Face-Centered Cubic Ti Cluster Assemblies Prepared by Plasma-Gas-Condensation

Abstract

Ti cluster assemblies have been prepared by a plasma-gas-condensation cluster deposition system, where the Ar gas flow rate (R_Ar) was fixed and the He gas flow rate (R_He) was varied. The microstructure of Ti clusters has been observed by a transmission electron microscope (TEM). The mean Ti cluster size, d_m, decreases with increasing R_He. Ti clusters are hexagonal close-packed (hcp) for d_m>15 nm, while they are face-centered cubic (fcc) for d_m<15 nm. The critical cluster size between the fcc and hcp phases is much larger than the theoretically predicted value and that (5–7 nm) for mechanically milled Ti powders. The fcc phase formation is attributed to the negative internal pressure in small Ti clusters. Moreover, Ti cluster assemblies have been prepared via the above-mentioned procedures with slight introduction of oxygen gas in the deposition chamber, and investigated by high resolution TEM, electron diffraction, and optical transmittance measurements. No clear core-shell morphology is observed in the partially oxidized Ti cluster assemblies.