Effect of mill type on the size reduction and phase transformation of gamma alumina

The influence of stress modes and comminution conditions on the effectiveness of particle size reduction of a common catalyst support; γ-Alumina is examined through a comparative assessment of three different mill types. Air jet milling is found to be the most effective in reducing particle size from a d90 of 37 µm to 2.9 µm compared to planetary ball milling (30.2 µm) and single ball milling (10.5 µm). XRD and TEM studies confirm that the planetary ball mill causes phase transformation to the less desired α-Alumina resulting in a notable decrease in surface area from 136.6 m2/g to 82.5 m2/g as measured by the BET method. This is consistent with the large shear stresses under high shear rates prevailing in the planetary ball mill when compared to the other mill types. These observations are consistent with a shear-induced phase transformation mechanism brought about by slip on alternate close packed oxygen layers from a cubic close packed to a hexagonal close packed structure

Carbothermal production of beta'-sialon from alumina, silica and carbon mixture

Mixtures of pure nanomete?-sized amorphous silica and ~/-alumina with the atomic ratio Si:AI = 1 were reduced by a stoichiometric amount of carbon between 1100 and 1450~ in flowing nitrogen in order to produce 13'-sialon powder. Using aqueous suspensions of starting materials, compacts with different microstructures were prepared for reaction. Silica reduction to SiO occurred at a temperature as low as 1300 ~ and part of it was removed with flowing nitrogen. Carbothermal reaction involving nitrogen stated at 1350 ~ and Si2N20 was found as an intermediate together with SiC, resulting in 13'-sialon formation. Loss of silica from the system led to AIN formation. Decomposition of lY-sialon into sialon polytypoids (15R, 12H) was observed as a result of sialon and AIN reaction at 1450 ~ The reaction rate of sialon formation was slowed down compared to the carbothermal reduction of kaolin because of the lack of impurities. The microstructure of the reacted pellets influenced the reaction products, and the narrow pore size distribution as well as good homogeneity enhanced J3'-sialon formation