Stoichiometric SiC Research Articles

Novel SiC synthesis method applicable to SiC solid phase sintering

AbstractMost of the present production processes of SiC sintered bodies require some powder mixing using a mechanical milling process (ball milling, and so on). In this case, relatively long hours are required, and there is the problem of contamination during the preparation process. To avoid these problems, we developed a new process for obtaining a self‐sinterable, stoichiometric SiC powder, whose precursor material is water‐soluble; the precursor material was synthesized from aqueous silica and citric acid containing a small amount of aluminum compound. In order to obtain the stoichiometric SiC composition, the above aqueous precursor material was adequately cured in air (200°C‐400°C); subsequently carbonization reaction (~800°C) in nitrogen atmosphere, carbothermal reduction (~1600°C) in argon atmosphere, and pressureless sintering (~1900°C) were performed. Among these processes, the curing process (cross‐linking process) is very important for obtaining the equivalent composition (silica and carbon) for the subsequent carbothermal reduction. In this study, the adequate curing temperature and suitable preparation condition for the carbothermal reduction were investigated for the production of stoichiometric self‐sinterable SiC powder. The pressureless sintered body achieved using the obtained SiC powder demonstrated a desirable trans‐crystalline fracture behavior.

Highly effective free‐radical‐catalyzed curing of hyperbranched polycarbosilane for near stoichiometric SiC ceramics

AbstractLiquid hyperbranched polycarbosilane (LHBPCS) is an excellent preceramic polymer of SiC. Before thermolysis, curing of LHBPCS is favorable for higher ceramic yield and molding. In order to avoid some unfavorable factors arising from traditional thermal curing, we introduced a free‐radical initiator tert‐butyl peroxybenzoate (TBPB) to trigger the crosslinking of allyl group on LHBPCS in this study. FT‐IR, NMR and DSC measurements indicated only allyl group was consumed during the cross‐linking process. The cross‐linking reaction rate was monitored by a rotational rheometer, and results confirmed LHBPCS could be cured in 4 minutes with the help of 1 wt% TBPB at 80°C. TGA and SEM demonstrated the catalytic‐cured LHBPCS had a high ceramic yield of 79.0 wt% and dense structure. The effect of TBPB on crystallization behavior of the obtained SiC was also explored by XRD, and little impact was detected. After 1600°C thermolysis, a near stoichiometric and crystalline SiC with ceramic yield of 77.5 wt% was obtained.

Fabrication of nearly stoichiometric polycrystalline SiC fibers with excellent high‐temperature stability up to 1900°C

AbstractDue to the extensive applications of SiC fiber‐reinforced composite materials in the fields of aviation, aerospace, and nuclear power, there are increasing demands for SiC fibers with both excellent mechanical performance and high‐temperature stability. In this work, nearly stoichiometric polycrystalline SiC fibers were fabricated using amorphous Si–C–Al–O fibers with excess carbon and oxygen (C/Si = 1.34, O content: 7.74 wt%). The nearly stoichiometric composition (C/Si = 1.05) of the product fibers was achieved by thermal decomposition of the starting fibers. The fibers were well‐crystallized with grain sizes of ~200 nm due to sintering at a high temperature of 1900°C. The fibers exhibited a high tensile strength and a high elastic modulus and were composed of SiC grains with twins and stacking‐faults, exhibiting intragranular fracture behavior. Furthermore, the fibers maintained their original tensile strength after being maintained at 1800°C for 5 hour or at 1900°C for 1 hour under an inert atmosphere, and they exhibited a high strength retention (97%) after exposure at 1300°C for 1 hour under air. The high‐temperature stability and creep resistance of the fibers were comparable to that of commercial Hi‐Nicalon S and Tyranno SA fibers.

