Preparation of highly porous ZrB2/ZrC/SiC composite monoliths using liquid precursors via direct drying process

We developed a simple liquid precursor method for the syntheses of porous ZrB₂/ZrC/SiC composite monoliths. Furfuryl alcohol (FA), zirconium n-butoxide, tetraethyl orthosilicate and boric acid are used as the raw materials. By combining the polymerization of FA and gelation of inorganic sols, porous hybrid monoliths are prepared by direct drying the wet gels. The inorganic and organic polymers possibly form interpenetrated network which provides the robustness for the wet gel to withstand the severe changes during dessication. When heat-treated at 1600 °C, hybrid gels are converted into porous ZrB₂/ZrC/SiC monoliths. The microstructure of the ZrB₂/ZrC/SiC monoliths can be easily tailored by controlling the synthesis conditions. The porosities of the ZrB₂/ZrC/SiC monoliths can be tuned around 74.3–81.6%, while the average pore diameters can be tuned ranging from 1.0 to 8.5 μm with pretty narrow distribution. The compressive strengths of such highly porous ceramics are in the range of 1.2–1.9 MPa.     

Fabrication and properties of Cf/Ta4HfC5-SiC composite via precursor infiltration and pyrolysis

In this study, a C_f/Ta₄HfC₅-SiC ultra-high-temperature ceramic matrix composite exhibiting a homogeneous phase distribution was successfully fabricated via precursor infiltration and pyrolysis processing. Initially, the pyrolysis and solid solution mechanisms exhibited by the Ta₄HfC₅ precursor were investigated and characterized through TG-MS and XRD analysis. The as-fabricated C_f/Ta₄HfC₅-SiC composite exhibited a density and open porosity of 2.84 g/cm³ and 10.62 vol%, respectively. It also exhibited outstanding mechanical properties, with a flexural strength of 339 ± 20 MPa and fracture toughness of 11.56 ± 0.77 MPa·m^1/2. The C_f/Ta₄HfC₅-SiC composite demonstrated strong ablation resistance under a heat flux of 5 MW/m² at ~2400℃, with corresponding linear and mass recession rates of 5.33 μm/s and 6.18 mg/s, respectively. The combination of strong mechanical properties and ablation resistance provides a solid basis for the use of the C_f/Ta₄HfC₅-SiC composite in a new generation of ultra-high-temperature materials.     

Effects of SiC/HfC ratios on the ablation and mechanical properties of 3D Cf/HfC-SiC composites

Effects of SiC/HfC ratios on the ablation and mechanical properties of 3D C_f/HfC–SiC composites by precursor impregnation and pyrolysis (PIP) process were investigated systematically. Both strength (flexural and compressive strength) and modulus increase as the SiC/HfC ratio are improved. The compact and stiff HfC-SiC matrix in addition to the carbon fiber and PyC interphase with less reaction damage accounts for the improved mechanical properties of C_f/HfC-SiC with higher SiC/HfC ratios. Meanwhile, both weight loss and erosion depth of C_f/HfC-SiC are improved with the increased SiC/HfC ratios. Therefore, in order to balance the ablation and mechanical properties, an appropriate SiC/HfC ratio should be considered.     

Seed-mediated growth of ZrC whisker and its toughening effect on ZrB2-SiC-C ceramic

ZrC whiskers (ZrC_w) hold great promise in improving the strength and toughness of ultra-high temperature ceramics (UHTCs) without reducing their high-temperature stability. However, obtaining high quality ZrC_w has been challenging. Herein, we propose a novel method for easily synthesizing catalyst-free ZrC_w by seed-mediated growth technique, in which single crystal ZrC nanoparticle (ZrC_np) was used as seed crystal and ZrO₂-C-NaF mixture was used as precursor system. The effect of ZrC_np, NaF, and, synthesis temperature on the growth of ZrC_w was studied and the reaction process was analyzed based on the experimental results. The vapor-solid (V-S) growth mechanism was proved to be the dominating growing mechanism. Subsequently, the synthesized ZrC_w were added to a ZrB₂-SiC-C ceramic. Compared with the baseline, ZrC_w reinforced ZrB₂-SiC-C exhibited a remarkable combination of high strength and high toughness (592 ± 30 MPa and 7.1 ± 0.8 MPa·m^1/2). This novel synthesis method of ZrC_w may be applicable to the synthesis of other carbide ceramic whiskers and enrich the design of high performance ultra-high temperature ceramics.     

