Synthesis of SiC ceramic fibers from nuclear reactor irradiated polycarbosilane ceramic precursor fibers

Polycarbosilane (PCS) ceramic precursor fibers are irradiated in a nuclear reactor and pyrolyzed under inert atmosphere. Bridge structure of Si–CH₂–Si is formed in the irradiated products by the rupture of Si–H bonds and succeeding cross-linking. When irradiated at the neutron fluence of 2.2 × 10¹⁷ cm⁻² under N₂ atmosphere, the gel content and ceramic yield at 1,273 K of PCS fibers are up to 80% and 94.3%, respectively, and their pyrolysis products are still fibrous, which illuminates that the infusibility of PCS fibers has been achieved. FT-IR spectra indicate that the chemical structure of pyrolysis products is very similar to that of pure SiC, while X-ray diffraction curves suggest that β-SiC microcrystals are formed in the fibers, and their mean grain size is about 7.5 nm. The oxygen content (1.69–3.77 wt%) is much lower than that of conventional SiC fibers by oxidation curing method (about 15 wt%). Tensile strength of the SiC fibers is up to 2.72 GPa, which demonstrates that their mechanical properties are excellent. After heat-treated at 1,673 K in air for an hour or at 1,873 K under Ar gas atmosphere for 0.5 h, their external appearance is still undamaged and dense, and their tensile strength decreases to a small extent, which verifies that heat resistance of the SiC fibers is eximious.     

Hot pressed ZrB2–SiC–C ultra high temperature ceramics with polycarbosilane as a precursor

ZrB₂–SiC–C ultra high temperature ceramics (UHTCs) have been produced by hot pressing pyrolyzed mixtures of ZrB₂ and polycarbosilane (PCS). Samples with SiC contents of 0%, 5% and 16% in volume derived from PCS were prepared. The phase composition, microstructure and mechanical properties were characterized for composites hot pressed at 2073 K for 60 min under the pressure of 20 MPa in an argon atmosphere. Analysis showed that the addition of PCS improved the relative density from 78% (without PCS addition) to ∼ 100% (with 16% SiC derived from PCS addition). Hardness and fracture toughness of the composite were also improved.     

Synthesis and pyrolysis of polysilazane as the precursor of Si3N4/SiC ceramic

By co-ammonolysis of MeHSiCI₂ and Me₂SiCl₂ and following thermal polymerization, polysilazanes as the precursors to Si₃N₄/SiC ceramic fibre were synthesized. The tendency to cross-link in thermal polymerization is suppressed as the result of the introduction of Me₂SiCl₂ as a starting material. The reactions occurring during thermal polymerization of ammonolysis products are discussed based on infrared (i.r.) and ¹H nuclear magnetic resonance (NMR) spectra analysis. The pyroiysis process of polysilazanes is examined in nitrogen and in an NH₃ atmosphere. It is shown that the formation of SiC results from the pyrolysis of Si-CH₃ groups in polysilazanes by means of the formation of Si-CH₂-Si intermediate bonds. The structure and properties of ceramics derived from the pyrolysis of polysilazane and polycarbosilane in N₂ or NH₃ atmosphere are compared and discussed. Si-N-C ceramic, which was proved to be a composite of -Si₃N₄ and -SiC, exhibits better thermal stability at temperatures higher than 1300°C.     

Synthesis of SiC fibre with low oxygen content and high tensile strength using a polyblend precursor

SiC fibre with low oxygen content and high tensile strength was first synthesized in our laboratory. The SiC fibre was obtained by using a polyblend of polycarbosilane (PC) and hydroxy-terminated-polybutadiene (HTPB) as a precursor. It was found that PC could react with HTPB to form cross-linked polymers at temperatures around 260 ° C, so the HTPB can be used as a curing agent. Consequently, the need for oxygen to be introduced in the air-curing process is reduced and SiC fibre with low oxygen content and higher tensile strength can be made. The chemical compositions, the oxidation resistance and chemical stability of the SiC fibre were also studied here.     

Structural evolutions from polycarbosilane to SiC ceramic

The pyrolysis process of a polycarbosilane into a microcrystalline silicon carbide ceramic has been followed up to 1700 ° C mainly by means of solid state²⁹Si and¹³C nuclear magnetic resonance, transmission electron microscopy and X-ray diffraction analysis. A structural model has been proposed for the amorphous silicon carbide phase that is formed during the pyrolysis process. The ceramic obtained at high temperature is formed by a mixture of -SiC and -SiC; however, some difficulties in the identification of the crystalline phases have been pointed out.     

