• Top of Page

• Abstract

• Results and Discussion

• Conclusions

• Methods

• References

Results and Discussion

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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 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 1b,c illustrates the schematic setups for thermal IR stealth application of KNA films and KNA/PCM films. A free-standing 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 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 freeze-drying 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 (S_BET) 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 S_BET of 365.99 m²/g (Table S1) but comparable to or higher than that of cellulose aerogels, SiC nanowire aerogels, and CNT aerogels with S_BET of about 100–300 m²/g,(35−37) 78 m²/g,(38) and 184 m²/g,(39) respectively.

Figure 2. Characterizations and properties of KNA films (ρ = 29.35 mg/cm³). (a,b) SEM images of surface morphology (a) and cross-sectional 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).

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 solution(35) 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 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 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 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.

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.

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/PEG_0.3, KNA/PEG_0.5, 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 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, 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.

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.

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 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 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.

Figure 5. Infrared stealth synergistic performance of KNA films and KNA/PEG films. (a–c) Thermal images of hot target covered by one-layer 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.

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 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.

• Reference QuickView

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• Abstract

• Results and Discussion

• Conclusions

• Methods

• References

References

This article references 53 other publications.

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   IR thermal emission from metals has important energy applications in thermophotovoltaics, radiative cooling, and lighting.  Unfortunately, the emissivity of flat metal films is close to zero because the screening effect prevents metals' fluctuating currents from emitting to the far field.  As a result, metal films are often used as reflecting mirrors instead of thermal emitters.  Recently, nanostructured metals, such as metamaterials, have emerged as an interesting way to enhance and to spectrally control thermal emission based on plasmonic resonant effects.  However, they require sophisticated lithog.  Here, the authors proposed and exptl. demonstrated a completely different mechanism to achieve spectrally selective metallic emitters based on a tunneling effect.  This effect allows a simple flat metal film to achieve a near-unity emissivity with controlled spectral selectivity for efficient heat-to-light energy conversion.  (c) 2015 American Institute of Physics.

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6. 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, DOI: 10.1038/416061a

   [Crossref], [PubMed], [CAS]

   6.

   Coherent emission of light by thermal sources

   Greffet, Jean-Jacques; Canninati, Rnmi; Joulain, Karl; Mulet, Jean-Philippe; Mainguy, Stiphane; Chen, Yong

   Nature (London, United Kingdom)        (2002),416(6876),     61-64CODEN:NATUAS;        ISSN:0028-0836.                (Nature Publishing Group)

   A thermal light-emitting source, such as a black body or the incandescent filament of a light bulb, is often presented as a typical example of an incoherent source and is in marked contrast to a laser.  Whereas a laser is highly monochromatic and very directional, a thermal source has a broad spectrum and is usually quasi-isotropic.  However, as is the case with many systems, different behavior can be expected on a microscopic scale.  It was shown recently that the field emitted by a thermal source made of a polar material is enhanced by >4 orders of magnitude and is partially coherent at a distance of the order of 10 to 100 nm.  By introducing a periodic microstructure into such a polar material (SiC) a thermal IR source can be fabricated that is coherent over large distances (many wavelengths) and radiates in well defined directions.  Narrow angular emission lobes similar to antenna lobes are obsd. and the emission spectra of the source, depends on the observation angle-the so-called Wolf effect.  The origin of the coherent emission lies in the diffraction of surface-phonon polaritons by the grating.

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7. 7.

   Inoue, T.; Zoysa, M. De; Asano, T.; Noda, S. Realization of Dynamic Thermal Emission Control. Nat. Mater. 2014,  13,  928, DOI: 10.1038/nmat4043

   [Crossref], [PubMed], [CAS]

   7.

   Realization of dynamic thermal emission control

   Inoue, Takuya; Zoysa, Menaka De; Asano, Takashi; Noda, Susumu

   Nature Materials        (2014),13(10),     928-931CODEN:NMAACR;        ISSN:1476-1122.                (Nature Publishing Group)

   Thermal emission in the IR range is important in various fields of research, including chem., medicine, and atm. science.  Recently, the possibility of controlling thermal emission based on wavelength-scale optical structures has been intensively investigated with a view towards a new generation of thermal emission devices.  However, all demonstrations so far have involved the 'static' control of thermal emission; high-speed modulation of thermal emission has proved difficult to achieve because the intensity of thermal emission from an object is usually detd. by its temp., and the frequency of temp. modulation is limited to 10-100 Hz even when the thermal mass of the object is small.  The authors exptl. demonstrate the dynamic control of thermal emission via the control of emissivity (absorptivity), at a speed four orders of magnitude faster than is possible using the conventional temp.-modulation method.  This approach is based on the dynamic control of intersubband absorption in n-type quantum wells, which is enhanced by an optical resonant mode in a photonic crystal slab.  The extn. of elec. carriers from the quantum wells leads to an immediate change in emissivity from 0.74 to 0.24 at the resonant wavelength while maintaining much lower emissivity at all other wavelengths.

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8. 8.

   Hutchins, M. G.; Butt, N. S.; Topping, A. J.; Gallego, J.; Milne, P.; Jeffrey, D.; Brotherston, I. Infrared Reflectance Modulation in Tungsten Oxide Based Electrochromic Devices. Electrochim. Acta 2001,  46,  1983– 1988, DOI: 10.1016/S0013-4686(01)00374-7

   [Crossref], [CAS]

   8.

   Infrared reflectance modulation in tungsten oxide based electrochromic devices

   Hutchins, M. G.; Butt, N. S.; Topping, A. J.; Gallego, J.; Milne, P.; Jeffrey, D.; Brotherston, I.

   Electrochimica Acta        (2001),46(13-14),     1983-1988CODEN:ELCAAV;        ISSN:0013-4686.                (Elsevier Science Ltd.)

   Tungsten oxide thin films prepd. by reactive magnetron sputtering are investigated for their use in IR reflectance modulation electrochromic devices.  The temp. dependence of the charge capacity and assocd. reflectance modulation are studied for W oxide films deposited in the temp. range 100-350°C.  Films were cycled in either 1 M lithium triflate in propylene carbonate or 1 M H2SO4 electrolytes.  The charge capacity of films of thickness ∼200 nm is obsd. to decrease with increasing deposition temp. from ∼50 to less than 25 mC cm-2 for films prepd. at the highest temps.  IR spectral optical properties were measured and the optical consts., nλ and kλ, then derived using an optical model.  For all films prepd. at 200°C and above the optical consts. are increased by ion insertion.  The dependence of nλ and kλ on the quantity of charge inserted for films prepd. at different substrate temps. is established.  The data enabled the design of variable reflectance devices utilizing either the charge dependence of nλ or of kλ.  Examples of such devices prepd. exptl. are presented.

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9. 9.

   Huang, Y.; Boriskina, S. V.; Chen, G. Electrically Tunable Near-Field Radiative Heat Transfer via Ferroelectric Materials. Appl. Phys. Lett. 2014,  105,  244102, DOI: 10.1063/1.4904456

   [Crossref], [CAS]

   9.

   Electrically tunable near-field radiative heat transfer via ferroelectric materials

   Huang, Yi; Boriskina, Svetlana V.; Chen, Gang

   Applied Physics Letters        (2014),105(24),     244102/1-244102/5CODEN:APPLAB;        ISSN:0003-6951.                (American Institute of Physics)

   The authors explore ways to actively control near-field radiative heat transfer between two surfaces that relies on elec. tuning of phonon modes of ferroelec. materials.  Ferroelecs. are widely used for tunable elec. devices, such as capacitors and memory devices; however, their tunable properties have not yet been examd. for heat transfer applications.  The authors show via simulations that radiative heat transfer between two ferroelec. materials can be enhanced by over two orders of magnitude over the blackbody limit in the near field, and can be tuned ≤16.5% by modulating the coupling between surface phonon polariton modes at the two surfaces via varying external elec. fields.  The authors then discuss how to maximize the modulation contrast for tunable thermal devices using the studied mechanism.  (c) 2014 American Institute of Physics.

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10. 10.

    Schuller, J. A.; Taubner, T.; Brongersma, M. L. Optical Antenna Thermal Emitters. Nat. Photonics 2009,  3,  658, DOI: 10.1038/nphoton.2009.188

    [Crossref], [CAS]

    10.

    Optical antenna thermal emitters

    Schuller, Jon A.; Taubner, Thomas; Brongersma, Mark L.

    Nature Photonics        (2009),3(11),     658-661CODEN:NPAHBY;        ISSN:1749-4885.                (Nature Publishing Group)

    Optical antennas are a crit. component in nanophotonics research and were used to enhance nonlinear and Raman cross sections and to make nanoscale optical probes.  In addn. to their receiving' properties, optical antennas can operate in broadcasting' mode, and were used to modify the emission rate and direction of individual mols.  In these applications the antenna must operate at frequencies given by existing light emitters.  Using thermal excitation of optical antennas, the authors bypass this limitation and realize emitters at IR frequencies where sources are less readily available.  Specifically, the thermal emission from a single SiC whisker antenna is attributable to well-defined, size-tunable Mie resonances.  Also, the authors derive a fundamental limit on the antenna emittance and argue theor. that these structures are nearly ideal black-body antennas.  Combined with advancing progress in antenna design, these results could lead to optical antenna emitters operating throughout the IR frequency range.

