Mechanical strengthening, stiffening, and oxidation behavior of pentatwinned Cu nanowires at near ambient temperatures

The complex effects of near ambient temperature exposure, i.e. 20-150 degrees C, on the oxidation and the mechanical properties of thermal solution grown faceted Cu nanowires were investigated. The mechanical behavior was quantified with experiments on individual Cu nanowires using a MEMS-based method for nanoscale mechanical property studies. The elastic modulus of pristine Cu nanowires with diameters 300-550 nm was 117 +/- 1.2 GPa which agreed very well with polycrystalline bulk Cu, while the ultimate tensile strength was more than three times higher than bulk Cu, averaging 683 +/- 55 MPa. Annealing at just 50 degrees C resulted in marked strengthening by almost 100% while the elastic modulus remained unchanged. Heat treatment in ambient air distinguished three different regimes of oxidation, namely the (a) formation of a thin passivation oxide at temperatures up to 50 degrees C, (b) formation of thermal oxide obeying an Arrhenius type process for Cu+ migration at temperatures higher than 70 degrees C, which was accelerated by grain boundary diffusion resulting in activation energies of 0.17-0.23 eV, and (c) complete oxidation following the Kirkendall effect at temperatures higher than 150 degrees C and for prolonged exposure times, which did not obey an Arrhenius law. Notably, the formation of a weaker and more compliant thermal Cu2O did not compromise the effective strength and elastic modulus of oxidized Cu nanowires: experiments in Ar at temperatures higher than 70 degrees C showed mechanical strengthening by similar to 50% and ultimate stiffening to similar to 190 GPa, which is near the upper limit for the elastic modulus of single crystal Cu in the <111> direction.