Ti-40Al-2B合金微观组织和初生TiB2生长特征

用原位自生法制备了Ti-40Al-2B(wt%)复合材料,并用XRD、SEM对复合材料的相组成和微观组织,特别是TiB₂的形貌进行了研究。结果表明:该合金由TiAl和TiB₂两相组成。其中TiB₂颗粒以初生的块状和共晶的片状或细棒状形式共存。初生TiB₂呈六面棱柱体,端面有清晰的生长台阶,其显露晶面分别为(0001)和｛101 - 0｝。结合晶体生长理论分析认为:TiB₂的生长单元为一个硼原子和六个钛原子组成的三棱柱,硼原子位于三棱柱的中心,生长方式以台阶式生长为主。其中｛112 - 1｝面生长速率最快,｛101 - 0｝面生长速率最慢,导致TiB₂形成以｛101 - 0｝面为棱柱面的六面棱柱结构。      

Semiconducting CdSe thin films made by tarnishing

The properties of thin CdSe films made by the tarnishing reaction of selenium vapour on cadmium are described. Scanning electron microscopy, X-ray crystallographic and semiconductor photoelectrochemical data were obtained for these films. Depending on the preparation of the substrate, highly oriented polycrystalline films can be prepared. The unstable cubic form of CdSe exists as a transition layer between hexagonal cadmium and hexagonal CdSe. Impurities in the cadmium and the selenium appear to be rejected from CdSe, as the measured donor densities in the semiconducting films are very low and independent of the cadmium and selenium purities.There is evidence that the donor density is not uniform in these films. This is inferred from the fact that good Mott-Schottky plots can be obtained from impedance measurements of the film-electrolyte junction, whilst the grain size is incompatible with such an observation. It is suggested that the hexagonal phase of the CdSe is intrinsic and that the cubic phase is doped by non-stoichiometry.     

Comparative study of hexagonal and cubic GaN growth by RF-MBE

The epitaxial growth of both cubic and hexagonal GaN epilayers is considered here with the aim of comparing their physical properties. In particular, the growth mechanisms at the first stages of growth will be dealt with together with the quality of the growth front. The optical characteristics of the epilayers will be compared by reference to the structure of the defects present within the different types of layers.     

Growth of ZnO prisms on self-source substrate

ZnO whisker arrays have been prepared by a self-source procedure of the substrate under hydrothermal conditions. Zinc foils were used as a substrate and zinc-ion source for direct growth of ZnO whiskers. SEM observations revealed that the as-prepared ZnO whiskers exhibit hexagonal prism-like shape showing well-defined crystallographic facets and hexagonal pyramid-like sharp tips. The growth of ZnO whiskers in the initial stage has been recorded, and the growth mechanism was discussed in detail.     

A simple method for the growth of high-quality GaN nanobelts

A simple method using two-step growth technology to successfully synthesize the high-quality single crystalline GaN nanobelts was employed in this paper. The as-prepared products are studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The results of XRD and the selective area electron diffraction (SAED) patterns indicate that the reflections of the samples can be indexed to the hexagonal GaN phase with single-crystal structure. From the SEM morphology, we can see that the width of the nanobelts is about 800 nm, and the ratio of thickness to width is about 1/10. The maximum length is up to several tens of micrometers. In the HRTEM image, the clear lattice fringes indicate the growth of good-quality hexagonal single-crystal GaN nanobelts. Finally, the growth mechanism is also briefly discussed.     

雪晶生成过程机理

雪花的形成包括晶核的形成和雪晶的生长两个阶段.晶核是雪晶形成的关键,冰晶的分子结构决定了雪晶的六角对称形状,生长环境的温度和湿度是影响雪晶生长和形状的主要因素.实验研究表明,碘化银微粒对雪晶的形成具有极好的促进作用,雪晶的生长是一个非常缓慢的过程.     

Controlled synthesis of spherical α-NiS and urchin-like β-NiS microstructures

Spheres and self-assembled wires of α-NiS (the hexagonal phase) have been successfully synthesized by a facile mix solvothermal route (glycol and water). Varying the volume ratio between glycol and water, and leaving the other conditions unchanged, the urchin-like β-NiS (the rhombohedral phase) microstructures were also prepared. The as-obtained products were characterized by means of X-ray powder diffraction patterns (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The TEM results show that the wires of α-NiS are constructed by a number of spheres of α-NiS as primary building units. On the basis of the TEM results, a phenomenological elucidation of the growth mechanism of α-NiS architectures has been presented as the directional aggregating and the Ostwald ripening. Moreover, the optical properties of the α-NiS and β-NiS microstructures were investigated by the room-temperature ultraviolet-visible (UV-Vis) absorption spectroscopy and the photoluminescence (PL) spectroscopy.     

