Adsorption and penetration of hydrogen in W: A first principles study

In BCC crystals, such as Tungsten (W), slippage has been observed on the (1 1 0) and (1 1 2) planes. In this work, hydrogen diffusion paths from three different W surfaces ((1 0 0), (1 1 0) and (1 1 2)) have been calculated using first principles density functional theory. Equilibrium sites for H at the surfaces are identified. The energetics for H penetration from the surfaces to the solute site in the bulk is calculated. It is found that for our low surface coverage of H (3.4 × 10¹⁴ H/cm²), approximately 2 eV is required for an H atom to penetrate any of the W surfaces considered in this study.     

The structural and mechanical properties of CdN compound: A first principles study

First principles calculations are performed to investigate the structural, elastic, and mechanical properties of CdN for various structures: NaCI, CsCl, ZnS, wurtzite, WC, CdTe, NiAs, and CuS. The local density and generalized gradient approximations are used for modeling exchange–correlation effects. Our calculations indicate that CuS (B18) structure is energetically the most stable among the considered structures. The some basic physical properties such as lattice parameters, bulk modulus, and second-order elastic constants are calculated. We have also predicted the shear modulus, Young’s modulus, Poison’s ratio, Debye temperature, and sound velocities. Our structural and some other results are consistent with the available theoretical data.     

The first principles study on PtC compound

We have studied structural, thermodynamic, elastic, and electronic properties of platinum carbide (PtC) in zinc-blende and rock-salt structures by performing ab initio calculations within the LDA approximations. Particularly, we have focused on the structural and the pressure dependence of elastic moduli and related quantities. The other basic key properties, such as the lattice constant, cohesive energy, the phase transition pressure, bulk modulus and its pressure derivative are also repeated and compared with the other available experimental and theoretical works.     

Effect of coordination on bond properties: A first principles study

We have used density functional theory to obtain the binding curves for a variety of hypothetical periodic structures of Al, Si, Pb, Sn and Au. Upon examining the resulting database of results for equilibrium bond lengths and radial force constants (within a nearest-neighbour model), we find that both decrease smoothly as coordination is reduced. The effect of dimensionality appears to be small. We find that the force constants at equilibrium vary as the inverse eighth power of the equilibrium bond length. We also find evidence that the force constants are sensitive only to the bond length, and not to the coordination number. We believe these results will be useful in formulating interatomic potentials, e.g., for nanosystems.     

Electrocatalytic and photocatalytic performance of noble metal doped monolayer MoS2 in the hydrogen evolution reaction:A first principles study

To maximize the catalytic performance of MoS₂ in the hydrogen evolution reaction,we investigate the electrocatalytic and photocatalytic performance of monolayer MoS₂ doped with noble metal(Ag,Au,Cu,Pd,and Pt)using first principles calculation combined with the climbing image nudged elastic band method.We find the band gap of the monolayer MoS₂ is reduced significantly by the noble metal doping,which is unfavorable to improving its photocatalytic performance.The optical absorption coefficient shows that the doping does not increase the ability of the monolayer MoS₂ to absorb visible light.The monolayer MoS₂ doped with the noble metal is not a potential photocatalyst for the hydrogen evolution reaction because the band edge position of the conduction band minimum is lower than
-4.44 eV,the reduction potential of H⁺/H₂.Fortunately,the band gap reduction increases the electron transport performance of the monolayer MoS₂,and the activation energy of water splitting is greatly reduced by the noble metal doping,especially the Pt doping.On the whole,noble metal doping can enhance the electrocatalytic performance of the monolayer MoS₂.     

A study on the hot workability of wrought NiTi shape memory alloy

The hot workability of a wrought 49.8 Ni-50.2 Ti (at pct) alloy was assessed using the hot compression tests in temperature range of 700-1000 °C, strain rate of 0.001-1 s⁻¹, and the total strain of 0.7. The constitutive equations of Arrhenius-type hyperbolic-sine function was used to describe the flow stress as a function of strain rate and temperature. The preferable regions for hot workability of the alloy were achieved at Z (Zener-Holloman parameter) values of about 10⁹-10¹³ corresponding to the peak efficiency of 20-30% in the processing map. However, a narrow area in the processing map including the deformation temperature of 1000 °C and strain rate of 1 s⁻¹ is inconsistent with the related Z values. A flow instability region was observed at high Z values. Further instability regions were found at low temperature of 700 °C and low strain rates of 0.01-0.001 s⁻¹ as well as at high temperature of 1000 °C and high strain rate of 1 s⁻¹. The apparent feature of flow curves, the low value of peak efficiency, the similarity between the estimated apparent activation energy of deformation and that of the self diffusion of Ti in Ni, and the stress exponent of higher than 5, suggested that dynamic recovery (DRV) is the dominant restoration phenomenon during the hot working of the alloy.     

Dielectric properties of nanoscale multi-component systems: A first principles computational study

A new method has been developed within the framework of density functional theory to aid in the study of the dielectric properties of multi-component systems, with explicit treatment of surface and interface effects. Specially, we have determined the position dependent dielectric constant profiles for Si–SiO₂ and SiO₂-polymer systems. We find that at regions close to surfaces and interfaces, the dielectric constant is enhanced compared to the corresponding bulk values. In interior regions, the dielectric constant approaches the corresponding bulk values. The calculated optical and static dielectric constant values of these systems are in excellent agreement with experimental results, and other more involved computational treatments.     

