Network Formers Research Articles

3D laser ablation-ICP-mass spectrometry mapping for the study of surface layer phenomena – a case study for weathered glass

In this work, a multi-elemental 3D laser ablation-ICP-mass spectrometry mapping procedure for high-resolution depth information retrieval to investigate surface layer phenomena is presented. The procedure is based on laser drilling on a virtual grid on the surface, followed by extraction of depth maps along the z-axis (for each element monitored). Using a burst of 50 laser pulses at 1 Hz on each point of the grid, a penetration rate of ca. 150 nm per pulse (in glass) was obtained and a lateral resolution in the order of the laser beam diameter. By ultrafast ICP-MS monitoring of individual ablation pulses (58 ms for a set of 19 elements) using a laser ablation cell with fast signal washout (less than 0.5 s for whole laser pulse), the corresponding peak areas could be consistently integrated, resulting in spatial elemental data associated with individual pulses. The usual laser drilling limitations such as pulse mixing and signal tailing are avoided with this approach. After manipulation of the spatial elemental datasets and quantification, stacks of 50 2D depth maps (for each element monitored) were produced which could be visualized as volume images or time-lapse movies. As a proof of concept, this approach was successfully used to investigate the degradation mechanisms of a medieval, weathered glass artifact by colocalization analysis of selected cross-sectional 2D elemental images in arbitrary planes of the volume images. It was shown that degradation must have started as a result of dealkalinization leading to depletion of alkalis/earth alkalis in glass surface layers and enrichment of network formers (Si and Al), and subsequent worsening by cracking and formation of corrosion pits and so-called spatiotemporal Liesegang rings indicative of radial leaching.

Raman Spectroscopy, X-Ray, SEM, and DTA Analysis of Alkali-Phosphate Glasses ContainingWO3and Nb2O5

New phosphate glasses in the quaternary system (50-x) A2O-xWO3-10 Nb2O5-40 P2O5, withx= 0; 30 and A = Li or Na were prepared by the melt quenching method. The effect on the crystallization behaviour of the glass due to the introduction of WO3into the glass composition and, consequently, the diminishing of the molar amount of the alkaline oxide and the decreasing of the molar ratio between network modifiers and network formers (M/F) was studied. The prepared glasses were heat-treated in air, at 550°C, 600°C, and 650°C for 4 hours. The structure, of the obtained samples, was studied by differential thermal analysis (DTA), X-ray powder diffraction (XRD), and Raman spectroscopy and the morphology by scanning electron microscopy (SEM). It was found that the replacement of Li2O or Na2O by WO3reduces the number of the crystallised phases. In the lithium-niobiophosphate glasses, the presence of WO3promotes the formation of NbOPO4instead of the LiNbO3phase and reduces the formation of ortho- and pyro-phosphate phases. The thermal treatments affect the arrangements of the network structure of the AW40-glasses.

CO2 solubility in primitive martian basalts similar to Yamato 980459, the effect of composition on CO2 solubility of basalts, and the evolution of the martian atmosphere

