Effective Encapsulation of Paraffin Wax in Carbon Nanotube

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Materials and Interfaces

Effective Encapsulation of Paraffin Wax in Carbon Nanotube Agglomerates for a New Shape-Stabilized Phase Change Material with Enhanced Thermal-Storage Capacity and Stability Liang Han, Xilai Jia, Zhimin Li, Zhou Yang, Ge Wang, and Guoqing Ning Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02159 • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018

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Industrial & Engineering Chemistry Research

Table of Content Graphic

Carbon nanotube agglomerates with interweaved porous nanostructure were developed to load paraffin for high-performance shape-stabilized phase change material.

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Effective Encapsulation of Paraffin Wax in Carbon Nanotube Agglomerates for a New Shape-Stabilized Phase Change Material with Enhanced Thermal-Storage Capacity and Stability Liang Hana,b, Xilai Jiaa,*, Zhimin Lia,b, Zhou Yanga, Ge Wanga, and Guoqing Ningb a

Beijing Key Laboratory of Function Materials for Molecule & Structure Construction,

School of Materials Science and Engineering, University of Science and Technology Beijing,

Beijing

100083,

People’s

Republic

of

China.

Email

addresses:

[email protected] (X. Jia) b

State Key Laboratory of Heavy Oil Processing, China University of Petroleum,

Beijing, Changping 102249, People’s Republic of China.

Abstract Seeking new applications of carbon nanotubes (CNTs) will be important since large-scale production has made them available in many applications. Herein, aggregated

CNTs

(A-CNTs)

with

interweaved

pores

were

prepared

for

shape-stabilized phase change materials (PCMs). Owning to the capillary force from the pores, paraffin was readily encapsulated into A-CNTs, and formed a new kind of shape-stabilized PCMs. As-prepared paraffin/A-CNT composites had optimal paraffin loading approaching 88 wt.% and offered a storage capacity of 172.14 J g-1 that exceeded the vale based on mixing rule. To confirm the structure effectiveness, A-CNTs were dispersed into random nanotubes; as-formed composite PCMs displayed obviously reduced loading and storage capacities. The enhanced performance was ascribed to the effective encapsulation of paraffin in the interweaved pores of A-CNTs. Moreover, the composites with improved thermal conductivities were attained, and displayed enhanced thermal-storage response. The results indicated that as-prepared composites were promising candidates for thermal management systems. Keywords: carbon nanotube; shape-stabilized phase change material; paraffin; nanocomposite material ACS Paragon Plus Environment

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1. INTRODUCTION Energy storage and management has always been a critical theme for the development of a sustainable world. In particular, thermal energy storage and management systems have wild applications, such as in energy-saving buildings, solar energy storage, waste heat recycling, etc. With the increasing energy consumption, it requires for an efficient utilization of thermal energy sources. Phase change materials (PCMs) provide the potential to make thermal energy savings,1-3 since they can absorb significant amounts of energy as latent heat and release it into the surroundings during a phase change process over a defined temperature range. PCMs could be classified into the types of solid-liquid and solid-solid, and others based on the way of phase change.4 During the past years, solid-liquid PCMs (e.g., paraffin wax,5-6 polyethylene glycol (PEG),7 octadecanoic acid,8 etc.) have been extensively studied,9 and applied in thermal energy storage because of the advantages of large latent heat and low cost. On the other hand, the direct utilization of solid-liquid PCMs has the problems of leakage and poor thermal conductivity, which directly affect the heat storage capacity and efficiency. To solve the problems, shape-stabilized PCMs that are mostly composed of a supporting material and a phase change working substance have become a research hotspot in latent heat energy storage.10-14 It usually was achieved by impregnating PCMs into porous materials (e.g., metal foams15, expanded graphite (EG) ,16 graphite foams,17 polymer networks,18-19 etc.) to overcome the aforementioned disadvantages.20-21 Due to the low cost, high thermal conductivity, and lower density compared to metals, carbon materials are considered as one important kind of supporting materials.22-24 For example, through adsorbing liquid paraffin into the expanded graphite, their composite was obtained and attained a melting latent heat of 126.4 J g-1 when it was melted to liquid25. Recently, nanocarbon materials such as carbon nanotubes (CNTs)26-29 and graphene11, 30-32 have also received increasing attentions in efforts to develop new thermal storage materials considering to the higher surface areas compared to EG, high thermal conductivity, and strong interactions with PCMs.



