A thermal grooving study of relative grain boundary energies of nickel in polycrystalline Ni and in a Ni/YSZ anode measured by atomic force microscopy

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 $T=750{}^{\circ }$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 ${\gamma }_{\text{GB}}/{\gamma }_{\text{S}}$ of nickel is $0.475\pm 0.013$ for high-angle grain boundaries, $0.217\pm 0.010$ for low-angle grain boundaries, $0.157\pm 0.013$ for $\Sigma 3$ grain boundaries and $0.019\pm 0.002$ for twin boundaries.