Transition aluminas, metastability

Transition Aluminas. Metastable crystalline aluminas (there are several series) which contain minor amounts of OH- ions. They are usually produced by gradual heating of alumina hydrates. Transition Scarp. A rib mark when a crack changes from one mode of growth to another. 

Interpretation. It is assumed that the incomplete occupancy of tetrahedral sites could be responsible for the metastability of transition aluminas. It is possible to retard the diffusion of Al3+ cations which are responsible for the structure transformations by introducing bivalent ions such as those of Cd, Ca, Zn, Mg, Ni, and Cu into the tetrahedral position in the spinel cubic lattice. 

Aluminum oxide, or alumina, ciystalUzes in the corundum stracture (mineralogical term) to form sapphire monociystals. We are referring here to white sapphire, like the one used for the manufacture of scratchproof glasses for watches, whereas in gemmology sapphire is the blue sapphire, which is alumina with a little Ti" mby being alumina colored in red by Cr +. This is the alpha variety (Al203-a), which is the stable variety, but we will see that the preparation of alumina involves various metastable varieties hydrated aluminas and transition aluminas. 

The radiative transitions of the previous descriptions have all been spontaneous Relaxation from the excited state to the ground state and emission of photons occur without external aid. In contrast, a stimulated emission occurs when the half-life of the excited state is relatively long, and relaxation can occur only through the aid of a stimulating photon. In stimulated emission, the emitted photon has the same direction as, and is in phase with, the stimulating photon. The example of Cr +-doped AI2O3 that we utilized earlier for our description of the color of ruby works equally well for a description of stimulated emission. Recall that the presence of chromium in alumina alters the electronic structure, creating a metastable state between the valence and conduction bands. Absorption of a blue-violet photon results in the excitation of an electron from... 

The stable crystalline form is corundum (a-alumlna). Kappa-alumina is one of several structurally related, metastable forms. These occur In "active alumina" and are much studied (5) due to their Importance in adsorbents and catalysts. Llppens and Steggerda (5) summarized the classifications of metastable forms, their crystal structures and conditions of formation by dehydration. Okuralya et al. (6) recently published an extensive study on the formation and structural relations of 1C-AI2O3 and its precursors le -AlgOg and tohdlte. Kappa-alumina belongs to the "nearly anhydrous", high-temperature classification (5) based on its formation temperture of 900 to 1000 C (somewhat lower in vacuo). The Irreversible transition x+o is thermally activated at 1200 C (2, 5). Thermochemical studies cited In this table presume that the samples of x-Al Og were essentially anhydrous. 

Several types of A1203 have been reported for alloys forming alumina layers [1-4]. At high temperatures the metastable (transition) oxides y, 5, and 0-Al2O3 will transform to the a-AI203 according to the following sequence [3,5] ... 

The mass gain curves from the thermogravimetry experiments at 1000°C are represented in Fig. 5. All investigated systems show a very fast initial mass gain corresponding to the transitional fast growth of metastable, non-protective alumina modifica-... 

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

Thus the form of the shrinkage curve of the compacts from the nanopowders of the metastable forms of alumina is determined by the processing of the polymorph transitions of alumina to stable a-AbOs. At the staring stage (t < Ta) the material consists of the y, 5, and 0 phases mixture. The polymorph transition (y + 8 + 0) a starts at Ta temperature and can... 

Compared to amorphous systems, structural evolution in crystalline systems may be dominated by the effects of phase transformations [190] which often occur in conjunction with dehydration or when metastable transitional phases are involved. The most thoroughly studied crystalline gel system is alumina prepared by the Yoldas process [47,191,192]. (See Chapter 2.) Xerogels, which are often completely transparent, are typically composed of... 

Goodboy and Fleming (1984) point out that the term alumina is a misused expression for aluminum oxides and hydroxides that exist in at least flve thermodynamically stable phases and many more metastable transition forms. The commercial activated alumina desiccants arc generally manufactured by a process involving the following. steps (Woo.sley, 1990) ... 

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... 

The water-copper system falls well between the other two. This also corresponds to qualitative observations of fluidization quality in such systems. Although transition points are relatively sharp, the bubbles that result remain very small and there is no evidence of metastable behaviour, referred to in Chapter 9 with reference to the phenomenon of premature bubbling in gas-fluidized systems, and analysed in Chapter 14 on the basis of the unlinearized particle bed model equations the manifestations of instability remain significantly less pronounced than is the case for the air-alumina system. 

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