蓝宝石晶体生长过程中粘锅现象的实验研究

采用泡生法生长蓝宝石晶体的过程中,经常遇到粘锅问题.通过不同炉次的实验研究,分别分析了放肩阶段、等径生长阶段、收尾阶段等生长过程中产生粘锅的原因.指出温场的对称性是消除粘锅现象的最重要方式之一.同时,在放肩结束段及收尾阶段,为避免粘锅,可采用适当增大拉速及增加功率的措施.     

近化学计量比铌酸锂晶体中机械孪晶的消除

在用掺钾的助熔剂提拉法生长近化学计量比LiNbO3晶体时,长出的晶体尾部很容易出现孪晶裂纹现象,这种孪晶被认为是机械孪晶.这种孪晶的起因是由于晶体生长的收尾和从熔体中提起晶体阶段,在晶体底部产生直径的缩进,在缩进产生的台阶处出现应力集中,从而造成孪晶的成核.通过采用适当的收尾过程,以避免直径台阶的出现,可以很好地消除这种孪晶及其引起的裂纹.     

冷心放肩微量提拉法大尺寸蓝宝石单晶生长过程的模拟分析

利用数值模拟方法计算了冷心放肩微量提拉法(SAPMAC)蓝宝石晶体生长过程.结合晶体直径变化、裂纹出现位置与延续方向、晶体透明性等实验现象,通过与提拉法、温梯法、坩埚移动法等相对比,分析了冷心放肩微量提拉法晶体生长各阶段的工艺特点,并根据模拟计算结果对晶体生长系统和晶体生长控制工艺进行了改进.分别利用增大热交换器的散热参数、降低加热温度、改进降温曲线、调节外加轴向和径向温度梯度的方式来实现对晶体生长的引晶、放肩、等径和收尾控制.通过实验比较证明了改进后的晶体生长系统和晶体生长控制工艺能够生长出性能较好的大尺寸蓝宝石晶体.     

基于有限元方法研究AT切石英晶体谐振器的动态特性

基于有限元方法研究石英晶体谐振器的谐响应振动特性.通过改变AT切石英晶片尺寸,分析不同尺寸情况下石英晶体各轴向的振动位移分布情况,可了解AT切石英晶体谐振器的厚度剪切振荡模态和寄生振动模态分布情况.仿真结果与实际测试结果对比,证明了采用这种仿真方法有助于石英晶体谐振器的设计.     

硅晶体生长速率与过冷度关系的分子动力学模拟研究

基于Tersoff势函数描述硅原子间的相互作用,运用分子动力学方法模拟研究了不同过冷度条件下硅晶体凝固生长速率.结果发现,在一定过冷度范围内,硅晶体的生长速率随过冷度的增大而呈先快速增大后缓慢减小的趋势,并最终趋于不生长.同时,运用Wilson-Frenkel模型从理论上对硅晶体生长速率与过冷度关系进行了预测,分子动力学模拟结果与Wilson-Frenkel模型预测结果基本吻合,表明了硅晶体沿[100]方向的生长是一种扩散型生长.     

泡生法生长大尺寸蓝宝石晶体的生长速率研究

采用晶体生长数值模拟软件CrysMAS对泡生法生长蓝宝石晶体过程的温场进行了研究,利用数值模拟结果来调节晶体生长实验的生长速率,成功的长出了质量为91.3 kg的高质量蓝宝石晶体,并将模拟结果与实验结果进行了对比.结果表明:数值模拟结果能很好的反映出晶体在不同时刻的生长状态.晶体在生长初期应保持较低的生长速度,在等径生长阶段,晶体的最大生长速度应低于1002 g/h.     

K2Al2B2O7晶体四倍频特性理论研究

基于非线性光学晶体的谐波理论,采用龙格-库塔数值模拟方法计算模拟,详细分析了不同条件下生长的KABO晶体的四倍频频率变换特性.在高重频、高平均功率条件下,进一步分析了不同KABO晶体倍频转换效率与晶体长度的关系和在一定晶体长度下输出与输入功率的关系.理论与已有实验结果对比证明KABO晶体是一种可实现大功率266 nm紫外光输出、实现实用化的紫外倍频晶体.     

流固耦合二维声子晶体的数值模拟和实验研究

为了研究流固耦合声子晶体的特性,本文应用有限元方法进行数值模拟并通过实验进行了验证.以二维钢柱/水正方晶格声子晶体为研究对象,利用ABAQUS有限元软件计算得到了完好周期和线缺陷情况下的传输谱和声压分布.同时利用脉冲响应技术进行实验测量,得到了以上两种情况下的带隙范围.结果表明,两种方法结果吻合得很好,说明有限元方法在计算流固耦合声子晶体问题上的优越性.     

