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Woven Kevlar® Fiber/Polydimethylsiloxane/Reduced Graphene Oxide Composite based Personal Thermal Management with Freestanding Cu-Ni Core-shell Nanowires Ankita Hazarika, Biplab K Deka, Do-Young Kim, Hoon Eui Jeong, Young-Bin Park, and Hyung Wook Park Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b02408 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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Woven Kevlar® Fiber/Polydimethylsiloxane/Reduced Graphene Oxide Composite based Personal Thermal Management with Freestanding Cu-Ni Core-shell Nanowires Ankita Hazarika, Biplab K. Deka, DoYoung Kim, Hoon Eui Jeong, Young-Bin Park, Hyung Wook Park* Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan, Republic of Korea, 44919 E-mail address: [email protected] (H.W. Park).

ABSTRACT: Thermotherapy is a widespread technique that provides relief for muscle spasms and joint injuries. A great deal of energy is used to heat the surrounding environment, and heat emitted by the human body is wasted on our surroundings. Herein, a woven Kevlar® fiber (WKF)-based personal thermal management device was fabricated by directly growing vertical copper-nickel (Cu-Ni) nanowires (NWs) on the WKF surface using a hydrothermal method. The treated WKF was combined with reduced graphene oxide (rGO) dispersed in polydimethylsiloxane (PDMS) to form composites using vacuum-assisted resin transfer molding (VARTM). This WKF-based personal thermal management system contained a conductive network of metallic NWs and rGO that promoted effective Joule heating and reflected back the infrared (IR) radiation emitted by the human body. It thus behaved as a type of thermal insulation. The CuNi NWs were synthesized with a tunable Ni layer on Cu core NWs to enhance the oxidation resistance of the Cu NWs. The combined effect of the NW networks and rGO enabled a surface temperature of 70°C to be attained on application of 1.5 V to the composites. The Cu3Ni1-WKF/PDMS provided 43% more thermal insulation and higher IR reflectance than bare WKF/PDMS. The absorbed impact energy and tensile strength was highest for the Cu1Ni3- and rGO-integrated WKF/PDMS samples. Those Cu-Ni NWs having higher Ni contents displayed better mechanical properties and those with higher Cu contents showed higher Joule heating performance and IR reflectivity at a given rGO loading. The composite shows sufficient breathability and very high durability. The high flexibility of the composites and their ability to generate sufficient heat during various human motions ensures their suitability for wearable applications.

KEYWORDS: Thermal management, Kevlar, Cu-Ni nanowires, reduced graphene oxide, wearable heater, mechanical properties. 1

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There is increasing demand for various types of wearable devices with safety and healthcare applications, such as for personal health monitoring, fitness tracking and medical diagnosis.1,2 A wearable heater is an efficient approach for physiotherapy because it can relieve pain due to joint injuries by providing localized thermal therapy and preserving the warmth of the body; the heater should be sufficiently flexible to adapt to human motions.3,4 Muscle spasms and inflammation can be alleviated, and joint stiffness decreased to a considerable extent, by the therapeutic action of heat provided by a wearable heater because of improved blood flow and collagen tissue flexibility.5 Traditional devices for delivering thermal therapy include heat wraps and packs, but these are often uncomfortable due to their heavy weight, stiffness, non-uniformity and inconsistent heating.6,7 In an indoor environment, more than 50% of the body heat produced is dissipated through infrared (IR) radiation. This generated thermal energy is wasted. “Personal thermal management” aims to harvest energy from the human body to maintain the body temperature to some desired value.8,9 Recent research has allowed fabrication of flexible wearable heaters by incorporating carbon nanotubes (CNTs), metallic nanoparticles/nanowires (NPs/NWs) and metal meshes.10–12 Metallic NWs are particularly promising because they can be formed into a conducting network.13–15 These have very good conductivity, particularly while being pulled or stretched, and their one-dimensional structure forms better electrical percolation networks than nanoflakes or NPs. NWs have been coated onto textiles using various approaches.9,16,17 However, direct contact of highly electrically conductive NWs with human skin poses a risk to human health and safety when an electric voltage is applied. Moreover, the high thermal conductivity of NWcoated cloth when in direct contact with skin diminishes thermal insulation. Furthermore, NWs can be removed by abrasion from the fabric during application. Silver (Ag) NWs have been identified as a prospective candidate.18,19 Although they are more conductive than CNTs, the cost of Ag NWs is a major obstacle for industrial applications. By contrast, copper (Cu) is relatively inexpensive and has only 6% lower conductivity than Ag. The substitution of Ag NWs with Cu NWs in flexible wearable heaters has been widely investigated.20,21 However, a major obstacle to using Cu NWs in heating devices is their 2


