【作者】 曹亚超；

【导师】 李明伟；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2014， 博士

【摘要】 硫脲硫酸锌（Zn[CS(NH2)2]3SO4，ZTS）晶体是一种性能优良的金属有机配合物型非线性光学晶体。它具有非线性系数高、激光损伤阈值高、光透过范围宽以及角度灵敏性低等优点，能够广泛应用于二次谐波激光发生器、电光调制器和高功率激光频率转换等高科技领域。另外，ZTS晶体还具有物理化学性质稳定、机械强度高且生长所需原料价格低廉等优点，因此ZTS晶体的生长和制备越来越受到人们的重视。目前，国内外学者对ZTS晶体的研究主要集中于对各种条件下（包括无机物、有机物的掺杂和pH值的改变等）生长出的晶体的性能的检测分析上，而对ZTS晶体微观生长过程方面的研究较少。虽然目前通过掺杂可以提高ZTS晶体的某些性能，但快速生长优质大尺寸ZTS晶体仍存在很多困难。因此，如何提高晶体的质量以及在保证晶体质量的前提下尽可能提高其生长速度，应引起研究人员的重视。众所周知，晶体的生长过程对晶体质量进而对晶体性能有重要影响，而晶体生长过程的实质就是生长基元通过生长界面进入晶格位置的过程，因而通过对ZTS晶体生长过程中晶体表面所发生的微观现象的观察研究，可以获得ZTS晶体生长过程方面的信息，加深对其微观生长过程的认识，了解其生长过程中的微观机理，从而指导人们控制晶体的生长过程，进行高质量的ZTS晶体生长。原子力显微镜（atomic force microscopy，AFM）正是这样一种可以帮助人们对微观现象进行观察的工具。AFM通过检测探针与样品表面的原子间的作用力可以获得样品的表面微观形貌，并且可在溶液环境下工作，因此可以利用AFM实时地研究晶体生长的表面形貌和生长机理。本文利用热科学的基础理论，通过开展ZTS晶体成核实验和ZTS晶体生长过程的AFM实时扫描实验等研究，对不同生长条件下，ZTS生长溶液的稳定性以及生长过程中晶体表面微观形貌的变化、晶体生长机制、缺陷形成等方面进行了深入探讨，主要内容为：①研究了不同过饱和比下ZTS溶液的成核过程，测定了不同过饱和比下溶液的诱导期。根据经典均匀成核理论，针对ZTS过饱和溶液均匀成核的情况计算得到了固-液界面张力等热力学参数。通过对表面熵因子的计算，确定了诱导期实验中ZTS晶体的微观生长机制。②利用AFM对不同生长条件下生长中的ZTS晶体（100）面表面微观形貌的实时变化过程进行研究，获得了不同条件下的界面微观形貌，并对台阶的运动进行了分析。ZTS晶体（100）面主要的形貌特征为台阶面，生长台阶分为基台阶、聚并台阶和“准聚并台阶”；台阶生成源不同，台阶高度不同，台阶的推移速度与台阶高度成反比。实时观察到一些未曾报道的现象：扇形台阶的推移；晶体从“生长死区”恢复生长的过程；聚并台阶的形成过程；各向异性的台阶推移。另外，对平行台阶列的稳定性进行了分析。③通过AFM实时扫描实验研究了30℃下ZTS晶体（100）面微观形貌随过饱和度的变化，确定在低过饱和度下，刃位错处易于成核从而可反复提供单分子台阶，成核的刃位错具有“记忆功能”；较高的过饱和度下，多核生长为主要生长机制；二维核在[010]和[001]方向上的推移速度不同，呈现出明显的各向异性；计算得到了[010]和[001]方向上二维核台阶推移的台阶动力学系数和台阶活化能。同时发现，台阶边缘的不连续处往往优先生长。④运用AFM实时扫描实验得到的微观数据，对30℃时低过饱和度（0.0150.025）下ZTS晶体（100）面的台阶动力学进行了研究，测算了ZTS晶体（100）面的水平推移速度和台阶的实际推移速度，发现此生长条件下ZTS晶体（100）面溶质边界层内的质量输运主要受体扩散机制控制。⑤利用AFM实时研究了30℃时不同浓度L-丙氨酸掺杂下ZTS晶体（100）面的微观形貌，分析了掺杂对（100）面表面形貌和台阶运动的影响，计算得到不同掺杂浓度下台阶的推移速度和台阶动力学系数。利用AFM对过饱和度5%，掺杂浓度2mol%时ZTS晶体（100）面生长过程进行实时观察发现，优先成核位置的依次形成导致成核具有交替性，并且优先成核位置被覆盖后依然能保持其活性。⑥利用光学显微镜和原子力显微镜，发现ZTS晶体中存在多种缺陷，主要包括位错、空洞、包裹体、开裂和楔化等；利用AFM观察到凹坑、包裹体等缺陷形成的实时过程，探讨了缺陷的形成机制，提出了减少晶体缺陷的相应措施。更多还原

