Nanofibrous Kevlar Aerogel Films and Their Phase-Change

Jan 30, 2019 - Herein, flexible, foldable, and robust Kevlar nanofiber aerogel (KNA) films with high porosity and specific surface area were fabricate...
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Nanofibrous Kevlar Aerogel Films and Their Phase-Change Composites for Highly Efficient Infrared Stealth Jing Lyu,† Zengwei Liu,† Xiaohan Wu,† Guangyong Li,† Dan Fang,† and Xuetong Zhang*,†,‡ †

Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, P.R. China Department of Surgical Biotechnology, Division of Surgery & Interventional Science, University College London, London NW3 2PF, United Kingdom

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‡

S Supporting Information *

ABSTRACT: Infrared (IR) stealth is essential not only in high technology and modern military but also in fundamental material science. However, effectively hiding targets and rendering them invisible to thermal infrared detectors have been great challenges in past decades. Herein, flexible, foldable, and robust Kevlar nanofiber aerogel (KNA) films with high porosity and specific surface area were fabricated first. The KNA films display excellent thermal insulation performance and can be employed to incorporate with phase-change materials (PCMs), such as polyethylene glycol, to fabricate KNA/PCM composite films. The KNA/PCM films with high thermal management capability and infrared emissivity comparable to that of various backgrounds demonstrate high performance in IR stealth in outdoor environments with solar illumination variations. To further realize hiding hot targets from IR detection, combined structures constituted of thermal insulation layers (KNA films) and ultralow IR transmittance layers (KNA/PCM) are proposed. A hot target covered with this combined structure becomes completely invisible in infrared images. Such KNA/PCM films and KNA−KNA/PCM combined structures hold great promise for broad applications in infrared thermal stealth. KEYWORDS: infrared stealth, Kevlar nanofibers, aerogel, free-standing films, phase-change materials, thermal management

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PCM and undergoes an insulator-to-metal transition around 68 °C, which has been demonstrated as a tunable thermal emissivity material, attributing to its large negative differential thermal emittance across the transition temperature.11 However, these research efforts have encountered multiple problems, such as sustained electric consumption, narrow spectral window,7 slow response speed,8 low tunability,9 and rigid substrates.7 On the other hand, modulating temperature including thermal insulation and heat flux manipulation4,12−14 has been proposed to hide infrared radiation and defeat thermal IR cameras. However, thermal insulators such as blankets are generally thick and heavy, which can cause heat buildup. Thermal cloaks which can manipulate heat diffusion are unable to realize large-scale manufacture. Some phase-change materials have been explored as tunable infrared emission materials, and other PCMs, especially

nfrared stealth technology has attracted increasing attention in decades due to its commercial, military, and scientific value. Considering that thermal cameras can only detect and visualize targets that have infrared radiance characteristics that are different from their surroundings. The Stefan−Boltzmann law (eq S1) reveals that the radiated thermal energy per unit area is directly proportional to the emissivity and the fourth power of the thermodynamic temperature.1,2 Therefore, infrared thermal stealth can be realized via two approaches by modulating thermal emissivity or temperature. Emissivity engineering is regarded as a promising way to realize thermal infrared stealth. For example, the thermal emission of targets can be regulated by manufacturing the surface with micro/nanostructures such as plasmonic metasurfaces,3 photonic crystals,4 photonic cavities,5 and gratings.6 However, static micro/nanostructures do not endow targets with the ability of tunable thermal emissivity. The dynamic control of infrared emission via quantum wells,7 electrochromic dyes,8 ferroelectric materials,9 plasmonic resonators,10 or phase-change materials11 (PCMs) has also been investigated. For example, vanadium dioxide is a typical © XXXX American Chemical Society

Received: November 22, 2018 Accepted: January 25, 2019

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Figure 1. (a) Schematic description of the preparation of Kevlar nanofiber aerogel (KNA) film and its phase-change composite film (KNA/ PCM). (b,c) Schematic representation of the infrared stealth of KNA/PCM composite films to targets in a simulated outdoor environment with solar illumination variations (b) and KNA−KNA/PCM combined structures to hot targets (c).

Figure 2. Characterizations and properties of KNA films (ρ = 29.35 mg/cm3). (a,b) SEM images of surface morphology (a) and crosssectional morphology (b) of KNA (derived from 2.0 wt % Kevlar nanofiber dispersion). The inset in (b) is the photograph of the flexible KNA film. (c) Nitrogen adsorption−desorption isotherm of KNA. The inset is the pore volume of KNA. (d,e) Stress−strain curves of KNA films prepared from different thickness (d) and after treatment with different temperature (e). (f) Fatigue reliability test for the KNA film with cyclic tensile stretch. (g,h) Thermal insulation property of one-layer KNA film (g) and three-layer KNA films (h) at high temperature (300 °C).

