Natural Microtubule-Encapsulated Phase-Change Material with

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Natural Microtubule-Encapsulated Phase-Change Material with Simultaneously High Latent-Heat Capacity and Enhanced Thermal Conductivity Shaokun Song, Tingting Zhao, Wanting Zhu, Feng Qiu, Yuqi Wang, and Lijie Dong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 22, 2019

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ACS Applied Materials & Interfaces

Natural Microtubule-Encapsulated Phase-Change Material with Simultaneously High Latent-Heat Capacity and Enhanced Thermal Conductivity

Shaokun Song, Tingting Zhao, Wanting Zhu,* Feng Qiu, Yuqi Wang, Lijie Dong*

Center for Smart Materials and Devices, State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, and School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, P. R. China * Corresponding author, E-mail address: [email protected] (Wanting Zhu) [email protected] (Lijie Dong)

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Abstract: It is of critical importance to exploit high-performance phase-change materials (PCM) for thermal-energy storage. Present form-stable PCM suffer from the defects in low PCM loading, poor form-stability, low thermal conductivity, and complicated approaches. We prepared a novel microtubule-encapsulated phase-change material (MTPCM) by encapsulating lauric acid (LA) into kapok fiber (KF) microtubules that had been pre-coated with silver nanoparticles. The measured melting and freezing temperatures were 43.9 and 41.3 °C for the LA/KF MTPCM, and 44.1 and 42.1 °C for the LA/KF@Ag MTPCM, respectively. After being heated, the MTPCM can retain its original solid state without leaking, even under a pressure of 500 times the gravity of MTPCM itself, which shows that the encapsulated phase-change material can undergo a solid–liquid transition microscopically while retaining it macroscopic solid state. The latent heats of fusion were found to be 153.5 J/g for the LA/KF MTPCM and 146.8 J/g for the LA/KF@Ag MTPCM, which is up to 86.5% and 82.7% that of pristine LA, respectively. This thermal-energy storage capacity is much higher than reported values in recent literature, which tend to be ≤ 60%. In contrast with the penalty of a 3.8% decrease in latent-heat capacity, the remarkable 92.3% increase in thermal conductivity caused by the introduction of silver nanoparticles, is more pronounced. The thermoregulatory capacity-analysis results show that the thermal transfer efficiency of LA/KF@Ag MTPCM has been enhanced significantly by 15.8% and 23.5% in terms of thermal-energy storage and release, compared with that of the LA/KF MTPCM. Moreover, the LA/KF@Ag MTPCM exhibits a robust thermal, chemical, and morphological reliability after 2000 thermal cycles, which makes it favorable for repetitive thermal-energy storage/retrieval applications. The high latent heat, suitable phase-change temperature, outstanding form-stability, robust thermal reliability, enhanced thermal-transfer efficiency, and the inherited advantages of KF and nanosilver provide potential for the novel application of MTPCM in solar thermal-energy storage, waste-heat recovery, intelligent thermo-regulated textiles, and infrared stealth of important military targets. Keywords: Thermal energy storage; Nature microtubules; Kapok fibers; Lauric acid; microtubule-encapsulated phase-change material


