【作者】 于永贵；

【导师】 王继扬； 张怀金；

【作者基本信息】 山东大学 ， 材料物理与化学， 2009， 博士

【摘要】 功能晶体材料具有光、电、热、声、磁等多种特殊的物理效应,被广泛应用于微电子、通讯、医疗、军事、科研教育、勘探等众多领域。如今,激光、能源、信息、航天等高新技术的发展,对功能晶体又提出了新的和更高的要求,使它们成为当前材料科学与工程发展的前沿领域和研究热点。在各种晶体生长方法中,提拉法因具有易观察、生长周期短、可控性好等优点成为晶体制备的首选方法。而熔体中晶体的生长是一个简单而又复杂的液-固相变过程,熔体在一定条件下于固/液界面处定向凝固,使生长单元有规则排列形成单晶。因此,固/液界面的状态及形状直接影响到晶体质量的好坏。同时,晶体生长过程是一个释放潜热的过程,生长系统内的各单元因传导、对流、辐射等存在热量交换;熔体在浮力、压力、磁场、晶体旋转和坩埚旋转等条件下也会产生不同类型的对流,对流状态会影响到熔体中的质量传输和热量传输。所有这些现象可最终归结为晶体生长的两大基本问题,传质和传热,它们决定着晶体生长的整个过程,是影响晶体质量的根本原因。于是,人们通过实物模拟和数值模拟的方法去了解这些热量、动量、质量传输过程。但实物模拟因其众多的不足无法使人们去充分认识这些过程。而数值模拟只要模型选取和条件设置合理,就可以对生长系统中的温度场、速度场及浓度场等给出详细的描述;对生长界面形状及其发展做出准确预测;对晶体中的缺陷形成和分布做出合理的解答,提供给人们更多的关于质量传输和热量传输的信息,认识各种生长条件对晶体生长的影响,为寻求最佳的生长工艺条件,提高晶体质量和成品率提供了参考,同时也大大缩短了试验周期,节约了成本。因此,数值模拟技术已经成为当今研究和改进晶体生长方法的重要工具,并已在工业化单晶的生产实践中发挥了巨大作用。本论文主要是通过CGSim软件对提拉法晶体生长中的质量传输、热量传输进行了模拟研究,分析了各生长参数对晶体生长过程和晶体质量的影响。同时根据实用要求,生长了电光Q开关用光学级硅酸镓镧（LGS）晶体,掺杂稀土钕离子硅酸镓镧（Nd:LGS）晶体和掺杂钕离子钙镓石榴石（Nd:CNGG）晶体,结合软件对晶体生长过程遇到的问题进行了数值模拟,针对性提出了相应的解决方案,对所生长晶体的质量、热学性质、光学性质和激光性能进行了研究。主要包括以下几方面的工作:一、软件模拟介绍了CGSim软件的基本模块及其所使用的主控方程和边界条件;针对LGS晶体和CNGG晶体开展了数值模拟工作。模拟了各生长参数对生长系统中传质和传热过程的影响,包括:晶体旋转速度对晶体生长界面的影响,提拉速度对加热功率的影响,晶体直径和坩埚尺寸对熔体中对流的影响,坩埚旋转在晶体生长中的作用,Marangoni对流在熔体表面处的作用及熔体液面高度对熔体中对流行为和生长界面的影响等。二、晶体生长1.光学级硅酸镓镧（LGS）晶体,掺钕硅酸镓镧（Nd:LGS）晶体生长以高纯原料La₂O₃,Ga₂O₃,SiO₂,Nd₂O₃通过固相反应合成了晶体生长用多晶料。采用提拉法并在改装的晶体生长自动等径程序控制技术的基础上,选取适当的工艺参数分别生长了光学级LGS电光晶体和掺钕浓度为1at%的Nd:LGS激光晶体。讨论了温场、原料配比、固液界面、后热器、生长工艺参数等因素对晶体生长的影响及提高晶体质量的途径与方法。在数值模拟的基础上针对晶体生长中出现的相关缺陷进行了相应分析并提出解决方案,主要包括:生长核心与生长界面。凸界面生长易产生小面而在晶体中心部位形成生长核心,采用平界面生长可以有效消除该核心。散射颗粒。晶体在放肩部位及转等径生长之前由于杂质不能有效从晶体下方排出而在晶体中形成散射颗粒,影响了晶体的光学均匀性,采取放肩时加大晶体转速或坩埚旋转等措施可以减少散射。生长条纹。晶体生长过程中的功率波动等会在引起晶体生长速率的变化,带来生长条纹,因此必须保持控温装置和机械装置的稳定工作。肩部开裂。平放肩晶体生长过程中,由于晶体肩部直接裸露于较低温的生长环境中而增加了晶体的热散失,进而造成晶体肩部存在较大的温度梯度,易引起开裂。加大后热器高度可以有效降低晶体肩部处的温度梯度,避免开裂。2.Nd:CNGG晶体的生长以高纯原料CaCO₃,Ga₂O₃,Nb₂O₅,Nd₂O₃通过固相反应合成了晶体生长用多晶料,结合对Nd:CNGG晶体的模拟结果,于坩埚内半满的熔体条件下,同样采用提拉法生长了无序结构Nd:CNGG激光晶体。钕离子掺杂浓度为0.5at%。三、LGS晶体质量检测及Nd:LGS和Nd:CNGG晶体性能测试1.LGS晶体质量检测电光Q开关用LGS晶体必须具备高的光学质量,主要通过静态消光比测试了晶体的光学均匀性。通过对晶体不同部位的X射线荧光试验分析了晶体组分的均匀性,利用XRD粉末试验计算了晶体晶格参数的变化情况,所有这些试验结果均显示所长LGS晶体具有较高的光学质量和组分均匀性,可满足于电光应用。2.Nd:LGS晶体性能测试2.1分凝测试:利用X射线荧光对晶体中的Nd离子进行了分凝测试,结果表明钕离子在晶体中的分凝系数约为0.87,因而,LGS晶体可进行较高浓度的掺杂。2.2热学性质是激光晶体需要考虑的重要参数。系统测量了Nd:LGS晶体的比热、热膨胀、热扩散和热传导性。差热扫描量热计（DSC）测得晶体室温下的比热为0.376J/g·K,热机械分析仪研究了晶体在30-500℃不同方向的热膨胀性,计算了其线性热膨胀系数,分别为α₁₁=6.05×10^-6/K,α₃₃=4.24×10^-6/K。用激光脉冲法测定了晶体的热扩散系数为λ₁₁=0.658mm²/s,λ₃₃=0.813mm²/s,并根据密度及比热计算了晶体的热导率,室温下晶体的热导率分别为k₁₁=1.42 W/m·K和k₃₃=1.75W/m·K,高于玻璃的热导率,因此,Nd:LGS晶体可应用于中等功率激光系统中。2.3光谱测试:激光晶体的光谱特性决定了该晶体应用范围和激光特性。我们测量了Nd:LGS晶体的吸收和发射光谱,发现晶体对σ偏振方向的吸收远大于对π方向的吸收,在808 nm处有强的吸收峰,其偏振吸收系数分别为3.50×10^-20 cm²（π偏振）和8.1×10^-20cm²（σ偏振）,其半峰宽度为20 nm,约为掺Nd钒酸盐和Nd:YAG晶体的10倍;其发射光谱表明:在1066 nm处,Nd:LGS晶体具有最强的发射峰,其光谱宽度从1020nm到1120 nm,与Nd掺杂玻璃相差不大,除此之外,还存在904nm和1343nm处的发射。2.4激光性能首次对Nd:LGS大功率LD泵浦的激光性能进行了研究,获得最大输出功率为2.25W,光光转化效率为28.7%的连续激光输出,并对其激光光束进行了表征;首次以Cr:YAG为饱和吸收体实现了该晶体的被动调Q激光输出,得到了最大平均输出功率为0.54 W、最大单脉冲能量为175μJ、最短脉冲宽度为23.4ns、最大峰值功率为5.02kW脉冲激光输出。由于LGS晶体已经被证明是一种优秀的电光晶体,我们认为Nd:LGS晶体应该是一种兼具电光和激光的自调制激光晶体,进一步的实验仍在进行中。3.Nd:CNGG晶体性能测试3.1晶体的热学性质差热扫描量热计（DSC）测得室温下晶体的比热为0.595 J/g·K,热机械分析仪测量了晶体于30-500℃范围内的热膨胀性,计算了其线性热膨胀系数为7.88×10^-6/K,激光脉冲法测得室温下晶体的热扩散系数为1.223 mm²/s,同样根据密度和比热计算了晶体的热导率为3.43 W/m·K。3.2晶体的热光系数热透镜效应是高功率激光下必须考虑的因素。以808m的LD为泵浦源,采用平-平腔,测量了晶体沿<111>方向的热透镜焦距大小,通过对热透镜焦距随泵浦功率的变化曲线进行拟合,得到Nd:CNGG晶体的热光系数为9.2×10^-6K^-1,约为Nd:GdVO₄晶体(2.7×10^-6K^-1)的3倍。3.3激光性能:以LD作为泵浦源,对0.5at%的Nd:CNGG晶体进行了激光试验。实现了该晶体较大功率的激光输出,其输出功率为1.91W,斜效率为28%,光转化效率为18.6%。另外,我们推断通过对谐振腔的进一步优化,有望获得更高功率的激光输出。更多还原

