The wetting of silicon carbide surfaces

Graphite and silicon carbide (SiC) are important materials of fuel elements in High Temperature Reactor-Pebble-bed Modules (HTR-PM) and it is essential to analyze the source term about the radioactive products adsorbed on graphite and SiC surface in HTR-PM. In this article, the adsorption behaviors of activation product Cobalt (Co) on graphite and SiC surface have been studied with the first-principle calculation, including the adsorption energy, charge density difference, density of states, and adsorption ratios. It shows that the adsorption behaviors of Co on graphite and SiC both belong to chemisorption, with an adsorption energy 2.971 eV located at the Hollow site and 6.677 eV located at the hcp-Hollow site, respectively. Combining the charge density difference and density of states, it indicates that the interaction of Co-SiC system is stronger than Co-graphite system. Furthermore, the variation of adsorption ratios of Co on different substrate is obtained by a model of grand canonical ensemble, and it is found that when the temperature is close to 650 K and 1700 K for graphite surface and SiC surface, respectively, the Co adatom on the substrate will desorb dramatically. These results show that SiC layer in fuel element could obstruct the diffusion of Co effectively in normal and accidental operation conditions, but the graphite may become a carrier of Co radioactivity nuclide in the primary circuit of HTR-PM.

Silicon carbide (SiC) is a promising material for power electronic devices; however, the fabrication of low-cost and high-quality single-crystal SiC wafers with a smooth surface remains difficult, due to its high mechanical hardness and chemical stability. We report an abrasive-free electrochemical mechanical polishing (AF-ECMP) for a SiC surface using Ce film deposited on a polishing pad. The SiC surface is oxidized by electrolysis between the SiC (anode) and the polishing plate (cathode), the resulting oxide is then removed by interaction with the Ce film. The excess oxide produces a rough surface during continuous electrolysis on the SiC surface. The periodic electrolysis improves the surface quality, and electrolysis conditions such as duty ratio, duty cycle, and applied voltage are optimized. The abrasive ECMP, using CeO2 particles, broadens the initial scratches on SiC, whereas no scratch broadening is observed by the AF-ECMP. Despite a lower material removal rate (MRR) than the abrasive ECMP, the surface roughness was improved by the AF-ECMP with less polishing time. AF-ECMP produces an atomically smooth SiC surface with a roughness of ∼0.5 nm Ra, which is promising for low-cost and environment-friendly finishing process for SiC.

Aluminum nitrate (Al(NO3)3) and sodium polystyrene sulfonate (PSS) were used to improve the dispersion of fine silicon carbide (SiC) powders. Effects of modification parameters on the viscosity of modified SiC slurry were studied by orthogonal experiments. Modified SiC slurry with the solid loading of 50 vol% reached the lowest viscosity of 34 MPa s. The adsorption processes of PSS on the as‐received and Al(NO3)3 premodified SiC surface were investigated. The results indicated that the adsorption between PSS and premodified SiC surfaces was a high affinity type and was mainly controlled by active sites on an SiC surface. The Langmuir model and the pseudo‐second‐order model could better fit the adsorption isotherm and kinetics data, respectively. The contact angle decreased from 32.8 to 15.2° and the wettability was improved by modification. The isoelectric point of modified SiC powder shifted to the acidic region and the maximum zeta potential was obtained at pH 11. Sedimentation results also showed that a stable dispersed suspension of modified SiC was achieved at pH 11. Density–pressure curves demonstrated that the flowability and formability of SiC powder were improved by modification. The dispersion effect of PSS on SiC and Al2O3 composite powder was verified by viscosity and sedimentation results.

As a new post-implantation activation annealing of Silicon Carbide (SiC), we propose the Si-vapor ambient anneal using Tantalum Carbide / metal Tantalum composite materials (TaC/Ta). In this technique, semi-closed TaC/Ta container which can supply Si-vapor ambient is used, and Si vapor compensates thermal desorption Si atoms from the SiC surface above 1500°C and can maintain the original surface morphology by controlling a process temperature and Ar back pressure. Therefore the Si-vapor ambient anneal is able to simplify the process of conventional activation anneal methods using refractory cap-layers for protecting SiC surface from thermal damage of Si-atom desorption. Experiments were performed under Ar 1.3kPa at 1600/1700°C for 5min optimized conditions in a 6inch TaC/Ta container, and the Al+ ion-implanted 4H-SiC properties after annealing were characterized by atomic force microscopy (AFM), Rutherford Back-scattering Spectrometry (RBS) channeling method, and four-point probe method. According to evaluation, there was no roughening of SiC surface from AFM topographic images and recovery of crystallinity at the ion-implanted layer was equivalent to by the conventional cap-layer method from RBS channeling measurement. The sheet resistance of 12kΩ/ at 1700°C equal to the typical Al+ ion implanted p-type SiC is confirmed by four-point probe method.