To determine the influence of basalt composition on the CO2 solubility in martian lavas, we investigated experimentally a synthetic melt based on the martian meteorite Yamato 980459 (Y 980459), an olivine-phyric shergottite and a picritic rock (19 wt% MgO) thought to be a near-primary liquid derived from high-temperature (>1540 °C) partial melting of the martian mantle. Experiments were performed in a piston-cylinder apparatus at 1–2 GPa and 1600–1650 °C. CO2 contents in quenched glasses were determined using Fourier transform infrared spectroscopy (FTIR) and range from 0.45–1.26 wt%. Despite large differences in FeO* and MgO contents, the CO2 solubilities in Y 980459 are similar to that in a less primitive synthetic martian basalt based on the Humphrey rock and to a Hawaiian tholeiite. The lack of enhanced solubility in Fe2+- and Mg2+-rich melts is likely owing to the complex structural role of these cations in silicate melts, acting partly as network formers, rather than network modifiers. The small sensitivity of CO2 solubility to compositional variations among martian and tholeiitic basalts means that the experimentally determined solubilities may be applicable to a wide spectrum of martian magmatic products. Using experimentally determined CO2 solubilities of Y 980459 and Humphrey allows the calibration of the thermodynamic parameters governing dissolution of CO2 vapor as carbonate in martian basalts. This relation facilitates calculation of the CO2 dissolved in magmas derived from graphite-saturated martian basalt source regions as a function of P , T , and f O2. The hot conditions in the source of Y 980459, 1540 ± 10 °C, and 1.2 ± 0.5 GPa, are plausible for plume-related magmas forming the giant Tharsis volcanic complex, which accounts for 50% of martian igneous activity since stabilization of the primordial crust. If oxygen fugacity in the sources of hot Tharsis magmatism were equivalent to that at the iron-wustite buffer (IW) or 1 log unit above (IW+1), respectively, then the entire Tharsis event would outgas 30–300 mbars of CO2 to the martian atmosphere, which is far from the 2 bars required to stabilize an equable climate in the late Noachian and early Hesperian epochs. This mismatch could be reconciled if significant martian igneous activity derived from comparatively oxidized mantle sources (i.e., IW+2) similar to those responsible for the nakhlite meteorites.

Electrical conductivity and local structure of lithium tin iron vanadate glass

A relationship between electrical conductivity (σ) and local structure of 15Li2O·10Fe2O3·xSnO2·(70–x)V2O5·5P2O5 glass (x = 0–20 mol%), abbreviated as xLFSVP glass, was investigated by 57Fe- and 119Sn-Mossbauer spectroscopies, differential thermal analysis (DTA) and dc-four probe method. A small increase in quadrupole splitting (Δ) for FeIII was observed from 0.70 to 0.74± 0.02 mm s − 1 with an increase of “x”, whereas isomer shift (Δ) values of 0.40±0.01 mm s − 1 were independent of “x”. This result suggests that local distortion of FeIIIO4 tetrahedra was slightly increased in SnO2-containing vanadate glasses, which was reflected as an increase in glass transition temperature (Tg) from 266 to 285±5 °C. A slope of 675 K / (mm s − 1) obtained in ‘Tg vs. Δ plot’ proved that FeIII occupied the site of network former (NWF). An isothermal annealing of 10LFSVP glass at 500 °C for 100 min resulted in a marked decrease of Δ from 0.72 to 0.56±0.02 mm s − 1, indicating that local distortion of FeO4 tetrahedra was reduced by the structural relaxation of 3D-network. In contrast, identical δ and Δ values of 0.07±0.01 and 0.53±0.02 mms − 1, respectively, were observed in 119Sn-Mossbauer spectra of 10LFSVP glass before and after the annealing. These results indicate that SnIVO6 octahedra are loosely bound in the glass matrix as a network modifier (NWM). A marked increase in σ from 7.4 × 10 − 7 to 9.1 × 10 − 3 S cm − 1 was observed in 20LFSVP glass after the isothermal annealing, indicating that structural relaxation of 3D-network evidently causes a marked increase in σ.

Thermodynamic investigation of the CaO–Al2O3–SiO2 system at high P and T through polymer chemistry and convex-hull techniques

The system CaO–Al2O3–SiO2 (CAS) is explored in the pressure–temperature (P, T) range 0–2GPa and 1000–3000K with the aim of defining the complex topology of the liquidus surface at various isobaric conditions and assessing the role of P on the stability fields and melting behavior of the various solids nucleating in the system. Calculations are carried out by coupling a generalized polymeric approach that reduces the system to two interacting sub‐lattices (Network Formers and Network Modifiers; NF, NM) with an improved and generalized convex-hull procedure that conforms mathematically the equipotential loci at the various T, P conditions. The thermodynamic model operates through a Toop's asymmetric deconvolution (interactions within the NM sub‐lattice unaffected by NF; interactions within the NF sub‐lattice affected by NM) of the bulk Gibbs free energy of mixing. Mixing energies (chemical and strain) are calculated with a polymeric model where the individual properties of the oxides composing the NF and NM matrixes are determined by their Lux–Flood acid–base properties and weighted on the basis of their electrical equivalent fractions. The convex-hull procedure locates points on the liquidus by lifted Delaunay triangulation. The isobaric liquidus at P=1bar (105Pa) is in reasonable agreement with the experimental observations. As far as we know isobaric sections at higher P conditions based on calculus have never been produced in literature. Our results indicate that the primary phase fields of anorthite and gehlenite shrink progressively with increasing P and a primary phase field of grossular appears at high P predating on the fields of the neighboring phases (gehlenite, rankinite, anorthite and wollastonite). Increasing P also causes the progressive disappearance of the liquid miscibility gap at high SiO2 content. Moreover the congruent melting of anorthite becomes incongruent. The fields of rankinite (Ca3Si2O7), tri-calcium aluminate (Ca3Al2O6) and grossite (CaAl4O7) disappear at P≥1GPa.

