A systematic kinetic investigation on the chemical synthesis of BaTiO 3 particles from aqueous solutions of BaCl 2 and TiCl 4 at T < 100°C and at pH 14 has been performed. Initially, a viscous suspension of a Ti-rich gel phase is obtained at room temperature. Later, formation of BaTiO 3 is induced by heating above 70°C and the gel phase is gradually converted to the crystalline perovskite. The isothermal formation kinetics of BaTiO 3 and the evolution of crystal size and particle size during the course of reaction are significantly influenced by temperature, concentration, and barium-to-titanium ratio of the solution. The early stages of reaction (yield < 1%) are dominated by primary nucleation, and slow formation of single nanocrystals of BaTiO 3 was observed by HRTEM. At a later stage, formation of polycrystalline particles occurs by secondary nucleation of BaTiO 3 on the surface of already existing crystals. During this stage, the reaction rate increases by 1 order of magnitude. When the yield exceeds 50%, nucleation becomes less important and the reaction is dominated by growth. Final particles have a diameter in the range 0.3-1.6 µm, depending on the processing parameters.

The incorporation of Er3+ into BaTiO3 ceramics was investigated on samples containing 0.25, 0.5, 1, 2, 8, and 10 at.% of dopant, after sintering at 1350–1550°C in air. For Er3+ concentrations ≤1 at.%, dense and large‐grained ceramics with low room‐temperature resistivity (102–103Ω·cm) were obtained. The observed properties are largely independent of stoichiometry. Simultaneous substitution of Er3+ at both cation sites, with higher preference for the Ba site, is proposed. The behavior of heavily doped ceramics depends on stoichiometry. When Ba/Ti < 1, the electrical properties change from slightly semiconducting to insulating as Er concentration increases from 2 to 8 at.%. The ceramics have tetragonal perovskite structure and contain a large amount of Er2Ti2O7 pyrochlore phase. On the other hand, when Ba/Ti > 1, the ceramics are insulating, fine‐grained, and single phase. In this case, incorporation of Er3+ predominantly occurs at the Ti site, with oxygen vacancy compensation. Incorporation is accompanied by a significant reduction of tetragonality and by expansion of the unit cell. The different results indicate that Er3+ solubility at the Ba site does not exceed 1 at.%, whereas solubility at the Ti site is at least 10 at.%. However, the incorporation of Er3+ and the resulting properties are also strongly affected by sintering conditions.

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.

Polymers with different hydrophilic groups [polyvinylpyrrolidone (PVP), ammonium polyacrylate (APA), and hydroxypropylmethyl cellulose (HPMC)] and surfactants [cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzensulfonate (SDBS)] were used as additives to modify the crystallization of β-Ni(OH) 2 in hydrothermal conditions. Marked morphological changes in the β-Ni(OH) 2 particles were observed depending on the additive concentration and on the duration of the hydrothermal treatment. The final morphology is the result of a complex, time-dependent self-assembly and growth process. Well-defined particles with sizes from submicrometer range to a few micrometers corresponding to hexagonal lamellae, hexagonal tabular mesocrystals, rosette-and flowerlike aggregates of lamellae, hexagonal prismatic mesocrystals, and acicular nanocrystals were easily obtained after a short time (2-24 h) aging at 150-200 °C. With PVP and CTAB, there is evidence of a growth process dominated by self-assembly of nanocrystals to produce mesocrystals. The formation of spherical superstructures (with SDBS, up to 70 µm in diameter) and hollow spheres (with PVP) is observed at long times (>24 h) as a result of solvent-mediated recrystallization processes, like Ostwald ripening. The overall results show that hydrothermal synthesis of β-Ni(OH) 2 in the presence of polymers with hydrophilic groups and surfactants is a versatile tool for crystal morphogenesis.

No abstract

No abstract

No abstract

No abstract