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Tetragonal zirconia ceramics have excellent electrical and mechanical properties. However, the physical mechanisms involved in the stabilization of the metastable tetragonal phase at room temperature are not well known yet. This phase can be retained in ceramic specimens provided that the grain size is smaller than a critical value (about 1 micron). In the case of powders, it has been established that the crystallite size must be nanometric (the surface free energy has a very important role), but the influence of stresses and defects are not fully understood. The aim of this work was to study the stability of yttria or ceria-doped tetragonal zirconia nanopowders synthesized by different gelcombustion routes. These methods are based on the gelling of a nitrate solution of the desired metals and some organic fuel (glycine, citric acid, etc.) followed by an intense combustion due to the exothermal redox reaction between nitrate ions and the fuel. The powders were characterized by X-ray diffraction applying the Rietveld method. A conjunct analysis of crystallite size and microstrain was carried out employing the multiple-line integral breadth method and assuming Lorentzian and Gaussian components for both effects. The crystallite size profile presented both components, being the Lorentzian the most important. On the contrary, the microstrain profile had a negligible Gaussian component, which is an interesting result since it is generally represented by a Gaussian function. It was also found that both, crystallite size and microstrains, affect the stability of the tetragonal phase.

X-ray line broadening study of tetragonal zirconia nanopowders synthesized by gel-combustion