Effects of process parameters on ultrafine SiC synthesis using induction plasmas

A study is reported of the formation of ultrafine SiC powder through the reaction of elemental silicon and CH₄ in an induction plasma. The reaction route used involved in the first place the vaporization of a fine elemental silicon powder axially injected into the center of the discharge followed by the carburization reaction through the coinjection of CH₄. The powder obtained was composed of a mixture of α- and β-SiC with varying amounts of free carbon and free silicon. The particle size distribution was typically in the range of 40–60 nm with a corresponding specific surface area of 30–50 m²/g. A parametric study showed that the quality of the powder obtained varied with the plasma plate power and the position of the injection probe. The plasma gas composition employed was found to influence the proportions of α- and β-SiC in the synthesized SiC powder. With an Ar/N₂ mixture as the plasma gas, the ratio of the α to β phases was less than 1.0, whereas the ratio was greater than 1.5 when using a mixture of Ar/H₂ as plasma gas. The Si powder feed rate and the input C/Si molar ratio in the injected reactants significantly affected both the formation of the SiC and the free Si and free C content in the synthesized powder. Lining the cylindrical reactor wall with graphite resulted in improved conversion of Si to SiC. The weight fraction of the powder collected at different sections of the reactor system varied with the reactor operating conditions. The experimental results support the view that the formation mechanism for ultrafine SiC is dominated by the reaction of Si vapor with the thermal decomposition products of CH₄.     

Structure of (O→Si)-(acetoxymethyl)trifluorosilane in three phase states and in solutions

According to the IR spectroscopy data, the molecules of (O→Si)-(acetoxymethyl)trifluorosilane having in the liquid state and in polar media the intramolecular bond C=O→Si, exist in the gas phase in the temperature range 438–538 K in the equilibrium with the molecules with tetracoordinate silicon atom. This allowed to determine experimentally the enthalpy of formation of the intramolecular bond C=O→Si for the gas phase to be ΔH = 2.2±0.1 kcal mol⁻¹. In the solid state at 110 K and in the CS₂ solution, along with molecule with the C=O→Si bond, the dimers exist, which include both tetra- and pentacoordinate silicon atom. The data of quantum-chemical calculations (B3LYP/6-311G**) show that the shortest intermolecular bond Si-F→Si is realized in the associate formed by the molecules in the ap,sp- and sp,sp-forms, and the longest one, when both components are in the sp,sp-forms.     

A study of silicon distribution in silicon-containing pyrocarbons

Summary The silicon distribution in silicon-containing pyrocarbon obtained by simultaneous pyrolysis of methane and silicon tetrachloride in the temperature range 1160–1630° C was followed by electron-probe microanalyser. Deposits obtained at about 1200° C contain about 4%w/w silicon in the form of silicon carbide. Very fine particles of SiC are homogeneously dispersed over the whole layer. Between 1300 and 1400° C larger lenticular inclusions of silicon carbide are formed which serve as nuclei for further crystallization of both silicon carbide and pyrocarbon. At the highest temperatures used (1600° C) only the solid solution containing 0.2%w/w silicon in pyrocarbon is deposited. It is supposed that the distribution of silicon in pyrocarbon is a result of temperature dependence of the nucleation and growth processes of silicon carbide.

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Zusammenfassung Die Verteilung des Siliciums in siliciumhältigen, durch gleichzeitige Pyrolyse von Methan und Siliciumtetrachlorid bei 1160 bis 1630° C erhaltenen Pyrokohlenstoffproben wurde mit einer Mikrosonde verfolgt. Bei etwa 1200° C erhaltene Abscheidungen enthalten etwa 4% (g/g) Silicium als Carbid. Sehr kleine SiC-Partikel sind homogen über die ganze Schichte verteilt. Zwischen 1300 und 1400° C bilden sich größere, linsenförmige Einschlüsse von SiC, die als Kristall isationskeime sowohl für Siliciumcarbid wie für Pyrokohlenstoff wirken. Bei der höchstangewandten Temperatur (1600° C) wird nur die feste Lösung von 0,2% (g/g) Silicium in Pyrokohlenstoff abgeschieden. Daher wird angenommen, daß die Siliciumverteilung das Ergebnis der Temperaturabhängigkeit der Keimbildung und des Fortschreitens der Siliciumcarbidabscheidung ist.

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Presented at VIth International Symposium on Microtechniques, Graz, 7–11 September 1970.     

