Yttria-stabilized zirconia (YSZ) is one of the most important ceramics due to its unique combination of properties: strength, fracture toughness, hardness, high temperature stability, ionic conductivity at high yttria content, etc. It is thus a key material in many fields from thermal barrier coatings to biomedical devices. The control of particle size, yttrium content and distribution of yttrium play a key role in optimizing its properties. A promising route towards the control of both structural and microstructural characteristics of YSZ is the preparation of nanostructured ceramics using sintering processes that afford lower densification temperatures than those required by conventional sintering (i.e. >1400°C). One option for optimizing the nanostructure is to use nanopowders with a fine control of their particles size, crystallinity, purity, composition and surface chemistry as raw material. In this context, the technology using fluids in supercritical media represents an interesting alternative as it allows the continuous synthesis of high-quality nanocrystals in a single step. In addition, solvent assisted sintering processes such as Hydrothermal Sintering (HS) and Cold Sintering Process (CSP) are promising low temperature sintering processes (<500°C), targeting densification while avoiding grain growth. Therefore, the combination of high-quality nanocrystals produced with the supercritical fluid technology and solvent assisted sintering processes such as HS and CSP, is a promising way to achieve nanostructured YSZ dense ceramics at low temperature thanks to an increased chemical reactivity.

Here, we first introduce the innovative synthesis process of YSZ nanoparticles using supercritical fluids. Nanocrystalline particles with different yttrium content were successfully synthesized and were characterized by a set of advanced characterization probes in order to evaluate the quality of the produced nanomaterials, in terms of size, surface chemistry and composition, using total X-ray scattering (PDF analysis), XPS analysis, as well as TEM-HR, Raman and infrared spectroscopies. In a second part, our results obtained with HS and CSP using supercritical synthesized YSZ nanocrystals will be presented to show the potentiality of these "new building blocks" in both a single step and two step sintering approaches at low temperature. Using low temperature sintering approaches, a better trade-off between mechanical strength, Low Temperature Degradation (LTD) resistance and optical properties are expected. Preliminary investigations focused on the mechanical behavior, the study of distribution of yttrium in the grains and at the grain boundaries, as well as the stability under accelerated aging will be discussed.