Chemical vapour-deposited silicon nitride

Pyrolytic Si₃N₄ has been deposited on a graphite substrate, using a mixture of SiCl₄, NH₃ and H₂. The pyrolysis is performed with deposition temperatures of 1100 to 1500° C, total gas pressures of 5 to 300 Torr, and flow rates of H₂=700, NH₃=60 and SiCl₄ (liq.)=0.8 cm³ min^–1. Massive amorphous and crystalline pyrolytic forms of Si₃N₄ are prepared at a maximum thickness of 4.6 mm. The effects of deposition conditions on some properties of the deposited products and the dependence of formation of amorphous or crystalline deposits on deposition temperature and total pressure were investigated. The surface and cross-sectional structures show growth cones and oriented crystals which are strongly dependent on the deposition conditions. The thin deposits are translucent; the thick deposits vary in colour from white to black. The silicon content is close to the theoretical composition and independent of the deposition conditions, while the oxygen content increases with decreasing deposition temperature and total pressure. No segregation of silicon and nitrogen at cone boundaries was found.     

Fe-catalyzed synthesis of SiC nanofibers from methyltrichlorosilane

We have studied Fe-catalyzed chemical vapor deposition of silicon carbide nanofibers via thermal decomposition of methyltrichlorosilane, CH₃SiCl₃, in hydrogen at temperatures from 1100 to 1350°C and the effects of synthesis temperature and time and gas (CH₃SiCl₃ + H₂) flow rate on the growth rate of SiC nanofibers. In the temperature range 1100–1350°C, the activation energy for nanofiber growth is 120 kJ/mol. The SiC nanofibers have the stoichiometric composition and consist of single-crystal β-SiC (cubic structure).     

化学气相沉积碳化硅的热力学分析

根据吉布斯自由能最小原理, 采用FACTSAGE计算软件, 重点对MTS/H₂体系化学气相沉积碳化硅进行了均相平衡计算,评价了体系中主要化合物对沉积碳化硅的作用. 结果表明,低温和高压下, SiCl₄和CH₄的含量最多, 不饱和物质和自由基的含量非常少, 温度的升高和压力的下降可显著提高不饱和物质和自由基的浓度; 高温和低压下, SiCl₂和C₂H₂可能是形成碳和硅的主要先驱体, 其它稳定物质如碳氢化合物、有机硅化合物和硅烷等由于浓度太小和表面反应粘结系数低, 对碳化硅的沉积可以不予考虑; 体系中几乎没有含Si--C和Si--Si键的物质, 说明碳化硅是经过碳和硅独立形成, 二者的相对速率决定了碳硅比.     

Pressure dependence of morphology and phase composition of SiC films deposited by microwave plasma chemical vapor deposition on cemented carbide substrates

SiC films were deposited on cemented carbide substrates by employing microwave plasma chemical vapor deposition method using tetramethylsilane (Si(CH₃)₄) diluted in H₂ as the precursor. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and scratching technique were used to characterize morphology, composition, phases present and adhesion of the films. Experimental results show that the deposition pressure has great influence on morphologies and phase composition of the films. In sequence, SiC films with a cauliflower-like microstructure, granular films with terrace-featured SiC particles coexisting with Co₂Si compound and clusters of nanometer SiC nanoplatelets appear as a function of the deposition pressure. In terms of plasma density and substrate temperature, this sequential appearance of microstructures of SiC films was explained. Adhesion tests showed that among the three types of films studied, the films with the terrace-featured SiC particles have relatively higher adhesion. Such knowledge will be of importance when the SiC films are used as interlayer between diamond films and cemented carbide substrates.     

SiC fibre by chemical vapour deposition on tungsten filament

A CVD system for the production of continuous SiC fibre was set up. The process of SiC coating on 19 μ m diameter tungsten substrate was studied. Methyl trichloro silane (CH₃SiCl₃) and hydrogen reactants were used. Effect of substrate temperature (1300–1500°C) and concentration of reactants on the formation of SiC coating were studied. SiC coatings of negligible thickness were formed at very low flow rates of hydrogen (5 × 10⁻⁵ m³/min) and CH₃SiCl₃ (1.0 × 10⁻⁴ m³/min of Ar). Uneven coatings and brittle fibres were formed atvery high concentrations of CH₃SiCl₃ (6 × 10⁻⁴ m³/min of Ar). The flow rates of CH₃SiCl₃ and hydrogen were adjusted to get SiC fibre with smooth surface. The structure and morphology of SiC fibres were evaluated.     

