Synthesis of Poly(silyne-co-hydridocarbyne) for Silicon Carbide Production

Pre-ceramic polymers have previously been shown to be polymeric precursors to silicon carbide, diamond and diamond-like carbon. Here, we report the synthesis of a pre-ceramic polymer, poly(silyne-co-hydridocarbyne), which was electrochemically synthesized from one monomer containing both silicon and carbon in its structure. The polymer is soluble in common solvents such as CHCl₃, CH₂Cl₂ and THF. Since the polymer contains both silyne and carbyne on its backbone, it can be easily converted to silicon carbide upon heating under an ambient inert atmosphere, or to SiO₂ under ambient air atmosphere. Poly(silyne-co-hydridocarbyne) was characterized with UV/Vis spectroscopy, FTIR, ¹H-NMR, GPC and Raman spectroscopy. Conversion of the polymer to SiC ceramic was accomplished by heating at 1000 and 750°C under an argon atmosphere and characterized with optical microscopy, SEM, X-Ray and Raman spectroscopies.     

Behavior of HfB<Subscript>2</Subscript>-SiC (10, 15, and 20 vol %) ceramic materials in high-enthalpy air flows

HfB₂–SiC ceramic samples containing 10, 15, and 20 vol % silicon carbide were prepared by spark plasma sintering. The samples were characterized by X-ray powder diffraction, SEM, and other methods. Their densities and calculated porosities were determined. The behavior of the materials under heating by a subsonic dissociated air flow was studied on a VGU-4 high-frequency inductive plasmatron. The average surface temperatures of the 10 and 15 vol % SiC samples were shown to increase up to 2550–2675°C during heating, due to the generation of surface localities having temperatures of 2600–2700°C (the initial surface temperature was ~1700–1900°C) and the progressive growth of these regions in area. The overall time during which the average surface temperatures of these samples were higher than 2000°C, was about 31–32 min. For the 20 vol % SiC sample, heat removal (when the sample touched a water-cooled holder) was shown to influence the surface temperature and surface temperature distribution. The variation in gas-phase composition over the central area of the sample surface during an experiment was studied using emission spectroscopy. Explanations are proposed to the variation of boron and silicon concentrations in the course of exposure to high-enthalpy flows. The elemental and phase compositions were determined and the microstructures were studied on the surface and sections of samples after long-term (~40-min) exposure to high-enthalpy air flows.     

Production of HfB<Subscript>2</Subscript>–SiC (10–65 vol % SiC) Ultra-High-Temperature Ceramics by Hot Pressing of HfB<Subscript>2</Subscript>–(SiO<Subscript>2</Subscript>–C) Composite Powder Synthesized by the Sol–Gel Method

The formation of HfB₂–SiC (10–65 vol % SiC) ultra-high-temperature ceramics by hot pressing of HfB₂–(SiO₂–C) composite powder synthesized by the sol–gel method was studied. By the example of HfB₂–30 vol % SiC ceramic, it was shown that the synthesis of nanocrystalline silicon carbide is completed at temperatures of as low as ≥1700°C (crystallite size 35–39 nm). The production of the composite materials with various contents of fine silicon carbide at 1800°C demonstrated that the samples of the composition HfB₂–SiC (20–30 vol % SiC) are characterized by the formation of SiC crystallites of the minimum sizes (36–38 nm), by the highest density (89%), and by higher oxidation resistance during heating in an air flow to 1400°C.     

The effect of the degree of ionicity of ceramic materials on their wettability by melted sodium chloride

The sessile drop method has been used to study the wettability of hexagonal boron nitride, sapphire, quartz, and polycrystalline silicon carbide by melted sodium chloride in a reducing He–H₂ atmosphere. Melted NaCl completely spreads over sapphire and quartz surfaces and form finite contact angles equal to 51° ± 10° and 77° ± 5° on silicon carbide and hexagonal boron nitride, respectively. The calculated works of salt adhesion to the ceramic substrates increase with the magnitude of the ionic component of ceramic material surface energy.     