The chemical durability of glass and graphite–glass composite doped with cesium oxide

The role of temperature in determining the chemical stability of a waste form, as well as its leach rate, is very complex. This is because the dissolution kinetics is dependent both on temperature and possibility of different rate-controlling mechanisms that appear at different temperature regions. The chemical durability of Alumina-Borosilicate Glass (ABG) and Glass–Graphite Composite (GGC), bearing Tristructural Isotropic (TRISO) fuel particles impregnated with cesium oxide, were compared using a static leach test. The purpose of this study is to examine the chemical durability of glass–graphite composite to encapsulate coated fuel particles, and as a possible alternative for recycling of irradiated graphite. The test was based on the ASTM C1220-98 methodology, where the leaching condition was set at a temperature varying from 298K to 363K for 28days. The release of cesium from ABG was in the permissible limit and followed the Arrhenius’s law of a surface controlled reaction; its activation energy (Ea) was 65.6±0.5kJ/mol. Similar values of Ea were obtained for Boron (64.3±0.5) and Silicon (69.6±0.5kJ/mol) as the main glass network formers. In contrast, the dissolution mechanism of cesium from GGC was a rapid release, with increasing temperature, and the activation energy of Cs (91.0±5kJ/mol) did not follow any model related to carbon kinetic dissolution in water. Microstructure analysis confirmed the formation of Crystobalite SiO2 as a gel layer and Cs+1 valence state on the ABG surface.

Molecular dynamics simulations of Y in silicate melts and implications for trace element partitioning

Element partitioning depends strongly on composition and structure of the involved phases. In this study, we use molecular dynamics simulations to investigate the local environment of Y as an exemplary trace element in four silicate melts with different compositions and thus varying degrees of polymerization. Based on these structural results, we propose a mechanism which explains the observed partitioning trends of Y and other rare-earth elements between crystals and melts or between two melts. With our computational approach, we found a systematic correlation between melt composition and Y coordination as well as Y―O bond lengths, a result which was corroborated by EXAFS spectroscopy on glasses with the same compositions as the simulated melts. Our simulations revealed, moreover, the affinity of Y for network modifiers as second-nearest neighbors (Ca in this study) and the tendency to avoid network formers (Si and Al). This is consistent with the observation that Y (and other rare-earth elements) in general prefer depolymerized to polymerized melts in partitioning experiments (see, e.g., Schmidt et al. (2006)). Furthermore, we used the method of thermodynamic integration to calculate the Gibbs free energy which governs Y partitioning between two exemplary melts. These more quantitative results, too, are in line with the observed partitioning trends.

Impact of soda-lime borosilicate glass composition on water penetration and water structure at the first time of alteration

In this study, the impact of soda-lime borosilicate glass composition and particularly the effect of charge compensators such Ca and Na and, of network formers such Si and Zr, on water penetration and water structure at the first time of alteration were investigated. Two non-destructive techniques were combined: the Fourier transform infrared spectroscopy in attenuated total reflection geometry to precise the predominant alteration mechanisms and assess the water structure in altered zone and the grazing incidence X-ray reflectometry to determine the thickness of the altered glass zone allowing to calculate the water diffusion coefficients through the glasses. The results of glass alteration at pH=3 and 30°C have shown that hydrolysis was the predominant mechanism after few seconds for glass having a high amount of non-binding oxygen. For the other glasses, which for the diffusion was the limiting reaction, the calculated water diffusion coefficients were comprised between 10−21 and 10−19m².s−1 and vary as a function of glass composition. An activation energy of 76.9kJ.mol−1 was calculated and appears to be higher than inert gas diffusion through the glass highlighting that water molecules strongly interact with the glass matrix.

