A Polygonal Approximation for General 4-Contours Corresponding to Weakly Simple Curves

Journal of Mathematical Imaging and Vision - The paper proposes a polygonal approximation for closed 4-paths obtained from standard contour following under 4-connectivity. Those 4-contours generate...     

Random linear-quadratic mathematical models: Computing explicit solutions and applications

This paper deals with the study of linear random population models and with a random logistic model (where parameters are random variables). Assuming appropriate conditions, the stochastic processes solutions are obtained under closed form using mean square calculus. Expectation and variance expressions for the stochastic processes solutions are given and illustrative examples are included.     

The effect of alloy composition and welding conditions on columnar-equiaxed transitions in ferritic stainless steel gas-tungsten arc welds

The columnar-equiaxed transition (CET) was investigated in full penetration gas-tungsten arc (GTA) welds on ferritic stainless steel plates containing different amounts of minor elements, such as titanium and aluminum, for a range of welding conditions. In general, the fraction of equiaxed grains increased, and the size of the equiaxed grains decreased with increasing titanium contents above 0.18 wt pct. At a given level of titanium, the equiaxed fraction increased, and the size of the equiaxed grains decreased with increased aluminum content. The CET was ascribed to heterogeneous nucleation of ferrite on Ti-rich cuboidal inclusions, since these inclusions were observed at the origin of equiaxed dendrites in the grain refined welds. Titanium-rich cuboidal inclusions, in turn, were found to contain Al-Ca-Mg-rich inclusions at their centers, consistent with observations by previous investigators for other processes. The welding conditions, in particular, the welding speed, were observed to affect the occurrence of the CET. Increasing the welding speed from 3 to 8 mm/s increased the equiaxed fraction noticeably, but a further increase in speed to 14 mm/s had a smaller additional effect. A finite element model (FEM) of heat transfer was used to examine the role of the welding conditions on the local solidification conditions along the weld pool edge. The results are compared with existing models for the CET. Formerly Postdoctoral Fellow, Department of Mechanical Engineering, University of Waterloo     

Numerical simulation of the cold gas dynamic spray process

A computational fluid dynamic (CFD) model of the cold gas dynamic spray process is presented. The gas dynamic flow field and particle trajectories within an oval-shaped supersonic nozzle as well as in the immediate surroundings of the nozzle exit, before and after the impact with the target plane, are simulated. Predicted nozzle wall pressure values compare well with experimental data. In addition, predicted particle velocity results at the nozzle exit are in qualitative agreement with those obtained using a side-scatter laser Doppler anemometer (LDA). Details of the pattern of the particle release into the surroundings are visualized in a convenient manner. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Simulation of a Commercial Semiregenerative Reforming Plant Using Feedstocks with and without Benzene Precursors

This paper describes the use of kinetic and reactor modeling to simulate the behavior of a commercial semi‐regenerative reforming unit when two feedstocks (with and without benzene precursors) are employed. The feed without benzene precursors was prepared by fractionation of a typical reforming feed. A cut point of 88 °C was selected because most of the C₅–C₆ light naphtha is excluded. This cut point was not deep enough to remove the main benzene precursor, cyclohexane, but other precursors, i.e. methylcyclopentane and n‐hexane, were almost totally eliminated. It was decided not to increase this cut point value beyond 88 °C because this would reduce the flowrate of the feedstock to the reforming unit, which is not convenient due to refineries gasoline production policies. It has been shown that naphtha fractionation is a very good method for reducing the content of benzene precursors in reforming feedstocks, and consequently an important decrease in reformate benzene concentration can be achieved.     

Phase Stability of Al-5Fe-V-Si Coatings Produced by Cold Gas Dynamic Spray Process Using Rapidly Solidified Feedstock Materials

In this study, aluminum alloy Al-5Fe-V-Si (in wt.%) feedstock powder, produced by rapid solidification (RS) using the gas atomization process, was selected to produce high-temperature resistant Al-alloy coatings using the cold gas dynamic spraying process (CGDS). The alloy composition was chosen for its mechanical properties at elevated temperature for potential applications in internal-combustion (IC) engines. The CGDS spray process was selected due to its relatively low operating temperature, thus preventing significant heating of the particles during spraying and as such allowing the original phases of the feedstock powder to be preserved within the coatings. The microstructure and phases stability was investigated by means of Scanning Electron Microscopy, transmission electron microscopy, X-ray diffraction and differential scanning calorimetery techniques. The coatings mechanical properties were evaluated through bond strength and microhardness testing. The study revealed the conservation of the complex microstructure of the rapid solidified powder during the spray process. Four distinct microstructures were observed as well as two different phases, namely a Al₁₃(Fe,V)₃Si silicide phase and a metastable (Al,Si) _x (Fe,V) Micro-quasicrystalline Icosahedral (MI) phase. Aging of the coating samples was performed and confirmed that the phase transformation of the metastable phases and coarsening of the nanosized precipitates will occurs at around 400 °C. The metastable MI phase was determined to be thermally stable up to 390 °C, after which a phase transformation to silicide starts to occur.     

ZnO:Co diluted magnetic semiconductor or hybrid nanostructure for spintronics?

We have studied the influence of intrinsic and extrinsic defects in the magnetic and electrical transport properties of Co-doped ZnO thin films. X-ray absorption measurements show that Co substitute Zn in the ZnO structure and it is in the 2+ oxidation state. Magnetization (M) measurements show that doped samples are mainly paramagnetic. From M vs. H loops measured at 5 K we found that the values of the orbital L and spin S numbers are between 1 and 1.3 for L and S = 3/2, in agreement with the representative values for isolated Co 2+. The obtained negative values of the Curie–Weiss temperatures indicate the existence of antiferromagnetic interactions between transition metal atoms.     

Dielectric permittivity and conductivity spectra of La0.5Ca0.5MnO3 thin films presenting electric field induced metal-insulator transition

Electric behavior of polycrystalline La_0.5Ca_0.5MnO₃ thin films, pulsed laser deposited on a (100) silicon substrate, is reported and discussed. An electrically induced metal-insulating transition around 150 K is found, which is voltage and thickness dependent. At low temperatures, the film conductivity is non-Ohmic and moderate electric fields lead to resistivity switching towards metaestable low-resistive states. Impedance spectroscopy measurements were also performed in order to determine the film dielectric permittivity and conductivity and to estimate the characteristic metal-semiconductor interface parameters. The obtained results show that the fraction of ferromagnetic metallic regions does not change when a voltage is applied, and that the mechanism responsible for the low temperature metal insulator transition and the conduction behavior is the appearance of connective paths due to the enhanced mobility of carriers activated by the electric field.