Controllable crystal growth of Mg(OH)2 hexagonal flakes and their surface modification using graft polymerization

Mg(OH)₂ crystals with excellent flame retardance in the application of polymer materials are always in demand. Herein, regular and well-dispersed Mg(OH)₂ hexagonal flakes were hydrothermally prepared with the existence of polyethylene glycol (PEG) and then modified by surface grafting-polymerization of methyl methacrylate (MMA) monomers. The results showed that the morphology and dispersity of Mg(OH)₂ relied on the precise control of the reaction parameters including hydrothermal conditions, the molecular weight and additive amount of PEG. PEG with a molecular weight of 8000 exhibited an enhanced directing role due to its more appropriate length of molecular chains and intensive interaction with the formed Mg(OH)₂ crystallites. The molecular chains of PEG-8000 can be preferentially adsorbed onto the (0 0 1) and (1 0 1) planes of Mg(OH)₂ crystallites and sub-micro hexagonal flakes with low-polarity were consequently assembled. The optimal conditions for preparing Mg(OH)₂ hexagonal flakes with higher crystallinity and more regular morphology were determined to be hydrothermal treatment at 120 °C for 12 h with 3 wt% PEG-8000. The dimension and decomposition temperature of the end products were 400 ~ 500 nm and 388 °C, respectively. The surface graft-modified Mg(OH)₂ hexagonal flakes exhibited high hydrophobicity with a water contact angle of 148°, indicating an excellent compatibility with polymers.     

KDP晶体生长习性与快速生长研究

发现由五氧化二磷合成的KDP晶体生长原料中含有大量还原性亚磷酸盐杂质．实验表明,亚磷酸盐对晶体锥面生长有显著的抑制作用．低浓度时造成柱面扩展,在高浓度时则使整个晶体的生长速度变慢．这种原料以适当方法提纯后,用“点籽晶”技术进行快速生长实验,平均生长速度达到13．7mm／天．     

Aligned Growth of Millimeter‐Size Hexagonal Boron Nitride Single‐Crystal Domains on Epitaxial Nickel Thin Film

Atomically thin hexagonal boron nitride (h‐BN) is gaining significant attention for many applications such as a dielectric layer or substrate for graphene‐based devices. For these applications, synthesis of high‐quality and large‐area h‐BN layers with few defects is strongly desirable. In this work, the aligned growth of millimeter‐size single‐crystal h‐BN domains on epitaxial Ni (111)/sapphire substrates by ion beam sputtering deposition is demonstrated. Under the optimized growth conditions, single‐crystal h‐BN domains up to 0.6 mm in edge length are obtained, the largest reported to date. The formation of large‐size h‐BN domains results mainly from the reduced Ni‐grain boundaries and the improved crystallinity of Ni film. Furthermore, the h‐BN domains show well‐aligned orientation and excellent dielectric properties. In addition, the sapphire substrates can be repeatedly used with almost no limit. This work provides an effective approach for synthesizing large‐scale high‐quality h‐BN layers for electronic applications.     

On the growth of KDP and KDP:Ni single crystals by the gel technique

Kinetic aspects of KDP and KDP:Ni single crystal growth in the gel technique are examined; the growth is controlled by volume diffusion. The growth rates of KDP and KDP:Ni are in the same range as those frequently encountered in industrial crystallization.     

Primary and secondary crystallization processes of WSe2 films

WSe₂ thin films are obtained by annealing tungsten and selenium constituents in thin-film form, under selenium pressure. Thin films have been investigated by X-ray analysis, scanning and transmission electron microscopy, and by microprobe analysis. Annealing temperature and time were used as parameters. It is shown that stoichiometric thin films, crystallized in the hexagonal structure, are systematically obtained. The grain-size evolution with increasing temperature and/or annealing time is interpreted in terms of primary and second crystallization processes (crystallization and recrystallization).     

不同籽晶DKDP晶体生长和光学性能研究

籽晶是影响DKDP晶体生长和光学性能的一个重要因素.通过传统降温法,分别利用Z片和[101]晶片作为籽晶,从氘化程度为85%的溶液中生长DKDP晶体并对加工样品进行了相关测试.研究了不同籽晶对DKDP晶体的生长和光学性能的影响.实验表明,[101]晶片籽晶所得DKDP晶体能有效缩短生长周期,晶体损伤阈值提高明显,但光学均匀性和透过性能有所下降.     

