Chemical Vapor Deposited Silicon Carbide Research Articles

Analysis of gas-cooled micro modular reactor (MMR) fuel

The gas-cooled micro modular reactor (MMR) is a novel concept with a compact fuel form consisting of thousands of tri-isotropic fuel (TRISO) particles dispersed in a silicon carbide matrix. The MMR fuel has a high volume packing fraction and is horizontally placed within graphite channels. This study demonstrates that the horizontal arrangement of MMR fuel within graphite channels resulted in a non-uniform gap between the fuel and graphite brick, leading to an "off-center" temperature distribution within the fuel, and resulting in differences in TRISO performance even within sub-zones positioned symmetrically in the radial direction. Given the complexity of the fuel structure and the challenges involved in simulation, a multi-scale and multi-physics coupled analysis method of MMR fuel was realized in this work by implementing physical models into the open-source finite element platform MOOSE. Thermal-mechanical analysis and failure analysis within sub-zones of the typical design MMR fuel during normal operation were conducted through simulation. The results indicate that higher-temperature sub-zones exhibit increased internal pressure within TRISO and show greater tensile stress in the Chemical Vapor Deposited Silicon Carbide (CVD-SiC) layer. Regarding fuel failure analysis, this work found limited impact from Pd penetration and kernel migration. Pressure-induced particle failure, closely associated with stress in CVD-SiC, emerges as the primary failure criterion. The probability of fuel failure is relatively low, and interfacial cohesive parameters have an impact on the SiC stress and the fuel failure probability at the highest level.

Damage characteristics and material removal mechanisms of CVD-SiC in nanotests: A comparative with sintered SiC

High sensitivity to crack and damage of chemical vapor-deposited silicon carbide (CVD- SiC) restricts the high-efficiency and low-damage manufacturing of CVD-SiC components. This paper aims to reveal the damage and material removal mechanisms of CVD-SiC after being scratched by diamond abrasives. The mechanical properties, scratching force, depth of ductile-brittle transition, residual depth and material removal damages of the CVD-SiC were discussed by the indentation, nano-scratching experiments, and compared with sintered silicon carbide (SSiC) ceramics. The results demonstrated that the mechanical properties of CVD-SiC are better than SSiC. In terms of hardness and fracture toughness, CVD-SiC demonstrates a 10– 20% and 20– 30% higher level of performance than SSiC, respectively. SSiC exhibits a brittleness level approximately 30% higher than that of CVD-SiC. SSiC ceramics are prone to crack deflection, intergranular fracture and transgranular fracture during indentation, indicating a higher propensity for brittle fracture in SSiC. The crack propagation in CVD-SiC is relatively regular. Both materials undergo the ductile, ductile-brittle transition and brittle stage, and the fluctuation variability of CVD-SiC material is notably lower than that of SSiC during the brittle stage. Compared with SSiC, the residual depth, groove width and transverse force of CVD-SiC are smaller under the same normal load. The critical depth of ductile-brittle transition of CVD-SiC is about 2 times that of SSiC in experimental and theoretical, indicating that CVD-SiC easily achieves ductile grinding. The mechanical properties, crack system and crystal structure of the CVD-SiC and SSiC materials cause the divergences in brittle removal. The radial-median crack system dominates the material removal of CVD-SiC. The disparity of deformations on the inside and outside of the crystal in the brittle stage leads to significant fluctuation variability of force and depth of SSiC during the scratching.

Optical polishing and characterization of chemical vapour deposited silicon carbide mirrors for space applications

Optical polishing and characterization of chemical vapour deposited silicon carbide mirrors for space applications

Lowest Concentration of Chlorine Trifluoride Gas for Cleaning Silicon Carbide Chemical Vapor Deposition Reactor

A dilute chlorine trifluoride gas at less than 1% was possible for the cleaning of a silicon carbide chemical vapor deposition (CVD) reactor. For 20 min, the chlorine trifluoride gas at the concentrations of 0.5%–1% in ambient nitrogen at atmospheric pressure could detach the 30 μm-thick particle-type polycrystalline silicon carbide CVD film from the susceptor which had a coating film made of a purified pyrolytic carbon (PPyC). While the PPyC film had some damage due to the shallow fluorine diffusion, it could be recovered without any pit formation by annealing in ambient nitrogen containing a trace amount of oxygen at atmospheric pressure for 10 min at the temperature of 845 °C.