As reported by Wang et al.,33 a nanocomposite consisting of multi-walled CNTs (MWCNTs) and paraffin was prepared, which increased by 35-40% the thermal conductivity of the composite when only adding 2.0 wt.% nanotubes. However, the randomly dispersed CNTs in paraffin as the supporting material usually was not fixed,34 and have low capacity to absorb the phase change working substance for shape-stabilized PCMs. Nanostructured CNTs with well-designed architectures (e.g., CNT sponge35) have proved to have high capacity to absorb phase change working substance. But for actual applications, it needs to make the CNT architectures at low cost. In the work, aggregated CNTs (A-CNT) with interweaved porous structure were prepared and loaded with paraffin for shape-stabilized PCMs with improved storage capacities and efficiency. Scheme 1 illustrates the route towards the paraffin/A-CNT nanocomposite for thermal energy storage. A-CNTs were first synthesized through a chemical vapor deposition process of propylene (C3H6) on a catalyst (Fe/Mo/MgO) in a fluid bed reactor. Then, melted paraffin was controllably absorbed into the CNT aggregates owning to the capillary force from the pores of A-CNTs. As-prepared paraffin/A-CNT composites have optimal mass ratio approaching 88 wt.% paraffin, that can offer a melting latent heat of 172.14 J g-1 with no leakage. To demonstrate the effectiveness of the aggregated structure, A-CNTs were dispersed into random nanotubes using acid oxidation, which offering a reduced paraffin loading and melting latent heat, that is, only 143.81 J g-1 at 75 wt.% paraffin. It has been noticed that CNTs were commonly modified to disperse into paraffin and other matrixes for higher thermal conductivities of their nanocomposites.36-37 The modifications (e.g., strong acid oxidation) often destroy the CNT length and introduce structural defects, which usually reduces its thermal conductivity and structural stability. By comparison, the strategy presented here utilizes the capillary force from the pore structure of pristine A-CNTs, which enables the simple yet efficient compounding of paraffin and CNTs, with no destroy of CNTs. To the best of our knowledge, this was the first work to develop such a kind of nanotube architecture for shape-stabilized PCMs with improved thermal storage performance. ACS Paragon Plus Environment

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2. EXPERIMENTAL SECTION Materials. Ethyl alcohol (A.R.) was purchased from Beijing Chemical Works. Fe(NO3)3·9H2O (A.R.), (NH4)6Mo7O24·4H2O (A.R.), MgO (A.R.), paraffin, and HNO3 (A.R.) were purchased from Sinopharm Chemical Reagent Co., Ltd. Preparation of Fe/Mo/MgO catalyst. 10 g of MgO powder was mixed with ultrapure water by ultrasonic and boiled for 24 hours at 94 oC. MgO content was 0.1 w/w in water. Next, Fe(NO3)3·9H2O (9.09 g) and (NH4)6Mo7O24·4H2O (0.79 g) were dissolved in ultrapure water respectively, and mixed together. The mixed solution was slowly dropped into as-prepared MgO slurry by magnetic stirring. The mixed slurry was aged 4-16 hours at 160 °C. After filtration, drying, and grinding, and calcination at 550 °C for 30 min, Fe/Mo/MgO catalyst was obtained. Preparation of A-CNTs in a fluidized bed reactor. The preparation of A-CNTs was carried out in a vertically quartz tube using a CVD method. After loading 1.0 g of Fe/Mo/MgO catalyst into the reactor, it was heated in flow argon (Ar, 500 mL min-1) atmosphere at 10 °Cmin-1. Once up to 720 °C, the flow rate of Ar was turned on 1200 mL min-1. Meanwhile, C3H6 (1000 mL min-1) was introduced into the reactor. The reaction time was kept 50 min; and then C3H6 was turned off and the reactor cooled to room temperature. For comparison, as-prepared A-CNTs were purified by acid washing (concentrated HNO3, 5 hours and 70 °C) to remove Fe/Mo/Mg layers catalyst, and to destroy the aggregated structure. After filtration and freeze drying, the collected powders was obtained and donated as o-CNT. Preparation of PCMs. Paraffin/A-CNT nanocomposite PCMs was prepared by impregnation method. Paraffin was dispersed in ethyl alcohol in a 25 mL beaker and heated to 80 °C. Then, A-CNT or o-CNT was added into beaker with magnetic stirring continuously. After the ethyl alcohol evaporated completely, the black solid PCMs were obtained. For paraffin/A-CNT composites, the loading ratio of paraffin was 75 wt.%, 80 wt.%, 85 wt.%, 88 wt.%, and 90 wt.%, respectively. For comparison, 75 wt. and 80 wt.% paraffin/o-CNT nanocomposites were also prepared.



