Determination of Grain Boundary Diffusion Coefficients in C-regime by Hwang-Balluffi method: Silver Diffusion in Pd
Résumé
Diffusion controlled processes play a crucial role in the degradation of technical materials. At low temperatures the most significant of them is the diffusion along grain boundaries. In thin film geometry one of the best methods for determining the grain boundary (GB) diffusion coefficient of an impurity element is the Hwang-Balluffi method, in which a surface sensitive technique is used to follow the surface accumulation kinetics. Results of grain boundary diffusion measurements, carried out in our laboratory by this method in three different materials systems (Ag/Pd, Ag/Cu and Au/Ni) are reviewed. In case of Ag diffusion along Pd GBs the surface accumulation was followed by AES method. The data points can be well fitted by an Arrhenius function with an activation energy Q=0.99eV. 1) Introduction Understanding the mechanisms of atomic movements in details is vastly important for modern material science/engineering especially in case of nanoscale applications. The manufacturing as well as the use of these devices (when degradation by diffusion may happen) rarely involve high temperature treatments (>0.5T melt). At lower temperatures the diffusion through shortcuts (grain boundaries, dislocations, etc.) plays the major role, so the need for reliable grain boundary (GB) diffusion data is high. For GB diffusion the most usual classification distinguishes three main kinetic regimes [1]. Type A kinetics is the case when the bulk diffusion fields of the GBs are overlapping and the system is characterized by an effective diffusion coefficient derived from the volume and grain boundary diffusion coefficients. In B kinetics there is also a flow from the grain boundary into the grains, but there is no overlapping of bulk diffusion penetrations. Measurements in this regime give the so called triple product P=D gb k' (D gb is the GB diffusion coefficient, and k' are the GB width and the segregation factor at the GB/bulk interface, k'=c gb /c o , respectively), provided that the bulk diffusion coefficient is known. The most interesting is the C kinetics regime where the diffusion is strictly limited to the grain boundaries; this provides the most straightforward method for the determination of D gb. This approach however requires very low diffusion depths and the use of the standard tracer technique is very difficult especially if the GB density is low. The Hwang-Balluffi arrangement [2] is a good approach to realize the C regime by measuring the surface accumulation due to the diffusion through a polycrystalline thin layer. In most cases surface sensitive techniques (AES, XPS, etc.) are used to follow the accumulation kinetics (see also [3]).
Loading...