at 70 °C for 12 h. The obtained white product was milled to a fine powder, referred to as “NH2-HPNT”.
The pore size distributions and Brunauer-Emmett-Teller (BET) specific surface area were measured by a nitrogen adsorption apparatus (ASAP 2020, USA) at 77 K. The external surface morphology and microstructure of prepared supporting materials and corresponding composites PCMs were characterized by scanning electron microscope instrument (SEM, Tescan Mira3 LMU, Czech). In order to determine their compatibilities, the composite PCMs were subjected to X-ray diffractometry (XRD, Bruker-AXS D8, Germany) using Cu Kα radiation (0.15406 nm). The XRD patterns were acquired under the following conditions: voltage, 40 kV; current, 40 mA; scanning range, 3–80°; scan rate, 4°/min; and step size, 0.02°. The chemical groups were determined by Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, USA) in the range of 600–4000 cm−1 with a resolution of 4 cm−1 at room temperature. The thermal properties of stearic acid and the form-stable composite PCMs were examined by differential scanning calorimeter (DSC, DSC214 Polyma, Germany) at a heating/cooling rate of 5 °C/min under nitrogen in the 40–80 °C temperature range. The thermal stabilities of the prepared form-stable PCM composites were determined by the thermogravimetric analyser (TGA, TGA-DSC I/1600 HT, USA) under an inert nitrogen atmosphere at 40 mL/min and a 10 °C/min heating rate. The thermal conductivity was measured by a thermal conductivity meter (XIATECH TC 3000, China) at room temperature. Thermal cycling experiments were conducted using a thermal cycler (BIOER model TC-25/H, China). The cycling process included melting of the composite PCM at 80 °C for 10 min and cooling at 40 °C for the same amount of time.
2.4. Preparation of form-stable composite PCMs The HPNT/SA and NH2-HPNT/SA form-stable composite PCMs used herein were manufactured by the vacuum impregnation method. Both form-stable composite PCMs were prepared in the same manner; hence, only the process for the preparation of NH2-HPNT/SA is described. A 7 g sample of SA and 3 g of NH2-HPNT were uniformly ground together, after which the mixture was placed in a filter flask and vacuum evacuated to −0.1 MPa for 10 min. The flask was then placed in a thermostat-controlled water bath and heated at 90 °C for 30 min. Afterwards, the vacuum was removed in order to force the liquid SA into the NH2-HPNT porous network. The NH2-HPNT/SA prepared in this manner was slowly cooled to room temperature and removed from the flask. Then the entire composite was placed in an oven at 90 °C for 48 h on filter paper in order to remove the excess PCM. The filter paper was changed every 2 h until no further leakage of melted PCM from the composite PCM was observed. Leakage testing was carried out as follows: Each composite PCM was first ground into a fine powder and pressed into a 25-mm-diameter disc. The disc was then placed on a filter paper and heated in an oven at 90 °C for 30 min. The seepage test results are shown in Fig. S1.
3. Results and discussion 3.1. Characterisation of the supporting materials In order to investigate the absorbability of supporting materials, the properties of the pore structure of supporting materials were measured by BET and N2 adsorption and desorption. Fig. 2 illustrates the N2 3
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Fig. 2. (a) Nitrogen adsorption-desorption isotherms and (b) corresponding pore size distributions of the selected supporting matrix: HPNT and NH2-HPNT.
indicating that more SA was loaded on the NH2-HPNT, which is in accordance with the results of TG analysis.
Table 1 Textural properties of Kaol, HPNT and NH2-HPNT. Samples
Surface area (m2/g)
Average pore diameter (nm)
Total Pore Volume (cc/g)
Kaol HPNT NH2-HPNT
19.6 310.8 249.8
16.4 14.0 20.2
0.08 1.09 1.26
3.3. Chemical compatibilities of the form-stable composite PCMs The chemical compatibilities of the components in the prepared form-stable composite PCMs were characterised by XRD and FTIR spectroscopy. Fig. 4a displays the XRD patterns of SA, HPNT, HPNT/SA, NH2-HPNT, and NH2-HPNT/SA. Pure SA exhibits three characteristic diffraction peaks at 6.7°, 21.7° and 24.2° in its XRD pattern [28], while HPNT exhibits some weak broad peaks corresponding to SiO2 and AlOOH nanosheet (Fig. S3). The characteristic peaks of NH2-HPNT are similar to those of HPNT, indicating that the structure of NH2-HPNT was not affected by organic modification with APTES. A comparison of the energy-dispersive X-ray spectra (data not shown here) of HPNT before and after modification reveals the presence of elemental nitrogen in NH2HPNT, which indicates that APTES was successfully grafted onto the surface of HPNT. SA was then impregnated into HPNT and NH2-HPNT. Compared with the XRD patterns of the supporting matrixes, the patterns of the composite PCMs exhibit diffraction peaks of pure SA in addition to the peaks of the supporting matrixes. These results demonstrate that the crystal structure of SA was not destroyed by impregnation. Fig. 4b shows the FTIR spectra of SA, HPNT, HPNT/SA, NH2-HPNT, and NH2-HPNT/SA. The absorption bands at 2917 and 2849 cm−1 in the spectrum of SA are attributed respectively to the asymmetric and symmetric stretching vibrations of eCH2 groups. The active absorption band at 1702 cm−1 is assigned to the stretching vibration of the carboxylic acid (C]O), while bands observed at 1472 cm−1 and 719 cm−1 represent the in-plane scissoring motions of eCH2 groups and out-ofplane bending motions of eCH groups. The bands at 3690 and 3620 cm−1 in the spectrum of raw Kaol are assigned to the hydroxyl stretching vibrations of inner-surface hydroxyls and inner hydroxyls, respectively. The multiple bands located at 1200–1000 cm−1 region are due to the SieO stretching vibrations, while the band at 911 cm−1 corresponds to AleOH bending vibrations, and the bands at 789 and 678 cm−1 are due to AleO stretching vibrations. The spectrum of HPNT displays a visible band at 1064 cm−1 associated with the SieO stretching vibration. A new band is observed at 1587 cm−1 following incorporation SA in the spectrum HPNT/SA, consistent with chemical reactions between HPNT and SA. From the mechanism analysis, the new band can be assigned to the group of AlOOCe. In addition, bands corresponding to the main functional groups of SA are also present in the spectrum of HPNT/SA, with only slight shifts observed. Compared with that of HPNT, the spectrum of NH2-HPNT exhibits almost no change following incorporation of APTES. Clearly, no significant new band appeared in the spectrum of the NH2-HPNT/SA composite PCM, indicating no chemical reaction between NH2-HPNT and SA. On the
adsorption-desorption isotherms and pore diameter distribution curves of HPNT and NH2-HPNT. Table 1 summarizes the textural properties of the corresponding samples. The isotherms of HPNT and NH2-HPNT displays Ⅳ type isotherms with a hysteresis loop at above 0.65 P/P0 that reflects the presence of mesopores (Fig. 2a). Two hysteresis loops of HPNT and NH2-HPNT at the P/P0 range of 0.6–1.0 are H3 type, which indicates that the presence of slitlike pores [37]. Moreover, in comparison with HPNT, the hysteresis loops shape of NH2-HPNT is more similar to the type H3, implying that more ink-bottle type pores transformed into slitlike pores after the modification treatment. Fig. 2b shows the corresponding pore diameter distribution of the received supporting matrix. The pore diameter distribution curves of HPNT and NH2-HPNT are very similar, which shows multimodal distribution, implying that the existence of micropore, mesopore and microporous. Thus, both HPNT and NH2-HPNT can be defined as hierarchical pores materials. Regarding the BET surface area, the supporting HPNT has the maximum value of 310.8 m2/g. After HPNT was modified by APTES, the specific surface area decreased to 249.8 m2/g, while the pore value volume increased from 1.09 to 1.26 cc/g. Compared with natural Kaol (Table 1), the textural properties of the prepared supporting materials significantly increased, which is beneficial to the immobilisation of SA for preventing the leakage. 3.2. Morphology and microstructure of composite PCMs Fig. 3 shows the SEM images of the prepared supporting materials and corresponding composite PCMs. As illustrated from Fig. 3a, HPNT shows a loose layered structure, which is assembled by SiO2 nanosheet and smaller and thinner AlOOH nanosheet (Fig. S2a). Compared with Kaol (Fig. S2b), the pseudo-hexagonal shaped platelet structure disappeared. When HPNT was modified by APTES, more and bigger sheets aggregations were observed in Fig. 3b, resulting in a decrease of specific area surface. It is in conformity with the results of BET analysis. It is clear that the SA was well dispersed into the porous networks of the received supporting materials (Fig. 3c and d). The surface tension and capillary force exhibits between supporting materials and SA could prevent leakage behavior during the melting process. In comparison with HPNT/SA, the morphology of HPNT still can be obviously found, 4
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Fig. 3. SEM images of (a) HPNT, (b) NH2-HPNT, (c) HPNT/SA and (d) NH2-HPNT/SA.
