P. C. Rivas

Phase structure and thermal evolution in coating films and powders obtained by sol-gel process: Part II. ZrO 2 –2.5 mole% Y 2 O 3

Yttria-stabilized cubic zirconia powders and coatings produced by the sol-gel method have been investigated by Perturbed Angular Correlation Spectroscopy (PAC). Results indicate that the metastable cubic phase is retained during heating and cooling cycles. The hyperfine interaction that describes this cubic phase, once crystallized, exhibits two components in a constant ratio of 4:1. The components represent different vacancy configurations. For the fast movement of oxygen vacancies starting at 750{degree}C, which is reflected by the damping of the hyperfine pattern, an activation energy of 0.96 eV was determined. {copyright} {ital 1997 Materials Research Society.}

Hyperfine interaction of ZrO2 — Tetragonal phase

The quadrupole hyperfine interaction of the tetragonal phase of the zirconium oxide has been measured at 1523 K using the time-differential perturbed angular correlation technique. The electric field gradient at zirconium sites was determined to beVzz=(17.5±0.4)·1017 Vcm−2 and axially symmetric.

Time‐differential perturbed angular correlations investigation of the (NH4)2ZrF6 thermal decomposition

Via the TDPAC technique and complementary x‐ray diffraction analysis, the thermal decomposition of (NH4)2ZrF6 has been observed to occur in the range 400–800 K according to the thermolysis scheme (NH4)2ZrF6→485 KNH4ZrF5→560 KNH4Zr2F9 →620 KZrF4→760 KZrO2. Assuming that the first transformation takes place through the simple chemical reaction (NH4)2ZrF6→NH4ZrF5+NH4F, the corresponding activation energy has been determined to be Ea=(82±6) kJ/mol. The hyperfine parameters of the NH4Zr2F9 decomposition product have been determined between 14 and 560 K. A complete analogy with the isomorphous compound NH4Hf2F9 was observed.

TDPAC investigation on thermally related HfF4.3H2O and HfO2

The hyperfine quadrupole interaction of HfF4.3H2O at Hf sites is investigated from 14 to 820 K. No transitions have been found. After the complete dehydration of this compound at 393 K, chemical reactions take place which give rise to hafnium oxifluorides and metastable forms of hafnium oxide. Heating treatments at increasing temperatures make HfO2 turn into its monoclinic phase.

On the kinetics of chemical reactions in hafnium tetrafluorides using time differential-perturbed angular correlations

Abstract Quadrupole hyperfine interactions in HfF 4 and HfF 4 ·3H 2 O determined by time differential-perturbed angular correlations technique are reported. Changes in the hyperfine interactions of HfF 4 ·3H 2 O with temperature are interpreted in terms of a dehydration process. The reversal process is studied at room temperature as a function of time. Assuming that the hydration obeys a first-order rate law, a rate constant k = 0.49 ± 0.10 d −1 is obtained.

Nanostructural study of sol-gel-derived zirconium oxides

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 Zr 4 + 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.

ZrO2 phase structure in coating films and powders obtained by sol-gel process

Zirconia coating film and powder obtained by the sol-gel route using zirconium n-propoxide as starting material and acid catalyst were investigated by the Perturbed Angular Correlations method, X-Ray Diffraction and Differential Scanning Calorimetry and Differential Thermal Analyses. The hyperfine interaction, measured after annealing the samples at increasing temperatures up to 1100°C, allowed to distinguish five different zirconium neighborhoods. Two of them describe rather disordered material which, on heating, crystallizes to the tetragonal phase and end finally in monoclinic zirconia. As compared with the powder, the film exhibits a minor fraction of an unidentified ordered form and a higher and more stable fraction of tetragonal phase. In addition, the tetragonal to monoclinic conversion takes place at higher temperatures and with a larger activation energy.

TDPAC study on the thermal stability of zirconium tetrafluoride

The thermal evolution of the hyperfine interaction in α-ZrF4 is investigated between 293 K and 788 K. The first change, observed ay 534 K, is attributed to the appearance of the non-stoichiometric compound ZrO1.3F1.4. During the transformation, the relative fraction of the latter shows an Arrhenius behaviour with an activation energyEa=(22.6±5.5) kJ/mol. At approximately 700 K, ZrO1.3F1.4 gives rise to the monoclinic phase of ZrO2 and to β-ZrF4.

TDPAC investigation on NH4Hf2F9

Time-differential perturbed angular correlation (TDPAC) measurements in NH4Hf2F9 were performed between 15 and 580 K. The compound was found to be stable in the whole temperature range and no phase transitions were observed. Experimental results could be explained assuming two equally populated quadrupole interactions which suggest a neat inequivalence between the two sites occupied by the hafnium atoms in the molecule.

The thermal behaviour of the quadrupole hyperfine interaction in (NH4)2HfF6

A TDPAC investigation has been accomplished on (NH4)2 HfF6 between 14 and 620 K. A phase transition was observed below 390 K. The low temperature phase is characterized by two non-equivalent sites for hafnium atoms in a 1∶1 ratio. The high temperature phase, on the other hand, is depicted by a unique quadrupole interaction. Above 400 K, the compound decomposes successively to NH4 HfF5, NH4 Hf2F9 and HfF4. An enthalpy of 76±4 kJ/mol could be determined for the (NH4)2 HfF6→NH4 HfF5+NH4F chemical reaction. The hyperfine interaction and thermal evolution of (NH4)2 HfF6 was found to be quite similar to that of (NH4)2 ZrF6.