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

Abstract

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 μm size. These were agglomerates having elongated fibrillar morphology. SU-reactions resulted in the direct formation of amorphous powders of 2–7 μm 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%.