Carbon-rich SiC Research Articles

Synthesis and broadband electromagnetic wave absorption for lightweight porous SiC/Si3N4 composite nanowires

As contemporary aerospace and radar-detecting technologies evolve, hypersonic vehicle components must avoid electromagnetic wave (EMW) detection in extremely harsh environments. In this study, lightweight porous SiC/Si3N4 composite nanowires (SiC/Si3N4 NWs), exhibiting excellent performance of broadband electromagnetic wave absorption (EWA), were successfully prepared via a facile electrostatic spinning and heat treatment in nitrogen atmosphere using phenol-formaldehyde resin (PR) and silica solution as precursors. The nanowire (NW) was composed of carbon-rich SiC and Si3N4 nanograins, and its microstructure and phase composition were regulated by carbon thermal reduction temperature. The EWA performance of the nanowires obtained through carbothermal reduction at 1300 °C, 1400 °C，and 1500 °C was investigated. The nanowires with 15 wt% obtained at 1500 °C were added to a paraffin matrix and the effective absorption bandwidth (EAB) was observed to cover almost the entire C, X, and Ku bands (4.2–18 GHz). Moreover, for a coating thickness of only 2.1 mm, the EAB was as high as 7.12 GHz (10.88–18 GHz), covering the entire Ku-band and achieving a minimum value of reflection loss (RLmin) of - 48 dB at 6.5 GHz. The 3D network structure of SiC/Si3N4 NWs with large specific surface areas and numerous mesopores extended the electromagnetic wave (EMW attenuation route and effectively improved the EWA performance through interfacial synergy between the carbon-rich SiC and Si3N4 nanograins). The strong absorption and wide EAB of the porous SiC/Si3N4 NWs rendered them highly suitable lightweight EMW absorbers in harsh, high-temperature environments.

Effect and mechanism analysis of free carbon on the oxidation behaviour of SiC fibres

Continuous SiC fibres have received widespread attention in the field of aero-engines due to their excellent mechanical properties and oxidation resistance. Free carbon is one of the main factors affecting the oxidation behaviour of SiC fibres, but its role and influence are still unclear. The oxidation behaviour of two types of SiC fibres with similar oxygen contents and crystallite sizes but different carbon-silicon ratios was investigated in this study. Free carbon did not significantly influence strength retention, but positively affected elastic modulus retention. The oxidised carbon-rich SiC fibres exhibited a thinner, smoother, and denser oxide scale. The oxidation rate of the carbon-rich fibres was lower than that of near-stoichiometric fibres after oxidising at 1200 °C. The oxidation activation energies of both were 170.44 ± 15.80 and 234.45 ± 24.94 kJ/mol, respectively. Free carbon can inhibit oxidation scale crystals at the beginning of oxidation while promoting crystallisation scale as oxidation intensifies. The oxidation scale crystals of carbon-rich fibres are mainly controlled by the interface, whereas those of near-stoichiometric fibres are mainly controlled by diffusion. A stress expansion zone was formed at the interface of the oxidised SiC fibres. Free carbon can enhance the internal stress of the stress expansion zone, resulting in lower residual compressive stress on the surface of carbon-rich SiC fibres. These results are significant for improving the oxidation resistance of SiC fibres by retaining free carbon.

In-situ growth and relevant mechanisms of thick SiC nanowhiskers from hybrid silicon sources

SiC nanowhiskers were prepared from photovoltaic silicon waste and quartz sand (as a hybrid silicon source) and coconut fiber (as a carbon source). The phase evolution of the reaction was investigated using X-ray diffraction (XRD) and the microstructure of the product powders was observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the temperature range of 1300–1500 °C, SiC whiskers were grown on the surfaces of the carbonized coconut fibers through vapor-solid (VS) mechanism. Above 1100 °C, straight SiC whiskers were also generated in the residual powders by vapor-liquid-solid (VLS) mechanism. The number of SiC whiskers increased significantly with increased reaction temperature. At the optimum reaction temperature of 1500 °C, thick carbon-rich SiC whiskers with straight and dendrite shape covered the whole surface of the carbonized coconut fiber. We have developed a novel, clean, and sustainable method of preparing SiC nanowhiskers. It provides a new way to recycle valuable resources from photovoltaic waste and enables the high-value utilization of agricultural wastes.

