Optimum design of carbon/carbon ablative property based on genetic algorithm

An optimum design model has been proposed for carbon/carbon ablative property based on genetic algorithm,in which the optimum parameters are the number of woven satins,K of fiber bundles,layers per unit height,the average distance of puncture fibers in Z direction and Ply Stacking angle,and the constraint conditions are the density and diameter of carbon fibers and the density of carbon matrix.The results demonstrate that after optimization,the overall height of the ablative carbon/carbon surface is reduced by 56.5%,the standard deviation is reduced by 34.9% and the surface roughness is reduced by 12.6%,which suggests the remarkable improvement of ablative homogeneity.The present investigation can provide practical methodology for the optimum design of carbon/carbon ablative property and the development of new carbon/carbon composites.     

Micro contact and stick-slip number between AFM probe tip and sample surface

Atomic force microscope (AFM) has become a main instrument in characterization of micro topography and micro tribology of surfaces. Under its contact mode, AFM probe tip slides on the surface in the direction perpendicular to its longitudinal axis to get topography and lateral force images simultaneously. During the scanning, the AFM tip is loaded by both a normal force and a lateral force, which make the probe bend and twist. In their pioneering work, Mate et al.[1] used AFM to measure m…     

In-situ TEM study of the dynamic behavior of the graphene-metal interface evolution under Joule heating

The dynamic behavior of the interface between few layer graphene(FLG) and tungsten metal tips under Joule heating has been studied by in-situ transmission electron microscopy(TEM) method. High-resolution and real-time observations show the tungsten tip ‘swallow' carbon atoms of the FLG and ‘spit' graphite shells at its surface. The tip was carbonized to tungsten carbide(WC, W_2 C and WC_x) after this process. A carbon diffusion mechanism has been proposed based on the diffusion of carbon atoms through the tungsten tip and separation from the surface of the tip. After Joule heating, the initial FLG-metal mechanical contact was transformed to FLG-WCx-W contact, which results in significant improvement on electrical conductivity at the interface.     

Edge Effects and Coupling Effects in Atomic Force Microscope Images

The AFM images were obtained by an atomic force microscope (AFM) and transformed from the deformation of AFM micro cantilever probe. However, due to the surface topography and surface forces applied on the AFM tip of sample, the deformation of AFM probe results in obvious edge effects and coupling effects in the AFM images. The deformation of AFM probe was analyzed,the mechanism of the edge effects and the coupling effects was investigated, and their results in the AFM images were studied. It is demanstrated by the theoretical analysis and AFM experiments that the edge effects make lateral force images more clear than the topography images, also make extraction of frictional force force from lateral force images mare complex and difficult. While the coupling effects make the comparison between topography images and lateral force images mare advantage to acquire precise topography information by AFM.     

In-situ Observation Technique of Propagation of Micro/nano Cracks

To observe cracks formation and propagation of materials,an in-situ tensile test system integrated with AFM and CCD imaging tools is developed in the present study.A left-right combination precision ball screw which can realize the equal motion of the sample’s two ends resulting in a constant position during imaging is employed.A linear encoder and a bridge circuit type strain gauge force sensor are used to measure the displacement and the force,respectively,which are utilized to calculate the strain and stress during the imaging.Based on this sytem,the mechanical properties of the sample and the deformation behavior of the materials can be obtained simultaneously.The magnetic tape is employed as the sample to be tensiled and observed by the developed system.AFM and CCD observation results verify that the in-situ tensile test system is feasible to observe the crack formation and propagation from micron to nano scale.This technique can also be used in other materials research fields such as failure of nano composites,local deformation of the sample surface under the tensile force and so on.     

Study of Slope Reinforcement Force Based on FLAC

A slope will slide if the unbalanced force does not tend to zero when the stability of the slope is analyzed with the help of FLAC. Thus the ultimate reason of slope sliding is the unbalanced force determined by FLAC. The slope will remain stable if the unbalanced force is counterbalanced by a reinforcement force which is produced by a suitable reinforcement method. In this paper, the stability of the slope was analyzed by using FLAC, and the unbalanced force of the slope was obtained through the FISH function in FLAC. According to the equilibrium conditions, the relationship between the reinforcement force and unbalanced force was derived and accordingly the reinforcement force was determined. The reinforcement design was adopted by using pre-stressed anchor bars on the basis of the reinforcement force. An example is used to show that the effect of slope reinforcement based on the reinforcement force is safe and economical. The method doesn＇t need to suppose a sliding surface to obtain the reinforcement force, and it is also clear in physical meaning. So this method realized the organic unification of the stability analysis and the slope reinforcement.     

