Porous characteristics of carbon-coated silica gel beads prepared by liquid phase deposition of organic colloidal nanoparticles

Carbon-coated silica gel beads could be prepared by immersing porous silica gel beads in a resorcinol–formaldehyde (RF) aqueous solution and then following by drying and pyrolysis in nitrogen atmosphere. With the proposed method, deposition of colloidal nanoparticles formed during the sol–gel polycondensation of the RF solution was successfully prepared on the mesopores of the silica gel beads. Both size and concentration of the colloidal particles could be controlled by the gelation time and the mixing ratio (V_EtOH/V_RF) of the RF solution to ethanol. The porous characteristics of the prepared carbon-coated silica gel beads and the amounts of deposited carbon could be controlled by changing V_EtOH/V_RF. Without any significant decrease in mesoporosity, the maximal carbon contents of carbon-coated silica gel beads could be increased up to 18 wt% in the inner part and 53 wt% on the outer surface, respectively.     

Structural model of gelation processes of a sodium silicate sol destabilized by calcium ions: combination of SAXS and rheological measurements

This study deals with the physico-chemical processes involved in the formation of basic fractal silica gels derived from a sodium silicate sol destabilized by calcium ions. Using small-angle X-ray scattering and dynamic rheological measurements, structural and viscoelastic properties have been investigated in situ during aggregation and gelation processes. The experimental results lead to a consistent model that describes the structural features and aggregation mechanisms involved in the formation of these gels.     

Hafnium silicate dielectrics fabricated by RF magnetron sputtering

Structural and composition properties of hafnium silicate layers fabricated by RF magnetron sputtering were studied by means of spectroscopic ellipsometry, X-ray diffraction, transmission electron microscopy and attenuated total reflection infrared spectroscopy with respect to the deposition parameters and post-deposition annealing treatment. The variation of the deposition conditions allows the temperature of amorphous-crystalline phase transformation of pure hafnium oxide layers to be controlled. It is shown that the silicon incorporation in oxide matrix prevents the formation of interfacial silicon oxide layer and plays a major role in the stability of the structure of hafnium based layers remaining an amorphous state upon annealing at 900–950 °C.     

Borophosphate glass-ceramic scaffolds by a sodium silicate bonding process

A borophosphate glass with the mol% composition 25Na₂O-25CaO-5P₂O₃-45B₂O₅ was melted. The crystalline phase rheanite crystallized spontaneously during cooling the sample. Porous glass-ceramic scaffolds were prepared by bonding glass particles with size distributions in the range of approximately 100-500 μm by 0.1 M Na₂SiO₃ solutions. The scaffolds porosities were 40 ~ 60% and their compressive strengths were 0.1 ~ 0.4 MPa. The conversion of the binding scaffolds to hydroxyapatite (HA) was investigated by measuring the weight loss after soaking the sample in a solution of 0.25 M K₂HPO₄ with a pH value of 9.0 at 37 °C as a function of time. Hydroxyapatite (HA) was observed by XRD and SEM after soaking for 7 days. Cell attachment, spreading and proliferation on both unconverted scaffolds and on scaffolds coated with HA were determined by MTT [3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide] assay method using Murine osteoblastic-like MC3T3-E1 cells. The scaffolds are biocompatible and their biocompatibility is enhanced after the formation of a HA layer.     

On the constitution of silicate groupings in binary lead silicate glasses

Direct chemical methods have been used to determine the type and the percentage of silicate groupings present in binary lead silicate glasses within the composition range 4PbO · SiO₂PbO · SiO₂. The results obtained indicate that the constitution and the relative amounts of various silicate groupings change with the changing total amount of SiO₂ in the glass. Glasses containing small amounts of SiO₂ are characterized by the presence of few low-molecular silicate units; increasing the percentage of SiO₂ promotes the formation of higher polymerized silicate groupings. The results are discussed with regard to stoichiometry, and are consistent with Hägg's concept of glassiness.     

Evidence of structural anisotropies in silicate glass fibres by ESR

The ESR spectra of Cr³⁺ and Cr⁵⁺ in soda-lime-silicate glass fibres were investigated with respect to defined parameters of the fibre drawing process from a nozzle. The following results concerning the structure of the fibres were obtained: Although the short-range order of the Cr³⁺ and Cr⁵⁺ ions is identical in the structure of the bulk glass as well as in the fibre glass, changes of the covalent portion of the bonds between the Cr⁵⁺ ion and its oxygen ligands were observed. The fibre glass structure is a frozen-in deformed or orientated (this depends on definition) silicate glass network, which influences the bonds in the square bipyramidal [Cr⁵⁺O₆] ⁷⁻ complexes in the following manner: the σ-bonds between Cr⁵⁺ and the four oxygens in the square plane of the bipyramid are more covalent in the fibre than in the bulk glass. They are also orientation-dependent: more covalent for those of the randomly distributed complexes with their c-axes perpendicular to the fibre axis than parallel to it. The π-bond of the CrO³⁺ oxygen is less covalent in the fibre than in the bulk glass and is also orientation-dependent: more covalent for those complexes with their c-axes parallel to the fibre axis than perpendicular to it.

