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Single-step synthesis of ultra-fine barium titanate powder with a crystallinity as high as 90% and without barium carbonate contamination has been successfully performed under supercritical conditions using a continuous-flow reactor in the temperature range 150–380 °C at 16 MPa. To synthesize this bimetallic oxide, alkoxides, ethanol and water were used. The influence of the synthesis parameters on the BaTiO3 powder characteristics was investigated. The results show that the water to alkoxide precursor ratio, the reactor temperature and the Ba:Ti molar ratio of alkoxide precursor play a major role in the crystallization of pure and well-crystallized BaTiO3 nanoparticles. The continuous mode of operation without post-treatments for powder washing, drying or crystallization increase the industrial interest.

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A method based on a seeded growth process was developed to coat ferroelectric nanoparticles with a dielectric silica shell. This method, applied to size-polydispersed (Ba0.6Sr0.4)TiO3 particles (BST, mean diameter 150 nm), allows the control of the silica shell thickness from 2 to 80 nm with an accuracy of 1−2 nm. The morphology and surface physical chemistry of the core−shell were studied by transmission electron microscopy, photon correlation spectroscopy, and zeta potential measurements. A size-sorting procedure consisting of several cycles of centrifugation was optimized to extract the BST@silica nanoparticles of the required size for dielectric properties tuning. Upon sintering, dielectric measurements showed that the ferroelectric transition was maintained in the dense nanocomposites.

Supercapacitor behavior has been reported in a number of oxides including reduced BaTiO3 ferroelectric ceramics. These so-called giant properties are however not easily controlled. We show here that the continuous coating of individual BaTiO3 grains by a silica shell in combination with spark plasma sintering is a way to process bulk composites having supercapacitor features with low dielectric losses and temperature stability. The silica shell acts both as an oxidation barrier during the processing and as a dielectric barrier in the final composite.

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