Papers by Keyword: Diffusion Anisotropy

Abstract: Studies of tracer diffusion in silicides of iron and molybdenum mainly from our laboratory are reviewed. For three compositions of the DO3-structured Fe3Si tracer diffusion data of Fe and Ge are available. Fe diffusion is relatively fast and increases with Si content. Ge diffusion, which mimics Si self-diffusion, is fairly slow and almost independent of composition. A Mössbauer study of Fe diffusion revealed nearest-neighbour jumps. Positron annihilation and high pressure diffusion data indicate that, in addition to isolated vacancies, vacancy pairs also contribute to Fe diffusion. Ge and Si diffusion is presumably restricted to the Si sublattice. Tracer diffusion of Fe and Ge has been studied for B20-structured FeSi with the conventional tracer technique. Tracer diffusion studies after implantation of 31Si (half-life 2.6 hours) performed at the IGISOL facility in Jyväskylä, Finland, show that Ge and Si diffusion have similar diffusivities. Fe diffusion rates are considerably slower than those of Si and Ge. Tracer data of Mo, Ge and Si diffusion are available for both principal directions of the tetragonal C11b-structured molybdenum disilicide (MoSi2). For all three kinds of atoms diffusion perpendicular to the tetragonal axis is faster than parallel to it. Mo diffusion in both principal directions is by orders of magnitude slower than Si and Ge diffusion. The huge asymmetry between Mo and Si or Ge suggests that diffusion of both constituents is restricted to their own sublattice. Positron annihilation studies show formation of thermal vacancies on the Si sublattice. This suggests that Si and Ge diffusion proceed by a vacancy mechanism in the Si sublattice of MoSi2. The anisotropy ratio of Si self-diffusion is attributed to a high mobility of vacancies in the Si double layers perpendicular to the tetragonal axis.

Abstract: Diffusion tensor imaging (DTI) provides quantitative information in the magnetic resonance imaging on the diffusion anisotropy about the integrity of white matter tracts. The aim of this study was to measure diffusion anisotropy and to correlate with upper-limb motor recovery in four chronic hemiparetic patients with thalamic hemorrhage. Fractional anisotropy (FA) was measured in the posterior limb of internal capsules (IC) in the affected side and in the corresponding contralateral normal side before and after 6 week motor training. The correlation between motor recovery of the affected hand and the ratio of FAs of the affected side and the contralateral normal side were investigated. Both FA ratio and motor function significantly increased after the training. A significant correlation was found between motor recovery and FA in the posterior limb of IC. The degree of impairment in diffusion anisotropy of hemiparetic patients might be helpful in prognosis of the motor function.

Abstract: The diffusion coefficients of 44Ti, 63Ni and 59Fe in γ-TiAl single crystals have been measured by ion-beam sputter-sectioning technique, while those of In have been measured using ion implantation technique and secondary ion mass spectroscopy (SIMS) in order to clarify the diffusion anisotropy: the diffusion perpendicular and parallel to the [001] axis. The diffusion of Ti and In perpendicular to the [001] axis is faster than that parallel to the [001] axis. However, the diffusion anisotropies of Fe and Ni show opposite trend to those of Ti and In, namely the diffusion parallel to the [001] axis is faster than that perpendicular to the axis. The predominant process of diffusion perpendicular to the [001] axis has been discussed from a viewpoint of activation energy using the expression of the diffusion coefficients in L10-ordered alloys.

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.