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.     

化学气相沉积炭/炭复合材料研究进展

炭/炭复合材料被成功用于许多领域,主要用作抗烧蚀、热防护和刹车材料,如飞机的刹车盘、导弹的头锥等.化学气相沉积是制备炭/炭复合材料的关键技术,本文阐述了炭/炭复合材料化学气相沉积工艺原理,论述了化学气相沉积热解炭机理的研究现状,介绍了不同种类化学气相沉积工艺的特点,以及化学气相沉积工艺计算机数值模拟技术的研究进展.提出了炭/炭复合材料化学气相沉积技术的研究方向和发展趋势.     

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。     

C/C复合材料高温纯化与致密化技术研究

为实现光伏设备中炭素热场材料的循环使用,对硅化腐蚀后的C/C复合材料采用先纯化、再沉积的工艺路线,研究了不同工艺对C/C复合材料体积密度、抗折强度、物相组成以及微观元素分布的变化情况.结果表明:采用高温还原纯化法可大幅降低C/C复合材料中存在的Si元素,且纯化次数越多,纯化效果越明显.与此同时,在经过气相沉积后,材料密...     

CVD法制备C/C复合材料沉积炉流场数值模拟北大核心

化学气相沉积(CVD)法是目前制备C/C复合材料的首选方法,沉积炉内流场的均匀性直接影响目标产物的质量及产量。本文建立了CVD法制备C/C复合材料的沉积炉内气体流动仿真模型,采用计算流体力学方法对沉积炉内流场进行模拟,并研究主要工艺参数对沉积炉内流场分布的影响规律。结果表明,产品区域气体呈现低速均匀的流动特征,真空度增大会导致沉积炉内气体流速显著增加,但对流场均匀性几乎无影响;采用各路相同的送气方式时沉积炉内流场分布比中心增大的送气方式更均匀;较小进气量可以使沉积炉内流场更均匀。     

C/C-SiC-HfC复合材料的微观结构及力学性能研究北大核心

采用炭纤维和陶瓷粉末混编技术将HfC粉引入到炭纤维预制体中,制备了含有HfC超高温陶瓷粉末的炭纤维预制体;随后采用化学气相渗透(CVI)和先驱体浸渍裂解工艺(PIP)制备了密度高达1.94 g/cm^(3)的C/C-SiC-HfC复合材料,分析了复合材料的微观结构及力学性能。结果表明,制备的复合材料主要由C、SiC及HfC等物相组成,复合材料的平均弯曲强度达到了78.3 MPa,平均压缩强度为127.9 MPa;断裂过程中,断口处出现明显的裂纹扩展、纤维拔出及脱粘现象,从而使得材料呈现出一定的假塑性断裂特征。     

化学气相渗透法制备三维针刺C/SiC复合材料的烧蚀性能

用化学气相渗透法制备了三维针刺碳纤维增强碳化硅陶瓷基复合材料,复合材料的平均密度为2.15 g/cm3,气孔率为16.0%.用氧乙炔焰研究了复合材料的烧蚀性能,用扫描电镜分析了烧蚀表面的形貌,用表面能谱分析了烧蚀产物的成分.复合材料的线烧蚀率和质量烧蚀率分别为0.03mm/s和0.004 7 g/s.在烧蚀中心区,烧蚀最严重,表层只有C纤维骨架,且C纤维呈针状,复合材料的烧蚀以升华和冲刷为主.在烧蚀过渡区,垂直于烧蚀面的C纤维表现出端部锐化、根部细化的特性,平行于烧蚀面的C纤维呈针状,复合材料的烧蚀以氧化和机械剥蚀为主.烧蚀边缘烧蚀不明显,烧蚀产物和SiC基体熔融后覆盖在烧蚀表面,阻碍了复合材料的进一步烧蚀,复合材料的烧蚀以氧化为主.     

