Giant thermal rectification efficiency by geometrically enhanced asymmetric non-linear radiation.

Thermal rectification is an asymmetric heat transport phenomenon where thermal conductance changes depending on the temperature gradient direction. The experimentally reported efficiency of thermal rectification materials and devices, which are applicable for a wide range of temperatures, is relatively low. Here we report a giant thermal rectification efficiency of 218% by maximizing asymmetry in parameters of the Stefan-Boltzmann law for highly non-linear thermal radiation. The asymmetry in emissivity is realized by sputter-depositing manganese (ε = ∼0.38) on the top right half surface of a polyurethane specimen (ε = ∼0.98). The surface area of the polyurethane side is also dramatically increased (1302%) by 3D printing to realize asymmetry in geometry. There is an excellent agreement between the experimentally measured temperature profiles and finite element simulation results, demonstrating the reliability of the analysis. Machine learning analysis reveals that the surface area is a dominant factor for thermal rectification and suggests novel light-weight designs with high efficiencies. This work may find applications in energy efficient thermal rectification management of electronic devices and housings.

Covalently Functionalized Leakage-Free Healable Phase-Change Interface Materials with Extraordinary High-Thermal Conductivity and Low-Thermal Resistance.

Phase-change materials (PCMs) have received considerable attention to take advantage of both pad-type and grease-type thermal interface materials (TIMs). However, the critical drawbacks of leaking, non-recyclability, and low thermal conductivity (κ) hinder industrial applications of PCM TIMs. Here, leakage-free healable PCM TIMs with extraordinarily high κ and low total thermal resistance (Rt ) are reported. The matrix material (OP) is synthesized by covalently functionalizing octadecanol PCM with polyethylene-co-methyl acrylate-co-glycidyl methacrylate polymer through the nucleophilic epoxy ring opening reaction. The OP changes from semicrystalline to amorphous above the phase-transition temperature, preventing leaking. The hydrogen-bond-forming functional groups in OP enable nearly perfect healing efficiencies in tensile strength (99.7%), κ (97.0%), and Rt (97.4%). Elaborately designed thermally conductive fillers, silver flakes and multiwalled carbon nanotubes decorated with silver nanoparticles (nAgMWNTs), are additionally introduced in the OP matrix (OP-Ag-nAgMWNT). The nAgMWNTs bridge silver-flake islands, resulting in extraordinarily high κ (43.4 W m-1  K-1 ) and low Rt (30.5 mm2  K W-1 ) compared with PCM TIMs in the literature. Excellent heat dissipation and recycling demonstration of the OP-Ag-nAgMWNT is also carried out using a computer graphic processing unit. The OP-Ag-nAgMWNT is a promising future TIM for thermal management of mechanical and electrical devices.

Solid-state thermal rectification of bilayers by asymmetric elastic modulus.

A highly efficient thermal rectification applicable to large panels still needs to be developed. Here, we experimentally achieve a high thermal rectification efficiency of 33% by carefully engineering elastic modulus asymmetry in a centimeter-scale bilayered silver-graphene oxide sponge. The thermal conduction primarily occurs in the out-of-plane direction, and the forward heat flow direction is from the hard silver to the soft graphene oxide. Surprisingly, the forward heat flow direction is reversed when a silver layer is formed on a harder polystyrene foam. The forward direction is always from the harder side to the softer side, and the asymmetry in elastic modulus is suggested as a possible mechanism based on the one-dimensional Frenkel-Kontorova (FK) model. The finite element analysis indicates that other mechanisms such as temperature-dependent thermal conductivity and radiation asymmetry cannot explain the high rectification efficiency. This scalable work over a wide temperature range may find immediate industrial applications.

Invariable resistance of conductive nanocomposite over 30% strain.

