Stochastic Aspects of Mass Transport in Gas Diffusion Layers

The relationship between the 3D morphology of gas-diffusion layers (GDL) of HT-PEFCs and their functionality is analyzed. A stochastic model describing the microstructure of paper-type GDL is combined with the Lattice-Boltzmann method (LBM) to simulate gas transport within the GDL microstructure. Virtual 3D microstructures representing paper-type GDL are generated by a stochastic model, where the binder morphology is systematically modified. On these structures, single phase single component gas flow is computed by the LBM. Quality criteria evaluating the spatial homogeneity of gas supply are introduced and related to the binder morphology. The spatial homogeneity of the gas supply is analyzed by a parametrized stochastic model describing the gas flow at the exit of the GDL. This approach gives insight into the spatial structure of the gas flow at the GDL exit. The quality of gas supply is quantified by characterizing size and arrangement of regions with high gas supply. This stochastic gas flow model predicts the quality of gas supply for further binder morphologies. Analyzing the quality criteria and the stochastic evaluation of the spatial structure of the gas flow field at the GDL exit, it is found that the binder morphology has an essential influence on the gas supply.     

First‐order Raman scattering of the MAX phases Ta4AlC3, Nb4AlC3, Ti4AlN3, and Ta2AlC

Herein, we report on the Raman spectra of the following ternary hexagonal carbides and nitrides (MAX phases): Ta₄AlC₃, Ta₂AlC and Ti₄AlN₃. We also present the Raman‐active modes of α‐ and β‐Ta₄AlC₃, Nb₄AlC₃ and Ti₄AlN₃, – also referred to as the 413 MAX phases – as predicted from first principles calculations using density functional theory. We compare the obtained experimental and calculated results with previous studies on Ta₂AlC and Ti₄AlN₃. The vibrational behavior associated with the Raman‐active modes for the 413 phases has been identified for the first time. In general, the agreement is good between theory and experiment. The experimental and calculated results indicate that the modes at low wavenumbers ‐ dominated by the Al atoms ‐ are a weak function of chemistry and the differences in energy can be traced to variations in the reduced mass. The modes at higher wavenumbers are dominated by the C and N atoms and show a strong dependence on the unit cell chemistry, with the Ta–C bond being stiffer than the Nb–C bond, which is in turn stiffer than Ti–N. Copyright © 2011 John Wiley & Sons, Ltd.     

On (essentially) non-oscillatory discretizations of evolutionary convection–diffusion equations

Finite element and finite difference discretizations for evolutionary convection–diffusion–reaction equations in two and three dimensions are studied which give solutions without or with small under- and overshoots. The studied methods include a linear and a nonlinear FEM-FCT scheme, simple upwinding, an ENO scheme of order 3, and a fifth order WENO scheme. Both finite element methods are combined with the Crank–Nicolson scheme and the finite difference discretizations are coupled with explicit total variation diminishing Runge–Kutta methods. An assessment of the methods with respect to accuracy, size of under- and overshoots, and efficiency is presented, in the situation of a domain which is a tensor product of intervals and of uniform grids in time and space. Some comments to the aspects of adaptivity and more complicated domains are given. The obtained results lead to recommendations concerning the use of the methods.     

Lateralization based on interaural differences in the second-order amplitude modulator

Second-order amplitude modulation is a relatively slow variation of the modulation depth of a first-order amplitude modulation with higher frequency. In contrast to first-order modulation, which appears as a physical component in the stimulus spectrum after half-wave rectification, second-order modulation is not necessarily demodulated by the auditory periphery. For binaural processing of second-order amplitude modulated stimuli it is unknown whether interaural time differences (ITDs) in the second-order modulation result in a lateralized percept. Thus, second-order modulation can serve as a tool to investigate whether demodulation of interaurally delayed components is a prerequisite for lateralization. In most of the psychoacoustic experiments presented here, a 25 Hz sinusoidally amplitude-modulated (SAM) 160 Hz tone was either transposed to 4 kHz by half-wave rectifying this SAM waveform before multiplication with a 4 kHz tone (TSAM), or by adding an offset before multiplication (SAMAM). The experiments revealed an inability to lateralize the SAMAM based on ITDs in the 25 Hz component, whereas subjects could lateralize the TSAM. Given that only the TSAM results in a demodulated 25 Hz component after peripheral auditory processing, this result supports the hypothesis that demodulation is a prerequisite for lateralization, which has consequences for temporal modulation processing in models of binaural interaction.     