SiC陶瓷纤维高聚物先驱体的研究进展

先驱体转换法是迄今为止高性能SiC陶瓷纤维最为成功的一种工业化方法，而高聚物先驱体的结构与性能则是该法的关键。本文主要综述了国内外SiC陶瓷纤维高聚物先驱体分子设计方面的研究动态及最近的发展趋势。     

具有优异纺丝性的陶瓷先驱体含铝聚碳硅烷的合成及表征

选择适当分子量的低分子量聚碳硅烷与Al(AcAc)3在自制常压高温合成装置中合成了含铝碳化硅纤维的先驱体--聚铝碳硅烷(polyaluminocarbosilane,PACS).并对PACS进行了软化点测试、傅立叶红外光谱(FT-IR)分析、凝胶渗透色谱(GPC)测试、元素分析以及可纺性研究.由中等分子量聚碳硅烷为原料合成出的软化点为194.8～220.1,Si-H键含量为0.857,数均分子量为2353,分子量分布呈"双峰"分布的PACS,经熔融纺丝得到了直径为5μm、表面光滑、直径均匀的原丝,表现出了优异的纺丝性能.     

An approach to the mechanical behaviour of SiC/SiC and C/SiC ceramic matrix composites

The mechanical behaviour of two woven composites C/SiC and SiC/SiC was investigated at room temperature. The non-linear load-displacement curves and the damaging process were closely related to the specific structure of the composites, consisting of a network of impregnated bundles of fibres. The damage in the bundles proceeded by multiple cracking in the matrix before fibre failure, and dictated the response to the applied load. Other mechanisms, consisting mainly of distortions in bundles and their framework, induced a residual deformation and an energy dissipation. The behaviour was characterized according to the damaging process. Stress-electric strain curves revealed a mechanical response similar to those observed in unidirectional composites, although some effect of the specimen geometry on the curves was observed. Residual strains were similar in tensile and bending conditions. The work of fracture was consistently described by a volumetric rate of energy absorption, related to the applied strain, but the respective contributions of different damage mechanisms could not be determined.     

Synthesis,characterization, and ceramic conversion of a liquid polymeric precursor to SiBCN ceramic via borazine-modified polymethylsilane

A liquid polymeric precursor to SiBCN ceramics was synthesized via dehydrogenation of polymethylsilane and borazine. The resulting precursor, its ceramic conversion, and its pyrolytic products were investigated via viscosity testing, gel permeation chromatography, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, thermogravimetric analysis, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results show that Si–H bonds in polymethylsilane react with N–H or B–H bonds in borazine to form a cross-linked polymer with a viscosity of 850 mPa s. The ceramic yield of the precursor is 89 wt% after treatment at up to 1000 °C. This is 51 wt% higher than the original polymethylsilane. The polymer-ceramic conversion is complete at 800 °C and further heating at 1200 °C induces partial crystallization, which forms β-SiC crystals. The final pyrolytic product is composed of β-SiC and Si₃N₄ crystals after heat treatment at 1600 °C. The introduction of boron into the ceramic can inhibit the growth of β-SiC crystals and improve densification of the ceramic products, thus aiding the high-temperature properties of the final products. Its appropriate viscosity, good thermal curability, and high ceramic yield make this precursor appropriate for use in preparation of high-performance SiBNC ceramics for high-temperature applications.     

Joining of SiC ceramic to Ni-based superalloy with functionally gradient material fillers and a tungsten intermediate layer

Joining of ceramics to metals is of great interest from both technical and economical points of view. SHS welding of SiC ceramic to Ni-based superalloy has been achieved with TiC-Ni functionally gradient materials as filler and a tungsten sheet as an intermediate layer. The obtained welded strength of the welded sample is 60% of the strength of the SiC ceramic measured under the identical conditions. This value is 25% greater than that without the tungsten intermediate layer. Within the range tested in this study, the strength of welded samples increases with increasing thickness of the tungsten layer. The presence of the tungsten intermediate layer can effectively improve the distribution of process-induced thermal stresses in the joints and reduce the concentration of residual thermal stresses in the ceramic near the welding seam. Microstructural study reveals that diffusion takes place at the interfaces, which promotes bonding at the interfaces.     

Elevated-temperature stress rupture in interlaminar shear of a Hi-Nic SiC/SiC ceramic matrix composite

Assessments of stress rupture in a gas-turbine grade, melt-infiltrated Hi-Nicalon^TM SiC continuous fiber-reinforced SiC ceramic matrix composite (CMC) were made in interlaminar shear at 1316 °C in air. The composite exhibited appreciable life limiting behavior with a life susceptibility parameter of n_s = 22–24, estimated based on a proposed phenomenological life prediction model together with experimental data. The phenomenological life model was in good agreement in prediction between the stress rupture and the constant stress-rate data, validating its appropriateness in describing the life limiting phenomenon of the CMC coupons subjected to interlaminar shear. The results of this work also indicated that the governing mechanism(s) associated with failure in interlaminar shear would have remained almost unchanged, regardless of the type of loading configurations, either in stress rupture or in constant stress rate.     