The dependence of the electrical resistance on materials' geometry determines the performance of conductive nanocomposites. Here, we report the invariable resistance of a conductive nanocomposite over 30% strain. This is enabled by the in situ-generated hierarchically structured silver nanosatellite particles, realizing a short interparticle distance (4.37 nm) in a stretchable silicone rubber matrix. Furthermore, the barrier height is tuned to be negligible by matching the electron affinity of silicone rubber to the work function of silver. The stretching results in the electron flow without additional scattering in the silicone rubber matrix. The transport is changed to quantum tunneling if the barrier height is gradually increased by using different matrix polymers with smaller electron affinities, such as ethyl vinyl acetates and thermoplastic polyurethane. The tunneling current decreases with increasing strain, which is accurately described by the Simmons approximation theory. The tunable transport in nanocomposites provides an advancement in the design of stretchable conductors.

Content Swapping: A New Image Synthesis for Construction Sign Detection in Autonomous Vehicles.

Construction signs alert drivers to the dangers of abnormally blocked roads. In the case of autonomous vehicles, construction signs should be detected automatically to prevent accidents. One might think that we can accomplish the goal easily using the popular deep-learning-based detectors, but it is not the case. To train the deep learning detectors to detect construction signs, we need a large amount of training images which contain construction signs. However, collecting training images including construction signs is very difficult in the real world because construction events do not occur frequently. To make matters worse, the construction signs might have dozens of different construction signs (i.e., contents). To address this problem, we propose a new method named content swapping. Our content swapping divides a construction sign into two parts: the board and the frame. Content swapping generates numerous synthetic construction signs by combining the board images (i.e., contents) taken from the in-domain images and the frames (i.e., geometric shapes) taken from the out-domain images. The generated synthetic construction signs are then added to the background road images via the cut-and-paste mechanism, increasing the number of training images. Furthermore, three fine-tuning methods regarding the region, size, and color of the construction signs are developed to make the generated training images look more realistic. To validate our approach, we applied our method to real-world images captured in South Korea. Finally, we achieve an average precision (AP50) score of 84.98%, which surpasses that of the off-the-shelf method by 9.15%. Full experimental results are available online as a supplemental video. The images used in the experiments are also released as a new dataset CSS138 for the benefit of the autonomous driving community.

Non-oxidized bare copper nanoparticles with surface excess electrons in air.

Copper (Cu) nanoparticles (NPs) have received extensive interest owing to their advantageous properties compared with their bulk counterparts. Although the natural oxidation of Cu NPs can be alleviated by passivating the surfaces with additional moieties, obtaining non-oxidized bare Cu NPs in air remains challenging. Here we report that bare Cu NPs with surface excess electrons retain their non-oxidized state over several months in ambient air. Cu NPs grown on an electride support with excellent electron transfer ability are encapsulated by the surface-accumulated excess electrons, exhibiting an ultralow work function of ~3.2 eV. Atomic-scale structural and chemical analyses confirm the absence of Cu oxide moiety at the outermost surface of air-exposed bare Cu NPs. Theoretical energetics clarify that the surface-accumulated excess electrons suppress the oxygen adsorption and consequently prohibit the infiltration of oxygen into the Cu lattice, provoking the endothermic reaction for oxidation process. Our results will further stimulate the practical use of metal NPs in versatile applications.

Highly Conductive Strong Healable Nanocomposites via Diels-Alder Reaction and Filler-Polymer Covalent Bifunctionalization.

Healable stretchable conductive nanocomposites have received considerable attention. However, there has been a trade-off between the filler-induced electrical conductivity (σ) and polymer-driven mechanical strength. Here significant enhancements in both σ and mechanical strength by designing reversible covalent bonding of the polymer matrix and filler-matrix covalent bifunctionalization are reported. A polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene grafted with maleic anhydride forms the strong reversible covalent bonding with furfuryl alcohol through the Diels-Alder reaction. Small (7.5 nm) and medium (117 nm) nanosatellite particles are generated by in situ etching of silver flakes, enabling electron tunneling-assisted percolation. The filler-polymer covalent bifunctionalization is achieved by 3-mercaptopropanoic acid. Altogether, this results in high σ (108 300 S m-1 ) and tensile strength (16.4 MPa), breaking the trade-off behavior. A nearly perfect (≈100%) healing efficiency is achieved in both σ and tensile strength. The conductive nanocomposite figure of merit (1.78 T Pa S m-1 ), defined by the product of σ and tensile strength, is orders of magnitude greater than the data in literature. The nanocomposite may find applications in healable strain sensors and electronic materials.

