Form-stable Solar-thermal Heat Packs Prepared by Impregnating

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Applications of Polymer, Composite, and Coating Materials

Form-stable Solar-thermal Heat Packs Prepared by Impregnating Phase Change Materials within Carbon-coated Copper Foams Qinxian Ye, Peng Tao, Chao Chang, Lingye Zhou, Xiaoliang Zeng, Chengyi Song, Wen Shang, Jianbo Wu, and Tao Deng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17492 • Publication Date (Web): 26 Dec 2018 Downloaded from http://pubs.acs.org on December 27, 2018

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ACS Applied Materials & Interfaces

Form-stable Solar-thermal Heat Packs Prepared by Impregnating Phase Change Materials within Carbon-coated Copper Foams Qinxian Ye,†,‖ Peng Tao, *, †,‖Chao Chang,† Linye Zhou,† Xiaoliang Zeng, *,‡ Chengyi Song,† Wen Shang,† Jianbo Wu,† Tao Deng*,†

†State Key Laboratory of Metal Matrix Composites, School of Materials Science and

Engineering, Shanghai Jiao Tong University, Shanghai, 200240, China ‡Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen 518055,

China KEYWORDS: Heat pack, Solar-thermal energy, Phase change materials, Thermal comfort, Form stability

ACS Paragon Plus Environment

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ABSTRACT

The heat packs that are based on solid-liquid transition of phase change materials (PCMs) have been pursued as a promising way to provide heating for human body comfort and thermotherapy owning to their large heat storage capacity and near-constant heat-releasing temperature. Current heat packs, however, suffer from leakage, slow charging and poor heat-releasing performance due to the flow of liquid PCMs and their low thermal conductivity. Here, we report a strategy for preparing high-performance PCM-based solar-thermal heat packs through impregnating organic PCMs within carbon-coated copper foams (CCFs). The porous structure and hydrophobic surface of CCF help effectively confine the melted liquid PCM within the composite heat pack without leakage. The carbon coating layer efficiently converts incident solar light into heat, which is rapidly transferred along the three-dimensional thermal conductive network of CCF and stored within the PCM. In the discharging process, the CCF network facilitates the extraction of the heat stored within the PCM. In contrast to neat PCM pack within which only a small portion of PCM that is in contact with human skin contributes to thermal comfort, all the PCMs within the CCF-based composite heat pack concertedly release the stored heat. Such release significantly increases the extractable thermal energy and prolongs the usable healing duration for thermotherapy.

INTRODUCTION As one of the classic and popular physiotherapies, thermal therapy has been widely used to reduce swelling of human tissues, mitigate body pain, allay tiredness or treat stiff muscles,1-3 and magnetic hyperthermia has been intensively investigated as a promising way to precisely treat tumors within human bodies.4, 5 So far, several approaches have been developed to generate local


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heating on human body and provide thermal comfort to alleviate the painful symptoms. Among them, the heat pack that contains materials with high specific heat capacity such as blue-colored silica gel or water is the most common approach.6, 7 Controlling the application temperature of the sensible heat packs, however, is difficult and the heating duration for such pack is often short due to the low energy density of the storage medium. Joule heating is another way to provide thermal comfort through electrically heating the pad, wrap or blanket.8-10 Besides electrodes and resistors, regulating the heating temperature often needs thermostat and other complicated electrical controllers. Such requirements would limit the widespread applications of this approach. Additionally, it may cause burning and other accidents if the thermostat malfunctions. High-capacity heat packs driven by renewable solar energy are the promising candidate to meet the therapeutic requirement and mitigate such risks. In recent years, solar-thermal technology has regained broad research interest for enabling many important heat-related applications.11-14 In particular, the rapid development of new solar-thermal conversion materials15,

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and solar-

thermal storage materials such as graphene-based solar-thermal fuels17-20 and high-temperature solar-thermal fluids21,

