Stoichiometric Silicon Carbide Research Articles

TRISO coated fuel particles with enhanced SiC properties

The silicon carbide (SiC) layer used for the formation of TRISO coated fuel particles is normally produced at 1500–1650 °C via fluidized bed chemical vapor deposition from methyltrichlorosilane in a hydrogen environment. In this work, we show the deposition of SiC coatings with uniform grain size throughout the coating thickness, as opposed to standard coatings which have larger grain sizes in the outer sections of the coating. Furthermore, the use of argon as the fluidizing gas and propylene as a carbon precursor, in addition to hydrogen and methyltrichlorosilane, allowed the deposition of stoichiometric SiC coatings with refined microstructure at 1400 and 1300 °C. The deposition of SiC at lower deposition temperatures was also advantageous since the reduced heat treatment was not detrimental to the properties of the inner pyrolytic carbon which generally occurs when SiC is deposited at 1500 °C. The use of a chemical vapor deposition coater with four spouts allowed the deposition of uniform and spherical coatings.

Control of stoichiometry, microstructure, and mechanical properties in SiC coatings produced by fluidized bed chemical vapor deposition

Stoichiometric silicon carbide coatings the same as those used in the formation of TRISO (TRistructural ISOtropic) fuel particles were produced by the decomposition of methyltrichlorosilane in hydrogen. Fluidized bed chemical vapor deposition at around 1500 °C, produced SiC with a Young’s modulus of 362 to 399 GPa. In this paper we demonstrate the deposition of stoichiometric silicon carbide coatings with refined microstructure (grain size between 0.4 and 0.8 μm) and enhanced mechanical properties (Young’s modulus of 448 GPa and hardness of 42 GPa) at 1300 °C by the addition of propene. The addition of ethyne, however, had little effect on the deposition of silicon carbide. The effect of deposition temperature and precursor concentration were correlated to changes in the type of molecules participating in the deposition mechanism.

A review of the development of three generations of small diameter silicon carbide fibres

Three generations of small diameter ceramic fibres based on polycrystalline silicon carbide have been developed over a period of thirty years. This has been possible due to studies into the relationships between the microstructures and properties of the fibres. A variety of techniques have been employed by research teams on three continents. The fibres are made by the conversion of polymer precursors to ceramic fibres and all three generations are presently produced commercially. The nature of the precursor and the techniques used for cross-linking have been varied in order to optimise both properties and cost of manufacture. It has been possible to improve the characteristics of the fibres as the processes involved in the cross-linking of the precursor fibres have been better understood and the mechanisms governing both room temperature and high temperature behaviour determined. The result is that, although first generation fibres were limited by a low Young's modulus at room temperature and by creep and instability of the structure at temperatures far lower than those limiting the behaviour of bulk silicon carbide, the third generation fibres shows many of the characteristics of stoichiometric silicon carbide. This remarkable improvement in characteristics has been due to a thorough understanding of the materials science governing the behaviour of these fibres which are reinforcements for ceramic matrix composite materials.

The Formation of Smooth, Defect-free, Stoichiometric Silicon Carbide Films from a Polymeric Precursor

AbstractSilicon carbide (SiC) materials, which are used in a variety of applications, are often produced using powder processing, sintering or bulk crystal growing techniques. The formation of silicon carbide films or shaped products, via these methods, is often extremely difficult and/or requires high temperatures. Here we report the synthesis and characterization of a polymeric precursor, Polymethylsilyne (PMSy), and it subsequent conversion to â-SiC. The polymer is simple to synthesize and is easily manipulated in air. The ceramic produced from PMSy is extremely pure, stoichiometric SiC and is produced in high yields (up to 85%). The ceramic films that can be produced from PMSy on a variety of substrates, from ceramics to metals, are again stoichiometric SiC and are smooth, continuous and defect free; possibly enabling the use of these films in electronic applications.

On the Role of Vacancies in Pore Formation in the Course of Anodizing of Silicon Carbide

Experimental data on the preparation of stoichiometric nanoporous silicon carbide are analyzed. Theoretical calculations are performed under the assumption that nanopores are formed through the vacancy diffusion mechanism. The results obtained confirm the hypothesis that the formation of pores with a steadystate radius of several tens of nanometers in silicon carbide can be associated with the diffusion and clustering of vacancies. The experimental data indicating that the proposed mechanism of formation of nanoporous silicon carbide correlates with the existing model of formation of porous silicon carbide with a fiber structure are discussed. This correlation can be revealed by assuming that nanopores are formed at the first stage with subsequent transformation of the nanoporous structure into a fiber structure due to the dissolution of the material in an electrolyte.

