The EFG process applied to the growth of silicon ribbons*

The development of edge-defined, film-fed growth (EFG) techniques for silicon ribbons has prompted analyses of appropriate die materials, die shapes, meniscus shapes and thermal gradients. The requirement for high electronic quality of the ribbons, in conjunction with the high effective solute distribution coefficients expected for EFG, narrows the choice of die materials primarily to graphite and fused silica. The shape of the meniscus between crystal and die has been calculated. This shape, together with contact angle information, is used to determine design criteria for both graphite and silica dies. The effect of crystal growth on the temperature gradient ahead of the growth interface has been calculated in terms of the ribbon thickness and an empirical parameter characterizing the radiating environment of the growth apparatus. Numerous 25 mm wide × 0.3 mm thick ribbons from carbon dies and a few small ribbons from silica dies have so far been grown. The ribbon quality is discussed. Work supported by the National Science Foundation.     

激光直接制造金属零件过程的闭环控制研究

对激光直接制造金属零件过程中熔池温度和熔覆层厚度的变化进行了研究,指出对确定的材料,当激光功率增大到一定值时,熔池内的金属溶液大多已达到了热饱和,温度累积效应并不显著,熔池温度基本保持平稳,而在制造过程中若零件由于工艺不稳定而产生凹凸点,在多次层叠制造后,凹处越凹、凸处越凸,严重影响零件的制造精度。所以通过传感器来直接监测金属零件的熔覆高度,进而通过控制送粉量来保证制造过程中熔覆高度的稳定性比起温度控制来更具有实际意义。提出了熔覆高度的检测方案,并对送粉量的闭环控制系统进行了研究,对送粉量的时间延迟提出了相应的对策。结果表明对熔覆层高度的检测和对送粉量的调节能够提高激光直接制造过程的稳定性和制造精度。     

Dynamics of liquid meniscus bridge of a vibrating disk: consideration of flow rheology

A modified Reynolds equation, which includes the effect of flow rheology, is derived to describe the flow behaviour of lubricant between the space of a magnetic head slider and a disk. Under the assumptions of a small vibration of the spacing, and zero contact angle of the liquid?solid interface, the dynamics of a liquid meniscus disk of finite radius is analysed. The time-dependent modified Reynolds equation is linearised, and solved, under the boundary condition considering Laplace pressure. The results show that the pressure and load carrying capacity consist of three terms, that is, the static meniscus force term, the spring term by the dynamic Laplace pressure, and time-dependent damping term by the flow rheology of the fluid. The flow rheology affects the static meniscus forces and the damping forces significantly as compared to the spring forces. The effects of flow rheology on the load carrying capacity are also discussed.     

半导体激光束整形器的设计与分析

随着加工技术的不断发展，特殊曲面的光学加工已成为可能，它可以采用由特殊曲面构成的光束整形器来实现任意波面的变换，本文根据能量平衡和光程相等的原理，导出一组平行入射光束的整形器面形设计公式。根据这组方式，设计了用于准直与均匀半导体激光束的整形器，通过计算机模拟，给出了光束整形结果，并具体分析了整形器厚度和折射率对整形光束性能的影响。     

基于光线追迹的LED裸眼三维显示技术准直性研究

在LED集成成像裸眼三维显示中,LED大屏幕的单个像素点的发散角很大,使用平凸及双凸透镜,图像的重构光线发散且相邻透镜单元间的像素串扰大,导致立体场景再现的过程中图像不清晰,影响重构三维图像的质量。根据光线追迹原理,分析了LED集成成像的成像过程,研究不同形状透镜阵列与LED集成成像相邻透镜单元间像素点间串扰的关系,仅使用单个凹凸透镜有效地控制了重构光线的发散问题,使通过透镜的重构光线更加汇聚准直,光线平行出射,有效地减小了相邻透镜间的像素串扰,提高了成像质量。通过比较字母模型成像的仿真结果,凹凸透镜非常适用于LED集成成像,成像的效果优于其他形状透镜。     

On instability of low-frequency waves in a stream-magnetized plasma system

The interaction of low-frequency electromagnetic waves propagating at an arbitrary angle to the static magnetic field with a nearly synchronous charged stream along the field in a cold plasma is analyzed. The temporal growth of the waves is found to increase with the signal frequency, the stream density, and the propagation angle off the field line. Explicit forms of maximum growth rate are derived for VLF and ELF waves, and a numerical example is given for a typical ionospheric plasma near the magnetic equator.     

