Field-driven redistribution of “guest protons” within protonated silicate glasses

Protonated layers of lithium silicate glasses generated by electrolysis were penetrated by lithium ions from appropriate anodic solutions during a second electrolysis. As the electric fields were applied in the same direction during both electrolyses, the number of protons within the glasses was unchanged. As is made apparent in the discussions of the local resistivities and the resulting profiles of the electric fields within the electrolysed membranes, the steady state of the protonated layer was disturbed by the lithium ions making the migrating front of the less mobile protons unstable. The protons were diluted, while simultaneously the layer thickness was increased.

A remarkable change in the ir spectrum observed after the second electrolysis and indicating that Beer's law is not valid is explained by the tendency of protons to occupy preferred sites. It is proposed that mixed twin siloxy groups containing one lithium ion and one proton with different bond energies and thus different probabilities for leaving the entity act as proton traps and function as single sites for migrating lithium ions. The effect was observed with several lithium silicate glasses but not with a lithium aluminum silicate glass. Protons introduced as probes for alkali into glasses may thus contribute information on the glass structure.     

Free energy profiles of Al and La cation distribution in silica and soda silicate glasses

The factors that control the distribution of Al³⁺ and La³⁺ cations in silica and soda silicate glasses is examined by using molecular dynamics (MD) simulations. In particular, the response of the glass network to the presence of metal oxide is probed using a liquid state theory that treats the glass network as a solvent and the metal cation as a solute. MD simulations are used to obtain the mean solvent-solute and solute-solute force. The trajectory used to determine the free energy is analyzed to determine the stable configurations of a cation pair. Details of determining the potential of mean force for an Al cation pair in silica and silicate glass is presented. A comparison of these results with those previously calculated for a La cation pair in the same glass systems is given. The results reveal that there are differences in how the network accommodates the two different size cations. It is found that for the potential used here, based on the Vessal potential, the network wraps itself around the larger La cation forming a solvent shell, whereas, the smaller Al cation is incorporated into the network backbone. In silica and soda silicate glasses, La ion pairs cluster to form La-O-La linkages. In contrast, the glasses favor a separated state of the Al ion pair.     

High-chromium containing lithium silicate glasses

Chromium as a transition metal oxide is incorporated into lithium silicate glass compositions. It supplies to these glasses adequate absorption and reflectance characteristics for electromagnetic radiation. This will make it possible to attain appropriate materials to apply as absorbers of solar energy. Properties of the glass and glass ceramics produced are investigated.     

Europium environment and clustering in europium doped silica and sodium silicate glasses

The local environment and distribution of rare earth ions are important to the optical properties of rare earth doped oxide glasses. In this paper, we report studies of the structures of europium doped (around 1 mol% Eu₂O₃) silica and sodium silicate glasses using molecular dynamics simulations. By using effective partial charge potentials, systems with over 24,000 atoms were modeled in order to obtain better statistics of rare earth ion distribution. The simulated glass structures were validated by comparing the calculated neutron and X-ray structure factors with experimental data. It was found that europium ions have higher coordination number (5.9 versus 4.8) and more symmetric environments in sodium silicate glasses than in the silica glass. Rare earth ion clustering has been characterized in detail and it was found that the clustering probability of europium ions in sodium silicate glass is consistently less than that predicted from a random distribution, while the probability of clustering in pure silica glass is higher than that of random distribution at the 1 mol% doping level.     

Partial ordering of glass networks adjacent to simulated glass–crystal interfaces

Interfaces between sodium lithium borosilicate glasses and the (100) and (110) surfaces of MgO and CaO crystals were simulated using a melt-quench procedure, employing classical pair potentials and molecular dynamics. The density of network forming species within the glass at these interfaces was considered as a function of distance from the plane of the interface and the positions of network formers were calculated in relation to sites in the crystal surface.For each interface, a strong correlation was found between the position and orientation of borate and silicate coordination polyhedra within the glass and particular positions of ions in the crystal surface, indicating that different partial ordering of the glass had occurred. In addition, examination of oxygen density profiles revealed a sequence of consistently spaced layers of increased density extending into the glass for all interfacial systems.     

