气体滞留时间对微波热解CVI工艺制备C/C复合材料性能的影响

以炭毡为预制体, N₂为稀释气体, 甲烷为炭源前驱体, 其分压为10kPa, 沉积温度为1100℃的工艺条件下,研究了不同的气体滞留时间(0.05、0.1、0.15、0.2s)对微波热解CVI工艺制备炭/炭复合材料的致密化速率、样品的体积密度及其密度均匀性的影响, 并对其组织结构进行了观察. 分析了气体的滞留时间对微波热解CVI工艺制备炭/炭复合材料的影响规律及组织结构的变化. 结果表明: 采用微波热解CVI工艺在1100℃90h内制备出体积密度为1.70g·cm^-3的炭/炭复合材料, 在滞留时间为0.15s时预制体呈现从内到外逐步致密的规律. 同时, 随着滞留时间的延长, 热解炭的组织结构从低织构到中等织构变化.     

多元耦合场 CVI法炭/炭复合材料制备及结构分析

为了降低成本,以液化石油气作碳源气体,炭毡作增强体,利用多元耦合场CVI方法快速制备了炭/炭复合材料．研究表明,炭纤维预制体在650℃较低温度条件下沉积15h,密度达到了1．71g·cm~(-3)；采用偏光显微镜研究了热解炭的显微结构．结果表明,在同一试样中存在粗糙层、光滑层和带状结构的热解炭,并且材料密度均匀性较好．同时分析了致密化的工艺过程并讨论了热解炭沉积机理．     

前驱体对CVI制备C/C复合材料的影响

分别以天然气和丙烯为前驱体制备了C/C复合材料.考察了密度和密度分布,借助偏光显微镜观察了沉积炭的显微结构,研究表明,天然气制备的C/C复合材料的密度大于丙烯制备的C/C复合材料的密度;在前60h以天然气为前驱体时C/C复合材料的致密化速率比以丙烯为前驱体时低,60h后致密化速率发生逆转;以天然气为前驱体所得C/C复合材料的密度梯度小,结构为单一的粗糙层;而以丙烯为前驱体所得C/C复合材料的密度梯度大,结构为光滑层.     

模压法制备C/C复合材料的研究

对模压法制备 Ｃ／ Ｃ复合材料的坯体模压工艺过程、 Ｃ／ Ｃ复合材料的致密化过程及 Ｃ／ Ｃ复合材料的结构和性能的相关性进行了研究。结果表明,物料中纤维含量及模压的温度、压力是影响初坯体成型及其密度的关键;除工艺条件外,原料的组成也是影响 Ｃ／ Ｃ复合材料致密化的重要因素;对 Ｃ／ Ｃ 复合材料力学性能研究的结果表明, Ｃ／ Ｃ复合材料的密度对材料的力学性能有很重要的影响。     

煤油蒸气为前驱体热梯度CVI法制备C/C复合材料

以炭毡为预制体,煤油蒸气为前驱体,利用两个热源分别加热预制体的上下表面,形成两个热梯度和双沉积面,第三个热源加热前驱体保证反应气体的供给,采用这种改进的化学液相气化渗入法快速制备了炭/炭(C/C)复合材料.对C/C复合材料的密度和气孔率进行了表征,并通过XRD,SEM等方法对其石墨化程度、显微结构进行了研究.结果表明:C/C复合材料的密度随沉积温度的升高呈线性增加,而气孔率逐渐减小,体积密度为0.2g/cm3的预制体在1100℃沉积3h后密度达到1.72g/cm3.2200℃热处理后,C/C复合材料的d002显著降低,具有较高的石墨化程度.C/C复合材料中的炭纤维被环状的热解炭所包围.沉积过程中前驱体较短的对流和扩散路径以及预制体中存在的上下热梯度和相应的双沉积面是材料快速制备的主要因素.     

