Neutron Reflectometry: A Tool to Investigate Diffusion Processes in Solids on the Nanometer Scale

Authors


  • The authors would like to thank U. Geckle and M. Bruns for preparation of the silicon nitride isotope multilayers, E. E. Haller, D. Bougeard, and H. Bracht for supplying the Ge isotope multilayers, and M. Horisberger for sputtering the Fe isotope multilayers. We also thank N. P. Lalla for carrying out the TEM measurements on Fe. This work is based on experiments performed at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland and at the Geesthacht Neutron Facility GENF, GKKS Institute, Germany. This research project has been supported by the European Commission under the 6th Framework Program through the Key Action: Strengthening the European Research Area, Research Infrastructures, contract no.: RII3-CT-2003-505925. Financial support of the German Research Foundation (DFG) is gratefully acknowledged

Abstract

The investigation of self-diffusion for the characterization of kinetic process in solids is one of the most fundamental tasks in materials science. We present the method of neutron reflectometry (NR), which allows the detection of extremely short diffusion lengths in the order of 1 nm and below at corresponding low self-diffusivities between 10−25 and 10−20 m2 s−1. Such a combination of values cannot be achieved by conventional methods of diffusivity determination, like the radiotracer method, secondary ion mass spectrometry, quasielastic neutron scattering, or nuclear magnetic resonance. Using our method, the extensive characterization of materials which are in a non-equilibrium state, like amorphous or nanocrystalline solids becomes possible. Due to the small experimentally accessible diffusion length microstructural changes (grain growth and crystallization) taking place simultaneously during the actual diffusion experiment can be avoided. For diffusion experiments with NR isotope multilayers are necessary, which are chemical homogeneous but isotope modulated films. We illustrate the basic aspects and potential of this technique using model systems of different classes of materials: single crystalline germanium, amorphous silicon nitride, and nanocrystalline iron.

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