Leaching of some lithium silicate glasses and glass-ceramics by HCl

The leaching of some binary and ternary lithium silicate glasses and their respective glass-ceramics by HCl is reported.The leaching rate of lithium silicate glasses gradually decreases with the decrease of the percentage of Li₂O or by the introduction of small amounts of a third component, e.g. Al₂O₃, MgO, ZnO or B₂O₃. With a further increase in the proportions of B₂O₃ or ZnO the rate of leaching increases. The rate of leaching is also substantially modified by the conversion of glasses into glasses-ceramics.The results obtained are discussed in terms of the effects of the different ions on the rate of the interdifussion of the lithium and hydrogen ions in the glass and the leached layer, the phase separation developed in the glass, the type and concentration of crystalline phases developed in glass-ceramics and the composition of the residual glass phase.     

Structural and optical properties of lithium barium bismuthate glasses

Structural and optical properties of some ion-conducting lithium barium bismuthate glasses have been studied. The structure of these glasses has been explored from the compositional variation of the density, the molar volume and the glass transition temperature. The density and the molar volume decrease with the increase of lithium ion content in these glasses keeping the barium content fixed. The optical studies in the ultraviolet–visible (UV–VIS) and infrared (IR) regions for all these glasses show a sharp cut-off and a large transmitting window, which make them appealing candidates for the different spectral devices.     

Chromium doping of silicate glasses by field-assisted solid-state ion exchange

Field-assisted solid-state ion exchange (FASSIE) is a suitable way to dope glasses with metallic ions. This approach is a promising technique for the production of glass waveguides containing either bivalent or trivalent ions, allowing the doping of glass surfaces with multivalent ions which could not diffuse into the glass matrix by means of the usual thermal ion-exchange process in molten salt baths. In this paper, results on the diffusion of chromium in silicate glasses are presented. A metallic chromium film deposited on the top of the glass substrates was used as the metal ions supplier. The doped layers were investigated by secondary ion mass and Rutherford backscattering spectrometries, as well as by micro-Raman spectroscopy. Chromium entered the glass matrices for some hundreds of nanometers, depending on the process temperature and the applied electric field. Strong compositional modification of the treated glasses was detected, related to alkali and alkali-earth elements distribution. For field-assisted solid-state diffusion, borosilicate glasses seem to be more stable matrices than the soda-lime silicate ones.     

Compositional dependence of the first sharp diffraction peaks in alkali silicate glasses: A molecular dynamics study

The compositional dependence of the first sharp diffraction peaks (FSDPs) of lithium, sodium and potassium silicate glasses were studied by calculating the neutron static structure factors. The advantage of using molecular dynamics simulations is that the contributions of partial structure factors to the FSDPs can be easily determined and provides the basis for detailed analysis. Examination of the correlations of the FSDP with the short and medium range structure reveal that the position and the shape of FSDP strongly depend on the type and concentration of alkali oxide in alkali silicate glasses. The characteristic repeating distances and the characteristic correlation lengths both decrease with increasing alkali oxide concentration indicating a decrease in the intermediate range order. In the lithium silicate glasses, the characteristic correlation length increases with lithium oxide concentration that is anomalous in that the trend is opposite to the other alkali silicate glasses. This anomaly is explained by the high field strength of lithium ions that increases the intermediate range order of the silicon oxygen network.     

Electronic state of sulfide-based lithium ion conducting glasses

Electronic states of the sulfide-based lithium ion conducting glasses were calculated by the DV-Xα cluster method. The cluster models were constructed by the coordination number reported by experimental methods and the bond length estimated from the ionic radii of each ion. The movement of the Li ion was simulated by several model clusters with different positions of the moving ion. The relationship between ionic conductivity and the differential total bond overlap population (DBOP) was discussed for the sulfide-based glasses in the systems Li₂S–SiS₂–Al₂S₃ and Li₂S–SiS₂–P₂S₅. In these glasses, the DBOP with the movement of the lithium ion had good negative correlations with the ionic conductivities and positive correlations with the activation energies obtained by the experimental measurements. In any cases, the smaller change of the total bond overlap population of the moving cations played an important role for the fast ion movement in the superionic conducting glasses. This bonding state of the moving cations is one of the characteristics of the electronic state in the sulfide-based lithium ion conducting glasses.     

