化学气相沉积(CVD)炭/碳复合材料(C/C)研究现状

主要介绍了炭／炭复合材料（C／C）的化学气相沉积（CVD0制备方法及其影响因素，以及热解炭的组织与沉积机理。概括了VCD的基本方法，如等温法、压差法、热梯度法等。分析了温度、气流速率、气体浓度、预制体及孔隙的形状大小状况等对CVD过程的影响。列举了热解炭的组织，包括光滑层、粗糙层、各向同性体碳以及它们的变体，并对其生长特征及性能进行了较详细的描述。综述了热解炭的沉积机理，典型的有分子沉积、固态沉积、液滴沉积机理等。     

炭/炭复合材料化学液气相沉积致密化技术研究

介绍了炭/炭复合材料化学液气相沉积工艺的基本原理.化学液气相沉积工艺与热梯度化学气相沉积工艺在本质上是相同的,不同之处是化学液气相沉积用液态烃作前驱体,提高了原料的利用率,加快热解炭沉积的速率.研究了不同沉积温度对炭/炭复合材料微观组织结构的影响.采用偏光显微镜观察了材料的粗糙层、光滑层热解炭微观组织结构.研究发现用环己烷作前驱体当沉积温度为1 100℃时可以得到粗糙层结构的热解炭.     

化学液气相沉积制备炭/炭复合材料的机理及研究进展

介绍了化学液气相沉积研究开发的背景、工艺设备装置及沉积机理；分析了该工艺的实质；概述了该工艺存在的问题及国内外对该工艺的研究进展。     

化学气相沉积热解炭机理及其化学动力学

综述了化学气相沉积热解炭的沉积机理、化学动力学的发展状况及一些基本理论问题，着重介绍了K.J.Huettinger等提出的沉积模型。     

以环己烷为前驱体制备炭/炭复合材料

以环己烷为前驱体利用化学液气相沉积工艺,采用针刺炭纤维毡为预制体,制备了具有光滑层和粗糙层结构的炭/炭复合材料。利用金相显微镜、高分辨扫描电子显微镜进行了材料的微观组织结构的分析,分析了在不同位置不同织构热解炭的形成机理。同时阐述了化学液气相沉积工艺原理。实验结果表明,通过调整工艺参数,利用化学液气相沉积工艺可以制备具有不同微观组织结构的炭/炭复合材料。     

化学液相气化渗透法制备炭/炭复合材料研究的进展

综合评述了化学液相气化渗透(chemical liquid vapor infiltration, CLVI)法制备炭/炭(C/C)复合材料的研究.概述了CLVI法的工艺特点及其快速致密的原理,并分别从预制体形状、发热体加热方式、前驱体种类等方面介绍了近年来各种改进的CLVI工艺.分析了制备方式、温度以及前驱体种类等对热解炭微观结构的影响.展望了CLVI法制备C/C复合材料的发展趋势.综合分析表明:目前,CLVI法尚不能满足应用于工业化生产的要求,今后将向多试样沉积、低能量消耗、前驱体高利用率等方向发展.     

热解炭的化学气相沉积机理和组织形貌

以石油液化气作为沉积气体，在900～1300℃温度内研究热解炭在炭纤维上的沉积机理及组织形貌。研究发现：热解炭在纤维表面上通过形核、长大，大晶粒吞并小晶粒的过程，形成宏观上均匀平整而微观上呈球状突起的表面形貌。热解炭以层状微晶(主导)和球状微晶(辅助)两种模式同时生长，最终得到层状结构。在微观上热解炭由若干个的炭层组成，每一层的炭层内含有若干小的并与微晶尺寸相当的亚层。热解炭中的微晶排列不规整，层状和球状微晶均可通过位置的移动调整和脱氢来优化结构。     

镍催化制备炭/炭复合材料

利用催化化学气相沉积法制备炭／炭复合材料，研究了反应温度、前驱体气体含量、催化剂含量和时间对所制备的炭／炭复合材料密度的影响，采用扫描电镜观察分析了基体碳的形貌。结果表明，利用催化剂镍可制取密度达1．594g／cm^3的炭／炭复合材料，并有晶须状基体碳生成。在各种工艺参数中，对炭／炭复合材料的密度影响最大的是温度和前驱体气体，其次为催化剂含量，最后是时间。     

