Study of phase transformations during calcination of aluminum hydroxides by selected area electron diffraction

Abstract

Bayerite, fl-AI(OH)3, and boehmite, 7-A1OOH, both with crystallite sizes of some tenths of a micron, were used for this investigation.The transformation of bayerite to T-alumina and of boehmite to 7-and &alumina showed a strict pseudomorphosis. 7-and 7-alumina have spinel lattices that differ in disorder: T-alumina has a strong one-dimensional disorder of the cubic close-packed stacking; for 7-alumina the oxygen sublattice is fairly well ordered, whereas the tetrahedral A1 lattice is strongly disordered.The crystallographic texture of 7-and 7-alumina could be related to their microporous texture and this in its turn to the length of the OH bonds in the hydroxides.The formation of the well ordered super spinel structure ~-A120 3 is suppressed by obstructing the diffusion of the A1 ions either by the surface of numerous pores (T-alumina from bayerite or 7-alumina from gelatinous boehmite) or by incorporating large ions such as Na ions (y-alumina from crystalline boehmite). In these cases the disordered spinel type aluminas are transformed at a high temperature to 0-and a-Al~O 3 by complete recrystallization to small particles which still have some preferred orientation.

Structures and transformation mechanisms of the η, γ and θ transition aluminas

“…Therefore, the third endothermic peak of bayerite, around 773 K, must result from the decomposition of boehmite to ~,-alumina. Consequently, ~7-alumina obtained from the dehydroxylation of bayerite inevitably contains a very small amount of y-alumina which confirms the finding of Lippens & DeBoer (1964). Notice that no significant thermal activity is observed in the DTA around 1273 K where the transition from both ~7-and y-to 0-alumina takes place.…”

“…Stumpf, Russell, Newsome & Tucker (1950) determined the cubic lattice constant of ~7-alumina to be 7.9/k. Lippens & DeBoer (1964) found, by selected-area electron diffraction, that rt-alumina is somewhat tetragonally deformed with the c/a ratio varying between 0.985 and 0.993, and y-alumina is more tetragonally deformed with the c/a ratio varying between 0.983 and 0.987, but that the oxygen sublattice of y-alumina is fairly well ordered, much more so than that of ~7-alumina. The strong anisotropy of the shrinkage in the a and b axes of boehmite, they believed, is the cause of the more pronounced tetragonal character of y-alumina.…”

“…Surface areas resulting from this pore system can easily exceed 300 m 2 g-1 (Wefers & Misra, 1987) and the majority of the new surfaces come from the (111) crystallographic plane of the resulting spinel-type lattices of ~7-and y-alumina. Lippens & DeBoer (1964) assumed, in view of the lamellar texture, that the cubic close-packed oxygen layers in the ~7 structure are one-dimensionally disordered in the direction parallel to the c axis of the precursor bayerite (which is the [111] direction of the spinel). Ervin (1952) observed that the X-ray powder diffraction patterns of all the transition aluminas have in common the strong line at d = 1.39 A (20 = 67-3°), see Figs.…”

On the Micro-, Meso-, and Macroporous Structures of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers

“…A universal dependence of the t thickness on the partial pressure of nitrogen was found for various adsorbate surface types (14,15). de Boer et al conducted an extensive study of the t-curve method for various porous materials (15)(16)(17)(18)(19)(20)(21) and determined the actual pore radius to be the sum of the radius calculated by the Kelvin equation and the thickness of the adsorbed t layer at each given partial pressure. An important refinement of the Kelvin equation using the t-thickness correction was performed by Barrett, Joyner, and Halenda (BJH) (22).…”

Observation of Inclusion in Aluminum Deoxidized Iron.

“…For the g and d-alumina, the identification was easier because they often show characteristic diffraction patterns. 13) The identification of the inclusion from the diffraction pattern was performed for inclusions where the diameter exceeded about 0.05 mm because the diffraction patterns of very fine inclusions were not clear. By contrast, the electric beam could not pass through the large inclusions (a few mm thickness).…”