不同过饱和度下DKDP晶体生长和缺陷的研究

过饱和度是影响DKDP晶体生长和质量的关键因素. 本文采用“点籽晶”快速生长技术在不同的过饱和度下从氘化程度为75%的溶液中生长DKDP晶体并选取部分样品进行同步辐射X射线形貌术和粉末X射线衍射测试. 研究了不同过饱和度下DKDP晶体的生长和缺陷. 实验证明, DKDP晶体可以在的过饱和度下实现快速生长, 但晶体的缺陷随着过饱和度的增大而增加.     

氧化碲大单晶的加压提拉生长

本文叙述了加压单晶炉的结构,重点介绍了传动机构及利用压力下的模拟体等径控制系统实现 TeO_2大单晶生长的直径自动控制。生长出了等径度和质量良好的以[110]、[100]等为提拉生长方向的 TeO_2大单晶,晶体直径为φ35～85mm,等径长度35～100mm,晶体的最大重量达1.4kg。本文还讨论了影响等径生长优质 TeO_2晶体的某些因素。经多次实验证明,我们设计研制的加压单晶炉是一种既经济又有实用价值的装置。     

ZnO晶体生长新方法研究

基于ZnO-PbF₂高温溶液体系的相关系和析晶行为, 发展了一种通气诱导成核的助熔剂-坩埚下降法生长技术. 通过优化生长参数, 获得了尺寸为φ25mm×5mm的ZnO晶体. 该晶体具有纤锌矿结构, 晶格常数a=0.3252nm, b=0.5209nm. X射线定向确认其择优取向生长方向为[0001]. 实验结果表明, 助熔剂-坩埚下降法是ZnO晶体生长的一条新途径.     

ZnO准一维纳米结构生长形态的演化机理

利用气相生长系统,通过调控实验参数,制备了多种形貌的ZnO准一维结构,如纳米条带、[011-0]和[21-1-0]取向的单侧生齿的梳状纳米条带、微米尺度的梳状结构,由多节状六角棱柱和八角棱柱组装成的微米条带等.通过X射线衍射、扫描电子显微镜及其所加载的能谱分析和背散射电子衍射仪、高分辨透射电子显微镜等分析技术, 对其中具有代表性的介观结构进行了系统的形貌分析和细致的结构解析.分析出基本的结构单元及其复合体, 揭示了显微尺度下ZnO晶体的外形多样性以及其形态演化中的关联和规律,即ZnO纳米条带、梳状结构和多节状微米条带具有晶体结构上的同一性.     

Comparison of Cellular Automaton and Phase Field Models to Simulate Dendrite Growth in Hexagonal Crystals

A cellular automaton (CA)-finite element (FE) model and a phase field (PF)-FE model were used to simulate equiaxed dendritic growth during the solidification of hexagonal metals.In the CA-FE model,the conservation equations of mass and energy were solved in order to calculate the temperature field,solute concentration,and the dendritic growth morphology.CA-FE simulation results showed reasonable agreement with the previously reported experimental data on secondary dendrite arm spacing (SDAS) vs cooling rate.In the PF model,a PF variable was used to distinguish solid and liquid phases similar to the conventional PF models for solidification of pure materials.Another PF variable was considered to determine the evolution of solute concentration.Validation of both models was performed by comparing the simulation results with the analytical model developed by Lipton-Glicksman-Kurz (LGK),showing quantitatively good agreement in the tip growth velocity at a given melt undercooling.Application to magnesium alloy AZ91 (approximated with the binary Mg-8.9 wt% Al) illustrates the difficulty of modeling dendrite growth in hexagonal systems using CA-FE regarding mesh-induced anisotropy and a better performance of PF-FE in modeling multiple arbitrarily-oriented dendrites growth.     

A hydrothermal approach to flake-shaped CdS single crystals

The morphological transformation process from CdS nanorods to hexagonal CdS flakes was investigated in detail by transmission electron microscope (TEM). The CdS flakes were liable to form with the alkalinity being increased and the reaction time being prolonged. Using this transformation, hexagonal CdS flakes with diameters of 0.3 cm were grown via a recrystallization process in 6 mol/L sodium hydroxide solution at 250 °C. And the formation mechanism of CdS flakes is suggested based on the growth habits of polar crystals under proper basic hydrothermal conditions.