Formation Of Silicon Carbide Research Articles

Preparation of silicon carbide–silicon nitride composite foams from pre-ceramic polymers

A new method of forming silicon carbide–silicon nitride composite foams is presented. These are prepared by immersing a polyurethane foam in a polysilane precursor solution mixed with Si3N4 powder to form a pre-foam followed by heating it in nitrogen at >900°C. X-ray diffraction patterns indicate that a SiC–Si3N4 composite was formed after sintering the ceramic foam at >1500°C. Micrographs show that most of these foams have well-defined open-cell structures and macro-defect free struts. The shrinkage is reduced considerably due to the addition of Si3N4 particles.

Carbothermal Reactions of Quartz and Kaolinite with Coal Char

Reactions of quartz and kaolinite separately with coal char were examined at temperatures up to 1600 °C in argon and nitrogen atmospheres, by using thermogravimetric analysis combined with off-gas analysis. X-ray diffraction analysis was utilized to identify the reaction products. The carbothermal reactions of quartz and kaolinite began, respectively, at temperatures as low as 1200 and 1150 °C, with the evolution of CO. The formation of reaction products was quite dependent on heating conditions. Under Ar, the main product(s) were β-SiC for quartz, and β-SiC and α-Al2O3 for kaolinite. Above 1550 °C, Al2O3 gradually formed Al4C3. Under N2, the carbothermic nitridation occurred for both quartz and kaolinite in company with the formation of SiC. For quartz, the products were SiC and hexagonal β-Si3N4 at 1600 °C; however, no crystalline nitrides were distinctly observed at 1400 °C. For the case of kaolinite, β-Si3N4 was appreciably formed at 1400 °C and it disappeared at 1600 °C; above 1500 °C, AlN was formed from Al2O3. Similar changes of crystalline mineral phases were examined on heating three coals in the same manner. Silicon carbide (SiC) and aluminum nitride (AlN) were the major products in the coals treated under nitrogen at 1600 °C for 10 min. The formation of these compounds could be attributed primarily to the carbothermal reactions of kaolinite and quartz inherent in coal.

Molecular-dynamics simulations of energetic C60 impacts on (2×1)-(100) silicon

Single impacts of energetic C60 clusters on (2×1)-(100) silicon substrates are studied by molecular-dynamics simulations. The role of impact energies and internal cluster energy are investigated in detail. Six different energy regimes can be identified at the end of the ballistic phase: At thermal energies below 20 eV the fullerene cages undergo elastic deformation, while impinging on the surface, and are mostly chemisorpted on top of the (2×1)-dimer rows. Between 20 and 100 eV the cage structure is preserved after the collision, but the cluster comes to rest within a few monolayers of the silicon surface. At energies of 100–500 eV the cluster partially decomposes and small coherent carbon caps are embedded in the surface. At higher energies up to 1.5 keV complete decomposition of the fullerene cluster occurs and an amorphous zone is formed in the subsurface area. At energies greater than approximately 1.5 keV craters form and above 6 keV sputtering becomes significant. In all cases the substrate temperature is of minor influence on the final result, but the projectile temperature is important for impacts at lower energies (<1.5 keV). For high energy impacts the ballistics resemble that of single atom impacts. Nearly 1:1 stoichiometry is obtained for impact energies around 1 keV. These results reveal an interesting possibility for controlled implantation of C in Si at high local concentrations, which might allow the formation of silicon carbide.

In situ study of the formation of SiC thin films on Si(111) surfaces with 1,3-disilabutane: Adsorption properties and initial deposition characteristics

The initial formation of silicon carbide by the interaction of 1,3-disilabutane (DSB) with a Si(111) surface has been studied in the temperature range of 100–1200 K using the low-energy Cs+ reactive scattering and x-ray photoelectron spectroscopy (XPS). By combining the results of these two techniques, adspecies in the intermediate states during the decomposition of DSB were qualitatively identified and the binding energies and full widths at half maximum of their C(1s) peaks were characterized. At 100 K, 1,3-DSB was found to adsorb on the surface as the C2H8Si2 species as well as CH4Si and to condense molecularly on the monolayer of C2H8Si2 adspecies. XPS indicates that the molecular species desorbs mostly at 200 K and completely at 300 K. Up to 600 K, the C2H8Si2 adspecies are converted to CH4Si with increasing temperature and then above this temperature the CH4Si species decomposes to form the SiC film. The intensity variations of Si(2p) and total C(1s) peaks and the analysis by curve fitting of the C(1s) peaks suggest that one C2H8Si2 adspecies makes only one CH4Si adspecies by breakage of the central C–Si bond rather than two CH4Si adspecies.