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sensitivity to moisture and oxygen compared with Ag NWs. Therefore, enhancing the oxidation resistance of Cu NWs is essential for their effective use in heating devices. The binary metallic Cu-Ni NWs have about 100-times higher resistance to oxidation than Ag NWs, in addition to outstanding stability.22 Since the electrical conductivity of Ni is much less than that of Cu, the Ni:Cu ratio should be tuned in such a way that the electrical conductivity of the NW does not deteriorate while retaining its oxidation resistance. Reduced graphene oxide (rGO) nanosheets (NSs) have very high thermal conductivities (3,000–5,000 W m1

K-1) and can enable uniform heating when incorporated into a polymer matrix in an electric heater

application.23 Kevlar® fiber, an aramid-based fiber, is commonly used for body armor applications due to its high strength. Some attempts have been made to fabricate soft materials as body armor to replace conventional body armors, which are stiff, heavy and uncomfortable.24,25 Lightweight, flexible body armor allowing personal thermal management is one example of a future smart wearable material. In this work, vertically aligned bimetallic CuxNiy (x, y = 1, 2, 3) alloy NWs were synthesized for the first time on the surface of woven Kevlar® fiber (WKF) using a hydrothermal technique, and were formed into a composite with polydimethylsiloxane (PDMS) resin containing dispersed rGO by vacuum-assisted resin transfer molding (VARTM). The Cu-Ni NWs and rGO granted outstanding mechanical strength and stable heating performance to the composite device. Besides preventing body-generated heat from escaping into the environment, and thereby reducing the amount of indoor heating required, the synergistic effect of the Cu-Ni NW network and rGO offers promising Joule heating performance by facilitating an increase in skin temperature from ambient to 70°C within a short period of time, and upon application of a fairly low voltage (1.5 V). The flexibility of the heater affords maximum comfort to the wearer and can provide efficient thermotherapy for joints by preserving body warmth via transferring heat uniformly to the skin. The fabricated wearable heater also has great potential for lightweight and flexible body armor because of its robust mechanical strength and ability to maintain body warmth. Minimizing the loss of body heat, or

3


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even increasing the temperature of the body to a preferred level, would be particularly advantageous in cold regions.

RESULTS AND DISCUSSION We fabricated a WKF-based personal thermal management device that prevents the loss of body heat by reflecting IR radiation back toward the body. It also directly increases the body temperature without wasting energy on heating the surroundings; these features are achieved without sacrificing wearability. Figure 1 (a) is a schematic diagram of the composite fabrication process. PDMS with rGO were inserted to the vacuum chamber through inlet and the excess resin was flowed out through outlet. The NW grown fiber sheets are shown in the figure that were placed just above the release fabric and the resin flows through the fiber sheets in the chamber and cured at room temperature for 48 h. The introduction of Cu-Ni NW networks in bare WKF/PDMS is largely responsible for the transformation of its electrically insulating properties into effective electric heating and radiation insulating properties. The NWs thus play a key role in improving the final properties of the composites. The unique advantage of vertically aligned NWs is that they were firmly affixed to the base with providing continuous pathway for carrier transport and have well defined parallel channels for easy transfer of electrons, which enhances the electrical conductivity.26,27 The fabricated sample was a two layered composite where electrodes were attached on the top and the bottom WKF sheet with grown NWs. Thus, voltage was easily transferred through the continuous path provided by the NWs between the two WKF sheets. Furthermore, the synthesized NWs were mostly perpendicular to the base. However, some NWs were aligned at diverse angles with respect to the base with providing electrical cross points. The electrical conduction takes place throughout the sample in the vertical direction along the growth of the nanowires from one sheet to other and homogenous temperature distribution is obtained due to the uniform growth of nanowires throughout the fiber surface and proper distribution of rGO in the resin. The electrical conduction path is vertical and is provided by the vertically grown nanowires. During fabricating the composites, vacuum and pressure were applied in the 4


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VARTM process to produce efficient contact at the junction of the NWs. Moreover, vertically aligned wire-shaped nanomaterials significantly improved mechanical properties by producing a functionally graded material interface to lessen the stress concentration between two phases on application of external force. There are numerous advantages of the hydrothermal method during the growth of the vertically aligned NWs. This eco-friendly process can be performed at low cost, remarkable low temperature, and homogenous particle size distribution. It also allows uniform growth of the NWs throughout the fiber sheet. The morphologies of the NWs can be tuned through varying temperature, concentration and growth time.28 Table S1 provides different experimental conditions with results and the electrical conductivity of NW grown WKFs. Morphological and microscopic analyses of the NWs were performed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) (Figures 1(b)-(d) and 2). TEM analysis revealed the Cu-rich core and Ni-rich outer shell of the NWs, i.e., a coating of Ni enveloped the Cu core, analogous to a coaxial cable structure, with a Ni shell ca. 5-nm thick (Figure 1b). This morphology played a pivotal role in enhancing the oxidation resistance of the NWs. The characteristic d-spacing for the Cu (111) and Ni (111) planes were 0.201 and 0.195 nm, respectively. The appearance of clear lattice fringes corresponding to a distance of 0.197 nm between the Cu (111) and Ni (111) planes confirmed that the synthesized NW was a Cu-Ni alloy.29 Energy-dispersive X-ray spectroscopy (EDS) mapping measurements confirmed the coexistence of Cu and Ni and revealed a homogenous distribution of elements across the side wall of the Cu NWs, indicating a uniform coating of Ni metal. The NWs with different Cu:Ni ratios displayed vertically aligned growth on the WKF surface in all cases. The NWs were firmly anchored to the base fibers and their tips crossed each other to form a networked structure, thereby promoting electrical contacts and flexibility. The NWs typically grew in the energetically favorable growth orientation that is perpendicular to the base surface, but were aligned at diverse angles with respect to the substrate. The likely growth mechanism of the NWs involves formation of 5


