Raman spectroscopic study of bioactive silica based glasses

A study of the structure and bonding configuration of the bioactive glasses in the system Na₂O–CaO–P₂O₅–SiO₂ by Fourier transform Raman spectroscopy is presented. The assignment of the Raman lines, the changes in the Si–O–Si bond environment and the identification of the non-bridging silicon–oxygen groups (Si–O–NBO) for a wide range of silicate glasses are discussed. The frequency shifting and intensity variations of the Raman lines as a function of the bioactive glass composition are attributed to a decrease of the local symmetry originated by the addition of alkali and alkali earth oxides to the vitreous silica network. Correlation plots for the quantification of the Si–O–NBO groups as a function of the glass composition are also presented. These Raman analyses contribute to a better knowledge of the structural role of the network modifiers in the bioactive glasses and, as a consequence, improve the understanding of the bioactive process and the chemical routes of the CaP layer formation when exposed body fluids.     

Materials investigation of multi-walled carbon nanotubes doped silica gel glass composites

Multi-walled carbon nanotubes (MWNTs) doped silica gel glass matrix nano-composites were successfully prepared by sol–gel technique. Morphology of the composites was characterized by scanning electrical microscope and transmittance electrical microscope images. Pore structure of net MWNTs, silica gel glass matrix and resulted MWNTs doped composites were studied and compared. The results show that MWNTs are well dispersed in the gel glass matrix and sol–gel processing does not appear to affect the morphology of MWNTs. Pore structure of the silica matrix is changed by the introduction of MWNTs.     

Luminescence and defects of Yb3+-doped sol–gel silica glasses

The optical properties of Yb-doped sol–gel silica glasses were studied by optical absorption and radio-luminescence (RL) measurements, that revealed the typical absorption and emission pattern of Yb³⁺ ions. Moreover, RL bands in the 1.5–3.5 eV interval were also observed, and related to defects of the silica matrix. The RL intensity temperature dependence, investigated in the 10–320 K interval, evidenced the presence of the SiO₂ self-trapped exciton emission at 2.2 eV, whose intensity was rapidly quenched by temperature increasing. At variance, the Yb³⁺ emission intensity increased markedly by temperature increasing. This phenomenon is interpreted by considering a competitive role of point defects in free carrier trapping, evidenced by parallel wavelength resolved thermally stimulated luminescence measurements.     

Sol–gel synthesis and optical properties of size controlled Indium Arsenide nanocrystals embedded in silica glasses

III–V semiconductor Indium Arsenide (InAs) nanocrystals embedded in silica glasses was synthesized by combining the sol–gel process and heat treatment in H₂ gas. The size of InAs nanocrystals can be easily controlled via changing the In and As content in the starting materials and the heating temperature in a H₂ gas atmosphere. Absorption measurements indicate a blue shift in energy with a reduction on the In and As content in the SiO₂ gel glasses as a result of quantum confinement effects. A near-infrared photoluminescence with peak at 3.40 μm was observed at 6 K under 514.5 nm Ar⁺ laser excitation from InAs nanocrystals embedded in the silica gel glasses.     

Infiltration under isostatic pressure of porous silica glasses with silica sols

In this work a SiO₂ matrix with more than 50% porosity was developed and infiltrated with a pure silica sol under isostatic pressure, as a prior step to the immobilization of radioactive waste using this technique. The silica glass was prepared through the acidic leaching of phase-separated and partially-sintered sodium–borosilicate glass powder compacts. Phase separation was promoted at different stages of the sintering process to obtain different total porosity or pore size distributions, which in all cases showed macro, meso and micropores. Infiltration leads to a significant increase in weight, reflecting the initial porosity of the substrates. Porosimetry techniques (Hg porosimetry and N₂ adsorption isotherms) show that the silica sol fills practically all the pores with diameters over 3 nm. Preliminary sintering tests show that the infiltration technique lowers the sintering temperature by more than 150 °C.     

Distribution of hydroxyl groups on the silica gel surface

An experimental method for the investigation of the structure of surface hydroxyl coverage has been developed using the vapour of evaporable halides. It was found that the reaction of the surface hydroxyl groups with TiCl₄ proceeds to completion. A possibility for a quantitative characterization of the hydroxyl groups density was found on the basis of the proved applicability of the binomial distribution for the investigated.     

Study on the gel casting of fused silica glass

Slip casting process is usually applied for the forming of fused silica products. Segregation always occurs and it will results in density deviation. By using gel casting process, green is fabricated by means of in situ polymerization with a three-dimensional network, holding the particles together and eliminating the tendency of migration. To prepare gel casting slurries, premix solutions were composed of acrylamide and N,N′-methylenebisacrylamide. Solid loading was kept 60% and the average particle size of silica powder 8–8.5 μm. Lactic acid was introduced as a dispersant to regulate the pH value 3–4. Mechanism of the dispersant was investigated by studying ζ-potential at different pH. Ammonium persulfate (NH₄)₂S₂O₄ was added as an initiator. Gelation took place with the help of initiator at 50–60 °C. Nanometer silica was introduced to boost sinterability so that the density and bending strength of fused silica ceramics have been increased to 2.1 g/cm³ and 40 Mpa, respectively.     

