Friction and wear remain as the primary modes of mechanical energy dissipation in moving mechanical assemblies; thus, it is desirable to minimize friction in a number of applications. We demonstrate ...that superlubricity can be realized at engineering scale when graphene is used in combination with nanodiamond particles and diamondlike carbon (DLC). Macroscopic superlubricity originates because graphene patches at a sliding interface wrap around nanodiamonds to form nanoscrolls with reduced contact area that slide against the DLC surface, achieving an incommensurate contact and substantially reduced coefficient of friction (∼0.004). Atomistic simulations elucidate the overall mechanism and mesoscopic link bridging the nanoscale mechanics and macroscopic experimental observations.
An experiment was conducted to study the effect of high temperature stress on the antioxidant enzyme activity in five wheat genotypes viz., PBW 343, PBW 175, HDR-77, HD 2815 and HD 2865. There was ...significant increase in the activity of superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) in the late and very late planting and at all stages of plant growth, i.e., vegetative, anthesis and 15 days after anthesis (DAA), however glutathione reductase (GR) and peroxidase (POX) activity decreased under late and very late plantings compared to normal planting. In general HD 2815, HDR-77 showed relatively higher SOD, APX, GR, CAT and POX activity in the late plantings compared to PBW 343, PBW 175 and HD 2865. Significant reduction in chlorophyll content and increase in membrane injury index were observed in all genotypes with age, and also under late and very late sowings at all the stages of plant growth. However HD 2815 and HDR-77, which showed highest activity of various antioxidant enzymes under late and very late sowing also showed minimum reduction in chlorophyll content and lower membrane injury index, indicating the amelioration of high temperature stress induced oxidative stress by antioxidant enzymes. Various antioxidant enzymes showed positive correlation (r) with chlorophyll content and negative with membrane injury index at most of the stages in the five wheat genotypes.
Conformational transitions in thermo-sensitive polymers are critical in determining their functional properties. The atomistic origin of polymer collapse at the lower critical solution temperature ...(LCST) remains a fundamental and challenging problem in polymer science. Here, molecular dynamics simulations are used to establish the role of solvation dynamics and local ordering of water in inducing conformational transitions in isotactic-rich poly(N-isopropylacrylamide) (PNIPAM) oligomers when the temperature is changed through the LCST. Simulated atomic trajectories are used to identify stable conformations of the water-molecule network in the vicinity of polymer segments, as a function of the polymer chain length. The dynamics of the conformational evolution of the polymer chain within its surrounding water molecules is evaluated using various structural and dynamical correlation functions. Around the polymer, water forms cage-like structures with hydrogen bonds. Such structures form at temperatures both below and above the LCST. The structures formed at temperatures above LCST, however, are significantly different from those formed below LCST. Short oligomers consisting of 3, 5, and 10 monomer units (3-, 5-, and 10-mer), are characterized by significantly higher hydration level (water per monomer ∼ 16). Increasing the temperature from 278 to 310 K does not perturb the structure of water around the short oligomers. In the case of 3-, 5-, and 10-mer, a distinct coil-to-globule transition was not observed when the temperature was raised from 278 to 310 K. For a PNIPAM polymer chain consisting of 30 monomeric units (30-mer), however, there exist significantly different conformations corresponding to two distinct temperature regimes. Below LCST, the water molecules in the first hydration layer (∼12) around hydrophilic groups arrange themselves in a specific ordered manner by forming a hydrogen-bonded network with the polymer, resulting in a solvated polymer acting as hydrophilic. Above LCST, this arrangement of water is no longer stable, and the hydrophobic interactions become dominant, which contributes to the collapse of the polymer. Thus, this study provides atomic-scale insights into the role of solvation dynamics in inducing coil-to-globule phase transitions through the LCST for thermo-sensitive polymers like PNIPAM.
During the last few years, graphene's unusual friction and wear properties have been demonstrated at nano to micro scales but its industrial tribological potential has not been fully realized. The ...macroscopic wear resistance of one atom thick graphene coating is reported by subjecting it to pin‐on‐disc type wear testing against most commonly used steel against steel tribo‐pair. It is shown that when tested in hydrogen, a single layer of graphene on steel can last for 6400 sliding cycles, while few‐layer graphene (3–4 layers) lasts for 47 000 cycles. Furthermore, these graphene layers are shown to completely cease wear despite the severe sliding conditions including high contact pressures (≈0.5 GPa) observed typically in macroscale wear tests. The computational simulations show that the extraordinary wear performance originates from hydrogen passivation of the dangling bonds in a ruptured graphene, leading to significant stability and longer lifetime of the graphene protection layer. Also, the electronic properties of these graphene sheets are theoretically evaluated and the improved wear resistance is demonstrated to preserve the electronic properties of graphene and to have significant potential for flexible electronics. The findings demonstrate that tuning the atomistic scale chemical interactions holds the promise of realizing extraordinary tribological properties of monolayer graphene coatings.
