Structure of organic-inorganic hybrid low-melting glasses from Si NMR and ab initio molecular orbital calculations

The structure of organic-inorganic hybrid glass precursor Me₂Si(OPO(OH)₂)₂ and low-melting glasses SnO-Me₂SiO-P₂O₅ has been examined by ²⁹Si static and MAS NMR, respectively. The ²⁹Si MAS NMR spectra of SnO-Me₂SiO-P₂O₅ were deconvoluted into three Gaussian peaks, whose chemical shifts were located at −3.1, −10.2 and −18.1 ppm. Ab initio molecular orbital calculations have also been carried out for the clusters modeling the structure of precursor and glasses. From the calculations of ²⁹Si NMR shielding constants, the experimental chemical shifts at −10.2, −3.1 and −18.1 ppm were assigned to ²⁹Si atoms in P-O-Si(Me)₂-O-P network linkage, in P-O-Si(Me)₂-OH terminating structure and in P-O-Si(Me)₂-O-Si network linkage, respectively. Based on the area ratio of the assigned three peaks, the network structures around Si constructing SnO-Me₂SiO-P₂O₅ low-melting glasses were discussed in detail.     

Multinuclear NMR study of phosphosilicate gels derived from POCl3 and Si(OC2H5)4

Solid state ¹H, ²⁹Si and ³¹P MAS NMR have been used to investigate the microstructure of phosphosilicate gels prepared by a modified sol-gel method involving hydrolysis of silicon precursors in a solely aqueous environment at 50 °C. Gels with molar compositions 5, 10, 20 and 30 mol% P₂O₅ in P₂O₅-SiO₂ were studied. After drying to 400 °C the gels have very similar structures formed by a siloxane framework containing silanol groups and trapped molecules of orthophosphoric acid together with a very small amount, of pyrophosphoric acid. Unlike the gel samples previously synthesized by the hydrolysis of the silicon precursor in alcoholic solution at room temperature, the co-polymerization of phosphorus and silicon is much reduced. Although co-polymerization increases with phosphorus content, it still represents less than 50% of the phosphorus in the 30 mol% P₂O₅ gel. Furthermore there is no evidence for six-coordinated silicon in the glassy matrix.     

Investigation of TiO2SiO2 glasses by X-ray absorption spectroscopy

The coordination and nearest-neighbor bond distances of Ti in a series of TiO₂SiO₂ glasses have been quantitatively determined using a combination of XANES and EXAFS measurements about the Ti K-edge at 4966 eV. The glasses covering the range 0.012 to 14.7 wt% TiO₂ were prepared by flame hydrolysis of predetermined mixtures of SiCl₄ and TiCl₄ vapors. At TiO₂ concentrations below ～0.05 wt%, Ti is found in a rutile-like octahedral coordination. With increased TiO₂ content in the glass, a two-site model applies, in which Ti is found predominately in a fourfold site. About ～9 wt% TiO₂, t6he sixfold/fourfold ratio increases appreciably and eventually at ～15 wt% TiO₂, crystalline TiO₂ segregates out as a second phase. The average TiOSi bond angle in these glasses was estimated to be ～159 ° which is slightly larger than the most probable value of 152 degrees for SiOSi in vitreous SiO₂. Within the accuracy of the edge shift measurements all Ti in the glass is in 4+ valence. Finally, various physical properties such as density, optical transparency and thermal expansion are correlated in light of the new structural data for this interesting binary silicate glass system.     

Characterization of a mould flux glass

A simplified mould flux glass composition used for the continuous casting of steel was synthesized and then characterized using X-ray powder diffraction (XRD) differential thermal analysis (DTA) and ¹⁹F and ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR). DTA showed the glass to have a low glass transition temperature and to crystallize readily at 600 °C. XRD of the heat-treated glass showed it to crystallize to cuspidine. ¹⁹F MAS-NMR showed the principal fluorine species to be F-Ca(n) with no evidence of Si-F or Al-F species. Fluoride ions therefore, complex calcium in this glass, rather than forming non-bridging fluorines. The network connectivity of the glass was calculated on this basis and found to be 2.07 this would be expected to correspond to a Q² Si species which was supported by the ²⁹Si data that gave a chemical shift of −78 ppm corresponding to Q² Si.     

