March 2001

New sol-gel route for low temperature synthesis of a-Al2O3
by N. Bahlawane
Fukuoka University Institute 'FIRECS', 
Fukuoka-ken, Japan

New sol-gel route has been recently developed for the preparation of pure a-Al2O3 since 850oC. The anhydrous preparation procedure allows a particular crystallization pattern that avoids the formation of q-Al2O3, well known for its relative high thermal stability. This structure (q-Al2O3) is a kind of energetic trap since annealing over 1100oC is needed to get "over it, to" pure a-Al2O3 phase.

In this particular case, both of g and a-Al2O3 were simultaneously formed since 800oC and the complete crystallization was attained starting 850oC. This behavior was attained thanks to the low and close activation energy of the amorphous  g-Al2O3 and g-Al2O3  to a-Al2O3 transformations.

The sol preparation does not involve any foreign element other than C and H that oxidize easily to CO and H2O, facilitating the ionic diffusion necessary for the crystallization process. The prepared sol experienced a linear crystallization kinetic that allows getting pure a-Al2O3 at temperatures starting 850oC. The sol with adjustable viscosity can be prepared within 30min involving two steps. The raw material is first dispersed in the organic solvent and then peptized using the organic acid that plays also a stabilizer role. Sol prepared using this process is stable over 20 months in a non-controlled atmosphere (neither gelation nor precipitation) that may well authorizes its commercialization and its on-line industrial use.

XRD patterns of the dry sol after annealing at different temperatures


Kinetic of the a-Al2O3 formation over 800oC


In addition to the powder synthesis, this sol was successfully used to deposit homogeneous and crack-free a-Al2O3 thin film on various substrates (silica, silicon and carbon).

For more information contact: 

N. Bahlawane
Fukuoka University Institute 'FIRECS'
10 Koyo machi, Wakamatsu, Kitakyushu, Fukuoka-ken, 808-0002 Japan. 
Tel: 81-93-751-9975
Fax: 81-93-751-9976
E-Mail : bahlawan@bronze.ocn.ne.jp


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