Preparation and properties of SiOC ceramic modified carbon fiber needled felt preform composites

SiOC ceramic modified carbon fiber needled felt preform composites (C_f/SiOC) with densities of 0.4 and 0.7 g/cm³ were prepared by precursor infiltration and pyrolysis (PIP) method using methyltrimethoxysilane (MTMS) and dimethyldimethoxysilane (DMDMS) as precursors. The densification behavior was investigated through scanning electron microscopy (SEM) analysis of microstructure of C_f/SiOC composites undergoing different PIP times. The results indicate that with increase of PIP times, a great amount of SiOC ceramic was introduced into the preform, completely covering on the carbon fibers and occupying the open pores. The thermal performance, mechanical properties, and oxidation resistance of the composites were studied via various tests. The results illustrate that after two-time PIP procedure, thermal conductivities of the composites are 0.41–2.54 and 1.28–4.04 W/(m·K) in z direction and x/y plane, respectively, at RT-1500 °C. The compressive strengths of the composite arrive at 2.1 MPa in z direction and 7.8 MPa in x/y plane, which are almost 3.5 times and 6.5 times, respectively, counterparts of the raw preform. The incorporation of SiOC ceramic can remarkably improve anti-oxidation ability of the composites at 600 °C. The oxidation weight loss is merely 2.1 wt% after 60-min oxidation at 600 °C.     

Microstructure dependent ablation behaviour of precursor derived SiOC ceramic foam for high temperature applications

Ablation behaviour of poly(hydridomethylsiloxane) derived open and closed porous structured SiOC ceramic foams was evaluated using oxy-acetylene flame at 1500 °C for various time durations. X-ray diffraction and scanning electron microscopy analyses of ablated SiOC ceramic foams revealed the formation of a thin protective SiO₂ layer inhibiting further oxidation. The closed porous structured SiOC ceramic foams exhibited very low mass ablation rate in contrast to open porous structured SiOC ceramic foams owing to the differences in thermal energy dissipation mechanism. The feasibility of the plausible foam reduction reactions pertaining to the ablation mechanism was further investigated by computing the Gibbs energy and HR-TEM analysis. The study corroborated the significance of tailoring the microporous structured SiOC ceramic foams as potential thermal protection material for high temperature applications.     

In-situ formation of titanium carbide in carbon-rich silicon oxycarbide ceramic for enhanced thermal stability

Silicon oxycarbide (SiOC) ceramic has attracted great attention as fascinating candidate of high-temperature material, however, its thermal stability is significantly limited by the phase separation at high temperature. Here, a TiC/SiOC ceramic was prepared by pyrolysis of a tetrabutyl titanate modified carbon-rich polysiloxane (TBT/PSO) precursor. The TiC phase is in-situ formed by the carbothermal reaction of TBT-derived amorphous TiO₂ phase with excess free-carbon phase during pyrolysis, and its size and amount increase with the pyrolysis temperature. The SiC phase appears at a higher temperature than the TiC phase and is hindered by the increased Ti content in the TBT/PSO precursor. Thus, the TiC/SiOC ceramic exhibits better thermal stability and crystallization resistance than the TiC-free SiOC ceramic under the thermal treatment (1500 °C) in argon atmosphere. The in-situ formation of metal carbide into the carbon-rich SiOC ceramic would further expand its application at high temperature environments.     

Oxidation and Crack Healing Behavior of a Fine‐Grained Cr2AlC Ceramic

This work reports the oxidation and crack healing behavior of a fine‐grained (~2 μm) Cr₂AlC MAX phase ceramic. The oxidation behavior was investigated in the temperature range 900°C–1200°C for times up to 100 h. The material showed a good oxidation resistance, owing to the formation of a dense and thin α‐Al₂O₃ layer. The microstructure, composition and thickness of the oxide scale were characterized. Its oxidative crack healing behavior as a function of temperature, healing time, and initial crack size was studied systematically. The material showed excellent healing behavior. The main crack healing mechanism is the filling of the crack by oxides well adhering to the crack faces. The crack geometry before and after healing was characterized by X‐ray tomography. Three‐point bend tests showed the dependence of strength recovery at 1100°C as a function of initial crack length and healing time.     

