Surface compositional studies of heat reduced lead silicate glass

SIMS and AES compositional analysis of the surface layer for a lead silicate glass reduced at three different temperatures was carried out and the depth distribution for some components to a depth of 120 nm was obtained. It was found that by increasing the reduction temperature the layer located just below the surface becomes rich in silicon oxides, a potassium accumulation takes place on the surface and the depth distribution of the other chemical components exhibits a maximum, the location of which depends on the reduction temperature.     

Charge trapping and release in electron and gamma irradiated lead silicate glasses

The thermally stimulated release of charge in electrically polarized and electron and gamma irradiated glasses containing 30 to 50 mol% PbO was investigated between 290 and 450 K. The release of trapped change in irradiated glasses produced a thermally stimulated current peak which had the same activation energy as the depolarization current peak in electrically polarized specimens. The irradiation induced dielectric breakdown of these glasses was correlated with the quantity of trapped charge and the relaxation time for charge release determined from the thermally stimulated depolarization current.     

Mechanism of deleading of silicate glass by 0.5 N HNO3

Mechanism of removal of lead from silicate glass containing 68.5 wt% PbO by 0.5 N HNO₃ was investigated by incorporation of the chemical-analyses/weight-loss data into shrinking-core model (SCM) and minimization of the difference. Scanning electron microscopy (SEM), emission spectrometry with inductively coupled plasma (ICP), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDS) were used to determine the compositional changes of the lead-silicate glass (LSG) samples. Dual inter-diffusion chemical reaction mechanisms having respective activation energies of 83.49 and 47.80 kJ/mol dominated the deleading process.     

Changes in surface morphology of silicate glass induced by fast electron irradiation

Binary potassium-silicate glass was irradiated with a defocused electron beam. During the irradiation the alkali ions migrate from the surface into the depth and alkali ions depleted layer is created near the surface. Such changes in the chemical composition are also accompanied with changes of the glass structure and finally result in the volume changes of the irradiated glass. This was directly studied using atomic force microscope (AFM). A series of exposures with energy of electrons of 7–50 keV and with different doses were performed. For low doses the irradiated glass is continuously depressed with the increasing dose, indicating this way the structural changes leading to the volume compaction. It is suggested the compaction is caused by the relaxation processes of the silica subnetwork. A further increase of the electron dose causes a formation of the small bump inside the center of the depression. The bump arises with the dose and finally exceeds the surrounding surface. It is suggested that the expansion is connected with the migration of alkali ions and the formation of Si–O–O–Si bonds which result in the formation of new rings with new space requirements.     

Molecular dynamics study of the influence of mobile cations on the reconstruction of an irradiated silicate glass

The effect of ballistic collisions in a simplified nuclear glass was investigated by molecular dynamics. Systematic results were obtained in the 0–16 keV energy range. Following a damage peak, reconstruction of the glass structure was observed in terms of the overall degree of polymerization. The reconstruction was facilitated by the presence of mobile cations. The dynamics of restoration of the SiO₄ tetrahedrons during displacement cascades and during the formation of a structure from a random configuration can be fitted to curves corresponding to the same analytic formula. This similarity allowed us to examine the influence of mobile cations (alkalis or alkaline earths) on the formation of SiO₄ entities in smaller systems formed from random configurations. The formation rate of SiO₄ tetrahedrons accelerates with the Na₂O or CaO percentage to reach a maximum rate above about 10 mol% Na₂O or CaO. This threshold corresponds to the disappearance in the glass structure of zones comprising a central bridging oxygen surrounded by two first-neighbor bridging oxygen rings. No mixed alkali effect was observed in systems containing both Na and Cs because the formation of SiO₄ does not require long-range diffusion of mobile cations.     

