Papers by Keyword: Tracer Diffusion

Abstract: Tracer diffusion in non-stoichiometric B2 intermetallic compounds having antistructural disorder is investigated using the six-jump-cycle (6JC) as a fundamental diffusion unit. For non-stoichiometric compositions, the antistructural atoms are assumed to be isolated and located at one of the six [110]-type and [100]-type sites (as only these sites are involved in the 6JC or 2JC). The jump frequencies for the 6JC involving a perfectly ordered configuration are calculated in terms of a four-frequency-model, using the meanfirst- passage concept of Arita et al. The jump frequency of an antistructural atom at [110] or [100]-type sites is taken to be the harmonic mean of frequencies of two successive nearestneighbour jumps of the same kind of atoms. The expressions for the tracer diffusion coefficients are derived for both atomic components at deviations from stoichiometry, assuming that the 6JC mechanism is valid. The results are compared with Monte Carlo simulations based on single vacancy jumps and found to be in fair agreement for compositions close to stoichiometry.

Abstract: The tracer diffusivities of 22Na and 45Ca in two high-quality standard silica glasses have been measured in the temperature range between 473 and 783 K. The temperature dependences of the tracer diffusion coefficients in both glasses follow Arrhenius laws. The diffusion of 22Na is six to seven orders of magnitude faster than the diffusion of 45Ca. The ionic conductivity was determined by impedance spectroscopy and the conductivity diffusion coefficient Ds was deduced from the dc conductivity via the Nernst-Einstein relation. The temperature dependences of Ds for both glasses follow also Arrhenius functions. The activation parameters and pre-exponential factors for tracer diffusion and for conductivity diffusion were determined. The activation enthalpy of 22Na diffusion is almost equal to the activation enthalpy of the dc conductivity. We conclude that the conductivity of standard glasses is due to the motion of Na ions. The diffusivities of 22Na and 45Ca in soda-lime glasses increase with increasing Na2O content.

Abstract: The tracer diffusion coefficients of Fe and Pd have been measured in FePt and FePt3 by standard radiotracer experiments. In FePt3 the diffusion coefficients are lower than extrapolations of those in the high-temperature disordered state, exhibiting curved Arrhenius plots below the transformation temperature. The diffusion of Pd is faster than that of Fe, and noticeable dependence on composition is found for the latter. In the L10 ordered FePt, the diffusion of Fe has been verified to be slower in the direction of the tetragonal axis, but the case of Pd appears more complex. The diffusion coefficients of Fe and Pd are similar in magnitude in the L10 ordered state.

Abstract: The diffusion coefficient of In in TiAl has been measured using ion implantation technique and secondary ion mass spectrometry. The diffusion coefficients of Fe and Pd in FePt have been measured at two compositions by radioactive tracer method. In order to clarify diffusion anisotropy, single crystal of each alloy was used. The In diffusion perpendicular to the [001] axis is faster than that parallel to the [001] axis. Such trend is similar to Ti diffusion previously measured in our group. The diffusion of Fe in FePt perpendicular to the [001] axis is faster than that parallel to the [001] axis at each composition, while the anisotropy of the Pd diffusion is different with composition. The predominant process of the diffusion in perpendicular to the [001] has been discussed on the basis of the expressions of the diffusion coefficients.