Thermal behaviour of bioactive alkaline-earth silicophosphate glasses

The thermal behaviour (sinterability and first crystallization) of a series of alkaline-earth silicophosphate glasses has been studied by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The samples were prepared from a base bioactive glass of the system CaO–P₂O₅–SiO₂–CaF₂, by (a) slightly changing the F/O ratio; (b) replacing part of the CaO by SrO or MgO; and (c) increasing the total alkaline earth concentration by MgO additions. The results show that the addition of MgO is the most effective way of improving sinterability. In these samples, a decrease of the glass transition temperature, together with an increase in the temperature of the first crystallization, is observed. The difference between both temperatures is proposed to be an adequate indicator of the sinterability. The initial stages of the first crystallization (which produces an oxo-fluorapatite), and its composition dependence, are discussed in terms of the results of sinterability, and the classical theory of nucleation.     

Wet chemical etching of silicate glasses in hydrofluoric acid based solutions

The etching of silicate glasses in aqueous hydrofluoric acid solutions is applied in many technological fields. In this review most of the aspects of the wet chemical etching process of silicate glasses are discussed. The mechanism of the dissolution reaction is governed by the adsorption of the two reactive species: HF and HF ₂ ^- and the catalytic action of H⁺ ions, resulting in the breakage of the siloxane bonds in the silicate network. The etch rate is determined by the composition of the etchant as well as by the glass, although the mechanism of dissolution is not influenced. In the second part of this review, diverse applications of etching glass objects in technology are described. Etching of SiO₂ and doped SiO₂ thin films, studied extensively for integrated circuit technology, is discussed separately.     

Thermal expansion of lead silicate glasses

The thermal expansion, when heated from 77 to 273 K, of two samples of lead silicate glass, containing 21 and 28.5 mol % PbO, has been measured. The temperature dependence of the thermal expansion coefficient,α, is in qualitative agreement with the expansion behaviour of sodium silicate glass. However, the addition of ∼ 21 mol % PbO to silica is required to produce an increase in the magnitude ofα comparable to the addition of only ∼ 10 mol % Na₂O. The differences in the magnitudes ofα for lead and soda glasses are considered in the right of previous proposals for their structures.     

Neutron-induced autoradiography of lithium silicate glasses

Immiscibilities between silica and lithium disilicate were investigated. Alpha particles ejected from ⁶Li atoms by thermal neutrons were used to autoradiograph surfaces representing diffusion-zone cross-sections. Etchant-enlarged alpha tracks in nitrocellulose disclosed the distribution of lithium. For autoradiography, procedural details were established, suitable operating conditions were selected, and the method was calibrated. For diffusion, adequate couples had to be prepared. Subsequent experiments indicated phase separations along an 885° C isotherm of the silica-lithium disilicate system. There was evidence of metastable compositions near 30, 25, 20, 16, 11, and 7 mol % of lithia in silica. These results have been given a very tentative interpretation: The first two compositions may correspond to metastable extensions of liquidus boundaries, projected through a eutectic point of the system. The third and sixth compositions are probably points on a liquid-liquid miscibility boundary, and the fourth and fifth compositions could be on an associated spinodal boundary.     

Luminescence of electron-irradiated nanoporous silicate glasses

It is experimentally demonstrated that the point electron-beam irradiation of a nanoporous silicate glass leads to the appearance of or significant increase in the UV-excited luminescence from the entire sample in the spectral intervals of 500–700 and 900–950 nm. The observed effect is related to (i) partial reduction of SiO₂ and H _x SiO _y phases by electrons that spread via through pores in the glass and (ii) an increase in the number of Si-H and Si-OH bonds.     

The wetting of three component silicate glasses on platinum

Measurements of the contact angles of sessile drops of alkali silicate glasses, containing other oxide additions, on platinum have been used to study the role of the glass composition in the wetting processes. A model has been proposed which relates the degree of wetting to the competition between the substrate metal and cationic and anionic groups in the molten glass for the valency electron of non-bridging oxygen ions. An empirical relationship between equilibrium contact angle and the affinity for oxygen ions of cations in the glass supports this model.     

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.     

