Compositional effects in the dissolution of multicomponent silicate glasses in aqueous HF solutions

The dissolution rate of multicomponent silicate glasses in a 2.9m aqueous HF solution is investigated as a function of its composition. The glasses studied are composed of SiO₂, B₂O₃, Al₂O₃, CaO, MgO, ZnO, Na₂O and K₂O, covering the compositions of most of the technologically important glasses. Unlike many physical properties, no linear relations are observed between the composition of the glass and its dissolution rate. The dissolution rate of a multicomponent silicate glass is found to be largely determined by two factors: The degree of linkage or connectivity of the silicate network and the concentration of SiO₂ in the glass. It is proposed that the dissolution of the glasses is preceded by the leaching of alkali and alkaline earth components present in the glass, followed by the subsequent dissolution of the leached layer. Probably fluorine species will diffuse into the leached layer to enhance the dissolution rate. Analysis of the activation energy data indicates that in some corrosive glasses the leaching itself becomes rate determining.     

The dissolution of Na2O-MgO-CaO-SiO2 glass in aqueous HF solutions

The dissolution (or etching) of a multicomponent (Na₂O-MgO-CaO-SiO₂) silicate glass in aqueous HF solutions is studied. The solutions were chosen in the systems HF-HNO₃-H₂O, HF-HCl-H₂O and HF-H₂SO₄-H₂O, and the temperatures varied from 25 to 60° C. SEM micrographs of the glass surface after etching show an orange peel surface structure which develops during etching and which originates from surface flaws. The dissolution rate of the glass was found to increase with higher HF concentration, higher strong-acid concentrations and higher temperatures. The dissolution rate is determined by the reaction of HF molecules and HF₂ ions with the Si-O-Si grouping surrounding the SiO₄ tetrahedron. In the multicomponent glass some of these bonds are non-bridging due to the presence of Na₂O, CaO and MgO, increasing the dissolution rate significantly. H⁺ ions introduced by adding strong acids to the etch solution adsorb on the surface and catalyse the dissolution reaction. Several models used to describe the relation between the dissolution rate and the H⁺ concentration are discussed.     

Molar extinction coefficients of the cupric ion in silicate glasses

The absorption characteristics of Cu²⁺ ions in binary alkali silicate and ternary silicate glasses were investigated. The molar extinction coefficients of the Cu²⁺ ion were determined in a series of binary alkali silicate and a ternary soda-lime-silica glasses for the actual intensities of the observed bands at their wavelength maxima. The absorption maxima due to Cu²⁺ ions were found to shift towards the longer wavelengths with increasing ionic radii of the alkali ions, in the present series of glasses. The base glass compositions were selected as30R₂O·70SiO₂ and 20Na₂O·10CaO·70SiO₂ where R⁺=Li⁺, Na⁺ and K⁺ ions. The results obtained are discussed in the light of a ligand field approach.     

The a.c. conductivity of CuO-containing lead silicate glasses

The a.c. conductivity of copper-free and copper-containing lead silicate glasses of the base composition 80PbO–20SiO₂ (wt %), equivalent to 51.5PbO–48.5SiO₂ (mol %), was measured at different temperatures and frequencies. The ratio Cu²⁺/Cu_total as a function of CuO content in the glasses studied was quantitatively determined by ESR. The results were used to derive the mechanism of conduction in lead silicate glasses. Values of a.c. conductivity, dielectric loss and dielectric constant decreased with the addition of CuO up to 0.25 g/100 g glass and increased with further additions of CuO. This may be due to the gradual decrease in the ratio Cu²⁺/Cu_total (as indicated by ESR) with increasing CuO content, through reduction of some Cu²⁺ ions to Cu⁺ ions, which have higher mobility. The a.c. conductivities agreed with the Anderson and Stuart model and indicated that the conduction mechanism was ionic in character.     

