Crystallite size growth and the derived dilatometric effect during θ- to α-phase transformation of nano-sized alumina powders

Abstract

The existence of critical and primary crystallite sizes ( d cα=17 nm and d p=51 nm), in which the primary crystallites are mono-crystallite particles formed during θ- to α-phase transformation of the ultrafine alumina powders are examined. θ-Al 2O 3 powders were CIPed into compacts, thermally treated at 2°C, 5°C, and 10°C/min heating rates and coordinated with an analysis of their thermal expansion characteristics. Both the direct measurement and the thermal expansion/shrinkage characteristic of the compacts reflect that the primary crystallite is a mono-crystallite particle. The thermal expansion profile can be divided into three steps. A thermodynamic model relating the free energy change and the crystallite size is formulated. The need for a critical size can be attributed to the surface tension and the free energy change per unit volume. The existence of the primary size can then result from the occurrence of strain energy induced by the coalescence of the critical crystallites.