Thermal,structure and morphological properties of lithium disilicate glasses doped with copper oxide and their glass–ceramic derivatives

This research aims to investigate and compare the structural and the morphological properties of both lithium disilicate glasses doped with copper oxide and their glass–ceramic derivatives. Density measurements were measured for all samples by Archimedes method at room temperature. Differential scanning calorimetric analysis was used to determine the glass transition temperature (T_g) and crystallization temperature (T_c) for all glasses. The glass transition temperature was observed to decrease with increasing CuO concentration indicating the formation of non-bridging oxygen bonds in the glass network. X-ray analysis patterns reveal the appearance of crystalline lithium metasilicate phase as the main phase within the glass–ceramic derivatives, and their crystallite sizes were observed to decrease as the CuO increased. Experimental infrared absorption data indicate the existence of characteristic vibrational bands due to structural building SiO₄ units in resemblance to the same vibrations observed from traditional crystalline silicates. Scanning electron microscopic investigations show the vitreous nature for lithium disilicate glasses and the distinct crystalline morphological features for the corresponding glass–ceramic derivatives.     

Effect of strontium substitution on the structure,ionic diffusion and dynamic properties of 45S5 Bioactive glasses

Strontium ions can promote bone growth and enhance bioactivity in bioactive glasses that find critical biomedical applications. In this paper, the effect of SrO/CaO substitution on the structure, ionic diffusion, and dynamic properties of 45S5 bioactive glasses has been studied using constant pressure molecular dynamics simulations with a set of effective partial charge potentials. The simulated structure models were validated by comparing with experimental neutron diffraction results. It was found that the SrO/CaO substitution leads to an increase of glass density and decrease of oxygen density, a measure of compactness of the glass, in excellent agreement with available experimental data. On the other hand, the substitution does not significantly change of the medium range glass structures as characterized by silicon and phosphorous Q_n distributions, network connectivity, and ring size distributions. The diffusion and dynamic behavior of these glasses and their melts were also determined by calculating the mean square displacements and velocity autocorrelation functions. It was found that the diffusion energy barriers of sodium, calcium and strontium ions remain nearly constant with respect to the level of substitution. However, strontium ions do influence the diffusion behaviors of calcium and sodium ions at high temperature, as evidenced from their velocity autocorrelation functions. Like calcium and sodium, strontium ions only contribute to the lower frequency (around 100 cm^? 1) of the vibrational spectra the substitution has little effect on high frequency features and the general shape of the vibrational density of states. These results suggest that the increase of the dissolution rate in strontium containing glasses are mainly due to the increase of free volume and the non-local effect that weakens the silicon-oxygen network due to strontium ions.     

The corrosion of zircon and zirconia refractories by molten glasses

The corrosion of ZrO₂-containing refractories by molten glasses has been investigated experimentally in relation to the compositions of the glasses. For a zircon refractory, zircon crystals contacting the glass decomposed into ZrO₂ crystals and glassy phase at higher temperatures. The decomposition temperature was changed with the glass compositions. Alkali components in the glasses were confirmed to be most corrosive for the zircon refractory. For a ZrO₂ refractory, compositions of the glass-refractory interfaces were nearly saturated with ZrO₂. The corrosion rate of the ZrO₂ refractory was considered to be controlled by the transport rate of zirconium away from the interface.

The concentration of CaO in glasses, as well as that of alkali components, was found to be very effective for increasing the corrosion rate of ZrO₂ refractories.     

Structural properties of lithium metaphosphate glasses by ab initio molecular electronic structure calculations

Following a theoretical determination of cluster-models for lithium metaphosphate glass, a theoretical method for the prediction of structural data such as radial and bond distribution functions is presented. These are calculated and compared to experimental data for this particular glass. Useful conclusions are drawn regarding the general use of molecular electronic structure methods for the determination of the structure of glasses.     

