Short-Range and Medium-Range Order in Lithium Silicate Glasses, Part I: Diffraction Experiments and Results

Silicate glasses of the composition (Li₂O) _x (SiO₂)_100−x ( x = 0, 20, 33.3, and 40) have been prepared and characterized by X-ray and neutron diffraction experiments using the technique of isotopic substitution. Thus, at each composition, a set of independent diffraction patterns allows the discussion of the short-range and medium-range structure. For the short-range structure, average coordination numbers and atomic-pair distances are obtained from the pairdistribution functions. For the medium-range structure, i.e., for atomic-pair distances >0.5 nm, an increase of the amplitudes of the oscillations in the pair-distribution function is observed with increases in the Li₂O content.     

Boson Peak and Medium-Range Order Structure of Alkali Borate Glasses

The boson peaks in the spectra of alkali borate glasses are investigated. The results obtained are interpreted within a model of Einstein oscillators localized at medium-range order clusters with hinged bonds. Inferences are drawn regarding the statistical characteristics and self-organization of medium-range order structures in glasses, their vibrational spectrum, and the relation to the parameters of alkali ions.     

29Si MAS-NMR Study of the Short-Range Order in Lithium Borosilicate Glasses

²⁹Si MAS-NMR measurements have been made on a series of lithium borosilicate glasses of general composition R Li₂O.B₂O₃· K SiO₂. At low alkali contents ( R < 1), the ²⁹Si resonance envelope is broadened and indicates a distribution of Si sites. As R increases above 1, the FWHM of the ²⁹Si resonance narrows considerably to that representative of a single chemical site. Simultaneously, the average chemical shift of the resonance shifts upfield in agreement with the trends found in the binary lithium silicate glass system. Using the chemical shifts for the individual Q species in the binary system it was found that very good agreement between the chemical shifts of the binary glasses and the ternary glasses examined here could be achieved if a model of proportional sharing of the added oxygen (from lithia) between silicate and borate units was used. In contrast to the ¹¹B NMR studies of these same glasses, the ²⁹Si NMR data are quantitatively best-fit if it is assumed that the proportional sharing of the oxygen from the added lithia begins at R = 0. Models of sharing developed from the ¹¹B NMR studies of these glasses, where proportional sharing above a certain fixed (independent of K ) or variable (dependent on K ) minimum R ₀, have been reexamined and were quantitatively shown through residual analysis to give consistently poorer fits to our data. At present the reasons for the discrepancy between the two sets of NMR data are unknown.     

Physical Properties and Structure of Silicate Glasses: I, Additive Relations in Alkali Binary Glasses

Correlations between certain physical properties, P , and all eighteen compositional ratios between the five ionic parameters of the system were studied with a computer for the five alkali silicates. Linear relations were found between P and the alkali/silica, alkali/oxygen, and silica/alkali ratios for certain regions of the glass field when the P's are expressed in proper molar terms. Specific volume and heat of formation data indicate three linear regions with two sharp break points between. The log electrical resistivity and the volatilization loss show two such regions and only the second break point. The break points, while structural in origin, must therefore occur for different reasons. The break points are the same for all P's and coincide with the eutectics. The constants of the linear relations are the partial molar quantities associated with P. Break points are not found in the molar refractivity data, so that no change in ligancy for these systems is indicated.     

A Study of the Short-Range Order in Multicomponent Glasses Manufactured from Mine Waste

The short-range order of multicomponent glasses in the diopside–orthoclase–apatite system with different contents of the initial components is investigated by X-ray diffraction upon heat treatments at different temperatures. It is found that the short-range order parameters change considerably when the apatite content in the melt exceeds 20 wt %. In glasses of this composition, heat treatment at certain temperatures leads to the precipitation of the crystalline apatite phase.     

Mechanical Relaxations in Mixed Alkali Silicate Glasses: II,Discussion*

The internal friction of mixed Li-Na, Li-K, Li-Rb, Li-Cs, Na-K, Na-Rb, Na-Cs, and K-Rb silicate glasses is interpreted on the basis of the various explanations proposed for the mixed alkali effect. A direct correlation was found between the mechanical loss peak due to the stress-induced movement of the alkali ions, i.e. the alkali peak, and the electrical loss properties. The large mechanical loss peak appearing when the alkalis are mixed was attributed to an interaction between dissimilar alkali ions such that an elastic dipole is formed. The absence of a comparable electrical loss peak was interpreted as indicating that the elastic dipole is electrically inactive. In glasses containing equimolar quantities of two alkalis, the activation energy and height of the new loss peak, i.e. the mixed peak, were more closely related to the size difference than to the mass difference of the alkali ions.     

