Sol-Gel Synthesis and Microstructural Characterization of Silicon Oxycarbide Glass Sheets with High Fracture Strength and High Modulus

A synthesis route to silicon oxycarbide glass sheets (thickness 40 to 1000 m and area up to 20 × 35 cm2) has been developed for the first time starting from a methyl modified sol containing nano-particulate SiO2 and having a solid content of 70 wt%. The gel sheets obtained by casting and drying of this sol were sintered in N2 atmosphere at temperatures between 900–1650°C. Only by the incorporation of colloidal silica (0.10–0.35 mole per mole alkoxide) to the sol could crack-free, large area glass sheets be obtained. Fracture strength (three point bending) was found to attain a maximum (200–300 MPa) for the sheets sintered at 1000°C. Young's modulus attained a peak value between 120 and 130 GPa for the sheets sintered at 1200°C. HR-TEM studies showed an amorphous and homogeneous matrix up to a sintering temperature of 1200°C, whereas at 1450°C and 1650°C, crystallites of SiC and lamellar graphite were formed. It is concluded that addition of colloidal silica to the sol does not lead to inhomogeneities after sintering and therefore does not decrease the strength and elastic modulus.     

Silicon Oxycarbide Glasses Derived from Polymer Precursors

This paper describes a new predictive model to estimate compositions of ceramic materials from the structure of the preceramic polymer. The polymers are prepared by sol-gel methods and have M and T silicone functionality. The ceramic predictive model works well for highly branched silsesquioxanes, which are some of the most common ceramic precursors. This report describes the control of ceramic SiOC compositions which in turn make the materials useful as: (1) matrices for ceramic matrix composites and; (2) anodes for rechargeable lithium ion batteries.     

Preparation and Structure of Silicon Oxycarbide Glasses Derived from Polysiloxane Precursors

Silicon oxycarbide phases with different O/Si ratios were prepared by pyrolysis of highly cross-linked polysiloxane precursors. Their structure was investigated using quantitative ²⁹ Si solid-state NMR and X-ray photoelectron spectroscopy (XPS). The prominent part played by the redistribution reactions on the ceramic yield, the composition of the oxycarbide phases and their structure was illustrated. In a large range of compositions the oxycarbide phases formed by pyrolysis at moderate temperature (<1000°C) may be described as a purely random arrangement of CSi₄ tetrahedra and Si–O–Si bridges.     

Surface Chemistry and Structure of Silicon Oxycarbide Gels and Glasses

In this study, the surface chemistry and structure of methyl-substituted silica gels and porous oxycarbide glasses were investigated. FTIR was used to measure the relative concentration of Si−CH₃ and Si−OH as a function of the degree of methyl-substitution and the pyrolysis temperature. The gels and glasses were further heated, dehydrated or hydrated, in situ, within the FTIR spectrometer. In the temperature range of 800–850°C, high surface area oxycarbide glasses were created with no detectable surface hydroxyl groups. Oxycarbide glasses synthesized in argon at 700°C displayed a weak band for surface hydroxyl groups and reversible physisorption of water, while those synthesized at 850/900°C showed a complete absence of surface hydroxyl groups and the formation of vicinal silanols upon chemisorption of water. Isolated silanols were observed upon heat treatment in vacuum. Formation of aromatic carbon species was found to correlate with the decomposition of the methyl groups. The oxycarbide surface is quite stable to densification (presumably due to elemental carbon on the pore surfaces). In the absence of oxygen, porous silicon oxycarbide glass powders maintain surface areas >200 m²/g at 1200°C. However, oxidizing species in the atmosphere deplete the aromatic carbon species, and the glasses lose surface area.     

High Temperature Behavior of a Gel-Derived SiOC Glass: Elasticity and Viscosity

The high temperature behavior of a sol-gel derived silicon oxycarbide glass containing 12 at.% carbon has been characterized by means of creep and in-situ ultrasonic echography measurements. Temperature induced changes include structural relaxation and densification from 1000 to 1200°C, and crystallization to form a fine and homogeneous -SiC/glass-matrix nanocomposite with 2.5 nm large crystals above 1200°C. Young's modulus measurements clearly reveal a consolidation of the material upon annealing below 1200°C. Crystallization is almost complete after few hours at 1300°C and results in a significant increase in Young's modulus. The viscosity of the oxycarbide glass is much higher than that of fused silica, with two orders of magnitude difference at 1200°C, and the glass transition temperature ranges from 1320 to 1370°C.     