The mechanism of extraordinary wear resistance of just one atom thick graphene layer on steel is revealed. A single layer of graphene is able to reduce steel wear by 3–4 orders of magnitude. The wear‐life of graphene significantly increases when tested in hydrogen environment. Hydrogen plays a crucial role in preventing graphene from wear‐induced damage by passivating carbon dangling bonds.
The degradation of intrinsic properties of graphene during the transfer process constitutes a major challenge in graphene device fabrication, stimulating the need for direct growth of graphene on ...dielectric substrates. Previous attempts of metal-induced transformation of diamond and silicon carbide into graphene suffers from metal contamination and inability to scale graphene growth over large area. Here, we introduce a direct approach to transform polycrystalline diamond into high-quality graphene layers on wafer scale (4 inch in diameter) using a rapid thermal annealing process facilitated by a nickel, Ni thin film catalyst on top. We show that the process can be tuned to grow single or multilayer graphene with good electronic properties. Molecular dynamics simulations elucidate the mechanism of graphene growth on polycrystalline diamond. In addition, we demonstrate the lateral growth of free-standing graphene over micron-sized pre-fabricated holes, opening exciting opportunities for future graphene/diamond-based electronics.
Schematic representation for synthesis of silver nanoparticles. Display omitted
► Environmentally benign and efficient method ► Stable silver NPs better for catalysis ► Elemental silver NPs instead ...of their oxides ► Effective capping of silver NPs by sodium dodecyl sulfate in aqueous medium ► Excellent catalyst towards reduction of p-nitrophenol to p-aminophenol at RT.
Supported silver nanoparticles were synthesized via in situ sol–gel followed by reduction method with dextrose as reductant and sodium dodecyl sulfate as stabilizer. The synthesized nanoparticles were characterized by X–ray diffraction, transmission electron microscopy, Fourier transform Infra-Red spectroscopy and UV–visible measurements. The XRD peaks confirm the metallic face-centered cubic silver particles. The formation of silver nanoparticles was confirmed from the appearance of surface plasmon absorption maxima at 412nm; which shifted to the longer wavelengths after supported on titania host lattice. TEM showed the spherical nanoparticles with size in the range of 18–23nm. An efficient and simple method was reported for the reduction of 4-nitrophenol using titania-supported silver nanoparticles at room temperature. The reaction was first order with respect to the concentration of 4-nitrophenol with higher efficiency. Titania supported silver nanoparticles are reusable and stable heterogeneous catalyst.
Size-segregated aerosols (nine stages from < 0.43 to > 11.3 µm in diameter) were collected at Cape Hedo, Okinawa, in spring 2008 and analyzed for water-soluble diacids (C2–C12), ω-oxoacids (ωC2–ωC9), ...pyruvic acid, benzoic acid, and α-dicarbonyls (C2–C3) as well as water-soluble organic carbon (WSOC), organic carbon (OC), and major ions (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3−, SO42−, and MSA−). In all the size-segregated aerosols, oxalic acid (C2) was found to be the most abundant species, followed by malonic and succinic acids, whereas glyoxylic acid (ωC2) was the dominant oxoacid and glyoxal (Gly) was more abundant than methylglyoxal. Diacids (C2–C5), ωC2, and Gly as well as WSOC and OC peaked at fine mode (0.65–1.1 µm) whereas azelaic (C9) and 9-oxononanoic (ωC9) acids peaked at coarse mode (3.3–4.7 µm). Sulfate and ammonium were enriched in fine mode, whereas sodium and chloride were in coarse mode. Strong correlations of C2–C5 diacids, ωC2 and Gly with sulfate were observed in fine mode (r = 0.86–0.99), indicating a commonality in their secondary formation. Their significant correlations with liquid water content in fine mode (r = 0.82–0.95) further suggest an importance of the aqueous-phase production in Okinawa aerosols. They may also have been directly emitted from biomass burning in fine mode as supported by strong correlations with potassium (r = 0.85–0.96), which is a tracer of biomass burning. Bimodal size distributions of longer-chain diacid (C9) and oxoacid (ωC9) with a major peak in the coarse mode suggest that they were emitted from the sea surface microlayers and/or produced by heterogeneous oxidation of biogenic unsaturated fatty acids on sea salt particles.