Thermal Shock Behavior of Silicon Oxycarbide Foams

Silicon oxycarbide (SiOC) ceramic foams, obtained from the pyrolysis of a preceramic polymer, were subjected to thermal multiple cycles from 800°–1200°C to room temperature in a water bath. Flexural and compression strengths, as well as elastic modulus, were characterized before and after quenching. Excellent thermal shock and cycling resistance behavior was observed, with only moderate strength and stiffness degradation. The phase assemblage of the foam remained unchanged, and no crack formation in the foams was observed. However, microstructural characterization revealed the development of porosity in the struts and cell walls due to the oxidation of residual carbon in the amorphous SiOC material, thereby contributing to a small decrease in stiffness after quenching.     

SiOC Ceramic Foams through Melt Foaming of a Methylsilicone Preceramic Polymer

A process for the production of SiOC ceramic foams has been for the first time developed through melt foaming of a siloxane preceramic polymer with the help of a blowing agent, followed by pyrolysis under an inert atmosphere. The raw material consisted of a methylsilicone resin, a catalyst (which accelerated the cross-linking reaction of the silicone resin) and a blowing agent (which generated gas above 210°C). Methylsilicone resin foams were obtained through controlling the melt viscosity around 210°C, temperature where the blowing agent started to decompose, by varying the initial molecular weight of the preceramic polymer and the amount of the catalyst. The obtained SiOC ceramic foams exhibited excellent oxidation stability up to 1000°C, as shown by thermal gravimetric analysis (TGA). As expected, the mechanical properties of the SiOC ceramic foams varied as a function of their bulk density, possessing a flexural strength up to 5.5 MPa and a compression strength up to 4.5 MPa. The main steps in the process, namely foaming and pyrolysis, were analyzed in detail. The viscosity change was analyzed as a function of temperature by the dynamic shear measurement method. The pyrolysis process of foams was analyzed by TGA coupled with infrared spectroscopy (IR).     

先驱体转化法制备SiOC多孔木质陶瓷

用杉木木粉浸渍高分子先驱体有机硅树脂,改变烧结温度和保温时间制备 SiOC 多孔木质陶瓷。用 X 射线衍射、扫描电子显微镜、红外光谱及热分析对样品的物相及微观结构进行了表征,并用四探针法测试样品的电学性能。结果表明：样品由 SiOC 玻璃相、α-石英和自由碳组成,化学结构主要含有 Si—O,Si—O—R,Si—C,C=C 和 C—H2。α-石英颗粒呈球状分布在样品的孔洞表面。样品在 Ar 中具有较好的热稳定性。但由于 SiOC 玻璃相和自由碳的氧化反应,样品在 O2中的质量损失较大。当烧结温度达 1200 ℃,保温 1h 时,样品具有较低的体积电阻率（0.03 ·cm）。     

In-situ pyrolyzed polymethylsilsesquioxane multi-walled carbon nanotubes derived ceramic nanocomposites for electromagnetic wave absorption

