Using a recently developed procedure combining isothermal and non-isothermal current measurements space charge trapping and transport in LDPE was successfully studied. Unaged, thermally and electrically aged samples were investigated. The samples were conditioned before each measurement in order to obtain reproducible results. In the non-isothermal measurements appeared a broad peak (40degreesC to 50degreesC) that was possible to decompose into two or three peaks (35, 45 and 65degreesC). At even higher temperature another peak was sometimes present (85degreesC) depending on the prior sample conditioning. The space charge is trapped near the surface in deep traps (maximum depth of approximate to 15 mum). Relaxation times, mobilities and activation energies have been calculated for different charging/discharging conditions. For unaged samples the reproducibility of the results was poor while for the aged polyethylene it was quite good, meaning that aging helps conditioning. In the electrically aged LDPE there is a decrease of conductivity and the broad peak of the non-isothermal spectra shows a slight shift towards higher temperatures when compared with the data found in the thermally aged polymer.

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The thermal decomposition of 2-azidoacetamide (N3CH2CONH2) has been studied by matrix-isolation infrared spectroscopy and real-time ultraviolet photoelectron spectroscopy. N2, CH2NH, HNCO, CO, NH3, and HCN are observed as high-temperature decomposition products, while at lower temperatures, the novel imine intermediate H2NCOCHNH is observed in the matrix-isolation IR experiments. The identity of this intermediate is confirmed both by ab initio molecular orbital calculations of its IR spectrum and by the temperature dependence and distribution of products in the photoelectron spectroscopy (PES) and IR studies. Mechanisms are proposed for the formation and decomposition of the intermediate consistent both with the observed results and with estimated activation energies based on pathway calculations.

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The cytochrome c nitrite reductase is isolated from the membranes of the sulfate-reducing bacterium Desulfovibrio desulfuricans ATCC 27774 as a heterooligomeric complex composed by two subunits (61 kDa and 19 kDa) containing c-type hemes, encoded by the genes nrfA and nrfH, respectively. The extracted complex has in average a 2NrfA:1NrfH composition. The separation of ccNiR subunits from one another is accomplished by gel filtration chromatography in the presence of SDS. The amino-acid sequence and biochemical subunits characterization show that NrfA contains five hemes and NrfH four hemes. These considerations enabled the revision of a vast amount of existing spectroscopic data on the NrfHA complex that was not originally well interpreted due to the lack of knowledge on the heme content and the oligomeric enzyme status. Based on EPR and Mossbauer parameters and their correlation to structural information recently obtained from X-ray crystallography on the NrfA structure {[}Cunha, C. A., Macieira, S., Dias, J.M., Almeida, M.G., Goncalves, L. M. L., Costa, C., Lampreia, J., Huber, R., Moura, J. J. G., Moura, I. & Romano, M. (2003) J. Biol. Chem. 278, 17455-17465], we propose the full assignment of midpoint reduction potentials values to the individual hemes. NrfA contains the high-spin catalytic site (-80 mV) as well as a quite unusual high reduction potential (+150 mV)/low-spin bis-His coordinated heme, considered to be the site where electrons enter. In addition, the reassessment of the spectroscopic data allowed the first partial spectroscopic characterization of the NrfH subunit. The four NrfH hemes are all in a low-spin state (S = 1/2). One of them has a g(max) at 3.55, characteristic of bis-histidinyl iron ligands in a noncoplanar arrangement, and has a positive reduction potential.

The gene encoding cytochrome c nitrite reductase (NrfA) from Desulfovibrio desulfuricans ATCC 27774 was sequenced and the crystal structure of the enzyme was determined to 2.3-Angstrom resolution. In comparison with homologous structures, it presents structural differences mainly located at the regions surrounding the putative substrate inlet and product outlet, and includes a well defined second calcium site with octahedral geometry, coordinated to propionates of hemes 3 and 4, and caged by a loop non-existent in the previous structures. The highly negative electrostatic potential in the environment around hemes 3 and 4 suggests that the main role of this calcium ion may not be electrostatic but structural, namely in the stabilization of the conformation of the additional loop that cages it and influences the solvent accessibility of heme 4. The NrfA active site is similar to that of peroxidases with a nearby calcium site at the heme distal side nearly in the same location as occurs in the class II and class III peroxidases. This fact suggests that the calcium ion at the distal side of the active site in the NrfA enzymes may have a similar physiological role to that reported for the peroxidases.

