Microstructure and properties of Cu-Ti alloy infiltrated chopped Cf reinforced ceramics composites

Novel friction composites (C/C-Cu₅Si-TiC) were prepared via reactive melt infiltration (RMI) of Cu-Ti alloy into porous C/C-SiC composites. The microstructure, physical properties and tribological behaviors of the novel material were studied. Results were compared to conventional C/C-SiC composites produced by liquid silicon infiltration(LSI). The resultant composite showed the microstructure composed of Cu₅Si matrix reinforced with TiC particles and intact C/C structures. Most importantly, the composite did not present traces of free Si. As a result, the C/C-Cu₅Si-TiC composite showed higher flexural strength, impact toughness and thermal diffusivity in comparison to C/C-SiC composites. Tribological properties were measured using 30CrSiMoVA as a counterpart. In general, the C/C-Cu₅Si-TiC composites showed lower coefficient of friction(COF), but higher wear resistance and frictional stability. The improved wear resistance of the C/C-Cu₅Si-TiC composites is credited to the formation of friction films from Cu₅Si matrix. Other deformation and wear mechanisms are also described considering the microstructural observations.     

Manufacture and thermomechanical characterization of wet filament wound C/C-SiC composites

The paper presents manufacture of C/C-SiC composite materials by wet filament winding of C fibers with a water-based phenolic resin with subsequent curing via autoclave as well as pyrolysis and liquid silicon infiltration (LSI). Almost dense C/C-SiC composite materials with different winding angles ranging from ±15° to ±75° could be obtained with porosities lower than 3% and densities in the range of 2 g/cm³. Thermomechanical characterization via tensile testing at room temperature and at 1300°C revealed higher tensile strength at elevated temperature than at room temperature. Thus, C/C-SiC material obtained by wet filament winding and LSI-processing has excellent high-temperature strength for high-temperature applications. Crack patterns during pyrolysis, microstructure after siliconization, and tensile strength strongly depend on the fiber/matrix interface strength and winding angle. Moreover, calculation tools for composites, such as classical laminate and inverse laminate theory, can be applied for structural evaluation and prediction of mechanical performance of C/C-SiC structures.     

Microstructures and properties of Si-Zr alloy based CMCs reinforced by various porous C/C performs

C/C-SiC composites were fabricated via Si-Zr reactive alloyed melt infiltration using various C/C preforms with different porosities as reinforcements. The influence of preform porosities on the microstructure, mechanical strength and ablation resistance of the as-prepared composites were investigated. The results indicated that microstructure and properties of the C/C-SiC composites seriously depended on C/C preform porosities. The composites were mainly composed of carbon, SiC and ZrSi₂ phases, while some residual silicon still existed in the composites prepared with very large porosity preforms. Flexural strength of the composites firstly increased with increasing C/C preform porosities, then reached the highest value, 307?MPa, and finally turned to decrease with the further increasing of preform porosities. Densities of the composites increased with increasing preform porosities, while open porosities were generally small below 7%. Linear ablation rates of the composites firstly sharply decreased with increasing preform porosities and then slightly decreased to reach a balance value. In a word, C/C preform porosity was of great significance for reactive melt infiltration of C/C-SiC composites. Densities, microstructure, mechanical strength and ablation resistance of the resulting composites should be comprehensively taken into consideration to choose an optimal preform porosity for fabrication of C/C-SiC composites.     

Microstructure and mechanical properties of reaction bonded B4C-SiC composites: The effect of polycarbosilane addition

Reaction bonded B₄C-SiC composites were prepared by infiltrating silicon melt into porous B₄C-SiC green preforms at 1500 °C in vacuum. The porous green preform was obtained from a mixture of polycarbosilane (PCS) and particle size graded B₄C after pre-sintering at 1600 °C. For the first time, PCS was used to adjust the phase composition and microstructure of the reaction bonded boron carbide composites. It is indicated that the addition of PCS and its content has a significant influence on the microstructure as well as the mechanical properties of the subsequent reaction bonded B₄C-SiC composites. For the B₄C-SiC composite with 5 wt% PCS added, a flexural strength of 319±12 MPa, and an elastic modulus of 402±18 GPa can be achieved, which is 23% and 15% higher than those of the composite without PCS addition, respectively. While, with the higher content of PCS addition, the mechanical properties of the composites are decreased drastically due to the large amount of residual Si agglomeration in the composites. The reaction mechanisms as well as their microstructure evolution processes correlated with the mechanical properties of the reaction bonded B₄C-SiC composites are further discussed in our work.     