Preparation of sol–gel based nano-structured hybrid coatings: effects of combined precursor’s mixtures on coatings morphological and mechanical properties

Nanostructured hybrid clearcoats were produced using sol–gel based organic/inorganic precursors. To this end, clearcoats containing mixtures of a network former (tetraethyl ortosilicate; TEOS) and a network modifier (methacryloxy propyltrimethoxysilane; MEMO) and their hydrolyzed forms were prepared. Scanning electron microscope and atomic force microscope were utilized in order to investigate the morphology of different sol–gel prepared films. The mechanical properties of the bulk and surface of the clearcoats were studied by dynamic mechanical thermal analysis and nanoindentation techniques. The scratch resistance of the hybrid clearcoats was also studied by a laboratory carwash simulator. Decrease in glass transition temperature (Tg), cross-linking density, hardness and elastic modulus were seen for the clearcoat composed of hydrolyzed network modifier combined with hydrolyzed and non-hydrolyzed network former. However, a reverse effect was seen when the hydrolyzed TEOS and non-hydrolyzed MEMO was used. No improvement was observed using hydrolyzed MEMO in the clearcoat composition. Generally, results revealed improved mechanical properties of the clearcoat when the hydrolyzed network former was used together with non-hydrolyzed MEMO. Greater phase separation and silica cluster formation were seen for the clearcoats loaded with hydrolyzed MEMO compared with hydrolyzed TEOS.

Crystal growth in non-isochemical, highly viscous liquids and percolation theory

In terms of percolation theory, the glass structure can be described as an arbitrarily formed network with rigid and floppy regions. It is already known, that the maximum size of a floppy region can be calculated from the concentration of network formers and network modifiers. By contrast to previous studies, the crystallization mechanism in subsequently annealed glasses is described. It was assumed that nucleation in highly viscous liquids, i.e. slightly above the glass transition temperature, takes place if the size of a floppy region is at least as large as the size of the critical nucleus. Once nucleation has occurred, the crystals start to grow. Considering a non-isochemical system, certain components are enriched near the growing crystal during crystallization. It is stated, that the liquid near the crystal will first have a floppy structure enabling instantaneous crystal growth to some extent. During crystal growth, the network connectivity will increase until rigidity is reached in the shell and the residual components are shoved away by the growing crystal.

The role of lead oxide on structural and physical properties of lithium diborate glasses

Pseudo-binary (100−x)Li2B4O7–xPb3O4, with x=0–70mol% PbO have been prepared and their properties investigated. The glass transition temperature, density and molar volume have been determined as a function of composition. The values of Tg and the molar volume decrease non-linearly while the density increases as the Pb3O4 content is raised. Infrared spectra of the glasses reveal that a strong network consisting of diborate units breaks up by the addition of Pb3O4. The absorption bands below 620cm−1 show that PbO is one of the network formers of the glasses 70⩾Pb3O4⩾10; as they can be associated with vibrations of (PbO4)2− grouping. PbO plays a dual role in the glass network. The calculated values of N4 [the fraction of borons which are tetrahedral] slightly decrease with PbO content up to 30mol% and then increase with Pb3O4 content up to 50mol%, then followed by a decrease as the Pb3O4 content rises further. The Vickers hardness of the glasses varies as a function of the PbO content in the same manner as the variation of N4. The dc conductivity decreases with the Pb3O4 concentration up to about 30mol% and then increases thereafter.