Mechanical Properties of Chemically Vapor-Infiltrated Silicon Carbide Structural Composites with Thin Carbon Interphases for Fusion and Advanced Fission Applications

Fast fracture properties of chemically vapor-infiltrated silicon carbide matrix composites with Hi-NicalonTM Type-S near-stoichiometric silicon carbide fiber reinforcements and thin pyrolytic carbon interphase were studied. The primary emphasis was on preliminary assessment of the applicability of a very thin pyrolytic carbon interphase between fibers and matrices of silicon carbide composites for use in nuclear environments. It appears that the mechanical properties of the present composite system are not subject to strong interphase thickness effects, in contrast to those in conventional non-stoichiometric silicon carbide-based fiber composites. The interphase thickness effects are discussed from the viewpoints of residual thermal stress, fiber damage, and interfacial friction. A preliminary conclusion is that a thin pyrolytic carbon interphase is beneficial for fast fracture properties of stoichiometric silicon carbide composites.

Smooth Continuous Films of Stoichiometric Silicon Carbide from Poly(methylsilyne)

A new synthesis of the silicon‐network‐backbone polymer poly(methylsilyne) gives a material that is easily converted by pyrolysis to smooth continous films of stoichiometric silicon carbide (see Figure). The films are adherent to the silicon or alumina substrates, and show root mean square roughness of 169 Å over a 500 μm range. Applications in electronics are envisaged.

Effects of short-range disorder upon electronic properties of a-SiC alloys

Molecular dynamics (MD) simulations, based on an empirical potential approach, have provided detailed information about chemical ordering and the structural short-range order in stoichiometric amorphous silicon carbide (a-SiC). Recursion band structure calculations based on amorphous geometries obtained from the MD simulations have enabled to establish the mechanism of an influence of homopolar bonds, three-fold (T 3) and five-fold (T 5) coordinated defects, strongly disordered four-fold coordinated sites and atoms, which are first nearest neighbors (FNN) of these defects (T 4) on the distribution of electronic states. We have found that electronic states at the middle of the gap can be associated with these kinds of defect with the exception of antisite defects (like-atom or homopolar bonding). It is the problem of chemical ordering in the stoichiometric silicon–carbon alloy that is the main subject of the present work. In contrast to crystalline SiC, in a-SiC, the resonance states at the valence band top, associated to Si–Si homonuclear bonds, split for the low symmetry of the amorphous surrounding, which gives rise to the additional split states at the band gap bottom. As a result, in the amorphous material, the decrease of a degree of chemical ordering is accompanied by narrowing the band gap.

CVD of SiC from Methyltrichlorosilane. Part II: Composition of the Gas Phase and the Deposit

Part I of the paper was concerned with the influence of the surface area/volume ratio, [AS/VR], temperature, and residence time on the deposition rate of silicon carbide from a 1:4 methyltrichlorosilane (MTS)/hydrogen mixture. In Part II, the composition of the gas phase and the deposit are investigated. It is shown that deposition rate maxima at about 900 °C and 1025 °C, frequently reported in the literature, result from a temperature-selective deposition of both SiC and Si. This selectivity is determined by the [AS/VR] ratio because it controls the interaction of the gas phase and surface reactions. At a sufficiently high [AS/VR] ratio, deposition of free silicon is completely suppressed and the rate maxima disappear simultaneously. Increasing the temperature generally favors deposition of stoichiometric silicon carbide. Results for the gas phase composition are used to discuss changes of gas phase chemistry responsible for the deposition of free silicon, as well as of stoichiometric silicon carbide. Higher chlorosilanes, as well as carbochlorosilanes, are postulated as intermediates of silicon and silicon carbide deposition, respectively.

Silicon carbide and amorphous carbon film formation by plasma immersion ion implantation: a comparison of methane and toluene as plasma forming gases

In the present study a comparison of two different hydrocarbon precursor gases, namely methane and toluene, on the formation of silicon carbide (SiC) and amorphous carbon (a-C:H) films by plasma immersion ion implantation (PIII) of silicon is made. The samples are analyzed by using Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS) for their element composition. XPS and Raman spectroscopy are applied to determine the bonding of silicon and carbon. Depending on the number of high voltage pulses the samples have been treated with, which corresponds to the ion fluence, it is possible to form stoichiometric SiC films without C layers on top of the wafers if methane is used as plasma forming species. For the same process conditions, CH 4 PIII always results in thicker SiC and thinner a-C:H films compared with C 7H 8 PIII. SiC bond formation is proven by XPS. Raman spectroscopy shows the formation of a-C:H films for high pulse numbers for the case of a methane plasma, but already for small pulse numbers if toluene is used for plasma forming species.

