2种离子固化剂改善黄土抗剪强度和抗渗性的研究

为了探讨CONAID和LUKANG 2种离子固化剂的固土性能及其影响因素,对不同固化剂掺量、养护龄期、压实度和含水率的固化黄土进行了直剪试验和渗透试验。研究结果表明:随着固化剂掺量、养护龄期和压实度增大,2种固化土的内摩擦角和黏聚力呈上升趋势,渗透系数呈下降趋势,且规律比较接近。其中LUKANG固化剂对黄土抗剪强度和抗渗性的改善效果优于CONAID固化剂。建议在施工过程中选用LUKANG固化剂,掺量宜取0.01%。为了达到更好的抗剪和抗渗效果,应尽量延长固化土的养护龄期,增加压实度。

胶体体系的固化及分离过程的计算机模拟

胶体体系是自然界很重要的组成部分，广泛应用于工业生产和日常生活中。胶体体系在固化的过程中，会形成有序的晶体结构或无序的玻璃态结构。对胶体的结晶和玻璃化的认识对研究相变理论，纳米材料以及自组装有着重要的意义。事实上，在这一领域存在着许多悬而未决的问题。另一方面，胶体的分离也是胶体科学研究中不可或缺的步骤。特别是近年来微流器件技术的进步为流体动力学色谱分离方法带来了进一步发展的机会，这为胶体的分离提供了非常有效的实验手段。然而，由于实际应用过程的复杂性，理论方法还不足以深入探讨胶体粒子体系固化和分离的本质。作为一种可以直接反映物理过程的研究方法，计算机模拟成为一种有效的研究胶体粒子固化和分离的手段。 硬球模型是一种最简单的胶体粒子体系，能够很好地描述具有体积排斥作用的胶体粒子的行为，因此，对于胶体粒子体系固化问题，我们采用分子动力学方法研究了硬球粒子体系的固化过程。此外，对于胶体和高分子体系的分离过程，我们主要采用耗散分子动力学（dissipative particle dynamics DPD）进行研究。论文的主要内容包括： 1)采用硬球分子动力学模型，研究了胶体颗粒在不同的压缩速率下得到的堆积态的结构异质性。我们应用Lubachevsky-Stillinger方法，在不同的压缩速率下得到了一系列的结构受限的硬球堆积状态。这些体系的有序度随着压缩速率的减小而增加，而体系的密度则是先增加后减小又增加，在中间压缩速率时出现最小值。通过测量体系中的局域序参量分布，粒子位置的均方根涨落分布和直接观察受限粒子的位置，发现这些状态中包含有无规区域和结晶区域，存在显著的结构异质性。特别的是低密度状态中许多小的有序区域互相连接成贯穿体系的逾渗网络，说明晶区之间的结构不匹配是生成这些状态的重要机理。 2)用DPD方法模拟了线性高分子链和星形高分子链在穿越纳米孔道的输运过程。对复合管道中小管的半径分别为2.5 和2.0 的两种情况进行了研究。在各种情况下我们都发现了存在一个临界流量Jc，只有当流量大于Jc的时候高分子才能进入纳米孔洞。对于线性高分子，长链的高分子进入同样粗细的小管所需要的临界流量比短链大。线性链和星形链在半径是2.0小管子中的临界流量比通过半径2.5的管子的临界流量大，或者近似相同。在半径2.5的小管中，星形高分子的临界流量要大于同等尺寸的线性高分子。此外，我们没有观察到不同接枝数的星形高分子的临界流量之间的差异。出现这种情况的原因可能是DPD模型中粒子间的相互作用势能很软，所以不能很好地体现出不同接枝数的星形高分子在位阻上的差异。 3）我们利用DPD方法模拟了圆柱形管道中线性高分子链的输运过程。比较了模拟得到的高分子保留时间和DG理论预测值之间的差异。发现分子量越大的高分子对DG理论的偏离越显著。计算了高分子链在流动过程中伸长的情况，发现了链的伸长率S与Weissenberg number 有着明确的标度关系。与狭缝中的情况类似，对于圆柱型管道中流动的线性高分子，如果采用修饰过的Peclet number，则能够更好地满足Aris-Taylor分散的标度关系。 4)我们建立了纳米粒子在DPD模拟的动力学模型，研究了在本体溶液中纳米粒子的自扩散系数和速度关联函数对与胶体粒子的半径定性关系。发现纳米粒子的扩散系数随粒子半径的增加而减小。纳米粒子的半径越大，其速度关联函数的衰减越慢，同样大小纳米粒子的质量越大，其速度关联函数衰减得越慢。我们进一步模拟了在狭缝中纳米粒子的输运过程。通过观察流体受限方向的流速曲线，我们发现由于与器壁之间的体积排斥作用，使得纳米粒子远离流体的边界。对Aris-Taylor分散的研究结果表明，在λ不大的时候，应用不同的Peclet numbers 对分散方程结果的影响不大。

