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Rung-Je Yang, Pei-Ching Yu, Chih-Cheng Chen, Fu-Su Yen
Journal of the European Ceramic Society, Volume 32, Issue 10, August 2012, Pages 2153-2162.
Department of Resources Engineering, National Cheng Kung University, No. 1 University Rd., Tainan 70101, Taiwan, ROC
Department of Mechanical Engineering, Far East University, No. 49, Jhonghua Rd., Shinshih Dist., Tainan 74448, Taiwan, ROC
Abstract
The reversal of the {Alpha}- to θ-Al2O3 phase transformation and the induced microstructure evolution of boehmite-derived discrete nanosized {Alpha}-crystallites are examined. Three categories of {Alpha}-crystallites smaller than 100 nm were examined and found to have similar behavior: (1) pre-existing {Alpha}-crystallites, (2) {Alpha}-crystallites formed in situ during the calcination of θ-crystallites of sizes near the critical size, 25 nm, and (3) {Alpha}-crystallites formed in situ by the thermal treatment of as-received θ-crystallites. The {Alpha}-crystallite may transform back to the θ-phase above 800 °C. The backwards θ-crystallite may also re-transform to the {Alpha}-phase again. Because of the density difference between {Alpha}- and θ-Al2O3, the strain involved in the volume expansion and shrinkage during the phase transition eventually results in the formation of a twinned and/or mosaic structure for the θ- and {Alpha}-crystallites. A strain release model representing the microstructure evolution of the {Alpha}- to θ-phase and the θ- to {Alpha}-Al2O3 phase transformation is proposed.