SiC formation on Si(100) via C 60 precursors

The interaction between C 60 molecules and the Si(100) surface and the preparation of silicon-carbide thin films by thermal reaction of C 60 molecules with the Si(100) surface have been investigated using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, reflection high-energy electron diffraction and atomic force microscopy measurements. The effects of annealing temperature and C 60 coverage on the SiC formation will be discussed. It is found that the C 60 molecules bond covalently with silicon, and the number of bonds increase upon increasing the annealing temperature. Annealing at T≥830°C entails the formation of stoichiometric silicon carbide clusters that coalesce to form a continuous SiC layer when the C 60 coverage is greater than one monolayer. Deep pits acting as silicon diffusion channels are present with dimensions that increase with the amounts of C 60. The interaction of C 60 with the SiC surface was also investigated. It is found that a similar covalent interaction is present in the two systems C 60/Si and C 60/SiC.

Experimental study of atmospheric pressure chemical vapor deposition of silicon carbide from methyltrichlorosilane

A comprehensive study of the chemical vapor deposition of SiC from methyltrichlorosilane at atmospheric pressure was conducted in this study; its main objectives were to identify the range of operating parameters in which stoichiometric SiC could be deposited and the generation of reliable kinetic data that could be used for the design of atmospheric pressure processes of chemical vapor deposition or chemical vapor infiltration of SiC. Deposition experiments were conducted in a hot-wall, cylindrical reactor at temperature ranging from 1273 to 573 K on flat graphite substrates or thin molybdenum wires aligned with the axis of the reactor. The obtained results showed that the deposition rate and the deposit stoichiometry varied markedly with the distance from the entrance of the reactor. The deposition rate exhibited, depending on the reaction temperature, one or two pronounced maxima before the beginning of the isothermal zone of the reaction, whereas the deposit stoichiometry showed an abrupt transition from almost silicon to stoichiometric silicon carbide after the first maximum. Experiments with HCl added in the feed showed that the presence of HCl could cause complete suppression of the deposition of silicon and lead to smoother variation of the SiC deposition rate with the residence time in the reactor. It is believed that this effect could be exploited to improve the uniformity of SiC deposition in chemical vapor deposition reactors or in the interior of porous preforms.

Neutron irradiation induced amorphization of silicon carbide

This paper provides the properties of bulk stoichiometric silicon carbide which has been amorphized under neutron irradiation. Both high purity single crystal hcp and high purity, highly faulted (cubic) chemically vapor deposited (CVD) SiC were irradiated at approximately 60°C to a total fast neutron fluence of 2.6 × 10 25 n/m 2. Amorphization was seen in both materials as evidenced by TEM, electron diffraction and X-ray diffraction techniques. Physical properties for the amorphized single crystal material are reported including large changes in density (−10.8%), elastic modulus as measured using a nanoindentation technique (−45%), hardness as measured by nanoindentation (−45%), and standard Vickers hardness (−24%). Similar property changes are observed for the amorphized CVD SiC. Using measured thermal conductivity data for the CVD SiC sample, the critical temperature for amorphization at this neutron dose and flux, above which amorphization is not possible, is estimated to be greater than ∼125°C.

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.

Nanometric powder of stoichiometric silicon carbide produced in square-wave modulated RF glow discharges

SiC nanometric powder has been obtained in square-wave modulated radiofrequency glow discharges from CH 4 and SiH 4 gas mixtures. Chemical and structural characterization revealed that the as-deposited SiC:H powder underwent spontaneous oxidation when exposed to atmosphere. To stabilise the powder chemically, we carried out a thermal treatment under vacuum (10 −4 Pa) consisting of heating to 800°C (20°C/min). The effects on the structure of the powder were examined by FTIR, EA, XPS and from optical transmittance measurements. They can be summarized as follows: dehydrogenation of the powder that induces the formation of a SiC carbidic network and chemical stability under atmospheric conditions, further confirmed by exposure to air for more than 6 months. In addition, TEM images showed that the annealed powder presented a short-range order in β-SiC units, but there was no evidence of size changes due to sinterization or compactaction phenomena.

