Porous Silicon Oxycarbide Glasses

High-surface-area silicon oxycarbide gels and glasses were synthesized from mixtures of methyldimethoxysilane (MDMS) and tetraethoxysilane (TEOS) through acidic hydrolysis and condensation. A surface area of ∼275 m²/g and an average pore size of ∼30 Å was obtained for a 50% MDMS-50% TEOS glass at 800°C under a flowing argon atmosphere. The average pore size was increased by aging the precursor gels in ammonium hydroxide. The increased average pore size and the higher strength of the mesoporous gel network enhanced the surface-area stability of the glasses; in this case, surface areas >200 m²/g were retained at 1200°C under an argon atmosphere. ²⁹Si MAS NMR spectra revealed that an oxycarbide structure was established in the mesoporous glasses obtained after pyrolysis of the aged gels. The role of carbon was demonstrated by comparing the surface-area stability of the oxycarbide glasses with that of pure silica and that of oxycarbide glasses where all the carbon groups were removed through low-temperature plasma-oxidation treatments. In the absence of carbon, the thermal stability of the surface area decreased dramatically.     

Chemical Durability of Silicon Oxycarbide Glasses

Silicon oxycarbide (SiOC) glasses with controlled amounts of Si—C bonds and free carbon have been produced via the pyrolysis of suitable preceramic networks. Their chemical durability in alkaline and hydrofluoric solutions has been studied and related to the network structure and microstructure of the glasses. SiOC glasses, because of the character of the Si—C bonds, exhibit greater chemical durability in both environments, compared with silica glass. Microphase separation into silicon carbide (SiC), silica (SiO₂), and carbon, which usually occurs in this system at pyrolysis temperatures of >1000°–1200°C, exerts great influence on the durability of these glasses. The chemical durability decreases as the amount of phase separation increases, because the silica/silicate species (without any carbon substituents) are interconnected and can be easily leached out, in comparison with the SiOC phase, which is resistant to attack by OH⁻ or F⁻ ions.     

Structural Characterization and High-Temperature Behavior of Silicon Oxycarbide Glasses Prepared from Sol-Gel Precursors Containing Si-H Bonds

Silicon oxycarbide glasses have been synthesized by inert atmosphere pyrolysis at 1000°C of gel precursors obtained by cohydrolysis of triethoxysilane, HSi(OEt)₃, and methyl-diethoxysilane, HMeSi(OEt)₂. The oxycarbide structures have been carefully characterized by means of different techniques such as ²⁹Si magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopies, X-ray diffraction (XRD), and chemical analysis. Experimental results clearly indicate that, depending on the composition of the starting gels, the resulting oxycarbide glass either is formed by a pure oxycarbide phase or contains an extra carbon or silicon phase. By increasing the temperature up to 1500°C, the oxycarbide glasses display compositional and weight stability; however, the amorphous network undergoes structural rearrangements that lead to the precipitation of nano-sized β-SiC crystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500°C for the samples in which the presence of Si-Si bonds was postulated at 1000°C.     

Crystallization Behavior of Novel Silicon Boron Oxycarbide Glasses

Homogeneous silicon boron oxycarbide (Si-B-O-C) glasses based on SiO _x C_{4– x} and BO _y C_{3– y} mixed environments were obtained by pyrolysis under inert atmosphere of sol–gel-derived precursors. Their high-temperature structural evolution from 1000° to 1500°C was followed using XRD, ²⁹Si and ¹¹B MAS NMR, and chemical analysis and compared with the behavior of the parent boron-free Si-O-C glasses. The XRD study revealed that, for the Si-O-C and the Si-B-O-C systems, high-temperature annealing led to the crystallization of nanosized β-SiC into an amorphous SiO₂-based matrix. NMR analysis suggested that the β-SiC crystallization occurred with a consumption of the mixed silicon and boron oxycarbide units. Finally, by comparing the behavior of the Si-O-C and Si-B-O-C glasses, it was shown that the presence of boron increased the crystallization kinetics of β-SiC.     

Gel Precursor to Silicon Oxycarbide Glasses with Ultrahigh Ceramic Yield

This paper reports on the synthesis and the pyrolysis transformation of a sol–gel precursor to an oxycarbide glass with an extremely high ceramic yield (up to 97 wt%). This unusually high ceramic yield is achieved by adding a small amount of boron alkoxide to a precursor gel containing Si—H and Si—CH₃ functionalities. The role of boron in the pyrolysis process is investigated using various spectroscopic techniques and by analyzing the pyrolysis gases with a mass spectrometer. The experimental results suggest a catalytic effect of the boron atom in promoting a partial oxidation of the Si—H bonds to Si—O bonds, with a corresponding reduction of the weight losses that are associated with the evolution of silanes during pyrolysis. Chemical analysis and XRD investigation indicate that the presence of boron does not modify the formation of the expected oxycarbide network.     

