Control of the γ-alumina to α-alumina phase transformation for an optimized alumina densification

In this work, we studied the aptitude to sintering green bodies using gamma-Al2O3 transition alumina as raw powder. We focused on the influence of the heating rate on densification and microstructural evolution. Phase transformations from transition alumina gamma ->delta ->theta ->alpha-Al2O3 were studied by in situ X-rays diffraction from the ambient to 1200 degrees C. XRD patterns revealed coexistence of various phase transformations during the heating cycle. DTA and dilatometry results showed that low heating rate leads to a significant reduction of the temperature of the alpha-Al2O3 alumina formation. Around 1190, 1217 and 1240 degrees C were found when using 5, 10 and 20 degrees C/min of heating rate, respectively. The activation energy for theta-Al2O3 -> alpha-Al2O3 transformation calculated by Kissinger and JMA equations using dilatometry method were 464.29 and 488.79 kJ/mol, respectively and by DTA method were 450.72 and 475.49 kJ/mol, respectively. In addition, the sintering of the green bodies with low heatin grate promotes the rearrangement of the grains during theta-Al2O3 -> alpha-Al2O3 transformation, enhancing the relative density to 95% and preventing the development of a vermicular structure. (C) 2016 SECV. Published by Elsevier Espana, S.L.U.