SiC Clusters Research Articles

Finding possible transition states of defects in silicon-carbide and alpha-iron using the dimer method

Energetic primary recoil atoms from ion implantation or fast neutron irradiation produce isolated point defects and clusters of both vacancies and interstitials. The migration energies and mechanisms for these defects are crucial to successful multiscale modeling of microstructural evolution during ion-implantation, thermal annealing, or under irradiation over long periods of time. The dimer method is employed to search for possible transition states of interstitials and small interstitial clusters in SiC and α-Fe. The method uses only the first derivatives of the potential energy to find saddle points without knowledge of the final state of the transition. In SiC, the possible migration pathway for the C interstitial is found to consist of the first neighbor jump via a Si site or second neighbor jump, but the relative probability for the second neighbor jump is very low. In α-Fe, the possible transition states are studied as a function of interstitial cluster size, and the lowest energy barriers correspond to defect migration along 〈1 1 1〉 directions. However, this paper addresses whether migrating interstitial clusters can thermally change their direction, and the activation energies and corresponding mechanisms for changing the direction of these clusters are determined.

Wear Behaviour and Friction Property of SiCp/Al Composites at Elevated Wear Environment Temperature

The effects of sliding speed and wear environment temperature (hereafter denoted as temperature) on the wear behaviour and friction coefficient of SiC particle reinforced A16061 composites (SiCp/Al) were studied using wear and friction test of a ring on disc type, and difference of wear mechanism in terms of temperature was also examined by a microscopic observation of worn surfaces. Thin surface layer (TSL) formed on the worn surface during the experiment was enhanced with increase of temperature. However, such a TSL was cracked due to point contact between the counter material and coarse SiC particles (and/or SiC clusters) existing on the worn surface of the SiCp/Al. Contour of worn surfaces became rougher with increase of temperature and sliding speed. Wear mechanism of the AA6061 changed from abrasive wear into adhesive wear at 300°C while for the SiCp/Al, wear mechanism did not changed in the test conditions. That is, abrasive wear was maintained up to 300°C. Due to the presence of SiC particles, Friction coefficient of the SiCp/Al was larger than that of the AA6061 regardless of the temperature. However, scatter of friction coefficient against temperature was smaller in the SiCp/Al than in the AA6061. Furthermore, the weight loss, the difference of specimen weight before and after the experiment, under the severest test condition was less in the SiCp/Al than in the AA6061.

A comprehensive study of SiC growth processes in a VPE reactor

We performed an experimental study of the effect of the gas phase composition on the growth mechanism of 3C-SiC on Si(100) by atmospheric-pressure vapour phase epitaxy at 1350°C. Silane and propane diluted in hydrogen were used as precursors for the growth. We demonstrate the existence of an equilibrium partial pressure of carbon above the growing surface, which ensures a mirror-like morphology. For too low a carbon partial pressure (C/Si ratio in the gas phase lower than 2.7 with a growth rate of 3 μm h −1), the layer morphology and crystalline quality quickly degrade. For too high a carbon partial pressure (C/Si ratio higher than 5 with the same growth rate), SiC clusters form on the growing layers. We propose a mechanism of formation for these clusters taking into account the interactions between the C and Si species in the hot boundary layer.

Formation of Si clusters in electron-irradiated SiC studied by electron energy-loss spectroscopy

Abstract Electron-irradiation damage in 6H-SiC was examined at room temperature by means of transmission electron microscopy and electron energy-loss spectroscopy (EELS). A detailed analysis of extended energy-loss fine structure (EXELFS) gave unambiguous evidence of the formation of direct Si–Si bonding after prolonged irradiation, where the sample still maintained the well-defined 6H structure, while no appreciable indication of direct C–C bonding was observed. These results suggest that C atoms should be predominantly displaced away compared to Si atoms and the residual Si atoms form small clusters. The changes in the energy-loss near-edge structure (ELNES) were consistent with the appearance of Si clusters.

Properties of silicon and silicon-carbon cluster assembled films

Si-and SiC cluster-assembled films have been produced in a laser vaporization source and analyzed in terms of electronic structure and vibrational density of states. In small silicon clusters (nanometer size range), we put on evidence five-fold rings into the films. In the case of SiC clusters, we observe the lack of the chemical order.

