Self-diffusion Coefficients Of Components Research Articles

Self-Diffusion Coefficients of Components in Liquid Binary Alloys of Noble Metals

An accurate determination of transport coefficients in liquids, such as diffusivity, is crucial for studying fundamental chemical processes, for constructing and verifying model theories of liquid, and for the optimization of technological processes. However, a reliable experimental determination of the diffusivity is a difficult and sometimes nearly impossible task. In this regard, the development of model theories that allow calculating characteristics of atomic transport is of special interest. Here, the concentration dependencies of the self-diffusion coefficients of the components in Cu-Ag, Cu-Au, and Ag-Au liquid alloys at T = 1423 K and T = 1573 K are calculated in the framework of the linear trajectory approximation in conjunction with the square-well model and the semi-analytical representation of the mean spherical approximation. We reveal that peculiarities in the behavior of the obtained dependencies are related to the peculiarities of the phase diagrams of the alloys under consideration. Additionally, we verify our calculation method on Al80-Cu20 and Al80-Au20 liquid alloys. The results obtained are in good agreement with available experimental and molecular-dynamic simulation data. In the cases when the experimental information is not available, the presented results can be considered as predictive to estimate the quantities under consideration approximately.

Spinodal Decomposition of an Alloy with a Strong Concentration Dependence of the Mutual Diffusion Coefficient

It is shown that the kinetics of decomposition of an alloy with a strong concentration dependence of the mutual diffusion coefficient is qualitatively different from the widely known kinetics of spinodal decomposition, since it includes the formation of metastable (in the kinetic sense) precipitates of the intermediate composition. Concentration dependence of the mutual diffusion coefficient is particularly due to the difference in impurity diffusion coefficients, as well as component self-diffusion coefficients, and it increases as temperature decreases. The calculations use an expression for the flow of component atoms in the absence of the substance flow, obtained previously under the kinetic theory of vacancy diffusion ("hole gas method"). Keywords: spinodal decomposition, mutual diffusion coefficient, precipitates of intermediate composition.

Physicochemical Properties and Electrochemical Stabilities of Highly Concentrated Electrolytes

Introduction 5 V class cathode active materials have been investigated intensively to increase the energy density of lithium batteries. However, the high electrode potential promotes the decomposition and side reactions of electrolytes, active material, binder, and conductive agent of the composite cathode, leading to low coulombic efficiency and high over potentials1,2). One approach to overcome this problem is to enhance the oxidative stability of the electrolyte. Our group has reported that the equimolar mixture of lithium bis (trifluoromethanesulfonyl)amide (Li[TFSA]) and triglyme (G3) or tetraglyme (G4) with high lithium salt concentration (≈3 mol dm−3) shows the properties similar to room temperature ionic liquids such as high thermal stabilities and low vapor pressure. In addition, the oxidative stability was enhanced by increasing the lithium salt concentration in Li[TFSA]/G3 and Li[TFSA]/G4 electrolytes owing to the lowering of HOMO energy level of glymes coordinating to the lithium cation3). The enhancement of oxidative stability in the highly concentrated electrolytes can be expected in other solvents. In this work, the physicochemical properties and electrochemical stabilities of highly concentrated electrolytes based on sulfone solvent will be mainly reported. Experimental Electrolytes were prepared by mixing LiBF4 and sulfolane (SL) in specific molar ratio under Ar atmosphere. The interaction of lithium cation and SL was observed by Raman spectroscopy using RMP-330, JASCO. The oxidative stabilities of electrolytes were investigated by linear sweep voltammetry (LSV) with three electrodes cell consisting of Pt disk working electrode, Pt coil counter electrode, and reference electrode of Li metal in 1 M Li[TFSA]/G3 with liquid junction. The scanning rate was 1 mV s−1 and the operation temperature was 30 °C. Result and Discussion Fig. 1 shows the concentration dependence of Raman spectra for LiBF4/SL in the range of 650-700 cm−1, corresponding to the C-S-C stretching vibration of SL4). The band gradually shifted to higher wave number with the increase of lithium salt. This suggests the population of uncoordinated (free) SL is decreasing with increase of lithium salt concentration. The LSV results for the mixtures of LiBF4:SL=1:7 (1.4 mol dm−3) and 1:1.35 (5.8 mol dm−3) are shown in Fig. 2. In the case of 1.4 mol dm−3 LiBF4/SL, oxidative current were observed at around 4.3 V (vs. Li/Li+). On the other hand, the highly concentrated electrolyte, 5.8 mol dm−3 LiBF4/SL, was stable up to 5.0 V (vs. Li/Li+). Apparently, the oxidative stability was enhanced by the increase of lithium salt concentration. The enhancement of oxidative stability of the SL molecule was attributed to the strong interaction between SL and Li+ ion. The lone pairs of oxygen atoms of SL are attracted to Li+ ion, and the electrons of the lone pairs become hard to be extracted. In the 5.8 mol dm−3 LiBF4/SL electrolyte, almost all the SL molecules coordinate to Li+ ions and uncoordinated SL scarcely exists, resulting in the extremely high oxidative stability of the electrolyte. Other physicochemical properties such as viscosity, ionic conductivity, and self-diffusion coefficients of components of electrolyte solutions will be also reported. Acknowledgment This research was supported by Japan Science and Technology Agency (JST) - Advanced Low Carbon Technology Research and Development Program - Specially Promoted Research for Innovative Next Generation Batteries (ALCA-SPRING) of Japan.

