Norimasa Umesaki

Raman spectra of K2O–B2O3 glasses and melts

The structures of K2O–B2O3 glasses and melts have been studied by high-temperature Raman spectroscopy. With an increase in the K2O content and with increasing temperature, the boroxol rings, which only consist of BO3 triangular units, were converted into pentaborate groups which consist of BO4 tetrahedral (O=bridging oxygen atom) and BO3 units in the 10 and 20 mol% K2O–B2O3 glasses/melts. In the 30 mol% K2O–B2O3 glass/melt, the Raman spectra indicated a structural change in the borate species from BO4 tetrahedral units to metaborate, BO2O−, triangular units with non-bridging oxygen with increasing temperature. The fraction of four-coordinated boron atoms, N4, obtained from deconvolution of the Raman bands was gradually reduced above the glass transition temperature and converged to a constant value over 1400 K. This phenomenon could be explained by a thermodynamic process.

Characterization of plasma-sprayed zirconia coatings by X-ray diffraction and Raman spectroscopy

Three zirconia ceramics, ZrO2−4.5mol.%Y2O3, ZrO2−12mol.%Y2O3 and ZrO2−8mol.%MgO, were investigated using X-ray diffraction before and after plasma spraying. The amounts of monoclinic, tetragonal and cubic phases existing in the three zirconia ceramics were determined from the integrated intensity ratios in the {400} and {111} regions of the X-ray diffraction patterns. The tetragonal-to-monoclinic transformation in the plasma-sprayed coatings caused by indentation, fracture and/or heat treatment with a laser beam was examined using Raman spectroscopy. As a result, this spectroscopic technique is shown to be a powerful tool for the observation of this transformation.

A structural study of rapidly quenched glasses in the system Li2OSiO2

Abstract Raman spectra of 4 glasses in the system Li2O SiO2 (41.3 ≦ Li2O ≦ 61.3 mol%) prepared by rapid quenching were measured. The proportions of SiO4 units with 1, 2, 3 and 4 non-bridging oxygens per Si were determined for these glasses from the quantitative analysis of their Raman spectra. X-ray structural analysis of the Li2O SiO2 glasses showed that the average atomic distance of Si O pair was elongated with increasing Li2O content due to the weakening of the Si O bond.

The structure of pressure-compacted vitreous germania

Abstract Neutron diffraction measurements have been performed on normal and pressure-compacted vitreous GeO2 using the LAD spectrometer at the ISIS spallation pulsed neutron source and the SLAD diffractometer at NFL Studsvik. Four cylindrical GeO2 samples, which had been subjected to pressures between 0 (ambient pressure) and 6 GPa, were studied. The first peak of the diffraction pattern shifts to higher scattering vector magnitude, Q, with increasing density (preparation pressure), indicating a modification of the medium range order resulting from a compaction of the network cages. The real space correlation function, T(r), shows that the mean Ge–O bond length only changes very slightly for glasses prepared at pressures up to 6 GPa, inferring that GeO6 octahedra are not present in significant numbers in the glass following pressure release. However changes in the first Ge–Ge peak with increasing density indicate a progressive reduction of the mean Ge–O–Ge bond angle.

Surface treatment of plasma-sprayed ceramic coatings by a laser beam

Abstract A CO2 laser beam with a power of 2000 W has been employed to modify the surface layer of a few plasma-sprayed alumina and zirconia coatings. The effect of the laser treatment for the coatings was examined by the use of several microanalytical techniques. We found that the coating layer became more dense as a result of the laser treatment. We also verified that the crystal phases in the coating layer were controlled by the laser treatment.

Neutron scattering from PbOGeO2 glasses

Abstract High-resolution neutron-diffraction data have been collected on PbO GeO 2 glasses and on GeOP 2 for comparison. These neutron data have revealed the existence of 6-fold coordinated Ge (GeO 6 octahedra) by virtue of the shift in the first peak in the total correlation function T ( r ) and increase in coordination. The neutron results also indicate that PbO exits as PbO 4 pyramids, as found in the orthorhombic form of PbO crystal, in PbO GeO 2 glasses.

