Modeling of ammonothermal growth of gallium nitride single crystals

Ammonothermal growth of GaN crystals with a retrograde solubility has been modeled and simulated here using fluid dynamics, thermodynamics and heat transfer models. The nutrient is considered as a porous media bed and the flow in the porous charge is simulated using the Darcy-Brinkman-Forchheimer model. The resulting governing equations are solved using the finite volume method. For the case of retrograde solubility, the charge is put above the baffle. The temperature difference between the dissolving zone and growth zone is found smaller than that applied on the sidewall of autoclave. The baffle opening has a strong effect on the nutrient transport and supersaturation of GaN species in the growth zone.     

Meshless local Petrov-Galerkin method for continuously nonhomogeneous linear viscoelastic solids

A meshless method based on the local Petrov-Galerkin approach is proposed for the solution of quasi-static and transient dynamic problems in two-dimensional (2-D) nonhomogeneous linear viscoelastic media. A unit step function is used as the test functions in the local weak form. It is leading to local boundary integral equations (LBIEs) involving only a domain-integral in the case of transient dynamic problems. The correspondence principle is applied to such nonhomogeneous linear viscoelastic solids where relaxation moduli are separable in space and time variables. Then, the LBIEs are formulated for the Laplace-transformed viscoelastic problem. The analyzed domain is covered by small subdomains with a simple geometry such as circles in 2-D problems. The moving least squares (MLS) method is used for approximation of physical quantities in LBIEs.     

用于大功率半导体激光器的新型焊料

在大功率半导体激光器列阵及叠阵的组装中,焊料的选择是极其关键的,因为焊料直接参与对激光器的导电、导热激光器所需的电流全部从焊料通过,而半导体激光器列阵或叠阵工作时电流是很大的,可达50A～100A。同时半导体激光器工作时产生的热量非常大,如焊料的导热性不好,由于电流的热效应,就会在焊料上产生巨大的热量,使焊料熔化。文中研制了一种新型的焊料,这种焊料在两层铟之间蒸镀几层金,焊料由钨/镍/金/铟/铜等多层金属构成。利用这种焊料研制出脉冲功率达100W的半导体激光器列阵。     

T-stress near the tips of a cruciform crack with unequal arms

The T-stress near the tips of a crack of cross shape embedded in an isotropic elastic solid is analyzed. The integral transform technique is employed to convert the associated boundary value problem to a system of singular integral equations. According to the stress difference method, T-stresses can be expressed as a sum of an integral involving crack opening displacement (COD) and applied loading at infinity. Obtained results indicate that, in addition to applied loading, T-stresses at the horizontal (vertical) crack tips depend on the COD of the vertical (horizontal) crack surface. COD plays a leading role in determining T-stresses, in particular for a cruciform crack of two crack-arm lengths of the same order. Moreover, T-stresses for a single-crack limiting case can be recovered from the present results as the length of one arm approaches zero. For a biaxial tension of the same magnitude, T-stresses are present for a cruciform crack, but absent for a single crack. Finally, for several cases of interest, T-stresses for a cruciform crack are evaluated and compared with those for a single crack, and the influence of the ratio of two crack-arm lengths b/a and the COD on the T-stress of a cruciform crack is examined.     

Bioelectrodynamics in living organisms

This article introduces an interdisciplinary subject of bioelectrodynamics in living organisms and its related research challenges and opportunities. Bioelectrodynamics in living organisms is aimed to reveal critical roles of electromagnetism and mechanics in biology, to correlate biophysical functions of living organisms with biochemical processes at the cellular level, and to introduce theoretical basis and methodology, such as modeling and simulations, for stimulating technical innovations and promoting technology development in biomedicine as well as for the study of human healthcare issues related to environments among others in our modern society. The article reviews some important issues in bioelectrodynamic modeling. This includes the modeling of living cells, blood, bones and soft tissues that may have unique properties, such as active control, regulation and remodeling capabilities that are completely different from those of conventionally man-made materials. Possible biological effects and potential biomedical usages of endogenous and exogenous electromagnetic fields and mechanical stresses in living organisms are also reviewed, which indicate promising future of biomedical imaging and therapeutic methods based on bioelectrodynamic techniques. The fact that living organisms may have well-organized structures, actively controlled actions and responses, extremely sensitivity in electromagnetic fields and mechanical actions, and amazing signal amplification functions may not only cause complexity and variety of the biological world, but also create opportunities for technical innovations in biomedicine to improve future quality of human life.     

