硅熔化特性的分子动力学模拟–-不同势函数的对比研究

分别采用Stillinger-Weber (SW)势、修正的成熟原子嵌入模型(MEAM)势、 Tersoff势和HOEP (highly optimized empirical potential)势来描述硅原子间相互作用, 运用分子动力学方法对比模拟研究了四种势函数的硅晶体的体熔化和表面熔化特性. 结果表明: 四种势函数均能反映出硅的热膨胀、高温熔化和熔化时吸热收缩等基本物理规律. 但综合对比发现, Tersoff势和MEAM势相对更适合描述硅的熔化和凝固过程, SW势次之, HOEP势则不适合描述硅的熔化和凝固过程. 关键词： 硅 势函数 熔化 分子动力学     

应变效应对金属Cu表面熔化影响的分子动力学模拟

采用Mishin镶嵌原子势，通过分子动力学方法模拟了金属Cu 的(110)表面在不同应变下的熔 化行为，分析了表面熔化过程中系统结构组态和能量的变化以及固液界面迁移情况.金属Cu 的(110)表面在低于热力学熔点的温度下发生预熔化，准液体层的厚度随温度升高而增加.当 温度高于热力学熔点时，固液界面的移动速度与温度成正比，外推得到热力学熔点为1380K ，与实验结果1358K吻合良好.应变效应（包括拉伸和压缩）导致热力学熔点降低，并促进表 面预熔化进程.在相同温度条件下，准液体层的厚度随应变绝对值的增加而增大.应变效应导 致的固相自由能增加是金属Cu(110)表面热稳定性下降的主要因素，且表面应力和应变方向 的异同也会影响表面预熔化的进程. 关键词： 表面预熔化 热力学熔点 表面应力 分子动力学     

α-B晶体的Lennard-Jones对势和对势型多体势构建

精确原子间相互作用势函数的建立是分子动力学模拟的核心. 针对α-B晶体(R3m群), 分别构建Lennard-Jones (L-J)型对势和对势型多体势, 这两类势函数的构建仅需考虑晶体的原子平均结合能和几何构型参数. 前者取最近邻原子间距的势函数为最小值; 后者采用负指数(L-J型)函数和分段插值多项式形式来分别构造势函数, 在近邻处为势能极小值, 相邻极小值位置的中点引入势垒. 针对完整的α-B 晶体和偏离完整α-B晶体结构, 首先利用第一性原理计算结果来确定势函数的参数, 然后借助分子动力学的能量最小化方法优化结构, 并对这两类势函数以及Tersoff 势函数进行比较. 结果表明: L-J对势和Tersoff势的结果与α-B晶体构型有较大偏离; 而对势型多体势, 无论初始晶体构型完整与否, 其结果与完整α-B晶体构型比较一致.     

氮化硼、碳化硅、锗纳米管的熔化与拉伸力学特性

该论文采用Tersoff势的分子动力学方法分析了单壁(5,5)氮化硼、碳化硅、锗纳米管的熔化与轴向拉伸力学特性,讨论了三种纳米管熔化与轴向拉伸力学性能的差异.研究表明:氮化硼管熔化后呈现为网状,碳化硅管为疏松的不规则的团状,锗管呈现为紧密排布的近似球状;相同温度下,碳化硅及氮化硼纳米管的熔点、比热容以及熔化热却均远高于锗管,但系统能量却远低于锗管;三种纳米管中,氮化硼管的抗变形抗能力最大,锗管的抗变形与抗载荷能力最小,而氮化硼、碳化硅管的抗载荷能力相当.     

氮化硼、碳化硅、锗纳米管的熔化与拉伸力学特性

该论文采用Tersoff势的分子动力学方法分析了单壁(5,5)氮化硼、碳化硅、锗纳米管的熔化与轴向拉伸力学特性,讨论了三种纳米管熔化与轴向拉伸力学性能的差异。 研究表明：氮化硼管熔化后呈现为网状,碳化硅管为疏松的不规则的团状, 锗管呈现为紧密排布的近似球状;相同温度下,碳化硅及氮化硼纳米管的熔点、比热容以及熔化热却均远高于锗管,但系统能量却远低于锗管;三种纳米管中,氮化硼管的抗变形抗能力最大,锗管的抗变形与抗载荷能力最小,而氮化硼、碳化硅管的抗载荷能力相当。     

