Effect of interface types on the heat-resistance of Cansas-II SiCf /SiC composites

The heat-resistance of the Cansas-II SiC/CVI-SiC mini-composites with a PyC and BN interface was studied in detail. The interfacial shear strength of the SiC/PyC/SiC mini-composites decreased from 15 MPa to 3 MPa after the heat treatment at 1500 °C for 50 h, while that of the SiC/BN/SiC mini-composites decreased from 248 MPa to 1 MPa, which could be mainly attributed to the improvement of the crystallization degree of the interface and the decomposition of the matrix. Aside from the above reasons, the larger declined fraction of the interfacial shear strength of the SiC/BN/SiC mini-composites might also be related to the gaps in the BN interface induced by the volatilization of B₂O₃·SiO₂ phase, leading to a significant larger declined fraction of the tensile strength of the SiC/BN/SiC mini-composites due to the obvious expansion of the critical flaws on the fiber surface. Therefore, compared with the CVI BN interface, the CVI PyC interface has better heat-resistance at high temperatures up to 1500 °C due to the fewer impurities in PyC.     

Study of Thermal Radiation Properties of Pyrolytic Carbon Protective Coatings for Monocrystalline Silicon Furnace

The thermal radiation properties of pyrolytic carbon （PyC） protective coatings for monocrystalline silicon furnace prepared by different processes were tested. The changes of normal emissivity of carbon materials caused by PyC protective coatings were discussed, and the influence of phase structure and surface appearance on the thermal radiation properties was investigated. The results show that the thermal radiation properties of PyC protective coatings with the wave band of 5 -25 I~m are better than C/C substrate, further, normal spectral emissivity of CVD PyC coating remains basically at 0. 85 - 0. 90, and the normal total emissivity can reach 0. 89, which shows high thermal radiation performance. For resin PyC coating and CVD PyC coating, the degree of graphitization are 44. 53% and 16. 28% respectively, and the R value of Raman spectrum are 0. g64 and 1. 384 respectively. Relatively disorder graphite structure of the latter causes various vibration modes, and the spectral emissivity is better, so the thermal radiation property of CVD PyC coating is excellent. A lot of spherical particles exists on the surface of the CVD PyC coating, and the more interface and spacing of particles reduce the number of particles per unit volume. Therefore, the scattering of thermal radiation is strongly strengthened, and the spectral emissivity is higher.     

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.     

Architectural engineering inspired method of preparing Cf/ZrC–SiC with graceful mechanical responses

Inspired by grouting technique in architectural engineering, an innovative method of slurry injection and vacuum impregnation was put forward to introduce nanosized ZrC–SiC ceramics into PyC modified 3-D needle-punched carbon fiber preform homogeneously and continuously, followed by spark plasma sintering to prepare C_f/ZrC–SiC with graceful mechanical responses. The composite possessed improved fracture toughness and work of fracture at 5.37 ± 0.25 MPa∙m^1/2 and 951 ± 12 J/m², 50% and nearly one order of magnitude higher than those of ZrC–SiC composite, respectively, with rivaling flexural strength at 177 ± 8 MPa synchronously. A graceful fracture mode was embodied in an obviously extended yield plateau with increased failure displacement by 300%. This enhancement was attributed to the uniform and continuous combination of ZrC–SiC with carbon fiber preform as well as protection and interface tailoring from PyC coating. The study offered an easy and effective method of preparing 3-D fiber reinforced ceramic matrix composites.     

Improved thermal shock resistance of SiCnw/PyC core-shell structure-toughened CVD-SiC coating

In-situ SiC nanowire (SiCnw)/pyrolytic carbon (PyC) core-shell structures were introduced to mainly improve the thermal shock performance of chemical vapor deposition (CVD)-SiC coating on carbon/carbon (C/C) composites. The microstructure, phase composition, and mechanical properties of the CVD-SiC coating toughened by SiCnw/PyC core-shell structures were studied as well. The results show that the introduction of SiCnw/PyC core-shell structures can effectively alleviate the mismatch of coefficient of thermal expansion (CTE) between SiC coating and C/C substrate, thus enhancing the thermal shock resistance of the coating. Furthermore, the increased numbers of interfaces in the SiC coating owing to the addition of core-shell structures are beneficial to the mechanical properties of the coating after thermal shock test.     

Effects of PyC shell thickness on the microstructure,ablation resistance of SiCnws/PyC-C/C-ZrC-SiC composites

SiC nanowires/pyrocarbon (SiCnws/PyC) core-shell structure toughened C/C-ZrC-SiC composites were fabricated by CLVD process,and the influences of PyC shell thickness on the microstructure and ablation resistance of the composites were researched.The results presented that SiCnws/PyC core-shell structure had a linear shape,and the composites became dense with the increasing PyC thickness.When the thickness of PyC shell increased from 0 to 2.4 μm,the density and thermal conductivity of the composites was improved gradually,but the coefficient of thermal expansion (CTE) decreased firstly and then increased.After the ablation test for 90 s,the ablation rates of the composites decreased continuously as the PyC thickness increased from 0 to 1.4 μm,but increased when the PyC thickness was up to 2.4 μm.Especially when the PyC thickness was 1.4 μm,the linear and mass ablation rates of the composites were 71.25 ％ and 63.01 ％ lower than those of the composites without PyC shell.The reasons behind the remarkable improvement of anti-ablation property were that the proper PyC thickness could alleviate the CTE mismatch to promote the formation of complete oxide coating,improve the thermal conductivity to reduce heat corrosion and enhance the capability to limit the mechanical erosion.     