Iron acetylacetonate (Fe(acac)₃) modified polymethylsilsesquioxane (PMS), simplified as PMS(Fe), was firstly obtained from PMS and Fe(acac)₃ via the condensation reaction. Multi-walled carbon nanotubes (MWCNTs) were then introduced to fabricate the corresponding MWCNTs/SiC nanocrystals/amorphous SiOC ceramic composites via pyrolyzed process. Owing to the catalytic effect of iron and heterogeneous nucleation promoted by MWCNTs, SiC nanocrystals were separated from SiOC amorphous ceramic matrix under 1400?°C. When the mass fraction of MWCNTs was 9?wt%, the obtained MWCNTs/SiC nanocrystals/amorphous SiOC ceramic composite (C9) demonstrated high microwave-absorbing properties. The minimum reflection loss (RL_min) and effective absorption bandwidth (EBA) of the obtained C9 at X-band (8.2–12.4) reached ?61.8?dB and 2.6?GHz (a thickness of 2.19?mm), respectively. Compared with other polymer-derived ceramics (PDCs), the RL_min was higher and the required thickness was thinner. This excellent microwave-absorbing property was due to the interfacial polarization relaxation generated between nanocrystals (MWCNTs & SiC) and amorphous SiOC, and the formed complete conductive networks inside the ceramic composites.     

Structural Design of Polymer‐Derived SiOC Ceramic Aerogels for High‐Rate Li Ion Storage Applications

SiOC ceramic aerogels with different porosity, pore size, and specific surface area have been synthesized through the polymer‐derived ceramic route by modifying the synthesis parameters and the pyrolysis steps. Preceramic aerogels are prepared by cross‐linking a linear polysiloxane with divinylbenzene (DVB) via hydrosilylation reaction in the presence of a Pt catalyst under highly diluted conditions. Acetone and cyclohexane are used as solvent in our study. Wet gels are subsequently supercritically dried with CO₂ to get the final preceramic aerogels. The SiOC ceramic aerogels are obtained after a pyrolysis treatment at 900°C in two different atmospheres: pure Ar and H₂ (3%)/Ar mixtures. The nature of the solvent has a profound influence of the aerogel microstructure in terms of porosity, pore size, and specific surface area. Synthesized SiOC ceramic aerogels have similar chemical compositions irrespective of processing conditions with ~40 wt% of free carbon distributed within remaining mixed SiOC matrix. The BET surface areas range from 215 m²/g for acetone samples to 80 m²/g for samples derived from cyclohexane solvent. The electrochemical characterization reveals a high specific reversible capacity of more than 900 mAh/g at a charging rate of C (360 mA/g) along with a good cycling stability. Samples pyrolyzed in H₂/Ar atmosphere show a high reversible capacity of 200 mAh/g even at a high charging/discharging rate of 20 C. Initial capacities were recovered after whole cycling procedure indicating their structural stabilities resisting any kind of exfoliations.     

Growth of multi-morphology amorphous silicon oxycarbide nanowires during the laser ablation of polymer-derived silicon carbonitride

Multi-morphology amorphous SiOC nanowires were successfully prepared within the interfacial interstices between the unaffected SiCN ceramic and the bracket during the laser ablation of polymer-derived SiCN ceramic in a low-pressure argon atmosphere. Laser irradiation experiments were performed using a continuous-wave CO₂ laser, and the gas source for the growth of amorphous SiOC nanowires was provided by the laser ablation of the SiCN ceramic. X-ray photoelectron spectroscopy shows that the amorphous SiOC nanowires possess a SiO₂ dominated nanostructure, and the formation of amorphous SiOC nanowires is attributed to the good diffusivity of CO in SiO₂. The morphologies of the amorphous SiOC nanowires include straight nanowires, beaded nanowires, helical nanowires, and branched nanowires, and these are determined by the flowing state of the reactant gases, the laser power, and the surface morphology of the SiCN ceramics. Each amorphous SiOC nanowire with specific morphology can be uniformly distributed in separate regions, which makes it possible to control the growth of amorphous SiOC nanowires in different morphologies.     