organic PCMs including paraffins, stearic acid, and polyethylene glycol (PEG), have been investigated for use in direct thermal management, attributed to their large latent heat during phase transitions and outstanding heat storage capacity.15−17 Aerogels are synthetic materials with ultralow density (0.1−800 mg·cm−3), high continuous porosity, and extremely large specific surface area, making them of immense

importance in various applications, such as thermal insulation,18−21 acoustic insulation,22 water purification,23,24 membrane separation, and supercapacitors.25,26 Aerogels have also been applied as porous scaffolds to nanoconfined PCMs to improve their thermophysical properties while maintaining their energy storage capacity.27,28 Function materials with multiple responsiveness have also been developed from B

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ultralow, increasing gradually with the concentration of Kevlar nanofiber dispersion from 13.03 kg/m3 for 0.5 wt % to 29.35 kg/m3 for 2.0 wt %. This implies that the concentration of Kevlar nanofiber dispersion has a decisive effect on the density. The structure and morphology of KNA (ρ = 29.35 mg/cm3) were characterized by the SEM, as shown in Figure 2a (cross section) and Figure 2b (surface), where Kevlar nanofibers interconnected to form three-dimensional (3D) porous networks. The formation of the connections is probably due to the nanofibers entangled through intra-nanofiber hydrogen bonds during the sol−gel process, and these connections make the present KNA a well-interconnected 3D porous architecture instead of a simple nanofibers stack.34 In addition, Figure S2 demonstrates that the concentration of Kevlar nanofibers plays a significant role in modulating the morphology of the KNA films. At low concentrations (e.g., 0.5 wt %), the Kevlar nanofibers in the obtained aerogel film are loosely connected to form a network structure with relatively large pores. With the increase of Kevlar nanofiber concentration, the pore size and distribution of the aerogel film decrease due to the increased aggregation and entanglement of the nanofibers resulting in a dense morphology. Therefore, the morphology of the KNA films also can be tailored efficiently by adjusting the concentration of the Kevlar nanofiber dispersions. It was interestingly found that using tertiary butyl alcohol (TBA)− water cosolvents (TBA to water with mass ratio of 1:1, 1:2, and 1:4) to replace water in Kevlar nanofiber hydrogel can eliminate the need for freezing before vacuuming in a freezedrying process. Because the cosolvent can freeze immediately under low pressure and results in more uniform morphology (Figure S3), which was attributed to a quicker freezing during vacuum and the smaller crystal size of TBA−water. There was no significant difference in specific surface areas (SBET) and pore volumes between KNA films fabricated from pure water and TBA−water cosolvent via freeze-drying (Table S1). TBA− water with a mass ratio of 1:1 was chosen as a typical solvent system for the preparation of KNA films, except for special instructions. The specific surface area of this KNA film was 272.5 m2/g, and the pore volume was 0.83 cm3/g (Figure 2c and its inset), which is lower than that of KNA film prepared via supercritical drying with SBET of 365.99 m2/g (Table S1) but comparable to or higher than that of cellulose aerogels, SiC nanowire aerogels, and CNT aerogels with SBET of about 100− 300 m2/g,35−37 78 m2/g,38 and 184 m2/g,39 respectively. Mechanical properties are of great importance for practical application, especially for those free-standing thin films. To evaluate the mechanical performances of the KNA films, uniaxial tensile test was carried out at ambient conditions and extreme situations. The film with 50 or 100 μm thickness showed weak mechanical properties, whereas films thicker than 150 μm show reducing trend in tensile strength with increasing thickness, which may be attributed to the porous structures (Figure 2d). Therefore, the films with thickness of 150 μm and prepared from 2 wt % Kevlar nanofiber solution showed the optimal mechanical properties with a tensile strength of 1.27 MPa and a strain of 12.9%, exceeding those of cellulose aerogel prepared from the 2 wt % initial cellulose solution35 or other aerogels made by nanofibers.40 The initial concentration of Kevlar nanofiber solution also has a significant effect on the mechanical properties. As shown in Figure S4a, with the increase of initial concentration of Kevlar nanofiber solution, the ultimate stress and the tensile strain increased gradually, which was attributed to the enhanced proportion of skeleton

aerogels due to the extraordinary capillary force which can incorporate other components into their porous frameworks.29 Herein, Kevlar nanofiber aerogel (KNA) films were fabricated through dispersing of Kevlar into dimethyl sulfoxide (DMSO), spin-coating/blade-coating, sol−gel processing, and subsequent freeze-drying. The obtained aerogel films are flexible and have high porosity with outstanding thermal insulation property and extraordinary capillary force, which were employed to incorporate with PCMs and followed by a hydrophobic coating to fabricate phase-change composite films (KNA/PCM). This composite exhibited the following superiorities: (1) a high thermal management capability originating from excellent energy storage and release properties with high phase-change enthalpy and effective prevention leakages; (2) an infrared emissivity of 0.94, comparable with the value of most backgrounds; (3) an ultralow IR transmittance of a wide waveband of 3−15 μm. Therefore, the KNA/PCM with tunable heat capacity and phase-change enthalpy demonstrated high performance in IR stealth according to outdoor environments such as solar illumination variation. In most thermal stealth situations, a hot target usually needs to be hidden in a relative cool background, then a combined structure constituted of thermal insulation layers (KNA) and IR absorption surface layers (KNA/PCM) was proposed to hide hot targets from IR detection. It is believed that this work will find wide applications not only in fundamental material science but also in high technology and modern military for thermal infrared stealth.

