Marek Liška

VISCOSITY OF TITANIA-BEARING SODIUM SILICATE MELTS

Abstract Viscosity measurements made between 10 2 and 10 12 dPa s are reported for six (25 − x )Na 2 O · x TiO 2 · 75SiO 2 glasses ( x = 0, 1, 2.5, 5, 7.5 and 10). The methods of falling ball, rotational viscosimetry and dilatometric axial deformation rate measurement were used to cover the studied viscosity range. The values of dilatometric glass transition temperature are also reported. Obtained temperature and compositional dependencies of viscosity confirmed the validity of the Adam-Gibbs viscosity equation. The individual molar free hindrance energies B e and the configurational entropies S conf ( T g ) resulted from the least-squares regression treatment for each glass composition. Both the molar free hindrance energies and the configurational entropies depend on the TiO 2 content in a similar fashion, indicating the changes in the titanium structural role in the compositional range studied. These results are in qualitative agreement with the analogous dependence of glass transition temperature.

Molecular dynamic modelling of potassium transport in a potassium-silicate glass irradiated by fast electrons

Abstract The binary potassium-silicate glass has been prepared from an ionic liquid by molecular dynamics (MD) using 900 ions interacting via Born–Mayer type pair potentials. The standard MD procedure was adapted so that an electric field and/or momentum transfer, corresponding to the elastic scattering of primary electrons, could be applied to a particular set of ions. Results have shown that the electric field itself should cause dielectric breakdown to drive the ions out of their sites. If the electric field is accompanied by momentum transfer from the primary electron, then potassium ions can be released from their sites and can be further driven by the macroscopic electric field, created by trapped electrons in the glass. The depth and width of the potassium potential well has been estimated by studying the motion of ions in a high electric field.

The structure of MD simulated cryolite melt

Abstract Molecular dynamics (MD) simulations have been performed for molten cryolite (Na3AlF6), employing Born-Mayer-type pair potential functions. The structure of the simulated liquid is characterized by the partial pair radial distribution functions and also by the distribution of Voronoi and direct polyhedra. The results obtained are compared with those obtained from the Monte Carlo (MC) calculations of Qiu and Xie and with X-ray crystal data. The MD results agree well with the equilibrium crystal structure, while significant differences between the MD and MC simulations are attributed to various sources. Both the MD and MC simulated melt structures do not confirm Gilberts dissociation scheme for the hexafluoroaluminate anion.

Thermodynamic model and structure of CaO–P2O5 glasses

The distribution of Q-units of CaO–P2O5 glasses was described by the thermodynamic model of Shakhmatkin and Vedishcheva. The glass was considered as the ideal solution of CaO, P2O5, CaP2O6, Ca2P2O7, and Ca3P2O8. In the first step, molar Gibbs energies of considered species were taken from the FACT thermodynamic database. The obtained result was compared with 31P solid-state NMR study of Roiland. It was shown that the calculated values were in fairly good agreement with the experimental values. After that, the nonlinear regression treatment was used for optimization of molar Gibbs energies by minimizing the sum of squares of deviations between experimental and calculated Q-distribution. In such a manner, the non-ideality of the system was reflected. In the studied case, no significant improvement of obtained results was achieved by this procedure—thus, the ideal solution assumption included in the thermodynamic model of Shakhmatkin and Vedishcheva holds very well for the studied binary glasses.

Mn2+ EPR investigation of sol-gel process in silica xerogels at 77 K

Abstract The EPR spectrum of the Mn 2+ ion has been used as a diagnostic tool at liquid-nitrogen temperature (77 K) to investigate the structural evolution of SiO 2 xerogels as a function of temperature during their calcination, as well as during subsequent aging. The positions of the Mn 2+ hyperfine lines were used to estimate the g -factor, the hyperfine interaction constant ( A ) and the zero-field splitting parameter ( D ) for the Mn 2+ ion in the various xerogels. The values of the parameters g , A of the xerogels were found to be, within experimental error, independent of the firing temperature as well as that of aging. However, the splitting of the forbidden hyperfine lines, and thus the value of D , was found to be significantly different in the samples prepared at firing temperatures of 360°C and below from those prepared at firing temperatures of 440°C and above; they remained independent of aging.

Thermal Properties and Related Structural Study of Oxide Glasses

Glass is accompanying people from the early times of mankind. First it was the natural glass generated by the volcanic processes and the glass “produced” by the impact of meteors on the earth. During formation of the earth, highly siliceous melts of rocks froze to natural glasses such as obsidians. After some time the people start the glass melting. Glass was first produced by man about 4,000 years ago in ancient Egypt. From this time the need of the knowledge of glass composition, structure and properties is dated. These are the typical questions answered by the glass chemistry [1–6].

Voronoi polyhedra analysis of MD simulated silicate glasses

The relationship between the volume and the surface of Voronoi polyhedra (VP) appeared to be an effective tool for the characterization of the structure of MD simulated silicate glasses. This feature was demonstrated on the series of binary potassium-silicate glasses with the alkali content ranging from 5 to 20 mol%. Moreover, the temperature dependence of the slope of the above dependence, when expressed for various types of central atoms (Si, O and K in our case) was studied. Each type of central atom is characterised by its own iron line linearly relating the volume of VP to its properly powered surface, so that new geometrical constrains to the VP was found.

Enthalpic analysis of melts in the Ca2MgSi2O7CaSiO3 system

Abstract Relative enthalpies of the Ca 2 MgSi 2 O 7 CaSiO 3 system melts have been measured by combination of high-temperature drop calorimetry and solution calorimetry of the same sample in the temperature interval 1760–1930 K. From these results, the relative enthalpy-temperature-composition relation has been found by regression analysis. The enthalpy of mixing of melts is zero at any composition within the experimental errors of measurement. The heat capacities of melts do not depend on temperature and are a linear function of composition. They are discussed with values calculated employing Stebbins et al.s, Richet and Bottingas, and Lange and Navrotskys models.

The bimodality in the SiOSi bond angle distribution in simulated sodium silicate systems

Molecular dynamic (MD) simulations were performed for systems Na 2 O·SiO 2 and Na 2 O·2SiO 2 employing both Born-Mayer (BM) and Pauling pair potential functions (PPFs) with commonly used parametrizations. The influence of these PPFs on the structure of the simulated systems was observed. The main difference is represented by the presence of some silicon atoms coordinated by five oxygen atoms that gives rise to an additional small peak at about 110° in the SiOSi bond angle distribution in case of BM PPFs.

Molecular orbital calculation of spin-orbit splittings in some halogeno-compounds

Abstract CNDO and INDO calculations incorporating the spin—orbit operator in the Hamiltonian have been carried out to obtain the spin—orbit splittings of ionization potentials of some halogen compounds. Two sets of spin—orbit coupling constants were tested. The calculated splittings are compared with photoelectron spectroscopy experimental results from the literature.