The constant improvement in performance and reliability of ceramics requires an optimization of each elaboration stage to meet multi-criteria specifications. A fundamental understanding of structure-(nano)microstructure-properties relationships is essential, especially for nanostructured ceramics in which the parameters of particle size, surface free energies and surface stresses impact phase stability, densification and ultimately the physical properties. Customized microstructures and properties achieved through the tuning of chemical reactivity (including surface chemistry) of the starting powder and defect chemistry are both actively explored pathways. In this context, how can unconventional sintering processes, like Spark Plasma Sintering (SPS), Cold Sintering Process (CSP) and Hydro/Solvothermal sintering (HS), be synergistically combined with chemistry-based strategies to control the densification of advanced ceramics at low temperature?
 
To respond to this question, three case studies will be described. The first is focused on BaTiO3 for which the breakthroughs recently obtained in terms of properties control (size dependent) and reduced sintering temperature (< 300°C), provide a highly compelling illustration.  A second relevant model case is ZnO, where the contribution of chemistry and point defects play an undeniable role in densification and electronic properties, but no structural sensitivity to particle size is observed. It is therefore clear that chemistry driven mechanisms are enhanced in ZnO using SPS, CSP and HS. Third, based on our recent results, the densification of zirconia will then be described in more detail. Zirconia, whose polymorphism and grain size control leads to specific properties, is indeed a prime material for exploring the potential of SPS, CSP and HS. Considering that CSP and HS are based on dissolution precipitation mechanisms induced by pressure solution creep as driving forces for densification, the low solubility of zirconia is an issue that needs to be tackled by alternative strategies. Our first approach was to explore the starting particles reactivity, using either hydroxides as precursors or yttria stabilized zirconia (YSZ) nanoparticles synthesized in supercritical fluid media. Using zirconium hydroxide, the stabilization of amorphous, monoclinic or tetragonal zirconia depends on mechano-chemical equilibrium, within a narrow temperature window driven by a complex dehydration/crystallization/densification sequence. Both the process and the associated experimental conditions play a key role in the stabilization of different zirconia forms at temperatures below 400°C.  Next, the elaboration in supercritical fluid media of YSZ particles (< 10nm), whose structure is tuned by the yttrium content, will be described and their behavior during sintering discussed. Our second approach, was to explore a chemical pathway leading to increased point defects density. Diverse SPS sintering conditions (atmosphere, pressure, thermal ramps, …) are shown to impact the densification of YSZ below 950°C.
 
Through these illustrations, the complementarity between the three unconventional sintering processes, appears as an additional lever to better understand the mechanisms and to optimize densification at low temperature. Chemistry and sintering have always been intimately linked; for the advanced processes discussed in this work, they become indissociable even at very low temperatures. This invisible meeting zone has allowed many breakthroughs, but it also raises issues and future challenges which will be discussed in this presentation.