Fluorine incorporation in silica glass by the MCVD process: Study of fluorine incorporation zone,evaluation of optical properties and structure of the glass

A theoretical approach was made to find out a complete fluorine incorporation zone on a ternary diagram which serves as a useful graphical representation to select the flows of the supplied reagents for incorporation of the suitable amount of fluorine into cladding glass of optical fiber preform made by the MCVD process using CCl₂F₂ as a source of fluorine under oxygen abundance, oxygen deficiency and intermediate oxygen state conditions. The possible mechanism for incorporation of fluorine into cladding glass of optical fiber is also evaluated on the basis of the thermodynamical data. The fluorine incorporation mechanism in silica glass by the MCVD process is found to be dependent on the CCl₂F₂/SiCl₄ ratio in the input gas mixture. Fluorine doping is found to be effective for removing the strained Si–O–Si bonds, which govern the optical transparency in deep ultra-violet (DUV) and vacuum ultra-violet (VUV) regions. The maximum refractive index depression of ?0.5 × 10^?3 is obtained with incorporation of fluorine into silica cladding glass by the MCVD process using CCl₂F₂ as a dopant precursor with suitable flow of SiCl₄ vapor along with O₂ through backward deposition pass. The structure of fluorine doped silica glass preform samples containing 1.70–1.79 mol% fluorine incorporated by the MCVD process based on the analyses of ¹⁹F MAS spectra done by high-resolution ¹⁹F NMR spectroscopy reveal the presence of two distinct types of fluorine environments. The majority of the fluorine environments are formed in SiO_1.5F polyhedral and less abundant species is observed to be highly unusual, yielding a fivefold coordinated silicon of the type SiO₂F polyhedral which become increased with increasing the fluorine content.     

Germanosilicate and alkali germanosilicate glass structure: New insights from high-resolution oxygen-17 NMR

We present here triple-quantum, magic-angle spinning (3QMAS) NMR spectra for ¹⁷O in a SiO₂–GeO₂ binary glass, and for two sodium germanosilicate glasses, all with Si/Ge ratios of 1. In the binary germanosilicate, three NMR peaks are partially resolved, and correspond to the three types of bridging oxygens, Si–O–Si, Si–O–Ge, and Ge–O–Ge. Peak areas indicate that the relative abundances of these species are close to those expected for random mixing of the Si and Ge in the network. In a sodium germanosilicate glass with a relatively low Na content (Na₂O ≈ 8 mol%), the spectra demonstrate the formation of significant fractions of both nonbridging oxygens bonded to Si, and of oxygens bonded to Ge in five- or six-coordination. At higher Na content (Na₂O ≈ 31%), most or all Ge is four-coordinated and network modification is dominated by the formation of NBO on Si and on Ge. Models of physical properties of alkali germanosilicates, in which modifier oxides are distributed between the Si and Ge components of the network in proportion to the Si/Ge ratio, are thus supported, as is extensive mixing of Si and Ge.     

Inelastic light scattering studies of silicon chemical vapor deposition (CVD) systems

Raman and resonance fluorescence spectra, determined by inelastic light scattering measurements, are used to identify molecular species and to measure their concentration gradients on a fine spatial scale throughout a CVD reactor. Raman spectra are also analyzed to give gas temperature and tempetature profiles. The temperature profiles near the leading edge of a horizontal rf heated susceptor in a laminar flow system are adequately described by using Lévêque's solution to an energy balance equation, assuming temperature-independent fluid properties. Raman spectra at room temperature of some of the compounds commonly used as source materials for Si epitaxial growth, SiCl₄, SiCl₃H, SiCl₂H₂ and Si₂Cl₆ indicate that these species are all detectable at the 10–100 ppm level and are distinguishable from each other. Measurements at 500–1300°C of these compounds reveal the presence of a common species, SiCl_x (probably SiCl₂), which exhibits a resonance flourescence spectrum at least 1000 times more intense than typical Raman spectra. SiCl_x density profile measurements above the susceptor indicate a concentration boundary layer thickness of 0.7-0.8 cm for one set of experimental conditions. SiCl_x density measurements as a function of suspector temperature are observed to vary over a range of 4 to 5 orders of magnitude, and are much higher for a SiCl₂H₂ input than for a SiCl₄ input.     

