Numerical Studies on the Parametric Ion-Acoustic Instability

A numerical treatment of the kinetic dispersion equation for ion-acoustic waves in a plasma exposed to a high frequency pump field is presented. Maximum growth rates for both the decay and the purely growing instability are calculated as functions of the pump frequency and the pump power. A static electron drift velocity is included.     

The dissociation of the 2Π fluoroethylene anions

Dissociative electron attachment spectroscopy is employed to study formation of stable negative ion fragments in the fluoroethylenes. It is found that in each case the energy of the first resonance leading to F^? and other stable negative ion fragments coincides with the energy of the temporary Π anions. Tetrafluoroethylene gives fragments by cleavage of the double bond.     

Zur Bildungskinetik der F-Folgezentren in KCl

The process of F-center aggregation under light irradiation, which involves ionic movement at low temperatures (observable down to — 60°C), is not at all understood in its mechanism. It is the aim of this work to evaluate quantitatively the kinetics of this process for different F-aggregate centers. In part I the assoziation of F-centers in KCl crystals with isolated Na⁺ or Li⁺ ions was thoroughly investigated as the clearest model case of F-center-aggregation. The reaction product in these crystals after light irradiation, an F-center associated to a Na⁺ or Li⁺ ion as nearest (100) neighbor (F _A -center), is well established in its model and can be detected by its double peak absorption structure. By optical measurements of the rate of F _A -center formation in dependence on light-intensity, time, Na⁺ or Li⁺-concentration, F→F′ conversion rate and temperature, the kinetics of this reaction could be evaluated in a simple equation of bimolecular type. The analysis leads to the conclusion, that either the anion vacancy or the F′-center must be regarded as a unit of high thermal mobility (activation energy 0·6±0·05 eV, jump frequency about 10² sec^?1 at room temperature) which diffuses randomly in the lattice and can be captured by a Na⁺ or Li⁺ ion.     

Finite Ion Temperature Effects on Strongly Non-Linear Ion Acoustic Solitons

The influence of finite ion temperature on the properties of strongly non-linear ion acoustic solitary waves is discussed within the framework in Sakanaka's ion fluid model.     

The relaxation of concentrated polymer solutions

The focus of this paper is on the viscoelastic properties of concentrated polymer solutions and polymer melts. Dynamic mechanical measurements were performed on various polystyrene/ethylbenzene solutions with polymer concentrations ranging from 40% up to 100% and temperatures from Tg+30°C up to 70°C (230°C for polymer melts). The basis polymers are two commerical grade polystyrenes (BASF) with M _W = 247 kg/mol and 374 kg/mol, respectively. To avoid solvent loss due to evaporating during the measurements, a special sealing technique was used.A phenomenological model which describes quantitatively the relaxation spectrum of concentrated polymer solutions from the flow regime up to the glass transition regime is developed. The relaxation data of the respective polymer melt and the glass transition temperature of the solution are the only input parameters needed. The temperature dependence is described by a universal, concentration invariant WLF-equation. The relaxation spectra are divided into two parts accounting for the entanglement and the segmental relaxation modes, respectively. The relaxation strength related to the flow and entanglement regime scale with c ^2.3, whereas the segmental relaxation strength does not alter with concentration. All relaxation times change with concentration proportional to c ^3.5. Flow curves can be calculated from these relaxation spectra and thus, our results are useful for engineering applications.Roman Symbols a _T Time temperature superposition shift - factor - a _c Time concentration superposition - shift factor in the flow regime - a _c Time concentration superposition - shift factor in the glassy regime - b _T Modulus temperature superposition - shift factor - b _c Modulus concentration shift factor - in the flow regime - b _c Modulus concentration shift factor - in the glassy regime - B Virial coefficients - c Polymer mass fraction kg/kg - c ₁ WLF-parameter - c2 WLF-parameter K - g Relaxation strength of a relaxation Pa mode - G(t) Relaxation modulus Pa - G Storage modulus Pa - G Loss modulus Pa - GN Plateau modulus of linear flexible Pa polymers - (x) Delta function: (0) = 1, - (x<>0)=0 - h() Damping function - H() Relaxation spectrum Pa - J ₀ ^N Recoverable compliance Pa^–1 - m Mass kg - M _c Critical molecular weight kg/mol - M _e Entanglement molecular weight kg/mol - M _w Weight average molecular weight kg/mol - M Number of datapoints - n Scaling exponent - N Number of discrete relaxation modes - T Temperature °C - T _g Glass transition temperature °C - V Volume 1 Greek Symbols Scaling exponent - _f Thermal expansion coefficient K^–1 - Scaling exponent - Shear deformation - Shear rate s^t–1 - Relaxation time s - _c Characteristic relaxation time of thes Cross model - _e Entanglement relaxation time s - Viscosity Pa s - ₀ Zero shear viscosity Pa s - ₀ First normal stress coefficientPa s2 - Segmental friction coefficient - Frequency rad/s Indices f Flow and entanglement regime - g Glass transition regime - i Count parameter - p Polymer - ref Reference state - s Solvent Dedicated to Prof. Dr. J. Meissner on the occasion of his retirement from the chair of Polymer Physics at the Eidgenössische Technische Hochschule (ETH) Zürich, Switzerland     

On the Relation between Fronts and High-Shear Layers in Wall Turbulence

Direct numerical simulation results of turbulent channel flow are analyzed in order to examine the relation between two kinds of near-wall flow structures, namely the instantaneous shear layers and the fronts which are derived from two-point statistics of the streamwise velocity component. The near-wall shear layers are analyzed by flow visualizations and conditional sampling, while the fronts are examined by means of space-time correlations and spatial two-point correlation functions. The present study focuses on the analysis of the propagation speed and the spatial shape of the structures. Concerning the propagation speed it is shown that the results obtained from flow visualizations are in close agreement with the propagation velocities derived from space-time correlation functions. The comparison of VISA results for the instantaneous shear with spatial structures obtained from two-point correlations of the streamwise velocity and the shear gives evidence that the fronts are intimately related to the pronounced near-wall shear layers.     

Spiro linked compounds for use as active materials in organic light emitting diodes

Spiro-linkage of low molecular weight entities as a new structural concept for the design of new active materials for electroluminescent applications is presented. These spiro linked compounds result in nonpolymeric organic glasses with high thermal stability as can be derived from their high glass transition temperatures (T_g), and characterized by differential scanning calorimetry. Blue emitters based on spiro linked oligophenyles are presented. These compounds are soluble in common organic solvents and show high photoluminescence quantum efficiency in the solid state and high morphologic stability with glass transition temperatures up to 250°C. Charge transport materials based on spiro linked versions of 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) for electron transport, and spiro linked versions of triphenyldiamin derivatives (TPD) for hole transport show improved morphologic properties with nearly unchanged electronic properties compared to the parent compounds. High quality amorphous films can be prepared with the spiro compounds by vapor deposition as well as by simple spin coating.