Defect and Diffusion Forum Vols. 289-292

Abstract: Spinel-type structured Li4+xTi5O12 (0 6 x 6 3 ) is actually one of the most promising anode materials for Li ion batteries. In its nanostructured form it is already used in some commercially available Li ion batteries. As was recently shown by our group (Wilkening et al., Phys. Chem. Chem. Phys. 9 (2007) 1239), Li diffusivity in microcrystalline Li4+xTi5O12 with x = 0 is rather slow. In the present contribution the Li conductivity in nanocrystalline samples of the electronic insulator Li4Ti5O12 prepared by different routes is investigated using impedance spectroscopy. The mean crystallite size of the samples is about 20 nm. The ionic conductivity of nanocrystalline Li4Ti5O12 obtained by mechanical treatment is higher by about two orders of magnitude compared to that found for a material which was prepared following a sol-gel method. The latter resembles the behaviour of the microcrystalline sample with an average particle size in the μm range rather than that of a nanocrystalline ball milled one with a mean crystallite size of about than 20 nm. The larger conductivity of the ball milled sample is ascribed to a much higher defect density generated when the particle size is reduced mechanically.

Abstract: Depending on the thermodynamic, structural and diffusion properties of the system, a thin deposit dissolves into a substrate by different mechanisms. In this communication these different behaviours, investigated by surface analytical techniques (AES, XPS, STM, UPS, etc) [ - ], are reviewed. The experiments were also supported by computer simulations. The obtained results are compared and it is summarized how different parameters influence the dissolution of a thin film in a substrate. Furthermore, it is show that i) the volume dissolution kinetics is different on the atomic-/nano-scale than on the microscopic scale due to the diffusion asymmetry ii) the volume and GB diffusion in one measurement can be separated and iii) pure (C-kinetic) GB diffusivities can be determined from thin film kinetics measurements performed under adequate conditions.

Abstract: It is known that the diffusion of Ag and Cu in Cd1 xZnxTe crystals exhibits unusual concentration profiles depending strongly on the external vapor pressure of Cd during diffusion. Recent experiments show that the dopant Na forms qualitatively the same diffusion profiles including the phenomenon of uphill diffusion. Also the transition elements Ni and Co show a strong dependence of the diffusion behavior on the external Cd pressure, but the shapes of the concentration profiles differ significantly from those known for Ag and Cu. The different behavior of Ag, Cu, and Na, on the one hand, and Ni and Co, on the other hand, are proposed to be connected to the respective charge states of the dopants at interstitial positions in Cd1 xZnxTe. For the dopants K and Au, unusual diffusion properties have not been observed. The respective diffusion coefficients are DK = 1.2(2)•10 10 cm2/s (750 K) and DAu = 8(2)•10 8 cm2/s (800 K).

Abstract: An analysis of multilayered assemblies set up with multicomponent alloys selected in a single phase field has been recently developed on the basis of a matrix of constant interdiffusion coefficients. This analysis employs a transfer matrix method and is applicable to a study of evolution of concentration profiles and diffusion paths as a function of time for multilayered diffusion assemblies (MDAs) where any number of finite layers is sandwiched between two bulk terminal alloys. The analysis is utilized in this study to simulate concentration profiles and diffusion paths for MDAs assembled with (fcc) Cu-Ni-Zn alloys with two terminal alloys, A and B, sandwiching an alloy layer C in the middle. For short diffusion times the diffusion path of the ternary MDA, A/C/B, corresponds to two segments corresponding to the diffusion paths of the infinite diffusion couples, A/C and C/B. At longer times the diffusion zones of the two individual couples overlap and the diffusion path of the MDA varies continuously with time. The evolution of the concentration profiles and diffusion paths is presented and each intermediate path configuration is associated with a unique ratio of the middle layer thickness to the square root of diffusion time. The simulated concentration profiles clearly show the development of uphill diffusion and zero-flux planes (ZFP) for the individual components due to diffusional interactions among the components. At very long times, the diffusion path of the MDA approaches that of the infinite couple A/B between the two terminal alloys.

