非连续边界层对圆柱声学共鸣频率的影响研究

基于声学一阶微扰理论,建立了低压下由于气 固界面的温度阶跃和速度滑移带来的边界层不连续性对圆柱声学共鸣频率的扰动规律,进一步发展了圆柱声学共振频率的非连续边界层修正模型,计算了非连续边界层对4种惰性气体、不同声学模式和不同压力及温度的影响。分析研究显示,气体处于低压状态时,温度阶跃和速度滑移会使声学共振频率发生偏移,不引入声能损耗。在50 kPa时,非连续边界层对声学共鸣频率的影响可达10×10^-6,表明非连续边界层修正项对于10^-6不确定度水平的尖端声学共鸣测量,是一个重要的不确定性来源。     

基于修正单层梁理论的夹层梁最大弯曲正应力计算

目的计算夹层梁横截面的最大弯曲正应力。方法将夹层梁等效成等截面均质单层梁,进而推导出了理论计算公式,并在此基础上进行了三点弯曲试验的算例研究。结果当破坏载荷与夹层梁横截面的尺寸一定时,随着芯层与总厚度比的增加,修正单层梁理论计算的最大正应力值逐渐增加,而单层梁理论计算的结果为恒定值。对于同样结构的夹层梁,随着芯层弹性模量与表层模量比的增加,修正单层梁理论计算的最大正应力与单层梁理论的差异值越来越小。结论修正单层梁理论与层合梁理论计算的结果是一致的,该方法可有效进行最大弯曲正应力的预测与计算。     

考虑芯层横向变形的粘弹性复合材料夹层板结构的声振特性分析

夹层板结构具有很高的比强度和比刚度。若芯层采用粘弹性阻尼材料，夹层板结构还具有良好的隔振和隔声特性，因此在工程结构中得到广泛应用。以往的夹层板理论大多忽略了芯层的横向正应变和横向正应力，在分析芯层较厚的夹层板或者夹层结构的高频振动问题时由于不能体现芯层的横向压缩变形，往往显得不够合理。针对这一不足，构造了一个复合材料夹层板单元：夹层板的上下面板采用基于一阶剪切变形理论的Mindlin假定以及层合板理论进行分析；采用文献[6，7]中提出的Timoshenko层合厚梁理论构造了单元每边的转角和剪应变场，消除了Mindlin板单元当板厚变小时的剪切锁死问题；假定芯层的位移沿厚度方向线性变化，并用上下面板的自由度表示，最终形成以上下面板自由度表示的系统总的运动方程。该单元不仅考虑了芯层的横向剪切变形，还考虑了芯层的横向压缩变形。数值计算结果表明：无论对于静力问题、动力问题还是声辐射等问题，考虑芯层的横向压缩变形是合理的，也是有必要的。     

Deposition of Bioactive Layer on NiTi Alloy by Chemical Treatment

A simple chemical method was developed for inducing bioactivity on NiTi alloys(50 at.pct by Ni/Ti).A layer of calcium phosphate was deposited on the surface to improve biocompatibility of thealloy.NiTi alloys were first etched in HNO3 aqueous solution,and then treated with boiling diluted NaOH solution.A rough surface was created and a thin TiO2 layer was formed on the surface.Pre-calcification was then introduced by immersing the treated NiTi alloys in supersaturated Na2HPO4 solution and supersaturated Ca(OH)2 solution in turn before calcification in simulated body fluid (SBF).A dense and uniform bonelike calcium phosphate(Ca-P) bioactive layer was formed on the surfaces of the specimen,which would improve their biocompatibility.Morphology and element analysis on NiTi surfaces during the treatments were investigated in detail by means of environment scanning electron microscopy(ESEM),energy dispersion X-ray spectroscopy(EDXS),and X-ray diffraction (XPD).     

