Lead silicate glass structure: New insights from diffraction and modeling of probable lone pair locations

Structures of binary PbO-SiO₂ glasses have been studied in detail over the compositional range 35 to 80 mol% PbO using high-resolution neutron diffraction, high-energy X-ray diffraction, static ²⁰⁷Pb NMR, and structural modeling. The changes in the local environment of Pb(II) are subtle; it has a low coordination to oxygen (∼3 to 4) plus a stereochemically active electron lone pair and, thus, behaves as a glass network forming (or intermediate) cation over the entire composition range. This conclusion contradicts previous reports that Pb(II) is a network modifier at low concentrations, and is supported by an analysis of lead and alkaline earth silicate glass molar volumes. The Pb-O peak bond length shortens by 0.04 Å with increasing PbO content, indicating stronger, more covalent bonding, and consistent with an increase in the number of short (≤ 2.70 Å) Pb-O bonds, from 3.3 to 3.6. This is accompanied by increased axial symmetry of the Pb(II) sites, and is interpreted as a gradual transition toward square pyramidal [PbO₄] sites such as those found in crystalline PbO polymorphs. An attendant decrease in the periodicity associated with the first sharp diffraction peak (FSDP) toward that of β-PbO, accompanied by increases in the correlation lengths associated with the plumbite network (FSDP) and silicate anions (neutron prepeak), provides evidence of increased intermediate-range order and has implications for the glass forming limit imposed by crystallization. Pb(II) electron lone pairs occupy the natural voids within the silicate network at low PbO contents, while at high PbO contents they aggregate to create voids that form part of the plumbite network, analogous to the open channels in Pb₁₁Si₃O₁₇ and the layered structures of α- and β-PbO. Si-O and Pb-O bond lengths have been correlated with ²⁹Si and ²⁰⁷Pb NMR chemical shifts, respectively. This is the first time that such correlations have been demonstrated for glasses and attests to the accuracy with which pulsed neutron total scattering can measure average bond lengths.     

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.     

Second-harmonic generation and structural rearrangements in multicomponent antimonite glasses by electrothermal poling

Electrothermal poling is shown here to effectively induce second-order nonlinear effects in heavy-metal oxide antimonite glasses. In M₂O–PbO–WO₃–Sb₂O₃ (M = Li, Na, K) glasses, the poling-induced second-harmonic generation intensity is five times larger than in silica (Infrasil) for M = Na, twice as large as in silica for M = Li, and smaller than in silica for M = K. X-ray photoelectron spectroscopy suggests that antimony ions exist predominantly in the trivalent oxidation state in the studied glass samples. Raman and infrared spectroscopy confirm that the glass network is comprised of SbO₃, WO₄, WO₆, and PbO₄ units—with some SiO₄ moieties due to leaching from the silica crucible. The WO₄ units appear to exist in two distinct sites, as evidenced by comparison of the vibrational spectra of alkali–tungsten–antimonite glasses with those of previously reported crystalline tungstate phases. The alkali type influences the equilibrium between tetrahedral tungstate anions, [WO₄]²⁻, and the isomeric partially polymerized octahedral tungstate units, [WØ₄O₂]²⁻ (Ø denotes a bridging oxygen). Raman spectroscopy line scans were used to track near-surface structural changes on the anode side of poled glasses. They reveal that the tungstate equilibrium is also affected by poling. At the anode side, the population of partially polymerized [WØ₄O₂]²⁻ species increases at the expense of anionic [WO₄]²⁻ species. This yields a net increase in the average bond length of the network forming constituents, which is commensurate with poling-induced structural changes observed in other systems experimentally and computationally.     