金属钨熔化过程的分子动力学模拟

本文使用分子动力学方法对金属钨的熔化过程进行了数值模拟,分析了钨在熔化过程中的结构、系统内能变化以及表面熔化过程固-液界面变化情况,初步分析了表面熔化现象的机理。模拟过程采用嵌入原子模型(EAM)描述原子间相互作用,模拟结果表明,嵌入原子模型适合于计算固-液相变过程,表面熔化过程是由表面处最外层原子的不稳定性触发的。对于均匀熔化过程,晶体在4700 K下发生固-液相变;对于表面熔化过程,计算获得了不同温度(3800～4800 K)下的熔化速度,拟合出熔化速度公式,得到的表面熔化热力学熔点与已有实验结果基本符合。     

支撑Co团簇熔化行为的分子动力学模拟

运用分子动力学结合退火方法,模拟研究了所含原子数介于400～20000之间的自由及支撑Co团簇的熔化行为.模拟中团簇Co原子间相互作用采用半经验的Gupta多体相互作用势.讨论了不同衬底势(Lennard-Jones势与Morse势)对支撑团簇熔化行为的影响.结果表明:对于给定原子数目的支撑Co团簇,在衬底势强度相同时,两种衬底势下对支撑团簇熔点及预熔化区间给出了一致的描述;随衬底势强度增加,支撑团簇熔点升高,且两种衬底势所描述的支撑Co团簇也类同于自由Co团簇都表现出较好的熔点与尺寸依赖的线性关系.     

热力学视角下“晶体的熔化与凝固实验”问题探究

"探究固体熔化时温度的变化规律"实验观察到了许多与书中规律不符的现象,对于学生准确理解课程内容有较大的阻碍,比如过冷现象、凝固点与熔点不相等的现象.本文将从热力学的相变、晶体生长的角度对上述现象的物理学本质进行分析,从而找到更优化的实验方案.     

5种金属高压熔化曲线的理论计算

利用从头算法确定了基于对势的平均场模型中的未知势参数, 从而确定了势函数, 然后利用得到的势函数构建平均场, 进一步计算了5种金属材料 Al, Cu, Ni, Na, K在熔点处原子振动的自由体积. 计算的结果表明, 熔化曲线上原子振动的自由体积与原子晶胞体积之比是常数这一假设并不普遍适用; 对该假设修正后可以比较准确地计算材料的熔化曲线. 关键词： 高压熔化 自由体积理论     

不同势下铱团簇结构和熔化行为的分子动力学模拟

采用分子动力学方法及淬火技术,结合Gupta势和Sutton-Chen势,模拟研究了包含13,14,55,56,147及148个原子数的铱团簇的熔化行为. 结果表明,Gupta势和Sutton-Chen势对所研究的Ir团簇的基态几何结构和熔化行为给出了基本一致的描述：两种不同势给出了完全相同的基态几何结构;两种势给出的Ir团簇熔点及预熔化区间随团簇尺寸的变化关系基本一致;对于小Ir_n团簇（n=13,14）两类势均表现出比热峰值相对于均方根键长涨落饱和值滞后的现象. 但是 关键词： 铱团簇 分子动力学 Gupta势和Sutton-Chen势 熔化     

High undercooling of bulk water during acoustic levitation

The experiments on undercooling of acoustically levitated water drops with the radius of 5-8 mm are carried out, and the maximum undercooling of 24 K is obtained in such a containerless state. Various factors influencing the undercoolability of water under acoustic levitation are synthetically analyzed. The experimental results indicate that impurities tend to decrease the undercooling level, whereas the dominant factor is the effect of ultrasound. The stirring and cavitation effects of ultrasound tend to stimulate the nucleation of water and prevent further bulk undercooling in experiments. The stirring effect provides some extra energy fluctuation to overcome the thermodynamic barrier for nucleation. The local high pressure caused by cavitation effect increases the local undercooling in water and stimulates nucleation before the achievement of a large bulk undercooling. According to the cooling curves, the dendrite growth velocity of ice is estimated, which is in good agreement with the theoretical prediction at the lower undercooling. The theoretical calculation predicts a dendrite growth velocity of 0.23 m/s corresponding to the maximum undercooling of 24 K, at which the rapid solidification of ice occurs.     

