Investigation of vapour-liquid nucleation properties for spherical and chain-like fluids by density functional theory

The excess Helmholtz free energy functional for nonpolar chain-like molecules is formulated in terms of a weighted density approximation （WDA） for short-range interactions and a Weaks Chandler Andersen （WCA） approximation and a Barker Henderson （BH） theory for long-range attraction. Within the framework of density functional theory （DFT）, vapour liquid interracial properties including density profile and surface tension, and vapour-liquid nucleation properties including density profile, work of formation and number of particles are investigated for spherical and chain- like molecules. The obtained vapour liquid surface tension and the number of particles in critical nucleus for Lennard- Jones （L J） fluids are consistent with the simulation results. The influences of supersaturation, temperature and chain length on vapour liquid nucleation properties are discussed.

Influence of Neodymium on Amorphizability of RS Al-Fe-V-Si-Nd Alloys:An Investigation Using Time Dependent Nucleation Theory

Time dependent nucleation theory was applied to calculate the incubation time required for α Al nucleation in rapid solidified (RS) Al Fe V Si Nd alloys. The nucleation rates were calculated as a function of temperature, and the critical cooling rates required for the formation of amorphous α Al at different neodymium concentrations were calculated too. The addition of neodymium increases the amorphizablity of α Al by increasing the incubation time and decreasing the nucleation rate and the critical cooling rate. The calculations are fitted to experimental results when liquidus temperatures are estimated from an approximation, which treats Al Fe V Si Nd as quasi binary Al Fe system.

The experimental investigation of microcracks nucleation in typical tectonics

he evolution and nucleation of microcracks in typical tectonics are investigated in the experiment of fracture of marble specimen. The change of state during nucleation of microcracks is observed. The controlling effect of tectonics on evolution of microcracks is analyzed by using thc damage mechanics theory. These characteristics can be analogized to kilometer meters as the first effect of earthquake precursors. These studies may be helpful to interpret-the foreshock or general foreshock in the moderate or short stage before strong earthquakes. The other physical precursors are second or third effect. The local density of microcracks increasing abruptly may be helpful to interpret the phenomenon that part precursor records appear catastrophic jump. The part out of nucleation where some microcracks heal and the density change reversibly may be helpful to interpret the phenomenon that some precursors records appear reverse change. The area difference of microcracks accumulation and evolution in different part of typital tectonics is studied. This difference may be helpful to interpret the characteristics (including the area) of earthquake preparation of diffcrent tectonics, and further to interpret the difference of the precursors beween plate edge and intraplate. These differences may be introduced by the scholars with different points of view as to discuss about the existence of precursors before earthquakes. However, when the precursor records are studied, one must notice the geology background in different areas.

Suppression of ice nucleation in supercooled water under temperature gradients

Understanding the behaviours of ice nucleation in non-isothermal conditions is of great importance for the preparation and retention of supercooled water. Here ice nucleation in supercooled water under temperature gradients is analyzed thermodynamically based on classical nucleation theory(CNT). Given that the free energy barrier for nucleation is dependent on temperature, different from a uniform temperature usually used in CNT, an assumption of linear temperature distribution in the ice nucleus was made and taken into consideration in analysis. The critical radius of the ice nucleus for nucleation and the corresponding nucleation model in the presence of a temperature gradient were obtained. It is observed that the critical radius is determined not only by the degree of supercooling, the only dependence in CNT, but also by the temperature gradient and even the Young's contact angle. Effects of temperature gradient on the change in free energy, critical radius,nucleation barrier and nucleation rate with different contact angles and degrees of supercooling are illustrated successively.The results show that a temperature gradient will increase the nucleation barrier and decrease the nucleation rate, particularly in the cases of large contact angle and low degree of supercooling. In addition, there is a critical temperature gradient for a given degree of supercooling and contact angle, at the higher of which the nucleation can be suppressed completely.