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a stable complex through reaction of the initial precursors of the growth solution, copper chloride dihydrate (CuCl2·2H2O) and nickel(II) acetylacetonate (Ni(acac)2), with hexadecyl amine (HDA). A certain proportion of the Ni2+ ions would be reduced to Ni0 atoms at the processing temperature, and the Ni0 atoms would quickly reduce Cu2+ ions to Cu0 atoms by galvanic replacement reactions.30 The Cu0 atoms would lead to the formation of Cu nuclei and then to multi-twinned crystal seeds that would finally grow into NWs with increasing reaction time. As the reaction progresses, the concentration of Cu2+ ions would dramatically decrease because of the formation of Cu NWs, thereby leading to the deposition of Ni0 atoms on the surface. Once the Ni0 atoms exceed the critical nucleation concentration, Ni clusters would accumulate on the {100} side wall of the Cu NW surfaces and the Ni clusters would eventually form Ni NPs.31 It was clearly evident from SEM images that NWs having higher Ni contents had higher surface roughness due to aggregation of more Ni NPs on the core Cu NWs (Figure 2a–e). The synthesized NWs had a very high aspect ratio with a length up to several micrometers and a diameter ranging from 127 to 186 nm (Figure 2f). The crosssectional and top view image of the WKF/Cu-Ni/rGO/PDMS composite samples showed the presence of fine-nanopores sufficient for breathability of the samples (Figure S1). The crystalline phases of the as-synthesized NWs grown on the surface of the WKF were determined by X-ray diffraction (XRD) (Figure S 2a). The molecular crystal structure of Kevlar fiber is monoclinic and displayed diffraction peaks at 2θ = 20.7° and 23.1° corresponding to the (110) and (200) planes of simple cubic structure; these peaks were observed previously for Cu-Ni NWs grown on WKF.32 The NWs were free of contaminating oxides or secondary phases and were composed of only metallic phases according to the XRD patterns. The two sets of diffraction peaks appearing in the diffractograms were attributed to the bimetallic crystalline phase structure of Cu and Ni in the NWs. The diffraction peaks at 2θ = 43.47° and 44.38° for Cu and Ni corresponded to (111) reflections of a face-centered cubic structure. The diffraction peaks at 2θ = 50.62° and 51.54° for Cu and Ni, respectively, were attributed to the (200) plane while peaks at 73.34° and 77.18° corresponded to the (220) reflection plane. The Cu/Ni peak intensity ratio increased with increasing Cu/Ni molar ratio in the NW growth solution. 6


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X-ray photoelectron spectroscopy (XPS) provided further insight into the elemental valence states of the Cu-Ni NWs. The full-scan spectrum over binding energies ranging from 0 to 1,200 eV displayed peaks corresponding to C 1s, O 1s, Ni 2p and Cu 2p (Figure S 2b). The appearance of C and O elements in the spectra was consistent with exposure of the samples to air and the elemental composition of WKF. Peaks corresponding to Cu 2p3/2 and 2p1/2 appeared at 932.0 and 953.3 eV, respectively, with a spin-orbit coupling energy of 21.3 eV; these peaks confirmed the presence of Cu0 (Figure S 2c). The low-intensity shakeup satellites were energy-loss peaks related to plasmon excitation of the metal.33 Peaks at 852.9 and 874.1 eV, assigned to Ni 2p3/2 and Ni 2p1/2, respectively, had a spin-orbit coupling energy of 21.2 eV; two shake-up satellites were also observed (Figure S 2d). The Ni 2p3/2 peak at 852.0 eV corresponded to Ni (0) and peaks at 855.3 and 856.0 eV were attributed to Ni(II) and Ni(III), respectively.34 Ni (II) and Ni (III) formed oxides and hydroxides as the surface layers of Ni was partly oxidized due to the synthetic process during XPS and exposure to air and the results were analogous to literature.29 Personal thermal management devices should be efficient heat providers. The electric heating performance of a WKF composite was investigated by applying three different voltages (0.5, 1.0, 1.5 V) to the sample to induce resistive Joule heating. Figure 3A–C shows the time-dependent temperature profiles, and Figure 3D the average surface temperatures, of the samples during heating. The surface temperature distribution was monitored with an IR camera. The surface temperature of the WKF/PDMS substrate did not increase, while the Cu-Ni NW- and rGO-containing samples showed a homogenous temperature distribution in the heating zone. The color of IR images changed from dark blue for neat WKF/PDMS to yellow, red and then white upon application of the electric voltage for the Cu-Ni NW- and rGO-containing WKF/PDMS samples. The increasing surface temperature with increasing electric voltage demonstrated the temperature controllability and efficient electric heating performance of the composite samples. The heating profiles of all of the samples consisted of three distinct sections, i.e., heating, high-temperature and cooling. The steady-state temperature increased with increasing applied voltage because the power (P) was distributed to the composite as Joule heat according to the formula, P = V2/R, where V is the voltage and R 7


