Influence of Point Defects in a Surface Layer on the Strength Characteristics of Glasses

The influence of the nature and the concentration of defects in a surface layer on the radiation resistance and the microhardness of silicate glasses is studied by photoemission spectroscopy. The investigation is performed with two types of silicate glasses: the K8 optical glass irradiated with fast electrons and an industrial sheet glass with a thermally polished surface. It is established that the radiation resistance and microhardness of glasses are determined by the content of structural defects of a particular nature. The radiation resistance of the surface of K8 optical glass decreases with an increase in the concentration of radiation E ^– ₄-centers, which are representative of the density of band-tail localized electron states recharged by irradiation. The microhardness of the studied glasses with different treatment of their surface depends linearly on the number of defect centers at the nonbridging and radiation-damaged bridging bonds of the silicon–oxygen network.     

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.     

Radiation hardening of silica glass through fictive temperature reduction

The aim of this article was to report the effects of γ-radiation on type-I Infrasil silica glass with different fictive temperatures, T_f, for harsh environment applications. Radiation-induced attenuation in the visible range is found to be much lower in low fictive temperature samples. Photoluminescence experiments show that glasses with higher fictive temperatures have a higher nonbridging oxygen hole centers defect concentration generated by irradiation. In addition, electron paramagnetic resonance studies reveal higher E’ point defects, AlOHC, and hydrogen(II) defects in high T_f samples. In general, we find that the γ-radiation “hardness” of Infrasil301 silica glass becomes significantly higher with decreasing fictive temperature.     

Alkali silica and pozzolanic reactions in concrete. Part 1: Interpretation of published results and an hypothesis concerning the mechanism

This paper presents an hypothesis on the mechanism of alkali silica and pozzolanic reactions in concrete based on the competition mechanism of the kinetics of dissolution of silica and that of crystallisation of CSH. With the contact of siliceous particles with cement solution OH⁻, Ca⁺⁺, Na⁺, K⁺ are adsorbed on the silica surface. The adsorption of OH⁻ provokes the dissolution of silicium atom on the silica surface. Ca⁺⁺ is adsorbed more strongly so first it reacts with dissolving silica to form CSH. If the rate of crystallisation of CSH is higher than that of dissolution of silica, CSH can be formed on the surface of silica and dissolution stops. Otherwise, dissolution goes on and K⁺, Na⁺ can penetrate into the structure leading to the formation of alkali silicate gel. Expansion of concrete depends on the quantity and viscosity of alkali silicate gel produced, and on the mechanical strengths and porosity of concrete. Influences of different parameters like pH of the solution fineness, quantity and type of silica, temperature are also discussed.     

The role of alkali cations in zeolite synthesis from silicate solutions containing N,N,N-trimethyl-1-adamantammonium cations

Zeolite synthesis from aqueous N,N,N-trimethyl-1-adamantammonium (TMAA⁺)-alkali (Na⁺, K⁺, Rb⁺, and Cs⁺) silicate mixtures is studied using X-ray diffraction, elemental analysis, scanning electron microscopy, and²⁹Si magic angle spinning (MAS),¹H-¹³C cross-polarization (CP) MAS and¹H-²⁹Si CP MAS NMR spectroscopies. SSZ-24 forms in the presence of potassium cations, and SSZ-31 crystallizes in the presence of sodium cations. This is the first report of SSZ-31 synthesis from Na-TMAA silicate mixtures. Unknown silicates form in the presence of rubidium and cesium cations, whereas no crystalline material is observed in synthesis mixtures devoid of alkali cations. The alkali cations do not appear to serve as templates or void fillers during zeolite crystallization, nor do they stabilize soluble silicate anions which serve as building blocks during zeolite crystallization. Rather, the alkali cations appear to regulate the transformation of the amorphous synthesis gel into either crystalline zeolite or other silicate phases.     

Pulsed cathodoluminescence of two-alkali sodium potassium silicate glasses

Alkali silicate glasses of variable composition 22xNa₂O · 22(1?x)K₂O · 3CaO · 75SiO₂ with equimolecular replacement of sodium ions by potassium ions are investigated using pulsed cathodoluminescence. It is revealed that localized electronic states interact with vibrations of two types, namely, polarization vibrations of the silicon-oxygen network with the frequency v₀ = 820 cm^?1 and bending vibrations of the modifier sublattice. At low concentrations of one of the alkali components (x < 0.1), bending vibrations are observed at two frequencies. These frequencies coincide with those of the corresponding vibrations in one-alkali systems containing Na (530 cm^?1) and K (520 cm^?1). At higher concentrations (x in the range ～0.14–0.86), there occur bending vibrations of the cationic sublattice with a frequency of 420 cm^?1. This can be interpreted as a luminescence analog of two-alkali (mixed-alkali) effect.     

