Growth of α‐Al₂O₃ Within a Transition Alumina Matrix

Abstract

The growth of a-A1203 from a planar specimen of thermally grown y-alumina on a molybdenum transmission electron microscope grid was studied. The a-A1203 grows into the transition alumina matrix and then thickens via a ledge growth mechanism. Faceted M o crystallites cause pinning of a-A1203 ledges and are larger on a-A1203 than on the transition alumina matrix. 14853NDERSTANDING the surface of a!-AIZO3 uis important in studying sintering, catalysis, and solid-state reactions because factors such as surface diffusion and the structure and migration of grain boundaries and phase boundaries each depend on the crystallography of the surface. It has been shown, for example, that grain boundaries tend to facet so that the boundary plane lies parallel to the basal plane in one grain or the other.' This observation can be explained by the basal-plane facet being of lower energy or by noting that, because of the near close-packing of the oxygen ions, an interface will move more slowly in the direction normal to the basal plane than in a direction which is inclined to this plane.Similarly, the y-and 7-aluminas having a high surface area are used widely as adsorbents, catalysts, and catalyst supports. Transformation through 6 , 0-, and eventually to a!-A1203 is accompanied by a catastrophic loss of surface area' and changes in the surface chemistry3 that can

Kinetics of the Amorphous →γ→α Transformations in Aluminum Oxide: Effect of Crystallographic Orientation

“…For both the a → ␥ and the ␥ → ␣ transformations, the transformation velocity is slowest for the C-plane sapphire and highest for the R-plane sapphire, which reflects the underlying symmetry of the substrate surface. This finding, together with the evidence for the transformation occurring via layer-by-layer growth and electron microscopy observations of ledge growth, 5 suggests that the velocity is determined by the differences in ledge and step mobilities on the individual surfaces.…”

“…[1][2][3][4][5][6] The kinetics of the transformations are of importance for several reasons. In the manufacture of ␣-alumina powders via the thermal decomposition of boehmite (AlO(OH)), it is generally desirable for the transformation to occur as fast as possible and at as low a temperature as possible, to minimize the energy that is used in the process.…”

“…A LUMINUM OXIDE can exist in an astonishing variety of crystallographic forms and, depending on the processing route that is used to produce them, can undergo a variety of different transformations when heated until the stable, high-temperature ␣-alumina (corundum) structure is formed. [1][2][3][4][5][6] The kinetics of the transformations are of importance for several reasons. In the manufacture of ␣-alumina powders via the thermal decomposition of boehmite (AlO(OH)), it is generally desirable for the transformation to occur as fast as possible and at as low a temperature as possible, to minimize the energy that is used in the process.…”

Synthesis of ι‐Al2O3 from a Mixture of Aluminum Nitrate and Carboxymethyl Cellulose

“…A lumina is an important material used in a wide range of applications, including refractories, structural materials, electronic packaging, catalysts, and sensors. Alumina precursors, depending on the starting materials, processing route, presence of impurities, and heating conditions, undergo a variety of transitions until the most stable structure (α‐Al 2 O 3 ) is formed at a high temperature 1–7 . It is worth noting that the formation of ι‐Al 2 O 3 is not usual in ordinary alumina phase transitions using alumina salts and hydroxides and until now little research work on the preparation of ι‐Al 2 O 3 has been reported.…”

“…Alumina precursors, depending on the starting materials, processing route, presence of impurities, and heating conditions, undergo a variety of transitions until the most stable structure (a-Al 2 O 3 ) is formed at a high temperature. [1][2][3][4][5][6][7] It is worth noting that the formation of i-Al 2 O 3 is not usual in ordinary alumina phase transitions using alumina salts and hydroxides and until now little research work on the preparation of i-Al 2 O 3 has been reported. Clarification comes from the systematic work on i-Al 2 O 3 that was reported by Cameron,8 who showed that extrapolation of lattice parameters of mullite solid solution (Al 412x Si 2À2x O 10Àx ) to x 5 1 (zero Si) gives the unstable phase known as i-Al 2 O 3 (i-alumina), which may be an end member of a sillimanite-mullite-alumina series.…”