Functionalized carbon nanotubes as phase change materials with enhanced thermal, electrical conductivity, light-to-thermal, and electro-to-thermal performances

Accepted Manuscript Functionalized carbon nanotubes as phase change materials with enhanced thermal, electrical conductivity, light-to-thermal, and electro-to-thermal performances Ruirui Cao, Sai Chen, Yuzhou Wang, Na Han, Haihui Liu, Xingxiang Zhang PII:

S0008-6223(19)30322-7

DOI:

https://doi.org/10.1016/j.carbon.2019.04.005

Reference:

CARBON 14089

To appear in:

Carbon

Received Date: 19 February 2019 Revised Date:

26 March 2019

Accepted Date: 2 April 2019

Please cite this article as: R. Cao, S. Chen, Y. Wang, N. Han, H. Liu, X. Zhang, Functionalized carbon nanotubes as phase change materials with enhanced thermal, electrical conductivity, light-to-thermal, and electro-to-thermal performances, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.04.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

enhanced

thermal,

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Functionalized carbon nanotubes as phase change materials with electrical

conductivity,

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electro-to-thermal performances

light-to-thermal,

and

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Functionalized carbon nanotubes as phase change materials with enhanced thermal, electrical conductivity, light-to-thermal, and electro-to-thermal performances

Zhang1,2,3*

State Key Lab of Separation Membranes and Membrane Processes, Tianjin 300387,

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1

China

Tianjin Municipal Key Lab of Advanced Fiber and Energy Storage Technology,

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2

Tianjin 300387, China 3

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Ruirui Cao1,2,3, Sai Chen1,2,3, Yuzhou Wang1,2,3, Na Han1,2,3, Haihui Liu1,2,3, Xingxiang

School of Material Science and Engineering, Tianjin Polytechnic University, Tianjin

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300387, China

*Corresponding Author

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Tel/Fax: +86-022-83955238. E-mail: [email protected].

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ACCEPTED MANUSCRIPT Abstract:

Light-

and

electro-driven

hexadecyl

acrylate-functionalized

single/multi-wall carbon nanotubes (HDA-g-SWCNT, HDA-g-MWCNT) solid-solid phase change materials (SSPCMs) were fabricated via a green Diels-Alder reaction.

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Both HDA-g-SWCNT and HDA-g-MWCNT show enhanced thermal and electrical conductivities, appropriate phase change temperature, almost no supercooling degree and effective phase change enthalpy. HDA was covalently grafted onto the surface of

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SWCNT/MWCNT. The phase transition enthalpy (∆Hh) and phase transition

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temperature (Thp) in the heating process, crystallization enthalpy (∆Hc) and crystallization temperature (Tcp) in the cooling process of HDA-g-SWCNT are 52 J/g, 36.7°C, 51 J/g, and 23.7°C, respectively. The ∆Hh, Thp, ∆Hc, and Tcp of HDA-g-SWCNT are 40 J/g, 38.0°C, 39 J/g, and 26.8°C, respectively. The

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HDA-g-SWCNT/HDA-g-MWCNT can effectively convert electric or light energy into thermal energy under electric field or solar illumination. Meanwhile, the electrical conductivity of HDA-g-SWCNT and HDA-g-MWCNT films reached up to

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718 and 389 S/m, respectively. The phase change property and enhanced thermal

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conductivity of HDA-g-SWCNT and HDA-g-MWCNT enable them to be used as a heat spreader for electronic cooling applications. Furthermore, the HDA-g-SWCNT and HDA-g-MWCNT exhibited good thermal stability, great thermal reliability, and shape stability, potentially leading to new energy systems with multi-responsive performance for electronic devices, solar energy utilization, and thermal management.

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ACCEPTED MANUSCRIPT 1. Introduction

Energy conversion and storage processes are accompanied by the dissipation of large amounts of thermal energy [1]. Phase change materials (PCMs) are reusable

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energy storage materials that can absorb significant amounts of energy as latent heat and release it into the surrounding environment during the phase change process over

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a defined temperature range [2]. Therefore, latent heat storage of PCMs is an extremely promising way to utilize thermal energy coming from the surrounding

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environment, solar irradiation, and waste heat produced by vehicles and electronic products [3, 4]. Nevertheless, the widespread utilization of organic PCMs is limited by their fatal shortcomings, such as the flow of liquid during melting, low thermal conductivity, electrical insulation, and shortage of multiple driving strategies [5].

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Therefore, organic PCMs have to enhance their thermal and electrical conductivity, light- and electro-driven performances and shape-stabilized property so that they

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could play a significant role in future energy conversion and storage.

