A viable approach to prepare 3C-SiC coatings by thermal MOCVD using commercial grade precursors

A series of 3 C-SiC coatings were prepared by organometallic chemical vapor deposition (MOCVD) using precursor solution containing a varying proportion of commercial-grade hexamethyldisiloxane (HMDSO) and n-hexane. The phase composition, bonding state, and microstructure of 3 C-SiC coatings were studied in detail by grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The microstructure and mechanical properties of the optimal 3 C-SiC coating were characterized by scanning transmission electron microscopy (STEM) and nanoindentation, respectively. Our results revealed that the amount of undesired graphite phase can be significantly reduced in the 3 C-SiC coating by introducing hydrogen gas in the reaction chamber alongside increasing the ratio of HMDSO/n-hexane in the precursor mixture. The STEM results revealed that the optimal coating was predominantly composed of nano-crystalline 3 C-SiC grains alongside a small amount of amorphous graphite. The hardness and elastic modulus of the optimal coating were 38.19 GPa and 363.2 GPa, respectively.     

Facile joining of SiC ceramics with screen-printed polycarbosilane without pressure

Joining of SiC ceramics was successfully achieved at a relatively low temperature of 1500 °C without any pressure using pure polycarbosilane (PCS) as the joining material, which was distributed homogenously on the surface of SiC monolith through a screen printing method. The XRD pattern shows that the pyrolysis product of PCS is single-phase SiC. The interlayer thickness of the SiC joint is approximately 2 μm. This ultra-thin interlayer with lower possibility of the existence of defects contributes to the average shear strength of 105.8 ± 10.4 MPa, higher than that of other works using other preceramic polymers to the best of our knowledge. Due to the simplicity, low cost and high joining strength, the screen printing method using PCS as the joining material has good practicality in SiC ceramics joining.     

Microwave synthesis of worm-like SiC nanowires for thin electromagnetic wave absorbing materials

Thin thickness is always the pursuit of excellent electromagnetic wave absorbing materials. Herein, SiC nanowires with worm-like morphology were synthesized by microwave heating the mixture of expanded graphite and silica. The worm-like SiC nanowires exhibit an excellent microwave absorption ability at a thin thickness. With the filling ratio of SiC nanowires increases in the matrix, the dielectric loss and microwave absorbing ability are significantly enhanced; meanwhile the number of absorption peaks is gradually increased, and the absorption peaks also move toward a thinner thickness. When the nanowires filling ratio was 40?wt%, the minimum reflection loss reached down to ?35.2?dB and the effective absorption (RL?<??10?dB) bandwidth was 1.8?GHz?at a thickness of 1.3?mm. The possible growth mechanism of the worm-like SiC nanowires is that the intermediate reaction gas phases, SiO and CO, were confined in the relatively independent tiny pores of expanded graphite. This resulting in an excessive local gas phase pressure, which causes the nanowire growth direction changes randomly.     

Interconnected SiC–Si network reinforced Al–20Si composites fabricated by high pressure solidification

The microstructures and mechanical properties of the interconnected SiC–Si network reinforced Al–20Si composites solidified under high pressures were investigated. The results demonstrate that the complete interconnected SiC–Si network can be obtained by high pressure solidification, and the connected micron-sized pores are uniformly distributed in the interconnected SiC–Si network. The compressive strength and microhardness of the SiC/Al–20Si composites solidified under 3 GPa were 723 MPa and 229 HV_0.05, respectively. Furthermore, the fracture process of SiC/Al–20Si composites was studied by in situ TEM tensile testing. The result shows that the crack first initiated and propagated at the Al/Si interface under an external load, and the SiC particles in the interconnected SiC–Si network can effectively hinder the crack propagation, thus enhancing the strength.     

Static and dynamic oxidation behaviour of silicon carbide at high temperature

SiC has extensive applications in high-temperature oxidation environments. However, few studies have investigated the differences between the static and dynamic oxidation behaviour. In this study, the static and dynamic oxidation of SiC were investigated in air and in plasma wind tunnels, respectively. The results demonstrated that the activation energy of static oxidation was ～68.02 kJ/mol at 1300–1600 ℃, which was approximately ten times that of dynamic oxidation ～7.05 kJ/mol at 1290–1534 ℃. The observed Si-O-C transition layer located at the SiO₂/SiC interface, and its thickness after dynamic oxidation for 300 s was thicker than that after static oxidation for 30 h. In dynamic oxidation, high-speed flowing atomic oxygen reacted directly with SiC, whereas molecular oxygen needed extra energy to break the OO bond and react with SiC in static oxidation. Atomic oxygen also migrated easier in the amorphous SiO₂ coating, contributing to a thicker Si-O-C layer and lower activation energy.     

A generalized CFD model for evaluating catalytic separation process in structured porous materials

A hybrid multiphase model is developed to simulate the simultaneous momentum, heat and mass transfer and heterogeneous catalyzed reaction in structured catalytic porous materials. The approach relies on the combination of the volume of fluid (VOF) and Eulerian–Eulerian models, and several plug-in field functions. The VOF method is used to capture the gas–liquid interface motion, and the Eulerian–Eulerian framework solves the temperature and chemical species concentration equations for each phase. The self-defined field functions utilize a single-domain approach to overcome convergence difficulty when applying the hybrid multiphase for a multi-domain problem. The method is then applied to investigate selective removal of specific species in multicomponent reactive evaporation process. The results show that the coupling of catalytic reaction and interface species mass transfer at the phase interface is conditional, and the coupling of catalytic reaction and momentum transfer across fluid–porous interface significantly affects the conversion rate of reactants. Based on the numerical results, a strategy is proposed for matching solid catalyst with operating condition in catalytic distillation application.     