Kinetics and Mechanism of the Interaction of Alkali Calcium Silicate Glasses with Salt Melts in the KNO3–Pb(NO3)2 System

The interaction of alkali calcium silicate glasses with salt melts in the KNO₃–Pb(NO₃)₂ system is investigated at temperatures of 420–520°C. The chemical composition of crystalline coatings formed upon treatment contains both components of the initial glass (SiO₂, 9–12 wt %; CaO, 0.8–1.2 wt %) and components of the salt melt (PbO, 82–89 wt %). The treatment temperature is the main factor affecting the structure of the modified surface layer. The mechanism of the interaction of alkali calcium silicate glasses with salt melts is analyzed. According to this mechanism, the interaction involves the ion exchange (with the participation of Na⁺, K⁺, Ca²⁺, and Pb²⁺ ions), crystallization of modified surface layers, and incorporation of Pb _x O _y nanoparticles (formed in the salt melt) into the coating structure.     

Effects of Al:Si and (Al + Na):Si ratios on the properties of the international simple glass,part II: Structure

High-alumina containing high-level waste (HLW) will be vitrified at the Waste Treatment Plant at the Hanford Site. The resulting glasses, high in alumina, will have distinct composition-structure-property (C-S-P) relationships compared to previously studied HLW glasses. These C-S-P relationships determine the processability and product durability of glasses and therefore must be understood. The main purpose of this study is to understand the detailed structural changes caused by Al:Si and (Al + Na):Si substitutions in a simplified nuclear waste model glass (ISG, international simple glass) by combining experimental structural characterizations and molecular dynamics (MD) simulations. The structures of these two series of glasses were characterized by neutron total scattering and ²⁷Al, ²³Na, ²⁹Si, and ¹¹B solid-state nuclear magnetic resonance (NMR) spectroscopy. Additionally, MD simulations were used to generate atomistic structural models of the borosilicate glasses and simulation results were validated by the experimental structural data. Short-range (eg, bond distance, coordination number, etc) and medium-range (eg, oxygen speciation, network connectivity, polyhedral linkages) structural features of the borosilicate glasses were systematically investigated as a function of the degree of substitution. The results show that bond distance and coordination number of the cation-oxygen pairs are relatively insensitive to Al:Si and (Al + Na):Si substitutions with the exception of the B-O pair. Additionally, the Al:Si substitution results in an increase in tri-bridging oxygen species, whereas (Al + Na):Si substitution creates nonbridging oxygen species. Charge compensator preferences were found for Si-[NBO] (Na⁺), ^[3]B-[NBO] (Na⁺), ^[4]B (mostly Ca²⁺), ^[4]Al (nearly equally split Na⁺ and Ca²⁺), and ^[6]Zr (mostly Ca²⁺). The network former-BO-network former linkages preferences were also tabulated; Si-O-Al and Al-O-Al were preferred at the expense of lower Si-O-^[3]B and ^[3]B-O-^[3]B linkages. These results provide insights on the structural origins of property changes such as glass-transition temperature caused by the substitutions, providing a basis for future improvements of theoretical and computer simulation models.     

Structural origin of the weak germanate anomaly in lead germanate glass properties

Binary PbO–GeO₂ glasses have been studied in detail from 5 to 75 mol% PbO using high-resolution neutron diffraction, high-energy X-ray diffraction, 207-Pb NMR, pycnometry, and thermal analysis. The Ge–O coordination number displays a broad maximum n_GeO = 4.14(3) close to 27 mol% PbO. This is smaller than the maximum n_GeO = 4.3 reported in CaO–GeO₂ glasses but occurs at a similar composition. This structural behavior appears to explain the relatively weak germanate anomaly manifest in lead germanate glasses, for example as a maximum in the measured atom number density and a plateau in the glass transition temperatures. The structural role of Pb(II) is complex. On the one hand, short covalent Pb–O bonds and small Pb–O coordination numbers of ∼3 to 4 indicate glass network former character for Pb(II), associated with a stereochemically active electron lone pair. On the other hand, the presence of some GeO₅ or GeO₆ units, in addition to the majority GeO₄ tetrahedral species, indicates some modifier character of Pb(II) at low PbO contents, giving rise to the observed weak germanate anomaly, as well as elongation and enhanced ionicity of the Pb–O bonds. Overall, the observed structural behavior of Pb(II) in lead germanate glasses appears as intermediate between that observed in lead silicate and lead borate glasses. Despite rapid quenching, at low PbO contents, the glasses studied exhibited nanoscale heterogeneity, evidenced by small-angle X-ray scattering consistent with the early stages of spinodal decomposition.     