Tunable solid-state thermal rectification by asymmetric nonlinear radiation.

Thermal rectification is a direction-dependent asymmetric heat transport phenomenon. Here we report the tunable solid-state thermal rectification by asymmetric nonlinear far-field radiation. The asymmetry in thermal conductivity and emissivity of a three-terminal device is realized by sputtering a thin metal film (radiation barrier: niobium, copper, or silver) on the top right half of a polyethylene terephthalate strip (emitter). Both the experiment and finite element analysis are in excellent agreement, revealing a thermal rectification ratio (TR) of 13.0% for the niobium-deposited specimen. The simulation demonstrates that the TR can be further increased to 74.5% by tuning asymmetry in thermal conductivity, emissivity, and surface area. The rectification can also be actively controlled, by gating the environmental temperature, resulting in a maximum TR of 93.1%. This work is applicable for a wide range of temperatures and device sizes, which may find applications in on-demand heat control and thermal logic gates.

High-Temperature Skin Softening Materials Overcoming the Trade-Off between Thermal Conductivity and Thermal Contact Resistance.

The trade-off between thermal conductivity (κ) and thermal contact resistance (Rc ) is regarded as a hurdle to develop superior interface materials for thermal management. Here a high-temperature skin softening material to overcome the trade-off relationship, realizing a record-high total thermal conductance (254.92 mW mm-2 K-1 ) for isotropic pad-type interface materials is introduced. A highly conductive hard core is constructed by incorporating Ag flakes and silver nanoparticle-decorated multiwalled carbon nanotubes in thermosetting epoxy (EP). The thin soft skin is composed of filler-embedded thermoplastic poly(ethylene-co-vinyl acetate) (PEVA). The κ (82.8 W m-1 K-1 ) of the PEVA-EP-PEVA interface material is only slightly compromised, compared with that (106.5 W m-1 K-1 ) of the EP core (386 µm). However, the elastic modulus (E = 2.10 GPa) at the skin is significantly smaller than the EP (26.28 GPa), enhancing conformality and decreasing Rc from 108.41 to 78.73 mm2 K W-1 . The thermoplastic skin is further softened at an elevated temperature (100 °C), dramatically decreasing E (0.19 GPa) and Rc (0.17 mm2 K W-1 ) with little change in κ, overcoming the trade-off relationship and enhancing the total thermal conductance by 2030%. The successful heat dissipation and applicability to the continuous manufacturing process demonstrate excellent feasibility as future thermal management materials.

Intertube Aggregation-Dependent Convective Heat Transfer in Vertically Aligned Carbon Nanotube Channels.

The heat transfer of carbon nanotube fin geometry has received considerable attention. However, the flow typically occurred over or around the pillars of nanotubes due to the greater flow resistance between the tubes. Here, we investigated the forced convective heat transfer of water through the interstitial space of vertically aligned multiwalled carbon nanotubes (VAMWNTs, intertube distance = 69 nm). The water flow provided significantly a greater Reynolds number (Re) and Nusselt number (Nu) than air flow due to the greater density, heat capacity, and thermal conductivity. However, it resulted in surface tension-induced nanotube aggregation after the flow and drying process, generating random voids in the nanotube channel. This increased permeability (1.27 × 10-11 m2) and Re (2.83 × 10-1) but decreased the heat transfer coefficient (h, 9900 W m-2 K-1) and Nu (53.77), demonstrating a trade-off relationship. The h (25,927 W m-2 K-1) and Nu (153.49) could be further increased, at an equivalent permeability or Re, by increasing nanotube areal density from 2.08 × 1010 to 1.04 × 1011 cm-2. The area-normalized thermal resistance of the densified and aggregated VAMWNTs was smaller than those of the Ni foam, Si microchannel, and carbon nanotube fin array, demonstrating excellent heat transfer characteristics.

Electron tunneling of hierarchically structured silver nanosatellite particles for highly conductive healable nanocomposites.