Ligand-Field Spectra and Structure of Nickel in Silicate Glasses

The spectra of nickel dissolved in silicate and aluminosilicate glasses with alkali and alkaline-earth modifiers were studied systematically as functions of composition, annealing and temperature, concentration of Ni²⁺, and redox conditions during melting. Although reducing conditions produced a gray scattering phase, only Ni²⁺ was observed as a dissolved species. Several glasses contained one of two Ni²⁺ species which corresponded to previously proposed 4- and 6-coordinated species. Therefore, both octahedral and tetrahedral calculations could be made from directly observed bands free of overlapping. The results of these calculations suggest that the postulated tetrahedral species may instead be cubic (8-coordinated). In addition, mixing of the Ni²⁺³ T _{1 g} ( P ) level with Ni²⁺ 4 p and/or ligand electron transfer bands was observed. Examination of the conditions that could produce the 8-fold site on the basis of stereochemical and crystal-chemical principles indicated that the packing requirements of the alkali-oxygen, alkaline-earth-oxygen, and silicate polyhedra may preclude sites other than a large 6-fold site or an even larger cubic (8) site. Thus, silicate glasses can be considered to consist of highly structured but disordered atomic arrangements.     

Influence of Chemical and Phase Equilibria on the Medium-Range Order and Physicochemical Properties of Chalcogenide Glasses1

The problem associated with the influence of chemical and phase equilibria in melts on the properties of formed glasses is analyzed experimentally and theoretically by the example of chalcogenide glasses prone to glass formation. It is proved that inorganic glasses should be treated as microinhomogeneous systems formed by a set of nanometer-sized pseudophases that differ in character. The equivalentometry method has revealed that, in inorganic glasses, there exist at least two types of the pseudophases which have different chemical compositions and are responsible for microinhomogeneous structure of vitreous materials. It is established that the pseudophases in glasses are fragments composed of elementary substances and are also stable and metastable compounds. The concept of a variable number of components in vitrifying melts is introduced for the first time. This concept and the eutectoidal model of the vitreous state of matter allow one to reveal the types of chemical and phase equilibria whose freezing in glasses results in the formation of a set of the pseudophases responsible for glass microinhomogeneity. The regularities revealed make it possible to put forward the principles of analysis of composition–property diagrams for vitreous systems.     

Constitution of Mixed-Alkali Phosphate Glasses: II, Constitution of Variable Lithium Constant Sodium-Potassium Phosphate Glasses

In order to study the cationic influence of lithium, if any, the constitution of variable lithium constant sodium-potassium phosphate glasses was determined using paper chromatographic methods. It was possible to prepare glasses with very low ñ, the number average chain length value. Glasses with ñ values of 2, 1.75, and 1.5 could be prepared for glasses with Li:Na:K atomic ratios of 1:1:1 and 2:1:1. Attempts to prepare a glass with ñ= 1.25 were unsuccessful. At higher lithium concentrations, it was not possible to prepare glasses with low ñ values. The constitution of lithium phosphate glasses was reinvestigated. A comparison of the constitutions of mixed-cation phosphate glasses and lithium glasses indicates a small but definite cationic effect. The significance of these results in relation to Van Wazer's polymer theory and distribution laws is discussed.     

Experimental and Simulation Investigations of Mechanical Properties and Gamma Radiation Shielding of Lithium Cadmium Gadolinium Silicate Glasses Doped Erbium Ions

Silicon - Melt-quenching method used to fabricate SiO2-Li2O - CdO – Gd2O3 – Er2O3 glass system. The XRD diffractometer procedure is used to check the status of these samples. The...     

Mesostructure of Calcium Silicate Hydrate (C-S-H) Gels in Portland Cement Paste: Short-Range Ordering, Nanocrystallinity, and Local Compositional Order

Transmission electron microscopy studies of calcium silicate hydrate gels in portland cement were performed. Selected area electron diffraction studies revealed diffuse rings, indicating the presence of short-range ordering. Investigations of various regions within the gel found strong fluctuations in the diffuseness of the rings; corresponding local compositional probes showed the presence of large fluctuations in the local calcium:silicon ratio. Lattice imaging studies were then performed to determine the nature of the short-range ordering These investigations revealed nanocrystalline regions within an amorphous matrix. The local composition within the nanocrystalline regions is believed to be homogeneous. However, strong fluctuations in the local compositional order parameter between various regions would seemingly exist. Optical diffractograms taken from individual nanocrystalline regions demonstrated the coexistence of both jennite and 1.4-nm tobermorite structural elements, supporting the essential features of the Taylor nanophasic model.     

Microstructure of an industrial char by diffraction techniques and Reverse Monte Carlo modelling

The disordered microstructure of an industrial carbonaceous char has been characterised using X-ray, neutron and electron diffraction techniques to obtain pair correlation functions. Whilst consistency has been observed between all three techniques, electron diffraction has been found to have advantages by allowing data to be collected over a large range of scattering angles as well as without interference from crystalline regions within the char. Based on the experimentally obtained pair correlation functions, Hybrid Reverse Monte Carlo (HRMC) simulations have also been performed to generate experimentally consistent nanoscale models of the disordered carbonaceous char. The model char structures comprise disordered and buckled graphitic sheets containing a small percentage of interstitial diamond-like atoms between the sheets.     

