Authors: Lyudmila N. Paritskaya, Yuri S. Kaganovsky, V.V. Bogdanov
Abstract: The phenomenon of low-temperature homogenization (LTH) during interdiffusion is studied under condition a t Dv £ 2 / 1 ) ( (Dv is the bulk diffusion coefficient, a is the lattice parameter) using nano-objects of binary Cu-Ni and Cr-Ni systems compacted from nano-powders and produced by mechanical alloying. Two stages of LTH were detected: at the first stage (t £ 103 s) the volume fraction of solution rapidly grows; at the second stage (t > 103 s) the volume fraction of solutions grows slowly with practically constant average solution concentration. The first stage of LTH correlates with active grain growth caused by small size (l) of structural element and nonequilibrium structure of nano-objects. Obtained results are analyzed theoretically in terms of interdiffusion along migrating GBs due to grain growth at the first stage and DIGM mechanism at the second stage. It is shown that the GB concentration distribution during interdiffusion along migrating GBs and the kinetics of LTH depend on a parameter l/l where 2 / 1 ) / ( b b V sD d l= is the
characteristic diffusion length. The mechanisms and criteria of LTH are proposed.
Abstract: In the drying of porous media, the mass transport occurs in the pores as well as on the
surface of the solid. The mechanisms involved can take place simultaneously, influenced by the
predominant one and can change depending on the moisture content. In this work, the moisture
effective diffusivity was estimated in solids with distinct structural properties in order to verify the
predominant mechanisms according to the moisture content, analyzing the influence of the physical
properties. The materials studied were NaY Zeolite, Kaolin, Silica and Alumina. The results of
diffusion coefficient present a minimum at low moisture content that can be related to pore size.
Authors: Boris A. Gizhevskii, Anatoly Yakovlevich Fishman, E.A. Kozlov, Tatiana Eugenievna Kurennykh, S.A. Petrova, I.Sh. Trakhtenberg, Vladimir Borisovich Vykhodets, V.B. Vykhodets, Robert Grigorievich Zakharov
Abstract: The kinetics of the isotope exchange between gaseous oxygen enriched with the 18O
isotope and two LaMnO3+δ oxide samples – a nanopowder and a bulk nanocrystal – has been
studied. The 18O isotope concentration has been measured by the acceleration nuclear microanalysis
method. The coefficients of the volume and the nanograin boundary self-diffusion of oxygen have
been evaluated at 500 °C. They are equal to 3.5·10
−20 and 1.5·10
−13 cm2/s, respectively.
Authors: Hervé Bulou, Christine Goyhenex, Carlo Massobrio
Abstract: This paper highlights the role played by diffusion processes to achieve a better characterization of structure and dynamics in atomic-scale studies of materials. Two classes of examples are presented. In the first, we take advantage of diffusion coefficients to assess the performances of different exchange-correlation functionals employed within the framework of density functional theory. By calculating the diffusion coefficients one is able to make a choice on the functional best suited to describe a prototypical disordered system, liquid GeSe2. In the second class of examples, we rely on classical molecular dynamics to describe diffusion mechanism on nanostructured substrates. The migration of a Co adatom on a stepped Pt(111) surface is analyzed in detail and correlated to the value of the different diffusion barriers. The diffusion behavior of Au adatoms on the reconstructed Au(111) substrate is described in terms of diffusion isotropy and anisotropy, by comparison with the case of Co/Au(111). Taken altogether, these studies exemplify the close link between diffusion properties, a realistic description of materials and the current level of performances of atomic-scale simulations methods.
Authors: Jaromír Drápala, Alena Struhařová, Daniel Petlák, Vlastimil Vodárek, Petr Kubíček
Abstract: Problems of reactive diffusion at the solid phase and melt contact were studied theoretically and experimentally. The main intention was to calculate the time course of the solid phase dissolving in the case of cylindrical dissolving. These calculations were carried out on the assumption for the rate constant of dissolving K = const. In our work we give heed especially to the dominating process, which is the solid metal A dissolved in the melt B. During the dissolving the melt B saturates with the metal A and the process is influenced by convections which are characteristic for the given experimental configuration. A theoretical description of the kinetics of the solid phase dissolving in the melt will be presented for the case of cylindrical dissolving. The aim is to derive a relation for the interface boundary movement c(t) in dependence on time and a time course of growth of the element A concentration in the melt B. There are problems with accurate determination of the interface boundary movement after certain heating times of specimens, when it is observed experimentally, since intermetallic phases create in the original A metal at both the diffusion and cooling and some phases segregate at the solidifying melt cooling. The main intention was an experimental study of the copper dissolving in the tin melt. Experiments aimed to the determination of the Cu wires (diameters from 0.5 to 3.5 mm) dissolution in the solder melt were carried out at various selected temperatures and times. Rapid growth of phases in the metal A and determination of the thickness of layers with these phases pose considerable time demands to X-ray micro-analyses (WDX, EDX) of specimens after their long-time heating.