近化学计量比LiNbO3晶体电畴结构观察和机理探讨

采用双坩埚提拉法(DCCZ)生长了各种不同成分的近化学计量比LiNbO3晶体,并用腐蚀法观察了其电畴结构.结果表明,化学成分对未经极化处理晶体的电畴结构起决定性作用,当Li2O含量处于49.4mol%附近时,晶体z面电畴呈现特殊的三次对称反畴;当晶体中Li2O含量为49.7mol%时,晶体为完全单畴.本文对其形成机理进行了探讨,认为在由顺电相向铁电相转变时,局部铁电畴的极性方向与该处沿z轴方向的温度梯度正负密切相关,z轴生长晶体时,由于相变发生所处位置离生长界面的距离受LiNbO3晶体计量比影响,所处温场固有温梯也随之不同,在此基础上解释了不同成分晶体的电畴结构形成原因.最后讨论了控制铁电畴结构的工艺措施.     

近化学计量比LiNbO3晶体电畴结构观察和机理探讨

采用双坩埚提拉法(DCCZ)生长了各种不同成分的近化学计量比LiNbO₃晶体, 并用腐蚀法观察了其电畴结构. 结果表明, 化学成分对未经极化处理晶体的电畴结构起决定性作用, 当Li₂O 含量处于49.4mol%附近时, 晶体z面电畴呈现特殊的三次对称反畴; 当晶体中Li₂O含量为49.7mol%时, 晶体为完全单畴. 本文对其形成机理进行了探讨, 认为在由顺电相向铁电相转变 时, 局部铁电畴的极性方向与该处沿z轴方向的温度梯度正负密切相关, z轴生长晶体时, 由于相变发生所处位置离生长界面的距离受LiNbO₃晶体计量比影响, 所处温场固有温梯也 随之不同, 在此基础上解释了不同成分晶体的电畴结构形成原因. 最后讨论了控制铁电畴结构的工艺措施.     

快速生长KDP晶体中的包裹体

利用底部热能输入晶体生长装置进行了KDP晶体快速生长,晶体生长速度达25mm/d.利用激光透射成像法、断面显微观察、SEM及电子能谱对快速生长晶体中的各种包裹体进行了观察,分别观察到了平行于生长面的层状包裹及其分布、线形排列的液相包裹以及微观包裹体的形貌、尺度和分布等,讨论了快速生长KDP晶体中包裹体出现的条件,分析了这些包裹体形成的原因.     

非完整形态KH2PO4晶体薄表面层生长特性

实时观察了非完整形态的KH2PO4 (KDP)晶体在过饱和溶液中以薄表面层生长形式恢复其结晶学形态的过程.提出了晶体形态恢复的“最小多面体原理”,即:在自由生长系统中,对于非完整形态的KDP晶体,当其以薄表面层形式恢复其结晶学完整形态时,薄表面层将选择相应的奇异面方向生长,使晶体形态最终恢复为一个由各结晶学显露面所围成的体积最小的凸多面体.利用PBC理论分析了生长基元在非结晶学显露面上的附着情况并阐述了锥顶处薄表面层倒垂生长的原因.结果表明,薄表面层形成与晶体非完整结晶学形态及不均匀水动力学条件相关联.柱面凹角与非正常棱边及Z切片正常棱角均可诱发产生薄表面层,且薄表面层生长终止于其所在奇异面的正常结晶学晶棱.     

基于结构性改变的光子晶体光纤光栅研究

利用多极法结合耦合模理论对一种基于结构性改变的光子晶体光纤光栅进行了研究,建立了其结构模型,理论分析了此种成栅工艺原理.计算了七层三角形对称排布空气孔包层有效折射率随波长变化情况,比较了不同光子晶体光纤结构成栅效果的差异.研究结果表明,光子晶体光纤包层中空气孔周期性塌缩可以形成光栅,空气孔排布层数,空气孔占空比、空气孔塌缩层数和塌缩程度等参数对光子晶体光纤光栅的传输特性有重要影响,得出了谐振波长和谐振带宽与上述参数之间的变化关系,并给出这种变化关系的解释.     

棒状纳米微晶纤维素诱导制备花状TiO2纳米晶体

以棒状纳米微晶纤维素(Nanocrystalline cellulose,NCC)为形貌诱导剂,TiCl4为原料,采用水解法在70℃的温度下反应4～6 h,制备了TiO2花状纳米晶体.采用TEM、HRTEM、XRD和FTIR对不同条件下制得的TiO2晶体的微观形貌、晶粒尺寸和晶相组成进行了表征,探讨了TiCl4的用量及反应时间对晶体形貌和晶型的影响,并对其形成机理进行分析.纳米微晶纤维素表面富含大量羟基,可与TiO2之间形成氢键连接,促使TiO2在其表面的异质成核和生长,同时纳米微晶纤维素在TiO2表面的吸附作用,改变了各晶面的表面能和生长速度,使TiO2沿着[110]方向优先生长形成一维针状物,针状物再进一步聚集形成花状聚集体.以甲基橙为目标污染物,测试了所得TiO2纳米花状晶体的光催化性能.结果表明,随着TiCl4用量的增多及反应时间的延长,所制得的TiO2纳米晶体中金红石含量增多,形貌逐渐复杂化,光催化性能下降.     