Fig. 4. (a) XRD patterns and (b) FTIR spectra of SA, HPNT, HPNT/SA, NH2-HPNT, and NH2-HPNT/SA.
Fig. 5. TGA and DTG curves of SA, support materials and the prepared composite PCMs. 5
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basis of these results, it could be concluded that it is necessary to determine compatibility between the supporting material and the organic molecule through the combined use of XRD and FTIR spectroscopy. 3.4. Thermal stabilities of the form-stable composite PCMs The thermal stabilities of the composite PCMs play essential roles for thermal energy-storage applications. The thermal stabilities of the HPNT/SA and NH2-HPNT/SA composites were evaluated by TGA. Fig. 5 shows the TGA and DTG curves of SA and the prepared composite PCMs, with the 5%-weight-loss temperature (T-5 wt%), the maximum decomposition peak temperatures (Tpeak1, Tpeak2, and Tpeak3), and the charred residues at 700 °C listed in Table 1. Fig. 3a and Table 1 reveal that the thermal-degradation-onset temperatures (the 5%-weight-loss temperature) of SA and the composite PCMs exceeded 200 °C; hence, the prepared composite PCMs are very thermally stable in the working temperature range. In detail, pristine SA was observed to degrade in one step in the 200–300 °C temperature range, with a maximum decomposition peak temperature (Tpeak1) of 277.3 °C. However, Fig. 3a&b show that the prepared composite PCMs decompose over two or three stages; the HPNT/SA and NH2-HPNT/SA samples exhibit weight losses between 320 and 600 °C, which is mainly due to water evaporation from the supports and dehydroxylation. In comparison, the maximum decomposition rate of NH2-HPNT/SA is observed at 283.5 °C, higher than that of HPNT/SA, indicating that the NH2-HPNT/SA composite is more thermally stable. The impregnation ratio of SA in the composite PCMs is very important to determine the thermal energy-storage efficiencies. The TG and DTG curves of HPNT and NH2-HPNT are shown in Fig. 5. In the case of HPNT, the mass loss is 21% when the measuring temperature changes from 30 to 700 °C, which is caused by the removal of the absorbed water and hydroxyl groups. However, it is determined that the total weight loss percentages are about 22% for NH2-HPNT, up to the same temperature of 700 °C. The different mass loss between HPNT and NH2-HPNT is due to the existence of APTES. Table 2 reveals weight-loss percentages to 700 °C of about 99.64%, 67.5%, and 71.8% for SA, HPNT/SA, and NH2-HPNT/SA, respectively. Based on the mass loss of the composite PCMs and the supporting matrix, the SA loading (M %) in the composite PCMs was calculated using Eq.1:
M=
MCPCM MSA
Msupport Msupport
× 100(%
Fig. 6. DSC curves of pure SA and the prepared composite PCMs.
in Table 3. The melting and freezing temperatures of pure SA are determined to be 68.6 °C and 66.4 °C, respectively (Fig. 6, Table 3); in addition, pure SA exhibits melting and freezing enthalpies of 194.3 and 198.6 J/g, respectively. Table 3 reveals that the melting temperatures of the HPNT/SA and NH2-HPNT/SA composite PCMs are 68.5 °C and 68.3 °C, respectively. Small differences in the melting temperatures of the composite PCMs were observed compared with that of pure SA. However, the freezing temperatures of HPNT/SA and NH2-HPNT/SA are found to be 60.2 °C and 63.7 °C, respectively, indicating that the composite PCMs have lower phase-transition temperatures than pure SA. Based on the difference between the melting and solidifying temperature (Table 3), the extent of supercooling could be calculated as 2.2 °C for SA, 8.3 °C for HPNT/SA, and 4.6 °C for NH2-HPNT/SA, respectively. The results indicate that HPNT/SA exhibits the highest extent of supercooling, and the degree of supercooling of the APTESmodified NH2-HPNT is observed to be significantly lower, and also could be directly observed (Fig. 6). The literature reports that the phase-transition-temperature changes of composite PCMs are strongly related to pore radius and the interactions between the PCM and the supporting matrix [38,39]; hence, the changes in phase-change temperature observed here are possibly due to physical interactions between SA molecules and the inner wall surfaces of the supporting materials, as well as SA confinement in microscopic pores. After modified with APTES, the pore structure and wall property of NH2-HPNT had a positive effect on the reducing of supercooling. The latent heats associated with the melting and freezing processes are 95.7 and 100.3 J/g for the HPNT/SA composite, and 118.6 and 111.8 J/g for the NH2-HPNT/SA composite (Fig. 6, Table 3). The melting and freezing enthalpies of the composite PCMs are lower than those of the pure SA since the addition of the supporting matrixes could decrease the weight percentage of SA and interfere with its crystallisation. In comparison, NH2-HPNT/SA had higher thermal capacity than HPNT/SA, which was attributed to the superior pore property of NH2-HPNT. Furthermore, confinement effects contributed to the lower enthalpy. To assess the heat-storage capacities of the composite PCMs, the thermal energy-storage efficiency (η) was calculated using Eq. (2):
(1)
where MSA is the mass ratio of pristine SA, MCPCM, and Msupport refer to the mass ratio of the obtained composite PCM and the corresponding supporting matrix, respectively. The mass loadings of SA in HPNT and NH2-HPNT were calculated to be 59.15% and 63.65%, respectively. The high SA loadings within these form-stable composite PCMs benefit considerably from their distinctive hierarchical porous structures. 3.5. Thermal properties of the form-stable composite PCMs Thermal properties, including phase change temperature and thermal capacity, are crucial thermal energy-storage factors for a composite PCM. The thermal properties of pure SA and the as-prepared composite PCMs were investigated by DSC, the results are displayed in Fig. 6 for SA, HPNT/SA, and NH2-HPNT/SA, and detailed thermalproperty data of pure SA and the as-prepared composite PCMs are listed
=
HcompositePCM HpureSA
× 100(%)
(2)
Table 2 Thermal properties of the prepared composite PCMs. Sample
T-5 wt%(°C)
Tpeak1(°C)
Tpeak2(°C)
Tpeak3(°C)
Charred residue at 700 °C (%)
SA HPNT/SA NH2-HPNT/SA
219.3 216.0 227.6
277.3 265.0 283.5
– 353.3 354.3
– 452.0 405.3
0.36 32.5 28.2
6
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Table 3 Thermal energy-storage properties of pure SA and the prepared composite PCMs as determined by DSC. Sample
SA HPNT/SA NH2-HPNT/SA
M(%)
Heating period
100 59.15 63.65
Cooling period
Tm (°C)
ΔHm (J/g)
ηm (%)
Tf (°C)
ΔHf (J/g)
ηf (%)
68.6 68.5 68.3
194.3 95.67 118.6
100 83.2 95.9
66.4 60.2 63.7
198.6 100.3 111.8
100 85.4 88.4
which is lower than that of NH2-HPNT/SA (95.9%), indicating that APTES modification improves the crystallinity of the SA in NH2-HPNT. Compared with the other composite PCM, the NH2-HPNT/SA composite is unquestionably the most promising latent heat-storage material. The thermal properties of NH2-HPNT/SA were compared with those of other reported composite PCMs (Table 4). Considering the importance of leakage in thermal energy-storage applications, leakage testing data are also summarized in Table 4. The data indicate that, after examined through serious leakage-testing experiments, NH2-HPNT/SA, still exhibited excellent thermal performance. Therefore, the NH2-HPNT/SA composite is obviously advantageous over the reported PCMs and may potentially become a preferential thermal energy-storage material.
