Toshiharu Fukunaga

DENSITY DEPENDENCE OF STRUCTURE OF SUPERCRITICAL CARBON DIOXIDE ALONG AN ISOTHERM

Neutron diffraction experiments for supercritical CO2 have been carried out over a wide range of Q (0.018≤Q≤30 A−1) at ρ*=ρ/ρc=1.5, 1.2, 0.77, and 0.34 along an isotherm at 310 K (T*=T/Tc=1.02). The measurement enabled us to obtain quantitatively reliable radial distribution functions of the fluid including both short‐ranged structure and long‐ranged density fluctuation. The structure factor and radial distribution function showed the structural change from the attraction‐predominant gaslike structure to the repulsion‐predominant liquidlike one with increasing fluid density. With respect to the long‐ranged structure, almost linear Ornstein–Zernike–Debye plots were obtained for S(Q) of the fluids at all densities. A plot of correlation length against reduced density seems to have a maximum at the critical density. A ratio, α(r), of the density fluctuation produced by the correlation within r to that to the infinity presented a new aspect of the density fluctuation. Molecular dynamics simulation has also be...

Amorphization in an immiscible CuV system by mechanical alloying and structure observed by neutron diffraction

Abstract Copper and vanadium atoms are immiscible even in the liquid state and the system is characterized by a positive heat of mixing. A mixture of elemental copper and vanadium powders was subjected to mechanical alloying. The mixing process down to an atomic level during mechanical alloying was studied by neutron diffraction and differential scanning calorimetry measurements. We take full advantage of the negligibly small scattering length of the vanadium atom in the neutron diffraction measurements. Some particular Bragg peaks of the f.c.c. copper crystal diminish quickly whereas the remaining peaks become broadened with increasing milling time. A halo pattern dominates after 60 hours of milling, though the first peak remains still rather sharp. The peaks of RDF(r) corresponding to those of the f.c.c. copper crystal are broadened with increasing milling time and approach gradually the characteristic of an amorphous state.

Amorphization of immiscible CuTa system by mechanical alloying and its structure observation

Abstract Cu and Ta atoms are immiscible with each other even in the liquid state and the Cuue5f8Ta alloy system is characterized by a positive heat of mixing. Utilizing the neutron and X-ray diffraction techniques as tools for the structural analyses and DTA, scanning electron microscopy and EPMA as supplementary tools, we could show that the amorphization proceeds slowly but definitely by mechanical alloying in the Cuue5f8Ta alloy system.

Mechanical alloying process of Fe-Cr powders studied by magnetic measurements

A mechanical alloying process for a mixture of elemental Fe and Cr powders with the Cr compositions 20–70 at.u2009% was investigated through the measurements of x‐ray diffraction, magnetization, and 57Fe Mossbauer spectrum. We show that magnetic studies provide more detailed information about the alloying process occurring during ball milling than the conventional diffraction techniques in this particular system. A final product after ball milling was identified as a high‐temperature phase of the α solid solution, regardless of compositions studied. Powders subjected to milling in Ar gas atmosphere for 200 h were further ball milled in N2 atmosphere. The presence of N2 gas has caused a partial amorphization. The amorphous phase thus produced is found to be paramagnetic at room temperature.

Neutron and X-ray diffraction studies of PbOGa2O3 and Bi2O3Ga2O3 glasses

Abstract The structure of 50PbO·50GaO1.5 and 80BiO1.5·20GaO1.5 glasses was investigated by neutron and X-ray diffraction. Most Ga3+ ions are four-coordinated in both glasses. PbO3 trigonal pyramids and PbO4 square pyramids are present in 50PbO·50GaO1.5 glass. The stronger glass-forming ability in PbOue5f8GaO1.5 is attributed to (PbO3, PbO4)n infinite spiral chains as in crystalline PbSiO3. Bi3+ ions form BiO5 and BiO6 units in 80BiO1.5·20GaO1.5 glass as in α-Bi2O3. This glass may be constructed by (BiO5, BiO6)n layers and GaO33− chains. It is assumed that in these glasses nearly all oxygen atoms bonded to Pb2+ or Bi3+ ions are three- or four-coordinated and most oxygen atoms bonded to Ga3+ ions are three-coordinated. It is found that the structure of the present ‘non-conventional’ glasses are contrary to Zachariasens rules for glass formation.

Structural defects in mechanically ground graphite

Abstract Highly ordered synthetic graphite was mechanically ground in a planetary ball mill. The interlayer spacing d002 was found to increase in the grinding process with the reduction of the crystalline size La, Lc. The specific surface area increased to over 600 m2/g at the maximum. The result of Raman spectroscopy shows that the ground graphite yields a smaller value of full width at half maximum (FWHM) of the 1350 cm−1 peak than the heat-treated carbon for a given value of La. This indicates the difference in the type of structural defects involved between the heat-treated carbon and ground graphite. A further suggestion of this result is that the mechanically ground graphite has more uniform boundary and a less-defective structure of hexagonal graphite lattice within the layer planes than heat-treated carbon.

Structure of liquid benzene and naphthalene studied by pulsed neutron total scattering

The structure factor Sm(Q) of liquid benzene and naphthalene have been measured by means of pulsed neutron total scattering. Both intramolecular and intermolecular structures are discussed. The intermolecular structures are analyzed by taking account of orientational correlations introduced between the nearest neighbors and a packing of uncorrelated molecules. The experimental Sm(Q) and the radial distribution function of liquid benzene are reproduced reasonably well by introducing a perpendicular orientation just between the nearest neighbors. The analysis of liquid naphthalene suggests an orientation somewhat similar to that in liquid benzene.

Solid state amorphization in the CuTa alloy system

Abstract The mechanical alloying (MA) technique was applied to the Cuue5f8Ta system, which has been known to be immiscible and never amorphized by the conventional rapid quenching technique. The possibility of the solid state amorphization of Cu30Ta70 powders was examined and it was found to proceed positively during MA. Experimental evidence that amorphization had taken place was demonstrated through X-ray diffraction, thermal analysis and extended X-ray absorption fine structure (EXAFS) studies. The present results suggest the importance of an energetically up-hill process through ball milling for amorphization of the system with a positive heat of mixing.

A neutron scattering study of the structure of supercritical carbon dioxide

Abstract The structure factor S(Q) of supercritical carbon dioxide has been measured over a wide range of the scattering vector Q (0.018–30 A−1) using two neutron apparatuses. The measured S(Q) has been Fourier transformed into the neutron weighted radial distribution function, GN(r). This measurement enables us to analyze the short-range and long-range structures simultaneously in the supercritical fluid.

Neutron diffraction and solid-state MAS-NMR studies of the structure of Y-AI-Si-O-N oxynitride glasses

Abstract Neutron diffraction and magic angle spinning (MAS) 27Al and 29Si NMR have been applied to study the local structure of glasses in the system Y-Al-Si-O with and without addition of AlN. 27Al MAS NMR indicated that in the Y-Al-Si-O-N oxynitride glass there is no Al-N bonding and aluminium atoms occupy various sites within the glass structure. The first peak of the radial distribution function curves obtained by neutron diffraction was deconvoluted into four peaks which are attributable to Si-O, Si-N(Al-O(4)), Al-O(5) and Al-O(6) pairs and the coordination numbers of Si, N, Al and Y were estimated. The glass network is assumed to consist mainly of corner-sharing AlO4 tetrahedra and SiO3N tetrahedra. 86% of N atoms are three-coordinated and 14% of N atoms are two-coordinated in the present oxynitride glass.