Comparison of Microwave Hybrid and Conventional Heating of Preceramic Polymers to Form Silicon Carbide and Silicon Oxycarbide Ceramics

Six different preceramic polymers were pyrolyzed via conventional and microwave hybrid heating; these polymers provide a range of carbon content and local atomic coordination. The products were compared with each other using X‐ray diffractometry and transmission electron microscopy. Nanocrystalline β‐SiC was the principal crystal phase detected, and the amount and size of the nanocrystals increased as the processing temperature increased. Differences were observed in the amount and size of the β‐SiC nanocrystals and the graphitization of residual carbon between the microwave hybrid heating and the conventional oven heating of polycarbosilanes. Conventional heating of a high‐carbon polysiloxane in an oven (in flowing argon) produced a greater amount of β‐SiC from carbothermal reduction at high temperature. Microwave hybrid heating led to better β‐SiC nanocrystal development for polyureasilazane.

C–SiC–Si gradient films formed on silicon by ion beam assisted deposition at room temperature

Ion beam assisted deposition (IBAD) of carbon under medium energy (35 keV) Ar ion bombardment onto a silicon target results in the formation of silicon carbide. The formation of a-C–SiC–Si gradient films is dependent on the ion/atom arrival ratio ( I/ A). All films are deposited at room temperature. The gradient layers formed are examined for their composition using RBS. Depending on the I/ A ratios, mixed interfaces of different widths, where silicon carbide exists, are formed. The SiC bonding in the mixed region is proven by XPS. The film formed on top of some samples contains amorphous carbon regions as shown by Raman spectroscopy. The surface of the layers grown is rather smooth with a roughness of several nanometres.

Production of Silicon Carbide from Rice Husks

This paper reports the formation of ß‐silicon carbide by the process of pyrolyzing rice husks (RH). The sequence of transformations and the mechanism of formation of crystalline silica, carbon, and silicon carbide from RH were investigated in detail. The structural evolution of silicon carbide particulates during the conversion process was critically examined by various techniques. Studies using X‐ray diffraction and infrared spectroscopic techniques revealed that a temperature of 1500°C is desirable for the completion of the reaction between RH silica and carbon, thereby leading to the formation of ß‐silicon carbide. Except for the gradual change in the morphology leading to the formation of dense crystals of ß‐SiCp, formation of alpha‐SiCp was not evidenced in the present studies. The fine particle size of ß‐SiC, formed at lower reaction temperatures, apparently precluded the transformation to alpha‐SiC.

Phase identification in reactively sintered molybdenum disilicide composites

The objective of this research was to identify and quantify the phases formed in reactively sintered MoSi 2 and MoSi 2 composites made from carbon coated MoSi 2 powders. The purpose of adding carbon was to form silicon carbide particles (SiC p ) in situ and reduce the inherent presence of SiO 2 in MoSi 2 , The carbon additions were made via polymeric coatings on MoSi 2 particles by two processes. Phenolic Resin Based Carbon by Solvent Evaporation (PRBCSE) and Aqueous Dispersion Flocculation (ADF). The sintering temperatures and times ranged from 1600 to 1800°C, and from 1 to 100 h, respectively. The addition of carbon did reduce the presence of SiO 2 and there was formation of SiC p . However, the SiC p formation was less than expected, a maximum of 1.4 vol.% SiC p was formed. The Nowotny phase (Mo 4 8 Si 3 C, 0.6 ) was observed in the sintered samples. The in situ formation of SiC p would increase the toughness of MoSi 2 by serving as a reinforcement. The reduction in the amount of SiO 2 would reduce the incidence of grain boundary sliding caused by viscous flow of SiO 2 at elevated temperatures.

Behavior of strained Si1−yCy (0⩽y⩽0.02) layers grown on silicon during wet oxidation

We have investigated the properties of strained Si1−yCy(0⩽y⩽0.02) alloy layers grown on silicon after wet oxidation between 750 and 1100 °C. Both secondary ion mass spectrometry and x-ray photoelectron spectroscopy prove the presence of carbon in the oxide. The growth rates and refractive indices of the oxides obtained from ellipsometry show no significant differences compared with oxides grown on silicon. Infrared absorption measurements reveal decreasing stress in the oxides with rising oxidation temperature and a slight dependence of the stress in the oxide upon the carbon concentration. For the Si1−yCy layer, x-ray diffraction measurements show a decrease of the substitutional carbon concentration with increasing oxidation temperature and duration. This loss of substitutional carbon is enhanced near the SiO2/Si1−yCy interface. The formation of silicon carbide precipitates in the Si1−yCy layer is demonstrated by infrared absorption. For short oxidation times, we find an increase of the intensity of the local vibrational absorption mode of substitutional carbon. We observe discrepancies between the determination of the substitutional carbon concentration in the x-ray diffraction and infrared absorption measurements.

