Molecular dynamics simulation of mechanical properties of nanocrystalline platinum: Grain-size and temperature effects

A series of molecular dynamics simulations has been carried out to study the mechanical properties of nanocrystalline platinum. The effects of average grain size and temperature on mechanical behaviors are discussed. The simulated uniaxial tensile results indicate the presence of a critical average grain size about 14.1 nm, for which there is an inversion of the conventional Hall-Petch relation at temperature of 300 K. The transition can be explained by a change of dominant deformation mechanism from dislocation motion for average grain size above 14.1 nm to grain boundary sliding for smaller grain size. The Young's modulus shows a linear relationship with the reciprocal of grain size, and the modulus of the grain boundary is about 42% of that of the grain core at 300 K. The parameters of mechanical properties, including Young's modulus, ultimate strength, yield stress and flow stress, decrease with the increase of temperature. It is noteworthy that the critical average grain size for the inversion of the Hall-Petch relation is sensitive to temperature and the Young's modulus has an approximate linear relation with the temperature. The results will accelerate its functional applications of nanocrystalline materials.     

Study of mechanical properties of amorphous copper with molecular dynamics simulation

The formation and mechanical properties of amorphous copper are studied using molecular dynamics simulation. The simulations of tension and shearing show that more pronounced plasticity is found under shearing, compared to tension. Apparent strain hardening and strain rate effect are observed. Interestingly, the variations of number density of atoms during deformation indicate free volume creation, especially under higher strain rate. In particular, it is found that shear induced dilatation does appear in the amorphous metal.     

Molecular dynamics simulation of thermodynamic properties of YAG

In this paper we study the thermodynamic properties of Y分子动力学;热力学;扩散;热学YAG, diffusion, elastic constant, molecular dynamicsProject supported by the National Natural Science Foundation of China (Grant No~10744002).2/2/2007 12:00:00 AMIn this paper we study the thermodynamic properties of Y3Al5O12 （YAG） by using molecular dynamic method combined with two- and three-body potentials. The dependences of melting process, elastic constant and diffusion coefficient on temperature of crystal YAG are simulated and compared with the experimental results. Our results show that anion O has the biggest self-diffusivity and cation Y has the smallest self-diffusivity in a crystal YAG. The calculated diffusion activation energies of ions O, Al and Y are 282.55, 439.46, 469.71kJ/mol, respectively. Comparing with experimental creep activation energy of YAG confirms that cation Y can restrict the diffusional creep rate of crystal YAG.     

Magnetic properties of Ni doped gallium nitride with vacancy induced defect

Investigations have been carried out to study the ferromagnetic properties of transition metal (TM) doped wurtzite GaN from first principle calculations using tight binding linear muffin-tin orbital (TBLMTO) method within the density functional theory. The present calculation reveals ferromagnetism in nickel doped GaN with a magnetic moment of 1.13 μ_B for 6.25% of Ni doping and 1.32 μ_B for 12.5% of nickel doping, there is a decrease of magnetic moment when two Ni atoms are bonded via nitrogen atom. The Ga vacancy (V_Ga) induced defect shows ferromagnetic state. Here the magnetic moment arises due to the tetrahedral bonding of three N atoms with the vacancy which is at a distance of 3.689 Å and the other N atom which is at a distance of 3.678 Å .On the other hand the defect induced by N vacancy (V_N) has no effect on magnetic moment and the system shows metallic character. When Ni is introduced into a Ga vacancy (V_Ga) site, charge transfer occur from the Ni ‘d’ like band to acceptor level of V_Ga and formed a strong Ni–N bond. In this Ni–V_Ga complex with an Ni ion and a Ga defect, the magnetic moment due to N atom is 0.299 μ_B .In case of Ni substitution in Ga site with N vacancy, the system is ferromagnetic with a magnetic moment of 1 μ_B.     

Molecular dynamics simulation of thermodynamical properties of copper clusters

The melting and freezing processes of Cu_N (N=180, 256, 360, 408, 500, 628 and 736) nanoclusters are simulated by using micro-canonical molecular dynamics simulation technique. The potential energies and the heat capacities as a function of temperature are obtained. The results reveal that the melting and freezing points increase almost linearly with the atom number in the cluster increasing. All copper nanoclusters have negative heat capacity around the melting and freezing points, and hysteresis effect in the melting/freezing transition is derived in Cu_N nanoclusters for the first time.     

Molecular dynamics study of the mechanical properties of palladium nanocontacts

The mechanical properties of extended palladium nanocontacts have been investigated by the molecular dynamics method. The characteristic interatomic distances in the contacts have been determined and the process of the formation of palladium atomic contacts undergoing breaking has been studied for the (100), (110), and (111) orientations of the contact-surface interfaces.     

