【作者】 郭怀新；

【导师】 韩杰才； 张明福；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2013， 博士

【摘要】 由于Nd:YAG晶体材料具有优异的激光性能以及良好的热学特性和机械性能，自上世纪60年代以来，钕掺杂钇铝石榴石晶体就成为固体激光器中占主要地位的工作物质。近年来，随着激光二极管泵浦激光晶体的激光器技术的发展，使得Nd:YAG激光材料在通信、材料加工、激光医疗领域广泛应用，特别是随着激光二极管作为惯性约束核聚变择优泵浦源的出现，使其军事上应用更为突出。Nd:YAG晶体已成为“三大基础激光晶体”之一，作为中、高功率激光器应用方向的主要支撑材料。这些领域向着高能量、大功率固体激光器方向发展，而目前传统的生长方法存在核心、侧心及Nd³⁺离子梯度等缺陷问题难以满足这些领域对大尺寸、高品质及高光学均匀性Nd:YAG激光晶体迫切需求。本文针对上述问题，在晶体尺寸和品质方面进行提升，利用水平定向结晶法（HorizontalDirectional Solidification, HDS）生长板状Nd:YAG激光晶体，计算分析了其生长过程中固液界面的变化形式以及熔体的对流形式；探讨了Nd³⁺离子在晶体中的分布规律及其与对流的关系；分析了生长出Nd:YAG激光晶体结构品质及在生长过程中出现的缺陷类型、分布规律和形成原因，提出了相应的改进措施；最后测试水平定向结晶法生长Nd:YAG晶体的光学性能，利用Judd-Ofelt理论预估其吸收截面、荧光寿命、发射截面等参数，预测了其激光输出特性。采用水平定向结晶法，设计合理的工艺参数，生长出舟型板状的大尺寸板状Nd:YAG激光晶体，尺寸达220mm×140mm×25mm，晶体的物相分析测试、晶体结构的摇摆曲线和透射电镜测试表明Nd:YAG激光晶体的结构完整性优良；晶体干涉条纹和消光比的测试表明晶体的干涉条纹为零级，晶体中无核心和侧心，晶体品质优良；退火后研究表明退火有利于提高其结构完整性。热学和力学性能的测试分析表明水平定向结晶法生长的Nd:YAG晶体的基本物理性能优异。针对水平定向生长Nd:YAG晶体过程中固液界面的热质传递发生变化，导致的固液界面结晶速率和坩埚移动速率不同步问题，推导出水平定向结晶法生长大尺寸Nd:YAG激光晶体过程中固液界面的实际结晶速率公式，对固液界面的结晶速率进行了数值计算分析，结果表明在晶体生长过程中实际的固液界面结晶速率是不断变化的，且与坩埚尺寸、温场梯度及熔体厚度等因素有关；并详细模拟计算了这些因素对固液界面实际结晶速率的影响。优异的热质传递是生长高质量大尺寸Nd:YAG激光晶体的重要条件之一，因此对水平定向生长Nd:YAG激光晶体的熔体对流进行了计算分析，明确在水平定向结晶法生长Nd:YAG晶体过程中的主要对流机制。研究了掺杂Nd³⁺离子在晶体中的沿生长方向的分布规律，及分布规律和温度梯度的关系；研究了掺杂离子(Nd³⁺)在晶体中的垂直于生长方向的分布规律及分布机制；结果显示其分布优于其他生长方法，表明该方法更适合生长Nd:YAG晶体。采用光学显微镜和扫描电镜等实验方法，对Nd:YAG晶体宏观缺陷及微观缺陷进行观测和研究，表明其缺陷主要为散射颗粒（气泡、小的夹杂物及大的包裹体）和位错。详细的研究了散射颗粒的形貌、在晶体中分布规律及形成原因。结合化学腐蚀法及X射线相貌分析法测试了水平定向结晶法生长Nd:YAG激光晶体的位错类型、分布规律；探讨了其形成原因，结果表明晶体中心区域的品质要好于边缘区域，中心区域的位错密度相对较低。表征了水平定向结晶法生长Nd:YAG激光晶体的吸收光谱和荧光光谱。结果表明在808nm处吸收谱带峰值有较大的半峰宽，荧光光谱呈带状的连续分布，中心波长谱图峰值1064.59nm对应于跃迁峰值的理论值1064.15nm，匹配非常好，表明水平定向结晶法生长的Nd:YAG晶体光学质量良好。应用Judd-Ofelt理论求得最佳唯象强度参数分别为Ω₂=0.5022×10^-20cm²,Ω₄=3.3506×10^-20cm²和Ω₆=5.1653×10^-20cm²，同时预估了激光晶体的吸收截面、荧光寿命、发射截面等参数；并测试了实际的激光输出性能，获得了较为优异的功-功转换效率高达37.44%。更多还原