Formation of Silicon Carbide in the Surface Layer of Metals by Dual High Energy Ion Implantation

The dual ion implantation of silicon and carbon into copper (99.9%), iron (99.9%), SKD11 steel and SUS304 austenitic stainless steels were carried out by the MeV energy ion accelerator. The cross sectional images of implanted layer were observed by scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). The microstructure of surface layer was analyzed using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It was shown that silicon carbide were formed by the dual ion implantation into metals. The hardness of sample was measured by the Nano-Indenter. Consequently, it was understood that the hardness of implanted substrate was improved by dual ion implantation.

Mechanism of Formation of Silicon Carbide from Phenol Resin -Tetraethyl Orthosilicate Hybrids with the Addition of Triethyl Borate

Hybrid Gel Precursors for SiC fine particles were synthesized from the liquid mixtures composed of tetraethylorthosilicate, triethyl borate and phenol resin. After the addition of catalyst, the turbid dispersion was converted intothe transparent solution and was gelled under continuous stirring. The synthesized gels with various compositions weredried under vacuum, crushed and pyrolyzed with an appointed conditions to yield the precursors for SiC powder.NMR study of the synthesized precursors indicates the formation of Q2 and Q3 units in the dried precursors, which would correspond to the low dimensional Si-O-Si networks. After the pyrolysis at 1273K, the single signal assigned to Q4 is observed, which corresponds to three-dimensional Si-O-Si network. The SiC precipitation process in these precursorsat 1873K was investigated in terms of X-ray diffraction analysis and powder morphology. The composition of the startingmixture hardly influenced the mixture transparency during the gelation process and the silicon environments in theobtained precursors. The morphology and the size of the resulting powders, however, strongly depend on the resin and borate content in the starting mixtures.

High-Temperature Evolution of SiC/C Products Derived from Preceramic Polymers

The products formed during thermolysis of organosilicon polymers containing unsaturated carbon-carbon bonds have been studied within the temperature range 20–1700°C by means of XRD, 13C and 29Si MAS NMR, and Raman spectroscopy. These products are distinguished by their structural instability. A continuous evolution of the products formed during organic–inorganic thermal transformation involves (i) ordering of the local surroundings of silicon atoms, the formation of a silicon carbide crystalline phase, and the transition from β to α and (ii) consumption of carbon for the completion of silicon carbide formation, carbon phase formation, and ordering.

Molecular Dynamics Simulation for Understanding Nucleation Mechanism on Amorphous Region of Silicon Substrate Surface.

In order to understand nucleation mechanism of diamond film fabricated by chemical vapor deposition method on silicon substrate, molecular dynamics simulation of a process forming silicon carbide layer on amorphous region of silicon substrate surface is conducted. The fact that silicon crystal surface scratched by various grit such as diamond contains many dislocations and amorphous region has been known, and silicon carbide layer as the interface between diamond film and silicon substrate is confirmed by high-resolution transmission electron microscopy. In this paper, molecular dynamics simulation using the Tersoff potential and temperature control method with introducing cut-off temperature for simulation has been carried out for 200 ps, where 4200 silicon atoms and less than 1000 carbon atoms in the cell are considered. The action of carbon atoms and silicon substrate in the simulation are shown, so one of processes forming silicon carbide layer on amorphous region of silicon substrate surface is found.

Formation of silicon carbide and amorphous carbon films by pulse biasing silicon to a high voltage in a methane electron cyclotron resonance microwave plasma

Silicon was pulse biased to −45 kV in a methane plasma generated by microwave excitation in the electron cyclotron resonance (ECR) mode. Hydrocarbon ions were accelerated in the electrical field and implanted into the silicon. Rutherford backscattering (RBS) measurements showed that it is possible to incorporate a concentration of up to 95 at.% C into the Si. Cross-section transmission microscopy (XTEM) showed that the resulting surface layer was amorphous. Annealing at 1250 °C resulted in the formation of 10 to 60 nm thick, crystalline SiC layers.

Synchrotron radiation induced SiC formation on Si substrate employing methanol and H radical

A novel system for a material processing was developed using synchrotron radiation (SR) induced reaction. This system consisted of a SR, a radical source with a microwave plasma for an injection of radicals, and a CO2 laser for heating a substrate. Film formation was performed using this system. CH4 and CH3OH were used as a source gas and H radicals were injected by the radical source during film formation. The synthesized films were characterized by an atomic force microscope, a micro Auger electron spectroscopy, and a Fourier transform infrared absorption spectroscopy. CH3OH gas was found to be dissociated by an irradiation of SR. When CH3OH gas was employed without H radical injection, a carbon film was formed only in the area of SR irradiation on Si substrate. Using SR irradiation employing CH3OH gas with H radical injection, nuclei of silicon carbide (SiC) were successfully formed in the area of SR irradiation on Si substrate at a temperature of 800 °C. The mechanism of SiC nucleation induced by SR irradiation is discussed.

