Showing papers by "Vincent Ji published in 2015"

Hydrogen adsorption and storage on palladium-decorated graphene with boron dopants and vacancy defects: A first-principles study

Abstract: The geometric stability and hydrogen capacity of Pd-decorated graphene with experimentally realizable boron dopants and various vacancy defects including single carbon vacancy (SV), “585”-type double carbon vacancy (585 DCV) and “555-777”-type double carbon vacancy (555-777 DCV) are investigated using the first-principles calculations based on density functional theory (DFT). It is found that among the four types of defective structures, Pd′s binding energies on SV and 585 DCV defect graphene sheets exceed the cohesive energy of the Pd metal bulk, thus Pd atoms are well dispersed above defective graphene sheets and effectively prevent Pd clustering. Up to three H2 molecules can bind to Pd atom on graphene with B dopants, SV and 555-777 DCV defects. For the cases of Pd-decorated graphene with B dopants and 555-777 DCV defect, a single H2 or two H2 are molecularly chemisorbed to Pd atom in the form of Pd–H2 Kubas complex, where the stretched H–H bond is relaxed but not dissociated. Out of two adsorbed H2, the third H2 binds to Pd atom by small van der Waals (vdW) forces and the nature of bonding is very weak physisorption. Different from above two cases, three H2 are all molecularly chemisorbed to Pd atom with stretched H–H bond for Pd-decorated SV defect graphene, the hybridization of the Pd-4d orbitals with the H2-σ orbitals and the electrostatic interaction between the Pd cation and the induced H2 dipole both contribute to the H2 molecules binding, and the binding energies of 0.25–0.41 eV/H2 is in the range that can permit H2 molecules recycling at ambient conditions.

Residual stress distribution and microstructure in the friction stir weld of 7075 aluminum alloy

Abstract: In order to study the relationship between residual stress (RS) and the microstructure of friction stir weld (FSW), RS profiles through thickness in the un-welded aluminum alloy 7075 plate and in middle layer of its FSW joint were determined nondestructively by the short-wavelength X-ray diffraction (SWXRD) and neutron diffraction. Microstructure and mechanical properties of the FSW joint were also studied by optical microscopic analysis, and microhardness and tensile strength measurements. RS profiles measured by the two methods had the same distribution trend. The maximum tensile RS tested by SWXRD and neutron diffraction in transverse and longitudinal direction occurred in the weld nugget. Microhardness in the direction perpendicular to the weld line showed a “W” shape distribution. Position of the local maximal extremum of RS in thermo-mechanically affected zone corresponded to that of minimal microhardness. The grain-refined strengthening caused by the recrystallization in the weld nugget kept the joint from fracturing at this region notwithstanding the maximum tensile RS. And the tensile fracture occurred near the boundary of welding zone and thermo-mechanically affected zone where minimum of hardness and maximum of RS appear at the same position.

The Effect of Y/Ti Ratio on Oxide Precipitate Evolution in ODS Fe-14 Wt Pct Cr Alloys

Abstract: Fe-14Cr-1W oxide dispersion strengthened ferritic alloys have excellent mechanical properties at high temperatures due to a high density of thermally stable Y-Ti-O nanoclusters, which are rather stable at high temperature. The aim of this work was to study the evolutions of Y-Ti-O precipitation at high temperature as a function of nominal content of Y, Ti, and O. Different Fe-14Cr alloys were characterized by small-angle neutron scattering technique under magnetic field as well as transmission electron microscopy investigations before and after annealing at 1573 K to 1723 K (1300 °C to 1450 °C). Significant differences are observed depending on the Y/Ti/O ratios. For Ti/Y >1, the coarsening kinetics is low even at 1723 K (1450 °C). For Ti/Y <1, very large size particles are observed after annealing at 1573 K (1300 °C). We have shown that the oxygen content should be strictly controlled to avoid significant coarsening at 1573 K (1300 °C) and above.

