Dynamic Behavior of Rotation Transmission Nano-System in Helium Environment: A Molecular Dynamics Study.

The molecular dynamics (MD) method is used to investigate the influence of the shielding gas on the dynamic behavior of the heterogeneous rotation transmission nano-system (RTS) built on carbon nanotubes (CNTs) and boron nitride nanotube (BNNT) in a helium environment. In the heterogeneous RTS, the inner CNT acts as a rotor, the middle BNNT serves as a motor, and the outer CNT functions as a stator. The rotor will be actuated to rotate by the motor due to the interlayer van der Waals effects and the end effects. The MD simulation results show that, when the gas density is lower than a critical range, a stable signal of the rotor will arise on the output and the rotation transmission ratio (RRT) of RTS can reach 1.0, but as the gas density is higher than the critical range, the output signal of the rotor cannot be stable due to the sharp drop of the RRT caused by the large friction between helium and the RTS. The greater the motor input signal of RTS, the lower the critical working helium density range. The results also show that the system temperature and gas density are the two main factors affecting the RTS transmission behavior regardless of the size of the simulation box. Our MD results clearly indicate that in the working temperature range of the RTS from 100 K to 600 K, the higher the temperature and the lower the motor input rotation frequency, the higher the critical working helium density range allows.

Enhanced mechanical properties of 4H-SiC by epitaxial carbon films obtained from bilayer graphene.

Graphene exhibits excellent mechanical properties under atomically thin thickness, which made it very suitable for nanoelectromechanical systems that had high requirements for the thickness of coatings. The epitaxial bilayer graphene on the 4H-SiC (0001) surface presents high stiffness and hardness comparable to diamond. However, due to structural transition occurring at the nanoscale, it is difficult to elucidate reinforcement mechanisms using experimental methods. Here, we applied molecular dynamics simulations to study nanoindentation of epitaxial carbon-film-covered 4H-SiC (0001) surfaces. Because a weak interaction potential existed between graphene layers at indentation depth (h < 0.8 Å) that far smaller than interlayer distance, the epitaxial bilayer graphene does not allow the 4H-SiC to exceed its intrinsic stiffness. When the indentation depth h ≥6.45 Å, the sp3 hybridized bonds formed on the interlayer of graphene, which leads to fewer amorphous atoms in the sample of 4H-SiC and exhibits stronger stiffness, in comparison with bare 4H-SiC. This strongly suggests the existence of sp3 bonds contributing to the surface strengthening. Meanwhile, we found that the comprehensive mechanical properties of nanocomposites with hydrogenated diamond-like films were superior to those of nanocomposites with other carbon films at high temperatures.

Cobalt vacancy-originated TiMnCoCN compounds with a self-adjusting ability for the high-efficiency acidic oxygen evolution reaction.

Oxygen evolution reaction (OER) electrocatalysts in acidic media, except for precious IrO2, have difficulty realizing good electrocatalytic activity and high electrochemical stability simultaneously. However, the scarcity of IrO2 as an acidic OER electrocatalyst impedes its large-scale application in hydrogen generation, organic synthesis, nonferrous metal production and sewage disposal. Herein, we report the design and fabrication of a nanoporous TiMnCoCN compound based on the nanoscale Kirkendall effect, possessing an intriguing self-adjusting capability for the oxygen evolution reaction (OER) in a 0.5 M H2SO4 solution. The nanoporous TiMnCoCN compound electrode for the acidic OER displays a low overpotential of 143 mV for 10 mA cm-2 and exhibits no increase in potential over 50,000 s, which is ascribed to the self-adjusting ability, Carbon/nitrogen (C/N) incorporation and nanoporous architecture. The concentration of inert TiO2 on the reconstructed surface of the compound can self-adjust with the change in OER potential via a cobalt-dissolved vacancy approach according to the stabilization requirement. In this work, the self-reconstruction law of surface structure was discovered, providing a novel strategy for designing and fabricating nonnoble OER electrocatalysts with superior catalytic performance and robust stability in acidic media.

