Temperature reduction of θ- to α-phase transformation induced by high-pressure pretreatments of nano-sized alumina powders derived from boehmite

Abstract

θ- to α-phase transformation of three different nano-sized θ-Al 2O 3 powders pre-treated with high uniaxial pressure (250–750 MPa) was examined. During phase transformation, the presence of critical crystallite sizes of θ- ( d cθ=∼25 nm) and α-Al 2O 3 ( d cα=∼17 nm) and the primary size of α-Al 2O 3 ( d p=∼45 nm) is required. The high pressure induces a removal of the agglomerate state among the θ-Al 2O 3 crystallites and an increase in bulk density of the green compact. The former eventually creates homogeneity in the interspacing of the θ-particles (=crystallites) that results in a narrowed exothermic peak on the differential thermal analysis (DTA) profile while the latter brings the particles closer in distance, thus resulting in a reduction of the transformation temperature. And the de-agglomeration effect presumably abates the finger growth of α-Al 2O 3 crystallites. The estimated lowest temperature for the completion of θ- to α-Al 2O 3 phase transformation using θ-powders derived from boehmite is 1050°C at a heating rate of 10°C/min. And the process duration is about 3.5 min or 35°C on the DTA profile.