The unique attractor in the theory of Ostwald ripening

It is shown that time-independence of the inverse rate constant implies a time-independent probability distribution of particles sizes, and that this probability distribution arises from the form of the asymptotic characteristic curves of the solution to the continuity equation. From this an expression for the distribution is derived, valid for finite volume fraction.     

To the theory of Ostwald ripening in metallic alloys

The mechanism of growth (dissolution) of nanoparticles controlled simultaneously by the matrix diffusion, diffusion along dislocations, and the rate of the passage of atoms through the interphase boundary (by the rate of formation of interatomic bonds) has been studied in alloys of CuNiAl and AlLi metallic systems at the stage of Ostwald ripening. An expression for the nanoparticle-size distribution function has been obtained, which was calculated based on the dependence of the rate of growth on three fluxes, i.e., diffusion flux j_V, dislocation flux j_d, and kinetic flux j_i. A comparison of the experimental histograms with the theoretically calculated curves indicates their good agreement, which can indicate the opportunity of the realization in practice of the proposed mechanism of growth of nanoparticles in alloys of metallic systems.     

The development of spatial correlations during Ostwald ripening: a test of theory

The coarsening of solid-Sn particles in a Pb–Sn liquid was studied under microgravity conditions. Spatial correlation functions were measured on plane sections in a low-volume fraction system undergoing Ostwald ripening. The correlation functions changed with time in a way that indicated that the microstructure initially consisted of clusters of particles and evolved into one which was more dispersed. The model by Akaiwa and Voorhees (AV) was used to study the effect of spatial correlations on the ripening process. We found that the initially highly correlated structure had no observable effect on the evolution of particle size distributions, but did have an effect on the coarsening rate of the system. Specifically, we determined that a structure consisting of clusters of particles coarsened faster than a system with a random, spatial arrangement of non-overlapping particles. We also found that the approach of the microstructure towards the steady-state regime could be monitored more sensitively using spatial correlations rather than using particle size distributions. The spacial correlations and the particle size distributions measured from the experiment agreed well with those calculated from the AV simulations using the initial experimental correlations and size distribution.     

Transient electroluminescence in multilayer organic light-emitting diodes: experiment and theory

We have investigated a multilayer organic light-emitting diode (OLED) with 1,3,5-tris(N,N-bis-(4,5-methoxy-phenyl)-aminophenyl)-benzene (TAPB) acting as hole transporting layer (HTL) and tris(8-hydroxy-quinolinolato) aluminium (Alq₃) as electron transporting layer (ETL). Positive charge carriers in the HTL were detected optically as a function of the applied bias. Furthermore, we investigated the DC-characteristics of current and brightness as well as the onset behaviour of the electroluminescence (EL) as a function of the applied bias. An analytical model is developed to describe the observed carrier concentrations as well as the current–voltage characteristics and the transient EL measurements quantitatively.     

Phase-field simulation of 2-D Ostwald ripening in the high volume fraction regime

The microstructural evolution and kinetics of Ostwald ripening were studied in the high volume fraction regime by numerically solving the time-dependent Ginzburg–Landau (TDGL) and Cahn–Hilliard equations. It is shown that the growth exponent m is equal to 3, independent of the volume fraction, and the kinetic coefficient k increases as the volume fraction increases. The shape of size distributions changes significantly with increasing volume fraction of the coarsening phase; the skewness changes continuously from negative to positive while the kurtosis decreases in the low fraction regime and increases in the high volume fraction regime.     

Quantitative predictions of the trans-interface diffusion-controlled theory of particle coarsening

Data on the coarsening of γ′ (Ni₃Al) precipitates in binary Ni–Al alloys are re-examined quantitatively in light of the theory of trans-interface diffusion-controlled (TIDC) coarsening, which predicts time-dependent behavior of the type 〈r〉ⁿ ≈ k_Tt for the growth of precipitates of average radius 〈r〉 and X ≈ κ_Tt^–1/n for the depletion of solute concentration in the matrix, X. The exponent n is intimately related to the width of the precipitate–matrix interface, δ, which depends on r as δ ∝ r^m (m = n ? 2). The scaled distribution of particle sizes (PSD) also depends on n, while the rate constants k_T and κ_T depend on the thermophysical constants of the alloy system. In Ni–Al alloys n = 2.4, determined from analyzing three different sets of PSDs. Quantitative analysis yields interfacial free energies and chemical diffusion coefficients that agree exceptionally well with extant data. The TIDC theory is the only theory that is consistent, both qualitatively and quantitatively, with the entirety of the data.     