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11. 11.

    Xiao, L.; Ma, H.; Liu, J.; Zhao, W.; Jia, Y.; Zhao, Q.; Liu, K.; Wu, Y.; Wei, Y.; Fan, S.; Jiang, K. Fast Adaptive Thermal Camouflage Based on Flexible VO2/Graphene/CNT Thin Films. Nano Lett. 2015,  15,  8365– 8370, DOI: 10.1021/acs.nanolett.5b04090

    [ACS Full Text ], [CAS]

    11.

    Fast Adaptive Thermal Camouflage Based on Flexible VO2/Graphene/CNT Thin Films

    Xiao, Lin; Ma, He; Liu, Junku; Zhao, Wei; Jia, Yi; Zhao, Qiang; Liu, Kai; Wu, Yang; Wei, Yang; Fan, Shoushan; Jiang, Kaili

    Nano Letters        (2015),15(12),     8365-8370CODEN:NALEFD;        ISSN:1530-6984.                (American Chemical Society)

    Adaptive camouflage in thermal imaging, a form of cloaking technol. capable of blending naturally into the surrounding environment, has been a great challenge in the past decades.  Emissivity engineering for thermal camouflage is regarded as a more promising way compared to merely temp. controlling that has to dissipate a large amt. of excessive heat.  However, practical devices with an active modulation of emissivity have yet to be well explored.  In this letter we demonstrate an active cloaking device capable of efficient thermal radiance control, which consists of a vanadium dioxide (VO2) layer, with a neg. differential thermal emissivity, coated on a graphene/carbon nanotube (CNT) thin film.  A slight joule heating drastically changes the emissivity of the device, achieving rapid switchable thermal camouflage with a low power consumption and excellent reliability.  It is believed that this device will find wide applications not only in artificial systems for IR camouflage or cloaking but also in energy-saving smart windows and thermo-optical modulators.

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12. 12.

    Shen, L.; Zheng, B.; Liu, Z.; Wang, Z.; Lin, S.; Dehdashti, S.; Li, E.; Chen, H. Large-Scale Far-Infrared Invisibility Cloak Hiding Object from Thermal Detection. Adv. Opt. Mater. 2015,  3,  1738– 1742, DOI: 10.1002/adom.201500267

    [Crossref], [CAS]

    12.

    Large-Scale Far-Infrared Invisibility Cloak Hiding Object from Thermal Detection

    Shen, Lian; Zheng, Bin; Liu, Zuozhu; Wang, Zuojia; Lin, Shisheng; Dehdashti, Shahram; Li, Erping; Chen, Hongsheng

    Advanced Optical Materials        (2015),3(12),     1738-1742CODEN:AOMDAX;        ISSN:2195-1071.                (Wiley-VCH Verlag GmbH & Co. KGaA)

    Natural animals such as crotaline and boid snakes have the ability to detect IR radiation, which is emitted by any living object while it is invisible to human eyes.  To evade thermal detection, various attempts, such as the stealth or low observability technologies, are generally made to reduce the IR signature.  However, the validity of these technologies is background dependent.  In this paper, the first, large-scale, far-IR, unidirectional invisibility cloak in air environment is exptl. demonstrated.  The cloak constructed from germanium can smoothly guide the IR wave around the hidden object when facing the incident wave direction.  The cloak works for full polarization in a broad far-IR spectrum.  This work provides a new way to manipulate IR radiation for future applications.

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13. 13.

    Schittny, R.; Kadic, M.; Guenneau, S.; Wegener, M. Experiments on Transformation Thermodynamics: Molding the Flow of Heat. Phys. Rev. Lett. 2013,  110,  195901, DOI: 10.1103/PhysRevLett.110.195901

    [Crossref], [PubMed], [CAS]

    13.

    Experiments on transformation thermodynamics: molding the flow of heat

    Schittny, Robert; Kadic, Muamer; Guenneau, Sebastien; Wegener, Martin

    Physical Review Letters        (2013),110(19),     195901/1-195901/5CODEN:PRLTAO;        ISSN:0031-9007.                (American Physical Society)

    It was recently shown theor. that the time-dependent heat conduction equation is form invariant under curvilinear coordinate transformations.  Thus, in analogy to transformation optics, fictitious transformed space can be mapped onto (meta)materials with spatially inhomogeneous and anisotropic heat-cond. tensors in the lab. space.  On this basis, we design, fabricate, and characterize a microstructured thermal cloak that molds the flow of heat around an object in a metal plate.  This allows for transient protection of the object from heating while maintaining the same downstream heat flow as without object and cloak.

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14. 14.

    Xu, H.; Shi, X.; Gao, F.; Sun, H.; Zhang, B. Ultrathin Three-Dimensional Thermal Cloak. Phys. Rev. Lett. 2014,  112,  54301, DOI: 10.1103/PhysRevLett.112.054301

    [Crossref], [CAS]

    14.

    Ultrathin three-dimensional thermal cloak

    Xu, Hongyi; Shi, Xihang; Gao, Fei; Sun, Handong; Zhang, Baile

    Physical Review Letters        (2014),112(5),     054301/1-054301/5CODEN:PRLTAO;        ISSN:0031-9007.                (American Physical Society)

    We report the first exptl. realization of a three-dimensional thermal cloak shielding an air bubble in a bulk metal without disturbing the external conductive thermal flux.  The cloak is made of a thin layer of homogeneous and isotropic material with specially designed three-dimensional manufg.  The cloak's thickness is 100 μm while the cloaked air bubble has a diam. of 1 cm, achieving the ratio between dimensions of the cloak and the cloaked object 2 orders smaller than previous thermal cloaks, which were mainly realized in a two-dimensional geometry.  This work can find applications in novel thermal devices in the three-dimensional phys. space.

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15. 15.

    Han, G. G. D.; Li, H.; Grossman, J. C. Optically-Controlled Long-Term Storage and Release of Thermal Energy in Phase-Change Materials. Nat. Commun. 2017,  8,  1446, DOI: 10.1038/s41467-017-01608-y

    [Crossref], [PubMed], [CAS]

    15.

    Optically-controlled long-term storage and release of thermal energy in phase-change materials

    Han Grace G D; Li Huashan; Grossman Jeffrey C

    Nature communications        (2017),8(1),     1446        ISSN:.

    Thermal energy storage offers enormous potential for a wide range of energy technologies.  Phase-change materials offer state-of-the-art thermal storage due to high latent heat.  However, spontaneous heat loss from thermally charged phase-change materials to cooler surroundings occurs due to the absence of a significant energy barrier for the liquid-solid transition.  This prevents control over the thermal storage, and developing effective methods to address this problem has remained an elusive goal.  Herein, we report a combination of photo-switching dopants and organic phase-change materials as a way to introduce an activation energy barrier for phase-change materials solidification and to conserve thermal energy in the materials, allowing them to be triggered optically to release their stored latent heat.  This approach enables the retention of thermal energy (about 200 J g(-1)) in the materials for at least 10 h at temperatures lower than the original crystallization point, unlocking opportunities for portable thermal energy storage systems.

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16. 16.

    Wang, X.; Li, G.; Hong, G.; Guo, Q.; Zhang, X. Graphene Aerogel Templated Fabrication of Phase Change Microspheres as Thermal Buffers in Microelectronic Devices. ACS Appl. Mater. Interfaces 2017,  9,  41323– 41331, DOI: 10.1021/acsami.7b13969

    [ACS Full Text ], [CAS]

    16.

    Graphene Aerogel Templated Fabrication of Phase Change Microspheres as Thermal Buffers in Microelectronic Devices

    Wang, Xuchun; Li, Guangyong; Hong, Guo; Guo, Qiang; Zhang, Xuetong

    ACS Applied Materials & Interfaces        (2017),9(47),     41323-41331CODEN:AAMICK;        ISSN:1944-8244.                (American Chemical Society)

    Phase change materials, changing from solid to liq. and vice versa, are capable of storing and releasing a large amt. of thermal energy during the phase change, and thus hold promise for numerous applications including thermal protection of electronic devices.  Shaping these materials into microspheres for addnl. fascinating properties is efficient but challenging.  In this regard, a novel phase change microsphere with the design for elec.-regulation and thermal storage/release properties was fabricated via the combination of monodispersed graphene aerogel microsphere (GAM) and phase change paraffin.  A programmable method, i.e., coupling ink jetting-liq. marbling-supercrit. drying (ILS) techniques, was demonstrated to produce monodispersed graphene aerogel microspheres (GAMs) with precise size-control.  The resulting GAMs showed ultralow d., low elec. resistance, and high sp. surface area with only ca. 5% diam. variation coeff., and exhibited promising performance in smart switches.  The phase change microspheres were obtained by capillary filling of phase change paraffin inside the GAMs and exhibited excellent properties, such as low elec. resistance, high latent heat, well sphericity, and thermal buffering.  Assembling the phase change microsphere into the microcircuit, we found that this tiny device was quite sensitive and could respond to heat as low as 0.027 J.

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17. 17.

