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Transition Aluminas (transition + alumina)

Distribution by Scientific Domains

Polymers and Materials Science	100%

Selected Abstracts

Metastable alumina formation during oxidation of FeCrAl and its suppression by surface treatments

MATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 12 2005
E. N'Dah
Abstract The influence of various surface treatments of industrial FeCrAl grades was investigated in order to reduce the formation of transition aluminas during thermal oxidation in the 800,950°C temperature range. High temperature gas phase annealing in H2 -H2O mixtures promoted the initial formation of an alpha thin film and no transition alumina formed during subsequent oxidation at lower temperatures, showing very low weight gains compared to non-treated grades. Such a treatment was shown to be efficient for isothermal oxidation in oxygen of laboratory foil specimens but also for cyclic air exposure of fiber mats in near-real operating conditions. Surface modification by application of a slurry TiO2 coating before oxidation was also shown to greatly reduce the amount of transition alumina, observed by X-Ray Diffraction and Laser Induced Optical Spectroscopy. For both treatments, the optimal conditions were determined and the influence on oxidation rate was assessed. [source]

Influence of Diaspore Seeding and Chloride Concentration on the Transformation of "Diasporic" Precursors to Corundum

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2001
Richard L. Smith
"Diasporic" precursors derived from sols formed by the controlled hydrolysis of aluminum sec-butoxide in the presence of HCl yielded mixtures of corundum and amorphous alumina when calcined between 500° and 800°C. The fraction of corundum in the calcined products depended on the HCl/alkoxide ratio used during hydrolysis and was maximized at 64 wt% when the molar ratio was 1. Precursors formed from sols hydrolyzed in the presence of HNO3 rather than HCl yielded only amorphous products or transition aluminas when treated below 900°C. The corundum yield of the precursors was enhanced when they were seeded with diaspore (,-AlOOH) crystals. Precursors synthesized with an HCl/alkoxide ratio of 1 and seeded with 7.6 × 1016 diaspore seeds/(mol of Al2O3) were transformed to phase-pure corundum within 12 h at 700°C. Based on studies of the phase evolution of the precursors during calcining, it was concluded that the diaspore seeds promoted conversion by acting as corundum nuclei once they decomposed at ,450°C. [source]

Influence of a TiO2 surface treatment on the growth and adhesion of alumina scales on FeCrAl alloys

MATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 5 2008
A. Galerie
Abstract Deposits of TiO2 on FeCrAl alloys were obtained by surface TiO2 slurry application or by immersion of samples in tetraisopropylorthotitanate (TIPT) solution followed by air dry which gave the thinnest coatings. Isothermal oxidation of treated samples showed strong modification compared to non-treated ones, particularly in the temperature range of 850,925,°C where parabolic rate constants rapidly decreased when alloys were TiO2 treated. SEM surface observation, X-ray diffraction and ruby fluorescence showed that the presence of TiO2 promoted the formation of ,-Al2O3 whereas non-treated samples exhibited large amounts of transition aluminas. An interesting effect of the rapid change from metastable to stable ,-alumina was a strong increase of scale adhesion, determined by tensile testing, from 300,400 to 2000 J/m2 for scales grown at 850,°C on Aluchrom YHfAl. This was explained not only by the change from outward to mainly inward growth but also by the volume reduction at the transition to alpha transformation. [source]