Insights into the Oxidation Chemistry of SiOC Ceramics Derived from Silsesquioxanes

We have undertaken a systematic study of the oxidation chemistry for a range of SiOC ceramics derived from silsesquioxane polymeric precursors. This study examines the oxidation for 500 hours at 600, 800, 1000 and 1200°C for four SiOC powders. The material changes upon oxidation were characterized qualitatively by color change and optical microscopy and quantitatively by weight and composition change. In this study we employ a very easy method that uses the weight change upon oxidation and a carbon analysis after oxidation to arrive at the composition of the oxidized SiOC. Combined these qualitative and quantitative techniques have shown that on oxidation at 800 and 600°C the SiOC composition is more rapidly changed to that of silica than oxidation over the same time frame at 1000 or 1200°C. The data indicates that this difference is due to the relative rates of oxidation of the excess carbon versus the Si—C bonds in the SiOC. At lower temperatures initially the carbon oxidation predominates which leads to higher porosity throughout the material and an increase in the surface area with eventually complete oxidation to silica. At higher temperatures the Si—C bond oxidation rate is comparable to the rate of oxidation of carbon. This allows a silica-like surface to build up on the SiOC, which slows all subsequent reactions due to the necessity to diffuse O2 in and COx out of the bulk. Under these oxidation conditions materials that originally contain high amounts of excess carbon are more quickly oxidized to silica than those that contain minimal amounts of excess carbon, as confirmed by elemental analysis and optical microscopy. Regardless of the time or temperature of the oxidation conditions no materials were found to be completely stable to oxidation. SiOC materials with low levels of excess carbon showed the best resistance to change upon oxidation.     

ARXPS-analysis of sputtered TiC, SiC and Ti0.5Si0.5C layers

Summary TiC, SiC and Ti_0.5Si_0.5C layers have been deposited by magnetron sputtering in Argon at bias voltages between 0 and 1500 V. AES and ARXPS analyses show that TiC and Ti_0.5Si_0.5C, at bias voltages below 1000 V, are C-rich (+20%) and contain TiC crystallites of diameter below about 10 nm and have a metal-like resistance of about mcm. The excess C segregates to the surface of TiC nanocrystallites showing an XPS C 1s level shift similar to Li-graphite or doped fullerenes. The doped carbon (carbidic) interface layer, higher deposition rate and better mechanical strength seem to be interrelated. Magnetron sputtered SiC is X-ray amorphous and insulating, grows more slowly, has reduced mechanical strength and does not contain excess C. The ARXPS analysis of Ti_0.5Si_0.5C layers allows the modelling of the TiC nanocrystallites embedded in interfacial carbon and defective SiC.     

Chiral vanadyl salen complex anchored on supports as recoverable catalysts for the enantioselective cyanosilylation of aldehydes. Comparison among silica, single wall carbon nanotube, activated carbon and imidazolium ion as support

The activity and enantiomeric excess (ee) (in some cases >85%) obtained for the asymmetric addition of trimethylsilyl cyanide to aldehydes using different heterogeneous chiral catalysts are compared. A library of recoverable catalysts was developed by immobilization of a chiral vanadyl salen complex having a terminal carbon-carbon double bond onto a series of scaffolds including silica, single-wall carbon nanotubes, activated carbon and room-temperature ionic liquids. The covalent linkage has been achieved by radical initiated addition of mercapto groups to CC. The highest enantiomeric excesses, similar to those obtained in the homogeneous phase, were achieved using silica as support or with the homogeneous tetra-tert-butyl salen catalyst dissolved in an imidazolium ionic liquid. The use of silica as support permits an easier separation and reuse of the catalyst from the reaction media.     

Hollow Silicon Carbide Nanostructures

Hollow silicon carbide nanostructures with length up to 1 m have been produced for the first time along with threadlike structures. The preparation of SiC nanostructures from silicon and carbon at 1000-1100 °C was carried out without prior gasification of these elements. The growth of SiC nanostructures involves a step featuring atomization of silicon and carbon at such low temperatures. The growth of SiC nanotubes upon the reduction of carbon by SiO₂ in the initial period of the preparation proceeds with their predominant formation as bundles. Silicon carbide may correspond to the highly textured -modification.     

