Numerical Simulation of Transient Thermal Field in Laser Melting Process

Numerical simulation of thermal field was studied in laser processing. The 3-D finite element model of transient thermal calculation is given by thermal conductive equation. The effects of phase transformation latent are considered. Numerical example is given to verify the model. Finally the real example of transient thermal field is given.

圆柱绕流旋涡运动及离散涡方法

<正> 圆柱绕流问题最早是Stokes于1901年开始研究的。最近15年来,以圆柱非定常分离流为代表的非流线型物体绕流及伴随的旋涡运动受到很大重视。这主要是因为它与近代海洋工程、航空工程及工业空气动力学中不少急待解决的流体力学问题有关。例如,海洋平台桩柱、导管等所受的波浪力分析中,当结构物特征尺寸与波浪的波长相比较小(且K-C数在5—30范围内)时,柱后的旋涡脱落及它所诱导的力(引起构件的振动)是主要应考虑的问题,要给出正确的预计。导弹无侧滑大攻角飞行时背风面分离涡面的不对称运动产生不希望有的偏航力,分析其产生的机理,预计其大小并设法抑制这种力十分重要。此外还有气动噪声及高耸建筑物的风振问题等等。这些复杂问题的解决常常要以低速高雷诺数圆柱非定常分离流研究为基础。

花楸属的系统学初步研究

花楸属隶属于蔷薇科苹果亚科，全世界约有100种左右，主要分布在北半球的亚洲、欧洲和北美洲。广义的花楸属包括下述六个亚属，即Subg. Aria, Subg. Chamaemespilus, Subg. Micromeles, Subg. Torminalis, Subg. Cormus, Subg. Sorbus。由于缺乏野外工作和标本不够丰富等原因，部分亚属的分类处理尚不能令人满意。同时，该属的范围及其属下部分类群的系统位置存在一定的争议。本文对广义花楸属的属下类群进行了研究，通过分支分析和表征分析以及分子系统学两方面的研究，对该属的属下类群的系统位置进行了探讨。 1、分支分析和表征分析 选取了广义花楸属（Sorbus L.）属下6个亚属的34种植物作为研究对象，并以Pyrus, Crataegus, Photinia, Malus和Cotoneaster作为外类群来研究花楸属属下类群的分类关系，对该属典型的30个形态性状数据进行编码，进行了分支分析和表征分析。结果表明，广义花楸属是一个非单系类群，外类群完全镶嵌于其中。广义花楸属属下的6个亚属各自也不能形成单系，只有花楸亚属（Subg. Sorbus）形成不严格的单系。因此，我们初步认为，花楸属应该仅包含复叶类群（Subg. Sorbus），或者还会有Subg. Cormus。由于其余的4个亚属尚不能形成规则的单系或聚类，因此进一步的研究宜采用分类群的更多性状或运用分子系统学的途径。 2、分子系统学 通过对花楸属23个种类的两个DNA片段：核nrDNA的ITS和叶绿体cpDNA的trnL-F进行简约分析，构建了广义花楸属5个亚属及其与近缘属的系统发育树。结果显示，由于外类群大多镶嵌于广义花楸属中，故花楸属不能形成一个单系。其中单叶类群Subg. Aria聚类较明显，并且得到了很高的支持率，而且该亚属与外类群Crataegus和Cotoneaster的关系要比它与其它亚属的关系更近，至于其是否是独立的属还有待进一步的研究。该属中的复叶类群Subg. Sorbus在图中并不能形成严格的单系，其分类位置值得重新考虑。

固氮蓝藻培养和应用的结果与展望

<正> 固氮蓝藻是一类低等原核植物。它能进行固氮作用,固定空气中分子态氮成为氮素化合物;又能进行光合作用,将二氧化碳变为碳素化合物并放出氧气,为自身的固氮提供能源和还原剂。由于具有以上功能,它们适应性很强往往是贫瘠土壤的先驱植物,因此,很早就引起了人们的重视。早在1889年Frank首先发现生长着蓝藻的土壤在光照下,氮化合物的含量有所增加;1901年Beijerinck发现在仅有一点土壤的缺氮水中的小链鱼腥藻

中国枝角类四新记录亚种(鳃足纲:双甲目)

本文描述了采自四川等省的、我国首次发现的4个枝角类亚种,即美弧网纹溞Ceriodaphnia pulchella pseudohamata Bowkiewcz,1925,西方笔纹溞Graptoleberis testudinaria occidentalis Sars,1901,无常平直溞Pleuroxus laevis incertus Brehm,1934以及宽尾平直潘P.aduncus latic-audatus Brehm,1933。解剖观察描述了盘肠溞科枝角类的头孔,并首次将我国淡水枝角类的分类

Strain-induced anodization of SiGe/Si multiple layers to form high density SiGe/Si heterogeneous nanorods

Nation Natural Science Foundation of China 50672079 60676027 60837001 60776007; National Basic Research Program of China (973 Program) 2007CB613404; China-MOST International Sci & Tech Cooperation and Exchange 2008DFA51230

Structural study of YSi1.7 layers formed by channeled ion beam synthesis

High quality YSi1.7 layers (chi(min) of Y is 3.5%) have been formed by 60 keV Y ion implantation in Si (111) substrates to a dose of 1.0 x 10(17)/cm(2) at 450 degrees C using channeled ion beam synthesis (CIBS). It shows that, compared to the conventional nonchanneled ion beam synthesis, CIBS is beneficial in forming YSi1.7 layers with better quality due to the lower defect density created in the implanted layer. Rutherford backscattering/channeling and x-ray diffraction have been used to study the structure and the strain of the YSi1.7 layers. The perpendicular and parallel elastic strains of the YSi1.7 epilayer are e(perpendicular to) = -0.67% +/- 0.02% and e(parallel to) = +1.04% +/- 0.08%. The phenomenon that a nearly zero mismatch of the YSi1.7/Si (111) system results in a nonpseudomorphic epilayer with a rather large parallel strain relative to the Si substrate (epsilon(parallel to) = +1.09%) is explained, and the model is further used to explain the elastic strain of epitaxial ErSi1.7 and GdSi1.7 rare-earth silicides. (C) 1998 American Vacuum Society.

Gasoline-range hydrocarbon synthesis over cobalt-based Fischer-Tropsch catalysts supported onSiO2/HZSM-5

Two types of SiO2 with different mesopore size and HZSM-5 zeolite were used to prepare hybrid supported cobalt-based catalysts. The textual and structural properties of the catalysts were studied using N-2 physisorption, X-ray diffraction (XRD), and H-2 temperature-programmed reduction (TPR) techniques. Fischer-Tropsch synthesis (FTS) performances of the catalysts were carried out in a fixed-bed reactor. The combination effects of the meso- and micropores of the supports as well as the interaction between supports and cobalt particles on FTS activity are discussed. The results indicate that the catalyst supported on the tailor-made SiO2 and HZSM-5 hybrid maintained both meso- and micropore pores during the preparation process without HZSM-5 particles agglomerating. The mesopores provided quick mass transfer channels, while the micropores contributed to high metal dispersion and accelerated hydrocracking/hydroisomerization reaction rate. High CO conversion of 83.9% and selectivity to gasoline-range hydrocarbons (C-5-C-12) of 55%, including more than 10% isoparaffins, were achieved simultaneously on this type of catalyst.