SiOC Ceramic Monoliths with Hierarchical Porosity

SiOC glass monoliths possessing hierarchical porosity were produced by a one-pot processing method. Periodic mesoporous organosilica (PMO) particles were embedded into a foamed siloxane preceramic polymer. After pyrolysis at 1000°C in inert atmosphere, open celled, permeable SiOC ceramic monoliths with a high amount of pores, ranging in size from hundred of micrometers to a few nanometers, were obtained. The components possessed a specific surface area of 137 m²/g, indicating the retention of most of the mesopores after the pyrolytic conversion of the PMO precursor particles. These fillers converted to truncated rhombic dodecahedral SiOC mesoporous micron-sized grains, homogeneously distributed throughout the SiOC cellular matrix. The produced porous ceramics possessed compression strength of about 1.7 MPa, which is adequate for their use in several engineering applications.     

Polymer-Derived SiOC/ZrO2 Ceramic Nanocomposites with Excellent High-Temperature Stability

Polymer-derived SiOC/ZrO₂ ceramic nanocomposites have been prepared using two synthetic approaches. A commercially available polymethylsilsesquioxane (MK Belsil PMS) was filled with nanocrystalline zirconia particles in the first approach. The second method involved the addition of zirconium tetra( n -propoxide), Zr(OⁿPr)₄, as zirconia precursor to polysilsesquioxane. The prepared materials have been subsequently cross-linked and pyrolyzed at 1100°C in argon atmosphere to provide SiOC/ZrO₂ ceramics. The obtained SiOC/ZrO₂ materials were characterized by means of X-ray diffraction, elemental analysis, Raman spectroscopy as well as transmission electron microscopy. Furthermore, annealing experiments at temperatures from 1300° to 1600°C have been performed. The annealing experiments revealed that the incorporation of ZrO₂ into the SiOC matrix remarkably increases the thermal stability of the composites with respect to crystallization and decomposition at temperatures exceeding 1300°C. The results obtained within this study emphasize the enormous potential of polymer-derived SiOC/ZrO₂ composites for high-temperature applications.     

Processing and characterization of particles reinforced SiOC composites via pyrolysis of polysiloxane with SiC or/and Al fillers

Polysiloxane loaded with SiC as inert filler, and Al as active filler, was pyrolyzed in nitrogen to fabricate SiOC composites, and the processing and properties of the filled SiOC composites were investigated. Adding SiC fillers could reduce the linear shrinkage of filler-free cured polysiloxane in order to obtain monolithic SiC/SiOC composites. The flexural strength of SiC/SiOC composites reached 201.3 MPa at a SiC filler content of 27.6 vol.%. However, SiC/SiOC composites exhibited poor oxidation resistance, thermal shock resistance and high temperature resistance. Al fillers could react with hydrocarbon generated during polysiloxane pyrolysis at 600 °C and N₂ at 800 °C to form Al₄C₃ and AlN, respectively. The volume expansions resulting from these two reactions were in favor of the reduction in linear shrinkage and the improvement in flexural strength of SiC/SiOC composites. The flexural strength of Al-containing SiC/SiOC composites was 1.36 times that of SiC/SiOC composites without Al at an Al filler content of 20 vol.%. The addition of Al fillers remarkably improved the high temperature resistance and oxidation resistance of SiC/SiOC composites, but not thermal shock resistance.     

Oxidation resistant ceramic foam from a silicone preceramic polymer/polyurethane blend

Silicon oxycarbide (SiOC) ceramic foams, produced by the pyrolysis of a foamed blend of a methylsilicone preceramic polymer and polyurethane (PU) in a 1/1 wt.% ratio, exhibit excellent physical and mechanical properties. The proposed process allows to easily modify the density and morphology of the foams, making them suitable for several engineering applications. However, it has been shown that, due to residual carbon present in the oxycarbide phase after pyrolysis, the foams are subjected to an oxidation process that reduces their strength after high temperature exposure to air (12 h 1200°C). A modified process, employing the same silicone resin preceramic polymer but a much lower PU content (silicone resin/PU=5.25/1 wt.% ratio), has been developed and is reported in this paper. Microstructural investigations showed that carbon rich regions deriving from the decomposition of the polyurethane template are still present in the SiOC foam, but have a much smaller dimension than those found in foams with a higher PU content. Thermal gravimetric studies performed in air or oxygen showed that the low-PU containing ceramic foams display an excellent oxidation resistance, because the carbon-rich areas are embedded inside the struts or cell walls and are thus protected by the dense silicon oxycarbide matrix surrounding them. SiOC foams obtained with the novel process are capable to maintain their mechanical strength after oxidation treatments at 800 and 1200°C (12 h), while SiOC foams obtained with a higher amount of PU show about a 30% strength decrease after oxidation at 1200°C (12 h).     

