The effect of Nb or Ta substitution into the M1 phase of the MoV(Nb,Ta)TeO selective oxidation catalyst

We use aberration corrected high-angle annular dark field (HAADF) imaging to systematically study, atomic column by atomic column, the effects of substituting Nb or Ta into the M1 phase of the MoV(Nb,Ta)TeO propane (amm)oxidation catalyst. The HAADF results indicate that the x,y coordinates of the metal sites within the M1 framework are unaffected by the substitution of either Nb or Ta for Mo. The HAADF analysis of the Ta-substituted catalyst demonstrated that the Ta preferentially substitutes into the pentagonal bipyramidal site, and by analogy, we anticipate that Nb substitutes similarly. Compositional analysis of the entire framework suggests that Ta/Nb behaves as a director of V among the octahedra that link the pentagonal rings, and the variable V occupancy may be correlated with variations in catalytic activities and selectivities. Finally, HAADF imaging provided evidence of coexistence of Ta-rich and Ta-poor domains. Similar phase segregation behavior may be present in Nb-substituted specimens, but would be very difficult to detect.     

Simulations of delamination in CFRP laminates: Effect of microstructural randomness

Due to their high specific strength and stiffness, fibre-reinforced composite materials are being increasingly used in structural applications where a high level of performance is important (e.g. aerospace, automotive, offshore structures, etc.). Performance in service of these composites is affected by multi-mechanism damage evolution under loading and environmental conditions. For instance, carbon fibre-reinforced laminates demonstrate a wide spectrum of failure mechanisms such as matrix cracking and delamination. These damage mechanisms can result in significant deterioration of the residual stiffness and load-bearing capacity of composite components and should be thoroughly investigated. The delamination failure mechanism is studied in this paper for a double cantilever beam (DCB) loaded in mode I. Several sensitivity studies are performed to analyse the effects of mesh density and of parameters of the cohesive law on the character of damage propagation in laminates. The microstructural randomness of laminates that is responsible for non-uniform distributions of stresses in them even under uniform loading conditions is accounted for in the model. The random properties are introduced with the use of Weibull’s two-parameter probability density function. Several statistical realisations are carried out which show that the effect of microstructure could significantly affect the macroscopic response emphasizing the need to account for microstructural randomness for accurate predictions of load-carrying capacity of laminate composite structures.     

The impact of thermal conductivity and diffusion rates on water vapor transport through gas diffusion layers

Proper water management in a hydrogen-fueled polymer electrolyte membrane (PEM) fuel cell is critical for performance and durability. A mathematical model has been developed to elucidate the effect of thermal conductivity and water vapor diffusion coefficient in the gas diffusion layers (GDLs). The fraction of product water removed in the vapor phase through the GDL as a function of GDL properties/set of material and component parameters and operating conditions has been calculated. The current model enables identification of conditions wherein condensation occurs in each GDL component. The model predicts the temperature gradient across various components of a PEM fuel cell, providing insight into the overall mechanism of water transport in a given cell design. The water condensation conditions and transport mode in the GDL components depend on the combination of water vapor diffusion coefficients and thermal conductivities of the GDL components. Different types of GDLs and water transport scenarios are defined in this work, based on water condensation in the GDL and fraction of water that the GDL removes through the vapor phase, respectively.     

Combined biochemistry and histocytochemistry as a tool to investigate Ecto-ATPase in the cardiac muscle

Ecto-ATPase (ecto-adenosine triphosphatase), a key enzyme of cardiac metabolism, is responsible for modulation of the concentration of extracellular nucleotides in the heart. We present methodology consisting of the combined use of biochemical and histocytochemical techniques to study its properties. Using samples from essentially the same preparation, we applied biochemistry and histocytochemistry to determine biochemical characteristics of ecto-ATPase and an in situ localization of its reactivity. Our results indicated that detected enzyme resists fixation, depends on divalent ions, and hydrolyzes ATP, but not AMP or ADP-beta-S. Reaction product of the enzyme activity was found confined to the extracellular surface of the plasma membrane of cardiac myocytes and endothelial cells due to the corresponding orientation of the enzyme active sites. Experiments using an inhibitor justified specificity of the reaction. When used together with molecular biological and immunocytochemical techniques, the present methodological approach should be capable of yielding important information about the actual ability of ecto-ATPase to operate.     

Angle- and size-dependent characteristics of incoherent Raman and fluorescent scattering by microspheres. 1. General expressions

A dipole model is used to simulate incoherent Raman and fluorescent scattering by microspheres. The use of the addition theorem for spherical harmonics circumvents the need to evaluate double sums in the final formulas, thereby drastically reducing computational effort. Special attention is paid to consideration of backscattering geometry, which is important for lidar applications. The formulas derived for backscattering geometry decrease the computation time for size parameter x approdximately 100 by a factor of 200 compared with the time for calculations performed at other angles.     

Inversion with regularization for the retrieval of tropospheric aerosol parameters from multiwavelength lidar sounding

We present an inversion algorithm for the retrieval of particle size distribution parameters, i.e., mean (effective) radius, number, surface area, and volume concentration, and complex refractive index from multiwavelength lidar data. In contrast to the classical Tikhonov method, which accepts only that solution for which the discrepancy reaches its global minimum, in our algorithm we perform the averaging of solutions in the vicinity of this minimum. This averaging stabilizes the underlying ill-posed inverse problem, particularly with respect to the retrieval of number concentration. Results show that, for typical tropospheric particles and 10% error in the optical data, the mean radius could be retrieved to better than 20% from a lidar on the basis of a Nd:YAG laser, which provides a combination of backscatter coefficients at 355, 532, and 1064 nm and extinction coefficients at 355 and 532 nm. The accuracy is improved if the lidar is also equipped with a hydrogen Raman shifter. In this case two additional backscatter coefficients at 416 and 683 nm are available. The combination of two extinction coefficients and five backscatter coefficients then allows one to retrieve not only averaged aerosol parameters but also the size distribution function. There was acceptable agreement between physical particle properties obtained from the evaluation of multiwavelength lidar data taken during the Lindenberg Aerosol Characterization Experiment in 1998 (LACE 98) and in situ data, which were taken aboard aircraft.     

Software Distributed Shared Memory: a VIA‐based implementation and comparison of sequential consistency with home‐based lazy release consistency

A Distributed Shared Memory (DSM) system provides a distributed application with a shared virtual address space. This article proposes a design for implementing the DSM communication layer on top of the Virtual Interface Architecture (VIA), an industry standard for user‐level networking protocols on high‐speed clusters. User‐level communication protocols operate in user mode, thus removing the operating system kernel's overhead from the critical communication pass, and significantly diminishing communication overhead as a result. We analyze VIA's facilities and limitations in order to ascertain which implementation trade‐offs can be best applied to our development of an efficient communication substrate optimized for DSM requirements. We then implement a multithreaded version of the Home‐based Lazy Release Consistency (HLRC) protocol on top of this substrate. In addition, we compare the performance of this HLRC protocol with that of the Sequential Consistency (SC) protocol in which a Multi View (MV) memory mapping technique was used. This technique enables a fine‐grained access to shared memory, while still relying on the virtual memory hardware to track memory accesses. We perform an ‘apple‐to‐apple’ comparison on the same testbed environment and benchmark suite, and investigate the effectiveness and scalability of both protocols. Copyright © 2005 John Wiley & Sons, Ltd.