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1. Introduction The conflict between growing demands for energy and the exhaustion of conventional fossil fuel, and the impacts of post-combustion on the environment, have become prominent problems in human society in the twenty-first century.1,2 Thermal-energy-storage (TES) systems that use phase-change materials (PCMs) store, transport, and use large amounts of latent heat via a reversible phase transition.3,4 PCMs have been considered potential candidates to relieve the energy crisis and environmental pollution, solve the energy supply/demand mismatch in time and space, and increase the energy-utilization efficiency. To date, PCM has been investigated extensively and applied for various purposes, such as solar thermal-energy storage,5 peak load shifting,6 waste-heat recovery,7 intelligent temperature-control building,8 re-writeable Blu-Ray disks,9,10 thermostat electronic devices,11 and smart thermo-regulated textiles.12 Fatty acids, as an excellent organic PCM, have shown great potential for TES application for their reducibility and ready availability from vegetable and animal oils, varied and adjustable phase-change temperature, large latent-heat storage capacity, robust chemical and thermal stability, noncorrosivity, nontoxicity, and low cost.13,14 However, flowing PCMs that are generated during solid-to-liquid phase transitions are difficult to handle and the triggered leakage issue has restricted their application in TES significantly.15-17 Earlier attempts at using PCMs have been confined to special closed tanks or heat-exchange pipes to prevent leakage, which increases the related expense, the thermal resistance between the PCM and environment, and hinders its practical accessibility.18 More recently, a newly developed technique, termed form-stable PCM (FSPCM), has sparked enormous interest because of its desirable characteristics, such as its form stability during phase change, direct use without additional encapsulation, cost-effectiveness, and easy preparation with desirable shapes.19-21 To date, considerable efforts have been devoted to achieve varied FSPCMs for specific applications, including polymer networks,22 porous inorganic clay (attapulgite,3 kaolinite,5 silica,6 diatomite,23 halloysite,7,8,24 and perlite25), carbon-based adsorbents (carbon,26 expanded graphite,27,28 and carbon nanotubes29), microcapsules,30 and aerogels.31,32



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Despite the superiority of the FSPCM, there remains numerous defects. For instance, the thermal-energy storage capacity of most FSPCM accounts for approximately 30%–60% of pristine PCM, which originates from the contradiction between the low PCM loading content and the poor form-stability of supporting material, especially under an exterior pressure.33,34 Beside, the low thermal conductivity of the organic PCM and supporting material leads to hysteresis of thermal dissipation, which suppresses the possible application and working efficiency.35-37 Most of all, previous supporting materials have been non-renewable, or prepared with complicated approaches, which associated with harmful solvents and byproducts.38,39 Recent developments in green and sustainable technology have yielded increased interest in renewable resources. Kapok fiber (KF), which is a type of microtubular-structure natural plant fiber with a void content as high as 80%– 90%,40 has inherent advantages of low density, high hollowness, and large adsorption capacity.41 KF is a renewable resource, and grows naturally in the tropical and subtropical regions of America, Africa, Asia, and South Pacific. It is an innovative product and will become a promising research topic for developing simple, efficient method to fabricate a natural microtubule-based FSPCM with a high latent heat, excellent form stability, and enhanced thermal conductivity. We propose the encapsulation of a PCM in optimized KF microtubules to fabricate a novel natural microtubule-encapsulated PCM (MTPCM). To improve the thermal-transfer efficiency of the MTPCM, the KF microtubules were pre-coated with silver nanoparticles (AgNP), using poly-dopamine as a binding platform. The inherited large hollow microtubular structure and the successful construction of continuous thermal conductive pathways help contribute to the obtained MTPCM with an ultra-high PCM encapsulation ratio, large thermal-energy storage capacity, excellent form stability, robust thermal reliability and enhanced thermal-transfer efficiency. AgNP across microtubules provides the novel MTPCM with broad potential for outstanding photocatalytic, antibacterial, and electrically conductive properties. Therefore, the green, low-cost, and simple-processing MTPCM with a distinguished comprehensive performance exhibits great potential in TES for renewable and sustainable development.


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2. Experimental 2.1 Materials KF was from Sichuan, China. The KF was oven-dried at 80°C overnight to remove moisture, and was cut into small segments. LA and anhydrous ethanol were from Sinopharm Chemical Regent

Co.,

Ltd,

China.

