Elasticity of ion stuffing in chemically strengthened glass

The chemical strengthening of glass involves the stuffing of large ions into network sites previously occupied by smaller ions. Typically this involves an exchange of Li⁺ or Na⁺ ions in the glass for larger K⁺ ions from a molten salt bath. This swapping of ions creates compressive stress in the surface of the glass, significantly increasing the strength of the final glass product. The magnitude of this compressive stress is governed by the linear network dilation coefficient (LNDC), which defines the amount of linear strain per unit of ion substitution. However, the amount of strain attainable through ion exchange is much smaller compared to what is expected from as-melted versions of the same final glass composition. This effect, which we have termed the “network dilation anomaly,” is a result of the different local environment around the invading ion species in as-melted versus ion-exchanged glasses. A remaining question concerns the nature of the network strain due to ion stuffing. Using molecular dynamics simulations, we show that the strain induced by ion stuffing is entirely elastic. In other words, when a reverse ion exchange is performed to swap the original ions back into the glass, the initial volume of the as-melted glasses is entirely recovered. Moreover, we show that the local structural environment around the alkali ions is restored to the as-melted conditions. The elastic nature of ion stuffing demonstrates that the network dilation anomaly is not a result of plasticity, but rather a failure to achieve the full amount of expected elastic strain during ion exchange. The elasticity itself consists of both instantaneous and delayed contributions.     

Rayleigh wave study of surface diffusion and stress relaxation in glass

Surface wave propagation on ion-exchanged glass surfaces has been systematically studied. The glass system was an alkali-alumino silicate glass, and the ion-exchange involved the replacement of lithium ions by sodium ions. Accurate velocity measurements were carried out on systems ion-exchanged for various times and at various temperatures. The effects of density and modulus changes on the velocity have been separated. Stress build-up and relaxation have been observed which correlate well with shear viscosity measurements. In addition, the diffusion coefficient for the Na⁺ ? Li⁺ exchange has been determined at a variety of temperatures.     

Kinetics of ion exchange in glasses

The kinetics of $\text{K}{}^{\mathrm{+}}\text{?}\text{Na}{}^{\mathrm{+}}$ exchange in two glass systems, 20Na₂O·(60?x)B₂O₃· (20 + x)Si₂ (where x = 0, 15, 30 and 45 mol%) and Na₂O·3SiO₂, were studied as a function of glass composition, salt bath composition, exchange temperature and time The distribution of K in the glass specimens after exchange in molten KNO₃ was determined with an electron probe. Stresses in these speciments were measured photoelastically. The interdiffusion coefficient $\text{D}$ for ion exchange was calculated as a function of local composition in the glass using the Boltzmann-Matano method. The strong variation of $\text{D}$ in any particular glass approximated that predicted by a mixed alkali model (as advanced by Lacharme), where the glass in the ion-exchanged region approximates a composite of stacked layers of mixed alkali glasses with a gradually varying alkali ratio. The small discrepancy between the experiment and the mixed alkali model was partly, but not fully, reconciled by considering the strains in the glasses. The observation which remained unexplained was that the calculated stress profiles did not show perfect agreement, both in magnitude and in shape, with the experimentally measured stress profiles. It appeared that the kinetics of ion exchange in the glasses were also influenced by a network relaxation process which may have occurred well below the glass transition temperature.     

Influence of relaxation on migration of ions during ion exchange by electrolysis in glass

After the first electrolysis of an ordinary sheet glass causing Na⁺ → K⁺ ion exchange below its strain point, the glass was subjected to the second electrolysis under the reverse electric field. In the second electrolysis, some of the potassium ions were removed from the previously ion-exchanged layer, while potassium ions were simultaneously introduced into the opposite surface (the anode side). The initial resistivity and the slope of the straight line expressing the relation of resistivity to the quantity of electricity passed through at the second electrolysis were determined numerically. These observations agreed fairly well with theoretical values obtained by taking stress relaxation into account.     

Optical spectroscopy of silver ion-exchanged As-doped glass

Soda-lime-silicate glass containing arsenic oxide and undoped soda-lime-silicate glass (blank) are prepared by melting from pure sand (iron concentration lower than 0.01 wt%). The effect of arsenic on the optical properties of the glass with and without silver ion exchange at 325 °C for various times is investigated by optical absorption and photoluminescence spectroscopy. Emission/excitation spectra of silver ion exchanged glass allow differentiation of three stages in the silver incorporation into the glass network. First and second stages are only observed in the undoped glass ion exchanged for short times. Such stages are associated with the presence of isolated Ag⁺-ions and Ag⁺-Ag⁺ pairs, respectively. The third stage appears in the undoped glass ion exchanged for times longer than 10 min and in the arsenic-doped glass even for exchange times as short as 1 min. Then, this stage is characterised by molecular mixed species formed with Ag⁺ and Ag⁰, which coexist with nanoparticles of metallic silver. The presence of those Ag⁰-aggregates gives a yellow colour to the glasses, which show the well-know absorption band at about 400 nm due to surface plasmon resonance.     

