Synthesis of graphene on silicon carbide substrates at low temperature

A method for the synthesis of millimeter-scaled graphene films on silicon carbide substrates at low temperatures (750 °C) is presented herein. Ni thin films were coated on a silicon carbide substrate and used to extract the substrate’s carbon atoms under rapid heating. During the cooling stage, the carbon atoms precipitated on the free surface of the Ni and formed single-layer or few-layer graphene. The result shows that the number of graphene layers might be further controlled by appropriate process conditions. In contrast to the epitaxial graphene synthesis on single crystal silicon carbide, the graphene prepared here are continuous over the entire Ni-coated area, and can be stripped from the substrate much more easily for further characterization. The large-scaled, low temperature and transferable features of our method suggest the potential for future graphene-based applications.     

Spark plasma sintering of graphene reinforced silicon carbide ceramics

Silicon carbide (SiC) ceramics have superior properties in terms of wear, corrosion, oxidation, thermal shock resistance and high temperature mechanical behavior, as well. However, they can be sintered with difficulties and have poor fracture toughness, which hinder their widespread industrial applications. In this work, SiC-based ceramics mixed with 1 wt% and 3 wt% multilayer graphene (MLG), respectively, were fabricated by solid-state spark plasma sintering (SPS) at different temperatures. We report the processing of MLG/SiC composites, study their microstructure and mechanical properties and demonstrate the influence of MLG loading on the microstructure of sintered bodies. It was found that MLG improved the mechanical properties of SiC-based composites due to formation of special microstructure. Some toughening mechanism due to MLG pull-out and crack bridging of particles was also observed. Addition of 3 wt% MLG to SiC matrix increased the Vickers hardness and Young's modulus of composite, even at a sintering temperature of 1700 °C. Furthermore, the fracture toughness increased by 20% for the 1 wt% MLG-containing composite as compared to the monolithic SiC selected for reference material. We demonstrated that the evolved 4H-SiC grains, as well as the strong interactions among the grains in the porous free matrices played an important role in the mechanical properties of sintered composite ceramics.     

Densification of liquid phase sintered silicon carbide

The densification and phase formation of liquid phase sintered silicon carbide (LPSSiC) with 10 wt.% additives were investigated. The ratio of the Al₂O₃/Y₂O₃-additives was changed between 4:1 and 1:2. Densification was carried out by hot pressing and gas pressure sintering. The different densification routes result in different major grain boundary phases—aluminates in gas pressure sintered materials and silicates in hot pressed samples. Thermodynamic calculations were carried out to determine the amount of liquid phase during densification and for the interpretation of the results.     

Effect of self-developed graphene lubricant on tribological behaviour of silicon carbide/silicon nitride interface

The research presented in this paper aims to investigate the effectiveness of different surface roughness and lubrication conditions on the interfacial tribological properties between silicon carbide (SiC) and silicon nitride (Si₃N₄) ceramics, particularly for providing insight into the mechanisms of how graphene reduces the friction and wear rate. The worn groove topography and surface composition were characterised in detail with 3D laser measuring microscopy and X-ray photoelectron spectroscopy. The tribological test results on the UMT-TriboLab show that a smooth initial surface is more likely to obtain a low friction coefficient and wear rate under water lubrication. The proper initial surface roughness for SiC and Si₃N₄ ceramics is approximately Ra 10?nm, and it will be lower in an alcohol or graphene aqueous solution. A large load does not worsen the tribological behaviour of a Si₃N₄ ball sliding against a SiC disk, and it reduces the friction coefficient and wear rate. Among the five lubrication states of dry friction, dry graphene lubrication, water lubrication, graphene solution lubrication, and self-developed graphene lubrication, the self-developed graphene lubricant can exhibit an ultra-low friction coefficient of 0.009 and ultra-low wear rate of 1.69?×?10^?7?mm³/N·m. The excellent tribological property of the graphene-coated ceramic surface helps the prepared lubricant to decrease the friction coefficient effectively. Furthermore, the graphene film can protect the SiC from being oxidised by water under the tribo-activated action, and therefore, lead to ultra-low wear rate under low friction condition. Alcohol improves the tribological property of the self-developed graphene lubricant, mainly because of the good wettability between graphene and ethanol. The self-developed graphene lubricant can be applied in water-lubricated ceramic bearings and motorised precision spindles.     

