Laurent Dessemond

Model for Ion-Blocking at Internal Interfaces in Zirconias

Abstract The model stresses the role played by the distribution of the current lines around the blockers and by the dielectric properties of these blockers. When the matrix conductivity is low enough, isolated spaces appear on both sides of the blockers. This model is consistent with most of the experimental features of ion-blocking in conducting ceramics, including the decrease of the blocking factor with temperature and the similarity between the bulk and blocked conductance activation energies. The admittance diagram parameters can be correlated to the geometrical parameters of the blockers. Additional relevant results obtained with alumina-zirconia composites reveal the limits of the model.

Dense ceramics of NaNbO3 produced from powders prepared by a new chemical route

Abstract A sodium niobate precursor was prepared by evaporating a solution of an oxalato-niobium complex, sodium nitrate, oxalic acid and ammonium hydroxide. The thermal evolution of the precursor powder, to the point of formation of the NaNbO 3 phase, was followed by thermal analysis and powder X-ray diffraction. Calcination of the precursor above 500°C for 5 h in air yielded a pure single phase NaNbO 3 . The non-isothermal densification behaviour was followed by dilatometric measurements. Highly sinterable powders were prepared by this new chemical route. Densities higher than 95% of the theoretical one were reached after sintering at 1250°C for 2 h in air.

AC impedance study of corrosion films formed on zirconium based alloys

Abstract Oxide films formed by water oxidation (360°C) of two Zr alloys, Zircaloy-4 (Zy-4) and ZrNb(1%)O(0.13%), were studied by impedance spectroscopy (IS) in gaseous atmospheres. Results show that capacitances are frequency dispersed, in agreement with Jonscher’s law of dielectric relaxation. A good correlation was found between oxide thickness calculated from both IS measurements at room temperature and weight gain and those estimated by metallographic examinations. The electrical characteristics of a 2 μm thick film formed on ZrNb(1%)O(0.13%) were investigated as a function of the temperature (25–280°C) at constant oxygen partial pressure (0.3 Pa). Cole–Cole diagrams suggest a frequency-temperature equivalence: low frequency points at low temperatures perfectly superimpose on high frequency points at higher temperatures. This resulting 14 decade ‘master’ curve so obtained can be characterized by the activation energy of the angular frequency ω close to 0.8 eV. An equivalent circuit based on an association in series of two layers with different dielectric properties was proposed. By fitting the curves obtained at different temperatures with only a parallel resistance, the thicknesses of the two layers were estimated to be 1.5 and 0.5 μm. Finally, both the Arrhenius diagrams of the total conductivity and the dispersion factor are characterized by a breakdown temperature point with two activation energy values.

Electrical degradation of LSM–YSZ interfaces

Abstract The impedance of an interface between La 1− x Sr x MnO 3 ( x =0.55−0.6) and yttria-stabilized zirconia has been analyzed as a function of time during high temperature annealings in air, and at medium temperatures. For curing below 1100°C, an improvement of intimate contacts between the electrode and the electrolyte materials is evidenced. Even at 1100°C, the electrode characteristic proper shows a marked degradation which increases with time. It mainly comes from an additional semicircle located on the low frequency side of the electrode impedance diagram. The additional low frequency satellite is ascribed to an alteration of the electrode reaction mechanism itself. The interface properties are shown to degrade only from 1200°C. Up to 1200°C, the degradation of both contributions appears reversible when cooling down the electrode. This is interpreted in terms of a narrowing of the contact area between the insulating phase and yttria-stabilized zirconia. Above 1200°C, the interface aging obeys linear laws. As could be expected, the growth of a reaction product along the LSM–YSZ interface results in an increased blocking effect. A remaining degradation of the reaction rate is also detected. The results suggest that the cathodic degradation mainly proceeds in the vicinity of the triple phase boundary (TPB) line, while the alteration of interface properties acts on a larger electrode area. The formation and the growth of an insulating layer results in ohmic and polarization losses.

In situ characterization of Zircaloy-4 oxidation at 500 °C in dry air

Abstract The in situ oxidation of Zircaloy-4 at 500 °C in dry air was investigated by thermogravimetric analysis (TGA) and electrochemical impedance spectroscopy (EIS). The coating of the alloy by a platinum film as electrode material was observed as not to modify the oxidation kinetic properties. After an initial cubic rate law, a transition to a quasi-linear curve occurs. The independence of the oxidation behavior to the Pt coupling is compatible with oxygen diffusion as the rate-determining step. During the pre-transition step, the rest potential of the cell Pt/oxide/Zy-4, the color of the oxide and the modulus of the single EIS signature indicate the high non-stoichiometry of the oxide. The kinetic transition was proposed to be correlated to the degradation of the film into a partially porous layer. This alteration of the oxide is associated to the appearance of a 1.2 V constant rest potential and the modification of the impedance diagrams in two high modulus contributions. The Cole–Cole representation has been used to demonstrate that the time variation of impedance spectra is related to the oxide growth. An equivalent circuit including two RC loops in series, whose capacitances are frequency dispersed, was proposed to be related to the film structure. Fitted data show that the thickness of the assumed protective layer of the film, close to the metal–oxide interface, is time independent in agreement with a constant oxidation rate. Finally, electrical properties of this inner layer were found to be quite different in pre- and post-transition stage.

Micromodeling of Functionally Graded SOFC Cathodes

One approach to improve the performances of a solid oxide fuel cell (SOFC) cathodes is to use a composite electrode that typically consists of a two-phase porous mixture of a solid electrolyte (yttria-stabilized zirconia) and an electrocatalytic material. The best electrochemical performances with such a type of cathode are obtained when the composition varies within the composite cathode called graded electrode. The present modeling has been performed in order to give a complete description of the electrode microstructure, as well as the process occurring therein. A one-dimensional, homogeneous model and a discrete element method was used. The present study proposes a one-dimensional dc and ac model that takes into account mass and charge conservation, transport of species, and reaction kinetics. The influence of transport parameters and microstructure on the shape of both polarization curves and impedance diagrams is discussed.

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.

Effects of mechanical damage on the electrical properties of zirconia ceramics

Abstract The capability of impedance spectroscopy to characterize cracks in solids has been evaluated. Measurements were performed on yttria-stabilized zirconia. Cracks were induced by Vickers indentation with loads in the 0–625 N range. Two regimes have been observed. Under loads lower than approximately 200 N unexpected results indicate a predominant decrease of the specific resistivity of the material and of the blocking coefficient at the grain boundaries. At higher loads the anticipated observations were made: the blocking coefficient and the grain-boundary capacitive effect indicate an increase of the internal surface density with increasing loads. The transition between the two regimes is marked by a very high value of the grain-boundary blocking effect.