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Characterizations. As-prepared materials were characterized by scanning electron microscopy (SEM, JSM 6700F, JEOL), transmission electron microscopy (TEM, JEM 2100, JEOL), Powder X-ray diffraction (XRD MAC Science Co. Ltd., Japan), Brunauer-Emmett-Teller

(BET)

measurements

by

N2

adsorption-desorption

(Micromeritics ASAP 2020). XRD was conducted on M21X diffractometer using Cu-Kα radiation (λ=1.541 Å) source at a scanning rate of 10°/min. Fourier transform infrared spectroscopy (FT-IR) was performed on Nicolet 6700 spectrometer. The pore properties were obtained by the adsorption branch of the isotherms, calculated using the Barrett-Joyner-Halenda (BJH) method and cylindrical model. Thermogravimetric Analysis (TGA) was conducted on NETZSCH STA449F3 thermal gravimetric analyzer at a heating rate of 10 °C min-1 from 40 to 800 °C under N2 atmosphere. Differential scanning calorimetry (DSC) was carried out on scanning differential calorimeter (Pyris 6, PerkinElmer) with materials heated and cooled at a rate of 10 °C min-1 between 10 °C and 80 °C under N2 atmosphere. Thermal conductivity of samples was measured by the guarded hot plate method, and was conducted on a transient hot disk thermal constants analyzer (Hot Disk TPS2500, Hot Disk AB company, Sweden). 3. RESULTS AND DISCUSSION Morphology and structure of A-CNTs. Figure. 1a and b displays the representative SEM images of the A-CNTs, showing that they are aggregated into round-shapes. The sizes of the granular aggregates were averaged at a diameter of 1.8 µm. The morphology looks like a cocklebur. This structure was similar to that of agglomerated CNTs produced by Prof. Wei’ group.38 The growth density of such kind of CNTs in the center was high, that leads to entangled and compacted internetworks of the center. As confirmed by low-magnification TEM images (Figure. 1c), the dense and interweaved core networks of CNTs were clearly observed for A-CNTs, with some nanotubes spread out from the catalyst center. The diameter of the nanotubes is 20-40 nm (inset of Figure. 1c). Those thin nanotubes interweaved into continuous pores, which have strong capillary force due to the nanosize. In the previous work,39-40 the