Fe为过渡层的Co/Cu/Co三明治巨磁电阻效应的研究

用超高真空电子束蒸发方法成功制备了以不同厚度Fe为过渡层的Co/Cu/Co三明治巨磁电阻样品,与以Cr为过渡层的Co/Cu/Co三明治巨磁电阻样品比较,样品的矫顽力大大减小,因而样品的磁灵敏度有了较大地提高。当Fe过渡层的厚度为7nm时样品的磁电阻值最大。另外,温度更强烈地影响以Fe为过渡层样品的磁电阻值,在77K下样品的磁电阻曲线表现出明显的不对称性,它来源于低温下fcc Fe过渡层的反铁磁性转变。     

Experimental Investigation of Strain Concentrations Caused by Inserts in Sandwich Beams

Abstract:  This paper presents an experimental study of the strain concentrations developing across the insert–core junctions in a sandwich beam with an implanted stiff insert into its core. Strain measurements were carried out at several positions on the skins and across the insert–core junction, for several sandwich beam specimens loaded in 3-points bending and having inserts with various geometries. The materials used were typical marine glass/polyester composite and PVC foams and the specimens were manufactured with the vacuum infusion method. A full material characterization was performed for the composite skins. The experimental results demonstrated a pure linear bending response of the specimens, as well as significant strain concentration factors across the insert–core junction, reaching values up to 3.3. Strain concentration factors measured for butt inserts were lower than those measured on the tension side of the specimens with scarf inserts. Transverse strains measurements verified that all specimens were in a plane stress state.     

Distribution of Residual Stresses Induced by Axially Symmetric Plastic Strains in a Layer

By using the direct method of integration of the resolving equations of an axially symmetric problem of the theory of elasticity for a layer in stresses, we find residual stresses for a given field of plastic strains. For plastic strains symmetric about the median surface of the layer, we performed the numerical analysis of the distribution of residual stresses. It is shown that stresses undergo significant redistributions over the thickness of the layer as the drop of plastic strains in the indicated direction increases.     

TiO2/层状矿物复合材料的研究进展

综述了国内外在纳米TiO2/层状硅酸盐矿物系列复合材料领域的研究动态,重点介绍了目前材料制备过程中混合加热、水解-沉淀和溶胶-凝胶(sol-gel)法3种常见的固载技术以及离子掺杂等表面修饰手段.同时对该领域今后的研究方向进行了展望:在保持复合材料优良光催化性的前提下,对制备技术和修饰手段进行创新和改进,并进一步研究了材料失活机理.     

Control of Velocity Profiles in a Boundary Layer

The control of average velocity in the turbulent boundary layer of a flat plate is investigated theoretically and experimentally. The control is exercised by injecting jets of air into the incident flow. Interpolation expressions are derived for the distribution of average velocities, dependent on the control parameters.     

铸铁基—碳化钨粒子表面复合层性能的研究

本文就铸铁基一碳化钨粒子表面复合层性能进行了研究。试验表明,复合层的磨损速度取决于基体的磨损速度,后者与其硬度有较好的对应关系,但复合层内基体的磨损速度比单一材料小。碳化钨粒子的溶解速度在铸铁中低于钢,它导致界面区成分的变化。可用合金元素来增强粒子与基体的结合,提高复合层的寿命。文中并对试验结果进行了较深入的理论分析。     

Al/Fe液-固界面扩散反应层生长动力学分析

采用镶嵌式技术制备了Al/Fe扩散偶,在铝熔点以上铁熔点以下进行扩散热处理,对Al/Fe液-固界面扩散反应层的生长动力学进行了分析,并建立了生长动力学方程。结果表明,Fe2Al5是热处理保温过程中唯一生成的新生相。在Fe2Al5连续单相层形成之前,其生长受Al原子和Fe原子的化学反应控制;一旦连续的Fe2Al5单相层形成,其生长则主要依赖于Al原子沿其晶界的扩散控制,且伴随着其晶粒尺寸的长大。在800℃以下热处理,可忽略晶粒长大对原子扩散的影响,其生长动力学方程为:y=2020.96exp(-78490/RT)t0.25。但当热处理温度超过铁熔点的0.7倍后,则不能忽略晶粒长大的影响,应适当减小生长动力学方程中的生长指数值。     