Micro-fabrication techniques applied to aluminosilicate glass surfaces: Micro-indentation and wet etching process

Micro-indentation and HF etching were explored as micro-fabrication techniques applied to glass surfaces. The effects of the aluminosilicate glass composition and of the etching conditions on the etching rate were investigated. It was found that the etching rate increased with increasing the ratio of Al₂O₃ to SiO₂ in the aluminosilicate glass. Etching parameters, such as pH, concentration and temperature of HF acid, had effects on etching rate. However, the effects of these parameters were much smaller at indented area than at non-indented area. The results indicated that the etching rate difference between the two areas, which is one of the key factors in the micro-fabrication technique, could be controlled with these parameters. And the phenomena can be well explained in terms of etching and leaching mechanism of aluminosilicate glass.     

Raman spectroscopic investigation of sodium borosilicate glass structure

Raman spectra of sodium borosilicate glasses with a wide range of Na₂O/B₂O₃ ratios were systematically measured. Variations of the spectra with glass composition were studied to interpret the implied distribution of Na⁺ ions between silicate and borate units. When Na₂O/B₂O₃ is less than 1, all Na⁺ ions are associated with borate units as indicated by the absence of the 1100 cm⁻¹ band of Si-O⁻ non-bridging bond stretching. For the (1−x)Na₂O · SiO₂ ·xB₂O₃ glass withx≦0.4 the peak-height ratio of the 950 cm⁻¹ band to the 1080 cm⁻¹ band was used to analyse semiquantitatively the distribution of the Na⁺ ions between silicate and borate units. Sodium ions are divided between silicate and borate units approximately in proportion to the amount of SiO₂ and B₂O₃ present in these glasses. Some of the high sodium content glasses were crystallized and their spectra were compared with the bulk glass spectra. The distribution of Na⁺ ions in the glass was quite different from their distribution after crystallization. Spectra of high silica glasses that had been heat-treated for phase separation indicated exclusion of borate units from the silica network and the formation of borate groups. For high boron content glasses, no change was observed on heat treatment. Raman bands due to borate groups seem to be little affected by their environments. Also affiliated with the Department of Geosciences.     

Kinetics and mechanisms of the conversion of silicate (45S5), borate, and borosilicate glasses to hydroxyapatite in dilute phosphate solutions

Bioactive glasses with controllable conversion rates to hydroxyapatite (HA) may provide a novel class of scaffold materials for bone tissue engineering. The objective of the present work was to comprehensively characterize the conversion of a silicate bioactive glass (45S5), a borate glass, and two intermediate borosilicate glass compositions to HA in a dilute phosphate solution at 37°C. The borate glass and the borosilicate glasses were derived from the 45S5 glass by fully or partially replacing the SiO₂ with B₂O₃. Higher B₂O₃ content produced a more rapid conversion of the glass to HA and a lower pH value of the phosphate solution. Whereas the borate glass was fully converted to HA in less than 4 days, the silicate (45S5) and borosilicate compositions were only partially converted even after 70 days, and contained residual SiO₂ in a Na-depleted core. The concentration of Na⁺ in the phosphate solution increased with reaction time whereas the PO₄^3– concentration decreased, both reaching final limiting values at a rate that increased with the B₂O₃ content of the glass. However, the Ca²⁺ concentration in the solution remained low, below the detection limit of atomic absorption, throughout the reaction. Immersion of the glasses in a mixed solution of K₂HPO₄ and K₂CO₃ produced a carbonate-substituted HA but the presence of the K₂CO₃ had little effect on the kinetics of conversion to HA. The kinetics and mechanisms of the conversion process of the four glasses to HA are compared and used to develop a model for the process.     

MAS-NMR support for Hench model in the case of bioactive glass microspheres

The structural changes occurred in bioactive glass microspheres belonging to the system SiO₂–Na₂O–P₂O₅–CaO–K₂O–MgO incorporating yttrium were investigated before and after soaking in simulated body fluid (SBF) by X-ray diffraction (XRD) and ³¹P and ²⁹Si magic angle sample spinning nuclear magnetic resonance (MAS-NMR). The addition of yttrium to the bioactive glass composition induces changes in the behavior of the glass microspheres in SBF. The XRD analysis proves that after the immersion in SBF a crystalline hydroxyapatite-like phase is developed on the microspheres surface. The ²⁹Si and ³¹P MAS-NMR results show that silicate species with two and three bridging oxygens per SiO₄ tetrahedra and PO₄ monomeric units are present in the glass structure. After immersion in SBF, new silicate species with four bridging oxygens appear as result of silica-gel layer formed on microspheres surface. The formation of crystalline hydroxyapatite-type layer is reflected by the occurrence of narrow components in ³¹P MAS-NMR spectra. The NMR results support the Hench model for bioactive glasses behavior in biological environments.     