    Aftab, W.; Huang, X.; Wu, W.; Liang, Z.; Mahmood, A.; Zou, R. Nanoconfined Phase Change Materials for Thermal Energy Applications. Energy Environ. Sci. 2018,  11,  1392– 1424, DOI: 10.1039/C7EE03587J

    [Crossref], [CAS]

    17.

    Nanoconfined phase change materials for thermal energy applications

    Aftab, Waseem; Huang, Xinyu; Wu, Wenhao; Liang, Zibin; Mahmood, Asif; Zou, Ruqiang

    Energy & Environmental Science        (2018),11(6),     1392-1424CODEN:EESNBY;        ISSN:1754-5706.                (Royal Society of Chemistry)

    A review.  Phase change materials (PCMs) have been extensively characterized as const. temp. latent heat thermal energy storage (TES) materials.  Nevertheless, the widespread utilization of PCMs is limited due to the flow of liq. PCMs during melting, phase sepn., supercooling and low heat transfer rate.  In order to overcome these inherent problems and to improve thermo-phys. properties, the confinement of PCMs at the nanoscale has been identified as a versatile strategy, which ensures the encapsulation of PCMs in much smaller nano-containers.  Such strategies including core-shell, longitudinal, interfacial and porous confinement have been widely presented in recent years to efficiently encapsulate PCMs in nanospaces and are presenting attractive ways to enhance thermal performance.  This review summarizes the recent advancement and crit. issues of nanoconfinement technologies of PCMs from the point of view of material design.  In addn., the potential applications of nanoconfined PCMs in diverse fields, including energy conversion and storage, thermal rectification and temp. controlled drug delivery systems, are presented in detail.  Finally, the major drawbacks assocd. with nanoconfined PCMs and their prospective solns. are also provided.

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18. 18.

    Yu, Z.-L.; Yang, N.; Apostolopoulou-Kalkavoura, V.; Qin, B.; Ma, Z.-Y.; Xing, W.-Y.; Qiao, C.; Bergström, L.; Antonietti, M.; Yu, S.-H. Fire-Retardant and Thermally Insulating Phenolic-Silica Aerogels. Angew. Chem., Int. Ed. 2018,  57,  4538– 4542, DOI: 10.1002/anie.201711717

    [Crossref], [PubMed], [CAS]

    18.

    Fire-Retardant and Thermally Insulating Phenolic-Silica Aerogels

    Yu, Zhi-Long; Yang, Ning; Apostolopoulou-Kalkavoura, Varvara; Qin, Bing; Ma, Zhi-Yuan; Xing, Wei-Yi; Qiao, Chan; Bergstroem, Lennart; Antonietti, Markus; Yu, Shu-Hong

    Angewandte Chemie, International Edition        (2018),57(17),     4538-4542CODEN:ACIEF5;        ISSN:1433-7851.                (Wiley-VCH Verlag GmbH & Co. KGaA)

    Energy efficient buildings require materials with a low thermal cond. and a high fire resistance.  Traditional org. insulation materials are limited by their poor fire resistance and inorg. insulation materials are either brittle or display a high thermal cond.  Herein we report a mech. resilient org./inorg. composite aerogel with a thermal cond. significantly lower than expanded polystyrene and excellent fire resistance.  Co-polymn. and nanoscale phase sepn. of the phenol-formaldehyde-resin (PFR) and silica generate a binary network with domain sizes below 20 nm.  The PFR/SiO2 aerogel can resist a high-temp. flame without disintegration and prevents the temp. on the non-exposed side from increasing above the temp. crit. for the collapse of reinforced concrete structures.

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19. 19.

    Cuce, E.; Cuce, P. M.; Wood, C. J.; Riffat, S. B. Toward Aerogel Based Thermal Superinsulation in Buildings: A Comprehensive Review. Renewable Sustainable Energy Rev. 2014,  34,  273– 299, DOI: 10.1016/j.rser.2014.03.017

    [Crossref], [CAS]

    19.

    Toward aerogel based thermal superinsulation in buildings: A comprehensive review

    Cuce, Erdem; Cuce, Pinar Mert; Wood, Christopher J.; Riffat, Saffa B.

    Renewable & Sustainable Energy Reviews        (2014),34(),     273-299CODEN:RSERFH;        ISSN:1364-0321.                (Elsevier Ltd.)

    A review.  Aerogel is a kind of synthetic porous material, in which the liq. component of the gel is replaced with a gas.  Aerogel has specific acoustic properties and remarkably lower thermal cond. (≈0.013 W/m K) than the other com. insulating materials.  It also has superior phys. and chem. characteristics like the translucent structure.  Therefore, it is considered as one of the most promising thermal insulating materials for building applications.  Besides its applications in residential and industrial buildings, aerogel has a great deal of application areas such as spacecrafts, skyscrapers, automobiles, electronic devices, clothing etc.  Although current cost of aerogel still remains higher compared to the conventional insulation materials, intensive efforts are made to reduce its manufg. cost and hence enable it to become widespread all over the world.  In this study, a comprehensive review on aerogel and its utilization in buildings are presented.  Thermal insulation materials based on aerogel are illustrated with various applications.  Economic anal. and future potential of aerogel are also considered in the study.

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20. 20.

    Randall, J. P.; Meador, M. A. B.; Jana, S. C. Tailoring Mechanical Properties of Aerogels for Aerospace Applications. ACS Appl. Mater. Interfaces 2011,  3,  613– 626, DOI: 10.1021/am200007n

    [ACS Full Text ], [CAS]

    20.

    Tailoring Mechanical Properties of Aerogels for Aerospace Applications

    Randall, Jason P.; Meador, Mary Ann B.; Jana, Sadhan C.

    ACS Applied Materials & Interfaces        (2011),3(3),     613-626CODEN:AAMICK;        ISSN:1944-8244.                (American Chemical Society)

    A review.  Silica aerogels are highly porous solid materials consisting of three-dimensional networks of silica particles and are typically obtained by removing the liq. in silica gels under supercrit. conditions.  Several unique attributes such as extremely low thermal cond. and low d. make silica aerogels excellent candidates in the quest for thermal insulation materials used in space missions.  However, native silica aerogels are fragile at relatively low stresses.  More durable aerogels with higher strength and stiffness are obtained by proper selection of silane precursors and by reinforcement with polymers.  This paper first presents a brief review of the literature on methods of silica aerogel reinforcement and then discusses our recent activities in improving not only the strength but also the elastic response of polymer-reinforced silica aerogels.  Several alkyl-linked bis-silanes were used in promoting flexibility of the silica networks in conjunction with polymer reinforcement by epoxy.

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    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXis1altL0%253D&md5=4a4886f5b4d64ada61e19b8df0c6ca31
21. 21.

    Kobayashi, Y.; Saito, T.; Isogai, A. Aerogels with 3D Ordered Nanofiber Skeletons of Liquid-Crystalline Nanocellulose Derivatives as Tough and Transparent Insulators. Angew. Chem., Int. Ed. 2014,  53,  10394– 10397, DOI: 10.1002/anie.201405123

    [Crossref], [PubMed], [CAS]

    21.

    Aerogels with 3D Ordered Nanofiber Skeletons of Liquid-Crystalline Nanocellulose Derivatives as Tough and Transparent Insulators

    Kobayashi, Yuri; Saito, Tsuguyuki; Isogai, Akira

    Angewandte Chemie, International Edition        (2014),53(39),     10394-10397CODEN:ACIEF5;        ISSN:1433-7851.                (Wiley-VCH Verlag GmbH & Co. KGaA)

    Aerogels of high porosity and with a large internal surface area exhibit outstanding performances as thermal, acoustic, or elec. insulators.  However, most aerogels are mech. brittle and optically opaque, and the structural and phys. properties of aerogels strongly depend on their densities.  The unfavorable characteristics of aerogels are intrinsic to their skeletal structures consisting of randomly interconnected spherical nanoparticles.  A structurally new type of aerogel with a three-dimensionally ordered nanofiber skeleton of liq.-cryst. nanocellulose (LC-NCell) is now reported.  This LC-NCell material is composed of mech. strong, surface-carboxylated cellulose nanofibers dispersed in a nematic LC order.  The LC-NCell aerogels are transparent and combine mech. toughness and good insulation properties.  These properties of the LC-NCell aerogels could also be readily controlled.

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22. 22.

    Pierre, A. C.; Pajonk, G. M. Chemistry of Aerogels and Their Applications. Chem. Rev. 2002,  102,  4243– 4266, DOI: 10.1021/cr0101306

    [ACS Full Text ], [CAS]

    22.

    Chemistry of Aerogels and Their Applications

    Pierre, Alain C.; Pajonk, Gerard M.

    Chemical Reviews (Washington, DC, United States)        (2002),102(11),     4243-4265CODEN:CHREAY;        ISSN:0009-2665.                (American Chemical Society)

    A review of the chem. of aerogels and their industrial and other applications is presented.

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23. 23.

    Si, Y.; Fu, Q.; Wang, X.; Zhu, J.; Yu, J.; Sun, G.; Ding, B. Superelastic and Superhydrophobic Nanofiber-Assembled Cellular Aerogels for Effective Separation of Oil/Water Emulsions. ACS Nano 2015,  9,  3791– 3799, DOI: 10.1021/nn506633b

    [ACS Full Text ], [CAS]

    23.