Raman spectroscopic study of alkali silicate glasses and melts

Abstract Raman scattering measurements have been performed on M 2 O · 4SiO 2 (M: K and Rb) glasses at room temperature and in the corresponding melts to study the structural relationship between the glasses and the melt. The fractions of SiO 3 2− chain, Si 2 O 5 2− sheet and SiO 2 0 three-dimensional network units in the glasses and melts were determined from the Raman results. It is found that above the glass transition temperature T g the neutral SiO 2 0 species are gradually destroyed and the highly charged Si 2 O 5 2− sheets created as the temperature increased.

Structural analysis of sodium silicate glasses containing TiO2 by pulsed neutron scattering

By the use of the time-of-flight pulsed neutron total-scattering spectrometer HIT at the Japanese National Laboratory for High Energy Physics, measurement of structure factorsS(Q) for glasses of type 10Na2O·(90−x)SiO2·xTiO2 (x=0, 5 and 20 mol%) was made over a wide range of scattering vectors up toQ (=4π sinθ/λ) ~ 500 nm−1. High-resolution real-space information was obtained from the Fourier transform ofS(Q). The influence of TiO2 on the nearest-neighbour Si-O bond length of the silicate anions existing in the glass samples studied is different from that of alkali oxides such as Na2O. It is concluded that most of the Ti4+ ions predominantly substitute for Si4+ ions in their tetrahedrally coordinated sites, leading to the formation of TiO6 octahedra.

Short-range structure of alkaline-earth borate glasses by pulsed neutron diffraction and molecular dynamics simulation

Abstract The structure of vitreous MO· n B 2 O 3 (M=Ca and Ba; n =2, 3 and 4) has been studied by pulsed neutron diffraction measurement with the help of molecular dynamics (MD) simulation. The first and second peaks assigned to the nearest-neighbor B–O and O–O correlations, respectively, in the obtained total pair distribution functions shifted little with increasing MO content, while the asymmetry of the first peak increased significantly with MO content; these results are different from those for potassium borate glasses. It was inferred that the BO 3 and BO 4 units are better defined in these alkaline-earth borate glasses than those in the potassium borate glasses. Both the full-width at half maximum (FWHM) of the first peak and the average co-ordination number of O around B clearly increased as MO content increased which is the same behavior with alkali borate glasses. The fraction of four-co-ordinated B showed a larger deviation from x /(1− x ) for CaO–B 2 O 3 glasses than that for BaO–B 2 O 3 glasses which shows a different dependence of the deviation on cation size from that in alkali borate glasses, and is in good agreement with the results from MD simulation.

High-energy X-ray diffraction studies of non-crystalline materials

Abstract High-energy ( E ≥ 30 keV) X-ray diffraction with the latest generation synchrotron sources as well as the introduction of advanced insertion devices: wiggler and undulator, has created new approaches to the quantitative study of the structure of non-crystalline materials because of several improvements: higher resolution in real space due to a wide range of Q, smaller correction terms (especially for absorption correction), reduction of truncation errors, the feasibility of running under extreme environments, including high-temperatures and high-pressures, and of obtaining a direct comparison between X-ray and neutron diffraction data. Recently, this technique has been combined with neutron diffraction with a pulsed source to provide more detailed and reliable structural information not previously available. This article reviews and summarizes recent results obtained from high-energy X-ray diffraction on several oxide glasses: SiO2, B2O3, MgP2O6 and PbSiO3, using bending magnet beamlines at Super Photon ring-8 (GeV) (SPring-8). In particular, it addresses the structural models of oxide glasses obtained by the reverse Monte Carlo (RMC) modelling technique using both the high-energy X-ray and neutron diffraction data.