Effect of soft substrate on the indentation damage in silicon carbide deposited on graphite

Indentation-induced damage is investigated in silicon carbide (SiC) deposited on graphite substrate. The SiC films have been grown by LPCVD (Low Pressure Chemical Vapor Deposition) method using MTS (CH₃SiCl₃) as a source gas and H₂ as a diluent gas to provide highly dense deposited layer and strong interfacial bonding. The elastic-plastic mismatch is very high to induce distinctive damages in the coating and the substrate layer. The specimens with various coating thicknesses are prepared by changing CVD condition or mechanical polishing. Indentation damages with different sizes are introduced by controlling indentation load in Nanoindentation, Vickers indentation and Hertzian indentation test. Basic mechanical properties such as hardness, toughness, elastic modulus are evaluated against coating thickness. Mechanical properties are sensitive to the indentation load and coating thickness. The results indicate that coating thickness has a vital importance on the design of hard coating/soft substrate system because the soft substrate affects on the mechanical properties.     

Microstructure and growth mechanism of SiC whiskers on carbon/carbon composites prepared by CVD

The SiC whiskers of good quality are expected to act as the reinforcing element in the ceramic coatings for C/C composites. Using CH₃SiCl₃(MTS) and H₂ as the precursors, SiC whiskers were prepared on the surface of C/C composites by chemical vapor deposition (CVD) at normal atmosphere pressure. XRD, SEM and TEM analyses show that the whiskers are β-SiC structure and their diameters are several-hundred nanometers. With the descending MTS concentration in the depositing room, the purity of the as-prepared whiskers increases and the diameters of the whiskers decrease. The investigation of growth mechanism shows the CVD-SiC whiskers grown up by the vapor–solid (VS) growth process.     

Oxidation kinetics of SiC deposited from CH3SiCl3/H2 under CVI conditions

Oxidation tests were performed on SiC deposits prepared from CH₃SiCl₃/H₂ under chemical vapour infiltration conditions, at temperatures ranging from 900–1500 °C under a flow of pure oxygen at 100 kPa (passive oxidation regime). The kinetics of growth of the silica layer were established from thickness measurements performed by spectroreflectometry. They obey classical parabolic laws from which rate constants are calculated. Within 1000–1400 °C, the oxidation process is thermally activated with an apparent activation energy of 128 kJ mol^–1. Above 1400 °C and below 1000 °C, an increase in the activation energy is observed which is thought to be related to a change in the mechanism of the oxygen transport across the silica layer forT>1400 °C and tentatively to stress effects forT<1000 °C. The kinetics data are compared to those measured on silicon single crystals (used as a standard) and to other reported data on SiC.     

Production of submicron SiC particles by d.c. thermal plasma: a systematic approach based on injection parameters

Aiming at producing high temperature structural ceramics, ultra-fine SiC powders were synthesized by the gas phase reaction of silicon tetrachloride with methane in a d.c. thermal plasma. The influence of parameters as the SiCl₄ feeding rate, C/Si and H₂/C molar ratios and internal pressure on the powder properties were investigated. The SiC powders were characterized by chemical analysis, Fourier transform infrared spectroscopy, X-ray diffraction and scanning electronic microscopy. The experimental set-up allows the production of β-SiC powders at a rate of 200 g h⁻¹ with particle size around 0.1 μm. The main impurities in the as-produced powder handled at ambient atmosphere are: oxygen (1.8–2.5%) and free carbon (3–4%). Interesting relationships were found between the SiCl₄ feeding rate and the H₂/C molar ratio and between the C/Si molar ratio and the internal pressure. The internal pressure plays a major role in controlling the particle size.     