Preparation of in situ grown silicon carbide whiskers onto graphite for application in Al2O3-C refractories

C-SiC composite powders were prepared by salt-assisted synthesis from Si powders, graphite, and a molten salt medium (NaCl and NaF) with the molar ratio of Si/C =?1/2 at 1300?°C for 3?h. After the C-SiC composite powders part and complete replacement of the graphite, the mechanical properties, oxidation resistance and slag-corrosion resistance of the Al₂O₃-C materials were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), as well as with dedicated equipment. The results indicated that SiC whiskers, with lengths of 10–50?nm, formed on the surface of the flake graphite, and the activation energy of oxidation of the C-SiC composite powder increased by 45.72?kJ?mol^?1 as compared to that of flake graphite. Furthermore, the decarburization area and slag erosion area of the Al₂O₃-C material decreased after 3?wt% of C-SiC composite powder was substituted for the flake graphite. Meanwhile, the cold modulus of rupture was maintained when 3?wt% of C-SiC composite powder was added. This improved both the oxidation and slag resistance of the Al₂O₃-C materials.     

The effects of high-temperature annealing on microstructure and performance of Cf/C-SiC composites modified by FeSi2

FeSi₂ modified C/C-SiC composites (C/C-SiC-FeSi₂) are fabricated by chemical vapor infiltration (CVI) combined with reactive melt infiltration (RMI) with FeSi75 alloy. The effects of high-temperature annealing (1600?°C, 1650?°C, 1700?°C) on the microstructure and performance of C/C-SiC-FeSi₂ are investigated. With the elevation of annealing temperature, the porosity of the composites and the content of SiC increase due to the evaporation of liquid Si and the further reaction of Si and C. The mechanical performance gradually decreases due to the catalytic graphitization of the carbon fiber, the high porosity and the thermal residual stress (TRS) caused by thermal mismatch of different phases. The coefficient of thermal expansion and thermal diffusivity slightly decrease with increasing annealing temperature for the increase of porosity. However, the friction performance of the heat treated materials at high braking speed are greatly improved attributing to the increase of SiC content and the capturing and storage function of pores on hard particles.     

A sustainable bio-adsorbent for thermal energy storage for space heating applications

Thermal energy storage is an emerging technology that allows the storage of heat when it is available, which can be used later. One of the available technologies for thermal energy storage is the adsorption of moisture from air by adsorbents. Several adsorbents have been studied in the literature for this application, but there is a need for a sustainable adsorbent that can be eco-friendly, cost effective, and available for scale-up for commercialization of the technology. The current paper focused on the synthesis of a flax shives-based composite (equal weight percent of flax shives and salt hydrates) prepared by the impregnation method and its application in thermal energy storage. The composite showed durability, stability, and reasonable energy storage density with a very low cost per unit of energy. The structural characterization of the hybrid was performed by scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX). The thermal energy storage density, as well as the charging/discharging characteristics were measured using a laboratory-scale thermal energy storage apparatus. The flax/CaCl₂/LiCl hybrid showed reasonable energy storage density at 74 kWh/m³ for 50% inlet relative humidity after regeneration at 120°C. Although the energy storage density was not high, the flax/CaCl₂ composite was found to be the most cost-effective material, as it showed the lowest cost per energy stored at 0.98 CAD/kWh at 50% relative humidity (RH) after regeneration at 120°C.     

TiAl/B4C marine material—Fabrication, mechanical and corrosion properties

A novel TiAl/B₄C intermetallic/ceramic matrix composite was fabricated by high energy ball milling followed by hot press sintering. The microstructure of TiAl/B₄C composite was studied and its mechanical properties and corrosion resistance behaviors were investigated. The bending strength and fracture toughness of the composite reached 437.3 MPa and 4.85 MPa m^1/2, respectively, i.e., much higher values than the monolithic phase B₄C. The potentiodynamic polarization measurements and electrochemical impedance spectroscopy results indicated the excellent corrosion resistance of such composite material.     

