The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close-packed oxygen sublattice (either face-centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face-centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed.

Metastable alumina polymorphs : Crystal structures and transition sequences

The present review discusses the most important aspects to take into consideration to improve the properties of γ-Al2O3 as a support for catalytic applications. We show that the synthetic route to γ-Al2O3 is the starting point that determines the micro- and macrostructure of the oxide and, consequently, allows control of the support characteristics. The relevance of the adequate structural characterization of the oxide as well as of its surface sorption behavior through the proton-affinity distributions, are considered. The usefulness of the latter during supported catalyst preparation, for catalytic active sites characterization, and for stability evaluation after calcination are discussed for the Co/γ-Al2O3 system in particular. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

γ-Alumina as a Support for Catalysts: A Review of Fundamental Aspects

The oxidation behaviour of NiAl-I. Phase transformations in the alumina scale during oxidation of NiAl and NiAl-Cr alloys

Thermally grown oxide scales that form beneath ZrO 2 top coats play an important role in determining the performance of thermal barrier coatings. Numerous factors, including the composition of both the alloy substrate and the bond coat, affect adhesion of the α -Al 2 O 3 scale. Three areas of focus in the formation of an `ideal', adherent scale encompass: (1) migration of Al and other elements in the metal substrate, (2) segregation of elements to the metal–scale interface and the scale grain boundaries, and (3) generation of stresses in the scale. Examples of the effects of reactive elements, Pt, indigenous S, and reaction temperature on scale adhesion are discussed.

Substrate and bond coat compositions: factors affecting alumina scale adhesion

Oxidation of NiAl and FeAl

Cross sections of oxide scale/(Ni-Al) intermetallics were prepared by a new method and studied using primarily transmission electron microscopy (TEM) The cross sections were prepared by encasing an oxidized metal specimen sandwich in a low-melting-temperature zinc alloy Observations of oxidized zirconium-doped β-NiAl cross sections revealed crystallographic voids beneath an adherent Al2O3 scale The oxide-metal interface was incoherent, but a high dislocation density in the metal near the interface suggested that a large tensile stress was induced by the attached oxide scale A duplex Al2O3-NiAl2O4 scale formed on zirconium-doped and zirconium/boron-doped γ′-Ni3Al alloys Additional results are presented involving oxidation mechanisms and oxide-metal interface structures

TEM studies of oxidized NiAl and Ni3Al cross sections

Transient oxidation of NiAl

Cubic to monoclinic phase transformations in alumina

Conventional electron microscopy, analytical electron microscopy, high resolution electron microscopy and high resolution SIMS have been used to investigate the effect of the reactive elements, Yand Zr, on the oxide scale formation on NiAl. Polycrystalline NiAl samples, doped with either 0. 1 wt% Yor 0.2 wt% Zr, were oxidized in air at 1200°C. 18O tracer experiments in conjunction with high resolution SIMS suggest that the reactive elements reduce the outward diffusion of cations. Energy dispersive X-ray spectroscopy on a dedicated STEM showed that the reactive elements segregate to the grain boundaries in the oxide scale and to the metal/oxide interface. The amount at the oxide scale grain boundaries was calculated to be 0.2 monolayers for both Zr and Y doped NiAl. The amounts of segregation were calculated to be 0.15 monolayers (Zr-doped) and 0.07 monolayers (Y-doped) at the metal/oxide interface. The presence of sulfur was detected in Y-rich particles in the NiAl close to the interface.



Untersuchungen zur Segregation von Y und Zr bei der Oxidation von NiAl



Konventionelle, analytische und hochauflosende Transmissionselektronenmikroskopie (TEM) sowie hochauflosende Sekundarionenmassenspektrometrie (SIMS) wurden benutzt, um den Effekt von reaktiven Elementen auf die Oxidation von NiAl zu untersuchen. Polykristalline NiAl-Proben, dotiert mit 0,1 Mas.-% Y oder 0,2 Mas.-% Zr, wurden bei einer Temperatur von 1200°C oxidiert. 18O Tracer-Experimente in Verbindung mit hochauflosenden SIMS-Untersuchungen deuten darauf hin, das reaktive Elemente die nach ausen gerichtete Diffusion von Kationen reduzieren. Energiedispersive Rontgenspektroskopie mit einem dedizierten STEM zeigte, das die reaktiven Elemente an den Korngrenzen in der Oxidschicht und an der Metall/Oxid-Grenzflache segregieren. Der Anteil von Y bzw. Zr an den Korngrenzen in der Oxidschicht betragt 0,2 Monolagen. Demgegenuber wurden an der Metall/Oxid-Grenzflache 0,15 Monolagen (Zr-dotiert) bzw. 0,07 Monolagen (Y-dotiert) gefunden. In Y-reichen Teilchen im NiAl nahe der Metall/Oxid-Grenzflache konnte Schwefel nachgewiesen werden.

M. Rühle

Papers

TEM studies of oxidized NiAl and Ni3Al cross sections

Transient oxidation of NiAl

Cubic to monoclinic phase transformations in alumina

Segregation studies of oxidized Y and Zr doped NiAl

TEM Investigations on the Oxidation of NiAl