Dopamine

hydrochloride,

silver

nitrate,

2-

-2(hydroxymethyl)-1,3-propanediol (Tris) and hydrochloric acid (HCl, 36%) were from Sigma-Aldrich, USA. All materials were analytic-reagent grade and were used as received without further purification. Distilled water was made in our laboratory. 2.2 Preparation of KF@Ag microtubules Silver-coated KF microtubules (KF@Ag) were prepared as follows. Initially, dopamine hydrochloride was dispersed ultrasonically into the Tis-HCl buffer solution (0.01 mol/L, pH 8.5) for 10 min to form a homogeneous aqueous solution (2 g/L). KF was immersed into the dispersion and stirred for 8 h at room temperature, rinsed with deionized water, and dried in a vacuum oven at 60°C overnight. The obtained gray poly-dopamine-coated kapok fiber (P-KF) was added to the silver-nitrate solution (10 g/L) and subjected to UV irradiation with stirring for 30 min. Silver-deposited P-KF (KF@Ag) was washed thoroughly with distilled water and dried in a vacuum oven. 2.3 Preparation of LA/KF@Ag MTPCM The LA/KF@Ag MTPCM was prepared by a simple vacuum-impregnation technique.23,42,43 In brief, a fixed amount of LA and KF@Ag microtubule mixture (mass ratio, 20:1) was heated to 60°C and maintained for 1 h to ensure that the molten LA had absorbed into the microtubules by capillary force. The mixture was transferred into a vacuum system, exhausted to 0.1 MPa and aspirated to facilitate PCM inhalation into the microtubules. The procedure was repeated at least 3 times before cooling to room temperature. The fabricated composite PCM was washed thoroughly using ethanol to guarantee the complete removal of free LA, and vacuum dried at 60°C overnight. The complete fabrication process is illustrated schematically in Fig. 1. For comparison, composite PCMs that



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fabricated using KF and KF@Ag microtubules as different microcontainers were termed LA/KF MTPCM and LA/KF@Ag MTPCM, respectively.

Fig. 1. Schematic illustration of LA/KF@Ag MTPCM preparation. 2.4 Characterization The chemical-structure evolution of KF was examined by Fourier-transform infrared spectrophotometry (FTIR, Thermo Nicolet Nexus, USA) in a wavenumber range of 400–4000 cm-1 using KBr pellets. The sample crystallization behavior was investigated by X-ray diffractometry (XRD, D/Max-IIIA, Rigaku, Japan), using Cu-Kα radiation in the 2θ range of 10–60°. The surface morphology of the KF and MTPCM microtubules was observed by field-emission scanning-electron microscopy (SEM, JSM-7500F, JEOL, Japan). Prior to observation, all samples were sputter-coated with gold to avoid charge accumulation. The internal microtubule morphology was studied by using a polarizing optical microscope (POM, XPL-40, PUDA, China). The POM was equipped with a color-sensitive plate that allowed the digital camera to capture images that were developed and analysed. Differential-scanning calorimetry measurement (DSC, Pyris 1, Perkin Elmer, USA) was used to determine the phase-change properties, including the phase-change temperatures and latent heats. Prior to testing, the DSC calorimeter was calibrated with indium as the reference. Sample analysis was conducted on approximately 5 mg of material each time with a


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heating and cooling rate of 10°C/min for 0–100°C under nitrogen. The phase-change temperatures, which correspond to the extrapolated onset temperature, were obtained by drawing a line at the maximum slope point from the DSC peak and by extrapolating a baseline on the same side as the leading edge of the peak. The latent heats were calculated based on the integrated area under the peak. The thermal conductivity was measured by using a thermal-constant analyser (TPS2500S, Hot Disk, Sweden). The pore-size distribution and total porosity were determined by mercury-intrusion porosimetry analysis (AutoPore IV-9500, pore size 6 nm–360 μm, Micromeritics, USA). An applicability test was conducted to investigate the thermal reliability and thermoregulation performance of the prepared MTPCM. Thermal-cycling tests were performed primarily to determine the thermal reliability of the MTPCM in a programmable constant-temperature and humidity chamber (FH-80R, Dongguan City Development Environment Test Equipment Co. Ltd, China). The thermal cycling consisted of 2000 consecutive melting/freezing cycles for 0–100°C. The thermoregulation capability was determined from the heating and cooling curves of a plastic tube, which was filled uniformly with the same amount of sample. In the experimental section, the self-designed setup was placed in a water bath at 60°C heating, and then allowed to cool in a 10°C environment. The central temperature of the tube during heating and cooling was recorded by an inserted thermocouple.