Quantification of Short and Medium Range Order in Mixed Network Former Glasses of the System GeO2–NaPO3: A Combined NMR and X-ray Photoelectron Spectroscopy Study

Glasses in the system xGeO2–(1–x)NaPO3 (0 ≤ x ≤ 0.50) were prepared by conventional melting–quenching and characterized by thermal analysis, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and 31P nuclear magnetic resonance (MAS NMR) techniques. The deconvolution of the latter spectra was aided by homonuclear J-resolved and refocused INADEQUATE techniques. The combined analyses of 31P MAS NMR and O-1s XPS lineshapes, taking charge and mass balance considerations into account, yield the detailed quantitative speciations of the phosphorus, germanium, and oxygen atoms and their respective connectivities. An internally consistent description is possible without invoking the formation of higher-coordinated germanium species in these glasses, in agreement with experimental evidence in the literature. The structure can be regarded, to a first approximation, as a network consisting of P(2) and P(3) tetrahedra linked via four-coordinate germanium. As implied by the appearance of P(3) units, there is a moderate extent of network modifier sharing between phosphate and germanate network formers, as expressed by the formal melt reaction P(2) + Ge(4) → P(3) + Ge(3). The equilibrium constant of this reaction is estimated as K = 0.52 ± 0.11, indicating a preferential attraction of network modifier by the phosphorus component. These conclusions are qualitatively supported by Raman spectroscopy as well as 31P{23Na} and 31P{23Na} rotational echo double resonance (REDOR) NMR results. The combined interpretation of O-1s XPS and 31P MAS NMR spectra shows further that there are clear deviations from a random connectivity scenario: heteroatomic P–O–Ge linkages are favored over homoatomic P–O–P and Ge–O–Ge linkages.

A model to calculate the viscosity of silicate melts

Abstract Our recently developed model for the viscosity of borosilicate melts is extended to describe and predict the viscosities of melts containing alkali oxides. In addition to the two model parameters that are required for each B2O3–MO x melt, where MO x is a basic oxide, three more parameters are needed when MO x is an alkali oxide to account for the formation of clusters near the tetraborate composition. The additional parameters represent the size and Gibbs energy of formation of these clusters and their contribution to the activation energy of the viscous flow. A general algorithm for the calculation of the viscosity is presented which summarizes the application of the viscosity model to melts that can contain two network formers, SiO2 and B2O3, any basic oxide and amphoteric oxides exhibiting the Charge Compensation Effect such as Al2O3. The predictive ability of the model is tested on all ternary subsystems of the B2O3–Na2O–K2O–CaO–MgO–PbO–ZnO–Al2O3–SiO2 system containing both an alkali oxide and B2O3 for which experimental data are available and on several multicomponent glass-forming melts around commercial glass compositions.

Preparation and characterization of a titanium bonding porcelain

The titanium bonding porcelain was synthesized through normal melting-derived route using borate–silicate system. The porcelain was characterized by thermal expansion, X-ray diffraction, scanning electron microscope and cytotoxicity tests. The results of X-ray diffraction showed that the main phase of the bonding porcelain was SnO2. The SnO2 microcrystals precipitated from the glass matrix when the SnO2 content was increased. The thermal expansion coefficient of bonding porcelains decreased with the increasing concentration of SiO2. The thermal expansion coefficient of bonding porcelains first decreased slightly with the increasing of B2O3 concentration (from 0wt% to 10wt%) and then increased to about 9.4×10−6/°C(from 10wt% to 12wt%). As an intermediate, B2O3 can act as both network formers and modifiers, depending on the relationship between the concentration of basic oxides and intermediates. The Vickers hardness of bonding porcelains increased with the increase of SnO2 concentration. When SnO2 concentration was 6wt%, only Si and Sn elements attended the reaction between titanium and porcelain and mainly adhesive fracture was found at Ti-porcelain interface. When SnO2 concentration was 12wt%, failure of the titanium–porcelain predominantly occurred in the bonding porcelain and mainly cohesive fracture was found at Ti-porcelain interface. The methyl thiazolyl tetrazolium assay results demonstrated that the cytotoxicity of the titanium porcelain was ranked as 0.

