Structural and morphological evolution of powders nanostructured ceramics: transitional aluminas

Abstract

This work aims with the study of the transformation of boehmite into transitional aluminas. Boehmite was obtained by a sol-gel method from an aluminium hazardous waste. The thermal behaviour of boehmite was followed by thermogravimetry and differential thermal analysis to determine the transformation temperatures.  By calcinations of boehmite at temperatures ranging between 250-1000ºC, transitional aluminas (?, ? , ?-Al₂O₃)  were synthesized and characterized by XRD, TEM and FTIR. All the transitional aluminas exhibit nanometric crystallite size, ranging from 2.5-15nm. ?-Al₂O₃ was obtained as a nanostructured material at 500ºC with a cell parameter a=7.923Å. ?-phase stars to appear at 850ºC with a crystallite size of 6nm and cell parameters a=5.672Å and c=24.600Å. For ?-Al₂O₃ the cell parameters, in Å, were a=11.817, b=2.912, c=5.621 and ?=103.8º. The progressive conversion of the transitional phase ?-Al₂O₃ into the stable polymorph ?-alumina, takes place gradually and a four-phases region is achieved at 1000ºC, where coexist with other transitional phase such as  ?- and ?-Al₂O₃.