Spectroscopic characterization of alkali borosilicate glasses containing iron ions

Structural properties of alkali borosilicate glasses containing iron ions were investigated using infrared, laser Raman and Mössbauer spectroscopy. Two types of glasses were prepared: SRL-type with the composition 18.5 wt% Na₂O, 10.0 wt% B₂O₃, 52.5 wt% SiO₂, 4.0 wt% Li₂O, 10 wt% TiO₂ and 5.0 wt% CaO, and sodium borosilicate glass with the composition 16.7 wt% Na₂O, 18.7 wt% B₂O₃ and 64.6 wt% SiO₂. Raman spectroscopy showed that orthosilicates are the dominant amorphous phase in the SRL-type of glass. Incorporation of iron in the SRL-type of glass induced polymerization of silicate units and -Si-O-Fe- copolymerization. It was concluded that different amorphous phases are simultaneously present in the SRL-type of glass containing iron ions. Interpretation of the Raman spectra is given. Incorporation of iron ions into the sodium borosilicate glass also affected the corresponding IR spectra. The valence state of iron and its coordination were determined by⁵⁷Fe Mössbauer spectroscopy.     

A study of the formation of amorphous calcium phosphate and hydroxyapatite on melt quenched Bioglass® using surface sensitive shallow angle X-ray diffraction

Melt quenched silicate glasses containing calcium, phosphorous and alkali metals have the ability to promote bone regeneration and to fuse to living bone. These glasses, including 45S5 Bioglass^® [(CaO)_26.9(Na₂O)_24.4(SiO₂)_46.1(P₂O₅)_2.6], are routinely used as clinical implants. Consequently there have been numerous studies on the structure of these glasses using conventional diffraction techniques. These studies have provided important information on the atomic structure of Bioglass^® but are of course intrinsically limited in the sense that they probe the bulk material and cannot be as sensitive to thin layers of near-surface dissolution/growth. The present study therefore uses surface sensitive shallow angle X-ray diffraction to study the formation of amorphous calcium phosphate and hydroxyapatite on Bioglass^® samples, pre-reacted in simulated body fluid (SBF). Unreacted Bioglass^® is dominated by a broad amorphous feature around 2.2 Å^?1 which is characteristic of sodium calcium silicate glass. After reacting Bioglass^® in SBF a second broad amorphous feature evolves ~1.6 Å^?1 which is attributed to amorphous calcium phosphate. This feature is evident for samples after only 4 h reacting in SBF and by 8 h the amorphous feature becomes comparable in magnitude to the background signal of the bulk Bioglass^®. Bragg peaks characteristic of hydroxyapatite form after 1–3 days of reacting in SBF.     

Comparative investigation on the effect of alkaline earth oxides on the intensity of absorption bands due to Cu<Superscript>2+</Superscript>, Mn<Superscript>3+</Superscript> and Cr<Superscript>3+</Superscript> ions in ternary silicate glasses

Absorption characteristics of Cu²⁺, Mn³⁺ and Cr³⁺ ions in ternary silicate (20Na₂O·10RO·70SiO₂, where R=Ca, Sr, Ba) glasses were investigated. The intensities of absorption bands due to Cu²⁺ ion was found to increase with increasing ionic radii of the alkaline earth ions whereas it was found to decrease in case of Mn³⁺ and Cr³⁺ ions with increasing ionic radii of the alkaline earth ions. The results were discussed in the light of relation between linear extinction coefficients of these ions and coulombic force of alkaline earth ions. The change in intensities of Cu²⁺, Mn³⁺ and Cr³⁺ ion is attributed due to change in silicate glass compositions.     

