Plasma-enhanced chemical vapor deposition of carbon films using dibromoadamantane

Carbon thin films are prepared from adamantane and dibromoadamantane by using a plasma-enhanced chemical vapor deposition method. Deposition rate for dibromoadamantane is approximately two times higher than that for pure adamantane. Infrared spectra of the films indicate that adamantane units are incorporated in the films, although higher electron temperature results in disorder in the films. The films prepared from dibromoadamantane have higher thermal stability, higher hardness and Young modulus than those from pure adamantane. Permittivity (= 3-4) of the dibromoadamantane films is higher than that (= 2-3) of pure adamantane films, which is regarded as a result of incorporation of bromine atoms and C=C bonds having higher polarizability according to the structural analysis of the films. Possible solution methods are proposed for reducing inclusion of such unfavorable chemical species and chemical bonds.     

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.     

Synthesis of phosphorus doped carbon nanotubes using chemical vapor deposition

Phosphorous-doped carbon nanotubes (PCNTs) was prepared via two-step methodology employing chemical vapor deposition, by using available starting materials and catalyst. First, CNTs was produced from acetylene gas at 750 ºC and then, PCNTs have been prepared with total yield of 44% by recooking of the prepared CNT with Ph₃P at 600 ºC. The product was characterized with FESEM, TEM and EDS analyses, which confirmed its nanotube shape and the presence of phosphorous atom. The high thermal stability of the product was obtained from TGA analysis, showing only 16.5% weight loss up to 890 ºC. The Raman spectrum of the product showed the I_D/I_G ration equal to 0.84. Moreover, the catalytic potency of the product has been examined in ORR electrochemical reaction using CV and LSV diagrams. The results confirmed appropriate catalytic activity and high stability of the product for this process.     

Catalytic chemical vapor deposition of carbon nanotubes using Ni-La-O precursors

Carbon nanotubes (CNT) are synthesized by catalytic chemical vapor deposition with different compositions of Ni-La-O catalyst precursors obtained by citric acid complexometry. Only two compounds: LaNiO₃ (perovskite-type crystal structure, hexagonal system) and La₂NiO₄ (spinel-type crystal structure, orthorhombic system) in the obtained Ni-La-O catalyst precursors have the ability to grow CNT. Moreover, CNT obtained with the two different crystal structure catalyst precursors have different characteristics: different yield, pattern and oxidation resistance performance.     

微波热解CVI法制备C/C复合材料

在传统CVI工艺的基础上,提出了一种新的炭/炭复合材料沉积致密化技术-微波热解CVI工艺.该工艺采用微波炉加热炭毡预制体,预制体自身发热,并通过控制微波场强分布和热传导过程产生温度梯度,加上微波对极性分子的极化作用和对热解反应和表面沉积反应的催化作用,使预制体从中心至表面逐层快速致密.通过考察炭毡预制体经微波加热后的温度场分布和沉积样品的体积密度变化和径向密度分布,观察材料的微观结构,分析了预制体的致密化过程.结果表明:微波热解CVI工艺在1075℃～1150℃的沉积温度下,以甲烷为碳源前驱体,经90 h的热解沉积,成功制备出体积密度为1.70 g/nc3的炭/炭复合材料,平均致密化速率达到0.0189g/(cm3·h);避免了表面结壳现象,热解炭沿着纤维表面层状生长;采用该工艺制备了结构均匀、主要为中等织构的热解炭.     

Nonlinear logistic regression mixture experiment modeling for binary data using dimensionally reduced components

This article motivates, presents, and illustrates an approach using nonlinear logistic regression (NLR) for modeling binary response data from a mixture experiment when the components can be partitioned into groups used to form dimensionally reduced components (DRCs). A DRC is formed from a linear combination of the components in a group having similar roles and/or effects of the same sign, where the linear combinations over all groups are normalized so that the DRC proportions sum to one. Linear combinations of a particular form provide for quantifying the effects of the remaining components in a group relative to a chosen component. This reason, plus dimensional reduction, are the primary motivations for the proposed DRC mixture experiment modeling approach. NLR is required because models expressed in terms of the DRCs are nonlinear in the parameters that specify the linear combinations. A method for obtaining nonparametric tolerance limits on the probability of a “success” for the binary response variable using a bootstrap approach is also presented. Finally, the article shows how DRCs provide for visualizing data and modeling results that otherwise would be impossible. A real database on the presence or absence of nepheline crystals in simulated nuclear waste glass is used to illustrate the NLR modeling and nonparametric tolerance limit approaches. The methodology is general and can be applied to other applications.     

