Thermally Induced Formation of Transition Aluminas from Boehmite

Abstract

Structural changes occurring during the conversion of boehmite over the transient phases g-, d-, and u-Al 2 O 3 to corundum were investigated. During the whole transition process, the specific surface area was reduced. The X-ray and electron diffraction experiments revealed that the transition from g-to u-Al 2 O 3 is continuous and that it proceeds over different d-states rather than over a distinct d-Al 2 O 3 phase. The reduction of the specific surface area was most pronounced prior to the onset of the phase transition from g-Al 2 O 3 to the first d-state. In g-Al 2 O 3 approximately 45% of Al cations were found to be located on tetrahedral sites within a slightly tetragonal distorted oxygen sublattice.

Structure model of γ-Al2O3based on planar defects

“…The progress of the continuous transition -Al 2 O 3 ! -Al 2 O 3 , the formation of several intermediate states between the metastable phases -Al 2 O 3 and -Al 2 O 3 , and the completed formation of -Al 2 O 3 at 975 o C were reported by Rudolph et al (2017).…”

“…1). Their size varies between a few hundred nanometres and several micrometres (Rudolph et al, 2017). After annealing, this particle shape was preserved, but the corresponding crystallographic directions changed according to the orientation relationship ½010 boehmite k ½110 ð001Þ boehmite k ð001Þ :…”

“…Because of its large surface area and high catalytic activity, metastable -Al 2 O 3 is often used as a functional constituent of catalytic converters or surface coatings. However, as -Al 2 O 3 is only stable below 700 o C (Paglia et al, 2004;Rudolph et al, 2017), it must be stabilized for high-temperature applications. All intermediate alumina phases possess a slightly distorted cubic close-packed (c.c.p.)…”

Structure of boehmite-derived γ-alumina and its transformation mechanism revealed by electron crystallography

“…From the previous published work, the tetrahedral site fraction was reported as 29% by Paglia et al (2003), or in the 31-34% range depending on processing temperature in another reference (Paglia et al, 2004). Rudolph et al (2017) found 45% Al fraction occupying tetrahedral sites. In fact, the Al 3+ cation location can be experimentally determined directly using 27 Al nuclear magnetic resonance (NMR) spectroscopy (Prins, 2020).…”

Regularities of the influence of microarc oxidation of aluminum alloys on the phase-structural state of the formed oxide coatings and the peculiarities of γ-AL2O3 → α-AL2O3 polymorphic transformation during their annealing

“…Different distribution of vacancies over cation sites: one of the reasons why different structural descriptions of -Al 2 O 3 can be found in the literature for samples obtained by different technologies, with different crystallite sizes, etc. [13,14].…”