θ-Crystallite growth restraint induced by the presence of α-crystallites in a nano-sized alumina powder system

Abstract

The restraint of θ-Al 2O 3 from crystallite growth induced by the presence of α-crystallites in the nano-sized alumina powder system is examined. The θ-crystallite powder was obtained by calcination of boehmite. α-Crystallites present in the θ-powder system, either through addition externally to boehmite or introduced internally by θ-crystallites that transform to α-during θ- to α-phase transformation of the boehmite, affect the rate of θ-crystallite growth. Small amounts of external α-crystallite addition initiates the formation of a subsystem in the boehmite precursor in which the formed θ-crystallite grows faster, bringing about part of the powder system to experience a temperature reduction of θ- to α-phase transformation on DTA profiles. With large amounts of α-crystallite addition, another subsystem in which the rate of θ-crystallite growth slows down occurs. α-Crystallites introduced spontaneously during phase transformation, although with small amounts of the α-crystallite presence, less than 10–20%, whose effect can be eliminated, show similar effects of restraining θ-crystallites from growth. Activation energy calculations demonstrated that, once the α-crystallite appeared, more than ∼100 kJ/mol was required for θ-crystallite growth.