A Grand Canonical Monte Carlo Study of Adsorption and Capillary Phenomena in Nanopores of Various Morphologies and Topologies: Testing the BET and BJH Characterization Methods

We report a Grand Canonical Monte Carlo simulation study of Ar adsorption at 77 K in silica nanopores having various morphologies/topologies. Both the morphological and topological disorders are shown to significantly affect the capillary condensation phenomenon. In the case of an ellipsoidal pore, we observe that the filling mechanism is similar to that of a cylindrical pore having the same section area but with a lower condensation pressure. We show that the adsorption/desorption hysteresis loop is asymmetrical for the pore with constrictions while it is symmetrical for the regular cylindrical pore. Moreover, the Ar adsorption isotherm for the constricted pore reproduces the main features of that for the fully disordered Vycor‐like porous matrix. The results for the different pore geometries (having no direct interface with the gas reservoir) indicate that the desorption occurs through cavitation at a pressure driven by the smallest void size. We also consider the validity of the BET and BJH methods for the different porous matrices. Except for the Vycor‐like matrix, the BET surface assessed from Ar adsorption isotherm at 77 K always significantly overestimates the intrinsic surface of the pore (even for a planar surface). The disagreement between the BET surface and the geometrical surface is found to increase as the confinement increases (cylindrical pore) and/or the shape of the pore becomes asymmetrical (ellipsoidal pore). Interestingly, the best agreement between the BET and the geometrical surface is found in the case of the pore with a constriction, i.e. a system that exhibits a surface with both negative and positive curvature regions. This idea is supported by the results for the Vycor‐like matrix, which has a distorted surface (many negative curvature regions combined with positive curvature regions): for this disordered porous matrix, the BET surface is found to underestimate the intrinsic surface. Finally, we show that the pore size determined using the BJH method always underestimates the pore size, in agreement with previous experimental and simulation studies.     

MD simulation of the effect of contact area and tip radius on nanoindentation

Nanoindentation,namelydepth-sensingindentation(DSI),involvesforcingarigidindenterwithknowngeometryintothesurfaceofamaterialwhilecontinuouslymonitoringtheloadontheindenter,thedisplacementoftheindenterintothesurface,andthetimeoftheexperiment.Thedepthisthenusedtocalculatetheprojectedareaofcontactforthepurposeofcalculatingthehardnessandelasticmodulus.Infact,variouserrorsareassociatedwiththisprocedure.Oneofthemcomesfromthemeasurementofthepenetrationdepth.Ideally,thepenetrationdepthshouldbecalcula…     

无源北斗导航定位系统初始校频的一种新方法

根据小波分析理论，针对无源北斗导航定位系统初始校频问题，提出了一种基于小波分析的信号特征提取算法。仿真结果表明该方法比较简单而切实可行，能够有效地对信号进行趋势估计，从而精确算得频标准确度。     

含有杂质的金属须的生长和强度

在金属须的生长过程中有意地搀入杂质,以研究杂质对于金属须的生长方式和强度的影响。把1:5的FeCl₂和CuCl混合起来在氢气中进行还原,得到一根直径13.6微米而拉断强度为262公斤/毫米²的铜金属须,这远比直径相同的“纯”铜金属须的强度为高。搀钴的铜金属须的强度也大大提高,但是搀银的铜金属须的强度却不发生显著的变化。对于这种金属须作了金相观测,由此推论到金属须的强化主要是由于沉淀硬化或弥散硬化。此外,还根据沿轴线的螺型位错的机构讨论了杂质对于促进金属须生长的效应。把较高比例(例如1:2)的氯化亚铁搀入氯化亚铜并且在800℃以上的温度在氢气中进行还原,得到了直径达到800微米的混合金属须。这种金属须具有一个心轴,是由一种金属的单晶金属须所构成的,心轴的外面包围着由铜铁紧密搀合所形成的机械混合体。初步的实验指出这种金属须的强度相当高。     

由汽态还原生长金属须的机制

进行了用蒸汽还原法生长铜须和铁须的实验。研究了须的生长机制,为的是找出能够生出又粗又长而强度又极高的金属须的有效措施。实验指出,当生长槽材料的晶体结构与生长须的金属的晶体结构相同时,可以促进须的生长。由此所得的结论是,须的生长是通过沿轴线的螺型位错的机制。观测了所生的铜须和铁须在生长槽上的分布和排列取向的各种方式,由此推论到这些须是由顶端生长的。此外,实验还指出,氯化亚铜(或亚铁)蒸汽是优先在须的顶端还原的,这可能是由于螺型位错在须顶端所产生的表面台阶对于这种还原有一种催化作用。观测了直径50—120微米的大铜须的生长情况,发现了片状生长、堆垛生长和层状生长。这种特殊生长方式可能与一维和二维成核生长的交互更迭发生有关。讨论了通过适当选择和控制生长条件和方式的途径来获得基本上不含位错的大块晶体的可能性。     

