Interaction of sodium with simple glasses

The discolouration of vitreous silica by sodium was viewed as a surface phenomenon, hence techniques such as electron spectroscopy for chemical analysis (ESCA) and infra-red reflection spectroscopy (IRRS) along with a variety of other analytical tools were used to investigate the interaction. When vitreous silica was exposed to sodium vapour at about 300 C, the formation of Si-O^– bonds were observed. In addition sodium which had diffused into the glass was found to be in an ionic state. At a higher temperature of exposure to sodium, about 450 C, crystalline sodium metasilicate (and possibly silicon) was formed. Using the experimental results presented here and elsewhere, a model has been developed based on the formation of sodium metasilicate and non-stoichiometric silicon oxide when vitreous silica is exposed to sodium vapour. The discolouration phenomenon is attributed solely to the formation of non-stoichiometric silicon oxide.     

Gamma ray interaction with copper-doped sodium phosphate glasses

Copper-doped sodium phosphate glasses of various compositions and with varying copper contents were prepared. UV-visible and infrared spectroscopic studies were measured before and after successive gamma irradiation. Experimental results indicate that copper ions occupy different local sites depending on the host glass composition and concentration of copper ions. The changes in UV, visible and infrared spectral data, are discussed in relation to the structural evolution caused by the change in composition and states of copper ions. Copper ions have been found to show a shielding behavior toward the effects of progressive gamma irradiation causing a retardation of the growth of the induced defects caused by irradiation.     

Interaction of Fecralloy with sodium disilicate glass

Interactions of sodium disilicate glass with Fecralloy, Fe-6Cr-4Al-0.25Y, have been analysed in terms of the Pask chemical equilibrium theory. Sessile drops assumed contact angles of 38 to 47‡ at 900 to 1100‡C on polished substrates following interactions that led to significant weight losses. Up to 7% of aluminium and much smaller amounts of iron, chromium manganese and yttrium were taken into the glass. On pre-oxidized substrates, coated with alumina, near-perfect wetting was achieved but once more aluminium was taken into the glass and weight losses suffered. These data can be interpreted as resulting from redox reactions between the aluminium in the alloy and the sodium monoxide in the glass.     

Viscosity properties of sodium borophosphate glasses

The viscosity behavior of (1 − x)NaPO₃−xNa₂B₄O₇ glasses (x = 0.05-0.20) have been measured as a function of temperature using beam-bending and parallel-plate viscometry. The viscosity was found to shift to higher temperatures with increasing sodium borate content. The kinetic fragility parameter, m, estimated from the viscosity curve, decreases from 52 to 33 when x increases from 0.05 to 0.20 indicating that the glass network transforms from fragile to strong with the addition of Na₂B₄O₇. The decrease in fragility with increasing x is due to the progressive depolymerization of the phosphate network by the preferred four-coordinated boron atoms present in the low alkali borate glasses. As confirmed by Raman spectroscopy increasing alkali borate leads to enhanced B-O-P linkages realized with the accompanying transition from solely four-coordinated boron (in BO₄ units) to mixed BO₄/BO₃ structures. The glass viscosity characteristics of the investigated glasses were compared to those of P-SF67 and N-FK5 commercial glasses from SCHOTT. We showed that the dependence of the viscosity of P-SF67 was similar to the investigated glasses due to similar phosphate network organization confirmed by Raman spectroscopy, whereas N-FK5 exhibited a very different viscosity curve and fragility parameter due to its highly coordinated silicate network.     

Some electrical properties of sodium silicate glasses modified with titania

A number of glass samples were prepared from mixtures of SiO₂, TiO₂ and Na₂CO₃ (for Na₂O) and their electrical characteristics were measured and interpreted in terms of an electrical energy gap somewhat smaller than the optical gaps previously reported for these glasses. Capacitance measurements showed that one effect of increasing the TiO₂ content was to increase the permittivity. Traditional sodium silicate glasses conduct electricity by ions, but the admixture of TiO₂ brings a significant electronic contribution. Some measurements at high electric fields were made and evidence of the applicability of small polaron theory to the results is presented.     