Molecular simulation studies on natural gas hydrates nucleation and growth:A review

How natural gas hydrates nucleate and grow is a crucial scientific question.The research on it will help solve practical problems encountered in hydrate accumulation,development,and utilization of hydrate related technology.Due to its limitations on both spatial and temporal dimensions,experiment cannot fully explain this issue on a micro-scale.With the development of computer technology,molecular simulation has been widely used in the study of hydrate formation because it can observe the nucleation and growth process of hydrates at the molecular level.This review will assess the recent progresses in molecular dynamics simulation of hydrate nucleation and growth,as well as the enlightening significance of these developments in hydrate applications.At the same time,combined with the problems encountered in recent hydrate trial mining and applications,some potential directions for molecular simulation in the research of hydrate nucleation and growth are proposed,and the future of molecular simulation research on hydrate nucleation and growth is prospected.

Electrodeposited PtNi nanoparticles towards oxygen reduction reaction:A study on nucleation and growth mechanism

In this work,highly monodispersed Pt-Ni alloy nanoparticles were directly deposited on carbon substrate through a facile electrodeposition strategy in the solvent system of N,N-dimethylformamide(DMF).A series of carbon supported Pt-Ni alloy electrocatalysts were synthesized under different applied electrode potentials.Among all as-obtained samples,the Pt-Ni/C electrocatalyst deposited at-1.73 V exhibits the optimal specific activity up to 1.850 mA cm^(-2)at 0.9 V vs.RHE,which is 6.85 times higher than that of the commercial Pt/C.Comprehensive physiochemical characterizations and computational evaluations via density functional theory were conducted to unveil the nucleation and growth mechanism of PtNi alloy formation.Compared to the aqueous solution,DMF solvent molecule must not be neglected in avoiding particle agglomeration and synthesis of monodispersed nanoparticles.During the alloy co-deposition process,Ni sites produced through the reduction of Ni(Ⅱ)precursor not only facilitates Pt-Ni alloy crystal nucleation but also in favor of further Pt reduction on the Ni-inserted Pt surface.As for the deposition potential,it adjusts the final particle size.This work provides a hopeful extended Pt-based catalyst layer production strategy for proton exchange membrane fuel cells and a new idea for the nucleation and growth mechanism exploration for electrodeposited Pt alloy.

基于分子束外延技术可控制备Bi原子团簇的研究

本研究基于分子束外延(MBE)技术在Si(111)衬底表面成功制备金属Bi原子团簇.首先,分别在100℃、125℃、150℃、175℃、200℃的生长温度下,制备了大小均一、密度不同的Bi原子团簇.实验结果表明,可以通过改变生长温度来精细控制Bi原子团簇的密度,当温度升高100℃,密度从1.05×10^(11)cm^(-2)降低至2.5×10^(7)cm^(-2),实现对团簇密度4个数量级的可控调节,并且发现Bi原子团簇密度对生长温度的依赖性符合经典成核理论.其次,分别在10 s、15 s、20 s的沉积时长下,制备了密度相同、尺寸各异的Bi原子团簇.实验结果表明,可以通过改变沉积时长来精细控制Bi原子团簇的尺寸:当沉积时长增加10 s,高度和直径分别从8.5 nm和65 nm增大到13.7 nm和100 nm,实现对团簇尺寸在10 nm高度、80 nm直径范围的可控调节,并且发现Bi原子团簇尺寸对沉积时长的依赖性符合晶体生长动力学.与分子束外延制备传统的Ⅲ族(Al,Ga,In)原子团簇做对比,这些结果可以为制备Ⅴ族原子团簇提供实验参考和指导,从而促进纳米级含Bi材料的制备.

Cerium oxide inclusion nucleation mechanism based on classical nucleation theory and two-step nucleation

It is challenging to assess the mechanism responsible for the nucleation of inclusions in metals at high temperatures.The present work therefore systematically investigates the nucleation of cerium oxide inclusions according to classical nucleation theory and a two-step nucleation mechanism.The nucleation rates and nucleation radii of these inclusions are obtained,and the results demonstrate a considerable difference between theoretical and experimental values.On the basis of a two-step nucleation mechanism,(CeO_(2))_(n) and(Ce_(2)O_(3))_(n)(n=1-6)clusters were constructed and the thermodynamic properties of both these clusters and of cerium oxide nanoparticles were analyzed.In addition,the entropies and heat capacity changes of cerium oxides were determined using first principles calculations and are found to be consistent with literature data.The present data indicate that the cerium oxide inclusion nucleation pathway can be summarized as[Ce]+[O]→(CeO_(2))n/(Ce_(2)O_(3))_(n)→(Ce_(2)O_(3))_(n)→(Ce_(2)O_(3))_(2)→core(Ce_(2)O_(3)crystal)-shell((Ce_(2)O_(3))_(2) cluster)nanoparticles→(Ce_(2)O_(3))bulk.

