Numerical modelling of a low power non-transferred arc plasma reactor for methane conversion

Thermal flow characteristics and the methane conversion reaction in a low power arc plasma reactor for efficient storage and transport of methane, which is the main component of shale gas, were simulated. The temperature and velocity distributions were calculated according to the type of discharge gases and arc current level by a self-developed magnetohydrodynamics (MHD) code and a commercial ANSYS-FLUENT code; the transport of chemical species was analyzed as including the chemical reactions of methane conversion. The simulated results were verified by the comparison of calculated and measured arc voltages with permissible low error as under 4%. Three C₂ hydrocarbon gases with ethane (C₂H₆), ethylene (C₂H₄), and acetylene (C₂H₂) were selected as the converted species of methane from experimental data. The mass fraction of C₂ hydrocarbons and hydrogen as the product of the conversion reaction at the reactor was also calculated. Those values show good agreement with the actual experimental results in that the major conversion reaction occurred in C₂H₂ and hydrogen, and the conversions to C₂H₆, C₂H₄, and hydrogen were minor reactions of methane pyrolysis conversion.     

Relation between etching profile and voltage–current shape of sintered SiC etching by atmospheric pressure plasma

Sintered silicon carbide (SiC) was etched by a dielectric barrier discharge source. A high voltage bipolar pulse was used with helium gas for the plasma generation. One stable filament plasma was generated and could be used for SiC etching. As the processing gas (NF₃) mixing rate increased, the width and depth of the etching profile became narrower and deeper. The differentiated V–Q Lissajous method was used for measuring the capacitances (C_eq) of the electrode after the plasma turned on. The width of the etching profile was proportional to C_eq. As the current peak value I_smx of the substrate current increased, the volume removal rate of SiC increased. The etch depth was proportional to the ratio of I_smx to C_eq. Additionally, because of the different characteristics of the plasma disks on SiC substrate by the voltage polarity, the etching profile was unstable. However, in high NF₃ mixing process, the etching profile became stable and deeper.     

Etching characteristics of thin SiON films using a liquefied perfluorocarbon precursor of C6F12O with a low global warming potential

Perfluorocarbon gas is widely used in the semiconductor industry. However, perfluorocarbon has a negative effect on the global environment owing to its high global warming potential (GWP) value. An alternative solution is essential. Therefore, we evaluated the possibility of replacing conventional perfluorocarbon etching gases such as CHF₃ with C₆F₁₂O, which has a low GWP and is in a liquid state at room temperature. In this study, silicon oxynitride (SiON) films were plasma-etched using inductively coupled CF₄ +C₆F₁₂O+O₂ mixed plasmas. Subsequently, the etching characteristics of the film, such as etching rate, etching profile, selectivity over Si, and photoresist, were investigated. A double Langmuir probe was used and optical emission spectroscopy was performed for plasma diagnostics. In addition, a contact angle goniometer and x-ray photoelectron spectroscope were used to confirm the change in the surface properties of the etched SiON film surface. Consequently, the etching characteristics of the C₆F₁₂O mixed plasma exhibited a lower etching rate, higher SiON/Si selectivity, lower plasma damage, and more vertical etched profiles than the conventional CHF₃ mixed plasma. In addition, the C₆F₁₂O gas can be recovered in the liquid state, thereby decreasing global warming. These results confirmed that the C₆F₁₂O precursor can sufficiently replace the conventional etching gas.     

深层搅拌水泥土基底加固层的三维随机有限元分析

在软土基坑开挖过程中,围护结构的最大侧向位移往往出现在开挖面以下。常用的防范方法是采用深层搅拌法对设计深度以下2~3 m的软土层进行水泥加固处理,形成水泥土加固层以减少围护结构的侧向位移。由于水泥土的强度具有显著的空间变异性,工程设计中通常采用较为保守的整体强度设计值。考虑深层搅拌桩形成过程中的两种主要不确定性因素:桩身位置不确定性和桩身内部水泥土强度不均匀性,将水泥土加固层的强度模拟为随机场,且将其作为材料变量结合有限元仿真模拟进行受力分析。分析结果表明水泥土加固层的设计强度可表示为钻心取样强度平均值减去t倍标准差,对系数t进行了详细探讨,所得结果可为今后类似工程设计提供参考依据。     

Multipoint scanning dual‐detection confocal microscopy for fast 3D volumetric measurement

We propose a multipoint scanning dual‐detection confocal microscopy (MS‐DDCM) system for fast 3D volumetric measurements. Unlike conventional confocal microscopy, MS‐DDCM can accomplish surface profiling without axial scanning. Also, to rapidly obtain 2D images, the MS‐DDCM employs a multipoint scanning technique, with a digital micromirror device used to produce arrays of effective pinholes, which are then scanned. The MS‐DDCM is composed of two CCDs: one collects the conjugate images and the other collects nonconjugate images. The ratio of the axial response curves, measured by the two detectors, provides a linear relationship between the height of the sample surface and the ratio of the intensity signals. Furthermore, the difference between the two images results in enhanced contrast. The normalising effect of the MS‐DDCM provides accurate sample heights, even when the reflectance distribution of the surface varies. Experimental results confirmed that the MS‐DDCM achieved high‐speed surface profiling with improved image contrast capability.     

Phase‐shifting differential interference contrast microscope with Savart shear prism and rotatable analyser

A novel differential interference contrast microscope (DICM) is proposed in this research. It is constituted by inserting a Savart shear prism between the objective and sample of a polarising microscope having a rotatable analyser as the phase‐shifter, and it is with the ability to enhance image contrast using the principle of shearing interferometry. This letter is to introduce the configuration, interpret the interference patterns and present the experimental setup of the DICM. In addition, this letter is to display the experimental results from the uses of the setup; the results demonstrate the validity and ability of the DICM.     

SMT production scheduling: a generalized transportation approach

A generalized transportation model is first formulated for the scheduling of Surface Mount Technology (SMT) production. Its dual form is next discussed. By specially coding the SMT scheduling problem, it is unnecessary to develop the constraint matrix for the dual. The simplex method is not applicable for this problem due to its sparse data structure. An efficient algorithm for the dual model is then developed from the idea of the revised simplex method. To illustrate the algorithm, a numeric example is presented.     

Performance improvement using time delays in multivariable controller design

A new controller for linear multivariable ordinary systems is suggested in which distributed delays are included in the feedback loop. A general design approach to obtain this type of controller is suggested. It is shown that the resulting closed-loop system is asymptotically stable under some sufficient conditions. Sufficient conditions are derived under which the disturbance attenuation, the robustness against parameter variations, and the time-delay stability margins are improved by the proposed controller. The state feedback tracking controller and the dual-state observer are obtained by modification of the proposed controller.     

A SHAPE AND SIZE OPTIMIZATION ALGORITHM FOR MACHINE TOOL ELEMENT DESIGN

A shape and size optimization algorithm is developed to incorporate the shape and size variables in a recently developed machine tool simulation and design methodology known as Integrated Machining Process Design Simulator (IMPDS). This shape optimization algorithm, using coordinates of master nodes, parameters of geometric equations, coordinate linking and symmetry option approaches presents a general and flexible way of controlling and optimizing the structural shape of machine tool elements. An application of the proposed shape and size optimization algorithm indicates that the shape and size parameters have significant effects on the structural characteristics and the dynamic behavior of machine tool elements.