    Superelastic and Superhydrophobic Nanofiber-Assembled Cellular Aerogels for Effective Separation of Oil/Water Emulsions

    Si, Yang; Fu, Qiuxia; Wang, Xueqin; Zhu, Jie; Yu, Jianyong; Sun, Gang; Ding, Bin

    ACS Nano        (2015),9(4),     3791-3799CODEN:ANCAC3;        ISSN:1936-0851.                (American Chemical Society)

    Fibrous, isotropically bonded elastic reconstructed (FIBER) aerogels with a hierarchical cellular structure and superelasticity have been prepd. by combining electrospun nanofibers and the freeze-shaping technique.  The intrinsically lamellar electrospun nanofibers were assembled into elastic bulk aerogels with tunable porous structure and wettability on a large scale.  The resulting FIBER aerogels exhibited the integrated properties of ultralow d. (< 30 mg cm-3), rapid recovery from 80% compression strain, superhydrophobic-superoleophilic wettability, and high pore tortuosity.  The FIBER aerogels can effectively sep. surfactant-stabilized water-in-oil emulsions, solely using gravity, with high flux (max. of 8140 ± 220 L m-2 h-1) and high sepn. efficiency, which match well with the requirements for treating the real emulsions.  The synthesis of FIBER aerogels also provides a versatile platform for exploring the applications of nanofibers in a self-supporting, structurally adaptive, and 3D macroscopic form.

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24. 24.

    Fan, Y.; Ma, W.; Han, D.; Gan, S.; Dong, X.; Niu, L. Convenient Recycling of 3D AgX/Graphene Aerogels (X = Br, Cl) for Efficient Photocatalytic Degradation of Water Pollutants. Adv. Mater. 2015,  27,  3767– 3773, DOI: 10.1002/adma.201500391

    [Crossref], [PubMed], [CAS]

    24.

    Convenient Recycling of 3D AgX/Graphene Aerogels (X =Br, Cl) for Efficient Photocatalytic Degradation of Water Pollutants

    Fan, Yingying; Ma, Weiguang; Han, Dongxue; Gan, Shiyu; Dong, Xiandui; Niu, Li

    Advanced Materials (Weinheim, Germany)        (2015),27(25),     3767-3773CODEN:ADVMEW;        ISSN:0935-9648.                (Wiley-VCH Verlag GmbH & Co. KGaA)

    We report a novel 3D AgX/GA (X =Br, Cl) structure, in which the AgX NPs are firmly and uniformly distributed throughout the surface of the 3D GA's hierarchically porous structure.  The recycling process gets very easy just with tweezers directly taking the 3D photocatalytic composite from one reaction system to another, which does not introduce any centrifugation, sonication, and drying processes.  In addn., due to their light wt., the 3D AgX/GAs are capable of being suspended in the aq. pollutant soln. without any assistance of external force and the hierarchically porous morphol. ensures that the photocatalyst NPs have effective contact with the contaminant mols.  Beyond those benefits of exptl. operation, the introduction of a GA substrate also improves the photocatalytic performance of Ag halide semiconductor significantly.  Compared with the pristine AgX, the preferable photocatalytic property of AgX/GAs is studied and confirmed through 2 degrdn. processes, the oxidative degrdn. of Methyl orange (MO) and the redn. of Cr(VI).  No matter the convenient mode of operation or the promoted photocatalytic performance of 3D AgX/GAs, they both provide robust support for the future industrial application of photocatalysts.

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25. 25.

    Zhu, C.; Liu, T.; Qian, F.; Han, T. Y.-J.; Duoss, E. B.; Kuntz, J. D.; Spadaccini, C. M.; Worsley, M. A.; Li, Y. Supercapacitors Based on Three-Dimensional Hierarchical Graphene Aerogels with Periodic Macropores. Nano Lett. 2016,  16,  3448– 3456, DOI: 10.1021/acs.nanolett.5b04965

    [ACS Full Text ], [CAS]

    25.

    Supercapacitors based on three-dimensional hierarchical graphene aerogels with periodic macropores

    Zhu, Cheng; Liu, Tianyu; Qian, Fang; Han, T. Yong-Jin; Duoss, Eric B.; Kuntz, Joshua D.; Spadaccini, Christopher M.; Worsley, Marcus A.; Li, Yat

    Nano Letters        (2016),16(6),     3448-3456CODEN:NALEFD;        ISSN:1530-6984.                (American Chemical Society)

    Graphene is an atomically thin, two-dimensional (2D) carbon material that offers a unique combination of low d., exceptional mech. properties, thermal stability, large surface area, and excellent elec. cond.  Recent progress has resulted in macro-assemblies of graphene, such as bulk graphene aerogels for a variety of applications.  However, these three-dimensional (3D) graphenes exhibit physicochem. property attenuation compared to their 2D building blocks because of one-fold compn. and tortuous, stochastic porous networks.  These limitations can be offset by developing a graphene composite material with an engineered porous architecture.  Here, we report the fabrication of 3D periodic graphene composite aerogel microlattices for supercapacitor applications, via a 3D printing technique known as direct-ink writing.  The key factor in developing these novel aerogels is creating an extrudable graphene oxide-based composite ink and modifying the 3D printing method to accommodate aerogel processing.  The 3D-printed graphene composite aerogel (3D-GCA) electrodes are lightwt., highly conductive, and exhibit excellent electrochem. properties.  In particular, the supercapacitors using these 3D-GCA electrodes with thicknesses on the order of millimeters display exceptional capacitive retention (ca. 90% from 0.5 to 10 A·g-1) and power densities (>4 kW·kg-1) that equal or exceed those of reported devices made with electrodes 10-100 times thinner.  This work provides an example of how 3D-printed materials, such as graphene aerogels, can significantly expand the design space for fabricating high-performance and fully integrable energy storage devices optimized for a broad range of applications.

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26. 26.

    Hao, P.; Zhao, Z.; Leng, Y.; Tian, J.; Sang, Y.; Boughton, R. I.; Wong, C. P.; Liu, H.; Yang, B. Graphene-Based Nitrogen Self-Doped Hierarchical Porous Carbon Aerogels Derived from Chitosan for High Performance Supercapacitors. Nano Energy 2015,  15,  9– 23, DOI: 10.1016/j.nanoen.2015.02.035

    [Crossref], [CAS]

    26.

    Graphene-based nitrogen self-doped hierarchical porous carbon aerogels derived from chitosan for high performance supercapacitors

    Hao, Pin; Zhao, Zhenhuan; Leng, Yanhua; Tian, Jian; Sang, Yuanhua; Boughton, Robert I.; Wong, C. P.; Liu, Hong; Yang, Bin

    Nano Energy        (2015),15(),     9-23CODEN:NEANCA;        ISSN:2211-2855.                (Elsevier Ltd.)

    Graphene-based nitrogen self-doped hierarchical porous carbon aerogels were synthesized for supercapacitor electrode application by using chitosan as a raw material through a carefully controlled aerogel formation-carbonization-activation process.  The as-synthesized N-doped graphene-based carbon aerogels contained both macropores and mesopores from the aerogel prepn. and carbonization process, and micropores from the chem. activation, confirmed by TEM, SEM, BET, etc.  Because chitosan is a nitrogen-contg. renewable biopolymer, the carbon aerogel derived from chitosan in this work was N-self-doped.  The carbonized carbon aerogel was composed of a graphene framework and amorphous carbon, and the ratio between these two components was controlled by the activation temp.  With an increase in activation temp., the amorphous carbon was etched away gradually, and a stable graphene portion remained to form a framework.  Accordingly, the performance of the graphene-based carbon aerogel as a supercapacitor varied with increasing activation temp.  Electrochem. investigation measurements showed that the N-doped graphene-based hierarchical porous carbon aerogel represents a good electrode candidate for construction of a solid sym. supercapacitor, which displays a high specific capacitance of about 197 F g-1 at a c.d. of 0.2 A g-1.  In addn., the solid state supercapacitor displayed excellent cyclability with a capacitance retention of about 92.1% over 10,000 cycles.  The excellent energy storage ability of the chitosan-derived hierarchical graphene-based carbon aerogels is ascribed to the high cond. of the graphene framework with nitrogen doping and the high storage ability of amorphous carbon with variable pore size and distribution.

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27. 27.

    Huang, X.; Liu, Z.; Xia, W.; Zou, R.; Han, R. P. S. Alkylated Phase Change Composites for Thermal Energy Storage Based on Surface-Modified Silica Aerogels. J. Mater. Chem. A 2015,  3,  1935– 1940, DOI: 10.1039/C4TA06735E

    [Crossref], [CAS]

    27.

    Alkylated phase change composites for thermal energy storage based on surface-modified silica aerogels

    Huang, Xinyu; Liu, Zhenpu; Xia, Wei; Zou, Ruqiang; Han, Ray P. S.