Annealing of silicon carbonitride nanostructured thin films: interdependency of hydrogen content, optical, and structural properties

Silicon carbonitride (SiCxNy) materials, inspired by their outstanding multifunctional properties, are finding increasing applications in a variety of fields, including as next generation solar cells and hard coatings. Hydrogenated SiCxNy thin films, along with binary submatrices of stoichiometric SiC and SiN3 as a reference, were deposited using electron cyclotron resonance plasma-enhanced chemical vapor deposition. We described a comparative study of the effects of post-deposition thermal annealing, from 300 to 1200 °C, on the evolution of hydrogen-rich a-SiC1.2N0.7:H1.4 thin films. Concurrently, two featured annealing temperatures (500 and 900 °C) were found to have the significant influence on the morphology, optical, and microstructural properties of the films. During annealing the amorphous phase of SiC1.2N0.7:H1.4 thin films was fully maintained according to the transmission electron microscopy and X-ray diffraction analyses. The hydrogen density was quantitatively analyzed employing two different experimental techniques, elastic recoil detection and Fourier transform infrared spectroscopy, showing the associate annealing temperatures of hydrogen desorption, breaking of all hydrogen-terminated bonds, and depletion of all hydrogen content. The refractive indices and optical band gap of the films were calculated using variable angle spectroscopic ellipsometer. Thermal annealing resulted in hydrogen desorption and consequently layer densification along with an increase in the refractive index. During the annealing process, first the optical band gap widened, and then narrowed due to hydrogen out-diffusion or chemical bond restructuring, depending on the annealing temperature. In addition, Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy were performed. These findings are discussed in the context of the interdependency of the hydrogen desorption and thermally induced changes in chemical bonds, mass density, and optical properties.

Direct formulation of nanocrystalline silicon carbide/nitride solid ceramics

We developed a new in situ reaction method to synthesize SiC and Si3N4 ceramic solids from meltable precursor compositions into shaped ceramic composites with nanocrystalline grains. The process uses Si powder and 1,2,4,5-tetrakis(phenylethynyl)benzene, which readily react above 1400 °C to form the SiC and Si3N4 crystallites in the presence of argon and nitrogen, respectively. X-ray diffraction analysis, Raman spectroscopy and density measurements indicated the formation of near stoichiometric SiC and Si3N4 within the shaped solid. Further characterization of electrical conductivity and oxidative stability of the prepared ceramics analyzed the influence of nanoscale features on intrinsic properties of resulting composites. The hardness and elastic modulus values for the synthesized SiC determined by nanoindentation varied in the range of 25–46 GPa and 300–440 GPa.

Accelerating the crosslinking process of hyperbranched polycarbosilane by UV irradiation

A liquid hyperbranched polycarbosilane (LHBPCS) with stoichiometric C/Si ratio but without unsaturated groups was synthesized. Different from traditional thermal crosslinking, ultraviolet (UV) irradiation crosslinking was taken. The molecular weight, the consumption of SiH group and ceramic yield of LHBPCS showed an increase trend with increasing the UV irradiation time. After 30min of UV irradiation, 71.8wt% ceramic yield was obtained. In addition, extra divinyldimethylsilane was added into LHBPCS. Under UV irradiation, both the SiH group and vinyl group of divinyldimethylsilane were consumed. But the reaction extend of vinyl group was much faster than that of SiH group. Compared with pure LHBPCS, the mixture of LHBPCS and 5wt% divinyldimethylsilane gave a higher ceramic yield of 79wt% after 30min of UV irradiation. By heating the crosslinked LHBPCS to 1000°C, a near stoichiometric SiC ceramic was got. It exhibited excellent thermal stability at 1400°C in air.

Conversion Process of Amorphous Si-Al-C-O Fiber into Nearly Stoichiometric SiC Polycrystalline Fiber

Tyranno SA (SiC-polycrystalline fiber, Ube Industries Ltd.) shows excellent heat-resistance up to 2000℃ with relatively high mechanical strength. This fiber is produced by the conversion process from a raw material (amorphous Si-Al-C-O fiber) into SiCpolycrystalline fiber at very high temperatures over 1500℃ in argon. In this conversion process, the degradation reaction of the amorphous Si-Al-C-O fiber accompanied by a release of CO gas for obtaining a stoichiometric composition and the subsequent sintering of the degraded fiber proceed. Furthermore, vaporization of gaseous SiO, phase transformation and active diffusion of the components of the Si-Al-C-O fiber competitively occur. Of these changes, vaporization of the gaseous SiO during the conversion process results in an abnormal SiC-grain growth and also leads to the non-stoichiometric composition. However, using a modified Si-Al-C-O fiber with an oxygen-rich surface, vaporization of the gaseous SiO was effectively prevented, and then consequently a nearly stoichiometric SiC composition could be obtained.

