Preparation of ultrafine particles by mechanical processing

The grinding and aggregation behavior of MoO3, WO3, and alpha -Al2O3 powder s in ball mills (specific power from 10 to 100 W/g) was studied. Is was sho wn using optical-beating measurements that the smallest particle size attai nable by mechanical processing is similar to7 nm. The powders prepared by d ry grinding contained no more than 5 wt % of 10-nm particles. According to particle-size analysis, after 30 s of grinding, the particles aggregated. T he separation of 10-nm particles from the suspension was found to be accomp anied by an increase in average particle size to 32 nm. Under the assumptio n that the instability of fine (5-50 nm) metal-oxide particles is caused by surface hydroxyls, a procedure was elaborated by which alpha -Al2O3 powder s containing similar to 50 wt % of 20-nm particles can be prepared.