Electroless nickel plating on the surface of carbon fibers

The electroless nickel plating on the surface of carbon fibers was prepared by pretreating the carbon fibers in order to increase their conductivity,and consequently enhance the EMI shielding effectiveness of the composites.The relationship between the performance of depositing coat and pH value,temperature,reaction time and the way of agitation was studied.Results show that the depositing quality is stable under pH between 4.5 and 5.0,temperature between 75 ℃ and 85 ℃,reaction time for 10 min and air agitation.The uniform and compact nickel layer deposited on carbon fibers was proved by XRD and SEM,and the electrical resistivity of carbon fibers with nickel coating was tested.Results indicate that the electrical resistivity of carbon fibers with electroless nickel plating is decreased by an order of magnitude compared with that of carbon fibers.It means that nickel coating can greatly improve the electromagnetic interference shielding properties of carbon fibers.     

Characters of surface deformation and surface wave in thermal capillary convection

In the field of fluid mechanics, free surface phenomena is one of the most important physical processes. In the present research work, the surface deformation and surface wave caused by temperature difference of sidewalls in a rectangular cavity have been investigated. The horizontal cross-section of the container is 52 mm×42 mm, and there is a silicon oil layer of height 3.5 mm in the experimental cavity. Temperature differ- ence between the two side walls of the cavity is increased gradually, and the flow on the liquid layer will develop from stable convection to un-stable convection. An optical diag- nostic system consisting of a modified Michelson interferometer and image processor has been developed for study of the surface deformation and surface wave of thermal capillary convection. The Fourier transformation method is used to interferometer fringe analysis. The quantitative results of surface deformation and surface wave have been calculated from a serial of the interference fringe patterns. The characters of surface deformation and surface wave have been obtained. They are related with temperature gradient and surface tension. Surface deformation is fluctuant with time, which shows the character of surface wave. The cycle period of the wave is 4.8 s, and the amplitudes are from 0 to 0.55 μm. The phase of the wave near the cool side of the cavity is opposite and correlative to that near the hot side. The present experiment proves that the surface wave of thermal capillary convection exists on liquid free surface, and it is wrapped in surface deformation.     

Numerical simulation on effects of embedded crack on rock fragmentation by a tunnel boring machine cutter

Based on the simplification of cutting process, a series of numerical simulations were conducted using a 2-D discrete element method to explore the effects of embedded cracks with different dip angles on the rock fragmentation process, cutting characteristics and breaking efficiency. The results show that the simulated results are in a good agreement with previous theoretical study. The main crack propagates to the top tip of embedded crack, except when the dip angle is 90°. Side cracks which are more fully developed in the rocks containing embedded cracks tend to propagate towards the free surface. According to the history of vertical cutting force, it is shown that the peak force is decreased by embedded cracks. The study on cutting efficiency was conducted by combining the quantity of crack and cutting energy. And the results show that breaking efficiency can be treated as a decreasing or a increasing function when the dip angle is less or larger than 30°, respectively. Breaking efficiency is higher than that in intact rock when the dip angle is larger than 45°.     

Deformation characterization of conductive particles in anisotropically conductive adhesive simulated by FEM

The deformation behavior and the contact area of conductive particles in anisotropically conductive adhesives (ACA) were investigated by finite element method (FEM). The solid conductive particles are made of pure Ni and Cu. The results indicate that the deformation of the conductive particles is inhomogeneous during fabrication. When the reduction in height is small the deformation concentrates in the area near the contact area. As the reduction in height increases, the strain in the area near the contact area increases, and the metal flows toward the circumference, resulting in the increase of the contact area between the conductive particles and pad. The higher the degree of deformation, the larger the contact area. The regression equations were offered to express the relations between the bounding force and the contact area or the reduction in height. An approach of how to obtain the maximum contact area in ACA was discussed.     

