Influence of Growth Temperature on Surface Morphologies of GaN Crystals Grown on Dot-Patterned Substrate by Hydride Vapor Phase Epitaxy

This paper studies the influence of growth temperatures in the range 825 to 1050_°C on the surface morphologies of GaN crystals grown on a SiO₂ dot-patterned substrate using Epitaxy Lateral Overgrowth (ELO) and Hydride Vapor Phase Epitaxy (HVPE) techniques. A lower growth temperature of 850_°C prompts the formation of GaN hexagonal pyramidal crystals with a higher fraction of {1 ${\bar 1}$ 01}}(0001) facet areas than those grown at high temperatures (>1000°C). In a subsequent coalescent (or lateral growth) process, a high temperature of 1050°C is applied to the original GaN hexagonal pyramidal crystals, and the morphologies of the GaN layers are inspected. It is established that the original {1 ${\bar 1}$ 01} faceted morphology of the hexagonal pyramids changes to an irregularly-shaped surface comprising {1 ${\bar 1}$ 01}, {11 ${\overline 2}$ 2} and high index facets, and that the nature of the surface morphology is influenced by the growth time and the application (or not) of Ga precursor support. Hence, the results show that the coalescence and planarization of the GaN layer can be controlled through an appropriate specification of the process parameters. At low temperatures in the region of 850°C, high index facets are observed on the tops of a small percentage of the hexagonal GaN columnar crystals. It is proposed that this phenomenon is caused by a reduction in the surface diffusion length of the precursors, e.g. NH₃ and GaCl, at lower temperature, which in turn, reduces the probability of desorption and increases the lifetime.     

Modeling and control of micropositioning systems using Stewart platforms

The modeling and control of a 6‐DOF Stewart micropositioning system with each leg actuated by a respective piezoelectric actuator are considered in this paper. The 12 multi‐DOF passive joints are assumed to be well designed and fabricated so that guaranteed guiding precision and lack of backlash can be obtained. The dynamics model of the micropositioning system is derived first, and then a composite control strategy consisting of moving platform model‐based feedback linearization and two sets of simple SISO fuzzy systems is proposed. By considering the internal axial forces of the six legs on the six spherical joints at the moving end as the virtual control inputs of the moving platform, feedback linearization can be easily used to derive the desired control forces for the moving platform. The corresponding desired linear displacement of each piezoelectric actuator can then be computed based on the derived leg model, and each piezoelectric actuator's control voltage can be generated by the first set of independent leg fuzzy controls. The second set of fuzzy controls is suggested for the further enhancement of robustness with respect to uncertainty. Computer simulations are presented to illustrate the effectiveness of the suggested micropositioning control strategy. ©2000 John Wiley & Sons, Inc.     

Crack Growth Prediction by Manifold Method

The prediction of crack growth is studied by the manifold method. The manifold method is a new numerical method proposed by Shi. This method provides a unified framework for solving problems dealing with both continuums and jointed materials. It can be considered as a generalized finite-element method and discontinuous deformation analysis. One of the most innovative features of the method is that it employs both a physical mesh and a mathematical mesh to formulate the physical problem. The physical mesh is dictated by the physical boundary of a problem, while the mathematical mesh is dictated by the computational consideration. These two meshes are interrelated through the application of weighting functions. In this study, a local mesh refinement and auto-remeshing schemes are proposed to extend the manifold method. The proposed model is first verified by comparing the numerical results with the benchmark solutions, and the results show satisfactory accuracy. The crack growth problems and the stress distributions are then investigated. The manifold method is proposed as an attractively new numerical technique for fracture mechanics analysis.     