RESULTS AND DISCUSSION Preparation and Characterization of KNAs and KNA/ PCM Films. Details on the fabrication process are schematically described in Figure 1a. The highly flexible and foldable KNA film was fabricated from Kevlar through dissolving it in DMSO, sol−gel procedure, and freeze-drying process. The Kevlar nanofiber dispersion with different concentration (0.5, 1.0, 1.5, and 2.0 wt %) was prepared according to previously reported methods.30−33 Blade-coating was employed for the preparation of KNA films, which makes the film preparation very versatile in controlling the thicknesses of the coatings and realizes the continuous and large-scale aerogel film fabrication. The Kevlar nanofiber hydrogel films were formed by immersing coated films into deionized water to remove the DMSO and protonate nanofibers. Freeze-drying was applied to acquire the corresponding KNA films, which is simpler, more efficient, and more convenient than typical supercritical drying. After PCMs were adequately absorbed and a hydrophobic barrier layer was coated with fluorocarbon resin, the multifunctional KNA/PCM composite film was finally fabricated. Figure 1b,c illustrates the schematic setups for thermal IR stealth application of KNA films and KNA/PCM films. A freestanding KNA/PCM film was covered on the targets in a simulated outdoor environment with solar illumination variations. The thermal camera was employed to record the IR images (Figure 1b). When the targets were operating machines or any part of them had temperatures higher than their backgrounds, KNA films were introduced as thermal insulation layers between KNA/PCM and the targets (Figure 1c). The as-prepared KNA films are highly flexible, which can be rolled up (inset in Figure1a) and even folded (Figure S1) without any damage, making this aerogel film easy to collect and store for further use. The density of the KNA film was C

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Figure 3. Characterizations and properties of KNA/PEG. (a) Photographs of the folding and releasing process of KNA/PEG. (b) Hydrophobic surface property of the KNA/PEG. (c) XRD patterns of KNA, pure PEG, and KNA/PEG composite. (d−f) SEM images of KNA/PEG with different PEG loading contents. (g) TG curves of Kevlar, KNA, and KNA/PEG. (h) Stress−strain curves of solid KNA/PEG with different thickness. (i) Stress−strain curves of melted KNA/PEG with different thickness.

heating plate (inset in Figure 2g), then the temperatures of the heating plate and the film outer surface were simultaneously collected with thermocouples. As illustrated in Figure 2g, the temperatures increased rapidly until reaching the plateau at approximate 302 °C for the heating plate and 222 °C for the outer surface of the KNA, which means that the temperature difference reached about 80 °C. The theory calculated temperature difference is 70.5 °C (eq S2), smaller than the experimental results, because the heat transfer between KNA film and air was neglected in theory calculation. Benefiting from the ultralight and ultrathin KNA films, we can further enhance the thermal insulation performance by applying double- or triple-layer KNA films without increasing too much weight or thickness. For example, temperature difference is as high as 135 °C between the heating plate and the aerogel surface for the triple-layer KNA films (Figure 2h). Moreover, the KNA films exhibited high thermal insulation at medium (50 °C) and low temperatures (−175 °C), as shown in Figure S5. These results indicated that the thermal insulation property of KNA films is superior to a thermally insulating textile inspired by polar bear hair.42 The extraordinary specific surface area and pore volume render the strong capillary force throughout the KNA films to absorb PCMs (such as PEG1.5k). In order to protect the hydrophilic KNA and PEG against moisture or water penetration, a hydrophobic protection layer of fluorocarbon resin (FC) with a thickness of several nanometers was coated

structure, especially manifested on the surface (Figure S2) and gradually increased density with increased solution concentration. We further investigated the high- and low-temperature performance of the KNA films. The same batch of KNAs was treated in liquid nitrogen, 100, 200, and 400 °C for 30 min before tensile test. As shown in Figure 2e, for 100 or 200 °C thermal treatment, there is a negligible decrease in mechanical strength and elasticity, compared to that of the original KNA film. The in situ tensile test under 100 and 140 °C also confirmed that (Figure S4b). The tensile strength of liquid nitrogen or 400 °C treated KNA films was maintained at about 0.8 MPs, which means that this film can withstand some extreme environments. The dynamic stability is also crucial for aerogels to be widely applied. Therefore, KNAs were tested by a cyclic tensile stretch method in ramp strain mode with a ramp rate of 0.05% per second until reaching the maximum value of 5.0%. A typical result is shown in Figure 2f, where there was no apparent degradation of mechanical strength or elasticity after 20 cycles of stretching. The typical aerogel structure of KNA films endows them with excellent thermal insulation performance, which exhibited a thermal conductivity of 0.036 W/m·K (density of 29.35 mg/ cm3) at room temperature. This value is comparable to that of cellular aerogel (0.0304 W/m·K with a density of 9.6 mg/ cm3)40 and other thermal insulators.41 The thermal insulation property of KNA films was further characterized by temperature−time curves. The KNA film encapsulated an electric D

