YSZ System Research Articles

A new alternative representation of impedance data using the derivative of the tangent of the phase angle: Application to the YSZ system and composites

The aim of this work is to propose a new alternative representation of impedance data using the derivative of the tangent of the phase angle, which allows enhanced discrimination between processes with relaxation frequencies that are very close. The new representation allows discrimination between overlapped processes within a factor of 2 in their relaxation frequencies for process with similar strength. Equations for the simplified behaviour of the impedance data have been proposed to obtain all the parameters of the processes involved in the impedance spectrum. This new alternative representation has been applied to bulk and grain boundary responses of YSZ with very satisfactory results. It has also been applied to the qualitative study of impedance data of a CuO composite showing the usefulness of this representation to discriminate different electrode processes. This approach provides an ab initio method of identify the contributing components to an electrochemical impedance spectrum with quite remarkable resolution. It is suggested that if this method is applied to provide starting parameters for non-linear least squares fitting using constant phase elements, then problems due to correlation of parameters and identification of components can be minimised.

State-space modeling of the anodic SOFC system Ni, H 2–H 2O∣YSZ

A simplified anodic solid oxide fuel cell (SOFC) system, Ni, H 2–H 2O∣YSZ, is investigated using the so-called state-space modeling (SSM) approach that combines experiments, modeling, and simulations. The approach is first described in general. Then, an electrochemical model is established for the Ni, H 2–H 2O∣YSZ system. The kinetic constants necessary for the simulations are estimated on the basis of literature data. The model is validated by fitting the simulated to the experimental impedance spectra, which were measured under well-defined operating conditions, simulated directly from the assumed electrochemical model and the estimated kinetic constants. Simulations were also carried out varying the triple phase boundary (TPB) length, the partial pressure of hydrogen and of water, and the overpotential. A comparison of the simulated with the experimental data proves the feasibility of the approach for investigating the electrochemistry of SOFC anodes. However, it comes out that the assumed model requires modifications. An alternative model is proposed for further investigations.

La 2Zr 2O 7 formation and oxygen reduction kinetics of the La 0.85Sr 0.15Mn yO 3, O 2(g)|YSZ system

Nucleation and growth of lanthanum zirconate (LZO) between (001) single crystals of 9.5 mol% Y 20 3-stabilized ZrO 2 (YSZ) and porous La 0.85Sr 0.15Mn y O 3± δ ( y=0.95…1.10) perovskites were investigated. The perovskites were screen-printed on the solid electrolyte and sintered in air at 1373 K for short periods. High resolution transmission electron microscopy, electrochemical impedance spectroscopy, and atomic force microscopy were employed for interface characterization. The manganese concentration in the perovskite affects the onset of nucleation and the growth rate of lanthanum zirconate. Excess lanthanum oxide within the perovskite reacts immediately with YSZ to form dense LZO layers. Layer growth kinetics is controlled by bulk diffusion of cations. Stoichiometric perovskite leads to the reductive formation of cubic LZO islands at the cathode/electrolyte interface after a few minutes of sintering. The A-site deficient perovskite is characterized by the reductive dissolution of Mn into YSZ in the early sintering stage and by a pronounced surface diffusion of cations. After the critical Mn concentration in the perovskite is reached, reductive nucleation of LZO originates. Island growth is controlled by surface diffusion of cations and the supply of lanthanum. The electrochemical properties of the cathode/YSZ interfaces are strongly influenced by the formation of LZO at the triple phase boundary. Charge transfer, dissociation of adsorbed molecular oxygen ions, and surface diffusion of atomic oxygen are discriminated from electrochemical impedance data.

YBCO step edge junctions on various substrates

We have fabricated YBCO 90/spl deg/ grain boundary junctions on step edges in NdGaO/sub 3/ and in deposited dielectric (CeO/sub 2/ on YSZ and SrTiO/sub 3/ on MgO) in order to compare junction performance to our standard, LaAlO/sub 3/. Average I/sub c/R/sub n/ values at 77 K in the 300-400 /spl mu/V range were measured for 2 /spl mu/m step edge junctions on NdGaO/sub 3/, LaAlO/sub 3/, and SrTiO/sub 3//MgO. Junction I/sub c/ is greatly reduced with the CeO/sub 2//YSZ system. I/sub c/R/sub n/ values in the 300-400 /spl mu/V range were measured at 65 K for 4 /spl mu/m junctions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>