"渗氮/物理气相沉积"复合涂层研究的新进展

介绍了"渗氮/物理气相沉积(PVD)"复合涂层在涂层结构设计、工艺与性能、失效与强化机制等方面研究的最新进展.与单一涂层相比,"渗氮/PVD"复合涂层表现出更好的物理力学性能,明显提高了工模具的耐用性.渗氮层的引入不仅显著地提高了基材表面的承载能力和膜/基结合力,而且还提高了钢表面的疲劳强度、耐磨性能、热冲击及化学抗力,尤其是在承受高载荷情况下的失效抗力.经过十几年的研发,复合涂层已开始进入商业应用阶段,在高速钢切削刀具和各种精密工模具上有广阔的应用前景.     

铝合金化学镀Ni-P/Ni-B双层复合镀层及其性能研究

基于雷达波导管内表面电镀银层工作时极易被臭氧(O3)氧化腐蚀,从而影响到使用可靠性.为了解决雷达波导管内表面既具有优良导电性能又耐臭氧腐蚀的技术要求,对铝合金化学镀Ni-P/Ni-B双层复合合金镀层的操作方法、工艺条件及主要性能进行了研究,结果表明,若用Ni-P/Ni-B化学镀层替代电镀银层,导电性能有所降低,而耐臭氧腐蚀性能明显增强.     

镁含量和硅对铁-锌铝镁合金固-液扩散偶中Fe-Al反应层的影响

在热浸镀锌中,铁基表面Fe-Al化合物层的形成会影响镀层的生长和质量。将Fe/(Zn-11%Al-3%Mg)和Fe/(Zn-11%Al-x%Mg-0.2%Si)扩散偶在600℃下进行25min的固-液扩散实验,利用扫描电子显微镜(SEM)和能谱仪(EDS)研究了镁含量和硅对铁-锌铝镁合金固-液界面Fe-Al合金层形成的影响。结果表明,Fe/(Zn-11%Al-3%Mg)固-液扩散偶反应层由FeAl3和Fe2Al5相层组成;随着Mg含量的增加,Fe/(Zn-11%Al-x%Mg-0.2%Si)扩散偶中反应层的厚度呈现先增加后减少再增加的变化趋势,当镁含量为3%时反应层厚度最薄;Fe/(Zn-11%Al-3%Mg)扩散偶中Fe-Al反应层的平均厚度比Fe/(Zn-11%Al-3%Mg-0.2%Si)扩散偶中反应层的厚度大60μm,证明Si元素起到抑制Fe-Al反应层形成的作用。研究结果为解释Super Dyma合金镀层中不形成明显的Fe-Al抑制层提供了实验依据。     

Characterisation and Assessment of Load Response, Failure and Fatigue Phenomena in Sandwich Structures Induced by Localised Effects: A Review

Abstract:  The objective of the paper is to provide an overview of the mechanical effects, which determine the occurrence and severity of localised bending effects in sandwich structures, and to provide a survey of the available structural sandwich models, with special emphasis on their ability to describe local bending effects. The presentation includes a brief survey of the various structural models, including classical, 'first-order shear', 'high-order' and continuum mechanics-based models. Moreover, the paper focuses on and addresses the experimental characterisation and assessment of local effects in sandwich structures based on realistic engineering practice, examples including sandwich panels with core materials of different stiffness (core junctions), sandwich plates with inserts and junctions between sandwich panels of different curvature. The issues of general load response (global and local) as well as failure and fatigue of such sandwich structures subjected to out-of-plane and in-plane loads are discussed in some detail, with the inclusion of recent theoretical and experimental results.     

