Papers by Author: Ondrej Gedeon

Abstract: One of the general symptoms of ageing is a volume change due to the moisture exposure, known as an irreversible moisture expansion (IME). It is assumed IME of fired clay-based materials is governed by the presence of inner-surface and interlamellar OH-groups, present in clay minerals and assumed to be also abundant in amorphous intergranular phases. IME was measured on a series of clay-based ceramic bodies with a variable content of clay raw minerals under two different hydrothermal conditions. IR spectroscopy was used to correlate IME with the increasing amount of inner and interlamellar hydroxyl groups.

Abstract: The ab-initio molecular dynamics (MD) calculations of the yttrium - aluminate binary system 23Y2O3·77Al2O3 were performed with help of the Vienna ab-initio simulation package VASP. The thermodynamic model (TD) of glasses of the studied binary system was constructed. Both the Y/O and Al/O coordination numbers calculated from the partial pair radial distribution function (PP RDF) obtained for MD simulated melt at the temperature of 2500 K were significantly lower than the corresponding coordination numbers obtained from the TD model. The published neutron diffraction and X-ray diffraction studies give the results close to the results of the present ab-initio MD simulation. The TD model resulted in higher Al/O and Y/O coordination when compared with experimental results.

Abstract: This work aims to explore possible applications of the ab initio molecular dynamics (MD) in modeling of the soda-lime-silica (NCS) glass and melt doped with admixtures. Preparation of the basic glass (15.8 wt.% Na2O, 10.5 wt.% CaO, and 73.7 wt.% SiO2) by the MD simulation from scratch is described. The structure analysis of the NCS glass is presented in the form of total and partial radial distribution functions (RDF), coordination numbers, and fractions of Qn units. The reasonable first neighbor distances were obtained, even if a rather small basis set of electronic wavefunctions and softer pseudopotentials for atomic core regions were applied. All major discrepancies in the first neighbor distances can be easily explained, and the results can be improved if needed. The Qn distribution shows higher disproportionation of Q3 than NMR and Raman experimental data, however, it is lower than previous classical MD simulations.

Abstract: Alkali-silicate glass irradiated with electrons of the medium energy becomes to be fully depleted of alkali ions after some time. The chemical changes are expected to be accompanied with significant changes of glass structure. The changes in structure of 15K2O•85SiO2 glass were simulated by ab-initio MD. Experimental conditions were imitated by the following procedure. First, glass was prepared by the numerical cooling from the high-temperature melt. Second, alkali ions were removed. Third, silica structure was relaxed at temperatures 300 K and 1000 K. Structural changes are characterized in terms PP RDF’s and coordination numbers. It was found peroxide bonds were created from the dangling bonds, coming from non-bridging oxygen.