Functionally graded materials (FGM’s), particularly in the form of Al-Si metal matrix composites (MMC’s) selectively reinforced at the surface with SiC particles, are advanced materials, combining high wear resistance with high bulk toughness or even a thermal barrier at the surface. Centrifugal casting is one of the most effective methods for processing this type of MMC, but accurate control of the ceramic particles distribution/gradient in the metallic matrix has not yet been completely attained. In this work, precursor Al/SiC composites were prepared by rheocasting, using SiC particles and an Al-10Si–2.2 Mg alloy. Morphology of the SiC particles was previously characterized by laser interpherometry and SEM. Differing grain sizes were selected as reinforcing elements. The MMC’s were then molten and centrifugally cast in order to produce the FGM composites, whose structure and properties were investigated by XRD, quantitative image analysis of optical micrographs and longitudinal hardness profiles. Therefore, it was possible to evaluate the influence of the particle grain size on the structure and properties of the FGM. Apart from the evaluation of the effects of particle grain size per se, its influence when combined with differing casting conditions are reported as well.

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The concept of functionally graded material (FGM), may be considered as a model particularly interesting to be applied in components for the automotive industry, if reliability and cost can be controlled in an advantageous way. In fact, coupling superior superficial wear resistance with a significant bulk toughness, without compromising important weight savings, by using Al-Si metal matrix composites (MMC’s) selectively reinforced at the surface with SiC particles, is likely to be considered as a innovative advance to that industrial field, if adequate production techniques are developed. Casting under a centrifugal force may well be considered as one of the most effective methods for processing Al-based FGM´s. A primary problem to be faced when producing MMC´s reinforced with ceramic particles is related with the imperfect wetting of the ceramic particles by the molten matrix alloy. A first consequence of defective wetting may be the formation of ceramic-ceramic, ceramic-gas and/or metal-gas interfaces, instead of the desired metal-ceramic interface. Secondly, wetting phenomena play an essential role regarding the physical, chemical and mechanical characteristics of the metal/ceramic interface. A general consequence of these aspects may be related with the degradation of the material properties, be it mechanical, chemical, or thermal in nature. The present work refers to an X-ray microtomography experiment aiming at the elucidation of some aspects regarding particle distribution in SiCp-reinforced functionally graded aluminium composites. Precursor composites were produced by rheocasting. These were then molten and centrifugally cast in order to produce the FGM composites. From these, small cylindrical samples were extracted and observed by X-ray microtomography at the European Synchrotron Radiation Facility (ESRF). The 3D tomographic images were obtained in edge-detection mode (phase-contrast mode), and an adequate segmentation procedure was employed to isolate the pores and SiC particles from the Al matrix. This has allowed a study of the relations between the matrix, the SiC particles, and locally intervening porosities of varying shapes, aiming at a better understanding of the mechanisms involved.

A novel iron(III) porphyrin disulphide derivative have been successfully immobilised on gold surfaces by self-assembly. The redox response of the modified electrodes was compared with the obtained for a similar iron porphyrin in solution, confirming the immobilisation of the metalloporphyrin. The gravimetric data obtained by electrochemical quartz crystal microbalance (EQCM) during adsorption allowed an estimation of the electrode coverage, providing further evidence for the formation of the porphyrin SAM. The modified electrodes were also measured by conventional and imaging ellipsometry. The electrocatalytic activity of the two modified electrodes was tested for the reduction of the molecular oxygen. (C) 2002 Elsevier Science B.V. All rights reserved.

The present work refers to an X-ray microtomography experiment aiming at the elucidation of some aspects regarding particle distribution in SiC-particle-reinforced functionally graded aluminium composites.
Precursor composites were produced by rheocasting. These were then molten and centrifugally cast to obtain the
functionally graded composites. From these, cylindrical samples, around 1 mm in diameter, were extracted, which were then irradiated with a X-ray beam produced at the European Synchrotron Radiation Facility.
The 3-D images were obtained in edge-detection mode. A segmentation procedure has been adapted in order to
separate the pores and SiC particles from the Al matrix. Preliminary results on the particle and pore distributions are presented.

The present study reports low velocity impact tests on composite laminate plates reinforced either with Kevlar 29 or Dyneema. The tests are produced using a Rosand Precision Impact tester. The experimental results obtained for Kevlar 29 are simulated numerically. The deflection history and the peak of the impact force are compared with experimental data and used to calibrate the numerical model.

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