A General Vapor-Induced Coating Approach for Layer-controlled Organic Single Crystals

2D organic semiconductor crystals (2D OSCs) are vital for high-performance electronic and optoelectronic devices owing to their unique material merits. However, it is still challenging to fabricate high-quality and large-scale ultrathin 2D OSCs with controllable molecular layers due to the disordered molecular deposition and uncontrollable mass transport in solution-processing fabrication. Here, a vapor-induced meniscus modulating strategy for preparing unidirectional and stable Marangoni flow to guide contactless meniscus evolution is reported, which ensures uniform mass transport and ordered molecular deposition to achieve high-quality ultrathin 2D OSCs. Both the surface tension difference and the substrate wettability are critical to meniscus formation, which results in various meniscus deformation states and film morphologies. Based on the optimized vapor-solvent system, ultrathin 2D OSCs of C₈-BTBT with precise layer definition are prepared controllably. The discrepancies in liquid film height and solute concentration are decisive in controlling the molecular scale thickness ranging from mono to a few layers. Moreover, the layer-dependent electronic and optoelectronic properties of the ultrathin films are systematically investigated. Notably, high-performance polarization-sensitive solar-blind photodetectors are achieved with a dichroic ratio of photocurrent up to 2.26, and the corresponding polarimetric image sensor exhibits superior solar-blind polarization imaging capability thanks to the high crystalline quality.     

PMMA疏水性表面的飞秒激光制备研究

利用飞秒激光加工系统对PMMA表面制备微结构,调节激光的加工次数和微结构的尺寸参数,研究PMMA表面的润湿性机理.激光制备出光栅结构和方柱结构,采用超景深三维显微镜和接触角测量仪对微结构表面形貌和润湿性进行测量分析.研究了不同的激光加工次数和微结构尺寸对表面润湿性的影响,结果表明:PMMA表面微结构的接触角随光栅结构间...     

稳定提拉法晶体生长界面的装置和方法研究

为了解决传统提拉单晶体生长界面不稳定的难题,该文在传统全自动提拉单晶炉等径控制理论的基础上,通过原料补充装置,不断添加与晶体生长量相等的晶体原料至坩埚内,以稳定晶体生长液面的高度不变;再通过光学放大和电荷耦合器件(CCD)成像装置测量晶体实时直径的变化,以此变化率调整晶体旋转速度,最终使晶体生长界面始终维持在一个相对稳定的理想状态,从而保证晶体外形符合设定要求和内部品质的优良。     

激光净成形制造金属零件过程稳定性研究

研究了金属零件的激光净成形过程的稳定性，利用闭环控制系统对成形过程进行实时监测和闭环控制，并通过该系统研究了工艺参数对成形过程的影响，发现当实际堆积厚度大于预设的堆积厚度时，成形过程能够自动维持在稳定的状态;而当实际堆积厚度小于预设的堆积厚度时，实际堆积厚度将逐渐减小，因而成形过程逐渐偏离稳定状态，最终导致成形过程无法继续进行。通过实验设计系统研究了工艺参数对成形特性的影响规律，并在实验过程中进行闭环控制，实时调整预设堆积厚度与实际堆积厚度相等，得到了最大堆积厚度的传递函数。参考传递函数合理选择工艺参数，成功制造出了具有很好宏观外形和表面质量的镍基高温合金零件。     

近场纳米光纤探针腐蚀制备技术概述

纳米光纤探针是扫描近场光学显微镜的常用关键器件,化学腐蚀法制备近场光学光纤探针是基于腐蚀液弯液面的高度随光纤芯径的动态变化过程。从流体力学杨一拉普拉斯方程出发,研究了弯液面高度随光纤芯径大小的变化关系,分析了影响腐蚀法制备纳米光纤探针的相关因素,追踪了利用化学腐蚀制备纳米光纤探针的最新进展,介绍了静态腐蚀法、动态腐蚀法和选择腐蚀法等多种方法,并分析了每种方法的特点,作了比较和总结。     

Dynamic flow measurements of capillary underfill through a bump array in flip chip package

This study investigated the dynamic variations of flow and meniscus during underfill process using flow visualization techniques to understand physics of capillary flows. For the quantitative flow visualization, a high speed micro particle image velocimetry (μPIV) was applied to a transparent flip chip specimen with arrayed bump structure. As an underfill liquid, glycerin was filled into the flip chip specimen by capillary action. The present visualization technique offers time-varying movement of meniscus and phase-locked velocity fields frozen to the meniscus position. To observe the dynamic contact angle between parallel plates, an in situ measurement technique was developed in the present study. Then, the filling time was compared with analytical models. From this experiment, it was found that the meniscus velocity and the contact angle vary in-phase according to the position of meniscus. The phase-locked velocity fields show velocity gradients on the meniscus surface which gives rise to the breakdown of equilibrium contact angle. Consequently, the detailed filling time has different behavior from the analytical models.     

Electromagnetic wave scattering by a system of two spheroids ofarbitrary orientation

An exact solution to the problem of the scattering of a plane electromagnetic wave by two perfectly conducting arbitrarily oriented prolate spheroids is obtained by expanding the incident and scattered electric fields in terms of an appropriate set of vector spheroidal eigenfunctions. The incident wave is considered to be a monochromatic, uniform plane electromagnetic wave of arbitrary polarization and angle of incidence. To impose the boundary conditions, the field scattered by one spheroid is expressed in terms of its spheroidal coordinates, using rotational-translational addition theorems for vector spheroidal wave functions. The column matrix of the scattered field expansion coefficients is equal to the product of a square matrix which is independent of the direction and polarization of the incident wave, and the column matrix of the known incident-field expansion coefficients. The unknown scattered-field expansion coefficients are obtained by solving the associated set of simultaneous linear equations. Numerical results for the bistatic and backscattering cross sections for prolate spheroids with various axial ratios and orientations are presented     