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Recently, some researchers have introduced black thermal fillers or organic dyes, such as nitrogen-doped porous carbon [6], Ti4O7 [7], carbon fiber [8], graphene [9], hierarchical graphene foam [10], Cu2O-Cu-MWCNTs [11], graphene oxide (GO)/boron nitride (BN) hybrid porous scaffolds [12], CNTs [13], and dye-linked polyurethane [14], into organic PCMs and fabricated a series of light-driven shape-stabilized

PCMs.

In

addition,

other

researchers

have

introduced

polyurethane@graphite foam [15], three-dimensional carbon aerogels [16], BN/GO 3

ACCEPTED MANUSCRIPT hierarchically interconnected porous scaffolds [17], and anisotropic graphene aerogels into organic PCMs and fabricated a series of light- and electro-driven shape-stabilized PCMs [18]. However, there are some unavoidable shortcomings in these studies: (ⅰ)

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in these studies, paraffin and polyethylene glycol (PEG) were used as phase change working substrates in order to obtain higher thermal enthalpy, however, the higher phase change temperatures and bigger supercooling degree of paraffin and PEG

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cannot meet the demand of low temperature applications, such as thermo-regulated

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fiber and smart textiles; (ⅰ) these light- and/or electro-driven shape-stabilized PCMs were fabricated by physical blending method, such as vacuum impregnation, melt blending; there is only physical interaction between paraffin or PEG and the supporting fillers, such as van der Waals force, π-π conjugation, and hydrogen bond,

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which can still cause leakage problem in long-term use; (ⅰ) most of the preparation processes are complex; (ⅰ) the electrical conductivity of these electro-driven shape-stabilized PCMs is still very low. Therefore, it is necessary to find better

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preparation methods and suitable PCMs to fabricate the light- and electro-driven

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shape-stabilized PCMs with enhanced properties.

Acrylic alkyl esters are a class of organic PCMs, which have superior properties

such as high thermal enthalpy, low or almost no supercooling degree, and adjustable phase change temperature by changing the number of carbon atoms in the side chain [19]. CNTs, as a kind of carbon materials, have received significant attention due to their excellent physical properties, such as low density, high strength, large surface area, high electrical, and high thermal conductivity [20]. A facile method for the 4

ACCEPTED MANUSCRIPT functionalization of carbon materials is the Diels-Alder (DA) reaction, because it does not generate byproducts, is environmentally friendly, simple and can be performed under mild conditions [21]. The possibility of the DA reaction has been studied

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theoretically since 2002 [22]. It has been proven experimentally that the DA reaction can be performed on carbon materials, such as GN, GO, and carbon nanotubes, with the assistance of microwaves [23], ultrasound [24], ball-milling [25], or

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heating-stirring [26].

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Therefore, in this study, hexadecyl acrylate (HDA) with appropriate phase change temperature was used as phase change working substrate; SWCNT and MWCNT were employed as the supporting materials, thermal and conductive fillers. At the same time, we presented a novel strategy to fabricate functionalized CNTs with

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HDA by a solvent-free Diels-Alder (DA) reaction [24, 27, 28]. In addition, HDA, with electron withdrawing substituents, is highly reactive in DA reactions. More

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importantly, it has not yet been reported that HDA was covalently grafted onto the surface of CNTs by DA reaction to fabricate SSPCMs with light- and electro-driven,

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enhanced thermal conductivity and excellent electrical conductivity. The fabricated HDA-g-SWCNT and HDA-g-MWCNT have efficient light-driven (light-to-thermal) performance with the light-to-thermal conversion efficiency of 79.1% and 64.8%, respectively, and excellent electro-driven (electro-to-thermal) performance. The electrical conductivity of HDA-g-SWCNT and HDA-g-MWCNT films researched up to 718 S/m and 389 S/m, respectively. The thermal conductivity of HDA-g-SWCNT and HDA-g-MWCNT were about 134% and 339% higher than that of HDA, 5

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The

phase

change

temperature

of

HDA-g-SWCNT

and

HDA-g-MWCNT were about 30°C, which are within the range of the human comfort temperature. Besides, the fabricated HDA-g-SWCNT and HDA-g-MWCNT SSPCMs

which greatly expand the application field of organic PCMs.

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2. Experimental procedures

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exhibited great thermal reliability as well as thermal, structural, and shape stability,

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2.1 Materials

Short single-wall carbon nanotubes (SWCNT, CNT400, outside diameter: 1–2 nm, length: 0.5–2 µm, 95%) and short multi-wall carbon nanotubes (MWCNT, CNT402, outside diameter <8 nm, length: 0.5–2 µm, 98%) were purchased from

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Beijing Deke Daojin Science and Technology Co., Ltd. Hexadecyl acrylate (HDA) was purchased from Zhejiang Kant Chemical Co., Ltd and used as received.

