THERMODYNAMIC ANALYSIS AND EXPERIMENTAL VERIFICATION FOR SYNTHESIZING SILICON NITRIDE NANOPARTICLES USING RF PLASMA CVD

Silicon nitride nanoparticles were synthesized by radio-frequency (RF) plasma chemical vapor deposition (PCVD) using silicon tetrachloride and ammonia as precursors, and argon as carrier gas. By assuming chemical thermodynamic equilibrium in the system, a computer program based on chemical thermodynamics was used to calculate the compositions of the system at different initial concentrations and final temperatures. At first, five elements and thirty-four species were considered. The effects of temperatures, and concentrations of ammonia, hydrogen and nitrogen on the equilibrium compositions were analyzed. It was found that the optimal reaction temperature range should be 1200 to 1500 K to obtain the highest conversion and yield of Si3N4. The inlet position of ammonia should be lower than that of silicon tetrachloride, and both should be located at the tail of the plasma torch. The best moleratio of ammonia to silicon tetrachloride was found to be about 6. Later, the influences of water (.and oxygen) were considered, and 17 additional species were included in the computations. It was found that oxygen or water content in the raw materials should be as low as possible in order to have high nitride content in the produced Si3N4. Nitrogen or hydrogen might be used to replace some or even all the argon to improve the yield of silicon nitride and reduce the cost. The ratio of ammonia to silicon tetrachloride should be high enough to obtain high conversion, but not excessively high to reduce the oxygen content due to the existence of water in ammonia. The simulated results were verified bv experiments.     

Analysis of non-symmetrical flapping airfoils

Simulations have been done to assess the lift, thrust and propulsive efficiency of different types of non-symmetrical airfoils under different flapping configurations. The variables involved are reduced frequency, Strouhal number, pitch amplitude and phase angle. In order to analyze the variables more efficiently, the design of experiments using the response surface methodology is applied. Results show that both the variables and shape of the airfoil have a profound effect on the lift, thrust, and efficiency. By using non- symmetrical airfoils, average lift coefficient as high as 2.23 can be obtained. The average thrust coefficient and efficiency also reach high values of 2.53 and 0.61, respectively. The lift production is highly dependent on the airfoil＇s shape while thrust production is influenced more heavily by the variables. Efficiency falls somewhere in between. Two-factor interac- tions are found to exist among the variables. This shows that it is not sufficient to analyze each variable individually. Vorticity diagrams are analyzed to explain the results obtained. Overall, the S1020 airfoil is able to provide relatively good efficiency and at the same time generate high thrust and lift force. These results aid in the design of a better ornithopter＇s wing.     

Inorganic nanocrystals：From molecular design to systematic engineering

Solution-based routes have been widely applied in the synthesis of nanostructures.It is desirable to choose or design precursors,ligands,and solvent molecules at the molecular level to allow the synthesis of lowdimensional nanocrystals with various shapes and sizes.The increasing requirements for the integration of the properties of nanocrystals have produced a high demand for the rational design and fine control of complex structures,in terms of both the composition and the structure.To meet this demand,researchers have developed new synthetic strategies to produce more complex and more functional nanocrystals.Typical procedures involve systematic engineering,which focuses on the whole synthetic procedure,instead of just the molecular details.This review focuses mainly on the work of Yadong Li’s group over the last decade.     

A 17-node quadrilateral spline finite element using the triangular area coordinates

Isopaxametric quadrilateral elements are widely used in the finite element method. However, they have a disadvantage of accuracy loss when elements are distorted. Spline functions have properties of simpleness and conformality. A 17onode quadrilateral element has been developed using the bivaxiate quaxtic spline interpolation basis and the triangular area coordinates, which can exactly model the quartic displacement fields. Some appropriate examples are employed to illustrate that the element possesses high precision and is insensitive to mesh distortions.     