    Journal of Materials Chemistry A: Materials for Energy and Sustainability        (2015),3(5),     1935-1940CODEN:JMCAET;        ISSN:2050-7496.                (Royal Society of Chemistry)

    We alkylated silica aerogels to make them hydrophobic for effective impregnation and storage of a phase change material.  As a result of this surface modification treatment, the aerogel scaffold exhibited an av. increase of 20.9-34.7% in the phase change material uptake with an improved thermal energy storage capacity and stability.  For the light-to-thermal energy conversion expts., we carbonized the treated aerogels and obsd. that they readily attained temps. above the m.p. of the phase change material.  Therefore, the carbonized phase change material-impregnated scaffold possesses enhanced thermal energy storage and release property via a phase change response in the encapsulated phase change material.

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28. 28.

    Huang, X.; Xia, W.; Zou, R. Nanoconfinement of Phase Change Materials within Carbon Aerogels: Phase Transition Behaviours and Photo-to-Thermal Energy Storage. J. Mater. Chem. A 2014,  2,  19963– 19968, DOI: 10.1039/C4TA04605F

    [Crossref], [CAS]

    28.

    Nanoconfinement of phase change materials within carbon aerogels: phase transition behaviours and photo-to-thermal energy storage

    Huang, Xinyu; Xia, Wei; Zou, Ruqiang

    Journal of Materials Chemistry A: Materials for Energy and Sustainability        (2014),2(47),     19963-19968CODEN:JMCAET;        ISSN:2050-7496.                (Royal Society of Chemistry)

    We systematically investigated the thermal energy storage properties and host-guest interactions in a phase change composite based on octadecanol/carbon aerogels.  Due to the nanoconfinement effect induced by the carbon aerogels, the loaded active materials show distinct phase transition behavior to the free octadecanol, in which the solid-to-solid and solid-to-liq. phase change processes occurred at a lower temp. and the interval between these two processes became larger.

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29. 29.

    Li, G.; Hong, G.; Dong, D.; Song, W.; Zhang, X. Multiresponsive Graphene-Aerogel-Directed Phase-Change Smart Fibers. Adv. Mater. 2018,  30,  1801754, DOI: 10.1002/adma.201801754

    [Crossref]

    There is no corresponding record for this reference.
30. 30.

    Yang, M.; Cao, K.; Sui, L.; Qi, Y.; Zhu, J.; Waas, A.; Arruda, E. M.; Kieffer, J.; Thouless, M. D.; Kotov, N. A. Dispersions of Aramid Nanofibers: A New Nanoscale Building Block. ACS Nano 2011,  5,  6945– 6954, DOI: 10.1021/nn2014003

    [ACS Full Text ], [CAS]

    30.

    Dispersions of Aramid Nanofibers: A New Nanoscale Building Block

    Yang, Ming; Cao, Keqin; Sui, Lang; Qi, Ying; Zhu, Jian; Waas, Anthony; Arruda, Ellen M.; Kieffer, John; Thouless, M. D.; Kotov, Nicholas A.

    ACS Nano        (2011),5(9),     6945-6954CODEN:ANCAC3;        ISSN:1936-0851.                (American Chemical Society)

    A review.  Stable dispersions of nanofibers are virtually unknown for synthetic polymers.  They can complement analogous dispersions of inorg. components, such as nanoparticles, nanowires, nanosheets, etc. as a fundamental component of a toolset for design of nanostructures and metamaterials via numerous solvent-based processing methods.  As such, strong flexible polymeric nanofibers are very desirable for the effective utilization within composites of nanoscale inorg. components such as nanowires, carbon nanotubes, graphene, and others.  Here stable dispersions of uniform high-aspect-ratio aramid nanofibers (ANFs) with diams. between 3 and 30 nm and up to 10 μm in length were successfully obtained.  Unlike the traditional approaches based on polymn. of monomers, they are made by controlled dissoln. of std. macroscale form of the aramid polymer, i.e., well-known Kevlar threads, and revealed distinct morphol. features similar to carbon nanotubes.  ANFs are successfully processed into films using layer-by-layer (LBL) assembly as one of the potential methods of prepn. of composites from ANFs.  The resultant films are transparent and highly temp. resilient.  They also display enhanced mech. characteristics making ANF films highly desirable as protective coatings, ultrastrong membranes, as well as building blocks of other high performance materials in place of or in combination with carbon nanotubes.

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31. 31.

    Lyu, J.; Wang, X.; Liu, L.; Kim, Y.; Tanyi, E. K.; Chi, H.; Feng, W.; Xu, L.; Li, T.; Noginov, M. A.; Uher, C.; Hammig, M. D.; Kotov, N. A. High Strength Conductive Composites with Plasmonic Nanoparticles Aligned on Aramid Nanofibers. Adv. Funct. Mater. 2016,  26,  8435– 8445, DOI: 10.1002/adfm.201603230

    [Crossref], [CAS]

    31.

    High Strength Conductive Composites with Plasmonic Nanoparticles Aligned on Aramid Nanofibers

    Lyu, Jing; Wang, Xinzhi; Liu, Lehao; Kim, Yoonseob; Tanyi, Ekembu K.; Chi, Hang; Feng, Wenchun; Xu, Lizhi; Li, Tiehu; Noginov, Mikhail A.; Uher, Ctirad; Hammig, Mark D.; Kotov, Nicholas A.

    Advanced Functional Materials        (2016),26(46),     8435-8445CODEN:AFMDC6;        ISSN:1616-301X.                (Wiley-VCH Verlag GmbH & Co. KGaA)

    Rapidly evolving fields of biomedical, energy, and (opto)electronic devices bring forward the need for deformable conductors with constantly rising benchmarks for mech. properties and electronic cond.  The search for conductors with improved strength and strain have inspired the multiple studies of nanocomposites and amorphous metals.  However, finding conductors that defy the boundaries of classical materials and exhibit simultaneously high strength, toughness, and fast charge transport while enabling their scalable prodn., remains a difficult materials engineering challenge.  Here, composites made from aramid nanofibers (ANFs) and gold nanoparticles (Au NPs) that offer a new toolset for engineering high strength flexible conductors are described.  ANFs are derived from Kevlar macrofibers and retain their strong mech. properties and temp. resilience.  Au NPs are infiltrated into a porous, free-standing aramid matrix, becoming aligned on ANFs, which reduces the charge percolation threshold and facilitates charge transport.  Further thermal annealing at 300 °C results in the Au-ANF composites with an elec. cond. of 1.25 × 104 S cm-1 combined with a tensile strength of 96 MPa, a Young's modulus of 5.29 GPa, and a toughness of 1.3 MJ m-3.  These parameters exceed those of most of the composite materials, and are comparable to those of amorphous metals but have no vol. limitations.  The plasmonic optical frequencies characteristic for constituent NPs are present in the composites with ANFs enabling plasmon-based optoelectronic applications.

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32. 32.

    Lyu, J.; Hammig, M. D.; Liu, L.; Xu, L.; Chi, H.; Uher, C.; Li, T.; Kotov, N. A. Stretchable Conductors by Kirigami Patterning of Aramid-Silver Nanocomposites with Zero Conductance Gradient. Appl. Phys. Lett. 2017,  111,  161901, DOI: 10.1063/1.5001094

    [Crossref], [CAS]

    32.

    Stretchable conductors by kirigami patterning of aramid-silver nanocomposites with zero conductance gradient

    Lyu, Jing; Hammig, Mark D.; Liu, Lehao; Xu, Lizhi; Chi, Hang; Uher, Ctirad; Li, Tiehu; Kotov, Nicholas A.

    Applied Physics Letters        (2017),111(16),     161901/1-161901/5CODEN:APPLAB;        ISSN:0003-6951.                (American Institute of Physics)

    Materials that are both stretchable and elec. conductive enable a broad spectrum of applications in sensing, actuating, electronics, optics and energy storage.  The materials engineering concept of stretchable conductors is primarily based on combining nanowires, nanoribbons, nanoparticles, or nanocarbons with rubbery polymers to obtain composites with different abilities to transport charge and alter their nanoscale organization under strain.  Although some of these composites reveal remarkably interesting multiscale reconfigurability and self-assembly phenomena, decreasing conductance with increased strain has restricted their widespread implementation.  In a broader phys. sense, the dependence of conductance on stress is undesirable because it requires a correlated change of elec. inputs.  In this paper, we describe highly conductive and deformable sheets with a cond. as high as 230 000 S cm-1, composed of silver nanoparticles, infiltrated within a porous aramid nanofiber (ANF) matrix.  By forming a kirigami pattern, consisting of a regularized network of notches cut within the films, their ultimate tensile strain is improved from ∼2% to beyond 100%.  The use of ANFs derived from well-known ultrastrong Kevlar fibers imparts high mech. performance to the base composite.  Importantly, the conductance of the films remains const., even under large deformation resulting in a material with a zero conductance gradient.  Unlike other nanocomposites for which strain and conductance are strongly coupled, the kirigami nanocomposite provides a pathway to demanding applications for flexible and stretchable electronics with power/voltage being unaffected by the deformation mode and temp.  (c) 2017 American Institute of Physics.

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33. 33.