Effect of electron irradiation on Na-K silicate glass investigated using X-ray photoelectron spectroscopy and pattern recognition method

Alkali silicate glasses and melts play an important role in material science. Electron interaction with glasses is important for radioactive waste deposition, where electrons of various energies lead to irreversible changes. These changes are caused mainly by ionization and ballistic interaction of electrons with atoms, introducing structural disorder, changes in atomic composition and chemical state, accompanied by alkali ions diffusion. The Na-K silicate glass (5Na₂O · 10K₂O · 85SiO₂), pristine and electron irradiated (doses from 25 C m⁻² to 20 236 C m⁻²) are investigated using X-ray photoelectron spectroscopy (XPS) and the pattern recognition (PR) and fitting procedures. Changes of composition and chemical state of atoms dependent on electron dose are analyzed. At low doses (100-300 C m⁻²), decrease followed by increase of O and Si concentrations was observed. Surface segregation, probable desorption, and in-bulk diffusion of K and Na ions (doses of about 50 C m⁻² and 2000 C m⁻², respectively) were observed. This was accompanied by changes in the chemical state of K atom, where with an electron dose increasing content of elemental K form accompanied by decreasing potassium peroxide form were observed. No difference in chemical state of Si and O atoms was visible under electron irradiation dose to 20 236 C m⁻², within the sensitivity of the applied method.     

Gas phase nitridation of silicate fiber glass materials with ammonia:Three case studies

Three types of the leached fiber glass materials of silicate origin modified with Zr, Al and rare earth metals (REM) were nitridated with ammonia at temperatures ranging from 673 to 973 K. DRIFTS (Diffuse Reflectance Infrared Fourier-Transform Spectroscopy) and H/D exchange using ND₃ were applied for characterization of the formed NH_x species. Along with nitridation of silanol groups their dehydroxilation as well as hydrolysis of NH_x species take place. In situ DRIFTS study showed that the limiting steps are slow chemical reactions, but not a diffusion of NH₃ or H₂O molecules in the bulk of glasses. The concept on strained siloxane bridges formed due to dehydroxylation of two adjacent silanol groups was used for explanation of the reaction kinetics features.     

Low energy and low dose electron irradiation of potassium–lime–silicate glass investigated by XPS. I. Surface composition

X-ray induced photoelectron spectroscopy has been used to study the influence of low-energy electron beam on the pristine potassium–lime–silicate glass surface prepared by fracturing in situ under ultrahigh vacuum. Relatively low-energy electron beam of 1600 eV with low-electron beam current density of 0.02–0.22 A/m² and low-electron dose of 29–5200 C/m² was used. The expected modified near-surface region thickness is in this case comparable with the mean sampling depth of the analytical tool. Therefore, possible changes and modifications due to electron irradiation could be recorded with high sensitivity. The freshly fractured glass surface was found to be significantly enriched with potassium, and slightly with calcium. As a consequence of the lowest electron dose irradiation used, the potassium signal substantially increased by a factor 1.24 relative to the value found for the fresh surface. For higher doses used, the potassium signal continuously increased with the dose to a maximum and decreased thereafter. This variation was accompanied with the qualitative opposite behaviour of calcium signal. The concentrations of the other elements present in the glass, oxygen and silicon, varied only slightly with the electron dose. They can be considered to be constant within experimental uncertainty. In agreement with experimental results, a model assuming mobility of only two most mobile cations, potassium and calcium, was suggested. The models assuming one layer and two layers on the bulk were developed. Their results reproduce well experimental findings: (i) the formation of a potassium-rich surface layer, and (ii) the opposite-like signal variation of calcium in comparison with potassium.     

Correlation between the 2.7 eV and 4.3 eV photoluminescence bands in silica glass

Our previous studies have reported the excitation energy dependence of the 2.7 and 4.3 eV photoluminescence (PL) bands in oxygen deficient silica glass at low temperature (∼20 K). An oxygen vacancy (O₃SiSiO₃) was thought to be the origin of the two PL bands. In order to verify the origin of the 2.7 and 4.3 eV PL bands in silica glass, we measured the PL band of various thermally heat treated silica glasses. In the sample after heat treatment, we did not observe the 4.3 eV PL band, though we did observe the 2.7 eV PL band. These results suggest that these two PL bands do not have a common origin.     

Phase evolution on heat treatment of sodium silicate water glass

Heat treatment of sodium silicate water glass of the nominal composition Na₂O/SiO₂ = 1:3 was carried out from 100 °C up to 800 °C and the advancement of the resulting phases was followed up by powder X-ray diffraction, scanning electron microscopy and thermogravimetry along with differential thermal analysis. The water glass, initially being an amorphous solid, starts to form crystals of β-Na₂Si₂O₅ at about 400 °C and crystallizes the SiO₂ modification cristobalite at about 600 °C that coexists along with β-Na₂Si₂O₅ up to 700 °C. At 750 °C Na₆Si₈O₁₉ appears as a separate phase and beyond 800 °C, the system turns into a liquid.     