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Hydroquinone (HQ, AR) and N, N-dimethylformamide (DMF, AR) were provided by Guangfu Fine Chemical Research Institute and used as received.

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2.2 Fabrication of HDA-g-SWCNT and HDA-g-MWCNT

The functionalized CNTs (HDA-g-SWCNT and HDA-g-MWCNT) were

fabricated by a solvent-free Diels-Alder (DA) reaction. The detailed fabrication process was as follows: a mixture of SWCNT or MWCNT (1 g) and HDA (100 g, 337 mmol) were placed in a glass beaker and sonicated for 2 h to form a homogeneous suspension. Then, the suspension was poured into a 250-mL three-necked flask 6

ACCEPTED MANUSCRIPT followed by the introduction of 0.5 g HQ. Under N2 flow (0.15 MPa) protection and mechanical stirring (250 rpm), the suspension was reacted for 12 h in a 180°C oil bath. Then, the resultant suspension was rinsed extensively with DMF and filtered through

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a 0.45-µm PTFE membrane. Finally, the obtained products were dried to a constant weight in a 75°C vacuum oven.

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2.3 Characterization

Raman spectra (excited at 638 nm) were obtained using a laser Raman

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spectrometer (XploRA PLUS, Horiba, Japan). X-ray photoelectron spectroscopy (XPS) were taken on a Thermo Fisher K-alpha XPS spectrometer. Surface wettability was measured with a contact angle meter (DSA100, Krüss, Germany) to obtain the

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static water contact angle (WCA). The water droplets used in the measurements were 2 µL in volume. For each sample, five measurements were collected to obtain the average WCA value. X-ray diffraction (XRD) patterns were obtained using a Rigaku

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D/MAX-gA diffractometer with a filtered Cu Kα radiation source (λ = 0.15406 nm) in

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the range of 3-40°(2θ) and a scanning speed of 2°/min at room temperature. The phase change properties and crystallization processes of the specimens were calculated by differential scanning calorimetry (DSC, NETZSCH 200 F3, Germany) in the range of -30-120°C at a rate of ±10 °C/min in a highly purified nitrogen atmosphere.

Thermogravimetric

analysis

(TGA)

was

conducted

on

a

thermogravimetric analyzer (TG, NETZSCH STA449F3, Germany) in the range of 40-600°C at a heating rate of 10 °C/min in a highly purified nitrogen atmosphere. 7

ACCEPTED MANUSCRIPT Thermal reliability of 100 heating/cooling cycles was conducted by a programmable controller (Giant Force) with temperature changing from 0 to 80°C. After that, the thermal reliability and the structural stability were determined by DSC analysis before

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and after 100 heating/cooling cycles. The shape-stabilized property was studied using a visual observation method, the specimen disk with a diameter 12 mm × thickness 1 mm was put into an 80°C oven for 1 h. Importantly, the heat-treated temperature was

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higher than the phase change temperature of HDA-g-SWCNT and HDA-g-MWCNT.

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Heat storage and release performance were conducted by a handy thermometer (DP-700B/E, RKC Instrument Inc., Japan) and two water baths. The temperature of two water baths was set at 75°C and about 0°C. The detailed measurement process was shown in the Supplementary Information. To measure the light-to-thermal

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conversion properties of the fabricated specimens, the specimens were irradiated through an infrared lamp (E27, Elitech, China) at room temperature (about 25°C). A set of photographs were taken by an infrared thermal imager (FLIR, E8 Wifi, USA) to

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record the temperature distribution with the extension of infrared radiation time.

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Thermal conductivity was measured using a hot disk thermal constant analyzer (TPS 3500, Hot Disk Company, Sweden) by a transient plane heat source method at room temperature. Before measurement, all the specimens were made into disks with diameter 20 mm × thickness 2 mm. Electrical conductivity was measured by Hall-effect measurement system (Hall8800, SWIN, Taiwan) at room temperature. Before the test, all the samples were vacuum-filtered to form a film and then naturally dried. 8

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3. Results and Discussion

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Figure 1. Schematic of the fabrication of (a) HDA-g-SWCNT and HDA-g-MWCNT via the Diels-Alder (DA) reaction. µ-Raman spectra of (b) SWCNT , HDA-g-SWCNT, MWCNT, and HDA-g-MWCNT. Full-scale XPS spectra of (c) SWCNT, HDA-g-SWCNT, MWCNT, and HDA-g-MWCNT. C 1s XPS spectra of (d, e) SWCNT and HDA-g-SWCNT, MWCNT, and HDA-g-MWCNT.