Resolvent Krylov subspace approximation to operator functions

We consider the approximation of operator functions in resolvent Krylov subspaces. Besides many other applications, such approximations are currently of high interest for the approximation of φ-functions that arise in the numerical solution of evolution equations by exponential integrators. It is well known that Krylov subspace methods for matrix functions without exponential decay show superlinear convergence behaviour if the number of steps is larger than the norm of the operator. Thus, Krylov approximations may fail to converge for unbounded operators. In this paper, we analyse a rational Krylov subspace method which converges not only for finite element or finite difference approximations to differential operators but even for abstract, unbounded operators whose field of values lies in the left half plane. In contrast to standard Krylov methods, the convergence will be independent of the norm of the discretised operator and thus of the spatial discretisation. We will discuss efficient implementations for finite element discretisations and illustrate our analysis with numerical experiments.     

On-line solid-phase extraction high-performance liquid chromatography-tandem mass spectrometry for the quantitative analysis of tacrolimus in whole blood hemolyzate

Tacrolimus is an immunosuppressive drug essential for preventing organ rejection after transplantation. Since tacrolimus strongly binds to erythrocytes, therapeutic monitoring requires its quantification in whole blood lyzate, representing one of the most difficult to analyze biological fluids due to its high protein load. In this communication, we report on the successful combination of whole blood hemolysis employing ionic liquids, followed by sample preparation by means of on-line solid phase extraction (SPE) using restricted access materials (RAM), which permitted the efficient removal of hemoglobin and other large biomolecules. Among six different tested RAM columns, highest hemoglobin depletion and analyte extraction efficiency was obtained with a polymer-based, glycoprotein-coated RAM stationary phase (Biotrap 500 MS) operated at an alkaline pH of 10.7. Analyte quantification was performed by high-performance liquid chromatography-selected reaction monitoring tandem mass spectrometry (HPLC-SRM-MS/MS). The ability to quantify tacrolimus in therapeutically relevant concentrations in whole blood hemolyzates was demonstrated via external calibration with lower limits of detection and quantification of 2.00 and 7.23 ng mL(-1), respectively. Moreover, the investigation of heparin-pretreated blood samples during blood sampling led to an increase in sensitivity for the analyte, while the method appeared to be more robust with ethylenediaminetetraacetic acid as anticoagulant.     

Perturbation theory of selfadjoint matrices and sign characteristics under generic structured rank one perturbations

For selfadjoint matrices in an indefinite inner product, possible canonical forms are identified that arise when the matrix is subjected to a selfadjoint generic rank one perturbation. Genericity is understood in the sense of algebraic geometry. Special attention is paid to the perturbation behavior of the sign characteristic. Typically, under such a perturbation, for every given eigenvalue, the largest Jordan block of the eigenvalue is destroyed and (in case the eigenvalue is real) all other Jordan blocks keep their sign characteristic. The new eigenvalues, i.e. those eigenvalues of the perturbed matrix that are not eigenvalues of the original matrix, are typically simple, and in some cases information is provided about their sign characteristic (if the new eigenvalue is real). The main results are proved by using the well known canonical forms of selfadjoint matrices in an indefinite inner product, a version of the Brunovsky canonical form and on general results concerning rank one perturbations obtained.     

3‐Oxa‐6,8‐diaza‐1,2:4,5‐dibenzocycloocta‐1,4‐dien‐7‐one: a three‐dimensional network assembled by hydrogen‐bonding, π–π and edge‐to‐face interactions

The title compound, C₁₃H₁₀N₂O₂, is the first structure in which the urea moiety is incorporated into an eight‐membered ring. Two mol­ecules are found in the asymmetric unit, which are almost identical in their conformation and their hydrogen‐bond pattern. The carbonyl O atom acts as a double acceptor for the NH groups of two adjacent mol­ecules. In this way, infinite tapes are formed, which are connected viaπ–π and edge‐to‐face interactions in the second and third dimension. This hierarchical order of interactions is confirmed by molecular mechanics calculations. Force‐field and semi‐empirical calculations for a single mol­ecule did not find the envelope conformation present in the crystal, indicating instead a C_s conformation. Only with a model consisting of a hydrogen‐bonded dimer or a larger hydrogen‐bonded section was a conformation found that was similar to the one present in the crystal.