Table 4 Comparing the thermal properties of the composite PCM prepared in this study with those of previously reported composite PCMs. Composite PCM
Tm (°C)
ΔHm (J/g)
Leakage Test
References
Paraffin/kaolin Paraffin/kaolin Lauric acid/kaolin Capric acid/kaolin PEG600/kaolin Heptadecane/kaolin Stearic acid/diatomite Stearic acid/halloysite CA–LA/kaolin Stearic acid/NH2-HPNT
62.4 50.57 43.7 30.71 15.16 11.61 52.3 53.46 16.96 68.3
119.49 94.8 72.5 27.23 32.8 34.63 57.1 93.97 42.36 118.6
No Yes Yes No No No Yes No Yes Yes
[19] [20] [35] [36] [36] [36] [40] [41] [42] This work
3.6. Thermal conductivity of the composite PCMs In the practical application, thermal conductivity plays a fundamental role for composite PCMs. Fig. 7 shows the thermal conductivities of SA, HPNT/SA, and NH2-HPNT/SA composites. The thermal conductivity values were measured as 0.25, 0.42 and 0.40 W/ m∙k for SA, HPNT/SA, and NH2-HPNT/SA, respectively. The results indicate that the thermal conductivity of HPNT/SA and NH2-HPNT/SA were increased as about 68% and 60% compared with that of SA, respectively. It also can be found that after modified with APTES, the thermal conductivity of NH2-HPNT/SA was nearly changed implying that the modification has little effects on the heat transfer rate. Combined with the results of DSC analysis, it can be concluded that the NH2HPNT/SA has a good potential used in thermal energy storage. 3.7. Thermal reliabilities of the form-stable composite PCMs Thermal reliability is an essential parameter for a PCM used as a TES material in buildings. In order to determine the thermal reliabilities of the prepared composite PCMs, they were subjected to 200 heatingcooling cycles. Fig. 8 shows DSC curves of the composite PCMs before and after 100 and 200 thermal cycles, while the thermal properties of these composites are listed in Table 5. Fig. 8 reveals that the phasetransition temperatures of the composite PCMs shift to the lower values during 200 repeated thermal cycles, but the changes are not significant.
Fig. 7. Thermal conductivities of SA, HPNT/SA and NH2-HPNT/SA composites.
where HpureSA and HcompositePCM are the latent heats of pure SA and the composite PCM, respectively, and β is the SA loading in the composite, which was calculated by TGA. The thermal energy-storage efficiency of HPNT/SA during melting was found to be 83.2% (Table 3),
Fig. 8. DSC curves for (a) HPNT/SA and (b) NH2-HPNT/SA at various thermal cycles. 7
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nature of this chemical reaction, SA was incorporated in SiO2 and AlOOH nanosheet. The FTIR spectrum of the SiO2-nanosheet/SA-composite PCM exhibits no new bands (Fig. S5). Additionally, the spectrum of the AlOOH-nanosheet/SA exhibits a new band at 1587 cm−1, indicating that AlOOH nanosheet chemically interacts with SA. The chemical reaction can be described as:
Table 5 Thermal characteristics of the composite PCMs during thermal cycling. Samples
Cycle number
Tm (°C)
ΔHm (J/g)
Tf (°C)
ΔHf (J/g)
HPNT/SA
0 100 200 0 100 200
68.5 68.3 68.1 68.3 68.2 67.7
95.67 29.87 21.51 118.6 107.8 107.4
60.2 59.4 59.4 63.7 62.2 62.6
100.3 26.29 22.74 111.8 108.2 100.5
NH2-HPNT/SA
AlO-OH + HO-CO(CH2)16CH3 = AlO-CO(CH2)16CH3 + H2O
(3)
which means that covalent bonds are formed between SA molecules and the AlOOH nanosheet of HPNT, which hinder the molecular motion of the SA PCMs and affect the latent heat storage capacity, as demonstrated in this study. On the other hand, more SA molecules become grafted to the surfaces of the AlOOH nanosheet with an increasing number of thermal cycles, leading to a significant decrease in the latent heat of HPNT-SA. After modification with APTES, the thermal storage capacity and durability of the composite was considerably improved due to the eOH groups of the AlOOH nanosheet first reacting with APTES, leaving SA molecules to readily crystallize (Fig. 9). Pore size is well known to play a critical role in a composite PCM; however, the effect of the interactions between the supporting material and the PCM on the thermal performance of the composite is also very important.
In detail, the melting and freezing temperatures changed by −0.4 and −0.8 °C for HPNT/SA, and −0.5 and −2.5 °C for NH2-HPNT/SA, which indicate that the phase-transition temperatures of composite PCMs change negligibly in TES applications. However, the latent heat of melting changed by −77.5% for HPNT/SA and −9.4% for NH2HPNT/SA, while the latent heat of freezing changed by −77.3% for HPNT/SA and −10.1% for NH2-HPNT/SA over 200 repeated thermal cycles. Obviously, the latent heats of HPNT/SA decreased very significantly, so HPNT/SA is not sufficiently stable as a TES material. Besides, the microstructure of NH2-HPNT/SA after 200 thermal cycles (Fig. S4) exhibited negligible change in comparison with that of NH2HPNT/SA (Fig. 3d), indicating that the HPNT reliability could be further improved by subsequent modification with APTES, such as in NH2HPNT/SA.
4. Conclusions Hierarchical porous nanosheet (HPNT) was successfully synthesized by a template-free structural-reorganization method using Kaol as raw material. HPNT was modified by grafting with (3-aminopropyl) triethoxysilane (APTES) and used as a supporting material for the preparation of novel SA-based shape-stable composite PCM. The maximum mass fraction of stearic acid in HPNT/SA and NH2-HPNT/SA could reach 59.15% and 63.65%, respectively. NH2-HPNT exhibits the highest loading capacity for SA, and the modification with APTES could improve the capacity of HPNT. NH2-HPNT/SA starts to melt at 68.3 °C with a phase-change enthalpy of 118.6 J/g, and solidify at 63.7 °C with a phase-change enthalpy of 111.8 J/g. The thermal conductivity of NH2-HPNT/SA is 0.4 W/m∙k and increase by approximately 60% compared with raw SA. Also, the as-prepared composite PCMs are thermally stable, and NH2-HPNT/SA is significantly more reliable than HPNT/SA after 200 thermal cycles. NH2-HPNT/SA, comprehensively remarkable compared with HPNT/SA, exhibits higher mass, higher enthalpy, and superior reliability. The results demonstrate that the synthesized NH2-
3.8. Proposed mechanism for improving the thermal properties of composite PCM According to the results detailed above, APTES modification could significantly improve the thermal properties of the composite PCM, such as thermal energy-storage efficiency and durability. As is known, the thermal properties of a PCM are affected by pore size, structure, and functional groups. Pan et al. synthesised a type of microencapsulated phase-change composite material based on a stearic acid core and a boehmite (AlOOH) shell [43], and showed that not only is the phasechange temperature of the composite lower than that of stearic acid, but the heat capacity was also lower than the theoretical value. They attributed these changes in phase-change temperature and heat capacity to the confinement conditions. However, the FTIR analysis in this work clearly showed that SA interacts with HPNT. In order to explore the
Fig. 9. Schematic illustration showing the mechanism for the improvement of the thermal properties of a composite PCM. 8
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HPNT could be a promising supporting material for thermal energy storage applications.