Post-implantation annealing of SiC studied by slow-positron spectroscopies

The effects of post-implantation annealing of damage in 6H-SiC caused by ion implantation at two different fluences have been studied by monoenergetic positron Doppler broadening and lifetime techniques. The measurements are supported by new calculations of positron lifetimes in vacancy clusters in SiC. At both fluences two defected layers are identified and characterized by depth and defect type as a function of annealing temperature. The results indicate that it is impossible to remove the radiation damage by annealing at temperatures up to .

Can we rationalize the structure of small silicon-carbon clusters?

A theoretical study of \(\) clusters with \(\) using density functional theory is presented. Tests of various functionals demonstrate that local spin density approximation (LSDA) is the most adequate functional for the study of these systems. Structures, vibrational frequencies, and IR intensities of the lowest energy isomer of the studied clusters obtained using LSDA are described, and the unusual properties of the Si-C clusters are discussed. A quantitative analysis of the obtained structures was carried out, and relations between the coordinations, interatomic distances, and angles observed in the Si-C clusters were obtained through introduction of the notion of coordination. This analysis also shows that the carbon atoms mainly exhibit sp and sp2 hybridizations, and that a majority of silicon atoms do not hybridize. This study is the fi rst step of the implementation of a semi-empirical potential, which would describe the moderately small Si-C clusters.

Formation of carbon nanocapsules with SiC nanoparticles prepared by polymer pyrolysis

Carbon nanocapsules with SiC nanoparticles were produced by thermal decomposition of polyvinyl alcohol with SiC clusters at 500 °C in Ar gas atmosphere. High-resolution electron microscopy also showed the formation of carbon hollow structures such as nanoparticles, polyhedra, and clusters. The present work indicates that the pyrolysis of polymer materials with clusters is a useful fabrication method for the mass-production of carbon nanocapsules at low temperatures compared to the ordinary arc discharge method.

STM study ofC2H2adsorption on Si(001)

We present here a scanning tunneling microscope study of the initial bonding structure and subsequent reaction mechanism of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ with the Si(001) surface. Upon exposure of the sample at room temperature to 0.2 L of ${\mathrm{C}}_{2}{\mathrm{H}}_{2}$ (approximately 20% coverage) adsorption of the molecule on alternate dimer pairs is observed, leading to either a local $2\ifmmode\times\else\texttimes\fi{}2$ or $c(2\ifmmode\times\else\texttimes\fi{}4)$ structure. In the filled-state image, a local minimum is observed in the center of the reacted dimer pairs, while the unreacted dimer pairs maintain the normal bean-shaped contour of the clean surface. The molecule forms an overlayer with either local $2\ifmmode\times\else\texttimes\fi{}2$ or $c(4\ifmmode\times\else\texttimes\fi{}2)$ order, leading to a saturation coverage of 0.5 monolayers. Upon annealing the substrate at 775 K the surface becomes disordered and the steps are no longer visible. After further annealing at 875 K, SiC clusters are formed and the $2\ifmmode\times\else\texttimes\fi{}1$ structure is again seen between the clusters. For a starting coverage of 20%, annealing to higher temperatures around 1100 K leads to pinning of the step movement by the SiC clusters. For a starting coverage of 0.5 monolayer, annealing at 1100 K results in faceting of the surface. Further annealing at 1275 K creates anisotropic facets that are oriented along the [1\ifmmode\bar\else\textasciimacron\fi{}10] direction with a typical aspect ratio of approximately 4 to 5. These facets act as nucleation sites for subsequent carbonization and SiC growth.

Effect of microstructural features on the ageing behaviour of Al–Cu/SiC metal matrix composites processed using casting and rheocasting routes

In the present study, microstructural variation in Al–2 wt% Cu/SiC composites was accomplished by synthesizing them using conventional casting and partial liquid phase casting (rheocasting) routes. Microstructural characterization studies conducted on the rheocast composite samples revealed a finer grain size, minimal porosity, uniform distribution of SiC particulates, and a superior matrix – particulate interfacial integrity when compared to the conventionally cast composite samples. Furthermore, the results of interfacial characterization studies revealed that the presence of porosity associated with either individual SiC particulate or SiC clusters significantly influence the constitutional characteristics of the interfacial region. Results of ageing studies revealed an accelerated ageing kinetics in case of rheocast samples when compared to the conventionally cast composite samples. The results of ageing studies were finally rationalized in terms of the difference in microstructural characteristics of the rheocast and conventionally cast composite samples.