基于散射单元的声子晶体振动带隙研究

硬质芯体及其包覆层构成的复合结构称为散射单元,散射单元按照特定的排列方式可以构成声子晶体.文章采用有限元方法分别计算了立方散射单元构成的一维声子晶体及简单立方结构三维声子晶体的振动特性,结果表明,两种声子晶体的振动带隙有较好的一致性,因此,对三维声子晶体振动带隙的研究可以简化为对一维声子晶体振动带隙的研究.叠加层数、芯体材料对一维声子晶体振动带隙的影响在文中进行了讨论.最后,声子晶体的振动特性测试结果验证了文中的结论.     

ADP晶体的全方位生长装置

本论文研究的全方位生长装置能解决常规的ADP晶体的单向生长问题.在这套装置中,籽晶完全位于溶液中央,籽晶可以首先恢复其理想的结晶学外形.在晶体生长过程中,晶体在各个方向的生长均为自由生长,且育晶器中央的溶液稳定性也明显高于育晶器顶部和底部.这些因素都有利于ADP晶体的优质快速生长.实验所得晶体的质量通过透过率、晶体内缺陷位错密度、高分辨率X射线衍射表征,并将所得结果与常规方法进行了比较.     

ZnGeP2晶体轴向成分均匀性对其红外光学性质的影响

运用X射线衍射(XRD)和X射线光电子能谱(XPS)对垂直布里奇曼法生长的ZnGeP2晶体的轴向组成进行分析,发现晶体的成分不均匀。采用Rietveld全谱拟合精修定量分析各物相的相对含量,发现晶体轴向组成分布存在较大差异。晶体肩部和尾部含有Ge和Zn3P2杂相,而主体部分为单相ZnGeP2、质量相对较高。傅里叶变换红外光谱(FTIR)的测试结果表明,晶体轴向成分的差异对其红外透过率有较大影响。经过适当的热处理工艺,可以提高晶体的均匀性,改善其光学性能。     

红外LED用GaAs单晶的垂直梯度凝固制备研究

GaAs单晶是当前光电子器件的主要衬底材料之一，在红外LED中有着重要应用。但杂质浓度高、迁移率低等缺点会严重影响红外LED器件性能。为生产出低杂质浓度、高迁移率、载流子分布均匀、高利用率的红外LED用掺硅垂直梯度凝固(VGF)法GaAs单晶，本文研究了热场分布、合成舟和炉膛材质、工艺参数对单晶的成晶质量、杂质浓度、迁移率、载流子分布的影响。利用CGSim软件对单晶生长热场系统进行数值模拟研究，温区一至温区六长度比例为8∶12∶9∶5∶5∶7时，恒温区达到最长，位错密度达到1 000 cm^-2以下，成晶率达到85%。采用打毛石英合成舟进行GaAs合成，用莫来石炉膛替代石英炉膛，可以获得迁移率整体高于3 000 cm²/(V·s)的GaAs单晶，满足红外LED使用要求。对单晶生长工艺参数展开研究，采用提高头部生长速度、降低尾部生长速度的方式提高单晶轴向载流子浓度均匀性，头尾部载流子浓度差降低33%,尾部迁移率从2 900 cm²/(V·s)提高到3 560 cm²/(V·s)。单晶有效利用长度提高33...     

生长气压对分子束外延β-Ga2O3薄膜特性的影响

本文采用分子束外延技术在具有6°斜切角的c面蓝宝石衬底上外延β-Ga₂O₃薄膜,系统研究了生长气压对薄膜特性的影响。X射线衍射谱和表面形貌分析表明,不同生长气压下所外延的薄膜表面平整,均具有(201)择优取向。并且,其结晶质量和生长速率均随生长气压增大而逐渐提高。通过X射线光电子能谱分析发现,生长气压增大使得氧空位的浓度大幅下降,高价态Ga比例增大,最终使得O/Ga原子数之比接近理想Ga₂O₃材料的化学计量比值。利用Tauc公式和乌尔巴赫带尾模型进行计算,结果表明随着生长气压的增大,样品的光学带隙由4.94 eV增加到5.00 eV,乌尔巴赫能量由0.47 eV下降到0.32 eV,证明了生长气压的增大有利于降低薄膜中的缺陷密度,提高薄膜晶体质量。     

Kinetic studies on the influence of temperature and growth rate history on crystal growth