3. Results and Discussion 3.1 KF@Ag microtubule morphology The surface-morphology changes between the KF and KF@Ag microtubules were examined by SEM. Fig. 2(a,b) shows that KF presents a cylindrical open-ended structure with a smooth surface. The measured KF external diameter and wall thickness were 18–22 m and 0.5–1 m through the vertical axes. It can be estimated that the hollow volume ratio of internal channels using external diameter of 20 μm and wall thickness of 0.5 μm (external and internal diameters 20 and 19 μm, respectively), is about 90 vol%, indicating the large adsorption capacity. Typical SEM images



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of the KF@Ag in Fig. 2(c,d) show that the KF morphology was well retained, whereas the 50–80 nm AgNP was anchored uniformly on the exterior KF wall without agglomeration. Elemental silver was clearly visible from the EDAX spectrum in Fig. 2(e). The corresponding elemental composition of the selected area, which is shown in the table in Fig. 2(e), illustrates that the quantitative signals for C, O, and Ag are 32.6 wt%, 24.1 wt%, and 43.3 wt%, respectively. Therefore, the UV irradiation has no significant influence on the morphological structure of the KF except for accelerating the AgNP formation.

Fig. 2. SEM images of KF (a,b) and KF@Ag (c,d); (e) corresponding EDAX pattern. 3.2 Chemical structure characterization FTIR analysis was used to monitor the chemical-structure evolution of intermediates. It has been widely reported that KF exhibits extremely hydrophobic properties with microtubular surface, which mainly composite of wax from the composition of 35% cellulose, 22%xylan, and lignin. 40,41 Fig. 3(a) compares the FTIR spectra of LA, KF, P-KF, KF@Ag, and LA/KF@Ag MTPCM. For


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P-KF and KF@Ag, the band around 3426 cm-1, which is ascribed to the stretching vibration of O-H, become wider because of the overlap of N-H stretching vibration peak in poly-dopamine.44 Moreover, the intensity of the peaks around 1739 cm-1, which is associated with ketones, carboxylic groups and esters in lignin and acetyl ester groups in xylan, increases obviously after the poly-dopamine coating. Except for that, the shape and frequency of the absorption peaks agree with those of KF, indicating the low quantity of poly-dopamine and AgNP. As for unmodified KF, it exhibits non-wettability in silver nitrate solution, which hinders the AgNP coating. Whereas, the P-KF was transferred into hydrophilic after the poly-dopamine modification. Meanwhile, the polymer layer plays as role of binding platform for AgNP.

Fig. 3. (a) FTIR spectra of LA, KF, P-KF, KF@Ag, and LA/KF@Ag MTPCM; (b) XRD patterns of LA, KF, KF@Ag, and LA/KF@Ag MTPCM. In the FTIR spectrum of LA, multiple absorption peaks at 2923 and 2854 cm-1 are attributed to the C-H stretching vibration of -CH3 and -CH2. 26,29 The broad peak at 3300–2800 cm-1 is caused by -OH stretching vibration and the intensive absorption peak at 1710 cm-1 is the characteristic absorption peak for the stretching vibration of the C=O group. Compared with the FTIR spectra of KF@Ag and LA, the shape and relative intensity of all characteristic peaks of MTPCM is similar to LA other than KF@Ag, which suggests a high total LA content in the composite. Beyond that, any new significant peak is not observed in the spectrum of MTPCM, which indicates no chemical



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reaction among its constituents. Therefore, the FTIR results show that a good physicochemical compatibility exists between the KF@Ag microtubule and LA. XRD analysis was performed to examine the formation of AgNP and LA crystals, and whether they retain the crystal form in MTPCM, as shown in Fig. 3(b). KF presents a broad diffraction peak at a 2θ of ~22.2°, which is ascribed to amorphous cellulose and lignin. For KF@Ag, the new characteristic diffraction peak of face-centered cubic AgNP at 38.0° is clearly distinguishable, which is indexed to JCPDS 87-0718. The XRD pattern of LA displays multiple intensive diffraction peaks at a 2θ of 9.8°, 21.7°, 23.5°, and 24.32°, which reveals its highly crystalline nature (PDF 00-008-0528).45 The XRD pattern of MTPCM exhibits a stack of characteristic crystalline peaks of KF@Ag and LA, which indicates that AgNP and LA are crystallized normally in MTPCM. Like the FTIR results, the XRD pattern of MTPCM is close to that of LA, especially in crystalline region.

