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Structural, thermal, and rheological characteristics of pyrolytically staged condensates in high-pressure pyrolysis of permethylpolysilane
The SiCf/SiC composites have been manufactured by a hybrid route combining chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) techniques. A relatively low deposition rate of CVI SiC matrix is favored ascribing to that its rapid deposition tends to cause a ‘surface sealing’ effect, which generates plenty of closed pores and severely damages the microstructural homogeneity of final composites. For a given fiber preform, there exists an optimized value of CVI SiC matrix to be introduced, at which the flexural strength of resultant composites reaches a peak value, which is almost twice of that for composites manufactured from the single PIP or CVI route. Further, this optimized CVI SiC amount is unveiled to be determined by a critical thickness t0, which relates to the average fiber distance in fiber preforms. While the deposited SiC thickness on fibers exceeds t0, closed pores will be generated, hence damaging the microstructural homogeneity of final composites. By applying an optimized CVI SiC deposition rate and amount, the prepared SiCf/SiC composites exhibit increased densities, reduced porosity, superior mechanical properties, increased microstructural homogeneity and thus reduced mechanical property deviations, suggesting a hybrid CVI and PIP route is a promising technique to manufacture SiCf/SiC composites for industrial applications.
This work describes a physics‐based model to simulate the polymer infiltration and pyrolysis (PIP) manufacturing process for ceramic matrix composites (CMCs). Models have been developed to characterize volumetric distribution of constituents and track porosity inside the composite at different PIP stages utilizing test data from TGA and DSC characterization of a commercial preceramic polymer. Laboratory experiments were done using C/SiC CMC specimens manufactured with a variable number of PIP cycles in order to obtain inputs for the models, and the analytical results have been shown to agree with porosity determined from physical measurements.
Silicon carbide fiber-reinforced silicon carbide matrix composites (SiC/SiC CMCs) are promising candidates for hot gas components in jet engines. Three common manufacturing routes are chemical vapor infiltration, reactive melt infiltration (RMI) and polymer infiltration and pyrolysis (PIP). A combination of the processes seems attractive: the remaining porosity after PIP process can be closed by subsequent siliconization, resulting in a dense material. This work describes a new approach of a combined PIP and RMI process. SiC/SiC CMCs were manufactured by PIP process using Hi-Nicalon Type S fibers. An additional RMI was carried out after a reduced number of PIP cycles. Microstructure was examined via μCT, SEM and EDS. Bending strength was determined to 433 MPa; strain to failure was 0.60 %. The overall processing time was reduced by 55 % compared to standard PIP route. The hybrid material contained 70 % less unreacted carbon than material produced by LSI process alone.
Ultra‐high temperature ceramics (UHTCs) represent an emerging class of materials capable of providing mechanical stability and heat dissipation upon operation in extreme environments, e.g., extreme heat fluxes, chemically reactive plasma conditions. In the last few decades, remarkable research efforts and progress were done concerning the physical properties of UHTCs as well as their processing. Moreover, there are vivid research activities related to developing synthetic access pathways to UHTCs and related materials with high purity, tunable composition, nano‐scaled morphology, or improved sinterability. Among them, synthesis methods considering preceramic polymers as suitable precursors to UHTCs have received increased attention in the last few years. As these synthesis techniques allow the processing of UHTCs from the liquid phase, they are highly interesting, e.g., for the fabrication of ultra‐high temperature ceramic composites (UHT CMCs), additive manufacturing of UHTCs, etc. In the present review, UHTCs are in particular discussed within the context of their physical properties as well as energetics. Moreover, various synthesis methods using preceramic polymers to access UHTCs and related materials (i.e., (nano)composites thereof with silica former phases) are summarized and critically evaluated.