IR and Raman spectroscopic studies of sol–gel derived alkaline-earth silicate glasses

IR and Raman spectroscopies have been utilized to study the structure and vibrational modes of sol–gel-derived binary silicate glasses. The present study is motivated by the immense geological significance and focuses on the MO–SiO ₂ (M = Ca, Mg) binary systems in an effort to unveil the role of the CaO and MgO modifiers when incorporated to the 3D silica structure. Glasses in the composition range x =0, 0·1, 0·2, 0·3 and 0·4 prepared by the sol–gel method were compared with the corresponding glasses formed by appropriate mixing of SiO ₂ and MO powders through melting and fast cooling. The vibrational spectra of the sol–gel-derived glasses have revealed considerable changes in relative intensities as a function of the MO mole fraction. These changes signify structural modifications on the silica network. The population of the Q ³ species was found to increase for both modified silicate systems. The rate of increase is more pronounced in the CaO–SiO ₂ glasses. The extent of network depolymerization in the porous glass is higher at the same content of alkaline earth oxide compared to the bulk glass. The results are indicative of a more ‘defective’ nature of the sol–gel glasses compared to the corresponding melt-quenched ones.     

Raman microprobe investigation of oxygen and carbon dioxide dissolution from bubbles in silicate glasses containing antimony oxide

The effect of addition of antimony oxide and/or sodium nitrate to silicate glass compositions upon the changes in the relative concentrations of oxygen and carbon dioxide in their bubbles from heat treatment was investigated using the Raman microprobe technique. The addition of antimony oxide to these glasses increased the relative rates of oxygen dissolution from their bubbles with respect to glasses containing no refining agents. These increases were closely related to the absolute amounts of Sb³⁺ ions that were present in the glasses. The relative rates were faster for glasses containing antimony oxide than for glasses containing the same molar amounts of arsenic oxide. The higher Sb³⁺/Sb⁵⁺ ratios for glasses containing antimony oxide with respect to the As³⁺/As⁵⁺ ratios for glasses containing arsenic oxide caused the relative rates of oxygen dissolution to be dramatically greater for the former glasses. In contrast to an earlier investigation with silicate glasses containing arsenic oxide, the addition of sodium nitrate to glasses containing antimony oxide using a similar glass preparation did not significantly change their relative rates of oxygen dissolution.     

Do properties of bioactive glasses exhibit mixed alkali behavior?

The effect of substituting K₂O for Na₂O on the physical and chemical properties of 15 glasses in the system Na₂O–K₂O–CaO–P₂O₅–SiO₂ was studied for three series: low (52 mol% SiO₂), medium (60 mol% SiO₂) and high (66 mol% SiO₂) silica. The SiO₂ content expressed as weight-% varied from 46 to 64 wt%, thus suggesting that the compositions were either bioactive or biocompatible. The crystallization tendency and sintering behavior were studied using differential thermal analysis and hot stage microscopy. Formation of silica- and hydroxy-apatite-rich layers were studied for glass plates immersed in static simulated body fluid. The release of inorganic ions into Tris buffer solution was analyzed using inductively coupled plasma optical emission spectrometer in dynamic and static conditions. Substitution of K₂O for Na₂O suggested mixed alkali effect (MAE) for the thermal properties with a minimum value around 25% substitution. With increased share of K₂O in total alkali oxides, the hot working window markedly expanded in each series. Silica and hydroxyapatite layers were seen only on the low silica glasses, while a thin silica-rich layer formed on the other glasses. In each series, greater dissolution of alkali and alkali earth ions was seen from K-rich glasses. Clear MAE and preferential ion dissolution were recorded for medium and high silica series, while for low silica glasses, the initial MAE dissolution trends become rapidly covered by other simultaneous surface reactions. MAE enables designing especially low silica bioactive glasses for improved hot working properties and medium and high silica glasses for controlled dissolution.     

Evidence for a threshold in the biosolubility of aluminosilicate vitreous fibers

Two series of aluminosilicate glasses have been synthesized with the nominal composition (64 − x) SiO₂–x Al₂O₃–36 Na₂O/CaO with x varying from 9 to 19 mol%. They have been corroded in static conditions in a solution that mimics in a simplified manner the intracellular medium of the lung alveolar macrophages (37 °C, pH 4.6, citric acid). The original and corroded glasses have been studied by ²⁷Al and ²⁹Si MAS NMR. Both series display a sharp increase in the silicon dissolution rate with the alumina content. The glass network dissolves extremely slowly, whereas the release of excess sodium is very fast, for the glasses with low alumina content. On the opposite, the glasses with high alumina content dissolve much more rapidly in a nearly congruent manner. The crossover between the two behaviors occurs for x = 13, which corresponds to 33% of aluminum in the glass-former network. The sharp crossover from slow to fast network dissolution is explained in terms of connectivity of the silica sub-network. Above a certain amount of alumina, the silicon sub-network is no more percolating and the corroded glass breaks up into colloids. The sharpness of the transition and the relatively low alumina content required for fast dissolution are related to a structural feature of the aluminosilicate glasses, namely the aluminum self-avoidance that decreases the connectivity of the silica sub-lattice.     