Formation of silicon nanocrystals in silicon carbide using flash lamp annealing

During the formation of Si nanocrystals (Si NC) in SixC1−x layers via solid-phase crystallization, the unintended formation of nanocrystalline SiC reduces the minority carrier lifetime and therefore the performance of SixC1−x as an absorber layer in solar cells. A significant reduction in the annealing time may suppress the crystallization of the SiC matrix while maintaining the formation of Si NC. In this study, we investigated the crystallization of stoichiometric SiC and Si-rich SiC using conventional rapid thermal annealing (RTA) and nonequilibrium millisecond range flash lamp annealing (FLA). The investigated SixC1−x films were prepared by plasma-enhanced chemical vapor deposition and annealed at temperatures from 700 °C to 1100 °C for RTA and at flash energies between 34 J/cm2 and 62 J/cm2 for FLA. Grazing incidence X-ray diffraction and Fourier transformed infrared spectroscopy were conducted to investigate hydrogen effusion, Si and SiC NC growth, and SiC crystallinity. Both the Si content and the choice of the annealing process affect the crystallization behavior. It is shown that under certain conditions, FLA can be successfully utilized for the formation of Si NC in a SiC matrix, which closely resembles Si NC in a SiC matrix achieved by RTA. The samples must have excess Si, and the flash energy should not exceed 40 J/cm2 and 47 J/cm2 for Si0.63C0.37 and Si0.77C0.23 samples, respectively. Under these conditions, FLA succeeds in producing Si NC of a given size in less crystalline SiC than RTA does. This result is discussed in terms of nucleation and crystal growth using classical crystallization theory. For FLA and RTA samples, an opposite relationship between NC size and Si content was observed and attributed either to the dependence of H effusion on Si content or to the optical absorption properties of the materials, which also depend on the Si content.

Effect of substrate temperature on structural and linear and nonlinear optical properties of nanostructured PLD a-SiC thin films

In this paper, the effect of substrate temperatures (Ts) on the structural, linear and non-linear optical properties of nanostructured amorphous Silicon Carbide (a-SiC) thin films deposited onto fused silica by pulsed-laser deposition technique is reported. The structural and compositional properties were studied by X-ray diffraction, Raman spectroscopy and Fourier transform Infrared spectroscopy. A transition from Silicon-rich SiC to nearly stoichiometric SiC was observed with increasing Ts. The absorption coefficient (α), refractive index (n), optical band gap and thickness of the films were determined from UV–vis-IR spectra. Static refractive indices of the films were found to vary from 2.99–2.54 with the increasing Ts. The optical band gap of the films was found to be increasing from 1.5eV to 2.33eV with increase in Ts. The third order non-linear optical susceptibility (χ(3)) of the a-SiC thin films estimated from Z-scan studies was found to be of the order of 10−3 esu.

Polymer-Derived Ceramic Fibers

SiC-based ceramic fibers are derived from polycarbosilane or polymetallocarbosilane precursors and are classified into three groups according to their chemical composition, oxygen content, and C/Si atomic ratio. The first-generation fibers are Si-C-O (Nicalon) fibers and Si-Ti-C-O (Tyranno Lox M) fibers. Both fibers contain more than 10-wt% oxygen owing to oxidation during curing and lead to degradation in strength at temperatures exceeding 1,300°C. The maximum use temperature is 1,100°C. The second-generation fibers are SiC (Hi-Nicalon) fibers and Si-Zr-C-O (Tyranno ZMI) fibers. The oxygen content of these fibers is reduced to less than 1 wt% by electron beam irradiation curing in He. The thermal stability of these fibers is improved (they are stable up to 1,500°C), but their creep resistance is limited to a maximum of 1,150°C because their C/Si atomic ratio results in excess carbon. The third-generation fibers are stoichiometric SiC fibers, i.e., Hi-Nicalon Type S (hereafter Type S), Tyranno SA, and Sylramic™ fibers. They exhibit improved thermal stability and creep resistance up to 1,400°C. Stoichiometric SiC fibers meet many of the requirements for the use of ceramic matrix composites for high-temperature structural application. SiBN3C fibers derived from polyborosilazane also show promise for structural applications, remain in the amorphous state up to 1,800°C, and have good high-temperature creep resistance.