Blue, Yellow-Green and Red Simultaneous Emission from SiO2 Matrices Co-Implanted with Si and C

AbstractDual Si+C implantations at high doses (3×1017 cm−2) were performed in thermal SiO2 layers. The C implantation energy was fixed at 60 keV while the Si ones were performed at different energies (150 keV and 60 keV) with the aim of synthetizing carbon rich luminescent centers. Annealings up to 1100 °C were performed in an inert N2 atmosphere either for short times in a rapid thermal processing furnace and up to two hours in a conventional furnace. Evidence of the formation of silicon and carbon-rich clusters was provided by XPS, SIMS, Raman and TEM experiments. Photoluminescence experiments under blue and ultraviolet excitations showed intense emission bands peaking at 1.5–1.7 eV (red), 2.20 eV (yellowgreen) and 2.75 eV (blue). The relative intensity and spectral shape of the emission bands was found to depend strongly upon implantation and annealing conditions and offered the possibility of tuning the desired dominant emission wavelength. The possible formation of SiC clusters and even SiC crystallites at the SiO2/Si interface could explain the origin of the highly intense blue emission band found for these samples.

A microstructural investigation of the mechanisms of tensile creep deformation in an Al 2O 3/SiC w composite

The tensile creep behaviour of an SiC w (25%) reinforced alumina composite was investigated using scanning electron microscopy (SEM) and transmission electron microscopy and automatic image analysis in SEM. The creep tests were carried out in air in the ranges 1100–1300 °C and 11–67 MPa. Each creep test was performed at a constant temperature. The material had a stress exponent of about three for all temperatures and an approximate activation energy of 650 kJ mol −1. The creep resistance of this composite is poorer than that of similar composites studied earlier. Microstructural examination revealed the microstructure to be extremely inhomogeneous consisting of spherical whisker-rich clusters (20–100 μm) surrounded/separated by Al 2O 3 rich rims (10 μm). The secondary creep rate is dominated by a damage accumulation process namely cavitation and crack growth in both the SiC clusters and the Al 2O 3 rims. Final fracture seems to occur through the alumina rich regions. The lower creep resistance of this composite compared to that of similar composites is attributed primarily to the inhomogeneity of the as-received material.

Adsorption and decomposition of C60 molecules on Si(111) surfaces

Adsorption of C60 molecules on Si(111)-(7×7) has been studied using scanning tunneling microscopy under ultrahigh vacuum conditions at a variety of temperatures and coverages. At submonolayer coverages, isolated C60 molecules were found to adsorb preferentially in the areas near the three midadatoms within a half unit cell of (7×7) symmetry. At monolayer coverage, the adsorbate–adsorbate interaction changes the molecular bonding sites and causes the local ordering of the C60 adlayer. Thermal annealing to temperatures above 800 °C causes the decomposition of C60 molecules on the Si surface and the formation of SiC clusters.

In-situ FTIR emission spectroscopy in a technological environment: chemical vapour infiltration (CVI) of SiC composites

A method has been established to detect transient species inside a hot wall technological reactor. The CVI plant is used to produce fibre reinforced composite materials with ceramic matrix, in particular to infiltrate carbon fibre woven structures with SiC. The infiltration has been carried out at about 1000°C under reduced pressure with a mixture of CH 3SiCl 3 (methyltrichlorosilane, MTS) and H 2 as the SiC precursor. To investigate the gas reactions near the preform the emissivity has been measured by FTIR spectrometry. Several gaseous species could be detected including MTS, SiCl 2, (SiCl 3) n=1,2 SiCl 4, HSiCl 3, CH 4, CH 3Cl and HCl, also possible indications for CH 3 radicals and SiC clusters have been found. The interpretation of the multicomponent high temperature emission spectra has been supported by investigating reference spectra of individual components at working temperature. Band profile changes caused by both temperature and precursor decay have been detected. After radiation calibration, concentration changes and the degree of MTS decay could be roughly estimated. The spectroscopical results are evaluated in comparison with kinetic models of the MTS decay.

