Elaboration and Characterization of Mullite Refractory Products from Moroccan Andalusite
Enrichment of schist mined in Morocco resulted in an extract essentially made of andalusite, quartz and muscovite, which was investigated with the aim to synthesize mullite refractory materials. X-ray diffraction analysis was used to assess phase transformations upon heating at different temperatures. Mullite formation starts as early as 1200℃ and seems almost total at 1450℃. This process correlates with a weak shrinkage (3.5% at 1450℃) and the formation of an important glass phase. Green compacts obtained by unidirectional pressing the extract powder were heated under air in the temperature range 1200℃ - 1600℃. Chemical and mechanical properties of these materials were investigated and correlated with their microstructure.
Mullite formation in clays and clay-derived vitreous ceramics
Recent studies of mullite formation in clays and clay-based vitreous ceramic systems are reviewed. In such systems, mullite crystals grow in a viscous aluminosilicate liquid, as primary 2:1 mullite in pure clays and as more acicular secondary 3:2 mullite in the presence of alkali fluxes. The stoichiometry of the mullite formed is difficult to determine since the crystals are embedded in an aluminosilicate glass matrix. However, it is likely the mullites formed have a range of Al/Si ratios as the local liquid composition varies significantly due to the heterogeneity of the microstructure. More fluid (fluxed with alkali and iron oxides) aluminosilicate liquids encourage growth of mullite crystals as does firing in oxidising atmosphere due to reducing atmosphere causing formation of Fe metal so removing the fluxing iron oxides from the aluminosilicate melt.
Negatively charged phospholipids trigger the interaction of a bacterial Tat substrate precursor protein with lipid monolayers.
Folded proteins can be translocated across biological membranes via the Tat machinery. It has been shown in vitro that these Tat substrates can interact with membranes prior to translocation. Here we report a monolayer and infrared reflection-absorption spectroscopic (IRRAS) study of the initial states of this membrane interaction, the binding to a lipid monolayer at the air/water interface serving as a model for half of a biological membrane. Using the model Tat substrate HiPIP (high potential iron-sulfur protein) from Allochromatium vinosum, we found that the precursor preferentially interacts with monolayers of negatively charged phospholipids. The signal peptide is essential for the interaction of the precursor protein with the monolayer because the mature HiPIP protein showed no interaction with the lipid monolayer. However, the individual signal peptide interacted differently with the monolayer compared to the complete precursor protein. IRRA spectroscopy indicated that the individual signal peptide forms mainly aggregated β-sheet structures. This β-sheet formation did not occur for the signal peptide when being part of the full length precursor. In this case it adopted an α-helical structure upon membrane insertion. The importance of the signal peptide and the mature domain for the membrane interaction is discussed in terms of current ideas of Tat substrate-membrane interactions.