Figure 1a presents the schematic of the fabrication of HDA-g-SWCNT and HDA-g-MWCNT via a simple and efficient solvent-free Diels-Alder (DA) reaction 9

ACCEPTED MANUSCRIPT without a catalyst. The mainly reaction in the fabricated process is that the conjugated double bonds of SWCNT/MWCNT (4π electron nucleophile, dienes) react with the unsaturated double bond of HDA (2π electron electrophile, dienophiles) to form [4+2]

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cycloaddition products [29]. The whole reaction process is accompanied by the transformation of sp2 hybridized carbons of SWCNT/MWCNT to sp3 hybridized carbons [30]. At the same time, HDA molecules were grafted onto the surface of

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SWCNT/MWCNT.

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To confirm covalent functionalization, the SWCNT, HDA-g-SWCNT, MWCNT, and HDA-g-MWCNT were analyzed using Raman and XPS (Figure 1b and Figure 1c, respectively). The ID/IG ratio of HDA-g-SWCNT and HDA-g-MWCNT changed from 0.290 of SWCNT to 1.335 and 1.367 of MWCNT to 1.579, respectively. The

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increasing ID/IG ratio indicates that some sp2 hybridized carbons of SWCNT/MWCNT converted to sp3 hybridized carbons in the DA reaction process. The O/C ratio of

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HDA-g-SWCNT and HDA-g-MWCNT changed from 0.038 to 0.084 and 0.027 to 0.082, respectively, compared with SWCNT and MWCNT (Figure 1c). In addition,

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the C 1s XPS spectra of SWCNT, HDA-g-SWCNT, MWCNT, and HDA-g-MWCNT are presented in Figure 1d and Figure 1e, respectively. Compared with SWCNT and MWCNT, a new peak of O-C=O group was observed in the C 1s XPS spectra of HDA-g-SWCNT and HDA-g-MWCNT. More importantly, the characteristic peak of C=C/C-C of HDA-g-SWCNT and HDA-g-MWCNT exhibited an obvious shift from 284.60 to 285.05 eV (0.45 eV) and 284.47 to 284.86 eV (0.39 eV) versus SWCNT and MWCNT, respectively. Combined with the FT-IR results as seen in Figure S1, it 10

ACCEPTED MANUSCRIPT can be confirmed that the DA cycloaddition between SWCNT/MWCNT and HDA

Figure

2.

WCA data

of

(a)

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was successful, i.e., the HDA-g-SWCNT and HDA-g-MWCNT were fabricated.

SWCNT,

HDA-g-SWCNT,

MWCNT,

and

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HDA-g-MWCNT. The digital photographs of SWCNT (top) and HDA-g-SWCNT (bottom) (b) dispersed in various solvents. XRD patterns of (c) SWCNT,

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HDA-g-SWCNT, MWCNT, and HDA-g-MWCNT.

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As seen in Figure 2a, the SWCNT and MWCNT show a low WCA at 84.3° and 32.3°, respectively, illustrating good hydrophilic properties. The WCA of HDA-g-SWCNT and HDA-g-MWCNT are 114.1° and 117.0°, respectively, illustrating improved hydrophobic property.

The hydrophobic property of

HDA-g-SWCNT and HDA-g-MWCNT is mainly because of the hydrophobic of HDA, which was grafted onto SWCNT/MWCNT. Figure 2b shows the results of dispersing the SWCNT (top) and HDA-g-SWCNT (bottom) at room temperature in 11

ACCEPTED MANUSCRIPT various solvents with sonication. Due to the hydrophilicity—as well as the strong π-π stacking and van der Waals interactions, the SWCNT agglomerated in most ester organic solvents (Figure 2b), such as ethyl acetate and methyl methacrylate [30].

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However, the solubility of the HDA-g-SWCNT was greatly improved after being functionalized with HDA. Figure 2b shows that HDA-g-SWCNT could form homogeneous stable dispersions in ethyl acetate, methyl methacrylate and other ester

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organic solvents. Meanwhile, the results of the UV-Visible spectroscopy (Figure S3)

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also validate this conclusion [31].