Preparation and properties of the three-dimensional highly thermal conductive carbon/carbon-silicon carbide composite using the mesophase-pitch-based carbon fibers and pyrocarbon as thermal diffusion channels

The novel three dimensional highly thermal conductive carbon/carbon-silicon carbide (C/C-SiC) composite was successfully prepared using the mesophase-pitch-based carbon fibers and pyrocarbon as the thermal diffusion channels. The results show that the highly thermal conductive C/C-SiC composite with 221.1 W m^?1K^?1 in the ablation direction exhibits a smaller temperature gradient, and the surface temperature is 470 °C lower than that of the lowly thermal conductive C/C-SiC composite. The mass and linear ablation rates of the highly thermal conductive C/C-SiC composite are 0.56 mg·cm^?2 s^?1 and 0.11 μm·s^?1, respectively.     

Thermal conductivity of porous SiC composite ceramics derived from paper precursor

Biomorphic porous SiC composite ceramics were produced by chemical vapor infiltration and reaction (CVI-R) technique using paper precursor as template. The thermal conductivity of four samples with different composition and microstructure was investigated: (a) C-template, (b) C-SiC, (c) C-SiC–Si₃N₄ and (d) SiC coated with a thin layer of TiO₂. The SiC–Si₃N₄ composite ceramic showed enhanced oxidation resistance compared to single phase SiC. However, a key property for the application of these materials at high temperatures is their thermal conductivity. The later was determined experimentally at defined temperatures in the range 293–373 K with a laser flash apparatus. It was found that the thermal conductivity of the porous ceramic composites increases in the following order: C-template < C-SiC < C-SiC–Si₃N₄ < SiC–TiO₂. The results were interpreted in regard to the porosity and the microstructure of the ceramics.     

Influence of h-BN as additive on microstructure and oxidation mechanism of C/C-SiC composite

Due to the favorable self-healing performance, hexagonal boron nitride (h-BN) as additive in the matrix can significantly influence the oxidation behavior and the kinetic characteristics of C/C-SiC composites. In this work, C/C-SiC composites modified by h-BN (C/C-BN-SiC) were prepared by low-temperature compression molding (LTCM), pyrolysis and liquid silicon infiltration (LSI). Microstructure, oxidation behavior and kinetic characteristics of the C/C-BN-SiC composites were investigated compared with the C/C-SiC composite. Because h-BN is non-wetted by liquid silicon, the h-BN flakes in the matrix can obstruct and prolong the flow path of silicon, and protect the carbon fibers from corrosion to a certain extent. The oxidation kinetics of composites occur in low and high temperature domains, with different oxidation-controlling mechanisms, and the addition of h-BN can hinder the inward diffusion and lead to the decline of carbon recession and apparent activation energy.     

Evaluation of dynamic modulus measurement for C/C-SiC composites at different temperatures

The determination of elastic properties at application temperature is fundamental for the design of fibre reinforced ceramic composite components. An attractive method to characterize the flexural modulus at room and high temperature under specific atmosphere is the nondestructive Resonant Frequency Damping Analysis (RFDA). The objective of this paper was to evaluate and validate the modulus measurement via RFDA for orthotropic C/C-SiC composites at the application temperature. At room temperature flexural moduli of C/C-SiC with 0/90° reinforcement were measured under quasi-static 4-point bending loads and compared with dynamic moduli measured via RFDA longitudinally to fibre direction. The dynamic modulus of C/C-SiC was then measured via RFDA up to 1250°C under flowing inert gas and showed an increase with temperature which fitted with literature values. The measured fundamental frequencies were finally compared to those resulting from numerical modal analyses. Dynamic and quasi-static flexural moduli are comparable and the numerical analyses proved that bending modes are correctly modeled by means of dynamic modulus measured via RFDA. The nondestructive RFDA as well as the numerical modeling approach are suitable for evaluation of C/C-SiC and may be transferred to other fibre reinforced ceramic composite materials.     