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Declaration of Competing Interest We declare that we have no conflict of interest. Acknowledgments This work was supported by the National Key R&D Program of China (2017YFB0310903), the National Science Fund for Distinguished Young Scholars (51225403), the Hunan Provincial Science and Technology Project (2018WK4023, 2016RS2004 and 2015TP1006), and the Central South University Graduate Independent Exploration Innovation Program (2017zzts191). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114417. References [1] T. Kousksou, P. Bruel, A. Jamil, T. El Rhafiki, Y. Zeraouli, Energy storage: applications and challenges, Sol. Energy Mater. Sol. Cells 120 (2014) 59–80. [2] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al-Hallaj, A review on phase change energy storage: materials and applications, Energy Convers. Manage. 45 (2004) 1597–1615. [3] S. Raoux, Phase change materials, Annu. Rev. Mater. Res. 39 (2009) 25–48. [4] Y. Lin, Y. Jia, G. Alva, G. Fang, Review on thermal conductivity enhancement, thermal properties and applications of phase change materials in thermal energy storage, Renew. Sustain. Energy Rev. 82 (2018) 2730–2742. [5] D. Rozanna, T.G. Chuah, A. Salmiah, T.S.Y. Choong, M. Sa'ari, Fatty acids as phase change materials (PCMs) for thermal energy storage: a review, Int. J. Green Energy 1 (2005) 495–513. [6] M.M. Kenisarin, Thermophysical properties of some organic phase change materials for latent heat storage. A review, Sol. Energy 107 (2014) 553–575. [7] Y. Zhao, X. Min, Z. Huang, Y. Liu, X. Wu, M. Fang, Honeycomb-like structured biological porous carbon encapsulating PEG: a shape-stable phase change material with enhanced thermal conductivity for thermal energy storage, Energy Build. 158 (2018) 1049–1062. [8] X. Zhang, H. Liu, Z. Huang, Z. Yin, R. Wen, X. Min, Y. Huang, Y. Liu, M. Fang, X. Wu, Preparation and characterization of the properties of polyethylene glycol@ Si3N4 nanowires as phase-change materials, Chem. Eng. J. 301 (2016) 229–237. [9] F. Cheng, X. Zhang, R. Wen, Z. Huang, M. Fang, Y. Liu, X. Wu, X. Min, Thermal conductivity enhancement of form-stable tetradecanol/expanded perlite composite phase change materials by adding Cu powder and carbon fiber for thermal energy storage, Appl. Therm. Eng. 156 (2019) 653–659. [10] Q. Zhao, L. Fu, D. Jiang, J. Ouyang, Y. Hu, H. Yang, Y. Xi, Nanoclay modulated oxygen vacancies of metal oxide, Commun. Chem. 2 (2019) 1–10. [11] M. Long, Y. Zhang, P. Huang, S. Chang, Y. Hu, Q. Yang, L. Mao, H. Yang, Emerging nanoclay composite for effective hemostasis, Adv. Funct. Mater. 28 (2018) 1704452–1704461. [12] Z. Yan, L. Fu, X. Zuo, H. Yang, Green assembly of stable and uniform silver nanoparticles on 2D silica nanosheets for catalytic reduction of 4-nitrophenol, Appl. Catal. B: Environ. 226 (2018) 23–30. [13] Q. Zhao, L. Fu, D. Jiang, Y. Xi, H. Yang, A nanoclay-induced defective g-C3N4 photocatalyst for highly efficient catalytic reaction, Chem. Commun. 54 (2018) 8249–8252. [14] Z. Yan, H. Yang, J. Ouyang, A. Tang, In situ loading of highly-dispersed CuO nanoparticles on hydroxyl-group-rich SiO2-AlOOH composite nanosheets for CO catalytic oxidation, Chem. Eng. J. 316 (2017) 1035–1046. [15] Y. Zhang, A. Tang, H. Yang, J. Ouyang, Applications and interfaces of halloysite nanocomposites, Appl. Clay Sci. 119 (2016) 8–17. [16] X. Li, Q. Yang, J. Ouyang, H. Yang, S. Chang, Chitosan modified halloysite nanotubes as emerging porous microspheres for drug carrier, Appl. Clay Sci. 126 (2016) 306–312. [17] K. Peng, L. Fu, H. Yang, J. Ouyang, A. Tang, Hierarchical MoS2 intercalated clay
9
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Highly stable hierarchical porous nanosheet composite phase change materials for thermal energy storage Xiaochao Zuoa,b,c, Zhaoli Yana, Kai Houa, Huaming Yanga,d,e, , Yunfei Xib,c, ⁎
T
⁎
a
Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, QLD 4001, Australia c Institute for Future Environments, Science and Engineering Faculty, Queensland University of Technology, Brisbane, QLD 4001, Australia d Hunan International Joint Lab of Mineral Materials, Central South University, Changsha 410083, China e Key Lab of Clay Mineral Functional Materials in China Building Materials Industry, Central South University, Changsha 410083, China b
HIGHLIGHTS
GRAPHICAL ABSTRACT
porous nanosheets • Hierarchical (HPNTs) were synthesized from kaolinite.
HPNTs were modified by grafting • The with (3-aminopropyl)triethoxysilane (APTES)
HPNT (NH -HPNT) • APTES-modified was used a substrate for phase-change 2
materials.
stearic acid-based (SA-based) • Novel phase-change materials were prepared.
-HPNT is a promising supporting • NH material for thermal energy-storage 2
applications.
ARTICLE INFO
ABSTRACT
Keywords: Hierarchical porous nanosheet Kaolinite Stearic acid Phase change material Thermal energy storage
Thermal energy storage is considered as an effective strategy for improving energy efficiency, and phase change materials (PCMs) are promising in that regard. However, the leakage in the melting process and low thermal conductivity of phase change materials are a big challenge for their practical application. Herein we prepared an amino-functional hierarchical porous nanosheet (NH2-HPNT) by the template-free structural reorganisation of natural clay mineral (kaolinite) and subsequent modification with (3-aminopropyl) triethoxysilane (APTES). NH2-HPNT was hybridized with stearic acid (SA) to produce novel form-stable composite phase change material (PCM). The effect of the NH2-HPNT microstructure on the thermal properties of the composite PCM was investigated. The samples were characterized by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and differential thermogravimetry (DTG). After serious leakage testing, the maximum amount of SA loaded onto NH2-HPNT without leakage was determined to be 63.5 wt%. NH2-HPNT/SA composite PCM melts at 68.3 °C with a phase change enthalpy of 118.6 J/g, and solidifies at 63.7 °C with that of 111.8 J/g. The as-prepared composite PCM is
⁎ Corresponding authors at: Centre for Mineral Materials, School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China (H. Yang). School of Earth, Environmental and Biological Sciences, Queensland University of Technology, Brisbane, QLD 4001, Australia (Y. Xi). E-mail addresses: [email protected] (H. Yang), [email protected] (Y. Xi).
https://doi.org/10.1016/j.applthermaleng.2019.114417 Received 26 July 2019; Received in revised form 18 September 2019; Accepted 19 September 2019 Available online 19 September 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.
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thermally stable. In comparison with SA, the thermal conductivity of NH2-HPNT/SA increase by up to 60%. Additionally, thermal cycling experiments showed that NH2-HPNT/SA is significantly more reliable than HPNT/ SA after 200 heating cycles. The mechanism responsible for improving the thermal properties of the composite PCMs is proposed. The results indicate that NH2-HPNT could be a promising supporting material for PCMs in thermal energy storage applications.
1. Introduction
[36]. The proportion of capric acid, PEG600, and heptadecane impregnated in Kaol was determined by seepage to 17.5, 21, and 16.5 wt %, respectively. In conclusion, Kaol as a supporting material for PCMs has received significant attention. However, the loading of PCMs in raw Kaol leads the Kaol-based form-stable composite PCM to exhibit low thermal energy-storage efficiency, which is due to the insufficient space to adsorb the more PCMs. Therefore, it would be interesting for improving the pore property of low-cost Kaol via preparation of hierarchical porous material. Hierarchical porous nanosheet synthesised using low-cost Kaol as a starting material, to the best of our knowledge, has not been studied in thermal energy storage systems. Herein this work firstly reported the preparation of hierarchical porous nanosheet and further fabricated an amino-functional hierarchical porous nanosheet (NH2-HPNT) using (3aminopropyl) triethoxysilane (APTES) as a surface modifier to improve the thermal properties of form-stable composite PCM. Finally, an NH2HPNT/SA form-stable composite PCM was prepared by incorporating stearic acid into the pores of NH2-HPNT using the vacuum-impregnation method. The preparation procedure and sample designations are shown schematically in Fig. 1. X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were used to detailedly characterise the prepared materials. Also, thermal cycling experiments were used to determine the thermal reliabilities of the composite PCMs.
