Control of Polymorphism and Mass-Transfer in Al2O3 Scale on Alumina Forming Alloys

2010 ◽  
Vol 638-642 ◽  
pp. 888-893 ◽  
Author(s):  
Satoshi Kitaoka ◽  
Tsuneaki Matsudaira ◽  
Masashi Wada ◽  
Tomohiro Kuroyama
Keyword(s):  
Mass Transfer ◽  
Oxygen Partial Pressure ◽  
Partial Pressure ◽  
Oxygen Potential ◽  
Oxygen Permeability ◽  
Al2o3 Scale ◽  
Lower Oxygen Partial Pressure ◽  
Lower Oxygen ◽  
Potential Gradients ◽  
Conicraly Alloy

The transformation from metastable polymorphs to stable alpha-Al2O3 in the scale formed on a CoNiCrAlY alloy is accelerated under lower oxygen partial pressure (PO2), where both Al and Cr in the alloy are simultaneously oxidized, resulting in the formation of a dense and monolithic alpha-Al2O3 scale. Under higher PO2, where all components of the alloy are oxidized, the transformation is retarded and (Co,Ni)(Al,Cr)2O4 is also produced. The oxygen permeability in polycrystalline alpha-Al2O3 wafers exposed to steep oxygen potential gradients is evaluated at high temperatures to investigate the complicated mass-transfer phenomena through the scale formed on the alloy. The diffusion of Al and O species, which are responsible for the oxygen permeation along the grain boundaries of Al2O3, is dependent on the formation of an oxygen potential gradients. For Lu-doped Al2O3 polycrystals, it was found that Lu depressed the mobility of oxygen, but did not directly influence the migration of Al.

scholarly journals Kinetic decomposition of Ni2SiO4 in oxygen potential gradients

1987 ◽  
Vol 2 (3) ◽  
pp. 338-344 ◽  
Author(s):  
K. T. Jacob ◽  
A. K. Shukla
Keyword(s):  
Oxygen Partial Pressure ◽  
Partial Pressure ◽  
Oxygen Potential ◽  
Pressure Ratio ◽  
Metallurgical Society ◽  
Low Oxygen ◽  
Critical Oxygen ◽  
Kinetic Decomposition ◽  
Potential Gradients ◽  
Partial Pressure Ratio

Nickel orthosilicate (Ni2SiO4) has been found to decompose into its component binary oxides in oxygen potential gradients at 1373 K. Nickel oxide was formed at the high oxygen potential boundary, while silica was detected at the low oxygen potential side. Significant porosity and fissures were observed near the Ni2SiO4/SiO2 interface and the SiO2 layer. The critical oxygen partial pressure ratio required for decomposition varied from 1.63 to 2.15 as the oxygen pressures were altered from 1.01 ⊠ 105 to 2.7X 10−4 Pa, well above the dissociation pressure of Ni2SiO4. Platinum markers placed at the boundaries of the Ni2SiO4 sample indicated growth of NiO at the higher oxygen potential boundary, without any apparent transport of material to the low oxygen potential side. However, significant movement of the bulk Ni2SiO4 crystal with respect to the marker was not observed. The decomposition of the silicate occurs due to the unequal rates of transport of Ni and Si. The critical oxygen partial pressure ratio required for decomposition is related both to the thermodynamic stability of Ni2SiO4 with respect to component oxides and the ratio of diffusivities of nickel and silicon. Kinetic decomposition of multicomponent oxides, first discovered by Schmalzried, Laqua, and co-workers [H. Schmalzried, W. Laqua, and P. L. Lin, Z. Natur Forsch. Teil A 34, 192 (1979); H. Schmalzried and W. Laqua, Oxid. Met. 15, 339 (1981); W. Laqua and H. Schmalzried, Chemical Metallurgy—A Tribute to Carl Wagner (Metallurgical Society of the AIME, New York, 1981), p. 29] has important consequences for their use at high temperatures and in geochemistry.

Über die Instabilität von NiTiO3 unter der Wirkung eines Sauerstoffpartialdruck-Gradienten Instability of NiTiO3 in an Oxygen-Potential Gradient

1981 ◽  
Vol 36 (10) ◽  
pp. 1211-1214 ◽  
Author(s):  
W. Laqua
Keyword(s):  
Oxygen Partial Pressure ◽  
Partial Pressure ◽  
Oxygen Potential ◽  
Potential Gradient ◽  
Simultaneous Action ◽  
Partial Pressures ◽  
Quantitative Explanation

Abstract If a NiTiO3 poly crystal is exposed to an oxygen-potential gradient -established by the simultaneous action of two different oxygen partial pressures -it will be decomposed into its component oxides NiO and TiO2 despite the fact, that the compound is stable at both the lower and the higher oxygen partial pressure. A quantitative explanation of this phenomenon will be given below.