Healable conductive materials have received considerable attention. However, their practical applications are impeded by low electrical conductivity and irreversible degradation after breaking/healing cycles. Here we report a highly conductive completely reversible electron tunneling-assisted percolation network of silver nanosatellite particles for putty-like moldable and healable nanocomposites. The densely and uniformly distributed silver nanosatellite particles with a bimodal size distribution are generated by the radical and reactive oxygen species-mediated vigorous etching and reduction reaction of silver flakes using tetrahydrofuran peroxide in a silicone rubber matrix. The close work function match between silicone and silver enables electron tunneling between nanosatellite particles, increasing electrical conductivity by ~5 orders of magnitude (1.02×103 Scm-1) without coalescence of fillers. This results in ~100% electrical healing efficiency after 1000 breaking/healing cycles and stability under water immersion and 6-month exposure to ambient air. The highly conductive moldable nanocomposite may find applications in improvising and healing electrical parts.

Enhanced Stability of MAPbI3 Perovskite Solar Cells using Poly(p-chloro-xylylene) Encapsulation.

We demonstrated an effective poly(p-chloro-xylylene) (Parylene-C) encapsulation method for MAPbI3 solar cells. By structural and optical analysis, we confirmed that Parylene-C efficiently slowed the decomposition reaction in MAPbI3. From a water permeability test with different encapsulating materials, we found that Parylene-C-coated MAPbI3 perovskite was successfully passivated from reaction with water, owing to the hydrophobic behavior of Parylene-C. As a result, the Parylene-C-coated MAPbI3 solar cells showed better device stability than uncoated cells, virtually maintaining the initial power conversion efficiency value (15.5 ± 0.3%) for 196 h.

Significantly enhanced phonon mean free path and thermal conductivity by percolation of silver nanoflowers.

Soft thermal interface materials (TIMs) composed of thermally conductive fillers and polymer matrixes have been widely employed for thermal management in electronic and energy devices. However, the thermal conductivity (κ) of TIMs is significantly smaller than the intrinsic κ of fillers due to the large interfacial thermal contact resistance between fillers. Here we achieve a very efficient thermal percolation network of flower-shaped silver nanoparticles (silver nanoflowers, Ag NFs) in soft polyurethane (PU) matrix TIMs. A record high κ (42.4 W m-1 K-1) is achieved compared with soft isotropic TIMs in the literature. Ag nanoflake-PU and Ag nanosphere-PU TIMs provide significantly smaller κ (7.9 and 15.0 W m-1 K-1) at an identical filler concentration (38 vol%). Surprisingly, the phonon transport of the Ag NF-PU TIM dramatically increases (κlat = 22.2 W m-1 K-1) compared with Ag nanoflake-PU and Ag nanosphere-PU (κlat = 0.2 and 1.2 W m-1 K-1) TIMs. Kinetic theory reveals that the phonon mean free path (39.6 nm) is significantly increased for the Ag NF-PU TIM by the active coalescence of metallic Ag NFs. The hierarchically structured Ag NFs construct an excellent thermal percolation network in soft isotropic TIMs.

A Superior Method for Constructing Electrical Percolation Network of Nanocomposite Fibers: In Situ Thermally Reduced Silver Nanoparticles.

Nanocomposite fibers, composed of conductive nanoparticles and polymer matrix, are crucial for wearable electronics. However, the nanoparticle mixing approach results in aggregation and dispersion problems. A revolutionary synthesis method by premixing silver precursor ions (silver ammonium acetate) with polyvinyl alcohol is reported here. The solvation of ions-prevented aggregation, and uniformly distributed silver nanoparticles (in situ AgNPs, 77 nm) are formed after thermal reduction (155 °C) without using additional reducing or dispersion agents. The conductive fiber is synthesized by the wet spinning technology. After careful optimization, flower-shaped silver nanoparticles (AgNFs, 350-450 nm) are also employed as cofillers. The addition of in situ AgNPs (9.5 vol%) to AgNFs (30 vol%) increases electrical conductivity by 1434% (2090 to 32 064 S cm-1 ) through the efficient construction of percolation networks. The in situ AgNPs provide significantly higher conductivity compared with other secondary nanoparticle fillers. The gaseous byproducts dramatically increase flexibility with a moderate compromise in tensile strength (55 MPa). The particle-free ion-level uniform mixing of silver precursors, followed by in situ reduction, would be a fundamental paradigm shift in nanocomposite synthesis.