Recycling of waste red mud for fabrication of SiC/mullite composite porous ceramics

SiC/mullite composite porous ceramics were fabricated from recycled solid red mud (RM) waste. The porous ceramics were formed using a graphite pore forming agent, RM, Al(OH)₃ and SiC in the presence of catalysts. The influence of firing temperature and the pore-forming agent content on the mechanical performance, porosity and the microstructure of the porous SiC ceramics were investigated. Optimal preparation condition were determined by some testing. The results indicated that the flexural strength of specimens increased as a function of firing temperature and a reduction in graphite content, which concomitantly decreased porosity. The ceramic prepared under optimal conditions having 15?wt% graphite and sintered at 1350?°C, demonstrated excellent performance. Under optimal preparation conditions the flexural strength and porosity of the ceramic were 49.4?MPa and 31.4%, respectively. Scanning electron microscopy observation result showed that rod-shape mullite grains endowed the samples with high flexural strength and porosity. X-ray diffraction analysis indicated that the main crystallization phases of the porous ceramics were 6H-SiC, mullite, cristobalite and alumina. This work demonstrates that RM can be sucessfully reused as a new raw material for SiC/mullite composite porous ceramics.     

Synthesis of nanocrystalline TaC powders via single-step high temperature spray pyrolysis from solution precursors

Nanocrystalline powders for high temperature ceramics (HTC) and ultrahigh temperature ceramics (UHTC) are important materials for aerospace and other important industrial applications. In this study, a low cost, single-step synthesis method for nanocrystalline HTC and UHTC powders is reported, which is based on high temperature spray pyrolysis (HTSP) process. The synthesis starts with solution of oxide and carbon precursors dissolved in common organic solvents, which are broken into fine droplets (e.g., via nebulizer). The droplets then go through processes of solvent removal, thermolysis, and rapid in situ carbothermal reduction (CTR), all in one single pass in a tube furnace operated at high temperature. The synthesis method was demonstrated successfully using the example of tantalum carbide (TaC) from precursors of tantalum chloride (TaCl₅) and phenolic resin in a single-step HTSP process with maximum temperature of 1650 °C in one pass that finished within minutes, yielding agglomerated nanocrystalline TaC UHTC powders.     

New approach to evaluate the influence of compressive stress on the oxidation of non-oxide ceramics

The oxidation process of non-oxide ceramics (NOCs) is accompanied with the formation of the compressive stress generated within the oxide film, which in turn affects the oxidation behavior. To shed light on the influence of the compressive stress, a modified model combining the stress equilibrium equation and RPP model has been herein proposed. This modified model can not only quantify the effect of stress evolution on the oxidation process but also accurately describe the relationship between oxidation fraction and other factors including time and temperature, etc. The comparison between experimental data and theoretical calculations for the oxidation of different NOCs verifies the validity of the modified model. It is predictable that this modified model can be applied to treat the oxidation behavior of different NOCs with the consideration of compressive stress.     

基于核应用下碳化硅陶瓷及其复合材料的连接研究进展

SiC陶瓷及其复合材料凭借其自身固有的核辐射下的稳定性而有望成为新一代核裂变以及未来核聚变反应堆中重要的结构材料.能否满足核应用环境下各种苛刻条件而实现完美连接是其能够得到最终应用的关键.本文综述了目前国际上基于核应用上SiC陶瓷及其复合材料的几种连接工艺的发展情况.     

A facile method to prepare ZrC nanofibers by electrospinning and pyrolysis of polymeric precursors

Zirconium carbide (ZrC) is one of the most attractive ultra-high temperature ceramics due to its excellent properties. ZrC nanofibers were fabricated via electrospinning and pyrolysis of a novel polymeric precursor, Polyzirconosaal (PZSA), with the addition of polyvinylpyrrolidone (PVP) as the spinning aid. The polymer PZSA was prepared from the chemical reaction between Polyzirconoxane (PZO) and Salicyl alcohol. The as-spun PZSA/PVP fibers were converted to ZrC nanofibers with a diameter ~200 nm after carbothermal reduction at 1300 °C in argon. The obtained ZrC nanofibers maintained its excellent fibrous morphology. The microstructures exhibited that nanoscale ZrC particles dispersed in the fibers containing free carbon. The average crystallite size of ZrC particles using Scherrer method was 42 nm. The obtained ZrC nanofibers were characterized by XRD, SEM and TEM. The current material would be particularly useful for applications such as catalyst support, filters, gas storage, supercapacitors, and phase change material support in thermal management systems.     

