Structure and thermal behavior of nanocrystalline boehmite

First, the structural features of nanocrystalline boehmite synthesized by hydrolysis of aluminum sec-butoxide according to the Yoldas method are reported. The nanosized boehmite consists of rectangular platelets averaging 8 by 9 nm and 2–3 nm in thickness which contain a large excess of water. Dehydration by heating under vacuum induced an increase in the specific surface area, down to a minimum water content ( 0.2 H2O per Al2O3); values up to 470 m2/g can be reached. However this enlargement of specific surface area only results from water loss, the surface area remaining constant. The particle morphology, the excess of water, as well as the specific surface area, depend on the amount of acid used for the peptization during the synthesis. Second, a comprehensive investigation of the dehydration kinetics is presented. The simulations of the non-isothermal experiments at constant heating rates show that thermally stimulated transformation of nanocrystalline boehmite into alumina can be accurately modeled by a 4-reaction mechanism involving: (I) the loss of physisorbed water, (II) the loss of chemisorbed water, (III) the conversion of boehmite into transition alumina, (IV) the dehydration of transition alumina (loss of residual hydroxyl groups). The activation energy of each step is found to be very similar for experiments done in various conditions (heating rate, atmosphere, kind of sample,…)

Tracking the weathering of basalts on Mars using lithium isotope fractionation models

An edited version of this paper was published by AGU. Copyright (2015) American Geophysical UnionLithium (Li), the lightest of the alkali elements, has geochemical properties that include high aqueous solubility (Li is the most fluid mobile element) and high relative abundance in basalt-forming minerals (values ranking between 0.2 and 12 ppm). Li isotopes are particularly subject to fractionation because the two stable isotopes of lithium - 7Li and 6Li - have a large relative mass difference (∼15%) that results in significant fractionation between water and solid phases. The extent of Li isotope fractionation during aqueous alteration of basalt depends on the dissolution rate of primary minerals - the source of Li - and on the precipitation kinetics, leading to formation of secondary phases. Consequently, a detailed analysis of Li isotopic ratios in both solution and secondary mineral lattices could provide clues about past Martian weathering conditions, including weathering extent, temperature, pH, supersaturation, and evaporation rate of the initial solutions in contact with basalt rocks. In this paper, we discuss ways in which Martian aqueous processes could have lead to Li isotope fractionation. We show that Li isotopic data obtained by future exploration of Mars could be relevant to highlighting different processes of Li isotopic fractionation in the past, and therefore to understanding basalt weathering and environmental conditions early in the planet's historyData supporting our models and calculations are available as supporting information. The research leading to these results is a contribution from the Project ‘icyMARS’’, funded by the European Research Council, Starting Grant no 307496. This work was also partially supported by the European FEDER program and the Spanish Ministry of Science (MICINN) through the project CGL2011–30079. Comments by R. James and four anonymous reviewers helped us to clarify and strengthen our wor

Hodge Decomposition and the Shapley Value of a Cooperative Game

From the Washington University Senior Honors Thesis Abstracts (WUSHTA), Spring 2016. Published by the Office of Undergraduate Research. Joy Zalis Kiefer, Director of Undergraduate Research and Associate Dean in the College of Arts & Sciences; Lindsey Paunovich, Editor; Kristin G. Sobotka, Undergraduate Research Coordinator; Jennifer Kohl. Mentor: Ari Ster