Fig. 4. (a) TG analysis of KF and KF@Ag microtubules, (b) mercury-intrusion porosimetry analysis of KF and LA/KF@Ag MTPCM. TG analysis was used to evaluate quantitatively the thermal stability of KF and AgNP content. The TG curves in Fig. 4(a) show that the small weight loss of 5.9 wt% for KF and 4.4 wt% for KF@Ag before 100°C is caused by the removal of adsorbed water. KF and KF@Ag present a significant mass loss for 250–350°C because of the effect of depolymerization, dehydration, and decarburization, which is far higher than the working temperature of PCM. A maximum mass loss


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occurs at 309.5°C, and the charred residue at 500°C is 2.94 wt% for KF. The thermal stability of KF@Ag is enhanced slightly and the maximum mass loss temperature increases to 315.3°C. Besides, the residual mass of KF@Ag (6.95 wt%) is larger than that of KF, ascribed to the fact that AgNP does no decompose from 100–500°C. Hence, it can be deduced that the AgNP content is 4.01 wt% by subtracting the charred residue of KF from the charred residue of KF@Ag. Furthermore, the mercury-intrusion porosimetry analysis was used in order to investigate the impacts of LA impregnation on the internal microtubule structure (porosity). The cumulative volume curves in Fig. 4(b) indicate one level pore volume for KF and LA/KA@Ag MTPCM, and the diameter larger than 30 μm decreases to some extent. The internal microtubule void decreases significantly by encapsulating LA within the microtubule channels, which is an approximate 12-times reduction in the total volume of pores ˂ 30 μm. The total intrusion volume decreases significantly from 19.41 ml/g to 3.73 ml/g, and the overall porosity decreases from 91.8% to 79.9%. The intrusion volume decreases reveals that the LA takes up as high as 81 vol% in the internal channels of microtubules and the reserved space is left to be 19 vol%. To verify whether KF microtubules can prevent the melted LA from overflowing when solid– liquid phase-transition occurs within the channel, an additional leakage test was conducted. Leakage is most likely to happen at the end of the KF microtubule and once it happens, the enormous stored thermal energy will lose meaning. The leakage test was performed by heating the LA/KF@Ag MTPCM above the melting temperature of the LA under pressure from a steel block, which provides 500 times the gravity of MTPCM itself. The leakage was observed by using a digital camera, as shown in Fig. 5(a–c). As expected, the MTPCM retains its original fiber state and does not allow liquid leakage into the filter paper even after being pressed for 1 h, which shows that the PCM can be well imprisoned in the interior KF microtubule channel by strong capillary action and interfacial bonding.



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Fig. 5. (a–c) Leakage test of MTPCM; (d,e) SEM images of MTPCM; POM images of (f) KF, (g) KF@Ag and (h,i) MTPCM (magnification: f, g and h ×400; I ×100). SEM and POM analysis were applied to observe the morphological changes of the MTPCM. The SEM images in Fig. 5(d,e) show that the MTPCM microtubule exhibits a cylindrical and smooth morphology that is similar to pristine KF without visible solid matter in the interspace and exterior surface, and confirms the complete removal of all free LA during preparation. The POM images in Fig. 5(f–i) show that the KF and KF@Ag microtubule have a smooth, thin wall and a transparent structure. The POM images of the MTPCM show that the interior channel with a large aspect ratio is filled mostly with PCM in the middle of the KF microtubule by the deeper contrast. The large aspect ratio of microtubule means that there exists strong capillary action and interfacial bonding for PCM. In addition, KF is a type of natural fiber based on lignin and cellulose, which provide a favourable stiffness and rigidity for the microtubule. Furthermore, the 19 vol% reserved space of internal channels enables LA to expand sufficiently and will not leak from the open ends of the long microtubules when heated or pressed, and thus the influence of the open tube end can be


