Zirconia

Zirconia, or zirconium oxide (dioxide), has aroused keen interest from the very beginning of its identification and preparation. In fact, this oxide presents several allotropic varieties depending on temperature; moreover, pressure or mechanical stress can act on the structure, making it a material of choice for studying crystallographic transformations, especially as one of these is of the martensitic type (i.e. with shearing) accompanied by strong thermal hysteresis. This oxide is a refractory ceramic, both chemically and thermally. Its physical properties, particularly in terms of electrical and electronic conduction, are remarkable. Insulating in its pure state with low n-type semiconduction, this oxide can become a pure ionic conductor if doped. The range of applications for this oxide is vast: solid electrolyte, vein for magnetohydrodynamics (MHD), fuel cell, oxygen probe... With this in mind, its preparation methods have given rise to numerous studies, ranging from sol-gel processes to solar furnace melting, via more traditional precipitation or co-precipitation processes. This oxide also owes part of its interest to the fact that it is the corrosion product formed on zirconium alloy cladding in pressurized water reactors (PWRs). The mechanical behavior of the zirconia layer formed on these claddings during operation, and therefore the degree of protection afforded by this layer, determines the long-term performance of the fuel bundles as a whole (breakaway or desquamation phenomena).

From this description, it can be seen that zirconia is a typical material whose properties in use are closely linked to structural aspects (crystallography and point defects). These aspects will be discussed and described in the remainder of this article.