Phosphate glass dissolution in aqueous solutions

Rates of aqueous dissolution are reported for a series of phosphate glasses having the composition (50 ? X)M₂O·(X)CaO·50P₂O₅. Dissolution reactions involving the consumption of H⁺ and OH^? were monitored using pH stat titration techniques, solution analyses using inductively coupled plasma emission, and depth profiling of corroded glass surfaces for H and other elements using elastic recoil detection analysis and Rutherford backscattering. These analytical results indicate that the phosphate glasses dissolve uniformly due to acid- or base-catalyzed hydration of the polymeric phosphate network. The rate and nature of this hydration appears to be controlled by the surface pH and/or charge which develops at the glass/ solution interface. The implication of these results on dissolution models for both the phosphate and silicate glasses is discussed.     

Investigation on sol–gel silica coatings for the protection of ancient glass: Interaction with glass surface and protection efficiency

To hinder the phenomenon of weathering of ancient stained glass, the present work proposes the application of sol–gel coatings to the glass surface. Previous investigations [1], [2], [3], [4], [5] and [6], in fact, show that sol–gel silica coatings do not change the appearance of artistic glasses when deposited on their surface. Moreover, the film thickness is so small (around 200 nm) and its composition and structure so compatible with that of the original glass, that the characteristics of the coating and original glass are not distinguishable. In this work, several recipes used to produce sol–gel coatings have been tested in order to understand their behavior when adopted for covering ancient weathered glass. The coatings are made of sol–gel silica prepared with two different catalysts: H⁺, Pb²⁺ and without catalyst. All the investigated samples show a good adhesion of the coating to the glasses used to simulate the behavior of ancient artefacts. The sol–gel silica coatings have been studied before and after accelerated ageing to test the resistance of the protective coatings to weathering. Another important index to test of the efficiency of the sol–gel coatings for the protection of an ancient glass is the lead ion mobility. In ancient stained glass, in fact, this element is present in the metallic lead network, in the grisaille paintings and constitutes a main component of many glass tesserae. The action of water on this highly mobile ion involves the degradation of the glass itself and the release of the ion in the rain solution. Ageing tests show the efficiency of H⁺ and Pb²⁺ catalyzed coatings and the inefficiency of the non-catalyzed sol–gel layers.     

Electrical conductivity of silicate glasses with tetravalent cations substituting Si

The effects of substituting Si by M^4 + cations in soda-lime silica glasses were analyzed by impedance spectroscopy in the frequency range of 1 Hz–1 MHz. The glass composition was (mol%) 22Na₂O·8CaO·65SiO₂·5MO₂, M = Si, Ti, Ge, Zr, Sn, and Ce. Although the Na⁺ concentration in the glasses is constant, the Zr-containing glass exhibits the highest dc conductivity and the lowest activation energy, while the Ce-containing glass exhibits the lowest conductivity. The activation energies obtained experimentally agree with those obtained by a theoretical equation proposed by Anderson and Stuart. The differences in electrical conductivity presented by the several M-containing glasses are attributed to the effect that the M^4 + ion has on the mobility of the diffusing Na⁺ ion.     

Changes of physical properties of glass surfaces exposed to KNO3 vapors

Modifications of glass surfaces were studied after exposure of samples to an atmosphere resulting from the decomposition of molten KNO₃. The diffusion coefficient of K⁺ ions migrating into the surfaces of float glass and synthesized glasses doped with up to 5 wt% SnO₂ was calculated by the Boltzmann–Matano technique. The Vickers hardness and the refractive index increase with exposure time. Infrared spectra show that the migration of K⁺ is responsible for an increase in the number of non-bridging oxygens in the exposed samples. The spectra of the synthesized glasses present evidences that their surfaces undergo crystallization during the exposure. All results lead to the conclusion that the presence of tin in the glasses hinders the diffusion of K⁺ ions, thus affecting the Vickers hardness, the refractive index and the infrared spectra. It is shown that the exposure method can be used as an alternative process to promote the K⁺ migration into glass surfaces.     

