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The densification of zirconia at very low temperatures (<400 °C) requires enhanced synergy between thermodynamics and kinetics. This work demonstrates an efficient single step approach combining amorphous hydrated zirconia and Spark Plasma Sintering at 350 °C and 600 MPa. The resulting zirconia ceramics exhibit a cohesive nanostructure with small average grain sizes (20 nm) and a predominantly monoclinic structural polymorph confirmed by both X-ray scattering analyses and High Resolution Transmission Electronic Microscopy. Remarkable Vickers hardness of 3.8 GPa for high level of porosity (30%) is explained by the density and homogeneous distribution of grains boundaries and meso /microporosities. Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract: Supercritical fluids have been used to prepare alumina powders. This versatile technique allows the tuning of the crystallinity of the obtained powders by a simple modification of the fluid''s nature. Amorphous or boehmite alumina could be successfully synthesised using either CO2/Ethanol or H2O/Ethanol, respectively. The two powders present very different sintering paths, which have been correlated to the grain morphology and crystalline state of the powders. We have established that the alumina elaborated in CO2/ethanol exhibited a significant decrease of the γ-Al2O3 →α-Al2O3 transition temperature resulting in a lower densification temperature. [Copyright &y& Elsevier]

For microwave agile devices such as tunable resonators or phase shifters, low loss components exhibiting electric-field dependent dielectric properties are seeked. Ferroelectric/low loss dielectric composites are good candidates to improve dielectric properties of BaxSr1-xTiO3 (BST) for such applications. Nanosized ferroelectric powders have been successfully encapsulated in a thin surrounding shell made of a binary oxide. The conservation of the ferroelectric transition in the composite has been shown while preserving the size of the ferroelectric cores after sintering.The dielectric properties of the composite can be tuned by changing the relative core/shell sizes, the intercore distance, the nature of the dielectric phase. Various synthesis routes have been used according to the required design of the final nanocomposite. Dielectric performances are presented and compared among the different nano-ferroelectric core–shell based particles obtained. [Copyright &y& Elsevier]

Spark plasma sintering (SPS) is an efficient tool to obtain highly densified ferroelectric-dielectric ceramic composites with clean interfaces and tunable properties. Dielectric MgO and ferroelectric Ba0.6Sr0.4TiO3 (BST) were combined in two-dimensional multilayer and three-dimensional random powders design. Their unmodified BST Curie temperature proves the suppression of interdiffusion while dielectric losses are below 0.5% and the tunability is 40% at room temperature. The composites and pure BST with similar densities (>95%) were obtained, owing reliable comparison of their dielectric properties. Such SPS ceramics can be used as experimental input for simulation and are potential candidates for high frequency applications. [ABSTRACT FROM AUTHOR]

The re-oxidation kinetics of BaTiO 3 ceramics sintered by Spark Plasma Sintering (SPS) was investigated using in-situ impedance spectroscopy. Thanks to the flexibility of the SPS process, the grain size of the dense ceramics was tuned from 0.5 μm to 10 μm. The re-oxidation kinetics are found to be very fast regardless of the grain size and a full re-oxidation of the ceramics are achieved after 20 h of exposure to an ambient environment at only 600 °C. The residual density of charge carriers is reduced when using finer starting powders. SPS ceramics made with micrometer size grains demonstrate a residual charge-carrier density that is one tenth that of ceramics made from 10 μm particles. Grain-boundary conduction is dominant through fine-grain SPS ceramics. This latter feature is similar to BaTiO 3 sintered using the conventional route with 10 μm size grain. Finally, the critical grain size for optimal dielectric permittivity is found to shift from 0.7 μm in standard ceramics to 1.5 μm in SPS ceramics. [ABSTRACT FROM AUTHOR]