Effects of carrier gas on densification of porous carbon-carbon composites during chemical vapor infiltration

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Ablation properties of C/C composites with various needled preforms prepared by isothermal chemical vapor infiltration

The ablation properties of C/C composites with four different needled preforms prepared by isothermal chemical vapor infiltration (ICVI),which are super-thin mat lay-up,0°/90° weftless fabric lay-up,0°/45° weftless fabric lay-up and 0°/45° twill fabric lay-up,were quantitatively evaluated by performing the ablation tests with an engine torch.And their ablation discrepancies were analyzed according to the surface characteristic,porosity and thermal diffusivity.The results show that the 0°/45° weftless composite has a flat eroded surface with no obvious macroscopic pits.Its thickness and mass erosion rates are decreased by about 46.8% and 34.8%,25.0% and 27.5%,and 17.5% and 19.4% compared with those of the mat,the 0°/90° weftless and the 0°/45° twill composites,respectively.The ablation properties are mainly controlled by the thermo-chemical effect (oxidation),and a little by the thermo-mechanical effect (mechanical denudation).The needling fiber bundles play an important role in accelerating the ablation process and resulting in the heterogeneous ablation.     

Electrical resistance and microstructure of latex modified carbon fiber reinforced cement composites

The electrical resistance,flexural strength,and microstructure of carbon fiber reinforced cement composites(CFRC) were improved greatly by adding water-redispersible latex powder.The electrical resistance of CFRC was investigated by two-probe method.The input range of CFRC based strain sensors was therefore increased,whereas electrical resistance was increased and remained in the perfect range of CFRC sensors.The analysis of scanning electron microscopy indicated that elastic latex bridges and a latex layer existed among the interspaces of the adjacent cement hydration products which were responsible for the enhancement of the flexural strength and electrical resistance.The formation mechanism of the elastic latex bridges was also discussed in detail.The continuous moving of two opposite interfaces of the latex solution-air along the interspaces of the adjacent hydrated crystals or colloids was attributed to the formation of the elastic latex bridges.     

Numerical simulation of isothermal chemical vapor infiltration process in fabrication of carbon-carbon composites by finite element method

The chemical vapor infiltration process in fabrication of carbon-carbon composites is highly inefficient and requires long processing time. These limitations add considerably to the cost of fabrication and restrict the application of this material. Efforts have been made to study the CVI process in fabrication of carbon-carbon composites by computer simulation and predict the process parameters, density, porosity, etc. According to the characteristics of CVI process, the basic principle of FEM and mass transport, the finite element model has been established. Incremental finite element equations and the elemental stiffness matrices have been derived for the first time. The finite element program developed by the authors has been used to simulate the ICVI process in fabrication of carbon-carbon composites. Computer color display of simulated results can express the densification and distributions of density and porosity in preform clearly. The influence of process parameters on the densification of prefo     

Effect of nanofibers at surface of carbon fibers on microstructure of carbon/carbon composites during chemical vapor infiltration

Before densification by chemical vapor infiltration,carbon or SiC nanofibers were grown on the surface of carbon fibers by catalytic chemical vapor deposition using electroplated Ni as catalyst.The modification and mechanism of nanofibers on the pyrocarbon deposition during chemical vapor infiltration were investigated.The results show that the nanofibers improve the surface activity of the carbon fibers and become active nucleation centers during chemical vapor infiltration.They can induce the ordered deposition of pyrocarbon and adjust the interface bonding between pyrocarbon and carbon fibers during the infiltration.     