Study of the chemical reactions' influence on semiconductor films structure formation by the Monte-Carlo method

The investigation by the Monte-Carlo method of the growth of the silicon epitaxial film at a chloride CVD system has allowed to find out the composition of adsorption layer, the micromechanism of the reactions of Si atoms building-in into the growing crystalline layer and the growth conditions influence on the growth rate and film surface roughness. The change of adsorptive layer composition in the system SiCl₄—HCl—H₂ (fraction of adaatoms, silicon atoms built-in a crystal and molecules SiCl₂) depending on temperature has been determined. The change of silicon film growth rate depending on temperature and concentration change of SiH₂Cl₂ has been established and the contribution of growth mechanism (with participation of adatom, silicon atoms and molecules SiCl₂) into the total rate of film growth has been shown.     

New evidence for tetrahedral triclusters in aluminosilicate glasses

The formation of thermodynamically stable 3/2-mullite (3 Al₂OAl₃·2 SiO₂) was investigated by scanning electron microscopy using reaction couples consisting of 2/1-mullite (2 Al₂O₃·1 SiO₂) plus SiO₂ glass, or Na₂O-SiO₂ glass, respectively. The mullite substrates were partially dissolved, thus leading to Al incorporation in the siliceous phases. In both reaction couples thin layers of stoichiometric 3/2-mullite form on the 2/1-mullite substrates. However, the major mullitization steps are different: The 2/1-mullite/SiO₂ reaction couple gives rise to 3/2-mullite crystallization within the bulk of the glass, whereas epitactic growth of c-axis orientated 3/2-mullite needles on the 2/1-mullite substrate was observed in the presence of Na₂O-SiO₂ glass. The differences in mullite nucleation were attributed to the existence or non-existence of tetrahedral triclusters in the as-reacted non-crystalline Al₂O₃-SiO₂ and Na₂O-Al₂O₃-SiO₂ phases, respectively. Triclusters of (Si,Al)O₄-tetrahedra in the Al₂O₃-SiO₂ glass may act as nuclei for 3/2-mullite crystallization in the bulk of the glass since these structural units also occur in mullite. In Na₂O-Al₂O₃-SiO₂ glasses triclusters are absent, and epitactical 3/2-mullite formation on the mullite substrate becomes more favorable energetically.     

A comparative FTIR study of thermal and photo-polymerization processes in hybrid sol-gel films

Controlling the organic polymerization in organic-inorganic hybrids is a key point in the development of new materials with high homogeneity of the nanostructure. The main difficulty is related with the achievement of a simultaneous control of the organic and inorganic network formation. Thermal and photocuring represent the main routes to form the organic chains when polymerizable organic groups are present in the hybrid materials. In the present work hybrid organic-inorganic films were synthesized from 3-methacryloxypropyltrimethoxysilane (MPTMS) cohydrolyzed with tetraethylorthosilicate (TEOS) and N-[(3-trimethoxysilyl)propyl]ethylenediamine (TMESPE) or 3-(triethoxysilyl)-propylamine (TESPA). This an example of basic catalyzed hybrid material with a polymerizable methacrylate functionality whose micro-structure is modified by the amine groups. FTIR spectroscopy was used to compare the effects of thermal or photo-induced polymerization on the materials. TESPA and TMESPE showed a different catalytic effect on the condensation of the inorganic network, with TMESPE the more efficient one. The presence of a more extended silica backbone reduced the curing efficiency in TMESPE derived samples. UV curing was also very effective in catalyzing the inorganic condensation of un-reacted species still present in the film after the deposition. A photo-induced polymerization of the inorganic side was observed in the hybrid films. Thermal polymerization in TMESPE films induces the reaction between the secondary amine and CO bonds in MPTMS, this reaction is, instead, not observed in films cured by UV radiation.     

Cation clustering and formation of free oxide ions in sodium and potassium lanthanum silicate glasses: nuclear magnetic resonance and Raman spectroscopic findings

Alkali silicate glasses containing lanthanum oxide are useful model systems for understanding the structural role of rare earth cations in optical and other types of materials. We report ²⁹Si and Raman spectra of sodium and potassium silicate glasses, both with added La₂O₃ and with La₂O₃ substituted for Na₂O or K₂O on an equal-oxygen basis. In the former series, silicate speciation changes show the formation of more non-bridging oxygens (NBO) as more of the network-modifying La₂O₃ is added. In the latter series, however, in which the nominal ratio of NBO to Si is constant, silicate speciation changes indicate that the actual ratio decreases significantly as La is substituted for 3 Na or K. The simplest explanation of this finding is that up to several percent of the oxygen in the La-rich glasses is not bonded to any Si, but instead forms `free oxide' ions that are part of La-rich domains. Although the size of these domains remains unconstrained, the lack of evidence for phase separation and continuity of trends in structure with composition suggests that the metastable liquid structure at the glass transition contains substantial intermediate-range heterogeneity.     

