Moisés Naranjo

Sintering of Al/AlN composite powder obtained by gas–solid reaction milling

Abstract Attrition milling in ammonia gas was used to obtain aluminum composite powder by partial nitriding of Al powder. The composite powder is very hard, thermally stable and shows excellent sinterability. The reason for the high sinterability is not clear at this stage of the research. Samples consolidated by cold pressing and vacuum sintering achieve a hardness of 167 HB and a bending strength of 920 MPa.

Structure-directing effect of phyllosilicates on the synthesis of y-Y2Si2O7. Phase transitions in Y2Si2O7

This paper describes the low temperature hydrothermal synthesis of y-Y2Si2O7 from a layered silicate (2 ∶ 1 clay), a precursor which acts as a structure-templating agent. Both the transformations undergone by the clay during the hydrothermal process and the new products formed at different reaction stages have been characterised at both long and short range orders. Secondly, the influence of the aluminium content of the clay on the stability of y-Y2Si2O7 has been investigated. For this purpose, another layered clay, containing nominally no Al, has been submitted to the same hydrothermal treatment as the previous one. The results indicate that the amount of aluminium present in the starting material assists the stabilisation of y-Y2Si2O7. The behaviour of y-Y2Si2O7 with increasing temperature has been studied and the sequence of polymorphs and transition temperatures are reported and compared with those found in the literature.

Hydration properties of synthetic high-charge micas saturated with different cations: An experimental approach

Abstract An understanding of the interaction mechanisms between exchangeable cations and layered silicates is of interest from both a basic and an applied point of view. Among 2:1 phyllosilicates, a new family of swelling high-charge synthetic micas has been shown to be potentially useful as decontaminant. However, the location of the interlayer cations, their acidity and the water structure in the interlayer space of these silicates are still unknown. The aim of this paper was therefore to study the hydration state of the interlayer cations in the interlayer space of high-charge expandable micas and to evaluate the effect that this hydration has on the swelling and acidity behavior of these new materials. To achieve these objectives, three synthetic micas with different charge density total layer charges (ranging between 2 and 4 per unit cell) and with five interlayer cations (Na+, Li+, K+, Mg2+, and Al3+) were synthesized and their hydration state, interlayer space, and acidity analyzed by DTA/TG, XRD, and 1H MAS NMR spectroscopy. The results showed that the hydration state depends on both the layer charge and the nature of the interlayer cation. A high participation of the inner-sphere complexes in the highly charged confined space has been inferred and proposed to induce Brønsted acidity in the solid.

Wear of aluminium-base materials processed by mechanical milling in air or ammonia

Abstract The tribological behaviour of mechanically alloyed (MA) aluminium-base materials has been studied as a function of processing methods. In particular, a new sintered MA aluminium material, milled under an ammonia atmosphere (MA Al-NH 3 ) and consolidated by a single cycle of cold compaction and sintering, is compared with conventional MA aluminium (MA Al-air), milled in confined air and consolidated by a double cycle of compaction and sintering. The wear behaviour of the aluminium-base materials has been studied in a pin-on-disk tribometer, sliding against AISI 52100 steel pins. Dry wear tests have been carried out under variable load and temperature, to show that milling conditions, specifically milling atmosphere, have a strong influence both on mechanical properties and wear resistance. The higher wear resistance of MA Al-NH 3 with respect to MA Al-air increases with the severity of the contact conditions. Wear mechanisms are discussed from SEM observation of wear tracks, wear debris morphology and transfer tribolayers.