Ultra-fine particles of amorphous TiO2 studied by means of positron annihilation spectroscopy

Abstract

Microscopic structures and mechanical properties of amorphous TiO2 ultra-fine particles have been studied by positron annihilation spectroscopy. A positron lifetime of about 1.5 ns was found as the longest lifetime component τ3 which is attributed to the ortho-positronium annihilation in open space. The lifetime τ3 and the corresponding intensity I3 was independent of atmosphere: vacuum and various gasses. Accordingly, a micro-void is contained inside the particle as the open space. An intermediate lifetime τ2 was approximately 0.4 ns. The intensity I2 was strongly affected by the atmosphere. No effect was found for the particles, the surface of which was coated by silicon dioxide film. This component is arisen from the positron annihilation on the particle surface and the particles contain the surface cracks. When the volume of particles packed in an ampoule was decreased by 1/4 times by compression, the longest component as well as the intermediate one disappeared and only the shortest-lifetime component τ1 of about 0.24 ns remained. To conclude, the particles are so soft that the micro-voids and the surface cracks are collapsed by compression.