High pressure and low temperature sintering of bulk nanocrystalline TiO2

Bulk n-TiO2 samples with a relative density as high as 95% and a grain size less than 50 nm were fabricated by hot-pressing at temperatures as low as 400 °C and at pressures up to 1.5 GPa. During hot-pressing, the anatase phase transformed to the rutile phase and the amount of transformation increased with sintering pressure. The grain size in both the anatase and the rutile phase increased with sintering pressure at a constant temperature but the grain size of the transformed phase is always smaller than that of the starting material. We believe that the smaller grain size of the rutile phase is related to multiple nucleation events in the anatase phase during sintering at very high pressure. The average grain size increased from 27 nm in the original powder to only 45 nm in the compact after hot-pressing. Analysis of the grain size and closed porosity by transmission electron microscopy suggested that closed pores at grain boundary triple junctions might also retard the grain boundary migration and thus prevent grain growth. A competing mechanism is also proposed in which the rate of grain growth is controlled by the pressure effect on the bulk diffusion rate and interface energy.