The solubilities of MWCNT and HDA-g-MWCNT in solvents are displayed in Figure S4, and the Hansen solubility parameters of the chosen solvents are listed in Table S1. The Hansen solubility parameters of SWCNT, MWCNT, HDA-g-SWCNT,

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and HDA-g-MWCNT were calculated, and they are also listed in Table S1. The Hansen parameters were calculated as described previously [32]. The δ values of

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SWCNT and MWCNT are 23.24 MPa1/2 and 21.28 MPa1/2, respectively, which is consistent with previous reports [33]. The δ values of HDA, HDA-g-SWCNT, and

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HDA-g-MWCNT were 19.18 MPa1/2, 19.89 MPa1/2, and 20.62 MPa1/2, respectively. Versus SWCNT and MWCNT, the δ values of HDA-g-SWCNT and HDA-g-MWCNT became smaller and closer to that of HDA. That is why they are well dispersed in ethyl acetate, methyl methacrylate, and other ester organic solvents.

Figure 2c displays the XRD patterns of SWCNT, HDA-g-SWCNT, MWCNT, and HDA-g-MWCNT. The corresponding XRD data are listed in Table S2. For 12

ACCEPTED MANUSCRIPT HDA-g-SWCNT and HDA-g-MWCNT, the XRD patterns show the characteristic peak of HDA at 21.440° (2θ, D=10.805 nm) and 21.380° (2θ, D=11.844 nm),

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respectively, which correspond to the (110) diffraction peak of HDA [34].

Figure 3. DSC curves of (a) HDA-g-SWCNT and HDA-g-MWCNT. TGA curves of (b) SWCNT, MWCNT, HDA, HDA-g-SWCNT, and HDA-g-MWCNT. DSC curves (c) and FT-IR spectra (d) of HDA-g-SWCNT after 1 and 100 thermal cycling treatments. Panel (e) is photographs of HDA-g-SWCNT and HDA-g-MWCNT before 13

ACCEPTED MANUSCRIPT and after thermal treatment in an oven (80°C, 1 h).

Figure 3a displays the DSC curves of HDA-g-SWCNT and HDA-g-MWCNT. The relevant DSC data are listed in Table 1. As displayed in Figure 3a, both

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HDA-g-SWCNT and HDA-g-MWCNT have strong endothermic/exothermic peaks in the heating/cooling processes, which indicates that they have obvious phase change

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property. As shown in Figure 3e, even after 1 h at 80°C, the HDA-g-SWCNT and HDA-g-MWCNT still kept their original shape without any leakage. This is mainly

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because the HDA molecules were covalently grafted onto the surface of SWCNT/MWCNT rather than physical adsorption. Therefore, HDA-g-SWCNT and HDA-g-MWCNT are solid-solid PCMs and have excellent shape stability. In addition, it can be concluded that, in the heating/cooling processes, HDA molecules which were

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covalently grafted onto the surface of SWCNT/MWCNT in HDA-g-SWCNT and HDA-g-MWCNT undergo phase transition process (rather than melting process) from

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crystalline phase to amorphous phase and phase transition process (crystallization process) from amorphous phase to crystalline phase, respectively. The phase transition

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enthalpy (∆Hh) and phase transition temperature (Thp) in the heating process, crystallization enthalpy (∆Hc) and crystallization temperature (Tcp) in the cooling process of HDA-g-SWCNT are 52 J/g, 36.7°C, 51 J/g, and 23.7°C, respectively. The ∆Hh, Thp, ∆Hc, and Tcp of HDA-g-SWCNT are 40 J/g, 38.0°C, 39 J/g, and 26.8°C, respectively. Table 1 shows the ∆Hh and ∆Hc of HDA-g-SWCNT are higher than that of HDA-g-MWCNT even under the same reaction conditions. This is mainly because the carbon atoms on the outmost wall of CNT can react with HDA molecules. There 14

ACCEPTED MANUSCRIPT are more reactive sites on SWCNT than on MWCNT at the same dosage, i.e., there are more HDA molecules grafted onto SWCNT than that onto MWCNT during the DA reaction. This can be verified from the following TGA data.

cycling treatments) and HDA-g-MWCNT. Thoa

Thpb

∆Hc

(J/g)

(oC)

(oC)

(J/g)

HDA-g-SWCNT

52

26.9

36.7

HDA-g-SWCNT-100 cycles

54

26.7

36.2

HDA-g-MWCNT

40

29.2

38.0

Tcpd

∆Te

(oC)

(oC)

(oC)

51

30.2

23.7

3.3

51

31.1

25.7

4.4

39

32.4

26.8

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a

Tcoc

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∆Hh

Specimen

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Table 1. Phase change properties of HDA-g-SWCNT (before and after thermal

Onset temperature of phase transition point in the heating process. b Peak temperature of phase

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transition point in the heating process. c Onset temperature of crystallization point in the cooling process. d Peak temperature of crystallization point in the cooling process. e ∆T = Tho-Tco.

