过热合金熔体的几种物性滞后效应

过热处理的合金熔体会使熔体结构状态发生不可逆变化。这种不可逆变化与合金熔体中结构不均匀性有关 ,即熔体过热处理引起非均匀形核中心数量的不可逆变化 ,导致熔体粘滞性 η和溶质扩散系数DL 出现滞后效应     

共晶Ni_(53)Si_(47)合金的液-固态结构相关性

利用高温液态X射线衍射仪研究了共晶Ni53Si47合金的液态结构。在衍射强度曲线的小角部分发现预峰,且预峰的强度随温度的降低而增强;研究表明,熔体中原子团簇结构稳定,有序度随温度降低而提高;通过纳米晶粒模型进一步研究了该合金的液-固态结构相关性,指出熔体衍射强度曲线上的预峰是化合物NiSi液态结构的一种体现,该合金液态结构与固态结构具有一定的相关性。     

脉冲电场对Al-10%Cu熔体活度的影响

采用分配系数法,以Pb-10%Cu熔体为平衡相,结合Miedema生成热模型,考察了电脉冲作用前后Al-10%Cu熔体Cu组元活度的变化。研究表明,脉冲电场作用下,体系溶质Cu的分配系数由0.2876变为1.6541,Pb-10%Cu合金中Cu原子有向上部Al-10%Cu熔体扩散的倾向。计算结果表明,分配体系下部Pb-Cu合金中溶质Cu的活度由原来的0.1143变为0.0385,基于溶质Cu在体系上下部活度相等的事实,得出上部Al-Cu熔体溶质Cu的活度变化。     

迁移预测模型中扩散系数的研究

详细介绍了3种典型的半经验化的扩散系数公式,分析其使用范围和局限性.同时,对通过直接计算机模拟得到扩散系数的分子动力学方法进行介绍.与半经验的扩散系数方程相比,分子动力学方法直接地给出了小分子在聚合物中的扩散系数,而且揭示了影响扩散的分子机理.     

过热合金熔体的几种物性滞后效应

过热处理的合金熔体会使熔体结构状态发生不可逆变化，。这种不可逆变化与合金熔体中的结构不均匀性有关，即熔体过热处理引起非均匀形核中心数量的不可逆变化，导致熔体粘滞性η和熔质扩散系数DL出现滞后效应。     

NiCrAlYSi涂层与镍基高温合金基体互扩散行为研究

采用电弧离子镀方法在镍基高温合金DZ125上沉积NiCrAlYSi涂层,通过对不同氧化时间后Al和Cr原子浓度分布曲线的分析,运用Boltzmann-matano方法,计算了Al和Cr元素在1373K分别加热0.5,2h和5h的互扩散系数,并拟合了这三个时间段的计算结果.结果表明:相同温度下,Al和Cr的互扩散系数分别随Al和Cr的原子浓度增加而增大.随氧化时间的延长,Al的互扩散系数随原子浓度的变化先增大然后基本不变,Cr的互扩散系数则逐渐减小;伴随着元素间互扩散行为的增强,涂层中的Al和Cr向基体扩散,基体合金元素Ni,Co,Mo,Ti和W则向涂层扩散,但涂层中Mo和Ti的含量相对较少.由于元素间互扩散行为,涂层中各元素的含量将趋向于更加均匀.     

脉冲电场对Al-1O%Cu熔体活度的影响

采用分配系数法,以Pb-10%Cu熔体为平衡相,结合Miedema生成热模型,考察了电脉冲作用前后Al-10%Cu熔体Cu组元活度的变化.研究表明,脉冲电场作用下,体系溶质Cu的分配系数由0.2876变为1.6541,Pb-10%Cu合金中Cu原子有向上部Al-10%Cu熔体扩散的倾向.计算结果表明,分配体系下部Pb-Cu合金中溶质Cu的活度由原来的0.1143变为0.0385,基于溶质Cu在体系上下部活度相等的事实,得出上部Al-Cu熔体溶质Cu的活度变化.     

铅铜合金真空蒸馏分离的研究

通过计算铅铜的饱和蒸气压、分离系数及气液相平衡图等热力学数据从理论上分析真空蒸馏分离Pb-Cu合金的可行性,计算结果表明:利用Pb与Cu饱和蒸气的差别采用真空蒸馏可以有效实现铅铜合金的分离.结合计算结果对含Cu5％～ 20％(质量比)的合金进行了实验研究,考察了蒸馏温度,蒸馏时间,熔体深度对合金分离效果的影响.实验结果表明:在蒸馏温度1373 K,蒸馏时间30min,料层厚度6mm的条件下,含铜量为5％ ～20％的铅铜合金经一次真空蒸馏后,得到Pb含量为99.9％的铅,铜含量为99.99％的铜,能够同时获得纯度较高的金属Pb与Cu.     