    Lyu, J.; Zhao, X.; Hou, X.; Zhang, Y.; Li, T.; Yan, Y. Electromagnetic Interference Shielding Based on a High Strength Polyaniline-Aramid Nanocomposite. Compos. Sci. Technol. 2017,  149,  159– 165, DOI: 10.1016/j.compscitech.2017.06.026

    [Crossref]

    There is no corresponding record for this reference.
34. 34.

    Zhu, J.; Yang, M.; Emre, A.; Bahng, J. H.; Xu, L.; Yeom, J.; Yeom, B.; Kim, Y.; Johnson, K.; Green, P.; Kotov, N. A. Branched Aramid Nanofibers. Angew. Chem., Int. Ed. 2017,  56,  11744– 11748, DOI: 10.1002/anie.201703766

    [Crossref], [PubMed], [CAS]

    34.

    Branched Aramid Nanofibers

    Zhu, Jian; Yang, Ming; Emre, Ahmet; Bahng, Joong Hwan; Xu, Lizhi; Yeom, Jihyeon; Yeom, Bongjun; Kim, Yoonseob; Johnson, Kyle; Green, Peter; Kotov, Nicholas A.

    Angewandte Chemie, International Edition        (2017),56(39),     11744-11748CODEN:ACIEF5;        ISSN:1433-7851.                (Wiley-VCH Verlag GmbH & Co. KGaA)

    Interconnectivity of components in three-dimensional networks (3DNs) is essential for stress transfer in hydrogels, aerogels, and composites.  Entanglement of nanoscale components in the network relies on weak short-range intermol. interactions.  The intrinsic stiffness and rod-like geometry of nanoscale components limit the cohesive energy of the phys. crosslinks in 3DN materials.  Nature realizes networked gels differently using components with extensive branching.  Branched aramid nanofibers (BANFs) mimicking polymeric components of biol. gels were synthesized to produce 3DNs with high efficiency stress transfer.  Individual BANFs are flexible, with the no. of branches controlled by base strength in the hydrolysis process.  The extensive connectivity of the BANFs allows them to form hydro- and aerogel monoliths with an order of magnitude less solid content than rod-like nanocomponents.  Branching of nanofibers also leads to improved mechanics of gels and nanocomposites.

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35. 35.

    Wan, J.; Zhang, J.; Yu, J.; Zhang, J. Cellulose Aerogel Membranes with a Tunable Nanoporous Network as a Matrix of Gel Polymer Electrolytes for Safer Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2017,  9,  24591– 24599, DOI: 10.1021/acsami.7b06271

    [ACS Full Text ], [CAS]

    35.

    Cellulose Aerogel Membranes with a Tunable Nanoporous Network as a Matrix of Gel Polymer Electrolytes for Safer Lithium-Ion Batteries

    Wan, Jiqiang; Zhang, Jinming; Yu, Jian; Zhang, Jun

    ACS Applied Materials & Interfaces        (2017),9(29),     24591-24599CODEN:AAMICK;        ISSN:1944-8244.                (American Chemical Society)

    Cellulose aerogel membranes (CAMs) are proposed as a matrix for gel polymer electrolyte to the fabrication of lithium-ion batteries (LIBs) with superior thermal stability.  The CAMs are obtained from a cellulose-ionic liq. soln. via a dissoln.-regeneration-supercrit. drying route.  The presence of high porosity, the nanoporous network structure, and numerous polar hydroxyl groups benefits the quick absorption of liq. electrolytes for gelation of the CAMs and improves the ionic cond. of the gelled CAMs.  LIBs assembled with the gelled CAMs display excellent electrochem. performance at room temp., and more importantly, the intrinsic thermal resistance of cellulose allows the LIBs to run stably for at least 30 min at working temps. as high as 120 °C.  The CAMs, with their excellent thermal stability, are promising for the development of highly safe, cost-effective, and high-performance LIBs.

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36. 36.

    Toivonen, M. S.; Kaskela, A.; Rojas, O. J.; Kauppinen, E. I.; Ikkala, O. Ambient-Dried Cellulose Nanofibril Aerogel Membranes with High Tensile Strength and Their Use for Aerosol Collection and Templates for Transparent, Flexible Devices. Adv. Funct. Mater. 2015,  25,  6618– 6626, DOI: 10.1002/adfm.201502566

    [Crossref], [CAS]

    36.

    Ambient-Dried Cellulose Nanofibril Aerogel Membranes with High Tensile Strength and Their Use for Aerosol Collection and Templates for Transparent, Flexible Devices

    Toivonen, Matti S.; Kaskela, Antti; Rojas, Orlando J.; Kauppinen, Esko I.; Ikkala, Olli

    Advanced Functional Materials        (2015),25(42),     6618-6626CODEN:AFMDC6;        ISSN:1616-301X.                (Wiley-VCH Verlag GmbH & Co. KGaA)

    The application potential of cellulose nanofibril (CNF) aerogels has been hindered by the slow and costly freeze- or supercrit. drying methods.  Here, CNF aerogel membranes with attractive mech., optical, and gas transport properties are prepd. in ambient conditions with a facile and scalable process.  Aq. CNF dispersions are vacuum-filtered and solvent exchanged to 2-propanol and further to octane, followed by ambient drying.  The resulting CNF aerogel membranes are characterized by high transparency (>90% transmittance), stiffness (6 GPa Young's modulus, 10 GPa cm3 g-1 specific modulus), strength (97 MPa tensile strength, 161 MPa m3 kg-1 specific strength), mesoporosity (pore diam. 10-30 nm, 208 m2 g-1 sp. surface area), and low d. (≈0.6 g cm-3).  They are gas permeable thus enabling collection of nanoparticles (for example, single-walled carbon nanotubes, SWNT) from aerosols under pressure gradients.  The membranes with deposited SWNT can be further compacted to transparent, conductive, and flexible conducting films (90% specular transmittance at 550 nm and 300 Ω .box.-1 sheet resistance with AuCl3-salt doping).  Overall, the developed aerogel membranes pave way toward use in gas filtration and transparent, flexible devices.

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37. 37.

    Nemoto, J.; Saito, T.; Isogai, A. Simple Freeze-Drying Procedure for Producing Nanocellulose Aerogel-Containing, High-Performance Air Filters. ACS Appl. Mater. Interfaces 2015,  7,  19809– 19815, DOI: 10.1021/acsami.5b05841

    [ACS Full Text ], [CAS]

    37.

    Simple Freeze-Drying Procedure for Producing Nanocellulose Aerogel-Containing, High-Performance Air Filters

    Nemoto, Junji; Saito, Tsuguyuki; Isogai, Akira

    ACS Applied Materials & Interfaces        (2015),7(35),     19809-19815CODEN:AAMICK;        ISSN:1944-8244.                (American Chemical Society)

    Simple freeze-drying of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibril (TOCN) dispersions in water/tert-Bu alc. (TBA) mixts. was conducted to prep. TOCN aerogels as high-performance air filter components.  The dispersibility of the TOCNs in the water/TBA mixts., and the sp. surface area (SSA) of the resulting TOCN aerogels, was investigated as a function of the TBA concn. in the mixts.  The TOCNs were homogeneously dispersed in the water/TBA mixts. at TBA concns. up to 40% wt./wt.  The SSAs of the TOCN aerogels exceeded 300 m2/g when the TBA concn. in the aq. mixts. was in the range from 20-50% wt./wt.  When a com. available, high-efficiency particulate air (HEPA) filter was combined with TOCN/water/TBA dispersions prepd. using 30% TBA, and the product was freeze-dried, the resulting TOCN aerogel-contg. filters showed superior filtration properties.  This was because nanoscale, spider-web-like networks of the TOCNs with large SSAs were formed within the filter.

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38. 38.

    Su, L.; Wang, H.; Niu, M.; Fan, X.; Ma, M.; Shi, Z.; Guo, S.-W. Ultralight, Recoverable, and High-Temperature-Resistant SiC Nanowire Aerogel. ACS Nano 2018,  12,  3103– 3111, DOI: 10.1021/acsnano.7b08577

    [ACS Full Text ], [CAS]

    38.

    Ultralight, Recoverable, and High-Temperature-Resistant SiC Nanowire Aerogel

    Su, Lei; Wang, Hongjie; Niu, Min; Fan, Xingyu; Ma, Mingbo; Shi, Zhongqi; Guo, Sheng-Wu

    ACS Nano        (2018),12(4),     3103-3111CODEN:ANCAC3;        ISSN:1936-0851.                (American Chemical Society)

    Ultralight ceramic aerogels with the property combination of recoverable compressibility and excellent high-temp. stability are attractive for use in harsh environments.  However, conventional ceramic aerogels are usually constructed by oxide ceramic nanoparticles, and their practical applications have always been limited by the brittle nature of ceramics and vol. shrinkage at high temp.  Silicon carbide (SiC) nanowire offers the integrated properties of elasticity and flexibility of 1-dimensional (1D) nanomaterials and superior high-temp. thermal and chem. stability of SiC ceramics, which makes it a promising building block for compressible ceramic nanowire aerogels (NWAs).  Here, we report the fabrication and properties of a highly porous 3-dimensional (3D) SiC NWA assembled by a large no. of interweaving 3C-SiC nanowires of 20-50 nm diam. and tens to hundreds of micrometers in length.  The SiC NWA possesses ultralow d. (∼5 mg/cm3), excellent mech. properties of large recoverable compression strain (>70%) and fatigue resistance, refractory property, oxidn. and high-temp. resistance, and thermal insulating property (0.026 W/m/K at room temp. in N2).  When used as absorbents, the SiC NWAs exhibit an adsorption selectivity of low-viscosity org. solvents with high absorption capacity (130-237 g/g).  The successful fabrication of such an attractive material may provide promising perspectives to the design and fabrication of other compressible and multifunctional ceramic NWAs.