以丙烯为碳源气的化学气相沉积模型

以丙烯为碳源气,考察了热解炭在化学气相沉积中的生长过程:包括丙烯裂解后的脱氢、断链、歧化以及加成等复杂的化学过程,还包括裂解产物在气相中通过在自由活化点上(基体表面或聚合芳香烃网平面边缘处)的加成反应以及在基体表面通过物理吸附进行堆积的过程。不同结构热解炭的生成,主要取决于丙烯裂解后的中间产物在基准面进行加成反应后的产物。     

Ni/SiO2催化制备炭/炭复合材料研究

利用常规化学气相渗透工艺，在针刺炭布预制体中添加3．5％，4．O％Ni／SiO2负载型金属催化剂，以丙烯作碳源气体，在750-900℃下，经过100h的沉积，炭／炭（C／C）复合材料的密度达到1．65g／cm^3，其催化沉积炭的速率比不舍催化剂时提高了3倍以上。该材料经高温处理后，氧化失重率低，氧化起始温度高。应用扫描电镜（SEM），X射线衍射分析（XRD）和光学显微镜观察了基体炭的形貌，分析了催化沉积炭和抗氧化机理。实验结果证明，用该催化化学气相渗透法制备C／C复合材料，周期短，成本低，抗氧化性能好。     

A double layer nanostructure SiC coating for anti-oxidation protection of carbon/carbon composites prepared by chemical vapor reaction and chemical vapor deposition

A double layer nanostructure SiC coating was prepared by chemical vapor reaction and chemical vapor deposition to protect carbon/carbon composites from oxidation. The obtained dense coating reveals a typical crystalline structure and combines well with the substrate. The outer layer of the coating consists of SiC nanocrystals and nanowires, whereas the inner layer is mainly composed of SiC microcrystals, nanocrystals and nanowires. The oxidation and cyclic thermal shock test performed at 1400 °C in air demonstrates that the prepared dense nanostructure coating has excellent anti-oxidation behavior and thermal shock resistance at high temperature. After 400 h oxidation and 34 cycles of thermal shock from 1400 °C to room temperature, the weight loss of the coated sample is only 1.67%. In the oxidation process, the amorphous silica formed at the beginning of the oxidation crystallizes to cristobalite as oxidation time increased. The formation of cristobalite resulted in micro-cracks formed along grain boundaries in the cyclic thermal shock test. As only cracks are formed on the coating surface, it can be concluded that the formation of the penetration cracks may be the reason for the weight loss of the SiC coated composite.     

化学气相沉积法制备碳纳米管的研究进展

从催化剂、碳源气体及反应器的选择等方面综述了化学气相沉积法制备碳纳米管的研究进展 ,讨论了碳纳米管的合成机理。指出催化合成碳纳米管的研究难点在于管径的有效调控和大批量生产 ,今后的研究方向应为单层碳纳米管的有效合成     

化学气相沉积致密针刺C/C复合材料的结构及力学性能研究

以T300炭纤维无纬布、网胎为原材料,层叠针刺成型炭纤维预制体,并采用化学气相沉积工艺对预制体进行致密,制成密度为1.55 g/cm3的针刺C/C复合材料。对针刺C/C复合材料的微观结构进行了观察分析,并对材料力学性能进行了测试。结果表明:化学气相沉积致密的针刺C/C复合材料呈现出以层间大量垂直纤维束为节点的类钉板状网状结构,这种特殊结构使材料层间结合更好,材料整个结构更加紧密;针刺C/C复合材料内部纤维被沉积形成的热解炭所包裹,热解炭的织构类型为光滑层(SL)和粗糙层(RL)并存;针刺C/C复合材料的各项力学性能均达到了较高水平,并且高温力学性能比常温力学性能有了很大幅度的提高。     

Carbon/carbon composites prepared by chemical vapour deposition

This paper summarizes the results of the authors' work on composites prepared by chemical vapour deposition (CVD), using propylene as a source of carbon and various substrates (cellulose carbon, natural graphite, different grades of carbon fibres bonded by phenolic or CVD carbon). The equations relating

1. (i) open porosity P with infiltration rate (− dP/dt = kP).