多元耦合场CVI法快速致密化炭/炭复合材料研究

以液化石油气为碳源气体,采用多元耦合场CVI工艺方法快速制备了炭/炭复合材料.在自制冷壁CVI炉中,使用普通炭毡作为炭纤维预制体,设置特殊的导电发热层,沉积温度为650～1050℃,系统的气氛压力为0.1～30kPa,流量为0.1～0.5m3/h,沉积时间12h的条件下可将预制体一次性快速增密至1.75g/cm3.XRD分析表明:该材料经过2300℃,2h高温石墨化处理,其石墨化度(g)可达到61.3%,晶粒尺寸达到16.1nm.PLM分析表明所得材料偏光形貌表现为光滑层(SL)结构,SEM形貌照片测算可知热解炭沉积速率在6.6μm/h以上.分析了炭/炭致密化的过程和热解炭的沉积机理,说明多元耦合场加速了热解炭的沉积,缩短了致密化时间,降低了成本.     

载气对C/C复合材料CVI热解炭显微结构的影响

在1000℃,0.5 kPa丙烯分压条件下,以N₂、H₂为载气和无载气情况下于热梯度CVI炉中制备了C/C复合材料,研究了载气特别是氢气对材料基体热解炭显微结构的影响。通过偏光显微镜测定材料不同区域CVI热解炭的消光角,分析热解炭显微结构在试样中的分布规律。结果表明: N₂在CVI过程中充当惰性角色,不影响热解炭的显微结构; H₂可以调节热解炭沉积时的微环境气氛,更有利于得到粗糙层结构热解炭。试样增密延长炭源气滞留时间,引起带状结构热解炭中显微结构的突变。借助氢气作为载气,可以实现对C/C复合材料中热解炭显微结构的更优控制。      

薄膜沸腾CVI双热源加热法制备C/C复合材料及其致密化特性

采用薄膜沸腾CVI以双热源加热的方法在900~1200℃下热解二甲苯前驱体增密二维针刺炭毡预制体,30~35h内制备出密度1.70g/cm3~1.73 g/cm3的C/C复合材料。研究致密化过程中热解炭基体的沉积速率变化规律,应用排水法和偏光显微镜分别测试材料的密度及热解炭层的厚度。结果表明,当沉积温度由900~1000℃升高至1100~1200℃时,沉积前沿的厚度拓宽,热解炭的初始沉积速率增大,但高沉积温度下预制体边缘将优先完成致密化,导致材料的平均密度由1.72~1.73g/cm3降低至1.70,致密化均匀性变差,材料轴向和径向方向的密度偏差高于0.04g/cm3。上热源开多个轴向通孔可使沉积前沿的厚度减小,前驱体在预制体内的传输效率提高,进而改善较高沉积温度下材料的致密化效果。     

基于数值模拟的热梯度CVI制备C/C 复合材料致密化行为

根据热梯度化学气相渗透(CVI)工艺制备C/C复合材料的特点, 建立了均相与非均相反应的多场耦合数学模型。以2D炭毡为预制体, 天然气为前驱体, 炉压为100 kPa的工艺条件下, 通过计算获得了预制体致密化过程中密度的演变规律; 分析了沉积温度及气体流量对致密化的影响, 获得了合理的沉积温度和气体流量范围。致密化100 h后, 预制体整体密度的计算值与实验值基本一致, 径向密度分布的模拟值与实验值呈相同的变化规律, 验证了模型的可靠性和模拟的预测能力。      

CLVD法制备炭毡/炭复合材料

用化学液气相沉积(CLVD)法制务炭毡／炭复合材料，用电镜扫描（SEM）观察了材料的微观结构，并分析了预制件内部温度梯度的建立过程，结果表明：该法制备的炭毡／炭复合材料致密均匀、结构理想，其预制体内部温度梯度的大小与前驱体的沸点有关。     

微波辅助化学气相渗透法快速制备C／C复合材料

以炭毡为预制体,甲烷为炭源前驱体,沉积温度为1000℃～1150℃的工艺条件下,从温度梯度,密度梯度和沉积动力学方面,研究了制备炭/炭复合材料的微波热解CVI工艺特点,分析了微波热解CVI工艺的沉积机理.结果表明:采用微波热解CVI工艺可制备出体积密度为1.84g·cm-3的炭/炭复合材料,平均致密化速率达0.063g·cm-3·h-1.温度梯度的存在,使预制体实现了从内至外逐步沉积;微波的引入,增加了纤维表面的有效活性点,提高了表面反应速率;微波对化学反应具有一定的催化作用.     

