Study of cluster-assembled nanophase copper using NMR
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R.W. Siegeland Y.X. Liao Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received 13 May 1993; accepted 17 September 1993)
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 Knight-shifted frequency for copper metal, which arises from only the central 1/2 to —1/2 transition. The broadening of the central line is associated with a distribution of Knight shifts. A very broad background is observed on either side of that peak, associated with broadening due to internal electric field gradients. 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. The strain within the grains is estimated to be 0.7%. Line shape measurements as a function of spin echo delay time 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 line narrowing after an anneal at 450 °C consistent with other nanophase metals which show grain growth above 40-50% of the absolute melting temperature. The dependence of NMR linewidth on average grain diameter is estimated.
I. INTRODUCTION Cluster-assembled nanostructured materials with an average grain size of 10 nm or less, known as "nanophase" materials, exhibit significantly different mechanical properties compared to their large-grained counterparts. Nanophase metals typically exhibit an increase in strength and hardness associated with the smaller grain sizes (for a review of many of these results, see Ref. 1). Increases in hardness as much as a factor of 5 have been observed in single phase materials. This paper reports room temperature nuclear magnetic resonance (NMR) measurements on cluster-assembled copper samples. Some of these results were reported previously.2 NMR probes the sample noninvasively, and the NMR shifts and relaxation times provide information on the atomic and electronic structure surrounding an atom. Since nanophase materials contain a large number of atoms in grain boundaries and possibly at free surfaces, a local technique such as NMR can be hoped to be quite useful to characterize the distribution of sites in these materials. Dickenscheid et al? have used NMR relaxation time measurements to investigate self-diffusion in nanophase copper and found an activation enthalpy for diffusion in good agreement with previous tracer studies4 and measurements on copper surfaces.5 The samples used for the NMR self-diffusion study had a density 80% that of the large-grained copper indicating a large fraction of 336
internal free surface associated with porosity exists in the material. II. EXPERIMENTAL A. Samples The samples were prepared using
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