Quasi-aligned ZnO nanotubes grown on Si substrates

Quasi-aligned ZnO nanotubes have been grown on silicon substrates by metalorganic chemical vapor deposition without using any catalyst. Two kinds of ZnO nanotubular structures were found: Nanotubes with single walls and nanotubes with double walls. The nanotubes were grown along the [001] direction. Room-temperature photoluminescence measurements of the ZnO nanotubes indicate strong ultraviolet emission and weak green emission. A new growth mode for these ZnO nanotubes is proposed, which can be used to prepare other nanotubular structures. (c) 2005 American Institute of Physics.

Metal catalysis-free, direction-controlled planar growth of single-crystalline alpha-Si3N4 nanowires on Si(100) substrate

Single-crystalline alpha-Si3N4 nanowires are controlled to grow perpendicular to the wet-etched trenches in the SiO0.94 film on the plane of the Si substrate without metal catalysis. A detailed characterization is carried out by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photoluminescence at 600 nm from alpha-Si3N4 nanowires is attributed to the recombination at the defect state formed by the Si dangling bond N3 equivalent to Si-center dot. The growth mechanism is considered to be related to the catalysis and nitridation of SiO nanoclusters preferably re-deposited around the inner corner of the trenches, as well as faster Si diffusion along the slanting side walls of the trenches. This simple direction-controlled growth method is compatible with the CMOS process, and could facilitate the fabrication of alpha-Si3N4 nanoelectronic or nanophotonic devices on the Si platform.

基于动态抢占阈值的实时调度

具有抢占阈值的调度算法集非抢占调度和纯抢占调度的特点 ,既减少了由于过多的随意抢占造成的CPU资源浪费 ,又保证了一定的任务截止期错失率及CPU资源利用率 已有的工作基本集中于讨论任务集完全给定 ,任务数、任务的优先级及任务的抢占阈值在调度前已完全确定 ,而且要求不同的任务具有不同的优先级 提出的具有抢占阈值的调度算法 ,完全放松了对这些条件的限制 ,即任务的个数不确定 ,任务的优先级及其抢占阈值在调度过程中可以动态地变化 最后以常用的LSF调度策略为例 ,结合动态的抢占阈值进行仿真 仿真结果表明 ,对于不确定的任务集、任务优先级和抢占阈值 ,利用具有抢占阈值的动态调度算法 ,降低了任务截止期错失率、提高了CPU的有效使用率

一种任务优先级的综合设计方法

提出了一种基于优先级表设计的调度算法.将任务的相对截止期和空闲时间这两个特征参数结合起来,综合设计任务的优先级表,使得截止期越早或空闲时间越短,任务的优先级越高,而且任务的优先级由相对截止期和空闲时间惟一确定.对于任意一个任务,可通过对设计的优先级表进行二元多点插值获得相应任务的惟一优先级.与传统的EDF和LSF算法进行仿真比较,仿真结果表明,通过优先级表设计方法来确定任务的优先级,提高了任务调度的成功率,降低了任务截止期的错失率.该方法可应用于实时系统中实时任务的动态调度中.

改进的最小空闲时间优先调度算法

最小空闲时间优先(least slack first,简称LSF)算法结合任务执行的缓急程度来给任务分配优先级.任务所剩的空闲时间越少,就越需要尽快执行.然而,LSF算法造成任务之间的频繁切换或严重的颠簸现象,增大了系统开销,并限制了其应用.在调度策略中设置抢占阈值可以减少任务之间的切换,但现有的抢占阈值设置方法因受到固定优先级的限制而不适用于LSF算法.为了减轻LSF算法的颠簸现象,基于抢占阈值的思想,提出适用于LSF算法的抢占阈值分配方法,动态地给每个任务配置抢占阈值.任务的抢占阈值是随着任务执行的缓急程度不同而动态地变化的,而且不受任务个数的限制.仿真结果表明,通过对LSF算法的改进,任务之间的切换大大减少,同时降低了任务截止期错失率.该改进型算法对设计和实现实时操作系统具有一定的参考价值.