Large-scale simulations of Ostwald ripening in elastically stressed solids. II. Coarsening kinetics and particle size distribution

Ostwald ripening of misfitting second-phase particles in an elastically anisotropic solid is studied by large-scale simulations. The coarsening kinetics for the average particle size are described by a t^1/3 power law with a rate constant equal to its stress-free value when the particles are fourfold symmetric. However, the rate constant increases when the elastic stress is sufficient to induce a large number of twofold-symmetric particles. We find that interparticle elastic interactions at a 10% area fraction of particles do not affect the overall coarsening kinetics. A mean-field approach was used to develop a theory of Ostwald ripening in the presence of elastic stress. The simulation results on the coarsening kinetics agree well with the theoretical predictions. The particle size distribution scaled by the average particle size is not time invariant, but widens slightly with an increasing ratio of elastic to interfacial energies. No time-independent steady state under scaling is found, but a unique time-dependent state exists that is characterized by the ratio of elastic energy to interfacial energy.     

Ostwald ripening and grain growth in Ti(C,N)-based cermets during liquid phase sintering

After liquid phase sintering, core-rim microstructure occurs in the grains of Ti(C,N)-based cermets. The mechanisms of the microstructural evolution during different sintering stages were investigated here. Our theoretical analyses show that the rim thickness of a grain is relatively independent of its initial radius when rim formation is dominated by Ostwald ripening during dissolution-reprecipitation stage, whereas it increases linearly with its initial radius when grain growth during subsequent cooling stage is the dominant mechanism. A geometric analysis via cross-sectioning technique is developed to identify dominant rim formation mechanism. Experiments with Ti(C,N)-based cermets show that rim formation is dominated by grain growth at low sintering temperatures and by Ostwald ripening at sufficiently high sintering temperatures.     

Modern charge density studies: the entanglement of experiment and theory

This tutorial review article is intended to provide a general guidance to a reader interested to learn about the methodologies to obtain accurate electron density mapping in molecules and crystalline solids, from theory or from experiment, and to carry out a sensible interpretation of the results, for chemical, biochemical or materials science applications. The review mainly focuses on X-ray diffraction techniques and refinement of experimental models, in particular multipolar models. Neutron diffraction, which was widely used in the past to fix accurate positions of atoms, is now used for more specific purposes. The review illustrates three principal analyses of the experimental or theoretical electron density, based on quantum chemical, semi-empirical or empirical interpretation schemes, such as the quantum theory of atoms in molecules, the semi-classical evaluation of interaction energies and the Hirshfeld analysis. In particular, it is shown that a simple topological analysis based on a partition of the electron density cannot alone reveal the whole nature of chemical bonding. More information based on the pair density is necessary. A connection between quantum mechanics and observable quantities is given in order to provide the physical grounds to explain the observations and to justify the interpretations.     

Analysis for the wear resistance anisotropy of diamond cutting tools in theory and experiment

In this work, the dynamic micro-mechanical strengths of diamond crystal are deduced in theory, including the tensile, shearing and compressive strengths. The calculated results reveal that the dynamic micro-mechanical strengths have great anisotropy, but the tensile strengths are less than the shearing and compressive ones in any orientation of any plane. Subsequently, a novel evaluation factor is proposed, which integrates from the theoretical tensile strength in the orientation of flank face paralleling to the cutting direction and the theoretical tensile strength in the orientation of rake face paralleling to the chip flowing direction. And then as expected, the anisotropy of the resistance to wear of diamond cutting tools can be predicted exactly through comparing the evaluation factor. Theoretical analyses indicate the larger the evaluation factor, the greater the wear resistance of diamond cutting tool is. Finally, the cutting experiments are carried out on the (1 1 1) silicon wafers, and the sampled data are well consistent with the theoretical predictions, which validates that the proposed evaluation factor is suited for predicting the anisotropy of the resistance to wear of diamond cutting tools.     

矩形薄钢板高温态横向皱曲的解析与实验研究

承受非均匀拉伸矩形薄钢板的横向皱曲现象,既存在于汽车零件冲压成形制造中,也常见于薄带钢生产制造(尤其连续退火和热镀锌)中,是一个未完全解决的力学问题。文章认为矩形板宽度方向局部区域张力集中诱导生成的压应力是这一力学现象发生的主因,通过利用伽辽金变分与泛函分析原理给出了一种收敛性好、速度快的求解矩形方板弹塑性状态下应力场与屈曲临界应力的一种迭代算法,讨论了板的几何尺寸、拉伸非均匀度、材料性能对弹塑性状态前屈曲应力场的影响,根据临界载荷最小原则对皱曲区域进行了合理优化,同时结合有限元仿真和常温态非均匀拉伸实验,验证了解析计算模型的合理性。     

Pre-mould processing technique for syntactic foams: Generalised modelling, theory and experiment

Pre-mould processing for syntactic foams made of starch and ceramic hollow microspheres was studied. A statistical model to relate various parameters such as volume expansion rate (VER) of initial bulk volume of microspheres (IBVMS), microsphere size, microsphere volume fractions, minimum inter-microsphere distance (MID), and mixing ratio of microspheres was developed for theoretical relations. The statistical model consists of cubic cells which can optionally be simple cubic (SC) or face centered cubic (FCC) or body centered cubic (BCC) cells. The theoretical relations were verified with experimental/numerical data for various mixtures of microspheres and found to be capable of predicting effects of microsphere size and mixture ratio of microsphere size groups on VER under various conditions arising from starch content in binder and IBVMS. Volume shrinkage for mixture of microspheres and binder after moulding but before forming syntactic foam was found to be high for high contents of small microspheres in mixture.     

