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 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. A comprehensive literature review of surface and grain boundary energies of nickel is given, however, the values in literature do not meet the required 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.