导模法生长微孔蓝宝石晶体工艺及性能研究

采用导模法生长了内孔径470 μm的蓝宝石晶棒和内孔径160 μm板形蓝宝石晶体。基于求解拉普拉斯方程的数值解, 得到生长界面处微孔的熔体膜轮廓曲线, 采用插入钼丝的方法设计了生长模具以形成和维持晶体内的微孔尺寸, 同时解决了微孔蓝宝石长晶生长过程中的两个难点: (1)获得高质量的蓝宝石晶体; (2)在蓝宝石晶体中形成并维持所需的内孔尺寸。所生长的晶体透明完整、无开裂、双晶摇摆曲线测定显示其衍射半峰宽为3.8°, 具有良好的结晶质量     

在多工位炉上CeO2:Bi12SiO20单晶生长的研究(Ⅱ)——空间生长实验部分

掺CeBi12SiO20单晶在神舟3号(SZ-3)飞船上成功地进行了空间生长,得到晶体尺寸为φ10mm×40mm.将空间生长的晶体和地面生长晶体对比发现空间生长晶体的外观同地面生长晶体有明显差异.分析测试空间和地面晶体的X射线摇摆曲线、吸收曲线和喇曼光谱,结果表明空间生长掺CeBSO晶体的结构完整性优于地面生长的晶体,掺Ce对BSO晶体光学性能的影响空间制备晶体要大于地面制备的晶体     

BiB3O6晶体的褐色缺陷

利用同步辐射白光形貌术和透射电子显微镜,对BiB3O6晶体的缺陷进行研究.靠近籽晶部位,可以观察到包藏、位错和生长扇面边界,而远离籽晶的位置,没有发现任何微观的缺陷.通过实验观察,孪晶和生长扇面边界可能是导致晶体中褐色区域的形成原因.提出了消除缺陷的方法.     

氧化锌纳米晶体的生长及生长机理分析

采用沉淀法生长了不同粒径且性能良好的氧化锌纳米晶体,探讨了晶粒尺寸与煅烧温度的变化关系,研究了ZnO纳米晶体的生长机理.通过实验验证了在中间产物Zn(OH)2中加入NH4HCO3后,ZnO纳米晶体实现了局域生长,很好地阻碍了晶核的过度生长,从而使生成的ZnO纳米晶体保持很小的粒径.煅烧温度较低时,ZnO纳米晶体的生长模式为正常扩散生长,煅烧温度在800℃时,发生了竞争生长现象.     

Interactions Between Bubble and Interface During KTa1?x NbxO3 Crystal Growth

The generation of bubbles and its interaction with the interface during melting and growth process of po-tassium tantalate niobate (KTa1?xNbxO3) crystals were visualized by a high temperature in-situ observation system. It was found that bubbles are generated mainly from the solid-liquid interface during melting, rather than from the melt. Bubbles with radii smaller than 0.7 r (where r is the mean radius of bubbles) arise mostly from nucleation at the interface while radii larger than 1.5 r are the result of coalescence. The existence of the bubble not only lowers the growth velocity of the near interface, but also affects the structure of the crystal. The effect of a bubble on the growing interface depends on their dimension ratio together with the moving speed of the interface. There are three typical kinds (hat-, sphere- and ellipsoid-shaped) of vapor inclusion morphologies being demonstrated. The analysis of the bubble behavior can promote the understanding of the formation of inclusion defects in KTN crystal growth process.     

Ultrasound effect on the Czochralski growth of corundum crystals

We have studied the effect of ultrasound in the range 100–400 kHz on the Czochralski growth of corundum single crystals in a carbon-containing medium. The results demonstrate that sonication during crystal growth influences the size and density of gas bubbles in the melt. At frequencies above 350 kHz, sonication contributes to the bubble rejection at the growth front, thereby reducing the pore density in the crystal.     

Photographic studies of light induced nucleation of boiling at the interface of a solid and superheated liquid helium I

We report the results of photographic studies which show that light induces the nucleation of vapour bubbles in superheated liquid helium I. We believe that these bubbles are associated with the cooling that we have previously observed to result from a flash of visible light applied to the surface of a bismuth crystal heater-thermometer immersed in the superheated liquid. We have used this photographic technique to study vapour bubble growth dynamics and compare our results with the theory.     

Sliding and sticking vapour bubbles under inclined plane and curved surfaces

In flow boiling heat is transferred by the combined effects of nucleate boiling, with local generation of bubbles, and evaporative and convective cooling by the passage of bubbles generated elsewhere. In this study, nucleate boiling was eliminated by measuring the heat transfer near injected steam bubbles sliding under an inclined plate heated to low superheats, using liquid crystal thermography combined with high speed video recording and computerised image analysis. Heat was transferred by evaporation of the thin liquid film between the bubble and the wall and by enhanced convection in a wake region wider than the bubble and many bubble diameters long. Evaporation was the dominant mechanism for large, easily deformed, slow-moving bubbles. For small, faster-moving bubbles the reduction in evaporation was offset by an improvement in convection.     