2. (ii) apparent density of the composites d_app with P (d_app = nP + q).

3. (iii) composite properties Y with d_app (Y = ad_app^b) have been confirmed for the systems studied up to the highest composite densities attained. The constants k, n, q, a and b vary with infiltration conditions, nature of the substrate (involving bonding carbon in the skeleton), heat treatment (HT) conditions and geometrical factors.

For a given set of conditions, infiltration rate increases with temperature and the partial pressure of propylene, but decreases with residence time. Contrary to the composite properties, the kinetics of infiltration are not influenced by the nature of the substrate, except for the initial stage before the substrate is coated by the CVD carbon, but depends on the shape and size of open pores. The influence of the nature of the carbon used to bond the fibres may be of particular importance for composite properties, the latter being also influenced by the state of the surface of the fibres. Among the geometrical factors, the studied effect of fibre content has been found to strongly influence the infiltration rate, in agreement with predictions, as well as the composite properties. The influence of HT on composite properties can be explained by the resulting structural changes in the substrate and the matrix and by the effect of stress relaxation.

The chemical vapor deposition of carbon on carbon fibers

The relations between chemical vapor deposition (CVD) parameters and the resultant pyrolytic carbon microstructures have been examined for matrix deposition in fibrous carbon substrates. The parameters considered are temperature (1200–1450°C), pressure (20–630 Torr), C/H ratio ($\text{1}\text{4}\text{–}\text{1}\text{14}$), total flow rate (2–16) 1/min), and carbon felt density (0·12–0·23 g/cm³). Most of the data obtained are in agreement with a CVD model for carbon; where agreement is not obtained, it is surmised that the assumptions of the model may not be satisfied.     

Growth of nanodiamond/carbon-nanotube composites with hot filament chemical vapor deposition

The growth of diamond/carbon-nanotube (CNT) composites by Hot Filament Assisted Chemical Vapor Deposition (HFCVD) was investigated. The growth was achieved by pre-dispersing commercially available multiwalled CNTs onto a Si(100) surface and subsequently growing diamond nanoparticles over this layer. The diamond/CNT composites were characterized with SEM, TEM, and Raman Spectroscopy. It was found that in a flow of 1% CH₄ in H₂ (a typical condition for microcrystalline diamond growth using HFCVD) most of the CNTs are destroyed by the harsh growth conditions. Reduction in the etching of the CNTs was achieved by reducing the H₂ partial pressure in the precursor flow. There exists an optimal between 2–5% of CH₄ in H₂ wherein the CNTs are not destroyed and the resulting diamond film retains a high percentage of its sp³ structure. The TEM analyses showed that nanometer sized diamond particles nucleate on the surface of the CNTs and grow radially outward. The retention of the CNT structure and the direct growth of the diamond on the CNTs, the two factors necessary for good load transfer between a matrix and reinforcement, suggest the possibility of using this material as a structural composite. Based on the characterization of the composite, a growth mechanism for diamond onto the CNTs was proposed.     

The chemical vapor infiltration of exfoliated graphite to produce carbon/carbon composites

Chemical vapor infiltration was used for the production of carbon/carbon composites based on exfoliated graphite and pyrolytic carbon. Two different exfoliated graphites compacted to densities of 0.05–0.4 g/cm³ were used as a preform. The influence of the synthesis conditions (temperature, pressure, time etc.) on the degree of infiltration, the pyrolytic carbon morphology and the C/C composite characteristics was examined using Raman spectroscopy, scanning electron microscopy and low-temperature nitrogen adsorption.     

Plasma-enhanced chemical vapor deposition carbon nanotubes for ethanol gas sensors

Carbon nanotubes (CNTs) have been fabricated by microwave plasma-enhanced chemical vapor deposition for detecting the presence of ethanol vapor. The conductance of the CNTs decreases when the sensors are successively exposed to ethanol vapor at room temperature. The surface of the CNTs was modified in oxygen plasma to elevate the detection sensitivity for ethanol. Successful utilization of CNTs in gas sensors may open a new window for the development of novel nanostructure gas devices.