A Fourier transform pulsed-gradient spin echo nuclear magnetic resonance self-diffusion study of microemulsions and the droplet size determination

Self-diffusion of all components in two different microemulsions has been studied by the Fourier-transform pulsed-gradient spin echo 1H nuclear magnetic resonance technique. The analysis of hydrodynamic and direct droplet-droplet interactions in microemulsions allows the simultaneous application of the Stokes-Einstein equation for surfactant and oil. The result of this analysis is a simple relation connecting self-diffusion coefficients of components and the size of droplets. It is shown that because of partial solubilization of water in oil the droplet radius can be significantly different from the expected radius.

The relationship between component self-diffusion coefficients and interdiffusion coefficient for pseudobinary systems

A general relationship between the interdiffusion ( D) and component self-diffusion coefficients ( D i ∗) for pseudobinary systems is derived using the basic definitions of the diffusion coefficients and the constraints appropriate for pseudobinary systems. The Hg 1 − x ,Cd x Te (MCT) system is used as an example of such a system. The relationship between D and the intrinsic chemical diffusion coefficient ( D i,) is the same as for a binary system and is described by the Darken equation. In pseudobinary systems there are relevant diffusion driving forces in addition to neutral chemical potential gradients and the relationships between D i and D i ∗ and D and D i ∗ are complicated by the diffusion of a common non-metal species. The physical meaning of the resulting equation is explained by the limiting behavior of the general equation. A Darken-type equation is obtained when there is only a neutral chemical potential gradient for metal diffusion species such as the case of a common non-metal species moving much faster than the metal species. However, when the common non-metal species diffuses more slowly than the metal components, the resulting interdiffusion coefficient is related to the D i ∗ by a Nernst-Planck-type equation, since the electroneutrality and/or the conservation of lattice sites requires the flux of metals to be coupled. The slow movement of the common non-metal species plays the same role as lack of attainment of vacancy equilibrium in binary systems. Our experimental results of the D and D i ∗ values in the MCT system support our theoretical analysis.

Diffusion in FeNiCr alloys with an F.C.C. lattice

The volume self-diffusion coefficients of 51Cr and 59Fe in 16 FeNiCr alloys with an f.c.c. lattice were measured using the modified Gruzin method in the temperature ranges 1298–1548 K for iron and 1286–1536 K for chromium. The self-diffusion coefficients, pre-exponential factors and activation energies for iron and chromium are tabulated. The concentration dependence obtained cannot be expressed by a simple mathematical relation; it is of higher order with a local minimum near the composition Fe-55 wt.%Ni-20 wt.%Cr. A critical analysis of the results obtained as well as previous knowledge made it possible to find the following relation between the self-diffusion coefficients of components which are valid over the whole temperature and concentration ranges examined: D Ni ∗: D Fe ∗: D Cr ∗ = 1:1.2:2 From the thermodynamic activity measurements, values of the thermodynamic diffusion factors in the ternary FeNiCr system and further values of the interdiffusion coefficients in this system were calculated. In this way a complete description of the diffusion processes in the ternary system was obtained. A comparison of the calculated interdiffusion coefficients with the experimental values found by other researchers was made.

Application of the Hard Sphere Theory to the Diffusion of Binary Liquid Alloy Systems

On the basis of the van der Waals concept of Ascarelli and Paskin the hard sphere theory of self diffusion is extended to binary liquid alloys. Using only the melting temperature of the pure metals and the densities, component self-diffusion coefficients and, with the help of Darken’s equation, mutual diffusion coefficients were calculated. Agreement with experimental results is good in (Bi, Sn), and excellent in (Sn, Zn) and (Li, Ag). Impurity diffusion in liquid Cu, Sn and Pb is predicted better than by the theory of Protopapas et al. Deviations in (Hg, Zn) and (Li, Pb) are tentatively attributed to strong particle interactions in one component (Hg) or in the alloy (Li, Pb).

Diffusion and thermodynamic properties of Ni-V system

A complex analysis of the selfdiffusion, interdiffusion and vaporization data in the Ni-V system is presented in this paper. Full descriptions of diffusion processes in this system were given. Temperature dependence of the selfdiffusion coefficients of components in the solid solution region fulfil the Arrhenius law. Concentration dependences of the selfdiffusion characteristics (D,D0,ΔH) have the extremes in the region of 25 at. % of vanadium (minimum for logD=f(x), maximum for logD0=f(x) andΔH=f(x)).