Ultrasonic velocity in sodium borate glasses

Ultrasonic velocities in sodium borate glasses are measured as a function of composition at a frequency of 10 MHz and at a temperature of 298 K by making use of the ultrasonic pulse echo overlap method. Elastic properties of these glasses are analysed in terms of the elastic internal energy due to deformation; elastic resistances of the network-former, B₂O₃, and the modifier, Na₂O, are obtained as a function of composition from the plot ofMV ² againstx ₂, whereM is the molar mass of sodium borate glasses,V the velocity of sound andx ₂ the mole fraction of Na₂O. The elastic resistances of B₂O₃ and Na₂O are as follows: (i) forx ₂<0.33, the elastic resistance of B₂O₃ is smaller than that of Na₂O; (ii) atx ₂=0.33, the elastic resistances of B₂O₃ and Na₂O are equal; (iii) forx ₂>0.33, the elastic resistance of B₂O₃ is greater than that of Na₂O; (iv) atx ₂≈0.15, the elastic resistances of B₂O₃ and Na₂O become respectively maximal and minimal; (v) atx ₂≈0.23, the elastic resistances of B₂O₃ and Na₂O become respectively minimal and maximal; (vi) abovex ₂=0.36, the elastic resistance of Na₂O becomes negative.     

Ultrasonic studies on irradiated sodium borate glasses

The ultrasonic velocity and attenuation coefficient have been measured at the frequency 4 MHz and at room temperature using the pulse echo technique. The effect of γ-radiation (1–10 Gy) on the ultrasonic attenuation, the internal friction and elastic properties inside the modified sodium borate glasses was studied. The used modification oxides are CaO, ZnO, MgO, BaO, and CdO. The results indicate the possibility for using modified sodium borate glasses as γ-ray dosimeters in the low γ-radiation doses.     

Interaction between bioactive glasses and human dentin

This study explores the interaction between bioactive glasses and dentin from extracted human teeth in simulated oral conditions. Bioactive glasses in the Na₂O–CaO–P₂O₅–SiO₂ and MgO–CaO–P₂O₅–SiO₂ systems were prepared as polished disks. Teeth were prepared by grinding to expose dentin and etching with phosphoric acid. A layer of saliva was placed between the two, and the pair was secured with an elastic band and immersed in saliva at 37 °C for 5, 21 or 42 days. The bioactive glasses adhered to dentin, while controls showed no such interaction. A continuous interface between the bioactive glass and dentin was imaged using cryogenic-scanning electron microscopy (SEM). However, after alcohol dehydration and critical point drying, fracture occurred due to stresses from dentin shrinkage. SEM investigations showed a microstructurally different material at the fractured interface. Chemical analyses revealed that ions from the glass penetrated into the dentin and that the surface of the glass in contact with the dentin was modified. Microdiffractometry showed the presence of apatite at the interface. Bonding appears to be due to an affinity of collagen for the glass surface and chemical interaction between the dentin and glass, leading to apatite formation at the interface.     

EXAFS and MAS NMR studies of sodium molybdophosphate and sodium tungstophosphate glasses

The local order around molybdenum and tungsten atoms in various sodium molybdophosphate and sodium tungstophosphate glasses has been investigated using extended X-ray absorption fine structure (EXAFS). Both molybdenum and tungsten atoms are present in six-coordinated environment in these glasses. Magic angle spinning nuclear magnetic resonance (MAS NMR) of³¹P suggests that metaphosphate or neutral [POO_3/2] groups are present in these glasses.     

Optical absorption of vanadium in sodium diborate glasses

The optical absorption in blown films and bulk specimens of sodium diborate glasses containing vanadium oxide based on the system [Na₂O-2B₂O₃]_1?x [V₂O₅] _x , wherex = 0, 0.05, 0.10, 0.15 and 0.20, has been studied in the wavelength range 200 to 900 nm. The fundamental absorption edge has been analysed in the light of the existing models. It is found that glasses containing a higher content of vanadium have fewer defects when compared with those containing a low vanadium content. The absorption spectra are interpreted as due solely to pentavalent vanadium.     

Structural study of sodium silicate glasses and melts

We report an X-ray diffraction study and modeling of the structure of sodium silicate glasses and melts that are used as binders in the production of the latest generation of ceramic welding fluxes. Experimental data and reverse Monte Carlo (RMC) simulations results are analyzed in comparison with results for a sodium silicate monolith and the water glass prepared from it. The RMC results agree well with structure factor and radial distribution function analysis data. The sodium silicate glasses and melts are shown to have a number of structural features, which have much in common with their crystalline counterparts. The high capacity of the silicate glasses and melts for water is due to the presence of nanopores formed (largely) by six-membered rings of silicon-oxygen tetrahedra and partially by sodium-oxygen octahedra.     

Preparation of sodium trisilicate glasses of varying water content

Various properties of glasses are affected by small amounts of structural water. This water is known to be present in essentially all silicate glasses. Much of the differing values reported for various properties, such as electrical resistivity and diffusivity, probably is caused by using glass samples with varying water content. We have developed a technique for making glasses with nearly any desired level of water content.     