HOMOTOPY CONTINUATION ALGORITHM FOR STRUCTURAL DAMAGE IDENTIFICATION

I. INTRODUCTIONStructural damages due to the loss of sti?ness such as crack, localization bulking have remarkablein?uences on the physical properties of structure such as deformation, stress, frequency and modelshape. So the change of these properties can be used to identify the damage location and degree ofstructures. The damage identification techniques based on these properties’ changes have attractedmuch attention in recent years, and many approaches have been developed[1??5].Nowaday…     

THEORETICAL STUDIES ON MICROBUCKLING MODE OF ELASTIC MEMORY COMPOSITES

Elastic memory composites （EMCs） have great potential applications in future deployable space structures due to their high packaging strain and shape memory characteristics. Microbuckling of compressed fibers is the primary deformation mechanism of such structures to receive a higher packaging strain than that of traditional fiber-reinforced composites. In order to have a better understanding of such deformation mechanism, the microbuckling model of EMC laminates under bending is analyzed firstly. Then the theoretical critical microbuckling parameters are predicted, which are compared with experimental observations and other models.     

A plastic indentation model for sandwich beams with metallic foam cores

Light weight high performance sandwich composite structures have been used extensively in various load bearing applications.Experiments have shown that the indentation significantly reduces the load bearing capacity of sandwiched beams.In this paper,the indentation behavior of foam core sandwich beams without considering the globally axial and flexural deformation was analyzed using the principle of virtual velocities.A concisely theoretical solution of loading capacity and denting profile was presented.The denting load was found to be proportional to the square root of the denting depth.A finite element model was established to verify the prediction of the model.The load-indentation curves and the profiles of the dented zone predicted by theoretical model and numerical simulation are in good agreement.     

SHOCK INITIATION OF EXPLOSIVES INVESTIGATED WITH SMALL PARTITION EXPERIMENT AND NUMERICAL SIMULATION

In order to investigate the shock ignition of high energy solid explosives by shock waves,we carry out Lagrangian experiments with 2-D Lagrangian technique which uses composite manganin-constantan(CMC).The efects of the shock sensitivity of pressed solid high explosives,TNT,and the efect of the lateral rarefaction wave were studied.Based on the measured pressure histories and the radial displacements,we formulate the Ignition and Growth reactive flow models for the pressed TNT.The shock initiation process simulated by Ignition and Growth model agreed well with experimental data.This pressed TNT model can be applied to shock initiation scenarios which are highly unpredictable and have not been or cannot be tested experimentally.     

COAL COMBUSTION EFFICIENCY IN CFB BOILER

The carbon content in the fly ash from most Chinese circulating fluidized bed (CFB) boilers is much higher than expected, thus directly influencing the combustion efficiency. In the present paper, carbon burnout was investigated both in field tests and laboratory experiments. The effect of coal property, operation condition, gas-solid mixing, char deactivation,residence time and cyclone performance are analyzed seriatim based on large amount of experimental results.A coal index is proposed to describe the coal rank, defined by the ratio of the volatile content to the coal heat value, is a useful parameter to analyze the char burnout. The carbon content in the fly ash depends on the coal rank strongly. CFB boilers burning anthracite, which has low coal index, usually have high carbon content in the fly ash. On the contrary, the CFB boilers burning brown coal, which has high coal index, normally have low carbon content.Poor gas-solid mixing in the furnace is another important reason of the higher carbon content in the fly ash. Increasing the velocity and rigidity of the secondary air could extend the penetration depth and induce more oxygen into the furnace center. Better gas solid mixing will decrease the lean oxygen core area and increase char combustion efficiency.The fine char particles could be divided into two groups according to their reactivity. One group is “fresh“ char particles with high reactivity and certain amount of volatile content. The other group of char particles has experienced sufficient combustion time both in the furnace and in the cyclone, with nearly no volatile. These “old“ chars in the fly ash will be deactivated during combustion of large coal particles and have very low carbon reactivity. The generated fine inert char particles by attrition of large coal particles could not easily burn out even with the fly ash recirculation. The fraction of large coal particles in coal feed should be reduced during fuel preparation process.The cyclone efficiency controls the particle residence time in CFB loop, especially for that of the fine particles. So the cyclone efficiency, especially the cut size, will greatly influence the carbon content in the fly ash.     

Nanofluids: Stability, phase diagram, rheology and applications

There is no doubt about the potential technological significance of nanofluids. The promising application areas have been identified as effective heat transfer fluids, contrast agents in magnetic resonance imaging, magnetohyperthermia treatment, precursors to high performance nanocomposites and ordered nanostructures. However, commercial applications are rare, in part due to the limited understanding of the nanofluid fundamentals such as colloid stability, phase diagrams and rheology. This paper intends to provide a brief overview of the scientific disciplines that are important to nanofluids, and the interconnection among different disciplines in order to gain a perspective on the future development of this intriguing area.     