Growth and characterization of carbon nanofibers by a technique of polymer doped catalyst and hot-filament chemical vapor deposition

Carbon nanostructures have been prepared from the catalytic conversion of polyethylene glycol using a rapid immersion in hot-filament system fed with ethanol, hydrogen and argon. Fiber structures of external diameter about 30 nm have been observed by field emission scanning electron microscopy (FESEM). Raman measurements indicate high degree of C-C sp² ordering which suggests that the samples correspond to CNTs of good tube crystallinity. The samples presented remarkable field emission properties. Lowest threshold field achieved for electron emission was 1.0 V/μm.     

Structure control of carbon nanotubes using radio-frequency plasma enhanced chemical vapor deposition

Carbon nanotube structures such as tube diameter, growth site, and formation density are controlled using radio-frequency (RF, 13.56 MHz) plasma enhanced chemical vapor deposition (RF-PECVD) method. We have produced uniformly well-aligned multi-walled carbon nanotubes (MWNTs) grown over the large scale area and linearly arrayed MWNTs grown in a selected area without any highly-sophisticated patterning process. In our RF-PECVD experiment, furthermore, individually grown single-walled carbon nanotubes (SWNTs) or their thin bundles are synthesized for the first time within the scope of the PECVD methods. These results indicate that PECVD method provides the high potential for the further development of nano-technology.     

Synthesis of sulfur-doped carbon nanotubes from sulfur powder using chemical vapor deposition

Sulfur-doped carbon nanotubes (SCNT) with high sulfur contents have been prepared using chemical vapor deposition (CVD) from sulfur powder, acetylene gas and Fe/CaCO₃ (as catalyst). In this regard, various growth's temperatures were examined to investigate its effects on the structure and the sulfur content of prepared SCNT. The best condition was obtained at 700°C and using 4 g of sulfur powder. The product was characterized using FE-SEM and EDS, which showed rode-like SCNT with about 40 nm diameter and 10.75% sulfur content. Moreover, TEM, Raman and XPS analyses were employed to obtain more details related to the product. The XPS results confirmed the presence of sulfur atoms, which covalently incorporated in the carbon framework. Finally, the catalytic ability of the product in oxygen evolution reaction was examined and the results showed high stability and low oxygen production rate for this product.     

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.     

Hot-wire chemical vapor deposition of carbon nanotubes

Hot-wire chemical vapor deposition (HWCVD) has been employed for the continuous gas-phase generation of both carbon multi-wall and single-wall nanotube (MWNT and SWNT) materials. Graphitic MWNTs were produced at a very high density at a synthesis temperature of 600 °C. SWNTs were deposited at a much lower density on a glass substrate held at 450 °C. SWNTs are typically observed in large bundles that are stabilized by tube–tube van der Waals’ interactions. However, transmission electron microscopy analyses revealed only the presence of isolated SWNTs in these HWCVD-generated materials.     

Synthesis of binary and triple carbon nanotubes over Ni/Cu/Al2O3 catalyst by chemical vapor deposition

Novel binary and triple carbon nanotubes (CNTs) with one common catalytic particle encapsulated have been synthesized using Ni/Cu/Al₂O₃ catalyst, which was produced by a sol-gel method. But when using Ni/Al₂O₃ as catalyst, a mass of common CNTs, that is, one CNT with one catalytic particle encapsulated, was obtained. The results showed that copper-element doping to the Ni/Al₂O₃ catalyst played a key role in the synthesis of CNTs, signifying a novel approach to modify the Ni/Al₂O₃ catalyst. Based on the transmission electron microscopy observations, a simple growth mechanism was developed to describe the growth of the binary or triple CNTs, which could be well explained by a diffusion segregation process.     