The morphology of ceramic phases in BxC-SiC-Si infiltrated composites

The present communication is concerned with the effect of the carbon source on the morphology of reaction bonded boron carbide (B₄C). Molten silicon reacts strongly and rapidly with free carbon to form large, faceted, regular polygon-shaped SiC particles, usually embedded in residual silicon pools. In the absence of free carbon, the formation of SiC relies on carbon that originates from within the boron carbide particles. Examination of the reaction bonded boron carbide revealed a core-rim microstructure consisting of boron carbide particles surrounded by secondary boron carbide containing some dissolved silicon. This microstructure is generated as the outcome of a dissolution-precipitation process. In the course of the infiltration process molten Si dissolves some boron carbide until its saturation with B and C. Subsequently, precipitation of secondary boron carbide enriched with boron and silicon takes place. In parallel, elongated, strongly twinned, faceted SiC particles are generated by rapid growth along preferred crystallographic directions. This sequence of events is supported by X-ray diffraction and microcompositional analysis and well accounted for by the thermodynamic analysis of the ternary B-C-Si system.     

Preparation of a liquid boron‐modified polycarbosilane and its ceramic conversion to dense SiC ceramics

A boron‐modified ethynylhydridopolycarbosilane (B‐EHPCS) was successfully prepared via the hydroboration reaction of ethynylhydridopolycarbosilane (EHPCS) with 9‐borabicyclo‐[3.3.1]nonane (9‐BBN). The as‐synthesized B‐EHPCS with a branched structure was characterized by means of gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). The structural evolution of ceramic conversion of B‐EHPCS was investigated by solid‐state NMR. The ¹³C magic angle spinning (MAS) NMR results indicated that the C?C and C?C groups of B‐EHPCS take part in the hydrosilation cross‐linking at a relatively low temperature (170°C). According to the ²⁹Si MAS NMR analysis, the CSiH₃ end groups are most reactive hydride functionality involved in the hydrosilation cross‐linking. With increasing curing temperature, the C₂SiH₂ and CSiH₃ units are completely consumed, while C₃SiH units remain even after curing at 600°C. The TGA results show the 1200°C ceramic yield of B‐EHPCS reaches 86%, which is 10% higher than that of the parent EHPCS (76%). At high temperatures, the introduction of <1 wt% boron significantly inhibits silicon carbide (SiC) crystallization. The 1800°C ceramics derived from B‐EHPCS are found to be significantly denser than that from EHPCS. Copyright © 2010 John Wiley & Sons, Ltd.     

LaCrO<Subscript>3</Subscript> powder from lanthanum trisoxalatochromate(III) (LTCR) precursor

Lanthanum chromite LaCrO₃, an important catalyst and interconnect material used in solid oxide fuel cell was prepared from lanthanumtrisoxalatochromate(III) hydrate [LaCr(C₂O₄)₃]·9H₂O (LTCR) employing microwave heating technique. The compound LTCR heated in microwave heating system gave pure LaCrO₃ at 500°C within one hour. However LTCR heated in silicon carbide furnace yielded LaCrO₃ at 900°C. BET surface area of LaCrO₃ prepared by microwave and conventional heating techniques were found to be 2.8 and 1.2 m² g⁻¹, respectively. Thermogravimetry, differential thermal analysis and X-ray diffraction techniques were used to optimize the conditions for the microwave processing of the precursor.     

How xerogel carbonization conditions affect the reactivity of highly disperse SiO<Subscript>2</Subscript>–C composites in the sol–gel synthesis of nanocrystalline silicon carbide

A transparent silicon polymer gel was prepared by sol–gel technology to serve as the base in the preparation of highly disperse SiO₂–C composites at various temperatures (400, 600, 800, and 1000°C) and various exposure times (1, 3, and 6 h) via pyrolysis under a dynamic vacuum (at residual pressures of ~1 × 10^–1 to 1 × 10^–2 mmHg). These composites were X-ray amorphous; their thermal behavior in flowing air in the range 20–1200°C was studied. The encapsulation of nascent carbon, which kept it from oxidizing in air and reduced the reactivity of the system in SiC synthesis, was enhanced as the carbonization temperature and exposure time increased. How xerogel carbonization conditions affect the micro- and mesostructure of the xerogel was studied by ultra-small-angle neutron scattering (USANS). Both the carbonization temperature and the exposure time were found to considerably influence structure formation in highly disperse SiO₂–C composites. Dynamic DSC/DTA/TG experiments in an inert gas flow showed that the increasing xerogel pyrolysis temperatures significantly reduced silicon carbide yields upon subsequent heating of SiO₂–C systems to 1500°C, from 35–39 (400°C) to 10–21% (1000°C).     

