Kirkendall效应和空心纳米晶体的人工合成

<正>5反Kirkendall效应[9,28]经典的Kirkandall效应反映的是合金中组元元素在成分浓度梯度下引发空位的净流量,以一定的扩散速度越过扩散偶界面。或解释为:在退火的扩散偶条件下,在溶质原子的区域中诱发产生空位点缺陷的浓度改变。反Kirkendall效应则倒过来为[9]:由空位(或间隙原子)的扩散迁移区域中诱发产生溶     

固态扩散过程中的Kirkendall效应(英文)

在二十世纪四十年代,柯肯达尔(Kirkendall)发现,在二元固溶体中,扩散过程不能简单地用一种扩散系数来描述它,人们必须考虑到两种物质的扩散系数的不同。他的发现改变了以往的扩散理论和数据的处理方式。基于他的这种扩散理论成功解释了Kirkendall平面的行为。但是,多相扩散和扩散过程中组织形貌的演变是很复杂的,需要从物理化学的途径才能解决。从理论和工艺的角度来看,二元或多元体系中的相互作用是关键问题。本文综述了Kirkendall效应的发现以及最近的进展,及其在材料科学中的应用前景。     

纳米晶体的结构模型及性质研究

结合分子静力学的原子弛豫技术提出了一对纳米晶体的原子结构模型及性质进行研究的计算机模拟方法。并以纳米铁晶为模型材料，对纳米晶体的各种结构特性进行了研究，分析讨论了各种因素对结构特性的影响，模拟结果表明，纳米晶体晶界区域的结构既不同干非晶态，也不同于气态，它是一种具有一定短程有序度的特殊结构。此外，晶内原子结构与理想晶体结构也并不完全相同。     

纳米晶体材料的研究现状

本文综述国内外在纳米晶体材料研究领域的最新研究进展，包括纳米晶体材料的制备、微观结构特征、热稳定性、结构性能关系及纳米晶体材料的应用。     

纳米晶体Se的微观结构特征

本文采用Ｘ射线衍射和透射电子显微镜研究了纳米晶体Ｓｅ的微观结构特征，通过测定不同晶粒尺寸的纳米晶体Ｓｅ的点阵参数ａ和ｃ，发现纳米晶体Ｓｅ中存在晶格畸变，畸变量与晶粒尺寸有关，得到了纳米晶体Ｓｅ中晶胞体积变化与晶粒尺寸的关系，并由点阵参数计算出纳米晶体Ｓｅ的键长，用非晶态Ｓｅ中的无规链折叠的晶化机制解释了纳米晶体Ｓｅ中晶格畸变现象。     

纳米晶体材料热稳定性的研究进展

纳米晶体材料独特的结构特征使其具有不同于传统多晶材料的优异性能,如何提高纳米晶体材料的热稳定性,避免其过度粗化,是近年来材料领域研究的热点课题。本文综述了国内外对纳米晶体材料热稳定性的研究进展,简要介绍了纳米晶体材料的微观结构特点,着重分析了溶质原子、第二相颗粒和微观应力等因素对纳米晶体材料晶粒长大的影响规律,介绍了纳米晶体材料的热力学和动力学稳定机制。     

纯镍纳米晶体的晶格膨胀

本文研究了用高频溅射制备的纯Ｎｉ纳米晶体的晶格结构特征，利用Ｘ射线衍射技术精确测定了晶粒尺寸为６－１５ｎｍ的Ｎｉ纳米晶体的晶格常数，发现纳米晶体的晶格常数均大于完整Ｎｉ单晶的平衡晶格常数，且ａ值随晶粒尺寸减小而显著增大，利用纳米晶体的热力学状态对这种晶格膨胀效应进行了解释。     

非晶态合金向纳米晶体的相转变

利用非晶态合金的晶化过程可以形成纳米尺寸晶粒的多晶体材料（即纳米晶体），这种制备纳米晶体的方法近年来得到了广泛重视和深入研究，本文对这种制备方法和非晶态合金向纳米晶体的相转变动力学，热力学及转变机理等方面进行综述。     

金属纳米晶体熔化与过热的等效模型

从Hill微系统热力学理论出发,根据已建立的金属纳米晶体结合能的等效模型与相应熔化热力学模型,建立了简单、实用的金属纳米晶体熔化与过热的等效模型．模型不仅预测了随表面原子百分数的增大自由表面纳米晶体熔点与熔化熵的减少及包覆纳米晶体熔点与熔化熵的增加,还对纳米晶体产生过冷和过热的条件进行了分析．对金属纳米微粒、纳米线及纳米薄膜的熔点、熔化熵增大或减少的预测值与实验结果相一致．     

Interaction between the Kirkendall effect and the inverse Kirkendall effect in nanoscale particles

A simple kinetic model of the interaction between the Kirkendall effect and the inverse Kirkendall effect in hollow nano-shell formation by solid-state reaction is presented. The difference in vacancy concentrations at the inter-phase boundaries (and corresponding vacancy gradients) is taken into account as well as the curvature effect on the driving force of the reaction. A criterion for nano-shell formation is proposed.     