The present research effort was undertaken to develop a new generation of SiC fiber- reinforced engineered ceramic matrix composites (E-CMCs). In contrast to traditional CMCs with a brittle SiC matrix, an E-CMC is designed to consist of a matrix engineered to possess sufficient high temperature plasticity to minimize crack propagation, relatively high fracture toughness, and self-healing capabilities to prevent oxygen ingress to the BN-coated fibers through surface-connected cracks. The present paper discusses the bend strength, isothermal oxidation, microstructures and self-healing properties of several silicide-behaved engineered matrices. Based on the oxidation tests, where it was observed that some of the matrices exhibited either catastrophic oxidation (“pesting”) or spalling of the oxide scale, two engineered matrices, CrSi2/SiC/Si3N4 and a CrMoSi/SiC/Si3N4, were down-selected for further investigation. Four-point bend tests were conducted on these two engineered matrices between room temperature and 1698 K. Although these matrices were brittle at low temperatures, it was observed that the bend strengths and bend ductility increased at high temperatures as the silicide particles became more ductile, which was qualitatively consistent with the theoretically expected behavior that crack blunting at these particles should increase the matrix strength. Additional studies were conducted to study the effects of different additives on the self-healing properties of the engineered matrices, which helped to identify the most effective additives.
In this investigation, terpolymers, copolymers, and homopolymer of acrylonitrile with dimethylaminopropyl acrylamide (DMAPA), itaconic acid (IA) viz., poly(acrylonitrile-ran-3-dimethylaminopropyl acrylamide-ran-itaconic acid) [P(AN-DMAPP-IA)], poly(acrylonitrile-co-3, dimethylaminopropyl acrylamide) [P(AN-DMAPP)] were synthesized with varying amounts of comonomers using solution polymerization process. The chemical structure, composition, bonding network were determined employing infrared, 1H and, 13-carbon nuclear magnetic resonance spectroscopic techniques. Molecular characteristics of as-synthesized polymers such as different kinds of average molecular weights, molecular weight distribution were estimated applying solution viscometry and size exclusion chromatography. The influence of comonomers (DMPAA, IA) on the thermal stabilization characteristics of acrylonitrile terpolymers in comparison with copolymers and homopolymers of acrylonitrile were studied using differential scanning calorimetry (DSC), hyphenated thermal techniques (thermal gravimetry coupled with differential thermal analyzer).The DSC curves of P(AN-DMAPP-IA) exhibit a distinct broader bimodal peaks with thermal exotherm initiating at as low as 165 °C, and followed by two peaks with temperature difference of 42 °C, releasing the evolved heat at a release rate of 0.7–0.11 J g⁻¹s⁻¹over 10 min as compared to 1.2, 7.5 J g⁻¹s⁻¹ in 4.5, 2 min as observed in P(AN-DMAPP), polyacrylonitrile, respectively. The thermal stability of P(AN-DMAPP-IA) and P(AN-DMAPP), as evidenced by TGA-DTA was found to be higher than PAN homopolymers. Specific heat capacity measurements confirmed the DSC results. Bulk densities of P(AN-DMAPP-IA) were in the range 0.31–0.35 g/cc. These results confirm the low-temperature stabilization characteristics and suitability of P(AN-DMAPP-IA) as low cost carbon fiber precursor polymers. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46381.
A series of liquid polycarbosilanes were successfully synthesized by a sequential Grignard coupling reaction of (chloromethyl)triethoxysilane and vinylmagnesium bromide, followed by reduction with lithium aluminium hydride. The use of non-corrosive, easily handleable and storage stable (chloromethyl)triethoxysilane as starting material is an attractive advantage for this route. The as-synthesized polymers containing Si−CH=CH2 and Si − H groups are storage stable and can be easily cured in an inert atmosphere. The ceramic yield of the cured polycarbosilane and the C:Si ratio of the derived ceramic were varied by adjusting the mole ratio of starting materials in the feed. Among them, the polymer with a mole ratio of Si−CH=CH2 to Si − H of 1:10 has the highest ceramic yield at 89.6% after curing at 220 °C and the C:Si ratio of the derived ceramic is 1.48. This kind of polycarbosilane shows high potential as an SiC ceramic precursor in the fabrication of an SiC matrix by the precursor infiltration pyrolysis process. © 2015 Society of Chemical Industry
Characterizations of a liquid polycarbosilane used as SiC matrix precursor were investigated by TG-DTA, FTIR, XRD, etc, which indicated the feasibility of using LPCS as precursor for CLVD process to prepare C/SiC composites. The results show that the inorganic conversion of LPCS to SiC is almost completed at 900 °C, and the crystallization of β-SiC appears at 855 °C approximately. As the temperature increases, the deposit becomes more pure and the crystallinity of β-SiC also increases. The atomic ratio of C/Si in the deposit attained at 1200 °C is near-stoichiometric, the crystallite size of β-SiC is about 33.4 nm.