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39. 39.

    Worsley, M. A.; Kucheyev, S. O.; Kuntz, J. D.; Hamza, A. V.; Satcher, J. H.; Baumann, T. F. Stiff and Electrically Conductive Composites of Carbon Nanotube Aerogels and Polymers. J. Mater. Chem. 2009,  19,  3370– 3372, DOI: 10.1039/b905735h

    [Crossref], [CAS]

    39.

    Stiff and electrically conductive composites of carbon nanotube aerogels and polymers

    Worsley, Marcus A.; Kucheyev, Sergei O.; Kuntz, Joshua D.; Hamza, Alex V.; Satcher, Joe H., Jr.; Baumann, Theodore F.

    Journal of Materials Chemistry        (2009),19(21),     3370-3372CODEN:JMACEP;        ISSN:0959-9428.                (Royal Society of Chemistry)

    Highly elec. conductive and mech. stiff composites of polymers and single-walled carbon nanotubes (SWNT) were developed.  Conductive SWNT-based nanofoams (aerogels) were used as scaffolds to fabricate composites with poly(dimethylsiloxane).  The resulting composites possess elec. cond. over 1 S cm-1 and exhibit ∼300% increase in the elastic modulus with as little as 1 vol% nanotube content.

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40. 40.

    Si, Y.; Yu, J.; Tang, X.; Ge, J.; Ding, B. Ultralight Nanofibre-Assembled Cellular Aerogels with Superelasticity and Multifunctionality. Nat. Commun. 2014,  5,  5802, DOI: 10.1038/ncomms6802

    [Crossref], [PubMed], [CAS]

    40.

    Ultralight nanofibre-assembled cellular aerogels with superelasticity and multifunctionality

    Si, Yang; Yu, Jianyong; Tang, Xiaomin; Ge, Jianlong; Ding, Bin

    Nature Communications        (2014),5(),     5802pp.CODEN:NCAOBW;        ISSN:2041-1723.                (Nature Publishing Group)

    Three-dimensional nanofibrous aerogels (NFAs) that are both highly compressible and resilient would have broad technol. implications for areas ranging from elec. devices and bioengineering to damping materials; however, creating such NFAs has proven extremely challenging.  Here we report a novel strategy to create fibrous, isotropically bonded elastic reconstructed (FIBER) NFAs with a hierarchical cellular structure and superelasticity by combining electrospun nanofibers and the fibrous freeze-shaping technique.  Our approach causes the intrinsically lamellar deposited electrospun nanofibers to assemble into elastic bulk aerogels with tunable densities and desirable shapes on a large scale.  The resulting FIBER NFAs exhibit densities of >0.12 mg cm-3, rapid recovery from deformation, efficient energy absorption and multifunctionality in terms of the combination of thermal insulation, sound absorption, emulsion sepn. and elasticity-responsive elec. conduction.  The successful synthesis of such fascinating materials may provide new insights into the design and development of multifunctional NFAs for various applications.

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41. 41.

    Li, T.; Song, J.; Zhao, X.; Yang, Z.; Pastel, G.; Xu, S.; Jia, C.; Dai, J.; Chen, C.; Gong, A.; Jiang, F.; Yao, Y.; Fan, T.; Yang, B.; Wågberg, L.; Yang, R.; Hu, L. Anisotropic, Lightweight, Strong, and Super Thermally Insulating Nanowood with Naturally Aligned Nanocellulose. Sci. Adv. 2018,  4, eaar3724 DOI: 10.1126/sciadv.aar3724

    [Crossref], [PubMed]

    There is no corresponding record for this reference.
42. 42.

    Cui, Y.; Gong, H.; Wang, Y.; Li, D.; Bai, H. A Thermally Insulating Textile Inspired by Polar Bear Hair. Adv. Mater. 2018,  30,  1706807, DOI: 10.1002/adma.201706807

    [Crossref]

    There is no corresponding record for this reference.
43. 43.

    Li, H.; Fang, G.-Y. Experimental Investigation on the Characteristics of Polyethylene Glycol/Cement Composites as Thermal Energy Storage Materials. Chem. Eng. Technol. 2010,  33,  1650– 1654, DOI: 10.1002/ceat.201000102

    [Crossref], [CAS]

    43.

    Experimental investigation on the characteristics of polyethylene glycol/cement composites as thermal energy storage materials

    Li, Hui; Fang, Gui-Yin

    Chemical Engineering & Technology        (2010),33(10),     1650-1654CODEN:CETEER;        ISSN:0930-7516.                (Wiley-VCH Verlag GmbH & Co. KGaA)

    The polyethylene glycol/cement composites as thermal energy storage materials were prepd. by blending polyethylene glycol and cement.  In composite materials, polyethylene glycol (PEG) is used as the phase change material for thermal energy storage and cement acts as the supporting material.  A Fourier transformation IR spectroscope (FT-IR), x-ray diffractometer (XRD), and scanning electronic microscope (SEM) were used to det. the chem. structure, the crystalloid phase, and microstructure of the polyethylene glycol/cement composites, resp.  The thermal properties and thermal stability were investigated by a differential scanning calorimeter (DSC) and a thermogravimetry analyzer (TGA).  The SEM results showed that the polyethylene glycol was well dispersed in the porous network of the cement.

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44. 44.

    Wang, Y.; Tang, B.; Zhang, S. Single-Walled Carbon Nanotube/Phase Change Material Composites: Sunlight-Driven, Reversible, Form-Stable Phase Transitions for Solar Thermal Energy Storage. Adv. Funct. Mater. 2013,  23,  4354– 4360, DOI: 10.1002/adfm.201203728

    [Crossref], [CAS]

    44.

    Single-Walled Carbon Nanotube/Phase Change Material Composites: Sunlight-Driven, Reversible, Form-Stable Phase Transitions for Solar Thermal Energy Storage

    Wang, Yunming; Tang, Bingtao; Zhang, Shufen

    Advanced Functional Materials        (2013),23(35),     4354-4360CODEN:AFMDC6;        ISSN:1616-301X.                (Wiley-VCH Verlag GmbH & Co. KGaA)

    The development of solar energy conversion materials is crit. to the growth of a sustainable energy infrastructure in the coming years.  A novel hybrid material based on single-walled carbon nanotubes (SWNTs) and form-stable polymer phase change materials (PCMs) is reported.  The obtained materials have UV-vis sunlight harvesting, light-thermal conversion, thermal energy storage, and form-stable effects.  Judicious application of this efficient photothermal conversion to SWNTs has opened up a rich field of energy materials based on novel SWNT/PCM composites with enhanced performance in energy conversion and storage.

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    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXnsV2isb0%253D&md5=49ce0dcef2dcdde6a8c145aaad113b01
45. 45.

    Yang, J.; Yu, P.; Tang, L.-S.; Bao, R.-Y.; Liu, Z.-Y.; Yang, M.-B.; Yang, W. Hierarchically Interconnected Porous Scaffolds for Phase Change Materials with Improved Thermal Conductivity and Efficient Solar-to-Electric Energy Conversion. Nanoscale 2017,  9,  17704– 17709, DOI: 10.1039/C7NR05449A

    [Crossref], [PubMed], [CAS]

    45.

    Hierarchically interconnected porous scaffolds for phase change materials with improved thermal conductivity and efficient solar-to-electric energy conversion

    Yang, Jie; Yu, Peng; Tang, Li-Sheng; Bao, Rui-Ying; Liu, Zheng-Ying; Yang, Ming-Bo; Yang, Wei

    Nanoscale        (2017),9(45),     17704-17709CODEN:NANOHL;        ISSN:2040-3372.                (Royal Society of Chemistry)

    An ice-templating self-assembly strategy and a vacuum impregnation method were used to fabricate polyethylene glycol (PEG)/hierarchical porous scaffold composite phase change materials (PCMs).  Hierarchically interconnected porous scaffolds of boron nitride (BN), with the aid of a small amt. of graphene oxide (GO), endow the composite PCMs with high thermal cond., excellent shape-stability and efficient solar-to-elec. energy conversion.  The formation of a three-dimensional (3D) thermally conductive pathway in the composites contributes to improving the thermal cond. up to 2.36 W m-1 K-1 at a relatively low content of BN (ca. 23 wt%).  This work provides a route for thermally conductive and shape-stabilized composite PCMs used as energy storage materials.

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46. 46.

    Anghel, E. M.; Pavel, P. M.; Constantinescu, M.; Petrescu, S.; Atkinson, I.; Buixaderas, E. Thermal Transfer Performance of a Spherical Encapsulated PEG 6000-Based Composite for Thermal Energy Storage. Appl. Energy 2017,  208,  1222– 1231, DOI: 10.1016/j.apenergy.2017.09.031

    [Crossref], [CAS]

    46.