Surface Kinetic Mechanisms of Epitaxial Chemical Vapour Deposition of 4H Silicon Carbide Growth by Methyltrichlorosilane-H2 Gaseous System.

The chemical vapour deposition (CVD) technique could be used to fabricate a silicon carbide (SiC) epitaxial layer. Methyltrichlorosilane (CH3SiCl3, MTS) is widely used as a precursor for CVD of SiC with a wide range of allowable deposition temperatures. Typically, an appropriate model for the CVD process involves kinetic mechanisms of both gas-phase reactions and surface reactions. Here, we proposed the surface kinetic mechanisms of epitaxial SiC growth for MTS-H2 gaseous system where the MTS employed as the single precursor diluted in H2. The deposition face is assumed to be the Si face with a surface site terminated by an open site or H atom. The kinetic mechanisms for surface reactions proposed in this work for MTS-H2 gaseous system of epitaxial growth of SiC by CVD technique from mechanisms proposed for H-Si-C-Cl system are discussed in detail. Predicted components of surface species and growth rates at different mechanisms are discussed in detail.

Grinding of silicon carbide for optical surface fabrication, Part 1: surface analysis.

This paper presents a study of the grinding of three different grades of silicon carbide (SiC) under the same conditions. Surface topography is analyzed using coherent scanning interferometry and scanning electron microscopy. The study provides a baseline understanding of the process mechanics and targets effective selection of process parameters for grinding SiC optics with near optical level surface roughness, thus reducing the need for post-polishing. Samples are raster and spiral ground on conventional precision machines with metal and copper-resin bonded wheels under rough, medium, and finish grinding conditions. Material microstructure and grinding conditions affect attainable surface roughness. Local surface roughness of less than 3 nm RMS was attained in both chemical vapor deposition (CVD) and chemical vapor composite (CVC) SiC. The tool footprint is suitable for sub-aperture machining of a large freeform optics possibly without the need for surface finish correction by post-polishing. Subsurface damage will be assessed in Part 2 of this paper series.

Experimental and FEM based investigation of the influence of the deposition temperature on the mechanical properties of SiC coatings

Scanning electron microscopy shows that the microstructure, in particular the overall grain size, of chemical vapor deposited silicon carbide coatings depends on the deposition temperature. So far, the influence of the microstructure on the mechanical properties of such coatings is not well described in literature. To investigate the influence of the deposition temperature on the mechanical properties of the coating, nanoindentation is used in this work. Since the measurement results of nanoindentation can be affected by the substrate material, the contribution of the substrate material is taken into account utilizing a finite element model. The model is then employed to generate information about elastic and plastic properties of the coating by inverse simulation. To evaluate the fracture toughness of the coating, the generated material model is used in a cohesive-zone based formulation of the fracture process during indentation at higher loads. The results of this model allow determining the fracture toughness of silicon carbide coatings deposited at different temperatures.

Recovery Method of Pyrolytic Carbon Surface after Exposure to Chlorine Trifluoride Gas

An annealing process was developed for the practical recover of a purified py-rolytic carbon (PPyC) surface from damage, such as color change and peeling, due to fluorination during exposure to chlorine trifluoride gas at high tempera-tures for the cleaning of a silicon carbide chemical vapor deposition reactor. The PPyC surface was annealed at 900°C for 10 min in ambient nitrogen con-taining oxygen at the concentrations of 0.01% - 20%. The rainbow-like colored surface returned to the dark gray color of the original PPyC. Simultaneously, the thin peeled films disappeared. The Raman spectra showed that the PPyC surface chemical bonding returned to that of the original one. The oxygen concentration as low as 0.01% could recover the PPyC surface along with re-ducing the surface pit formation, when the combination of the exposure to chlorine trifluoride gas and the recovery was repeated.