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The TGA curves of HDA, SWCNT, MWCNT, HDA-g-SWCNT, and HDA-g-MWCNT are presented in Figure 3b. The TGA data are summarized in Table

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2. As seen in Figure 3b and Table 2, there is only one mass loss stage, and the thermal stability temperatures of HDA-g-SWCNT and HDA-g-MWCNT are all above 300°C. Therefore, HDA-g-SWCNT and HDA-g-MWCNT have excellent thermal stability. The grafting rate of HDA molecules can be estimated according to the TGA data as also shown in Table 2. For HDA-g-SWCNT and HDA-g-MWCNT, the grafting rates of the HDA molecules on the unit mass of SWCNT/MWCNT were calculated based on a previous study [19]. As seen in Table 2, the grafting rate of HDA-g-SWCNT is 15

ACCEPTED MANUSCRIPT higher than that of HDA-g-MWCNT. This explains why the ∆Hh and ∆Hc of HDA-g-SWCNT are all higher than that of HDA-g-MWCNT. Meanwhile, HDA-g-SWCNT and HDA-g-MWCNT showed long-term thermal reliability and

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chemical structure stability: There were hardly any changes to the DSC curve after 100 thermal cycles; its thermal enthalpy was well-retained without obvious decay (e.g., HDA-g-SWCNT). These data are in Figure 3c, Figure 3d and Table 2. Therefore,

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HDA-g-SWCNT and HDA-g-MWCNT have excellent thermal stability, thermal

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reliability, chemical structure stability, shape stability, and show promise for smart fibers, energy-saving buildings, and battery thermal management systems.

Table 2. Thermal stability of HDA, SWCNT, MWCNT, HDA-g-SWCNT, and HDA-g-MWCNT. T5%

HDA-g-SWCNT

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MWCNT

HDA-g-MWCNT

WRe

Grafting

Grafting density

(°C)

(%)

(%)

rate (%)

(mmol/g)

196.45

100.00

0.00

—

—

—

4.68

95.34

—

—

312.35

69.79

30.21

215.59

7.265

—

4.55

95.45

—

—

339.02

46.31

53.69

77.78

2.622

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HDA SWCNT

WLoss

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Specimen

T5%: the temperature at 5 % mass loss; WLoss: the percentage content of mass loss; WRe: the residual mass fraction.

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Figure 4. Heat storage process (a, c) and heat release process (b, d) of SWCNT and HDA-g-SWCNT, MWCNT, and HDA-g-MWCNT, respectively. Light-to-thermal conversion curves of (e) HDA-g-SWCNT and HDA-g-MWCNT (100 mW/cm2). Comparison of thermal conductivity of (f) HDA-g-MWCNT from this study and other 17

ACCEPTED MANUSCRIPT recently reported PCMs. FLIR camera images of (g) HDA-g-MWCNT (patterned letter “Ƶ”) and HDA-g-SWCNT (patterned letter “X”).

The temperature-time curves of the samples during the heating/cooling processes

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with temperature changing from 0°C to 75°C are shown in Figure 4. We also show the curves of SWCNT and MWCNT under the same test conditions. Figure 4a and 4c

show

that

HDA-g-SWCNT/HDA-g-MWCNT

the

corresponding

was

temperature

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Figure

significantly

lower

than

that

of

of

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SWCNT/MWCNT at the same time during heating process. Moreover, the corresponding temperature of HDA-g-SWCNT/HDA-g-MWCNT was significantly higher than that of SWCNT/MWCNT with the same time during cooling process (Figure 4b and Figure 4d). Therefore, both HDA-g-SWCNT and HDA-g-MWCNT

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have latent heat storage and release (phase change property) versus SWCNT and MWCNT. In addition, during the cooling process, the heat release curves of

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HDA-g-SWCNT and HDA-g-MWCNT all appeared to have a thermal buffering plateau, i.e., HDA-g-SWCNT and HDA-g-MWCNT have heat preservation effect,

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and they can be applied to thermal management systems for electronic devices.