XPS study of carbon nitride films deposited by hot filament chemical vapor deposition using carbon filament

Amorphous carbon nitride, a-CN_x, thin films were deposited by hot filament CVD using a carbon filament with dc negative bias voltage on the substrate. The effects of the negative bias and the filament components on the binding structure of the films are investigated by XPS. The composition ratio of graphite to amorphous carbon in the filaments affects the bonding structure of carbon and nitrogen in the films, although the nitrogen content in the films is almost same as 0.1. The nitrogen content in the films changes from 0.1 to 0.3 as the negative bias changes from 0 to − 300 V.     

Plasma-enhanced chemical vapor deposition of multiwalled carbon nanofibers

Plasma-enhanced chemical vapor deposition is used to grow vertically aligned multiwalled carbon nanofibers (MWNFs). The graphite basal planes in these nanofibers are not parallel as in nanotubes; instead they exhibit a small angle resembling a stacked cone arrangement. A parametric study with varying process parameters such as growth temperature, feedstock composition, and substrate power has been conducted, and these parameters are found to influence the growth rate, diameter, and morphology. The well-aligned MWNFs are suitable for fabricating electrode systems in sensor and device development.     

Controllable synthesis of carbon microcoils/nanocoils by catalysts supported on ceramics using catalyzed chemical vapor deposition process

Carbon microcoils (CMCs) and carbon nanocoils (CNCs) were prepared by the catalytic pyrolysis of acetylene using Ni-based or Fe-based catalysts supported on molecular sieves by the catalyzed chemical vapor deposition (CCVD) process. The growth pattern, morphology and structure of the CMCs and CNCs (to be called by a joint name ‘carbon coils’) were examined in detail. By using a ceramic supporter, the anisotropy of the catalysts could be utilized and carbon coils could be more effectively obtained compared to non-supported alloy catalysts. Furthermore, the morphologies of the carbon coils could be controlled. The Raman spectra indicated that the structures of all these carbon coils were nanocrystalline phases in amorphous networks in spite of the different catalysts and preparation conditions.     

Vacuum-UV emitter using low-pressure discharge in helium-water vapor mixture

The spectral characteristics of emission from plasma of periodic-pulsed capacitive discharge in a mixture of water vapor with helium have been studied in a spectral range of 140–315 nm. Dependences of the intensity of the characteristic emission lines of hydroxy groups in He-H₂O plasma on the partial pressure of water vapor at a helium pressure of p(He) = 2.6 kPa are presented. The results provide a basis for the creation of simple sources of vacuum-UV radiation with a cheap working medium based on water vapor.     

催化化学气相沉积过程中硫元素引发的炭形貌衍变

以甲苯为碳源,二茂铁为催化剂,噻吩为生长促进剂,通过化学气相沉积方法得到了多分叉结构的炭.借助SEM、XRD和EDX考察了硫元素对产物的形貌和微观结构的影响.结果表明,当甲苯中的噻吩体积分数在0.01%～1%变化时,产物形貌由树状逐渐衍变为分支状.树状炭南较长而笔直的多分叉炭纤维构成,而分支状炭则由较短且弯曲的炭纤维构成.XRD结果表明硫元素对产物的微观结构没有明显的影响.     

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.     

Manufacturing polymer/carbon nanotube composite using a novel direct process

A direct process for manufacturing polymer carbon nanotube (CNT)-based composite yarns is reported. The new approach is based on a modified dry spinning method of CNT yarn and gives a high alignment of the CNT bundle structure in yarns. The aligned CNT structure was combined with a polymer resin and, after being stressed through the spinning process, the resin was cured and polymerized, with the CNT structure acting as reinforcement in the composite. Thus the present method obviates the need for special and complex treatments to align and disperse CNTs in a polymer matrix. The new process allows us to produce a polymer/CNT composite with properties that may satisfy various engineering specifications. The structure of the yarn was investigated using scanning electron microscopy coupled with a focused-ion-beam system. The tensile behavior was characterized using a dynamic mechanical analyzer. Fourier transform infrared spectrometry was also used to chemically analyze the presence of polymer on the composites. The process allows development of polymer/CNT-based composites with different mechanical properties suitable for a range of applications by using various resins.