Microstructural evolution in sintering of ALOOH gels

Abstract

Materials derived by precipitation or polymerization chemistry (e.g., “sol-gel” methods) are usually obtained in noncrystalline or otherwise metastable phases, and transformation to more thermodynamically stable phases generally occurs by a nucleation and growth process. In a fully reconstructive transformation, such as occurs in the alumina system, the activation energy for nucleation may be higher than that for simple short-range diffusion. Hence nucleation frequency can be a controlling factor in the development of microstructurc. The efficacy of seeding as a method of microstructural and phase control in solution-derived or so-called sol-gel materials has been clearly demonstrated for the alumina system. The epitaxial nature of this phenomenon is explored, using the polarizing microscope to follow the crystallographic orientation of the transformed material as the transformation proceeds, showing that this is epitaxial in nature, and that the nucleation frequency in unseeded material is relatively low (∼ 1010 cm−3). The microscope was then used to demonstrate the effect on nucleation frequency of seeding with materials selected to be isostructural, isotypic, and having little or no similarity to the corundum structure. Using these and other methods, the seeding phenomenon in alumina gels is shown to result from epitaxial growth of the stable corundum phase on isostructural or isotypic nuclei in the solid state. This approach is applied to formulate hypotheses for the mechanisms by which some of the previously reported effects of seeding, e.g., enhanced densification and microstructural refinement, can be understood and to formulate a set of generalizations for its potential application to other systems.

Defect formation by order coalescence in vermicular grains during alumina phase transformation

“…The typical grain growth features of a-Al 2 O 3 obtained from calcination of boehmite or heating of c-Al 2 O 3 reported by other studies are confirmed [9][10][11][12].…”

“…The formation of a single-crystal vermicular microstructure is difficult to understand as these grains consist of sub-units of similar size and with the same crystal orientation on both sides of the mutual necking. It has been proposed that a-Al 2 O 3 nucleation from h-phase was sporadic rather than uniform, leading to exaggerated grain growth and vermicular microstructure [10]. Others speculate that the phase transformation from boehmite might not proceed by a single-crystal to single-crystal transition [11].…”

Influence of nitrogen addition on oxyacetylene flame chemical vapor deposition of diamond as studied by solid state techniques and gas phase diagnostics

“…Underneath the diamond peak there is a band that can be assigned to the D band of graphitic carbon, 36,37 which is located at 1357 cm Ϫ1 . 36,38 The band is strongest for the 80 and 100 sccm samples, which may be caused by the very strong secondary nucleation on the crystallites of these samples. 37 As follows from the overlay of the spectra in Fig.…”

Interfacial reactions in K2Ti6O13(w)/Al2O3/Al composite

“…The reason for the occurrence of this transformation at such a low temperature may be the contribution of sol -gel byproducts of aluminium nitrate in the gel 17 and the seeding effect of the fine KTiO whiskers. 18 Figure 3 shows the TEM morphology of the sol -gel processed a-Al 2 O 3 coating on the whisker surfaces. It can be seen that a dense alumina coating was uniformly deposited on the whisker surfaces.…”