Preparation of Boron Carbide from BF<Subscript>3</Subscript> and BCl<Subscript>3</Subscript> in Hydrogen Plasma of Arc RF Discharge

A comparative study was carried out of the process of plasma chemical deposition of boron carbide from hydrogen plasma containing the mixtures of BF₃ + CH₄ and BCl₃ + CH₄ sustained by RF arc (13.56 MHz) discharge. It was shown that in the case of synthesis of B₄C from a mixture of BF₃ + CH₄, carbon and complex coordination compound [X₃B]^?H⁺ (R₃B·FH) are formed as the by-products of condensed products. In the case of synthesis of B₄C from the BCl₃ + CH₄ mixture, the only condensed product is carbon. Mechanisms for the formation of boron carbide on the surface of heated electrodes are proposed. The main feature of these mechanisms is the preliminary deposition of a graphite layer from CH₄ and then the precipitation of boron with the participation of the radicals BF₂, BF and BCl. B₄C samples were obtained and the impurity composition, morphology and structure of bulk boron carbide samples obtained using both of its halides were studied. It was found that in both cases a carbon phase is present in boron carbide samples. The main impurities entering the B₄C, in the case of using a mixture of BF₃ + CH₄, is silicon, and in the case of a mixture of BCl₃ + CH₄, is tungsten.     

Sol‐Gel‐Process in the Ammono‐System – a Novel Access to Silicon Based Nitrides

Controlled coammonolysis of elementalkylamides in aprotic organic solvents at low temperatures have been shown to result in the formation of polyazanes. The synthetic procedure developed may be addressed as “sol‐gel‐route in the ammono system”. Pyrolysis of these novel polymer precursors gave access to multinary nitrides. For the model systems Si(NHMe)₄/B(NMe₂)₃, Si(NHMe)₄/Ti(NMe₂)₄, and Si(NHMe)₄/Ta(NMe₂)₅ polymeric boro‐, titano and tantalosilazanes were obtained. Pyrolysis in ammonia at 1000 °C yielded amorphous silicon boron nitride, silicon titanium nitride and silicon tantalum nitride powders; further heating of the nitride powders at 1500 °C in nitrogen atmosphere led to the formation of partly crystalline composites of α‐Si₃N₄ and amorphous silicon boron nitride for the Si/B/N system, a composite of finely dispersed TiN and amorphous silicon titanium nitride for the Si/Ti/N system, and crystalline TaN and amorphous silicon nitride for the Si/Ta/N system. Furthermore, the structure and pyrolysis chemistry of the polymeric intermediates, as well as the morphology of the pyrolysis products, were studied by NMR, MAS‐NMR, FT‐IR, DTA‐TG‐MS, XRD, SEM, EDX and elemental analyses.     

Identification of selective oxidation of TiC/SiC composite with X-ray diffraction and Raman spectroscopy

Open cell 3D titanium carbide/silicon carbide (TiC/SiC) composite was oxidised to titanium oxide/silicon carbide (TiO₂/SiC) following different temperature profiles in a thermal gravimetric analysis (TGA) instrument in continuous air-flow and static air (oven) environments. The TiC oxidation to anatase, starting at temperatures over 450°C, was confirmed by Raman spectroscopy and X-Ray diffraction (XRD). By increasing the temperature, the mass fraction of anatase diminished, while the mass fraction of rutile increased. SiC oxidation started at 650°C when a mixture of TiO₂/SiO₂/SiC could be observed by Raman, XRD and HRTEM.     

Stoichiometric silicon carbide from borate‐catalyzed polymethylsilane–polyvinylsilane formulations

Polymethylsilane (PMS) and polyvinysilane (PVS) were prepared by Wurtz condensation of chlorosilanes and characterized by spectroscopy (¹H, ¹³C and ²⁹Si NMR, and infrared), viscosity and GPC analysis. Mixtures of the PMS and PVS were prepared and stabilized with 2,6‐di‐t‐butyl‐4‐methylphenol (BHT; 0.5 wt%) to which was added a catalytic amount of tris­(trimethylsilyl)borate, B(OSiMe₃)₃ (BTMS; 2 wt% by weight). The resulting liquid materials were pyrolyzed to 950 °C and to 1400 °C under argon. The formulation, composed of 60% PMS / 40% PVS / 2% BTMS, was pyrolyzed and gives nearly stoichiometric silicon carbide in 73% yield. The pyrolyzate was analyzed spectroscopically at intermediate stages in order to study its thermal transformations and the influence of the boron catalyst. The ceramic obtained from the formulation 60% PMS / 40% PVS / 2% BTMS shows good stability at 1500 °C under oxygen. Copyright © 1999 John Wiley & Sons, Ltd.     