KCl溶液结晶辅助表征4H-SiC衬底中的微划痕

4H-SiC晶圆表面的微划痕会使外延层中产生大量缺陷,但其线宽低于光学表征技术的极限,无法以无损光学检测的方法获得微划痕在晶圆上的分布规律。本研究基于经典成核理论,利用微划痕上的高指数晶面,通过溶液干燥析晶的方法,使KCl晶体优先成核于高表面能的高指数晶面。实验结果表明:当KCl溶液的浓度为0.013 mol/L时,溶液蒸发后的KCl晶体嵌于微划痕的沟壑内,使微划痕可以间接地被光学方法检测。当KCl溶液的浓度大于0.013 mol/L时,KCl晶体会结晶于没有微划痕的区域,干扰对微划痕的表征。本方法制备的KCl晶体仅吸附成核于4H-SiC表面,不与4H-SiC发生化学反应,通过RCA清洗工艺即可完全去除表面的KCl晶体,不影响晶圆的后续加工工艺。

Asymmetric nucleation processes in spontaneous mode switch of active matter

Flocking and vortical are two typical motion modes in active matter.Although it is known that the two modes can spontaneously switch between each other in a finite-size system,the switching dynamics remain elusive.In this work,by computer simulation of a two-dimensional Vicsek-like system with 1000 particles,we find from the perspective of the classical nucleation theory that the forward and backward switching dynamics are asymmetric:going from flocking to vortical is a one-step nucleation process,while the opposite is a two-step nucleation process,with the system staying in a metastable state before reaching the final flocking state.

聚丙烯成核剂的开发与进展

聚丙烯(PP)是一种典型的半结晶热塑性聚合物,由于PP熔体的结晶速率较慢,易形成较大尺寸的球晶,其制品的光泽度和透明性较差,限制了PP在透明包装和日用品等领域的发展。目前,主要通过改变PP晶型的结晶行为达到改性PP的目的;而成核剂与PP共混改性在保留了PP原有特点的同时,透明性济表面光泽度均得到明显改善,为PP改性常用的方法。基于此,总结了PP结晶的机理,并且,综述了目前PP成核剂的研究现状,涉及的α成核剂种类包括无机类、枞酸类、环状羧酸盐类、山梨醇缩醛类及磷酸酯盐类成核剂;β成核剂种类包括稠环类、二元羧酸盐类、酰胺类、稀土类成核剂及复配型成核剂。

Constrained lattice density functional theory and its applications on vapor–liquid nucleations

Vapor-liquid phase transition occurs via a nucleation process, and depending on the role of foreign objects, nucleation can be either homogeneous or heterogeneous. In this review, we focus on the recently developed constrained lattice density functional theory （CLDFT） and its applications on vapor-liquid nucleation. We also review the recent theoretical advance on the stability of nanobubbles. Based on CLDFT, a pinning and supersaturation mechanism has proposed to interpret the surprising stability of surface nanobubbles. The mechanism can interpret most characteristics of nanobubbles. More importantly, the mechanism suggests that the critical nucleus can be stabilized under the condition of contact line pinning. Thus, CLDFT studies provide an alternative way to measure the critical nucleus that is difficult to measure experimentally in the bulk solution, through stabilizing it with surface roughness or heterogeneities.

天然气水合物生成动力学机理的研究

对天然气的气 水体系进行了研究,提出了一种新的天然气水合物生成机理,认为水合物晶核的生长经历了溶解、水合物骨架形成、扩散和吸附4个阶段,很好地解释了水合物组成的不确定性.据此机理对水合物在成核以后生成的动力学方程进行了推导,引入了有效面积分率、压力指数、过冷度校正参数等,得到了气 水体系水合物生成的改进的动力学方程.用遗传算法对方程中的参数进行拟合的结果表明,由方程计算所得的水合物生成速率与实验结果的最大误差为15.9%,最小误差为1.6%,平均误差为9.6%,说明拟合结果准确.