Gamma irradiation-induced effects in sodium diborate glasses doped with some transition metals

Sodium diborate glasses (Na₂B₄O₇) doped with 0.5 mol % Fe, Cu and V were irradiated by -rays at room temperature at a dose rate of 175 Gyh^–1 for 8 and 30 h. The spectral absorption of these glasses before and after irradiation was measured in the range 185–2500 nm and was compared with that of the base glass (Na₂B₄O₇). The spectral measurements were used to explore the effect of Fe, Cu and V in low quantities on the formation of colour centres in the base glass and, in particular, the sensitivity of these glasses to low and high doses of -rays.     

Properties and structure of semiconducting sodium iron germanoborate glasses

Several properties of semiconducting Na₂B₄O₇–GeO₂–Fe₂O₃ glasses have been measured. These properties include Mössbauer, density, d.c. conductivity, thermal expansion coefficient, glass transition temperature (T _g), and softening temperature (T _s). The d.c. conductivity was found to decrease as the iron content increases while the activation energy increases with increasing iron content in the glasses. From the conductivity temperature relation, it was found that the small polaron hopping model was applicable at temperature above _D/2 (_D: the Debye temperature); and the electrical conduction at T>_D/2 was due to non-adiabatic small polaron hopping of electrons between iron ions. The parameters obtained from the fits of the experimental data to this model appear reasonable and are consistent with the glass composition. Dilatometric measurements indicated that the thermal expansion coefficient decreases with the increase of iron content whereas both of the (T _g) and the (T _s) increase with increasing iron content. Different behaviors were observed in the Fourier transform infrared (FTIR) absorption spectra as a result of the progressive replacement of GeO₂ by Fe₂O₃. In the case of 20 and 25 mol % GeO₂, simultaneous existence of [GeO₄] and [GeO₆] polyhedra can be observed in the glass network. Below 20 mol % GeO₂, all germanium participating into the glass network is tetrahedrally coordinated, that is [GeO₄]. It is also indicated that, with the increase of iron content, boron attains an enhanced state for its coordination number to increase from 3 to 4, and consequently the tetrahedral [BO₄] units increase at the expense of the trigonal [BO₃] units, indicating the increase of the bridging character. The results of the measured properties were correlated with those from the FTIR spectroscopy.     

Reaction of sodium calcium borate glasses to form hydroxyapatite

This study investigated the transformation of two sodium calcium borate glasses to hydroxyapatite (HA). The chemical reaction was between either 1CaO · 2Na₂O · 6B₂O₃ or 2CaO · 2Na₂O · 6B₂O₃ glass and a 0.25 M phosphate (K₂HPO₄) solution at 37, 75 and 200 °C. Glass samples in the form of irregular particles (125–180 μm) and microspheres (45–90 and 125–180 μm) were used in order to understand the reaction mechanism. The effect of glass composition (calcium content) on the weight loss rate and reaction temperature on crystal size, crystallinity and grain shape of the reaction products were studied. Carbonated HA was made by dissolving an appropriate amount of carbonate (K₂CO₃) in the 0.25 M phosphate solution. X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy were used to characterize the reaction products. The results show that sodium calcium borate glasses can be transformed to HA by reacting with a phosphate solution. It is essentially a process of dissolution of glass and precipitation of HA. The transformation begins from an amorphous state to calcium-deficient HA without changing the size and shape of the original glass sample. Glass with a lower calcium content (1CaO · 2Na₂O · 6B₂O₃), or reacted at an elevated temperature (75 °C), has a higher reaction rate. The HA crystal size increases and grain shape changes from spheroidal to cylindrical as temperature increases from 37 to 200 °C. Increase in carbonate concentration can also decrease the crystal size and yield a more needle-like grain shape.     

Interaction of a simple Prandtl-Meyer wave with a weakly vortexed layer

The problem of interaction of a centered wave of rarefaction with a shear layer is solved in the case of a small flow vorticity in the shear layer. The solution is found in the form of an asymptotic series with respect to a small parameter of the problem. A system of equations derived in the zero approximation describes the flow in a simple wave. A uniformly applicable first-order expansion is constructed using the method of deformed coordinates.     

Ultrasonic investigation and infrared absorption of some sodium borate glasses

Ultrasonic velocities and absorption measurements in sodium borate glasses containing Na₂O have been made using the pulse echo technique at 4, 5 and 6 MHz. Results showed an increase in velocity as the sodium oxide content (mol%) increased. The ultrasonic absorption results showed the presence of very high and well-defined peak which shifts its position to lower temperatures with increasing frequency. This suggests some sort of relaxation process and the activation energy of this process increased with increasing Na₂O concentration (mol%). The infrared absorption spectra of sodium borate glasses confirmed the results obtained by ultrasonic investigations.