Preparation of Ultrafine Particles by Mechanical Processing

Abstract

The grinding and aggregation behavior of MoO3 , WO3 , and α-Al2O3powders in ball mills (specific power from 10 to 100 W/g) was studied. Is was shown using optical-beating measurements that the smallest particle size attainable by mechanical processing is ∼7 nm. The powders prepared by dry grinding contained no more than 5 wt % of 10-nm particles. According to particle-size analysis, after 30 s of grinding, the particles aggregated. The separation of 10-nm particles from the suspension was found to be accompanied by an increase in average particle size to 32 nm. Under the assumption that the instability of fine (5–50 nm) metal-oxide particles is caused by surface hydroxyls, a procedure was elaborated by which α-Al2O3powders containing ∼50 wt % of 20-nm particles can be prepared.