Thermal conductivity of liquid phase sintered silicon carbide

This study investigates the thermal diffusivity and conductivity of YAG/AlN-alloyed LPS-SiC as a function of composition and temperature using the laser-flash technique. Maxwell's model for the thermal conductivity of composites with spherical inclusions is adapted to typical features of the LPS-microstructure and its predictions are compared with experimental data. The results indicate that the thermal conductivity of LPS-SiC is controlled by (i) concentration of impurity atoms in the SiC-phase, (ii) fraction of bulk oxide phase and (iii) amorphous interphases which act as thermal resistance barriers.     

Micro-segregations in liquid phase sintered silicon carbide ceramics

The densification behaviour of LPSSiC ceramics with different amount of secondary phases was investigated during Field Assisted Sintering (FAST). In the densified materials micro-segregations were found with dimensions of several 100 μm. Sometimes such segregations were found in gas pressure sintered materials. The investigation of the state of crystallisation by EBSD and XRD revealed that these micro-segregations are connected with the formation of large YAG (Yttrium aluminium garnet) crystals. The mobility of yttrium in the grain boundaries was investigated by measuring concentration profiles in diffusion couples. The high diffusion coefficient determined at 1850 °C (10^?6 cm/s) indicates that the observed segregations are caused by the crystallisation kinetics of the secondary phases during cooling.     

Exceptional micromachining performance of silicon carbide ceramics by adding graphene nanoplatelets

The electrical discharge machining (EDM) performance of silicon carbide (SiC) ceramics containing graphene nanoplatelets (GNPs) is investigated for the first time. Under fine machining conditions, the material removal rate (MRR) dramatically increases up to 186% when 20 vol.% of GNPs are added to SiC ceramics, leading to reductions on the electrode wear rate of 132%. The EDMed nanocomposites exhibit surface roughness ≤ 0.8 μm. This outstanding EDM response of the graphene nanocomposites as compared to monolithic SiC is explained by their enhanced transport properties, establishing a direct dependence of MRR with the electrical conductivity. EDM performance of the nanocomposites also depends on the testing direction for materials with low GNPs connectivity (≤ 10 vol.%). Melting/evaporation are the main removal mechanisms, thermal spalling also operating for low thermal conducting materials. The employ of EDM on SiC/graphene nanocomposites allows machining microparts with a fine dimensional precision, opening new opportunities for SiC-based microcomponents.     

Sintering behaviour of silicon carbide matrix composites reinforced with multilayer graphene

The scope of this paper includes preparation and characterisation of dense silicon carbide matrix composites reinforced with multilayer graphene (MLG). Application of graphene as a reinforcement phase should simultaneously improve mechanical properties of SiC matrix composites and act as one of the sintering activators. In the present work the mechanical properties and the microstructure changes of samples sintered with different additions of graphene (0.5, 1, 2, 3, 4 wt%) and boron (0.3, 1 and 2 wt%) were examined. The composites were consolidated at two different temperatures (1800 °C and 1900 °C) using the Spark Plasma Sintering method (SPS). Reference samples with the addition of graphite as a source of carbon (1 and 3 wt%) were also sintered in the same conditions. The abovementioned amounts of graphite are an optimal content which is essential to obtain high density of samples [1], [2], [3], [4], [5], [6], [7], [8], [9]. The influence of MLG on density, mechanical properties and phase structure of the sintered samples were investigated. A high rate of densification for the composites with 0.3 wt% of B and 1 wt% of MLG sintered at 1900 °C was observed. Moreover, these composites showed the highest average of microhardness (2663 HV0.5) and single-phase structure.     

Heteroepitaxial nucleation and growth of graphene nanowalls on silicon

Heteroepitaxial nucleation of {0 0 2} graphene sheets on {1 1 1} facets of plasma treated (1 0 0) silicon by direct-current plasma enhanced chemical vapor deposition in methane–hydrogen gas mixtures is confirmed by high-resolution transmission electron microscopy. Lattice mismatch by 12% is compensated by tilting the graphene {0 0 2} with respect to silicon {1 1 1} and matching the silicon lattice with fewer graphene layers. The interlayer spacing of graphene sheets near the silicon surface is 0.355 nm, which is larger than that of AB stacked graphite and confirmed as AA stacked graphitic phase. Subsequent growth of standing graphene nanowalls is characterized by scanning electron microscopy and Raman scattering (633 and 514 nm excitation). The Raman peaks of D-band, G-band, and 2D-band are discussed in correlation with SEM images of graphene nanowalls. A strong Raman peak corresponding to silicon–hydrogen stretch vibration is detected by 633 nm excitation at the early stage of graphene nucleation, indicating the silicon substrate etched by hydrogen plasma. With these analyses, the growth mechanism is also proposed in this paper.     