Design of a Silicon Carbide Chemical Vapor Deposition Reactor Cleaning Process Using Chlorine Trifluoride Gas Accounting for Exothermic Reaction Heat

A silicon carbide chemical vapor deposition reactor cleaning process was designed by managing and utilizing the temperature increase due to the exothermic reaction heat produced by the chemical reaction between chlorine trifluoride gas and a particle-type polycrystalline silicon carbide layer. The main issues were (i) the initial susceptor temperature, (ii) the exothermic reaction heat, and (iii) the heat transport from the susceptor surface for reducing the peeling of the susceptor coating film. The important parameter was the initial susceptor temperature for performing the moderate etching of the silicon carbide layer and the local etching at the contact point between the silicon carbide particles and the susceptor surface. The 30-μm-thick particle-type polycrystalline silicon carbide layer could be detached and cleaned in two minutes.

Progresses in Synthesis and Application of SiC Films: From CVD to ALD and from MEMS to NEMS.

A search of the recent literature reveals that there is a continuous growth of scientific publications on the development of chemical vapor deposition (CVD) processes for silicon carbide (SiC) films and their promising applications in micro- and nanoelectromechanical systems (MEMS/NEMS) devices. In recent years, considerable effort has been devoted to deposit high-quality SiC films on large areas enabling the low-cost fabrication methods of MEMS/NEMS sensors. The relatively high temperatures involved in CVD SiC growth are a drawback and studies have been made to develop low-temperature CVD processes. In this respect, atomic layer deposition (ALD), a modified CVD process promising for nanotechnology fabrication techniques, has attracted attention due to the deposition of thin films at low temperatures and additional benefits, such as excellent uniformity, conformability, good reproducibility, large area, and batch capability. This review article focuses on the recent advances in the strategies for the CVD of SiC films, with a special emphasis on low-temperature processes, as well as ALD. In addition, we summarize the applications of CVD SiC films in MEMS/NEMS devices and prospects for advancement of the CVD SiC technology.

Microstructural and ablative properties of graphite nozzles with SiC coating deposited by CVD technique

The development of efficient, reliable and affordable propulsion units is one of the main objectives in the development of aerospace technology. Typically the final cost of the vehicle is deeply affected by this subsystem. In this study, a hybrid combination of the ablative chamber is presented where graphite nozzles coated with chemical vapor deposited Silicon Carbide (CVD-SiC) is submitted to the ablative environment, generated by a DC plasma torch operating at a power of 35 kW and homogeneous heat flow of 0.75 MW m−2. The ablative properties of the samples were evaluated by measuring the weight loss as a function of the exposure time, weight loss of 0.11% after 70 s of exposition due to the formation of SiO gas was observed. The microstructure characteristics of SiC coating before and after ablation tests were carried out by SEM and XRD showing that it goes to scale oxidation to forming of SiO2 (β-quartz) and SiC (β to α) phase transformation. Whereas the ablation mechanism showed to be dependent on the initial coating thickness and exposure time.

Surface Erosion of Plasma-Facing Materials Using an Electrothermal Plasma Source and Ion Beam Micro-Trenches

Erosion characteristics of tungsten-alternative plasma-facing materials (PFMs) were tested under high heat flux conditions in the electrothermal plasma source facility at Oak Ridge National Laboratory. The PFMs of interest are high-purity β-3C chemical vapor deposition silicon carbide (SiC) and the MAX phases Ti3SiC2 and Ti2AlC [MAX = chemical formula Mn+1AXn, where M is an early transition metal (such as Ti or Ta), A is an A-group element (such as Si or Al), and X is carbon or nitrogen]. An erosion analysis method was developed using a combination of focused ion beam microscopy and scanning electron microscopy, carving micro-trench geometries into polished sample surfaces. Samples of SiC, Ti3SiC2, and Ti2AlC were exposed to the electrothermal plasma source alongside tungsten and monocrystalline silicon. Samples were exposed to a Lexan polycarbonate (C16H14O3) electrothermal plasma stream in a He environment, at a specified impact angle, with infrared camera diagnostics. Edge localized mode–relevant heat fluxes of 0.9 to 1 GW/m2 over 1-ms discharges were generated on the target surfaces. Tungsten samples exhibited pronounced melt-layer formation and deformation, with measured molten pits 2 to 10 μm in diameter and melt-layer depths of up to 7 μm deep. Surface erosion rates for Ti3SiC2 and Ti2AlC ranged from 80 to 775 μm/s and 85 to 470 μm/s, respectively. Both MAX phases exhibited extreme surface fracture and material ejection, with damage depths past 4 μm for Ti2AlC and 11 μm for Ti3SiC2. SiC displayed the best performance, in one case surviving 15 consecutive electrothermal plasma exposures with an average erosion rate of about 29 μm/s and no surface fracturing. SiC erosion rates ranged from 23 to 128 μm/s.