Model for analyzing optical properties of silicate glasses

A new model for analyzing optical properties of silicate glass materials including borosilicates has been developed. The model is based on computing refractive-index and density values of a given optical glass whose oxide composition is known in terms of weight fractions. The refractive-index variation with wavelength has also been used to predict the chromatic behavior of these glasses. The model has been compared with the existing chromatic model and was found to give more accurate values for borosilicates.     

The effect of vanadium oxides on the crystallisation of silicate glasses

The influence of vanadium oxides as catalysts for nucleation and crystal growth in CaO-MgO-Al₂O₃-SiO₂ glasses has been investigated. The effect of varying the total vanadium content and the ratio of oxidised to reduced vanadium ions has been observed. No internal nucleation was observed, but the rate of growth of crystals of anorthite and wollastonite from the surface was increased by additions of V₂O₅ and V₂O₃. The values of the growth rates and the analysed V⁵⁺ content in both oxidised and reduced glasses suggest that V⁵⁺ ions are the most active species. Increasing concentrations of V₂O₅ in the glass gave maxima in the growth rates between 2 and 4 wt % for crystallisation temperatures between 900 and 980° C.     

Compositional effects on the fracture behaviour of alkali–silicate glasses

The two-point bend strength and the fracture toughness of a series of soda–potassia–silicate and soda–potassia–calcia–silicate glass fibres have been measured. There is a clear variation of mean strength with composition for the soda–potassia–silicate glasses, however, there is much less variation of mean strength with composition for the soda–potassia–calcia–silicate glasses. There is also a greater variation of fracture toughness with composition for the soda–potassia–silicate glasses than for the soda–potassia–calcia–silicate glasses. The mean strength, fracture toughness and inferred flaw sizes for the soda–potassia–calcia–silicate glasses are all less than the equivalent values for the soda–potassia–silicate glasses. These results are related to the structural models and durability of the glasses tested.     

Microstructural evolution in nanocrystalline alumina containing silicate glasses

Pure nanocrystalline -alumina powders were coated with different fractions (5, 10, and 15 vol%) of SiO₂-SrO glass using the sol-gel technique. The isostatically cold pressed powders were pressureless sintered in air for 5 h in the temperature range of 1250°C to 1550°C. The relative densities were ranged between 60 to 90% of the theoretical and were composition dependent. The density was increased with the sintering temperature. In pure alumina, the to phase transformation went to completion by sintering at 1250°C. However, in the glass-coated samples, transition -alumina was mostly retained after sintering at the same temperature. Pure nanocrystalline alumina sintered at 1350°C exhibited vermicular structure with isolated pores. The microstructure of the low glass-containing samples exhibited nanocrystalline to submicron size grains arranged in platelet-shaped clusters. Samples with higher glass contents exhibited also micron-size needle-shape grains of strontium aluminate.     

Spherulitic growth and recrystallization in barium silicate glasses

The growth and recrystallization of spherulites formed in barium disilicate glasses between 700 and 900° C has been studied by electron microscopy and electron diffraction. Spherulites formed at 700° C consist of fibrillar (～ 100 Å diameter) monoclinic crystals in confocal arrangement with preferred crystallographic growth axes. High temperature (900° C) spherulites are composites of radially oriented plate-shaped orthorhombic crystals with lateral growth of epitaxially nucleated fibrillar monoclinic crystals. At intermediate temperatures “axialites”, consisting of a single orthorhombic “midrib” crystal with monoclinic fibrillar side-growths, grow in competition with the low temperature spherulite morphology. The monoclinic fibrillar phase is believed to be an intermediate metastable structure which is able to grow more rapidly than the orthorhombic phase via cellular transformation in the presence of impurities. Brief comparison is made between the observed morphologies and theories for interface instability and cellular crystallization. Recrystallization, induced mainly by the large interfacial area of spherulite fibrils, produces faulted and twinned monoclinic grains which transform slowly to the orthorhombic stable crystal phase. A glassy intercrystalline residue becomes more prominant with grain growth.     

