Shizhong Yang

Nitrogen-Doped Fullerene as a Potential Catalyst for Hydrogen Fuel Cells

We examine the possibility of nitrogen-doped C60 fullerene (N-C60) as a cathode catalyst for hydrogen fuel cells. We use first-principles spin-polarized density functional theory calculations to simulate the electrocatalytic reactions on N-C60. The first-principles results show that an O2 molecule can be adsorbed and partially reduced on the N-C complex sites (Pauling sites) of N-C60 without any activation barrier. Through a direct pathway, the partially reduced O2 can further react with H(+) and additional electrons and complete the water formation reaction (WFR) with no activation energy barrier. In the indirect pathway, reduced O2 reacts with H(+) and additional electrons to form H2O molecules through a transition state (TS) with a small activation barrier (0.22-0.37 eV). From an intermediate state to a TS, H(+) can obtain a kinetic energy of ∼0.95-3.68 eV, due to the Coulomb electric interaction, and easily overcome the activation energy barrier during the WFR. The full catalytic reaction cycles can be completed energetically, and N-C60 fullerene recovers to its original structure for the next catalytic reaction cycle. N-C60 fullerene is a potential cathode catalyst for hydrogen fuel cells.

Texture of Nanocrystalline Nickel: Probing the Lower Size Limit of Dislocation Activity

Strength Under Pressure Above a lower cutoff value, shrinking the grain size of a metal tends to strengthen it because the overall increase in grain boundaries limits the activity of the dislocations as the material undergoes plastic deformation. Chen et al. (p. 1448) explore the question of whether this restriction in dislocation activity occurs when a metal is subjected to high pressures. Foils of nickel made from particles of different sizes were subjected to high pressures inside a diamond anvil cell. An increase in pressure extended dislocation activity to smaller grain sizes, indicating that pressure compensates for the inhibition of dislocation activity in small volumes. A metal subjected to high pressures retains the activity of dislocations, even for very small grain sizes. The size of nanocrystals provides a limitation on dislocation activity and associated stress-induced deformation. Dislocation-mediated plastic deformation is expected to become inactive below a critical particle size, which has been proposed to be between 10 and 30 nanometers according to computer simulations and transmission electron microscopy analysis. However, deformation experiments at high pressure on polycrystalline nickel suggest that dislocation activity is still operative in 3-nanometer crystals. Substantial texturing is observed at pressures above 3.0 gigapascals for 500-nanometer nickel and at greater than 11.0 gigapascals for 20-nanometer nickel. Surprisingly, texturing is also seen in 3-nanometer nickel when compressed above 18.5 gigapascals. The observations of pressure-promoted texturing indicate that under high external pressures, dislocation activity can be extended down to a few-nanometers-length scale.

Senary refractory high entropy alloy MoNbTaTiVW

Abstract The design approach and validation of a single phase senary refractory high entropy alloy (HEA) MoNbTaTiVW was presented in the present study. The design approach was to combine phase diagram inspection of available binary and ternary systems and Calculation of Phase Diagrams prediction. Experiments using X-ray diffraction and scanning electron microscopy techniques verified a single phase microstructure in body centred cubic lattice for MoNbTaTiVW. The observed elemental segregation agrees well with the solidification prediction using the Scheil model. The lattice constant, density and microhardness were measured to be 0.3216 nm, 4.954 GPa and 11.70 g cm− 3 respectively. The atomic size difference, the Ω parameter, enthalpy of mixing and entropy of mixing for MoNbTaTiVW HEA are 3.1%, 11.1, − 3.4 kJ mol− 1 and +13.39 J K− 1 mol− 1 respectively.

Senary Refractory High-Entropy Alloy HfNbTaTiVZr

Discovery of new single-phase high-entropy alloys (HEAs) is important to understand HEA formation mechanisms. The present study reports computational design and experimental validation of a senary HEA, HfNbTaTiVZr, in a body-centered cubic structure. The phase diagrams and thermodynamic properties of this senary system were modeled using the CALPHAD method. Its atomic structure and diffusion constants were studied using ab initio molecular dynamics simulations. The microstructure of the as-cast HfNbTaTiVZr alloy was studied using X-ray diffraction and scanning electron microscopy, and the microsegregation in the as-cast state was found to qualitatively agree with the solidification predictions from CALPHAD. Supported by both simulation and experimental results, the HEA formation rules are discussed.

Detecting grain rotation at the nanoscale

Significance The plastic deformation of nanomaterials has long been wrapped in mystery. Grain rotation is suggested to be a dominant mechanism of plastic deformation for ultrafine nanomaterials. However, the in situ observation of grain rotation has been made possible only for coarse-grained materials. Here we report the in situ high-pressure detection of grain rotation at the nanoscale. The surprising observation is that the texture strength of the same-sized platinum drops rapidly with decreasing grain size of the nickel medium, indicating that more active grain rotation occurs in the smaller nickel nanocrystals. Insight into these processes provides a better understanding of the plastic deformation of nanomaterials at a few-nanometer length scale. It is well-believed that below a certain particle size, grain boundary-mediated plastic deformation (e.g., grain rotation, grain boundary sliding and diffusion) substitutes for conventional dislocation nucleation and motion as the dominant deformation mechanism. However, in situ probing of grain boundary processes of ultrafine nanocrystals during plastic deformation has not been feasible, precluding the direct exploration of the nanomechanics. Here we present the in situ texturing observation of bulk-sized platinum in a nickel pressure medium of various particle sizes from 500 nm down to 3 nm. Surprisingly, the texture strength of the same-sized platinum drops rapidly with decreasing grain size of the nickel medium, indicating that more active grain rotation occurs in the smaller nickel nanocrystals. Insight into these processes provides a better understanding of the plastic deformation of nanomaterials in a few-nanometer length scale.