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aggregates are difficult to be destroyed even after high-pressure compression. The robust structure was important for shape-stabilized PCMs. For comparison, A-CNTs were treated with HNO3 oxidation to destroy the interweaved structure. Figure. 1d displayed that A-CNTs were dispersed into random nanotubes. As expected, the porous structure was then confirmed by the N2 adsorption-desorption isotherms shown in Figure. 1e. A type-IV isotherm with significant nitrogen uptake at higher relative pressure is consistent with a hierarchically porous networks (see pore size distribution in Figure. 1f). This kind of A-CNTs exhibits a Brunauer-Emmett-Teller (BET) surface area of 181.84 m2g-1 and pore diameter distribution around at 3.2 nm. On the other hand, the dispersed o-CNT gave a similar BET surface area of 198.21 m2 g-1 and pore size distribution, enabling the comparison of thermal storage properties more solid. Morphology and structure characterizations of paraffin/A-CNTs. Figure 2 displays the results of leakage test of paraffin/A-CNT composites compared to that of paraffin/o-CNT composites. The leakage test was performed at 80 °C. Each sample was put on the filter papers, respectively. After put it into the oven, the weight of the each sample is measured with an electronic balance every 30 minutes. The initial mass is M0 and the weight of the sample at a time was expressed as Mt. When the weight of two measurements is equal, the weight is the final weight (Me). The leakage rate is calculated as formula below: Lr =

M0 −Me M0

× 100%. There was no obvious leakage

as increasing the mass fraction of paraffin from 75 wt.% to 88 wt.% for paraffin/A-CNT composites, as also observed in the optical photograph of Figure 2a-d. The leakage begin to appear as the paraffin ratio reach 90 wt.% (Figure 2e). Therefore, the optimized paraffin content was 88 wt.%, which was one of high level for shape-stabilized PCMs. On the other hand, when using o-CNTs, the maximum loading ratio of paraffin was only 75 wt.% (Figure 2f, g). This was due to the destruction of the compacted and interweaved pore structure of A-CNTs by strong acid oxidation.



The microstructures of paraffin/A-CNT composites containing 75 wt.% and 88 wt.% paraffin were compared in Figure 3. The microstructure of 75 wt.% paraffin/A-CNT was exhibited in Figure 3a,b. The entangled networks of A-CNTs were still observed at this loading amount of paraffin. As increasing the paraffin concentration (e.g., 88 wt.%, Figure 3c, d), the shape of entangled networks become not so obvious, while the pores formed from interconnected nanotubes was fulfilling by paraffin, with a little of paraffin observed on the surfaces of the A-CNTs. Based on this and the results of the leakage test, the optimized paraffin content was 88 wt.% for paraffin/A-CNT PCMs. The XRD patterns of A-CNTs, paraffin, and paraffin/A-CNTs shape-stabilized PCMs are displayed in Figure 4a. A-CNTs exhibit the typical peak at 26.8o, corresponding to the (002) diffraction of graphitic structure. For pure paraffin, clear characteristic peaks are located at 21.5o and 23.8o. Meanwhile, those characteristic peaks of paraffin were also observed in composite PCMs. It has been found that if the pore was too small, the phase change behavior of PCMs would be highly affected.41-42 In this work, from the XRD results, it indicated that the pore structure of A-CNTs has not changed the crystallization of paraffin, which is important to ensure the relative free phase change process of paraffin in the PCMs. Figure 4b compares the FT-IR curves of A-CNTs, paraffin, and paraffin/A-CNTs composites. For A-CNTs, there is only weak absorption peaks at 3300 cm-1, which could be attributed to vibration of H2O absorbed. As for the FT-IR curve of paraffin, the absorption peaks are observed at 2923 (-CH2-stretching vibration), 2826 (-CH3stretching vibration), 1466 (-CH2- in-plane bending vibration), 1371 (-CH3- in-plane bending vibration), and 725 cm-1 (-(CH2)n- rocking vibration). No obvious differences are observed for the peaks of paraffin and paraffin/A-CNTs PCMs, except that the band intensities of the nanocomposite PCMs increase with the increase of paraffin content. Thus, the chemical structures of paraffin were not changed due to the absorption of interconnected networks of A-CNTs. Thermal storage properties of paraffin/A-CNTs. DSC tests were carried out to evaluate the effect of A-CNT ratio on the melting/freezing enthalpies and ACS Paragon Plus Environment

