温度梯度对碲锌镉单晶体生长过程热应力场的影响

采用单晶位错研究的热弹性模型,计算模拟了垂直布里奇曼法碲锌镉单晶生长过程中的热应力场,研究了炉膛温度梯度对晶体内热应力的影响.计算结果表明:径向上晶体边缘与坩埚壁接触位置处的热应力远大干晶体中心处的热应力;轴向上晶体底部位置的热应力远大于晶体顶部的热应力.在晶体底部边缘与坩埚接触的位置出现最大热应力值σmax.当炉膛温度梯度从5K/cm增加到20K/cm,晶体内的热应力显著提高,σmax从41.83MPa增加到79.88MPa;当温度梯度超过20K/cm进一步增加时,晶体内的热应力增加很少,σmax仅增加了约5.3%.     

液封浮区法生长GaAs晶体中的热应力

用有限元法对微重力环境下液封浮区(LEFZ)法生长的3英寸GaAs单晶中的热应力进行求解.假设晶体处于准定常状态且为轴对称的各向同性线弹性体.分析了液封厚度、晶体和进料棒转速对晶体中热应力分布的影响.     

泡生法蓝宝石不同生长阶段热应力的数值模拟研究

利用计算机模拟对泡生法蓝宝石单晶各生长阶段的热应力进行了分析,发现晶体中最大热应力位于籽晶与新生晶体交界处或靠近固液界面的晶体边缘,其次位于晶体肩部。在此基础上,针对某一生长阶段讨论了加热器位置和坩埚形状对晶体中热应力分布的影响。结果表明:固液界面处的热应力随着加热器轴向中点与熔体轴向中点的距离L的增加先减后增,当L为190mm时,晶体中热应力最小;随着坩埚底部倒角半径的增加,固液界面处热应力呈上升的趋势。     

Lu_xY_(1-x)AlO_3:Ce晶体的缺陷研究

通过提拉法制备了LuxY1-xAlO3:Ce晶体样品.采用偏光显微镜、场发射扫描电镜、电子探针等手段,并结合热应力分析,研究了LuxY1-xAlO3:Ce晶体中的开裂、铱金包裹体、气泡、孪晶、解理等宏观缺陷.结果表明,不同形态晶体内热应力合力的方向导致不同的开裂面的取位方式,加强保温、建立适应晶体形态的合适温场等有助于获得完整晶体.高温熔体及保护气氛中微量氧对铱坩埚的熔蚀与氧化,使得晶体中存在有铱金包裹体,降低炉腔内的氧含量、采用高纯度铱金压铸制作坩埚等措施有助于减少晶体内的铱金包裹体.适当提高熔体温度以降低粘度、降低炉内气压等措施有助于减少晶体中的气体包裹物.低镥含量LuxY1-xAlO3:Ce晶体中,a、b轴方向的晶胞参数非常接近,在应力诱导作用下晶格参数互换易导致在110方向上形成孪晶.     

工作在放电环境中的石英晶体膜厚监控仪

石英晶体胰厚监控仪已广泛应用于蒸发镀膜,离子刻蚀厚度的精细控制和真空系统污染监测等方面.但一般的石英晶体膜厚监控仪使用的蒸发型探头却不能在溅射系统放电区中正常工作.实践表明,在溅射系统中,由阴极靶发射的二次电子碰撞测量晶体而引起的热应力是导致石英晶体膜厚监控仪测量失效的主要原因.为此,对测量探头做了改造,设计了一种带偏转磁场的溅射型测量探头,使石英晶体膜厚监控仪能在平面直流磁控溅射镀膜系统中稳定地作原位置膜厚监控.为了对测量计算公式进行修正,用实验确定了修正因子.     

闪烁晶体的生长与宏观缺陷研究

用提拉法生长了Lu2Si2O7：Ce晶体，讨论了晶体生长过程中的几个问题；（1）熔体挥发；（2）晶体开裂；（3）层状包裹．生长过程中LPS和SiO2均存在挥发，其中后者占主导；LPS挥发不会造成组分偏析，因此对生长过程没有负面的影响．接种温度偏低导致晶体出现多晶化和晶体中较大的热应力是促使开裂发生的主要原因．层状包裹现象的出现主要是由于该晶体的结晶温度范围狭窄，熔体容易出现组分过冷，以及生长设备的温控系统精度不高等造成的．     

水热法生长复合钛宝石激光晶体

水热法生长了复合钛宝石(Al₂O₃/Ti:Al₂O₃)激光晶体.对外层Al₂O₃X射线ω扫描表明其晶体质量很高.外层生长的Al₂O₃和内部的钛宝石激光棒结合牢固、紧密,因而能适应高强度泵浦所带来的激光介质内部的热应力.复合激光晶体界面层带来的光损耗很小.就我们所知,这是首次用水热法生长复合激光晶体.     

