Current numerical simulations of laser materials processing usually simplify any process model to a great extent in order to allow for short computation times. This significantly decreases their flexibility and ability to simulate the great variation of today's processes with their subtle but important differences. The simulation presented in this paper can be said to be truly three dimensional as opposed to other reported work that uses symmetric boundary conditions. This enables the investigators to simulate real laser beams. In contrast to the (well-documented) Marangoni flow profile, the authors will show results that do not use the usual simplifying assumptions of flat surfaces. Preliminary output from the simulation deals with the transient coupled velocity and pressure profile and temperature distribution and hence the heat affected zone (HAZ). From this, conclusions can be drawn with regard to improving process efficiency, especially in laser cutting. It will be shown that the traditional perception of equating higher processing speeds with better processing efficiency does not hold in all cases. In fact, the opposite may well hold true. However, to demonstrate this the actual process of producing a part needs to be fully understood. A process may influence the workpiece material properties beneficially when it is performed at reduced speeds (material hardening or softening). The investigators contend that numerical modeling of the above can only be achieved credibly using high performance computing methods.
This paper describes the implementation and numerical evaluation of a transient 3D computer simulation of the CO2 laser cutting process. Utilizing Crank-Nicolsen-Finite- Difference equations for the solution of the Fourier heat transfer equation with Newtonian convection, the temperature distribution is predicted. For high accuracy the mesh is of non-equidistant nature, following a Weibull Distribution for the grid spacing. A parallel computation solver is used, based on Divide-and-Conquer Gaussian elimination for banded matrices, to calculate the nodal temperatures using a cluster of two HP J5000 workstations. Included in the solution is the behavior of the material during phase change, while the open structure of the developed software allows incorporation of effects such as surface oxidation, radiation and limited convective flow. The main area of interest is the cutting capability with respect to varying material thickness, cutting speed, power of the laser, laser mode, focal spot diameter and material properties, as well as the effect of these parameters on the quality of the cut.
Organic light-emitting diodes (OLEDs) represent a promising technology for flat-panel displays. Doped ?-conjugated polymer layers have been demonstrated to improve hole injection and lifetime of OLED devices. In this paper, we demonstrate that the work function of such polymers can be continuously adjusted by means of electrochemical doping. This allows to control the hole current through organic semiconducting devices. Injection-limited or bulk-limited device performance can be obtained with the identical semiconductor. We use a combinatorial approach to speed up the optimization process.
This paper describes the implementation and numerical evaluation of a transient three-dimensional computer simulation of the CO2 laser cutting process. Utilising Crank-Nicolsen-Finite-Difference equations for the solution of the Fourier heat transfer equation with Newtonian convection, the temperature distribution is predicted. For high accuracy the mesh is of non-equidistant nature, following a Weibull Distribution for the grid spacing. A parallel computation solver will be used, based on Aztec (a parallel iterative library for solving linear systems -www.sandia.gov), to calculate the nodal temperatures using a cluster of two HP J5000 workstations. Included in the solution is the behaviour of the material during phase change, whilst the open structure of the developed software allows incorporation of effects such as surface oxidation, radiation and limited convective flow. The main area of interest is the cutting capability with respect to varying material thickness (e.g. tailored blanks), cutting speed, power of the laser, laser mode, focal spot diameter and material properties, as well as the effect of these parameters on the quality of the cut. Further developments will also be outlined in this paper.
Homologous series of mixed oligoheterocycles based on end- capped oligothiophenes ECnT 1 were synthesized by introduction of electronegative heteroatoms like oxygen and nitrogen into the conjugated (pi) -system. This led to novel structures 2-11 in which thiophene units of the parent compounds are substituted by other heterocycles with more pronounced acceptor character. Other moieties like phenylsubstituted thiophenes, benzo(c)thiophene, and spiro- bithiophenes have also been implemented resulting in oligothiophenes 12-16. The characterization of the optical and electrochemical properties clearly reveals the influence of the heteroatoms on the electronic properties. Thus e.g., due to the electron withdrawing character of the central heterocycle oxidation of the oligomer is rendered more difficult while reduction is facilitated. In some cases, a hypsochromic shift of the longest wavelength absorption and emission is observed and additionally a significant enhancement of the fluorescence quantum yield in solution and in the solid state. The HOMO/LUMO energy differences determined from the optical measurement correspond qualitatively well with the values obtained from electrochemical data. First experiments on single layer organic light emitting diodes show that these mixed oliogoheterocycles can be used as emitting materials.
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