Kinetics of chemical vapor infiltration of carbon nanofiber-reinforced carbon/carbon composites

Preforms containing 0, 5, 10, 15 and 20 wt.% carbon nanofibers (CNFs) were fabricated by spreading layers of carbon cloth, and infiltrated by using the technique of isothermal chemical vapor infiltration (ICVI) at the temperature of 1100 °C under the total pressure of 1 kPa and with the flow of the mixture of propane/nitrogen in a ratio of 13:1. The infiltration rates increased with the rising of CNF content, and after 580 h of infiltration, the achievable degree of pore filling was the highest when the CNF content was 5 wt.%, but the composite could not be densified efficiently as the CNF content ranged from 10 to 20 wt.%. An analysis of the results, based on the effective diffusion coefficient and on the in-pore deposition rates, shows that the CNFs, due to their higher aspect ratio, accelerate overgrowth at pore entrances and thus lead to incomplete pore filling.     

Fabrication of SiCf/SiC composites by chemical vapor infiltration and vapor silicon infiltration

Three-dimensional (3D) silicon carbide fiber reinforced silicon carbide matrix (SiC_f/SiC) composites, employing KD-1 SiC fibers (from National University of Defense Technology, China) as reinforcements, were fabricated by a combining chemical vapor infiltration (CVI) and vapor silicon infiltration (VSI) process. The microstructure and properties of the as prepared SiC_f/SiC composites were studied. The results show that the density and open porosity of the as prepared SiC_f/SiC composites are 2.1 g/cm³ and 7.7%, respectively. The SiC fibers are not severely damaged during the VSI process. And the SiC fibers adhere to the matrix with a weak interface, therefore the SiC_f/SiC composites exhibit non-catastrophic failure behavior with the flexural strength of 270 MPa, fracture toughness of 11.4 MPa·m^1/2 and shear strength of 25.7 MPa at ambient conditions. Moreover, the flexural strength decreases sharply at the temperature higher than 1200 °C. In addition, the thermal conductivity is 10.6 W/mk at room temperature.     

Kinetics of thermal gradient chemical vapor infiltration of large-size carbon/carbon composites with vaporized kerosene

Large-size samples of carbon/carbon composites were prepared using thermal gradient chemical vapor infiltration with kerosene precursor at 950, 1020, 1100, 1180 and 1250 °C. The temperature gradient, kinetics and density distribution of these samples were studied and the microstructure of pyrolytic carbon was examined by polarized light microscopy. The results show that the initial infiltration rate increased from 5.81 to 21.32 g min^?1 by increasing deposition temperature from 950 to 1250 °C. The densification kinetics relied on deposition temperature and competition between reaction and diffusion, and the diffusion mechanism transformed from bulk to Knudsen diffusion regime. The calculated apparent activation energy is about 68.2 kJ mol^?1. The temperature range 1020–1100 °C is appropriate for fabricating composites with high final bulk density due to high degree of pore filling and the density of sample S3 infiltrated at 1100 °C is the highest among all investigated samples.     

Mechanical and tribological properties of 2-D carbon/carbon composites densified through pulse chemical vapor infiltration

This paper presents the mechanical and tribological properties of 2-D carbon/carbon composites (C/C) fabricated by the Pulse Chemical Vapor Infiltration (PCVI) process. In the PCVI process, various fabrication temperatures and different reactant pressures were adopted to investigate the influence of processing condition on physical properties, microstructure and mechanical properties. In the densification process by PCVI, holding time and pulse number are two parameters which significantly affect physical properties (such as density, porosity, and weight gain) and mechanical properties (such as interlaminar strength and wear properties). It is found that the wear properties of carbon/carbon composites can be improved obviously after the densification by 1000 pulses. Effects of initial open porosity on density of the fabricated composites are also studied. In this work, tribological performance of the specimens fabricated through the Isothermal Chemical Vapor Infiltration (ICVI) process and the PCVI process under different pulse cycles are compared. Scanning Electronic Microscopy (SEM) was used to examine the morphology of worn surface. The relationship between the tribological performance and surface morphology was studied.     