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helps increase the energy conversion efficiency and address the solar

intermittency issue. Compared to sensible heat storage materials, phase change materials (PCMs) such as salt hydrates, paraffin, and fatty acid possess much higher energy density due to the large latent heat from the solid-liquid phase transition.23, 24 Unlike sensible storage media, these PCMs can release their latent heat within a relatively stable temperature range and the phase transition temperature is tunable as well. PCM-based solar-thermal packs could be the suitable candidate to provide thermal comfort for people living at poorly-facilitated regions or for field applications.25 Current PCMs, however, generally have the following challenges: (1) they have poor solar absorption


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and thus cannot directly convert solar energy into heat;26-28 (2) they typically have a low thermal conductivity, which limits melting of the solid PCM during the charging process and prevents extraction of stored thermal energy during the discharging process;28 (3) once melted the liquid PCMs tend to have the leakage problem.29 To address these issues, in the past high thermal conductivity solar-absorbing carbon materials such as graphite,30-34 carbon nanotubes,35,

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graphene37-41 and carbon fibers42 had been intentionally blended with PCMs to improve solar absorption and thermal conductivity. The enhancement of thermal conductivity is limited due to the large interfacial thermal resistance between the PCM matrix and the filler. Impregnating PCMs within three-dimensional (3D) porous carbon foams has been shown to be able to simultaneously realize direct solar-thermal storage, high thermal conductivity and leakage-proof capability.29,

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These carbon foams, however, are usually fabricated through relatively

complicated processes such as vapor deposition or slow self-assembly processes,31-35, 39, 40 which prevent scalable production of both the foams and the PCM composites for practical applications. In comparison to carbon foams, commercially available metal foams also have high thermal conductivity and can be produced with large sizes at low cost. Previously, they had been used together with PCM to improve heat transfer performance during charging and discharging processes of the PCM.43-45 The resultant composites still had leakage problem and could not directly harness solar-thermal energy. In this work, we report a facile way to fabricate metal foam-supported PCM heat packs that are form-stable, have high thermal conductivity and can be directly charged by solar illumination. We also explore their thermal comfort applications in this work. Such solar-thermal composite heat packs were prepared by impregnating PCMs within porous-structured copper foam that has been surface-etched by hot alkali solution followed by coating with mixed carbon


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nanoparticles and polydimethylsiloxane (PDMS). The small pore size together with hydrophobic surface of the carbon-coated copper foam (CCF) effectively prevents the leakage of melted paraffin PCM. The coated carbon layer efficiently absorbs the broadband solar light thus allowing direct charging of the heat pack with solar illumination. The 3D copper network with high thermal conductivity provides the paths for rapid heat transfer during charging and discharging processes. As a result, the CCF-PCM composite heat packs have demonstrated increased extractable solar-thermal energy and significantly prolonged healing time than the neat PCM. The enhancement in heat-releasing performance has also been confirmed by theoretical simulations. Finally, we showed that the multifunctional CCF could also be applied to impregnate other PCMs with different phase change temperature for thermal comfort applications. EXPERIMENTAL SECTION Materials. Copper foams with 100 pores per inch (100 PPI) and a porosity of ~90% was ordered from Kunshan Jiayisheng Electronics Co., Ltd. Paraffin wax was purchased from Aladdin Reagent Co., Ltd. Hydrochloric acid (HCl), (NH4)2S2O8, ethanol and chloroform were purchased from Sinopharm Chemical Reagent Co., Ltd. Potassium hydroxide (KOH) was purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. Polydimethylsiloxane (PDMS) silicone elastomer and curing agent (Sylgard 184) were bought from Dow Corning. Carbon nanoparticles (C, Vulcan® XC-72R) were purchased from Cabot. Preparation of PCM Composites. In a typical experiment, the copper foam was cut into a cuboid (2 cm × 2 cm × 0.8 cm) and ultrasonically cleaned in 4M HCl and DI water, respectively. The clean foam was then immersed within an aqueous etching solution of 3.2 M KOH and 0.48