Synthesis of Silicon Carbide Fibers by Sol-Gel Processing

Silica-phenolic resin hybrid fibers with carbon-to-silicon atomic ratios of 2.6 to 5.4 have been prepared from ethanol solutions of tetraethoxysilane, phenolic resins, water, and hydrochloric acid with a tetraethoxysilane-H₂O−HCl molar ratio of 1:2:0.01 by sol-gel processing. The hybrid fibers have been heated at 1500°C in Ar for carbothermal reduction to convert them into silicon carbide fibers. The effects of the holding time at 1500°C and the carbon-to-silicon atomic ratio of the hybrid fibers on the free carbon content in the silicon carbide fibers have been investigated. It has been found that the conversion is complete by the heat-treatment for more than 2 h. The silicon carbide fibers with a free carbon content of ca. 2 wt% have been obtained from the hybrid fibers with the ratios of 2.6 to 4.3.     

Organoboron compounds,synthesis of allyl(dialkyl)boranes and diallyl(alkyl)boranes

Highly reactive allyl(dialkyl)-, crotyl(dialkyl)-, 3,3-dimethylallyl(dialkyl)-(= prenyl(dialkyl), and diallyl(alkyl)-boranes were prepared by allylation of esters R₂BOR′, RB(OR′)₂ or thioesters R₂BSR′ (R = alkyl) using allylic derivatives of aluminium, magnesium or boron in exchange reactions.The titled compounds are stable up to 100°C and do not symmetrize even on heating at 100°C for a long time. PMR spectroscopy data show that the characteristic feature of these compounds is a permanent allyl rearrangement, the rate of which increases with an increase in temperature. For allyl(diethyl)-borane at 100°C and 125°C the rates are equal to 2500 and 5000 sec^?1 respectively; activation energy of the rearrangement amounts to 11.8±0.2 kcal mol^?1.The boronallyl bonds in unsymmetrical allyl(alkyl)boranes readily split under the action of water and alcohols, protonolysis being accompanied by allyl rearrangement, crotyl and prenyl compounds are converted into 1-butene or 3-methyl-1-butene, respectively.     

Die Perthioborate RbBS3, TIBS3 und TI3B3S10

The Perthioborates RbBS₃, TIBS₃, and Tl₃B₃S₁₀ . RbBS₃ (P2₁/c, a=7.082(2) Å, b=11.863(4) Å, c=5.794(2) Å, β=106.54(2)°) was prepared as colourless, plate-shaped crystals by reaction of stoichiometric amounts of rubidium sulfide, boron, and sulfur at 600°C and subsequent annealing. TlBS₃ (P2₁/c, a=6.874(3) Å, b=11.739(3) Å, c=5.775(2) Å, β=113.08(2)°) which is isotypic with RbBS₃ was synthesized from a sample of the composition Tl₂S · 2 B₂S₃. The glassy product which was obtained after 7 h at 850°C was annealed in a two zone furnace for 400 h at 400→350°C. Yellow crystals of TlBS₃ formed at the warmer side of the furnace. Tl₃B₃S₁₀ (P1 , a=6.828(2) Å, b=7.713(2) Å, c=13.769(5) Å, α=104.32(2)°, β=94.03(3)°, γ=94.69(2)°) was prepared as yellow plates from stoichiometric amounts of thallium sulfide, boron, and sulfur at 850°C and subsequent annealing. All compounds contain tetrahedrally coordinated boron. The crystal structures consist of polymeric anion chains. In the case of RbBS₃ and TlBS₃ nonplanar five-membered B₂S₃ rings are spirocyclically connected via the boron atoms. To obtain the anionic structure of Tl₃B₃S₁₀ every third B₂S₃ ring of the polymeric chains of MBS₃ is to be substituted by a six-membered B(S₂)₂B ring.     

Preparation of HfB<Subscript>2</Subscript>/SiC composite powders by sol–gel technology

Sol–gel technology was used to chemically modify the surface of HfB₂ powders with highly dispersed silicon carbide using two carbothermy protocols: (1) under heating to 1500°С in flowing argon (100 mL/min) without exposure and (2) under dynamic vacuum conditions (p ~ 1 × 10^–5–1 × 10^–6 MPa) at 1400°С with 4-h exposure. The phase composition and microstructural features of the thus-prepared HfB₂/xSiC (x = 10–65 vol %) composite powders were studied. The products prepared by the second protocol showed an enhanced oxidation resistance in the range 20–1400°C in flowing air compared to individual HfB₂.     