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Figure 4. Infrared stealth property of KNA/PEG under outdoor environment. (a) Schematic representation of the infrared stealth mechanism of KNA/PEG under outdoor environment. (b) DSC curves of KNA/PEG1.5k and KNA/PEG10k. (c−f) Thermal images displayed on the infrared thermal camera for exposure to sunlight (c,e) and turning off the sunlight (d,e). The sunlight was controlled by a solar simulator. Scale bar: 2 cm.

on the KNA/PEG composite film.29 This composite film can be folded and released as desired above melting temperature (e.g., KNA/PEG1.5k, ≥46 °C) without any damage to its mechanical or thermal properties (Figure 3a), which can conformably wrap around objects with arbitrary shape. Water contact angle measurements were performed at room temperature to evaluate the hydrophobic surface property of KNA/ PCM before and after FC coating. As shown in Figure 3b and Figure S6, the water droplets displayed nonwetting performance on the surface of a KNA/PCM film with FC coating, and the contact angle was approximate 113.8°, confirming the hydrophobic nature of the film surface, whereas the KNA/ PCM film without FC coating was hydrophilic (32.4°), which was inherited from hydrophilic PEG and KNA. Furthermore, the FC coating is able to withstand a cyclic melting− solidification process without the decrease in hydrophobicity (Figure S7). This hydrophobic property endows KNA/PCM films with the ability of waterproof and more widespread applied environments. The X-ray diffraction (XRD) patterns of the KNA, PEG, and KNA/PEG composites are shown in Figure 3c. The sharp and intense peaks occurring at around 19.27 and 23.32° in the XRD pattern were diffraction peaks of PEG because of its regular crystallization.43,44 As expected, the diffraction peaks of KNA/PEG were consistent with that of pure PEG, indicating that the PEG kept the original crystalline structure, but the crystallinity of PEG decreased after being confined within the films. In order to reveal the structure of KNA/PEG in which PEG had been trapped within the KNA films, they were subjected to scanning electron microscopy (SEM) analysis. The KNA/ PCM films with different PEG contents were prepared from

30, 50 wt % PEG aqueous solution, and the melting pure PEG (i.e., 100 wt %), designated as KNA/PEG0.3, KNA/PEG0.5, and KNA/PEG, respectively. As demonstrated in Figure 3d,e, PEG was spread homogeneously throughout the KNA matrix, establishing a strong connection between Kevlar nanofibers. A hierarchical layering structure was formed that was attributed to the subsequent freeze-drying. The SEM image of KNA/ PEG showed that the pores were completely filled with PEG (Figure 3f), resulting in an extraordinary PEG loading (>90 wt %), which is much highere than that of previously reported PCM composites.45,46 For determining the thermal stability of KNA/PCM film and the amount of PEG in the composite film, thermogravimetric analysis (TGA) was performed in nitrogen at a heating rate of 10 °C/min. As illustrated in Figure 3g, the main decomposition process of KNA films appeared at 495−645 °C, which is similar to the thermal decomposition behavior of bulk Kevlar material (500−650 °C),30,47 indicating the extraordinary thermal stability. The thermal decomposition of the KNA/ PCM film proceeded rapidly at 300−450 °C, which is attributed to the decomposition of the PEG.48 Judging by the weight loss, the PEG content is about 94.3 wt % in the KNA/PCM hybrids. The bendable and foldable KNA/PCM films (above melting temperature) consisting of PEG throughout the aerogel networks exhibited outstanding strength and elasticity. As illustrated in Figure 3h, KNA/PCM films exhibited typical stress and strain profiles in the solid state. The ultimate strength increased from 1.57 to 2.85 MPa, and the tensile strain improved from 17.5 to 22.2% with the thickness of KNA/PCM increased from 100 to 200 μm. These values E