Microstructural and Dielectric Characterization of Sol-Gel Derived Silicon Oxycarbide Glass Sheets

Thin (50—1000 m) silicon oxycarbide glass sheets were synthesized by the pyrolysis of gel sheets obtained from a methyl-modified silica sol containing colloidal silica under inert atmosphere between 900 and 1450°C. The microstructure of these glass sheets was investigated with the help of high resolution scanning and transmission electron microscopy (HR-SEM and HR-TEM), X-ray diffraction and Raman spectroscopy and their dielectric properties were determined. The surface morphology as observed with HR-SEM did not exhibit a notable temperature dependence. HR-TEM studies showed that the glass sheets sintered up to 1200°C are amorphous, whereas those sintered at 1450°C contain uniformly dispersed crystallites of SiC and graphite. X-ray diffraction studies were found in agreement with the HR-TEM results. Raman spectroscopy showed that free carbon is present as an amorphous phase till a temperature of 1000°C, whereas at temperatures 1200°C, the presence of graphitic carbon was observed. Silicon oxycarbide glass sheets heat treated at temperatures up to 1200°C, showed a dielectric constant between 4.1 ± 0.11 and 4.6 ± 0.15 in the frequency range from 75 kHz to 5 MHz, with corresponding losses between 0.0008 and 0.1100. Such silicon oxycarbide glass sheets sintered at 1200°C could find an application as substrates for electronic packaging.     

Amphiphilic Patchy Composite Colloids

A new approach to fabricate patchy silica/polymeric gel composite colloids with amphiphilic performance is reported. The amphiphilic performance is rendered by selectively modifying the silica framework with a silane that contains an oleophilic alkyl chain. The patchy composite colloids are dispersible in both water and oil, and may be used as a solid particle surfactant. The modified silica framework can also assist other functional materials to disperse in a desired media. The corresponding silica/carbon composite colloids become amphiphilic after a sequential activation of carbon and modification of silica, and meanwhile possess as good electron conductivity as the as‐prepared silica/carbon composite colloids.

     

Carbon and SiC macroscopic beads from ion-exchange resin templates

A method for preparing carbon and SiC macroscopic beads using ion-exchange resins as a macrotemplate that determines the macroshape and the pore structure of the product materials is reported. First, silicates are ion-exchanged into the resins to prevent the resin from collapsing during subsequent carbonization and allow them to be used as precursors for SiC formation. SiC is prepared via carbothermal reduction of carbon/silica composite beads obtained upon carbonization of the resin/silicate in an inert atmosphere. Finally, silica is removed by HF etching. Very high-surface area (1670-2026 m2 g-1) micro- or micro-/mesoporous carbon beads and relatively high-surface area (35-63 m2 g-1) macro- and meso-/macroporous SiC beads were prepared by the described method. The pore structure and the macroshape of the particles were controlled by the type of ion-exchange resins employed, gel or macroreticular.     

Silica grafted with silanized carbon dots as a nano-on-micro packing material with enhanced hydrophilic selectivity

Carbon dots derivatized from N-(β-aminoethyl)-γ-aminopropyl-methyldimethoxysilane (AEAPMS) were coated onto silica microparticles. These particles (Sil-CDs) are shown to be an excellent stationary phase for use in hydrophilic interaction chromatography. Analytes including sulfonamides, nucleosides and bases, flavones and amino acids can be well separated on this stationary phase. Compared to a silica stationary phase functionalized with AEAPMS only, the Sil-CDs show enhanced separation performance. The selectivity factors of three nucleosides and bases (1.02–1.09) and four sulfonamides (1.04–1.11) on AEAPMS functionalized silica stationary phase were improved to 1.10–1.20 and 1.13–1.15 respectively on Sil-CDs stationary phase. This is attributed to the higher number of surface functional groups due to the introduction of carbon dots. The successful application of the Sil-CDs stationary phase highlights the potential of carbon dots as a modified material in chromatography.

Open image in new window

Graphical Abstract Schematic presentation of the preparation of silanized carbon dots coated onto silica microparticles. The material represents a new stationary phase for hydrophilic interaction chromatography. It shows improved separation performance compared to a silane-only functionalized silica stationary phase.

     

Gas-Phase Reactions of Formation of Silicon Carbide Nanofilaments from Silicon and Carbon Powders

We propose and study an original route for low-temperature synthesis of SiC filaments from Si and C powders. We discuss a mechanism for the nucleation and growth of SiC, involving atomization of silicon and carbon at temperatures below 1000 °C. We assume that the nanodroplets at the ends of SiC filaments observed under the electron microscope belong to silica, as the center for dissolution of atomic carbon.     