【Abstract】 Neodymium-doped yttrium aluminum garnet （Nd:YAG） crystal has become themost suitable and dominant solid-state laser material since the1960’s. It is primarilyattributed to its excellent laser properties and outstanding thermal and mechanicalproperties. Recently, along with the development of pump technology of lasercrystal using laser diode, Nd:YAG single crystal has been widely applied in thefields of communication, material processing and laser medical treatment.Especially with the prosperous of the pumping source of the inertial confinementfusion （ICF） using laser diode, the field of military application has been moreprominent. The Nd:YAG crystal has become one of the "three basic laser crystals"and the main pillar of medium and high power laser applications. The increasingdemands for high-energy and high-power solid laser technology in those areas causethe Nd:AYG crystal struggles to satisfy the clients’ demands, however, the centralcores, lateral core and Nd³⁺concentration gradient of Nd:YAG crystal grown byexisting technologies cannot settle the problem. This article addresses the aboveproblems by using the horizontal directional solidification （HDS） method to growNd:YAG single crystal. The mode of solid-melt interface and mechanism of meltflow patterns were computed, Nd³⁺ion distribution and the relation with meltconvection were discussed. The quality and morphology of the structure,constituents and formation reasons of defects in crystal were analysised. Finally, theJudd-Ofelt theory was introduced, and absorption cross section, radiative life timeand emission cross section were estimated based on the absorption and fluorescencespectra, in order to predict the laser output property.The large-plate Nd:YAG laser crystal with a size of220mm×140mm×25mmwas successfully grown by HDS method based on reasonable process parameters.The analytical tests of phase, X-ray rocking curves and TEM shown that thestructural integrity of Nd:YAG laser crystal was excellent. The results ofinterference pattern and extinction ratio indicated that the quality of crystal withoutcentral cores and lateral cores was excellent, and the relations between structuralintegrity and anneal were studied as well. Meanwhile, analysis of thermal andmechanical properties also shown that the basic physical performances of crystalgrown by HDS method were good.For the change of heat mass transfer of solid-melt interface in the Nd:YAGcrystal growth process, which resulted in the inconformity of the crystallizationfront velocity and crucible pulling rate, the formula of crystallization front velocityin Nd:YAG crystal growth process by HDS method was derived and numerical analyzed. The results indicated that the crystallization front velocity was influencedby crystal thickness, crucible size and temperature gradient, furthermore, the effectswere calculated and analyzed in detail.One of the important conditions for high-quality large Nd:YAG laser crystal isthe excellent thermal mass transfer. Therefore, the mechanism of melt flow patternsin Nd:YAG crystal growth process by HDS method was derived and numericalanalyzed, in order to make clear of the main mechanism of the melt flow patterns.Meanwhile, the distribution of Nd³⁺ions in crystal was studied, and the relation withtemperature gradient was discussed as well. The results indicated that thedistribution was superior to that of other growth methods, and the HDS method wasmore suitable for Nd:YAG crystal growth.The growth defects of HDS-grown Nd:YAG crystal were carried out by meansof laser scanning confocal microscope and scanning electron microscope. Theresults manifested that scattering particles （bubbles, small inclusions and largeinclusions） and dislocations were the main growth defects, and the distribution andformal reasons of growth defects were discussed. Meanwhile, the distribution,density and categories of dislocations in grown crystal were also analyzed bychemical etching and XT methods, which indicated that the quality of crystal centerwas better than that of the crystal edge, and the dislocation density of the center wasrelatively lower.Absorption and fluorescence spectra of HDS-grown Nd:YAG crystal werecharacterized and analyzed. The results shown the maximum absorption at808nmwith a wide FWHM and strong emission peak at1064.59nm was coincident withtheoretical value （1064.15nm）, which proved the well optical quality of HDS-grownNd:YAG crystal. The intensity parameters were obtained to be Ω₂=0.5022×10^-20cm²,Ω₄=3.3506×10^-20cm²and Ω₆=5.1653×10^-20cm²based on the Judd-Ofelt theory, andabsorption cross section, radiative life time and emission cross section and otherparameters were estimated as well. Meanwhile, the laser conversion efficiency ofoutput power-input power was37.44%.更多还原