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is the total resistance. The presence of Cu-Ni NW and rGO improved the electrical conductivity and resulted in a rapid increase of the surface temperature from room temperature to the equilibrium state within a very short time, regardless of the applied voltage. The bimetallic NWs were uniformly interconnected into a conductive network within the composites35 and the presence of rGO NSs further enhanced the electrical conductivity by forming contiguous connections with the NWs. The incorporation of a small amount of rGO improved the thermal conductivity and assisted in attaining a homogenous temperature distribution across the film.23 The Joule heating performance of the WKF/CuxNiy/PDMS/rGO samples followed the order: Cu3Ni1 ˃ Cu2Ni1 ˃ Cu1Ni1 ˃ Cu1Ni2 ˃ Cu1Ni3. Those NWs with low Cu and high Ni contents generated low surface temperatures at a given applied voltage because of the lower conductivity of Ni. Cu has a conductivity (2 × 106 S m−1) twice that of Ni; NWs having a large Ni content had grainy, rough surfaces that reduced the conductivity of the composites.22 The thin coating of Ni on the bimetallic NWs provided oxidation resistance without sacrificing the final properties of the composites. The optimized metal ratio of the bimetallic NWs was Cu3Ni1, which provided improved oxidation resistance and stability to the composites. Besides the ratio of Cu:Ni in the nanowires, the Joule heating performance was also influenced by the surface area, thickness, nanowire length and diameter, contact resistance between nanowires, contact resistance between the textile and electrode. The NWs with small diameter and high aspect ratio could further enhance the conductivity of the composites. 22, 36, 37 For Cu-Ni, the conductivity is relatively high for very small diameter and NWs with large diameter could influence the stability of the composites adversely in high current or high temperatures. Small diameter of the NW is advantageous to reduce the amount of charge carrier scattered by the NW. The high diameter of nanowire could decrease electrical conductivity as Cu-Ni NW with increased diameter has grainy rough surface that enhanced charge scattering from its surface. It was observed that NWs with higher Ni content has high aggregation of Ni nanoparticles on the surface of Cu NW from SEM micrographs. This phenomenon induced higher surface roughness and increased diameter and low aspect ratio by decreasing surface area of interaction. Considering all the factors, Cu3Ni1 displayed very efficient Joule heating compared to other 8


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composition of the NWs, as shown in Figure 3. The parallel passage of NWs aided in continuous conduction of charge and the interconnection between them at the tip, which also influenced the final electric heating performance of the composites. The implementation of high pressure with vacuum during fabricating composites by VARTM led to effective electrical contact between the nanowires at the tip. The charge carrier collided at the junctions of NWs under applied electric voltage and their motions were accelerated. Thus, heat was also released from the nanowire junction point because of the inelastic collision.38 The heating performance also depends on the quality of the formed contact between the textile and electrodes. The contact area of the electrodes was comparatively very small with the fiber sheet relative to the surface of the sample and all the yarns in the fiber sheet have effective contact during measurements. Therefore, the resistance contribution from the electrode region to the total resistance was negligible. The heating profiles also indicated that the electric heating performance of a composite was affected by the presence of rGO; WKF/Cu3Ni1/PDMS/rGO had better heating performance than WKF/Cu3Ni1/PDMS. The samples required only 1.5 V to attain 70°C; this low voltage should not pose a risk to humans and, additionally, all of the conductive components in the sample were enveloped by the insulating PDMS matrix. Efficient transformation of electrical energy into Joule heating, indicated by achieving an adequate surface temperature upon the application of a low voltage, was attributed to the better conductivity provided by the Cu-Ni NWs and rGO network in the samples. Earlier, the electric heating performance of CNT included fabric up to 90 °C under an applied voltage of 40 V was reported due to high contact resistance.39 In our study, temperature of the composites samples raised to 70 oC on application of only 1.5 V suggesting low contact resistance and efficient electric heating performance of the composites is demonstrated. The electrical resistance of the composite samples is provided in Table S2. The temperature profile of WKF/Cu3Ni1/PDMS/rGO composite under bending, folding and rolling conditions has been provided in Figure S3. It has been observed that the Joule heating performance of the composite remains unaffected under various mechanical deformations. IR images were taken of samples under bending, folding and rolling conditions analogous to human motions. The samples displayed good 9