A molecular dynamics study of 3C-SiC/epoxy nanocomposite as a metal-free friction material

The tribological, mechanical, and thermal properties of an epoxy crosslinking network incorporated with 3C-SiC nanoparticles, serving as a metal-free friction material were investigated by molecular dynamics simulations. The considered models encompass pure epoxy materials with 35%, 50%, 65%, and 80% crosslinking degrees, as well as 3C-SiC/epoxy composite materials at the same crosslinking levels. Glass transition temperature (T_g) of the eight models was analyzed, and the result shows augmenting crosslinking density and addition of 3C-SiC nanofillers improve T_g of all models. Fractional free volume of each model was quantified to reflect the features of epoxy materials and influence the properties at the atomic scale. Frictional force, normal force, and coefficient of friction (COF) were calculated to elucidate the tribological performance of the epoxy-based materials. The introduction of 3C-SiC nanofillers reduces COF. With nanofillers, higher crosslinking degree brings lower COF except 80% crosslinking degree, while without nanofillers, higher COFs are obtained with 35% and 80% crosslinking degrees. 3C-SiC/epoxy composite with heightened crosslinking degree demonstrates superior Young's modulus, elevated tensile stress, and relatively smaller strain. Thermal conductivity analysis highlights the positive impact of both increased crosslinking density and incorporation of 3C-SiC nanofillers on heat transfer. Temperature elevation further enhances thermal conductivity.     

Formation mechanism of Si-Y-C ceramic matrix by reactive melt infiltration using Si-Y alloy and properties of C/Si-Y-C composites

Herein, Si-Y eutectic alloy were introduced into porous C/C preform by reactive melt infiltration (RMI) to prepare C/Si-Y-C composite. Phase compositions and their distributions in the as-prepared composites were investigated. Result indicated that four main regions were found in the typical zone in Si-Y-C matrix, i.e., amorphous carbon, polycrystalline SiC doped with YSi2, amorphous SiC and single crystal YSi2. Based on the reaction between Si-Y alloy and C/C preform and microstructural observations, a model regarding to microstructure formation mechanism was proposed to reveal reaction process. Moreover, improved flexural strength, fracture toughness, thermal diffusivity and thermal conductivity of C/Si-Y-C composite were achieved compared to C/C-SiC.     

Oxidation resistance,ablation resistance and in situ ceramization mechanism of Al-coated carbon fiber/boron phenolic resin ceramizable composites modified with Ti3SiC2

Inherent defect of easy oxidation limited further application of carbon fiber/phenolic resin composites in hostile environments. Herein, a combined strategy of matrix modification and fiber coating was proposed to fabricate a novel ceramizable composite containing Al-coated carbon fibers and Ti₃SiC₂ toward thermal protection materials (TPM), which offered a promising solution to challenge facing long-term thermal protection and load-bearing subject to severe oxidation corrosion and ablation in hypersonic vehicle applications. Oxidation resistance, mechanical strength evolution, phase evolution, microstructure evolution and mechanical strength failure mechanism at elevated temperatures were studied based on thermogravimetric analysis, static ablation test, mechanical test, X-ray diffraction analysis, and scanning electron microscopy coupled with energy dispersive X-ray analysis. The resulting composites exhibited outstanding oxidation resistance, with residue yield at 1600 °C and flexural strength at 1400 °C as high as 87.7% and 31.7 MPa, respectively. It was found that dense multiphase ceramics formed by reactions between Ti₃SiC₂, O₂, pyrolytic carbon (PyC) and N₂, acted as oxygen barriers and self-healing agents during static ablation. Besides, the resulting composites exhibited satisfactory ablation resistance and the linear ablation rate was as low as 0.00853 mm/s. Furthermore, ablation mechanisms were revealed based on phase identification, microstructure characterization and thermodynamic calculation analysis. It was revealed that multiphase ceramics composed of PyC, Al coatings, Ti₃SiC₂, TiC, Al₂OC and AlB₂ contributed great to the ablation resistance during oxyacetylene ablation.     

