Alumina metastable

Furthermore there is a need of detailed information about the formation of metastable alumina modifications during the isothermal oxidation of p-NiAl and about the influence of reactive elements on the. alumina modifications. At temperatures below 900°C and in the initial state the formation of -y-Al203 was reported by Doychak [14] on Zr-doped 0-NiAl and on undoped 0-NiAl by Brumm and Grabke [4]. The initial formation of -Al2Oa on 0-NiAl and its transformation to a-Al203 at temperatures higher than 950°C was reported by Brumm and Grabke [4], Rybicki and Smialek [8] and Doychak [14]. 

The formation and conversion of the metastable alumina modification is also indicated by the time dependent changes of the parabolic rate constant kp. From initially high values corresponding to the fast growth of the metastable alumina modification, the kp values decrease to a lower level corresponding to the slow growth of the a-... 

From Fig. 6 it is evident that the dopant elements have an influence on the transformation period of the metastable alumina phase to a-Al203. The longest transformation period is observed for p-NiAl + Ce with 150h, followed by (3-NiAl+Y with lOOh and undoped (3-NiAl with 50 h. The shortest transformation period is observed for (3-NiAl+Hf with 25 h. 

Values of the oxidation rate constants kp were determined from the slope of plots of the square of mass gain per area versus time. It is possible to separate the regions for the growth of metastable alumina modifications (I) in the first hours of the experiments and for protective a-alumina growth (II) after longer oxidation times. The... 

P"Ga203, though indexed in the monoclinic system, has many similarities with the spinel structure with an approximately cubic close-packed oxygen lattice. From their X-ray powder-diffraction patterns the 8- and 0- phases appear to be more crystalline than the y-phase. All these structural studies generally neglect the fact that the metastable aluminas are not pure oxides and contain water in the form of hydroxyl species [24,25]. 

Levin, I. and Brandon, D. (1998) Metastable alumina polymorphs crystal structures and transition sequences. J. Am. Ceram. Soc., 81 (8), 1995-2012. 

Number (site/nm ) and Distribution of Lewis Acid Sites on Different Metastable Aluminas as a Function of the Pretreatment Temperature by Using TMP and CO Adsorption (Ref. 107)... 

The development at FZJ Jiilich of a gas annealing treatment for FeCrAIRE alloys is described in detail in [23]. The twofold objectives of such a treatment in Ar -I- H2 mixtures were (a) the formation of a stable, thin, a-Al203 layer to prevent growth of transitional metastable aluminas at low temperatures, and (b) the removal of deleterious non-metallic alloy impurities (such as C, S, etc.). Pre-oxidation was undertaken primarily at 1200°C. Raman fluorescence spectrometry was used to establish the oxide phase composition, which... 

Was] Wasai, K., Mukai, K., Thermodynamic Analysis on Metastable Alumina Formation in Alu-... 

Ruberto C, Yourdshahyan Y, Lundqvist BI (2002) Stability of a flexible polar ionic crystal surface metastable alumina and one-dimensional surface metallicity. Phys Rev Lett 88(22) 226101... 

Ruberto C, Yourdshahyan Y, Lundqvist BI (2003) Surface properties of metastable alumina a comparative study of kappa- and alpha-Al203. Phys Rev B Condens Matter 67(19) 195412 Lin Y, Becerra-Toledo AE, Silly F, Poeppelmeier KR, Castell MR, Marks LD (2011) The (2 X 2) reconstructions on the SrTiOa (001) surface a combined scanning tunneling microscopy and density functional theory study. Surf Sci 605(17-18) L51-L55 Becerra-Toledo AE, Marshall MSJ, Castell MR, Marks LD (2012) c(4 x 2) and related structural units on the SrTi03 (001) surface scanning tunneling microscopy, density functional theory, and atomic structure. J Chem Phys 136(21) 214701... 

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