Development of wound SiCBNx/SiNx/SiC with near stoichiometric SiC matrix via LSI process

In order to develop SiC/SiC ceramic matrix composites with a low porosity, liquid silicon infiltration (LSI) was chosen as a technique, characterized by short processing times. To create the matrix, a tailored phenolic resin containing ß-naphthol units was synthesized and infiltrated in wound fiber preforms, thermally cured, pyrolysed and siliconized. The aim is to obtain a high carbon yield during pyrolysis and to create a dense carbon foam between the SiC fibers. Instead of a classic liquid silicon infiltration via capillary forces through a carbon block-like microstructure, this foam promotes a softer infiltration with less fiber degradation and a maximized carbon conversion to near stoichiometric SiC. To protect the SiC fibers from an attack of the liquid silicon and to simultaneously provide a weak fiber matrix bonding, a BNx/SiNx fiber coating was chosen. The coating was applied by means of low-pressure chemical vapor deposition (LPCVD) using gaseous chlorine free precursors.

Biomorphous SiC ceramics prepared from cork oak as precursor

Porous ceramic materials of SiC were synthesized from carbon matrices obtained via pyrolysis of natural cork as precursor. We propose a method for the fabrication of complex-shaped porous ceramic hardware consisting of separate parts prepared from natural cork. It is demonstrated that the thickness of the carbon-matrix walls can be increased through their impregnation with Bakelite phenolic glue solution followed by pyrolysis. This decreases the material’s porosity and can be used as a way to modify its mechanical and thermal characteristics. Both the carbon matrices (resulted from the pyrolysis step) and the resultant SiC ceramics are shown to be pseudomorphous to the structure of initial cork. Depending on the synthesis temperature, 3C-SiC, 6H-SiC, or a mixture of these polytypes, could be obtained. By varying the mass ratio of initial carbon and silicon components, stoichiometric SiC or SiC:С:Si, SiC:С, and SiC:Si ceramics could be produced. The structure, as well as chemical and phase composition of the prepared materials were studied by means of Raman spectroscopy and scanning electron microscopy.

Influence of Free Carbon Elimination on Microstructure and Properties of SiC Fibers

To prepare SiC fibers with different free carbon contents, polycarbosilane (PCS) fibers cured with unsaturated hydrocarbons were pyrolyzed at 1000°C under controlled hydrogen/nitrogen atmosphere and subsequently heat treated at 1500°C under nitrogen atmosphere. The process of carbon removal during pyrolysis was investigated using chemical elemental analysis, FTIR, and AES analysis. The microstructure and properties were examined by SEM, TEM, XRD, density measurements, tensile tests and resistivity measurements. The results show that the carbon content in SiC fibers decreases with H2 concentration increasing. The hydrogen atmosphere suppresses H2 evolution and helps to remove excess carbon as CH4 during pyrolysis. Although thin carbon-enriched films are present on the fiber surfaces, the distribution of silicon and carbon is uniform in the fiber cores. The microstructure and properties of the resulting SiC fibers are very dependent on their C/Si chemical compositions. The β-SiC grain size increases with a decrease in the carbon content because the excess carbon aggregates at the grain boundary and impedes the grain growth. Moreover, the removal of free carbon also results in fiber densification, decrease of porosity and improvement of fiber specific resistivity, tensile strength and tensile modulus. Therefore, the nearly stoichiometric SiC fiber has good comprehensive performance.

Can silicon carbide serve as a saturable absorber for passive mode-locked fiber lasers?

The study presents a novel demonstration of a passively mode-locked erbium-doped fiber laser (EDFL) that is based on a silicon carbide (SixC1−x) saturable absorber. When the C/Si composition ratio is increased to 1.83, the SixC1−x film transforms from two-photon absorption to nonlinear saturable absorption, and the corresponding value reaches −3.9 × 10−6 cm/W. The Si-rich SixC1−x film cannot mode lock the EDFL because it induced high intracavity loss through two-photon absorption. Even when a stoichiometric SiC is used, the EDFL is mode locked, similar to an EDFL operating under weak nonlinear-polarization-rotation condition. A C-rich SixC1−x film containing sp2-orbital C–C bonds with a linear absorbance of 0.172 and nonlinear absorbance of 0.04 at a 181 MW/cm2 saturation intensity demonstrates nonlinear transmittance. The C-rich SixC1−x saturable absorber successfully generates a short mode-locked EDFL pulse of 470 fs. The fluctuation of the pulse-train envelope dropps considerably from 11.6% to 0.8% when a strong saturable-absorption-induced self-amplitude modulation process occurs in the C-rich SixC1−x film.