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ignored. Hence, the complete encapsulated state of the PCM, the desirable mechanical strength of the microtubules, the large aspect ratio and the considerable reserved space of the MTPCM allows the PCM to undergo solid–liquid transition microscopically while maintaining a solid state macroscopically, even under an exterior pressure. It is easy to speculate that the microtubular microcontainer that is coated with AgNP plays an important role as a thermal-conductive pathway, which endows the final MTPCM with a high thermal-energy storage capacity and an enhanced thermal conductivity. 3.3 Thermophysical properties of LA/KF@Ag MTPCM

Fig. 6. (a) DSC curves of LA, LA/KF, and LA/KF@Ag MTPCM; (b) thermal-conductivity comparison of KF, KF@Ag, LA/KF, and LA/KF@Ag MTPCM. On the other hand, from the integrated area of the endothermic and exothermic peaks provides the latent heats of melting and freezing as 177.4 and 178.8 J/g for LA, 153.5 and 150.2 J/g for LA/KF MTPCM, and 146.8 and144.3 J/g for LA/KF@Ag MTPCM, respectively. Theoretically, the locked PCM content in the MTPCM is critical because the supporting material does not undergo phase transition and the PCM accounts for the latent thermal heat of the final MTPCM. Therefore, the calculated mass fraction of the PCM, which is obtained by dividing the latent heat of MTPCM by the latent heat of LA, achieves an unprecedented high encapsulation ratio of up to 86.5 wt% for the LA/KF MTPCM and 82.7 wt% for the LA/KF@Ag MTPCM. Especially, Table 1 presents the



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thermal properties and encapsulation ratio of the MTPCM prepared in this research with those of different FSPCM available in most recent and relevant references. In view of the reported results, it can be found that the MTPCM demonstrates the highest latent heat as well as largest encapsulation ratio compared with those of different FSPCMs in previous studies. It is the large hollow volume ratio of KF and high encapsulation ratio of PCM that derive the high latent heat storage capacity of MTPCM. Moreover, MTPCM is flexible, low-density, abundant resource, easy to shape, and thus, is more competitive in TES applications. Table 1. Comparison of encapsulation ratio and thermal properties of MTPCM prepared in this work with those of recently reported FSPCM. FSPCM

Melting point (°C)

Freezing point (°C)

Latent heat (J/g)

Encapsulation ratio (wt%)

