枝晶研究的发展现状

本文主要介绍了枝晶研究的发展现状,主要包括枝晶形貌研究和生长理论研究两个方面.枝晶形貌方面:对生长形态选择问题,比较符合实际的解是"最大质量沉积速率"选择规律;枝晶分枝程度与过冷度和晶体对称性有关,分枝反映晶体的对称性.生长理论方面主要介绍三种生长模型:Zener近似因子模型,扩散限制聚集(DLA)模型和成核限制聚集(NLA)模型.并探讨了需进一步研究的两个方面:具对称结构的枝晶的生长机理和枝晶形貌的统一规范化描述.     

固液界面形貌及其转变

凝固时的固液界面通常为平面、胞状、枝晶状。在某些条件下结晶,会形成双胞结构、三胞结构、倾斜枝晶、密集分枝和退化枝晶等不规则界面。本文介绍了不规则界面的形貌特征,讨论了界面能各向异性、压力和温度对界面形貌的影响以及规则界面与不规则界面的相互转变等问题。当晶体沿着某些特定位向生长时,界面能接近于各向同性,晶体以密集分枝方式生长;当界面能各向异性时,大多数晶体以规则枝晶方式生长。在VF工艺中,低压下得到密集分枝界面;压力增加,界面以枝晶方式生长;压力进一步增加,重新得到密集分枝界面。     

Di-An-Ab体系快速冷却结晶的枝晶研究

对透辉石-钙长石-拉长石(Di-An-Ab)三元体系在不同降温速度条件下形成的枝晶进行了研究.其中,透辉石枝晶为规则树枝状,主枝和分枝明显;斜长石枝晶呈放射纤维状集合.枝晶形貌随降温速度变快没有明显变化,枝晶晶体结构随降温速度不同而有改变,降温速度减小,透辉石枝晶晶胞参数a值呈增加趋势,b、c呈减小趋势,斜长石枝晶b值呈减少趋势,c值呈增加趋势,两者β值呈增加趋势,但改变不大.随着降温速度增大,透辉石枝晶、斜长石枝晶硅氧骨干中,铝硅比值增大,铝更大程度地替代硅.另外还发现,降温越快,枝晶结晶数量越少.     

ZnWO4单晶衬底上ZnO薄膜的晶核发育与形貌分析

ZnWO4单晶的a晶面与氧化锌c晶面晶格匹配很好,是制备氧化锌薄膜的优良衬底.本文采用溶胶-凝胶法在ZnWO4单晶衬底上制备出透明的ZnO薄膜.通过光学显微镜对薄膜晶核发育过程和形貌进行了详细的分析.实验结果表明:在结晶刚开始,系统将经历成核--长大的过程,随着生长过程的进行,在主晶轴上(一次轴)上又长出二次轴、三次轴等等,最后逐渐形成树枝状晶核.由于ZnO晶核是在非平衡条件下生长的,故在晶核发育过程中又出现了三种不同的生长形态--成核生长、枝晶生长和分形生长.     

Di-Ab-An体系远离平衡枝状形貌的演化及枝体结晶学各向异性特征研究

对钠长石(Ab)-钙长石(An)-透辉石(Di)三元体系跨相界线的不同成分点在不同的降温速度条件下快速结晶形成的枝状形貌进行了研究.透辉石为枝晶,斜长石为球粒晶.当降温速度慢于1℃/min后,枝状形貌变为板-柱状形貌(即近平衡形貌).我们的实验结果中没有观察到如E.Brener 和H.Muller等人提出的形貌演化图一样的演化规律.当成分点靠近同结线后,出现两晶共结形成的"中空结构".通过光学显微镜下测试其光学性质发现,透辉石沿c轴方向延长,斜长石沿a轴延长,与晶体结构中的强键链方向一致,即快速结晶时晶体沿着结构中负离子配位多面体共角顶、共棱形成的强键链方向生长形成枝体.     

Li2O-ZnO-Al2O3-SiO2系微晶玻璃的烧结特性

采用收缩率、X射线衍射、扫描电镜对Li2O-ZnO-Al2O3-SiO2 (LZAS)系微晶玻璃的烧结特性、析晶特性及烧结后微晶玻璃试样的结构进行了研究.结果表明:LZAS微晶玻璃在530～560℃烧结时,析出的主晶相为γⅡ-LZS,γ0-LZS和方石英为次晶相.在590℃烧结时开始析出β-石英固溶体.620℃以上温度烧结时,析出的主晶相为γ0-LZS和β-石英固溶体,方石英为次晶相.LZAS系微晶玻璃的烧结属于粘滞性流动烧结,试样在470～590℃的温度区间完成烧结,但烧结温度高于530℃时由于试样析晶导致烧结收缩出现"滞缓"现象,590℃以上温度烧结时试样出现了流散.     