Microstructural and Spectroscopic Studies of Sol-Gel Derived Silica-Titania Waveguides

Silica-titania planar waveguides were prepared via the sol-gel method from acid-catalyzed solutions of firstly, ÿ-Glycidoxypropyltrimethoxysilane mixed with tetrapropylorthotitanate (labeled as GT), and secondly, ÿ-Glycidoxypropyltrimethoxysilane mixed with both tetrapropylorthotitanate and tetraethoxysilane (labeled as GTT). Atomic force microscopy, thermal gravimetric analysis, differential thermal analysis, UV-visible spectroscopy, Fourier transform infrared spectroscopy, and Raman spectroscopy were used to study the structural and optical properties of the waveguide films prepared from the two types of sols. The obtained results showed that in both cases, crack-free and highly transparent silica-titania films with a thickness of more than 0.5 m could be obtained by a single spin-coating process after a heat treatment at 500°C. The GT derived films showed more shrinkage and a higher refractive index after annealing as compared to the GTT derived films. When such films were deposited on a silica-on-silicon substrate to act as a surface planar waveguide, the light propagation loss was measured to be about 0.9 dB/cm and 1.3 dB/cm respectively. Raman spectroscopy results indicated that the GTT derived waveguide films with 0.5 molar titanium content contained amorphous carbon phase after being heated at above 500°C in air directly.     

Sol-Gel Synthesis and Structure of Borosilicon Oxycarbide Glasses

Existing sol-gel polymerisation/pyrolysis routes employing two separate boron containing metal alkoxide precursors have been applied to form multi-component silicon oxycarbide glasses based on a borosilicon oxycarbide (B–Si–O–C) system. 29Si, and 11B magic angle spinning nuclear magnetic resonance (MAS-NMR) was used to characterize the atomic structure of the gels, glasses and glass ceramics. Structural changes such as glass formation and crystal nucleation were identified by X-ray diffraction (XRD). Transmission electron microscopy (TEM) provided images of crystal nucleation and growth.     

Sol-Gel Derived Barium-Strontium Titanate Films

Sol-gel techniques for the preparation of barium-strontium titanate (BST) films are discussed. The evolution of film microstructure during heat treatment, and the dielectric properties of BST films prepared from alkoxide solutions and from alkoxide solutions modified by 2-ethylhexanoic acid were studied. It is shown that the extent of the modification of the precursors by 2-ethylhexanoic acid changes the precursor molecular complexity governing the microstructure of the films and their electrical properties.     

Control of the Composition and Structure of Silicon Oxycarbide and Oxynitride Glasses Derived from Polysiloxane Precursors

The reactions involved in the preparation of silicon oxycarbide and oxynitride glasses by pyrolysis of polysiloxane precursors respectively under argon and ammonia are reviewed. The influence of the composition and structure of the precursor and of these pyrolysis reactions on the pyrolysis yield, the composition, and the structure of the glass is discussed. The free-carbon content of the glass depends on the substituents in the precursor and on the nature of the pyrolysis atmosphere. The composition of the oxynitride or the oxycarbide phase depends on the O/Si ratio of the precursor. The structure of these phases is not directly related to the structure of the precursors, but rather depends on their composition and on the pyrolysis temperature.     

Dielectric and Ferroelectric Properties of Sol-Gel Derived YMnO3 Films

There has been considerable interest in ferroelectric (FE) films especially for non-volatile memories and ultra high density DRAM applications. Such FE films typically consist of lead zirconate titanate (PZT) with novel oxide contacts, or layered perovskite such as Sr2Bi2TaO9. Recently, there have been reports of sputtered YMnO3 films which exhibit a single polarization axis and do not contain any volatile species of Pb or Bi. Single crystal YMnO3 exhibits satisfactory polarization (6 C/cm2) and low coercive field (<20 kV/cm). Additionally, the dielectric constant of YMnO3 is quite low (<30) which should facilitate ferroelectric switching. In this study, sol-gel derived YMnO3 films were prepared on platinized Si wafers and their dielectric and ferroelectric properties were characterized. Their electrical properties will be discussed with respect to Y/Mn stoichiometry ratio, hexagonal phase development and processing conditions. The potential of YMnO3 as a material in non-volatile memories is evaluated.     

CaTiO3:Zn溶胶-凝胶法制备、结构及介电性能

采用溶胶-凝胶法制备了CaTiO3:Zn纳米粒子,透射电镜图显示平均粒径为25 nm.Zn的掺杂位置对于陶瓷相组成和烧结特性有很大影响.Ca1-xZnxTiO3和CaTiO3+zZnO样品的Zn以Zn2TiO4相形式存在;而CaZnyTi1-yO3-δy=0.01)样品中的Zn进入Ti位形成固溶体,无明显的降温效果,当Zn量增至0.05和0.1时,出现ZnO相.ZnO和Zn2TiO4第二相的存在均能明显促进陶瓷烧结.CaTiO3:Zn超细粉体可在较低温度下致密烧结(≤1 250 ℃).1 250℃烧结的CaZnyTi1-yO3-δ(y=0.01)陶瓷具有较好的介电性能:介电常数ε=157,品质因数Q×f=6 819 GHz,谐振频率温度系数(τ)f=7.51×10-4℃-1.     

