Alumina powders from aluminium nitrate-urea and aluminium sulphate-urea reactions - The role of the precursor anion and process conditions on characteristics

Precursor powders for alumina were prepared by aluminium nitrate-urea (NU-) and aluminium sulphate-urea (SU-) reactions. The role of process conditions on the reactions and characteristics of the resultant powders were studied. NU-reactions resulted in an amorphous gel that upon prolonged refluxing at a higher pH crystallized into boehmite powders of 0.3-0.8 mu m size. These were agglomerates having elongated fibrilla morphology. SU-reactions resulted in the direct formation of amorphous powders of 2-7 mu m size which were agglomerates of spherical particles. Upon prolonged refluxing at higher pH these powders were also found to undergo crystallization into boehmite, with a little modification of their particle size distribution and morphology. The alpha alumina powders formed by calcination of these precursors at 1673 K exhibited the same size distribution and morphological features as that of the precursors. At 1823 K, NU-based powder compacts sintered to above 95% T.D., while SU-based powder compacts could sinter only to about 85%. (C) 1996 Elsevier Science Limited and Techna S.r.l.