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temperatures of the paraffin/A-CNT composites. Figure 5a exhibits the first lap of DSC cycle curves of paraffin and as-prepared paraffin/A-CNT composites. We can see that the temperatures of onset (Ts), peak (Tp) and end (Te) of phase change of paraffin/CNTs are similar. The corresponding property parameters are presented in Table 1. It indicated that the use of A-CNTs has not obviously changed the phase transition of paraffin. In the DSC curves, two peaks are present on both heating and cooling curves. The left peak corresponds to the solid-solid phase change at a lower temperature, while the second peak corresponds to the solid-liquid phase change. The temperatures during melting are nearly from 49 to 60 °C. Then, the temperatures during solidification are nearly from 46 to 56 °C. Figure 5b shows the relationship of the latent heat and paraffin load ratio. The latent heat of pure paraffin was 193.17 J g-1. As changing the mass fraction of A-CNT, the latent heat was 146.39, 156.80, 165.79, 172.14 J g-1 for 75, 80, 85, and 88 wt.% paraffin/A-CNT PCMs. The expected latent heat was calculated based on latent heat of pure paraffin melting (193.17 J g-1) and corresponding paraffin loading ratios based on mixture rule. Interestingly, the latent heat of as-prepared paraffin/A-CNT was higher than that of expected latent heat, suggesting the obvious enhancements of the storage capacities and efficiency based on the interweaved porous structure of A-CNTs. However, the latent heat of paraffin/o-CNT composite was143.81 J g-1, a little smaller than that of expected latent heat. The phase change performance of the prepared nanocomposite PCMs was evaluated using loading ratio (R) and heat storage efficiency (E) following equation (1) and (2): R= E=

,

,

× 100%

, ,

, ,

× 100%

(1) (2)

where Hm,comp and Hf,comp are the melting and freezing latent heat of paraffin/A-CNT PCMs; Hm,PCM and Hf,PCM correspond to the melting and freezing latent heat of pure paraffin respectively. As shown in the Figure 5c, the loading ratio and heat storage efficiency are almost consistent as varying the loading percentage of paraffin of the composite. Taking 88 wt.% paraffin/A-CNT PCM as an example, the loading ratio



was calculated to be 89.11%, and storage efficiency reached 89.01%. This indicated that almost all PCMs in the composite materials can effectively perform heating and cooling during the phase transition process, suggesting high reversible efficiency. By comparison, paraffin/o-CNT PCMs displayed a reduced loading ratio and heat storage efficiency (e.g., 75 wt.% paraffin/o-CNT). This phenomenon was also observed in some other shape-stabilized PCMs35. It may be because of molecular interactions between paraffin and the supporting materials on nanoscale. As shown in Figure 5d, we suppose that extensive C-H…π interactions may be formed between paraffin and A-CNT surfaces, since this pristine nanotubes have huge π electrons on the surfaces, while C-H bonds exist in paraffin molecules. However, for the oxided random nanotubes, surface functional groups (e.g., C=O, COOH, etc.) that have been introduced on o-CNT surfaces during the oxidation treatment will bring on space repulsion and disturb the crystallization of paraffin molecules, which finally reduced the heat storage efficiency. Again, the destruction of the aggregated structure of A-CNT leads to decreased thermal energy storage performance. Figure 6 compares the DSC cycle stability of pure paraffin and as-prepared shape-stabilized PCMs. A small changes were observed between the initial cycle and the 10th and 50th curve of paraffin (Figure 6a); while all as-prepared paraffin/A-CNT PCMs exhibit excellent circulation stability within 50 cycles (Figure 6b-e). To further demonstrate the cycle stability, FT-IR spectra and XRD patterns of 88 wt.% paraffin/A-CNT before and after 50 thermal cycles were compared (Figure 7). The results indicated that there was no obvious change in the functional group and surface property after the cycles. Also, the cycle did not change the XRD patterns of paraffin in the composites. All those results indicated high chemical stability of the paraffin/A-CNT PCMs. The latent heat of 88 wt.% paraffin/A-CNT during cycling is shown in Table 2, which indicates that the molten heat of the as-prepared PCMs was only slightly decreased within 50 cycles. Also, long-term cycling stability was measured. As shown in Figure 6g and h, the paraffin/A-CNT-88% composite displayed high cycling stability during the 300 cycles. The high cycling stability was also displayed in other paraffin/A-CNT PCMs (Figure 6f).The thermal storage ACS Paragon Plus Environment

