Papers by Author: Dominique Gosset

Abstract: In the frame of the development of new nuclear reactors (Gen-IV project), materials able to withstand very high temperatures are required. Among the components of the cores, the transition metal carbides could be favourably considered as fission products barriers or inert matrix for the fuel. They are highly refractory, have good neutron properties and fission products retention. Unfortunately, first they are brittle and second the elaboration process of bulk materials requires drastical conditions (temperature, pressure). We have then undertaken the comparison of different materials to investigate the potentialities of nano-sized microstructures for the improvement of the mechanical properties and to determine the impact of different elaboration routes to obtain high density articles. The elaboration process of the nano-sized powders is presented elsewhere [M. Dollé et al., this meeting]. Here, we compare different sintering processes (uniaxial hot-pressing, spark plasma sintering) and we present a new process able to produce high density zirconium carbide articles with soft conditions. We show that the sintering of co-ground zirconium and graphite powders lead to high density, homogeneous, low grain-size materials at temperatures as low as 1500°C. At the contrary, the nano-powders appear to be more difficult to densify that classical micro-sized powders.

Abstract: Zirconia can be considered to be one of the most important ceramic materials because of its large range of industrial applications (catalysis, coatings, spacecraft shielding, paint additives, oxygen sensors, fuel cells, nuclear fuel matrices, an alternative high permittivity material to replace silicon oxide as a gate dielectric in MOS devices). Many of these applications require the use of zirconia in a nanocrystalline form. It is now well established that a monoclinc to tetragonal phase transition is trigged by the grain size of zirconia. The mechanism of this phase transition in zirconia is not yet clearly understood. Several experiments point out that the thermodynamic properties of nanocrystalline solids are particle-size dependent. Size-related effects like the reduction of the melting temperature and displacement of the phase boundaries can be predicted. Zirconia can be considered a textbook example for describing these effects. In this ceramic several polymorphic transformations occur with the change of external parameters (Temperature, pressure, …). In this paper, the behaviour of the tetragonal to monoclinic martensitic phase transition within Landau theory framework in particular will be discussed, pointing out the peculiar effects related the small grain size of the nanoparticles. Neutron diffraction experiments will illustrate the of these arguments and provide some insight to the understanding of the behaviour of nanocrystals in severe environments, such as in nuclear reactors or in space applications.