Crystallization experiments of sucrose were performed in a batch crystallizer to study the effect of temperature and growth rate history on the crystal growth kinetics. In one of the growth methods adopted, the isothermal volumetric growth rate (R_V) is determined as a function of supersaturation (S) at 35, 40 and 45 ºC. In the other, crystals are allowed to grow at constant supersaturation by automatically controlling the solution temperature as the solute concentration decreased. Using the latter method R_V is calculated as the solution is cooled. The obtained results are interpreted using empirical, engineering and fundamental perspectives of crystal growth. Firstly, the overall activation energy (E_A) is determined from the empirical growth constants obtained in the isothermal method. The concept of falsified kinetics, widely used in chemical reaction engineering, is then extended to the crystal growth of sucrose in order to estimate the true activation energy (E_T) from the diffusion‐affected constant, E_A. The differences found in the isothermal and constant supersaturation methods are explained from the viewpoint of the spiral nucleation mechanism, taking into account different crystal surface properties caused by the growth rate history in each method. Finally, the crystal growth curve obtained in the batch crystallizer at 40 ºC is compared with the one obtained in a fluidized bed crystallizer at the same temperature. Apparently divergent results are explained by the effects of crystal size, hydrodynamic conditions and growth rate history on the crystallization kinetics of sucrose. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influences of ethanol on the thermodynamics and kinetics in the crystallization of xylitol

The metastable zone and crystal growth rates of xylitol in water were investigated under the influence of small volume fractions of ethanol. The utilization of low amounts of ethanol is necessary to help adding slightly soluble ingredients during the fabrication of xylitol products, e.g. in food industry. Therefore, the metastable zone and crystal growth rates were determined by means of an ultrasound measurement technique and batch crystallization experiments. It can clearly be seen that the addition of ethanol is lowering the solubility of xylitol in water. The higher the volume fraction of ethanol, the lower the solubility. The metastable zone width is increased whereas the smallest investigated ethanol content results in the widest metastable zone. The crystal growth of xylitol is inhibited by the addition of ethanol whereby the crystal growth rates are decreasing with increasing ethanol content.     

基于数据驱动的晶体直径模型辨识方法研究

直拉法硅单晶生长是一个多场多相耦合、物理变化复杂,且具有大滞后和非线性现象的过程,基于单晶硅生长系统内部机理所构建的机理模型由于存在诸多假设而无法应用于工程实际。因此,本文以现有CL120-97单晶炉拉晶车间的长期、海量晶体生长数据为基础,忽略炉内复杂的晶体生长环境,对影响晶体直径的拉晶参数进行关联性分析及特征量化,探寻拉晶数据中所蕴藏的规律信息,进而建立基于数据驱动的BP神经网络晶体直径预测模型,并针对现有BP神经网络易陷入局部极小值的问题,采用遗传算法对BP神经网络的阈值和权值进行优化,以提高晶体直径预测的准确性。通过实际拉晶数据对模型预测结果进行验证,结果表明,对任意选取的8组拉晶数据进行直径预测,预测的平均相对百分比误差为0.095 71%,证明该模型对于等径阶段晶体直径的预测是可行的。     

Rim formation on crystal faces growing in confinement

A crystal face growing from solution while exerting a normal force on a confining surface is often observed to develop a growth rim surrounding a hollow core. The interpretation has been that this is a manifestation of steady state growth due to the balance between the concentration gradient and stress gradient along the confined crystal surface. In this paper, we present experimental results which show that the growth rim is instead formed as a consequence of faceted growth on the confined surface. Steady state growth is not ensured by a gradient in normal stress, but rather a gradient in step density along the crystal face. The loaded crystal surfaces display a high degree of roughness, and the stress is not uniformly distributed across the surface, but transmitted at discrete asperities. We discuss the implications of these findings for the interpretation of previous experimental results, and for the thermodynamics of crystal growth subject to normal stress.     

PVT法生长SiC过程生长界面形状对热应力的影响

PVT法生长SiC过程中晶体内部的热应力是其位错产生的主要原因,而生长界面的形状对晶体热应力及缺陷的产生都有一定影响.本文对不同生长界面晶体的温场及应力场进行了数值分析,结果显示相对于凸出及平整界面的晶体,微凹界面晶体的轴向温差最小,同时产生缺陷的切应力Τrz及引起开裂的径向正应力σrr值都为最小.     

Study on crystallization kinetics and the crystal internal defects of HMX

In this paper, the solubility data of HMX (1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane) in acetone from 323.15 K to 293.15 K were accurately measured by use of the laser‐monitoring observation technique. Intermittent dynamic method was utilized to study crystallization kinetics of HMX in acetone. The data of crystallization kinetics were obtained by moment analysis, and the parameters of the growth rate and nucleation rate equations were derived by using multiple linear least squares method. Subsequently, growth rate and nucleation rate at different conditions were calculated according to these equations. In addition, Optical Microscopy Images (qualitative) and Particle Apparent Density (quantitative) experiments were applied to study the crystal internal defects of HMX under different crystallization conditions. It can be found that the crystal apparent density of HMX is in the range of 1.8993 g·cm⁻³ to 1.9017 g·cm⁻³, very close to the theory density of HMX; the internal defects and the crystal size do not increase after 25 °C, from which we predict that the HMX crystal growth reaches the steady growth segment. These results suggest that the nucleation rate is a significant factor influencing the crystal internal defects, and larger nucleation kinetics can reduce crystal internal defects.