雪晶成核和生长机理研究

在自然界 ,雪晶的形成是一个非常复杂的过程 ,要经历成核、生长、融并等阶段。探索雪晶的成核和生长机理对人工造雪技术的发展具有重要的作用。雪晶的形态是由冰晶的晶格结构和雪晶的生长环境所决定的。通过对均相成核和异相成核过程中自由能变化的分析 ,确定了晶核形成所需的临界半径和成核势垒 ,为研究晶核的形成过程提供了理论依据。依据雪晶质量生长率与水蒸汽通量之间的关系 。

水汽在燃煤PM_(2.5)表面异质核化特性数值预测

根据Fletcher经典异质核化理论建立数值实验平台,将水汽在不可溶微粒表面异质核化临界过饱和度模拟结果与文献中实验结果进行了对比,二者符合较好。在此基础上,结合燃煤PM2.5物化性质及实测的表面张力与接触角数据,数值预测了水汽及润湿剂溶液蒸气在燃煤PM2.5表面异质核化晶核形成自由能、临界晶核半径、成核速率、临界饱和度。结果表明,添加润湿剂能降低临界晶核形成自由能,减小临界晶核半径,使成核速率急剧增加,成核临界饱和度显著降低,极大地改善了水汽的核化凝结能力,但不同润湿剂的作用效果不同,所选用的5种润湿剂中,以SDS与Silanolw22效果最佳。

典型构造微破裂集结的实验研究

通过大理岩破裂实验 ,考察了典型构造中微裂纹的演化、集结的过程和特征 .根据损伤理论 ,分析了构造对微破裂演化的控制作用 .根据微破裂集结临界条件的相似原理 ,认为这些特征可以类推到公里尺度 ,它们有助于解释地震中短期阶段的前震或广义前震分布 ,作为地震前兆的一次效应 .其它物理效应为二次或三次效应 .微破裂集结时局部密度突然加大 ,有助于解释部分前兆记录突跳现象 .非集结部分出现部分微裂纹愈合 ,密度反向变化 ,有助于解释一些前兆记录在短临阶段记录中出现反向的现象 .考察了不同构造的不同部位微破裂萌生和演化的范围差异 ,这些差异有助于解释不同地质构造孕震区域的孕震过程 ,从而可解释板缘和板内地震前兆记录的差异 .这种差异可能被不同观点学者分别引用 ,以论证地震是否有前兆记录的问题 .然而 ,在考察不同地区的前兆记录时 ,必须注意不同地区的地质背景 .

深过冷Ni-31.44%Pb偏晶合金快速凝固行为

采用熔融玻璃净化和循环过热相结合的方法研究了Ni 31.44%Pb偏晶合金宽过冷区间的凝固组织演化规律;从形核热力学和动力学两方面分析过冷熔体中稳定相(α)和亚稳相(L2)两相的竞争形核规律。研究结果表明:过冷Ni 31.44%Pb偏晶合金在快速凝固阶段本质上是以枝晶方式生长,首先形成α枝晶骨架,再辉重熔后分布于枝晶间的残余液相按照正常凝固模式进行分相/偏晶等后续反应。

剪切流场中气泡成核理论的研究

针对经典成核理论在描述流场中气泡成核行为时的局限性 ,根据缝模流道中的剪切成核实验结果 ,提出了剪切流场下描述气泡成核的剪切能成核理论 ,并与文献中其它剪切促进气泡成核的解释进行了比较 ,从而进一步完善了剪切流场中的气泡成核理论。

固态化合物对钢液非均质形核的触媒作用

对钢液中各种固态化合物基底与形核相(δ-Fe和γ-Fe)的价电子结构进行了计算,基于静电作用理论和经验电子理论提出了基底和形核相之间的特征参量△ρ,并对△ρ与钢液非均质形核触媒作用的关系进行了分析和讨论.结果表明:基底和形核相之间的特征参量△ρ越大,非均质形核作用越好.

材料相变过程中的形核理论

形核过程作为一级不连续相变中的重要阶段,对其研究具有重要的理论和应用价值.从经典理论的提出,到现今各种经典理论的修正以及非经典形核理论的建立,已有近一个世纪的时间.立足于对形核理论的理解,从热力学和动力学两个方面对相变中的形核理论进行了阐述,并对一些重要公式进行了重新推导和诠释;结合实际相变过程,对形核理论中仍存在的问题及可能的解决方法进行了探讨.