低压环境固着盐水液滴蒸发析晶过程实验研究

实验研究了低压环境下固着盐水液滴在不同基底表面(铜、载玻片和聚四氟乙烯)的蒸发析晶过程,分析了表面性质和环境压力的影响。结果表明,低压环境下易在接触线处析出白色盐晶。铜表面由于表面能较大,接触面上覆盖盐晶体,液滴蒸发过程接触直径几乎不变,接触角逐渐减小。在载玻片表面,当环境压力较高时,液滴蒸发造成接触线收缩,伴随盐晶体的生长和移动接触角波动。在聚四氟乙烯表面,接触面处易产生气泡,气泡的生长和爆裂导致接触角明显波动。Pe数可以揭示液滴蒸发过程外部传质扩散和内部离子扩散的相对大小。研究成果有助于指导海水淡化的工业应用。     

Structural analysis of thermokarst lake ice in Beiluhe Basin,Qinghai–Tibet Plateau

The high altitude and cold climate of the Qinghai–Tibet Plateau (QTP) means that rivers and lakes are covered by ice for 6–7 months of the year. The ice cover is a major influence on environmental and ecological conditions, since it alters the heat and energy exchange with local atmosphere, as well as determines the design and construction of civil buildings. However, research on these ice covers has not been conducted previously. For the first time, the present work focuses on in-situ observations of ice growth and decay, and the inner structural analysis of ice samples from a thermokarst lake, Beiluhe basin, in the central plateau. Ice was sampled on two occasions during fast growth and slow growth periods. Ice crystals, densities and gas bubbles were investigated and classified into ice types. Vertical profiles of the ice crystal type, crystal size, fabrics, gas bubble shape and size, gas content, as well as ice density were determined. The formation mechanism was discussed based on QTP meteorology and stratigraphy.     

功率超声在金属熔体成形中的作用效应及其可视化研究进展

主要介绍功率超声在金属熔体成形领域中的作用,详述了超声在铸造成形、辅助焊接成形中细化晶粒、除气除杂、强化焊缝、减小残余应力、促进界面润湿等方面的应用和作用机理。针对超声空化气泡的高速摄影及金属熔体中晶体生长的同步辐射X射线成像观察的研究进展,提出将高速摄影与同步辐射X射线成像结合起来,通过原位观察方法研究超声波与液态金属媒质的相互作用机理,为进一步促进超声在金属熔体成形中的应用提供理论指导。     

Diffusion-controlled growth of multi-component gas bubbles

Theoretical solutions for some important problems involving diffusion-controlled growth of gas bubbles in liquids in conditions of spherical symmetry are presented. It is shown that bubbles in systems containing several independently diffusing gases always approach an asymptotic composition and a parabolic relation between size and time. Solutions for this asymptotic regime have been obtained analytically for growth from zero size and numerically for growth from finite initial size; the two solutions agree well for sufficiently large sizes. The numerical methods can deal with transient growth from finite size, including the behaviour of bubbles of non-equilibrium initial composition. The differences between initial and equilibrium compositions make it easy to understand why the transient behaviour of bubbles can involve an initial period of shrinkage before the asymptotic regime is established.     

Controlled growth of CsI〈Tl〉 single crystals

CsI〈Tl〉 crystals have been grown by the axial-heat-flux-close-to-the-phase-interface (AHP) method in a purpose-designed apparatus at an argon overpressure. The furnace and crystallizer of the apparatus have been designed to ensure considerable (5 to 100 K/cm) axial and low (within 1 K/cm) radial temperature gradients at the growth interface. The effects of the melt layer thickness, temperature gradient, and activator concentration in the melt on the crystal quality have been studied. The results demonstrate that reducing the AHP heater temperature for even a short time (by 2°C in 3 min) markedly raises the actual growth rate, from 2 to 15 mm/h, and leads to entrapment of bubbles of various diameters and TlI inclusions.     

Observation and analysis of the detachment frequency of coalesced bubbles in pool boiling liquid nitrogen

The detachment frequency of coalesced bubbles is one of the important parameters in the boiling heat transfer model at high heat flux. Although some researches on detachment frequency of coalesced bubbles have been done for common liquids, the work on cryogenic liquids is relatively insufficient. A visual experimental apparatus was built for observing the boiling in liquid nitrogen. The growth process of coalesced bubbles was recorded by a high-speed camera, emphasizing on the detachment frequency of coalesced bubbles from the circular, horizontal heating surfaces. The effects of the heating surface material, the surface diameter and the heat flux on the detachment frequency were analyzed. The available empirical correlations from common liquids are compared with our experimental data to check their applicability in liquid nitrogen, where the detachment frequency is 13.47 s⁻¹.