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M (NH4)2S2O8 at 65 oC under stirring for 20 min. Afterwards, the etched foam was rinsed by ethanol and DI water for 5 times followed by drying within an oven. The surface of obtained foam was further deposited with a black PDMS coating by immersing it within chloroform dispersion (0.1 wt%) of mixed silicone elastomer and curing agent (10:1 weight ratio), and carbon nanoparticles (0.7 wt%). The sample was heated at 60 oC for 24 h to cure the PDMS coating. To prepare the PCM composites, the carbon-coated copper foam was immersed within melted paraffin wax for 5 min before being taken out and cooled to room temperature. Characterization and Property Measurement. The microstructure of pre-cleaned, alkalietched, carbon-coated copper foams and PCM composites was examined by a field-emission scanning electron microscopy (SEM, Sirion 200). A high-speed camera (X-Stream XS-4, IDT, US) was used to record the contact angle of a water droplet (10 µL) on different foam surfaces. Form stability test was carried out by comparing the weight of PCM composite before and after heating in an oven at 100 oC for 20 min. The fusion enthalpy of paraffin and PCM composites was measured by a Differential Scanning Calorimeter (DSC, Netzsch, 204F1). A Thermo Gravimetric Analyzer (TGA, Pyris 1) was used to characterize thermal stability of the PCM composites. The absorption spectra were collected by an UV-Vis Spectrometer (PerkinElmer, Lambda 950, USA) using the Universal Reflectance Accessory (PerkinElmer, USA) as the reference sample. All infrared (IR) images were captured by an IR camera (FLIR T620, FLIR Systems, Inc., USA). To measure the thermal conductivity of PCM composite, the sample was sandwiched between a cooling plate (Shanghai Bilon Equipment) at 10 oC and an electrical heating plate (2 cm × 2 cm) with tunable heating power. The thermal conductivity was calculated based on the Fourier’s law.


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Charging and discharging performance measurement. To compare the charging and discharging behavior, the neat paraffin and PCM composite samples were placed within the same quartz container. The simulated solar light generated by a xenon lamp (JYANG HID) was directly shed onto the sample. The simulated solar light was concentrated by a Fresnel lens and the illumination power density was measured by a power meter (Aulight, Beijing). The whole sample was placed on a silicone sheet to avoid rapid heat loss from the bottom. Time-sequential IR images of the PCM sample were taken to monitor the charging and discharging processes. To evaluate the discharging performance on human skin, all the samples were placed in contact with a cooling plate that was maintained at 35 oC to imitate human skin temperature. The PCM samples were fully melted by heating them at 70 oC in an oven. The time for the sample to be cooled down to 40 oC was used as the useful healing duration when evaluating the heatreleasing performance. To explore its potential practical thermal comfort application on human bodies, the PCM composite was bended to fit human arm’s shape and charged under simulated solar illumination of 8 kW/m2 for 5 min before being applied on a human arm to test its heatreleasing performance. RESULTS AND DISCUSSION Design of Form-stable PCM Heat Pack. As schemed by Figure 1, the composite heat pack is essentially a porous CCF that is impregnated with PCM. In the composite, the CCF plays several key roles: (1) the coated carbon at the surface is a good solar absorber, which can efficiently convert solar energy into heat; (2) the porous structure of the CCF also helps trapping the incident solar light; (3) the small pore size and surface hydrophobicity of the CCF generate strong capillary force to prevent the leakage of melted PCM; (4) the copper network with high


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thermal conductivity provides the fast heat-transfer path to timely conduct the solar-thermal energy into PCM during the charging process and to release the stored heat during the discharging process. With these integrated features, the CCF-PCM composites can directly harvest solar energy and quickly store the solar-thermal energy in the PCM as desired latent heat. The stored latent heat can be effectively released along the high thermal conductivity network. By taking advantage of the bendability of the CCF and stable heat-releasing temperature of the PCM, the charged composite heat pack can be directly applied onto human arms or wrists to provide thermal comfort and thermotherapy at desired temperatures. Furthermore, PCMs with different phase transition temperature can be loaded within the CCF to satisfy diverse applications that have different requirements on the heat-releasing temperature.

