Wetting layer thickness and early evolution of epitaxially strained thin films

We propose a physical model which explains the existence of finite thickness wetting layers in epitaxially strained films. The finite wetting layer is shown to be stable due to the variation of the nonlinear elastic free energy with film thickness. We show that anisotropic surface tension gives rise to a metastable enlarged wetting layer. The perturbation amplitude needed to destabilize this wetting layer decreases with increasing lattice mismatch. We observe the development of faceted islands in unstable films.     

On the dependence of surface undulation on film thickness

This paper investigates the dependence of surface undulation on a film thickness considerably greater than the critical value of a thin film system. It considers that surface tension and residual stress are the main cause of surface undulation. The study found that there is a critical undulation wavelength that minimizes the free energy of a thin film system, that this critical wavelength depends on the film thickness, and the effect of undulation amplitude is insignificant. The research also found that the surface undulation has a negligible influence on the residual stresses in the thin film system.     

Electromigration-Induced Instability of the Interface between Solid Conductors

A model has been developed to describe the influence of vacancy electromigration in the bulk of joined conducting materials under applied electric current on the shape stability of a flat interface between them. A system of equations is formulated and solved to describe the relationship between changes in the interface profile and mechanical stresses arising in it due to ion and vacancy fluxes, induced by a small spatially periodic perturbation of the interface. Criteria of the perturbation amplitude growth with time, i.e., the shape instability conditions for the interface, are determined. A more detailed analysis and estimation are performed for two special cases: in the first case the interface is between two similar materials, and in the other the mobility of ions and vacancies in one of the materials can be neglected. Perturbation wavelength ranges are determined and studied analytically for these cases; within the intervals the bulk vacancy electromigration is the main factor that leads to the growth of perturbation amplitude and mechanical stresses along the interface with time. Conditions for the existence of such ranges and dependences of their boundaries on the current direction and current density are determined. Particular wavelength and current density ranges of the interface instability are estimated. The estimates show that the interface instability due to bulk electromigration is possible under reasonable (experimentally and practically) conditions in terms of temperature (～100°C), current density (～10¹⁰ ? 10¹² A/m²), and perturbation wavelength (～10¹ ? 10³ μm). The obtained results may be useful, e.g., for improving the reliability and lifetime of micro- and nanoelectronic components.     

凝固界面形态演化的实验研究

本文采用典型的透明模型合金丁二腈-1.1wt%乙醇进行定向凝固实验,对从平界面失去稳定性到建立起新的稳定的界面形态之间时间相关的形态演化过程进行了详细的直接观察研究。发现:1)平界面失稳初始扰动波长在Mullins-Sekerka(缩写为MS)稳定性理论所预言的波长范围内,但与理论预言的振幅发展最快的波长不一致;2)平界面失稳初期,扰动振幅发展速率确实是振幅的线性函数,但在具体数值上实测值比理论值小得多;3)胞晶和枝晶形态演化过程具有不同的特征,导致这种差别的主要原因是稳态一次间距与初始扰动波长之间相对尺 关键词：     

Nonlinear Saturation Amplitude in Classical Planar Richtmyer–Meshkov Instability

The classical planar Richtmyer–Meshkov instability(RMI) at a fluid interface supported by a constant pressure is investigated by a formal perturbation expansion up to the third order,and then according to definition of nonlinear saturation amplitude(NSA) in Rayleigh–Taylor instability(RTI),the NSA in planar RMI is obtained explicitly.It is found that the NSA in planar RMI is affected by the initial perturbation wavelength and the initial amplitude of the interface,while the effect of the initial amplitude of the interface on the NSA is less than that of the initial perturbation wavelength.Without marginal influence of the initial amplitude,the NSA increases linearly with wavelength.The NSA normalized by the wavelength in planar RMI is about 0.11,larger than that corresponding to RTI.     

Elastic-plastic analyses on the residual stresses and curvature of the film/substrate bilayer system

During thermal cycling, the residual stresses are often generated in the film/substrate bilayer due to the material mismatch between the substrate and the film. If the thickness of the film is relatively high, the thermal residual stresses in it may be of different signs. When the film is subjected to elastic-plastic deformation, two plastic zones with different thicknesses may be generated in the film at a significantly high temperature difference. In this paper, a theoretical model which reflects the complete history of thermal residual stresses and curvatures in the elastoplastic film/substrate bilayer system is developed. Solutions are derived to estimate the residual stresses and curvature in the film as functions of temperature difference. The case of Al/Si system is used to illustrate the implementation of this model. Results show that the critical temperature difference at which the second plastic zone near the film surface is generated near the Al film surface is dependent on the film thickness. The strain hardening of the film has an obvious influence on the magnitude of residual stresses within the film at high temperature difference.     

