A continuum model for yttria-stabilized zirconia incorporating triple phase boundary,lattice structure and immobile oxide ions

A continuum model for yttria-stabilized zirconia (YSZ) in the framework of non-equilibrium thermodynamics is developed. Particular attention is given to (i) modeling of the YSZ-metal-gas triple phase boundary, (ii) incorporation of the lattice structure and immobile oxide ions within the free energy model and (iii) surface reactions. A finite volume discretization method based on modified Scharfetter-Gummel fluxes is derived in order to perform numerical simulations. The model is used to study the impact of yttria and immobile oxide ions on the structure of the charged boundary layer and the double layer capacitance. Cyclic voltammograms of an air-half cell are simulated to study the effect of parameter variations on surface reactions, adsorption and anion diffusion.

     

3D electrothermal simulations of organic LEDs showing negative differential resistance

Organic semiconductor devices show a pronounced interplay between temperature-activated conductivity and self-heating which in particular causes inhomogeneities in the brightness of large-area OLEDs at high power. We consider a 3D thermistor model based on partial differential equations for the electrothermal behavior of organic devices and introduce an extension to multiple layers with nonlinear conductivity laws, which also take the diode-like behavior in recombination zones into account. We present a numerical simulation study for a red OLED using a finite-volume approximation of this model. The appearance of S-shaped current–voltage characteristics with regions of negative differential resistance in a measured device can be quantitatively reproduced. Furthermore, this simulation study reveals a propagation of spatial zones of negative differential resistance in the electron and hole transport layers toward the contact.     

Second moment relationships for waiting times in queueing systems with poisson input

For theM/G/1 queue there are well-known and simple relationships among the second moments of waiting time under the first-in-first-out, last-in-first-out, and random-order-of-service disciplines. This paper points out that these relationships hold in considerably more general settings. In particular, it is shown that these relationships hold forM/G/1 queues with exceptional first service,M/G/1 queues with server vacations, andM/G/1 queues with static priorities.     

Elektrische Eigenschaften polymerer Festkörper in starken elektrischen Feldern

Zusammenfassung Dichte und energetische Lage lokalisierter Niveaus in der Oberfläche und im Volumen und deren Besetzungswahrscheinlichkeit für Excesselektronen oder Defektelektronen bestimmen die elektrostatische Rufladung polymerer Festkörper beim Kontakt mit Metall. Es lassen sich Anreicherungsrandschichten für Excesselektronen und Verarmungsrandschichten für Defektelektronen im Polymeren je nach elektrischem Potential des kontaktierenden Metalls nachweisen. Ursache für diese Randschichten ist der unterschiedliche physikalische Zustand der Oberfläche relativ zum Volumen des Polymeren. Die energetische Lage der lokalisierten Niveaus wird durch die zwischen-molekulare Wechselwirkung bestimmt und hängt daher vom Ordnungszustand der Oberfläche und des Volumens ab.Bei Polystyrol verdecken intermolekulare lokalisierte Niveaus den Einfluß intramolekularer Niveaus. Die feldinduzierte Änderung des physikalischen Zustands der Oberfläche und des Volumens (ferroelektrische Erscheinungen) beeinflußt die energetische Situation der lokalisierten Niveaus stark. Drei diskrete Haftstellenarten für Excesselektronen lassen sich bei Polystyrol nachweisen, die energetisch tiefste hängt vom Ordnungszustand und daher auch von der Feldstärke ab. Polystyrol zeigt einen starken elektrischen Feldeffekt und einen feldstärkeabhängigen Imaginärteil der Dielektrizitätskonstanten. Hinweise auf einen photokapazitiven Effekt konnten gefunden werden.

