Advanced Materials Research Vols. 39-40

Abstract: Basicity of glass is still a challenge in spite of various attempts to measure or calculate it. The values assigned for basicity of glasses, either calculated or experimentally determined, are not always in full agreement with actual facts, and discrepancies among the theoretical ones are not unusual. For instance, SiO2 is described by a single basicity value even if the polymorphs of SiO2 are quite different. Only few attempts were made to face this challenge. Present paper deals with a novel approach on theoretical ionicity / basicity based on electronic energy levels or band structure of solids. Another major adjustment takes into account the possibility of decomposing ionicity of complex chemical bondings into a sum of binary bondings. Considering the distribution of the interbonding angles specific for vitreous systems, it is possible to estimate both a local ionicity (basicity) of bondings and a global (mean) basicity of glass. The variation of basicity (ionicity) with temperature is also presented, this finding being able to open a new view on thermal history of glass considered through chemical character of bondings.

Abstract: Although Poisson's ratio (ν) is a macroscopic elastic parameter it depends much on the fine details of the atomic packing. Glasses exhibit a wide range of values for  from 0.1 to 0.4 which correlate to the glass network polymerisation degree, hence reproducing at the atomic scale what is observed in cellular materials at the macroscopic scale[1]. As for pure oxide glasses, we found in various multi-component glasses built on ionic-, covalent- or Van der Waals bonds that an increase of Poisson’s ratio corresponds to a decrease of the atomic network crosslink degree[2]. Noteworthy, an extension of this analysis to the case of metallic glasses correlate the recently proposed cluster-like network structure for these glasses[3,4]. A general feature is that a highly cross-linked atomic network results in a glass with a low atomic packing density (large free volume fraction), as exemplified with the case of amorphous silica. The lower the atomic packing density is and the larger the volume change the glass experiences under high pressure (1 to 25 GPa). Indentation experiments with sharp indenters (such as the Vickers one) give birth to hydrostatic stresses of the same order of magnitude and thus induce glass densification. There is hence a direct correlation between ν (reflecting the packing density) and the indentation behavior[5].

Abstract: Specific features of two global glass property databases, Interglad and SciGlass, are described. It is shown that the Interglad database is more convenient for specialists who use commercial glasses in practice and are to know the properties of these glasses and particular details of their usage, whereas the SciGlass database is more helpful for scientists who investigate glass properties and develop new glasses. Some prospects of further development of glass property databases are characterized.

Abstract: The features of anomalous behavior of silica glass doped with titanium oxide are discussed. The low thermal expansion of silica glass is considered on the base of the idea about polymorphous transformations in meta-stable glass structures. The analogy of these transformations to structural transitions of martensite type in metallic alloys is assumed. The effect of TiO2-doping on the structure and the intrinsic strength of silica glass fibers is studied. The structural (intrinsic) strength has been found to decrease by about 15% compared to that of non-doped silica glass. This decrease of strength is assumed to be accounted for the incorporation of a part of titanium ions into glass structure with six-fold coordination resulting in weakening the connectivity of a glass anionic network.

Abstract: The ternary tellurite glass structure (TeO2-ZnO-Na2O) was analysed using the Raman spectroscopy in the range of 300-1000 cm-1. The influence of the concentration of alkali ion oxide on the structural change of tellurite glasses was discussed. It is evident that the network of structural units changes significantly with TeO2:Na2O ratio, which then influences the glass thermal properties. Glass density, molar volume, glass transition temperature, crystallization temperature and viscosity have been measured and calculated. These results provide clear evidence and explanation of the glass structural change.

Abstract: The mathematical model of the stress-strain curve of the strand of glass fibers was proposed and applied on the experimental data obtained for E glass fibers. The model reflects the lognormal continuous distribution of the unstrained lengths of glass fibers and the Weibull distribution of the fibers strength. The regression treatment of experimental data provided the statistically robust estimates of the parameters of the lognormal length distribution, of the Young modulus, and of the parameters of the Weibull glass fibers strength distribution. It was shown that neglecting of the continuous unstrained length distribution leads to serious errors in estimates of the fiber strength distribution.

Abstract: In present paper there are presented such correlations for the silica-alkali systems (M2O – SiO2) where reliable thermodynamic data exists from the thermal equilibrium diagrams. It was observed that with the saturation of SiO2 tetrahedron with metal ions, the difference in basicity between the reactants is diminished, in the order SiO2 → disilicate → metasilicate → orthosilicate, and that confirms the role of driving force for these reactions of the acid-base interaction between oxides and the intermediate silicates formed. Also, there were proposed some correlations between the standard formation Gibbs free energy, (Gº298), and basicity percentage, pB, for the binary compounds from the same oxide systems. Considering the fact that for a system at equilibrium, reaction constant Kp depends on the reaction free Gibbs energy (rG0 298) and temperature, a correlation between basicity difference through basicity percentage, pB, and rG0 298 is proposed.

Abstract: Viscosity, density, thermal expansion, glass transition temperature, refractive index, molar refraction, and chemical durability of selected compositional series of Na2O-K2O-ZnO-CaOZrO2- SiO2 glasses containing 1 and 3 mol. % ZrO2 were measured and interpreted according to the network-forming / -modifying activities of particular oxides. The influence of ZrO2 / SiO2, ZnO / CaO, and K2O / Na2O substitution on measured physical and chemical properties was discussed.

Abstract: Glasses have been synthesized in the system La2O3-PbO-MnO-B2O3 varying the MnOcontent in large compositional range between 10 and 70 mol%. Glass-ceramics containing La0,6Pb0,4MnO3 phase have been obtained directly by melt quenching method. Transition and crystallization temperatures have been determined by DTA. The increasing of the B2O3-content improves the thermal stability of the glasses. By TEM analysis, different type microheterogeneous structures have been observed in compositions rich in B2O3.

Abstract: Lithium-lead phosphate and borophosphate glasses were studied in compositional series of xLi2O-(50-x)PbO-50P2O5, xLi2O-(50-x)PbO-10B2O3-40P2O5 and xLi2O-(50-x)PbO-20B2O3- 30P2O5. All glasses crystallize on heating within the range of 400-610oC. Their glass transition temperature decreases with increasing Li2O content in the region of x = 0-30 mol% Li2O and then slowly increases in all series of glasses reflecting thus changes in the bond strength of the glass structural network. Density, molar volume and chemical durability of the glasses decrease with increasing Li2O content. The 31P MAS NMR and Raman spectra show that at all glass series with the replacement of PbO by Li2O the ratio of Q2/Q1 units, present in the structure, slightly increase. The 11B MAS NMR spectra of glasses with 10 mol% B2O3 reveal the presence of only BO4 units in the glass structure, whereas the spectra of the glasses with 20 mol% B2O3 reveal also the presence of small amount of BO3 units.