Figure 1. Schematic of form-stable CCF-PCM heat packs charged by direct solar irradiation for thermal comfort application. Fabrication and Characterization of CCF-PCM Heat Pack. As schemed by Figure 2a, the CCF-PCM heat pack is fabricated by the etching of the copper foam with hot alkali solution, hydrophobic surface modification and back-filling of PCM. To achieve sufficient capillary force for the minimization of potential leakage, we selected copper foams with an average pore


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diameter of ~200 µm as the skeleton to contain paraffin, which has a high latent heat (194.2 J/g) and a suitable phase change temperature (~60 oC). The pre-cleaned copper foam possesses a smooth surface and the water droplet (10 µL) forms a hemisphere on the surface. Figure 2b shows that after treatment with the alkali solution the yellow-colored copper foam becomes black as the smooth copper surface has been oxidized into rough nanostructured surface of Cu(OH)2 needles.47, 48 The formation of hydrophilic Cu(OH)2 and increased surface roughness together makes the surface superhydrophilic. The water droplets thus are quickly absorbed by the surface. The oxidized copper foam was further coated with carbon black nanoparticles dispersed within uncured PDMS. After curing, the cross-lined PDMS coating robustly binds the solarabsorbing carbon nanoparticles onto the copper foam. In the meanwhile, the PDMS coating can also protect the rough surface and change the surface from superhydrophilic to superhydrophobic. The added carbon nanoparticles help increase the surface roughness, and enhance light absorption capability. As shown by Figure 2b, the CCF surface becomes superhydrophobic and has achieved a water contact angle of 155o. Such superhydrophobicity not only helps confine the hydrophobic PCM liquid from leaking out but also facilitates loading of PCM through the impregnation method. It was found that 53 wt% of paraffin wax can be loaded within such porous-structured CCF skeleton (Figure 2b).


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Figure 2. (a) Schematic steps for preparation of CCF-PCM heat pack. (b) Optical images, SEM images and contact angles of surface of Cu foam, oxidized Cu foam, CCF and CCF-PCM.


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To evaluate the form stability of the heat pack, the sample was heated to 100 oC and maintained at the temperature for 10 min to ensure full melting of paraffin. As presented by Figure 3a, the CCF-PCM sample maintained the original shape, and no leakage of the loaded paraffin was observed. As a comparison, the neat paraffin was fully melted and the liquid paraffin spread out in the petri dish. We further tested the leakage-proof performance by measuring the weight change of the CCF-PCM sample after repeated heating and cooling. No obvious weight change of the sample was observed after continuously running for 20 cycles (Figure 3a). The observed form stability of the CCF-PCM sample should be attributed to the strong capillary force resulted from the small pore size and superhydrophobicity of the CCF that is rendered by the hydrophobic PDMS coating on the rough surfaces of the oxidized copper foam.49 DSC measurement in Figure 3b shows that CCF-PCM and neat paraffin have nearly the same melting temperature (61.9 oC and 62.2 oC) and solidification temperature (56.8 oC and 56.9 oC), which implies that the porous foam does not affect normal solid-liquid phase change of paraffin. The fusion enthalpy of the composite sample with 53 wt% loading of paraffin is 103.3 J/g, which is close to the calculated fusion enthalpy (102.9 J/g) based on the rule of mixture by taking into account the weight fraction of paraffin in the composite.50 TGA measurement (Figure S2) shows that below 200 oC both the neat paraffin and the CCF-PCM composite sample have negligible weight loss. Above the decomposing temperature the weight loss rate of the composite sample is slightly lower than the neat paraffin, which implies that the CCF does not sacrifice thermal stability of the PCM. The paraffin weight loss of CCF-PCM composite after decomposition in TGA measurement (60%) is slightly higher than the actual filling ratio (53%) is due to the fact that there is volume shrinkage of the impregnated paraffin during solidification process and the


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TGA sample was a small cuboid (~10 mg) taken from the center part of the composite with less shrinkage. Effective solar absorption is critical to the efficient harvesting of solar-thermal energy. As shown in Figure 3c, the pristine copper foam has poor light absorption capability, with an average absorption of ~34 %. After surface oxidation treatment, the light absorption capability has been significantly improved, but in the IR range the absorption quickly decays with increasing wavelengths, which matches the optical spectrum of CuO in this wavelength range.51 By contrast, the CCF has achieved full band absorption of solar light ranging from 250 to 2500 nm with a high absorptance of ~97%, which should be attributed to the good solar absorption capability from the added carbon nanoparticles in the external coating layer. Meanwhile, the porous structure of the CCF can also help trap the incident light.


