The nature and thermal evolution of nanoconfigurations around cations in a sol-gel derived solid solution of a ZrO 2 15 mol % Fe 2 O 3 nominal concentration have been investigated by means of the hyperfine techniques of the Mo ¨ssbauer effect at the Fe sites and perturbed angular correlation spectroscopy (PAC) at the Zr sites, assisted by X-ray diffractometry and thermal analyses. The resulting substitutional solid solution is metastable tetragonal zirconia and exhibits a solubility of 12 mol % of Fe 2 O 3 up to temperatures near 750 °C. The hyperfine experiments allowed the establishment of the existence of two nonequivalent sites for each of the probe cations. The most disordered one reflects the influence of the oxygen vacancy introduced after Fe 3+ substitution for Zr 4+ , which is shared by host and dopant cations. The other tetragonal nanoconfiguration, characterized by PAC for the first time, has been thought to involve a direct interaction between the nearest Fe and Zr ions of the largely distorted cation network.
Powders and coatings of zirconia doped with 2.5 mole% yttria have been produced via the sol-gel route. The phase structure and subsequent thermal evolution in heating and cooling cycles have been investigated using mainly perturbed angular correlations spectroscopy. Thermal analyses and XRD as a function of temperature have also been performed to obtain complementary information. Upon heating, the amorphous gels crystallized into the tetragonal structure and showed the same hyperfine pattern and thermal behavior as observed in tetragonal zirconia obtained by the ceramic route: the two configurations of vacancies around zirconium ions denoted as t1 and t2 forms and their mutual t1 → t2 transformation. While the powder sample exhibited an incipient thermal instability above 1000 °C and underwent completely the t2 form to m–ZrO2 transition during subsequent, gradual cooling below 500 °C, the coating retained the tetragonal phase within the whole temperature range investigated. Hyperfine results suggest that the tetragonal phase stabilization is favored by the highly defective nature of the t1 form and consequently hardened by the availability of oxygen. The PAC derived activation energy for the fast diffusion of the oxygen vacancies inherent to the t2 form was determined as 0.54 ± 0.14 eV.
Sol-gel-derived powder samples of zirconia (ZrO 2 ) prepared via the dissolution of zirconium n-propoxide in methanol, ethanol, and 2-propanol have been characterized mainly using perturbed angular correlations spectroscopy, as a function of temperature. Results indicate that the nanostructures and subsequent thermal evolution are alcohol dependent: the shorter the alcohol chain, the more hydrolyzed the product. ZrO 2 powder that has been obtained using ethanol as the solvent is the product that exhibits the better stabilization of the metastable tetragonal phase (t-phase). It undergoes a clear and detailed t 1 -form 3 t 2 -form 3 monoclinic ZrO 2 thermal transformation and shows the highest activation energy against the transformation to the monoclinic phase.
This paper deals with a perturbed angular correlation investigation of the microstructure and subsequent thermal evolution of a commercial ZrO2‐2. 8 mol% Y2O3 ceramic powder and of a pellet obtained upon sintering the powder at 1450°C. The perturbed angular correlation results, complemented by those of X‐ray diffraction and Raman spectrometry, indicate that the metastable tetragonal phase exhibits two hyperflne interaction forms instead of the interaction associated with the conventional t‐ZrO2 phase, both having different oxygen vacancy configurations around zirconium sites. While the powder sample exhibits mainly a very defective tetragonal form, the pellet exhibits a slightly distorted one as the major form. As temperature is increased, a reversible transformation between both forms occurs involving a redistribution of the oxygen vacancy. Once the transition is completed, the oxygen vacancy movement is described by a fast relaxation regime with an activation energy Eact= 0. 50 ± 0. 04 eV.
Monoclinic ZrO, has been milled for various times down to a crystallite size of 145 A. Using the time-differential perturbed-angular-correlation technique, it was possible to associate the resulting growing amount of disordered material with two different, very distributed and millingtime-dependent hyperfine interactions: one (Y) of quadrupole frequency similar to that of tetragonal zirconia after long milling, the other ( X ) one of a quadrupole frequency similar to that of monoclinic zirconia at shorter milling times. Upon annealing, all samples showed the recovery of the crystalline monoclinic phase at the expense of the disordered structures. In samples milled for shorter times, the ( Y ) interaction emerges as an ordered crystallization product, upon annealing at temperatures which depend nearly linearly on the crystallite size.
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