Directivity of the intramolecular O→Si bond in pentacoordinate organosilicone compounds

According to the data of X-ray diffraction analysis and quantum chemical calculations, in the hypervalent silicon compounds where the coordination cycle is closed by the C=O→Si-F fragment, the O→Si interatomic distance is governed by the orientation of the silicon atom determined by the C=O→Si angle. This is an indication of a directivity of the coordination bond O→Si; a regular variation in the nature of the latter is reflected in the observed dependence of the O→Si distance on the C=O→Si angle.     

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.     

Controlled Synthesis of β-SiC Nanopowders with Variable Stoichiometry Using Inductively Coupled Plasma

In the growing field of nanomaterials, SiC nanoparticles arouse interest for numerous applications. The inductively coupled plasma (ICP) technique allows obtaining large amount of SiC nanopowders from cheap coarse SiC powders. In this paper, the effects on the SiC structure of the process pressure, the plasma gas composition, and the precursor nature are addressed. The powders were characterized by X-ray diffraction (XRD), Raman and fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) and high resolution electron microscopy (HREM), chemical analyses, BET and photon correlation spectroscopy (PCS) measurements. Whatever the precursor (α- or β-SiC), the nanoparticles were crystallised in the cubic β-SiC phase, with average sizes in the 20–40 nm range. Few residual grains of precursor were observed, and the decarburization due to the reductive Ar–H₂ plasma lead to the appearance of Si nanograins. The stoichiometry of the final product was found to be controllable by the process pressure and the addition of methane.     

Amorphization of Silicon during Mechanical Treatment of Its Powders: 1. Process Kinetics

The kinetics of silicon amorphization in the process of mechanical treatment of powders in a vibrating micromill was studied by the X-ray diffraction. The treatment was carried out in the argon atmosphere, the apparatus energy intensity was equal to 18 W/g, and the amount of consumed energy (dose) was as high as 510 kJ/g (14 MJ/mol). The analysis of the shape of X-ray diffraction patterns and the dynamics of the changes in silicon atomic structure were described within the framework of three-fraction model. Fraction 1 composed of large crystalline blocks comprising particles of initial powder; the second fraction is represented by nanocrystalline blocks with the dimensions of not less than 8 nm; and the third fraction is an amorphous phase. A decrease in the content and sizes (from 10²to 25 nm) of initial microcrystals of fraction 1 is accompanied by the formation of X-ray amorphous phase 3. Nanocrystalline blocks of fraction 2 are none other than the intermediate products. They are first accumulated synchronously with the amorphous phase and then disintegrated with a decrease in their sizes from 8 to 4 nm. At the initial stage of experiment, at the dose up to 15 kJ/g and the degree of amorphization up to 40%, the energy yield of the formation of amorphous phase amounts to 1 ± 0.1 mol/MJ. At the end of experiment (the dose varies from 20 to 510 kJ/g), the yield drops by tens of times, and the content of amorphous phase reaches 70–80%.     

Ablation properties of C/C–SiC composites tested on an arc heater

Carbon fiber-reinforced carbon and silicon carbide (C/C–SiC) composites were fabricated by a combination of chemical vapor infiltration and liquid silicon infiltration. Ablation properties of C/C–SiC composites and C/C composites with similar technique were tested on a high-pressure arc heater. The results show that ablation properties of C/C–SiC composites are more severe than those of C/C composites. Ablation of C/C–SiC composites includes oxidation, sublimation of SiC (Si), and mechanical denudation. Oxidation and sublimation of SiC (Si) lead to the enlarged ablation rates between carbon fibers and matrices, which finally cause serious ablation of C/C–SiC composites.     

Chemical Vapor Deposition of Silicon Carbide and Silicon Nitride—Chemistry's Contribution to Modern Silicon Ceramics

Pure silicon carbide and silicon nitride have valuable properties in bulk pore-free form; however, their industrial exploitation has hardly been possible so far. Neither compound can be melted or sintered in pure form; hot pressing or sintering at normal pressure requires the presence of additives; and the reaction-sintering process in which only Si and C or Si and N are employed as additives affords porous materials.–The novel process of chemical vapor deposition has partly overcome the drawbacks of the previous methods. In the new process SiC is produced, e.g., by pyrolysis of CH₃SiCl₃, and Si₃N₄ by reaction of SiCl₄ with NH₃. This technique can also be used for pore filling in objects made of SiC and Si₃N₄ (gas phase impregnation) and for producing extremely fine SiC and Si₃N₄ (gas phase impregnation) and for producing extremely fine SiC and Si₃N₄ powder and SiC monofilaments suitable as components for SiC composites. Moreover, gas phase impregnation can also give fiber composites.     

