Elisabeth Djurado

Deposition and characterization of nanocrystalline tetragonal zirconia films using electrostatic spray deposition

Abstract Pure tetragonal and nanocrystalline 2 mol.% Y 2 O 3 -doped ZrO 2 thin films are deposited on stainless steel substrates by electrostatic spray deposition (ESD) technique. Tetragonal phase is evidenced by Raman spectrometry and X-ray diffraction in an as-deposited film at 400°C. Good homogeneity in composition is confirmed using wavelength dispersion scanning. The surface microstructure of dense to porous thin films is investigated varying both deposition parameters: substrate temperature and deposition time.

Structural changes of rare-earth-doped, nanostructured zirconia solid solution

A systematic study of the influence of the rare-earth (Sc, Yb, Y, Gd, Sm) dopant content and the ionic radius on the structural changes was undertaken on nanocrystalline-doped zirconia ceramics, which present the same microstructure. Tetragonal single-phased ceramics which were prepared by sintering of nano-powders obtained by spray pyrolysis at 1500 °C for 2 h in air, are characterized by an average grain size as low as 60 nm. Their densities are about 96% of the theoretical values. A good homogeneity in composition and in grain size was obtained for all samples. This exceptionally low grain growth during the sintering was related to the high purity of the materials and also to the morphology of the powders. Tetragonal solid solution domains were determined from Differential Thermal Analysis (DTA), X-ray Diffraction (XRD), and Raman spectrometry measurements. The size of the solid solution domain depends on the dopant ionic radius and on the grain size.

Metastable forms of the tetragonal phase in compositionally homogeneous, nanocrystalline zirconia–ceria powders synthesised by gel-combustion

The crystal structure of compositionally homogeneous, nanocrystalline ZrO2–10, 30, 50, 70 and 90 mol% CeO2 powders synthesised by a pH-controlled nitrate–glycine gel-combustion process has been studied by X-ray diffraction and Raman spectroscopy. All the powders with a CeO2 content up to 70 mol% exhibited the tetragonal phase (P42/nmc space group), whereas the ZrO2–90 mol% CeO2 solid solution presented the cubic phase (Fmm space group). The axial ratio c/a decreased with increasing CeO2 content and became unity for ZrO2–70 mol% CeO2 powders. By analysing X-ray diffraction data using the Rietveld method, it was found that this material exhibited the t″-form of the tetragonal phase (the oxygen atoms were displaced from their ideal sites of the cubic phase along the c axis). Raman spectroscopy study confirmed the results found by X-ray diffraction. The morphology of the nanopowders was also evaluated.

Low temperature phase transformation of nanocrystalline tetragonal ZrO2 by neutron and Raman scattering studies

Abstract The structural properties of three, undoped, nanocrystalline, tetragonal zirconia powders, characterized by 6.9, 13 and 18 nm average crystallite size, respectively, have been studied at low temperature by neutron diffraction and Raman spectrometry. Raman spectrometry has shown that the 6.9 nm sample undergoes a continuous and reversible phase transition, towards the monoclinic structure below 175 K. This transformation could be at the origin of the appearance of components near 22.5 and 35 meV in the inelastic neutron spectrum when the temperature is lowered to 100 K. Such a transformation is not observed for the 13 and 18 nm samples. This transformation seems to depend on the crystallite size and on the kinetics of thermal treatments. This could explain why the transformation of the 6.9 nm zirconia could not be induced by the slow cooling and heating rates of the cryostat used in neutron diffraction. This transition has some similarities with infrared, low temperature measurements on cerium-doped zirconia reported in the literature. The neutron diffraction data have allowed the determination of thermal expansion coefficients: β a varies from 0.76 to 0.78 and β b from 0.80 to 0.91×10 −5 K −1 for the 6.9 and the 18 nm zirconia, respectively.

Materials synthesis and characterization of 8YSZ nanomaterials for the fabrication of electrolyte membranes in solid oxide fuel cells

Two different nanosized materials were synthesized by two preparation methods, namely a sonochemical technique and a spray pyrolysis process. The powder properties, the sintering behavior and the resulting crystallinity with respect to their utilisation for solid oxide fuel cell electrolytes were investigated. While the spray pyrolysis provides crystallized powder without any organic residue, the sonochemical powder is amorphous with some organic residue. Crystallization begins in the 400–500 °C temperature domain. The particle sizes vary between 10 and 50 nm for the sonochemical powder and between 50 nm and 1 μm for the spray pyrolysis powder, as determined by SEM analyses. Crystal growth of both powders starts between 800 and 1000 °C. After heating and sintering at 1300 °C the resulting grain sizes of the spray pyrolysis powder are 5 times larger than those of the sonochemical powder (about 250 nm and 1.2 μm, respectively). Additionally, the maximum rate of grain growth for the spray-pyrolysed material at 97 nm/h is even higher compared with 22 nm/h for the sonochemical powder.

