Search

The aim of this work is to present the methodology to characterize deformation and contact damage initiation and evolution in porous bulk and film components used in solid oxide fuel cells, based on indentation and simulation. Spherical indentation tests at a broad range of loads (50-10000 mN) were carried out on porous bulk and film electrodes with different levels of porosity, and on bilayer system. An axisymmetric model based on the Gurson model used for porous materials was developed to simulate the indentation processes. Elasticity and hardness of each component were reliably determined via both experiments and modelling. Inverse analysis via comparison of experimental indentation response curves and simulation-generated curves shows a very different relation between hardness and yield stress, compared with dense materials. Cracking behaviour was examined and appropriately explained by FEM results. Further insight of the deformation and damage behaviour was also obtained based on microstructural study using FIB-SEM. Overall, the study shows that the model developed in this work is highly applicable for the description the deformation and damage characteristics in porous bulk and film ceramics. [ABSTRACT FROM AUTHOR]

The aim of this work was to present the methodology for reliably determining the elastic moduli of partially sintered porous SOFC cathode films by appropriate nanoindentation experiments and analysis. The 3D microstructures of the porous crack-free perovskite LSCF films, which were prepared by sintering at 900-1200°C, were reconstructed based on dual-beam FIB/SEM tomography, followed by FEM to compute the elastic moduli. Detailed analysis of the nanoindentation results was also made to estimate the film-only elastic modulus for each film. The experiments showed that reliable measurements of the true film properties were obtained by data extrapolation provided that indentation depth was 0.1-0.2 of the film thickness. Effects from surface roughness and substrates responsible for the data deviation were taken into account. It was found that the elastic modulus increased from 32-122GPa as the sintering temperature raised from 900-1200°C and the porosity decreased from 47-15%. Further insights were gained into the porosity evolution, plastic deformation and substrate effect in the indented films with the aid of FIB/SEM. Excellent agreement was found when comparing the elastic moduli derived from nanoindentation and FEM, implying the present method could be an effective tool for investigating the way in which the mechanical properties of porous ceramic films is influenced by their microstructures. [ABSTRACT FROM AUTHOR]

The transfer of chromium from metallic components into the cathode of a solid oxide fuel cell (SOFC) can severely degrade its performance and is known as 'chromium poisoning'. In this study we investigate the relationship between the amount of chromium introduced into La0.6Sr0.4Co0.2Fe0.8O3-δ, LSCF (6428), cathodes and their electrochemical performance, and clarify further the poisoning mechanism. LSCF cathodes were screen printed onto Ce0.9Gd0.1O1.95 electrolyte pellets and infiltrated with Cr(NO3)3 solutions to different Cr levels up to 2 wt%. Electrochemical impedance spectroscopy at 500--800°C showed that even very low levels of Cr give a significant increase in cathode polarization resistance, which increases with Cr concentration. The impedance response was analyzed using the model of Adler, Lane and Steele to extract oxygen self-diffusion (DO) and surface exchange (kO) parameters for the LSCF. The results show that Cr reduces both DO and kO, the latter being the more affected. However, the activation energies for polarization resistance, DO and kO are not significantly affected by Cr content. This indicates that the Cr poisoning mechanism involves the de-activation of sites for oxygen exchange on the LSCF surface and that the cathode's residual activity is by means of remaining active sites. [ABSTRACT FROM AUTHOR]

Abstract: The oxygen tracer diffusion coefficient (D⁎) has been measured for 9 mol% scandia 2 mol% yttria co-doped zirconia solid solution, (Y2O3)2(Sc2O3)9(ZrO2)89, using isotopic exchange and line scanning by Secondary Ion Mass Spectrometry, as a function of temperature. The values of the tracer diffusion coefficient are in the range of 10−8–10−7 cm2 s−1 and the Arrhenius activation energy was calculated to be 0.9 eV; both valid in the temperature range of 600–900 °C. Electrical conductivity measurements were carried out using 2-probe and 4-probe AC impedance spectroscopy, and a 4-point DC method at various temperatures. There is a good agreement between the measured tracer diffusion coefficients (D⁎, Ea=0.9 eV) and the diffusion coefficients calculated from the DC total conductivity data (D σ , Ea=1.0 eV), the latter calculated using the Nernst–Einstein relationship. [Copyright &y& Elsevier]

