The recent development of cold sint

The recent development of cold sintering techniques has shown a great efficiency in the preparation of dense samples at extremely low temperatures [1–3]. The process is generally assumed to proceed through a pressure solution creep mechanism achieved under uniaxial high pressure (typically > 100 MPa to GPa range) and mild temperatures (< 500°C). Although it appears that sintering mechanisms induce a dissolution step followed by a precipitation step,their precise description is not provided yet [4]. This way, dense bulk materials were obtained in various chemical media. A majorchallenge concerns materials with very low solubility in the experimental conditions (typically ~400°C under uniaxial pressure 100-800 MPa [5,6]). To densify such materials, alternative strategies have been developed, showing successful results. Ionic liquid was used to densify BaTiO3 in the absence of water, with densities reaching 98% of theoretical density at temperatures as low as 300°C without further heat treatment [7]. Another solution consists in activating sintering through a decomposition-reactive sintering. Following this route lead to the preparation of pure zirconia from hydroxide precursors Zr(OH)4 in just a few minutes at 350°C [8]. Recent work on BaTiO3 from Ba(OH)2, 8H2O precursors also showed high densification rate (95% relative density) without formation of BaCO3 [9]. This route offers a great potential for the preparation of metal oxides from low temperature decomposing hydroxides. Moreover, this also offers opportunities for the preparation of dense materials with very low decompositiontemperatures [10] such as sulfates [11], carbonates [5,12], phosphates [13,14] but also oxides [6]. Alumina represents one of the most used oxide material, especially under the most stable phaseα-Al2O3. It also presents a large subset of metastable crystalline phases (γ , δ, θ, κ, ε, η, χ) [15]. Among alumina crystalline phases,γ -Al2O3 is of particular interest for catalyst support application. Its lack of stability makes it difficult to densify into monolithic ceramics. However, densification could be performed under extremely high pressure, at 3 GPa - 500°C using a specific design [16]. Another possibility for γ -Al2O3 is to use it to promote densificationof α-Al2O3 phase through a phase transformation enhanced sintering [17,18].