Core–shell structure of strontium titanate self-grown by a hydrothermal process for use in grain boundary barrier layers

A perovskite shell of strontium titanate (SrTiO₃) has been successfully self-grown on the surface of a titanium (Ti) particle by a hydrothermal process using Sr(OH)₂·8H₂O and Ti powder with or without alkaline (NaOH) solution at a temperature of 150–200°C for 3 h. The obtained strontium titanate (ST) shell contained spherical grains. The shells' crystallinity, thickness and grain size increased with an increase in the hydrothermal growth temperature. The obtained shell grain size was estimated by transmission electron microscopy to be in the range 50–120 nm. The thickness of the spherical grain shell was about 400 nm for 3 h hydrothermal growth at 180°C and 600 nm for 3 h at 200°C. There was a transition zone of about 200 nm between the ST shell and the Ti core. The results are ascribed to diffusion of the Sr ions through the shell into the core to react with dissolved Ti ions in an oxygen-deficient environment. The self-grown ST–Ti was annealed in air after sintering at 1350°C in a reducing C/CO atmosphere. This produced a core–shell structure with an ST grain boundary barrier layer. The sintered and annealed body exhibited the good nonlinear resistance properties of a varistor.