冷轧取向硅钢二次再结晶过程的能量分析

冷轧取向硅钢片高温加热时,二次晶粒形核于极表面附近的区域,核的长大方式与冷轧形变率有关。本文按二次冷轧工艺、H1-B(一次冷轧)工艺下二次晶核的长大特点,分别采用能量分析方法导出了各种组织参量,如:初次晶粒半径、夹杂因子及二次晶核临界尺寸间的定量关系。计算结果与文献实验数据符合。 关键词：     

在疲劳载荷下含铜4%的铝合金中的位错钉扎和解脱

【摘　要】用Al-4％Cu合金进行了恆应变扭转疲劳试验,观察到ΔE在起始阶段下降直至接近零值,经过一定的疲劳循环数以后又重新上升。金相观测的结果指出,ΔE的重新上升对应着滑移区的突然变得集中。这有力地证明了我们以前所作的假设,即局部化滑移区的出现是引起ΔE的一种基本过程。在疲劳载荷下的T_m在超始阶段上升,但上升至最高值并保持此值经过一定的疲劳循环数以后,出现骤然的下降,这正对应着ΔE的重新上升。这种现象的出现可能是反映着在前期疲劳载荷下被钉紥的位错得到了骤然的解脱。初步认为,位错的被钉紥和随后得到解脱的过程,是由于在疲劳初期所形成的溶质铜原子气团沿着位错线的某些有利地段富集成核,同时使位错线上的另一些地段从溶质原子中解脱出来。随着富集核的逐惭增大,和其间的得到了解脱的位错段的长度的逐渐增加到某一临界值,便在疲劳载荷下出现了位错“雪崩”的现象,产生了大量的新位错,这使T_m骤然降低,并且导致局部滑移地区的形成,从而使ΔE重新增加。     

二元金属须的强度和形变特点

测量了直径20—300微米的大量的Fe鬚、Fe-Cu鬚、Fe-Co鬚、Cu鬚和Cu-Co鬚的室温和高温拉断强度,结果指出,弥散型的二元金属鬚如Fe-Cu鬚和Cu-Co鬚的强度较高于单元金属鬚,而固溶型的二元金属鬚如Fe-Co鬚则否。根据应力应变曲线的测量和金相观测的结果,认为弥散型的Fe-Cu鬚的强度所以较高,可能是由于高强度的铁铜弥散混合层的出现,而它的范性所以较好,可能是由于这种鬚具有范性较好的铜心轴。讨论了弥散型和固溶型二元金属鬚在变粗当中所包含的基本过程,以及与它们所表现的强度和形变特点的联系。并且指出,生长弥散型混合鬚的原理和方法给制备强度较高和范性较好的复合材料提供了一个值得参考的途径。 关键词：     

铁锰合金在正、反马氏体型相变温度范围内出现的位错内耗峰

用扭摆法测量了含锰18.5％的铁锰合金的内耗,在正和反马氏体型相变温度范围内都观察到一个稳定的内耗峯。在降温时,正马氏体型相变区域内的内耗峯出现在100℃左右;在重新加热时,反马氏体型相变区域内的内耗峯出现在200℃左右。对于这两个变为稳定以后的内耗峯进行了系统的研究,它们的共同特征是:1.内耗峯的位置与测量内耗所用的振动频率无关;2.稳定内耗峯的高度,随着测量内耗所用的应变振幅的增大而显著地升高;3.随着对于试样的加热和冷却循环次数的增加,内耗峯的位置离相变开始点的温度越远,而且内耗峯的高度也下降。从以上的特点可以看出,内耗峯的位置与频率无关表示产生内耗的基本过程不是受热激活所控制的。此外,内耗与振幅有关这一现象进一步指出,产生内耗的机构不是一种滞弹性效应。电子衍衬金相观测已经指出,作为扩展位错的组成部分的堆垜层错可以作为马氏体型相变的核,因此,这种内耗现象可能是由于扩展位错的应力感生加宽和收缩所引起的。这种模型可以统一地解释在铁锰合金中所观察到的各种内耗现象。     

Anomalous temperature dependence of surfactant self-assembly from aqueous solution

The self-assembly of surfactant molecules from aqueous solutions exhibits anomalous and as yet unexplained temperature dependence. These effects include (a) a decrease in surfactant solubility, (b) an increase of the critical micelle concentration, and (c) increasing self-assembly on solid surfaces with increasing temperature near ambient temperature. These are the opposite of what one intuitively expects based on increasing molecular motion with temperature, and of what is observed for simpler fluids. Using a lattice gas theory that allows for inhomogeneity, hydrogen bonding, and micelle formation, we show conclusively that all of these phenomena arise from hydrogen bonding between water and the surfactant headgroups. While theory and experiment show very good qualitative agreement when such H bonding is included, its omission leads to complete reversal of the temperature dependence.