The effect of alumina on the Sn/Sn redox equilibrium and the incorporation of tin in Na2O/Al2O3/SiO2 melts

Glasses with the basic compositions 10Na₂O · 10CaO · xAl₂O₃ · (80 − x)SiO₂ (x=0, 5, 15, 25) and 16Na₂O · 10CaO · xAl₂O₃ · (74 − x)SiO₂ (x=0, 5, 10, 15, 20) doped with 0.25-0.5 mol% SnO₂ were studied using square-wave-voltammetry at temperatures in the range from 1000 to 1600 °C. The voltammograms exhibit a maximum which increases linearly with increasing temperature. With increasing alumina concentration and decreasing Na₂O concentration the peak potentials get more negative. Mössbauer spectra showed two signals attributed to Sn²⁺ and Sn⁴⁺. Increasing alumina concentrations did not affect the isomer shift of Sn²⁺; however, they led to increasing quadrupole splitting, while in the case of Sn⁴⁺ both isomer shift and quadrupole splitting increased. A structural model is proposed which explains the effect of the composition on both the peak potentials and the Mössbauer parameters.     

Electrical properties of Al2O3 film deposited at low temperatures

Amorphous Al₂O₃ films were deposited on p-Si by rf magnetron sputtering to investigate their potential as a gate dielectric in organic thin film transistors (OTFTs). The deposition was performed at room temperature, 200 and 300 °C using Al₂O₃ and Al targets. Achieved Al₂O₃ films have higher capacitance values than thermally grown SiO₂ as characterized by capacitance-voltage measurements. It is also found from current-voltage and roughness measurements that the leakage current and the surface roughness can be least when the films are deposited at room temperature. The capacitance of the film obtained from the Al₂O₃ target appears higher than that of the Al₂O₃ film from the Al target while the results of electrical breakdown are opposite. These room temperature processes are promising for applications to the gate dielectrics of organic TFTs.     

Density, viscosity and surface tension of 50RO-50P2O5 (R: Mg, Ca, Sr, Ba, and Zn) glass melts

The density, surface tension and viscosity of the 50RO-50P₂O₅ (R: Mg, Ca, Sr, Ba and Zn) glass melts have been measured over the range, 1073-1623 K. The effects of R cations on these properties have been investigated. The density of the melt was found to increase in the order, R: Mg<Ca<Zn<Sr<Ba, with increasing molar weight of cation. The surface tension in the temperature range of 1373-1473 K increased approximately with cation in the order, R: Zn<Ba<Mg<Ca<Sr. The viscosity and the negative, temperature coefficient of surface tension increased in the order, R: Ba<Sr<Ca<Zn<Mg. All melts exhibited negative temperature coefficients of surface tension. The effect of Mg and Zn cations on the properties were different to that for Ba, Sr, and Ca cations and this is discussed using bulk glass data published in a previous report. The features of Mg and Zn metaphosphate glass melts, that is high values of viscosity, and temperature coefficient of surface tension, are related to the small Oxygen Coordination Number of the cations (=4) when compared with those of Ca, Sr, Ba metaphosphate glass melts.     

Effect of Al2O3 on phase separation of SiO2-Nd2O3 glasses

The immiscibility boundary and the critical point of SiO₂-Nd₂O₃ system glass were determined as a function of Al₂O₃ addition. The critical temperature of the immiscibility boundary was observed to decrease with the addition of Al₂O₃. Using the regular solution model, the observed decrease of the immiscibility boundary was directly related to the decrease of the concentration fluctuation of Nd₂O₃ in SiO₂. It is concluded that the Al₂O₃ addition to Nd₂O₃ containing silica glass is beneficial in decreasing the concentration quenching effect, deterioration of the optical efficiency due to clustering of rare earth element, because Al₂O₃ addition diminishes the concentration fluctuation of Nd₂O₃ in silica glass.     

Characterization of chemosynthetic Al2O3-2SiO2 geopolymers

Pure chemosynthetic Al₂O₃-2SiO₂ geoploymers displaying positive alkali-activated polymerization properties and high compressive strength at room temperature were effectively fabricated utilizing a sol-gel method. The molecular structure of the precursor powder and resulting geopolymers were investigated by X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analysis. In addition, the mechanical and alkali-activated polymerization properties of these materials were also studied. NMR data revealed that the chemosynthetic powders began to contain 5-coordinated Al atoms when the calcination temperatures exceeded 200 °C. These calcined powders were capable of reacting with sodium silicate solutions at calcination temperatures exceeding 300 °C, which is, however, much lower than the temperature required to convert kaolin to Metakaolin.     

