Masataka Nakata

Positron-Annihilation Studies of Frank-Kasper-Type Quasicrystals of the Mg–Zn–Ga–Al and the Mg–Zn–Rare-Earth System

We have performed positron-annihilation experiments for a Frank-Kasper (F-K)-type icosahedral quasicrystal, 2/1 and 1/1 approximant crystals of Mg–Zn–Ga–Al, and the stable F-K-type icosahedral quasicrystals of Mg–Zn–RE (RE=rare-earth metal, Y, Ho and Gd) alloys. It is shown that a high density of vacant centers in triacontahedral clusters exists in both icosahedral and approximant phases of Mg–Zn–Ga–Al. Furthermore, in the present study, we report a very interesting finding that stable non-Al-based quasicrystals such as the icosahedral quasicrystal Mg–Zn–RE system also contain a high density of structural vacancies, similar to the stable Al-based quasicrystals. In particular, it is estimated experimentally that the structural vacancy densities for icosahedral quasicrystal Mg34Zn58Y8 and Mg40Zn52Ho8 are 3 ×1020 and 4 ×1020 cm-3, respectively. These results provide important information on the structural modeling of the stable icosahedral quasicrystal Mg–Zn–RE, which is a recent research topic of the icosahedral quasicrystals.

Open-Nano Pores in Natural Minerals Studied by Positron Lifetime Spectroscopy

Positron lifetime spectroscopy was conducted for natural minerals to investigate open-nano pores. Positronium (Ps) formation occurs with ortho-positronium (o-Ps) intensities ranging from a few % to 20 % and size distributions of nano pores are evaluated from o-Ps lifetimes. The results will be discussed with respect to formation processes of nano pores influenced by earths environment.

Evidence for Enhanced Matrix Diffusion in Geological Environment

Molecular diffusion in rock matrix, called as matrix diffusion, has been appreciated as a static process for elemental migration in geological environment that has been acknowledged in the context of geological disposal of radioactive waste. However, incomprehensible enhancement of matrix diffusion has been reported at a number of field test sites. Here, the matrix diffusion of saline water at Horonobe, Hokkaido, Japan is highlighted directly probing angstrom-scale pores on a field scale up to 1 km by positron–positronium annihilation spectroscopy. The first application of positron–positronium annihilation spectroscopy to field-scale geophysical research reveals the slight variation of angstrom-scale pores influenced by saline water diffusion with complete accuracy. We found widely interconnected 3 A pores, which offer the pathway of saline water diffusion with the highly enhanced effective matrix diffusion coefficient of \(4\times 10^{-6}\) cm2 s-1. The present findings provide unambiguous evidence that ...

Atomic transfer through interfacial free volumes in Sn65.4Bi34.6 eutectic systems

Atomic transfer through the interfaces upon Bi precipitation is specifically investigated with respect to vacancy-sized free volumes for a Sn65.4Bi34.6 eutectic alloy of a highly heterogeneous system by making full use of backscattering electron imaging, small-angle x-ray scattering, x-ray diffraction, and positron annihilation spectroscopy. Bi-rich particles of ~100 nm with segregated nanocrystallites of ~30 nm are observed for the alloy cooled at a cooling rate of 10−1 K s−1. Bi particles and nanocrystallites intergrow with each other up to ~1 µm and ~55 nm, respectively, by an extremely slow cooling of 10−5 K s−1. Regardless of the cooling rate, high concentrations of vacancy-sized free volumes are found to be located at interfaces among short-range ordered phases. For the alloy cooled at 10−1 K s−1, the free volumes are dominantly surrounded by Bi atoms. Decreasing the cooling rate down to 10−5 K s−1 changes the free volumes with Bi-rich chemical surroundings to an Sn-rich environment, which directly indicates atomic transfer through the interfaces upon Bi precipitation. The present results demonstrate that the kinetics of vacancy-sized free volumes in the interfaces plays an important role in understanding the precipitation mechanism in nanostructured heterogeneous materials.

Mechanism of Bi Precipitation in Sn65.4Bi34.6 Eutectic System

The macroscopic Bi precipitation process in highly heterogeneous Sn65.4Bi34.6 eutectic alloy is investigated on an atomic scale by positron annihilation spectroscopy. High concentration of vacancy-sized free volumes are located at interfaces among short-range ordered phases. Along with Bi precipitation, the chemical surroundings around the free volumes are changed from Bi- to Sn-rich environment and the concentration of free volume increases. The results demonstrate that free volume-intermediated atomic diffusion at the interfaces is responsible for macroscopic Bi precipitation in Sn65.4Bi34.6.

Interlayer Spaces Associated with Hydration and Dehydration in Synthetic Saponite

Positron annihilation lifetime spectroscopy was conducted for a synthetic saponite to investigate hydration and dehydration behaviour occurring in nanoscale interlayer spaces. Long positron lifetime of ~ 24 ns was observed with the intensity of ~ 8 % when the sample is evacuated at ~ 10−5 Torr. The radius of open nanospace evaluated from the positron lifetime is ~ 10 A, which corresponds to the interlayer spaces of saponite. The open nanospace increases up to 13 % after baking at 423 K for 8 h under the vacuum. The present results imply that positron annihilation spectroscopy is powerful tool to investigate hydration and dehydration behavior through the local structural changes in the interlayer spaces of clay minerals.

Atomic-Scale Structural Evolution upon Crystallite Nucleation and Growth in Amorphous Fe78B13Si9

The structural evolution upon nucleation and growth of nanocrystals was investigated for Fe78B13Si9 alloy. Upon the onset of α-Fe nanocrystallization, free volumes dominantly surrounded by Fe atoms are developed in the intergranular amorphous phase. Along with the growth of Fe-based nanocrystallites, free volumes in the intergranular amorphous phase enlarge and Fe atoms in the surroundings of the free volumes get partially replaced by B and Si. The results suggest that nanocrystallization is associated with a local structural relaxation in the metastable intergranular amorphous phase.

Dissolution of Ir and Os in Hydrothermal 1N and 3N HCl Solutions at 150.DEG.C..

Experiments on dissolution of Ir and Os in 1N and 3N HCl solutions were carried out using a Teflon reaction vessel maintained at 150°C over 150 days. We obtained high concentrations of Jr and Os, in relatively low pH, high oxygen fugacity and highly saline conditions. For instance, Jr concentration in a solution with 3N HCl as the starting condition reaches about 1, 800ppm, and Os concentrations in solutions with 1N and 3N HCl as the starting conditions reach, in both cases, about 800ppm. These high concentrations of Jr and Os are believed to be due to the stable existence of Ir4+ and Os4+ chloro-complexes in the solution which was clarified by UV and visible absorption spectra, and due to degassing of H2 from the system, which gives a rise to the decomposition of H2O. On the basis of the results of the present experimental work and those on the platinum dissolution in HCl solution obtained by Honma et al., we suggest that if the conditions for the formation of hydrothermal origin of Pt-group metals deposits are present, conditions of low pH, high Cl- concentration and oxygen fugacity are favorable for the formation of Pt-group metals, and chloro complexes of Pt-group metals are stable in the ore fluids.