Improved thermal conductivity and mechanical properties of SiC/SiC composites using pitch-based carbon fibers

Pitch-based carbon fibers were assembled in horizontal and thickness directions of SiC/SiC composites to form three-dimensional heat conduction networks. The effects of heat conduction networks on microstructures, mechanics, and thermal conductivities were investigated. The results revealed the benefit of introducing heat conduction networks in the densification of composites. The maximum bending strength and interlaminar shear strength of the modified composites reached 568.67 MPa and 68.48 MPa, respectively. These values were equivalent to 18.6% and 69.4% increase compared to those of composites without channels. However, channels in thickness direction destroyed the continuity of fibers and matrix, creating numerous defects. As the volume fraction of heat conduction channels rose, the pinning strengthening effect of channels and influence of defects competed with each other to result in first enhanced mechanical properties followed by a decline. The in-plane thermal conductivity was found anisotropic with a maximum value reaching 86.20 W/(m·K) after introducing pitch-based carbon unidirectional tapes. The thermal conductivity in thickness direction increased with volume fraction of pitch-based carbon fibers and reached 19.13 W/(m·K) at 3.87 vol% pitch-based carbon fibers in the thickness direction. This value was 90.75% higher than that of composites without channels.     

Effects of aligned carbon nanotube microcolumns on mechanical and thermal properties of C/SiC composites prepared by LA-CVI methods

Herein, C/SiC-CNTs composites were prepared by laser assisted chemical vapor infiltration (LA-CVI) method combined with vacuum impregnation. Density, mechanical property and thermal conductivity of as-prepared composites were then investigated by various analytical methods. Scanning electron microscopy (SEM) revealed good dispersion of CNTs in C/SiC-CNTs between composites layers and directional heat transfer channels. This formed unique three-dimensional connected networks, reinforcing multi-scale composites matrix. Average density and bending strength of composites were estimated to 2.35 g cm⁻³ and 598 MPa, respectively, which is 20.5% and 27.2% higher than those of CVI-C/SiC composites. The comparison between theoretical thermal conductivity and experiments revealed that the overall thermal conductivity of LA-CVI-C/SiC-CNTs composites (150.42 W m⁻¹ K⁻¹) was nearly 25 times higher than that of CVI-C/SiC composites.     

Effect of heat transfer channels on thermal conductivity of silicon carbide composites reinforced with pitch-based carbon fibers

In this study, silicon carbide (SiC) composites reinforced with pitch-based carbon fibers and composed of heat transfer channels were fabricated by combining chemical vapor infiltration and reactive melting infiltration method. It was observed that the internal heat conduction skeleton of pitch-based carbon fibers was sequentially formed. The thermal conductivities from room temperature to 500 °C along through-thickness direction and in-plane direction were investigated. The results showed that C_pf/SiC composites with heat transfer channels possessed excellent thermal conductvity in two directions, and the thermal conductivity increased with increasing volume content of heat transfer channels. The thermal conductivity in through-thickness direction reached 38.89 W/(m·K), and that for in-plane direction reached 112.42 W/(m·K). Theoretical calculations were empolyed to study the temperature dependence of the C_pf/SiC composites. The variations in slope A′ and intercept B′ values of fitted curves were in good agreement with the experimental results. To verify the reliablilty of the theoretical model, the C_pf/SiC composites were heated at 1650 °C for 2 h and the thermal conductivity exhibited further improvement due to the formation of more perfect crystalline structure. Thermal conductivity through thickness direction improved to 43.49 W/(m·K), and that in in-plane direction improved to 142.49 W/(m·K), which could be identified by the theoretical model. Finally, the leading edge model was established by using ABAQUS finite element analysis software to evaluate the potential application of the composites. Owing to the outstanding thermal conductivity, the leading edge obtained by using C_pf/SiC composites in this study exhibited lower temperature gradient and a more uniform temperature distribution. Moreover, less thermal stress and displacement were generated during heating process.     