Atomic layer deposition of Al2O3, ZrO2, Ta2O5, and Nb2O5 based nanolayered dielectrics

Ta₂O₅, Ta-Nb-O, Zr-Al-Nb-O, and Zr-Al-O mixture films or solid solutions were grown on Si(1 0 0) substrates at 300 °C by atomic layer deposition. The equivalent oxide thickness of Ta₂O₅ based capacitors was between 1 and 3 nm. In Zr-Al-O films, the high permittivity of ZrO₂ was combined with high resistivity of Al₂O₃ layers. The permittivity, surface roughness and interface charge density increased with the Zr content and the equivalent oxide thickness was between 2.0 and 2.5 nm. In the Zr-Al-Nb-O films the equivalent oxide thickness remained at 1.8-2.0 nm.     

Crystallization of SiO2Al2O3MgO gel glasses

Basic glasses are prepared by chemical polymerization in a sol-gel process. Nucleation and crystallization of these glasses are analyzed in dependence of the composition of the basic glasses and “additions” of TiO₂ and LiO₂. A comparison of gel glasses with conventionally molten glasses is made. Gel glass-ceramics are prepared as bulk materials and thin coatings.     

Thermally modulated differential scanning calorimetry of a glass of composition 45Na2O-40B2O3-10Al2O3-5In2O3

The kinetics of structural relaxation and of glass transition of the 45Na₂O-40B₂O₃-10Al₂O₃-5In₂O₃ glassforming melt is studied by means of standard DSC and of temperature modulated DSC. In this way the dependence of the fictive temperature on cooling rate is determined simultaneously with the determination of the dependence of the dynamic glass transition temperature on modulation frequency. Both sets of data are fitted together in terms of the equation of Ritland-Bartenev. It was found that the activation energy of the structural relaxation exhibits a moderate dependence on temperature with the dimensionless fragility parameter α=3.3 (for strong systems, α is about 1 and increases to about 8 for some very fragile polymeric systems).     

A partial molar volume for La2O3 in silicate melts

The densities of several La-bearing silicate melts distributed along binary joins have been measured using the double-bob Archimedean method. The results show that the addition of La₂O₃ leads to an increase in the melt density. From these density data, the partial molar volume of La₂O₃ in silicate melts has been determined. Distinct values for partial molar volume of La₂O₃ have been obtained (i.e., 44.01 and 37.04 cm³/mol at 1273 K for the La₂O₃-Na₂O-SiO₂ and La₂O₃-CaO-MgO-Al₂O₃-SiO₂ systems, respectively) indicating a compositional dependence of the partial molar volume of La₂O₃. Nevertheless, a single value for the partial molar volume of La₂O₃ (i.e., 43.47 cm³/mol at 1273 K) suffices to describe the molar volume of melts within errors in the entire multi-component system La₂O₃-Na₂O-CaO-MgO-Al₂O₃-SiO₂ and for La₂O₃ concentrations not greater than 7 mol%. In addition, this work points out that the molar volumes behave ideally within the composition range investigated (i.e., linear variation of the molar volume along the binary joins). Distinct values for the partial molar volume of La₂O₃ obtained in this study and their differences with the molar volumes of molten La₂O₃ given in the literature raise the possibility however that this ideality is not maintained within the entire system.     

Diffusion and viscosity in molten Pd40Ni40P20 and Pd40Cu30Ni10P20 alloys

The self diffusion of ⁶²Ni and the shear viscosity in liquid Pd₄₀Ni₄₀P₂₀ and Pd₄₀Cu₃₀Ni₁₀P₂₀ have been measured. The used methods were the long-capillary technique for diffusion measurement and the gas-film-levitation for viscosity measurement. The temperature dependence of diffusion in the equilibrium melt follows the prediction of the mode-coupling theory. The Stokes-Einstein relation describes well the momentum and mass transport in both melts. The ⁶²Ni diffusion is equal in both alloys whereas the normalized bulk viscosity is higher in Pd₄₀Cu₃₀Ni₁₀P₂₀. Thermodynamic and structural considerations are invoked to propose a qualitative explanation for this behavior.     

