The effect of initial oxidation state on crystallization of basaltic glass

Iron-rich silicates mined as basalt may be processed as glass fibers and, woven to textile mats, it may be used as heat insulation. High-temperature stability is however a limiting factor because at least three process of alteration occur upon heat treatment that affect mechanical and chemical properties, micron to nano-crystallization, oxidation and cation-enrichment at the glass surface with a few microns depth. We here evaluate the crystallization behavior of synthetic Fe³⁺-rich basalt (SB) heat treated in air (negligible oxygen potential) in comparison with previously studied Fe²⁺-rich natural basalt (NB) heat treated in air and in Ar (i.e., high and low oxygen potential, respectively). Initial crystals growing in Fe²⁺-rich basalt are micron-sized dendrites, and with temperature of heat treatment they become increasingly granular and are identified as pyroxene. Pyroxene in SB form smaller dendrites; crystallization textures and nucleation and growth rates, derived from crystal size distribution, are independent of the temperature of heat treatment in the range between 850 and 1030 °C. Here, the activation energy of pyroxene vanishes, though crystallization rates are smaller than those of NB. Whereas in NB pyroxene growth occurs diffusion limited, in SB it is likely to be limited by the attachment of ions to new crystal surface.     

Mössbauer and optical spectroscopic study of temperature and redox effects on iron local environments in a Fe-doped (0.5 mol% Fe2O3) 18Na2O-72SiO2 glass

Local environments of ferric and ferrous irons were systematically studied with Mössbauer (at liquid helium temperature) and ultraviolet-visible-near infrared spectroscopic methods for various 18Na₂O-72SiO₂ glasses doped with 0.5 mol% Fe₂O₃. These were prepared at temperatures of 1300-1600 °C in ambient air or at 1500 °C under reducing conditions with oxygen partial pressures from 12.3 to 0.27×10⁻⁷ atmospheres. The Mössbauer spectroscopic method identified three types of local environments, which were represented by the Fe³⁺ sextet, the Fe³⁺ doublet, and the Fe²⁺ doublet. The Fe³⁺ sextet ions were assigned to ‘isolated’ octahedral ions. Under reducing conditions, the octahedral Fe³⁺ ions were readily converted into octahedral ferrous ions. The Fe³⁺ doublet exists both in octahedral and tetrahedral environment, mainly as tetrahedral sites in the reduced samples. The tetrahedral ions were found stable against reduction to ferrous ions. The Fe²⁺ doublet sites existed in octahedral coordination. Combining results from both spectroscopic studies, the 1120- and 2020-nm optical bands were assigned to octahedral ferrous ions with a different degree of distortion rather than different coordinations. Further, we assigned the 375-nm band to the transition of octahedral ferric ions that are sensitive to the change of oxygen partial pressure in glass melting and 415-, 435-, and 485-nm bands to the transitions of the tetrahedral ferric ions that are insensitive to oxidation states of the melt. The effect of ferric and ferrous ions with different coordination environments on the glass immiscibility was elucidated.     

Study of color and structural changes in silver painted medieval glasses

Silver was introduced into medieval glass by an ancient painting process using different clay minerals (ochre, illitic, montmorillonitic, and kaolinitic clays). The colorimetric properties, studied by UV-Vis spectroscopy, were dependent on the clay mineral as a result of different concentration of Ag ions diffused into the glass surface. TEM results showed the well known formation of silver nanoclusters which give the yellow coloration of the glass. The obtained results showed that clay properties such as specific surface area, pore volume and iron concentration (Fe₂O₃), are important factors that affect the yellow coloration. It is also observed that Fe₂O₃ acts as an oxidant agent for silver atoms providing the Ag₂O formation. This oxide cannot diffuse into the glass structure and avoid the ion-exchanged process. After Ag ion diffusion some structural changes occur in the glass as it has been shown by Raman spectroscopy. It is observed that the diffusion process leads to depolymerization of the glass network as it is determined by analyzing the Qⁿ components of Raman spectra. Two Raman bands at 148 and 244 cm⁻¹ assigned to Ag-O bonds can be associated to the presence of Ag₂O on the glass painted surface.     

