Growth of Silicon Carbide Bulk Crystals by Physical Vapor Transport Method and Modeling Efforts in the Process Optimization

Silicon carbide bulk crystals were grown in an induction-heating furnace using the physical vapor transport method. Crystal growth modeling was performed to obtain the required inert gas pressure and temperatures for sufficiently large growth rates. The SiC crystals were expanded by designing a growth chamber having a positive temperature gradient along the growth interface. The obtained 6H-SiC crystals were cut into wafers and characterized by Raman scattering spectroscopy and X-ray diffraction, and the results showed that most parts of the crystals had good crystallographic structures.

Novel Mechanical Behavior Of Zno Nanorods

A novel stress-strain relation with two stages of linear elastic deformation is observed in [0 0 0 1]-oriented ZnO nanorods under uniaxial tensile loading. This phenomenon results from a phase transformation from wurtzite (WZ, P6(3)mc space group) to a body-centered tetragonal structure with four-atom rings (denoted as BCT-4) belonging to the P4(2)/mnm space group. The analysis here focuses on the effects of nanorod size and temperature on the phase transformation and the associated mechanical behavior. It is found that as size is increased from 19.5 to 45.5 angstrom, the critical stress for nucleation of the transformation decreases by 25% from 21.90 to 16.50 GPa and the elastic moduli of the WZ- and BCT-4-structured nanorods decrease by 24% (from 299.49 to 227.51 GPa) and 38% (from 269.29 to 166.86 GPa), respectively. A significant temperature effect is also observed, with the critical stress for transformation initiation decreasing 87.8% from 17.89 to 2.19 GPa as temperature increases from 300 to 1500 K. (c) 2007 Elsevier B.V. All rights reserved.

日冕磁环中虹吸流动的稳定性

在日冕磁环中观测到速度为100km/sec的等离子体流动。本文采用平面和柱面模型讨论了这个虹吸流动的稳定性。用数值方法计算了色散方程,得到了不稳定性增长率与扰动波的频率。主要结论是:磁场是一个致稳因素,流速是一个不稳定因素。本文讨论了日冕磁环中是否存在定常的高速虹吸流动的问题。

The plane problem of collinear rigid lines under arbitrary loads

The elastic plane problem of collinear rigid lines under arbitrary loads is dealt with. Applying the Riemann-Schwarz symmetry principle integrated with the analysis of the singularity of complex stress functions, the general formulation is presented, and the closed-form solutions to several problems of practical importance are given, which include some published results as the special cases. Lastly the stress distribution in the immediate vicinity of the rigid line end is examined.

利用遗传算法进行水平井水平段长度优化设计

在油藏开发过程中，随着水平井水平段长度的增加，水平井段与油藏的接触面也随之增加，同时井筒内流体的流动阻力也增加，从而导致水平井的产液量减少，即水平井的最终产量是2个相互制约的因素共同作用的结果。利用遗传算法全局寻优的特点，结合一种计算无限导流和有限导流情况下水平井产量的井筒／油藏耦合模型，提出了一种用于水平井水平段长度的优化设计方法。该方法可以在不同的油藏地质模型中使用。

伴随微孔洞生长的裂纹弹塑性定常扩展

从传统的断裂过程区模型出发研究微孔洞的萌生和生长对材料总体断裂韧性的影响，通过采用Gurson模型，建立塑性增量本构关系，然后针对定常扩展情况直接进行分析。孔洞对材料断裂韧性的影响由本构关系刻划，而在孔洞汇合模型中，上述影响则由断裂过程区的改变来体现。研究结果显示出，微孔洞的萌生与生长将使材料断裂韧性降低并随着初始孔洞体积百分比的增大，裂纹呈脆性扩展且发生损伤破坏。

微重力下不可燃气体对火焰沿薄燃料表面传播的影响

数值模拟研究表明,不可燃气体对火焰传播有重要的影响。当不可燃气体为N_2时,在分析中可不考虑辐射再吸收的影响,热传导是火焰传播的主要驱动力。而当不可燃气体为CO_2时,辐射再吸收对火焰结构和传播速度有较大的影响。在静止微重力下,火焰对燃料表面的热辐射和热传导在火焰传播机理中的作用几乎相同,火焰传播速度具有最大值。随着逆流速度的增大,辐射再吸收对火焰传播的影响逐渐减弱,热传导又成为火焰传播的主要驱动力。