With the developing economy and improving living stands, the demand for energy has increased dramatically in recent years, which has led to a variety of problems that include the exhaustion of fossil fuels, environmental pollution, and increasing levels of carbon dioxide emissions. Therefore, improving energy-use efficiency and environmental protection are important issues. Thermal energy storage (TES), which includes sensible heat storage, latent heat storage, and thermochemical storage, is considered to be one of the most effective methods for improving energy efficiency [1]. Compared with other methods, latent heat storage is a more-promising heat-storage technology by virtue of the advantages associated with phase change materials (PCMs), which have high energy-storage densities and isothermal characteristics [2,3]. PCMs are mainly divided into two types (inorganic and organic) based on their chemical compositions [1,2,4]. In addition to the abovementioned advantages of PCMs, organic PCMs have many other desirable properties, such as a variety of phase-change temperatures, negligible supercooling behavior, low vapor pressures in the melt, chemical inertness and stabilities, self-nucleation, a lack of phase segregation, and their commercial availabilities [5,6]. However, the disadvantages of their low thermal conductivities and the leakage of organic PCMs during the phase change process have limited their applications. To overcome the two problems, considerable efforts have been devoted to developing form-stable composite PCMs without leakage and with high conductivity [7–9]. Clay minerals with porous structures and considerable specific surface areas have been widely used for the synthesis of advanced functional materials [10–17], and especially integrated with organic PCMs to prepare form-stable composite PCMs [18]. Pore confinement and mutual interactions between the mineral pore surface and PCM molecules could restrict the leakage during the PCM phase-change process. On the other hand, due to the high thermal conductivity of the natural mineral, the thermal conductivity of the organic material is enhanced significantly after incorporation into the clay [19,20]. To date, clay minerals have been used to prevent organic PCMs leakage and increase the thermal conductivity, including perlite [21,22], sepiolite [23,24], diatomite [25], bentonite [26], montmorillonite [27,28], attapulgite [29], opal [30], and hydroxyapatite [31], etc. Among various clay minerals, kaolinite (Kaol), Al2Si2O5(OH)4, is a layered magnesium aluminum silicate with a 1:1 crystalline structure belonging to kaolin group of minerals. Globally, Kaol is distributed widely, especially in China, and has been extensively used to manufacture paper, ceramics, inks, and paints, as well as additives in the production of rubber and polymers [32–34]. In recent years, a new application that uses Kaol as a support matrix for PCMs has been explored. Li et al. incorporated paraffin into three different kinds of kaolin (platelet kaolin, layered kaolin, and rod kaolin) [20], and revealed that the layered kaolin had relatively good thermal properties and thermal conductivity (0.78 W/(m K)). In addition, Lv et al. studied the effect the Kaol particle size on the thermal properties of composite PCMs, showing that the highest thermal conductivity was exhibited by the Kaol-based composite PCM with the largest particle size [19]. A lauricacid/intercalated-Kaol form-stable composite PCM was reported by Song et al. [35]; the phase-change temperature and latent heat of this material were determined to be 43.7 °C and 72.5 J/g. Sari fabricated three Kaol-based PCMs using capric acid, PEG600, and heptadecane
2. Experimental 2.1. Materials Kaolinite (Kaol) was supplied by the China Kaolin Clay Co., Ltd., China. Stearic acid (SA) was obtained from the Tianjin Kemiou Chemical Reagent Co., Ltd., China. Hydrochloric acid, ammonium hydroxide, ethanol, and methylbenzene, were purchased from the Sinopharm Chemical Reagent Co., Ltd., China. Methanol was supplied by the Xilong Chemical Reagent Co., Ltd., China. APTES (98%) was purchased from the Aladdin Reagent Co., Ltd., China. All reagents were used as received. 2.2. Preparation of HPNT Kaol was first converted to anhydrous-phase metakaolinite by heating in a muffle furnace at 700 °C for 2 h (Fig. 1). An 8-g sample of metakaolinite and 400 mL of 2 M HCl were added to 500-mL flask. The mixture was placed in a water bath at 85 °C with stirring for 8 h. After standing and cooling to room temperature, the precipitate was collected by centrifugation and washed repeatedly (~5×) with distilled water. The precipitate was then dispersed in 100 mL of ethanol in distilled water (1:1 v/v) followed by sonication for 2 h (400 W, 24 kHz). The solution was then mixed with about 300 mL of a leached aluminum-ion solution, and the leached Al ions were then reprecipitated by the gradual addition of 28% ammonium hydroxide with vigorous stirring until the desired pH of 8.6 was reached. The suspension was transferred to Teflon-lined stainless steel autoclave and then placed in an oven at 180 °C for 12 h. The white product was collected by centrifugation and repeatedly washing with distilled water. The sample was then dried at 100 °C for 12 h and ground into a fine powder to obtain hierarchical porous nanosheet, referred to as “HPNT”. 2
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Fig. 1. Schematic diagram depicting the preparation of NH2-HPNT and the vacuum-impregnation method for the preparation of the NH2-HPNT/SA composite.
2.3. Modification of HPNT
2.5. Characterisation
A 0.5-g sample of HPNT powder was dispersed in 25 mL of fresh methanol by ultrasonication for 30 min, after which 50 mL of toluene was added into to the suspension with vigorous stirring. The methanol was then evaporated under a flow of argon at 75 °C for 1.5 h. A 5-mL aliquot of APTES was added dropwise to the suspension, vigorously stirred for 1 h under the same conditions. After naturally cooling to room temperature, the precipitate was collected by centrifugation and washed three times with fresh toluene and ethanol, then dried in a vacuum drying oven at 70 °C for 12 h. The obtained white product was milled to a fine powder, referred to as “NH2-HPNT”.
The pore size distributions and Brunauer-Emmett-Teller (BET) specific surface area were measured by a nitrogen adsorption apparatus (ASAP 2020, USA) at 77 K. The external surface morphology and microstructure of prepared supporting materials and corresponding composites PCMs were characterized by scanning electron microscope instrument (SEM, Tescan Mira3 LMU, Czech). In order to determine their compatibilities, the composite PCMs were subjected to X-ray diffractometry (XRD, Bruker-AXS D8, Germany) using Cu Kα radiation (0.15406 nm). The XRD patterns were acquired under the following conditions: voltage, 40 kV; current, 40 mA; scanning range, 3–80°; scan rate, 4°/min; and step size, 0.02°. The chemical groups were determined by Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, USA) in the range of 600–4000 cm−1 with a resolution of 4 cm−1 at room temperature. The thermal properties of stearic acid and the form-stable composite PCMs were examined by differential scanning calorimeter (DSC, DSC214 Polyma, Germany) at a heating/cooling rate of 5 °C/min under nitrogen in the 40–80 °C temperature range. The thermal stabilities of the prepared form-stable PCM composites were determined by the thermogravimetric analyser (TGA, TGA-DSC I/1600 HT, USA) under an inert nitrogen atmosphere at 40 mL/min and a 10 °C/min heating rate. The thermal conductivity was measured by a thermal conductivity meter (XIATECH TC 3000, China) at room temperature. Thermal cycling experiments were conducted using a thermal cycler (BIOER model TC-25/H, China). The cycling process included melting of the composite PCM at 80 °C for 10 min and cooling at 40 °C for the same amount of time.
2.4. Preparation of form-stable composite PCMs The HPNT/SA and NH2-HPNT/SA form-stable composite PCMs used herein were manufactured by the vacuum impregnation method. Both form-stable composite PCMs were prepared in the same manner; hence, only the process for the preparation of NH2-HPNT/SA is described. A 7 g sample of SA and 3 g of NH2-HPNT were uniformly ground together, after which the mixture was placed in a filter flask and vacuum evacuated to −0.1 MPa for 10 min. The flask was then placed in a thermostat-controlled water bath and heated at 90 °C for 30 min. Afterwards, the vacuum was removed in order to force the liquid SA into the NH2-HPNT porous network. The NH2-HPNT/SA prepared in this manner was slowly cooled to room temperature and removed from the flask. Then the entire composite was placed in an oven at 90 °C for 48 h on filter paper in order to remove the excess PCM. The filter paper was changed every 2 h until no further leakage of melted PCM from the composite PCM was observed. Leakage testing was carried out as follows: Each composite PCM was first ground into a fine powder and pressed into a 25-mm-diameter disc. The disc was then placed on a filter paper and heated in an oven at 90 °C for 30 min. The seepage test results are shown in Fig. S1.
3. Results and discussion 3.1. Characterisation of the supporting materials In order to investigate the absorbability of supporting materials, the properties of the pore structure of supporting materials were measured by BET and N2 adsorption and desorption. Fig. 2 illustrates the N2 3
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Fig. 2. (a) Nitrogen adsorption-desorption isotherms and (b) corresponding pore size distributions of the selected supporting matrix: HPNT and NH2-HPNT.
indicating that more SA was loaded on the NH2-HPNT, which is in accordance with the results of TG analysis.