Role of in-situ formed free carbon on electromagnetic absorption properties of polymer-derived SiC ceramics

In order to enhance dielectric properties of polymer-derived SiC ceramics, a novel single-source-precursor was synthesized by the reaction of an allylhydrido polycarbosilane (AHPCS) and divinyl benzene (DVB) to form carbon-rich SiC. As expected, the free carbon contents of resultant SiC ceramics annealed at 1600 °C are significantly enhanced from 6.62 wt% to 44.67 wt%. After annealing at 900–1600 °C, the obtained carbon-rich SiC ceramics undergo phase separation from amorphous to crystalline feature where superfine SiC nanocrystals and turbostratic carbon networks are dispersed in an amorphous SiC(O) matrix. The dielectric properties and electromagnetic (EM) absorption performance of as-synthesized carbon-rich SiC ceramics are significantly improved by increasing the structural order and content of free carbon. For the 1600 °C ceramics mixed with paraffin wax, the minimum reflection coefficient (RCmin) reaches –56.8 dB at 15.2 GHz with the thickness of 1.51 mm and a relatively broad effective bandwidth (the bandwidth of RC values lower than –10 dB) of 4.43 GHz, indicating the excellent EM absorption performance. The carbon-rich SiC ceramics have to be considered as harsh environmental EM absorbers with excellent chemical stability, high temperature, and oxidation and corrosion resistance.

Electrospinning synthesis of SiC/Carbon hybrid nanofibers with satisfactory electromagnetic wave absorption performance

In this paper, silicon carbide/carbon (SiC/C) hybrid nanofibers were synthesized via electrospinning and high-temperature pyrolysis. As-prepared nanofibers possess a high aspect ratio and a scaly surface. The electromagnetic (EM) parameters and EM wave absorption properties of the SiC/C composite nanofiber were studied focusing on the polarization loss mechanism and on the impedance matching in the range from 2 GHz to 18 GHz. The 30 wt% composite nanofibers in the paraffin matrix show satisfactory dielectric loss value and a minimal reflection loss (RL) of approximately −36 dB at 6.8 GHz with a 3 mm thickness. Moreover, the maximum effective absorption (<−10 dB) bandwidth (EAB) is approximately 4.1 GHz (12.6–16.7 GHz) with a 1.5 mm thickness, which could cover most of the Ku-band. The above results show that the prepared carbon-rich SiC nanofiber-filled composite is a promising EM wave absorbing material with excellent EM wave absorbing ability.

Continuous SiCN Fibers with Interfacial SiC xN y Phase as Structural Materials for Electromagnetic Absorbing Applications.

SiCN ceramics are one of the most important electromagnetic wave (EMW) absorbing materials for application in harsh environments, but research studies on optimizing phase distribution in SiCN ceramics for excellent EMW absorbing properties are still lacking. Herein, continuous SiCN fibers with an interfacial SiC xN y phase were prepared through nanochannel diffusion-controlled nitridation of polycarbosilane fibers with an NH3 gas flow. The existence of the interfacial SiC xN y phase distributed between the carbon-rich SiC phase and Si3N4 phase can improve the impedance matching and efficiently promote the production of macroscopic dipole moments in the heterointerfaces of SiC xN y-SiC and SiC xN y-Si3N4 for an enhanced multifarious polarization relaxation loss. The EMW absorption properties can be further improved by optimizing the microstructure with a continuous carbon-rich SiC phase for possessing an excellent conductive loss by converting the EMW energy into current flow. Finally, under the synergy of the interfacial SiC xN y phase and the continuous carbon-rich SiC phase, SiCN fibers can present excellent EMW absorption properties with extremely strong absorption ability (reflection loss of -63.7 dB), ultrathin thickness (1.78 mm), and wide effective absorption bandwidth (4.20 GHz). These obtained SiCN fibers also possess excellent mechanical properties with the tensile strength higher than 2.0 GPa and excellent high-temperature stability up to 1500 °C. This work provides a strategic method for optimizing the microstructure of SiCN ceramics for admirable EMW absorption properties, and the obtained SiCN fibers can be used as reinforcements of ceramic matrix composites for stealth applications under harsh environments.

Nanochannel Diffusion-Controlled Nitridation of Polycarbosilanes for Diversified SiCN Fibers with Interfacial Gradient-SiC xN y Phase and Enhanced High-Temperature Stability.