High-Temperature Reactor Cleaning Using Chlorine Trifluoride Gas for Silicon Carbide Chemical Vapor Deposition

The high-temperature cleaning of a silicon carbide chemical vapor deposition reactor was developed using chlorine trifluoride gas and purified pyrolytic carbon (PyC) as the susceptor coating material. The purified PyC could be exposed to the chlorine trifluoride gas without causing serious damage at temperatures up to 570°C, which was in the range providing a sufficiently high silicon carbide etching rate. The spontaneous temperature increase at the susceptor surface due to the exothermic reaction heat was moderated by means of adding nitrogen gas. Thus, the particle-type silicon carbide layer could be cleaned while preventing peeling of the purified PyC surface and obtaining a practical etching rate. Additionally, the peeled surface of the purified PyC could be recovered by annealing at 900°C in ambient nitrogen containing oxygen.

ABAQUS analysis of the SiC cladding fuel rod behavior under PWR normal operation conditions

Silicon Carbide (SiC) cladding is anticipated to be superior to zirconium-based cladding with regard to tolerating light water reactor (LWR) accidents, but it is also crucial that the cladding performance under normal operation conditions be well understood. This paper focuses on the modeling of thermomechanical behavior of a two-layer SiC cladding in Pressurized Water Reactor (PWR) fuel rod under normal operation conditions. For this purpose, a set of ABAQUS subroutines has been written to incorporate material properties and behavior models of Chemical Vapor Deposited (CVD) monolithic SiC, Chemical Vapor Infiltration (CVI) SiCf/SiC Ceramic Matrix Composite (CMC) and UO2. Especially, apart from UO2 irradiation behaviors and fuel rod phenomena models (gas pressure, gap heat transfer, etc.) which have been extensively studied, an alternative swelling model of SiC dedicated for transient simulation and a pseudo-ductile behavior model of CVI SiCf/SiC have been implemented. Both models are important for conducting best estimate analysis of the fuel rod in-pile behavior. Then after benchmarking with the BISON fuel performance analysis code, the ABAQUS model is applied to a downscaled PWR fuel rod with two-layer SiC cladding design, considering different cladding thicknesses and covering the conditions of simple constant power levels, power ramps, power operation mode, reactor hot standby and cold shutdown. The effect of swelling differential between CVD SiC and CVI SiCf/SiC is also discussed. With the help of finite element analysis (FEA), the tightly coupled thermomechanical behavior of SiC cladding fuel rod can be well studied.

Quick and Practical Cleaning Process for Silicon Carbide Epitaxial Reactor

In order to develop a quick and practical cleaning process for the silicon carbide chemical vapor deposition reactor, the pyrolytic carbon-coated susceptor was used. The 30-μm-thick silicon carbide film was formed on the susceptor; the film was cleaning by chlorine trifluoride gas at 460 °C for 15 min. The remained fluorine was removed by the annealing at 900 °C in ambient hydrogen. The pyrolytic carbon surface did not suffer from any damage, because the pyrolytic carbon film surface morphology after the cleaning process was the same as that before the silicon carbide film deposition.