Table 1 Textural properties of Kaol, HPNT and NH2-HPNT. Samples
Surface area (m2/g)
Average pore diameter (nm)
Total Pore Volume (cc/g)
Kaol HPNT NH2-HPNT
19.6 310.8 249.8
16.4 14.0 20.2
0.08 1.09 1.26
3.3. Chemical compatibilities of the form-stable composite PCMs The chemical compatibilities of the components in the prepared form-stable composite PCMs were characterised by XRD and FTIR spectroscopy. Fig. 4a displays the XRD patterns of SA, HPNT, HPNT/SA, NH2-HPNT, and NH2-HPNT/SA. Pure SA exhibits three characteristic diffraction peaks at 6.7°, 21.7° and 24.2° in its XRD pattern [28], while HPNT exhibits some weak broad peaks corresponding to SiO2 and AlOOH nanosheet (Fig. S3). The characteristic peaks of NH2-HPNT are similar to those of HPNT, indicating that the structure of NH2-HPNT was not affected by organic modification with APTES. A comparison of the energy-dispersive X-ray spectra (data not shown here) of HPNT before and after modification reveals the presence of elemental nitrogen in NH2HPNT, which indicates that APTES was successfully grafted onto the surface of HPNT. SA was then impregnated into HPNT and NH2-HPNT. Compared with the XRD patterns of the supporting matrixes, the patterns of the composite PCMs exhibit diffraction peaks of pure SA in addition to the peaks of the supporting matrixes. These results demonstrate that the crystal structure of SA was not destroyed by impregnation. Fig. 4b shows the FTIR spectra of SA, HPNT, HPNT/SA, NH2-HPNT, and NH2-HPNT/SA. The absorption bands at 2917 and 2849 cm−1 in the spectrum of SA are attributed respectively to the asymmetric and symmetric stretching vibrations of eCH2 groups. The active absorption band at 1702 cm−1 is assigned to the stretching vibration of the carboxylic acid (C]O), while bands observed at 1472 cm−1 and 719 cm−1 represent the in-plane scissoring motions of eCH2 groups and out-ofplane bending motions of eCH groups. The bands at 3690 and 3620 cm−1 in the spectrum of raw Kaol are assigned to the hydroxyl stretching vibrations of inner-surface hydroxyls and inner hydroxyls, respectively. The multiple bands located at 1200–1000 cm−1 region are due to the SieO stretching vibrations, while the band at 911 cm−1 corresponds to AleOH bending vibrations, and the bands at 789 and 678 cm−1 are due to AleO stretching vibrations. The spectrum of HPNT displays a visible band at 1064 cm−1 associated with the SieO stretching vibration. A new band is observed at 1587 cm−1 following incorporation SA in the spectrum HPNT/SA, consistent with chemical reactions between HPNT and SA. From the mechanism analysis, the new band can be assigned to the group of AlOOCe. In addition, bands corresponding to the main functional groups of SA are also present in the spectrum of HPNT/SA, with only slight shifts observed. Compared with that of HPNT, the spectrum of NH2-HPNT exhibits almost no change following incorporation of APTES. Clearly, no significant new band appeared in the spectrum of the NH2-HPNT/SA composite PCM, indicating no chemical reaction between NH2-HPNT and SA. On the
adsorption-desorption isotherms and pore diameter distribution curves of HPNT and NH2-HPNT. Table 1 summarizes the textural properties of the corresponding samples. The isotherms of HPNT and NH2-HPNT displays Ⅳ type isotherms with a hysteresis loop at above 0.65 P/P0 that reflects the presence of mesopores (Fig. 2a). Two hysteresis loops of HPNT and NH2-HPNT at the P/P0 range of 0.6–1.0 are H3 type, which indicates that the presence of slitlike pores [37]. Moreover, in comparison with HPNT, the hysteresis loops shape of NH2-HPNT is more similar to the type H3, implying that more ink-bottle type pores transformed into slitlike pores after the modification treatment. Fig. 2b shows the corresponding pore diameter distribution of the received supporting matrix. The pore diameter distribution curves of HPNT and NH2-HPNT are very similar, which shows multimodal distribution, implying that the existence of micropore, mesopore and microporous. Thus, both HPNT and NH2-HPNT can be defined as hierarchical pores materials. Regarding the BET surface area, the supporting HPNT has the maximum value of 310.8 m2/g. After HPNT was modified by APTES, the specific surface area decreased to 249.8 m2/g, while the pore value volume increased from 1.09 to 1.26 cc/g. Compared with natural Kaol (Table 1), the textural properties of the prepared supporting materials significantly increased, which is beneficial to the immobilisation of SA for preventing the leakage. 3.2. Morphology and microstructure of composite PCMs Fig. 3 shows the SEM images of the prepared supporting materials and corresponding composite PCMs. As illustrated from Fig. 3a, HPNT shows a loose layered structure, which is assembled by SiO2 nanosheet and smaller and thinner AlOOH nanosheet (Fig. S2a). Compared with Kaol (Fig. S2b), the pseudo-hexagonal shaped platelet structure disappeared. When HPNT was modified by APTES, more and bigger sheets aggregations were observed in Fig. 3b, resulting in a decrease of specific area surface. It is in conformity with the results of BET analysis. It is clear that the SA was well dispersed into the porous networks of the received supporting materials (Fig. 3c and d). The surface tension and capillary force exhibits between supporting materials and SA could prevent leakage behavior during the melting process. In comparison with HPNT/SA, the morphology of HPNT still can be obviously found, 4
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Fig. 3. SEM images of (a) HPNT, (b) NH2-HPNT, (c) HPNT/SA and (d) NH2-HPNT/SA.
Fig. 4. (a) XRD patterns and (b) FTIR spectra of SA, HPNT, HPNT/SA, NH2-HPNT, and NH2-HPNT/SA.
Fig. 5. TGA and DTG curves of SA, support materials and the prepared composite PCMs. 5
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basis of these results, it could be concluded that it is necessary to determine compatibility between the supporting material and the organic molecule through the combined use of XRD and FTIR spectroscopy. 3.4. Thermal stabilities of the form-stable composite PCMs The thermal stabilities of the composite PCMs play essential roles for thermal energy-storage applications. The thermal stabilities of the HPNT/SA and NH2-HPNT/SA composites were evaluated by TGA. Fig. 5 shows the TGA and DTG curves of SA and the prepared composite PCMs, with the 5%-weight-loss temperature (T-5 wt%), the maximum decomposition peak temperatures (Tpeak1, Tpeak2, and Tpeak3), and the charred residues at 700 °C listed in Table 1. Fig. 3a and Table 1 reveal that the thermal-degradation-onset temperatures (the 5%-weight-loss temperature) of SA and the composite PCMs exceeded 200 °C; hence, the prepared composite PCMs are very thermally stable in the working temperature range. In detail, pristine SA was observed to degrade in one step in the 200–300 °C temperature range, with a maximum decomposition peak temperature (Tpeak1) of 277.3 °C. However, Fig. 3a&b show that the prepared composite PCMs decompose over two or three stages; the HPNT/SA and NH2-HPNT/SA samples exhibit weight losses between 320 and 600 °C, which is mainly due to water evaporation from the supports and dehydroxylation. In comparison, the maximum decomposition rate of NH2-HPNT/SA is observed at 283.5 °C, higher than that of HPNT/SA, indicating that the NH2-HPNT/SA composite is more thermally stable. The impregnation ratio of SA in the composite PCMs is very important to determine the thermal energy-storage efficiencies. The TG and DTG curves of HPNT and NH2-HPNT are shown in Fig. 5. In the case of HPNT, the mass loss is 21% when the measuring temperature changes from 30 to 700 °C, which is caused by the removal of the absorbed water and hydroxyl groups. However, it is determined that the total weight loss percentages are about 22% for NH2-HPNT, up to the same temperature of 700 °C. The different mass loss between HPNT and NH2-HPNT is due to the existence of APTES. Table 2 reveals weight-loss percentages to 700 °C of about 99.64%, 67.5%, and 71.8% for SA, HPNT/SA, and NH2-HPNT/SA, respectively. Based on the mass loss of the composite PCMs and the supporting matrix, the SA loading (M %) in the composite PCMs was calculated using Eq.1:
M=
MCPCM MSA
Msupport Msupport
× 100(%
Fig. 6. DSC curves of pure SA and the prepared composite PCMs.