The ability of the glass network to form [Cr⁵⁺O₆] ⁷⁻ or CrO³⁺ complexes increases with the following increasing parameters of the fibre drawing process: melting temperature, oxygen partial pressure, mean cooling rate, and tensile stress. This fact is primarily a consequence of the shift of the redox-equilibrium and of the difficulty for the structural change from an octahedral symmetry of the Cr³⁺ and Cr⁵⁺ to a tetrahedral symmetry of the Cr⁶⁺ and vice versa during the fibre drawing process. The [Cr⁵⁺O₆] ⁷⁻ complex is highly unstable and is an intermediate state.     

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.     

Volume fluctuations in potassium silicate glass studied by molecular dynamics

Potassium-silicate glass, 0.05K₂O·0.95SiO₂, was prepared by molecular dynamics under three different regimes (cooling rates, simulating box sizes) to study their influence to temperature regimes of volume fluctuations. Partial volume fluctuations (VF) were introduced with the help of Voronoi polyhedra tessellation. Dynamic and static fluctuations were suggested as a measure for time- and space-related fluctuations. Separation of the dynamic and static VFs anticipates glass transition. The glass transition was related with the change of the temperature course of the static VFs. The dynamic VFs changed their temperature regimes well below the glass transition and the transition temperature was related with the change of the transport regime. Glass transition cages the atoms but Voronoi polyhedra are definitely shaped well below the glass transition temperature.     

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 study of sol-gel silicate glasses by IR and Raman spectroscopies

A study of the structure and bonding configuration of sol-gel silicate glasses by Raman and infrared spectroscopies is presented. Moreover, a review of the Raman lines and infrared bands assignment, the identification of the non-bridging silicon-oxygen groups and the ring structures are also demonstrated. The evolution of the changes of the bonding configuration in the composition and the stabilization temperature of the bioactive glasses is discussed in terms of the structural and textural characteristics of the glasses. Raman and infrared analyses contribute to the improvement in understanding of the local symmetry for sol-gel silicate glasses. infrared spectroscopy has allowed to identify the vibration bands of the hydroxyl groups associated with various configurations of the terminal silanol bonds on the glass surface and the free molecular water in the glass matrix. Raman analysis has provided an alternative method of quantifying the network connectivity grade and predicting the textural properties of the sol-gel silicate glasses.     

Study of sodium silicate melt and glass by infrared reflectance spectroscopy

Infrared reflectance spectra and the derived absorption spectra of a Na₂O-1.41 SiO₂ glass and of the parent melt are reported. Polymers with 2 bridging oxygens per silicon (chains) and 3 bridging oxygens (sheets) are identified. Spectra of glass and melt are qualitatively similar. Melt absorption bands show a 10–30 cm^?1 shift to lower frequencies. They are broader and less intense than those of the glass which does not agree with Sweet and White's reflectance measurements on similar glass and melt. The small differences between glass and melt can be attributed to the thermal expansion of the medium.     

Density fluctuations in silicate glasses

The potential use of inorganic glasses as waveguides at optical wavelengths has precipitated the need to understand the relationship between intrinsic optical loss and glass composition. The contribution to optical loss due to scattering from density fluctuations, is analyzed and discussed for various homogeneous (non-phase-separated) silicate glasses. The results show that binary alkali silicates in certain composition ranges form homogeneous glasses with density fluctuations less intense than that of pure silica glass. As third components added to these binary compositions alkali oxides (different from the one already present in the base glass), alkaline earth oxide (CaO) and Al₂O₃ cause further reductions in the magnitude of the density fluctuations.     

Strontium environment transition in tin silicate glasses by neutron and X-ray diffraction

The effect of Sr modifier atoms on the structure of stannosilicate glasses of composition (Sr0)_x(SnO)_0.5−x(SiO₂)_0.5, with 0 ⩽ x ⩽ 0.15, has been studied using Mössbauer spectroscopy and neutron and X-ray diffraction. The tin is mostly in the Sn²⁺ state. The Sr–O bond length undergoes a step decrease from (2.640 ± 0.005) Å to (2.585 ± 0.005) Å as x increases from 0.10 to 0.15, indicating a decrease in co-ordination number from 8 to 7. A Sn–Sn distance of 3.507 ± 0.005 Å is revealed by a first-order difference calculation from the x = 0 sample. This is too short to be consistent with significant edge sharing of [SnO₃] trigonal pyramids.     

Incorporation of silver into soda-lime silicate glass by a classical staining process

Silver was introduced into soda-lime silicate glass by a classical staining process under varying conditions of temperature and time. The spectroscopic features of the stained glasses were investigated by studying absorption, photoluminescence and electron paramagnetic resonance properties. The diffusion parameters (diffusion coefficient and diffusion activation energy) of silver ions were calculated based on elemental analysis data from inductively coupled plasma (ICP) spectrometry, and line scan profiles from energy dispersion X-ray analysis (EDX).     