Silicon carbide obtaining with DC arc-discharge plasma: synthesis, product characterization and purification

With the increased demands of environmental protection, recycling/utilization of industrial byproducts has attracted much attention from both industry and academic communities. In this work, silicon carbide (SiC) was successfully synthesized from industrial waste silica fume (SF) during metallic silicon production. Following this, Ni nanoparticles with many defects were supported on the as-obtained SiC by conventional impregnation method. The results showed that defect-rich Ni nanoparticles were dispersed onto the surface of SiC. The as-obtained Ni/SF-SiC exhibited an enhanced metal-support interaction between Ni and SiC. Furthermore, the density functional theory (DFT) calculations showed that the H2 and CO adsorption energy on Ni vacancy (VNi) sites of Ni/SF-SiC were 1.84 and 4.88 eV, respectively. Finally, the Ni/SF-SiC performed high catalytic activity with CO conversion of 99.1% and CH4 selectivity of 85.7% at 350 °C, 0.1 MPa and a gas hourly space velocity (GHSV) of 18,000 mL·g−1·h−1. Moreover, Ni/SF-SiC processed good catalytic stability in the 50 h continuous reaction.

An investigation was conducted to examine the friction properties and mechanical behavior of single-crystal silicon carbide (0001) surface sliding against itself and against polycrystalline titanium. The results indicate hexagon-shaped pits of silicon carbide and the formation of platelet hexagon-shaped wear debris of silicon carbide due to cleavages of both prismatic and basal planes as a result of silicon carbide sliding against itself. The fracturing of silicon carbide also occurs near the adhesive bond to titanium. The wear debris produced by brittle fracture plows the titanium and transfers to it. Further, the silicon carbide wear debris, which adhered and transferred to titanium, plows the silicon carbide surface and transfers back to it.

Fabrication of grating structures on surfaces of wide-bandgap semiconductors, namely silicon carbide (SiC) and gallium nitride (GaN), was achieved using a femtosecond laser and a phase mask. The phase mask was used to produce stable interference patterns from the focused femtosecond laser to form the grating struc- tures on the bulk materials. The effects of the irradiation power and time on the Bragg grating morphology that was formed on the SiC surface were studied. By optimizing the fabrication parameters, we successfully pro- duced grating structures with uniform periods of 1.07 μm on SiC and GaN. The threshold powers necessary for grating structure formation on wide-bandgap semiconductors were investigated. It was found that the thresh- old powers for SiC and GaN were much smaller than those for silica glass. The reason for this difference is that the absorption of the incident laser light in SiC and GaN is a lower-order nonlinear absorption process compared to that in silica glass. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.OE.54.12.126106)

Silicon carbide surfaces were heated to 1500 C in a vacuum and analyzed at room temperature with X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The basic unit of the surfaces was considered as a plane of a tetrahedron of either SiC4 and CSi4 composition. AES spectra were obtained from 250-1500 C, with an analysis depth of 1 nm revealed the presence of little Si and mostly graphite. XPS analysis depth was 2 nm or less, and Si was found in the second 1 nm. Sliding friction tests with single-crystal silicon carbide in contact with iron in a vacuum were characterized by a stock-slip value. The coefficient of friction increased with increasing temperature up to 400 C, then decreased with increasing temperature from 400-600 C. Reheating surfaces to 800 C after preheating them to that temperature produced no changes in AES readings. It is concluded that the maximum density of silicon and silicon-carbide is at 800 C, and the higher the sliding temperature, the more metal that is transferred.

The controlled creation of defects in silicon carbide represents a major challenge. A well-known and efficient tool for defect creation in dielectric materials is the irradiation with swift (E(kin) ≥ 500 keV/amu) heavy ions, which deposit a significant amount of their kinetic energy into the electronic system. However, in the case of silicon carbide, a significant defect creation by individual ions could hitherto not be achieved. Here we present experimental evidence that silicon carbide surfaces can be modified by individual swift heavy ions with an energy well below the proposed threshold if the irradiation takes place under oblique angles. Depending on the angle of incidence, these grooves can span several hundreds of nanometres. We show that our experimental data are fully compatible with the assumption that each ion induces the sublimation of silicon atoms along its trajectory, resulting in narrow graphitic grooves in the silicon carbide matrix.