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flexibility and stable heating, suitable for wearable purposes even when the wearer is moving (Figure 3 E). The fabricated heater had satisfactory performance with very good stability during various mechanical disruptions, including bending and folding. The feasibility of the heater to provide warmth or articular thermotherapy was demonstrated by holding a sample in the hand while applying a voltage of 1.5 V; the temperature progression in real-time is attached as a video clip (Movie S1). The thermal stability and durability of a composite was assessed by 15 repeated on/off cycles at an applied voltage of 1.5 V (Figure 3 F). Steady, rapid heating of the composite was observed without any deformation or degradation of properties. The Cu-Ni NWs and rGO provided steady electrical conductivity, ensuring effective heating performance of the composite. The observed rapid and uniform heat distribution is very important for practical applications. The thermal management device needs to provide, as well as confine, heat. The radiative insulation provided by normal cloth is meagre due to its high emissivity, and hence it cannot prevent body heat from dissipating into the surrounding environment. Significant heat loss is caused by the incessant emission and absorption of IR radiation from the human body. The WKF/PDMS material provides more radiative insulation than normal cloth and the effect is enhanced by the presence of Cu-Ni NWs combined with rGO. The long NWs interlocked in the woven fiber surface cross each other to form networks, and the spaces between the NW networks are much smaller than the wavelength of IR radiation emitted from the human body (9.5 µm).40 This enables a large portion of the radiation to be reflected back to the body. The thermal emission of objects is described by the Stefan−Boltzmann law (Equation 1): j = εσT4

(1)

where j, ε, σ and T denote the total energy flux, material emissivity, Stefan−Boltzmann constant and temperature, respectively. Materials having high emissivity have high radiating ability at the same temperature. The ε values for bulk Cu and Ni are 0.02 and 0.07, respectively, but rGO and WKF have high emissivity values of ca. 0.9.41 The Cu-Ni NWs have much lower emissivity than WKF and thus provide much more insulation. Passive radiative heating analysis of the WKF/PDMS composites incorporating 10


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different amounts of Cu-Ni NWs and rGO was done by placing them on a hot plate in the open air. The schematic representation of the test has been provided in Figure S4. The test was performed in a closed room that has ambient environment with well controlled relative humidity and temperature of 23oC. The temperature of the hot plate was maintained at 35°C to replicate human body temperature. An air gap was generated between the hot plate and the sample by enclosing the hot plate in a WKF sheet and placing the sample on top of the fiber sheet. The air gap was constant between the two surface during each measurement because the bottom fiber sheet was fixed to the hot plate and all the composite samples were of uniform thickness and dimensions. The spaces between the NW networks were much smaller than the wavelength of IR radiation emitted from the human body (9.5 µm). This enabled a large amount of the radiation to be returned to the human body. The formation of the air gap between the bare WKF sheet and WKF composites was necessary to avoid the conductive loss of heat because Cu-Ni NWs have higher thermal conductivity than WKF sheet. Thus, the Cu-Ni NWs incorporated composites possessed high thermal conductivity than that of the WKF sheet. Hsu et al also studied the radiation insulation of AgNW cloth by the similar method.9 A greater difference in temperature between the sample and the hot plate implied better thermal insulation of a sample. The WKF/Cu-Ni/PDMS composite was a good thermal insulator because the Cu-Ni NW network acted as heat insulator for the composites. Samples incorporating Cu3Ni1 had the best thermal insulation behavior, with a temperature difference of 5.15°C, whereas WKF/PDMS gave a temperature difference of 3.6°C (Figure 4a). The reduction in radiation loss enabled WKF/Cu3Ni1/PDMS to contribute 43% more insulating value than WKF/PDMS. Cu-Ni alloys with higher Cu and lower Ni contents were better insulators against radiative heat loss because Cu has a lower emissivity than Ni. The high radiative insulation was also influenced by the uniform growth density of the Cu3Ni1 NWs on the surface of the fiber sheet. The surface of the WKF sheet was uniformly covered by the NWs, as shown in the SEM micrographs. The density of nanowires on single fiber filament determined from SEM images was obtained as around 61/µm2. This will be an effective surface coverage of the fiber sheet with metallic nanowires. Although rGO has a high emissivity, the addition of a small amount of rGO 11


