Papers by Author: A.A. Kodentsov

Abstract: A diffusion-controlled growth of intermetallic phases and the role of the Kirkendall effect in morphological evolution of the product phase layers can be described in terms of an alternative theory considering chemical reactions at the interphase interfaces. Application of this “physicochemical” treatment to diffusional growth of intermediate phases with fairly wide homogeneity ranges is illustrated by the example of interaction in the Ag-Zn system. The model is purely phenomenological, and its use is convenient, since no explicit assumption of the underlying diffusion mechanism is required.

Abstract: Formation of diffusion zone morphologies periodic in time and space during metalceramic reactions is considered as a manifestation of the Kirkendall effect. In a diffusion-controlled interaction, the Kirkendall marker plane can bifurcate, which is attributed to diverging vacancies fluxes in the reaction zone. When the Kirkendall plane is present in a phase layer, it attracts in situproduced inclusions of “secondary-formed phase”, which, in turn, can result in a highly patterned microstructure.

Abstract: Since conventional production of high-temperature materials involves high investments and costly consumption of both energy and time, reaction engineering methodology combined with near-net shaping is often the answer to problems associated with the fabrication of advanced materials. Over the last decades, the number of different reaction–based processing methods for near-net-shaped ceramics has gradually increased. In this review, different reactive processing techniques and their potential for near-netshaping are treated, e.g. SHTS (self-supporting high temperature synthesis), the Lanxide method DIMEX®, reaction bonding (RB), reactive processing of Alumina-Aluminide Alloys (3A) and Al2O3-Al alloyed metal composites (3AMC). In addition to their potential for near-net shaping, other advantages to reactive processing routes are recognized to be reduced processing temperatures, reduced glassy phase formation at the grain boundaries, fine grained microstructures and improved mechanical strength. Since the exothermic reactions constitute the base for reactive processing of high quality materials in an economic way, control of these reactions is essential. The process flows are described together with characteristic features of process and materials. In addition, specific aspects of reaction-based synthesis will be illustrated with examples from own work in the area of reaction bonding of silicon nitride and alumina.

Abstract: There is now a considerable body of experimental evidence to indicate that in a volumediffusion controlled interaction the Kirkendall plane need not be unique. The Kirkendall plane can microstructurally be stable as well as unstable (it does not exist!). Under predictable circumstances, it can also bifurcate and even trifurcate. This can be rationalised in terms of Kirkendall velocity construction as well as from a purely chemical point of view considering diffusion-controlled interactions at the interphase interfaces. The physico-chemical approach is also used to explain significance of the Kirkendall effect in the morphogenesis of interdiffusion systems.