Surface nature of nanoparticle gold/iron oxide aerogel catalysts

The aerogel catalysts investigated are constituted by two chemically different nanoparticle systems consisting of the gold phase and the iron oxide support. High-resolution transmission electronic microscopy (HRTEM) showed an increased surface roughness for aerogel particles with higher gold loading. X-ray photoelectron spectroscopy (XPS) revealed increases in the surface coverage of hydroxyl groups and the Fe²⁺/Fe³⁺ ratio due to the addition of gold, and showed the transition of gold from oxidized to metallic states due to calcination. In the presence of gold species, the Fe³⁺ satellite structure in XPS was not produced. The crystallinity of maghemite as the support was found quite stable with respect to gold addition and thermal treatment. The aerogels were evaluated for methanol oxidation carried out in an ambient flow reactor. The oxidation activity enhanced with decreasing catalyst pretreatment temperatures and with increasing gold loadings up to 5 wt%. A wide selectivity pattern formed between dimethyl ether and carbon dioxide products. The size of gold particles and the status of surface gold species played a crucial role in the catalytic conversion of methanol. The oxidized gold was more active than the metallic gold towards the total combustion to carbon dioxide. The surface nature has been proven to transform from strong Lewis acidic to high basic characters due to the formation of reactive hydroxyl groups near by the gold sites.     

Precipitation of nanometer-sized SnO2 crystals and Sn depth profile in heat-treated float glass

The effect of oxygen diffusion from the atmosphere on tin depth profile in the bottom face of a soda-lime-silica float glass at temperatures above T_g was investigated. The heat treatment was performed in ¹⁸O₂/N₂ and argon (Ar) atmospheres. The significant diffusion of tin to the surface was observed for the glass heat-treated in ¹⁸O₂/N₂ atmosphere, resulting in the formation of a tin-enriched layer near the surface region. It was found that the tin was supplied from the region shallower than the ‘hump’ which is commonly observed in the tin profile of a commercial soda-lime-silica float glass. No significant change in the tin depth profile was observed for the glass heat-treated in Ar atmosphere. These results indicate that ¹⁸O diffusion into the glass, which causes the change in chemical state of tin from Sn²⁺ to Sn⁴⁺, induces the significant diffusion of tin. Furthermore, the precipitation of crystalline SnO₂ particles with a diameter of ∼1 nm was clearly recognized in the tin-enriched layer. This fact indicates that a phase separation was induced by the oxygen diffusion into the glass. Consequently, Sn²⁺ may be supplied to the surface in order to compensate for the marked decrease in Sn²⁺ concentration in the glass system. The significant diffusion of tin to the surface was suppressed by increasing the iron content in the glass. This suppression was ascribed to the increase in Sn⁴⁺ concentration as a result of the redox reaction between tin and iron because the diffusion coefficient of Sn⁴⁺ is much smaller than that of Sn²⁺.     

On the oxidation mechanism of microcrystalline silicon thin films studied by Fourier transform infrared spectroscopy

Insight into the oxidation mechanism of microcrystalline silicon thin films has been obtained by means of Fourier transform infrared spectroscopy. The films were deposited by using the expanding thermal plasma and their oxidation upon air exposure was followed in time. Transmission spectra were recorded directly after deposition and at regular intervals up to 8 months after deposition. The interpretation of the spectra is focused on the Si-H_x stretching (2000-2100 cm⁻¹), Si-O-Si (1000-1200 cm⁻¹), and O_xSi-H_y modes (2130-2250 cm⁻¹). A short time scale (< 3 months) oxidation of the crystalline grain boundaries is observed, while at longer time scales, the oxidation of the amorphous tissue and the formation of O-H groups on the grain boundary surfaces play a role. The implications of this study on the quality of microcrystalline silicon exhibiting no post-deposition oxidation are discussed: it is not sufficient to merely passivate the surface of the crystalline grains and fill the gap between the grains with amorphous silicon. Instead, the quality of the amorphous silicon tissue should also be taken into account, since this oxidation can affect the passivating properties of the amorphous tissue on the surface of the crystalline silicon grains.     