Changes in surface morphology of silicate glass induced by fast electron irradiation

Binary potassium-silicate glass was irradiated with a defocused electron beam. During the irradiation the alkali ions migrate from the surface into the depth and alkali ions depleted layer is created near the surface. Such changes in the chemical composition are also accompanied with changes of the glass structure and finally result in the volume changes of the irradiated glass. This was directly studied using atomic force microscope (AFM). A series of exposures with energy of electrons of 7–50 keV and with different doses were performed. For low doses the irradiated glass is continuously depressed with the increasing dose, indicating this way the structural changes leading to the volume compaction. It is suggested the compaction is caused by the relaxation processes of the silica subnetwork. A further increase of the electron dose causes a formation of the small bump inside the center of the depression. The bump arises with the dose and finally exceeds the surrounding surface. It is suggested that the expansion is connected with the migration of alkali ions and the formation of Si–O–O–Si bonds which result in the formation of new rings with new space requirements.     

Light scattering dynamic study of the gelation process

Light scattering is a choice method for studying the dynamic properties of fractal structures and growth processes. We present preliminary light scattering measurements to characterize the formation of small silica particles prepared in highly basic TMOS/methanol/water sols and the transformation of sol into gel employing digital clipped autocorrelation spectroscopy. The light scattering is due to gel clusters.     

Rheology of the gelation of fluorine-doped silica sols

Dynamic oscillatory shear measurements are used to probe the gelation kinetics of aqueous sols composed of either particulate silica or silicon alkoxide solution. Unlike steady shear measurements, these dynamic tests do not alter the structure and kinetics of the sol-gel process. The dynamic storage moduli of both systems show sharp transitions at the onset of gelation. However, the gelation kinetics of the two systems are very different; the modulus of alkoxide system remains unchanged until the gel point is reached, whereas that of the particulate system increases with time even below the gel point as the colloid cluster grow. Unlike the alkoxide gel, the particulate gel reverses to a sol upon shearing. The gelation kinetics of a resheared sample is slower than that of a fresh sample. The structures associated with the sol-gel transition for this particulate system have been monitored using freeze fracture microscopy and correlate with rheological observations. The gelation times for both sols are varied by changing the fluoride ion content.     

Characterization of alkoxy-derived iron silicate

Amorphous iron silicates, x Fe₂O₃ · (1 ? x) SiO₂ (0.1 ? x ? 0.4), were prepared by calcining alkoxy-derived gels obtained by the hydrolysis of metal alkoxide mixtures. Structural studies based on X-ray diffratometry. Mössbauer effect and infrared spectroscopy indicated that the materials were indeed amorphous up to 500°C in the composition range x ? 0.3 and that the structure is formed by SiOFe bonds. Crystallization took place on calcining the samples with the composition x ? 0.3 above 600°C. Fayalite deposited at 600°C and hematite and α-cristobalite deposited above 700°C. In the case of the composition x = 0.4 the product was amorphous up to 300°C and only hematite was detected above 400°C.     

Structure and photoluminescence of amorphous silicate composites containing ZnO particles synthesized from layered sodium silicate

This study has been conducted to synthesize ZnO/silicate composites from layered sodium silicate (δ-Na₂Si₂O₅). For this purpose, various techniques have been employed to characterize the composites. X-ray diffraction (XRD) and Fourier transform infrared spectra (FT-IR) indicate that ZnO/silicate composites are amorphous. Scanning electron microscope equipped with energy dispersive X-ray (SEM-EDX) was used to estimate the chemical compositions of the composites. Photoluminescence (PL) spectra of ZnO/silicate composites present a significant blue-shift of peak position and increasing intensity. The size of ZnO embedded in silicates was estimated to be 10–20 nm by transmission electron microscope (TEM). Thermogravimetric and differential thermal analyses (TG–DTA) suggest that the ZnO/silicate composites are different from the ZnO and layered sodium silicate. All these results confirmed that the amorphous ZnO/silicate composites formed and the photoluminescent property of ZnO embedded in the silicate was obviously improved in comparison with the uncovered ZnO.     

Structure and lattice dynamics of binary lead silicate glasses investigated by infrared spectroscopy

The polar lattices dynamics of seven binary lead silicate glasses have been studied by infrared spectroscopy. The analysis of the reflectivity spectra with a dielectric function model, based on a modified Gaussian profile, allows a quantitative evaluation of the presence of lead cations within different structural sites. From the role of the lead cations versus the degree of polymerization of the silicate network and the comparison with literature results, we may to give a scenario for explaining the observed structural evolution of the glass matrix and more particularly the drastic change occurring around 45% of lead content. Below this threshold, lead cations act only as modifiers of the silicate network. Above, the glass structure is deeply modified; a lead network involving around 10% of the lead content appears in glasses whose composition is just above the threshold and progressively grows at the expense of the silicate network with the increase of lead content. For high lead content, lead cations can act as modifiers of the silicate network or as network formers. Results also show that the analysis of far infrared measurements combined with the knowledge of the UV edge optical response is very promising to characterize the local disorder around cations in glasses.