热解含铁聚硅氧烷原位生长硅氧碳纤维制备硅氧碳复合材料

将铁氯化物混入聚硅氧烷前驱体进行交联成型和热解,利用热解中在聚硅氧烷中形成的孔隙和在孔隙中形成的铁颗粒为催化剂,在硅氧碳陶瓷基体中原位生长出硅氧碳纳米纤维,制备出硅氧碳陶瓷和硅氧碳纤维复合材料。用扫描电子显微镜观察材料断面,结果显示：在硅氧碳陶瓷基体中生长出纳米纤维,部分纤维取向分布,纤维紧贴于硅氧碳陶瓷基体,二者呈良好结合;能谱分析显示纤维中含硅、氧和碳,证实其为硅氧碳。所制得的硅氧碳陶瓷和硅氧碳纤维的复合结构不同于通常热解纯聚硅氧烷形成的单相的硅氧碳结构,在硅氧碳基体中的硅氧碳纤维是在聚硅氧烷前驱体中引入的铁催化剂在热解过程中通过催化聚硅氧烷一维生长形成的,该过程可用于发展一步法原位制备纳米纤维前驱体陶瓷复合材料。     

A new precursor of liquid and curable polysiloxane for highly cost-efficient SiOC-based composites

Polymer-derived SiOC matrix composites are very promising structure ceramics in moderate temperature (<?1200?°C) application, in view of their outstanding comprehensive performance and satisfying costs. Herein, we developed a new precursor of liquid and curable polysiloxane (LC-PSO) with Si-H and vinyl side groups to meet the requirements of precursor infiltration and pyrolysis (PIP) route for preparing highly cost-efficient composites. It was found that the crosslink structure was completely built below 150?°C through hydrosilylation reaction, thus converting the liquid precursor into solid state and rendering the cured product with a high ceramic yield of 81.1%. The addition reaction was also greatly inhibited under 5?°C in order that the LC-PSO was able to store for long time. We revealed that although the SiOC ceramic underwent structural rearrangement and slow crystallization during pyrolysis, no mass loss was observed below 1400?°C before the carbothermal reaction was initiated. The prepared carbon fiber-reinforced SiOC (2D-Cf/SiOC) composite verified the feasibility of directly using LC-PSO as precursor for PIP process, and the mean flexure strength and modulus of the composite were 253.3?MPa and 33.3?GPa, respectively. Our work presents the great potential of LC-PSO in fabricating highly cost-efficient CMCs for moderate temperature application.     

Fabrication of SiOC Ceramic Microparts and Patterned Structures from Polysiloxanes via Liquid Cast and Pyrolysis

SiOC ceramic microparts and patterned microstructures are fabricated from their metal masters using polysiloxanes as a precursor, which consists of polyhydromethylsiloxane (PHMS) and 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane (D₄Vi), via mold transfer, liquid cast, crosslink, and pyrolysis. The mixed liquid of PHMS and D₄V was cast with the polydimethylsiloxane (PDMS) negative molds transferred from the masters and solidified via hydrosilylation of PHMS and D₄Vi under controlled heating. Because both the precursors and mold materials are polysiloxanes, their chemical similarities ensure the excellent contact between the two phases that allows for the precise duplication of the master microstructures into the polysiloxanes. Strategies were developed for the use of a two-step controlled heating method and the use of polysiloxane as a support in the processes of demolding, crosslinking, and pyrolysis in order to ensure the bonding qualities of both the crosslinked bodies and the pyrolyzed microstructures. Through this route, we obtained dense and crack-free SiOC ceramic microngears and arrayed holes of well-duplicated microstructures with a resolution down to the submicrometers. Moreover, the polysiloxanes allow the direct imprints with the metal masters to form inversed microstructures of SiOC ceramics, as demonstrated by the formation of microchannels and various motifs of SiOC ceramics from their metal counterparts.     

