Memoria (del) Simposium Internacional Sobre Medicina Veterinaria Preventiva, Monterrey, N.L. 25 al 27 de mayo de 1992

UANL

Foro mundial de educación, Porto Alegre, 24 al 27 de Octubre de 2001 : Carta de Porto Alegre por la Educación Pública para todos.

Resumen de los autores

Primer encuentro del profesorado madrileño de Ciencias Sociales : del 17 al 27 de abril de 1989.

Primer Encuentro del Profesorado Madrileño de Ciencias Sociales que tiene como objetivo fundamental generar propuestas innovadoras y estimular la elaboración de nuevos materiales curriculares entre el profesorado. Para ello, además de las conferencias, comunicaciones y presentación de experiencias didácticas prácticas, se exponen diferentes materiales y recursos a través de talleres de trabajo (A quien corresponda, Jugando a través del tiempo y del espacio, Viajando por Madrid, El ordenador, Taller de radio, Madrid ayer y hoy) con el proposito de difundir sus posibilidades educativas y al mismo tiempo proponer actividades lúdicas que sirvan de apoyo al trabajo de las áreas, potencien técnicas de trabajo en equipo y fomenten la preocupación por su entorno.

Conferencia de Ministros Europeos de Educación : Bruselas, del 25 al 27 de noviembre de 1969.

Continúa la publicación con el título : Revista de información de la Comisión Nacional Española de Cooperación con la UNESCO

Segundas Jornadas Uruguayas de estudios clásicos - 25 al 27 de abril de 2002 - Montevideo

Fil: Buisel, María Delia. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Segundas Jornadas Uruguayas de estudios clásicos - 25 al 27 de abril de 2002 - Montevideo

Fil: Buisel, María Delia. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Segundas Jornadas Uruguayas de estudios clásicos - 25 al 27 de abril de 2002 - Montevideo

Fil: Buisel, María Delia. Universidad Nacional de La Plata. Facultad de Humanidades y Ciencias de la Educación; Argentina.

Characterization of the acidic properties of mesoporous aluminosilicates synthesized from leached saponite with additional aluminum incorporation

The acidic properties of hexagonal mesoporous aluminosilicates synthesized via a new successful short time synthesis route using leached saponite and a low concentration of surfactant are thoroughly investigated. The resulting aluminosilicate mesoporous materials with high Si/Al ratios of around 11 have a maximal surface area of 1130 m(2)/g, a pore volume of 0.92 cm(3)/g, and a narrow pore size distribution at around 3 nm. The replacement of the sodium ions, present as counterions in the synthesized aluminosilicates, with protons imparts useful catalytic acidity. This acidity is extensively studied with FTIR spectroscopy after adsorption of ammonia and cyclohexylamine, while deuterated acetonitrile differentiates between Bronsted and Lewis acidity. Al-27 NMR spectroscopy determined the coordination of the aluminum in the FSM materials. Simultaneously the effect of an additional Al incorporation, utilizing sodium aluminate, aluminum nitrate, and aluminum isopropoxide is studied. From an acidic point of view, the incorporation with Al(NO3)(3) appears to be the most optimal, as the sample has a very high amount of acid sites (1.3 mmol/g). Investigating the nature of the acid sites it is found that in all samples except the one incorporated with Al(NO3)(3), more Bronsted than Lewis sites are present, both sites being quite acidic as they resist desorption temperatures up to 300 degreesC. Probing the coordination and location of the Al atoms, all the catalysts appeared to have mostly tetrahedral aluminum, up to 95% of the total Al amount for the proton exchanged AI(NO3)(3) incorporated sample.

Corrosion resistance of aged die cast magnesium alloy AZ91D

The corrosion behaviour of die cast magnesium alloy AZ91D aged at 160degreesC was investigated. The corrosion rate of the alloy decreases with ageing time in the initial stages and then increases again at ageing times greater than 45 h. The dependence of the corrosion rate on ageing time can be related to the changes in microstructure and local composition during ageing. Precipitation of the beta phase (Mg17Al12) occurs exclusively along the grain boundaries during ageing. The beta phase acts as a barrier, resulting in a decreasing corrosion rate in the initial stages of ageing. In the later stages, the decreasing aluminium content of alpha grains makes the alpha matrix more active, causing an increase in the corrosion rate. Electrochemical testing results also confirm the combined effects of the changes in alpha and beta phases on the corrosion resistance of the aged die cast AZ91D alloy. (C) 2003 Elsevier B.V. All rights reserved.

The effect of aluminium content on the mechanical properties and microstructure of die cast binary magnesium-aluminium alloys

This paper investigates the relationship between mechanical properties and microstructure in high pressure die cast binary Mg-Al alloys. As-cast test bars produced using high pressure die casting have been tested in tension in order to determine the properties for castings produced using this technique. It has been shown that increasing aluminium levels results in increases in yield strength and a decrease in ductility for these alloys. Higher aluminium levels also result in a decrease in creep rate at 150 degrees C. It has also been shown that an increase in aluminium levels results in an increase in the volume fraction of eutectic Mg17Al12 in the microstructure.

Structural models for yttrium aluminium borate laser glasses: NMR and EPR studies of the system (Y(2)O(3))(0.2)-(Al(2)O(3))(x)-(B(2)O(3))(0.8-x)

The structure of laser glasses in the system (Y(2)O(3))(0.2){(Al(2)O(3))(x))(B(2)O(3))(0.8-x)} (0.15 <= x <= 0.40) has been investigated by means of (11)B, (27)Al, and (89)Y solid state NMR as well as electron spin echo envelope modulation (ESEEM) of Yb-doped samples. The latter technique has been applied for the first time to an aluminoborate glass system. (11)B magic-angle spinning (MAS)-NMR spectra reveal that, while the majority of the boron atoms are three-coordinated over the entire composition region, the fraction of three-coordinated boron atoms increases significantly with increasing x. Charge balance considerations as well as (11)B NMR lineshape analyses suggest that the dominant borate species are predominantly singly charged metaborate (BO(2/2)O(-)), doubly charged pyroborate (BO(1/2)(O(-))(2)), and (at x = 0.40) triply charged orthoborate groups. As x increases along this series, the average anionic charge per trigonal borate group increases from 1.38 to 2.91. (27)Al MAS-NMR spectra show that the alumina species are present in the coordination states four, five and six, and the fraction of four-coordinated Al increases markedly with increasing x. All of the Al coordination states are in intimate contact with both the three-and the four-coordinate boron species and vice versa, as indicated by (11)B/(27)Al rotational echo double resonance (REDOR) data. These results are consistent with the formation of a homogeneous, non-segregated glass structure. (89)Y solid state NMR spectra show a significant chemical shift trend, reflecting that the second coordination sphere becomes increasingly ""aluminate-like'' with increasing x. This conclusion is supported by electron spin echo envelope modulation (ESEEM) data of Yb-doped glasses, which indicate that both borate and aluminate species participate in the medium range structure of the rare-earth ions, consistent with a random spatial distribution of the glass components.