Ricardo Caruso

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.}

Controlled preparation and characterization of multilayer sol-gel zirconia dip-coatings

We describe a method for controlled preparation of sol-gel ceramic multilayer coatings obtained by dip-coating. The proposed fabrication routine guarantees obtaining crack-free multilayer coatings with control of the thickness and porosity of individual layers. The procedure is applied to obtain a 10-layer ZrO 2 ‐3 mol% Y 2 O 3 coating deposited on AISI 310 stainless steel. The microstructure of this coating was investigated by using x-ray powder diffractometry and atomic force microscopy, and special consideration was given to the mechanical characterization of this thin film using ultramicrohardness indentation tests. Our results suggest that these zirconia coatings have good properties for use as protection barriers. Implications concerning general tailoring design of sol-gel dip-coatings are also considered.

Synthesis of precursors for chemical solution deposition of PZT thin films

Abstract Lead zirconate titanate precursors involved in the sol–gel synthesis of PZT thin films were prepared from alkoxides of Pb, Zr and Ti, with the same alcoholic radical (methoxyethoxide), obtained by dehydration and alcoholysis of lead acetate, and by alcoholic exchange of zirconium n-propoxide and titanium ethoxide. In this investigation, 1H and 13C NMR spectroscopic techniques and FTIR analysis have been used to study the reaction mechanisms and identify the compounds obtained. The titanium and zirconium precursors present a complete alkoxy group exchange, whereas lead acetate is partially alkoxylated. A precursor solution of PZT has been obtained with minimal quantity of acetate groups and residual water. The molecular structures for the Zr, Ti, and Pb precursors and for the PZT precursor have been proposed.

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.

Theoretical study of the molecular structure for zirconium complexes

Abstract This work constitutes a study guided to the design of the molecular geometry of ZrO 2 gels aided by computer-based calculations (Density-Functional Theory). The electronic and spectroscopic properties of two zirconium complexes, [Zr(OH) 6 ] 2− and [Zr(OH) 5 (OCH 2 CH 2 CH 3 )] 2− , are explored. Vibration frequencies and properties of nuclear magnetic resonance are theoretically studied.

Influence of ion nitriding process on the properties of zirconia coating deposited on stainless steel

ZrO2-3 mol % Y2O3-coated AISI 310 stainless steel was nitrided using plasma produced by dc pulsed discharge in a mixture of N2 and H2 at an equilibrium temperature of 450oC. Profile chemical analyses with GDOS (glow discharge optical spectroscopy) and XRD showed the formation of Nitrogen/Y-ZrO2 solid solution. Nitrogen atoms enter in the zirconia substituting to the oxygen atoms in the network. Although, the nitrogen content measured in zirconia coating is greater than the allowed maximum nitrogen content in doped-zirconia solid solution, the formation of oxynitrides could not be detected. The phase structure was investigated by X-ray diffractometry The mechanical properties were studied by means ultramicrohardness indentation tests. The nitruration improves the mechanical responses of the ZrO2-3 mol% Y2O3 -coated AISI 310 stainless steel.

Preparation of YBa2Cu3O7−x superconducting films: influence of the chemical composition on the sintering

Abstract The influence of the cationic ratio Y:Ba:Cu on the kinetics of densification of the YBa 2 Cu 3 O 7− x (YBCO) superconductor ceramic was studied. The powders were prepared from organic precursors and the nominal compositions were: (A) stoichiometric composition YBa 2 Cu 3 O 7− x , (B) composition on the tie line joining YBa 2 Cu 3 O 7− x –BaCuO 2 phases, and (C) composition in the triangle YBa 2 Cu 3 O 7− x –Y 2 BaCuO 5 –CuO. The phases present in these powders heated to 890°C were detected by X-ray diffraction (XRD), while the ratio Y:Ba:Cu was measured by neutronic activation analysis (NAA). The reactions between 900°C and 1070°C under O 2 atmosphere were determined by differential thermal analysis (DTA) at a heating rate of 10°C/min. Compacts of different powders were obtained by uniaxial pressing and were then studied by dilatometric experiments under similar conditions as established in DTA measurements. The dilatometric curves were applied to different sintering models to obtain the validity ranges and activation energies corresponding to the different densification mechanisms that operate in the initial and intermediate stages of sintering.

Comparative study of sol-gel derived 2.5% and 11.3% Y2O3−ZrO2 by perturbed angular correlations spectroscopy

Powders and thin films of zirconia doped with 2.5 and 11.3 mole% Y2O3 have been fabricated via the sol-gel route. The phase structure and its thermal evolution have been investigated using mainly the hyperfine nuclear technique of Perturbed Angular Correlations. Both resulting structures, tetragonal and cubic, exhibited at least two different configurations around zirconium sites. The tetragonal powder involves an activation energy for vacancy fast diffusion lower than the cubic powder. Conclusions have also been drawn regarding the thermal stability of the ceramics under different heat treatments.

Interdiffusion Phenomena of Zirconia-Nitride Layers on Coated AISI 310 Steel

Duplex coatings on stainless steel American Iron and Steel Institute (AISI) 310 were prepared by a double process consisting of a plasma nitriding treatment and a multilayer ZrO2-3 mol % Y2O3 coating grown by chemical solution deposition. In this paper we investigate the interdiffusion phenomena between the elements and the deposited phases. The zirconia coating process over the nitrided steel initiated strong diffusion events. Hydrogen diffuses easily throughout the coating and acts as a reducing agent. The layer of zirconia works as a sieve, retaining nitrogen and allowing the passage of hydrogen. The contribution of this study show the interdiffusion at the interfaces of elements in samples subjected to treatments known as duplex coatings.