ATOMIC STRUCTURE AND RELATED PROPERTIES OF NANOCRYSTALLINE MATERIALS

H. Gleiter

Forschungszentrum Karlsruhe, D-76021 Karlsruhe, Germany

The atomic structure of nanocrystalline materials has been a subject of considerable interest for more than a decade. The interest was motivated in part by speculations about structural differences of the boundaries between nanometer-sized crystallites as compared to boundaries in coarse- grained polycrystals or bicrystals. Furthermore, detailed knowledge about the boundary structure is a prerequisite for the understanding of the properties of these materials in terms of their atomic arrangement. At present, a remarkable body of experimental and theoretical studies on this problem exists, from which the following picture emerges. The atomic structure of nanocrystalline materials seems to depend on the nature of the material (chemical bonds). Even relatively small variations (e.g. between Si and diamond) may affect the structure severely. The other important parameters seem to be the grain size and the preparation method. For grain sizes below a few nm, the boundary structures in nm-sized materials seem to deviate significantly from the ones of coarse-grained polycrystals. Depending on the preparation procedure and/or the time-temperature history, dramatic differences in the atomic structure were revealed by measurements of various properties. An attempt will be made to review the state of the art and to indicate areas in which further research seems to be needed in order to gain a solid basis for technological applications of these materials.