In this contribution, a review of the use of solid state NMR in the field of Si-based Polymer Derived Ceramics is presented. This synthetic approach allows to prepare a large variety of precursors leading to a wide range of ceramics, both in terms of composition and microstructure: multinuclear solid state NMR is obviously a pertinent spectroscopic tool of investigation since its versatility provides a detailed description of the structure of the polymeric precursors and their pyrolysis products. In a first introductory section, the relevant nuclei that can be probed in such systems are presented with their advantages and drawbacks, as well as the main NMR sequences that have been used to obtain as much structural information as possible in terms of local environments and connectivities. All along this article, examples are then given to illustrate which solid state NMR techniques are currently available to characterise the structure of a large variety of PDCs screening Si-C, Si-C-O, Si-/Ti/B/Al-C-O, Si-C-N, and Si-B/Al/Ti/Zr-N systems.
Pyrolysis process accompanying polydimethylsilane to polycarbosilane transformation and the decomposition behaviors of polydimethylsilane (PDMS), polycarbosilanes (PCS Mk-I/II and III) were investigated applying evolved gas analysis and pyrolysis–gas chromatography-mass spectrometry. Volatile fractions evolved during the reactions were recorded as specific pyrograms. The degradation of the polymer skeleton under argon leads to the evolution of methyl silane and higher linear oligomers in the molar mass range 42 ̶ 348 Da; the escape of hydrogen starts simultaneously. Based on the results, specific pyrolysis patterns and decomposition behavior of PDMS, PCS Mk-III were established. Mass spectral data revealed the structural and compositional identification of molecular moieties evolved during the pyrolysis reactions. Thermal data confirmed and complemented the pyrolysis data.
Most polycrystalline SiC-based fibers were prepared at a sintering temperature higher than 1600 °C. In this work, a ZrO2 reinforced SiC-ZrB2 polycrystalline fiber was prepared at 1400 °C via the polymer-derived ceramic method from a new Zr- and B-containing polycarbosilane. The morphology and microstructure of the polycrystalline nanocomposite fiber were studied using XRD, XPS, EDS, SEM, and TEM. The results showed that t-ZrO2 was formed at relatively lower temperature (<1000 °C). The most interesting result is that the polycrystalline nanocomposite SiC-ZrB2 was generated after heat-treatment at 1400 °C, producing an excellent ZrO2 reinforced SiC-ZrB2 polycrystalline fiber. The present study provides a novel strategy for the fabrication of SiC-based polycrystalline fiber at a relatively low temperature.
SiBCN ceramics have excellent high-temperature stability and anti-oxidation properties, and they are candidates for materials used at extreme conditions. In this paper, amorphous SiBCN(O) ceramics were obtained through pyrolysis of cross-linked polyborosilazanes. The element content could be readily tuned through adjusting the structures of polymer precursors. The cerminization process and microstructure evolution process were systematically investigated. The SiBCN(O) ceramics remain amorphous before 1400 °C. The carbothermal reduction was suppressed because of the presence of boron. The SiBCN(O) ceramics showed 95.7% and 99.2% carbon residual in argon and air at 1600 °C, indicating high-temperature stability and good anti-oxidation properties. These SiBCN(O) precursors/ceramics could be potentially used in composites or protective coatings for harsh environments.