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Figure 3. (a) Leakage tests of CCF-paraffin and neat paraffin. (b) DSC curves of paraffin and CCF-paraffin composite. (c) Optical absorption spectra of copper foam, oxidized copper foam and CCF. (d) Measurement of thermal conductivity of CCF-paraffin composite through a differential steady state method. As schemed by Figure 3d, the thermal conductivity of CCF-PCM composites was evaluated by placing the sample in between a hot and a cold plate, and measuring the temperature gradient under varying heating power input (𝑄) with an IR camera. Based on the measured temperature gradient, the thermal conductivity (k) can be calculated by the following equation:52, 53 ∆𝑇

𝑄 = 𝐴𝑘∆𝑥 + 𝑄loss

(1)

where A is the top surface area of the sample, ∆𝑇/∆𝑥 is the temperature gradient between the top surface and the bottom surface, 𝑄loss is the heat loss from the heat source to environment. Through linearly fitting the measured data points under different heating power input, the thermal conductivity of the CCF-PCM heat pack was estimated to be 3.94 W/m K. Such improvement of thermal conductivity is also evidenced by the uniform temperature distribution of the composite sample as shown by the IR image in Figure 3d. Based on the aforementioned results, it can be seen that the porosity of the copper foam is one of key parameter that simultaneously affects the effective loading of PCM, the leakage-proof capability, and the thermal conductivity of the composites. In general, a large porosity would enable loading more PCM, but the melted PCM liquids would leak out due to the small capillary confinement force in the foam with large-sized pores. In the meanwhile, the large porosity would result in reduced effective thermal conductivity of the PCM-CCF composites due to the lower copper loading. On the contrary, if the porosity is too small, the amount of PCM loaded into the


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composite is limited, which will sacrifice the solar-thermal energy storage capacity. It was found that the copper foam with a porosity of 90% and an average pore diameter of ~200 µm is suitable for loading large fractions of PCM without causing leakage. Charging Performance. To investigate charging performance, the CCF-PCM composite was placed on a silicone rubber cushion, subjected to solar illumination with a power density of 2 kW/m2 for 20 min and then was cooled by natural cooling for 30 min. The temperature evolution of the composite was recorded by both an IR camera and three thermocouples located at top surface, center and bottom surface of the sample. The neat paraffin and paraffin covered by a black aluminum foil (paraffin-Al) were used as the control samples. Figure 4a shows that the neat paraffin was not melted during the whole charging process. Thermocouple measurement indicates a temperature rise of ~10 oC for the neat paraffin sample due to its low absorption of solar light (Figure 4b). For the paraffin-Al sample, the solar-thermal heating is localized at the top surface at the initial charging period (10 min) and the melted zone slowly expands with prolonged charging time. Figure 4c shows that the temperature at top surface rapidly rose up as soon as the light was shined onto the sample and reached to ~85 oC while the center temperature and bottom surface temperature slowly increased to about 60 oC. A small drop of the surface temperature was observed at ~70 oC, which was due to loosen contact between the black aluminum foil and paraffin caused by volume expansion during the solid-liquid phase transition. Although the black aluminum can efficiently convert the incident light into heat, the low thermal conductivity of paraffin wax limits the transfer of generated heat to the center and bottom of the sample, which in turn causes serious heat losses and slow charging. As a result, the surface temperature immediately dropped down once the light source was removed (Figure 4c).


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In contrast, there was negligible temperature difference for the CCF-paraffin composite during both the heating and cooling period. Figure 4d shows that the paraffin was fully melted after charging for 10 min as evidenced by the changed heating rate in the temperature curves, which indicates the transition from latent heat storage to sensible heat storage. The IR image in Figure 4a clearly shows the rapid charging of the CCF-PCM sample and full melting of paraffin within 10 min. Further charging leads to continuous temperature rise as the solar-thermal energy was stored as sensible heat in the melted paraffin. Owing to the efficient conversion of incident solar light into heat by the carbon coating layer, the whole CCF-PCM sample was heated to ~85 oC, which is much higher than the temperature of neat paraffin sample. Meanwhile, facilitated by the improved heat transfer capability from the increased thermal conductivity, the whole CCF-PCM sample was uniformly heated. Unlike separated temperature distribution profiles in the neat paraffin sample and paraffin-Al sample, the temperature curves are almost overlapped in the CCF-PCM sample in both heating and cooling processes.