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(0.5%) did not adversely affect the heat insulation performance because integration of the Cu-Ni NWs in the composite overcame the drawbacks associated with rGO incorporation. The difference of ∆T in the two samples (WKF/Cu3Ni1/PDMS and WKF/Cu3Ni1/rGO/PDMS) is also less than 0.5. The addition of rGO induced high impact resistance and tensile strength to the composites. Further, the Joule heating performance was also improved by adding rGO to the composites. Only 0.5 % of rGO was added in all the composites. In our previous work, we have optimized the amount of rGO added in the WKF composites. A maximum of 1.5 % of rGO can be added, which contributed to improving properties.42 However, in our present study, we have added the minimum amount of rGO so as to obtain optimized mechanical strength, Joule heating and radiation insulation. The WKF/Cu3Ni1/PDMS composite was a better heat insulator than WKF/Cu3Ni1/rGO/PDMS. These results together demonstrate an efficient heat-insulating ability of the composites that could be exploited in energy-saving wearable devices. Thermal images of WKF/Cu3Ni1/PDMS and WKF/PDMS confirmed the radiative heating property (Figure 4a insets). Samples were placed in the hand to maintain thermal equilibrium with the body temperature. The superior heat-insulating performance of WKF/Cu3Ni1/PDMS compared with WKF/PDMS was evident from the difference in colors between the two samples. The WKF/Cu3Ni1/PDMS sample appeared to be colder than the WKF/PDMS one despite being held at the same temperature. The NW network and lower Ni content of the NWs improved the thermal insulation value. Fourier transform-infrared spectroscopy (FT-IR) was used to measure the total IR reflectance of the composite sample. Data was acquired over the wavelength range of 2–15 μm; diffuse gold film was used as the reference (Figure 4b). The IR reflectivity of normal cotton cloth is quite low (8.4%).43 The WKF displayed an average reflectivity of 77.1% and the formation of composites with PDMS increased the reflectivity to 84.2% (relative to that of the human body). The average reflectance of all the other samples was measured. It was observed that the IR reflectivity values of WKF/Cu1Ni3/PDMS/rGO, WKF/Cu1Ni2/PDMS/rGO, WKF/Cu1Ni1/PDMS/rGO, WKF/Cu2Ni1/PDMS/rGO, WKF/Cu3Ni1/PDMS/rGO and WKF/Cu3Ni1/PDMS were 98.12%, 98.28%, 98.32%, 98.31%, 98.43% and 98.47% respectively. The 12


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IR reflectivity of 98.5 % is also reported for nanoporous polyethylene coated Ag film weighted over human body radiation by Cai et al.43 The introduction of Cu-Ni NWs into WKF/PDMS enhanced the IR reflectivity because the NW networks reflected the IR radiation back into the body. All of the WKF/PDMS composites containing Cu-Ni NWs and rGO exhibited similar IR reflectivity, with only minor differences (Figure 4b insets). The addition of a small amount of rGO to the composite have little effect on the performance of the composite samples due to the high reflectance of the Cu-Ni NWs. The Ni shell on the surface of the core Cu NWs improved the oxidation resistance of the NWs. The IR reflectivity of the samples was highest for the Cu3Ni1-incorporated sample and lowest for the Cu1Ni3-incorporated one. Although all metals are highly IR reflective, Cu possesses slight higher IR reflectivity than Ni. Since Ni was incorporated to avoid oxidation of Cu NWs, it is suggested that a thin layer of Ni is sufficient to prevent oxidation of the NWs. Furthermore, the surface roughness increased with increment of Ni content in the nanowires, which might also affect the IR reflectivity. The WKF/Cu3Ni1/PDMS/rGO composite had a lower IR reflectivity than WKF/Cu3Ni1/PDMS because of the high reflectivity of Cu-Ni than rGO. The high IR reflectivity of WKF/Cu3Ni1/PDMS indicated that the Cu:Ni ratio also influenced the IR reflectivity. Both sides of The fabricated two layered composite were same. The surface emissivity of the samples was negligible as the sample displayed very high reflectance. All the infrared radiation emitted by the body was reflected back toward the body to reduce heat loss, which significantly improves the radiation insulation performance. The composites also displayed very high IR reflectance (Figure 4b). Hsu et al. fabricated Ag NW-embedded cloth which reflected human body infrared radiation back to the body and showed that IR reflectance of Ag NW cloth was higher than normal cloth.9 Similar results were also obtained in previous literatures where radiative warming effect was achieved by reflecting back the IR radiation toward the body.8,9,44 The IR transmittance was further measured (Figure S5) and the average transmittance were around 22.8 % for WKF, 15.4% for WKF/PDMS and 1.2 % for WKF/Cu-Ni/PDMS/rGO composites (weight based on human body radiation). The reflectance and transmittance results demonstrated that the composites were effective IR reflectors and would be suitable for thermal management applications. 13


