M. C. Caracoche’s research while affiliated with National University of La Plata and other places

The Perturbed Angular Correlations technique was used to determine the configurations around Zirconium ions and their thermal behavior in non-aggregated sub-micron zirconia spherical particles. Three residues containing- Zr surroundings were determined for the non-crystalline starting particles, which were identified under the assumption of a certain chemical reactions sequence during synthesis. While the one made up mainly by hydroxyl groups was common to both samples, the two involving mainly organic residues were particle size dependent. Upon crystallization, both samples stabilized in the t'- and t- tetragonal forms and the Xc-cubic form but their amounts and temperatures of appearance were different. On heating, the structure of the smaller particles became gradually monoclinic achieving total degradation upon the subsequent cooling to RT.

Solid solutions of ZrO2 and HfO2 are potential electrolyte materials for intermediate-temperature SOFC because both are oxygen-ion conductors. The main challenge for these compounds is to reduce the relatively high value of the activation energies vacancies diffusion, which is influenced by several factors. In this work the thermal evolution of CaO-HfO(2) materials have been investigated. (CaO) y-Hf(1-y)O(2-y) (y = 0.06, 0.14 y 0.2) coatings and powders were synthesized by chemical solution deposition (CSD). Films were deposited onto alumina substrates by Dip Coating technique, the burning of organic waste was carried out at 500 degrees C under normal atmosphere and then the films were thermally treated at intervals of temperature rising to a maximum temperature of 1250 degrees C. By means Glazing Incidence X-ray Diffraction (phi-2 theta configuration) the phases were studied in the annealed films. On the other hand, the thermal evolution and crystallization process of powders were analyzed in-situ by HT-XRD. The phenomena crystallization occurred in films and powders were analyzed. The activation energies of diffusion of oxygen vacancies of HfO2-14 mole% CaO and HfO2-20 mole% CaO films were measured from the thermal evolution of the relaxation constant measured by Perturbed Angular Correlation Technique.

Nowadays, some widely used inorganic pigments (vanadium blue, praseodymium yellow, and iron-coral pink) are based on ZrSiO4 stoichiometry where the color is prevalently obtained by substitutional doping. This work deals with provided structural information on pure and praseodymium-doped zircon powders synthesized by the traditional ceramic route in order to evaluate the location of the dopant ion and the modification introduced in the zircon lattice. The information obtained by using the extremely localized perturbed angular correlation technique was compared with Rietveld structure refinement data. The kinetic of zircon pigment formation was followed by environmental scanning electron microscopy.

The hyperfine quadrupole interaction at Hf sites in films and powders of 14 mol% CaO–HfO2 and 20 mol% CaO–HfO2 has been determined as a function of temperature. Results indicate the formation of a cubic solid solution and other microstructures assigned to the ϕ1 (CaHf4O9) and ϕ2 (Ca6Hf19O44) phases. Dynamical effects on the electric field gradient reveal the existence of oxygen vacancies movements in the solid solution. The thermal behavior of the relaxation constant observed in films allowed the determination of activation energies of 0.54 eV and 0.70 eV for the 14 mol% and 20 mol% CaO doped hafnias, respectively. The influence of the microdomains and the stability of the cubic solid solution are discussed.

A Zr(10 mol % Pr)O2 powder obtained by high-energy ball milling has been investigated at nanoscopic scale using primarily the Perturbed Angular Correlations technique. The aim has been to determine the nanoconfigurations around Zr4+ cations present in the solid solution and their thermal evolution with the intention of providing knowledge on the stability of the system. Results indicate that the milled product is a substitutional cubic solid solution described by two hyperfine interactions: a highly disordered interaction due to oxygen vacancies located very close to Zr4+ and an ordered interaction probably depicting a charge distribution including Pr3+ as nearest neighbor to Zr4+ probes. On cooling from high temperatures, monoclinic zirconia appears mostly at the expense of the oxygen defective cubic form. A gradual cooling indicates that destabilization of the solid solution takes place around 500°C. Thermal cycling leads to increasing amounts of the monoclinic phase.