A molecular dynamics study on melting point and specific heat of Ni<Subscript>3</Subscript>Al alloy

Using the Embedding Atom Method (EAM) for highly undercooled Ni₃Al alloy, the melting point and the specific heat were studied by a molecular dynamics simulation. The simulation of melting point was carried out by means of the sandwich method and the NVE ensemble method, and the results show a good agreement, whereas are larger than the experimental value of 1663 K. This difference is attributed to the influence of surface melting on experimental results, which causes the smaller measurements compared with the thermodynamic melting point. The simulated specific heat of Ni₃Al alloy weakly and linearly increases with the increase of undercooling in the temperature range from 800 K to 2000 K. Supported by the National Natural Science Foundation of China (Grant No. 50395101)     

Molecular dynamics of binary metal nitrides and ternary oxynitrides

Ternary systems incorporating metals with oxygen and nitrogen are examined using Tersoff potentials. The apparent success of treating some binary nitride systems using the Tersoff potential is used as a way forward to obtain a new parameter set incorporates atomic features into a series of Tersoff potential for binary nitrides and ternary oxynitrides.     

Metastable phase separation and rapid solidification of undercooled Co_(40)Fe_(40)Cu_(20) alloy

The metastable liquid phase separation and rapid solidification behaviors of Co_(40) Fe_(40) Cu_(20) alloy were investigated by using differential thermal analysis(DTA) in combination with glass fluxing and electromagnetic levitation(EML) techniques. The critical liquid phase separation undercooling for this alloy was determined by DTA to be 174 K. Macrosegregation morphologies are formed in the bulk samples processed by both DTA and EML. It is revealed that undercooling level, cooling rate, convection, and surface tension difference between the two separated phases play a dominant role in the coalescence and segregation of the separated phases. The growth velocity of the(Fe,Co) dendrite has been measured as a function of undercooling up to 275 K. The temperature rise resulting from recalescence increases linearly with the increase of undercooling because of the enhancement of recalescence. The slope change of the recalescence temperature rise versus undercooling at the critical undercooling also implies the occurrence of liquid demixing.     

Formation of NI80P20 solid solution phase under pressure

A Ni-P solid solution phase was obtained by quenching of melts under a pressure of 4.5 GPa. This was considered as a metastable high pressure phase. Despite the lack of thermodynamic parameters for Ni₈₀, P₂₀ alloy under pressure, the degree of undercooling, nucleation frequency and crystal growth velocity were calculated. We conclude that metastable phases with the same composition as the melting phase, such as supersaturated solid solution phase and amorphous phase, are easily prepared by high-pressure quenching.     

Molecular dynamics study of anisotropic growth of silicon

Based on the Tersoff potential, molecular dynamics simulations have been performed to investigate the kinetic coefficients and growth velocities of Si(100),(110),(111), and(112) planes. The sequences of the kinetic coefficients and growth velocities are μ_((100)) μ_((110)) μ_((112)) μ_((111))and v_((100)) v_((110)) v_((112)) v_((111)), respectively, which are not consistent with the sequences of the interface energies, interplanar spacings, and melting points of the four planes. However,they agree well with the sequences of the distributions and diffusion coefficients of the melting atoms near the solid–liquid interfaces. It indicates that the atomic distributions and diffusion coefficients affected by the crystal orientations determine the anisotropic growth of silicon. The formation of stacking fault structure will further decrease the growth velocity of the Si(111) plane.     

MEAM study of carbon atom interaction with Ni nano particle

Carbon, Ni, and C-Ni alloy modified embedded atom method (MEAM) potentials were developed to study the initial process of carbon nanotube growth on Ni catalyst particles. The MEAM potentials were used to study the atomistic interaction between a carbon atom and a fcc Ni nano particle, both on the particle surfaces and inside the Ni nano particles. The result shows that surface carbon atom is more stable than those in the bulk and sub-surface interstitial positions. Carbon atoms are expected to diffuse from the bulk to the surface, and the single walled and double-walled carbon nanotubes would be more favorable to form on Ni nano particle catalyst. The carbon and Ni nano particle interaction calculation shows that the corner and the edge of the particle are the energetically more favorable sites for the carbon adatom. The carbon nanotube may grow from the corner and edge of the particle.     

Evaporation of atoms from femtosecond laser-heated gallium arsenide

The surface temperature of a GaAs crystal irradiated with 150 fs laser pulses is determined from the Maxwell velocity distribution of the evaporated atoms. The crystal is strongly superheated, and melting is observed to occur at a temperature 600–1000 K above the equilibrium melting point.