Boehmite coating on θ-Al2O3 particles via a sol–gel route

θ-Al2O3 powders in slurry form were coated by boehmite nanoparticles prepared from alumina sol that consists of aluminum tri-sec-butoxide (Al(OC4H9)3) as a precursor. The θ-Al2O3 powders presented an iso-electrical point (IEP) at pH ∼ 9.2, and formed stable suspensions without notable gravity settling at acidic conditions (pH 3–4). Boehmite nanoparticles with a mean particle size ca. 68 nm were derived from the hydrolysis of Al(OC4H9)3 sol. Well-dispersed boehmite/θ-Al2O3 aqueous mixtures consisting of 3 and 9 wt.% of boehmite were then prepared by addition of θ-Al2O3 powder into the Al(OC4H9)3 sol followed then by vigorous agitation at 90 °C and in a pH range 3–4. IR and particle-size measurement both revealed possible coating of boehmite on the θ-Al2O3 particles. From TEM observation, “thickness” of the boehmite layer did not vary much with the boehmite concentration. Agglomeration of the particles became nonetheless apparent as the boehmite concentration was increased. The boehmite coating on θ-Al2O3 surface inhibited grain growth during the θ- to α-phase transformation upon heating. Mean grain size of the boehmite-coated alumina was substantially smaller than that of the un-coated one.