非均匀圆柱壳中传播孤立波的动力学稳定性

利用积分因子法对非均匀圆柱壳中传播孤立波的动力学稳定性进行详细的数值研究.以高斯波扰动、简谐波扰动和随机扰动作为初始扰动,考察受到这三种不同扰动的孤立波能否较长时间保持波形结构和传播速度而稳定传播的问题.结果表明：孤立波的稳定传播与扰动幅度、宽度和波长有关,只在受到幅度、宽度和波长都足够小的扰动下非均匀圆柱壳中传播的孤立波才能够表现出较强的抗干扰性,具有良好的动力学稳定性.     

相位调制边带失衡问题研究

 高功率激光系统中一般利用相位调制器对单纵模激光进行频谱展宽以避免受激布里渊散射，当后继系统中的滤波器存在中心波长偏差时，则会导致相位调制产生的边带失衡，使光信号出现较为严重的高频起伏。基于前端系统模拟平台对此问题进行了数值模拟研究，结果表明滤波后的高频起伏随滤波器带宽变窄和中心波长偏差增大而加剧。分析表明0.2 nm带宽的滤波器出现0.05 nm左右的波长偏差时，将引起24.3%左右的顶部起伏。实验中根据分析将滤波器带宽从0.2 nm加大到0.4 nm，成功将脉冲顶部起伏从25.2%减小到了4.4%，对前端系统的研制起到了很好的指导作用。     

Measurement of modulational instability gain of second-order nonlinear optical eigenmodes in a one-dimensional system

We have investigated the amplification of a spatially periodic perturbation applied to a wide fundamental beam launched near phase matching for second-harmonic generation in a lithium niobate film waveguide. We measured the gain coefficient for the modulational instability of quadratic eigenmodes as a function of periodicity, intensity, and wave-vector mismatch. Excellent agreement with theory was obtained.     

Weakly nonlinear analysis of the morphological stability of a two-dimensional cylindrical crystal

We present the first weakly nonlinear analysis of the morphological stability of a two-dimensional cylindrical crystal growing from solution in an arbitrary regime (with the growth rate proportional to supersaturation). A quadratic (with respect to the perturbation amplitude) correction to the critical radius of a stable crystal determined in the linear theory is obtained in an analytical form and studied as a function of the per-turbation frequency and the growth regime. It is established that an increase in the perturbation amplitude vir-tually always leads to a decrease in the critical radius. Factors accounting for this nontrivial effect are consid-ered.     

Interfacial diffusion relaxation of internal stresses in solids

The relaxation of internal stresses due to interfacial diffusion in a two-phase solid is studied theoretically with the help of the onsageristic approach of irreversible thermodynamics. In this note we derive an expression for the rate at which internal stresses associated with misfit caused by bonding a flat surface of one material to a rough surface of another. The two phases are treated as isotropic clastic substances. It is assumed that the components of only of the solids are capable of leaving their positions of migrating along the interface. The driving force for this process is minimization of total energy-clastic plus interfacial energy. We show that the time constant for relaxing these stresses is proportional to the cube of the wavelength of the roughness.     

镀含弱吸收膜两层膜系长周期光纤光栅谐振特性及其优化

采用严格的耦合模理论,建立了两层膜系长周期光纤光栅复特征方程,用微扰法对复特征方程进行求解,结果和D.V.Ignacio的结论相符.求得的谐振波长表明其随膜层厚度和膜层折射率变化明显,受消光系数影响很小.重点提出了两种途径来优化膜层参量组合,以获得较大的谐振波长偏移量,计算结果显示,参量组合取值为(h_3=122.76 nm,h_4=400 nm)和(n_3=1.572 2,h_4=400 nm)时,谐振波长偏移量分别为10.35 nm和11.74 nm,远高于只镀一层敏感膜LPFG的偏移量2.78 nm,从而证明敏感膜层厚度(h_4)较大的参量组合可提高传感器的灵敏度.     

Ripples in tapped or blown powder

We observe ripples forming on the surface of a granular powder in a container submitted from below to a series of brief and distinct shocks. After a few taps, the pattern turns out to be stable against any further shock of the same amplitude. We find that the wavelength of the pattern is proportional to the amplitude of the shocks. Starting from considerations involving air flow through the porous granulate and avalanche properties, we build up a semiquantitative model which satisfactorily fits the set of experimental observations of either tapped or blown powder.     

Higher accuracy analytical approximations to a nonlinear oscillator with discontinuity by He's homotopy perturbation method

He's homotopy perturbation method is used to calculate higher-order approximate periodic solutions of a nonlinear oscillator with discontinuity for which the elastic force term is proportional to sgn(x). We find He's homotopy perturbation method works very well for the whole range of initial amplitudes, and the excellent agreement of the approximate frequencies and periodic solutions with the exact ones has been demonstrated and discussed. Only one iteration leads to high accuracy of the solutions with a maximal relative error for the approximate period of less than 1.56% for all values of oscillation amplitude, while this relative error is 0.30% for the second iteration and as low as 0.057% when the third-order approximation is considered. Comparison of the result obtained using this method with those obtained by different harmonic balance methods reveals that He's homotopy perturbation method is very effective and convenient.     