Synthesis and thermal performance of polymer precursor for ZrC ceramic

A novel ZrC preceramic precursor (PZC) was compounded via liquid phase chemical reaction without any organic solvent choosing ZrOCl₂·8H₂O and polyvinyl alcohol as Zr source and C source, respectively. The composition and structure of ZrC precursor were analysed through XRD, FT-IR, XPS and SEM. The results showed both Zr-O-C bonds and Zr-O bonds existed in the precursor. The results observed by SEM showed that many irregular particles were generated, whose particle sizes were mainly in the range of 0.2–3 μm. In addition, particle aggregation can be easily observed. Besides, the thermal property and pyrolysis process of PZC were studied. In accordance with XRD, the initial temperature of the earliest detection of ZrC in pyrolysis products of PZC was 1300 °C. Monoclinic ZrO₂ and tetragonal ZrO₂ can be observed at this temperature as well. Ulteriorly, when the pyrolysis temperature was risen up to 1500 °C, only ZrC ceramic can be found.     

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.     

Pursuing enhanced oxidation resistance of ZrB2 ceramics by SiC and WC co-doping

The oxidative degradation of ZrB₂ ceramics is the main challenge for its extensive application under high temperature condition. Here, we report an effective method for co-doping suitable compounds into ZrB₂ in order to significantly improve its anti-oxidation performance. The incorporation of SiC and WC into ZrB₂ matrix is achieved using spark plasma sintering (SPS) at 1800?°C. The oxidation behavior of ZrB₂-based ceramics is investigated in the temperature range of 1000?°C–1600?°C. The oxidation resistance of single SiC-doped ZrB₂ ceramics is improved due to the formation of silica layer on the surface of the ceramics. As for the WC-doped ZrB₂, a dense ZrO₂ layer is formed which enhances the oxidation resistance. Notably, the SiC and WC co-doped ZrB₂ ceramics with relative density of almost 100% exhibit the lowest oxidation weight gain in the process of oxidation treatment. Consequently, the co-doped ZrB₂ ceramics have the highest oxidation resistance among all the samples.     

Synthesis of a meltable polyzirconosilane precursor for SiZrNC multinary ceramics

Polymeric precursors with refractory metal in the main chain while still possess meltable/soluble characteristics are highly desired for ultra-high temperature ceramics (UHTCs). Herein, we proposed a stepwise synthesis of polyzirconosilane (PZCS) with Zr-C-Si-N polymeric chain. The reaction mechanism was discussed based on the frontier molecular orbital theory and atom transfer radical polymerization, which was initiated and propagated by a low oxidation state active specie of Cp₂Zr(II). The PZCS precursor has excellent soluble and meltable properties with a softening point of 80.8 ~ 89.2 ^oC, which could be spun into green fibers by melt spinning technique. The ceramization process of PZCS was studied, and the resulted SiZrNC multinary ceramics were composed of ZrC/SiC nanocrystals embedded by graphitized carbon phase. The excellent mouldable properties, oxygen-free compositions and high Zr content of PZCS make it an ideal precursor for the preparation of UHTCs matrixes and fibers.     

Laser ablation behavior of SiHfC-based ceramics prepared from a single-source precursor: Effects of Hf-incorporation into SiC

Laser ablation test of SiHfC-based ceramic nanocomposites as well as ceramic matrix composites (CMCs) was conducted by exposure to a CO₂ laser beam in air. Laser ablation behavior and possible degradation mechanisms of dense monolithic HfC/SiC ceramic nanocomposites as well as of C_f/SiHfC CMCs were investigated. Dense SiC monoliths and C_f/SiC CMCs were exposed to same laser ablation conditions and considered as reference materials. The evolution of microstructure and chemical/phase composition of the studied ceramics was addressed by scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX) as well as by X-ray diffraction. The results reveal that from the center to the edge of the damaged region of the materials three sections with different surface morphologies and ablation mechanisms are identified. The comparation between the SiC-based monoliths and CMCs with and without Hf demonstrates the positive effects of Hf-incorporation on their laser ablation resistance.     

Pressure‐less joining of C/SiC and SiC/SiC by a MoSi2/Si composite

A MoSi₂/Si composite obtained in situ by reaction of silicon and molybdenum at 1450°C in Ar flow is proposed as pressure‐less joining material for C/SiC and SiC/SiC composites. A new “Mo‐wrap” technique was developed to form the joining material and to control silicon infiltration in porous composites. MoSi₂/Si composite joining material infiltration inside coated and uncoated C/SiC and SiC/SiC composites, as well as its microstructure and interfacial reactions were studied. Preliminary mechanical strength of joints was tested at room temperature and after aging at service temperatures, resulting in interlaminar failure of the composites in most cases.     

