【作者】 姚淑华；

【导师】 刘宏； 王继扬；

【作者基本信息】 山东大学 ， 材料学， 2009， 博士

【摘要】 铌酸锂（LN）是一种重要的多功能晶体,具有优良的压电性能、非线性光学性能、电光及光折变性能等。能用提拉法生长出大尺寸晶体,而且易加工。因此,铌酸锂晶体是一种广泛应用重要功能材料。LN晶体是一种典型的非化学计量比晶体。一般条件下采用提拉法生长的同成分铌酸锂晶体（CLN）是在固液同成分配比（[Li]/[Nb]=48.6/51.4）熔体中生长的,Li离子的缺失造成晶体中存在大量的空位缺陷,使LN晶体的许多物理性能受到很大影响。人们十分希望长出低缺陷浓度的近化学计量比铌酸锂（NSLN）晶体。与传统的同成分铌酸锂晶体相比,NSLN晶体中缺陷明显减少,许多重要的功能性质都要优于同成分铌酸锂晶体。然而,对于一般应用来说,CLN晶体已可满足要求,故目前广泛生长和使用的铌酸锂晶体大多为CLN晶体,按质量和用途区别,可以分为声学级和光学级铌酸锂晶体。声学级的铌酸锂晶体的生长和应用技术已经十分成熟,光学级及掺杂同成分铌酸锂晶体在实际应用中还存在一些问题,如组分均匀性,光学均匀性等。本论文对近化学计量比铌酸锂晶体及其掺杂晶体的生长、结构、缺陷及性质进行了系统的研究。同时探讨了光学级及掺杂同成分铌酸锂晶体在电光方面的应用。本论文的主要研究工作与LN晶体的生产和应用单位结合,具有重要的理论和应用价值。本论文的主要研究内容包括:一、湿化学法合成铌酸锂粉体:针对掺杂元素的不均匀和加料的需求,采用湿化学法合成了铌酸锂及掺杂铌酸锂粉体,合成的铌酸锂粉末具有较低的熔点,在用于双坩埚连续加料方面有重要的前景。二、晶体生长:在总结现在国际上生长化学计量比铌酸锂晶体的几种主要方法的基础上,采用大坩埚生长小晶体和连续加料法利用自行研制的悬挂坩埚连续加料系统从富锂熔体中生长出了较高质量的近化学计量比铌酸锂晶体。从晶体生长热力学和动力学的观点出发,系统讨论了在生长过程中影响晶体生长和晶体质量的主要因素。认为合理的温场,优质籽晶,合适工艺参数和控制锂从熔体的挥发是生长高质量晶体的前提条件。三、晶体的组分:铌酸锂晶体的[Li]/[Nb]比对晶体的性质有很大影响,大尺寸铌酸锂晶体组分的均匀性对铌酸锂晶体的产业化生产及其应用尤其重要,为此我们通过紫外吸收光谱,OH^-吸收谱,对铌酸锂晶体的组分及其均匀性进行了详细的分析。认为造成组分分布不匀的主要原因是由于Li元素的挥发。初步表征了Mg:NSLN晶体的组分,沿生长方向Mg:NSLN晶体的紫外吸收边没有变化,并详细分析了其中的原因。测定了晶体的红外吸收谱,发现在NSLN晶体红外吸收谱中存在3466 cm^-1处的尖吸收峰和3478 cm^-1处的伴峰。Mg:NSLN和Mg（6mol%）:CLN晶体的OH^-红外吸收峰均位于3534 cm^-1左右,说明晶体中Mg²⁺离子已经进入正常Nb位,可以初步判断所生长的掺镁晶体的掺镁量达到抗光损伤阈值的浓度。四、晶体的结构研究:采用高温X射线粉末衍射法研究了晶体结构随温度的变化。发现在温度1073 K以上沿z轴出现收缩现象。结合高温拉曼研究,研究了晶体E（TO₁）和A1（TO₁）模随温度的变化,从NbO₆结构分析铌酸锂晶体出现的反常现象。五、晶体的缺陷研究:采用同步辐射白光貌相术,结合光学显微术和其他分析技术,对铌酸锂晶体的面缺陷（孪晶）、线缺陷（位错）、体缺陷（包裹体）等进行了观测和研究,探讨缺陷的形成机制,为生长优质晶体提供了依据。并通过高能X射线研究了大尺寸晶体的质量。六、铌酸锂晶体的畴结构:采用化学腐蚀法对铌酸锂晶体的畴结构进行了研究,发现我们生长的NSLN晶体并没有完全单畴化,在少数区域有反转畴的存在。铌酸锂晶体的单畴化程度依赖晶体的组分。Mg:NSLN晶体中发现区域性畴结构。在Mg:CLN晶体中观察到同心环状结构,对不同位置进行了OH^-红外光谱放入研究,发现掺镁对畴的形成有一定影响。Mg²⁺离子的极化对Mg:NSLN晶体的畴结构有一定影响。七、铌酸锂晶体的热学性质:包括晶体的熔点、居里点、热膨胀、比热、热扩散系数及热导率等热学性质,并根据晶体热学性质,结合晶体的结构的特点,分析了热学性能对晶体生长和光学应用的影响。八、晶体的基本物理性质及应用探索:测量了铌酸锂晶体的吸收、透过以及折射率,通过NSLN晶体电滞回线测得了晶体的矫顽场。利用本论文生长的晶体,制备了周期极化LN晶体。根据电光Q开关应用对铌酸锂晶体的要求,探索铌酸锂晶体生长、组分、均匀性与电光应用的关系,测试了铌酸锂晶体的动态消光比、半波电压、吸收边以及微量元素的分析,指出组分均匀光学质量好的CLN晶体可以满足电光Q开关的应用要求。但是有不同来源的光学级CLN晶体不能完全符合使用要求,在实验基础上指出了这些铌酸锂晶体存在的问题。并提出相应的解决方案。为了提高LN晶片制备器件的成品率和光刻精度,我们对CLN晶片在还原气氛下进行了还原处理。更多还原