Luminescence of modified nonbridging oxygen hole centers in silica and alkali silicate glasses

This paper reports on the results of the investigation of the cathodoluminescence spectra of silica and alkali silicate glasses upon excitation with a pulsed electron beam (energy, 180 keV; current density, 700 A/cm²; pulse duration, 2 ns). The luminescence band observed in the energy range 2.4–2.6 eV is assigned to modified structural defects of the ≡Si-O·/Me ⁺ type. These defects are revealed under high-density electronic excitation and, unlike the known L centers in alkali silicate glasses, are interpreted as a variety of nonbridging oxygen hole centers (defects of the dangling bond type) subjected to a disturbing action of the nearest neighbor alkali metal cations. The cathodoluminescence of similar centers is observed in neutron-irradiated silica glasses with lithium impurities; alkali silicate glasses with Li, Na, and K cations; and glasses in the two-alkali Na-K systems. It is established that the energy of the radiative transition of a modified nonbridging oxygen hole center, namely, ≡Si-O·/Me ⁺, depends on the alkali cation type.     

Processes in solid state at anodic oxidation of a lead electrode in H2SO4 solution and their dependence on the oxide structure and properties

During anodic oxidation of the lead electrode in H₂SO₄ solution in PbO potential region an anodic deposit is formed containing a dense layer of tet-PbO and a porous layer of PbSO₄. The rate of oxidation of Pb to tet-PbO is determined from the transport of O^2? ions through the tet-PbO layer by a vacancy mechanism.On illumination of the electrode with white light photoelectrochemical processes proceed in the anodic layer leading to the transformation of tet-PbO into PbO_n where (1 < n < 2). PbO_n is a solid electrolyte with semiconductor properties. The processes of photoelectrochemical oxidation of tet-PbO to αPbO₂ are discussed on the basis of the band energy scheme of semiconductors. These photoelectrochemical investigations show that at oxidation of tet-PbO to αPbO₂ in solid state the highest energy barrier is the band gap.In the PbO₂ potential region the PbO_n transforms into a semiconductor with electrone conductivity. The anodic corrosion of the Pb electrode in this region is discussed on the basis of semiconductor properties of the oxide. The corrosion proceeds in two stages. During the first Pb is oxidized to tet-PbO and at the second stage tet-PbO is oxidized in solid state to αPbO₂. For the second process the highest energy barrier is the band gap. Its overcoming is realized by surface states forming at the oxide/solution interface during the reactions of oxygen evolution. These are O^? radicals and O atoms. They penetrate into the oxide layer by an oxygen vacancy mechanism and oxidize the Pb to tet-PbO as well as the tet-PbO to αPbO₂. The processes in solid state in the PbO_n layer are favoured by the layer crystal structure and by the similar shape of the unit cells of tet-PbO and αPbO₂.     

Dependence of the phase composition of the anodic layer on oxygen evolution and anodic corrosion of lead electrode in lead dioxide potential region

At potentials more positive than 1300 than 1300 mV with respect to a Hg/Hg₂SO₄ electrode the partial currents of lead corrosion and oxygen evolution in 7_n H₂SO₄ follow the Tafel's dependence. X-ray investigations and wet analyses of the anodic layer show that it consists of tet-PbO and α PbO₂. Besides, at potentials more negative than 1530 mV, β PbO₂ forms at the oxide/solution interface.It is established that lead anodic corrosion proceeds in two stages. During the first Pb is oxidized to tet-PbO. During the second stage tet-PbP is oxidized to PbO₂. If this process is performed in solid state, α PbO₂ forms. If PbO is dissolved and then oxidized, β PbO₂ crystals are formed. The oxidation of tet-PbO to α PbO₂ proceeds at different rates in the bulk of the oxide and at its surface with the solution. At potentials more positive than 1530 mV the oxidation of tet-PbO to α PbO₂ at the surface of the oxide with the solution leads to the dropping off of part of the oxide layer.The oxidation of Pb and PbO is carried out under the action of O atoms and O^? radicals which evolve at the oxide/solution interface and penetrating the oxide reach the metal/oxide surface.     