NaCl枝状形貌的制备及生长机理探讨

采用溶剂蒸发法,在不同的条件下分别获得了具有四次对称和二次对称的Na Cl枝晶、"Z"字形枝蔓晶以及主要由枝晶呈放射状分布的球粒晶。借助于X射线衍射仪、光学显微镜、扫描电镜以及电子背散射衍射仪(EBSD),对Na Cl枝状形貌进行了测试分析。结果表明,Na Cl球粒晶主要由主干沿着<111>方向生长的一系列枝晶组成。Na Cl晶体(111)面网由同号离子组成,且钠离子层与氯离子层相间分布,层与层之间的联结力强,且(111)的面网间距最小,是导致Na Cl枝晶优先沿<111>方向生长的主要原因。溶剂的蒸发速度等环境因素的变化对Na Cl枝状形貌的形成具有很大的影响。     

K2SO4晶体枝状形貌及结晶学模型

生长了具有枝状形貌的K2SO4晶体.基于单偏光显微镜下的形貌观察和背散射电子衍射分析,测定了枝晶的生长方向,建立了K2SO4晶体枝状形貌的结晶学模型.研究表明,K2SO4晶体的生长方向为[111]方向和[(1)-(1)1]方向,且两者夹角为114.4°;发育的晶面为(1(1)0)面,是由[111]晶向和[(1)-(1)-1]晶向组成的晶面.K2SO4枝晶具有两种生长模型,当其作为单枝生长时,一个< 111>方向的枝体连续发育做主枝,另一个<111>方向的枝体不连续发育作为侧枝;当其多枝同时生长时,<111>方向既可作为一个主枝,又可以作为其它主枝的一个侧枝.两种模型均指示出了K2SO4枝晶生长的不对称性.该研究揭示了结晶学方向在枝晶形成过程中的控制作用.     

电化学沉积法制备镍枝晶

采用电化学沉积法制备了镍枝晶沉积物,考察了不同的试验参数对枝晶生长过程以及微观形貌的影响.结果表明,枝晶具有典型的自相似性,枝晶容易在高的电流密度下生长,电流密度高,提高了阴极过电位,有利于枝晶形成;电解液里Ni2浓度的增加,沉积层分型更加明显,制备的镍枝晶逐渐稀疏,分叉也相应减小.随着电解液温度的提高,沉积层由开放型向致密型转化.这是因为在很高电流密度下,很容易发生析氢反应,释放大量气泡;金属离子在气泡之间的空隙被还原形成多孔的沉积层.     

热处理制度对LZAS系微晶玻璃析晶行为的影响

采用烧结法制备了Li2O-ZnO-Al2O3-SiO2 (LZAS)系微晶玻璃.采用DTA曲线初步确定了LZAS系微晶玻璃的成核和晶化温度范围,再用传统方法确定其最佳热处理制度.分别研究了核化温度、核化时间和晶化温度对LZAS系微晶玻璃析晶行为的影响.此外,还对比分析了两步和一步热处理制度对此玻璃析晶行为的影响.研究结果表明:核化温度和核化时间严重影响LZAS微晶玻璃的结晶度,对析出物相种类影响较小;晶化温度对LZAS微晶玻璃析出晶体的种类、晶粒的大小和晶体尺寸的均匀性有重要影响;两步法和一步法热处理对析出晶相的种类和材料的微观结构影响不大,但经历两步法热处理析出晶体的数量更多;LZAS系微晶玻璃在热处理过程中产生了2种相变:方石英转化为β-石英固溶体,γⅡ-LZS转化为γ0-LZS.     

On the inhomogeneity of the transition surface layer of the solid core of the earth

Different geophysical data and conclusions of theoretical models, which can give information about the behavior of the solid and liquid cores of the Earth as well as about the existence of a transition layer as a temperature-hysteresis region at a relatively weak first-order phase transition, are compared. It is concluded that liquid inclusions inevitably exist in this region; these inclusions are involved (due to the complex convective processes occurring in the liquid core) in the transport of light materials from some areas of the solid-core surface. The porosity and permeability of the transition layer determine the seismic acoustic inhomogeneities in these areas, which contact the convective flows in the liquid core. In particular, this explains the well-known ??east-west?? effect. Obviously, the model of the crystalline core is not the only possible alternative for a model of a core with a metallic glasslike structure.     

On the role of the porous shell of the solid core of the earth in the anomalous heat and mass flow to the mantle

The model of the pressure-induced first-order phase transition of a metal melt to the metallicglass state considers a thermodynamically nonequilibrium porous near-surface shell of the solid core of the Earth, which contacts cyclonic vortices in the liquid core. Anomalous flows of heat and light-material mass to the mantle from the solid core at these contact points are calculated. These anomalous flows are shown to be comparable with the observed ones under the assumption of a rapid increase in the melt viscosity at pressures of 1–10 Mbar, which is characteristic of a solid core. In this case, the porous layer permeability may be very low.     

