High Resolution Electron Microscopy of Interfaces in Topotaxial and Epitaxial Reactions
- PDF / 5,739,237 Bytes
- 10 Pages / 420.48 x 639 pts Page_size
- 39 Downloads / 206 Views
HIGH RESOLUTION ELECTRON MICROSCOPY OF INTERFACES IN TOPOTAXIAL AND EPITAXIAL REACTIONS U. DAHMEN, J. DOUIN, C.J.D. HETHERINGTON AND K.H. WESTMACOTI, National Center for Electron Microscopy, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720. ABSTRACT In the study of interfaces HREM is most useful when the interface is viewed The edge-on while both crystals are accurately aligned along low index zone axes. formation of such interfaces by epitaxy or topotaxy is the natural means of obtaining structures that can be usefully analyzed by HREM. Furthermore, there is intense interest in understanding the atomic structure of such interfaces in a variety of technologically important materials. This contribution addresses such structures produced by thermal decomposition, precipitation reactions and ionized-clusterbeam deposition, and reports on the structural investigation of symmetrical and asymmetrical grain boundaries, precipitate/matrix interfaces, internal defect structure of precipitates and nanocrystalline composites. INTRODUCTION The optimum interface for characterization by high resolution electron microscopy (HREM) is one in which low index zone axes in the two crystals are parallel to each other and lie in the interface. Only for this special geometry is it possible to obtain simultaneously structure images of both crystals adjacent to the interface while the boundary itself is viewed edge-on. This imposes stringent 1) the orientation conditions on the crystallography of such boundaries: relationship between the two adjacent crystals must be special, and 2) the boundary, although it does not have to be planar, must lie in the zone of the parallel zone axes. It might appear as though these conditions would severely limit the useful application of high resolution microscopy in the study of interfaces. However, because symmetrical geometries correspond to extrema in energy[I], special boundaries are favored over general boundaries. Due to their relative simplicity, symmetrical geometries are also preferred by theoreticians who model interface structure. It is thus fortunate that the specialized boundaries that can be characterized by high resolution microscopy are precisely the ones that allow the critical comparison between theory and experiment. This is particularly relevant for grain boundaries where modelling of atomic relaxations is presently limited to rational boundaries with distinct structural repeat units[2]. HREM is clearly most useful if the interface is edge-on, and most of the figures in this paper show edge-on boundaries. However, the potential of HREM for studying inclined or even face-on boundaries should not be overlooked[3]. Even if relaxations at the interface are obscured, rigid shifts (by a fraction of a unit cell) of the lattices across an inclined boundary can still be detected directly, through a change in the Moir6 or the convergent beam diffraction pattern[4]. In the case of general heterophase interfaces between different crystal structures, especially in situatio
Data Loading...