2D-C/C-SiC复合材料强粒子冲蚀下的失效分析

本文采用化学气相渗透(CVI)工艺制备了2D针刺预制体增强的C/C-SiC复合材料,并对材料密度、力学性能以及强粒子冲蚀下的烧蚀机理和破坏机制进行了分析。结果表明,C/C-SiC复合材料在强粒子冲蚀下的破坏机制主要为机械冲蚀和颗粒侵蚀,其次是冲蚀过程中伴随的少量氧化。材料内层间孔、束间孔以及针刺孔的存在加剧了C/C-SiC复合材料破坏。研究发现,通过改变预制体结构来实现材料力学性能的均衡,并提高材料密度以减少材料的孔隙率将成为该使用环境下的材料设计原则     

Laser ablation resistance and mechanism of Si-Zr alloyed melt infiltrated C/C-SiC composite

Ablation resistance of C/C-SiC composite prepared via Si-Zr alloyed reactive melt infiltration was evaluated using a facile and economical laser ablation method. Linear ablation rates of the composite increased with an increase in laser power densities and decreased with extended ablation time. The C/C-SiC composite prepared via Si-Zr alloyed melt infiltration presented much better ablation resistance compared with the C/SiC composite prepared by polymer infiltration and pyrolysis process. The good ablation resistance of the composite was attributed to the melted ZrC layer formed at the ablation center region. Microstructure and phase composition of different ablated region were investigated by SEM and EDS, and a laser ablation model was finally proposed based on the testing results and microstructure characterization. Laser ablation of the composite experienced three distinct periods. At the very beginning, the laser ablation was dominated by the oxidation process. Then for the second period, the laser ablation was dominated by the evaporation, decomposition and sublimation process. With the further ablation of the composite, chemical stable ZrC was formed on the ablated surface and the laser ablation was synergistically controlled by the scouring away of ZrC melts and evaporation, decomposition and sublimation process.     

输油管道保温技术及应用研究进展

综述了国内外各类保温材料和保温管道结构的技术现状及应用研究进展;分析了不同保温材料的隔热性能、力学性能、微观结构等特点,包括实心保温材料、无机及有机发泡材料、复合保温材料和相变储能材料;介绍了各类结构的输油管道的加工工艺、结构设计等特点,包括管中管结构、单壁管、技术管系统和复合保温软管;并对目前保温输油管道的材料及结构的应用和发展进行了讨论。     

太阳盐/钢渣定型复合相变储热材料的制备与性能研究

全球范围内的能源短缺和环境污染问题迫使人们积极开发可再生新能源。储热技术是解决新能源不稳定性问题的关键技术。相变材料是重要的储热介质之一。熔盐相变材料因其储热密度高,可操作温度范围广的优势,成为储热材料领域研究的热点。为解决熔盐液相易泄漏、低导热和高成本的问题,选择钢渣为基体材料,制备了太阳盐/钢渣定型复合相变储热材料,并通过扫描电子显微镜(SEM),热重–差示扫描量热法(TG–DSC),闪射法导热仪(LFA)和X射线衍射仪(XRD)对复合材料的微观结构、热性能和化学相容性进行了测试与表征。结果表明,钢渣与熔盐质量比5:5的复合材料定型效果最优。复合材料结构紧密;钢渣与熔盐化学相容性良好;复合材料潜热为64.0 kJ/kg,100~500℃内储热密度为945 kJ/kg,热导率高达2.23 W/(m?K)。太阳盐/钢渣复合相变储热材料不仅有利于储热技术的大规模应用,而且为钢铁工业废弃物回收利用提供了良好的参考,对节约资源、保护环境以及提高经济效益具有重要的意义。     

Microstructural evolution and kinetics analysis of aluminum borate ceramics via solid-state reaction synthesis

Aluminum borate ceramics (ABCs) with a skeleton structure of rod-like crystals were established via a solid-state reaction synthesis. The influence of heat treatment systems on the phase composition and microstructural evolution of an anisotropic grain were firstly studied. Next, the impact of boric acid content on the activation energy of reaction, microstructure, mechanical properties, and neutron shielding capability were investigated. The results demonstrate that orthorhombic Al₁₈B₄O₃₃ formed between 900°C and 1000°C which was the stable phase during heating treatment. In addition, increasing the boric acid content was favorable for reducing the activation energy of forming aluminum borate, resulting in increased in situ formation and growth of aluminum borate whiskers in fired samples. Next, mircropores could be generated by the decomposition of boric acid at high temperatures, which had a considerable influence on the physical properties of the sample. Furthermore, the Monte Carlo particle transport program simulation and neutron shielding experiments showed that the prepared ABCs could shield neutron effectively manifested as possessing a high neutron absorption cross-section in the case of porous materials. Moreover, ABCs have potential application value as thermal insulation material in the nuclear energy field.