Features of the magnetic state of the layered Fe-V nanostructure of the superconductor-ferromagnet type

The magnetic state of the V(39 nm)/20 [V(3 nm)/Fe(3 nm)] nanostructure has been investigated by polarized neutron reflectometry in the temperature range from 1.6 to 30 K in magnetic fields from 0.2 to 15 kOe. The data obtained indicate that the superconductivity of vanadium layers may affect magnetic ordering both over the depth of the structure and in its plane.     

Study of the influence of the rare-earth elements on the properties of lead iodide

Lead iodide (PbI₂) shows excellent electronic properties for detection of ionizing radiation. We report the introduction of rare-earth elements and other elements as admixtures during synthesis to study their influence on the quality of single crystals. Synthesized material as well as single crystals have been characterized by measurements of electrical resistivity and low-temperature photo luminescence and index of refraction. The structural quality with respect to polytypes was analysed by electron back scatter diffraction. Makyoh topography was applied for surface studies.     

Influence of the Crystal Structure of the Nucleus on the Morphology of t-ZnO Tetrapods

Crystallography Reports - A new approach to simulating the morphology of hierarchical nanocrystals has been proposed on the example of ZnO nanotetrapods. Within this approach, twinning is...     

Effect of the temperature on the TMOS sono-hydrolysis and a study of the structure of the resulting aged sonogels

The effect of temperature on the oxalic acid catalyzed sono-hydrolysis of tetramethoxysilane (TMOS) was studied by means of a heat flux calorimetric method. The activation energy of the process was measured as (24.5 ± 0.8) kJ/mol in the temperature range between 10 and 50 °C. The structural characteristics of the resulting sonogels, after long period of aging in saturated conditions, were studied by means of small angle X-ray scattering. The structure can be described as formed by ∼2.7 nm mean size mass fractal-like aggregates (clusters) of primary silica particles of ∼0.3 nm mean size, all imbibed in a liquid phase. The average mass fractal dimension of the clusters was found to be 2.58. The primary particle density was estimated as 2.23 g/cm³, in good agreement with the value frequently quoted for fused silica. The volume fraction of the clusters in the saturated sonogels was estimated as about 28%. The moment in which the meniscus of the liquid phase penetrates into the clusters under rapid evaporation process has been detected by an inflection in the first derivative of the curve of weight loss in a simple thermogravimetric test.     

Ordering of cations in the voids of the anionic framework of the crystal structure of catapleiite

The pseudohexagonal crystal structure of the mineral catapleiite Na_1.5Ca_0.2[ZrSi₃(O,OH)₉] · 2(H₂O,F) from the Zhil’naya Valley in the central part of the Khibiny alkaline massif (Kola Peninsula, Russia) is studied by X-ray diffraction (XCalibur-S diffractometer, R = 0.0346): a = 20.100(4), b = 25.673(5), and c = 14.822(3) Å; space group Fdd2, Z = 32, and ρ_calcd = 2.76 g/cm³. Fluorine atoms substituting part of H₂O molecules in open channels of the crystal structure have been found for the first time in the catapleiite composition by microprobe analysis. The pattern of distribution of Na and Ca atoms over the voids of the mixed anionic framework consisting of Zr-octahedra and three-membered rings of Si-tetrahedra accounts for the pronounced pseudoperiodicity along the a and c axes of the pseudohexagonal unit cell and for the lowering of crystal symmetry to the orthorhombic one. It is shown that part of the hydrogen atoms of water molecules is statistically disordered; their distribution correlates with the pattern of the population of large eight-vertex polyhedra by Na and Ca atoms.     

Investigations of the Chemical Composition of the Metal Finds from the Levadki Necropolis

Crystallography Reports - Copper-based alloy items of the III century A.D., found in Burial no. 182 of the Late-Scythian Levadki necropolis in the submountain Crimea, have been investigated. The...     

Computer Modelling of the Light Propagation through the Complex Anisotropy Systems - the way of a Determination of the Optical Parameters of the Liquid Crystal Displays

A mathematical and numerical method for an analysis of a liquid crystal displays (LCD) has been presented. The assumptions of the calculations of a light propagation through the display are shown. The real conditions of a work of these displays have been taken into account. Basing of this mathematical model a computer program for a numerical modelling of the optical parameters of LC displays working in real conditions has been worked out. This program makes possible to obtain the most important parameters of a display such as: contrast ratio, luminance in on- and off-state, spectral characteristics of light and its colour coordinates in these both states. The results obtained from this program have been verified experimentally. Very high conformity of these results has been obtained.