Er-doped silica–hafnia films for optical waveguides and spherical resonators

In this paper we present some result on sol–gel derived silica–hafnia systems. In particular we focus on fabrication, morphological and spectroscopic assessment of Er³⁺-activated thin films. Two examples of silica–hafnia-derived waveguiding glass ceramics, prepared by top–down and bottom–up techniques are reported, and the main optical properties are discussed. Finally, some properties of activated microspherical resonators, having a silica core, obtained by melting the end of a telecom fiber, coated with an Er³⁺-doped 70SiO₂–30HfO₂ film, are presented.     

Chemical attack of optical glass surface by formic acid vapor

The surface deterioration of glass by the attack of formic acid vapor was observed by an optical microscope, an ellipsometer, an X-ray diffractometer and an electron microprobe X-ray analyzer. The refractive index and thickness of the anomalous surface film formed on the glass were measured by means of ellipsometry. In the weathering of SF-3 and KzFS-2 glasses, the presence of water vapor in the atmosphere is necessary to produce stains on the glass surface. In the SF-3 glass with a SiO₂ network, the diffusion of Pb²⁺ and the formation of fine crystalline lead formate play important roles. In the KzFS-2 glass with a B₂O₃ network, both the destruction of the B₂O₃ network and the formation of fine crystals of boric acid play important roles. The diffusion of Pb²⁺ is not so important.     

Properties and microstructure of silica glass incorporated with tributyl phosphate by sol–gel method

An organic phosphate species tributyl phosphate (TBP) was incorporated into sol–gel-derived glass matrix. TBP could be directly added to the hydrolyzed silica source from tetraethylorthosilicate (TEOS) and immobilized in silica glass matrix. TBP was stably immobilized in silica glass matrix even in the case where the weight ratio of TBP to silica was unity, and where the volume fraction of the glass sample occupied by TBP moiety was as large as 69%. The glass sample showed an appearance of hard glassy solid even at such a large fraction of TBP which is an organic solvent in the neat state at room temperature. The FT-IR spectrum showed that TBP was immobilized in silica glass in an intact state without chemical bonding with the siloxane network. The Vickers hardness was large enough even at higher weight ratios of TBP to silica to be measured as data indicating that the immobilized TBP molecules could play a promotive role in forming the siloxane bonding. The wide-angle X-ray scattering experiments revealed that the siloxane bonding was expanded by TBP molecules entrapped in the siloxane network. Furthermore, TBP molecules are dispersed in the siloxane network in the molecular scale.     

Raman and ESR study of sol–gel materials from ZnO–TiO2–B2O3 system

The paper studies the materials from ZnO–TiO₂–B₂O₃ ternary system, obtained by sol–gel method [1] and [2], starting from organic and inorganic precursors. The obtained samples are investigated by FTIR and Raman spectroscopy, which provide structural information, at molecular level. FTIR absorption maxima are identified and discussed according to literature data. Raman spectra are acquired by a Raman Jasco NRS-3100 spectrometer, at 532 nm wavelength and put in evidence characteristic vibration modes for all three oxide components. ESR spectra were plotted with the aid of a JES-FA 100-JEOL Japan spectrometer and titanium surrounding is investigated.     

Fluorescence of copper, manganese and antimony ions in phosphate glass host

The paper presents a study based on luminescence characteristics of phosphate glasses containing Cu²⁺, Mn⁴⁺ and Sb³⁺. The glass samples obtained by a wet chemical route belong to Li₂O–BaO–Al₂O₃–La₂O₃–P₂O₅ oxide system. The oxide composition of the glass samples is calculated to obtain a vitreous network composed of metaphosphate chains bonded by modifier ions (Li⁺, Ba²⁺ and La³⁺) and fluorescent ions. The absorption spectra of the samples were acquired in the UV domain in order to establish the excitation wavelength for each fluorescent ion. The absorption peaks of Sb³⁺ ion are ranged at 285 nm and 250 nm, Mn²⁺ ion at 280 nm and 365 nm, Cu²⁺ ion at 295 nm and 313 nm. The luminescence peaks of Cu²⁺, Mn⁴⁺ and Sb³⁺ ions are found in the visible domain at different wavelengths, depending on the oxidation state and coordination symmetry of each fluorescent ion. The fluorescence of Sb³⁺ ion has a strong signal at 450 nm and a weak one at 465 nm, Mn²⁺ ion shows a fluorescence peak at 600 nm and the pair Cu²⁺/Cu⁺ ions reveals a fluorescence emission at 460 nm.     