NMR studies of oxide-based glasses

Solid-state nuclear magnetic resonance (NMR) spectroscopy offers an array of options for exploring structural order of amorphous materials both over short (≲0.3 nm) and medium (≲1 nm) ranges. We review the advances reported in the literature for characterizing the structures of oxide-based glasses over roughly the past 15 years. Besides describing the current understanding of basic short-range structural aspects of binary and ternary glass systems that typically involve a single glass modifier and one (silicates and phosphates) or two (e.g., aluminosilicates, aluminophosphates and silicophosphates) network formers, we focus on illustrating the progress made for revealing intermediate-range structural order, such as extracting information about connectivities among the basic building blocks in the glass networks and how more extended structural motifs may be identified. By assuming only relatively basic background in solid-state NMR and/or the structure of glasses, we hope this review will appeal to newcomers of either area.

Aluminum coordination in rhyolite and andesite glasses and melts: Effect of temperature, pressure, composition and water content

The coordination state of the network formers Si and Al is one of the primary parameters describing the structure of silicate glasses and melts. In this experimental study, we use 27Al MAS NMR spectroscopy to investigate the effect of temperature, pressure and composition on the coordination of Al in dry and hydrous rhyolite and andesite glasses, quenched from pressures of up to 3.5GPa. The spectra were collected at a high magnetic field of 20 Tesla to increase spectral resolution and allow for a direct quantification of the different Al coordinations. The disorder around [4]Al increases with increasing fictive temperature and pressure and with decreasing water content or peralkalinity. Al is predominantly (>98%) in fourfold coordination in all investigated samples. Nevertheless, systematic trends in Al coordination have been observed as a function of pressure, composition and water content. For the investigated pressure range, the proportion of [5]Al increases with increasing pressure, but decreases with increasing water content and proximity to the metaluminous join for rhyolite glasses. Within analytical uncertainty, the amount of [5]Al is independent of fictive temperature. The addition of water to andesite glasses does not decrease the amount of [5]Al, in contrast to the rhyolite glasses. Based on the glass data, we conclude that the majority of Al and Si will be present in fourfold coordination in metaluminous dry and hydrous rhyolite magmas at the temperature and pressure conditions relevant for the crust and the subducting slab (up to 3.5GPa). However, consistent with previous predictions from model systems, more higher-coordinated Al may be present in andesite and peralkaline or peraluminous rhyolite melts, although the exact concentrations cannot be predicted accurately from the available data. Compared to more depolymerized melts, rhyolite melts can accommodate a higher degree of densification without a significant increase in Al coordination.

Electrical and physicochemical properties of some Ag2O-containing lithia iron silica phosphate glasses

The present paper reports the influence of Ag2O addition at the expense of Li2O on the local structure of xAg2O·(30 − x)Li2O·10Fe2O3·10SiO2·50P2O5 glass matrix (with x = 0, 0.5, 1, 1.5 and 2 mol %). The phosphate structural units of the network former are assessed from Fourier transform infrared (FT-IR) spectroscopy. The addition of Ag2O to the glass network matrix ≤1 mol % leads to the occurrence of a depolymerization process of the phosphate structure and consequently, to the appearance of a distortion of the PO4 tetrahedra. When the content of Ag2O is increased from 1.5 up to 2 mol %, a remarkable polymerization process has been observed. The density, molar volume, microhardness and chemical durability have been investigated in order to study the effect of Ag2O/Li2O replacements on the physicochemical properties the studied glasses. The AC electrical properties are affected to a great extent with composition. These results are related to the internal structure of the glass samples. The conductivity, dielectric constant and dielectric loss of the studied glasses were studied using the frequency response in the interval 100 Hz–100 kHz and the effect of compositional changes on the measured properties was investigated. Measurements showed that the electrical responses of glass samples were different and complex for interpretation. The increase of Ag2O addition at the expense of Li2O contents (from 0 to 1 mol %) led to increase the conductivity, dielectric constant and dielectric losses of samples. The addition of more Ag2O at the expense of Li2O (from 1.5 to 2 mol %), resulting into decreasing the conductivity, the dielectric constant and dielectric losses of the studied glasses. The experimental data of the glass samples were argued to the internal structure of the glasses and the nature and role-played by weakening or increasing the rigidity of the structure of the sample. It could be concluded, therefore, that the AC electrical properties of the samples were influenced by the distribution of its constituents, connectivity, and number of free charges.

