The chemistry of ceramic grain boundaries
The arrangement of ions and grain boundary structures in ceramics have been made clearer by high resolution electron microscopy and computer simulations. Oxide grain boundaries are more open than metal boundaries. Calculations indicate that there are at least some low energy sites for all solutes. Widespread segregation at boundaries and boundary chemistry different from the bulk is thus expected; it is experimentally found to be the case. Boundaries in ceramic oxides generally have an electrical charge and associated space charge region which also affect boundary chemistry. Many properties of ceramics are affected by boundary chemistry and boundary charge. The application of chemistry to ceramic materials has mostly been focussed on developing and understanding the composition and structure of each of the phases present and how they developed. This provides the essential basis for ceramic studies of how the arrangement of these phases, the microstructure, develops and how that microstructure affects and controls ceramic properties. It is increasingly being recognized that the boundary region between grains is an essential constituent of the microstructure. The composition and structure—— the chemistry, if you like——of the grain boundary region must be a necessary constituent of the further development of high technology ceramics. In the last few years both experimental techniques and theoretical methods have been developed which make it practical to investigate the chemistry of the boundary regions in a way that has not heretofore been possible. It is my purpose to discuss a few of these recent developments to illustrate how this new field of chemistry is being nucleated and its importance for the continuing development of modern ceramics. GRAIN BOUNDARY STRUCTURE Any understanding of grain boundary chemistry must ultimately depend on knowing the arrangement of ions in the boundary and the existence of dislocations and point defects in the boundary structure. In forming an arbitrarily oriented grain boundary between two crystals, one can imagine adjusting the position of the crystals adjoining the boundary, describing the orientation of the boundary plane, allowing translation of the crystals to their minimum relative energy positions and letting the atoms in and near the boundary relax to a minimum total energy. This requires eight macroscopic parameters to define a particular boundary, and it is clear that the specification of boundary structure is a complex problem in the general case. This difficulty is accentuated by the fact that most information about detailed boundary structures has been developed from computer simulation studies applied principally to metals and that most have pertained to special grain boundaries with structures having relatively short wavelength periodicity (1—5). Our present view and description of boundary structures are largely based on Bollmann's 0—lattice theory (6,7). At particular orientations, special low—energy boundaries occur in which there is a significant periodic structure that can be described locally with ordered arrays of compact polyhedra (8—10). These can be classified as having coincident site lattice matching in which the two adjacent crystals have a fraction of their atoms located on the three—dimensional coincident site lattice. Or, they may be described in terms of near coincident site lattice matching, in which a two—dimensional matching is achieved across a boundary plane with the inclusion of a network of grain boundary dislocations (11). Experimental observations with gold and germanium have shown these characteristics (12,13). Recent studies in magnesium oxide (14) and in aluminum oxide (15) make it clear that boundaries of these ionic materials with crystal misorientations near those corresponding to special boundaries of low energy do indeed consist of areas of special boundary, plus arrays of grain boundary dislocations (14,15). Faceted boundaries that are presumed to be