热镀Ni3Al耐蚀合金镀层液态冷凝过程的分子动力学模拟研究

钢铁表面热镀Ni3Al耐蚀合金过程中，涉及Ni3Al合金在钢铁表面由液态冷凝为镀层，冷却速度对结构及性能影响很大。采用F－S多体势对耐蚀液态合金Ni3Al在不同冷却速度下的微观结构及其转变机制进行了分子动力学模拟，得到了不同冷速下各温度的双体分布函数；采用HA键型指数法对其结构进行了分析，结果表明：Ni3Al的结构及能量转变受冷速影响较大，快冷时形成非晶，而慢冷时出现明显结晶，晶体的形成过程中有能量突变。     

非晶合金熔体的原子扩散

非晶合金熔体的扩散是描述非晶合金熔体动力学行为的重要参数,不同于一般的金属熔体,非晶合金熔体的扩散行为具有自己独特的性质,如表现出典型的慢扩散和复杂的温度依赖关系等。由于技术、理论上的原因,目前无论是国内还是国际上,对非晶合金熔体扩散的研究尚处于不成熟的阶段。主要介绍了扩散系数的几种比较可行的测量方法,其中包括最近本课题组在传统长管法和切单元法基础上开发的滑动剪切技术,该技术能够有效消除加热阶段的扩散,是熔体扩散系数测量的方法的一项进步。同时,基于前人的测量技术和理论模型,对非晶合金熔体的扩散研究进行了系统的总结和讨论。目前能较好地描述一般熔体原子扩散的模型:Arrhenius关系、VFT方程、Tn关系、Darken公式及S-E关系,在非晶合金熔体中都表现出很大的局限性。尽管MCT理论能预言熔体原子扩散的动力学行为,且得到了实验证实,但是其自身亦存在一些难以克服的问题。     

A Molecular Dynamics Simulation of the Diffusivity of O2 in Supercritical Water

Equilibrium molecular dynamics simulations were carried out to study the diffusion process of oxygen in supercritical water (SCW). Both infinite-dilution diffusion and Maxwell–Stefan (MS) mutual diffusion coefficients were calculated. The differences between the simulated Maxwell–Stefan diffusion coefficients and those predicted by the Darken equation were examined. It suggests that the velocity cross correlation function plays an important role in the oxygen–SCW mutual diffusion. The Darken equation may not be valid in predicting the Maxwell–Stefan diffusion coefficients for this mixture.     

Atomic scale structure and dynamics characteristics simulation of amorphous SrTiO3 by molecular dynamics

The atomic scale structure and dynamics characteristics of amorphous SrTiO₃ was simulated by molecular dynamics with potential function including Coulomb interaction, short range repulsion potential, Van der Waals interaction and Morse potential. From the energy and volume’s dramatic increase during heating, the melting point was estimated to be about 2440 K, in good agreement with the experimental value. The amorphous SrTiO₃ was obtained by quenching the liquid to room temperature. The correlation function and coordination numbers of the crystalline, liquid, and amorphous states were analyzed. The diffusion coefficients at various temperatures calculated from the auto correlation function of velocity verified that the melting occurred at 2440 K.     

Atomic scale structure and dynamics characteristics simulation of amorphous SrTiO3 by molecular dynamics

The atomic scale structure and dynamics characteristics of amorphous SrTiO₃ was simulated by molecular dynamics with potential function including Coulomb interaction, short range repulsion potential, Van der Waals interaction and Morse potential. From the energy and volume’s dramatic increase during heating, the melting point was estimated to be about 2440 K, in good agreement with the experimental value. The amorphous SrTiO₃ was obtained by quenching the liquid to room temperature. The correlation function and coordination numbers of the crystalline, liquid, and amorphous states were analyzed. The diffusion coefficients at various temperatures calculated from the auto correlation function of velocity verified that the melting occurred at 2440 K.     