Liquid phase sintered silicon carbide (LPS-SiC) ceramics having remarkably high oxidation resistance in wet air

Numerous studies on the oxidation characteristics of common silicon carbide ceramics have revealed very good oxidation resistance in dry atmospheres, provided that the oxygen partial pressure is sufficiently high. On the contrary, as other SiO₂-passivated ceramics, SiC shows poor oxidation resistance in moist atmospheres. In this contribution, the oxidation behaviour at 1400 °C of fully dense SiC samples with Lu₂O₃–AlN and Lu₂O₃–Ho₂O₃ sintering additives is investigated. Similar to results recently reported for a novel Si₃N₄ material, it is demonstrated that Lu₂O₃ yields greatly improved oxidation behaviour as compared to other liquid phase sintered SiC materials. In particular, the Lu₂O₃–Ho₂O₃ additive leads to passive oxidation under a 0.01 MPa partial pressure of water.     

Pressureless sintering of multilayer graphene reinforced silicon carbide ceramics for mechanical seals

ABSTRACT

The silicon carbide (SiC) ceramics containing multilayer graphene derived from graphite exfoliation were successfully prepared by pressureless sintering, and the effect of graphene content on the sintering behaviours, microstructure, mechanical, tribological, electrical and thermal properties was investigated in detail. The bulk density, bending strength and hardness of the composite ceramics gradually decrease with the increase of graphene content, but the friction, conductance and thermal conductance properties are improved obviously. When the graphene content reaches 5?wt-%, the dry friction coefficient of 0.22, electrical conductivity of 2724.14 S?1?m?1 and thermal conductivity of 8.5?W?(m?1?K?1) can be obtained, indicating good comprehensive mechanical, tribological, electrical and thermal properties. This multilayer graphene reinforced silicon carbide ceramic is a promising seal material instead of SiC seal materials containing graphite to be applied in next-generation mechanical seals.     

Enhanced electromagnetic interference shielding properties of silicon carbide composites with aligned graphene nanoplatelets

Graphene nanoplatelets (GNPs) were successfully incorporated into silicon carbide (SiC) ceramic matrix in a self-aligned pattern and the obtained materials displayed extremely high value of shielding effectiveness (SE) over 40?dB by adding only 3?wt.% GNPs, which was the highest SE value in all SiC-based composites reported in literature up to now. It was found that the texture distribution of GNPs was crucial to achieve the high electromagnetic interference shielding performance of SiC/GNPs composites, which can contribute to the significant improvement of both absorption and reflection. The improved absorption originated from the formation of network of mini capacitors comprised of self-aligned GNPs and multiple reflections. The improvement of reflection was attributed to the high electrical conductivity of the composite due to the introduction of GNPs. These results indicate that SiC/GNPs composites can be used as high-performance ceramic-based EMI shielding materials.     

Friction and wear behaviour of silicon carbide/graphene composites under isooctane lubrication

The tribological performance of silicon carbide (SiC)/graphene nanoplatelets (GNPs) composites is analysed under oscillating sliding tests lubricated with isooctane, looking to explore their potential as components for gasoline direct injection (GDI) engines. High graphene filler contents (20?vol.% of GNPs) are required to substantially reduce the friction coefficient of SiC ceramics, attaining decreases on friction up to 30% independently of the applied load. For all materials and testing conditions a mild wear regime is evidenced. SiC/20?vol.% GNPs composite also enhances the wear resistance up to 35% at low load, but the addition of GNPs produces a deleterious effect as the load augments. The tribological behaviour depends on the formation and destabilization of a solid lubricant carbon-based tribofilm and strongly correlates with the mechanical properties of the tested materials.     

Joining of silicon carbide ceramics using a silicon carbide tape

This paper reports the joining of liquid-phase sintered SiC ceramics using a thin SiC tape with the same composition as base SiC material. The base SiC ceramics were fabricated by hot pressing of submicron SiC powders with 4 wt% Al₂O₃–Y₂O₃–MgO additives. The base SiC ceramics were joined by hot-pressing at 1800-1900°C under a pressure of 10 or 20 MPa in an argon atmosphere. The effects of sintering temperature and pressure were examined carefully in terms of microstructure and strength of the joined samples. The flexural strength of the SiC ceramic which was joined at 1850°C under 20 MPa, was 343 ± 53 MPa, higher than the SiC material (289 ± 53 MPa). The joined SiC ceramics showed no residual stress built up near the joining layer, which was evidenced by indentation cracks with almost the same lengths in four directions.     