Reference

PEG/Fe3O4–GNS

-

-

100

54.1

2

PEG/mesoporous silica

52.0

-

88.2

70.0

6

PEG/PU-halloysite

52.6–57.4

27.3–38.1

83.8–118.7

69.5–79.3

8

SA/polypropylene aerogel

59.0

47.7

143.3

77.2

14

SA/carbon-nanotube

29.0–30.0

22.0–22.5

76.3–111.8

54.2–79.5

21

PEG/diatomite

58.8–59.8

38.6–41.0

51.5–110.7

30.0–63.0

23

LA/carbon nanotubes

30.1–37.8

-

0-54.6

15.5–54.8

29

Paraffin/vermiculite

48.9

53.0

101.1

53.9

37

Coconut oil/xGnP

26.9

18.3

82.3

74.5

43

PEG/PU

43.8–49.9

26.5–38.4

79.6–98.2

46.3–56.2

46

Mg(NO3)2·6H2O/ PECA

87.0

86.0

83.2

51.9

47

PEG/PDA@BN

45.9

37.7

117.4

70.0

48

LA/KF MTPCM

43.9

41.3

153.5

86.5

This work

LA/KF@Ag MTPCM

44.1

42.1

146.8

82.7

This work


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Thermal conductivity of PCM is another important parameter in thermal-energy storage applications. The thermal conductivity of samples in Fig. 6(b) was determined with a thermal constant analyser at room temperature. The measured thermal conductivity is 0.0293 W/mK for KF, which indicates the intrinsic low thermal transfer efficiency of the natural microtubule. However, the thermal conductivity of KF@Ag is enhanced prominently to 0.0552 W/mK. By introduction of AgNP, the thermal conductivity was further increased to 0.0902 W/mK for LA/KF@Ag MTPCM, which is a significant enhancement compared with the LA/KF MTPCM of 0.0469 W/mK. The thermal conductivity improvement for unfilled microtubules and filled MTPCM was ascribed to the heat transfer mode is thermal radiation for KF, while it translates into conduction for MTPCM. In contrast with the penalty of a 3.8% decrease in the latent-heat capacity of LA/KF@Ag MTPCM, the remarkable 92.3% increase in thermal conductivity caused by the coated AgNP, is more pronounced. The enormous latent-heat capacity of LA/KF@Ag MTPCM is inherited from the unique thin wall, large hollow structure and excellent absorption feature of the KF microtubule. The significant enhancement in thermal conductivity results from the AgNP coated on the exterior microtubule wall, which acts as a heat-conductive bridge, facilitates the construction of a continuous heat-conduction network or efficient percolating pathway for heat flow, and leads to an increase in transfer efficiency. Furthermore, KF is a green, naturally abundant, and renewable resource, whereas AgNP has photocatalytic, antibacterial, and electricity conductive properties, and thus LA/KF@Ag MTPCM is promising for widespread thermal-energy storage applications. 3.3 Thermal reliability of LA/KF@Ag MTPCM For efficient TES applications, it is critically important that a perfect MTPCM should exhibit a high thermal-energy storage capacity, an enhanced thermal conductivity, and a robust reliability with respect to a large number of thermal circulations. The thermal reliability of the MTPCM in this work was determined by performing 2000 consecutive melting/freezing cycling tests. Fig. 7(a) shows the latent heat of fusion of the LA/KF@Ag MTPCM normalized by first heating cycle before and after the different numbers of thermal cycling. The inset shows that the heating/freezing loops



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of MTPCM after different thermal cycles coincide well with those of the uncycled sample, and the positions and relative areas of the melting and freezing peaks maintain a stable invariability after 2000 thermal cycles. The melting and freezing temperatures change 1.9°C, and the latent heats of melting and freezing fluctuate 0.43% during the thermal cycling, which are both at a reasonable level and negligible for TES applications. As shown in Fig. 7(b), the shape and frequencies in the FTIR spectrum of cycled MTPCM fit well with the uncycled sample, which indicates an unchanged chemical structure over 2000 repeated cycles.

Fig. 7. (a) Latent heat of fusion of MTPCM normalized by first heating cycle (inset shows DSC curves after different thermal cycles), (b) FTIR spectra and (c) thermal conductivity of MTPCM before and after thermal cycling, (d) SEM image of MTPCM after thermal cycling. The thermal conductivity comparison of the LA/KF@Ag MTPCM before and after thermal cycling in Fig. 7(c) indicates no obvious difference. In addition, the representative SEM image after thermal cycling test, Fig. 7(d), proves that no molten LA leaked out from the open ends of the microtubule after thermal cycling, which can be ascribed to the strong capillary force and surface-bonding interaction, and the reserved space. On the basis of the above results from the


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thermal-cycling test, it can be concluded that the novel MTPCM exhibits a robust thermal, chemical and morphological reliability over the 2000 thermal cycles, which is favorable for use in repetitive thermal-energy storage/retrieval applications. 3.4 Thermoregulatory capacity of LA/KF@Ag MTPCM