The role of MgO on the structural properties of CaO–Na2O(MgO)–P2O5–CaF2–SiO2 derived glass ceramics

The effect of increasing MgO/Na₂O replacements (on mole basis) on the crystallization characteristics of glasses based on the CaO–Na₂O(MgO)–P₂O₅–CaF₂–SiO₂ system were studied by using DTA, XRD, and SEM. The crystallization characteristics of the glasses, the type of crystalline phases formed and the resulting microstructure were investigated. The main crystalline phases formed after controlled heat-treatment of the base glass were diopside, wollastonite solid solution, fluoroapatite and sodium calcium silicate phases. The increase of MgO at the expense of Na₂O led to decrease the amount of sodium calcium silicate phase. The Vicker's microhardness values (5837–3362 MPa) of the resulting glass–ceramics were markedly improved by increasing the MgO-content in the glasses. The obtained data were correlated to the nature and concentration of the crystalline phases formed and the resulting microstructure.     

EQCM study of electrodeposited PbO2: Investigation of the gel formation and discharge mechanisms

Lead dioxide thin films were electrodeposited on gold substrates and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mass change occurring upon immersion in a H₂SO₄ electrolyte and during electrochemical reduction was observed in situ by electrochemical quartz crystal microbalance (EQCM). A hydrated PbO₂ gel-type layer is formed at the surface of electrodeposited PbO₂. The concentration of the H₂SO₄ electrolyte does not affect the composition of the gel nor the amount of lead dioxide involved in the hydration process. It is established that 1.3 × 10⁻⁷ mol cm⁻² of β-PbO₂ are hydrated at the surface of an electrodeposited film and that the hydration reaction occurs according to the following reaction: PbO₂^(crystal) + xH₂O ↔ (PbO(OH)₂·(x − 1)H₂O)^(gel), where x = 8.1. The mass change occurring during the first and subsequent discharge of PbO₂ was recorded. It is shown that both PbO₂^(crystal) and PbO(OH)₂·(7.1)H₂O)^(gel) are reduced to PbSO₄ during the first discharge.     

Evaluation of novel composition (SiO2–CaO–Na2O–P2O5, SrO–CuO) degradable amorphous silicate glasses properties and comprehensive characterization of the thermal features

The current work presents and discusses the findings of a comprehensive study on the structural, chemical and thermal properties of SrO and CuO incorporated SiO₂–CaO–Na₂O–P₂O₅ amorphous silicate glass with a novel composition. Here, fundamental features (experimental density, oxygen density, and hardness) of all glasses were determined and chemical as well as phase composition of the glasses was verified with XRF and XRD, respectively. Moreover, the thermal behavior (viscos flow and crystallization kinetics) of amorphous silicate glass was investigated by non-isothermal methods using DTA analysis. The activation energies of glass transition (E_g) were calculated in the range of 546–1115 kJ/mol by Kissinger method, whereas the activation energies of crystallization (E_c) were calculated in the range of 164–270 kJ/mol by three different methods (Kissinger, Ozawa, Yinnon and Uhlmann). Avrami exponent (n) values ranged from 1.17 to 3.28 demonstrated that amorphous silicate glasses have different crystallization mechanism. Working temperature, which is one of the parameters indicating glass stability, increased with the incorporation of Sr and Cu from 187 °C to 245 °C. The initial dissolution measurement has been applied to study the degradability behavior of Sr and Cu incorporated amorphous glasses in vitro. Quantitative evaluation of Si⁴⁺ (0.156–0.373 kV), Ca²⁺ (0.043–0.332 kV), Na⁺ (0.044–0.329 kV), P⁵⁺ (0.057–0.289 kV), Sr²⁺ (0.134–0.385 kV), and Cu²⁺ (0.090–0.203 kV) depending on the ion activation energy (E_a-ion) and ion concentration at different temperature values (24, 37 and 55 °C) was performed in contact with Tris-HCl solution by ICP-OES analysis. The results revealed that investigated glasses were degradable and incorporation of Sr and Cu affected the glass initial dissolution. Overall, investigated glasses are suitable for various application such as hot-working production, glass-ceramic manufacturing, and glass or glass-ceramic scaffolds fabrication, due to wide working temperature ranges and high crystallization tendencies of the developed glasses.     

