Nanocrystalline apatites analogous to bone mineral are very promising materials for the preparation of highly bioactive ceramics due to their unique intrinsic physico-chemical characteristics. Their surface reactivity is indeed linked to the presence of a metastable hydrated layer on the surface of the nanocrystals. Yet the sintering of such apatites by conventional techniques, at high temperature, strongly alters their physico-chemical characteristics and biological properties, which points out the need for "softer" sintering processes limiting such alterations. In the present work a non-conventional technique, spark plasma sintering, was used to consolidate such nanocrystalline apatites at non-conventional, very low temperatures (T<300 degrees C) so as to preserve the surface hydrated layer present on the nanocrystals. The bioceramics obtained were then thoroughly characterized by way of complementary techniques. In particular, microstructural, nanostructural and other major physico-chemical features were investigated and commented on. This work adds to the current international concern aiming at improving the capacities of present bioceramics, in view of elaborating a new generation of resorbable and highly bioactive ceramics for bone tissue engineering.

Spark plasma and flash sintering process characteristics together with their corresponding sintering and densification mechanisms and field effects were briefly reviewed. The enhanced and inhibited grain growth obtained using these field-assisted densification techniques were reported for different ceramic nanoparticle systems and related to their respective densification mechanisms. When the densification is aided by plastic deformation, the kinetics of grain growth depends on the particles' rotation/sliding rate and is controlled by lattice and pipe diffusion. When the densification is aided by spark, plasma, and the particles' surface softening, grain growth kinetics is controlled by viscous diffusion and interface reactions. Grain growth in both cases is hierarchical by grain rotation, grain cluster formation and sliding, as long as the plastic deformation proceeds or as long as plasma exists. Densification by diffusion in a solid state via defects leads to normal grain growth, which takes over at the final stage of sintering. Various field effects, as well as the effect of external pressure on the grain growth behaviour were also addressed.

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible.

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Cuboidal LiF microcrystal powder was densified by spark plasma sintering at different pressures up to 500°C. Densification at pressures above the yield stress occurred by plastic deformation and strain hardening. Densification at 2 MPa, below the yield stress, occurred by particle rearrangement assisted by viscous flow at the particle surfaces. Scanning electron microscopy examination of the fracture surfaces of the partially dense specimens revealed partial melting of the particle surfaces due to the plasma. The onset temperature for plasma formation was 180°C.

Spark plasma sintering (SPS) is a breakthrough process for powder consolidation assisted by pulsed current and uniaxial pressure. In order to model the temperature variations of the tools during a SPS cycle, the Graphite-Papyex-Graphite contact phenomena are studied experimentally and modeled by finite element calculations. Compared to conducting materials, the thermo graphic image of an insulating sample (alumina) shows strongly localized heating along the Papyex implying contact effects are predominant. The aim of this modeling study is to determine the main contact phenomena due to Papyex. It is based on numerous experimental data and studies the case of alumina sintering. Finally the contact model is confronted to experimental thermal images.

Christophe Fast and easy preparation of few-layered-graphene/magnesia powders for strong, hard and electrically conducting composites. (2018) a b s t r a c tComposite powders were prepared by the chemical vapor deposition (CH 4 /Ar atmosphere) of carbon in the form of 2e8 layers few-layered-graphene (FLG) covering the MgO powder grains, without any mixing step. The composites were consolidated to nearly full (99%) density by spark plasma sintering with no or little damage to the FLG. The FLG is located along the MgO grain boundaries, as opposed to be dispersed as discrete particles or flakes. This causes a dramatic hindrance of the MgO grain growth, the average grain size being considerably lower for the sample with 2.08 vol% carbon (200 nm) than for pure MgO (3.7 mm). The samples are investigated by Raman spectroscopy, scanning and transmission electron microscopy. The composites are electrically conducting with a percolation threshold below 0.56 vol%. Compared to pure MgO, the composites are simultaneously stronger (345 vs 200 MPa) and harder (9.8 vs 3.8 GPa). This could arise from reinforcement mechanisms such as crack-deflection and crack-bridging by FLG, but also from MgO grain refinement.