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Boehmite compacts and boehmite gels (seeded and unseeded) were annealed at various temperatures in nitrogen and 1 atm water vapor to determine the influence of water vapor on boehmite transformation to new phases, changes in porosity, and morphology. Water vapor was found to accelerate the phase transformations markedly compared to dry N2 treatment. The catalyzing effect of water vapor may be due to its interactions on grain surfaces and generation of fast diffusion paths, resulting in nucleation and growth of new phases. This result shows that control of the furnace atmosphere is a useful variable in thermally activated processes.

I. INTRODUCTION The sequence of events in the thermal decomposition of alumina hydrates yields seven crystalline modifications of nearly anhydrous alumina: 7, 6, rj, 0, x> x, and a.1'2 The transformation temperature and the rate of change depend upon the heating rate, phase purity, particle size, seeding, and time of annealing, as well as the atmosphere in the furnace. The basic sequence of the occurrence of transformations is boehmite to 7, to 6, to 0, and finally to a-Al 2 O 3 . 2 The structures of the various forms of alumina have been investigated by a large number of authors. Most of the published papers,2"6 however, did not take into account the composition of furnace atmosphere although the equilibrium constant of alumina dehydroxylation reactions depends on partial pressure of water vapor which is inherently present in the system (thermodynamic point of view). There are papers describing the influence of furnace atmosphere composition on the rate of decomposition and sintering processes as well as on the physical parameters of various calcined products from the kinetic point of view.7"10 The composition of the gaseous phase in the furnace during calcination of alumina hydrates was mentioned by only a few workers.2'3'11 In general, calcination and subsequent thermal treatments carried out by previous investigators were without any control of the furnace atmosphere. On the other hand, it is well known that high pressure water catalyzes the progress toward the equilibrium phase so that at even modest pressures of 100 atm or slightly more, a-Al 2 O3 can be made easily at as low as 500 °C. 12 This communication presents several laboratory experimental results that demonstrate changes in phase development when boehmite powder compacts

^Current address: Department of Silicate Chemistry, Slovak Technical University, CS-812 37 Bratislava, Czechoslovakia. ^Also with the Department of Agronomy.

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444 http://journals.cambridge.org

J. Mater. Res., Vol. 7, No. 2, Feb 1992

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as well as seeded and unseeded boehmite gels are heated in controlled environments of dry nitrogen (reference atmosphere) and water vapor at 1 atm. II. EXPERIMENTAL Pellets were made of 1.2 g of boehmite powder, Disperal [Condea Chemie, 160 m2 g"1; d « 7 nm; loss on ignition (LOI) = 22.65 wt. % ] , pressed in a steel die (diameter 12.5 mm, pressure 150 MPa). All pellets were measured and weighed