Crystallization kinetics of 1Na2O · 2CaO · 3SiO2 · glass monitored by electrical conductivity measurements

We investigated the kinetics of crystal nucleation, growth, and overall crystallization of a glass with composition close to the stoichiometric 1Na₂O · 2CaO · 3SiO₂. The nucleation and subsequent growth of sodium-rich crystals in this glass decreases the sodium content in the glassy matrix, drastically hindering further nucleation and growth. Compositional changes of the crystals and glassy matrix at different stages of the crystallization process were determined by EDS. These compositional variations were also monitored by electrical conductivity measurements, carried out by impedance spectroscopy, in glassy, partially, and fully crystallized samples. The electrical conductivity of both crystalline and glassy phases decreases with the increase of the crystallized volume fraction. Starting at a crystallized volume fraction of about 0.5, the crystalline phase dominates the electrical conductivity of the sample. This behavior was corroborated by an analysis of the activation energy for conduction. We show that electrical conductivity is highly sensitive and can indicate compositional shifts, changes in the spatial distribution of mobile ions in the glassy matrix. Conductivity measurements are thus a powerful tool for the investigation of complex heterogeneous systems, such as partially crystallized glasses and glass-ceramics.     

Memory effect in the glass transition region of silicate glass based on light scattering data

The experimental results of the changes of the visible light scattered intensity after temperature jumps in the glass transition region of silicate glass were presented. The influence of the thermal prehistory on the height and location of the scattered intensity maximum was studied. The conditions corresponding to the maximum value of the scattered intensity were determined. The non-monotone character of the dependence of the limit height of the maximum as a function of the stabilization temperature was obtained. It was shown that the relaxation times of the increasing of the intensity considerably exceeded the structural relaxation times. These features were similar to the results obtained for the phosphate glass earlier. The universal character of the non-linear coupling of the laser irradiation with the glass structure formed in the process of development of the maximum intensity was found.     

Properties and structure of (50 − x)BaO–xZnO–50P2O5 glasses

The thermal, mechanical, chemical properties and the structure of (50 − x)BaO–xZnO–50P₂O₅ (0 ⩽ x ⩽ 50 mol%) glasses were investigated. For these glasses, the density (ρ), glass transition temperature (T_g), dissolution rate (DR), ³¹P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra and Fourier-transformed infrared (FTIR) spectra were determined. As BaO was replaced by ZnO, all the properties were similarly decreased in density, Young’s modulus, T_g and water resistance. FTIR analyses revealed a shortening of phosphate chains by the shift of (P–O–P)_as band to a higher wave number owing to the substitution ZnO of BaO. The NMR spectra showed that the replacement of BaO by ZnO decreased the concentration of Q²-tetrahedral sites and increased that of Q¹-tetrahedral sites.     

Effect of organic acid vapors on the alteration of soda silicate glass

Organic acids were previously shown to be involved in the alteration of historic soda silicate glasses in humid atmospheres under museum storage conditions. The present study investigates the role of these pollutants on the visual, compositional and structural modification of soda silicate glasses. Replica glasses aged in humid or humid/acidic atmospheres under accelerated conditions were examined and compared using light microscopy, electron microprobe and Raman spectroscopy. The characteristic modifications induced by each atmosphere are described. In the acid polluted atmosphere the leaching process created a layer that retained the transparency of the glass but with a chemical structure hydrated and more polymerized following the loss of alkali and the associated non-bridging oxygens, and the formation of new bridging bonds. In an unpolluted humid atmosphere, the dissolution process caused disruption of the silicate network at the glass surface and formation of an opaque gel layer. The hydrated silicate species and the cations in this gel layer subsequently polymerized to form a new amorphous material, hydrated and more depolymerized than the original glass. This investigation confirms that organic acid pollutants are responsible for the modifications observed on altered historic soda silicate glasses in the collections of the National Museums of Scotland.     