Fabrication of nano-sized grains by pulsed laser surface melting

In this work, the formation and characterization of nano-sized grains on the modified surfaces of GCr15 and H13 steels have been investigated. The material was processed by pulsed laser surface melting (LSM) under different depths of de-ionized water. The microstructures and phases of the melted zones were examined by x-ray diffraction, environmental field emission scanning electron microscopy and high resolution transmission electron microscopy. The results indicate that LSM under water can successfully fabricate nano-scaled grains on the surfaces of steel, due to the rapid solidification and crystallization by heterogeneous nucleation. The elemental segregation of chromium and activated heterogeneous nucleation mechanism of austenite in liquid metal play a key role in the formation of nano-sized grains at high cooling rates. This one-step technique provides us a new way to prepare uniform nano-scaled grains, and is of great importance for further understanding the growth of nano-materials under extreme conditions.

Thermal analysis of microstructural evolution of Al2O3 surface modified layers

Solidification behavior and microstructural evolution of surface modified layers in plasma cladding technique are studied via numerical simulations. Both the coupling effect of temperature and solid volume fraction are considered in the proposed thermal analytical model, by which the transient temperature distributions are calculated and the shape of melting pool is determined. Furthermore, we perform microscopic thermal analysis on the nucleation and growth behaviors of ceramic hardening phases and dendrites, as well as the kinetics of related two-phase flow systems. By comparing with experimental observations, the evolution mechanisms of the morphology of Al2O3 ceramic hardening layer are explained. Based on the above results, a relationship among the scanning velocity of plasma stream, dendritic growth rate and the advancing speed of solid/liquid interface is found, and an energy criterion is proposed for predicting the pushing/engulfing transition of ceramic particles by grain growth fronts. (C) 2009 Elsevier B.V. All rights reserved.

一种镍基单晶合金的凝固行为

采用等温凝固方法研究了单晶镍基合金的凝固区间,利用DSC测试了合金的凝固曲线.结果表明:实验合金的液相线温度约为1380℃,固相线温度约为1310℃.合金的凝固顺序为: Lγ,L MC;γγ′;Lγ+MC.单晶合金的铸态组织中,W偏析于枝晶干, Ti,Cr,Mo和Ta偏析于枝晶间,偏析程度为: Mo>Ti>Cr>Ta, Al和Co几乎不发生偏析.1314℃1382℃1361℃1325℃

Microstructure and Elevated Temperature Properties of Die-cast AZ91-xNd magnesium alloy

The effect of Nd addition on the microstructure and mechanical properties of a die-cast AZ91 alloy was investigated in the present work. The results show that the die-cast AZ91 alloy is composed of alpha-Mg matrix and gamma-Mg17Al12 phase. Nd addition into the AZ91 alloy leads to the formation of rare earth containing intermetallic phase. Al4Nd phase forms when Nd content is less than or equal to 1.0 wt.%. Al2Nd phase appears simultaneously when Nd content reaches to 3.0 wt.%.

Temperature dependence of the distribution of the first passage time: Results from discontinuous molecular dynamics simulations of an all-atom model of the second beta-hairpin fragment of protein G

More than 22 000 folding kinetic simulations were performed to study the temperature dependence of the distribution of first passage time (FPT) for the folding of an all-atom Go-like model of the second beta-hairpin fragment of protein G. We find that the mean FPT (MFPT) for folding has a U (or V)-shaped dependence on the temperature with a minimum at a characteristic optimal folding temperature T-opt*. The optimal folding temperature T-opt* is located between the thermodynamic folding transition temperature and the solidification temperature based on the Lindemann criterion for the solid. Both the T-opt* and the MFPT decrease when the energy bias gap against nonnative contacts increases. The high-order moments are nearly constant when the temperature is higher than T-opt* and start to diverge when the temperature is lower than T-opt*. The distribution of FPT is close to a log-normal-like distribution at T* greater than or equal to T-opt*. At even lower temperatures, the distribution starts to develop long power-law-like tails, indicating the non-self-averaging intermittent behavior of the folding dynamics. It is demonstrated that the distribution of FPT can also be calculated reliably from the derivative of the fraction not folded (or fraction folded), a measurable quantity by routine ensemble-averaged experimental techniques at dilute protein concentrations.

External field induced crystallization of poly(3-dodecylthiophene)

In order to investigate the effect of external field on the crystallization behavior of poly(3-dodecylthiopliene) (P3DDT), the samples were recrystallized with different electrostatic field intensity, different pressure and different solidification direction in temperature gradient field. Measurements of differential scanning calorimetry and X-ray diffraction were operated to characterize these samples for analysis. The results suggest that after recrystallization, whether the external field is added or not, a more compact packing of molecular chains in P3DDT could be obtained without the change of the crystal structure model. Moreover, the addition of electrostatic field has greater effects on the crystallization of rigid main chains than on that of flexible side chains, Merely great pressure field can effect the rearrangements of molecular chains greatly. As for the temperature gradient field induced crystallization, different oriented solidification direction will lead to different effects on the compact degree and perfect degree of molecular chains packing,

