Defect and Diffusion Forum Vol. 277

Abstract: With the aim to study the regularities of surface diffusion in coadsorbed layers, we investigated diffusion of lithium on the (112) surfaces of Mo and W precovered with submonolayers of dysprosium and strontium, which have substantially lower mobilities than lithium. Experiments were carried out using scanning contact-potential microscopy, and Li diffusion parameters were extracted from diffusional evolution of coverage profiles. Dy and Sr preadsorbed in amounts of ∼10–1 of a monolayer were found to reduce the diffusion rate of Li by orders of magnitude. The strong impact of coadsorbates with low mobility on Li diffusion can be caused by important role of collective mechanisms in surface diffusion, which entails pronounced pinning effects, as well as by the possibility of formation of surface alloys and surface vitrification.

Abstract: Long–time scale molecular dynamics simulation in combination with the embedded atom method is used to investigate the effect of surface segregation phenomena at 1000 K on the structure of Pd alloy nanoparticles (of diameter of ∼ 4.5 nm) containing ∼ 30 at. % Ni. A core–shell f.c.c. structure was chosen as the initial state wherein a core of Ni atoms is surrounded by shell of Pd atoms. It is found that such nanoparticles form a surface–sandwich structure by interdiffusion. In this structure, the Ni atoms, which mostly accumulate in a layer just below the surface and at the same time are located in the centres of interpenetrating icosahedra to generate a subsurface shell as a Kagomé net. Meanwhile, the Pd atoms occupy the vertices of the icosahedra and cover this Ni layer from inside and outside as well as being located in the core of the nanoparticle forming (according to alloy composition) a Pd–rich solid solution with the remaining Ni atoms. The total atomic fraction involved in building up the shell of the nanoparticle in the form of the Ni Kagomé net layer covered on both side by Pd atoms can be estimated at ∼ 70 %.

Abstract: The problem of grain boundary diffusion for a case of boundary grain with periodic heterogeneity diffusion properties is considered. Dependence of Laplace transform images of impurity concentration on various diffusion conductivity places is built. The dimensionless parameters, forming the system of grain boundary diffusion regimes are determined. The space (ln x,lnt,1/T) is divided into areas in which the ratio ln ln ~ ln ln ( / ) ln eff o C r x−k t− Q RT − A is maintained, when the parameters of diffusion regime are constant. The values of parameters , , , eff o r k Q A fully specify the diffusion regimes. On this base the new concept for the definition of grain boundary diffusion regimes is carried out and the method for construction of simplified tracer concentration profile is proposed.

Abstract: The new way of calculation of segregation enthalpy on the basis of grain boundary diffusion data is offered. It proceeds from the primary data on an interval "B"- regime of grain boundary diffusion. For data processing we used the expression for coefficient of grain boundary diffusion got by Borisov and Lubov according to Fisher model for a tail part of the concentration curve. To approve this approach the data of tellurium diffusion in grain boundary of polycrystalline silver are considered.