模糊动态抢占调度算法

针对不确定任务特征,提出应用模糊理论进行动态抢占调度.用语言模糊集来描述任务的不确定特征和不同的优先级等级,利用最大隶属度原理确定任务的优先级等级,采用优先调度高优先级等级任务的调度策略提高重要任务的调度成功率,实现具有不确定任务特征的抢占调度.与传统的EDF和LSF算法相比较,仿真表明,所提算法能够提高重要任务的调度成功率,并降低重要任务的截止期错失率;同时,任务间的平均切换次数大大小于LSF的平均切换次数,而与EDF保持相当.该方法可应用于计算机控制系统的控制任务调度,并借鉴于其它具有不确定任务特征或具有有限优先级等级的实时调度问题研究中.

Boundary conditions at the solid-liquid surface over the multiscale channel size from nanometer to micron

The boundary condition at the solid surface is one of the important problems for the microfluidics. In this paper we study the effects of the channel sizes on the boundary conditions (BC), using the hybrid computation scheme adjoining the molecular dynamics (MD) simulations and the continuum fluid mechanics. We could reproduce the three types of boundary conditions (slip, no-slip and locking) over the multiscale channel sizes. The slip lengths are found to be mainly dependent on the interfacial parameters with the fixed apparent shear rate. The channel size has little effects on the slip lengths if the size is above a critical value within a couple of tens of molecular diameters. We explore the liquid particle distributions nearest the solid walls and found that the slip boundary condition always corresponds to the uniform liquid particle distributions parallel to the solid walls, while the no-slip or locking boundary conditions correspond to the ordered liquid structures close to the solid walls. The slip, no-slip and locking interfacial parameters yield the positive, zero and negative slip lengths respectively. The three types of boundary conditions existing in "microscale" still occur in "macroscale". However, the slip lengths weakly dependent on the channel sizes yield the real shear rates and the slip velocity relative to the solid wall traveling speed approaching those with the no-slip boundary condition when the channel size is larger than thousands of liquid molecular diameters for all of the three types of interfacial parameters, leading to the quasi-no-slip boundary conditions.

Fabrication of Silicon-Based Template-Assisted Nanoelectrode Arrays and Ohmic Contact Properties Investigation

A convenient fabrication technology for large-area, highly-ordered nanoelectrode arrays on silicon substrate has been described here, using porous anodic alumina (PAA) as a template. The ultrathin PAA membranes were anodic oxidized utilizing a two-step anodization method, from Al film evaporated on substrate. The purposes for the use of two-step anodization were, first, improving the regularity of the porous structures, and second reducing the thickness of the membranes to 100 similar to 200 nm we desired. Then the nanoelectrode arrays were obtained by electroless depositing Ni-W alloy into the through pores of PAA membranes, making the alloy isolated by the insulating pore walls and contacting with the silicon substrates at the bottoms of pores. The Ni-W alloy was also electroless deposited at the back surface of silicon to form back electrode. Then ohmic contact properties between silicon and Ni-W alloy were investigated after rapid thermal annealing. Scanning electron microscopy (SEM) observations showed the structure characteristics, and the influence factors of fabrication effect were discussed. The current voltage (I-V) curves revealed the contact properties. After annealing in N-2 at 700 degrees C, good linear property was shown with contact resistance of 33 Omega, which confirmed ohmic contacts between silicon and electrodes. These results presented significant application potential of this technology in nanosize current-injection devices in optoelectronics, microelectronics and bio-medical fields.

SIMULATION OF LATERAL CONFINEMENT IN VERY NARROW CHANNELS

A theoretical study is presented of the lateral confinement potential (CP) in the very narrow mesa channels fabricated in the conventional two-dimensional (2D) electron gas in GaAs-AlxGa1-xAs heterostructures. The ID electronic structures are calculated in the framework of the confinement potential: V(x) = m* omega0(2)x2/2 for Absolute value of x walls are more realistic than this complex potential. The experimental result for a wire of W(str) = 250 nm cannot be explained within the composite-well model including these two cases. Thus how to explain the experiment for this wire remains an open question.

On the soft wall guiding potentials in realistic quantum waveguides

A transfer matrix method is presented for the study of electron conduction in a quantum waveguide with soft wall lateral confinement. By transforming the two-dimensional Schrodinger equation into a set of second order ordinary differential equations, the total transfer matrix is obtained and the scattering probability amplitudes are calculated. The proposed method is applied to the evaluation of the electron transmission in two types of cavity structure with finite-height square-well confinement. The results obtained by our method, which are found to be in excellent agreement with those from another transfer matrix method, suggest that the infinite square-well potential is a good approximation to finite-height square-well confinement for electrons propagating in the ground transverse mode, but softening of the walls has an obvious effect on the electron transmission and mode-mixing for propagating in the excited transverse mode. (C) 1996 American Institute of Physics.