Effects of Difference in Ionic Radii on Chemical Ordering in Mixed‐Cation Silicate Glasses: Insights from Solid‐State 17O and 7Li NMR of Li–Ba Silicate Glasses

Understanding of the extent of cation disorder and its effect on the properties in glasses and melts is among the fundamental puzzles in glass sciences, materials sciences, physical chemistry, and geochemistry. Particularly, the nature of chemical ordering in mixed‐cation silicate glasses is not fully understood. The Li–Ba silicate glass with significant difference in the ionic radii of network‐modifying cations (~0.59 Å) is an ideal system for revealing unknown details of the effect of network modifiers on the extent of mixing and their contribution to the cation mobility. These glasses also find potential application as energy and battery materials. Here, we report the detailed atomic environments and the extent of cation mixing in Li–Ba silicate glasses with varying X_BaO [BaO/(Li₂O + BaO)] using high‐resolution solid‐state nuclear magnetic resonance (NMR) spectroscopy. The first ¹⁷O MAS and 3QMAS NMR spectra for Li–Ba silicate glasses reveal the well‐resolved peaks due to bridging oxygen (Si–O–Si) and those of the nonbridging oxygens including Li–O–Si and mixed {Li, Ba}–O–Si. The fraction of Li–O–Si decreases with an increase in X_BaO and is less than that predicted by a random Li–Ba distribution. The result demonstrates a nonrandom distribution of Li⁺ and Ba⁺ around NBOs characterized by a prevalence of the dissimilar Li–Ba pair. Considering the previously reported experimental results on chemical ordering in other mixed‐cation silicate glasses, the current results reveal a hierarchy in the degree of chemical order that increases with an increase in difference in ionic radius of the cation in the glasses [e.g., K–Mg (~0.66 Å) ≈Ba–Mg (~0.63 Å) ≈Li–Ba (~0.59 Å) > Na–Ba (~0.33 Å) > Na–Ca (~0.02 Å)]. The ⁷Li MAS NMR spectra of the Li–Ba silicate glasses show that the peak maximum increases with increasing X_BaO, suggesting that the average Li coordination number and thus Li–O distance decrease slightly with increasing X_BaO, potentially leading to an increased activation energy barrier for Li diffusion. Current experimental results confirm that the degree of chemical ordering due to a large difference in ionic radii controls the transport properties of the mixed‐cation silicate glasses.     

Structure and disorder in Pb-Na metasilicate (PbO:Na2O:2SiO2) glasses: A view from high-resolution 17O solid-state NMR

Knowledge of the structure of lead (Pb)-bearing silicate glasses, such as degree of polymerization and arrangement among cations, provides improved prospects for understanding their macroscopic properties. Despite the importance, the detailed disorder in Pb-bearing silicate glasses with varying composition (i.e., Pb/alkali content) has not been systematically explored. Here, we reveal the first unambiguous structural information of PbO-Na₂O-SiO₂ glasses with varying PbO content [i.e., X_PbO = PbO/(Na₂O + PbO)], which are the fundamental model system for multicomponent Pb-bearing glasses, using high-resolution ¹⁷O solid-state NMR. ¹⁷O NMR spectra clearly show the resolved multiple oxygen sites, such as Na-O-Si, Si-O-Si, and [Na,Pb]-O-Si. As X_PbO increases, the fraction of [Na,Pb]-O-Si peak increases markedly at the expanse of substantial reduction in the fraction of Na-O-Si/total NBO. This trend indicates the relative predominance of the dissimilar pairs around non-bridging oxygen (NBO) and, therefore, can be explained well with the pronounced chemical ordering among Na⁺ and Pb²⁺. These results confirm that Pb is primarily a network-modifier in the glasses studied here. Atomic environments around both NBO and BO are affected by the change in Na/Pb ratio, while topological disorder due to cation mixing around NBO is much more prominent in Pb endmember. The structural details of short-range configurations around oxygen in alkali Pb-silicate glasses provide atomistic insights for understanding the properties of Pb-bearing multicomponent silicate glasses.     