Chemical vapor deposition of Si/SiC nano-multilayer thin films

Stoichiometric SiC films were deposited with the commercially available single source precursor Et3SiH by classical thermal chemical vapor deposition (CVD) as well as plasma-enhanced CVD at low temperatures in the absence of any other reactive gases. Temperature-variable deposition studies revealed that polycrystalline films containing different SiC polytypes with a Si to carbon ratio of close to 1:1 are formed at 1000°C in thermal CVD process and below 100°C in the plasma-enhanced CVD process. The plasma enhanced CVD process enables the reduction of residual stress in the deposited films and offers the deposition on temperature sensitive substrates in the future. In both deposition processes the film thickness can be controlled by variation of the process parameters such as the substrate temperature and the deposition time. The resulting material films were characterized with respect to their chemical composition and their crystallinity using scanning electron microscope, energy dispersive X-ray spectroscopy (XRD), atomic force microscopy, X-ray diffraction, grazing incidence X-ray diffraction, secondary ion mass spectrometry and Raman spectroscopy. Finally, Si/SiC multilayers of up to 10 individual layers of equal thickness (about 450nm) were deposited at 1000°C using Et3SiH and SiH4. The resulting multilayers features amorphous SiC films alternating with Si films, which feature larger crystals up to 300nm size as measured by transmission electron microscopy as well as by XRD. XRD features three distinct peaks for Si(111), Si(220) and Si(311).

Investigation of Different Phenolic Resins and their Behavior during Pyrolysis to Form SiC/C-Composites

Specific phenolic resin samples have been developed as the carbon precursor for SiC/C composites. Liquid phenolic resins suitable for fiber-infiltration in the resin transfer moulding (RTM) process are synthesized by using versatile combination of the aromatic component (phenol, naphthalen-2-ol) with various formaldehyde equivalents such as methanal, 1,3,5,7tetraazatricyclo [3.3.1.13,7] decane (urotropine), and 1,3,5-trioxane, under different reaction conditions. Room temperature liquid resoles (RTLR) are obtained by using an excess of the formaldehyde component over phenol (≥2) under basic conditions. Upon heating RTLR can form a crosslinked network even without addition of a hardening reagent. In addition, novolacs are synthesized under acidic conditions using a phenol/formaldehyde ratio ≥1. Nitrogen-containing resins contain nitrogen due to reaction of phenol with urotropine. Novolacs and nitrogen-containing resins are solids at room temperature and not self-curing. To infiltrate these both resins into SiC fibers in the RTM process, they are dissolved in 2furanmethanol (furfuryl alcohol FA) and urotropine which is added as curing-agent. Both, the molecular weight and the amount of the dissolved phenolic resin have an influence on the viscosity and the carbon yield after pyrolysis which is important for this application. The aim was to create different phenolic resins for the fabrication in the RTM process and to characterize the carbon after pyrolysis with respect to the structure and porosity as these are key parameters to generate a stoichiometric SiC matrix by LSI.

Structural and optical properties of silicon nanocrystals embedded in silicon carbide: Comparison of single layers and multilayer structures