Thermophysical Investigations of the Polymorphous Phases of Calcium Carbonate

1. Results of thermodynamic and kinetic investigations for the different crystalline calcium carbonate phases and their phase transition data are reported and summarized (vaterite: V; aragonite: A; calcite: C). A→C: T _tr=455±10°C, Δ_tr H=403±8 J mol^–1 at T _tr, V→C: T _tr=320–460°C, depending on the way of preparation,Δ_tr H=–3.2±0.1 kJ mol^–1 at T _tr,Δ_tr H=–3.4±0.9 kJ mol^–1 at 40°C, S _V ^Θ= 93.6±0.5 J (K mol)^–1, A→C: E _A=370±10 kJ mol^–1; XRD only, V→C: E _A=250±10 kJ mol^–1; thermally activated, iso- and non-isothermal, XRD 2. Preliminary results on the preparation and investigation of inhibitor-free non-crystalline calcium carbonate (NCC) are presented. NCC→C: T _tr=276±10°C,Δ_tr H=–15.0±3 kJ mol^–1 at T _tr, T _tr – transition temperature, Δ_tr H – transition enthalpy, S ^Θ – standard entropy, E _A – activation energy. 3. Biologically formed internal shell of Sepia officinalis seems to be composed of ca 96% aragonite and 4% non-crystalline calcium carbonate. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Thermodynamic and experimental study of the interaction of silicon and carbon monoxide: Synthesis of silicon carbide nanofibers

The reaction of crystalline silicon with carbon monoxide to produce silicon carbide was studied. Thermodynamic simulation of the equilibrium phase composition of the nSi-mCO system was carried out in the range 300–2000 K (27–1727°C). Conditions required for silicon carbide was carried out applying various experimental modes (n, m, and T) and possible pathways of the reactions were determined. Interaction between crystalline silicon and carbon monoxide formation in a temperature range of 1000–1450°C. The order of the reaction in CO was found to be close to unity. Silicon carbide nanofibers with thicknesses of from 5 to 100 nm were synthesized and characterized by powder X-ray diffraction, mass-spectral elemental analysis, and scanning electron microscopy. A possibility of synthesizing high-purity silicon carbide fibers were experimentally evaluated.     

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.     

Surface morphology of Ge‐modified 3C‐SiC/Si films

The influence of Ge deposition prior to carbon interaction with 3° off‐axis Si(111) substrates on the structural and morphological properties of the formed silicon carbide (SiC) layer is studied. In situ reflection high‐energy electron diffraction (RHEED) and X‐ray diffraction (XRD) revealed the formation of the cubic silicon carbide (3C‐SiC) modification. In situ spectroscopic ellipsometry measurements revealed a decreasing 3C‐SiC thickness with increasing Ge predeposition. Atomic force microscopy (AFM) studies revealed that the surface overlayer morphology is mainly formed by periodic step arrangements whose relevant geometric parameters, i.e. lateral separation, height and terrace width, depend on the Ge content. Besides the changes of the step morphology, the surface roughness and the grain size and the strain of the formed 3C‐SiC decreases with increasing germanium precoverage. Copyright © 2008 John Wiley & Sons, Ltd.     

Structure of mechanically activated high-energy Al + polytetrafluoroethylene nanocomposites

The structure of high-energy Al/polytetrafluoroethylene nanocomposites prepared by mechanochemical synthesis is studied by X-ray diffraction analysis, scanning electron microscopy, atomic force microscopy, and chemical analysis. It is revealed that the composite consists of aluminum particles with sizes of 100–150 nm separated by the polymer layers. The formation of nanocomposite is accompanied by the accumulation of dislocations with the density ρ = (4 ± 1.5) × 10¹⁰ cm⁻². Upon the shock-wave initiation of activated samples, Al + (-C₂ F₄-) → AlF₃ + C reaction propagates in detonation-similar regime at supersonic speed. The velocity of detonation is the highest at the stoichiometric component ratio.     