Degree of Polymerization of Aluminosilicate Glasses and Melts

This paper presents the results of analyzing the data available in the literature on the structure and properties of silicate glasses and melts that contain Ti⁴⁺, Al³⁺, and Fe³⁺ cations in addition to alkali and alkaline-earth cations. It is established that the aforementioned multivalent cations in glasses and melts have a coordination number of four and play the role of network-formers. Aluminosilicate glasses and melts with the mole fraction ratio Al₂O₃/M ₂(M)O = 1 are of special interest. For these glasses, the structure is considered to be completely polymerized and, contrary to traditional concepts, their properties depend on the concentration ratio Al₂O₃/SiO₂. Taking into account that the structure of aluminosilicate glasses involves unusual structural units (such as triclusters) and a certain number of nonbridging oxygen atoms, a formula is proposed for calculating the degree of polymerization. The proposed formula is used to calculate the degree of polymerization for a number of Na₂O · Al₂O₃ · mSiO₂ glasses and the CaO · Al₂O₃ · 2SiO₂ glass. It is demonstrated that the calculated degrees of polymerization correlate with the experimentally measured viscosities of the relevant melts.     

Hydration and reaction mechanisms on sodium silicate glass surfaces from molecular dynamics simulations with reactive force fields

Recent development of reactive force fields have enabled molecular dynamics simulations of interactions between silicate glasses and water at the atomistic scale. While multicomponent silicate glasses encompass a wide variety of compositions and properties, one common structural feature in these glasses is the combination of the network structure that is made up of silica tetrahedra linked through corner sharing interspersed with network modifiers like alkali and alkaline-earth ions that break up the Si–O–Si linkages by forming nonbridging oxygen. In reactions with water, ion exchange between alkali ions in the glass and proton or hydronium in the solution, as well as hydrolysis reaction of the Si–O–Si linkages and subsequent silanol formation, is observed and well documented. We have used a set of recently developed reactive force field to investigate the reactions between water and the surfaces of silica and sodium silicate glasses of different compositions for reactions up to 8 nanoseconds. Our results indicate sodium leaching into water and diffusion of water molecules up to 25 Å into the glass surface. We examined the structural and compositional changes inside the glass and around the diffused ions and use these to explain the rates of silanol formation at the surface. We also observed proton transport in the glass which has an indirect influence on the silanol formation rates. While the surface of the glass was rough to start with, it undergoes further modification into a hydrated gel-like structure in the glass for up to 5 Å in the higher alkali containing glasses. It was found that the leached sodium ions remain close to the interface and that fragments of silicate network from the surface is capable of dislodging from the bulk glass and enter the aqueous solution. These simulations thus provide insights into the formation and structure of an alteration layers commonly observed in multicomponent silicate glasses corroded in aqueous solutions.     

Stannous-Stannic Equilibrium in Molten Binary Alkali Silicate and Ternary Silicate Glasses

The stannous-stannic equilibrium in binary alkali silicate and ternary silicate glasses was studied by equilibrating glassmelts with air at 1400°C. The Sn²⁺-Sn⁴⁺ equilibrium shifts more toward the oxidized state with increasing ionic radii of the alkali ions or with increasing concentration of the alkali ions in the same series of glasses. The slope of the straight lines obtained on plotting log (Sn⁴⁺)/(Sn²⁺)( pO₂ )^n/2 vs mol% R₂O increased in the order Li→Na→K. In ternary silicate glasses having the base glass composition 20Na₂O·10RO·70SiO₂, the Sn²⁺-Sn⁴⁺ equilibrium shifts more toward the reduced state, with increasing bond strength between the divalent cations and the nonbridging oxygens. With increasing temperature, the equilibrium shifts more toward the reduced state.     

A Theoretical Study of the Half-Sandwich Silicon(II) Cation [(Me<Subscript>5</Subscript>C<Subscript>5</Subscript>)Si]<Superscript>+</Superscript> and of the Cationic Complex [(Me<Subscript>5</Subscript>C<Subscript>5</Subscript>)Si (DME)]<Superscript>+</Superscript>

Ab-initio calculations have been performed for the half-sandwich cation [(Me₅C₅)Si]⁺ and its DME complex [(Me₅C₅)Si(DME)]⁺. For these cations, the ground state energies, the complexation energy, the frontier orbitals, the vertical singlet-triplet excitation energies, and the natural atomic charges have been calculated. In both cations, the “lone-pair” at silicon does not represent the HOMO. The nature of the weak dative O → Si bond can best be described in terms of electrostatic and attractive dispersion interactions. The DME coordination destabilizes the cluster orbitals and slightly enhances the positive charge at silicon.     