Amorphous phase separation and crystallization in a lithium silicate glass prepared by the sol-gel method

A 10Li₂O---90SiO₂ (mol%) gel-glass has been prepared by using tetramethyl orthosilicate and lithium iso-propoxide as starting materials. The phase separation and crystallization behaviour was compared with the corresponding conventionally melted glass using DTA, X-ray diffraction and TEM. The same crystallization phase was found in both the gel glass and melted glass upon heating above 650°C. However, the rate of crystallization in the gel-glass was higher than in the melted glass. TEM revealed amorphous phase separation in the gel glass and melted glass. However, the morphologies were different, an interconnected microstructure being observed in the gel glass and a droplet structure in the melted glass. These differences can be partly attributed to differences in OH content. Other potential influencing factors are also considered. After 650°C for 2 h lithium disilicate crystals were observed in the volume of the gel glass by TEM. As the crystals grew they absorbed Li₂O from the surrounding lithia-rich amorphous phase so that silica-rich (lithia depleted) diffusion zones formed around them.     

Diffusion-controlled reaction of water with glass

A model in which the diffusion of molecular water controls the rate of reaction of water with glass is considered. As a result of a number of questionable aspects of this model, it is concluded that the ion exchange mechanism is still the best one for the diffusion-controlled reaction of water with alkali silicate glasses.     

Sodium ion transport in high purity borosilicate glasses

²²Na diffusion coefficients and dc electrical conductivity were measured in 0.7 B₂O₃·0.3 SiO₂ (mole ratio) glasses containing small amounts of Na₂O (111 ppm to 2.6 mol%). Correlation factors near unity were obtained from the Nernst-Einstein equation for these glasses. These values are in agreement with a previously proposed free “interstitial” alkali ion diffusion mechanism in low alkali content glasses. It is concluded from the low values of the sodium diffusion coefficients and dc conductivity in these glasses (relative to similar silicate and germanate glasses) that the Na⁺ are bound strongly to the borate-rich glass network and that the number of dissociated Na⁺ at any given time is small.     

Sensitivity of structural results to initial configurations and quench algorithms of lead silicate glass

The sensitivity of resulting structures to starting configurations and quench algorithms were characterized using molecular dynamics (MD) simulations. The classical potential model introduced by Damodaran, Rao, and Rao (DRR) Phys. Chem. Glasses 31 (1990) 212 for lead silicate glass was used. Glasses were prepared using five distinct initial configurations and four glass forming algorithms. In previous MD work of bulk lead silicate glasses the ability of this potential model to provide good structural results were established by comparing to experimental results. Here the sensitivity of the results to the simulation methodology and the persistence of clustering with attention to details of molecular structure are determined.     

Investigation of surface layers,formed on glass electrode membranes in aqueous solutions,by means of an ion sputtering method

A method of measuring concentration profiles in glass surface layers by photon emission during ion sputtering, that offers a depth resolution of 30 to 50 Å, is applied to leached surface layers of pH-electrode membranes. Two steady state layer regions different with respect to ion diffusion are found at low pH. Within the outer layer the interdiffusion coefficient is constant. Within the transition layer interdiffusion coefficients are observed with a minimum that is two orders of magnitude lower than the lithium diffusion coefficient within the glass and is explained by a low diffusion coefficient of interdiffusing protons within a highly glass-like structure. The explanation is supported by the similar magnitude of the diffusion coefficient obtained here and that of protons which have replaced lithium ions within this glass in an electric field. The corresponding maximum resistivity agrees with measurements by Wikby.Surface layer properties and additional experimental results suggest that the equilibrium determining the pH-membrane potential is the dissociation of SiOH-groups at the immediate layer surface rather than an ion exchange of glass alkali and solution hydrogen ions.     

Correlation effects on alkali ion diffusion in binary alkali oxide glasses

The correlation factor in the Nernst-Einstei equation for low sodium Na₂OGeO₂ glass determined from dc conductivity and ²²Na diffusion coefficient measurements was found to be near unity. Values of the correlation factor were also compiled from the literature for higher alkali content germanate glasses as well as for sodium borate and alkali silicate glasses. In all three systems the correlation factor was found to depend primarily on the alkali content in the glass. Specifically, uncorrelated ionic diffusion ($\text{? ? 1}$) occurs in low alkali glasses while correlated motion ($\text{? < 1}$) takes place at higher alkali concentrations. This observation is consistent with the theory that many “holes” exist in low alkali glasses through which the diffusing cation can randomly jump.