Optimized ablation resistance behavior and mechanism of C/SiC composites with high thermal conductive channels

Aimed to enhance the high-temperature service performance of C/SiC composites in high-speed aircraft thermal protection system, in this article, pitch-based carbon fibers were used to construct high thermal conductive channels to improve the heat transfer capability of C/SiC composites. The results revealed that the as-prepared composites equipped with 4.7 times higher thermal conductivity than that of conventional C/SiC composites. The oxyacetylene flame ablation test confirmed that the constructed high thermal conductive channels, which quickly conducted the heat flow from the ablation center area to other areas is the main reason of as-prepared composites exhibiting a very impressive ablation resistance property. Briefly, the ablation temperature of the as-prepared composite surfaces considerably dropped by about 300°C compared with conventional C/SiC composites, while the linear ablation rate and mass ablation rate of the composites are 1.27 μm/s and 0.61 mg/s respectively, which is superior to many recent reports, demonstrating that this article provides a simple but highly effective measure to improve the ablation resistance property of C/SiC composites.     

Effect of initial density on thermal conductivity of new micro-pipeline heat conduction C/SiC composites

C/SiC composites prepared by chemical vapor infiltration technique (CVI) have been regarded as thermal structural materials widely. However, these composites still suffer from poor functional properties like low thermal conductivity, especially in thickness direction of the composites, limiting their large-scale applications. Herein, mesophase pitch based carbon fiber (MPCF) and continuous wave laser machining were utilized to construct highly effective heat conductive micro-pipelines within CVI C/SiC composite. The effect of initial density on the final density and thermal conductivity of the as-obtained MPCF-C/SiC composites were investigated. The results revealed that higher initial density would directly enhance the thermal conductivity and reduce the negative impact of the bottle-neck effect. At temperatures between 100°C and 500°C, MPCF-C/SiC composites preserved more than threefold of the thermal conductivity (340%) when compared to reference C/SiC composites. This work provides a highly effective route for enhancing the thermal conductivity of C/SiC, which would broaden their future applications.     

Effect of high-temperature heat treatment on the microstructure and mechanical behavior of PIP-based C/C-SiC composites with SiC filler

C/C-SiC composites were fabricated by a combined process of chemical vapor deposition (CVD), slurry infiltration(SI), and precursor infiltration and pyrolysis (PIP). The microstructure and mechanical behavior were investigated for the dense C/C-SiC composites before and after high-temperature heat treatment. The results indicated that the sintering of the SiC matrix and the migration of the SiC matrix/fiber bundles weak interface occurred after high-temperature heat treatment at 1900 ℃. The SiC sintering resulted in an increase in the flexural strength of the C/C-SiC composites from 298.9 ± 35.0 MPa to 411.1 ± 57.3 MPa. The migration of the weak interface changed the direction of crack propagation, making the fracture toughness of the C/C-SiC composites decrease from 13.3 ± 1.7 MPa⋅m ^1/2 to 9.02 ± 1.5 MPa⋅m ^1/2.     

Thermal Conductivity of Vapor-Liquid-Solid and Vapor-Solid Silicon Carbide Whisker-Reinforced Lithium Aluminosilicate Glass-Ceramic Composites

The thermal conductivities of two lithium aluminosilicate glass-ceramic matrix composites reinforced with 30 vol% of either SiC VS (rice hull) whiskers or SiC VLS (vapor-liquid-solid) whiskers were determined from room temperature to 500°C. Because of the preferred alignment of the whiskers, the thermal conductivity values normal to the hot-pressing direction were found to be significantly higher than those in the parallel direction. The composites with the VLS whiskers exhibited higher thermal conductivity values than those with the VS whiskers. An analysis of the room-temperature data showed that the thermal conductivity values parallel to the hot-pressing direction were higher than those predicted from theory, even for whiskers with infinite thermal conductivity and perfect interfacial thermal contact. This effect was attributed to a significant contribution of percolation to the total heat flow as a result of direct whisker-to-whisker contact. For both types of whiskers, the interfacial thermal conductance and thermal conductivity values (at ∼6.5 × 10⁵ W/(m²-K) and 200 W/(m·K), respectively) inferred from the composite thermal conductivity values perpendicular to the hot-pressing direction were essentially the same. It was concluded that the order of magnitude difference in thickness for the two whisker types was primarily responsible for the differences in thermal conductivity measured for these two composites.     