The material behavior of Polymer Infiltration and Pyrolysis based SiC/SiCN composites is studied and the characteristic thermal and mechanical properties in on- (0/90 ◦) and off-axis (±45 ◦) direction are summarized. The tensile properties are determined at room temperature and 1300 ◦C. Based on the ratio of Young’s modulus and strength between on- and off-axis loading, a new approach for the classification of Weak Matrix Composites (WMC) and Weak Interface Composites (WIC) is proposed, which seems to be reasonable for various CMCs. Even without fibre coating mechanical behavior of SiC/SiCN is similar to that of WIC. In order to explain this, a microstructure model is developed and confirmed by analysis of fracture surface. The effect of temperature on the tensile properties is investigated through analysis of residual thermal stresses. Even though at 1300 ◦C the strength is slightly lower, the fracture strain increased significantly from RT to 1300 ◦C.
The influences of polymerization conditions, viz. the kind of catalyst, solvent medium and temperature, on thermal, molecular and structural characteristics of carbon fiber precursor polymer were investigated. The dependence of the exothermic peak temperature (Tpk) and heat release rate on the kind of polymerization process was established. An aqueous redox slurry polymer had narrow exothermic peaks (Tpk = 294–297 °C) and relatively high heat release rates (∆H/∆T = 0.96–3.3 J g−1 s−1), and polymers prepared in solid state, solution and bulk had a broader exotherms with Tpk in the range 264–318 °C and low heat release rates in the range 1.15–3.9 J g−1 s−1. Dynamic and thermomechanical analyses indicate three glass transition temperature ranges, i.e., 42–54 °C, 80–98 °C, and 140–142 °C, which are similar irrespective of catalysts. Branching tendency in aqueous redox slurry polymer was notable beyond the intrinsic viscosity of 250 cm3/g. Bulk densities of the polymers were found to be in the range 0.25–0.45 g cm−3.
The straightforward synthesis of polycarbosilanes (PCS) was achieved via the polycondensation of chlorosilanes and chloromethanes using magnesium (Mg) and titanium(IV) tetrachloride (TiCl4). In this polycondensation, TiCl4 acts as an efficient catalyst and the combined use of Mg and TiCl4 is necessary to produce PCS. The resulting PCS are viscous colored oils that are soluble in organic solvents. The number-average molar mass (Mn; 1.0–1.5 kg mol⁻¹) and molar-mass dispersity (ÐM; 1.3–1.9) values of the resulting PCS were determined by size-exclusion chromatography (SEC) using polystyrene standards. The ¹H, ¹³C{¹H}, and ²⁹Si{¹H} NMR as well as IR spectra of the resulting PCS revealed that they consist of Si–C, Si–H, Si(H)–H, C–C, and Si–Si bonds. The structures of the resulting PCS vary depending on the structures of the monomers and their initial molar ratios. The ceramic yields from the obtained PCS (23-70%) depend on the original structures of the PCS. A plausible mechanism of this polycondensation is proposed based on the results of polycondensations and model reactions.
Synthetic route to β-SiC micro powders through selective solvent precipitation (SSP) of high-molecular-weight (HMF) polycarbosilane. (PCS) was developed. PCS (>3000 Da) was obtained through pyrolysis polycondensation of Polydimethylsilane. Spectral data confirmed that PCS could be fractionated and precipitated during the process without disturbing its molecular structure. Fractionated fine PCS powders were converted into crystalline β-SiC particles through ceramization at 1200-1450°C under argon. XRD patterns revealed a strong and sharp peak at a diffraction angle of 35.8°, thereby confirm the β-SiC structure and purity. SEM analysis confirmed the particle morphology and aggregation of β-SiC particles. Particle size analysis results show that the SiC particles' size is in the range of 10-100 μm. Thermal data prove the high thermal stability of PCS-HMF. β-SiC particles were thermally inert up to 1000 ºC and became reactive beyond 1000 ºC.