Calculation of the mutual diffusion coefficient by equilibrium and nonequilibrium molecular dynamics

A nonequilibrium molecular dynamics method for the calculation of the mutual diffusion coefficient for a mixture of hard spheres is described. The method is applied to a 50-50 mixture of equidiameter particles having a mass ratio of 0.1 for the two species, at a volume of three times close-packing. By extrapolating the results to the limit of vanishing concentration gradient and infinite system size, we obtain a value in statistical agreement with the result obtained using a Green-Kubo molecular dynamics procedure, which is also described. The non-equilibrium calculation yields a mutual diffusion coefficient which decreases slightly with increasing concentration gradient. The Green-Kubo timecorrelation function for mutual diffusion displays a slow decay with time, qualitatively similar to the long-time tail which has been predicted by the hydrodynamic theory of Pomeau.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Modeling benzene sorption and diffusion in a γAl<Subscript>2</Subscript>O<Subscript>3</Subscript>catalyst

The behavior of the benzene--Pt--Sn/γ-Al₂O₃ catalytic system was simulated with the software Cerius² from Accelrys Inc. The Grand Canonical Monte Carlo (GCMC) method was used to assess the adsorption isotherms under conditions of constant pressure and an equilibrium number of benzene molecules. With the results of GCMC simulation it was possible to locate the adsorption centers on the micropore surface at low temperature and a homogeneous benzene monolayer surface at elevated temperature. The results obtained with the GCMC method were then used to simulate the molecular dynamics of the catalytic system and to assess the coefficients of benzene diffusion. Elevated temperature was found to limit molecular motion across the micropore to a large extent. From the temperature-dependence plots of the diffusion coefficients it can be inferred that if the number of molecules is constant, the diffusion in the micropore is a molecular one.     

Self Diffusion and Binary Maxwell–Stefan Diffusion in Simple Fluids with the Green–Kubo Method

Self-diffusion coefficients and binary Maxwell–Stefan diffusion coefficients were determined by equilibrium molecular dynamics simulations with the Green–Kubo method. The study covers five pure fluids: neon, argon, krypton, xenon, and methane and three binary mixtures: argon+krypton, argon+xenon, and krypton+xenon. The fluids are modeled by spherical Lennard-Jones pair-potentials, with parameters which were determined solely on the basis of vapor-liquid equilibrium data. The predictions of the self-diffusion coefficients agree within 5% for gas state points and about 10% for liquid state points. The Maxwell–Stefan diffusion coefficients are predicted within 10%. A test of Darken's model shows good agreement.     

Modeling benzene sorption and diffusion in a γAl2O3catalyst

The behavior of the benzene--Pt--Sn/-Al₂O₃ catalytic system was simulated with the software Cerius² from Accelrys Inc. The Grand Canonical Monte Carlo (GCMC) method was used to assess the adsorption isotherms under conditions of constant pressure and an equilibrium number of benzene molecules. With the results of GCMC simulation it was possible to locate the adsorption centers on the micropore surface at low temperature and a homogeneous benzene monolayer surface at elevated temperature. The results obtained with the GCMC method were then used to simulate the molecular dynamics of the catalytic system and to assess the coefficients of benzene diffusion. Elevated temperature was found to limit molecular motion across the micropore to a large extent. From the temperature-dependence plots of the diffusion coefficients it can be inferred that if the number of molecules is constant, the diffusion in the micropore is a molecular one.     

On the possible mechanism of crystal growth from melt under zero gravity conditions

A possible mechanism of the growth of semiconductor single crystals from melt under zero gravity conditions is considered. The results of computer simulation performed by the molecular dynamics method for a thin layer of melt on a single crystal surface are presented. The main characteristics of the melt component dynamics suggest a mechanism whereby the crystal grows to a significant extent due to the attachment of atoms or small atomic clusters, which accounts for the perfection of the crystal structure being grown.     

The glass transition and thermodynamics of liquid and amorphous TiO(2) nanoparticles

The glass transition and thermodynamics of spherical liquid TiO(2) nanoparticles, with different sizes ranging from 2 to 5?nm, have been studied in a model under non-periodic boundary conditions. We use the pairwise interatomic potentials proposed by Matsui and Akaogi. Models have been obtained by cooling from the melt via molecular dynamics (MD) simulation. The structural properties of liquid nanoparticles at 3500?K have been analyzed in detail through the partial radial distribution functions (PRDFs), coordination number distributions, bond-angle distributions and interatomic distances. Moreover, we also show the radial density profile in nanoparticles. Calculations show that size effects on the structure of a model are significant and that liquid TiO(2) nanoparticles have a distorted pentahedral network structure with the mean coordination numbers Z(Ti-O)≈5.0 and Z(O-Ti)≈2.5, while amorphous TiO(2) nanoparticles have an octahedral network structure. The temperature dependence of the surface structure and surface energy of the nanoparticles has been obtained and is presented. In addition, the size dependence of the glass transition temperature and the temperature dependence of the diffusion constant of atomic species have been found and are discussed.