Structural investigations of copper doped B2O3-Bi2O3 glasses with high bismuth oxide content

Raman and infrared spectroscopy have been employed to investigate the 99.5%[xB₂O₃(1−x)Bi₂O₃]0.5%CuO glasses with different Bi/B nominal ratios (0.07?x?0.625) in order to obtain information about the competitive role of B₂O₃ and Bi₂O₃ in the formation of the glass network. The glass samples have been prepared by melting at 1100 °C and rapidly cooling at room temperature. In order to relax the structure, to improve the local order and to develop crystalline phases the glass samples were kept at 575 °C for 10 h. The influence of both Bi₂O₃ and CuO on the vitreous B₂O₃ network as well as the local order changes around bismuth and boron atoms in as prepared and heat treated samples was studied. Structural modifications occurring in heat treated samples compared to the untreated glasses have been observed.     

WO3-based electrochromic system with hybrid organic–inorganic gel electrolytes

Thin metal oxide films for a WO₃-based symmetric electrochromic system with a nickel oxide layer as the counter electrode have been prepared by spray pyrolysis on SnO₂:F coated soda-lime float glass, at a temperature of 670–720 °C and using metal acetylacetonates as precursors. The films have been characterized for composition and morphology by scanning electron microscopy equipped with an X-ray energy dispersive analyzer (SEM/EDAX), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Electrochromic properties have been examined in the electrochemical cells of a smart window arrangement using lithium ion doped sol–gel derived organic–inorganic hybrid materials as electrolytes. Hybrids with room-temperature ionic conductivities of 10^?4–10^?3 Ω^?1 cm^?1 have been synthesized from tetraethyl orthosilicate (TEOS) with an addition of 35 mass % of organic compounds. Coloration-bleaching of WO₃ films with lithium ions from hybrid electrolytes has resulted in the desired modulating the properties in the visible and near infrared spectrum range. An XPS analysis has shown the presence of a lower oxidized tungsten oxide phase (WO_2.92) in the WO₃ film.     

Heteroepitaxial growth of β-SiC on silicon substrate using SiCl4-C3H8-H2 system

Single crystals of β-SiC were prepared on Si substrates at a temperature around 1390°C with the standard conditions: H₂ ≈ 1 1/min, SiCl₄≈3 ml/min, C₃H₈≈1 ml/min, deposition period≈10 min. The dependences of the growth rate and the crystallinity on the substrate temperature were studied. By detailed reflection electron diffraction analyses, the crystallinity of β-SiC with 1 μm thickness was found to be better for the layer on the (100) and (110)Si substrates than for that on the (111)Si substrate. An activation energy of 25kcal/mole was obtained for the formation of β-SiC. Optimum conditions to obtain thicker β-SiC films are discussed.     

Formation of silica/epoxy hybrid network polymers

Epoxy-based inorganic–organic hybrid polymers, for use as a matrix in coatings, have been prepared from 3-glycidoxypropyltrimethoxysilane by a sol–gel process. The precursor molecule possesses both epoxy and silicon alkoxide functionality and so interlinked inorganic–organic networks can be formed. Diethylenetriamine was used to open the epoxy rings and form the organic network to an extent determined by the initial ratio of amine to epoxy groups. The materials were cured either at room temperature or with an additional heat treatment at 150 °C. Structural characterisation of the cured hybrid materials was performed using a combination of Raman, and ²⁹Si and ¹³C MAS NMR spectroscopies. These show that the formation of the two networks does not occur independently and the rate or extent of organic cross-linking has a direct effect on the extent of the inorganic network formation, and vice-versa.     

Effects of addition of nitrogen on bioglass properties and structure

Bioglasses have been developed for use in surgery because of their ability to form a hydroxy-carbonate apatite (HCA) layer on their surface which facilitates bonding to natural bone. However, they do not have sufficient strength for use in load-bearing situations and therefore improving their mechanical properties would allow their use in more robust applications. The purpose of this work was to study the effects of nitrogen addition on the physical and mechanical properties and the structure of oxynitride bioglasses based on the system Na₂O–CaO–SiO₂–Si₃N₄. The density, glass transition temperature, hardness and elastic modulus were measured and observed to increase linearly with nitrogen content. These increases are consistent with the incorporation of N into the glass structure in three-fold coordination with silicon which results in extra cross-linking of the glass network. The characterization of these oxynitride bioglasses using solid state nuclear magnetic resonance ²⁹Si MAS NMR and infrared spectroscopy have shown firstly that all the N atoms are bonded to Si atoms and secondly that this increase in rigidity of the glass network can be explained by the formation of SiO₃N, SiO₂N₂ tetrahedra and Q⁴ units with extra bridging anions at the expense of Q³ units. The oxynitride bioglasses in simulated body fluid form a hydroxy-carbonate apatite (HCA) layer on their surfaces showing that bioactivity is retained.     