The energy storage of PCMs has been realized via a light-thermal route in recent

studies [6, 10, 35, 36]. In our study, the light-to-thermal conversion performance of HDA-g-SWCNT and HDA-g-MWCNT were investigated via an infrared lamp at 100 mW/cm2, and the temperature-irradiation time curves of the samples were recorded via a handheld paperless recorder. The light-to-thermal conversion curves of the 18

ACCEPTED MANUSCRIPT samples are shown in Figure 4e. As shown in Figure 4e, the temperature of the samples increases sharply under infrared irradiation; it is higher than that of HDA by nearly 7.4°C (HDA-g-SWCNT) and 3.3°C (HDA-g-MWCNT) under the same

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irradiation times. Meanwhile, HDA-g-SWCNT and HDA-g-MWCNT all exhibited a thermal buffering plateau indicating that a large amount of thermal energy was absorbed by the PCMs in the heating phase change process. In addition, the energy conversion

efficiency (η) of HDA-g-SWCNT and

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light-to-thermal

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HDA-g-MWCNT were calculated based on a previous study, and the calculated results are 79.1% and 64.8%, respectively [6]. After the light irradiation was removed, the temperatures of HDA-g-SWCNT and HDA-g-MWCNT decreased rapidly relative to HDA. This phenomenon indicated that HDA-g-SWCNT and HDA-g-MWCNT

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have higher thermal conductivity than that of HDA.

Table 3. Thermal conductivity properties of HDA-g-SWCNT and HDA-g-MWCNT. Specimen

Thermal

Thermal

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Conductivity Diffusivity

Specific

Thermal

Heat

Effusivity

(mm2/s)

(MJ/(m3 K)) (Ws1/2/(m2 K))

HDA-g-SWCNT

0.4675

0.4291

1.089

713.6

HDA-g-MWCNT

0.8770

1.3950

0.629

742.6

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(W/(m K))

The thermal conductivity data of HDA-g-SWCNT and HDA-g-SWCNT are listed in Table 3. SWCNT/MWCNT acts as a heat transfer pathway and provides continuous channels for phonon transfer—this leads to rapid heat transfer between HDA-g-SWCNT/HDA-g-SWCNT and the external environment [37]. The thermal 19

ACCEPTED MANUSCRIPT conductivity of HDA is about 0.20 W/(m K) [38], and the intrinsically low thermal conductivity will lead to a large temperature gradient and insensitivity to temperature change [39]. In non-steady state or transient conditions, the ability of materials to

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transfer thermal energy is determined by the thermal diffusivity, which is proportional to thermal conductivity. Materials with a high thermal diffusivity exhibit fast responses to thermal changes in the environment; thus, they are preferred when it

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comes to transferring stored heat [40].

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Table 3 shows that the thermal diffusivity of HDA-g-SWCNT and HDA-g-MWCNT are 0.4291 mm2/s and 1.3950 mm2/s, respectively. In addition, the thermal conductivity of HDA-g-SWCNT and HDA-g-MWCNT are 0.4675 W/(m K) and 0.8770 W/(m K), which are 134% and 339% higher than that of pure HDA,

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respectively. Meanwhile, the thermal conductivity of HDA-g-MWCNT is much higher than most recently reported results (Figure 4f). In summary, the thermal

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diffusivity values of HDA-g-SWCNT and HDA-g-MWCNT are higher than that of pure HDA. The significant improvement in thermal conductivity and thermal

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diffusivity enables HDA-g-SWCNT and HDA-g-MWCNT to be heat spreaders for electronic cooling applications [40]. More importantly, HDA-g-SWCNT and HDA-g-MWCNT are PCMs that can absorb the thermal energy generated during the operation of electronic devices.

The above data shows that under infrared irradiation, HDA-g-SWCNT and HDA-g-MWCNT would create a significant temperature increase—this could be 20

ACCEPTED MANUSCRIPT captured with an IR thermal camera. Here, we prepared a patterned letter “Ƶ” and “X” with HDA-g-MWCNT and HDA-g-SWCNT, respectively. Figure 4g displays the FLIR camera images of HDA-g-MWCNT and HDA-g-SWCNT. With increasing the

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irradiation time, the “Ƶ” and “X” patterns became increasingly brighter, i.e., the temperature of the “Ƶ” and “X” patterns increased. In addition, the temperature changes with increasing irradiation time are shown in Figure 5; a temperature increase

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of over 43°C (HDA-g-MWCNT and HDA-g-SWCNT) was measured within 180 s.

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Therefore, HDA-g-MWCNT and HDA-g-SWCNT have an enhanced thermal conductivity and high-performance light-to-thermal conversion (light-driven) property. These not only greatly improved the rate of thermal response and the working efficiency of PCMs [41], but they also expanded the potential applications, including

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light-to-thermal imaging in real-time or for time-resolved responses [42, 43].

Figure 5. FLIR camera images of (a, b) HDA-g-MWCNT (patterned “Ƶ”) and (c, d) HDA-g-SWCNT (patterned “X”) at different irradiation times.