in Table 3. The melting and freezing temperatures of pure SA are determined to be 68.6 °C and 66.4 °C, respectively (Fig. 6, Table 3); in addition, pure SA exhibits melting and freezing enthalpies of 194.3 and 198.6 J/g, respectively. Table 3 reveals that the melting temperatures of the HPNT/SA and NH2-HPNT/SA composite PCMs are 68.5 °C and 68.3 °C, respectively. Small differences in the melting temperatures of the composite PCMs were observed compared with that of pure SA. However, the freezing temperatures of HPNT/SA and NH2-HPNT/SA are found to be 60.2 °C and 63.7 °C, respectively, indicating that the composite PCMs have lower phase-transition temperatures than pure SA. Based on the difference between the melting and solidifying temperature (Table 3), the extent of supercooling could be calculated as 2.2 °C for SA, 8.3 °C for HPNT/SA, and 4.6 °C for NH2-HPNT/SA, respectively. The results indicate that HPNT/SA exhibits the highest extent of supercooling, and the degree of supercooling of the APTESmodified NH2-HPNT is observed to be significantly lower, and also could be directly observed (Fig. 6). The literature reports that the phase-transition-temperature changes of composite PCMs are strongly related to pore radius and the interactions between the PCM and the supporting matrix [38,39]; hence, the changes in phase-change temperature observed here are possibly due to physical interactions between SA molecules and the inner wall surfaces of the supporting materials, as well as SA confinement in microscopic pores. After modified with APTES, the pore structure and wall property of NH2-HPNT had a positive effect on the reducing of supercooling. The latent heats associated with the melting and freezing processes are 95.7 and 100.3 J/g for the HPNT/SA composite, and 118.6 and 111.8 J/g for the NH2-HPNT/SA composite (Fig. 6, Table 3). The melting and freezing enthalpies of the composite PCMs are lower than those of the pure SA since the addition of the supporting matrixes could decrease the weight percentage of SA and interfere with its crystallisation. In comparison, NH2-HPNT/SA had higher thermal capacity than HPNT/SA, which was attributed to the superior pore property of NH2-HPNT. Furthermore, confinement effects contributed to the lower enthalpy. To assess the heat-storage capacities of the composite PCMs, the thermal energy-storage efficiency (η) was calculated using Eq. (2):
(1)
where MSA is the mass ratio of pristine SA, MCPCM, and Msupport refer to the mass ratio of the obtained composite PCM and the corresponding supporting matrix, respectively. The mass loadings of SA in HPNT and NH2-HPNT were calculated to be 59.15% and 63.65%, respectively. The high SA loadings within these form-stable composite PCMs benefit considerably from their distinctive hierarchical porous structures. 3.5. Thermal properties of the form-stable composite PCMs Thermal properties, including phase change temperature and thermal capacity, are crucial thermal energy-storage factors for a composite PCM. The thermal properties of pure SA and the as-prepared composite PCMs were investigated by DSC, the results are displayed in Fig. 6 for SA, HPNT/SA, and NH2-HPNT/SA, and detailed thermalproperty data of pure SA and the as-prepared composite PCMs are listed
=
HcompositePCM HpureSA
× 100(%)
(2)
Table 2 Thermal properties of the prepared composite PCMs. Sample
T-5 wt%(°C)
Tpeak1(°C)
Tpeak2(°C)
Tpeak3(°C)
Charred residue at 700 °C (%)
SA HPNT/SA NH2-HPNT/SA
219.3 216.0 227.6
277.3 265.0 283.5
– 353.3 354.3
– 452.0 405.3
0.36 32.5 28.2
6
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Table 3 Thermal energy-storage properties of pure SA and the prepared composite PCMs as determined by DSC. Sample
SA HPNT/SA NH2-HPNT/SA
M(%)
Heating period
100 59.15 63.65
Cooling period
Tm (°C)
ΔHm (J/g)
ηm (%)
Tf (°C)
ΔHf (J/g)
ηf (%)
68.6 68.5 68.3
194.3 95.67 118.6
100 83.2 95.9
66.4 60.2 63.7
198.6 100.3 111.8
100 85.4 88.4
which is lower than that of NH2-HPNT/SA (95.9%), indicating that APTES modification improves the crystallinity of the SA in NH2-HPNT. Compared with the other composite PCM, the NH2-HPNT/SA composite is unquestionably the most promising latent heat-storage material. The thermal properties of NH2-HPNT/SA were compared with those of other reported composite PCMs (Table 4). Considering the importance of leakage in thermal energy-storage applications, leakage testing data are also summarized in Table 4. The data indicate that, after examined through serious leakage-testing experiments, NH2-HPNT/SA, still exhibited excellent thermal performance. Therefore, the NH2-HPNT/SA composite is obviously advantageous over the reported PCMs and may potentially become a preferential thermal energy-storage material.
Table 4 Comparing the thermal properties of the composite PCM prepared in this study with those of previously reported composite PCMs. Composite PCM
Tm (°C)
ΔHm (J/g)
Leakage Test
References
Paraffin/kaolin Paraffin/kaolin Lauric acid/kaolin Capric acid/kaolin PEG600/kaolin Heptadecane/kaolin Stearic acid/diatomite Stearic acid/halloysite CA–LA/kaolin Stearic acid/NH2-HPNT
62.4 50.57 43.7 30.71 15.16 11.61 52.3 53.46 16.96 68.3
119.49 94.8 72.5 27.23 32.8 34.63 57.1 93.97 42.36 118.6
No Yes Yes No No No Yes No Yes Yes
[19] [20] [35] [36] [36] [36] [40] [41] [42] This work
3.6. Thermal conductivity of the composite PCMs In the practical application, thermal conductivity plays a fundamental role for composite PCMs. Fig. 7 shows the thermal conductivities of SA, HPNT/SA, and NH2-HPNT/SA composites. The thermal conductivity values were measured as 0.25, 0.42 and 0.40 W/ m∙k for SA, HPNT/SA, and NH2-HPNT/SA, respectively. The results indicate that the thermal conductivity of HPNT/SA and NH2-HPNT/SA were increased as about 68% and 60% compared with that of SA, respectively. It also can be found that after modified with APTES, the thermal conductivity of NH2-HPNT/SA was nearly changed implying that the modification has little effects on the heat transfer rate. Combined with the results of DSC analysis, it can be concluded that the NH2HPNT/SA has a good potential used in thermal energy storage. 3.7. Thermal reliabilities of the form-stable composite PCMs Thermal reliability is an essential parameter for a PCM used as a TES material in buildings. In order to determine the thermal reliabilities of the prepared composite PCMs, they were subjected to 200 heatingcooling cycles. Fig. 8 shows DSC curves of the composite PCMs before and after 100 and 200 thermal cycles, while the thermal properties of these composites are listed in Table 5. Fig. 8 reveals that the phasetransition temperatures of the composite PCMs shift to the lower values during 200 repeated thermal cycles, but the changes are not significant.
Fig. 7. Thermal conductivities of SA, HPNT/SA and NH2-HPNT/SA composites.
where HpureSA and HcompositePCM are the latent heats of pure SA and the composite PCM, respectively, and β is the SA loading in the composite, which was calculated by TGA. The thermal energy-storage efficiency of HPNT/SA during melting was found to be 83.2% (Table 3),
Fig. 8. DSC curves for (a) HPNT/SA and (b) NH2-HPNT/SA at various thermal cycles. 7
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nature of this chemical reaction, SA was incorporated in SiO2 and AlOOH nanosheet. The FTIR spectrum of the SiO2-nanosheet/SA-composite PCM exhibits no new bands (Fig. S5). Additionally, the spectrum of the AlOOH-nanosheet/SA exhibits a new band at 1587 cm−1, indicating that AlOOH nanosheet chemically interacts with SA. The chemical reaction can be described as:
Table 5 Thermal characteristics of the composite PCMs during thermal cycling. Samples
Cycle number
Tm (°C)
ΔHm (J/g)
Tf (°C)
ΔHf (J/g)
HPNT/SA
0 100 200 0 100 200
68.5 68.3 68.1 68.3 68.2 67.7
95.67 29.87 21.51 118.6 107.8 107.4
60.2 59.4 59.4 63.7 62.2 62.6
100.3 26.29 22.74 111.8 108.2 100.5
NH2-HPNT/SA
AlO-OH + HO-CO(CH2)16CH3 = AlO-CO(CH2)16CH3 + H2O
(3)
which means that covalent bonds are formed between SA molecules and the AlOOH nanosheet of HPNT, which hinder the molecular motion of the SA PCMs and affect the latent heat storage capacity, as demonstrated in this study. On the other hand, more SA molecules become grafted to the surfaces of the AlOOH nanosheet with an increasing number of thermal cycles, leading to a significant decrease in the latent heat of HPNT-SA. After modification with APTES, the thermal storage capacity and durability of the composite was considerably improved due to the eOH groups of the AlOOH nanosheet first reacting with APTES, leaving SA molecules to readily crystallize (Fig. 9). Pore size is well known to play a critical role in a composite PCM; however, the effect of the interactions between the supporting material and the PCM on the thermal performance of the composite is also very important.