Synthesis and characterization of polyvinylsilazane as a precursor for Si3N4 based ceramic materials

This investigation focused on the synthesis and characterization of a polyvinylsilazane (PVSZ) polymer. This material shows promise as a precursor for silicon nitride/silicon carbide based ceramic materials. The polymer was synthesized via the ammonolysis of vinyltrichlorosilane (VTS) in tetrahydrofuran (THF). The polymer was characterized using Fourier Transform Infrared (FT-IR) spectroscopy, ¹H/¹³C nuclear magnetic resonance (NMR) spectroscopy, and gel permeation chromatography (GPC). This polymer was found to have a molecular weight of around 880 g/mol, as determined by GPC. Thermal cross-linking, up to 1500 °C in an inert atmosphere, produced a ceramic material with a char yield of around 85%. The gas by-products from the partial decomposition of the polymer to a ceramic material were characterized using residual gas analysis (RGA). The crystallite phases of the ceramic char, pyrolyzed in different atmospheres, were determined by X-ray powder diffraction (XRD). Lastly, the ability of the polymer to form a free standing monolithic structure as well as the ability to be drawn into fibers was evaluated.     

Preparation of a hyperbranched polycarbosilane precursor to SiC ceramics following an efficient room-temperature cross-linking process

Room-temperature cross-linking of a hyperbranched polycarbosilane (HBPCS) with divinylbenzene (DVB) in the presence of the cyclohexanone peroxide–cobaltous naphthenate (CHP–CN) initiator system was studied. According to the Fourier transform infrared spectroscopy (FT-IR) and ¹H nuclear magnetic resonance (¹H NMR) results, the cross-linking reaction occurred via the vinyl polymerization. The GPC analysis confirmed the molecular weight of the cross-linked HBPCS significantly increased. Thermal behaviors of cross-linked HBPCS and original HBPCS were investigated by thermal gravimetric analysis-differential thermal analysis (TGA–DTA). The TGA results indicated that the ceramic yield of HBPCS remarkably increased by the cross-linking treatment. For the HBPCS/10 wt% DVB system, the maximum of reaction degree of HBPCS was obtained, which might be responsible for the highest ceramic yield of 70.1 wt% at 1000 °C. However, the ceramic yield of the non-crosslinked HBPCS was only 45 wt% at 1000 °C. The evolution of crystal structure of SiC as a function of pyrolysis temperature was traced by means of X-ray diffraction (XRD) and FT-IR. With the pyrolysis temperature increasing, the β-SiC peaks became sharper and the grain size also grew larger. As the DVB content increased, the intensity of β-SiC peaks significantly reduced, indicating smaller β-SiC grain size.     

Nonlinear softening as a predictive precursor to climate tipping

Approaching a dangerous bifurcation, from which a dynamical system such as the Earth's climate will jump (tip) to a different state, the current stable state lies within a shrinking basin of attraction. Persistence of the state becomes increasingly precarious in the presence of noisy disturbances. We argue that one needs to extract information about the nonlinear features (a 'softening') of the underlying potential from the time series to judge the probability and timing of tipping. This analysis is the logical next step if one has detected a decrease of the linear decay rate. If there is no discernible trend in the linear analysis, nonlinear softening is even more important in showing the proximity to tipping. After extensive normal-form calibration studies, we check two geological time series from palaeo-climate tipping events for softening of the underlying well. For the ending of the last ice age, where we find no convincing linear precursor, we identify a statistically significant nonlinear softening towards increasing temperature.     

Solvothermal-assisted exfoliation process to produce graphene with high yield and high quality

Monolayer and bilayer graphene sheets have been produced by a solvothermal-assisted exfoliation process in a highly polar organic solvent, acetonitrile, using expanded graphite (EG) as the starting material. It is proposed that the dipole-induced dipole interactions between graphene and acetonitrile facilitate the exfoliation and dispersion of graphene. The facile and effective solvothermal-assisted exfoliation process raises the low yield of graphene reported in previous syntheses to 10 wt%–12 wt%. By means of centrifugation at 2000 rpm for 90 min, monolayer and bilayer graphene were separated effectively without the need to add a stabilizer or modifier. Electron diffraction and Raman spectroscopy indicate that the resulting graphene sheets are high quality products without any significant structural defects.      

Thermal stability of ceramic fibre in a CVI-processed SiC matrix composite

The thermal stability of the HPZ fibre in a chemical vapour infiltration (CVI)-processed SiC matrix composite was studied. The mechanical properties and fracture behaviour of the untreated and SiC-coated fibres after thermal exposure at different temperatures and atmospheres were characterized. The results show that, below 1000 °C, the strength degradation is negligible. However, severe degradation occurs at temperatures above 1000 °C due to the evolution of CO, SiO, and other gaseous species. Also, pyrolytic C-coating is needed to tailor the interfacial bond strength in the HPZ/SiC composite.