Sol-Gel Synthesis of Ge Nanocrystals-Doped Glass and Its Photoluminescence

Ge nanocrystal-embedded SiO₂ glasses were prepared by a sol-gel process. The glasses synthesized through the hydrolysis of Si(OC₂H₅)₄ and GeCl₄ were heated in H₂ gas atmosphere at 500 to 800°C, in which Ge⁴⁺ ions were reduced to precipitate nanosized Ge crystals with the size smaller than 10 nm diameter. Glasses doped with Ge nanocrystals of diameter of ≈5 nm showed the optical absorption edge at ≈2.8 eV and a broad photoluminescence exhibiting the peak at around 2.2 eV. Large Ge crystals precipitated by heating above 800°C showed no photoluminescence.     

Synthesis of Ag-Colloids in Sol-Gel Derived SiO2-Coatings on Glass

Ag colloid-containing coatings on soda lime glass and fused silica are prepared via the sol-gel process. To incorporate Ag⁺-ions in the coatings homogeneously, they are stabilized by a functionalised silane (aminosilane) and then mixed with the basic sol prepared from 3-glycidoxypropyl trimethoxysilane (GPTS) and tetraethoxysilane (TEOS). Crack-free and transparent coatings with a thickness of 0.5 to 1.2 m, are obtained by heat treatment between 120°C and 600°C. The Ag-colloid formation was monitored by UV-VIS spectroscopy as a function of temperature. The investigations reveal that the substrate has a deciding influence on the Ag-colloid formation caused by alkali diffusion from the substrate into the coating. High resolution transmission electron microscopy (HRTEM) investigations prove that poly-crystalline Ag_xO_y-nanoparticles are formed during thermal densification in the coatings and that this change is accompanied by a vanishing of the yellow colour of the coatings. A post-heat treatment in a reducing atmosphere (90% N₂, 10% H₂) turns back the yellow colour and single-crystalline Ag-colloids can be detected by HRTEM. A suitable choice of the temperature and time conditions allows the control of the colloid size during heat treatment in a reducing atmosphere. For comparison, ion-exchange experiments have been carried out which showed that a spontaneous Ag-colloid formation was achieved in the soda lime substrate at 400°C. Since Ag containing SiO₂-coatings remained colourless after thermal treatment between 400°C and 600°C in air, on soda lime substrates, a remarkable diffusion of Ag⁺ into the substrate was excluded.     

Synthesis and Characterization of Gelatin-Siloxane Hybrids Derived through Sol-Gel Procedure

A new type of inorganic-organic hybrid materials incorporating gelatin and 3-(glycidoxypropyl) trimethoxysilane (GPSM) was prepared through sol-gel processing. A solid-state ²⁹Si NMR analysis indicated that all the methoxy silane groups of GPSM were polymerized to yield —Si—O—Si— bridging bonds. An amino acid analysis confirmed grafting reactions of GPSM against gelatin chains. The increasing GPSM/gelatin ratio stimulated gel formation, phase separation, and the density of GPSM-crosslinking of the gelatin chains as well as it changed the micro- and macro- structures and the viscoelastic properties of the final products.     

Preparation of Silicon Oxycarbide Ceramic Films by Pyrolysis of Polymethyl- and Polyvinylsilsesquioxanes

The sol-gel reactions of methyl- and vinyltrimethoxysilane under controlled conditions provided the corresponding polysilsesquioxanes and flexible thin films on further condensation of the precursors. Silicon oxycarbides containing a large amount of free carbon 58 mol% and 90 mol% were obtained in high yields 90 wt% on pyrolyzing the precursors of polymethyl-and polyvinylsilsesquioxane at 1400°C under N₂ atmosphere, respectively. Moreover, pyrolytic polycondensation of polyvinylsilsesquioxane was found to give the silicon oxycarbide ceramic films without deformations and cracks. The composition and properties of films were investigated.     

Immobilization and Characterization of Lactate Dehydrogenase on TEOS Derived Sol-Gel Films

Physical adsorption and physical entrapment techniques have been utilized for the immobilization of lactate dehydrogenase (LDH) on tetraethylorthosilicate (TEOS) derived sol-gel films. The enzyme (LDH) activity has been assayed as a function of time, temperature, pH and pyruvate concentration. The results of photometric measurements used for monitoring the reaction yield a response time of about 1 min, linearity over a concentration range of 0–1.5 × 10^-3 M and detection limit of 5 × 10^-5 M. The TEOS sol-gel films containing LDH have been found to be stable for about 30 days at temperatures 4 to 10°C.     