Nanowear pretreatment of AFM tips for reasonable friction force

In a nanoscale friction test, wear of an atomic force microscope (AFM) tip is inevitable. The shape of the worn tip influences the friction force measured. In order to eliminate the influence, nanowear pretreatment should be conducted for the AFM tip. In our study, pretreatment of three kinds of tips, i.e. Si₃N₄ tip, Si tip and silica colloidal tip, was performed using AFM. The results show that the shape of the tips changes with the increase of sliding distance, which leads to the variety of friction force. Whereas, when the tip gets blunt, the shape of tip tends to become stable and the friction force becomes stable correspondingly. To a certain degree, it reveals that the pretreated tips can be applied to study the friction force of samples.     

扫描隧道显微镜探针作用力的测量

我们用石英振子频移法成功地对ＳＴＭ探针与被测表面间的作用力进行了测量,进一步证实了在ＳＴＭ测量时探针与被测表面之间存在着一种作用力的预言,为最终解明该现象的本质提供了一种新的方法。     

新型除锈爬壁机器人附壁建模与仿真

设计了1种永磁真空混合附壁的船舶壁面除锈爬壁机器人,该机器人负载大、本体重,机器人的附壁面法向存在水射流反冲力和真空负压压力.建立了机器人下滑和后翻两静态模型,结合船壁面法向的3种受力状态,分别对下滑模型和后翻模型进行了分析,并将两模型永磁单元所需吸附力进行了对比.仿真和实验结果表明,真空负压提高机器人附壁能力明显,可以较大地降低永磁吸附单元所需吸附力,减小机器人负载,较低的真空负压可实现辅助永磁良好附壁,在保证灵活运动的前提下吸附可靠.     

Investigation of atomic structure ahead of crack tip by STM and AFM

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外加电场对云母表面纳尺度摩擦行为的影响

利用原子力显微镜（AFM）研究了施加恒定外电场时,AFM针尖与云母表面间的摩擦力变化情况．结果表明,对样品基底施加一定偏压后,由于针尖与云母表面间的静电作用增强,使得针尖与样品间的总相互作用势增加,从而导致针尖与云母间的摩擦力比未施加偏压时大,且摩擦力大小随电压升高而增大．尤其是施加负偏压时,摩擦力增加更为显著,且大于正偏压时的摩擦力．     

Study on microscale adhesion between solid surfaces with scanning probe

The adhesion between two parallel solid surfaces is of great interest with the rapid development of micro-nano devices and instruments. The adhesion forces between a flat tip with a diameter ～1.7 μm and some surface have been determined by recording the force-displacement curves with an atomic force microscope (AFM). The flat tip is used to prevent wear and mimic the adhesion between two parallel surfaces. The free energy of the solid surface is calculated by the contact angles between the probe liquids and the surface. The adhesion force between parallel solid surfaces cannot be predicted by the theory of thermodynamic surface free energy. The adhesion measurements were carried out under ambient conditions, in a nitrogen-filled glove box, under distilled water, and under potassium chloride (KCl) solution. The outcome shows that the real contact area without the applied load is only a small proportion of the apparent contact area. The measurement stability and repeatability of adhesion by the AFM depend on the surface characterization, measurement methods and the environment. Under different environments, there are different interactions and factors affecting the adhesion force, and the dominant interactions and factors may be different too. The various interactions and factors are mutually coupled to determine the final adhesion force.     

利用AFM对单晶硅表面加工化学反应理论分析

以原子力显微镜为工具,在空气条件下对单晶硅片表面加工得到下凹的三维微结构。给出了AFM金刚石针尖在对单晶硅的微加工中的化学反应和加工模型。对加工条件下发生的化学反应及其原理进行了理论分析。用化学热力学定律、化学动力学原理和相关的热力学数据自由能、生成热、化学键能、活化能等分析加工过程中发生的反应是空气中O2与单晶硅Si反应生成二氧化硅的过程;金刚石针尖磨损发生的反应主要是空气中O2与金刚石针尖C反应生成CO2的过程。