【Abstract】 Zinc tris(thiourea) sulphate (Zn[CS(NH2)2]3SO4, ZTS) crystal is an excellentmetaorganic coordination compound of the nonlinear optical crystal, which has highnonlinear optical coefficient, high laser damage threshold, wide range of lighttransmission and low angular sensitivity, eta. Due to its excellent properties, it can bebroadly used in many high technology fields, such as secondary harmonic lasergeneration, electro optics modulated, high-power laser frequency conversion and so on.In addition, ZTS crystal has some other significant advantages, such as stablephysicochemical properties, high mechanical strength, and the low expensive for rawmaterial synthesis, thus, the growth and preparation of ZTS crystal have been paid moreattention.Currently, the emphasis focus on analysising and testing of the performance ofZTS crystal grown under in variety of growth conditions (including kinds of inorganicor organic materials doping and different pH value and so on), while the micro growthprocesses of ZTS crystal was few studied. Although some performances of ZTS crystalcan be improved by doping, there are still many difficulties for growing large-size ZTScrystal with high-quality at a rapid growth speed. Therefore, how to improve the qualityand increase the growth rate of ZTS crystal should arouse attention of researchers. It iswell known that crystal growth process has significant influences on crystal quality andresultantly affect the performance of the crystal. The crystal growth process is indeed aprocess of growing units entering into the lattice position via the growth interface.Therefore, the study of crystal growth interface micromorphology should enableresearchers to achieve more information about the crystal growth process and deepenthe understanding of microscopic phenomena associated with crystal growth andunderstand the microscopic mechanism of the growth process so as to guide researchersto optimize the crystal growth process and ultimately produce high-quality ZTS crystals.The atomic force microscopy (AFM) is just a relatively new and powerful instrumentthat can help people to investigate the microscopic phenomenon of crystal growth. Theoperational principle of AFM is that a sensitive tip scanning over the surfaces to test theatomic force between the tip and the surfaces. For its ability of working in liquids, itprovides us an opportunity to observe in-situ the growth surfaces morphology andunderstand the growth mechanisms at a nanometer scale. In this paper, the nucleation and real-time growth process of ZTS crystal werestudied under different conditions based on the theory of thermal science. The solutionstability was discussed and the surface morphology of ZTS crystal (100) surface beinggrown under different conditions were recorded and analyzed. The main works can besummarized as flows:①The effects of different supersaturations on the nucleation of ZTS solution werestudied. The induction periods with different supersaturations were measured.According to the classical homogeneous nucleation theory, nucleation parameters arecalculated by using the experimental results. By the calculation of the surface entropyfactor, the crystal surface growth model of ZTS in these experiments is confirmed.②Through the investigating on the growth of (100) face of ZTS crystal by in situAFM under different growth conditions, the micro morphologies of the interface wereobtained and the movement of steps were analyzed. The (100) surface exhibits steppedsurface characteristics. The growth step patterns consist of three types, namely theelementary steps, macrosteps and quasi-macrosteps. Steps derived from differentsources have different heights and the step speed inversely proportional to the stepheight. A fan-shaped steps phenomenon which was never reported before was observedby in situ AFM. The growth process of recovering from the “dead zone” and stepbunching phenomenon were recorded. A phenomenon of step advancement showstypical anisotropy was observed. In addition, the stability of the parallel step trains wasanalyzed.③Growth morphology of the (100) face of ZTS crystals, grown under differentsupersaturations at30℃were investigated by in situ AFM. It is found that at lowersupersaturations, nucleation easily occurred at the outcrops of edge dislocations whichcould act as persistent sources of monomolecular growth steps, and the edge dislocationsites appeared to exhibit a “memory function”. At higher supersaturations, however,poly-nucleation becomes the primary growth mechanism. The rates of step advancementin the [010] and [001] directions of the two-dimensional nuclei were different, as itensues from the symmetry anisotropy. The activation energy of the two-dimensionalnuclei along with the step kinetic coefficients in the [010] and [001] directions werecalculated. The result also showed that the position on the step edge where the edge isdiscontinuity is the preferential sites for growth.④Through investigating on the kinetics of step flow of the (100) face of ZTScrystal by using in situ AFM at lower supersaturations at30℃, the horizontal direction growth velocities of the (100) face were estimated. The real step flowing rates of ZTSare also calculated, and we find that the mass transfer is controlled by bulk diffusion inthe growth of steps.⑤The growth morphologies of the (100) face of ZTS crystals under differentconcentrations of L-alanine doped were investigation by using in situ AFM at the samesupersaturations at30℃. The effects of the dopant on step morphology and stepmovement were analyzed. The step velocity and step kinetic coefficients under differentdoping concentrations were calculated. The results of in situ AFM study on ZTS crystal(100) surface with supersaturation at5%with the doping concentration of2mol%showed that preferential nucleation sites sequentially formed which lead to thenucleation appears alternating, and the preferential nucleation sites still able to maintainits activity after it be covered.⑥By optical microscope and atomic force microscopy，we found that there aremany kinds of defects in ZTS crystals. The main defects of ZTS crystals are growthdislocations, hollow cores, inclusions, cracks and tapering. The real-time process of theformation of pits and inclusions were recorded and the formation mechanisms werediscussed. According to the distinct formation mechanisms results, some methods wereproposed to decrease these defects.更多还原