The INFLUENCES OF FLUORIDES ON THE TRANSFORMATION OF Α -ALUMINA CRYSTALS

Abstract

Influence of addition of aluminum fluoride (AlF 3) and ammonium fluoride (NH 4 F) on transformation of α-Al 2 O 3 from boehmite was investigated. Transformation of α-Al 2 O 3 occurred at about 1050°C and 950°C, when boehmite contained AlF 3 and NH 4 F additives respectively, which was 150°C and 300°C lower than that without any additives. The formed α-Al 2 O 3 consisted of clear, hexagonal plate-like particles about 2-3 µm in diameter and 200 nm in thickness. However, the formed hexagonal platelet particles gradually became blunt at edges and decreased in sizes at the higher calcinating temperatures. The transfer of α-Al 2 O 3 morphology was due to the diminution and disappearance of fluorine. The fluoride coated on the surfaces were firstly decomposed and then fluorine entered the lattices of crystals was volatilized. The fluoride with tiny particles attached on the surface had a same chemical bonding of F-Al-O with that in the lattices of crystals.

Lowering the synthesis temperature of α‐Al2O3 by increasing the content of pentacoordinate aluminum in the precursors

“…These "seeds" serve to increase nucleation sites, thereby reducing the formation temperature of α-Al 2 O 3 , allowing the synthesis of α-Al 2 O 3 at approximately 850 • C. Additionally, several additives, especially fluorides like AlF 3 , Zn 2 F, and NH 4 F have demonstrated efficacy in lowering the calcination temperature of α-Al 2 O 3 . [16][17][18] The reduction of the transformation temperature was mainly attributed to the formation of intermediate compound of AlOF, which was considered to accelerate the mass transportation from transition alumina to α-Al 2 O 3 . This method allows the synthesis of α-Al 2 O 3 at approximately 900 • C. The molten salt method stands out as another widely employed technique for temperature reduction.…”

Low-Temperature Fabrication of Plate-like α-Al2O3 with Less NH4F Additive

“…It is well known that fluorine-containing compounds can significantly reduce the synthesis temperature of α-Al 2 O 3 , and they are also considered effective mineralization agents for the fabrication of plate-like α-Al 2 O 3 [ 5 , 7 , 8 , 14 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 ]. Hence, coupled with the low cost and simple process of α-Al 2 O 3 , the addition of fluorine-containing mineralization agents to alumina precursors has been used by many researchers for the preparation of plate-like α-Al 2 O 3 .…”