Diversified SiCN fibers with gradient-SiC xN y phase in the interfacial regions between the major phases of carbon-rich SiC phase and Si3N4 phase were prepared via nanochannel diffusion-controlled nitridation of polycarbosilane fibers under different NH3 flow rates. The obtained fibers with excellent mechanical properties showed a different nanostructure and improved high-temperature behavior compared with polysilazane- and polysilylcarbodiimide-derived SiCN ceramics. The enhanced high-temperature properties could be contributed to the inhibition of carbothermal reduction of the Si3N4 phase by the gradient-SiC xN y phase in the interfacial region between the Si3N4 phase and carbon-rich SiC phase. Meanwhile, a suitable amount of interfacial SiC xN y phase as well as the fine distributed microstructure can be helpful to inhibit the high-temperature crystallization of both the SiC phase and Si3N4 phase. Additionally, a nanostructural model has been proposed to understand the effect of interfacial gradient-SiC xN y phase and compositional-dependent high-temperature behavior of obtained SiCN fibers. Our findings provide a novel strategy to prepare SiCN-based ceramic materials with excellent high-temperature stabilities, which we expect to possess great potential in structural and (multi)functional applications at high temperatures and under harsh environments.

In situ growth of one-dimensional carbon-rich SiC nanowires in porous Sc2Si2O7 ceramics with excellent microwave absorption properties

For enhancing the absorption ability of dielectric and electromagnetic wave (EMW), C-rich SiC NWs /Sc2Si2O7 ceramics are successfully fabricated through in-situ growth of SiC nanowires (NWs) into porous Sc2Si2O7 ceramics by precursor infiltration and pyrolysis (PIP) at 1400 °C in Ar. SiC NWs are in-situ formed in the pore channels via a vapor-liquid-solid (VLS) mechanism, the relative complex permittivity increases notably with the content of absorber (C-rich SiC NWs), which tune the microstructure and dielectric property of C-rich SiC NWs/Sc2Si2O7 ceramics. Meanwhile, the minimum reflection coefficient (RC) of C-rich SiC NWs/Sc2Si2O7 ceramic decreases from −9.5 dB to − 35.5 dB at 11 GHz with a thickness of 2.75 mm, and the effective absorption bandwidth (EAB) covers the whole X band (8.2–12.4 GHz) when the content of absorber is 24.5 wt%. The results indicate that Sc2Si2O7 ceramics decorated with SiC NWs and nanosized carbon have a superior microwave-absorbing ability, which can be contributed to the Debye relaxation, interfacial polarization and conductivity loss enhanced by in-situ formed SiC NWs and nanosized carbon phases. The C-rich SiC NWs /Sc2Si2O7 ceramics can be a promising microwave absorbing materials within a broad bandwidth.

Microstructures, dielectric response and microwave absorption properties of polycarbosilane derived SiC powders

Carbon-rich SiC powders with high dielectric loss were prepared via pyrolysis of polycarbosilane (PCS). The effects of pyrolysis temperature on microstructures, dielectric response and microwave absorption properties in X-band (8.2–12.4GHz) of PCS-derived SiC powders were investigated. The PCS-derived SiC powders are mainly composed of SiC nanocrystal, turbostratic carbon and amorphous phase (SiC and/or C). The size of SiC nanocrystals and the graphitization degree of carbon both increase with the elevation of pyrolysis temperature. Furthermore, the residual carbon is transformed from amorphous into turbostratic structure with a phenomenon of regional enrichment. Moreover, the relative complex permittivity increases notably with the higher pyrolysis temperature. Meanwhile, the dielectric loss tangent increases from 0.19 to 0.57, while the microwave impedance decreases from 73.20 to 53.58. The optimal reflection loss of −35dB for PCS-derived SiC powders is obtained when the pyrolysis temperature is 1500°C, which exhibits a great application prospect in microwave absorbing materials.

Conductive SiC ceramics fabricated by spark plasma sintering

Bulk SiC ceramics were fabricated by spark plasma sintering, and the effects of Y2O3 addition on the structural and electrical properties were investigated. Raman spectroscopy revealed that SiC specimens with low Y2O3 content (0.01 and 0.05wt%) contained a graphitic phase (1580 and 2710cm−1) which was insignificant for high Y2O3 content (1.0 and 5.0wt%). Carbon-rich SiC specimens exhibited p-type conduction character, while others exhibited n-type character. The p- and n-type SiC specimens had electrical resistivities around 10−2Ωcm but exhibited photoluminescence spectra quite different from each other. The p-type character is primarily attributable to silicon vacancies (VSi), while the n-type character is ascribed to nitrogen substitution in carbon sites (NC) of the zincblende lattice. The change in conduction type with yttria addition in the SiC specimen can be understood in terms of a competition between the densities of VSi and NC.