A sheath-core shaped ZrO2-SiC/SiO2 fiber felt with continuously distributed SiC for broad-band electromagnetic absorption

Bubble characterization and bubble population on SiC material surface in subcooled boiling flow

Slurryless electrochemical mechanical polishing (ECMP) is very effective in the polishing of silicon carbide (SiC) wafers. To achieve a high material removal rate (MRR) of SiC wafer using ECMP with low electrical energy loss, charge utilization efficiency in the anodic oxidation of the SiC surface was investigated and the underlying mechanism was clarified by modeling the anodic oxidation system of SiC in 1 wt% NaCl aqueous solution. The charge utilization efficiency in the anodic oxidation of SiC was found to be constant when the current density was less than 20 mA cm−2 and significantly decreased when the current density was greater than 30 mA cm−2, resulting in a significant reduction in the MRR. Modeling of the anodic oxidation system indicates that the charge utilization efficiency depended on the potential applied on the SiC surface: the oxidation of SiC occupied the dominant position in the anodizing system when the potential is lower than 25 V vs Ag∣AgCl, charge utilization efficiency greatly decreased when the applied potential was greater than 25 V owing to the occurrence of oxidations of the H2O and Cl−. This research provides both a theoretical and practical foundation for using ECMP to polish SiC wafers.

Key to the safe operation of nuclear reactors is the understanding of materials degradation due to neutron damage. Ion implantation is often used as a surrogate for nutron damage when screening nuclear candidate materials. Ion implantation results in a thin damage layer, the mechanical properties of which are often difficult to determine. In this study a micromechanical test regime is developed in a model material, 6H single crystal silicon carbide (SiC). This test technique is then applied to gold ion irradiated zirconium nitride (ZrN). Micromechanical test samples are often prepared using a focused ion beam. However, ion beam milling has the potential to damage the crystal structure of a material and introduce residual stress. Therefore, a range of cutting strategies were used to assess the effects of focused ion beam cutting on the modulus and strength of SiC cantilevers. The effects of sample size were also explored. Gallium ion milling resulted in amorphisation of the surface of the SiC crystal micro cantilevers. The thickness of the amorphous zone was then reduced using low voltage cleaning. Low voltage cleaning did not, however, result in increased mechanical performance as other unintended consequences such as cantilever edge rounding occurred. SiC exhibited a plastic deformation threshold of around 0.3 × 0.3 µm but did not exhibit a significant size effect. Nanoindentation was used as a benchmark test to compare to the mechanical properties gathered during micro bend testing. Under indentation conditions, a size effect was identified in hardness and modulus but not in fracture toughness. Modulus results from indentation, and micro bend testing was comparable when ion damage was accounted for.Hot pressed ZrN samples were ion implanted with gold ions. Microstructural characterisation, nanoindentation and micromechanical tests were performed in the ion implanted zone. Microstructural characterisation identified a dual phase microstructure consisting of ZrN and Zr2ON2. The implanted layer consisted of implanted gold ions followed by a network of dislocations centred around a depth of 1.20 µm. High-resolution electron backscatter diffraction (HR-EBSD) identified that tensile surface stresses and compressive subsurface stress had been introduced. Nanoindentation linked ion implantation to increased hardness and no modification in modulus. Micromechanical testing indicated a reduction in modulus and strength. This work highlighted the need to understand sample size effect and ion damage on micro mechanical tests if they are to be used for screening nuclear materials.

An investigation was made of laser-assisted (by radiation from a copper vapour laser) etching of polycrystalline silicon carbide in air and in liquid media (water, dimethyl sulfoxide). The maximum etching rate of ceramic SiC in air reached 0.24 μm pulse-1 and in dimethyl sulfoxide it was 0.07 μm pulse-1 when the energy density was 16 J cm-2 in both cases. Scanning electron microscopy and x-ray structure analysis were used to study changes in the morphology and chemical composition of the etched surface of SiC. Etching in air produced partly amorphised silicon carbide and microcrystals of free Si with a characteristic size of about 30 nm. The surface of silicon carbide subjected to laser-assisted etching was capable of chemical reduction of copper from a solution used for chemical metallisation. Copper coatings formed in this way adhered strongly to the surface (adhesion force up to 30 N mm-2).