    Thermal transfer performance of a spherical encapsulated PEG 6000-based composite for thermal energy storage

    Anghel, E. M.; Pavel, P. M.; Constantinescu, M.; Petrescu, S.; Atkinson, I.; Buixaderas, E.

    Applied Energy        (2017),208(),     1222-1231CODEN:APENDX;        ISSN:0306-2619.                (Elsevier Ltd.)

    A polymeric phase change composite material (70 wt% polyethylene glycol, PEG, 6000)-epoxy resin (29 wt%) with aluminum nanopowder (1 wt%) as filler, P60-E, was developed and thermally tested first in a spherical macro capsule in order to be used in thermal energy storage (TES) systems in constructions with low energy consumption.  Since the thermal behavior of the phase change component, PEG 6000, is highly influenced by its crystn. behavior, structural and thermal data were correlated.  Consequently a high crystallinity degree of 82.6%, found by X-ray diffraction (XRD), for the PEG 6000 component is analogous with values obtained from integrated Raman spectra and DSC data (latent heat of -113.6 J/g) collected at a cooling rate of 0.4 °C/min.  Both exptl. and math. modeling of PEG 6000 solidification in the P60-E nanocomposite was conducted using a single spherical test cell.  The heat transfer during solidification assumes time evolution of both liq. and the two solid radial fronts corresponding to cryst. chains of PEG and amorphous counterpart of PEG and epoxy resin in the P60-E composite.  Good agreement between exptl. values and calcd. theor. curves was found by using a two-front solids model.

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47. 47.

    Tanner, D.; Fitzgerald, J. A.; Phillips, B. R. The Kevlar Story—an Advanced Materials Case Study. Angew. Chem., Int. Ed. Engl. 1989,  28,  649– 654, DOI: 10.1002/anie.198906491

    [Crossref]

    There is no corresponding record for this reference.
48. 48.

    Tang, H.; Butchosa, N.; Zhou, Q. A Transparent, Hazy, and Strong Macroscopic Ribbon of Oriented Cellulose Nanofibrils Bearing Poly(Ethylene Glycol). Adv. Mater. 2015,  27,  2070– 2076, DOI: 10.1002/adma.201404565

    [Crossref], [PubMed], [CAS]

    48.

    A Transparent, Hazy, and Strong Macroscopic Ribbon of Oriented Cellulose Nanofibrils Bearing Poly(ethylene glycol)

    Tang, Hu; Butchosa, Nuria; Zhou, Qi

    Advanced Materials (Weinheim, Germany)        (2015),27(12),     2070-2076CODEN:ADVMEW;        ISSN:0935-9648.                (Wiley-VCH Verlag GmbH & Co. KGaA)

    This study presents for the first time the covalent grafting of PEF onto the surfaces of cellulose nanofibrils.  This work demonstrates how these individual cellulose nanofibrils bearing PEG can be aligned into macroscopic ribbons with remarkably improved fibril orientation and mech. properties compared to previous studies.

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49. 49.

    Kuang, Q.; Zhang, D.; Yu, J. C.; Chang, Y. W.; Yue, M.; Hou, Y.; Yang, M. Toward Record-High Stiffness in Polyurethane Nanocomposites Using Aramid Nanofibers. J. Phys. Chem. C 2015,  119,  27467– 27477, DOI: 10.1021/acs.jpcc.5b08856

    [ACS Full Text ], [CAS]

    49.

    Toward Record-High Stiffness in Polyurethane Nanocomposites Using Aramid Nanofibers

    Kuang, Qingxia; Zhang, Dan; Yu, Jae Chul; Chang, Young-Wook; Yue, Mingli; Hou, Ying; Yang, Ming

    Journal of Physical Chemistry C        (2015),119(49),     27467-27477CODEN:JPCCCK;        ISSN:1932-7447.                (American Chemical Society)

    Elastomers such as polyurethanes usually possess low stiffness, and the addn. of traditional fillers typically results in a moderate improvement.  Aramid nanofibers (ANFs) represent one of the most promising nanoscale building blocks for high-performance nanocomposites.  In this work, waterborne polyurethanes (PUs) have been reinforced with ANFs using two soln. processing methods, namely, layer-by-layer (LBL) assembly technique and the vacuum-assisted flocculation (VAF) method.  Record-high modulus of 5.275 GPa and ultimate strength of 98.02 MPa are obtained among all the reported PU based nanocomposites.  We attribute such achievement to the similar mol. structures of ANFs with PUs which ensures a high affinity made possible by the manifold interfacial interactions.  The formation of multiple hydrogen bonds due to the presence of amide groups with appropriate spacing in both components is confirmed by the computer simulation.  Compared with the VAF method, it is found that LBL assembly allows a better load transfer, resulting in higher ultimate strength and stiffness.  The VAF method shows advantages in improving the ultimate strength at low loadings of ANFs.  We believe our work may not only lead to a new practical combination within the field of composite materials but also provide important implications for the future design of nanocomposites based on the innovative nanofillers.

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50. 50.

    Moghimi, M. J.; Lin, G.; Jiang, H. Broadband and Ultrathin Infrared Stealth Sheets. Adv. Eng. Mater. 2018,  20,  1800038, DOI: 10.1002/adem.201800038

    [Crossref]

    There is no corresponding record for this reference.
51. 51.

    Mandal, J.; Fu, Y.; Overvig, A.; Jia, M.; Sun, K.; Shi, N.; Zhou, H.; Xiao, X.; Yu, N.; Yang, Y. Hierarchically Porous Polymer Coatings for Highly Efficient Passive Daytime Radiative Cooling. Science 2018,  362,  315– 319, DOI: 10.1126/science.aat9513

    [Crossref], [PubMed], [CAS]

    51.

    Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling

    Mandal, Jyotirmoy; Fu, Yanke; Overvig, Adam C.; Jia, Mingxin; Sun, Kerui; Shi, Norman N.; Zhou, Hua; Xiao, Xianghui; Yu, Nanfang; Yang, Yuan

    Science (Washington, DC, United States)        (2018),362(6412),     315-319CODEN:SCIEAS;        ISSN:0036-8075.                (American Association for the Advancement of Science)

    Passive daytime radiative cooling (PDRC) involves spontaneously cooling a surface by reflecting sunlight and radiating heat to the cold outer space.  Current PDRC designs are promising alternatives to elec. cooling but are either inefficient or have limited applicability.  A simple, inexpensive, and scalable phase inversion-based method is presented for fabricating hierarchically porous poly(vinylidene fluoride-co-hexafluoropropene) [P(VdF-HFP)HP] coatings with excellent PDRC capability.  High, substrate-independent hemispherical solar reflectances (0.96 ± 0.03) and long-wave IR emittances (0.97 ± 0.02) allow for subambient temp. drops of ∼ 6° and cooling powers of ∼96 W per square meter (W m-2) under solar intensities of 890 and 750 W m-2, resp.  The performance equals or surpasses those of state-of-the-art PDRC designs, and the technique offers a paint-like simplicity.

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52. 52.

    Kazemi Moridani, A.; Zando, R.; Xie, W.; Howell, I.; Watkins, J. J.; Lee, J.-H. Plasmonic Thermal Emitters for Dynamically Tunable Infrared Radiation. Adv. Opt. Mater. 2017,  5,  1600993, DOI: 10.1002/adom.201600993

    [Crossref]

    There is no corresponding record for this reference.
53. 53.

    Pielichowska, K.; Pielichowski, K. Phase Change Materials for Thermal Energy Storage. Prog. Mater. Sci. 2014,  65,  67– 123, DOI: 10.1016/j.pmatsci.2014.03.005

    [Crossref], [CAS]

    53.

    Phase change materials for thermal energy storage

    Pielichowska, Kinga; Pielichowski, Krzysztof

    Progress in Materials Science        (2014),65(),     67-123CODEN:PRMSAQ;        ISSN:0079-6425.                (Elsevier Ltd.)

    A review.  Phase change materials (PCMs) used for the storage of thermal energy as sensible and latent heat are an important class of modern materials which substantially contribute to the efficient use and conservation of waste heat and solar energy.  The storage of latent heat provides a greater d. of energy storage with a smaller temp. difference between storing and releasing heat than the sensible heat storage method.  Many different groups of materials have been investigated during the tech. evolution of PCMs, including inorg. systems (salt and salt hydrates), org. compds. such as paraffins or fatty acids and polymeric materials, e.g. poly(ethylene glycol).  Historically, the relationships between the structure and the energy storage properties of a material have been studied to provide an understanding of the heat accumulation/emission mechanism governing the material's imparted energy storage characteristics.  This paper reviews the present state of the art of PCMs for thermal energy storage applications and provides an insight into recent efforts to develop new PCMs with enhanced performance and safety.  Specific attention is given to the improvement of thermal cond., encapsulation methods and shape stabilization procedures.  In addn., the flame retarding properties and performance are discussed.  The wide range of PCM applications in the construction, electronic, biomedical, textile and automotive industries is presented and future research directions are indicated.

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