Organosilicon polymers with bis-acetylenic units: Conductivity and ceramisation studies

The transformation poly[(silylene)diacetylene] into ceramic have been studied and SiC obtained with a quantitative yield: all the silicon atoms present in the starting material are converted into SiC. The key step is the crosslinkage of C₄ units into a carbon matrix in which the R₁SiR₂ moieties are in the vicinity one to each other allowing the quantitative transformation into SiC. The carbon content of the resulting ceramic depends on the nature of R₁ and R₂ groups. The excess of carbon has been used for the carboreduction of oxides (TiO₂, ZrO₂, HfO₂, etc…) and mixed ceramics with interpenetrating network have been obtained: SiC / TiC, SiC / ZrC, etc… Under nitrogen atmosphere, the ceramisation leads to different systems such as SiC / TiN, SiC / AlN, etc…     

Sorbents for liquid chromatography based on the footprint principle

Summary The possibility of synthesizing normal-phase chromatographic sorbents with selectivity and specificity for an organic compound was investigated using two approaches. The generation of footprints, imprinted adsorption sites reflecting the shape of an added template molecule, was attempted by imprinting aluminum ion-doped silica gel and by modifying silica gel with diazomethane. In both cases phenanthrene was used as template molecule. Surprisingly, the diazomethane-modified silica gels showed a stronger retention for all polycyclic aromatic compounds than unmodified silica up to a carbon load of ca. 1.5%. The different gels synthesized according to the first principle showed somewhat different selectivity towards various polycyclic aromatic compounds. In neither case was a specificity toward phenanthrene observable.     

Boron carbide-copper infiltrated cermets

Porous boron carbide preforms, prepared with and without excess carbon, were infiltrated with a Cu-Si alloy. Contrary to unalloyed copper, the Cu-Si alloy wets and infiltrates the porous preforms. A thermodynamic analysis of the B-C-Cu-Si system indicated that a Si content of the alloy above 15 at% leads to the formation of SiC. At higher Si content, the composition of boron carbide in contact with the melt also changes towards higher boron content. Metallographic examination validated these conclusions. The SiC compound forms preferentially around the free carbon particles in preforms containing excess carbon, and also in the vicinity of carbide that did not contain any excess carbon. Eventually, SiC, a product of the reaction between the carbide and the melt, forms a continuous barrier that impedes completion of the reaction and accounts for the limited increase of hardness as a result of lengthy heat treatments.     

Study of modified silicas by inverse gas chromatography. Part I: Influence of chain length on grafting ratio

Summary Silica has been modified by an esterification reaction using either n-alcohols or - diols. The grafting ratios were evaluated by elemental analysis of carbon or by the weight loss of the grafted silica heated to 700°C. The grafting ratio can be easily fixed by changing the silica to reactant impregnation ratios. In the case of n-alcohols, the grafting ratios do not vary monotonously with the number of carbon atoms of the grafts and the density of the grafted layer is 25% higher for - diols than for n-alcohols. This observation needs further investigation.     

Intermediate Chemical State in Multicomponent Systems: Si and C Dopants in Aluminum Nitride

The tightbinding band method is employed to study stabilization of a complex doped system upon clustering of various dopants using as an example aluminum nitride doped with silicon and carbon. The dopants form a fragment of their proper phase (SiC) embedded in an isostructural matrix (AlN). This effect is attributed to the formation of a local chemical state intermediate between the compositions of the matrix and the binary phases formed by the dopants.     

Preparation of octadecyl modified column gels using heat-treated silicas and their retention behaviour in high-performance liquid chromatography

Summary The effect of silanol groups on three types of octadecylmodified column gels using heat-treated silicas by calcination has been studied by high-performance liquid chromatography. After heat-treating at 180°C, 500°C and 950°C, the silicas treated with octadecyldimethylchlorosilane were used for the measurement of physical and chemical analysis. From elemental carbon analysis data, the reactive silanol group concentrations, _OH(s), were determined to be 2.0 in the 180°C treated silica, 2.1 in the 500°C treated silica and 1.6 in the 950°C treated silica, respectively (original silica: mean pore diameter 116 Å, specific surface area 298 m²/g, pore volume 1.22 ml/g, particle size 5.0 m). The separation factors, , of pyridine versus phenol were measured to be 0.79 on 180°C treated silica, 0.91 on 500°C treated silica and 1.98 on 950°C treated silica, using acetonitrile-water mixtures as the eluent. And then, on the basis of the physico-chemical and chromatographic data, the three types of octadecyl modified column gels using heat-treated silicas by calcination have been compared.     

