The structure of oxide glasses studied by high-energy x-ray diffraction

The use of high‑energy x‑ray diffraction techniques with the latest generation synchrotron sources has created new approaches to study quantitatively the structure of noncrystalline materials. Recently, this technique has been combined with neutron diffraction at pulsed source to provide more detailed and reliable structural information not previously available. This article reviews and summarises recent results obtained from the high energy x‑ray diffraction on several oxide glasses, SiO2, B2O3 and PbSiO3, using bending magnet beamlines at SPring‑8. In particular, it addresses the structural models of the oxide glasses obtained by the reverse Monte Carlo (RMC) modelling technique using both the high energy x‑ray and neutron diffraction data.

Intrinsic strength and the structure of glass

Intrinsic strength is the strength of a glass without extrinsic flaws. Knowledge of the composition dependence of the intrinsic strength is important both technologically for the development of high strength glass compositions and theoretically as it may provide important structural information. Intrinsic strength is easy to define but difficult to measure experimentally. In this paper we discuss intrinsic (σ*) strength measured in normal laboratory conditions as well as the inert intrinsic (σ0*) strength. We define these terms in a rigorous manner and indicate how to gauge if they have been measured by developing a set of criteria. Based on these criteria, we tabulate those values of intrinsic strengths that have been measured to date.

Effect of the Ba/K ratio on structural, dielectric and energy storage properties of BaO–K2O–TiO2–P2O5 glass-ceramics

xBaO–(20−x)K2O–30TiO2–50P2O5 with (0≤x≤20 mol%) glasses were successfully elaborated by the melt quenching while their related glass-ceramics were developed by controlled crystallisation. Density and molar volume measurements, differential thermal analysis and Raman spectroscopy were carried out to examine the glassy structure, the results revealed that the addition of BaO increases the reticulation and reinforces the glass network by the creation of strengthened linkages. X-ray diffraction has identified the formation of MTi2(PO4)3 with M=(K, Ba0·5) in all the glass-ceramics (GC) and the precipitation of a secondary BaTiP2O8 phase when x increase beyond 10 mol%. The dielectric properties of the glass-ceramics were studied by impedance spectroscopy, it showed that the addition of BaO induces an enhancement of both thermal and frequency stability of the dielectric parameters (εr and tanδ). The glass-ceramic with 5 mol% of BaO GC-(x=5) presents the highest dielectric constant and the lowest dielectric loss. The P-E hysteresis loops were recorded at room temperature and the energy storage parameters of the glass-ceramics were determined. These parameters were significantly improved by the increase of the BaO content and the optimum parameters were obtained for GC-(x=5). The dielectric and energy storage parameters were discussed according to the structure data.

Generalised kinetics of overall phase transition useful for glass crystallisation when assuming non-isothermal conditions

The modelling of reaction kinetics is a fashionable subject of publications. We developed an analogue of the KJMA equation under non-isothermal conditions α(T)=1−exp(−(T/θ)N) that describes the dependence of degree of transformation α(T) at a constant rate, q, of heating with characteristic temperature θ(q) and power N, proportional to the Avrami parameter, n. This equation is valid even when the activation energy of the process is not constant. We demonstrate that reliable information about the activation energy is obtained when the experimental data are plotted in coordinates: logq (heating rate) against logTp (peak temperature).

XAFS studies of glass structure

X-ray absorption fine structure (XAFS) spectroscopy has become one of the most important techniques to characterise the local coordination of specific atomic species present in condensed matter. The possibility of investigating the structure of noncrystalline materials has made XAFS very attractive for the study of the short range structure of the glassy state. The XAFS acronym denotes the structure present above the x-ray absorption edges; such a structure is distinguished as x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS). XANES carries information on the atomic arrangement around the absorbing species: distances and bonding angles. XANES also provides information on the electronic states in the proximity of the conduction band: different chemical environments may be identified from known features in the spectrum. EXAFS provides information on the average distance and radial distribution function of the nearest neighbours of the absorbing species. In the last decade, the enormous progress of experimental techniques and the excellent quality of experimental spectra, consequent to the development of the synchrotron radiation facilities, have stimulated the evolution of the XAFS theory and analysis procedures. This paper is a survey on the theoretical development of XAFS and on the methods for determining the short range information in disordered materials. Applications of XAFS to some vitreous systems are reported.