人工金红石单晶体中包裹体结构分析

采用焰熔法生长金红石单晶体时,有时发现在晶体中部生长有透明、半透明间杂的包裹体,且包裹体中有平行条纹．利用ＳＥＭ和ＸＲＤ对其（１１０）面分析研究发现,包裹体是由基本为（１１０）面取向的小晶体构成的多晶体,有的小晶体受到张应力作用,有的受到压应力作用;包裹体（１１０）面中的平行条纹为晶体的层错线或晶界,取向为［００１］,它形成于热应力造成的沿（１１０）面的解理．     

多层薄膜的热应力研究

薄膜的应力对薄膜性能有着重要的影响.考虑了各个膜层弹性模量不同的情况,建立了多层薄膜热应力的数理模型,计算了典型多层薄膜体系中应力的分布.如果使用0级近似,每层薄膜中的热应力是由基底和膜层之间的热失配决定的,而不受其它膜层的约束;而使用一级近似,每一膜层中的热应力受其它膜层的影响.这种方法为多层薄膜应力的评价提供了依据.     

激光金属沉积成形过程热应力的数值模拟

根据有限元分析中的"单元生死"技术,利用APDL编程实现了对多道多层激光金属沉积成形过程热应力的三维数值模拟.模拟采用了Gauss热源模型,并引入了沿长边的平行往复扫描方式.计算结果表明,熔池区域以及试样与基板相邻区域是高热应力区,试样内部的热应力较小;试样沿厚度增加方向的热应力在沉积过程中幅值很大并且以拉伸应力为主,是导致试样产生裂缝的主要原因;沉积过程中沉积开始的位置对热应力的分布和强度影响很大,同一沉积层首道各节点热应力值几乎是末道各节点热应力值的一倍.在与模拟过程相同的条件下,实际成形试样裂缝的产生和发展规律与模拟结果相符.     

单晶体材料内应力研究进展

随着科学技术的发展,晶体的力学性质,如弹性、脆性、硬度和解理性等引起了人们的重视。晶体材料应力分布取决于很多复杂因素(加热、退火、提拉、切割、搬运以及生长过程中的各种力学因素)。从应力表征、硬度及断裂韧性的测试方法入手,回顾并总结了晶体材料力学参数的表征手段,阐述了晶体开裂的分布规律及原因。其中光测方法(主要包括光弹法、X射线衍射法等)因其对晶体材料无任何机械损伤、检验灵敏度高而应用广泛:光弹性是光学晶体材料的重要特性,利用光弹仪测定光程差,根据平面光弹性的应力-光学定律确定主应力差;X射线衍射方法测定样品中宏观应力具有无污染、测量精度高等特点。压痕实验和划痕实验是表征晶体硬度的主要手段,结合化学腐蚀和光学观测方法可以有效探讨晶体开裂的微观机理。     

Micro-stress distribution within polycrystalline aggregate

Summary The three-dimensional Poisson-Voronoi model, which is topologically equivalent to the microstructure of real ceramics and metals, has been used to study the stress distribution within a simulated polycrystalline aggregate having 200 grains. Micro-stresses such as the maximum principal stress, maximum shear stress, first invariant of stress, and Von-Mises stress are found to vary systematically with the anisotropy of single crystal.     

Piezospectroscopic study of mechanical stress in Al2O3:Cr under swift heavy ion irradiation

The spatial distribution of mechanical stresses in Al₂O₃:Cr single crystal irradiated with (1 ÷ 3) MeV/amu Kr, Xe and Bi ions has been studied by using laser confocal scanning microscopy technique. The stress level as a function of the ion penetration depth has been evaluated from depth-resolved photostimulated R-line luminescence spectra exploiting the piezospectroscopic method. As it was found, the stress field generated by swift heavy ion irradiation is composed of stresses with maximal magnitude comparable with the ultimate stress limit of ruby crystals. Experimental data are discussed in the framework of a model considering the Cr³⁺ atoms as individual piezosensors.     

稀土影响高温合金GH907性能的显微机理研究

研究了稀土对GH907合金的力学和物理性能的影响规律,优化了可善GH907合金缺口持久性能,高温抗氧化性能的合金体系,通过比较标准GH907合金和稀土GH907合金的显微组织和界面结构,提出了稀土提高合金性能的微观机理,稀土主要存在于合金的片状相中,使其含量,形态,分布和晶体结构发生改变,与基体之间界面晶格失配和界面应力大小大减小。     

The nucleation and suppression of transformation twinning in barium sodium niobate

It has already been established that the microtwinning associated with the tetragonal to orthorhombic phase transition in barium sodium niobate can be suppressed by cooling from above the transformation temperature under an applied stress. The work reported here indicates that the quotation of specific stress values for this process is misleading since, for any specimen, there is an optimum range of applied stress over which complete suppression of the microtwinning is achieved. This range is shown to be dependent upon the particular twin density and distribution which is determined by the presence of other crystal defects and by the composition of the material. Means of reducing the twin density are also discussed.An anomalous expansion behaviour in the [001] crystal direction, detected by a dilatometric technique, is consistent with the observed tendency to crack as the material is cooled through the Curie point.     