加卸载响应比理论用于矿震预测的初步研究

加卸载响应比(LURR)理论是～种前景很好的中期地震预测方法，通常在强烈地震发生前的数月至l～2年I．URR出现高值，因而I。URR可以作为强烈天然地震的前兆，用此方法曾经成功地预测过Northridge地震(1 994年1月17日，M6．7，美国加州)，Nanto地震(1 996年9月11日，M6．6，日本)及不少发生在中国的天然地震。用房山煤矿1 992年8月至1993年7月的微震资料,计算了全年内7组M>2．1矿震前的加卸载响应比γ值，其中5组矿震前γ值均明显大于l(γ≥2 9)。以上结果表明，加卸载响应比理论有可能用于矿震的预测。

Edge and Size Effects in Micro Composite Structure

Because of the load transfer effect of interface layer, the stress distribution inside the composite structure of film/substrate can be very different from the Timoshenko's model. In this paper, we give the derivation and analysis of such load transfer effect of shear-lag (S-L) model. The micro-structure size (boundary conditions) effect together with interface load transfer effect becomes more and more important as the microstructure size including the three dimensions of thickness, width and length shrinks. The microstructure size is also responsible for the so-called edge-induced stress. The edge effect and difference of S-L model and Timoshenko model are also demonstrated.

郭永怀文集

Short-range-order effects on intrinsic plasticity of metallic glasses

Through a systematical analysis of the elastic moduli for 137 metallic glasses (MGs) and 56 polycrystalline metals, we use a simple model developed by Knuyt et al. [J. Phys. F: Met. Phys. 16 (1986) p.1989; Phil. Mag. B 64 (1991) p.299] based on a Gaussian distribution for the first-neighbor distance to reveal the short-range-order (SRO) structural conditions for plasticity of MGs. It is found that the SRO structure with dense atomic packing, large packing dispersion and a significant anharmonicity of atomic interaction within an MG is favorable for its global plasticity. Although these conditions seem paradoxical, their perfect matching is believed to be a key for designing large plastic bulk MGs not only in compression but also in tension.

Simultaneous three-photon absorption induced ultraviolet upconversion in Pr3+:Y2SiO5 crystal by femtosecond laser irradiation

Near-infrared to ultraviolet upconversion luminescence was observed in the Pr3+ :Y2SiO5 crystal with 120 fs, 800 mn infrared laser irradiation. The observed emissions at around 270 nm and 305 nm could be assigned to 5d -> 4f transitions of Pr3+ ions. The relationship between the upconversion luminescence intensity and the pump power of the femtosecond laser reveals that the UV emission belongs to simultaneous three-photon absorption induced upconversion luminescence. (c) 2007 Elsevier B.V. All rights reserved.

Fabrication of microfluidic optical waveguides on glass chips with femtosecond laser pulses

We describe the fabrication of microfluidic channel structures on the surface of a borosilicate glass slide by femtosecond laser direct writing for optical waveguide application. Liquid with a variable refractive index is fed into the microchannel, serving as the core of the waveguide. We demonstrate that either a multimode or a single-mode waveguide can be achieved by controlling the refractive index of the liquid. (C) 2007 Optical Society of America

Spectroscopic properties and Judd-Ofelt theory analysis of Dy3+ doped oxyfluoride silicate glass

Dy3+ doped oxyfluoride silicate glass was prepared and its optical absorption, 1.3 mu m emission, and upconversion luminescence properties were studied. Furthermore, the Judd-Ofelt [Phys. Rev. 127, 750 (1962); J. Chem. Phys. 37, 511 (1962)] intensity parameters, oscillator strengths, spontaneous transition probability, fluorescence branching ratio and radiative lifetime were calculated by Judd-Ofelt theory, while stimulated emission cross section of H-6(9/2)+F-6(11/2)-> H-6(15/2) transition was calculated by McCumber theory [Phys. Rev. A. 134, 299 (1964)]. According to the obtained Judd-Ofelt intensity parameters Omega(2)=2.69x10(-20) cm(2), Omega(4)=1.64x10(-20) cm(2), and Omega(6)=1.64x10(-20) cm(2), the radiative lifetime was calculated to be 810 mu s for 1.3 mu m emission, whose full width at half maximum and sigma(e) were 115 nm and 2.21x10(-20)cm(2), respectively. In addition, near infrared to visible upconversion luminescence was observed and evaluated. The results suggest that Dy3+ doped oxyfluoride silicate glass can be used as potential host material for developing broadband optical amplifiers and laser applications.