Synthesis of aligned carbon nanotubes by thermal chemical vapor deposition

Single crystal silicon was found to be very beneficial to the growth of aligned carbon nanotubes by chemical vapor deposition with C₂H₂ as carbon source. A thin film of Ni served as catalyst was deposited on the Si substrate by the K575X Peltier Cooled High Resolution Sputter Coater before growth. The growth properties of carbon nanotubes were studied as a function of the Ni catalyst layer thickness. The diameter, growth rate and areal density of the carbon nanotubes were controlled by the initial thickness of the catalyst layer. Steric hindrance between nanotubes forces them to grow in well-aligned manner at an initial stage of growth. Transmission electron microscope analysis revealed that nanotubes grew by a tip growth mechanism. Funded by the National Natural Science Foundation of China (No. 50435030)     

The growth of carbon nanotube with chemical vapor deposition under different process parameters

Whether the active catalytic species are in a liquid,solid phase,surface premelting or surface processes during CNT or other nanowire growth are controversial.In order to explore the mechanism for catalytically grown carbon nanotube (CNT),the mechanism for CNT grown under different temperatures was proposed tentatively.With ethanol chemical vapor deposition (CVD),carbon n.anotubes (CNTs) were synthesized controllably on Si substrates using cobalt (Co) as a catalyst.The effects of the Co particle size,growth...     

Synthesis of carbon nanotubes using Cu-Cr-O as catalyst by chemical vapor deposition

A novel powder catalyst Cu-Cr-O applied to the synthesis of carbon nanotubes(CNTs) was developed, which was prepared via ammonia precipitation method. Techniques of thermo-gravimetric/ differential scanning calorimeter(TG-DSC), X-ray diffraction(XRD) as well as scanning electron microscopy(SEM) and transmission electron microscopy(TEM) have been employed to characterize the thermal decomposition procedure, crystal phase and micro structural morphologies of the as-synthesized materials, respectively. The results show that carbon nanotubes are successfully synthesized using Cu-Cr-O as catalyst when the precursors are calcined at 400, 500, 600, and 700 ℃. The results indicate that the calcination of the Cu-Cr-O catalyst at 600 ℃ is an effective method to get MWCNT with few nano-tube defects or amorphous carbons.     

Design,preparation, and structure of particle preforms for Si3N4（p）/Si3N4 radome composites prepared using chemical vapor infiltration process

A particle preform was designed and prepared by conglomerating and cold-pressed process, which was condensed by chemical vapor infiltration （CVI） process to fabricate silicon nitride particles reinforced silicon nitride composites. The conglomerations are of almost sphericity after conglomerated. There are large pores among the conglomerations and small pores within themselves in the preform according to the design and the test of pore size distribution. The pore size of the preform is characterized by a double-peak distribution. The pore size distribution is influenced by conglomeration size. Large pores among the conglomerations still exist after infiltrated Si3N4 matrix. The conglomerations, however, are very compact. The CVI Si3N4 looks like cauliflowershaped structure. 2008 University of Science and Technology Beijing. All rights reserved.     

炭黑/水性聚氨酯复合材料的制备与气敏性能

以环境友好的水性聚氨酯（WPU）为基体,以炭黑（CB）为导电填料,采用乳液共混法制备了低逾渗值的炭黑/水性聚氨酯（CB/WPU）导电复合材料,研究其在不同有机蒸汽环境中的气敏响应性能.研究发现,CB/WPU复合材料对有机溶剂蒸汽的气敏响应速率快,当复合材料置于饱和有机溶剂蒸汽环境时,电阻迅速增大,60 s左右电阻即可达到最大值,然后趋于稳定值;当复合材料转移到干燥空气中时,电阻急剧下降,直到回复初始电阻值.另外,CB/WPU复合材料具有广谱的气敏响应特征,表现为对极性和非极性有机溶剂蒸汽均呈现较大的气敏响应度.从而表明,CB/WPU复合材料是一种气敏响应度大、响应速度快、具有广谱气敏响应性能等综合性能优异的气敏导电复合材料.     

Design,preparation,and structure of particle preforms for Si_3N_(4(P))/Si_3N_4 radome composites prepared using chemical vapor infiltration process

A particle preform was designed and prepared by conglomerating and cold-pressed process,which was condensed by chemical vapor infiltration(CVI)process to fabricate silicon nitride particles reinforced silicon nitride composites.The conglomera- tions are of almost sphericity after conglomerated.There are large pores among the conglomerations and small pores within them- selves in the preform according to the design and the test of pore size distribution.The pore size of the preform is characterized by a double-peak distribution.The pore size distribution is influenced by conglomeration size.Large pores among the conglomerations still exist after infiltrated Si_3N_4 matrix.The conglomerations,however,are very compact.The CVI Si_3N_4 looks like cauliflower- shaped structure.     