Fig. 8. (a) Schematic illustration of temperature-monitoring system; (b,c) thermoregulatory curves of KF, LA/KF, and LA/KF@Ag MTPCM. To estimate the applicable performance of the LA/KF@Ag MTPCM in terms of temperature regulation, the thermoregulatory capacity was analyzed from a heating and cooling temperature comparison of a plastic tube, which was filled uniformly with the same amount of KF, LA/KF, and LA/KF@Ag MTPCM. The self-designed setup was subjected to a simulated exterior environment temperature change from 60°C to 10°C for heating and cooling, as illustrated schematically in Fig. 8(a). The corresponding heating and cooling curves of the tube center are shown in Fig. 8(b,c). Evidently, the KF shows the fastest heating and cooling rates without thermoregulatory capacity, whereas the MTPCM shows characteristic plateau regions of the PCM around 40°C during heating



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and cooling, which indicates that the MTPCM can achieve the harvesting, transportation, and utilization of large amounts of thermal energy and undergo a reversible phase transition. Nevertheless, the AgNP results yield a significant difference in thermal-energy storage and release time, which is defined by measuring the time between 35 and 45°C. Accordingly, the thermal-energy storage and release time is determined to be 247 and 277 s for the LA/KF MTPCM, and 208 and 220 s for the LA/KF@Ag MTPCM, respectively. These results show that the thermal transfer efficiency in thermal-energy storage and release has been enhanced significantly by 15.8% and 23.5% for the LA/KF@Ag MTPCM, compared with that of the LA/KF MTPCM. Therefore, if we consider the high latent heat, suitable phase-change temperature, outstanding form stability, robust thermal reliability, enhanced thermal conductivity, applicable thermoregulatory capacity, the MTPCM with green, renewable, low-cost, flexible, easy processing advantages has widespread potential in applications such as solar thermal-energy storage, waste-heat recovery, intelligent thermo-regulated textiles, and infrared stealth of important military targets.

4. Conclusions A novel microtubule-encapsulated PCM was prepared by encapsulating LA into KF microtubules that had been pre-coated with AgNP to improve the thermal transfer efficiency. The melting and freezing temperatures were 43.9 and 41.3°C for the LA/KF MTPCM, and 44.1 and 42.1°C for the LA/KF@Ag MTPCM, respectively. After being heated, the MTPCM retained its original solid state without leaking, even under a pressure of 500 times the gravity of MTPCM itself, which demonstrates that the MTPCM can allow the encapsulated PCM to undergo a solid– liquid transition microscopically while retaining its macroscopic solid state even under an exterior pressure. The latent heats of melting and freezing were 153.5 and 150.2 J/g for LA/KF MTPCM, and 146.8 and 144.3 J/g for LA/KF@Ag MTPCM, respectively, which indicates an unprecedented high up to 86.5% and 82.7% that of pristine LA, respectively. This thermal-energy storage capacity is much higher than the reported values in recent literatures, which tend to be less than 60%. In


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contrast with the penalty of a 3.8% decrease in latent-heat capacity, however, the remarkable 92.3% increase in thermal conductivity caused by the AgNP coating, is more pronounced. The thermoregulatory capacity test results demonstrate that the thermal transfer efficiency of the LA/KF@Ag MTPCM has been enhanced significantly by 15.8% and 23.5% in thermal-energy storage and release, compared with that of the LA/KF MTPCM. Moreover, the LA/KF@Ag MTPCM exhibits a robust thermal, chemical, and morphological reliability over 2000 thermal cycles, which favors repetitive thermal-energy storage/retrieval applications. Therefore, based on the high latent heat, suitable phase-change temperature, outstanding form-stability, robust thermal reliability, enhanced thermal conductivity, and applicable thermoregulatory capacity the MTPCM with advantages of green, renewable, low-cost, flexible and easy processing has widespread potential in applications such as solar thermal-energy storage, waste-heat recovery, intelligent thermo-regulated textiles and infrared stealth of important military targets.

Acknowledgements We gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51706166 and 51773163), the Innovation Group Project of Natural Science Foundation of Hubei Province (2016CFA008), and the 973 Program (No. 2010CB227105).

Conflicts of interest There are no conflicts to declare.



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Natural Microtubule-Encapsulated Phase-Change Material with

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