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Figure 4. (a) Time-consequent IR images of neat paraffin wax, paraffin-Al and CCF-paraffin composite at different charging time. (b-d) Temperature profiles at the top surface, center, bottom of neat paraffin wax (b), paraffin-Al (c) and CCF-paraffin composite (d). Thermal Comfort Applications. To imitate thermal comfort applications on human skins, the discharging behavior of charged sample was monitored. Both the neat paraffin wax and CCFparaffin composite samples were heated to 70 oC and then placed on a cooling plate that was set to 35 oC to mimic the skin temperature. The temperature at the bottom of the neat paraffin sample that is in contact with human skin quickly dropped while other parts of the paraffin sample remained hot after discharging for 90 s (Figure 5a). Such discharging behavior implies that the charged thermal energy in neat paraffin wax could not be effectively released to provide thermal comfort due to its low thermal conductivity. In contrast, the CCF-paraffin sample uniformly and gradually releases the charged thermal energy with significantly prolonged healing period as the thermally conductive copper network facilitates heat transfer from all the PCM impregnated within the foam towards human skin. Figure 5c shows that starting from the same charged state it took only 16 s for the neat paraffin wax to cool down to the healing temperature of 40 oC while the CCF-paraffin composite could maintain over the healing temperature for 313 s, prolonging the useful healing time by more than 18 times. Even though with the same sample size more thermal energy was stored in the neat paraffin sample (~975 J) than the CCF-paraffin composite (~925 J), the extractable heat for thermal comfort in the CCFparaffin composite was significantly higher than the neat paraffin sample. As the thickness of the foam increases, the increased solar-thermal energy storage capacity and prolonged healing effect by the CCF-PCM composite over the neat PCM would be more obvious. Limited by the low thermal conductivity of the neat paraffin sample, during the charging process only a small


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portion of PCM is melted. Similarly, only the melted PCM that is close to human skin can contribute to the effective heat releasing. In contrast, the solar-thermal energy can be rapidly charged into the CCF-PCM composites during the solar charging process and all the stored thermal energy within the composite could be extracted due to the high thermal conductivity of CCF. But it should be note that for wearable thermal therapy applications the foam should not be too thick in order to maintain its good bendability. As a general template to simultaneously achieve non-leakage and high thermal conductivity, we also demonstrated that CCF can be used to load other organic PCMs such as stearic acid and myristic acid that have different phasechange temperatures (Figure S3). Figure 5d demonstrates similar prolonged heat-releasing duration with different heat-releasing temperature for CCF-stearic acid and CCF-myristic acid composite as compared to the neat stearic acid and myristic acid PCMs. With the higher phasechange temperature of stearic acid, the CCF-stearic acid composite can achieve a longer effective healing duration than the CCF-paraffin composite.


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Figure 5. (a) IR images of neat paraffin and CCF-paraffin composite at different discharging time on simulated human skin at 35 oC. (b) Prolonged healing period of CCF-paraffin composite over neat paraffin on imitated human skin. (c) Improved heat extraction of CCF-stearic acid and CCF-myristic acid composites. After confirming the advantage for thermal comfort applications, we further applied the CCFparaffin composite on human arms to test its feasibility and practicality. The readily availability and mechanical flexibility of copper foams with different sizes allows us to fabricate CCF-PCM composites with a large size, and bend them to fit the curvature of human wrist without leakage of PCM (Figure 6a). In this case, the heat pack was charged under direct solar illumination (8 kW/m2). Figure 6b shows that after illumination for 5 min the CCF-paraffin surface temperature reached ~70 oC. The charged composite was attached on a human arm to provide thermal comfort. The heat-releasing curve in Figure 6c presents that the charged CCF-paraffin could provide thermal comfort above 40 oC on the arm for more than 10 min. The IR image in Figure 6d presents that the charged CCF-paraffin composite uniformly released thermal energy to the skin, which indicates the stored solar-thermal energy in the whole composite has been effectively conducted out for providing thermal comfort. Furthermore, the uniform solar-absorbing coating and high thermal conductivity of the CCF-PCM composites allow for rapid charging of the thermal pack with a small spot size of concentrated sunlight from any direction, which would facilitate daily use of the heat pack for thermal comfort applications.