The state of magnesium in silicate glasses and melts

The properties of silicate glasses and melts containing magnesium are analyzed in comparison with the properties of glasses and melts in which magnesium is replaced by aluminum. In particular, the properties of the glass and the melt of the diopside composition CaMgSi₂O₆ are analyzed in comparison with the properties of the glass and the melt of the anorthite composition CaAl₂Si₂O₈. It is demonstrated that the properties of aluminosilicate and magnesium silicate glasses and melts differ not so strongly as should be expected upon replacement of modifier ions by network-former ions. By using the parameters γ _n characterizing the cation field strength, it is shown that Mg²⁺ cations can fulfill both the function of network formers like Al³⁺ cations and the function of modifiers like Ca²⁺ cations. The degree of depolymerization of the glass and the melt of the composition CaMgSi₂O₆ is estimated to be 0.4–0.5 from the dependences of the change in the relative density (d − d ₀)/d at different pressures on the degree of depolymerization NBO/T (the ratio of the number of nonbridging oxygen atoms to the number of network-former cations) for silicate glasses and the dependence of the isothermal bulk modulus K _t on the quantity NBO/T for silicate melts.     

Full color white light,temperature self-monitor,and thermochromatic effect of Cu+ and Tm3+ codoped germanate glasses

Lighting sources with full-color visible output are widely preferred in practical applications. In addition, modern lighting sources also tend to be intelligentized, and the intelligentization asks for smart luminescence materials. In this work, we attempt to develop novel full-color emitting material with temperature sensing and thermochromatic ability. To this end, the Cu²⁺ is successfully reduced to Cu⁺ which is incorporated into the germanate glasses. The glasses are prepared via a melt-quenching technique using graphite powders as reducing reagent. The supper-broadening of the excitation and the emission spectra of Cu⁺ in the germanate glasses are observed. Full-color emission is realized by introducing Tm³⁺ as co-dopant to provide the blue component in the spectra. The energy transfer behavior between Cu⁺ and Tm³⁺ is investigated, and it is found that these two luminescence centers are independently existent without energy transfer between them. The chromatic properties of the Cu⁺/Tm³⁺ co-doped glasses are tuned by Tm³⁺ concentration and excitation wavelength. The temperature sensing based on the fluorescence intensity ratio technique is demonstrated, and a constant sensitivity for the temperature detection is obtained. Moreover the thermochromatic property is also investigated, and it is found that the studied Cu⁺/Tm³⁺-doped glasses exhibit excellent thermochromatic performance.     

Effects of alkali metal ion on imprinting GRIN microstructure in GeS2-Ga2S3-MCl (M=Na,K, Cs) glasses for visible to mid-infrared microgratings

Gradient refractive index (GRIN) micro-optics present unique opportunities for control of the chromatic properties, new degrees of freedom for optical design as well as the potential for use in new optical system applications. GRIN microgratings were imprinted in GeS₂-Ga₂S₃-MCl (M = Na, K, Cs) chalcohalide glasses by microthermal poling, and the effects of the type and concentration of alkali cations on their performance were investigated. Two effective imprinting formation regions of the GRIN microstructure based on the poling saturation voltage (U_s) and glass composition are observed at fixed poling time and temperature. The U_s increases from 140 to 750 and 2600 V in accordance with the activation energy (E_a) of alkali ions (Na⁺ to K⁺ and Cs⁺) increasing from 45.15 to 58.62 and 92.58 kJ/mol for studied samples. The saturated numbers of diffraction order (N_s) of the gratings in these samples are 7, 9 and 6, respectively, the highest number being provided by the K⁺-containing sample. This is in accordance with imprinting-induced phase differences (0.14λ, 0.19λ and 0.09λ) measured in the fabricated samples containing Na⁺, K⁺ and Cs⁺ ions. Furthermore, the U_s of samples decreases from 1500 to 300 V with four concentrations of K⁺ from 10 to 30%, associated with their E_a of K⁺ decreasing from 69.62 to 53.46 kJ/mol, while N_s increases from 8 to 14, which is attributed to the increase of the phase difference in the GRIN structures. The controllable GRIN microstructures are realized by adjusting the type and concentration of alkali cations in chalcohalide glasses, which is expected to drive the design of broadband GRIN microgratings.     

Regularities of Sol-Gel Processes in the Preparation of Polysilicates

The regularities of sol-gel processes in the preparation of potassium polysilicate based on silica hydrosol (Ludox AS-40) are studied. The potassium hydroxide–silica molar ratio (silicate modulus M = [SiO₂]/[Me₂O]) is set by introducing potassium hydroxide into the sol. The addition of alkali at the initial stage induces sol coagulation. The mechanism of alkali effect is considered taking into account the reactivity of silica (the polycondensation and depolymerization reactions) and the aggregative stability of the colloidal system.     

