Complex thermal evolution of size-stabilized tetragonal zirconia

Zirconia nanopowder with doping YO1.5 contents between 0 and 1 mol% were synthesized by the Pechini method. The crystallite dimensions of the powder, around 10 nm, allows for the size stabilization of the tetragonal polymorph over the thermodynamically stable monoclinic one. As the nanopowders are heated to 1200 °C and subsequently cooled back to room temperature, a complex evolution of the phase composition occurs. Upon heating the tetragonal phase transforms slowly into the monoclinic one and the transition cannot be completed before entering the stability range of the tetragonal phase (above 1150 °C). Upon cooling, on the other hand, the reaction is considerably faster and the complete transformation into the monoclinc phase occurs in a narrow temperature range. Rietveld analysis of the high temperature X-ray patterns revealed as, during heating, the transition is mainly controlled by microstructural parameters and in particular it is triggered by the release of RMS microstrain. Upon cooling, on the other hand, the transition is kinetically controlled by the doping content.