Decisive role of Au layer on the oriented growth of ZnO nanorod arrays via a simple aqueous solution method

Vertically aligned arrays of ZnO nanorod were synthesized on the Au/SiO₂/Si(1 0 0) substrate by a simple aqueous solution growth process, without pre-prepared ZnO seed layer. For comparison, glass and SiO₂/Si were also used as substrates, and the results show that the Au layer plays a decisive role in orienting the growth of the ZnO nanorod. The effects of other growth parameters, including Zn²⁺ concentration and growth time, on morphology, density, and orientation of the ZnO nanostructure were also studied and with longer reaction time, a new structure namely ZnO nanotip was obtained. Moreover, the growth mechanism of ZnO nanorod arrays grown on the Au/SiO₂/Si substrate was proposed.     

Novel polyether–inorganic hybrid mesoporous silica synthesized through in situ incorporation of organic functionality

A new polyether–inorganic hybrid mesoporous silica has been synthesized through in situ incorporation of hydroquinone (HQ) moiety bridging between two silica units using tetraethyl orthosilicate (TEOS) as silica source in the presence of the self assembly of cationic surfactant under acidic pH. Samples synthesized with TEOS:HQ mole ratio of 2.0 and 4.0 showed good organic loading, mesoporosity and stability. Decreasing this ratio to 1.0 resulted in a organic-rich mesoporous hybrid material, which collapsed during template removal, whereas, increasing this ratio to 8.0 resulted in very poor incorporation of hydroquinone in the mesoporous silica. XRD and N₂ sorption data suggested the mesopore structure. TEM images indicated the wormhole like structure of these mesoporous samples. Solid state NMR data suggested the existence of (–O–C₆H₄–O)₂Si¹(OSi)₂ and (–O–C₆H₄–O)Si¹(OSi)₃ sites in addition to (Si¹(OSi)₄) sites. UV–Visible and FT-IR data suggested the incorporation of hydroquinone species and Si–O–Ph bonding in the samples.     

Sol-gel chemistry approach in the preparation of precursors for the substituted superconducting oxides

Sol-gel processing of ceramic materials for advanced applications involves several steps starting from precursor synthesis and ending up with multicomponent metal oxides. A simple sol-gel synthesis technique has been refined to prepare the precursors for the superconducting (Y_1−xSc_x)Ba₂Cu₄O₈ and (Y_1−xGa_x)Ba₂Cu₄O₈ compounds. The amorphous gel powders were characterized by powder X-ray diffraction, infrared spectroscopy, thermal analysis and elemental analysis. A systematic characterization of precursor gels led us to predict the approximate composition and the chemical reactions involved during gelation. The stability and high level of homogeneity obtained for the gels make them suitable as processable precursors to substituted (Y_1−xSc_x)Ba₂Cu₄O₈ and (Y_1−xGa_x)Ba₂Cu₄O₈ superconducting compounds.     

Optical and electrical properties of chlorinated and hydrogenated amorphous silicon prepared by glow discharge

Chlorinated and hydrogenated amorphous silicon films were prepared by glow discharge of a SiCl₄/H₂ mixture. Infrared spectra of these films show that, in addition to the hydrogen induced bands, two new modes appear at 545 cm^?1 (SiCl stretching) and 500 cm^?1 (Si TO modes induced by chlorine). Observation of the 545 cm^?1 band proves that chlorine is able to act as a dangling bond terminator in an amorphous silicon matrix. A good agreement is found between the total amount of chlorine determined by electron microprobe analysis and the value estimated from the integrated strength of the SiCl stretching mode. The relatively high value of the optical band gap (1.80 eV) of our material containing only 5 at.% bonded hydrogen shows that chlorine plays a major role in the optical gap value. Electrical conductivity, photoconductivity and luminescence properties are qualitatively similar to that of a: SiH films.