Finite Element Analysis and Computational Fluid Dynamics Verification of Molten Pool Characteristics During Selective Laser Melting of Ti-6Al-4V Plates.

The finite element (FE) method is used to characterize the thermal gradient, solidification rate, and molten pool sizes of Ti-6Al-4V plates in the process of selective laser melting (SLM). The results are verified by using the computational fluid dynamics (CFD) simulation. The proposed FE model contains a series of toolpath information that is directly converted from a G-code file, including hatch spacing, laser power, layer thickness, dwell time, and scanning speed generated by using Slic3r software from a CAD file. A proposed multi-layer, multi-track FE model is used to investigate the influence of the laser power, scanning speed, and scanning path on the microstructure in the Ti-6Al-4V plate built via SLM. The processing window is also determined based on the proposed FE model. The FE results indicate that, with a decrease in the laser power and an increase in the scanning speed, the morphology of the crystal grains, showing fully columnar crystals, gradually deviates from the fully equiaxed region. The formed grains are dependent on the laser power, scanning speed, and deposition position, but they are not sensitive to the scanning path, and with the deposition from the bottom layer to the top layer, the size of the formed grains is gradually increasing, which shows a good agreement with the experimental results.

Bicontinuous Nanoporous Nitrogen/Carbon-Codoped FeCoNiMg Alloy as a High-Performance Electrode for the Oxygen Evolution Reaction.

The kinetics of the oxygen evolution reaction (OER) in aqueous electrolytes is relatively slow, which seriously limits the energy efficiency of electricity-to-hydrogen conversion. Herein, a bicontinuous nanoporous FeCoNiMg alloy is prepared by high heat sintering method based on the nanoscale Kirkendall effect and the surface is codoped with nitrogen and carbon elements by the nitrocarburizing method (denoted NC-FeCoNiMg). The three-dimensional (3D) nanoporous NC-FeCoNiMg alloy electrode achieves superior electrocatalytic performance for the OER in alkaline media, delivering a low Tafel slope (34.6 mV dec-1) and small overpotentials (235 and 290 mV at 10 and 100 mA cm-2, respectively). Under consecutive high current densities, the NC-FeCoNiMg electrode still exhibits excellent long-term stability, and the OER activity even increases after testing for 100 h at a high current density of 1000 mA cm-2. Comprehensive studies reveal that the N/C codoping of the inner and outer surfaces dramatically improves the electrocatalytic activity of the NC-FeCoNiMg electrode. This work demonstrates an efficient nanoarchitectural construction and a surface modulation strategy to increase the electrocatalytic activity and stability of transition-metal-based electrodes for the OER, holding great promise for fulfilling the requirements for the large-scale production of clean hydrogen energy.

Strain in a platinum plate induced by an ultrahigh energy laser boosts the hydrogen evolution reaction.

The ligand and the strain near the active sites in catalysts jointly affect the electrocatalytic activity for the catalytic industry. In many cases, there is no effective strategy for the independent study of the strain effect without the ligand effect on the electrocatalytic activity for the hydrogen evolution reaction (HER). Laser shock peening (LSP) with a GW cm-2 level power density and a 10-30 ns short pulse is employed to form compressive strain on the surface and in the depth direction of a platinum (Pt) plate, which changes the inherent interatomic distance and modifies the energy level of the bonded electrons, thereby greatly optimizing the energy barrier for the HER. The crystal lattice near the surface of the LSP Pt plate is distorted by the strain, and the interplanar spacing decreases from 0.225 nm in the undeformed region to 0.211 nm in the deformed region. The specific activity of the LSP Pt has an increase of 2.9 and 6.4 times in comparison with that of the pristine Pt in alkaline and acidic environments, respectively. This investigation provides a novel strategy for the independent study of the strain effect on the electrocatalytic activity and the improvement of electrocatalysts with high performance in extensive energy conversion.