Processing and properties of 2D SiC/SiC composites by precursor infiltration and pyrolysis

Two-dimensional plain-weave silicon carbide fiber fabric reinforced silicon carbide (2D-SiC/SiC) composites were molded by stacking method and densified through precursor infiltration and pyrolysis (PIP) process. SiC coating was deposited as the fiber/matrix interphase layer by chemical vapor deposition (CVD) technique. Fiber/matrix debonding and relatively long fiber pullouts were observed on the fracture surfaces. Additionally, the flexural strength and elastic modulus of the composites with and without fiber/matrix interphase layer were investigated using three-point bending test and single-edge notched beam test. The results show that the fiber fraction and the porosity of 2D-SiC/SiC composites with and without coating are 27.2% (volume fraction) and 11.1%, and 40.7% (volume fraction) and 7.5%, respectively. And the flexural strength and elastic modulus of 2D-SiC/SiC composites with and without coating are 363.3 MPa and 127.8 GPa, and 180.2 MPa and 97.2 GPa, respectively. With a proper thickness, the coating can effectively adjust the fiber/matrix interface, thus causing a dramatic increase in the mechanical properties of the composites. Foundation item: Project(NCET-07-0228) support by the New Century Excellent Talents in University     

Synthesis and oxidation behavior of boron-substituted carbon powders by hot filament chemical vapor deposition

Boron-substituted carbon powder, B _x C_1−x with x up to 0.17, has been successfully synthesized by hot filament chemical vapor deposition. The boron concentration in prepared B _x C_1−x samples can be controlled by varying the relative proportions of methane and diborane. X-ray diffraction, transmission electron microscopy, and electron energy loss spectrum confirm the successful synthesis of an amorphous BC₅ compound, which consists of 10–20 nm particles with disk-like morphology. Thermogravimetry measurement shows that BC₅ compound starts to oxidize approximately at 620°C and has a higher oxidation resistance than carbon. Supported by the National Natural Science Foundation of China (Grant Nos. 10474083, 50472051, 50532020, 50672081) and the National Basic Research Program of China (Grant No. 2005CB724400)     

Improved oxidation resistance of chemical vapor reaction SiC coating modified with silica for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation, a SiC coating modified with SiO2 was prepared by a complex technology. The inner SiC coating with thickness varying from 150 to 300 μm was initially coated by chemical vapor reaction (CVR): a simple and cheap technique to prepare the SiC coating via siliconizing the substrate that was exposed to the mixed vapor (Si and SiO2) at high temperatures (1 923?2 273 K). Then the as-prepared coating was processed by a dipping and drying procedure with tetraet...     

Resistance Responses of Carbon Fiber Cement to Cycled Compressive Stresses

1Introduction Thestudyofthecement basedmaterialswithcarbon fibershaslastedmorethanonedecade.Carbonfiberisa high performanceandmulti functionalreinforcementma terial.Itpossessesaseriesofexcellentcharacteristics,suchaslowthermalexpansion,highelectricalcondu…     

碳纳米纤维／镍管复合材料的制备

采用化学镀法在化学纤维布表面覆盖均匀镍层,通过热处理去除基体后获得中空镍纤维管;将其置于化学气相沉积装置中,通过调整合适的氢气和甲烷流量比及气压条件制备了以中空微米镍纤维管为主体结构、碳纳米纤维（CNF）以及金属管体结构为存储介质的碳纳米纤维／镍管复合纤维材料．运用扫描电子显微镜（SEM）分析中空镍纤维及复合纤维管表面形貌,x射线衍射（XRD）对复合纤维管晶相组成进行表征．结果表明,利用模版法制备出的中空镍纤维管孔径在10μm左右,管壁厚约0．5μm;化学气相沉积制备过程中,当微波功率500W,氢气和甲烷流量比100：6,气压4．0kPa,沉积时间5min时,复合纤维管外壁和端口内壁均匀沉积长径比较大且直径均匀分布的碳纳米纤维,碳纤维直径约50nm;复合纤维成分为碳纳米纤维、镍和三镍化磷合金相,其中碳纳米纤维表现为石墨相．表面覆盖有碳纳米纤维的镍管复合材料,增加了材料自身的吸附存储和导电性能,可应用于多相催化、电容存储和电极材料等领域．     