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properties of paraffin/A-CNT composites are compared with related work of shape-stabilized PCMs.33,

43-50

As shown in Table 3, the composite have the

comparable or even higher thermal storage properties compared to paraffin-based composites

such

as

paraffin/diatomite/MWCNTs,43

paraffin/graphite

foam,44

paraffin/graphite oxide,45 and other shape-stabilized PCMs.51-53 This indicated that A-CNT have the potential to be used as supporting materials for shape-stabilized PCMs. Thermal properties of paraffin/ACNTs. Figure 8 displays the optical photographs of the PCM samples deposited at 60 oC. Paraffin wax was completely melted into liquid in 1 hour of heating. On the contrast, there were no obvious shape changes for four kinds of composite materials after 1 hour. In the heat of 10 h and 24 h, paraffin/A-CNT-88% became slightly softer than before heating, while other three composites remained the same conditions. The stability of the composite comes from the capillary action of the A-CNTs on melted paraffin molecules, which gave them high shape stability. Also, the thermal stability of the composite PCMs was confirmed by TGA analysis. Figure 9 shows the TGA curves of A-CNT, paraffin, and paraffin/A-CNT PCMs. As seen from the TG curve, ACNT was still stable over 750 °C in N2. This property makes it excellent as supporting materials to load solid-liquid PCMs. The onset temperatures of weight loss for paraffin/A-CNTs are obviously higher than that of pure paraffin. In detail, the onset temperatures increase from 332 °C to 366 °C, 343 °C, 369 °C, and 359 °C respectively with reducing the content of A-CNTs. This was different from lots of previous reports, where the onset temperatures of weight loss were similar to the pure PCMs. This is due to the adsorption capacity of interconnected pores of A-CNTs on paraffin, including capillary force and intermolecular forces. The results indicated preferable thermal stability of the as-prepared PCMs, which can prevent leakage effectively. The thermal properties of paraffin and as-prepared paraffin/A-CNT PCMs are listed in Figure 10. After being pressed into a cylinder shape with a diameter of 6 mm and a thickness of 4 mm under 14 MPa pressure, the measured densities of the samples are 0.91, 0.92, 0.90, 0.84, and 0.80 g cm-3 for paraffin, 75 wt.%, 80 wt.%, 85 wt.% and 88 ACS Paragon Plus Environment


wt.% paraffin/A-CNT composites respectively. When the paraffin/A-CNT PCMs or paraffin was placed in a heated oven, a sensor inserted into them was used to measure the temperature changes of the PCMs (Figure 10a). It can be seen that the thermal conductivities of the paraffin/A-CNT composites clearly improved compared to the pure paraffin (Figure 10b). As expected, decreasing the fraction of A-CNT, the thermal conductivity of as-prepared paraffin/A-CNT also decreased gradually at room temperature. For pure paraffin, the thermal conductivity was only 0.28 W m-1 K-1. As for 88 wt.% paraffin/A-CNT composite, thermal conductivity s were increased to 0.71 W m−1 K−1, 2.5 times that of pure paraffin. Therefore, the as-prepared PCMs presented highly improved thermal conductivity with high loading of paraffin. This could be ascribed to the high graphitic degree of carbon nanotubes, which can improve thermal conductivities of the composites. Based on the improved thermal conductivities, time-course of the output temperature of 75 wt.% and 88 wt.% paraffin/A-CNT PCMs was faster than paraffin (Figure 10c). This was also the same in temperature-fall period. In order to test the thermal properties of the PCMs under phase transition temperature, we test the heat transfer performance at higher temperature (for example, 65 °C). Figure 10d-e shows the measured temperature-development trends of 75 wt.% paraffin/A-CNT and 88 wt.% paraffin/A-CNT with heating/cooling. We can see that there are obvious temperature plateaus both 75 wt.% paraffin/A-CNT PCMs and 88 wt.% paraffin/A-CNT PCMs during the heating and cooling process due to the occurrence of phase changes. Furthermore, the rate of 75 wt.% paraffin/A-CNT is faster than 88 wt.% paraffin/A-CNT in the heating and cooling processes. In the heating process (Figure 10d), it takes 103 s and 200 s respectively, when the temperature ranges from 51 °C to 56 °C for 75 wt.% and 88 wt.% paraffin/A-CNTPCMs. Meanwhile, in the cooling process (Figure 10e), times of 116 s and 164 s are required for 75 wt.% and 88 wt.% paraffin/A-CNT PCMs, respectively, to decrease the temperature from 55 °C to 50 °C. The result is consistent with the thermal conductivity test results, which shows that the addition of the nanotubes greatly improves the thermal conductivity of the PCMs. 4. CONCLUSION ACS Paragon Plus Environment

