Effect of mechanochemical activation on the thermal reactions of boehmite (g-AlOOH) and g-Al 2 O 3 K.J.D. MacKenzie a,* , J. Temuujin b , M.E. Smith c , P. Angerer d , Y. Kameshima e a Department of Materials, University of Oxford, Oxford OX1 3PH, UK b Institute of Chemistry and Chemical Technology, Mongolian Academy of Sciences, Ulaanbaatar 51, Mongolia c Department of Physics, University of Warwick, Coventry CV4 7AL, UK d Institute for Materials Research, German Aerospace Centre, 51147 Koln, Germany e Department of Inorganic Materials, Tokyo Institute of Technology, Tokyo, Japan Received 5 January 2000; received in revised form 4 April 2000; accepted 6 April 2000 Abstract Boehmite (g-AlOOH) and g-Al 2 O 3 were ground for 60 min and the changes in their structure and thermal reactions were monitored by X-ray powder diffraction, thermal analysis and 27 Al MAS NMR. Grinding does not cause the boehmite to dehydrate, but causes a decrease in the intensity of its X-ray re¯ections, and the formation of an amorphous phase containing tetrahedral Al and another Al site with a resonance at 36 ppm (sometimes attributed to pentacoordinated Al). The ground material thermally transforms to corundum at 9408C via g-Al 2 O 3 , by contrast with the unground control, which forms corundum at 11958C via g and d-Al 2 O 3 . Grinding g-Al 2 O 3 decreases the intensity of its X-ray re¯ections and induces the formation of some corundum without further heating. Complete transformation to corundum occurs at 9278C, compared with 11938C in the unground material which transforms via d-Al 2 O 3 . The relationship between the mechanochemical development of the 36 ppm 27 Al NMR resonance in aluminas and their hydroxyl and adsorbed water content is discussed. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Mechanochemical activation; Thermal reactions; Alumina 1. Introduction g-Al 2 O 3 is one of several metastable alumina polymorphs which exist in the cubic system with face-centred cubic (fcc) oxygen packing. The spinel structure adopted by g-Al 2 O 3 contains 16 octahedral and 8 tetrahedral cations site per unit cell in which 62.5% of the available Al ions occupy octahedral sites and the remainder are tetrahedral [1]. To satisfy the Al 2 O 3 stoichiometry, 8/3 vacancies are assumed to be distributed randomly over the tetrahedral sites [1]. When heated, g-Al 2 O 3 transforms to the stable form, hexagonal a-Al 2 O 3 (corundum) via the transition aluminas d and y-Al 2 O 3 . d-Al 2 O 3 is of tetragonal or orthorhombic symmetry, having a spinel superlattice in which the cation vacancies are ordered; like its g-phase precursor it is based on fcc oxygen packing, but with a higher degree of ordering of the interstitial cations [1]. y-Al 2 O 3 has monoclinic symmetry and is Thermochimica Acta 359 (2000) 87±94 * Corresponding author. Tel.: 44-1865-273700; fax: 44-1865-273789. E-mail address: kenneth.mackenzie@materials.ox.ac.uk (K.J.D. MacKenzie) 0040-6031/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0040-6031(00)00513-X