Ultrafine oxide powders prepared by electron beam evaporation

Several ultrafine oxide powders (MgO, CaO, Al2O3, SiO2, ZrO2, CeO2, U3O8) have been prepared by evaporation of bulk oxide with a focused beam of electrons, followed by condensation of the vapour. Rates of evaporation were dependent on the temperature of the heated zone and the corresponding vapour pressure of the oxide, and agreed with those predicted by the Hertz-Langmuir equation. The microstructure of the powders was affected by the pressure and type of gas present during condensation of the vapour. Particles, which were nucleated in the vapour phase, were unagglomerated and had an extremely small size (≲10 nm) which was dependent on the rate of evaporation; at very low rates, sizes approaching the critical nucleus (≲3 nm) were attained.