Scientific Papers in SCI

The influence of firing temperature on the ceramic properties of illite-chlorite-calcitic clays has been investigated. Three samples of the same clay deposit have been selected. Weight loss, dimensional changes, water absorption, bulk density, open porosity, flexural and compressive strengths, initial capillary water absorption rate and thermal conductivity have been determined as a function of firing temperature in the range 900-1200 degrees C with 1 h of soaking time. The microstructures of the fired samples have been examined by SEM and the phase evolution studied by XRD. The water absorption capacity decreased from similar to 22% at 900 degrees C/1 h to a maximum of 12% at 1200 degrees C/1 h with a maximum linear shrinkage of similar to 2.7%. The open porosities decreased from similar to 22% at 900 degrees C/1 h up to similar to 20% at 1200 degrees C/1 h as an effect of progressive sintering with higher densification degree of the ceramic bodies. The flexural strength reached a maximum value of similar to 34 MPa at 1200 degrees C/1 h. In contrast, the compressive strengths increased by firing up to a maximum of similar to 114 MPa at 1200 degrees C/1 h. The thermal conductivity increased slightly as increasing firing temperature with a maximum value of 0.582 W/m.K in samples fired at 1200 degrees C/1 h. The Ryshkevitch-Duckworth equation was applied and the results indicated that compressive strength is related linearly with open porosity. A linear correlation was found between thermal conductivity and open porosity. The microstructural evolution by SEM indicated that there is a change of the fired samples at 1100 degrees C as compared to SEM observations at 900 and 1000 degrees C. There is an increase of contacts between particles and layered structures associated to dehydroxylated clay minerals (illite and chlorite), quartz particles and pores developed by firing. At 1200 degrees C/1 h, the microstructures have changed associated to the higher degree of vitrification in the fired sample, with consolidation of the material, interparticle and neck contacts with formation of vitrified bridges. The formation of closed and large open pores of several sizes has been achieved by firing. Small particles were observed as a fine precipitation of crystals in the vitrified structures associated to anorthite, hematite and quartz relicts. This change in microstructure allowed deduce that the compressive strength increased upon firing, with maximum values of this ceramic property at 1200 degrees C. The ceramic bodies were more sintered by firing and the open porosity decreased progressively. Brickmaking is the main application of these fired illite-chlorite calcitic clays. These clays fired at 900-1100 degrees C, with 1 h of soaking time, could be applied in the fabrication of clay roofing tiles, tiles and even porous ceramic supports with small variations on shrinkage and porosity, good flexural strengths and high compressive strengths. Samples fired at higher temperatures, 1100 degrees C/1 h, could be applied as ceramic bricks showing a medium porosity (similar to 20%). They show almost the same bulk density when they are fired at lower temperatures (900 degrees C). Samples fired at higher temperatures (1150-1200 degrees C/1 h) could be applied as dark ceramic products. This investigation was interesting because a better knowledge of illite-chlorite-calcitic clays applied as ceramic raw materials has been achieved.

An almost single phase 14 M modulated martensite is obtained in melt spun ribbon of Ni55Fe17Ga26Co2 Heusler alloy. The effect of thermal treatments on the stability of the reverse martensitic transformation from 14 M modulated martensite to austenite phase in this system has been investigated by both non -isothermal and isothermal treatments. Heating above martensitic transformation promotes a continuous reduction of the martensitic transformation temperature, which stabilizes the austenite phase at room temperature and induces the precipitation of the gamma phase. However, thermal treatments at tem-peratures between the austenite start and finish temperatures induce the decoupling of the austenite formation in a subsequent heating. The two successive reverse martensitic transformations could be as-cribed to the untransformed martensite in the previous interrupted heating and to the new martensite formed during cooling.

In this study, palladium is proposed as an active site for formic acid dehydrogenation reaction. Pd activity was modulated with Co metal with the final aim of finding a synergistic effect that makes possible efficient hydrogen production for a low noble metal content. For the monometallic catalysts, the metal loadings were optimized, and the increase in the reaction temperature and presence of additives were carefully considered. The present study aimed, to a great extent, to enlighten the possible routes for decreasing noble metal loading in view of the better sustainability of hydrogen production from liquid organic carrier molecules, such as formic acid.

Sputtering of silicon in a Helium magnetron discharge has been reported as a bottom-up procedure to obtain amorphous Si films containing high amounts of gas-filled nanopores. Here we compare the microstructure and composition of Si-He nanocomposite films deposited by magnetron sputtering (MS) with 4He in DC or RF and 3He in RF operation modes. Electron microscopy (SEM and TEM), X-ray diffraction (XRD) and ion beam analysis (IBA) have been used to analyze the films and to investigate the in-situ and ex-situ thermal evolution. Depending on deposition conditions different in depth compositions, nanopore size and shape distributions, porosity and He content could be obtained. The presence of impurities (i.e. oxygen) has shown to promote He diffusivity reducing He accumulation. The start temperature of He-release varied in the range 473-723 K without films crystallization. Films grown in RF mode reached contents of 32 and 29 at% of 4He and 3He and were respectively stable up to 573 and 723 K both in vacuum and under inert gas flow. In-situ p-EBS (proton Elastic Back Scattering) allowed monitoring the He release accompanied by blistering/delamination effects visualized by SEM. These results show the potentiality of annealing to hold nano-porous structures after liberation of trapped gas.