Excitation d'ondes acoustiques par une flamme

Acoustic waves have been excited in a flame seeded with potassium. We show that the amplitude of the excited wave is proportional to the frequency of a harmonic perturbation imposed on the plasma. This suggests a means of ultrasonic excitation, the limitations of which are analysed.

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Laboratoire Associé au Centre National de la Recherche Scientifique (C.N.R.S.).     

基于XRD的镀锌钝化膜残余应力试验研究

利用XRD技术测试了镀锌钝化膜结合界面的残余应力，同时通过电解抛光法检测了其厚度方向残余应力的分布规律，分析了残余应力对镀锌钝化膜结合强度的影响. 试验结果表明，镀锌钝化膜的残余应力均表现为压应力，并随着基体表面残余应力的增大而减小；钝化膜在2—10μm厚度方向的残应力为-274.5—-428.3MPa，其应力为梯度分布；镀锌钝化膜与基体的界面结合强度与其残余应力成反比，减小薄膜残余应力，有利于提高镀锌钝化膜与基体的结合强度. 关键词： X射线衍射法(XRD) 镀锌钝化膜 结合强度 残余应力     

红外薄膜中热应力的研究

由于红外薄膜材料和基板热膨胀系数显著不同,所以在高温基板上镀膜后降温将产生热应力,进而引起边界分层破裂现象,影响薄膜器件的牢固性.对薄膜厚度、杨氏模量和热膨胀系数对薄膜分层破裂的影响进行了研究,同时分析了薄膜设计对减小分层破裂的作用.这对减小红外薄膜系统因热应力引起的分层破裂现象具有实际应用价值. 关键词： 多层介质薄膜 红外 热应力 分层破裂     

Destabilization of a Saffman-Taylor fingerlike pattern in a granular suspension

We study the Saffman-Taylor instability in a granular suspension formed by micrometric beads immersed in a viscous liquid. When using an effective viscosity for the flow of the suspension in the Hele-Shaw cell to define the control parameter of the system, the results for the finger width of stable fingers are found to be close to the classical results of Saffman-Taylor. One observes, however, an early destabilization of the fingers that can be attributed to the discrete nature of the individual grains. Classically, the threshold of destabilization is linked to the noise in the cell and is thus difficult to quantify. We show that the grains represent a "controlled noise" and produce an initial perturbation of the interface with an amplitude proportional to the grain size. The finite amplitude instability mechanism proposed by Bensimon et al. allows us to link this perturbation to the value of the threshold observed.     

侧照明实验用平面调制微靶的制备及参数测量

 介绍了用于侧照明实验研究瑞利-泰勒流体力学不稳定性(Rayleigh-Taylor instability)实验的聚苯乙烯和铝平面调制微靶的制备工艺。通过图形转移，靶型切割和装配工艺的控制，结合靶参数测量，获得了厚度在几十μm、表面具有波长50 μm和55 μm、振幅从几到几十μm、微靶宽度约为200 μm、剖面正弦调制图形清晰的两种靶型。     

爆轰加载下高纯铜界面Rayleigh-Taylor不稳定性实验研究

金属界面不稳定性是内爆物理压缩过程中关注的重要问题,与传统流体界面不稳定性具有显著区别.由于相关理论和实验诊断技术的限制,目前该问题的研究还明显不足.为加深对金属界面不稳定性扰动增长行为的认识,本文建立了爆轰加载下高纯铜界面Rayleigh-Taylor不稳定性研究的实验诊断技术和数据处理方法,得到了扰动发展早期不同时刻界面扰动增长的X光图像.实验结果分析表明:在爆轰产物的无冲击加载条件下扰动波长基本保持不变,而初始扰动幅值越大,界面扰动增长的趋势就越明显;同时随着样品前界面扰动的不断发展,在样品的后自由面也出现了与前界面初始相位相反的扰动特征,即样品前界面扰动为波谷的位置所对应的后界面先运动而逐渐演变为波峰,而前界面扰动为波峰的位置所对应的后界面则演变为波谷;在5.26μs时刻,界面扰动幅值增长为初始值的700%左右,应变率达到了约105/s.结合数值模拟研究表明:在此情况下常用的Steinberg-Cochran-Guinan模型在一定程度上低估了高纯铜材料强度的强化特性,无法准确地描述强度对界面扰动增长的制稳作用,从而导致数值模拟结果要大于实验测量结果.