Surface self-diffusion was investigated via thermal grooving experiments on nickel bicrystals in combina-tion with numerical simulations of the groove profiles. The objective of this work was the clarification of the dominant mechanism of grain boundary grooving in nickel at T = 750 degrees C and the determination of the respective diffusion coefficients with high accuracy. The literature values of surface self-diffusion co-efficients in nickel exhibit high scatter most likely associated with adsorption of impurities, which makes a comparison between different sets of data impractical and drawing conclusions about surface diffusion anisotropy impossible. In this work, thermal grooving experiments were performed on nickel bicrystals with different grain boundary and surface orientations at T = 750 degrees C in forming gas. The groove profiles were measured by means of atomic force microscopy. Surface diffusion was clearly identified as the dom-inant mechanism of grain boundary grooving. Asymmetrical grain boundary grooving by surface diffusion was modelled numerically. From the comparison of simulated and measured groove profiles, the surface diffusion coefficients were determined with high accuracy and for the first time the anisotropy of surface diffusion was quantitatively measured. The surface self-diffusion in nickel is highly anisotropic, varying between DS = 0 . 3 x 10 -21 m 3 / s and DS = 12 . 0 x 10 -21 m 3 / s, whereby the surface diffusion near { 100 } sur-faces is an order of magnitude slower than near { 110 } and { 111 } surfaces. The present work demonstrates that thermal grooving is an excellent tool to study the surface diffusion and to determine the respective diffusion coefficients with high accuracy from the comparison of simulated and measured groove profiles.(c) 2022 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Grain boundary grooves of nickel were studied in Ni polycrystals and in Ni/YSZ (nickel/yttria-stabilized zirconia) anode microstructures of an solid oxide fuel cell (SOFC) in order to determine the relative grain boundary energies of nickel. Reliable material parameters are necessary for realistic simulations to model the coarsening of nickel grains in SOFC anodes. However, the reported values in literature do not meet the requirements for accuracy and the experimental conditions differ strongly from the conditions within an anode. In this work, the measurement approach for atomic force microscopy was optimized to ensure the required accuracy in measuring grain boundary grooves; the thermal grooving experiments were performed at T=750∘C in dry and humid atmosphere. The resulting distributions of measured dihedral angles and relative grain boundary energies are identical in the polycrystal and the anode microstructure and are independent of annealing time and humidity. For the first time, precise values of the relative grain boundary energies of nickel are determined with high accuracy under operating conditions of an SOFC anode. The mean value of the relative grain boundary energies γGB/γS of nickel is 0.475±0.013 for high-angle grain boundaries, 0.217±0.010 for low-angle grain boundaries, 0.157±0.013 for Σ3 grain boundaries and 0.019±0.002 for twin boundaries.

In this study, cathode performance of cost-effective inert substrate-supported solid oxide fuel cells fabricated by a single step cosintering process is investigated. The polarization resistance of cosintered inert substrate-supported cathode symmetrical cells (ISC) is compared with the polarization resistance of electrolyte-supported symmetrical cells (ESC) prepared by post- and cosintering. ESC prepared by cosintering have similar polarization resistance than ESC prepared by post-firing due to the addition of pore formers. However, the implementation of a porous inert substrate increases the polarization resistance. Analysis of electrochemical impedance spectra could exclude a gas-phase diffusion limitation due to the porous substrate. Time-of-Flight secondary ion mass spectrometry (ToF-SIMS) reveals an accumulation of zinc, magnesium and silicon on the inner pore surface of the cathode layer. Thermodynamic calculations confirm desorption of these elements from the silicate substrate during the cosintering. In addition, a zinc manganite spinel is detected in the cathode layer via confocal Raman spectroscopy, which indicates a reaction between the cathode material and zinc. The larger cathode polarization resistance of the inert substrate-supported cell is attributed to microstructural changes and the coverage of the cathode surface.