Susceptor Coating Materials Applicable for SiC Reactor Cleaning

In order to develop a cleaning process for the silicon carbide chemical vapor deposition reactor, the susceptor coating materials are developed for protecting the susceptor from the etching by chlorine trifluoride gas. The chlorine trifluoride gas does not give a serious damage to the pyrolitic carbon film at the temperatures lower than 480 °C, at which temperature the quick and practical reactor cleaning process is expected to be possible

Incorporation of dopants in epitaxial SiC layers grown with fluorinated CVD chemistry

Fluorinated chemistry in chemical vapor deposition (CVD) of silicon carbide (SiC) with SiF4 as Si precursor has been shown to fully eliminate the formation of silicon clusters in the gas phase, making SiF4 an interesting Si precursor. However, before a fluorinated CVD chemistry can be adopted, the effect of fluorine on the dopant incorporation must be understood since dopant incorporation is of paramount importance in semiconductor manufacturing. Here, the authors present dopant incorporation studies for n-type doping with N using N2 and p-type doping with Al using TMAl in fluorinated CVD of homoepitaxial SiC. The precursors used were SiF4 as Si precursor and the source of F together with CH4 as C precursor. The authors find that it is possible to control the doping in SiC epitaxial layers when using a fluorinated CVD chemistry for both n- and p-type materials using the C/Si ratio as in standard SiC CVD. However, large area doping uniformity seems to be a challenge for a fluorinated CVD chemistry, most likely due to the very strong Si–F and Al–F bonds.

Quick Cleaning Process for Silicon Carbide Chemical Vapor Deposition Reactor

A quick cleaning process was developed for a silicon carbide chemical vapor deposition reactor. For this purpose, the stability of the susceptor coating film made of pyrolytic carbon was evaluated by means of exposing it to 100% chlorine trifluoride gas for 10 min at various temperatures. The original surface morphology of the pyrolytic carbon film was maintained under 480°C. The fluorine atoms incorporated into the pyrolytic carbon film were removed by annealing at 900°C either in ambient hydrogen or in ambient nitrogen. Finally, the 30-μm thick silicon carbide film formed on the pyrolytic carbon was successfully cleaned by the chlorine trifluoride gas either at 400°C for 30 min or at 460°C for 15 min and by additional annealing in ambient nitrogen at 900°C.

Investigating routes toward atomic layer deposition of silicon carbide: Ab initio screening of potential silicon and carbon precursors

Silicon carbide (SiC) is a promising material for electronics due to its hardness, and ability to carry high currents and high operating temperature. SiC films are currently deposited using chemical vapor deposition (CVD) at high temperatures 1500–1600 °C. However, there is a need to deposit SiC-based films on the surface of high aspect ratio features at low temperatures. One of the most precise thin film deposition techniques on high-aspect-ratio surfaces that operates at low temperatures is atomic layer deposition (ALD). However, there are currently no known methods for ALD of SiC. Herein, the authors present a first-principles thermodynamic analysis so as to screen different precursor combinations for SiC thin films. The authors do this by calculating the Gibbs energy ΔG of the reaction using density functional theory and including the effects of pressure and temperature. This theoretical model was validated for existing chemical reactions in CVD of SiC at 1000 °C. The precursors disilane (Si2H6), silane (SiH4), or monochlorosilane (SiH3Cl) with ethyne (C2H2), carbontetrachloride (CCl4), or trichloromethane (CHCl3) were predicted to be the most promising for ALD of SiC at 400 °C.

Novel orthogonal velocity polishing tool and its material removal characteristics from CVD SiC mirror surfaces.

A new and patented polishing tool called Orthogonal Velocity field Tool (OVT) was built and its material removal characteristics from Chemical Vapor Deposition Silicon Carbide (CVD SiC) mirror surfaces were investigated in this study. The velocity field of OVT is produced by rotating the bicycle type tool in the two orthogonal axes, and this concept is capable of producing a material removal foot print of pseudo Gaussian shapes. First for the OVT characterization, we derived a theoretical material removal model using distributions of pressure exerted onto the workpiece surface, relative speed between the tool and workpiece surface, and dwell time inside the tool- workpiece contact area. Second, using two flat CVD SiC mirrors that are 150 mm in diameter, we ran material removal experiments over machine run parameter ranging from 12.901 to 25.867 psi in pressure, from 0.086 m/sec to 0.147 m/sec tool in the relative speed, and 5 to 15 sec in dwell time. Material removal coefficients are obtained by using the in-house developed data analysis program. The resulting material removal coefficient varies from 3.35 to 9.46 um/psi hour m/sec with a mean value of 5.90 ± 1.26(standard deviation). We describe the technical details of the new OVT machine, the data analysis program, the experiments, and the results together with the implications to the future development of the machine.