Effects of axial tensile and bending strains on the critical current of Bi-2223 superconducting tapes

Abstract The effects of axial tensile and bending strains on the critical current of Ag and AgMn/Bi-based tapes produced by the powder-in-tube method have been studied at 77 K. The observed degradation of critical current due to tensile strain effects is irreversible. AgMn sheath and multifilament are beneficial to improve the tensile and bending strain tolerance of Bi-2223 superconducting tapes. In the case of Ag/Bi-based tapes, the e irr ≈0.15%, while that of AgMn/Bi-based tapes is 0.34%. The larger tensile and bending strain tolerance of AgMn/Bi-2223 superconducting tape may result from better mechanical properties.

The AC loss of transport current in (Bi, Pb)-2223 superconducting tapes

Abstract The self-field AC loss of the untwisted (Bi,Pb)-2223 tapes made with different matrices, number of filaments, critical currents and process techniques was measured in the frequency range from 20.8 to 363 Hz at 77 K. The losses of different matrix (Bi,Pb)-2223 tapes under different tensile strain were investigated at 50 Hz, 77 K. We find that: (1) in the case of I m / I c I m / I c >0.4, the eddy current loss must be taken into consideration; (2) above the critical current, the loss, which is shown resistive, is independent of current frequency, and differences in processing have a major impact; (3) below the critical current, the losses, P , increase in proportion with I m / I c with slopes less than 3, which is not in accordance with the Bean model; (4) during the elastic stage the self-field AC losses of AgMn and Ag matrix (Bi,Pb)-2223 tapes do not change, while in yielding stage the unstable relations of AC loss vs. axial tensile strain maybe caused by the serrated curve of stress vs. tensile strain relation, and the losses of both AgMn and Ag matrix tapes increase more in the plastic stage.

Effects of the hydroxyl group on phenyl based ligand/ERRγ protein binding.

Bisphenol-A (4,4′-dihydroxy-2,2-diphenylpropane, BPA, or BPA-A) and its derivatives, when exposed to humans, may affect functions of multiple organs by specific binding to the human estrogen-related receptor γ (ERRγ). We carried out atomistic molecular dynamics (MD) simulations of three ligand compounds including BPA-A, 4-α-cumylphenol (BPA-C), and 2,2-diphenylpropane (BPA-D) binding to the ligand binding domain (LBD) of a human ERRγ to study the structures and energies associated with the binding. We used the implicit Molecular Mechanics/Poisson–Boltzmann Surface Area (MM/PBSA) method to estimate the free energies of binding for the phenyl based compound/ERRγ systems. The addition of hydroxyl groups to the aromatic ring had only a minor effect on binding structures and a significant effect on ligand/protein binding energy in an aqueous solution. Free binding energies of BPA-D to the ERRγ were found to be considerably less than those of BPA-A and BPA-C to the ERRγ. These results are well correlated with those from experiments where no binding affinities were determined in the BPA-D/ERRγ complex. No conformational change was observed for the helix 12 (H-12) of ERRγ upon binding of these compounds preserving an active transcriptional conformation state.

Structural and mechanical stability of dilute yttrium doped chromium

Rare earth element doping of chromium is much desired for various applications, but is technically difficult because of dopant segregation. Using a room temperature mechanical alloying method, dilute yttrium doping into nanosized chromium was achieved. Synchrotron-based high-pressure X-ray diffraction indicated that the Cr-Y alloy (Cr0.97Y0.03) was stable at up to 39 GPa, and the bulk modulus was 203 ± 2.6 GPa. The experimental results were consistent with first-principles density functional theory simulation. The diffraction line broadening profiles indicated the deformation anisotropy of the nanoalloy. This study suggests that Cr0.97Y0.03 alloy is promising for ultrahigh stress applications such as airplane engines and land-based turbines.

Unexpected pressure induced ductileness tuning in sulfur doped polycrystalline nickel metal

The sulfur induced embrittlement of polycrystalline nickel (Ni) metal has been a long-standing mystery. It is suggested that sulfur impurity makes ductile Ni metal brittle in many industry applications due to various mechanisms, such as impurity segregation and disorder-induced melting etc. Here we report an observation that the most ductile measurement occurs at a critical sulfur doping concentration, 14 at.% at pressure from 14 GPa up to 29 GPa through texture evolution analysis. The synchrotron-based high pressure texturing measurements using radial diamond anvil cell (rDAC) X-ray diffraction (XRD) techniques reveal that the activities of slip systems in the polycrystalline nickel metal are affected by sulfur impurities and external pressures, giving rise to the changes in the plastic deformation of the nickel metal. Dislocation dynamics (DD) simulation on dislocation density and velocity further confirms the pressure induced ductilization changes in S doped Ni metal. This observation and simulation su...