In-Situ Chemical Vapor Growth of Carbon-Rich Silicon Carbide Fibers Upon Pyrolysis of Polyzirconosilane

Carbon-rich SiC fibers were prepared in a cost-effective way using in-situ chemical vapor growth upon pyrolysis of polyzirconosilane at 1200 °C under nitrogen atmosphere, with 0.5-1 μm in diameter and a few milimeters in length and a yield of about 30 %. The empirical formula of the fibers is Si1C6.6O0.1, and both silicon and carbon atoms inside the fibers are found to be uniformly distributed, and the free carbon atoms show a turbostratic state. The carbon-rich SiC fibers show ideal thermo-oxidation resistance at 1200 °C in air. Chemical vapor growth of ceramic fibers upon preceramic pyrolysis can be used to prepare high performance ceramic fibers on a large scale cost- effectively.

Structural evolutions in polymer-derived carbon-rich amorphous silicon carbide.

The detailed structural evolutions in polycarbosilane-derived carbon-rich amorphous SiC were investigated semiquantitatively by combining experimental and analytical methods. It is revealed that the material is comprised of a Si-containing matrix phase and a free-carbon phase. The matrix phase is amorphous, comprised of SiC4 tetrahedra, SiCxOx-4 tetrahedra, and Si-C-C-Si/Si-C-H defects. With increasing pyrolysis temperature, the amorphous matrix becomes more ordered, accompanied by a transition from SiC2O2 to SiCO3. The transition was completed at 1250 °C, where the matrix phase started to crystallize by forming a small amount of β-SiC. The free-carbon phase was comprised of carbon nanoclusters and C-dangling bonds. Increasing pyrolysis temperature led to the transition of the free carbon from amorphous carbon to nanocrystalline graphite. The size of the carbon clusters decreased first and then increased, while the C-dangling bond content decreased continuously. The growth of carbon clusters was attributed to Ostwald ripening and described using a two-dimensional grain growth model. The calculated activation energy suggested that the decrease in C-dangling bonds is directly related to the lateral growth of the carbon clusters.

Wide visible and unique NIR fluorescence from SiC nanocrystals embedded in carbon rich SiC matrix derived from liquid polycarbosilane

A novel epitaxial growth of SiC nanocrystals embedded in carbon rich amorphous SiC films deposited using liquid polycarbosilane as precursor. These films exhibit wide visible and unconventional NIR fluorescence with exceptionally short lifetimes of 146 ps and 138 ps. Fabrication of LEDs is a potential application of these materials.

20pPSA-45 Structure of carbon-rich SiC(0001)-(√3x√3) surface investigated by LEED I-V analysis

20pPSA-45 Structure of carbon-rich SiC(0001)-(√3x√3) surface investigated by LEED I-V analysis

Atomic-Scale Morphology and Electronic Structure of Manganese Atomic Layers Underneath Epitaxial Graphene on SiC(0001)

We report the fabrication of a novel epitaxial graphene(EG)/Mn/SiC(0001) sandwiched structure through the intercalation of as-deposited Mn atoms on graphene surfaces, with the aid of scanning tunneling microscope, low energy electron diffraction, and X-ray photoelectron spectroscopy. We found that Mn can intercalate below both sp(3)-hybridized carbon-rich interface layer and monolayer graphene, along with the formation of various embedded Mn islands showing different surface morphologies. The unique trait of the sandwiched system is that the strong interaction between the carbon-rich interface layer and SiC(0001) can be decoupled to some degrees, and contemporaneous, an n-doping effect is observed by mapping the energy band of the system using angle-resolved photoemission spectroscopy. Moreover, what deserves our special attention is that the intercalated islands can only evolve below monolayer graphene when a bilayer coexists, accounting for an intriguing graphene thickness-dependent intercalation effect. In the long run, we believe that the construction of graphene/Mn/SiC(0001) systems offers ideal candidates for exploring some intriguing physical properties such as the magnetic property of two-dimensional transition metal systems.

Dependence of chemical composition and bonding of amorphous SiC on deposition temperature and the choice of substrate

Amorphous SiC has superior mechanical, chemical, electrical, and optical properties which are process dependent. In this study, the impact of deposition temperature and substrate choice on the chemical composition and bonding of deposited amorphous SiC is investigated, both 6 in. single-crystalline Si and oxide covered Si wafers were used as substrates. The deposition was performed in a standard low-pressure chemical vapour deposition reactor, methylsilane was used as the single precursor, and deposition temperature was set at 600 and 650 °C. XPS analyses were employed to investigate the chemical composition, Si/C ratio, and chemical bonding of deposited amorphous SiC. The results demonstrate that these properties varied with deposition temperature, and the impact of substrate on them became minor when deposition temperature was raised up from 600 °C to 650 °C. Nearly stoichiometric amorphous SiC with higher impurity concentration was deposited on crystalline Si substrate at 600 °C. Slightly carbon rich amorphous SiC films with much lower impurity concentration were prepared at 650 °C on both kinds of substrates. Tetrahedral Si–C bonds were found to be the dominant bonds in all deposited amorphous SiC. No contribution from Si–H/Si–Si but from sp 2 and sp 3 C–C/C–H bonds was identified.