Fabrication and optimization of 3D-Cf/HfC-SiC-based composites via sol-gel processing and reactive melt infiltration

In this work, 3D-C_f/HfC-SiC-based composites were fabricated and optimized via reactive melt infiltration (RMI) of Si into porous C_f/HfC-C preforms prepared by a sol-gel processing. The physical and chemical processes involved during the fabrication were identified and analyzed in details. It is revealed that fibers and interphase of the composites can be eroded during carbothermal reduction process, which can be further aggravated during RMI, with the formation of Hf-containing substance on the fibers surface. The fibers and interphase degradation is mainly induced by the reactions between HfO₂ and C/SiC interphase layers at elevated temperatures. Accordingly, a two-step carbothermal reduction treatment was proposed for the optimization of the fabrication procedure. As a result, less fiber/interphase erosion and improved mechanical properties are achieved in the composites, with the bending strength increased by ～49 % (from 214.1 ± 15.7 MPa to 319.0 ± 26.0 MPa).     

Preparation,structural evolution,and performance of heat-resistant organosilicon polymer adhesives for joining SiC ceramics

There is an urgent need for heat-resistant adhesives with high bonding strength in order to able to fabricate large and complex SiC components for aeronautical and astronautical applications. In this study, heat-resistant organic adhesives prepared using an organosilicon polymer and inorganic additives (B₄C and SiO₂) were used successfully to bond SiC ceramics. The prepared adhesives were characterised through shear strength tests as well as using thermogravimetry-differential scanning calorimetry, Fourier-transform infrared spectroscopy, X-diffraction analysis, and scanning electron microscopy. The adhesives exhibited high room-temperature shear strengths (greater than 15 MPa) after being subjected to heat treatments at 200–1200°C. Further, the high-temperature shear strengths of the adhesives at 200, 400, 600, 800, and 1000°C were 10.5, 10.1, 7.7, 8.6, and 8.4 MPa, respectively. The high performance of the adhesives indicated that they should be suitable for joining SiC-based materials for use in high-temperature applications.     

Low-temperature fabrication of porous ZrC/C composite material from molten salts

In this work, biological ceramic-wood porous ZrC/C materials were prepared in a KCl-KF molten salt reaction medium, using Zr as a metal source and a C template as C source. The effects of reaction temperature and salts/Zr mole ratio on the formation of porous biomorphic ZrC/C ceramics were investigated. The phase compositions and morphological structures of the C templates and porous ZrC/C ceramics were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and pore size analysis. The results showed that the ZrC grains were equiaxed and enhanced the oxidation resistance of the material. Furthermore, a possible reaction mechanism for the formation of porous biomorphic ZrC/C ceramics in molten salt is proposed.     

Synthesis of the ternary metal carbide solid-solution ceramics by polymer-derived-ceramic route

The polymer-derived-ceramic (PDC) route has been widely used to fabricate the transition-metal carbides (TMCs). Previously reported works focused mainly on the synthesis of the single or binary TMCs, while the synthesis of the ternary or more component TMCs was rarely reported. Herein, a class of the ternary TMCs, namely (Nb_1/3Zr_1/3Ta_1/3)C solid-solution ceramics, was successfully synthesized via PDC route for the first time. The as-synthesized ceramics exhibited the particle-like morphology with an average particle size of ~250 nm and showed a single rock-salt crystal structure of metal carbides. At the same time, they had high compositional uniformity from nanoscale to microscale. In addition, they possessed low-oxygen impurity content of 0.79 wt% and moderate-carbon impurity content of 8.98 wt%. Such work provides a novel route to fabricate the ternary or more component TMCs.     

Improved dielectric and electromagnetic interference shielding properties of ferrocene-modified polycarbosilane derived SiC/C composite ceramics

In order to enhance the dielectric and electromagnetic interference shielding (EMI) properties, the SiC/C composite ceramics were fabricated by pyrolysis of ferrocene-modified polycarbosilane. The microstructure evolutions, dielectric properties, EMI and microwave absorption properties of SiC/C composite ceramics were investigated. The increases of both ferrocene contents and annealing temperatures led to the increases of crystallizations of SiC and carbons. Crystallized carbons including carbon nanowires, turbostratic carbons, onion-like carbons and graphene-like carbons were obtained in the materials. The carbon nanowires were longest when the 5 wt.% ferrocene-modified polycarbosilane was annealed at 1250 °C. These carbons played a more important role than SiC in the increases of dielectric and EMI properties. The average real and imaginary permittivities of materials increased from 4.4 and 0.7 to 38.9 and 39.6, respectively. The materials exhibited high total shielding effectiveness, high absorption shielding effectiveness and low reflection shielding effectiveness, which were 36.6, 30.1 and 6.5 dB, respectively.