The effect of diluent gases on the growth behavior of CVD SiC films with temperature

Silicon carbide films have been grown onto graphite substrates by low pressure chemical vapor deposition using MTS (CH₃SiCl₃) as a source precursor and H₂ or N₂ as a diluent gas. The experiments were performed at fixed conditions of a flow rate of 100 sccm for each MTS and carrier gas, a flow rate of 300 sccm for diluent gas addition, and a total pressure of 5 torr. The effect of temperature from 900°C to 1350°C and the alteration of diluent gas species on the growth rate and structure of deposits have been studied. The experimental results showed that the deposition rate increased with increasing deposition temperature irrespective of diluent gases and reactant depletion effect increased especially at H₂ diluent gas ambient. At MTS-H₂ system, the deposition mechanism changed from chemical reaction to mass transfer controlled reaction with temperature. Otherwise, For MTS-H₂-N₂ system, surface chemical reaction controlled the growth process at whole deposition temperature ranges. For N₂ addition, surface morphology of leaf-like structure appeared, and for H₂, faceted structure at 1350°C. The observed features were involved by crystalline phase of -SiC and surface composition with different gas ambient.     

Deposition process of Si-B-C ceramics from CH3SiCl3/BCl3/H2 precursor

Si-B-C coatings have been prepared by chemical vapour deposition (CVD) from CH₃SiCl₃/BCl₃/H₂ precursor mixtures at low temperature (800-1050 °C) and reduced pressures (2, 5, 12 kPa). The kinetics (including apparent activation energy and reaction orders) related to the deposition process were determined within the regime controlled by chemical reactions. A wide range of coatings, prepared in various CVD conditions, were characterized in terms of morphology (scanning electron microscopy), structure (transmission electron microscopy, Raman spectroscopy) and elemental composition (Auger electron spectroscopy). On the basis of an in-situ gas phase analysis by Fourier transform infrared spectroscopy and in agreement with a previous study on the B-C system, the HBCl₂ species was identified as an effective precursor of the boron element. H_xSiCl_(4−x), SiCl₄ and CH₄, derived from CH₃SiCl₃, were also shown to be involved in the homogeneous and the heterogeneous reactions generating silicon and carbon in the coating. A correlation between the various experimental approaches has supported a discussion on the chemical steps involved in the deposition process.     

Synthesis of ultra-fine SiC powders in a d.c. plasma reactor

Synthesis of SiC powder in a 15 kW d.c. plasma reactor by using the reaction systems of SiCl₄+CH₄ and CH₃SiCl₃+CH₄ was studied. The powder produced was characterized by X-ray diffraction and electron microscopy (SEM and TEM). Specific surface area was determined by the BET method using a sorptograph. The oxygen content was analysed by thermogravimetric method. Powder characteristics with respect to plasma conditions were analysed. The SiC powder generated was sintered in the presence of boron and carbide.     

A hexane solution deposition of SnS2 films from tetrabutyltin via a solvothermal route at moderate temperature

SnS₂ films have been deposited on glass and alumina plate substrates by the reactions between an organotin precursor [tetrabuyltin, (CH₂CH₂CH₂CH₃)₄Sn] and carbon disulfide in n-hexane at the temperature range 180-200 °C for 10-40 h. The reaction system was oxygen free and applied at a moderate temperature. The films so prepared were characterized by techniques of X-ray diffraction, Scanning electron microscopy, Raman and Mössbauer spectroscopies. The films deposited on glass as well as on alumina plate have an average thickness of 30 μm, but have different rose-like morphologies, which are influenced by both the anisotropic growths of crystals and the different substrate structures. Photoluminescence measurements show that the films have an emission peak at approximately 590 nm.     

The growth characteristics of chemical vapour-deposited β-SiC on a graphite substrate by the SiCl4/C3H8/H2 system

Silicon carbide has been grown at 1300–1800°C by chemical vapour deposition using the SiCl₄/C₃H₈/H₂ system on a graphite substrate. The effect of C₃H₈ flow rate and deposition temperature on the growth characteristics and structure of the deposit has been studied. The experimental results show that the degree of film density is changeable from a dense plate to a porous one with increasing C₃H₈ flow rate. The activation energy increases with increasing C₃H₈ flow rate. The grain size of the polycrystalline β-SiC becomes coarser when the C₃H₈ flow rate and the deposition temperature are increased. The preferred orientation of the deposited SiC layers changes from (111) to (220) on increasing deposition temperature from 1300°C to 1400°C. The deposition mechanism is also discussed.     