Role of oxygen in surface kinetics of SiO2 growth on single crystal SiC at elevated temperatures

Understanding surface kinetics of SiO₂ growth on single crystal SiC at elevated temperatures is crucial to fabricate high-performance SiC-based devices. However, the role of oxygen in the evolution mechanism of SiC surface at atomic scale has not been comprehensively elaborated. Here, we reveal the manipulation effect of oxygen on the competitive growth of thermal oxidation SiO₂ (TO-SiO₂) and thermal chemical vapor deposition SiO₂ (TCVD-SiO₂) on the 4H-SiC substrate at 1500 °C. TO-SiO₂ is formed by the thermal oxidation of SiC, in which the substrate undergoes layer-by-layer oxidation, resulting in an atomically flat SiC/TO-SiO₂ interface. TCVD-SiO₂ growth includes the sublimation of Si atoms, the reaction between sublimated Si atoms and reactive oxygen, and the adsorption of gaseous Si_xO_y species. A relatively high sublimation rate of Si atoms at SiC atomic steps causes the transverse evolution of the nucleation sites, leading to the formation of nonuniform micron-sized pits at the SiC/TCVD-SiO₂ interface. The low oxygen concentration favors TCVD-SiO₂ growth, whose crystal quality is much better than that of TO-SiO₂ due to the high surface mobility in the thermal CVD process. We further achieve the epitaxial growth of graphene on 4H-SiC in an almost oxygen-free reaction atmosphere. Additionally, ReaxFF reactive molecular dynamic simulation results illustrate that the decrease in oxygen concentration can promote the growth kinetics of SiO₂ on single crystal SiC from being dominated by thermal oxidation to being dominated by thermal CVD.     

Tribological performance of UV picosecond laser multi-scale composite textures for C/SiC mechanical seals: Theoretical analysis and experimental verification

The performance of mechanical seals can be improved by using multi-scale composite textures with spiral grooves and micro-dimples, but without a clear texture function mechanism, it is difficult to optimize the textures on the sealing surface. This research established a mathematical model based on the mass-conservative JFO cavitation boundary to analyze the mechanical seal performance of multi-scale composite micro-textures. We use the multi-grid method for numerical solutions, investigate the hydrodynamic lubrication characteristics of composite-textured mechanical seals, and inspect the coupling effect of micro-dimples and spiral grooves. We also analyzed the influence of the geometrical parameters of composite textures on the sealing performance and optimized the sealing surface textures using theoretical analysis. The numerical analysis showed that the multi-scale composite-textured mechanical seal produced a coupling effect between the micro-dimples and spiral grooves, which improved the lubrication of the sealing pair. Eventually, C/SiC mechanical seal bench tests confirmed the tribological improvement of composite textures compared with the un-textured seal under various sealing liquid pressures and rotation speeds. Through comparing the Stribeck curve, the multi-scale composite textured C/SiC mechanical seals have a lower and more stable friction torque than the un-textured.     

Application of the pack cementation process on SiC/SiC ceramic matrix composites

The potentials and limitations of a halide-activated pack cementation process on SiC/SiC Ceramic Matrix Composites for the development of bond coats as part of environmental barrier coating (EBCs) systems were investigated. Different pack compositions using chromium, aluminum and alloys of these elements were tested and the kinetics of coating formation were examined in addition to their microstructure. The results and their analogy to diffusion couples were discussed and it was shown that coating elements which form silicides and carbides are promising candidates for coatings deposited on SiC/SiC via pack cementation. Based on such considerations a two-step pack cementation was proposed, which used chromium, one of the suitable elements, in a first step, to finally achieve an alumina-forming coating. The oxidation resistance of the developed coating was tested via thermogravimetric analysis and compared to the uncoated material. The coating protected the fiber-matrix interface of the SiC/SiC Ceramic Matrix Composites from oxidation.     

Influence of SiC dispersion and Ba(Sr) substitution on the thermoelectric properties of Ca3Co4O9+δ

Ca₃Co₄O_9+δ is a typical p-type thermoelectric oxide material with a low thermal conductivity. In this study, double-layered oxide samples Ca(Ba,Sr)₃Co₄O_9+δ dispersed with different SiC contents were obtained via the traditional solid phase reaction method. The effects of different elemental substitutions and SiC dispersion contents on the microstructure and thermoelectric properties of the samples were studied. The double optimisation of partial substitution of Ca-site atoms and SiC dispersion considerably improved the thermoelectric properties of Ca₃Co₄O_9+δ. Through the elemental substitution, the resistivity of the Ca₃Co₄O_9+δ material was reduced. Conversely, introducing an appropriate amount of SiC nanoparticles enhanced phonon scattering and was crucial in reducing its thermal conductivity. After double optimisations, the dimensionless thermoelectric figure of merit (ZT) values of both Ca_2.93Sr_0.07Co₄O_9+δ + 0.1 wt% SiC and Ca_2.9Ba_0.1Co₄O_9+δ + 0.1 wt% SiC achieved an optimum value of 0.25 at 923 K.