Research on maximizing the diamond content of diamond/SiC composite

Diamond content is a key factor affecting diamond/SiC composite performance, especially thermal and mechanical properties, but the composite with high diamond content manufacturing is still challenging issues. Hot mold pressing combined with liquid silicon infiltration to make diamond/SiC composites with high diamond content and relative density has been proposed in this paper. In addition, the effect of diamond particle size on the maximization of diamond content as well as properties of the composites were evaluated. The experiment shows that the content of diamond in the composites increases with the increase of the diamond particle size. When the particle size of diamond is 400 µm, the volume fraction of diamond reaches 59.08%. The highest thermal conductivity (d_dia= 300 µm) and highest bending strength (d_dia= 50 µm) are 616.77 W/m K (It is the maximum TC of diamond/SiC prepared by pressureless infiltration at present) and 380 MPa, respectively. This work provides a novel and efficient preparation method for further improving the thermal conductivity of diamond/SiC composites.     

Effect of SiC coating and heat treatment on the thermal radiation properties of C/SiC composites

The effects of SiC coating and heat treatment on the emissivity were investigated for 2D C/SiC composites prepared by CVI in the 6–16 μm range. SiC coating had an obvious effect on the spectral emissivity of the composites but caused just 5% difference in the total emissivity. A radiation transport model was applied to explain those changes caused by SiC coating. Heat treatment affected the thermal radiation properties of the composites through the microstructure evolution. Base on the complementary analytical techniques, the changes in the emissivity were attributed to a good graphitization degree of carbon phases, large β-SiC grain sizes and high α-SiC content resulting in high emissivity.     

Three-dimensional micro-pipelines high thermal conductive C/SiC composites

Carbon/silicon carbide (C/SiC) composites are usually regarded as thermal protective system materials and widely applied in hypersonic vehicles or ramjet. However, poor thermal conductivity of C/SiC composites, leading to severe heat concentration and thermal stress during the high-speed operation of hypersonic vehicle, limits their broad-range of practical applications. Modification with high thermal conductive fillers is an optional method; however, controllable dispersion and orientation of the fillers to construct continuous and ordered heat conductive channel has been proven to be a challenging task. Herein, based on high thermal conductivity fibers, a three-dimensional micro-pipeline preform was developed for the preparation of structure–function integrated C/SiC composites. The technical feasibility of the method, the characteristics of microstructures, and the thermal conductivity and bending strength of the as-obtained composites were systematically studied. Results revealed that the thermal conductivities of as-obtained composites reached 150.2 and 46.7 W m⁻¹ K⁻¹ for in-plane and out-of-plane direction, respectively. The bending strength obtained herein is 264.4 MPa, which is lower than that of polyacrylonitrile C/SiC composites. However, the fine control over the component and microstructure or densification could provide a higher value in the future research. In sum, the proposed method provides a convenient and feasible approach to prepare high thermal conductive C/SiC composites.     

碳纳米管/聚合物基导热复合材料研究进展

碳纳米管（CNTs）由于优异的轴向导热性能,是目前制备高热导率聚合物基复合材料的一类重要填料。本文综述了近年来CNTs增强聚合物复合材料的研究进展,探讨了CNTs/聚合物复合材料的导热机理以及CNTs用量、尺寸及结构、表面改性、混杂CNTs粒子和聚合物基体结构等因素对CNTs/聚合物复合材料热导率的影响。同CNTs/聚合物的电导率相比,热导率远低于预期值,归因于CNTs/树脂界面间的声子频率失配现象导致了声子在界面的散射及很高的界面接触热阻,从而降低了体系热导率。分析和总结了改善体系热导率的方法和措施,采用特殊工艺使CNTs在基体内形成特殊的隔离结构或者取向结构是CNTs/聚合物导热复合材料的未来研究及发展方向。     

Additive-free low temperature sintering of amorphous SiBC powders derived from boron-modified polycarbosilanes: Toward the design of SiC with tunable mechanical,electrical and thermal properties

Additive-free SiC ceramics are prepared from polymer-derived powders with Si, C and B elements homogeneously distributed on the atomic level by rapid hot-pressing at 1750 °C and 1800 °C under argon and a load of 50 MPa. As-sintered samples display a Vickers hardness in the range of 9.6 ± 0.5 GPa–17.3 ± 1.9 GPa and an elastic modulus varying from 137 ± 3.4 GPa to 239 ± 6 GPa, both depending on the sample phase composition, crystallinity and porosity. Accordingly, the electrical conductivity changes from 340 to 3900 S/m whereas the thermal conductivity varies from 17.7 to 45.1 W/m⋅K as a function of these characteristics. Thus, we demonstrated that a polycarbosilane containing 0.7 wt.% of boron could produce boron-doped SiC powders that demonstrate tailored sinterability at temperatures as low as 1750 °C to form nearly dense SiC ceramics with adjusted hardness, Young’s modulus, electrical and thermal conductivities.     

