Growth Thermodynamics of Nanoscaled α-Alumina Crystallites

Abstract

The growth thermodynamics of nanoscaled α-Al2O3 crystallites after its formation from θ-Al2O3 phase transition by thermal treatment was examined. The crystallite growth characterized with quantized size increments can be performed through coalescence processes. The α-Al2O3 nuclei with sizes 17−20 nm (d cα) coalesced to form α-crystallites of sizes 45−50 nm (d p), then by coalescence of which the stable α-crystallites (d s) with minimum sizes 75−80 nm were obtained. Coalescence of the d s crystallites leads to formation of the crystallites with vermicular growth. A growth thermodynamic model describing the three-staged size growth was proposed. It provides well-coincided calculations with the data observed. The surface free energy of θ-Al2O3, which is being reported for the first time, is 2.16 J/m2.

Growth mechanism of single-crystal α-Al2O3 nanofibers fabricated by electrospinning techniques

“…[27][28][29][30] Thus, the thermodynamic stability of the crystallites is related to their volume and surface free energies. 26 Based on this, the diameter and length that make up the volume and surface of the nanofiber crystallites could presumably experience certain limitations. The length-to-diameter ratios of the nanofibers were briefly examined.…”

“…(9) and (8) is proposed in this study using values of γ and Gv that have been reported previously. 26,40,41 The surface and volume free energies ofAl 2 O 3 are taken as 2.16 J/m 2 and −73.316 × 10 9 J/m 3 , and those of ␣-Al 2 O 3 are 2.64 J/m 2 and −73.429 × 10 9 J/m 3 . Fig.…”

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

“…The transformation is always accompanied with the formation of vermicular microstructures, substantial grain growth and, hence, a great surface area loss. Better understandings of both the a-phase formation and grain growth during the process are of crucial importance to be able to prepare nanocrystalline a-Al 2 O 3 powders, but neither process is well clarified [3].…”

Disperse Equiaxed α‐Al2O3 Nanoparticles Without Vermicular Microstructure

“…4). As a-Fe 2 O 3 has the corundum structure such as a-Al 2 O 3 , the a-Al 2 O 3 /a- nanoparticles, 16 which may stabilize a-Al 2 O 3 nanoparticles embedded in a-Fe 2 O 3 matrix and conduce to the formation of a-Al 2 O 3 nanoparticles smaller than 12 nm. Because of a too low content of a-Al 2 O 3 in the composite, the XRD pattern of the product calcined at 770°C for 2 h does not show any diffraction peaks of a-Al 2 O 3 .…”

“…[11][12][13][14][15][16] The vermicular microstructure will lead to large pores and nonuniform packing in green compacts which will trigger differential densification, reduce densification rate, and limit the final density of sintered bodies during sintering. 11-16 a-Al 2 O 3 nucleation from transition aluminas will lead to exaggerated grain growth and a vermicular microstructure (a catenulate and/or reticular microstructure of many primary particles sintered or agglomerated together).…”