Abstract: The manufacture of solid oxide fuel cells (SOFCs) involves fabrication of a multilayer ceramic structure, for which constrained sintering is a key processing step in many cases. Defects are often observed in the sintered structure, but their formation during sintering is not well understood. In this work, various ceramic films were fabricated by screen printing and a variety of defects observed. Some films showed “mud-cracking” defects, whereas others presented distributed large pores. “Mud cracking” defects were found to originate from a network of fine cracks present in the green film and formed during drying and binder burn-out. Control of these early stages is essential for producing crack-free films. In order to investigate how defects evolve during sintering, artificial cracks were introduced in the green films using indentation. It was observed that crack opening always increased during constrained sintering. In contrast, similar initial cracks could be closed and healed during co-sintering. [Copyright &y& Elsevier]

In this study, high-resolution focused ion beam sectioning assisted with SEM imaging was used to study the indentation microstructures of porous bulks and films of a solid oxide fuel cell cathode material (La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ) sintered at different temperatures. The crack morphologies and pore-filling densification caused by crushing of particle networks were studied in details. Analysis showed distinct permanent deformation mechanisms of the indentation microstructures between porous bulks and films. Whilst remarkable porosity gradient was found for the porous films under both Berkovich and spherical indenters, the porous bulks were found to behave more like dense materials. Results also showed that radial cracks induced by Berkovich indentation on the bulks could not generate observable pop-in/pop-out events in the loading-unloading curves. However, when indenting with the spherical indenter on a thick film, the shear sliding of the particle networks immediately under the indenter could cause phenomenal disruption in the loading unloading curves shown. [ABSTRACT FROM AUTHOR]

Mechanical properties of porous SOFC electrodes are largely determined by their microstructures. Measurements of the elastic properties and microstructural parameters can be achieved by modelling of the digitally reconstructed 3D volumes based on the real electrode microstructures. However, the reliability of such measurements is greatly dependent on the processing of raw images acquired for reconstruction. In this work, the actual microstructures of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ (LSCF) cathodes sintered at an elevated temperature were reconstructed based on dual-beam FIB/SEM tomography. Key microstructural and elastic parameters were estimated and correlated. Analyses of their sensitivity to the grayscale threshold value applied in the image segmentation were performed. The important microstructural parameters included porosity, tortuosity, specific surface area, particle and pore size distributions, and inter-particle neck size distribution, which may have varying extent of effect on the elastic properties simulated from the microstructures using FEM. Results showed that different threshold value range would result in different degree of sensitivity for a specific parameter. The estimated porosity and tortuosity were more sensitive than surface area to volume ratio. Pore and neck size were found to be less sensitive than particle size. Results also showed that the modulus was essentially sensitive to the porosity which was largely controlled by the threshold value. [ABSTRACT FROM AUTHOR]

Abstract: The conductivity and elastic modulus of (CeO2)1− x (YO1.5) x for x values of 0.10, 0.15, 0.20, 0.30, and 0.40 were investigated by experiments and molecular dynamics simulations. The calculated conductivity exhibited a maximum value at approximately 15 mol% Y2O3; this trend agreed with that of the experimental results. In order to clarify the reason for the occurrence of the maximum conductivity, the paths for the transfer of oxygen vacancies were counted. The numerical result revealed that as the content of Y2O3 dopant increases, the number of paths for the transfer of oxygen vacancies decreases, whereas the number of oxygen vacancies for conductivity increases. Thus, the trade-off between the increase in the number of vacancy sites and the decrease in the vacancy transfer was considered to be the reason for the maximum conductivity occurring at the Y2O3 dopant content of approximately 15 mol%. The calculated elastic modulus also exhibited a minimum value at approximately 20 mol% Y2O3, which also agreed with the experimental results. It was shown that the Y–O–Y bonding energy increased with the increasing content of Y2O3 dopant. Thus, the trade-off between the increase in the number of vacancy sites and that in the Y–O–Y bonding energy was considered to be the reason for the minimum elastic modulus occurring at the Y2O3 dopant content of approximately 20 mol%. [Copyright &y& Elsevier]