In detail, the melting and freezing temperatures changed by −0.4 and −0.8 °C for HPNT/SA, and −0.5 and −2.5 °C for NH2-HPNT/SA, which indicate that the phase-transition temperatures of composite PCMs change negligibly in TES applications. However, the latent heat of melting changed by −77.5% for HPNT/SA and −9.4% for NH2HPNT/SA, while the latent heat of freezing changed by −77.3% for HPNT/SA and −10.1% for NH2-HPNT/SA over 200 repeated thermal cycles. Obviously, the latent heats of HPNT/SA decreased very significantly, so HPNT/SA is not sufficiently stable as a TES material. Besides, the microstructure of NH2-HPNT/SA after 200 thermal cycles (Fig. S4) exhibited negligible change in comparison with that of NH2HPNT/SA (Fig. 3d), indicating that the HPNT reliability could be further improved by subsequent modification with APTES, such as in NH2HPNT/SA.
4. Conclusions Hierarchical porous nanosheet (HPNT) was successfully synthesized by a template-free structural-reorganization method using Kaol as raw material. HPNT was modified by grafting with (3-aminopropyl) triethoxysilane (APTES) and used as a supporting material for the preparation of novel SA-based shape-stable composite PCM. The maximum mass fraction of stearic acid in HPNT/SA and NH2-HPNT/SA could reach 59.15% and 63.65%, respectively. NH2-HPNT exhibits the highest loading capacity for SA, and the modification with APTES could improve the capacity of HPNT. NH2-HPNT/SA starts to melt at 68.3 °C with a phase-change enthalpy of 118.6 J/g, and solidify at 63.7 °C with a phase-change enthalpy of 111.8 J/g. The thermal conductivity of NH2-HPNT/SA is 0.4 W/m∙k and increase by approximately 60% compared with raw SA. Also, the as-prepared composite PCMs are thermally stable, and NH2-HPNT/SA is significantly more reliable than HPNT/SA after 200 thermal cycles. NH2-HPNT/SA, comprehensively remarkable compared with HPNT/SA, exhibits higher mass, higher enthalpy, and superior reliability. The results demonstrate that the synthesized NH2-
3.8. Proposed mechanism for improving the thermal properties of composite PCM According to the results detailed above, APTES modification could significantly improve the thermal properties of the composite PCM, such as thermal energy-storage efficiency and durability. As is known, the thermal properties of a PCM are affected by pore size, structure, and functional groups. Pan et al. synthesised a type of microencapsulated phase-change composite material based on a stearic acid core and a boehmite (AlOOH) shell [43], and showed that not only is the phasechange temperature of the composite lower than that of stearic acid, but the heat capacity was also lower than the theoretical value. They attributed these changes in phase-change temperature and heat capacity to the confinement conditions. However, the FTIR analysis in this work clearly showed that SA interacts with HPNT. In order to explore the
Fig. 9. Schematic illustration showing the mechanism for the improvement of the thermal properties of a composite PCM. 8
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HPNT could be a promising supporting material for thermal energy storage applications.
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Declaration of Competing Interest We declare that we have no conflict of interest. Acknowledgments This work was supported by the National Key R&D Program of China (2017YFB0310903), the National Science Fund for Distinguished Young Scholars (51225403), the Hunan Provincial Science and Technology Project (2018WK4023, 2016RS2004 and 2015TP1006), and the Central South University Graduate Independent Exploration Innovation Program (2017zzts191). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.applthermaleng.2019.114417. References [1] T. Kousksou, P. Bruel, A. Jamil, T. El Rhafiki, Y. Zeraouli, Energy storage: applications and challenges, Sol. Energy Mater. Sol. Cells 120 (2014) 59–80. [2] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al-Hallaj, A review on phase change energy storage: materials and applications, Energy Convers. Manage. 45 (2004) 1597–1615. [3] S. Raoux, Phase change materials, Annu. Rev. Mater. Res. 39 (2009) 25–48. [4] Y. Lin, Y. Jia, G. Alva, G. Fang, Review on thermal conductivity enhancement, thermal properties and applications of phase change materials in thermal energy storage, Renew. Sustain. Energy Rev. 82 (2018) 2730–2742. [5] D. Rozanna, T.G. Chuah, A. Salmiah, T.S.Y. Choong, M. Sa'ari, Fatty acids as phase change materials (PCMs) for thermal energy storage: a review, Int. J. Green Energy 1 (2005) 495–513. [6] M.M. Kenisarin, Thermophysical properties of some organic phase change materials for latent heat storage. A review, Sol. Energy 107 (2014) 553–575. [7] Y. Zhao, X. Min, Z. Huang, Y. Liu, X. Wu, M. Fang, Honeycomb-like structured biological porous carbon encapsulating PEG: a shape-stable phase change material with enhanced thermal conductivity for thermal energy storage, Energy Build. 158 (2018) 1049–1062. [8] X. Zhang, H. Liu, Z. Huang, Z. Yin, R. Wen, X. Min, Y. Huang, Y. Liu, M. Fang, X. Wu, Preparation and characterization of the properties of polyethylene glycol@ Si3N4 nanowires as phase-change materials, Chem. Eng. J. 301 (2016) 229–237. [9] F. Cheng, X. Zhang, R. Wen, Z. Huang, M. Fang, Y. Liu, X. Wu, X. Min, Thermal conductivity enhancement of form-stable tetradecanol/expanded perlite composite phase change materials by adding Cu powder and carbon fiber for thermal energy storage, Appl. Therm. Eng. 156 (2019) 653–659. [10] Q. Zhao, L. Fu, D. Jiang, J. Ouyang, Y. Hu, H. Yang, Y. Xi, Nanoclay modulated oxygen vacancies of metal oxide, Commun. Chem. 2 (2019) 1–10. [11] M. Long, Y. Zhang, P. Huang, S. Chang, Y. Hu, Q. Yang, L. Mao, H. Yang, Emerging nanoclay composite for effective hemostasis, Adv. Funct. Mater. 28 (2018) 1704452–1704461. [12] Z. Yan, L. Fu, X. Zuo, H. Yang, Green assembly of stable and uniform silver nanoparticles on 2D silica nanosheets for catalytic reduction of 4-nitrophenol, Appl. Catal. B: Environ. 226 (2018) 23–30. [13] Q. Zhao, L. Fu, D. Jiang, Y. Xi, H. Yang, A nanoclay-induced defective g-C3N4 photocatalyst for highly efficient catalytic reaction, Chem. Commun. 54 (2018) 8249–8252. [14] Z. Yan, H. Yang, J. Ouyang, A. Tang, In situ loading of highly-dispersed CuO nanoparticles on hydroxyl-group-rich SiO2-AlOOH composite nanosheets for CO catalytic oxidation, Chem. Eng. J. 316 (2017) 1035–1046. [15] Y. Zhang, A. Tang, H. Yang, J. Ouyang, Applications and interfaces of halloysite nanocomposites, Appl. Clay Sci. 119 (2016) 8–17. [16] X. Li, Q. Yang, J. Ouyang, H. Yang, S. Chang, Chitosan modified halloysite nanotubes as emerging porous microspheres for drug carrier, Appl. Clay Sci. 126 (2016) 306–312. [17] K. Peng, L. Fu, H. Yang, J. Ouyang, A. Tang, Hierarchical MoS2 intercalated clay
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