Glass and glass-ceramic matrix composites are a special group of ceramic materials, which are the object of continuous academic and industrial research, due to the wide range of properties and superior performance that these materials can achieve and that allow their use in a wide range of applications. They consist of a glass or semi-crystalline matrix (glass-ceramic, developed from controlled crystallization of a glass) incorporating a “reinforcing” phase. Depending on the morphology of the second phase, composites can be classified as dispersion or particle reinforced, laminated reinforced and fiber-reinforced. Varying the microstructure and chemical composition of the glass/glass-ceramic matrix and the nature and distribution of reinforcements, the properties of the composites can be tuned to meet different requirements. The materials developed may be transparent or opaque and they can exhibit almost zero thermal expansion and high fracture toughness. In addition, they are usually resistant to thermal shock and have a high impact resistance and temperature/oxidation capability. Unlike polycrystalline ceramic matrices, glasses show viscous flow, which enables them to be shaped and densified at lower temperatures than their ceramic matrix counterparts which leads to cost-effective manufacturing. This article will review the field of glass and glass-ceramic matrix composites in terms of systems investigated, fabrication technologies and properties.
Thermally induced Kumada rearrangement of –Si–Si– linear chains in polysilanes to –Si–C– chain was conducted in the liquid–vapor phase, followed by isothermal treatments at 410, 460 °C and pressure of 15 kgf cm−2 leading to polycarbosilane(PCS) with higher silane (–Si–H) content in the range 0.46–0.58 mass%. Molecular and thermal changes during the oligomer to polymer transformation were investigated, applying spectral and thermal techniques. FTIR, Raman, and 1H, 13C, and 29Si-NMR analytical results established the chemical structural formula, –Si–Si–, –Si–C–, –Si–H bonding networks, and evolution of –Si–H functionality in as-synthesized polycarbosilane during the thermal transformations. FTIR and 29Si-NMR studies followed the increase in silane content (–Si–H). Raman data revealed the formation and disappearance of –Si–Si– functional group as the transformation progresses. Average molecular mass increased proportionally with polymerization reaction time. Thermogravimetric studies at 1400 °C confirmed a polymer to ceramic conversion (ceramic yield) of as-synthesized PCS increased with the increase in mass average molecular mass and found to be as high as 88% mass. The formation of a high purity green β-SiC powder on heat treatment at 1500 °C confirmed the high molecular polycarbosilane.
Silicon carbide fiber-reinforced silicon carbide matrix composites (SiC/SiC CMCs) are promising candidates for components in the hot gas section of jet engines, as they exhibit high temperature resistance and low density compared to their metal alloy counterparts. Three common manufacturing routes are chemical vapor infiltration, reactive melt infiltration (RMI) and polymer infiltration and pyrolysis (PIP). This work describes a new approach of a combined PIP and RMI process. A combination of the processes seems attractive: the remaining porosity after PIP process can be closed by subsequent siliconization, resulting in a dense material. SiC/SiC CMCs were manufactured by PIP process using Hi-Nicalon Type S fibers. Generally, the processing of SiC/SiC, produced solely by PIP route, is rather time-consuming and the composites show a certain residual porosity. In order to obtain a dense matrix and to reduce the processing time, an additional RMI with silicon alloy is carried out after a reduced number of PIP cycles. To protect the fibers during the siliconization, a CVD fiber coating was applied. Microstructure was examined via microCT, SEM and EDS. Bending strength was determined to 433 MPa; strain to failure was 0.60%. The overall processing time was reduced by 55% compared to standard PIP route. The hybrid material contained 70% less unreacted carbon than material produced by LSI process alone.