二氧化碳蒸汽压方程的比较与改进

对二氧化碳几种常用的饱和蒸汽压方程的精度进行了比较,认为文献中作为对比态方程的热力学参数对比变换的变量不具有足够的完备性,而导致在远离临界点的近三相点处精度不足.文中采用完备的参量对比变换形式,并具体给出了二氧化碳蒸汽压的完备对比态变量的拟合方程.经检验,该方程的计算值与实验数据之间的误差在±0.5%以内,标准偏差为9.40852×10-4,具有足够的工程计算精度.     

Fabrication of micro carbon pillar by laser-induced chemical vapor deposition

Argon ion laser was used as the induced light source and ethane(C2H4) was selected as the precursor gas,in the variety ranges of laser power from 0.5 W to 4.5 W and the pressure of the precursor gas from 225×133.3 Pa to 680×133.3 Pa,the experiments of laser induced chemical vapor deposition were proceeded for fabrication of micro carbon pillar.In the experiments,the influences of power of laser and pressure of work gas on the diameter and length of micro carbon pillar were investigated,the variety on averaged growth rate of carbon pillar with the laser irradiation time and moving speed of focus was discussed.Based on experiment data,the micro carbon pillar with an aspect ratio of over 500 was built through the method of moving the focus.     

Effect of Post-spinning Modification on the PAN Precursors and Resulting Carbon Fibers

The impregnation of a special grade PAN precursor,fibers wus carried out in a 8 wt% KMnO4 aqueous solution to obtain modified PAN precursor fibers. The effects of modification on the chemical stncture and the mechanical properties of precursor fibers thermally stabilized and their resulting carbon fibers u＇ere characterized by the combiination use of densities, wide-angle X-ray diffraction （WAXD）, X-ray photoelectron speetroscopy （XPS）, elemental analysis （ EA ）, Fourier traasform infrared spectroscopy （FT-IR） and scanning electron microscope （SEM）, etc.KMnO4 as a strong oxidizer can swell, oxidize and corrode the skin of a precursor.fiber, and transform C≡N groups to C≡N ones, meamchile , it can decreuse the crystal .size increuse the orientation index and the costallinity index, furthermore it can increuse the densities of modified PAN precursors and resuhing thermally stabilized fibers. As a result, the carbon fibers developed from modified PAN fibers show an improvement in tensile strength of 31.25 % and an improvement in elongation of 77.78 % , but a decrease of 16. 52% in Young＇s modulus.     

多向编织炭/炭的复合材料烧蚀性能

利用以星型交流电弧加热器为核心的地面模拟系统对三维5D编织炭/炭复合材料的烧蚀行为进行了考核,通过对材料烧蚀表面温度、形貌的在线实时监测及烧蚀后微观结构的观察,研究了该类材料的烧蚀行为.研究发现:三维5D编织炭/炭复合材料的烧蚀是热化学烧蚀和机械剥蚀的综合作用,构件边缘区域以机械剥蚀为主,中心区域表现为热化学烧蚀和机械剥蚀相互耦合,没有明确的分界;基体炭与炭纤维的抗烧蚀性能相差较大,炭纤维的抗热化学烧蚀性能、抗机械剥蚀性能明显高于基体炭,烧蚀后试件烧蚀表面仅剩下凸起的纤维束骨架结构;三维5D编织炭/炭复合材料的烧蚀性能表现出较强的各向异性,在轴向体现为单根纤维失去周围基体支持而发生剥离,抗烧蚀性能相比较好;在径向体现为烧蚀表面脱层,抗烧蚀性能相比较差.