Analysis of waveguide silica glasses using Raman microscopy

A spectroscopic method to determine dopant concentrations in silicas used in silica on silicon planar waveguides has been developed. Raman spectroscopic measurements in the range 740 cm⁻¹–1370 cm⁻¹ of cross-sections of the glass layers identified correlations between simple, rapidly calculated, spectral features related separately to each of the three dopants, boron, phosphorous and germanium, and the wt% analyses results for these dopants from inductively coupled plasma mass spectrometry (ICP-MS) measurements on fragments from the respective wafers. The calibration wafers comprised a set of monitor wafers with dopant concentrations spanning the ranges used in devices. The Raman-based analyses were able to determine boron and phosphorous wt% s in boro-phosphosilicate cladding glasses with accuracies of ≈0.1 wt% and germanium wt% s in core glasses with an accuracy of at least ≈0.3 wt% (small batch size). The method, which performed successfully in blind tests, provides a spatially resolving and rapid alternative to ICP-MS analyses of monitor wafers. Exploratory face-on measurements were performed on device wafers. Spectra of the cladding, core and underlayer were obtained from AWG samples. The effects of the confocal volume’s finite size and refractive index differences were observed. Exploratory measurements using UV Raman excitation showed potential advantages for cladding glass analyses.     

Diffusion of gases in porous silica gel

Gas diffusion in porous silica gels prepared by the sol-gel process is studied at room temperature. It is shown from the measurement of helium or oxygen gas diffusion in the gels that the gas diffusion is limited by the average pore diameter of the gel, R_a; that is, the mean free path of gas in a porous gel can be regarded as equal to R_a. The results also indicate that the gas diffusing length is about three times larger than the geometrical thickness of the sample gel. Some adsorption of oxygen gas appears to take place on the silica surface of the gel at room temperature.     

Electron paramagnetic resonance of silica glasses implanted with nickel

Silica substrates were implanted with Ni⁺ ions to nominal fluences (F) ranging from 10¹⁵ to 6×10¹⁶ cm⁻² at energy E=160 keV. The anisotropic spectrum of electron paramagnetic resonance with g_||=2.49±0.03 and g_⊥=2.17±0.03 was observed at 77 and 295 K for the samples implanted to F=(1-3)×10¹⁵ cm⁻² and was attributed to Ni⁺ ions with ground state |xy〉 or |x²−y²〉. The single line with g=2.29 and ΔH_pp∼14 mT was found at 77 and 295 K for the samples implanted to F=(4-8)×10¹⁵ cm⁻² and was assigned to clusters of Ni⁺ ions coupled by exchange interactions. Starting from F=8×10¹⁵ cm⁻² and at higher fluences the single line with g∼2.2 and ΔH_pp∼30-40 mT was observed at room temperature. The behavior of this line as function of measurement temperature indicates its superparamagnetic origin earlier studied by Isobe et al. Optical absorption spectra and transmission electron microscopy confirm the results obtained by other authors concerning the formation of particles of metallic nickel of nanometer size in silica glass implanted with Ni⁺ to F>10¹⁶ cm⁻².     

Fabrication of fluorine-doped silica glasses by the sol-gel method

Fluorine-doped silica glasses are produced by the sol-gel method for optical fiber preforms. In order to dope fluorine into silica glass, fluorinated silicon alkoxide, Si(OC₂H₅)₃F, is titrated into SiO₂ sol solutions. The fluorine content in silica glass depends on: the fluorine concentration in the gel, the specific surface area of SiO₂ particles and the heating rate in the sintering process. Fluorine-doped silica glass with a maximum relative refractive index difference of −0.93% is obtained. Using this technique, optical fibers with a triangular refractive index profile are fabricated with a minimum optical loss of 1.6 dB/km at 1.69 μm wavelength.     

Crystal-field spectroscopy of Eu3+ doped silica glasses

Fourier transform absorption spectroscopy in the 500–6000 cm^? 1 and 9–300 K ranges is applied to monitor the effects produced by Eu³⁺ incorporation into sol–gel silica samples doped with concentration increasing from 0.001 to 10 mol%. The aim is to investigate the formation of aggregates by exploiting the Eu³⁺ crystal-field transitions. Complementary microreflectance and Raman spectra are also measured in the range of silica intrinsic vibrational modes to confirm the hypothesis of matrix modification induced by increasing doping levels. Evidences of clustering are found for high Eu³⁺ concentrations. Up to 3 mol% the crystal-field line intensities gradually increase and the OH^? content smoothly decreases. A further increase to 10 mol% causes drastic, remarkable changes, i.e. sharp crystal-field lines appear which narrow by lowering the temperature. Furthermore, the OH^? related bands are no longer detectable. For concentrations up to 3 mol% the aggregates are amorphous as the silica matrix, while for the Eu³⁺ 10 mol% sample they show a rather ordered structure.     