Colors and Magnetic Properties of Iron in Glasses of Various Types and Their Implications Concerning Structure: I, High Alkali Silicate Glasses

The effect of increase of alkali and the replacement of soda by potash and lithia was studied in glasses containing iron and melted under strongly reducing conditions. An increase of alkali in these glasses causes an increase in the absorption throughout the visible spectrum; this is attributed, from magnetic and optical measurements, to Fe2+04 incorporated in the molecular network of the glass structure and having a very low magnetic susceptibility as found in some black spinels (hercynite).     

Internal Friction of Simple Alkali Silicate Glasses Containing Alkaline-Earth Oxides: II, Interpretation and Discussion

The results of internal friction measurements on simple three-component glasses that were reported in Part I of this paper are discussed. From a consideration of the basic equations governing internal friction and examination of the results of other investigators, a basis of interpretation is developed. It is shown that changes in peak-temperature position can be explained by changes in the activation energy of the process causing the peak. A change in peak height not accompanied by a change in the shape of the peak is shown to be due to a change in the concentration of the relaxation mechanisms. Using this information the changes in the peaks due to composition variations are explained on a structural basis. A mechanism is also suggested to explain the new high-temperature peak. A critical examination of the measurements on alkali-free and phosphate glasses leads to the conclusion that it may be necessary to re-evaluate some of the concepts concerning the low-temperature peak in alkali silicate glasses.     

The Impact of PbF2-Based Glasses on Radiation Shielding and Mechanical Concepts: An Extensive Theoretical and Monte Carlo Simulation Study

Journal of Inorganic and Organometallic Polymers and Materials - This work aimed to investigate the impact of Lead-fluoride based glasses via theoretical and simulation techniques on mechanical and...     

Interfacial shear strength studies using the single-filament-composite test. Part II: A probability model and Monte Carlo simulation

A Monte Carlo simulation is developed to provide a foundation for interpreting experimental data from the single-filament-composite test. The main focus is on developing an improved procedure for arriving at a realistic value for the shear strength of the fiber-matrix interface, but it is also shown how the test may be used to characterize the strength distribution of fibers at a length scale much shorter than is achievable in standard tension tests. The simulation is based on the widely used Poisson/Weibull probability model for fiber failure that characterizes the strength in terms of the random flaws distributed along the fiber. The primary mechanical model for stress buildup at the fiber end is the same as that assumed by most authors and assumes a constant interfacial shear stress in this shear transfer zone reminiscent of a yielding plastic matrix. We also, however, consider a bilinear model that allows for a zone of debonding with a constant shear stress lower than in the primary “plastic” zone. Simulation results are cast in terms of nondimensional variables and tabulated to allow for wide applicability. Sample size and confidence interval issues are also discussed.     

DSMC-LES方法数值模拟鼓泡流化床内气泡和颗粒流动行为

基于稠密气体分子动力学和气固两相流体动力学,建立流化床稠密气固两相离散颗粒运动-碰撞解耦模型,采用直接模拟蒙特卡罗方法(DSMC)模拟颗粒间的碰撞,采用考虑颗粒脉动流动对气相湍流流动影响的大涡模拟(LES)研究气相湍流,单颗粒运动满足牛顿第二定律,颗粒相和气相相间作用的双向耦合由牛顿第三定律确定.数值模拟流化床中颗粒流动以及气泡的生成、长大和破碎过程,获得颗粒轴向和径向速度的概率密度分布,及颗粒浓度分布.计算结果表明床内气泡的形成造成床内颗粒的循环,使得流化床内颗粒具有不同的轴向和径向脉动速度,颗粒分速度分布近似服从高斯分布.颗粒温度随颗粒浓度增加,达到最大值后,随颗粒浓度增大而下降.流化床颗粒浓度脉动主要是低频部分,高频分量较低,表明在流化床内颗粒浓度脉动频率低,能量高,颗粒浓度脉动主频率为0.4～1.0Hz,其值与Pain et al.获得的颗粒浓度脉动主频率基本吻合.     

Pulsed Cathodoluminescence and Vibrational Structure of Localized Electronic States in Alkali Silicate Glasses

The cathodoluminescence of localized electronic states (L intrinsic centers) in glasses of the composition (mol %) 22Me ₂O · 3CaO · 75SiO₂ and Me ₂O · 3SiO₂ (Me = Li, Na, K) is investigated upon excitation with a pulsed electron beam (180 keV, 700 A/cm², 2 ns). The luminescence spectra recorded in a pulsed periodic mode contain bands of L centers of two types whose occurrence reflects the formation of fragments with different degrees of atomic ordering in the microstructure of glasses. The line spectra with separations between the lines ₀ = 820 cm^-1 and ₁ = 520-640 cm^-1 are measured in a single-pulse mode. The effect revealed is attributed to the manifestation of the vibronic interactions during radiative relaxation of the triplet state of L centers. It is demonstrated that, according to the mechanism of cathodoluminescence, electronic excitations interact with local vibrations of nonbridging oxygen atoms and phonon modes of the glass network. The corresponding interactions are classified as vibronic(₀) and electron-phonon (₁) interactions.