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The coating of BaTiO3particles with a different perovskite and the subsequent consolidation to dense ceramics retaining a radial composition gradient within the single grains are presented and discussed. A shell of SrTiO3or BaZrO3was directly grown on the surface of BaTiO3spherical templates suspended in aqueous solution by means of a precipitation process making use of inorganic precursors. The overall composition and the particle size can be tailored over a wide range. Densification of the resulting core−shell particles was realized using spark plasma sintering or conventional sintering. Dense ceramics with locally graded structure can be only obtained by a careful choice of the sintering conditions, that is, controlling the interdiffusion between core and shell. The final materials show strongly modified dielectric properties in comparison to both the parent compounds and the homogeneous solid solutions. The proposed approach is generic and suggests a new avenue to create functional and structural polycrystalline materials with locally graded structure by the controlled sintering of core−shell particles.

Cubic SrTiO3 particles are obtained by precipitation from an aqueous gel suspension. The gel suspension is prepared by hydrolyzing a TiOCl2 solution with NaOH and adding SrCl2. The addition of citric acid leads to spherical particles. The particle size can be tailored in the range 80−1400 nm by varying the temperature and the concentration. A careful control of the synthesis conditions is essential for producing particles with a narrow size distribution. The results of HRTEM and ED investigations provide strong evidence that the oriented aggregation of small (4−5 nm) nanocrystals is the dominant growth mechanism for the formation of the observed SrTiO3 particles. The primary nanocrystals self-assemble in a highly oriented fashion, producing defective single-crystal particles. The above results show that the directional aggregation process can be controlled by changing the temperature and concentration of the suspension as well as by adding organic molecules, obtaining SrTiO3 particles with a controlled size and shape.