The Gamma to Alpha Transformation in Thin Film Alumina

Abstract

Many forms of low temperature alumina have been identified [1]. These are collectively referred to as transitional forms. The particular form depends strongly on the starting material (e.g., hydrated, thermally oxidized, alkoxide, etc.), the impurities present, and on thermal history. Although different forms of transition alumina are sometimes referred to as phases, none can be considered a true polymorph of corundum, i.e., α-alumina. Transformations from one form to another which occur upon heating are not reversible, although they are quite reproducible [2]. These different transitional structures are therefore considered to comprise different states of reordering. Figure 1 is a compendium of powder x-ray diffraction patterns from all known transitional aluminas and several aluminum hydroxides [3]. It i.s obvious that there are several crystallographic spacings which are nearly common to all forms.

Metallic Character of the Al2O3(0001)-(√31 × √31)R ± 9° Surface Reconstruction

“…Creating the Al-terminated surface by fracture using the double-cantilever-cleavage technique is difficult or impossible . However, as mentioned earlier, the (0001) surface can grow in during Al 2 O 3 formation via aluminum oxidation . In section IIIA, we discuss specifics of the energetics and atomic relaxations calculated for this surface.…”

“…15 However, as mentioned earlier, the (0001) surface can grow in during Al 2 O 3 formation via aluminum oxidation. 16 In section IIIA, we discuss specifics of the energetics and atomic relaxations calculated for this surface.…”

A Nanoscale Mechanism of Fatigue in Ionic Solids

“…Figure 1 displays the bulk hexagonal R-Al 2 O 3 unit cell and a location between adjacent Al planes parallel to (0001) where nanocracks are introduced. The direction of crack formation was chosen because alumina has been shown to grow with surfaces parallel to the (0001) planes 41 upon aluminum oxidation, and the particular crack surface termination chosen is the most stable low-index surface of alumina. [42][43][44] Cases where lattice strain is introduced through compression or expansion in the direction of crack propagation are more indicative of a realistic scenario of materials failure since the ideal crystalline lattice will be strained at a crack front.…”

Effects of Oxidation on the Nanoscale Mechanisms of Crack Formation in Aluminum

“…More specifically, we calculate the dependence of the crystal energies upon tensile stress applied perpendicular to the face‐centered cubic (fcc) Al (111) planes and hexagonal α ‐Al 2 O 3 (0001) planes, also known as the basal plane of alumina. These planes were chosen since the most energetically favorable, low‐index surfaces of aluminum are those parallel to the (111) planes7 and Al 2 O 3 formed from the oxidized metal has been shown to grow with surfaces parallel to the (0001) planes 8…”