Dysprosium (III)-doped novel silicate glasses

Dysprosium-doped optical materials are currently becoming a matter of interest mainly for their utilisation in the MIR spectral region. We report here on specially designed dysprosium-doped silicate glasses, whose optical properties were characterised by VIS and NIR measurements of their absorption and transmission spectra.The planar waveguides were fabricated in these glasses employing Ag⁺ and K⁺ ion exchange (IE), and their optical properties were ascertained by dark mode spectroscopy. The chemical compositions of the exchanged layers were determined using electron microscope analysis (EMA), where the effect of the bivalent glass modifiers (such as Ca²⁺, Mg²⁺ and Zn²⁺ and their mixtures) in the glass matrix on the properties of the exchanged layers – both chemical (with different permeabilities of the matrix for diffusing ions) and optical (waveguiding) were examined. The role of bivalent modifiers in the permeability of the glasses has been described and explained on the basis of the concept of optical basicity. The relations yielding a guideline to specify the contributions of the particular modifiers to the resultant depths of the IE layers have been suggested as well.     

Compositional control of the photoelastic response of silicate glasses

The stress-induced birefringence (termed photoelastic response) in oxide glasses has important consequences for several applications. In this work, we provide new insights into the structural origins of the photoelastic response of silicate glasses by determining the composition dependence of the stress optic coefficient (C) of forty-nine silicate glasses containing different alkali and alkaline earth oxides. We find that the value of C decreases with increasing modifier-to-silica ratio and increases with alumina-to-silica ratio. The scaling of stress optic coefficient with composition can be predicted based on the average ratio of bond metallicity to cation coordination number in the glass, which varies as a function of composition. This is evidence that the details of the glass network structure need to be considered in order to account accurately for the composition dependence of C, a result that is consistent with a previously proposed empirical model and with topological constraint theory. Our results enable an improved control of the photoelastic response of silicate glasses through compositional design.     

An intrinsic luminescence in binary lead silicate glasses

The low-temperature photoluminescence (PL) of xPbO · (1 − x)SiO₂ glasses (x = 0.20-0.75) was studied at Т = 10 K. The recorded PL-spectra are a superposition of three spectral components with maxima located at 1.8 eV (identified as Pb 6p → metal-bridging O2p radiative electron transition, the “R”-band), 2.0 eV (Pb 6p → non-bridging O2p, the “O”-band) and 2.55 eV (Pb 6p → Pb 6s, the “B”-band), respectively. It was found the essential link for “R”, “O” and “B” PL-bands with chemical composition x of the glasses under study. These concentration dependences are expressed as mutual PL-intensity variations for each recorded luminescence band that allowed to determine their origin. The shape of established dependences well coincides with numerical data on NBO- and MBO-density of chemical bonding, reported previously.The overall PL-manner within the temperature range of 10-295 K is described by an empirical Street’s law. It was shown that experimental photoluminescence quenching curves may be precisely approximated as a superposition of Mott relationships for nonequivalent luminescence centers. The obtained distribution of PL-centers on the activation energy for luminescence quenching reflects the essential donation of the low-energy states into the overall PL-process. The width of this energy distribution affects by the type of PL-emission band and the disordering degree in the arrangement of local PL-centers of a certain kind.     

Second-order non-linear optical studies on CdS microcrystallite doped lead silicate glasses

Second-order non-linear optical effect of CdS microcrystallite doped lead silicate glass is investigated in this paper. The second harmonic generation (SHG) from the base glass and CdS microcrystallite doped glasses were observed, and the effects of the heat treatments and the thermal poling temperature on the crystallization of CdS and second-order harmonic (SH) intensity were discussed, respectively. It is indicated that samples doped with CdS microcrystallite showed larger SH intensity than that of the base glass. Longer heat treatment time, higher heat treatment temperature and thermal poling temperature cannot get a good SH intensity in glasses.     

Two opposite effects of sodium on elastic constants of silicate binary glasses

(1 - x)SiO₂-xNa₂O glasses have been studied by Brillouin scattering when sodium oxide molar concentration x, varies in the range 0.05 < x < 0.44. Non-linear dependence of elastic constants on sodium oxide concentration is shown unambiguously for the first time and the compressive elastic constant C₁₁ experienced a non-monotonic behaviour with a minimum value around x = 0.15. These results are compared with those obtained previously in the case of other alkali silicate glasses (Li, K). The variations of elastic constants of these glasses with the alkali metal nature and concentration are discussed on the basis of relations with the molar density of silicon atoms and the force constants between modifiers and non-bridging oxygens.