Understanding the role of water in governing the kinetics of the self-assembly processes of amphiphilic peptides remains elusive. Here, we use a multistage atomistic-coarse-grained approach, ...complemented by circular dichroism/infrared spectroscopy and dynamic light scattering experiments to highlight the dual nature of water in driving the self-assembly of peptide amphiphiles (PAs). We show computationally that water cage formation and breakage near the hydrophobic groups control the fusion dynamics and aggregation of PAs in the micellar stage. Simulations also suggest that enhanced structural ordering of vicinal water near the hydrophilic amino acids shifts the equilibrium towards the fibre phase and stimulates structure and order during the PA assembly into nanofibres. Experiments validate our simulation findings; the measured infrared O-H bond stretching frequency is reminiscent of an ice-like bond which suggests that the solvated water becomes increasingly ordered with time in the assembled peptide network, thus shedding light on the role of water in a self-assembly process.
Aerosol liquid water (ALW) can serve as an aqueous-phase medium for numerous chemical reactions and consequently enhance the formation of secondary aerosols in a highly humid atmosphere. However, the ...aqueous-phase formation of secondary organic aerosols (SOAs) is not well understood in the Indian regions, particularly in tropical peninsular India. In this study, we collected total suspended particulate samples (n = 30) at a semiarid station (Ballari; 15.15°N, 76.93°E; 495 m asl) in tropical peninsular India during the winter of 2016. Homologous series of dicarboxylic acids (C2–C12), oxoacids (ωC2–ωC9), pyruvic acid (Pyr), and glyoxal (Gly) were determined by employing a water-extraction of aerosol and analyzed using capillary gas chromatography (GC). Results show that oxalic acid (C2) was the most abundant organic acid, followed by succinic (C4), malonic (C3), azelaic (C9), and glyoxylic (ωC2) or phthalic (Ph) acids. Total diacids-C accounted for 1.7–5.8 % of water-soluble organic carbon (WSOC) and 0.6–3.6 % of total carbon (TC). ALW, estimated from the ISORROPIA 2.1 model, showed a strong linear relationship with sulfate (SO42−), C2, C3, C4, ωC2, Pyr, and Gly. Based on molecular distribution, specific mass ratios (C2/C3, C2/C4, C2/Gly, and Ph/C9), linear relationships among the measured organic acids, ALW, organic (levoglucosan and oleic acid), and inorganic (SO42−) marker compounds, we emphasize that diacids and related organic compounds, especially C2, majorly form via aqueous-phase oxidation of precursor compounds including aromatic hydrocarbons (HCs) and unsaturated fatty acids (FAs) originated from biomass burning and combustion-related sources. The present study demonstrates that sulfate driven ALW largely enhances the formation of SOAs via the aqueous-phase reactions over tropical peninsular India during winter.
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•Sources and formation processes of organic aerosols were investigated at a tropical semi-arid site in peninsular India.•Molecular distribution of diacids was characterized by C2 > C4 > C3 > C9 > ωC2 > Ph.•ALW robustly correlated linearly with SO42−, C2/C3, C2/C4, C2/Gly.•Sulfate-driven ALW largely enhances the formation of C2 via the aqueous-phase reactions.
Stress rupture tests of normalized and tempered P92 (9Cr–0.5Mo–1.8W) steel were performed in the range of 135–215MPa at 650°C. Effect of tempering temperature in the range of 740–780°C on the creep ...rupture life was investigated. Resulting rupture times varied from 100 to 3000h, and creep rate by one order of magnitude. In the high stress regime, lower tempering temperature resulted in the highest rupture time due to initial high dislocation density and fine laths. However, at lower stresses, highest rupture time was observed for highest tempering temperature. Formation of Laves phase (Fe2Mo, Fe2W) adjacent to M23C6 carbides was responsible for increase in rupture time. Back scattered electron imaging (BSE) in scanning electron microscopy (SEM) was used to identify Laves phases, and study their distribution. Reduction in dislocation density and coarsening of laves phase precipitates result in decrease in stress exponent value ‘n’ at higher test temperatures of 650°C.
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