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whereas the temperatures of the background and KNA/PCM increase gradually due to the relatively high thermal capacity. The phase transition of KNA/PCM can lead to temperature growth platform and energy storage in PCM, which suppresses the thermal radiance, thus merging with the background. After withdrawing the solar irradiation (e.g., night time), the temperature of the target would decrease rapidly, whereas the temperature of the KNA/PCM decreases gradually due to the releasing process of heat energy (Figure S9). Therefore, the temperature and infrared radiance of KNA/PCM are always consistent with those of backgrounds. Polyethylene glycol was chosen as the phase-change filler because the melting point of PEG is dependent on the molecular weight and may vary in a wide temperature range (4−70 °C), with high enthalpy of fusion in the range of 115− 180 J/g. An increase in the molecular weight of PEG causes an increase in the melting temperature and the heat of phase transition.53 Different polyethylene glycols with different melting temperatures were chosen to incorporate with KNA films according to the specific application conditions. Herein, KNA/PEG1.5k and KNA/PEG10k were applied as an infrared stealth layer to cover the targets. The thermal storage and phase-transition properties of these KNA/PCM composite films were characterized by differential scanning calorimetry (DSC). As shown in Figure 4b, the melting onset and peak temperatures of KNA/PEG1.5k were 38.7 and 49.9 °C, respectively, whereas those values of KNA/PEG10k increased to 55.9 and 66.5 °C, respectively. As expected, the melting enthalpy of KNA/PEG10k (179.1 J/g) was higher than that of KNA/PEG1.5k (162.9 J/g), owing to the increased molecular weight of the PEG. Compared to pure PEG, the KNA/PEG shows a downshift of the onset point, an upshift of the termination point, and a decrease of melting enthalpy, as shown in Figure S10, which were attributed to the confinement of the Kevlar nanofiber networks that decreased the PEG10k crystal size and crystallinity degree (∼96% for KNA/PEG) and the great interaction between PEG and Kevlar nanofibers. Other organic PCMs such as PEG4k, eicosane (C20), and stearic acid (SA) with high phase-transition enthalpy were also used to impregnate KNA films to obtain KNA/PCM composite films, showing different melting temperatures (Figure S11). Figure 4c−f shows thermal images on the screen captured by the infrared thermal camera after turning on and off the sunlight. For comparison, a target and KNA/PEG1.5k film were placed at the same condition. Once the sunlight is turned on, the target can be detected and imaged by an IR camera because of the higher radiant temperature than KNA/PEG1.5k film and their background due to the low thermal capacity. The average radiant temperature difference between the KNA/ PEG1.5k film and background was smaller than 0.5 °C (Figure 4c). The temperature difference between KNA/PEG1.5k film and background tested by thermal couples was always less than 2 °C. Turning off the light irradiation, the target, KNA/ PEG1.5k film, and the backgrounds started to decrease in temperature. The high-temperature reduction ratio of the target resulted in the target clearly appearing in the thermal images, whereas the KNA/PEG1.5k is integrated into the background (Figure 4d). Figure 4e shows the thermal image when the target was covered by the KNA/PEG1.5k film under solar illumination. As expected, the target covered by the KNA/PCM became undetectable, completely blending into its surroundings. In addition, the target was still invisible after

indicated the superior strength and elasticity of KNA/PCM, compared to that of KNAs with the same thickness. It is accessible that more effective interfaces between Kevlar nanofiber matrix and PEG filler are instrumental in higher mechanical performances, which can be attributed to the strong adhesion force (hydrogen bonding) from the abundant hydrogen and oxygen on Kevlar nanofibers and PEG molecules (Figure S8).33,49 Further increasing the thickness of the composite film resulted in a noticeable improvement in strength but a reduction in elasticity. At the melting state of PEG, the form-stable KNA/PCM film maintained its shape and dimension. As expected, the ultimate strength was decreased compared to that of the composite film in the solid state. However, the tensile strain dramatically increased to about 38%. This mechanical behavior also confirms the strong interaction between Kevlar nanofibers and PEG throughout the aerogel networks regardless of the melted state of PEG. Infrared Stealth Property of KNA/PEG in an Outdoor Environment. Any object that has a surface temperature higher than absolute zero emits thermal radiation, and the peak wavelength of the radiation is determined by the surface temperature.12 For most living creatures and instruments in operation, the exterior surface temperatures mostly range within −80 to 500 °C, which means the radiation peaks are distributed in 3−15 μm range (eq S3). Unfortunately, 3−5 and 8−14 μm wavebands are the atmospheric transmission windows,50,51 within which the IR radiation can easily pass through the atmosphere and be received by thermal cameras. If the radiant temperature difference between targets and their surroundings is greater than 4 °C, the radiation contrast would identify the targets in thermal images. Therefore, one tremendous challenge for thermal IR stealth is to blend targets into their surroundings in infrared imaging to evade IR thermal detection. In some military and industrial requirements for thermal infrared stealth, the targets are placed in an outdoor environment with solar illumination variations, or the targets are much hotter than the surroundings. In the first case, we simplify the outdoor environment to with and without sunlight irradiation (1.0 Sun), and the sunlight was controlled by a solar simulator. The solar illumination variations lead to environments and targets storing or releasing energy, whereas the difference in thermal capacity, infrared emissivity, and other thermal properties between targets and their surroundings results in great differences in thermal IR radiation. Therefore, the targets would be detected easily with thermal cameras. One general strategy of infrared stealth is applying low thermal emissivity coatings to decrease the radiation intensity;3,52 however, the thermal radiation between targets (with low thermal emissivity coatings) and their backgrounds only matches at certain temperatures. The emissivity of KNA/PEG films was approximately 0.94, comparable with those of most common backgrounds (Table S2). Then, controlling the surface temperature of KNA/PEG to synchronize with their backgrounds was required. Phase-change materials exhibit a high phase-transition enthalpy with the ability to store or release large amounts of energy as latent heat during melting and solidification. Figure 4a illustrates the working principle of the KNA/PEG phase-change composite film as an infrared stealth layer. Under the illumination of sunlight (e.g., day time), the temperature of the target might increase rapidly via absorbing the solar irradiation, resulting in strong infrared radiance, F