Two stages for the evolution of crystallite size and texture of electrodeposited Ni–ZrC composite coating

Abstract: In this work, the evolution of crystallite size and crystalline preferred orientation (texture) of the electrodeposited Ni–ZrC composite coating has been investigated by using Rietveld X-ray diffraction (XRD) line profile analysis. The influences of both ZrC micron- and nano-particle concentrations on the microstructure evolution of the composite coating have been studied. Pole figures reconstructed by Rietveld refinements and measured by XRD and relative texture coefficients are all applied to characterize the variation of texture. With increasing ZrC particle concentrations in electrolyte from 1 to 40 g/L, the evolution of both crystallite size and texture can be divided to two stages, which can be named as texture-modified and crystallite-refined stage. At the first stage, strong depressing of fiber texture synchronizes with the decreasing of crystallite size along direction, which reduces more quickly than those along and directions. At the second stage, the strong fiber texture has been totally depressed and the crystallite shape changes from anisotropy to isotropy. And the crystallite sizes along , and directions all decrease gradually with the increasing of ZrC concentration. Compared with the ZrC micron-particles, ZrC nano-particles lead the composite coating with smaller crystallite size, weaker texture, higher microstrain and higher dislocation density at the same particle concentration. The evolution of crystallite size and texture is ascribed to the incorporation of the second phase ZrC particles into the coating.

Structural, electronic and magnetic properties of the Si chains doped zigzag AlN nanoribbons

Abstract: The structural, electronic and magnetic properties of the zigzag Si chains substituting for zigzag Al–N chains in hydrogen terminated hexagonally AlN nanoribbon with 7 zigzag Al–N chains across the ribbon width (7-ZAlNNR) have been investigated systematically using the first-principles calculations. The longer Si–Si bond length leads to a decrease (increase) in armchair (zigzag) Al–N bond length. The pure 7-ZAlNNR is a non-magnetic indirect semiconductor with a large indirect band gap of 2.979 eV. The Si chains doped 7-ZAlNNR is always metallic regardless of the Si chains doping concentrations, while the induced magnetic moment depends on the Si chains doping concentrations, increasing from 0.11 μ B for Si 2 –7-ZAlNNR to 0.45 μ B for pure 7-ZSiNR. The very low Si chains doping concentration makes the Si 1 –7-ZAlNNR a zero magnetic moment. Compared with the total DOS of the pure 7-ZAlNNR, the total DOS of the Si chains doped 7-ZAlNNR is not only broadened and shifted towards the low energy region but also transformed to metal. The magnetic moments of the Si chains doped 7-ZAlNNR are resulted from the zigzag Si chains but are not from zigzag Al–N chains because the Si–Si bond is a covalent σ bond with nonbonding π electrons while the Al–N bond is an ionic bond. Furthermore, on zigzag Si chains, the magnetic moments are mainly localized on the edge Si atoms decaying parallel (antiparallel) to the same (different) inner sub-lattices. With increasing Si chains doping concentration, both spin channels enlarge their distribution regions and thus magnetic moments.

Effects of vertical strain on zigzag graphene nanoribbon with a topological line defect

Abstract: We report the elastic, electronic and magnetic properties of naked ZGNRs with topological line defects (LD-ZGNRs) lying symmetrically on the ribbon's middle under vertical strains at four types of line defect atoms by using a first-principles approach. By changing the position and size of the local deformation of the ribbon, the optimal position is obtained. Moreover, an apparent spin-splitting of the energy band is obtained when a local deformation is created by the vertically applied strain.

Characterization of the deformation texture after tensile test and cold rolling of a Ti-6Al-4V sheet alloy

Abstract: The deformation texture after cold rolling and tensile test of an industrial Ti-6Al-4V sheet alloy was studied using X-ray diffraction. The alloy was subjected to a cold rolling to different thickness reductions (from 20% to 60%) and then tensile tests have been carried out along three directions relatively to the rolling direction (0°, 45° and 90°). The experimental results were compared to the existing literature and discussed in terms of active plastic deformation mechanisms.

Effect of Humidity on High Temperature Oxidation of AISI 430 Stainless Steel

Abstract: In order to investigate the high temperature oxidation behavior of AISI 430 stainless steel in atmosphere with humidity, the thermogravimetric analysis (TGA) has been conducted for different duration under air with variable absolute humidity (from 0% to 5%) at 800 °C and 900 °C. XRD has been used to determine residual stresses in each layer and in substrate. It has been found that the oxidation kinetics and residual stresses were affected by the water vapor. After oxidation, the oxide scale composed of an inner Chromia (Cr2O3) layer and an outer Mn1.5Cr1.5O4 spinel layer, while breakaway oxidation happened with the introduction of water vapor at 900 °C. The residual stresses in each of oxide layer are in compression and their levels varied with oxidation conditions.