晶粒尺寸对CoSb3化合物热电性能的影响

系统地研究了晶粒尺寸对CoSb3化合物热电性能的影响规律,结果表明晶粒尺寸对CoSb3化合物的晶格热导率κp、电导率σ、能隙宽度Eg和Seebeck系数α有显著影响.当晶粒尺寸由微米尺度减小到纳米尺度时,晶格热导率κp显著降低,Seebeck系数α有较大幅度的增加,能隙宽度Eg变宽,电导率σ有一定程度的下降.平均晶粒尺寸为200nm的CoSb3化合物在温度为700K时,ZT值达到0.43,比平均晶粒尺寸为5000 nm的试样增加了4倍.     

Effect of oxygen vacancy defect on the magnetic properties of Co-doped ZnO

The influence of oxygen vacancy on the magnetism of Co-doped ZnO has been investigated by the first-principles calculations.It is suggested that oxygen vacancy and its location play crucial roles on the magnetic properties of Co-doped ZnO.The exchange coupling mechanism should account for the magnetism in Co-doped ZnO with oxygen vacancy and the oxygen vacancy is likely to be close to the Co atom.The oxygen vacancy (doping electrons) might be available for carrier mediation but is localized with a certain length and can strengthen the ferromagnetic exchange interaction between Co atoms.     

Molecular dynamics simulation on the elastoplastic properties of copper nanowire under torsion

Influences of different factors on the torsion properties of single crystal copper nanowire are studied by molecular dynamics method. The length, torsional rate, and temperature of the nanowire are discussed at the elastic-plastic critical point. According to the average potential energy curve and shear stress curve, the elastic-plastic critical angle is determined. Also, the dislocation at elastoplastic critical points is analyzed. The simulation results show that the single crystal copper nanowire can be strengthened by lengthening the model, decreasing the torsional rate, and lowering the temperature. Moreover, atoms move violently and dislocation is more likely to occur with a higher temperature. This work mainly describes the mechanical behavior of the model under different states.     

Molecular dynamics study of mechanical properties of carbon nanotube-embedded gold composites

The mechanical properties of nano-single crystal gold and carbon nanotube-embedded gold (CNT/Au) composites under axial tension were investigated using molecular dynamics (MD) simulation method. The interactions between atoms were modeled using the many-body tight-binding (TB) potential and the empirical Tersoff potential coupled with the Lennard-Jones (L-J) potential. We get the yield strain and the yield stress of nano-single crystal gold 0.092, 5.74 GPa, respectively. The computational results show that the increase in Young's modulus of the long CNT-embedded gold composite over pure gold is much large. From the simulation, we also find that the yield stress and the yield strain of short CNT-embedded gold composite are evidently less than that of the nano-single crystal gold.     

高温下钙蒙脱石膨胀特性的分子动力学模拟

高温下蒙脱石的膨胀特性在核废料深部封存、二氧化碳封存及页岩气开发等应用中有着重要影响,但相关机理尚不明确.本工作使用分子动力学模拟为技术手段计算5 MPa和298—500 K等条件下,1.40—4.00 nm晶面间距(d)的一系列饱和钙蒙脱石的膨胀压力.以模拟所得的数值结果为依据,基于水化效应、双电层效应和离子关联效应等模型推演膨胀压力随温度与d的变化规律,并与相应的实验数据进行对比.模拟结果表明,当d较小时,因为高温会弱化水化力的强度,钙蒙脱石膨胀压力震荡的幅度降低,同时水化力作用的d的范围减小.当d较大时,因为高温强化离子关联效应,膨胀压力降低,同时双电层力的作用的d的范围增加.在较高温度和较大d时,膨胀压力为收缩力,阻碍膨胀.这些膨胀压力的变化规律与前期钠蒙脱石体系的研究类似.然而,通过对比两种蒙脱石体系的模拟结果,发现两种体系存在显著的差异—钙蒙脱石比钠蒙脱石更难膨胀到较大的d.此模拟结果与前人实验观测的结果相符.我们进一步将此差异归于钙蒙脱石的离子关联效应要远大于钠蒙脱石.有别于分子模拟中对于离子关联效应的精确描述,连续化的Poisson-Boltzmann方程因为忽略了离子关联效应,从而无法表达出与两种体系模拟结果都相吻合的膨胀压力变化规律.     

Molecular dynamics simulation of the surface properties of nanocrystalline uranium dioxide

The molecular dynamics method is used to simulate the nanosized UO₂ crystals. The phase-transition temperatures are calculated for the nanosized crystals of the uranium dioxide. It is demonstrated that the melting point and the temperature of the transition to the superionic state (melting of the anion sublattice) of the crystals decrease with decreasing sizes. In particular, melting point (T _m ～ 2300 K) for the cubic nanocrystal with a size of 3.3 nm is lower than the melting point of the single crystal by almost 1000 K. The calculated surface energies are in agreement with the experimental results. The dependence of the surface energy on the size of the UO₂ nanocrystals is obtained. The effect of the nanocrystal temperature on the surface energy is studied. The temperature dependence of the thickness of the melt layer is obtained in the framework of the model of the heterogeneous melting. The parameters and dependences can be used for the further analysis of the microstructure properties of nuclear fuel in working systems.     