Effect of Oxygen Potential Gradient on Mass Transfer in Polycrystalline α-Alumina at High Temperature

2016 ◽  
Vol 879 ◽  
pp. 966-971 ◽  
Author(s):  
Satoshi Kitaoka ◽  
Tsuneaki Matsudaira ◽  
Tsubasa Nakagawa ◽  
Naoya Shibata ◽  
Yuichi Ikuhara
Keyword(s):  
Mass Transfer ◽  
Diffusion Coefficients ◽  
Oxygen Potential ◽  
Oxygen Permeability ◽  
Electronic Conductivity ◽  
Potential Gradient ◽  
High Oxygen Partial Pressure ◽  
Self Diffusion ◽  
Permeability Constants ◽  
Effect Of Oxygen

The oxygen permeability of polycrystalline α-alumina wafers, which served as model alumina scales formed on heat-resistant alloys, was evaluated at a temperature of 1873 K. Mass transfer along grain boundaries (GBs) in an alumina wafer exposed to a large oxygen potential gradient (dμO), where both oxygen and aluminum mutually diffuse along GBs, was analyzed using 18O2 and SIMS. 18O was concentrated at GB ridges on the high oxygen partial pressure (PO2(hi)) surface and along the GBs near the PO2(hi) surface. 18O adsorbed on the surface diffused almost immediately to surface GBs, resulting in the formation of new alumina by reaction with aluminum diffusing outward along the GBs. Oxygen GB diffusion coefficients in the vicinity of the PO2(hi) surface were determined from the 18O depth profile along each GB for the 18O map of the cross section of the exposed alumina wafer. The oxygen GB diffusion coefficients were comparable to the values calculated from the oxygen permeability constants assuming an electronic conductivity and were obviously lower than those of oxygen GB self-diffusion without an oxygen potential gradient.

Mass transfer through a single grain boundary in alumina bicrystals under oxygen potential gradients

2010 ◽  
Vol 46 (12) ◽  
pp. 4407-4412 ◽  
Author(s):  
Tsuneaki Matsudaira ◽  
Satoshi Kitaoka ◽  
Naoya Shibata ◽  
Tsubasa Nakagawa ◽  
Yuichi Ikuhara
Keyword(s):  
Mass Transfer ◽  
Grain Boundary ◽  
Oxygen Potential ◽  
Potential Gradients ◽  
Single Grain

Control of Oxygen Permeability in Alumina under Oxygen Potential Gradients at High Temperature by Dopant Configurations

2014 ◽  
Vol 97 (7) ◽  
pp. 2314-2322 ◽  
Author(s):  
Satoshi Kitaoka ◽  
Tsuneaki Matsudaira ◽  
Masashi Wada ◽  
Tomohiro Saito ◽  
Makoto Tanaka ◽  
...  
Keyword(s):  
High Temperature ◽  
Oxygen Potential ◽  
Oxygen Permeability ◽  
Potential Gradients

Mixed Conductivity and Oxygen Permeability of (BaxSr1-x)0.98Fe1-yMyO3-&[delta] ( x = 0.1-0.3, y = 0.1-0.4 and M = Ga, Al )

2006 ◽  
Vol 972 ◽  
Author(s):  
Kang Chun Yong ◽  
Hajime Kusaba ◽  
Hisahiro Einaga ◽  
Yasutake Teraoka
Keyword(s):  
Crystal Structure ◽  
Pressure Gradient ◽  
Oxygen Partial Pressure ◽  
Partial Pressure ◽  
Partial Oxidation ◽  
Oxygen Permeability ◽  
Ionic Conductivities ◽  
Methane Partial Oxidation ◽  
Substitution Effects ◽  
Mixed Conductivity

AbstractSubstitution effects of Ga and Al for Fe in (BaxSr1-x)0.98FeO3-δ were studied with respect to crystal structure, conductivity and oxygen permeability. The solubility limits of Ga and Al in (BaxSr1-x)0.98Fe1-yMyO3-δ (M=Ga, Al) were between 20 and 30 mol%. The substitution of Ga and Al caused decreases in electronic and oxide ionic conductivities (mixed conductivities) but the enhancement of the reduction tolerance. Sintered membranes of the Ga- and Al-substituted oxides showed oxygen permeability under the He/air oxygen partial pressure gradient and were stable under the condition of methane partial oxidation at 900 °C.

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