Relationships between DTA and DIL characteristics of nanosized alumina powders during θ- to α-phase transformation

The dynamic characteristics observed on differential thermal analysis (DTA) and dilatometric (DIL) profiles during theta- to alpha-phase transformation of nano-sized Al2O3 powders were examined using three sets of powder compacts of different theta-sizes and forming pressures. It is found that the time needed for performing the transformation of one theta-crystallite of critical size to one alpha-nucleus (d(ctheta)- to d(calpha)-Al2O3) at a heating rate of 10degreesC/min is around 1.5 min. The formation reaction does not depend on the theta-crystallite size of the powder system nor does it on the bulk density of the compact. The action of forming primary crystallites, d(p)-Al2O3 from alpha-nuclei is brought about in a simple manner if the number of d(calpha)-nuclei is sufficient to form a d(p)-crystallite. And if the theta-crystallites reach the critical size of phase transformation, d(ctheta) at first deflection point on DIL profiles (T-d) simultaneously, d(p)-Al2O3 crystallites would have climax of formation at the temperature close to T-p of the exotherm on DTA. In this case, a quasi-homogeneous nucleation-growth mechanism could be obtained. The preliminary estimated temperature of the exothermic peak can be lower than 10-15degreesC. And the temperature interval of exothermic peak can be small than 30degreesC. The ultimate relationship between DTA/DIL profiles of theta-powder compacts is formulated. (C) 2002 Elsevier Science B.V. All rights reserved.