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To assure the comfortability of the wearer, the device should be able to permit vaporizing sweat from the skin’s surface. The breathability is usually determined by the interconnected nanowires network structure, and porosity of the sample. The nano-porous structure of the composite (Figure S 1) can help in moisture transmission but high porosity will have adverse effect on the windproof property of the composite as it allows cold wind to penetrate the region between the device and the skin in extreme climate conditions. Many complexities of interactions might occur at the interface between fibers and the matrix. The fabricated composites by the VARTM process did not contain any macro/micro pores due to the infusion of resin through vacuum. However, the presence of nanowires at the interface of woven fiber sheet and polymer matrix with dispersed rGO acted as a smart cushion at the interfacial region of WKF and polymer matrix.45,46 The nano-pores at the interfacial region were responsible to the slight breathability of the composites and contributed to high mechanical strength due to releasing applied forces. The presence of nano-pores at the interface led to small breathability of the composites and contributed to improving properties of the final composites. The water vapor transmission rate of the samples was checked by noting the weight gains of the desiccant mass that were filled inside glass bottle with open-top cap (Fisher) and the samples were utilized in sealing the desiccant. The test of all the samples was carried out at the same period of time. The water vapor transmission rate of bare WKF was highest among all the samples. On forming composite with PDMS the transmission rate decreased steeply because PDMS decreased the porosity of the sample. After the introduction of the nanowires and rGO in the composite the water vapor transmission rate improved which indicated that the WKF/CuNi/rGO/PDMS composites contained fine nano-pores and interconnected channels necessary for transmission of water vapor from perspiration by natural diffusion and convection. WKF/CuNi/rGO/PDMS samples incorporated with different amount of Cu-Ni NW displayed almost similar water vapor transmission rate (Figure S 6a). The composite samples containing PDMS was waterproof and, at the same time, breathable as the size of water vapor molecule is ~0.2 nm which can easily pass through the nano-pores present in the samples. Since the device is designed for special application like flexible smart bullet proof vest especially in cold regions, 14


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therefore highly porous structure and extremely breathability close to that of normal cotton cloth is not very essential. The durability of WKF/Cu3Ni1/PDMS/rGO composite was investigated by measuring the relative resistance of the sample during the wash tests and six washing cycles were performed having washing times of 1200 min. The electrical resistance measurements were checked after the samples were thoroughly dried. The resistance of the composites remained constant after each washing cycles (Figure S 6b) suggesting high durability of the composites ideal for personal thermal management. Low-velocity impact resistance testing was performed to evaluate safeguarding performance; the results of absorbed energy as a function of time are illustrated in Figure 5a. Impact energy consists of absorbed and rebounded energies. Initially, energy is absorbed by elastic deformation upon impact. The absorbed energy propagates throughout the sample on termination of the impact and the generation of friction in the sample leads to absorption of some of the energy. The remaining energy is absorbed via various damage mechanisms when the energy level exceeds the elastic limit. The absorbed energy was higher for Cu-Ni NW-containing composites than WKF/PDMS; the higher impact resistance was attributed to the networked structure of the NWs within the composite. The absorbed energy increased with increasing Ni amount and decreasing Cu content in the Cu-Ni NWs. This resulted in WKF/Cu1Ni3/PDMS/rGO having the highest impact resistance (127.5% increase), and WKF/Cu3Ni1/PDMS the lowest impact resistance, among the composites. The high surface area of rGO present in the samples also contributed to the increased impact resistance. The very high interfacial interactions within the homogenous dispersion of rGO in PDMS helped to mitigate damage within the samples.42 The reinforcement provided by Cu-Ni NWs and rGO in the samples was evaluated by their tensile properties; the stress–strain curves are shown in Figure 5b. The WKF/PDMS sample had the lowest strength and Cu-Ni NWs grown on the WKF improved this property. The large aspect ratio of the bimetallic Cu-Ni NWs and the rGO NSs effectively reinforced the composites. The high length of the individual NWs facilitated the formation of the networked structure within a composite that restricted polymer chain movement within the samples.47 The WKF/Cu1Ni3/PDMS/rGO composite had a 95.6%15