Structural,thermal and dielectric properties of AlN–SiC composites fabricated by plasma activated sintering

Highly dense AlN–SiC composites with various SiC additions (0–50?wt-%) were fabricated at 1800°C by plasma activated sintering. The effect of SiC addition on structural, thermal and dielectric properties as well as microwave absorbing performance of the composites was investigated. The thermal conductivity decreases with increasing SiC addition, from 68.7 W (m?K)^?1 for 0?wt-% SiC to 19.38?W (m?K)^?1 for 50?wt-% SiC. On the contrary, the permittivity and dielectric loss increase gradually, from 7.6–8.5 to 22–26.7 and from 0.02–0.1 to 0.2–0.53, respectively. AlN–SiC composite with better thermal and dielectric properties in 30?wt-% SiC, whose thermal conductivity and dielectric loss are found to be 24.88?W (m?K)^?1 and 0.15–0.74, respectively. Furthermore, the composite exhibits microwave absorbing performance with the minimum reflection loss (RL) of ?16.5 dB at 15.5 GHz and the frequency range of 2.6 GHz for RL below ?10 dB (90% absorption).     

Heat treatment effects on microstructure and properties of CVI SiC/SiC composites with Sylramic™-iBN SiC fibers

Chemical-vapor-infiltrated (CVI) SiC/SiC composites with Sylramic?-iBN SiC fibers and CVI carbon, BN, and a combination of BN/C interface coating were heat treated in 0.1-MPa argon or 6.9-MPa N₂ at temperatures to 1800 °C for exposure times up to 100 hr. The effects of thermal treatment on constituent microstructures, in-plane tensile properties, in-plane and through-the-thickness thermal conductivities, and creep behavior of the composites were investigated. Results indicate that heat treatment affected stoichiometry of the CVI SiC matrix and interface coating microstructure, depending on the interface coating composition and heat treatment conditions. Heat treatment of the composites with CVI BN interface in argon caused some degradation of in-plane properties due to the decrease in interface shear strength, but it improved creep resistance significantly. In-plane tensile property loss in the composites can be avoided by modifying the interface composition and heat treatment conditions.     

炭前驱体形态对C/C复合材料导热系数的影响

利用热塑性中间相沥青为黏结剂,短炭纤维.增强体,一步热压成型制备C/C导热复合材料.采用SEM和偏光显微镜观察等分析手段,研究了2∶1,2.5∶1和3∶1三种不同管径比对C/C复合材料的影响.结果表明:通过热压模具空腔结构的改变可以引起炭前驱体挤出形态的变化,使得轴向基体炭有序生长与短炭纤维增强体呈现有序排列,其中间相液晶分子垂直和平行于模压压力方向均排列成纤维状长程有序结构,短切纤维呈现出与压力平行方向排布.当空腔管径比为3:1,轴向导热系数由86.2 W/(m·K)增大至115.5 W/(m· K),各向异性比由1.6减小为1.2.由此所得块体C/C复合材料具有显著的二维取向结构,轴径向导热系数趋于平衡.     

Oxidation and erosion resistance of multi-layer SiC nanowires reinforced SiC coating prepared by CVD on C/C composites in static and aerodynamic oxidation environments

A multi-layer SiC nanowires reinforced SiC (SiCnws-SiC) coating was prepared in-situ on carbon/carbon (C/C) composites by three chemical vapor deposition (CVD) processes. The microstructure and phase composition of the nanowires fabricated on the first-layer SiCnws-SiC coating and the coatings were examined by SEM, TEM, and XRD. The bamboo-like SiC nanowires with a 50?nm diameter and a length of several tens of micrometers are straight, randomly orientated and distributed like a net on the first-layer SiCnws-SiC coating. The growth direction is [111], and the growth mechanism is VS. The multi-layer SiCnws-SiC coating has three layers: the thickness of the first-layer is roughly 400?µm, and the outer two layers are about 200?µm. Each layer has a sandwich structure. The isothermal oxidation and erosion resistance of the multi-layer SiCnws-SiC coating were investigated in an electrical furnace and a high temperature wind tunnel. The results indicated that the weight loss of the multi-layer SiCnws-SiC coated C/C composites was only 1.8% after oxidation in static air at 1773?K for 361?h. Further, the coated sample failed due to fracture of the coating at the clamping position (i.e. 80?mm) after erosion at 1873?K for 130?h in the wind tunnel. The weight loss of the coated C/C composites occurred due to the formation of penetrating cracks in the coating during the oxidation thermal shock. The maximum bending moment and the larger clamping force caused the coating fracture and resulted in intense oxidation of the substrate and the failure of the specimen.     

