Experimental proof of Joule heating-induced switched-back regions in OLEDs

Organic light-emitting diodes(OLEDs)have become a major pixel technology in the display sector,with products spanning the entire range of current panel sizes.The ability to freely scale the active area to large and random surfaces paired with flexible substrates provides additional application scenarios for OLEDs in the general lighting,automotive,and signage sectors.These applications require higher brightness and,thus,current density operation compared to the specifications needed for general displays.As extended transparent electrodes pose a significant ohmic resistance,OLEDs suffering from Joule self-heating exhibit spatial inhomogeneities in electrical potential,current density,and hence luminance.In this article,we provide experimental proof of the theoretical prediction that OLEDs will display regions of decreasing luminance with increasing driving current.With a two-dimensional OLED model,we can conclude that these regions are switched back locally in voltage as well as current due to insufficient lateral thermal coupling.Experimentally,we demonstrate this effect in lab-scale devices and derive that it becomes more severe with increasing pixel size,which implies its significance for large-area,high-brightness use cases of OLEDs.Equally,these non-linear switching effects cannot be ignored with respect to the long-term operation and stability of OLEDs;in particular,they might be important for the understanding of sudden-death scenarios.

A BIE-BASED DtN-FEM FOR FLUID-SOLID INTERACTION PROBLEMS

In this paper, we are concerned with the coupling of finite element methods and bound- ary integral equation methods solving the classical fluid-solid interaction problem in two dimensions. The original transmission problem is reduced to an equivalent nonlocal bound- ary value problem via introducing a Dirichlet-to-Neumann mapping by the direct boundary integral equation method. We show the existence and uniqueness of the solution for the corresponding variational equation. Numerical results based on the finite element method coupled with the standard Galerkin boundary element method, the fast multipole method and the NystrSm method for approximating the DtN mapping are provided to illustrate the efficiency and accuracy of the numerical schemes.

An Optimization Method in Inverse Elastic Scattering for One-Dimensional Grating Profiles

Consider the inverse diffraction problem to determine a two-dimensional periodic structure from scattered elastic waves measured above the structure.We formulate the inverse problem as a least squares optimization problem,following the two-step algorithm by G.Bruckner and J.Elschner[Inverse Probl.,19(2003),315–329]for electromagnetic diffraction gratings.Such a method is based on the Kirsch-Kress optimization scheme and consists of two parts:a linear severely ill-posed problem and a nonlinear well-posed one.We apply this method to both smooth(C2)and piecewise linear gratings for the Dirichlet boundary value problem of the Navier equation.Numerical reconstructions from exact and noisy data illustrate the feasibility of the method.

OPTIMAL AND PRESSURE-INDEPENDENT L2 VELOCITY ERROR ESTIMATES FOR A MODIFIED CROUZEIX-RAVIART STOKES ELEMENT WITH BDM RECONSTRUCTIONS

Nearly all inf-sup stable mixed finite elements for the incompressible Stokes equations relax the divergence constraint. The price to pay is that a priori estimates for the ve- locity error become pressure-dependent, while divergence-free mixed finite elements de- liver pressure-independent estimates. A recently introduced new variational crime using lowest-order Raviart-Thomas velocity reconstructions delivers a much more robust modi- fied Crouzeix-Raviart element, obeying an optimal pressure-independent discrete H1 ve- locity estimate. Refining this approach, a more sophisticated variational crime employing the lowest-order BDM element is proposed, which also allows proving an optimal pressure- independent L2 velocity error. Numerical examples confirm the analysis and demonstrate the improved robustness in the Navier-Stokes case.

Vibrations of a laboratory-scale gas-stirred ladle with two eccentric nozzles and multiple sensors

During ladle stirring,a gas is injected into the steel bath to generate a mixing of the liquid steel.The optimal process control requires a reliable measurement of the stirring intensity,for which the induced ladle wall vibrations have proved to be a potential indicator.An experimental cold water ladle with two eccentric nozzles and eight mono-axial accelerometers was thus investigated to measure the vibrations.The effect of the sensors'positions with respect to the gas plugs on the vibration intensity was analyzed,and experimental data on several points of the ladle were collected for future numerical simulations.It is shown that the vibration root-mean-square values depend not only on process parameters,such as gas flow rate,water,and oil heights,but also on the radial and axial positions of the sensors.The vibration intensity is clearly higher,close to the gas plumes,than in the opposite side.If one of the nozzles is clogged,the vibration intensity close to the clogged nozzle drops drastically(-36 to-59%),while the vibrations close to the normal operating nozzle are hardly affected.Based on these results,guidelines are provided for an optimized vibration-based stirring.

On a Fast Integral Equation Method for Diffraction Gratings

The integral equation method for the simulation of the diffraction by optical gratings is an efficient numerical tool if profile gratings determined by simple crosssection curves are considered.This method in its recent version is capable to tackle profile curves with corners,gratings with thin coated layers,and diffraction scenarios with unfavorably large ratio period over wavelength.We discuss special implementational issues including the efficient evaluation of the quasi-periodic Green kernels,the quadrature algorithm,and the iterative solution of the arising systems of linear equations.Finally,as an example we present the simulation of echelle gratings which demonstrates the efficency of our approach.

DETECTION OF ELECTROMAGNETIC INCLUSIONS USING TOPOLOGICAL SENSITIVITY

In this article, a topological sensitivity framework for far-field detection of a diamet- rically small electromagnetic inclusion is established. The cases of single and multiple measurements of the electric far-field scattering amplitude at a fixed frequency are tak- en into account. The performance of the algorithm is analyzed theoretically in terms of its resolution and sensitivity for locating an inclusion. The stability of the framework with respect to measurement and medium noises is discussed. Moreover, the quantitative results for signal-to-noise ratio are presented. A few numerical results are presented to illustrate the detection capabilities of the proposed framework with single and multiple measurements.

A Review of Transparent and Artificial Boundary Conditions Techniques for Linear and Nonlinear Schrodinger Equations

In this review article we discuss different techniques to solve numerically the time-dependent Schrodinger equation on unbounded domains.We present in detail the most recent approaches and describe briefly alternative ideas pointing out the relations between these works.We conclude with several numerical examples from different application areas to compare the presented techniques.We mainly focus on the one-dimensional problem but also touch upon the situation in two space dimensions and the cubic nonlinear case.