Influence of the urea content on the YSZ hydrothermal synthesis under dilute conditions and its role as dispersant agent in the post-reaction medium

The effect of the urea content, under mild and dilute conditions, on the extension of the YSZ hydrothermal reaction, on the crystalline zirconia phases obtained and, on its primary particle size has been studied. The key role of the urea as a dispersant agent for the synthesized YSZ nanoparticles has also been investigated in terms of particle size distribution and zeta potential. The latest study has been performed in the post-reaction medium. Pure nanocrystalline YSZ is obtained when a basic pH (∼10) is achieved during the synthesis. In addition, the urea is protonated at 5 < pH < 7 in the post-reaction medium and it allows its specific adsorption on the surface of the particles by electrostatic and steric mechanisms, keeping a stable suspension. In those conditions, the measured average particle size is 20 nm and the agglomeration factor (Fag) is 2. However, by HR-TEM particles with a size even less than 5 nm are observed.