STUDY ON THE RESPONSE TO LOW-VELOCITY IMPACT OF A COMPOSITE PLATE IMPROVED BY SHAPE MEMORY ALLOY

Improvement from the pseudo-elastic effect of shape memory alloy (SMA) on the low-velocity impact (LVI) resistance of a composite plate is investigated by the finite element method (FEM).The stiffness matrix of the dynamic finite element equation is established step by step and the martensite fraction is obtained at each time step.The direct Newmark integration method is employed in solving the dynamic finite element equation,while the impact contact force is determined using the modified Hertz's law.It is found that SMA can effectively improve the performance of a composite structure subjected to low-velocity impact.Numerical results show that the deflection of a SMA-hybrid composite plate has been reduced approximately by thirty percent when the volume fraction of the embedded SMA reaches 0.3.     

拟弹性SMA对复合材料板抗低速冲击响应性能的影响

以形状记忆合金(SMA)纤维增强复合材料板为研究对象,根据SMA拟弹性曲线的特性,建立了一种SMA拟弹性应力-应变关系的分段线性化模型;在此基础上,采用分步能量平衡法,求解了SMA增强复合材料板受低速冲击时的横向位移和应力,分析了SMA的拟弹性特性对复合材料板低速冲击性能的影响.研究结果表明,SMA的能量吸收特性能有效地增强复合材料板抗低速冲击能力,板的最大位移和最大应力都明显减少.冲击速度为10m/s的情况下,板的最大挠度和应力降低了18%左右;冲击速度为25 m/s的情况下,板的最大挠度和应力降低了42%左右.     

低速冲击下复合材料合板的响应过程模拟

运用一种精度较高的高阶位移模型分析了复合材料层合板在低速击下的响应过程,该位移模型能够同时考虑层间正应力和横向剪应力,另外,采用修正的Hertz接触定律与Newmark积分方法相结合,建立了冲击接触的有限元模型,本文的数值模拟计算结果与解析结果相比较证明了该方法的有效和精确性。     

冰载荷下不锈钢复合板挠度特性分析

以复合板中面的挠度响应作为不锈钢复合板抗冲击性能的评价指标,基于能量法和经典层合板理论,考虑层间结构参数设计,通过横向载荷下的弯曲平衡微分方程,建立冰载荷下不锈钢复合板挠度响应简化解析模型。该分析模型将整个动态响应分析过程分为冰载荷计算分析和动力学方程求解两个阶段。分析了冰载荷模型的面倾角、冲击速度和碰撞位置对冰载荷的影响,确定极端工况参数,汇总接触面的节点力数据;分析了层厚比对挠度响应的影响规律;基于LS-DYNA有限元仿真以及数值算例分析,对比挠度响应仿真结果和解析计算值,验证了本文简化解析模型的准确性,研究结果对不锈钢复合板抗冲击性能分析和评估具有一定的参考价值。     

形状记忆合金增强复合材料连接件模型的实验研究

本文对形状记忆合金（ＳＭＡ）增强复合材料连接件模型进行了初步的实验分析和计算,结果表明：ＮｉＴｉＳＭＡ丝产生的回复应力对连接件模型孔应变有明显的影响,埋入ＮｉＴｉ丝的复合材料连接件模型的拉伸破坏载荷有所提高,本研究为改善复合材料连接的强度问题作了有益的探索。     

复合材料层压板低速冲击响应尺度效应数值模拟研究

为了研究尺度效应对于复合材料层压板在低速冲击作用下的动态响应和冲击损伤的影响，基于相似理论，建立了三种不同尺寸的层压板受冲击的三维有限元模型。在该模型中，针对层压板的面内损伤，采用改进的Chang-Chang准则进行预测；针对层压板内层间分层损伤，则使用Cohesive界面单元进行模拟。一旦复合材料层压板在低速冲击作用下产生损伤，则对出现损伤的区域进行材料参数退化。采用该模型对三种不同尺寸的层压板的冲击过程进行有限元分析，并将不同冲击速度下的冲击响应进行比较，得出了如下结论：在层压板内未发生冲击损伤时，冲击产生的挠度和冲击力与相似理论解十分吻合，一旦出现冲击损伤，则冲击力的变化与相似理论解有所差别；如果两个缩放模型的冲击速度之比等于缩放比例的平方根，则两个模型中的相对分层尺寸基本是相同的，这个结果与已有的实验结果吻合；而对冲击后面内损伤的分析表明，其损伤尺寸不符合这一相似规律。     