Atomistic understanding of the network dilation anomaly in ion-exchanged glass

Chemically strengthened glasses are of increasing technological importance for personal electronic devices, tablet computers, and a variety of other applications. However, there are many unexplained phenomena associated with the physics of the ion exchange process used for strengthening. One of the most puzzling of these is the anomalous behavior of the network dilation coefficient, i.e., the parameter governing the amount of linear strain of the glass per unit of alkali ions exchanged, which is inevitably a factor of 2–4 higher for as-melted glasses as compared to chemically strengthened versions of the same glass compositions prepared via ion exchange. In this paper, we investigate the atomistic origin of this discrepancy between as-melted and ion-exchanged glasses based on molecular dynamics simulations of a series of alkali tetrasilicate glasses, viz., xNa₂O·(20 ? x)K₂O·80SiO₂ (mol%). The network dilation coefficient of the ion-exchanged glasses is dependent on composition and ranges from 30% to 54% of the ideal value obtained from the as-melted glasses. This anomalous behavior of the network dilation coefficient is explained in terms of different local environments between sodium and potassium sites in the glass network and a two-stage relaxation process of the local potassium environment after ion exchange.     

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.     

Up-conversion enhancement in Er3 +-Ag co-doped zinc tellurite glass: Effect of heat treatment

The melt quenching method was used to synthesize the Ag⁰ nanoparticles and Er^3 + ions co-doped zinc tellurite glass. The glasses were characterized by differential thermal analyzer, UV–VIS-IR absorption, photoluminescence spectroscopy and TEM imaging. Heat treatment at different annealing time intervals above the glass transition temperature was applied to reduce the Ag⁺ ions to Ag⁰ NPs. The influence of heat treatment on structural and optical properties is examined. Intense and broad up-conversion emissions of silver are recorded in the visible region. Up-conversion luminescence spectra revealed three major emission peaks at 520, 550 and 650 nm originating from ²H_11/2, ⁴S_3/2 and ⁴F_9/2 levels, respectively. An efficient enhancement in visible region is observed for samples containing silver NPs. The absorption plasmon peaks are evidenced around 560 and 594 nm. The effect of localized surface plasmon resonance and the energy transfer from the surface of silver NP to trivalent erbium ions are described as the sources of enhancement.     

Strengthening of soda-lime-silica glass by surface treatment with sol–gel silica

In this study, the failure resistance of soda-lime-silica glass was increased by surface treatment with sol–gel silica. Samples annealed and ion-exchanged in KNO₃ for 24 h at 450 °C were considered. Sol–gel silica coating was carried out by dipping the glass samples into a sol suspension prepared by hydrolysis of Si(OEt)₄ in ethanol/water solution. The deposited layer was consolidated in air for 24 h and subjected to mild thermal treatment at 300 °C for 1 h. The surface treatment increased the fracture resistance of annealed glass of about 35 MPa; conversely, ion-exchanged specimens showed an average increase of about 90 MPa. The strengthening effect induced by the surface treatment was attributed to the reduction of the effective crack length generated by the silica coating. The different strength increase between annealed and ion-exchanged samples is discussed in terms of fracture toughness which, for ion-exchanged glass, is not constant, due to the presence of the surface residual stresses and thus the reduction of the crack length due to the silica coating determines a higher strength increase than for annealed glass.     

Activation volumes and site relaxation in mixed alkali glasses

Evidence is presented for site relaxations occurring in mixed alkali (cation) glasses based on activation volumes, V_A(σ) = −RT[d ln σ/dp]_T, which are determined for sodium aluminoborate glasses of varying Na₂O content, and for mixed alkali glasses where Na⁺ is partially replaced by Li⁺, K⁺ or Cs⁺ ions. In accordance with the ‘updated’ dynamic structure model, activation volumes are identified here with local expansions that accompany the opening up of C′ sites to admit incoming ions. ‘Anomalous’ increases in activation volume in mixed cation glasses correlate with the size of minority (guest) cations. This anomaly is interpreted in terms of a ‘leader follower’ mechanism that involves dynamic coupling between the faster (majority) and slower (minority) cations. Because of mismatch effects in mixed cation glasses this coupling requires the opening up of additional cation sites by the slower follower cations. The resulting disturbances in the glass network are responsible for many characteristic features of the mixed alkali effect, including the appearance of high temperature internal friction peaks and observed minima in glass transition temperatures and melt viscosities.     

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.     