Effect of porous NiTi alloy on bone formation: A comparative investigation with bulk NiTi alloy for 15 weeks in vivo

The purpose of this study is to investigate the effect of porous NiTi alloy on bone formation with a bulk NiTi alloy as a contrast. The porous NiTi alloy prepared by element powder sintering under Ar protection has a porosity of 45% and a mean pore size of 130 μm, and the pores are highly interconnected. The porous and bulk NiTi alloys were bilaterally implanted into the femurs of rabbits for 15 weeks. The bone-implant interface and bone ingrowth were evaluated by undecalcified histological examination under light and fluorescent microscope as well as environmental scanning electron microscope (ESEM). The results show: osteoblasts are very active with fast proliferation and no adverse tissue reaction occurs for the porous NiTi alloy after 15 weeks implantation; porous NiTi alloy has better osteoconductivity and osteointegration than the bulk one; the osteoblasts can ingrow the pores of porous NiTi implant, and direct bone-implant interface can be observed by fluorescent light microscope.     

A first principles technique for the analysis of alloy phase stability in random binary alloys

In this communication we introduce the augmented-space recursion method coupled with the orbital peeling technique, as a powerful tool for the calculation of effective cluster interactions, useful for the study of alloy phase stability. An application to the well studied PdV system has been carried out. This work was done in collaboration with Tanusri Saha and Indra Dasgupta, S N Bose National Centre for Basic Sciences, Calcutta     

Experimental study of one‐dimensional superelastic capability of a nearly equi‐atomic NiTi shape memory alloy

A series of experimental studies have been carried out on nearly equi‐atomic NiTi shape memory alloy wires. The effects of fatigue cycles, displacement rates as well as testing temperatures on the superelastic capabilities have been studied. Under cycling loading, the threshold stresses for martensitic transformation decrease and the residule strains increase. Saturation is reached after 100 cycles. With increasing displacement rates, the critical stresses required for the martensitic transformation increase and the slopes of the upper plateau in the stress‐strain curves rise. The dissipated energy increases rapidly with increasing displacement rate, reaches a maximum value at around 6.0mm/min and then decreases as the displacement rate continues to increase. Tests also show that the threshold stresses characterizing the forward/reverse transformations increase linearly with increasing test temperature.     

Biocompatibility study of plasma-coated nitinol (NiTi alloy) stents

The authors aimed to assess the surface modification effects of plasma coatings on biocompatibility of nitinol intravascular stent in terms of anticoagulation, haemocytolysis rate, hydrophilicity, cytotoxicity and so on. In order to improve their surface adhesive properties to endothelial cells, NiTi alloy intravascular stents were treated and coated using a low-temperature plasma deposition technique. It was found that plasma coating changed the surface morphology of the stents to a micron-level surface roughness in the range of 1-5 microm. In comparison with the untreated control, the plasma-treated NiTi alloy intravascular stents showed increased surface hydrophilicity and enhanced anticoagulation property. Testing results on plasma-coated NiTi stents indicated that they complied with the standard of national biologic safety evaluation of medical apparatus and instrument (GB/T16886-1997, People's Republic of China) in terms of haemocytolysis rate, cytotoxicity and pyretogen.     

等离子体沉积非晶SiOx:H纳米膜层改性医用NiTi合金研究

采用等离子体沉积技术在医用NiTi合金表面制备非晶SiOx:H纳米膜层,并对改性后材料表面化学及膜层晶体结构等进行表征.结果表明,通过控制TMS和O2两种单体流速比和沉积时间,可控制材料表面亲疏水性和SiOx:H纳米膜层厚度.反应沉积的SiOx:H具有非晶的结构.其最优化条件为;μ(TMS):μ(O2)=1:4,系统压力3.33Pa,D.C 5W,沉积时间4min.所制备的膜层厚度约为45nm,表面去离子水接触角为30～40°.SiOx:H纳米膜层改性NiTi合金表面可提高材料的抗腐蚀性能.     

Mechanical properties of U-0.95 mass fraction of Ti alloy quenching and aging treatment:a first principles study

First principles plane wave pseudopotential method was executed to calculate the mechanical properties with respect to the uranium-0.95 mass fraction of titanium(U-0.95 mass fraction of Ti) alloy for quenching and aging,including the elastic modulus,the value of shear modulus to bulk modulus(G/B) and the ideal tensile strength.The further research has also been done about the crack mechanism through Griffith rupture energy.These results show that the elastic moduli are 195.1 GPa for quenching orthorhombic a phase and 201.8 GPa for aging formed Guinier-Preston(G.P) zones,while G/B values are0.67 and 0.56,respectively.With the phase change of uranium-titanium(U-Ti) alloy via the quenching treatment,the ideal tensile strength is diverse and distinct with different crystal orientations of the anisotropic α phase.Comparison of quenching and short time aging treatment,both of the strength and toughness trend to improve slightly.Further analysis about electronic density of states(DOS) in the electronic scale indicates that the strength increases continuously while toughness decreases with the aging proceeding.The equilibrium structure appears in overaging process,as a result of decomposition of metastable quenching a phase.Thereby the strength and toughness trend to decrease slightly.Finally,the ideal fracture energies of G.P zones and overaging structure are obtained within the framework of Griffith fracture theory,which are 4.67 J/m2 and 3.83 J/m2,respectively.These results theoretically demonstrate strengthening effect of quenching and aging heat treatment on U-Ti alloy.     