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Figure 6. (a) Photograph of a bended CCF-paraffin composite heat pack. (b) IR image of charged CCF-paraffin composite heat pack after solar illumination for 5 min. (c) Temperature evolution profile of the heat pack during the discharging on human skin. (d) IR image of CCFparaffin composite heat pack performing thermal healing on human arm.

Theoretical Simulation of Discharging Process. To gain theoretical insight on the enhanced discharging performance as a result of increased thermal conductivity, we simulated the heatreleasing process (see Supporting Information). The sample is divided into numerous small cuboids to simulate the discharging process (Figure S4a). Both the neat paraffin and the composite sample were uniformly heated to 65 oC initially. The charged pack releases heat


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through conduction from the bottom and dissipate heat through convection from the other surfaces. After discharging for 5 min, Figure 7a shows that the CCF-paraffin sample has a uniform temperature of 40 oC. For the neat paraffin sample, Figure 7b shows strong temperature stratification and only the bottom portion of the paraffin (~25% of the whole sample) is cooled down. More than 50% of the paraffin is still above the melting temperature, indicating that these portions of charged thermal energy could not be released to the contact surface to maintain the heat-releasing temperature for thermal comfort. In this case, the bottom section of melted paraffin transforms into solid after heat releasing, but the low thermal conductivity of the solid paraffin (~0.2 W/m K) limits further extraction of the stored heat. This portion of unreleased thermal energy is gradually lost to the environment. Based on the simulated temperature distribution, we further calculated the heat-releasing efficiency for the neat paraffin and CCFcomposite. After discharging for 600 s, ~74 % of the stored heat within CCF-paraffin composite was released, but only ~20 % of stored heat in the neat paraffin was extracted out (Figure 4c). It was noticed that the heat loss of CCF-composite (~18.2 %) is lower than that of the neat paraffin (~24.3 %) during the discharging process. The remained large portion of heat in the neat paraffin (~56.2 %), will finally be lost to the environment. We further extracted the simulated temperature evolution profiles at the bottom, central and top surface of the sample as a function of discharging time. Figure 7d shows that in the CCF sample the paraffin at the bottom section releases the sensible heat in melted paraffin in the first 10 s, undergoes liquid-to-solid phase transition, and transits to sensible heat release starting from 200 s, which was evidenced by the abrupt change of heat-releasing rates. Owing to the effective heat conduction along the CCF network, the composite sample concertedly released the stored heat. In the neat paraffin sample, only the temperature at the bottom section quickly dropped, and


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the top and central paraffin still maintained at high temperatures above the melting point (Figure 7e).

Figure 7. (a) Simulated temperature distribution of the CCF-paraffin composite heat pack after discharging for 300 s. (b) Simulated temperature distribution of the neat paraffin after discharging for 300 s. (c) Simulated heat extraction from the neat paraffin and the CCF-paraffin composite after discharging for 600 s. (d) Simulated temperature profiles of CCF-paraffin composite heat pack at the top, center, bottom as a function of discharging time. (e) Simulated temperature profiles of neat paraffin wax at the top, center, bottom as a function of discharging time.