Self-healing of Al2O3 containing Ti microparticles

This work explores the possibility of using embedded micron-sized Ti particles to heal surface cracks in alumina and to unravel the evolution of the crack filling process in case of pure solid-state oxidation reactions. The oxidation kinetics of the Ti particles is studied and the results are applied in a simple model for crack-gap filling. An activation energy of 136?kJ/mol is determined for the oxidation of the Ti particles having an average particle size of 10?µm. The almost fully dense alumina composite containing 10?vol% Ti has an indentation fracture resistance of 4.5?±?0.5?MPa?m^1/2. Crack healing in air is studied at 700, 800 and 900?°C for 0.5, 1, and 4?h and the strength recovered is measured by 4-point bending. The optimum healing condition for full strength recovery is 800?°C for 1?h or 900?°C for 15?min. Crack filling is observed to proceed in three steps i.e., local bonding at the site of an intersected Ti particle, lateral spreading of the oxide and global filling of the crack. It is discovered that, although significant strength recovery can be attained by local bonding of the intersected particles, full crack filling is required to prevent crack initiation from the damaged region upon reloading. The experimental results observed are in good agreement with the predictions of a simple discrete crack filling/healing model.     

Enhanced microwave-absorption properties of polymer-derived SiC/SiOC composite ceramics modified by carbon nanowires

Novel microwave-absorbing SiOC composite ceramics with dual nanowires (carbon nanowires (CNWs) and SiC nanowires) with high performances were fabricated by using the polymer-derivation method and heat treatment in Ar atmosphere. The introduction of CNWs in the amorphous SiOC ceramics promotes the ceramic crystallization into SiC nanoparticles and SiC nanowires at lower annealing temperatures, which leads to multi-phases and multiple nano heterogeneous interfaces. The distinctive architectures largely increase the interfacial and dipole polarizations of the composite ceramics. The CNWs/SiC/SiOC composite ceramics exhibit excellent microwave-absorption properties in the Ku band (12.4–18 GHz). The minimum reflection coefficient (RC) is -24.5 dB at a thickness of 1.8 mm, while the maximum effective absorption bandwidth (EAB, the corresponding frequency band in which RC is smaller than -10 dB) is 4.8 GHz at a thickness of 1.9 mm, which make the CNWs/SiC/SiOC composite ceramics promising electromagnetic-wave-absorbing materials.     

Preparation of Micro‐/Mesoporous SiOC Bulk Ceramics

Micro‐/mesoporous SiOC bulk ceramics with high surface area and bimodal pore size distribution were prepared by pyrolysis of polysiloxane in argon atmosphere at 1100°C–1400°C followed by etching in hydrofluoric acid solution. Their thermal behaviors, phase compositions, and microstructures at different nano‐SiO₂ filler contents and pyrolysis temperatures were investigated by XRD, SEM, DSC, and BET. The SiO₂ fillers and SiO₂‐rich clusters in the SiOC matrix act as pore‐forming sites and can be etched away by HF. At the same time, the SiO₂ filler promotes SiOC phase separation during the pyrolysis. The filler content and pyrolysis temperature have important effects on phase compositions and microstructures of porous SiOC ceramics. The resulting porous SiOC bulk ceramic has a maximum specific surface area of 822.7 m²/g and an average pore size of 2.61 nm, and consists of free carbon, silicon carbide, and silicon oxycarbide phases.