高温下钙蒙脱石膨胀特性的分子动力学模拟

高温下蒙脱石的膨胀特性在核废料深部封存、二氧化碳封存及页岩气开发等应用中有着重要影响,但相关机理尚不明确.本工作使用分子动力学模拟为技术手段计算5 MPa和298—500 K等条件下,1.40—4.00 nm晶面间距(d)的一系列饱和钙蒙脱石的膨胀压力.以模拟所得的数值结果为依据,基于水化效应、双电层效应和离子关联效应等模型推演膨胀压力随温度与d的变化规律,并与相应的实验数据进行对比.模拟结果表明,当d较小时,因为高温会弱化水化力的强度,钙蒙脱石膨胀压力震荡的幅度降低,同时水化力作用的d的范围减小.当d较大时,因为高温强化离子关联效应,膨胀压力降低,同时双电层力的作用的d的范围增加.在较高温度和较大d时,膨胀压力为收缩力,阻碍膨胀.这些膨胀压力的变化规律与前期钠蒙脱石体系的研究类似.然而,通过对比两种蒙脱石体系的模拟结果,发现两种体系存在显著的差异—钙蒙脱石比钠蒙脱石更难膨胀到较大的d.此模拟结果与前人实验观测的结果相符.我们进一步将此差异归于钙蒙脱石的离子关联效应要远大于钠蒙脱石.有别于分子模拟中对于离子关联效应的精确描述,连续化的Poisson-Boltzmann方程因为忽略了离子关联效应,从而无法表达出与两种体系模拟结果都相吻合的膨胀压力变化规律.     

Molecular dynamics simulation on elastoplastic properties of the void expansion in nanocrystalline copper

Influences of different factors on the elastic-plastic properties of nanocrystalline copper containing a void are studied by the molecular dynamics method. The radius of the circular plate is 30a, while the radius of the void is 5a (a is 0.3615 nm for the lattice constant of bulk copper). The effects of crystal orientation, the void ellipticity, loading rate, and temperature of nanocrystalline copper are discussed. The elastic-plastic deformation of nanocrystalline under inner pressure is investigated in this research. The plastic zone is determined according to the dislocation nucleation from the edge of the void. The simulation results show that there are different deformation mechanisms under different crystal orientations, and the nanocrystalline copper can be strengthened by changing the void shape, decreasing the loading rate, and lowering the temperature. And the plastic zone initiation and growth are further explained. The change of different conditions has a great influence on plastic zone.     

Molecular dynamics simulation of nanoscale mechanical behaviour of ZnO under nanoscratching and nanoindentation

Molecular dynamics is used to simulate the mechanical behaviour of zinc oxide under nanoscratching and nanoindentation. The effects of indenter speed and substrate temperature on the structure-phase formation, slip vector, radial distribution function, and residual stresses are investigated. Simulation results show that the dislocation loops nucleate and propagate, forming a body-centred tetragonal lattice structure along the slip direction due to high local stress. Furthermore, the dislocation loops nucleate and propagate due to the resolved shear stress along the 45° slip direction under nanoscratching. The average mean biaxial stress and the normal stress of the O layers are –9.35 and –4.36 GPa, respectively, and those of the Zn layers are –0.80 and –0.30 GPa, respectively. This may be attributed to the energetic O atoms, with which unstable atoms have high activation.     

Molecular dynamics simulation of defect formation in an aluminum crystal under low-energy ion bombardment

Atomic collision cascades initiated by Ar and Xe ions (with energies of 25, 40, and 50 eV) normally incident on the Al(100) crystal surface at a crystal temperature of 300 K have been simulated by the molecular dynamics technique. The formation of vacancies and radiation-adsorbed and interstitial atoms in a cascade is discussed. It is demonstrated that the numbers of surface and bulk vacancies formed in cascades under bombardment of the Al(100) surface by Xe ions reach two maxima within 0.2–0.3 and 0.7–1.0 ps after the cascade initiation, whereas the number of vacancies generated under bombardment by Ar ions reaches one maximum within 0.2–0.3 ps after the cascade initiation.     

Molecular dynamics study on the thermal conductivity and mechanical properties of boron doped graphene

We investigated the effects of boron atoms substitution on the thermal conductivity and mechanical properties of single-layer graphene using the non-equilibrium molecular dynamics (NEMD) simulations. By performing the uniaxial tension simulations, we observed that substituted boron atoms slightly decrease the elastic modulus and tensile strength of graphene. On the other hand, it was observed that only 0.75% concentration of boron atoms in graphene reduces the thermal conductivity of graphene by more than 60% and leads to vanishing chirality effect.     

气体分子与纳米粒子碰撞的分子动力学仿真分析

当气体分子与纳米粒子碰撞的时候,纳米粒子传输理论预测到当纳米粒子的直径由小变大时,碰撞会由镜面反射转化为漫反射.文章利用分子动力学仿真研究了气体分子与纳米粒子碰撞的过程.在验证了这种转化存在同时,又探讨了碰撞转化的机理,即漫反射的起因.仿真结果揭示了漫反射的起因是由于纳米粒子对气体分子的吸附作用.这种吸附作用是由于纳米粒子对能量的容纳特性而产生的.