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Figure 6. (a, b) Digital photos of the electrical conductivity experimental design. (c) FLIR camera image of the test spline (HDA-g-SWCNT film) under 30 V electric field.

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(d, e) Test spline of the HDA-g-SWCNT film under 30 V electric field.

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Based on the previous studies, the electrical conductivity of HDA-g-SWCNT and HDA-g-MWSCNT films were obtained by I-E curves [41, 44]. The HDA-g-SWCNT and HDA-g-MWSCNT films had outstanding electrical conductivity with ~ 718 S/m and 389 S/m, respectively. As we all known, for an individual SWCNT/MWCNT, if covalent functionalized with sp3 rehybridization, its electrical conductivity will be dropped significantly even by a few orders. As a result, compared to

SWCNT/MWCNT,

the

electrical

conductivity of 22

HDA-g-SWCNT and

ACCEPTED MANUSCRIPT HDA-g-MWCNT is bound to decrease significantly. However, in this study, the fabricated HDA-g-SWCNT and HDA-g-MWCNT are used as SSPCMs. Compared to HDA organic SLPCM, the electrical conductivity of HDA-g-SWCNT and

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HDA-g-MWCNT SSPCMs has been greatly improved and are much higher than the value of this kind PCMs reported in the recent literature [16, 45]. To further study the conductivity of the fabricated samples, we have made a simple experiment (Figure 6).

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The optical photographs of the LED lamp luminescence in the test circuit (taking

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HDA-g-SWCNT film as an example) are presented in Figure S5. As seen in Figure 6b, Figure S5, and Video 1, the LED lamp in the test circuit shines when the voltage is applied. Meanwhile, the LED lamp gradually brightened with increasing voltage in the test circuit. These results confirmed that HDA-g-SWCNT and HDA-g-MWCNT

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films have an excellent conductivity. Moreover, the FLIR camera images of HDA-g-SWCNT are shown in Figure 6c, Figure 6d, Figure 6e, and Video 2. As seen in Figure 6d and Figure 6e, the temperature of HDA-g-SWCNT film increased by

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~2.8°C when the test circuit was continuously energized for 30 s. When the electric

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field was removed, the temperature of HDA-g-SWCNT film decreases rapidly to room temperature. The entire dynamic testing process was shown in Video 2. Therefore, HDA-g-SWCNT and HDA-g-MWCNT have a certain electro-to-thermal (electro-driven) energy storage and release performance that effectively expands the utility of PCMs to include thermal buffer in integrated circuits.

4. Conclusions

23

ACCEPTED MANUSCRIPT Hexadecyl

acrylate

(HDA)-grafted

single/multi-wall

carbon

nanotube

(HDA-g-SWCNT, HDA-g-MWCNT) were fabricated by a green Diels-Alder reaction. The HDA molecules were grafted on the surface of SWCNT/MWCNT. This improves

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the shortcomings of HDA including leakage during work, low thermal conductivity, electrical isolation, and shortage of multiple driving strategies. The ∆Hh and ∆Hc are 52 J/g and 51 J/g for HDA-g-SWCNT and 40 J/g and 39 J/g for HDA-g-MWCNT.

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The phase change temperature of HDA-g-SWCNT and HDA-g-MWCNT were about

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30°C, which are within the range of the human comfort temperature. The thermal conductivity of HDA-g-SWCNT and HDA-g-MWCNT are 0.4675 and 0.8770 W/(m K)—these values are about 134% and 339% higher than that of HDA, respectively. The excellent thermal properties allow HDA-g-SWCNT and HDA-g-MWCNT to be

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efficient heat spreaders for thermal management in electronics. Moreover, the electrical conductivity of HDA-g-SWCNT and HDA-g-MWCNT films are 718 S/m and 389 S/m, respectively; they also exhibit great thermal stability, thermal reliability,

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structural stability, and shape stability. In addition, HDA-g-SWCNT and exhibit

excellent

light-to-thermal

performance

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HDA-g-MWCNT

with

the

light-to-thermal conversion efficiency of 79.1% and 64.8%, respectively, and electro-to-thermal performance. Therefore, our study offers the possibility to develop new types energy systems with multi-responsive performance for electronic devices, solar energy utilization, energy-saving buildings, and thermal management.

Acknowledgment

24

ACCEPTED MANUSCRIPT This work was supported by the New Materials Research Key Program of Tianjin (No. 16ZXCLGX00090) and the National Key Research and Development

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Program of China (No. 2016YFB0303000).

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