Phase Control in Nanophase Materials Formed from Ultrafine Ti Or Pd Powders

ABSTRACTThe effect of He gas pressure during evaporation and post-evaporation O2 exposure on phase formation in nanophase materials has been examined. Ultrafine powders of Ti and Pd were prepared by the inert gas-condensation technique and bulk nanophase samples were formed by consolidation of these powders with or without prior exposure to O2. Evaporation of Ti in He pressures greater than 500 Pa followed by exposure to O2 results in the formation of ultrafine powders of crystalline rutile (TiO2) which are compacted to form nanocrystalline TiO2. Surprisingly, reducing the He pressure used during evaporation to less than approximately 500 Pa results in the formation of ultrafine powders of an amorphous phase. Room temperature consolidation of this powder under vacuum, however, results in nanocrystalline Ti being formed if the powder is not exposed to O2 prior to consolidation, and a mixture of rutile and an unidentified crystalline phase if the powder has been previously exposed to O2. Further reduction of the He pressure during evaporation of Ti to less than approximately 10 Pa results in the formation of crystalline Ti having a film-like morphology rather than than the desired ultrafine particles. Experiments on Pd evaporated in 10 to 6000 Pa of He have yielded ultrafine powders and consolidated samples of only the crystalline fcc phase, regardless of the He pressure