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Figure 5. Infrared stealth synergistic performance of KNA films and KNA/PEG films. (a−c) Thermal images of hot target covered by onelayer KNA (a), three-layer KNA (b), and five-layer KNA (c). (d−f) Thermal images of hot target covered by one-layer KNA−KNA/PEG (d), three-layer KNA−KNA/PEG (e), and five-layer KNA−KNA/PEG (f). (g) Temperature−time curves of bare surface and KNA film, KNA/ PEG covered surface under heating. (h) Fourier transform infrared spectra of KNA and KNA/PEG films with different thickness. (i) Schematic representation of the infrared stealth mechanism of KNA and KNA/PEG to the hot target.

thermocouple decreased significantly (Figure S15) owing to the high thermal insulating ability, the infrared radiation temperature decreased slowly. Then, KNA/PCM combined with a KNA structure was proposed. We designed the combined structures as KNA/PCM−KNA when KNA films were on top, and KNA−KNA/PCM when KNA/PCM was on top. The infrared thermal images of hot targets covered with KNA/PCM−KNA are shown in Figure S16, where the shape of the hot target was still dimly visible even covered with KNA/PCM and five-layer KNA film. Figure 5d−f and Figure S14d−f show infrared thermal images of electric heating plates covered with one-layer, three-layer, and five-layer KNA−KNA/ PCM combined structure. Obviously, the KNA−KNA/PCM combined structures displayed infrared thermal stealth properties better than that of the corresponding KNA films and KNA/PCM−KNA. The covered region of the heating plate by five-layer KNA−KNA/PCM became invisible in its infrared image (Figure 5f). Accordingly, this IR stealth system would work on hotter targets by increasing the thickness or number of thermal insulating layers. The applied boundary of the thermal source is about 180 °C, which is the highest long-term working temperature for KNA films. In order to figure out the excellent infrared stealth mechanism of KNA−KNA/PCM, we tested and compared the dynamic temperature−time curves of the heating plate during heating−cooling cycles when covered with one-layer KNA or KNA/PCM films, respectively (Figure 5g). When the electric heating plate was heated from 25 °C to reach the equilibrium temperature of about 80 °C, the temperature of the KNA surface only increased to 68 °C, reaching the equilibrium, demonstrating the excellent thermal insulation property. The equilibrium temperature of the KNA/PCM

turning off the sunlight (Figure 4f). The infrared stealth performance of the KNA/PEG composite originating from the excellent thermal management was much better than that of thermal insulating KNA film (Figure S12) and comparable with that of adaptive thermal stealth films.11 Moreover, the fabrication process of KNA/PCM was simpler and more economic than that of previously reported infrared stealth materials.3,50 Infrared Stealth Property of KNA-KNA/PEG to Hot Targets. When the target continually generates heat (for example, electric heating plate), the covered KNA/PCM film will be heated up eventually, resulting in thermal radiation higher than that of the backgrounds (Figure S13). Therefore, thermal insulating layers are desired to block heat transfer and thus reduce the surface temperature. The thin KNA films with high thermal insulating performance are chosen as the insulating layer. For a convenient observation, the KNA films covered only part of the heating plates. When the power source was turned on, the electric heating plate started to increase the temperature until reaching an equilibrium temperature (∼40 or ∼60 °C), while the environment temperature was about 25 °C. Apparently, the target covered by a thin KNA film displayed thermal radiance stronger than that of its background (Figure 5a and Figure S14a). To enhance the thermal IR stealth performance, three-layer and five-layer KNA films were applied to cover the hot targets. Figure 5b,c and Figure S14b,c show the corresponding infrared thermal images. With the addition of layers of KNA films, the thermal radiation contrast between the covered part and backgrounds decreased gradually; however, the target covered by five-layer KNA films maintained thermal radiation higher than that of the background. Although the top surface temperature collected by G