Large amplitude free vibration analysis of thermally post-buckled composite doubly curved panel embedded with SMA fibers

Thermal post-buckled vibration of laminated composite doubly curved panel embedded with shape memory alloy (SMA) fiber is investigated and presented in this article. The geometry matrix and the nonlinear stiffness matrices are derived using Green–Lagrange type nonlinear kinematics in the framework of higher order shear deformation theory. In addition to that, material nonlinearity in shape memory alloy due to thermal load is incorporated by the marching technique. The developed mathematical model is discretized using a nonlinear finite element model and the sets of nonlinear governing equations are obtained using Hamilton’s principle. The equations are solved using the direct iterative method. The effect of nonlinearity both in geometric and material have been studied using the developed model and compared with those published literature. Effect of various geometric parameters such as thickness ratio, amplitude ratio, lamination scheme, support condition, prestrains of SMA, and volume fractions of SMA on the nonlinear free vibration behavior of thermally post-buckled composite flat/curved panel been studied in detail and reported.     

Nonlinear finite element analysis of shape memory alloy (SMA) wire reinforced hybrid laminate composite shells

This study introduces a non-linear finite element analysis approach to the procedure of modeling hybrid laminate composite shells with embedded shape memory alloy (SMA) wire subjected to coupled structural and thermal loading. Numerical analyses of SMA wire reinforced composite laminates were carried out by synergizing the non-linear laminate shell element with Brison's model of the SMA constitutive law. To verify the proposed procedure, the present illustrative applications involve rectangular laminated panels clamped along one side. Analysis results were compared with corresponding experimental results from a prior study. Several test cases that depend on the volume fraction of SMA, temperature, and ply angles are presented to illustrate the highly entangled thermo-mechanical behavior of shape memory alloy hybrid composites (SMAHCs). The results of the numerical analysis show the ability of the suggested procedure to compute the thermo-mechanical behavior of a SMAHC in accordance with the SMA's internal phase transformations induced by stress and temperature variation and demonstrate very good agreement with experimental results.     

Micro–macro analysis of shape memory alloy composites

The aim of the paper is to develop a micro–macro approach for the analysis of the mechanical behavior of composites obtained embedding long fibers of Shape Memory Alloys (SMA) into an elastic matrix. In order to determine the overall constitutive response of the SMA composites, two homogenization techniques are proposed: one is based on the self-consistent method while the other on the analysis of a periodic composite. The overall response of the SMA composites is strongly influenced by the pseudo-elastic and shape memory effects occurring in the SMA material. In particular, it is assumed that the phase transformations in the SMA are governed by the wire temperature and by the average stress tensor acting in the fiber. A possible prestrain of the fibers is taken into account in the model.Numerical applications are developed in order to analyze the thermo-mechanical behavior of the SMA composite. The results obtained by the proposed procedures are compared with the ones determined through a micromechanical analysis of a periodic composite performed using suitable finite elements.Then, in order to study the macromechanical response of structural elements made of SMA composites, a three-dimensional finite element is developed implementing at each Gauss point the overall constitutive laws of the SMA composite obtained by the proposed homogenization procedures. Some numerical applications are developed in order to assess the efficiency of the proposed micro–macro model.     

Numerical Simulation of the Fall of a Body with a Corrugated Bottom on Water

A numerical model is proposed for the potential flow of an ideal incompressible fluid produced by impact of a body with concave bottom on water. Compression of the entrapped air is taken into account. The algorithm is based on joint solution of the equations of motion for the body and the fluid by the finite difference method with approximation in time. At each time, the boundaryvalue problem for the Laplace equation is solved by the boundaryelement method. Calculation results are given. The effects of the air layer, dimensions and shape of the corrugations, initial velocity, and other parameters on the impact process are shown.