Fabrication of agar-gel microelectrodes and their application in the study of ion transfer across the agar-water 1,2-dichloroethane interface

In this paper, we describe a simple procedure to make agar-gel microelectrodes by filling micropipettes. These microelectrodes were used to study K+ transfer across the agar-water \ 1,2-dichloroethane interface facilitated by dibenzo-18-crown-6 (DB18C6), and the transfer of tetraethylammonium (TEA(+)). The results observed were similar to those obtained at micro-liquid \ liquid interfaces. The effect of various amounts of agar in the aqueous phase was optimized and 3% agar was chosen based on the potential window and solidification time. The different shapes of micro-agar-gel electrodes were prepared in a similar way. The fabricated agar-gel microelectrodes obey the classical micro-disk steady-state current equation, which is different from the behavior of a normal micropipette filled with aqueous solution without silanization. (C) 2001 Elsevier Science B.V. All rights reserved.

化学冷铁——焓变涂料的研究

焓变涂料是以在一定的温度区间内可发生吸热化学反应的物质为基料配制的,将其涂在砂型(芯)表面,可代替常规冷铁.它对解决难于放置冷铁的串皮芯部位的激冷问题,效果明显.焓变涂料激冷作用的大小,取决于基料物质比焓变的大小以及涂层厚度等因素。

Metallic foam processing from the liquid state

Cox, S.J., Bradley, G. and Weaire, D. (2001) Metallic foam processing from the liquid state: the competition between solidification and drainage. Eur. Phys. J. AP 14:87-97. Sponsorship: This research was supported by the Prodex programme of ESA, and is a contribution to ESA contract C14308/AO-075-99. SJC was supported by Enterprise Ireland and a Marie Curie fellowship. GB was supported by the HPC Programme of TCD.

Does an incremental filling technique reduce polymerization shrinkage stresses

It is widely accepted that volumetric contraction and solidification during the polymerization process of restorative composites in combination with bonding to the hard tissue result in stress transfer and inward deformation of the cavity walls of the restored tooth. Deformation of the walls decreases the size of the cavity during the filling process. This fact has a profound influence on the assumption-raised and discussed in this paper-that an incremental filling technique reduces the stress effect of composite shrinkage on the tooth. Developing stress fields for different incremental filling techniques are simulated in a numerical analysis. The analysis shows that, in a restoration with a well-established bond to the tooth-as is generally desired-incremental filling techniques increase the deformation of the restored tooth. The increase is caused by the incremental deformation of the preparation, which effectively decreases the total amount of composite needed to fill the cavity. This leads to a higher-stressed tooth-composite structure. The study also shows that the assessment of intercuspal distance measurements as well as simplifications based on generalization of the shrinkage stress state cannot be sufficient to characterize the effect of polymerization shrinkage in a tooth-restoration complex. Incremental filling methods may need to be retained for reasons such as densification, adaptation, thoroughness of cure, and bond formation. However, it is very difficult to prove that incrementalization needs to be retained because of the abatement of shrinkage effects.

Numerical modelling of solder joint formation

Solder materials are used to provide a connection between electronic components and printed circuit boards (PCBs) using either the reflow or wave soldering process. As a board assembly passes through a reflow furnace the solder (initially in the form of solder paste) melts, reflows, then solidifies, and finally deforms between the chip and board. A number of defects may occur during this process such as flux entrapment, void formation, and cracking of the joint, chip or board. These defects are a serious concern to industry, especially with trends towards increasing component miniaturisation and smaller pitch sizes. This paper presents a modelling methodology for predicting solder joint shape, solidification, and deformation (stress) during the assembly process.

The numerical modelling of multi-physics phenomena: The shape casting process

In this paper a computer simulation tool capable of modelling multi-physics processes in complex geometry has been developed and applied to the casting process. The quest for high-quality complex casting components demanded by the aerospace and automobile industries, requires more precise numerical modelling techniques and one that need to be generic and modular in its approach to modelling multi-processes problems. For such a computer model to be successful in shape casting, the complete casting process needs to be addressed, the major events being:-• Filling of hot liquid metal into a cavity mould • Solidification and latent heat evolution of liquid metal • Convection currents generated in liquid metal by thermal gradients • Deformation of cast and stress development in solidified metal • Macroscopic porosity formation The above phenomena combines the analysis of fluid flow, heat transfer, change of phase and thermal stress development. None of these events can be treated in isolation as they inexorably interact with each other in a complex way. Also conditions such as design of running system, location of feeders and chills, moulding materials and types of boundary conditions can all affect on the final cast quality and must be appropriately represented in the model.

PHYSICA - A software environment for the modelling of multi-physics phenomena

A brief description of a software environment in FORTRAN77 for the modelling of multi-physics phenomena is given. The numerical approach is based on finite volume methods but extended to unstructured meshes (ie. FV-UM). A range of interacting solution procedures for turbulent fluid flow, heat transfer with solidification/melting and elasto-visco-plastic solid mechanics are implemented in the first version of PHYSICA, which will be released in source code form to the academic community in late 1995.