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higher tensile strength than the WKF/PDMS sample. The tensile properties followed the order: Cu3Ni1 ˂ Cu2Ni1 ˂ Cu1Ni1 ˂ Cu1Ni2 ˂ Cu1Ni3. The high specific surface area coupled with high stiffness promoted potential interface coupling with the polymer and WKF, which led to improved load transfer within the composites. The enhanced tensile strength and high average elongation at break indicated robust performance and excellent flexibility, properties that are important for their application as a wearable heater for body-warming thermotherapy. In conclusion, the suitability of Cu-Ni NWs and rGO in a WKF-based composite was demonstrated in a flexible electric heater and thermal-insulating device. NWs having different Cu:Ni ratios were synthesized by coating Ni on the surface of Cu NWs. The coated NWs were then grown vertically on the surface of WKF by a hydrothermal technique. A dispersion of rGO (0.5%) in PDMS resin and Cu-Ni NWs grown on the WKF substrate were formed into composites using VARTM. The layer of Ni acted as a protective coating and preserved the Cu core NWs from oxidation. The formation of Cu-Ni NWs was confirmed by TEM studies and their vertical growth on the fiber surface was evident on SEM analyses. The crystalline phases of the Cu-Ni alloy were identified by XRD and the chemical states were determined by XPS analyses. The high conductivity of the NWs and rGO enabled efficient Joule heating performance of the composite, as revealed by a rapid temperature response and uniform temperature distribution at low applied voltages. The integration of Cu3Ni1 NWs with WKF/PDMS with a fixed amount of rGO exhibited the highest Joule heating among the Cu-Ni NW composites. Passive radiating heating experiments showed that WKF/Cu3Ni1/PDMS had a higher thermal insulating ability and IR reflectance than WKF/PDMS. However, addition of rGO to WKF/Cu3Ni1/PDMS decreased the thermal insulation and IR reflectance. The Cu-Ni NW- and rGO-added samples had better impact resistance and tensile properties than the neat WKF/PDMS samples. The WKF/PDMS samples containing Cu1Ni3 NWs with a fixed loading of rGO displayed the highest impact resistance (127.5%) and tensile strength (95.6%). Those NWs with higher Ni contents also had higher mechanical strengths, while NWs with higher Cu contents exhibited better Joule heating and passive heating performance. The exceptional flexibility, electric heating performance, thermal 16


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stability, breathability and washability of the WKF composites make them excellent candidates for articular thermotherapy and wearable applications. The localized heating effect induced by the NWs can diminish power demand and reduce our reliance on fossil fuels for heating. The high safeguarding performance evident from their high mechanical strength and flexibility also makes such composites promising for body armor applications requiring thermal insulation and electric heating; the required applied voltage could be provided by a small battery. Our results suggest widespread applications in efficient personal thermal management systems.

Supplementary Information. Characterization, SEM of the composite, details of XRD and XPS analysis, heating profiles under folding, bending and rolling conditions, electrical and thermal conductivity, water vapor transmission rate and wash test are provided. The video clip shows the real-time temperature rise of the heater when held in the hand on applying 1.5 V to the sample (Movie S1). The rapid, homogenous heating and stable electrical conductivity of the composite makes it suitable for heating and articular thermotherapy applications.

Acknowledgement

This work was supported by the Mid-career Researcher Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF2018R1A2B3007806) and the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT (NRF-2017R1A5A1015311) REFERENCES (1) Zang, Y.; Zhang, F.; Di, C. A.; Zhu, D. Mater. Horiz. 2015, 2, 140−156. (2) Yi, F.; Wang, J.; Wang, X.; Niu, S.; Li, S.; Liao, Q.; Xu,Y.; You, Z.; Zhang, Y.; Wang, Z. L. ACS Nano 2016, 10, 6519−6525. (3) Zhang, M.; Wang, C.; Liang X.; Yin Z.; Xia, K.; Wang, H.; Jian, M.; Zhang, Y. Adv. Electron. Mater. 2017, 3, 1700193. 17


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FIGURES

Graphical Abstract

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Figure 1 (a). Schematic diagram for preparation of WKF/Cu-Ni/rGO/PDMS composite (b) TEM image of Cu-Ni NW(c) high resolution micrograph of Cu-Ni nanowire (d) STEM image and elemental mapping of Cu and Ni for the Cu3Ni1 NW.

Figure 2. Scanning electron micrograph images of (a) Cu1Ni3, (b) Cu1Ni2, (c) Cu1Ni1, (d) Cu2 Ni1 and (e) Cu3Ni1 nanowires (NWs). (f) High-resolution image of a Cu3Ni1 NW showing the length and diameter.

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Figure 3. (A) - (C) Time-dependent temperature profile during Joule heating of WKF composites (D) Average temperature and IR images under 1.5 V applied voltage of (a) WKF/PDMS (b) WKF/Cu1Ni3/PDMS/rGO (c) WKF/Cu1Ni2/PDMS/rGO (d) WKF/Cu1Ni1/PDMS/rGO (e) WKF/Cu2Ni1/PDMS/rGO (f) WKF/Cu3Ni1/PDMS (g) WKF/Cu3Ni1/PDMS/rGO (E) IR images of WKF/Cu3Ni1/PDMS/rGO composite under bending, folding and rolling condition (F) On/off responses for WKF/Cu3Ni1/PDMS/rGO under 1.5 V.

Figure 4. (a) An open-air hot plate method for heat transfer analysis measuring the temperature drop in the WKF composites. The inset shows IR images of WKF/PDMS and WKF/Cu3Ni1/PDMS held in the hand. (b) Reflectance measurement of WKF and its composites using a Fourier transform-infrared microscope. 23


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Figure 5. (a) Energy–time response curves and (b) tensile stress–strain curves of WKF/PDMS composites containing Cu-Ni NWs and rGO.

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