Mechanical strengthening, stiffening, and oxidation behavior of pentatwinned Cu nanowires at near ambient temperatures
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2014 IOP Publishing Ltd. The complex effects of near ambient temperature exposure, i.e. 20-150 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 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 C, (b) formation of thermal oxide obeying an Arrhenius type process for Cu + migration at temperatures higher than 70 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 C and for prolonged exposure times, which did not obey an Arrhenius law. Notably, the formation of a weaker and more compliant thermal Cu 2 O did not compromise the effective strength and elastic modulus of oxidized Cu nanowires: experiments in Ar at temperatures higher than 70 C showed mechanical strengthening by 50% and ultimate stiffening to 190 GPa, which is near the upper limit for the elastic modulus of single crystal Cu in the <111> direction.