Structure-property correlation in (50 − x)Na2O–50P2O5–xCoCl2 glassy systems

Ternary sodium–cobalt–phosphate glasses of the composition (50 − x)Na₂O–50P₂O₅–xCoCl₂ with x varying between 0 and 15 mol% prepared by melt quenching have been characterized by Fourier transform infrared (FT-IR) spectroscopy and X-ray diffraction (XRD) techniques. Thermal (T_g, T_c) and electrical properties have been investigated. Infrared spectra reveal the formation of metaphosphate glasses (Q² tetrahedral units) with symmetric bridging oxygen (P–O–P) and non-bridging oxygen (P–O⁻). The spectra also indicate the formation of P–O–Co bonds in the metaphosphate glasses that replace P–O⁻–Na⁺ bonds. The results of thermal studies correlate with these FT-IR findings and support the formation of P–O–Co bonds and an increased cross-link density with increasing CoCl₂. This results in enhanced chemical durability and increased T_g and T_c of the glasses. The electrical conductivity parameters upon changing the composition have been correlated with structural changes in the glass matrix.     

Effect of preparation procedure on redox states of iron in soda-lime silicate glass

The redox state of iron in soda-lime silicate glass was determined by analysis of the optical absorption bands due to Fe²⁺ and Fe³⁺ states. When raw materials containing Fe₂O₃ were heated gradually to 1400 °C, the total-Fe content of the glass was 9.4% Fe²⁺, but rapid heating at 1400 °C increased the Fe²⁺ content to 11.7%. The oxygen activity (aO₂) in the corresponding melts was measured using zirconia and Pt electrodes. The value increased with increasing temperature in the gradually heated sample and reached log(aO₂) = 0.03 at 1400 °C, but was about 2.5 times lower in the rapidly heated sample at log(aO₂) = ?0.37. After SnO addition to the raw material, oxygen activity depended strongly on heating speed: log(aO₂) at 1400 °C fell as low as ?1.8 with rapid temperature increase but was about ?0.2 or higher with gradual heating. The Fe²⁺ content of the cooled glass was consistent with the oxygen activity of the melts. The effect of heating speed was attributed to the formation of a melt layer on the surface of the raw material.     

Vacuum ultraviolet excitation of the 2.7 eV emission band in neutron irradiated silica

The temperature dependence of the photoluminescence induced at 2.7 eV by ultraviolet (UV) and vacuum ultraviolet (VUV) excitation of neutron irradiated (10²¹ n/m² and 10²² n/m²) KU1 and KS-4V high purity silica, with different OH content, have been studied. Commercial silica Infrasil 301 has also been studied for comparison. At the highest neutron fluence and at the same temperature, the three irradiated silica grades show similar excitation spectra. Two close UV excitation bands, which show opposite temperature dependence, are observed at 4.8 and 5.1 eV. The 4.8 eV band, related to the triplet–singlet transition in SiODCs(II), decreases on decreasing temperature from 300 to 10 K and the band at 5.1 eV, probably related to SiODCs(I), is observed only at very low temperatures (～10 K). An important VUV excitation structure, observed at low temperature, could also be related to SiODCs(I). A shift of the irradiated bands is detected at low temperature.     

White light emission from Eu3 +/Tb3 +/Tm3 + triply-doped aluminoborate glass excited by UV light

The Eu^3 +/Tb^3 +/Tm^3 + triply-doped glasses with the composition of CaO―Al₂O₃―B₂O₃―RE₂O₃ (RE = Eu,Tb,Tm) have been synthesized by melt quenching method. The photoluminescence of these Eu^3 +/Tb^3 +/Tm^3 + triply-doped glasses (CaAlB:RE^3 +) were studied and the emission spectra combining with blue, green and reddish orange bands were observed. Under 360 nm wavelength excitation the white light emission is achieved when the concentration (x) of Tm^3 + in Ca_0.931 ?xAlB:Eu^3 +_0.038,Tb^3 +_0.031,Tm^3 +_x glass is in the range of 0.0013-0.011 per mol matrix. In addition, the energy transfer (ET) between Tb^3 + and Eu^3 + ions in Eu^3 +/Tb^3 +/Tm^3 + triply-doped glasses was validated and the electric dipole–dipole interaction is responsible for the ET process of Tb^3 + → Eu^3 + at low concentrations. Hence, the Eu^3 +/Tb^3 +/Tm^3 + triply-doped aluminoborate glass could be a potential candidate for white LEDs.