Fast mass transport-assisted convective heat transfer through a multi-walled carbon nanotube array.

The recently reported fast mass transport through nanochannels provides a unique opportunity to explore nanoscale energy transport. Here we experimentally investigated the convective heat transport of air through vertically aligned multi-walled carbon nanotubes (VAMWNTs). The flow through the unit cell, defined as an interstitial space among four adjacent nanotubes (hydraulic diameter = 84.9 nm), was in the transition (0.62 ≤ Knudsen number ≤ 0.78) and creeping flow (3.83 × 10-5 ≤ Reynolds number (Re) ≤ 1.55 × 10-4) regime. The constant heat flux (0.102 or 0.286 W m-2) was supplied by a single-mode microwave (2.45 GHz) instantly heating the VAMWNTs. The volume flow rate was two orders of magnitude greater than the Hagen-Poiseuille theory value. The experimentally determined convective heat transfer coefficient (h, 3.70 × 10-4-4.01 × 10-3 W m-2 K-1) and Nusselt number (Nu, 1.17 × 10-9-1.26 × 10-8) were small partly due to the small Re. A further increase in Re (2.12 × 10-3) with the support of a polytetrafluoroethylene mesh significantly increased h (5.48 × 10-2 W m-2 K-1) and Nu (2.37 × 10-7). A large number of nanochannels in a given cross-section of heat sinks may enhance the heat dissipation significantly.

Unusual strain-dependent thermal conductivity modulation of silver nanoflower-polyurethane fibers.

Thermal management of stretchable and wearable electronic devices is an important issue in enhancing performance, reliability, and human thermal comfort. Here, we constructed a unique experimental setup which investigated the strain-dependent thermal conductivity. The thermal conductivity of flower-shaped silver nanoparticle (silver nanoflower)-polyurethane (Ag-PU) composite fibers was systematically investigated as a function of strain. The strain-dependent temperature distribution of the Joule-heated fiber was measured using an infrared camera, and the thermal conductivity was obtained from the 1-dimensional Fourier's conduction model. There was a monotonic decrease in both lattice and electronic thermal conductivity with stretching at 25 °C. However, there was an initial increase in lattice and total thermal conductivity in the low strain region (<10%), when the fiber was stretched at 45 °C, although the electronic thermal conductivity decreased monotonically. The softening of the polymer at increased temperatures enhanced Poisson's ratio. Resultantly, the fiber cross-sectional area and radial-direction inter-particle distance between silver nanoflowers decreased. This could increase the thermal transport in conductive fibers by modulating the interfaces between silver nanoflowers and polyurethane. A further stretching decreased the lattice thermal conductivity due to the significantly increased axial distance between silver nanoflowers and the decreased filler fraction. The weft-knitted fabric also demonstrated an increased thermal conductance in the low strain region (≤30%) at 45 °C.

Silver Nanoflower Decorated Graphene Oxide Sponges for Highly Sensitive Variable Stiffness Stress Sensors.

Soft conductive materials should enable large deformation while keeping high electrical conductivity and elasticity. The graphene oxide (GO)-based sponge is a potential candidate to endow large deformation. However, it typically exhibits low conductivity and elasticity. Here, the highly conductive and elastic sponge composed of GO, flower-shaped silver nanoparticles (AgNFs), and polyimide (GO-AgNF-PI sponge) are demonstrated. The average pore size and porosity are 114 µm and 94.7%, respectively. Ag NFs have thin petals (8-20 nm) protruding out of the surface of a spherical bud (300-350 nm) significantly enhancing the specific surface area (2.83 m2 g-1 ). The electrical conductivity (0.306 S m-1 at 0% strain) of the GO-AgNF-PI sponge is increased by more than an order of magnitude with the addition of Ag NFs. A nearly perfect elasticity is obtained over a wide compressive strain range (0-90%). The strain-dependent, nonlinear variation of Young's modulus of the sponge provides a unique opportunity as a variable stiffness stress sensor that operates over a wide stress range (0-10 kPa) with a high maximum sensitivity (0.572 kPa-1 ). It allows grasping of a soft rose and a hard bottle, with the minimal object deformation, when attached on the finger of a robot gripper.

