Papers by Keyword: Kirkendall Effect

Abstract: Periodic layered morphology may occur during displacement solid-state reactions in ternary (and higher-order) silicide and other material systems. This periodic layered structure consists of regularly spaced layers (bands) of particles of one reaction product embedded in a matrix phase of another reaction product. The number of systems that is known to produce the periodic layered structure is rather small but increasing and includes metal/metal and metal/ceramic semi-infinite diffusion couples. The experimental results on different systems, where the periodic pattern formation has been observed are systematized and earlier explanations for this peculiar diffusion phenomenon are discussed. Formation of the reaction zone morphologies periodic in time and space can be considered as a manifestation of the Kirkendall effect accompanying interdiffusion in the solid state. The patterning during multiphase diffusion is attributed to diverging vacancy fluxes within the interaction zone. This can generate multiple Kirkendall planes, which by attracting in situ-formed inclusions of “secondary-formed phase” can result in a highly patterned microstructure.

Abstract: Thermodynamic and diffusion models are given to describe morphological evolution of the reaction zone during diffusion-limited interaction between non-oxide Si-containing ceramics (SiC and Si₃N₄) and transition metals (Cr, Mo, Ti, Ni, Co, Pt). In the case of diffusion-controlled process in the ternary metal-ceramic systems, reaction phenomena can be rationalized using chemical potential diagrams. However, in some cases, a periodic layered morphology is found in the transition zone, which is not fully understood, and it is difficult to predict a priori. Silicide formation in systems based on dense Silicon Nitride and non-nitride forming metals can be explained by assuming a nitrogen pressure build-up at the contact surface. This pressure determines the chemical potential of Silicon at the interface, and hence, the product phases in the diffusion zone. Traces of Oxygen in the ambient atmosphere might affect the interaction in non-oxide ceramic/transition metal systems. The thermodynamic stability of the condensed phases in the systems where volatile species may form can be interpreted using predominant area-type diagrams.

Abstract: In binary systems Kirkendall shift is a well-known phenomenon. We investigated nanoscale diffusion in the framework of a recently published continuum model [Erdélyi and Schmitz, Acta. Mater. 60 (2012) 1807]. In thin films the usual vacancy creation and annihilation mechanisms, leading to the Kirkendall shift on larger scales, cannot operate in the same way. On this length-scale the characteristic distances between vacancy sinks and sources can be comparable to the dimension of the sample, causing differences in the development of the Kirkendall effect. Our group recently reported results in simulating nanoscale Kirkendall shift. In present work we show how using conventional method for velocity reconstruction used in multifoil experiments can be misleading if the distribution of vacancy sinks and sources is not uniform.

Abstract: The paper discusses influence of main structural factors, as the type of crystal structure and net constant, on macroscopic features of self-diffusion and interdiffusion, in an attempt to provide a consistent description of diffusion that would involve all relevant physical effects, such as Kirkendall effect, dependence of diffusion on vacancy concentration, crystallography, and concentration of individual elements.Basing on the model structure, which is a Simple Regular net along with a corresponding Wigner-Seitz cell, the principal assumptions of the proposed model are presented as well as its implications, in order to determine general relations, which can further be used in numerical calculations involving diffusion streams. Subsequently, example calculations are performed for a simplified case of a two-dimensional Simple Regular net.The conclusions reached through this line of reasoning are then extrapolated to more complicated cases of diffusion – FCC, BCC, and HCP nets – which makes possible to relate particular material structures with diffusion rates. The connection of Wigner-Seitz cells with diffusion constitutes a good demonstration of dependence of the diffusion stream on the crystallographic orientation as well on the corresponding diffusion anisotropy for complex networks.

Abstract: In this paper three methods for calculating Kirkendall plane position after diffusion process are described. Bifurcation of K-plane is discussed. Kirkendall planes shift is simulated in hypothetical binary system where solid solution occur and binary system, where two interphases occur. Kirkendall plane shift was estimated by marker conservation method, which is described. It was shown that bifurcation of Kirkendall plane can occur in multiphase systems.

Abstract: The Kirkendall effect appears due to the unbalanced diffusion fluxes causing the vacancy flux. There are several numerical methods that allow to predict the position of Kirkendall plane after the diffusion couple annealing. In this work for the first time the entropy density distribution is used to estimate the trajectory of the Kirkendall plane. The entropy density distribution is calculated with use of the bi-velocity method, which combines: (1) the volume continuity, (2) the conservation of mass, (3) momentum and (4) entropy-density. The method is applied to simulate the diffusion in Ni-Pd diffusion couple.

Abstract: The three-dimensional dandelion-like zinc oxide nanostructure composed of single-crystal ZnO nanorods was prepared by means of Kirkendall effect while using solvothermal method and taking Zn powder as the source of zinc. In the paper, the composition, structure and appearance of the sample were studied by means of XRD, SEM, RDS, SEAD, TEM, HRTEM and other methods. The method was widely used as a new method for the large-scale preparation of nano/micro zinc oxide material.

Abstract: We consider the kinetics of chemical interdiffusion along the grain boundaries in stressed thin metal film attached to inert substrate. We show that the kinetics of stress relaxation in the film can be either accelerated or slowed down if compared with the same kinetics in a single-component film, depending on the difference of intrinsic GB diffusion coefficients of the two components. In the case of faster matrix atoms the tensile stress in the film significantly increases beyond its initial value at the beginning of interdiffusion process, while in the case of faster diffuser atoms the compressive stresses develop in the film at the intermediate stages of stress evolution.

Abstract: Ni/Si multilayer contact structures to 4H-SiC after subsequent annealing steps are investigated with electron microscopy methods. After high temperature annealing step, specific defects in the contact structures are observed. The influence of phase transformations during annealings on the morphology on the contacts is discussed and the explanation of formation mechanism of voids within contact layer is proposed.

Abstract: We report on the influence of titanium thickness on the structural and electrical properties of annealed Ti/Ni ohmic contacts on highly doped n-type 3C-SiC. Electrical analysis by means of circular transfer length method demonstrate that an interlayer of titanium with thickness in the range of 25-150 nm has no significant influence on specific contact resistance. However, from a structural point of view, the formation of nickel silicides as well as Ti₃SiC₂ is severely affected by the titanium thickness. Moreover, the Kirkendall effect due to the reaction between Ni and SiC is influenced by the titanium thickness. In fact, Scanning Electron Microscopy analysis demonstrates that the adjunction of titanium affects the distribution of Kirkendall voids in the contact. Current maps determined by conductive Atomic Force Microscopy reveal significant variation of uniformity according to the titanium thickness.