钨/316L不锈钢的瞬间液相扩散连接

采用Cu-5Ni(质量分数,%)合金箔为中间层,在加压15 MPa、连接温度1 120℃、保温10~360 min的工艺条件下对纯钨/316L不锈钢进行瞬间液相扩散连接。利用OM、SEM、EDS和电子万能试验机等研究接头的微观组织、成分分布、力学性能及断口特征。结果表明:保温10和30 min对应的接头组织由分界明显的富铁层和富铜层两层构成;保温时间增至180 min时,接头组织中的富铜层变薄、变分散,而富铁层则变厚、且局部和不锈钢奥氏体晶粒粘接;当保温时间达到360 min后,接头区和不锈钢母材较好地实现了组织与成分均匀化,钨母材中则形成2~3μm厚的Cr、Fe元素扩散带,接头抗剪强度达到213 MPa,断裂失效主要发生在钨母材中。     

基于正交试验和灰色系统理论的拼焊板前纵梁成形优化

影响拼焊板冲压成形的工艺参数较多,难以精确建立工艺参数与成形质量之间的关系。以压边力、拉延筋高度、凸筋圆角半径、凹筋圆角半径为自变量,进行四因素四水平正交试验,模拟拼焊板前纵梁拉延成形过程,获得最大减薄率和最大焊缝移动量的数据。利用灰色系统理论,分别计算成形工艺参数对单目标函数的关联系数和多目标函数的关联度,将多目标转换为以关联度为目标的单目标。进一步计算各成形工艺参数的平均关联度,将优化的压边力、拉延筋截面几何参数进行有限元模拟验证,指导设计、试模,成形的质量得到明显提高。     

Reducing the thermal expansion anisotropy in Mo5Si3 by Nb and V additions: theory and experiment

Mo₅Si₃ exhibits a high anisotropy of its coefficients of thermal expansion (CTEs) in the a and c directions, i.e., CTE(c)/CTE(a)=2.0. The high CTE anisotropy is due to the existence of prominent atom chains along the c-axis. Ternary alloying additions that effectively stretch the atom chains are likely to reduce the bonding directionality/strength and CTE anisotropy. First-principles local-density-functional calculations were carried out for two alloy systems, MoNb₄Si₃ and Mo₄VSi₃. Indeed, we find a significant reduction of the CTE ratio to a value of 1.5 for MoNb₄Si₃ and a value close to 1.0 for Mo₄VSi₃. The decrease in the CTE anisotropy mainly comes from the reduction in CTE in the c-direction. While a conventional approach to reduce the CTE would involve an increase in lattice rigidity by increasing the bond strength, our strategy focuses on the reduction of driving force for thermal expansion by decreasing the bond directionality.     

On the mechanism of diffusion-induced recrystallization: Comparison between experiment and molecular dynamics simulations

Recent experimental results on diffusion-induced recrystallization (DIR) in size-mismatched thin film metallic diffusion couples are summarized in this paper. As the most striking feature, newly formed grains reveal a preferred concentration which is characteristic for a given couple. Based on a suggested thermoelastic interpretation, stress in front of a migrating grain boundary is calculated from observed characteristic compositions. A remarkable relation between derived stresses and shear strength of respective parent matrix is discovered: plane stress in the diffusion zone amounts consistently to about 80% of the ideal shear strength. To elucidate this relation, molecular dynamics simulations are performed in the copper–gold system applying the embedded-atom method. Simulations indicate a break of coherency within the nanometric diffusion zone when a critical diffusor concentration is reached. Both experiment and simulation show in close agreement that maximum stress in the diffusion zone is of the order of the ideal shear strength and hence far above the yield strength of the material. It is deduced that the limit of coherency controls the observed characteristic concentration levels of DIR.     

An encounter between 4e cognition and attachment theory

This paper explores a constructive revision of the conceptual underpinnings of Attachment Theory through an encounter with the diverse elements of 4e cognition. Attachment relationships involve the development of preference for one or a few carers and expectations about their availability and responsiveness as a haven of safety and a base from which to explore. In attachment theory, mental representations have been assigned a central organising role in explaining attachment phenomena. The 4e cognition approaches in cognitive science raise a number of questions about the development and interplay of attachment and cognition. These include: (1) the nature of what Bowlby called ‘internal working models of attachment’; (2) the extent to which the infant–carer dyad functions as an extension of the infant's mind; and (3) whether Bowlby's attachment control system concept can be usefully re-framed in enactive terms where traditional cognitivist representations are: (3i) substituted for sensorimotor skill-focused mediating representations; (3ii) viewed as arising from autopoietic living organisms; and/or (3iii) mostly composed from the non-contentful mechanisms of basic minds? A theme that cross-cuts these research questions is how representations for capturing meaning, and structures for adaptive control, are both required to explain the full range of behaviour of interest to Attachment Theory researchers.