高性能C/C复合材料制备工艺研究

对制备C/C复合材料的化学气相渗透工艺进行了系统的实验研究,着重分析了热解碳的沉积过程。研究表明,在化学气相渗透的初始阶段,热解碳主要在碳纤维表面沉积,并与碳纤维之间形成了界面结合;随后,热解碳的沉积继续填充碳纤维预制体内部的气孔。这一过程有助于缓解纤维与陶瓷基体之间的界面应力。研究表明,通过调节热解碳的沉积时间可以得到具有一定密度梯度的C/C复合材料。     

三维针刺C/SiC复合材料无纬布纤维方向对材料力学性能的影响

通过化学气相渗透法结合反应熔体浸渗法制备了三维针刺C/SiC复合材料,采用扫描电子显微镜观察材料的显微结构,并研究了无纬布纤维方向对材料力学性能的影响.结果表明,三维针刺C/SiC复合材料由O°无纬布层、短纤维胎网层、90°无纬布层以及针刺纤维束组成,无纬布层纤维方向对材料性能有显著影响.试样的拉伸强度和弯曲强度随着无纬布纤维方向与试样长度方向的夹角θ(0 ～45°)值的增大而减小,面内剪切强度和冲击韧性随θ角的增大而增大.     

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。     

Electrochemical properties of N-doped hydrogenated amorphous carbon films fabricated by plasma-enhanced chemical vapor deposition methods

Nitrogen-doped hydrogenated amorphous carbon thin films (a-C:N:H, N-doped DLC) were synthesized with microwave-assisted plasma-enhanced chemical vapor deposition widely used for DLC coating such as the inner surface of PET bottles. The electrochemical properties of N-doped DLC surfaces that can be useful in the application as an electrochemical sensor were investigated. N-doped DLC was easily fabricated using the vapor of nitrogen contained hydrocarbon as carbon and nitrogen source. A N/C ratio of resulting N-doped DLC films was 0.08 and atomic ratio of sp³/sp²-bonded carbons was 25/75. The electrical resistivity and optical gap were 0.695 Ω cm and 0.38 eV, respectively. N-doped DLC thin film was found to be an ideal polarizable electrode material with physical stability and chemical inertness. The film has a wide working potential range over 3 V, low double-layer capacitance, and high resistance to electrochemically induced corrosion in strong acid media, which were the same level as those for boron-doped diamond (BDD). The charge transfer rates for the inorganic redox species, Fe^2+/3+ and Fe(CN)₆^4−/3− at N-doped DLC were sufficiently high. The redox reaction of Ce^2+/3+ with standard potential higher than H₂O/O₂ were observed due to the wider potential window. At N-doped DLC, the change of the kinetics of Fe(CN)₆^3−/4− by surface oxidation is different from that at BDD. The rate of Fe(CN)₆^3−/4− was not varied before and after oxidative treatment on N-doped DLC includes sp² carbons, which indicates high durability of the electrochemical activity against surface oxidation.     

Effect of additives on mechanical properties of oxidation-resistant carbon/carbon composite fabricated by rapid CVD method

The optimum composition of additives was obtained by the orthogonal design test method for oxidation-resistant carbon/carbon composites (C/C) fabricated by the rapid CVD method. The effect of additives on mechanical properties was examined. The additives used in this test included silicon carbide, silicon nitride, and metal borides. Additives doped into the matrix of C/C increase not only the initial oxidation temperature from 400 to 657°C (64%), but also its flexural strength from 121 to 254 MPa (110%), and flexural modulus from 25 to 45 GPa (96%). The increase of mechanical properties is considered to be due to the formation of a metallic carbon–boron compound in the microstructure.     

Effects of needle-punched felt structure on the mechanical properties of carbon/carbon composites

The effects of needle-punched felt structure, including mass ratio of non-woven cloth to short-cut fiber web, PAN-based carbon fiber types of non-woven cloth and thickness of unit (one layer of non-woven cloth and short-cut web was named as a unit), on the flexural properties of C/C composites from pressure gradient CVI are discussed. Results show that flexural strength and modulus increase when mass ratio of non-woven cloth to short-cut fiber web changes from 7:3 to 6:4 and that PAN-based carbon fiber types of non-woven cloth strongly influence the flexural properties. The strength of C/C composites is not linear with the strength of non-woven cloth carbon fiber because of the important interface between carbon fiber and matrix carbon. It is suitable to choose T300 or T700 as reinforcing carbon fiber for C/C composites in the present study. An optimum unit number per cm of the needle-punched felts for higher flexural properties exists.     

