Pore structure of bulk tungsten carbide powder catalysts

Bulk tungsten carbide catalysts are prepared by direct carburization/reduction of tungsten trioxide in methane-hydrogen mixtures. The catalytic properties of such catalysts have been studied by several authors. The porous structure of these catalysts is studied by adsorption of N₂, Kr, CF₄ and neohexane. Adsorption isotherms and hysteresis loops for the catalysts suggest the presence of a microporous structure made of parallel plates distant approximately by 20 Å. These results are compared to those obtained using such catalysts for hydrogen oxidation and where condensation in the porous structure was observed.     

Fast computation of minimal elementary decompositions of metabolic flux vectors

The concept of elementary flux vector is valuable in a number of applications of metabolic engineering. For instance, in metabolic flux analysis, each admissible flux vector can be expressed as a non-negative linear combination of a small number of elementary flux vectors. However a critical issue concerns the total number of elementary flux vectors which may be huge because it combinatorially increases with the size of the metabolic network. In this paper we present a fast algorithm that randomly computes a decomposition of admissible flux vectors in a minimal number of elementary flux vectors without explicitly enumerating all of them.     

Lyapunov exponential stability of 1-D linear hyperbolic systems of balance laws

Explicit boundary dissipative conditions are given for the exponential stability in $L^2$-norm of one-dimensional linear hyperbolic systems of balance laws ${?}_t\xi +\Lambda {?}_x\xi ?M\xi =0$ over a finite interval, when the matrix $M$ is marginally diagonally stable. The result is illustrated with an application to boundary feedback stabilisation of open channels represented by linearised Saint–Venant–Exner equations.     

Comparison of fuzzy c-means classification,linear mixture modelling and MLC probabilities as tools for unmixing coarse pixels

Three different 'soft' classifiers (fuzzy c-means classifier, linear mixture model, and probability values from a maximum likelihood classification) were used for unmixing of coarse pixel signatures to identify four land cover classes (i.e., supervised classifications). The coarse images were generated from a 30m Thematic Mapper (TM) image; one set by mean filtering, and another using an asymmetric filter kernel to simulate Multi-Spectral Scanner (MSS) sensor sampling. These filters collapsed together windows of up to 11 11 pixels. The fractional maps generated by the three classifiers were compared to truth maps at the corresponding scales, and to the results of a hard maximum likelihood classification. Overall, the fuzzy c-means classifier gave the best predictions of sub-pixel landcover areas, followed by the linear mixture model. The probabilities differed little from the hard classification, suggesting that the clusters should be modelled more loosely. This paper demonstrates successful methods for use and comparison of the classifiers that should ideally be extended to a real dataset.     

The influence of oxide surface layers on bulk electron probe microanalysis of oxygen—application to Ti-Si-O compounds

This paper discusses a generalized method to measure with the electron probe microanalyzer (EPMA) the oxygen in a material containing a surface oxide layer. The continuum background is the most difficult to measure, particularly for materials in which oxygen-free samples cannot be produced. The method depends on the preparation of either oxygen-free samples or well characterized oxygen-containing samples. Specific application of the method to the Ti-Si-O system is discussed. In addition, measurements of oxide surface-layer thickness of 3.6–8.0 nm on Ti and Ti-Si compounds were obtained using EPMA and a scanning Auger microprobe (SAM). The nature of the oxide surface layers was shown using x-ray photoelectron spectroscopy (XPS).     

A fuzzy c-means classification of elevation derivatives to extract the morphometric classification of landforms in Snowdonia, Wales

The increasing global coverage of high resolution/large-scale digital elevation data has allowed the study of geomorphological form to receive renewed attention by providing accessible datasets for the characterisation and quantification of land surfaces. Digital elevation models (DEMs) provide quantitative elevation data, but it is the characterisation and extraction of geomorphologically significant measures (morphometric indices) from these raw data that form more informative and useful datasets. Common to many geographical measures, morphometric measures derived from DEMs are dependent on the scale of observation. This paper reports results of employing a fuzzy c-means classification for a sample DEM from Snowdonia, Wales, with a number of morphometric measures at different resolutions as input, and morphometric classification of landforms at each resolution as output. The classifications reveal that different landscape components or morphometric classes are important at different resolutions, and that morphometric classes exhibit resolution dependency in their geographical extents. Examination of the scale dependency and behaviour of morphometric classifications of landforms at different resolutions provides a fuller and more holistic view of the classes present than a single-scale analysis.     

Solid-state Reactions and Phase Relations in the Ti-Si-O System at 1373 K

Reactions between Ti and SiO₂ were studied at 1373 K (1100°C) under vacuum conditions using planar diffusion couples. A method to correct for the presence of surface oxide was developed which led to improved oxygen measurements with the electron probe microanalyzer. An isothermal section through the Ti-Si-O phase diagram at 1373 K was determined using measured diffusion paths and phase compositions in equilibrated alloys. The experimentally determined isothermal section was compared to isothermal sections calculated using thermodynamic data. In addition the sequence of reaction layers formed in the diffusion couples is discussed in terms of thermodynamic activity diagrams.     

Control of wheeled mobile robots not satisfying ideal velocity constraints: A singular perturbation approach

We propose in this paper a singular perturbation formulation for mechanical systems not exactly satisfying velocity constraints. We particularize to ‘wheeled mobile robots’ which do not satisfy ideal pure rolling and nonslipping constraints. We study the robustness of static-state linearizing feedback laws which have been obtained to solve a point tracking problem in the ideal case when constraints were satisfied.