Structural investigation of iron phosphate glasses using molecular dynamics simulation

The current study reports the first molecular dynamics models of iron phosphate glasses. Models were made for xFeO-(100 − x)P₂O₅ glasses with x = 30, 40 and 50 and for xFe₂O₃-(100 − x)P₂O₅ glasses with x = 30 and 40. This study also looks at the effects of mixed Fe²⁺/Fe³⁺ contents. The models are in good agreement with experimental results for nearest-neighbour distances and coordination numbers, and in reasonable agreement with X-ray and neutron diffraction structure factors. As expected the models contain a tetrahedral phosphate network with P-O distances of 1.50 ± 0.01 Å. The network connectivity is dominated by the expected Qⁿ (where n is the number of bridging oxygen) corresponding to the O:P ratio. These are average Qⁿ of 2.3 for 40FeO and 1.0 for 40Fe₂O₃ glasses respectively. Interestingly a small amount of non-network oxygen is found to be present in the 40Fe₂O₃ glass model. The Fe-O coordination is close to 4.5 in both FeO and Fe₂O₃ glass models, with Fe-O bond lengths of 2.12 Å and 1.89 Å respectively. The greater durability of xFe₂O₃-(100 − x)P₂O₅ glasses can be attributed to the lower content of P-O-P bonds and higher bond valence across Fe-O-P bonds. For 40Fe₂O₃ glass the Fe-Fe correlation shows a main peak at 5-6 Å in good agreement with the result from magnetic scattering which was interpreted in terms of speromagnetic order.     

The solubility and activity determination of NiO in the SiO2-NiO-FeO system

A structural model for the liquid phase of silicate systems was used to obtain the SiO₂-NiO-FeO phase diagram and the NiO activities at 1573 and 1673 K. The NiO activities were determined experimentally by equilibrating a pure nickel crucible with an iron-silicate slag and CO-CO₂ gas mixture to produce pO₂ = 10⁻⁶ and 10⁻¹⁰ atm. The NiO activities obtained by the structural model are in good agreement with those obtained experimentally for strongly reduction conditions (10⁻¹⁰ atm), due to the structural model consider only the iron in the divalent state. The Fe³⁺/Fe²⁺ ratio and the nickel oxide solubility were increased when the oxygen partial pressure and the temperature were increased.     

Oxygen bombardment effects on average crystallite size of sputter-deposited ZnO films

Zinc oxide (ZnO) film was deposited on a glass substrate by rf magnetron sputtering with O₂/Ar as working gases. Structural properties of the films were characterized by XRD. Average crystallite size in the films was strongly dependent on both the gas flow ratio of O₂/Ar and rf-power at a constant deposition pressure. During the deposition, energetic species in the plasma were in situ monitored using optical emission spectroscopy. An inverse correlation was observed between the average crystallite size and the emission intensity ratio of I_O^∗/I_Ar. Bombardment of atomic oxygen to the growing surface played an important role in determining the average crystallite size in the films. The average crystallite size could be controlled by the emission intensity ratio of I_O^∗/I_Ar.     

Sodium tracer diffusion in glasses of the type (CaO·Al2O3)x(2 SiO2)1−x

Tracer diffusion coefficients of the radioactive isotope Na-22 were measured in glasses of the type (CaO·Al₂O₃)_x(2 SiO₂)_1−x to study the diffusion of sodium as a function of glass composition, x, temperature and initial water content. The diffusion of Na-22 in glasses diffusion-annealed in dry air can always be well described by a single tracer diffusion coefficient, but sometimes not in samples annealed in common air. It was found that the sodium tracer diffusion coefficient decreases by about six orders of magnitude when the glass composition x changes from 0 to 0.75 at 800 °C. The temperature dependence of the diffusion of sodium seems to decrease as the silica content increases. Variations of the initial water content in some of the glasses investigated did not very significantly influence the rate of the tracer diffusion of sodium.     