Numerical study of an unsteady free convective magnetohydrodynamic flow of a dissipative fluid along a vertical plate subject to a constant heat flux

A new method is presented to solve the transient free convection MHD flow of a dissipative fluid along a semi-infinite vertical plate with mass transfer, the surface of which is exposed to a constant heat flux. The non-linear system of partial differential equations is numerically solved by means of the network simulation method, based on the thermo–electric analogy. This method permits the direct visualisation and evolution of the local and/or integrated transport variables (temperatures, velocities, concentrations and fluxes) at any point or section of the medium. At the same time, the solution for both transient and steady-state problems is obtained, the only requirement being finite-difference schemes for the spatial variable, while its programming does not involve manipulation of the sophisticated mathematical software that is inherent in other numerical methods. The technique is always stable and convergent. Velocity, temperature and concentration profiles, local skin-friction, local Nusselt and local Sherwood numbers are plotted for air. The influence of the viscous dissipation, buoyancy ratio parameter, Schmidt number and magnetic parameter on heat and mass transfer and on the time needed to reach the steady-state are discussed.     

Multistep impregnation method for incorporation of high amount of titania into SBA-15

A multistep impregnation method was employed to incorporate high amount of titania into the mesoporous SBA-15 silica. No damage to the SBA-15 silica mesostructures was caused by the loading of titania in every cycle. The existence of titania small nanodomains were confirmed to be present by Raman spectra and UV–vis DRS measurements. High dispersion of them was realized via this method according to the results of low-angle X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and N₂ sorption measurements. Importantly, no blockage of mesostructures was acknowledged with titania content up to 24.4 wt.%. In comparison, normally used one-step impregnation method led to serious blockage of mesopores as the results of formation of bulk titania particles in the mesochannels. Photo-activity test for the removal of oestrogen showed the superiority of the materials synthesized by multistep impregnation method to one-step impregnation method.     

Semiconductor/porous silica glass nanocomposites via the single-source precursor approach

The utilization of single-source molecular precursor approach to obtain II–VI and IV–VI semiconductors encapsulated in porous Vycor glass (PVG) is described. The procedure is based on the impregnation of cadmium and lead(II) diethyl-dithiocarbamate complexes, Cd(S₂CNEt₂)₂ and Pb(S₂CNEt₂)₂, inside the porous environment of PVG followed by a thermal treatment of the glass. The pyrolysis of the impregnated precursor gives rise to binary semiconductors CdS and PbS, respectively. The impregnation step is driven by interactions between the precursors and active sites located at glass pore surfaces. After completing the impregnation–decomposition cycle, it was found that the active glass sites were regenerated, making new cycles possible. The amount of encapsulated semiconductor increases linearly as a function of the number of cycles. Nanocomposites obtained after 1–10 cycles were prepared and characterized by optical spectroscopy, X-ray diffraction powder and transmission electron microscopy.     

Formation of lanthanum beryllate real structure under different crystallization conditions

The aim of this paper is to characterize the major structural defects of lanthanum beryllate single crystals grown by the Czochralski method, including those doped with rare-earth elements, and to reveal their relationship to specific properties of the crystal structure of La₂Be₂O₅ and with their crystallization conditions. As a basic method for research, we used transmission X-ray topography. It was established that the defect state of La₂Be₂O₅ crystals prepared by this method can be caused by different types of dislocations and their ordered assemblies, solid-phase inclusions of crucible metal and eutectically co-crystallizing phases, as well as by face growth sectors with elevated content of dopant. We show a possibility for growing single crystals of doped lanthanum beryllate having a minimum quantity of structural defects that could be suitable for manufacture of high quality laser rods.     

Simulation of cyclic stress/strain evolutions for multiaxial fatigue life prediction

This study deals with simulation for cyclic stress/strain evolutions and redistributions, and evaluation of fatigue parameters suitable for estimating fatigue lives under multiaxial loadings. The local cyclic elastic–plastic stress–strain responses were analyzed using the incremental plasticity procedures of ABAQUS finite element code for both smooth and notched specimens made of three materials: a medium carbon steel in the normalized condition, an alloy steel quenched and tempered and a stainless steel, respectively. Emphasis is on the studying of ‘intelligent’ material behaviors to resist fracture, such as stress redistribution and relaxation through plastic deformations, etc. For experimental verifications, a series of tests of biaxial low cycle fatigue composed of tension/compression with static and cyclic torsion were carried out on a biaxial servo-hydraulic testing machine (Instron 8800). Different multiaxial loading paths were used to verify their effects on the additional cyclic hardening. The comparisons between numerical simulations and experimental observations show that the FEM simulations allow better understanding on the evolutions of the local cyclic stress–strain and it is shown that strong interactions exist between the most stressed material element and its neighboring material elements in the plastic deformations and stress redistributions. Based on the local cyclic elastic–plastic stress–strain responses, the energy-based multiaxial fatigue damage parameters are applied to correlating the experimentally obtained lives. Improved correlations between the predicted and the experimental results are shown. It is concluded that the improvement of fatigue life prediction depends not only on the fatigue damage models, but also on the accurate evaluations of the cyclic elasto-plastic stress/strain responses.