Low Dielectric Constant Organosilicate Films Prepared by Sol-Gel and Templating Methods

Low dielectric constant organosilicate films with controllable microstructure have been successfully synthesized by multiple-step sol-gel process and templating method, which are the two basic methods to establish porous network in the films. Ultra-low dielectric constant (k) of around 2.0 can be achieved for both films. The microstructure such as porosity, pore interconnection and pore size of the two types of the films have been studied and compared. It has been found that the sol-gel films have a higher level of porosity comparing to the templating films to obtain the same k value. The sol-gel film has a majority of closed pores with pore size around 5 nm. The templating film has a closed pore structure with pore size around 10 nm. Preliminary results present a very positive prospective for intermetal dielectric applications.     

Preparation and Characterization of Germanium Sulfide Based Sol-Gel Planar Waveguides

Germanium sulfide based glass films have been deposited by spin-coating onto single crystal silicon wafers and silica glass disks, using the reaction between GeCl₄, either pure or doped with of 5 mol% of SbCl₃, dissolved in toluene, with H₂S. The films, heat-treated under different conditions, were characterized by X-ray diffraction, infrared spectroscopy, X-ray photoemission spectroscopy, mechanical profilometry and ellipsometry. Oxide contamination was found in these films, but this was reduced or even eliminated by a heat-treatment in H₂S gas, at 270°C. A maximum film thickness of 1.3 m was achieved and the refractive indices of the films were in the range of 1.9–2.8 at 633 nm. Propagated light was observed by butt-coupling, for waveguides deposited on silica glass disks, at = 633 nm. Optical losses in the range of 1.1–1.9 dB/cm were measured at this wavelength, for different regions of different planar waveguides. The present method has achieved reasonably low loss and low levels of oxide contamination, which are promising for active applications.     

Fourier Transform and Multi Dimensional EPR Spectroscopy for the Characterization of Sol-Gel Derived Hydroxyapatite

Calcium hydroxyapatite powder was prepared by sol-gel method using calcium acetate and PO(OC₂H₅)₃ as initial compounds, and alcohol (methyl, ethyl, and propyl-alcohol) as solvent. Homogeneous solution and gels were prepared using a molar ratio of Ca/P=1.67. The evolution of the structure was detected by X-ray diffraction, IR, and FT-n(=1,2,3)D-EPR spectroscopy. The dried gels exhibit a signal characterized by a central line and two satellites. The 2D spectrum (ESEEM vs. field sweep) showed the same modulation for the central line. The FT-EPR spectrum vs. field sweep 2D-spectrum indicated that the satellites are due to an hfs splitting with water. The central region of this 2D spectrum is influenced by P and H in a concentration ratio of [H]/[P]=2.5. The ESEEM spectrum was simulated assuming the equation V_mod=V_mod(P)^mV_mod(H)ⁿ for two spin systems S=1/2 and I=1/2. This simulation gave form andn the values of 2 and 5, respectively. This finding suggests the structure: for the unpaired state. It appears that one ethyl group does not hydrolyse in the gelation process. The ESEEM spectra of hydroxyapatite exhibit a modulation generated by P, H and Ca atoms.     

Atomic Hydrogen Induced Chemical Vapor Deposition of Silicon Oxycarbide Thin Films Derived from Diethoxymethylsilane Precursor

Amorphous silicon oxycarbide (a-SiOC:H) films produced by remote plasma RPCVD from diethoxymethylsilane (DEMS) were characterized in terms of their basic properties related to the coatings deposited using conventional plasma enhanced PECVD method. The effect of substrate temperature (T_S) on the growth rate, chemical composition, structure, and properties of resulting a-SiOC:H films is reported. Film growth is an adsorption-controlled process, wherein two mechanisms can be distinguished with a transition at about T_S=70°C. Depending on the temperature, films of different nature can be obtained, from polymer-like to highly crosslinked material with C-Si-O network. The chemical structure of a-SiOC:H films was characterized by FTIR, ¹³C and ²⁹Si solid-state NMR, and X-ray photoelectron spectroscopes. The a-SiOC:H films were also characterized in terms of their density, refractive index, surface morphology, conformality of coverage, hardness, adhesion to a substrate, and friction coefficient. The films were found to be morphologically homogeneous materials exhibiting good conformality of coverage and small surface roughness. Their refractive index exhibits anomalous effect revealing a minimum value at T_S=125°C. Due to their exceptional physical properties a-SiOC:H films produced by RPCVD from DEMS precursor seems to be useful as potential dielectric materials or coatings for various encapsulation applications.