Modeling of gas phase nucleation during silicon carbide chemical vapor deposition

An advanced model taking into account silicon cluster formation in the gas phase is suggested. 2D simulation is carried out at the growth parameters typical for chemical vapor deposition (CVD) of SiC in a vertical reactor. It is found that two main parameters have a significant effect on the nucleation and transport of the clusters in the gas phase: the input flow rate of silane and the thermophoretic force. Using a special criterion, the growth conditions favorable for parasitic graphite and silicon phases formation are determined. The obtained results are in good agreement with experimental data.     

In situ growth of silicon carbide nanowires from anthracite surfaces

Silicon carbide nanowires (SCNWs) were grown from anthracite fine surfaces through a simple one-step carbothermal process with silicon powder as the Si precursor. This straightforward and fast formation of SCNWs made it possible to maintain the binding of briquetted waste anthracite fines at very high temperatures as an alternative fuel in foundry cupola furnaces. This SCNW mechanism could thus provide the crucial hot crushing strength in the cupola heat zone and melt zone. Progressive thermal tests exhibited that the formation of the SCNWs started from 1100 ?C, and was favored at 1400 ?C. No extra metal catalyst was needed for the growth of the SCNWs. Characterizations were performed by XRD, SEM, EDS, TEM, and SAED. The SCNWs were 30-60 nm in diameter and were typically grown by stacking the (1 1 1) lattice plane of 3C-SiC along the [1 1 1] direction. Many non-epitaxial branches of the nanowires were also formed through this one-step process as observed by TEM. The results suggest that the SCNWs were most likely grown through the vapor-solid mechanism.     

Properties of liquid phase pressureless sintered silicon carbide obtained without sintering bed

High density pressureless sintered silicon carbide bodies with yttria and alumina as sintering aids were obtained without sintering bed (LPSSC-NB). Sintering behavior of this material was studied between 1850 °C and 1950 °C and it was compared to the liquid phase sintered SiC material obtained using sintering bed (LPSSC-B). Sintered density was 97% of the theoretical density (T.D.) at 1875 °C. Mechanical properties like fracture toughness, hardness, flexural strength were determined and compared to other SiC-based materials. In this manner we were able to demonstrate that silicon carbide could successfully be sintered by means of liquid phase mechanism also without sintering bed. This fact opens liquid phase sintered silicon carbide to a wide range of industrial application.     

Liquid poly(silylacetylene)siloxane resin as a novel precursor of silicon carbide and silicon oxycarbide ceramics

Liquid preceramic poly(silylacetylene)siloxane resin was synthesized via a two-step protocol including organometallic condensation and hydrolysis reactions. The preceramic resin was well soluble in acetone, toluene, and tetrahydrofuran (THF), etc. By thermal cure at 180–250 °C a hard monolithic solid was formed through radical polymerization of secondary ethynyl groups. The poly(silylacetylene)siloxane resin was processed easily to various nonporous shapes to silicon carbide (SiC) and silicon oxycarbide (SiCO). SiCO ceramic was obtained at a yield of >75% by pressureless pyrolysis at 900–1200 °C; while SiC ceramic was obtained at 1500 °C at a yield of ≈67%. The molar ratio of Si/C in the SiC was found at 1:1.1–1:3, based on ICP-MS elemental analysis. X-ray diffraction (XRD) results revealed the typical β-SiC structure in the poly(silylacetylene)siloxane derived SiC ceramics. The SiC ceramics exhibited high thermo-oxidation resistance at elevated temperatures in air atmosphere.     

Effect of grain growth on the thermal conductivity of liquid-phase sintered silicon carbide ceramics

The effect of grain growth on the thermal conductivity of SiC ceramics sintered with 3 vol% equimolar Gd₂O₃-Y₂O₃ was investigated. During prolonged sintering at 2000 °C in an argon or nitrogen atmosphere, the β → α phase transformation, grain growth, and reduction in lattice oxygen content occurs in the ceramics. The effects of these parameters on the thermal conductivity of liquid-phase sintered SiC ceramics were investigated. The results suggest that (1) grain growth achieved by prolonged sintering at 2000 °C accompanies the decrease of lattice oxygen content and the occurrence of the β → α phase transformation; (2) the reduction of lattice oxygen content plays the most important role in enhancing the thermal conductivity; and (3) the thermal conductivity of the SiC ceramic was insensitive to the occurrence of the β → α phase transformation. The highest thermal conductivity obtained was 225 W(m K)⁻¹ after 12 h sintering at 2000 °C under an applied pressure of 40 MPa in argon.