Residual Stresses and Deformation Behaviour in a SiC_p/Al Composite

Experiments were conducted to determine theresidual stresses with X-ray diffraction in the ma-trix of a SiC/Al composite after different thermaltreatments,and to investigate the stress-straincharacteristics and fracture behaviour of the com-posite.It was found that there existed a tensileresidual stress in the matrix and both thermal cy-cling between room temperature and 350℃ and lowtemperature treatment in liquid nitrogen reducedthe residual stress.The results of the strengthdifferential effect and Bauschinger effect were con-sistent with the results of residual stressmeasurements.The tensile residual stresses in the Almatrix enhanced the strength differential effect.Themagnitude of Bauschinger effect is greater for a testinitially started in compression than that in tension.     

Bandgap Tuning with Thermal Residual Stresses Induced in a Quantum Dot

Lattice distortion induced by residual stresses can alter electronic and mechanical properties of materials significantly. Herein, a novel way of the bandgap tuning in a quantum dot (QD) by lattice distortion is presented using 4‐nm‐sized CdS QDs grown on a TiO₂ particle as an application example. The bandgap tuning (from 2.74 eV to 2.49 eV) of a CdS QD is achieved by suitably adjusting the degree of lattice distortion in a QD via the tensile residual stresses which arise from the difference in thermal expansion coefficients between CdS and TiO₂. The idea of bandgap tuning is then applied to QD‐sensitized solar cells, achieving ≈60% increase in the power conversion efficiency by controlling the degree of thermal residual stress. Since the present methodology is not limited to a specific QD system, it will potentially pave a way to unexplored quantum effects in various QD‐based applications.     

An Axisymmetric Numerical Model for Simulating Kinetically—Limited Growth of a Cylindrical Rod in 3D Laser—induced Chemical Vapor Deposition

Laser-induced chemical vapor deposition（LCVD） is an important process for freeform microfabrication of high aspect ratio prototypes.The system consists of a laser beam focused onto a movable substrate in a vacuum chamber.Heat from the laser at or near the focal spot of the beam causes gas in the chamber to react.As a result ,solid-phase reaction products are deposited on the substrate to from the microstructure.In this paper,we develop a numerical model for simulating growth of an axisymmetric cylindrical rod by pre-specifying the surface temperatures.The solution using least squares is obtained by minimizing the sum of square deviations between the pre-specified surface temperatures and the calculated temperatures from the heat equation with a given laser power as a heat source.Model predictions of the laser power over growth time helped in optimizing the growth process.Rods grown based on the predicted laser power from the numerical model were very close to being cylindrical in shape.Ways to further improve the model are being investigated.     

High‐Performance Integrated Self‐Package Flexible Li–O2 Battery Based on Stable Composite Anode and Flexible Gas Diffusion Layer

With the rising development of flexible and wearable electronics, corresponding flexible energy storage devices with high energy density are required to provide a sustainable energy supply. Theoretically, rechargeable flexible Li–O₂ batteries can provide high specific energy density; however, there are only a few reports on the construction of flexible Li–O₂ batteries. Conventional flexible Li–O₂ batteries possess a loose battery structure, which prevents flexibility and stability. The low mechanical strength of the gas diffusion layer and anode also lead to a flexible Li–O₂ battery with poor mechanical properties. All these attributes limit their practical applications. Herein, the authors develop an integrated flexible Li–O₂ battery based on a high‐fatigue‐resistance anode and a novel flexible stretchable gas diffusion layer. Owing to the synergistic effect of the stable electrocatalytic activity and hierarchical 3D interconnected network structure of the free‐standing cathode, the obtained flexible Li–O₂ batteries exhibit superior electrochemical performance, including a high specific capacity, an excellent rate capability, and exceptional cycle stability. Furthermore, benefitting from the above advantages, the as‐fabricated flexible batteries can realize excellent mechanical and electrochemical stability. Even after a thousand cycles of the bending process, the flexible Li–O₂ battery can still possess a stable open‐circuit voltage, a high specific capacity, and a durable cycle performance.