Electronic relaxation in the PbOSiO2Fe2O3 glass system

The a.c. and d.c. conductivities of PbOSiO₂ glass with 0–10 mole% Fe₂O₃ additions were measured in the temperature range 77–700 K. Two relaxation processes were noted. The relaxation mechanism at lower temperatures is due to polaron hopping between ferric-ferrous ion pairs with a distribution of relaxation times which is energy independent. The second relaxation phenomenon, at higher temperatures is polaron hopping along iron ion chains in the glass of which the pairs responsible for the first process are a part. A theoreticalmodel is proposed and additional evidence for this model is presented in the form of magnetic susceptibility and electron spin resonance (esr) data.     

Glasses and glass-ceramics from naturally occurring CaOMgOAl2O3SiO2 materials (II) crystallization behavior

Although shale glasses exhibit surface devitrification when heat treated, the addition of Cr₂O₃ to the glass enables volume crystallization to occur. The mechanisms and kinetics of crystallization for both processes are presented and found to correlate well with existing theories; the Swift equation for surface crystallization and the Johnson-Mehl equation for volume crystallization. Phase separation in the Cr₂O₃-containing glasses promotes the formation of an iron-chrome spinel as nuclei and leads to the development of a fine-grained glass-ceramic.     

Nano-crystallization in LaF3–Na2O–Al2O3–SiO2 glass

Aiming at tailoring optical properties, the precipitation of LaF₃ nano-crystals in LaF₃–Na₂O–Al₂O₃–SiO₂ glass-ceramics is studied thoroughly on the nano-scale using advanced transmission electron microscopic techniques. Nano-sized phase-separation droplets enriched in lanthanum and silicon are formed already in the base glass. Within these less than 20 nm large droplets, LaF₃ crystallizes upon heat treatment. The nano-crystallization mechanism revealed is self-limited since growth is restricted by the size of the droplets. An average crystallite size of around 12 nm is achieved with a narrow size distribution since the phase-separation droplets also contain silicon not incorporated into the growing crystal. Instead, excess silicon relocated to the periphery of the pre-existing phase-separation droplets forms a diffusion barrier around the LaF₃ nano-crystals preventing further crystal growth and/or ripening.     

Cobalt ortho- and para-substituted tetraphenylporphyrins inserted in SiO2 gels

The sol-gel method, generally, allows the trapping of important organic and biological systems in inorganic networks. Free and metallic porphyrins feature a wide range of important electrical, optical, and catalytic properties. To fix them in gels it is necessary to substitute an appropriate reactive group in the periphery of the macrocycle, which thus becomes more soluble in water or alcohols. The substituents more frequently employed have been -SO₃⁻, -Na⁺, -COOH and pyridinium salts. We have tried to increase the number, type, and spatial position of such substituents, to evaluate their influence in the quality and properties of the final materials. In this work we report the results obtained with the Co(II) complexes of the meso-5,10,15,20-tetraphenylporphyrin macrocycle, in which the amino, -NH₂(basic) or hydroxyl, -OH (acid) groups have been placed in the ortho- and para-positions of the phenyl groups. The insertion of cobalt porphyrins was made possible by the addition of small quantities of pyridine, methanol or dimethylformamide (DMF). Maxima in the transparency and strength of the materials were obtained with the para-amino substituted porphyrin, when methanol and pyridine were employed as the solvent media. Our results reveals that methanol has no influence on the morphology of the silica network, but DMF changes it drastically. The procedure here described has been successfully extended to trap other free and metallic porphyrinic systems.     

Some characteristics of glass in the Al2O3---B2O3---SiO2 system from the gel

The colorless and transparent glasses in the Al₂O₃---B₂O₃---SiO₃ system with high B₂O₃ and SiO₂ content were prepared from gels at low temperature. Their IR spectra not only revealed the evolution of the gel to glass conversion, but also showed that the formation of mixed bonds in the glasses obtained did not show any effect due to the B₂O₃ content. The accuracy of the glass composition is dependent upon the SiO₂/B₂O₃ molar ratio. The higher the ratio, the less the deviation of the analyzed compositions of the resulting glasses from their original calculated values. It is obvious that the higher the ratio, the lower the thermal expansion coefficient and the higher the transformation temperature of the glass, and the temperature at which the thermal contraction reaches an equilibrium is higher.     