Density of MgO–Al2O3–SiO2, MgO–CaO–SiO2 and MgO–CaO–Al2O3–SiO2 glasses and the structural role of MgO

Density and molar volume of MgO–Al2O3–SiO2, MgO–CaO–SiO2 and MgO–CaO–Al2O3–SiO2 glasses have been correlated with the structure by following the change in type and concentration of structural units with composition. It is assumed that for (MgO+CaO)≥Al2O3 all Al atoms form AlO4 tetrahedra. The type of modified units of SiO2 (Q3 and Q2) are determined by the ratio [(MgO+CaO)−Al2O3]/SiO2. In the case of (MgO+CaO)<Al2O3, there is a fraction of Al2O3 that forms units based on the AlO3/2 formula. The entire SiO2 content would be in the form of Q4 units. Any of the structural units has its own constant volume that is independent of the type of glass. The agreement between calculated and experimental density and molar volume strongly suggests that MgO has only a modifier role in the studied glasses. Formation of assumed MgO4, MgO5 and MgO6 polyhedra can be due to change in coordination of oxygen, rather being looked as structural units.

Corrosion resistance of CaO–Al2O3–SiO2 glasses used for flame spraying on concrete

Concrete components can severely suffer from corrosion during their service lifetime. To achieve higher durability, glass coatings can be applied onto the concrete surface for protection purposes. Current solutions for protection measures are either sticking thin glass panes onto the concrete or applying plastic layers using flame spraying. The aim of this work was to develop an optimised glass composition for protective layers, in order to be suitable for flame spraying on concrete. The main advantage of this process is the deposition of continuous and joint-free glass layers onto complex geometries. Dense glass layers provide a much higher corrosion resistance and lifetime compared to polymer layers. The challenge encountered by these glass coatings resides in the contact with strong acidic fluids on one side and with the alkaline concrete on the other. Therefore, a new glass appropriate for flame spraying has been developed. The composition of this glass is cheaper compared to established reference glass products while providing comparable alkaline resistance. As the acid resistance of this glass is not sufficient, in order to ensure the desired requirements of an adequate corrosion protection, a multilayer glass coating was developed, in which the first layer consists of the alkaline-resistant CaO–Al2O3–SiO2 glass and the second layer of an acid-resistant glass.

Letter to the Editor: The extraction of atomic co-ordination numbers from x-ray diffraction data for network glasses

It has been demonstrated that both the detailed electron distributions of the constituent atoms/ions and non-Gaussian Gaussian peaks in T(r) can seriously limit the accuracy of co-ordination numbers extracted from x-ray diffraction data, whether the fit is performed in real or reciprocal space. In general, however, it is preferable to perform the fit in real space, where differences in shape between the experimental and fitted peaks are much more obvious, together with the effect of any experimental uncertainties, as represented by the noise in the resulting correlation function in the region below the first true peak. Note, also, that in reciprocal space the effects of a non-Gaussian peak shape on the extrapolation of the sinusoidal envelope to zero Q, from a high-Q fit to the interference function, Qi(Q), apply equally to neutron diffraction data, similarly limiting the accuracy of the resulting co-ordination numbers.

Structure and glass transition temperatures of tellurite glasses

Tellurite glasses, made from the conditional glass former TeO2, show potential for use in optical applications. Alkali and alkaline earth tellurite glasses, along with single component, rapidly cooled, TeO2 are reported and studied here. Thermal properties of boron, potassium, lithium, sodium, rubidium, cesium, barium, and strontium tellurites were obtained via differential scanning calorimetry and related to structural changes observed using Raman spectroscopy. Additionally, coordination numbers of barium and strontium tellurites versus amount of modifier are also calculated. By understanding the thermal properties and coordination numbers of alkali and alkaline earth tellurites, the goal is to better elucidate the structure of amorphous TeO2.