Delay of tetragonal-to-monoclinic transition in water vapour due to nanostructural effect

The ageing behaviour of ultrafine yttria-doped single-phased tetragonal zirconia was investigated at 400 °C in dry air and in water vapour by complex impedance analysis and Raman spectroscopy. Pellets sintered at 1500 °C for 2 h in air were characterized by a 60 nm average grain size, a unimodal distribution of 300 nm average aggregate size, a density of 97% of theoretical one and pore sizes lower than 20 nm. No degradation of the electrical properties of dense pellets was detected in both gaseous environments during 1000 h at 400 °C. Simultaneously, no monoclinic signature was detected by Raman spectroscopy. Furthermore, tests were investigated in conditions close to those of IT-SOFC: 700 °C in water vapour during 1000 h. In these conditions, a conductivity decrease of 5% was recorded and less than 2 mol% of monoclinic zirconia was observed by Raman spectroscopy.

Dopant size effect on structural and transport properties of nanometric and single-phased TZP

AbstractElectrical properties of doped nanocrystalline ceramics of xR 2 O 3 –ZrO 2 with 1Vx mol% final contentV3.5 of Sc 2 O 3 ,Yb 2 O 3 ,Y 2 O 3 ,Gd 2 O 3 and Sm 2 O 3 were studied. Rare-earth oxide dopants were selected for their lower valence (+3) compared to Zr 4+ and for an increase in their ionic radii ranging from 0.087 to 0.109 nm, respectively, from Sc 3+ to Sm . This systematic study isfocused on compositions defined as the minimal dopant concentration at which the tetragonal single-phase is stabilized. Theseceramics are characterized by a good purity (quantity of Si less than 0.1 wt.%) and are found to be perfectly stable intemperature, keeping their microstructure constant. Impedance spectroscopy was used to determine the tetragonal zirconiamatrix electrical contribution and the internal interface blocking effect. Both specific and blocking conductivities decrease whenthe dopant ionic radius is increased. The influence of space-charge layers on the increasing blocking effect was suggested.D 2002 Elsevier Science B.V. All rights reserved.

Phase stability of nanostructured tetragonal zirconia polycrystals versus temperature and water vapor

Abstract Phase changes of nanocrystallized undoped and 2 mol% Y 2 O 3 -doped tetragonal zirconia polycrystals (2Y-TZP) prepared by spray pyrolysis were characterized by XRD, Raman spectrometry versus temperature in dry and humid air. 2Y-TZP powders characterized by the finest crystallites and 5 wt% OH amount are remarkably stable during cooling in dry air (DTA). This excellent stability was confirmed after 12 h annealing in dry as well as in humid air at 350°C. Only a ≈9 wt% monoclinic zirconia was detected in those latter conditions for undoped TZP. The transformation is not greatly enhanced by humidity during annealings. The effect of tetragonal deterioration of 2Y-TZP was investigated by impedance spectroscopy at 300°C up to 160 h in dry air. A small addition of Al 2 O 3 particles inhibits the tetragonal to monoclinic transformation in dry air.

Low-temperature oxygen electrode reaction on bismuth ruthenium oxides/stabilized zirconia

Abstract Bi2Ru2O7 and Bi3Ru3O11, have been tested as electrodes on stabilized zirconia at temperatures as low as 500 K. The oxides have been characterized by thermogravimetry under controlled oxygen partial pressure at high temperature and by potentiometric measurements after controlled cathodic polarization. The oxygen electrode reaction as functions of oxygen pressure and temperature has been studied by impedance spectroscopy. At low temperature, the dependence of the polarization resistance on the oxygen pressure was analyzed in terms of charge transfer as a rate controlling step.

Synthesis and structural characterization of a new system: ZrO2Y2O3RuO2

Abstract New compounds belonging to the pseudo-ternary system ZrO 2 Y 2 O 3 RuO 2 have been synthesized from nitrates as precursors. The general formula was [(ZrO 2 ) 0.91 (Y 2 O 3 ) 0.09 ] 1 − x , (RuO 2 ) x with 0 ≤ x ≤ 20 mol % RuO 2 . In order to avoid thermal decomposition and to obtain the best sample crystallinity, the last firing was performed at 900 °C. X-ray diffraction measurements provide evidence of a solid solution limit at 10 ≤ x ≤ 72.5 mol % RuO 2 with the cubic fluorite-type structure. Microstructural analyses show samples with high porosity and heterogeneity in grain size and ruthenium composition.