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Summary The density and the energetic situation of localized levels at the polymer surface and bulk and their occupancy probability of excesselectrons or defect-electrons determine the electrostatic charging of polymer solids associated with the contact of metals. Accumulation layers of excesselectrons and depletion layers of defectelectrons can exist near the polymer surface depending on the electrical potential of the contacting metal. These layers are caused by the difference between the physical states of the polymer surface and bulk. The energetic situation of the localized levels is determined by intermolecular interactions and therefore it depends on the state of order of the surface and the bulk.In polystyrene e. g. the contribution of intramolecular localized levels is covered by the influence of intermolecular localized levels. Therefore the energetic situation of these levels depends on field-induced change of the state of order of the polystyrene surface and bulk (ferroelectric phenomena). Three discrete localized levels of excesselectrons could be identified in polystyrene. The level with the lowest energy depends on the state of order and on the electric fieldstrength. Polystyrene exhibits a strong electric field effect and a field depending imaginary part of its dielectric constant. There are hints to photocapacitive effects in polymers.

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Mit 5 Abbildungen     

Cover Picture

The cover picture shows the synthesis of novel conjugated macrocycles assembled from oligothiophenes bearing terminal acetylene groups. Under pseudo-high-dilution conditions the oxidative cyclooligomerization first gives the oligothiophenediynes, the precursors to the new class of alpha-cyclo[n]thiophenes. The detailed structure of macrocycles with up to 76 ring members and cavities of up to 3 nm could be investigated by means of X-ray structure analysis, scanning tunneling microscopy, and quantum chemical calculations (see the molecular model top right). The novel rings combine the excellent electronic properties of the corresponding linearly conjugated oligomers with the possibility of complexing large organic guest molecules or other objects (the tower of the Cathedral at Ulm represents a nanometer-sized, rodlike entity), which should have new fundamental properties and applications. The background shows the image obtained by scanning electron microscopy of a self-assembled and perfectly ordered monolayer of macrocycles on a graphite surface. More on these fascinating nanometer-sized rings can be found in the communication by P. B?uerle et al. on p. 3481 ff.     

Catalysis of O-(para-nitrophenyl) O,O-dimethyl thiophosphate decomposition by chromonic mesophases based on polymerized ionic amphiphiles

The formation of a nematic chromonic mesophase in aqueous solutions of quaternized poly(ethylene imine) has been found using polythermal polarization microscopy. The reaction kinetics of O-(para-nitrophenyl) O,O-dimethyl thiophosphate hydrolysis has been studied by spectrophotometry, and a comparative analysis of the effects of lyotropic liquid crystals constructed as hexagonal and chromonic mesophases on this kinetics has been performed. It has been found that the hydrolysis of the substrate in a nematic chromonic mesophase is accelerated due to the concentration of the reactants.     

On kaonic deuterium. Quantum field-theoretic and relativistic covariant approach

We study kaonic deuterium, the bound K^-d state A_{K d}. Within a quantum field-theoretic and relativistic covariant approach we derive the energy level displacement of the ground state of kaonic deuterium in terms of the amplitude of K^-d scattering for arbitrary relative momenta. Near threshold our formula reduces to the well-known DGBT formula. The S-wave amplitude of K^-d scattering near threshold is defined by the resonances (1405), (1750) and a smooth elastic background, and the inelastic channels K^-d NY and K^-d NY, where Y = ^±,^{0} and ^{0}, where the final-state interactions play an important role. The Ericson-Weise formula for the S-wave scattering length of K^-d scattering is derived. The total width of the energy level of the ground state of kaonic deuterium is estimated using the theoretical predictions of the partial widths of the two-body decays A_Kd NY and experimental data on the rates of the NY pair production in the reactions K^-d NY. We obtain _{1s} = (630±100)eV. For the shift of the energy level of the ground state of kaonic deuterium we predict _{1s} = (325±60)eV.     

A continuously tunable GaAs diode laser with an external resonator

Design and characteristics of an external cavity single mode GaAs diode laser are described which allow continuous wavelength tuning over 50 GHz without mode hops. Tuning is achieved by synchronous tilting of both a thin solid etalon and a Brewster plate inside the laser resonator. Stabilizing the laser frequency to an external Fabry-Perot interferometer yields a frequency stability of 1 MHz at output powers up to 20 m W.