Electrical conductivity and local structure of barium manganese iron vanadate glass

Local structure and electrical conductivity of semiconducting 20BaO·10Fe2O3·x MnO2·(70 − x)V2O5 glass (x = 0 − 30), abbreviated as xBFMV, were investigated by means of 57Fe-Mossbauer spectroscopy, differential thermal analysis (DTA) and DC four-probe method. Mossbauer spectrum of these vanadate glasses consists of a doublet with an identical isomer shift (δ) of 0.38±0.01 mm s − 1, indicating that distorted FeO4 tetrahedra constitute the structural units with distorted VO4 tetrahedra and VO5 pyramids. Quadrupole splitting (Δ) gradually increases from 0.70±0.02 to 0.87±0.02 mm s − 1 with an increase in the MnO2 content, indicating an increased local distortion of FeIIIO4 tetrahedra. DTA study of these glasses showed a gradual increase of glass transition temperature (T g) from 329±5 to 411±5°C, showing an improved thermal durability. ‘T g vs. Δ plot’ yielded a straight line with a large slope of 707°C(K)/mm s − 1, proving that FeIII played a role of network former (NWF). An isothermal annealing of 10BFMV glass at 500°C for 1000 min resulted in a marked increase in the electrical conductivity (σ) from (4.5±3.9) × 10 − 7 to (1.4±0.3) × 10 − 2 S cm − 1 and a decrease in the activation energy for the electrical conduction (E a) from 0.33 ± 0.07 to 0.11±0.01 eV, while Δ of FeIII decreased from 0.76±0.02 to 0.49±0.02 mm s − 1. These results suggest that decrease in the distortion of FeIIIO4 tetrahedra involved with the structural relaxation causes an increase in the probability of electron hopping from VIV or VIII to VV.

Characterization of the Products of the Clay Mineral Thermal Reactions during Pulverization Coal Combustion in Order to Study the Coal Slagging Propensity

Silicate liquid phases dominate the primary particle bonding or sintering mechanisms during boiler slagging. Clay minerals, classified as phyllosilicates in the silicate system, are the main inorganic matter in coal, with the aluminosilicates forming from them in coal ash. The high NBO/T (the ratio of nonbridge oxygens to tetrahedrally coordinated network formers) matter, rich with network modifiers resulting from the further reactions of dehydroxylated kaolinite and illite at high temperatures, is of special interest because they cause slagging. In this study, fly ash samples were prepared from two coals in a drop tube furnace system (referred as DTF ash samples) at 1050, 1150, and 1250 °C and then chemically separated using hydrochloric acid. The products of the DTF ash samples before and after chemical separation were analyzed by elemental and phase analyses to characterize the high NBO/T matter in the ash samples and to investigate the relationship between their formation and the combustion temperatur...

Sodium diffusion in boroaluminosilicate glasses

Understanding the fundamentals of alkali diffusion in boroaluminosilicate (BAS) glasses is of critical importance for advanced glass applications, e.g., the production of chemically strengthened glass covers for personal electronic devices. Here, we investigate the composition dependence of isothermal sodium diffusion in BAS glasses by ion exchange, inward diffusion, and tracer diffusion experiments. By varying the [SiO 2]/[Al 2O 3] ratio of the glasses, different structural regimes of sodium behavior are accessed. We show that the mobility of the sodium ions decreases with increasing [SiO 2]/[Al 2O 3] ratio, revealing that sodium is more mobile when it acts as a charge compensator to stabilize network formers than when it acts as a creator of non-bridging oxygens on tetrahedrally-coordinated silicon and trigonal boron. The impacts of both the addition of iron and its redox state on the sodium diffusivity are explored in terms of the structural role of ferric and ferrous ions. By comparing the results obtained by the three approaches, we observe that both the tracer Na diffusion and the Na–K interdiffusion are significantly faster than the Na inward diffusion. The origin of this discrepancy could be attributed to the fact that for sodium inward diffusion, the charge compensation for electron holes is a rather slow process that limits the rate of diffusion.