【Abstract】 Functional crystal matetials have been widely used in the fields of microelectronics, communication, medical treatment, military, exploration, science reaserch and education for its unique optical properties, electronic properties, thermal properties and magnetic properties With the development of such high technology as lasers, energy, communication and spaceflight, furthur demands on crystal materials have been put forward, making them the preceding domain and research hot point both in the fields of material science and engineering development.The Czochraski method is a preferred one among all the crystal growth methods for its short growth period, convenient observation and controllability. Crystal growth is a process of transformation from liquid sphase to solid sphase and achieved by the regularly solidating of grow species at the growth interface whose profile has much influence on the shape and quality of the as-grown crystal. Release of latent heat occurs during crystal growth and heat exchanges exist between different components of growth system by conduction, convection and radiation, and various type of melt convections, which influence the transportation of mass and heat, can be induced under the influence of buoyancy, pressure, magnetic field , crystal and crucible rotation. The above mentioned phenomena of heat transfer and mass transfer are the key factors that determine both the process of crystal growth and crystal quality. Transparent liquids simulation and numerical simulation are usually applied to the study of heat transfer and mass transfer during crystal growth. But, for some shortages of transparent fluids simulation, people still can not know much about the real process of crystal growth. While as long as appropriate model and parameters were adopted, numerical simulation can provide detail description on the thermal field, velocity field and concentration field of the melt; accurate prediction for the shape of growth interface and reasonable explaination on the formation and distribution of defects, which can be very helpful in understanding the mechanism of heat transfer and mass transfer, as well as the influence of various growth paramaters on the crystal growth and quality with less time and economical cost. As a result, numerical simulation has become key tools in the research and improving of crystal growth, and play an important role in commercial crystal production. In this work, using a numerical simulation software of CGSim, the heat transfer, mass transfer and influence of growth parameters on crystal growth were investigated. LGS, Nd:LGS and Nd:CNGG crystals were grown by Czochralski method, problems occurred during crystal growth were numerically simulated and corresponding resolvents were proposed. Characterization on crystal quality, thermal properties, optical properties and laser performance were also carried out. The outline is shown as follows:1. Numerical simulationThe constituent modules, governing equations and boundary conditions of the CGSim software were introduced. Various physical effect on the mass and heat transfer, including influence of crystal rotation rate on the growth interface, crystal and crucible size on the behavior of melt flow, role of cucible rotation and Marangoni flow on crystal growth, were systematically simulated.2. Crystal growth2.1 Growth of LGS crystal with optical quality and Nd³⁺-doped