Formation of the spinels MgAl2O4, NiAl2O4, and ZnAl2O4 from α-alumina and MgO, NiO, or ZnO was studied at several temperatures by measuring the fraction of reaction completed as a function of time. Apparent activation energies (in kJ mole-1) were MgAl2O4, 185; NiAl2O4, 300; and ZnAl2O4, 230. At 1373 K, the rates of spinel formation followed the sequence MgAl2O4 > ZnAl2O4 > NiAl2O4. The results are discussed in terms of coordination preference of the involved divalent cations.

The formation of zinc aluminate from zinc oxide and α-alumina in air has been studied at 1173, 1273, 1373, 1573 and 1673 K by measuring the fraction of reaction completed as a function of time. Two values of the apparent activation energy were found, Ea = 230 kJ mol−1 for T < 1400 K and Ea = 430 kJ mol−1 for T ⪢ 1400 K. These were interpreted in terms of a change in the reaction mechanism, solid-solid in the low-temperature range and solid-gas for T ⪢ 1400 K.

Zirconium dioxide, prepared by dehydration of a zirconia gel, was calcined in a vacuum at temperatures ranging from 573 to 1173K, in order to investigate the evolution of specific surface area and porosity as a function of the calcination temperature. The results are compared with those previously obtained for samples heated in air.

AbstractZirconium dioxide, prepared by thermal decomposition of zirconium nitrate, was heated in air at temperatures ranging from 573 to 1023 K. An analysis of nitrogen adsorption‐desorption isotherms on the resulting materials allowed determination of the corresponding specific surface area and porous texture. All oxides calcined within the temperature range 573–873 K were found to be basically mesoporous; the most frequent pore radius increasing from 3 to 10.5 nm as the temperature was raised. BET surface areas decreased across the same temperature range, from 94 down to 20 m2 g−1. The sample fired at 1023 K showed a BET surface area smaller than 5 m2 g−1.