Emanation Thermal Analysis of SiC Based Materials

Results of emanation thermal analysis (ETA) characterizing microstructure changes of SiC based materials during heat treatment in argon are demonstrated. This method made it possible to reveal fine changes of the texture of SiC nano-sized powders, SiC micro-sized powders and SiC whiskers under in situconditions of the heating. ETA curves can serve as fingerprints of the respective samples.This revised version was published online in November 2005 with corrections to the Cover Date.     

Ultrahigh‐Density Carbon Nanoring Arrays on Silicon Wafer through Templated Solution Deposition Method

Ultrahigh‐density carbon nanoring arrays on a silicon wafer are achieved by a novel templated solution deposition method. Initially the silica nanodot arrays obtained from a nanoporous thin film are used as a template to direct the surface dewetting of a phenolic precursor, while further curing and calcination of the phenolic precursor, followed by etching of the silica arrays, results in large area carbon nanoring arrays with a diameter as small as 25 nm. This study provides a simple and robust chemical route to fabricate complex nanoring arrays with ultrahigh density of about one terabit per square inch.

     

Electrical conductivity of carbon black filled ethylene-vinyl acetate copolymer as a function of vinyl acetate content

The electrical conductivity of carbon black filled ethylene-vinyl acetate copolymer was measured as a function of carbon and vinyl acetate (VAc) content. For the composites whose matrices contain less than 32 wt% ofVAc content, a sharp break point of the relation between carbon content and conductivity was observed. The conductivity jumps as much as ten orders of magnitude at the break point. The critical carbon content corresponding to the break point can essentially be predicted by our previous model. This model was derived under certain assumptions, the most important of which is that when the interfacial excess energy introduced by carbon particles into the polymer matrix reaches a universal value,g ^*, the carbon particles begin to coagulate so as to avoid any further increase of energy and to form networks which facilitate electrical conduction. On the other hand, for the composites whose matrices contain more than 32 wt% ofVAc content, a sharp break of the relation between the carbon content and conductivity cannot be observed and conductivity increases continuously with increasingVAc content. In this region ofVAc content, carbon particles were dispersed well in theVAc rich matrices. This is because the presence of polar groups in aVAc component enhances its bonding to conductive fillers. In this case, the interfacial excess energy,g, seems to be the caseg0. Better dispersibility of fillers in this region ofVAc content can be shown from an electron micrograph (TEM).     

Thermal stability of high surface area silicon carbide materials

The synthesis of mesoporous silicon carbide by chemical vapor infiltration of dimethyl dichlorosilane into mesoporous silica SBA-15 and subsequent dissolution of the silica matrix with HF was investigated. The influence of the synthesis parameters of the composite material (SiC/SBA-15) on the final product (mesoporous SiC) was determined. Depending on the preparation conditions, materials with specific surface areas from 410 to 830 m² g⁻¹ and pore sizes between 2 and 10 nm with high mesopore volume (0.31-0.96 cm³ g⁻¹) were prepared. Additionally, the thermal stability of mesoporous silicon carbide at 1573 K in an inert atmosphere (argon) was investigated, and compared to that of SBA-15 and ordered mesoporous carbon (CMK-1). Mesoporous SiC has a much higher thermal textural stability as compared to SBA-15, but a lower stability than ordered mesoporous carbon CMK-1.     

Porous Silicon Carbide/Carbon Composite Microspherules for Methane Storage

Porous silicon carbide/carbon(SiC/C)microspherules were prepared by the controlled heating treatment of polymer and silica hybrid precursors over 1000℃in Ar/H_2 stream.The resultant SiC/C composite shows improved physical properties such as excellent mechanical strength,regular physical form, and high packing density.Such improvement overcomes the main inherent problems encountered when using activated carbons as absorbents without sacrificing porosity properties.N_2 sorption analysis shows that the SiC/C composite has a BET surface area of 1793 m~2/g and a pore volume of 0.92 ml/g.Methane adsorption isotherm is determined by the conventional volumetric method at 25℃and up to 7.0 MPa.On volumetric basis,the SiC/C composite microspherules show methane storage of 145(V/V)at 3.5 MPa and 25℃.The combination of excellent physical properties and porosity properties in this SiC/C composite lends a great possibility to develop a competitive storage system for natural gas.