Preparation and properties of porous glass using fly ash as a raw material

The porous glasses were prepared by a conventional phase separation method using coal fly ash as a raw material, and the properties of these porous glasses were investigated. The composition of coal fly ash is basically composed of SiO₂–Al₂O₃–Fe₂O₃–CaO system and the SiO₂–B₂O₃–Al₂O₃–CaO–Na₂O system of glass was chosen as base glass composition. The pore diameter increases proportional to cube root of heating time (t^1/3), however, the early stage of phase separation is not clear. It is estimated that the rate determining step may be the diffusion process of structural units involving oxygen ions and the phase separation may take place by the nucleation and growth mechanism, and the relatively larger pores of above 1 μm can be obtained easily. The chemical composition of porous glasses is SiO₂–B₂O₃–Al₂O₃(–CaO–Na₂O). A relatively large amount of fly ash (>40%) can be used successfully for the preparation of porous glass.     

Interaction of silica glass with molten mixtures of lead and potassium nitrates

The formation of lead–silicate coatings onto the surface of silica glass after its treatment in molten mixtures of Pb(NO₃)₂–KNO₃ and Pb(NO₃)₂–Cu(NO₃)₂–KNO₃ at 420–520 °C was investigated. The coating formation was supported by the appearance and adsorption/sedimentation of lead–oxide cations and nano-particles, from the salt melt, onto the treated glass surface. These particles, deposited onto the substrate, interacted with the structure of silica glass and formed an amorphous lead–silicate coating exhibiting a chemical composition gradient from PbO to SiO₂. The admixture of Cu(NO₃)₂ in the abovementioned salt mixture favored higher rates of lead oxide sedimentation (co-sedimentation with copper oxide). The external potion of this coating was progressively crystallized during treatment in the molten mixture of Pb(NO₃)₂–Cu(NO₃)₂–KNO₃ due to the nucleation of Cu₂O crystals supporting the formation of Pb₈Cu(Si₂O₇)₃, Pb₃O₄ and 2PbO · SiO₂.     

Effects of humidity on the weathering of glass

The weathering of 26 glass compositions was investigated under static conditions of 30%, 50%, 75%, 90% and 98% relative humidity (RH) at 50°C and a cycling condition between 77% and 98% RH at 50°C. The cycling condition was found to be less severe than static 98% RH. The effect of humidity level on weathering was studied and showed that water adsorption increases with time and humidity, whereas alkali generation increases with time, but not always with humidity. Significant weathering occurs for some glasses at 30% RH with the quantitative effect being almost as great as at 90% RH. Visual weathering was observed for some glasses when the level of generated alkali was less than 0.5 μg/cm².Various techniques for evaluating the weathering of glass under humid conditions were investigated. Visual appearance was judged to be the best evaluation method for routine weathering tests. It represents the effect of most concern and is sufficiently precise. Electron microscopy, measurement of sorbed water and alkali generated are principally used for research studies. Weight change and haze measurement lack sensitivity and reproducibility. A standard test was defined. The conditions are 98% RH at 50°C for 12 weeks. The visual ranking system is used. The glasses were generally ranked in the same order by the various evaluation techniques.     

Effect of tin-oxide on the physical properties of soda-lime–silica glass

The effect of tin-oxide on the physical properties of soda-lime–silica glasses was investigated. Glasses containing tin-oxide concentrations ranging from 0 to 3 mol% were synthesized in the laboratory. In some of the glasses, an attempt was made to control the ratio of Sn²⁺ to Sn⁴⁺ present in the glass. Dilatometry, beam-bending viscosity, and sonic resonance experiments were performed on the glasses to determine the role of tin on the thermal expansion, glass transition temperature, dilatometric softening temperature, annealing temperature, strain temperature, and elastic modulus of the glass. Mössbauer spectroscopy was used to determine the presence and relative amount of Sn²⁺ and Sn⁴⁺ in the glasses. The data suggest that the substitution of relatively small amounts of tin for a modifier species in the glass composition results in an increase in the network connectivity. In addition, the results suggest that this increase in network connectivity is more apparent in glasses containing higher relative amounts of Sn⁴⁺ compared to Sn²⁺.     

Variation of optical basicity with probe ion,for Cs2O + B2O3 and Li2O + B2O3 glasses

Optical basicities (Γ) for Cs₂O + B₂O₃ and Li₂O + B₂O₃ glasses have been measured as a function of glass composition, using Tl⁺, Pb²⁺ and Bi³⁺ probe ions. The three probe ions register different values of Γ for glasses of given composition (and also for pure B₂O₃ glass and water). The divergence decreases as the alkali metal ion size decreases.For the Li₂O + B₂O₃ glasses, ideal (calculated) optical basicities agree within experimental precision with experimental values registered by Pb²⁺ (Γ_Pb²⁺) up to about 15 mol% Li₂O. For higher Li₂O contents, and for the Cs₂O + B₂O₃ glasses, ideal optical basicities agree less well with Γ_Pb²⁺, but show similar trends with composition to those shown by Γ_Pb²⁺.     