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ACS Nano surface was approximately 70 °C, and noticeable temperature plateaus were observed for KNA/PCM during heating and cooling processes, corresponding to the thermal storage and release, respectively. The temperature plateau, as labeled in red frame (Figure 5g), corresponds to a latent heat storage process, where the phase of PEG10k was changed from solid to liquid to store the energy, which is consistent with the DSC curve in Figure 4b. During the cooling process, the thermal release process appeared, as labeled in the blue frame (Figure 5g), where the state of PEG10k transformed from liquid to solid to release the energy, which corresponds to the DSC analysis result in Figure 4b. Furthermore, Fourier transform infrared (FT-IR) spectroscopy analysis was conducted on KNA films and KNA/PCM composite films with different thickness to measure optical characteristics such as transmission and reflection (Figure 5h and Figures S17−S19). The spectrographs covered a wide range of wavelength from 3 to 15 μm, which includes both atmospheric windows of 3−5 and 8−14 μm. The results demonstrate that the average infrared transmittance of KNA/ PCM is very low and shows a tendency to decrease with increasing thickness. With the broad wavelength ranging from 3 to 15 μm, the average infrared transmittance of 50 μm thick KNA/PCM is 7.4% and decreased to 1.7% with the thickness increasing to 250 μm. It is noteworthy that the IR transmittance of melting KNA/PCM was almost identical to that of solid state (Figure S17). The infrared transmittance of KNAs is very high (the average infrared transmittance with 50 μm in thickness is 81.1%), which may be attributed to the porous structure (Figure 5h and Figure S18). The KNA/PCM film also exhibited ultralow average infrared reflection in the desired bands (Figure S19), which signified the high absorptivity.3 Therefore, a superior approach to realizing IR stealth for hot targets would be a combined structure of KNA−KNA/PCM, including the thermal insulating layers and IR absorption layer to disturb IR propagation toward detectors. The thermal stealth performance of this combined structure to hot targets not only surpasses the performance of thermal insulating KNA layers, IR absorption KNA/PCM layer, or KNA/PCM−KNA structures but also is comparable or surpasses the property of the materials described previously.2,11,50 This stealth mechanism is schematically shown in Figure 5i. The KNA films act as thermal insulated layers to prevent effective heat transfer. However, IR radiation from the hot target can partially pass through these KNA films and reach the KNA/PCM layer. The KNA/PCM film can further reduce the temperature to approach the environment temperature and more importantly absorb most of the emissivity from targets and KNA films. Therefore, the thermal radiation contrast between the covered target and the background is too low to be detected.

PEG10k) and an infrared emissivity of 0.94, which was comparable with that of most backgrounds. The KNA/PCM composite films with thermal management and comparable IR emissivity to backgrounds demonstrated high performance in IR stealth in outdoor environments with solar illumination variation, where the target covered with a KNA/PCM film can blend their thermal appearance into the background. Furthermore, KNA/PCM exhibited an ultralow average transmittance of a wide waveband of 3−15 μm (1.7% with 250 μm in thickness). A combined structure constituted of thermal insulation layers (KNA films) and IR absorption surface layer (KNA/PCM) was proposed to hide hot targets from IR detection. Compared with the other IR stealth materials, the target coated with KNA−KNA/PCM combined structure exhibits better infrared stealth performance due to the combination of excellent thermal insulation and ultralow infrared transmittance. The KNA/PCM films and the KNA− KNA/PCM combined structures with extraordinary infrared thermal stealth performances exhibit great potential for future applications in military and industrial fields and provide a more effective solution for infrared stealth technology.

METHODS Materials. Kevlar 1000D was purchased from Dongguan SOVETL Co., Ltd. DMSO was obtained from Sinopharm. Potassium hydroxide (KOH), polyethylene glycol (PEG1.5k, PEG10k), eicosane (C20), stearic acid (SA), and tert-butyl alcohol were all purchased from Aladdin Company and used as received. E-pure deionized water (18.2 MΩ·cm−1) was obtained from a Millipore Milli-Q system. Preparation of Kevlar Nanofiber Aerogel Films. Kevlar 1000D was dissolved in DMSO to obtain a nanofiber solution (0.5−2.0 wt %) via previously reported methods.30 Blade-coating was applied to deposit the Kevlar nanofiber, followed by submerging in deionized water for hours to generate Kevlar nanofiber hydrogel films. The obtained hydrogel films were subsequently transferred to mixed solvents of deionized water and tert-butyl alcohol for solvent exchange and finally freeze-dried for more than 12 h at −50 °C with a freezedryer. Preparation of Kevlar Nanofiber Aerogel/Phase-Change Material Films. Phase-change materials including SA, C20, and PEG10k were heated (80 °C) to completely melt in a vacuum oven. Then KNA films were immersed in melting PCM and kept in a vacuum oven at 80 °C for another 6 h (saturated adsorption was reached). Subsequently, the excess PCM adhered on the film surface was removed by transferring the samples to filter paper and repeating several times. After being cooled to room temperature and coated with fluorocarbon resin, the nanocomposite films were obtained. Characterization on Morphology and Mechanical Properties. The morphology of the KNA aerogel films and KNA/PCM films was characterized by a scanning electron microscope (Hitachi S4800) with an acceleration voltage of 5−10 kV. The specific surface area of the aerogels was determined by the Brunauer−Emmett−Teller method, based on the amount of N2 adsorbed at pressures 0.05 < P/ P0 < 0.3. The pore size distribution and average pore diameter of the aerogels were inspected with BJH nitrogen adsorption and desorption method (ASAP 2020, Micromeritics, USA). The contact angle was performed using an optical angle meter system (OCA 15EC, Data Physics Instruments GmbH). XRD patterns were collected at ambient temperature using a D8 Advance spectrometer (Bruker AXS) with Cu Kα generated at 40 kV and 40 mA over an angular range of 5−70° (2θ). TGA was carried out using a TG 209F1 Libra (NETZSCH) analyzer with a heating rate of 10 K·min−1 in a nitrogen atmosphere. The tensile stress−strain curves were recorded by using an Instron 3365 tensile testing machine. Characterization on Thermal Insulation Properties. Thermal conductivity was measured using TC 3010L thermal conductivity meter (Xi’an Xiatech Electronic Technology Co., Ltd.). DSC analysis