Characterization of a mould flux glass

A simplified mould flux glass composition used for the continuous casting of steel was synthesized and then characterized using X-ray powder diffraction (XRD) differential thermal analysis (DTA) and ¹⁹F and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR). DTA showed the glass to have a low glass transition temperature and to crystallize readily at 600 °C. XRD of the heat-treated glass showed it to crystallize to cuspidine. ¹⁹F MAS-NMR showed the principal fluorine species to be F-Ca(n) with no evidence of Si-F or Al-F species. Fluoride ions therefore, complex calcium in this glass, rather than forming non-bridging fluorines. The network connectivity of the glass was calculated on this basis and found to be 2.07 this would be expected to correspond to a Q² Si species which was supported by the ²⁹Si data that gave a chemical shift of −78 ppm corresponding to Q² Si.     

Effect of boron addition on the structure and properties of iron phosphate glasses

Effects of boron addition on the glass forming characteristics, structure and properties of iron phosphate glasses with nominal compositions of xB₂O₃-(40−x)Fe₂O₃-60P₂O₅ (x = 2-20, mol%) and xB₂O₃-(100−x)[Fe₂O₃-60P₂O₅] (x = 2-20, mol%) have been investigated by DTA, XRD, IR and Mössbauer spectroscopy. Although there were some weak local surface crystallizations on especially most of the glasses in group B, all of the compositions formed glass. DTA spectra showed two exothermic peaks corresponding to crystallizations along with an endothermic glass transition peak. T_g increased with increasing B₂O₃ content for the glasses in the first series which indicates that the addition of B₂O₃ increases the thermal stability of glasses in this series while the opposite is observed in the second series. The dissolution rates of boron containing bulk glasses were found to be around 10⁻⁹ gr/cm² min which are comparable to that of the base iron phosphate glass. When the B₂O₃ content was above 14%, new bands related to BO₄ tetrahedral groups have been observed in the IR spectra. The Mössbauer isomer shift values of boron doped glasses were found to be a little lower than that of base glass but both iron ions had distorted octahedral coordination in all glasses. The fraction of Fe²⁺ ions in glasses (Fe²⁺/∑(Fe²⁺ + Fe³⁺)) was found to be 23% for the base glass while it was 10-22% for the boron doped glasses.     

Redox kinetics of iron in alkali silicate glasses exposed to ionizing beams: Examples with the electron microprobe

We report herein on changes in the Fe³⁺/ΣFe ratios induced by the electromigration of alkali ions (Alk⁺s) in natural alkali silicate glasses by measuring the shift of the Fe Lα emission peak at the electron microprobe. The Fe³⁺ reduction to Fe²⁺ classically occurs by electron transfer (Fe³⁺ + e^? → Fe²⁺). The inward diffusion (to the bulk) of Alk⁺s is correlated with the outward diffusion (to the surface) of electrons transferred from a Fe²⁺ site to a neighboring Fe³⁺ site. This reduction process is somewhat different when iron is found at low amounts in glasses. In the latter case, Fe³⁺ is an efficient electron trap and its reduction to Fe²⁺ occurs by direct capture of a free electron. The Fe²⁺ oxidation is induced by the formation and the outward diffusion of O^2? interstitial ions produced at the sites of paired nonbridging oxygens after the departure of the charge compensating Alk⁺s. The accumulation of free oxygens beneath the surface makes Fe³⁺-rich oxide phases to precipitate as separate nanometer sized particles. Outgassing of atomic oxygens as bubbles is also observed.     