Molecular dynamics calculations of defect energetics in β-SiC

The molecular dynamics (MD) method is used to calculate defect energetics in β-silicon carbide. Many-body interaction effects in this covalent material are accounted for by using a hybrid of two-body and three-body potentials. Pearson's potential is modified to accurately fit the sublimation energy of β-SiC, and interatomic potentials among silicon, carbon, and helium atoms are also developed. A microcrystal is constructed to represent the computational cell, and external forces are applied to its boundaries so that it behaves as a part of an infinite medium. The potential energy for the unperturbed computational cell is first calculated. The cell is then set at a chosen defect configuration and relaxed, and the potential energy of the relaxed cell is computed. The difference between the potential energies of the unperturbed cell and that of the defect-containing cell is used to calculate the formation energies of point defects and defect clusters in SiC. Binding energies, and migration energies of point defects are then deduced. Preexponential factors of point defect diffusion coefficients are derived from the calculated potential energy profile. Activation energies and preexponential factors of thermal diffusion through the vacancy mechanism are compared to corresponding experimental data.

The polycarbosilane-to-Si xC 1− x conversion studied by inelastic neutron scattering and infrared absorption

Polycarbosilane (PC) is a precursor which is converted to Si x C 1− x fibers by pyrolysis in an inert gas atmosphere. The changes in the atomic vibrational spectrum during the conversion process from PC to Si x C 1− x have been examined by means of inelastic neutron scattering (INS) and infrared absorption (IR). The existence of transverse-optical(TO)- and longitudinal-optical(LO)-like phonon modes due to amorphous SiC clusters was established in the original and pyrolyzed PC by INS measurements. After pyrolyzing at 700–800°C, these modes appear distinctly around 730 and 930 cm −1, respectively, in the INS spectra. Pyrolysis at 1000°C makes these TO- and LO-like phonon modes sharper.

Synthesis of Sic Clusters in a Nonthermal Microwave Plasma

ABSTRACTA new technique has been developed to condense nanoparticles (clusters) of SiC and other nonoxide materials in a nonthermal microwave plasma. The experimental rationale and approach is described. It utilises silane and acetylene precursors in an Ar diluent, with the synthesis carried out in a microwave plasma at a pressure of ∼1 torr. The plasma conditions have been characterised, as have the effect of important parameters (gas ratios, flow rates and pressure) on the product nanoparticles. The resultant nanoparticles have been characterised ex-situ by TEM, Auger, and XPS spectroscopy. SiC particles collected are ∼5nm in size, with cubic, hexagonal and rhombohedral polytypes observed. Silicon nitride has been synthesised similarly by using silane and nitrogen as the precursor gases. Implications of the new technique are discussed.

Study of the polymer to ceramic evolution induced by pyrolsis of organic precursor

The structure of a precursor: the polycarbosilane, has been followed during pyrolysis up to 1600°C. A continuous evolution is observed, leading to a nucleation of SiC clusters followed at higher temperature by a growth of the SiC crystalline phase. The structure keeps the memory of the precursor up to 1400°C.

光ＣＶＤ法によるＳｉＣエピタキシャル薄膜の作製と光照射効果

Epitaxial SiC films grow on α-Al2O3(0001) substrates under the irradiation of UV light from an ArF excimer laser or a deuterium discharge lamp. The irradiation of laser beam is effective to assist the formation of the interface between the SiC film and the Al2O3 substrate. Epitaxial SIC films which are very adhesive to the substrate, are grown at a low temperature. Irradiation to the substrate is essential to these effects. At an initial stage of film growth, nucleation density is increased and the contact between SiC and α-Al2O3 is improved by the irradiation. The irradiation of the light with the wavelength shorter than about 380nm is effective to obtain the adhesive epitaxial films. This wavelength is discordant with the light absorption range for the reaction gases(Si2H6, C2H2), Al2O3 and SIC. The light absorbents which exist only at the substrate surface (as possible examples, SIC clusters and surface states of Al2O3) improve the film growth on the substrate.

Synthesis and analysis of buried SiC layers in monocrystalline silicon

SiC layers were fabricated by implantation using 10–40 keV carbon ions in Si(100) and Si(111) wafers, with subsequent annealing between 800 and 1100°C. Computed and experimental IR curves were compared and the thickness of the film were determined from transmission data. The computed thickness were compared with twice the straggling width. The results lead to the conclusion that the fabricated films are continuous rather than SiC clusters in a silicon matrix.