CONCLUSIONS In conclusion, a flexible, foldable, and robust Kevlar nanofiber aerogel film that can withstand some extreme environments was fabricated through blade-coating, sol−gel processing, and the subsequent freeze-drying method. The typical aerogel structure of KNAs endowed them with excellent thermal insulation performance (0.036 W/m·K in thermal conductivity) and extraordinary capillary force, which were employed to incorporate with PCMs to fabricate KNA/PCM composite films. This KNA/PCM demonstrated excellent energy storage properties (phase-change enthalpy of 179.1 J/g for KNA/ H

DOI: 10.1021/acsnano.8b08913 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano and specific heat capacity were performed on a DSC 200F3 NETZSCH with a heating and cooling rate of 10 K·min−1 in a nitrogen atmosphere. The temperature−time curves of KNA films and KNA/PCM films were collected by a Keysight 34970 Data Acquisition instrument equipped with thermocouples. Characterization on Infrared Stealth. FT-IR spectra were determined by a Nicolet 6700 FT-IR spectrometer over a wavelength range of 3−15 μm. Infrared thermal images were taken with a MinIR (M1100150) camera. It should be mentioned that the thermal IR images and temperature−time curves were collected in an open environment with thermal convection and thermal radiation, which would dissipate the heat naturally.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b08913. Stefan−Boltzmann law, Wien’s displacement law and heat conduction equations, digital photos, SEM images, surface volume, pore size, tensile behavior, and thermal insulation properties of KNA films, water contact angle, temperature−time, surface temperature, DSC, FT-IR, and thermal images measurements (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], [email protected]. ORCID

Xuetong Zhang: 0000-0002-1268-9250 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51572285), the National Key Research and Development Program of China (2016YFA0203301), the Royal Society Newton Advanced Fellowship (NA170184), the Natural Science Foundation of Jiangsu Province (BK20170428), China Postdoctoral Science Foundation (2018M642352), and Jingsu Planned Projects for Postdoctoral Research Funds (2018K070C). The authors thank Wei Wang from Suzhou Institute of Nanotech and Nanobionics for drawing the schematic diagram of the preparation of KNA film and KNA/PCM composite. REFERENCES (1) Salihoglu, O.; Uzlu, H. B.; Yakar, O.; Aas, S.; Balci, O.; Kakenov, N.; Balci, S.; Olcum, S.; Süzer, S.; Kocabas, C. Graphene-Based Adaptive Thermal Camouflage. Nano Lett. 2018, 18, 4541−4548. (2) Peng, L.; Liu, D.; Cheng, H.; Zhou, S.; Zu, M. A Multilayer Film Based Selective Thermal Emitter for Infrared Stealth Technology. Adv. Opt. Mater. 2018, 6, 1801006. (3) Xie, X.; Li, X.; Pu, M.; Ma, X.; Liu, K.; Guo, Y.; Luo, X. Plasmonic Metasurfaces for Simultaneous Thermal Infrared Invisibility and Holographic Illusion. Adv. Funct. Mater. 2018, 28, 1706673. (4) Han, T.; Bai, X.; Thong, J. T. L.; Li, B.; Qiu, C.-W. Full Control and Manipulation of Heat Signatures: Cloaking, Camouflage and Thermal Metamaterials. Adv. Mater. 2014, 26, 1731−1734. (5) Wang, Z.; Luk, T. S.; Tan, Y.; Ji, D.; Zhou, M.; Gan, Q.; Yu, Z. Tunneling-Enabled Spectrally Selective Thermal Emitter Based on Flat Metallic Films. Appl. Phys. Lett. 2015, 106, 101104. (6) Greffet, J.-J.; Carminati, R.; Joulain, K.; Mulet, J.-P.; Mainguy, S.; Chen, Y. Coherent Emission of Light by Thermal Sources. Nature 2002, 416, 61−64. I

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