Investigation of gel porosity clogging during glass leaching

A 5-oxide glass (62.5SiO₂, 16.6B₂O₃, 13.1Na₂O, 6.0CaO, 1.8ZrO₂) was leached at 90 °C at a high glass-surface-area-to-solution-volume ratio (SA/V = 80 cm^?1). Its dissolution rate diminished over time until it became unmeasurable. The alteration layer was characterized by ²⁹Si isotopic tracing in the leaching solution. ToF-SIMS elemental profiles showed that glass dissolution ceased due to clogging of the gel porosity at the gel/solution interface. One of the hypotheses proposed to account for the rate drop observed during borosilicate glass alteration is based on morphological changes in the alteration gel over time. Monte Carlo modeling of glass alteration, especially with simple glasses, indicates a clogging of the porosity on the external portion of the gel (near the solution/gel interface) after densification of the layer by silicon precipitation, but this phenomenon had never previously been directly observed experimentally. The initial results obtained by isotopic tracing provide new data that appears to confirm this hypothesis.     

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.     

Fluorogermanate glass with reduced content of OH-groups for infrared fiber optics

The hydroxyl (OH⁻) impurities in germanium oxide (GeO₂)-based glasses cause a strong absorption band between 2.4 and 3.6 μm, which makes this region of the electromagnetic spectrum inaccessible for signal transmission. To eliminate the OH⁻ absorption, PbF₂ was employed by partially replacing the PbO content in the following compositions: 56GeO₂−(31-x) PbO–9Na₂O–4Ga₂O₃–xPbF₂ (x = 0, 3, 6, 9, 15, 25, 31). The removal of OH^- ions in relation to the concentration of PbF₂ incorporated in the glass-forming liquid was systematically studied. Differential thermal analysis (DTA) and Fourier transform infrared (FTIR) spectroscopy were respectively used for the analysis of glass devitrification and optical properties. The results of FTIR suggest that over 99% OH⁻ impurity was removed, as compared to glasses without a purification process. An optimized core/cladding glass pair was then selected and the rod-in-tube technique was used. Fiber drawing conditions were established using the measured viscosity data and the devitrification range. The low OH⁻ absorption fiber has demonstrated the transmission window up to 4 μm with a loss of 2.34 dB/m at 1.49 μm.     

Tensile strength of boric oxide glass in vacuum and at very low humidities

The tensile strength of a glass fiber of pure boric oxide was measured at low humidities and in vacuum. The strength increased from 60 kg/mm² in a relative humidity (RH) range of 7% to 120 kg/mm² at 0.4% RH, levelling off to 0.02% RH which was the lower limit of humidity in this experiment. At relative humidities higher than 0.4% weathering and stress corrosion were observed, while at relative humidities lower than 0.4%, weathering was not observed but the decrease of the strength with a dynamic loading duration was observed, although the behavior was somewhat different from the typical stress corrosion. The strength measured in vacuum was almost equal to that measured at relative humidities lower than 0.4%, although the data were more scattered. The strength of boric oxide glass was much lower than that of silica glass, even when weathering and stress corrosion were depressed to the extreme. This weakness of boric oxide glass may be attributed to its layer structure.     

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.     

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.     

Vacancy-induced densification of silica glass

We investigate vacancy-induced densification of silica glass using molecular dynamics simulations. Equilibration of defective glasses initially with various concentrations of vacancies yields glasses denser than the intact glass. The structural and vibrational properties of the densified glasses are characterized. Densification is related to structural changes induced by atomic rearrangement near vacancies, and increases with the concentration of vacancies. Vacancies may cluster and form voids, and the maximum densification for void-structured glasses occurs at a critical radius of about 0.44 nm. The glasses densified by vacancies and by simulated UV-laser irradiation display nearly identical structural and vibrational properties. These results appear to support the Douillard–Duraud point defect model as a common mechanism for radiation densification of silica glass.     

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.     

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.     

Development of multifunctional photoactive self-cleaning glasses

Glasses for architecture must have many functions in addition to their transparency. For example, the glasses with the functions, of self-cleaning, light control, UV reduction, anti-bacterial, energy conversion, and so on, will be used in buildings in the near future. This paper reviews some results on multifunctional photoactive glasses based on multi-layer coatings containing TiO₂ film and other functional coatings developed by us recently. The self-cleaning of glasses can be realized by coating the photoinduced super-hydrophilic nanoporous thin films based on TiO₂ photocatalysts via sol–gel route. A new method to enhance the photocatalytic activity of TiO₂ thin films direct coated on soda-lime glass was developed by treating the films in acidic solutions. The films also have good photoinduced anti-bacterial properties. The doping of a small amount of silver into the TiO₂ porous film can enhance its anti-bacteria effect without UV light irradiation. The TiO₂ thin films by appropriate heat-treatment can operate as self-cleaning glass in the visible light region. The UV reduction self-cleaning glasses are prepared by magnetron sputtering two layers of TiO₂–CeO₂ and TiO₂ thin films on soda-lime glasses. The TiO₂–CeO₂ thin films can cut all of UV light through adjusting the ratio of TiO₂ and CeO₂. The TiO₂/TiN/TiO₂ type multi-layer coated on glass substrate can act as low-E self-cleaning glass. The potential water-repellent coating based on TiO₂ is discussed finally.