Structural evolutions in Ti and TiO2 powders by ball milling and subsequent heat-treatments

In this study, nanocrystalline Ti and TiO2 powders were synthesized by wet milling of Ti powder within water and reactive milling of Ti powder under oxygen atmosphere, respectively. The wet milling processes were performed at 300 rpm milling speed for different milling durations (6, 12, 24, and 48 h). Subsequent heat-treatments of the as-milled powders were performed at 200–800 °C for 1 h in an air atmosphere. Structural characteristics of the as-milled and the heat-treated powders were investigated by X-ray diffraction (XRD). The crystallite size, lattice strain and lattice parameters of the powders were also calculated. It has been realized that during the wet milling process, elemental Ti powder undergoes an hcp to fcc polymorphic transformation, which is related to grain refinement and plastic strain. In addition, TiO2 powders are produced after heat-treatments at 600 and 800 °C. Reactive milling of the elemental Ti powder in pressurized oxygen atmosphere leads to nanocrystalline TiO2 powders with an average crystallite size of about 10 nm after 48 h milling time. The subsequent heat-treatments of the as-milled TiO2 powders results in the increase of the crystallite size up to 70–150 nm. In order to understand the thermal behavior of the as-milled powders, thermal analysis (DTA) and thermogravimetric analysis (TGA) were employed. The activation energy of the nanocrystalline TiO2 powders was determined by the Kissinger method is 128±3 kJ/mol.