Pressure pulsed chemical vapour infiltration of SiC to two-dimensional-Tyranno/SiC-C preforms

Preforms of two-dimensional Tyranno fibre (SiC base) of 7×20×1.3 mm³ were chemically vapour infiltrated with SiC at 850–1050 °C from a gas mixture of CH₃SiCl₃ (6%)-H₂ using pressure pulses between below 0.3 kPa and 0.1 MPa. Above 900 °C, films grew on the macrosurface dominantly. At 850 °C, residual porosity decreased to about 10% after 10⁵ pulses, and three point flexural strength reached about 200 MPa. X-ray diffractograms (XRDs) on the surface showed the deposits to be -SiC only.     

Preparation of SiC foams by CVI-R technique with SiCl4/H2/CH4 system

Silicon carbide foams were prepared by the chemical vapor infiltration-reaction (CVI-R) of SiCl₄/H₂/CH₄ with carbon foam derived from mesophase pitch (MP), which had not only high bending strength but also low bulk density. The influence of the CH₄/SiCl₄ ratio in reaction atmosphere on the properties of as-prepared silicon carbide foams was investigated in detail. As the CH₄/SiCl₄ ratio was 0.25, resultant foam possessed the highest bending strength of 17.13 MPa. At the same time, correlations between properties and microstructure are also discussed.     

Correlations between gas phase supersaturation,nucleation process and physico-chemical characteristics of silicon carbide deposited from Si-C-H-Cl system on silica substrates

In the CH₃SiCl₃-H₂ CVD system, from which SiC-based films are prepared, the supersaturation of the gas phase increases when temperature and total pressure decreases and when a diffusion-controlled kinetic process is changed in a reaction-controlled one. These conditions variations seem to induce a transition from a growth regime to a nucleation regime, as evidenced by a study of the initial stages of the deposition. A transition from a crystallized film with columnar crystals to a nanocrystalline deposit is also reported on the basis of accurate experiments using TEM and related techniques.     

Pressure-pulsed chemical vapour infiltration of SiC into two-dimensional-Tyranno/SiC particulate preforms from SiCl4–CH4–H2

SiC was infiltrated in two-dimensionally woven Tyranno/SiC particulate preforms from SiCl₄–CH₄–H₂ using pressure-pulsed chemical vapour infiltration (PCVI) in the temperature range 1348–1423 K. Above 1373 K, only β-SiC was deposited, whereas, at 1348 K, Si codeposition was found. At 1423 K, a macrosurface film was formed in the early stage of PCVI. At 1373 K, residual porosity decreased from 30% to 7.5% irrespective of the sample size. Three point flexural strength increased with decreasing residual porosity and increasing fibre volume fraction in the sample. Flexural strength of the sample having 48% fibre volume fraction reached about 325 MPa after 5 × 10⁴ pulses of CVI at 1373 K. Inter-laminar shear strength of the sample obtained at 1373 K reached 40 MPa at 7 × 10⁴ pulses. This revised version was published online in November 2006 with corrections to the Cover Date.     

Reinforcement and antioxidizing of porous carbon by pulse CVI of SiC

In order to reinforce and antioxidize porous carbon, chemical vapour infiltration (CVI) of SiC was investigated using a repetitive cycle of evacuation of vessel and instantaneous source-gas filling. From a source gas of 5% CH₃SiCl₃-H₂, a temperature range of 1273 to 1373 K was considered to be suitable to infiltrate SiC into a deep level, and surface deposition was enhanced at above 1373 K which led to pore blockage. With 3000 pulses, flexural strength was improved from 35 to about 90 MPa. Several specimens were exposed to air at 1573 K for 1070h during which the specimens were cooled to room temperature between four and seven times. SiC felt was also obtained by oxidation of a carbon skelton after pulse CVI.     

Pulsed chemical vapour infiltration of SiC to three-dimensional carbon fibre preforms

Three-dimensional carbon fibre preforms were infiltrated with silicon carbide from a gas system of CH₃SiCl₃-H₂ using a process of pressure pulsed chemical vapour infiltration. To infiltrate to a deep level, the temperature had to be lowered to 870–900°C, and the hold time per pulse below 1.0 s. Three-dimensional carbon fibre preforms partly filled with SiC fine powder were compared with those without filler. The weight of the preforms increased linearly with increasing number of pulses up to 10⁵ when no filler was present. However, the weight increase slowed down above 8×10⁴ pulses when the filler was used. Preforms with and without SiC filler showed three-point flexural strengths of 160 and 80 MPa after CVI of 10⁵ pulses, respectively. In order to improve the strength, a denser filling of SiC powder is necessary.