LGS laser crystalPolycrystalline materials of LGS and Nd:LGS was synthesized by solid-phase reaction with 99.99% purified starting reagents of La₂O₃,Ga₂O₃,SiO₂. and La₂O₃, Ga₂O₃, SiO₂, Nd₂O₃, respectively. Optical quality LGS electro-optical crystal and Nd:LGS laser crystal were grown by Czoahralski method with auto diameter controll （ADC） technique. Influence of thermal field, starting composition of raw materials, solid/liquid interface and afterheater on crystal growth was discussed.Defects listed below occurred in the crystal growth were analyzed with the assistance of numerical simulation and approaches for problems resolving were proposed:Core and the growth section boundaries: A core defect easily occurres in the middle part of as-grown crystal by the formation of some facets, which can be avoided by crystal growth with flat interface.Scattering particles: Scattering particles are usually formed before boule stage of crystal growth for that such particles can not be efficiently expelled from the region under crystal. Such scattering can be reduced by the crucible rotation or the increase of crystal rotation rate.Growth striation: Growth striations can be introduced by the variation of growth speed which induced by the fluctuation of power input, so to ensure a stable growth environment is beneficial for striation reducing. Shoulder crack: During the period of crystal growth with flat shouldering, the shouder part of crystal was exposed directly upward to the coolest portion of growth enclosure, this increases the heat loss throgh crystal, which results in large thermal gradient in the shoulder part of the crystal, corresponding the crack. Increasing the height of afterheater can decrease the thermal gradient in the shoulder part of as-grown crystal, such crack can be avoided dramaticallly..2.2 Growth of Nd:CNGG crystalPolycrystalline material for Nd:CNGG single crystal growth was aslo synthesized by solid-phase reaction with 99.99% purified starting reagents of CaCO₃,Ga₂O₃,Nb₂O₅,Nd₂O₃. On the basis of numerical simulation, Disordered Nd:CNGGcrystal with Nd³⁺-doping concentration of 0.5at% was grown by Czoahralski method from half-loaded melt.3. Investigation on the crystal quality of LGS and thermal and laser properties of Nd:LGS and Nd:CNGG crystals3.1 Determination of crystal quality of LGSLGS crystal with high optical quality is necessary for applications of electro-optical Q-switch.. Optical homogeneity was examined by measurement of static extinction ratio, and that of composition was investigated by X-ray fluorescence experiment and variation in cell parameters with XRD powder experiments. All the results show that the as-growgn LGS crystal have both high optical quality and composition uniformity, which is qualified for eletro-optical Q-switch applications.3.2 Study on properties of Nd:LGS crystal3.2.1 Segregation measurementThe segregation coefficient of Nd³⁺ in Nd:LGS crystal was determined to be 0.87 by X-ray fluorescence analysis, and hence, LGS crystal can be highly Nd³⁺-doped.3.2.2 Thermal propertiesImportant properties of Nd:LGS crystal such as the specific heat, thermal expansion, thermal diffusion and thermal conductivity have been systematically determined. The specific heat was measured to be 0.376 J·g^-1·K^-1 at room temperature by a Differential Scanning Calorometer（DSC）. Thermal expansion coefficients were measured to beα₁₁=6.05×10^-6/K andα₃₃=4.24×10^-6/K