Toshio Yokokawa

The structural groups of alkali silicate glasses determined from 29Si MAS-NMR

Abstract Lithium, sodium and potassium silicate glasses containing 20–56 mol% alkali oxide were investigated by 29 Si nuclear magnetic resonance (MAS-NMR) spectroscopy. In the spectrum of each sample, at least two to four distinct peaks were identified. The distributions of SiO 4 structural units, Q n , where n is the number of bridging oxygen atoms bound to other Si atoms, were determined as a function of composition. The equilibrium constants of the reactions, 2Q n ⇌ Q n −1 + Q n +1 ( n = 3, 2, 1), were determined. The reaction proceeds to the right direction as cationic power of alkali ion ( Z / r ) increases (Li + >Na + >K + ) at the same alkali oxide concentration. The apparent equilibrium constants of the above reactions are discussed along with a proposed thermodynamic model. The 29 Si chemical shifts assigned to each structural unit increase linearly with alkali oxide contents. The slope of these lines decreases as the numbers of attached bridging oxygen (BO) atoms decrease. The average chemical shifts also increase linearly with an increase of alkali content. A close relationship between the average chemical shifts and the theoretical optical basicity was observed.

Fe3+/Fe2+ redox equilibrium in the molten Na2O—B2B3 system by linear sweep voltammetry

Voltammetry of Fe3+/Fe2+/Fe0 was carried out in Na2O—B2O3 melt. The redox equilibrium constant of Fe3+/Fe2+ and the diffusion coefficient of the Fe3+ ion were estimated from the voltammograms. The equilibrium constant increases with the increasing concentration of Na2O in the solvent. It was larger than that in Na2O—SiO2 and Na2O—P2O5 by 0.6—0.9 on a logarithmic scale at the same solvent basicity. The square root of the ratio of diffusion coefficients, (DFe3+/DFe2+)12, was between 1 and 2. As little of 0.001 to 0.004 mol. % Na2SO4 present as an impurity caused the appearance of several peaks in the voltammograms including a large anodic peak when the mol. % of the Fe2O3 was larger than 0.1. The existence of a complex of Fe2+ and S4+ was suggested, although its stability constant seemed quite small.

Molecular dynamics calculations for boron oxide and sodium borate glasses

Abstract Molecular dynamics (MD) calculations were carried out for boron oxide and sodium borate glasses using the interionic potentials derived by the MD calculations of crystals. It is shown that the calculated composition dependence of the fraction of the boron ions having tetrahedral coordination is in good agreement with the results measured by NMR. The pair distributions and bond angle distributions around 3- and 4-fold coordinated boron ions showed remarkable differences from each other and gave the particular local structures. It is concluded that the network is developed at first with an increase of Na2O content and the maximum development is shown near 33 mol% Na2O. Above 33 mol% Na2O, the network of the sodium borate glass starts to disintegrate and a significant increase of the non-bridging oxygen ion is found.

Enthalpies of mixing in the liquid state: Tl+Te and Bi+Te

Enthalpies of mixing of thallium + tellurium and bismuth + tellurium in the liquid state have been measured by means of high temperature calorimetry. These systems have large negative enthalpies of mixing. The minimum in the enthalpy of mixing occurs in each case at the composition corresponding to a compound which is stable in the solid state, namely Tl2Te and Bi2Te3.

E.m.f. measurements of molten oxide mixtures III. Sodium oxide + silicon dioxide

Abstract The relative partial molar quantities of binary molten mixtures of Na2O + SiO2 were evaluated from e.m.f. measurements of the melts containing from 45 to 77.5 moles per cent of SiO2. In addition to a steep change of the activity, a substantial inflection of the partial molar enthalpy and entropy at the composition of 60 moles per cent of SiO2 was observed.

Enthalpies of mixing in the liquid state III. In + Te and Sb + Te

Enthalpies of mixing of indium + tellurium and antimony + tellurium in the liquid state have been measured. The minima in the enthalpy of mixing appeared at the composition corresponding to the compounds, In 2 Te 3 and Sb 2 Te 3 which are stable in the solid state. The absolute values of the minima in the enthalpies of mixing increase with the difference in the electronegativities of the component elements in the order, Sb 2 Te 3 , Bi 2 Te 3 , and In 2 Te 3 .

E.m.f. measurements of molten mixtures of lithium oxide +, sodium oxide +, and potassium oxide + boron oxide

Abstract Partial molar Gibbs free energies, enthalpies, and entropies for the binary oxide mixtures: Li 2 O + B 2 O 3 , Na 2 O + B 2 O 3 , and K 2 O + B 2 O 3 were determined by means of concentration cells at high temperature. Positive partial molar excess enthalpies and entropies of B 2 O 3 were observed for high mole fractions of B 2 O 3 . These are discussed in terms of the dipole-dipole interaction.

Ce4+Ce3+ redox equilibrium in Na2O-B2O3 melts by linear sweep voltammetry

Abstract New semi-empirical equations were presented to study the redox equilibria of transition metal ions in molten oxides by linear sweep voltammetry. These equations gave the relation between the peak potential and the polarographic half-wave potential, and contained the Randles-Sevcik equation as an extreme case. The measurements of Ce 4+ Ce 3+ equilibrium in several Na 2 O-B 2 O 3 melts were used to verify the equations. The agreement between the electrochemically estimated value of the equilibrium and the other estimated from the chemical analysis of the quenched sample was quite satisfactory. The relation between the Ce 4+ Ce 3+ equilibrium and the composition of the melt was discussed as the functions of the basicity.

Studies of NiO dissolved in alkali silicate melts based on redox potential and visible absorption spectra

The redox behavior of Ni2+ ions in alkali silicate melts, xR2O · (1 - x)SiO2, (R = Na, K and Cs and x in the range 0.175 – 0.45), was investigated using linear sweep voltammetry and differential pulse polarography. The half-wave potential of Ni2+Ni0 depends on the alkali species and contents. It shifted to the positive side when the alkali species changed from sodium through potassium and cesium at a fixed ratio of alkali oxide to silica, indicating that NiO loses its thermodynamic stability in this order. Based on visible absorption spectra, it is shown that the Ni2+ ion exists in tetrahedral and octahedral coordinations in the alkali silicate glasses. The relative ratio of tetrahedral to octahedral species of Ni2+ also depends on the alkali species, and the content of the tetrahedral species of Ni2+ in potassium or cesium silicate is greater than that in sodium silicate. From these two results, it is assumed that, while NiO dissolves in the silicate melts as a weak base, some Ni2+ ions change into a tetrahedral coordination in a strongly basic solvent such as cesium silicate.

Molecular dynamics study on the shear viscosity of molten Na2O•2SiO2

Abstract This paper describes the applicability of molecular dynamics simulation to the shear viscosity of silicate melts. Simulations of molten Na 2 O·2SiO 2 have been performed on 756 and 189 particle systems to evaluate the viscosity by using three different methods: the Green-Kubo integrand, the Stokes-Einstein equation, and the non-equilibrium molecular dynamics (NEMD) simulation. Of these methods, the Green-Kubo treatment and the Stokes-Einstein equation, adopting the SiO 4 4− anion for the diffusing unit, gave the most reasonable results, and they qualitatively reproduced the temperature dependence of viscosity. The simulated values of viscosity were consistent with experimental values within an order of magnitude at temperatures, higher than 1500 K, but were smaller by more than one order of magnitude at lower temperatures. The NEMD simulation of shear flow showed a strong shear-thinning features and failed to reproduce the viscosity value at the experimental shear rate region. The discrepancy between the simulated and experimental values is attributed to the short sampling time and the small system size in the MD simulation.