Influence of low-gravity level on crystal-growth in floating-zone

The influence of low gravity level on crystal growth in the floating zone, which involves thermocapillary convection, phase change convection, thermal and solutal diffusion, is investigated numerically by a finite element method for the silicon crystal growth process. The velocity, temperature, concentration fields and phase change interfaces depending on heating temperature and growth rates are analyzed. The influence of low gravity level on the concentration is studied especially. The results show that the non-uniformities of concentration are about 10(-3) for growth rate nu(p) = 5.12 x 10(-8) m/s, 10(-2) for nu(p) = 5.12 x 10(-7) m/s and relatively larger for larger growth rate in the gravity level g = 0-9.8 m/s2. The thermocapillary effect is strong in comparison with the Bridgman system, and the level of low gravity is relatively insensitive for lower growth rates.

A study on stratification of concentration in crystal-growth of alpha-liio3 by holographic-interferometry

A new thermoplastic-photoconductor laser holographic recording system has been used for real-time and in situ observation of alpha-LiIO3 crystal growth. The influence of crystallization-driven convection on the concentration stratification in solution has been studied under gravity field. It is found that the stratification is closely related to the seed orientation of alpha-LiIO3 crystal. When the optical axis of crystal seed C is parallel to the gravity vector g, the velocity of the concentration stratification is two times larger than that in the case of C perpendicular-to g. It needs 40 h for the crystalline system of alpha-LiIO3 to reach stable concentration distribution (expressed as tau) at 47.6-degrees-C. The time tau is not sensitive to the seed orientation. Our results provide valuable data for designing the crystal growth experiments ia space.

Crystal-growth in floating zone with phase-change and thermosolutal convections

Floating zone crystal growth in microgravity environment is investigated numerically by a finite element method for semiconductor growth processing, which involves thermocapillary convection, phase change convection, thermal diffusion and solutal diffusion. The configurations of phase change interfaces and distributions of velocity, temperature and concentration fields are analyzed for typical conditions of pulling rates and segregation coefficients. The influence of phase change convection on the distribution of concentration is studied in detail. The results show that the thermocapillary convection plays an important role in mixing up the melt with dopant. The deformations of phase change interfaces by thermal convection-diffusion and pulling rods make larger variation of concentration field in comparison with the case of plane interfaces.

Preliminary-results of gaas single-crystal growth under high gravity conditions

GaAs single crystals have been grown under high gravity conditions, up to 9g0, by a recrystallization method with decreasing temperature. The impurity striations in GaAs grown under high gravity become weak and indistinct with smaller striation spacings. The dislocation density of surcharge-grown GaAs increases with increase of centrifugal force. The cathodoluminescence results also show worse perfection in the GaAs grown at high gravity than at normal earth gravity.

Extremum principle of crystal plasticity

An analytical method for determining slip shear rate under prescribed stress rate or prescribed strain rate has been presented on the basis of the incremental theory of crystal plasticity. The problem has been reduced to a quadric convex programming.In order to analyse the plastic response of crystals subjected to external load, two new extremum principles are proposed. They are equivalent to the boundary-value problem of crystal plasticity. By the new extremum principles, the slip shear rates are independent function which can be obtained from the variational equation.

YAG激光诱导放电通道伏安特性研究

研究了在空气、油膜和绝缘树脂薄膜这三种放电介质中的伏安特性,得出了在同一介质中,通道电极间电压不随其它初始条件改变而变化,只与放电介质对通道箍缩能力成正比,在本实验中薄膜的通道电压最高80.4V,空气中最低26.0V。通道中电流波形与放电介质有关,同样的高低压组合、相同放电脉宽下在空气和油膜中为矩形波,而在薄膜中为三角波,峰值电流变化不大。电流波形不随脉宽的改变而变化,当增大初始高压端或低压端输入时,电流随之增加,并且低压端输入对提高峰值电流更有效果。

铁基抗高温磨损激光熔覆涂层强韧设计和研究 Ⅱ:Fe-Cr-C-W-Ni激光熔覆层的微观组织及性能

利用ＯＭ、ＳＥＭ、ＴＥＭ研究了Ｆｅ－Ｃｒ－Ｃ－Ｗ－Ｎｉ激光熔覆涂层熔覆态及其高温时效态的微观组织结构。结果表明激光熔覆层组织细小，具有强韧两相组成（奥氏体和Ｍ_７Ｃ_３碳化物）的微观结构特征，高温时效处理组织中有Ｍ_（２３）Ｃ_６、Ｍ_６Ｃ、Ｍ_２Ｃ等新碳化物形成。显微硬度和冲击磨损实验证实了激光熔覆态和峰值时效态熔覆层均具有良好的力学性能。