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.     

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.     

On the Glass Formation in the Na2O–SiO2–H2O System

The glass formation in the Na₂O–SiO₂–H₂O system is considered in the framework of the free volume theory of glasses. The boundaries of the glass transition range in the given system are determined from the dependences of the specific volume and the degree of connectivity on the water content in the system. The differences in the structure of hydrated glasses prepared by different methods are revealed. It is demonstrated that the molecular packing coefficients of the structures formed by drying of sodium silicate solutions are smaller than those of glasses prepared by hydration of anhydrous glasses. The densities are determined and the molar volumes are calculated for hydrated glasses in the Na₂O–SiO₂–H₂O system with a water content of 12–23 wt % (33–51 mol %) at a constant molar ratio SiO₂ : Na₂O = 2.8. The correlation relationship for the partial molar volume of water as a component of hydrated glasses is proposed as a function of the water content in the system. The use of this dependence allows one to calculate the molar volumes (densities) of hydrated glasses with different water contents.     

Influence of Al2O3/SiO2 ratio in multicomponent LNAS glasses and glass-ceramics on the crystallization behavior,microstructure and mechanical performance

Transparent glass-ceramics containing eucryptite and nepheline crystalline phases were prepared from alkali (Li, Na) aluminosilicate glasses with various mole substitutions of Al₂O₃ for SiO₂. The relationships between glass network structure and crystallization behavior of Li₂O–Na₂O–Al₂O₃–SiO₂ (LNAS) glasses were investigated. It was found that the crystallization of the eucryptite and nepheline in LNAS glasses significantly depended on the concentration of Al₂O₃. LNAS glasses with the addition of Al₂O₃ from 16 to 18 mol% exhibited increasing Q⁴ (mAl) structural units confirmed by NMR and Raman spectroscopy, which promoted the formation of eucryptite and nepheline crystalline phases. With the Al₂O₃ content increasing to 19–20 mol%, the formation of highly disordered (Li, Na)₃PO₄ phase which can serve as nucleation sites was inhibited and the crystallization mechanism of glass became surface crystallization. Glass-ceramics containing 18 mol% Al₂O₃ showed high transparency ～84% at 550 nm. Moreover, the microhardness, elastic modulus and fracture toughness are 8.56 GPa, 95.7 GPa and 0.78 MPa m^1/2 respectively. The transparent glass-ceramics with good mechanical properties show high potential in the applications of protective cover of displays.     

Optical Properties and Effect of Gamma Irradiation on Bismuth Silicate Glasses Containing SrO, BaO or PbO

Optical and FT Infrared spectroscopic measurements have been utilized to investigate and characterize binary bismuth silicate glass together with derived samples by replacements of parts of the Bi₂O₃ by SrO, BaO, or PbO. This study aims to justify and compare the spectral and shielding behavior of the studied glasses containing heavy metal ions towards gamma irradiation. The study also aims to measure or calculate the optical energy band gap of these glasses. The replacements of parts of Bi₂O₃ by SrO, BaO or PbO caused some changes within the optical and infrared absorption spectra due to the different housing positions and physical properties of the respective divalent Sr2+, Ba2+, Pb2+ ions. The stability of both the optical and infrared spectra of the studied bismuth silicate glass and related samples towards gamma irradiation confirm some shielding behavior of the studied glasses and their suitability as radiation shielding candidates.     