三维碳化硅纳米线增强碳化硅陶瓷基复合材料的电磁屏蔽性能

碳化硅纳米线具有优异的电磁吸收性能, 三维网络结构可以更好地使电磁波在空间内被多次反射和吸收。通过抽滤的方法制备得到体积分数20%交错排列的碳化硅纳米线网络预制体。然后采用化学气相渗透工艺制备热解炭界面和碳化硅基体, 并通过化学气相渗透和前驱体浸渍热解工艺得到致密的SiCNWs/SiC陶瓷基复合材料。甲烷和三氯甲基硅烷分别是热解炭和碳化硅的前驱体, 随着热解碳质量分数从21.3%增加到29.5%, 多孔SiCNWs预制体电磁屏蔽效率均值在8~12 GHz (X)波段从9.2 dB增加到64.1 dB。质量增重13%的热解碳界面修饰的SiCNWs/SiC陶瓷基复合材料在X波段平均电磁屏蔽效率达到37.8 dB电磁屏蔽性能。结果显示, SiCNWs/SiC陶瓷基复合材料在新一代军事电磁屏蔽材料中具有潜在应用前景。     

CVD PyC对炭泡沫结构及性能的影响

以AR中间相沥青为原料,采用中间相沥青自发泡法在发泡压力为0.1、3.0MPa,发泡温度为450℃的条件下制备了两种不同体积密度的炭泡沫CF-1和CF-2.将CF-1经过10h和70h化学气相沉积热解炭(CVDPyC)处理后得到炭泡沫CF-1-PC1和CF-1-PC2.测定了炭泡沫的抗压强度和导热系数,利用SEM和光学显微镜观察了炭泡沫的孔结构,考察了CVD PyC对炭泡沫结构及性能的影响.研究结果表明,CVD PyC处理可以增加炭泡沫韧带宽度,封填孔壁微裂纹;沥青炭和热解炭之间无明显界面,结合良好;经过CVD PyC处理后得到的CF-1-PC1和CF-1-PC2的体积密度、抗压强度、导热系数分别为, 0.196g·cm-3、1.89MPa、0.314W·m-1·K-1和0.461g·cm-3、11.93MPa、1.581W·m-1·K-1.     

热解碳涂层对Cf/Al复合材料热膨胀性能的影响

采用真空挤压吸渗工艺制备了不同厚度热解碳（PyC）涂层的碳纤维增强铝基（C_f/Al）复合材料,研究了热解碳涂层对C_f/Al复合材料热膨胀性能的影响。结果表明,热解碳涂层均匀而致密,可有效地保护碳纤维;对C_f/Al复合材料的热膨胀性能进行测试表明,与碳纤维表面无涂层的C_f/Al复合材料相比,当热解碳涂层的厚度约为70 nm时,碳纤维表面具有热解碳涂层的C_f/Al复合材料的热膨胀系数减小了24%;在一定的厚度范围内（70~250 nm）,随着热解碳涂层厚度的增加,碳纤维表面具有热解碳涂层的C_f/Al复合材料界面结合强度和滑移阻力逐渐减小,热膨胀系数逐渐增大。     

热解炭(PyC)/BN复合粉的制备及其吸波性能

以尿素、硼酸和热解炭(PyC)粉为原料, 利用化学转化法, 在1750℃、N₂气氛下反应6 h, 制备了PyC/BN复合粉。采用XRD、XPS、SEM、DTA/TGA对PyC/BN复合粉的物相组成、化学成分、表面形貌及抗氧化性能进行了表征, 并用矢量网络分析仪测试了PyC/BN复合粉在2~18 GHz频段内的吸波性能。结果表明: 生成的h-BN颗粒在PyC粉表面形成包裹层, 其尺寸为几十至几百纳米。相比于PyC粉, PyC/BN复合粉的抗氧化性能明显提高, 在1200℃时其质量仍剩余50%以上, 而PyC粉在940℃时已被氧化完全。同时, 由于波阻抗增大, 电子位移极化、界面极化、多重散射和λ / 4干涉相消作用, 使得PyC/BN复合粉在相同厚度下(d=2 mm), 其反射率峰值达到-27.2 dB, 反射率小于-10 dB的频宽为2.6 GHz; 且随着厚度的增加, 其反射率峰值均小于-10 dB, 并出现多重吸收峰。PyC/BN复合粉可作为理想的轻质、耐高温、强吸收的吸波剂应用于微波领域。