The effect of substrate temperature on the properties of nanostructured silicon carbide films deposited by hypersonic plasma particle deposition

Nanostructured silicon carbide films have been deposited on molybdenum substrates by hypersonic plasma particle deposition. In this process a thermal plasma with injected reactants (SiCl4 and CH4) is expanded through a nozzle leading to the nucleation of ultrafine particles. Particles entrained in the supersonic flow are then inertially deposited in vacuum onto a temperature-controlled substrate, leading to the formation of a consolidated film. In the experiments reported, the deposition substrate temperature Ts has ranged from 250°C to 700°C, and the effect of Ts on film morphology, composition, and mechanical properties has been studied. Examination of the films by scanning electron microscopy has shown that the grain sizes in the films did not vary significantly with Ts. Micro-X-ray diffraction analysis of the deposits has shown that amorphous films are deposited at low Ts, while crystalline films are formed at high Ts. Rutherford backscattering spectrometry has indicated that the films are largely stoichiometric silicon carbide with small amounts of chlorine. The chlorine content decreases from 8% to 1.5% when the deposition temperature is raised from 450°C to 700°C. Nanoindentation and microindentation tests have been performed on as-deposited films to measure hardness, Young's modulus and to evaluate adhesion strength. The tests show that film adhesion, hardness and Young's modulus increase with increasing Ts. These results taken together demonstrate that in HPPD, as in vapor deposition processes, the substrate temperature may be used to control film properties, and that better quality films are obtained at higher substrate temperatures, i.e. Ts≈700°C.

Atomic force microscopy study of microcrystalline SiC fabricated by ion beam synthesis

Ion beam synthesis of silicon carbide (SiC) layers was performed by metal vapor vacuum arc implantation. Subsequent furnace annealing was carried out in nitrogen at various temperatures to form stoichiometric SiC layers. The implanted layers were characterized by various techniques. Rutherford backscattering spectrometry analysis revealed the carbon distribution and formation of the SiC layers. Fourier transform infrared spectroscopy spectra showed absorption peaks produced by the amorphous and β-phase crystalline SiC layers. The crystallinity was increased by high temperature annealing. The dependence of the surface morphology and dynamics of the crystallization on annealing temperature and time was studied by atomic force microscopy (AFM) for the first time. At about 900 °C, nanocrystalline SiC was formed on the sample surface and contained columnar grains with a full width of half maximum of tens of nanometers and a height of 10 nm. A conducting AFM was used to study the electrical properties of the formed SiC layer on the nanometer scale.

Synthesis of epitaxial 3C-SiC by C 60 carbonization of silicon on sapphire

C 60 molecule deposition onto commercial SOS (silicon on sapphire) wafers with (001) silicon on r-plane (11̄02) sapphire results in the formation of silicon carbide (SiC), if substrate temperatures of 810 °C are exceeded. Elastic recoil spectroscopy indicates the formation of a stoichiometric SiC layer. X-ray pole figure measurements show that the main part of the resulting cubic 3C-SiC grows epitaxially with the underlaying silicon. Cross-sectional transmission electron microscopic investigations indicate as well that the 3C-SiC exhibits an epitaxial relationship to the Si matrix and that the resulting SiC grains grow columnarly.

Formation of the interface between c-Si(111) and diamond-like carbon studied with photoelectron spectroscopy

The hydrogen-terminated Si(111) surface was used as a substrate to grow amorphous hydrogenated carbon (a-C:H) films by means of layer-by-layer plasma deposition. Electronic and structural properties of the forming interface were determined by in situ core level and valence band photoelectron spectroscopy. From the core level spectra the gradual formation of an initially carbon-rich and eventually stoichiometric silicon carbide interface layer is inferred with a final thickness of 18Å. The silicon core level data indicate that the substrate is hydrogen-passivated owing to the plasma exposure. On the final heterostructure, the valence band edge of the diamond-like carbon is found to be 0.4 eV below that of the interfacial a-SiC:H and also below that of the silicon, with the offset amounting to 0.7 ± 0.1 eV.

A swelling model for stoichiometric SiC at temperatures below 1000°C under neutron irradiation

A simple phenomenological model for the saturation swelling below 1000°C of neutron-irradiated silicon carbide (SiC) is presented in this paper. Under fast neutron irradiation, SiC is known to undergo volumetric expansion (swelling) which quickly saturates at a fast fluence of approximately 10 25 n/m 2 for irradiation temperatures below 1000°C. A previous model due to Balarin attributes swelling to lattice dilation as a result of single point defects. We show in this paper that the experimentally observed linear temperature dependence of saturation swelling can be explained in terms of the formation and growth of small interstitial clusters, resulting directly from collision cascades initiated by energetic neutrons. These loops grow by absorption of mobile carbon interstitials and their composition is subject to stoichiometry constraints, requiring absorption of slower silicon interstitials. Because of cascade re-solution events, the density of loops decreases sharply with temperature as a result of overlap of cascades with larger size loops at higher temperatures. The average radius of these loops increases with temperature. Volumetric swelling is shown to obey a linear temperature dependence as a consequence of the strong decrease in density and the simultaneous increase in average radius, and to saturate with fluence. The model is shown to be consistent with experimental observations. In the temperature range below 500–600°C, swelling seems to be dominated by single point defects, or defect clusters containing only a few atoms, in accordance with the explanation offered by Balarin.