催化化学气相渗透法制备C/C复合材料

为了提高基体炭的沉积效率和抗氧化性能，采用常规化学气相渗透法催化制备C／C复合材料。研究了Ni／ZSM-5负载型催化剂对热解炭的致密化速率和性能的影响，并借助偏光显微镜（PLM）、扫描电子显微镜（SEM）和X射线衍射法（XRD）表征了热解炭微观结构。结果表明：在添加Ni／ZSM-5的炭布预制体中，热解炭有较快的沉积速率，其微观结构和抗氧化性能均与催化剂有关。     

Effect of interface structures on the fracture behavior of two-dimensional carbon/carbon composites by isothermal chemical vapor infiltration

The interface structures and fracture behavior of the two-dimensional carbon/carbon composites by isothermal vapor infiltration have been investigated. The results show that the graphene layers exhibit long-range order in high/textured pyrocarbon matrix and are curved in about 5-nm interface region of the fiber/high-textured. Some globular nanoparticles are formed on the fiber surface and the high-textured layer about 10 nm exists in the interface of the fiber/low-textured. The graphene layers stacks are scrolled and folded in the medium-textured and they are waved together in the interface of the fiber/medium-textured. The pseudo-plastic fracture behavior of the two-dimensional carbon/carbon composites is resulted from the dominant high-textured matrix and a moderate interfacial bonding force. A strong adhesion of the fiber/low-textured and the thicker fiber increased by surrounding low-textured layer result in the increasing flexural strength. The single medium-textured and a very strong bonding force of the fiber/medium-textured lead to the brittle fracture behavior.     

脉冲FCVI制备炭/炭复合材料的微观结构及力学性能

采用脉冲强制流动热梯度化学气相渗透（IFCVI）法制备了毡基炭／炭复合材料。借助偏光显微镜及扫描电子显微镜观察了基体热解炭的微观组织结构及断口形貌特征；用弯曲实验测定了材料的力学性能。结果表明：采用脉冲FCVI，经1000℃～1250℃，100h致密化，2300℃热处理后，炭／炭复合材料的密度可达1．7g／cm^3，弯曲强度为125．4MPa，挠度为0．61mm。该工艺致密化速率快，所制备材料的密度分布均匀、力学性能好。研究表明，温度是影响材料组织结构的主要因素，高温条件下有利于粗糙层热解炭组织的生成，而低温有利于光滑层热解炭组织的生成，一般因沉积环境复杂多变，常得到混合型组织。     

丙烯CVI致密C/C复合材料的模拟研究

通过模拟法对丙烯化学气相渗透(cvI)制备炭/炭复合材料(C/c)的工艺过程进行研究.基于16种反应物,建立了由34个基元反应组成的均相反应模型.引用双孔隙演化模型、非均相反应模型,对整个丙烯CVI模型进行耦合模拟.模拟结果表明,CVI法C/C复合材料的密度均匀性受沉积气体滞留时间、致密温度和致密时间等因素影响;致密化过程可分为微孔隙快速致密和大孔隙致密两个阶段,丙烯初级热解产物的浓度分布与这两个阶段关系密切;其中大分子气体苯的浓度分布对CVI沉积速度和C/C复合材料密度均匀性的影响明显,当苯浓度分布最高区严重变窄时,表明致密化过程结束,致密表面将出现结壳现象.验证实验与模拟数据吻合较好.     

CVI法制备2D C/C复合材料

采用预制体直接加热模式的CVI工艺在25．5h内制备出24mm厚的2DC／C复合材料，分析了该工艺快速致密的机理，并观察和测试了材料的微观结构和力学性能。结果表明：该工艺能在较短时间内制备出结构均匀、力学性能较好的C／C复合材料，是一种制备C／C复合材料较为理想的工艺。其快速致密机理可认为：自由基磁吸引作用、自由基电沉积作用和自由基脱氢聚合三个方面。