Electric conductivity of glasses in the system Na2O/CaO/SiO2/Fe2O3

The addition of polyvalent transition metal ions to the usually insulating traditional soda-lime-silica glasses can lead to semiconducting properties. We report on synthesis of glasses and glass-ceramics in a soda-lime-silicate based system containing Fe₂O₃ in the concentration range from 5 to 30 mol%. Two sub-systems were considered, in one of them the ratio [Na₂O]/[Fe₂O₃] was varied while in the other one, the ratio [SiO₂]/[Fe₂O₃] was changed. The phase composition of the synthesized products was characterized by X-ray diffraction and energy dispersive X-ray analysis, while the electrical properties were studied by impedance spectroscopy. Partially crystallized non-reduced samples are semiconducting even at room temperature while the glassy samples (both reduced and non-reduced) exhibit semiconducting properties at temperatures equal or larger than 100 °C. An attempt is done to predict the physical approximation explaining the conduction process in the glasses.     

Thermodynamic modeling of the Al2O3–B2O3–SiO2 system

A complete literature review, critical evaluation, and thermodynamic modeling of the phase diagrams and thermodynamic properties of all oxide phases in the ternary Al₂O₃–B₂O₃–SiO₂ system at 1 bar pressure are presented. The molten oxide phase is described by the Modified Quasichemical Model and the Gibbs energy of the mullite solid solution is modeled using the Compound Energy Formalism. A set of optimized internally consistent thermodynamic functions for all the phases is presented. With the thermodynamic dataset, all available and reliable thermodynamic and phase equilibrium data can be reproduced within experimental error limits from 25 °C to above the liquidus temperatures. In addition, the reasonable predictions obtained for phase relations in the experimentally unexplored composition ranges suggest that the thermodynamic database can be used along with appropriate Gibbs energy minimization routines to calculate thermodynamic properties, phase equilibria, and phase diagrams of interest.     

Study of the structural insertion of Al in the Al2O3-SiO2 and Nd2O3-Al2O3-SiO2 glass systems

Glasses in the Al₂O₃-SiO₂ and Nd₂O₃-Al₂O₃-SiO₂ systems were prepared by the sol-gel method. The gel-glass evolution was studied using near-infrared (NIR) and ultraviolet-visible (UV-VIS) spectrophotometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR). The results obtained indicate a relationship between the sample compositions, the treatment temperature and the Al coordination. In the samples of the Nd₂O₃-Al₂O₃-SiO₂ system the densification of the structure, when the treatment temperature increases, leads to the segregation of neodymium oxide particles.     

Structure and electrical conductivity of new Li2O-CeO2-B2O3 glasses

Fourier transformation infrared spectra, density and DC electrical conductivity of 30Li₂O · xCeO₂⋅(70 − x)B₂O₃ glasses, where x ranged between 0 and 15 mol%, have been investigated. The results suggested that CeO₂ plays the role of network modifier up to 7.5 mol%. At higher concentrations it plays a dual role; where most of ceria plays the role of network former. The density was observed to increase with increasing CeO₂ content. The effect on density of the oxides in the glasses investigated is in the succession: B₂O₃ < Li₂O < CeO₂. Most of CeO₂ content was found to be associated with B₂O₃ network to convert BO₃ into B O₄⁻ units. The contribution of Li⁺ ions in the conduction process is much more than that due to small polarons. The conductivity of the glasses is mostly controlled by the Li⁺ ions concentration rather than the activation energy for CeO₂ > 5 mol%. Lower than 5 mol% CeO₂ the conductivity is controlled by both factors. The dependence of W on BO₄⁻ content supports the idea of ionic conduction in these glasses.     

Supercritical fluctuations in the B2O3PbOAl2O3 system

A small-angle X-ray diffraction study of the supercritical region of the B₂O₃PbOAl₂O₃ immiscibility surface is presented. This is the first time that diffraction methods have been applied to the study of critical opalescence in a system of oxides at high temperatures. Debye's theory is shown to be applicable and, from the spectra correlation lengths and the range of molecular interactions l is calculated. The topography of the supercritical region is determined and the relationship between l and the main interatomic distances in the melt discussed.