Snezana Boskovic

Mechanical activation of the gamma to alpha transition in Al2O3

Abstract The phase transformation of mechanically treated γ-Al2O3 for grinding times up to 600 min was studied. The grinding was performed under a centrifugal force, through impact and friction, at a speed of 1000 min−1. After mechanical activation up to 120 min, the powder was heated in the temperature range 800–1100°C, from 1 to 4 hours. The phase composition of samples that were only milled and of those that were milled and heated was followed using an X-ray method. The change in the specific surface area of the starting powder subjected to grinding was measured. In the case of powders mechanically treated up to 120 min and not heated afterwards, the γ→δ-Al2O3 transition was observed with the appearance of small amounts of the α phase. After 600 min of grinding the previously formed δ phase was transformed into O-Al2O3. In the case of both mechanically treated and unground powders which were heated afterwards, the phase transition to the α phase could be described by the scheme γ → δ → O → α-Al2O3. It developed at lower temperatures in the case of powders ground for a longer time. The kinetics of α-phase formation was also dependent on the time of previous mechanical treatment.

Influence of mechanical activation and fluorine ion on forsterite formation

Abstract The influence of mechanical activation of the mixture basic MgCO 3 –amorphous SiO 2 on reaction of forsterite formation was studied with and without the presence of fluorine ion. Beneficial influence of mechanical activation on the reaction in the mixture without fluorine ion is a consequence of the change of the granulometry during the intense milling. Mechanical activation in the presence of fluorine ion affects the mechanism of forsterite formation via different compounds from the humite group. With non-activated mixture dominant transition phase is hondrodite, while with activated mixture, clinohumite is the transition compound in the process of formation of end reaction products. Fluorine ion is released from the system by hydrolysis.

Decrease of the MgA12O4 formation temperature

Mixtures of A12O3 and (MgCO3)4·Mg(OH)2·5H2O, mechanically activated by intense grinding in a laboratory vibrating mill for up to 120 min, were additionally heat treated from 800 to 1250°C. It was shown that more intense particle size reduction of the crystallites of basic MgCO3 from 39 to 13 nm took place during activation, while the Al2O3 crystallites became finer from 32 to 23 nm. The BET specific surface area of ground spinel mixtures increases up to 30 min of activation and decreases thereafter. With prolonged activation time the temperature of reaction termination drops after the same isothermal heating time. The beneficial effect of the mechanical activation of spinel mixtures on the development of the chemical reaction is due to both the particle size reduction of the reactants and the partial decomposition of basic MgCO3. The positive influence of intense grinding on the reaction product yield is more pronounced at lower temperatures.

Transition of γ-Al2O3 into α-Al2O3 during vibro milling

Abstract The results obtained in studying the phase transition of γ- into α-Al2O3 during intense mechanical activation by vibro milling are presented in this paper. It was found that depending on experimental conditions, the γ→α-Al2O3 transition through δ- and θ-phases, can be initiated. X-ray and SEM analyses of activation products show that the phase transition is preceded by intense reduction of γ-phase particles. The produced α-Al2O3 crystallites have a distorted lattice and are larger than the initial crystallites in the starting powder. The change of specific surface area (Sp) with activation time shows the presence of high internal porosity of γ-Al2O3 which disappears during the complex process of the phase transition. An attempt was made to present the specific surface area change via its γ- and α-phase contributions. In this respect, detailed analyses of γ-Al2O3 particle size reduction along with reduction of their internal porosity and agglomeration of ground powder were performed, which explain the change of the specific surface area with activation time.