Synthesis and structural characterization of carbon-rich SiC xO y derived from a Ni-containing hybrid polymer

Carbon-rich silicon oxycarbide ceramics were obtained by controlled pyrolysis at 950, 1300 and 1500 °C from hybrid polymeric precursors based on poly(methylsiloxane) and divinylbenzene with different molar ratios, in the presence or absence of nickel acetate. By increasing the pyrolysis temperature changes in the local molecular structure were observed by 29Si NMR, with the segregation of thermodynamically stable SiO 2 and SiC phases, in addition to formation of graphitic carbon nanoclusters. Ceramics without Ni were amorphous at 950 °C, while those obtained at 1300 °C showed the presence of a β-SiC phase. The presence of Ni in the polymeric precursor induced the formation of β-SiC, cristobalite silica, graphitic carbon, metallic Ni, NiO and Ni 2Si, and also reflected in denser materials. In general, the resulting ceramics without Ni did not present measurable porosity, whereas Ni-containing ceramics at 1500 °C showed behavior typical of mesoporous materials.

The Effect of Excess Carbon on the Crystallographic, Microstructural, and Mechanical Properties of CVD Silicon Carbide Fibers

ABSTRACTSilicon carbide (SiC) fibers made by chemical vapor deposition (CVD) are of interest for organic, ceramic, and metal matrix composite materials due their high strength, high elastic modulus, and retention of mechanical properties at elevated processing and operating temperatures. The properties of SCS-6™ silicon carbide fibers, which are made by a commercial process and consist largely of stoichiometric SiC, were compared with an experimental carbon-rich CVD SiC fiber, to which excess carbon was added during the CVD process. The concentration, homogeneity, and distribution of carbon were measured using energy dispersive x-ray spectroscopy (SEM/EDS). The effect of excess carbon on the tensile strength, elastic modulus, and the crystallographic and microstructural properties of CVD silicon carbide fibers was investigated using tensile testing, x-ray diffraction, scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

Epitaxial Growth of 3C-SiC on Si(111) Using Tetraethylsilane and Prevention of Void Formation at Heterointerfaces

The surface morphology and crystalline quality of 3C-SiC films on Si(111) substrates were investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy, where tetraethylsilane (TES, Si(C2H5)4) was first used as a safety source for the growth of SiC films. While TES was expected to be the safest in various organosilane sources, it was essentially anticipated that the high C/Si ratio in TES results in deposition of carbon-rich SiC films, preventing single crystal formation. In raising the temperature after carbonization, voids appeared at the interface of SiC/Si, causing the formation of hillocks on the grown SiC films. The formation of voids and hillocks was prevented by adjusting the TES flow rate and time in the heating process to the growth temperature, leading to the growth of SiC films with a good surface morphology. As a result, it was shown that single crystal epitaxial films could be obtained in the growth of SiC films using TES, and yet a high density of stacking faults resided at the SiC/Si interface.

Properties of a-Si:H TFTs using silicon carbonitride as dielectric

Silicon carbonitride (SiC X N Y ) thin films have been deposited by magnetically confined 13.56 MHz PECVD, using SiH 4, CH 4, and NH 3 as gas sources. The ternary system (Si, C, N) was exploited in order to determine the production conditions which lead to a material suitable for gate dielectric application in TFTs. RBS and ERD measurements were performed to determine the elemental composition of the films. SiC X N Y materials produced with low SiH 4 concentration (20%) presenting resistivities greater than 10 14 Ω cm were found to be promising gate dielectrics and were applied to a-Si:H TFT devices. The performance of these TFTs was compared to those made with standard SiN X dielectric and also with SiC X dielectric. Results show that threshold voltage and leakage current decrease with increasing carbon and hydrogen concentrations, while the mobility presents a maximum of 0.43 cm 2 V −1 s −1 for the sample with the composition SiC 0.3N 1.3:H 0.93, corresponding to gas percentage of SiH 4=20%, CH 4=20% and NH 3=60%. Small amounts of carbon in the SiN X network can improve the mobility in more than 13%. Carbon-rich SiC X N Y presents good off-state characteristics, like low threshold voltage and leakage current, but lower on-state performance due to its lower dielectric constant and bulk resistivity.