曲面GRIN光学元件近轴光学参数的通用算法

本文利用矩阵和常数ＫＧ方法推导了曲面梯度折射率光学元件的近轴参数，如焦距、象方截距，焦截距，线放大率与主点位置，角放大率与节点位置的表达式，并讨论了势物面形径向对称曲面ＧＲＩＮ光学元件的成象特性，当度为半周期的整数时，发现了其有趣的现象，得出了一些有益的结论。     

Theoretical studies on a new single cristalline ribbon growth method

In order to obtain single crystalline ribbons from raw materials, we suggest a new method. From a narrow melted zone, formed along a generative line of a turning cylinder of polycrystalline material, a ribbon of material is continuously pulled in a direction perpendicular to the generative line and making a small angle with the horizontal plane. We have studied the application of the method to the silicon ribbon. We have studied the heat flux and temperatures profiles, pointing out relations between melted zone configuration, supplied power and rotation rate. The study of the melted zone hydrostatic equilibrium configurations and their influence on the ribbon growth characteristics and stability, improves on previous results for the melted zone central area and determines the shape of the floating zone lateral sides. The main advantages of the method for the production of silicon solar cells or in the space metallurgy field are discussed. Work supported by ‘Solar Energy Program’ (PIRDES) of the C.N.R.S.     

Energy efficient cosine similarity measures according to a convex cost function

We propose a new family of vector similarity measures. Each measure is associated with a convex cost function. Given two vectors, we determine the surface normals of the convex function at the vectors. The angle between the two surface normals is the similarity measure. Convex cost function can be the negative entropy function, total variation (TV) function and filtered variation function constructed from wavelets. The convex cost functions need not to be differentiable everywhere. In general, we need to compute the gradient of the cost function to compute the surface normals. If the gradient does not exist at a given vector, it is possible to use the sub-gradients and the normal producing the smallest angle between the two vectors is used to compute the similarity measure. The proposed measures are compared experimentally to other nonlinear similarity measures and the ordinary cosine similarity measure. The TV-based vector product is more energy efficient than the ordinary inner product because it does not require any multiplications.     

Effect of Cooling Rate on Microstructural Homogeneity and Grain Size of <Emphasis Type="Italic">n</Emphasis>-Type Si-Ge Thermoelectric Alloy by Melt Spinning

n-Type Si-Ge thermoelectric alloys were prepared using a melt spinning (MS) process, and then the microstructures of the samples were investigated. The alloys studied were ribbon shaped with a thickness of about 30 μm. Scanning electron microscopy (SEM) along with energy-dispersive spectrometry (EDS) and x-ray diffraction (XRD) showed a predictable, homogeneous, fine-grained microstructure at the high cooling rate, different from those of samples created by slow solidification (SS).     

Analysis of impurity concentration distributions in pulled semiconductor crystals

InSb and GaSb single crystals were grown by the Czochralski method. The impurity concentration distribution in the melt and crystal was investigated experimentally and computationally as a function of the interface segregation coefficient, the shape of the solid-liquid interface and the growth rate. It was found that the impurity concentration distribution in the melt was largely influenced by the interface segregation coefficient; the impurity concentration in the crystal increased as the shape of the solid-liquid interface became more planar and also as the growth rate increased; the impurity concentration was more markedly influenced by the growth rate than by the shape of the solid-liquid interface. When the growth rate was large, the impurity concentration increased and the shape of the solid-liquid interface was concave toward the melt near the crystal periphery. When the growth rate was small, the impurity concentration decreased and the shape of the solid-liquid interface was convex toward the melt.     

高功率微波弹最大毁伤效能分析

提出了高功率微波弹等功率密度曲面“腰”、“毁伤截面”等概念。建立了高功率微波弹的最大毁伤面积分析过程:结合微波弹的功率、增益和微波脉冲的半角宽度等参数确定等功率密度曲面的表达式;对等功率密度曲面进行曲面等效建模,相对于传统的锥形建模,文中的方法误差更小。采用微波弹为参考,旋转地面的坐标系对毁伤截面的面积进行了分析。结合公开的高功率微波弹型号和雷达设备型号,对微波弹以不同角度打击雷达设备的最大毁伤面积和相应的微波弹起爆高度以及微波弹对雷达网攻击的最佳匹配角进行了分析,验证了文中方法的可行性。     

Structural defects in laser crystallized silicon ribbons and their influence on photovoltaic behaviour

Analysis of structural defects in laser crystallized silicon sheets has been carried out. It is found that (non-equilibrium) defect structure strongly depends on growth parameters such as thermal profiles, ambient gases and the operational laser parameters. Laser crystallization with intense beams can lead to surface defects which are formed by convection flow in the melt (Marangoni effect). Characteristic defects due to various growth ambients are outlined. Influences of ribbon defects on photovoltaic performance are also described. By properly controlling growth conditions, high quality sheets have been grown which yield solar cell efficiency ∼ 13%, and better performance can be predicted based on projected improvements in growth conditions.