Bending of a Bimetallic Beam Due to the Kirkendall Effect

The time-dependent bending of single-phase and two-phase bimetal strips due to interdiffusion is computed. The model couples simple beam theory and diffusion with bending due to the creation and/or annihilation of vacancies necessitated by unequal lattice diffusion rates of the two metals. The single-phase analysis employs a Fourier method for the diffusion analysis and predicts a beam curvature that is initially proportional to time and later reaches a constant value. The two-phase analysis, which involves a moving interphase boundary, employs an error function solution for the diffusion problem to model early times and a numerical solution for later time. Unlike the single-phase results, linear behavior is obtained at early time only if the original interface is centered in the beam. In general, the curvature initially is proportional to the square root of time. The numerical solution gives the gradual transition of the curvature to a constant value at late time. In some cases, nonmonotonic time dependence is obtained for the curvature for the two-phase beam.     

The discovery and acceptance of the Kirkendall Effect: The result of a short research career

In the 1940s, it was a common belief that atomic diffusion took place via a direct exchange or ring mechanism that indicated the equality of diffusion of binary elements in metals and alloys. However, Ernest Kirkendall first observed inequality in the diffusion of copper and zinc in interdiffusion between brass and copper. This article reports how Kirkendall discovered the effect, now known as the Kirkendall Effect, in his short research career. Editor's Note: A hypertext-enhanced version of this article can be found at http://www.tms.org/pubs/journals/JOM/9706/Nakajima-9706.html. Also some of the artwork employed here was photographically reproduced from existing publications. As a result, the quality of the images is sometimes less than ideal.     

Effect of isothermal aging on intermetallic compounds and Kirkendall void growth kinetics of Au stud bumps

Intermetallic compounds (IMCs) and Kirkendall void growth kinetics at various interfaces in Au stud bumps were studied in terms of isothermal aging at 120 °C, 150 °C, and 180 °C for 300 h. Phases of AuSn, AuSn₂, and AuSn₄ form at the interface of Au studs and Sn after a reflow. The thickness of an Au-Sn IMC was quantified with an image analyzer as a function of aging temperature and time. The growth of the Au-Sn IMC increases linearly with the square root of the aging time at 120 °C, 150 °C, and 180 °C. The growth of the Au-Sn IMC at 180 °C differs from the growth of the Au-Sn IMC at 120 °C and 150 °C because of excessive Au consumption by the Au studs. Kirkendall voids form at the interface of the Au stud and the Au-Sn IMC and increase linearly with the square root of the aging time. The apparent activation energies for the growth of the Au-Sn IMC and the Kirkendall voids were determined to be 0.57 eV and 0.2 eV, respectively, from measurement of the thickness of the Au-Sn IMCs and the width of the Kirkendall voids in relation to the aging temperature and time.     

复合材料中空心颗粒的团聚效应

采用有限元分析方法对空心颗粒增强金属基复合材料进行研究,分析空心颗粒团聚对复合材料性能的影响。结果表明,空心颗粒的加入,可以改善金属基复合材料的力学性能。当空心颗粒体积分数较小时,其团聚程度对复合材料的弹性模量影响不大。随着团聚颗粒体积分数的提高,复合材料的屈服强度先增大后降低。     

Analysis of the Kirkendall effect,marker migration and pore formation

Some aspects of Kirkendall and lattice plane migration are studied by means of numerical simulations. The simulations are performed using individual mobilities assessed in this work and thermodynamic data available in literature. The simulations show that the position and stability of the Kirkendall plane as well as a number of other interesting features can be predicted using numerical simulation software in combination with assessed thermodynamic and kinetic data.     

中空冒口保温效果研究

依据空气绝热性,采用冒口中空的方式,减少冒口传热,提高冒口的保温效果。在实验模拟中,测定刚玉砖内外壁和中空气隙温度。由传热学热平衡理论和外壁温度初步确定最佳中空气隙为25 mm,但其保温效果并不明显。为进一步探究,提出在内外壁间以25 mm为一个单位插入隔热板的改进措施。结果表明,插入隔热板,可以大幅度降低外壁温度,提高保温效果。当插入一个隔热板时,可使热流量降低约50%。随着插入隔热板数量的增加,保温效果也逐渐提高。     

空心钢锭锭型结构对空心钢锭凝固过程的影响

对空心钢锭凝固过程的温度场进行了有限元分析 ,对不同锭型结构的芯部换热系数、耐火材料厚度对最终凝固位置、内筒壁最高温度及缩孔疏松的影响进行了对比 ,并讨论了耐火材料典型部位的温度随凝固时间的变化