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CONCLUSIONS In summary, we reported a strategy to prepare form-stable PCM-based heat packs driven by solar light with fast charging and discharging performance through impregnating organic PCMs into carbon-coated metallic foams. The multifunctional foam has enabled direct and fast charging of the thermal pack through solar-thermal conversion, helped prevent leakage issue by confining the liquid PCM within the pack, and significantly prolonged the heat-releasing duration of the PCM by efficiently conducting the stored latent heat along the three-dimensional high thermal conductivity network. Combining the advantages such as form-stability, high thermal conductivity, mechanical bendability, high energy storage capacity and near-constant heatreleasing temperatures, it is demonstrated such composite heat pack could be applied in providing thermal comfort with much higher energy utilization efficiency and longer healing duration than the neat PCM heat pack. The energy utilization efficiency and healing duration could be further improved by slowing down the heat releasing to both the surrounding environment and the human body. To this end, the heat pack could be encapsulated by thermal insulation materials to suppress the heat loss to ambient environment. Additionally, a low thermal conductivity polymer pad could be placed in between the heat-releasing pack and human body to reduce the heat-releasing rate. Considering the general applicability of our approach to prepare PCM heat packs with different heat-releasing temperature through the facile fabrication process it is expected that the PCM composites would hold the potential to expand solar-thermal applications into thermal therapy and other fields. ASSOCIATED CONTENT The supporting information is available free of charge on the ACS Publications website at DOI:


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SEM images of Cu foam, oxidized Cu foam, CCF and CCF-paraffin composite; TGA curves of neat paraffin and CCF-paraffin composite; DSC curves of pure stearic acid and pure myristic acid; Theoretical simulation of temperature distribution of neat paraffin heat pack and CCF-PCM composite heat pack (PDF) AUTHOR INFORMATION Corresponding Author * Email: [email protected]. * Email: [email protected]. *Email: [email protected].

Author Contributions ‖Q. Y and P. T. equally contributed to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the support from National Key R&D Program of China (2017YFB0406000), National Natural Science Foundation of China (Grant No: 51873105, 51403127, 51521004 and 51420105009), Shanghai Rising-Star Program (Grant No: 18QA1402200), “Chen Guang” project from Shanghai Municipal Education Commission and Shanghai Education Development


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Foundation under Grant No. 15CG06 and Guangdong Provincial Key Laboratory (2014B030301014).

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Figure 1. Schematic of form-stable CCF-PCM heat packs charged by direct solar irradiation for thermal comfort application. 270x110mm (150 x 150 DPI)


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Figure 2. (a) Schematic steps for preparation of CCF-PCM heat pack. (b) Optical images, SEM images and contact angles of surface of Cu foam, oxidized Cu foam, CCF and CCF-PCM. 280x389mm (150 x 150 DPI)



Figure 3. (a) Leakage tests of CCF-paraffin and neat paraffin. (b) DSC curves of paraffin and CCF-paraffin composite. (c) Optical absorption spectra of copper foam, oxidized copper foam and CCF. (d) Measurement of thermal conductivity of CCF-paraffin composite through a differential steady state method. 292x220mm (150 x 150 DPI)


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Figure 4. (a) Time-consequent IR images of neat paraffin wax, paraffin-Al and CCF-paraffin composite at different charging time. (b-d) Temperature profiles at the top surface, center, bottom of neat paraffin wax (b), paraffin-Al (c) and CCF-paraffin composite (d). 458x256mm (150 x 150 DPI)



Figure 5. (a) IR images of neat paraffin and CCF-paraffin composite at different discharging time on simulated human skin at 35 oC. (b) Prolonged healing period of CCF-paraffin composite over neat paraffin on imitated human skin. (c) Improved heat extraction of CCF-stearic acid and CCF-myristic acid composites. 374x272mm (150 x 150 DPI)


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Figure 6. (a) Photograph of a bended CCF-paraffin composite heat pack. (b) IR image of charged CCFparaffin composite heat pack after solar illumination for 5 min. (c) Temperature evolution profile of the heat pack during the discharging on human skin. (d) IR image of CCF-paraffin composite heat pack performing thermal healing on human arm. 226x170mm (150 x 150 DPI)



Figure 7. (a) Simulated temperature distribution of the CCF-paraffin composite heat pack after discharging for 300 s. (b) Simulated temperature distribution of the neat paraffin after discharging for 300 s. (c) Simulated heat extraction from the neat paraffin and the CCF-paraffin composite after discharging for 600 s. (d) Simulated temperature profiles of CCF-paraffin composite heat pack at the top, center, bottom as a function of discharging time. (e) Simulated temperature profiles of neat paraffin wax at the top, center, bottom as a function of discharging time. 295x193mm (150 x 150 DPI)


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Form-stable Solar-thermal Heat Packs Prepared by Impregnating

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