Silica encapsulated hemoglobin and myoglobin powders prepared by an aqueous fast-freezing technique

Hemoglobin and myoglobin were encapsulated in silica gels and powders. Protein encapsulated powders were fabricated via the condensation of silicic acid around the protein, followed by a fast freezing with liquid nitrogen, and subsequent thawing. The fast-freezing technique led to high surface area stable silica encapsulated protein powders. Transmission UV-vis spectroscopy techniques were used to verify that neither protein was damaged during gelling or freezing processes. Both hemoglobin and myoglobin gels and powders retained their biological activity and were able to bind cyano ligands while in the oxidized Fe³⁺ state and carbon monoxy ligands while in the reduced Fe²⁺ state. Kinetics experiments showed that the rates of binding of CO and CN⁻ to the proteins in the silica gel versus a buffer solution are decreased by 30-45%. This result was likely due to mass transfer effects associated with diffusion through the gel network. Hemoglobin/silica powders were successfully stabilized in the Fe²⁺ oxidation state by addition of the amino acid l-cysteine.     

Chemical diffusion and crystalline nucleation during oxidation of ferrous iron-bearing magnesium aluminosilicate glass

Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM) have been used to evaluate the mechanism and kinetics of oxidation of a Fe²⁺-doped MgO---Al₂O₃---SiO₂ glass (with nominal composition along the enstatite-cordierite-liquid divariant) which was heat treated in air under the time and temperature ranges 10–150 h and 700–800°C, respectively. The results clearly demonstrate that oxidation occurs by a cation diffusion process: specifically, the divalent cations diffuse from the interior of the glass to the free surface where they subsequently react with environmental oxygen to form a two-phase, MgO---(Mg, Fe)₃O₄ crystalline layer which covers the (divalent cation-depleted) glass. Oxidation of some Fe²⁺ within the glass occurs via the inward flux of electron holes (a counterflux to the divalent cation diffusion required to maintain charge neutrality of the glass); this internal oxidation results in the fine-scale ( 1–5 nm), homogeneous nucleation of crystalline (Mg, Fe)₃O₄ within the divalent cation-depleted layer of the glass. Chemical diffusion of an oxygen species is thus demonstrated to be a slower, parallel kinetic process which is not required for oxidation to occur in this material. A first-order analysis of oxidation kinetics in the glass is presented.     

An easy and economic method for preparing completely reduced basalt glass

Four basalt glass samples were prepared by fusing basalt rock (powder) with different amounts of sulfur in a platinum crucible at 1550 °C for 30 min. Each melt was quenched in air. Sulfur addition to the basalt powder was changed from 0 to 5, 10 and 15 wt%. The prepared glass samples were pulverized for measuring the Mössbauer spectra by the constant acceleration method. The basalt rock spectrum can be analyzed into four peaks; two sites due to Fe³⁺ with octahedral (Oh) and tetrahedral (Td) symmetry, and the other two due to Fe²⁺ with Oh and Td symmetry. Pure basalt glass (sulfur-free) consists of four doublets; two of them represent Fe²⁺(Oh) sites and the third represents Fe²⁺(Td); while the fourth doublet belongs to Fe³⁺(Td). The sample containing 5 wt% sulfur has four iron sites also, although there is a slight difference in the relative absorption area when compared with sulfur-free sample. The fraction of Fe³⁺ in the 5% sulfur sample was estimated to be only 7.1%; i.e., the fraction of Fe²⁺ was 92.9%. Three iron sites present in the 10% sulfur sample, two of them represent Fe²⁺ with (Oh) symmetry, while the third one represents Fe²⁺(Td) site. Mössbauer spectrum of 15 wt% sulfur sample is essentially the same as that of the sample which contains 10 wt%. It is noteworthy that the sulfur content shows a linear relationship with the Fe²⁺ fraction which is calculated from the Mössbauer spectra of basalt glasses. 7.5 wt% of sulfur is large enough to completely reduce the iron in basalt glass. The reduction of glasses could occur easily and economically using sulfur as a reducing agent. This method is a very easy and economic method for the preparation of completely reduced oxide glass.     