Correlation between the hierarchical structures and nanomechanical properties of amyloid fibrils.

Amyloid fibrils have recently been highlighted due to their excellent mechanical properties, which not only play a role in their biological functions but also imply their applications in biomimetic material design. Despite recent efforts to unveil how the excellent mechanical properties of amyloid fibrils originate, it has remained elusive how the anisotropic nanomechanical properties of hierarchically structured amyloid fibrils are determined. Here, we characterize the anisotropic nanomechanical properties of hierarchically structured amyloid fibrils using atomic force microscopy experiments and atomistic simulations. It is shown that the hierarchical structure of amyloid fibrils plays a crucial role in determining their radial elastic property but does not make any effect on their bending elastic property. This is attributed to the role of intermolecular force acting between the filaments (constituting the fibril) on the radial elastic modulus of amyloid fibrils. Our finding illustrates how the hierarchical structure of amyloid fibrils encodes their anisotropic nanomechanical properties. Our study provides key design principles of amyloid fibrils, which endow valuable insight into the underlying mechanisms of amyloid mechanics.

Enhanced crystallinity of CH3NH3PbI3 by the pre-coordination of PbI2-DMSO powders for highly reproducible and efficient planar heterojunction perovskite solar cells.

Solution processable CH3NH3PbI3 has received considerable attention for highly-efficient perovskite solar cells. However, the different solubility of PbI2 and CH3NH3I is problematic, initiating active solvent engineering research using dimethyl sulfoxide (DMSO). Here we investigated the pre-coordination of PbI2-DMSO powders for planar heterojunction perovskite solar cells fabricated by a low-temperature process (≤100 °C). Pre-coordination was carried out by simple mechanical mixing using a mortar and pestle. The composition of PbI2-DMSO x (x = 0, 1, or 2) in the powder mixture was investigated by gradually increasing mechanical mixing time, and a dominant composition of PbI2-DMSO1 was obtained after a 10 min mixing process. The pre-coordinated PbI2-DMSO powders were then blended with CH3NH3I in DMF to make the CH3NH3PbI3 film by toluene-assisted spin-coating and heat treatment. Compared with the one-step blending of CH3NH3I, PbI2, and DMSO in DMF, the pre-coordination method resulted in better dissolution of PbI2, larger grain size, and pinhole-free morphology. Consequently, absorption, fluorescence, carrier lifetime, and charge extraction were enhanced. The average open-circuit voltage (1.046 V), short-circuit current (22.9 mA cm-2), fill factor (73.5%), and power conversion efficiency (17.6%) were increased by 2-12% with decreased standard deviations (13-50%), compared with the one-step blending method. The best efficiency was 18.2%. The simple mechanical pre-coordination of PbI2-DMSO powders was very effective in enhancing the crystallinity of CH3NH3PbI3 and photovoltaic performance.

Silver nanoflowers for single-particle SERS with 10 pM sensitivity.

Surface-enhanced Raman scattering (SERS) has received considerable attention as a noninvasive optical sensing technique with ultrahigh sensitivity. While numerous types of metallic particles have been actively investigated as SERS substrates, the development of new SERS agents with high sensitivity and their reliable characterization are still required. Here we report the preparation and characterization of flower-shaped silver (Ag) nanoparticles that exhibit high-sensitivity single-particle SERS performance. Ag nanoflowers (NFs) with bud sizes in the range 220-620 nm were synthesized by the wet synthesis method. The densely packed nanoscale petals with thicknesses in the range 9-22 nm exhibit a large number of hot spots that significantly enhance their plasmonic activity. A single Ag NF particle (530-620 nm) can detect as little as 10-11 M 4-mercaptobenzoic acid, and thus provides a sensitivity three orders of SERS magnitude greater than that of a spherical Ag nanoparticle. The analytical enhancement factors for single Ag NF particles were found to be as high as 8.0 × 109, providing unprecedented high SERS detectivity at the single particle level. Here we present an unambiguous and systematic assessment of the SERS performances of the Ag NFs and demonstrate that they provide highly sensitive sensing platforms by single SERS particle.