Composition Dependence Of Diffusion Coefficient Research Articles

Predicting atomic diffusion in concentrated magnetic alloys: The case of paramagnetic Fe-Ni

Predicting atomic diffusion in concentrated magnetic systems is challenging due to thermal magnetic effects and complex magnetochemical interplay. We propose an efficient approach via kinetic Monte Carlo using ab initio parametrized models. We demonstrate its accuracy in the case of Fe-Ni alloys, where we successfully predict and explain the weak composition dependence of diffusion coefficients due to a compensation of distinct contributions of their constituents. The diffusion-behavior difference between the paramagnetic and the magnetic ground states is elucidated, evidencing the role of magnetic disorder.

Composition dependence of tracer diffusion coefficients in Fe–Ga alloys: A case study by a tracer-diffusion couple method

The problem of estimation of the tracer diffusion coefficients is solved by utlizing a novel radiotracer-diffusion couple technique in the absence of a suitable radioisotope of one of the components and reliable thermodynamic parameters. This is demonstrated by generating reliable and reproducible mobility data in the alloys of the Fe–Ga system with a strong composition dependence of the diffusion coefficients. Tracer- (59Fe) and inter-diffusion are simultaneously measured in three couples Fe/Fe-16 Ga, Fe/Fe-24 Ga and Fe-16 Ga/Fe-24 Ga at 1143 K in at.%). The results obtained for the couples with different end-members are in an excellent agreement with each other for the overlapping composition intervals. The influence of the molar volume on the measured tracer- (59Fe) and inter-diffusion coefficients is evaluated. Using thermodynamic calculations, the Ga tracer diffusion coefficient and the vacancy wind factor are determined via the Darken-Manning relation for the composition range of 0–24 at.% Ga. The results in this study confirm the reliability of the radiotracer-diffusion couple technique for producing highly accurate diffusion data. As a result, this helped for optimizing the mobility description of the bcc phase of the Fe–Ga system. The Ga tracer diffusion coefficients are further estimated via experimental determination of the ratio of the Fe and Ga tracer diffusivities at the Kirkendall marker planes and utilizing the Fe tracer diffusion coefficients measured directly by the radiotracer method.

Diffusion Coefficients of a Highly Nonideal Ternary Liquid Mixture: Cyclohexane–Toluene–Methanol

To better understand diffusion phenomena in highly nonideal ternary liquid mixtures, cyclohexane–toluene–methanol is studied by equilibrium molecular dynamics (EMD) simulation. Intradiffusion and Maxwell–Stefan (MS) diffusion coefficients, being strictly kinetic properties, are predicted by EMD over the entire composition range at ambient conditions. The thermodynamic contribution to the Fick diffusion coefficients is studied with an excess Gibbs energy model. Predictive results from the combination of these two approaches are in convincing agreement with experimental Fick diffusion coefficient data. Different aspects determining the composition dependence of diffusion coefficients, such as their behavior at the binary limits, hydrogen bonding, and stability criteria, are discussed. While the intradiffusion coefficients exhibit only a weak composition dependence, the MS diffusion coefficients are strongly affected by the nonideality of the present mixture. Fick diffusion coefficients reveal pronounced dif...

Bis(2-ethylhexyl)phosphoric acid/bis(2-ethylhexyl)amine mixtures as solvent media for lithium-ions: A dynamical study

Abstract The self-diffusion coefficient, the spin-lattice relaxation times and ionic conductivity of lithium ions in liquid mixtures composed of bis(2-ethylhexyl)amine (BEEA) and bis(2-ethylhexyl)phosphoric acid (HDEHP) have been thoroughly investigated as a function of composition and temperature by NMR spectroscopy and conductometry. While the temperature and composition dependence of diffusion coefficients of lithium ions follow the same trend observed for those of the surfactant molecules, the spin-lattice relaxation times of lithium ions and 1 H are remarkably different. The observed behavior has been interpreted in terms of lithium ions diffusion occurring through its association with the negatively charged HDEHP species, whereas its rotational dynamics being decoupled from that of surfactant molecules, gives T 1 values lower than those of surfactant molecules and characterized by a different temperature dependence.

Parabolic vs linear interface shift on the nanoscale

It was shown very recently that diffusion nonlinearity, caused by the strong composition dependence of diffusion coefficients, can lead to surprising effects on the nanoscale: a nonparabolic shift of interfaces (both in ideal and phase separating systems) and sharpening of an initially diffuse interface in ideal systems. Some of these can not be interpreted even qualitatively from Fick’s classic equations. For instance, the nonparabolic shift of an interface at the very beginning is a consequence of the violation of Fick’s first equation on the nanoscale, and the transition from this to the classic parabolic behavior depends on the strength of the nonlinearity and the value of the solid solution parameter V (proportional to the heat of mixing). Experimental and theoretical efforts to explore the above phenomena are summarized in this paper.

Nonparabolic nanoscale shift of phase boundaries in binary systems with restricted solubility

Computer simulations were used to study the interplay of the diffusion asymmetry ~composition dependence of diffusion coefficient! and the phase-separation tendency ~chemical effect! in the kinetics of the interface shift during dissolution in a binary system with restricted solubility. We have found that—on nanoscale, taking into account only the diffusion asymmetry—the shift of the chemically sharp interface is not proportional to the square root of the time as would be expected from Fick’s laws but to t kc, where 0.25,k c,1 ~deviations from the parabolic law!. In ideal systems 0.5<kc<1, but with increasing mixing energy ( V) the interface shift returns to the parabolic law ( kc’0.5), and at very large V values k c can be even less than 0.5. This effect is a real ‘‘nanoeffect,’’ because after dissolving a certain number of layers ~long time or macroscopic limit!, the interface shift returns to the parabolic behavior. It is also illustrated that these phenomena can be observed experimentally as well.

Al Composition Dependence of Pt, Mn and V Diffusion in Ll2 - Type Ordered Ni3Al

The Al composition dependence of diffusion coefficients of Pt (substituting for Ni atoms on face centered sites in the Ni 3 Al, referred to as α-sites), Mn (for Al atom on corner sites, β-sites) and V (for both the atoms) atoms in the Ni 3 Al phase was studied by using the diffusion couple method. The diffusion coefficients of Pt, Mn and V elements increase with increasing Al composition of Ni 3 Al. These composition dependences are consistent with that of vacancy concentrations at α-sites in Ni 3 Al calculated by Kim. The activation energy of V diffusion is by about 110 kJ/mol higher than those of Pt and Mn. This high activation energy includes the increase in the total bond energy due to the jump of V atom from β- to α-sites. The above results verified that the V atom jumps to an adjacent vacant α-site and then it diffuses randomly through α-sites until it jumps to a vacant β-site.

Auger electron spectroscopy studies of interdiffusion in electrodeposited amorphous NiP alloys

AbstractThe effect of annealing on the Auger electron depth profiles of electrodeposited amorphous Ni100‐yPy/Ni100‐xPx (15 &lt; x &lt; 32 at.%, y &gt; x) thin film couples is studied at temperatures in the range 190–280 °C. Diffusion coefficients are evaluated by means of the Boltzmann–Matano method. The composition dependence of diffusion coefficients is explained by the role of free volumes in amorphous materials on interdiffusion.