Experimental study on the characteristic of the NS-GT cut quartz crystal resonator oscillating in the sub-resonant frequency

We previously reported that the dynamic photo-elastic method was a very effective measuring technique for the stress distribution of vibrating quartz crystal resonators. The existence of a twisted asymmetrical vibration mode has been verified experimentally when the NS-GT cut quartz crystal resonator was vibrating in the main resonant frequency (MRF). A MRF and a sub-resonant frequency (SRF) of the NS-GT cut quartz resonator were defined as follows. If a mechanical standing wave was in the x' or y' direction of the resonator, the former was MRF vibration and the latter was SRF vibration, respectively. In this paper, stress distributions of two samples of the NS-GT cut quartz crystal resonator, one of which had a thickness of 80 mum and the other 150 mum, were measured by the dynamic photo-elastic method when the resonators were vibrating in each SRF. Thereafter, vibration modes of those resonators were estimated by the experimental data of stress distributions. We find that the vibration mode of the 80-mum resonator had a simple mechanical standing wave on the y' direction and the vibration mode of the 150-mum resonator was combined with a shearing mode in the SRF vibration. From the experiment, we decided that vibration modes of the NS-GT cut quartz crystal resonator were composed of the longitudinal stress T(3)' belonging to the z' direction of the plate and of the shearing stress T(5)' when the plate thickness was thickened and the resonator was oscillating in the SRF.     

Finite Element Analysis of Mono- and Bi-Adhesively Bonded Functionally Graded Adherend

The stress concentration at the end of bonded lap joints is a major concern in the design and application of adhesive joints, and, therefore, many research works have been carried out to reduce the stress level in the bond line. Most of the proposed methods focus on changing adhesive geometries or properties to achieve an optimized model. In this paper, the stress and strain distribution for adherend with functionally graded properties was analyzed to investigate the effect of the adherend material properties and the type of joint on the stress distribution within bond line. The effect of ceramic volume fraction of the functionally graded materials (FGMs) on the stress concentration has been studied. Also, bi-adhesive joint is used as an alternative stress reduction technique for the joint. Results show that using bi-adhesively joint technique together with high-ceramic volume fraction FGMs can significantly reduce the shear and peel stress in the lap joint.     

Comparative study on cracking behavior of sol-gel silica antireflective coating for high-powered laser system

Potassium dihydrogen phosphate (KDP) crystal is a kind of very crucial nonlinear optics for high-powered laser equipment as frequency conversion (doubler, tripler or quadrupler) components. In this paper, investigation on cracking behavior of sol-gel silica antireflective (AR) coatings has been performed using KDP crystal as substrate. Cracking induced by heating or moisturizing post-treatment is respectively studied, in which distinct cracking behaviors have been observed. It has been interestingly found that either temperature or humidity saltation can result in coating crack generation, but the corresponding residual stress demonstrates in completely reverse ways, giving rise to distinctive crack morphology on both crack edge and bottom. The origination and the derivation of such stress manner have been stated based on the intrinsic porous structure of sol-gel silica coating, and the critical cracking condition has been suggested dependent on the relationship between the accumulated residual stress against elastic recovery capability of such coating.     

Manufacturing polymer/clay nanocomposites through elongational flow technique

A novel processing technique that employs continuous elongational flow to fabricate polymer/clay nanocomposites has been developed and evaluated in this work. A self-made vane mixer has been used to supply the continuous elongational flow, while high-density polyethylene (HDPE) and organic montmorillonite (O-MMT) were used as the polymer matrix and clay, respectively. The morphology of resultant nanocomposites has been carefully revealed and studied by examining wide-angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM). Intercalation effect and dispersion of O-MMT layers have been investigated by the morphology study. The results indicate that the elongational flow has a great potential in melt intercalation of O-MMT, and can lead to an orderly O-MMT layers’ distribution. Thermal properties of as-mixed nanocomposites that prepared under elongational flow have been determined by the differential scanning calorimetry (DSC), which demonstrates that the introduction of O-MMT nano-sheets is bad for the crystal of HDPE matrix. The universal tensile test shows how O-MMT layers affect the mechanical properties of nanocomposites, including the tensile strength and elongation at break. The strain–stress relationship reveals that with continually adding O-MMT layers, the tensile strength increases at first, and then decreases. While the elongation at break shows the same trend.