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In summary, CNT nanoaggregates with interweaved porous structure were prepared through a chemical vapor deposition process. Based on the capillary force from the interweaved pores of A-CNTs, melted paraffin was controllably absorbed into the CNT nanoaggregates, and form a new kind of shape-stabilized PCMs. As-prepared paraffin/A-CNT composites have the optimal encapsulation of paraffin approaching 88 wt.%, that can offer a melting latent heat of 172.14 J g-1 with no leakage. Also, the composite PCMs show high cycling stability for 50 cycles. By comparison, when A-CNT was dispersed into random nanotubes using acid-assisted dispersion, it displayed a reduced melting latent heat, only offering 143.81 J g-1 at 75 wt.% paraffin loading, further confirmed the effective nest of A-CNTs as supporting material for PCMs. The enhanced capacities and stability of paraffin/A-CNT PCMs were ascribed to the effective encapsulation of paraffin in the interweaved structure of A-CNTs and the enhanced interactions between paraffin and pristine nanotube surfaces; while for the oxided nanotubes, surface functional groups and defects that had been introduced during dispersion would bring on space repulsion and disturb the crystallization of paraffin, which finally reduced the heat storage performance. In addition, higher thermal conductivity was attained for the composite PCMs. The results indicated that A-CNTs have the potential to be used as supporting material for shape-stabilized PCMs. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (FRF-AS-17-007) and partially supported by the National Natural Science Foundation of China (No. 51502347 and No. 51436001). REFERENCES 1.

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Scheme 1 Schematic representation of the synthesis of the A-CNTs by CVD and the compounding of A-CNTs and paraffin for nanocomposite PCMs.


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Figure 1. (a, b) Representative SEM and (c) TEM images of the A-CNT; (d) SEM image of the o-CNT; (e) N2 adsorption-desorption isotherms and (f) pore size distribution of A-CNT and o-CNT.



Figure 2. (a-e) Leakage test as increasing the mass fraction of paraffin/A-CNT PCMs from 75 wt.% to 80, 85, 88.and 90 wt.%; (f, g) leakage appearing for 75 and 80 wt.% paraffin/o-CNT PCMs.

Figure 3. SEM microstructures of (a, b) 75 wt.% and (c, d) 88 wt.% paraffin/A-CNT shape-stabilized PCMs.


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Figure 4. (a) XRD and (b) FTIR curves of paraffin, A-CNT, and as-obtained paraffin/A-CNT PCMs.

Figure 5. (a) The first lap DSC curves of paraffin and as-obtained paraffin/A-CNT PCMs; (b) latent heat of paraffin/A-CNT PCMs vas the loading ratio of paraffin in the nanocomposites; (c) energy storage efficiency (left) and encapsulation ratio (Hm,comp/Hm,PCM, right) of paraffin/A-CNT and paraffin/o-CNT PCMs; (d) illustration of the interactions of paraffin molecules with surfaces of A-CNT and o-CNT in contact.