Visible to near-infrared down-conversion luminescence in Tb3 + and Yb3 + co-doped lithium–lanthanum–aluminosilicate oxyfluoride glass and glass-ceramics

Tb^3 + and Yb^3 + co-doped oxyfluoride glasses were fabricated in a lithium–lanthanum–aluminosilicate matrix (LLAS) by a melt-quench technique. Glass-ceramics were obtained by appropriate heat treatment of the as-prepared glasses. Visible to near-infrared down-conversion luminescence was studied for glass and glass-ceramic samples with different Yb^3 + concentrations. It has been found that the luminescence intensity at 940–1020 nm from Yb^3 + ions increases while the emission lifetime of Tb^3 + ions decreases in the glass-ceramic compared to that in the as-prepared glass, which indicates that the energy transfer efficiency increases in the glass-ceramics compared to that in the as-prepared glass. The down-conversion luminescence also increased for increasing Yb^3 + concentration from 1 mol% to 2 mol%, but decreased for the sample with a high Yb^3 + co-doping concentration of 8 mol%, which is attributed to the concentration quenching.     

IR-spectral study of vanadate vitreous systems

Some structural conclusions have been reached concerning vanadate glasses according to the influence of the Mⁿ⁺-ions on the vibrations of the isolated V-O-bonds and the location of the bands in the IR-spectra. They may shift with the compositions or preserve their positions.Ions of predominant electrostatic interactions are distributed between the chains and the layers in the glass structure, they interact directly with the isolated V-O bonds. As a result of this the transition from VO₅ into VO₄ is achieved directly or through intermediate complexes.It is likely that ions which participate in covalent bonds fit into vanadate chains of the glass network in substitution positions.     

Copper redox behavior, structure and properties of copper lead borate glasses

Copper oxidation states, structure and properties of xCuO · (50-x)PbO · 50B₂O₃ glasses were investigated. Both infrared (IR) and ¹¹B magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies were employed to determine the tetrahedral BO₄ fraction in the glasses as a function of CuO content. IR study indicates that the replacement of Pb²⁺ by Cu²⁺ ions increases the BO₃ units by converting BO₄- containing groups into ring type metaborate groups. The oxidation states of copper ions in the glasses have been studied using both X-ray photoelectron spectroscopy (XPS) and the wet chemical method. For high CuO containing (?30 mol%) glasses, high Cu⁺ ion concentrations (Cu⁺/Cu_tot.>0.3) result in a relatively slow disproportionation of B⁴-containing groups because of the small coordination number of Cu⁺ compared to Cu²⁺ ions. Effects of both glass structure and redox states of copper ions on glass properties including density, Vickers’ hardness, coefficient of thermal expansion, and chemical durability have been discussed.     

Novel properties of glass–metal nanocomposites

Nanoparticles of silver and copper were grown at the glass–crystal interfaces within a silicate glass by reducing the ion-exchanged glass–ceramic concerned. By controlling the reduction treatment a wide range of surface resistivity e.g., from 0.2 to 10¹⁰ Ω/sq. could be generated. Silver nanowires of diameter ∼40 nm were grown within the pores of a silica gel. They exhibited single electron tunneling as evidenced by conductance maxima at definite intervals of the applied voltage. Silver nanowires of diameter 0.5 nm were grown within the crystal channels of fluorophlogopite mica which were first precipitated in a silicate glass. The nanowires when broken gave giant dielectric permittivity (∼10⁷) to the composite. Copper core–copper oxide shell and iron core–iron oxide shell nanostructures respectively were generated within a silica gel. The core–shell structure exhibited electrical conductivity several orders of magnitude higher than that of the precursor gel. An interfacial amorphous phase contributed to this increase in electrical conductivity. Glass–ceramics containing BaTiO₃ and nanoparticles of silver showed a five order of magnitude decrease in electrical resistance as the relative humidity was changed from 25% to 75%. Arrays of metal nanoparticles (silver or copper) grown within a silicate glass exhibited a diode-like behavior. This was explained as arising due to formation of metal–semiconductor nanojunctions – metal particles smaller than 3 nm behaving like a semiconductor. The examples reviewed here show that exploiting the void spaces available in an oxide glass nanophases of a wide variety could be grown within and novel properties generated. This approach could be promising in imparting new functionality to conventional glasses.     

Synthesis of two-dimensional metallic silver using sodium beta-alumina crystal channels

Sodium beta-alumina (β-NaAl₁₁0₁₇) crystals were grown within a gel containing Na₂O and Al₂O₃. The glass–crystal composite was put through a Na⁺ ? Ag⁺ ion-exchange reaction. The ion-exchanged glass–crystal was then subjected to an electrodeposition process. Nanosheets of metallic silver were found to have grown within the β-NaAl₁₁0₁₇ channels which usually contain the mobile sodium ions. The DC electrical resistance of the composites was caused due to charge transport in the two-dimensional crystal planes. The average silver layer thickness was ～0.6 nm and the interlayer separation ～1.13 nm.