The Martensitic Transformation and Mechanical Properties of Ti6Al4V Prepared via Selective Laser Melting.

This article investigated the microstructure of Ti6Al4V that was fabricated via selective laser melting; specifically, the mechanism of martensitic transformation and relationship among parent ß phase, martensite (α') and newly generated ß phase that formed in the present experiments were elucidated. The primary X-ray diffraction (XRD), transmission electron microscopy (TEM) and tensile test were combined to discuss the relationship between α', ß phase and mechanical properties. The average width of each coarse ß columnar grain is 80⁻160 µm, which is in agreement with the width of a laser scanning track. The result revealed a further relationship between ß columnar grain and laser scanning track. Additionally, the high dislocation density, stacking faults and the typical ( 10 1 ¯ 1 ) twinning were identified in the as-built sample. The twinning was filled with many dislocation lines that exhibited apparent slip systems of climbing and cross-slip. Moreover, the α + ß phase with fine dislocation lines and residual twinning were observed in the stress relieving sample. Furthermore, both as-built and stress-relieved samples had a better homogeneous density and finer grains in the center area than in the edge area, displaying good mechanical properties by Feature-Scan. The α' phase resulted in the improvement of tensile strength and hardness and decrease of plasticity, while the newly generated ß phase resulted in a decrease of strength and enhancement of plasticity. The poor plasticity was ascribed to the different print mode, remained support structures and large thermal stresses.

Cobalt-Iron Oxide Nanoarrays Supported on Carbon Fiber Paper with High Stability for Electrochemical Oxygen Evolution at Large Current Densities.

Here, we demonstrate that nonprecious CoFe-based oxide nanoarrays exhibit excellent electrocatalytic activity and superior stability for electrochemical oxygen evolution reaction (OER) at large current densities (>500 mA cm-2). Carbon fiber paper (CFP) with three-dimensional macroporous structure, excellent corrosion resistance, and high electrical properties is used as the support material to prevent surface passivation during the long-term process of OER. Through a facile method of hydrothermal synthesis and thermal treatment, vertically aligned arrays of spinel Co xFe3- xO4 nanostructures are homogeneously grown on CFP. The morphology and the Fe-doping content of the CoFe oxide nanoarrays can be controlled by the Fe3+ concentration in the precursor solution. The arrays of CoFe oxide nanosheets (NSs) grown on CFP (Co2.3Fe0.7O4-NSs/CFP) deliver lower Tafel slope (34.3 mV dec-1) than CoFe oxide nanowire (NW) arrays grown on CFP (Co2.7Fe0.3O4-NWs/CFP) in alkaline solution, owing to higher Fe-doping content and larger effective specific surface area. The Co2.3Fe0.7O4-NSs/CFP electrode exhibits excellent stability for OER at large current densities in alkaline solution. Moreover, the morphology and structure of CoFeO nanoarrays are well preserved after long-term testing, indicating the high stability for OER.

Fabrication and cyto-compatibility of Fe3O4/SiO2/graphene-CdTe QDs/CS nanocomposites for drug delivery.

Synthesis of magnetic Fe3O4/SiO2/graphene-CdTe QDs/chitosan nanocomposites (FGQCs) is investigated with respect to their potential of improving the drug loading content above that of magnetic/fluorescent bifunctional nanocomposites. To evaluate the performance of the FGQCs, their surface morphology was thoroughly assessed. The in vitro interaction between the FGQCs and heptoma cell line smmc-7721 cells was observed for the first time by TEM ultrathin section imaging. At an excitation wavelength of 365 nm, the graphene-QDs exhibit a strong luminescence in aqueous environments. The loading content and entrapment efficiency of the FGQCs were 70% and 50%, respectively. The cytotoxicity of this novel drug delivery system was evaluated in vitro using heptoma cell line smmc-7721 and quantified by the 3-(4,5-dimethylthiazol-z-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The results show that FGQCs are a promising new multifunctional material for drug delivery in biological and medical applications.