【Abstract】 Lithium niobate（LN） is an important multi-functional crystal,possessing good piezo-electric,nonlinear-optic,electro-optic and photorefractive properties.Lithium Niobate have been widely grown mainly by Czochralski method,and it is easy to be cut and polished for the fabrication of desired elements due to its excellent mechanical property.Based on the above merits,lithium niobate has been widely used in various areas and is still a promising crystal for new applications.Lithium niobate is a typical non-stoichiometry crystal.The deficiency of lithium ion in the crystal induced a number of vacancies reduction of some important function properties.Stoichiometric lithium niobate crystal（SLN） with low vacancy density has been grown and attracted many attentions recent years.Compared NSLN crystal with CLN crystal,the defect density of SLN crystal decreases obviously.In the meantime, many functional properties of SLN are better than those of CLN.The CLN crystal is widely used to fabricate the acoustic devices and called to be acoustic grade LN crystal. And those used to fabricate optics elements are called optical grade LN crystal.The application of acoustics grade LN crystal is very succeed,however,the applications of optical grade and doped CLN crystal has encountered some problems due to its component and refractive index homogeneous.In this thesis,we focused our attention to the crystal growth,determination of composition,observation of defects and basic properties of NSLN crystals.And we investigated the CLN and doped crystals for fabrication of electro-optic Q-switch.The main content of this thesis is described as follows:1.The synthesis of LiNbO₃ nanopowders through a wet chemical route:In order to reduce doped elementary inhomogeneity,undoped and doped LiNbO₃ powder have been successfully prepared by using commercial Nb（OH）₅ and nontoxic DL-Malic Acid as raw materials at relatively low temperatures through a wet chemical route.The powder prepared is suitable to be used as feeding powder in the double crucible technique growth of NSLN crystals due to its low melting point.2.Crystal growth:On the understanding of crystal growth methods of NSLN in this field,we adopted CZ technology to use a large crucible to grow a small SLN crystal.A modified suspend quazi double-crucible technology with continuous powder feeding system was designed so as to keep a composition homogeneity in grown crystal.By this technology,NSLN crystals with high optical quality were successfully grown from a rich lithium melt.The factors that determine quality of a crystal during the growth were analyzed,on the basis of thermodynamics and dynamics theories of crystal growth.In our understanding,the optimal thermal field,high quality seed,the optimal parameters and controlling lithium volatilization from the melt surface are the prerequisite for the crystal growth with high perfection.3.Crystal components:The ratio value of[Li]/[Nb]in LiNbO₃ crystal have great influence on crystal quality,And the components homogeneity of the large size LN crystal is very important in industrialization production.Therefore,components the analysis of crystal components is necessary.Here,crystals compositions were assessed by measuring the UV absorption edge and OH^- absorption.The reason of components inhomogeneity is volatilization of Li₂O.The absorption edge wavelength of Mg:SLN crystal have not obviously changed along Z axis,and the reason is discussed.The OH^- infrared absorption peak for NSLN crystal was located at 3466 cm^-1 with a shoulder peak at the high wavenumber side.The absorption peak for Mg:SLN （2mol%） and Mg:CLN（6 mol%） crystal were located at 3534 cm^-1,which indicated indirectly that the Mg²⁺ began to enter the Nb site as the substitiution of Nb⁵⁺.MgO doping concentration in the as-grown crystal has exceeded optical damage resistance threshold concentration. 4.Crystal structures and defectsHigh temperature X-ray diffraction,thermal expansion and Raman scattering measurements have been conducted to investigate the structural behavior of a nearly stoichiometric LiNbO₃.Abnormal expansion is observed within the temperature range of 1021 K to 1373 K.This abnormal behavior was discussed from E（TO1） and A1（TO1） modes Raman shifts at different temperature.5.Crystal defects:Defects of LN crystals have been observed by white-beam synchrotron radiation topography（WBSRT） technique,the chemical etching method and optical microscopy. The main defects in LN crystals were twins,inclusions and dislocations.Based on the observations of growth defects in LN crystals,the formation mechanisms of these defects were discussed.In order to avoid or decrease these defects,we take some corresponding steps according to the above discussions.The perfection of the crystal was assessed by high-energy X-ray diffraction.It could be helpful for the high quality LN single crystal growth.6.Crystal domain structure:Domain structure has also been observed by chemical etching method in acid solution of HF and HNO₃.The results showed that NSLN crystal possessed an incomplete single domain structure.The regional single domain structure and ring domain structure were found in Mg:NSLN crystal.Crystal components and polarization of Mg ion have an effect on the domain structure.7.Crystal thermal properties:The investigation of thermal properties including melt point,T_c,specific heat, thermal expansion coefficient,density changing along the temperature,thermal diffusion coefficient and thermal conductivity.The influence of these properties on growth and applications were also discussed.8.Basic physical properties:The transmission and absorption properties of NSLN crystals were studied.The refractive indices and optical homogeneity of LN crystals were measured.The ferroelectric hysteresis was obtained and the polarization mechanism was also discussed.At present,many of CLN crystal can only work stably at room temperature and light power laser.In order to suit the width temperature range high stable Q switch, we have analyzed the extinction ratio,half wave voltage,UV absorption edge and trace element of LN crystals.We reduced CLN slice to eliminate the pyroelectric discharge in SAW device manufacture.更多还原