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2Mg-Fe mixtures produced by high-energy ball milling were consolidated into bulk form by hot extrusion at different processing temperatures (573 K (300 °C), 623 K (350 °C) and 673 K (400 °C)), aiming to evaluate their influence on the structure and microstructure of bulk materials and their consequent influence on the hydrogen sorption properties. In spite being in the nanosize range, the highest the processing temperature, the larger the grain sizes. However, the nanometric grain size remained after any hot extrusion condition, as estimated by Rietveld refinement. The pinning effect of Fe on Mg grain boundaries explained this effect. In the first absorption (activation), powders showed a hydrogen storage capacity of ∼4.53 wt%, while the extruded samples (bulk materials) reached almost the same capacity during the period of hydrogenation (∼94% of the maximum hydrogen storage capacity for Mg 2 FeH 6 - 5.5 wt%). The smallest crystallite sizes and highest surface area for hydrogenation explain the good performance of powders. However, when comparing only extruded samples, it was observed that the highest capacity and the lowest incubation times were mainly related to grain sizes and to the favorable texture along (002) plane of αMg. The desorption temperature of bulk materials was very similar to that of powders, which is good considering the lower surface area of bulk materials. [ABSTRACT FROM AUTHOR]

Abstract: Carbon-containing refractories are widely used in the steelmaking process due to their outstanding properties and, in order to improve their oxidation resistance, the so-called antioxidants have often been used. Al4SiC4 is pointed out as a novel additive that presents suitable properties such as Al, but without its drawbacks. Therefore, the effect of Al4SiC4 addition to Al2O3 yph name="sbnd" />SiO2 es designed for lining blast furnace troughs was investigated in this work. Apparent porosity, oxidation, thermogravimetric, X-ray diffraction, hot elastic modulus tests and thermodynamic calculations were carried out in order to better understand the antioxidant effects and reaction mechanisms. Additionally, the collected results were compared with those from the compositions containing other commonly used antioxidants (Si, B4C and sodium borosilicate glass). The performance of the novel additive proved to be limited as most of the carbon source used reacted earlier than the Al4SiC4 action. As a consequence, intense carbon oxidation, along with the thermal expansion mismatch among the phases during the cooling step, intensified the deterioration of the evaluated refractory material. [Copyright &y& Elsevier]

Abstract: Even though many investigations have focused on describing the kinetics of both MgO hydration and Mg(OH)2 decomposition, few studies have addressed the understanding of the mechanisms to control or to modify the reaction of magnesia with water. Silica is the most applied anti-hydration additive in MgO-containing castables. The use of silico-phosphates (SiO2 rich and presenting higher reactivity and solubility) seems to be a promising alternative to induce a faster generation of magnesium–silicate–hydrated gels on the MgO grain surface, inhibiting brucite formation. In the present work, the performance of a commercial phosphate chemical binder (combined or not with silica fume) in Al2O33MgO refractory castables and MgO aqueous suspensions was evaluated. Thermodynamic simulations, thermogravimetric measurements, X-ray diffraction, splitting tensile and hot elastic modulus tests were carried out to understand the additive performance and the characterization of the castables properties. According to the simulated Pourbaix diagrams, two different gels [Mg3Si2O5(OH)4 or Mg3Si4O10(OH)2] can be formed in the structure of the samples (depending on the Mg:Si molar ratio), halting the MgO hydration. Due to the high amount of MgO (6wt%) contained in the designed Al2O33MgO castables, the chrysotile-like phase [Mg3Si2O5(OH)4] should be the main compound formed during the refractories' processing. Moreover, the blend of silica fume and the phosphate-based additive consisted of the most effective route to stop the Mg(OH)2 formation so that castables with high flowability, mechanical strength levels and elastic modulus values can be prepared. [Copyright &y& Elsevier]