Precise XPS depth profile of soda-lime-silica glass using C60 ion beam

Ar ion sputtering is one of the most accepted techniques for depth profiling in practical X-ray photoelectron spectroscopy (XPS) analysis, while this technique is known to be inadequate for quantitative analysis of glass including mobile ions such as soda-lime-silica glass. For the precise depth profiling on XPS depth analysis, three methods, (i) coating of conductive thin film on glass surface, (ii) sample cooling at the temperature of −130 °C and (iii) buckminsterfullerene (C₆₀) ion sputtering, were investigated to suppress the migration of mobile ions in glass. In our best knowledge, it is the first time to succeed the precise XPS depth analysis of soda-lime-silica glass (70.4SiO₂, 0.9Al₂O₃, 7.3MgO, 7.8CaO, 13.6Na₂O in mol%) without compositional change by using C₆₀ ion sputtering. The precise analysis revealed that the ion implantation during Ar ion sputtering principally resulted in the compositional change due to the migration of mobile ions.     

Czochralski-growth of germanium crystals containing high concentrations of oxygen impurities

Oxygen-containing germanium (Ge) single crystals with low density of grown-in dislocations were grown by the Czochralski (CZ) technique from a Ge melt, both with and without a covering by boron oxide (B₂O₃) liquid. Interstitially dissolved oxygen concentrations in the crystals were determined by the absorption peak at 855 cm⁻¹ in the infrared absorption spectra at room temperature. It was found that oxygen concentration in a Ge crystal grown from melt partially or fully covered with B₂O₃ liquid was about 10¹⁶ cm⁻³ and was almost the same as that in a Ge crystal grown without B₂O₃. Oxygen concentration in a Ge crystal was enhanced to be greater than 10¹⁷ cm⁻³ by growing a crystal from a melt fully covered with B₂O₃; with the addition of germanium oxide powder, the maximum oxygen concentration achieved was 5.5×10¹⁷ cm⁻³. The effective segregation coefficients of oxygen in the present Ge crystal growth were roughly estimated to be between 1.0 and 1.4.     

Preparation and the growth mechanism of zinc blende structure tin sulfide films by successive ionic layer adsorption and reaction

Zinc blende structure tin sulfide (SnS) films have been prepared by successive ionic layer adsorption and reaction (SILAR) method. It is found that both the annealing and addition of NH₄Cl or NaCl to cation solution during the SILAR process can promote the crystallization of SnS films. The growth mechanism for this novel structure considered that the initial distribution of cations on the substrate surface during the cation adsorption process is the crucial factor that determines the structure of the final production. Because Cl⁻ anions can complex with Sn²⁺ cations, when the concentration of Cl⁻ in the cation precursor solution increases, more [SnCl]⁺ complex ions are adsorbed on the substrate. These [SnCl]⁺ ions assemble more orderly than Sn²⁺ ions because of the polarity of ions, so crystallized SnS films can be obtained when NH₄Cl or NaCl is added to the cation solution.     

Electron spin resonance and magnetic studies on CaO-SiO2-P2O5-Na2O-Fe2O3 glasses

Room temperature electron spin resonance (ESR) spectra and temperature dependent magnetic susceptibility measurements have been performed to investigate the effect of iron ions in 41CaO · (52 − x)SiO₂ · 4P₂O₅ · xFe₂O₃ · 3Na₂O (2 ? x ? 10 mol%) glasses. The ESR spectra of the glass exhibited the absorptions centered at g ≈ 2.1 and g ≈ 4.3. The variation of the intensity and linewidth of these absorption lines with composition has been interpreted in terms of variation in the concentration of the Fe²⁺ and Fe³⁺ in the glass and the interaction between the iron ions. The magnetic susceptibility data were used to obtain information on the relative concentration and interaction between the iron ions in the glass.