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.

化学气相沉积工艺制备绳状纳米碳管的研究

用硝酸铁作催化剂,乙炔作碳源气体,高纯氮气作稀释气体,在750℃下化学气相沉积生长了绳状纳米碳管,用高分辨扫描电镜观察了所得绳状纳米碳管的形貌.纳米碳管的直径为100～200nm,长度为10～20 μm.文中还提出了绳状纳米碳管的生长机理.     

碳/碳复合材料力学性能的研究进展

本文综述了碳/碳复合材料力学性能的研究进展,包括碳纤维、基体炭、界面性能、制备工艺及工艺参数等对碳/碳复合材料力学性能的影响。同时简单介绍了当今单向碳/碳复合材料力学性能的表征手段。希望对碳/碳复合材料力学性能的研究及应用提供帮助。     

HfB2 coating on C/C composites prepared by chemical vapor deposition: Thermodynamics and experimental investigation

A thermodynamic calculation on the HfB₂ coating prepared by chemical vapor deposition (CVD) through HfCl₄-BCl₃-H₂-Ar system was performed, together with the relevant verification experiments. The calculation results indicated that HfB₂ coating could be obtained above 900 °C with the ratios of BCl₃/HfCl₄ and H₂/HfCl₄ higher than 1 and 12, respectively. The experimental results demonstrated that the deposition temperature, H₂ and BCl₃ flow rates had significant effects on the grain size, growth rate and phase composition of HfB₂ coatings. A dense and uniform HfB₂ coating was prepared at 1150 °C with a BCl₃/HfCl₄ ratio of 3 and a H₂/HfCl₄ ratio of 20, whose mass and linear ablation rates were 15.61 mg/s and 15.58 μm/s under oxyacetylene flame.     

Chemical vapor deposition and infiltration processes of carbon materials

The chemical vapor deposition (CVD) and the chemical vapor infiltration (CVI) processes of carbon materials are reviewed starting from the historical aspects and including the latest developments in the preparation of C/C composites. Our presentation is based on an analysis of the different types of reactors, of the composite materials with different types of pyrocarbon as matrices and a comparison between the different processes. In particular, the classical isothermal-isobaric technique and temperature or pressure gradient reactors, which lead to a higher deposition efficiency, are compared. A complementary aspect is the structural and physical analysis of the deposited pyrocarbons: they are considered as reproducible metastable phases which are obtained under non-equilibrium thermodynamic conditions. The final relevant point concerns the relationship between the process parameters and the type of pyrocarbon. In particular, the so-called rough laminar microstructure, useful for most composite applications, is described.     

The influence of thermal gradient on pyrocarbon deposition in carbon/carbon composites during the CVI process

A thermal gradient CVI process was investigated. A graphite heater in the center of a carbon felt disk preform was heated by Joule heating to a temperature of 900 °C, the carbon felt had a low thermal conductivity, and the rapid natural gas flow cooled the exterior surface of the preform. The rate constant of the chemical vapor deposition reaction increased exponentially with increasing temperatures with pyrocarbon being formed only in the designated deposition zone. Plugging of surface pores in the preforms, which often occurs in the isothermal CVI technology was unusual in this thermal gradient CVI process. As the deposition process went on, the deposited zone moved progressively towards the outside surface of the preform. The electrical resistance between two electrodes decreased gradually while the power of the thermal gradient CVI furnace increased non-linearly with the progressive densification. The temperature distribution in the thermal gradient furnace changed non-linearly with time and position. The relationship between temperature and position in the deposition zone followed the classical Fourier law. The microstructure of pyrocarbon at different positions was discussed.