Note on the multiple scattering in an IEM model

The authors derive the multiple scattering expression within the framework of an IEM model for rough surface scattering. The complementary field coefficients are rederived based on a new surface slope expressions which are dependent on spatial variables. This leads to a more complete expression of the multiple scattering terms, thus allowing the authors to account for multiple effects more accurately. Numerical calculations and comparisons with numerical simulation are provided to demonstrate the results     

Sensitivity of off-nadir zenith angles to correlation betweenvisible and near-infrared reflectance for use in remote sensing ofaerosol over land

Cloud absorption radiometer (CAR) multispectral and multiangular data, collected during the Smoke, Clouds, and Radiation-Brazil (SCAR-B) Experiment, was used to examine the ratio technique, the official method for remote sensing of aerosols over land from the moderate resolution imaging spectroradiometer (MODIS) data, for view angles from nadir to 650 off-nadir. The strategy used is to first select a pristine, low aerosol optical thickness flight, and then to compute ratios of reflectance at 0.47 and 0.68 μm to corresponding values at 2.20 μm, separately for backward and forward scattering directions. Similarly, the authors analyzed data from high turbidity flights for comparison purposes. For both flights, they removed the effects of atmospheric absorption and scattering using 6S, a radiative transfer code, and then recomputed the ratios again for different values of aerosol optical thickness. Finally, they analyzed bidirectional reflection function (BRF) data to examine the dependence of the ratio technique on the relative azimuth angle. Results of this analysis show that a relationship between visible reflectance and near infrared (IR) reflectance exists for view angles from nadir to 400 off-nadir, and that simple parametric relationships can be derived     

Nonconvex economic dispatch by integrated artificial intelligence

This paper presents a new algorithm by integrating evolutionary programming (EP), tabu search (TS) and quadratic programming (QP) methods to solve the nonconvex economic dispatch problem (NED). A hybrid EP and TS were used for quality control, and Fletcher's quadratic programming technique for solving. EP and TS determines the segment of a cost curve used, which is piecewise quadratic natured. Operation constraints are modeled as linear equality or inequality equations, resulting in a typical QP problem. Fletcher's QP was chosen to enhance the performance. The fitness function is constructed from priorities without penalty terms. Numerical results show that the proposed method is more effective than other previously developed evolutionary computation algorithms     

Enhanced performance for photocatalytic hydrogen evolution using MoS2/graphene hybrids

A MoS₂/graphene hybrid (MSG) is synthesized by microwave hydrothermal method. Both of the charge transfer resistance and the photocurrent are tuned in graphene modified MoS₂ by enhancing photocatalytic nature, where the charge transfer resistance significantly decreases from 36,000 Ω–8.49 Ω and the photocurrent promotes from 0.29 mA cm^?2 to 16.47 mA cm^?2. In this article, the result reveals that the appropriate modification of graphene can reach the maximum yield of hydrogen gas. In addition, the appropriate conditions, such as the concentration of 0.32 M formic acid and the MoS₂ photocatalyst with 0.8 wt% graphene (MSG_0.8) dose of 0.013 g L^?1, can complete the outstanding photocatalytic hydrogen evolution, where the hydrogen evolution using MSG_0.8 composite photocatalyst has the maximum yield of 667.2 μmol h^?1 g^?1.     

Manifestations of strain-relaxation in the structure of nano-sized Co-2 × 2 islands grown on Ag/Ge(111)-√3 × √3 surface

We have examined strain-relaxation of Co-2 × 2 islands grown on the Ag/Ge(111)-√3 × √3 surface by analyzing scanning tunneling microscopy images. We have found that the Co-2 × 2 islands commonly adopt a more compact arrangement as compared to that of the Ge(111) substrate, however they differ in a degree of an atomic compactness. We have not found a distinct relation between strain-relaxation and the island height. Three groups of islands have been identified upon analyzing a correspondence between strain-relaxation and the island size: (i) small islands (not bigger than 80 nm²) with a high atomic compactness, displaying fixed inter-row distances, (ii) small islands with unfixed distances between atomic rows, and (iii) big islands (bigger than 80 nm²) with fixed inter-row distances, but with a less compact atomic arrangement compared to that of the first two groups. We propose a model to account for the relation between the relaxation and the island size.     

Corner blending of free-form N-sided holes

Geometric modeling requires constructing blends between surfaces to meet manufacturing specifications, reduce stress concentrations in designs, and enhance aesthetics. Industrial engineers may design parts using different surface types to satisfy design requirements. Surface blending integrates the diverse representations. Because most CAD/CAM software uses parametric representations for curves and surfaces, blending techniques for parametric surfaces are more urgently needed than methods for implicit surfaces. The authors discuss a new method for parametric surface blending. They apply their corner blending technique to three- to six-sided holes and discuss corner blending of three sided holes for three different cases, each presenting different blending challenges