The improved plasticity of NiTi alloy via electropulsing in rolling

NiTi alloys were cold-rolled and electroplastic-rolled (EP-rolled) respectively, and the effect of rolling parameters on working plasticity was studied. NiTi alloy is easy to be embrittled and cracked in cold rolling, and the number of edge cracks is increased with higher thickness reduction. Electropulsing can enhance the ductility of NiTi alloy in rolling. In rolling, the resistance of deformation can be decreased, and deformation degree at the same roll gap increases because NiTi alloy develops recrystallisation and the hardness is decreased.     

NiTi合金动态断裂过程的实验和数值方法研究

为获得Ni Ti合金的动态起裂韧度和动态扩展速度与动态加载率之间的定量变化规律。利用分离式霍普金森压杆(SHPB)测试系统对单边三点弯曲试样(SENB)进行冲击加载试验,采用实验-有限元相结合的方法,获得动态断裂参数随时间的变化规律。SENB试样裂纹起裂时刻和裂纹扩展速度由粘贴在裂纹尖端的裂纹扩展计(CPG)测定。采用上述方法和数据获得Ni Ti合金的I型动态起裂韧度和动态扩展速度。实验结果表明:裂纹扩展计测得的起裂时刻与粘贴在同一试样上的监裂应变片测得的结果基本相符,因此可以利用裂纹扩展计代替传统的监裂应变片来监测裂纹起裂时刻,并获得Ni Ti合金的起裂韧度。同时,可以利用裂纹扩展计(CPG)获得裂纹动态扩展过程,绘制出裂纹扩展速度与时间的关系曲线,从而探讨Ni Ti合金的动态断裂韧度和裂纹扩展速度与动态加载率之间的定量变化规律。     

X-ray diffraction study of the phase transformations in NiTi shape memory alloy

The phase transformations occurring in heat-treated NiTi shape memory alloys have been studied through the analysis of variation in integrated peak area (integrated intensity) with temperature, under the XRD peak profiles in the transformation temperature range. For this purpose, integrated peak area under the prominent peak corresponding to (110) plane of the austenitic phase has been chosen. The results so obtained are compared with those got from the DSC method. The XRD method is found to be more sensitive.     

Silicon adsorption in defective carbon nanotubes: a first principles study

The electronic and structural properties of an (8, 0) single-walled carbon nanotube (SWNT) with a single vacancy and interacting with a Si atom are studied using first principles calculations based on the density-functional theory. Initially, the Si atom is positioned in the site above the vacancy, with its position fixed until the nanotube geometry is fully relaxed. After that, the Si atom approaches the tube and it is shown that one C atom is displaced outwards forming a bump. The final configuration, as well as each step of the process, is studied in detail and the resulting band structures and the total charge densities are systematically analysed.     

Shock compaction of NiTi alloy powder

The shock recovery experiment for the equiatomic NiTi alloy powder was performed by the flyer impact technique. The powder samples with the initial density of 70% of full density were shock-treated in the dyer velocity range 0.65 to 1.7 km sec^?1. At the optimum flyer velocity of 1.3 km sec^?1, the powder sample is compacted up to 99.5% of the full density. With increasing flyer velocity, new pores are formed in the melted layer instead of the disappearance of initial interstices. Effects of the mechanical deformation and the annealing brought by the shock treatment evidently appear in the temperature dependence of electrical resistivity accompanied with the martensitic transformation. The shock state and the relaxation of the heterogeneous temperature by the shock treatment are estimated, which indicate the annealing condition caused by shock-loading and the formation of new pores.     

A first principles study on the full-Heusler compound Mn2CuSb

Electronic structure calculations from first-principles are employed to design half-metallic ferrimagnets susceptible of finding applications in spintronics. The electronic structure and magnetism properties of a new Mn-based Heusler alloy Mn₂CuSb are studied and both possible L2₁ structures (CuHg₂Ti and AlCu₂Mn types) are taken into account. It is found that the CuHg₂Ti-type structure is energetically more favorable than the AlCu₂Mn-type structure and presents half-metallic character. Calculations show that their total spin moment is ?2.00μ_B for a wide range of equilibrium lattice constants and the total spin magnetic moment is attributed mainly to the two Mn atoms. Simultaneously, the small spin magnetic moments of Cu and Sb atoms are parallel and the compound is ferrimagnets. The total spin moment mainly origins from the antiparallel configurations of the Mn partial moments. The CuHg₂Ti-type Mn₂CuSb alloy keeps a 100% of spin polarization of states at the Fermi level. Our findings suggest that Mn₂CuSb may be a promising material for future spintronic applications.