NMR study of cluster-assembled nanophase copper

摘要

(sup 63)Cu and (sup 65)Cu NMR spectra from cluster-assembled nanophase copper with an average grain size between 5 and 10mn show a broadened peak, at the normal Knight-shifted frequency for copper meta which arises from only the central 1/2 to -1/2 transition. A very broad background is observed on either side of that peak. Some samples exhibit a second broad peak at a position normally associated with non-metallic copper. Pulsed NMR measurements of the central peak show that virtually all the copper signals are significantly broadened and have a spin-spin relaxation time longer than larger grained copper samples. Line shape measurements, using spin echoes, as a function of delay between rf excitation and measurement show there are a number of copper sites with longer relaxation times which have a significantly larger broadening. Those sites are tentatively identified as being at or near a grain boundary or free surface. A small orientation effect is observed indicating an anisotropy within the samples. An isochronal anneal of one sample showed significant, but not complete, line narrowing after an anneal at 450C consistent with other nanophase metals which show grain growth above 40--50% of the melting temperature.