Mechanical,thermal, and dielectric properties of SiCf/SiC composites reinforced with electrospun SiC fibers by PIP

Electrospun unidirectional SiC fibers reinforced SiC_f/SiC composites (e-SiC_f/SiC) were prepared with ∼10% volume fraction by polymer infiltration and pyrolysis (PIP) process. Pyrolysis temperature was varied to investigate the changes in microstructures, mechanical, thermal, and dielectric properties of e-SiC_f/SiC composites. The composites prepared at 1100 °C exhibit the highest flexural strength of 286.0 ± 33.9 MPa, then reduced at 1300 °C, mainly due to the degradation of electrospun SiC fibers, increased porosity, and reaction-controlled interfacial bonding. The thermal conductivity of e-SiC_f/SiC prepared at 1300 °C reached 2.663 W/(m∙K). The dielectric properties of e-SiC_f/SiC composites were also investigated and the complex permittivities increase with raising pyrolysis temperature. The e-SiC_f/SiC composites prepared at 1300 °C exhibited EMI shielding effectiveness exceeding 24 dB over the whole X band. The electrospun SiC fibers reinforced SiC_f/SiC composites can serve as a potential material for structural components and EMI shielding applications in the future.     

Fabrication of improved flexural strength C/SiC composites via LA-CVI method using optimized spacing of mass transfer channels

The traditional chemical vapor infiltration (CVI) method still faces massive challenge in improving the densification owing to its unavoidable bottleneck effect. Herein, laser assisted-chemical vapor infiltration (LA-CVI) was introduced to fabricate C/SiC composites with mass transfer channels. As a result, the densities of the C/SiC composites were improved due to the dense band formed during the LA-CVI process. Also, with different spacing of mass transfer channels, C/SiC composites exhibited enhanced degree of densification varying from 2.10 to 2.23 g/cm³. When the spacing of channels was 3 mm, the maximum value of flexural strength reached 528 ± 12 MPa. Additionally, micro-CT and finite element analysis were empolyed to investigate dense band and density gradient in detail. The results show that C/SiC composites prepared via LA-CVI method with suitable spacing of channels had improved density and great flexural strength. The proposed method provides a novel route for the preparation of ceramic matrix composites with high density.     

针刺炭纤维预制体结构单元对C/C复合材料性能的影响

采用6K炭纤维无纬布/网胎交替叠层及12K炭纤维无纬布/网胎交替叠层,在针刺工艺,致密化、热处理工艺完全相同的情况下,制备了密度为1.8g/cm3的热解炭/树脂炭双元基体的两种C/C复合材料产品,考察了针刺预制体结构单元对C/C复合材料性能的影响.结果表明,两种C/C复合材料的热学（垂直方向导热系数）、电学性能及石墨化度基本相当;而针刺6K炭纤维无纬布/网胎预制体C/C复合材料的拉伸、弯曲、压缩、层间剪切强度分别为127MPa,189MPa,263MPa,24.6MPa;其平行方向导热系数为54.6W/m＆#183;K,比常规针刺12K炭纤维无纬布/网胎预制体C/C复合材料相应提高了38％,32.2％,32.8％,38.9％,21％,彰显了细化针刺预制体结构单元对C/C复合材料力学性能的显著影响.     

碳纳米管材料导热性能的实验研究

本文对碳纳米管与环氧树脂(Epoxy-EP)复合材料的导热性能进行了定量的研究,探索了CNTs/EP复合材料的制备方法,运用Hotdisk热常数分析仪研究了CNTs/EP复合材料的导热系数;利用CNTs/EP两相复合材料的导热理论模型得到了室温下单壁碳纳米管(Single-Wall Carbon Nanotubes-SWCNTs)的导热系数为3980 W/(m.K),双壁碳纳米管的导热系数(Double-Wall Carbon Nanotubes-DWCNTs)为3580 W/(m.K),以及多壁碳纳米管(Multi-Wall Carbon Nanotubes-MWCNTs)的导热系数为2860 W/(m.K)。