Figure 6. DSC curves of (a) paraffin and (b-e) as-obtained paraffin/A-CNT composite PCMs within 50 cycles; (f) cycling stability of the latent heat for paraffin/A-CNT composite PCMs. (g) DSC curves of 88 wt.% paraffin/A-CNT PCM in 300 cycles; (h) cycling stability of 88 wt.% paraffin/A-CNT PCM for the 300 cycles.

Figure 7. (a) XRD curves and (b) FT-IR spectra of 88 wt.% paraffin/A-CNT before and after 50 thermal cycles.


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Figure 8. Optical photographs of paraﬃn and paraﬃn/A-CNT composite PCMs deposited at 60 °C at increasing time.

Figure 9. TGA curves of A-CNT, paraffin, and as-obtained paraffin/A-CNT PCMs.



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Figure 10. (a) Schematic equipment for measuring time-course of the output temperature; (b) thermal conductivity of pure paraffin and paraffin/A-CNT PCMs; (c) time-course of the output temperature of paraffin/A-CNT PCMs compared to pure paraffin when they are placed in 40 °C heating oven and in the temperature-fall period; (d, e) time-dependent temperature variations of 75 wt.% and 88 wt.% paraffin/A-CNT PCMs. Table 1. Thermal characteristics of the paraffin/A-CNT composite PCMs for melting and freezing processes. Melting process Freezing process PCM

Ts/Te (°C)

Tp (°C)

Hm (J/g)

Ts/Te (°C)

Tp (°C)

Hf (J/g)

Paraffin/A-CNT-75% Paraffin/A-CNT-80% Paraffin/A-CNT-85% Paraffin/A-CNT-88%

50.01/59.69 49.87/59.67 50.13/60.19 49.81/60.00

57.54 57.31 57.96 57.79

146.39 156.80 165.79 170.84

56.52/46.56 56.64/46.67 56.78/46.59 56.85/46.85

53.04 53.08 53.10 53.45

146.66 155.62 163.97 169.78


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Table 2. Cycling stability of 88 wt.% paraffin/A-CNT composite PCMs. Melting process Freezing process The rate Capacity The rate Capacity Number Hm Hf of heat Retention of heat Retention (J/g) (J/g) loss (%) (%) loss (%) (%) First circle 170.84 100 169.78 100 10-th circle 169.90 0.55 99.45 168.80 0.58 99.42 50-th circle 169.41 0.83 99.17 167.99 1.05 98.95



Table 3. Thermal storage property comparison of paraffin/A-CNT PCMs and related references. Hm of Hm,comp/ Hm of Percentage PCMs paraffin Hm,PCM References composite (wt.%) (J/g) (%) PCM (J/g) paraffin/diatomite/MWCNT 47.25 89.4 201.5 44.37 ref 43 paraffin/ MWCNT 98 163.8 165.3 99.09 ref 33 paraffin/graphite foam 76 109.70 143.70 76.34 ref 44 paraffin/graphite oxide 48.3 63.76 131.92 48.33 ref 45 paraffin/carbon foam 90.8 164.98 189.54 87.04 ref 46 paraffin/graphite nanoplatelet 95 128.8 131.5 97.95 ref 47 paraffin/expanded graphite 92 170.3 188.2 90.49 ref 48 paraffin/silicondioxide/expanded 72 104.41 209.33 49.88 ref 49 graphite paraffin/expanded vermiculite 53.2 101.14 187.31 53.60 ref 50 paraffin/A-CNT 75 146.39 193.17 75.77 This work paraffin/A-CNT 80 156.80 193.17 81.17 This work paraffin/A-CNT 85 165.79 193.17 85.82 This work paraffin/A-CNT 88 172.14 193.17 89.11 This work


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Effective Encapsulation of Paraffin Wax in Carbon Nanotube

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