Kristall- und Molekülstruktur von 2.5-Dijod-3.4-diäthyl-1.2.5-thiadiborolen

Crystal arid Molecular Structure of 2,5-Diiodine-3,4-diethyl-1,2,5-thiadiborolene The title compound crystallizes in the triclinic space group P1 with a = 8.22, b = 8.33, c = 9.74₅ Å, α = 109.1°, β = 107.1°, γ = 102.9° and two molecules per unit cell. The two molecules are associated by a center of symmetry, forming a four-membered B–S ring, which leads to differently coordinated boron atoms in the thiadiborolene ring. The bond lengths and angles for the boron atoms are similar to those known for trigonally and tetrahedrally bonded boron, respectively. The five-membered ring is nearly planar.     

Oxidation Behaviour of A Boron Carbide Based Material in Dry and Wet Oxygen

The oxidation behaviour of a B₄C based material was investigated in a dry atmosphere O₂(20 vol.%)-CO₂(5 vol.%)-He and also in the presence of moisture H₂O (2.3 vol%) as boron oxide is very sensitive to water vapour. The mass changes of samples consisting of a chemical vapour deposit of B₄C on silicon nitride substrates were continuously monitored in the range 500–1000°C during isothermal experiments of 20 h. The stability of boron oxide formed by oxidation of B₄C was also studied in dry and wet atmospheres to explain the kinetic curves. In both atmospheres, oxidation is diffusion controlled at 700 and 800°C and enhanced by water vapour. At 900°C and higher temperatures, boron oxide volatilisation and consumption by reaction with water vapour modifies the properties of the oxide film and the material is no more protected. At 600°C, B₄C oxidation is weak but the process remains diffusion controlled in dry conditions as boron oxide volatilisation is negligible. However, in the presence of water vapour, B₂O₃ consumption rate is significant and mass losses corresponding to this consumption and to the combustion of the excess carbon are observed. This revised version was published online in August 2006 with corrections to the Cover Date.     

An integrated low-power thin-film CO gas sensor on silicon

An approach to an integrated semiconductor gas sensor is presented. The major reasons considered for developing a semiconductor oxide gas sensor on silicon are the accurate local temperature control of the sensing area and the low level of the heating power required, together with an appropriate integrated structure. Thermal loss measurements show that the integrated gas sensor can operate up to 400 °C with less than 200 mW heating power. Depending on the deposition conditions, catalyst addition or surface conditioning, the SnO_x thin films are known to have an optimal sensitivity to CO between 250 °C and 400 °C. The sensitivity for CO gas and the response time of the device are presented for sputtered thin films of SnO_x, deposited on top of an isolated resistive heater, separated from silicon by a thin thermally-isolating membrane.     

Impact of a Subsonic Dissociated Air Flow on the Surface of HfB<Subscript>2</Subscript>–30 vol % SiC UHTC Produced by the Sol–Gel Method

A new method, which included the sol–gel synthesis of a HfB₂–(SiO₂–C) reactive composite powder and its subsequent consolidation by hot pressing (1700°C, 30 MPa, 15 min) with simultaneous carbothermic synthesis of nanocrystalline silicon carbide, was used to produce HfB₂–SiC ultra-high-temperature ceramic material promising for using in an air atmosphere at temperatures above 2000°C. Its elemental and phase compositions, as well as its microstructure were investigated. The density and calculated porosity were 7.6 g/cm³ and 13.5%, respectively. The behavior of a cylindrical sample of the material was studied on long-term (40 min) exposure to a subsonic dissociated air flow in a high-frequency induction plasmatron. The change in the temperature of the surface of the material was examined in the context of its relationship with the HfB₂ and SiC oxidation and the evaporation of the oxidation products. The phase composition and microstructure were determined in regions of the oxidized surface of a HfB₂–SiC sample containing 30 vol % SiC that were heated on exposure to high-enthalpy flows to 2600–2700°C and in regions the temperature of which was 1850–1950°C. By scanning electron microscopy, the thickness, microstructure, and composition of the oxidized layer were found.