In situ examination of the chemical etching of SiO2-Si structures using an atomic force microscope

First results are reported of in situ visualization of the chemical etching of P⁺-ion implanted SiO₂-Si structures in an aqueous HF solution using an atomic force microscope. The rates of SiO₂ etching were determined and the kinetics of the photostimulated chemical etching of Si were investigated. Pis’ma Zh. Tekh. Fiz. 24, 81–86 (November 12, 1998)     

Thermally stimulated polarization and depolarization current (TSPC/TSDC) techniques for studying ion motion in glass

The feasibility of studying ionic motion in glass using thermally stimulated polarization (TSPC)/depolarization current (TSDC) techniques was investigated with 4Na₂O-96SiO₂ and 30PbO-70SiO₂ (mol %) glasses. The TSPC peaks in these glasses were dependent on glass composition and attributed to bulk polarization. The high temperature background TSPC is shown to be due to the d.c. conductivity, whereas the TSPC/TSDC peaks in the 4Na₂O glass are attributed to shorter range Na⁺ motion.     

Bioactivity of Fe2O3-containing CaO-SiO2 glasses: in vitro evaluation

In order to reveal conditions for obtaining bioactive and ferrimagnetic glass-ceramics useful as thermoseeds for hyperthermia treatment of cancer, the effects of additives on the bioactivity of an Fe₂O₃-CaO-SiO₂ glass were investigated by examining apatite formation on the surfaces of the glasses in a simulated body fluid with ion concentrations nearly equal to those in human blood plasma. A 3Fe₂O₃, 100CaO · SiO₂ (in weight ratio) glass did not form an apatite layer on its surface in the fluid, but glasses of the same compositions with 3Na₂O, B₂O₃ and/or P₂O₅ added (in weight ratio) formed an apatite layer. This indicates that bioactive and ferrimagnetic glass-ceramics could be obtained from Fe₂O₃-containing CaO-SiO₂ glasses with Na₂O, B₂O₃ and/or P₂O₅ added. Apatite formation on the surfaces of the glasses with the additives are interpreted in terms of the dissolution of the calcium and silicate ions from the glasses.     

On improving the track development properties of soda glass detectors

A detailed study of track development properties of soda glass detectors is carried out by varying the etching parameters, i.e. etching time, etchant temperature and the composition of the etching solution. The etch product layer (EPL) which mainly limits the sensitivity of these detectors is dissolved chemically as soon as it is formed. The largest observable etch pit diameter by the use of this method is almost double the one obtained using HF. Room temperature (39 ± 1)°C appears to be the optimum temperature for etching of soda glasses in this solution. The spectrometric response of soda glasses to fission fragments of ²⁵²Cf has also been studied, using this solution. The energy resolution of soda glass detectors is seen to improve and become comparable with the energy resolution of phosphate glass detectors.     

Optical absorption and infrared studies of some silicate glasses containing titanium

The optical absorption and infrared (IR) spectra of mixed alkali silicate glasses in the system 3SiO₂–(1 − x)Na₂O–xK₂O + 2.5–20 g of TiO₂ were measured. Absorption bands due to Ti³⁺ ions, at 500 and 570 nm, were observed in the spectrum of Na₂O-free glasses. The intensities of such bands were followed with the variation in TiO₂ content from 2.5 to 20 g (per 100 g of glass). The incorporation of titanium oxide as Ti³⁺ in the Na₂O-free glass, conferring a violet hue to it, was explained on the basis of the acidity–basicity character of the glass. The IR measurements have been used to explore the structural changes throughout the compositional variations. This revised version was published online in November 2006 with corrections to the Cover Date.