Formation of silicon carbide and nitride by ECR microwave plasma immersion ion implantation

Silicon was pulse biased to −45 kV in a methane or nitrogen plasma, generated by electron cyclotron resonance microwave excitation. Hydrocarbon or nitrogen ions were accelerated in the electrical field and implanted into the silicon. The depth distribution of the implanted ions was measured by Rutherford backscattering spectrometry (RBS) before and after annealing. For the case of nitrogen implantation, one observes a saturation concentration of incorporated nitrogen, which is below the value of stoichiometric Si 3N 4. In contrast to that, carbon implantation can result in far overstoichiometric C:Si ratios and therefore in the formation of an amorphous carbon rich (95 at.%) phase. Annealing of the wafers results in partial oxidation of the surface layers and in the formation of crystalline phases. The structure of the implanted silicon wafers has been determined by cross-section transmission electron microscopy (XTEM) for the as implanted and annealed samples. For both implanted ion species, carbon and nitrogen, the implantation resulted in an amorphization of the wafer surface. No crystalline phases except silicon could be detected by electron diffraction. Annealing of the carbon implanted silicon results in the formation of a continuous, 50 nm thick polycrystalline β-SiC layer. For the case of nitrogen implantation, the formation of a 25 nm thick, continuous α-Si 3N 4 film below a 50 nm thick amorphous surface zone has been observed.

Stable blue-green and ul traviolet photoluminescence from silicon carbide on porous silicon

β-Silicon carbide layers have been prepared by high temperature pyrolysis of polyimide Langmuir-Blodgett films on porous silicon substrate in vacuum. The formation of silicon carbide is confirmed by the IR and XRD spectra. It is found that photoluminescence still exists and appears in the blue-green and ultraviolet regions after thermal treatment at 900°C. These results indicate that the silicon carbide layers, which are formed, are responsible for the blue-green luminescence.

The mechanical properties and microstructure of SiC–AlN particulate composite

Silicon carbide (SiC) and aluminium nitride (AlN) were found to form a solid solution at temperatures above 1800°C. Based on this interesting result, the composite was fabricated by mechanical mixing of the SiC and AlN powders, and hot pressed under 40 MPa at 1950°C in an argon atmosphere. The objective was to achieve full density and minimize solid solution formation. During the sintering process, the SiC–AlN system passed through three steps to form the solid solution at the end. First, the AlN particle is vaporized from its surface; next, the evaporated AlN is deposited on the surface of the SiC grains and the AlN particle, accompanied by a reduction in its size, and finally, partial SiC and AlN solid-solution formation on the boundary of the SiC grains. Because of the AlN deposition and solid-solution formation at the boundary of SiC grains, a barrier layer was present on the surface of SiC grains. This leads to the formation of fine grains. The toughening mechanism is thought to be by thermal residual stresses, due to the difference between the coefficients of thermal expansion of the matrix SiC and that of the dispersed AlN particles, and crack deflection around the SiC grains. Therefore, it is that which improves the mechanical properties of the SiC–AlN composite. © 1998 Chapman & Hall

Carbon incorporation in Si1−yCy alloys grown by molecular beam epitaxy using a single silicon–graphite source

Pseudomorphic Si1−yCy alloys on silicon (100) were grown by molecular beam epitaxy using a single effusion source of silicon contained in a graphite crucible, producing carbon concentrations of y=0.008. The behavior of carbon incorporation using this source was studied as a function of growth temperature using x-ray diffraction and infrared spectroscopy, and was compared to previous studies, where Si1−yCy was grown from separate silicon and graphite sources. An increased energy barrier for the surface diffusion of carbon was observed using the single silicon–graphite source. An infrared absorption mode near 725 cm−1, observed for growth temperatures up to 700 °C, was attributed to a transitional phase between the loss of substitutional carbon and the formation of silicon carbide precipitates.

Analysis of reactive melt infiltration in the processing of ceramics and ceramic composites

A model describing the reactive melt infiltration (RMI) of liquid silicon into a carbonaceous preform is solved and the simulation is compared with experimental results. The finite-element method is used to solve the equations in arbitrary geometries. The model is consistent with experimental results in that it predicts the infiltration rate in a reacting and nonreacting system and the magnitude and functional form of the temperature rise in the sample caused by the exothermic reaction of silicon with carbon to form silicon carbide. The model is particularly sensitive to two parameters, the pore neck diameter ( d 1) as defined by an ideal repeating pore unit, and the SiC reaction barrier thickness ( δ 0) which forms when silicon reacts with carbon to form a SiC skin around the particles. The sensitivity of the solution to these two parameters is demonstrated. It is suggested that d 1 and δ 0 be chosen based on experimental measurements of infiltration rate and temperature excursion during silicon infiltration.