Zirconia can be considered one of the most important ceramic materials because of the large range of industrial applications (catalysis, coatings, spacecraft shieldings, paint additives, oxygen sensors, fuel cells, nuclear fuel matrix, alternative high permittivity material to replace silicon oxyde as gate dielectric in MOS devices, …).
Many of these applications require the use of zirconia in a nanocrystalline form. Several experiments point out that the thermodynamic properties of nanocrystalline solids are particle-size dependent. Size-related effects like lowering of the melting temperature and displacement of the phase boundaries can be predicted.
Zirconia can be considered a textbook example for describing these effects. It undergoes several polymorphic transformations with the change of external parameters (T, p, …). We will discuss in particular the behaviour of the tetragonal to monoclinic martensitic phase transition within the Landau theory framework, pointing out the peculiar effects related to the small grain size of the nanoparticles. Neutron diffraction experiments will illustrate the validity of these arguments and provide some insight to understand the behaviour of zirconia nanocrystals in severe environments as in nuclear reactors or in space applications.