Abstract
In the Ti-rich region of the SrO-TiO2 system, a temperature-composition range is found to exhibit limited solubility of strontium, the maximum limit being 0.5 mol % Sr. It is also found that a two-phase region exists between this solubility limit and SrTiO3, in agreement with existing phase diagrams. When the strontium concentration is small (⩽ 0.5 mol %), strontium segregates to the grain boundaries to form a second phase of SrTiO3 below the solubility limit. Above 1200‡ C, this second phase is continuous; shape instability causes it to break up into discrete particles below 1200‡ C. At higher strontium concentration (about 2 mol % Sr), nucleation of SrTiO3 crystallites occurs both inside the TiO2 grains and at the grain boundaries. The morphology of these crystallites after long time annealing suggests that the {110} plane has the fastest growth rate. The appropriate range of composition and heat treatment to form a continuous SrTiO3 grain boundary layer is discussed in relation to the optimum heat treatment to develop the best nonlinear electrical properties of varistor devices.
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