STUDY OF CLUSTER-ASSEMBLED NANOPHASE COPPER USING NMR

Cu NMR spectra from cluster-assembled nanophase copper with an average grain size between 5 and 10 nm show a broadened peak, at the normal K night-shifted frequency for copper metal, which arises from only the c entral 1/2 to -1/2 transition. The broadening of the central line is a ssociated with a distribution of Knight shifts. A very broad backgroun d is observed on either side of that peak, associated with broadening due to internal electric field gradients. Pulsed NMR measurements of t he central peak show that virtually all the copper signals are signifi cantly broadened and have a spin-spin relaxation time longer than larg er-grained copper samples. The strain within the grains is estimated t o be 0.7%. Line shape measurements as a function of spin echo delay ti me show there are a number of copper sites with longer relaxation time s which have a significantly larger broadening. Those sites are tentat ively identified as being at or near a grain boundary or free surface. A small orientation effect is observed indicating an anisotropy withi n the samples. An isochronal anneal of one sample showed significant l ine narrowing after an anneal at 450 degrees C consistent with other n anophase metals which show grain growth above 40-50% of the absolute m elting temperature. The dependence of NMR linewidth on average grain d iameter is estimated.