Formation of functional phosphosilicate gels from phytic acid and tetraethyl orthosilicate

Phosphosilicate gels with high phosphorus content (P mol% > Si mol%) have been prepared using phytic acid as the phosphorus precursor, with tetraethyl orthosilicate (TEOS). It is shown that the structure of phytic acid is maintained in both the sols and those gels dried at a low temperature (i.e. ≤120 °C). Solid state ²⁹Si and ³¹P NMR suggest that the gel network is primarily based on tetrahedral silicon and that phosphorus is not chemically incorporated into the silicate network at this point. X-ray diffraction shows the gel to be amorphous at low temperatures. After heat treatment at higher temperatures (i.e. up to 450 °C), P–O–Si linkages are formed and the silicon coordination changes from tetrahedral to octahedral. At the same time, the gel crystallizes. Even after this partial calcination, ³¹P NMR shows that a large fraction of phytic acid remains in the network. The function of phytic acid as chelating agent is also maintained in the gels dried at 120 °C such that its ability to absorb Ca²⁺ from aqueous solution is preserved.     

Deposition of Silicon Carbide Thin Films from 1,1-Dimethyl-1-Silacyclobutane

1,1-Dimethyl-1-silacyclobutane was used as a single-source precursor to deposit SiC thin films on Si(100) and Si(111) by low-pressure chemical vapor deposition (LPCVD). Polycrystalline β-SiC thin films were grown at temperatures 1100 and 1200°C. At temperatures between 950 and 1100°C, amorphous thin films of silicon carbide were obtained. The films were studied by X-ray diffraction (XRD), infrared spectroscopy (IR), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and electron diffraction (ED).     

Quantum chemical study of silicon pentacoordination in (Aroyloxymethyl)trifluorosilanes and their analogs

Pentacoordinated silicon compounds of the series 4-XC₆H₄C(O)O(CH₂)_mSi(CH₃)_3-nF_n (m = 1, 2; n= 1,2,3) with an intramolecular 0→Si bond are studied by ab initia and semiempirical (AMI) quantum chemical methods. The results are compared with published experimental data. The C₆H₅C(O)OCH₂SiF₃ molecule is calculated in an RHF approximation using the 6–31G*basis set. The total energy of the molecule for its geometry optimization is calculated by the MP2 method including electron correlation. This leads to considerably improved agreement between the calculated coordination energy (25.3 kJ/mole) and the experimental value (28.5 kJ/mole). The geometry and the dipole moment calculated by both ab initio (HF/6-31G*//HF/6-31G*, MP2/6-31G*//MP2/6-31G*) methods and by the AMI method are in satisfactory agreement with the experimental data.     

Covalent attachment of organic monolayers to silicon carbide surfaces

This work presents the first alkyl monolayers covalently bound on HF-treated silicon carbide surfaces (SiC) through thermal reaction with 1-alkenes. Treatment of SiC with diluted aqueous HF solutions removes the native oxide layer (SiO2) and provides a reactive hydroxyl-covered surface. Very hydrophobic methyl-terminated surfaces (water contact angle theta = 107 degrees ) are obtained on flat SiC, whereas attachment of omega-functionalized 1-alkenes also yields well-defined functionalized surfaces. Infrared reflection absorption spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy measurements are used to characterize the monolayers and show their covalent attachment. The resulting surfaces are shown to be extremely stable under harsh acidic conditions (e.g., no change in theta after 4 h in 2 M HCl at 90 degrees C), while their stability in alkaline conditions (pH = 11, 60 degrees C) also supersedes that of analogous monolayers such as those on Au, Si, and SiO2. These results are very promising for applications involving functionalized silicon carbide.     

聚铝硅烷与聚碳硅烷共混先驱体制备SiC(Al)纤维

采用聚铝碳硅烷和聚碳硅烷共混制备含铝碳化硅的先驱体,并与直接合成得到的聚铝碳硅烷进行了比较.元素分析表明,共混法能够有效控制聚铝碳硅烷中的铝含量,且共混聚铝碳硅烷先驱体Si—H键含量更高.流变性能研究表明,共混获得的聚铝碳硅烷先驱体黏流活化能从255kJ/mol降至200kJ/mol,先驱体的可纺性提高,所以原纤维的平均直径从19μm降至12μm.预氧化后聚铝碳硅烷原纤维经1800℃一步烧成可得到致密的SiC(Al)纤维;XRD研究表明,纤维中的铝起到抑制碳化硅晶粒长大的作用.