SiC fibers can be obtained by the spinning, curing, and heat treatment of polycarbosilane (PCS); however, the properties of the PCS precursor must be considered to set the correct spinning conditions. Although many studies have focused on the synthesis conditions, the characterization (in particular, the structural characteristics) of PCS fibers, and the polymer itself has limitations. In this study, PCS was prepared in two steps, and the growth of the polymer with respect to the reaction conditions was analyzed. We found that PCS is formed and grown by the rearrangement and subsequent condensation reactions of polydimethylsilane (PDMS). Further, fiber formation was affected by the reaction temperature, time, and pressure. Three types of PCS were obtained under different synthetic conditions, and they were all characterized. Regardless of the structural similarity of the PCS fibers (based on the spectroscopic analysis), the polymers showed different thermal and rheological properties. Our findings will be important in improving the production of PCS fibers (and subsequent SiC fibers) with finely controlled properties.
The goal of achieving higher thermal efficiency in nuclear power systems, whether fission or fusion based, has invariably led to the study and development of refractory metals, ceramics, and their composites. Silicon carbide materials, owing to their favorable neutronic and high-temperature properties, have seen extensive study for over half a century in support of this goal. Currently, our community has a relatively deep understanding of the irradiation effects on this system and has developed irradiation-hardened materials that are currently in use for fission reactor fuels and available as structural composites for next generation reactors. Outside of the nuclear arena SiC has also enjoyed significant development with a wide range of ordinary and high-value product now in use including very high temperature commercial aerospace installations such as turbine engines. The paper presents a brief history of the development of SiC, focused on but not limited to irradiation applications that has led to our present understanding of the system for nuclear application.
A low-viscous liquid polycarbosilane (LPCS) is being proposed as a silicon carbide (SiC) precursor for chemical liquid vapour deposition (CLVD) process. The LPCS was characterized by Gel permeation chromatography (GPC), Fourier transformation infrared (FT-IR) spectroscopy, Nuclear magnetic resonance (NMR) spectroscopy, GC-MS and Thermogravimetric analysis (TGA). Spectroscopic investigations together with Gas chromatography Mass spectrum (GC-MS) indicated that the synthesized LPCS is a mixture of cyclic/linear silanes and carbosilanes. TGA showed the complete evaporation of the LPCS below 250 °C, a suitable property for application as a CLVD precursor. Ceramic conversion of LPCS at different temperatures (900 °C, 1100 °C and 1300 °C) under argon, indicated the formation of nano-sized crystallites of β-SiC at 1300 °C.
Polycarbosilane (PCS) is a typical ceramic precursor for the fabrication of SiC fibers and SiC matrix composites. However, it is still costly and not widely employed in industry, mainly due to its low yield by typical chemosynthesis methods.
In this work, we reported for the first time a novel method to significantly increase the PCS yield from 51.3% to 62.8% by recycling the liquid by-products (LBP), which was mainly made up of low-molecular-weight PCS, as revealed by FT-IR, GPC, and NMR analysis. The results showed that recycling LBP did not alter the chemical structure of the PCS products, making it very promising for mass production and application of PCS at the industrial scale.
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.
The detailed structure of poly(dimethylsilylene) was analyzed by 29Si and 13C CP/MAS NMR spectra and theoretical calculations. The end groups were assigned to methoxy and hydroxy groups. This assignment was also supported by the NMR measurement of model compounds. Branched structures are not present. A trace amount of siloxane bonds is contained. The degree of polymerization was estimated to be ca. 580–750 by the 29Si CP/MAS NMR spectra.
This chapter reviews the fundamentals of synthetic approaches and the processing of non-oxide, silicon-based precursor-derived ceramics (PDCs) to reveal possibilities of microstructure development and to give hints on their potential application. It has been shown that the basis for microstructure design of PDCs is the type of macromolecule, its chemical composition, and molecular structure. Therefore, first, synthetic procedures that deliver different types of organometallic, silicon-based, non-oxide ceramic precursors are evaluated. Next, thermolysis reactions are considered, as the structure of the ceramic materials is also widely determined by the structure of the molecular precursors. Further, phase reactions controlling the high-temperature stability of these materials are discussed and finally, recent trends in technologically relevant applications are reported.
Kumada rearrangement of polydimethylsilane using a catalytic process
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