Abstract: We present an experimental study by Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) of the dissolution of about one monolayer of silicon previously deposited at room temperature on Cu (001). The isochronal dissolution has been recorded in the temperature range [50-320°C] (annealing rate 1.5°C/min). The plateau observed in the kinetics dissolution for temperatures between 95°C and 240°C, reveals the formation of an intermetallic two dimensional superficial phase thermally stable in this range of temperature. On the plateau, LEED patterns show the formation of a (5x3) superstructure. Above 255°C, we observe a very fast dissolution of the surface alloy characteristic of a first-order surface transition. Isothermal dissolutions kinetics have been recorded above and under the surface transition temperature (250°C and 270°C). From these measurements, we have evaluated bulk diffusion coefficients of Si in Cu assuming a local equilibrium. The diffusion coefficients measured within this hypothesis at 250°C and 270°C are respectively higher and lower than those extrapolated from high temperature measurements for Ge in Cu.

Abstract: Self diffusion in liquid titanium was measured at 2000K by quasielastic neutron scattering (QNS) in combination with container less processing via electromagnetic levitation. At small wavenumbers q the quasielastic signal is dominated by incoherent scattering. Up to about 1.2 °A−1 the width of the quasielastic line exhibits a q2 dependence as expected for long range atomic transport, thus allowing to measure the self diffusion coefficient DTi. As a result the value DTi = (5.3± 0.2)× 10−9 m2s−1 was obtained.With a molecular dynamics (MD) computer simulation using an embedded atom model (EAM) for Ti, the self diffusion coefficient is determined from the mean square displacement as well as from the decay of the incoherent intermediate scattering function at different q. By comparing both methods, we show that the hydrodynamic prediction of a q2 dependence indeed extends up to about 1.2 °A−1. Since this result does not depend significantly on the details of the interatomic potential, our findings show that accurate values of self diffusion coefficients in liquid metals can be measured by QNS on an absolute scale.

Abstract: Diffusion of alkaline-earth ions in mixed cation glasses of the composition xA2O•(3-x)MO•4SiO2 (x = 0.0, 0.1, 0.3, 0.4 and 1.0; A = Na, K; M = Ca, Ba) was investigated by means of the radiotracer diffusion technique below the respective glass transition temperatures. The mobility of alkaline-earth ions increases with the alkali content in all analyzed glass systems with no decrease in the diffusion activation energy, but a raise in the pre-exponential factor. A distinct dependency of the activation energy of the alkaline-earth ions on the type and content of the alkali ions in the glass is observed. Additional experiments with thin glass films derived by the sol-gel technique reveal an analogous diffusion behaviour. This demonstrates that the dynamic of alkaline-earth ions in mixed cation glasses does not depend on the way of glass preparation.

Abstract: Recent results on diffusion in nanostructured materials are reviewed. The analysis highlights the importance of the proper account for a hierarchic microstructure which is often formed in nanostructured materials. The diffusion kinetics is such a material requires a special consideration in dependence on the temperature, diffusion time and the segregation level of the solute. Pressure-less sintering results in clustering of nanograins with significantly enhanced diffusivity of the inter-agglomerate boundaries. Severe plastic deformation produces a broad spectrum of high-angle grain boundaries (GBs) with different kinetic properties. The majority of the high-angle GBs reveals diffusivities very similar to those of general high-angle GBs in their well-annealed coarse-grained counterparts. Nevertheless, considerably faster short-circuit diffusion paths are detected, too. The origin, geometric arrangement, structural and kinetic properties of these high-diffusivity paths are comprehensively investigated and discussed.

Abstract: Capabilities of the emission nuclear gamma resonance (NGR) spectroscopy in studying grain boundaries in metals are considered. It is demonstrated that this method enables to evaluate grain-boundary segregation factors, to determine grain-boundary diffusion mechanisms, to estimate an effective diffusion coefficient in near-boundary areas, etc. Besides, this method permits to evaluate quantitatively differences in the properties of grain boundaries of polycrystalline and nanostructured materials.

Abstract: A model that considers diffusion in nanocrystalline materials undergoing recrystallization was developed. Application of this model enabled us deriving 63Ni radiotracer diffusion coefficients along the grain boundaries in ultrafine grain copper produced by equal channel angular pressing from the experimentally measured radiotracer penetration profiles.