ON THE DETERMINATION OF THE CONSTITUTION OF GLASSES1

This method of determination of the constituents of glasses is entirely new in its way. It is derived from the author's experiments on the causes of the surface devitrify-cation of glasses. It is quite natural to produce crystals in glasses when heated several times at high temperatures. The glasses are super cooled liquids and are very viscous at low temperatures. But, when they are heated at higher temperatures, they attain low viscosities to produce crystals in them. In general, the chemical composition of the crystals produced (primary phase) in ordinary glasses without B₂O₂ is SiO₂. And for production of an elementary body of crystals of silica, there should be at least three neighboring molecules of silica moving within some ranges of speed. The motion of such molecules in glasses is set by two causes: the one is that quantity of energy given to the molecules by heating, and the other is that lowering of viscosity of the medium by heating. Heating glasses to certain temperatures is the exclusive cause of the crystal production of glasses. This has been already explained by many persons and no ambiguity has been left unsolved. The author has given another important cause for crystal production on glass surfaces and has described the interpretation. ³ 1 Received August 11, 1926.
From these views, the author has arbitrarily defined devitrify-cation in five degrees. They are seen in accompanying photographs. It has been determined that these degrees of devitrify-cation were entirely coincident with the quantities of dissolved silica in glasses. The experiments have been conducted for a series of glasses of alkali silicates and alkali lead silicates. From the results of these experiments, the author has determined the constitutions of those silicates in glasses to be meta-disilicates and the double compounds to consist of those meta-disilicates. As the result of the determination of the constituents of alkali silicates and of alkali lead silicate glasses, it is pointed out that the compositions of crucibles to melt glasses of different silica content, should be either rich Al₂O₃: or rich SiO₂ according to the dissolving power of glasses for silica.     

On the critical displacement of excited kinetic units in liquids and glasses

The critical displacement of an atom (a group of atoms) in inorganic glasses Δr _m, which corresponds to the maximum of the interatomic attractive force, is calculated using available data on the surface tension and elastic constants. It is found that the critical atomic displacement Δr _m is close in order of magnitude to the linear dimension of the activation volume of atomic excitation v _h ^1/3 for glasses in the As-S and Ge-As-S systems with a chain structure and is considerably less than the value of v _h ^1/3 for alkali silicate glasses and glasses in the Cd-As system with a structure involving ionic sublattices. A relationship for calculating the activation volume of the atomic excitation from data on the glass transition temperature and elastic constants is derived within the model of an excited state.     

Reduction‐Induced Inward Diffusion and Crystal Growth on the Surfaces of Iron‐Bearing Silicate Glasses

We investigate the sodium inward diffusion (i.e., sodium diffusion from surface toward interior) in iron containing alkaline‐earth silicate glasses under reducing conditions around T_g and the induced surface crystallization. The surface crystallization is caused by formation of a silicate‐gel layer first and then the growth of silica crystals on the glass surface. The type of alkaline‐earth cations has a strong impact on both the glass transition and the surface crystallization. In the Mg‐containing glass, a quartz layer forms on the glass surface. This could be attributed to the fact that Mg²⁺ ions have stronger bonds to oxygen and lower coordination number (4–5) than Ca²⁺, Sr²⁺, and Ba²⁺ ions. In contrast, a cristobalite layer forms in Ca‐, Sr‐, and Ba‐containing glasses.     

Effect of Gamma Radiation on Optical and EPR Absorption Spectra of Phosphate and Fluoride Glasses Containing Lead

The induced optical and EPR absorption spectra of phosphate and fluoride glasses containing lead are investigated. It is revealed that exposure to gamma radiation leads to the formation of radiation-induced defects responsible for the induced absorption band with a maximum at 12500–13500 cm⁻¹ and the EPR signal in the form of an almost symmetric line with a g factor of 1.999 and a linewidth of ≈26 Oe. Analysis of the intensities of the absorption bands and the EPR signals in the spectra of glasses with low terbium, tin, and carbon contents and the study of their thermal bleaching demonstrate that the color centers are electron traps, whereas the paramagnetic centers are hole-trapping centers. Examination of the change in the parameters of the absorption bands in the spectra of glasses with different R ₂O contents (R = Na, K, Rb, Cs) makes it possible to determine the location of the color centers associated with the Pb⁺ ions in the structure. It is established that the glasses under investigation are characterized by the nonlinear absorption of radiation at a wavelength of 1.06 μm. The mechanism of formation of radiation-induced defects is considered. Original Russian Text Copyright ? 2005 by Fizika i Khimiya Stekla, Bocharova, Karapetyan.