YAG激光毛化技术进展

二十世纪八十年代，当比利时冶金研究中心(CRM)开发出CO_2激光毛化冷轧辊技术后，尝试用YAG激光进行轧辊毛化一直吸引着众多的研究者，这是因为YAG(1.06μm)激光波长比CO_2(10.6μm)激光波长短一个量级，材料对YAG激光有更高的吸收率，并用YAG激光可以聚焦到更小的光斑尺寸，同时使用电信号驱动的声光开关技术便于对毛化分布进行可设定控制。但是用传统声光调制的YAG激光虽然可以碇以很高的脉冲频率(>30kHz)，但单脉冲有量仅为10mJ左右，难以达到辊面毛化粗糙度的要求，因此人们认为YAG激光用于毛化的主要困难是脉冲能量太小。

3D Finite Volume Scheme for Czochralski Crystal Growth

A general three-dimensional model is developed for simulation of the growth process of silicon single crystals by Czochralski technique. The numerical scheme is based on the curvilinear non-orthogonal finite volume discretization. Numerical solutions show that the flow and temperature fields in the melt are asymmetric and unsteady for 8’’ silicon growth. The effects of rotation of crystal on the flow structure are studied. The rotation of crystal forms the Ekman layer in which the temperature gradient along solid/melt surface is small.

3D Finite Volume Scheme for Czochralski Single Crystal Growth

Czochralski (Cz) technique, which is used for growing single crystals, has dominated the production of single crystals for electronic applications. The Cz growth process involves multiple phases, moving interface and three-dimensional behavior. Much has been done to study these phenomena by means of numerical methods as well as experimental observations. A three-dimensional curvilinear finite volume based algorithm has been developed to model the Cz process. A body-fitted transformation based approach is adopted in conjunction with a multizone adaptive grid generation (MAGG) technique to accurately handle the three-dimensional problems of phase-change in irregular geometries with free and moving surfaces. The multizone adaptive model is used to perform a three-dimensional simulation of the Cz growth of silicon single crystals.Since the phase change interface are irregular in shape and they move in response to the solution, accurate treatment of these interfaces is important from numerical accuracy point of view. The multizone adaptive grid generation (MAGG) is the appropriate scheme for this purpose. Another challenge encountered is the moving and periodic boundary conditions, which is essential to the numerical solution of the governing equations. Special treatments are implemented to impose the periodic boundary condition in a particular direction and to determine the internal boundary position and shape varying with the combination of ambient physicochemical transport process and interfacial dynamics. As indicated above that the applications and processes characterized by multi-phase, moving interfaces and irregular shape render the associated physical phenomena three-dimensional and unsteady. Therefore a generalized 3D model rather than a 2D simulation, in which the governing equations are solved in a general non-orthogonal coordinate system, is constructed to describe and capture the features of the growth process. All this has been implemented and validated by using it to model the low pressure Cz growth of silicon. Accuracy of this scheme is demonstrated by agreement of simulation data with available experimental data. Using the quasi-steady state approximation, it is shown that the flow and temperature fields in the melt under certain operating conditions become asymmetric and unsteady even in the absence of extrinsic sources of asymmetry. Asymmetry in the flow and temperature fields, caused by high shear initiated phenomena, affects the interface shape in the azimuthal direction thus results in the thermal stress distribution in the vicinity, which has serious implications from crystal quality point of view.

Research on Q-Switched Nd : YAG laser engraving

IN this paper, the engraving process with Q-Switched Nd:YAG laser is investigated. High power density is the pre- requisition to vapor materials, and high repetition rate makes the engraving process highly efficient. An acousto- optic Q-Switch is applied in the cavity of CW 200 W Nd:YAG laser to achieve the high peak power density and the high pulse repetition rate. Different shape craters are formed in a patterned structure on the material surface when the laser beam irradiates on it by controlling power density, pulse repetition rate, pulse quantity and pulse interval. In addition, assisting oxygen gas is used for not only improving combustion to deepen the craters but also removing the plasma that generated on the top of craters. Off-focus length classified as negative and positive has a substantial effect on crater diameters. According to the message of rotating angle positions from material to be engraved and the information of graph pixels from computer, a special graph is imparted to the material by integrating the Q- Switched Nd:YAG laser with the computer graph manipulation and the numerically controlled worktable. The crater diameter depends on laser beam divergence and laser focal length. The crater diameter changes from 50 micrometers to 300 micrometers , and the maximum of crater depth reaches one millimeter.

MATLAB在Cr涂层微裂纹组态分析中的应用

镀Cr身管在使用时普遍存在涂层剥落问题。本文结合我国某新型号机枪枪管的研制工艺,对涂层微裂纹组态、控制因素及其与破坏模式的关系进行研究。鉴于微裂纹的纳米级宽度,本文通过特殊的金相制样方法将其扩宽,从而得到反映微裂纹组态的低倍照片。应用MATLAB对微裂纹特征进行统计分析,得出微裂纹密度、长度等特征参数与涂层失效模式间的对应关系,同时证实基体激光离散预处理对微裂纹组态有控制作用。