by thermal-mechanical analyzer at a temperature range of 30-500℃. Thermal diffusion coefficient was measured by laser flash method, and thermal conductivity was calculated to be k₁₁=1.42 W/m·K and k₃₃=1.75W/m·K with the measured data of specific heat, thermal diffusion coefficient and density, which was higher than that of glass, making Nd:LGS crystal suitable for such applications in medium and high laser system.3.2.3 Optical propertiesThe optical properties of a laser crystal determine its application regimes and laser performznce. The absorption and emission spectra of Nd:LGS have been measure. From the absorption spectra, it has been found that theσpolarized absorption is stronger than that ofπpolarized and the strongest one is centered at 588 ran. The full width at half maximum at 808 nm is about 20 nm wider which is over 10 times that of Nd doped vanadate and YAG crystals. From the emission spectra, we found that there is a strong emission peak centered at 1065.5 nm. The crystal has a wider emission band from 1020 nm to 1120 nm, which is comparable with that of Nd:Glass. Beside this emission peak, there are also two peaks centered at 904 nm and 1342 nm, respectively.3.2.4 Laser propertiesFor the first time to our knowledge, the high-power LD pumped laser performance of Nd:LGS has been studied. The highest cw laser power was obtained to be 2.25 W with the slope efficiency of 28.7%, and the laser beam has been characterized. With a Cr:YAG as the saturable absorber, the passive Q-switched Nd:LGS laser was achieved for the first time to our knowledge. The maximum average output power, largest pulse energy, shortest pulse width and highest peak power were 0.54 W, 175μJ, 23.4 ns and5.02 kW, respectively. The LGS crystal have been identified to be a excellent electro-optical crystal, we believed that the Nd:LGS should be a self-Q-switched laser crystal, and the experiments are being proceeded.3.3 Properties of Nd:CNGG laser crystal3.3.1 Thermal propertiesThe specific heat was measured to be 0.36 J·g^-1·K^-1 at room temperature by a Differential Scanning Calorometer（DSC）, and themal expansion coefficients was measured to be 7.88×10^-6/K by thermal-mechanical analyzer at a temperature range of 30-500℃. Thermal diffusion coefficient was obtained by laser flash method from 29 -300℃and has a value of 1.223 mm²/s at room temperature. Thermal conductivity was also calculated to be 3.43 W.m^-1.K^-1 at room temperature. 3.3.2.Thermal-optical coefficient of Nd:CNGG crystalThe effect of thermal-focusing lense must be considered for high power laser systems. Based on a flat-flat cavity, the distance of thermal-focusing lense was measured along a <111> derection oriented Nd:CNGG sample using a LD centered at 808 nm as pump source. Through functional fitting on the distance of thermal-focusing lense vs incident power curve, the thermal-optical coefficient was obtained to be 9.2×10^-6K^-1, which is almost three times that of Nd:GdVO₄ (2.7×10^-6K^-1).3.3.3 Laser propertiesLaser properties of Nd:CNGG crystal was investigated based on a flat-concave cavity using a LD pump source centered at 808 nm. The continous-wave output power, slope efficiency and optical-optical conversion efficiency were obtained to be 1.91 W, 28% and 18.6%, respectively. We believed that laser performance can be further improved with modified cavity based on the fact that the output power was still not saturated.更多还原