Fluorescence Hydrogen Peroxide Probe Based on a Microstructured Polymer Optical Fiber Modified with a Titanium Dioxide Film

A highly sensitive microstructured polymer optical fiber (MPOF) probe for hydrogen peroxide was made by forming a rhodamine 6G-doped titanium dioxide film on the side walls of array holes in an MPOF. It was found that hydrogen peroxide only has a response to the MPOF probe in a certain concentration of potassium iodide in sulfuric acid solution. The calibration graph of fluorescence intensity versus hydrogen peroxide concentration is linear in the range of 1.6 x 10(-7) mol/L to 9.6 x 10(-5) mol/L. The method, with high sensitivity and a wide linear range, has been applied to the determination of trace amounts of hydrogen peroxide in a few real samples, such as rain water and contact lens disinfectant, with satisfactory results.

Research on the mechanics of underwater supersonic gas jets

An experimental research was carried out to study the fluid mechanics of underwater supersonic gas jets. High pressure air was injected into a water tank through converging-diverging nozzles (Laval nozzles). The jets were operated at different conditions of over-, full-and under-expansions. The jet sequences were visualized using a CCD camera. It was found that the injection of supersonic air jets into water is always accompanied by strong flow oscillation, which is related to the phenomenon of shock waves feedback in the gas phase. The shock wave feedback is different from the acoustic feedback when a supersonic gas jet discharges into open air, which causes screech tone. It is a process that the shock waves enclosed in the gas pocket induce a periodic pressure with large amplitude variation in the gas jet. Consequently, the periodic pressure causes the jet oscillation including the large amplitude expansion. Detailed pressure measurements were also conducted to verify the shock wave feedback phenomenon. Three kinds of measuring methods were used, i.e., pressure probe submerged in water, pressure measurements from the side and front walls of the nozzle devices respectively. The results measured by these methods are in a good agreement. They show that every oscillation of the jets causes a sudden increase of pressure and the average frequency of the shock wave feedback is about 5-10 Hz.

Effects of convection and solid wall on the diffusion in microscale convection flows

The diffusive transport properties in microscale convection flows are studied by using the direct simulation Monte Carlo method. The effective diffusion coefficient D is computed from the mean square displacements of simulated molecules based on the Einstein diffusion equation D = x2 t /2t. Two typical convection flows, namely, thermal creep convection and Rayleigh– Bénard convection, are investigated. The thermal creep convection in our simulation is in the noncontinuum regime, with the characteristic scale of the vortex varying from 1 to 100 molecular mean free paths. The diffusion is shown to be enhanced only when the vortex scale exceeds a certain critical value, while the diffusion is reduced when the vortex scale is less than the critical value. The reason for phenomenon of diffusion reduction in the noncontinuum regime is that the reduction effect due to solid wall is dominant while the enhancement effect due to convection is negligible. A molecule will lose its memory of macroscopic velocity when it collides with the walls, and thus molecules are hard to diffuse away if they are confined between very close walls. The Rayleigh– Bénard convection in our simulation is in the continuum regime, with the characteristic length of 1000 molecular mean free paths. Under such condition, the effect of solid wall on diffusion is negligible. The diffusion enhancement due to convection is shown to scale as the square root of the Péclet number in the steady convection regime, which is in agreement with previous theoretical and experimental results. In the oscillation convection regime, the diffusion is more strongly enhanced because the molecules can easily advect from one roll to its neighbor due to an oscillation mechanism. © 2010 American Institute of Physics. doi:10.1063/1.3528310

Study on Mechanics Parameters of Unloaded Rock Mass about the Underground Powerhouse of Jin-ping Hydroelectric Station

After the excavation of Jinping underground cavern, a strong expansion appears along the unloading direction of the rock mass, mainly in the type of tensile rupture, accompanied by shear destruction, unloading resulted in significant deterioration of mechanical properties of rock. Based on the in-site investigation of rock mass structure, via analyzing the acoustic testing data, we identify the unloading range of the side walls and the division of rock types, and carry out with the solution of rock mechanical parameters about different unloading zone, providing geological foundation for the supporting design of the following design of the side walls, at the same time, providing reference for the selection of mechanical parameters of other underground excavation engineering with similar geological conditions.