FTIR and ultrasonic investigations on modified bismuth borate glasses

Studies on xRO · 30Bi₂O₃ · (70−x)B₂O₃ glasses have been carried out (0 ? x ? 30 mol%, R = Zn, Ba). Elastic properties and Debye temperature have been investigated using sound velocity measurements at 4 MHz. The ultrasonic parameters along with the IR spectroscopic studies have been employed to explore the role of divalent cations in the structure of the studied glasses. Analysis of infrared spectra indicates that RO is preferentially incorporated into the borate network, forming BO₄ units. It is assumed that Bi₂O₃ enters the structure in the form of BiO₆ only. The change of density and molar volume with RO content reveals that BO₄ units linked to R²⁺ cations are denser than those linked to positive sites in the Bi₂O₃ network. Predicted values of four co-ordinated boron put forward questions about the reliability of assignment of structural units that Bi₂O₃ may form.     

Periodic precipitation of barium molybdate in silica gel

The mechanism of formation of the rhythmic pattern of precipitate of barium molybdate in one-dimensional Liesegang phenomena is investigated quantitatively. They rhythmicity is found to be influenced by the molarity of inner and outer electrolytes and pH of the gel medium. The ‘time-law’, ‘spacing-law’ and ‘Δx-law’ are verified.     

Raman and infrared spectra on silica gel evolving toward glass

Infrared and Raman spectra of gels obtained from specially prepared solutions of Si(OC₂H₅)₄ (TEOS) which have been thermally treated in the 40–800°C temperature range, are reported and discussed with reference to the spectra of fused quartz.The results show that the gel to glass transformation is an hydrolytic polycondensation process, which takes plase gradually and is practically completed in the samples treated at 800°C.Particularly revealing is the behaviour of the bands due to OH (associated and/or unassociated H-bonded) stretching modes, including those of water, which are weakened by increasing temperature; the behaviour of the band due to the SiOH stretching mode; the behaviour of the network bands, some of which become more intense with increasing temperature, due to asymmetric and symmetric stretching and the bending SiOSi modes.In the Raman spectra of gels heated at different temperatures two peaks also appear at ≈600 and ≈490 cm^?1, which are also present in the Raman spectrum of fused quartz, they are related to network defects of the glass structure, for which no definite interpretation has yet been given in the literature.     

Growth of manganese tartrate trihydrate in silica gel

Growth of manganese tartrate trihydrate crystals in silica gel media with manganese chloride and manganese sulphate as the reactant is attempted. The reason for the occurrence of platelets along with dendrites in the case of manganese sulphate is discussed. IR, X-ray, atomic absorption and thermogravimetric techniques are employed to characterize the grown crystals and the results are discussed.     

Energy transfer from Bi3+ sensitizing the luminescence of Eu3+ in clusters embedded into sol–gel silica glasses

Y³⁺(La³⁺), Eu³⁺ and Bi³⁺ ions co-doped sol–gel silica glasses were synthesized. Photoluminescence spectra show that there is energy transfer from Bi³⁺ ions to the emission band of Eu³⁺ ions. The co-dopants Y³⁺ or La³⁺ have strong effects on the local structure and luminescence of Eu³⁺ ions. For 0.5 mol% Eu³⁺ ions doped glasses, the co-doping of 1 mol% Bi³⁺ and 1 mol% Y³⁺ is the most appropriate for the sensitization from Bi³⁺ to Eu³⁺. The sensitization effectiveness from Bi³⁺ ions to Eu³⁺ ions was studied by changing the amount of Bi³⁺ and Y³⁺, and clusters containing rare earth ions and Bi³⁺ ions dominate the energy transfer processes. The comparison of luminescent R-values (the intensity ratio of ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ in Eu³⁺ ions) between glasses containing La³⁺ and containing Y³⁺ verifies the formation of clusters in sol–gel glasses. As a favorable configuration for energy transfer, the accurate design and synthesis of clusters-contained glasses may provide a new kind of luminescent materials.     

Reaction sintering of gel derived ceramic composites

Fibrous SiC/SiO₂ matrix monolithic composites which have previously been fabricated using the sol-gel technique showed difficulty in densification. To enhance the sinterability of the SiC/SiO₂ composites, reactive ceramic species including furfuryl alcohol, alumina, and chromia were added to SiO₂ sol which was obtained by hydrolyzing tetraethoxysilane (TEOS). Physical and mechanical properties of these composites as a function of the reactive additives and of temperatures are presented.