The outstanding demonstration of quantum confinement in Si nanocrystals (Si NC) in a SiC matrix requires the fabrication of Si NC with a narrow size distribution. It is understood without controversy that this fabrication is a difficult exercise and that a multilayer (ML) structure is suitable for such fabrication only in a narrow parameter range. This parameter range is sought by varying both the stoichiometric SiC barrier thickness and the Si-rich SiC well thickness between 3 and 9nm and comparing them to single layers (SL). The samples processed for this investigation were deposited by plasma-enhanced chemical vapor deposition (PECVD) and subsequently subjected to thermal annealing at 1000–1100°C for crystal formation. Bulk information about the entire sample area and depth were obtained by structural and optical characterization methods: information about the mean Si NC size was determined from grazing incidence X-ray diffraction (GIXRD) measurements. Fourier-transform infrared spectroscopy (FTIR) was applied to gain insight into the structure of the Si–C network, and spectrophotometry measurements were performed to investigate the absorption coefficient and to estimate the bandgap E04. All measurements showed that the influence of the ML structure on the Si NC size, on the Si–C network and on the absorption properties is subordinate to the influence of the overall Si content in the samples, which we identified as the key parameter for the structural and optical properties. We attribute this behavior to interdiffusion of the barrier and well layers. Because the produced Si NC are within the target size range of 2–4nm for all layer thickness variations, we propose to use the Si content to adjust the Si NC size in future experiments.

Effect of Curing Methods on the Mechanical Property of SiC Fiber Prepared via Hydrogenation

Two different polycarbosilane (PCS) fibers were prepared by a new oxygen-free curing method and Chemical Vapor Curing (CVC), respectively. Nearly stoichiometric SiC fiber was prepared via hydrogenation in H2-N2 mixture and subsequent sintering in inert gases. The green PCS fiber and nearly stoichiometric SiC fiber were characterized by FT-IR, XRD, SEM etc. Meanwhile, the effect of hydrogenation on the thermal stability of SiC fibres cured by CVC method was studied. It was found that the oxygen-free curing method can effectively decrease the content of oxygen in the final SiC fiber, while the hydrogenation treatment can dramatically diminish the content of free carbon, so the nearly stoichiometric SiC fiber with C/Si atomic ratio being 1.06 can be obtained. After the heat-treated at 1600°C in inert gases, the strength retaining ratio of the oxygen-free cured SiC achieved 68%, demonstrating that the nearly stoichiometric SiC fiber possesses excellent high-temperature stability and higher anti-oxidation property.

High-Temperature Chemical Vapor Deposition for SiC Single Crystal Bulk Growth Using Tetramethylsilane as a Precursor

SiC single bulk crystals were grown using a high-temperature chemical vapor deposition (HTCVD) method, with SiH4 and hydrocarbons as the source materials. SiH4 is a pyrophoric gas, which frequently causes fatal accidents in experiments. In this study, therefore, we propose the use of a HTCVD method using tetramethylsilane (TMS), a cheap and safe precursor, for growing SiC bulk crystals. Although TMS contains C four times more than Si in its chemical formula, a stoichiometric SiC layer was successfully synthesized from TMS in the presence of a high concentration of H2 based on the thermodynamic process design. 6H-SiC single crystals were successfully grown on a 6H-SiC seed crystal using the same process conditions. The resulting single crystalline layer was evaluated by rocking curve analysis by X-ray diffraction, which showed that the crystal properties of the grown SiC layer were improved compared to the seed crystals. This suggests that the TMS-based HTCVD method is feasible for practical use in SiC bul...

Surface damage on 6H–SiC by highly-charged Xeq+ ions irradiation

Surface damage on 6H–SiC irradiated by highly-charged Xeq+ (q=18, 26) ions to different fluences in two geometries was studied by means of AFM, Raman scattering spectroscopy and FTIR spectrometry. The FTIR spectra analysis shows that for Xe26+ ions irradiation at normal incidence, a deep reflection dip appears at about 930cm−1. Moreover, the reflectance on top of reststrahlen band decreases as the ion fluence increases, and the reflectance at tilted incidence is larger than that at normal incidence. The Raman scattering spectra reveal that for Xe26+ ions at normal incidence, surface reconstruction occurs and amorphous stoichiometric SiC and Si–Si and C–C bonds are generated and original Si–C vibrational mode disappears. And the intensity of scattering peaks decreases with increasing dose. The AFM measurement shows that the surface swells after irradiation. With increasing ion fluence, the step height between the irradiated and the unirradiated region increases for Xe18+ ions irradiation; while for Xe26+ ions irradiation, the step height first increases and then decreases with increasing ion fluence. Moreover, the step height at normal incidence is higher than that at tilted incidence by the irradiation with Xe18+ to the same ion fluence. A good agreement between the results from the three methods is found.