Mechanical Characterization of Sol–Gel-Derived Silicon Oxycarbide Glasses

Silicon oxycarbide glasses have been fabricated, in the shape of thin rods suitable for flexural test experiments, by pyrolysis in an inert atmosphere at 1000° and 1200°C of solgel precursors containing Si–CH₃ and Si–H bonds. The amount of carbon in the silicon oxycarbide network has been controlled by varying the carbon load in the precursor gel. Density and surface area analysis revealed that all of the samples pyrolyzed at 1200°C were well-densified silicon oxycarbide glasses whereas for the glasses treated at 1000°C, compositions with low carbon loads showed the presence of a residual fine porous phase. The elastic modulus ( E ), flexural strength (MOR), and Vickers hardness ( H_v ) increase markedly with the amount of carbon in the oxycarbide glasses reaching the maximum values ( E ∼ 115 GPa, MOR ∼ 550 MPa, and H_v ∼ 9 GPa) for samples with the highest carbon content. The experimental elastic modulus values of the silicon oxycarbide glasses compare well with the theoretical estimations obtained using the Voigt and Reuss models assuming the disordered network formed by the corresponding thermodynamic compositions.     

Local Structure of Alkaline-Earth Boroaluminate Crystals and Glasses: II, 11B and 27Al MAS NMR Spectroscopy of Alkaline-Earth Boroaluminate Glasses

¹¹B and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of alkaline-earth boroaluminate glasses show that the structure of these glasses are far more complicated than previously thought. The relative concentrations of 3- and 4-coordinated boron vary as found by previous investigators using continuous-wave NMR methods, but the ²⁷Al NMR spectra indicate the presence of Al in 4-, 5-, and 6-fold coordination, in contrast to previous assignments. Analysis of the data based on local charge balance considerations provides a qualitative model that correctly predicts compositional variations of the NMR peak intensities and ²⁷Al chemical shifts for a wide range of boroaluminate glass compositions.     

29Si MAS-NMR Study of the Short-Range Order in Lithium Borosilicate Glasses

²⁹Si MAS-NMR measurements have been made on a series of lithium borosilicate glasses of general composition R Li₂O.B₂O₃· K SiO₂. At low alkali contents ( R < 1), the ²⁹Si resonance envelope is broadened and indicates a distribution of Si sites. As R increases above 1, the FWHM of the ²⁹Si resonance narrows considerably to that representative of a single chemical site. Simultaneously, the average chemical shift of the resonance shifts upfield in agreement with the trends found in the binary lithium silicate glass system. Using the chemical shifts for the individual Q species in the binary system it was found that very good agreement between the chemical shifts of the binary glasses and the ternary glasses examined here could be achieved if a model of proportional sharing of the added oxygen (from lithia) between silicate and borate units was used. In contrast to the ¹¹B NMR studies of these same glasses, the ²⁹Si NMR data are quantitatively best-fit if it is assumed that the proportional sharing of the oxygen from the added lithia begins at R = 0. Models of sharing developed from the ¹¹B NMR studies of these glasses, where proportional sharing above a certain fixed (independent of K ) or variable (dependent on K ) minimum R ₀, have been reexamined and were quantitatively shown through residual analysis to give consistently poorer fits to our data. At present the reasons for the discrepancy between the two sets of NMR data are unknown.     

Nitrogen Adsorption onto Silane-Derived Silicon Powders

Nitrogen chemisorption onto silane-derived silicon powder was studied at atmospheric pressure in N₂, using thermogravimetric analysis (TGA) to examine the reaction kinetics and using Fourier transform infrared (FTIR) spectroscopy to characterize the surface chemistry. Above 700°C the initial Si—H surface is transformed to an Si—N surface; the latter is characterized by a broad infrared absorption band at 830 to 840 cm⁻¹. Isothermal TGA data exhibit a sigmoidal shape which is apparently caused by simultaneous hydrogen desorption and nitrogen adsorption. By holding samples under vacuum at 900°C before nitridation the adsorption reaction was isolated. The kinetics of this reaction are well described by a simple first-order model with an activation energy of 140 kJ/mol, and with very low sticking probabilities (between 10⁻¹³ and 10⁻¹⁰).     

Spectroscopic Properties and Local Structure of Eu3+ in Ge–Ga–S–CsBr (or CsCl) Glasses

Spectroscopic properties and local structure of Eu³⁺ in Ge–Ga–S–CsBr (or CsCl) glasses were investigated using fluorescence measurements and several spectroscopic methods. Fluorescence from Eu³⁺:⁵D₀→⁷F₂ was observed only from glasses with CsBr/Ga ratios greater than unity and disappeared at temperatures above 140 K. Phonon sideband (PSB) spectra revealed that Eu³⁺ ions are located next to halogen ions, which form part of well-structured complexes such as EuCl₃, tetrahedral [GaS_3/2Cl]⁻, subunits and/or Ga₂Cl₆. These new bonds showed reduced coupling strength compared with Eu³⁺–S bonds in Ge–Ga–S glass. Fluorescence line narrowing experiments showed little site-to-site variation of Eu³⁺ ions.