Environmental influence of high Na2O additions on microstructure and ligand field parameters of Cr3+ inside borosilicate glass

A glass system based on the Na₂O/B₂O₃-doped CrO₃ borosilicate has been prepared by the melt quenching technique. The structure, color, optical absorbance and ligand field parameters were investigated for a wide range of Na₂O additives (20–60 mol%). All X-ray photoelectron spectroscopy (XPS) profiles were used to study the chemical shift states of the glass-constituting elements. Fourier transform infrared (FTIR) analyses explored the internal structure and subnetwork units. Furthermore, from the FTIR results, we concluded the transformation of trigonal borate units (BO₃) to tetrahedral borate units (BO₄) and the possibility of transformation from B₃-O-Si linkages to B₄-O-Si linkages. Despite the fixed CrO₃ content, the doped glasses showed a color transition from green to yellow with additional Na₂O content. The increased intensity of the band at 451–427 nm and the decreased intensity of the band at 619–627 nm are the main reasons for this color transformation. The optical absorption spectra confirmed the existence of Cr³⁺ and Cr⁶⁺ states. A decreasing behavior for the crystal field splitting (10Dq) and an increasing behavior for Racah parameter (B) were obtained with further Na₂O additives. The decreasing behavior of 10Dq was attributed to reduced oxygen concentrations with more Na₂O/B₂O₃ substitutions. The increasing behavior of B reflects the tendency of the bond between the Cr cations and their oxygen ligands towards an ionic nature. Moreover, the Dq/B values indicated that Cr³⁺ cations are in high-field positions for the glass sample containing 20 mol% Na₂O, and Cr³⁺ cations are in intermediate field positions for the glass sample containing 30 mol% Na₂O. However, for the glass samples doped with 40, 50 and 60 mol% Na₂O glass samples, Cr³⁺-cations are in weak field positions. These results of (Dq/B) recommend the glass sample doped with 20 mol% Na₂O for tunable laser applications.     

An ultrasonic study on ternary xPbO–(45-x)CuO–55B2O3 glasses

A series of xPbO–(45-x)CuO–55B₂O₃ glasses (5 ≤ x ≥ 40 mol %) were prepared by the melt-quenching technique. The X-ray diffraction (XRD) patterns of the prepared glasses are found to have amorphous structure. An extensive ultrasonic study has been made to explore the structural role of PbO and CuO in the borate network. Various elastic properties were calculated from the measured data of density and ultrasonic velocity. Ultrasonic velocity and elastic moduli revealed broad humps at about 20 mol % PbO, which are attributed to the borate anomaly. Below 20 mol % PbO, all Pb²⁺ ions are considered to be entering the borate network as a glass modifier. This results in the transforms the borate network from an open structure to a denser three-dimensional structure due to BO₃ → BO₄ conversion. Beyond 20 mol, addition of PbO results in the formation of metaborate, pyroborate, and orthoborate units with NBOs. This weakness the glass structure and decrease both ultrasonic velocity and elastic moduli. The elastic properties were predicted and quantitatively analyzed by taking into account the effect of boron coordination number on the compositional and structural parameters involved in Makishima–Mackenzie's theory, ring deformation model and bond compression model. An excellent agreement between the computed theoretical and experimental elastic moduli, micro-harness and Poisson's ratio was achieved for majority of samples.     

The dissolution behavior in alkaline solutions of a borosilicate glass with and without P2O5

There are a variety of applications for glasses in alkaline environments, including glass fibers and glass‐coated steel to reinforce concrete structures. To understand how a simple glass reacts in such environments, the dissolution behavior of a 25Na₂O–25B₂O₃–50SiO₂ (mol%) glass, doped with and without 3 mol% P₂O₅, in pH 12 KOH and pH 12 KOH saturated with Ca²⁺ ions was studied. Ca²⁺ ions in the solution significantly reduce the glass dissolution rate by forming a passivating calcium silicate hydrate (C–S–H) gel layer on the glass surface. When these corroded glasses were then exposed to Ca‐free KOH, the C–S–H layer redissolves into the undersaturated solution and the glass dissolution rate increases. For phosphate‐doped borosilicate glass, PO₄^3? units released from the dissolving glass react with Ca²⁺ ions in saturated solutions to form crystalline hydroxylapatite on the glass surface, but this layer does not protect the glass from corrosion as well as the C–S–H does. The nature of the C–S–H layer was characterized by Raman spectroscopy, which reveals a gel layer constituted mainly of silicate anions.