Zirconia powders, prepared by a sol-gel method using zirconium n-propoxide (ZNP) and different H2O/ZNP and alcohol/ZNP ratios, were investigated by the perturbed-angular-correlation method, along with X-ray diffraction (XRD), Raman spectroscopy, differential thermal analysis (DTA), and thermogravimetric analysis (TGA) techniques. The hyperfine interaction was measured after annealing the samples at increasing temperatures up to 1080°C. Two disordered structures, giving tetragonal patterns in XRD and Raman analysis, were identified in as-produced powders, which consisted of chains of fully hydrolyzed species and chains containing residual organic groups. As the annealing temperature was increased, crystallization occurred, producing the tetragonal and monoclinic phases of zirconia. All powders were essentially monoclinic at the highest temperatures. Those representing low H2O/ZNP ratios retained a large amount of tetragonal phase over a wide temperature range and transformed to monoclinic with a higher activation energy.

By using Perturbed Angular Correlation Spectroscopy, a suitable technique to explore internal fields at nanoscopic scale, the electric field gradients at Zr4+ sites in the Pr2Zr2O7 compound have been determined as a function of temperature. Three nonequivalent nanoconfigurations are present, which have been interpreted with the aid of point charge model calculations. Two of them correspond to pyrochlore - oxygen defective and perfect structures -, and the third one to the pyrochlore-related defect fluorite structure. The most abundant interaction is a disordered and fluctuating electric field gradient, which describes the oxygen defective pyrochlore. As temperature increases, its gradual and reversible transformation towards the perfect form is observed. Below 750 degrees C the oxygen vacancies movement, which exhibits an activation energy of 0.14 eV, is assumed to be due to vacancies jumping among 48f equivalent sites. At higher temperatures the movement is interpreted as the fast diffusion of oxygen vacancies involving 48f and 8b sites, thus giving place to anionic disorder. The activation energy for this movement has been determined to be of 0.85 eV. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

A nanocrystalline ZrO2–10 mol% Y2O3 powder synthesized by a nitrate–citrate gel-combustion process and stabilized in the metastable t′′-form of the tetragonal phase has been characterized using mainly the Perturbed Angular Correlations technique. The determined hyperfine interaction allows a clear distinction between the zirconium surroundings in the t′′-form and those already known of the other forms of the tetragonal phase. Oxygen vacancies movement is revealed as a slow diffusion involving a very small activation energy. As the powder is heated, a transformation is observed to occur around 560°C, which is consistent with a t′′-to-cubic transition.

The thermal behavior of the hyperfine interaction at Zr sites on two sol-gel derived 6 mole% CaO-ZrO2 powders, obtained from the particulate and non-particulate regimes, has been studied by Perturbed Angular Correlations and complementary techniques. The aim was to get experimental support on the thermal stability of the obtained material in order to see the advantages of the different preparation regimes. The results could be interpreted in terms of the different microstructures and nanoscopic configurations exhibited by the resulting powders. After crystallization both powders showed the hyperfine nanoconfigurations of metastable tetragonal zirconia. In the sample obtained following the hydrolysis and the condensation processes via a particulate regime, the undesirable phase transformation towards the monoclinic form of zirconia is inhibited up to higher temperatures.

Two sol-gel derived zirconia powders were prepared at pH = 0.5 and pH = 5.5. They were investigated as a function of temperature using mainly perturbed angular correlation spectroscopy. The aim was to elucidate the relationship between the nanoscopic configurations around Zr4+ ions and the morphology and structure of the powders. The highly porous material resulting from the solution at higher pH could be described mainly by defective and disordered, very hydrolyzed tetragonal arrays. As temperature increased, the amount of these arrays decreased while they became increasingly asymmetric, thus suggesting their superficial localization. The easy removal of hydroxyls led to the early appearance of the monoclinic phase. The gel obtained from the precursor at pH = 0.5 was entirely described by configurations still involving organic residues. After their calcination, the powder underwent a well-defined two-step hydroxyl removal thermal process leading to the crystallization of the tetragonal and the monoclinic phases. The thermal stability of the metastable tetragonal phase in the investigated powders seems to be controlled by their different capability to absorb oxygen.