The activation energy for bulk self-diffusion of Cu is approximately 210 kJ/mol. Estimate the activation energy of Cu diffusion along grain boundaries when the deviation from the high-temperature Arrhenius behavior in the self-diffusion coefficient is observed at 500 ºC in polycrystalline Cu with a grain size of 120 μm. (Hint: Assume a grain boundary width of 0.5 nm as well as that the pre-exponential term of the diffusion coefficient are in a first approximation the same for bulk and grain boundary diffusion).