The periplasmic hydrogenase of Desulfovibrio vulgaris (Hildenbourough) is an all Fe-containing hydrogenase. It contains two ferredoxin type [4Fe-4S] clusters, termed the F clusters, and a catalytic H cluster. Recent X-ray crystallographic studies on two Fe hydrogenases revealed that the H cluster is composed of two sub-clusters, a [4Fe-4S] cluster ([4Fe-4S]H) and-a binuclear Fe cluster ([2Fe]H), bridged by a cysteine sulfur. The aerobically purified D. vulgaris hydrogenase is stable in air. It is inactive and requires reductive activation. Upon reduction, the enzyme becomes sensitive to O-2 indicating that the reductive activation process is irreversible. Previous EPR investigations showed that upon reoxidation (under argon) the H cluster exhibits a rhombic EPR signal that is not seen in the as-purified enzyme, suggesting a conformational change in association with the reductive activation. For the purpose of gaining more information on the electronic properties of this unique H cluster and to understand further the reductive activation process, variable-temperature and variable-field Mossbauer spectroscopy has been used to characterize the Fe-S clusters in D. vulgaris hydrogenase poised at different redox states generated during a reductive titration, and in the GO-reacted enzyme. The data were successfully decomposed into spectral components corresponding to the F and H clusters,and characteristic parameters describing the electronic and magnetic properties of the F and H clusters were obtained. Consistent with the X-ray crystallographic results, the spectra of the H cluster can be understood as originating from an exchange coupled [4Fe-4S] - [2Fe] system. In particular, detailed analysis of the data reveals that the reductive activation begins with reduction of the [4Fe-4S]H cluster from the 2+ to the If state, followed by transfer of the reducing equivalent from the [4Fe-4S]H subcluster to the binuclear [2Fe]H subcluster. The results also reveal that binding of exogenous CO to the H cluster affects significantly the exchange coupling between the [4Fe-4S]H and the [2Fe]H subclusters. Implication of such a CO binding effect is discussed.

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The changes occurring during the heating of a borosilicate glass have been investigated by differential thermal analysis, dilatometric analysis and thermomechanical analysis. The thermal properties of this glass, such as glass transition temperature, dilatometric softening temperature and linear thermal expansion coefficient, have been determined. Viscosity measurements in the temperature range 898-1048 K were performed in a thermomechanical analyser equipped with a penetration attachment for isothermal measurement, and from the temperature dependence of viscosity a value of 290 kJ mol-1 was obtained for the activation energy for viscous flow. Devitrification of the glass was observed, specifically in finely powdered glass samples, where the precipitation of cristobalite was identified by X-ray diffraction. Glass powder compacts sintered by viscous flow and cristobalite precipitation reduced strongly the shrinkage rate. © 2001 Elsevier Science B.V.

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Working teams in enterprise environment are considered as the most advanced forms of work organisation. This means the forms that can improve productivity quality of working life. Nevertheless, it prevails a slow development and dissemination of these advanced organisational forms in European companies. The reason for that lays in a complex linkage factors from social values to the economical pressures. But other factors are also related to the national systems of education training, to the different systems of industrial relations and technology policy.

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This paper deals with the determination of plasma impedance and ion density in r.f. plasmas using different mixtures of silane with methane or ethylene and r.f. powers. The aim is to correlate these parameters with carbon and hydrogen contents of the films produced. The data achieved show that the best carbon incorporation is achieved using ethylene gas mixtures, under low gas mixture concentration, where the substrate also sustains a low ion bombardment. The data also show that particulates in the plasma can be more easily formed in the ethylene-based processes. © 2001 Published by Elsevier Science B.V.

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Water trees result from ac electrical aging of the polymeric insulation of medium and HV power cables in a humid or wet environment. As suggested by their name, they arise from penetration of water in the polymer. Visual observation with the help of an optical microscope shows tree (bush) type structures. This suggests that water trees might be fractal objects. Calculation of the fractal dimension from experimental samples may confirm the fractal characteristics and also give information on the damage caused to the polymer. In this work images of water trees taken under the optical microscope, dyed by methylene blue and etched for scanning electron microscopy (SEM), were studied in order to estimate the fractal dimension using a box-counting algorithm. The photographs, made using an optical microscope (scale of 100 mum), of the dyed samples were obtained from laboratory-aged low-density polyethylene (LDPE) specimens using accelerated techniques. Different field amplitude and frequency and also time of aging were used and the dimension values were compared. SEM images resulting from aged cross-linked polyethylene (XLPE) cables revealed a structure at a different scale (similar to 3 mum). Each photograph was analyzed to compare regions with and without water trees.

Water trees result from ac electrical aging of the polymeric insulation of medium and HV power cables in a humid or wet environment. As suggested by their name, they arise from penetration of water in the polymer. Visual observation with the help of an optical microscope shows tree (bush) type structures. This suggests that water trees might be fractal objects. Calculation of the fractal dimension from experimental samples may confirm the fractal characteristics and also give information on the damage caused to the polymer. In this work images of water trees taken under the optical microscope, dyed by methylene blue and etched for scanning electron microscopy (SEM), were studied in order to estimate the fractal dimension using a box-counting algorithm. The photographs, made using an optical microscope (scale of 100 mum), of the dyed samples were obtained from laboratory-aged low-density polyethylene (LDPE) specimens using accelerated techniques. Different field amplitude and frequency and also time of aging were used and the dimension values were compared. SEM images resulting from aged cross-linked polyethylene (XLPE) cables revealed a structure at a different scale (similar to 3 mum). Each photograph was analyzed to compare regions with and without water trees.

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