Advanced Materials Research Vols. 39-40

Abstract: An overview is given of recent advances in the experimental physics of supercooled liquids and glasses. These are described in the context of measurements of non-ergodicity, viscosity and low frequency dynamics. Particular attention is paid to recently reported correlations between melt fragility and the vibrational and mechanical properties of the ensuing glass. Low frequency vibrations are discussed in relation to the Boson peak that characterises the glassy state. Throughout the over-arching role of density fluctuations in glass formation is stressed together with the experimental tools now available to characterise heterogeneity.

Abstract: The heterogeneous character of the glass structure is, from long-time, in the attention of researchers. Some calculation methods for obtaining information concerning the nature and distribution of nano aggregates in glass were published in the middle of the 20th century. The experimental methods borrowed from other fields of chemistry are hardly useful the results are mostly qualitative and their use to select the calculation method giving closer to reality data is practically impossible. For these reasons, a new combined experimental and calculation method was imagined, allowing the obtainment of quantitative data about the concentration and distribution of nano-aggregates in glasses. By means of the method for experimental determination of the basicity of glass, with Cu2+ as probe ion, relationships were deduced allowing obtaining the composition and the amount of each nano-aggregate in glass from the recorded maximum of the charge transfer transition absorption. The results obtained for some silicate glasses, compared with the theoretical calculated data, seem to validate this approaching way. By means of the new experimental method, the direct following of the technological parameters influence on glass structure and properties become possible.

Abstract: The structure of a seven oxide aluminoborosilicate simplified nuclear glass, bearing a high amount of neodymium or lanthanum oxide (16 wt%), alkali and alkaline earth cations is studied. Nd3+ or La3+ are supposed to simulate the trivalent lanthanides and minor actinides present in nuclear wastes. In the studied glass composition, lanthanide ions have a modifying role and are located in highly depolymerized regions of the structure as shown by neodymium optical absorption and EXAFS spectroscopies. Both alkali and alkaline earth cations are present around Nd3+ ions enabling their stabilization in glass structure near non-bridging oxygen atoms (NBOs). We show that both the nature of alkali R+ and alkaline earth R'2+ cations and the K = [R'O]/([R2O]+[R'O]) ratio can greatly influence the structure of the aluminoborosilicate glass network. Three glass series were prepared for which: (i) K ratio was varied from 0 to 0.5 (Na+ and Ca2+ being respectively the only alkali and alkaline-earth cations), (ii) the nature of R+ cation was varied from Li+ to Cs+ (Ca2+ being the only alkaline earth cation and K = 0.3), (iii) the nature of R'2+ cation was varied from Mg2+ to Ba2+ (Na+ being the only alkali cation and K = 0.3). 27Al MAS NMR spectroscopy results show that (AlO4)- units are preferentially charge compensated by alkali cations rather than by alkaline-earth cations. Both R+ and R’2+ cations can compensate (BO4)- units. Nevertheless, whereas the proportion N4 of (BO4)- units increases with the size of R'2+ cations, the evolution of N4 with R+ cation size for glasses of the R series is not monotonous. The evolution of sodium ions distribution trough glass structure is followed by 23Na MAS NMR spectroscopy.

Abstract: Fluorine containing calcium aluminosilicate glasses are widely used for a number of technological applications including dental cements, mould fluxes in steel making and in a variety of glass-ceramic systems. Despite of their importance these systems remain quite poorly understood with respect to their composition. To address this question a glass composition corresponding to the equimolar binary system anorthite−fluorite (Ca2Al2Si2O8−CaF2) was chosen as a base point for two series of compositions. One of the series is designed on the anorthite stoichiometry and considered as classically charge balanced. Another series starts from the fluorine free composition of the anorthite−lime (Ca2Al2Si2O8−CaO) stoichiometry and, therefore, is characterized by a disrupted network with at least one non-bridging oxygen (NBO) attached to silicon. A multinuclear 19F, 27Al, 29Si solid state NMR study of the glasses was undertaken. It is shown that in both series fluorine is predominantly coordinated by calcium, F−Ca(n), and in addition interacts with aluminium forming Al−F−Ca(n) complexes, where n denotes the number of first neighbouring calcium cations. Small amounts of high coordinated aluminium grows with increasing fluoride content in both glass series. However, the high coordinated aluminium may not be solely due to the formation of the Al−F−Ca(n) complexes. Glasses of the first series displayed systematic upfield shift of 29Si NMR resonance while substituting fluoride for oxide, starting from the fluorine free composition. This upfield shift is interpreted as the lack of cations in the network, due to formation of the F−Ca(n), which drives silicon network to polymerize toward a higher Qn structure. Contrary to the first series, the 29Si NMR resonance remains constant for fluorine containing compositions of the second series but differs downfield from the initial anorthite glass. The latter is explained by the excess of cations in the network due to addition of the fluorite resulting in formation of NBO on the silicon. Binding of fluorine with silicon is considered negligible in these systems. Thus, fluorine and calcium both define the degree of network polymerization and are considered as a cause for the changes in silicon and aluminium networks.

Abstract: Formation processes and morphology of nanostructured glass ceramic materials containing finely dispersed lead sulfide crystalline phase has been studied by X-ray technique. The spatial ordering of PbS nanocrystals has been revealed which manifests itself by interference effects in X-ray scattering. We have ascertained that important morphology feature of the system is nearly monodisperse size distribution of nanocrystals.